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The effects of biochar and redox conditions on soil Pb bioaccessibility to people and waterfowl
Citation:
Plunkett, S., C. Eckley, T. Luxton, AND M. Johnson. The effects of biochar and redox conditions on soil Pb bioaccessibility to people and waterfowl. CHEMOSPHERE. Elsevier Science Ltd, New York, NY, 294:133675, (2022). https://doi.org/10.1016/j.chemosphere.2022.133675
Impact/Purpose:
Biochar can improve plant growth and reduce metal bioavailability to plants in metal-contaminated soil but the ability of biochar to reduce gastrointestinal bioaccessibility of heavy metals to people and waterfowl is not known. In this project, lead (Pb) contaminated soil from three sites with distinct contaminant legacies and hydrology (well-drained upland soils and submerged wetland soil) were amended with 2% biochar (17 unique chars) and incubated for six months. Results show that the impact of biochar on Pb bioaccessibility varies between upland and wetland soils. In upland soils, select biochar amendments improve nutrient availability and reduce soil Pb gastrointestinal bioaccessibility to some extent (e.g. 1758 vs 1504 mg kg-1 bioaccessible Pb pre vs post amendment). In the water-saturated wetland soil, Pb bioaccessibility doubled, on average, for both treated and control samples over the course of the experiment. The increase is a result of low solubility (low toxicity) reduced Pb species in the wetland oxidizing to more soluble (and more toxic) forms of Pb upon being exposed to the air. These results demonstrate how remediation efforts intended to reduce Pb toxicity could inadvertently exacerbate the hazard if they aerate soils with reduced Pb species (e.g. by mixing or draining sediment etc.). We expect these findings to inform remediation strategies for redox-sensitive media like wetlands. This research seeks to inform federal, state and tribal agencies charged with cleaning up Pb-contaminated sites, including the EPA, by illustrating the remedial potential as well as the limitations of using biochar to reduce soil Pb toxicity, and by suggesting adaptations to improve risk assessment in redox-sensitive media like wetland soils.
Description:
Biochar can improve soil fertility and reduce lead (Pb) bioavailability to plants in metal-contaminated soil, but the ability of biochar to reduce gastrointestinal bioavailability of soil Pb remains unknown. In this study, 17 biochars were evaluated as in situ amendments for three soils with distinct sources of Pb contamination (smelter emissions, ceramics waste, mining waste), hydrology (upland, well-drained soil vs submerged wetland soil), and receptors (human vs waterfowl). Biochars were made from blends of 30% manure (poultry litter or dairy manure) and 70% lignocellulosic material (wheat straw or grand fir shavings) and pyrolyzed at 300, 500, 700, and 900° C. Soils were amended with 2% biochar (w/w) and incubated for 6 months. A suite of standard (e.g. EPA Method 1340) and experimental soil Pb bioaccessibility assays were used to assess the impact of treatment on Pb bioaccessibility. Biochar amendments to upland soils were associated with modest reductions in Pb bioaccessibility with a maximum reduction of 254 mg kg-1 bioaccessible Pb (from 78 to 68% bioaccessibility as a percent of total) according to EPA Method 1340 pH 2.5. However, no treatment in this study reduced Pb bioaccessibility to a threshold that is protective of human health (< 400 mg kg-1 bioaccessible Pb). In the aqueous wetland soil, sample redox status had a greater impact on Pb bioaccessibility than any amendment. Low-solubility Pb sulfides in this soil oxidized over the course of the study and no treatment was able to offset the increase in Pb bioaccessibility caused by oxidation. Remediation efforts in soils with reduced Pb species may inadvertently exacerbate Pb bioaccessibility if they alter redox status (i.e., oxidize reduced Pb species) to favor more soluble Pb forms. The impact of redox status on Pb bioaccessibility was only evident when soil bioaccessibility assays were adapted to preserve sample redox status. Pb bioaccessibility estimates for wetlands and other low-oxygen environments must maintain low-oxygen sample processing and analytical conditions to avoid severely distorting the results.
URLs/Downloads:
DOI: The effects of biochar and redox conditions on soil Pb bioaccessibility to people and waterfowl