Biological processes: The effects of initial pH, percentage inoculum and nutrient enrichment on the solubilization of sediment bound metals

Abstract

Biological solubilization of metals in highly contaminated sediments (Lachine Canal, Montreal, P.Q., Canada) was tested in 500 mL batches. The biological process uses the leaching capacity of Thiobacilli. Batch experiments were performed to determine the influence of initial pH, the percentage of inoculum, and the addition of nitrogen and phosphate on the efficiency of the process. Similar metal recoveries were obtained at either of the initial pH values (pH 4.0 and pH 4.5). The addition of 20% inoculum (v/v) appears to result in acceptable yields over a short time period (24–48 hours). Solubilization of Cu is strongly correlated with the presence of bacteria (r² increase with time up to 0.90 after 53h). In the case of Zn (within first 6 hours) and Pb (at the beginning), the chemical environment appears to be the main factor controlling solubilization of these elements (r² up to 0.99). The addition of nutrients had no affect on the production of Thiobacilli, but the addition of NH₄ reduced the solubilization of Zn, Pb (r² up to 0.90) and at the beginning, the solubilization of Cu (r²=0.67). While the addition of PO₄ ³⁻ diminished the solubilization of Zn, Pb and Cu (r² up to 0.96). Upon addition of substrate (FeSO₄.7H₂O), the growth of Thiobacilli already present in the sediments is favored. The application of this process to sediments appears feasible, as Zn and Cu levels were at acceptable levels following treatment. Further studies are necessary to improve the removal of Pb.