Persulfate Oxidation Coupled With Microbial Sulfate Reduction as a Combined Remedy
Shayan Moghadam, Mahsa
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Groundwater contamination by petroleum hydrocarbon (PHC) compounds including the high impact and persistent aromatic compounds such as benzene, toluene, ethyl benzene and xylene (BTEX) poses serious risk to human health and the environment. The coupling or sequential use of different remediation technologies, also referred to as a “treatment train”, has become an emerging strategy for the treatment of PHC-contaminated sites. Minimizing clean-up cost and time as well as maximizing the overall treatment efficiency are the primary goals of combined remedies. Coupling in situ chemical oxidation (ISCO) and enhanced bioremediation (EBR) is an example of a plausible treatment train. The general concept behind an integrated ISCO/EBR system is the use of chemical oxidation to target the bulk of the contaminant mass near the source, followed by the enhancement of biological processes to “polish” the remaining mass in the source and the downgradient plume. Persulfate (S2O82-) is a persistent but yet aggressive oxidant which can rapidly destroy a wide variety of PHC compounds. Persulfate degrades complex organic compounds into simpler and more bioavailable organic substrates and produces sulfate, an electron acceptor. The anaerobic environment that is created is ideal for sulfate reduction to be enhanced. Therefore, enhanced bioremediation under sulfate reducing conditions is expected to dominate the mass removal processes following the consumption of persulfate. To assess the viability and performance of a persulfate/EBR treatment train, the role of the intertwined mass removal processes (e.g., persulfate oxidation vs sulfate reduction) and the impact of persulfate on indigenous microbial processes need to be quantified. Hence, a pilot-scale trial was conducted in a 24 m long experimental gate at the University of Waterloo Groundwater Research Facility at CFB Borden over a period of 13 months. After a quasi steady-state plume of dissolved benzene, toluene and xylene (BTX) was established in the gate, two persulfate injection episodes were conducted to create a chemical oxidation zone. As this chemical oxidation zone migrated downgradient it was extensively monitored as it transitioned into an enhanced bioremediation zone. Mass loss estimates and geochemical indicators were used to identify the distinct transition between the chemical oxidation and enhanced biological reactive zones. Compound specific isotope analysis (CSIA) was used to delineate the dominant mass removal process, and to track the fate of the sulfate. Molecular biology tools, including specific metabolite detection and quantitative polymerase chain reaction analysis were used to understand the effect of persulfate on the population and activity of the indigenous microorganisms with a focus on the SRB community. A modelling tool was developed to simulate the coupled processes involved in a persulfate/EBR treatment train, and to quantify the impact of various parameters on the performance of this treatment system. The existing BIONAPL/3D model was enhanced (BIONAPL/PS) with the capabilities of simulating the majority of processes involved in a persulfate/EBR treatment train including: density-dependent advective-dispersive transport, persulfate decomposition, sulfate production, chemical oxidation, and biodegradation of PHC compounds under various redox conditions. The BIONAPL/PS model formulation was validated against observations from a series of column experiments designed to mimic various phases of a persulfate/EBR treatment train, and then was used to capture the observations from the pilot-scale trial. This latter effort was aimed to evaluate the model capability to simulate a complex system with multiple components within a dynamic flow system. The modelling tool was also used to evaluate options for performance optimization. Multiple lines of evidence from the pilot-scale trial confirmed that the BTX plume was degraded with this persulfate/EBR treatment train (>70% BTX mass removed). Chemical oxidation was the dominant mass removal process in the vicinity of the persulfate injections (i.e., ChemOx zone), whereas enhanced bioremediation (including enhanced microbial sulfate reduction and methanogenesis) dominated BTX degradation in the downgradient portions of the experimental gate (i.e., the EBR zone). The transformation of the ChemOx zone into the EBR zone was also observed following depletion of persulfate from the system. The population and activity of SRB communities which were temporarily inhibited in the ChemOx zone immediately after persulfate injection, rebounded and increased by three (3) orders of magnitude after persulfate depletion. This significant enhancement in the microbial population was linked to increased sulfate concentrations, and the breakdown of complex substrates into simpler, more bioavailable compounds. The data also demonstrated that once flow in the experimental gate was stopped, the activity and population of the SRB community decreased as a result of the lack of sulfate, and methanogenic activity increased. In general, the data collected confirmed that the activity of both SRB and methanogens was enhanced under the geochemical conditions created following persulfate injection. BIONAPL/PS provided a suitable platform in which the complex processes involved in a persulfate/EBR treatment train could be captured including the degradation of PHC compounds following persulfate injection, formation of ChemOx and EBR zones, depletion of persulfate, and the generation and consumption of sulfate. Benchmarking of BIONAPL/PS against data from the pilot-scale trial highlighted the impact of persulfate on the subsequent sulfate reduction process. It was shown that aerobic degradation and sulfate reduction acted sequentially as the dominant mass removal process during the plume generation phase; however, immediately after persulfate injection, sulfate reduction was inhibited in the ChemOx zone and persulfate oxidation dominated the removal of BTX mass. Upon the depletion of persulfate, microbial sulfate reduction was re-established and became the dominant mass removal process at this location. Persulfate oxidation was responsible for the majority (78%) of the mass loss that occurred in the vicinity of the persulfate injections, followed by sulfate reduction (21%) and aerobic biodegradation (1%). Alternatively, it was observed that microbial sulfate reduction was responsible for most of the mass removal at a downgradient location with an increased rate that corresponded to the arrival of high sulfate concentrations. Reaction kinetics, transport parameters and design options (i.e., persulfate concentration, and injection period/interval and rate) were identified as the key factors which influence the overall system performance. It was also found that a less aggressive persulfate treatment step (i.e., lower dosage, duration and extent) improves the overall treatment efficiency by minimizing the inhibitory effect of persulfate on the subsequent microbial processes. Results from this research effort indicated that persulfate oxidation coupled with enhanced bioremediation appears to be a viable approach to treat dissolved PHC compounds in situ. The inhibitory impact of persulfate on the population and activity of indigenous microbial communities (including SRB) was shown to be short term. Stable isotope analysis of BTX and sulfate, and monitoring of process-specific functional genes and intermediate metabolites proved useful to evaluate system performance and to identify temporal changes in the dominant degradation pathway. For an effective persulfate/EBR treatment train, a carefully balanced design which takes into account the interactions among the physical, chemical and biological processes is required. The combination of experimental and modelling efforts provided key insights into an effective design of a combined persulfate/EBR remedy, and lessons learned will be useful for remediation engineers and scientists.