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TECHNICAL REPORTS
Heavy Metals in the Environment

Contamination Time Effect on Lead and Cadmium Fractionation in a Tropical Coastal Clay

^a Div. of Environ. and Water Resources Eng., School of Civil and Environ. Eng., Nanyang Technol. Univ., Nanyang Ave., Singapore 639798
^b Div. of Geotech. and Transp. Eng., School of Civil and Environ. Eng., Nanyang Technol. Univ., Nanyang Ave., Singapore 639798

* Corresponding author (cttlim{at}ntu.edu.sg)

Received for publication January 26, 2001.

	ABSTRACT

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The capability of a tropical coastal clay to immobilize lead (Pb) and cadmium (Cd) was investigated in laboratory batch sorption tests conducted under acidic, neutral, and slightly alkaline conditions. The contact time was extended to 65 d. The distribution of Pb and Cd among various sorbed phases was examined using a sequential extraction technique. The sorbed phases were fractionated into the exchangeable, carbonate, reducible, organic, and residual fractions. There were only small changes in the total Pb and Cd sorption beyond a 1-d sorption period. The metal fractionation results show that the amount of Pb and Cd in various fractions changed with sorption time, and the changes were pH-dependent. These changes could be attributed to mineral dissolution and transformation or redistribution of the sorbed phases. Transformation of the sorbed phases resulted in increasing Pb and Cd retention in the more persistent fractions with time, at the expense of reductions in the loosely bound fractions. Nevertheless, Pb and Cd fractionation in the solid phase appeared to reach equilibrium within the 65-d sorption period. These Pb and Cd fractionation results reflect the effect of contamination time on the heavy metal lability and bioavailability in the subsurface environment.

Abbreviations: ICP-ES, inductively coupled plasma emission spectrometer

	INTRODUCTION

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CLAY DEPOSITS can be found in estuaries, coastal plains, continental shelves, and offshore islands of various parts of the world. These geographic zones are usually the economic, industrial, commercial, and residential hubs of regions. In the recent decades, many waste landfill facilities have been constructed on the coastal and offshore clay deposits, in view of their strategic locations. The clay deposits found in these zones have been regarded as excellent natural hydraulic barriers. In addition, exposure to the marine environment during initial deposition gave the clays favorable geochemical properties such as high alkalinity and acid buffer capacity for heavy metal immobilization (Lim, 1998).

Clay deposits, besides serving as excellent hydraulic barriers, can attenuate contaminant migration via geochemical processes such as adsorption, precipitation, and coprecipitation. The actual modes of heavy metal retention depend on the species of heavy metal, soil constituents, leachate pH and composition, redox conditions, and numerous physical parameters of the soil–waste system. pH has been regarded as a master variable regulating the mobility of metals. A large volume of literature is available on the influence of pH on metal sorption in various natural soils and synthetic soil constituents (e.g., Harter, 1983; Yong et al., 1993; Holm and Zhu, 1994; Lee et al., 1998; Papini et al., 1999; Coles et al., 2000). Most of these metal sorption studies were conducted within a short artificial contamination period (e.g., 24 h) in the laboratory.

This study investigated the possible changes of Pb and Cd sorption and fractionation (in a single-element system) with time in a tropical clay, to understand the long-term fate of these heavy metals in the soil. Clay contamination was effected using the laboratory batch sorption technique and the contaminated clay sample was examined using a sequential extraction technique, which has been widely used for analysis of metal fractionation in various contaminated geomaterials (Sheppard and Thibault, 1992; Ramos et al., 1994; Yarlagadda et al., 1995; Lo and Yang, 1998; Plassard et al., 2000). The main advantage of conducting a laboratory sorption test is that the mass balance can be checked and the environmental variables sensitive to sorption processes can be controlled and defined explicitly.

	MATERIALS

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Background of Singapore Clay
The geological formation and mineralogy of coastal and marine clay of Singapore have been reported by previous investigators (Pitts, 1984; Jaritngam, 1996). The Singapore clay is of Late Pleistocene and Recent deposits, and is part of the clay deposit commonly found in coastal plains of Southeast Asia. It occurs over an area covering about 25% of Singapore Island. The clay is kaolinite-rich, pale gray to dark blue–gray in color, silty, and has a small percentages of shell fragments disseminated throughout.

Clay Sample and Geochemical Properties
An unpolluted natural clay sample was obtained from the clay deposit in the southern part of Singapore Island. Extensive characterization of the clay has been carried out (Lim, 1998). The natural clay pH was 8.4. The oxidizable organic matter content was 3%. Cation exchange capacity of the clay measured at various pH values (with Na as the index cation) increased fairly steadily from 14 cmol_c kg^-1 (at pH 3.0) to 23 cmol_c kg^-1 (at pH 9.0). The clay contained an alkalinity equivalent to 54 g CaCO₃ kg^-1. The iron (Fe) content was 28.1 g kg^-1. Jaritngam (1996) reported that the kaolinite content in the clay from the southern part of Singapore Island exceeds 50%, while quartz and montmorillonite are the other important minerals in the soil. The Pb and Cd concentrations were both <2 g kg^-1 (Lim, 1998), determined by microwave digestion of the clay sample, and subsequent analysis of the liquid using Inductively Coupled Plasma Emission Spectrometer (ICP-ES). The detection limits for Pb and Cd analysis with this equipment were approximately 200 ppb and 20 ppb, respectively.

	EXPERIMENTAL METHODS

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Sorption Tests
Metal sorption by the clay was determined using a batch sorption technique. The sorption tests were carried out in triplicate. Two grams of the air-dried clay sample ground to pass a 150-µm sieve was added to each 60-mL polypropylene centrifuge bottle. The clay sample in each bottle was first suspended in 40 mL of 0.5 M sodium acetate (NaOAc) buffer previously adjusted to the desired pH. This step was essential to minimize fluctuation in the suspension pH during sorption tests. After pH equalization, the suspension was centrifuged and the supernatant decanted. The pH-stabilized clay sample was then washed with 0.05 M NaCl solution and the supernatant decanted to remove the remaining acetate buffer entrapped during centrifugation. For each of the pH-stabilized clay samples, 40 mL of a synthetic leachate was added. The clay/solution ratio was 1:20. Two types of synthetic leachates were used, one with 500 µM CdCl₂ and 0.05 M CaCl₂ and the other with 500 µM PbCl₂ and 0.05 M CaCl₂. The heavy metal loading in the clay suspensions was therefore 10 mmol kg^-1. Acidified blanks were included to account for any heavy metal losses not associated with the clay.

The sorption tests were conducted during periods of 1, 7, 15, 30, 50, and 65 d at various pH values between 4 and 8. The clay suspensions contained in the centrifuge bottles were shaken with a reciprocating shaker operated at 35 oscillations min^-1 at room temperature of 23°C ± 2. The suspension pH values were adjusted by adding HCl or NaOH. For sorption tests with 1-d duration, the suspension pHs were adjusted at every 2- to 4-h intervals for the first 8 h, and 2 h before the end of the tests. For sorption tests with 7-d duration, the suspension pH values were adjusted on a daily basis. For sorption tests with duration longer than 7 d, the pHs of the clay suspensions were adjusted on the basis of 2- to 4-d intervals.

At the end of sorption equilibration, the pH of each equilibrated suspension was measured. After centrifugation of the suspension to obtain a clear supernatant, the supernatant was filtered through a 0.45-µm Whatman membrane filter. The filtrate was then acidified with HCl and the concentrations of dissolved metals were measured by ICP-ES. The concentrations of the metal elements, which form the solid constituents of the clay sample (e.g., clay minerals, oxides, and carbonate compounds), were also measured to estimate the degree of mineral dissolution at various pH values throughout the heavy metal sorption equalization periods.

Sequential Extraction of Sorbed Metals
After the clay suspensions were centrifuged and the supernatants decanted at the end of sorption tests, the contaminated clay samples were washed with 20 mL distilled water to remove the Pb and Cd entrapped in occluded solution in the clay samples during centrifugation. Concentrations of the heavy metals in the various sorbed fractions were then determined using a sequential extraction technique.

The sequential extraction scheme adopted in this study was a minor modification of the procedure proposed by Tessier et al. (1979). The fractions were operationally defined as follows:

1. Exchangeable—contaminated clay samples extracted with 40 mL 1 M CaCl₂ buffered to the equilibration pH measured at the end of the sorption tests (to prevent redistribution of sorbed phases due to pH change during this extraction stage). The extraction was carried out at 23 ± 2°C for 2 h.
   
2. Carbonate—residue from exchangeable extraction extracted with 40 mL 1 M NaOAc buffered at pH 5 for 5 h at 23 ± 2°C.
   
3. Reducible—residue from carbonate extraction extracted with 40 mL 1 M NH₂OH·HCl in 25% (v/v) HOAc at 70°C for 6 h.
   
4. Organic—residue from reducible extraction extracted with 10 mL H₂O₂ adjusted to pH 2 with 0.02 M HNO₃ for 2 h at 85°C with intermittent agitation, followed by a second aliquot of 5 mL H₂O₂ (adjusted to pH 2 with HNO₃) heated again to 85°C for 3 h. After cooling, 10 mL of 3.2 M NH₄OAc in 20% HNO₃ was added and the sample was diluted with deionized distilled water to 40 mL and agitated continuously for 30 min.
   
5. Residual—residue from organic extraction digested by microwave heating with HNO₃ (90%), HCl (37%) or HClO₄ (if Pb was extracted), HF (50%), and H₂O₂ (30%).
   

The concentrations of Pb and Cd in the extractants at the end of the sequential extractions were analyzed using ICP-ES. The mass recovery achieved was computed. In this study, the operationally defined reducible fraction which used 1 M NH₂OH·HCl in 25% (v/v) HOAc at 70°C was introduced to replace the Fe/Mn oxide fraction (which was proposed by Tessier et al., 1979), to exclude the components of Fe-oxides that were not extracted. These components included most of the crystalline Fe-oxides such as hematite, goethite, and magnetite (Chao and Zhou, 1983).

	RESULTS AND DISCUSSION

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Results of Time-Dependent Sorption and Fractionation
Table 1 shows the total Pb and Cd sorption by the clay at various pH values at the end of various sorption periods. The standard deviations of the triplicate sorption measurements were generally low, with the computed coefficients of variation ranged from 0.1 to 11.7%. The amount of Pb and Cd sorbed did not vary significantly with time at all the pH values investigated. Nevertheless, it appeared that low pH could marginally decrease metal sorption with time. For instance, the Cd sorption decreased from 16.1 ± 0.9% at pH 4 at the end of the 1-d sorption period to 12.3 ± 0.9% at the end of the 65-d sorption period. Complete Pb sorption occurred at pH 7 and 8, while Cd sorption may be complete only at pH > 8.

View larger version (32K): [in this window] [in a new window]   Fig. 1. Changes of Pb retention in various fractions of clay with time: (a) exchangeable, (b) carbonate, (c) reducible, (d) organic, and (e) residual fractions.

Figure 1d shows that there was almost no change in the organic fraction with time. The organic fraction was the smallest among the five fractions. Experimental variations might have masked the time-dependent Pb sorption in this fraction. For the residual fraction, the percentage decreased with time at pH 4 only, as shown in Fig. 1e. At pH 6, the residual fraction increased with time. At pH 5, no significant change was observed in the amount of Pb retained in this fraction. The coefficients of variation in this fraction ranged from 1.8 to 12.1%.

Figure 2 shows a comparison of the Pb distribution among various fractions of sorbed phases in the clay at the end of the 1-d and 65-d sorption periods. The data show that although there were changes in the amount of Pb sorbed in some fractions with time, distribution of Pb in the clay did not significantly change with time. The exchangeable fraction remained dominant in the acidic conditions throughout the 65-d sorption period, while the carbonate fraction remained the most significant fraction around neutral and under alkaline conditions. The reducible and organic fractions did not contribute significantly to the total Pb sorbed on the clay.

View larger version (45K): [in this window] [in a new window]   Fig. 2. Lead fractions in clay after sorption in 0.05 M CaCl2 medium: (a) 1-d sorption period, and (b) 65-d sorption period.

View larger version (30K): [in this window] [in a new window]   Fig. 3. Changes of Cd retention in various fractions of clay with time: (a) exchangeable, (b) carbonate, (c) reducible, (d) organic, and (e) residual fractions.

The Cd fractionation data measured at the end of the 1- and 65-d sorption periods are plotted in Fig. 4 . The figure illustrates that the distribution of Cd in the clay did not appear to change significantly with time. The carbonate fraction remained by far the most dominant fraction in the alkaline condition (e.g., pH 8), throughout the 65-d sorption period. Under acidic conditions, the exchangeable and residual fractions were dominant.

View larger version (38K): [in this window] [in a new window]   Fig. 4. Cadmium fractions in clay after sorption in 0.05 M CaCl2 medium: (a) 1-d sorption period, and (b) 65-d sorption period.

	DISCUSSION

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The use of a batch sorption test in this study eliminated the rate-limiting physical processes such as intra-aggregate diffusion process and diffusion in micropores, which could mask the interpretation of the time-dependent metal sorption and fractionation results. The use of low clay/solution ratio (i.e., 1:20) and the fine clay powder (avg. diam. <150 µm) maximized contact between added Pb and Cd and the clay sorption sites. While the batch sorption experiment introduced an artificial sorption environment that would weaken the usefulness of the data, it did allow insight into the geochemical characteristics of time-dependent metal sorption and fractionation in the clay.

By minimizing intra-aggregate diffusion and other rate-limiting physical processes in this study, the observed changes in Pb and Cd fractions with time could be attributed to several geochemical phenomena. One of these could be time-dependent mineral dissolution (van Grinsven et al., 1986; Bloom and Erich, 1987; Simard et al., 1992). Mineral dissolution would result in the release of the associated metals. Figure 5 shows the concentrations and percentages of the released Al, Si, Mg, and Fe, as measured at the end of sorption experiments. The released Si and Al could be associated with dissolution of amorphous Al and Si oxides and the predominant minerals in the clay, i.e., kaolinite [Al₂Si₂O₅(OH)₄] and montmorillonite [Al₄Si₈O₂₀(OH)₄]. The release of Fe was primarily due to the dissolution of amorphous or poorly crystalline Fe-oxides, which are relatively more soluble than the crystalline Fe-oxides. Magnesium could be released by the dissolution of magnesium carbonate compounds (such as dolomite [CaMg(CO₃)₂]) and montmorillonite minerals [MgAl₃Si₈O₂₀(OH)₄] with isomorphous substitution of Mg for Al (Mitchell, 1993; Blyth and Freitas, 1988). Mineral dissolution was enhanced at low pH values, as depicted in Fig. 5. Under acidic conditions, significant percentages of Fe (up to 16%) were released, compared with Al (<1.5%) and Si (<0.26%). Therefore, the degree of dissolution of aluminosilicates (clay minerals) was insignificant compared with the dissolution of Fe oxides.

View larger version (34K): [in this window] [in a new window]   Fig. 5. Measured amounts of metals released into dissolved phase at the end of sorption periods up to 65 d: (a) Al, (b) Si, (c) Mg, and (d) Fe.

Sorbed phase transformations could be operative for all pHs, but become predominant at higher pH values only. At high pH values, all Pb and Cd in the aqueous phase could be sorbed by the clay (as shown in Table 1). The exchangeable sorption and precipitation could be the instantaneous sorption mechanisms. The loosely bound sorbed phases then transformed to the less reversibly sorbed phases (formed via specific sorption) that would increase over time. Under acidic conditions, mineral dissolution predominated, and the accumulation of Pb and Cd in the reducible fractions was somewhat offset by simultaneous release of the retained heavy metals from the dissolving Fe oxides. Meanwhile, mineral dissolution also caused the decrease in the exchangeable fractions of sorbed Pb and Cd with time. This would be manifested as a net decrease in the total sorption of Pb and Cd with time at acidic conditions, as depicted by Table 1.

	CONCLUSIONS

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The sorption of Pb and Cd to the clay and their solid phase fractionation assist prediction of their bioavailability and toxicity in the subsurface environment. The sorption of both Pb and Cd by the tropical clay showed two distinctive time-dependent behaviors, depending on the pH. Under mildly acidic conditions, both Pb and Cd sorption decreased with time, due to dissolution of clay constituents. At higher pH values, there were only marginal increases in the heavy metal retention over time, until a maximum sorption was reached. This could be the result of various rate-limiting retention processes.

The Pb and Cd fractionation data monitored during the 65-d period revealed transformation of the heavy metals from the loosely bound sorbed phases (i.e., the exchangeable and carbonate fractions) to the strongly bound phases (i.e., the reducible and residual fractions). This finding implies that there would be gradually increasing proportions of Pb and Cd retained in the more persistent clay phases, which are bound by nonreversible sorption mechanisms. This phenomenon is especially important when the clay pH is within the range that commonly exists in the natural subsurface environment, i.e., pH 4 to 8. One can imply from this finding that polluted sites that have been subjected to long-standing metal contamination will likely be more difficult to remediate due to strong contaminant retention than recently contaminated sites.

	REFERENCES

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