Phenolic compounds exist in water bodies due to the discharge of polluted wastewater from industrial, agricultural and domestic activities into water bodies. They also occur as a result of natural phenomena. These compounds are known to be toxic and inflict both severe and long-lasting effects on both humans and animals. They act as carcinogens and cause damage to the red blood cells and the liver, even at low concentrations. Interaction of these compounds with microorganisms, inorganic and other organic compounds in water can produce substituted compounds or other moieties, which may be as toxic as the original phenolic compounds. This chapter dwells on the sources and reactivity of phenolic compounds in water, their toxic effects on humans, and methods of their removal from water. Specific emphasis is placed on the techniques of their removal from water with attention on both conventional and advanced methods. Among these methods are ozonation, adsorption, extraction, photocatalytic degradation, biological, electro-Fenton, adsorption and ion exchange and membrane-based separation.

Photocatalytic approaches in the visible region show promising potential in photocatalytic water splitting and water treatment to boost water purification efficiency. For this reason, developing cost‐effective and efficient photocatalysts for environmental remediation is a growing need, and semiconductor photocatalysts have now received more interest owing to their excellent activity and stability. Recently, several metal oxides, sulfides, and nitrides‐based semiconductors for water splitting and photodegradation of pollutants have been developed. However, the existing challenges, such as high over potential, wide band gap as well as fast recombination of charge carriers of most of the semiconductors limit their photocatalytic properties. This review summarizes the recent state‐of‐the‐art first‐principles research progress in the design of effective visible‐light‐response semiconductor photocatalysts through several modification processes with a focus on density functional theory (DFT) calculations. Recent developments to the exchange‐correlation effect, such as hybrid functionals, DFT + U as well as methods beyond DFT are also emphasized. Recent discoveries on the origin, fundamentals, and the underlying mechanisms of the interfacial electron transfer, band gap reduction, enhanced optical absorption, and electron–holes separation are presented. Highlights on the challenges and proposed strategies in developing advanced semiconductor photocatalysts for the application in water splitting and degradation of pollutants are proposed.

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Innovative characteristics of hydrogels such as swellability, modifiability and hydrophilicity make them materials of choice for water treatment and other applications. Hydrogels have shown excellent adsorptive performance for different types of water pollutants comprising toxic dyes, nutrients and heavy metals. Among different types of hydrogel based materials, hydrogelnanomaterials combination represent a highly viable method to further improve the properties of hydrogel for numerous applications. The combination of hydrogel and nanomaterials leads to the development of hybrid hydrogel with multifunctional network. This novel combination gives synergistic effect to the newly formed novel hydrogel materials. In this article, we briefly review the recent progress in gelatin based hydrogel nanocomposites with particular emphasis on wastewater treatment along with biomedical applications.

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