High-iron biosolids compost-induced changes in lead and arsenic speciation and bioaccessibility in co-contaminated soils.

The safety of urban farming has been questioned due to the potential for contamination in urban soils. A laboratory incubation, a field trial, and a second laboratory incubation were conducted to test the ability of high-Fe biosolids-based composts to reduce the bioaccessibility of soil Pb and As in situ. Lead and As bioaccessibility were evaluated using an in vitro assay. Changes in Pb, As, and Fe speciation were determined on select samples after the second laboratory incubation using μ-X-ray fluorescence mapping followed by μ-X-ray absorption near-edge structure (XANES). A compost with Fe added to wastewater treatment residuals (Fe WTR compost) added to soils at 100 g kg decreased Pb bioaccessibility in both laboratory incubations. Mixed results were observed for As. Composts tested in the field trial (Fe added as Fe powder or FeCl) did not reduce bioaccessible Pb, and limited reductions were observed in bioaccessible As. These composts had no effect on Pb bioaccessibility during the second laboratory incubation. Bulk XANES showed association of Pb with sulfates and carbonates in the control soil. μ-XANES for three points in the Fe WTR amended soil showed Pb present as Fe-sorbed Pb (88 and 100% of two points) and pyromorphite (12 and 53% of two points). Bulk XANES of the Fe WTR compost showed 97% of total Fe present as Fe. The results of this study indicate that addition of high-Fe biosolids compost is an effective means to reduce Pb accessibility only for certain types of Fe-rich materials.