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Advancement and New Challenges in Heterogeneous Photocatalysts for Industrial Wastewater Treatment in the 21st Century

Sadanand Pandey1*, Tanushri Chatterji2, Edwin Makhado3, Abbas Rahdar4, Elvis Fosso-Kankeu5 and Misook Kang1┼

1Department of Chemistry, College of Natural Science, Yeungnam University, Daehak-Ro, Gyeongsan, Gyeongbuk, Republic of Korea

2School of Bioscience, IMS Ghaziabad (University Courses Campus), Uttar Pradesh, India

3Department of Chemistry, School of Physical and Mineral Sciences, University of Limpopo, Polokwane, Sovenga, South Africa

4Department of Physics, University of Zabol, Zabol, Iran

5Department of Mining Engineering, College of Science Engineering and Technology, University of South Africa, Florida Science Campus, South Africa

Abstract

From the last few decades, heterogeneous photocatalysts have flourished significant consideration especially concerning energy and the environment. Heterogeneous photocatalysts play a vital role in the cleavage of solar water and in the removal of environmental pollutants, including organic and inorganic species from aqueous or gas phase systems in environmental remediation, drinking water treatment, industrial, and health care setups. The current chapter starts with a brief introduction on the background of industrial wastewater and the advancement of wastewater treatment processes through advanced oxidation processes (AOPs), comparing the importance of AOPs technology for water treatment. The recent development of heterogeneous photocatalysts for the treatment of minor pollutant concentrations in water/air is also reviewed. The chapter also focuses on the mechanisms of heterogeneous photocatalysis, the impact of various designs of photoreactors with the review of the published literature, which includes various types and designs of photocatalytic reactors. It is our hope that readers will get an overview of the requirements guiding the usage of suitable photoreactors. Finally, the chapter ends with a discussion of the personal perspectives that can provide new insights into the future development and prospects of heterogeneous photocatalysts for industrial wastewater.

Keywords: Photoreactors, heterogeneous photocatalysts, advanced oxidation processes, water treatment

1.1 Introduction

The existence of all living beings on this planet depends on water. It covered about 71% of the earth's surface but almost 2.5% is specified as freshwater. The limited amount of fresh water is used and then recycled to support the growing population. A rapid population growth, the increase in industrialization and material production inflicts the influx of anthropogenic pollutants into the water environment, engendering a potential threat to human health and the ecological environment. The increased usage of water by various industrial sectors has inescapably led to a rise in the generation of wastewater. Numerous modern industries, such as textile, paper printing, leather, food, mining, electroplating, cosmetics, and other chemical industries, discharge highly noxious chemicals into water sources. The main causes of water pollution lies in improper disposal and extensive usage of organic products that majorly include pharmaceuticals and personal care products, detergents, plasticizers, and dyes. Furthermore, hazardous substances are toxic, carcinogenic, and nonbiode-gradable, making them a major threat to society. These classes of pollutants are becoming more complex and challenging to treat. Traditional methods for remediation of water gradually can no longer meet the requirement to treat complex contaminated water. For these reasons, researchers have focused on finding some emergent strategies to assist in removing these species of contaminants from wastewater.

Enormous attempts have been made to remediate organic products from wastewater, which include electrocoagulation/degradation process, membrane filtration [1], electrocoagulation [2], chemical coagulation [3], chemical precipitation [4], adsorption system [5], and advanced oxidation processes (AOPs) [6] have shown off a good performance in wastewater treatment and purification. The latter is a highly efficient treatment method owing to its fast reaction speed, simple technology and relatively no secondary pollution. Many factors like low efficiency, side product formation, and high-energy consumption are encouraging us to search for innovations in AOP.

Heterogeneous photocatalysis (HPC), the Fenton process, sonolysis, the ozonation process, and radiation-induced degradation are the AOPs, which exhibited great potential as a solution for decontaminating the aquatic environment. These techniques have shown enhanced efficacy for decaying, nonselective performance, and mineralizing organic toxins at relatively reduced concentrations without producing secondary pollution. The AOPs accomplish mineralization of organic compounds and sometimes inorganic compounds also to carbon dioxide and mineral acids [7]. For several years now, HPC has been one of the most promising approaches for the breakdown of organic compounds and metal ions in industrial wastewater. This process is based on aqueous phase hydroxyl radical chemistry and pair of lower-energy radiation or light source with semiconductors as photocatalysts. The technique has proven to be a viable alternative to solving environmental problems, overcoming many of the limitations of traditional industrial wastewater treatment methods. This emerging trend treatment promotes water purification, which includes decontamination, detoxification, discoloration, deodorization, and simultaneous degeneration of the pollutants.

The HPC is defined as the alteration in the rate of a chemical reaction or its onset, which is regulated by the action of ultraviolet, visible or infrared radiation in the presence of a substance called a photocatalyst. This photocatalyst consumes light and undergoes chemical conversion. The factors which accelerate the rate of photocatalysis are light intensity, pH, and modified photocatalyst [8]. The efficacy of HPC could be enhanced by the use of different semiconductors due to its advanced oxidation process. Preparation of HPCs by semiconductor oxides is one of the promising methods and acts for remediation of many organic and inorganic pollutants from water and air [9]. On the grounds of their unique combination of physical and chemical properties, and their low cost and photostability under irradiation [10], Titanium oxide (TiO2) nanomaterial, provide a wide variety of possible applications. Few studies revealed its effectiveness studies related to air cleaning and water purification. For environmental applications, visible light-harvesting nanomaterials will be increasingly applied in combination with different advanced oxidative processes (AOPs) technologies [11].

The effectiveness of HPCs in the removal of organic compounds from polluted soil is quite remarkable. The stringent method is the action of TiO2 under UV irradiation and solar light is noted. On the contrary, the difficulty of removing simple deposition of the photocatalyst on the soil is also observed. The reason behind this is that light cannot penetrate deeper to induce the process of photocatalysis. Hence, the degradation of pollutant is restricted to a maximum of 4cm in contaminated soil. To overcome, the polluted soil is missed with the photocatalyst followed by the exposure to irradiation light. In the previous studies, it was reported that heterogeneous photocatalytic degrade the pesticide Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea), atrazine, 2-chlorophenol, and 2,7-dichlorodibenzodioxin present in the soil samples [12].

Nowadays, the process of HPCs is implemented for remediation of environmental problems including air, water, wastewater treatment [13], disinfection processes [14, 15]. Additionally, it is being also used for energy production by degrading biomass, hydrogen generation by water splitting, treating oil spills and chemical synthesis [16]. Recently, Shukla et al. (2021) reviewed the current advances in heterogeneous micro-photocatalytic reactors for wastewater treatment [17]. Contrary to previous chapters, this one discusses the advancements in wastewater treatment using HPCs. The photocatalytic degradation of organic pollutants by employing recently developed HPCs has been comprehensively discussed. Mechanism of HPCs, effects of different designs of photoreactors and the parameters required to conduct the photocatalytic process are discussed. Lastly, the future challenges for wastewater treatment via photocatalytic degradation are also considered.

1.2 Development of Heterogeneous Photocatalysts

Various advanced oxidation processes (AOPs) have been continually explored for the decomposition of organic pollutants, such as dyes, surfactants, phenolic substances, personal care products, pharmaceuticals, hydrocarbons, endocrine disruptors, fertilizers, and pesticides. Most of these organic contaminants are usually active at low concentrations. Several studies have been focused on the photocatalytic remediation method to eliminate the abovementioned organic contaminates because this approach is clean, sustainable, and it completely decomposes/degrade pollutants or converts them into nontoxic forms. In this direction, the degradation of these pollutants has spurred great interest in the remediation of wastewater, and this has led to a rise in the development of different HPCs. The benefit of HPCs lies in the...