Biodegradation of Naphthenic Acids in Microbial Fuel Cell type bioreactors
Canada has the third largest oil reserves in the world. Close to 96% of these reserves are located in oil sands deposits (CAPP, 2015). Extraction of bitumen from these oil sands is carried out by alkaline hot water process (Clark Process) which results in the generation of large volumes of waters contaminated with naphthenic acids (NAs). These waters are referred to as Oil Sands Process Water (OSPW) and they are maintained in large tailing ponds due to their toxicity and a zero-discharge policy enforced by the Government. Given the environmental challenges associated with OSPW and tailing ponds, several physicochemical and biological treatments have been evaluated as remediation option. Previous studies in our research group have successfully achieved biodegradation of model NAs in conventional bioreactors of various configurations under aerobic and anoxic conditions (Paslawski et al., 2009; Huang et al., 2012; D’Souza et al., 2014; Gunawan et al., 2014; Dong and Nemati, 2016). Against this background, the current work offer an alternative treatment approach based on anoxic biodegradation of NAs in Microbial Fuel Cells (MFCs). MFCs are unconventional bioreactor in which biodegradation of a contaminant occurs with concomitant generation of energy. In the present study, biodegradations of a linear (octanoic acid) and a cyclic NA (trans-4-methyl-1-cyclohexane carboxylic acid, trans-4MCHCA) were evaluated in MFCs. Firstly, biodegradation of individual NAs (100, 250 and 500 mg L-1) was carried out in batch operated MFCs with either graphite rod or granular graphite electrodes. Maximum biodegradation rates in the single rod electrode MFCs were achieved during the biodegradation of NAs with highest concentration (1.56 and 2.46 mg L-1 h-1 for trans-4MCHCA and octanoic acid, respectively). This trend was also observed in MFCs with granular electrodes, where the removal of 500 mg L-1 of each individual compound led to the highest biodegradation rates, with values of 7.2 and 22.78 mg L-1 h-1 for trans-4MCHCA and octanoic acid, respectively. Regardless of the type of employed electrodes, biodegradation of the linear NA occurred at a rate much faster than that of its cyclic counterpart. Moreover, sequential batch operation of MFCs enhanced the biodegradation rate of both compounds. In continuously operated MFCs with granular electrodes, biodegradation of each individual NAs (trans-4MCHCA or octanoic acid) was assessed at initial concentrations of 100, 250 and 500 mg L-1, with the maximum biodegradation rate again achieved with the highest NA concentration (36.5 and 49.9 mg L-1 h-1 for trans-4MCHCA and octanoic acid, respectively). Interestingly, the highest current and power densities were attained when the biodegradation rate was at the level, with the values being 481.5 mW m-3 and 4296.3 mA m-3 for trans-4MCHCA, and 963.0 mW m-3 and 6000.0 mA m-3 for octanoic acid. Co-biodegradation of linear and cyclic NAs was also studied using mixtures of NAs with different concentrations in two MFC configurations: batch-wise operated with single rod electrodes and continuously operated with granular electrodes. In batch MFCs with single rod electrodes, it was observed that biodegradation of the linear NA occurred first and biodegradation of the cyclic NA occurred only after complete exhaustion of the linear compound. Contrary to what observed in the batch MFC with rod electrodes, in the continuous flow MFC with granular electrodes biodegradation of cyclic and linear NAs occurred simultaneously, with the biodegradation rate of the linear NA being much faster than that of the cyclic NA.
Naphthenic Acids, Microbial Fuel Cell, OSPW, Bioremediation
Master of Science (M.Sc.)
Chemical and Biological Engineering