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

Freezing-Enhanced Dissolution of Iron Oxides: Effects of Inorganic Acid Anions

Dissolution of iron from mineral dust particles greatly depends upon the type and amount of copresent inorganic anions. In this study, we investigated the roles of sulfate, chloride, nitrate, and perchlorate on the dissolution of maghemite and lepidocrocite in ice under both dark and UV irradiation and compared the results with those of their aqueous counterparts. After 96 h of reaction, the total dissolved iron in ice (pH 3 before freezing) was higher than that in the aqueous phase (pH 3) by 6–28 times and 10–20 times under dark and UV irradiation, respectively. Sulfuric acid was the most efficient in producing labile iron under dark condition, whereas hydrochloric acid induced the most dissolution of the total and ferrous iron in the presence of light. This ice-induced dissolution result was also confirmed with Arizona Test Dust (AZTD). In the freeze–thaw cycling test, the iron oxide samples containing chloride, nitrate, or perchlorate showed a similar extent of total dissolved iron after each cycling while the sulfate-containing sample rapidly lost its dissolution activity with repeating the cycle. This unique phenomenon observed in ice might be related to the freeze concentration of protons, iron oxides, and inorganic anions in the liquid-like ice grain boundary region. These results suggest that the ice-enhanced dissolution of iron oxides can be a potential source of bioavailable iron, and the acid anions critically influence this process

Production of Molecular Iodine and Tri-iodide in the Frozen Solution of Iodide: Implication for Polar Atmosphere

The chemistry of reactive halogens in the polar atmosphere plays important roles in ozone and mercury depletion events, oxidizing capacity, and dimethylsulfide oxidation to form cloud-condensation nuclei. Among halogen species, the sources and emission mechanisms of inorganic iodine compounds in the polar boundary layer remain unknown. Here, we demonstrate that the production of tri-iodide (I₃^–) via iodide oxidation, which is negligible in aqueous solution, is significantly accelerated in frozen solution, both in the presence and the absence of solar irradiation. Field experiments carried out in the Antarctic region (King George Island, 62°13′S, 58°47′W) also showed that the generation of tri-iodide via solar photo-oxidation was enhanced when iodide was added to various ice media. The emission of gaseous I₂ from the irradiated frozen solution of iodide to the gas phase was detected by using cavity ring-down spectroscopy, which was observed both in the frozen state at 253 K and after thawing the ice at 298 K. The accelerated (photo-)­oxidation of iodide and the subsequent formation of tri-iodide and I₂ in ice appear to be related with the freeze concentration of iodide and dissolved O₂ trapped in the ice crystal grain boundaries. We propose that an accelerated abiotic transformation of iodide to gaseous I₂ in ice media provides a previously unrecognized formation pathway of active iodine species in the polar atmosphere