Sustainable wastewater treatment using highly efficient nanocatalysts

With globalization and rapid increase of population, especially in developing countries, the crisis of fresh water availability and its quality has become one of the major concerns of this decade. Contamination of natural fresh water resources by harmful substances makes the water undrinkable and toxic to ecosystems. Therefore, wastewater management has recently gained global attention as a major challenge for preserving natural water resources. To meet the basic requirements of such a huge population in developing countries, which is continuously growing, rapid growth across various industries and the agricultural sector has been observed. Because of the tremendous growth of small and large industries over the past two decades, water pollution has increased rapidly in developing nations such as India, Pakistan, and Bangladesh. In this context, wastewater treatment has become crucial in recent times, particularly for developing nations. Waste water treatment in India has drawn the attention of citizens and the government, and necessary actions have been taken to eliminate toxic pollutants from waste water and protect natural water resources and groundwater. Because hazardous chemicals have long-term negative effects on ecosystems, the environment, and human health, it is imperative that they be removed from wastewater. It is observed that the pigment and dye industries, among other emerging sectors, have grown to be a significant source of wastewater containing hazardous contaminants. In addition, dye effluents are produced in large quantities due to the extensive use of non-biodegradable, hazardous dyes (Rhodamine B, Methylene Blue, Rose Bengal) in specific industries such as textile, polymer, leather, cosmetics, notepaper, decoration, and food. These harmful dyes can cause water pollution when they come into contact with fresh water. If left untreated, these toxic pollutants in wastewater, then they can lead to significant environmental degradation as well as health problems for people. It was reported that even at low concentrations, these toxic dyes can significantly harm the human body's skin, eyes, respiratory system, and nervous system. Thus, it is crucial to remove all these harmful organic dyes from the wastewater. 

          Over the past few decades, a number of methods have been developed for treating wastewater that contains toxic dyes. In this regard, photocatalysis has emerged as an effective solution for degrading harmful organic dyes compared to other available water-treatment procedures. It is observed that photocatalysis with nanosized catalysts, due to their high specific surface area, is often favored for the degradation of toxic dyes. Photocatalysis is a process in which photogenerated electron-hole (exciton) pairs, generated by light of specific wavelengths, actively participate in oxidation and reduction reactions. It is a straightforward and economical method for reducing harmful dyes using nanosized catalysts. It was observed that nanoparticles with a mean size below 10 nm and quantum dots, due to their high specific surface area, porous structure, and semiconducting nature with a bandgap between 1.5–3.5 eV, are suitable for photocatalysis applications. Several ferrimagnetic spinel cubic ferrite nanoparticles (CoFe2O4, NiFe2O4, ZnFe2O4, and MnFe2O4), along with various quantum dots (TiO2, ZnS, ZnSe, and SnO2), have shown excellent photocatalytic activity for degrading toxic dyes under xenon radiation and sunlight. Hence, photocatalytic reduction of toxic dyes using nanosized catalysts and light of particular wavelengths can be a decisive step towards wastewater treatment. 

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