Electrochemical Oxidation of Carboxylic Acids in the Presence of Manganese Chloride

Abstract

Carboxylic acids are the main byproducts in effluents after treating source pollutants with advanced oxidation processes (AOPs). This work developed a feasible electrochemical method for mineralizing oxalic acid and citric acid (50 ppm, pH i = 3). A reactor was mounted with RuO 2 /IrO 2 coated Ti-DSA electrodes (titanium dimensional stable anode). The selected chloride electrolytes (NaCl, MgCl 2 and MnCl 2 , [Cl − ] = 20 mM) mineralized completely oxalic acid through the production of active chlorine. However, the highest total organic carbon (TOC) removal efficiency was obtained using MnCl 2 and was aided by the MnO 2 deposited electrochemically on the anode. In the chlorine-free electrolysis process, the performance in terms of TOC removal of the Ti-DSA electrodes that had MnO 2 on the anode depended on the electrical conditions. Without electricity, MnO 2 directly mineralized oxalic acid; with electricity, TOC removal from the citric acid solution improved substantially. The changes in the Mn 2+ concentration suggest that MnO 2 dissolved by organic compounds could be recovered and reused through formation of electrolytic manganese dioxide (EMD).In recent years, advanced oxidation processes (AOPs) have been applied to remove hazardous and environmentally undesirable chemicals from industrial wastewater. Among AOPs, the Fenton technique, which has many merits, such as cost effectiveness and simple operation, can efficiently generate hydroxyl radicals by catalyzing hydrogen peroxide with ferrous ions. However, the high reaction constant (k = 4.3 × 10 9 L mol −1 s −1 ) between the hydroxyl radical and chloride ion, which is a co-existing ion in Fenton chemicals (also frequently in wastewater), typically prohibits the hydroxyl radical from mineralizing the intermediates that are from the oxidation of target aromatic compounds. 1 These intermediates are generally carboxylic acids, and are the major parts of total organic carbon (TOC) residues in end waters from a Fenton process (oxalic acid and citric acid reacting with hydroxyl radicals have reaction constants of 1.4 × 10 6 and 5 × 10 7 L mol −1 s −1 , respectively). 2 Although the photo-or electrical assistance can extend the life span of hydroxyl radicals to improve the mineralization of intermediates. 3-5 Biodegradation has been used in the treatment of carboxylic acids in oil sand process water. 6 However, the costs of sludge disposal and reagents in present methods still have to be reduced. Carboxylic acids are known as agriculturally inhibitors of crops, and accumulates in natural soils during manure followed by water logging. The phytotoxicity of carboxylic acids may potentially cause another environmental problems after the Fenton oxidation of aromatic pollutants. 7 Direct and indirect electrochemical oxidation of organic substances is extensively exploited to overcome the scavenging effect of chloride ions in Fenton reactions. [8][9][10] In the presence of chloride ions, electrochemical chlorine is produced by anodic oxidation, and hydrogen is generated at...

Experimental correlation of Mn3+cation defects and electrocatalytic activity of α-MnO2– an X-ray photoelectron spectroscopy study

“…† In brief, by PXRD, a tetragonal a-phase (ICDD PDF 00-044-0141, space group I4/m, a ¼ b ¼ 9.7847 Å) could be identied for MnO 2 -SF (Fig. S30 †), 17,67,68 a mixed hexagonal 3-(ICDD PDF 00-030-0820, space group P63/mmc, a ¼ b ¼ 2.8 Å, c ¼ 4.45 Å) 187,188 / orthorhombic g-phase (ICDD PDF 00-014-0644, a ¼ 6.36 Å, b ¼ 10.15 Å, c ¼ 4.09 Å) for commercial MnO 2 activated (Fig. S31 †), [189][190][191][192][193][194][195] an orthorhombic spinel phase (ICDD PDF 00-024-0508, space group Pcab, a ¼ 9.4161 Å, b ¼ 9.4237 Å, c ¼ 9.4051 Å) [196][197][198] for Mn 2 O 3 (Fig.…”