An overview of biogenic metallic nanoparticles for water treatment and purification: the state of the art

An ever-increasing world population is a blessing to the economy but also a burden on the environment as more waste is generated, degrading ecological matrices (Hlongwane et al. 2019; Borah et al. 2020; Ojha 2020). The consequences of human activity on the environment are indescribably devastating, and the toxicities of pollutants are inexpressibly overwhelming (Azeez et al. 2018; Yaqoob et al. 2020). Water is an indispensable natural resource that is crucial for the continued existence of living things. The obligation to preserve its quality is a call to preserve human health. However, many water sources receive loads of municipal, industrial, and agricultural wastes, degrading their quality (Sharma & Negi 2020). The effects of these would be more damning in developing countries where little or no attention is paid to providing quality water (Thangadurai et al. 2020; Azeez 2021). These wastes, whether biodegradable or not, constitute health hazards, inducing genotoxic, hepatotoxic, carcinogenic, and teratogenic effects on vital organs in the human body. Heavy metals, polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), pesticides, grease, oil, sewage, nylons, and iron-fillings among others are intolerable to the human body, causing different health disorders (Miklos et al. 2018; Borah et al. 2020).

Several methods have been reported for the remediation and purification of water pollution, but their shortcomings have limited their application. Methods involving physical, chemical, and biological processes, such as precipitation, ion-exchange, coagulation/flocculation, photochemical degradation, reverse osmosis, membrane filtration, adsorption, and electrochemical processes have been employed (Manasa & Mehta 2020; Sachidhanandham & Periyasamy 2020; Saleh et al. 2020; Soni et al. 2020; Mathur et al. 2022). Toxic sludge from coagulation, disposal of huge amounts of waste in precipitation, formation of secondary metabolites, and lack of sensitivity for many pollutants in biological processes, lack of affinity, specificity, cost-effectiveness, and fouling in membrane filtration as well as lower sorption efficiency of some adsorbents are some of the contending factors that militate against continuous use of these processes (Hlongwane et al. 2019; Sachidhanandham & Periyasamy 2020; Sharma & Negi 2020). Moreover, meeting stringent standards of water quality with these conventional remediation processes appears to be time-consuming and less economical as they require a complement of other methods to achieve clean water useful for all human activities (Azeez et al. 2020; Thangadurai et al. 2020).

Adsorption is a cost-effective, easy, and eco-friendly technology that provides the most effective option for polluted water remediation with suitable adsorbents. It is suitable for inorganic, organic, and biological pollutants and is not limited by biodegradability (Miklos et al. 2018; Mustapha et al. 2020). Different adsorbents prepared from natural sources and functionalized ones have proven to be effective. Various low-cost adsorbents, including activated carbons, clay, biochar, zeolites, and other polymeric materials, are used for remediation. However, these are not without burdens as large quantities would be required to achieve excellent results due to their macroaggregated nature coupled with expensive production and regeneration (Azeez et al. 2018; Hlongwane et al. 2019; Ojo et al. 2019; Saxena et al. 2020). Therefore, finding more suitable, efficient, biodegradable, safer, environmentally friendly, and economical alternatives to meet humans’ various water needs would yield better results (Xu et al. 2018; Kamali et al. 2019).

Nanotechnology is presently being deployed in cancer diagnostics, drug delivery, metal detection, and remediation, and applied in electronics and agriculture for the enhancement of insect-resistant plants (Adebayo et al. 2019; Siddhardha & Parasuraman 2019; Lateef et al. 2020). Recent developments in nanotechnology have provided significant relief in overcoming the limitations of conventional water remediation methods together with a long-term opportunity to rid environmental matrices of pollutants (Sannino et al. 2017; Gebre & Sendeku 2019; Kamali et al. 2019; Azeez 2021). Nanotechnology offers a variety of options to meet the demands of remediating polluted water with intrinsically reactive, flexible, magnetic, biodegradable, and size-dependent nanostructured materials that possess improved adsorptive properties in addition to controllable properties to suit particular purposes (Azeez et al. 2018; Nnaji et al. 2018; Yadav et al. 2020; Yaqoob et al. 2020) Excellent adsorptive efficiencies predominantly reported for wastewater remediation using nanomaterials as compared to conventional adsorbents confirm their usefulness for purification (Saikia et al. 2019; Thangadurai et al. 2020; Nasrollahzadeh et al. 2021). This is in addition to easier regeneration of nanomaterials after the adsorption process, simpler application, catalytic properties, and safer usage with mild or no adverse environmental effects due to their biodegradable nature (Ahmed et al. 2018).

Nanoparticles transform, photodegrade, adsorb, and detoxify different pollutants especially hitherto recalcitrant ones (Kumar et al. 2013; Azeez et al. 2020; Azeez 2021). Nanomaterials such as single and multi-walled carbon nanotubes, metallic nanoparticles and nanocellulose were reported to be excellent at removing dyes, organic pollutants, pesticides, heavy metals, and endocrine-disrupting chemicals from water (Nnaji et al. 2018; Chen et al. 2019; Kamali et al. 2019; Attatsi & Nsiah 2020; Yakar et al. 2020; Yusuf 2020). Unlike other methods of remediation that provide only remediation, nanoparticles assist in the reduction/extermination of harmful pathogens concurrently because they possess antioxidant, antibacterial, antiviral, and catalytic properties (Saikia et al. 2019; Elegbede & Lateef 2020; Lateef et al. 2020). These features are unique and required for environmental conservation. Although associated toxicities from some nanoparticles have been reported, especially those prepared via physical and chemical methods, the green/biological method of synthesis is eco-friendly and typically devoid of toxicities because it uses biological macromolecules to reduce and cap the synthesis (Akintayo et al. 2020; Badmus et al. 2020; Lateef et al. 2020). Green/biogenic nanoparticles are environmentally safe but adversely affect pathogenic bacteria (Nasrollahzadeh et al. 2021). Green synthesized copper oxide nanoparticles (CuONPs) and calcium oxide nanoparticles (CaONPs) performed better than chemically mediated nanoparticles in a study published by Muthuvel et al. (2020) and Thakur et al. (2021). Furthermore, Panchal et al. (2022) and Jayapriya et al. (2020) demonstrated improved photocatalytic degradation performance of green generated silver nanoparticles-doped magnesium oxide nanoparticles (Ag–MgONPs) over similar chemically synthesized nanocomposites. This review discusses the performance of green produced metallic nanoparticles for total remediation. It focuses on waste materials and without complementary procedures or contention with secondary pollutants.

The majority of reviews of metallic nanoparticles focus on chemically synthesized ones, focusing on their adsorption capacities and toxicities. Accordingly, this review evaluates the current status of biogenic metallic nanoparticles for water treatment and purification in difference to chemically synthesized ones. This review contributes to knowledge for assessing appropriate metallic nanoparticles for different pollutants as more metallic nanoparticle characteristics were reviewed in this manuscript. Metallic nanoparticles are discussed specifically in terms of their adsorption capacity, purification techniques, and mechanisms of action.

This review examines articles published in the period between 2015 and 2022 on the application of nanoparticles for water remediation. It focuses on green synthesized metallic nanoparticles as adsorbents, disinfectants, and catalysts for the degradation of water pollutants. This is to avoid the toxicity of chemically engineered nanoparticles.

The indispensability of water for human activities is an impulse to preserve its quality and degrade pollutants received by it. Water is an essential requirement for all life activities, from drinking to preparing food, agriculture, industries, and sanitation, and if any of these is affected, it affects the quality of life (Manasa & Mehta 2020; Srivastav & Ranjan 2020). The presence of pollutants belonging to various classes in water renders it unfit for drinking and undesirable for use in many other activities. Deaths resulting from the consumption of polluted water are on the rise (Ahmed et al. 2018; Yadav et al. 2019). The health challenges associated with the consumption of contaminated water are too dangerous to overlook. More so, when such water is used in agriculture, these pollutants, particularly the recalcitrant, can bioaccumulate through the food chain. Many of these pollutants are caused intentionally or unintentionally. There are numerous types of pollutants found in water, ranging from particulate, inorganic, organic/chemical and biological, covering heavy metals, fungi, bacteria, plastics, fillings, dyes, pharmaceuticals, medical, effluents, and pathogens (Miklos et al. 2018; Harja & Ciobanu 2020; Sharma & Negi 2020; Yaqoob et al. 2020).

Particulate pollutants comprise fine and coarse suspended particles in water and can sometimes act as vehicles for transferring other pollutants due to their charged surfaces. They cause turbidity in water which irritates fish gills, prevents light penetration, and reduces dissolved oxygen levels, ultimately leading to the death of aquatic organisms. When such fish are consumed by humans, they can cause respiratory illnesses (Simeonidis et al. 2018; Azeez et al. 2020; Yaqoob et al. 2020; Azeez 2021).

Inorganic pollutants include heavy metals, radionuclides, acids, anions, and cyanide. Their presence in water is undesirable and poses a major environmental challenge. They are nonbiodegradable and persistent in the environment. The accumulation of heavy metals, radionuclides, and cyanide in water is detrimental to aquatic lives. Undesirably, acid drain increases their rate of accumulation (Manasa & Mehta 2020). The most detrimental of this group is heavy metals. Heavy metals such as cadmium, lead, arsenic, mercury, and chromium are highly toxic without any known biological usefulness rather are carcinogenic, teratogenic, and mutagenic, causing malfunctioning of the kidney, skin, liver, lungs, and nervous system in humans when water containing these pollutants is consumed (Harja & Ciobanu 2020). When used for agriculture, they disrupt photosynthesis and induce oxidative stress in plants, causing significant retardation in physiological and morphological traits (Borah et al. 2020; Srivastav & Ranjan 2020).

Organic/chemical pollutants are carbon-containing comprising dyes, agrochemicals (pesticides), VOCs, PAHs, plastics, organic solvents, polychlorinated biphenyls (PCBs) and petroleum products, among others (Prasse & Ternes 2010; Kumar et al. 2019; Saxena et al. 2020; Yakar et al. 2020). They are highly reactive, soluble in water, and their presence is associated with hepatoxicity, carcinogenicity, nephrotoxicity, and genotoxicity of vital organs in humans, in addition to inducing imbalances in the morphological functions of plants (Wang et al. 2019). They are diverse, persistent, and difficult to remediate. They can undergo transformation into secondary pollutants, producing worse effects (Salimi et al. 2017; Chokkalingam et al. 2019; Saleh et al. 2020).

Biological pollutants are waterborne pathogens known to cause severe and fatal health challenges such as diarrhea, dysentery, infections, cholera, fever, and malfunctions of the kidney, pulmonary, and urinary tracts. Their examples include Entamoeba histolytica, Pseudomonas spp., Shigella sonnei, Salmollena, Vibrio cholera, Escherichia coli, and Giardia intestinalis. They cause deadly diseases, and produce secondary toxins during disinfection, and in many instances, conventional disinfectants are not so effective against them (Prasse & Ternes 2010; Wang et al. 2019; Yaqoob et al. 2020).

Adsorption remains the most effective method for water remediation, depending on the structure of adsorbents, whether they are macro, micro, or nanostructured. It is a technically feasible and eco-friendly process, particularly for pollutants too stable for biological processes. Pollutants of various types are trapped physically or chemically within or on the porous surfaces of adsorbents (Liu et al. 2019; Ojo et al. 2019; Sachidhanandham & Periyasamy 2020). Activated carbon is the most widely used adsorbent. Although it is expensive, it has been largely utilized for adsorption and has been found to be highly efficient. It has a large surface area, is vastly porous, and has a high adsorption capacity. These and other properties qualify activated carbons as commonly used adsorbents (Chen et al. 2019; Hlongwane et al. 2019). Due to the expense of commercial activated carbon, cheaper alternative sources have been used instead. Activated carbons or functionalized adsorbents produced from agricultural wastes (seeds, pods, silk, grass, leaves, feathers, stalks, peels, epicarp, sawdust), crustaceans (chitosan, perlite, hydroxyapatite, periwinkle), and biomass are presently finding higher patronage than purchased activated owing to better reusability/regeneration, ease of design and operation, cost-effectiveness, and sensitivity to toxic wastes (Xu et al. 2018; Ojo et al. 2019; Saleh et al. 2020; Azeez 2021). However, as promising as these adsorbents are, large quantities and lengthy experimental time are needed to achieve excellent adsorptive results. Moreover, they only serve as adsorbents and are not involved in detoxification, disinfection, and catalytic degradation of pollutants (Dimapilis et al. 2018; Nnaji et al. 2018; Husein et al. 2019).

Nanoremediation of water involves the application of intrinsically surface reactive nanomaterials with imbued properties that enable photocatalysis and mediate degradation, transformation, and detoxification of pollutants. Nanoparticles with sizes between 1 and 100 nm are the most representative of all nanomaterials (Lateef et al. 2015, 2016a; Gebre & Sendeku 2019; Hlongwane et al. 2019). Their versatility as adsorbents offers reliable remediation of pollutants devoid of stoichiometric calculation of macro or microporous adsorbents to transform pollutants into innocuous substances (Hlongwane et al. 2019). Although there are concerns about toxicity from the application of nanoparticles, green synthesized nanoparticles are safer and eco-friendlier, having no adverse aftereffects from usage (Prasse & Ternes 2010; Azeez et al. 2018; Pedata et al. 2019; Yoganandham Suman et al. 2020).

For water remediation, nanoparticles perform more functions (photocatalysis and disinfection) than adsorption since the pollutants’ types and their concentration in water are diverse (Cerrón-Calle et al. 2019; Sebastian et al. 2020). Their basic adsorption function is enhanced by their surface morphology, surface reactivity, and magnetism, while their disinfection power, aided by their ability to rapidly release ions that inactivate pathogens, is incomparable with conventional disinfectants. For water disinfection, chlorine, ozone, chlorine dioxide, chloroamines, and ultra-violet (UV) radiation are frequently used, but concerns about the formation of toxic byproducts, ineffectiveness against all pathogens, and the requirement of a complimentary process are their shortcomings (Ahmed et al. 2017; Dimapilis et al. 2018; Simeonidis et al. 2018; Husein et al. 2019). Nanoparticles have exceptional functionalities needed to inactivate pathogens through disruption of cell membrane, and the generation of reactive oxygen species (Ollier et al. 2020; Azeez 2021).

Although nanoparticles have been synthesized using chemicals as reducing agents, health implication for the applications of these types of nanoparticles has been reported. Chemically synthesized TiO₂, Fe₃O₄, ZnO, and MgO were reported toxic to the hemoglobin, alveola cells, dermal fibroblast cells, and aquatic lives (Saif et al. 2016; Azeez et al. 2018; Akintayo et al. 2020; Badmus et al. 2020). Hence, the limitations of their application in water remediation. To circumvent reported and possible toxicities from chemically produced nanoparticles, a green synthesis approach that uses biological materials as reductants provides a better alternative (Elegbede et al. 2019; Marimuthu et al. 2020).

Green synthesis of nanoparticles is a one-pot process that involves the use of extracts of plants’ parts (seeds, shells, leaves, roots, barks), microorganisms, cobwebs, enzymes, and other biological materials to mediate zero-valent nanoparticles (Lateef et al. 2015, 2016a; Elegbede et al. 2018; Akintayo et al. 2020; Yusuf 2020). They are more beneficial than nanoparticles produced by physical and chemical methods because of their less or no toxicity and because they are strengthened by properties such as antibacterial, antioxidant, and larvicidal that inactivate intolerable pathogens in water (Azeez et al. 2018; Elegbede et al. 2019; Lateef et al. 2020; Azeez 2021).

These are zero-valent metallic nanoparticles that have been reduced and capped to zero charge by biological macromolecules in the extract in green synthesis or chemically reduced. Their precursors are metal and metal oxide salts (Gebre & Sendeku 2019; Gomes et al. 2019; Attatsi & Nsiah 2020; Marimuthu et al. 2020). They enjoy morphological features, special affinity, and high adsorptive efficiency. They can be synthesized either as single metal or bimetallic nanoparticles. The existence of many reactive functional moieties, especially hydroxyl, amine, carboxyl, and phenolic groups on the surfaces of nanoparticles, enhances their adsorptive efficiency (Saikia et al. 2019; Azeez et al. 2020; Yadav et al. 2020; Rani et al. 2021). The moieties may be present as protonated or deprotonated species depending on pH to support electrostatic interaction between nanoparticles and oppositely charged pollutants (Mathur et al. 2022). They provide an unrivaled advantage for remediation due to their known properties of faster adsorption kinetics, easy delivery, and suitability for all classes of pollutants, whether anionic or cationic (Hlongwane et al. 2019; Wang et al. 2019; Sebastian et al. 2020). They have been successfully deployed in the remediation of water, soil, and air (Sannino et al. 2017). In many instances, they outperformed conventional adsorbents (Table 1). Silver, gold, zinc, copper, iron, titanium, manganese, magnesium, cobalt, nickel, and silver–gold nanoparticles are commonly used as adsorbents. They have been found to be effective for heavy metal removal and degradation of dyes.

CuONPs are low-cost adsorbents that possess a large band gap for photocatalytic degradation of pollutants. This is in addition to the large surface area, effective selectivity, excellent stability, highly sensitive, high adsorption efficiency for molecular oxygen, and low toxicity (Santhoshkumar et al. 2019; Cuong et al. 2022). Selvam et al. (2022), Thakur & Kumar (2019), Muthuvel et al. (2020), Zeebaree et al. (2021), Batool et al. (2019) have demonstrated the applications of CuONPs for the degradation of methylene blue, methyl orange, and congo red achieving high percentage decolorization. In another application, CuONPs were found to be highly effective for the adsorption of Cr(IV), methylene blue, As(III), and degradation of 4-nitrophenol and congo red effectively (Bordbar et al. 2017; Dubey & Sharma 2017; Singh et al. 2017; Vidovix et al. 2019; Cuong et al. 2022).

Studies by Sree et al. (2020) and Meshkatalsadat et al. (2022) showed that CaONPs biosynthesized from eggshell and Crataegus pontica C.Koch extract effectively photocatalyzed Rhodamine B, methylene blue, and Toluidine blue achieving complete degradation in many cases. Mohamed et al. (2021) reported the applications of CaONPs and CaONPs/carbon from eggshell for the degradation of methylene blue with 88.04 and 99.76% performance, respectively. Moreso, eggshell-derived CaONPs were applied for the remediation of bromocresol green, and oil spill and proven to be more effective than commercially available CaO (Osuntokun et al. 2018; Thakur et al. 2021).

Biogenic nanoparticles of selenium (SeNPs), palladium (PdNPs), and platinum (PtNPs): SeNPs have been shown to effectively adsorb heavy metals like Zn, Ni, Cu, Hg, and Cd due to their amorphous nature following a ligand-like adsorption process (Jain et al. 2015). PdNPs and PtNPs have been shown to exhibit catalytically degrade dyes, 4-nitrophenol, and converted Cr(VI) to Cr(III) owing to the dissociation of hydrogen molecular into radicals that initiate the process (Siddiqi & Husen 2016; Lebaschi et al. 2017; Garole et al. 2019; Kadam et al. 2020; Şahin Ün et al. 2021).

Magnesium oxide nanoparticles (MgONPs) are excellent adsorbents that have attracted the attention of industrialists due to their crystalline nature, physicochemical properties resistance to corrosion, heat, acid and alkali, excellent thermal conductivity, high porosity, and good mechanical strength. They have been studied for the adsorption of dyes and heavy metals (Co²⁺, Cd²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Mn²⁺) recording excellent adsorption of over 60% of the pollutants (Srivastava et al. 2015; Abinaya et al. 2021; Nguyen et al. 2021).

Conventional water remediation methods currently being used do not offer catalytic degradation of pollutants, but metallic nanoparticles do. This ensures the total degradation of pollutants with environmentally stable and lowly toxic nanoparticles by providing a shift from single degradation at a time to multi-degradation of pollutants (Mustapha et al. 2020; Yadav et al. 2020; Mathur et al. 2022). They are target-specific, especially for recalcitrant pollutants within a short treatment time into safe byproducts that are usually beneficial for other activities. Metal nanoparticles can be used as catalysts in a variety of ways. Catalysis: these include AgNPs, AuNPs and Ag-AuNPs. Photocatalysis: these include ZnONPs, CuONPs, TiO₂NPs, Fe₂O₃NPs, and Fenton-based catalysis (Fe₃O₄NPs) (Azzouz et al. 2018; Zhang et al. 2018; Adebayo et al. 2019; Pasinszki & Krebsz 2020; Mpongwana & Rathilal 2022).

Catalysis is an innovative technique of nanoparticles providing more advantageous environmental remediation in addition to adsorption to transform pollutants into innocuous forms. It is a redox process that involves donation or the removal of electrons to degrade pollutants, especially dyes (Lateef et al. 2015; Shanker et al. 2016; Elegbede et al. 2018). Numerous reports have described the catalytic degradation and decoloration ability of dyes by AgNPs and AuNPs with exceptional results, achieving about 96% decoloration of methyl orange, methylene blue, congo red, eosin Y, cresol green, rhodamine B, methyl red, methyl green, and p-nitrophenol (Table 2). This involves breakage of chromophoric functional groups responsible for color formation to degrade dyes (Bogireddy et al. 2016; Edison et al. 2016; Mata et al. 2016; Ojo et al. 2016; Bonigala et al. 2018; Veisi et al. 2018; Liu et al. 2018a; Muzaffar & Tahir 2018; Lateef et al. 2020). They were also reported to degrade organic compounds like 4-nitrophenol, 2-nitroaniline, and 4-nitroaniline (Mata et al. 2016; Francis et al. 2018). Catalysis is also useful for the degradation of persistent organic compounds whereby an electron transfer to pollutants initiates their degradation/transformation to nontoxic molecules (Bogireddy et al. 2016; Owaid et al. 2019; Attatsi & Nsiah 2020). Depending on the metallic composition and ingredients, catalysis can be done with mono, bi, and trimetallic nanoparticles for efficient degradation of organic pollutants (Miri et al. 2022). Studies by (Ojo et al. 2016; Elegbede et al. 2019; Dobrucka 2021) reported the applications of bimetallic alloys of Ag-AuNPs and trimetallic Au/CuO/ZnO for the catalytic degradation of organic dyes.

Fenton-based catalysis works on the oxidative degradation of organic pollutants mediated by zero-valent iron under aerobic conditions without illumination in a Fenton reaction (Nizamuddin et al. 2018). Fenton catalysts are promising candidates for overcoming the barrier of catalyst loss during the Fenton reaction. At pH below 5, OH⁻ radicals are generated and are responsible for the degradation of organic pollutants (Shanker et al. 2016; Liu et al. 2019). This technique has been used to degrade halogenated and nonhalogenated compounds (Pasinszki & Krebsz 2020).

CuONPs have been demonstrated for extensive adsorption applications such as photocatalysis of methylene blue and methyl orange which resulted in over 90% degradation (Batool et al. 2019; Vidovix et al. 2019; Mali et al. 2020; Zeebaree et al. 2021; Selvam et al. 2022). Bordbar et al. (2017) reported immediate total discoloration of rhodamine B, methylene blue, and conversion of 4-nitrophenol to 4-aminophenol upon the addition of nanocomposite of biogenic CuONP/clinoptilolite.

Magnesium oxide nanoparticles (MgONPs), nanosheet and magnesium oxide-coated nanocomposites are known for their high adsorption capacity for heavy metals owing to their polymorphic structure and large surface areas (Purwajanti et al. 2015; Cai et al. 2017). Panchal et al. (2022) and Jayapriya et al. (2020) reported the application of nanocomposites of silver nanoparticles-doped-MgO nanoparticles mediated with Aloe vera and Musa paradisiaca bract extracts and obtained over 90% photodegradation of methylene blue, o-nitrophenol, and phenol.

Silver nanoparticles (AgNPs) have been established as potent antipathogenics possessing the ability to control the presence of pathogens in water. This is thought to be due to AgNPs’ antimicrobial, antifungal, and antibacterial properties, which exhibit toxicity on pathogens, resulting in cell lysis (Elegbede et al. 2018; Akintayo et al. 2020; Azeez 2021). Also, the rapid release of Ag⁺ from AgNPs plays a major role. Ionic silver (Ag⁺) is known to be toxic to microorganisms, creating an oxidative burst that damages pathogen cells via disruption of electron transport chains, and microbial cellular homeostasis. This eventually compromises cell integrity and leads to pathogens’ death (Azzouz et al. 2018; Marimuthu et al. 2020). Silver nanoparticles effectively inhibited Aspergillus fumigatus, A. niger, Staphylococcus aureus, Escherichia coli, Klebsiella granulomatis, and Pseudomonas aeruginosa (Lateef et al. 2015, 2016a, 2016b; Ojo et al. 2016; Ojha 2020).

Titanium oxide nanoparticles (TiO₂NPs) exert their disinfecting power through photocatalysis. Reactive radicals generated are responsible for the disruption of biochemical and physical activities, creating ruptures in the gene expression of pathogens’ cells that eventually lead to their death (Singh et al. 2019). They also possess antimicrobial activity, which is due to the release of Ti ions that bind to the cell inhibiting its activities (Gomes et al. 2019; Yaqoob et al. 2020).

Zinc oxide nanoparticles (ZnONPs) induce their antimicrobial properties via photocatalysis. The free radicals generated during photocatalysis cause damage to the cell wall and membrane, ensuing disruption of the intracellular activities and inactivation of the adenylyl cyclase enzyme that is mostly responsible for the pathogenicity of microbes (Shanker et al. 2016; Singh et al. 2019). The antimicrobial activity of ZnONPs comes from the release of Zn ions that inhibit cell molecules by binding to them.

As water disinfectants, CuONPs have antibacterial and fungicidal effects derived from the release of Cu that binds efficiently with the bacteria wall, thereby causing it to rupture and interact with the sulfhydryl group of protein to denature and cause death (Mali et al. 2020; Selvam et al. 2022). It has also been noted that CuONPs have been found to produce an electrical current that weakens and causes holes in cell walls when they come into contact with the cell wall's outer and inner membranes. Once the CuONPs are inside the cell, they interrupt the cell metabolism, nutrient distribution, and enzyme activities (Santhoshkumar et al. 2019; Muthuvel et al. 2020; Nieto-Maldonado et al. 2022).

CaONPs have demonstrated antimicrobial and antifungal properties restricting the cell viability of microorganisms causing apoptosis (Ramola et al. 2019; Yoonus et al. 2021).

Magnetic nanoparticles are another option for the remediation of water as they provide magnetic properties to remove solids, heavy metals, and dyes (Sannino et al. 2017). Magnetic nanoparticle allows easy recovery of adsorbent from solutions with the assistance of a magnetic field to avoid the import of conventional methods of filtration and centrifugation (Saif et al. 2016; Galdames et al. 2020). Magnetic nanoparticles are inexpensive, have a large surface area, high adsorption ability, high porosity, high environmental stability, and are nontoxic (Lu & Astruc 2018; Nizamuddin et al. 2018). The most commonly used magnetic nanoparticles are magnetite (Fe₃O₄), maghemite (Fe₂O₃), and hematite (α-Fe₂O₃). They have different mechanisms for the removal of pollutants since they have variable oxidation states (Liu et al. 2019; Yang et al. 2019; Pasinszki & Krebsz 2020). They are characteristically excellent for the removal of dye (methylene blue, rhodamine B, malachite green), heavy metals (Zn²⁺, Pb²⁺, Cr⁶⁺, Hg²⁺), and chlorinated pollutants (Gómez-Pastora et al. 2017; Lu & Astruc 2018; Yang et al. 2019; Yakar et al. 2020).

Water issues are diverse, as are the pollutants found in it, which are of great concern due to their toxic effects on plants, animals, and humans. Due to these health challenges, disruption of water quality and limitations of conventional techniques, nanoparticles have played a major role in the removal of different classes of pollutants, but not without challenges. Possible challenges that could arise from the applications of nanoparticles for water remediation are:

Metallic nanoparticles have found valuable applications in a variety of human care products, food, and as water purification adsorbents. However, there is a genuine concern about safety as some nanoparticles have been reported to induce cell homeostasis, inhibit cell proliferation, DNA damage, disruption of protective enzyme activities, and damage to key organs in fishes and animals (Ahmed et al. 2017; Cardozo et al. 2019; Pedata et al. 2019; Akintayo et al. 2020; Badmus et al. 2020; Yoganandham Suman et al. 2020). There is also an operational concern about leaching into the bodies of water that can accumulate to cause harmful effects on aquatic animals. Dissolution of nanoparticles in water may release ions that are toxic to aquatic organisms (Yadav et al. 2020). These toxicities from experimental reports are associated with mostly nanoparticles synthesized with chemicals used as reducing agents. This is equally dependent on size, shape, concentration, aggregation, dissolution, surface charge, and generation of reactive oxygen radicals (Yoganandham Suman et al. 2020; Azeez 2021). Positively charged nanoparticles and those with rod-like shapes dissolve faster and are more toxic. Exposure to nanoparticles can lead to systemic or local toxicity causing inflammation of vital organs, suppression of immunity, thrombolysis, neurotoxicity, and genotoxicity, probably arising from the generation of reactive free radicals that suppress immune functions (Yoganandham Suman et al. 2020).

The use of green synthesis has drastically reduced the toxic nature of metallic nanoparticles. Green synthesized nanoparticles are eco-friendly, nontoxic, and cost-effective because biological macromolecules used as reductants are easily sourced (Elegbede et al. 2018; Harja & Ciobanu 2020; Marimuthu et al. 2020). Green synthesized nanoparticles have been tested for safety on onions, bulbs, cell lines, and blood, with little, or no known toxicity (Sebastian et al. 2020). Green synthesized nanoparticles appear to aid blood clothing, boost immunity, and suppress free radical production in numerous studies (Lateef et al. 2015, 2016a, 2016b).

The production cost of nanoparticles has been a subject of discussion owing to the affordability of metal or metal oxide salts needed for their synthesis. This is adequately catered for by the green synthesis procedure, which is simple, cost-effective, and biocompatible with various applications (Elegbede & Lateef 2020). Additionally, the absence of secondary pollution is an advantage over cost. Besides, the application of nanoparticles is not as expensive as conventional methods. The review by Mpongwana & Rathilal (2022) shows that it is economically feasible and sustainable to adapt metallic nanoparticles for water remediation due to their properties and ease of regeneration. However, this is nanoparticle-specific as nanoparticles such as magnetic nanoparticles are highly effective but could be toxic after use.

Advantages enjoyed by metallic nanoparticles are their sizes, shapes, and other morphological characteristics. These properties, however, tend to fade over time as nanoparticles aggregate into larger sizes with impeded movement, robbing them of much of their effectiveness (Azeez 2021). Another problem faced as adsorbents is difficulty in regenerating and separating them from solutions, which could constitute major environmental challenges. Silver nanoparticles were easily regenerated after decolorization of rhodamine B (Veisi et al. 2018). This is sorted by using magnetic nanoparticles that can easily be separated from the solution by magnetic force (Yang et al. 2019; Yakar et al. 2020).

The functions of nanoparticles are time-bound like every other technique. For instance, the antimicrobial activities of AgNPs are aided by the release of Ag⁺ and they slow down once there is a loss of Ag⁺ (Azeez 2021). This can be controlled by maintaining a controllable release of nanoparticles in solution since only a small portion is needed at a time.

Most experiments done on the use of nanoparticles for adsorption, photocatalysis, and disinfection have been on a laboratory-scale, and much of it has not been replicated in the industry. The applicability of metallic nanoparticles on a large scale, as being done in food and agriculture, should be encouraged for water remediation (Elegbede & Lateef 2020).

The disposal of pollutants into water bodies and existential global warming has resulted in insufficient high-quality water for humans, and many will face waterborne diseases as well as cancer from exposure to pollutants. The worst-case scenarios have occurred in developing/underdeveloped countries, where epidemics of cholera and metal poisoning are common daily. Despite this, little attention is paid to it. This is in addition to the use of certain conventional/traditional techniques and a lack of efficient infrastructure to support water remediation. It is often necessary to perform more than one treatment to achieve comprehensive remediation using conventional procedures, which has many limitations and is rarely cost-effective. The development of metallic nanoparticles with properties that make them suitable for disinfection, degradation of all pollutants, separation of solids, and adsorption of inorganic and organic pollutants is ongoing. Due to their size range (1–100 nm), shape, reactive surfaces, surface plasmon resonance, magnetism, optical activities, antibacterial characteristics, and quick release of ions, their use in water remediation is highly desirable to ensure comprehensive water remediation. Multiple reports demonstrate that metallic nanoparticle deployment is economically feasible and sustainable to combat water pollution when based on a detailed analysis of the economic feasibility of biogenic metallic nanoparticles. Despite these facts, there are still some limitations in terms of environmental bioaccumulation, industrial usability, and regeneration of biogenic metallic nanoparticles. In order to fully harness the potential of metallic nanoparticles for water purification, future research must critically consider how these limitations can be corrected.

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

The authors declare there is no conflict.