Mild Photocatalysis Removes Microbial Inhibition and Enables Effective Biological Treatment of Naphthenic Acids

Bitumen extraction involves large volumes of water known as oil sands process-affected water (OSPW). OSPW contains naphthenic acids (NAs), a class of aliphatic and alicyclic carboxylic acids that can be toxic and recalcitrant to natural attenuation. Solar photocatalysis (PC) with buoyant photocatalysts (BPCs) is a promising passive treatment since it converts NAs to more hydrophilic forms or CO2, depending on the solar dose. Although BPCs exhibit strong reactivity, NA mineralization requires impractical treatment times. Biodegradation is another promising passive treatment, but the toxicity and structural complexities of NAs limit its effectiveness. We hypothesized that BPC pre-treatments may improve NA biodegradation since partial oxidation can lower toxicity and improve biodegradability. Thus, simulated OSPW was pre-treated under different PC exposure durations before a biological treatment stage to understand how PC impacts NA chemical speciation and biodegradation kinetics. Two day PC pre-treatments removed microbial inhibitions, which enabled mineralization and >99.9% removal of acid-extractable organics in the secondary biological treatment. Mineralization was achieved earlier in the combined PC + biotreatment than by photocatalysis alone, and the microbial growth rate was accelerated by 23-fold compared to the non-pre-treated water. Therefore, BPCs can improve NA biodegradability and accelerate mineralization through a passive hybrid-treatment process without chemical or energy inputs.