Published online 9 January 2007
Published in J Environ Qual 36:53-60 (2007)
DOI: 10.2134/jeq2006.0134
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Heavy Metals in the Environment
Single-Element and Competitive Metal Mobility Measured with Column Infiltration and Batch Tests
Vasileios Antoniadisa,*,
John D. McKinleyb and
Wan Y. W. Zuhairic
a Dep. of Agricultural Development, Democritus Univ. of Thrace, Pantazidou 193, GR-682 00, Orestiada, Greece
b School of Planning, Architecture, and Civil Engineering, Queen's Univ. Belfast, David Kier Building, Stranmillis Rd., Belfast, BT9 5AG, UK
c School of Environmental Sciences and Natural Resources, Faculty of Science and Technology, Univ. Kebangsoon Malaysia, 43600, Bangi, Slangor, Malaysia
* Corresponding author (vasilisrev{at}yahoo.com)
Received for publication April 5, 2006.
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ABSTRACT
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The distribution coefficient, Kd, is often used to quantify heavy metal mobility in soils. Batch sorption or column infiltration tests may be used to measure Kd. The latter are closer to natural soil conditions, but are difficult to conduct in clays. This difficulty can be overcome by using a laboratory centrifuge. An acceleration of 2600 gravities was applied to columns of London Clay, an Eocene clay sub-stratum, and Cu, Ni, and Zn mobility was measured in centrifuge infiltration tests, both as single elements and in dual competition. Single-element Kd values were also obtained from batch sorption tests, and the results from the two techniques were compared. It was found that Kd values obtained by batch tests vary considerably depending on the metal concentration, while infiltration tests provided a single Kd value for each metal. This was typically in the lower end of the range of the batch test Kd values. For both tests, the order of mobility was Ni > Zn > Cu. Metals became more mobile in competition than when in single-element systems: Ni Kd decreased 3.3 times and Zn Kd 3.4 times when they competed with Cu, while Cu decreased only 1.2 times when in competition with either Ni or Zn. Our study showed that competitive sorption between metals increases the mobility of those metals less strongly bound more than it increases the mobility of more strongly bound metals.
Abbreviations: BAT, batch adsorption test • BTC, breakthrough curve • CIT, centrifuge infiltration test
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INTRODUCTION
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HEAVY METAL bioavailability in soil has received much attention because it is linked to risks concerning ground water quality and human food chain contamination. The behavior of heavy metals in soil is controlled by a number of chemical and physical processes, including adsorption onto colloidal surfaces, and transportation of metals by advection, diffusion, and dispersion. The extent of metal sorption depends on the nature of the metal, and the properties of secondary clay minerals present in the soil (McBride et al., 1997). Thus, it has long been suggested that ‘heavy’ soils, i.e., high clay content soils, tend to bind and immobilize heavy metals, so that many industries regard the disposal of their wastes in clayey soils a safe option (Sharma and Lewis, 1994). However, this simplification is not always true, since the transport of contaminants even in clayey soils can be significant and needs to be addressed and quantified.
Very often the sorption of heavy metals is measured in the laboratory using batch adsorption tests (BAT), and can be quantified by parameters obtained by simple adsorption isotherm curve fit models such as the Langmuir model described as follows:
 | [1] |
where S (mg kg–1) is the equilibrium concentration of the solute on the solid phase, C (mg L–1) is the equilibrium concentration of the solute in the liquid phase, k (L mg–1) represents the Langmuir bonding term related to interaction energies, and Q (mg kg–1) denotes the Langmuir monolayer capacity. From this model, it is possible to obtain the effective distribution coefficient, Kd (L kg–1), which describes equilibrium partitioning of a metal between solid and liquid phases, and thus can be used as an index of heavy metal mobility in the soil systems. The value of Kd is determined from the following equation:
 | [2] |
From this equation it can be deduced that the higher the Kd value of a metal, the higher the affinity of this metal for the solid phase, and, thus, the lower the metal mobility in solution. For a linear adsorption isotherm, Kd is constant and independent of the metal solution concentration. However, for a nonlinear isotherm, which can be fitted to the Langmuir equation, Kd is concentration-dependent, and increases with the decrease of metal concentration in the equilibrium solution, according to the equation:
 | [3] |
where k, Q, and C are the parameters described in Eq. [1]. This means that it is difficult to pick one Kd value that adequately represents the sorption, if the sorption isotherm is not linear over the range of interest of metal concentration.
Another problem concerning BAT is that they do not represent real field conditions in two main ways. First, they are conducted at relatively high and widely varying soil/solution ratios, which range from as low as 1:2.5 (Filius et al., 1998) and 1:25 (Tsadilas et al., 1997) to as high as 1:204 (Atanassova, 1999) and 1:500 (Bibak, 1997). This makes conditions different from field situations and results difficult to compare, because the retention capacities of soils are altered when soil/solution ratios vary (Harter and Naidu, 2001). Second, as shown by the same workers, BAT can only account for metal sorption, ignoring important physical processes that may also have significant contribution in metal mobility such as advection and diffusion, when metals are transported through soil. If decision-making relies solely on measurements conducted by BAT, heavy metal contamination may not be properly assessed.
Column infiltration tests have also been used for assessing metal Kd values and metal mobility. Infiltration tests do not have the above problems. They produce only one Kd value per test, without the need for very low added metal concentrations which create analytical difficulties in highly sorptive soils. Moreover, infiltration tests account for a variety of chemical and physical processes, which all contribute to the adsorption–desorption and transportation phenomena of heavy metals in soils (Voegelin et al., 2003). Although infiltration tests are more representative of natural conditions, they may be technically more challenging to run and time-consuming, especially for clayey soils. This technical difficulty can be overcome with the use of small laboratory centrifuges (Mitchell, 1998). Centrifuges apply stresses to the soil column, which permit the systematic examination of heavy metal movement in the soil under controlled and reproducible testing conditions, even in clayey soils, without departing significantly from the natural soil conditions. Centrifuge infiltration tests (CIT) are also relatively simple to run (McKinley et al., 1998).
Even though it is well recognized that multi-element contamination incidents are more often encountered in ecosystems than monometal, workers who employ infiltration tests often study the leaching and mobility of only one metal at a time, even in experiments where more than one heavy metal is added to the systems (e.g., Hinz and Selim, 1994; Camobreco et al., 1996). Moreover, multi-element BAT have indicated that metal sorption decreases when compared with similar monometal sorption tests (e.g., Harter, 1992). This evidence has not been examined for CIT. We are not aware of any published literature containing experimental data from infiltration tests in which the differences between metal behavior in monometal tests and in competitive multi-element tests are systematically addressed.
Currently, practical environmental policy considerations are often based on data obtained from BAT alone. A sound comparison of data obtained from both BAT and CIT can be beneficial in drawing better conclusions, but studies of this sort are also lacking in the literature. Moreover, there is a scarcity of published data concerning studies of multi-element infiltration through reactive clays, like those used as landfill clay barriers, because such tests are difficult to run due to the extremely low hydraulic conductivity of those soils. Therefore, the objectives of this study were (i) to examine the sorption behavior of Cu, Ni, and Zn in a clay using CIT, and compare this to sorption behavior assessed by BAT, using Kd values as indices of metal mobility and (ii) to investigate the competitive effect of multi-element sorption using CIT and compare this with the monometal sorption behavior. The heavy metal ions studied were Cu, Ni, and Zn, because they are among the most important in their group and represent a variety of behaviors in soils. While they are all toxic at high concentrations to living organisms, Cu is typically strongly bound onto soil solids, while Ni and Zn are usually more mobile in the soil environment (Alloway, 1995).
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MATERIALS AND METHODS
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London Clay samples recovered from tunnelling work beneath the River Thames were used for this study. London Clay is a low permeability stiff to very stiff, fissured clay, sedimented as a marine mud in the Eocene epoch (Dewhurst et al., 1998). Selected clay properties are summarized in Table 1. The blocks of clay were broken up, allowed to air-dry and passed through a 2-mm sieve.
Centrifuge Infiltration Tests
Centrifuge infiltration tests were conducted to investigate the sorption and transport of Cu, Ni, and Zn through the clay. The experiments studied the metals both without competition, and in pairs, so that the effect of competition could be assessed. In the single-element infiltration tests, a solution of each metal as its nitrate salt was infiltrated at a concentration of 500 mg L–1. This concentration was chosen to permit good delineation of the breakthrough curves without significant analytical difficulties. In the tests where competition was assessed, three different combinations of metal solutions were used: (i) 500 mg Cu L–1 and 500 mg Ni L–1, (ii) 500 mg Cu L–1 and 500 mg Zn L–1, and (iii) 500 mg Ni L–1 and 500 mg Zn L–1. Each test, both the single element and the competitive, was replicated to account for experimental variability. Clay samples weighing 2 g were placed into the centrifuge infiltration cell. The cell had an upper section with a porous base, which retained the clay, and a lower section in which the leachate was collected (Fig. 1). In use, the cell resembled a falling head permeameter. The centrifuge equipment permitted sixteen infiltration cells to be used at once. Dried clay was slurried with deionized water, left overnight so that it may be saturated, and then placed in the infiltration cell. Initially the cells were run until one pore volume of water was collected. The centrifuge was then stopped, the supernatant fluid replaced with leachant solutions, and infiltration tests were performed at a constant temperature of 21°C, at 2600 gravities. Periodically, the centrifuge was stopped and fresh leachant was added on the top of the cell, to replace fluid passing through the clay; the collected leachate was removed at the same time. This leachate was acidified with 5% HNO3 and then stored in a refrigerator (at 4°C), until the concentrations of heavy metals were determined by inductively coupled plasma–optical emission spectroscopy (ICP–OES). When the metal breakthrough was complete, desorption tests were also performed by infiltrating 0.01% HNO3 into the same clay columns. The leachates were then treated as explained above. The pH values of the leachates were monitored, but they were all close to pH 7, and are not reported.
The number of pore volumes required for the leachate concentration to rise to half of the leachant concentration is a measure of the contaminant mobility, approximately equal to the unitless retardation factor, Rd (Shackelford and Redmond, 1995). For the tests reported here, this concentration was 250 mg L–1. A second estimate for Rd was calculated by fitting the breakthrough curve (BTC) using the CXTFIT model (Toride et al., 1995). The model describes one-dimension solute transport according to the classical advection-dispersion equation. The two methods of measuring Rd showed a remarkable agreement, but in the Results and Discussion section the effective Rd derived from CXTFIT was preferred as it is believed to be more robust. The Kd was then calculated, to be able to compare results from CIT and BAT, according to the equation:
 | [4] |
where n (cm3 cm–3) is the soil porosity, and 
b (g cm–3) is the bulk density. Table 2 presents the CIT parameters, some of which were obtained by CXTFIT.
Batch Adsorption Tests
The BAT investigated the amount of Cu, Ni, and Zn sorbed onto the clay, using the approach described by Roy et al. (1992). Monometal adsorption isotherms were obtained by weighing 1 g of clay into acid-washed polyethylene bottles before addition of 200 mL of Cu, Ni, and Zn as solution in their nitrates. This soil/solution ratio of 1:200 was chosen to be similar to the amount of leachant that was infiltrated through London Clay in the CIT. The added metal concentrations were 0, 10, 25, 50, 200, 300, 400, and 500 mg L–1. All tests were done in duplicates. Samples were equilibrated for 24 h on a reciprocating shaker at constant room temperature (20 ± 2°C), and then centrifuged until the supernatant was clear. Copper, Ni, and Zn concentrations in the supernatant were then measured by atomic adsorption spectrometry. Total metal retained was calculated from the difference between that added and that measured in the equilibrium solution. The pH values of the equilibrium solution in all tests were monitored, but they did not show any significant differences to the initial soil pH. The adsorption isotherm fitted better in the Langmuir model, which is described in Eq. [1].
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RESULTS AND DISCUSSION
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Centrifuge Infiltration Tests
Single-Element Infiltration
The BTC (Fig. 2a for Cu, Fig. 3a for Ni, and Fig. 4a for Zn) were assymetrical, displaying a relatively slow breakthrough front and prolonged tailing. This indicates that the transport of all three metals was a nonequilibrium process, meaning that a flowing and a stagnant phase of water were both present in the infiltration columns. The ß value, the partitioning between the mobile and the immobile water phase, is shown in Table 2 for all the CIT. The Rd values obtained from the CXTFIT model were 204 for Cu, 131 for Ni, and 168 for Zn, indicating that the order of metal mobility recorded in this study was Ni > Zn > Ni. This is similar to other studies that measured metal mobility in soils (Gao et al., 1997; Gomes et al., 2001). In the desorption process Cu had a recovery rate of only 49%, i.e., the concentration of Cu in the leachant solutions, instead of decreasing back to 0 mg L–1, remained at 256 mg L–1, while Zn had a 70% recovery, and Ni had an almost complete recovery of 95%. This means that in highly sorptive soils some percentage of the added metals, especially those of lower mobility, can be irreversibly sorbed onto the solid phases (Gray et al., 1999).
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Fig. 2. Copper breakthrough curves, representing Cu concentrations eluted in the centrifuge infiltration tests: (a) breakthrough of Cu as a single component (without competition), (b) breakthrough of Cu when in competition with Ni, and (c) breakthrough of Cu when in competition with Zn. The arrows signify the end of the infiltration of the metal solution, and the beginning of the desorption process.
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Fig. 3. Nickel breakthrough, representing Ni concentrations eluted in the centrifuge infiltration tests: (a) breakthrough of Ni as a single component (without competition), (b) breakthrough of Ni when in competition with Cu, and (c) breakthrough of Ni when in competition with Zn. The arrows signify the end of the infiltration of the metal solution, and the beginning of the desorption process.
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Fig. 4. Zinc breakthrough, representing Zn concentrations eluted in the centrifuge infiltration tests: (a) breakthrough of Zn as a single component (without competition), (b) breakthrough of Zn when in competition with Cu, and (c) breakthrough of Zn when in competition with Ni. The arrows signify the end of the infiltration of the metal solution, and the beginning of the desorption process.
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Competitive Infiltration
The effect of competition was studied in three different combinations of metal pairs, i.e., Cu with Ni, Cu with Zn, and Ni with Zn. Competitive infiltration increased metal mobility in all cases. The Kd value for Cu obtained by the BTC, was 63.4 L kg–1 when alone, and decreased by 17% when in competition with Ni, and by 18% when in competition with Zn (Table 3). On the other hand, Ni Kd decreased by a factor of 3.4 when in competition with Cu, and Zn by a factor of 3.2. Nickel (Fig. 3c) and Zn (Fig. 4c) when infiltrated as a pair, decreased their Kd compared with their monometal state 2.6 times in the case of Ni and 2.3 times in the case of Zn. Similar rates of decrease of the Cu, Ni, and Zn Kd values due to competition are also evident in other works (Harter, 1992; Bibak, 1997; Echeverria et al., 1998), although there is a considerable variation in the initial monometal Kd values reported in these studies (Table 3). This shows that, although metal mobility increases when metals are in competition for the common adsorption sites of the soil, the least mobile elements (like Cu) increase their mobility much less than do the more mobile metals. Copper has higher affinity for solid phases than Ni and Zn, and this is probably associated with its higher electronegativity (1.9 vs. 1.8 for Ni and 1.6 for Zn). Thus, Cu resisted being exchanged into the solution in the presence of similar concentrations of Ni and Zn. The fact that competition affected Cu sorption less than Ni and Zn suggests that Cu was preferentially bound to specific adsorption sites which did not overlap with that of Ni and Zn. On the other hand, Ni and Zn did not seem to bind preferentially to different sites; thus, they were exchanged relatively easily. In batch systems, competition seems to affect metal adsorption only when high metal concentrations are introduced to the soil system (Saha et al., 2002). In column systems, where the steady-state condition is adsorption site saturation by the given metal, competition affects metal adsorption even if the added metal concentration is low.
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Table 3. Distribution coefficients, Kd, of Cu, Ni, and Zn as single elements, and in competition, as reported by various works.
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The effect of competition on metal mobility was also evident in metal desorption. Copper when in competition with Ni (Fig. 2b) and Zn (Fig. 2c), remained strongly bound onto the clay, and exhibited a recovery rate of only 36 (Cu with Zn) and 17% (Cu with Ni). This shows that Cu became more dominant, and was irreversibly sorbed onto the clay to a larger extent during the competitive desorption process. On the other hand, the BTC of Ni with Cu (Fig. 3b), and Zn with Cu (Fig. 4b), exhibited a leftward shift of their front compared with their single-element infiltration, which according to Kookana et al. (1994) indicates decreased metal affinity.
Batch Adsorption Tests
Metal distribution coefficients were also measured with BAT for comparison with the data obtained from metal BTC. The Langmuir curve fit model described best the isotherms of the BAT for all three metals (Fig. 5). The parameters k and Q of the Langmuir model, as well as the R2 values of the curve fitting, are presented in Table 4. From these parameters, the metal Kd values were calculated using Eq. [3]. These Kd values were the basis for the comparison of metal mobility between the CIT and BAT. The distribution coefficient, Kd, was dependent on the equilibrium metal concentration in solution, C, and this resulted in a considerable range of Kd values, which are presented in Table 5. The varying Kd values are a distinct disadvantage of predicting mobility from the BAT. For Cu, the Kd values ranged from 18.2 to 2910.3, Ni Kd values from 8.5 to 81.1, and Zn Kd values from 10.3 to 183.1 (units in L kg–1). Comparing these values with those back-calculated from the estimated retardation factor for the CIT, which gave Kd values of 63.4 for Cu, 40.7 for Ni, and 52.1 for Zn (units in L kg–1), it is evident that the CIT Kd values were at the lower end of the range of the BAT Kd values, except for Ni, which was only slightly lower than the middle of the BAT Kd range.
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Fig. 5. Sorption isotherms for Cu, Ni, and Zn fitted with the Langmuir curve fit model. On the y axis S represents the equilibrium concentration of metal sorbed onto the soil solids, and on the x axis C represents the equilibrium concentration of metal remaining in the soil solution. Open squares represent the results from one of the replicate adsorption tests, closed squares the results from the other.
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Table 4. Summary of the Langmuir parameters for the adsorption of Cu, Ni, and Zn onto London Clay, obtained by the batch adsorption tests.
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Table 5. Distribution coefficients, Kd, calculated for each added metal concentration in the batch adsorption experiment, according to the Langmuir curve fit parameters.
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The differences in predicted metal mobility are associated with the fact that experimental conditions for batch tests do not always account for all retention reactions that occur during metal transport. In batch systems desorption species are not removed and may thus induce secondary reactions, such as precipitation, with the studied metal ions, which decrease metal concentration in eqilibrium solution, causing an increase in metal Kd. On the other hand, column tests study metal sorption in an open system, and thus such interferences are not likely, since desorption species are continuously leached out of the system. Moreover, in batch tests all soil surfaces, even those in the soil micropores, which abound in clayey materials, are exposed to ion exchange reactions, and thus equilibrium is achievable within a few hours. In column systems nonequilibrium conditions are common, both chemical (easily and noneasily accessible sorption domains) and physical (stagnant and flowing water phases). These are affected by rate-limited diffusive mass transfer phenomena between the two regions. The two nonequilibrium types are practically indistinguishable when evaluating macroscopic metal transport with column infiltration tests (Seuntjens et al., 2001). In our column systems, all particles were not exposed for interaction with metal ions, due to such nonequilibrium conditions (indicated by the ß and 
values, Table 2). However, this situation better replicates metal movement under field conditions, especially through clays (Harter and Naidu, 2001). Thus, if decision making relies solely on measurements conducted with batch tests, heavy metal mobility may not be successfully assessed because the overestimation of Kd by batch techniques may lead to unnecessarily optimistic judgments concerning metal retention.
This difference of predicted metal mobility between BAT and CIT is in agreement with Celorie et al. (1989), who found similar trends in their work, in which they compared batch tests to infiltrations tests, and suggested that the higher the added metal concentration, the greater the overestimation of Kd, adding that BAT tend to overestimate metal Kd values, which in turn predict low metal mobilities. However, they reported CIT Kd values that were lower than the whole range of BAT Kd values. This, they concluded, can lead to unnecessary optimism concerning the risks of heavy metal contamination in soils. Although this Kd overestimation was evident in our work, it was not demonstrated as clearly as in the Celorie et al. (1989) study. There are two possible reasons for this. First, this may be the result of the use of relatively low soil/solution ratios in the BAT, which means that relatively small quantities of clay were used and high absolute quantities of heavy metals were added in the soil system. This probably led to the overestimation of metal mobility, which was expressed by the lower Kd values than normally expected. In their early work with a similarly clayey soil Griffin and Au (1977) dealt with the adsorption of Pb, a metal with a behavior in the soil environment similar to Cu. They found that the higher weights of clay adsorbed larger absolute amounts of the metal. Observing in various works the soil/solution ratios used, one can see that there is credibility in this ‘solids effect’. The Kd values found in our study are similar to those reported by Bibak (1997) who used a soil/solution ratio of 1:500 and reported a Kd of 37.9 L kg–1 for Cu and a Kd of 14.3 L kg–1 for Zn (Table 3). Conversely, Yuan and Lavkulich (1997) found Kd values of 2400 L kg–1 and 1200 L kg–1 for Cu and Zn, respectively, using a much higher soil/solution ratio of 1:40. Second, another explanation may be the fact that Celorie et al. (1989), although they claimed to have used a fine-grained soil, performed the leaching in a relatively nonreactive clay comprised mainly of kaolinite, with a cation exchange capacity (CEC) of only 2.0 cmolc kg–1, in which sorption would be low, so mobility increased during competition more significantly. It should be stressed that the two above-mentioned explanations are linked together. Low soil/solution ratios in this study were used in the BAT to represent the large amounts of solution that had to be infiltrated through the London Clay in the CIT. Large amounts of solution were used because the infiltration and leaching of metals were slow due to London Clay being a very reactive, high-affinity clay.
Moreover, BAT Kd values similar to CIT can only be obtained if low added metal concentrations are used, which is not always easy, especially in highly sorptive soils. On the other hand, for all given added metal concentrations, the order of mobility was Ni > Zn > Cu, which was the same as the sequence predicted from the CIT.
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CONCLUSIONS
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When carefully evaluated, CITs can provide important data on metal sorption affinity and mobility, since even difficult clayey soils can be successfully examined. In CITs, the solids concentration and the confining stresses are very similar to natural soil conditions. The ability to examine a soil closely under such conditions is critical in evaluating solute transport more effectively. Since the method is relatively rapid compared to infiltration tests where no extra gravitational force is imposed to the soil columns, and it accounts for a variety of important soil processes that may control metal mobility, the comparison between CITs and BATs can be useful in calibrating transport models and assessing the risk of metals leaching through the soils at contaminated areas. Comparison of the two methods is necessary because batch sorption isotherms are not always well described by the linear partitioning model and this leads to a range of plausible effective Kd values. Thus, it is unclear which Kd to choose to appropriately evaluate metal mobility. Comparison is also important, because nonequilibrium water flow characteristics, unaccounted for in batch tests, significantly increase metal mobility even in soils of high retention capacity, and these characteristics are successfully simulated with column techniques.
This study showed that BATs and CITs predicted the same order of metal mobility as a result of monometal sorption, which was Ni > Zn > Cu, although CIT Kd values were at the lower end of the range of the BAT Kd values. For competitive sorption CIT, the three metals exhibited higher mobility than in monometal CIT, but Cu was much less affected than Ni and Zn. Copper Kd decreased only by a factor of 1.2 when competing with either Ni or Zn, while Ni and Zn, when competing with Cu, decreased their Kd value by a factor of 3.4 (for Ni) and 3.2 (for Zn). This shows that competition has a much more profound effect on mobile elements and further enhances the contamination risks. Since the vast majority of the contamination cases are multi-element, risk assessment should carefully consider multicomponent metal infiltration studies.
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ACKNOWLEDGMENTS
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This work was sponsored by the Engineering and Physical Sciences Research Council, grant number GR/M27067. This support is gratefully acknowledged. We also wish to thank Dr. David L. Rowell from the Department of Soil Science, The University of Reading, UK, for his valuable comments on the manuscript.
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INTRODUCTION
