Metal Sulfides in Aged-Coarse Sands Tailings Facilitate Naphthenic Acids Removal from Oil Sands Process Water

Abstract

The use of natural substrates for oil sands process water (OSPW) reclamation offers advantages such as onsite availability and scalability. This study evaluated potential of aged and fresh coarse sand tailings (CST) towards removal of classical naphthenic acids (NAs) from a real OSPW obtained from an oil sands’ tailing ponds in Alberta (NAs: 4.87 mg/L). Aged-CST achieved superior removal efficiencies of NAs (96.5%), aromatics (>90%), and acid-extractable organics (∼95%), compared to fresh-CST, which showed limited removal (∼34.3%) similar to conventional slow sand filters (∼30–45%). Although limited surface area of both CST materials (∼1.82 m²/g) was not conducive to physical adsorption, the oxidation of metal sulfides in aged-CST enhanced the chemical reactivity, surface heterogeneity, and microbial activity, facilitating efficient adsorption, precipitation, and biodegradation of NAs. Kinetics modelling indicated that aged-CST strongly fit the pseudo-second order (R² = 0.969, k₂ = 0.003 g mg⁻¹ h⁻¹) and Elovich model (R² = 0.876, 1/b = 1.713 mg g⁻¹), indicating chemisorption as dominant removal mechanism, while fresh-CST exhibited poor fits and limited performance. Fourier-transform infrared spectroscopy and synchronous fluorescence spectroscopy analyses revealed that intensities of hydroxyl groups, aliphatic, carboxylic, and ester compounds significantly increased in aged-CST after filtration. A labelled isotope desorption study using Lauric-D23 acid cross-verified that adsorption and precipitation (∼65%) with metal sulfides were key mechanisms, while remaining ∼35% were chemically transformed by-products, as indicated by mass balance. Microbial community analysis showed that aged-CST had higher microbial richness (Chao1 ∼1000) compared to fresh-CST (∼500, respectively). Hydrocarbon-degrading bacteria (e.g., Rhodococcus and Sphingomonas) and acidophilic bacteria (Bryobacter, Candidatus Solibacter) were dominant in aged-CST, facilitating NAs biodegradation. BE-SPME analysis confirmed successful removal (∼86%) of bioavailable organics removing toxicity. This study highlights aged-CST as a viable natural substrate for OSPW reclamation, offering insights into its fate and opportunities for resource recovery.