Factors that affect the degradation of naphthenic acids in oil sands wastewater by indigenous microbial communities

The acute toxicity of wastewater generated during the extraction of bitumen from oil sands is believed to be due to naphthenic acids (NAs). To determine the factors that affect the rate of degradation of representative NAs in microcosms containing wastewater and the acute toxicity of treated and untreated wastewater, the effects of temperature, dissolved oxygen concentration, and phosphate addition on the rate of (CO2)-C-14 release from two representative naphthenic acid substrates, (linear) U-C-14-palmitic acid (PA) and (bicyclic) decahydro-2-naphthoic acid 8-C-14 (DHNA), were monitored. Tailings pond water (TPW) contained microorganisms well adapted to mineralizing both PA and DHNA: PA was degraded more quickly (10-15% in 4 weeks) compared to DHNA (2-4% in 8 weeks). On addition of phosphate, the rate of NA degradation increased up to twofold in the first 4 weeks, with a concurrent increase in the rate of oxygen consumption by oil sands TPW. The degradation rate then declined to levels equivalent to those measured in flasks without phosphate. The observed plateau was not due to phosphate Limitation. Decreases in either the dissolved oxygen concentration or the temperature reduced the rate. Phosphate addition also significantly decreased the acute toxicity of TPW to fathead minnows. In contrast, Microtox(R) analyses showed no reduction in the toxicity of treated or untreated TPW after incubation for up to 8 weeks at 15 degrees C.