A combination of bioleaching and bioprecipitation for deep removal of contaminating metals from dredged sediment.

A linked microbial process comprising bioleaching with sulfate-oxidizing bacteria and bioprecipitation with sulfate-reducing bacteria operating sequentially was investigated to deeply remove contaminating metals from dredged sediment. The results showed that sediment bioleaching resulted in a sharp decrease in sediment pH from an initial pH ∼ 7.6 to pH ∼ 2.5 within 10-20 days, approximately 65% of the main heavy metals present (Zn+Cu+Cr) were solubilized, and most of the unsolubilized metals existed in residual form of sediment. The acidic leachate that resulted from sediment bioleaching was efficiently stripped of metal sulfates using a bioprecipitation reactor when challenged with influent as low as pH ∼ 3.7. More than 99% of Zn(2+), 99% of Cu(2+) and 90% of Cr(3+) were removed from the leachate, respectively, due to the formation of ZnS, Cu(2)S and CrOOH precipitates, as confirmed by SEM-EDS and XRD detection. It was also found that alkalization of bioleached sediment using Ca(OH)(2) excluded the risk of sediment re-acidification. The ability of the combined process developed in this study to deeply remove heavy metals in insoluble sulfides or hydroxides forms makes it particularly attractive for the treatment of different types of metal contaminants.