Catalytic post-treatment of homogeneous Ag–S2O8 and Cu–H2O2 advanced oxidation processes: Revealing the importance of capacitive-Faradaic electrochemical structure of Pt/AC

Abstract

Implementation of homogeneous advanced oxidation processes (HAOPs) catalysed by Cu and Ag metals is limited due to high residual concentrations of oxidants and dissolved metals. The proposed post-treatment process starts with recirculation of H2-enriched HAOP-treated water through the Pt-loaded activated carbon (AC). Hydrogen oxidation reaction (HOR) on Pt results in degradation of oxidants while silver and/or copper ions are reduced and deposited on the catalytic media. The subsequent aeration of the catalyst results in oxygen reduction reaction (ORR) and oxidation of the metals back into ionic forms. This allows the homogeneous metal catalysts to be reused in multiple HAOP cycles.The capacitive-Faradaic electrochemical structure of the Pt/AC catalyst with Pt acting as a Faradaic electrode and AC as a capacitive electrode plays an important role in the process. The HOR on Pt/AC results in accumulation of electrons in electrical double layer of AC. These electrons can be effectively used in a subsequent H2-free reduction of metal ions and oxidants. Thus, the degradation of H2O2 and S2O82− and reduction of Cu2+ and Ag + ions proceed much faster if the H2-step of the process is split into two sub-processes: (i) hydrogenation of Pt/AC catalyst in an inert Na2SO4 solution to pre-charge the Pt/AC with electrons originating at HOR, followed by (ii) catalytic reduction of oxidants and metal ions by the capacitively stored electrons.The process was investigated using Ag–S2O82− ([S2O82−]0 = 25 mM; [Ag+]0 = 50 mg/L) and Cu–H2O2 ([H2O2]0 = 100 mM, [Cu]0 = 635 mg/L) solutions. Total degradation of oxidants was achieved within <2 h and complete metals’ recovery was obtained. Several cycles of HAOP experiments were performed for the degradation of 1,4-dioxane (10 mg/L) and phenol (25 mg/L) using Ag– S2O82− and Cu–H2O2 HAOPs, respectively. The residual oxidants were completely degraded in all experiments. The reactivation of Pt/AC catalysts inhibited by phenol derivatives was achieved by hydrogenation in Na2SO4 solution (i.e., using the pre-charge approach).