The increase in population demands for industrialization and urbanization which led to the introduction of novel hazardous chemicals in our environment. The most significant parts of these harmful substances found in water bodies remain in the background, causing a health risk to humans and animals. It is critical to remove these toxic chemicals from the wastewater to keep a cleaner and greener environment. Hence, wastewater treatment is a challenging area these days to manage liquid wastes effectively. Therefore, scientists are in search of novel technologies to treat and recycle wastewater, and nanotechnology is one of them, thanks to the potential of nanoparticles to effectively clean wastewater while also being ecologically benign. However, there is relatively little information about nanocatalysts’ applicability, efficacy, and challenges for future applications in wastewater purification. This review paper is designed to summarize the recent studies on applying various types of nanocatalysts for wastewater purification. This review paper highlights innovative work utilizing nanocatalysts for wastewater applications and identifies issues and challenges to overcome for the practical implementation of nanocatalysts for wastewater treatment.

Textile wastewater is among the most polluted types of industrial waste. Wastewater treatment in the textile industry is notoriously difficult because of the use of complex chemicals and dyes in the textile production steps, and conventional methods are not enough to treat these. Textile wastewater, known for its high pollution levels, poses challenges for treatment due to complex chemicals and dyes. A comparative study was conducted on simple ozonation and catalytic ozonation (CO) using alum to degrade methylene blue (MB). The authors analyzed various factors like time, pH, catalyst loading and ozone dosages during the study. Results showed that catalytic activity in ozonation depends on pH and ozone flow. The maximum MB elimination was achieved at pH 6.6 and 200 V ozone flow. Scanning electron microscopy (SEM) was used to characterize the surface morphology of the catalyst, Fourier transform infrared spectroscopy (FTIR) was used to identify the important functional groups, and energy dispersive X-ray spectroscopy (EDS) was used to characterize the catalyst's elemental composition. Compared to simple ozonation, CO showed higher removal in the initial phase. Real textile wastewater analysis confirmed the effectiveness of alum catalysts in achieving significant removal of MB (87%) through this novel cost-efficient process. Graphical Abstract

The ample increase in water scarcity and depletion of natural resources due to their overconsumption and the contamination of water sources becomes more challenging day by day. This challenging situation has pushed the scientific community to cope with it by providing alternative solutions. Therefore, it is indeed important to conduct a sustainable study on recycling wastewater for a particular purpose. Taking this into account, an effort was made to develop a novel hybrid treatment system that applied both conventional and advanced oxidation treatment processes. In this sustainable study, an integrated system was designed for the effective treatment followed by the recycling of automobile service station wastewater (ASSWW) which comprised sedimentation (sed), catalytic ozonation, adsorption, and filtration. In the current investigation, two catalysts/adsorbents, the granular activated carbon (GAC) and rice husk (RH) were employed individually and in combination for the first time in the studied hybrid process and their performance was compared and evaluated. The obtained results revealed that the hybrid system combination-I (Sed–O₃/GAC) was more efficient than combination-II (Sed–O₃/RH); the maximum removal efficiency of COD was 100% and 80%, respectively. In addition, the hybrid system combination-III (Sed–O₃/RH + GAC) was more economical and efficient than others by employing 35% of each absorbent in the adsorption column. Moreover, this efficient Sed–O₃/RH + GAC system has a maximum removal efficiency 99%, 100%, 99%, 100%, (89%, 99%, 100%) and 100% for turbidity, COD, BOD₅, fecal coliform, potentially toxic metals (Cd, Pb, As), oil and grease, respectively, at optimized conditions (O₃ = 82.5 mg/L; contact time = 18 min and catalyst dose of GAC and RH = 200 g each). Furthermore, the treated water sample complied with the WWF-recommended Irrigation Water Quality Guidelines (IWQGs) for class D. The increase in biodegradability (BOD₅/COD ratio) was observed from 0.41 to 0.83. Therefore, the proposed efficacious hybrid system may be employed for the recycling of ASSWW for irrigation purposes.

Clean water is a necessity for all life to survive and flourish. However, natural waters are being continuously contaminated due to the release of waste streams in water. Hence, it is important to remove pollutants from wastewater to fulfill human needs. Conventional treatment methods are neither efficient nor economical for wastewaters that especially contain refractory toxic pollutants. This requires novel techniques like Advanced oxidation processes (AOPs), that may successfully degrade persistent micropollutants more efficiently. In this study, an azo dye Acid Red 1 was removed by three AOPs, namely Photocatalytic oxidation, Ozonation and Photocatalytic Ozonation, by employing heterogenous catalysts. TiO₂ was used as photocatalyst, whereas Fe-Zeolite has been further added as Ozonation catalyst. The study revealed that photocatalysis degraded only 28% Acid red dye after 15 min, whereas for ozonation, the degradation percentage was 95% in same time. In combined photocatalytic ozonation process using TiO₂, 95% degradation was achieved in just 9 min and treatment time further reduced to 5 min when Fe-zeolite was added. Optimization studies for initial concentration, UV intensity and catalyst loading were performed. Finally, rate constants and Electrical Energy per Order (EEO) values were determined for all AOPs, and mechanism was proposed.