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Journal of Environmental Quality 31:813-821 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Heavy Metals in the Environment

Metal Immobilization in Soils Using Synthetic Zeolites

Leonard A. Oste{dagger}, Theo M. Lexmond and Willem H. Van Riemsdijk*

Department of Environmental Sciences, Wageningen Univ., P.O. Box 8005, 6700 EC Wageningen, the Netherlands

* Corresponding author (willem.vanriemsdijk{at}bodsch.benp.wau.nl)

Received for publication March 28, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In situ immobilization of heavy metals in contaminated soils is a technique to improve soil quality. Synthetic zeolites are potentially useful additives to bind heavy metals. This study selected the most effective zeolite in cadmium and zinc binding out of six synthetic zeolites (mordenite-type, faujasite-type, zeolite X, zeolite P, and two zeolites A) and one natural zeolite (clinoptilolite). Zeolite A appeared to have the highest binding capacity between pH 5 and 6.5 and was stable above pH 5.5. The second objective of this study was to investigate the effects of zeolite addition on the dissolved organic matter (DOM) concentration. Since zeolites increase soil pH and bind Ca, their application might lead to dispersion of organic matter. In a batch experiment, the DOM concentration increased by a factor of 5 when the pH increased from 6 to 8 as a result of zeolite A addition. A strong increase in DOM was also found in the leachate of soil columns, particularly in the beginning of the experiment. This resulted in higher metal leaching caused by metal–DOM complexes. In contrast, the free ionic concentration of Cd and Zn strongly decreased after the addition of zeolites, which might explain the reduction in metal uptake observed in plant growth experiments. Pretreatment of zeolites with acid (to prevent a pH increase) or Ca (to coagulate organic matter) suppressed the dispersion of organic matter, but also decreased the metal binding capacity of the zeolites due to competition of protons or Ca.

Abbreviations: CA, cyclonic ashes (formerly called beringite) • DOC, dissolved organic carbon • DOM, dissolved organic matter • ICP–OES, inductively coupled plasma with optical emission spectrometry


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
COMMON soil remediation technologies are suitable for relatively small and heavily contaminated sites, but are financially and physically inefficient in large, moderately contaminated areas. New techniques are being developed to treat these areas, such as immobilization and phytoremediation (Vangronsveld and Cunningham, 1998). In situ immobilization reduces negative effects of contaminants by adding an immobilizing chemical to the soil. The additives must obviously possess a high binding capacity, but they should not impair soil structure, soil fertility, or the ecosystem. Hence, evaluation of potential immobilizing agents should not only include the metal binding capacity, but also possible side effects.

Many additives have been screened for their potential to immobilize heavy metals in soils. Many of them are alkaline materials. Examples of alkaline additives include lime (Hooda and Alloway, 1996; Singh and Myhr, 1998), zeolites (Czupyrna et al., 1989; Gworek, 1992; Edwards et al., 1999), incinerator ashes (Shende et al., 1994; Vangronsveld et al., 1996), Fe rich (an iron oxide released during the industrial production of TiO2 pigment; Chlopecka and Adriano, 1997), gravel sludge (Lothenbach et al., 1998), hydroxyapatite (Boisson et al., 1999), and Thomas basic slag (Mench et al., 1994). In addition to binding sites on the surface of the immobilizing material, an increase in soil pH also contributes to the immobilization of heavy metals in soil by making existing sites in the soil (present at the surface of clay, iron oxides, organic matter, etc.) more reactive toward metal binding due to a decreased proton competition. An increase in pH and the addition of new sites do not always produce similar effects. Oste et al. (2001) showed that the addition of lime (pH increase) and manganese oxides (addition of new sites) could decrease the free Cd concentration in soil pore water to the same extent, but that the Cd uptake by earthworms was significantly more reduced by MnO2 compared with lime. Furthermore, an increase in pH may increase the dissolved organic matter concentration and microbial activity (Haynes and Naidu, 1998) and might change the ecosystem with respect to vegetation (Roem and Berendse, 2000) and soil organisms (Korthals et al., 1996; Edwards and Bohlen, 1996). Nevertheless, a pH increase is often effective in reducing metal leaching and plant uptake. Alkaline additives can thus improve the quality of many contaminated soils.

This study is mainly focused on the use of zeolites as immobilizing agents. Zeolites are crystalline alumino silicate minerals. They are generally formed in nature when water of high pH and high salt content interacts with volcanic ash causing a rapid crystal formation. Zeolites are also industrially synthesized. Generally, the starting point is crystallization from an inhomogeneous gel, created from a silica source and an alumina source combined with water under high pH conditions. Control of the SiO2 to Al2O3 ratio in this gel qualifies the final framework composition. Also, process parameters, such as exact pH, temperature, and mixing and drying speed, as well as salt impurities, influence the final product (Dyer, 1998). The framework consists of [SiO4]4- and [AlO4]5- tetrahedra linked at all corners. The building structure of these tetrahedra determines the type. If there is a prefix "zeolite" in the name, this material is synthetically produced. The letter (e.g., A, P, X, Y) refers to the structure. The framework is open and contains channels and cavities in which cations and water molecules are located. The channel structure of zeolites is responsible for their function as a molecular sieve, but is also important for "selective" cation exchange. The selectivity for different ions is determined by several factors (Dyer, 1988): the size and state of solvation of the ions, the charge (Si to Al ratio) and geometry of the framework, the number of cation sites available for occupation inside the framework, and the temperature.

Natural zeolites have been used for a long time in Japan to improve soil quality. Farmers add the zeolites to the soil to control soil pH and to improve ammonium retention (Dwyer and Dyer, 1984). Natural zeolites are also used in wastewater treatment to remove ammonium ions and heavy metals (Zamzow et al., 1990). Weber et al. (1984) investigated the effect of a natural zeolite (clinoptilolite) on heavy metal uptake by plants from an agricultural field. They did not observe a reduction in heavy-metal uptake in sorghum [Sorghum bicolor (L.) Moench] even at an addition rate of approximately 6.5%. Other studies concerning the addition of natural zeolites to soil also show little or no effect on the availability of metals (Mineyev et al., 1990; Chlopecka and Adriano, 1996, 1997; Baydina, 1996).

Synthetic zeolites have yielded better results. Czupyrna et al. (1989) selected a large number of additives and the zeolite type A appeared to immobilize heavy metals very strongly in batch and column experiments, certainly when the application was combined with addition of FeSO4. Singh et al. (2000) measured high adsorption of Cd to the synthetic zeolites A, P, and Y, compared with natural zeolite in a batch experiment that was carried out in 0.01 M NaClO4 at pH 6 to 7. Gworek (1992) added 1% synthetic zeolites A and X to a Cd-contaminated loamy soil (pHKCl = 7.8) and grew lettuce (Lactuca sativa L.) and ryegrass (Lolium perenne L.). The Cd uptake in both plants was significantly reduced in the treated soils. Zeolite X performed slightly better compared with zeolite A. Zinc uptake was also reduced after adding zeolite A to a loamy sandy soil (pHKCl = 6.4) (Gworek, 1994). The effect of zeolites on soil pH was not reported, but particularly in the Cd-contaminated soil (pH 7.8), it is unlikely that an increase in pH could have strongly contributed to the reduction in plant uptake. Other work on synthetic zeolites has been done by Lepp and co-workers (Rebedea and Lepp, 1994; Rebedea et al., 1997; Edwards et al., 1999). Rebedea and Lepp (1994) used zeolites A and P to evaluate metal uptake by plants under field and greenhouse conditions, and metal leaching in soil columns. The pot experiment showed that zeolites significantly reduced metal uptake, but the reduction was partly caused by a pH increase, a side effect of zeolite addition. Edwards et al. (1999) compared zeolites and lime to quantify the contribution of ion exchange and alkalization. Two soils were treated with three levels of zeolites A, P, and X and watered to field capacity. After 3 mo, soil solution samples were taken using a displacement technique. One liming level was applied, and a rough estimation indicates that the reduction in the soil solution independent of the alkalization effect is up to a factor of 2 for Cd and Zn.

Although the binding capacity of synthetic zeolites can be very high, an increase in soil pH without a simultaneous addition of Ca can raise the DOM concentration in the soil (Oste et al., 2002). An increase in DOM might increase metal leaching (e.g., McCarthy and Zachara, 1989; Temminghoff et al., 1997).

Therefore, this study has two main objectives. The first one is to evaluate the Cd and Zn binding capacity and stability of synthetic zeolites. The second objective is to assess the effect of zeolites on leaching of DOM and metals bound to DOM, and the possibilities to limit leaching, without losing the metal binding capacity of zeolites.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils
The soils used in this study originated from the Kempen, an area in the southern portion of the Netherlands contaminated with airborne Cd and Zn emitted by a number of Zn smelters in the region. The molar ratio of Cd to Zn in these soils was approximately 1:100 (Boekhold et al., 1992a; Wilkens and Loch, 1997). Soils could be classified as sandy, siliceous, mesic Typic Haplaquod, and characteristics are presented in Table 1. The percentage of organic carbon was determined after wet oxidation by sulfochromic acid (Walinga et al., 1992). The detailed procedure of citrate–dithionite extractable iron (Fed) was described by Buurman et al. (1996)(p. 32–35). Clay content was determined by sieve and pipet; unbuffered 0.01 M BaCl2 was used for the cation exchange capacity (CEC) and the pH was measured in 0.01 M CaCl2. "Total" metal concentrations were determined by aqua regia extraction: 2 g of soil and 16 mL of a concentrated HCl–HNO3 mixture, in a ratio of 3:1 by volume, were boiled under reflux for 2 h. All methods are described by Houba et al. (1995).


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  		Table 1. Characteristics of soils and additives.

 
Binding of Cadmium and Zinc to Zeolites in a Batch System
We tested one natural and six synthetic zeolites in a batch experiment. A mordenite-type (MOR), a faujasite-type (FAU), zeolite A [zeo A(PQ)], and zeolite X (zeo X) were obtained from The PQ Corporation (Valley Forge, PA). Zeolite P (zeo P) and another zeolite A [zeo A(Cf)] were produced by Crosfield Company (Eijsden, the Netherlands). All synthetic zeolites were supplied in the Na form, and had an average particle size diameter of 3 to 10 µm. Natural zeolite, clinoptilolite (CLIN), was provided by Minerais de la Mediterranée (Balaruc-les-Bains, France), and had a slightly coarser texture. The binding capacity was determined at three concentrations of Cd and Zn by shaking 0.2 g of zeolite in 180 mL of an artificial soil solution adjusted to pH 5 or 6.5 with 0.1 M HNO3 at 20°C. The pH was measured with a pH electrode (Radiometer, Copenhagen, Denmark). The artificial soil solution contained 0.5 mM K, 0.5 mM Na, 0.3 mM Mg, and 1 mM Ca all added as chloride salts, resulting in an ionic strength of 5 mM. Cadmium and Zn were added to this solution in a molar ratio of 1:100. Added Zn levels were 1.5, 5, and 10 µM, whereas Cd levels equaled 0.015, 0.05, and 0.1 µM. After a shaking period of approximately 2 wk, the suspensions were centrifuged at 1800 x g for 15 min. After acidification, the supernatant was analyzed for Ca, K, Mg, and Na by inductively coupled plasma with optical emission spectrometry (ICP–OES; Spectro Analytical Instruments, Kleve, Germany). Zinc was analyzed by flame atomic absorption spectrometery (FAAS) with Smith Hieftje background correction (Instrumental Laboratory-S11; Thermo Jarrell Ash, Breda, the Netherlands), and Cd by graphite furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction (SpectrAA 300-Zeeman; Varian, Sunnyvale, CA).

Zeolite A(PQ) was chosen to determine more detailed adsorption characteristics for Cd and Zn in a 1:100 ratio. Again 0.2 g of zeolite was added to 180 mL 0.005 M salt solution, containing 0, 0.156, 0.313, 0.625, 1.25, or 5 µM ZnCl2 and adjusted to 0.005 M with CaCl2. The pH was 5, 5.7, and 6.5. After shaking for 2 wk, centrifuging, and acidifying, the samples were analyzed for Zn by FAAS, and for Cd by GFAAS.

Stability of Zeolites as Influenced by pH
Zeolite P and zeolites A(Cf) and A(PQ) were used in this experiment. Five grams of zeolite were suspended in 50 mL of demineralized water and pH adjusted to lower values with 2.5 M HNO3. The pH ranged from 9 to 4. The experimental procedure was similar to the previous experiment, except that in this experiment Al, Ca, K, Mg, and Na were measured by ICP–OES.

Effects of Zeolites and Other Alkaline Materials on Dissolved Organic Carbon in Soil Suspensions
A batch experiment was designed to study the solid-solution partitioning of organic matter after zeolite A addition to a soil. Cyclonic ashes, formerly called beringite (provided by Dr. J. Vangronsveld, LUC, Diepenbeek, Belgium), were also included in this experiment, because they contain much Ca. Characteristics of this material are presented in Table 1. These two treatments were compared with addition of NaOH or Ca(OH)2. Three grams of soil K1 (Table 1) were suspended in a solution containing 0.03 M NaNO3 and alkaline material in a 40-mL centrifuge tube (not air-tight). NaOH was added as a solution (0.03 M) up to 3 mL; other materials were added as a suspension (strongly stirred). Maximum additions were 0.35% Ca(OH)2, 25% zeolite A(PQ), and 25% cyclonic ashes. The highest additions increased the pH to 9, which required fairly high additions of zeolite and cyclonic ashes. This is caused by the high salt level of these materials, suppressing an increase in pH, a phenomenon that does not take place in column experiments. The tubes were shaken for 96 h, and centrifuged. The supernatant was analyzed for DOC (SK12 TOC/DOC analyzer; Skalar, Breda, the Netherlands) instead of DOM. In the remainder, it is assumed that DOM consists of 50% DOC. Acidified samples were analyzed for Al, Ca, K, Na, and Zn by ICP–OES. To be sure that anoxic conditions did not occur during the experiment, we also analyzed for iron. No increase was measured in the soluble iron concentrations in the untreated soils, confirming that the conditions stayed aerobic.

Effects on Leaching of Dissolved Organic Carbon from Zeolite-Treated Soil
We conducted an experiment using four columns: untreated soil, and soils treated with 0.5% zeolite A(PQ), 2.5% zeolite A(PQ), and 0.3% lime [Ca(OH)2]. Soil K4 (Table 1) was mixed with additives and moistened to a water content of 0.18 L/kg. The soil was left at room temperature in the dark for 4 wk and then dried. The column experiments were carried out according to the Dutch protocol for leaching tests (Nederlands Normalisatie Instituut, 1995). The columns (5 cm internal diameter, 22 cm long) were completely filled with approximately 650 g of dry soil and leached with ultrapure water (acidified to pH 4 with HNO3) from the bottom to the top at a speed of 15 mL/h. Total liquid to solid ratio (v/w) over the whole experiment is 10, corresponding to approximately 30 pore volumes. A glass fiber pre-filter (Schleicher & Schuell, Dassel, Germany), to prevent blockage, and a 0.45-µm membrane filter (Schleicher & Schuell), to separate the solid and dissolved phase, were fixed at the top of the column. The protocol prescribes collection of the leachate in seven fractions, but we collected approximately 100 fractions with an increasing volume. All fractions were analyzed for pH, and for DOC, Al, Ca, and Na by ICP–OES. Zinc and Cd were measured by inductively coupled plasma mass spectrometry (Elan 6000; PerkinElmer, Wellesley, MA).

Effects on Leaching of Dissolved Organic Carbon from Soil Mixed with Pretreated Zeolites
The experimental setup is similar to the first column experiment. In addition to an untreated soil column, and a 0.1% CaOH2 treatment, there were three columns containing soil with 0.5% pretreated zeolite. The pretreatments comprised (i) addition of acid to adjust to soil pH, (ii) addition of Ca, and (iii) addition of acid and Ca. The acid treatment included the addition of concentrated HNO3 to a zeolite suspension up to pH 5.8. The Ca treatment comprised adding 1 mol Ca/kg zeolite and stirring the suspension overnight. Subsequently, the zeolite was washed three times with 0.001 M Ca(NO3)2. The combined Ca–H treatments received the same treatments as the Ca-treated zeolite, but also concentrated HNO3 was added in each step (also in the first step).

Modeling Organic Matter Partitioning
Oste et al. (2002) developed an approach in which DOC concentrations in batch experiments were related to the electrical Donnan potential of humic acid ({psi}). In this approach, the soil organic matter is considered a gel phase, exhibiting both specific binding and nonspecific ion exchange. Specific metal–DOM complexes were calculated using the Non Ideal Competitive Adsorption (NICA) model. Ion exchange was calculated using an electrostatic model that considers the organic matter as a gel with a Donnan potential. The average {psi} was determined by speciation calculations using ECOSAT, a computer code for speciation and transport in soil–water systems (Keizer and van Riemsdijk, 1999). We refer to the model as the NICA–Donnan model (Kinniburgh et al., 1999). Sodium, Ca, Zn, DOM, and pH as determined in the supernatant were used as input variables in the model. Generic NICA–Donnan parameters for humic acid were obtained from Milne (2000) and Milne et al. (2001). The relationship between DOC and {psi} obtained for the Kempen soil (K1) by Oste et al. (2002) is:

[1]

We used this equation to describe the DOC in soil suspensions treated with NaOH, Ca(OH)2, zeolite A, and cyclonic ashes. For each tube, we calculated the Donnan potential in the supernatant, and calculated the DOC concentration using Eq. [1].


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Selection of Zeolites
The batch experiments without soil were designed to select the most effective zeolites with respect to metal binding at moderate pH. Table 2 shows the percentage of added metal that is bound to the zeolites. Approximately 2 wk was required for the zeolite suspensions to attain required pH values. To prevent destruction of the zeolite framework caused by an overdose of acid, HNO3 was added in small increments. However, it was observed that if the required pH was reached on a certain day, the pH had increased again by the following day. It takes time to remove the alkalinity within the zeolite framework. This phenomenon was also observed by Singh et al. (2000). We continued adjusting the pH until the change over 24 h was less than 0.1 pH unit. Although we started with an artificial soil solution, the large cation exchange capacity of the zeolites and soluble salts in the material changed the initial solution completely, resulting in a strongly increased Na concentration. The adsorption values in Table 2 are averages of three Cd–Zn levels, indicating that the binding rates vary widely between different zeolites. The amount of metals bound to the natural zeolite (clinoptilolite) is almost negligible, but also the mordenite-type and faujasite-type do not show a high metal binding capacity. Zeolites A, P, and X displayed the higher adsorption values. Zeolite X shows a strong decrease in Zn binding at pH 5, whereas zeolite P shows a strong decrease in Cd binding at pH 5. Zeolite A(PQ) seems to have a slightly higher binding capacity than zeolite A(Cf), but the differences were not significant (t test, p > 0.05). Based on these results, the stability experiment was conducted with both zeolites A, because of their high binding capacity, and with zeolite X, because of its preference for Cd.


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  		Table 2. Percentages of applied Cd, Zn, and Ca bound by different zeolites at pH 5 and 6.5.

 
Stability
The stability experiment showed that the Al concentration increased at low pH values (Fig. 1) . The measured concentrations indicated a supersaturated solution, possibly because of alumino silicate colloids. The maximum dissolved Al concentrations in Fig. 1 accounted for only 5% of the total Al present in the system, but damage to the surface layer might strongly affect the binding behavior. Singh et al. (2000) found that Cd sorption to zeolite A increased from less than 10% at pH 4 to more than 95% at pH 5.8. They used X-ray diffraction (XRD) to characterize the zeolites. The XRD patterns confirmed the destruction of the mineral structure under acidic conditions. Although zeolite X appeared to be more stable, we considered zeolite A to be at adequate stability for further experimental work. Other factors that affected this decision included the slightly higher binding capacity of zeolite A(PQ), particularly for Zn (Table 2), and the cheaper price, which would favor its use in contaminated land remediation. A rough indication of the price of zeolite A is $0.80 per kg (A. Dube, The PQ Corporation, personal communication, 1998).



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  		Fig. 1. Aluminum released by adding HNO3 to a zeolite suspension.

 
Cadmium and Zinc Binding by Zeolite A at pH 5, 5.7, and 6.5
Zinc and Cd adsorption by zeolite A(PQ) was highly pH dependent (Fig. 2) , though the effects of pH on metal binding were most pronounced between pH 5.7 and 6.5, whereas little difference was observed between pH 5 and 5.7. It might be possible that the dissolution behavior of the zeolite between pH 5 and 6 (Fig. 1) is responsible for this difference. The analytical detection limit made it impossible to measure lower Cd concentrations (Fig. 2b).



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  		Fig. 2. Adsorption of Zn (a) and Cd (b) on zeolite A(PQ) at three pH levels.

 
We compared the Cd sorption characteristics of this zeolite with a Kempen soil using a pH-dependent Freundlich equation (Boekhold et al., 1992b). If the Cd loading on the solid phase was similar, the Cd concentration in solution was at least 100 times lower in a zeolite suspension compared with a soil suspension at pH 5, and 10-4 mol Cd/kg. The difference between soil and zeolite could rise to a factor of 2000 at pH 6.5, and 10-2 mol Cd/kg, clearly demonstrating the potential effects of zeolites as a remediation material.

Effects of Zeolites on Disolved Organic Matter in Soils
Figure 3a presents the DOC concentrations as influenced by an increase in pH as realized by different materials. Addition of NaOH increases the DOC concentration by a factor of 5 when the pH shifts from 6 to 8, whereas no increase in DOC was observed if the pH was increased with Ca(OH)2. Figure 3a also shows that zeolite A acts like NaOH and that the Ca-rich cyclonic ashes (CA) behave similarly to lime. The various treatments affect the Ca concentration in solution differently (Fig. 3b). The addition of CA leads to a rise in Ca concentration, whereas addition of zeolites or NaOH leads to a decrease in soluble Ca. The effects on the soluble Zn concentration are shown in Fig. 3c. Initially, Zn decreases in all treatments (Fig. 3c), but only the CA treatment shows a continuous decrease to almost zero. Addition of Ca(OH)2 does not decrease the Zn concentration to less than 5 mM. The differences between CA and Ca(OH)2 can partly be attributed to the decrease in organic matter, but probably also to the metal binding capacity of the CA at high addition rates (10, 20, and 25% in case of the three highest data points). Treatment with NaOH or zeolites induces an increased Zn concentration in solution above pH 6.5 and 7 respectively. The increase in DOM can explain the increased Zn concentrations at higher pH values.



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  		Fig. 3. Dissolved organic carbon (DOC) (a), Ca (b), and Zn (c) measured in solution in suspensions containing soil K1 and different additives. {diamond}, NaOH; {square}, Ca(OH)2; {circ}, cyclonic ashes; {triangleup}, zeolite A.

 
Each measured DOC value, as shown in Fig. 3a, can also be calculated with Eq. [1]. The lines in Fig. 4 represent the calculated data points that are connected to each other. The predictions are in perfect agreement with the data in the case of the CA and lime treatments, whereas the calculations of the NaOH treatment are reasonable. The line in Fig. 4 representing the zeolite treatment shows a higher calculated DOC concentration between pH 6 and 8 than the NaOH line. This is not surprising, since zeolites decrease the Ca concentration, leading to a more negative Donnan potential, and thus a higher DOC concentration (Eq. [1]). In contrast to the calculations, the measured data show a slightly lower DOC concentration in the zeolite treatment compared with the NaOH treatment. Apparently, the zeolite addition produces other effects on the DOC than changes in the pH and the concentration of cations in solution that are accounted for in our calculations. However, general trends also are well predicted for the zeolite treatments.



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  		Fig. 4. Measured (symbols) and calculated (lines) dissolved organic carbon (DOC) concentrations in suspensions containing soil K1 and different additives. {diamond}, NaOH; {square}, Ca(OH)2; {circ}, cyclonic ashes; {triangleup}, zeolite A.

 
Leaching from Zeolite-Treated Soil Columns
Figure 5a presents the pH in the leachate of the first column experiment. The zeolite-treated columns show an elevated pH during the whole experiment compared with the untreated soil. The lime-treated column started above pH 8, but at the end the pH had decreased to 7.6. Figures 5b to 5f show cumulative amounts of leached elements, because the differences in concentration between the first few pore volumes and those at the end of the experiment are large and difficult to show in one graph.



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  		Fig. 5. (a) pH measured in the leachate of the first column experiment; (b–f) cumulative amounts of dissolved organic carbon (DOC), Na, Ca, Zn, and Cd leached from the columns. {diamond}, no additives; {square}, 0.3% Ca(OH)2; {triangleup}, 0.5% zeolite A; {circ}, 2.5% zeolite A.

 
The results of the batch experiments regarding DOC (Fig. 3a) are confirmed by the results of our first column experiment. Figure 5b shows that addition of zeolite A (Na form) increased DOC leaching. The DOC concentrations in the first tube were up to 14 g/L compared with 0.4 g/L in the control soil. After 3 pore volumes, the amount of DOC leached from the 2.5%-zeolite-treated column was still a factor of 6 higher than from the untreated column (Fig. 5b). In the later stages of the experiment, the differences in DOC concentrations between the 2.5%-zeolite treatment and the control were less than a factor of 2. Apparently, a change in ionic composition of the solution generated an immediate increase of DOC, and subsequently the situation became more or less stable again. The lime treatment did not show increased DOC leaching, although the pH was 8 in the first few pore volumes, presumably because Ca in the lime prevented the dispersion of organic matter (Fig. 5d).

Increased Zn and Cd leaching was observed in the soil with 2.5% zeolite A as a result of DOC leaching (Fig. 5e,f). This was particularly the case in the first part of the experiment. In the second part, the cumulative amounts of Cd and Zn leached from the untreated column show a slightly steeper slope, meaning a higher concentration in the leachate in the untreated soil than in the treated soil. Two possible reasons can be forwarded for reduced metal leaching from the zeolite-treated columns at the end of the experiment. First, high leaching in the beginning removed approximately 1.25% of the total Cd and 1% of the total Zn. This is the most mobile fraction and it might be possible that the remaining fraction is more difficult to extract. Sequential extraction data available in the literature have shown that this can be expected for heavy metals. Another explanation may be the effect of the binding capacity of the zeolite. The zeolite addition decreases the free metal concentration, due to its binding capacity and alkalinity. When the DOC concentration is back to more normal levels, it will also lead to decreased Zn leaching in the later stage of the experiment.

Leaching of Ca (Fig. 5d) was always less in the 0.5%-zeolite treatment compared with the control, although DOC leaching was much higher. At 2.5% zeolite, Ca leaching in the beginning was higher than in the control, due to the very high DOC levels. In contrast, the total amount leached at the end of the experiment was well below that from the control, due to the Ca binding capacity of the zeolites. The high Ca binding capacity was not surprising, because these zeolites had been developed to soften laundry washwater (The PQ Corporation, 1992).

Total leaching of Cd and Zn from the 2.5%-zeolite-treated soil was much greater than the metals leached from the control, indicating that these metals tended to be associated with DOC, and that the soluble DOC had a greater binding power than the zeolite. The effect of the metal binding can be explained if the free metal ion concentration in the leachates is calculated, assuming that the DOC behaves similarly to humic acid. The results (Fig. 6) show that the concentrations of free Cd, Zn, and Ca in the zeolite-treated soils are much lower than in the control over the whole range of the experiment. The low free metal concentrations may explain the reduced uptake by plants observed in other studies (Rebedea et al., 1997; Gworek, 1992, 1994).



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  		Fig. 6. Free ionic concentrations of (a) Ca, (b) Zn, and (c) Cd based upon speciation calculations in the column leachates. {diamond}, no additives; {square}, 0.3% Ca(OH)2; {triangleup}, 0.5% zeolite A; {circ}, 2.5% zeolite A.

 
Leaching from Soil Mixed with Pretreated Zeolites
The dispersion of organic matter can be prevented if the pH increase is reduced or if the Ca level is high enough (as in case of lime and cyclonic ashes). To realize these conditions, we pretreated zeolite A with acid and/or Ca. The results of the column experiment with pretreated zeolites are presented in Fig. 7 . The pH differences between the various treatments (Fig. 7a) are less than observed in Fig. 5a. The effluent pH for the columns containing H-treated zeolites is almost equal to the pH in the untreated column, but Ca and Ca–H treated zeolites still increase the pH by approximately 0.5 units. The amount of DOC leached from the columns is close to the control soil (Fig. 7b). In the first experiment, 0.3% lime did not affect DOC leaching, but here 0.1% lime slightly decreased DOC leaching. The behavior of the soil mixed with H-treated zeolite A is unexpected. The pH of this column deviates from the Ca- and Ca–H-treated zeolites, but also leaching of DOC, Na, and Ca is different (Fig. 7b–d). This must be attributed to the pretreatment procedure. The number of acid additions and the amounts per addition probably affected the behavior of the pretreated zeolite. Figures 7e and 7f show that metal leaching from the control soil is higher after approximately 10 pore volumes (steeper slope), suggesting that treatment of a soil with zeolite will reduce leaching on a longer term. However, the liming treatment was the most successful treatment in reducing Zn and also performed well in case of Cd. The competition with Ca and protons that was present in the pretreated zeolites made the zeolites less effective with respect to metal binding. This becomes even more clear if the free ionic concentrations are calculated (Fig. 8) . The overall differences between treated and untreated soils are much smaller in Fig. 8 compared with Fig. 6.



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  		Fig. 7. (a) pH measured in the leachate of the second column experiment; (b–f) cumulative amounts of dissolved organic carbon (DOC), Na, Ca, Zn, and Cd leached from the columns. {diamond}, no additives; {square}, 0.1% Ca(OH)2; +, 0.5% Ca-treated zeolite A; x, 0.5% H-treated zeolite A; –, 0.5% Ca–H-treated zeolite A.

 


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  		Fig. 8. Free ionic concentrations of (a) Ca, (b) Zn, and (c) Cd based upon speciation calculations in the column leachates. {diamond}, no additives; {square}, 0.1% Ca(OH)2; +, 0.5% Ca-treated zeolite A; x, 0.5% H-treated zeolite A; –, 0.5% Ca–H-treated zeolite A.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Synthetic zeolite A has a high capacity to bind heavy metals, and the alkaline character of the material also increases metal sorption to the soil. Sodium-saturated zeolite A can bind high amounts of Ca, decreasing the competition of Ca in the soil. This study did not reveal whether the effect of a decreased proton and Ca concentration will be sustained over a long period. If the pH decreases again, the binding to the zeolites will decrease as well, because metal binding appeared to be very pH dependent between pH 5 and 6.5.

Moreover, the synthetic zeolites used in this study induced dispersion of soil organic matter, leading to higher metal leaching from soil columns. In field or laboratory experiments without leachate collection, organic matter leaching will not be noticed. Although total metal leaching increases, zeolites can decrease the free metal concentration in the soil solution, leading to a decreased metal uptake by plants.

Pretreatment of zeolites with H or Ca can prevent high DOM leaching. However, this limits the binding of Cd and Zn. If reduced leaching to deeper soil layers is required, particularly in poorly buffered soils, zeolites are not suitable. However, a small amount of zeolites may work well in soils with a high acid buffering capacity and a pH > 6, particularly to reduce metal uptake by plants.

In general, alkaline additives have to be evaluated whether they contain enough Ca to prevent increased DOM leaching from soils as a result of their alkalinity. A high amount of Ca, as in the cyclonic ashes, reduces DOM leaching, resulting in decreased metal leaching.


   ACKNOWLEDGMENTS
 
The authors would like to thank Arie van de Berg, Monique Driessen, Gerdine Gaikhorst, and Egbert Nab for technical assistance, and Simon Maasland for preparing the columns. They also acknowledge Gerlinde Roskam and Roger Saanen for conducting part of the experiments.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
{dagger} Present address: Institute for Inland Water Management and Waste Water Treatment, P.O. Box 17, 8200 AA Lelystad, the Netherlands. Back


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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