Integrated Plume Treatment Using Persulfate Coupled with Microbial Sulfate Reduction

The integration or sequential use of different remediation technologies, also referred to as a combined remedy, has become an emerging strategy for the treatment of contaminated sites. Coupling chemical oxidation using persulfate with enhanced bioremediation (EBR) under sulfate reducing conditions is a plausible combined remedy. To characterize the role of the mass removal processes (e.g., chemical oxidation vs. sulfate reduction) and to quantify the impact of persulfate on indigenous microbial processes in a combined persulfate/EBR treatment system, a pilot-scale field experiment was conducted in a 24-m long sheet pile-walled gate over a period of approximately 400 d. After dissolved benzene, toluene, and o-xylene (BTX) quasi steady-state plumes were developed, two persulfate injection episodes were performed 10 d apart to create a chemical oxidation (ChemOx) zone. High-resolution monitoring was conducted to observe the migration of the ChemOx zone and transition into an EBR zone. Mass loss estimates and geochemical indicators were used to identify the distinct transition between the ChemOx and enhanced biological reactive zones. Compound specific isotope analysis (CSIA) was used to distinguish the dominant mass removal process, and to investigate the occurrence of microbial sulfate reduction. BTX metabolites and reverse-transcriptase quantitative polymerase chain reaction analyses of expressed biodegradation genes (as mRNA) were also used to characterize the response of indigenous microorganisms (especially sulfate-reducing bacteria) to the added persulfate. Multiple lines of evidence supported the conclusion that chemical oxidation was the dominant mass removal process in the vicinity of the injection zone, while enhanced biodegradation dominated BTX degradation in the downgradient portions of the system. The CSIA and supporting molecular biological data were critical in documenting temporally and spatially distinctive zones in this system that were dominated by either chemical-oxidation or anaerobic-biodegradation processes. Initially, persulfate had an inhibitory impact on the activity of the indigenous microbial community, but this was followed by a substantial rebound of microbial activity to above baseline levels. The results from this investigation demonstrate that the suite of diagnostic tools employed can be used to distinguish between chemical oxidation using persulfate and the subsequent effects of the produced sulfate.