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Journal of Environmental Quality 30:1206-1213 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Heavy Metals in the Environment

Immobilization of Cesium-137 and Uranium in Contaminated Sediments Using Soil Amendments

John C. Seaman*, T. Meehan and P.M. Bertsch

Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Lab., Univ. of Georgia, Aiken, SC 29802

* Corresponding author (seaman{at}srel.edu)

Received for publication March 31, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Batch and dynamic leaching methods were used to evaluate the effectiveness of hydroxyapatite (HA), illite, and zeolite, alone and in combination, as soil additives for reducing the migration of cesium-137 (137Cs+) and uranium (U) from contaminated sediments. Amendment treatments ranging from 0 to 50 g kg-1 were added to the sediment and equilibrated in 0.001 M CaCl2. After equilibration, the treatment supernatants were analyzed for 137Cs+, U, PO4, and other metals. The residual sediments were then extracted overnight using one of the following: 1.0 M NH4Cl, 0.5 M CaCl2, or the Toxicity Characteristic Leaching Procedure (TCLP) extractant. Cesium was strongly sorbed to the contaminated sediments, presumably due to interlayer fixation within native illitic clays. In fact, 137Cs+ was below detection limits in the initial equilibration solutions, the CaCl2 extract, and the TCLP solution, regardless of amendment. Extractants selective for interlayer cations (1.0 M NH4Cl) were necessary to extract measurable levels of 137Cs+. Addition of illitic clays further reduced Cs+ extractability, even when subjected to the aggressive extractants. Zeolite, however, was ineffective in reducing Cs+ mobility when subjected to the aggressive extractants. Hydroxyapatite was less effective than illite at reducing NH+4–extractable Cs+. Hydroxyapatite, and mixtures of HA with illite or zeolite, were highly effective in reducing U extractability in both batch and leaching tests. Uranium immobilization by HA was rapid with similar final U concentrations observed for equilibration times ranging from 1 h to 30 d. The current results demonstrate the effectiveness of soil amendments in reducing the mobility of U and Cs+, which makes in-place immobilization an effective remediation alternative.

Abbreviations: HA, hydroxyapatite • ICP–MS, inductively coupled plasma–mass spectrometry • SRL, Savannah River Laboratory • SRS, Savannah River Site • TCLP, Toxicity Characteristic Leaching Procedure


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
FOUR unlined basins located in the northeast corner of the U.S. Department of Energy's Savannah River Site (SRS), Aiken, SC, known as the Savannah River Laboratory (SRL) basins, received low-level radioactive wastewater from 1954 until 1982 (Westinghouse Savannah River Company, 1996). The basins acted as a series of sequential overflow catchments receiving wastewater containing tritium (3H), cesium-137 (137Cs+), and other radioactive and nonradioactive wastes resulting from SRL activities. As a result, the shallow sediments ({approx}0.3 m depth) within the basins had elevated levels of 137Cs+, U, and other contaminants that warranted remedial action. The use of chemical stabilizing agents to reduce plant uptake and contaminant migration prior to in-place disposal is an attractive alternative to conventional excavation and long-term storage of the sediments.

Laboratory studies often focus on reducing the mobility of one specific contaminant metal, for example, Cr(VI), Pb, or U. Most sites, however, consist of mixtures of various contaminants, such as Cs+ and U, which display different chemical properties that influence their mobility and toxicity in the environment, and therefore require specific treatment scenarios that account for such differences. The choice of stabilizing agents typically relies on an understanding of the physicochemical mechanisms affecting the speciation and fate of the target contaminant or a chemical analog in the natural environment. For instance, the use of apatite [Ca5(PO4)3X; X = halide, hydroxyl] to reduce the solubility of lead (Pb) is based on the observation that such minerals often act as sinks for many transition and heavy metals, metalloids, and radionuclides (Nriagu, 1974; Wright, 1990; Wright et al., 1987). To date, numerous studies have demonstrated the ability of apatite to react with Pb to form highly insoluble Pb phosphates (Berti and Cunningham, 1997; Chen et al., 1996; Laperche et al., 1997; Ma et al., 1994a,b; Ma et al., 1993; Ruby et al., 1994; Zhang et al., 1997).

In batch studies evaluating in situ immobilization of contaminants in similar highly weathered soils (Arey et al., 1999; Seaman et al., 2001), minor additions (5 g kg-1) of hydroxyapatite (HA) significantly reduced both the water-soluble and TCLP-extractable U, as well as a number of other environmentally important metals, including Pb and nickel (Ni). However, competition from HA-derived PO4 for oxyanion sorption sites can enhance the mobility of contaminants such as Cr(VI) and As (Seaman et al., 2001). Despite its high solubility, Cs+, like K+, can be selectively adsorbed and strongly bound to zeolites and certain 2:1 phyllosilicate clays (Sawhney, 1966), often resulting in Cs+ contamination being restricted close to the point of deposition (Mahara, 1993). The 137Cs+ distribution within the sediments underlying the SRL basins reflects such limited mobility (Westinghouse Savannah River Company, 1996).

The addition of clays to contaminated soils can result in a number of physical and chemical changes that could affect contaminant fate and transport. A reduction in the solubility or bioavailability of contaminants may simply reflect an increase in the cation exchange capacity or mineral surface area for coarse-textured soils. However, addition of certain clay minerals, such as vermiculite and illite, to contaminated soils may be effective in immobilizing 137Cs+ through mechanisms other than simple cation exchange (Poinssot et al., 1999). Illites and vermiculites, 2:1 phyllosilicate clays possessing a relatively high layer charge, have the ability to strongly sorb certain elements and molecules within the clay interlayer, which are similar in size and hydration energy to K+, such as NH+4 and Cs+, thereby significantly reducing their mobility and bioavailability in the soil environment. Such interlayer sorption is often referred to as interlayer fixation due to the relatively slow kinetics of release. Greenhouse and field studies have demonstrated that 137Cs+ sorption by such materials can reduce uptake by plants (Chelishchev, 1995; Leppert, 1990). Illitic materials may be more effective stabilizing agents due to their ability to fix Cs+ under a range of moisture conditions compared with vermiculites, which often fix interlayer cations only upon drying.

Because exchange with interlayer cations is generally considered a slow process that may not be readily reversible (Comans et al., 1991; Comans and Hockley, 1992; Komarneni, 1978; Sawhney, 1966; Zygmunt et al., 1998), fixation may be an ideal means of limiting 137Cs+ mobility until it has time to decay (t1/2 = 30.17 yr). In fact, the bioavailability of 137Cs+ at the landscape scale is generally controlled by the kinetics of the interlayer fixation process (Smith et al., 1999). Other factors such as soil moisture content and the presence of organic acids and cations of similar ionic radius, such as K+ and NH+4, can also affect Cs+ sorption and fixation by soil clays (Comans et al., 1991; Comans and Hockley, 1992; Hsu and Chang, 1994; Staunton and Roubaud, 1997). The relative effectiveness of interlayer fixation as a stabilization method depends on factors such as background solution composition (i.e., predominant cation, concentration, etc.), soil pH, clay mineralogy, and organic matter content (Staunton and Roubaud, 1997).

Site-specific data under a range of potential field conditions (i.e., organic matter content, soil mineralogy and texture, etc.) is needed to predict the long-term efficacy of such remediation strategies. Therefore, the objective for the present study was to evaluate the ability of minor additions (5–50 g kg-1) of illitic clays and zeolite, alone and in combination with HA, to reduce the potential mobility of Cs+, U, and other contaminants prior to excavation and long-term storage of the sediment or in situ treatment and storage in place.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Sediment Characterization
Two sediments, hereafter referred to as Sediment A and Sediment B, were collected from the SRL basins on the U.S. Department of Energy's Savannah River Site, located near Aiken, SC (Table 1). The 137Cs+ content in the sediments and various extracts was determined by gamma spectrometry (Packard [Meriden, CT] Minimax {gamma}, Auto-Gamma 5000 Series). Total U and other metals were determined using an HF/HNO3–based, microwave-assisted digestion method (USEPA, 1996) followed by inductively coupled plasma–mass spectrometry (ICP–MS) (Elan 6000; PerkinElmer, Norwalk, CT) based on the quality assurance–quality control protocols outlined in USEPA Method 6020 (USEPA, 1994). Uranium and 137Cs+ were also extracted from the nonamended sediments using the USEPA standardized Toxicity Characteristic Leaching Procedure (TCLP; USEPA, 1992). The TCLP extraction is commonly used as a regulatory indicator of contaminant leachability under slightly acidic (pH {approx} 4.93), weak ligand extraction (acetate) conditions thought to be analogous to the leaching environment of municipal landfills. The more aggressive, pH 2.9 TCLP extraction (TCLPpH2.9), which is generally used for wastes that possess a greater buffering capacity, was also performed (Buonicore, 1996).


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  		Table 1. Select physical and chemical properties of Sediments A and B from the Savannah River Laboratory (SRL) basins on the Savannah River Site (SRS).

 
Batch Equilibration Procedures
A series of batch equilibration experiments was conducted to evaluate the effectiveness of the HA, illite (Silver Hill Illite; Clay Minerals Repository, Columbia, MO), and zeolite (Clinoptilolite; Steelhead Specialty Minerals, Spokane, WA), alone and in combination, in reducing contaminant solubility and extractability. Amendment levels ranging from 0 to 50 g kg-1, corresponding to 0 to 5% by weight, were added to 5 g of basin sediment in replicate 50-mL centrifuge tubes, and then equilibrated on a reciprocating shaker for 2 to 30 d in a 0.001 mol L-1 CaCl2 background solution. Combined treatments consisted of equal mass fractions of the two amendments added to the sediment (e.g., 50 g kg-1 combined = 50 g kg-1 HA + 50 g kg-1 illite). After equilibrating for a prescribed time interval, the samples were centrifuged and the filtered supernatant, operationally defined as the equilibration solution, was analyzed for 137Cs+, U, PO4, and other metals. The sediments were then extracted overnight with one of the following: 0.5 mol L-1 CaCl2, 1.0 mol L-1 NH4Cl, or the TCLP extractants. The Ca2+– and NH+4–based extractants were used to discriminate between 137Cs+ retained by simple cation exchange sites (i.e., Ca2+ extractable) and a fraction of the Cs+ "fixed" within clay interlayers or sorbed to highly specific sites within the zeolites (i.e., NH+4 extractable = exchangeable + "weakly" fixed). The 137Cs+ content in the extracts and equilibrating solutions was determined by gamma spectrometry as described above. In addition, batch supernatants were acidified (1% nitric acid) and analyzed for metals including U and strontium-88 (88Sr) by ICP–MS. Strontium-88 was used as a surrogate for predicting the behavior of 90Sr due to the low levels of 90Sr present in the sediment samples. Preliminary kinetic experiments evaluating extraction times as long as 8 d using nonamended sediments indicated that 1.0 mol L-1 NH4Cl-extractable 137Cs+ leveled off within the first hour of shaking.

Frayed-Edge Sites in Amended Sediments
Rapid fixation and specific sorption of 137Cs+ in soils has been attributed to frayed-edge sites present on illitic clays. The concentration of frayed-edge sites can be determined by measuring Cs+ sorption in the presence of silver–thiourea (AgTU), a compound that supposedly blocks planar cation exchange sites from participating in Cs+ sorption, thereby restricting Cs+ to clay interlayer and edge sites that the AgTU cannot reach due to molecular size constraints (Cremers et al., 1988; Cremers and Pleysier, 1973). In the present study, the concentration of frayed-edge sites (FES) for the basin sediments, the illitic amendment materials, and the treatments involving illite were determined using a modified AgTU method (Cremers et al., 1988; Cremers and Pleysier, 1973). A frayed edge site concentration of {approx}0.04 meq 100 g-1 (0.04 cmol kg-1) was determined for the illitic materials used in the current study, which is generally consistent with previously reported values for other illitic materials (Comans et al., 1991; Poinssot et al., 1999).

Column Leaching Procedures
Air-dried sediments mixed with the appropriate amendment were packed to uniform bulk densities of {approx}1.43 g cm-3 in 2.5-cm ID chromatography columns (Kontes, Vineland, NJ). Layers of Ottawa sand (20-30 Mesh, Fisher Scientific, Fair Lawn, NJ), 0.5 cm in depth, were included above and below the sediment to disperse flow throughout the cross section of the column. Packed columns were mounted vertically and continuously leached from the bottom (i.e., inlet) to reduce the potential for side-wall flow. Each column was saturated from the inlet and leached continuously at 0.2 ± 0.02 mL min-1 (Darcy velocity {approx} 0.59 m d-1) for 50 pore volumes using one of three solution treatments: 0.001 mol L-1 CaCl2, 0.001 mol L-1 CaCl2 + 0.0005 mol L-1 NH4Cl, or 1.0 mol L-1 NH4Cl. The leaching solution combining CaCl2 and NH4Cl was selected to represent the "typical" ground water leaching environment on the SRS, with the NH+4 concentration chosen to be analogous to the combined K+ + NH+4 concentration found in SRS precipitation (0.009 mM) or ground water from nonimpacted water-table wells (0.017 mM) (Strom and Kaback, 1992). The volumetric water content ({theta} {approx} 0.40) or pore volume (PV) was assumed to be equivalent to the column porosity. The electrical conductivity (EC) and pH of the effluent were monitored continuously using flow-though cells, and effluent samples were collected and filtered for chemical analysis as described previously.

Thermodynamic Modeling
Equilibrium modeling was conducted using the USEPA's MINTEQA2 geochemical code (v. 3.11) updated with the Nuclear Energy Agency's thermodynamic database for uranium (Turner et al., 1993; USEPA, 1997). The degree of saturation with respect to a specific solid phase ({Omega}) is:

where IAP is the ion activity product and Ksp is the solubility product. To readily compare the degree of saturation for a set of potential solid phases, the saturation index (SI) is then defined as the log {Omega}. An SI of <0 (i.e., negative value) indicates that the solution is undersaturated while an SI >0 indicates that the solution is supersaturated with respect to a specified solid phase. An SI of 0 indicates that the solution is in equilibrium with a given solid phase. To calculate the saturation indices for various U-phosphate mineral phases, modeling was performed using HA as an infinite solid without allowing secondary phases to precipitate that would alter the resulting solution chemistry. The solution composition (i.e., U, K, and PO4) and pH were based on results from the initial apatite–soil equilibrations in the background solution, 0.001 mol L-1 CaCl2. Therefore, Ca2+ and Cl- concentrations were assumed to reflect the background solution.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Batch Cesium Results
Consistent results were observed in batch experiments using both of the SRL basin materials, Sediments A and B. Therefore, the discussion will be restricted to Sediment A, the material with the higher total 137Cs+ and U content. Since the basin sediments contain some micaceous clays, the strong Cs+ retention in such materials was demonstrated by its negligible release (<0.11 Bq gm-1) during the initial sample equilibration (0.001 mol L-1 CaCl2) or subsequent TCLP extraction (Table 1), even in the absence of any amendment (i.e., control sediment). In fact, significant extraction accounting for {approx}10 to 15% of the total 137Cs+ in the soil occurred only when the sediments were subjected to 1.0 mol L-1 NH4Cl (Fig. 1), suggesting that Cs+ retention in the unamended sediments was restricted to interlayer sites that are not accessible to Ca2+. Illite and HA were moderately effective at reducing NH+4–extractable Cs+, with retention increasing with increasing amendment level (Fig. 1). Slightly higher levels of 137Cs+ were extracted from zeolite-amended samples compared with the control. Although the differences are slight, this suggests that the zeolite acts as a sorptive sink for 137Cs+ during the initial equilibration, which was then released during the subsequent 1.0 mol L-1 NH+4 extraction, even though detectable levels of Cs+ were never observed in the equilibration solutions. The Sr batch results further support such a mechanism.



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  		Fig. 1. Effect of single reagent-grade amendments on 1.0 mol L-1 NH4Cl-extractable 137Cs+ after equilibrating for 30 d in 0.001 mol L-1 CaCl2. Error bars indicate the standard deviation of the means.

 
Batch Strontium-88 Results
Strontium-88 (88Sr), which can be readily analyzed by ICP–MS, was used as a surrogate for predicting the behavior of 90Sr. Zeolite and HA were effective at reducing the solution-phase Sr concentration during the initial sample equilibrations (Fig. 2A). In fact, the Sr concentrations were below the ICP–MS detection limits at the two highest zeolite amendment rates (20 and 50 g kg-1) and the highest HA amendment rate (50 g kg-1). The HA treatments were effective at increasing the sediment pH, while the zeolite and illite treatments had little influence on pH (Fig. 2B). When extracted with 1.0 mol L-1 NH4Cl, however, high levels of Sr were recovered from the zeolite-amended sediment (Fig. 2C). Like the Cs+ results reported earlier, this suggests that the zeolite was highly effective at scavenging Sr during the initial equilibration, sorbing much more than was initially solubilized in nonamended control samples. However, when extracted with 1.0 mol L-1 NH4Cl, Sr accumulated by zeolite was released in much greater concentrations than observed for the other two, initially less-effective amendments.



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  		Fig. 2. Effect of various single reagent-grade amendments (hydroxyapatite, illite, zeolite) on 88Sr concentration (A) and pH (B) after equilibrating for 30 d in 0.001 mol L-1 CaCl2. Figure C is the subsequent 1.0 mol L-1 NH4Cl-extractable Sr after equilibration. Error bars indicate the standard deviation of the means.

 
Batch Uranium Results
Uranium retention by the sediment increased (i.e., a decrease in solution-phase U concentration) in an apparent linear manner with increasing addition of illite and zeolite during the initial equilibration in 0.001 mol L-1 CaCl2 (Fig. 3A). This may be attributed to an increase in the cation exchange capacity (CEC) of the sediments resulting from the amendments, with enhanced U retention limited to external and/or readily accessible exchange sites. Such a hypothesis is further supported by the fact that both extractants, 0.5 mol L-1 Ca2+ and 1.0 mol L-1 NH+4, were effective at removing U during the final extraction tests (Fig. 3C, 0.5 mol L-1 Ca; extraction data not shown). In fact, zeolite, the material with the greater CEC, was more effective than illite at reducing the solution-phase U concentration during the initial equilibration (Fig. 3A), resulting in greater U extraction from the zeolite-treated sediments during subsequent extraction with 1.0 mol L-1 NH4Cl (Fig. 3C). These results suggest that the "extractable" U exists in the more-soluble U(VI) valence state as uranyl (UO2+2), the primary species in natural systems under aerobic, mildly acidic conditions (pH {approx} 5) (Langmuir, 1997).



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  		Fig. 3. Effect of various single reagent-grade amendments (hydroxyapatite, illite, zeolite) on U concentration (A) and pH (B) after equilibrating for 30 d in 1.0 mM CaCl2. Figure C is the subsequent 1.0 mol L-1 NH4Cl-extractable U after equilibration. Error bars indicate the standard deviation of the means.

 
In contrast, U solubility and extractability decreased dramatically with HA addition in the equilibration (0.001 mol L-1 CaCl2) and extraction solutions (0.5 mol L-1 Ca2+ or 1.0 mol L-1 NH+4) (Fig. 3A,C). In fact, the lowest HA treatment level, 5 g kg-1, was essentially as effective as the higher amendment rates, 20 and 50 g kg-1, at reducing U solubility. Solution-phase U levels in the HA-amended sediments were well below the saturation point for various autunite mineral phases, as indicated by the highly negative saturation indices (Table 2). Although uranium phosphates such as autunite are thought to control U solubility in phosphate-rich systems, this has not been observed for highly weathered, contaminated soils. Transmission electron microscopy–energy dispersive X-ray analysis of HA-amended soils similar to those in the present study showed that U, Ni, and other contaminant metals tend to be associated with secondary aluminum- and iron-rich phosphates, rather than residual unreacted apatite (Arey et al., 1999; Seaman et al., 2001). In fact, Seaman et al. (2001) found that the equilibrium solutions for these batch studies were very close to the saturation state (i.e., log {Omega} = 0) for variscite (AlPO4 2H2O), an Al-phosphate phase thought to control PO4 activity in highly weathered soil systems (Lindsay, 1979; Nriagu, 1974).


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  		Table 2. MinteqA2 input parameters and calculated saturation indices of select uranyl (UO2+2) phosphates for Sediments A and B amended with hydroxyapatite (HA) and equilibrated in 0.001 mol L-1 CaCl2 for 30 d.

 
The pH of the sediment increased, presumably due to alkalinity released during HA dissolution (Fig. 3B), while the addition of illite and zeolite had little influence on pH. Solution-phase equilibrium U concentrations in the HA-amended samples were consistently less than 1.0 µg L-1 (<1 ppb); <6 µg kg-1 of sediment on the scale of Fig. 3A when accounting for sample size and equilibrium solution volume. Data in Fig. 3 represent 30-d equilibrations; however, similar results were obtained for HA-amended sediments, which had been equilibrated for much shorter time periods (i.e., 2 d), indicating rapid, solid-phase U redistribution in the presence of HA (data not shown). In addition, HA was equally effective at reducing U solubility and extractability in batch studies evaluating treatment mixtures of HA with illite or zeolite (Fig. 4). As reported for the single-amendment treatments, the HA combined treatments resulted in a similar increase in batch equilibration pH (Fig. 4B) and decrease in 1 M NH4–extractable U (Fig. 4C).



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  		Fig. 4. Effect of various combined amendments (hydroxyapatite [HA] + illite and HA + zeolite) on U concentration (A) and pH (B) after equilibrating for 30 d in 0.001 mol L-1 CaCl2. Figure C is the subsequent 1.0 mol L-1 NH4Cl-extractable U after equilibration. Note that the results for equilibrium (A) and extractable (B) U for the two treatments are essentially identical. Error bars indicate the standard deviation of the means.

 
Column Cesium Results
Batch results were used to constrain the various treatment scenarios evaluated in column leaching experiments. Detectable levels of 137Cs+ in the column effluents, >18.6 Bq L-1, were only observed when the amended and unamended sediments were leached with 1.0 mol L-1 NH4Cl (Fig. 5A). With continued leaching, effluent 137Cs+ for the 1.0 mol L-1 NH4Cl leaching solution soon fell below the detection limit as well. Cesium in effluents from columns leached with either the 0.001 mol L-1 CaCl2 or the "typical" SRS leaching solution (0.001 mol L-1 CaCl2 + 0.0005 mol L-1 NH4Cl) were below the detection limit (18.6 Bq L-1) throughout the course of leaching, again consistent with the batch equilibration results. The NH+4 concentration for the "typical" SRS ground water was chosen as a conservative (i.e., more-aggressive leaching environment) representation of the concentrations of cations (such as NH+4 + K+) that such materials are likely to encounter on the SRS. Therefore, the concentration is twice the average concentration of NH+4 + K+ detected in ground water from the water-table aquifer and an order of magnitude greater than the combined concentration of those elements in SRS precipitation (Strom and Kaback, 1992). Therefore, Cs+ leachability in the presence of such solutions would probably be somewhat greater than the typical case. However, Cs+ in the column effluents never exceeded the detection limit throughout leaching for {approx}50 pore volumes.



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  		Fig. 5. Effluent 137Cs+ (A) and U (B) concentration for hydroxyapatite [HA]-amended and unamended Sediment A columns leached with 1.0 mol L-1 NH4Cl, 0.001 mol L-1 CaCl2, or 0.001 mol L-1 CaCl2 + 0.0005 mol L-1 NH4Cl. Note that the 137Cs+ levels for all but the 1.0 mol L-1 NH4Cl leachates were below detection limits (<18.6 Bq L-1) regardless of amendment treatment. Error bars indicate the standard deviation of the means.

 
Hydroxyapatite, illite, and zeolite (50 g kg-1) were ineffective at reducing Cs+ leaching, with similar effluent mass recoveries ({approx}34 Bq of 137Cs+) observed for the amended and control columns when leached with 1.0 mol L-1 NH4Cl. This reflects {approx}14% of the total 137Cs in the soil column, which is consistent with the amount extracted by 1.0 mol L-1 NH4Cl for batch control treatments with no soil amendments. The negligible treatment effect observed in the column experiments most likely reflects both the slow kinetics associated with interlayer fixation processes, with insufficient column residence times ({approx}1.25 h) to observe such sorption, and the necessity to use aggressive leaching solutions that further inhibit fixation through competition for the same interlayer sites.

Column Uranium Results
Hydroxyapatite addition was effective at reducing U leachability compared with unamended control columns, with {approx}0.01% of the total U leached from the column within the first 60 pore volumes. When unamended columns were leached with 1.0 mol L-1 NH4Cl, high levels of U, {approx}500 µg L-1, were initially detected in the effluent, but eventually leveled off to {approx}25 µg L-1 over the remaining course of leaching (Fig. 5B). Although this accounted for {approx}1.7% of the total U in the column, this was a much greater amount than leached from the HA-treated samples ({approx}0.01%) over the same time frame. The more-dilute 0.001 mol L-1 CaCl2 solution was somewhat less effective at enhancing U leaching, with the effluent concentration gradually increasing to {approx}25 µg L-1, presumably due to the lesser competition from the dilute leachate with uranyl for cation exchange sites. When HA was added, however, effluent U concentrations were consistently below 1 µg L-1, regardless of the major cation (NH+4 vs. Ca2+) or concentration of the leaching solution. In addition to reducing U leaching, HA effectively increased the pH of the column effluent. The potential use of HA as an underlying barrier to U migration was also evaluated in column leaching experiments. However, when the reagent grade powder was used as an underlying barrier, the pozolonic (e.g., cement like) nature of the material, even when mixed with sand, resulted in clogging that precluded long-term leaching experiments. In preliminary studies, industrial grade HA composed of coarser particles was effective at reducing U leaching when incorporated within the contaminated sediments or applied as an underlying barrier, but higher relative application rates were required due to the lesser reactivity when compared with reagent-grade HA.


   SUMMARY
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
The results clearly illustrate the utility of soil amendments in reducing the potential leachability of a number of important contaminants. Such amendments may be surface-applied, then incorporated within shallow contaminated sediments as an in-place disposal method or added to contaminated soils and sediments prior to long-term storage as a means of reducing the leaching hazard should containment systems fail. Hydroxyapatite was highly effective at reducing U solubility and extractability in both batch and column experiments, indicating that such immobilization was extremely rapid. The ability of illite to reduce 137Cs+ mobility through interlayer fixation was demonstrated in batch studies despite the high degree of fixation within native illitic clays. Column results for 137Cs+ were less compelling due to the limited solution residence times and the necessity to use unrealistically aggressive leaching solutions to observe detectable levels of 137Cs+ in the leachate, regardless of amendment treatment. Similar limitations were even more evident when evaluating the zeolite as a sediment amendment. The effectiveness of zeolite in reducing Sr solubility and extractability was clearly demonstrated in batch experiments due to the higher native solubility of that element compared with 137Cs+. However, the concentrated NH+4–extractant required to evaluate 137Cs+ sorption in batch studies precluded sorption to the zeolite, despite the fact that enhanced Cs+ and Sr extractability by 1.0 mol L-1 NH4Cl for zeolite-amended sediments suggested that the mineral acted as a sorbent and sink for such elements during batch equilibration. The current results clearly indicate the importance in choosing realistic batch and column leaching conditions to evaluate the effectiveness of chemical immobilization strategies.


   ACKNOWLEDGMENTS
 
This research was supported by the U.S. Department of Energy (DOE) Centers for Excellence in Water Research and partially by Financial Assistance Award Number DE-FC09-96SR18546 from the DOE to the University of Georgia Research Foundation. The authors would like to acknowledge the thoughtful comments of Dr. C. Strojan and Dr. V. Vulava on an early version of the manuscript and the laboratory assistance of J. Logan and T. Parker.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 




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J. C. Seaman, J. M. Hutchison, B. P. Jackson, and V. M. Vulava
In Situ Treatment of Metals in Contaminated Soils with Phytate
J. Environ. Qual., January 1, 2003; 32(1): 153 - 161.
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