Strategies to maximize the microbial leaching of lead from metal-contaminated aquatic sediments

Different strategies designed to increase the removal of Pb were tested during the application of a biological remediation procedure to treat highly-contaminated aquatic sediments. The use of FeCl2 instead of FeSO4•7H2O as a substrate for thiobacilli bacteria did not interfere with the biological solubilization process which occurred in sediments acidified to pH 4 with H2SO4. With FeCl2, twice as much Pb was solubilized (5 mg Pb l−1) compared to the same strain acclimated to FeSO4•7H2O. With FeCl2, the solubilization of other trace metals (Cu,Zn) was similar to that obtained with the strain acclimated to FeSO4•7H2O. If the sediments were acidified with HCl, rather than H2SO4, then the solubilization of Pb increased to 11 mg Pb l−1: this was five times greater than when a strain acclimated to FeSO4•7H2O was used in sediments acidified with H2SO4. Theoretical calculations with MINEQL+ (chemical equilibrium program) predicted 10.8 mg Pb l−1 under similar conditions. This biological solubilization process was generally poorly adaptable to changes in the source of acidity (i.e. HCl vs H2SO4). The solubilization of Cu remained relatively low (solubilization of only 35% after 72 h) since the inadequately low levels of sulphate prevented the Thiobacillus bacteria from properly developing. Finally, the higher concentration of chloride ions gained through an increased level of total solids (7% rather than 3% TS) did not increase the soluble concentration of Pb. The soluble concentration of Pb remained at 11 mg Pb l−1 (46%), whereas theory predicted a concentration of 24.1 (100%) mg Pb l−1. Most of the experimental results seemed to follow the theoretically predicted outcome for Pb solubilization. There is more work to do to optimize the process. However, this work reveals that it seems possible to develop an economical process for the removal of Pb from contaminated sediments.