Co-biodegradation of linear and cyclic naphthenic acids in a circulating packed bed bioreactor

Abstract

Large volumes of oil sand process water are generated as part of the Clarke caustic hot water process used for extraction of bitumen from shallow oil sand reserves. These process waters contain naphthenic acids in high concentrations (40-120 mg/L) which are persistent in the environment for decades. The toxic nature of naphthenic acids has been found to endanger aquatic biota and terrestrial habitat. Reclamation of these oil sand process waters has also come to the forefront due to the increasing future demand for water consumption in the oil sand industry and the need for sustainable use of water. Bioremediation as a cost effective technology for treatment of these process affected waters is gaining impetus. In earlier works in our group, the biodegradation of single naphthenic acids in the circulating packed bed bioreactor was studied in the batch and continuous modes. In this work, a circulating packed bed bioreactor has been used to study the batch and continuous co-biodegradation of different combinations of the four model naphthenic acids (one linear, and three cyclic compounds of different molecular structures) namely octanoic acid, trans 4-methylcyclohexanecarboxylic acid (4MCHCA) and a mixture of cis and trans isomer of 4-methylcyclohexaneacetic acid (4MCHAA). The circulating packed bed bioreactor effectively biodegraded all the candidate NAs with rates as high as 401.1 mg/L-h for octanoic acid, 208.8 mg/L-h for trans-4MCHCA, 4.5 mg/L-h and 10.2 mg/L-h for cis-4MCHAA and trans-4MCHAA, respectively. The maximum removal rates of the cyclic naphthenic acids were found to be much lower than that of octanoic acid irrespective of the presence of the other compound in the mixture. The data in this study also suggests that removal rate of NAs in the mixture was influenced by geometric isomerism of the compounds where biodegradation of cis isomer was much slower than that of its trans counterpart. Moreover, increase in the carbon number (presence of additional methyl group) resulted in lower removal rate. Another important finding of this work was that co-biodegradation of octanoic acid with trans-4MCHCA and 4MCHAA has no impact on trans-4MCHCA but co-biodegradation enhanced the removal rate of cis-4MCHAA which is the most recalcitrant of the three compounds used in this study by 23%.