Urea Degradation by Electrochemically Generated Reactive Chlorine Species: Products and Reaction Pathways

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 TiO_2 (BiOx/TiO_2) 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 (Cl_2–•) ions facilitate the urea transformation primary driven by free chlorine. Quadrupole mass spectrometer analysis of the reactor headspace revealed that N_2 and CO_2 are the primary gaseous products of the oxidation of urea, whose urea-N was completely transformed into N_2 (91%) and NO_3– (9%). The higher reaction selectivity with respect to N_2 production can be ascribed to a low operational ratio of free available chlorine to N. The mass-balance analysis recovered urea-C as CO_2 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

Electrochemical treatment of human waste coupled with molecular hydrogen production

We have developed a wastewater treatment system that incorporates an electrolysis cell for on-site wastewater treatment coupled with molecular hydrogen production for use in a hydrogen fuel cell. Herein, we report on the efficacy of a laboratory-scale wastewater electrolysis cell (WEC) using real human waste for the first time with semiconductor electrode utilizing a mixed particle coating of bismuth oxide doped titanium dioxide (BiO_x/TiO_2). A comprehensive environmental analysis has been coupled together with a robust kinetic model under the chemical reaction limited regime to investigate the role of various redox reactions mediated by chloride present in human waste. The oxidative elimination of the chemical oxygen demand (COD) and ammonium ion can be modelled using empirical, pseudo-first-order rate constants and current efficiencies (CE). In combination with an anaerobic pre-treatment step, human waste containing high-levels of COD, protein, and color are eliminated within 6 hours of batch treatment in the WEC. The reactor effluent has a residual inorganic total nitrogen (TN) concentration of [similar]40 mM. The CE and specific energy consumption were 8.5% and 200 kWh per kgCOD for the COD removal, 11% and 260 for kWh per kgTN for the TN conversion. The CE and energy efficiencies (EE) for hydrogen production were calculated to be 90% and 25%, respectively

Bi_xTi_(1−x)O_z Functionalized Heterojunction Anode with an Enhanced Reactive Chlorine Generation Efficiency in Dilute Aqueous Solutions

Ir_(0.7)Ta_(0.3)O_y/Bi_xTi_(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 valence-state 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_xTi_(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_xTi_(1–x)O_z in the surface layers undergoes oxidation and a subsequent dissolution

Tailoring Heterojunction Architecture on IrO2 Based Dimensionally Stable Anodes for Environmental Applications

1

Wastewater Electrolysis Cell for Environmental Pollutants Degradation and Molecular Hydrogen Generation

This study proposes a wastewater electrolysis cell (WEC) for on-site treatment of human waste coupled with decentralized molecular H2 production. The core of the WEC includes mixed metal oxides anodes functionalized with bismuth doped TiO2 (BiOx/TiO2). The BiOx/TiO2 anode shows reliable electro-catalytic activity to oxidize Cl- to reactive chlorine species (RCS), which degrades environmental pollutants including chemical oxygen demand (COD), protein, NH4+, urea, and total coliforms. The WEC experiments for treatment of various kinds of synthetic and real wastewater demonstrate sufficient water quality of effluent for reuse for toilet flushing and environmental purposes. Cathodic reduction of water and proton on stainless steel cathodes produced molecular H2 with moderate levels of current and energy efficiency. This thesis presents a comprehensive environmental analysis together with kinetic models to provide an in-depth understanding of reaction pathways mediated by the RCS and the effects of key operating parameters. The latter part of this thesis is dedicated to bilayer hetero-junction anodes which show enhanced generation efficiency of RCS and long-term stability. Chapter 2 describes the reaction pathway and kinetics of urea degradation mediated by electrochemically generated RCS. The urea oxidation involves chloramines and chlorinated urea as reaction intermediates, for which the mass/charge balance analysis reveals that N2 and CO2 are the primary products. Chapter 3 investigates direct-current and photovoltaic powered WEC for domestic wastewater treatment, while Chapter 4 demonstrates the feasibility of the WEC to treat model septic tank effluents. The results in Chapter 2 and 3 corroborate the active roles of chlorine radicals (Cl•/Cl2-•) based on iR-compensated anodic potential (thermodynamic basis) and enhanced pseudo-first-order rate constants (kinetic basis). The effects of operating parameters (anodic potential and [Cl-] in Chapter 3; influent dilution and anaerobic pretreatment in Chapter 4) on the rate and current/energy efficiency of pollutants degradation and H2 production are thoroughly discussed based on robust kinetic models. Chapter 5 reports the generation of RCS on Ir0.7Ta0.3Oy/BixTi1-xOz hetero-junction anodes with enhanced rate, current efficiency, and long-term stability compared to the Ir0.7Ta0.3Oy anode. The effects of surficial Bi concentration are interrogated, focusing on relative distributions between surface-bound hydroxyl radical and higher oxide.</p

Unraveling electrochemical chlorination of ammoniacal water

1

Nickel based electrocatalysts for environmental and energy applications

1

Self-Doped TiO2 Nanotube Array for Photoelectrochemical Water Treatment: Effects of Cathodization Condition

1