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Several sources of water pollution are causing negative consequences to marine life. The organisms that are more affected are fishes and marine mammals since they are at the top of the food chain. They are directly exposed to high levels of toxins in water and/or they feed on other fishes that are contaminated. Unfortunately, the main cause of the contaminations, and thus of the fish deaths, come from human activities, such as industry, agriculture, municipal wastewater and solid wastes. The present study is concerned with the effect of organic and inorganic pollutants on the survival of fish in water bodies. We introduce a nonlinear mathematical model by considering five interacting variables; organic pollutants, inorganic pollutants, bacteria, dissolved oxygen and fish in the water body. The model is analyzed using the stability theory of differential equations and to confirm the analytical findings, numerical simulations are performed. Our results suggest that to maintain water quality and to save fish life, the global community has to limit the release of organic and inorganic pollutants into the aquatic system.

In this paper, we introduce a model to study the effects of human populations on fish survival in aquatic media. Directly, this occurs by fishing. Indirectly instead this is related to other human actions that lead to organic pollution and consequently low dissolved oxygen(DO) levels, thereby harming the aquatic fauna. Mathematically, we consider various nonlinear processes involving human population, organic pollutants, bacteria, DO and fish population. In the present study, our aim is to investigate the effect of depleted level of DO on the survival of fish populations in such an aquatic system. The case study in consideration is represented by the Ulsoor lake, Bengaluru, India. Into it, huge amounts of sewage were discharged and resulted in reduction of DO level and massive fish mortality. Equilibria are analyzed for feasibility and stability, substantiated via numerical simulations. Global sensitivity analysis identifies the important parameters having a significant impact on the fish population. The Partial Rank Correlation Coefficients (PRCCs) values of fish population in the lake with respect to input parameters of the system show that the growth rate of humans in the lake watershed has maximum negative correlation while the growth in the fish population due to DO has maximum positive correlation with the density of fish population in the lake. The results show that increase in human population may decrease fish population in the system to very low values. However, by controlling additional dissolved organic loads coming from domestic sewage, farm waste and many other sources, the level of DO can be brought back to values that allow fish survival. Maintaining it at these levels would preserve the ecosystem.

Numerous sustainable water processing techniques have been widely investigated and are capable of boosting the quality of water. Among these techniques, photonanocatalysis has stood tall with great promises in the last few decades. Nonetheless, a major challenge in the environmental remediation of photocatalyst technology is to develop an ideal photocatalyst that must have excellent photocatalytic efficiency, large specific surface area, maximum harvesting of solar energy, high durability and recyclability. Due to their stability, low toxicity, low cost and superhydrophilicity TiO₂ has been used by researchers as an efficient photocatalyst for the degradation of organic pollutants. Unfortunately, it suffers greatly due to its high band gap with 3.2eV, insufficient visible light response, fast photogenerated electrons and holes recombination rate and serious agglomeration. Keeping these views, the present review highlights the principal results of studies on current practical synthesis and photocatalytic activity of graphene-based TiO₂ nanocomposite materials for the treatment of water. The amalgamation of graphene oxide (GO) and reduced graphene oxide (rGO) with nanoscale TiO₂ particles results in synergistic properties thereby tuning and increasing the functionality of the composite. In this regard, the review also addressed the progress and insight into graphene-based TiO₂ nanocomposite in photocatalytic removal of organic pollutants including basis mechanism, possible key strategies of the composite, and an overview of how to elevate efficacy. Finally, brief challenges and future perspectives in this field are also presented. Indeed, this work illustrated that graphene-based TiO₂ composite nanomaterials can be a green signal in the future of photocatalysis targeting water pollution remediation.

New methods for water treatment are required as a result from an increasing awareness in the reduction of the pollution impact in the environment. In the perspective of the photo-oxidation of organic pollutants present in water, the principal incentive for the preparation of heterogeneous photocatalysts is their easy recovery from the reaction mixture, which allows their reuse in successive runs, minimizing the loss of their original photocatalytic properties. Different types of supports can be used in the immobilization of photoactive species, such as porphyrins (Pors) and phthalocyanines (Pcs). This mini-review will consider the different methodologies for the immobilization of Pors and Pcs and their photocatalytic performance in the photodegradation of organic pollutants in water, addressing also their recycling ability in successive water treatments.

Ag-modified TiO₂ on activated carbon fibers (ACF) was synthesized and successfully used for the photocatalytic degradation of organic pollutants in this work. The physical properties of the materials were characterized using various methods such as X-ray diffraction, scanning electron microscope, Brunauer–Emmett–Teller, Fourier transform infrared, UV–Vis, electrochemical impedance spectroscopy and reusability tests. Besides, the effects of reaction parameters on photodegradation such as silver loading, light source, organic pollutant species and initial pollutant concentration were investigated. The results showed that Ag significantly increased the number of surface adsorption sites, improved the electron transmission rate and suppressed the complexation of e⁻ and h${}^{+}$. Under visible light, 1.5-Ag-TiO₂/ACF $[n(\mathrm{\text{Ag}}:{\mathrm{\text{TiO}}}_2)=1.5]$ revealed the best catalytic activity and reusability for dye removal and toluene adsorption. Based on the experimental results, a possible photocatalytic mechanism of Ag-TiO₂/ACF was proposed. This study will provide a theoretical basis for the application of Ag-TiO₂/ACF photocatalyst in the field of organic pollutants treatment.

In this paper, a nonlinear mathematical model is proposed and analyzed to study the role of dissolved oxygen (DO)-dependent bacteria on biodegradation of one or two organic pollutant(s) in a water body. In the case of two organic pollutant(s), it is assumed that the one is fast degrading and the other is slow degrading and both are discharged into the water body from outside with constant rates. The density of bacteria is assumed to follow logistic model and its growth increases due to biodegradation of one or two organic pollutant(s) as well as with the increase in the concentration of DO. The model is analyzed using the stability theory of differential equations and by simulation. The model analysis shows that the concentration(s) of one or both organic pollutant(s) decrease(s) as the density of bacteria increases. It is noted that for very large density of bacteria, the organic pollutant(s) may be removed almost completely from the water body. It is found that simulation analysis confirms the analytical results. The results obtained in this paper are in line with the experimental observations published in literature.

Keggin-type polyoxometalates is a kind of promising photocatalyst with adjustable light absorption range. However, the high recombination rate of photogenerated charge carriers hampered its photocatalytic efficiency. To solve this weakness, novel carbon quantum dots (CQDs) modified Cs₄PW₁₁O₃₉Fe(III)(H₂O) photocatalysts (CQDs/Cs₄PW₁₁Fe) have been synthesized via a hydrothermal process. Phase structure, morphology, optical and electronic properties of the as prepared photocatalysts were systematically characterized by XRD, SEM, TEM, DRS and surface photovoltage tests. It could be found that CQDs was well deposited on the surface of Cs₄PW₁₁Fe, which has broadened the light absorption of Cs₄PW₁₁Fe. The photodegradation efficiencies of the CQDs/Cs₄PW₁₁Fe composite photocatalysts for RhB under visible light irradiation were clearly enhanced. Trapping experiments were carried out to detect the photocatalytic mechanisms. The charge transfer and separation efficiencies of the samples were confirmed by electrochemical impedance spectroscopy tests.

Being available in water and air, organic contaminants have easy access to animal bodies to accumulate in the biological food chain, resisting chemical, biological, and photolytic degradation. Besides, they have the ability to travel great distances to end up being dispersed across a broad area, even in places where they have never been used. Furthermore, they pose a significant risk to both human society and the environment, which forces the international scientific community to plan and act to eradicate organic pollutants from the environment and establish a mechanism to stop their discharge. In this context, covalent organic frameworks (COFs)-based materials are found to be promising to control air and water pollutants because of their unique porous, and polymeric crystal structure. This study highlights the history, design, and applications of COFs for reducing organic pollution, as well as the obstacles and opportunities facing their widespread usage in environmental remediation today.