| The bioleaching process, which based on the catalytic oxidation and bio-acidification by sulfur-oxidizing bacteria (SOB) has been demonstrated to be a promising technology in removing heavy metals from contaminated soil. The method has advantages of high removal efficiency, mild reaction conditions, less acid consumption as well as low operation cost. But its practical application is hindered due to long leaching process with a large amount of acid wastewater containing heavy metals.This paper studied the impacts of initial pH, using sludge filtrate (SF) instead of SM medium and leaching methods (one-step and two-step leaching) on bioleaching by sequencing batch test on the shaking table. The effects of Thiobacillus and extracellular metabolic products were investigated. A 1.0 liter Stirred Tank Reactor (STR) was applied to study the performance of removal and recovery heavy metals out of acid soil leachate by mixing with the sulfide-rich effluent from sulfate bio-reduction and by gas sparging with biogenic sulfide. The main objective was to provide technology support for the practical application of soil bioleaching-bioprecipitatio. The results indicated that bioleaching of heavy metals from soil achieved highest performance at initial pH 5.0. 14 days later, pH decreased to 2.5 and the removal efficiency of Zn, Cu and Cd reached 98%, 73% and 84% respectively. Soil leaching using sludge filtrate instead of SM medium displayed that a SF/SM ratio of 30/70 had no substantial influence on the growth of Thiobacillus, acidification of soil and biooxidation of sulfur to sulfuric acid. By comparison with one-step leaching, two-step leaching (soil was added at maximum acid-production activity of Thiobacillus) could shorten the bioleaching process by 12 days. Finaly, the solublization of Zn, Cu and Cd was 81%, 82% and 98%. Besides, by comparison with the two methods of removing heavy metals from soil, chemical sulfuric acid leaching and bioleaching of sulfuric acid produced by Thiobacillus, it was discovered that O-H, peptidic bond ,C-O-C, et al. in protein and polysaccharide out of extracellular metabolic products could restrain heavy metals removal by complexation, while biomass facilitated solubilization of heavy metals. Bioleaching showed a higher performance than chemical leaching, for biomass adsorption made a more effective contribution to heavy metals removal. Experiment on heavy metals removal out of soil leachate by sulfide reduced from sulfate showed that at the 3/1 molar ratio of sulfide and metals, removal efficiency of Cu, Zn, Cd, Fe was 100%, 88%, 95% and 100%, respectively. Through the adjustment of pH and sparging of N2, Cu, Cd and Zn could be removed from soil leachate effectively. At pH of 2.0, 99% of Cu could be recovered, and the efficiency was higher with the decrease of pH, 95% of Cd and 93% of Zn could be precipited alone at pH of 3.0 and 4.0 respectively. The images of SEM-EDS and XRD showed that Cu was precipated as covellite (86.5% component was Cu and S), while Zn was precipated as wurtzite (78.3% component was Zn and S). |
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