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ENVIRONMENTAL RISK ASSESSMENT OF PHOSPHATE-BASED REMEDIAL TECHNOLOGY IN METAL CONTAMINATED URBAN AND MINING AREAS IN A SELECTED MISSOURI SUPERFUND SITE
Impact/Purpose:
In situ immobilization of lead in contaminated soils through phosphate amendments is being tested as a cost-effective remedial alternative to safeguard human health and ecosystem from the environmental contamination. Although the phosphate treatment has been proven to effectively reduce human exposure for contaminated metals, particularly lead, the long-term environmental risk assessment of the phosphate-based remedial technology itself has not been investigated and is largely unknown. Deployment of the immobilization technology using phosphate-based materials requires a comprehensive assessment that verifies the risk reduction to human and ecosystem by in situ soil treatment is long-term and environmental-safe in order to be approved by federal and local regulatory agencies and to become publicly acceptable.
Overall goal of this proposed project is to determine whether the health risk reduction and stabilization of soil metals by in situ phosphate treatment are nearly permanent or long-term, and the impact of the soil treatment on ecosystem is minimum. The proposed research tasks include: i) Long-term bioavailability assessment that include in vitro bioavailability test, phyto-availability test, and micro-toxicity test; ii) Leachability/Stability assessment under various chemical and biological conditions; iii) Identification of chemical species responsible for metal or phosphate stability and mobility; iv) Evaluation of soil microbial community alteration upon the soil treatment; and v) Long-term monitoring of water quality upon the soil treatment. This project will combine both field and laboratory investigations, and primarily focus on two pilot field treatment sites that have been established by the Missouri Department of Natural Resources and are located in the urban and mining areas, respectively, in the Jasper County Superfund Site, Missouri.
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In situ immobilization of lead in contaminated soils through phosphate amendments is being tested as a cost-effective remedial alternative to safeguard human health and ecosystem from the environmental contamination. Although the phosphate treatment has been proven to effectively reduce human exposure for contaminated metals, particularly lead, the long-term environmental risk assessment of the phosphate-based remedial technology itself has not been investigated and is largely unknown. Deployment of the immobilization technology using phosphate-based materials requires a comprehensive assessment that verifies the risk reduction to human and ecosystem by in situ soil treatment is long-term and environmental-safe in order to be approved by federal and local regulatory agencies and to become publicly acceptable.
Overall goal of this proposed project is to determine whether the health risk reduction and stabilization of soil metals by in situ phosphate treatment are nearly permanent or long-term, and the impact of the soil treatment on ecosystem is minimum. The proposed research tasks include: i) Long-term bioavailability assessment that include in vitro bioavailability test, phyto-availability test, and micro-toxicity test; ii) Leachability/Stability assessment under various chemical and biological conditions; iii) Identification of chemical species responsible for metal or phosphate stability and mobility; iv) Evaluation of soil microbial community alteration upon the soil treatment; and v) Long-term monitoring of water quality upon the soil treatment. This project will combine both field and laboratory investigations, and primarily focus on two pilot field treatment sites that have been established by the Missouri Department of Natural Resources and are located in the urban and mining areas, respectively, in the Jasper County Superfund Site, Missouri.
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Description:
This project provided important data on fundamental processes responsible for health and environmental risk reductions and environmental safety of the phosphate-based treatments in metal, specifically Pb, contaminated soils. By an integrated approach of environmental risk assessments on phosphate-treated mining sites, in situ soil treatments using phosphate amendments were documented to effectively reduce health and environmental risks of Pb-contaminated soils under field conditions through immobilization reactions. The efficacy of the risk reduction was long-term. Such risk reduction was primarily achieved by transforming labile metal species to relatively stable compounds as induced by the soil treatments. Phosphate-immobilized metals such as lead phosphates were chemically and biologically stable under the surface soil conditions. As a result of the treatments, the potential of soil metals contributing to surface or ground water was minimized. The phosphate treatments led to a minimal elevation of phosphate in water and had no adverse impacts on soil microbial community. These results suggest that in situ soil treatments using phosphate-based amendments would result in a long-term risk reduction of metal-contaminated soil and are environmentally safe, which could be potentially a practical, cost-effective remedial alternative to safeguard human and ecosystem from Pb contamination in moderate-elevated soils. Major findings from this project lead to three publications in refereed journals and two graduate thesis. More manuscripts are currently in preparation. The work has also been presented in a variety of invited and contributed talks by the PI, postdocs, or graduate students at international or national conferences.
Health and ecological risk assessment
Health and ecological risk assessment was performed by in vitro bioavailability (bioaccessibility) test, phyto-availability test, and micro-toxicity test. In vitro bioavailability is an estimate of metal bioavailability using a chemical extraction procedure that simulates metal dissolution in gastrointestinal tract, representing a potential risk to human health. Measurements indicated that in vitro Pb bioavailability of the P-treated soils was significantly, constantly lower than non-treated soils, regardless of soil matrixes, application methods, and phosphate amendments. The reductions were at a range of 60 to 80%. Among three application methods (rototilling, surface application, and pressure injection), the rototilling was most effective in mixing added phosphate with soil, resulting in a homogeneous distribution of phosphates in treated zone and achieving the highest reduction of in vitro Pb bioavailability. The reductions appeared increasing with higher amounts of phosphate applied. It was observed that the treatments also reduced, to some extent, in vitro Pb bioavailability of subsoil below the treated zone, which could be attributed to leached phosphates that reacted with subsoil Pb and stabilize soil Pb through the immobilization reactions. Among three sites, soil treatment using phosphoric acid at a rate of 1% resulted in higher reductions of the in vitro bioavailability in mill waste than urban soil as both compared with the control, and phosphate-enrich biosolids were equally effective as soluble phosphates in mine waste. Physical properties of soil matrix, level of contamination, and metal species or lability could be factors affecting the treatment efficacy on various soil matrixes. Although consistent reductions were achieved by the soil treatments in three soil matrixes, a temporal variation of in vitro bioavailability in soils was measured. However, the variation was consistent with a trend in both treated and non-treated soil, and the reductions by the treatments remained relatively constant through years.
Metal analyses of plant tissue grown in the sites demonstrated that the soil phosphate treatments induced lower tissue Pb concentrations, and the concentration was well correlated with in vitro bioavailable Pb in soil, with a R2 of about 0.7. This suggests that reductions of metal uptake by plants were primarily due to immobilization reactions induced by the treatments. The treatments induced the transformation of soil Pb to stable species, which reduced solubility of soil metals and availability to plant uptake. Measurements of micro-toxicity, an estimate of potential toxicological effect to microbial community, were found to present a similar pattern responding to the soil treatment as the plant uptake, in which lower micro-toxicity was observed in P-treated soils. Similarly, the reduction of soluble metals by the soil treatments would be primarily responsible for less toxicological effect to soil microbial community or more suitable environmental conditions for microbial growth.
Metal stability assessment
Metal stability assessments are essential for verifying the stability of metal phosphates formed after immobilization reactions and long-term treatment effect. The assessments were conducted by ex situ leachability test and simulated column leaching procedures. Ex situ leachability, an estimate of maximum amount of soil leachable metals and potential risk to surface or ground water quality, showed that the leachable Pb was significantly lower in P-treated soils than control, as a result of the treatments. The reductions were ranged from 30-70% depending on soil matrixes, although temporal variations were observed during the sampling periods. Moderate elevations of leachable P in treated soils were also measured, varying with amounts of phosphate applied. The P-enrich biosolid treatments in mine waste produced the least increase of leachable P in three soil matrixes. Immobilization reactions that transformed soil Pb to relatively stable species by the treatments could account for reduced Pb leachability due to low solubility of Pb compounds formed after treatments.
Simulated column leachings using both toxicity characteristic leaching procedure (TCLP) and precipitation simulated leaching procedures (PSLP) presented a similar trend as to the leachability test, but with much lower aqueous Pb and P concentrations than expected. Aqueous Pb and P in the leachates were both below the EPA criteria of water quality (1 and 10 ppm, respectively). The column leachings also indicated that aqueous Pb and P in leachate reached at the maxima after three pore volume of water and then gradually decreased as more water applied. In addition, the leaching study illustrated that aqueous Pb and P in leachate had not been enhanced at presence of plant growth. This suggest that plant roots played a very limited role in remobilizing soil metals, and the species of metal phosphates once formed were relatively stable under various chemical and biological conditions of surface soil. Under field conditions, much low Pb and P leaching would be anticipated because of surface interactions and adsorption by soil particles or minerals along the leaching pathway to deep ground.
Chemical speciation assessment
Chemical speciation of soil metals would provide an insight of immobilization reactions that verifies metal transformation and account for mechanisms responsible for metal risk reduction by the phosphate treatments. Chemical speciation was performed by sequential extraction procedures targeting five chemical groups from the most labile to least soluble forms and by solid-phase electron beam microprobe analyses with scanning microscopy (SEM) in conjunction of energy- or- wavelength-dispersive microspectroscopy (EDS or WDS). Fractionation analyses by sequential extraction procedures revealed the fraction of water soluble Pb (highly soluble form) present in studied soils, in either P-treated or control soils, were very limited. The fractions of exchangeable and carbonate metals (soluble or highly bioavailable form) were dominate species in control soils, accounting for about 50% of the total. After the phosphate treatments, there was a substantial increase (100-200%) of the residue fraction (the least soluble form) in the soils as compared with control, which accounted for 70-80% of the total. This study demonstrated that the immobilization reactions were carried out by treatment-inducing transformation from labile soil Pb (exchangeable and carbonate forms) to less soluble or less bioavailable forms (residues) while the fractions of oxide- and organic metals remained relatively constant. The transformations would primarily be responsible or account for the reductions of the Pb bioaccessibility, plant uptake, micro-toxicity, and leachability, which potentially lowered the risks to human health and ecosystem.
Electron-beam microprobe qualitative analyses confirmed that Pb-bearing solids in the P-treated soil particles contained P or P and Cl in the composition, an indication of the presence of lead phosphates or pyromorphites. Formation of lead phosphates or pyromorphites induced by the phosphate treatments would be a major contributor to the increased residue fractions. An additional study was conducted with focusing the reaction mechanisms of labile Pb pure compounds (PbCO3 and PbO) with phosphoric acid, in which initial formation of pyromorphites was found kinetically rapid, and then the rate was substantially reduced during four-week period of the reactions. Intermediate lead phosphates were identified on the reaction pathway of transformation to pyromorphites, which probably was responsible for limited formation of pyromorphites post the initial reactions. The formed lead phosphates could be accountable for incomplete transformation of soil Pb or partial formation of pyromorphites previously reported and might also explain the discrepancy between in vivo and in vitro measurements of soil Pb bioavailability.
Microbial community assessment
Soil microbial diversity and growth is an indicator of soil health. A microbial assessment was conducted to determine ecological safety of the phosphate treatments, in other words, whether the soil treatments have any adverse impacts on soil microbial properties or whether soil sustainability and productivity are maintained by the treatments. The assessment included microbial diversity or community analyses, microbial biomass measurements, and selected enzyme activity test. Microbial diversity analyses by the denatured gradient gel electrophoresis (DGGE) showed highly diverse microbial community in all soils, containing over 20 bands (bacterial genotypes) in each soil. As a result of the phosphate treatments, higher diversity microbial community was observed in P-treated soils as indicated by the band numbers. The UPGMA analyses demonstrate that P-treated soils contained significantly different DNA sequences from control soils, implying the variation of microbial species in two soils. Data suggest that the diversity enhancement by the phosphate treatments might result from soil conditions more appropriate for microbial growth and diversity. Enhanced microbial diversity was probably attributed to lower Pb solubility or toxicity from phosphate treatment. Greater numbers of bands were observed in summer season. Most dominant (dense) bands were common among all the soils regardless of P treatment. As identified by BioLog analyses, Brevundimona svesiicularis, Burkholderia gladioli, Burkholderia glumae, and Pseudomonas chloroaphis were dominate four species in soils, but the quantity varied with soil matrix and treatments. The presence of common bands across treatments would suggest that P treatment has limited effect on soil bacterial activity. Combined with the decrease in lead hazard due to the phosphate treatment, the application of phosphorus can be considered beneficial.
As an estimate of microbial biomass, total organic carbon (TOC) and total nitrogen were found varying over time in soils, with a slightly more rapid increase in P-treated soil (78%) versus control (60%). Similar trends were observed for the enzymatic phosphatase activities, with a 15% increase in activity for the treated soils versus as a 13% increase for the control soil. For the soils analyzed, the carbon to nitrogen ratio (C:N ratio) averaged between 15:1 and 25:1, indicating that microbes were at no time stressed for either carbon or nitrogen. The similarity in total organic carbon (TOC), total nitrogen (TN) and phosphatase activity for both the treated and control soils suggest that the treatment does not negatively affect the original microbial population.
Water quality monitoring
Surface waters were collected by a lysiometer installed below treated zone of selected plots while ground water collected from five wells near mine waste treatment site. Due to droughts during the sampling seasons, limited samples were collected. Similar to in vitro Pb bioavailability, in all cases, aqueous Pb concentrations in both surface and ground waters were significantly reduced by the phosphate treatments. 30% and 44% reductions by the H3PO4 treatment were achieved in the surface water of the urban soil and mill waste, respectively, and the reductions by the biosolid treatments in the mine waste were comparable. Aqueous Pb concentrations were found to decrease linearly with decreasing in vitro bioavailable Pb of the soil, accounting for 69% of aqueous Pb variance. Aqueous Pb levels met the EPA regulatory criteria. As expected, ground waters contained about half of aqueous Pb as in surface waters, which might result from adsorption of leachable Pb by soil components such as clays or Fe/Mn oxides during the pathway to ground water.
Aqueous phosphorus was environmentally concerned due to the potentials of the P enrichment in surface water or eutrophication. Aqueous P in waters indeed increased as a result of phosphoric acid treatments, measuring ~10 mg P L-1 of H3PO4-treated soil vs. ~2 mg P L-1 of control in surface water , aqueous P levels in both surface and ground waters of biosolid-treated mine waste were even lower. Data suggested that soluble P treatments would be primarily responsible for enhanced P leaching, and the P enrichment in waters was limited or minimal. Lower than expected P leaching under field conditions could be due to plant uptake or interactions of soil minerals during the pathway to ground. Water ecotoxicity, an estimate of toxicological effects of water to microbes, was significantly improved as a result of reduced aqueous Pb. Both in vitro bioavailable Pb and aqueous Pb appeared to influencing the ecotoxicity of water, accounted for 66-70% of ecotoxicity variance, suggesting that improvements of water ecotoxicity were primarily achieved by reduced aqueous Pb through phosphate-induced Pb immobilization in soil, which made aqueous environment less toxic and more suitable for microbial growth.
Overall, this project provide important insights and evidence verifying that the health and ecological risk reduction by in situ phosphate-based treatments in metal-contaminated soil would be long-term and environmentally safe, which could be applicable for remediation or site restoration of moderate contaminated soil or land. Soil treatments combined soluble P with biosolids are recommended in context of maximum risk reduction and minimal P enrichment in water.
URLs/Downloads:
URL2005 Progress Report
Final Progress Report
2006 Progress Report
2004 Progress Report