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TECHNICAL REPORTS
Heavy Metals in the Environment

Apatite and Phillipsite as Sequestering Agents for Metals and Radionuclides

^a Westinghouse Savannah River Company, Aiken, SC 29808
^b Savannah River Ecology Laboratory, Univ. of Georgia, Drawer E, Aiken, SC 29802

^* Corresponding author (formerly A. Chlopecka) (anna.knox{at}srs.gov)

Received for publication February 19, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Laboratory and greenhouse studies were conducted to quantify apatite and phillipsite (zeolite) sequestration of selected metal contaminants. The laboratory batch study measured the sorption of aqueous Co²⁺, Ba²⁺, Pb²⁺, Eu³⁺, and UO₂²⁺. Apatite sorbed more Co²⁺, Pb²⁺, Eu³⁺, and UO₂²⁺ from the spike solution than phillipsite, resulting in distribution coefficients (K_d values) of >200 000 L kg^-1. Phillipsite was more effective than apatite at sorbing aqueous Ba²⁺. Results from the laboratory study were used to design the greenhouse study that used a soil affected by a Zn–Pb smelter from Pribram, Czech Republic. Two application rates (25 and 50 g kg^-1) of phillipsite and apatite and two plant species, maize (Zea mays L.) and oat (Avena sativa L.), were evaluated in this study. There was little (maize) to no (oat) plant growth in the unamended contaminated soil. Apatite and, to a slightly lesser extent, phillipsite additions significantly enhanced plant growth and reduced Cd, Pb, and Zn concentrations in all analyzed tissues (grain, leaves, and roots). The sequestering agents also affected some essential elements (Ca, Fe, and Mg). Phillipsite reduced Fe and apatite reduced P and Fe concentrations in oat tissues; however, the level of these elements in oat leaves and grains remained sufficient. Sequential extractions of the soil indicated that the Cd, Pb, and Zn were much more strongly sorbed onto the amended soil, making the contaminants less phytoavailable.

Abbreviations: MANOVA, multivariate analysis of variance • TF, transfer factor

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
IN SITU IMMOBILIZATION of metals using inexpensive sequestering agents, such as minerals (e.g., apatite, zeolite, or clay minerals) or waste by-products (e.g., steel shots, Fe-rich biosolids), is an attractive alternative to many current remediation methods. The use of natural minerals in remediation techniques is based on well-recognized geochemical principles. The most common among these remediation minerals are apatite and zeolite. Apatite minerals form naturally and are stable across a wide range of geologic conditions (Wright et al., 1995). Apatite has been used primarily in association with remediating Pb-contaminated soils (Ma et al., 1994a,b, 1995; Xu and Schwartz, 1994; Xu et al., 1994). However, current research demonstrates the successful immobilization not only of Pb, but also of other metals and some radionuclides when phosphate minerals are added to a contaminated medium (Arey et al., 1999, Seaman et al., 2001; Suzuki et al., 1981, 1982).

Zeolites are crystalline, hydrated aluminosilicates of alkali and alkaline earth cations that possess infinite, three-dimensional crystal structures. The use of zeolites for pollution control depends primarily on their ion-exchange and retention properties. Clinoptilolite, chabazite, and phillipsite are natural zeolites that have been evaluated as sequestering agents for environmental cleanup (Ming and Mumpton, 1995; Tsitsishvili et al., 1992).

The goal of in situ immobilization is to reduce the risk associated with potentially bioavailable toxic metals. However, an important limitation of this approach is that unforeseen chemical or biological transformations may occur over time to reconvert the contaminant to its bioavailable form.

The efficacy of in situ immobilization techniques is usually evaluated with one of the following approaches: (i) batch studies, (ii) dynamic leaching studies, and (iii) plant growth studies. Batch studies evaluate contaminant solubility and migration potential under controlled steady state conditions. Many of these studies identify and characterize mineral phases or specific sorption processes with spectroscopic techniques (Chen et al., 1997; Ma et al., 1993; Ruby et al., 1994; Ryan et al., 2001; Xu et al., 1994; Zhang et al., 1997; Zhang and Ryan, 1998, 1999a,b). Dynamic leaching studies provide a means to evaluate kinetically limiting conditions (Seaman et al., 1999). Plant growth experiments provide a means for estimating bioavailability and long-term stability (Chlopecka and Adriano, 1996, 1997a,b; Laperche et al., 1997). These three types of studies reflect a general progression from well-defined systems to more complex systems, typical of the real world. Often, sequential extraction techniques, whereby a sequential series of increasingly more harsh extracts are used to operationally define how strongly a contaminant is sorbed to a soil (Tessier et al., 1979), are combined with these approaches. Together, these studies provide information about the contaminants' bioavailability, potential mobility, chemical liability, and sorption process (Arey et al., 1999; Chlopecka and Adriano, 1996, 1997a,b; Ma and Rao, 1997). The batch extraction methods provide means of rapidly screening numerous alternative treatment scenarios, especially when evaluating contaminant mobility, but the limitations of such methods necessitate the use of plant growth experiments and bioassays to assess biological availability (Hinton et al., 1998).

The objective of this study was to determine the influence of apatite and phillipsite additions on contaminant sequestration and plant growth. In a laboratory study, the tendency of these two sequestering agents to sorb Ba²⁺, Co²⁺, Eu³⁺, Pb²⁺, and UO²⁺₂ from an aqueous spike solution was measured. Results from the laboratory study were used to design a greenhouse study that evaluated the effect of apatite and phillipsite on Cd, Pb, and Zn bioavailability from a soil affected by a Zn–Pb smelter. Additionally, the effect of these two sequestering agents on the uptake of essential elements such as P, Ca, Mg, and Fe by oat was also determined.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Laboratory Batch Study
The effect of apatite from North Carolina and natural zeolite (phillipsite) on immobilization of Ba²⁺, Co²⁺, Eu³⁺, Pb²⁺, and UO²⁺₂ was examined in this study. Phillipsite was obtained from Steelhead Specialty Minerals (Spokane, WA) and had a pH of 9.05 (1:1 solid to H₂O) and a calcium carbonate equivalence (CCE) of 49 g kg^-1 (Table 1). X-ray diffraction analysis of the material indicated that it was highly crystalline and displayed strong peaks (8.191, 7.068, 5.366, 4.991, 4.097, 3.174, 2.949, 2.680, 1.961, 1.771, and 1.674 Angstroms [Å]) characteristic for phillipsite (Fig. 1) . The peaks at 3.262 and 2.387 Å were probably from quartz. The apatite was from North Carolina and had a pH of 7.8 (1:1 solid to H₂O) and CCE of 218 g kg^-1. In terms of specific mineral species, many of the peaks were close to hydroxyapatite (3.824, 3.427, 3.160, 3.063, 2.616, and 1.785 Å) and a few matched it exactly (2.782, 2.233, 1.874, 1.754, and 1.719 Å).

View larger version (30K): [in this window] [in a new window]   Fig. 1. X-ray diffraction pattern for apatite and phillipsite. CPS, counts per second; Deg., °2. Values adjacent to peaks are measured in Angstroms (Å).

The metal concentration data were used to calculate distribution coefficients (K_d values), defined as the ratio of the concentration of solute sorbed to the solid divided by its concentration in solution. The K_d (L kg^-1) was calculated with Eq. [1] (American Society for Testing and Materials, 1993):

[1]

where C_spike is the element concentration in the spike solution before the addition of the sequestering agent (µg L^-1), C_final is the element concentration in the solution after contact with the sequestering agent (µg L^-1), M_mineral is the sequestering agent mass (g), and V_spike is the volume of the spike solution (mL).

Greenhouse Bioavailability Study
Experimental Design
The soil used in this experiment was collected from the surface 10 cm near a Zn–Pb smelter located in Pribram, Czech Republic, 50 km southwest of Prague. The soil possessed the following properties: pH of 5.4 (1:1, soil to H₂O), 20 g kg^-1 organic matter (OM), and 4.8 cmol kg^-1 cation exchange capacity (CEC). The soil had the following metal concentrations (in mg kg^-1): 20 Cd, 1936 Pb, 2488 Zn, 34000 Fe, and 3800 Mn (Table 1).

The same phillipsite and apatite used in the laboratory study were mixed separately with the contaminated soil at two rates: 25 and 50 g kg^-1. Apatite rates were selected based on the molar ratio of phosphate released from apatite and Pb soil concentrations. For the purpose of comparison, phillipsite application rates were set equal to the apatite rates. There were four replicates for each treatment, and the 1-kg pots were arranged in a completely randomized design. After four weeks of soil equilibration, maize was planted and then harvested after six weeks of growth. After maize, oat was planted and grown to maturity. While culturing, the plants were fertilized with dissolved N–P–K (1:0.7:0.7) (applied as KNO₃ and NH₄H₂PO₄). The plants were watered with deionized water as needed. At the end of the growing period, the maize was separated into leaves and roots and the oat plants were separated into leaves and grain for subsequent chemical analyses.

Analytical Methods
The soil samples from all treatments were taken twice: before planting maize and after harvesting oat. Soil samples were dried at ambient temperature (about 21°C) and ground in a porcelain mortar to pass through a 2-mm sieve. The air-dried soil samples were then stored in paper bags for subsequent analysis. The <2-mm fraction was used for the determination of pH, organic matter, cation exchange capacity, and particle size distribution with standard methods. At the end of the experiment, a 1.5-g aliquot of a homogenized soil sample was used to conduct sequential extractions (Tessier, et al., 1979). The following extractions were conducted:

• Fraction 1: Exchangeable. Sample extracted with 12 mL of 0.5 M MgCl₂.
  
• Fraction 2: Bound to carbonate(s). Leached with 12 mL of 1 M NaOAc.
  
• Fraction 3: Bound to Fe–Mn oxides and hydroxides. Residue from the second extraction was extracted with 30 mL of 0.04 M NH₂OH·HCl in 25% (v/v) HOAc and the extracted solutions were diluted to 30 mL with deionized water.
  
• Fraction 4: Bound to organic matter and sulfide. The residue from the third extraction was treated with 4.5 mL of 0.02 M HNO₃, 12 mL of 30% H₂O₂, and 7.5 mL of 3.2 M NH₄OAc in 20% (v/v) HNO₃ and diluted to final volume of 30 mL with deionized water.
  
• Fraction 5: Residual phase. The samples were digested with aqua regia and HF.
  

The total metal content in the soil was determined independently by digesting 0.5 g of soil in a mixture of 1.5 mL of aqua regia and 5 mL of concentrated HF (40% v/v).

The X-ray diffraction (XRD) analyses of apatite and phillipsite were conducted with an X-ray diffractometer (X2 advanced diffraction system; Thermo ARL, Ecublens, Switzerland). Measurements were made with a step-scanning technique with a fixed time of 4 s/0.05°2. Data points were obtained from 5 to 90°2. The XRD analyses were performed with fine-ground samples.

Plant samples were washed thoroughly with deionized water, placed in paper bags, and dried to constant weight at 60°C. The dried tissues were finely ground and 0.5-g samples were digested with 10 mL of concentrated HNO₃ in a 125-mL Erlenmeyer flask with a reflux tube. Samples were boiled gently on a hot plate at 100 to 110°C until the organic matter was destroyed, and then 1 mL of H₂O₂ was added. Cooled samples were passed through #41 filter paper (Whatman, Maidstone, UK) and diluted to 50 mL with deionized water.

Soil and plant extracts were analyzed by atomic absorption spectroscopy (AAS) or by inductively coupled plasma–atomic emission spectroscopy (ICP–AES). The analytical accuracy of AAS and ICP–AES was ascertained by including control samples in each set of samples. A standard reference soil or plant material (San Joaquin Soil, SRM 2709 and Apple Leaves, SRM 1515; National Institute of Standards and Technology, Gaithersburg, MD) of known composition was also analyzed to check the accuracy of the method. In addition, duplicate samples from approximately 10% of the sample pool were run to check analytical precision. For quality control, matched matrix standards were used in all instances with blanks and spiked samples. Comparisons between standards and metal concentrations in the extracts agreed to within 5%. Mean fraction (four replicates) recoveries from the sequential extraction were satisfactory, as differences between total amount of metals in the soil and the sum of the metal fractions were <10%.

Statistical Analysis
The data were analyzed by Tukey's multiple range test (t tests) to determine whether the treatment means were significantly different. Additionally, since these data involved measuring several variables on the same experimental unit, multivariate analysis of variance, or MANOVA, was employed. Multivariate analysis of variance offers two major benefits over a series of univariate ANOVAs or multiple range tests (Johnson and Wichern, 1992). First, MANOVA looks at all response variables simultaneously and therefore takes into account the actual experimental design. Second, MANOVA avoids performing univariate multiple comparisons on the multivariate data separately, which could lead to false significance for similar reasons as performing independent t tests rather than multiple comparison tests in the univariate case. Performing a MANOVA in conjunction with the multiple range test increases confidence in the results by allowing an examination of possible interactions and correlations, by considering the full experimental design in the analysis, and by avoiding false significance due to performing several trials. However, if the MANOVA results are consistent with the multiple range tests, then we can be more confident that the multivariate nature of the data holds no surprises. Multivariate analysis of variance tests the null hypothesis of no difference between all treatments for all variables. Which means differ is determined with the matrix of mahalanobis distances, which gives the distance between the group means, and by examination of the univariate test results. The tests for treatment differences were performed in SAS (SAS Institute, 2000) and the matrix of mahalanobis distances was estimated in MATLAB (The MathWorks, 2001). The MANOVA results were consistent with Tukey's test results, as well as the univariate test results performed in conjunction with MANOVA.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Laboratory Batch Study
The concentration of each element (Co²⁺, Ba²⁺, Eu³⁺, Pb²⁺, and UO²⁺₂) in the spike solution was approximately 50 µg L^-1. After one week, apatite significantly reduced aqueous concentrations of each metal (Table 2). Apatite reduced aqueous Co, Eu, and U concentrations to a greater extent than phillipsite. It is likely that apatite lowered Pb and U concentrations by forming sparingly soluble phosphate precipitates (Seaman et al., 2001; Singh et al., 2001; Wright et al., 1995). Apatite distribution coefficients (K_d) for Eu, Pb, and U were >200 000 L kg^-1 (Table 2). A ranking of metals by their apatite K_d values is as follows: Eu, Pb, U > Co > Ba. The pH did not remain constant between the controls and the two mineral-addition treatments. The pH of the metal spike solution was 3.84 ± 0.01, apatite suspension was 4.89 ± 0.05, and the phillipsite suspension was 4.26 ± 0.05 (Table 2). The concomitant increase in pH with mineral addition is probably the result of the alkaline nature of apatite, as indicated by its relatively high calcium carbonate equivalents (Table 1).

View this table: [in this window] [in a new window]   Table 2. Metal sorption by apatite and phillipsite.

Greenhouse Bioavailability Study
The relative extractability of Cd, Pb, and Zn in the greenhouse study soil was determined by sequential chemical extractions. In the unamended soil (control), a significant portion of the Cd (23%) was found in the exchangeable form (0.5 M MgCl₂ extractable), implying that a large fraction of the metal was weakly bound to the soil (Fig. 2) . Importantly, 31% of the Cd was associated with the residual fraction (aqua regia and HF extractable), the fraction least likely to be bioavailable. Lead and Zn in the unamended soil had similar distributions: between 40 and 50% existed in the Fe- and Mn-oxide-bound fraction (NH₂OH·HCl extractable) and between 25 and 30% existed in the residual fraction (Fig. 2). However, in the exchangeable fraction, 11% of Zn and only 2% of Pb were found (Fig. 2).

View larger version (45K): [in this window] [in a new window]   Fig. 2. Sequential chemical extraction of Cd, Pb, and Zn from contaminated soil (control) and from contaminated soil amended with apatite and phillipsite at two levels of application (25 and 50 g kg-1). The percentage of each fraction was calculated based on the total element concentration in the soil.

View larger version (35K): [in this window] [in a new window]   Fig. 3. Effect of apatite or phillipsite on redistribution of Cd, Pb, and Zn fractions in contaminated soil at two levels of application (25 and 50 g kg-1).

The growth of both tested plants (maize and oat) was affected by high concentrations of metals in the contaminated soil. In the untreated soil, plant yield was very low. The yield in untreated soil of six-week-old maize was 0.94 g pot^-1 and of mature oat was 1.7 g pot^-1 (Fig. 4) . Plants in this treatment did not appear healthy (i.e., the plants had yellow leaves and poorly developed shoots). Apatite and phillipsite soil additions significantly enhanced plant yields (Fig. 4). No significant increase in biomass yield was observed between the 25 and the 50 g kg^-1 treatments of the apatite or phillipsite treatments. Additionally, grain yield following these treatments was obtained at 6.3 and 5.9 g pot^-1, respectively, for the second dose of apatite and phillipsite (Fig. 4).

View larger version (40K): [in this window] [in a new window]   Fig. 4. Yield of six-week-old maize and oat plants grown on contaminated soil (Ctrl) and on contaminated soil amended with two levels (25 and 50 g kg-1) of apatite (AP) or phillipsite (PZ); no oat grains were produced in the control.

View larger version (34K): [in this window] [in a new window]   Fig. 5. Cadmium, Pb, and Zn concentrations (mg kg-1) in maize grown on contaminated soil (Ctrl) and on contaminated soil amended with two levels (25 and 50 g kg-1) of apatite (AP) or phillipsite (PZ).

View larger version (20K): [in this window] [in a new window]   Fig. 6. Cadmium, Pb, and Zn concentration (mg kg-1) in oat plants grown on contaminated soil (Ctrl) and on contaminated soil amended with two levels (25 and 50 g kg-1) of apatite (AP) or phillipsite (PZ); no oat grains were produced in the control.

[2]

A low TF value indicates low metal bioavailability to plants or low metal translocation within the plant. In Fig. 7 , TF values are presented for the roots and leaves of six-week-old maize and for the leaves and grains of mature oat. The highest TF was observed for maize roots and the lowest TF values were in the oat grain (Fig. 7). The trends in the data (Fig. 7) suggest that although metals accumulated mostly in the roots, substantial Cd and Zn was translocated to the leaves and grains, especially in the untreated soil. For example, the Zn TF values in maize leaves in untreated soil were the highest (1.72). Both amendments significantly reduced TF values (P < 0.05) for each metal and the highest reduction occurred in maize roots and leaves and in oat leaves (Fig. 7).

View larger version (18K): [in this window] [in a new window]   Fig. 7. Transfer factor values (TF) for Cd, Pb, and Zn in maize and oat grown on contaminated soil (Ctrl) and on contaminated soil amended with two levels (25 and 50 g kg-1) of apatite (AP) or phillipsite (PZ); no oat grains were produced in the control.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

In the laboratory batch study, phillipsite effectively sequestered Pb, Ba, Eu, and Co, but not U, from contaminated media (Table 2). In the greenhouse bioavailability study, phillipsite significantly decreased the exchangeable fraction in the soil and the phytoavailability of Cd, Pb, and Zn. The effectiveness of phillipsite to sequester metals is attributed to its molecular structure, which provides large cation exchange properties to these minerals (Ming and Mumpton, 1989). Zeolites derive cation exchange capacity from Al substitution for Si with the size of the channel determining the preferred ionic radius (Breck, 1974). Several studies have demonstrated the ability of zeolite to reduce the mobility of Cs, Sr, Cu, Cd, Pb, and Zn (Chelishchev, 1995; Mumpton, 1984). However, zeolites are not effective sorbants for transuranic species, such as (UO₂), that are common on sites with elevated Cs and Sr levels (Vaniman and Bish, 1995).

A large percentage of Cd and Zn in the Czech Republic contaminated soil were found in the easily exchangeable fractions (Fig. 2). This high exchangeability of Cd and Zn is consistent with the findings of Hickey and Kittrick (1984) and Chlopecka et al. (1996). A similar distribution of forms was found for Cd near the Donana National Park in Spain (Ramos et al., 1994). Lead in this study's soil was less mobile, and the majority of Pb was associated with the Fe–Mn oxide fraction (Fig. 2). Chlopecka et al. (1996) also found that Pb was associated with the oxide fraction in their study of a contaminated Polish soil. Ma and Rao (1997) and Sposito et al. (1982) reported similar findings. In general, it seems that the oxide fraction is able to scavenge Pb in natural and contaminated soils, presumably by surface complexation. The results from sequential chemical extractions (Fig. 2) suggested that this soil could present a potential hazard to human health and the environment through plant uptake, food chain, leaching to ground water, or direct soil ingestion. Many research groups have demonstrated that soil amendments (apatite, zeolite, Fe oxides, Mn oxides, clay minerals, and some byproducts) can reduce the environmental risks of contaminated soils (Berti and Cunningham, 1997; Knox et al., 2000a, b; Ma and Rao, 1997). Application of these amendments should cause the contaminant to shift from forms with high availability (exchangeable, carbonate, or even oxide fractions) to those with low or no relative availability (sulfide or residual fractions). In the greenhouse bioavailability study, both sequestering agents reduced the easily exchangeable fraction of Cd, Zn, and Pb and increased the content of these elements in the more stable fractions (oxides, organic, or residual) (Fig. 3). Others (Berti and Cunningham, 1997; Ma and Rao, 1997, Ryan et al., 2001) have also shown that KH₂PO₄, apatite, or synthetic hydroxyapatite can convert Pb in contaminated soils from the exchangeable, carbonate, Fe–Mn oxide, and/or organic fractions to the residual fraction.

In the greenhouse bioavailability study, apatite, Ca₅ (PO₄)₃(F,Cl,OH), was added to soil mainly as a source of P, however, it also has a liming value in addition to supplying Ca. The liming action of apatite can occur through two processes. First, the apatite contains impurities of free CaCO₃ (Table 1), which itself can act as a liming agent. Second, the dissolution process of apatite in which it releases P and Ca into the soil consumes H⁺, thereby reducing the soil acidity. Phillipsite, due to its alkaline nature and high calcium carbonate equivalence, also had liming potential. Therefore, both sequestering agents significantly increased soil pH by 0.3 to 0.4 units. There were no significant differences (P < 0.05) in the change of pH between the phillipsite and apatite, or between the two rates of mineral amendment addition. Increased soil pH on adding the soil amendments probably played a role in reduction of mobility of the tested metals. Precipitation, which was probably an important sequestering process in the apatite systems, is greatly influenced by pH (Chen et al., 1997). Similarly, cation exchange, which was probably an important sequestering process in the phillipsite systems, is greatly influenced by pH (Ming and Mumpton, 1989).

Based on soil chemistry considerations and the literature, plant growth was probably diminished in the Czech soil primarily as a result of the elevated Cd and Zn concentrations, and less as a result of the elevated Pb concentrations (Alloway et al., 1990; Rebedea and Lepp, 1994; Kabata-Pendias and Pendias, 2001; Adriano, 2001). Toxicity prevented oat grain production on the untreated soil. On treating the soil with apatite and phillipsite, the toxicity was at least partially alleviated, yet grain Cd and Zn concentrations (Fig. 6) exceeded permissible levels (Kabata-Pendias and Pendias, 2001). Lead content in the Czech soil also was very high but it is well known that Pb is not relatively toxic to plants even at high concentrations in soil (Kabata-Pendias and Pendias, 2001). Cadmium toxicity is rare unless its level in plant tissues exceeds 20 mg kg^-1 (Alloway et al., 1990); however, Zn phytotoxicity is more common (Adriano, 2001). The Czech soil was contaminated with multiple metals, including Cd, Pb, and Zn. Therefore, the observed phytotoxicity was probably the interaction of these elements. Interaction of multiple metals on phytotoxicity is not fully understood and quantified (Adriano, 2001). What is well known is that toxicity levels of these metals vary with plant species and soil type.

Rebedea and Lepp (1994) also observed significant enhancement in the shoot biomass by grasses grown on metal-contaminated soils using synthetic zeolites. Similarly, the ameliorative effect of natural zeolite on barley yield was observed by Mineyev et al. (1989). Chlopecka and Adriano (1996)(1997a,b) demonstrated that apatite from North Carolina and clinoptilolite (natural zeolite) improved growth and yield of maize, barley, and radish while cultivated on contaminated soil with Cd, Pb, and Zn.

Zinc, but not Cd or Pb, is an essential plant nutrient. However, Cd and Pb exist in all soils and for that reason all crops contain at least trace amounts of these elements (Adriano, 2001). The concentrations of Cd, Pb, and Zn in both tested plants in the untreated Czech soil exceeded normal concentrations of Cd and Pb and the sufficient concentrations of Zn. As expected, the highest concentrations of Cd, Pb, and Zn occurred in the maize roots. Among the leaf tissues, the highest accumulation was observed for Zn in maize leaves, 3188 mg kg^-1, which is 53 times the sufficient concentration of Zn in maize tissues (Jones et al., 1991). The high content of studied elements in maize and oat leaves is inconsistent with the concept of the root–shoot barrier (Adriano, 2001). The sequestering materials significantly reduced Cd, Pb, and Zn in all parts of maize and oat. However, the highest reduction of these elements was observed in leaves of both plants and maize roots, followed by oat grain. Similarly, Chlopecka and Adriano (1997a) reported that the highest reduction of Cd and Pb by apatite and zeolite was observed in maize leaves.

Bioavailability of tested metals in contaminated soil and in remediated contaminated soil was also evaluated by TF (Eq. [2]), a well-known and widely used factor to describe metal bioavailability (Adriano, 2001; Kabata-Pendias and Pendias, 2001). In highly contaminated soils, bioavailability increases, as indicated by much greater TF values. For example, the TF values for Cd and Zn are in the neighborhood of 1 to 10 and for Pb from 0.01 to 0.1 (Kabata-Pendias and Pendias, 2001). This reconfirms the relatively higher bioavailability of Cd and Zn than Pb. In the plant study, the largest TF values were obtained for Zn and Cd in maize roots, followed by maize leaves in untreated soil. Both amendments reduced the TF value for each metal; however the highest reduction of TF values was obtained by apatite applied at 50 g kg^-1. For example, the TF values for maize roots and leaves for Cd and Zn were below 1. The low TF values for Cd and Zn in the Czech soil with the second dose of apatite confirm the low bioavailability of these elements, despite very high total concentrations of these elements in this soil.

Interactions between chemical elements may be antagonistic or synergistic. These interactions may refer to the ability of one element to inhibit or stimulate the adsorption of other elements in plants (Kabat-Pendias and Pendias, 2000). The antagonistic relationship between P and heavy metals (e.g., Be, Cd, and Pb) and between P and some micronutrients (e.g., Fe and Zn) is well known (Jones et al., 1991, Kabata-Pendias and Pendias, 2001). This study indicates that immobilization of the contaminants (Cd, Pb, and Zn) in soil by the sequestering agents did not result in deficiency of nutrients such as Fe, P, and Ca in oat tissues (Table 3). Magnesium concentrations in oat tissues were deficient (Table 3). Even though both sequestering agents increased Mg concentrations in oat tissues, they were still below the range of sufficient level (i.e., lower than 1500 mg kg^-1; Jones et al., 1991). Magnesium deficiency can be induced by high concentrations of either NH₄, K, or Ca in the rooting medium; Mg is the poorest competitor among these cations (Jones et al., 1991). Addition of both levels of apatite also reduced P concentration in oat leaves. On the other hand, the addition of phillipsite increased P content in oat leaves. The increase in the P content of grain was more strongly influenced by phillipsite than apatite (Fig. 3). While the reduction of P concentration in oat was significant (P < 0.05), its concentration remained within the range considered sufficient, 2.0 to 5.0 g kg^-1 dry wt. (Jones et al., 1991). The apatite used in this experiment contained more P and Ca than the phillipsite (Table 1). The high Ca content may explain the decrease in soluble P in the Czech soil (loamy sand) with apatite addition. A large content of Ca promotes desorption of adsorbed P, thereby enhancing its uptake by plants. However, high soil Ca can precipitate appreciable quantities of P, especially if the pH is high. This negative effect of Ca on P availability is more marked on light-texture soil, since the low clay content limits the positive effect of Ca on desorption of adsorbed P (Jones et al., 1991). In phillipsite treatments P content in oat could increase due to decreasing Al, rather than increasing P availability in soil (Haynes, 1984).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
These studies evaluated the use of apatite or phillipsite as sequestering agents to remediate a contaminated medium. Both the immediate effectiveness and long-term sustainability of using such soil amendments must be demonstrated to achieve widespread public and regulatory acceptance. Furthermore, it is necessary to evaluate the ability of prospective soil amendments to reduce contaminant bioavailability. Simple batch (i.e., sorption and extractability) and column techniques can be used to estimate the potential mobility of a given contaminant under various leaching scenarios. However, the evaluation of metal bioavailability is appreciably more difficult to quantify due to the large number and the highly interactive nature of the parameters controlling it.

Since certain sources of apatite and zeolite have been found not to be effective at sequestering soil metals, a laboratory batch study was conducted to screen the effectiveness of the specific minerals used in the subsequent bioavailability study. The bioavailability study showed that the amendments were very effective at redistributing the Cd, Pb, and Zn into fractions that were more strongly held by the soil. This redistribution of the metals resulted in less phytotoxicity, as indicated by greater plant growth and lower metal concentrations in the plant tissue. Apatite was consistently better than the phillipsite at reducing metal bioavailability. The lowest application rate, 25 g kg^-1 apatite, achieved a statistically significant improvement in reducing plant toxicity and metal tissue concentrations; only an incremental improvement was obtained at the higher apatite application rate of 50 g kg^-1. It may not only be less costly to use the lower application rate, but it may better for plant growth because the same sorption mechanisms that act to limit toxic metal availability can reduce the availability of certain plant essential micronutrients, such as Zn, Fe, or P. Furthermore, mineral amendments often contain metal impurities that may exceed environmental limits if applied at sufficiently high concentrations. Thus, it may prove most beneficial to periodically apply stabilizing–immobilizing agents to ensure continued effectiveness.

	ACKNOWLEDGMENTS

 
The authors would like to thank Dr. Milan Sanka from the Central Institute for Supervising and Testing in Agriculture, Brno, Czech Republic, for supplying the contaminated soil. The authors gratefully acknowledge Dr. Yi Yi of the University of Georgia, Dr. P.F. Ziemkiewicz of West Virginia University, and Dr. A. Pishko of Westinghouse Savannah River Company for reviewing this manuscript. This research was supported by Financial Assistance Award no. DE-FC09-96SR18546 from the U.S. Department of Energy to the University of Georgia Research Foundation. Westinghouse Savannah River Company is operated for the U.S. Department of Energy under contract DE-AC09-96SR18500.

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