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Carbon-Based Nanoadsorbents for the Removal of Emerging Pollutants

Carbon-Based Nanoadsorbents for the Removal of Emerging Pollutants Hindawi Adsorption Science & Technology Volume 2023, Article ID 3579165, 12 pages https://doi.org/10.1155/2023/3579165 Review Article Carbon-Based Nanoadsorbents for the Removal of Emerging Pollutants 1 2 3 4 M. Venkata Ratnam , P. Akilamudhan, K. Senthil Kumar , S. Narasimha Reddy , 5 6 7 K. Nagamalleswara Rao , Feroz Shaik , and D. M. Reddy Prasad Department of Chemical Engineering, Mettu University, Ethiopia Department of Chemical Engineering, Erode Sengunthar Engineering College, Tamil Nadu, India Department of Chemical Engineering, Kongu Engineering College, Tamil Nadu, India Department of Chemical Engineering, Rajalakshmi Engineering College, Rajalakshmi Nagar, Tamil Nadu, India School of Chemical Engineering, Vellore Institute of Technology, Tamil Nadu, India Department of Mechanical Engineering, Prince Mohammad Bin Fahd University, Saudi Arabia Petroleum and Chemical Engineering Programme Area, Faculty of Engineering, Universiti Teknologi Brunei, Gadong, Brunei Darussalam Correspondence should be addressed to M. Venkata Ratnam; venkata.rat@meu.edu.et Received 18 July 2022; Revised 9 November 2022; Accepted 6 April 2023; Published 15 April 2023 Academic Editor: S Rangabhashiyam Copyright © 2023 M. Venkata Ratnam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Emerging contaminants (ECs) are substances that have been detected in water but have not been thoroughly tested or regulated. Pesticides, cosmetics, pharmaceuticals, and other medications are examples of compounds in this category. Even at low quantities, these pollutants can harm human health and the environment; therefore, avoiding them is critical. The consequences of EC pollution on the endocrine, hormonal, and genetic systems are causing significant concern. Even with current best practices and available technology, it is difficult to totally eliminate ECs from municipal and industrial wastewater treatment plants. Adsorption has been the method of choice for EC removal since it is less costly, more effective, and easier to use. To treat ECs, newer generation nanoadsorbents are employed. Adsorption was greatly enhanced by functional changes to the adsorbent surface. Carbon nanostructures are widely used as adsorbents because of their outstanding surface properties, adaptability, large surface area, adjustable structural changes, and high chemical stability. This review reviews and examines recent research on the production and use of carbon-based nanoadsorbents. The emphasis is on carbon nanotubes, graphene, and graphene- derived adsorbents. It is being investigated if these adsorbents can be used to extract hormone-disrupting chemicals and other emerging pollutants. The sources and classification of these pollutants, treatment knowledge gaps, and novel prospects for increasing carbonaceous nanoadsorbent utilization were all explored. The environmental and health problems associated with EC use are also studied. 1. Introduction environment [2, 3]. Emerging contaminants (ECs) are che- micals (synthetic or natural) and microorganisms of any sort Water consumption is rising as a result of population growth that are not routinely monitored, have not previously been and rising living standards [1–5]. The accumulation of nox- examined, and may pose harm to ecosystems, human health, ious substances makes it more difficult to maintain the qual- and safety [6–8]. Hormone activity; damage to the skin, ity of the water supply. Personal care products, home brain, and neurological system; cancer; and ecological toxic- cleansers, perfluorinated compounds (PFCs), endocrine- ity are some of the most important health and environmen- disrupting compounds (EDCs), prescription drugs, and tal problems linked with ECs. Because of their androgenic or other commodities emit a wide range of chemicals into the estrogenic actions, endocrine-disrupting chemicals (EDCs) 2 Adsorption Science & Technology can cause damage to the body’s hormonal system even at usefulness. Even regenerated AC’sefficiency is inferior to low doses [9–11]. These contaminants may increase the that of fresh AC [1]. As a result, the quest for new adsor- number of cancers and antibiotic-resistant microorganisms bents has risen in recent years [1, 6, 7]. Nanotechnology is [12]. The concentration of ECs may vary substantially from being applied in a range of scientific sectors, including water one location to another depending on the country’s purification, as it advances. The usage of nanoscale adsor- manufacturing procedures. Because their presence has bents aids in the removal of water pollutants. These innova- harmed the water’s physicochemical qualities, immediate tive adsorbents can remove pollutants down to the atomic action is essential. Water samples have been shown to level, in addition to having a remarkable adsorption capac- include antimicrobials, steroid analgesics, profens, antidia- ity. Recent studies have concentrated on the development betic drugs, antidepressants, cytostatics, gastrointestinal of nanoscale adsorbents for EC removal [6, 25, 26]. The meds, and lipid controllers [13, 14]. adsorbent’s capacity to remove a wide spectrum of pollut- The treatment of these contaminants is critical owing to ants is enhanced by the surface functional groups [27–31]. their environmental impacts. Some ECs may go undetected The optimal adsorbent would have a large surface area and in water and wastewater treatment systems due to their specialized adsorption sites with high porosity. A substance’s extremely low concentrations. The ecotoxicological effects porous structure enhances its surface area and adsorption capabilities. Carbon nanomaterials are a type of porous and behavior of ECs have yet to be validated by a global rou- tine checking effort [5, 15]. The standard water treatment nanoadsorbent that has a great deal of potential for EC methods are intended to eliminate only the normal contam- removal. They may have been used in place of commercial inants while preserving basic water quality parameters [16]. activated carbon to remove various pollutants. There has been limited investigation towards removing Adsorption-friendly features of carbon nanoadsorbents ECs from aquatic environments. Traditional biological include ordered structure, high porosity, homogeneous pore removal of these contaminants in wastewater treatment size distribution, high specific surface area, chemical and plants is time-consuming and not necessarily successful thermal stability, and nontoxicity. Furthermore, these mate- since not all emergent pollutants can be eradicated. Photol- rials’ surfaces may be altered, making them into functional ysis, sonochemistry, ozonation, ultrasound, solar-powered materials with a greater capacity to remove different con- processes, photo-Fenton, photocatalysis, and electro- taminants. The essential components of the adsorption Fenton have all been studied recently [17]. These technolo- mechanism include interactions, hydrogen bonds, and elec- gies, however, are costly to operate and maintain, and they trostatic interactions. The presence of oxygen-containing require a lot of energy [17]. As a result, there is a need to functional groups in adsorbents also promotes adsorption. provide efficient and cost-effective solutions. Carbon nanotubes (CNTs), graphene, and graphene deriva- The adsorption approach may efficiently cure a wide tives are the principal carbon-based nanoadsorbents identi- range of contaminants. This technique is regarded as the fied which have been researched and critically appraised in most cost-effective, efficient, practical, and ecologically this article. This paper also includes a discussion on EC cat- friendly of the wastewater treatment technologies now in egorization, as well as their history, impacts, and potential use [18–22]. Adsorption is a well-known surface phenome- futures. non that may remove organic and inorganic micropollutants effectively. It is used to remove contaminants from water 2. Potential Sources and Impact on Health and after it has been treated chemically or biologically. Adsorp- Environment Due to ECs tion is becoming more used as a method of eliminating dis- solved pollutants that have withstood chemical oxidation or Sewage, solid waste generated by municipal solid waste col- biological treatment. Several scientists have spent the last lection and treatment facilities, and urban runoff are all decade studying the adsorption of ECs on activated carbon important sources of ECs [1]. There are several ECs in both (AC). Activated carbon (AC) is a porous carbonaceous sub- surface and groundwater. The concentrations of ECs in sur- stance that may be produced chemically or by pyrolysis from face water are typically lower than those reported shortly bamboo, coal, wood, nutshells, and other organic materials. after wastewater treatment facilities discharge. This is caused The source material as well as the method of activation has by dilution and other natural processes. Groundwater con- a large impact on the surface functional groups of ACs. centrations may worsen if the aquifer is contaminated. Envi- Adsorption using granular activated carbon (GAC) is dem- ronmental and water-related physicochemical factors, as onstrated by Rao et al. as a viable tertiary treatment for the well as longitude and latitude, may all influence how ECs simultaneous removal of five PPCPs from an aqueous solu- migrate, where they go, and how they appear in the environ- tion, including three hydrophilic (ciprofloxacin, acetamino- ment. The chemical purity and exposure dosage are influ- phen, and caffeine) and two hydrophobic (benzophenone enced by the source type. Many human actions contribute and Irgasan) PPCPs [23]. Using batch sorption studies and to the destruction of the environment. ECs were discharged commercial granular activated carbon as an adsorbent, the in large quantities into wastewater treatment facilities adsorption of six emerging pollutants from aqueous solu- (WWTPs) from industrial, commercial, and residential tions was investigated. Caffeine, clofibric acid, diclofenac, sources. The EC sources and paths are depicted in gallic acid, ibuprofen, and salicylic acid were chosen as typ- Figure 1. Heavy metals and organic compounds are present ical pollutants [24]. However, the problems in regeneration in sludge produced by physical and chemical processes in and higher pricing of activated carbon restrict its practical WWTPs. Excreta from the human body, as well as Adsorption Science & Technology 3 Animal farming Emerging Manure contaminants Households Runoff water Agriculture Hospital Industry Water bodies Waste water plants Figure 1: EC origin and pathways. abandoned, expired, or unused medications and medicine, with proper drainage, soil, and topography [14]. Pesticide made their way into the environment. A variety of chemical risks are often overlooked by people in developing nations. and microbiological components are non-biodegradable Pesticide usage and ineffective management are to blame. long-term contaminants. These pollutants can be found in Pesticide and biocide concentrations will be higher in the industrial, agricultural, and municipal effluents, as well as absence of monitoring data. industrial smoke [32]. When pharmaceutical waste and Polychlorinated biphenyls are largely suspended solids organic matter disintegrate in neutral conditions, microcon- because of their low vapor pressure, poor water solubility, taminants that might be toxic are produced. These micro- and high octane-water coefficient [33]. As body fat levels pollutants are present in the distribution of drinking water. rise, their half-life lengthens from weeks to months. These Organic contaminants in wastewater rise as leaching pollutants have been associated with neurological and endo- increases. This is harmful to people’s health. Pesticides in crine system malfunction in addition to increasing tumor groundwater can be reduced by replacing ecologically development [33]. The presence or absence of aryl hydrocar- acceptable materials for pesticides in fertilizer. Human bon receptors influences the toxicity of dioxin-like com- excretion, residual medicine disposal, and agricultural usage pounds. As a result of increased environmental awareness, were the chief sources of pharmaceuticals entering the atmo- industrially related synthetic dyes and hazardous wastewater sphere. These drugs were identified in both groundwater and effluents including colors have garnered more attention. The surface water. 90% of pharmaceutically active chemicals most efficient approach to avoid harmful contaminants is (PhACs) reach water bodies, according to research [33]. Pes- through environmental laws and regulations. Synthetic dyes ticide and insect-repellent compounds, lipid regulators, and with structurally diverse molecular structures, such as steroid and sunscreen components are all found in our anthraquinone and anthraquinone-based dispersions and homes and personal care items. Fluoride is a bactericide that metal complexes, are among the most commonly used and is commonly found in toothpaste, shampoo, soap, mouth- ecologically hazardous dyes [1]. Surfactants are synthetic wash, and even skin creams. Triclosan, an antibacterial substances that are widely used in the production of cleaning agent, is commonly included in deodorants and cosmetics agents, emulsions, paints, insecticides, and cosmetics across [33]. Benzophenone and its 2,4-dihydroxybenzophenone the world. Surfactant toxicity is mostly determined by their derivatives are used in sunscreen and UV cosmetic products. capacity to permeate marine cell membranes. Large-scale Because they are designed for outdoor use, most self-care surfactants include linear alkylbenzene sulfonates, lignin sul- products may be cleaned without affecting their structure fonates, fatty alcohol ethoxylates, and alkylphenol or quality. Toxins have a bigger impact at wastewater treat- ethoxylates. ECs have been demonstrated to be hazardous to both ment plants because they are more easily transferred into aquatic habitats [14]. Pesticides are used in farming opera- human and environmental health. Mistakes in glucose tions to reduce the spread of potentially harmful insects, metabolism and infertility have been related to a wide range weeds, and microorganisms such as fungi and bacteria [6]. of health issues. Infertility, pregnancy difficulties (such as Inequity in the use of antibiotics, biocides, and pesticides excessive cholesterol, fetal obesity, and low sperm quality), must be eliminated. Pesticides must be used in order to safe- memory loss and anemia, high blood pressure and apopto- guard the food supply. Biocides and insecticides are com- sis, and a range of other disorders are among these [34]. monly used on farms and in cities. Pesticides and biocides Medication usage has been associated with an increased risk may readily permeate the water supply and affect aquatic life of birth defects and developmental delays, as well as 4 Adsorption Science & Technology hormone imbalances and endocrine system malfunctions. ing water treatment systems used to remove EC. Materials ECs may be accumulating in humans and/or wildlife. To like graphene, carbon nanotubes (CNTs), clay minerals, sili- protect both human health and the environment, ECs and ceous adsorbents, and polymers like polyethylene tere- their adverse consequences must be studied and handled phthalate can replace AC in EC removal. Chemical or further. The deliberate or inadvertent dumping of dangerous thermal modification of the adsorbent’s surface can result chemicals into large bodies of water endangers the environ- in a multifunctional nanoadsorbent with improved capacity ment and human health. for EC absorption. Even at low concentrations (mg/L), nanoadsorbents were able to remove ECs. To top it all off, the nanoadsorbent dosage was small, and the removal time 3. Adsorption for EC Removal for ECs was quick (1–15 min). Figure 2 presents the over- In order to remove ECs, physical, biological, and chemical view of the EC adsorption. methods are used. Physical treatment with no biological or 3.1. Carbon Nanotubes (CNTs). Carbon nanotubes (CNTs) chemical materials has no influence on the biochemical are a potential adsorbent for the remediation of several characteristics of the ECs. Enzymatic breakdown and live ECs due to their large surface area, tiny size, and tremendous organisms are both included in biological treatment. Chem- porosity [11]. Carbon nanotubes (CNTs) have significant ical treatment entails the use of chemical compounds. potential to replace activated carbon in water treatment Adsorption, advanced oxidation processes, biological treat- technologies and are likely to do so in the near future. ments, and membrane separations are some of the most suc- Because of their open structure, CNTs have a larger surface cessful EC removal methods [35–37]. Because of its area, faster access to reactive sites, faster kinetics, and simplicity of use and minimal environmental impact, biolog- improved adsorption capacity [6]. Cost and development ical methods are the most extensively utilized technique. of sustainable production procedures, on the other hand, However, they are less effective due to limited biodegradabil- are hindering the widespread use of carbon nanotubes. The ity. High selectivity, high efficiency, simple processing, no most common CNT forms are single-walled CNTs, multi- need for harsh chemicals, high productivity, cost-effective- walled CNTs, and functionalized CNTs. When it comes to ness, easy posttreatment, and less disruptive are a few of adsorption, the morphologies of carbon nanotubes, such as the advantages of adsorption [38–41]. Adsorption is a sur- tube diameter and bundle shape, are crucial. Smaller carbon face phenomenon where pollutant molecules cling to an nanotubes with bigger specific surface areas and distinctive adsorbent owing to the van der Waals forces and electro- hollow and layered structures have a better potential for static interactions. Adsorbents and adsorbates interact in adsorption than larger carbon nanotubes [6]. As a result, two ways: chemically and physically. Through pores in the adsorbent, the adsorbate diffuses and interacts with the single-walled carbon nanotubes (SWCNTs) are less effective than multiwalled carbon nanotubes (MWCNTs) at adsorb- active sites when it comes into contact with the outer surface ing the adsorbate. The capacity of pollutants to adhere to [13]. In the adsorption process, the adsorbate and the adsor- surfaces is determined by how they interact with one bent’s physical and chemical properties play a major role. another. This implies that each pollutant has a unique capac- For example, changes in pH can affect adsorbent surface ity to adhere to surfaces. groups and pollutant charge [10]. The presence of functional Carbon nanotube surfaces’ wettability and hydrophilicity groups such as hydroxyl and carboxyl groups makes the are improved by adding functional groups [44–46]. Oxygen- adsorbent highly effective [13, 14]. Natural adsorbents like containing groups, such as hydroxyl, carbonyl, or carboxylic, clay and sand are ideal for adsorption since they are abun- are found in functionalized carbon nanotubes. To add func- dant and cheap. Industrial waste adsorbents can encapsulate a material in another substance. tional groups to carbon nanotubes, sulfuric acid (H SO ) 2 4 and nitric acid (HNO ) can be utilized. Carbonyl groups Activated carbon (AC) has been extensively studied for and oxygen levels on the surface of MWCNTs have been EC removal. However, adsorption using AC is expensive shown to have a significant influence on their maximal since activated carbon is seldom recovered. Normally, less adsorption capacity [45]. As a consequence, researchers cre- than 40% of the AC impregnated is reused. These factors sig- ated modified carbon nanotubes for use as an adsorbent. The nificantly limit the use of AC [42]. Biochar (BC) is a stable researchers employed oxidized MWCNTs in conjunction source of carbon that is produced by thermal or aqueous with a range of oxygen molecules to adsorb the antibiotic processes in low- or no-oxygen environments. It increases medication tetracycline from aqueous settings [46]. The the surface activity, porosity, and utility of biochars. Some Langmuir model calculated the maximum adsorption capac- biochars may be confused with activated carbon due to their ity (q ) of carbon nanotubes with 2.0%, 3.25%, 4.75%, and similarity. It is said that BC composites treated with nano- max particles enhance pollutant absorption. BC’s corrosive treat- 5.95% oxygen to be 217.8, 269.25, 217.56, and 210.43 mg/g, ment promotes oxygenated surface groupings [43]. ECs (like respectively. Another investigation validated the impact of raising the oxygen content from 2.0 to 5.9% on the sorption tetracycline and endocrine-disrupting compounds) can be taken up by modified biochar through hydrophobic, electro- limit of carbon nanotubes for ciprofloxacin expulsion [11]. Using the Langmuir isotherm model, q was calculated static, hydrogen bonding, and functional groups [1]. max Nanoadsorbents are adsorbents with a diameter of a few to be 150.6, 178.9, 206.0, and 181.2 mg/g for carbon nano- nanometers. Despite its limited application in industrial tubes containing 2.0%, 3.2%, 4.7%, and 5.9% oxygen, respec- adsorption, nanotechnology has great promise for improv- tively [43]. Adsorption of anti-infection medicines Adsorption Science & Technology 5 Personal care Industrial chemicals Trace metals chemicals Particulates House hold chemicals Pharmaceuticals Soaps, Disinfection chemicals Engineered nano Shampoos, Food preservatives Antibiotics, Surfactants, Particles, micro Tooth pastes, Pesticides Anti inflammatory Plastics UV filters Plasticizers Substances 𝜋-𝜋 Hydrogen interactions bonding EC molecules Adsorption Hydrophobic Electrostatic effects interaction Adsorbent Figure 2: Overview of EC adsorption. norfloxacin and ofloxacin onto functionalized carbon nano- on the attachment of CPX to MWCNTs. Ionic strength, on tubes has been studied [47]. The MWCNT and SWCNT the other hand, had no effect on CPX’s capacity to adsorb were altered to add beneficial groups such as hydroxyl onto MWCNTs. Electrostatic interactions appear to have a (-OH) and carbonyl (CO). significant role in adsorption [46]. Single-walled carbon In the adsorption of the antibiotics ofloxacin (OFL) and nanotubes were employed to remove bisphenol A (BPA) norfloxacin (NOR), there is a significant link between the and 17-estradiol (E2) from aqueous systems without and adsorption coefficients and the specific surface area of CNTs. with ammonium persulfate treatment. DFT calculations It is probable that structural properties had a significant revealed that two chemicals interact with sorbent structures. influence on the adsorption of OFL and NOR on CNTs via According to adsorption energy estimates, both sorbents an electron donor-acceptor mechanism [47]. Tetracycline preferentially adsorb E2 over BPA. The optimum geometric adsorption on MWCNTs is influenced by surface character- orientation of molecules in contact can have a significant istics and solution chemistry. The adsorption capacity and impact on adsorption behavior [48]. Ahmaruzzaman et al. coefficient of adsorption of tetracycline increased linearly synthesized CNTs from sunflower oil, a readily accessible with the surface oxygen concentration of MWCNTs. Water bioprecursor, which was then coated with SnO nanoparti- clusters formed during tetracycline adsorption due to the cles using Coccinia grandis extracts. The generated nano- dispersibility of the nanotubes. This contact is assumed to heterojunction displayed outstanding performance against be the source of the problem. Furthermore, interparticle arsenic, with a maximum adsorption capacity of and boundary layer diffusion might influence total tetracy- 106.95 mg/g. Furthermore, the SnO -CNT nano- cline adsorption onto 3.2% oxygen-containing carbon nano- heterojunctions showed catalytic activity in the reduction tubes. When the pH was between 3.3 and 8.0, the majority of of 4-nitrophenol [29]. tetracycline could adhere to carbon nanotubes. This CNTs may be used to make structures such as a one- occurred when water clusters, or H-bonds, formed on the dimensional hollow tube shape. Depending on the quantity carbon nanotubes [44]. of graphene layers, single-walled and multiwalled carbon Oxidized multiwalled carbon nanotubes were utilized as nanotubes can be created. Many pollutants are adsorbed adsorbents in a study to investigate the effect of oxygen con- on the surface of carbon nanotubes [11, 44–46]. CNTs can centration on the adsorption capabilities of ciprofloxacin have their base or sidewalls modified with different (CPX) [46]. The rise in oxygen content from 2.0% to 5.9% oxygen-containing functional groups to improve the surface appears to be increasing CPX’s adsorption capacity. The properties. MCNT, a magnetic material consisting of carbon interaction of electron donors and acceptors has been iden- nanotubes, has become a popular approach for enhancing tified as the fundamental reason for the lower expansion separation and purification efficiency. For example, because rate. The increased hydrophilicity and dispersion of the of its large surface area and capacity to be regenerated, adsorbent, as well as the suppression of water clusters, MCNT is perfect for the rapid separation of various environ- enabled CPX adsorption on oxidized MWCNTs. The alka- mental media. Sulfamethoxazole, carbamazepine, and keto- line atmosphere was demonstrated to have a negative impact profen were among the pharmaceutical pollutants that Emerging contaminants 6 Adsorption Science & Technology could be removed from water using carbonaceous adsor- ufactured utilizing both techniques [27]. The issue with gra- bents containing doped phosphorus (P). Adsorbents dem- phene is that due to its hydrophobicity, it is difficult to onstrated high removal rates (>99%) for all substances recycle. Graphene is also ineffectual for polar component tested. Adsorption was primarily controlled by π‐π and n- adsorption with hydrophilic chemical groups. Khalil et al. EDA interactions as well as H-bonds. Metal ions were dem- used porous graphene (PG) to extract the medicines atenolol onstrated to have no influence on the removal of pharma- (ATL), ciprofloxacin (CIP), carbamazepine (CBZ), gemfi- ceutical pollutants [34]. Table 1 summarizes the studies on brozil (GEM), diclofenac (DCF), and ibuprofen (IBP) from CNT adsorbents used to clear up EC. aqueous solutions. At trace concentrations, low PG dosages Even though carbon nanotubes are well known in many (100 mg/L) resulted in quick response times and high clear- industries and have significant promise for environmental ance efficiencies for all studied EC. EC mixes were examined remediation, there are a number of factors that prevent them to evaluate if PG might be utilized for tertiary therapy. from being employed more broadly. Scientists must cope Increasing the quantity of PG in water and wastewater sam- with production costs, toxicity, and environmental dangers. ples can aid in the removal of mixed ECs [56]. Table 2 sum- It is expected that CNTs can have safety criteria and risk marizes the studies on graphene adsorbents used to remove evaluations to determine how safe they are to use, which EC. might lead to additional CNT uses in the near future. Toxic organic contaminants are effectively removed by graphene and its functionalized compounds. The most sig- 3.2. Graphene-Based Adsorbents. Many scientists believe that nificant constraints of GO and GO-based nanomaterials graphene and graphene-based nanomaterials are the ideal are their high cost and difficulties in reusing. Because of options for water purification because of their high surface the nanomaterial’s high electrostatic interactions, reuse may be impossible. Only a few researchers have looked at area-to-volume ratio and other physical features, such as their capacity to receive electrons and resist pollutants [35, the reusability of graphene-based EC adsorbents. More 54]. According to the literature, nonelectrostatic interactions study is needed to determine how graphene impacts human are the primary means by which graphene-based nanomate- health and the environment. rials remove pollutants. Several researchers have attempted to alter the surface of graphene in order to make it more effi- 3.3. Miscellaneous Carbonaceous Nanoadsorbents. Fuller- cient and simpler to reuse [27, 35, 54, 55]. For the majority enes, carbon nanospheres, and carbon nanofibers are the of the ECs studied, reduced graphene oxide and graphene other carbon-based nanoadsorbents that have recently been were found to have lower adsorption capabilities than gra- employed for EC adsorption. The major contrast between phene oxide. This is because the surface has grown more CNTs and fullerenes is the carbon form. Fullerenes are fre- hydrophobic, with fewer oxygen functional groups, making quently found as hexagonal rings containing carbon atoms. it more difficult for ECs in water to adhere to it. With a Fullerene’s properties have been effectively used to increase wider surface area, ECs may adsorb in more places. As a its utility in the environmental domain. The detection and result, the material’s adsorption capability can be increased. capture of carbamazepine in an aqueous media were investi- Because of its delocalized electrons and vast surface area, gated theoretically using fullerene and its derivatives doped graphene is suited for the removal of organic compounds with B, Al, Ga, Si, Ge, N, and P. The fullerene derivatives comprising benzene rings and π‐π stacking. GO suspension doped with Al, Si, and Ga are the strongest candidates for was used to eliminate tetracycline, a prescription antibiotic. serving as sensors and uptaking carbamazepine in aquatic Tetracycline’s four aromatic rings each have a distinct func- conditions, according to DFT simulations [61]. Mesoporous tional group, such as phenol, aldehyde, ketone, and amino. It carbon nanospheres (MCNs) were employed to efficiently adheres to the GO surface via two mechanisms: interaction remove methyl orange (MO), rhodamine (RhB), and p- and cation bonding. The Langmuir model predicts a maxi- hydroxybenzoic acid (p-HBA), with a removal efficacy of mum adsorption capacity of 313 mg/g. When pH or Na more than 95% [62]. The greatest removal efficacy for tetra- concentrations were increased, tetracycline adsorption on cycline hydrochloride (TCH) and ciprofloxacin hydrochlo- GO was decreased [54]. ride (CPH) for hollow mesoporous carbon spheres Three-dimensional chitosan-gelatin aerogels containing (HMCSs) generated and modified for laccase (Lac) immobi- GO are mixed in two ways: coating and embedding. Toler- lization was 99.4% and 96.9%, respectively [63]. A zinc 2+ ance to lead (Pb ) was assessed, as well as its effectiveness oxide-coated carbon nanofiber composite was used as an against the fluoroquinolonic medications ofloxacin and cip- adsorbent to extract amoxicillin from ambient water matri- rofloxacin. Coating and embedding techniques demon- ces. The maximal adsorption capacity was determined to strated only a small influence on organic contaminant be 156 mg/g based on the results. Furthermore, the adsor- adsorption capacity, which varied from 5 to 8 mg/g, whereas bent was successfully tested on actual wastewater samples chitosan-gelatin control aerogels without GO showed no and shown to be reusable for up to fifteen cycles [64]. Acti- adsorption [27]. Kovtun et al. used coating and embedding vated carbon, multiwalled carbon nanotubes, and carbon processes to incorporate GO into three-dimensional nanofibers have been used to remove atenolol, caffeine, chitosan-gelatin aerogels. The fluoroquinolonic medications diclofenac, and isoproturon from ultrapure water and a 2+ ofloxacin and ciprofloxacin, as well as lead (Pb ), were used municipal wastewater treatment plant effluent [65]. A mag- to evaluate the produced adsorbents. There was just a little netic carbon nanofiber (MCF) composed of bacterial cellu- variation in pollutant removal between the adsorbents man- lose absorbed diclofenac from water. MCF is a porous Adsorption Science & Technology 7 Table 1: Removal of emerging contaminants by CNT-based adsorbents. Adsorbent Pollutant Adsorption capacity Reference Significant findings (i) 86% removal efficiency after 5 regeneration cycles (ii) Multifunctionality: catalytic effectiveness against SnO -CNT As (III) 106.95 mg/g [29] 4-nitrophenol, alizarin red S dye, and metronidazole pollutants. Antimicrobial activity against bacterial and fungal strains Tetracycline [44] Oxidized multiwalled carbon nanotubes with Adsorption capacity of CNTs-2:0%O < CNTs‐3:2%O qm/SSA continued to Ciprofloxacin different oxygen > CNTs‐4:7%O > CNTs‐5:9%O. [46] increase with increasing (CPX) contents oxygen content SWCNT, acidified Bisphenol A ammonium persulfate BPA: 19.4 mg/g and 8 mg/g, respectively, for SWCNT (BPA), 17β- [48] treated SWCNT (t- and t-SWCNT; E2: 27.2 mg/g estradiol (E2) SWCNT) MWCNTs with 15nm, 30nm, 50nm and Ofloxacin The structural and SWCNTs (hydroxyl (OFL) and hydrophobic characteristics [47] functionalized, carboxy norfloxacin of OFL and NOR influenced functionalized, and (NOR) their adsorption pure) SWCNT adsorbed more IBU and TCS than MWCNT; Ibuprofen IBU adsorption was higher SWCNTs and (IBU) and For SWCNT, IBU at pH 7: 232 mg/g; TCS at pH 7: [49] at pH 4, but TCS adsorption MWCNTs triclosan 558 mg/g was higher at pH 7; CNT (TCS) surface oxidation decreased adsorption CNT absorbed more CPX than activated carbon and Ciprofloxacin carbon xerogel; however, MWCNTs 150 mg/g [50] (CPX) oxidation and heat treatment had little effect on CNT adsorption DWCNTs adsorbed better than SWCNTs and SWCNTs and Perchlorate MWCNTs; the presence of 3.55 mg/g [51] MWCNTs (ClO ) additional ClO oxygen- 4 4 containing functional groups increased adsorption The adsorption of BPA and Bisphenol A E2 varied from 7.3 to 95% SWCNTs in the (BPA) and depending on the solution presence of natural [52] 17β-estradiol pH and the presence or organic matter (NOM) (E2) absence of NOM and SWCNTs Mixture of The adsorption was made MWCNT carboxyl four linear possible through 168 mg/g [53] functionalization alkyl benzene hydrophobic contact and the sulfonates creation of hydrogen bonds (mesopores and macropores) material having a specific sur- struct three-dimensional macrostructures. The adsorbent face area of 222.3 m /g. The diclofenac elimination was effec- was more effective at eliminating oxytetracycline (1729 mg/ tive (93.2%) and quick (20 min) [66]. Self-assembling two- g) and diethyl phthalate (680 mg/g) [67]. The literature stud- dimensional graphene oxide nanosheets and one- ies show that ECs can be successfully removed by adsorption dimensional carbon nanotubes were used to readily con- tests using fullerenes, carbon nanospheres, and carbon 8 Adsorption Science & Technology Table 2: Removal of emerging contaminants by GO-based adsorbents. Adsorbent Pollutant Adsorption capacity Reference Significant findings 5-8 mg/g for antibiotics for both Embedded GO Antimicrobial effects were found 2+ adsorbents. For Pb : 11.1 mg/g 2+ aerosols. Coated Ofloxacin, ciprofloxacin, and Pb [27] particularly for the GO-coated for embedded GO aerogels and GO aerosols aerogel materials 1.5 mg/g in coated GO ones Tetracycline strongly deposited on Graphene oxide Tetracycline antibiotics 313 mg/g [54] the GO surface via π‐π interaction (GO) and cation–π bonding. (i) Regeneration and reuse for four Atenolol (ATL), ciprofloxacin cycles Nanostructured (CIP), carbamazepine (CBZ), 8.87, 7.33, 14.63, 47.85, 91.59, [56] (ii) Heterogeneous adsorption porous graphene ibuprofen (IBP), diclofenac (DCF), and 9.26 mg/g, respectively described by the Toth and Sips and gemfibrozil (GEM) isotherm models Graphene oxide Metformin 96.7 mg/g [57] Graphene oxide Carbamazepine (CBZ) 9.2 mg/g [58] Could be reused for up to 8 times nanoplatelets According to DFT studies, the adsorption process is mostly Graphene oxide Acetaminophen (ACP), accompanied by size-related composite with carbamazepine (CBZ), bisphenol 13.7, 11.2, 13.2, 14.8, and diffusion, with a modest [59] activated carbon A (BPA), caffeine (CAFF), and 14.5 mg/g, respectively contribution from a synergetic mix and chitosan triclosan (TCS) of hydrophobic/hydrophilic, hydrogen bonding, electrostatic, and π‐π interactions Reduced graphene oxide (rGO)–cellulose Methylene blue 17 mg/g [60] nanocrystal sponge nanofibers. However, these studies provide a scant descrip- pKa, making it an important factor in organic molecule tion of adsorption processes and place little focus on adsor- adsorption [10]. This can be aided by increasing the pH, bent reusability. These materials have the potential to be which changes the interactions between adsorbents and sor- extremely useful for EC adsorption. bates by changing their hydrophobic and electrostatic prop- erties [44, 49]. A higher pH may also increase the ability of the adsorbate to donate electrons, potentially improving 3.4. Adsorption Mechanism and Influencing Factors. Polar organic molecules, such as carbon-based nanoadsorbents, the overall electron donor-acceptor interaction. The pH of the carbon nanotube surface can affect the protonation state exhibit hydrophobic effects, π-π interactions, hydrogen bonds, covalent bonds, and electrostatic interactions [44]. of the tetracycline molecule and the hydrophobicity of the The π-π interaction dominated benzene ring adsorption. adsorbate, thereby influencing adsorption interactions [54]. Triclosan, for example, has two aromatic rings and is thus The adsorption of tetracycline on GO varied greatly between more compatible with the CNT surface than ibuprofen pH ranges of 3 and 11. Tetracycline’s adsorption capacity varies with initial concentration. When the adsorption [49]. Electrostatic interactions can greatly aid adsorption. Depending on the pH of the solution, functional groups con- capacity falls to 133.62 mg/g, three times as much, tetracy- taining oxygen can be protonated or deprotonated [49]. Car- cline is removed. Tetracycline’s adsorption capacity decreased eightfold and fourteenfold over the pH range, bon materials absorb hydrophobic organic molecules as a result of hydrophobic interactions. Adsorption is most effec- depending on the initial concentration. Because adsorption and adsorption are electrostatically repelled, an increase in tive in carbon materials with a net charge density of zero. Furthermore, the benzene ring on the surface of carbon ionic strength facilitates adsorption. Increased ionic strength nanotubes can serve as an electron donor for organic mole- can make organic molecules more likely to precipitate from cules containing oxygen-containing functional groups [44, aqueous solutions and bind to nanoadsorbents. Different 49]. This enables hydrogen bonds to form. Adsorption is a concentrations of NaCl were added to the tetracycline and GO solutions to investigate the effect of ionic strength on good way to get rid of ECs in water because they have a lot of aromatic rings and a specific chemical makeup [18]. adsorption capacity. The adsorption capability decreases The pH of the solution affects the protonation and when NaCl is added. Tetracycline’s adsorption capability was reduced by more than half when NaCl concentrations deprotonation of pollutants, which is dependent on their Adsorption Science & Technology 9 were increased to 100 mmol/L. The ability of NaCl to bind to Data Availability tetracycline varies little between 8.33 mg/L and 33.33 mg/L All the data is available in the manuscript. in the range of 20-100 mmol/L [54]. Additional Points 4. Current and Future Challenges Highlights. (1) Adsorption is the most preferred method for There is a possibility that nanomaterials could remove EC removing ECs. (2) The use of nanoadsorbents may increase from water in an effective manner. It is possible that limiting the adsorption efficiency of ECs. (3) Carbon nanotubes, gra- the use of adsorbents will result in the creation of new phene, and their derivatives have the potential to replace the sources of pollution. It is difficult to develop adsorbents of commercially available adsorbent activated carbon, which high quality today that can be utilized in the most modern has limitations. (4) Functional modifications will play a water treatment processes. The adsorbents are susceptible major role in improvising the uptake of emerging to change if functional groups or structures are introduced contaminants. into the mix. This approach is more effective than others in the elimination of pollutants. 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Carbon-Based Nanoadsorbents for the Removal of Emerging Pollutants

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10.1155/2023/3579165
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Hindawi Adsorption Science & Technology Volume 2023, Article ID 3579165, 12 pages https://doi.org/10.1155/2023/3579165 Review Article Carbon-Based Nanoadsorbents for the Removal of Emerging Pollutants 1 2 3 4 M. Venkata Ratnam , P. Akilamudhan, K. Senthil Kumar , S. Narasimha Reddy , 5 6 7 K. Nagamalleswara Rao , Feroz Shaik , and D. M. Reddy Prasad Department of Chemical Engineering, Mettu University, Ethiopia Department of Chemical Engineering, Erode Sengunthar Engineering College, Tamil Nadu, India Department of Chemical Engineering, Kongu Engineering College, Tamil Nadu, India Department of Chemical Engineering, Rajalakshmi Engineering College, Rajalakshmi Nagar, Tamil Nadu, India School of Chemical Engineering, Vellore Institute of Technology, Tamil Nadu, India Department of Mechanical Engineering, Prince Mohammad Bin Fahd University, Saudi Arabia Petroleum and Chemical Engineering Programme Area, Faculty of Engineering, Universiti Teknologi Brunei, Gadong, Brunei Darussalam Correspondence should be addressed to M. Venkata Ratnam; venkata.rat@meu.edu.et Received 18 July 2022; Revised 9 November 2022; Accepted 6 April 2023; Published 15 April 2023 Academic Editor: S Rangabhashiyam Copyright © 2023 M. Venkata Ratnam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Emerging contaminants (ECs) are substances that have been detected in water but have not been thoroughly tested or regulated. Pesticides, cosmetics, pharmaceuticals, and other medications are examples of compounds in this category. Even at low quantities, these pollutants can harm human health and the environment; therefore, avoiding them is critical. The consequences of EC pollution on the endocrine, hormonal, and genetic systems are causing significant concern. Even with current best practices and available technology, it is difficult to totally eliminate ECs from municipal and industrial wastewater treatment plants. Adsorption has been the method of choice for EC removal since it is less costly, more effective, and easier to use. To treat ECs, newer generation nanoadsorbents are employed. Adsorption was greatly enhanced by functional changes to the adsorbent surface. Carbon nanostructures are widely used as adsorbents because of their outstanding surface properties, adaptability, large surface area, adjustable structural changes, and high chemical stability. This review reviews and examines recent research on the production and use of carbon-based nanoadsorbents. The emphasis is on carbon nanotubes, graphene, and graphene- derived adsorbents. It is being investigated if these adsorbents can be used to extract hormone-disrupting chemicals and other emerging pollutants. The sources and classification of these pollutants, treatment knowledge gaps, and novel prospects for increasing carbonaceous nanoadsorbent utilization were all explored. The environmental and health problems associated with EC use are also studied. 1. Introduction environment [2, 3]. Emerging contaminants (ECs) are che- micals (synthetic or natural) and microorganisms of any sort Water consumption is rising as a result of population growth that are not routinely monitored, have not previously been and rising living standards [1–5]. The accumulation of nox- examined, and may pose harm to ecosystems, human health, ious substances makes it more difficult to maintain the qual- and safety [6–8]. Hormone activity; damage to the skin, ity of the water supply. Personal care products, home brain, and neurological system; cancer; and ecological toxic- cleansers, perfluorinated compounds (PFCs), endocrine- ity are some of the most important health and environmen- disrupting compounds (EDCs), prescription drugs, and tal problems linked with ECs. Because of their androgenic or other commodities emit a wide range of chemicals into the estrogenic actions, endocrine-disrupting chemicals (EDCs) 2 Adsorption Science & Technology can cause damage to the body’s hormonal system even at usefulness. Even regenerated AC’sefficiency is inferior to low doses [9–11]. These contaminants may increase the that of fresh AC [1]. As a result, the quest for new adsor- number of cancers and antibiotic-resistant microorganisms bents has risen in recent years [1, 6, 7]. Nanotechnology is [12]. The concentration of ECs may vary substantially from being applied in a range of scientific sectors, including water one location to another depending on the country’s purification, as it advances. The usage of nanoscale adsor- manufacturing procedures. Because their presence has bents aids in the removal of water pollutants. These innova- harmed the water’s physicochemical qualities, immediate tive adsorbents can remove pollutants down to the atomic action is essential. Water samples have been shown to level, in addition to having a remarkable adsorption capac- include antimicrobials, steroid analgesics, profens, antidia- ity. Recent studies have concentrated on the development betic drugs, antidepressants, cytostatics, gastrointestinal of nanoscale adsorbents for EC removal [6, 25, 26]. The meds, and lipid controllers [13, 14]. adsorbent’s capacity to remove a wide spectrum of pollut- The treatment of these contaminants is critical owing to ants is enhanced by the surface functional groups [27–31]. their environmental impacts. Some ECs may go undetected The optimal adsorbent would have a large surface area and in water and wastewater treatment systems due to their specialized adsorption sites with high porosity. A substance’s extremely low concentrations. The ecotoxicological effects porous structure enhances its surface area and adsorption capabilities. Carbon nanomaterials are a type of porous and behavior of ECs have yet to be validated by a global rou- tine checking effort [5, 15]. The standard water treatment nanoadsorbent that has a great deal of potential for EC methods are intended to eliminate only the normal contam- removal. They may have been used in place of commercial inants while preserving basic water quality parameters [16]. activated carbon to remove various pollutants. There has been limited investigation towards removing Adsorption-friendly features of carbon nanoadsorbents ECs from aquatic environments. Traditional biological include ordered structure, high porosity, homogeneous pore removal of these contaminants in wastewater treatment size distribution, high specific surface area, chemical and plants is time-consuming and not necessarily successful thermal stability, and nontoxicity. Furthermore, these mate- since not all emergent pollutants can be eradicated. Photol- rials’ surfaces may be altered, making them into functional ysis, sonochemistry, ozonation, ultrasound, solar-powered materials with a greater capacity to remove different con- processes, photo-Fenton, photocatalysis, and electro- taminants. The essential components of the adsorption Fenton have all been studied recently [17]. These technolo- mechanism include interactions, hydrogen bonds, and elec- gies, however, are costly to operate and maintain, and they trostatic interactions. The presence of oxygen-containing require a lot of energy [17]. As a result, there is a need to functional groups in adsorbents also promotes adsorption. provide efficient and cost-effective solutions. Carbon nanotubes (CNTs), graphene, and graphene deriva- The adsorption approach may efficiently cure a wide tives are the principal carbon-based nanoadsorbents identi- range of contaminants. This technique is regarded as the fied which have been researched and critically appraised in most cost-effective, efficient, practical, and ecologically this article. This paper also includes a discussion on EC cat- friendly of the wastewater treatment technologies now in egorization, as well as their history, impacts, and potential use [18–22]. Adsorption is a well-known surface phenome- futures. non that may remove organic and inorganic micropollutants effectively. It is used to remove contaminants from water 2. Potential Sources and Impact on Health and after it has been treated chemically or biologically. Adsorp- Environment Due to ECs tion is becoming more used as a method of eliminating dis- solved pollutants that have withstood chemical oxidation or Sewage, solid waste generated by municipal solid waste col- biological treatment. Several scientists have spent the last lection and treatment facilities, and urban runoff are all decade studying the adsorption of ECs on activated carbon important sources of ECs [1]. There are several ECs in both (AC). Activated carbon (AC) is a porous carbonaceous sub- surface and groundwater. The concentrations of ECs in sur- stance that may be produced chemically or by pyrolysis from face water are typically lower than those reported shortly bamboo, coal, wood, nutshells, and other organic materials. after wastewater treatment facilities discharge. This is caused The source material as well as the method of activation has by dilution and other natural processes. Groundwater con- a large impact on the surface functional groups of ACs. centrations may worsen if the aquifer is contaminated. Envi- Adsorption using granular activated carbon (GAC) is dem- ronmental and water-related physicochemical factors, as onstrated by Rao et al. as a viable tertiary treatment for the well as longitude and latitude, may all influence how ECs simultaneous removal of five PPCPs from an aqueous solu- migrate, where they go, and how they appear in the environ- tion, including three hydrophilic (ciprofloxacin, acetamino- ment. The chemical purity and exposure dosage are influ- phen, and caffeine) and two hydrophobic (benzophenone enced by the source type. Many human actions contribute and Irgasan) PPCPs [23]. Using batch sorption studies and to the destruction of the environment. ECs were discharged commercial granular activated carbon as an adsorbent, the in large quantities into wastewater treatment facilities adsorption of six emerging pollutants from aqueous solu- (WWTPs) from industrial, commercial, and residential tions was investigated. Caffeine, clofibric acid, diclofenac, sources. The EC sources and paths are depicted in gallic acid, ibuprofen, and salicylic acid were chosen as typ- Figure 1. Heavy metals and organic compounds are present ical pollutants [24]. However, the problems in regeneration in sludge produced by physical and chemical processes in and higher pricing of activated carbon restrict its practical WWTPs. Excreta from the human body, as well as Adsorption Science & Technology 3 Animal farming Emerging Manure contaminants Households Runoff water Agriculture Hospital Industry Water bodies Waste water plants Figure 1: EC origin and pathways. abandoned, expired, or unused medications and medicine, with proper drainage, soil, and topography [14]. Pesticide made their way into the environment. A variety of chemical risks are often overlooked by people in developing nations. and microbiological components are non-biodegradable Pesticide usage and ineffective management are to blame. long-term contaminants. These pollutants can be found in Pesticide and biocide concentrations will be higher in the industrial, agricultural, and municipal effluents, as well as absence of monitoring data. industrial smoke [32]. When pharmaceutical waste and Polychlorinated biphenyls are largely suspended solids organic matter disintegrate in neutral conditions, microcon- because of their low vapor pressure, poor water solubility, taminants that might be toxic are produced. These micro- and high octane-water coefficient [33]. As body fat levels pollutants are present in the distribution of drinking water. rise, their half-life lengthens from weeks to months. These Organic contaminants in wastewater rise as leaching pollutants have been associated with neurological and endo- increases. This is harmful to people’s health. Pesticides in crine system malfunction in addition to increasing tumor groundwater can be reduced by replacing ecologically development [33]. The presence or absence of aryl hydrocar- acceptable materials for pesticides in fertilizer. Human bon receptors influences the toxicity of dioxin-like com- excretion, residual medicine disposal, and agricultural usage pounds. As a result of increased environmental awareness, were the chief sources of pharmaceuticals entering the atmo- industrially related synthetic dyes and hazardous wastewater sphere. These drugs were identified in both groundwater and effluents including colors have garnered more attention. The surface water. 90% of pharmaceutically active chemicals most efficient approach to avoid harmful contaminants is (PhACs) reach water bodies, according to research [33]. Pes- through environmental laws and regulations. Synthetic dyes ticide and insect-repellent compounds, lipid regulators, and with structurally diverse molecular structures, such as steroid and sunscreen components are all found in our anthraquinone and anthraquinone-based dispersions and homes and personal care items. Fluoride is a bactericide that metal complexes, are among the most commonly used and is commonly found in toothpaste, shampoo, soap, mouth- ecologically hazardous dyes [1]. Surfactants are synthetic wash, and even skin creams. Triclosan, an antibacterial substances that are widely used in the production of cleaning agent, is commonly included in deodorants and cosmetics agents, emulsions, paints, insecticides, and cosmetics across [33]. Benzophenone and its 2,4-dihydroxybenzophenone the world. Surfactant toxicity is mostly determined by their derivatives are used in sunscreen and UV cosmetic products. capacity to permeate marine cell membranes. Large-scale Because they are designed for outdoor use, most self-care surfactants include linear alkylbenzene sulfonates, lignin sul- products may be cleaned without affecting their structure fonates, fatty alcohol ethoxylates, and alkylphenol or quality. Toxins have a bigger impact at wastewater treat- ethoxylates. ECs have been demonstrated to be hazardous to both ment plants because they are more easily transferred into aquatic habitats [14]. Pesticides are used in farming opera- human and environmental health. Mistakes in glucose tions to reduce the spread of potentially harmful insects, metabolism and infertility have been related to a wide range weeds, and microorganisms such as fungi and bacteria [6]. of health issues. Infertility, pregnancy difficulties (such as Inequity in the use of antibiotics, biocides, and pesticides excessive cholesterol, fetal obesity, and low sperm quality), must be eliminated. Pesticides must be used in order to safe- memory loss and anemia, high blood pressure and apopto- guard the food supply. Biocides and insecticides are com- sis, and a range of other disorders are among these [34]. monly used on farms and in cities. Pesticides and biocides Medication usage has been associated with an increased risk may readily permeate the water supply and affect aquatic life of birth defects and developmental delays, as well as 4 Adsorption Science & Technology hormone imbalances and endocrine system malfunctions. ing water treatment systems used to remove EC. Materials ECs may be accumulating in humans and/or wildlife. To like graphene, carbon nanotubes (CNTs), clay minerals, sili- protect both human health and the environment, ECs and ceous adsorbents, and polymers like polyethylene tere- their adverse consequences must be studied and handled phthalate can replace AC in EC removal. Chemical or further. The deliberate or inadvertent dumping of dangerous thermal modification of the adsorbent’s surface can result chemicals into large bodies of water endangers the environ- in a multifunctional nanoadsorbent with improved capacity ment and human health. for EC absorption. Even at low concentrations (mg/L), nanoadsorbents were able to remove ECs. To top it all off, the nanoadsorbent dosage was small, and the removal time 3. Adsorption for EC Removal for ECs was quick (1–15 min). Figure 2 presents the over- In order to remove ECs, physical, biological, and chemical view of the EC adsorption. methods are used. Physical treatment with no biological or 3.1. Carbon Nanotubes (CNTs). Carbon nanotubes (CNTs) chemical materials has no influence on the biochemical are a potential adsorbent for the remediation of several characteristics of the ECs. Enzymatic breakdown and live ECs due to their large surface area, tiny size, and tremendous organisms are both included in biological treatment. Chem- porosity [11]. Carbon nanotubes (CNTs) have significant ical treatment entails the use of chemical compounds. potential to replace activated carbon in water treatment Adsorption, advanced oxidation processes, biological treat- technologies and are likely to do so in the near future. ments, and membrane separations are some of the most suc- Because of their open structure, CNTs have a larger surface cessful EC removal methods [35–37]. Because of its area, faster access to reactive sites, faster kinetics, and simplicity of use and minimal environmental impact, biolog- improved adsorption capacity [6]. Cost and development ical methods are the most extensively utilized technique. of sustainable production procedures, on the other hand, However, they are less effective due to limited biodegradabil- are hindering the widespread use of carbon nanotubes. The ity. High selectivity, high efficiency, simple processing, no most common CNT forms are single-walled CNTs, multi- need for harsh chemicals, high productivity, cost-effective- walled CNTs, and functionalized CNTs. When it comes to ness, easy posttreatment, and less disruptive are a few of adsorption, the morphologies of carbon nanotubes, such as the advantages of adsorption [38–41]. Adsorption is a sur- tube diameter and bundle shape, are crucial. Smaller carbon face phenomenon where pollutant molecules cling to an nanotubes with bigger specific surface areas and distinctive adsorbent owing to the van der Waals forces and electro- hollow and layered structures have a better potential for static interactions. Adsorbents and adsorbates interact in adsorption than larger carbon nanotubes [6]. As a result, two ways: chemically and physically. Through pores in the adsorbent, the adsorbate diffuses and interacts with the single-walled carbon nanotubes (SWCNTs) are less effective than multiwalled carbon nanotubes (MWCNTs) at adsorb- active sites when it comes into contact with the outer surface ing the adsorbate. The capacity of pollutants to adhere to [13]. In the adsorption process, the adsorbate and the adsor- surfaces is determined by how they interact with one bent’s physical and chemical properties play a major role. another. This implies that each pollutant has a unique capac- For example, changes in pH can affect adsorbent surface ity to adhere to surfaces. groups and pollutant charge [10]. The presence of functional Carbon nanotube surfaces’ wettability and hydrophilicity groups such as hydroxyl and carboxyl groups makes the are improved by adding functional groups [44–46]. Oxygen- adsorbent highly effective [13, 14]. Natural adsorbents like containing groups, such as hydroxyl, carbonyl, or carboxylic, clay and sand are ideal for adsorption since they are abun- are found in functionalized carbon nanotubes. To add func- dant and cheap. Industrial waste adsorbents can encapsulate a material in another substance. tional groups to carbon nanotubes, sulfuric acid (H SO ) 2 4 and nitric acid (HNO ) can be utilized. Carbonyl groups Activated carbon (AC) has been extensively studied for and oxygen levels on the surface of MWCNTs have been EC removal. However, adsorption using AC is expensive shown to have a significant influence on their maximal since activated carbon is seldom recovered. Normally, less adsorption capacity [45]. As a consequence, researchers cre- than 40% of the AC impregnated is reused. These factors sig- ated modified carbon nanotubes for use as an adsorbent. The nificantly limit the use of AC [42]. Biochar (BC) is a stable researchers employed oxidized MWCNTs in conjunction source of carbon that is produced by thermal or aqueous with a range of oxygen molecules to adsorb the antibiotic processes in low- or no-oxygen environments. It increases medication tetracycline from aqueous settings [46]. The the surface activity, porosity, and utility of biochars. Some Langmuir model calculated the maximum adsorption capac- biochars may be confused with activated carbon due to their ity (q ) of carbon nanotubes with 2.0%, 3.25%, 4.75%, and similarity. It is said that BC composites treated with nano- max particles enhance pollutant absorption. BC’s corrosive treat- 5.95% oxygen to be 217.8, 269.25, 217.56, and 210.43 mg/g, ment promotes oxygenated surface groupings [43]. ECs (like respectively. Another investigation validated the impact of raising the oxygen content from 2.0 to 5.9% on the sorption tetracycline and endocrine-disrupting compounds) can be taken up by modified biochar through hydrophobic, electro- limit of carbon nanotubes for ciprofloxacin expulsion [11]. Using the Langmuir isotherm model, q was calculated static, hydrogen bonding, and functional groups [1]. max Nanoadsorbents are adsorbents with a diameter of a few to be 150.6, 178.9, 206.0, and 181.2 mg/g for carbon nano- nanometers. Despite its limited application in industrial tubes containing 2.0%, 3.2%, 4.7%, and 5.9% oxygen, respec- adsorption, nanotechnology has great promise for improv- tively [43]. Adsorption of anti-infection medicines Adsorption Science & Technology 5 Personal care Industrial chemicals Trace metals chemicals Particulates House hold chemicals Pharmaceuticals Soaps, Disinfection chemicals Engineered nano Shampoos, Food preservatives Antibiotics, Surfactants, Particles, micro Tooth pastes, Pesticides Anti inflammatory Plastics UV filters Plasticizers Substances 𝜋-𝜋 Hydrogen interactions bonding EC molecules Adsorption Hydrophobic Electrostatic effects interaction Adsorbent Figure 2: Overview of EC adsorption. norfloxacin and ofloxacin onto functionalized carbon nano- on the attachment of CPX to MWCNTs. Ionic strength, on tubes has been studied [47]. The MWCNT and SWCNT the other hand, had no effect on CPX’s capacity to adsorb were altered to add beneficial groups such as hydroxyl onto MWCNTs. Electrostatic interactions appear to have a (-OH) and carbonyl (CO). significant role in adsorption [46]. Single-walled carbon In the adsorption of the antibiotics ofloxacin (OFL) and nanotubes were employed to remove bisphenol A (BPA) norfloxacin (NOR), there is a significant link between the and 17-estradiol (E2) from aqueous systems without and adsorption coefficients and the specific surface area of CNTs. with ammonium persulfate treatment. DFT calculations It is probable that structural properties had a significant revealed that two chemicals interact with sorbent structures. influence on the adsorption of OFL and NOR on CNTs via According to adsorption energy estimates, both sorbents an electron donor-acceptor mechanism [47]. Tetracycline preferentially adsorb E2 over BPA. The optimum geometric adsorption on MWCNTs is influenced by surface character- orientation of molecules in contact can have a significant istics and solution chemistry. The adsorption capacity and impact on adsorption behavior [48]. Ahmaruzzaman et al. coefficient of adsorption of tetracycline increased linearly synthesized CNTs from sunflower oil, a readily accessible with the surface oxygen concentration of MWCNTs. Water bioprecursor, which was then coated with SnO nanoparti- clusters formed during tetracycline adsorption due to the cles using Coccinia grandis extracts. The generated nano- dispersibility of the nanotubes. This contact is assumed to heterojunction displayed outstanding performance against be the source of the problem. Furthermore, interparticle arsenic, with a maximum adsorption capacity of and boundary layer diffusion might influence total tetracy- 106.95 mg/g. Furthermore, the SnO -CNT nano- cline adsorption onto 3.2% oxygen-containing carbon nano- heterojunctions showed catalytic activity in the reduction tubes. When the pH was between 3.3 and 8.0, the majority of of 4-nitrophenol [29]. tetracycline could adhere to carbon nanotubes. This CNTs may be used to make structures such as a one- occurred when water clusters, or H-bonds, formed on the dimensional hollow tube shape. Depending on the quantity carbon nanotubes [44]. of graphene layers, single-walled and multiwalled carbon Oxidized multiwalled carbon nanotubes were utilized as nanotubes can be created. Many pollutants are adsorbed adsorbents in a study to investigate the effect of oxygen con- on the surface of carbon nanotubes [11, 44–46]. CNTs can centration on the adsorption capabilities of ciprofloxacin have their base or sidewalls modified with different (CPX) [46]. The rise in oxygen content from 2.0% to 5.9% oxygen-containing functional groups to improve the surface appears to be increasing CPX’s adsorption capacity. The properties. MCNT, a magnetic material consisting of carbon interaction of electron donors and acceptors has been iden- nanotubes, has become a popular approach for enhancing tified as the fundamental reason for the lower expansion separation and purification efficiency. For example, because rate. The increased hydrophilicity and dispersion of the of its large surface area and capacity to be regenerated, adsorbent, as well as the suppression of water clusters, MCNT is perfect for the rapid separation of various environ- enabled CPX adsorption on oxidized MWCNTs. The alka- mental media. Sulfamethoxazole, carbamazepine, and keto- line atmosphere was demonstrated to have a negative impact profen were among the pharmaceutical pollutants that Emerging contaminants 6 Adsorption Science & Technology could be removed from water using carbonaceous adsor- ufactured utilizing both techniques [27]. The issue with gra- bents containing doped phosphorus (P). Adsorbents dem- phene is that due to its hydrophobicity, it is difficult to onstrated high removal rates (>99%) for all substances recycle. Graphene is also ineffectual for polar component tested. Adsorption was primarily controlled by π‐π and n- adsorption with hydrophilic chemical groups. Khalil et al. EDA interactions as well as H-bonds. Metal ions were dem- used porous graphene (PG) to extract the medicines atenolol onstrated to have no influence on the removal of pharma- (ATL), ciprofloxacin (CIP), carbamazepine (CBZ), gemfi- ceutical pollutants [34]. Table 1 summarizes the studies on brozil (GEM), diclofenac (DCF), and ibuprofen (IBP) from CNT adsorbents used to clear up EC. aqueous solutions. At trace concentrations, low PG dosages Even though carbon nanotubes are well known in many (100 mg/L) resulted in quick response times and high clear- industries and have significant promise for environmental ance efficiencies for all studied EC. EC mixes were examined remediation, there are a number of factors that prevent them to evaluate if PG might be utilized for tertiary therapy. from being employed more broadly. Scientists must cope Increasing the quantity of PG in water and wastewater sam- with production costs, toxicity, and environmental dangers. ples can aid in the removal of mixed ECs [56]. Table 2 sum- It is expected that CNTs can have safety criteria and risk marizes the studies on graphene adsorbents used to remove evaluations to determine how safe they are to use, which EC. might lead to additional CNT uses in the near future. Toxic organic contaminants are effectively removed by graphene and its functionalized compounds. The most sig- 3.2. Graphene-Based Adsorbents. Many scientists believe that nificant constraints of GO and GO-based nanomaterials graphene and graphene-based nanomaterials are the ideal are their high cost and difficulties in reusing. Because of options for water purification because of their high surface the nanomaterial’s high electrostatic interactions, reuse may be impossible. Only a few researchers have looked at area-to-volume ratio and other physical features, such as their capacity to receive electrons and resist pollutants [35, the reusability of graphene-based EC adsorbents. More 54]. According to the literature, nonelectrostatic interactions study is needed to determine how graphene impacts human are the primary means by which graphene-based nanomate- health and the environment. rials remove pollutants. Several researchers have attempted to alter the surface of graphene in order to make it more effi- 3.3. Miscellaneous Carbonaceous Nanoadsorbents. Fuller- cient and simpler to reuse [27, 35, 54, 55]. For the majority enes, carbon nanospheres, and carbon nanofibers are the of the ECs studied, reduced graphene oxide and graphene other carbon-based nanoadsorbents that have recently been were found to have lower adsorption capabilities than gra- employed for EC adsorption. The major contrast between phene oxide. This is because the surface has grown more CNTs and fullerenes is the carbon form. Fullerenes are fre- hydrophobic, with fewer oxygen functional groups, making quently found as hexagonal rings containing carbon atoms. it more difficult for ECs in water to adhere to it. With a Fullerene’s properties have been effectively used to increase wider surface area, ECs may adsorb in more places. As a its utility in the environmental domain. The detection and result, the material’s adsorption capability can be increased. capture of carbamazepine in an aqueous media were investi- Because of its delocalized electrons and vast surface area, gated theoretically using fullerene and its derivatives doped graphene is suited for the removal of organic compounds with B, Al, Ga, Si, Ge, N, and P. The fullerene derivatives comprising benzene rings and π‐π stacking. GO suspension doped with Al, Si, and Ga are the strongest candidates for was used to eliminate tetracycline, a prescription antibiotic. serving as sensors and uptaking carbamazepine in aquatic Tetracycline’s four aromatic rings each have a distinct func- conditions, according to DFT simulations [61]. Mesoporous tional group, such as phenol, aldehyde, ketone, and amino. It carbon nanospheres (MCNs) were employed to efficiently adheres to the GO surface via two mechanisms: interaction remove methyl orange (MO), rhodamine (RhB), and p- and cation bonding. The Langmuir model predicts a maxi- hydroxybenzoic acid (p-HBA), with a removal efficacy of mum adsorption capacity of 313 mg/g. When pH or Na more than 95% [62]. The greatest removal efficacy for tetra- concentrations were increased, tetracycline adsorption on cycline hydrochloride (TCH) and ciprofloxacin hydrochlo- GO was decreased [54]. ride (CPH) for hollow mesoporous carbon spheres Three-dimensional chitosan-gelatin aerogels containing (HMCSs) generated and modified for laccase (Lac) immobi- GO are mixed in two ways: coating and embedding. Toler- lization was 99.4% and 96.9%, respectively [63]. A zinc 2+ ance to lead (Pb ) was assessed, as well as its effectiveness oxide-coated carbon nanofiber composite was used as an against the fluoroquinolonic medications ofloxacin and cip- adsorbent to extract amoxicillin from ambient water matri- rofloxacin. Coating and embedding techniques demon- ces. The maximal adsorption capacity was determined to strated only a small influence on organic contaminant be 156 mg/g based on the results. Furthermore, the adsor- adsorption capacity, which varied from 5 to 8 mg/g, whereas bent was successfully tested on actual wastewater samples chitosan-gelatin control aerogels without GO showed no and shown to be reusable for up to fifteen cycles [64]. Acti- adsorption [27]. Kovtun et al. used coating and embedding vated carbon, multiwalled carbon nanotubes, and carbon processes to incorporate GO into three-dimensional nanofibers have been used to remove atenolol, caffeine, chitosan-gelatin aerogels. The fluoroquinolonic medications diclofenac, and isoproturon from ultrapure water and a 2+ ofloxacin and ciprofloxacin, as well as lead (Pb ), were used municipal wastewater treatment plant effluent [65]. A mag- to evaluate the produced adsorbents. There was just a little netic carbon nanofiber (MCF) composed of bacterial cellu- variation in pollutant removal between the adsorbents man- lose absorbed diclofenac from water. MCF is a porous Adsorption Science & Technology 7 Table 1: Removal of emerging contaminants by CNT-based adsorbents. Adsorbent Pollutant Adsorption capacity Reference Significant findings (i) 86% removal efficiency after 5 regeneration cycles (ii) Multifunctionality: catalytic effectiveness against SnO -CNT As (III) 106.95 mg/g [29] 4-nitrophenol, alizarin red S dye, and metronidazole pollutants. Antimicrobial activity against bacterial and fungal strains Tetracycline [44] Oxidized multiwalled carbon nanotubes with Adsorption capacity of CNTs-2:0%O < CNTs‐3:2%O qm/SSA continued to Ciprofloxacin different oxygen > CNTs‐4:7%O > CNTs‐5:9%O. [46] increase with increasing (CPX) contents oxygen content SWCNT, acidified Bisphenol A ammonium persulfate BPA: 19.4 mg/g and 8 mg/g, respectively, for SWCNT (BPA), 17β- [48] treated SWCNT (t- and t-SWCNT; E2: 27.2 mg/g estradiol (E2) SWCNT) MWCNTs with 15nm, 30nm, 50nm and Ofloxacin The structural and SWCNTs (hydroxyl (OFL) and hydrophobic characteristics [47] functionalized, carboxy norfloxacin of OFL and NOR influenced functionalized, and (NOR) their adsorption pure) SWCNT adsorbed more IBU and TCS than MWCNT; Ibuprofen IBU adsorption was higher SWCNTs and (IBU) and For SWCNT, IBU at pH 7: 232 mg/g; TCS at pH 7: [49] at pH 4, but TCS adsorption MWCNTs triclosan 558 mg/g was higher at pH 7; CNT (TCS) surface oxidation decreased adsorption CNT absorbed more CPX than activated carbon and Ciprofloxacin carbon xerogel; however, MWCNTs 150 mg/g [50] (CPX) oxidation and heat treatment had little effect on CNT adsorption DWCNTs adsorbed better than SWCNTs and SWCNTs and Perchlorate MWCNTs; the presence of 3.55 mg/g [51] MWCNTs (ClO ) additional ClO oxygen- 4 4 containing functional groups increased adsorption The adsorption of BPA and Bisphenol A E2 varied from 7.3 to 95% SWCNTs in the (BPA) and depending on the solution presence of natural [52] 17β-estradiol pH and the presence or organic matter (NOM) (E2) absence of NOM and SWCNTs Mixture of The adsorption was made MWCNT carboxyl four linear possible through 168 mg/g [53] functionalization alkyl benzene hydrophobic contact and the sulfonates creation of hydrogen bonds (mesopores and macropores) material having a specific sur- struct three-dimensional macrostructures. The adsorbent face area of 222.3 m /g. The diclofenac elimination was effec- was more effective at eliminating oxytetracycline (1729 mg/ tive (93.2%) and quick (20 min) [66]. Self-assembling two- g) and diethyl phthalate (680 mg/g) [67]. The literature stud- dimensional graphene oxide nanosheets and one- ies show that ECs can be successfully removed by adsorption dimensional carbon nanotubes were used to readily con- tests using fullerenes, carbon nanospheres, and carbon 8 Adsorption Science & Technology Table 2: Removal of emerging contaminants by GO-based adsorbents. Adsorbent Pollutant Adsorption capacity Reference Significant findings 5-8 mg/g for antibiotics for both Embedded GO Antimicrobial effects were found 2+ adsorbents. For Pb : 11.1 mg/g 2+ aerosols. Coated Ofloxacin, ciprofloxacin, and Pb [27] particularly for the GO-coated for embedded GO aerogels and GO aerosols aerogel materials 1.5 mg/g in coated GO ones Tetracycline strongly deposited on Graphene oxide Tetracycline antibiotics 313 mg/g [54] the GO surface via π‐π interaction (GO) and cation–π bonding. (i) Regeneration and reuse for four Atenolol (ATL), ciprofloxacin cycles Nanostructured (CIP), carbamazepine (CBZ), 8.87, 7.33, 14.63, 47.85, 91.59, [56] (ii) Heterogeneous adsorption porous graphene ibuprofen (IBP), diclofenac (DCF), and 9.26 mg/g, respectively described by the Toth and Sips and gemfibrozil (GEM) isotherm models Graphene oxide Metformin 96.7 mg/g [57] Graphene oxide Carbamazepine (CBZ) 9.2 mg/g [58] Could be reused for up to 8 times nanoplatelets According to DFT studies, the adsorption process is mostly Graphene oxide Acetaminophen (ACP), accompanied by size-related composite with carbamazepine (CBZ), bisphenol 13.7, 11.2, 13.2, 14.8, and diffusion, with a modest [59] activated carbon A (BPA), caffeine (CAFF), and 14.5 mg/g, respectively contribution from a synergetic mix and chitosan triclosan (TCS) of hydrophobic/hydrophilic, hydrogen bonding, electrostatic, and π‐π interactions Reduced graphene oxide (rGO)–cellulose Methylene blue 17 mg/g [60] nanocrystal sponge nanofibers. However, these studies provide a scant descrip- pKa, making it an important factor in organic molecule tion of adsorption processes and place little focus on adsor- adsorption [10]. This can be aided by increasing the pH, bent reusability. These materials have the potential to be which changes the interactions between adsorbents and sor- extremely useful for EC adsorption. bates by changing their hydrophobic and electrostatic prop- erties [44, 49]. A higher pH may also increase the ability of the adsorbate to donate electrons, potentially improving 3.4. Adsorption Mechanism and Influencing Factors. Polar organic molecules, such as carbon-based nanoadsorbents, the overall electron donor-acceptor interaction. The pH of the carbon nanotube surface can affect the protonation state exhibit hydrophobic effects, π-π interactions, hydrogen bonds, covalent bonds, and electrostatic interactions [44]. of the tetracycline molecule and the hydrophobicity of the The π-π interaction dominated benzene ring adsorption. adsorbate, thereby influencing adsorption interactions [54]. Triclosan, for example, has two aromatic rings and is thus The adsorption of tetracycline on GO varied greatly between more compatible with the CNT surface than ibuprofen pH ranges of 3 and 11. Tetracycline’s adsorption capacity varies with initial concentration. When the adsorption [49]. Electrostatic interactions can greatly aid adsorption. Depending on the pH of the solution, functional groups con- capacity falls to 133.62 mg/g, three times as much, tetracy- taining oxygen can be protonated or deprotonated [49]. Car- cline is removed. Tetracycline’s adsorption capacity decreased eightfold and fourteenfold over the pH range, bon materials absorb hydrophobic organic molecules as a result of hydrophobic interactions. Adsorption is most effec- depending on the initial concentration. Because adsorption and adsorption are electrostatically repelled, an increase in tive in carbon materials with a net charge density of zero. Furthermore, the benzene ring on the surface of carbon ionic strength facilitates adsorption. Increased ionic strength nanotubes can serve as an electron donor for organic mole- can make organic molecules more likely to precipitate from cules containing oxygen-containing functional groups [44, aqueous solutions and bind to nanoadsorbents. Different 49]. This enables hydrogen bonds to form. Adsorption is a concentrations of NaCl were added to the tetracycline and GO solutions to investigate the effect of ionic strength on good way to get rid of ECs in water because they have a lot of aromatic rings and a specific chemical makeup [18]. adsorption capacity. The adsorption capability decreases The pH of the solution affects the protonation and when NaCl is added. Tetracycline’s adsorption capability was reduced by more than half when NaCl concentrations deprotonation of pollutants, which is dependent on their Adsorption Science & Technology 9 were increased to 100 mmol/L. The ability of NaCl to bind to Data Availability tetracycline varies little between 8.33 mg/L and 33.33 mg/L All the data is available in the manuscript. in the range of 20-100 mmol/L [54]. Additional Points 4. Current and Future Challenges Highlights. (1) Adsorption is the most preferred method for There is a possibility that nanomaterials could remove EC removing ECs. (2) The use of nanoadsorbents may increase from water in an effective manner. It is possible that limiting the adsorption efficiency of ECs. (3) Carbon nanotubes, gra- the use of adsorbents will result in the creation of new phene, and their derivatives have the potential to replace the sources of pollution. It is difficult to develop adsorbents of commercially available adsorbent activated carbon, which high quality today that can be utilized in the most modern has limitations. (4) Functional modifications will play a water treatment processes. The adsorbents are susceptible major role in improvising the uptake of emerging to change if functional groups or structures are introduced contaminants. into the mix. This approach is more effective than others in the elimination of pollutants. 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