**Preface to "Innovative Materials and Methods for the Removal of Pollutants from the Environment"**

It is with great pleasure we welcome you to this *Molecules* Special Issue devoted to "Innovative Materials and Methods for the Removal of Pollutants from the Environment".

In the last decade, environmental pollution has reached worrying proportions, and intensive efforts have been devoted to seeking solutions that can be effectively employed for the removal of pollutants from different environmental compartments. This Special Issue aims to gather the most recent and significant contributions related to innovative materials and processes for environmental purposes, with special emphasis on the depollution of air, water, and soil. Biological approaches for pollutants removal are also considered as a valid alternative to contrast environmental threats.

The 18 articles (5 reviews and 13 research papers) composing this Special Issue can be thus considered as a representative selection of the current research and novel applications of inorganic, organic, or hybrid materials with porous or layered structures for the removal of pollutants through adsorption processes or (catalytic) transformation of noxious compounds into species with reduced environmental impact. Bioremediation approaches have been also proposed to treat contaminated media, for instance by altering environmental conditions to stimulate growth of microorganisms aiming to the degradation of toxic species.

Three of the five reviews articles report an overview of the available research strategies for the removal of pollutants from aqueous environments and industrial wastewaters. These contributions, together with research articles dedicated to water safety and depollution, allow to acquire knowledge about the research efforts currently carried out to preserve and improve quality of one of the most important natural resources in the world.

Most of the articles composing the Special Issue are related to the use of adsorption processes for the removal of toxic species from polluted water. In this respect, the use of nanomaterials for the removal of different classes of pollutants normally found in industrial wastewaters (i.e. dyes, heavy metals and pesticides) has been reviewed, also taking into account their possible regeneration and reuse strategies, thus designing future possible applications (S.K. Lakkaboyana et al., DOI: 10.3390/molecules26092799).

Among the different materials that can be employed for wastewater treatments, layered zirconium phosphate (ZrP) has proven to be really versatile materials and their possible use as sorbents for heavy metal cations or dye molecules in wastewater treatments has been reviewed. Special emphasis has been given to the the possibility of combining ZrP with polymers or other inorganic materials (i.e. photocatalytic species), aiming at having suitable systems for real and large scale applications (M. Pica, DOI: 10.3390/molecules26082392).

Chitosan and derived composites are attracting considerable interest as environmentally adsorbents and have the potential to remove water contaminants, such as heavy metal ions, cationic and anionic dyes, phenol and other organic molecules, such as antibiotics. The development of chitosan-based adsorbents is reviewed starting from the extraction of chitin from seafood wastes, followed by its conversion to chitosan, and then describing the emerging chitosan/carbon-based composite materials and their application for water pollutants removal (C. B. Breslin et al., DOI: 10.3390/molecules26030594).

Several research papers are related to materials for the removal of toxic molecules from aqueous media. Removing methylene blue (MB) dye from aqueous solutions was examined by the use of nickel molybdate (-NiMoO4) as adsorbent. The effect of different parameters, such as the pH, the adsorbent dose, the temperature, the contact time, and the initial dye concentration, have been studied, aiming at improving the removal conditions, thus indicating that -NiMoO4 nanosorbent is an outstanding candidate that can be exploited for removing MB dye in wastewater (S. Rakass et al., DOI:10.3390/molecules26051378).

The application of mesoporous silica monoliths for the removal of organic pollutants from water and gas phase have also been proposed. Besides the adsorption properties of the monoliths for gaseous toluene, as a model of volatile organic compounds (VOCs), the same monoliths have been considered for their potential application for the removal of Rhodamine B in water, showing good performances and stability of the siliceous sorbents in aqueous media (C. Bisio et al., DOI:10.3390/molecules26051316).

An eco-friendly approach for acetampirid pesticide removal based on use of micro- and mesoporous activated carbon (TPAC), synthesized via chemical treatment of tangerine peels, is also proposed. The effects of various parameters on the adsorption of acetampirid including adsorbent dose (0.02–0.2 g), pH 2–8, initial adsorbate concentration (10–100 mg/L), contact time (10–300 min), and temperature (25°C–50°C) were studied. The results of the presented study show that TPAC may be effectively used for the removal of acetamiprid pesticides from aqueous solutions (A. E.-G. E. Amr et al., DOI:10.3390/molecules25102339).

Another approach is related to the use of pineapple leaves (PAL) for the adsorption of rose bengal (RB) from contaminated water samples. Raw pineapple leaves (RPAL) and the thermally activated bio-waste leaves have been studied and the adsorption processes have been followed by means of a multitechnique approach (M. El-Azazy et al., DOI:10.3390/molecules25163752).

Another research paper reports the use of Carpathian clinoptilolite powder to remove ammonium from sludge water arising from the dewatering of anaerobic digested sludge with high loads of ammonium in wastewater treatment plants (WWTP). This paper aims also to recover ammonium as a valuable resource to substitute ammonia from the energy intensive Haber–Bosch process for fertilizer production (S. Wasielewski et al., DOI:10.3390/molecules26010114).

Wastewaters are characterized by a high content of phosphate and toxic metals. Many studies have confirmed the sorption affinity of alginate adsorbents for these ions. The adsorption of phosphate from effluent of sewage sludge on biodegradable alginate matrices cross-linked with Fe3+ ions (Fe Alg) was also investigated (H. Siwek et al., DOI:10.3390/molecules25173962).

A versatile synthetic approach has been developed to prepare polyvinyl alcohol (PVA)/nitrogen-doped carbon dots (CDs) composite film (PVA-CDs) for removal of toxic cadmium ions from waters. The PVA-CDs film possess synergistic functionalities through increasing the content of hydrogen bonds for chemisorption compared to the pure CDs. The film was successfully applied to real water samples with an outstanding removal efficiency for tap and drinking water, respectively (M.H. Issa, DOI:10.3390/molecules25153541).

Copper ferrite nano-particles (CuFe2O4) have been also modified with polyaniline to form CuFe2O4/PANI nano-composite and used as new adsorbents for the removal of the hazardous mercuric ions from aqueous solutions. Adsorption conditions have been modified and the method at the optimum conditions exhibited high performance with high mercury removal performances (S.S.M. Hassan et al., DOI:/10.3390/molecules25122721).

Continuous fixed-bed column adsorption system was also proposed for defluoridation of water using volcanic rocks, virgin pumice (VPum) and virgin scoria (VSco), as adsorbents. The effects of particle size of adsorbents, solution pH, and flow rate on the adsorption performance of the column were assessed at room temperature, constant initial concentration, and bed depth. The results revealed that the developed column is effective in handling water containing excess fluoride and the possiblity to regenerate confirmed that defluoridation of groundwater employing volcanic rocks is a safe and sustainable method (B. Lennartz et al., DOI:10.3390/molecules26040977).

Some of the articles that appear in this collection are related to the removal of toxic contaminants from soils. It is well-known that soil contamination by heavy metals threatens the quality of agricultural products and human health, so it is necessary to choose certain economic and effective remediation techniques to control the continuous deterioration of land quality.

An overview on the application of biochar (produced by the thermochemical transformation of plant- and animal-based biomass) for the remediation of heavy-metal-contaminated soil is proposed. Different conditions, such as biomass type, pyrolysis temperature, heating rate and residence time are the pivotal factors governing the performance characteristics of biochar. The potential risks of in-situ remediation strategy of biochar are expounded upon, which provides the directions for future research to ensure the safe production and sustainable utilization of biochar (T. Chen, DOI:10.3390/molecules25143167).

A research paper also deals with the use of biochars produced from Litchi branch for the removal of Cu from contaminated soils. The study demonstrates that BCs, especially those produced at a higher temperature, are promising for Cu immobilization and to release available silicon in soils. However, the remediation efficiencies were restricted by the soil type contamination status and remediation time (T. Chen et al., DOI:10.3390/molecules25184319).

The remediation of toxic components, such as radioactive waste, toxic heavy metals, and other harmful substances, under extreme conditions is quite difficult due to their restricted accessibility. Thus, novel treatment methods for the removal of toxic pollutants using extremophilic microorganisms that can thrive under extreme conditions have been investigated during the past several decades. Recent trends in bioremediation using extremophilic microorganisms and related approaches to develop them are reviewed, with relevant examples and perspective (Y.J. Choi et al., DOI:10.3390/molecules25214916).

Another contribution regards lignin fractionation by enzymatic hydrolysis is proposed as an effective approach to reduce the heterogeneity of lignin and improve the adsorption and recycle performances of lignin as a cationic dye adsorbent. The proposed fractionation method is proved to be a novel and efficient non-chemical modification approach that significantly improves adsorption capacity and recyclability of lignin (W. Sui et al., DOI:10.3390/molecules25112603).

A study to explore the potential phytoremediation of industrial NTO (3-nitro-1,2,4-triazol-5-one)-wastewater using vetiver grass (Chrysopogon zizanioides L.) is also proposed. Evidences that vetiver can effectively remove NTO from wastewater, and also translocated NTO from root to shoot are also presented (D. Sarkar et al., DOI:10.3390/molecules26010074).

Finally, some contribution is also related to air pollution causing serious challenges to modern society. A study proposes a non-thermal plasma (NTP) system for placement in the exhaust system of internal combustion engines to reduce the toxic contaminants (HC, CO, and NOx) of exhaust gases. The proposed NTP system, installed in the exhaust system, can significantly reduce air pollutants (M-H. Hsueh et al., DOI:10.3390/molecules25246041).

Of course, we wish to thank all the authors for their valuable contribution, as well as the reviewers for their work and relevant input. Finally, we would like to thank the editorial staff at *Molecules* for making this themed Special Issue possible.

We created, for this book, an original cover image, dedicated to the efforts of chemistry to defend the beauty of environment, represented by flowers, against every prejudice that considers chemistry an enemy of life. We hope this Special Issue makes enjoyable reading and that it can be a source of inspiration and future fruitful collaboration.

> **Chiara Bisio, Monica Pica** *Editors*

## **Current Trends in the Application of Nanomaterials for the Removal of Pollutants from Industrial Wastewater Treatment—A Review**

**Geetha Palani <sup>1</sup> , A. Arputhalatha <sup>2</sup> , Karthik Kannan <sup>3</sup> , Sivarama Krishna Lakkaboyana 4,\*, Marlia M. Hanafiah 5,6 , Vinay Kumar <sup>7</sup> and Ravi Kumar Marella <sup>8</sup>**


**Abstract:** In the recent decades, development of new and innovative technology resulted in a very high amount of effluents. Industrial wastewaters originating from various industries contribute as a major source of water pollution. The pollutants in the wastewater include organic and inorganic pollutants, heavy metals, and non-disintegrating materials. This pollutant poses a severe threat to the environment. Therefore, novel and innovative methods and technologies need to adapt for their removal. Recent years saw nanomaterials as a potential candidate for pollutants removal. Nowadays, a range of cost-effective nanomaterials are available with unique properties. In this context, nano-absorbents are excellent materials. Heavy metal contamination is widespread in underground and surface waters. Recently, various studies focused on the removal of heavy metals. The presented review article here focused on removal of contaminants originated from industrial wastewater utilizing nanomaterials.

**Keywords:** nanomaterials; rare earth metals; wastewater treatment; pollutants

#### **1. Introduction**

One of the common natural resources in the world is water, which is indispensable for the endurance of every human and human development. With the rapid increase of urbanization and industrialization, water usage is expanding quickly, and the problem of water shortage has become imperative to solve for developing economies. A huge quantity of polluted wastewater is released from various industries, which includes the manufacturing of batteries, mining, toxins, and electroplating. Wastewater toxins cause numerous antagonistic impacts on living creatures and to the surrounding environment [1,2]. It ended up being a more productive and more affordable tool for treating industrial wastewater. Contaminant types present in the industrial wastewater rely upon the production process [3]. Toxins of industrial wastewater usually include large constituents of organic compounds, increased pH level, harmful substantial metals, huge saltiness, and increased

**Citation:** Palani, G.; Arputhalatha, A.; Kannan, K.; Lakkaboyana, S.K.; Hanafiah, M.M.; Kumar, V.; Marella, R.K. Current Trends in the Application of Nanomaterials for the Removal of Pollutants from Industrial Wastewater Treatment—A Review. *Molecules* **2021**, *26*, 2799. https:// doi.org/10.3390/molecules26092799

Academic Editors: Chiara Bisio and Monica Pica

Received: 17 March 2021 Accepted: 27 April 2021 Published: 10 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

turbidity from the presence of impurities of inorganic compounds. Adsorption, flotation, chemical precipitation, membrane filtration, flocculation, and coagulation are included in the industrial wastewater treatment [4,5].These wastewater treatments are sometimes insufficient in eliminating explicit impurities, for example, harmful heavy metals, oil, and microorganisms.

Wastewater is generated from various sources such as residential areas, commercial areas, industrial properties, and agriculture lands. Composition of wastewater differs extensively, and it is majorly dependent on the source from which it is generated. Common constituents of wastewater are inorganic substances such as solutes, heavy metals, metal ions, ammonia, and gases, complex organic compounds such as excreta, plants material, food, protein, natural organic matter, and nitrate, and other pollutants present in surface water, ground water, and/or industrial water. Typically, industrial waste can be divided into two categories, hazardous and nonhazardous. Nonhazardous industrial wastes do not cause environmental and health hazards and are produced from cardboard, plastic, iron, glass, stone, and organic waste. In contrast, hazardous wastes are industrial waste that can be harmful to public health or the environment, such as flammable, biodegradable, and hazardous materials [6]. Industrial waste is classified as wastewater, solid waste, or air leaks. There is some overlap in the physical properties of the substances present in these three categories, as wastewater can contain suspended solids and suspended liquids, and precipitation of solid waste can include gas, liquid, and some liquids. Particles and air exposures may consist of a fluid that emits air fluid and a substance known as particle emission [7]. Industrial waste, which has a significant concentration of non-recyclable or recyclable metals, is usually a good candidate for landfill, which is the dumping of waste into the ground area. Figures 1 and 2 shows wastewater and industrial wastewater general classifications.

**Figure 1.** Anthropogenic wastes classification, (Reprinted with permission from [8]).

**Figure 2.** Industrial hazardous wastes general classification (reprinted with permission from [8]). F-list (wastes from common manufacturing and industrial processes), K-list (wastes from specific industries), P-list, and U-list (wastes from commercial chemical products).

When left untreated, these constituents may pose a threat to living beings and the environment, which makes it crucial to treat wastewater before disposal. Various physical, chemical, and biological treatment processes are used for wastewater treatment. Until now, a variety of research on nanomaterials was done to research heavy metal water treatments to find their applications, and they show incredible potential as an irreplaceable option to adsorb heavy metals from wastewater [8]. For the removal of heavy metals from polluted wastewater, these properties are very useful. According to the type of nanomaterial, wastewater treatment is classified into three fundamental groups [9]: nano-adsorbents, nanomembranes, and nano-catalysts. Some of the common sources of wastewater types are below [10]:


Industrial wastewater treatment needs different successive complicated studies to fulfill the standard of water reusability. In looking at nanomaterials in waste streams and possibilities for recovery, one of the first items that need to be understood is the amount of industrial nanomaterial being produced and how much is then discharged, as shown in Table 1 [11]. The amount discharged to waste was based on presumptive material flow and then back calculated by knowing the reported amount produced rather than the actual measurement of the true nanomaterials concentration in a waste stream. For example, iron, zinc, and copper oxides would likely not last very long in the nanoparticle form, as they are more soluble. On the other hand, TiO<sup>2</sup> is one of the least soluble among nanomaterials and would likely stay in the nanoparticle form for much longer. SiO<sup>2</sup> would also be very similar to TiO<sup>2</sup> with a low solubility.


**Table 1.** Nanomaterials and discharges (reprinted with permission from [11]).

One of the most significant advancements in the 21st century was nanotechnology. Nanomaterials criteria include well-organized structure, filtration competence, small in size, and high surface to volume ratio. Some special properties of nanomaterials under the nanoscale are effects on the surface region, large quantum tunnel effects, small size effects, and quantum effects. These properties add to their adsorption capacity and reactivity, which are unprecedented and are great for heavy metal ions removal [12,13]. Industrial pollution continues to be a major factor in worsening the environment around us, the water we use, the air we breathe, and the land we live in. The growing power of industrialization not only consumes large agricultural land but at the same time causes environmental degradation as well as land degradation.

For raw materials management and manufacturing related to human activities, materials are driven from the liquids of industrial wastewaters (IWW) [14,15]. IWW acts as one of the pollutants of environmental water. From a recent survey, a huge amount of industrial wastewater is mixed into lakes, beaches, and streams. In the end, this produces contamination complications and entanglements in the surrounding water and leads to the eco-system returning as a negative output that negatively impacts human life. Industrial wastewater results in human population spill, and the climate ends up being awful in many scenarios. Huge quantities of these kinds of wastewaters are naturally incredibly solid, highly inorganic, effectively biodegradable, or inhibitory potential. In regard to these qualities, total dissolved solids (TSS), biological oxygen demand (BOD), and chemical oxygen demand (COD) might be high [16]. Industrial wastewater comprises wastewater from each sector of industry that produces its own exact blend of impurities.

As with the industrial wastewater's shifting character, industrial wastewater processing must be arranged explicitly for the specific sort of produced liquid. The metal industries discharge heavy metals and some of their compounds; also, the electroplating industry is a critical cause of contamination [17]. Higher amounts of Ag compounds are produced by photograph handling workshops, and, at the same time, printing plants release inks and dyes. The chlorine substances generally rely upon mash and paper industries; generally, they contain chloride compounds and dioxins. A very large quantity of phenols and oils are released by the petrochemical industry. Food handling effluents of the plant are loaded with organic and solid issues. Commonly, industrial wastewater is categorized into two classes: (1) organic industrial wastewater and (2) inorganic industrial wastewater. The main compound, inorganic industrial wastewater, exists in steel and coal industries, non-metallic minerals industry, metals manufacturing surface processing, and in commercial adventures [18]. Generally, wastewater, solid substances, and oils are delivered, and they contain incredibly poisonous solutes. This type may impact gas washing wastewater or cyanide blast-furnace industrial wastewater of metal processing with alkaline or acids solutions in which wastewater exudes from the gas refinement of Al work, which contains fluoride.

According to local regulations, non-metallic minerals exist in tiny and normal sizes alongside metal handling plants and are situated so they may discharge their wastewater into municipal wastewater systems; their effluents should be treated before liberation. The contaminants that come from the chemical compounds/industries which act on a

large-scale by organic industrial wastewater mostly exploit substances for chemical responses [19]. The fluids incorporate substances of organics with variable properties and beginnings. The below-listed plants and industries primarily produce organic industrial wastewaters (OIW) [20]:factories manufacturing pharmaceuticals, beauty products, synthetic detergents, herbicides, and pesticides, leather and tanneries factories, textile factories, paper and cellulose manufacturing plants, factories related to oil-refining, and metal processing industry.

Now, the steel industry is viewed as a fundamental and crucial industry. Steel industries produce wastewater in huge quantities that contain many disintegrated chemicals in the sludge and undisclosed substances [20]. Producing iron out of its metals involves intensely decreased responses in impact heaters. Necessary cooling waters such as cyanide and ammonium hydroxides are soiled. Acidified rinse waters are present in wastewaters and waste corrosive blends. Regardless of the large number of plants working in acid plants recovery, where the mineral acid is reduced from the salts of iron, a huge amount of acid ferrous sulfate/chloride remains undisposed [21]. The paper production and wood-pulping products distribute polluters when natural fluids are delivered into emanating waters [22]. These liquids subsequently have extraordinary mutagenic effects that cause physiological weakness and damage aquatic organisms. The textile industry altogether additionally adds to water contamination in changing produced and natural threads into fabrics and different products. While fabricating many of the materials, wet chemical techniques are compulsory to legitimately sanitize, color, plan, or finalize the item [23]. This represents wastewater development, which usually contains loads to eliminate pollution from the crude materials themselves, yet excess reagent compounds are utilized in preparing such processes. Essential polluters in textile wastewaters are exceptionally chemical oxygen, heat, suspended solids, corrosiveness, and some dissolvable substances [24].

#### *Industrial Wastewater Treatment (IWT) Processes*

IWT is commonly arranged as physical, chemical, and organic processes. The usually embraced advances might be divided into [25]: (i) pre-treatment, (ii) primary; (iii) secondary and tertiary; (iv) refinement; and (v) purification. Usually, a basic level of treatment is based on size divisions utilizing physical methods, for example, filtration/sedimentation, for fundamental cleaning. More than 99% of removal can be achieved by tertiary treatment, which involves the final polishing of the effluent by toxic removal of pollutants to certain levels [26–28]. Wastewater produced by the primary process is not reasonable for release or reuse. The primary goal is to deliver the water suitable for optional and tertiary separations. A noticeable illustration of this is pH clarification/modification before membrane adsorption or separation or ion exchange. Separation processes, which are ordinarily viewed as basic treatments, include separation based on size, including the actual driving force that affects separation. Screening, cyclone separations, sedimentation, precipitations, thickening, centrifugation, and filtration are included in primary methods. Among the main homogenization/leveling, pre-treatments are meant to balance, and wastewater homogenization from inlets, particularly where industrial production creation occurs, is irregular and variable for consumption toxins [28].

Further developed processes of separation are utilized with immense changes in equipment and process nature in secondary and tertiary treatment stages. The separation process generally incorporates evaporation, absorption, distillation, extraction, ion exchange, biological processes, adsorption, crystallization, cavitations, and separation of membranes [29]. The process of separations involved here can be arranged depending on driving forces, such as thermal driving force, and pressure-driven processes, such as membrane separationmicrofiltration (MF), ultra filtration (UF), nano-filtration (NF), and electrical forces, e.g., electro-dialysis or RO. Physicochemical methods are the main processes of separation that assume an imperative function in the field of wastewater treatment. This main class incorporates a huge assortment of processes, such as flocculation/coagulation, cavitations, oxidations, separation reactions, and extractions [30]. The process of separation includes

ion exchange, and adsorption also comes under physicochemical methods of treatment for utilizing either electrostatic attraction or surface forces. Ion exchange, coagulation, and adsorption are methods of membranes separation that belong to charge-based separations, where the removal process is generally contaminated under charge neutralization and is applicable explicitly for the removal of charged bodies/ions from the solution. Based on the idea of the profluent, at least one process of separation is involved in meeting the end goals of water reuse/discharge [31]. Figure 3 shows the steps in wastewater treatment processes as a flow chart.

**Figure 3.** Flow chart showing steps in wastewater treatment processes.

#### **2. Removal of Dyes**

From industries, waters dyes and pesticides are being released frequently, and their findings, particularly at low concentrations, need the development of complex advances, for example, separation or filtration of compound combinations combined with detection utilizing multi-technique methods [32,33]. These estimations are subsequently tedious due to the many middle-level handling steps associated with preparing the sample. Utilization of silicon-graphene (sg) nanoporous composites takes into consideration an extreme cut of all these methods, as the compounds can pre-concentrate the analyte into the porous structure and widen the analyte signal if a proper method is utilized [34]. Table 2 shows the application and the harmfulness of various dyes [35,36].


**Table 2.** Application and harmfulness of various dyes, reprinted with permission from [35,36].

By using Raman spectroscopy [28], for example, steps in sampling the water can be removed. The specific hierarchal porous silicon graphene composites design and the thick coating formed shape permit a "lab-on-a-chip" device to be combined with the Raman scattering method. Samples of water can be straightforwardly stored over the silicon graphene coatings, and toxins can be identified with the utilization of Raman spectroscopy. Raman signal enhancement identified with the analysis can be achieved through pure graphene-enhanced Raman scattering effect (GERS) or a combination of the surface-enhanced Raman scattering (SERS), e.g., graphene-mediated, surface-enhanced Raman scattering (G-SERS) given by plasmonic nanoparticles [37,38]. The affectability of such devices can be obtained by molecular imprinting methods. This substance approach permits framing exceptionally tiny cavities in the porous silica structures with sub-atomic recognition capabilities. Using molecular templating techniques, silicon graphene is able to form thick films that can be utilized for detecting dyes that mostly show the largest analytical enhancement factor of 14.64 for Rhodamine 6G dyes for a concentration of 10−<sup>3</sup> M [39,40]. Additionally, the same team prepared porous SG templated films for the detection of paraoxon, an organophosphate pesticide, with the concentration of 10−<sup>5</sup> M. Aside from identification, composites of silicon graphene additionally react as sorbents for the equivalent molecules of the analyte. Adsorption of organ phosphorus pesticides (OPPs) on silicon graphene composites was researched [41].

The adsorption was helped because of the substance co-operations between the composite and the functional groups; moreover, the strong π bonding between composites and the phenyl ring preferred the adsorption. The performance of adsorption of silicon graphene compounds was explored for various pesticides, and the capacity of removal varied. As referred to recently, for this situation, the expansion of magnetic nanoparticles to silicon graphene composites assumes a significant part in facilitating the recovery of the composites. Additionally, with an extra modification in the hydrophobic surface, the adsorption sites of pesticides expand [42]. Dye adsorption onto silicon graphene composites is likewise preferred by a mixture of physicochemical adsorption routes that rely upon the sorbent physicochemical properties.

Silicon graphene nanocomposites of the multifunctional compound were prepared by Kubo et al., inducing a super paramagnetic nanoparticles material to the mesoporous silica embedding graphene oxide. The nanoparticle functionalization permitted a simple recovery of the composites by an outer magnet. Fe3O<sup>4</sup> addition, nonetheless, caused sharp diminishing of the area of the surface by 72% and ~15% of pore sizes, which decreased from 305 mg/g to 125 mg/g in regard to methylene blue (MB) removal capacity [43,44]. Considering this, amino group surface functionalization of the multifunctional silicon graphene nanocomposites might help increase the interactions for the pollutant removal.

The sorbent physical properties, e.g., pore size, surface area, and shape, give more sites for the dyes to diffuse and secure through the pores. The performance of adsorption is reliant additionally on the physicochemical properties of the dyes and the environmental media [28,45], such as different structure charges and dyes, which show unmistakable sorption conduct in various pH conditions.

#### **3. Heavy Metals Removal**

Heavy metals are those metals which have very high density and are highly poisonous, even at low concentrations. These heavy metals include mercury (Hg), cadmium (Cd), arsenic(As), chromium (Cr), thallium (Tl), zinc (Zn), nickel (Ni), copper (Cu), and lead (Pb). Municipal wastewater, industrial wastewaters, landfill leaches, mining wastes, and urban runoff are the main sources of contamination [46,47]. Industries are the major source of heavy metals in wastewater. The electroplating industry's wastewater contains a great number of metals such as Cd, Zn, Pb, Cr, Ni, C, Ag, Pd, and titanium. The wood processing industry is also a major source of metal in wastewater. Paint and enameling industries also release their effluents containing nickel into water bodies [48]. PCB manufacturing industries are also a very significant source of producing metal waste [49]. Nanomaterials of metal compounds displayed preference for heavy metals removal over activated carbon, e.g., titanium dioxide nanoparticles in arsenic adsorption and nanosized magnetite. The photo catalyst usage of, for example, nanoparticles titanium dioxide, was explored in detail to decrease the toxic metal ions in water. In a survey, titanium dioxide nanocrystalline showed adequacy in eliminating various types of As, and it was demonstrated to be the most viable photo catalyst aside from industrially accessible nanoparticles of titanium dioxide, which showed almost extreme efficiency of arsenic removal at a relatively neutral pH value [50]. A titanium dioxide nanocomposite and nanoparticles of titanium dioxide added on a graphene sheet was additionally utilized to decrease chromium VI to chromium III in daylight. Comparatively, chromium treatment was completed by utilizing nanoparticles of palladium in another survey; conventional technologies for heavy metal removal are shown in Figure 4. The removing capacity of arsenic (heavy metal) was likewise tested by utilizing Fe2O<sup>3</sup> and Fe3O<sup>4</sup> as adsorbents by most of the analysts. Removal of As was also additionally explored by utilizing a high, particular surface area of iron oxide nanocrystals [51].

Recently, this process was recognized widely for removal of heavy metals from wastewater. Many cheap adsorbents were developed recently. These adsorbents are widely used for treatment of wastewater containing heavy metals [52]. These adsorbents are derived from the waste products generated from industrial activities, waste generated from agriculture, and natural materials [53]. Adsorption can be defined as a mass transfer process which transfers the substance from the liquid phase to the surface of a solid and becomes bound by physical and chemical interactions. Figure 5, illustrates some conventional methods for removal of metal. It is a three-step treatment process: (1) the pollutant is transferred to the sorbent surface from bulk solution, (2) adsorption occurs on the particle surface, (3) transportation occurs within the sorbent particle. These techniques are very cost effective: adsorption on modified natural materials, adsorption on industrial by-products, bio-sorption [54].

**Figure 4.** Conventional technologies for heavy metal removal, reprinted with permission from [52].

**Figure 5.** Conventional methods for removal of metal, reprinted with permission from [52].

#### **4. Removal of Pesticides**

Low-cost adsorbents development for pesticide maintenance is a significant part of environmental sciences research. Wastes from industries such as carbon slurry, fly ash, and sludge are delegated as easy materials due to their minimal price, and local accessibility pesticides removal can be utilized as adsorbents. Fly ash, lignite, coal-fired, thermal, power station solid waste, is an easy adsorbent that demonstrated huge adsorption limit with regard to organic contaminations [55]. Some researchers explained the sorption of pesticides and fly ash capability and suggested its utilization for pesticides removal from wastewater samples and water. Coal fly ash altogether has a high maintenance limit with regard to metolachlor, atrazine, and metribuzin.

When compared with metribuzin and metolachlor, atrazine was the most sorbed. The fly ash sorption herbicide efficiency relied upon the herbicide concentration in the mixes, and the highest removal of herbicide was seen at lower concentrations. These are common experiences in samples of water. At some point, when the water enters, pollution of biodegradables goes through concentrated processes of biological disintegration. Pollution of bio-resistant items fundamentally presents, to a lesser extent, an issue on the off chance that they are inactive biologically or are inorganic, inert, material framing sludges that are environmentally appropriate for benthos production and biological processes improvement. Inorganic poisonous contamination [56] is caused by heavy metals, organic pesticides, and so on, where nature speaks to the most unsafe kind of bio-resistant pollution.

The health risk from unsafe and hazardous chemical substances present in drinking water is regularly characterized as the likelihood that an unfriendly impact on health will be caused by substance exposure. The microelements interact and transport particularly heavy metals; streams courses indicate the most complex aquatic systems. In the agricultural region, water pollution is mainly caused by fertilizers, pesticides, and waste from poultry processing plants, drainage from livestock farms, and so on. Particularly, pollution made by pesticides on the row by heavy metals from fertilizers and nitrates contamination is very dangerous. Water mineralization indicates groundwater contamination of soil improvement on territories. Large mineral exploitation stores frequently prompt water quality disintegration on the more extensive area around strip mines and mines [57]. A great deal of water is commonly cleared during the depositing of drainage, and it upsets the groundwater regime and disturbs the hydro-chemical equilibrium.

#### **5. Key Parameters for the Pollutant Removal**

Commonly, contaminations can exist in viscous crude oil forms that are mostly either miscible or immiscible in water or as heavy metals, dyes, and pesticides of dispersible molecules. From the accessible information, there is more to viscous oil capturing than the actual process, and it intensely relies upon the huge composites' porosity and density for dissemination of fluid density via pores. Fewer viscosity solvents stream into the pores easily, but highly viscous liquids are used to diffuse slowly. The multifunctional composites design can be heated up through various mechanisms by using Joule effect, which permits one to reduce the oil viscosity, allowing faster diffusion of oil into the pores and promoting quicker removal. On the account of water-dispersible contaminations, the elimination depends on electrostatic interactions and chemicals with the sorbent. These may be improved by expanding the number of dynamic sites such as surface area and charge by chemical functionalization.

Silicon graphene multifunctional composites can be surface-functionalized to enhance the modifications with the analyte. The technique of sub-atomic is a wiser approach to silicon graphene composites with improved performance [58,59]. The molecular cavities focus on molecules and improve the detecting capabilities of dyes. The molecular imprinting is likewise useful for designing nanocomposites of silicon graphene for removing heavy metals. Moreover, conditions of the environment play a critical function in the pollutant's adsorption.

In general, the process of sorption relies on the physical state and the physicochemical properties of the composite and the pollutant chemical composition in the water. The properties evaluation versus the performance of the material is significant due to the absence of broad availability of characterization data and associated sorption capacities, such as specific surface areas [60,61]. Regardless of these obstructions, we distinguished the patterns and were effective in building up connections that helped increase the performance of the composites.

#### **6. Nanomaterials Reuse and Retaining**

Nano-adsorbent reuse and materials recovery from an aqueous solution are profoundly hard and may cause problems to the environment; as the compounds are adsorbed, pre-treatment samples must be created, and the process of separation technologies must ensue [62]. As another option, ballistic electrons discharged from the nanostructured adsorbent material and the nonporous carbonaceous utilizing microwave irradiation may likewise annihilate the adsorbed compound. Nanomaterial holding and reutilization by membrane filtration are enabled device designs of nanotechnology, key features that allow persistent chemical usage because of the expense and general health concerns. Besides, membranes of ceramics are beneficial compared to polymeric membranes in photo catalytic applications, as they exceptionally oppose chemical oxidants and ultraviolet.

Nanomaterials can be controlled on different membranes and resins, thus separation can be removed further. Vast studies are needed to propel easy, budget free techniques for nanomaterials immobilization without influencing their performances. In any case, the liberation potential is required to be generally reliant on the separation process and the technique of immobilization employed. For nanomaterials, which liberate metal ions, their disintegration must be restrained carefully [63,64]. The nanomaterial liberation detection is a significant, specialized obstacle for hazardous assessment and remains challenging, retaining particular nanomaterials reutilization to correct the cost process. For a nanomaterial applicable to treat wastewater, there are two investigative requirements. Future research with good, consistent conditions is needed to utilize various nanomaterials efficiencies.

#### **7. Environmental Significance and Future Application**

As of now, there is no doubt of the nanomaterials usage efficiency in industrial wastewater treatment, however, nanomaterials impose a huge number of genuine cons because, during treatment processes, they may discharge into the surrounding environment, and they can withstand serious risks for a long time. In such a manner, there is a requirement for more studies and surveys to decrease the toxicity in the environment [40,65]. Among different metal oxide nanoparticles, titanium dioxide, for instance, is generally utilized and have some significant constraints and some toxic effects towards human health and the environment, which causes difficulty in creating a sustainable environmental pollutants removal system. In such a manner, initiatives for new findings and research should be investigated to overcome these difficulties, and several scientists and analysts are continuously attempting to defeat these obstructions [66]. A few researchers just determined a better approach to diminish the band gap to utilize solar energy proficiently [67]. More research is needed to build up the practical techniques for the synthesis of nanomaterials and to determine effective areas of application in discovering the material proficiency. Table 3 summarizes the advantages and the disadvantages of the different physicochemical technologies for treatment of rare-earth elements in water and wastewater.

The market requirement is to restrict the costs of the procedure concerning the environmentally friendly nature of the referenced nanomaterials. Critical accentuation for this utilization should be given to green technology by material by-products of agricultural wastes. Numerous works in this area are needed in order to improve. The main concentration of the existing research works is that their utilization is yet to occur in the research [68,69]. Only a restricted amount of studies for brief market analyses and economic

aspects is accessible. The principle market objective for the future is to improve the process of treatment on an industrial scale, which requires a significant monetary and technological method. In such a way, colleges and research labs can assume an effective part through a more official way to deal with the transfer of technology and copyright protection [70–72].

**Table 3.** Summary of the advantages and the disadvantages of the different physicochemical technologies for treatment of rare earth elements in water and wastewater [72].


#### **8. Conclusions**

Water safety is of critical importance among major areas in the universe because of population increase, droughts drawn out, environmental changes, etc. From the literature review, wastewater or water treatment utilizing nanomaterials is becoming a prominent field in research. When compared with other planets, water makes our planet better. However, the overall pure water accessibility is part of what is causing current and unsurprising demands for water. The resources of drinking water are not satisfying domestic developmental, fundamental, or basic needs in many regions of the world. In particular regions, there is a lack of pure water to satisfy the fundamental requirement for sanitation, and human water needs are positively a breaking point in regard to the well-being of humans and for other creatures in the world. Academicians, research institutes, research fellows, and young scientists must discover a new way to eliminate these constraints. This universe is confronting many difficulties in doing this, particularly given a fluctuating and environmental future; a rise in population is driving community enlargement, globalization, and urbanization. How to preeminently defeat these difficulties requires investigation in every aspect of water management.

Nanomaterial treatment for water pollutants is becoming a trend, and it is drastically improving in this advanced time because of entirely awful states of water and freshwater demand in the entire universe. A significant requirement for progressive innovation for water treatment draws near, explicitly to affirm good quality drinking water and to eliminate micro and macro contaminations and toxins. We must improve developments of industrial production through deftly replaceable approaches for water treatment. Nanotechnology has manifested incredible accomplishments for water decontamination, controlling diffi-

culties and making some progress for the future. Approaches of nanomaterials such as nanostructured catalytic membranes, nanosorbents, and so on are extremely productive, require less time, are eco-friendly techniques, and require less energy, however, every one of these techniques is inexpensive, and they are not utilized at this point for the industrial purpose of wastewater purification at an enormous scope.

Due to the high reaction rate, nanomaterials show high efficiency. In any case, there are a few shortcomings that should be avoided. There is no digital, computerized monitoring methods that offer predictable measurement on real-life facts regarding nanoparticles prevalence, which are available in limited quantities in water. Besides, to decrease hazards to health, international research universities and research institutes ought to plan legitimate terms and conditions to solve these circumstances. Moreover, the nanoengineered mechanical restriction for approaching water is that it is inconsistently adaptable to mass growths, and in today's scenario, few cases exist that are not obtrusive with moderate treatment approaches. Moreover, there are incredible requirements to incorporate some nanomaterials modifications, which include having large productivity and being cost effective, simple to deal with, and environmentally friendly. It is likewise important for wastewater treatment to grasp the economic difficulties and the commercialization of these innovations. Various uses of nanomaterial can give a gigantic proposal to flexible drinking water to the entire universe.

**Author Contributions:** Conceptualization, G.P., K.K.; methodology, G.P.; software, S.K.L.; validation, S.K.L., A.A. and M.M.H.; formal analysis, K.K.; investigation, G.P., K.K., S.K.L., V.K.; resources, M.M.H., A.A., K.K., R.K.M.; data curation, R.K.M.; writing—original draft preparation, G.P.; writing review and editing, G.P., K.K., S.K.L.; visualization, S.K.L.; supervision, A.A, K.K., S.K.L., M.M.H.; project administration, M.M.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Marlia M. Hanafiah was funded by the Universiti Kebangsaan Malaysia (DIP-2019-001; GUP-2020-034).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

