Advances in photocatalytic and electrocatalytic removal of chlorinated volatile organic compounds

Abstract

Chlorinated volatile organic compounds (Cl-VOCs), including chlorinated alkanes, chlorinated alkenes and chlorinated aromatic compounds and so on, is a typical kind of environmental pollutants. Due to their significant characteristics of high volatility and poor degradability, they can be transported over long distance and existed in the environmental medium for a long time. In addition, compared with ordinary VOCs, Cl-VOCs show a high toxicity and an obvious effect of mutagenesis, carcinogenesis and teratogenesis. Most of the Cl-VOCs have been listed as priority pollutants, and were monitored and controlled strictly. Therefore, the development of efficient purification and transformation technology for Cl-VOCs has attracted great attention all over the world. At present, a large number of Cl-VOCs purification and transformation technologies have been developed for different conditions and purposes, including recovery technologies (adsorption, absorption, membrane separation and condensation), destruction technologies (incineration, catalytic oxidation, photocatalytic degradation and biodegradation) and conversion technologies (dechlorination). For different types, concentrations and emission sources of Cl-VOCs, each technology shows different advantages and limitations. Among them, photocatalytic degradation and electrocatalytic dechlorination technologies have attracted extensive attention due to their advantages of highly catalytic efficiency, economic and environmental processes, and mild reaction conditions. Photocatalytic degradation mainly uses clean solar energy to stimulate the electronic transition of semiconductor materials, the excited electrons and generated holes reacted with adsorptive species on the material surface to form strong radicals. The Cl-VOCs molecules are attacked by the radicals and mineralized into inorganic molecules, like CO2, H2O and so on. Therefore, it is one of the most promising purification technologies for Cl-VOCs. The key of the development of photocatalytic degradation technology is the preparation of advanced photocatalysts. To accelerate the separation of photogenerated electrons and holes and improve the utilization of light energy, a series of photocatalysts have been developed, such as noble metal catalysts, transition metal catalysts and nonmetallic catalysts. Generally, the photocatalytic performance of catalyst is significantly influenced by the crystal structure, micro structure and electronic structure. Therefore, the methods of regulating physicochemical properties, semiconductor compounding (heterojunction), element doping have been used to design advanced catalyst. Furthermore, the analysis of the intermediate products and the prediction of possible degradation pathway can help us to understand the degradation reaction of Cl-VOCs. The highly environmental toxicity of Cl-VOCs mostly comes from the Cl group. The harm to environment will be greatly reduced if the Cl groups can be effectively removed. At the same time, the highly-valuable dechlorinated products can be recycled. Therefore, reductive dechlorination has become one of the most promising detoxification and remediation technologies for Cl-VOCs. Due to the high reduction potential of C-Cl bond, photocatalytic reduction of Cl-VOCs is difficult, and electrocatalytic reduction is regarded as the main method for dechlorination. According to the involved reductants, electrochemical dechlorination can be generally divided into direct electrochemical reduction method and electrocatalytic hydrodechlorination method. The developing of high performance electrocatalysts and exploring the reaction mechanism are the key points of electrochemical dechlorination technology. This paper aims to comprehensively introduce the research progress of photocatalytic degradation and electrocatalytic dechlorination technologies for Cl-VOCs purification and transformation, providing reference for the effective treatment and resource utilization of Cl-VOCs. Based on this, the mechanism of catalytic reaction and the factors affecting the activity of catalysts were summarized, which provided ideas for the rational design of advanced catalysts. Finally, the potential development trends and research directions in the field of Cl-VOCs purification and transformation were prospected.