Application of Advanced Oxidation Processes for Treatment of Naphthenic Acids in Oil Sands Process Water

Abstract

The large volume of oil sands process-affected water (OSPW) produced by the oil sands industries in Northern Alberta, Canada, is an environmental concern. The toxicity of OSPW has been attributed to a complex mixture of naturally occurring acids, including naphthenic acids (NAs). NAs are a broad range of alicyclic and aliphatic compounds that are persistent in the environment. This work focused on the application of advanced oxidation processes (AOPs): UV/H2O2 and O3/H2O2 for degradation of model NA compounds and OSPW NAs. Cyclohexanoic acid (CHA) was selected as a simple model naphthenic acid, and its oxidation and byproduct formation in the UV/H2O2 and ozonation processes was studied. The results indicated that in the UV/H2O2 process, the pH had no significant effect on the degradation, nor on the formation and degradation of byproducts in ultrapure water. A real OSPW matrix had a significant impact by decreasing the CHA degradation rate up to 82% relative to that in ultrapure water. Relative rate measurements using binary mixtures of model NA compounds confirmed that reactivity favoured compounds with more carbons, and also favoured NAs with one saturated ring, relative to the corresponding linear NA. However, for model compound with three rings, no increased reactivity was observed relative to mono-cyclic NA. The mechanism of ozonation of CHA was different at pH 3 and pH 9. At pH 9, oxo-CHA and hydroxy-CHA were both detected by LC-MS/MS, confirming the hydroxyl radical (•OH) pathway in which superoxide CHA radical is a possible intermediate. The results of the O3/H2O2 advanced oxidation process of OSPW NAs showed that in a semi-batch system, approximately 90% of extractable organic acids of OSPW were removed using O3/H2O2 process with 85 mg/L of O3 and O3 to H2O2 ratio of 0.3.