Bioremediation of heavy metal-contaminated soils by sulfate-reducing bacteria.

Many sewage irrigation areas in China have lost their agricultural productivity because they are polluted by cadmium (Cd). Use of sulfate-reducing bacteria (SRB) may have a positive ecological effect on the bioremediation of heavy-metal Cd-contaminated soils. A five-step Tessier sequential extraction procedure to analyze the geochemical speciation of metal Cd and investigate the bioremediation efficiency by SRB was adopted. The results indicate that the SRB can reduce the concentrations of the exchangeable fraction of Cd, which can be easily absorbed by plants. The removal is approximately 70%, which demonstrates bioremediation efficiency. In the contaminated soils before bioremediation, the exchangeable fraction of Cd is the dominate carrier and the fraction of Fe-Mn oxides is next in importance, accounting for 45-60% and 35-55% of the total Cd, respectively. After adding the SRB into the water-logged soils, the concentration of different geochemical speciation of Cd was determined every 7 days for 33 days. During this period, the total concentrations of Cd did not change, while the form of Cd changed significantly. The concentrations of the exchangeable fraction of Cd decreased significantly, which shows that the Cd bioavailability in the soils markedly decreased. The concentrations of the Fe-Mn oxides fraction of Cd rose conspicuously, ranging from 60% to 120%; the concentrations of the carbonate fraction of Cd and organic matter fraction of Cd both increased slightly. The residue fraction of Cd did not change. The scope of oxidation-reduction potential was from 150 mv to -450 mv, and the rate of reduction of sulfate decreased with the increase of total Cd concentration.