Mineralization of 4-methyl-1-cyclohexane carboxylic acid by heterogeneous catalytic ozonation

Abstract

This work studied the mineralization of an aqueous solution of a naphthenic acid model compound, 4-methyl-1-cyclohexane carboxylic acid (4MCH), by heterogenous catalytic ozonation in a semi-batch reactor. A catalyst screening was first conducted by evaluating total organic compound (TOC) removal of Cu, Ce, and Mn (1 and 5 wt%) dispersed on γ-Al2O3, TiO2, ZrO2, and MgO supports. Cu containing catalysts showed the highest TOC removal with Cu/MgO having the best mineralization effectiveness. The Cu/MgO catalysts were characterized by ICP-OES, BET, XRD, XPS, SEM, and TEM. CuO and Cu2O particles were formed on MgO with the majority of them being smaller than 5 nm. pH = 8.5 was identified as the optimum pH for total oxidation of 4MCH due to the right balance between the 4MCH oxidation and ozone decomposition rates. Phosphate and bicarbonate buffers had a different effect on the TOC removal of the Cu/MgO catalysts with better activities in the presence of the phosphate buffer. Mg and Cu leaching was observed during the oxidation of 4MCH, accompanied with an increase in pH. The reusability tests of Cu/MgO (1 wt%) showed that the catalyst retained its activity over 5 cycles with an average Mg and Cu leaching of 44 and 3 mg/L, respectively. The energy consumption cost of the treatment of 4MCH by catalytic ozonation was estimated to be around $13 per cubic meter of the wastewater.