Reducing Bioaccessibility of Lead and Arsenic in a Contaminated Urban Garden Soil.

Elevated levels of environmental contaminants such as lead (Pb) and arsenic (As) in urban soils, pose a significant problem to gardeners concerned about soil-plant-human transfer. Chemical sorbents containing high phosphorus (P) and iron oxy/hydroxides have been used to bind these contaminants rendering them immobile in the soil, and not available for plant root uptake. We investigated the effectiveness of a class A biosolid amendment (Tagro mix) and an iron oxyhydroxide (2 line-ferrihydrite) on reducing the bioaccessibility of Pb and As. An incubation study was conducted on a moderately acidic loamy sand surface soil (0-15cm) contaminated with Pb (166-172 mg/kg) and As (82-86 mg/kg), collected from an urban garden in Tacoma, WA. The experimental design was a completely randomized design with two factors [Tagro mix: 10% and 20%; Ferrihydrite (1%)] with three replication. A 200 g soil amendment adjusted to a pH of about 6.8-7.5 with calcium oxide and moistened with deionized water to 60% water holding capacity (WHC). The amendments were placed in 8 oz high density polyethylene containers, covered with clear plastic wrap and incubated for 30, 90 and 180 days in a precision low temperature incubator at 25 °C. Lead bioaccessibility was determined by a physiologically based extraction test (PBET), using a gastric solution (pH: 2.5). A two-step sequential extraction procedure (SEP) targeting the non-specifically bound and specifically bound As fractions was conducted on the soils. Preliminary results indicated that pH decreased slightly with increasing incubation time. Extractable As concentrations in Tagro (20%) + ferrihydrite amended soils were as low as 3.2 mg/kg. PBET extractable Pb was the lowest (2.95 mg/kg) for the Tagro (20%) + ferrihydrite amendments as compared to the unamended control (9.45 mg/kg).