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Published online 8 August 2008
Published in J Environ Qual 37:1733-1740 (2008)
DOI: 10.2134/jeq2007.0409
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Heavy Metals in the Environment

Potential Negative Consequences of Adding Phosphorus-Based Fertilizers to Immobilize Lead in Soil

Douglas W. Kilgoura,*, Rebecca B. Moseleya, Mark O. Barnetta, Kaye S. Savageb and Philip M. Jardinec

a Dep. of Civil Engineering, 208 Harbert Engineering Center, Auburn Univ., Auburn, AL 36849
b Dep. of Earth and Environmental Sciences, 2301 Vanderbilt Place, Vanderbilt Univ., Nashville, TN 37235
c Environmental Sciences Div., P.O. Box 2008, Oak Ridge National Lab., Oak Ridge, TN 37831-6038

* Corresponding author (mark.barnett{at}auburn.edu).

Received for publication August 2, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
A study of the potential negative consequences of adding phosphate (P)-based fertilizers as amendments to immobilize lead (Pb) in contaminated soils was conducted. Lead-contaminated firing range soils also contained elevated concentrations of antimony (Sb), a common Pb hardening agent, and some arsenic (As) of unknown (possibly background) origin. After amending the soils with triple superphosphate, a relatively soluble P source, column leaching experiments revealed elevated concentrations of Sb, As, and Pb in the leachate, reflecting an initial spike in soluble Pb and a particularly dramatic increase in Sb and As mobility. Minimal As, Sb, and Pb leaching was observed during column tests performed on non-amended control soils. In vitro extractions tests were performed to assess changes in Pb, As, and Sb bioaccessibility on P amendment. Lead bioaccessibility was systematically lowered with increasing P dosage, but there was much less of an effect on As and Sb bioaccessibility than on mobility. Our results indicate that although P amendments may aid in lowering the bioaccessibility of soil-bound Pb, it may also produce an initial increase in Pb mobility and a significant release of Sb and As from the soil, dramatically increasing their mobility and to a lesser extent their bioavailability.

Abbreviations: PBET, physiologically based extraction test • SI, saturation index • TSP, triple superphosphate


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
IN the USA, lead (Pb) has been identified as a contaminant of concern at approximately one half of the sites on the US national priority list (Ryan et al., 2004). At most Pb-contaminated sites, the ingestion of contaminated soil by children is the risk driver (Dudka and Miller, 1999; Salatas et al., 2004), and Pb poisoning is the most common and serious environmental health problem facing children in the USA (Ryan et al., 2004). There are numerous sources of Pb-contaminated soils in the USA, resulting from Pb mining, smelting, and industrial use as well as urban sources such as residual soil- and dust-bound contamination from the historical use of Pb-based paint, gasoline, and solder. Firing ranges are another significant and emerging source of Pb-contaminated soils (Cao et al., 2003a). There are well over 10,000 firing ranges in the USA, where approximately 4% of the 80,000 tons of Pb produced in the USA in the late 1990s was made into bullets and shot (ITRC, 2003). Lead concentrations in the topsoil at shooting ranges can reach as high as 50,000 mg kg–1 (Rooney et al., 1999). Lead mobilization in these firing range surface soils has also led to elevated concentrations in subsurface soils, further complicating the sites (Murray et al., 1997). Nonetheless, Pb remains the primary choice of ammunition in small-arms weapons, creating a growing need to expand the knowledge base for Pb contamination and remediation to minimize the adverse impacts of Pb at firing ranges (Cao et al., 2003b).

Due to the high costs of soil excavation and off-site remediation, in situ chemical stabilization with phosphorous (P)-based amendments has been thoroughly investigated as a more efficient and cost-effective method of site remediation (Hettiarachchi and Pierzynski, 2004). In situ stabilization can lower the mobility and toxicity of Pb, which is preferred at larger sites where metal stabilization is needed, such as shooting ranges (Wilson et al., 2006). Phosphorus-based fertilizers are effective in Pb remediation, especially where immediate site remediation is needed (Stanforth and Qiu, 2001). By applying P-based amendments to Pb-contaminated soils, insoluble Pb-P solids such as pyromorphites may form via the reaction

Formula 1[1]
where X represents OH–, Cl–, or F– forming hydroxo-, chloro-, or fluoro-pyormorphite, respectively.

Pyromorphites provide a highly insoluble species of Pb over a wide range of environmental conditions (Cotter-Howells and Caporn, 1996; Hettiarachchi et al., 2000; Lin et al., 2005; Ma et al., 1993), and field tests have been conducted at a number of contaminated sites (Brown et al., 2004; Chen et al., 2003; Farfel et al., 2005; Lin et al., 2005; Melamed et al., 2003). The use of P-based amendments at firing ranges has also been proposed (Cao et al., 2003a).

Although P-based amendments may be effective at immobilizing and reducing the bioavailability of Pb in contaminated soils, there are several potential drawbacks. Relatively high P dosages (e.g., ≥3%) may be required when applying amendments to soils to significantly immobilize Pb. However, high doses can leave an excess of P (Chen et al., 2006; Stanforth and Qiu, 2001; Tang et al., 2004), potentially leading to concerns over soil structure and eutrophication (Scheckel and Ryan, 2004; Yoon et al., 2007). In addition, many Pb-contaminated soils contain other toxic metals. For example, after Pb, As is the second most frequently occurring metal(loid) at contaminated sites in the USA (Davis et al., 2001), and As and Sb are the second and fifth most frequently occurring metals that exceed human health criteria at US Department of Defense sites (Salatas et al., 2004). The presence of co-contaminants can be of concern at firing ranges because Sb is a common alloy in Pb-based ammunition, where it is used as a hardening agent (Johnson et al., 2005). Competitive reactions between P and other contaminants such as Sb and As could potentially mobilize these contaminants and increase their availability.

Chemically, antimony (Sb) and arsenic (As) are metalloids, whereas P is a non-metal. As such, these elements may behave differently in the environment (e.g., Sb forms solid hydroxides, whereas As and P typically do not). Despite these differences, P, As, and Sb fall in the same group of elements and thus can display similar geochemical and toxicological behavior (Gal and Cuthbert, 2006; Wilson et al., 2004). For example, in oxic soil conditions, such as metal-contaminated sites where ingestion of surface soils by children is the major risk pathway, pentavalent oxyanions of P (phosphate), Sb (antimonate), and As (arsenate) are the thermodynamically stable species (Mitsunobu et al., 2005; Smedley and Kinniburgh, 2002; Zhang and Selim, 2005). Numerous studies have shown that strong reaction site competition from P increases the availability and mobility of As (Fayiga and Ma, 2006; Hongshao and Stanforth, 2001; Tao et al., 2006). An abundance of soluble P at metal-contaminated sites could create problems such as increased mobility and bioavailability of co-contaminant As and Sb.

The objectives of this study were to examine the potential relationships between Pb and co-contaminant Sb and As at small-arms firing ranges and to assess the potential for increased Sb and As mobility and bioaccessibility on the addition of P.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials
Bulk samples of Pb-contaminated surface soils were acquired from four locations in the southeastern USA: (i) an inactive small-arms federal security firing range located in eastern Tennessee, (ii) an active military small-arms firing range located in south-central Alabama, (iii) a small-arms police firing range in east-central Alabama, and (iv) a military munitions and small arms testing site in the southeast (referred to as Sites 1, 2, 3, and 4, respectively). Soil samples were collected from three locations at Site 1, three locations at Site 2, 10 locations at Site 3, and one location at Site 4. The concentrations of Pb, Sb, and As were measured for all samples, and the sample from Site 4 was used for additional experiments. The soils were typical silty-clay or silty-sand southeastern Ultisols with acidic to slightly acidic pH, high Fe-oxide contents in the subsoils (>2%), and variable quantities of organic matter depending on the soil horizon (2–3% and approximately 0.3% respectively in the A- and B-horizons).

Samples were taken from various areas at each firing range but primarily around bullet stops. Soil samples from all sites were collected from the upper 30-cm portion of the surface. Soils were sieved to <2 mm on site to remove large bullet fragments, gravel, and organic debris. Each soil sample was air dried, disaggregated, and homogenized using a mortar and pestle and sieved to <250 µm, the approximate particle size ingested by children (Hettiarachchi and Pierzynski, 2004). Samples were stored dry before use. Total soil metal concentrations were measured by USEPA Method 3050B or equivalent, a harsh acid soil digestion procedure.

Chemicals used in this research were trace-metal grade, and solutions were prepared with double-deionized water prepared with a Milli-Q system. Triple superphosphate (TSP), a common agricultural fertilizer and Pb-immobilizing agent made by reacting P-containing rock [e.g., fluoroapatite, Ca5(PO4)3F(s)] with orthophosphoric acid to form relatively soluble monocalcium orthophosphate [Ca(H2PO4)2] (Hettiarachchi et al., 2001) was obtained from a local gardening store (45% P2O5). The As and Sb concentrations in the TSP were below the detection limit (<2 mg kg–1). The Pb concentration in the TSP was 14 mg kg–1. Adding P-based amendments changes the soil's P content and potentially the soil pH, both of which can influence the mobility and bioavailability of Pb, Sb, and As. The amendments experiments described herein were designed to examine the overall effect of adding P-based amendments to soil, which are generally rototilled or mixed into the surface of the soil (Brown et al., 2004; Chen et al., 2003; Farfel et al., 2005; Ryan et al., 2004).

Laser Ablation
Several large (>2 mm) bullet fragments were collected from Site 3, split vertically with an industrial saw, and qualitatively analyzed for the presence of metals other than Pb. Bullet cores and jackets were analyzed. Bullet jackets are made primarily with a copper-based alloy that covers the bullet entirely (full metal jacket) or partially to keep the bullet intact as it exits the gun barrel; the bullet cores are primarily Pb. Chemical analysis of the bullet was performed using a New Wave UP 213 laser ablation system (New Wave Research, Fremont, CA) coupled with a PerkinElmer Elan DRC ICP–MS (PerkinElmer, Wellesley, MA), using an Ar plasma. Laser ablation products were collected along a 3-mm line (length) at a rate of 20 µm s–1 over a swath of 80 µm (width) and swept into the plasma with Ar gas. The ICP–MS operated in a simple mass scanning mode from 9 (F) to 80 (Hg) amu, providing a qualitative summary of elements present in the different parts of the bullet. Lead was not included in the scan because it is known to be present in high concentration and its analysis would swamp the detector. Ablation scans were performed on the bullet core and exterior jacket after a pre-ablation scan (120 µm swath, 5 Hz, 70 µm s–1) to remove surface contamination.

Column Leaching Tests
Column leaching tests were conducted in 1-cm-diameter glass columns at room temperature (22 ± 2°C) on the soil from Site 4. Four grams of contaminated soil was dry packed to a depth of 3.6 cm (±0.1 cm). Amended samples received a 1.0-g addition of TSP (5% P addition by weight) for a total packed mass of 5.0 g, which increased the packing depth relative to unamended controls by approximately 25%. The columns were slowly flushed from the bottom with 10–3 M CaCl2. Three different amendment methods/flow rates combinations (with one performed in duplicate to quantify variability) were tested (Table 1 ): TSP mixed and packed dry with soil at 15.3 cm h–1 infiltration rate (duplicate columns A1 and A2); TSP mixed and packed dry with soil at a 1.53 cm h–1 infiltration rate (column B); and TSP mixed with soil and water, aged for 1 wk, packed dry, and leached at a 15.3 cm h–1 infiltration rate (column C). The aging procedure for column test C was done to promote Pb–PO4 reaction before leaching might occur, for example, at a site where the amendment was rototilled into moist field soil. Control tests for each column were performed in the same manner but without TSP addition. Effluent samples were collected with a fraction collector and analyzed for pH, Sb, As, Pb, and PO4.


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  		Table 1. Quantitative analysis of column experiments for Site 4.

 
Mass balance checks on individual metals were performed by comparing the total mass leached out in the effluent plus the amount remaining in the soil at the conclusion of the experiment to the total amount of Pb, Sb, or As initially in the soil. The pre- and post- experiment concentrations of metal in soil were measured by acid digestion as described previously. Graphical integration was used to determine the total amount of metal in the column effluent. With rare exception, mass balance recoveries of 100 ± 10% were consistently achieved (Table 1).

Batch Amendment Experiments
Batch amendment experiments were conducted in 20 mL HDPE vials at room temperature (22 ± 2°C) on soils from Site 4. One gram of contaminated soil, along with 0 (control), 1, 2.5, and 5% P by weight (via TSP) were added along with 100% water content (1.0 g water per 1.0 g soil plus amendment). The samples were then roughly shaken for approximately 30 s on a Vortex shaking apparatus (Fisher Scientific, Waltham, MA). The aging process of the samples consisted of 24 h shaking at 250 rpm, after which the samples were left open to the atmosphere in a 100% humidity aging apparatus for 7 d. After aging, samples were air dried before in vitro extraction testing.

Bioaccessibility Extractions
A streamlined version (Kelley et al., 2002) of the physiologically based extraction test (PBET) originally developed by Ruby et al. (1996) was used to measure the bioaccessibility of As, Pb, and Sb in the soil. The PBET extraction was designed to simulate a fasting child's gastrointestinal tract. After aging, 0.1 g of each sample was added to a 15-mL polypropylene centrifuge tube along with 10 mL of PBET solution. The PBET solution was added at 37 ± 2°C, and the samples were immediately immersed and rotated at 30 rpm in a water bath at 37 ± 2°C. After 1 h, the samples were centrifuged for 5 min to aid with filtration and filtered into 20-mL HDPE vials with a 0.45-µm disk filter and syringe. The As, Sb, and Pb concentrations in the filtrates represented the bioaccessibility values for each respective metal.

Stock PBET solution was made using a 0.4 mol L–1 glycine (G48; Fisher Scientific) solution adjusted with trace-metal grade, 12.1 mol L–1 concentrated HCl to a pH of 1.5 ± 0.01 for As and Sb (the pH specified in the procedure) but at a pH of 2.3 ± 0.01 for Pb. Although the pH of the PBET extraction would ideally be the same for all elements, we used pH 2.3 for Pb to reflect the results of recent research indicating that an extraction pH of approximately 2.3 more accurately reflects reductions in Pb bioavailability due to P amendments (Brown et al., 2003; Brown et al., 2004; Ryan et al., 2004). However, we kept the specified pH of 1.5 for As and Sb because we had no basis for changing the extraction pH for these elements. The solution pH was adjusted at 37 ± 2°C using a pH meter calibrated with buffer solutions adjusted to 37 ± 2°C.

Analytical Methods
A PerkinElmer HGA-600 graphite furnace and a 3110 PerkinElmer atomic absorption spectrometer were used to analyze As and Sb concentrations in all aqueous samples. A Varian SpectrAA flame atomic absorption spectrometer (Varian, Palo Alto, CA) was used to analyze Pb concentrations in all aqueous samples. Phosphate concentrations in column effluents were measured with a Dionex DX-120 ion chromatograph (Dionex, Sunnyvale, CA). Column and batch aqueous samples that were not immediately analyzed were stored at 4°C until analysis.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Relationship between Lead, Antimony, and Arsenic
One of the objectives of this study was to examine the relationship between Sb and Pb at small-arm firing range sites because Sb is a common alloy in Pb bullets. Soil-metal concentrations for Sb and Pb from various surface soils taken from the firing ranges at Sites 1, 2, and 3 exhibited strong linear correlations (r2 > 0.9; P < 0.05) at all three sites (Fig. 1 ). It was not possible to assess the relationship of Sb to Pb at Site 4 because only a single sample was available from that site. Soil concentrations of As were also measured at Site 3, but no linear correlation (r2 > 0.104; P > 0.05) between As and Pb was observed at this site. The mean concentration of As for the surface soils collected at this site was 8.2 ± 0.3 mg kg–1, which is within the possible range of background (Smith et al., 1998).


Figure 1
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  		Fig. 1. Correlations between Sb and Pb at Sites 1 through 3 and As and Pb at Site 3. Strong correlations were observed between Sb and Pb at all three sites (r2 > 0.9; P < 0.05). No correlation was observed between As and Pb at Site 3 (r2 = 0.1037; P > 0.05).

 
To further examine the relationship between Pb and Sb at firing ranges, two bullets from Site 3 were collected, split with an industrial saw, and analyzed by laser ablation. As expected, the Cu-based bullet jacket showed different elemental signatures than the Pb-based bullet core. A few elements were detected in higher concentrations in the bullet core compared with the jacket, including Se, Sb, Ag, Te, and Au. Furthermore, Sb was the highest detected element in the bullet interior (excluding Pb), whereas As was detected at low levels compared with most elements. Therefore, if As was a co-contaminant in the Pb-based bullet core, it was only a minor one.

Column Leaching Tests
The mobility of Sb and As was examined by measuring the concentrations of Sb and As in leachate collected from columns of amended and unamended (control) soils. Unlike batch experiments, which have predominantly been used to study P-based Pb immobilization, column experiments indicate whether mobilized contaminants could potentially enter groundwater through the leaching processes. The mobility of Sb and As was initially expected to increase on P addition, due to the excess of soluble P, and the results confirmed this hypothesis (Table 1, Fig. 2a and 2b ). Among the three column tests, an average of 27.5% of the total Sb leached from the P-amended samples, compared with ≤2% from the unamended (control) samples. The concentrations of Sb in the initial sample (representing 6.3 cm of infiltration, approximately 1 pore volume) were very high (approximately 20 µmol L–1 in one column). Although gradually decreasing over the duration of the experiment, the concentration of Sb remained above the concentration in the effluent from the control column and the US drinking water standard. Considering the annual average rainfall infiltration in the Southeastern USA is about 630 cm (Viessman and Hammer, 1998), the >250 cm of leachate collected from each column is equivalent to approximately 2.5 yr worth of infiltration (although at a much faster rate).


Figure 2
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  		Fig. 2. (a) Antimony effluent concentration versus infiltration through amended and control columns C (Site 4). The results for columns A and B (not shown) were similar. The US drinking water maximum contaminant level for Sb (6 µg L–1) is shown for comparison. (b) Arsenic effluent concentration versus infiltration through amended and control columns C (Site 4). The results for columns A and B (not shown) were similar. The As drinking water maximum contaminant level (10 µg L–1) is shown for comparison. (c) Lead effluent concentration versus infiltration through amended and control columns C (Site 4). The results for columns A and B (not shown) were similar. The Pb drinking water maximum contaminant level (15 µg L–1) is shown for comparison. (d) Phosphate effluent concentration versus infiltration through amended and control columns C (Site 4). The results for columns A and B (not shown) were similar.

 
To assess the potential leachability of Sb, As, and Pb from the TSP, 1 g of TSP (the mass added to each column) was leached with 5 mL (the equivalent of approximately one column pore volume) of 10–3 M CaCl2 in batch mode for 48 h. These conditions reflect the maximum column effluent concentration due to the leaching of Sb, As, and Pb from the TSP. The batch TSP-leachable concentrations of As, Sb, and Pb were <5, 1.63 ± 0.06, and 0.21 ± 0.02 µM, respectively, well below the initial concentrations of these elements leached from the TSP-amended columns (Fig. 2). The consistent recovery of 100 ± 10% of the mass of As, Sb, and Pb initially present in the soils indicates that the contribution of TSP to the leachable Sb, As, and Pb concentrations were minor.

The mobility of As also increased greatly when P was applied to the contaminated soil columns (Table 1, Fig. 2b). An average of 17.9% of As leached from the amended samples, compared with an average of only 1.8% from the control columns, approximately a 10-fold increase. Like Sb, the highest As leaching occurred in the initial sample, with concentrations as high as 46 µmol L–1 (3400 µg L–1); however, As effluent concentrations declined more rapidly than the more gradual decrease observed in Sb concentrations (Fig. 2b). Nonetheless, the As concentrations measured in the effluent of the amended columns exceeded the US drinking water maximum contaminant level (10 µg L–1) for at least the first approximately 50 cm of infiltration for all columns. The source of the As in Soil 4 is unknown. At a concentration of 17.0 ± 0.7 mg kg–1, it is a little beyond the range of typical As background concentrations (≤15 mg kg–1) but well within the range of possible background concentrations (≤40 mg kg–1) (Smith et al., 1998). Furthermore, no association between Pb and As was noted in the firing range soils from Site 3. However, a column leaching experiment with a soil from Site 3 (not shown because the column subsequently clogged and the experiment was discontinued) exhibited an initial spike in As concentration to almost 20 µmol L–1 (1500 µg L–1). These results suggest that it might be possible to inadvertently mobilize As when applying P-based amendments at a Pb-contaminated site, potentially even As not associated with the Pb contamination.

As a percentage of the total concentration, much less Pb than Sb or As leached from the columns (Table 1, Fig. 2c). In fact, as a percentage of the total Pb, approximately the same amount of Pb leached from the control columns as from the amended columns, with the effluent concentration in the amended columns eventually decreasing below the effluent concentration in the control columns, reflecting the ultimate benefit of adding the P amendment. However, a significant spike (aproximately 15 µM in one of the columns) in Pb effluent concentration was observed in the effluent of at least the first sample from each of the amended columns, whereas no corresponding spike occurred in the control columns. This initial increase in Pb mobility is likely due to the initial drop in pH as a result of the dissolution of the TSP (Table 1). The initial leachate pH decreased from approximately 7 (control columns) to approximately 5 or lower in amended columns before gradually moving back toward a steady-state pH of approximately 6 to 7, depending on the column test. Adsorbed and/or solid-phase Pb may be desorbed and/or dissolved due to the initial decrease in soil pH, resulting to an initial increase in Pb mobility. Despite the initial increase in Pb mobility, the final decrease in Pb mobility and bioaccessibility may be enhanced as a result of the initial decrease in pH (Yoon et al., 2007; Zhang and Ryan, 1999a). At circumneutral soil pH conditions, the dissolution of contaminant Pb is minimal and inhibits the ultimate formation of pyromorphite. Between pH values 5 and 8, the dissolution of Pb is the rate-limiting factor in the formation of hydroxypyromorphite [Pb10(PO4)6(OH)2] (Laperche et al., 1996), with lower pH values generally resulting in faster hydroxypyromorphite formation, the desired end product.

Most of the total P added to the soil was flushed out over the course of the experiments, with the initial P concentrations being very high (approximately 0.75–1 mol L–1). The concentration of P remained above the effluent from the control columns (which remained below the detection limit) and ranged from approximately 0.25 to almost 2.0 mmol L–1 (Fig. 2d), even after >250 cm of infiltration. These P levels could cause concerns about eutrophication and continue to mobilize even background levels of As further down the soil column.

We were initially concerned that the dramatic increase in the mobility of Sb and As and large initial spike in Pb and P from Column A were due to the experimental conditions (i.e., packing the column dry and using a relatively rapid infiltration rate). However, decreasing the infiltration rate by a factor of 10 (Column B) and allowing the amendment to age in the wet soil for 1 wk before packing the column (Column C), mimicking a site where the amendment was rototilled into the soil (Farfel et al., 2005; Ryan et al., 2004), did not markedly change the results. Figure 2 shows the results from the latter scenario, where the amendment was added and aged for 7 d before packing. The results from the other columns (not shown) were similar.

After measuring the pH, the effluent samples were immediately acidified (1% nitric acid) to prevent further Pb-P precipitation from occurring in the sample vial. Effluent saturation indices (SIs) were calculated via Visual Minteq version 2.40 for the initial and final samples of Columns B and C (the Pb concentration dropped below the detection limit after aproximately 38 cm of infiltration in column C, so the last sample with detectable Pb was used to calculate the "final" SI for that column). Column B, which was leached over a 7-d duration, and Column C, where the amendment was aged in the soil for 7 d before packing, were the most likely to be at geochemical equilibrium. The initial effluents of both columns were well oversaturated with a variety of Ca- and Pb phosphates (Table 2 ). These very high SI values indicate that the samples were oversaturated with respect to Ca- and Pb phosphates (i.e., not at equilibrium) and/or possibly that some Ca- and Pb phosphates solids were being transported from the column. The first sample (6.3 cm) of Column C was analyzed before and after filtration with a 0.45-µm filter. Filtration reduced the P concentration slightly (approximately 20%) while reducing the Pb concentration by almost an order of magnitude, resulting in the filtrate being closer to but still supersaturated with respect to Pb phosphates while having a minimal impact on the SI of Ca phosphates (Table 2). Thus, at least initially, the effluent from this column was not at equilibrium, and some Pb phosphates solids were apparently being transported from the column.


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  		Table 2. Calculated saturation indices for the initial and final samples from Column B and C (Site 4).

 
As the experiment progressed and the effluent Pb concentrations dropped while the pH began to increase back toward the soil's native (control) value, these samples remained well supersaturated with respect to all the Pb-containing solids. However, the effluent had evolved considerably closer to equilibrium with PbHPO4(s) while moving only marginally closer to or further away from equilibrium with the other Pb phosphates. Both effluents remained supersaturated with respect to chloropyromorphite, even after the amendment had been in contact with the soil for 7 d. However, at least initially, some particulate Pb was being transported from the columns. In addition, Lang and Kaupenjohann (2003) recently noted that natural organic matter in soil may promote the formation and transport of Pb colloids. In terms of the increase in overall Pb mobility, whether the Pb was being mobilized as dissolved or colloidal Pb would not be significant. The geochemical equilibrium calculations for Pb, however, should be interpreted with the caveat that the calculations were based on the Pb being in the dissolved phase. Between the initial and final samples, the effluent from both columns moved closer to equilibrium with all of the Ca phosphates. In fact, the final effluent samples were very close to equilibrium to with respect to CaHPO4·2H2O and CaHPO4(s), the thermodynamically stable phase resulting from the dissolution of Ca(H2PO4)2 in 1 mmol L–1 CaCl2 solution.

Batch Amendment/Bioaccessibility Experiments
To assess potential changes in As and Sb bioavailability on amending the soils, batch experiments were conducted by adding various amounts of amendment to the soil from Site 4, which was used in the column leaching experiments. The pH in these samples decreased with increasing amendment dosage from 7.71 (control) to 5.54 (5% P) (Fig. 3 ). Adding P to soil can potentially affect the bioaccessibility of oxyanions like Sb and As in two different and opposing ways, by adding a competitive oxyanion, which would tend to increase bioaccessibility, and by lowering the soil's pH, which would tend to decrease bioaccessibility (Yang et al., 2003; Yang et al., 2002). The net direction of the effect (i.e., increasing or decreasing bioaccessibility) is probably a complicated function of the relative amounts of competitive oxyanions (Sb, As, P), adsorbents, and the change in pH.


Figure 3
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  		Fig. 3. Bioaccessibility values for As, Sb, and P versus P addition for Site 4.

 
Soil from Site 4 had average unamended bioaccessibility values of 37.5, 34.6, and 60.7% for Sb, As, and Pb, respectively. Upon the addition of P, the bioaccessibility of Pb and As decreased along with the pH, whereas the bioaccessibility of Sb increased. At the highest amendment level (5% P by soil weight), final bioaccessibility values were 52.7, 23.8, and 33.1% for Sb, As, and Pb, respectively. Similar results (not shown) were obtained from one of the soils from Site 3, indicating that although Sb and As bioaccessibility is affected on the addition of P amendments, there was a much lesser relative effect on bioaccessibility than on mobility.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
The strong linear relationship between Sb and Pb in soils samples from three firing ranges along with the high concentration of Sb in Pb bullet cores collected from one of these sites suggests that Sb can be a significant co-contaminant with Pb at firing ranges. Although As is often present with Pb and other heavy metals as a by-product (Fayiga and Ma, 2006), no relationship was observed between As and Pb at one of these sites, and As was detected at low levels compared with other elements in the bullets analyzed by laser ablation, suggesting that the presence of As is probably not associated with the presence of Pb (e.g., naturally occurring), at least at Site 3. Even a small fraction of As mobilized from the solid phase can result in a relatively large groundwater concentration (Radu et al., 2005), and in the USA it is not uncommon to find As concentrations in soils elevated above background levels, even in residential areas and public spaces (Belluck et al., 2003). Arsenic concentrations are high in groundwater at approximately 30% of contaminated sites in the USA, at least some of which is due to the mobilization of naturally occurring As due to the presence of other contaminants (Welch et al., 2000). These results suggest that the presence of As should be considered in assessing contaminated sites and potential remediation scenarios, even if As is not a known contaminant of concern.

Adding P amendments to Pb-contaminated soils may decrease the bioavailability of Pb, limiting its negative health impacts on children who consume the soil. However, the use of P as an amendment can have a significant impact on the mobility of fellow oxyanions Sb and As. The addition of a P-based amendment to a Pb-contaminated soil resulted in a tremendous increase in the mobility of Sb and As. Although Pb mobility exhibited no major changes relative to the total amount of Pb in the soil, Pb spikes observed in the initial 6.3 cm of infiltration in amended samples suggest the potential for Pb to leach further into the soil profile due to the initial decrease in soil pH due to P addition. The dissolution of native Pb-bearing minerals may be beneficial in overall Pb immobilization by increasing the kinetics of pyromorphite formation (Zhang and Ryan, 1999b). Applying a layer of rock phosphate beneath the P-amended soil at a Pb-contaminated site is a potential method of intercepting the initial increase in soluble Pb (Yoon et al., 2007), although the soil would presumably have to be removed to install such a layer, potentially mitigating some of the advantages of in situ treatment in the first place. Bioaccessibility changes in Sb and As were observed as a result of P amendments, although these changes were relatively minor (and not always detrimental) compared with the corresponding changes in mobility.

Our results complement the recent results of Spuller et al. (2007), who showed that in addition to Pb and Sb, Cu can be mobilized when adding P to Pb-contaminated soils. Together these studies indicate that the marked increase in the mobility of co-contaminants or even native elements (e.g., As) should be added to enhanced eutrophication as potential negative consequences of in situ Pb immobilization with P-based amendments.


   ACKNOWLEDGMENTS
 
Kent Hartzog assisted in preparing the final version of the manuscript. This research was sponsored by the Strategic Environmental Research and Development Program (SERDP) under the direction of Dr. Andrea Leeson.


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 Results and Discussion
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