Embed Size (px)
Citation preview
This article was downloaded by: ["Queen's University Libraries, Kingston"]On: 19 August 2013, At: 17:07Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Communications in Soil Science and PlantAnalysisPublication details, including instructions for authors and subscriptioninformation:http://www.tandfonline.com/loi/lcss20
Rare Earth Elements in SoilsZhengyi Hu a , Silvia Haneklaus b , Gerd Sparovek c & Ewald Schnug ba State Key Laboratory of Soil and Sustainable Agriculture, Institute of SoilScience, Chinese Academy of Sciences, Nanjing, P. R. Chinab Institute of Plant Nutrition and Soil Science (FAL), Braunschweig, Germanyc University of São Paulo, Padua Dias, BrazilPublished online: 15 Aug 2006.
To cite this article: Zhengyi Hu , Silvia Haneklaus , Gerd Sparovek & Ewald Schnug (2006) RareEarth Elements in Soils, Communications in Soil Science and Plant Analysis, 37:9-10, 1381-1420, DOI:10.1080/00103620600628680
To link to this article: http://dx.doi.org/10.1080/00103620600628680
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and ourlicensors make no representations or warranties whatsoever as to the accuracy, completeness, orsuitability for any purpose of the Content. Any opinions and views expressed in this publicationare the opinions and views of the authors, and are not the views of or endorsed by Taylor &Francis. The accuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liable for anylosses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilitieswhatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access and usecan be found at http://www.tandfonline.com/page/terms-and-conditions
Rare Earth Elements in Soils
Zhengyi Hu
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil
Science, Chinese Academy of Sciences, Nanjing, P. R. China
Silvia Haneklaus
Institute of Plant Nutrition and Soil Science (FAL),
Braunschweig, Germany
Gerd SparovekUniversity of Sao Paulo, Padua Dias, Brazil
Ewald Schnug
Institute of Plant Nutrition and Soil Science (FAL),
Braunschweig, Germany
Abstract: Rare earth elements (REEs) comprise a group of 17 elements with very similar
chemical and physical properties, which include scandium (Sc, Z ¼ 21), yttrium
(Y, Z ¼ 39), and the lanthanides with successive atomic numbers (Z from 57 to 71).
Lanthanides are the elements lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd),
terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium
(Yb), and lutetium (Lu). REEs are required in modern industry, and their use in agriculture
yielded positive effects in terms of crop yield and body weight of poultry. However, the
question of whether the use of REEs in agriculture yields an enrichment of these elements
in the environment remains open. It was the aim of this review to summarize the data
about REEs in soils with view to their content, fractions, availability, chemical
behavior, and translocation in soils and to elucidate further research needs.
Keywords: Chemical behavior, chemical speciation, lanthanides, rare earth elements,
translocation
Received 17 January 2005, Accepted 7 October 2005
Address correspondence to Zhengyi Hu, State Key Laboratory of Soil and Sustain-
able Agriculture, Institute of Soil Science, Chinese Academy of Sciences, No. 71, East
Beijing Road, Nanjing, 210008, P. R. China. E-mail: [email protected]
Communications in Soil Science and Plant Analysis, 37: 1381–1420, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0010-3624 print/1532-2416 online
DOI: 10.1080/00103620600628680
1381
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
INTRODUCTION
Rare earth elements (REEs) comprise a homogenous group of elements in the
periodic system. They include the elements Sc (Z ¼ 21) and Yb (Z ¼ 39) and
15 lanthanides with successive atomic numbers (Z) from 57 to 71 (Alina and
Henryk 1984). Promethium does not occur naturally in the earth’s crust,
whereas all other elements occur in parent materials. REEs can be divided
into two groups: light and heavy REEs. The distinction is based on their
physical and chemical properties and ion radius. The light REEs are La, Ce,
Pr, Nd, Pm, Sm, and Eu, and all other elements belong to the group of heavy
REEs. Actually, the REEs are not at all rare, as they account for 0.015% of
the earth’s crust and thus are as abundant as copper, lead, and zinc and occur
in even higher concentrations than tin, cobalt, silver, and mercury (Wang,
Yu, and Zhao 1989). REEs show similar chemical and physical properties
and represent a geochemically coherent group. REEs occur in nature predomi-
nately in þ3 valence; Ce, however, may be abundant in a stable tetrapositive
state, and Pr and Tb are known to form higher valence oxides. REEs show
an affinity for oxygen and are found in concentrated form in phosphorites as
well as argillaceous sediments (Alina and Henryk 1984).
The determination of REEs in environmental samples has been limited
because of the lack of sensitive analytical techniques, which allow a quantitat-
ive assessment of trace concentrations in various biological materials (Markert
1987, Markert et al. 1989). The toxicity of REEs is generally low (Haley
1979). For example, the toxicity of La in wheat is lower than that of Cu but
somewhat higher than Fe (Wheeler and Power, 1995). The increasing use of
REEs in the technological industry and beneficial effects in agricultural
production (Markert et al. 1989; Hu et al. 2004; He and Rambeck 2000;
Pang and Peng 2002; Buckingham et al. 1999) require a bias-free assessment
of possible implications when applying these elements regularly to the soil–
plant system. Pang and Peng (2002) completed a review about the application
of REEs in Chinese agriculture and their environmental behavior in soils.
The physiological and biochemical effects of rare earth elements on plants
and their agricultural significance were also reviewed (Hu et al. 2004).
Generally, the knowledge about REEs in soils is scarcely available to occiden-
tal scientists, as most reports are only available in the native languages,
Chinese and Russian (Peng and Zhu 2003). It was the aim of this review to
summarize all relevant studies about this subject and to highlight future
research needs in the field of REEs in order to acquire a global understanding
of their significance in agricultural and adjacent ecosystems.
Dispersion of REEs in Soils
REEs in soils mainly originate from parent materials (Liu 1988), but fertiliza-
tion is an important path of entry of REEs into soils. For instance, Australian
Z. Hu et al.1382
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
phosphate fertilizers contain on average 45.2 mg La kg21 and 61.0 mg Ce
kg21 (Meehan et al. 2001). In comparison, the total REE content of superpho-
sphate produced from Kola apatites is about 2.6 g kg21 (Todorovsky,
Minkova, and Bakalova 1997). With an annual application rate of 300 kg
ha21 of P-fertilizers, 30–170 g REEs ha21 will enter the soil (Todorovsky,
Minkova, and Bakalova 1997). A regular use of these fertilizers yielded a
significant increase in the REE content in cultivated soils (Todorovsky,
Minkova, and Bakalova 1997; Volokh et al. 1990).
The application of elemental REEs in agriculture is widely practiced in
China. REEs proved to increase yield and quality of agricultural crops (Hu
et al. 2004; Pang and Peng 2002; Buckingham et al. 1999; Wang and Zheng
2001). By 2001, 6.5 million ha of land in China was treated with REEs in
the form of foliar sprays and seed treatments or as an addition to solid or
liquid root media. In total, 11,000 tons of REEs were applied in the form of
chlorides and nitrates (Wang and Zheng 2001). So far, however, no infor-
mation is available about the long-term impact of REE-enriched soils on
soil life and plant growth and quality.
Phosphogypsum (PG) is an industrial by-product formed during the pro-
duction of phosphate fertilizers and regularly used in agriculture. PG consists
of more than 90% gypsum. Impurities include Al, P, F, Si, Mg and Fe as well
as trace elements, REEs, and certain naturally occurring radio-nuclides
(Ferguson 1988). In PG from Australia, 27.2 mg La kg21 and 39.0 mg Ce
kg21 were found (Meehan et al. 2001). In PG, REEs are enriched in the
fraction with a particle size ,20mm (Arocena, Rutherford, and Dudas 1995).
The application of sewage sludge ashes or incinerator bottom ashes in
particular bear the risk of an accumulation of Sc, Sm, and Eu in soils
(Kawasaki, Kimura, and Arai 1998; Zhang, Yamasaki, and Kimura, 2001).
The atmospheric deposition of REEs is another path of entry into soils. In
the western part of the Netherlands, the total REE concentration in atmos-
pheric particulate matter was 0.22–33.0 ng m23 (Wang et al. 2000). The con-
centration of REEs in air particles with a diameter of ,10mm was 36 ng m23,
and the soluble REE content was 0.69mg L21 in rainwater in the suburb of
Beijing, China (Wang, Wang, et al. 2001). The concentration of the REEs
La, Ce, Nd, Sm, Eu, Tb, Dy, Yt, and Lu in snow was even as high as
1333.7 mg L21 in the area around a phosphorus fertilizer plant in Russia,
and the La and Ce concentration in these samples was 454 and 660 mg L21
(Volokh et al. 1990).
CHEMICAL SPECIATION OF REES IN SOILS
Total REE Content
The total REE content in soils can be determined by employing a spectropho-
tometric method using chlorophosphonazop hippuric acid (l ¼ 675 nm) after
Rare Earth Elements in Soils 1383
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
alkaline digestion with NaOH and Na2O2 (Hu and Liu 1988). Alternatively,
individual REEs are determined by NAA or ICP-MS or ICP-AES in wet
digested soil samples (Wyttenbach et al. 1998; Song and Chen 1987; Wang
et al. 1988). The spectrophotometric method by chlorophosphonazop
hippuric acid was used to determine the REE content in 1225 soil samples
from China. The total REE content in these samples varied between 68 and
629 mg kg21, with a mean concentration of 181 mg kg21 (Table 1). In
1363, soil samples from Brazil, the REE content ranged from 51 to 182 mg
kg21, with a mean concentration of 77 mg kg21 (Table 2). The ranges for
the REE content determined in both countries were similar to those reported
by Bohn, McNeal, and O’Conner (1985) with 30 to 700 mg kg21 in 1985.
Summarizing the results of soil analysis from 398 soil samples taken in
seven countries, the mean total REE concentration was 149.5 mg kg21
(Table 3), thus lower than the average content found in China. Besides the
different origins, the analytical procedures employed may have contributed
to these differences.
The REE content decreases in dependence on the parent material in the
following order: granite . quaternary . basalt . purple sandstone . red
sandstone (Zhu and Liu 1988). Soils that developed from basic igneous
rock, acid igneous rock, sandstone, and shale rock usually have higher REE
contents, ranging from 174 to 219 mg kg21, than soils originating from
loess and calcareous rock, which show lower REE concentrations with 137
to 174 mg kg21 (Table 4). The data in Table 4 also show that red soils
(Ferralic Cambisol) and latosols (Rhodic Ferralsol) originating from granite
tend to have high REE concentrations. In the Ceveiro watershed in Brazil,
the REE content decreased in dependence on soil type in the following
order: Rhodic Paleudalf, Reservoir (sediments) . Typic Udorthent . Mollic
Paleudalf, Typic Eutrochrept, Typic Kandiaqualf, Typic Paleudalf, Typic
Paleudult, Alluvium, Arenic Endoaquult . Psammentic Paleudult, Arenic
Paleudalf . Arenic Paleudult (Table 2). The lowest REE content was in a
coarse soil texture, as Psammentic Paleudult, Arenic Paleudalf, and Arenic
Paleudult are sandy substrates. The land use affected the REE content in the
Brazilian soils, too. The REE content was higher where pasture was grown
than in agricultural (sugarcane, corn) and forest soils (Table 2). These differ-
ences could be due to REE off-take and/or uptake by plants, because trees
may enrich REEs during their life cycle and REEs were removed by harvest
products such as sugarcane and corn.
In Chinese soils, the mean REE content was 174 mg kg21, whereas that in
Germany, Australia, Japan, and Brail varied between 16 and 105 mg kg21
(Tables 2 and 5). Certain geographic differences in the REE content could
be verified for the samples from China. The REE content decreased tenden-
tiously from south to north. In the southern parts, the REE content was
higher than 200 mg kg21, whereas in the northern parts this value was never
exceeded (Table 1). After acid digestion, X-RF analysis of 96 soil samples
taken on the German Isle of Ruegen showed that the mean La, Ce, Pr, Nd,
Z. Hu et al.1384
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 1. Total REE concentration (mg kg21) in Chinese soils
Soils n Min Max Mean
Latosol/rhodic ferralsol� 16 18 343 250
Lateritic red earth/eutric vertisol 31 62 480 215
Dry laterite/calcic vertisol 6 65 286 163
Red earth/ferralic cambisol 101 82 629 231
Yellow soil/haplic acrisol 38 95 357 187
Purplish soil/chromic cambisol 34 126 362 196
Yellow brown earth/haplic luvisol 73 95 307 206
Brown earth/dytric cambisol 75 89 503 186
Dark brown earth/eutric cambisol 35 101 325 191
Cinnamon/haplic luvisol 102 103 371 192
Gray cinnamon/eutric cambisol 5 157 264 192
Black soil/haplic phaeozems 35 155 372 217
Chernozem/haplic chemozems 44 99 359 178
Chestnut soil/haplic kastanozems 38 74 205 149
Meadow soil/haplic phaeozem 58 70 333 179
Albic beached soil/albic luvisol 25 131 247 171
Cultivated loessial soil/calcaric regosol 15 126 210 170
Matured loessial soil/cumulic anthrosols 10 183 250 222
Dark loessial soil/cumulic haplustolls 8 104 186 190
Fluvo-aquic soil/calcaric cambisol 105 102 306 195
Bleached earth/albic luvisol 4 229 275 252
Sierozem/calcaric cambisol 5 143 195 178
Castanozem/haplic calcisols 13 101 195 147
Brown desert soil/calcaric fluvisals 16 68 273 148
Gray desert soil/cumulic—calcaric regosol 4 138 254 173
Gray brown desert soil/cumulic—calcaric regosol 14 95 308 169
Cultivated desert soil/anthrosol 16 78 214 159
Calcareous soil/chromic luvisol 15 91 520 322
Bog soil/umbric gleysols 20 107 270 179
Saline soil/gypsic soloncnaks 32 79 363 173
Alkali soil/calcic kastanozems 3 118 202 164
Wind sand soil/gleyic Arenosol 13 58 322 131
Paddy soil/hydrgric Anthrosols 58 69 409 226
Alp meadow soil/gelic Cambisols 18 94 336 176
Subalp meadow soil/calciudoll 25 111 257 178
Alp plain soil/calcicryids (US Soil Taxonomy, 1994) 23 93 277 188
Sub-alp plain soil/calcicryids
(US Soil Taxonomy, 1994)
17 122 336 188
Cold desert soil/gypsicryids (US Soil Taxonomy, 1994) 2 131 247 189
Alp desert soil/gypsicryids (US Soil Taxonomy, 1994) 9 154 271 198
Total 1225 68 629 181
Note: Data sources: Ran and Liu (1991), Ran and Liu (1994), Zhu and Liu (1988),
China Environmental Monitoring Station (1999), Ran (1991), and Zhu and Liu (1991).�Indicate FAO Soil Taxonomy/Unesco (1988).
Rare Earth Elements in Soils 1385
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 2. Mean REE content (mg kg21) in soils for the Ceveiro watershed (Brazil) separated by soil type, geology, and land use type (unpublished
data from authors)
Soil type Mean (n) Tukey tests Geology Mean (n) Tukey tests Land use type Mean (n) Tukey tests
Rhodic paleudalf 182 (13) �� Serrra geral 109 (115) A Reservoir 145 (44) A
Reservoir (sediments) 145 (38) A Corumbatai 87 (880) B Pasture 107 (123) A
Typic udorthent 115 (281) AB Piramboia 43 (374) C Sugarcane 72 (997) B
Mollic paleudalf 111 (20) B Corn 68 (5) B
Typic eutrochrept 98 (29) B Forest 68 (194) B
Typic kandiaqualf 90 (11) ��
Typic paleudalf 89 (104) B
Typic paleudult 85 (42) B
Alluvium 78 (26) BC
Arenic endoaquult 71 (28) BCD
Typic udipsamment 69 (15) ��
Typic dystrochrept 65 (14) ��
Psammentic paleudult 57 (47) CD
Arenic paleudalf 53 (31) CD
Arenic paleudult 51 (664) D
Total 77 (1363) 77 (1363) 77 (1363)
�Means with the same letter are not significantly different (p , 0.05).��Soil type excluded from mean comparison test for having fewer than 20 samples.
Z.Huet
al.
1386
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
and Sm content was 2.83, 4.62, 0.42, 1.43, and 0.22 mg kg21 (unpublished
data). These results and those of Market and Zhang (1991) reveal that the
REE content in German soils is distinctly lower than in other countries.
Individual REE Content
The results of soil analysis of 482 samples representing different soil types in
China showed a mean La, Ce, Nd, Sm, and Eu content of 41, 74, 7, 28, and
6 mg kg21, respectively. These five light REEs accounted for 90% of the
Table 3. Mean individual REE content (mg kg21) in soils of different origin
Elementsa Meanb nb Mean c Sample nc
La 28.91 902 31.26 398
Ce 59.61 902 65.25 398
Pr 7.28 430 6.68 398
Nd 23.88 902 25.05 398
Sm 5.09 772 4.91 398
Eu 1.01 698 1.05 398
Gd 4.99 438 4.63 398
Tb 0.75 698 0.70 398
Dy 4.23 450 4.00 398
Ho 0.81 400 0.81 398
Er 2.34 408 2.28 398
Tm 0.36 400 0.36 398
Yb 2.19 828 2.19 398
Lu 0.39 696 0.35 398
Y 18.94 503
REE 149.52
LRE 134.20
HRE 15.32
LRE/HRE 8.76
LRE/REE 0.90
aLRE ¼ light rare earth elements, HRE ¼ heavy rare earth elements, and REE ¼ all
rare earth elements.bData sources: Markert (1987), Wang et al. (1996), Ran and Liu (1994), Ichihashi,
Morita, and Tatsukawa (1992), Wyttenbach et al. (1998), Wang et al. (1988), Dudka
(1992), Zimny and Korzeniewska (1996), Mohamad and Rafek (1993), Liu, Tian,
and Li (1995), Gao Zhang, and Wang (1999a), Wang, Wang, and Zhang (1997), Sun
and Wang (1993), and Liu, Lao et al. (1997).cData sources: Zhu and Xing (1992b), Wutscher and Perkins (1993), Diatloff Ashe,
and Smith (1996), Markert and Zhang (1991), Yoshida et al. (1998), Shi et al. (1995),
Lu et al. (1986), Zhou et al. (1989), Liu, Wei, and Ten (1992), Yang et al. (1992), Jiang
et al. (1994), Yang et al. (1999a), Fu et al. (2000), Huang and Gong (2001).
Rare Earth Elements in Soils 1387
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
total REE content (Table 6). In comparison, the La, Ce, Nd, Sm, and Eu
content in the earth crust is about 39, 60, 8.2, 28, and 6 mg kg21, respectively
(Vinogradov 1959). These results reveal that the La and Ce content in Chinese
soils is higher than that in the earth’s crust. Basically, the concentration of
individual REEs depends on the parent material and soil type. Soils derived
from granite-gneiss and quartzite-mica rock tend to contain higher concen-
trations of REEs (Ure and Bacon 1978). On average, calcareous rock soils
(Chromic Luvisol) have the highest concentration of every individual REE.
Paddy soils (Hydrgric Anthrosols) contain the highest concentration of light
REEs (Table 6), and latesol soils (Rhodic Ferralsol) show the narrowest
ratio of light to heavy REEs (Table 6).
Based on physico-chemical properties and biological activation, five to
eight different chemical binding forms of REEs in soils can be distinguished
(Ran and Liu 1991; Zhu and Xing 1992a; Wang et al. 1996; Wang et al. 1998;
Land et al. 1999). Table 7 summarizes sequential extraction procedures used
to determine different binding forms of REEs in soils.
Water-soluble and exchangeable REEs are clearly defined through the
kind of extraction media, which only react with the surface of soil particles,
while the soil structure is not influenced. In comparison, all other forms of
REEs have to be seen as operational definitions, rather than true chemical
forms in the soil, because the extractants are not completely selective in
extracting different binding forms of REEs, so that transitions between
different forms are not rigid. For this reason Wang et al. (1996) attributed
adsorbed and carbonate bound REEs in one group, but a distinction
between REEs that are loosely and tightly bound to organic matter seems
feasible (Wang et al. 1998).
REEs in soils are predominately concentrated in minerals, such as
fluorocarbonates, phosphates, silicates, and oxides. The solubility of REEs
Table 4. Mean total REE content in soils from different
parent materials (Liu 1996)
Parent materials n
Mean content
(mg kg21)
Acid igneous rock 133 196
Neutral igneous rock 8 178
Basic igneous rock 5 216
Loess 70 174
Laterite 23 203
Sediment rock and shale 60 202
Sandstone 80 219
Lime rock 45 137
Purple sandstone 10 190
Sand-shale stone 21 174
Z. Hu et al.1388
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 5. REE content (mg kg21) in soils of selected countries
Country (sample no.)
Elements
Australiaa
(9)
Polandb
(52)
Switzerlandc
(6)
Germanyd
(5)
Swedene
(2)
Japanf
(77)
Malaysiag
(12)
USAh
(30)
Chinai
(279)
15.38 14.68 17.800 3.57 17.70 18.20 30.46 13.62 37.57
Ce 60.49 29.25 36.100 5.92 29.05 39.80 52.75 25.67 77.32
Pr 4.13 — — 0.95 7.22 4.53 — 2.45 7.87
Nd 14.63 11.23 15.000 2.53 13.50 17.60 28.75 9.98 29.31
Sm 2.76 2.03 2.820 0.54 3.05 3.67 4.88 1.4 5.75
Eu 0.64 0.54 0.513 0.12 0.77 0.96 0.96 0.37 1.17
Gd 2.58 — — 0.53 2.58 3.71 21.13 2.82 5.19
Tb 0.43 0.51 0.381 0.13 0.62 0.56 1.3 0.14 0.81
Dy 2.06 — — 0.50 2.57 3.29 4.98 0.7 4.66
Ho 0.21 — — 0.12 0.57 0.68 — 0.16 0.94
Er 0.79 — — 0.22 0.88 1.99 5.53 ,0.003 2.68
Tm 0.08 — — 0.04 0.25 0.29 — ,0.02 0.43
Yb 0.57 1.73 1.470 0.27 1.44 2.00 2.89 ,0.008 2.55
Lu 0.08 0.27 — 0.04 — 0.29 0.88 ,0.01 0.42
(continued )
Rare
Earth
Elem
ents
inSoils
1389
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 5. Continued
Country (sample no.)
Elements
Australiaa
(9)
Polandb
(52)
Switzerlandc
(6)
Germanyd
(5)
Swedene
(2)
Japanf
(77)
Malaysiag
(12)
USAh
(30)
Chinai
(279)
REE 104.83 15.48 97.57 176.67
LRE 98.03 13.63 84.76 158.99
HRE 6.80 1.85 12.81 17.68
LRE/HRE 14.42 7.37 6.62 8.99
LRE/REE 0.94 0.88 0.87 0.90
Note: LRE ¼ light rare earth elements, HRE ¼ heavy rare earth elements, and REE ¼ all rare earth elements.aDiatloff, Asher, and Smith (1996).bZimny and Korzeniewska (1996).cWyttenbach et al. (1998).dMarkert and Zhang (1991).eMarket (1987).fYoshida et al. (1998).gMohamad and Rafek (1993).hWutscher and Perkins (1993).iLu et al. (1986), Zhou et al. (1989), Liu, Wei, and Ten (1992), Yang et al. (1992), Jiang et al. (1994), Yang et al. (1999a), Fu et al. (2000), Huang
and Gong (2001).
Z.Huet
al.
1390
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 6. REE content in selected soils of China (mg kg21)
Soil n La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE LRE HRE
LRE/
HRE
LRE/
REE
Latesol/rhodic
ferralsol
13 30 54 6 20 4 0.58 4.09 0.70 4.12 1.01 3.28 0.38 3.09 0.47 132 115 17.1 6.7 0.87
Lateritic red
earth/eutric
vertisol
30 39 85 7 27 5 0.91 4.56 0.63 4.68 0.95 3.04 0.46 3.23 0.50 182 164 18.1 9.1 0.90
Red soil/
ferralic
cambisol
89 44 84 8 28 5 1.06 4.87 0.71 4.50 0.93 2.92 0.43 2.90 0.44 188 170 17.7 9.6 0.91
Yellow soil/
haplic acrisol
37 37 77 6 25 5 0.83 4.00 0.58 4.09 0.84 2.57 0.36 2.46 0.37 166 151 15.3 9.9 0.91
Purplish soil/
chromic
cambisol
29 45 69 7 28 5 1.12 4.70 0.63 4.00 0.83 2.38 0.34 2.23 0.34 171 155 15.4 10.1 0.91
Calcareous rock
soil/chromic
luvisol
16 55 114 13 48 9 1.88 8.62 1.21 7.68 1.51 4.44 0.61 4.08 0.59 270 242 28.7 8.4 0.89
Cinnamon/
haplic luvisol
45 39 72 8 29 6 1.12 4.90 0.71 4.36 0.92 2.63 0.37 2.41 0.37 172 155 16.7 9.3 0.90
Meadow/
haplic
phaeozem
39 36 63 7 26 5 1.05 4.64 0.58 3.92 0.84 2.41 0.36 2.36 0.37 154 138 15.5 8.9 0.90
Albic beached
soil/albic
luvisol
10 40 68 7 27 5 1.02 4.48 0.65 4.07 0.83 2.44 0.37 1.98 0.35 163 148 15.2 9.8 0.91
(continued )
Rare
Earth
Elem
ents
inSoils
1391
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 6. Continued
Soil n La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE LRE HRE
LRE/
HRE
LRE/
REE
Castanozem/haplic calcisols
13 36 49 5 20 4 0.86 3.67 0.47 3.39 0.72 2.05 0.30 2.40 0.29 128 115 13.3 8.6 0.90
Sierozem/
calcaric
cambisol
5 30 60 6 24 5 0.99 4.47 0.68 4.09 0.82 2.54 0.35 1.97 0.35 141 126 15.3 8.3 0.89
Fluvo-aquic
soil/calcaric
cambisol
51 40 68 6 24 6 1.13 5.00 0.64 4.30 0.89 2.55 0.37 2.21 0.36 161 145 16.3 8.9 0.90
Gray desert soil/
cumulic –
calcaric regosol
4 38 53 6 24 5 0.98 4.88 0.73 4.69 1.09 2.95 0.49 2.35 0.46 145 127 17.6 7.2 0.88
Gray brown desert
soil/Cumulic –
calcaric
Regosol
14 35 57 6 24 5 0.98 4.63 0.72 4.28 0.91 2.64 0.39 3.09 0.38 145 128 17.0 7.5 0.88
Z.Huet
al.
1392
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Brown desert soil/
gypsic
soloncnaks
14 36 52 5 23 4 0.86 4.42 0.62 3.78 0.85 2.33 0.35 2.52 0.34 136 121 15.2 7.9 0.89
Cultivated loessial
soil/Calcaric
Regosol
7 79 60 7 25 5 1 4.31 0.59 3.88 0.80 2.42 0.34 2.29 0.33 192 177 15.0 11.8 0.92
Dark loessial soil/cumulic
haplustolls
7 27 57 6 26 5 1.07 4.14 0.61 4.02 0.76 2.51 0.42 2.21 0.35 137 122 15.0 8.1 0.89
Paddy soil/
hydrgric
anthrosols
59 47 84 9 33 7 1.22 5.69 0.70 5.02 0.99 2.96 0.32 1.83 0.42 199 181 17.9 10.1 0.91
Total 482 41 74 7 28 6 1.07 4.89 0.67 4.44 0.92 2.75 0.38 2.53 0.40 174 157 17.0 9.3 0.90
Notes: Data sources: Ran and Liu (1991, 1994), China Environmental Monitoring Station (1999), Yang et al. (1999a), Fu et al. (2000).
LRE ¼ light rare earth elements, HRE ¼ heavy rare earth elements, and REE ¼ all rare earth elements.
Rare
Earth
Elem
ents
inSoils
1393
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 7. Sequential extraction procedures for determining different binding forms of
REEs in soils
Reference Number Forms of REEs Extractants
Ran and
Liu (1991)
1 Exchangeable 1 mol L21 M HAc-NH4Ac
(pH 7.0)
2 Carbonate bound 1 mol L21 HAc-NNH4Ac
(pH 5.0)
3 Mn-oxide bound 0.1 mol L21 NH2OH . HCl of
25% HAc solution (pH 2)
4 Bound to organic
matter
30% H2O2 (pH 2)
5 Bound to amorphous
Fe-oxide
0.25 mol L21 NH2OH .
HCl þ 0.25 mol L21 HCl
6 Bound to crystal
Fe-oxide
0.04 mol L21 NH2OH .
HCl þ 25% HAc
7 Residual HNO3 þ H2SO4
Zhu and
Xing (1992a)
1 Water soluble H2O
2 Exchangeable 1 mol L21 Mg(NO3)2
3 Carbonate bound
and adsorbed
HCl (pH 2-3)
4 Bound to organic
matter
30% H2O2 and 1 mol L21
Mg(NO3)2
5 Bound to
Fe/Mn-oxide
0.2 mol L21 NH2OH . HCl
of HCl (pH 2)
6 Residual Na2O2
Wang et al.
(1996)
1 Water soluble H2O
2 Exchangeable 1 mol L21 MgCl2 (pH 7)
3 Loosely bound to
organic matter
0.1 mol L21 K4P2O7 (pH 7.5)
4 Carbonate bound
and adsorbed
Dilute HCl (pH 2.8)
5 Bound to
Fe/Mn-oxide
0.04 mol L21 NH2OH . HCl
of 25%Hac solution
6 Tightly bound to
organic matter
After 2 h extraction of
0.02 mol L21 NH2OH .
HCl þ 30% H2O2 (pH 2),
add H2O2, finally add
3.2 M NH4Ac of 20%
HNO3 solution
7 Residual Weight after drying, directly
for analysis
Wang, Sun and
Tu (1998)
1 Water soluble H2O
2 Exchangeable 1 mol L21 MgCl2 (pH 7)
(continued )
Z. Hu et al.1394
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
in water derived from fluorocarbonates varies between 1025 and 1027 mol
L21, that from hydroxides is about 1026 mol L21, and that from phosphates
is in the range of 1024 to 1025 mol L21 (Department of Metal, Zhongshan
University 1978). Therefore, a limited amount of REEs exists in the water-
soluble state in the soil, which can be directly taken up by plant roots and
soil microorganisms or subjected to translocation processes in the soil. In
34 soils from China, the water-soluble REE content was on average 0.27 mg
kg21, which accounted for 0.18% of the total REE concentration (Zhu and
Xing 1992a). Wang et al. (1998) found similar values in their studies.
The exchangeable REE content in soils varies between traces and concen-
trations of up to 24.1 mg kg21 (Table 8). A maximum of 10.5% of the total
REE content was found to be exchangeable on a brown earth soil (Dytric
Cambisol) in China (Table 8). Red soils (Ferralic Cambisol) and brown
earth soils (Dytric Cambisol) showed the highest proportion of exchangeable
REEs with 4.5%, followed by yellow-brown earth soils (Haplic Luvisol) with
2.5% and black soils (Haplic Phaeozems), chernozems (Haplic Chernozems),
and Albic beached soil (Albic Luvisol) with values ,0.5% (Table 8). Similar
results were reported by Wang et al. (1998).
Table 7. Continued
Reference Number Forms of REEs Extractants
3 Loosely bound to
organic matter
0.1 mol L21 K4P2O7 (pH 7.5)
4 Carbonate bound 1 mol L21 NH4Ac (pH 5)
5 Adsorbed Dilute HCl (pH 2.8)
6 Fe/Mn-oxide bound 0.04 mol L21 NH2OH . HCl of
25%Hac solution
7 Tightly bound to
organic matter
0.02 mol L21 NH2OH .
HCl þ 30% H2O2 (pH 2)
8 Residual Weight after drying
Land et al.
(1999)
1 Exchangeable/adsorbed/carbonate bound
CH3COONa
2 Bound to Labile
organic matter
Na4P2O7 (pH 7.5)
3 Bound in amorphous
Fe-oxyhdroxides/Mn-oxides
0.25 mol L21 NH2OH . HCl
4 Bound in crystalline
Fe-oxides
1 mol L21H2OH . HCl
5 Bound to no-labile
organic matter/sulphides
KClO3.HCl
Rare Earth Elements in Soils 1395
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 8. REE content in different binding forms on different soil types
Soil type
Exchangeable RE
Carbonate
bound
Mn-oxide-
bound
Organic matter
bound
Amorphous
Fe-oxide bound
Crystalline
Fe-oxide bound ResidualData
sourcesmg kg21 % mg kg21 % mg kg21 % mg kg21 % mg kg21 % mg kg21 % mg kg21 %
Latosol/
rhodic
ferralsola
1.0 1.2 1.4 1.9 24.3 31.9 23.8 31.2 8.2 10.8 17.5 23.0 Ran and Liu
(1991)
4.1 1.7 2.7 1.1 10.9 4.5 94.6 38.9 41.0 16.9 90.0 37.0 Ran and Liu
(1994)
6.0 2.0 7.5 2.5 4.5 1.5 113.0 37.6 46.3 15.4 123.0 41.0
3.7a 1.6 3.9 1.8 13.2 12.6 77.1 35.9 31.8 14.4 76.8 33.6
Red soil/ferralic
cambisol
6.4 2.6 3.0 1.2 55.1 22.2 66.7 26.9 47.6 19.2 69.0 27.9 Ran and Liu
(1991)
24.1 3.9 3.6 0.6 91.5 14.7 127.0 20.4 64.0 10.3 313.0 50.2 Ran and Liu
(1994)
10.4 1.7 5.0 0.8 0.5 0.1 246.0 39.1 11.5 1.8 356.0 56.6 Liang et al.
(2000)
17.1 6.8 12.4 4.9 3.2 1.3 55.0 21.9 23.3 9.3 140.0 55.8
16.0 7.2 15.9 7.2 0.3 0.1 45.4 20.5 23.0 10.4 121.0 54.6
7.1 4.8 0.9 0.6 40.4 27.4 22.7 15.4 3.2 2.2 73.1 49.6
14.0a 4.5 6.8 2.6 31.8 11.0 93.8 24.0 28.8 8.9 178.7 49.1
Yellow brown
earth/hap-
lic luvisol
2.9 1.1 2.7 1.0 24.4 9.4 75.0 29.0 83.6 32.3 70.0 27.1 Ran and Liu
(1991)
21.4 7.7 12.2 4.4 27.7 9.9 83.6 30.0 93.0 33.3 41.0 14.7 Ran and Liu
(1994)
0.1 0.1 2.5 1.4 48.8 27.0 15.7 8.7 4.8 2.7 108.6 60.2 Liang et al.
(2000)
8.1a 2.9 5.8 2.3 33.6 15.5 58.1 22.6 60.5 22.8 73.2 34.0
Z.Huet
al.
1396
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Brown earth/dytric
cambisol
Trace Trace 2.1 1.1 36.4 18.2 75.0 37.4 51.9 25.9 35.0 17.5 Ran and Liu
(1991)
22.4 10.5 4.6 2.2 38.7 18.2 82.1 38.6 52.0 24.4 13.0 6.1 Ran and Liu
(1994)
11.2a 5.3 3.4 1.6 37.6 18.2 78.6 38.0 52.0 25.2 24.0 11.8
Black soil/
haplic
phaeozems
0.9 0.3 2.3 0.9 1.2 0.4 39.7 14.5 90.0 32.9 76.7 28.0 63.0 23.0 Ran and Liu
(1991)
2.0 0.7 8.3 2.9 1.3 0.5 53.8 18.5 87.6 30.1 72.8 25.0 65.0 22.4 Ran and Liu
(1994)
1.4a 0.5 5.3 1.9 1.2 0.4 46.8 16.5 88.8 31.5 74.8 26.5 64.0 22.7
Chernozem/
haplic
chernozems
1.3 0.4 3.2 1.0 2.5 0.7 41.6 12.2 114.0 33.5 99.4 29.2 78.0 22.9 Ran and Liu
(1991)
2.3 0.7 19.5 5.9 3.1 0.9 40.7 12.3 96.5 29.2 88.8 26.8 80.0 24.2 Ran and Liu
(1994)
0.1 0.1 2.7 1.7 0.1 0.1 56.8 35.2 4.9 3.1 2.3 1.4 94.6 58.6 Liang et al.
(2000)
1.2a 0.4 8.5 2.9 1.9 0.6 46.4 19.9 71.8 21.9 63.5 19.2 84.2 35.2
Albic
beached soil/
albic luvisol
0.1 0.1 11.9 8.1 0.1 0.1 10.2 6.9 28.2 19.3 2.8 1.9 93.0 63.6 Liang et al.
(2000)
Mean 7.9 3.1 1.8 0.6 4.6 1.9 33.4 15.0 79.4 28.6 46.0 17.2 97.5 33.6
aMean values.
Rare
Earth
Elem
ents
inSoils
1397
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
REEs may be incorporated into the lattice of other minerals by iso-
morphic exchange. Such an isomorphous replacement could take place
rather easily between REE ions and Ca2þ because of their similar ion
radius. Actually, Ca minerals, particularly rock-forming minerals such as
hornblende, augite, apatite, and fluorite, may have bound most REEs in the
earth’s crust during magmatism and hydatogenetic processes (Liu 1984), so
that nowadays a considerable amount of REEs in soil minerals exists in the
form of carbonates. Therefore, Zhu and Xing (1992a) suggested that
carbonate-bound REEs should be classified as an independent REE form in
soils. The content of REEs bound to carbonates is the range of 2.3 to 11.9
(mean 1.8) mg kg21. These values correspond with a proportion of 0.9%–
8.1% (mean 0.6%) of the total REE concentration (Table 8). Zhu and Xing
(1992b) found an even higher value with 14.0 mg kg21, corresponding with
9.2% of the total REE content in a set of 34 soils from China (Table 9).
The reason is that these authors used the extraction procedure developed by
Zhu and Xing (1992a), which reflects the carbonate-bound plus adsorbed
REE content. REEs are hydrolyzed in the soil, and the amorphous hydroxyl
groups, RE(OH)2þ, are adsorbed to the surfaces of clay minerals and Fe-Mn
oxides in soils. In 20 soils from China, the mean content of amorphous
REE hydroxyl was 79 mg kg21, accounting for 29% of the total REE
content (Table 8).
REEs are bound in colloidal form as Fe-Mn oxides. In general, REEs
bound in this form are not fully plant available. Only after changes of environ-
mental conditions, such as in a reducing wetland environment, are Fe-Mn
oxides reduced, and then REEs may be released. Zhu and Xing (1992b)
reported that about 22.9 mg kg21 of REEs was bound to Fe-Mn oxides
(Table 9). In another study carried out in China, the content of REEs bound
to crystalline ion oxides was 44.8 mg kg21 (Table 8).
REEs can be fixed in organic matter as chelates or organic sulfides, which
are released only under strong oxidizing conditions. Type and composition of
organic matter, therefore, has a strong influence on the chemical speciation of
REEs. REEs that are loosely bound to organic matter show a much higher bio-
logical activity than those being tightly bound (Peng and Wang 1995). Zhu
and Xing (1992b) reported that about 6.8 mg REEs kg21 or 4.5% of the
total REE content were bound in organic forms (Table 9). In other soils
from China, 32.5 mg of REEs kg21 were fixed in the organic matter (Table 8).
The residual form of REEs is fixed in the lattices of minerals and not plant
available. Wang et al. (1998) found that REEs in the residual form were dom-
inating with concentrations equaling 63–89% of the total REE content in
soils. Zhu and Xing (1992b) reported that REEs in the residual form were
as high as 106.9 mg kg21, accounting for 70% of the total REE content
(Table 9). The evaluation of the analytical results from another 20 soils in
China revealed REE concentrations in the residual form of up to 102.2 mg
kg21, accounting for 38% of the total REE content (Table 8). Differences
between studies are putatively linked to the analytical procedure that
Z. Hu et al.1398
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 9. Mean individual REE content in different binding forms and percentages of the total REE content in different soils of Chinaa; (Zhu and
Xing 1992a)
Element
Total
(mg kg21)
Water soluble Exchangeable
Carbonate and
specifically
adsorbed
Organic matter
bound
Fe-Mn oxide
bound Residual
mg kg21 % mg kg21 % mg kg21 % mg kg21 % mg kg21 % mg kg21 %
La 32.09 0.03 0.1 0.47 1.3 2.75 8.9 2.02 6.6 2.98 10.2 23.84 72.8
Ce 62.59 0.07 0.1 0.95 1.1 5.49 8.2 2.07 3.6 12.09 18.1 41.91 68.8
Pr 8.19 0.03 0.5 0.11 1.2 0.73 8.7 0.44 5.8 1.04 13.0 5.84 70.9
Nd 28.18 0.04 0.2 0.19 0.4 2.66 9.1 1.42 5.3 3.84 14.1 20.05 70.9
Sm 5.14 0.02 0.5 0.04 0.6 0.57 10.3 0.25 5.2 0.81 15.9 3.44 67.5
Eu 1.01 0.01 0.2 0.01 0.5 0.12 10.7 0.05 4.9 0.17 16.0 0.66 67.8
Gd 4.67 0.01 0.4 0.04 0.7 0.58 11.1 0.23 5.0 0.71 15.1 3.10 67.9
Tb 0.78 0.01 1.2 0.04 1.7 0.09 10.6 0.04 4.7 0.12 14.8 0.51 66.9
Dy 4.04 0.01 0.1 0.02 0.3 0.47 10.1 0.15 3.6 0.57 13.9 2.84 71.9
Ho 0.79 0.01 1.1 0.02 2.6 0.09 10.1 0.03 3.9 0.08 8.5 0.56 73.8
Er 2.28 0.01 0.3 0.01 0.5 0.22 8.8 0.06 2.6 0.25 10.8 1.73 77.0
Tm 0.46 0.01 2.9 0.02 3.7 0.04 8.9 0.03 6.5 0.05 10.9 0.30 67.0
Yb 2.21 0.01 0.3 0.01 0.7 0.18 7.3 0.05 2.3 0.20 9.9 1.76 79.5
Lu 0.42 0.01 0.4 0.01 0.7 0.03 6.9 0.01 1.9 0.04 8.5 0.34 81.6
Total/mean 152.83 0.27 0.2 1.94 1.3 14.03 9.2 6.84 4.5 22.93 15.0 106.87 69.9
aThe set of 34 soil samples included the following soil types: brown forest soil (dytric cambisol), dark forest soil (eutric cambisol), gray forest soil
(humic cambisol), chernozem (haplic chemozems), castanozem black soil (haplic calcisols), bog soil (umbric gleysols), fluvo-aquic soil (calcaric
cambisol), manured loessial soil (cumulic anthrosols), dark loessial soil (cumulic haplustolls), yellow cultivated loessial soil (cumulic-calcaric
arenosol), gray desert soil (cumulic-calcaric regosol), brown desert soil (calcaric fluvisals), residual saline soil (gypsic soloncnaks), yellow brown
earth (haplic luvisol), red soil (ferralic cambisol), lateritic red earth (eutric vertisol), latosol (rhodic ferralso), purplish soil (chromic cambisol),
and paddy soil (hydrgric anthrosols).
Rare
Earth
Elem
ents
inSoils
1399
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
was employed. So, the data of Zhu and Xing (1992b) include REEs bound in
crystalline Fe oxides.
Distribution Patterns of Individual REEs
The distribution pattern of individual REEs in different binding forms shows a
high variation in dependence on the soil type (Table 9). Generally, higher
concentrations of water-soluble and exchangeable elements were found for
those with an odd atomic number. In case of other binding forms, the
differences proved to be minor. Yb and Lu showed the highest and Tb and
Tm the lowest concentration of REEs in the residual form (Zhu and Xing
1992b).
The results summarized in Table 10 show that Ce had the highest pro-
portion of all REEs with about 33% in the various binding forms and Lu
the lowest with about 0.2%. The share of individual REEs varied highly but
decreased generally in the following order: Ce . La . Nd . Pr . Sm .
Gd . Dy . Er . Yb . Eu . Tb . Ho . Tm . Lu (Zhu and Xing 1992b).
Translocation of REEs in Soils
Translocation and leaching of REEs in soils bears the risk of groundwater con-
tamination and dispersion of REEs in the environment. Leaching rates of the
radioactive labeled isotopes, 141Ce and 147Nd, were studied in columns for
nine different types of soils from China (Zhang, Zhu, and Zhang 1995). In
this experiment, a 141Ce and 147Nd solution was added on top of the soil
columns at rates of 25% and 50% of the maximum REE adsorption. The
soil columns were leached with distilled water and/or 0.01 M CaCl2 for
48–72 h, simulating an annual rainfall of 600–1500 mm. Soil samples from
different depths and leachates were analyzed. The elements 141Ce and 147Nd
were translocated into depths of 6–10 cm and 4 cm, respectively, in acid
soils (Latosol/Rhodic Ferralsol, yellow-red earth/Ferralic Cambisol, and
paddy soil/Hydrgric Anthrosols). In the leachates, no 141Ce and 147Nd were
found, indicating that the added 141Ce and 147Nd were rather immobile,
even if added in high concentrations of 300–7500 mg kg21 soil. In other
soil column studies, exogenously applied REEs were found in the top soil
layers (0–5 cm and 0–20 cm) (Gao, Hong et al. 1999; Stokes et al. 2001).
From these experiments, it was concluded that the risk of groundwater
pollution by REEs through leaching is extremely low.
Under natural conditions, an annual translocation rate by precipitation of
about 1 cm was determined when REEs were applied in rates equivalent to
10% of the adsorption saturation on acid soils with a low adsorption
capacity. The translocation rate was about 0.5 cm on slightly acid soils with
a moderate adsorption capacity, whereas no translocation of REEs was
Z. Hu et al.1400
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 10. Percentage (%) of individual REEs from the total REE concentration in soils in dependence on the binding form
(Zhu and Xing 1992b)
Water soluble Exchangeable
Carbonate
adsorbed
Organically
bound
Fe-Mn oxide
bound Residual Total
La 11.1 24.2 19.6 29.5 13.0 22.3 21.0
Ce 25.9 49.0 39.1 30.3 52.7 39.2 41.0
Pr 11.1 5.7 5.2 6.4 4.5 5.5 5.4
Nd 14.8 9.8 19.0 20.8 16.7 18.8 18.4
Sm 7.4 2.1 4.1 3.7 3.5 3.2 3.4
Eu 0.0 0.5 0.9 0.7 0.7 0.6 0.7
Gd 3.7 2.1 4.1 3.4 3.1 2.9 3.1
Tb 3.7 2.1 0.6 0.6 0.5 0.5 0.5
Dy 3.7 1.0 3.3 2.2 2.5 2.7 2.6
Ho 3.7 1.0 0.6 0.4 0.3 0.5 0.5
Er 3.7 0.5 1.6 0.9 1.1 1.6 1.5
Tm 3.7 1.0 0.3 0.4 0.2 0.3 0.3
Yb 3.7 0.5 1.3 0.7 0.9 1.6 1.4
Lu Trace Trace 0.2 0.1 0.2 0.3 0.3
Rare
Earth
Elem
ents
inSoils
1401
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
found on alkaline soils with a high adsorption capacity (Zhu, Zhang, and
Zhang 1996).
The main factors influencing the translocation rate are soil pH for the HAc
extractable REEs, and Fe-Mn oxides for HCl- and HNO3-extractable REEs
(Steinmann and Stille 1997). The REE concentration in the HAc-extractable
phase increased with soil depth and increasing soil pH, which indicates that
these REEs were translocated from the surface and hence are of anthropogenic
origin (Steinmann and Stille 1997). The authors speculated that REEs were
transported in dissolved forms and then precipitated in carbonate complexes
in deeper soil layers, where the carbonate content and pH were correspond-
ingly higher (Steinmann and Stille 1997). Nagao et al. (1998) found that Eu
was not detected in leachates when the eluant was used without soluble
organic matter. The addition of dissolved organic material into the eluant
increased the relative Eu concentrations in this order: river fulvic acid ,
groundwater dissolved organic matter , river humic acid. The reason is
that the Eu(III) binding reactions in the humic acid is mainly controlled by
a class of functional groups possessing caroxy-like reactivities (Martinez,
Traina, and Logan 1998). These results imply that particularly the transloca-
tion of humic substances into water bodies is associated with a loss of REEs to
the environment.
Soil-Applied REEs
The chemical speciation of soil-amended REEs is linked to the soil type. REEs
applied to a Latosol (Rhodic Ferralsol), yellow-brown earth (Haplic Luvisol),
black soil (Haplic Phaeozems), and Cheronzem (Haplic Chernozems) were
mainly found in amorphous Fe-Mn oxides and bound to organic matter. The
REEs applied to a red earth soil (Ferralic Cambisol) that was low in pH and
low in amorphous Fe-Mn oxides were adsorbed and bound to Mn oxides
(Ran and Liu 1993a). Chen, Wang, and Tian (1995) found that on a red soil
(Ferralic Cambisol), soil-amended REEs were tightly bound to organic
matter rather than loosely, whereas on a yellow-brown earth (Haplic
Luvisol) twisted results were found. Only a low amount of adsorbed REEs
in the inert form was found on Chernozems (Haplic Chemozems), yellow-
brown soils (Haplic Luvisol), latosols (Rhodic Ferralsol), and red soils
(Ferralic Cambisol) (Ran and Liu 1993a), indicating that only a small
portion of adsorbed REE ions was transferred into the mineral lattice. In
contrast, a relatively high amount of adsorbed REEs was found in the inert
form on loess soils and dark brown soils (Eutric Cambisol) because of their
higher pH values. The distribution coefficient (percentage of individual
REEs of added REEs) of REEs added to a red soil (Ferralic Cambisol)
and a yellow-brown soil (Haplic Luvisol) decreased in the following order:
residual REE . exchangeable REEs . organic matter–bound REEs . Fe-
Mn oxide–bound REEs (Chen, Wang, and Tian 1995).
Z. Hu et al.1402
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
In field experiments in China, all soil-amended REEs were present in the
water soluble, exchangeable, carbonate-bound, Fe-Mn oxide–bound, organic
matter– and sulfide-bound form with variations of 1.5–13.9%, 35.2–70.3%,
and 19.1–60.8% of the total applied REE rate (Wen et al. 2001). Generally,
REEs applied to soils are transformed quickly (Liu et al. 1999). The share
of soluble and exchangeable forms of REEs decrease, however, rapidly over
time. In comparison, the part of REEs bound to organic matter increases
after a certain period of time (Liu et al. 1999). After soil application of
REEs, an enhanced fraction is bound to Fe-Mn oxides, but this share
declines after a peak value has been achieved (Liu et al 1999). REEs that
are adsorbed may precipitate in the form of hydroxides, dehydrated hydrox-
ides, or insoluble carbonate, and then desorption processes are limited (Zhu
et al. 1993). In comparison, the residual REE content is a stable fraction
(Liu et al. 1999).
Adsorption and Desorption of Soil-Applied REEs
Adsorption of REEs
In six soil samples of different soil types from China, less than 95% of the
added REEs were adsorbed (Zhu et al. 1993) to the oxides of clay minerals
and organic matter (Beckwith and Butler 1993; Brown, Franklin, and Miller
1969). The soil clay fraction (,2mm) consists of clay minerals such as
illite, caolinite and smectite and hydrous metal oxides, for example, Fe,
Mn, and Al. The Fe and Mn oxides can coprecipitate and adsorb cations
from the solution because of their pH-dependent charge (Alloway 1995). It
has been speculated that these metal oxides are primarily responsible for
accumulating REEs in soils (Peng and Wang 1996).
Fedorf and Fendorf (1996) found in their studies that La was adsorbed to
the oxides of minerals. In their experiments, the surface precipitation of La-
hydroxide at pH . 5 was obviously the dominant sorption mechanism to bir-
nessite. Above soil pH values of 6.5 to 8.0, surface precipitation was observed
for rutile and goethite, too. The sorption of La increased with increasing soil
pH values. The depletion of La from the solution by rutile and goethite at pH
values well below the ZPC (zero point of charge) indicates that these sorption
mechanisms differ from those of birnessite. In the case of rutile and goethite,
surface complexation of monomeric and small multinuclear mineral species
dominated the retention of La (Fedorf and Fendorf 1996). Dong et al.
(2001) identified the formation of bridged hydroxo lanthanide complexes to
be the main sorption mechanism for betonite, alumina on a red soil
(Ferralic Cambisol).
Organic matter contributes significantly to the adsorption of REEs
because of the dissociation of protons from carboxyl and phenolic groups of
humic polymers in the soil (Beckwith and Butler 1993). So, the adsorption
Rare Earth Elements in Soils 1403
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
of Ce to humic acids was distinctly higher than that of bentonite
(Brown 1969). The humic and fulvic acids proved to be relevant for the
absorption and desorption of Eu (Dong et al. 2001; Wang, Dong, et al.
2001a). On a sandy soil, the percentage of adsorbed Eu was as high as
95%, if no humic acids were abundant, whereas in the presence of humic
acids Eu(III)–humate complexes were formed and thus only 17–25% of Eu
was adsorbed in the sandy soil (Nagao et al. 1996). The complexation of
REEs (La, Ce, Pr, and Nd) with organic acids decreased in the following
order: citric acid . malic acid . tartaric acid . acetic acid ¼ Ca(NO3)2
(Shan, Lian, and Web 2002). This sequence is consistent with the order of
stability of the REE complexes formed by these organic acids (Shan, Lian
and Web 2002).
The sorption of applied REEs to MnO2 and FeOOH was fast and strong
(Koeppenkastrop and DeCarlo 1993) and agrees with other reports about
the enrichment of REEs in Fe and Mn concretions (Fleet 1984). REE adsorp-
tion reactions are very swiftly. Wang et al. (2000) reported that the adsorption
equilibrium of REEs (La, Gd, Y) was adjusted 0.5 h after the addition of the
REEs. These results were confirmed by Gao, Zhang, and Wang (1999b),
who concluded that the adsorption rate is fast at the beginning when it is con-
trolled by the diffusion rate.
The adsorption of individual REEs, such as Nd, La, and Yb, can be
described by Langmuir, Temkin, and Freundlich equations (Gao, Zhang,
and Wang 1999b; Wang and Zhan 1992; Jones 1997). Langmuir isotherm
models were used to describe the adsorption of mixed REEs to soils and
synthetic oxides (Ran and Liu 1993b). The significant positive relation-
ship between sorption isotherm parameters (Kf, A—constant related to
adsorption characteristic— and Smax—max amount of adsorption) and the
clay content indicates that the concentration and type of clay minerals
determine the adsorption of La in soils (Stokes et al. 1999). The highest
adsorption value (Qm) in soils for Ce and Nd was found to correlate posi-
tively with high illite (Zhang, and Zhang 1996), CEC, and amorphous
iron contents in the soils (Ran and Liu 1993b; Wang and Zhan 1992; Ran
and Liu 1992).
Soils and minerals show a different REE adsorption capacity. The
maximum REE adsorption capacity (Qm) decreased in this order: d-MnO2 �
amorphous iron . kaolinite ¼ black soil (Haplic Phaeozems) . chernozem
(Haplic Chemozems) . yellow-brown earth (Haplic Luvisol) . latosol
(Rhodic Ferralsol) ... red earth (Ferralic Cambisol) (Ran and Liu
1993b). The corresponding order for different soil types determined by Zhu
and Xing (1993) was loess soil . chernozem (Haplic Chemozems) .
yellow-brown soil (Haplic Luvisol) . dark brown soil (Eutric Cambisol) .
latosol (Rhodic Ferralsol) . red earth (Ferralic Cambisol).
The soil pH value is another important factor influencing the adsorption
of REEs (Wang, Dong, et al. 2001a; Gao, Zhang, and Wang 1999b). The
adsorption of REEs generally increases with increasing soil pH values,
Z. Hu et al.1404
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
because the surface of soil particles is charged with more OH2 ions, so that
dissolved REE ions can easily form complexes, such as Ln(OH)2þ,
Ln(OH)2þ, and Ln(OH)4
2 (Zhu and Xing 1993). The adsorption equipotential
points of La and Yb for soils and minerals are obtained at pH 6.3 and
pH 5.3, respectively. The equipotential point of La was found at pH 6.3
and that of Yb at pH 5.3, indicating that the adsorption capacity is higher
for La than that for Yb at pH values similar to the equipotential point,
whereas at other pH values inverse results were obtained (Gao, Zhang, and
Wang 1999b).
Wang et al. (2000) reported that the rate of La, Gd, and Y adsorption in
soils decreased with an increasing concentration of EDTA and that the
rate showed a negative relationship with the stability constants of the
REE-EDTA complexes.
Desorption of REEs
Information about desorption processes of REEs in soils is important with
view to their plant availability, translocation processes, and potential entry
into the food chain. The adsorption of REEs to clay minerals and Fe-Mn
oxides is high, whereas the desorption is very low because of their strong
specific binding (Ran and Liu 1993b). Desorption from d-MnO2 is equivalent
with that from amorphous iron (Ran and Liu 1993b).
Desorption of REEs is also related to the soil pH. REE desorption
decreases from 90% to 29.5% when the soil pH increases from 4.1 to 6.3
(Ran and Liu 1992). Similar results were reported by Wen et al. (2002),
who found that on an acid soil with a pH of 5.4 and an organic matter
content of 1.5% the relative desorption of La (89.9–98.5%), and Ce (57.6–
96.4%) was high. In contrast, on calcareous soils with high soil pH values
between 7.2 and 8.2 and an organic matter of 36.4%, the relative desorption
of La (27.6–53.6%), and Ce (1.09–50.8%) was only low.
Shan, Lian and Web (2002) reported that REEs (La, Ce, Pr, and Nd) were
desorbed easily in the presence of organic acids (citric acid, malic acid, tartaric
acid and acetic acid) to form stable complexes so that the adsorption will
decrease while desorption processes increase. The organic ligand EDTA-
promoted the desorption of REEs, which was proportional to its concentration
(Wang et al. 2000; Yang, Wang, and Sun 1999b).
The time required for establishing an equilibrium of desorption for Ce(III)
and the concentration of desorbed Ce depends on the soil type (Li et al. 2000).
The Elovich equation provided the best model for describing the desorption of
Ce(III) in a meadow soil (Haplic Phaeozem) and a black soil (Haplic
Phaeozems); the parabolic-diffusion equation (Elovich) gave best results for
a red soil (Ferralic Cambisol) and a yellow soil (Haplic Acrisol) (Li et al.
2000).
Rare Earth Elements in Soils 1405
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
REES IN SOILS AND PLANTS
Plant-Available REE Content
We previously discussed how the chemical behavior of natural and applied
REEs in soils may have a strong effect on the transfer of REEs into the food
chain. Besides this, the quantification of adsorption–desorption and transform-
ations processes is required for an ecoenvironmental evaluation of REEs.
The total REE content in soils can be regarded as a storage pool for REEs,
which does not reflect the potential availability for plant uptake. In China,
researchers used 1 mol L21 HAc–NaAc (pH 4.8) to extract soil REEs to
assess the plant-available REE content (Liu 1988). Usually, HAc–NaAc-
extractable REEs reflect the amount of soluble REEs or plant-available
REEs. The plant-available REE content in Chinese soils ranged from traces
that were below the detection limit to 208 mg kg21 with a mean value of
11.8 mg kg21 (n ¼ 1790) (Table 11). Besides paddy soils (Hydrgric Anthro-
sols) (17.1 mg REEs kg21), red soils (Ferralic Cambisol) proved to have the
highest plant-available REE content (18.8 mg REEs kg21). In comparison,
gray cinnamon (Eutric Cambisol), castanozem (Haplic Calcisols), bog soil
(Umbric Gleysols), gray desert soil (Cumulic—calcaric Regosol), subalp
meadow soil (Calciudoll), alp plain soil (Calcicryids, US Soil Taxonomy,
1994), and subalp plain soils (Calcicryids, US Soil Taxonomy, 1994) have dis-
tinctly lower plant available REE concentrations. The lowest plant-available
REE content was found on black soils (Haplic Phaeozems), Chernozem
(Haplic Chemozems), dark brown soils (Eutric Cambisol), gray sand soils
(Cumulic—calcaric Regosol), Shajiang soils (Gleyic Cambisol), meadow
soils (Haplic Phaeozem), Albic beached soil (Albic Luvisol), brown sand
soils (Calcaric Fluvisals), saline soils (Gypsic Soloncnaks), and alp meadow
soils (Gelic Cambisols) (Table 11).
The plant-available REE content in acid soils is usually higher than that in
calcareous soils. There is a significant negative correlation between soil pH
and the plant-available REE content in the range from soil pH 6 to 10. The
plant-available REE content increases with increasing clay and organic
matter content (.1%) of soils (Xiong et al. 2000).
Plant Uptake of Different Forms of REEs
The uptake of REEs varies strongly between plant species (Wyttenbach et al.
1994). Certain plants accumulate REEs (Wyttenbach et al. 1994; Aidid 1994;
Wutscher and Perkins 1993; Ichihashi, Morita, and Tatsukawa 1992; Takada
et al. 1998). So, some ferns contain up to 1 mg g21 La (Wyttenbach et al.
1994). In another species of Dicranopteris dichotoma, a total REE content
of greater than 3 mg g21 was determined so that these plant species may be
considered as REE accumulators (Wang et al. 1997; Zhang et al. 2000). In
Z. Hu et al.1406
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 11. Soluble REE content in selected soils of China (mg kg21) (Xiong et al.
2000)
Soils n Min Max Mean SD
Latosol/rhodic ferralsol 60 ,LLD 47.0 10.6 9.7
Lateritic red earth/eutric
vertisol
160 ,LLD 206 16.8 24.2
Red earth/ferralic cambisol 262 ,LLD 208 18.8 24.8
Yellow soil/haplic acrisol 39 ,LLD 25.4 6.9 7.6
Yellow brown soil/haplic
luvisol
60 ,LLD 56.6 9.8 10.7
Brown soil/dytric cambisol 72 ,LLD 27.0 5.1 6.8
Dark brown soil/eutric
cambisol
30 ,LLD 12.8 3.6 4.3
Albic beached soil/albic
cuvisol
4 ,LLD 6.0 1.5 3.0
Cinnamon soil/haplic luvisol 141 ,LLD 26.6 7.9 6.1
Matured loessial soil/cumulic anthrosols
36 3.3 32.6 13.2 5.9
Cultivated loessial soil/cumulic-calcaric arenosol
18 ,LLD 17.4 10.1 4.9
Dark loessial soil/cumulic
haplustolls
12 ,LLD 13.7 6.5 4.5
Gray cinnamon/eutric
cambisol
2 Trace ,LLD
Black soil/haplic phaeozems 47 ,LLD 9.2 2.3 2.3
Albic beached soil/albic
luvisol
18 ,LLD 12.5 5.2 4.1
Chernozem/haplic
chemozems
20 ,LLD 21.6 5.9 4.7
Chestnut soil/haplic
kastanozems
47 ,LLD 16.9 5.9 4.7
Castanozem/haplic calcisols 6 ,LLD ,LLD
Sierozem/calcaric cambisol 16 ,LLD 12.7 6.2 4.6
Gray desert soil/cumulic—calcaric regosol
2 ,LLD ,LLD
Gray brown desert soil/cumulic—calcaric regosol
5 ,LLD 8.2 5.5 3.3
Brown desert soil/calcaric fluvisals
12 ,LLD 8.4 2.3 3.2
Fluvo-aquic soil/calcaric cambisol
103 ,LLD 30.4 8.6 5.6
Shajiang balck soil/gleyic
cambisol
24 ,LLD 12.8 3.4 3.6
Cultivated desert soil/anthrosol
8 ,LLD 8.8 6.4 2.7
(continued )
Rare Earth Elements in Soils 1407
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
contrast, the REE content may be less than 10 ng g21 in spruce needles (Wyt-
tenbach et al. 1994; Zhang et al. 2000). Yet, the metabolic processes control-
ling the REE uptake by plants are completely unknown.
The soil/plant transfer factor for spruce and fern differ and depend on the
kind of REE and plant/soil factors (Wyttenbach et al. 1998). Nevertheless,
there seems to be no need for a fractionation of individual REEs in the soil
(Henke 1977; Lau and Weimer 1982; Miekeley, Casartelli, and Dotto
1994), which otherwise would have yielded additional analytical costs
(Wyttenbach et al. 1998).
REEs extracted by hydrofluoric acid (HF) were related to the REE content
in fern, whereas no such relationship was found for HCl/HNO3 (Fu, Akagi,
Table 11. Continued
Soils n Min Max Mean SD
Paddy soil/hydrgric
anthrosols
392 ,LLD 177 17.1 25.1
Dark meadow soil/haplic phaeozem
15 ,LLD 19.5 7.0 6.2
Meadow soil/haplic
phaeozem
32 ,LLD 7.7 2.4 2.2
Bog soil/umbric gleysols 7 ,LLD ,LLD
Shore saline soil/ 32 ,LLD 34.6 8.0 10.3
Saline soil/gypsic
soloncnaks
18 ,LLD 9.7 2.0 3.1
Black calcareous soil/typic
rendolls
3 ,LLD 5.5 2.8 2.8
Brown calcareous soil/ 31 ,LLD 38.4 10.2 9.1
Red calcareous soil/ferralic
cambisol
14 1.9 79.5 16.3 19.1
Purple soil/chromic
cambisol
22 ,LLD 22.5 4.9 6.0
Wind sand soil/gleyic
arenosol
3 ,LLD 8.9 6.0 4.9
Alp/sub-Alp desert soil/gypsicryids (US Soil
Taxonomy, 1994)
2 ,LLD 5.3 2.6 3.7
Alp meadow soil/gelic
cambisols
5 ,LLD 3.2 1.5 1.6
Sub-alp meadow soil/calciudoll
8 ,LLD ,LLD
Alp plain soil/calcicryids (ST, 1994)
1 ,LLD ,LLD
Sub-alp plain soil/calcicryids (ST, 1994)
1 ,LLD ,LLD
Total/Mean 1790 11.8
Z. Hu et al.1408
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
and Shinotsuka 1998). From these results, the authors concluded that fern takes
up REEs favorably from silicate minerals (Fu, Akagi, and Shinotsuka 1998).
REEs taken up by plants accumulate in the plant roots. Consequently,
more than 90% of the REEs can be found in the roots, and here 51.4% of
the total Ce and 32.5% of the total Nd content (Zhu and Chen 1984). The
REE concentration in plant tissues decreases in this order: root . leaves .
stems . flowers, fruits, seeds (Wen et al. 2001; Ma, Lao, and Liu 1996; Gu
et al. 2001).
Soil Properties Influencing the Accumulation of REEs in Plants
In general, a higher availability of REEs causes a higher REE uptake by plants.
The availability of REEs in soils is closely related to the water-soluble and
exchangeable fractions of REEs and thus dependent on physico-chemical soil
properties (Liang et al. 2000) such as pH, Eh, CEC, and clay content (Ran
and Liu 1994; Wutscher and Perkins 1993; Takada et al. 1998; Cao et al. 2001).
A significant negative relationship was found between soil pH and water-
soluble REE content (Liu 1988). Zhang and Shan (2001) also determined a
negative correlation between soil pH and exchangeable, carbonate and Fe-
Mn oxide–bound REEs. Consequently, liming reduced the REE concentration
in the soil solution (Diatloff, Asher, and Smith 1993).
With decreasing redox potential, the soluble REE content increased with
each Eh decrement (Ran and Liu 1994). A decrease in soil pH and Eh was
associated with an increasing availability of La, Ce, Gd, and Y (Cao et al.
2001). Under reducing soil conditions and at low pH values, the dissolution
of Fe-Mn oxyhydroxides releases REEs (Cao et al. 2001). Thereby, the
redox potential has a stronger influence on the release of Ce than on La,
Gd, and Y (Cao et al. 2001). This phenomenon could be related to changes
in the valences of Ce, Ce2þ, and Ce4þ side by side with oxidation and
reduction processes.
Solubility calculations suggested that CePO4 may be the component
controlling the concentration of Ce in the soil solution (Diatloff, Asher, and
Smith 1993). La and Ce were precipitated as phosphates at soil pH values
of greater than 4.0 (Diatloff, Asher, and Smith 1993). At a soil pH of
greater than 5.5, less than 10% of the added La or Ce can be expected to
remain plant available in the soil solution (Diatloff, Asher, and Smith
1995). H2PO42 plays a dominant role with view to the chemical speciation
of REEs in soils through changes in the Gibbs free energy of REEs (Wu
et al. 2001). Thus, the P status needs to be taken into account when
assessing the environmental risk of REEs in the soil–plant system.
The exchangeable Nd concentration in the soil and Nd uptake by
wheat seedlings was low when the free carbonate content was in the range
of 0.8–1.6 g kg21, whereas higher concentrations of 4.0 g kg21 had little
effect on soil and plant Nd contents (Xu et al. 2001).
Rare Earth Elements in Soils 1409
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
The results of a water culture experiment showed that SO422 amendments
to the nutrient solution, which contained 2.0 mg L21 of REEs, inhibited the
uptake of La3þ and Gd3þ into wheat roots, whereas that of Y3þ was
promoted. In stems and leaves, this treatment yielded an accumulation of
Gd3þ (Gu et al. 2000).
Soil and foliar-applied fulvic acid amendments of less than 0.4 mg C L21
to the root and less than 0.7 mg C L21 to stem and leaves increased the
REE concentration in wheat, whereas high application rates of more than
0.4 mg C L21 to roots and more than 1.5 mg C L21 to stem and leaves
decreased the uptake (Gu et al. 2001).
The REE uptake of one accumulator fern could be reduced by adding
chelating agents, whereas no effect was observed for nonaccumulator
species (Ozaki et al. 2002). A linear accumulation of REEs with increasing
plant age was found for Norwegian spruce (Wyttenbach et al. 1994), a fact
that is important when studying perennial crops.
Relevant for plant uptake in this context is also whether REEs are
naturally abundant or exogenously applied. So, 81–97% of the applied La
was plant available but only 25–56% of the naturally abundant element
(Stokes et al. 2001). The studies of Liu and Wang (2000) confirmed these
results. These findings need to be taken into account for predicting the REE
uptake of plants. The differences between natural and exogenously applied
REEs can be explained by their different chemical speciation. Exogenously
applied REEs cannot be found in residual form, a chemical species, which
is not plant available (Zhu and Xing 1992a; Wang et al. 1998).
Suitability of Soil Analytical Methods for Predicting the REE
Uptake of Plants
In previous investigations in China, various soil analytical methods were
tested with view to their suitability to predict the REE uptake of plants
(Zhu and Liu 1985). The HAc–NaAc (pH4.8) extractants were extensively
used to evaluate the status of plant-available REEs in soils during the past
decades (Xiong et al. 2000). Under controlled greenhouse conditions, Zhu
and Hu (1988) found that the HAc–NaAc (pH 4.8) extract was closely
related (r ¼ 0.9917) to the REE content in the stems of wheat. Sun et al.
(1998) and Li, Shan, and Zhang (2001) found a close relationship between
HCl-extractable REEs and the REE content in the shoots of wheat
(r ¼ 0.67–0.89). Another suitable extraction method proved to be 1.0 M
NH4NO3 (pH 7.0) (Zhai et al. 1999).
With view to individual REEs, soil extraction with HCl delivered satisfac-
tory results, too (Table 12). Zhang, Shan, and Li (2000) found that the extrac-
tion with 0.1 mol L21 malic–citric acid was closely related to the content of
individual REEs in ryegrass (r ¼ 0.72–0.96).
Z. Hu et al.1410
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Table 12. Correlation coefficients (r) for the relationships between extractable REEs in soils and the individual REE content in plants
REE Extractant Crop
Correlation coefficient (r)
N ReferenceRoot Shoot
La 0.1 mol L21 HCl Wheat 0.96�� 7 Sun et al. (1998)
Gd 0.96�� 7
Ce 0.98�� 7
Sm 0.97�� 7
La 0.1 mol L21 HCl Wheat 0.72� 0.70� 9 Li et al. (2001)
Ce 0.85�� 0.78� 9
Pr 0.81�� 0.81�� 9
Nd 0.77� 0.77� 9
Sm 0.85�� 0.89�� 9
Eu 0.76� 9
Gd 0.83�� 0.82�� 9
Tb 0.75� 9
Dy 0.78� 0.79� 9
Ho 0.73� 9
Er 0.79� 0.72� 9
Tm 0.75� 9
Yb 0.81�� 0.67� 9
Lu 0.76� 9
REEs (La-Lu) CH3COOH, DTPA ,0.47 ,0.55 9
REEs 0.1 mol L21Malic-citric acid Wheat 0.72-0.96 (significant) Zhang et al. (2000a)
Mehlich-3 and EDTA Significant
CH3COOH, DTPA, total content Not significant
REEs HAc-NaAc (pH 4.8) Wheat 0.9912�� 0.9917�� Zhu and Hu (1988)
�, �� Indicate significant levels at p , 0.05, p , 0.01, respectively.
Rare
Earth
Elem
ents
inSoils
1411
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
From different binding forms of REEs being tested, the exchangeable
REE content showed a close correlation with the Pr content in alfalfa
(r ¼ 0.8912) (Cao et al. 2000). The same experiment revealed that exchange-
able REEs, and those bound to carbonates and organic matter, strongly influ-
enced the plant availability of La and Nd. The plant availability of Ce, Gd, and
Dy were found to be related to the exchangeable and organic matter–bound
fraction, whereas that of Yb was associated with the exchangeable and
carbonate bound fraction (Cao et al. 2000). Therefore, the authors
concluded that plant availability differs between individual REEs (Cao et al.
2000).
FUTURE RESEARCH NEEDS
The following research needs from the viewpoint of soil science can be
addressed:
1. Soil inventory of REEs: Background values of REEs in soils are mainly
available for China (Xiong et al. 2000). Here, information about not
only the total REE concentration but also the distribution of individual
REEs and their chemical speciation in dependence on soil type and
climatic conditions in other countries is of prime interest.
2. Chemical behavior of REEs in the rhizosphere: The rhizosphere is a key
zone with view to the mechanism of soil nutrient dynamics and chemical
speciation of REEs (Zhu and Chen 1984; Zhu and Chen 1989; Ma, Lao,
and Liu 1996). Wang, Shan, and Zhang (2001) found that a discrimination
of REE species in the rhizosphere and nonrhizosphere provided more
precise information about plant availability. Here, differences between
the two zones could be contributed to the exudation of organic acids
from roots, because citric and malic acid have a strong capacity to form
complexes with REEs (Shan, Lian and Web 2002). The significance of
root–soil interactions for the uptake of REEs is important for establishing
risk assessments.
3. Physico-chemical characteristics of soil applied REEs: The use of REE
fertilizers is widely practiced in Chinese agriculture (Hu et al. 2004;
Wang and Zheng 2001). Other sources for REEs are phosphogypsum,
P-fertilizers, and sewage sludges (Meehan et al. 2001; Todorovsky,
Minkova, and Bakalova 1997; Volokh et al. 1990; Kawasaki, Kimura,
and Arai 1998; Zhang, Yamasaki, and Kimura 2001). Long-term appli-
cations of superphosphate (.30 years) caused a significant increase of
the total REE content in soils (Todorovsky, Minkova, and Bakalova
1997; Volokh et al. 1990). In experiments where the application of
REE fertilizers yielded no increase in the soil content (Liu, Wang, et al.
1997), the reasons are most likely the limited rate and time of experimen-
tation. Vertical and lateral translocation processes need to be followed up
under field conditions.
Z. Hu et al.1412
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
4. Effect of REEs on soil fertility: Soil microbial biomass, microorganism
species, enzymes, and soon are important characteristics of soil fertility.
The uncontrolled application of REEs on soils needs to be critically
evaluated because of possible adverse effects on soil fertility. La had,
for instance, stimulative effects on soil nitrification and P transformation
when applied in low doses, whereas high concentrations in the soil had an
inhibitory effect (Xu and Wang 2001). The no-observed-effect level with
view to ammonium oxidation and N mineralization in these studies was
432 and 443 mg La kg21, respectively (Xu and Wang 2001). Additionally,
La has a strongly inhibitory effect on the decomposition of phenolic
compounds in the soil (Chu et al. 2002). Little information about the
influence of individual REEs on soil microbiological parameters is
available (Xie et al. 2001). However, such detailed information is,
badly required for efficient soil protection.
ACKNOWLEDGMENTS
This work was jointly supported by the Max Planck Society (Germany) and
the Bilateral Chinese/German Co-operation of the Ministries of Agriculture.
REFERENCES
Aidid, S.B. (1994) Rare earth element abundance and distribution patterns in plantmaterials. The Journal of Radioanalytical and Nuclear Chemistry, 183: 351–358.
Alina, K.P. and Henryk, P. (1984) Trace Elements in Soil and Plants; CRC Press/Taylor and Francis CEC Press: Boca Raton, Florida, 315.
Alloway, B. (1995) Heavy Metals in Soils, 2nd Edition; Blackie Academic &Professional: UK.
Arocena, J.M., Rutherford, P.M., and Dudas, M.J. (1995) Heterogeneous distribution oftrace elements and fluorine in phosphogypsum. Science-of-Total-Environment,162 (2–3): 149–160.
Beckwith, R. and Butler, J. (1993) Aspects of the chemistry of soil organic matter. InSoils, an Australian Viewpoint; Division of Soils, CSIRO: Canberra, Australia,Chapter 35, 561–581.
Bohn, R.L., McNeal, B.L., and O’Conner, G.A. (1985) Soil Chemistry, 2nd Edition;John Wiley & Sons: New York.
Brwon, R., Franklin, R., and Miller, R. (1969) Reactions of 144Ce in solution and sus-pensions of soil humic acid and bentonite. Soil Science Society of America Jounal,33: 677–681.
Buckingham, S., Maheswaran, J., Meehan, B., and Peverill, K. (1999) The role ofapplications of rare elements in enhancement of crop and pasture production.Materials Science Forum, 315–317: 339–347.
China Environmental Monitoring Station. (1999)Background Value of Elements in Soilsof China (in Chinese); China Environmental Science Press: Beijing, China, 1–55.
Chen, Z.X., Wang, X.R., and Tian, L.C. (1995) Studies on the fractionation and avail-ability of added soluble rare earth elements in soil. Joumal of Rare Earths, 13 (1):42–46.
Rare Earth Elements in Soils 1413
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Chu, H.Y., Zhu, J.G., Xie, Z.B., Li, Z.G., Cao, Z.H., and Zeng, Q. (2002) Effect of
lanthaum on nitrification, phosphorus transformation and phenol decomposition in
red soil. Joumal of Rare Earths, 20 (1): 67–70.
Department of Metal, Zhongshan University. (1978) Constant of Rare Earth Elements
in Physical Chemistry (in Chinese); Metallurgical Industry Press: Beijing, 535.
Diatloff, E., Asher, C.J., and Smith, F.W. (1993) Use of GEOCHEM-PC to predict rareearth elements (REE) species in nutrient solutions. Plant and Soil, 155/156:
251–254.
Diatloff, E., Asher, C.J., and Smith, F.W. (1996) Concentration of rare earth elements
in some Australian soils. Australian Journal of Soil Research, 34: 735–747.
Diatloff, E., Asher, C.J., and Smith, F.W. (1995) Effects of rare earth elements (REEs)
on the growth and mineral nutrition of plants. In Proceedings of the Rare Earths in
Agriculture Seminar, Australian Academy of Technological Sciences and Engineer-
ing, eds; Brown Prior Anderson Pty Ltd.: Canberra, ACT, Australia, 11–23.
Dong, W.M., Wang, X.K., Bian, X.Y., Wang, A.X., Du, J.Z., and Tao, Z.Y. (2001)Comparative study on sorption/desorption of radioeuropium on alumina,
bentonite and red earth: effects of pH, ionic strength, fulvic acid, and iron oxides
in red earth. Applied Radiation and Isotopes, 54 (4): 603–610.
Dudka, S. (1992) The concentration of some scandium subgroup trace metals in soils of
Poland. Geoderma, 52 (3–4): 279–289.
Fedorf, S. and Fendorf, M. (1996) Sorption mechanism of lanthanide on oxide
minerals. Clays and Clay Mineral, 44 (2): 220–227.
Ferguson, F. (1988) Phosphogypsum-an overview. In Proceedings of the Second Inter-
national Symposium on Phosphogypsum; No. 01-037-055, Orlando, FL; Vol. 1,117–130.
Fleet, A. (1984) Aqueous and sedimentary geochemistry of the rare earth elements. In
Developments in Geochemistry 2: Rare Earth Element Geochemistry; Henderson, P.
(ed.); Elsevier: Amsterdam, Chapter 10, 30–95.
Fu, F., Akagi, T., and Shinotsuka, K. (1998) Distribution pattern of rare earth elements
in fern. Biological Trace Element Research, 64 (1): 13–26.
Fu, S.Z., Yan, C.L., Wu, S.Y., and Yang, X.K. (2000) Content and distribution charac-
teristics of rare earth elements in typical soils of Guizhou Province. Acta Pedologica
Sinica (in Chinese), 37 (1): 109–115.Cao, X.D., Chen, Y., Wang, X.R., and Deng, X.H. (2001) Effects of redox potential and
pH value on the release of rare earth elements from soil. Chemosphere, 44 (4):
655–661.
Cao, X.D., Wang, X.R., Zhao, G.W., Gao, X.D., and Wang, X.R. (2000) Assessment of
the bioavailability of rare earth elements in soils by chemical fractionation and
multiple regression analysis. Chemosphere, 40 (1): 23–28.
Gao, X.J., Zhang, S., and Wang, L.J. (1999a) Soil geochemistry of rare earth elements
(REE) in REE-riched landscape in southern Jiangxi Province. Acta Pedologica
Sonica (in Chinese), 36 (4): 492–498.
Gao, X.J., Zhang, S., and Wang, L.J. (1999b) The adsorption of La3þ and Yb3þ on soiland mineral and its environmental significance. China Environmental Science,
19 (2): 149–152.
Gao, Z., Hong, W., Xiong, B., Zheng, W., and Wu, Y. (1999c) A study of the migration
and adsorption kinetics of REEs in soils. Material Science Forum, 315–317:
520–524.
Gu, Z.M., Wang, X.R., Cheng, J., Wang, L.S., and Dai, L.M. (2000) Effects of sulfate
on speciation and bioavailability of rare earth elements in nutrient solution.
Chemical Speciation and Bioavailability, 12 (2): 53–58.
Z. Hu et al.1414
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Gu, Z.M., Wang, X.R., Gu, X.Y., Cheng, J., Wang, L.S., Dai, L.M., and Chen, Y.J.
(2001) Effects of fulvic acid on the bioavailability of rare earth elements and
GOT enzymes activity in wheat (Triticum Aestivum). Chemosphere, 44 (4):
545–551.
Haley, T.G. (1979) Toxity. In Handbook on the Physics and Chemistry of Rare Earths;
Gschneidner, K.A. and Eyring, L.R. (eds.); North Holland: Amsterdam, Oxford,New York, Toronto, Vol. 4, 555.
He, M.L. and Rambeck, W.A. (2000) Rare earth elements–a new generation of growth
promoters for pigs. Archives of Animal Nutrition, 53: 323–334.
Henke, G. (1977) Activation analysis of rare earth elements in opium and cannabis
samples. The Journal of Radioanalytical and Nuclear Chemistry, 39: 69–83.
Huang, C.M. and Gong, Z.T. (2001) Geochemical implication of rare earth elements in
progress of soil development. Journal of Rare Earths, 19 (1): 57–62.
Hu, C.P. and Liu, W.H. (1988) Measurement of TRE in Soils; National Bureau of
Standard of PRC: Beijing, GB 6260-68.Hu, Z.Y., Richter, H., Sparovek, G., and Schnug, E. (2004) Physiological and bio-
chemical effects of rare earth elements on plants and their agricultural significance:
a review. Journal of Plant Nutrition, 27 (1): 183–219.
Ichihashi, H., Morita, H., and Tatsukawa, R. (1992) Rare earth elements (REEs) in
naturally grown plants in relation to their variation in soils. Environmental
Pollution, 76: 157–162.
Jiang, Y.Q., Li, P., Chen, D.J., Yan, F., and Zhai, X. (1994) Rare earth elements in the
chief soils of Shandong Province. Laiyang Journal of Agricultural College (in
Chinese), 11 (1): 38–41.Jones, D.L. (1997) Trivalent metal (Cr, Y, Rh, La, Pr, Gd) sorption in two acid soils and
its consequences for bioremediation. European Journal of Soil Science, 48:
697–702.
Kawasaki, A., Kimura, R., and Arai, S. (1998) Rare earth elements and other trace
elements in wastewater treatment sludges. Soil Science and Plant Nutrition,
44 (3): 433–441.
Koeppenkastrop, D. and DeCarlo, E. (1993) Uptake of rare earth elements from
solution by metal oxides. Environ. Science and Technology, 27: 1796–1802.
Land, M., Ohlander, B., Ingri, J., and Thunberg, J. (1999) Solid speciation and fraction-ation of rare earth elements in a spodosol profile from northern Sweden as revealed
by sequential extraction. Chemical Geology, 160 (1–2): 121–138.
Lau, J.C. and Weimer, W.C. (1982) Behaviour of REE in geological and biological
systems. In The Rare Earths in Modern Science and Technology; McGrathy, G.J.
(ed.); Plenum Press: New Yok, Vol. 3, 351–535.
Li, D.C., Huang, S.B., Wang, W.H., and Peng, A. (2000) Study on kinetics of
adsorption-desorption of Ce(III) in different soils. Acta Scientiae Circumstantiae,
20 (5): 548–553.
Li, F.L., Shan, X.Q., and Zhang, S.Z. (2001) Evaluation of single extractants for
assessing plant availability of rare earth elements in soils. Communications in SoilScience and Plant Analysis, 32 (15&16): 2577–2587.
Liang, T., Zhang, S., Wang, L.J., Yan, B.Z., Wang, Y.Q., Sun, A.T., and Zhai, H.
(2000) Biogeochemical characteristics of rare earth elements in soil-plant system.
Acta Pedological Sinica (in Chinese), 37 (Supplement): 119–129.
Liu, Y.J. (1984) Element geochemistry (in Chinese); Science Press: Beijing, 548–549.
Liu, Z. (ed.); (1996) Rare earth elements in soil. In Microelements in Soils of China (in
Chinese); Jiangsu Science and Technology Publishing House: Nanjing, China,
293–329.
Rare Earth Elements in Soils 1415
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Liu, Z. (1998) Rare earth elements in soil. In Rare Earth Elements in Agriculture (in
Chinese); Guo, B.S., Zhu, W.M., Xiong, B.K., Ji, Y.J., Liu, Z. and Wu, Z.M.
(eds.); China Agricultural Science and Technology Press: Beijing, 22–44.
Liu, Y.L., Liu, J.H., Wang, Z.J., and Peng, A. (1999) Speciation transformation of
added rare earth elements in soil. Environmental Chemistry (in Chinese), 18 (5):
393–397.
Liu, C.S., Lao, X.R., Wang, W.X., and Chu, K.J. (1997a) Characteristics of REE in
soils and rocks of Shangdong Province. Chinese Journal of Soil Science (in
Chinese), 28 (1): 34–35.
Liu, S.J., Wang, L.J., Zhang, S., Gao, X.J., Wang, Y.Q., San, J.X., Chen, H.M., and
Qui, H.J. (1997b) Effect of long-term foliage-dressing rare earth elements on their
distribution, accumulation and translocation in soil-spring wheat system. Chinese
Journal of Applied Ecology, 8 (1): 55–58.
Liu, P.L., Tian, J.L., and Li, Y.Q. (1995) REE contents in topsoil on the loess plateau
and effects of REE applying on crops. Journal of Rare Earths (in Chinese), 13 (3):
221–224.
Liu, D.F. and Wang, Z.J. (2000) Dose-effect relationship for uptake of rare-earth
elements in corn after application as REE-fertilizer. Acta Pedologica Sinica (in
Chinese), 37 (3): 364–371.
Liu, Y.L., Wei, F.S., and Ten, E.J. (1992) Background characteristics of REE in granite
and basalt and soils originated from these parent material. Environmental Monitor-
ing in China, 8 (3): 18–21.
Liu, J.C., Su, Q.D., Wang, S.K., and Zhao, G.W. (1986) Concentration of rare earth
elements in the typical soils in Anhui Province. Anhui Journal of Agricultural
College (in Chinese), Special issue on Rare Earth Agricultural Application: 15–19.
Ma, Y.Z., Lao, X.R., and Liu, C.S. (1996) Study on distribution of REEs in peanut
treated by seed socking with REEs solution. Chinese Journal of Rare Earth (in
Chinese), 15 (5): 62–65.
Markert, B. (1987) Distribution of lanthanide elements in soil and plants. Phytochem-
istry, 26: 3167–3170.
Markert, B., Piehler, H., Leith, H., and Sugimae, A. (1989) Normalization and calcu-
lation of lanthanide element concentrations in environmental samples. Radiation
and Environment Biophysics, 28: 213–221.
Markert, B. and Zhang, D.L. (1991) Natural background concentration of rare earth
elements in a forest ecosystem. Science-of-the-Total-Environment, 103 (1): 27–35.
Martinez, G.A., Traina, S.J., and Logan, T.J. (1998) Evaluation of proton and europium
(III) binding reactions in a sewage sludge humic acid. Journal of Agriculture of the
University of Puerto Rico, 82 (3–4): 121–140.
Meehan, B., Peverill, J., Maheswaran, J., and Bidwell, S. (2001) The application of rare
earth elements in enhancement of crop production in Australia. In Proceedings of the
4th International Conference on Rare Earth Development and Application, Part 1;
Yu, Z.S., Yan, C.H., Xu, G.Y., Niu, J.K. and Chen, Z.H. (eds.); Metallurgical
Industry Press: Beijing, China, 224–250.
Miekeley, N., Casartelli, E.A., and Dotto, R.M. (1994) Concentration levels of rare
earth elements and thorium in plants from the Morro Do Ferro environment.
J. Radioanal. Nucl. Chem., 1982: 75–84.
Mohamad, H. and Rafek, A.G. (1993) The distribution of rare earth in tropical granitic
soils: a case study from Malaysia. Journal of Southeast Asian Earth Science,
8 (1–4): 617–625.
Z. Hu et al.1416
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Nagao, S., Rao, R.R., Killey, R.W.D., and Young, J.L. (1998) Migration between of
Eu(III) in sandy soil in the presence of dissolved organic materials. Radiochimica
Acta, 82: 205–211.
Nagao, S., Tanaka, T., Sakamoto, Y., and Seno, M. (1996) Sorption of europium(III)-
humate complexes onto a Sandy soil. Radiochimica Acta, 74: 245–249.
Ozaki, T., Ambe, S., Enomoto, S., Minai, Y., Yoshida, S., and Makide, Y. (2002)Multitracer study on the uptake mechanism of yttium and rare earth elements by
autumn fern. Radiochimica Acta, 90 (5): 303–307.
Pang, X., Li, D., and Peng, A. (2002) Application of rare-earth elements in the agricul-
ture of China and its environmental behavior in soil. Environmental Science and
Pollution Research, 9 (2): 143–148.
Peng, A. and Zhu, J. (2003) Environmental Chemistry and Ecological Effect of Rare
Earth Elements; China Environmental Science Publishing Company: Beijing.
Peng, A. and Wang, Z.J. (1995) Progress in recent studies on rare earth environmental
chemistry. Advances in Environmental Science (in Chinese), 3 (4): 22–25.Peng, A. and Wang, Z.J. (1996) Recent research progress on environmental chemistry
of rare earth elements, introduction. In Proceedings of the First Sino-Dutch
Workshop on the Environmental Behavior and Ecotoxicology of Rare Earth
Elements; Beijing, 125–131.
Ran, Y. (1991) Chemical behaviors and availability of REEs in soils. PhD Thesis,
Institute of Soil Science, Chinese Academy of Sciences (in Chinese).
Ran, Y. and Liu, Z. (1991) Content and fractions of rare earth elements in soils. In Pro-
ceedings of 2nd InternationalConference on Rare Earth Development and Appli-
cation (in Chinese); International Academic Publishers: Beijing; Vol. II, 988–994.Ran, Y. and Liu, Z. (1994) Concentration and distribution of REEs in mainly soils of
China. Joumal of the Chinese Rare Earth Society (in Chinese), 12: 248–256.
Ran, Y. and Liu, Z. (1992) Characteristics of adsorption and desorption of REEs on
main type soils of China. Joumal of the Chinese Rare Earth Society, 10 (4):
377–380.
Ran, Y. and Liu, Z. (1993a) Chemical speciation of exogenous REEs in main types of
soil of China. Journal of the Chinese Rare Earth Society, 14 (2): 31–35.
Ran, Y. and Liu, Z. (1993b) Adsorption and desorption of rare earth elements on soils
and synthetic oxides. Acta Scientiae Circumstantiae, 13 (3): 287–293.Shan, X.Q., Lian, J., and Web, B. (2002) Effect of organic acids on adsorption and
desorption of rare earth elements. Chemosphere, 47 (7): 701–710.
Shi, C.W., Zhao, L.Z., Guo, X.B., Gao, S., Li, J.H., and Yang, J.P. (1995) Background
values of REEs in soils of Shanxi Province. Shanxi Joumal of Agricultural Sciences
(in Chinese), 23 (2): 38–40.
Song, C.Z. and Chen, P.H. (1987) Determination of REES in soils by ICP-AES.
AnaIytical Chemistry (in Chinese), 16 (8): 717–723.
Steinmann, M. and Stille, P. (1997) Rare earth element behavior and Pb, Sr, Nd, isotope
systematics in a heavy metal contaminated soil. Applied Geochemistry, 12 (5):
607–623.Stokes, J., Meehan, B., Fowless, A., and Peverill, K. (2001) The bioavailability of
applied lanthanum in 7 different soils from Victoria, Australia. In Proceedings of
the 4th International Conference on Rare Earth Development and Application,
Part 1; Yu, Z.S., Yan, C.H., Xu, G.Y., Niu, I.K. and Chen, Z.H. (eds.); Metallurgical
Industry Press: Beijing, China, 255–261.
Stokes, J., Meehan, B., Fowless, A., and Peverill, K. (1999) The investigation of
lanthanum sorption in agricultural soils. Materials Science Forum, 315–317:
361–367.
Rare Earth Elements in Soils 1417
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Sun, J.X. and Wang, Y.Q. (1993) Study on the geochemical indicators of elements in
soil. Joumal of the Chinese Rare Earth Society (in Chinese), 11 (1): 73–82.
Sun, H., Wang, X.R., Wang, Q., Wu, H., and Yang, L.H. (1998) Study on the species
and the bioavailability of rare earth elements in soil and wheat system. China
Environmental Science, 18 (3): 268–271.
Takada, J., Sumino, T., Tanaka, Y., Nishimura, K., and Akahoshi, M. (1998) Corre-lation between terbium and the other rare earth elements in fern leaves. The
Journal of Radioanalytical and Nuclear Chemistry, 236 (1–2): 35–38.
Todorovsky, D.S., Minkova, N.L., and Bakalova, D.P. (1997) Effect of the application
of superphosphate on rare earth content in the soils. Science-Total-Environment,
203 (1): 13–16.
Ure, A.M. and Bacon, J.R. (1978) Comprehensive analysis of soils and rocks by spark-
source mass spectrometry. Analyst, 103: 807.
Vinogradov, A.P. (1959) The Geochemistry of Rare and Dispersed Elements in Soils;
Consultant Bureau, Inc.: New York, pp. 145.Volokh, A.A., Gorbnov, A.V., Gundorina, S.F., and Revich, B.A. (1990) Phosphorus
fertilizer production as a source of rare-earth elements pollution of the environment.
The Science of the Environment, 95: 141–148.
Wang, N., Cao, L., Cao, X.D., Wang, X.R., and Dai, L.M. (2000) The effects of organic
ligand EDTA on the adsorption and desorption of rare earth elements in soils. China
Environmental Science, 20 (4): 296–300.
Wang, X.K., Dong, W.M., Gong, Y.C., Wang, C.H., and Tao, Z.Y. (2001a) Sorption
characteristics of radioeuropium on bentonite and kaolinite. The Journal of Radio-
analytical and Nuclear Chemistry, 250 (2): 267–270.Wang, Z.W., Shan, X.Q., and Zhang, S.Z. (2001b) Comparison of speciation and bio-
availability of rare earth elements between wet rhizosphere soil and air-died hulk
soil. Analytical Chimical Acta, 441 (1): 147–156.
Wang, Z.J., Wang, C.X., Lu, P., and Zhu, W.Z. (2001c) Concentrations and flux of rare
earth elements in a semifield plot as influenced by their agricultural application. Bio-
logical Trace Element Research, 84 (1–3): 213–226.
Wang, S.M. and Zheng, W. (2001) The developing technology of rare earth application
on agriculture and biological field in China. In Proceedings of the 4th International
Conference on Rare Earth Development and Application, Part 1; Yu, Z.S.,Yan, C.H., Xu, G.Y., Niu, J.K. and Chen, Z.H. (eds.); Metallurgical Industry
Press: Beijing, China, 237–240.
Wang, Y.Q., Sun, J.X., and Tu, S.D. (1988) Concentrations and distributions of REE in
the soils from Xingjiang valley. Acta Scientiae Circumstantiae (in Chinese), 8 (2):
172–178.
Wang, Y.Q., Sun, J.X., Chen, H.M., and Guo, F.Q. (1996) Study on speciation of rare
earth elements in soil. Journal of Nuclear and Radiochemistry, 18 (3): 146–151.
Wang, Y.Q., Sun, J.X., Chen, H.M., and Guo, F.Q. (1997a) Study on contents and dis-
tribution characteristics of rare earths in natural plants by NAA. Journal of the
Chinese Rare Earth Society, 15 (2): 160–164.Wang, L.J., Wang, Y.Q., and Zhang, S. (1997b) Speciation of rare earth elements
in different types of soils in China. Journal of the Chinese Rare Earth Society
(in Chinese), 15 (1): 64–69.
Wang, Z.G, Yu, X.Y., and Zhao, Z.H. (1989) Rare Earth Elements Geochemistry
(in Chinese); Scientific Publishing Company: Beijing, China.
Wang, L.J., Zhang, S., Gao, X.J., Liu, S.J., Wang, Y.Q., Sun, J.X., Hu, A.T.,
Chen, H.M., and Gao, F.Q. (1998) Geochemical characteristics of rare earth
elements in different types of soils in China. Journal of Rare Earth, 16 (1): 51–67.
Z. Hu et al.1418
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Wang, C.X., Zhu, W., Wang, Z.J., Guicherit, R., Wang, C.X., and Wang, Z.J. (2000)
Rare earth elements and other metals in atmospheric particle matter in the western
part of the Netherlands. Water, Air and Soil Pollution, 121 (1–4): 109–118.
Wang, X.M. and Zhan, G.L. (1992) Adsorption of Nd by soil of main type of Anhui
Province. Journal of the Chinese Rare Earth Society (in Chinese), 10 (2): 159–162.
Wen, B., Yuan, D.A., Shan, X.Q., Li, F.L., and Zhang, S.Z. (2001) The Influence ofrare earth element fertilizer application on the distribution and bioaccumulation
of rare earth elements in plants under filed conditions. Chemical Speciation and
Bioavailability, 13 (2): 39–48.
Wen, B., Shan, X.Q., Lin, J.M., Tang, G.G., Bai, N.B., and Yuan, D.A. (2002) De-
sorption kinetics of yetuinm, lanthanum, and cerium from soils. Soil Science
Society of America Journal, 66 (4): 1198–1206.
Wheeler, D.M. and Power, I.L. (1995) Comparison of plant uptake and plant toxicity of
various ions in wheat. Plant and Soil, 172 (2): 167–173.
Wutscher, H.K. and Perkins, R.E. (1993) Acid extractable rare elements in Floridacitrus soils and trees. Comminications of Soil Science and Plant Analysis, 24:
2059–2068.
Wu, Z.H., Wang, X.R., Zhang, Y.F., Dai, L.M., and Chen, Y.J. (2001) Effects of apatite
and calcium oxyphosphate on speciation and bioavilability of exogenous rare earth
elements in the soil-plant system. Chemical Speciation and Bioavailability, 13 (2):
49–56.
Wyttenbach, A., Furrer, V., Schleppi, P., and Tobler, L. (1998) Rare earth elements in
soil and soil-grown plants. Plant and Soil, 199: 267–273.
Wyttenbach, A., Schleppi, P., Bucher, J., Furrer, V., and Tobler, L. (1994) The accumu-lation of the rare earth elements and of scandium in successive needle age classes of
Norway spruce. Biological Trace Element Research, 41: 13–29.
Xie, Z.B., Zhu, J.G., Chu, H.Y., Zeng, Q., Zhang, Y.L., and Cao, Z.H. (2001) Influence
of exogenous lanthanum on fertility parameters of red soil and paddy soil. Journal of
Rare Earths, 19 (3): 229–232.
Xiong, B.K., Chen, P., Guo, B.S., and Zheng, W. (2000) Rare Earth Element Research
and Application in Chinese Agriculture and Forest (in Chinese); Metallurgical
Industry Press: Beijing, China, 1–151.
Xu, X.K. and Wang, Z.J. (2001) Effects of lanthanum and mixture of rare earths onammonium oxidation and mineralization of nitrogen in soil. European Journal of
Soil Science, 52: 323–329.
Xu, Z.J., Li, D.C., Yang, J.H., and Peng, A. (2001) Effects of carbonate on exchange-
ability and bioavailability of exogenous neodymium in soil. Journal of Rare Earths,
19 (3): 233–237.
Yang, J.Z., Liao, I., Xiong, D.G., Yang, W.Z., and Wen, Y.F. (1992) Background
values and geochemical characteristics of REEs in soils of Sichuang Province.
Environmental Monitoring in China (in Chinese), 8 (3): 90–94.
Yang, Y.G., Liu, C.Q., Yuan, K.N., and He, Z.L. (1999a) Distribution of rare elements
in red soils in Southern China. GeoChimica (in Chinese), 28 (1): 70–79.Yang, L.H., Wang, X.R., and Sun, H. (1999b) The effect of EDTA on rare earth
elements bioavailability in soil ecosystem. Chemosphere, 38 (12): 2825–2833.
Yoshida, S., Muramatsu, Y., Tagami, K., and Uchida, S. (1998) Concentrations
of lanthanide elementals, Th, and U in 77 Japanese surface soils. Environment
International, 24 (3): 275–286.
Zhai, H., Yang, Y.G., Zheng, S.J., Hu, A.T., Zhang, S., and Wang, L.J. (1999) Selection
of the extractants for available rare earths in soils. China Environmental Science,
19 (1): 67–71.
Rare Earth Elements in Soils 1419
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
Zhang, S.Z. and Shan, X.Q. (2001) Speciation of rare earth elements in soil andaccumulation by wheat with rare earth fertilizer application. EnvironmentalPollution, 112 (3): 395–405.
Zhang, S.Z., Shan, X.Q., and Li, F.L. (2000a) Low-molecular-weight-organic-acid asextractant to predict plant bioavailability of rare earth elements. InternationalJournal of Environmental Analytical Chemistry, 76 (4): 283–294.
Zhang, Z.Y., Wang, Y.Q., Sun, J.X., and Li, F.L. (2000b) Chemical species of rareearth elements in RE hyperaccumulator Dicranopteris dichotoma. Chinese RareEarths (in Chinese), 21 (3): 42–58.
Zhang, L., Zhu, W.M., and Zhang, J.Z. (1996a) Determination of REE adsorption, des-orption, and diffusion by isotopic tracer. Journal of the Chinese Rare Earth Society(in Chinese), 14: 249–253.
Zhang, J.Z., Zhu, W.M., and Zhang, L.G. (1995) Accumulation, distribution, andmigration of 141Ce and 147Nd in soils of China. In Proceedings of the 3rd Inter-national Conference on Rare Earth Development and Application; Xu, G.X.,Xia, J.M., Yu, Z.S. and Yan, C.H. (eds.); Metallurgical Industry Press: Baotou,Inne Mongolia, China; Vol. II, 21–25.
Zhang, F.S., Yamasaki, S., and Kimura, K. (2001) Rare earth element content invarious water ashes and the potential risk to Japanese soils. Environment Inter-national, 27 (5): 393–398.
Zhou, S.M., Zhu, F.Y., Liu, B.Z., Yang, Q.P., Dong, Y.Z., and Xiao, Y. (1989) Inves-tigation and study of the background level of rare earth elements in Henan Province.Henan Journal of Science (in Chinese), 3–4: 158–180.
Zhu, Y.Y. and Chen, J.J. (1984) Effect of plant on absorption, distribution and accumu-lation of REEs. Rare metal (in Chinese), 4: 27–31.
Zhu, Y.Y. and Chen, J.J. (1989) Study on course of for plant uptake of REEs. RareMetal (in Chinese), 7: 91–96.
Zhu, W.M. and Hu, Z.Y. (eds.); (1988) Interrelation among soil soluble RE and N, P, Kin plant. In Proceedings of International Symposium on W-Ti-RE-Sb. Chinese RareEarth Society; Metallurgical Industry Press: Bao Tou, China, 1291–1295.
Zhu, Q.Q. and Liu, Z. (eds.); (1985) Soluble rare earth elements in soils of China. InNew Frontiers in Rare Earth Science and Applications (in Chinese); Chinese RareEarth Society, Science Press: Beijiug, 1511–1515.
Zhu, Q.Q. and Liu, Z. (1988) REEs in soils of eastern China. Journal of the ChineseRare Earth Society (in Chinese), 6 (4): 59–65.
Zhu, Q.Q. and Liu, Z. (1991) Content and distribution of rare earth elements in soils ofChina. In Proceedings of the International Symposium on the Role of Magnesiumand Micronutrients in Balanced Plant Nutrition; Metallurgical Industry Press:Chengdu, China, 94–99.
Zhu, J.G. and Xing, G.X. (1992a) Sequential extraction procedure for the speciation ofREEs in soils. Soils (in Chinese), 24 (4): 215–218.
Zhu, J.G. and Xing, G.X. (1992b) Forms of rare earth elements in soils: I. Distribution.Pedosphere, 2 (2): 125–134.
Zhu, J.G., Xing, G.X., Yamasaki, S., and Tsumura, A. (1993) Adsorption and deso-rption of exogenous rare earth elements in soils: I. Rate of forms of rare earthelements sorbed. Pedosphere, 3 (4): 299–308.
Zhu, W.M., Zhang, J.Z., and Zhang, L.G. (1996) Numerical simulation of RE migrationin soils. Journal of the Chinese Rare Earth Society, 14 (4): 341–346.
Zimny, H. and Korzeniewska, E. (1996) Lanthanide concentration in warsaw soils asan environment anthropization indicator. Polish Journal of Environment Studies,5 (4): 51–53.
Z. Hu et al.1420
Dow
nloa
ded
by [
"Que
en's
Uni
vers
ity L
ibra
ries
, Kin
gsto
n"]
at 1
7:07
19
Aug
ust 2
013
