Electrochemical Production of Hydrogen Coupled with the Oxidation of Arsenite

Abstract

The production of hydrogen accompanied by the simultaneous oxidation of arsenite (As(III)) was achieved using an electrochemical system that employed a BiO_x–TiO_2 semiconductor anode and a stainless steel (SS) cathode in the presence of sodium chloride (NaCl) electrolyte. The production of H_2 was enhanced by the addition of As(III) during the course of water electrolysis. The synergistic effect of As(III) on H_2 production can be explained in terms of (1) the scavenging of reactive chlorine species (RCS), which inhibit the production of H_2 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 (E_(cell)) of 3.0 V enhanced the production of H2 by 12% even though the cell current (I_(cell)) was reduced by 5%. The net effect results in an increase in the energy efficiency (EE) for H_2 production (ΔEE) by 17.5%. Furthermore, the value ΔEE, which depended on As(III) concentration, also depended on the applied E_(cell). For example, the ΔEE increased with increasing As(III) concentration in the micromolar range but decreased as a function of E_(cell). This is attributed to the fact that the reactions between RCS and As(III) are influenced by both RCS concentration depending on E_(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 H_2 production shows that an energetic byproduct can be produced during the remediation of a significant inorganic pollutant.