Dye and nitro-compound pollution has become a significant issue worldwide. The adsorption and degradation of dyes and nitro-compounds have recently become important areas of study. Different methods, such as precipitation, flocculation, ultra-filtration, ion exchange, coagulation, and electro-catalytic degradation have been adopted for the adsorption and degradation of these organic pollutants. Apart from these methods, adsorption, photocatalytic degradation, and chemical degradation are considered the most economical and efficient to control water pollution from dyes and nitro-compounds. In this review, different kinds of dyes and nitro-compounds, and their adverse effects on aquatic organisms and human beings, were summarized in depth. This review article covers the comprehensive analysis of the adsorption of dyes over different materials (porous polymer, carbon-based materials, clay-based materials, layer double hydroxides, metal-organic frameworks, and biosorbents). The mechanism and kinetics of dye adsorption were the central parts of this study. The structures of all the materials mentioned above were discussed, along with their main functional groups responsible for dye adsorption. Removal and degradation methods, such as adsorption, photocatalytic degradation, and chemical degradation of dyes and nitro-compounds were also the main aim of this review article, as well as the materials used for such degradation. The mechanisms of photocatalytic and chemical degradation were also explained comprehensively. Different factors responsible for adsorption, photocatalytic degradation, and chemical degradation were also highlighted. Advantages and disadvantages, as well as economic cost, were also discussed briefly. This review will be beneficial for the reader as it covers all aspects of dye adsorption and the degradation of dyes and nitro-compounds. Future aspects and shortcomings were also part of this review article. There are several review articles on all these topics, but such a comprehensive study has not been performed so far in the literature.

It is universally accepted that the financial advancement of a state is essentially dependent upon the energy sector as it is essential in the growth, development, and improvement of the farming, mechanical, and defense sectors.

Owing to the high cost of recycling waste, underdeveloped countries discharge industrial, agricultural, and anthropogenic effluents without pretreatment. As a result, pollutant-loaded waste enters water bodies. Among the diverse toxic contaminants, heavy metal ions are the most detrimental because of their chronic toxicity, non-degradability, prevalence, and bioaccumulation. The growing shortage of water resources demands the removal of heavy metal ions from wastewater. Three SDGs of the sustainability agenda of the United Nations appeal for clean water to protect life beneath water and on land depending on the water sources. Therefore, efficient environmentally friendly approaches for wastewater treatment are urgently required. In this regard, several methods have been developed for the removal of heavy metal ions from wastewater, including adsorption as the most widely used method owing to its eco-friendly, cost-effective, and sustainable nature. The present review discusses the progress in the preparation and application of various adsorbents based on carbon, micro-organisms, agricultural waste and inorganic materials for the extraction of toxic metal ions such as Pb²⁺, Cr⁶⁺, As³⁺, As⁵⁺, Hg²⁺ and Cd²⁺. Herein, we provide information on the role of the homogeneity and heterogeneity of adsorbents, kinetics of the adsorption of an adsorbate on the surface of an adsorbent, insights into adsorption reaction pathways, the mechanism of the sorption process, and the uptake of solutes from solution. The present review will be useful for researchers working on environmental protection and clean environment.

Photocatalytic degradation technology has received much attention from researchers in the last few decades, due to its easy and cost-effective nature. A lot of review articles have been published on dyes via photocatalytic degradation, but most of the review articles lack a detailed and in-depth photocatalytic degradation mechanism of dyes. Numerous review articles are available on photocatalysis. Here, in this review article, we are mainly focused on the complete and in-depth photocatalytic degradation mechanism of four commonly used dyes such as Malachite Green, Methylene Blue, Congo Red and Rhodamine B, which will be highly useful for the new researchers that work on dyes’ photocatalytic degradation. Initially, various aspects of dyes have been included in this review article, comprehensively. The main focus was on the covalent organic framework-based photocatalysts for dyes’ photocatalytic degradation, due to their porous nature and various unique properties. Various synthesis routes and the photocatalytic performance of covalent organic frameworks and composite of covalent organic frameworks have been highlighted in this review article. In the last section of this review article, the main stimulus was the four mentioned dyes’ properties, uses, and toxicity, and the photocatalytic degradation mechanism through various paths into environmentally friendly and less-harmful compounds in the presence of photocatalysts. Factors effecting the photocatalytic degradation, economic cost, challenges and future aspects of photocatalytic technology were also included in this review article. This review article will be highly useful for those researchers that work on the photocatalytic degradation of various dyes and search for the complete degradation of complex dye molecules.