Use of Urban Residuals to Reduce Pb and As Bioaccessibility.

The safety of urban farming has been questioned due to the potential for contamination in urban soils.  A laboratory incubation, field trial and 2^nd 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 2^nd laboratory incubation using µ- XRF mapping followed by µ-XANES. A compost with Fe added during wastewater treatment (Fe WTR compost) added to soils at 100 g kg^-1 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 FeCl₂) did not reduce bioaccessible Pb with limited reductions observed in bioaccessible As.  These composts also had no effect on Pb bioaccessibility during the 2^nd laboratory incubation. Bulk XANES showed association of Pb with sulfates and carbonates in the control soil. µ- XANES  for 3 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 Fe³⁺. 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.