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 TiO₂ (BiO_<i>x</i>/TiO₂) anode coupled with a stainless steel cathode at applied anodic potentials (<i>E</i>_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 <i>E</i>_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

Bi_<i>x</i>Ti_1–<i>x</i>O_<i>z</i> Functionalized Heterojunction Anode with an Enhanced Reactive Chlorine Generation Efficiency in Dilute Aqueous Solutions

Ir_0.7Ta_0.3O_<i>y</i>/Bi_<i>x</i>Ti_1–<i>x</i>O_<i>z</i> 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_<i>x</i>(·OH)) and higher valence-state oxides (>MO_<i>x</i>+1). An underlying layer of the mixed-metal oxide, Ir_0.7Ta_0.3O_<i>y</i>, was synthesized to serve as a primary Ohmic contact and electron shuttle. Binary thin films of Bi_<i>x</i>Ti_1–<i>x</i>O_<i>z</i> 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 <i>x</i> = 0.1 or 0.3 were twice of the η_RCS of the Ir_0.7Ta_0.3O_<i>y</i> 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_<i>x</i>(·OH) to >MO_<i>x</i>+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_0.7Ta_0.3O_<i>y</i>/TiO₂ was found to be minimal, while Bi_<i>x</i>Ti_1–<i>x</i>O_<i>z</i> in the surface layers undergoes oxidation and a subsequent dissolution

Phosphate Recovery from Human Waste via the Formation of Hydroxyapatite during Electrochemical Wastewater Treatment

Electrolysis of toilet wastewater with TiO₂-coated semiconductor anodes and stainless steel cathodes is a potentially viable onsite sanitation solution in parts of the world without infrastructure for centralized wastewater treatment. In addition to treating toilet wastewater, pilot-scale and bench-scale experiments demonstrated that electrolysis can remove phosphate by cathodic precipitation as hydroxyapatite at no additional energy cost. Phosphate removal could be predicted based on initial phosphate and calcium concentrations, and up to 80% total phosphate removal was achieved. While calcium was critical for phosphate removal, magnesium and bicarbonate had only minor impacts on phosphate removal rates at concentrations typical of toilet wastewater. Optimal conditions for phosphate removal were 3 to 4 h treatment at about 5 mA cm^–2 (∼3.4 V), with greater than 20 m² m^–3 electrode surface area to reactor volume ratios. Pilot-scale systems are currently operated under similar conditions, suggesting that phosphate removal can be viewed as an ancillary benefit of electrochemical wastewater treatment, adding utility to the process without requiring additional energy inputs. Further value may be provided by designing reactors to recover precipitated hydroxyapatite for use as a low solubility phosphorus-rich fertilizer

Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol Episodes

Haze events in China megacities involve the rapid oxidation of SO₂ to sulfate aerosol. Given the weak photochemistry that takes place in these optically thick hazes, it has been hypothesized that SO₂ is mostly oxidized by NO₂ emissions in the bulk of pH > 5.5 aerosols. Because NO₂(g) dissolution in water is very slow and aerosols are more acidic, we decided to test such a hypothesis. Herein, we report that > 95% of NO₂(g) disproportionates [2NO₂(g) + H₂O­(l) = H⁺ + NO₃^–(aq) + HONO (R1)] upon hitting the surface of NaHSO₃ aqueous microjets for < 50 μs, thereby giving rise to strong NO₃^– (<i>m</i>/<i>z</i> 62) signals detected by online electrospray mass spectrometry, rather than oxidizing HSO₃^– (<i>m</i>/<i>z</i> 81) to HSO₄^– (<i>m</i>/<i>z</i> 97) in the relevant pH 3–6 range. Because NO₂(g) will be consumed via R1 on the surface of typical aerosols, the oxidation of S­(IV) may in fact be driven by the HONO/NO₂^– generated therein. S­(IV) heterogeneous oxidation rates are expected to primarily depend on the surface density and liquid water content of the aerosol, which are enhanced by fine aerosol and high humidity. Whether aerosol acidity affects the oxidation of S­(IV) by HONO/NO₂^– remains to be elucidated

Electrochemical Transformation of Trace Organic Contaminants in Latrine Wastewater

Solar-powered electrochemical systems have shown promise for onsite wastewater treatment in regions where basic infrastructure for conventional wastewater treatment is not available. To assess the applicability of these systems for trace organic contaminant treatment, test compound electrolysis rate constants were measured in authentic latrine wastewater using mixed-metal oxide anodes coupled with stainless steel cathodes. Complete removal of ranitidine and cimetidine was achieved within 30 min of electrolysis at an applied potential of 3.5 V (0.7 A L^–1). Removal of acetaminophen, ciprofloxacin, trimethoprim, propranolol, and carbamazepine (>80%) was achieved within 3 h of electrolysis. Oxidation of ranitidine, cimetidine, and ciprofloxacin was primarily attributed to reaction with NH₂Cl. Transformation of trimethoprim, propranolol, and carbamazepine was attributed to direct electron transfer and to reactions with surface-bound reactive chlorine species. Relative contributions of aqueous phase ·OH, ·Cl, ·Cl₂^–, HOCl/OCl^–, and Cl₂ were determined to be negligible based on measured second-order reaction rate constants, probe compound reaction rates, and experiments in buffered Cl^– solutions. Electrical energy per order of removal (<i>E</i>_EO) increased with increasing applied potentials and current densities. Test compound removal was most efficient at elevated Cl^– concentrations present when treated wastewater is recycled for use as flushing water (i.e., ∼ 75 mM Cl^–; E_EO = 0.2–6.9 kWh log^–1 m^–3). Identified halogenated and oxygenated electrolysis products typically underwent further transformations to unidentifiable products within the 3 h treatment cycle. Identifiable halogenated byproduct formation and accumulation was minimized during electrolysis of wastewater containing 75 mM Cl^–

Spectroscopic Study on CdS/Ni/KNbO₃: Confirming Ni Effect to Photocatalytic Activity

Herein, we report the structural and photophysical properties of CdS/Ni/KNbO3 composites with a quantum yield for photocatalytic H2 generation that is CdS and Ni amount dependent. The nonstoichiometric KNbO3 (1:1.1) structure indicates the defect at the K site, which is Ni-occupied during its deposit process. It exhibits a tendency like a Ni-doped characteristic up to 0.1 wt % Ni and then forms a Ni cluster in case the Ni amount exceeds 0.1 wt %. The related structural and photophysical properties of CdS/Ni/KNbO3 are examined with Fourier transform infrared, X-ray diffraction, ultraviolet–visible absorption, and luminescence spectral analysis. It demonstrates the CdS/Ni/KNbO3 composites to be an efficient light conversion caused by efficient charge/electron transfer between KNbO3 and CdS via doped Ni. The photocatalytic activity of CdS/Ni/KNbO3 exhibits a CdS and Ni amount dependency. The best photocatalytic activity for H2 generation is obtained with 0.1 wt % Ni and 2.9 wt % CdS as it gradually declines with the excess Ni amount than 0.1 wt % caused by a formed Ni cluster

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

Confocal Fluorescence Microscopy of the Morphology and Composition of Interstitial Fluids in Freezing Electrolyte Solutions

Ice rheology, the integrity of polar ice core records, and ice−atmosphere interactions are among the phenomena controlled by the morphology and composition of interstitial fluids threading polycrystalline ice. Herein, we investigate how ionic impurities affect such features via time-resolved confocal fluorescence microscopy of freezing electrolyte solutions doped with a pH probe. We find that the 10 μM probe accumulates into 12 μm thick glassy channels in frozen water, but it is incorporated into randomly distributed <1 μm diameter inclusions in freezing 1 mM NaCl. We infer that morphology is largely determined by the dynamic instabilities generated upon advancing ice by the rejected solute, rather than by thermodynamics. The protracted alkalinization of the fluid inclusions reveals that the excess negative charge generated by the preferential incorporation of Cl⁻ over Na⁺ in ice is neutralized by the seepage of the OH⁻ slowly produced via H₂O → H⁺ + OH⁻ thermal dissociation

Superacid Chemistry on Mildly Acidic Water

The mechanism of proton transfer across water−hydrophobic media boundaries is investigated in experiments in which the protonation of gaseous <i>n</i>-hexanoic acid (PCOOH) upon collision with liquid water microjets is monitored by online electrospray mass spectrometry as a function of pH. Although PCOOH(aq) is a very weak base (p<i>K</i>_BH⁺ < −3), PCOOH(g) is converted to PC(OH)₂⁺ on pH < 4 water via a process that ostensibly retains some of the exoergicity of its gas-phase counterpart, PCOOH + H₃O⁺ = PC(OH)₂⁺ + H₂O, Δ<i>G</i> < −22 kcal mol⁻¹. The large kinetic isotope effects observed on H₂O/D₂O microjets, PC(OH)₂⁺/PC(OH)OD⁺ = 88 and PC(OH)OD⁺/PC(OD)₂⁺ = 156 at pD = 2, and their inverse dependences on pH indicate that PCOOH(g) hydronation on water (1) involves tunneling, (2) is faster than H-isotope exchange, and (3) is progressively confined to the outermost layers as water becomes more acidic. Proton transfers across steep water density gradients appear to be promoted by both dynamic and thermodynamic factors

UV/Nitrilotriacetic Acid Process as a Novel Strategy for Efficient Photoreductive Degradation of Perfluorooctanesulfonate

Perfluorooctanesulfonate (PFOS) is a toxic, bioaccumulative, and highly persistent anthropogenic chemical. Hydrated electrons (<i>e</i>_aq^–) are potent nucleophiles that can effectively decompose PFOS. In previous studies, <i>e</i>_aq^– are mainly produced by photoionization of aqueous anions or aromatic compounds. In this study, we proposed a new photolytic strategy to generate <i>e</i>_aq^– and in turn decompose PFOS, which utilizes nitrilotriacetic acid (NTA) as a photosensitizer to induce water photodissociation and photoionization, and subsequently as a scavenger of hydroxyl radical (^•OH) to minimize the geminate recombination between ^•OH and <i>e</i>_aq^–. The net effect is to increase the amount of <i>e</i>_aq^– available for PFOS degradation. The UV/NTA process achieved a high PFOS degradation ratio of 85.4% and a defluorination ratio of 46.8% within 10 h. A pseudo-first-order rate constant (<i>k</i>) of 0.27 h^–1 was obtained. The laser flash photolysis study indicates that <i>e</i>_aq^– is the dominant reactive species responsible for PFOS decomposition. The generation of <i>e</i>_aq^– is greatly enhanced and its half-life is significantly prolonged in the presence of NTA. The electron spin resonance (ESR) measurement verified the photodissociation of water by detecting ^•OH. The model compound study indicates that the acetate and amine groups are the primary reactive sites