Photoreactors in Advanced Oxidation Process E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
PHOTOREACTORS IN ADVANCED OXIDATION PROCESSES Unique book covering topics related to the evolving photoreactors concepts, design, and application as well as green synthesis of heterogenous photocatalysts which are the key aspects to facilitate the escalation of bench scale works toward industrial/commercial applications. In this book, the editors present the most up-to-date research on Advanced Oxidation Processes (AOPs) to make the argument that AOPs offer an eco-friendly method of wastewater treatment. In addition to an overview of the fundamentals and applications, it provides ample details of the reactive species involved in AOPs as well as reactor design concepts, thus providing readers with the necessary tools to better understand and implement these methods. Moreover, this book presents some conventional and novel photoreactors equipped with UV/vis lamps for working under solar radiation for wastewater treatment in a laboratory and on an industrial scale, which is an important focus of the book. Readers will find in this book: * In-depth coverage of the performance of sustainable eco-friendly and low-cost heterogeneous photocatalysts (biogenic photocatalysts); * A state-of-the-art fundamental review of parameters affecting photoreactor designs for the effective performance of reactive species; * Identifies, formulates, and analyzes developed processes and technologies to meet desired environmental protection needs of society and formulates solutions that are technically sound, economically feasible, and socially acceptable. Audience This book will be of interest to academic researchers and graduate students from the fields of environment, chemistry, and engineering, and professionals including environmental managers in industry, water treatment plants managers and operators, water authorities, government regulatory bodies, and environmentalists.
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Table of Contents
Cover
Series Page
Title Page
Copyright Page
Preface
Part I: ADVANCES IN PHOTOCATALYSTS SYNTHESIS
1 Advancement and New Challenges in Heterogeneous Photocatalysts for Industrial Wastewater Treatment in the 21st Century
1.1 Introduction
1.2 Development of Heterogeneous Photocatalysts
1.3 Mechanism of Action of Heterogeneous Photocatalysis
1.4 Recent Advances in Heterogeneous Photocatalyst
1.5 Heterostructure Photocatalysts for the Degradation of Organic Pollutants
1.6 Photoreactors
1.7 Photoreactors for the Degradation of Volatile Organic Compounds
1.8 Advantages and Disadvantages of Heterogeneous Photocatalysis
1.9 Conclusion
Acknowledgment
References
2 Role of Heterogeneous Catalysts for Advanced Oxidation Process in Wastewater Treatment
Abbreviations
2.1 Introduction
2.2 Effect of Pollutant
2.3 Type of Catalysts
2.4 Some Recent Heterogeneous Catalysts for Advanced Oxidation Process
2.5 Conclusions and Future Prospect
Acknowledgement
References
3 Green Synthesis of Photocatalysts and its Applications in Wastewater Treatment
3.1 Introduction
3.2 Photocatalysts and Green Chemistry
3.3 Limitations and Future Aspects
3.4 Conclusion
References
4 Green Synthesis of Metal Ferrite Nanoparticles for the Photocatalytic Degradation of Dyes in Wastewater
Abbreviations
4.1 Introduction
4.2 Metal Ferrite Nanoparticles
4.3 General Synthesis Methods of Metal Ferrites and Their Limitations
4.4 Biological Synthesis of Metal Ferrite Nanostructures
4.5 Plant-Derived Metal Ferrites as Photocatalysts for Dye Degradation
4.6 Challenges of these Materials and Photocatalysis
4.7 Conclusion: Future Perspectives
References
Part 2: ADVANCED OXIDATION PROCESSES
5 Selected Advanced Oxidation Processes for Wastewater Remediation
5.1 Introduction
5.2 Photocatalysis and Ozonation
5.3 Hybrid AOP Technologies
5.4 Membrane-Based AOPs
5.5 Conclusion and Future Perspectives
References
6 Advanced Oxidation Processes-Mediated Removal of Aqueous Ammonia Nitrogen in Wastewater
Abbreviations
6.1 Introduction
6.2 Basic Chemistry and Occurrence of Ammonia Nitrogen
6.3 Photocatalytic Technique for Removal of Aqueous Ammonia Nitrogen FromWastewater
6.4 Ozonation Technique for Removal of Aqueous Ammonia Nitrogen FromWastewater
6.5 Conclusion and Future Prospects
Acknowledgments
References
Part 3: DESIGN AND MODELLING OF PHOTOREACTORS
7 Recent Advances in Photoreactors for Water Treatment
7.1 Introduction
7.2 Photocatalysis Fundamentals and Mechanism
7.3 Configuration of Photoreactor
7.4 Types of Photoreactors
7.5 Photocatalytic Water Purification Using Photoreactors
7.6 Challenges for Effective Photoreactors
7.7 Conclusion
References
8 Design of Photoreactors for Effective Dye Degradation
Abbreviations
8.1 Introduction
8.2 Different Photoreactors Are Used for Wastewater Treatment
8.3 Photoreactors Designed to Work Under Visible-Light Irradiation Toward Wastewater Treatment
8.4 Current and Future Developments
References
9 Simulation of Photocatalytic Reactors
Abbreviations
9.1 Introduction
9.2 Modeling of Light Distribution
9.3 Photocatalysis Kinetics
9.4 Conclusion
References
10 The Development of Self-Powered Nanoelectrocatalytic Reactor for Simultaneous Piezo-Catalytic Degradation of Bacteria and Organic Dyes in Wastewater
Abbreviations
10.1 Introduction
10.2 Degradation Techniques
10.3 Characteristics and Properties of Piezoelectric Materials
10.4 Synthesis of Piezoelectric Materials
10.5 Challenges of Piezoelectric Nanomaterials/ Nanogenerators
10.6 Application of Piezoelectric Materials for Piezo-Electrocatalytic Degradation of Dyes and Bacteria in Wastewater
10.7 Conclusion and Future Perspectives
Acknowledgments
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Recent reviews on heterogeneous photocatalytic for degradation of or...
Table 1.2 Advantages and disadvantages of HPCs.
Table 1.3 Photodegradation efficiencies of some of HPCs on organic contaminant...
Table 1.4 Commonly used photoreactors for organic compounds.
Table 1.5 Types of reactors used for destruction of inorganic pollutants.
Chapter 2
Table 2.1 Published articles on application of AOP.
Chapter 3
Table 3.1 Analytical techniques used for the characterization of NPCs.
Table 3.2 UV-vis range (nm) and band gap energies of reported green synthesize...
Table 3.3 Accomplishment of various green synthesized NPCs in wastewater treat...
Chapter 4
Table 4.1 Green-derived metal ferrite nanoparticles using bacterial extracts.
Table 4.2
G
reen derived metal ferrite nanoparticles using fungal extracts an...
Table 4.3 Green-derived metal ferrite nanoparticles using plants extracts.
Table 4.4 Green-derived metal ferrite nanoparticles and their photocatalytic a...
Table 4.5 Effect of depositing noble and transition metal on metal ferrites fo...
Table 4.6 Carbon nanomaterials deposited on metal ferrites for photocatalytic ...
Table 4.7 Effect of coupling metal oxide semiconductors with metal ferrites fo...
Table 4.8 Biological applications of plant-derived metal ferrites.
Chapter 6
Table 6.1 Effect of aqueous ammonia nitrogen on aquaculture species.
Table 6.2 Various photocatalytic systems for the remediation of ammonia nitrog...
Chapter 7
Table 7.1 Characteristics of light sources [47].
Table 7.2 Merits and demerits of slurry photoreactor.
Table 7.3 Some recent examples of different photoreactors and their performanc...
Chapter 8
Table 8.1 Categories, sources, and application of electromagnetic radiation.
Table 8.2 Recommended dose of photocatalyst for various compounds and photorea...
Chapter 9
Table 9.1 Lamp emission models [24].
Table 9.2 Henyey-Greenstein asymmetry factor for Aeroxide P25 TiO
2
from differ...
Table 9.3 Experimental determination of absorption and scattering coefficients...
Chapter 10
Table 10.1 Types of anode materials with their respective oxidation potential ...
Table 10.2 Piezoelectric material properties and characteristics [24].
Table 10.3 Properties of piezopolymeric and ceramic materials [23].
Table 10.4 Performance comparison of PZT energy harvesters under different con...
Table 10.5 Deposition techniques used to prepare piezoelectric thin films and ...
Table 10.6 Allowed limits of dye effluents in the environment according to int...
List of Illustrations
Chapter 1
Figure 1.1 (a) Applications of heterogeneous photocatalysts (adopted from Ref ...
Figure 1.2 Mechanism of action of a photocatalyst.
Figure 1.3 Types of heterostructure semiconductor photocatalysts (adopted from...
Figure 1.4 Design of annular reactor (adopted from ref [89])
.
Figure 1.5 Internal structure of plate reactor (adopted from ref [90]).
Figure 1.6 Design of packed bed photoreactor (adopted from ref [89]).
Figure 1.7 Classification of CO
2
photoreactor designs cast-off for reduction o...
Figure 1.8 General layout of gas-phase photoreactors (adopted from ref [98]).
Chapter 2
Figure 2.1 Emerging pollutants in the environment.
Figure 2.2 The most popular AOPs for water and wastewater that have been teste...
Figure 2.3 Applications of heterogeneous photocatalysis.
Figure 2.4 Possible applications of graphene material.
Figure 2.5 Schematic diagram for illustrating the photocatalytic mechanism of ...
Figure 2.6 Photographs of the linen samples before bleaching, after water + ul...
Figure 2.7 Comparison of RhB photocatalytic degradation by original ZnCr-0.75S...
Chapter 3
Figure 3.1 Major water-polluting sources.
Figure 3.2 The 12 principles of green chemistry.
Figure 3.3 Mechanism of photocatalysis with methylene blue dye as an organic p...
Figure 3.4 The classification of methods used for NP synthesis.
Figure 3.5 Schematic representation for the synthesis of plant-based NPCs.
Figure 3.6 Biopolymer and their sources.
Figure 3.7 UV-visible absorbance of R6 colloid measured as a function of time ...
Figure 3.8 XRD pattern of synthesized ZnONPs (reproduced with permission from ...
Figure 3.9 (a) SEM analysis of PdNPs synthesized at optimized conditions (10 m...
Figure 3.10 (a), (b), (c) are HR-TEM images and (d) SAED image of AuNPs/GI (re...
Figure 3.11 AFM height image (10×10 μm) of AgNPs obtained using the C
4.5
compo...
Figure 3.12 FTIR spectra of CeO2 NPs, (1)
M. oleifera
peel extract, (2) CAN (3...
Figure 3.13 Particle size distribution curve of green synthesized TiO
2
NPs by ...
Figure 3.14 The N2 adsorption-desorption isotherm and Barrett–Joyner–Halenda (...
Chapter 4
Figure 4.1 Schematic diagram displaying the spinel structure of MFe
2
O
4
, indica...
Figure 4.2 Reactor utilized for the pyrolysis of biomass feedstock [18].
Figure 4.3 Synthesis of cobalt ferrite supported onto the bacterial cellulose ...
Figure 4.4 Biosynthesis of cobalt ferrite nanoparticles utilizing the fungal e...
Figure 4.5 Various types of organic dyes [1].
Chapter 5
Figure 5.1 Schematic representation of radical generation in a bubble cavity.
Figure 5.2 The degradation of an organic pollutant under sonophotocatalysis.
Chapter 6
Figure 6.1 Advanced oxidation processes for wastewater treatment.
Figure 6.2 Effect of oxidative stress on fruit cell.
Figure 6.3 Effect of ions on NH
3
degradation by La/Fe/TiO
2
in the aquatic syst...
Figure 6.4 The basic reaction mechanism of oxidation of ammonia nitrogen by O
3
Chapter 7
Figure 7.1 General mechanism of photocatalytic wastewater treatment. Reproduce...
Figure 7.2 Reactor configurations (A and B) and emission spectra of the differ...
Figure 7.3 Schematic diagram (a) and front side (b) of the pilot scale continu...
Figure 7.4 Schematic diagram of (a) annular photoreactor [79], and (b) tubular...
Figure 7.5 A simplified diagram of a closed-loop step photoreactor. Reproduced...
Figure 7.6 Approaches used for the effective implementation of photocatalysts ...
Figure 7.7 (a) Illustration of the assembled continuous magnetic aggregation b...
Chapter 8
Figure 8.1 Schematic diagram of experimental set-up with point irradiation [9]...
Figure 8.2 Experimental set-up photograph with 12 simultaneously. irradiated t...
Figure 8.3 Photocatalytic experimental setup tests under visible-light irradia...
Figure 8.4 Schematic diagram of Heraeus photoreactor: 1—UV/vis lamp (MPML) ins...
Figure 8.5 Installation of the reactor panel in the solar photoreactor [131].
Figure 8.6 MB photodegradation experiment using the proposed photoreactor: (a)...
Figure 8.7 (a) The design of the reactor panel as main part of FP photoreactor...
Figure 8.8 Schematic picture of the packed bed photoreactor working in continu...
Chapter 9
Figure 9.1 Lamp emission models (a) line source (b) surface source (c) volume ...
Figure 9.2 Scattering probability of (a) isotropic (ISO), diffuse reflectance ...
Figure 9.3 Experimental set up for (a) Extinction coefficient, (b) absorption ...
Figure 9.4 Absorption coefficient (κ
λ
), scattering coefficient (σ
λ
...
Figure 9.5 Validation methods. (a) Total transmittance. Reprinted from [7] wit...
Chapter 10
Figure 10.1 EAOPs degradation mechanism for organic dyes [19].
Figure 10.2 Natural existing piezoelectric materials [25].
Figure 10.3 Historic development of lead free piezoelectric materials [29].
Figure 10.4 Schematic representation of electrospinning process [39].
Figure 10.5 Hydrothermal synthetic flowchart [26].
Figure 10.6 SEM images for side view of (a) BaTiO
3
and top view of (b) PbTiO
3
...
Figure 10.7 Number of publications on piezoelectric, electrostatic and electro...
Figure 10.8 Percentage contribution of organic dyes by various industries [70]...
Figure 10.9 Pieozocatalytic mechanism of AO7 using strained BaTiO
3
[77].
Figure 10.10 Three recycling tests of piezocatalytic activity for (a) RhB and ...
Figure 10.11 SEM images of
E. coli
after piezocatalytic treatment for (a) 0 mi...
Figure 10.12 Survival percentages of bacteria during photocatalytic and piezo-...
Figure 10.13 Number of colony forming unit (CFU)/mL of (a)
S. aureus
and (b)
E
...
Figure 10.14 Piezo-photocatalytic degradation mechanism of
E. coli
by MoS
2
[86...
Figure 10.15 (a) Quantification of ROS levels (intensity of F/F
0
represents RO...
Guide
Cover Page
Series Page
Title Page
Copyright Page
Preface
Table of Contents
Begin Reading
Index
Also of Interest
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Photoreactors in Advanced Oxidation Processes
The Future of Wastewater Treatment
Edited by
Elvis Fosso-Kankeu
Department of Electrical and Mining Engineering, the University of South Africa, Pretoria, South Africa
Sadanand Pandey
Particulate Matter Research Center, Research Institute of Industrial Science & Technology (RIST), South Korea
and
Suprakas Sinha Ray
DST-CSIR National Centre for Nanostructured Materials, Pretoria, South Africa
This edition first published 2023 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2023 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-394-16629-9
Cover image: Pixabay.ComCover design by Russell Richardson
Preface
Global population growth along with ever-increasing industrialization are significantly compromising access to safe drinking water. Advanced oxidation processes (AOPs) for wastewater treatment are emerging as one of the most efficient, renewable green chemical technologies among those being discussed to address this problem. Recently, photochemical oxidation of organic and inorganic pollutants has become an attractive technique for water purification and wastewater treatment. In order to increase the water treatment efficiency, the selection of suitable biogenic and cheaper photo catalyst, as well as a light source and an oxidation system, are some of the key parameters required. It is well known that sufficient UV penetration into the radiated liquid (promotion of efficient conversion of incident photons to charge carriers) is of paramount importance, especially for an opaque environment, and UV radiation is effective when very close to the UV lamp surface. High mass transfer rates for efficient interaction between the pollutant and the photocatalyst and high oxygen uptake at the gas-liquid interface are vital requirements for practical applications. In this regard, designing a photoreactor for efficient wastewater treatment has been challenging. Many types of photoreactors have already been studied, reported on and patented in the literature.
In this book, we present the most up-to-date research on AOPs in order to make the argument that AOPs offer an eco-friendly method of wastewater treatment. In addition to an overview of the fundamentals and applications, it provides ample details of the reactive species involved in AOPs as well as reactor design concepts, thus providing readers with the necessary tools to better understand and implement these methods. Moreover, this book presents some conventional and novel photoreactors equipped with UV/vis lamps for working under solar radiation for wastewater treatment in a laboratory and on an industrial scale, which is an important focus of our book.
There have been numerous studies covering the modeling of light distribution in different photocatalytic reactors using simulation methods such as Six-Flux analysis and Monte Carlo analysis. These studies have reported novel methods of establishing the catalyst’s optical properties and validating the models. Therefore, this book also reviews these recent developments with respect to the modeling of light distribution and reaction kinetics in photocatalysis reactors.
Since the main objective of this book is to critically discuss and evaluate the chemical oxidation applications for industrial wastewater presented in this up-to-date review, it will be of interest to scientists and engineers in academia or industry working on projects related to the removal of organic pollutants from wastewater.
The editors are grateful to the reviewers who have contributed to improving the quality of the book through their constructive comments. The editors also thank the publisher for including this book in their series.
Elvis Fosso-Kankeu, Sadanand Pandey and Suprakas Sinha Ray
December 2022
Part IADVANCES IN PHOTOCATALYSTS SYNTHESIS
1Advancement 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 fact that they can regulate optical band gaps; increase the sorption threshold via combining semiconductors having different bang gaps. Moreover, they can improve the charge separation under light irradiation and lower the recombination rate of electrons and holes, which may be appropriate for the oxidation-reduction process and leads to enhance the efficacy of photocatalysis process [18, 19]. HPCs is a wide field that offers great potential for many applications. Figure 1.1a shows some of the potential applications of heterogeneous photocatalysts [20]. A significant number of HPCs have been developed for a broad array of photocatalytic applications, among which water and wastewater treatment could be mentioned. HPCs have witnessed extensive scientific attention for several years, predominately for the treatment of low load of pollutants in water or air [21, 22]. In the past 20 years, a developing trend of publications in heterogeneous photocatalysts for the decomposition of organic contaminants are shown in Figure 1.1b. Search results using keywords “heterogeneous photocatalysts for degradation of organic pollutants/contaminants” from the ScienceDirect database on June 30, 2021. The literature survey statistics show an increase in the number of publications per year considering heterogeneous photocatalysts for the elimination of organic pollutants in aqueous environment. Significant growth is noticed in last few years (2017–2021). The usage of HPCs in treating wastewater has gained enormous interest from academics; researchers have been encouraged to write a review in the field of photocatalytic technology. Among many publications on the HPCs for degradation of organic contaminants, several reviews have published concerning the use of HPCs for photodegradation of various organic pollutants. Table 1.1 shows some of the recent reviews concerning the use of heterogeneous photocatalysts for the photodegradation of organic contaminants of wastewater. Recent advancement in HPCs for water and wastewater treatment are cited here to give the readers an overview background on the progress to date. The next section discusses the mechanism of action of photocatalysis.
Figure 1.1 (a) Applications of heterogeneous photocatalysts (adopted from Ref [20]) and (b) The total number of publications per year on heterogeneous photocatalysts for degradation of organic pollutants/contaminants during the period 2010 to 2021 (using ScienceDirect database). *Data collected in June 2021.
1.3 Mechanism of Action of Heterogeneous Photocatalysis
The mechanism of action of photocatalysis is described as a combined action of light and catalyst, which accelerates a reaction and leads to chemical transformation [29, 30]. Semiconductors are generally used as catalysts for these purposes. The action of catalysis is modulated due to their specific electronic structure, which is characterized by a filled valence band (VB) and an empty conduction band [31]. The variation in the energy level between the conduction band (CB) and the valence band (VB) is called band gap energy. The band gap energy is normally relatively low approximately a few electron volts (eV). To activate the catalyst, a photon with sufficient energy could be used by transporting an electron from the filled valence band into the conduction band (Figure 1.2) [16]. Apart from the photocatalyst, the process of photocatalysis is a preferable as it does not require any additional reagents, and the catalytic reaction is initiated by light absorption. In addition, the semiconductor photocatalyst is activated by the consumption of a photon with ultra-band gap energy, ensuing in the advancement of an electron (e−) from the valence band (VB) into the CB and the instantaneous formation of a positive hole (h+) in the VB. This leads to the formation of e− and h+ pairs, known as charge separation. The combination of these products induces reduction and oxidation reactions with species adsorbed on the surface of photocatalyst. Elimination of pollutants from water bodies, e− in the CB can interact with adsorbed O2, creating a superoxide radical anion (O2·), whereas h+ in the can react with water adsorbed on the photocatalyst surface and lead to the formation of hydroxyl radicals (OH·) [49].
Table 1.1 Recent reviews on heterogeneous photocatalytic for degradation of organic contaminants in the aquatic environment.
Descriptions/titles
References
Recent advances on modelling of solar heterogeneous photocatalytic reactors applied for degradation of pharmaceuticals and emerging organic contaminants in water
[
23
]
An overview on nonspherical semiconductors for heterogeneous photocatalytic degradation of organic water contaminants
[
24
]
Recent developments in the use of metal oxides for photocatalytic degradation of pharmaceutical pollutants in water—a review
[
25
]
An overview of the recent advances of carbon quantum dots/metal oxides in the application of heterogeneous photocatalysis in photodegradation of pollutants towards visible-light and solar energy exploitation
[
26
]
Application of heterogeneous nano-semiconductors for photocatalytic advanced oxidation of organic compounds: a review
[
27
]
The application of heterogeneous visible light photocatalyst in organic synthesis
[
28
]
Heterogeneous photocatalysis and its potential applications in water and wastewater treatment: a review
[
20
]
The following are the four varied reactions, which are predicted for while the removal of pollutants photocatalytic degradation [32].
The reaction occurs while both species are adsorbed [
33
].
A nonbound radical reacts with an adsorbed organic species [
34
].
Figure 1.2 Mechanism of action of a photocatalyst.
An adsorbed radical reacts with a free organic species arriving at the catalyst surface [
35
].
A reaction occurs between two free species in the bulk solution [
36
].
Heterogeneous photocatalysis is a promising approach for treating polluted industrial wastewater possessing organic compounds. These new technologies are composed of advanced oxidation processes. A few of the major AOPs processes are H2O2/UV, O3/UV, H2O2/O3/UV, TiO2/UV and VUV. Under the UV illumination of 254nm, H2O2/UV photocatalysis takes place and H2O2 is cleaved into hydroxyl radicals (OH*). The hydroxyl radicals that are produced further degrade the organic compounds of wastewater by the mechanism of hydrogen abstraction, electrophilic addition and electron transfer. The rate of oxidation during photocatalysis is regulated by the hydroxyl radicals formed [37]. In the case of O3/UV, oxidation of organic compounds present in industrial wastewater takes place. In this system, UV radiation (254 nm) is irradiated from aqueous solution saturated with ozone. It is observed that the use of ozone separately increase accelerates the oxidative degradation rates but the limitation is low solubility in water and consequent mass transfer [38]. The mechanism of action of O3/H2O2/UV is similar to O3/UV process. The rate of degradation increases by the photochemical generation of hydroxyl radicals by adding H2O2. The vacuum UV of 190nm is emitted by an excimer lamp as a light source. In VUV, the generation of hydroxyl radicals is mainly generated from H2O. VUV photocatalysis is quite simple and advantageous as it requires no chemical usage.
1.4 Recent Advances in Heterogeneous Photocatalyst
Industrial wastewater contains a complex composition and dominant pollutants, such as ammonia, cyanide benzene, naphthalene, phenols, and cresols. Nowadays, the treatment of industrial wastewater has become a demanding topic. Therefore, various new technologies, such as membrane technology, electrochemical processes, and membrane bioreactor (MBR), for the treatment of industrial wastewater have been proposed and adopted [39]. In this context, the use of metal oxide-based photocatalysis is the method of choice. Since the discovery of HPCs by Fujishima and Honda in 1972, the use of semiconductors has received great attention [35]. The metal oxides used for photocatalysis are TiO2, WO3, ZnO, Fe3O4, V2O5, SnO2, ZrO2, Cu2O, and Ta2O5. Sulfides (e.g. SnS, CnS, CDs) and ferrites are successfully used to break down organic pollutants. Principles of HPCs have been well documented in the literature, with the emphasis on the electronic structure of semiconductors [18, 40–42]. TiO2, ZnO, and CdS are the main semiconductors that are used for heterogeneous photocatalysis.
The most common and frequently used semiconductor used in this process is TiO2 [43]. The treatment has been chosen due to biocompatibility, abundance, low cost, and well suited for the design of efficient photocatalysis. During the photodegradation of varied organic compounds like aromatic and aliphatic chlorinated hydrocarbons using TiO2 AOPs the following reactions occurs [43]:
Although semiconductors are ideal candidates for photocatalysis, they are associated with some inherent disadvantages that limit their photocatalytic performance, as well as their application prospects. Anatase TiO2, for example, can only be activated under UV light, which is less than 5% of the solar spectrum. To take advantage of the abundant visible light in the solar spectrum, it is of paramount importance to produce photocatalysts that operate with high efficiency under visible light. In the case of conventional TiO2, defects, such as quantum efficiency, low specific surface area, low use of visible light, recovery from the reaction medium, weak photoreductivity, limit its performance as photocatalysts. The HPCs have shown several advantages over the conventional homogeneous catalysts (Table 1.2). The ideal HPCs must have immediately recognizable properties, such as high activity, efficient recovery, low cost, photo stability, nontoxicity, chemical inertness, and high efficiency [44–48]. The applications of HPCs on a pilot scale are hampered by their poor consumption of visible light along with the high recombination rate of electron-hole pairs. Given the above challenges, the continuing exploration in the field of photocatalytic technology has led researchers to expand both the photoreaction and photoactivity of the visible light range in the solar spectrum.
The development of new semiconductors has received considerable attention for their application to eliminate organic pollutants from aqueous environments. In this context, efforts have been made to use semiconductors for various applications, including photocatalytic technology. For example, experiments, such as doping can be considered, since TiO is highlighted, since it has a band gap in the range of 3.0 to 3.2 eV. The2 photocatalytic performance of TiO2 could be improved by incorporating additional components into the semiconductor structure, which in turn promote the sensitivity of TiO2 to visible light. This could be possible by changing its electronic as well as optical properties. The technique increases the VB edge of TiO2 without changing the position of the CB edge, thereby lowering the band gap. The trending approach minimizes the electron-hole recombination method in some way and improves photocatalysis [49]. Other approaches include the formation of composites, precious metal deposition, nonprecious metal deposition, surface modification, and dye sensitization [50–53]. Several approaches have been tried to improve the inherited properties of conventional TiO2. Among the photocatalytic materials developed, ZnO is considered an alternative photocatalyst to TiO2, as it has almost the similar band gap energy, but has a high absorption efficiency over a wide range of the solar spectrum compared to TiO2 [54, 55]. In recent years, ZnO has attracted great interest as a potential photocatalyst for the decomposition of pollutants, including organic contents, due to its unique properties, such as long-term photo stability, excellent chemical stability, nontoxicity, and excellent charge transport binding energy at 60 meV. The modification of these semiconductors via the above approaches improves the surface structure to allow the absorption of light in the visible region of the solar spectrum. Table 1.3 shows some of the reported photocatalytic degradation efficiencies of the HPCs, with respect to organic pollutants. Overall, recently developed HPCs show a higher photocatalytic degradation performance for the elimination of organic pollutants. Coupling or modification of semiconductors has been shown to improve charge separation, reduce band gap energy, and reduce recombination rate.
Table 1.2 Advantages and disadvantages of HPCs.
Advantages
Disadvantages
Low-cost stability
Harvesting of visible light
Chemical inertness
Photocatalysis recovery from the mixture is not easy
High activity nontoxic
Difficult isolation
Stability in aqueous environment
High recombination rate of electron-hole pairs
Efficient recovery and reasonable recyclability
Poor electric adsorption and treatment of high concentration of organic pollutants
Table 1.3 Photodegradation efficiencies of some of HPCs on organic contaminants.
HPCs
Target pollutants
Reaction conditions
Time (min)
Degradation efficiency (%)
References
Nb
2
O
5
/ZnAL-LDH
Congo red
Visible light
390
85
[
56
]
Graphene-based TiO
2
Bisphenol A
Visible light
180
67.6
[
57
]
TiO
2
/WO
3
Methylene blueRhodamine B
Visible light
6020
10050
[
58
]
Sn/N-TiO
2
Zopiclone
UV-Vis light
120
91
[
59
]
ZnO-TiO
2
Azo dye
UV-Vis light
180
99
[
60
]
Sepiolite/BiOCl/ TiO
2
Tetracycline
Visible light
180
90
[
61
]
Sm(III), N, P-doped TiO
2
4-Chlorophenols
Visible light
120
100
[
62
]
C/N-doped TiO
2
Phenols
Visible light
150
87
[
63
]
Fe(III)-doped TiO
2
Nitrobenzene
Visible light
240
88
[
64
]
ZnSnO
3
Ciprofloxacin
Visible light
100
85.9
[
65
]
MOF-@rGO
Methylene blueRhodamine BMethyl Orange
SunlightSunlightSunlight
202020
939792
[
66
]
TiO
2
@LDH
Methylene bluePhenol
Visible lightUV light
6060
9590
[
67
]
SiO
2
-TiO
2
Phenol
Visible light
240
90
[
68
]
Cu-TiO
2
Chlorophenols
Visible light
144
98.9
[
69
]
Zr
+4
-TiO
2
4-Chlorophenols
UV-Vis light
480
37.4
[
70
]
TiO
2
@MIL-101
Methyl orange
UV light
30
99
[
71
]
Fe
2
O
4
@MIL-100(Fe)
Methylene blueMethylene blue
UV-VisVisible light
40200
100100
[
72
]
ZnO/CdS@ZIF-8
Rhodamine B Methylene blue
Visible light
120
6299.9
[
73
]
Fe
3
O
4
@rGO@ZnO
Metformin
Visible light
60
100
[
74
]
Ag-ZnFe
2
O
4
Oxytetracycline
Visible light
150
90.5
[
75
]
Ca/Zn-Al
2
O
3
Caffeine
UV light
70
98.5
[
76
]
GO-based TiO
2
Bromothymol blue Rose bengal
UV-Vis light
80
86 90
[
77
]
1.5 Heterostructure Photocatalysts for the Degradation of Organic Pollutants
The HPCs can regulate the optical band gap, increase the absorption threshold, improve charge separation and lowers the recombination rate of electrons and holes, which can be preferable for oxidation and reduction reaction, and proved to be highly effective in photocatalytic degradation [18, 78, 79]. The heterostructure photocatalysts can be prepared by a combination of two semiconductors with varied energy levels, these photocatalysts show improved activity in photocatalysis as compared to pure photocatalysts. Semiconductors that are having low bandgap energy and negative CB are coupled with semiconductors that are having large bandgap, and then the transfer of electrons occurs between semiconductors. Based on several components, heterostructure photocatalysts are termed binary, ternary, etc. Heterostructure can be categorized by alignments of the semiconductors like Type-I, Type-II, Z-scheme, and S-scheme (Figure 1.3).
The positions of VB and CB of semiconductor A must be more positive and negative than those of semiconductor B. Under a suitable source of light, photogenerated electrons and holes of semiconductor A are simultaneously moved to semiconductor B. The recombination of electron-hole pairs within the similar semiconductor surface is high, this form of heterostructure is called type I. On the other hand, in the type II heterostructure, photoinduced electron-hole pairs are prepared within semiconductor A and semiconductor B in the presence of light. The photogenerated electrons are moved from semiconductor A to semiconductor B, whereas photogenerated holes are transferred in the opposite direction. This led to the aggregation of electrons on the semiconductor B, whereas semiconductor A receives holes. It promotes oxidation and reduction sites on semiconductor A or semiconductor A. For this reason, the photogenerated charges are spatially separated. In the Z-scheme heterostructure, holes in semiconductor A react with electron donors to form electron acceptors. Then the higher energy level electron in the CB of semiconductor A and holes in the VB of semiconductor B contribute in the photoreduction or oxidation reactions. The S-scheme heterostructure include combination of reduction and oxidation photocatalysts. Here, the strong electrons and holes in the CB of the reduction photocatalyst or VB of the oxidation photocatalyst are reserved [80].
Figure 1.3 Types of heterostructure semiconductor photocatalysts (adopted from Ref [80]).
1.6 Photoreactors
The equipment used to carry out the photocatalytic process is known as a photoreactor. The photoreactors are designed accordingly to the reaction kinetics of photoreaction resulting in the production of intermediate products in a short period. The equipment is focused on the artificial or natural source of radiation for photocatalysis. The process of photocatalysis involves an advanced oxidation process that mineralizes the carbon containing compounds to water and carbon dioxide. Therefore, to carry out the advanced oxidation process, a semiconductor material is required. This semiconductor has a distinctive energy band gap level variating among its valence band and its conduction band, which is adequate to be overcome by the electrons excited by solar radiation [81].
Industrial wastewater is purified and degraded through the interaction of three phases. These three phases are the solid phase; the photocatalyst, the liquid phase; the contaminant and gas phase; oxygen. The intermediates and the radicals are short-tensioned. The photoreactors are designed to process the complex phenomena, a suitable interaction of the three phases under high turbulence, so that a proper reaction for industrial wastewater treatment takes place. The activation of the photocatalyst is controlled by the illumination of reactants and phases. Therefore, the management of the efficient operation of photoreactors is responsible for several reactions in several phases and the use of several phenomena occurring simultaneously [83].
The two most important parameters that influence the photocatalytic reactor are the kinetic reaction and the mass transport. Efficiency of the process enhanced by few of the factors, which include light source, intensity, pollutant concentration, humidity, temperature, surface area and activity of the photocatalyst [83]. In addition, approaches for photoreactor modeling and coupling of PC with other advanced oxidation processes (AOPs) using H2O2, O3, and peroxydisulfate are preferred. Photocatalysis takes into account only a few targets, as mentioned below [82]:
To change the photoactivity of catalysts in the visible light range or to accelerate the degradation rate;
Use of artificial UV light sources, UV polychromatic lamps, and solar light;
Recovery and deactivation of photocatalysts;
Configuration of photoreactors;
Photodegradation of impurities;
To verify the induced effects of photocatalytic treatment; such as the induction of bacterial resistance.
Apart from the abovementioned objectives, few parameters are required to scale and stipulates the use of a photoreactor. The parameters are mentioned below [83]:
Photocatalyst—type and particle size.
Dispersion of the photocatalyst (fixed or suspended).
Photocatalyst—type, content, and distribution.
Mass transfer.
Fluid dynamics based on laminar and turbulent flow.
Temperature regulation.
Reaction mechanism and reaction kinetics.
Furthermore, photoreactors are classified based on the types of pollutants they degrade. Few of them are described in this chapter.
1.7 Photoreactors for the Degradation of Volatile Organic Compounds
1.7.1 Annular Reactors
They consist of two or more concentric cylinders, usually made of Pyrex glass. The interior of the outer cylindrical tube surface is coated with the photocatalyst. The coating of the photocatalyst layer should be thin so that it can illuminate the radiation source. The central region of the cylindrical tube has the radiation source. In the case of a gas-phase reactor, a fluorescent black-light blue lamp is used as the light source and a P25 TiO2 thin film was applied to the internal glass surface. The efficacy of the photoreactor could be improved by the use of P25-TiO2 impregnated with glass fiber support amid two Pyrex glass tubes. The use of this fiberglass supports maximum exposure to UV radiation. Air flow is passed in the axial direction through the ring between the lamp and the tube. Therefore, it is called a ring reactor (Figure 1.4). In addition, the reactor is classified as a 1D and 2D model on the basis of ideal flow or laminar flow conditions. Photocatalytic reaction carried out by these reactors, restricted by internal mass transfer [83–88].
Figure 1.4 Design of annular reactor (adopted from ref [89]).
1.7.2 Plate Reactor
It is considered one of the less complex types of reactors against volatile organic compounds. Plate reactors are divided into two types (internal and external source), depending on their source of radiation. It is made of stainless steel, plexiglass or polycarbonate and appears in square or rectangular box shape. Photocatalyst substances are positioned at the bottom of the device and used in powder form. In the reactor type with the internal radiation source, the lamp is attached to the upper part. In case of reactor with external source use of quartz or borosilicate window is observed (Figure 1.5). The advantageous properties of the use of these reactors are that they are less complex, have a low-pressure drop, and achieve high reaction rates. The disadvantage of this reactor, on the contrary, is that it has a small surface area for the continuation of the reaction [83].
Figure 1.5 Internal structure of plate reactor (adopted from ref [90]).
1.7.3 Packed Bed Reactors
They consist of the tube-shaped usually composed of Pyrex glass or metal. Photocatalyst samples are positioned in the central area and the radiation source is located inside or outside the reactors (Figure 1.6). After numerous experiments in which material and radiation source were varied, it was concluded that the reactor could be optimized by theoretical prediction of the conversion factor, depending on volume, reaction, and molecular feed [83].
1.7.4 Honeycomb Monolith Reactors
These reactors are designed with several channels of circular or square cross-section. Inner walls of the channels coated with thin films of photocatalysts. For effective functioning, the radiation source is placed in front of the channels. Depending on the flow rate over the monolith, it was noted that an increase in the number of lamps should be followed with minimal distance between monolith and lamp, as this setup achieve an optimal configuration. Researches were also performed with computational fluid dynamics for a better outcome. Honeycomb monolith reactors are best suited for automobile combustion emission and nitrogen oxides reduction in power plant flue gases [83, 91].
Figure 1.6 Design of packed bed photoreactor (adopted from ref [89]).
1.7.5 Fluidized Bed Reactors
They consist of transparent containers. Their mechanism of action is based on the container through which air flows and is occupied with the photocatalyst bed. The light source is situated outside the apparatus. To achieve the best results, photocatalysts should be located near the high air flows used. Two-diode ultraviolet light-emitting diode (UV-LED) modules were used to intensify the photocatalytic oxidative dehydrogenation of cyclohexane in the gas phase. It was placed in front of the Pyrex windows for the emission of UV radiation [83, 92].
1.7.6 Batch Reactors
These reactors are comparatively less complex and are used specifically for the degradation of volatile organic compounds. They consist of a compartment made of Pyrex glass, and the photocatalyst is placed in the lower region of the compartment. The radiation source is aligned externally the reactor. In some studies, TiO2 has also been coated on fiberglass fabrics using the sol-gel process with fluorescent black light lamps to achieve better performance [83, 93]. Commonly used photoreactors for organic compounds are shown in Table 1.4.
Furthermore, photolytic reactors with TiO2 filter (500 µm) are used to inactivate bacteria (Bacillus subtilis and Penicillium citrinum). There is a fluorescent backlight lamp on the surface of the filter and glass slide. The use of photocatalytic HEPA filters (high-efficiency particle absorber filters) is also used for the disinfection of microorganisms in practice. The mechanism of action for inactivating bacteria is oxidative destruction to cell walls, membranes, enzymes, and nucleic acids by ROS [94–96].
Table 1.4 Commonly used photoreactors for organic compounds.
S. no.
Types of chemical reactions
Types of photoreactors
1
Disinfection of water and polluted water
Annular flow reactor, packed-bed reactor, honeycomb monolithic reactor, plate reactor, wall reactor, fixed-bed reactor slurry with the immersed and external light source.
2
CO
2
conversion
Annular flow reactor, packed-bed reactor, honeycomb monolithic reactor, plate reactor, batch reactor.
3
Treatment of wastewater
Double-skin sheet reactor (DSSR), Parabolic trough reactor (PTR), Compound parabolic collecting reactor (CPCR), Wall reactor, fixed-bed reactor slurry with immersed and external light source batch reactor.
4
Treatment of polluted air
Annular flow reactor, Packed-bed reactor, Honeycomb Monolithic reactor and Plate reactor.
5
Glycerol/biomass conversion and organic synthesis
Wall reactor, Fixed-bed reactor slurry with immersed and the external light source, Batch reactor.
6
Water splitting
Twin reactor, Batch reactor.
Photocatalysis of CO2 conversion could be carried out in types of systems
Two-phase system.
Three-phase system.
The former include gas photocatalysts and liquid photocatalysts. Into this gas mixture of CO2, H2O and methanol is added into the reactor and the CO2 reduction takes place. Various photoreactors used for processing are sludge, fixed bed, ring, fiberglass and honeycomb monolith (Figure 1.7). The main factors for efficient photocatalysis are quite similar, i.e., the convective mass transfer rate of CO2, the reaction rate, and the surface area of the photocatalyst [83].
In the conversion of inorganic pollutants, an oxidation process for degradation takes place. Therefore, their approach to the degradation of the pollutants is summarized in Table 1.5. Some other commonly used photoreactors are parabolic trough photoreactors and inclined plane photoreactors.
Figure 1.7 Classification of CO2 photoreactor designs cast-off for reduction of CO2 (adopted from ref [89]).
Table 1.5 Types of reactors used for destruction of inorganic pollutants.
Inorganic pollutants
Type of reactors
Mechanism of action
Flow reactor
Oxidation of nitrogen oxide in the gas phase
Fixed bed reactor
Oxidation of nitrogen oxide, sulphur oxide, hydrogen sulphide
Plate reactor
Oxidation of nitrogen oxide in the gas phase
1.7.7 Parabolic Trough Photoreactors
A parabolic trough photoreactor (PTP) is named because it has a parabolic reflector. This reflector accumulates the solar energy through radiation and focuses it on a photoreactor that is held on the focal line. The reflector consists of a transparent tube that contains wastewater with a suspended photocatalyst. In order to process the photocatalytic processes, the concentration ratio is kept between the range of 5 and 30. This type of photoreactor requires a tracking system in order to find out the concentrated radiation on the focal line. One of the advantages of using PTP is that this photoreactor concentrates the maximum radiations falling over it. On the contrary, due to its tracking system, which is the only axis and changes daily and seasonally, this is considered to be one of its limitations [97].
1.7.8 Inclined Flat Photoreactors
The inclined planar photoreactor (IPP) is designed as an inclined surface upon which the wastewater and the dispersed photocatalyst are allowed to flow. Exposure to the source of light. The mixture flows downward in a thin film formed by the action of gravity. The induction of turbulence depends on the inclined surface, which could be smooth or rough (step-wise) [97].
1.7.9 Gas Phase Photoreactors
This type of reactor is broadly categorized into two parts—the reactor structure and the light source. In the case of photoreactors used for purification of air, source of radiation focused on photocatalyst surface, photocatalyst with large area and mass transfer, low pressure drop and long residence time are indicated (Figure 1.8). Some of the appropriately designed common photoreactors are annular, plate, slurry, honeycomb, monolith, packed bed, and fluidized bed reactors. The other types include powder layer reactor, aerosol generator, optical fibers, etc. Many of these photoreactors are limited to the use of laboratory scale. Whereas, for commercial application for environmental applications design of efficient reactors are preferred [83].
Figure 1.8 General layout of gas-phase photoreactors (adopted from ref [98]).
1.8 Advantages and Disadvantages of Heterogeneous Photocatalysis
Keeping in view the latest technique of degradation of industrial wastewater, a few of its advantages and disadvantages are as follows [99, 100]:
Advantages:
stability of photocatalyst in aqueous system,
consumable reagents are not required,
nonselectivity of catalytic activity,
highly active and nontoxic,
destruction of organic and inorganic pollutants,
low-cost stability of the photocatalyst,
efficient recovery and reasonable recyclability,
cost-effective in case of using solar radiation as a light source.
Disadvantages:
high recombination rate of e
-
–h
+
pair,
lack of mass transfer limitations, in augmented rate,
difficulty in recovery from the mixture,
expensive while using UV radiation,
harvesting of visible light,
photocatalyst has poor electric adsorption and treatment of high concentration of organic pollutant,
complex O
3
/UV process.
1.9 Conclusion
The persistence of pollutants in industrial wastewater is an issue of concern to contemporary society. Among varied types of advanced techniques available, structured photocatalytic systems are a great alternative in the advancement for the treatment of industrial wastewater. However, to develop these structured photocatalytic systems more efficient for scale-up and effective management of industrial waste, the efficiency of the process of photocatalysis should be focused on. Moreover, many aspects of type and effective reactor design should be structured for the proper functioning of these photocatalytic systems. Parallelly, researchers should concentrate on nanomaterial-based photocatalysis for minimal or no environmental impacts and risks. Nevertheless, continual attempts are being made in this regard, which caters to various applications.
Acknowledgment
This research was supported by Development of Marine Microplastic Pollution Response and Management Technology of Korea Institute of Marine Science & Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (KIMST-20220035).
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