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Due to the significant progress in chemical and bioanalytical methods, the presence of a plethora of chemicals and biological contaminants in various water sources is well established. The nature of pollutants in water may be organic compounds, inorganic metal ions, nanomaterials, or others. These pollutants are present in water in environmentally relevant concentrations, usually in the ng/L and µg/L range. The source of pollution can be due to treated wastewater discharge in surface-water bodies or its reuse in irrigation, nonpoint pollution sources or agriculture runoff. Although their concentration levels are very low, their presence is challenging due to direct known and unknown adverse effects on humans, aquatic life, and indirect effects as they tend to bioaccumulate in food crops, water, and marine life. These pollutants are referred to as contaminants of emerging concern (CECs). According to the EPA's whitepaper, CECs include persistent organic pollutants (POPs), pharmaceuticals and personal-care products (PCCPs), endocrine-disrupting chemicals (EDCs), and nanomaterials. Large-scale biological treatment and disinfection processes are conventionally used all over the world to treat wastewater. Additionally, tertiary treatment options such as physicochemical processes and adsorption units may also be applied. Unfortunately, CECs are not effectively removed by conventional biological and physicochemical water-treatment processes. Thus, advanced water-treatment processes are being studied for adequately treating contaminated water as a requirement for reuse and recycling.

Moreover, antibiotic-resistant bacteria, pathogenic protozoans, and novel viruses are also a major concern as well as antibiotic-resistant genes, toxins, and CECs present in water resources. These should be carefully monitored, and, where possible, completely removed from the water matrix. Their presence and persistence, expensive detection techniques, regulation issues, undesirable effects on human health, and aquatic species along with unknown intermediate metabolites and their toxicities add further complexity.

Photochemical approaches to remove contaminants from water offer an exciting array of possibilities: covering chemical analysis, molecular-biology methods, and computational approaches important for the elucidation of mechanisms and evaluation of photochemical processes, this book focuses on advanced photochemical processes for the removal of CECs from water. Readers will find chapters on advanced photochemical-treatment prototypes and pilot plants, in addition to discussions on translation from lab- to field-scale implementation. Examples include highly transparent materials, integrated membranes, adsorbents, and photocatalysts for enhancing the photochemical processes by direct sunlight-induced reactions, photo-Fenton, and photoelectrochemical reactions. Discussing both the advantages and disadvantages of these technologies, this book is a great resource both for academicians and researchers working on this topic and for water-treatment professionals.

The book comprises 14 chapters. In Chapter 1, Hiwrale et al. discuss analytical techniques in measurement of ubiquitous CEC and their transformed products. In Chapter 2, Menacherry et al. discuss high-resolution mass spectrometry (HRMS), as an analytical technique to explore the identity of major reaction products and thereby, degradation mechanisms of CEC during photochemical AOPs. In Chapter 3, Bombaywala et al. discuss the ecology of antibiotic-resistant bacteria (ARB) and antibiotic-resistant-genes (ARGs) in treatment plants, and applications of photochemical water treatment for their removal. Casado et al. investigate modeling strategies for the simulation and design of photochemical processes for water treatment, in Chapter 4. In Chapter 5, Polo-López et al. evaluate and compare common and novel materials for the design of larger volume (>20 L) transparent solar water disinfection (SODIS) containers. In Chapter 6, Giannakis et al. discuss action mechanisms and the effect of different operational parameters involved in electro-Fenton and electrophoto-Fenton processes for the elimination of organic and microbial contaminants commonly present in urban wastewater effluent. The fundamentals of photoelectrocatalysis, reactor-design concepts, materials-selection criteria, and specialized considerations related to the pollutants, are examined by Fernandez-Ibanez et al. in Chapter 7. In Chapter 8, Abeledo-Lameiro et al. explore the current understanding of treatment technologies for the removal from water of Cryptosporidium and Giardia. These techniques include photochemical advanced oxidation processes (AOPs), ultrasound (a nonphotochemical AOP) and granular activated-carbon adsorption. Nair et al. discuss strategies for material fabrication and integration with the membrane system for photocatalytic membrane-based AOP to remove organic pollutants in water, in Chapter 9. In Chapter 10, Prakash et al., examine the mechanism, key factors, and performance of various carbonaceous adsorbent photocatalysts and their derivatives for the adsorption-photodegradation of pharmaceuticals. The performance of UV LED photolytic sources for the removal of organic and microbial CEC, and discussions about prototypes based on UV LED photolytic process for small-scale POU application, to large-scale pilot studies, are investigated by Nupur and Laxman et al., in Chapter 11. Laxman et al. examine photo- and electrochemical routes for H2O2 generation using various catalysts, and performance metrics such as yield rates, apparent quantum efficiency, solar-to-chemical conversion efficiency (for phototcatalytic materials) and Faradaic efficiency, kinetic parameters (for electrocatalytic materials), and methods for the quantification of H2O2 generated during photo/electrochemical processes in Chapter 12. Tripathy et al. present a cost analysis of photochemical and other advanced processes for graywater treatment, in Chapter 13, while Tyagi et al. present an evaluation of contaminant-removal technologies in wastewater-treatment plants in the context of the circular economy, in Chapter 14.

We thank all the authors for their contributions. We acknowledge the Editorial Board, Chemistry in the Environment Series, Royal Society of Chemistry, for this opportunity, and Dr Helen Armes and team for their kind support. Last but not the least, we thank the European Commission and the Department of Science and Technology (DST) of India.EU, grant agreement number 820718, for supporting this work.

Halan Prakash

Rita Dhodapkar

Kevin McGuigan

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