The immobilization of heavy metals in biosolids using phosphate amendments--comparison of EPA (6010 and 3051) and selective sequential extraction methods.

Biosolids are organic by-products from wastewater treatment plants that contain valuable nutrients but also represent a pollution problem. The amount of heavy metals in biosolids at high concentrations prevents their use as fertilizers. The aim of the present study is to immobilize heavy metals in biosolids using phosphate amendments. Analytical-grade potassium dihydrogenphosphate (KH(2)PO(4)) was used as the source of phosphate, and its ability to reduce heavy metals in biosolids (200 g) was evaluated according to five additional phosphate quantities: Control (C) (without phosphate), P1 (1.45 g), P2 (2.9 g), P3 (5.8 g), P4 (10.15 g), and P5 (14.5 g). The effect of phosphate treatments on metal extractability was evaluated using the selective sequential extraction (SSE) method, and total metal concentrations were analyzed by the (US Environmental Protection Agency) EPA 6010 and EPA 3051 methods. The total and extractable level of metals showed that Cu, Zn and Pb were the predominant elements in the biosolids analyzed in this study (control). SSE results showed that, on average, over 80% of the total metals present in the biosolids were found in the organic (F4) and residual (F5) fractions for Cu and Zn; Fe-Mn oxide (F3), organic (F4) and residual (F5) fractions for Cd; exchangeable (F1) and residual (F5) fractions for Pb; and exchangeable (F1), organic (F4) and residual (F5) fractions for Ni. The addition of phosphate substantially decreased the amount of metals (Cu, Zn, Cd, Pb, and Ni) in biosolids after a 10-day period; however, the amount of removal varied between the examined analytical techniques.