The effects of biochar and redox conditions on soil Pb bioaccessibility to people and waterfowl
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.