Solar photocatalytic degradation of model naphthenic acids mixtures by Bi2WO6 and Bi2WO6/NiO/Ag: Exploring the influence of inorganic fraction of oil sands process water

This study investigates the comparative photocatalytic degradation of six model naphthenic acids, key organic constituents responsible for the toxicity of oil sands process water, using the semiconductor photocatalysts Bi2WO6 and Bi2WO6/NiO/Ag in both buffered solution and the inorganic fraction of OSPW. The heterojunction catalyst (Bi2WO6/NiO/Ag) exhibited the highest removal efficiencies for these organic pollutants with k values 0.048, 0.036, 0.019, 0.283, 0.026, and 0.075 min(-1) for 1-adamantanecarboxylic acid, cyclohexanecarboxylic acid, tetrahydropyran-4-carboxylic acid, tetrahydro-2H-thiopyran-4-carboxylic acid, isonipecotic acid, and 4,5-dihydronaphtho[1,2-b]thiophene-2-carboxylic acid, respectively. Experimental analysis suggested that chloride and bicarbonate inhibited the photocatalytic degradation of the target naphthenic acids, while nitrate could accelerate the production of aminyl radicals, enhancing the degradation of isonipecotic acid. Notably, four additional nitrated and hydroxylated by-products were identified during 1-adamantanecarboxylic acid degradation in the presence of nitrate. The addition of the catalyst not only accelerated the degradation rate of 1-adamantanecarboxylic acid but also helped mitigate the formation of potentially toxic by-products. Additionally, the transformation products of 4,5-dihydronaphtho[1,2-b] thiophene-2-carboxylic acid were identified. Degradation pathways were further elucidated with the assistance of density functional theory calculations, identifying the beta-carbon as the initial reactive site, with the sulfur atom also playing a significant role in reactivity. These findings contribute to the understanding of the water matrix-induced transformation process of model naphthenic acids during photocatalytic treatment, enhancing the environmental applicability of photocatalytic treatments.