Impact of Anaerobic Biofilm Formation on Sorption Characteristics of Powdered Activated Carbon

Abstract

Subsurface contamination by petroleum hydrocarbons (PHCs) results from the leakage of petroleum products during extraction, processing or transport. It has been documented that activated carbon (AC) can effectively sorb organic compounds present in water. As a result, the use of carbon-based injectates (CBI) has gained popularity for use to treat groundwater impacted with PHCs in situ. CBI relies on the dynamic equilibrium between sorption, desorption and biodegradation of contaminants, leading to long-term treatment. The aim of this study was to investigate the influence of biofilm formation on the sorption characteristics of PHCs to powdered activated carbon (PAC). Specifically, the sorption performance of PAC for toluene was evaluated before and after an anaerobic methanogenic toluene-degrading microbial biofilm was developed on the PAC. Batch microcosm experiments were used to grow a biofilm on PAC surfaces (bio-coated PAC). The microbial culture used in the microcosm experiments actively degraded toluene as demonstrated by toluene reduction and the methane production. Confocal microscopy was conducted for qualitative visualization and quantitative analysis of the biofilms. The biofilm continually developed on the PAC surfaces and increased its mass and thickness over the 180-day long experimental period. The sorption characteristics of PAC without biofilm formation (fresh PAC) were compared to PAC samples removed from microcosms at Day 80 and Day 180. The change of sorption characteristics of PAC was evaluated based on best fit Freundlich isotherm parameters (Kf and nf). The value of Kf was reduced from 79.8 for fresh PAC to 50.2 and 47.7 for Day 80 and Day 180 bio-coated PAC, respectively. An increase in nf from 0.35 for fresh PAC to 0.42 for bio-coated PAC at Day 80 and 180 was also observed. These results show that the sorption performance of PAC was reduced when a microbial biofilm was developed. Although the biofilm growth was significant between Day 80 and Day 180, and a slight decrease in Kf was obtained for the bio-coated PAC at Day 180 compared to Day 80, the observed growth did not yield a statistically significant difference in the loss of sorption capacity between both time points. A simulated PAC barrier was used to demonstrate the iv impact that biofilm formation could have on potential performance deterioration. The biocoated PAC barrier performance was evaluated based on the percent reduction of breakthrough time as a function of incoming contaminant concentration. The breakthrough performance was found to deteriorate with the biofilm formation, with a greater impact at lower incoming concentrations (e.g., 60% reduction in breakthrough time for a concentration of 10 μg/L). The results of this study revealed that the sorption performance of PAC could be hampered by biofilm formation leading to fouling the AC pores (by biomass production and/or sorption of microbial by-products) which may affect its long-term effectiveness.