International Journal of Environment Science and Technology Center for Environment and Energy Research and Studies (CEERS) ISSN: 1735-1472 EISSN: 1735-2630 Vol. 4, Num. 2, 2007, pp. 169-176

International Journal of Enviornmental Science and Technology, Vol. 4, No. 2, Spring 2007, pp.169-176

Comparison of extractability of Cd, Cu, Pb and Zn with sequential extraction in contaminated and non-contaminated soils

¹Laboratory of Plant Physiology and Nutrition, Faculty of Agriculture, Iwate University, Ueda 3-18-8, Morioka 020-8550, Japan
²Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, Box 5003, No-1432 Aas, Norway
³Geo-Environmental Protection Department, Fujita Corporation, 2025-1 Ono, Atsugi 243-0125 Japan
⁴Department of Soil, Water and Environment, Dhaka University, Dhaka 1000, Bangladesh
*Corresponding author, Email: kashemab@iwate-u.ac.jp Tel.: +9118 621 6153; Fax: +9118 621 6153

Received 13 December 2006;
Revised 26 February 2007;
Accepted 7 March 2007
Available online 20 March 2007

Code Number: st07021

ABSTRACT: Various extraction procedures were employed for measuring extractable concentrations of potential toxic elements in soil. The extractability of Cd, Cu, Pb and Zn in four contaminated and four non-contaminated soils of Japan, was compared by single extraction (CaCl₂, DTPA, NH₄Cl, 0.1 M HCl and 1 M HCl ) and sequential extraction procedures [(six operationally defined chemical phases, viz. water soluble (F1), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5) and residual (F6) fractions)]. Extractability of metals from soils samples varied depending on metals and/or extractants used. Among the extractants, 1 M HCl extracted the largest proportion of Cd (79 to 96% of total), Cu (61 to 83%), Pb (51 to 99%) and Zn (23 to 52%) from soils followed by 0.1 M HCl, NH₄Cl, DTPA and CaCl₂. In all the extractants, the proportion of extractability of metals was higher in the contaminated soils than the non-contaminated soils. Regardless of soils and extractants, relative extractability was higher for Cd as compared to other three metals. The use of 1 M HCl may be recommended for first-level screening of soil contamination with heavy metals. The other four weak extractants are believed to provide a better assessment of bioavailable/mobile metals content in soils than 1 M HCl extractant. However, 0.1 M HCl mobilized all four metals irrespective of soil types, therefore, might be the best choice if only one extractant is to be used. The sequential extraction procedures showed 22 to 64% of total Cd was in the mobile fraction (sum of F1 to F3), while the corresponding values for Cu, Pb and Zn in this fractions were 2 to 23% suggesting higher mobility of Cd than other three metals. The single extraction procedures are simple and easy to perform and obtained results are comparable with sequential extraction procedure.

Key words: Contaminated soils, chemical extraction, heavy metals, single and sequential extraction

In many industrial countries, soils are affected by mine waste disposal, acid deposition and sewage sludge use that could provide large inputs of pollutants and especially heavy metals to the soils. The impact of contamination on the environment should be of scientific concern, in order to minimize the threat of soil and groundwater contamination (Matos, et al., 2001). Heavy metal contamination of arable soils through industrial activities is a serious problem in Japan. Rice fields at different locations of Japan have suffered by Cd and other heavy metals contamination. Asami, et al. (1994) investigated some contaminated and non-contaminated Japanese soils and found that the total Cd, Cu, Pb and Zn concentrations were in the range of0.7 to 8.8, 33 to 81, 31 to237 and 118 to738 mg/ kg, respectively, which indicated large anthropogenic contribution of metals in soils. A variety of in-situ (onsite) and ex-situ remedial methods have been used/ proposed for the restoration of soils contaminated with heavy metals, which warrant investigating the extractability of heavy metals in soils. Chemical extraction procedures are able to predict the changes in the heavy metal mobility or bioavailability in soils after amendments for metal remediation (Singh, 2006).

A large number of extracting solutions have been used to assess plant available trace elements (Gupta and Aten, 1993;He and Singh, 1993). Single extractants may broadlybe divided into threemain classes: (i) weak replacement of ion salts (MgCl₂, CaCl₂, NH4NO₃), (ii) dilute solutions of either weak acids (acetic acid) or strong acids (HCl, HNO₃) and (iii) chelating agents (DTPA, EDTA). The first type of extractants are able to release into solution metals which are associated with the exchange sites on the soil solid-phase and hence can be considered as bioavailable (McLaughlin, et al., 2000). The chelating agents, such as DTPA and EDTA, form complexes with free metal ions in solution and thus reduce the activities of the free metal ions in solution. The unbuffered salt solutions, (CaCl₂ and NH₄Cl), is rapid and simple procedure to extract bioavailable metals (Beckett, 1989). In some cases, salt solution does not represent plant available metals (Gupta and Aten, 1993). Use of 0.1 M HCl solution (CSTPA, 1980) may reflect bioavailability of metals. ANZECC and ARM-CANZ (2000) recommend the use of 1 M HCl-extractable metal content for first-level screening-the rational being that total metal analysis include all non-residual metals as well as metals in the silicate mineral matrix, which are not available. We assume 1M HCl will not attack the silicate matrix, results from such extraction procedure may be closer to bioavailable fraction than total analysis. The procedures of sequential extraction define more precisely the individual fractions of elements in soil. This procedure requires three to seven steps. Consequently, it requires skilled personal, adequate analytical facilities and is also time consuming. However, complementary use of both single and sequential extraction methods may provide information useful for both short-term and long -term biological uptake of heavymetals (Singh, 2006).

In the other part of the study (Kashem, et al., 2006) the chemical fractionation of Cd, Cu, Pb and Zn, is investigated but in the present study we made a comparison of individual fractions of sequential fractionation procedure with single extractants. Therefore, this study was designed to: (i) investigate the extractability of Cd, Cu, Pb and Zn in four contaminated and four non-contaminated soils from Japan, and (ii) compare the metal extractability by single extractants with the fractions of sequential extraction procedure.

Soil samplings

Four contaminated and four non-contaminated surface soil samples (0 to 15 cm) used in this study were collected from different locations of Japan in November 2005. Four contaminated soil samples were Miyagi-1 & 2, Akita and Gumma and non-contaminated soils were Takizawa, Rokuhara, Hachirogata and Isawa. The Miyagi-1 & 2, and Akita soils were contaminated with heavymetalsfrom irrigation with river waterpassing through mines after different types of industrial operations. The source of Gumma soil was the Annaka Zn mining industry The Gumma soil was located at the Gumma Prefecture of Kanto district and other seven soils were from different Prefectures of Thohoku district (Table 1). Rice and vegetables are the common agricultural crops. A part of each sample was air-dried, ground and passed through a 2 mm stainless steel screen before use. Some important characteristics of the soils used in this study are presented in Table 1. All analytical work of this study was performed at the Laboratory of Plant Physiology and Nutrition, Iwate University, Japan.

Soil analysis

Soil pH was measured in a 1:2 soil/water suspension. Soil organic carbon (OC) was determined by wet oxidation methods of Walkley and Black (1934). Cation exchange capacity (CEC) was determined by extraction with 1 M NH₄OAc (pH 7.0) (Soil Survey Laboratory Staff, 1992). The hydrometer method (Bouyoucos, 1962) was used for the particle size distribution. Total concentration of metals (Cd, Cu, Pb and Zn) in the soils was determined by the HITACHI 170-30 atomic absorption spectrophotometry (AAS) after digestion withnitric-perchloric acid mixture (3:1).

Five different extraction procedures were used to establish the extractability and to predict the phytoavilability of heavymetalsin soils (Table 2). Each extraction procedure was performed in duplicate for each sample in 50 mL polycarbonate centrifuged tubes except1 M HCl extract. All the extractswerecentrifuged for 10 minutes at 3000 rpm, and the supernatant filtered. In case of 1 M HCl, the supernatant was passed through a membrane filter (0.5 µm). A brief summary of the sequential extraction procedure used (Kashem and Singh, 2001) is discussed here. Two grams of soil, 2mm sieve fraction, was placedin a 50mL polycarbonate centrifuge tube and following extractions were performed sequentially.

(F1): Sample extracted with 20 mL of deionized water for 1 h at 20 ºC on a rolling table.

(F2): The residue from F1 extracted with 20 mL of 1 M NH₄OAc, pH 7 for 2 h at 20 ºC on a rolling table.

(F3): Residue from the F2 extracted with 20 mL of 1 M NH₄OAc, pH 5 for 2 h at 20 ºC on a rolling table.

(F4): Residue from F3 extracted with 20 mL of 0.04 M hydroxylamine hydrochloride (NH₂OH.HCl) in 25 % acetic acid (v/v) at pH 3, reaction time 6 h in a water bath at 80 ºC with occasional shacking.

(F5): Residue from F4 extracted with 15 mL of 30 % H₂O₂ (adj. pH 2), reaction time 5.5 h in a water bath at 80 ºC with occasional shaking. After cooling, 5 mL of 3.2 M NH₄OAc in 20 % (v/v) HNO₃ was added; sample was shaken on a rolling table for 0.5 h at 20 ºC and finallydiluted to 20 mLwith water.

(F6): The residue from F5 fraction, extracted with 7 M HNO₃, reaction time was 6 h in a water bath at 80 ºC with occasional shaking.

Duplicates analysis was performed for each sample. Metals present in extracts were determined by HITICHI 170-30 atomic absorption spectrophotometry (AAS). The results were statistically evaluated by Pearson correlation coefficients and the descriptive statistics was performed by Minitab program (Minitab Inc. 1992).

Soil properties

Soil pH ranged from 4.7 in the Miyagi-2 soil to 6.5 in the Isawa soil. Relatively lower values of pH in some of these soils may be assigned to acid effluent coming from near by industrial operations as well as high humus content (Table 1). Soil textural class varied from sand to sandy clay loam. The organic matter (OM) content and cation exchange capacity (CEC) varied considerably in these soils, for example, the soil from Rokuhara had 11.0%, while Isawa soil had only 0.3% of OM. Cation exchange capacity varied from 9 to 45 cmol/kg (Table 1) and it was positively correlated (p<0.01) with OM content. The total concentrations of Cd, Cu, Pb and Zn in the soils varied from 0.7 to 12.5, 23 to 133, 19 to 359 and 90 to 704 mg/kg, respectively. Generally the concentrations of most metals were 5 to 18 fold higher in the contaminated soils than the non-contaminated soils (Table 1).

Metals extractability

As expected, 1 M HCl was the most efficient extractant (Table 3). The recoveries (% of total) of Cd varied from 79 to 83% in the non-contaminated soils and from 83 to 96% in the contaminated soils. There was only a small difference(<10%) in Cdextractability between 1 M HCl and 0.1 M HCl extractant in contaminated soils, however, the extractability of Cd with 0.1 M HCl was substantially lower (up to 45%) in non-contaminated soils (<1 mg/kg). Amount of Cd extracted byNH₄Cl was21 to 90%. DTPAextracts metals that are thought to represent the plant-available fractions (Lindsay and Norvell, 1978; Mellum, et al., 1998). DTPA forms soluble complexes with metals, reducing the activity in the soil solution; therefore, ions desorbed from the surface and enter into the solution.

Table 1: Sampling locations and physio-chemical characteristics of the soils used

C-Contaminated soil; Nc-Non-contaminated soil; OM-Organic matter; CEC-Cation exchange capacity;
^a Total concentration of metals extracted with nitric-perchloric acid

Table 3: Heavy metal concentrations in soils after individual extractions (mg/kg) and percent of total in parentheses

Range = minimum to maximum; SD= Standard deviation; ND= not detected

The relative extractability of Cd with DTPA was 6 to 37% in non-contaminated soils and from 29 to 63% in contaminated soils. The CalCl₂ extracted relatively smaller amount (3-30%) of total Cd than those of other extractants (Table 3).

As soils were not contaminated with Cu, the extractability ofCu was relatively low compared to Cd, irrespective of the extractant used. However, 1 M HCl extracted highest proportion of Cu (60 to 83%), followed by 0.1 M HCl (3 to 33%) and DTPA (up to 18%) extractant (Table 3). Chloride salts, such as NH₄Cl and CaCl₂ were found to be veryweak extractantstorelease Cu (~ 1%). For Pb, only two acid (HCl) extractants recovered considerable amount from all soils, while 1 M HCl dominated (51- 99%) over 0.1 M HCl (6- 32%) as it did for Cd and Cu. The efficiency of other three extractants was not consistent and found to be unsatisfactory for non-contaminated soils (<38mg/kg) (Table 3). Zn was released with all extractants in varying amounts similar to Cd. The efficiency of Zn extraction was highest with 1 M HC (23 to 52%) and lowest with CaCl₂ (1 to 17%). Overall, the extractability of Zn with different extractants was in the order: 1 M HCl > 0.1 M HCl > NH₄Cl > DTPA > CaCl₂ as was the case for Cd, Cu and Pb with minor exceptions (Table 3).

Comparison of metal extractability of single extractants with sequential extraction

The chemical fractionation of Cd, Cu, Pb and Zn in contaminated and non-contaminated soil fractions was determined by the multi step sequential extraction procedures of Kashem, et al. (2006). This method fractionates metals into six operationally defined fractions such as: water soluble (F1), exchangeable (F2), carbonate-bound (F3), oxide-bound (F4), organically-bound (F5) and residual (F6) fractions. Cadmium was associated mostly in first three fractions (F1-F3) (mobile), while Cu, Pb and Zn in the last three (F4-F6) fractions. The mobile fraction of metals varied from 22 to64% for Cd, 2 – 4% for Cu, 8 – 15% for Pband 4 – 23% for Zn with the higher values in the contaminated soils as compared to non-contaminated soils. In this part, we compared the average efficiency of single extraction procedures with element recoveries in F1, F2, F3, F4, F5 and F6 fractions of sequential extraction (Fig. 1).

Evidently, 1 M HCl was the most efficient agent for releasing Cd (85%), followed by Cu (69%), Pb (65%) and Zn (39%) from these soils. These results indicate that 1 M HCl extracted most of the non residual (sum of F1 to F5) plus part of residual (F6) fraction of Cd, Cu and Pb, while this agent was not able to dissolve quantitatively the Zn from organic (F5) and F6 fraction (Fig. 1). Metals extracted with 0.1 M HCl exceeded the mobile (sum of F1, F2 and F3) fraction for all metals. On other hand, the efficiency of NH₄Cl was found similar to 0.1 M HCl for Cd extraction. Ammonium chloride extractable Pb and Zn were lower than Cd but the extractable portions of the former elements were equivalent to their mobile fraction in the soils. Ammonium chloride had very poor extractability for Cu because Cu was mostly associated with residual and organic fraction (Kashem, et al. 2006). The chelating agent (DTPA) also played significant role to extract metals equivalent to mobile fractions except Pb (Fig. 1). DTPA extracted very small amount (2%) of Pb that represents water soluble and part of exchangeable fraction of Pb. Calcium chloride (CaCl₂) was not found suitable for Cu and Pb extraction; however it extracted 13% of Cd and 7% of Zn that represent exchangeable (F1 + F2) fraction of the sequential extraction procedure.

Interrelationships among extractable, mobile, total metal concentrations, soil pH and CEC

A significant positive relationship (r = 0.26 to 0.98; p <0.05 and <0.001) between extractable, mobile and total Cd, Pb and Zn was observed (Table 4). As soil was not contaminated with Cu, hence its relationships were not found consistent and significant (Table 4). Soil pH and cation exchange capacity (CEC) showed significant negative relationships with extractable, mobile and total metal concentrations except extractable Cu. Soil organic matter did not show any significant relationship and hence the results are not reported. A similar relationship of pH and CEC with mobile fraction of sequential extraction was found in the other part of this study(Kashem, et al., 2006).

Table 4: Pearson correlation coefficients between concentration of metals, pH and CEC in soil

All values in the Table are significantly different at p d” 0.05 probability level; NS = non significant; E = extractable metal concentration measured by single extraction procedures; T=total metal concentration extracted with nitric-perchloric acid: M= mobile fraction of metal (sum of F1 to F3) extracted from Kashem, et al., (2006); CEC= cation exchange capacity.

DISCUSSION AND CONCLUSION

Elevated concentrations of total and extractable metals in the Miyagi, Akita andGumma soils weredue to anthropogenic sources of metal additions to these soils. The sources of metal contamination for Miyagi 1 and 2, and Akita soils were from irrigation with river water passing through mines after different types of industrial operations, while for the Gumma soil, the Annaka zinc mining industries contributed to metal contamination. The proportion Cd and Zn measured by different single extractants represented more than one soil metal fraction determined by the sequential extraction procedure (Kashem, et al., 2006). The total metal analysis (HNO₃:HClO₄) include all non-residual as well as metals present insilicatemineral matrix, thelaterfraction(silicate bound) is not considered bioavailable. In our study, 1 M HCl dissolved all non-residual and part of residual Cd, Cu andPband hence it overestimatedthe bioavailability because the residual fraction is not considered as plant available(Sims and Kline,1991; Narwaland Singh,1998). The exchangeable fraction is generally considered immediate nutrient reservoir for plants (Narwal and Singh, 1998). Metals associated with carbonates would be susceptible to pH changes, may be regarded as potentially phytoavailable. Heavy metals present in other chemical forms, such as oxides, organic and residual with very low solubility and high stability for biological activity, would not have direct bearings on their uptake by plants (Xian, 1989). On the other hand, 1 M HCl showed less variability among soils in the proportion extracted as compared to other extractants, such as 0.1 M HCl, NH₄Cl, DTPA and CaCl₂. The later extractants may provide more realistic estimate of bioavailability than 1 MHCl. Although metal extractability by 1M HCl in thesedimentsamplesfrom SydneyHarbour was high (>80% for Zn and Pb; >60% for Cd and Cu) even then McCready, et al.(2003) concluded that 1 M HCl may have application in evaluating potential bioavailability differences for these metals among sediments. The significant positive relationshipbetween metals extracted withsingle extractants and mobile fraction ofmetals from sequential extraction procedure suggests that both sequential and single extractants are useful to predict metals bioavailability (Table 4). The strong negative relationship of extractable metal concentration and soil pH indicates that metal extractability decreased with increasing soil pH, as observed by many other investigators (Singh, et al., 1995; Evans, et al., 1995). They found that DTPA and NH₄NO₃ extractable metals decreased with increasing soil pH. In general, soil pH and CEC was found to be an important predictors of metal extractability (Rieuwerts, et al., 2006), retention and movement in soils (Matos, et al., 2001; Udom, et al., 2004). Strong positive correlations of total Cd, Pb and Zn, with their extractable and mobile fractions and each other suggest overloading of geochemical system by contamination with these metals from anthropogenic sources. There wasa large variability in metal extraction depending on extractant, metal type and nature of the soil. Weak extractants showed larger variability among soil types with respect to metal content than did by 1 M HCl, and hence they may be used to assess metals bioavailability. Nature of individual metals also need to be considered for selecting extractants as they are not equally suitable for all metals. Metal extractability is higher in the contaminated soils as compared to non-contaminated soils, irrespective of the extractant used indicating greater potential bioavailability of metals in the contaminated soils. The results obtained from sequential extraction indicated that most Cd was associated in the mobile fraction,,while Cu in the organic and residual, Pb and Zn in the oxides and residual fractions. Metals associations with oxides and residual fractions are considered least extractable. It may be stated here that the comparsion of single and sequential extraction procedure provided an interesting insight in extracting mobile/ bioavialble fractions ofheavy metals in contaminated and non-contaminated soils in this study.

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