Green-synthesized nanocatalysts and nanomaterials for water treatment: Current challenges and future perspectives

Abstract

Numerous hazardous environmental pollutants in water bodies, both organic and inorganic, have become a critical global issue. As greener and bio-synthesized versions of nanoparticles exhibit significant promise for wastewater treatment, this review discusses trends and future prospects exploiting the sustainable applications of green-synthesized nanocatalysts and nanomaterials for the removal of contaminants and metal ions from aqueous solutions. Recent trends and challenges about these nanocatalysts and nanomaterials and their potential applications in wastewater treatment and water purification are highlighted including toxicity and biosafety issues. This review delineates the pros and cons and critical issues pertaining to the deployment of these nanomaterials endowed with their superior surface area, mechanical properties, significant chemical reactivity, and cost-effectiveness with low energy consumption, for removal of hazardous materials and contaminants from water; comprehensive coverage of these materials for industrial wastewater remediation, and their recovery is underscored by recent advancements in nanofabrication, encompassing intelligent and smart nanomaterials.

Graphical Abstract

1. Introduction

Nano-engineered materials, such as nanoadsorbents, nanometals, nanomembranes, and photocatalysts offer promising options for novel water technologies which can be adapted to customer-specific needs. A large majority of them are compatible with existing treatment technologies and can be integrated simply in the existing set-up. There are numerous contaminants in wastewater discharge which have adverse health effects namely pesticides, textile dyes, plasticizers, disinfection by-products, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and emerging pollutants such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), endocrine disrupting materials, pharmaceutical and personal care products (Bousselmi et al., 2004; Mozia et al., 2007; Rizzo et al., 2009). Innovative engineered nanomaterials are very encouraging for removal of these hazardous contaminants, as they have high surface areas and remarkable reactivity (Zhang et al., 2019). In this context, the development of greener protocols for the elimination of ionic metal species from water has witnessed profound interest (Iravani, 2011; Shukla and Iravani, 2017; Nadagouda and Varma, 2008; Moulton et al., 2010).

Nanotechnology and nanoscience, an area of research that has a progressed at a very fast pace, present numerous attractive options for water/wastewater treatment. Nowadays, nanostructured materials have garnered attention in the degradation as well as remediation of toxic organic/inorganic pollutants owing to unique physicochemical properties such as their high catalytic activity, high physical/chemical and thermal stability, large specific surface area, significant chemical reactivity, and strong electron transfer ability, among others (Pradhan et al., 2001; Sinha et al., 2013; Xu et al., 2019; Zhang et al., 2014). Indeed, nanomaterials and nanoparticles (NPs) are recently applied to address the environmental issues e.g. water contaminant treatment and/or environmental monitoring/sensing; they are considered as an excellent option, since the reactive nanostructures have potential features that render them more efficient to convert and/or remove hazardous/toxic pollutants into toxic-free substances. In general, nanostructured materials e.g. nanosorbents, nanoparticles (Pd, Au, Ag, Cu, Fe₃O₄, TiO₂, etc.), nanocatalytic membrane systems, are more efficient, require lesser time, environmentally-friendly and constitute low energy approaches but not all these systems are inexpensive or green, and hence are not applied yet to treat the wastewater on large scales. Consequently, there is an essential need to fabricate some green nanomaterials, which must be very effective, having high activity/efficiency, eco-friendly, green and easy to handle. In this respect, green-fabricated nanomaterials can be considered as good candidates for the photocatalysis application in practical water treatment systems, although still more elaborative studies should be performed regarding the application of these nanomaterials.

Organisms specifically fungi and bacteria are capable of surviving and multiplying under stressful conditions due to the presence of higher concentrations of toxic metals (Beveridge et al., 1996; Rouch et al., 1995). It appears that numerous reducing agents in organisms and biochemical trajectories lead to bioreduction of metal ions. In view of the critical function of these agents, there have been more studies pertaining to the role and appliance of genetically engineered and natural organisms in bioreduction of metal ions (Stephen and Macnaughtont, 1999). It has been realized that many organisms reduce various metals, metalloids and radio nuclides such as uranium(VI) (Lovley et al., 1991; Kashefi and Lovley, 2000; Bansal et al., 2004; Mukherjee et al., 2001; Fredrickson et al., 2000; Lloyd and Macaskie, 2000; Lovley and Phillips, 1992; Lovley et al., 1993) and technetium (VII) (Kashefi and Lovley, 2000; Fredrickson et al., 2000; Lloyd and Macaskie, 2000; Philipse and Maas, 2002; Lloyd and Macaskie, 1996) and trace metals including arsenic(V) (Sweeney et al., 2004; Laverman et al., 1995), chromium(VI) (Kashefi and Lovley, 2000; Fredrickson et al., 2000; Zhang et al., 1998, 1996; Wang, 2000; Lovley, 1993), cobalt(III) (Kashefi and Lovley, 2000; Zhang et al., 1996; Sastry et al., 2003; Slawson et al., 1992; Gorby et al., 1998; Caccavo et al., 1994), manganese(IV) (Kashefi and Lovley, 2000; Lovley, 2000), and selenium (VI) (Konishi et al., 2007; Oremland, 1994); majority of them being hazardous environmental contaminants. Therefore, these organisms can be utilized for removing metal and metal oxides contaminants from water and wastewaters (Lee et al., 2004; Grünberg et al., 2001; Lovley, 1995; Lovley and Coates, 1997). As an example, aquatic macrophytes exhibited great potential for eliminating heavy metals (Sood et al., 2012; Gunawardaha et al., 2016; Sarkar and Jana, 1986) which can be harnessed for producing metallic NPs, as well (Gunawardaha et al., 2016; Korbekandi et al., 2014).

The conventional physicochemical strategies for the fabrication of nanomaterials entail the participation of hazardous and volatile materials. This has prompted the researchers to design suitable bioinspired biogenic and greener strategies which are eco-friendly, safer, and cost-effective for the development of novel and efficient nano-scale adsorbents and catalysts which can be harnessed for eliminating and degrading various contaminants in water (Figs. 1 and 2). Indeed, the presence of various phenolic antioxidants in plants and other microorganisms serve as capping and reducing agents for the production of nanomaterials in varied shapes namely, flowers, wires, rods, and tubes.

Fig. 1.

Biogenic nanomaterials for wastewater treatment: notable advantages and challenges.

Fig. 2.

Categories of biogenic nanomaterials for appliances in wastewater treatment.

In this critical review, current trends and future prospects exploiting the application of green-synthesized nanocatalysts and nanomaterials for water and wastewater treatments are discussed. This encompasses advanced nanomaterials and development of novel nanosorbents attained via greener and sustainable processes for removing the contaminants and metal ions from aqueous solutions, including groundwater, drinking water, and wastewater treatment. Recent trends and forthcoming challenges pertaining to green-synthesized nanocatalysts and nanomaterials and their potential applications for treating and purifying wastewater are highlighted. The development of new ecofriendly treatment methods should be perceived as a critical element for the industries producing hazardous, toxic, and chemically-laden wastewater.

2. Mechanistic aspects

2.1. Mechanism for biological preparation of metal/metal oxide NPs

There are several eco-friendly and biological routes for the biogenic fabrication of nanomaterials using plants and microorganisms (Fig. 3) namely algae, bacteria, fungi, viruses, yeasts, and waste materials or fusion of such biogenic methods with alternative activation means such as microwave and ultrasound (Nasrollahzadeh et al., 2019a; 2019b; Singh et al., 2016). The presence of flavonoids, terpenoids, proteins, vitamins, phenolic acid, glycosides, carbohydrates, polymers, alkaloids and various antioxidants in such sources serve as capping/stabilizing and reducing agents for the production of sustainable nanostructures, namely nanoflowers, nanowires, nanorods, nanotubes, and nanoparticles. The biosynthesis of nanoparticles using plants and microorganisms as living organisms offers several environmental applications (Fig. 2) as exemplified by a simple and eco-friendly protocol deploying Parthenocissus quinquefolia leaf extract in presence of oxalic acid for the synthesis Fe, Cu-based nanoparticle adsorbents; they exhibit substantial adsorptive capacity for aqueous Malachite (Zhang et al., 2018).

Fig. 3.

Parameters for the fabrication of monodispersed and stable NPs. Reproduced with permission from Ref (Singh et al., 2016).

The historical utilization of organisms in the fabrication of bio (nano)materials dates back to 1980 by Beveridge et al., (Beveridge and Murray, 1980) when they evaluated the synthesis of gold NPs by using the Bacillus subtilis as an aerobic, gram-positive bacterium. Indeed, microorganisms have the capability to adsorb and accumulate metal ions, which can secrete a higher amount of enzymes by cell activities, thereby increasing the reduction of metal ions to their elemental form. The microbial generation of NPs depends on the presence of reductive enzymes/metabolites of the cell wall, and either their location on the cell or secretion of soluble enzymes (Fig. 4) (Sengani et al., 2017; Parandhaman et al., 2019; Nair and Pradeep, 2002). Indeed, enzymatic reduction processes have implicated the microbial enzymes/metabolites especially NADH and/or NADPH-dependent enzymatic reduction of metal ions to NPs (Parandhaman et al., 2019; Das et al., 2010).

Fig. 4.

(a) Mechanism of extracellular/intracellular synthesis of NPs by microbial enzymes and/or metabolites. (b) Lactobacillus bacterial cell can serve as support and reducing agent for the formation of NPs. Reprinted with permission from Refs (Sengani et al., 2017; Parandhaman et al., 2019; Nair and Pradeep, 2002).

Das, Marsili et al., (Das et al., 2012) recounted on the biosynthesis mechanism of Au NPs formation in the fungi mycelia of Rhizopus oryzae; a sizeable quantities of generated extra- or intracellular enzymes/proteins, apparently play a main function in the reduction of AuCl₄– ions to Au NPs and their subsequent stabilization by the capping activity of the enzymes. Consequently, two proteins (42 and 45 kDa) partake in the reduction of gold, whereas alternative protein of 80 kDa serve as capping entity in stabilization of the as-prepared Au NPs. Various enzymes namely nitrate reductase, sulfite reductase, keratinase, and alphaamylase, and also plant macro-enzymes possess the capability to help reduce and stabilize metal ions to NPs (Parandhaman et al., 2019; Durán et al., 2015). In the enzyme-assisted reduction (Durán et al., 2015), different amino acid residues bind to metal/metal oxide ions and then reduce them to metal/metal oxide NPs. These enzymes, secreted from microorganisms and plants inside or outside of the cell wall, are decidedly suitable for the bulk production of NPs via a facile and ecofriendly procedure (Thapa et al., 2017). Owing to the presence of the negative charge-bearing amino acids like glutamic or aspartic acid, the enzymes, peptides and proteins play a vital function in the reduction of metal ions to synthesize NPs. Further, polysaccharides can play a key role in the reduction of metal ions, which are largely available from plants and/or microorganisms; negative surface charge of polysaccharides in view of the presence of carboxylic or phosphoric groups, which can bind with the positively-charged metal/metal oxide ions via an electrostatic interactions, culminate in the formation of various metal/metal oxide NPs by the metal ions reduction (Banerjee et al., 2017).

In general, biological/biogenic approaches exploit plant polyphenols, microorganisms, algae, enzymes, and industrial and/or agricultural wastes. Among biomaterials/biomolecules, enzymes and their metabolites (e.g., proteins, polysaccharides, peptide chains, carbohydrates, and nucleic acids, among others) have been utilized as reducing/capping agents for the reduction of metal/metal oxide ions to generate assorted NPs and functionalization of NPs (metal/metal oxides, alloy, etc.) on inorganic supports. Coker et al., (Coker et al., 2010) described a novel and environmentally benign approach for the preparation of biogenic magnetite NPs (Fe₃O₄ NPs), and their subsequent decoration with Pd NPs using bacterium Geobacter sulfurreducens to reduce Fe³⁺-oxyhydroxide and Na₂PdCl₄ ions without modifying the surface of bio-mineral. Similarly, Lee et al., (Sureshkumar et al., 2010) synthesized a ferric/ferrous magnetic Ag nanocomposite (PMBC-Ag) as an easily recyclable heterogeneous nanocatalyst deploying bacterial cellulose (BC); Fe₃O₄ NPs gets precipitated, and integrated into the BC nanofibrous structure at alkaline pH, and then coated with a polydopamine layer via immersing in a dopamine solution. Subsequently, the PMBC-Ag was fabricated by incorporation of Ag NPs into the dopamine-amended magnetic BC (MBC) nanofiber by the reduction of Ag⁺ ions.

Furthermore, biomaterial can serve as an effectual support and host for the NPs. For example, Das et al., (Das et al., 2013) have shown that the cell-free protein extracts of R. oryzae can simply be anchored on the nanosilica surface (protein-conjugated nanosilica) and serve as an efficient template and/or host for growth of Ag NPs in situ on the nanosilica surface. Indeed, protein-based decoration of Ag NPs on the nanosilica surface (Ag@nanosilica) was accomplished by quickly adsorbing positively-charged Ag⁺ ions on the negatively-charged protein surface via an electrostatic (π-π stacking) contact. Microscopic studies have revealed that the nanosilica-supported stabilized fungal whole protein performed as both, the reducing and capping agent wherein the spherical Ag NPs (∼20 nm) were well-dispersed and stable over the whole surface of nanosilica; they exhibited an enhanced catalytic reduction of the 4-NP by this novel and recoverable Ag@nanosilica.

The biological capacity of plant-mediated synthesis of nanocatalysts and nanomaterials is remarkably enhanced owing to its environmentally benign nature and the unique single-step operation with a mechanism that entails synergistic reduction, stabilization and capping of the NPs. Overall, the mechanism of plant-mediated synthesis of nanomaterials employing diverse plants is presently under continual exploration. Various metal/metal oxide salts, including chlorides, acetates, and nitrates possess high reduction potentials because metals were attached to acetate and/or halogen and also have an electron donation tendency, which can enhancethe electron density of metals on their conjugative salts. The ionic forms of metals can be easily detached from anionic parts owing to the reduction process, which renders them stable via the use of plant extracts (Nasrollahzadeh et al., 2019c, a; Nasrollahzadeh et al., 2020a). For example, a likely mechanism for the Pd NP synthesis via the reduction of Pd²⁺ to Pd NPs using plant extract as a reducing/capping agent is presented in Fig. 5; biogenic synthesis of metal NPs using plants is a sustainable technique for generating NPs as shown below:

$$“\text{Metal}\text{salts}+\text{Plant}\text{sources}\to \text{Biocompatible}\text{metal}\text{NPs}+\text{Biocompatible}\text{by-products}”$$

Fig. 5.

Proposed reaction mechanism for the green-synthesized Pd nanomaterials. Redrawn from Ref (Nasrollahzadeh et al., 2020a).

A generalized view has been proposed for the biosynthesis of metal nanomaterials using the plant biomolecules, such as flavonoids and/or polyphenols for the reduction of the metal ions and the stabilization of the ensuing metal nanomaterials (Fig. 6) (Mittal et al., 2013; Huang et al., 2011a) as exemplified for the reduction/stabilization of PdCl₂ using OH groups of Delonix regia leaf extract that can reduce Pd(II) to Pd (0) (Fig. 7) (Dauthal and Mukhopadhyay, 2013).

Fig. 6.

Probable components of various plant extracts towards the reduction of metal ions to metal NPs. Redrawn from Ref (Mittal et al., 2013).

Fig. 7.

Gallic acid-assisted (a) Pd(II) reduction and (b) Pd NPs stabilization. Redrawn from Ref (Dauthal and Mukhopadhyay, 2013).

2.2. Mechanistic aspects for the degradation of various contaminants

Various physical, chemical, and biological technologies for treating the wastewater include ion-exchange, reverse osmosis, oxidation, adsorption, flocculation, sedimentation, membrane, ultra-filtration, and advanced oxidation processes (AOPs). Among these conventional technologies applied in pollution control, AOPs, namely the Fenton reaction, photocatalysis, ozonation, and/or combinations of these, are increasingly adopted in the degradation of organic pollutants, due to their great efficiency, easy handling, simplicity, and good reproducibility (Chong et al., 2010; Bremner et al., 2009). AOP includes in situ generation of highly reactive and nonselective chemical oxidants (e.g. ^•OH, H₂O₂, O₃, ^•O₂) to degrade non-biodegradable and resistant organic contaminants. Indeed, Fenton reaction using ^•OH radical is a sustainable, effective and low-cost technique for the treatment of water/wastewater, as shown below (Jaafara et al., 2019):

$$“{\text{Fe}}^{2+}+{\text{H}}_2{\text{O}}_2+{\text{H}}^{+}\to {\text{Fe}}^{3+}+{}^{\cdot }\text{OH}+{\text{H}}_2\text{O}”$$

(Jaafar et al. (2019)) have developed a series of quantum calculations based on the DFT (density functional theory) and ELF (electron localization function) for studying the degradation behavior of Neutral Red dye (NR), present in wastewater; mechanism of the Fenton reaction between the NR dye and free radicals (^•OH) for the degradation of NR dye (Fig. 8) were examined in aqueous medium. An electrophilic/nucleophilic free-radical interaction occurs between the nucleophilic center of NR dye, electrophilic center of hydroxyl radical, oxygen, and nitrogen b (N_b), leading to intermediate (I), which has one of the highest nucleophilic activation at the carbon a (C_a). Then, the second single bond can be formed via a nucleophilic attack of the C_a of the NR on the oxygen of the ^•OH radical leading to intermediate (II). The optimization of the NR dye structure and determination of global and local descriptors of chemical reactivity (e.g. chemical hardness, global/local electrophilicity, global nucleophilicity, chemical potential, and the local nucleophilicity indices) of ^•OH radical and NR dye were evaluated using the DFT technique.

Fig. 8.

Initial step proposed for the reaction of NR dye with hydroxyl radical. Redrawn from Ref (Jaafar et al., 2019).

Numerous pathways such as UV photolysis/photocatalysis, adsorption, reduction and (photo) degradation, have been deployed for treating the contaminants (Fig. 9) and removal organic/inorganic pollutants from groundwater, freshwater sediments, wastewater, etc (Eskandarloo et al., 2017).

Fig. 9.

Removal of pollutants by applying nanomaterials. Redrawn from Ref (Eskandarloo et al., 2017).

2.2.1. Photocatalytic degradation of organic pollutants

In general, nanomaterials either adsorb the contaminants or they degrade them by diverse catalytic methods e.g. assisted by NaBH₄, H₂O₂, and photocatalysis wherein green-synthesized NPs are excellent candidate for the photocatalytic water purification (Fig. 10) (Shivaji et al., 2020); toxic organic contaminants are decomposed into other products (Yaqoob et al., 2020) or complete mineralization of organic contaminants occurs to yield carbon dioxide, water, or some inorganic ions. Generally, a semiconductor e.g. TiO₂ would absorb the light that is higher or equal to the semiconductor band gap width, creating electron-hole pairs (e⁻-h⁺). The interaction on the surface of nanocatalyst with adsorbed species takes place in the reduction-oxidation (redox) reactions. Besides, h⁺_vb react with surface-bound water to form the ^•OH and concomitantly e⁻_cb selected using oxygen to generate a superoxide radical anion, as depicted below in equations.

Fig. 10.

Schematic representation and general mechanism for photocatalytic degradation of dye using green-synthesized NPs. Reproduced with permission from Ref (Shivaji et al., 2020).

A great deal of effort has recently been expanded to design novel and green photocatalytic materials based on metal-organic frameworks (MOFs), especially suited for their potential utilizations in the green degradation of toxic organic contaminants (Lin and Maggard, 2008; Yu et al., 2005; Liao et al., 2008; Toyao et al., 2013); various reports have appeared on the fabrication of MOF-based (photo)catalysts by transition metals to degrade highly toxic pollutants under visible, UV, or UV/vis light (Yu et al., 2005; Toyao et al., 2013). In this context, a MOF-5 was first suggested to behave as an effective photocatalyst (Fig. 11) (Alvaro et al., 2007); these MOFs possess a wide absorption band located in the range 500–840 nm that are assigned to delocalized electron living on a microsecond time scale, and most likely occupying a conduction band (CB),the actual CB energy value being estimated to be 0.2 V vs. NHE, with a 3.4 eV band gap (Fig. 11a). The strategy demonstrated comparable activities for the aqueous phenol degradation to that of a commercial TiO₂ or ZnO (Fig. 11b). As a result, a charge-separation state, with electrons in the CB and holes in valence bands (VB), rendersMOF-5 to function as an effective photocatalyst. Overall, like TiO₂, the phenol photodegradation could be occurring via a network of reactions, namely initial generation of radical cations by electron-transfer from phenol to the MOF-5 hole or the formation of oxygen active species (such as superoxide radical anions) via the reaction of oxygen with the photo-ejected electrons (Fig. 11c).

Fig. 11.

(a) Calculated values of the band gaps and position of the conduction and valence bands (CB and VB) for MOF-5 in comparison with those of commercial TiO₂. (b) A time conversion plot of the phenol disappearance (y axis represents “mol of phenol decomposed per g per mol”). (c) A possible mechanistic proposal towards the photodegradation of phenol utilizing MOF-5 photocatalyst. Reproduced with permission from Ref (Alvaro et al., 2007).

In a similar study, (Das et al. (2011)) developed a Zn₄O-containing doubly interpenetrated porous MOF (UTSA-38) with a band gap of 2.85 eV, which revealed good photocatalytic activity for the degradation of methyl orange (MO) in aqueous solutions under dark, visible and UV/vis light. The proposed mechanisms for the photodegradation of MO by UTSA-38 under UV or visible light irradiation are illustrated in Fig. 12; MO can be completely decomposed into colorless small molecules under UV light for 120 min.

Fig. 12.

Main pathways proposed for the photodegradation of MO by UTSA-38 under visible or UV/vis light irradiation. Reproduced with permission from Ref (Das et al., 2011).

2.2.2. Reduction of nitro compounds and dyes

Among different reducing agents, NaBH₄ has been extensively considered a favored water soluble reductant and preferred alternative to hydrogen sources in the reduction of toxic nitro compounds to significant and useful amino compounds in an aqueous medium. The NaBH₄ activation is a main process, which requires a metal substrate as active site since metal hydride complexes fabricated from the BH₄⁻ ions via π-π stacking interactions have been considered intermediates in this reduction reaction (Nasrollahzadeh et al., 2020b). The reduction of toxic 4-NP using NaBH₄ as a reductant was described in presence of Pd nanocatalyst stabilized amine modified zeolite (Pd NPs@Zeo) via π-π stacking interactions (Fig. 13) (Nasrollahzadeh et al., 2020b). Pd NPs@Zeo converts NaBH₄ to molecular H₂ and also BO₂⁻ dissociated on the surface nanocatalyst, wherein the adsorbed 4-NP interacts with the dissociated H₂ gas and the 4-NP reduction occurs in a step-wise manner to generate 4-aminophenol. As a result, the as-prepared aminophenol is finally desorbed from the surface of the nanocatalyst and the subsequent catalytic run starts afresh. Indeed, the main role of nanocatalyst is to adsorb the molecular H₂ and/or 4-NP in the close proximity to facilitate simple reduction.

Fig. 13.

Possible mechanistic pathway for the NaBH₄-assisted reduction of 4-NP by Pd NPs@Zeo. Reproduced with permission from Ref (Nasrollahzadeh et al., 2020b).

Inspired by the biosynthetic mineralization process, magnetically separable nanobiohybrid catalysts, Fe₃O₄@Ch-AuNPs and Fe₃O₄@Ch-PdNPs (Fig. 14), have been designed and fabricated via a three-step procedure (Parandhaman et al., 2016). The spherical Fe₃O₄ NPs (∼35 nm) were initially generated using Shewanella algae and then functionalized or coated with chitosan, followed by decoration with Pd and/or Au NPs to generate a water dispersible and reusable nanobiohybrid catalyst; they exhibited noteworthy activities for the reduction of 4-NP and photodegradation of dye (> 99 % conversion) in polluted water at room temperature. The reaction was suggested to occur by the adsorption and reduction of MB by Pd or AuNPs through an electron transfer process. The rate of the reactions followed pseudo-second-order rate kinetics; Fe₃O₄@Ch-PdNPs and -AuNPs took just 1 min under UV light to complete the MB reduction with an apparent rate constant (k_app) of 5.0 min⁻¹ and 4.0 min⁻¹. Besides, authors reported that the normalized rate constant (k_nor) values are 1.72×10² and 1.14 × 10² mmol⁻¹s⁻¹, respectively, representing superior catalytic activities of the synthesized Fe₃O₄@Ch-PdNPs and -AuNPs for the degradation of MB.

Fig. 14.

Biomolecule directed synthesis of a magnetite@chitosan-Au or Pd NPs and (HR)SEM images of well-dispersed spherically-shaped nanocomposite. Reproduced with permission from Ref (Parandhaman et al., 2016).

2.2.3. Adsorption of arsenic

Green-fabricated amorphous iron NPs (with the specific surface area of 51.1368 m² g⁻¹) was evaluated for removing highly toxic and carcinogenic arsenic (As) from polluted resources (Wu et al., 2019). Consequently, it was detected that arsenate was uniformly adsorbed on the surfaces of iron NPs; FTIR evaluation showed that the adsorption was predominantly through an FeOAs bond, while XPS analyses revealed that only As(V) was adsorbed. Thus, the suggested mechanism for arsenate removal is based on primarily iron NPs reacting with arsenate to produce a monodentate chelating ligand and then a bidentate binuclear complex. More investigations demonstrated that the maximum adsorption capacity of the prepared NPs for arsenate was about 14.617 mg g⁻¹, and the optimal pH range for adsorption of anionic arsenate was between 4 and 6 (Wu et al., 2019). The sorption kinetics was also examined, and the Langmuir adsorption isotherms indicated that As(V) adsorption by iron NPs best fit the regression coefficient (R_L² = 0.9903), thus validating the proposed chemisorption; the adsorption efficiency fitted the pseudo-second-order kinetic model well. Thus, the green-synthesis of iron NPs have high application potential towards elimination of As(V) and simplicity of their preparation.

An iron-based MOF, MIL-88B, was green-synthesized at room temperature wherein MIL-88B(Fe), with remarkable adsorption capacity of 156.7 mg g⁻¹ at a low dosage, was analyzed for eliminating arsenate in water; capacity for removing trace arsenate on MIL-88B(Fe) was ∼32.3 mg g⁻¹ at a low equilibrium concentration (6.4 μg L⁻¹), which satisfied the arsenic threshold for drinking water. The FTIR and XPS analyses validated that the arsenate molecules bonded with the oxygen molecules, coordinating with FeO clusters in the framework (Hou et al., 2018).

2.2.4. (Photo)degradation and adsorption of organic contaminants

Compared with conventional water treatment processes (e.g. adsorption, conventional oxidation process, etc.), (photo)degradation and Fenton-like reaction have been broadly utilized in the pollutants treatment (Lai et al., 2016; Huang et al., 2017; Wang et al., 2016; Khodadadi et al., 2017a). However, some drawbacks (such as low efficiency/activity, low oxidation rate, low pH levels, etc.) restrain the potential applications of these individual approaches to economically dispose the toxic contaminants. As an example, a bio-inspired strategy based on biomimetic photocatalytic systems over a combined g-C₃N₄-imidazole-hemin assisted by H₂O₂ showed excellent photocatalytic oxidation activities under solar irradiation (Chen et al., 2017).

In another study, a low-cost photocatalyst Bi₂WO₆ and an ecofriendly biomimetic material hemin together enabled the development of a novel and efficient hemin-modified Bi₂WO₆ composite via a facile solvothermal technique (Yi et al., 2018). Combining experimental and theoretical investigations showed an excellent catalytic activity with enhanced pH tolerance by using simulated-solar light (SSL)/H-Bi₂WO₆/H₂O₂ process. According to the experimental results, a plausible reaction mechanism for high photocatalytic activity/stability of the H-Bi₂WO₆ is suggested in Fig. 15; ^•O₂⁻, Fe(IV)=O, and ^•OOH active species played the key role in the SSL/H-Bi₂WO₆/H₂O₂ system for the RhB degradation.

Fig. 15.

Proposed mechanism for the H₂O₂-assisted H-Bi₂WO₆ photocatalytic degradation of organic pollutants under solar irradiation. Reproduced with permission from Ref (Yi et al., 2018).

3. Applications of green-synthesized nanomaterials for water and wastewater treatment

Green-synthesized and biogenic NPs can be explored for remediation in sewage systems, treatment plants, membrane bioreactors and the other state-of-the-art water purification devices to reduce or eliminate the perilous contaminated materials in water resources. However, the size control, stability, aggregation and sedimentation are still persistent challenges for the commercial appliances of biogenic NPs in treatment of effluents. Heavy metals removal and degradation of inorganic, organic, radioactive and pharmaceutical pollutants, nitro compounds (e.g. 4-NP as a toxic nitroarene), nitrate, phosphate, and also hazardous dyes such as methyl orange (MO), Congo red (CR), Eosin Y (EY), rhodamine B (RhB), methylene blue (MB), etc. have been undertaken via nano-adsorbents, nanocatalysts and nano-films in view of their high efficiency and greater surface area (Lapworth et al., 2012; Yadav et al., 2015; Gautam et al., 2015; Arora et al., 2014; Kim et al., 2007; Gawande and Jenkins-Smith, 2001; Tyagi et al., 2018; Chipasa, 2003).

3.1. Removal of organic and inorganic contaminants

One-step ambient temperature preparation of Au NPs on γ-Al₂O₃ supports (Fig. 16a) has been demonstrated by deploying polyphenols from bayberry tannin (BT) plant (Huang et al., 2011b); formation of Al₂O₃/BT/Au NPs entailed initial reduction of AuCl₄⁻ by BT as a reducing and stabilizing agent, which up on adsorption and glutaraldehyde-assisted self-crosslinking on the porous γ-Al₂O₃, generated a bridged structure. The activity of well-dispersed spherically-shaped gold NPs on γ-Al₂O₃ with a size of∼23 nm (Fig. 16b,c) was revealed for the reduction of 4-NP as an active and recyclable nanocatalyst assisted by NaBH₄-mediated reduction.

Fig. 16.

(a) Schematic diagram illustrating the biomolecule directed synthesis of Al₂O₃/BT/Au NPs, (b,c) (HR)TEM images of the well-dispersed spherically-shaped Au NPs on Al₂O₃ support. Reproduced with permission from Ref (Huang et al., 2011b).

Biogenic metal NPs from natural sources, such as algae, bacteria, plants and fungi have shown eminent capabilities for environmental applications, especially wastewater treatment. Earth-abundant elements should be considered first in these remediation endeavors. Iron NPs generated using aqueous green tea extract showed catalytic activity to degrade malachite green (Weng et al., 2013; Plachtová et al., 2018; Markova et al., 2014; Nadagouda et al., 2010) and their eco-toxicological impact has been evaluated (Markova et al., 2014). Additionally, the production, characterization and biocompatibility aspects of green tea-derived silver NPs have been reported (Markova et al., 2014). In another study, Anatase TiO₂ NPs doped with iron were produced via a greener method by applying aqueous extract of lemongrass (Cymbopogon citratus) (Solano et al., 2019), where these Fe-TiO₂ NPs exhibited potential for photocatalytic treatment of wastewater, and degradation of organic pollutants (Solano et al., 2019). Additionally, tea extract-facilitated biofabrication of Fe and Fe/Pd bimetallic NPs (∼20−30 nm) has been described for removing trichloroethane, a highly toxic chemical from water via reductive degradation mechanism (Smuleac et al., 2011).

The elimination of As(V) and As(III) by deployment of magnetic iron oxide NPs (∼5−25 nm) biosynthesized from tea waste has been described (Lunge et al., 2014); the elimination of trivalent and pentavalent arsenic (maximum adsorption capacities were about 188.69 and 153.8 mg g⁻¹ for As(III) and As(V), respectively) (Fig. 17) was illustrated (Lunge et al., 2014). Moreover, iron oxide NPs were fabricated by applying bio-reducing agents from eucalyptus extract wherein ensuing NPs were seized in chitosan to form a recyclable magnetic organic-nano iron hybrid for removal of arsenic from water (Martínez-Cabanas et al., 2016).

Fig. 17.

SEM images of the produced magnetic iron oxide NPs-Tea. Reproduced with permission from Ref (Lunge et al., 2014).

A reduced graphene oxide-silver NP hybrid nanocomposite was synthesized under greener conditions using aqueous extract of Brassica nigra (Karthik et al., 2020) which showed antibacterial activities and could be deployed as a photocatalytic agent for removing dyes; Dye, Direct blue-14 (DB-14) was employed to evaluate the adsorption productivity of the prepared nanocomposites. The unqualified recovery of adsorbent after the reaction and its unchanged efficiency for cyclic applications demonstrated that it may serve as an economically and eco-friendly photocatalyst (Karthik et al., 2020).

Ammonia and phosphate, in natural water resources can make remarkable deterioration of pristine water ecosystems because of eutrophication; thus, the innovative and cost-effective remediation methods are highly necessitated (Xu et al., 2020). In one study, the greener produced iron oxide NPs dispersed onto zeolite by eucalyptus leaf extracts, were applied to concurrently eliminate ammonia and phosphate from aqueous solutions; at primary concentration of 10 mg L⁻¹ each for two co-existing ions, the prepared material eliminated 43.3 % of NH₄⁺ and 99.8 % of PO₄³⁻. After optimization evaluations, the conditions for maximum adsorption capacity of the produced material for NH₄⁺ and PO₄³⁻ were 3.47 and 38.91 mg g⁻¹, respectively (Xu et al., 2020) (Fig. 18).

Fig. 18.

Green-produced iron oxide NPs dispersed onto zeolite by eucalyptus leaf extracts (EL-MNP@zeolite). Reproduced with permission from Ref (Xu et al., 2020).

In another investigation, water-soluble green-fabricated fluorescent carbon quantum dots (QDs) (∼260−400 nm), have been prepared by hydrothermal treatment using Tamarindus indica leaves (Bano et al., 2018). The ensuing QDs can be applied as sensitive probe for sensing Hg²⁺ with a detection boundary of 6 nM in the dynamic span of 0 to 0.1 μM; the feasibility of this detecting device was tested by using ‘real’ pond water samples for detection of Hg²⁺, and may be adaptable for additional analysis (Bano et al., 2018). A large assortment of biosynthesized metallic nanocatalysts deployed for the remediation and degradation of various pollutants in water or wastewater are presented in Tables 1 and 2.

Table 1.

Important biosynthesized metal and metal oxide-based nanocatalysts in the degradation of pollutants in water.

Entry	Nanocatalysts	Applications	Biogenic source	Refs.
1	Ag nanoparticles/clinoptilolite	Reduction of MB, MO, CR and RhB	Vaccinium macrocarpon fruit	(Khodadadi et al., 2017b)
2	rGO/Ag-Au NPs	Reduction of toxic Cr(VI)	Albizia Saman leaf	(Vellaichamy and Periakaruppan, 2016a)
3	Ag/zeolite nanocomposite	Reduction of MB, 4-NP, CR, RhB and MO	Euphorbia prolifera leaf	(Hatamifard et al., 2016)
4	AgPd NPs	Electrocatalytic reduction of H2O2	Lithodora hispidula (Sm.) Griseb. leaf	(Turunc et al., 2017)
5	Ag/RGO nanocomposite	Reduction of CR, 4-NP and RhB	Abutilon hirtum leaf	(Maryami et al., 2016)
6	ZnO-Ag nano custard apple	Degradation of MB	Pomegranate peel	(Kaviya and Prasad, 2015)
7	Ag/bentonite nanocomposite	Reduction of MB, 4-NP, CR and RhB	Euphorbia larica	(Sajadi et al., 2018)
8	Ag NPs	Photodegradation of bromo phenyl blue (BPB)	Cirsium japonicum	(Khan et al., 2016)
9	Ag-Mo/CuO NPs	Photodegradation of MB	Azadirachta indica leaf	(Rajendaran et al., 2019)
10	Au and Ag-Au NPs	Degradation of malachite green	Bacillus safensis	(Ojo et al., 2016)
11	Ag-SnO2 nanocomposites	Degradation of MB, Methyl Violet 6B, Rose Bengal and 4-NP	Saccharum officinarum	(Sinha et al., 2017)
12	Ag@AgCl NPs	Degradation of Victoria Blue B	Aquilaria agallocha (AA) leaf juice	(Devi et al., 2016)
13	RGO nanosponge/Ag-NP	Reduction of 4-NP	Tabebuia berteroi leaf	(Vellaichamy and Periakaruppan, 2016b)
14	Ag NPs	Degradation of RB-21, reactive Red-141 (RR-141) and Rhodamine-6G	Palm shell	(Vanaamudan et al., 2016)
15	Ag NPs	Reduction of 4-NP	Phoenix Dactylifera L. (date palm) leaf	(Aitenneite et al., 2016)
16	Ag-ZnO	Photodegradation of MB	Azadirachta indica (Neem) leaf	(Patil et al., 2016)
17	Ag/MgO nanocomposite	Reduction of MB, 4-NP, MO and 2,4-dinitrophenylhydrazine (2,4-DNPH)	Acalypha hispida	(Nasrollahzadeh et al., 2020c)
18	Ag nanocomposite hydrogels based on sodium alginate	Removal of MB	Mukia maderaspatna leaf	(Karthiga Devi et al., 2016)
19	Ag/polyphenols-modified graphene	Reduction of 4-NP	Green tea	(Wang et al., 2015)
20	Ag/ZnO in montmorillonite	Photodegradation of MB	Urtica dioica leaf	(Sohrabnezhad and Seifi, 2016)
21	Ag NPs	Reduction of 4-NP	Coleus forskohlii root	(Naraginti and Sivakumar, 2014)
22	Ag/TiO2 NPs	Photodegradation of MB	Rambutan (Nephelium lappaceum L.) peel	(Kumar et al., 2016a)
23	Ag-TiO2 nanopowders	Photodegradation of MB	Carambola	(Chowdhury et al., 2016)
24	Ag NPs	Degradation of malachite green	Extract of paper wasp (Polistes sp)	(Lateef et al., 2016)
25	Ag NPs	Degradation of RhB and MB	Parkia roxburghii leaf	(Paul et al., 2016)
26	Ag/AgCl NPs	Photodegradation of malachite green	Benincasa hispida (ash gourd) peel	(Devi and Ahmaruzzaman, 2016)
27	Ag NPs	Photodegradation of Putnam sky blue 39	Rosa ‘Andeli’ double delight petals aqueous extract (PERA)	(Suárez-Cerda et al., 2015)
28	Ag NPs/peach kernel shell	Reduction of MB, 4-NP and MO	Achillea millefolium L.	(Khodadadi et al., 2017a)
29	Ag NPs	Biosorbent to treat industrial effluents	Morinda Tinctoria leaf	(Vennila and Prabha, 2015)
30	Ag NPs	Photodegradation of MB	Polygonum minus	(Ullah et al., 2017)
31	Ag NPs	Photodegradation of Methyl Red (MR)	Piper pedicellatum C.DC leaf	(Tamuly et al., 2014)
32	Ag-Cr-AC nanocomposites	Removal of binary dye system of Reactive Red (RR) and CV	Azadirachta indica leaf	(Saad et al., 2017)
33	Ag/CN-TiO2@g-C3N4	Photodegradation of RhB	Bamboo leaf	(Jiang et al., 2014)
34	Ag/Hazelnut shell nanocomposite	Reduction of MO and CR	Origanum vulgare leaf
35	Ag NPs	Photodegradation of Coomassie Brilliant Blue G-250	Coccinia grandis leaf	(Arunachalam et al., 2012)
36	Ag NPs	Degradation of MB	Plectranthus amboinicus leaf	(Zheng et al., 2017)
37	Ag NPs	Photodegradation of MB	Biebersteinia multifida	(Miri et al., 2018a)
38	Ag/Cu NPs	Degradation of toxic chlorpyrifos pesticide	Carica papaya	(Rosbero and Camacho, 2017)
39	Ag NPs	Photodegradation of MB	Trichodesma indicum leaf	(Kathiravan, 2018)
40	Au@Ag@AgCl core-double shell NPs	Reduction of 2,4,6-trinitro phenol and photodegradation of ibuprofen and clofibric acid	Momordica Charantia leaf	(Devi and Ahmaruzzaman, 2017)
41	Ag NPs	Photodegradation of MB	Prosopis farcta fruit	(Miri et al., 2018b)
42	Ag NWs-rGO nanosheets	Reduction of 4-NP and 2-nitrophenol (2-NP)	Abelmoschus esculentus	(Gnanaprakasam and Selvaraju, 2014)
43	Ag@Fe bimetallic NPs	Degradation of bromothymol blue	Palm dates fruit	(Al-Asfar et al., 2018)
44	Ag and Au NPs	Reduction of 4-nitroaniline	Citrus aurantifolia peel	(Dauthal and Mukhopadhyay, 2015)
45	Ag NPs	Reduction of EY and CR	Synedrella nodiflora leaf	(Vijayan et al., 2018)
46	Au-Ag bimetallic nanocomposite	Reduction of 4-NP	Silybum marianum seed	(Gopalakrishnan et al., 2015)
47	Ag NPs	Degradation of MB	Trichodesma indicum leaf	(Kathiravan, 2018)
48	Ag NPs	Reduction of 4-NP	lavender leaf	(Kumar et al., 2016b)
49	Ag NPs	Reduction of 4-NP, MB, MO and MR	Stemona tuberosa Lour	(Bonigala et al., 2018)
50	Ag/HZSM-5 nanocomposite	Reduction of MB, CR, RhB and 4-NP	Euphorbia heterophylla leaf	(Tajbakhsh et al., 2016)
51	Ag NPs	Reduction of 4-NP	Ficus hispida Linn. f. leaf	(Ramesh et al., 2018)
52	Ag NPs	Reduction of poisonous nitro compounds	Extract of date palm	(Farhadi et al., 2017)
53	Ag NPs	Degradation of CR and MO	Salvia microphylla Kunth leaf	(Lopez-Miranda et al., 2018)
54	Ag NPs	Reduction of Eosin Blue (EB) and 4-NP	Sapindus mukorossi fruit	(Dinda et al., 2017)
55	Ag NPs	Reduction of 4-NP	Citrus maxima peel	(Huo et al., 2018)
56	Ag NPs/almond shell	Reduction of MB, RhB and 4-NP	Ruta graveolens sleeves	(Bordbar, 2017)
57	Ag NPs	Reduction of 4-NP	Allium ampeloprasum L. leaf	(Khoshnamvand et al., 2019)
58	Au, Ag and Ag/Au alloy NPs	Reduction of 4-NP	Guazuma ulmifolia L. bark	(Karthika et al., 2017)
59	Ag NPs	Photodegradation of MB	Mortiño berry	(Kumar et al., 2019)
60	Pd NPs	Reduction of 4-NP	Frimiana simplex	(Peng et al., 2019)
61	Pd NPs	Reduction of organic pollutant	Lagerstroemia speciosa	(Garole et al., 2019)
62	Natrolite zeolite/Pd nanocomposite	Reduction of MB, MO, CR, RhB and 4-NP	Piper longum fruit	(Hatamifard et al., 2015)
63	Pd-RGO nanocomposite	Degradation of dye pollutants	Rosa Canina fruit	(Nasrollahzadeh et al., 2020d)
64	Au-Pd	Reduction of 3-nitroaniline	Delonix regia	(Dauthal and Mukhopadhyay, 2016)
65	Pd NPs	Reduction of dyes	Anogeissus latifolia gum	(Kora and Rastogi, 2018)
66	Pd NPs	Degradation of Bismarck brown and antidandruff	Lagenaria siceraria	(Kalpana and Rajeswari, 2018)
67	Pd NPs	Photodegradation of MB	Andean blackberry	(Kumar et al., 2015)
68	Pd NPs	Cr(VI) reduction	Garcinia pedunculata Roxb	(Hazarika et al., 2017)
69	Pd NPs	Reduction of organic dyes	Terminalia arjuna	(Garai et al., 2018)
70	Pd/RGO	Reduction of various dyes	Artemisia abrotanum	(Hashemi Salehi et al., 2019)
71	Pd/perlite nanocomposite	Reduction of 4-NP, CR, RhB, MO and 2,4-DNPH	Euphorbia neriifolia L. leaf	(Maryami et al., 2017)
72	Pd NPs	Degradation of dyes	Pimpinella tirupatiensis	(Narasaiah et al., 2017)
73	Pd/walnut shell nanocomposite	Degradation of RhB, CR, and MB	Equisetum arvense L.	(Bordbar and Mortazavimanesh, 2017)
74	Pd/Fe3O4 nanocomposite	Degradation of Cr(VI), 4-NP and 2,4-DNPH	Hibiscus tiliaceus L.	(Nasrollahzadeh et al., 2018b)
75	Pd/bentonite nanocomposite	Degradation of 2,4-DNPH, Cr(VI), and 4-NP	Gardenia taitensis leaf	(Nasrollahzadeh et al., 2018c)
76	Pd NPs/sodium borosilicate glass	Reduction of 4-NP, 2,4-DNPH, MO, CR, MB, and Cr(VI)	Euphorbia milii	(Nasrollahzadeh et al., 2018d)
77	Pd NPs	Diatrizoate removal from hospital wastewater	S. oneidensis	(De Gusseme et al., 2011)
78	Cu/reduced graphene oxide/Fe3O4 nanocomposite	Reduction of 4-NP and RhB	Euphorbia wallichii leaf	(Atarod et al., 2015)
79	CuO/ZnO nanocomposite	Reduction of 4-NP and RhB	Melissa Officinalis L. leaf	(Bordbar et al., 2018)
80	Cu NPs	Degradation of MR	Peel extract of Citrus grandis	(Sinha and Ahmaruzzaman, 2015)
81	CuO NPs	Photodegradation of MB	Tinospora cordifolia	(Nethravathi et al., 2015)
82	Cu/ZnO NPs	Degradation of MB and CR	Euphorbia prolifera leaf	(Momeni et al., 2016)
83	Cu NPs	Degradation of Bismarck brown	Tridax procumbens leaf	(Kalpana et al., 2016)
84	Cu nanoflowers	Degradation of MB	Ficus benghalensis leaf	(Robati et al., 2016)
85	CuO NPs	Reduction of 4-NP	Tecoma castanifolia leaf	(Sharmila et al., 2016)
86	Cu/Fe3O4/eggshell nanocomposite	Reduction of MO, 4-NP, CR, RhB and MB	Orchis mascula L. leaf	(Nasrollahzadeh et al., 2016a)
87	Cu/Fe3O4 NPs	Reduction of 4-NP, CR and RhB	Morinda morindoides seeds	(Nasrollahzadeh et al., 2016b)
88	CuO nanocrystals	Degradation of MB, MO, MR, EY and reduction of 2-NP, 3-NP and 4-NP	Psidium guajava leaf	(Sreeju et al., 2017)
89	CuO NPs	Reduction of 4-NP	Fruit extract of plant Fortunella japonica	(Singh et al., 2017)
90	CuO NPs	Photodegradation of Acid Black 210 (AB)	Abutilon indicum	(Ijaz et al., 2017)
91	CuO NPs	Degradation of 4-NP	Rosehip	(Jafarirad et al., 2018)
92	CuO NPs	Reduction of CR, MB and 4-NP	Aglaia elaeagnoidea flowers	(Manjari et al., 2017)
93	CuO NPs	Photodegradation of RhB	Ferulago angulata (schlecht) boiss	(Mehr et al., 2018)
94	CuS NPs	Degradation of safranin O (SO)	Calotropis gigantean leaf	(Ayodhya and Veerabhadram, 2017)
95	CuO NPs/clinoptilolite	Degradation of 4-NP, RhB and MB	Rheum palmatum L. root	(Bordbar et al., 2017)
96	Cu-doped ZnO NPs	Degradation of Acid Black 234	Clerodendrum infortunatum and Clerodendrum inerme	(Khan et al., 2018)
97	Cu NPs	Removal of nitrate	Extract of Hibiscus sabdariffa flowers	(Paixão et al., 2018)
98	CdS	Removal of Cd	P. aeruginosa JP-11	(Raj et al., 2016)
99	Se	Removal of Zn(II)	Anaerobic microbial consortium	(Jain et al., 2015)
100	Se	Removal of Hg0	Citrobacter freundii Y9	(Wang et al., 2018)
101	Mn	Removal of Pb(II), Cd(II), and Zn(II)	Pseudomonas putida MnB1	(Zhou et al., 2015)
102	MgO	Removal of Ni(II),Pb(II) Cd(II), Cu(II), Zn(II),Co(II), Mn (II)	Acacia sp.	(Srivastava et al., 2015)
103	ZnO NPs	Degradation of Synozol Navy Blue-KBF textile dye	Trianthema portulacastrum	(Khan et al., 2019)
104	ZnO nano-flowers	Photodegradation of MB, EY and Malachite green (MG)	Panos	(Kaliraj et al., 2019)
105	ZnO NPs	Degradation of CR	Artocarpus Heterophyllus leaf	(Vidya et al., 2017)
106	ZnO NPs	Degradation of Alizarin Red-S	Carica papaya milk (CPM) latex	(Sharma, 2016)
107	ZnO NPs	Photodegradation of MB	Hydnocarpus alpina Wt	(Ganesh et al., 2019)
108	SnO2-ZnO	Degradation of MO	Gel of Aloe vera plant	(Sudhaparimala and Vaishnavi, 2016)
109	ZnO NPs	Degradation of RhB and MB	Seeds extract of Parkia roxburghii	(Paul et al., 2017)
110	ZnO/MgO nanocomposite	Degradation of MO, MB and 2-NP	Musa paradisiaca bract	(Maruthai et al., 2018)
111	ZnO/NiFe2O4 NPs	Photodegradation of MB	Mangifera indica leaves	(Adeleke et al., 2018)
112	ZnO NPs	Photodegradation of RhB	Cyanometra ramiflora leaf	(Varadavenkatesan et al., 2019)
113	ZnO NPs	Photodegradation of MB	Thymus vulgaris leaf	(Zare et al., 2019)
114	ZnO NPs	Degradation of MO, CR, RhB and MB	Abelmoschus esculentus mucilage	(Prasad et al., 2019)
115	Fe-ZnO NPs	Photodegradation of naphthalene	Amaranthus dubius aqueous leaf	(Muthukumar et al., 2017)
116	ZrO2/rGO nanocomposite	Photodegradation of RB 4 dye	Cinnamon	(Gurushantha et al., 2017)
117	Hollow microspheres Mg-doped ZrO2 NPs	Photodegradation of RhB	Aloe vera gel	(Renuka et al., 2016)
118	rGO/TiO2/Co3O4	Degradation of MB and CV	Shuteria involucrata leaf	(Ranjith et al., 2019)
119	α-Fe2O3/TiO2	Degradation of MB	Flax seed	(Mohamed et al., 2019)
120	SnO2 NPs	Photodegradation of MB, MO and erichrome black T	Erwinia herbicola	(Srivastava and Mukhopadhyay, 2014)
121	Au NPs	Reduction of 4-NP	Trichoderma viride and Hypocrea lixii	(Mishra et al., 2014)
122	Au NPs	Degradation of CR and MB	Cellular extract of Bacillus marisflavi	(Nadaf and Kanase, 2016)
123	Au NPs	decolorization of cationic Red X-GRL, Acid Orange II and Acid scarlet GR	Aspergillum sp. WL-Au	(Qu et al., 2017)
124	Au NPs	Reduction of 4-NP	Aspergillum sp. WL-Au	(Shen et al., 2017)
125	Manganese oxides	Removal of bisphenol A	Desmodesmus sp. WR1	(Wang et al., 2017)
126	nano‐MnOx	Oxidative degradation of 2-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol	Pseudomonas sp. G7	(Tu et al., 2015)
127	Dy2Ce2O7 nanostructure	Degradation of MO, and RhB and B nepthol	Vitis vinifera juice	(Zinatloo-Ajabshir et al., 2018a)
128	Ln2Sn2O7 nanostructure	Degradation of EY, eriochrome black T and methyl violet	Pomegranate juice	(Zinatloo-Ajabshir et al., 2018b)
129	Bio-Pt and bio-Pd nanocatlyst	Removal of ciprofloxacin, sulfamethoxazole and 17b-estradiol	Desulfovibrio vulgaris	(Martins et al., 2017)
130	Pd/Au	Dechlorination of diclofenac	Shewanella oneidensis MR-1	(De Corte et al., 2012)

Table 2.

Appliances of biogenic iron-based NPs in degradation of dyes and removal of heavy metals.

Entry	Biogenic source	Nanoparticle size & shape	Dye/metal removed	Refs.
1	Eucalyptus	20–80 nm, spherical	Swine wastewater treatment	(Wang et al., 2014a)
2	Mentha spicata	20–45 nm, core and shell morphology	As(III) & As(V)	(Prasad et al., 2014)
3	Sorghum bran	40–50 nm, amorphous	Bromothymol blue	(Njagi et al., 2010)
4	Green, Oolong and Black tea	20–40 nm, spherical	Monochlorobenzene	(Kuang et al., 2013)
5	Amaranthus dubis	60–300 nm, spherical	MO	(Harshiny et al., 2015)
6	Eucalyptus tereticornis, Melaleuca nesophila, Rosmarinus officinalis	40–60 nm, spherical	Azo dye	(Wang et al., 2014b)
7	Black tea	40–50 nm	Ametryn	(Ali et al., 2016)
8	Oak, mulberry and cherry	10–30 nm, spherical	As(III) & Cr(VI)	(Poguberović et al., 2016)
9	Amaranthus spinosus	58–530 nm, spherical	MB, MO	(Muthukumar and Matheswaran, 2015)
10	Eucalyptus	20–60 nm, spherical agglomerates	Direct black G	(Zhuang et al., 2015)
11	Omani leaf	15 ± 2 in length and 3.0 ± 0.2 nm dia, nanorod	Heavy oil viscosity treatment	(Al-Ruqeishi et al., 2016)
12	Aloe vera	100 × 20 nm, nanorod	As(V)	(Mukherjee et al., 2016)
13	Black tea	40–50 nm, spherical	Fluoride	(Ali et al., 2015)
14	Phyllanthus acidius	4.5–5.8 nm, spherical	AO7	(Gurushantha et al., 2015)
15	Grape leaf	10–100 nm, quasi spherical shape	Orange II dye	(Luo et al., 2016)
16	Eichhornia crassipes (water hyacinth)	20–80 nm, amorphous	Cr(VI)	(Wei et al., 2017)
17	Eucalyptus leaf	20 and 80 nm, amorphous	Cr(VI), Cu(II)	(Weng et al., 2016)
18	Green tea	5–15 nm, spherical	Bromothymol blue	(Hoag et al., 2009)
19	Eucalyptus	50–80 nm, spherical	Cr(VI)	(Madhavi et al., 2013)
20	Eucalyptus	40–60 nm, amorphous	Acid black 194	(Wang, 2013)
21	Green, Oolong and Black tea	40–50 nm, spherical	Malachite green	(Huang et al., 2014a)
22	Pomegranate	100–200 nm	Cr(VI)	(Rao et al., 2013)
23	Vine leaves, black tea, grape marc	15–45 nm	Ibuprofen	(Machado et al., 2013)
24	E. globules	80–90 nm, spherical	Phosphate	(Gan et al., 2018)
25	E. globules	∼80, spherical	Nitrate	(Wang et al., 2014c)
26	Aloe vera	5.5, Rod like	As(V)	(Mukherjee et al., 2016)
27	M. oleifera	250–474, spherical	Nitrate	(Katata-Seru et al., 2018)
28	C. sinensis	5–25, Cuboid/Pyramidal	As(V) & As(III)	(Lunge et al., 2014)
29	M. ferrooxydans	100–130, Rope like	As(III) & As(V)	(Andjelkovic et al., 2017)
30	E. globules	-, spherical	As(V)	(Martínez-Cabanas et al., 2016)
31	C. reticulata	∼50, spherical	Cd(II)	(Ehrampoush et al., 2015)
32	S. thermolineatus	25, distorted spherical	Cu	(Kandasamy, 2017)
33	S. jambos	5–60, Oval, spherical	Cr(VI)	(Xiao et al., 2016)
34	E. globules	50–80, spherical	Cr(VI)	(Madhavi et al., 2013)
35	P. granatum	100–200, irregular	Cr(VI)	(Rao et al., 2013)
36	C. sinensis, S. aromaticum, M. spicata, P. granatum	50–60, spherical	Cr(VI)	(Mystrioti et al., 2016)
37	C. (L.) Cuss	∼45.4, irregular	Cr(III) & Pb(II)	(Lingamdinne et al., 2017)
38	Oolong tea	40–50 nm, spherical	Malachite green	(Huang et al., 2014b)
39	Green tea	70–80 nm, spherical	Malachite green	(Huang et al., 2015)
40	Eucalyptus leaf	80 nm, spherical	Phosphate	(Cao et al., 2016)

3.2. Removal of pharmaceutical contaminants

Pharmaceutical contaminants, especially antibiotics in the natural water systems, pose different complications and hazardous effects for human health, wherein biogenic nanomaterials can be employed for remediation. For instance, tetracycline, as one of the highest applied antibiotics for human and veterinary applications, can be removed via deployment of nano zero-valent technology-based tactic (Yi et al., 2018; Gopal et al., 2020; Yi et al., 2019). The bimetallic nano zero-valent iron (nZVI)-Cu NPs were prepared using pomegranate rind extract for remediation purposes; tetracycline removal of 72 ± 0.5 % (initial tetracycline concentration 10 mg L⁻¹) has been reported with the nZVI-Cu concentration of 750 mg L⁻¹ at pH 7. To resolve the colloidal instability and enhance the tetracycline removal, bentonite-supported composite have been employed which displayed remarkable improvement in removal with a considerably decreased NP loading (Fig. 19) (Gopal et al., 2020). In another study, nickel-iron nanocomposite has been ecofriendly fabricated using polyphenol rich pomegranate (Punica granatum) peel extract, and ensuing nickel-iron was immobilized on to biocompatible and biodegradable alginate to produce nanocomposite beads (GS-NiFe beads) (Fig. 20) (Ravikumar et al., 2020). By using the optimized conditions (20 mg L⁻¹ of tetracycline initial concentration; 1000 mg L⁻¹ GS-NiFe concentration in beads; bead weight (wet): 20 % (WV⁻¹); interaction time 90 min), 99 % removal was attained in a batch reactor, with adsorption and degradation processes in the remediation. Additionally, the maximum removal capacity (487 ± 6.84 mg g⁻¹) was obtained under the reaction conditions: bed height: 15 cm; initial tetracycline concentration: 20 mg L⁻¹; and flow rate: 1 mL min⁻¹ (Ravikumar et al., 2020).

Fig. 19.

Bimetallic nZVI-Cu and bentonite supported green nZVI-Cu nanocomposite for removing tetracycline (TC). Reproduced with permission from Ref (Gopal et al., 2020).

Fig. 20.

(A) SEM image of GS-NiFe NPs (B) SEM image of GS-NiFe beads. Reproduced with permission from Ref (Ravikumar et al., 2020).

In an utilization of biogenic nanomaterials, green-synthesized Cu NPs were expeditiously produced using aqueous Tilia extract residues (Husein et al., 2019) and the ensuingbiogenic NPs were applied in the removal of three selected pharmaceutical drugs from wastewater samples; Diclofenac (Dic), Ibuprofen (Ibu), and Naproxen (Nap) could be eliminated 91.4, 74.4, and 86.9 %, respectively with 10.0 mg of Cu NPs at pH 4.5 and 298 k for 60 min. The data fitted well with Langmuir model with R², the values of 0.998, 0.998 and 0.977 for Dic, Nap, and Ibu, respectively; the maximum adsorption capacities being 36.0, 33.9, and 33.9 mg g⁻¹ for Dic, Nap, and Ibu, respectively. In order to provide useful information on the adsorption kinetic mechanism of non-steroidal anti-inflammatory drugs adsorption onto Cu NPs surface, diverse kinetic models were checked to analyze the kinetic data. Kinetic studies revealed that these sorption processes obeyed the pseudo-second-order model, while the thermodynamic parameters indicated the spontaneous and exothermic and/or physical nature of the adsorption (+38.3, +23.8, and +40.8 kJ mol⁻¹ for Dic, Ibu, and Nap, respectively) (Husein et al., 2019).

In yet another attempt, a new Fe₃O₄ nanosorbent was prepared using plant extracts of cucumber (Cucumis sativus), lemon (Citrus limon), and black grapes (Vitis vinifera) via a green approach (Stan et al., 2017). The as-prepared Fe₃O₄(cum), Fe₃O₄(lem), and Fe₃O₄(grp) nanosorbents were applied for the elimination of seven antibiotics such as piperacillin, sulfamethoxazole, tetracycline, tazobactam, trimethoprim, erythromycin, and ampicillin from water bodies. The Box-Behnken design method was applied to recognize the optimum conditions for the antibiotics removal; Langmuir, Freundlich, and/or Temkin adsorption isotherm models were the best fitted towards the adsorption of selected antibiotics with an excellent removal of > 90 % was observed for most of these antibiotics.

3.3. Membrane-based water treatment

MOFs have remarkable advantages, including low cost readily achieved raw materials, relatively non-toxic metal source with adequate biocompatibility, and desirable physicochemical characteristics (e.g., semiconductor properties, high porosity and framework flexibility), and thus they can be employed as promising alternatives for environmental remediation (Hou et al., 2018; Lee et al., 2014). Interestingly, a porous matrix membrane (PMM) was constructed using an eco-friendly method, by applying MOF particles as green template, which although is insoluble in polar organic solvents but can be simply washed away by water (Fig. 21) (Lee et al., 2014). Such systems have appliance potential for pressure-driven membranes processes and carbonaceous nano-fiber membranes removing and separating NPs with remarkable selectivity, or osmotically-driven membrane systems, including pressure-retarded osmosis and forward osmosis (Lee et al., 2014).

Fig. 21.

(A) PMMs fabrication strategy. (B) PMMs were fabricated by applying MOF as green template for treatment of water; the reported strategy was compared with traditional mixed matrix membranes. Reproduced (Adapted) with permission from Ref (Lee et al., 2014).

4. Current challenges and future perspectives

There is a vital need for the introduction of novel advanced water technologies to ensure a high quality of drinking water, with added capacity to eliminate micropollutants. Industrial production processes need to be strengthened via the use of flexible and adaptable water treatment systems. One of the most important advantages of nanomaterials, when compared with conventional water technologies, is their ability to integrate various properties, resulting in multifunctional systems such as nanocomposite membranes that enable both, the particle retention and elimination of contaminants. Furthermore, nanomaterials enable higher process efficiency due to their unique characteristics, such as a high surface area. However, some important drawbacks need to be pointed out at this stage. For instance, materials functionalized with NPs incorporated or deposited on their surface have risk potential, as NPs may be released to the environment where they can get accumulated over a longer period of time. In order to minimize the health risk, several national and international regulations and laws are being established. The main technical limitation of nano-engineered water technologies is that they are seldom adaptable for large-scale processes, and at present, in many cases are not competitive with conventional treatment technologies. Nevertheless, safer and earth-abundant nano-engineered materials offer great potential for innovations in the near future, particularly for the decentralized treatment systems, point-of-use devices, and heavily degradable contaminants. Biogenic NPs are promising materials due to the inherent greenness and sustainability of the production methods, and their good performance in the reduction of environmental contaminants (Gautam et al., 2019). Progress of advanced analytical and imaging technologies has paved various pathways for the assessment and measurement of nano-sized objects, especially for water treatment applications. In view of the applications of hazardous chemicals and materials for producing nano objects, chemical industry has been under stress to subrogate toxic reagents and harmful solvents; the main push has been to deploy biomolecules from organisms as an alternative to damaging synthetic chemicals to produce biocompatible nano objects. It appears that bioprepared nanomaterials can adsorb contaminants from aqueous watercourses or catalyze the degradation of organic pollutants into nontoxic categories. Biogenic nanomaterials are sustainable, relatively inexpensive, can be produced in an energy-efficient manner and ecologically reliable in view of their bio-renewable nature and could play significant roles in decontamination protocols for drinking and industrial wastewaters (Gautam et al., 2019). In terms of deployment of biogenic nanomaterials for water treatment and purification, some important future perspectives need to be considered:

1. The sustainability and toxicity issues need to be evaluated; more elaborative studies are required for application of these green-synthesized nanocatalysts and nanomaterials in industrial and commercial scales. On the other hand, applying nanomaterials may additionally contribute to the secondary pollution, and thus this critical issue should be addressed and evaluated comprehensively.

2. Although the production of these nanomaterials are simple and ecofriendly, some important and challenging aspects should be analyzed and optimized, including the effects of reaction parameters and stability issues, because these factors can modify the behavior of nanomaterials, morphologies and their pollutant removal performance. Further, the purification and extraction of the produced biogenic nanomaterials for additional applications are very important, and they should be isolated with high purity especially in the case of water treatment.

3. Further investigations are needed to find innovative nanohybrids and multifunctional nanomaterials to enhance their effectual usefulness.

4. The cost-effectiveness studies should be addressed to compare the fabrication of green-synthesized nanomaterials with the NPs prepared by conventional approaches.

5. The efficacy issues and evaluation of remedial performances are typically designed on laboratory scales, simulating the variable levels of realistic exposure conditions, but it is crucial to investigate and evaluate the results from realistic environmental conditions.

5. Conclusion

Green-synthesized and biogenic nanocatalysts and nanomaterials can cost-effectively and proficiently eliminate the inorganic, organic, pharmaceutical, and heavy metal pollutants from the aqueous streams. As low cost of production is imperative for their broader applications in wastewater treatment, future studies should be dedicated to refining the economic viability of these nanomaterials and evaluation of their interactive mechanisms in water treatment systems. Additionally, their potential toxicity to human health and the environment need to be thoroughly probed; comprehensive evaluations of their noxiousness are very critical to ensure their safer applications. Further studies are warranted to compare the relative performances of these nanomaterials in terms of energy usage and resource utilization and recognize favorable earth-abundant materials which merit additional developments.

Nanotechnology-facilitated wastewater treatment systems need to ensure not only to circumvent the main challenges encountered by existing technologies but also to tender innovative treatment abilities which can permit economical applications of unconventional water resources to recover and develop for the water supply. Applying green-synthesized nanocatalysts and nanomaterials for the remediation of pollutants and aqueous metal ions are significantly encouraging, but some important and critical issues pertaining to the toxicity and biosafety issues and their mechanistic aspects should be systematically and comprehensively evaluated; more elaborative studies are still demanded to find the low cost, high adsorption capacity, and high selectivity of the fabrication method, as well as the recyclability of green-fabricated nanocatalysts and nanomaterials.