Photocatalytic oxidation mechanism of arsenite on tungsten trioxide under visible light

Visible light-induced photocatalytic degradation of tetrabromobisphenol A on platinized tungsten oxide

In this study, we investigated the degradation of the flame retardant tetrabromobisphenol A (TBBPA) using platinized tungsten oxide (Pt/WO3), synthesized via a simple photodeposition method, under visible light. The results of degradation experiments show a significant enhancement in TBBPA degradation upon surface platinization of WO3, with the degradation rate increasing by 13.4 times compared to bare WO3. The presence of Pt on the WO3 surface stores conduction band electrons, which facilitates the two-electron reduction of oxygen and enhances the production of valence band holes (hVB+) and hydroxyl radicals (●OH). Both hVB+ and ●OH are significantly involved in the degradation of TBBPA in the visible light-irradiated Pt/WO3 system. This was verified through fluorescence spectroscopy employing coumarin as a chemical probe and oxidizing species-quenching experiments. The analysis of degradation products and their toxicity assessment demonstrate that the toxicity of TBBPA-contaminated water is significantly reduced after Pt/WO3 photocatalysis. The degradation rate of TBBPA increased with increasing Pt/WO3 dosage, reached an optimum at a Pt content of 0.5 wt%, but decreased with increasing TBBPA concentration. The decrease in degradation efficiency of Pt/WO3 was minor, both in the presence of various anions and after repeated use. This study proposes that Pt/WO3 is a viable photocatalyst for the degradation of TBBPA in water under visible light.

Inhibition of biofouling on reverse osmosis membrane surfaces by germicidal ultraviolet light side-emitting optical fibers

Biofouling of membrane surfaces poses significant operational challenges and costs for desalination and wastewater reuse applications. Ultraviolet (UV) light can control biofilms while reducing chemical usage and disinfection by-products, but light deliveries to membrane surfaces in spiral wound geometries has been a daunting challenge. Thin and flexible nano-enabled side-emitting optical fibers (SEOFs) are novel light delivery devices that enable disinfection or photocatalytic oxidation by radiating UV light from light-emitting diodes (LEDs). We envision SEOFs as an active membrane spacer to mitigate biofilm formation on reverse osmosis (RO) membranes. A lab-scale RO membrane apparatus equipped with SEOFs allowed comparison of UV-A (photocatalysis-enabled) versus UV-C (direct photolysis disinfection). Compared against systems without any light exposure, systems with UV-C light formed thinner-but denser-biofilms, prevented permeate flux declines due to biofouling, and maintained the highest salt rejection. Results were corroborated by in-situ optical coherence tomography and ex-situ measurements of biofilm growth on the membranes. Transcriptomic analysis showed that UV-C SEOFs down-regulated quorum sensing and surface attachment genes. In contrast, UV-A SEOFs upregulated quorum sensing, surface attachment, and oxidative stress genes, resulting in higher extracellular polymeric substances (EPS) accumulation on membrane surfaces. Overall, SEOFs that deliver a low fluence of UV-C light onto membrane surfaces are a promising non-chemical approach for mitigating biofouling formation on RO membranes.

Highly efficient visible light-induced photocatalytic oxidation of arsenite with nanosized WO3 particles in the presence of Cu2+ and CuO

Although WO₃ appears to be one of the extensively studied photocatalysts, the low response of pure WO₃ in aqueous solution under visible light limits its application remarkably. In this work, the enhancement of the efficiency of WO₃ for the visible light-driven photocatalytic oxidation of arsenite was explored using Cu²⁺ ion and CuO as a co-catalyst. While the addition of Cu²⁺ was found effective for the suppression of dissolution of WO₃, the efficiency of CuO appeared to be slightly lower. Significant improvement of the efficiency for the photocatalytic oxidation of As(III) with WO₃ was noted when Cu²⁺ ions and CuO were added. The optimized conditions were WO₃ in the presence of 10 mg L^-1 Cu²⁺ ion and 1 wt% CuO coupled with WO₃, respectively. The As(III) concentration of 10 mg L^-1 could be lowered to less than 0.1 mg L^-1 by the photocatalytic treatment. Acidic pH favours the oxidation of arsenite in the presence of Cu²⁺ whereas basic pH is suitable with CuO. Characterization techniques such as TEM, XPS, XRD and UV-DRS were used to characterize photocatalysts. The reactive species scavenger tests revealed that the photo-induced holes (h⁺) play a key role in the photocatalytic oxidation process while the effect of •OH is negligible. It was found that As(III) oxidation rate was remarkably suppressed in the nitrogen atmosphere. A mechanism for enhanced photocatalytic oxidation has been proposed based on the results of the reactive species scavenger tests. This research may contribute to the large-scale As(III) oxidation treatment in the groundwater.

Abiotic oxidation of arsenite in natural and engineered systems: Mechanisms and related controversies over the last two decades (1999-2020)

Abiotic oxidation of toxic As(III) to As(V) is being deemed as a necessary step for the overall arsenic decontamination in both natural and engineered systems. Direct oxidation of As(III) by chemical oxidants, such as ozone, permanganate, ferrate, chlorine and chloramine, or naturally occurring minerals like Mn, Fe oxides, seems straightforward. Both O₂ and H₂O₂ are ineffective for arsenite oxidation, but they can be activated by reducing substances like Fe²⁺, Fe⁰ to increase the oxidation rates. Photo-induced oxidation of As(III) has been demonstrated effective in Fe complexes or minerals, NO₃^-/NO₂^-, dissolved organic matter (DOM), peroxygens and TiO₂ systems. Although a variety of oxidation methods have been developed over the past two decades, there remain many scientific and technical challenges that must be overcome before the rapid progress in basic knowledge can be translated into environmental benefits. To better understand the trends in the existing data and to identify the knowledge gaps, this review describes in detail the complicated mechanisms for As(III) oxidation by various methods and emphasizes on the conflicting data and explanation. Some prevailing concerns and challenges in the sphere of As(III) oxidation are also pointed out so as to appeal to researchers for further investigations.

Photocatalytic oxidation of urea on TiO2 in water and urine: mechanism, product distribution, and effect of surface platinization

The photocatalytic oxidation of urea on TiO₂ in water was compared with that in urine. Despite the presence of other organic compounds in urine, the oxidation efficiency of urea on TiO₂ in urine was higher than that in water. This enhanced oxidation of urea in urine is ascribed to the higher production of ^•OH (primary oxidant for urea degradation) by the adsorption of PO₄^3- (one constituent of urine) on the TiO₂ surface. Among the various anions in urine, only PO₄^3- was adsorbed on the surface of TiO₂. Both the production of ^•OH and the oxidation of urea were enhanced in the presence of PO₄^3-. These results indicate that the enhanced ^•OH production by in situ surface phosphorylation is the reason for the increased oxidation of urea in urine. Surface platinization of TiO₂ enhanced the oxidation of urea in water. However, the oxidation efficiency of urea on Pt/TiO₂ in urine was lower than that in water. This behavior is due to the adsorption of PO₄^3- and SO₄^2- in urine on Pt deposits, which inhibits the adsorption of oxygen and the interfacial electron transfer to oxygen. The product distribution (i.e., the molar ratio of NO₃^- to NH₄⁺) in water was different from that in urine because the negatively charged surface of TiO₂ in urine attracts the positively charged area of carbamic acid (intermediate) and encourages its decomposition into NH₄⁺ and not into NO₃^-.

Efficient photocatalytic oxidation of arsenite from contaminated water by Fe2O3-Mn2O3 nanocomposite under UVA radiation and process optimization with experimental design

The efficiency of photocatalytic oxidation process in arsenite (As(III)) removal from contaminated water by a new Fe₂O₃-Mn₂O₃ nanocomposite under UV_A radiation was investigated. The effect of nanocomposite dosage, pH and initial As(III) concentration on the photocatalytic oxidation of As(III) were studied by experimental design. The synthesized nanocomposite had a uniform and spherical morphological structure and contained 49.83% of Fe₂O₃ and 29.36% of Mn₂O₃. Based on the experimental design model, in photocatalytic oxidation process, the effect of pH was higher than other parameters. At nanocomposite concentrations of more than 12 mg L^-1, pH 4 to 6 and oxidation time of 30 min, photocatalytic oxidation efficiency was more than 95% for initial As(III) concentration of less than 500 μg L^-1. By decreasing pH and increasing the nanocomposite concentration, the photocatalytic oxidation efficiency was increased. Furthermore, by increasing the oxidation time from 10 to 240 min, in addition to oxidation of As(III) to arsenate (As(V)), the residual As(V) was adsorbed on the Fe₂O₃-Mn₂O₃ nanocomposite and total As concentration was decreased. Therefore, Fe₂O₃-Mn₂O₃ nanocomposite as a bimetal oxide, at low doses and short time, can enhance and improve the efficiency of the photocatalytic oxidation and adsorption of As(III) from contaminated water resources. Furthermore, the energy and material costs of the UV_A/Fe₂O₃-Mn₂O₃ system for photocatalytic oxidation of 1  mg L^-1 As(III) in the 1 L laboratory scale reactor was 0.0051 €.

The Combination of Hydrogen and Methanol Production through Artificial Photosynthesis-Are We Ready Yet?

Because 100 % quantum efficiency for the photosynthetic production of H₂ from H₂ O under visible illumination has been achieved recently, the oxidation of H₂ O to O₂ remains the bottleneck to the overall water-splitting reaction. Oxidation of CH₄ to CH₃ OH might be combined with water reduction instead, so that H₂ and CH₃ OH chemical fuels can be simultaneously produced through a one-step process under solar illumination. This combination would be a promising approach towards a more sustainable future of chemistry, in which developing different strategies for artificial photosynthesis is of paramount importance. By using free and adsorbed HO^. radicals on the semiconductor surface, CH₄ can be activated to H₃ C^. radicals and converted into CH₃ OH, respectively, with great selectivity up to 100 %. The present lack of efficient photosynthetic systems for the formation of H₂ and CH₃ OH from abundant H₂ O and CH₄ motivates future research for basic science and industrial applications.

Comparision of photocatalysis and photolysis processes for arsenic oxidation in water

The oxidation of As(III) to As(V) in aqueous solution was evaluated using heterogeneous photocatalysis and photolysis. The influence of TiO₂ as catalyst in different crystalline (rutile, anatase) and commercial forms was evaluated in a batch reactor and an insignificant difference was observed between them. The process by photocatalysis reached up to 97% As(III) oxidation and no significant difference was observed comparing to results obtained by photolysis. The photolysis experiments (UV radiation only), also carried out in a batch system, showed a high oxidation rate of As(III) (90% in 20min). The influence of different matrices (well water, river water and public water supply) were evaluated. Additionally, the effect of As(V) concentration, generated during the oxidation process, was studied. Continuous photolysis experiments using only UV radiation were performed, resulting in a high As(III) oxidation rate. Using a flow rate of 5mLmin^-1 and an initial concentration of As(III) 200µgL^-1, gave an oxidation percentage of As(III) of up to 72%, showing a simple and economical alternative to the oxidation step of As(III) to As(V) in the treatment of water contaminated with arsenic.

Synthesis of AG@AgCl Core-Shell Structure Nanowires and Its Photocatalytic Oxidation of Arsenic (III) Under Visible Light

Ag@AgCl core-shell nanowires were synthesized by oxidation of Ag nanowires with moderate FeCl₃, which exhibited excellent photocatalytic activity for As(III) oxidation under visible light. It was proved that the photocatalytic oxidation efficiency was significantly dependent on the mole ratio of Ag:AgCl. The oxidation rate of As(III) over Ag@AgCl core-shell nanowires first increased with the decrease of Ag⁰ percentage, up until the optimized synthesis mole ratio of Ag nanowires:FeCl₃ was 2.32:2.20, with 0.023 mg L^-1 min^-1 As(III) oxidation rate; subsequently, the oxidation rate dropped with the further decrease of Ag⁰ percentage. Effects of the pH, ionic strength, and concentration of humic acid on Ag@AgCl photocatalytic ability were also studied. Trapping experiments using radical scavengers confirmed that h⁺ and ·O₂^- acted as the main active species during the visible-light-driven photocatalytic process for As(III) oxidation. The recycling experiments validated that Ag@AgCl core-shell nanowires were a kind of efficient and stable photocatalyst for As(III) oxidation under visible-light irradiation.

Efficiently Visible-Light Driven Photoelectrocatalytic Oxidation of As(III) at Low Positive Biasing Using Pt/TiO2 Nanotube Electrode

A constant current deposition method was selected to load highly dispersed Pt nanoparticles on TiO2 nanotubes in this paper, to extend the excited spectrum range of TiO2-based photocatalysts to visible light. The morphology, elemental composition, and light absorption capability of as-obtained Pt/TiO2 nanotubes electrodes were characterized by FE-SEM, energy dispersive spectrometer (EDS), X-ray photoelectron spectrometer (XPS), and UV-vis spectrometer. The photocatalytic and photoelectrocatalytic oxidation of As(III) using a Pt/TiO2 nanotube arrays electrode under visible light (λ > 420 nm) irradiation were investigated in a divided anode/cathode electrolytic tank. Compared with pure TiO2 which had no As(III) oxidation capacity under visible light, Pt/TiO2 nanotubes exhibited excellent visible-light photocatalytic performance toward As(III), even at dark condition. In anodic cell, As(III) could be oxidized with high efficiency by photoelectrochemical process with only 1.2 V positive biasing. Experimental results showed that photoelectrocatalytic oxidation process of As(III) could be well described by pseudo-first-order kinetic model. Rate constants depended on initial concentration of As(III), applied bias potential and solution pH. At the same time, it was interesting to find that in cathode cell, As(III) was also continuously oxidized to As(V). Furthermore, high-arsenic groundwater sample (25 m underground) with 0.32 mg/L As(III) and 0.35 mg/L As(V), which was collected from Daying Village, Datong basin, Northern China, could totally transform to As(V) after 200 min under visible light in this system.