1

HEAVY METAL POLLUTION OF SOIL ANDA NEW APPROACH TO ITS REMEDIATION:

RESEARCH EXPERIENCES IN JAPAN

Tomoyuki MakinoNational Institute for Agro-Environmental Science

3-1-3 Kannondai, Tsukuba, Japan

ABSTRACT

Rapid industrialization in the 1960s caused heavy metals such as cadmium (Cd) to severelypollute soil in Japan. The Agricultural Land Soil Pollution Prevention Law was enacted in 1970to cope with the heavy metal pollution. Cd, in particular, has been recognized as one of the mostdetrimental elements in Japan because of the so-called “itai-itai” disease caused through Cduptake. Recently, the Codex Alimentarius Commission proposed the maximum permissibleconcentration of Cd in polished rice and other relevant crops. Therefore, it is urgent for theJapanese government to evaluate the Cd uptake risk for Japanese citizens, and to minimize theCd risk by decreasing soil Cd contamination for food safety and for human health. This paper isan overview of the soil contamination of heavy metals in Japan, particularly of Cd. The naturalabundance level of heavy metals in Japanese soil is discussed referring to the regulations of soilheavy metal pollution. In addition, appropriate technologies to minimize soil Cd contaminationwere discussed and proposed on the following: (1) water management to reduce bioavailability ofsoil Cd to rice plants; (2) addressing and/or replacement of contaminated soil with non-pollutedsoil; (3) phytoremediation of the polluted soil by rice and other promising crops; and (4)chemical remediation of Cd-contaminated soil by washing it with chemicals such as iron salts.This paper also contains details on chemical remediation using iron chloride to alleviate the Cdcontaminated soil.

Key words: heavy metal, Cadmium, remediation, soil washing, rice

INTRODUCTION

Japanese arable soils, particularly paddy soilsin some regions, have been heavily pollutedwith cadmium (Cd) and other various heavymetals, owing to rapid industrialization duringthe 1960s. The Japanese government urgentlyenacted the Agricultural Land Soil PollutionPrevention Law in 1970 to cope with the heavymetal pollution, in which Cd, arsenic (As), andcopper (Cu) were the targeted hazardoussubstances for regulation. Cd, in particular, hasbeen recognized as one of the most detrimentalelements in Japan, because of the so-called“itai-itai” disease it caused (Kobayashi 1978).The law designated paddy fields as Cd-polluted, where unpolished rice grainscontaining more than 1 mg Cd kg –1 wereproduced. Ever since the law was in effect, thepolluted paddy soils have been remedied

mainly through unpolluted soil dressing and/orunpolluted soil replacement. However, theseremediable practices have become increasinglydifficult to implement because of their highcosts, and due to difficulty in obtaininguncontaminated soil.

In July 2006, the Codex AlimentariusCommission of the Joint FAO/WHO FoodStandards Program proposed 0.4 mg Cd kg–1 asthe maximum permissible concentration of Cdfor polished rice (Codex 2006). Therefore, it isurgent to develop cheaper, effective, andpromising technologies to resolve Cd pollutedpaddy soils, replacing soil dressing practices. Ithas been supposed that 34-50% of the Cdintake by Japanese citizens comes from rice(Kawada and Suzuki, 1998), hence, it is vital toalleviate Cd content in rice fields and ensurethe safety of this staple crop for the Japanesecitizens. This paper reviews the current status

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of the heavy metal contamination of Japanesearable soils and presents new approaches toprovide remedy to paddy soils contaminatedwith Cd.

CONCENTRATION OF HEAVY METALS INSOILS AND CROPS IN JAPAN

The natural abundance levels of some heavymetals in soils in Japan and around the worldare given in Table 1 (both the mean value andrange of the soil heavy metal concentrationswere presented). The ranges were very wide;the ratio of the highest value was a hundredtimes different from the lowest value. Theheavy metal concentration in Japanese soil wassimilar to that in the world soil, indicating thatthe natural abundance level of heavy metals inJapanese soil has been affected by factors notunique to Japan alone. The pollution of arable

soils by heavy metals is primarily caused bywastewater from mines used for irrigation waterto paddy fields, and by emissions fromnonferrous metal refining plants (Asami 2001).Humans are exposed to heavy metals byingesting crops grown in polluted soil anddrinking water contaminated with somehazardous heavy metals.

Codex (2005) also proposed the maximumpermissible level of Cd for other relevant cropssuch as wheat grain, edible roots and stem ofpotato, beans, leafy vegetables and some othervegetables (Table 2). To cope with newlyproposed Cd permissible standard, the Ministryof Agriculture, Forestry and Fisheries (MAFF)consulted with the Foodstuff Safety Committeeon revising the permissible Cd concentrationsin rice grains and other staple cops, ifnecessary (MAFF 2002). At the same time,MAFF has conducted a nationwide survey on

Table 1. Natural abundance of heavy metals in Japanese soil and brown rice(Average: mg kg-1)

Surface soil PaddySubstance Japan

a/World

c/soils

b/Brown rice

b/

Cr 58 63.7 64 ---(1.4-233) (1-1500)

Co 18 9.62 9 ---(0.2-61.3) (0.1-275)

Ni 26 22.92 39 0.19(0.2-107.4) (0.2-3240)

Cu 48 21.61 32 2.9(0.9-234.9) (1-323)

Zn 89 65.84 99 19(2.5-331) (3-770)

As 11b/

8.93 9 0.16(0.4-70) (0.07-95)

Mo 1.3 2.10 --- ---(0.1-8) (0.2-17.8)

Cd 0.3 0.48 0.45 0.07d/

(0.02-3) (0.01-4)

Hg 0.3b/

0.13 0.32 0.013(N.D.-5.4) (0.004-1.5)

Pb 24 29.85 29 0.19(1-1098) (1.5-286)

The values in the parenthesis mean ranges of heavy metal concentrations.a/Calculated from Yamasaki (2001), detailed data kindly provided.b/Iimura (1981).c/Calculated from Kabata-Pendias (2001).d/MAFF (2002).

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the current status of Cd content in somerelevant crops (Figs. 1 to 12). Table 3 showsthat many relevant crops contained Cd levelsmore than the Codex proposed values; fourcrops, in particular, exceeded more than 5% ofthe proposed value, which include aroid,burdock, gumbo and eggplant.

REGULATION OF HEAVY METALPOLLUTION OF SOILS IN JAPAN

The Japanese government has enacted thefollowing two laws to cope with arable soilpolluted with hazardous metals and otherchemical substances: 1) the Soil ContaminationCountermeasures Law; and 2) the AgriculturalLand Soil Pollution Prevention, which tookeffect in 2002 and in 1970, respectively. Inaddition to the two laws, various decrees andministerial ordinances have been issued.Targeted substances and the maximumpermissible concentrations of heavy metals

regulated by the two laws are summarized inTable 4 (MAFF 1970, MOE, 2003a, 2003b). Thedesignated standard for the hazardoussubstances under the Soil ContaminationCountermeasures Law are basically applicableto all types of soil including agricultural soil.However, the law is more often applied toformer sites of chemical and high-tech factories,where various harmful substances were onceused. In the long run, these sites may causehealth risk to people living around them.

The Cd standard designation concept ofthe Soil Contamination Countermeasures Lawis illustrated in Fig. 13. The law aims tocontrol heavy metal pollution of soils in termsof two factors, namely, soil Cd concentrationand Cd concentration in the soil leachate. Theformer may greatly affect the risk of Cd intakethrough direct ingestion, while the latter willminimize the risk of the Cd pollutedgroundwater to humans. A water extraction testwas performed to the leachate standard to

Table 2. International standard value of Cd concentration in crops adopted by Codex

Foods Values (mg/kg) Remarks

Polished rice 0.4Wheat grains 0.2Edible roots & stem 0.1 Except potatoPotato 0.1 StrippedBeans 0.1 Except soybeansLeaf vegetables 0.2Other vegetables 0.05 Except mushroom & tomato

Table 3. International standard value (Codex) and excess rate of Cd in staple crops(MAFF 2002)

Standard value Excess rateCrop (mg/kg) (%)

Rice1/ 0.4 0.3Wheat 0.2 3.1Spinach 0.2 3.0Aroid 0.1 9.9Burdock 0.1 5.6Carrot 0.1 1.5Green onion 0.1 1.0Onion 0.1 1.0Eggplant 0.1 7.3Gumbo 0.1 22.4

1/Codex: polished rice, MAFF: brown rice

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Fig. 1.Distribution of cadmium concentration in brownrice in Japan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Number of samples

Fig. 2.Distribution of cadmium concentration in wheatgrain in Japan.

Con

cent

ratio

n of

C

d (m

gkg

-1)

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 3.Distribution of cadmium concentration in grains(barley, rye and buckwheat) in Japan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 4. Distribution of cadmium concentration in soybean inJapan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 5. Distribution of cadmium concentration in spinach inJapan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 6. Distribution of cadmium concentration in cabbages(head) in Japan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 7. Distribution of cadmium concentration in Chinesecabbages in Japan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 8. Distribution of cadmium concentration in onionwelsh in Japan.

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determine the heavy metal concentrations in anunderground seepage (Table 4).

The Environmental Quality Standard(EQS) for Soil Pollution was enacted in August1991 (Table 4). Since February 1994, manysubstances have been added to EQS. Atpresent, it regulates a total of 25 hazardoussubstances. Guidelines for Investigation andPrevention of Soil and Groundwater Pollutionhave been effective since November 1994,ensuring efficient implementation of survey anddevelopment of prevention techniques basedon EQS. Administrative Guidelines for Pollutersare issued to urge people to voluntarily cleanup polluted soil. The analysis methods andvalues of the standards concurred with thepollutants in the leachate according to the SoilContamination Countermeasures Law and theAgricultural Soil Pollution Prevention Law(Table 4).

The Agricultural Land Soil PollutionPrevention Law designated Cd, arsenic (As),and copper (Cu) as hazardous substances tobe regulated. The maximum allowable limit ofCd in soil was decided through Cdconcentration in rice grains, but not by soil Cdconcentration, because the bioavailability of Cd

in soil is affected by many factors, like watermanagement for rice cultivation. Hence, usingsoil Cd content alone to determine themaximum allowable concentration is verydifficult. (Asami, 1981). As of 2005, 87.2% ofthe total polluted land (7,327 ha), designatedby the Agricultural Land Soil PollutionPrevention, was remedied with uncontaminatedsoil application and/or soil replacement (Fig. 14)(MOE, 2006). Fig. 15 shows a flow diagram forimplementation of the Agricultural Land SoilPollution Control Measures (MOE, 2007). Thecountermeasures used were mostly soildressing.

CONVENTIONAL CULTURAL PRACTICES TOALLEVIATE CD CONTAMINATION

IN RICE IN JAPAN

Water Management

Water management is a popular and cost-effective cultural practice for alleviating rice Cdcontamination in Japan. It influenced heavymetal content of paddy rice, particularly Cd.Hence, redox state of the paddy soil waspossible. Table 5 shows effect of water

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 9. Distribution of cadmium concentration in onion,bulb in Japan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 10. Distribution of cadmium concentration in taroexcept for skin in Japan.

Con

cent

ratio

n of

C

d (m

gkg

-1)

Number of samples

Fig. 11. Distribution of cadmium concentration inburdock in Japan.

Number of samples

Con

cent

ratio

n of

C

d (m

gkg

-1)

Fig. 12. Distribution of cadmium concentration incarrot in Japan.

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Table 4. Target substances and standards for heavy metal pollution of soils in Japan

Soil contamination Agricultural Land Soilcountermeasures law1/ Prevention Law1/

Soil concentration standard2/ Soil leachate standard3/,4/ Concentration standard4/

Substance (mg kg-1) (mg L-1) (mg kg-1)

Cd <150 <0.01 <1 in brown rice

As <150 <0.01 <15 in soil(paddy fields only)5/

Cu No designation No designation <125 in soil(paddy fields only)6/

Cr (VI) <250 <0.05 No designationPb <150 <0.01 No designationHg <15 <0.0005 No designation

alkyl Hg No designation Not detectable No designationSe <150 <0.01 No designation

1/The law also regulates fluorine, boron and hazardous organic substances.2/Extracted with 1M HCl, soil/solution (w/v) % = 3.3/Extracted with water, soil/water (w/v) = 0.1.4/Analysis methods and Standard values are identical with those in Environmental Quality Standard (MOE, 1991).5/Extracted with 1M HCl, soil/solution (w/v) = 0.2.6/Extracted with 1M HCl, soil/solution (w/v) = 0.2.

Fig.13. Concept of designation standard bythe Soil ContaminationCountermeasures Law (MOE 2006).

Fig.14. Areas where countermeasures areconducted for hazardous metalcontamination under theAgricultural Land Soil PreventionLaw (Revised from MOE 2006).

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management on Cd content of rice grains(Sakurai, NIAES, unpublished data). Cdabsorption by rice drastically decreasedthrough continuously submerging paddy fieldafter heading-time. Many Japanese scientistshave reported similar results.

It is most likely that a considerabledecrease in Cd absorption by rice undersubmerged condition is a decrease in the Cdsolubility due to the formation of Cd sulfides.When paddy field is flooded, the paddy soil israpidly reduced, and consequently, its redoxpotential (Eh) is shifted toward a reduced state(a sharp decrease in Eh), where sulfate ion canbe reduced to sulfide ion (Iimura, 1978). Thesulfide ion produced thus reacts with Cd toprecipitate out of soil solution as cadmiumsulfide. The precipitation of cadmium sulfide, inturn, lowers the Cd concentration in the soilsolution, lowering the amount of the Cdbioavailable to rice plants. The theoreticalexplanation based on physico-chemical equationwas summarized below.

Fig. 16 presents a relationship betweenredox potential (Eh) and pH for some heavymetal species, calculated by log K (Lindsay,1979). The diagram indicates that sulfate ionsare converted to hydrogen sulfide that lowersthe soil redox potential.

Supposed that the dominant sulfurspecies in soil solution are SO 4

2- , H2S, HS - 1

and S2-, the change in the relationship between{(SO4

2-) /((SO 42-)+(ÓH2S))} at pH7 and Eh is

expressed through a solid line in Fig. 17(Makino, 2002). The dashed line shows therelationship in a similar calculation at pH 6.The open circles show the measured extractableCd with 1 M ammonium acetate from the soilin a water-submerged incubation test. Theamount of Cd extracted with Eh was nicelyfitted to that of the calculated values,indicating the Cd extraction rate rapidlydecreased with an increase in the ratio ofÓH2S to total sulfur. The sulfide ion willprecipitate with Cd ion as cadmium sulfide(CdS), which is hardly soluble in water (Iimura1978).

Flooding from tilling to head formation inrice growth stage would be the most effectiveperiod to decrease the Cd content in ricegrains. It is highly recommended to keepflooding the paddy fields as late as possibletoward harvest time. However, the later theflooding keeps, the more difficult to operatemachine for harvest. Therefore, we have to finda way to lower bioavailable Cd and managemachine operation at the same time.

Soil Dressing

Soil dressing is simple and is one of the mostwidely used techniques for heavily

Fig.15. Outline of law on soil contamination prevention in agricultural land (MOE 2006).

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CdS formation is preceded in the following equation according to the redox equilibriumof sulfate and sulfide.

H2S(aq)+4H2O=SO42-+10H++8e- Eq.1 (log K0=-40.66, calculated from Lind

The Eq.1 can be rewritten using the Nernst’s equation and the log K0 value to thepH-Eh relation;

Eq.2H2S is obtained from the dissociation of HS- and S2- with the following dissociation con-

stants of K1 and K2.

K1 = (H+)(HS-)/(H2S) = 10-7.02 Eq.3K2 = (H+)(S2-)/(HS-) = 10-12.9 Eq.4

(SO42-)Eh=0.301-0.0739pH+0.00739log

(H2S)

Table 5. Effect of water management on Cd content in rice grains (Sakurai, NIAES,unpublished data).

Water management after heading-timeSoil Flooded Drained

Cd mg/kg

Soil A Trace 1.10Soil B Trace 0.68Soil C 0.09 0.23Soil D 0.16 0.33

Soil A,B: pot experiments, Soil C,D: field experiments.Each soil in different area of Japan is contaminated with Cd.

Pot test

Field test

1200

O2

NO2-H2O

N2O

MnO2

Mn2+

H2AsO42-

H3AsO3

H2AsO4-

H3AsO3

900

600

300

0

-300

-600

4 4.5 5 5.5 6 6.5 7 7.5 8

Eh(

mV

)

pH

SO42-

Fe2+

Fe(OH)3Mn3O4

Mn2+

S2-

1200

O2

NO2-H2O

N2O

MnO2

Mn2+

H2AsO42-

H3AsO3

H2AsO4-

H3AsO3

900

600

300

0

-300

-600

4 4.5 5 5.5 6 6.5 7 7.5 8

O2O2

NO2-NO2-H2O

N2O

MnO2

Mn2+

H2AsO42-

H3AsO3

H2AsO4-

H3AsO3

900

600

300

0

-300

-600

4 4.5 5 5.5 6 6.5 7 7.5 8

Eh(

mV

)

pH

SO42-

Fe2+

Fe(OH)3Mn3O4

Mn2+

S2-

Fig. 16. Eh-pH diagram of some chemical species.

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contaminated sites (Vangronsveld 1998). Thismethod has been adopted as a primarycountermeasure for Cd contamination inagricultural soil under the Agricultural LandSoil Pollution Prevention Law in Japan (Chapter3, Regulation of heavy metal pollution of soilsin Japan). Local managers who are responsiblefor preventing contamination, prefer thistechnique over other countermeasures, becauseof its low failure risk, its predictable time frame,and because it leaves sites in a relativelypristine condition. Below are several methodsto amend the polluted soils through soildressing (Yamada 2007):

Simple soil dressing (Fig. 18).Unpolluted soils are placed on top of thepolluted soil. Since the paddy fields amendedby this method are raised by 20 to 30 cmhigh, preparation of agricultural canal,agricultural road and paddy fields re-zoning areneeded.

Soil removal followed by new soildressing. Polluted surface soils are removedand discarded outside the paddy fields. Thenthe infertile subsurface soils are covered withunpolluted soil. The depth of polluted soil tobe removed is determined based on the degreeof soil pollution and plant root elongation.

Here, a total of the water soluble sulfides is described as Eq. 5.

SH2S=(H2S)+(HS-)+(S2-) Eq.5

By substituting the sulfide species in Eq.5 using Eq. 3 and Eq. 4, Eq.6 is obtained;

Eq.6

At pH 7, Eq.6 can be reduced to Eq.7;

Finally, combining Eq.1 and Eq.7, Eh can be expressed by hydrogen sulfide and sulfate ion (Eq.8):

Eq.7

Eh = -0.215 + 0.00739log(?H2S)

(SO42-)Eq.8

0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2 -0.3Soil Eh (V)

1.2

1

0.4

0.2

0

0.6

0.8

(SO

42- )/

{(SO

42- )+

(?H

2S)

}or

Cd

extr

actio

n ra

te

Fig.17. Relationships between soil Eh and rate of Cd extraction or that ofsulfate residue. The lines and open circles correspond with the(SO4

2-)/{(SO42-)+(SH2S)} and Cd extraction rate of the vertical line

(Makino 2002, modified from Iimura 1978).

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In-situ placement of polluted soils (Fig.19). After removing polluted surface soils,subsurface soil and subsoil are also temporarilyremoved. Then the polluted surface soil isburied into subsoil layers. After that, theremoved subsoil is placed back on the buriedsurface soil, placing the new unpolluted soil onthe top of the subsoil. However, it is difficultto apply this method to paddy fields wheresub-layer soil is also polluted and/or watertable is high.

Turning the soil layers upside down.The polluted surface soils are switched placeswith the unpolluted subsoil. This method isapplied to paddy fields where unpolluted soilis hardly present and subsoil is not polluted atall.

According to several follow-up surveys,soil dressing is a very effective and reliablepractice to decrease the Cd content in rice

grains when the newly-dressed unpolluted soillayer is 20–30 cm thick. However, this practiceis costly and becoming increasingly difficult toimplement because of scarce suitableuncontaminated soils.

NEW APPROACHES TO REMEDIATION OFCD-CONTAMINATED SOIL IN JAPAN

Phytoremediation

Recently, phytoremediation has come topeople’s attention as a cost-effective andenvironment-friendly technology to removevarious toxic materials from soils. There aredifferent types of phytoremediations such asphytoextraction, phytovolatilization,phytostabilization and rhizofiltration (Table 6).Phytoextraction is the most popular technologyand intensively examined of the four

Fig. 18. Simple soil dressing (modified from Yamada 2007).

Hardpan

Unpolluted soil

Subsurface soil

Subsoil

(Unpolluted soil)

Before worksDevelopment of

hardpan

Surface soil

Placement of subsoil

Fig. 19. In-situ placement of polluted soils (modified from Yamada 2007).

Before works Removal of polluted soil and excavation

Subsurface soil

Subsoil

Burying of polluted soil

Development of hardpan and placement of subsoil

Surface soil(Unpolluted subsoil)Hardpan

Unpolluted soil

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phytoremdiations in Japan. Fig. 20 describesthe conceptual scheme of phytoextraction forCd-contaminated paddy soils for plants.

A variety of plant species have beenstudied for their capacity of extracting Cd fromcontaminated soil, such as tall goldenrod or S .altissima ( Tatekawa e t a l . 1975), indianmustard or Brassica juncea (Yanai et. al. 2004),kenaf (Hibiscus cannabinus), okra o rAbelmoschus esculentu (Kurihara et al. 2005),sorghum or Sorghum bicolor (Kato e t a l .2004), hakusanhatazao (Arabidopsis halleri ssp.Gemmifera)( Nagashima et al. 2005), Asteraceae(Watanabe and Sasaya 2007), sugar beet (Betavulgaris L. ) (Ishikawa et al. 2006) and rice(Oryza sativa L.) (Murakami et al. 2007).

There have been reports that someJaponica-Indica hybrid and Indica rice varietypossess a considerably high capacity of soilCd absorption compared to other Japonica ricevarieties. Murakami (2007) reported, as shownin Fig. 21, a comparison of Cd extractionefficiency from soils through the rice cultivarMiyang 23. In this context, rice plants areconsidered to be one of the most promisingspecies for Cd phytoextraction fromcontaminated paddy soils. Although there are acouple of upland and/or perennial plant specieswith a possible high capacity of Cd extraction,it is difficult to vigorously grow these speciesin paddy soils. It also takes a long time tobring back the once converted soil suitable forthe upland species to its original paddycondition. On the other hand, any ricevarieties, either Indica, Japonica or their hybrid,

very easily adopt to Japanese paddy soils, andJapanese growers are familiar with almost allrelevant cultural practices for rice cultivation.After two or three times of Cd phytoextractionpractices, the paddy field remedied could beplanted with a commercial variety of rice.

Soil Washing

Soil washing is conventionally performed exsitu using an appropriate apparatus, in whichextracting reagents are used to removehazardous metals from soil into aqueoussolution (Elliott and Herzig 1999). Althoughsoil-washing techniques offer a great advantageof high Cd-removal efficiency for contaminatedsoils, they are considered difficult to directlyapply to agricultural land. Wastewater drainedduring the process of soil washing mightpollute surrounding areas such as agriculturalcanal, neighboring agricultural field,groundwater, etc.. However, Cd contaminatedpaddy fields are distributed nationwide andmost of the Cd contamination level in ricepaddy fields are less than 1 mg Cd kg–1 (oven-dry basis). This wide distribution of low levelsCd contamination makes it extremely difficult totransport contaminated soils to an ex s i tutreatment plant. In this context, soil washing ofcontaminated paddy fields should be conductedin situ. Since paddy fields possess impervioushardpan just below the subsurface layer whichhinders vertical movement of water, the washedsolution stays in the surface soil and does notpenetrate into subsoil layers and groundwater.

Table 6. Types of phytoremediation for inorganic constituents (Suthersan 2002)

Type of Process involved Contaminant treatedphytoremediation

Phytostabilization Plants control pH, soil gases, Proven for heavy metals inand redox conditions in soil mine tailing pondsto immobilize contminants.Humification of some organiccompounds is expected.

Rhizofiltration Compounds are taken up or Heavy metals and radionuclides(contaminant uptake) biosorbed by roots

(or sorbed to algae and bacteria).

Phytoextraction Metals and organic chemicals taken Nickel, zinc, lead, chromium,(phytoaccumulation or up by the plant with water, or by cadmium, selenium, otherhyperaccumulaition) cation pumps, sorption and other heavy metals radionuclides

mechanisms.

Phytovolatilization Volatile metals are taken up, Mercury and seleniumchanged in species, and transpired.

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So, an in-site technology of soil washingshould be fully utilized given such uniquecharacteristics of paddy field.

An in-situ soil washing method of paddyfields has to meet the following criteria(Makino et al. 2006, 2007):

1. Use of washing chemicals with highefficiency of Cd removal, but a minimaladverse impact on the paddy field and itssurrounding environment;

2. Cost-effective and environmentally soundoperation of the system;

3. Soil fertility of paddy field and its cropgrowth which are not greatly affected bythe washing treatment or can be easilycorrected by application of agriculturalmaterials; and

4. A lasting effect of washing for areasonably long period.Metal chelating agents, neutral salts, and

strong acids have been used for the soilwashing chemicals (Davis 2000). In particular,ethylenediamine tetraacetic acid (EDTA) hasbeen commonly used due to its efficient Cd

Fig. 20. Conceptual scheme of phytoextractionfor Cd-contaminated paddy soil(provided by Ono, NIAES).

Cd

cont

ent

in

soils

(m

g kg

-1)

Fig. 21. Changes in Cd content in soils with phytoextraction. The ricecultivar of Japonica-Indica hybrid (Miyang 23) was cultivated inpots. Soil Cd was determined by 0.1M HCl extraction (modifiedfrom Murakami et al. 2007).

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removal from contaminated soils (Nakashimaand Ono, 1979; Abumaizar and Smith 1999;Zeng e t a l . 2005). EDTA, however, is apersistent chemical and stays for a long timein the environment (Tandy et al. 2004). Somescientists therefore have used morebiodegradable chelating agents instead ofEDTA (Mulligan et al. 1999; Hong et al. 2002;Tandy et al. 2004; Chang et al. 2005; Kantarand Honeyman 2006). In case biodegradableagents are used, however, the cost becomesrelatively higher than that of the non-/lessdegradable counterparts.

Ogawa et al . (1985) used HCl to washsoil in a batch experiment and conducted pottests to confirm a decrease in Cdconcentrations of unpolished rice. The pollutedsoil treatment system, in which in situ soilwashing and on-site wastewater treatment arecombined, has never been applied to Cd-contaminated paddy fields.

Hereafter, we are to introduce a new soilwashing practice combined with on-sitewastewater treatment that completely satisfiesthe above mentioned four requirements(Makino, et al. 2006, 2007 and in press).

Materials and methodsSelection of washing chemicals:a. SoilsSamples of three soils were obtained from theplow layers of paddy fields of Nagano,Toyama, and Hyogo prefectures in Japan. Allof the soil samples were air-dried and passedthrough a 2-mm mesh sieve before chemicalanalysis. Soil pH was determined through aglass-electrode method (Horiba, PH81, Japan)containing a ratio of 1:2.5 of soil and eitherwater or 1 mol L–1 KCl. The total carbon (TC)and nitrogen (TN) contents of the soils weremeasured by a dry combustion method(Shimadzu, Sumigraph NC-900, Japan). The claycontent and clay mineralogy of the soils weredetermined by a pipette method (Gee andBauder, 1986) without prior removal of ironoxides, and by X-ray diffraction analysis (JEOLLtd., JDX-3530, Japan), respectively.

Soil was digested with a mixture of nitricand perchloric acids on a hot plate (Amacher1996). The Cd content in the digested solutionwas determined through inductively coupledplasma optical emission spectrometry (ICP-OES)(Varian Inc., Vista-Pro, USA). Table 8

summarizes relevant properties of the soils. TheCd-contaminated soils were obtained frompaddy fields polluted by the wastewater ofmines. The total Cd contents of soils fromNagano, Hyogo, and Toyama were determinedas 0.71, 4.65, and 1.21 mg kg–1 , respectively.These Cd concentrations were substantiallyhigher than the mean values in uncontaminatedsoils, which average 0.33 mg kg–1 in Japan and0.48 mg kg–1 in the world (Table 1).

b. Various chemicalsThree paddy soils were used for a Cdextraction test: Nagano soil (Fluvaquents),Toyama soil (Epiaquepts), and Hyogo soil(Fluvaquents). 10 g of each of the three paddysoils contaminated with Cd were shaken forone hour with 15 mL solutions of 0.02 Mc L–1

or 0.1 M c L–1 chemicals such as calciumchloride (CaCl 2), calcium acetate, magnesiumchloride, magnesium acetate, sodium chloride,sodium acetate, potassium chloride, potassiumacetate, iron (III) chloride (FeCl 3), disodiumethylenediamine tetraacetate (EDTA-2Na), citricacid, acetic acid, and hydrochloric acid. Thesoil-chemical mixtures in tubes were centrifugedfor 15 minutes at 3,000 rpm and thesupernatants were filtered in a disposablemembrane filter, with 0.2 ìm pore size. Afteradding 50 ìL of the concentrated nitric to 4.95mL of the filtrates, Cd levels were analyzedusing ICP-OES.

c. Metal saltsSame extraction procedure was conducted usingthe same three sample soils and 100 mM c o facids and metal salts varieties, such as HCl,HNO3, H2SO4 , FeCl3, MnCl 2, ZnCl2 , Fe(NO3)3 ,Mn(NO 3)2 , Zn(NO 3)2, Fe2(SO4)3, MnSO 4 , andZnSO4. Various ions extracted using FeCl3 ,Fe(NO3)3, and Fe2(SO4)3 were determined by thefollowing analytical methods with duplicate:ICP-OES for Na, K, Ca, and Mg, anddistillation (Mulvaney, 1996) with MgO forNH4

+ . An ion chromatograph (DX-320, DionexCorp., USA) was used to measure anions (Cl– ,NO3

–, PO43–, SO4

2–). Dissolved organic carbon(DOC) was analyzed using a total organiccarbon analyzer (TOC-5000, Shimadzu Corp.,Japan). Visual MINTEQ software was used toanalyze the ionic, DOC, and pH data sets toestimate Cd speciation in the extracts(Gustafsson 2004).

14

On-site soil washing:An on-site testing plot was prepared in apaddy field in Nagano Prefecture. The soilwashing procedure consisted of three steps: (1)chemical washing with FeCl3 solution, (2)following water washing to eliminate theremaining chemicals, and (3) on-site treatmentof wastewater by a portable purificationapparatus with a chelatin meterial (Fig. 22). Apart of the paddy field was bounded with 60-cm-high plastic boards, which were partiallyburied on the edge of the paddy field so thatthe upper two-thirds of each board remainedabove the ground surface. This boundaryprovided containment for additional water andchemicals in the paddy field. Soil washing wasconducted in the bounded area, whichencompassed about 100 m2.

a. Chemical washingFerric chloride (FeCl3) was applied to thebounded experimental field, and then addedwith agricultural water, creating a soil-solutionratio and a FeCl3 concentration of 1:1.5 and 15mM, respectively. These were the optimalvalues for soil washing in this paddy in apreliminary experiment. The soil solution wasmixed by a 13-metric-hp cultivator (KubotaCorp. B7000, Japan) until it turned into slurry.After the mixing, the slurry was allowed to restfor more than two hours, and then thesupernatant of the slurry was drained-off aswastewater. Chemical components of Cd in the

wastewater were analyzed, as mentioned in theselection of washing chemicals, specificallymetal salts.

b. Water washingThe experimental field was then filled withagricultural water until the water level reachedthe initial point. To eliminate residual Cd andCl, the soil solution was mixed for 1hr until itturned into slurry, allowed to rest for 2-5 hoursand then the supernatant of the slurry wasdrained-off as wastewater. This procedure wasrepeated three times, until the residual Clconcentration was reduced to lower than thetarget value for rice growth (500 mg L-1). Thesupernatant Cl concentration was measured bya Cl meter (IM-40S, DKK-TOA Corp., Japan).

c. On-site treatment of wastewaterThe wastewater produced by chemical andwater washing was pumped into the on-sitewastewater treatment system. The systemremoved Cd from the wastewater, and thendischarged the treated water to a canal. Analkaline treatment and chelating material withflocculent settling in the treatment system wereapplied to remove Cl from the wastewater. Thewastewater was sampled before and after thetreatment system. The concentrations of Cdand Cl were determined by ICP-OES and ionchromatography, respectively, in the wastewaterand the treated water.

Fig. 22. Conceptual diagram of on-site soil washing.

?Chemical washing

Extraction of soil Cd by FeCl3

?Water washing

Removing residual Cd and C1in the paddy water

?Wastewater treatment

Removing Cd in the wastewaterby chelating material

? Mixing? Soil sedimentation

? Mixing? Soil sedimentation

Cd removal

Chelating

compliance withenvironmentalstandards

FeCl3

Water treatment

Water Cd

Cd Cd

Cd

CdCd

CdCd

CdCd

Cd

CdCdCd

Cd Cd

Cd CdCd

CdCd

Cd

CdCd

CdCd

Paddy water

?

? ? Treatedwater

Discharge

15

Measurement of changes in soil Cd content:

The washed experimental area was divided intofour plots for a wet rice culture experiment.Two soil samples were collected from the Aphorizon in each plot, before and after thewashing. Soil samples were also collected fromfour control plots located in the unwashedexperimental area. All samples were air-driedand then passed through a 2-mm mesh sievebefore analysis. Four grams of soil from eachsample was placed in a 50-mL PP tube, and 20mL of 0.1 mol L-1 or 0.01 mol L-1 HCl solution,or 40 mL of 1 mol L-1 NH4NO3 solution, wasadded. The extracts of the soil solutionmixtures were sampled and filtrated. The Cdlevel in the filtrates was analyzed by ICP-OES.The concentrations of soil Cd measured by 0.1mol L-1 , 0.01 mol L-1 HCl and 1 mol L- 1

NH4NO3 solution were defined as that of acidsoluble, weakly soluble and exchangeablefraction, respectively.

Soil fertility:Soils were sampled from the washed andunwashed plot in the experimental site andwere air-dried. Total carbon, pH, and nitrogenin the soils were measured by same methodsdescribed above. Soil EC was analyzed usingthe electrode method (Mettler, MC126, USA)with a soil-water ratio of 1:5. Exchangeablecations and available phosphate were analyzedaccording to Thomas (1982), and Truog (1930),respectively. Available nitrogen was measuredby phosphate-buffer extraction method(Matsumoto et. al. 2000).

Rice cultivation:Two rice cultivars, ‘Akitakomachi’ and‘Kusahonami’, were transplanted, andharvested, in the experimental and controlplots. Akitakomachi is one of the popular ricevarieties in the region. On the other hand,Kusahonami has a high capacity for absorbingCd. Mature rice was manually harvested, takingtwo 1.65-m2 quadrilaterals in each subplot. Air-dried shoot and brown rice yield weremeasured. The brown rice yield was convertedto ordinary water concentration (150 g kg -1 dryweight). A part of the shoot material and ofthe brown rice was ground in a stainless steelvibration sample mill, and 1 g of eachgrounded sample was digested withconcentrated HNO3 followed by HClO4. Cd

concentration in the digested solution wasdetermined by ICP-OES.

Results and discussionSelection of washing chemicals:Hydrochloric acid, nitric acid, and EDTA-2Naextracted more Cd in soil than the neutral salts(Fig. 23). However, EDTA-2Na is difficult touse for practical purposes because of itspersistent nature in the environment andrelatively high cost. Nonetheless, both strongacids can cause serious soil acidification in thesoils with a low acid-buffering capacity.

Iron (III) chloride extracted nearly asmuch Cd as hydrochloric acid, nitric acid, andEDTA-2Na from the Nagano and Toyama soils,and more Cd from the Hyogo soil (Fig. 23).Iron is a major soil constituent and is lessenvironmentally harmful than the remainingchemicals. In addition, FeCl3 is less expensiveand easier to handle than both hydrochloricacid and EDTA-2Na, thus, FeCl3 was selectedas a promising washing chemical.

The Cd extraction capacity was comparedwith other metal salts to clarify the Cdextraction mechanism of FeCl3. The proportionof total soil Cd extracted by the washingchemicals (i.e., the Cd extraction efficiency)increased in the following order: Mn salts =Zn salts << ferric Fe salts in all the threesoils, with efficiencies ranging from 4-41%, 8-44%, and 24-66%, respectively (Fig. 24). Theamount of Cd extracted was negativelycorrelated with the extraction pH (Fig. 25),suggesting that extraction pH plays animportant role in determining the Cd extractionefficiency.

When metal salts are added to soils, thedissociated metal cations that may formhydroxide precipitates with releasing protonsaccording to the following equations(Hydrolysis):

(1) MmAn=mMn++nAm-

(2) Mn++nH2O=M(OH)n+nH+

[M(OH)n]{H+}n

(3) Kom=

{Mn+

}[H2O]n

where MmAn denotes a metal salt, metal cationas M (Fe, Zn, or Mn) and anion as A (Cl – ,NO3

–, or SO42–). m and n represent the charge

numbers of the anion and cation, respectively.Kom denotes the equilibrium constants

16

0.0

1.0

2.0

3.0

0.02 Mc0.1 Mc

Toyama soil

Nagano soil

Cd

conc

entr

atio

n in

the

extr

acts

(ppb)

Hyogo soil

0.0

0.2

0.4

0.6

Iron(?

) Chlo

ride

EDTA

-Na2

Acetic

Acid

Citric

Acid

Nitric Acid

Hydro

chloric

Acid

Calciu

m chl

oride

0.0

0.2

0.4

0.6

0.0

1.0

2.0

3.0

0.02 Mc0.1 Mc

Toyama soil

Nagano soil

Cd

conc

entr

atio

n in

the

extr

acts

(ppb)

Hyogo soil

0.0

0.2

0.4

0.6

Iron(?

) Chlo

ride

EDTA

-Na2

Acetic

Acid

Citric

Acid

Nitric Acid

Hydro

chloric

Acid

Calciu

m chl

oride

0.0

0.2

0.4

0.6

Fig. 23. Efficiency of Cd extractionwith various chemicalsfrom the three soils.

Fig. 24. Comparison of cadmium extraction efficiency from the three soils bymetal salts (gray bars) and strong acids (shaded bars) at 0.1 Mc. Theextraction pH is shown in the parenthesis. The error bars indicatestandard deviation (Makino et al. in press).

(expressed in terms of activities) for metal Mn+

in equation (2), which correspond to 2.88´10–4 ,3.31´10–13 , and 6.46´10 –16 for Fe3+ , Zn2+ , andMn2+ (Lindsay 1979).

The precipitation of the metal hydroxide(hydrolysis of the metal ion) generates protonsat a rate that depends on Kom, and theseprotons may decrease the extraction pH (Eqs.1-3). Figure 26 illustrates the theoreticalrelationships between pH and activity of metal

ions in the metal hydrolysis reactions at theequilibrium, with soil iron (calculated using Eq.3 and the Kom values). Ferric hydroxide hasaround pH 2 (Fig. 26), which is much lowerthan the original soil pH (H 2O) of the threesoils.

Thus, Fe-hydrolysis is associated with agreater decrease in soil pH compared to theother two metals. This indicates that protonrelease is a driving force of the Cd extraction

Cd extraction efficiency (% of total Cd)

0 10 20 30 40 50

Hyogo

17

by FeCl3, which results in a sharp decrease insoil pH. In another study, Cd was highlymobile under oxidizing and acidic conditions ofthese soils (Kabata-Pendias 2000). Heavy metalsolubilization was greatly enhanced byacidification, and at pH 1.3, it reached morethan 80% of the total Cd content of the soil(Dube and Galvez-Cloutier 2005). Our resultsand these previous reports endorse theeffectiveness of iron salts as washingchemicals to remove Cd from soil.

The Cd extraction efficiency of metalchlorides was greater than that of thecorresponding metal sulfates and nitrates in allsoils (Fig. 24). Extraction efficiency decreasedin the following order: chlorides > nitrates ˜sulfates, with values ranging from 41-75%, 14-63%, and 26-62%, respectively, in the Naganosoil. The results are similar for the other twosoils. To examine the factors, which result inthe difference of the extraction efficiencybetween the metal salts, we estimated the

Fig. 25. Relationships between extraction pH and the amounts of Cd extractedfrom the three soils using metal salts and the three strong acids.?: Hyogo soil : Nagano soil ? : Toyama soil (Makino et al., in press)

Fig. 26. Diagram of pH and metal activity to precipitatemetal hydroxides.(Makino et al., in press)

0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090.010

0 2 4 6 8 10pH

Act

ivit

ity

of

free

met

al io

ns

Fe(OH)3 Zn(OH)2 Mn(OH)2

Act

ivit

y o

f fr

ee

met

al

ion

s

18

relative abundance of dissolved Cd species in100 mM c iron salt solution by the VisualMINTEQ software (Gustafsson 2004). Fig. 27indicates that Cd–Cl complexes such as CdCl+

and CdCl2 (aq) accounted for 80% of the totaldissolved Cd in the Nagano soil at 100 mM cFeCl3 , versus values of 33% for Fe2(SO4)3 and9% for Fe(NO3)3. Similar trends were observedfor the other metal salts and soils (data notshown). Cadmium has a high capacity to formcomplexes with anions such as Cl–, SO4

2–, CO32–

, PO43– , organic acids, and fulvic acid (Traina

1999). Doner (1978) reported that Cd wasleached more rapidly in the presence of Cl –

than in the presence of ClO 4– . Sakurai and

Huang (1996) showed that the rate ofdesorption of Cd from a montmorillonite wasgreater with KCl than with KNO3. Smolders andMcLaughlin (1996) suggested that highconcentrations of Cl – might increase plant’suptake of Cd either by enhancing masstransport of Cd or by enhancing uptake of theCdCl+ complex through plant roots.Accordingly, the formation of stable Cd–Clcomplexes could inhibit resorption of theextracted Cd onto adsorption sites on thesurface of the soil particles. This inhibitionmechanism will improve the efficacy ofextraction with FeCl 3 compared to that withFe2(SO4)3 and Fe(NO3)3, because the proportion

of Cd complexes to the total dissolved Cdconcentration is high in the extracts withchloride salts.

On-site soil washing:Fig. 28 shows the profile of Cd concentrationin the pre-treated and treated wastewatersgenerated during the chemical washes and thewater wash. The Cd concentrations in thetreated wastewater were below the Japaneseenvironmental quality standard (0.01 mg Cd L-

1), demonstrating that the in situ treatmentsystem could treat the wastewater as expected.The Cl concentration was less than 500 mg L- 1

after three water washes. This concentration isthe threshold value for healthy rice crops.

Cadmium has a good capacity to formcomplexes with various anions, such as Cl – ,SO4

2– , CO32– , PO4

3– , organic acids, and fulvicacid (Traina 1999). Because paddy soilsreceive a wide variety of anions from differentsources, including irrigation water, fertilizer, andsoil amendments, the Cd extracted from soiladsorption sites may easily form complexeswith existing anions during the extractionprocess. To evaluate the kinds of Cdcomplexes that formed during the first chemical-wash process, we calculated the chemicalspecies of Cd that would be present in theextracts, using Visual MINTEQ software

Fe2(SO4)3 FeC13 Fe(NO3)3

100%

60%

40%

80%

20%

0%

Cd

spec

ies

(%

of

tota

l)

Fig. 27. Relative abundance of various Cd species in the extracts of the Naganosoil in the presence of the three iron compounds. The Cd species werecalculated using the Visual MINTEQ software (Gustafsson 2004) based onthe data set of cation, anion, pH, and dissolved organic carbon valuesobtained for the extracts (Makino et al., in press).

19

(Gustafsson 2004). The Cd-Cl complexes suchas CdCl+ and CdCl2 (aq) exceeded 70% of thetotal dissolved Cd at 0.1 mol L–1 CaCl2 (Fig.29). Determining the chemical species of Cdby means of MINTEQ software revealed theformation of Cd–chloride complexes, whichenhanced Cd extraction from the soils. Theformation of stable Cd-Cl complexes couldpromote Cd desorption from soils and inhibitresorption of extracted Cd onto adsorptionsites on the surface of the soil particles.

Verification of the washing effects of FeCl3 insitu on contaminated soil:

a. Soil Cd :The concentration of exchangeable Cd haschanged a little after FeCl3 washing, whereasacid-soluble Cd form decreased substantially(Fig. 30). Although the exchangeable Cdincreased with decreasing soil pH caused bythe washing treatment (data not shown),adjusting the pH to its initial level by addinglime could decrease the exchangeable Cdconcentration and maintain it at this level afterthe washing. Total Cd content of soildecreased substantially, to 55% of theunwashed value, compared to a value of 83%after CaCl 2 treatment in a field washing

experiment (Makino et al. 2007). These resultsindicate that FeCl 3 has a high Cd extractionefficiency in paddy soils. These results alsoappear to be the first practical example ofdetoxifying soils contaminated with Cd usingFeCl3 based on proton release, and throughgeneration of hydroxides and formation of Cd–Cl complexes.

b. Soil fertility:Fig. 31 summarizes the changes in soil fertilityproperties using soil washing. The pH(H 2O)and pH(KCl) significantly decreased after thewashing treatment. Although EC increased, itdid not reach a level that would affect plantgrowth. Exchangeable cations decreased dueto soil washing. The Mg and K deficit wascorrected by applying fertilizers to the washedsoil, restoring the Mg and K concentration inthe soil to approximately 70-80% of the valuein the unwashed soil (data not shown) duringthe growth period. Total carbon and totalnitrogen concentrations changed a little, whileavailable nitrogen and available phosphorusdecreased significantly after washing. Althoughthe extraction pH became very acidic byapplying FeCl3, the amount of soil Al releasedwas less than 1% against the total soil Al

Fig. 28. Profile of Cd concentration in the wastewater andtreated wastewater generated at on-site soil washing.

0.0

0.1

0.2

0.3

WasterwaterTreated wastewater

Chemicalwashing

Waterwashing #1

Wastewater standard 0.1

Environmental standard 0.01

Waterwashing #2

Waterwashing #3

0.0

0.1

0.2

0.3

WasterwaterTreated wastewater

Chemicalwashing

Waterwashing #1

Wastewater standard 0.1

Environmental standard 0.01

Waterwashing #2

Waterwashing #3

Cd

(mg

L-1)

Wastewater

20

(data not shown). This means that this in situsoil treatment is unlikely to cause serious soildamage such as clay mineral destruction.

c. Rice cultivationSoil washing had markedly positive effects onthe growth and yield of rice crops. Itconsiderably decreased the Cd concentrationsin the rice straw and unpolished rice, from 0.91

and 0.31 mg kg–1 in the unwashed soil to 0.18and 0.053 mg kg –1 in the washed soil,respectively. This reduction rate of plant Cd ishigher than that of soil Cd estimated based onthe amounts of the total and acid-soluble form.These results proved efficiency andeffectiveness of the soil washing method forremediation of Cd-contaminated paddy fields.

1595

3660.15

0.34

0

200

400

600

800

1000

1200

1400

1600

? ? ? ? 1 ? ? 2 ? ? 3

??

??

(mg/

l)0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Cd?

??

(mg/

kg)

? ? ? ?Cd? ? ?

Chemical speciation at Fe wash( Calculation with Minteq)

Cd+2CdCl+CdCl2 (aq)CdSO4 (aq)FACd+ (? ? ? ? )

Cum

ulat

ive

Cd

rem

oval

(mg/

kg)

Promotion of Cd extraction with Cd-Cl complexation

Clc

once

ntra

tion

(mg

L-1)

FeCl3wash Water-wash

#1 #2 #3

Cl

Cun

ulat

ive

Cd

rem

ova

l (m

g k

g-1)

1595

3660.15

0.34

0

200

400

600

800

1000

1200

1400

1600

? ? ? ? 1 ? ? 2 ? ? 3

??

??

(mg/

l)0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Cd?

??

(mg/

kg)

? ? ? ?Cd? ? ?

Chemical speciation at Fe wash( Calculation with Minteq)

Cd+2CdCl+CdCl2 (aq)CdSO4 (aq)FACd+ (? ? ? ? )

Chemical speciation at Fe wash( Calculation with Minteq)

Cd+2CdCl+CdCl2 (aq)CdSO4 (aq)FACd+ (? ? ? ? )

Cum

ulat

ive

Cd

rem

oval

(mg/

kg)

Promotion of Cd extraction with Cd-Cl complexation

Clc

once

ntra

tion

(mg

L-1)

FeCl3wash Water-wash

#1 #2 #3

Cl

Cun

ulat

ive

Cd

rem

ova

l (m

g k

g-1)

Promotion of Cd extraction with Cd-Cl complexation

Clc

once

ntra

tion

(mg

L-1)

FeCl3wash Water-wash

#1 #2 #3

Cl

Cun

ulat

ive

Cd

rem

ova

l (m

g k

g-1)

Fig. 29. Changes of Cl concentrations during soil washing, and Cdchemical speciation at the FeCl3- wash.

Fig. 30. Changes of soil Cd contents with washing treatment.

1MNH4NO3 0.01MHC1 0.1MHC1Extractant solvent

0.8

0.6

0.4

0.2

0.0

Cd

cont

ent

(mg

kg-1) Unwashed

Washed

21

Fig. 31. Comparison of soil fertilities before and after washing treatment. pH valuesare raw data. EC value means almost six-fold increase after the washing.

ACKNOWLEDGMENT

The author would like to thank the TaiheiyoCement Corp and Nagano Agricultural ResearchCenter for their cooperation on soil washingstudy. The study was supported in part by aGrant-in-Aid (Hazardous Chemicals) from theMinistry of Agriculture, Forestry, and Fisheriesof Japan (HC-04-1140-1).

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