Immobilization of biogenic metal nanoparticles on sustainable materials – green approach applied to wastewater treatment: a systematic review

Abstract

The application of the principles of green chemistry to the synthesis of metal nanoparticles (MNP) is a new emerging issue concerning sustainability. Together with the green nanotechnology, this is an evolving approach with innovative, reliable and sustainable solutions for applications in all fields of life. Those principles may be used on greener bio-inspired materials applied for the synthesis of MNP as well as for their immobilization on solid supports. This review is carried out for its relevance, covering the available literature on the green fabrication of MNP from different bio-resources, ranging from micro- to macromolecular levels (bacteria, fungi, yeasts, algae and plants) acting as reducing and capping agents and enabling the immobilization of biogenic MNP on sustainable materials to improve their performance when applied to wastewater treatment. Materials derived from renewable and earth-abundant sources (clays, natural zeolites, sediments, sands, rocks and volcanic glasses) or residues from food, industry and agro-forestry activity, activated carbon and biochar were found as sustainable supports with diverse applications such as disinfection of water or removal/degradation of various pollutants of wastewater. For the first time, a thorough review of the literature available on this topic has been carried out and the synthesis and immobilization of biogenic MNP on sustainable materials are reported, through two main routes: (i) bio-inspired fabrication of MNP, with the consideration of the adverse effects of their suspension status and (ii) supported MNP obtained with sustainable supports, with focus on natural or on waste materials, activated carbon and biochar, as well as on the obtained modified sustainable materials. Finally, an economic and environmental perspective as well as the challenges on the use of supported MNP will be presented.

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Environmental significance

Currently, there is growing effort on investigating the environmental pollution abatement issue. With the increasing need to develop sustainable materials/methods for the elimination or effective reduction of pollutants in wastewater and drinking water, nanotechnology appears as an innovative tool with excellent demonstrated potential for the purpose through more effective strategies than previously explored ones. Various bio-resources have shown their effectiveness in the biosynthesis of metal nanoparticles (MNP). However, the utilization of powder MNP in reactors shows several limitations, including the challenge of filtering the final suspensions due to the small particle size and their agglomeration. The immobilization of MNP on solid supports could be used to address these issues.

1. Introduction

The continuous growth of the global population combined with industrialization and urbanization has resulted in increased demand for chemicals, materials and energy.¹ The release of unwanted wastes into the environment from numerous anthropogenic sources has been inevitably accompanied by a lower quality of environmental systems.² There is an urgent need for the development of sustainable, efficient and cost-effective materials/methods for the elimination or effective reduction of pollutants and microorganisms in wastewater and drinking water.³

In recent years, nanotechnology has appeared as a new innovative tool with excellent potential for wastewater treatment through more effective strategies than previously explored ones.⁴ Nanotechnology is an evolving field with the creation and utilization of materials with structural features of bulk materials with at least one dimension in the nanoscale.⁵ Over the past few years, researchers have been developing and characterizing unique nanostructural morphologies such as nanowires, nanotubes, nanospheres, nanoparticles (NP), etc. with various practical applications in engineering, materials science, chemistry and biology.⁶ Among the enormous variety of nanostructured materials that have emerged, metal nanoparticles (MNP) are considered as the basic building blocks of nanotechnology and act as bridges between bulk materials and atomic or molecular structures.^7,8 MNP have gained prominence in technological advancements due to their tunable physicochemical characteristics such as melting point, wettability, electrical and thermal conductivity, catalytic activity, light absorption and scattering and surface area to volume ratio resulting in enhanced performance over their bulk counterparts.⁹ The nanostructured materials find their pioneering applications in diverged areas ranging from medicine, agriculture, electronic element fabrication, environmental waste remediation to several industrial domains.¹⁰

The fabrication of MNP can be broadly categorized into two approaches: top-down and bottom-up.¹¹ The top-down approach involves crushing or cutting the bulk materials into fine particles at the nanoscale, using processes such as arc discharge, ball milling, pulsed laser ablation, etc.^5,9 In the bottom-up approach, MNP are formed by assembling atom by atom, molecule by molecule or cluster by cluster.^5,12 This last approach includes numerous methods of MNP synthesis: coprecipitation, chemical reduction of metal salts, electrochemical and green synthesis methods.^13,14 In the chemical synthesis of MNP, the chemical reduction of metal ions from salt solution in the presence of strong bases (reducing agents), sodium borohydride (NaBH₄) or sodium hydroxide (NaOH) was achieved, followed by the addition of a stabilizing agent, also called the capping agent.⁸ However, the reagents employed as reducing agents and the solvents used to dissolve the stabilizers are commonly toxic substances that have adverse and deleterious health and environmental effects.^15,16 The physical MNP synthesis is mainly a top-down approach in which the material is reduced in size by various physical approaches.¹⁷ These conventional methods of MNP synthesis have various limitations such as the need for a huge amount of energy, specific and costly equipment, the production of flammable hydrogen gas and the use of toxic chemicals such as NaBH₄, organic solvents, and stabilizing and dispersing agents.¹⁸

The development of an environmentally friendly, renewable, easy-to-implement and cost-effective means of MNP synthesis has become one of the foremost demands in the field of environmental remediation.¹ The bio-inspired fabrication is cost-effective, easy to implement, reduces the chemical load into the environment and eliminates unnecessary processing during synthesis.^19,20

Although the green synthesis mitigates the environmental impact in production of MNP, these biogenic procedures still suffer from certain limitations. In the past years, solid materials have been used as supports for MNP to prevent agglomeration and to overcome the obstacles associated with stability and recovery. Additionally, the accumulation of MNP in the environment, after their intended use, also generates persistent MNP waste.²¹ The development of an environmentally friendly method for the synthesis of MNP with good dispersion and stability and their immobilization on effective and sustainable supports is highly desirable and it has progressively focused on green materials applied as supports for MNP for sustainable development.^22,23 The interest in sustainable materials applied to support the biosynthesized MNP in the field of wastewater treatment has become attractive.

The simultaneous use of bio-resources and sustainable materials as reducing/capping agents and supporting materials to obtain supported MNP is a significant green approach applied to wastewater treatment. Bearing this in mind, in the present study a systematic review was carried out to assess the literature currently available on the performance increase of biogenic MNP applied to wastewater treatment. In the following qualitative analysis of all research articles published in this field, the literature focusing on the use of sustainable materials as supports for biogenic MNP was thoroughly reviewed for the first time.

This systematic review focused on biogenic MNP immobilized on sustainable materials is divided into two different goals:

(1) Green fabrication of MNP from different bio-resources ranging from micro- to macromolecular levels (bacteria, fungi, yeasts, algae and plants – Fig. 1) acting as reducing and capping agents and a brief overview of the adverse effects of suspensions of MNP, emphasizing the need for solid-supported MNP.

Fig. 1 Bio-resources: bacteria, fungi, yeast, algae and plants and some examples of sustainable materials: clays, sands, rocks, activated carbon and residues from forest.

(2) Immobilization of biogenic MNP on sustainable materials derived from earth-abundant and renewable materials (clays, natural zeolites, sediments, sands, rocks and volcanic glass), residues from food, industry and agro-forestry activities, activated carbon or biochar derived from wastes and their use as green nanocomposites with diverse applications for removal or degradation of various pollutants or disinfection of water (Fig. 1).

2. Methodology for literature search

Using the search strings listed in Table 1, a literature search was performed on the Scopus database, according to the Preferred Reporting Items of Systematic Reviews and MetaAnalyses (PRISMA) 2020 statement.

Table 1 Search strings used to find the relevant literature on the Scopus database, and corresponding results

Search string	Document results
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND “natural material”) AND (LIMIT-TO (DOCTYPE, “ar”))	9
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND clay) AND (LIMIT-TO (DOCTYPE, “ar”))	79
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND zeolite) AND (LIMIT-TO (DOCTYPE, “ar”))	61
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND sand) AND (LIMIT-TO (DOCTYPE, “ar”))	11
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND waste) AND (LIMIT-TO (DOCTYPE, “ar”))	264
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND residues) AND (LIMIT-TO (DOCTYPE, “ar”))	70
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND “activated carbon”) AND (LIMIT-TO (DOCTYPE, “ar”))	141
TITLE-ABS-KEY (biogenic OR green OR biosynthesis AND nanoparticles OR composite OR nanocomposite OR bionanocomposite AND supported OR loaded OR attached OR imprinted AND biochar) AND (LIMIT-TO (DOCTYPE, “ar”))	40

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Briefly, the literature search combined the terms “biogenic” or “green” or “biosynthesis” and “nanoparticles” or “composite” or “nanocomposite” or “bionanocomposite” and “supported” or “loaded” or “attached” or “imprinted” with the terms for different sustainable materials (e.g., clay, zeolite, sand, waste, activated carbon, biochar, etc.). This literature search was performed from September 2021 to January 2023. Titles and abstracts of the retrieved articles were screened for relevance considering the following eligibility criteria: use of sustainable materials as supports for biogenic MNP with application in wastewater treatment and original studies. Exclusion criteria: full text in a language other than English, lack of access to the full article, articles without application in wastewater treatment or articles with MNP supported on non-sustainable materials. The classification of sustainable materials was performed according to our detailed analysis of the reported results.

2.1 Results

A total of 675 results were obtained by conducting a literature search on the Scopus database, based on the strings described in Table 1 (Fig. 2A).

Fig. 2 (A) Flowchart considered in the process of the study selection and (B) data collected from the 77 studies selected from qualitative analysis based on the criteria summarized in A.

The titles and abstracts of those articles were screened for relevance and duplicates were removed, leading to the selection of 77 articles for full-text reading. The type of sustainable material used as a support for biogenic MNP was analyzed first and the results showed that 48% of sustainable materials are from renewable sources (clays, natural zeolites, sediments, sands, rocks and volcanic glass), 27% are residues, 16% are activated carbon and biochar only represents 10% (Fig. 2B). Until 2017, few works were found on the application of these sustainable materials. Since then, the application of these materials has aroused the interest of researchers and an increase in the number of publications in this field is expected.

3. Bio-inspired fabrication of metal nanoparticles

In recent years, interest has grown for the implementation of environment friendly and economic remediation methods that guarantee low secondary impacts. Several green methods have been proposed and implemented for MNP synthesis. This field has received great attention due to its capability to design alternative, harmless, energy-efficient and less toxic routes towards the synthesis of these MNP. These routes have been associated with the rational utilization of various substances and limited use of toxic precursors consequently reducing the quantity of impurities and by-products.²⁴

The bio-inspired fabrication of MNP involves the use of prokaryotic/eukaryotic cells or extracted biomolecules that act as reducing, stabilizing and capping agents.¹³ The green synthetic processes comprise the use of either microorganisms such as bacteria,^10,25 fungi^26,27 and yeasts,^28,29 as well as algae^30,31 and plant biomass or plant extract.^32,33

Plants have been reported to contain a huge variety of biomolecules that work in harmony to obstruct cellular components from oxidative damage, thereby resulting in metal ion reduction to MNP.¹³ The first work on plants employed in the synthesis of MNP used Medicago sativa (alfalfa) which was able to synthesize Au-NP and Ag-NP.³⁴ Since then, more attention has been paid to plants. Most of the studies confer the production of MNP by plants that are known to be more stable than MNP synthesized by microorganisms.³⁵ Biosynthesis can be achieved by plant tissues, extract of leaves, stems, roots, fruits, bark peels and flowers, being from a sustainable perspective, i.e. the utilization of a renewable feedstock as a precursor material. The usage of plant extracts for the synthesis of MNP has drawn special attention due to their rapid, low-cost, biogenic and single-step method involved, making it an interesting and reliable alternative to conventional methodologies.⁶ Currently, the number of publications on the synthesis of MNP produced by plant extracts is increasing exponentially. Extracts derived from diverse plant species such as Psidium guajava (guava),³⁶Cymbopogon citratus (lemongrass),³⁷Trigonella foenumgraceum (fenugreek),³⁸Aloe vera (cactus),³⁹Hibiscus rosa sinensis (hibiscus),⁴⁰Citrus sinensis (orange),⁴¹Saussurea costus (costus)⁴² or Eucalyptus globulus (eucalyptus)⁴³ are some examples of successful usage in the green synthesis of MNP.

3.1 Adverse effects of suspension nanoparticles

Suspensions of MNP have been widely applied for the removal and/or degradation of various poisonous aqueous pollutants or the disinfection of water. However, the accumulation of MNP in the environment, after their intended use, also generates persistent pollution and can provoke harmful effects on living organisms.²¹

MNP disposal by incineration or by land fill processes leads to the accumulation of the NP in land or in subaquatic sediments or to dispersion in the atmosphere.⁴⁴ Shi et al.⁴⁵ studied the outcome of titanium dioxide (TiO₂) NP in wastewater treatment plants (WWTP) and although a part of the NP (>74%) was removed by the activated sludge, a significant concentration of them (27 to 43 μg L⁻¹) was found in the effluents, which is higher than natural background levels (less than 5 μg L⁻¹). After being released into the environment, the MNP can suffer different transformations such as agglomeration, sedimentation, oxidation, reduction, sulphidation, photochemical and biological mediated reactions.^46–48

The toxicity effects of citrate-coated Ag-NP in adult zebrafish in vivo are reported by Osborne et al.⁴⁹ The authors show that the particle size and dissolution of Ag⁺ ions from Ag-NP determined the toxic impact in the grills and intestines of zebrafish. Martin et al.⁵⁰ studied the effect of Ag-NP on a boreal lake, where the Ag accumulation was monitored in the tissues of yellow perch (Perca flavescens) and of northern pike (Esox lucius). It was found that both species accumulated Ag in their tissues for 2 years following exposure, with higher concentration detected in the liver. The toxic effects of Ag-NP of different sizes and surface functionalities were further investigated by Hou et al.⁵¹ on aquatic crustacean Daphnia magna. Shi et al.⁴⁵ also reported the presence of Ti in various fish tissues.

Studies to evaluate the phytotoxic impacts of suspended MNP have been also reported by Pradas Del Real et al.⁵² who studied the effect on wheat plants exposed to Ag in its different forms (Ag-NP, Ag₂S-NP and Ag⁺ ions). Various chemical transformations were observed on the epidermis and inside of the roots after the exposure.

The wide application of MNP is limited by many obstacles such as the difficulty of manipulating the suspension, the ultimate health risks of the dissipation of MNP into the environment and the instability of size due to aggregation.⁵³ Synthesis of MNP on a solid support has emerged as a prospective solution that may prevent the accumulation of MNP waste in the ecosystem through a recycling process.²¹

4. Supported metal nanoparticles as a green approach

The bio-inspired fabrication of MNP on solid supports has emerged as a novel strategy for the sustainable development of nanotechnology. The easy recovery and reuse of the supported MNP avoid their accumulation into the environment and provide stability to the MNP. It further prevents aggregation and leaching of metal ions and thus reduces the expected toxicity of MNP.²¹ Moreover, MNP powder is widely used in laboratory experiments for wastewater treatment, but for large-scale water treatment, immobilized MNP are much more desired.^54,55

A credible support-based material candidate should have high charge carrier mobility and thermal stability, high specific surface area, as well as a biocompatible nature.^56–58 Moreover, they should not be strong oxidant materials to restrict the production of harmful disinfectant by-products. The nature of the support will influence the performance kinetics and the leaching rate of the MNP.^59,60 Generally, supporting the MNP either by the usual chemical process or by green methods into suitable carriers enhances the surface area, adsorption capacity and mechanical stability.^61,62

The attachment of MNP to the supporting medium can be performed by: (1) in situ fabrication that involves the formation of a bond between metallic ions and the negatively charged functional groups of the supporting material, with the gradual reduction of the first that start nucleating on the solid support (Fig. 3) or (2) ex situ fabrication which includes the mixing of freshly pre-synthesized MNP with the supporting material by an electrostatic interaction between them (Fig. 3).^21,63,64

Fig. 3 Green approaches for synthesis of metal nanoparticles (MNP) supported on solid materials.

4.1 Synthetic supports

Different types of solid supports can be used for the immobilization of biogenic MNP and the most common are polymer-based membranes,^65,66 glass,⁶⁷ silica gel,⁶⁸ commercial activated carbon,⁶⁹ synthetic zeolites,^70–72 resins,⁷³ graphene oxides (GO)⁷⁴ and reduced graphene oxide (rGO).^75,76

Amongst the different available synthetic supports, polymer-based membranes have received special attention due to their open structure and high internal surface area which ensure a high MNP loading and easy active site accessibility.⁷⁷ Recently, Kalaivizhi et al.⁶⁵ described the removal process of Congo red and methylene blue by using biogenic ZnONP immobilized on polysulfone/polyurethane membranes. Arif et al.⁶⁶ studied the application of TiO₂-NP synthesized through a green route using a polyvinylidene fluoride polymer embedded in the extract of Cajanus cajan to develop a photocatalytic membrane. This membrane supporting biogenic TiO₂ acted as a synergistic separator to reject chromium (Cr⁶⁺) and further reduce the ion toxicity by photocatalytic reduction of the concentrated Cr⁶⁺, achieving >90% rejection and >85% reduction of Cr⁶⁺.

Zeolites acting as supports have been receiving considerable attention due to their high specific surface area, high catalytic activity, high thermal stability and exceptional structural properties with uniform distribution of pores and channels.⁷² Nizam et al.⁷⁰ reported the use of biosynthesized magnetic iron(III) oxide (Fe₂O₃)-NP immobilized in situ on 13X zeolite (Fe₂O₃/MS) in the photocatalytic degradation of methylene blue. The results demonstrated excellent performance of Fe₂O₃/MS on the photodegradation of the dye, with 99% efficiency achieved in 150 min. The authors observed an enhanced photocatalytic performance of Fe₂O₃/MS when compared to the pure synthesized Fe₂O₃-NP. Rostami-Vartooni et al.⁷¹ investigated the use of magnetically recoverable Fe₃O₄/HZSM-5 as a support for Ag-NP, synthesized in situ in the presence of Juglans regia L. leaf extract. The authors found that the Ag/Fe₃O₄/HZSM-5 nanocomposite showed high activity for the reduction of methyl orange and 4-nitrophenol, being reused three times without loss of activity. Also, Tajbakhsh et al.⁷² studied the efficiency of the Ag/HZSM-5 nanocomposite for the reduction of different organic dyes such as methylene blue, Congo red, rhodamine B and 4-nitrophenol. The green synthesized Ag/HZSM-5 nanocomposite prepared in situ using Euphorbia heterophylla leaf extract was found to be an efficient nanocatalyst with high regeneration potential.

Another class of materials, GO and rGO, has been extensively used as promising supports for the immobilization of green MNP. These materials have many advantages such as a large specific surface area, high adsorption capacity, good electrical conductivity and mechanical stability.^75,78 Naghdi et al.⁷⁴ reported the use of Cuscuta reflexa leaf extract for the biosynthesis of the Cu/GO/MnO₂ nanocomposite as a heterogeneous catalyst for the reduction of 4-nitrophenol, 2,4-dinitrophenylhydrazine, Congo red, methyl orange, methylene blue and rhodamine B. These authors reported a high catalytic activity of the Cu/GO/MnO₂ nanocomposite (100% degradation attained instantly) as a recyclable heterogeneous catalyst for the reduction of all organic dyes and nitro compounds tested. Xue et al.⁷⁵ evaluated the use of rGO-supported bimetallic Fe/Ni-NP (Fe/Ni–rGO) for the simultaneous removal of rifampicin and Pb²⁺ from aqueous solution. It is stated that Fe/Ni–rGO nanocomposites prepared by green synthesis using green tea extract were effective for the removal of mixed Pb²⁺ and rifampicin, with removal efficiencies of 81.9% and 94.3%, respectively. Ranjith et al.⁷⁶ synthesized a hybrid rGO–TiO₂/Co₃O₄ nanocomposite and studied the photocatalytic degradation of methylene blue and crystal violet. The results revealed that the presence of rGO significantly induced higher photocatalytic ability and the degradation of both dyes was more extensive under visible light irradiation.

For the sake of comparison and although the selected support is not a common material, the work of Harjati et al.⁷⁹ should be referred as they were able to synthesize green Fe₂O₃ supported on TUD-1 (Technische Universiteit Delft), using Parkia speciosa hassk pod extract as a bioreductor agent. The support is, in fact, a three-dimensional mesoporous silicate with a high specific surface area and with Brønsted acidic behavior. These biosynthesized Fe₂O₃-NP were immobilized on TUD-1 via the sol–gel method and their photocatalytic activity on bromophenol blue and methyl violet degradation was assessed.

Most of these nanocomposites were developed with expensive and sometimes not-readily available materials, which makes it difficult to scale-up their production to technologically relevant amounts. Alternative and sustainable supports have been recently employed on the preparation of supported MNP.

4.2 Sustainable supports

From the perspective of circular economy and green development, the application of natural materials, organic wastes, activated carbon or biochar as solid supports for biogenic MNP, in the treatment of wastewater is the most attractive path for environmental researchers to incorporate the principles of Green Chemistry.

Recently, several works in immobilized green MNP for environmental pollution remediation have diverted their focus on the direct or indirect immobilization of MNP on a number of sustainable materials. There is a variety of sustainable materials, meaning materials in high abundance and are renewable, agro-forest, food and industrial residues, which can either be utilized in pristine or in modified structures for supporting MNP. In this section, an exhaustive overview of the application of sustainable materials supporting biogenic MNP is presented, as well as their application in wastewater treatment for the degradation of various pollutants and for the inactivation of some microorganisms.

4.2.1 Natural materials. Green synthesis of MNP supported on natural materials (clay minerals, natural zeolites, sediments, sands, rocks and volcanic glass) forms an innovative approach for the preparation of these nanocomposites. These materials are widespread and abundant on earth and can be purchased at a low cost. Some can also be obtained as waste from exploitation deposits or from processing and purification processes. These accessible starting materials ensure that a reasonable quantity can be obtained and that there is potential for up-scaling.³Table 2 summarizes all works that highlight the ability of natural materials to act as stable supports for the immobilization of biogenic MNP for environmental pollution abatement.

Table 2 Natural materials as supports for biogenic MNP with applications in wastewater treatment

Support	Biogenic MNP	Bioreductor	Immobilization method	Shape and size	Wastewater application and recyclability	Ref.
Nano zerovalent iron (nZVI); room temperature (RT); not reported (n.r.).
Kaolin	nZVI	Ruellia tuberosa leaf extract	In situ with stirring	Spherical	Adsorption of reactive black 5	84
				20–40 nm	5 cycles
	TiO2/ZnWO4	Carica papaya seeds and Musa paradisiaca peels biomass	In situ with mechanical grinding for 15 min and calcined	Spherical	Photocatalytic degradation of ampicillin and sulfamethoxazole; artemether	3
				62–257 nm	5 cycles
	Ag/AgCl	Teucrium polium extract	In situ with stirring for 24 h	Spherical	Catalytic reduction of 4-nitrophenol	83
				19–22 nm	n.r.
Montmorillonite	Ag	Sida acuta leaf extract	In situ with stirring at room temperature for 24 h	n.r.	Adsorption of methylene blue	89
				15–20 nm	n.r.
	nZVI	Green tea extract	In situ with stirring	Spherical	Adsorption of Cr^6+	86
				15–30 nm	n.r.
	Ag/ZnO	Urtica dioica leaf extract	In situ with stirring at 80 °C for 4 h	Spherical	Photocatalytic degradation of methylene blue	87
				2–4 nm	4 cycles
Bentonite	Ag	Euphorbia larica extract	In situ with constant stirring at 80 °C for 4 h	Spherical	Catalytic reduction of 4-nitrophenol, Congo red, methylene blue and rhodamine B	92
				32 nm	5 cycles
	Fe	Different extracts	In situ with stirring for 30 min or overnight	n.r.	Adsorption of chlorfenapyr	93
	Ag				n.r.
	Cu	Thymus vulgaris leaf extract	In situ with constant stirring at 80 °C for 4 h	Spherical	Catalytic reduction of methylene blue and Congo red	94
				23–94 nm	5 cycles
				Spherical	Catalytic reduction of 4-nitrophenol	95
				56 nm	5 cycles
	SnO2	Ziziphus jujuba fruit extract	In situ with vigorous stirring at RT for 30 min and calcined	Spherical	Photocatalytic degradation of methylene blue and eriochrome black T	90
				18 nm	3 cycles
	Pd	Gardenia taitensis leaf extract	In situ with constant stirring at 80 °C for 4 h	Spherical	Catalytic reduction of Cr(VI), 4-nitrophenol and 2,4-dinitrophenylhydrazine	96
				76–97 nm	5 cycles
	ZnO	Jujube fruit extract	In situ with constant stirring at 80 °C for 5 h and calcinated	Spherical	Photocatalytic degradation of methylene blue and eriochrome black-T	97
				10–40 nm	3 cycles
	nZVI	Green tea extract	In situ with constant stirring at RT	Spherical	Adsorption of phosphate	98
				40–60 nm	n.r.
			In situ with constant stirring at RT	n.r.	Adsorption of Cr^6+	99
				40–80 nm	n.r.
			In situ with constant stirring at RT for 15 min	Irregular spherical	Fenton-like oxidation of reactive blue 238	100
				<50 nm	n.r.
			In situ with stirring for 1 h	n.r.	Adsorption of malachite green	101
				50–60 nm	2 cycles
		Eucalyptus leaf extract	In situ with stirring for 3 h	Irregular	Catalytic reduction of 4-nitrophenol	91
				10–60 nm	5 cycles
		Black tea extract	In situ with constant stirring at RT for 15 min	Irregular spherical	Fenton-like oxidation of reactive blue 238	102
				<50 nm	n.r.
	nZVI-Cu	Pomegranate rind extract	In situ with continuous purging of nitrogen gas and continuous stirring	Spherical	Adsorption of tetracycline	81
				60 nm	5 cycles
	Ag/Fe3O4	Salix aegyptiaca leaf extract	In situ with stirring at RT for 1 h	Spherical	Catalytic reduction of rhodamine B, methylene blue and methyl orange	103
	Pd/Fe3O4			15–65 nm	4 cycles
	Fe/Pd	Pomegranate peel extract	In situ with nitrogen gas atmosphere	Mostly spherical	Adsorption of tetracycline	104
				20–60 nm	3 cycles
	Fe/Ni			Spherical	Adsorption of tetracycline and Cu^2+	105
				70 nm	4 cycles
Sand	Fe/Ni	Punica granatum peel extract	In situ with orbital shaker for 1 h	Spherical	Adsorption of tetracycline	113
				161 nm	4 cycles
Silty	nZVI	Green tea leaf extract	In situ with stirring for 1 h	Cubical	Adsorption of phenol	114
				100 nm	n.r.
Diatomite	Ag	Alocasia macrorrhiza leaf extract	In situ with stirring at 80 °C for 4 h	Spherical	Catalytic reduction of 2,4-dinitrophenylhydrazine, nigrosin, 4-nitrophenol, methyl orange and Congo red	115
				32 nm	5 cycles
	Pt	Cinnamomum camphora leaf extract	In situ with stirring at 90 °C for 1 h	n.r.	Catalytic oxidation of benzene	116
				3.5 nm	n.r.
Perlite	Ag	Hamamelis virginiana leaf extract	In situ with stirring at 60 °C for 1 h	Spherical	Catalytic reduction of 4-nitrophenol and Congo red	117
				8–25 nm	4 cycles
	Cu	Euphorbia esula L. leaf extract	Ex situ with stirring at 100 °C for 15 h	Spherical	Catalytic reduction of 4-nitrophenol	118
				32 nm	3 cycles
	Pd	Euphorbia neriifolia L. leaf extract	In situ with continuous stirring and heated under reflux conditions for 3 h	Spherical	Catalytic reduction of 2,4-dinitrophenylhydrazine, 4-nitrophenol, rhodamine B, methyl orange and Congo red	119
				<18 nm	5 cycles
Clinoptilolite	Ag	Vaccinium macrocarpon fruit extract	In situ with constant stirring at 60–70 °C for 30 min	Spherical	Catalytic reduction of rhodamine B, methylene blue, methyl orange and Congo red	107
				15–30 nm	6 cycles
	CuO	Rheum palmatum L. root extract	In situ with constant stirring at 70 °C for 10 min	Spherical	Catalytic reduction of 4-nitrophenol, rhodamine B, methylene blue and methyl orange	109
				30 nm	5 cycles
Natrolite	Ag	Euphorbia prolifera leaf extract	In situ with stirring at 60 °C for 2 h	Semispherical	Catalytic reduction of 4-nitrophenol, rhodamine B, methylene blue, methyl orange and Congo red	108
				15 nm	7 cycles
	Cu	Anthemis xylopoda flower extract	In situ with stirring at 100 °C for 15 h	Spherical	Catalytic reduction of 4-nitrophenol, Congo red and methylene blue	110
				20 nm	5 cycles
	Pd	Piper longum fruit extract	In situ with stirring at 100 °C for 15 h	Spherical	Catalytic reduction of 4-nitrophenol, rhodamine B, methylene blue, methyl orange and Congo red	111
				12.5 nm	7 cycles
Natural zeolite	nZVI	Eucalyptus leaf extract	Ex situ with stirring	Spherical	Adsorption of ammonia and phosphate	112
				60 nm	n.r.

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Clays are layered phyllosilicate minerals that occur naturally in the earth's crust and are important constituents of soils. These inorganic materials have gained significant interest as solid supports for MNP due to their natural abundance, non-toxic nature, porosity, low cost, layered morphology, chemical inertness and mechanical stability.^80,81 Kaolin, which is one of the most common clay minerals, is low-cost, easily available, biogenic and has high thermal and mechanical stability.⁸² These properties make it a significant candidate for an MNP solid support. Alfred et al.³ developed an alternative method for biofabrication of kaolinite–TiO₂ nanocomposites doped with ZnWO₄ with effective activity on the photodegradation of artemether, ampicillin and sulfamethoxazole in water. These nanocomposites were prepared by biomass assisted synthesis routes, using Carica papaya seeds or Musa paradisiaca peels. The authors performed a pre-screening to evaluate the influence of TiO₂ and the impact of the different biomasses on the photocatalytic efficiency of the nanocomposites. The results showed that the presence of both biomass and TiO₂ is crucial for the photocatalytic efficiency of the nanocomposites. The nanocomposite prepared from Musa paradisiaca peels at 500 °C showed the highest efficiency for the degradation of the substrates in water, due to its relatively small particle size (62 nm), high porosity and consequent large surface area, with a high number of active sites needed for photocatalysis.³ Recently, Rakhshan et al.⁸³ produced a three-component Ag/AgCl/kaolinite bionanocomposite and a four-component Ag/AgCl/Fe₃O₄/kaolinite using the aqueous extract of Teucrium polium. Both showed relevant catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol, with a surplus for the four-component nanocomposite that can be easily separated with the aid of an external magnet. Kaolin was used as a support for nano zerovalent iron (nZVI) synthesized via a green method using the leaf extract of Ruellia tuberosa as a reducing agent.⁸⁴ The synthesized green nanocomposite was used for the degradation of reactive black 5 with high decolorization efficiency in acidic medium.

Montmorillonite is a very promising support for the preparation of nanocomposites, due to its particular characteristics such as high ion exchange capacity, swelling, intercalation and layered nanostructure that make it a suitable solid support for the green synthesis of MNP.^53,85–89 Yang et al.⁸⁶ successfully prepared a nanocomposite of nZVI/montmorillonite using a low-cost and environment friendly green synthesis via tea leaf extract. Although few studies propose the nanocomposite formation mechanism, the authors suggest that during the synthesis with green tea extract, the unique structure of montmorillonite with isolated exchangeable cations of Fe(III) results in nZVI particle formation, covered by organic matter (green tea extract) in the clay interlayer associated with negative charges (Fig. 4A).⁸⁶ The bio-synthesized composite showed a great removal capacity for Cr⁶⁺ and sufficient mobility under different soil conditions. Also, Sohrabnezhad and Seifi used montmorillonite to support green bimetallic Ag/ZnO-NP.⁸⁷ The attachment of MNP on the surface of montmorillonite and the presence of aluminum atoms in this clay structure not only prevent the loss of the photocatalyst during recovery but also assist in electron–hole separation during the photocatalytic process.⁸⁷

Fig. 4 Mechanism of composite synthesis: (A) nZVI/montmorillonite, reproduced from ref. 86 with permission from Elsevier, copyright 2021. (B) ZnS/eggshell membrane (ESM), reproduced from ref. 120 with permission from Royal Society of Chemistry, copyright 2014, (C) Fe_xO_y/biochar, reproduced from ref. 121 with permission from Elsevier, copyright 2020, and (D) Co/modified bentonite, reproduced from ref. 122 with permission from American Chemical Society, copyright 2020.

Bentonite is a member of the smectite family, which is found in many countries and originates from the activities of volcanoes.⁹⁰ This clay has an ordered structure, uniform pore volume, high exchange capacity, high specific surface area, thermal, chemical and mechanical stability and it is low-cost, which makes it a promising support.⁹¹ Bentonite is widely used as a support for green MNP such as Ag-NP,^92,93 Cu-NP,^94,95 SnO₂-NP,⁹⁰ Pd-NP,⁹⁶ ZnO-NP,⁹⁷ nZVI-NP,^91,98–102 bimetallic nZVI/Cu-NP,⁸¹ Ag/Fe₃O₄ and Pd/Fe₃O₄-NP,¹⁰³ Fe/Pd-NP¹⁰⁴ and Fe/Ni-NP.¹⁰⁵ The efficiency of tetracycline adsorption by green nZVI/Cu-NP in suspension and by bentonite supported green nZVI/Cu using pomegranate rind extract was studied by Gopal et al.⁸¹ The aggregation of bimetal NP was reduced and their dispersion onto bentonite increased. The increased surface area could be assigned to the certainty of bentonite as a supporting material that effectively reduces the aggregation of nZVI/Cu-NP and leads to equal distribution of bimetal NP on its surface.³ Issaabadi et al.⁹⁴ described the preparation of bentonite/Cu-NP using the aqueous extract of the leaves of Thymus vulgaris as a reducing agent and efficient stabilizer and studied the catalytic efficiency of the nanocomposite in the degradation of methylene blue and Congo red. They observed that the Thymus vulgaris leaf extract allows a fast and convenient preparation of the bentonite/Cu-NP and can reduce Cu²⁺ ions into Cu-NP without severe conditions. All the syntheses using green tea extract to produce bentonite supporting nZVI (ref. 98–101) were performed at RT. The biofabrication performed by Hassan et al.¹⁰⁰ was carried out in a short reaction period (15 min), but NaOH solution was added to adjust the pH, which makes the process less sustainable.

Zeolites are a particular group of aluminosilicate and microporous materials that have very good adsorption properties. The microporous structure with varying pore diameter can retain the substances to be separated depending on their molecular size and thus acts as molecular sieves. This attribute of zeolites qualifies them as reliable, cost-effective and eco-friendly supports for binding the MNP.¹⁰⁶ Natural zeolites have been used as sustainable supports for biogenic MNP such as Ag-NP,^107,108 Cu-NP,^109,110 Pd-NP¹¹¹ and nZVI-NP.¹¹² In a recent study, the green nZVI-NP dispersed onto zeolite produced with eucalyptus leaf extracts was applied to concurrently eliminate ammonia and phosphate from aqueous solutions.¹¹² This nanocomposite eliminated 43.3% of NH₄⁺ and 99.8% of PO₄³⁻, at a primary concentration of 10 mg L⁻¹ for each of the 2 co-existing ions. After optimization, the conditions for maximum adsorption capacity of the produced material for NH₄⁺ and PO₄³⁻ were 3.47 and 38.91 mg g⁻¹, respectively.

Other natural materials such as sand,¹¹³ sediments,¹¹⁴ rocks^115,116 and volcanic glass^117–119 have also been reported as solid supports for green MNP. Perlites are small pebbles of natural glass which contain a small amount of occluded water and are found in volcanic deposits.¹¹⁷ The reduction of nitroarenes and organic dyes in water was evaluated using green Pd-NP dispersed on perlite.¹¹⁹ The green Pd/perlite nanocomposite can be recovered and recycled several times without significant loss of activity. Likewise, Nasrollahzadeh et al.¹¹⁸ used perlite to immobilize Cu-NP synthesized using an aqueous extract of the leaves of E. esula L. Ex situ fabrication involves the mixing of freshly pre-synthesized Cu-NP along with perlite for 15 h. The particles exhibited spherical morphology with a low tendency to agglomeration. The Cu-NP/perlite shows favorable activity and separability on the catalytic reduction of 4-nitrophenol and can be reused several times without a decrease in the catalytic activity.

The in situ fabrication is widely used to synthesize composites since it warrants more uniformity of the NP on the support. Only two studies were performed ex situ using natural materials as solid supports and both reported a low tendency of NP to agglomerate.^112,118

4.2.2 Waste materials. In recent years, there has been a tremendous upsurge of interest in the application of waste materials for the removal of pollutants from aqueous media with focus on the application of waste materials as supports for biogenic MNP. Some residues from food, industry and agro-forestry have been reported as good supports for biogenic MNP. Table 3 summarizes all studies that highlight the ability of waste materials to act as stable supports for the immobilization of biogenic MNP for environmental pollution abatement.

Table 3 Waste materials as supports for biogenic MNP with applications in wastewater treatment

Support	Biogenic MNP	Bioreductor	Immobilization method	Shape and size	Wastewater application and recyclability	Ref.
Room temperature (RT); not reported (n.r.); not applicable (n.a.).
Eggshell	Cu	Orchis mascula L. leaf extract	In situ with vigorous shaking at 70 °C for 3 h	Spherical	Catalytic reduction of 4-nitrophenol, rhodamine B, methylene blue, methyl orange and Congo red	124
	Fe3O4
	Cu/Fe3O4			5–15 nm	7 cycles
	Pd	Barberry fruit extract	In situ with vigorous stirring and refluxed for 3 h	Spherical	Catalytic reduction of 4-nitrophenol, methylene blue, methyl orange and Congo red	126
				<20 nm	6 cycles
	SnO2/ZnO	Teucrium polium leaf extract	In situ with vigorous stirring at 70 °C for 8 h	Spherical	Adsorption of Hg^2+	125
				20–25 nm	3 cycles
	CuO	Pomegranate peel extract	Ex situ with reflux condition at 80 °C for 24 h	Spherical	Adsorption of aromatic compound from crude oil	127
					10 cycles
				30 nm	Catalytic reduction of 4-nitrophenol
					6 cycles
	Ag	Cacumen platycladi extract	In situ with impregnation for 6 h and reacted for 48 h	Spherical	Catalytic reduction of 4-nitrophenol	128
					5 cycles
				60 nm	Antibacterial activity against E. coli and S. aureus
					n.a.
	ZnO	Ferulago macrocarpa extract	In situ with stirring for 24 h and calcined	Spherical	Photocatalytic degradation of diazinon	129
					n.r.
				25 nm	Antibacterial activity against both human and fish pathogenic bacteria
					n.a.
Membrane of eggshell	MnO2	Membrane of eggshell	In situ with stirring at RT for 35 min	Spherical	Adsorption of tetracycline	130
				4.8 nm	n.r.
	ZnS		In situ with liquid impregnation method	Spherical	Photocatalytic degradation of methyl orange	120
				40 nm	4 cycles
	Au	Lagerstroemia speciose leaf extract	In situ with impregnation for 3 h	Spherical	Catalytic reduction of 4-nitrophenol	131
				20 nm	10 cycles
Almond shell	Pd	Almond hull extract	In situ with stirring at 100 °C	Spherical	Catalytic reduction of methylene blue, methyl orange and rhodamine 6G	132
				<20 nm	10 cycles
	Ag	Ruta graveolens sleeve extract	In situ with stirring at 70 °C for 2 h	Spherical	Catalytic reduction of 4-nitrophenol, rhodamine B and methylene blue	133
				10–15 nm	5 cycles
Walnut shell	Pd	Equisetum arvense L. leaf extract	In situ with vigorous shaking at 70 °C for 3 h	Spherical	Catalytic reduction of 4-nitrophenol, rhodamine B, methylene blue, methyl orange and Congo red	22
				5–12 nm	7 cycles
Peach kernel shell	Ag	Achillea millefolium L. extract	In situ with reflux conditions at 70 °C for 1.5 h	Spherical	Catalytic reduction 4-nitrophenol, methyl orange and methylene blue	4
				<20 nm	5 cycles
Seashell	Ag	Bunium persicum seed extract	In situ with stirring at 70 °C for 1 h	Spherical	Catalytic reduction of 4-nitrophenol, methylene blue, methyl orange and Congo red	140
				11 nm	5 cycles
	CuO	Rumex crispus seed extract	In situ with stirring at 60 °C for 20 min and calcination	Spherical	Catalytic reduction of 4-nitrophenol and Congo red	141
				8–60 nm	5 cycles
Pistachio shell	Ag	Cichorium intybus L. leaf extract	In situ with stirring at 75 °C for 2 h	Spherical	Catalytic reduction of 4-nitrophenol, methylene blue, methyl orange and Congo red	134
				20–50 nm	5 cycles
	Cu	Pistacia vera L. hull extract	In situ with sonicator bath at 60 °C	Spherical	Catalytic reduction of 4-nitrophenol, rhodamine B, methylene blue, and methyl orange	135
				15–45 nm	5 cycles
Hazelnut shell	Ag	Origanum vulgare leaf extract	In situ with stirring at 70 °C for 2 h	Spherical	Catalytic reduction of methyl orange and Congo red	136
				<30 nm	5 cycles
Wheat bran	Fe	Different extracts	In situ with stirring for 30 min or overnight	n.r.	Adsorption of chlorfenapyr	93
Rice bran	Ag				n.r.
Coconut palm spathe	Fe–TiO2	Cymbopogon citratus aqueous extract	Ex situ with doctor blade technique	Spherical	Photocatalytic degradation of cypermethrin	139
				13–16 nm	n.r.
Elaeagnus angustifolia seed	Ag@AgCl	Elaeagnus angustifolia leaf extract	In situ with stirring at 60 °C	Spherical	Photocatalytic degradation of methylene blue	137
				40 nm	3 cycles
Luffa sponge	Zn	Peroxidase enzymes from Euphorbia amygdaloides	Ex situ with ultrasonic batch for 30 min and lyophilized	n.a.	Adsorption of trypan blue	142
				20 nm	n.r.

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Eggshell waste is among the most abundant waste materials coming from food processing industries and it is mainly composed of calcium carbonate (CaCO₃).¹²³ The eggshell waste has been applied as a reliable support for biosynthesized Cu/Fe₂O₃-NP,¹²⁴ SnO₂/ZnO-NP,¹²⁵ Pd-NP,¹²⁶ Cu-NP,¹²⁷ Ag-NP¹²⁸ and ZnO-NP.¹²⁹ Nasrollahzadeh et al.¹²⁴ used for the first time waste materials as a support for biogenic MNP with catalytic activity for pollutant reduction. These authors produced Cu/eggshell, Fe₃O₄/eggshell and Cu/Fe₃O₄/eggshell nanocomposites from waste chicken eggshells using the aqueous extract of the leaves of Orchis mascula L. as a stabilizing and reducing agent. The immobilization of Cu and Fe₃O₄-NP increased the specific surface area, the number of pores on the surface of the eggshell and the catalytic activity compared with the MNP in suspension. Also, eggshell was used as an economic and environmentally friendly support for the preparation of the supported Pd-NP through a green and simple in situ reduction method using barberry fruit extract as a reducing and stabilizing agent.¹²⁶ The authors observed that Pd-NP were immobilized on the eggshell surface, which indicates the perfect combination of the Pd-NP and eggshell.

The eggshell membrane is the innermost portion of the eggshell and is mainly composed of a thin inner and a thick outer membrane which includes proteins such as collagen, sialoprotein, osteopontin, and a small amount of saccharides with an interwoven fibrous structure.¹²⁰ The use of the eggshell membrane as a support for biogenic MNP was found for MnO₂-NP,¹³⁰ ZnS-NP¹²⁰ and Au-NP.¹³¹ Wang et al.¹³⁰ used the eggshell membrane acting as both a support and reducing agent for MnO₂-NP. The clean eggshell membrane was cut into slices, soaked into potassium permanganate solution and stirred at room temperature. This nanocomposite showed a good capacity for decontamination of tetracycline hydrochloride and can be separated easily from the bulk solution by taking the membrane out to stop the degradation, instead of centrifugation or filtration. Zhang et al.¹²⁰ proposed a mechanism to explain the production of ZnS supported on the eggshell membrane. The authors show that when the membrane of the eggshell is immersed into Zn-precursor solution, Zn²⁺ can be tightly adsorbed by negative functional groups on the glycoprotein mantle of the eggshell membrane. By consecutive soaking of the new eggshell membrane into Na₂S solution, S²⁻ is moved toward active positions to achieve in situ nucleation of the ZnS nanocrystallites. With the repetition of this cycle, the ZnS-NP self-assembled spheroid-like nanocrystallites on the eggshell membrane do grow (Fig. 4B).

Agriculture wastes can be used as supports for biogenic MNP as is the case of the shells of almonds,^132,133 walnuts,²² peaches,⁴ pistachios,^134,135 hazelnuts,¹³⁶ wheat bran and rice bran⁹³ and seeds of Russian olives.¹³⁷ The catalytic performance of green Pd-NP supported on modified almond shells was studied.¹⁵³ The surface of almond shells was chemically modified using captopril and it was observed that the surface to volume ratio increased and the immobilized NP were more stable. Walnut shell is a low-cost, easily available biomaterial, which has an intrinsic pore structure and is composed of cellulose, hemicelluloses and lignin.¹³⁸ This material was used for the first time by Bordbar and Mortazavimanesh,²² in the in situ fabrication of Pd-NP by reduction of Pd ions adsorbed on the surface of the walnut shell using the aqueous extract of the leaves of E. arvense L. as a reducing agent. These nanocomposites showed high catalytic activity in the reduction of 4-nitrophenol, Congo red, methylene blue and rhodamine B in the presence of aqueous NaBH₄ at room temperature. Also, the pistachio shell powder is an effective support for immobilization and dispersion of Cu-NP using pistachio hull extract.¹³⁵ The nanocomposite of Cu-NP/pistachio shell has shown high catalytic activity in the reduction of organic dyes and could be easily recovered and reused several cycles.

Forestry wastes such as coconut palm spathe have been used as supports for green Fe–TiO₂-NP and used for photocatalytic degradation of cypermethrin, reaching more than 80% degradation.¹³⁹

Natural seashell waste was utilized to support Ag-NP by a simple and green method using B. persicum seed extract.¹⁴⁰ The Ag-NP/seashell nanocomposite showed great catalytic reduction of different organic dyes. Also, Rostami-Vartooni used seashell waste to support CuO-NP using Rumex crispus seed extract as a chelating and capping agent.¹⁴¹ This nanocomposite was reused five times for 100% reduction of 4-nitrophenol.

Luffa sponge is the dried vascular bundle from the fruit of a widely distributed cucurbit and it is an environmentally friendly and pollution-free support with a large specific surface area and was used to immobilize ZnO-NP to remove trypan blue azo dye.¹⁴² The ZnO-NP were obtained by using the peroxidase enzyme from Euphorbia amygdaloides plants and then were immobilized on the fiber of the luffa sponge by incubation in an ultrasonic bath for 30 min. The optimum removal of the dye was found at pH 7 and the equilibrium was attained within 30 min.

4.2.3 Activated carbon and biochar. Among the various types of sustainable supports, carbon-based materials derived from bio-resources are also utilized as suitable supports for biogenic MNP. Biochar and activated carbon are pyrogenic carbonaceous materials that are produced by thermochemical conversion of carbonaceous feedstock (pyrolysis or/and activation).¹⁴³Table 4 summarizes all studies that highlight the ability of biochar and activated carbon to act as stable supports for the immobilization of biogenic MNP for environmental pollution abatement.

Table 4 Biochar and activated carbon as supports for biogenic MNP with applications in wastewater treatment

Support	Biogenic MNP	Bioreductor	Immobilization method	Shape and size	Wastewater application and recyclability	Ref.
Nano zerovalent iron (nZVI); room temperature (RT); not reported (n.r.); not applicable (n.a.).
Biochar from carbonaceous material from sewage sludge	Ag	Camellia sinensis leaf extract	In situ with stirring for 12 h	Non-spherical	Catalytic reduction of methylene blue	145
				26 nm	n.r.
Biochar from Spartina alterniflora	ZnO	Spartina alterniflora extract	Ex situ with a water bath at 80 °C for 2 h and carbonized	Near-rodlike	Photocatalytic degradation of malachite green	146
				25–40 nm	n.r.
Biochar from jackfruit peel	Fe	Polysaccharide extract from mushroom	Ex situ with microwave irradiation	Spherical	Adsorption of phosphate and nitrate	147
				100 nm	5 cycles
Biochar from oak wood	nZVI	Tea polyphenol	In situ with stirring for 30 min	Spherical	Adsorption of Cr^6+	148
				90–200 nm	n.r.
Biochar from banana peel	Fe	Banana peel extract	In situ with continuous sonication for 1.5 h	Irregular	Adsorption of methylene blue	121
				n.r.	5 cycles
Biochar from kenaf bar	nZVI	Green tea extract	In situ with N2 atmosphere	Spherical	Catalytic reduction and oxidation of Cu^2+ and bisphenol A	149
				100 nm	3 cycles
Biochar from green tea residues				n.r.	Adsorption of p-nitrophenol	150
					5 cycles
Biochar from N. lappaceum peel	Fe	Nephelium lappaceum peel extract	In situ with stirring at RT for 30 min	Spherical	Adsorption of organochlorine pesticides	151
				20–80 nm	5 cycles
Activated carbon from Hildegardia barteri leaves	Ag	Ocimum gratissimum leaf extract	Ex situ with stirring for 2 h	Spherical	Adsorption of Congo red	153
				22–43 nm	5 cycles
Activated carbon from Corchorus olitorius stems	SnO2	Saccharum officinarum juice	In situ with constant stirring for 15 min and autoclaved	Irregular	Photocatalytic degradation of naproxen	154
				3 nm	5 cycles
Activated carbon from Delonix regia	ZnO	Tithonia diversifolia leaf extract	Ex situ with stirring at 150 °C for 1 h	Spherical	Adsorption of methylene blue	155
				9–29 nm	5 cycles
Activated carbon from palm coconut shell	Cu	Moringa oleifera leaf extract	In situ with stirring at RT for 36 h	n.r.	Adsorption of nitrate	156
				66–61	n.r.
Activated carbon from lemon pomace	Fe/Zn	Lemon leaf extract	In situ with stirring for 3 h	Spherical	Fenton-like oxidation of reactive red 2	157
				126 nm	n.r.
Activated carbon from corn straws	S-nZVI	Green tea extract	In situ with stirring for 2 h under nitrogen-flowing atmosphere	n.r.	Adsorption of Pb^2+	158
					n.r.
Activated carbon from worn tires	ZnO	Walnut peel extract	Ex situ with stirring for 2 h	Spherical	Adsorption of eosin Y and erythrosine B	160
				31–35 nm	5 cycles
		Walnut shell extract	Ex situ with stirring for 10 h	n.r.	Adsorption of reactive blue 19 and reactive black-5	159
					n.r.
Activated carbon from pomegranate peel	nZVI	Pomegranate peel extract	Ex situ with stirring for 2 h and placed in the oven for 10 h at 95 °C	Non-uniform	Adsorption of furfural	161
				11–35 nm	5 cycles
Activated carbon from sawdust	Cu	Green tea extract	Ex situ with stirring for 12 h at RT and then heated at 80 °C	n.r.	Adsorption of acridine orange	162
				200 nm	n.r.
Activated carbon from spent mushroom compost	Ag	Corncob extract	Ex situ with magnetic stirring and sonication	Spherical	Adsorption of levofloxacin	163
					n.r.
				10–25 nm	Antibacterial activity against both E. coli and S. aureus
					n.a.
Activated carbon from the spent home filtration system filters	Ag	Azadirachta indica leaf extract	Ex situ with vigorous stirring overnight at RT	Spherical	Catalytic reduction of 4-nitrophenol	164
					n.r.
	ZnO			20–300 nm	Antibacterial activity against 16 pathogenic bacteria
	Ag/ZnO				n.a.

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Biochar is an attractive and low-cost carbon-rich material derived from biomass and produced by pyrolysis under limited oxygen or by hydrothermal carbonization at high pressure. In order to produce biochar the pyrolysis temperature generally ranges from 300 to 1000 °C.¹⁴⁴ A variety of raw materials have been tested as precursors of biochar such as carbonaceous materials from sewage sludge,¹⁴⁵ invasive plants,¹⁴⁶ jackfruit peel,¹⁴⁷ oak wood,¹⁴⁸ banana peel,¹²¹ kenaf bar,¹⁴⁹ green tea¹⁵⁰ and rambutan peel,¹⁵¹ to serve as a support for biogenic MNP. The synthesis of a novel biochar-supported nZVI through a green method with green tea aqueous extract has been reported by Liu et al.¹⁴⁹

The nanocomposite showed high performance on the simultaneous removal of Cu²⁺ and bisphenol A by a combination of biochar–nZVI with a persulfate system, reaching removal rates of 96% and 98% within 60 min, respectively. The authors proposed a possible reaction mechanism for the synergistic reduction and oxidation. For the Cu²⁺ removal process, Cu²⁺ species could be firstly adsorbed and then suffer co-precipitation and complexation processes, being reduced into Cu⁺ and Cu⁰ species at the end through one electron reduction process. For the bisphenol A removal process, this could also be adsorbed on the biochar–nZVI surface, while Fe²⁺ would be released from the nanocomposite under acidic conditions which could activate the persulfate system for the production of SO₄²⁻. Subsequently, bisphenol A could be degraded into a series of products such as p-isopropenyl phenol, 4-isopropylphenol, 4-hydroxyacetophenone, p-hydroquinone, fumaric acid and 2-hydroxypropionic acid.¹⁴⁹ The synthesis of a green biochar/iron oxide composite, using a facile approach involving banana peel extract and biochar from the same banana peel, was achieved with an enhanced adsorption ability for methylene blue (862 mg g⁻¹).¹²¹ The authors proposed a mechanism in which the banana peel biochar with reductive biomolecules reacts with FeSO₄ to form a biochar/nZVI composite. Formerly, under the action of dissolved oxygen in water, nZVI is oxidized and hydrolyzed to form a biochar/Fe_xO_y composite (Fig. 4C).

Also Jing et al.¹⁴⁶ synthesized ZnO-NP using S. alterniflora extract loaded on biochar derived from S. alterniflora by a one-step carbonization method and showed 98.38% photocatalytic degradation of malachite green. Nayak et al.¹⁴⁷ fabricated a novel biocompatible nanocomposite by ultrasonication-assisted extraction of natural polysaccharides from mushrooms followed by its use in the synthesis of biogenic magnetite NP immobilized onto the biochar of jackfruit peel with a good adsorption capacity for phosphate (7.95 mg g⁻¹) and nitrate (5.26 mg g⁻¹).

Activated carbon is produced from environmental wastes with high carbon content.¹⁵² Lignocellulosic and coal materials such as tree,¹⁵³ jute stick,¹⁵⁴ flamboyant pods,¹⁵⁵ palm coconut,¹⁵⁶ lemon pomace,¹⁵⁷ corn straws,¹⁵⁸ worn tires,^159,160 pomegranate peel,¹⁶¹ sawdust,¹⁶² spent mushroom compost¹⁶³ and spent filters from home filtration systems¹⁶⁴ have been used as raw materials planned for the manufacture of activated carbon to act as a support for biogenic MNP.

The preparation of an efficient heterogeneous nanocomposite based on the immobilization of biogenic SnO₂-NP on activated carbon from Corchorus olitorius stems was reported and showed excellent photocatalytic activity towards degradation of naproxen.¹⁵⁴ Taha et al.¹⁶⁴ have reported the green synthesis of Ag-, ZnO- and Ag/ZnO-NP with posterior immobilization on activated carbon from the spent filters from home filtration systems and evaluated the antibacterial and the catalytic activity of the synthesized nanocomposites. The results showed that Ag-NP loaded on activated carbon have the best catalytic activity compared to the other nanocomposites, which was attributed to the good dispersion of Ag-NP on the surface of activated carbon.

4.3 Modified sustainable materials

Modified sustainable materials as efficient solid supports for the immobilization of biogenic MNP have been applied on the removal of organic or inorganic water contaminants.

The modification of natural bentonite by different chemical and physical processes including alkaline treatment, acid leaching, thermal treatment, polymer intercalation and organic modifications resulted in hybrid materials with high basal spacing, high surface area, high adsorption capacity and with more active functional groups with high affinity for organic pollutants.¹⁶⁵ Abdel Salam et al.¹²² modified natural bentonite by direct intercalation of its layers by cetyltrimethylammonium bromide (CTAB) and applied it as a carrier for green fabricated cobalt oxide-NP. The authors showed that the mechanism involved the intercalation of bentonite layers with CTAB chains, producing organically modified bentonite of high basal spacing and extended surface area, with different types of functional groups. Afterward, the green decoration of the modified clay by Co₃O₄-NP results in the formation of individual nanograins of cobalt oxide distributed on the surface of the clay, defining other functional groups to increase the overall adsorption capacity (Fig. 4D). This organic modification improved the bentonite surface area and its adsorption capacity, especially for the organic pollutant fixation, compared to those of the raw phase, which is considered a vital step for effective photocatalytic degradation and reduction processes. In addition, natural bentonite was modified by interchanging the Al polycation with Na⁺ and Ca²⁺ in the interlayer of the clay through a pillarization process followed by calcination.¹⁶⁶ The resulting pillared bentonite has a relatively large surface area and interlayer distance, has uniform pores and is not fluffy (i.e., it is non-swelling) which renders it thermally stable and allows it to be used as a catalyst.

The natural low-cost montmorillonite has proven to exhibit excellent properties in wastewater treatment. The modification of montmorillonite is performed to increase the surface area and to generate pores enabling it to act as a good support and stabilizer in the synthesis of MNP. Saikia et al.¹⁶⁷ modified montmorillonite (purified bentonite) with hydrochloric acid to increase the surface area by pore generation, in order to immobilize Pd-NP, using Ocimum sanctum leaf extract as a reducing agent. The supported Pd-NP were used in the catalytic hydrodechlorination of 4-chlorophenol in water under base free conditions and showed conversions up to 98%.

Natural zeolite is an abundant resource of aluminosilicate materials with a high cation exchange capacity. Kim et al.¹⁶⁸ tested the adsorption of Pb²⁺, Cd²⁺ and Zn²⁺ by immobilizing biogenic manganese oxide on natural zeolite pretreated with NaCl or NaOH to increase the adsorption ability.

Perlite, a naturally occurring glassy volcanic siliceous rock with both unique physical and chemical properties, has attracted a great deal of attention in different areas, especially when it is heated rapidly at 760–1100 °C. Shirkhodaie et al.¹⁶⁹ used expanded perlite supporting Fe₂O₃-NP with yellow pea as a reducing agent and ibuprofen as an organic agent. With perlite being heated, it expands up to 20 times its original volume and forms a light white mineral. It is chemically inert and nontoxic with different types of silanol groups and alumina hydrous oxide on its surface.

5. Economic and environmental perspective

In order to guarantee the highest quality drinking water, it is necessary to introduce new advanced technologies in this area. The ability to integrate different properties, resulting in multifunctional systems, is a major benefit for the use of nanomaterials as compared with traditional water technology.¹⁷⁰ In fact, the development of sustainable, efficient and cost-effective methods for the elimination or effective reduction of pollutants and microorganisms in wastewater and drinking water is highly required.³

Recently, the application of bio-resources for the development of green methods for the synthesis of MNP has gained widespread attention. Using non-toxic, biocompatible and environmentally benign methods for the synthesis of MNP associated with sustainable materials that may be recovered and reused has become of great interest. Various high-value products may be obtained with low economic and environmental impact and nature provides many useful materials with hidden potential to be applied in innovative processes. This environmental approach has been reflected in increased interest in clay minerals because they are abundant, easily found everywhere and present unique properties to act as MNP supports. They have a significant ability to immobilize pollutants, either by adsorption or ion exchange mechanisms, and usually present excellent mechanical stability.⁸¹ Among the more than four thousand known minerals, only a small part has been tested as a support for green NP. Clays such as sepiolite,¹⁷¹ rectorite,¹⁷² chrysotile¹⁷³ and halloysite¹⁷⁴ are found in the literature as sustainable supports for commercial MNP or chemically synthesized MNP.

Amongst natural materials, cork (the bark of the evergreen oak Quercus suber L.) is a particularly interesting and promising option not yet found as a support for biogenic MNP.^175,176 The use of cork is sustainable, as the tree is not harmed during the harvesting of the bark (the harvesting is performed every 9–13 years).¹⁷⁷

Wastes have been used as valuable and environmentally friendly supports for the immobilization of biogenic MNP. The concept of utilization of waste materials as a vector of circular economy introduces alternatives to maximize the reuse and recycling of wastes within the wastewater treatment process.^178,179 At present, large quantities of wastes are discharged as the end of the line, resulting in considerable environmental issues. Food waste in the world is estimated at 1.6 billion tons per year as a result of population growth.¹⁷⁸ For example, eggshell is one of the most common forms of food waste with a worldwide production of 50 000 t per year.¹²³ According to Food and Agriculture Organization (FAO) reports, almost 30 million tons of biomass from pistachio nuts are discarded annually in landfills by the pistachio nut processing industry.¹³⁵ According to Welter et al.,¹⁸⁰ more than 50% of fishing is thrown away as wastes resulting in around 20 million tons of residues being discarded worldwide each year from fisheries. These wastes, if not appropriately handled, will produce large quantities of greenhouse gases to be released into the atmosphere.¹⁸¹ Mostly, waste materials from the different sectors are disposed of by allowing them to rot at the source or in stockpiles.¹⁷⁸ This lack of proper treatment of wastes severely affects the ecosystems.¹⁸² Furthermore, there are numerous wastes that can be applied as sustainable supports such as kitchen wipe sponge,¹⁸³ palm trunk,¹⁸⁴ industrial waste (lithium silicon powder),¹⁸⁵ fly ash,¹⁸⁶ wood¹⁸⁷ and aloe vera.¹⁸⁸ The use of wastes as sustainable supports for biogenic MNP only implies washing, drying and crushing. In addition, activated carbon and biochar have been produced from wastes as precursors, making this approach an effective and economically valuable solution for environmental pollution. Activated carbon has been identified as one of the most promising materials of the 21st century used in chemical industries thanks to its unique properties, including morphology, high mechanical strength, adsorption capacity, durability and good chemical stability.¹⁸⁹ There is research effort to design and prepare affordable materials that may act as an alternative to the commercially available activated carbon.^161,190

Alternative supports including sustainable natural biopolymers such as cellulose, chitosan, alginate, dextran and starch have been recently employed for the preparation of supported MNP.^191,192 Biopolymers offer several advantages compared to traditional supports including low toxicity and cost, high biocompatibility, availability and abundance as well as delivering unique functions as per their use.¹⁹³

These natural or cheap precursor materials, therefore, ensure that a reasonable quantity of nanocomposite can be produced and that there is potential for upscaling. This is a condition that is not typically the case in current nanocomposites because they are prepared from rather expensive reagents and often through more complex processes that are not generally amenable for developing countries.³

The circular economy nowadays is the goal for research that seeks improvement in sustainable development. It aims at minimizing human and industrial pollution by turning waste and by-products into added value products.¹⁹⁴ From the perspective of circular economy and green development, the simultaneous utilization of low-cost materials and/or reuse of natural or organic waste materials as supports for green MNP in the treatment of wastewater is one of the most thrust areas for researchers and environmental scientists.¹⁸² This synergistic effect will enhance the reusability of material and eliminate the generation of secondary pollutants.

6. Applications in wastewater treatment

Green nanocomposites have been applied in wastewater treatment mainly as adsorbents and catalysts. Various studies propose the role of these nanocomposites in wastewater treatment.

Qu et al.¹⁵⁸ detailed the possible interactions associated with Pb(II) and sulfide-nZVI immobilized on activated carbon and proposed that the adsorption of metal occurred via the combination of electrostatic interactions, chemical precipitation, surface complexation and reductive actions (Fig. 5A). Soliemanzadeh and Fekri⁹⁹ showed that Cr(VI) was attracted to the surface of green nZVI supported on bentonite by monolayer adsorption with specific force rather than a simple electrostatic attraction.

Fig. 5 Mechanisms of green nanocomposite application in wastewater treatment. (A) Adsorption of Pb(II) by sulfide-nZVI supported in activated carbon, reproduced from ref. 158 with permission from Elsevier, copyright 2021, and (B) catalytic reduction of 4-nitrophenol by NaBH₄ and CuNP supported on pistachio shell, reproduced from ref. 135 with permission from Elsevier, copyright 2018.

Taghizadeh and Rad-Moghadam¹³⁵ proposed a mechanism of catalytic reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ and CuNP supported on pistachio shell (Fig. 5B). Firstly, the formation of 4-hydroxylaminophenol due to dissociation of NaBH₄ in the aqueous medium to generate borohydride ions is reported as well as the subsequent adsorption and diffusion of these ions on the surface of CuNP and adsorption of 4-nitrophenol onto the nanocomposite. The 4-hydroxylaminophenol is reduced to 4-aminophenol, then it departs from the surface of CuNP by physical desorption to create a free surface that enters into a new catalytic cycle.

Some studies were published with green nanocomposites applied as antibacterial agents.^{128,129,163,164} Lashkarizadeh¹²⁹ demonstrated that green ZnONP immobilized on eggshells had a significant microbial growth inhibition compared with the ZnONP in suspension. Many studies reported that the reduced size of MNP leads to an increased surface area with a good affinity for both amino and carboxyl groups on the cell surface, facilitating penetration into the cells.¹⁶⁴ A mechanism reported by Karadirek et al. indicates that the ions present in the MNP potentially leads to the destruction of the double helix chain structure of DNA.¹⁶³

The ideal nanocomposite ought to be easily separated from the reaction medium, recycled and reused for other runs without marked loss of activity. The reuse efficiency of the nanocomposite is demonstrated in most studies. It should be noted that the highest recyclability is found in studies that use wastes as solid supports for MNP. Some composites were reused several times (10 fold) without any significant loss in the activity.^127,131,132 In the case of the adsorption process, the reusability of nanocomposites implies the desorption of sorbate. Various authors reported the use of sodium hydroxide,^81,105 hydrochloric acid,¹¹³ ethylenediaminetetraacetic acid,¹²⁵ methanol,¹²⁷ ammonium hydroxide,¹⁴⁷ ethanol¹²¹ and n-hexane.¹⁵¹ Meanwhile in other wastewater applications, as catalysts, the reuse of the nanocomposite only implied the process of separation from the reaction mixture, which includes centrifugation,^131,132 filtration¹²⁷ and external magnetic separation.⁸²

Although the use of green nanocomposites on water treatment is becoming more popular and the results seem to be promising in this research field, it still presents several challenges. First, the high variability of bio-resources to biosynthesize MNP influences the characteristics of NP, since different bio-resources contain different concentrations of organic reducing agents, which implies a previous optimization of the synthesis process. It is important to understand the specific biosynthesis mechanism to standardize the methodology. The green nanocomposite application in wastewater treatment is mostly assessed with dyes at the lab-scale. In the future, it is important to contemplate the application of these materials with other hazardous and persistent pollutants as well as to conduct research at the pilot-scale in order to evaluate the system efficiency in real effluents.^3,81 In addition, the leaching rate of the green material and toxicity to the environment and human health are other challenges.¹⁹⁵ Further research is needed to address the challenges posed by nanomaterials.

7. Conclusions

The principles of Green Chemistry can be used on greener bio-inspired materials applied on the synthesis of MNP, as well on their immobilization on solid supports. A wider application of MNP is limited by some issues such as the difficulty of suspension manipulation, the ultimate health risks of MNP dissipation into the environment and size instability due to aggregation. Synthesis of MNP on a solid support has emerged as a sustainable solution that may prevent the accumulation of MNP waste in the ecosystem through a recycling process. This review focusses on sustainable materials to act as supports of biosynthesized MNP in the field of wastewater treatment. Materials derived from renewable and earth-abundant sources, such as kaolin, montmorillonite and bentonite, are used as sustainable supports. Residues from the food industry, such as eggshells or almond shells, have been explored as MNP supports. The nanocomposites are applied in wastewater treatment as adsorbents, catalysts or antibacterial agents for removal or elimination of various xenobiotic pollutants or water disinfection. Constant efforts are made to use sustainable materials as pristine or in modified structures for supporting MNP and future studies should be focused on such new green methodologies and combined techniques for the reuse of sustainable materials.

Abbreviations

CTAB	Cetyltrimethylammonium bromide
FAO	Food and Agriculture Organization
GO	Graphene oxide
MNP	Metal nanoparticles
NP	Nanoparticles
nZVI	Nano zerovalent iron
rGO	Reduced graphene oxide
RT	Room temperature
TUD-1	Technische Universiteit Delft
WWTP	Wastewater treatment plants

Author contributions

Verónica Rocha: conceptualization, methodology, data curation, writing – original draft; Ana Elisa Lago: writing – original draft; Bruna Silva: writing – original draft; Óscar Barros: writing – original draft; Isabel C. Neves: conceptualization, methodology, supervision, writing – review & editing, Teresa Tavares: supervision, validation and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Verónica Rocha, Ana Lago and Óscar Barros thank FCT (Portuguese Foundation for Science and Technology) for their PhD grants (SFRH/BD/141073/2018, SFRH/BD/132271/2017, SFRH/BD/140362/2018, respectively). This research work has been funded by national funds funded through FCT/MCTES (PIDDAC) over the projects: UIDB/50020/2020, Centre of Chemistry (UID/QUI/0686/2020), CEB (UIDB/04469/2020) and LABBELS (LA/P/0029/2020), and project BioTecNorte (operation NORTE-01-0145-FEDER-000004), supported by the Northern Portugal Regional Operational Program (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF).

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