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

Abstract

Bitumen extraction from the Athabasca oil sands involves large volumes of water known as oil sands process affected water (OSPW). OSPW contains naphthenic acids (NAs), a class of aliphatic and cyclic carboxylic acids that can be toxic and are recalcitrant to natural attenuation. A passive advanced oxidation process (P-AOP), such as solar photocatalysis (PC) with buoyant photocatalysts (BPCs, TiO2-coated buoyant microspheres), is promising for NA treatment, through conversion to more hydrophilic forms (partial oxidation) or to CO2 (complete mineralization), depending on the solar dose. Although BPCs exhibit strong reactivity, full NA mineralization can require impractical hydraulic retention times. Biodegradation is another promising passive approach, but biodegradation rates are ultimately inhibited by the toxicity and structural complexity of NAs. We hypothesized that biological NA removal kinetics could be enhanced through BPC pre-treatment, since partial oxidation can lower NA toxicity and improve their biodegradability. Different PC exposure durations were used to pre-treat simulated OSPW prior to a biological treatment stage (with natural microbial culture from OSPW), to understand their impacts on NA chemical speciation and biodegradation kinetics. PC pre-treatment (2 d) enabled full mineralization (to <3 mg/L COD) and >99.9% removal of acid-extractable organics (AEO) in secondary biological treatment (21 d). Mineralization was achieved earlier in the combined PC+bio treatment than by photocatalysis alone (33 d vs. >42.2 d), and microbial growth rate was accelerated 23-fold compared to the non-pre-treated water. Overall, BPCs can improve NA biodegradability to achieve mineralization through a fully passive combined treatment process, without chemical or energy inputs.