Photoelectrochemical reduction of Cr (VI) on plate-like WO3/BiVO4 composite electrodes under visible-light irradiation: characteristics and kinetic study

Fabrication of Ru-CoFe2O4/RGO hierarchical nanostructures for high-performance photoelectrodes to reduce hazards Cr(VI) into Cr(III) coupled with anodic oxidation of phenols

Dual-functional photo (electro)catalysis (PEC) is a key strategy for removing coexisting heavy metals and phenolic compounds from wastewater treatment systems. To design a PEC cell, it is crucial to use chemically stable and cost-effective bifunctional photocatalysts. The present study shows that ruthenium metallic nanoparticles decorated with CoFe₂O₄/RGO (Ru-CoFe₂O₄/RGO) are effective bifunctional photoelectrodes for the reduction of Cr(VI) ions. Ru-CoFe₂O₄/RGO achieves a maximum Cr(VI) reduction rate of 99% at 30 min under visible light irradiation, which is much higher than previously reported catalysts. Moreover, PEC Cr(VI) reduction rate is also tuned by adding varying concentration of phenol. A mechanism for the concurrent removal of Cr(VI) and phenol has been revealed over a bifunctional Ru-CoFe₂O₄/RGO catalyst. A number of key conclusions emerged from this study, demonstrating the dual role of phenol during Cr(VI) reduction by PEC. Anodic oxidation of phenol produces the enormous H⁺ ion, which appears to be a key component of Cr(VI) reduction. Additionally, phenolic molecules serve as hole (h⁺) scavengers that reduce e^-/h⁺ recombination, thus enhancing the reduction rate of Cr(VI). Therefore, the Ru-CoFe₂O₄/RGO photoelectrode exhibits a promising capability of reducing both heavy metals and phenolic compounds simultaneously in wastewater.

Carnation-like Morphology of BiVO4-7 Enables Sensitive Photoelectrochemical Determination of Cr(VI) in the Food and Environment

Hexavalent chromium, namely, Cr(VI), is a significant threat to ecological and food safety. Current detection methods are not sensitive to Cr(VI). A photoelectrochemical (PEC) sensor based on bismuth vanadate (BiVO₄) was developed for sensitive detection of Cr(VI). First, BiVO₄-X (X: the pH of the reaction precursor solution) was synthesized using a facile surfactant-free hydrothermal method. The BiVO₄-X morphology was well controlled according to pH values, showing rock-like (X = 1), wrinkled bark-like (X = 4), carnation-like (X = 7), and the collapsed sheet-like morphologies (X = 9, 12). BiVO₄-7 exhibited excellent photoelectric performance due to a proper band structure under visible light and a large specific surface area. Then, BiVO₄-7 was used to construct a PEC sensor to detect Cr(VI), which was demonstrated to have a low detection limit (10 nM) and wide detection range (2–210 μM). The BiVO₄-7 PEC sensor had a stable output signal, as well as excellent reproducibility, repeatability, and selectivity. We used the BiVO₄-7 PEC sensor to detect Cr(VI) in real environmental and food samples, resulting in a satisfactory recovery of 90.3–103.0%, as determined by comparison with results obtained using a spectrophotometric method. The BiVO₄-7 PEC sensor is promising for practical application to heavy metal detection in the food and environment.

Electrochemical recovery and high value-added reutilization of heavy metal ions from wastewater: Recent advances and future trends

Wastewater treatment for heavy metals is currently transitioning from pollution remediation towards resource recovery. As a controllable and environment-friendly method, electrochemical technologies have recently gained significant attention. However, there is a lack of systematic and goal oriented summarize of electrochemical metal recovery techniques, which has inhibited the optimized application of these methods. This review aims at recent advances in electrochemical metal recovery techniques, by comparing different electrochemical recovery methods, attempts to target recycling heavy metal resources with minimize energy consumption, boost recovery efficiency and realize the commercial application. In this review, different electrochemical recovery methods (including E-adsorption recovery, E-oxidation recovery, E-reduction recovery, and E-precipitation recovery) for recovering heavy metals are introduced, followed an analysis of their corresponding mechanisms, influencing factors, and recovery efficiencies. In addition, the mass transfer efficiency can be promoted further through optimizing electrodes and reactors, and multiple technologies (photo-electrochemical and sono-electrochemical) could to be used synergistically improve recovery efficiencies. Finally, the most promising directions for electrochemical recovery of heavy metals are discussed along with the challenges and future opportunities of electrochemical technology in recycling heavy metals from wastewater.

WO3-x horizontally-grown on TiO2(B) nanosheets for enhanced photo- and electro-chemical activity

WO3-x was deposited on TiO2(B) nanosheets prepared using TiCl4 to form layered heterostructures via a two-step solvothermal synthesis, in which the horizontal growth of WO3-x on TiO2(B) nanosheets was carried out using WCl6 and ascorbic acid as a reducer. Optimized preparation conditions allowed WO3-x/TiO2(B) layered heterostructures to be formed. The photo- and electro-chemical properties of layered heterostructures depended strongly on the amount of WO3-x. The WO3-x/TiO2(B) heterostructures demonstrated perfect catalytic performance in full solar-spectrum light and a fast degradation effect for dye and organic colorless pollutant. All target chemicals were degraded within 10 min using WO3-x/TiO2(B) samples as a photo-catalyst in the full solar-spectrum. The photo-assisted production kinetic of Cr(VI) ions were tested. The results indicate that the reproduction rate of Cr(VI) ions using WO3-x/TiO2(B) sample is three times higher than the initial TiO2 nanosheets. The result of the photo current and Mott-Shottky curve indicates that enhanced catalysis activity is ascribed to the surface of the metastable TiO2(B) with Ti3+ defects and oxygen vacancies as active sites for photocatalytic reaction. In addition, the loading of WO3-x greatly broadened the light absorption range of TiO2(B), meaning that the product responded in the full solar spectrum.

Semiconductor Electrode Materials Applied in Photoelectrocatalytic Wastewater Treatment—an Overview

Industrial sources of environmental pollution generate huge amounts of industrial wastewater containing various recalcitrant organic and inorganic pollutants that are hazardous to the environment. On the other hand, industrial wastewater can be regarded as a prospective source of fresh water, energy, and valuable raw materials. Conventional sewage treatment systems are often not efficient enough for the complete degradation of pollutants and they are characterized by high energy consumption. Moreover, the chemical energy that is stored in the wastewater is wasted. A solution to these problems is an application of photoelectrocatalytic treatment methods, especially when they are coupled with energy generation. The paper presents a general overview of the semiconductor materials applied as photoelectrodes in the treatment of various pollutants. The fundamentals of photoelectrocatalytic reactions and the mechanism of pollutants treatment as well as parameters affecting the treatment process are presented. Examples of different semiconductor photoelectrodes that are applied in treatment processes are described in order to present the strengths and weaknesses of the photoelectrocatalytic treatment of industrial wastewater. This overview is an addition to the existing knowledge with a particular focus on the main experimental conditions employed in the photoelectrocatalytic degradation of various pollutants with the application of semiconductor photoelectrodes.