Nanotechnology as an efficient and effective alternative for wastewater treatment: an overview

Rapid population growth, urbanization, accelerated development of industrialization, and agricultural practices contribute to the continued degradation of quality water supplies, resulting in a severe global problem (Rathi & Kumar 2021). Drinking water scarcity tops the list of today's most pressing environmental issues (Singh et al. 2022). About 25% of large cities experience some level of water stress, and 2.2 billion people worldwide do not have access to this vital liquid (Salehi 2022). The above issues call for the recycling and reclamation of wastewater. Reclaimed or treated wastewater should be used in the agricultural, industrial, and public use sectors, and treatments should be carried out according to the type of wastewater being treated and its degree of contamination (Baskar et al. 2022).

Wastewater contains different pollutants, such as heavy metals, antibiotics, pesticides, endocrine disruptors, dyes, and polycyclic aromatic hydrocarbons. These pollutants adversely affect the environment and human health (Morin-Crini et al. 2022). Various technologies, such as adsorption, chemical precipitation, photodegradation, and reverse osmosis, have been developed to solve this problem. However, these treatments have drawbacks, including high operating and implementation costs, low removal efficiency, and toxic residues (Sikiru et al. 2022).

Nanotechnology is one of the processes that have attracted great interest and are innovative for wastewater treatment (Marimuthu et al. 2023). Different nanomaterials have been targeted for reducing concentrations of toxic pollutants in wastewater due to their superior properties derived from the nanoscale effect. Nanomaterials possess enhanced reactivity, excellent adsorption properties, and a high surface-to-volume ratio (Nasrollahzadeh et al. 2021). Nanomaterials offer unparalleled opportunities to develop more efficient catalysts and redox systems for wastewater purification due to their ease of functionalization, quantum effect, high free-state mobility (Zahid & Abd-Elsalam 2021), and low energy consumption (Sheoran et al. 2022), additionally, development of various nano adsorbents, nanocatalysts, and nanomembranes has led to significant advances in wastewater remediation (Boulkhessaim et al. 2022).

Some reviews on the use of nanomaterials in water treatment have recently been published in the state of the art. Rajendran et al. (2022) studied adsorbent materials with nanotechnology-based techniques for removing heavy metals in wastewater. This article described the processing of these nanomaterials and their feasibility for detoxifying sewage by removing metal ions. Saravanan et al. (2022) critically reviewed nanomaterial synthesis, mainly green synthesis, and its applications in wastewater treatment to remove harmful pollutants such as dyes, heavy metals, and pesticides. Kumar & Kumar (2023) provided detailed information on producing cellulose-based nanomaterials for large-scale remediation of heavy metals in wastewater. The authors discussed nanocellulose, nanoxylan, and nanolignin production methods and their surface modification for heavy metal adsorption.

This review discusses different types of nanomaterials, such as metal oxides, carbon-based nanomaterials, and cellulose-based nanomaterials, for wastewater treatment. A brief characterization of the wastewater, its primary distribution sources, the waste generated, and an overview of the different types of pollutants present in this wastewater is made. This article also discusses the challenges of using nanomaterials in wastewater treatment and the effects they may have on the environment and human health.

Domestic wastewater includes materials added by a community during or after use. Therefore, they contain human body wastes and residues (Gupta et al. 2022). Domestic wastewater generally presents an objectionable appearance and dangerous consequences due to its content of pathogenic microorganisms.

Industrial processes also generate wastewater, which contains toxic and hazardous pollutants (Sousa et al. 2018) due to the products and production methods employed: industrial wastewater, insoluble and suspended solids, organic compounds, nutrient salts (⁠⁠, ⁠, and ⁠), corrosive substances (alkalis, solvents, and oil), cleaning agents, lubricants, disinfectants, and other hazardous substances (hydrocarbons, chlorinated molecules, organic halogen compounds, cyanides, and heavy metals) (Rosenwinkel et al. 2005; Barik et al. 2020). Among the industries that produce wastewater with a substantial number of pollutants are the textile industry, food processing industry, steel plants, coal mines, and the pharmaceutical industry. Each industry has terms and conditions for wastewater flow fraction discharged to the environment.

Agricultural activities are another vital source of wastewater. The main components of these agricultural wastewaters are toxic chemicals in the composition of fertilizers, pesticides, animal wastes, irrigation residues, residual veterinary drugs, and antibiotics (Wei et al. 2018), which variously affect surface waters. Agricultural effluents have high nitrogen, phosphorus, and chemical oxygen demand (COD) content which poses several environmental threats, so the values of these parameters should be reduced before discharging these effluents to water sources.

Wastewater can also come from other sources such as hospitals, dairy plants, and animal confinements, from street and parking lot waters that are caused by storms (storm sewage); these can include oil, fuel, insecticides, herbicides, pathogenic organisms, and residual sediments (Abedi-Koupai et al. 2006).

Preserving and protecting the environment is currently a significant challenge for humanity. In recent years, growing environmental awareness has intensified efforts to treat, monitor, and reuse non-conventional water sources, mainly wastewater, through efficient and high-end purification methods. The increasing presence of polluting macromolecules in wastewater requires introducing new technologies to degrade these macromolecules into smaller, less toxic molecules.

The most common residues, such as tires, branches, containers, plastics, balls, bags, and clothing, are removed in the primary phase through screening. For collection and removal, the waste is passed through a steel mesh with an opening called a light (Bayo et al. 2022). Once collected, they are valued, even recycled, or transformed by pyrolysis into an energy source, generating extensive use. In the case of RPBI, it can also be treated by screening and later final disposal in special containers for biological waste or materials that have been in contacts, such as gauze, gloves, bags, hoses, and sharps.

For its part, the secondary stage of wastewater treatment involves the degradation of polluting compounds and the death of microorganisms by physical, chemical, or biological methods. Once the process is generated, residual sludge or biosolids are obtained that are treated by biodegradation, aerobic, anaerobic, fertilization, and thermal depolymerization to obtain fertilizers or alternative sources of energy and valuable resources (Kumar et al. 2023).

The tertiary stage of the treatment train is carried out to remove heavy metals and some chemical remains. In the case of heavy metals, they can be removed by coprecipitation by coagulation. In this step, compounds such as iron salts, aluminum sulfate, and calcium oxide generate flocs where the metals adhere. Likewise, size exclusion filtration is implemented, where micro and manometric particles can be retained by a mesh with pores more minor than the size of the contaminant, thus achieving its removal. Finally, the hazardous waste obtained must be confined in special containers to avoid its reincorporation by leaching into natural bodies of water (Bayo et al. 2022).

Man plays an essential role in the issue of pollutants released in wastewater, either by industrial discharges, agricultural practices, or water use by the general population. Depending on their nature and basis, these are classified into inorganic, biological, and organic (Gupta et al. 2012). Table 1 shows a summary of the primary pollutants.

Table 1

Types of contaminants present in wastewater

Contaminant .		Properties .	Examples .	Distribution and transport routes .	Adverse effects on the environment and living beings .	Reference .
Biological		The primary organisms found in wastewater are fungi, bacteria, viruses, and protozoa. Bacteria are the most common microbial contaminants; most are of fecal origin.	Salmonella Klebsiella Escherichia coli Poliovirus Rotavirus Enterovirus Cryptosporidum ssp. Giardia ssp. Entamoeba histolytica	Human or animal excrement	Diarrhea, renal failure, colitis. Homolytic uremic syndrome. Diseases such as typhoid fever and salmonellosis. Legionnaire's disease.	Ahmed et al. (2016), El-Sayed (2020), Fatima (2021)
Inorganic		Heavy metals have a specific density greater than 5 g/cm3 and atomic weights between 63.5 and 200.6. Bioaccumulative. Toxic.	Cadmium (Cd) Lead (Pb) Chromium (Cr) Zinc (Zn) Mercury (Hg)	Industries in which electroplating and surface treatment are performed	Heavy metals increase susceptibility of the body to infections. Change in the synthesis and application of neurotransmitters in the body. Generation of reactive oxygen species (ROS) that can cause oxidative stress.	Verma & Dwivedi (2013), Akpor et al. (2014), Chuang et al. (2014), Kumar & Gopal (2015), Fowler & Galán (2018), Rajasurya & Surani (2019), Fu & Xi (2020)
Organic		Pesticides play a fundamental role in food safety. Depending on their use in agriculture, they can be classified into fungicides, insecticides, and herbicides and, according to the target organism, into algaecides, acaricides, and rodenticides. Bioaccumulative and persistent.	Glyphosate Dimethoate Parathion Malathion N-methylcarbamate DDT Endosulfan Aldrin Dieldrin Mirex Lindane	Subsurface drainage, leaching, runoff, and spray drift	Diabetes mellitus, respiratory disorders, neurological disorders, reproductive syndromes (sexual/genital), and oxidative stress. Development of cancers in both children and adults.	Zhou et al. (2015), Nsibande & Forbes (2016), Huang et al. (2018), Cosgrove et al. (2019), De Souza et al. (2020), Mojiri et al. (2020)
		Endocrine disruptors are artificial chemicals, and natural or synthetic exogenous agents can cause endocrine disruption in animals and humans and damage reproductive health.Bioaccumulative and persistent	4-nonylphenol (4-NP) 4-tert-octylphenol (4-t-OP) 4-alpha-coumylphenol (4-α-CP) Bisphenol A (BPA)	Air, water, food and food packaging, personal care products, household goods, detergents, fabrics and upholstery, medical equipment, and electronic products	Permanent and profound changes in the development of the nervous system. Deterioration of mental performance, together with alterations in the reproductive endocrine system.Alteration in the weight, height, and nutritional status of the newborn.Modification of the functioning of the endocrine system in natural populations.	Gross (2007), Scholz & Klüver (2009), Zhou et al. (2011), Wang et al. (2012), Janicki et al. (2016), Bliatka et al. (2017), Tursi et al. (2018), Encarnação et al. (2019), Er et al. (2019), Zhou et al. (2019a, 2019b)
		Pharmaceutical and personal care products are used by individuals for personal health or cosmetic reasons or used by agribusiness to improve the growth or health of livestock. They are considered emerging contaminants. Recalcitrant and persistent.	Illegal drugs Human and veterinary drugs (over-the-counter and prescription), as well as their downstream metabolites and conjugates Sunscreens Fragrances Soaps Moisturizers Lipsticks Insect repellents Shampoos	Wastewater, treated or untreated	Acute and chronic damage to humans.It affects plant development through two mechanisms of action: i) direct damage to the plant (reduction of the number and size of mature leaves, reduction of photosynthetic pigments, adverse effects on plant growth and development, inhibition of root elongation) ii) modification of plant-microorganism symbiosis and nutrient cycling in soils.	Cizmas et al. (2015), Dhodapkar & Gandhi (2019), Xie et al. (2019)
		Polycyclic aromatic hydrocarbons are ubiquitous organic pollutants produced by anthropogenic activities associated with industrialization and urbanization and by natural sources. Their chemical structure consists of two or more aromatic rings fused without heteroatoms, with some alkyl substitutions. They are toxic pollutants with mutagenic and bioaccumulative properties.	Acenaphthene AcenaphthyleneAnthracene Benzo(a)anthracene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(g,h,i)perylene Benzo(a)pyrene Chrysene Dibenzo(a,h)anthracene Fluorene Fluoranthene Indeno[1,2,3-cd] Pyrene Naphthalene Phenanthrene Pyrene	Stormwater, sediments, sewage, and crustaceans	Decrease the number of leaves, root length, biomass, and plant germination. Damage to human body organs. Carcinogenic.	Bojes & Pope (2007), Ma et al. (2010), Olivella et al. (2011), Hussain et al. (2018), Ozaki et al. (2019), Zhou et al. (2019a, (2019b), Onydinma et al. (2021), Sajid et al. (2021)
		Surfactants are used in numerous industrial processes. An essential cleaning product. They are classified into cationic detergents, anionic detergents, zwitterionic detergents, and nonionic detergents. Resistant to biodegradation and bioaccumulative.	Methylestersulphonate Olefinsulphonate Alkyl benzene sulphonates Alkyl ether sulphates Isotridecanolethoxylates Benzalkonium chloride N-hexadecyltrimethy Ammonium chloride	Wastewater treatment plants are partially aerobically degraded and partially absorbed by sewage sludge.	Decreased adsorption of dissolved oxygen by aquatic organisms. Modification of temperature, salinity, turbidity, and pH. The gradual destruction of soil structure. Decrease in plant germination.	Bianchetti et al. (2015), Giagnorio et al. (2017), Mousavi & Khodadoost (2019), Lu & Astruc (2020)
		Chromophores and auxochromes form synthetic aromatic compounds called dyes. Toxic, non-biodegradable nature and complex structures.	Congo red Rose bengal Crystal violet Indigo carmine Acid orange Methyl orange	Textile industry	Carcinogenic substances. Disturbance of gas solubility, which in turn affects the gills of aquatic organisms. Modification of photosynthesis.	Jain & Sikarwar (2008), Carmen & Daniela (2012), Padhi (2012), Shanker et al. (2017), Tara et al. (2020), Elgarahy et al. (2021)

Contaminant .		Properties .	Examples .	Distribution and transport routes .	Adverse effects on the environment and living beings .	Reference .
Biological		The primary organisms found in wastewater are fungi, bacteria, viruses, and protozoa. Bacteria are the most common microbial contaminants; most are of fecal origin.	Salmonella Klebsiella Escherichia coli Poliovirus Rotavirus Enterovirus Cryptosporidum ssp. Giardia ssp. Entamoeba histolytica	Human or animal excrement	Diarrhea, renal failure, colitis. Homolytic uremic syndrome. Diseases such as typhoid fever and salmonellosis. Legionnaire's disease.	Ahmed et al. (2016), El-Sayed (2020), Fatima (2021)
Inorganic		Heavy metals have a specific density greater than 5 g/cm3 and atomic weights between 63.5 and 200.6. Bioaccumulative. Toxic.	Cadmium (Cd) Lead (Pb) Chromium (Cr) Zinc (Zn) Mercury (Hg)	Industries in which electroplating and surface treatment are performed	Heavy metals increase susceptibility of the body to infections. Change in the synthesis and application of neurotransmitters in the body. Generation of reactive oxygen species (ROS) that can cause oxidative stress.	Verma & Dwivedi (2013), Akpor et al. (2014), Chuang et al. (2014), Kumar & Gopal (2015), Fowler & Galán (2018), Rajasurya & Surani (2019), Fu & Xi (2020)
Organic		Pesticides play a fundamental role in food safety. Depending on their use in agriculture, they can be classified into fungicides, insecticides, and herbicides and, according to the target organism, into algaecides, acaricides, and rodenticides. Bioaccumulative and persistent.	Glyphosate Dimethoate Parathion Malathion N-methylcarbamate DDT Endosulfan Aldrin Dieldrin Mirex Lindane	Subsurface drainage, leaching, runoff, and spray drift	Diabetes mellitus, respiratory disorders, neurological disorders, reproductive syndromes (sexual/genital), and oxidative stress. Development of cancers in both children and adults.	Zhou et al. (2015), Nsibande & Forbes (2016), Huang et al. (2018), Cosgrove et al. (2019), De Souza et al. (2020), Mojiri et al. (2020)
		Endocrine disruptors are artificial chemicals, and natural or synthetic exogenous agents can cause endocrine disruption in animals and humans and damage reproductive health.Bioaccumulative and persistent	4-nonylphenol (4-NP) 4-tert-octylphenol (4-t-OP) 4-alpha-coumylphenol (4-α-CP) Bisphenol A (BPA)	Air, water, food and food packaging, personal care products, household goods, detergents, fabrics and upholstery, medical equipment, and electronic products	Permanent and profound changes in the development of the nervous system. Deterioration of mental performance, together with alterations in the reproductive endocrine system.Alteration in the weight, height, and nutritional status of the newborn.Modification of the functioning of the endocrine system in natural populations.	Gross (2007), Scholz & Klüver (2009), Zhou et al. (2011), Wang et al. (2012), Janicki et al. (2016), Bliatka et al. (2017), Tursi et al. (2018), Encarnação et al. (2019), Er et al. (2019), Zhou et al. (2019a, 2019b)
		Pharmaceutical and personal care products are used by individuals for personal health or cosmetic reasons or used by agribusiness to improve the growth or health of livestock. They are considered emerging contaminants. Recalcitrant and persistent.	Illegal drugs Human and veterinary drugs (over-the-counter and prescription), as well as their downstream metabolites and conjugates Sunscreens Fragrances Soaps Moisturizers Lipsticks Insect repellents Shampoos	Wastewater, treated or untreated	Acute and chronic damage to humans.It affects plant development through two mechanisms of action: i) direct damage to the plant (reduction of the number and size of mature leaves, reduction of photosynthetic pigments, adverse effects on plant growth and development, inhibition of root elongation) ii) modification of plant-microorganism symbiosis and nutrient cycling in soils.	Cizmas et al. (2015), Dhodapkar & Gandhi (2019), Xie et al. (2019)
		Polycyclic aromatic hydrocarbons are ubiquitous organic pollutants produced by anthropogenic activities associated with industrialization and urbanization and by natural sources. Their chemical structure consists of two or more aromatic rings fused without heteroatoms, with some alkyl substitutions. They are toxic pollutants with mutagenic and bioaccumulative properties.	Acenaphthene AcenaphthyleneAnthracene Benzo(a)anthracene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(g,h,i)perylene Benzo(a)pyrene Chrysene Dibenzo(a,h)anthracene Fluorene Fluoranthene Indeno[1,2,3-cd] Pyrene Naphthalene Phenanthrene Pyrene	Stormwater, sediments, sewage, and crustaceans	Decrease the number of leaves, root length, biomass, and plant germination. Damage to human body organs. Carcinogenic.	Bojes & Pope (2007), Ma et al. (2010), Olivella et al. (2011), Hussain et al. (2018), Ozaki et al. (2019), Zhou et al. (2019a, (2019b), Onydinma et al. (2021), Sajid et al. (2021)
		Surfactants are used in numerous industrial processes. An essential cleaning product. They are classified into cationic detergents, anionic detergents, zwitterionic detergents, and nonionic detergents. Resistant to biodegradation and bioaccumulative.	Methylestersulphonate Olefinsulphonate Alkyl benzene sulphonates Alkyl ether sulphates Isotridecanolethoxylates Benzalkonium chloride N-hexadecyltrimethy Ammonium chloride	Wastewater treatment plants are partially aerobically degraded and partially absorbed by sewage sludge.	Decreased adsorption of dissolved oxygen by aquatic organisms. Modification of temperature, salinity, turbidity, and pH. The gradual destruction of soil structure. Decrease in plant germination.	Bianchetti et al. (2015), Giagnorio et al. (2017), Mousavi & Khodadoost (2019), Lu & Astruc (2020)
		Chromophores and auxochromes form synthetic aromatic compounds called dyes. Toxic, non-biodegradable nature and complex structures.	Congo red Rose bengal Crystal violet Indigo carmine Acid orange Methyl orange	Textile industry	Carcinogenic substances. Disturbance of gas solubility, which in turn affects the gills of aquatic organisms. Modification of photosynthesis.	Jain & Sikarwar (2008), Carmen & Daniela (2012), Padhi (2012), Shanker et al. (2017), Tara et al. (2020), Elgarahy et al. (2021)

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As mentioned above, the composition of wastewater varies significantly depending on its origin. However, the primary wastewater pollutants can be present in all wastewater sources, even in very different concentrations (Bartolomeu et al. 2018). Suspended solids, biodegradable organic matter, nutrients (e.g., nitrogen and phosphorus compounds), non-biodegradable organic matter (e.g., pesticides and detergents), metals, dissolved inorganic solids, and pathogens tend to be present in all domestic and industrial wastewater as urban and agricultural runoff/wastewater (Bartolomeu et al. 2018). Depending on the concentration of these pollutants, some wastewater may be less hazardous than others. However, it is essential to remember that even if these substances are present in lower concentrations, they can cause long-term damage. In addition, the fact that wastewater treatment plants do not generally receive discharge water from a single source could pose severe problems in removing contaminants due to their complex composition.

Conventional water decontamination methods include chemical oxidation/reduction and physicochemical separation methods such as adsorption, membrane filtration, coagulation, precipitation, and ion exchange (Borovik et al. 2020). Although these technologies are effective, they also have significant limitations that must be addressed. Some of the environmental implications of the current processes are the need for high energy and chemical use and the production of large volumes of waste. In addition, conventional water decontamination methods lack selectivity towards contaminants (Lu et al. 2017). They cannot remove one compound over another, suggesting inefficiency and susceptibility to interference during operation (Malato et al. 2009). In conclusion, certain limitations of current water decontamination technologies establish the need to advance treatment methods with novel materials and processes.

Along with the development of nanotechnology applications in all fields of life, water treatment based on this science can potentially improve or even replace traditional technologies (Bishoge et al. 2018). Nanomaterials are one of the promising materials for water purification (Khan & Malik 2019). Generally, nanomaterials are materials with at least one dimension in less than 100 nm (Kreyling et al. 2010; Berekaa 2016; Goh et al. 2016). These materials possess unique functionality due to their smaller size and, more importantly, volume to specific surface area (VSSA) ratio. It has been proposed that materials with VSSA greater than or equal to 60 m²/cm³ should be nanomaterials (Kreyling et al. 2010). Nanomaterials exhibit a high specific surface area, providing a high potential for liquid-solid interactions while utilizing a small process footprint. One of the main advantages of nanomaterials is that they can be tailored and designed to have the necessary properties for efficient water treatment.

Different nanomaterials exist in organic, inorganic, carbon, and composites, which exhibit various physical, chemical, and biological characteristics (Barik et al. 2020). Some of the most explored nanomaterials are metal oxides, magnetic nanomaterials, carbon nanotubes (CNTs), metal nanoparticles, metal-organic frameworks (MOFs), layered double hydroxides (LDHs), graphene-based nanomaterials, and polymer-based nanomaterials. Organic nanomaterials include CNTs, fullerenes, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), graphene, and their derivatives. Most organic nanomaterials are carbon-based nanomaterials (Grimsdale & Müllen 2005). Nanomaterials are based on metals and metal oxides such as aluminum, zinc, copper, iron, aluminum oxide, iron oxide, titanium oxide, and ZnO are classified as inorganic nanomaterials (Landsiedel et al. 2010). Composite nanomaterials combine organic-organic nanomaterials, organic-inorganic nanomaterials, and inorganic-inorganic composite materials (Taylor-Pashow et al. 2010).

Metal oxide nanoparticles (MNP) have demonstrated extraordinary potential as an environmentally friendly, low-cost, and sustainable wastewater treatment technology. These nanoparticles possess short intraparticle diffusion distances and high specific surface area and are compressible without significant surface area reduction (Corsi et al. 2018). Their high adsorption capacity, easy separation, and regeneration make them technologically and economically cost-effective nanoparticles (Abdelbasir & Shalan 2019.) Moreover, some are superparamagnetic, producing adsorption performance superior to activated carbon (Corsi et al. 2018).

This adsorption process is mainly developed by complexation between dissolved metals and oxygen molecules on metal oxide clusters. The efficient adsorption of heavy metals on the outer surface of MNPs is known to be triggered along the walls by rate-limiting intraparticle diffusion (Ilankoon 2014). These nanoparticles possess high adsorption capacity and faster kinetics (Singh et al. 2019) due to many surface reaction sites (edges, vacancies, and corners), high specific surface area, and shorter intraparticle diffusion distance.

A reduction in nanomagnetic particle size increases the adsorption capacity up to 100-fold, which is attributed to the increase in specific surface area, suggesting a nanoscale effect (Nassar 2012). This effect has been attributed to the modification of nanoparticles on the magnetite surface and the development of new adsorption sites. MNPs have demonstrated their ability to remove heavy metals such as chromium, nickel, cadmium, copper, mercury, and arsenic (Abdelbasir & Shalan 2019).

Metallic nanoparticles such as magnesium, gold, and silver (Atacan et al. 2021) and metal oxides such as TiO₂ and MgO (Atacan et al. 2022) have shown high antimicrobial potential due to their small size and unique physical-chemical characteristics. Antimicrobial activity depends on materials that kill bacteria or slow their growth without being toxic to the surrounding tissue. Although still a matter of debate (Gharpure et al. 2020), the mechanism of action of the antimicrobial properties of metal nanoparticles and metal oxides are mainly attributed to ROS formation, inhibition of biofilm formation, and cell membrane damage (Atacan et al. 2022). The association of nanoparticles with cellular components of the microbial cytoplasm and cell membrane increases with a higher surface area to volume ratio due to smaller nanoparticle size (Gharpure et al. 2020).

Moreover, since DNA is composed of abundant sulfur and phosphorus elements, Ag NPs can interact with it and thus destroy it. This destruction is another explanation of cell death due to Ag NPs (Abdelbasir & Shalan 2019). Furthermore, the dissolution of Ag NPs releases antimicrobial Ag⁺ ions that can bind to the thiol groups of different vital enzymes, deactivate them and disrupt ordinary functions in the cell (Ferreira et al. 2015).

Most studies argue that the activity of Ag NPs is due to their nanoforms or the release of silver ions from these nanoforms (Nowack et al. 2011). Several studies have demonstrated the efficacy of Ag NPs in water purification; for example, a paper impregnated with silver nanoparticles was developed for water purification that showed antimicrobial activity against E. coli and Enterococcus faecalis bacteria (Dankovich & Gray 2011). In another study, a polysulfone ultrafiltration membrane impregnated with silver nanoparticles was created and proved to be quite effective. It was even able to remove viruses present in contaminated water. In it, Ag NPs evidenced their antimicrobial activity by employing mechanisms that included binding the nanoparticles to DNA and the sulfhydryl group of proteins. Then, inhibition of cell wall synthesis and cell division occurred, followed by destabilization and disruption of the outer membrane, inhibition of respiration, purine metabolism, and reduction of intracellular ATP levels (Cui et al. 2013; Khan et al. 2020).

Iron nanoparticles are widely used in pollutant reduction technologies (Gutierrez et al. 2017) for groundwater remediation and wastewater treatment. Specifically, zero-valent iron nanoparticles (Fe⁰, nZVI) possess strong reduction potential and can reduce various oxidation-labile pollutants (Khan et al. 2020).

Due to the incredibly tiny size and large specific surface area, nZVI has excellent adsorption qualities and is stable, reducing capacity (Xiong et al. 2015). These properties contribute to enhanced contaminant removal. Under anaerobic conditions, as shown in Equations (1) and (2), Fe⁰ can be oxidized with H₂O or H⁺ producing Fe²⁺ and H₂, potential pollutant-reducing agents. In the oxidation-reduction reaction between nZVI and contaminants, Fe²⁺ will be oxidized to Fe³⁺, and Fe(OH)₃ can be formed with increased pH.

In reduction, oxidation, adsorption, and precipitation (presence of DO) processes, nZVI has been successfully employed in the removal of a large number of contaminants, including halogenated organic compounds, nitroaromatic compounds, organic dyes (Ottofuelling et al. 2011), heavy metals, phenols and inorganic anions such as nitrates (Zhang 2003).

The utilization and synthesis of iron oxide nanoparticles (iron oxide NMs) with novel functions and properties have been widely studied due to their large paramagnetism, low cost, natural abundance (Leonel et al. 2021), nanoscale size and high surface volume (Ali et al. 2016). In particular, the ease of synthesis, modification, or coating and the potential to control or alter matter at the atomic scale can facilitate unparalleled adaptability. Furthermore, iron oxide nanoparticles, with chemical inertness, biocompatibility, and low toxicity, show extraordinary potential when merged with biotechnology (Dinali et al. 2017). Iron oxide nanoparticles prove are promising nanomaterials for wastewater treatment due to their strong adsorption capacity, easy separation, and enhanced stability (Fan et al. 2012). Current applications of these nanoparticles in wastewater treatment can be divided into two technologies: (a) technologies that use iron oxide NMs as photocatalysts to decompose or convert pollutants into a less toxic form (i.e., photocatalytic technologies) and (b) technologies that use iron oxide NMs as a kind of nanoadsorbent or immobilization carrier to improve the removal efficiency. However, it should be noted that many technologies can use both processes (Xu et al. 2012).

The adsorption process has been widely used to remove chemical contaminants from water. It has numerous advantages in terms of cost, flexibility, simplicity of design/operation, and is not sensitive to toxic contaminants (Rafatullah et al. 2010). Iron oxide NMs are currently being explored in the adsorption of organic contaminants, particularly in efficiently treating large-volume water samples. Studies have been conducted to determine the removal efficiency of organic contaminants using iron oxide NMs (Rani & Shanker 2020). For example, Fe₃O₄ hollow nanospheres were shown to be an effective sorbent for red dye (with a maximum adsorption capacity of 90 mg/g) (Iram et al. 2010). The saturation magnetization of the prepared nanospheres was 42 emu/g, which was sufficient for magnetic separation with a magnet (critical value 16.3 emu/g) (Ma et al. 2005). The results obtained in this study demonstrated that magnetic NMs technology represents a novel, promising and desirable alternative for the adsorption of organic contaminants.

The adsorption of organic contaminants takes place through surface exchange reactions. Contaminants can diffuse into the adsorbent and interact with functional groups when the occupation of functional surface sites is completed (Hu et al. 2011). To improve the adsorption capacity of NMs, surface modification and chemical treatment of the NMs are essential. An example illustrating the above is a study using carbon-coated Fe₃O₄ nanoparticles (Fe₃O₄/C) to remove trace amounts of PAHs (Zhang et al. 2010). The removal efficiency percentages of these compounds on Fe₃O₄/C nanosorbents were significantly increased compared to pure Fe₃O₄ nanoparticles, obtaining values above 90%. Moreover, this method allows carboxyl and hydroxyl groups to modify Fe₃O₄/C nanoparticles with a hydrophilic surface. In this way, the modified nanoparticles cannot only stably disperse in the solution of practical applications but also decrease the irreversible adsorption of analytes to overcome the desorption problem of carbon materials (Xu et al. 2012).

Heavy metal contamination is of great concern due to its toxicity, adverse effects on plants, animals, and humans, and its tendency to bioaccumulate even at relatively low concentrations. Currently, most laboratory-scale research and applications of different materials for wastewater treatment have focused on carbon nanotubes (Rodríguez et al. 2020), magnetic NMs, activated carbon, and zero-valent iron (Bhatti et al. 2020). Iron oxide magnetic NMs stand out, as they can treat a large volume of wastewater and are convenient for magnetic separation. Therefore, they are the most promising materials for treating heavy metals (Jiang et al. 2021).

The above stated was evidenced in a study by Nassar (2010), in which the maximum adsorption capacity of Pb(II) ions was found to be 36.0 mg/g by Fe₃O₄ nanoparticles, a result much higher than that reported for low-cost adsorbents. The small size of the Fe₃O₄ nanosorbents favored the diffusion of metal ions from the solution to the active sites on the adsorbent surface. In conclusion, the author recommended that the nanoadsorbents under study be used to remove and recover metal ions from wastewater effluents because of their efficiency and cost-effectiveness.

However, some phenomena may limit the effectiveness of the nanoadsorbents. Aggregation caused by high surface area-to-volume ratios of NMs could control some essential environmental processes, including ion adsorption (Baalousha 2009). In addition to aggregation, numerous interactions in wastewater and contaminant types affect metal adsorption. For example, phosphates can adsorb well and compete with metals at adsorption sites due to their high concentrations in wastewater (Feng et al. 2010). Exploring highly effective modification methods for NMs is a hot research field to improve the efficiency of nanoadsorbents. Surface modification, which inorganic binding layers or organic molecules can achieve, not only stabilizes nanoparticles and eventually prevents their oxidation but also provides specific functionalities that can be selected for ion adsorption and thus improve the adsorption capacity of heavy metals in water treatment processes.

However, iron oxide-based technology for heavy metal adsorption is complex. Many researchers acknowledge that more research is needed to advance the knowledge area of NM and emphasize that before its widespread application, questions about its health impact and environmental fate must be resolved (Bhateria & Singh 2019).

Carbon is a non-metallic element. The primary source of carbon comes from coal deposits. It ranks sixth in the list of most abundant elements and is the second most abundant element in the human body, oxygen being the first. Different types of nanomaterials based on this compound have been widely employed, in the last decades, in wastewater treatment, specifically in the removal of dyes and heavy metals due to its non-toxicity, abundance, ease of preparation, high surface area and porosity, stability structure, and high sorption capacity (Apul et al. 2013).

Depending on its nature, the adsorption process can be classified as physisorption and chemisorption. Physisorption occurs when the molecules in question, the adsorbate, and the adsorbent, are attracted to each other by Van der Waals interactions (Huang & Chen 2014), while chemisorption occurs when the molecules in question are attached to the surface of the adsorbent by a strong chemical bond (Krishnamurthy & Agarwal 2013). The performance of an adsorption process depends on the percentage removal of contaminants from water and is closely related to the adsorption capacity of the adsorbent used. The adsorption capacity of an adsorbent depends mainly on the surface characteristics (specific surface area), the active sites available on the surface, and the affinity towards the contaminants. Due to their large specific surface area along with their efficient active sites, carbon-based nanomaterials prove to be essential adsorbents (Ibrahim et al. 2016). The following will present the application of different types of carbon-based nanomaterials for the adsorption of organic and inorganic pollutants in wastewater.

In recent years, the production of adsorbents based on biomaterials, such as agricultural residues, has increased due to the abundance of these resources (Liu et al. 2018). In addition to being abundant, these agricultural sources are cheap to obtain, and the production cost to synthesize an adsorbent from these agricultural residues is relatively inexpensive. Moreover, these biomaterials are very effective at low metal concentrations and are easily reusable. Carbon-based adsorbents are synthesized from biomaterials such as sawdust, crab shells, bagasse, olive pit waste, and activated carbon cloth (Thines et al. 2017). All these adsorbents present good adsorption capacity in the removal of contaminants present in wastewater. However, the efficiency still needs to be higher. Therefore, the adsorption capacity of this activated carbon can be increased by increasing the specific surface area or reducing the particle size. In other words, this activated carbon can be modified into nanoporous activated carbon by functionalization to provide high pollutant removal efficiency (Thines et al. 2017).

Mangun et al. (2001) employed nanoporous activated carbon fibers (ACF) synthesized with an average pore size of 1.16 nm and surface areas within a range of 171–483 m²/g to determine the adsorption capacity of benzene, toluene, p-xylene, and ethylbenzene. The adsorption data obtained fitted well with the Freundlich adsorption isotherm, and ACFs were found to have a higher adsorption capacity than granular activated carbon (GAC).

Valderrama et al. (2008) tested and analyzed the use of GAC as an efficient adsorbent for the adsorption of PAH such as acenaphthene, anthracene, fluorene, fluoranthene, naphthalene, and pyrene from aqueous solution. The kinetic sorption of PAH from aqueous solutions on GAC was studied by applying the homogeneous particle diffusion model (HPDM) and the progressive shell model (SPM), in which the rate-determining step of PAH removal was the diffusion of the sorbent phase. The affinity of PAH was found to correlate with the hydrophobicity of the nonpolar fraction. The sorption affinity sequence presented pyrene ≈ fluoranthene > anthracene > fluorene > acenaphthene > naphthalene, corresponding with increasing hydrophobicity.

Teimouri et al. (2019) performed an eco-friendly transformation of agricultural wastes, walnut, and almond shells, into activated carbon by a low-cost technique employing microwaves. The obtained product was applied in treating wastewater contaminated by cationic dyes. To ensure the adsorbent's strength, the shell's particle size was less than 300 μm, more significant than those used commercially. The effects of zinc chloride impregnation ratio, activation power, and carbonization temperature on wastewater treatment were investigated in detail. The adsorbents that showed the highest efficiency, 99.0%, were obtained from the carbonized shells in a closed vessel. The amount of activated carbon produced from almond shells in wastewater treatment was less than that of the adsorbent produced from walnut shells, according to the maximum adsorption capacity calculated from the Langmuir isotherm, 114 mg/g, which meant savings from the economic point of view. This study showed that adsorbents could be obtained from agricultural raw materials by applying a simple, easy, and fast method with zinc chloride as a catalyst. The activated carbon with an average pore diameter of 2.4 nm obtained from almond shells improved the adsorbent's performance in wastewater treatment.

Carbon nanotubes (CNTs) have a more efficient adsorption capacity than activated carbon for many organic pollutants. This extraordinary adsorption capacity is mainly due to their large surface area and the various interactions of CNTs with the contaminants. The effective surface area for adsorption on particular CNTs exists on their outer surfaces (Chen et al. 2007). In solution, CNTs form loose aggregates due to the hydrophobic character of the graphitic surface, which reduces the effective surface area. However, CNTs aggregates have interstitial spaces and channels that are considered high sorption areas for organic molecules (Lin & Xing 2008). According to their structure, CNTs present two distinct forms: single-walled carbon nanotube (SWNT) (single layer of graphene sheets) and multi-walled carbon nanotube (MWNT) (multiple layers of graphene sheets) (Müller et al. 2007; Ali et al. 2014). These composites can be produced on a large scale by different methods, such as chemical vapor deposition, favoring production cost reduction.

Electrostatic forces facilitate the adsorption of some antibiotics (Liu et al. 2014) at appropriate pH values (Yang et al. 2008a, 2008b). Oxidized CNTs effectively adsorb metal ions due to electrostatic attraction forces and bond formation (Abdelbasir & Shalan 2019); their adsorption capacity increases remarkably with surface oxidation. Table 2 shows the adsorption capacity and surface area of some CNTs employed for metal ion removal.

CNTs have also shown exceptionally high sorption capacity and efficiency for Cd(II), Cr(VI), and Pb(II) present in water (Robati 2013). Surface characteristics govern sorption mechanisms, ion exchange, and electrochemical potential. The latter plays an essential role in multicomponent sorption, where redox reactions will likely occur on the adsorbent surface and between the different adsorbates (Sadegh et al. 2017).

Different studies have demonstrated the potential of these nanomaterials in the adsorption of heavy metal ions. One of them was conducted by Chen & Wang (2006); oxidized MWCNTs were employed for Ni(II) adsorption from an aqueous solution. It was reported that the predominant mechanism of Ni(II) adsorption at acidic pH was ion exchange and basic pH surface complexation. It was also evidenced that oxidized MWCNTs are promising materials for the preconcentration and solidification of heavy metal ions. Rao et al. (2007) verified the removal of divalent metal ions [Cd(II), Cu(II), Ni(II), Pb(II), Zn(II)] from aqueous solution by CNTs. This study showed these compounds as promising adsorbents for environmental protection applications due to their adequate desorption capacity of divalent metal ions.

Graphene (GO) is a two-dimensional (2D) one-atom-thick layer of sp2 hybridized carbon atoms (Shan et al. 2017) that form a hexagonal honeycomb-like crystal lattice via σ and π bonds. This material is the basis of all graphitic forms: graphite sheets (3D), when separated, become graphene; a rolled graphene layer forms a carbon nanotube (1D); while folded, graphene becomes fullerene (0D) (Sahu et al. 2021).

GO and its derived structures have become novel and efficient materials for wastewater treatment. Due to its characteristics (e.g., large specific surface area, many functional groups, and excellent charge carrier mobility), graphene-based materials are adsorbents for wastewater decontamination (Ali et al. 2019).

Upadhyayula et al. (2009) chemically modified graphene to improve the metal ion removal capacity. The authors used potassium hexafluorophosphate and 1-octyl-3-methylimidazolium hexafluorophosphate ionic liquids to modify graphene. The maximum sorption value recorded was 89.89%, described by the Freundlich and Langmuir pseudo-second-order models.

Zhao et al. (2011) reported that graphene oxide nanosheets synthesized from graphite using the modified Hummers method were used as adsorbents to remove Cd²⁺ and Co²⁺ from large volumes of aqueous solutions. The influence of the analyzed parameters, such as pH, ionic strength, and humic acid, on Cd²⁺ and Co²⁺, showed that the sorption of metal ions on graphene oxide nanosheets strongly depended on pH and weakly on ionic strength. In this regard, the presence of humic acid reduced the adsorption of these metal ions on graphene oxide nanosheets at pH < 8. The maximum adsorption capacities of Cd²⁺ and Co²⁺ on graphene oxide nanosheets at pH 6 were 106.3 and 68.2 mg/g, respectively, at a temperature of approximately 303 K. Based on this study, it was suggested that graphene oxide nanosheets result as an outstanding material for large-scale water treatment.

Sitko et al. (2013) reported the use of GO for the removal of Cd(II), Cu (II), Pb(II), and Zn(II), having the following order Pb(II) > Cd(II) > Zn(II) > Cu(II) in single metal systems, and Pb(II) > Cu(II) > >> Cd (II) > Zn(II) in binary metal systems. Sorption and kinetic studies showed monolayer sorption of the metal ions. A summary of heavy metal adsorption using graphene as an adsorbent is shown in Table 3.

In addition to heavy metal adsorption, multiple investigations have shown that graphene can be employed in the adsorption of dyes in aqueous solutions. Zhao & Liu (2009) demonstrated that modified expanded graphite (MEG) powder could be employed as an adsorbent for a cationic dye such as methylene blue (MB) in an aqueous solution. The results showed that the highest adsorption capacity is achieved at basic pH, higher initial dye concentration, and elevated temperature. The experiment data were fitted to the pseudo-second-order kinetic model, while the adsorption data were fitted to the Langmuir adsorption model with an adsorption capacity of 7.77 μg/g. Li et al. (2013) used graphene for the removal, also, of methylene blue. The authors reported maximum adsorption of 204.08 mg/g. They further reported that the sorption data described the Langmuir model, pseudo-second-order mechanism, and intraparticle diffusion. Othman et al. (2018) synthesized graphene oxide/magnetic iron oxide nanoparticles and employed them as synthetic methylene blue (MB) dye adsorbents in an aqueous solution and obtained a removal rate of 99.6%. The researchers concluded that the adsorption rate depends on pH, contact time, MB concentration, and adsorbent dosage.

Fullerenes are known as the milestone in the production of multiple carbon-based nanomaterials. They are a class of carbon allotropes (Thilgen & Diederich 2006) whose molecules have the shape of a hollow sphere, tube, or ellipsoid and represent convex closed polyhedra formed by an even number of coordinated carbon atoms. Their formula is C_20+m, where m is an integer (Burakov et al. 2018). Fullerenes are traditionally produced by arc discharge vaporization of graphite, chemical vapor deposition methods, and combustion processes (Manawi et al. 2018). Their low aggregation tendency and large specific surface area allow them to be used as adsorbents to remove heavy metals in industrial wastewater (Lucena et al. 2011). Several studies have been conducted in which fullerene has been used as an adsorbent for organic materials in contaminated waters.

Cheng et al. (2004) reported that naphthalene could be adsorbed using C₆₀ fullerene as an adsorbent. The researchers concluded that an enhanced dispersion of C₆₀ fullerene had a higher adsorption affinity towards naphthalene than activated carbon. A solid water distribution coefficient of 102.4 mL/g was obtained for poorly dispersed C₆₀. In contrast, 104.2–104.3 mL/g were obtained for dispersed C₆₀ samples. In general, C₆₀ fullerene presents a hydrophobic surface so that it could be a perfect sorbent for many organic compounds in water.

Yang et al. (2006) studied the extraction of polycyclic aromatic hydrocarbons (PAH) such as naphthalene, phenanthrene, and pyrene from aqueous solution using fullerenes, SWCNTs, and MWCNTs. The adsorption isotherms were fitted to the Polanyi-Manes model (PMM), and the adsorption capacity of these three adsorbents depended on the properties of the HPAs, such as molecular size, and the properties of the carbon nanomaterials, such as surface area, micropore volume and volume ratios between mesopores and micropores. Alekseeva et al. 2016 employed fullerene or fullerene combined with a polystyrene-based composite. They determined that the removal efficiency of Cu(II) was higher in the former case. The equilibrium isotherm of Cu²⁺ sorption was fitted to the Langmuir model, and the Cu²⁺ sorption capacity of the monomolecular layer was 14.6 mmol/g.

Studies have been carried out on modifying conventional sorbents with small amounts of fullerenes to improve their sorption capacity. Including fullerenes increases the hydrophobicity and improves the sorption capacity of heavy metals from aquatic environments. For example, after introducing 0.001–0.004% fullerenes into activated carbons, their sorption capacity for Pb(II) and Cu(II) increased by 1.5 and 2.5 times, respectively (Burakov et al. 2018).

Cellulose is one of the most abundant materials on Earth and constitutes the main component of all plant fibers (Olivera et al. 2016). It is the most promising biopolymer (Carpenter et al. 2015) due to its abundance, biodegradability, renewability, and low cost. Cellulose is a linear polysaccharide with long chains. Its molecular formula is (C₆H₁₀O₅)_n consisting of β-Dglucopyranose units linked by β-1,4-glycosidic bonds that form a dimer known as cellobiose, the central unit of cellulose. Cellulose fibers contain loosely packed amorphous chains and highly crystalline, compacted domains (Abouzeid et al. 2018).

Several nomenclatures have been used to describe the various forms of cellulose-derived nanomaterials. Some are nanocrystalline cellulose, cellulose nanograins, cellulose nanocrystals, nanocellulose, cellulose nanofibrils, nanofibrillated cellulose, bacterial cellulose, and any combination of the above. Several Canadian entities, the Technical Association of the Pulp and Paper Industry (TAPPI), the U.S. Department of Agriculture, and the U.S. Forest Service have standardized the classification of NCs into two main groups: cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) (Mohammed et al. 2018; She et al. 2018).

NCs represent a promising alternative as an adsorbent due to their high surface-to-volume ratio and inherent environmental inertness. Moreover, their easily functionalizable surface allows the incorporation of chemical element moieties that can increase the binding efficiency of contaminants to these nanomaterials (Abouzeid et al. 2018). Srivastava et al. (2012) reported the potential use of pristine and surface-functionalized CNCs for the adsorption removal of heavy metal ions. In this case, CNCs prepared from rice straw fibers were used for the adsorption of Cd(II), Pb(II), and Ni(II), and higher adsorption capacity was observed for CNCs derived from rice straw fibers rather than the original rice straw. This higher adsorption efficiency could have been influenced by the removal of amorphous domains in the cellulosic chains during the synthesis of CNCs, promoting the availability of more primary hydroxyl groups on their surface. In addition, acid hydrolysis of cellulosic fibers during the synthesis of CNCs introduced negatively charged sulfate ester groups on the surface of CNCs, acting as binding sites for heavy metal ions.

Yu et al. (2013) concluded that incorporating succinic acid groups on CNCs significantly increased the binding efficiency of Pb²⁺ and Cd²⁺ ions. The conversion of carboxylic acid groups to sodium carboxylates further improved their ability to remove these toxic metal ions from aqueous solutions.

CNFs have also been extensively studied to remove heavy metal ions from aqueous solutions. She et al. (2018) successfully developed CNFs aerogel adsorbents using cellulose nanofibrils crosslinked with poly (vinyl alcohol) (PVA) and acrylic acid (AA). The combination of PVA and AA conferred the CNFs aerogel with a solid three-dimensional porous structure and excellent adsorption properties. Cu2+ and Pb2+ metal ions were efficiently adsorbed. The adsorption data of these heavy metal ions were confirmed to fit the pseudo-second-order kinetic model and Langmuir isotherm. The functional groups C ═ O and -OH were the main adsorption sites. Mahfoudhi & Boufi (2020) employed a CNF-Al(OH)₃ modified aerogel for fluoride adsorption in an aqueous solution. The adsorbent was prepared by freeze-drying the cellulose nanofiber gel, impregnation with AlCl₃ solution, and washing with water. The adsorption capacity ranged from 20 to 35 mg/g, depending on the Al(OH)₃ loading. The CNF-Al(OH)₃ adsorbent was reused for five adsorption-desorption cycles without significant loss of adsorption capacity.

Several NC-based dye adsorbents have been developed. Eyley & Thielemans (2011) demonstrated the ion exchange capacity of imidazolium grafted CNCs for an anionic dye, Orange II. Pristine CNCs were cationically modified by grafting imidazolium groups using Cu(I)-catalyzed alkyne-azide cycloaddition. The amount of dye adsorbed by these functionalized CNCs was 0.28 mmol/g for Orange II. Meanwhile, Jin et al. (2015) used amino-functionalized CNCs (A-CNCs) as adsorbents for anionic dyes (Congo Red 4BS, Acid Red GR, and Reactive Yellow K-4G). A-CNCs were prepared by grafting ethylenediamine onto pristine CNCs oxidized with sodium periodate. The hydroxyl and primary amine groups of the A-CNCs served as active adsorption sites for anionic dye binding, and the qmax of acid red GR was found to be 1.39 mmol/g.

Like CNCs, CNFs have also been surface functionalized to enhance their adsorption capacity for various pollutants. Xie et al. (2011) prepared CNF-based nanostructured bisorbents for the adsorption removal of reactive dyes, B-4RFN yellow and B-RN blue. The CNF-based fabrics were modified with reactive polyhedral silsesquioxane (R-POSS) to introduce multi-N-methylol groups on their surface. Compared with the new CNF-derived fabrics, the R-POSS-modified CNF hybrid fabric showed a higher adsorption capacity of reactive dyes. This adsorption capacity is likely due to the cationic organic functional groups (-C-N-) and nanometer-sized cubic cores that act as active sites for the adsorption of these dyes.

Pei et al. (2013) developed surface quaternized CNFs (Q-CNFs) with high adsorption capacity for anionic dyes (Congo Red and Acid Green 25). Herein, Q-CNFs were prepared by mechanical disintegration of wood pulp fibers and pretreated with glycidyl trimethylammonium chloride (GTMAC). An optimized concentration of sodium hydroxide (NaOH) was used for this reaction to maximize the conversion of hydroxyl groups on the surface of the CNFs to quaternary ammonium groups. Q-CNFs with a trimethylammonium chloride content of 1.32 mmol/g could adsorb 0.95 mmol/g of Congo Red and 1.09 mmol/g of Acid Green 25.

Membranes, generally, are employed in the removal of contaminants from water and are classified as a physical process that works by the movement of particles based on different concentration levels or through a difference in particle size (Qu et al. 2013), such as metals, bacteria or viruses (Kunduru et al. 2017). The overall performance of a membrane is determined by two critical factors: selectivity and membrane permeability. A membrane with higher permeability decreases the amount of membrane area needed to treat water, ultimately leading to a decrease in the capital cost of the membrane. Similarly, a membrane with higher selectivity provides a purer product. Increased permeability and selectivity lead to lower energy consumption, essential for this pressure-driven process (Thines et al. 2017).

One of the main obstacles to the long-term stability of membranes is biofouling (Zhu et al. 2018). In this regard, developing specialty anti-biofouling is crucial in water and wastewater treatment membrane technology. Hydrophilic membranes have been developed to alleviate surface biofouling; however, more is needed to control long-term biofouling due to bacteria's continuous bioadsorption, growth, and regeneration on a polymeric membrane. Current research is geared toward designing antibacterial membranes by integrating biocidal nanomaterials through surface mixing and functionalization (Mansouri et al. 2016). These materials can offer additional desired properties to membranes, such as a smooth and hydrophilic surface, leading to increased membrane resistance to fouling. Although nanomaterials have shown promise for removing the microbial load from wastewater, the degree of removal of microorganisms depends mainly on the nature of the NPs. In general, nanomaterials act on microorganisms to remove them by various strategies: damaging microbial cell membranes by direct contact (e.g., chitosan NPs), discharging toxic metal ions (e.g., Ag+), and generating ROS (e.g., photocatalytic TiO₂) (Sarkar et al. 2017). However, despite their high antibacterial activity, problems such as the release of (heavy) metal ions that the membranes cannot reject and the leaching of nanomaterials from the antibacterial membrane will potentially pose a risk to the aquatic ecosystem due to the potential toxicity of some nanomaterials (Bystrzejewska-Piotrowska et al. 2009; Zhu et al. 2018). These considerations should be taken into account in antibacterial modifications using multifunctional nanomaterials. Additionally, that future research should focus on reasonable release rate control, aiming to inactivate bacterial strains effectively and prolong the shelf life of membranes.

Nanotechnology focuses on solving issues related to water purification and water quality. Nanomaterials are employed for their potential in microbial load control and disinfection. Specifically, nanofilters and nanomembranes stand out for their properties in drinking water production (Ahmed et al. 2014). Nanofilters, compared to conventional technologies, require less pressure, while TiO₂-modified filtration membranes produce high-quality water with higher flux. Other nanomaterials that stand out for their removal of bacteria, toxins, and organic contaminants involved in water purification are magnesium aluminum silicate clays and nanoporous polymers (Singh et al. 2021).

Several studies have focused on developing inorganic or polymeric nanometric membranes to decrease membrane saturation and increase water permeability (Daer et al. 2015). Inorganic nanoparticles improve the thermal and mechanical stability of polymeric membranes, reducing the negative impact of compaction. These processes have shown high potential in desalination and can be combined with other technologies, such as disinfection. Market introduction shortly is a latent possibility. However, desalination and long-term stability are critical challenges before noticeable commercialization (Villaseñor & Ríos 2018).

The fate of nanomaterials (NMs) in the environment is controlled by the combined effects of their physicochemical properties and their interactions with other contaminants (Maiti et al. 2016). Nanomaterials found in the environment can come from a variety of natural sources (e.g., volcanic eruptions, forest fires, soil erosion, weathering, clay minerals, and dust storms) or from intentional/unintentional anthropogenic activities (e.g., fossil fuel burning, mining/demolition, automobile traffic, and NM production and waste stream) (Kabir et al. 2018) (Figure 8).

Once NMs are released into the environment, they accumulate in different environmental matrices, e.g., air, water, soil, and sediments (Iavicoli et al. 2014). Figure 9 presents the transport pathway NMs can follow to reach different aquatic environments.

Although precise data are not currently available, it is known that the most significant quantities of nanomaterials in the environment end in the soil. In contrast, smaller quantities of nanoparticles are present in water and air. One of the soil's largest ‘sources’ of nanomaterials is the sludge generated in wastewater treatment plants. After wastewater treatment, the total contamination (including nanoparticles) accumulates in the sludge, which is then transported and handled in various ways. Studies so far have focused on the applicability of nanomaterials in various sectors, such as catalysis, sensing, photovoltaics, environment, and biomedicine. However, due to the lack of adequate strategy or methods for their disposal, the level of NMs in the environment is constantly increasing (Zekić et al. 2018).

In order to have technical data on the advantages and disadvantages of using nanomaterials, it is essential to conduct life cycle assessments and risk analyses before their widespread application because harmless bulk materials could be converted toxic and reactive substances at the nanometer level. In addition, studies on the adverse effects of NMs on higher animals and humans are limited. However, it has been shown that the toxicological profile of these structures in lower organisms and human cell lines does not support health (Zekić et al. 2018). Narrowing the knowledge gaps on the nature of nanomaterial interactions will lead to standards detailing their application, processing, impact on the environment (e.g., through tools such as life cycle analysis), exposure, and effects on human health (through ecological and health risk studies).

The transition from a linear economy to a circular economy is relatively recent. Eco-toxicological hazards and nanomaterials in waste streams are issued to be addressed soon. In addition, safety and sustainability assessments are needed due to their complexity and dynamic behavior in wastewater to make many natural opportunities for using nanotechnology and enable circularity.

Nanobioremediation is a novel and sustainable biological advancement treatment for cleaning-up (Enamala et al. 2019) by bacterial degradation, which can exploit them as the only source of carbon.

Nanoparticles have found an effective way of treating heavy metals through the technology known as nanotechnology. The technology used for cleaning up polluted sites with the help of nanotechnology is known as nanobioremediation; these technologies reduce the costs of cleaning up contaminated sites on a large scale and reduce the processing time.

Due to their remarkable and unique properties, it has shown potential application for the remediation of several pesticides and textile dyes. Recently it has shown positive results for the remediation of sodium dodecyl sulfate (SDS). One of the highly exploited surfactants in detergent preparation is anionic surfactants. The SDS is an example of anionic linear alkyl sulfate, which is utilized extensively in industrial washing, which results in the high effluent level of this contaminant and ubiquitously toxic to the environment. Nanobioremediation can play a role in the remediation of such recalcitrant pesticides from groundwater and wastewater (Yadav et al. 2022).

Deposits containing wastewater, as well as clandestine dumps, have generated negative impacts globally that result in the deterioration of the quality of natural water bodies for wild and human uses. Wastewater is caused by incorporating various substances and residues, which worsen water quality. Due to industrial development and the consumption of materials, wastewater bodies are increasingly complex to treat because of the heterogeneity of recalcitrant compounds found. Some of the most abundant and detected examples are pesticides, heavy metals, pharmaceuticals, colorants, and polycyclic aromatic hydrocarbons. Based on the above, the scientific society has paid vital attention to developing alternatives that, although they can mitigate the ravages, are sustainable, economical, and easily accessible. This review has shown a wide range of promising nanotechnologies tested to treat and improve the organoleptic characteristics of wastewater and drinking water, providing society and the environment with a safe and innocuous liquid. Nanotechnology has generated relevant contributions through various functional materials, such as graphene, metallic, cellulose materials, catalysts, nanotubes, polymers, nanoparticles, and metal oxides, which contribute unspecifically to the degradation of the pollutants above. Although nanomaterials have been considered an effective alternative, they also present several disadvantages, such as implementation at the industrial level, which has yet to allow an integral development in their use for the treatment of large volumes of wastewater since studies at these levels are scarce. It is also advisable to carry out toxicological studies to rule out nanomaterials as toxic, teratogenic, and, in severe cases, carcinogenic agents. However, no cases of intoxication have been reported due to their use and disposal. However, these disadvantages do not limit their potential as a promising platform focused on wastewater treatment. Nanomaterials possess favorable characteristics that make them ecologically based alternative processes based on the principles of green chemistry and circular economy, with the primary objective of solving the potential water pollution risks.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.