We have investigated electrochemical treatment of real domestic wastewater coupled with simultaneous production of molecular H 2 as useful byproduct. The electrolysis cells employ multilayer semiconductor anodes with electroactive bismuth-doped TiO 2 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•, Cl 2 − •) 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 H 2 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.

Ir 0.7 Ta 0.3 O y /Bi x Ti 1−x O z heterojunction anodes have been developed and characterized for reactive chlorine species (RCS) generation in dilute aqueous solution (50 mM NaCl). The primary objective of the research was to control the electro-stationary speciation of hydrous metal oxides between hydroxyl radical (>MO x (•OH)) and higher valencestate oxides (>MO x+1 ). An underlying layer of the mixed-metal oxide, Ir 0.7 Ta 0.3 O y , was synthesized to serve as a primary Ohmic contact and electron shuttle. Binary thin films of Bi x Ti 1−x O z were prepared from the thermal decomposition of an aqueous solution mixture of Ti/Bi complexes. With these core components, the measured current efficiency for RCS generation (η RCS ) was enhanced where the values observed for x = 0.1 or 0.3 were twice of the η RCS of the Ir 0.7 Ta 0.3 O y anode. At the same time, the rates of RCS generation were enhanced by factors of 20−30%. Partial substitution of Ti with Bi results in a positive shift in surface charge allowing for stronger interaction with anions, as confirmed by FTIR-ATR analysis. A kinetic model to describe the formate ion degradation showed that an increasing fraction of Bi in the composite promotes a redox transition of >MO x (•OH) to >MO x+1 . In accelerated life tests under conditions corresponding to a service life of 2 years under an operational current density of 300 A m −2 , dissociation of the Ti component from Ir 0.7 Ta 0.3 O y /TiO 2 was found to be minimal, while Bi x Ti 1−x O z in the surface layers undergoes oxidation and a subsequent dissolution.

This study investigated the transformation of urea by electrochemically generated reactive chlorine species (RCS). Solutions of urea with chloride ions were electrolyzed using a bismuth doped TiO2 (BiO x /TiO2) anode coupled with a stainless steel cathode at applied anodic potentials (E a) of either +2.2 V or +3.0 V versus the normal hydrogen electrode. In NaCl solution, the current efficiency of RCS generation was near 30% at both potentials. In divided cell experiments, the pseudo-first-order rate of total nitrogen decay was an order of magnitude higher at E a of +3.0 V than at +2.2 V, presumably because dichlorine radical (Cl2 –·) ions facilitate the urea transformation primary driven by free chlorine. Quadrupole mass spectrometer analysis of the reactor headspace revealed that N2 and CO2 are the primary gaseous products of the oxidation of urea, whose urea-N was completely transformed into N2 (91%) and NO3 – (9%). The higher reaction selectivity with respect to N2 production can be ascribed to a low operational ratio of free available chlorine to N. The mass-balance analysis recovered urea-C as CO2 at 77%, while CO generation most likely accounts for the residual carbon. In light of these results, we propose a reaction mechanism involving chloramines and chloramides as reaction intermediates, where the initial chlorination is the rate-determining step in the overall sequence of reactions.