Nanomaterials significance; contaminants degradation for environmental applications

Nanotechnology provides an innovative platform that is inexpensive, reasonable, having least chances of secondary contamination, economical, and an effective method to concurrently eradicate numerous impurities from contaminated wastewater. Presently, different researches have been conducted exhibiting versatile multifunctional nanoparticles (NPs) that concurrently confiscate several impurities existing in the water. Nanotechnology helps in eliminating impurities from water through the rapid, low-cost method. Pollutants such as 2,4-dichlorophenol (death-causing contaminant as it quickly gets absorbed via the skin), or industrial dyes including methyl violet (MV) or methyl orange (MO) causing water contamination were also concisely explained. In this mini-review, nanomaterials were critically investigated, and the practicability and effectiveness of the elimination of contaminations were debated. The analysis shows that a few of these processes can be commercialized in treating diverse toxins via multifunctional nanotechnology innovations. Hence, nanotechnology shows a promising and environmental friendly method to resolve the restrictions of current and conventional contaminated water treatment. We can progress the technology, without influencing and affecting the natural earth environment conditions.


Introduction
Nanomaterials contribute a significant role in the reduction of environmental contamination, industrial, factories, and agricultural wastes treatment. Environmental pollution can easily decline via green progression and production, emissions control, desulfurization/denitrification, and agriculture upgrading. The management of industrial and agricultural wastes encompasses the transformation of wastes products into goods, remediation groundwater via photocatalys is and nanomembranes. Nanomaterials modify physical properties due to their high specific surface area to volume ratio. The nanomaterials having nanoscale dimensions were used as adsorbents, catalysts, membranes, or additives to upsurge activity and competency due to their high specific surface areas. Hence, nanomaterials are more effective at handling environmental trashes [1].
There are different kinds of observable, non-observable pollution influencing one in daily life. Industrial pollution, water pollution, greenhouse gases (GHG) [2,3]. Ozone (O) dangers are common factors enhancing environmental pollution, generating atmospheric toxins polluting water resources (rivers, lakes, and oceans) land and air [4]. Industrial waste contaminates both water and soil [5]. One cannot see different kinds of toxins until they move in the surrounding atmosphere extending in the far-flung and widespread region. Factory pollution is observable, retaining stinking cloudy smoke emissions. The factory air pollutants comprise greenhouse gases due to fossil fuel burning. Factories also detoxify water and land via acidifying rain, chemical, or dumping toxic waste. Carbon dioxide (CO 2 ) is the most destructive greenhouse gas released in the air via the burning of fossil fuels. Sulfur dioxide also generates from burning fossil fuels, a significant source of causing acid purple. The inhaled ultrafine particles created on e-cigarettes and other recently developed items have caused serious health concerns. Many methods have been used to cope with contamination problems, including the elimination of toxins through filtration, adsorption, oxidation, or by biological agents. On the other side, pathogens that we historically did not know becomes the key component that triggers several terrible infections and diseases connected with skin and respiratory infections [23]. Many researchers are interested in the production of therapeutic agents due to metals among microbes and their ability to battle against microbial diseases. Recently, metallic nanomaterials have been the subject of a great deal of concern in the area of biomedical science owing to their potentially biomedical advantages.
Besides this, pathogenic microorganisms are also becoming a key problem of infectious agents among all demographic groups varying from adolescents to the elderly. The tolerance of pathogenic bacteria increases the probability of illnesses with pathogenic diseases in the gastrointestinal tract. The fact scientists have been creating antibacterial medicines that are immune to infection has pushed doctors to discover new and creative methods to suppress them. In the current status, NPs are being regarded favorably for treatment of various diseases, and even for the treatment of AIDs or HIV [24].

Motivation and objective of study proposed
It is a need in today's time to control toxins, contaminations, and take steps to decline pollutants for a living being. The present review focus on the recent progression and innovative ideas to control, regulate, and clean the toxins and contaminants generated from different pollution sources at a safer level for the benefit of living habitats. In this review article, the discussion starts with an overview of various pollutants classes, and particular focus is given on 2,4 DCP (Molecular formula: C 6 H 4 Cl 2 O) having a melting point of 45°C showing poor biodegradability (0%, four weeks) with water solubility 4.5 g l −1 (20°C)) having adverse effects on humans [25,26]. In the past, the immediate death of workers was reported if exposed to 2,4 DCP, i.e., worker (male) died instantly when exposed to broiling steam having 2,4-DCP (EPA, 2000). Earlier, it was reported that a worker suffers epileptic seizures in 20 min and expired after splashed (only 10% of the body) with pure molten 2,4-DCP. It shows that 2,4-DCP exposure instantly gets absorbed via skin which is deadly and harmful [27].
In nature, freshwater is a valued resource that can easily be contaminated. It is highly problematic to reinstate the quality of freshwater once it gets 'polluted. The water in streams, reservoirs, lakes or alongside the water that drifts continually in watercourses/channels, can easily be contaminated because it is used for water supplies, agriculture, industry, or recreation. The majority of diseases spread due to drinking contaminated water. Chlorophenol compounds (CPC) are poisonous, carcinogens, and extensively used as pesticides, wood preservatives, drugs, fungicides, insecticides, or in anticorrosive rust production organically polluting waters and soil continuously [28,29]. CPCs are non-biodegradable due to their physicochemical characteristics depending on chlorine substitutes. The chlorinated hydrocarbons biodegradation through microorganisms becomes problematic and difficult with growing chlorine substitutes on CPC [29,30]. As a consequence, it leads towards the accumulation of CPC, cause danger to human health via food chains. Furthermore, it can also initiate disorders in cellular bilayer phospholipids structure, generating cancer-causing, and mutagenic disease in humans [31]. It is considered toxic, harmful waste, and hazardous in US Clean Water Act as well as EU Directive 2455/2001/EC[ [32]].

Developed methodology
A variety of methods, often classified into three groups, make possible the removal of dye from wastewater i.e. i) physical shielding, which involves adsorption and ion diffusion; chemical processes such as electrical deterioration, chemical oxidation, photocatalytic degradation or ozonation; and (iii) natural degradation [33]. The physical adsorption system is a time-consuming process. Likewise, natural degradation methods are disadvantaged due to their poor degradation efficacy for certain dyes and expensive treatment procedures [34]. Photocatalysis seems to be a feasible substitution throughout the currently accessible strategies for the removal of pollutants [35]. Diverse semiconductors were used throughout the years as photocatalysts. The popular photocatalysts TiO 2 has indeed been widely known and has already been classified as a remarkable photocatalyst due to its strong flexibility and its exceptional corrosion resistance. The use of manganese oxide, namely birneitis was widely investigated due to its significant use in different fields, such as electrical condensers, aquatic oxygen therapy, catalytic oxidation of natural dies, and gas equipment. Thanks to the excellent possessions of electron transport and multiple adsorption and oxidation capabilities of MnO 2 , having bandwidths of 2.20-2,63 eV have fascinated significant interest for photocatalysis [36].

Metallic nanoparticles synthesis
Metallic nanoparticles were also considered to be able to be an effective antioxidant agent against pathogenic bacteria besides resolving water contamination issues. Despite the existence of numerous metallic nanoparticles such as lead, bronze, zinc, copper, titanium, and magnesium, investigations have been carried against such bacterial infections. Metals oxide possessing specific structural configurations make them conductive, insulator, or semiconductive. Nanomaterials were prepared via co-precipitation, thermal decomposition, microemulsion, hydrothermal or sonochemical processes. These nanomaterials can be used in lithium-based batteries, solar panels sensors, or as pigments and dyes beside medications. Although NPs are not as hazardous as arsenic and mercury, they are dose-dependent and become toxic at higher concentrations. The degradation of picric acid Pd@TiO 2 nanoparticles for anticancer activity [37]. Different methods have been used to synthesis metallic nanomaterials using top down or bottom up approaches as depicted in schematic figure 2.

Water contamination and its removal techniques
Owing to massive urbanization and industrialization globally, contaminated water is pervasive. Hazardous waste management is now a serious ecological problem that affects sea animals, water environments, and public health emitted from numerous industries such as textiles, manufacturing, leathers, food manufacturing and pharmaceutical goods, pharmacy, paper, maquillage, and rubber [38]. Cationic crystal dye (CV) was used in medical diagnostics due to its easy interaction with many other materials. Furthermore, inhaling excess CV or CR pigments may induce gastrointestinal system allergic reactions, vomiting, diarrhea, nausea, distorted vision, phlegm cell damage, or genetic mutation [38,39]. Photoelectrocatalytic dye degradation is an increasing strategy that attracts considerable interest in handling chemical dye-containing wastewater. Manufacturers processing significant quantities of toxins utilizing techniques like adsorbent, ultrafiltration, flocculation, filtration, solvent extraction, reverse osmosis, or biodegradation for wastewater treatment [39]. In contrast, some decaying microbes, including certain fish scales, eggshells, oyster shells, or biomaterials, have been used in numerous experiments to degrade impurities due to their cost-effective, environmental friendly nature and the ability for accelerated degradation of dangerous dyes [40].
Photocatalysis is a groundbreaking technique developed in light of the failure of the conventional water treatment techniques. The three key features comprising heterogeneous photocatalytic (mostly oxygen) reactions are selective i.e. selective wavelength photons, catalyst (especially the solid catalyst), and a strong oxidant. The participation of NPs must comply with all relevant requirements about the use of the metal nanomaterials, such as CuO, Ag, and others for biomedicine [41]. The use of non-metallic nanoparticles nevertheless encourages a safer, non-toxic approach to the degeneration of photocatalytic dyes. Researchers thus focus on the use of nanosized hydroxyapatite (HAp) in synthetic dyes deterioration. A significant inorganic material in all vertebrates is hydroxyapatite. It comprises around 70 percent of the chemical formula of apatite calcium phosphate, Ca 10 (PO 4 ) 6 (OH) 22 (HAp). HAp's biocompatibility, degradability, and ion exchange characteristics allow them ideal products for the implementation of cartilage, such as automated bone marrow transplants, orthopedic therapy, and regenerative medicine systems [42]. In science and nanotechnology uses, HAp's similarity to bone minerals renders it a biocompatible ceramic. The reduced crystallite size nanostructured HAp represents the biological HAp configuration. HAp may be processed using different processes, including hydrothermal processing, the chemical reaction of sol-gel, and the technique of wet chemical precipitation [43]. The usage of HAp for wastewater treatment and its potential to extract fluorides, heavy metal ions as well as dyes from aqueous solutions have been documented by several researchers. Selvam Sathiyavimal has documented the preparation and characterization of HAp from fish (Gibelion Catla) bones and its sustainable and environmental use as photocatalytic activity for deterioration of CV or CR dyes through natural daylight irradiation, taking into account the efficient application of Hap demonstrating 77% degradation of CV and 87% of CR within 75 min of exposure [38].
Pinaud et al [44] have recently established the strong photocatalytic activity of nanocomposite MnO2/TiO 2 to degrade methylene blue under visible light radiation. Effective photocatalysts for water oxidation were also found to be chemically shaped i.e. manganese-calcium oxides, but their structures at the atomic level are not clear. The load concentration and oxidation of manganese are affected by Ca 2+ ions [45] Lucht et al have shown that birthite frameworks with Sr, Ca, B and Al are capable of separating water. A sufficient lighting variance is needed for anhydrous Ca Birnesite. Rapid nanotechnological reforms are expected to illustrate the marketing trend for the usage of nanostructures in polluted water treatment [46]. Nanocomposites, utilizing single metal nanoparticles, are formed and vary considerably due to different physical, structural and optoelectronic properties [47]. These nanocomposites were considered to provide an outstanding remarkable dye degradation efficiency and energy conservation capacity for metal oxides and are thus labeled energy materials. Different studies reported the synthesis of mixed oxide catalysts used for different nanostructured materials. The method involves sol-gel, hydrothermal flammable spray, deposition of the chemical gas phase, or combustion. The microstructures and attributes of the product are therefore distinct and highly reliant on synthesis processes and conditions. The sol-gel mechanism, solvothermal growth, or ultrasonication approach also appears to be a better way to synthesize nanoparticles. The ultrasonication provides a simple and easy solution and is a valuable tool for the development of standardized materials which cannot be processed utilizing conventional methods [48]. Thus cleaning contaminated water could be carried out using different approaches mentioned in schematic figure 3.
Polluted water, including systems built around groundwater, profoundly impacts community life. Water supplies have indeed been polluted by poisonous chemicals as a consequence of the growing industrial revolution. In the long term, the difficulties in accessing clean drinking water is a massive concern, for even a significant period. Toxins that reach the water supplies have become so complex that it is challenging as an unbroken subject to deal with it in one cohesive direction. Pigments were considered as dyes frequently used in several applications, including fabrics, pharmaceutical drugs, nutrition, makeup, plastic, painting, ink, photography, wood, or automobile industries parts. A detrimental effect is due to the use of colored water infiltration systems because of their chemical and particle composition. In the aqueous phase, pigments throughout various proportions specifically refer to various categories, like oxidative, acidic, basic, dispersed, aldehyde, diazo, anthraquinone-based, or metal-complex pigments. The origins of such pigments consist of various components classified as toxic substances, like benzidine or naphthalene. Furthermore, once these dyes were broken down by microorganisms, the byproducts could still be lethal. Few pigments are naturally xenobiotics, however, in the wastewater stream withstand traditional removal techniques. There have been various strategies for the degradation of impurities in drinkable water that have already been introduced. After all, it's indeed essential to multiply the efficacy of treatment according to the considerations. Figure 4 represents the model explaining various factors which may impact the pollutant degradation from contaminated water.
In view of that, nanoparticles have attracted interest in different disciplines in recent years, such as bioengineering, photocatalysis, detectors, robotics or in optoelectronics [49]. The synthesis of metallic nanoparticles with controlled structural composition and practical benefits is the objective of researchers. Environmental friendly nanoparticles synthesis is now implemented to eliminate the occurrence of contamination and encourage sustainable development. In specific, throughout environmental sustainability, multiple concepts are often used to minimize the use of harmful and pathogenic contaminants or chemicals to synthesize sustainable and non-toxic nanomaterials. Natural resources such as plantations comprising leaves, wood, vegetables, flowers, seedlings, or microbes, such as bacteria, seaweeds, etc. can function as metallic and non-metallic nanostructures. Therefore, by removing the use of harmful chemical substances, natural resources have offered an eco-sustainable and environmentally friendly route to synthesize possible nanomaterials [50]. Amongst various metal oxide nanoparticles, zinc oxide and TiO 2 particles are the most commonly shown in beauty products, moisturizer creams and cleansers, and commercial applications. ZnO has significant antibacterial activity, in biological processes as well as the ecosystem, it's a much more stable nanomaterial due to its durability and long-term effectiveness against microbes [51]. Laser beam deposition (PLD), evaporation, molecular beam epitaxy (MBE), or sputtering are part of the physical technique for nanomaterials processing, whereas chemical synthesis is conducted utilizing spray pyrolysis, co-precipitation, hydroxylation, hydrolysis, advanced oxidation process (AOP), or sol-gel approaches [52].
The use of nanoparticles to remove dyes from contaminated water has risen to prominence over the last couple of years as they have a greater surface area, large adsorption properties, less diffusion resistance, and faster equilibrium rates. More primarily, several other research teams all around the world have investigated the effect of nanomaterials in wastewater treatment. The physical, chemical and biological properties of nanomaterials are preferable, helping to identify uses in numerous industries. Besides nanomaterials exhibit antimicrobial properties that promote their effects in the treatment of wastewater. Photocatalysis is a mechanism in the removal of dye effluents, in which electrons moved to conduction from the valence band when exposed to radiation, leading to the production of electron-hole pairs. The produced hydroxyl radical serves as a strong oxidizing agent and fully deteriorates the dye into anti harmful bi-products (CO 2 , H 2 O, etc.) [53]. The Advanced oxidation process (AOP) due to its efficient performance and stability, has been employed for handling these organic and inorganic pollutants without generating secondary left-over difficulties. In AOP, hydroxyl radicals (·OH) were generated for the oxidation process. The nanoscale zero-valent iron (ZVI) or metallic NPs (Pd/Fe, Cu/Fe, Ni/Fe) remain potential reductants besides ecologically biocompatible for the reduction of harmful waste.
Recently, Fangbai Li [54] and his colleagues fabricate Fe/Cu/Pd catalytic agents to analyze their catalytic activity. They use 4-nitrophenol (PNP) as an ideal composite and transform it into an aminophenol (useful chemical) via reduction reactions in the presence of NaBH 4 [55]. The experimental result demonstrates that PNP reduction via NaBH 4 was 25-fold advanced than that without NaBH 4.   heightened. It displays considerably enhanced reactivity of Fe/Cu@Pd in NaBH 4 presence in comparison to metallic constituent part (nZVI and Fe/Cu). It was experimentally proved that PNP could be competently reduced attained from various water sources, i.e., lake, pure water, and groundwater, having different flow rates. This innovative method offers the lessening of carbon-based impurities in aerobic surroundings, outspreading the nZVI-based method to wide-ranging applications in water cleaning. It also confirms the advantage of Fe/Cu@Pd for proficiently decreasing the electron-deficient compounds in the occurrence of NaBH 4 [19].
The dechlorination of 2,4-DCP via polyvinylpyrrolidone (PVP) coated Cu/Fe bimetal NPs has also been experimentally performed. The experimental analysis shows that the fraction of PVP to Cu/Fe should be 0.35, Cu percentage in Cu/Fe NPs was 41%. PVP helps in dispersing and stabilizing uniformly distributed Cu/Fe NPs. PVP also enhances the reactivity for removing 2,4-DCP via Cu/Fe NPs. A low pH and advanced temperatures, boost the dechlorination rate. These PVP-Cu/Fe NPs are easily usable due to their magnetic characteristics, which permit quick magnetic separation of the catalysts [57]. Earlier, oxidative degradation of 2,4-DCP via ethylenediaminetetraacetic acid (EDTA)@bimetallic Cu-Fe coordination was also investigated, which shows 100% degradation in 1 (pH range of 3∼7) [29] as 2,4-DCP is abundantly present in the groundwater contaminating it. Humic acid (HA) is also used for the dechlorination of chlorinated hydrocarbons via Pd/Fe bimetallic NPs. Different influencing aspects i.e., NO 3 − absorption or Pd concentration also impact the lessening of 2,4-DCP [58]. The experimental consequences propose that a low concentration of HA has a marked effect on 2,4-DCP dechlorination [58]. Many reductive substitutes have been employed for the photocatalytic reduction and remediation of polluted water using advanced technologies and small power consumption. The extensively cast-off semiconductive catalyst titanium dioxide (TiO 2 ) was restricted in practical applications due to its narrow UVlight response [59]. Wang et al report the water splitting of metal-free photocatalyst a metal-free polymeric graphitic carbon nitride (g-C 3 N 4 ),] [60]. Afterward, wide-ranging investigations have been carried out on environmental purification [61]. Though, the photocatalytic activity is restrained because of the fast recombination of photo-generated electrons. However, the photocatalytic efficacy was improved via two procedures ion doping [61,62] or treating with noble metals [63]. These methods simultaneously extend light absorbance as well as accelerate the charge separation process [64,65]. Heterojunction approach [66,67] and Z-scheme [68,69] coordination also supports in attaining the high charge separation. As long as the global environment is concerned, the development of dangerous chemicals, in particular heavy metals and dyes, is a disturbing aspect, attracting the attention of many scholars to address this global crisis. The increasing pollution of dyes in the aquatic ecosystem is listed as one of the most serious environmental issues. Dyes stay persistent and do not suffer normal deterioration in the climate, which consequently affects and causes the environment pollutant due to manufacturing operations i.e. paints, fabrics or in bioindustries, p-nitrophenol (4-NP) a pollutant penetrates the aquatic systems having adverse consequences. Besides this, the primary dyeing material used in numerous industries, such as wood, fiber, or MB which also causes eye illness.
Calcium magnesium oxide nanoflakes were used to remove organic toxins from waste synthesized via coprecipitation technique supported by ultrasonication. Their physical and chemical analysis was implemented to establish the physicochemical, structural, and functional properties and the composition of nanoflakes. HR-TEM test proves the diameter of nanoflakes from 10 to 30 nanometers with an average size of 25 nanometers. The photocatalytic function of flake and p-nitrophenol (4-NP) was also determined by the rate of removal of these compounds under UV radiation. Synthesized nanoflakes will decompose 4-nitrophenol and methylene blue in a short time suggestings the usage of CaMgO 2 nanoflakes to handle wastewater [70].
In recent times, Bismuth tungstate (Bi 2 WO 6 ) has generated consideration owing to visible irradiation compelled characteristics and fast oxidizability [71,72]. The conduction and valence band edge in Bi 2 WO 6 is at about −0.12 and 2.59 eV, and the g-C 3 N 4 band gap is −0.78 eV and 1.89 eV [73,74]. The experiments proved that photo-generated electrons in the Bi 2 WO 6 conduction band might quench with g-C 3 N 4 holes present in the valance band, developing the Z-scheme coordination as shown in figure 6(a). Xiao et al synthesize a composite photocatalyst (g-C 3 N4@Ag-Bi 2 WO 6 ) displaying Z-scheme [75]. The Ag takes part as an electron mediator, showing enhanced photoactivity during Rhodamine B degradation. Similarly, Ma et al [30] synthesis a Z-scheme g-C 3 N4@RGO-Bi 2 WO 6 which shows an outstanding reduction of 2,4,6-trichlorophenol (TCP) [76]. Though, the use of noble metals (electron mediators) restricted this approach practically at a commercial scale due to high cost. As a consequence, researchers investigate and concentrate on developing mediator-free Z-scheme [77][78][79].
Sintering and heating technique was used to synthesize composite based binary photocatalysts comprising of a graphitic carbon nitride and bismuth tungstate (g-C 3 N 4 /Bi 2 WO 6 ). The binary photocatalysts displayed heightened photocatalytic performances due to active declination in charge recombination and comprehensive spectrum absorption range under UV-vis light irradiation [80]. The schematic representation of the photogenerated charge carrier's separation and transference in g-C 3 N 4 /Bi 2 WO 6 under UV-vis irradiation is shown in figure 6(a). The as-prepared g-C 3 N 4 /Bi 2 WO 6 (samples mass ratios at 3:7) degraded 2,4-DCP entirely within 2 h (h). The operational and retrieval activity of g-C 3 N4/Bi 2 WO 6 showing that the binary photocatalyst retains its high photocatalytic activity even after five successive sequences, specifying the catalyst stability. The 2,4-DCP reaction pathways, intermediates, and finishing bi-products were investigated, which shows that intermediates comprise o-CP and p-CP, producing phenol (P) as a final product. The potential dechlorination pathway based on detection results was proposed in figure 6 Without illumination (dark conditions), isopropanol and 2,4-DCP adsorbed on surface of a binary catalytic agent. During illumination (UV-vis light), the dechlorination of 2,4-DCP into o-CP and p-CP takes place due to the binary composite catalyst which excites and produces electrons in the interim electrons conglomerate with protons and 2,4-DCP. Additionally, the produced o-CP and p-CP react with accumulated electrons and protons, developing the final last product, phenol ( figure 6(b)). Thus, these binary photocatalysts help in reducing electron-hole pairs recombination rate and enhancing photon efficacy. In the heterostructure system, the band arrangement shows a fundamental role in defining the movement of photo-generated carriers [73]. The conduction band (CB) and valence band (VB) potentials of g-C 3 N 4 are negative than that of Bi 2 WO 6 . In these circumstances, the charge carriers transference paths may follow two patterns: Firstly, in the heterojunction type, the generated electrons due to light exposure in g-C 3 N 4 CB transfer to Bi 2 WO 6 CB, whereas holes in VB of Bi 2 WO 6 transport to VB of g-C 3 N 4 , Secondly, via Z scheme in which the generated electrons due to light exposure in the Bi 2 WO 6 CB transferal to g-C 3 N 4 VB, leaving photo-generated electrons in CB of g-C 3 N 4 and holes in the VB of Bi 2 WO 6 . CeO 2 /g-C 3 N 4 complexes can be synthesized via a wet-chemical solution method, which displays improved visible-light activity for 2,4-DCP. It is due to an improved charge carrier's separation and transfer among the two constituent coordination. A graphic mechanism for 2,4-DCP degradation via CeO 2 /g-C 3 N 4 amalgams is suggested in figure 7. The C 3 N 4 valence band is positioned at 1.4eV and the CB is at −1.3 eV versus normal hydrogen electrode (NHE) [81]. According to UV-vis diffuse reflectance spectra, bandgap of CeO is nearly at 2.82 eV. The VB of CeO is at 2.03 eV (measured via XPS spectra), and the conduction band is at −0.79 eV. The work functions (Ф) of CeO 2 /g-C 3 N 4 were 4.69eV and 4.34 eV [82][83][84]. The heterojunction is formed between binary composites CeO 2 /g-C 3 N 4 generating charge carriers (electron-hole pairs) under visible-light. The band alignments of CeO 2 /g-C 3 N 4 are well-coordinated, which facilitates and enhances the electron-hole pair's separation.
Additionally, the CeO 2 /g-C 3 N 4 interface in composite offers a stage for charge carrier's passage between CeO and CN. The visible-light irradiation excited CB electrons of g-C 3 N 4, which transfer to CeO 2 CB, because of the higher work function of g-C 3 N 4 w.r.t CeO 2 . At the same time, the photo-induced holes of CeO 2 in VB transfer to the g-C 3 N 4 VB. The competent electron-hole pairs transmission among the two constituents may considerably diminish charge carriers recombination possibility. The photocatalytic cycle comprises 3h, after which the photocatalyst was used for a second time to degrade the fresh 2,4-DCP solution. Experimental results prove that CeO/CN composite shows consistency and remains unchanged even after four times repetition demonstrating stabliltiy [85].
Besides the physical and chemical techniques, the biological method of treating chlorophenols also gathered attention. Different microorganisms consume the chlorophenols as carbon or energy source [95,96] few of them includes Pseudomonas pickettii, Alcalilgenes eutrophus, Desulfomonile tiedjei, or Phanerochaete chrysosporium [96,97]. The activated sludge systems or bioaugmentation are used to biotreat the pollutants to improve the elimination of contaminants comprising of phenols, chloroaniline, chlorobenzoate, resin acid, or chlorinated phenols [95]. In bioaugmentation, the appropriate selection of bioaugmentation microorganisms and applicable bioaugmentation approach is significant to degrade the pollutant. Yi Qian combines the conventional activated sludge system with bioaugmentation for the deduction of 2,4-DCP efficiently, besides chlorophenols i.e., 4-monochlorophenol or 2,4,5-trichlorophenol [95]. The aerobic granular treatment of 2,4dichlorophenol and COD shows 96% and 95% degradation within 270 min; the sequencing batch reactor (SBR) nearly takes 80 d for stabilizing the granules [98]. Similarly, S.G Wang shows the formation of the pellets within 39 d showing 94% 2,4 DCP degradation which is a potential candidate for treating industrial waste water [99]. Similarly, 2,4-DCP abolished up to 52% and 78% in up-flow anaerobic sludge blanket (UASB). The aerobic suspended growth ASG reactor shows 2,4-DCP (86%) and COD (95%) eliminations within 26 h [100]. The recent development in degrading the organic pollutant 2,4 DCP, MB,MV and MO using different nanomaterial system is mentioned in table 1. Different factors influence the % removal efficiency of these toxin, such as catalyst concentration, pH, illumination source, temperature, amount of contaminant presence, time consumption, technique, or temperature. However, still, it is a need to design such kind of nanomaterial system which is economical, durable, showing persistent consistency, and degrade the organic pollutant in less time consumption.

In-depth analysis
Humans influence nature slowly, but its after-effects are permanent such as greenhouse warming. The air and noise pollution via vehicles or industries increases day by day slowly and continuously as everyone could feel the noise, smell the odorous goings-on, and observe the black smoke influencing the surrounding local community representing a worsening atmosphere. All of us need to live in an unpolluted environment having pure air, clean water, and fresh surrounding air including standard living material. Unluckily, all these practical demands may be challenging to accomplish for every individual at the same time due to industrialized progression. Human modification of earth is essential emerging and increasing. The majority of the land surface, water, and air have been transformed and polluted by human action and industrial development. The CO 2 concentration has increased by nearly 30% since the start of the industrial revolution in the atmosphere. The available water is used by civilization due to which one-quarter of the bird and animal species have been driven to extinction. Investigators have executed comprehensively, promising, and well-defined efforts in the arena of photocatalysis and decontaminating pollutants. The semiconductive NPs as photocatalysts are narrow because they respond  [22] only to UV-excitation. Until now, a lot of scope and effort is required for the synthesis of UV-Visible lightinduced nanomaterials that show enhanced catalytic activities; in less time, consumption, as well as its fabrication, should take place at a large scale to come across the requirements. Nanostructures having surface area ranging (<100 nm) display different distinctive properties that have never been observed in bulk. The significance of nanomaterials in every domain has been directly linked to high surface sites for physicochemical properties, that are unavailable in bulk. In the treatment of wastewater, there have been four main categories of functional nanoparticles, namely dendrimers, nanocomposites comprising metals, zeolites, or carbonate nanoparticles. Nanostructures comprising metals are effective as biocides and killing several microbes adhering both for gram-positive and negative groups. Besides, they help with the elimination of toxic metals including arsenic or halogen besides degradation of dissolved dyes as well as other toxic elements. Zeolites operate to eliminate contaminants from water as an ion-exchange medium. In an aqueous medium, carbonaceous materials could perhaps operate as sorbents for organic solutes. Through the use of bioactive nanoparticle-integrated membrane surface to eliminate hazardous chlorinated compounds is advancement in this respect to perform the catalytic activity in a short time, having various bioactive immobilized enzymes. Besides this, core/shell nanomaterials were indeed highly efficient materials with enhanced characteristics and attained progressive attention. By modifying core/shell physicochemical characteristics or amount, the characteristics of core/shell nanoparticles are quite diversified and adapted to various applications. The core/shell are dual-phase nanomaterials composed of various elements with the main core structure and an external shell. These particulates can reveal interesting characteristics increasing from the combination of core and shell, its configuration, and material design. These nanocomposites, have been used in multiple fields, based upon certain remarkable physical-chemical characteristics including therapeutics, metallurgy, photcatalyst or optoelectronics devices [132]. Figure 8 demonstrates the advantages of nanomaterials importance in today's life. Nanomaterials rely mostly on purpose to adjust the material properties at incredibly tiny dimensions to accomplish particular attributes, thereby enhancing the materials characteristics.

Conclusion
Nanotechnology offers a cost-effective way to both purify water and minimize overall costs by simultaneously eliminating harmful and unwanted impurities. Various investigations have been performed displaying multipurpose NPs that simultaneously eliminate numerous impurities in water. Nanotechnology was critically studied in this review, and the feasibility and effectiveness of eliminating contaminations were discussed. The aim of the research was to completely eliminate infectious microorganisms, inorganic compounds, and environmental contaminants. The future advanced effort is a prerequisite to initiate metal oxide response in the visible light. However, few researchers fabricate the binary metal oxide composite photocatalysts, but the output is quite low. The recognizing and degradation concerns of the hazardous and toxic chemicals should be determined and resolved. The incorporation of the nanomaterials for the removal of decontamination and toxins bring unexpected benefits due to intrinsic nanoscale dimensions, to induce info at atomic resolutions, enhance low signal levels, accelerate response times, and continuously regulate changes. Developing nanostructured instruments for the eco-friendly environment needs the engineering progressions in identifying and distinguishing mechanisms and configurations, for the future emerging requirements.
Henceforth, information on the diverse artificial techniques used for the synthesis of photocatalytic constituents should be considered necessary. Besides this, the exploration of new photocatalysts having preferred characteristics to persuade the oxidation reaction of organic substrates or the reduction of the pollutants under visible light irradiation should be invigorated. The population rise, associated with the quick resource consumption and constant industrial growth and agrarian advancement has led to surplus wastewater with compositional modifications texture, toxicity, and noxiousness due to different kinds of contaminants existing in wastewater. Currently, the challenges faced via wastewater management are primarily linked with the wastewater structural intricacy as it makes difficulties in treatment progresses via requiring different equipment and advanced technologies, thus causing prolong handling periods and higher working expenses.