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

Biochar can reduce lead (Pb) bioavailability to plants in metal-contaminated soil, but the ability of biochar to reduce the bioavailability of soil Pb to people and wildlife 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 biological 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 the treatments. The results showed that biochar amendments to upland soils resulted in modest reductions in gastrointestinal Pb bioaccessibility (maximum reduction from 78 to 68% bioaccessibility as a percent of total, EPA Method 1340 at pH 2.5). In the 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 this oxidation. The impact of redox status on Pb bioaccessibility was only evident when soil bioaccessibility assays were adapted to preserve sample redox status. This result highlights the importance of maintaining in situ redox conditions when processing/analyzing samples from low-oxygen environments and that soil remediation efforts should consider the role of redox conditions on Pb bioaccessibility.

Use of Routine Soil Tests to Estimate Pb Bioaccessibility.

Soil lead (Pb) hazard level is contingent on bioavailability, but existing assays that estimate Pb bioavailability for human health risks are too expensive or otherwise inaccessible to many people that are impacted by Pb-contaminated soil. This study investigated the use of routine soil nutrient tests to estimate soil-Pb bioaccessibility as a surrogate measure of Pb bioavailability. A silt loam soil was spiked to a target concentration of 2000 mg Pb kg-1 with Pb(NO3)2 and amended with H3PO4 (varying P-to-Pb molar ratios) and KCl (Cl-to-P molar ratio of 2:5) to generate soils with similar total Pb concentrations but a range of Pb bioavailability (and bioaccessibility). Soils were extracted using Mehlich 3, Mehlich 1, Bray P1, Olsen, and  micronutrient (DTPA) methods, and the results were compared to U.S. Environmental Protection Agency method 1340 data as well as to extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mehlich 3 and method 1340 treatment effect ratios were well-correlated ( r2 = 0.88, p ≤ 0.05), whereas Bray P1, DTPA, and Olsen results were more reflective of EXAFS data. Preliminary animal-feeding trials suggest that the Mehlich 3 is as effective as method 1340 at predicting the impact of P treatment on Pb relative bioavailability; however, both methods over-estimated the Pb hazard to mice in P-amended soil. Other routine soil tests that have heightened sensitivity to P amendment (e.g., Bray P1) may be promising candidates for Pb bioaccessibility assessment.