Cyclohexanoic acid breakdown by two-step persulfate and heterogeneous Fenton-like oxidation

Abstract

This work reports on the application of a two-step treatment consisting of persulfate (PS) and heterogeneous Fenton-like oxidation for the breakdown of cyclohexanoic acid (CHA, 50 mg L−1) as the model naphthenic acid (NA) of oil sands process affected wastewaters (OSPWs). Home-made activated carbon and γ-alumina-supported iron (Fe/AC and Fe/γ-Al2O3) catalysts were tested for the Fenton-like step, namely Catalytic Wet Peroxide Oxidation (CWPO). The two catalysts were compared varying the H2O2 dose and temperature on the treatment efficiency and their stability was checked in terms of iron loss by leaching. The catalysts before and after reaction were characterized by 77 K N2 adsorption-desorption, TXRF and XPS analyses. Results show that over 75% mineralization can be achieved by the combined system at 80 °C. The starting pH was the circumneutral natural value of OSPWs (≈8), which during the PS-oxidation step evolved towards the acid (≈3–3.5), optimum pH for the Fenton-like systems. This, together with the partial substitution of PS by cheaper H2O2 and the consequent lower introduction of sulfate into the water are the main advantages of the combined treatment compared to PS-oxidation alone. The time course of TOC in the CWPO step can be well described by pseudo-first and pseudo-second order kinetics for the Fe/AC and Fe/γ-Al2O3 catalysts, respectively. Characterization of the fresh and used catalysts shows negligible changes in porous texture for both of the catalysts, while XPS analyses indicate an apparent decrease of the proportion of inorganic oxygen and increase of carboxylic acid group for the AC-based one. That catalyst suffered higher iron leaching and yielded less effective H2O2 consumption. The amount of oxalic acid formed, the low pH value of the solution after the PS pretreatment and the metal-support interactions in the catalysts tested are experimentally demonstrated to be the main factors affecting iron leaching.