The effect of inoculum on the performance of sulfate-reducing columns treating heavy metal contaminated water

Sulfate-reducing permeable reactive zones (SR-PRZs) are a passive means of immobilizing metals and neutralizing the pH of mine drainage through microbially mediated reactions. In this bench-scale study, the influence of inoculum on the performance of columns simulating SR-PRZs was investigated using chemical and biomolecular analyses. Columns inoculated from two sources (bovine dairy manure (DM) and a previous sulfate-reducing column (SRC)) and uninoculated columns (U) were fed a simulated mine drainage and compared on the basis of pH neutralization and removal of cadmium, zinc, iron, and sulfate. Cadmium, zinc, and sulfate removal was significantly higher in SRC columns than in the DM and U columns, while there was no significant difference between the DM and U columns. Denaturing gradient gel electrophoresis (DGGE) analysis revealed differences in the microbial community composition among columns with different inocula, and indicated that the microbial community in the SRC columns was the first to reach a pseudo-steady state. In the SRC columns, a higher proportion of the DGGE band DNA sequences were related to microorganisms that carry out cellulose degradation, the rate-limiting step in SR-PRZ energy flow, than was the case in the other columns. The proportion of sulfate-reducing bacteria of the genus Desulfobacterium was monitored using real-time quantitative PCR and was observed to be consistently higher in the SRC columns. The results of this study suggest that the inoculum plays an important role in SR-PRZ performance. This is the first report providing a detailed analysis of the effect of different microbial inocula on the remediation of acid mine drainage.