Water is the founding fundamental of life and hence is a basic need of life. However, due to the ever-rising population, industrialization has emerged as a global issue. This problem has notably escalated in developing countries. Their citizens face problems such as floods, drought, and poor water quality. Due to poor water quality and sanitation problems, most health issues are caused by water-borne infections. In developing countries, untreated wastewater is released into water bodies or the ground, thereby polluting natural resources. This is due to the lack of sufficient infrastructure, planning, funding, and technologies to overcome these problems. Additionally, the urbanization of megacities in developing countries is highly accelerated, but it is disproportionate to the required resources for treating wastewater. Due to this biological oxygen demand (BOD): chemical oxygen demand (COD) ratio is increasing exponentially in developing countries compared to developed ones. Spreading awareness, education and supporting relevant research, and making stringent rules for industries can alone solve the water problem in developing countries.

Persistent organic pollutants (POPs) have become a major global concern due to their large amount of utilization every year and their calcitrant nature. Due to their continuous utilization and calcitrant nature, it has led to several environmental hazards. The conventional approaches are expensive, less efficient, laborious, time-consuming, and expensive. Therefore, here in this review the authors suggest the shortcomings of conventional techniques by using nanoparticles and nanotechnology. Nanotechnology has shown immense potential for the remediation of such POPs within a short period of time with high efficiency. The present review highlights the use of nanoremediation technologies for the removal of POPs with a special focus on nanocatalysis, nanofiltration, and nanoadsorption processes. Nanoparticles such as clays, zinc oxide, iron oxide, aluminum oxide, and their composites have been used widely for the efficient remediation of POPs. Moreover, filtrations such as nanofiltration and ultrafiltration have also shown interest in the remediation of POPs from wastewater. From several pieces of literature, it has been found that nano-based techniques have shown complete removal of POPs from wastewater in comparison to conventional methods, but the cost is one of the major issues when it comes to nano- and ultrafiltration. Future research in nano-based techniques for POP remediation will solve the cost issue and will make it one of the most widely accepted and available techniques. Nano-based processes provide a sustainable solution to the problem of POPs.

Both fossil and renewable fuel sources are used widely to produce electricity around the globe. The dependency on fossil fuels for energy leads to the depletion of reserves and various forms of pollution. Coal fly ash (CFA) is one of the most burning issues in the whole world due to its large amount of production in thermal power plants. Every year a million tons (MTs) of CFA are generated globally of which almost half is utilized in various forms, while the remaining half remains unused, leading to various types of pollution. Hence, there is an immediate requirement for CFA management approaches for the efficient and sustainable use of fly ash. In the present review, the authors emphasize the status of energy and its supply and demand. A detailed description of coal fly ash-based thermal power plants, fly ash production, and utilization is provided. Moreover, the current and emerging applications of CFA are also provided.

ZnO is a semiconductor material that has important physical and chemical properties, which are frequently and significantly enhanced by the addition of impurities, such as doping. A study of the structural properties of pristine and functionalized (i.e., doped with Antimony and Tungsten) ZnO nanoparticles has been conducted for the photocatalyst-based degradation of methylene blue (MB) dye under both Ultraviolet (UV) and solar light. Authors have used a 1% concentration of dopant for doping purposes. The synthesized materials were characterized for structural analysis, functional group identification, spectroscopic measurements, and morphological examination using X-ray diffraction (XRD), Fourier transform-infrared (FTIR), UV-Vis spectroscopy (UV-Vis), and Field emission scanning electron microscope (FESEM) techniques. XRD analysis confirmed that the synthesized-doped materials retained the wurtzite hexagonal structure with a purity of 99%. Transmission electron microscope (TEM) analysis data reveals the average size of pure ZnO-NPs was found to be 7 nm; after doping the size was found to be increased to 18 nm and 9.55 nm, respectively, for ZnO-W and ZnO-Sb. As per FESEM analysis results, minor morphological changes were observed after doping. The Ultraviolet Differential reflectance spectroscopy UV-DRS study revealed the confirmation of ZnO doping with antimony and tungsten, which exhibited a blue shift. The decrease in the band-gap on doping makes the ZnO-NPs more efficient for photocatalytic applications. The photocatalytic efficiency of pristine and doped ZnO-NPs catalysts for methylene blue photocatalytic degradation (PCD) was analyzed under both UV and solar irradiation. This study analyzed the effect of pH, nano-photocatalyst dose, and initial dye concentration (ICD) on the PCD of MB. The obtained analytical results showed that the ideal conditions for the PCD of MB dye are as follows: pH = 9, the quantity of the nano-photocatalyst used was 300 mg/L, and an initial MB dye dose of 10 ppm. These conditions lead to a PCD of about 91% of the MB dye by using ZnO-Sb nano-photocatalyst on exposure to solar radiation. The reusability study also revealed the stability of nano-photocatalysts. The current research may pave the way for the removal of hazardous dyes from wastewater discharged by many industries.

The textile industry generated a series of synthetic dyestuffs that threatened environmental protection. Azo dyes, widely utilized in textile, paper, fruit, leathers, cosmeceuticals and pharmaceutical fields, account for most of the dyestuffs made. Since they have colour fastness properties, stability, and susceptibility to oxidation, existing effluent treatment methods cannot entirely strip different dyes from effluents. Under certain environmental factors, bacteria decolourize and degrade dyes. The treatment process is cheap, environmentally safe, and can be used on various dyes. However, textile plant wastewater can produce many polluting chemicals and dyes. Environmental legislation is increasingly being enacted to regulate mainly azo-based dyes in the environment. The potential of the microbes for the decolourization of dyes and metabolizing them is long-known knowledge. The toxic components of dyes challenge a potential threat to all the living forms of life. Though both natural and synthetic dyes are used for the colourization of textiles, only synthetic ones are challenging to decolourize. Microbial-based bioremediation of dyes has been studied and reviewed primarily to accelerate dye degradation. The various piece of the literature revealed that the majority of these dye removal microbes belong to mainly white-rot fungi, a consortium of anaerobic bacteria. In addition to this, there are several (genetically engineered microorganisms) GEMs that remediate dyes efficiently. Here in the current review, the authors have tried to bridge the existing gap in the bioremediation of dyestuff. Moreover, the authors have also tried to provide the latest trend in this field. This study will surely benefit the industries and researchers related to dyestuffs by maintaining eco-friendly approaches.

Due to the increased demand for clothes by the growing population, the dye-based sectors have seen fast growth in the recent decade. Among all the dyes, methylene blue dye is the most commonly used in textiles, resulting in dye effluent contamination. It is carcinogenic, which raises the stakes for the environment. The numerous sources of methylene blue dye and their effective treatment procedures are addressed in the current review. Even among nanoparticles, photocatalytic materials, such as TiO₂, ZnO, and Fe₃O₄, have shown greater potential for photocatalytic methylene blue degradation. Such nano-sized metal oxides are the most ideal materials for the removal of water pollutants, as these materials are related to the qualities of flexibility, simplicity, efficiency, versatility, and high surface reactivity. The use of nanoparticles generated from waste materials to remediate methylene blue is highlighted in the present review.