Remediation of oil sands naphthenic acids by activated persulfate oxidation and biodegradation

Abstract

Bitumen extraction from surface mines in Alberta produces oil sands process-affected water (OSPW) that contains toxic organic contaminants such as naphthenic acid fraction compounds (NAFCs). Due to the large volumes of OSPW generated, cost-effective remediation strategies are vital. One option is coupling chemical oxidation with biodegradation, where limited amounts of oxidant are used to break down NAFCs into more bioavailable compounds which microorganisms can further degrade into non-toxic end products. Sodium persulfate is a low cost, powerful oxidant that may be less damaging to microorganisms compared to other oxidants. Coupling unactivated (21°C) and activated (30°C) persulfate with biodegradation for the remediation of commercially produced Merichem naphthenic acids (NAs) was previously studied. Results demonstrated that persulfate was primarily responsible for NA reduction while bacteria removed degradation by-products, leading to a significant decline in water toxicity.This study utilises OSPW from an active settling basin and Merichem NAs. Preliminary trials were conducted to determine the conditions of persulfate activation. Persulfate was added to OSPW at concentrations of 250, 500 and 1,000 mg/L and activated by heating to 40-60°C. NAFC removal was limited at 40°C, suggesting ineffective persulfate activation for the concentrations tested. At 60°C, 250 mg/L of persulfate removed 42.4% of OSPW NAFCs compared to 20.3% of Merichem NAs after 8 hours. OSPW NAFCs appear to be more reactive with persulfate than Merichem NAs, indicating a preference for oxidising the more complex, branched species. Increasing the persulfate concentration to 1,000 mg/L improved OSPW NAFC removal to 78.6% after 8 hours at 60°C. Despite increased NAFC removal at higher persulfate concentrations, it is more costly and previous results showed that increasing persulfate concentration past 250 mg/L led to declining microbial viability. Persulfate is a promising oxidant for NAFC remediation and coupling oxidation with biodegradation can provide more efficient and extensive clean-up than either option alone.