Electrochemical Regeneration of Free Chlorine Treated Nickel Oxide Catalysts for Oxidation of Aqueous Pollutants

Abstract

Nickel oxide upon free chlorine treatment has been used as a heterogeneous oxidant to eliminate aqueous organic pollutants, where oxygenation or chlorination could regenerate the oxidation capacity. In order to overcome problems associated with low regeneration efficiency and undesirable chloride ion generation, we herein investigate electrochemical regeneration for sustainable catalytic usage of the free chlorine treated nickel oxide, obviating a dosage of chemicals. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) characterized drop-casted catalysts with thermal decomposition to be NiO nanoparticles with amorphous nickel oxyhydroxide shells generated by free chlorine treatment. Along with an elevated open circuit potential of the active sites (NiOx(OH)y) near 0.75 V NHE (at pH 14), the chlorination was found to enhance the electrical conductivity of the catalysts. Galvanostatic anodization at current density < 4 mA/cm2 effectively restored the oxidizing power for repetitive degradations of 4-chlorophenol (4-CP). Despite greater conversion under the continuous anodization, periodic regeneration (10/60 on/off) showed far lower energy consumption (44.6 +/- 3.0 kW h/kg 4-CP) for 4-CP removal. The changes in XPS and X-ray absorption near edge structure (XANES) spectra, coupled with kinetic and electrochemical analyses, demonstrated that electron transfer towards Ni3+ with H abstraction would be the primary degradation mechanism. The stability and versatility were assessed by sequencing batch cycles and degradation of 2,4-dichlorophenol, carbofuran, and oxalic acid. Application in a real phenolic wastewater matrix noted 98% and 79% removal of chemical oxygen demand and total organic carbon after 12 h.