Review on electrical discharge plasma technology for wastewater remediation

This exhaustive review focuses on the fundamental principles and applications of heterogeneous electrochemical wastewater treatment based on Fenton’s chemistry reaction. The elementary equations involved in formation of hydroxyl radical in homogeneous electro-Fenton (EF) and photo electro-Fenton (PEF) processes was presented and the advantages of using insoluble solids as heterogeneous catalyst rather than soluble iron salts (heterogeneous electro-Fenton process) (Hetero-EF) was enumerated. Some of the required features of good heterogeneous catalysts were discussed, followed by the mechanisms of catalytic activation of H2O2 to reactive oxygen species (ROS) especially hydroxyl radical ( OH) by heterogeneous catalyst in Hetero-EF system. Extensive discussion on the two configuration of Hetero-EF system vis-a-vis added solid catalysts and functionalized cathodic materials were provided along with summaries of some relevant studies that are available in literature. The solid catalysts and the functionalized cathodic materials that have been utilized in Hetero-EF wastewater treatment were grouped into different classes and brief discussion on their synthesis route were given. Besides, the use of solid catalysts and iron-functionalized cathodic materials in bioelectrochemical system (BES) especially bioelectro-Fenton technology (BEF) using microbial fuel cells (MFCs) with concurrent electricity generation for Hetero-EF treatment of biorefractory organic pollutants was discussed. In the final part, emphasis was made on the challenges and future prospects of the Hetero-EF for wastewater treatment.

Heterogeneous electro-Fenton and photoelectro-Fenton processes: A critical review of fundamental principles and application for water/wastewater treatment

A large variety of electrochemical advanced oxidation processes (EAOPs) have been recently developed to remove organic pollutants from wastewaters to avoid their serious health-risk factors from their environmental accumulation and to reuse the treated water for human activities. The effectiveness of EAOPs is based on the in situ production of strong reactive oxygen species (ROS) like hydroxyl radical ( OH). Photoelectrocatalysis (PEC) has emerged as a promising powerful EAOP by combining photocatalytic and electrolytic processes. It consists in the promotion of electrons from the valence band to the conduction band of a semiconductor photocatalyst upon light irradiation, with production of positive holes. The fast recombination of the electron/hole pairs formed is avoided in PEC by applying an external bias potential to the photocatalyst that extracts the photogenerated electrons up to the cathode of the electrolytic cell. Organics can be oxidized directly by the holes, •OH formed from water oxidation with holes and other ROS produced between the electrons and dissolved O2. This paper presents a general and critical review on the application of PEC to the remediation of wastewaters with organic pollutants. Special attention is made over the different kinds of photocatalysts utilized and preparation methods of the most ubiquitous TiO2 materials. Typical PEC systems and main operation variables that affect the effectiveness of the degradation process are also examined. An exhaustive analysis of the advances obtained on the treatment of dyes, chemicals and pharmaceuticals from synthetic solutions, as well as of real wastewaters, is performed. Finally, research prospects are proposed for the future development of PEC with perspectives to industrial application.

Applied photoelectrocatalysis on the degradation of organic pollutants in wastewaters

Carbon nitride (g-C3N4) has attracted great attention for its wide applications in hydrogen evolution and photocatalytic degradation. In this study, phosphorus doped porous ultrathin carbon nitride nanosheets (PCN-S) were prepared successfully via the element doping and thermal exfoliation method. The prepared PCN-S was characterized by XRD, SEM, TEM, N2-adsorption-desorption measurement, FT-IR, XPS, UV–vis diffuse reflectance spectra, photoluminescence (PL), photocurrent response (I-t) and EIS. The results show that PCN-S owns regular crystal structure of g-C3N4, large specific surface areas and nanosheet structure with lots of in-plane pores on its surface, excellent chemical stability, and broad light response to the whole visible light region, which was attributed to the doping of phosphorus element. Under visible light irradiation, the photocatalytic reduction of Cr(VI) over different samples indicated that the P doping and porous nanosheet structure play an important role for the enhanced performance of PCN-S. The reason was that P element doping can broaden the visible light response region, and large specific surface areas from the porous nanosheet structure can provide quantities of active sites for the photocatalytic reaction. Then the detailed study on the PCN-S for simultaneous photocatalytic reduction of Cr(VI) and oxidation of 2,4-diclorophenol (2,4-DCP) was conducted. The experiments results show that low pH value and enough dissolved oxygen were found to promote Cr(VI) reduction and 2,4-DCP oxidation. The detailed photocatalytic mechanism was proposed. The strategies used in this study could provide new insight into the design of g-C3N4 based materials with high photocatalytic activity, and present potential for the treatment of Cr(VI)/2,4-DCP or other mixed pollutants in wastewater.

Insight into highly efficient simultaneous photocatalytic removal of Cr(VI) and 2,4-diclorophenol under visible light irradiation by phosphorus doped porous ultrathin g-C3N4 nanosheets from aqueous media: Performance and reaction mechanism

Single-atom dispersed Ag modified mesoporous graphitic carbon nitride (Ag/mpg-C3N4) hybrid was synthesized by co-condensation method and employed as a visible light photocatalyst. In the presence of peroxymonosulfate (PMS), Ag/mpg-C3N4 showed excellent performance for the degradation of bisphenol A (BPA). 100% BPA and 80% TOC could be removed with 0.1 g/L catalyst and 1 mM PMS under visible light (λ > 400 nm) within 60 min. The reason for the enhanced performance is possibly due to the synergistic effect of single-atom Ag and mpg-C3N4. On the one hand, more visible light could be captured with the introduction of Ag; on the other hand, the existence of PMS promotes the separation efficiency of photogenerated electron-hole pairs. Electron spin resonance (ESR) and free radicals quenching experiment suggest that the major reactive oxygen species (ROS) are sulfate radical (SO4 −), superoxide radicals (O2 −) and photogenerated holes, while the role of hydroxyl radicals ( OH) is insignificant in this process. The findings of this work highlighted the great potential of g-C3N4 as photocatalysts and elucidated a new opportunity for PMS remediation of contaminated water.

Yi Zhang

Papers

Synthesis of α-Fe2O3/TiO2 nanotube arrays for photoelectro-Fenton degradation of phenol

Fabrication of visible-light-active MR/NH2-MIL-125(Ti) homojunction with boosted photocatalytic performance

Facile synthesis of AgI/BiOI-Bi2O3 multi-heterojunctions with high visible light activity for Cr(VI) reduction.

Synergistic photosensitized removal of Cr(VI) and Rhodamine B dye on amorphous TiO2 under visible light irradiation

Fabrication of 3D Bi2O3-BiOI heterojunction by a simple dipping method: Highly enhanced visible-light photoelectrocatalytic activity