A Systematic Study of Solar-Assisted ZnS Catalysis for Treating Model Naphthenic Acid and Oil Sands Process Water: Effects of Synthesis Parameters, Kinetics, and Toxicity Evaluation

Abstract Oil sands operations produce large quantities of oil sands process water (OSPW), which contains persistent and toxic naphthenic acids (NAs). In this study, ZnS with different Zn/S molar ratios was synthesized and evaluated for the first time for the degradation of a single model NA (5-phenyl valeric acid, PVA), a mixture of seven classical NAs, and natural NAs in real OSPW. ZnS with a 1:2 Zn/S ratio exhibited the highest PVA degradation (>93%), following pseudo-first-order kinetics (k = 0.319 h–1). The enhanced activity was attributed to increased surface area and pore volume, smaller crystalline size, and reduced bandgap energy. PVA degradation byproducts and pathways were also evaluated. ZnS effectively degraded structurally diverse NAs, with cyclohexanepentanoic acid (CHPA) and decanoic acid (DA) showing the highest efficiencies, likely due to their high carbon numbers (C10 and C11) and alkyl-branched linear structures. In real OSPW, fluorophore organic compounds (FOCs) with two and three fused rings showed >87% degradation, while approximately 60% of classical NAs (O2–NAs) were removed. Treatment also significantly reduced the immunotoxicity and cytotoxicity of OSPW. Overall, these findings demonstrate the potential of solar-assisted ZnS photocatalysis for treating naturally occurring NAs in industrially affected waters.