Evaluation of heat-activated and unactivated persulfate oxidation coupled with biodegradation for naphthenic acid removal and acute toxicity reduction in oil sands process-affected water

Oil sands process-affected water (OSPW) contains complex mixtures of naphthenic acid fraction compounds (NAFCs) that contribute to acute toxicity. This study evaluated whether residual persulfate remaining after heat-activated oxidation (60ºC, 8 h) could be coupled with biodegradation by Pseudomonas fluorescens during a subsequent unactivated stage at room temperature (20ºC, 150 d). Commercial Merichem naphthenic acids (NAs) and aged OSPW were compared to assess the influence of water matrix complexity on oxidation efficiency and microbial viability. Heat-activated persulfate reduced OSPW-NAFCs by 45–89% at 250–1000 mg/L persulfate, increasing to 70–95% with continued reaction at room temperature, mostly within 30 d. Oxidation preferentially removed higher molecular weight, more unsaturated, O 2 -NAFCs, shifting the distribution toward more oxidized species. In OSPW, acute toxicity to Aliivibrio fischeri initially increased following activated oxidation, then declined to near background with continued reaction, consistent with progressive transformation of toxic intermediates. The addition of P. fluorescens did not improve NAFC removal and often hindered toxicity reduction, particularly in OSPW. Coupled treatments exhibited reduced persulfate reactivity, increased chemical oxygen demand, and elevated toxicity, which we hypothesize reflect oxidative stress, limited bioavailable carbon, and release of soluble microbial products that may scavenge oxidants. Only the 250 mg/L persulfate coupled treatment improved toxicity reduction relative to oxidation alone for Merichem NAs and coincided with higher viable cell counts. Overall, persulfate oxidation was effective for NAFC removal; however, simultaneous coupling with biodegradation was not effective for the OSPW tested and may be constrained by water chemistry, oxidative stress, and microbial viability.