Heavy Metal Pollution of Soil and a New Approach to Its Remediation: Research Experiences in Japan

Tomoyuki Makino
National Institute for Agro-Environmental Science
3-1-3 Kannondai, Tsukuba, Japan, 2009-04-13

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

Introduction

Japanese arable soils, particularly paddy soils in some regions, have been heavily polluted with cadmium (Cd) and other various heavy metals, owing to rapid industrialization during the 1960s. The Japanese government urgently enacted the Agricultural Land Soil Pollution Prevention Law in 1970 to cope with the heavy metal pollution, in which Cd, arsenic (As), and copper (Cu) were the targeted hazardous substances for regulation. Cd, in particular, has been recognized as one of the most detrimental elements 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 grains containing more than 1 mg Cd kg^_1 were produced. Ever since the law was in effect, the polluted paddy soils have been remedied mainly through unpolluted soil dressing and/or unpolluted soil replacement. However, these remediable practices have become increasingly difficult to implement because of their high costs, and due to difficulty in obtaining uncontaminated soil.

In July 2006, the Codex Alimentarius Commission of the Joint FAO/WHO Food Standards Program proposed 0.4 mg Cd kg^_1 as the maximum permissible concentration of Cd for polished rice (Codex 2006). Therefore, it is urgent to develop cheaper, effective, and promising technologies to resolve Cd polluted paddy soils, replacing soil dressing practices. It has been supposed that 34-50% of the Cd intake by Japanese citizens comes from rice (Kawada and Suzuki, 1998), hence, it is vital to alleviate Cd content in rice fields and ensure the safety of this staple crop for the Japanese citizens. This paper reviews the current status of the heavy metal contamination of Japanese arable soils and presents new approaches to provide remedy to paddy soils contaminated with Cd.

Concentration of Heavy Metals in Soils and Crops in Japan

The natural abundance levels of some heavy metals in soils in Japan and around the world are given in Table 1 (both the mean value and range of the soil heavy metal concentrations were presented). The ranges were very wide; the ratio of the highest value was a hundred times different from the lowest value. The heavy metal concentration in Japanese soil was similar to that in the world soil, indicating that the natural abundance level of heavy metals in Japanese soil has been affected by factors not unique to Japan alone. The pollution of arable soils by heavy metals is primarily caused by wastewater from mines used for irrigation water to paddy fields, and by emissions from nonferrous metal refining plants (Asami 2001). Humans are exposed to heavy metals by ingesting crops grown in polluted soil and drinking water contaminated with some hazardous heavy metals.

Codex (2005) also proposed the maximum permissible level of Cd for other relevant crops such as wheat grain, edible roots and stem of potato, beans, leafy vegetables and some other vegetables (Table 2). To cope with newly proposed Cd permissible standard, the Ministry of Agriculture, Forestry and Fisheries (MAFF) consulted with the Foodstuff Safety Committee on revising the permissible Cd concentrations in rice grains and other staple cops, if necessary (MAFF 2002). At the same time, MAFF has conducted a nationwide survey on the current status of Cd content in some relevant crops (Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, and Fig. 12). Table 3 shows that many relevant crops contained Cd levels more than the Codex proposed values; four crops, in particular, exceeded more than 5% of the proposed value, which include aroid, burdock, gumbo and eggplant.

Regulation of Heavy Metal Pollution of Soils in Japan

The Japanese government has enacted the following two laws to cope with arable soil polluted with hazardous metals and other chemical substances: 1) the Soil Contamination Countermeasures Law; and 2) the Agricultural Land Soil Pollution Prevention, which took effect in 2002 and in 1970, respectively. In addition to the two laws, various decrees and ministerial ordinances have been issued. Targeted substances and the maximum permissible concentrations of heavy metals regulated by the two laws are summarized in Table 4 (MAFF 1970, MOE, 2003^a, 2003^b). The designated standard for the hazardous substances under the Soil Contamination Countermeasures Law are basically applicable to all types of soil including agricultural soil. However, the law is more often applied to former sites of chemical and high-tech factories, where various harmful substances were once used. In the long run, these sites may cause health risk to people living around them.

The Cd standard designation concept of the Soil Contamination Countermeasures Law is illustrated in Fig. 13. The law aims to control heavy metal pollution of soils in terms of two factors, namely, soil Cd concentration and Cd concentration in the soil leachate. The former may greatly affect the risk of Cd intake through direct ingestion, while the latter will minimize the risk of the Cd polluted groundwater to humans. A water extraction test was performed to the leachate standard to determine the heavy metal concentrations in an underground seepage (Table 4).

The Environmental Quality Standard (EQS) for Soil Pollution was enacted in August 1991 (Table 4). Since February 1994, many substances have been added to EQS. At present, it regulates a total of 25 hazardous substances. Guidelines for Investigation and Prevention of Soil and Groundwater Pollution have been effective since November 1994, ensuring efficient implementation of survey and development of prevention techniques based on EQS. Administrative Guidelines for Polluters are issued to urge people to voluntarily clean up polluted soil. The analysis methods and values of the standards concurred with the pollutants in the leachate according to the Soil Contamination Countermeasures Law and the Agricultural Soil Pollution Prevention Law (Table 4).

The Agricultural Land Soil Pollution Prevention Law designated Cd, arsenic (As), and copper (Cu) as hazardous substances to be regulated. The maximum allowable limit of Cd in soil was decided through Cd concentration in rice grains, but not by soil Cd concentration, because the bioavailability of Cd in soil is affected by many factors, like water management for rice cultivation. Hence, using soil Cd content alone to determine the maximum allowable concentration is very difficult. (Asami, 1981). As of 2005, 87.2% of the total polluted land (7,327 ha), designated by the Agricultural Land Soil Pollution Prevention, was remedied with uncontaminated soil application and/or soil replacement (Fig. 14) (MOE, 2006). Fig. 15 shows a flow diagram for implementation of the Agricultural Land Soil Pollution Control Measures (MOE, 2007). The countermeasures used were mostly soil dressing.

Conventional Cultural Practices to Alleviate CD Contamination in Rice in Japan

Water Management

Water management is a popular and cost-effective cultural practice for alleviating rice Cd contamination in Japan. It influenced heavy metal content of paddy rice, particularly Cd. Hence, redox state of the paddy soil was possible. Table 5 shows effect of water management on Cd content of rice grains (Sakurai, NIAES, unpublished data). Cd absorption by rice drastically decreased through continuously submerging paddy field after heading-time. Many Japanese scientists have reported similar results.

It is most likely that a considerable decrease in Cd absorption by rice under submerged condition is a decrease in the Cd solubility due to the formation of Cd sulfides. When paddy field is flooded, the paddy soil is rapidly reduced, and consequently, its redox potential (Eh) is shifted toward a reduced state (a sharp decrease in Eh), where sulfate ion can be reduced to sulfide ion (Iimura, 1978). The sulfide ion produced thus reacts with Cd to precipitate out of soil solution as cadmium sulfide. The precipitation of cadmium sulfide, in turn, lowers the Cd concentration in the soil solution, lowering the amount of the Cd bioavailable to rice plants. The theoretical explanation based on physico-chemical equation was summarized below.

Fig. 16 presents a relationship between redox potential (Eh) and pH for some heavy metal species, calculated by log K (Lindsay, 1979). The diagram indicates that sulfate ions are converted to hydrogen sulfide that lowers the soil redox potential.

Supposed that the dominant sulfur species in soil solution are SO₄^2-, H₂S, HS^-1 and S^2-, the change in the relationship between {(SO₄^2-) /((SO₄^2-)+(ÓH₂ S))} at pH7 and Eh is expressed through a solid line in Fig. 17 (Makino, 2002). The dashed line shows the relationship in a similar calculation at pH 6. The open circles show the measured extractable Cd with 1 M ammonium acetate from the soil in a water-submerged incubation test. The amount of Cd extracted with Eh was nicely fitted to that of the calculated values, indicating the Cd extraction rate rapidly decreased with an increase in the ratio of ÓH2S to total sulfur. The sulfide ion will precipitate with Cd ion as cadmium sulfide (CdS), which is hardly soluble in water (Iimura 1978).

Flooding from tilling to head formation in rice growth stage would be the most effective period to decrease the Cd content in rice grains. It is highly recommended to keep flooding the paddy fields as late as possible toward harvest time. However, the later the flooding keeps, the more difficult to operate machine for harvest. Therefore, we have to find a way to lower bioavailable Cd and manage machine operation at the same time.

Soil Dressing

Soil dressing is simple and is one of the most widely used techniques for heavily contaminated sites (Vangronsveld 1998). This method has been adopted as a primary countermeasure for Cd contamination in agricultural soil under the Agricultural Land Soil Pollution Prevention Law in Japan (Chapter 3, Regulation of heavy metal pollution of soils in Japan). Local managers who are responsible for preventing contamination, prefer this technique over other countermeasures, because of its low failure risk, its predictable time frame, and because it leaves sites in a relatively pristine condition. Below are several methods to amend the polluted soils through soil dressing (Yamada 2007):

Simple soil dressing (Fig. 18). Unpolluted soils are placed on top of the polluted soil. Since the paddy fields amended by this method are raised by 20 to 30 cm high, preparation of agricultural canal, agricultural road and paddy fields re-zoning are needed.

Soil removal followed by new soil dressing. Polluted surface soils are removed and discarded outside the paddy fields. Then the infertile subsurface soils are covered with unpolluted soil. The depth of polluted soil to be removed is determined based on the degree of soil pollution and plant root elongation.

In-situ placement of polluted soils (Fig. 19). After removing polluted surface soils, subsurface soil and subsoil are also temporarily removed. Then the polluted surface soil is buried into subsoil layers. After that, the removed subsoil is placed back on the buried surface soil, placing the new unpolluted soil on the top of the subsoil. However, it is difficult to apply this method to paddy fields where sub-layer soil is also polluted and/or water table is high.

Turning the soil layers upside down. The polluted surface soils are switched places with the unpolluted subsoil. This method is applied to paddy fields where unpolluted soil is hardly present and subsoil is not polluted at all.

According to several follow-up surveys, soil dressing is a very effective and reliable practice to decrease the Cd content in rice grains when the newly-dressed unpolluted soil layer is 20_30 cm thick. However, this practice is costly and becoming increasingly difficult to implement because of scarce suitable uncontaminated soils.

New Approaches to Remediation of CD-Contaminated Soil in Japan

Phytoremediation

Recently, phytoremediation has come to people's attention as a cost-effective and environment-friendly technology to remove various toxic materials from soils. There are different types of phytoremediations such as phytoextraction, phytovolatilization, phytostabilization and rhizofiltration (Table 6). Phytoextraction is the most popular technology and intensively examined of the four phytoremdiations in Japan. Fig. 20 describes the conceptual scheme of phytoextraction for Cd-contaminated paddy soils for plants.

A variety of plant species have been studied for their capacity of extracting Cd from contaminated soil, such as tall goldenrod or S. altissima ( Tatekawa et al. 1975), indian mustard or Brassica juncea (Yanai et. al. 2004), kenaf (Hibiscus cannabinus), okra or Abelmoschus esculentu (Kurihara et al. 2005), sorghum or Sorghum bicolor (Kato et al. 2004), hakusanhatazao (Arabidopsis halleri ssp. Gemmifera)( Nagashima et al. 2005), Asteraceae (Watanabe and Sasaya 2007), sugar beet (Beta vulgaris L. ) (Ishikawa et al. 2006) and rice (Oryza sativa L.) (Murakami et al. 2007).

There have been reports that some Japonica-Indica hybrid and Indica rice variety possess a considerably high capacity of soil Cd absorption compared to other Japonica rice varieties. Murakami (2007) reported, as shown in Fig. 21, a comparison of Cd extraction efficiency from soils through the rice cultivar Miyang 23. In this context, rice plants are considered to be one of the most promising species for Cd phytoextraction from contaminated paddy soils. Although there are a couple of upland and/or perennial plant species with a possible high capacity of Cd extraction, it is difficult to vigorously grow these species in paddy soils. It also takes a long time to bring back the once converted soil suitable for the upland species to its original paddy condition. On the other hand, any rice varieties, either Indica, Japonica or their hybrid, very easily adopt to Japanese paddy soils, and Japanese growers are familiar with almost all relevant cultural practices for rice cultivation. After two or three times of Cd phytoextraction practices, the paddy field remedied could be planted with a commercial variety of rice.

Soil Washing

Soil washing is conventionally performed ex situ using an appropriate apparatus, in which extracting reagents are used to remove hazardous metals from soil into aqueous solution (Elliott and Herzig 1999). Although soil-washing techniques offer a great advantage of high Cd-removal efficiency for contaminated soils, they are considered difficult to directly apply to agricultural land. Wastewater drained during the process of soil washing might pollute surrounding areas such as agricultural canal, neighboring agricultural field, groundwater, etc.. However, Cd contaminated paddy fields are distributed nationwide and most of the Cd contamination level in rice paddy fields are less than 1 mg Cd kg^_1 (oven-dry basis). This wide distribution of low levels Cd contamination makes it extremely difficult to transport contaminated soils to an ex situ treatment plant. In this context, soil washing of contaminated paddy fields should be conducted in situ. Since paddy fields possess impervious hardpan just below the subsurface layer which hinders vertical movement of water, the washed solution stays in the surface soil and does not penetrate into subsoil layers and groundwater. So, an in-site technology of soil washing should be fully utilized given such unique characteristics of paddy field.

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

• 1. Use of washing chemicals with high efficiency of Cd removal, but a minimal adverse impact on the paddy field and its surrounding environment;
• 2. Cost-effective and environmentally sound operation of the system;
• 3. Soil fertility of paddy field and its crop growth which are not greatly affected by the washing treatment or can be easily corrected by application of agricultural materials; and
• 4. A lasting effect of washing for a reasonably long period.

Metal chelating agents, neutral salts, and strong acids have been used for the soil washing chemicals (Davis 2000). In particular, ethylenediamine tetraacetic acid (EDTA) has been commonly used due to its efficient Cd removal from contaminated soils (Nakashima and Ono, 1979; Abumaizar and Smith 1999; Zeng et al. 2005). EDTA, however, is a persistent chemical and stays for a long time in the environment (Tandy et al. 2004). Some scientists therefore have used more biodegradable chelating agents instead of EDTA (Mulligan et al. 1999; Hong et al. 2002; Tandy et al. 2004; Chang et al. 2005; Kantar and Honeyman 2006). In case biodegradable agents are used, however, the cost becomes relatively higher than that of the non-/less degradable counterparts.

Ogawa et al. (1985) used HCl to wash soil in a batch experiment and conducted pot tests to confirm a decrease in Cd concentrations of unpolished rice. The polluted soil treatment system, in which in situ soil washing and on-site wastewater treatment are combined, has never been applied to Cd-contaminated paddy fields.

Hereafter, we are to introduce a new soil washing practice combined with on-site wastewater treatment that completely satisfies the above mentioned four requirements (Makino, et al. 2006, 2007 and in press).

Materials and methods

Selection of washing chemicals:

a. Soils

Samples of three soils were obtained from the plow layers of paddy fields of Nagano, Toyama, and Hyogo prefectures in Japan. All of the soil samples were air-dried and passed through a 2-mm mesh sieve before chemical analysis. Soil pH was determined through a glass-electrode method (Horiba, PH81, Japan) containing a ratio of 1:2.5 of soil and either water or 1 mol L^_1 KCl. The total carbon (TC) and nitrogen (TN) contents of the soils were measured by a dry combustion method (Shimadzu, Sumigraph NC-900, Japan). The clay content and clay mineralogy of the soils were determined by a pipette method (Gee and Bauder, 1986) without prior removal of iron oxides, and by X-ray diffraction analysis (JEOL Ltd., JDX-3530, Japan), respectively.

Soil was digested with a mixture of nitric and perchloric acids on a hot plate (Amacher 1996). The Cd content in the digested solution was determined through inductively coupled plasma optical emission spectrometry (ICP-OES) (Varian Inc., Vista-Pro, USA). The Cd-contaminated soils were obtained from paddy fields polluted by the wastewater of mines. The total Cd contents of soils from Nagano, Hyogo, and Toyama were determined as 0.71, 4.65, and 1.21 mg kg^_1, respectively. These Cd concentrations were substantially higher than the mean values in uncontaminated soils, which average 0.33 mg kg^_1 in Japan and 0.48 mg kg^_1 in the world (Table 1).

b. Various chemicals

Three paddy soils were used for a Cd extraction test: Nagano soil (Fluvaquents), Toyama soil (Epiaquepts), and Hyogo soil (Fluvaquents). 10 g of each of the three paddy soils contaminated with Cd were shaken for one hour with 15 mL solutions of 0.02 M_c L^_1 or 0.1 M_c L^_1 chemicals such as calcium chloride (CaCl₂), calcium acetate, magnesium chloride, magnesium acetate, sodium chloride, sodium acetate, potassium chloride, potassium acetate, iron (III) chloride (FeCl₃), disodium ethylenediamine tetraacetate (EDTA-2Na), citric acid, acetic acid, and hydrochloric acid. The soil-chemical mixtures in tubes were centrifuged for 15 minutes at 3,000 rpm and the supernatants were filtered in a disposable membrane filter, with 0.2 ìm pore size. After adding 50 ìL of the concentrated nitric to 4.95 mL of the filtrates, Cd levels were analyzed using ICP-OES.

c. Metal salts

Same extraction procedure was conducted using the same three sample soils and 100 mM_c of acids and metal salts varieties, such as HCl, HNO₃, H₂SO₄, FeCl₃, MnCl₂, ZnCl₂, Fe(NO₃)₃, Mn(NO₃)₂, Zn(NO₃)₂, Fe₂(SO₄)₃, MnSO₄, and ZnSO₄. Various ions extracted using FeCl₃, Fe(NO₃)₃, and Fe₂(SO₄)₃ were determined by the following analytical methods with duplicate: ICP-OES for Na, K, Ca, and Mg, and distillation (Mulvaney, 1996) with MgO for NH₄⁺. An ion chromatograph (DX-320, Dionex Corp., USA) was used to measure anions (Cl^_, NO₃^_, PO₄^3_, SO₄^2_). Dissolved organic carbon (DOC) was analyzed using a total organic carbon analyzer (TOC-5000, Shimadzu Corp., Japan). Visual MINTEQ software was used to analyze the ionic, DOC, and pH data sets to estimate Cd speciation in the extracts (Gustafsson 2004).

On-site soil washing:

An on-site testing plot was prepared in a paddy field in Nagano Prefecture. The soil washing procedure consisted of three steps: (1) chemical washing with FeCl₃ solution, (2) following water washing to eliminate the remaining chemicals, and (3) on-site treatment of wastewater by a portable purification apparatus with a chelatin meterial (Fig. 22). A part of the paddy field was bounded with 60-cm-high plastic boards, which were partially buried on the edge of the paddy field so that the upper two-thirds of each board remained above the ground surface. This boundary provided containment for additional water and chemicals in the paddy field. Soil washing was conducted in the bounded area, which encompassed about 100 m².

a. Chemical washing

Ferric chloride (FeCl₃) was applied to the bounded experimental field, and then added with agricultural water, creating a soil-solution ratio and a FeCl₃ concentration of 1:1.5 and 15 mM, respectively. These were the optimal values for soil washing in this paddy in a preliminary experiment. The soil solution was mixed by a 13-metric-hp cultivator (Kubota Corp. B7000, Japan) until it turned into slurry. After the mixing, the slurry was allowed to rest for more than two hours, and then the supernatant of the slurry was drained-off as wastewater. Chemical components of Cd in the wastewater were analyzed, as mentioned in the selection of washing chemicals, specifically metal salts.

b. Water washing

The experimental field was then filled with agricultural water until the water level reached the initial point. To eliminate residual Cd and Cl, the soil solution was mixed for 1hr until it turned into slurry, allowed to rest for 2-5 hours and then the supernatant of the slurry was drained-off as wastewater. This procedure was repeated three times, until the residual Cl concentration was reduced to lower than the target value for rice growth (500 mg L^-1). The supernatant Cl concentration was measured by a Cl meter (IM-40S, DKK-TOA Corp., Japan).

c. On-site treatment of wastewater

The wastewater produced by chemical and water washing was pumped into the on-site wastewater treatment system. The system removed Cd from the wastewater, and then discharged the treated water to a canal. An alkaline treatment and chelating material with flocculent settling in the treatment system were applied to remove Cl from the wastewater. The wastewater was sampled before and after the treatment system. The concentrations of Cd and Cl were determined by ICP-OES and ion chromatography, respectively, in the wastewater and the treated water.

Measurement of changes in soil Cd content:

The washed experimental area was divided into four plots for a wet rice culture experiment. Two soil samples were collected from the Ap horizon in each plot, before and after the washing. Soil samples were also collected from four control plots located in the unwashed experimental area. All samples were air-dried and then passed through a 2-mm mesh sieve before analysis. Four grams of soil from each sample was placed in a 50-mL PP tube, and 20 mL of 0.1 mol L^-1 or 0.01 mol L^-1 HCl solution, or 40 mL of 1 mol L^-1 NH₄NO₃ solution, was added. The extracts of the soil solution mixtures were sampled and filtrated. The Cd level in the filtrates was analyzed by ICP-OES. The concentrations of soil Cd measured by 0.1 mol L^-1, 0.01 mol L^-1 HCl and 1 mol L^-1 NH₄NO₃ solution were defined as that of acid soluble, weakly soluble and exchangeable fraction, respectively.

Soil fertility:

Soils were sampled from the washed and unwashed plot in the experimental site and were air-dried. Total carbon, pH, and nitrogen in the soils were measured by same methods described above. Soil EC was analyzed using the electrode method (Mettler, MC126, USA) with a soil-water ratio of 1:5. Exchangeable cations and available phosphate were analyzed according to Thomas (1982), and Truog (1930), respectively. Available nitrogen was measured by phosphate-buffer extraction method (Matsumoto et. al. 2000).

Rice cultivation:

Two rice cultivars, `Akitakomachi' and `Kusahonami', were transplanted, and harvested, in the experimental and control plots. Akitakomachi is one of the popular rice varieties in the region. On the other hand, Kusahonami has a high capacity for absorbing Cd. Mature rice was manually harvested, taking two 1.65-m² quadrilaterals in each subplot. Air-dried shoot and brown rice yield were measured. The brown rice yield was converted to ordinary water concentration (150 g kg^-1 dry weight). A part of the shoot material and of the brown rice was ground in a stainless steel vibration sample mill, and 1 g of each grounded sample was digested with concentrated HNO₃ followed by HClO₄. Cd concentration in the digested solution was determined by ICP-OES.

Results and discussion

Selection of washing chemicals:

Hydrochloric acid, nitric acid, and EDTA-2Na extracted more Cd in soil than the neutral salts (Fig. 24). However, EDTA-2Na is difficult to use for practical purposes because of its persistent nature in the environment and relatively high cost. Nonetheless, both strong acids can cause serious soil acidification in the soils with a low acid-buffering capacity.

Iron (III) chloride extracted nearly as much Cd as hydrochloric acid, nitric acid, and EDTA-2Na from the Nagano and Toyama soils, and more Cd from the Hyogo soil (Fig. 23). Iron is a major soil constituent and is less environmentally harmful than the remaining chemicals. In addition, FeCl₃ is less expensive and easier to handle than both hydrochloric acid and EDTA-2Na, thus, FeCl₃ was selected as a promising washing chemical.

The Cd extraction capacity was compared with other metal salts to clarify the Cd extraction mechanism of FeCl₃. The proportion of total soil Cd extracted by the washing chemicals (i.e., the Cd extraction efficiency) increased in the following order: Mn salts = Zn salts << ferric Fe salts in all the three soils, with efficiencies ranging from 4-41%, 8-44%, and 24-66%, respectively (Fig. 25). The amount of Cd extracted was negatively correlated with the extraction pH (Fig. 26), suggesting that extraction pH plays an important role in determining the Cd extraction efficiency.

When metal salts are added to soils, the dissociated metal cations that may form hydroxide precipitates with releasing protons according to the Hydrolysis equation (Fig. 23).

The precipitation of the metal hydroxide (hydrolysis of the metal ion) generates protons at a rate that depends on Kom, and these protons may decrease the extraction pH (Eqs. 1-3). Figure 27 illustrates the theoretical relationships between pH and activity of metal ions in the metal hydrolysis reactions at the equilibrium, with soil iron (calculated using Eq. 3 and the K^om values). Ferric hydroxide has around pH 2 (Fig. 27), which is much lower than the original soil pH (H₂O) of the three soils.

Thus, Fe-hydrolysis is associated with a greater decrease in soil pH compared to the other two metals. This indicates that proton release is a driving force of the Cd extraction by FeCl₃, which results in a sharp decrease in soil pH. In another study, Cd was highly mobile under oxidizing and acidic conditions of these soils (Kabata-Pendias 2000). Heavy metal solubilization was greatly enhanced by acidification, and at pH 1.3, it reached more than 80% of the total Cd content of the soil (Dube and Galvez-Cloutier 2005). Our results and these previous reports endorse the effectiveness of iron salts as washing chemicals to remove Cd from soil.

The Cd extraction efficiency of metal chlorides was greater than that of the corresponding metal sulfates and nitrates in all soils (Fig. 25). Extraction efficiency decreased in the following order: chlorides > nitrates ~ sulfates, with values ranging from 41-75%, 14-63%, and 26-62%, respectively, in the Nagano soil. The results are similar for the other two soils. To examine the factors, which result in the difference of the extraction efficiency between the metal salts, we estimated the relative abundance of dissolved Cd species in 100 mM_c iron salt solution by the Visual MINTEQ software (Gustafsson 2004). Fig. 27 indicates that Cd_Cl complexes such as CdCl⁺ and CdCl₂ (aq) accounted for 80% of the total dissolved Cd in the Nagano soil at 100 mM_c FeCl₃, versus values of 33% for Fe₂(SO₄)₃ and 9% for Fe(NO₃)₃. Similar trends were observed for the other metal salts and soils (data not shown). Cadmium has a high capacity to form complexes with anions such as Cl^_, SO₄^2_, CO₃^2_, PO₄^3_, organic acids, and fulvic acid (Traina 1999). Doner (1978) reported that Cd was leached more rapidly in the presence of Cl^_ than in the presence of ClO₄^_. Sakurai and Huang (1996) showed that the rate of desorption of Cd from a montmorillonite was greater with KCl than with KNO₃. Smolders and McLaughlin (1996) suggested that high concentrations of Cl^_ might increase plant's uptake of Cd either by enhancing mass transport of Cd or by enhancing uptake of the CdCl+ complex through plant roots. Accordingly, the formation of stable Cd_Cl complexes could inhibit resorption of the extracted Cd onto adsorption sites on the surface of the soil particles. This inhibition mechanism will improve the efficacy of extraction with FeCl₃ compared to that with Fe₂(SO₄)₃ and Fe(NO₃)₃, because the proportion of Cd complexes to the total dissolved Cd concentration is high in the extracts with chloride salts.

On-site soil washing:

Fig. 29 shows the profile of Cd concentration in the pre-treated and treated wastewaters generated during the chemical washes and the water wash. The Cd concentrations in the treated wastewater were below the Japanese environmental quality standard (0.01 mg Cd L^-1), demonstrating that the in situ treatment system could treat the wastewater as expected. The Cl concentration was less than 500 mg L^-1 after three water washes. This concentration is the threshold value for healthy rice crops.

Cadmium has a good capacity to form complexes with various anions, such as Cl^_, SO₄^2_, CO₃^2_, PO₄^3_, organic acids, and fulvic acid (Traina 1999). Because paddy soils receive a wide variety of anions from different sources, including irrigation water, fertilizer, and soil amendments, the Cd extracted from soil adsorption sites may easily form complexes with existing anions during the extraction process. To evaluate the kinds of Cd complexes that formed during the first chemical-wash process, we calculated the chemical species of Cd that would be present in the extracts, using Visual MINTEQ software (Gustafsson 2004). The Cd-Cl complexes such as CdCl⁺ and CdCl₂ (aq) exceeded 70% of the total dissolved Cd at 0.1 mol L^_1 CaCl₂ (Fig. 30). Determining the chemical species of Cd by means of MINTEQ software revealed the formation of Cd_chloride complexes, which enhanced Cd extraction from the soils. The formation of stable Cd-Cl complexes could promote Cd desorption from soils and inhibit resorption of extracted Cd onto adsorption sites on the surface of the soil particles.

Verification of the washing effects of FeCl₃ in situ on contaminated soil:

a. Soil Cd :

The concentration of exchangeable Cd has changed a little after FeCl₃ washing, whereas acid-soluble Cd form decreased substantially (Fig. 31). Although the exchangeable Cd increased with decreasing soil pH caused by the washing treatment (data not shown), adjusting the pH to its initial level by adding lime could decrease the exchangeable Cd concentration and maintain it at this level after the washing. Total Cd content of soil decreased substantially, to 55% of the unwashed value, compared to a value of 83% after CaCl₂ treatment in a field washing experiment (Makino et al. 2007). These results indicate that FeCl₃ has a high Cd extraction efficiency in paddy soils. These results also appear to be the first practical example of detoxifying soils contaminated with Cd using FeCl₃ based on proton release, and through generation of hydroxides and formation of Cd_Cl complexes.

b. Soil fertility:

Fig. 32 summarizes the changes in soil fertility properties using soil washing. The pH(H₂O) and pH(KCl) significantly decreased after the washing treatment. Although EC increased, it did not reach a level that would affect plant growth. Exchangeable cations decreased due to soil washing. The Mg and K deficit was corrected by applying fertilizers to the washed soil, restoring the Mg and K concentration in the soil to approximately 70-80% of the value in the unwashed soil (data not shown) during the growth period. Total carbon and total nitrogen concentrations changed a little, while available nitrogen and available phosphorus decreased significantly after washing. Although the extraction pH became very acidic by applying FeCl₃, the amount of soil Al released was less than 1% against the total soil Al (data not shown). This means that this in situ soil treatment is unlikely to cause serious soil damage such as clay mineral destruction.

c. Rice cultivation:

Soil washing had markedly positive effects on the growth and yield of rice crops. It considerably decreased the Cd concentrations in the rice straw and unpolished rice, from 0.91 and 0.31 mg kg^_1 in the unwashed soil to 0.18 and 0.053 mg kg^_1 in the washed soil, respectively. This reduction rate of plant Cd is higher than that of soil Cd estimated based on the amounts of the total and acid-soluble form. These results proved efficiency and effectiveness of the soil washing method for remediation of Cd-contaminated paddy fields.

Acknowledgment

The author would like to thank the Taiheiyo Cement Corp and Nagano Agricultural Research Center for their cooperation on soil washing study. The study was supported in part by a Grant-in-Aid (Hazardous Chemicals) from the Ministry of Agriculture, Forestry, and Fisheries of Japan (HC-04-1140-1).

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Index of Images

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  Figure 1 Distribution of Cadmium Concentration in Brown Rice in Japan.

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  Figure 2 Distribution of Cadmium Concentration in Wheat Grain in Japan.

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  Figure 3 Distribution of Cadmium Concentration in Grains (Barley, Rye and Buckwheat) in Japan.<BR>

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  Figure 4 Distribution of Cadmium Concentration in Soybean in Japan.

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  Figure 5 Distribution of Cadmium Concentration in Spinach in Japan.

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  Figure 6 Distribution of Cadmium Concentration in Cabbages (Head) in Japan.

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  Figure 7 Distribution of Cadmium Concentration in Chinese Cabbages in Japan.

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  Figure 8 Distribution of Cadmium Concentration in Onion Welsh in Japan.

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  Figure 9 Distribution of Cadmium Concentration in Onion, Bulb in Japan.

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  Figure 10 Distribution of Cadmium Concentration in Taro except for Skin in Japan.

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  Figure 11 Distribution of Cadmium Concentration in Burdock in Japan.

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  Figure 12 Distribution of Cadmium Concentration in Carrot in Japan.

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  Figure 13 Concept of designation standard by the Soil Contamination Countermeasures Law (MOE 2006).

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  Figure 14 Areas where countermeasures are conducted for hazardous metal contamination under the Agricultural Land Soil Prevention Law (Revised from MOE 2006).

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

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  Figure 16 Eh-PH Diagram of Some Chemical Species.

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  Figure 17 Relationships between soil Eh and rate of Cd extraction or that of sulfate residue. The lines and open circles correspond with the (SO42-)/{(SO42-)+(ÓH2S)} and Cd extraction rate of the vertical line (Makino 2002, modified from Iimura 1978).

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  Figure 18 Simple Soil Dressing (Modified from Yamada 2007).

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  Figure 19 In-Situ Placement of Polluted Soils (Modified from Yamada 2007).

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  Figure 20 Conceptual Scheme of Phytoextraction for CD-Contaminated Paddy Soil

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  Figure 21 Changes in CD Content in Soils with Phytoextraction. the Rice Cultivar of Japonica-Indica Hybrid (Miyang 23) Was Cultivated in Pots. Soil CD Was Determined by 0.1M HCL Extraction (Modified from Murakami Et Al. 2007).

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  Figure 22 Conceptual Diagram of on-Site Soil Washing.

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  Figure 23 Hydrolysis Equation

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  Figure 24 Efficiency of CD Extraction with Various Chemicals from the Three Soils.

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  Figure 25 Comparison of Cadmium Extraction Efficiency from the Three Soils by Metal Salts (Gray Bars) and Strong Acids (Shaded Bars) at 0.1 MC. the Extraction PH Is Shown in the Parenthesis. the Error Bars Indicate Standard Deviation (Makino Et Al. in Press).

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  Figure 26 Relationships between Extraction PH and the Amounts of CD Extracted from the Three Soils Using Metal Salts and the Three Strong Acids.

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  Figure 27 Diagram of PH and Metal Activity to Precipitate Metal Hydroxides.

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  Figure 28 Relative Abundance of Various CD Species in the Extracts of the Nagano Soil in the Presence of the Three Iron Compounds. the CD Species Were Calculated Using the Visual Minteq Software (Gustafsson 2004) Based on the Data Set of Cation, Anion, PH, and Dissolved Organic Carbon Values Obtained for the Extracts (Makino Et Al., in Press).

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  Figure 29 Profile of CD Concentration in the Wastewater and Treated Wastewater Generated at on-Site Soil Washing.

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  Figure 30 Changes of CL Concentrations during Soil Washing, and CD Chemical Speciation at the Fecl3- Wash.

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  Figure 31 Changes of Soil CD Contents with Washing Treatment.

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  Figure 32 Comparison of Soil Fertilities before and after Washing Treatment. PH Values Are Raw Data. Ec Value Means Almost Six-Fold Increase after the Washing.

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  Table 1 Natural Abundance of Heavy Metals in Japanese Soil and Brown Rice

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  Table 2 International Standard Value of CD Concentration in Crops Adopted by Codex

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  Table 3 International Standard Value (Codex) and Excess Rate of CD in Staple Crops (Maff 2002)

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  Table 4 Target Substances and Standards for Heavy Metal Pollution of Soils in Japanfig.13. Concept of Designation Standard by the Soil Contamination Countermeasures Law (Moe 2006).

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  Table 5 Effect of Water Management on CD Content in Rice Grains (Sakurai, Niaes, Unpublished Data).

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  Table 6 Types of Phytoremediation for Inorganic Constituents (Suthersan 2002)<BR>

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