Anoxic biodegradation of a surrogate naphthenic acid coupled to reduction of nitrite

Impacts of bioreactor operating parameters on removal efficiency, biodegradation rate, molecular distribution, and toxicity of commercial naphthenic acids

Effects of naphthenic acids (NAs) concentration (50-200 mg NA L^-1; 35-140 mg TOC L^-1) and loading rate (1.4-1249 mg NA L^-1 h^-1; 1-874 mg TOC L^-1 h^-1) on removal efficiency, removal rate, and molecular distribution of NAs, and effluent toxicity were evaluated for biodegradation of commercial NAs mixture in circulating packed bed bioreactors (CPBBs). Increase of NAs concentration and loading rate (shorter residence times) increased the removal rate, while removal efficiency initially declined and then stabilized. The maximum biodegradation rates for 50, 100, 150, and 200 mg NA L^-1 were 128.0, 321.7, 430.2, and 630.0 mg TOC L^-1 h^-1 at loading rates of 218.5, 455.6, 673.5 and 874.0 mg TOC L^-1 h^-1, respectively, with removal efficiencies of 58.6, 70.6, 63.9 and 72.1%. Analysis of influent and treated effluents with gas chromatography-mass spectrometry showed that molecular weight and cyclicity (C and Z numbers) affected the biodegradation, with low molecular weight acyclic NAs (C = 6-12) were the most amenable to biodegradation and those with intermediate and high molecular weights (C = 13-22) and moderate cyclicity (Z = - 4, - 6) were the most recalcitrant. In the biofilm, Proteobacteria and Actinobacteria were the most abundant phyla, and Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria were the dominant classes. Toxicity analyses with Artemia salina and Vibrio fischeri (Microtox) showed that high influent concentrations and loading rates (short residence times) led to higher NAs residual concentration and effluent toxicity. To design and operate large-scale CPBBs, intermediate loading rates and residence times that result in high removal efficiency, reasonable removal rates, and low toxicity are recommended.

Co-biodegradation of naphthenic acids in anoxic denitrifying biofilm reactors

Anoxic co-biodegradation of linear and cyclic naphthenic acids (NAs) namely octanoic acid, trans-4-methyl-1-cyclohexane carboxylic acid (trans-4MCHCA), cis- and trans-4-methyl-1-cyclohexane-acetic acids (cis-4MCHAA and trans-4MCHAA) was investigated in denitrifying biofilm reactors. In all evaluated compositions, co-biodegradation of NAs was coupled to denitrification, with octanoic acid showing the fastest biodegradation rate (1180.4 mg L^-1 h^-1 at loading rate of 1180.4 mg L^-1 h^-1), followed by trans-4MCHCA (398.1 mg L^-1 h^-1 at loading rate of 435.8 mg L^-1 h^-1), trans-4MCHAA (25.7 mg L^-1 h^-1 at loading rate of 221.7 mg L^-1 h^-1), and cis-4MCHAA (5.3 mg L^-1 h^-1 at loading rate of 16.9 mg L^-1 h^-1). Biodegradation of octanoic acid and trans-4MCHCA were not influenced by the presence of recalcitrant NAs (cis- and trans-4MCHAA). Co-biodegradation of cis- and trans-4MCHAA with octanoic acid, trans-4MCHCA, or their combination enhanced the biodegradability of these recalcitrant NAs, with the positive impact being more pronounced for trans-4MCHCA. Finally anoxic co-biodegradation of NAs under denitrifying conditions proceeded at rates that were faster than the aerobic rates obtained in similar mixtures. Anoxic biodegradation, therefore, is an effective alternative for in situ treatment of oil sands process water in anoxic stabilization ponds amended with nitrate, or as an ex situ treatment approach in denitrifying bioreactors whereby the cost and technical challenges of aeration are eliminated.

Biodegradation of surrogate naphthenic acids and electricity generation in microbial fuel cells: bioelectrochemical and microbial characterizations

Waters contaminated with naphthenic acids (NAs) and associated tailings are one of the major environmental challenges associated with the processing of oil sands and production of heavy oil. In the current work biodegradation of linear and cyclic naphthenic acids, namely octanoic acid and 4-methyl-1-cyclohexane carboxylic acid (trans-4MCHCA), individually and in mixture were evaluated in microbial fuel cells (MFCs). In batch MFCs with single rod electrodes and freely suspended bacteria, biodegradation rate increased as NA initial concentration increased from 100 to 250 mg L^-1 with no further improvement when a concentration of 500 mg L^-1 was evaluated. During the co-biodegradation, diauxic microbial growth and preferential use of octanoic acid were observed. Moreover, the presence of octanoic acid enhanced the biodegradation of trans-4MCHCA. In the continuous flow MFCs with granular graphite electrodes and biofilm, increases in NA concentration and loading rate led to higher biodegradation rates and improvement of electrochemical output. Furthermore, MFC operated with octanoic acid outperformed its counterpart that was fed with trans-4MCHCA, with the maximum biodegradation rate, current and power densities for octanoic acid and trans-4MCHCA being 49.9 and 36.5 mg L^-1 h^-1, 6000.0 and 4296.3 mA m^-3, and 963.0 and 481.5 mW m^-3, respectively. Co-biodegradation of NAs in continuous flow MFCs with biofilm acclimated to octanoic acid or trans-4MCHCA revealed development of distinctly different microbial communities, simultaneous biodegradation of NAs albeit at faster rates for octanoic acid, and superior performance of MFC with the biofilm developed with trans-4MCHCA.

Modified biopolymers as sorbents for the removal of naphthenic acids from oil sands process affected water (OSPW)

Oil sands operations consume large volumes of water in bitumen extraction process and produce tailings that express pore water to the surface of tailings ponds known as oil sands process-affected water (OSPW). The OSPW is toxic and cannot be released into the environment without treatment. In addition to metals, dissolved solids, dissolved gases, hydrocarbons and polyaromatic compounds etc., OSPW also contains a complex mixture of dissolved organic acids, referred to as naphthenic acids (NAs). The NAs are highly toxic and react with metals to develop highly corrosive functionalities which cause corrosion in the oil sands processing and refining processes. We have chemically modified keratin biopolymer using polyhedral oligomeric silsesquioxanes (POSS) nanocages and goethite dopant to unfold keratinous structure for improving functionality. The untreated neat keratin and two modified sorbents were characterized to investigate structural, morphological, dimensional and thermal properties. These sorbents were then tested for the removal of NAs from OSPW. The NAs were selectively extracted and quantified before and after sorption process. The biosorption capacity (Q), rejection percentage (R%) and isotherm models were studied to investigate NAs removal efficiency of POSS modified keratin biopolymer (PMKB) and goethite modified keratin biopolymer (GMKB) from aliquots of OSPW.