Vertical Distributions of Trace Metals in Natural Soil Horizons from Japan. Part 1. Effect of Soil Types

VERTICAL DISTRIBUTIONS OF TRACE METALS IN NATURAL SOILHORIZONS FROM JAPAN. PART 1. EFFECT OF SOIL TYPES

YOKO FUJIKAWA∗, MASAMI FUKUI and AKIRA KUDOResearch Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka, Japan

(∗ author for correspondence, e-mail: [email protected])

(Received 12 November 1997; accepted 19 November 1998)

Abstract. To predict the long-term behavior of trace metals in a soil profile, we studied the ver-tical distributions of barium, zinc, copper, chromium, nickel, cobalt, lead, and the principal metals,aluminum, iron and manganese, in three soils with developed horizons obtained from rural areas inJapan. Total element analysis and selective extraction tests with various reagents were conductedto clarify the extractability of the metals at each sampling depth. Soil-b (Dystric Cambisols) had thehighest extractability of elements although the vertical distributions of its trace elements were similarto those of soil-d (Umbric Andosols), which had to lowest metal extractability of the three soils. Soil-KUR (Orthic Acrisols or Dystric Cambisols) was the oldest of the three soils and showed downwardmovement of some trace metals (chromium and nickel) and principal elements (Fe and Mn) that wasprobably induced by long-term weathering. The extractability of manganese, zinc and barium withwater was higher than other metals examined in all three soils. Lead and cobalt in soil-b and soil-KUR also were considered to have high extractability under long-term weathering processes. Zincand lead accumulated near soil surface showed higher extractability with every reagent used thanthose in deep layers of the three soils.

Keywords: horizon, selective extraction, soil, trace metal, vertical distribution

1. Introduction

A number of hazardous inorganic elements (e.g., lead, zinc, copper, chromium,nickel, cobalt, arsenic and selenium) have been emitted into atmospheric, terrestrialand aquatic environments as a result of such recent human activities as mining,smelting, manufacturing, municipal and industrial waste disposal and fossil fuelcombustion.

Vertical distributions of hazardous elements in soils have been well investig-ated to clarify the meachanisms of the contamination of soils and groundwater(existensively reviewed in Tiller, 1989; Ross, 1994). The chosen sites for theseprevious investigations were heavily polluted zones adjacent to industrial areas ormines (Merringtonet al., 1994; Scokartet al., 1983; Dumontetet al., 1990; Abdel-Sahebet al., 1994) or agricultural lands fertilized with manure contaminated byheavy metals (Williamset al., 1980; Dowdyet al., 1991; Dreiss, 1986). The resultsobtained for these sites reflect short term (a few decades) behavior of the elementsbecause the emissions have occurred only over the last few decades.

Water, Air, and Soil Pollution124: 1–21, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

2 Y. FUJIKAWA ET AL.

Long-term behavior of hazardous elements in the environment needs to be elu-cidated because recent emissions from human activities may pose adverse healtheffects to generations of people through pollution of water, air and soil. As for soilenvironment, long-term accumulation, leaching and plant uptake of elements in soilprofiles are important. The most appropriate means of predicting such behavior ofhazardous elements in soils over a period longer than a few decades is to investigatethe indigenous element distribution generated by long-term pedogenesis in naturalsoil profiles.

Indigenous elements are distributed within a soil profile as eluviated and accu-mulated zones develop (Buolet al., 1989), and are governed by such mechanismsas weathering, changes in soil formation factors (climate, landscape, parent mater-ials and living organisms) and bio-geochemical processes. A few studies have beenconducted to determine the long-term (centuries or millenniums) behavior of haz-ardous elements in soils. Studies by Bainet al. (1994) and Sjöström and Qvarfort(1992) showed that weathering rates of soil increased in recent times comparedto long-term weathering rates by measurement of distribution and extractabilityof principal elements (Ca, Mg, Na, K, Fe and Mn) in soil profiles. Koonset al.(1990) showed that trace elements (e.g. Co, Cr) were often associated with ironoxide minerals in the weathering of granite to saprolite. Kasimovet al. (1996)showed that 0.1 N HCl extractability of indigenous trace elements (Cr, Cu, Ni, Pb,Zn) in Russian soil differend considerably depending on soil types (e.g. dark-grayforest soil, leached chernozem, podzolized chernozem, floodland turf-gley soil,etc.) and horizons. Asamiet al. (1995) showed that exctactability of indigenoustrace elements Pb and Zn by ammonium acetate in soils is smaller than that ofanthropogenic elements.

The above studies suggest that (1) soil types, (2) horizons, (3) weathering,(4) iron-oxide minerals, and (5) origin of elements (anthropogenic or not), areimportant factors concerning trace element distribution and migration in soil pro-files. Although eluviation and accumulation of principal elements are known to bedependent on soil type and horizon (Buolet al., 1989), less is known about traceelements. Weathering of soil may affect mobility of trace elements. The fact thatiron-oxide minerals and origin of elements (anthropogenic or indigenous) affectstheir mobility in soil profiles suggests that chemical speciation of elements mustbe elucidated.

In order to clarify long-term eluviation and accumulation behavior of tracemetals governed by pedogenesis in Japan, we measured vertical concentration dis-tributions of the hazardous trace metals barium (Ba), zinc (Zn), copper (Cu), chro-mium (Cr), nickel (Ni), cobalt (Co) and lead (Pb), and the principal inorganicmetals which affect the behavior of these hazardous elements, aluminum (Al), iron(Fe) and manganese (Mn), in three Japanese soils of different morphology. Wetested extractability of these metals in soils by various reagents to elucidate theirchemical speciation. Effects of the soil type (brown forest soil, Ando-soil, red andyellow soil) on the distribution and speciation of these elements are considered.

VERTICAL DISTRIBUTIONS OF TRACE METALS IN NATURAL SOIL HORIZONS 3

Figure 1.Soil sampling sites in Japan.

2. Materials and Methods

2.1. SOILS

Samples were collected from three kinds of undisturbed, horizontally developedsoils from rural areas of Japan (Figure 1). Two pits of about 1 m depth wereexcavated in order to expose profile of the soil. Then fresh fallen leaves on thesoil surface were removed carefully so as not to disturb soft A horizon, and 3 to5 kg of samples were collected at 3–10 cm intervals to a depth of 70–90 cm.

Soil pH and Eh were measured at the site by 1:2.5 suspension in distilled water.Organic matter content of soil was measured by dry-combusting an oven-dried soilsamples at 375◦C for 16 hr and was calculated using the empirical equation forJapanese soil (Department of Agricultural Chemistry of Tokyo, 1988);

y = 0.458x − 0.4 (1)

wherey is the amount of organic carbon in the soil, andx the decrease in the soilmass after combustion.


Samples of each soil type were homogenized after removing large stones androots then sifted through a 2.0 mm plastic sieve. The sieved samples for the se-lective extraction tests were preserved moist in a dark place at 15◦C. For the totalelement analysis, samples were dried for 24 hr at 110◦C then ground to a powder(less than 200 mesh, approximately 80µm) in an agate mortar or a mortar coatedwith tungsten carbide to avoid contamination from grinding.

2.1.1. Soil-bClassified as brown forest soil (hereafter soil-b), which belongs to Dystric Cambisolsof FAO/UNESCO soil classification (United Nations, Food and Agriculture Organ-ization, 1988). It is typical of soils formed under deciduous broad-leaved forests ina cool-temperate climatic zones. Soil-b had a dark brown A horizon approximately15 cm thick and organic carbon content 2–8%, and brown B horizon which is about50 cm thick. Clay minerals such as montmorillonite was found in the deep layers.Soil was sampled in Shinrin Koen (forest park) in Naka-machi (population 45 000in 83.14 km2), Ibaragi Prefecture, Japan (Figure 1), which is located 30 km west toHitachi, one of the industrial centers of petrochemical and steel industry in Japan.The sampling site is in the middle of a relatively dry forest of Alnus Mill and Pinusdensiflora more than halfway up a hill. Its parent material is a mixture of tephradeposited several thousand years ago and materials transported by soil-creep, asjudged by the geographical location and landscape. The soil pH varied from 4.7 to5.4, Eh from 350 to 430 mV and density from 2.2 to 2.5 g cm−3 depending on thedepth.

2.1.2. Soil-dA dark-colored soil (hereafter soil-d), classified as Ando soil, which belongs tohumic Andosols of FAO/UNESCO soil classification (1988). It is typically foundin central Japan and the Kanto area, is comprised of a thick (sometimes morethan several meters), dark-colored A horizon enriched with organic matter suppliedfrom grass roots, and B horizon with allophanic properties. Soil-d had a A horizonthicker than 70 cm with organic carbon content 4 to 8%. The soil was sampledin Naka-machi, Ibaragi Prefecture, Japan. Its parent material is a tephra depositedseveral thousand years ago. The sampling site is relatively humid and flat, andcovered with Cryptomeria Japonica. The soil pH varied from 4.6 to 5.6, Eh from320 to 350 mV and density 1.6 to 1.7 g cm−3 depending on the depth.

2.1.3. Soil-KURClassified as red and yellow soil, Orthic Acrisols or Dystric Cambisols of FAO/UNESCO soil classification (1988). It is a typical diluvial soil found in southwestJapan, is well weathered and contains a little organic matter (organic carbon 1–2%. It is considered a relic of soil formed under a warmer, more humid climate 1million years ago. The soil was sampled at the site of the Kyoto University reactor(hereafter soil-KUR) in Kumatori-cho, Osaka Prefecture, Japan. Its parent material


is an old sediment belonging to the Plio-Pleistocene Osaka Group deposited 1million years ago (Itiharaet al., 1975). Soil-KUR had thin A horizon (less than10 cm) with organic carbon content of 2%, and yellowish orange B horizon. Thesampling site is in the middle of a hilly grassland covered with Pueraria lobata.The soil pH varied from 5.0 to 6.2, Eh from 240 to 400 mV and density from 2.2to 2.6 g cm−3 depending on the depth.

2.2. ANALYTICAL METHODS AND INSTRUMENTS

Analytical methods and instruments used in the trace metal analysis of the soils andleachates were (1) instrumental neutron activation analysis (INAA) done with theKyoto University reactor, irradiation under a flux of thermal neutrons 2.75×109

m−2 s−1 followed by gamma radioactivity measurements with a pure germaniumdetector; (2) inductively coupled plasma atomic emission spectrometry (ICP-AES)using ICPS-1000TR manufactured by Shimadzu Scientific Instruments, Inc.; (3)electrothermal atomization – atomic absorption spectrometry (ETAAS) with Zee-man background correction and a graphite furnace coated with high density graph-ite using Z-9000 manufactured by Hitachi Instruments Service Co.; and (4) induct-ively coupled plasma – mass spectrometry (ICP-MS) using HP-4500 manufacturedby Yokogawa Analytical Systems.

Principal and trace metals in the soils were analyzed using INAA (Al, Fe, Co),ICP-AES (Al, Mn, Fe, Ba) and ICP-MS (Zn, Cu, Cr, Ni, Co and Pb). Elementsin the extracting reagent were analyzed using ICP-AES (Al, Mn, Fe, Ba), ETAAS(Cr, Ni, Co, Cu, Pb) or ICP-MS (Ba, Zn, Cu, Cr, Ni, Co and Pb), depending on theconcentration of the element and the chemical characteristics of the extractant. Anelement sometimes was measured by multiple methods to check the precision ofthe analysis. Results obtained by the different analytical methods generally werein good agreement. Prior to analysis using ICP-AES and ICP-MS, the soil wasdecomposed in a MDS-2000 microwave oven manufactured by CEM Co.

2.3. REAGENTS

Nitric acid especially prepared for a class-100 environment (AA-100 grade, TamaChemical Co.), hydrofluoric acid (Suprapur grade, Merck) and perchloric acid (ul-trapure grade, Kanto Chemicals Co.) were used to decompose the soils, and boricacid (microselect grade, Fluka) was used to mask fluoride ions.

Palladium nitrate 99.999% (Aldrich) and magnesium nitrate (ultrapure grade,Kanto Chemicals Co.) were used as the matrix modifier in the analysis by ETAAS.

Mixed element standard solutions (Spex chemicals Co.) were used for the ICP-MS analysis, and single element solutions (1000 ppm each) for atomic absorptionspectrometry (Wako Chemicals Co.) were used for the ETAAS and ICP-AES ana-lyses. The extraction reagents were hydroxylamine hydrochloride (NH2OH-HCl),acetic acid, ammonium hydroxide solution (reagent grade, Wako Chemicals Co.)and hydrochloric acid (Suprapur grade, Merck).


2.4. TOTAL ELEMENT ANALYSIS

Soils ground to 200 mesh (approximately 80µm in diameter) were wet decom-posed with mixed acids (hydrofluoric acid, nitric acid and perchloric acid) in sealedTeflon vessels under microwave irradiation for the ICP-AES and ICP-MS meas-urements. Boric acid was then added to mask the free fluoride ion which attacksglasswares of ICP-AES and ICP-MS and to dissolve sparingly soluble fluorides(e.g CaF2). The recovery rates were 90–100% for the measurements of Zn, Cu, Cr,Ni, Co and Pb by ICP-MS for cerified values of standard soil and sediment samples,Mess-2 and Pacs-1 of the National Research Council, Canada, and NIST-2709 ofthe National Institute of Standard Technology, U.S.A. (Fujikawaet al., manuscriptin preparation). The recovery was 100–120% for the measurements of Al, Mn, Feand Ba by ICP-AES, based on recommended values of a standard rock sample,JA-2, of the Geological Survey of Japan. INAA was used with the dried soils.

2.5. SELECTIVE EXTRACTION TESTS

The extraction tests used are modification of those of Tessieret al. (1979) andFujikawa and Fukui (1991). All extractions were started by adding 30 mL of re-agent to a 12 g wet soil sample in a 50 mL Teflon centrifuge tube (Nalge Company).To extract the elements, the tubes were shaken in a water bath equipped with atemperature control system. Supernatants obtained after centrifugation for 30 minat 10 000 rpm, 4◦C were analyzed by ETAAS, ICP-AES or ICP-MS, dependingon the elements of interest. Palladium nitrate together with magnesium nitrate wasadded as the matrix modifier for ETAAS analysis of Pb with the NH2OH-HClextractant. To avoid extractant contamination by the vessels, the centrifuge tubeswere washed with detergent, soaked overnight in 1 N HCl then in 1 N HNO3 andrinsed with double distilled water prior to use.

The extracting procedure and characteristics of the fraction extracted were:

(1) Extraction with double distilled water for 3 weeks at 30◦C. The extractedfractions were those easily desorbed from the solid phase. The mechanism ofdesorption may be low sorption of the elements to the solid phase, high solu-bility of the elements in water or resuspension of the elements as colloids. Theextractablity of elements as governed by eluviation (transportation of dissolvedor suspended soil material within a soil by the downward or lateral movementof water) is reflected in this test.

(2) Extraction with 1 N ammonium acetate (pH 7) for 1 week at 30◦C. The ele-ments extracted were those exchangeable with NH+

4 or CH3COO− ions at pH7.

(3) Extraction with 0.2 M NH2OH-HCl v/v 25% acetic for 12 hr at 85◦C. Theextracted fraction is the sum of the elements associated with amorphous oxidesof iron or manganese that are easily dissolved by NH2OH-HCl (a mild redu-cing reagent) plus the elements adsorbed by ion exchange. Amorphous oxides


together with the trace metals associated with them are considered more sus-ceptible than crystalline minerals to dissolution by future weathering processesand anaerobic conditions brought about by change in the regional groundwaterflow.

(4) Extraction with 0.1 N HCl at 30◦C for 3 weeks. The elements extracted arethose susceptible to proton-promoted dissolution, including those exchange-able with H+ or Cl−. The extractant has often been applied to extract availableforms of Ni, Cu, Zn and Cd from soils (Page, 1982).

Differences of the extraction procedures from the published methods were (1)single-stage extraction rather than sequential extraction because extractants maymodify the mineral phase of the soil and affect the results of subsequent extrac-tions (Beckett, 1989); (2) adjustment of the period of extraction until transientequilibrium was obtained for the metal concentrations in the extractant in order toimprove the reliability of the method (Fujikawa and Fukui, 1991); (3) extraction ofamorphous iron and manganese oxides by reciprocal shaking with 0.2 M NH2OH-HCl in 25% acetic acid for 12 hr at 85◦C after experimenting with 0.04, 0.2 and2 M concentration of NH2OH-HCl; (4) moist storage of the soils because ovendrying may change the extractability of metals (Milleret al., 1986).

The period of extraction was adjusted by periodical measurement of the amountof some elements extracted from soil-KUR up to 3 weeks for double distilled water,0.1 N HCl and 1 N ammonium acetate. Equilibrium was attained after 3 weeksextraction with 0.1 N HCl (Ca, Mg, Cu and Ba) and 1 week extraction with 1 Nammonium Acetate (Ca, Mg, Pb). Extraction period therefore was set to be 3 weeksfor 0.1 N HCl and 1 week for 1 N ammonium acetate. As for double distilled waterextraction, amount of Ca, Fe and Al extracted increased with time and equilibriumwas not attained within 3 weeks. The Eh measurement showed that the extractionwas conducted under aerobic condition. In previous tests of extraction of radio-active cobalt and cesium sorbed on rocks with double distilled water (Fujikawaand Fukui, 1991), 120 day extraction was necessary for equilibrium to be attained.Extraction equilibrium may therefore be attained after longer extraction period incase of our soil samples, but such a long extraction may induce microbiologicalreactions which is beyond the scope of this study. Period of double distilled waterextraction therefore was set to be as long as practicable (i.e. 3 weeks).

Association between metals and the solid phase is stronger in the ascending or-der of double distilled water extraction < 1 N ammonium acetate (pH 7) extraction< 0.2 M NH2OH-HCl v/v 25% acetic acid extraction or 0.1 N HCl extraction. Inother words, the extracting reagent are stronger in the ascending order given above.Relative strength of 0.1 N HCl and NH2OH-HCl v/v 25% acetic acid is unknownat present.

A strong reagent extracts the fraction characteristic to it as well as the fractionextracted by a weaker extraction reagent, unless the reagents are applied sequen-tially (Beckett, 1989). To estimate the net amount of the elements extracted by a


reagent in a single stage extraction test such as ours, the amount extracted with areagent weaker than the reagent of interest is subtracted from the apparent amountextracted. Namely,

Eddw = Addw (2)

Eamm.ac. = Aamm.ac. − Addw (3)

ENH2OH−HCl = ANH2OH−HCl − Aamm.ac. (4)

whereEddw (Addw), Eamm.ac. (Aamm.ac.) and (ANH2OH−HCl) are the net (apparent)amounts extracted respectively in single-stage tests per unit mass of soil (mg/kg-soil) with double distilled water, 1 N ammonium acetate (pH 7) and 0.2 M NH2OH-HCl v/v 25% acetic acid.

As for 0.1 N HCl extraction test, the results were evaluated as they were be-cause the test was designed to examine the effect of proton-promoted dissolutionof the solid phase (i.e., susceptibility to acid rain), and by definition this fractionoverlaps with fractions extracted with double distilled water, ammonium acetateand NH2OH-HCl.

3. Results and Discussion

The vertical distributions of Al, Mn, Fe, Ba, Zn, Cu, Ni, Co and Pb (total concentra-tions) in soil-b, soil-d and soil-KUR are shown in Figure 2. As the result obtainedfrom two profiles as each sampling locations was very similar, the result from oneprofile was shown. Element-extraction findings for the three soils are shown inTables I (soil-b), II (soil-d) and III (soil-KUR) for extractions with double distilledwater, 1 N ammonium acetate (pH 7), 0.2 M NH2OH-HCl in 25% acetic and 0.1 NHCl.

3.1. FEATURES AND MECHANISMS OF THE VERTICAL DISTRIBUTIONS OF

TRACE ELEMENTS IN THE THREE SOILS

3.1.1. Soil-KURPeaks of Fe, Mn and the trace elements Cr and Ni appeared at a depth of 20–40 cmin soil-KUR (Figure 2).

The iron distribution mechanism probably is migration of small particles rich iniron down the soil profile and their accumulation at a 20–40 cm depth. This appearsto be caused by intensive weathering soil-KUR first was formed (Matsui, 1988) inthe diluvial age when the climate was warmer and the precipitation rate higher.

The distribution of Cr and Ni in soil-KUR is simular to that of Fe probablybecause iron oxide particles retain these metals during the weathering of parentmaterials as reported by Koonset al. (1980). The effect of trace element-organic

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9Figure 2.Vertical distribution of metal concentrations in three types of Japanese soils.


TABLE I

Fractions of metals (in %) extracted from soil-b by extracting reagents (%: extracted amount/totalamount in soil×100)

Depth ddw ammonium NH2OH- HCl ddw ammonium NH2OH- HCl

(cm) (%) acetate (%) HCl (%) (%) (%) acetate (%) HCl (%) (%)

Al in soil-b Mn in soil-b

1.5 0.01 0.01 5.3 2.0 4.0 15.0 24.3 54.2

6.5 0.01 0.01 6.9 2.8 0.6 4.9 33.8 55.1

15 0.01 0.01 7.5 3.0 0.3 1.8 33.5 50.0

25 0.00 0.00 6.3 3.0 0.3 0.8 29.2 39.6

35 0.00 0.00 5.5 3.1 0.1 0.4 32.9 32.0

45 0.00 0.00 3.5 3.0 0.0 0.5 27.5 25.6

55 0.00 0.00 2.3 2.0 0.0 0.5 32.0 24.1

65 0.00 0.00 3.1 2.7 0.0 1.7 32.2 9.1

Fe in soil-b Ba in soil-b

1.5 0.27 0.0 7.1 2.4 0.6 5.2 2.9 8.6

6.5 0.01 0.0 7.2 1.5 0.1 4.0 2.3 8.2

15 0.00 0.0 7.6 0.5 6.1 5.9 2.4 7.3

25 0.00 0.0 6.8 0.3 0.2 8.1 2.2 8.9

35 0.00 0.0 10.5 0.3 0.1 3.9 0.0 12.7

45 0.00 0.0 7.4 0.4 0.1 7.8 0.0 18.1

55 0.00 0.0 7.7 0.4 0.0 4.6 0.0 16.1

65 0.00 0.0 6.3 0.2 0.0 24.3 0.0 28.7

Zn in soil-b Cu in soil-b

1.5 0.27 1.5 11.5 16.5 0.00 0.3 0.4 2.1

6.5 0.18 0.0 18.2 23.5 0.01 0.4 1.6 5.9

15 0.12 0.0 12.6 18.0 0.00 0.4 3.4 6.9

25 0.00 0.0 9.1 19.6 0.00 0.4 3.6 7.9

35 0.00 0.0 6.1 4.2 0.00 0.6 5.1 10.8

45 0.00 0.0 2.2 14.2 0.00 0.4 6.1 15.8

55 0.00 0.0 8.5 11.1 0.00 0.5 6.8 17.1

65 0.00 0.0 8.3 13.3 0.00 0.1 3.6 8.3


TABLE I

(continued)



Cr in soil-b Ni in soil-b

1.5 0.01 0.01 0.3 0.9 0.31 0.51 4.1 8.2

6.5 0.00 0.00 0.9 0.8 0.00 0.13 4.1 13.1

15 0.00 0.03 1.9 0.7 0.00 0.30 3.7 12.2

25 0.00 0.01 2.9 1.0 0.00 0.00 3.5 12.0

35 0.01 0.01 2.8 1.4 0.00 0.00 2.5 11.4

45 0.00 0.01 5.3 9.8 0.00 0.18 1.7 18.6

55 0.00 0.00 4.0 4.1 0.00 0.05 2.4 5.1

65 0.00 0.00 3.6 1.5 0.00 0.00 1.3 0

Co in soil-b Pb in soil-b

1.5 0.12 0.22 6.0 24.5 0.04 1.02 18.9 1.0

6.5 0.00 0.07 13.3 32.0 0.01 0.61 31.9 0.6

15 0.00 0.00 20.0 30.1 0.00 0.29 37.7 0.3

25 0.00 0.00 19.5 28.1 0.00 0.06 13.7 0.1

35 0.00 0.00 14.4 20.4 0.00 0.00 10.1 0.0

45 0.00 0.06 10.9 18.0 0.00 0.40 9.4 0.4

55 0.00 0.14 19.1 29.2 0.00 0.00 9.7 0.0

65 0.00 0.05 10.7 7.7 0.00 0.08 8.6 0.1

chelates on trace element distribution in soil profiles is discussed in the Part II ofthis study (Fujikawaet al., submitted).

3.1.2. Soil-b and Soil-dTotal elemental analysis shows that the vertical distribution patterns of concentra-tions of some metals (Ba, Zn, Cu, Cr, Ni, Co and Pb) are similar in soil-b and soil-d(Figure 2). Namely, in both soils, Ba, Cu, Cr and Ni concentrations were almostconstant irrespective of the sampling depth, Zn and Pb concentrations were higherin shallow layer than in deep layer, and Co concentration was higher in deep layerthan in shallow layer.

The profiles obtained in the extraction tests, however, are very different betweensoil-b and soil-d. Soil-b has a more leachable fraction (%) in the shallow layer thansoil-d in terms of the extraction of Al, Mn, Fe, Zn, Cr, Ni and Co with doubledistilled water, the extraction of Mn, Zn, Ni, Co, Cr, Cu and Pb with ammoniumacetate, the extraction of Mn, Fe, Zn, Cu, Cr, Ni, Co and Pb with NH2OH-HCl


TABLE II

Fractions of metals (in %) extracted from soil-d by extracting reagents (%: extracted amount/totalamount in soil×100)



Al in soil-d Mn in soil-d

2.5 0.001 0.03 5.6 2.7 1.1 2.6 23.9 30.4

7.5 0.001 0.03 6.1 2.4 1.9 1.0 16.6 17.0

15 0.001 0.02 7.7 2.8 0.1 0.2 14.7 15.4

25 0.001 0.04 8.0 2.9 0.0 0.1 13.1 11.1

35 0.000 0.01 5.6 2.2 0.0 0.0 16.3 7.9

45 0.000 0.00 8.5 2.6 0.0 0.0 14.9 4.1

53.5 0.001 0.00 8.2 2.5 0.0 0.0 15.5 3.8

60 0.000 0.00 8.6 2.4 0.0 0.0 17.4 3.7

66.5 0.000 0.00 6.8 2.0 0.0 0.0 16.7 3.5

Fe in soil-d Ba in soil-d

2.5 0.00 0.00 5.1 0.21 0.09 4.2 1.9 6.8

7.5 0.00 0.00 3.3 0.11 0.19 5.7 2.5 7.7

15 0.00 0.00 3.5 0.06 0.28 5.4 2.6 10.1

25 0.00 0.00 3.3 0.01 0.43 7.1 1.8 12.2

35 0.00 0.00 3.5 0.01 0.23 6.8 6.1 9.0

45 0.00 0.00 3.1 0.01 0.09 7.1 2.6 9.9

53.5 0.00 0.00 3.0 0.01 0.02 5.9 4.7 9.9

60 0.00 0.00 3.6 0.01 0.00 7.2 3.6 9.4

66.5 0.00 0.00 3.3 0.01 0.01 6.7 4.8 8.1

Zn in soil-d Cu in soil-d

2.5 0.02 0.66 5.3 6.4 0.00 0.12 0.7 0.2

7.5 0.01 0.35 9.2 7.3 0.00 0.19 1.2 0.2

15 0.00 1.36 3.2 3.6 0.00 0.10 1.0 0.1

25 0.00 0.11 2.1 1.3 0.00 0.05 1.4 0.1

35 0.00 0.00 2.5 0.8 0.00 0.06 1.3 0.0

45 0.00 0.00 2.2 0.6 0.00 0.02 1.2 0.1

53.5 0.00 0.12 0.9 0.8 0.00 0.00 0.5 0.0

60 0.00 0.00 0.7 1.7 0.00 0.00 0.7 0.0

66.5 0.00 0.00 0.6 1.1 0.00 0.00 0.5 0.0


TABLE II

(continued)



Cr in soil-d Ni in soil-d

2.5 0.00 0.00 0.6 0.2 0.00 0.01 1.6 1.3

7.5 0.00 0.00 1.1 0.3 0.00 0.00 3.2 2.0

15 0.00 0.00 1.2 0.2 0.00 0.00 2.0 3.0

25 0.00 0.00 1.7 0.3 0.00 0.00 2.3 3.3

35 0.00 0.00 2.3 0.0 0.00 0.00 2.5 0.7

45 0.00 0.00 1.7 0.4 0.00 0.00 2.7 3.3

53.5 0.00 0.00 1.9 0.4 0.00 0.00 1.5 6.5

60 0.00 0.00 1.9 0.4 0.00 0.00 1.3 6.1

66.5 0.00 0.00 1.5 0.3 0.00 0.00 1.2 5.0

Co in soil-d Pb in soil-d

2.5 0.02 0.04 3.8 9.0 0.00 0.30 5.1 5.2

7.5 0.01 0.08 7.5 11.7 0.00 0.33 7.9 5.5

15 0.00 0.02 6.8 10.0 0.00 0.04 5.3 7.9

25 0.00 0.01 6.3 5.9 0.00 0.00 6.7 12.1

35 0.00 0.01 8.8 3.4 0.00 0.00 5.2 9.9

45 0.00 0.00 8.2 1.8 0.00 0.00 5.2 16.9

53.5 0.00 0.00 4.0 1.3 0.00 0.00 1.6 13.6

60 0.00 0.00 5.2 1.8 0.00 0.00 2.0 19.9

66.5 0.00 0.00 4.7 1.6 0.00 0.00 1.5 12.4

in 25% acetic acid, and the extraction of Mn, Fe, Ba, Zn, Cu, Cr, Ni and Co with0.1 N HCl (Tables I and II). Different vertical distributions of trace elements maydevelop in soil-b and soil-d profiles in the future as a consequence of the differencesin leachability.

3.2. METAL SOLUBILITY IN WATER IN THE THREE SOILS

Double distilled water extraction shows possible remobilization of elements inthe solid phase with infiltrating water because the chemical composition of theinfiltrating water was more similar to that of double distilled water extractant thanthat of the other extractants.

As shown in Tables I, II and III, the solubility of metals generally is higher inthe shallower regions of the soils, except for Ba in soil-d. The possible mechanisms


TABLE III

Fractions of metals (in %) extracted from soil-KUR by extracting reagents (%: extractedamount/total amount in soil×100)



Al in soil-KUR Mn in soil-KUR

1.5 0.00 0.00 0.52 1.04 2.36 8.03 34.68 47.82

4 0.00 0.00 0.54 1.24 0.72 3.17 26.43 37.15

7.5 0.00 0.00 0.48 1.00 0.13 1.08 26.50 37.77

12.5 0.00 0.00 0.52 1.19 0.03 0.64 33.77 34.86

17.5 0.00 0.00 0.44 1.04 0.03 0.55 39.18 29.44

25 0.00 0.00 0.36 0.75 0.02 0.36 33.91 27.83

35 0.00 0.00 0.41 0.87 0.01 0.16 28.21 39.40

45 0.00 0.00 0.45 0.88 0.01 0.00 14.12 20.19

55 0.00 0.00 0.43 0.99 0.00 0.00 24.17 31.95

65 0.00 0.00 0.38 0.86 0.00 0.17 22.69 27.39

75 0.00 0.00 0.35 0.92 0.00 0.14 9.74 16.56

85 0.00 0.00 0.30 0.90 0.00 0.00 5.81 18.45

Fe in soil-KUR Ba in soil-KUR

1.5 0.07 0.00 12.78 8.00 0.08 3.18 2.10 5.68

4 0.05 0.00 10.33 4.71 0.03 2.23 1.73 4.75

7.5 0.00 0.00 7.48 2.12 0.01 2.05 1.42 4.78

12.5 0.00 0.00 8.02 1.76 0.02 3.65 7.20 5.75

17.5 0.00 0.00 9.97 1.77 0.02 0.06 1.22 7.00

25 0.00 0.00 6.57 1.24 0.00 6.17 0.53 6.80

35 0.00 0.00 6.27 1.74 0.00 3.62 0.00 4.80

45 0.00 0.00 7.46 2.93 0.00 3.38 0.00 3.92

55 0.00 0.00 6.40 2.57 0.00 4.10 0.00 4.97

65 0.00 0.00 8.35 2.58 0.00 3.14 0.00 3.56

75 0.00 0.00 5.37 2.17 0.00 5.90 0.00 5.23

85 0.00 0.00 6.68 2.30 0.00 5.40 0.00 5.05

Zn in soil-KUR Cu in soil-KUR

1.5 0.00 1.72 29.01 31.41 0.00 0.11 1.00 6.73

4 0.00 1.03 21.58 24.39 0.00 0.25 1.66 9.21

7.5 0.00 0.54 13.89 15.44 0.00 0.10 1.91 8.38

12.5 0.03 0.56 14.45 16.74 0.00 0.23 3.37 9.28

17.5 0.02 0.56 14.76 18.64 0.00 0.10 2.61 8.90

25 0.00 0.16 11.85 14.87 0.00 0.21 4.24 10.14

35 0.00 0.00 9.22 13.54 0.00 0.18 4.54 10.73


TABLE III

(continued)



Zn in soil-KUR Cu in soil-KUR

45 0.00 0.00 8.37 9.72 0.00 0.19 4.79 11.82

55 0.00 0.00 5.40 9.45 0.00 0.17 3.34 9.56

65 0.00 0.03 5.21 9.62 0.00 0.00 3.05 7.72

75 0.00 0.53 3.20 12.56 0.00 0.04 3.06 7.60

85 0.00 0.37 2.73 12.43 0.00 0.04 3.36 6.98

Cr in soil-KUR Ni in soil-KUR

1.5 0.00 0.03 1.28 1.98 0.00 0.31 12.93 22.72

4 0.00 0.06 1.67 2.14 0.00 0.71 11.88 13.35

7.5 0.00 0.00 0.90 0.90 0.00 0.17 4.83 5.26

12.5 0.00 0.01 0.63 0.85 0.01 0.77 5.00 6.35

17.5 0.00 0.00 0.48 1.02 0.00 0.43 5.30 6.00

25 0.00 0.01 0.44 0.67 0.00 0.60 5.48 5.11

35 0.00 0.01 0.55 0.91 0.00 0.51 7.50 5.22

45 0.00 0.01 0.45 1.00 0.00 0.37 12.64 4.63

55 0.00 0.01 0.39 0.91 0.00 0.32 5.11 3.28

65 0.00 0.00 0.60 0.99 0.00 0.08 2.87 2.57

75 0.00 0.01 0.86 1.00 0.00 0.00 2.03 2.98

85 0.00 0.01 0.84 1.13 0.00 0.15 2.69 2.57

Co in soil-KUR Pb in soil-KUR

1.5 0.00 0.04 10.10 18.53 1.00 2.15 97.85 28.73

4 0.00 0.16 22.79 32.69 0.00 4.14 95.86 32.55

7.5 0.00 0.00 8.64 10.94 0.00 0.14 16.95 45.07

12.5 0.00 0.07 18.96 16.27 0.00 0.76 17.57 28.91

17.5 0.00 0.00 20.08 21.23 0.00 0.46 17.31 30.43

25 0.00 0.02 27.37 23.94 0.00 0.05 20.57 30.76

35 0.00 0.01 16.68 15.46 0.00 0.02 18.61 29.13

45 0.00 0.01 9.34 9.01 0.00 0.24 22.57 31.53

55 0.00 0.04 13.54 15.18 0.00 0.30 13.04 33.45

65 0.00 0.00 4.04 9.90 0.00 0.00 16.20 22.71

75 0.00 0.04 9.20 11.25 0.00 0.00 17.58 27.30

85 0.00 0.02 3.93 5.13 0.00 0.13 18.38 27.05


are (1) mineral components in the soils are more weathered in shallow layer thanin deep layer because of higher rainfall and biological activities, and more signi-ficant temperature variations; (2) the soil in the shallow layer is richer in organicmatter which may enhance desorption of metals as a soluble organic complex;(3) the extractant in the shallow layer contained more base cations (Na+, K+, Ca2+,Mg2+) which reduced the sorption of trace metals (i.e. increased the trace metalconcentration in the extractant) by increasing the competition for sorption sites.

Mn, Zn and Ba show ‘medium’ to ‘high’ (0.025 to 5%) solubility in all three Ja-panese soils and can be classified as metals of relatively high solubility. Fe solubil-ity in soil-b and soil-KUR is ‘low’ to ‘medium’ (0.07 to 0.3%) but is comparativelyhigh among the metals examined. Fe in water probably exists as a soluble organiccomplex rather than an inorganic ion, because free Fe2+ ions were consideredscarce under the aerobic conditions we used.

Soil-b had the highest element solubility of the three soils, principally owingto the release of the soluble metal-organic complex into the soil solution, becauseafter extraction the liquid phase had a slightly brown color caused by dissolvedorganic matter. Soil-d had the lowest element solubility, except for Ba, probablybecause of the high sorption capacity of metals by insoluble organic matter in thissoil.

The mobilities of the elements in soil-b (brown forest soil), Mn > Ni > Zn > Pb> Cr > Cu, are similar to those reported by Tyler (1978) for Swedish mor layersof spruce forest soils (Mn > Ni > Zn > Cu > Cr > Pb), although there must havebeen different pedogeneses (Buolet al., 1989) under a cool temperate region forest(soil-b) and a cold temperate one (the Swedish soil). The results for soil-d (Mn >Ba > Zn, Co > Al, Fe > Cr, Ni, Cu, Pb) and soil-KUR (Mn > Fe > Zn, Ba > Cr, Ni,Al > Cu, Pb, Co) are less similar to those of Tyler (1978) than the results for soil-b.In our soils, Mn and Fe were more soluble than reported by Plant and Raiswell(1983) (Zn > Cu, Co, Ni > Pb > Fe, Mn, Al, Cr).

3.3. ASSOCIATION OF METALS WITH AMORPHOUS IRON AND MANGANESE

OXIDES

Extraction tests with NH2OH-HCl in 25% ammonium acetate (Tessieret al., 1979)were designed to leach the amorphous oxides of iron and manganese, as well as thetrace metals associated with these oxides. Iron and manganese oxides retain tracemetals such as Co and Cr during the weathering of diabase and granite (Koonsetal., 1980). Secondary iron and manganese minerals also retain trace metals throughsorption (Chao and Theobald, 1976).

Metals extracted with acidified NH2OH-HCl are distributed unevenly over thesoil profile (Tables I, II and III):

(1) Soil-b shows a peak in the surface 0–15 cm layer (Pb, Ni, Zn and Mn) or oneat the 40–60 cm depth (Cu and Cr).


(2) Soil-d shows a peak in the surface 0–15 cm layer (Pb and Zn) but there are nodistinctive features for the distributions of the other elements.

(3) Soil-KUR shows a peak in the surface 0–15 cm layer (Pb, Zn, Cr, Fe and Mn)or a marked increase at the 20–40 cm depth (Cu and Co).

The high extractability found in the surface layer probably is due to the more ad-vanced stage of weathering in the shallow as opposed to the deep layer. Part of Pband Zn in the shallow layer probably are anthropogenic in origin (discussed later)and therefore are in more chemically labile forms than the indigenous elements inthe deep layers. Elevated total concentrations of Co at the 20–40 cm depth in soil-KUR may be due to the retention of once-eluted trace metals with amorphous ironoxides accumulated at that depth (i.e. absolute amount of NH2OH-HCl extractableFe is 2500–2700 mg kg−1 at the 20–40 cm layer compared to 1500–2000 mg kg−1

in other layers). The mechanism by which the peaks at 40–60 cm depth are formedin soil-b is not clear at present.

The order of metal mobility in the soil profile induced by long-term weatheringprocesses are considered to be reflected in the order of metal extractability withacidified NH2OH-HCl because such amorphous oxides are more chemically labilethan crystalline minerals.

The order of metal extractability with NH2OH-HCl in 25% acetic acid, ex-pressed as the ratio (in %) of the metal extracted to the total amount of that metalin a soil, is shown in Table V for the three soils. Metals extracted with the reagentare classified as highly extractable (≥ 10%), and not very extractable (< 10%).

The possible remobilization of Pb and Co in soil-b and soil-KUR due to long-term weathering or reducing conditions is higher than expected from the results ofdouble distilled water extraction tests (Table IV). Among the elements classified ashighly extractable in both soil-b and soil-KUR (Mn, Pb, Co and Zn), the order ofextractability of Pb and Co is markedly higher in NH2OH-HCl extraction (Table V)than in double distilled water extraction (Table IV).

Soil-d is much more resistant to NH2OH-HCl extraction (as it also is to doubledistilled water extraction) in comparison to soil-b and soil-KUR (Table V).

3.4. METAL EXTRACTABILITY IN THE THREE SOILS

The extractability of elements may be characterized in the following way:

Al: The ratio extracted with NH2OH-HCl inacetic from soil-b and soil-d wasas much as 8–9% (Tables I and II), probably because of the dissolution ofamorphous aluminum minerals (such as allophane) derived from the potentialparent material of these soils, a volcanic ash. Less Al was extracted withNH2OH-HCl from soil-KUR (Table III) because Al is much more crystallinein that soil than in soil-b and soil-d.


TABLE IV

Solubility (in %) of metals in water in three Japanese soils (%: extractedamount/total amount in soil× 100)

Relative solubility soil-b soil-d soil-KUR

High Mn (4.0%) Mn (2.0%) Mn (2.4%)

Medium Fe (0.3%) Ba (0.45%)

Ni (0.3%)

Zn (0.3%)

Ba (0.2%)

Co (0.1%)

Low Pb (0.04%) Zn (0.025%) Fe (0.07%)

Cr (0.015%) Co (0.015%) Zn (0.03%)

Al (0.01%) Al (0.001%) Ba (0.02%)

Cu (0.006%) Fe (0.0005%) Cr (0.009%)

Cr (0.0%) Ni (0.006%)

Ni (0.0%) Al (0.0006%)

Cu (0.0%) Cu (0.0%)

Pb (0.0%) Pb (0.0%)

Co (0.0%)

Mn: The ratio extracted with NH2OH-HCl in acetic acid (or HCl) from the threesoils was as much as 24–40% (or 50%) (Tables I, II and III). The ratios ofextraction with double distilled water and ammonium acetate are much higherthan those of the other elements (Tables I, II and III). Mn therefore may beleached by acidified rainwater and washed down soil profiles relatively easily.

Fe: The ratio extracted with NH2OH-HCl in acetic aid from the three soils was5–13% (Tables I, II and III). Relatively mobile in water (Table IV), but virtu-ally no ammonium acetate-exchangeable component exists in any of the soils(Tables I, II and III). More Fe was extracted by NH2OH-HCl than by 0.1 NHCl.

Ba: The amount extracted from the three soils with ammonium acetate was 5–30% and with HCl was 10–30% (Tables I, II and III). Ba may be leachedconsiderably upon acidification by rainwater.

Zn, Cu, Ni, Co, Cr and Pb: NH2OH-HCl-extractable and HCl-extractable com-ponents (up to 29%) were the principal extractable components in the three soils(Tables I, II and III). The apparent amounts extracted by NH2OH-HCl (i.e.ANH2OH−HCl in Equation (4)) and HCl were similar for Zn, Ba and Ni (soil-d), Ba,


TABLE V

Metals extracted with NH2OH-HCl from three Japanese soils. (The order of metalextracted with NH2OH-HCl in 25% acetic acid, expressed as the ratio of the metalextracted to the total amount of that metal in a soil (%), is shown. The contributionof the ammonium acetate – extractable component was subtracted from the amountextracted with acidified NH2OH-HCl according to Equation (4))

Relative extractability soil-b soil-d soil-KUR

High Pb (38%) Mn (24%) Pb (98%)

Mn (24%) Mn (39%)

Co (20%) Zn (29%)

Zn (18%) Co (27%)

Fe, Ni (13%)

Low Al, Fe (8%) Zn, Al, Co (9%) Ba (7%)

Cu (7%) Pb (8%) Cu (5%)

Cr (5%) Ba (6%) Cr (2%)

Ni (4%) Fe (5%) Al (0.5%)

Ba (3%) Ni (3%)

Ci (2%)

Cu (1%)

Zn, Cr and Ni (soil-KUR), and Zn (soil-b), which suggests that fractions of theseelements which may be leached by acidification of rainwater were associated withamorphous iron or manganese oxides. Of these elements, Zn had higher extractab-ility in water than the other metals, especially in soil-b. The extractability of Zn,Cu, Ni, Co and Pb (or Zn, Cu, Cr, Ni and Co) with NH2OH-HCl (or HCl) was muchless in soil-d than in soil-b and soil-KUR. Fractions of these elements extractablewith NH2OH-HCl or HCl may behave as follows under long-term weathering:re-adsorbed in amorphous iron or manganese minerals after being eluted; mech-anically washed down the soil profile with amorphous iron or manganese oxides,or both, as weathering proceeds; leached relatively easily as rainwater is acidifiedor a reducing condition develops in the soil.

3.5. VERTICAL DISTRIBUTION STATUS OF POTENTIALLY ANTHROPOGENIC

ELEMENTS IN THE THREE SOILS

The origin of the Pb accumulated in the surface 20 cm of the three Japanese soils(Figure 2) is considered to have been deposition from anthropogenic sources eventhough the soils were collected in rural areas. Measurement of the ratio of the stablelead isotope Pb-208/Pb-206 by ICP-MS showed that values in the surface 20 cm ofthese soils differed considerably from those obtained in a deeper region (Fujikawa


et al., manuscript in preparation). Because (1) this variation in the stable lead iso-topic ratio can only be caused by the presence of lead of different origin (Blais,1996; Facchetti, 1988; Mukaiet al., 1993) and (2) the source of lead depositedon land generally is anthropogenic rather than natural (Facchetti, 1988), the leadaccumulated in the surface layers of the three soils is considered anthropogenic.

Part of Zn accumulated near soil surface (approximately to the depth of 20 cmfrom surface) of Japanese soils may also be of anthropogenic origin having a chem-ical form different from that of indigenous zinc, because in every extraction test,zinc in the shallow layers was more extractable than that in the deep layers of thethree soils, as shown by double distilled water, ammonium acetate, NH2OH-HCl inacetic acid and 0.1 N HCl extractions (Tables I, II and III). None of the other ele-ments but Pb and Zn showed such uniform behavior. In fact, extractability of Zn byammonium acetate is reported to be high in polluted soils than in unpolluted soils(Asami et al., 1995). Anthropogenic emissions rate of Zn often exceeds naturalemission rate, according to Gallowayet al. (1982). Our sampling sites are rural butnot remote (30 km from industrial zones) and therefore the effect of anthropogenicemission on our site is possible even if the Zn concentration level in the soils is nottoxic. A more direct analytical method (e.g. isotopic ratio analysis of Pb) whichserves to identify the origin of Zn, however, is necessary for further investigation.

The higher extractability of potentially anthropogenic Pb in the shallow parts ascompared to the indigenous elements in the deep parts of our acid soils disagreeswith the findings of Tyler (1978) who reported low extractability of anthropogenicheavy metals in some polluted soils with high, probably owing to the difference insoil pH.

4. Conclusion

Three Japanese soils formed under different pedogeneses showed considerablydifferent trace element distribution in their soil profiles. Soil-b had the highestelement extractability although the vertical distribution of its trace elements wassimilar to that of soil-d. The metal extractability in solid-d was the lowest of thethree soils because of the low extractability of the metals with various reagents.Different vertical metal distribution profiles may develop in soil-b and soil-d in thefuture, as a result of differences in the metal extractability of these soils. Soil-KUR,considered the oldest of the soils, showed a downward movement of its metals (Crand Ni) with Fe and Mn that was induced by long-term weathering.

The extractability of Mn, Zn and Ba in water was relatively high in all threesoils. Pb and Co in soil-b and soil-KUR, as well as Mn, Zn and Ba, were consideredto be more mobile than the other metals under long-term weathering processes.

Anthropogenic Pb accumulated in the surface 20 cm of the three soils fromrural areas of Japan, the extractability of which every reagent used was higher thanthat of indigenous elements. Zn accumulated in the surface layers also showed


consistently high extractability than that in deep layers, suggesting the possibilitythat part of Zn in the topsoil was anthropogenic.

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Documents

Vertical Distributions of Trace Metals in Natural Soil Horizons from Japan. Part 1. Effect of Soil Types