Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece Heterogeneous photodegradation of industrial dyes: An insight to different mechanisms and rate affecting parameters Idrees Khan a, *, Khalid Saeed a , Nisar Ali b , Ibrahim Khan c , Baoliang Zhang d , Muhammad Sadiq e a Department of Chemistry, Bacha Khan University Charsadda, Khyber Pakhtunkhwa, Pakistan b Huaiyin Institute of Technology, School of Science Northwestern Polytechnical University, China c Center of Integrative Petroleum Research, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, 31261, Saudi Arabia d School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’an, 710129, China e Department of Chemistry, University of Malakand, Khyber Pakhtunkhwa, Pakistan ARTICLE INFO Editor: G.L. Dotto Keywords: Photodegradation Photocatalyst Photodegradation mechanism Type I mechanism Type II mechanism Z-Scheme ABSTRACT The removal of wastes through solar-induced technologies is gaining extensive momentum over the last few years. Photocatalytic (PC) removal of dyes and other waste products from the main stream and aquatic media is considering industrially feasible technologies. Photocatalytic degradation (PD) could economically convert complex molecules into smaller, non-toxic, and lower molecular weight species through photo-assisted redox reactions. Mechanistically, PD process is based on the production of highly reactive, photogenerated hydroxyl and superoxide anion radicals that attack dye molecules and completely mineralize it into more unaffected species, such as CO 2 and H 2 O. This advance technology has advantages over other conventional methods in terms of simplicity, complete pollutants mineralization, no harmful byproducts, cost-effectiveness and reducing contaminants at parts per million (ppm) and parts per billion (ppb) level. The aim of this review is to provide a thorough overview of heterogeneous PD with the literature support. The first section of the review will address the basic principle and materials utilization. This will be followed by a brief section on the different mechanisms reported for PD over the years with suitable examples. Finally, an overview will be provided to various para- meters, which influence the process and mechanism pathway. This review aims to not only educate the be- ginners in the field of PD but also target experienced researchers, who are involved in heterogeneous PD and predicting mechanistic studies. 1. Introduction Advanced oxidation processes (AOPs) are light-induced processes, developed for the treatment of toxic organic pollutants through strong redox processes with specific radicals generated in the process [1,2]. AOPs based on the in situ generation of highly reactive hydroxyl radi- cals ( % OH) that non-selectively react and degrade highly recalcitrant organic compounds [3]. Commonly applied AOPs include photo- catalysis, ozonation, Fenton process, microwave enhanced AOP, wet air oxidation, ultraviolet radiated electrochemical oxidation and H 2 O 2 oxidation [4]. Fig. 1 summarized the different possibilities offered by AOPs. Among these AOPs, heterogeneous photocatalysis has success- fully minimized a broad range of pollutants at ambient pressure and temperature without forming any harmful intermediates [5]. 1.1. Conventional treatment methods and limitations Today more than 10,000 dyes are commercially available and are using increasingly in textile, plastic, rubber, cosmetics, pharmaceutical, and food industries [7]. During the dyeing processes, these industries lost about 10–15 % of dyes directly to wastewater [8]. Water con- tamination by colouring materials such as dyes is a serious life-threa- tening and harmful to the surrounding environment, aquatic life, and human beings in particular [9–12] because most of the dyes are mu- tagenic, carcinogenic and non-biodegradable due to their bulky size and complex structures [13]. The uncontrolled contaminations also reduce aquatic plants photosynthesis by retarding penetration of light into the water [14]. Access to clean and toxic-free, harmful microbes free and carcinogenic agents free water is the foremost research challenge of modern science, which needs special attention to ensure human sur- vival in the future [15]. Controlling water pollution is a big challenge to the scientific community in terms of protection and conservation of https://doi.org/10.1016/j.jece.2020.104364 Received 28 June 2020; Received in revised form 29 July 2020; Accepted 5 August 2020 ⁎ Corresponding author. E-mail address: idreeschem_uom@yahoo.com (I. Khan). Journal of Environmental Chemical Engineering 8 (2020) 104364 Available online 15 August 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved. T