Electrochemical Production of Hydrogen Coupled with the Oxidation of Arsenite

The production of hydrogen accompanied by the simultaneous oxidation of arsenite (As­(III)) was achieved using an electrochemical system that employed a BiO_<i>x</i>–TiO₂ semiconductor anode and a stainless steel (SS) cathode in the presence of sodium chloride (NaCl) electrolyte. The production of H₂ was enhanced by the addition of As­(III) during the course of water electrolysis. The synergistic effect of As­(III) on H₂ production can be explained in terms of (1) the scavenging of reactive chlorine species (RCS), which inhibit the production of H₂ by competing with water molecules (or protons) for the electrons on the cathode, by As­(III) and (2) the generation of protons, which are more favorably reduced on the cathode than water molecules, through the oxidation of As­(III). The addition of 1.0 mM As­(III) to the electrolyte at a constant cell voltage (<i>E</i>_cell) of 3.0 V enhanced the production of H₂ by 12% even though the cell current (<i>I</i>_cell) was reduced by 5%. The net effect results in an increase in the energy efficiency (EE) for H₂ production (ΔEE) by 17.5%. Furthermore, the value ΔEE, which depended on As­(III) concentration, also depended on the applied <i>E</i>_cell. For example, the ΔEE increased with increasing As­(III) concentration in the micromolar range but decreased as a function of <i>E</i>_cell. This is attributed to the fact that the reactions between RCS and As­(III) are influenced by both RCS concentration depending on <i>E</i>_cell and As­(III) concentration in the solution. On the other hand, the ΔEE decreased with increasing As­(III) concentration in the millimolar range due to the adsorption of As­(V) generated from the oxidation of As­(III) on the semiconductor anode. In comparison to the electrochemical oxidation of certain organic compounds (e.g., phenol, 4-chlorophenol, 2-chlorophenol, salicylic acid, catechol, maleic acid, oxalate, and urea), the ΔEE obtained during As­(III) oxidation (17.5%) was higher than that observed during the oxidation of the above organic compounds (ΔEE = 3.0–15.3%) with the exception of phenol at 22.1%. The synergistic effect of As­(III) on H₂ production shows that an energetic byproduct can be produced during the remediation of a significant inorganic pollutant

Effects of Anodic Potential and Chloride Ion on Overall Reactivity in Electrochemical Reactors Designed for Solar-Powered Wastewater Treatment

We have investigated electrochemical treatment of real domestic wastewater coupled with simultaneous production of molecular H₂ as useful byproduct. The electrolysis cells employ multilayer semiconductor anodes with electroactive bismuth-doped TiO₂ functionalities and stainless steel cathodes. DC-powered laboratory-scale electrolysis experiments were performed under static anodic potentials (+2.2 or +3.0 V NHE) using domestic wastewater samples, with added chloride ion in variable concentrations. Greater than 95% reductions in chemical oxygen demand (COD) and ammonium ion were achieved within 6 h. In addition, we experimentally determined a decreasing overall reactivity of reactive chlorine species toward COD with an increasing chloride ion concentration under chlorine radicals (Cl·, Cl₂^–·) generation at +3.0 V NHE. The current efficiency for COD removal was 12% with the lowest specific energy consumption of 96 kWh kgCOD^–1 at the cell voltage of near 4 V in 50 mM chloride. The current efficiency and energy efficiency for H₂ generation were calculated to range from 34 to 84% and 14 to 26%, respectively. The hydrogen comprised 35 to 60% by volume of evolved gases. The efficacy of our electrolysis cell was further demonstrated by a 20 L prototype reactor totally powered by a photovoltaic (PV) panel, which was shown to eliminate COD and total coliform bacteria in less than 4 h of treatment