۱- Introduction

۲- Improvements on TiO2 photocatalytic activity

۳- Novel photocatalysts

۴- Photocatalysts for water and wastewater treatment

۵- Photocatalysts for endocrine disrupters and pesticides

۶- Anti-microbial applications of photocatalysts

۷- Metal organic frame works (MOFs)

۸- Conclusions

References

Abstract

Advanced Oxidation technologies (AOTs) are gaining attention as an effective waste water treatment methodology capable of degrading diverse spectrum of recalcitrant organic contaminants and microbes. Undoubtedly, photocatalysis is a promising AOT to alleviate the problem of water pollution. Despite recent research into other photocatalysts (e.g. ZnO, ZnS, Semiconductor-Graphene composites, perovskites, MoS2, WO3 and Fe2O3), titanium dioxide (TiO2) remains the most popular photocatalyst due to its low cost, nontoxicity and high oxidising ability. Moreover, titania photocatalysts can easily be immobilized on various surfaces and be scaled up for large scale water treatment. The current review aims to highlight recent advancements in photocatalytic AOTs with main emphasis on TiO2 photocatalysis. This review also discusses the use of TiO2 photocatalysis for water and waste treatment, treating contaminants of emerging concern (CECs), pesticides, endocrine disrupters (EDs) and bacteria using both UV and visible light irradiations. It was concluded that with efficient photoreactor configuration and further studies on the photocatalyst regeneration, TiO2 photocatalysis is a viable option for the reclamation of agricultural/irrigational waste water. Novel doped photocatalysts such as ZnS-CuS-CdS, carbon spheres/CdS, g-C3N4-Au-CdS, ZnS-WS2-CdS, C3N4-CdS and Pd-Cr2O3-CdS have also been discussed. Finally, the advances in the actively studied metal organic framework based photocatalysts that are emerging as effective alternate for metal oxide based photocatalysts is also discussed in detail.

Introduction

Photochemical AOPs are the most preferred because they offer the possibility of utilizing naturally available and renewable solar energy as light source for photochemical waste remediation, thereby making the process green and sustainable. TiO2 photocatalysis and photo-Fenton are two popular photochemical AOPs. Among them, TiO2 photocatalysis has gained particular interest because of heterogeneous nature that offers the possibility of catalyst reuse. In addition, it can operate at wide pH range unlike photo-Fenton process. TiO2 is environmentally benign, biocompatible, abundantly available, highly stable and low cost metal oxide photocatalyst with ability to efficiently degrade a spectrum of contaminants [1–۱۷]. TiO2 can be immobilized in to a variety of supports without much loss of its photocatalytic efficiency. This important feature favours the development of TiO2 photocatalytic process for constructing efficient photochemical reactors for air and water purification. Moreover, intense research on TiO2 to shift its optical response from UV to visible light has produced some interesting visible light active TiO2 materials that utilize much available visible light of solar radiation for water decontamination [3–۱۵,۱۸–۲۰]. The ability of TiO2 to act as a photocatalyst was first discovered approximately 90 years ago [19,21]. However, it did not become an extensively researched area until Fujishima and Honda discovered that TiO2 electrode could be used to photocatalytically split water in 1972 [19,21–۲۴]. In the decades since this discovery, there has been extensive research in understanding the photocatalytic process and attempting to improve the efficiency of using TiO2 as a photocatalyst [22]. There also has been a significant number of publications examining the applications photocatalysts, e.g. water or air decontamination and self-cleaning surfaces [19,25]. The reactions for heterogeneous photocatalysis occur at the surface of the semiconductor material.