A microcosm study on persulfate oxidation combined with enhanced bioremediation to remove dissolved BTEX in gasoline-contaminated groundwater.

The combination of persulfate (PS) oxidation with enhanced bioremediation (EBR) is a potential trend in remediating organic-contaminated groundwater. However, the impacts of PS on EBR presented in the transition zone between PS oxidation zone and EBR zone need further study. To better characterize the impacts and provide available indicators, PS oxidation and EBR with nitrate amended were performed through the microcosm experiments to remove dissolved benzene, toluene, ethylbenzene and xylene (denoted as BTEX) in gasoline-saturated groundwater. The results indicated that PS oxidation combined with EBR almost completely removed BTEX with the ratio of > 93% over the experiments, which is better than PS oxidation (54-97%) but still worse than EBR (100%). The removal velocities of BTEX in EBR, PS oxidation, and PS oxidation combined with EBR were 0.94, 0.1-0.16, and 0.1-0.54 mg/L/d, respectively. High concentration of PS, along with high-strength activation, made the pH decrease to 3.3-4.4 and the Eh increase to 141-203 mV, thus greatly inhibited microbial activities as well. In such circumstances, oxygen and nitrate could not be significantly used as electron acceptors by microbials. To reduce the impacts of PS oxidation on EBR, the PS/BTEX molar ratio of < 6 and the PS/Fe2+ molar ratio of > 1 may be appropriate in transition zone. The hydro-chemical indicators, including pH, Eh, and availability of electron acceptors such as oxygen and nitrate, could reflect the impacts of PS oxidation on bioprocesses. During in-situ chemical oxidation (ISCO), PS injection and PS activation by Fe2+ should be managed for decreasing the impacts on EBR, based on the PS/BTEX and PS/Fe2+ molar ratios.