Biologically synthesized Fe⁰-based nanoparticles and their application trends as catalysts in the treatment of chlorinated organic compounds: a review

Abstract

This review explores the advancements and trends in biologically synthesized Fe⁰-based nanoparticles (NPs) and their applications as catalysts in treating chlorinated organic compounds. The persistent nature and bioaccumulative characteristics of chlorinated organic compounds enable their accumulation in water, soil, and the food chain, leading to significant environmental and human health issues. The widespread presence of these toxic substances underscores the urgent need for effective treatment and remediation strategies. Biologically synthesized Fe⁰-based NPs are recognized for their considerable surface area, potent reduction properties, and environmental compatibility. These attributes render them a promising approach for the remediation of chlorinated compounds. This review categorizes synthesis methods into key groups: microorganisms, plant extracts, biological waste, and industrial–agricultural by-products. Recent studies highlight the promising applications of bio-NPs in environmental remediation, emphasizing their potential for sustainable and efficient treatment solutions. This analysis thoroughly examines current trends in the application and enhancement of nanoparticle activity, delineating various challenges and future prospects comprehensively. It offers well-defined research directions with high practical relevance, aiming to contribute to advancing knowledge and guiding future research endeavors in the field.

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Environmental significance

Chlorinated organic compounds pose significant environmental and health hazards due to their persistence, bioaccumulation, and toxic effects. Conventional treatment methods often fall short in effectively remediating these pollutants. Biologically synthesized Fe⁰-based nanoparticles offer a promising alternative, as they exhibit high reactivity, large surface area, and environmental compatibility. Their green synthesis using biological agents not only reduces the environmental impact associated with chemical synthesis methods but also enhances sustainability. This review highlights the potential of biogenic Fe⁰ nanoparticles as catalysts in the degradation of chlorinated organic compounds, contributing to advancing eco-friendly technologies for water and soil remediation. Their adoption could mark a substantial step forward in achieving cleaner and safer ecosystems.

1. Introduction

In the era of modern technology, the remarkable advancement of nanotechnology has led to significant scientific progress, unveiling vast potential applications across various fields such as medicine, the environment, energy, and industry. Among these, the application of nanomaterials to address environmental pollution issues has become a hot topic, garnering substantial interest.^1–3

The issue of pollution and the inherent risks of chlorinated organic pollutants have posed a persistent challenge, becoming a global concern and drawing attention from scientists for decades.^4–6 Control measures and numerous technologies are being researched, but they still face challenges in completely mitigating the destructive effects of these pollutants.⁶ Soil contamination with chlorinated organic chemicals is widespread, primarily resulting from industrial chemical waste or extensive pesticide use in agriculture.⁷ Meanwhile, the increasing presence of chlorinated organic compounds in aqueous environments has been acknowledged as a considerable threat to ecosystems. These substances' long-term persistence and bioaccumulation potential threaten the environment and human health.⁸ The application of metallic nanomaterials to remediate and eliminate chlorinated organic compounds has been extensively studied.^9,10 Fe⁰-based nanomaterials (also known as zero-valent iron/nZVI) possess the ability to catalyze the conversion of chlorinated organic compounds through a common mechanism, namely the hydro dechlorination (HDC) reaction.^11,12 Ševců et al.¹³ demonstrated that nZVI has the capacity to degrade two polychlorinated biphenyl (PCB) isomers, specifically PCB-28 and PCB-52, with their concentrations decreasing from 20.7 μg L⁻¹ to 3.0–5.8 μg L⁻¹ within 1 hour. Studies by Kim et al.¹⁴ and Dumestre et al.¹⁵ also highlighted nZVI's ability to degrade trichloroethylene (TCE) by over 99%. Similarly, Calderon et al.¹⁶ showed that polychlorinated dioxins and furans (PCDD/Fs) could be adsorbed onto the surface of nZVI, with total PCDD/F WHO-TEQ concentrations reaching up to 35 pg g⁻¹. This process effectively cleaves the C–Cl bond, forming less harmful or non-toxic by-products¹⁷ (Fig. 1).

Fig. 1 Mechanisms of reductive dechlorination by nZVI and nZVI/Pd.¹⁷

Typically, the synthesis of nanomaterials via physical (ball milling,^18,19 laser ablation,²⁰ mechanical grinding²¹) or chemical methods (using NaBH₄ reducing agent^22,23) is costly, time-consuming, and often generates highly toxic by-products, requiring high energy inputs and posing potential risks to human health.^24–26 To address these limitations, many researchers have developed a synthesis method that is more biocompatible, rapid, and cost-effective. The biological synthesis of NPs, also known as “green synthesis”, offers an alternative and more environmentally friendly approach to nanomaterial fabrication.^27,28 Green synthesis involves producing NPs without the use of hazardous or expensive chemicals.²⁹ Instead, natural resources are employed to create NPs, resulting in more environmentally friendly and biocompatible products. NPs synthesized through this method are commonly referred to as bio-NPs.^30,31 Recent studies have highlighted the potential of this approach for multi-faceted applications, particularly in the environmental field.

This review consolidates recent research on bio-synthesized Fe⁰-based NPs and their application in environmental pollution control, focusing on chlorinated organic compounds in water and soil. We will present some recent trends in nanotechnology fabrication, evaluate the advantages of green synthesis methods compared to traditional approaches, and discuss the future prospects of these materials in environmental applications.

2. Biological approach of NPs based on Fe⁰

NPs synthesized through biological approaches are commonly referred to as bio-NPs or bio-nanomaterials.^30,31 Iron NPs are characterized by their large surface area and strong reduction capability, and they are considered relatively safe and environmentally friendly.^32,33 Consequently, iron salts are frequently investigated in conjunction with biological agents to produce ‘green’ materials, minimizing negative environmental impacts and facilitating biodegradability. The trend in synthesizing Fe⁰-based nanomaterials biologically can be categorized into several groups: microorganisms, plant extracts, biological waste, and industrial–agricultural by-products.^31,34

2.1. Using microorganisms

The use of microorganisms for the biological synthesis of NPs is an innovative approach, leveraging natural microbial resources. Microbial nano-biosynthesis presents numerous advantages, including environmental friendliness and sustainability.^35,36 Furthermore, microbes provide enhanced control over the size, shape, and distribution of NPs because of their unique metabolic pathways and enzymatic systems.³⁷ The synthesis usually occurs under mild conditions, such as ambient temperature and pressure, which minimizes the need for specialized equipment and reduces energy consumption.³⁸ Besides, proteins and other biomolecules produced by microorganisms can serve as natural capping agents, stabilizing NPs and preventing their aggregation.³⁹ However, microbial nanotechnology still faces several challenging limitations, including slow production rates,⁴⁰ a limited variety of NPs that can be synthesized, the need for sterile conditions, and variability in the characteristics of the NPs produced.⁴¹ Moreover, the highly complex mechanism of biosynthesis of NPs is one of the limitations that has hindered the widespread research on this method. Fig. 2 summarizes some advantages and limitations of microbial nanoparticle synthesis.

Fig. 2 Advantages and limitations of microbial nanotechnology.

Microorganisms, which are microscopic in size, can be categorized into bacteria, fungi, yeasts, algae, and viruses.^42,43 They are considered ‘nano-factories’ for nanoparticle synthesis due to their ability to accumulate and detoxify heavy metals through various types of reductive enzymes.⁴² In microbial synthesis of Fe⁰-based nanoparticles, a wide range of microorganisms are utilized, including bacteria, fungi, algae, and yeasts. These microorganisms share common characteristics beneficial for nanoparticle synthesis, such as the ability to produce reductive enzymes and extracellular metabolites that aid in metal reduction and stabilization.⁴⁴ They also exhibit biocompatibility and environmental adaptability, allowing them to be effective and eco-friendly agents in synthesizing Fe⁰-based nanoparticles. Among these microorganisms, each group has unique attributes that further enhance the synthesis process. The suitability of Fe metal in the biosynthesis of NPs is attributed to its favorable properties, such as biocompatibility, availability, and the ability to facilitate various reduction and stabilization processes.⁴⁵ Iron NPs (Fe-NPs) are of particular interest due to their magnetic properties, which can enhance the efficiency of catalytic processes and environmental applications. Furthermore, Fe is often used in microbial synthesis due to its relatively low cost and the ease with which microorganisms can mediate its reduction to nanoparticle form. In the study by Dlamini et al.,⁴⁶ iron/copper (Fe/Cu) NPs, along with iron NPs (Fe-NPs) and copper NPs (Cu-NPs), were synthesized using a bioflocculant in an eco-friendly method. The Fe/Cu-NPs exhibited superior flocculation activity, achieving over 90% efficiency at a concentration of 0.2 mg mL⁻¹, while an optimal concentration of 0.4 mg mL⁻¹ was found for Fe-NPs. Additionally, these NPs demonstrated notable antimicrobial effects against both Gram-positive and Gram-negative bacteria. Safety tests indicated that the synthesized materials were non-toxic at low concentrations when tested on human embryonic (HEK293) and breast cancer (MCF7) cells, and they were also found to be biodegradable. In addition to bacteria, fungi, and yeasts are abundant and widely available natural sources that have attracted significant scientific interest. The synthesis of metallic NPs by fungi is facilitated by their ability to secrete molecules into the extracellular environment, reducing metal ions and stabilizing the resulting atom clusters. These processes are primarily driven by enzymes and other proteins. In the study by Alamilla-Martinez et al.,⁴⁷ Fe-NPs were synthesized by Alternaria alternata MVSS-AH-5, which had been previously isolated. TEM analysis revealed that the nanoparticle sizes ranged from 20 to 80 nm. Rojas-Avelizapa et al.⁴⁸ further demonstrated the ability of Alternaria alternata MVSS-AH-5 to produce siderophore organic acids. Siderophores are low-molecular-weight molecules known to be among the strongest Fe³⁺ chelators,⁴⁹ with exceptionally high affinity. While their primary role is to scavenge iron, they also form complexes with other essential elements. Another study by Tsilo et al.⁵⁰ explored the synthesis of Fe-NPs using a bioflocculant derived from the Pichia kudriavzevii yeast strain. These NPs were assessed for their flocculation and antimicrobial properties. The Fe-NPs showed significant antimicrobial activity against various Gram-positive and Gram-negative microorganisms. Another biological agent attracting scientific interest is algae, which are abundantly found in seaweed (Fig. 3a). Bensy et al.⁵¹ successfully synthesized iron NPs using a water extract from marine algae (Ulva lactuca) collected from the Tamilnadu coast, India. The green-synthesized NPs measured between 30–40 nm and exhibited strong biological activities. These NPs demonstrated anticancer effects against HeLa and Duke's Lymphoma of the Dog (DLD-1) cell lines and showed antimicrobial activity against S. aureus (24 ± 1 mm), E. coli (29 ± 1 mm), and S. typhimurium (31 ± 2 mm). Additional studies are summarized in Table 1.

Fig. 3 a) Nanoparticle synthesis using algae; b) intracellular biosynthesis of FeNPs in Pseudomonas putida;⁵² c) schematics of bacteria interaction with heavy metals and mechanisms of nanoparticle formation.⁵³

Table 1 Studies using microorganisms as sources for green synthesis of Fe⁰-based nanomaterials

NPs	Species	Particles size	Characterization	Shape	Ref.
Bacteria
Fe	Escherichia coli, Pseudomonas aeruginosa	18 ± 2 nm	FESEM, EDX, TEM, and UV-vis	Spherical	Crespo et al., (2017)^54
Fe	SRB strain WCA1	21 nm	SEM–EDS, and XRD	Spherical	Das et al., 2017 (ref. 55)
Fe	Proteus mirabilis strain 10B	11.7–60.8 nm	UV-vis, XRD, EDX, and TEM	Spherical	Zaki et al., 2019 (ref. 56)
Fe	Alcaligenes faecalis HCB2	50–60 nm	SEM, EDS, and FT-IR	Granular-like	Dlamini et al., 2020 (ref. 57)
Fe, Fe/Cu	Alcaligenes faecalis HCB2	<100 nm	FT-IR, TGA, SEM, TEM, UV-vis, and XRD	Spherical	Dlamini et al., 2020 (ref. 46)
Fe	Pseudomonas putida	1–4 nm	UV-vis, TEM, XRD, EDX, and VSM	Spherical	Zaki et al., 2021 (ref. 52)

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Fungi
Fe	Haemoglobin and myoglobin	2–5 nm	SEM, TEM, EDX, and XRD	Crystals	Sayyad et al., 2012 (ref. 58)
Fe	Fusarium oxysporum	20–40 nm	TEM and UV-vis	Spherical	Abdeen et al. 2013 (ref. 59)
Fe	Aspergillus oryzae	10–24.6 nm	DLS, TEM, HR-TEM and EDS	Spherical	Tarafdar et al., 2013 (ref. 60)
Fe	Alternaria alternata	5.4–12.1 nm	SEM, TEM, EDX, and UV-vis	Spherical	Yosri et al. 2015 (ref. 61)
Fe	Alternaria alternata	20–80 nm	TEM and UV-vis	Semi-oval/spherical	Alamilla-Martínez et al., 2019 (ref. 47)
Fe	Neurospora crassa	50 nm	SEM, EDX, XRD, FT-IR, and XPS	Granular	Li et al., 2020 (ref. 62)
Fe	Pleurotus florida	100 nm	SEM and UV-vis	Spherical	Manikandan et al., 2021 (ref. 63)

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Yeasts
Fe	Pichia kudriavzevii	58–79 nm	SEM–EDX, FT-IR, TEM, XRD, UV-vis, and TGA	Spherical	Tsilo et al., 2023 (ref. 50)

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Algae
nZVI	Chloroccum sp.	20–50 nm	TEM, DLS, and FT-IR	Spherical	Subramaniyam et al., 2015 (ref. 64)
Fe	Dictyota dichotoma	40–50 nm	SEM, FT-IR, and UV-vis	Cubic	Chandran et al., 2016 (ref. 65)
Fe	Chlorella sp.	5–50 nm	FE-SEM and XPS	Spherical	Subramaniyam et al., 2016 (ref. 66)
nZVI	Blue green alga	70–430 nm	SEM, UV-vis, and FT-IR	Agglomerated clusters	Karthika et al., 2019 (ref. 67)
Fe	Ulva lactuca	20–40 nm	FT-IR and SEM	Spherical	Bensy et al., 2022 (ref. 51)

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Microorganisms have developed mechanisms for metal sequestration to transport ions and regulate their availability. In the simplest scenario, metal ion sequestration occurs through adsorption.⁵³ The formation of NPs involves a complex mechanism, which Campaña et al.⁵³ have described, outlining the interactions between bacteria and metal ions in nanoparticle formation (Fig. 3c). During adsorption, deprotonated functional groups on the cell surface, such as carboxyl, phosphonate, amine, and hydroxyl groups, resulting in a net negative charge. This attracts metal cations and leads to non-specific binding of the metal to the cell surface.⁶⁸ The ultrastructural study of intracellular metal nanoparticle synthesis is often examined using Transmission Electron Microscopy (TEM). Nanoparticle formation is typically indicated by higher electron density, which appears darker in TEM images (Fig. 3b).

2.2. Plant extract

Plant extracts act as both reducing and stabilizing agents in nanoparticle synthesis. The key advantage of using plant extracts for metallic nanoparticle synthesis lies in their rapid reaction, mild conditions, and ability to prevent nanoparticle aggregation. The primary components in plant extracts include proteins, carbohydrates, vitamins, amino acids, flavonoids, alkaloids, terpenoids, ketones, tannins, aldehydes, amides, polysaccharides, polyphenols, carboxylic acids, and phenolic acids.^69,70 Among these, polyphenols are crucial, functioning as chelating/reducing agents and as capping agents for the NPs. This protects the particles from oxidation and aggregation, enhancing their stability and longevity. The mechanism of chelate bond formation between metals and polyphenols, resulting in a protective coating over the metal core, was proposed by Changyuan Xiao et al. (2020),⁷¹ illustrated in Fig. 4.

Fig. 4 The chelation reaction may occur between epicatechin and Fe²⁺.⁷¹

Plants such as Camellia sinensis (green tea), Aloe vera, Azadirachta indica, and extracts from various fruits, roots, and barks have long been studied for their role as reducing and stabilizing agents in the synthesis of Fe-NPs (Fig. 5). Machado et al. (2013) investigated approximately 26 different plant species for their ability to produce extracts capable of reducing Fe(III) in aqueous solutions to form nZVI.⁷² This study demonstrated that oak leaves, pomegranate leaves, and green tea leaves yielded extracts with the richest compositions. TEM analysis confirmed that the nZVI particles produced had an average size of 10–20 nm. In Karavasilis's (2019) study, green tea, pomegranate leaf extracts, and gallic acid were employed to synthesize GT-nZVI, PG-nZVI, and GA-nZVI for treatment in aqueous solutions.⁷³ The total polyphenol content (TPC) in the green tea and pomegranate extracts was relatively low, at 5.24 and 7.06 (g GAE L⁻¹), respectively. The GT-nZVI and PG-nZVI NPs exhibited very small sizes, ranging from 2.45 nm to 19.7 nm. A more recent study by Xiyao Liu et al. (2024) highlighted the simplicity and high reproducibility of using plant-based Fe-NPs. TEM analysis revealed spherical Fe-NPs ranging from 24–52 nm synthesized using pruned tea leaf extract.⁷⁴

Fig. 5 Schematic representation for plant-mediated biosynthesis of metallic nanoparticles.

In recent years, studies have emerged focusing on the green synthesis of bimetallic nanoparticles (BNPs). Combining two different metals exhibits multiple functions, enhanced selectivity, catalytic activity, and stability compared to monometallic NPs.^75,76 Fe⁰-based BNPs utilize two metals, one in a zero-valent state (Fe⁰ → Fe²⁺) and a noble or transition metal (typically Pd, Pt, Ni, Cu, …), which is added to ZVI. This results in the formation of galvanic cells, accelerating the production of electrons and atomic hydrogen.⁷⁷ A study on the one-step green synthesis of Fe/Ni BNPs using Eucalyptus leaf extract was conducted by Xiulan Weng et al. (2017).⁷⁸ The research identified over eight compounds in the Eucalyptus leaf extract that could serve as reducing agents and protective coatings confirmed through GC-MS analysis. Ahmed K. Hassan's et al. (2022) synthesized Fe/Cu NPs using Ficus leaf extract as a catalyst for the degradation of pollutants via a Fenton-like mechanism.⁷⁹ Another study by Inas A. Ahmed (2023) focused on the green synthesis of Fe/Cu NPs on an alginate limestone substrate (Fe/Cu/Alg-LS), which was utilized as an adsorbent to remove ciprofloxacin (CIP) and levofloxacin (LEV) from contaminated water. The maximum removal efficiencies for CIP (20 ppm) and LEV (10 ppm) were 97.3% and 100%, respectively. This material demonstrated significant potential as an adsorbent for water treatment applications.⁸⁰ Additional studies on the green synthesis of Fe⁰-based NPs using plant extracts are listed in Table 2 as follow.

Table 2 Several studies have synthesized Fe⁰-based NPs using plant extracts

NPs	Plant extract	Particles size	Functional group	Characterization	Shape	Ref.
Fe	26 different tree species	10–20 nm	NI	SEM and TEM	Spherical	Machado et al., 2013 (ref. 72)
Fe	Rosa damascene, Thymus vulgaris, and Urtica dioica	100 nm	–OH stretch, C–H stretch, C=C stretch, CH3 bend	SEM, FTIR and XRD	Different shapes	Fazlzadeh et al., 2017 (ref. 81)
Fe/Cu	Camellia sinensis	60–120 nm	O–H stretch, C=O stretch, C–O–C stretch	SEM, FTIR, and XRD	Spherical	Zhu et al. 2017 (ref. 82)
Fe/Ni	Eucalyptus	20–50 nm	N–H stretch, C–H stretch, C≡N stretch, C–C stretch	FTIR, TGA, SEM, TEM, EDS, XRD and XPS	Sphericaland irregular	Weng et al. 2017 (ref. 78)
Fe/Ni	Punica granatum peel	129 ± 7.23 nm	Phenolic and COOH functional groups	XRD, FE-SEM, FT-IR, and BET	Spherical	Ravikumar et al., 2018 (ref. 83)
AC-Fe/Zn	Citrus limon	126 nm	O–H stretch, C=C stretching in aromatic rings, C–CH3 deformation, Fe–O	FTIR, SEM, EDX, BET, and XRD	Spherical, agglomerated	Oruç et al. 2019 (ref. 84)
Fe/Pd	Eucalyptus	30–60 nm	NI	TEM, EDS, XRD, and XPS	Spherical	Lin et al., 2020 (ref. 85)
Fe	Mangifera indica	200–400 nm	C–H and CH2 bond vibration of aliphatic, C–O and C=C aromatic ring, Fe–O	SEM and FTIR	Spherical	Zulfikar et al., 2021 (ref. 86)
Fe/Ag	Salvia officinalis	27 ± 7 nm	O–H stretch, C=O stretch, C=C stretch, C–H bending, C–O stretch	SEM, TEM, XRD, FTIR, EDX, and TGA	Nearly spherical	Malik et al. 2021 (ref. 87)
Fe/Ag	Gardenia jasminoides	13 ± 6.3 nm	O–H stretch, C–H stretch, C=O and C=C stretch, C–O stretch	TEM, UV-vis, and EDX	Spherical	Padilla-Cruz et al. 2021 (ref. 88)
Fe/Cu	Ixora finlaysoniana	50–200 nm	C–H stretch, C=C stretch, C–O–C	UV-vis, FTIR, XRD, and SEM	Spherical	Younas et al. 2021 (ref. 89)
Fe	Camellia sinensis	NI	O–H stretch, C=O and C=C of the aromatic ring, C–O bond	UV-vis, FTIR, SEM and EDS	Amorphous forms	Eddy et al., 2022 (ref. 90)
Fe	Strychnos nux vomica (SN), Abrus precatorius (AP)	3–32 nm (AP-Fe NPs), 2.1–5 nm (SN-Fe NPs)	O–H and N–H stretch, C=O stretch, O–H bending, C–N stretch, N–H bending, Fe–C=O, Fe–O stretch	HR-TEM, UV-vis, and FTIR	Crystalline and amorphous forms	Puthukkara et al. 2022 (ref. 91)
Fe/Cu	Pomegranate peel	<100 nm	NI	NI	Irregularly spherical	Medina et al. 2020, Huang et al. 2023 (ref. 92 and 93)
Fe	Oolong tea, green tea, and black tea	39.5–50.5 nm	NI	XRD, SEM, TEM, FTIR, BET, EDS	Spherical	Benković et al. 2023 (ref. 94)
Fe/Cu, Fe/Ag	Annona muricata	NI	NI	XRD, FTIR, UV-vis, FE-SEM, and EDX	NI	Raja Nandhini et al. 2023 (ref. 95)
nZVI-SCM	Tobacco leaf extract	<100 nm	O–H/N–H stretch, amide II band, amide I band, Fe–O	XPS, SEM, FT-IR and XRD	Quasi spherical	Shi et al. 2023 (ref. 96)
Fe	Pruned tea leaves	17.9 nm	O–H stretch, C=C aromatic ring stretch, C=O of polysaccharide and N–H stretch, Fe–O stretch	XRD, FTIR, XPS, and TEM	Spherical	Liu et al., 2024 (ref. 74)
Fe/Cu, Fe/Ni	Camellia sinensis	37–45 nm (Fe/Cu), 11–22 nm (Fe/Ni)	NI	SEM, TEM, XRD, EDS, and XPS	Spherical	Son et al. 2024 (ref. 97)
Fe/Mg	Azadirachta indica	88 nm	C=O stretch, C=C aromatic, C–H bending	XRD, SEM, EDX and FTIR	Spherical	Qadir Ahmad et al. 2024 (ref. 98)

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Nano-scale Fe⁰-based NPs are the main nanomaterial used in environmental remediation processes. However, as with any remediation technique, the use of nanomaterials can also cause undesirable impacts on human health and the environment.⁹⁹ Martins et al.¹⁰⁰ conducted a comparative assessment of environmental and economic aspects involved in synthesizing nZVI via conventional methods using NaBH₄versus approaches utilizing natural extracts. Their study applied life cycle assessment (LCA) as a strategy for ecodesign, focusing on the environmental performance of these two synthesis techniques and identifying critical stages in each. Findings demonstrated that the green synthesis method exhibited substantially lower environmental impacts than the conventional approach, with reductions of around 50% in the first scenario and about 62% in the second. In the green synthesis, the primary environmental burden stems from the extraction process, primarily influenced by electricity consumption. Conversely, the critical stage in the traditional method is associated with reactant usage, particularly the production of sodium borohydride. Economically, the conventional synthesis is significantly more costly, approximately eight times more than the green synthesis.

Although using plant extracts in nanoparticle synthesis offers environmental benefits, this method still presents several limitations that necessitate continued research. The biological reaction process inherently depends on natural compounds in the plant extracts, making it challenging to control the size and morphology of the NPs.¹⁰¹ Additionally, the purity of the resulting product is relatively low, as impurities from the plant extracts can negatively impact the catalytic activity and adsorption efficiency of the material.¹⁰² The varying composition of different plant extracts can also lead to inconsistencies in the structure and properties of the NPs,¹⁰³ reducing their practical applications. Another drawback lies in reproducibility, as maintaining consistent conditions across multiple batches of synthesis with plant extracts is difficult, leading to variations in the final product. Nevertheless, considering the environmental impact of chemical synthesis, green methods present a promising, more sustainable alternative for future applications.

2.3. Agricultural and biowaste by-products

Biological and agricultural waste are abundant sources of phytochemicals, making them ideal for nanoparticle synthesis. These waste materials offer a sustainable and readily available resource for nanomaterial production while also helping to reduce environmental pollution.³¹ The synthesis process avoids the use of toxic chemicals, and the repurposing of waste plays a crucial role in promoting a circular economy. For example, one study demonstrated the synthesis of Fe-Zn activated carbon NPs from lemon pomace waste, which were applied as catalysts in water treatment by Fenton-like reaction.^31,84 Similarly, waste silkworm cocoons, a by-product of the protein-rich silk industry, were used to synthesize Fe/Cu alloy NPs.¹⁰⁴ Silkworm cocoon is a by-product of the silk industry. The cocoon is rich in protein, and its extract serves as a reducing and stabilizing agent. The protein extract acted as both the reducing and stabilizing agent. These NPs exhibited notable antimicrobial properties against selected bacterial strains and fungal pathogens, suggesting their potential use as protective agents in cement mortars. By inhibiting the growth of harmful organisms, they may also help prevent premature deterioration of the material. Palm waste was studied by Tesnim et al. for extraction and nanoparticle synthesis.¹⁰⁵ Several other studies following this trend are presented in Table 3.

Table 3 Synthesis of Fe⁰-based NPs using waste resources

NPs	Precursor	Particles size	Functional group	Characterization	Shape	Ref.
ARH-nZVI	Ash rice husk	10–15 nm	O–H from alcohol, H–O–H stretch, C–H bending, Si–O stretch	SEM, TEM, UV-vis, FTIR, and EDX	Spherical clusters	Dada et al., 2014 (ref. 106)
Fe	Waste tea	98 nm	–OH groups stretch, –S=O group stretch, –C=C stretch	SEM, EDX, FTIR and BSE	Slight clumping form	Gautam et al., 2018 (ref. 107)
Fe/Cu	Waste silkworm cocoon	50–60 nm	C–N stretch, C–N triple bond stretch and C–H stretch	UV-vis, FE-SEM, EDAX, XRD, and FTIR	Spherical	Dhruval et al., 2020 (ref. 104)
nZVI	Green tea bio-waste	100 nm	Silicate ions, Fe–O stretch	FTIR, SEM, XRD, and BET	Cubical	Kadhum et al., 2021 (ref. 108)
nZVI-LBC	Lemon residues	50 nm	OH vibration stretch, C–H and C=C of the aromatic ring, Si–O, C–O of ester, FeOOH	FTIR, XRD, TEM, XPS, VSM, and BET	Spherical	El-Monaem et al., 2022 (ref. 109)
Fe	Onion, potato, tea and moringa	NI	O–H stretch, C=O stretch, C=C, C–O	FTIR, XRD, XRF, and EDX	NI	Ahmed et al., 2023 (ref. 110)
nZVI-NC	Palm waste extract	91 nm	O–H stretch, C–C bonds in the phenolic groups, C–O–C stretch, Fe–O stretch	SEM, TEM, BET, FTIR, DLS and XRD	Spherical	Tesnim et al., 2023 (ref. 111)
nZVI	Palm petiole extract	45–90 nm	O–H stretch, C–O, C–O–H, C–O–C, Fe–O stretch	UV-vis, SEM, TEM, XRD, FTIR, and BET	Spherical	Tesnim et al., 2024 (ref. 105)
GO–nZVI	Sugarcane bagasse	NI	NI	FESEM, HR-TEM, EDX, and XRD	NI	Jha et al., 2024 (ref. 112)

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2.4. Comparison of chemically and biologically synthesized Fe⁰-based NPs

Each approach has distinct advantages and limitations; chemically synthesized Fe⁰ nanoparticles offer precise control over particle size and high reaction efficiency, while biologically synthesized nanoparticles are more environmentally sustainable, utilizing natural biological agents. Although chemically synthesized nanoparticles have shown promising results, their safety is often compromised due to the use of toxic reagents in the synthesis process. For instance, sodium borohydride (NaBH₄), a widely used reducing agent in metal NPs synthesis,^22,113,114 has been classified as harmful if inhaled or absorbed through the skin.¹¹⁵ The use of such toxic chemicals is common in synthesis since they tend to drive reactions that are favorable in terms of kinetics or thermodynamics.³⁴ While this can lead to high yield and reproducibility in metal NPs production, the chemical processes involved frequently contribute to an elevated toxicity profile of the final products, posing risks to both health and the environment. To address these issues, green nanotechnology prioritizes the use of environmentally benign agents and highlights biological materials as sustainable sources for reagents in nanoparticle synthesis. Several studies have been conducted to compare the effectiveness of these two synthesis methods. Ravikumar et al.¹¹⁶ report a comparative study of Cr(VI) removal using biologically synthesized nano zero-valent iron (BS-nZVI) using fresh neem leaves and chemically synthesized nZVI (CS-nZVI) using NaBH₄, both immobilized in calcium alginate beads. Under the optimized conditions concentration of nZVI = 1000 mg L⁻¹, contact time = ∼80 min, and initial concentration of Cr(VI) = 10 mg L⁻¹, the Cr(VI) removal by the immobilized BS-nZVI and CS-nZVI alginate beads was 80.04% and 81.08%, respectively. A study by Hammad et al.¹¹⁷ demonstrated that iron nanoparticles synthesized using Chlorella vulgaris microalga extract achieved a particle size of 4.8 nm, whereas those synthesized chemically had a particle size of 9.07 nm. The adsorption data for methylene blue (MB) for both materials followed the Langmuir isotherm model, with maximum adsorption capacities (q_max) of 29.14 mg g⁻¹ and 19.08 mg g⁻¹, respectively. This study highlights the superior adsorption capacity of biologically synthesized nanoparticles. Additionally, several studies have provided specific assessments of the environmental impact of Fe⁰-based NPs synthesized via these two methods. Bhuvaneshwari et al.¹¹⁸ examined their effects on algae and crustaceans to evaluate potential toxicity and nutrient transfer within the aquatic food chain. By evaluating factors such as ROS generation, oxidative enzyme activity, membrane damage, and biouptake, the study reveals significant differences in toxicity between CS and BS-nZVI. Daphnia showed greater sensitivity to CS-nZVI, while algae exhibited toxicity differences only at higher nZVI concentrations.

In conclusion, both chemically and biologically synthesized Fe⁰-based NPs offer distinct advantages and limitations. Chemically synthesized NPs provide better control over size and uniformity, often leading to higher catalytic efficiency; however, they involve toxic reagents that may pose environmental and health risks. In contrast, biologically synthesized NPs are produced through environmentally friendly processes using natural reducing agents, which often enhance their stability and compatibility in ecological applications. The choice between these synthesis methods should be guided by the specific requirements of the application, balancing factors such as environmental safety, cost, and performance.

3. Trends in the application of biological NPs based on Fe⁰ to treat chlorine organic compound

3.1. Targeted pollutant removal

3.1.1. Persistent organic pollutants. Persistent organic pollutants (POPs) have received considerable attention in the past few decades because of their persistence, long-distance migration, potential bioaccumulation, and latent toxicity for humans and wildlife.¹¹⁹ There is no doubt that POPs cause serious effects on the global ecosystem. Currently, 30 groups of POPs are regulated under the Stockholm Convention, with member countries committed to strengthening management, reducing usage, and ultimately phasing out their production and application. Chlorinated organic compounds within the POPs group (Cl-POPs), such as aldrin, chlordane, dichlorodiphenyl trichloroethane (DDT), lindane, PCBs, and dioxins/furans, are commonly found in pesticides and insecticides. These compounds are highly toxic¹²⁰ and possess the ability to bioaccumulate¹²¹ through the food chain, leading to adverse effects on immune systems, endocrine disruption, reproductive abnormalities, neurological damage, congenital disabilities, and other harmful outcomes for living organisms, including humans. Fundamentally, the research on synthesizing Fe⁰-based NPs as catalysts for transforming and removing Cl-POPs has been an ongoing trend. The general mechanism describing the treatment of chlorinated organic compounds using Fe⁰-based nanoparticles involves the hydrodechlorination (HDC) reaction. During HDC, electrons can transfer directly from the Fe⁰ source to chlorinated molecules, generating Fe²⁺ as a by-product.¹² This process breaks the C–Cl bonds, releasing Cl⁻ ions, as shown in the overall reaction equation:

2Fe⁰ + R–Cl + 3H₂O → 2Fe²⁺ + R–H + 3OH⁻ + H₂ + Cl⁻

Additionally, the incorporation of a second metal (commonly transition metals such as Pd, Pt, Ni, or Cu) into nZVI results in the formation of galvanic cells, which accelerate the production of atomic hydrogen. This atomic hydrogen is adsorbed onto the reducing catalyst, forming metal hydrides (M–H) that act to dehalogenate chlorinated organic compounds. The mechanism for the degradation of chlorinated organic compounds by the Fe/Me bimetallic catalytic system can be described in detail in the study by Liu et al.,¹²² as follows:

Fe⁰ + 2H⁺ → Fe²⁺ + H₂

M (M = Pd, Ni, Cu, …) + H₂ → M*2H·

M + RCl → M⋯R⋯Cl

M*2H· + M⋯R⋯Cl → R–H + H⁺ + Cl⁻ + 2M

In a pioneering study by Wang et al. (1997),¹²³ the complete degradation of PCBs Aroclor 1254 was demonstrated using nZVI and Pd/Fe NPs synthesized via borohydride reduction. Since then, this method has become widely adopted for the synthesis of highly active Fe⁰-based NPs. Numerous studies have emerged aiming to enhance the application of nano Fe⁰ by modifying it with surfactants,^124,125 anchoring nano particles onto substrates to form composites,^126,127 combining it with other metals to create multi-metal systems,^128–130 and developing biologically synthesized nano Fe⁰.

In recent years, the use of biologically synthesized nanomaterials for the treatment of Cl-POPs has garnered significant attention from the scientific community, with promising preliminary results indicating the potential of this approach. For instance, a 2019 study led by Yuanqiong Lin explored the green synthesis of Fe/Ni BNPs using Eucalyptus leaf extracts. These BNPs were applied for the simultaneous removal of triclosan (TCS) and Cu(II) from aqueous solutions, achieving notable removal efficiencies of 75.8% for TCS and 44.1% for Cu(II). This study demonstrated the versatility of Fe/Ni BNPs in treating complex pollutant systems under a variety of conditions.¹³¹ The mechanisms and effectiveness of this removal process are illustrated in Fig. 6. Another study highlighted Pomegranate peel extracts for the green synthesis of nZVI, which exhibited high efficacy in treating lindane (γ-HCH).¹³² In liquid medium, 0.1 g L⁻¹ of FeNPs successfully degraded approximately 99% of lindane within 24 hours. Moreover, treatment with the FeNPs significantly reduced lindane's cytotoxicity, suggesting its breakdown into less harmful and simpler by-products. Within the group of Cl-POPs, compounds such as PCBs and tetrachlorodibenzo-p-dioxin (TCDD), particularly 2,3,7,8-TCDD, are regarded as highly toxic. These substances pose severe health risks, including links to cancer, immune system disorders, and reproductive issues.¹²¹ The study conducted by Tlčíková et al. presented a solution for removing PCBs from the environment by employing biologically synthesized iron NPs derived from plant matrices. This approach was coupled with the use of bacteria, specifically Stenotrophomonas maltophilia (SM) and Ochrobactrum anthropi (OA), both of which were isolated from a PCB-contaminated site, the Strážsky canal in Slovakia.¹³³ This study also addresses the optimization of parameters for using nZVI in PCBs degradation, including reaction pH, oxygen requirements, and nZVI dosage. Experimental analyses revealed that sequential nanobiodegradation proved to be the most effective for PCBs degradation among the various approaches tested. Specifically, the combination of Fe-Bio from sage and Stenotrophomonas maltophilia (SM) resulted in a 75% degradation of PCBs, while Fe-Bio synthesized from Garcinia atroviridis (GA) at 0.3 g L⁻¹, combined with Ochrobactrum anthropi (OA), achieved a 92% PCB degradation rate. Recently, Toan et al. proposed a technological solution combining green-synthesized BNPs (Fe/Cu and Fe/Ni) using Camellia sinensis with 11 microbial strains to reduce 2,3,7,8-TCDD concentrations in soil at the Phu Cat airport, Vietnam.¹³⁴ Approximately 90% of the 2,3,7,8-TCDD was successfully treated in both laboratory and pilot-scale experiments. Table 4 summarizes several studies that highlight the potential of bio-nano Fe⁰-based systems for the treatment of Cl-POPs.

Fig. 6 Mechanism and efficiency of TCS and Cu(II) elimination by Fe/Ni BNPs.¹³¹

Table 4 The efficiency of removing POPs using bio-synthesized Fe⁰-based NPs

NPs	Precursor	POPs	Condition	Removal efficiency	Ref.
Fe/Ni	Eucalyptus leaves	Triclosan	pH = 2 ÷ 6, [TCS] = 10 mg L^−1, T = 303 K, [Fe/Ni] = 1.5 g L^−1	75.8–96.6%	Lin et al., 2019 (ref. 131)
G-Fe/Co	Ginkgo biloba L. extract	Triclosan	pH = 6.92, 5% Co wt, [NPs] = 0.56 g L^−1, t = 5 min	62.7–92.8%	Gao et al., 2019 (ref. 135)
Fe	Euphorbia cochinchinensis leaves	2,4-Dichlorophenol (2,4-DCP)	pH = 3 ÷ 6, [2,4-DCP] = 50 mg L^−1, T = 303 K, rotative speed: 250 r min^−1, [Fe NPs] = 1 g L^−1	>83.5%	Gan et al., 2018 (ref. 136)
Fe	Euphorbia cochinchinensis leaves	2,4-DCP	pH = 6.8, [2,4-DCP] = 50 mg L^−1, T = 313 K, rotative speed: 250 r min^−1, [Fe NPs] = 1 g L^−1	51.9%	Guo et al., 2016 (ref. 137)
Fe	Yeast extract	Dichlorvos	[Fe NPs] = 2000 mg L^−1, VH2O2 = 1 mL	∼93.6%	Mehrotra et al., 2017 (ref. 138)
Fe	Neem leaf extract	DDT	pH = 3, [DDT] = 500 ppm, t = 120 min	88–92%	Pushkar et al., 2019 (ref. 139)
nZVI	Pomegranate peel extracts	Lindane	[Fe NPs] = 0.1 g L^−1, t = 24 h	∼99%	Ningthoujam et al., 2023 (ref. 132)
Fe	Camellia sinensis extracts	PCBs	Fe-BNPs combined with Ochrobactrum anthropi bacteria	92%	Tlčíková et al., 2024 (ref. 133)
Fe/Cu, Fe/Ni	Camellia sinensis extracts	2,3,7,8-TCDD	[TCDD] = 5000 ppt, t = 30 days, pH = 5–7, [NPs] ≥ 40 ppm, combined with 11 microbial strains	90%	Toan et al., 2024 (ref. 134)

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3.1.2. Volatile organic compound (Cl-VOCs). Chlorinated volatile organic compounds (Cl-VOCs) are widely used as solvents and degreasers and in various commercial products. These compounds are common pollutants found in contaminated soil, air, and aquatic environments such as groundwater, rivers, and lakes.^140,141 Various methods have been proposed to address the removal of VOCs, generally categorized into two main groups based on their mechanisms: adsorption techniques and oxidation techniques,¹⁴² or a combination of both.¹⁴³ Adsorption techniques, a conventional approach, involve transferring VOCs from the air to a solid phase using adsorbents.¹⁴⁴ Although there is potential for adsorbing and treating Cl-VOCs through the HDC catalytic mechanism similar to that for POPs, research, and evidence regarding the effectiveness of biologically synthesized nano-based Fe⁰ for VOCs treatment remain limited.

In the study by Smuleac et al., a novel method was developed to prepare membranes incorporating reactive NPs (Fe and Fe/Pd) immobilized within a polymer film, specifically polyacrylic acid (PAA)-coated polyvinylidene fluoride (PVDF) membranes.¹⁴⁵ Instead of the commonly used sodium borohydride, a non-toxic “green” reducing agent, green tea extract (Camellia sinensis), was employed for nanoparticle synthesis. These membrane-supported NPs were effectively utilized for the degradation of TCE, a prevalent and significant pollutant. The degradation rate of TCE increased linearly with the amount of Fe immobilized on the membrane, with a surface-normalized rate constant k(SA) of 0.005 L m⁻² h⁻¹. Incorporating a second catalytic metal, Pd, to form a bimetallic Fe/Pd system enhanced the k(SA) value to 0.008 L m⁻² h⁻¹. The authors suggested that dechlorination could be carried out in a convective mode using a pump-and-treat approach or in a batch mode, where the membrane-immobilized NPs could be injected underground. Qianwei Li et al.⁶² developed a straightforward fungal biomineralization method to synthesize nZVI with a distinct N-doped branching structure (Fig. 7). This innovative approach demonstrated exceptional stability and facilitated efficient degradation of carbon tetrachloride (CCl₄) in aqueous solutions. The ureolytic fungus Neurospora crassa was cultured in a medium containing Fe²⁺ and urea, which precipitated iron carbonate biominerals. The resulting “iron coral” composite was characterized as a blend of zero-valent iron (Fe⁰), carbon iron (Fe_1.91C_0.09), and iron oxide (Fe₃O₄). The porous branching hyphal framework enhanced CCl₄ capture, while the N-doped sites likely accelerated electron transfer between CCl₄ and nZVI. Geochemical simulations supported the biomineral formation, and chemical analyses confirmed its significant efficacy in degrading CCl₄. In actual field conditions, some commonly encountered contaminants, such as dichloromethane (DCM) and 1,2-dichloroethane (1,2-DCA), exhibit minimal reactivity with nZVI. Salom et al.¹⁴⁶ explored the feasibility of integrating anaerobic dechlorinating bacteria, specifically Dehalobacterium and Dehalogenimonas, with nZVI as a combined treatment approach for detoxifying chlorinated methanes (e.g., chloroform-CF and DCM) and 1,2-DCA. The authors reported that the recovery of dechlorinating activity in Dehalobacterium and Dehalogenimonas indicates that combining nZVI with bioremediation techniques could be effective under field conditions, where dilution processes may mitigate the impact of potentially inhibitory soluble compounds. Additional studies on this topic are summarized in Table 5.

Fig. 7 Degradation of CCl₄ by nZVI with a distinct N-doped branching structure.⁶²

Table 5 Some typical studies on VOCs removal by bio-nano Fe⁰-based

NPs	Precursor	VOCs	Condition	Removal efficiency	Ref.
Fe, Fe/Pd	Green tea extract	TCE	pH = 7, t = 23 h, [TCE] = 30 mg L^−1, 8 mg Fe (3% wt Pd)	∼60%	Smuleac et al., 2011 (ref. 145)
nZVI	Green tea, black tea, oolong tea	Chlorobenzene (CB)	pH = 3, [GT-Fe NPs] = 0.6 g L^−1, [CB] = 50 mg L^−1, T = 313 K	81%	Kuang et al., 2013 (ref. 147)
nZVI	Virginia creeper extract	VOCl	t = 6 h, [nZVI] = 2500–10 000 g dm^−3	100%	Kozma et al., 2015 (ref. 148)
nZVI	Neurospora crassa	Carbon tetrachloride (CCl4)	—	∼75%	Li et al., 2020 (ref. 62)
nZVI-PS	Tea polyphenol extract	1,2-Dichlorobenzene	[1,2-DCB] = 28.6 mg kg^−1, [nZVI] = 67.2 mg L^−1, [PS] = 1.2 mmol L^−1, pH = 7.5	61.3%	Yan et al., 2022 (ref. 149)
nZVI	Anaerobic dechlorinating bacteria	Chloroform, DCM, 1,2-dichloroethane	[nZVI] = 1 gL^−1, pH = 7	100%	Salom et al., 2023 (ref. 146)

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According to Zhan et al.,¹⁵⁰ the bioprecipitation synthesis of sulfidated nZVI particles can be considered a “green synthesis” process. However, using NaBH₄ in the prior synthesis of nZVI particles introduces an expensive and toxic chemical, leading to potential secondary pollution for environmental. Therefore, we adopt the “semi-green” synthesis concept to describe this method. Studies on Fe⁰-based nanomaterials synthesized through semi-green processes have also demonstrated their effectiveness in treating POPs and VOCs. To remediate organic contaminants in natural waters and soils, ZVI-included biochar was synthesized via slow pyrolysis (Seok-Young Oh et al.¹⁵¹). The removal of 2,4-dinitrotoluene (DNT) and 2,4-DCP from water was achieved via sorption onto Fe⁰-embedded biochar. Results indicated that after 24 h, only 40% of DCP was removed from the solution. Regarding the sorbed amount, the total removal of DCP by Fe⁰ alone was just 23% in the same period. However, the removal efficiency of DCP increased with higher carbon content in the Fe⁰-embedded biochar. In another study, Junyi Huang et al.¹⁵² successfully synthesized an nZVI composite supported on zeolite through ion exchange and borohydride reduction methods, specifically to study its ability to activate persulfate (PS) for the degradation of TCE. The Z/nZVI composite exhibited excellent PS activation capacity (1.5 mM) and achieved 98.8% TCE degradation at pH = 7 within 120 min.

To assess the treatment efficiency of Fe⁰-based nanoparticles synthesized through chemical and biological methods, various studies have conducted comparative analyses on the removal of chlorinated organic compounds. Variations in synthesis techniques markedly influence reaction efficiency, degradation kinetics, and nanoparticle stability in pollutant exposure scenarios. Table 6 presents a comparison of treatment performances for different chlorinated organic compounds.

Table 6 Comparison of the treatment efficiency of Fe⁰-based nanoparticles synthesized via chemical and biological methods for chlorinated organic compounds

NPs	Precursor	Pollutants	Concentration	Effects	Ref.
Fe/Ni	Eucalyptus leaves	Triclosan	[TCS] = 10 mg L^−1, [Fe/Ni] = 1.5 g L^−1	75.8–96.6%	Lin et al., 2019 (ref. 131)
Fe/Ni	NaBH4	Triclosan	[TCS] = 10 mg L^−1, [Fe/Ni] = 2.2 g L^−1	63.9%	Ren et al., 2022 (ref. 153)
Fe	NaBH4	Triclosan	[TCS] = 10 mg L^−1, [Fe] = 2.0 g L^−1	55.4%
Fe	Euphorbia cochinchinensis leaves	2,4-DCP	[2,4-DCP] = 50 mg L^−1, [Fe NPs] = 1.0 g L^−1	>83.5%	Gan et al., 2018 (ref. 136)
S-nZVI	NaBH4	2,4-DCP	[2,4-DCP] = 20 mg L^−1, S/nZVI = 0.129 (mole ratio), [S-nZVI] = 2.0 g L^−1	91.9%	Yajun Li et al., 2022 (ref. 154)
Fe, Fe/Pd	Green tea extract	TCE	[TCE] = 30 mg L^−1, [Fe] = 0.4 g L^−1 (3% wt Pd)	∼60%	Smuleac et al., 2011 (ref. 145)
Fe	NaBH4	TCE	[TCE] = 0.15 mM, [Fe] = 5 g L^−1	>95%	Ahn et al., 2014 (ref. 155)
CaCO3@nZVI	KBH4	TCE	[CaCO3@nZVI] = 0.24 g L^−1, [TCE] = 25 mg L^−1	>80%	Zheng et al., 2023 (ref. 156)
Fe	Green tea, black tea, oolong tea	CB	[Fe] = 0.6 g L^−1, [CB] = 50 mg L^−1	81%	Kuang et al., 2013 (ref. 147)
Fe	NaBH4	Hexachloro benzene (HCB)	[HCB] = 2.0–2.8 mg g^−1, [Fe] = 1 mg g^−1	80%	Wang et al., 2023 (ref. 157)

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The application of biogenic Fe⁰ NPs in the remediation of VOCs and POPs presents a promising and sustainable approach. These NPs offer significant advantages in terms of reactivity, environmental compatibility, and scalability. Beyond Cl-VOCs and Cl-POPs, Fe⁰-based nanoparticles have also been recognized as a suitable option for the treatment and degradation of other pollutants, such as antibiotics and per- and polyfluoroalkyl substances (PFAS). In general, the mechanism of antibiotic degradation by Fe⁰ mainly includes adsorption and reduction, while promoting the biodegradation of antibiotics by affecting the microbial community. Fe⁰ can also be combined with persulfates to degrade antibiotics through advanced oxidation processes.¹⁵⁸ Specifically, the structure and chemical properties of Fe⁰ make it highly compatible with interactions with the resilient bonds in PFAS, resulting in high treatment efficiency.¹⁵⁹ However, further practical and large-scale studies are needed to comprehensively assess the material's capabilities in complex environmental conditions.

While challenges remain, particularly in optimizing the synthesis and enhancing these nanomaterials' long-term stability and reactivity, their potential to mitigate environmental pollution, especially for persistent organic compounds, is undeniable. Continued research into the synergistic effects and biocompatibility of biogenic Fe⁰ NPs will be crucial in advancing their practical deployment in environmental cleanup efforts.

3.2. Enhanced catalytic activity

3.2.1. Synergistic effects. Synergistic effects between nZVI and other catalysts have significantly enhanced catalytic activity, facilitating more rapid and efficient redox reactions. This collaboration not only boosts treatment efficiency but also opens new pathways for developing environmentally friendly materials with broad potential applications in the remediation of complex pollutants. However, the practical application of nZVI is often hindered by its aggregation and limited activity in aqueous media. Abdelfatah et al.¹⁶⁰ explored the synergistic effect of green-synthesized reduced graphene oxide (rGO) and nZVI in a composite that removes doxycycline from water. In this study, the rGO/nZVI composite was synthesized for the first time using a simple, environmentally friendly approach, leveraging Atriplex halimus leaf extract as both a reducing and stabilizing agent. This method adhered to green chemistry principles, minimizing the use of hazardous chemicals. The rGO/nZVI and nZVI particles exhibit a spherical shape with sizes ranging from 15 nm to 26 nm. Zeta potential analysis of the phytosynthesized nZVI, GO, and rGO/nZVI composite demonstrated their stability, indicated by surface negative charges of −20.8, −22, and −27.4 mV, respectively. Additionally, the specific surface area (S_BET) of nZVI increased significantly, from 47.45 m² g⁻¹ to 152.52 m² g⁻¹, following integration with rGO. The composite's performance in removing doxycycline was compared to that of pristine nZVI across various initial concentrations to assess the synergistic effect between rGO and nZVI. Under optimal conditions (25 mg L⁻¹ doxycycline, 25 °C, and 0.05 g of material), bare nZVI achieved 90% removal, while the rGO/nZVI composite reached an impressive 94.6%. This enhancement clearly demonstrates the beneficial interaction between nZVI and rGO, leading to superior adsorptive capacity. Nano zero-valent iron-modified biochar (nZVI/BC) has been demonstrated to effectively remediate contaminated environments, attributed to its abundant active sites and exceptional reducing properties. Anqi Chen et al.¹⁶¹ provide a thorough review of the applications of nZVI/BC in addressing organic pollution in environmental remediation, along with its underlying mechanisms. The review covers the efficacy of nZVI/BC in remediating soil and water polluted by antibiotics, pesticides, polycyclic aromatic hydrocarbons (PAHs), and dyes. The mechanisms behind the adsorption and chemical degradation of pollutants by nZVI/BC are examined in detail, as well as the synergistic degradation mechanisms involved in nZVI/BC-advanced oxidation processes (AOPs) and nZVI/BC–microbial interactions. The enhanced degradation of organic compounds through synergistic effects in nZVI/BC-AOPs systems¹⁶¹ is illustrated in Fig. 8.

Fig. 8 Schematic diagram of organic pollutants degradation by nZVI/BC-AOPs.¹⁶¹

In Xiangyu Wang's study, a cost-effective, eco-friendly, robust, and efficient synthesis method was established for nZVI-based composites.¹⁶² The nZVI@chitin-modified ZSM-5 (nZVI@C-ZSM) composite was synthesized through a simple and green approach, where nZVI was loaded onto alkali-modified ZSM-5 molecular sieves, using chitin as a surfactant and binder. The nZVI@C-ZSM material has a surface area of approximately 52.22 m² g⁻¹ and a pore size of 18.48 nm. The presence of chitin on nZVI@C-ZSM results in more prominent surface wrinkles, while the coating of fine particles on nZVI@C-ZSM is significantly reduced due to the protective effect of chitosan. This composite demonstrated remarkable performance in the removal of tetracycline, achieving a removal efficiency of 97.72% within 60 min. Compared to pristine nZVI, the nZVI@C-ZSM composite exhibited twice the removal efficiency, which suggests that the composite significantly improved the dispersion and stability of nZVI. This improvement allowed for more reactive sites to generate reactive oxygen species (ROS), greatly enhancing the catalytic activity and durability while minimizing the risk of secondary pollution¹⁶² (Fig. 9).

Fig. 9 Proposed synergistic mechanism of the components in nZVI@C-ZSM.¹⁶²

Bimetallization strategies lead to materials that combine two different metals exhibiting enhanced and modified properties with respect to a monometallic system. BNPs based on Fe⁰ offer significant advantages over their monometallic counterparts due to the synergistic effects arising from electronic, mechanical, and structural modifications.³⁴ Wang et al.¹⁶³ investigated the debromination of BDE-47 using six bimetallic Fe/M systems (M: Cu, Ni, Pd, Ag, Pt, Au). They found that the additive metals alone were inactive without Fe, highlighting iron's critical reductive role. Debromination rates followed the order: Fe/Pd > Fe/Ag > Fe/Cu > Fe/Ni > Fe/Au > Fe/Pt ≈ n-ZVI. Furthermore, each system exhibited distinct mechanisms-Fe/Ni, Fe/Pd, and Fe/Pt employed H-transfer, while Fe/Ag followed electron-transfer, and Fe/Cu and Fe/Au used both pathways, producing different final products. Fang et al.¹⁶⁴ reported that S-nZVI particles were synthesized from steel pickling waste liquor via chemical deposition and were used to remove BDE209 in a water/tetrahydrofuran (4/6, v/v) solution. The crystalline structure of S-nZVI differed from conventional nZVI, though the BET surface area of S-nZVI remained comparable to that of nZVI. The authors concluded that, by considering both the cost and the importance of reclaiming iron resources, S-nZVI offered better compensation than other metals. S-nZVI has garnered increasing attention due to its simple production process, high reactivity toward a variety of contaminants, including TCE, diclofenac, cadmium, and chromate, and especially its selectivity in water pollution treatment. S-nZVI nanoparticles with an optimal S/Fe molar ratio demonstrate significantly higher contaminant removal efficiency than unmodified nZVI.¹⁶⁵ Qu et al.¹⁶⁶ successfully synthesized S-nZVI on a hydrophilic porous activated carbon substrate (S-nZVI@HPAC) using green tea extract as a reducing agent. The S-nZVI@HPAC with an atomic S/Fe ratio of 0.16 was found to enhance Pb(II) uptake through synergistic effects of electrostatic attraction, chemical precipitation, complexation, and reduction. Despite these advancements, S-nZVI synthesis remains predominantly reliant on chemical methods utilizing NaBH₄ as a reducing agent. A series of studies^167–170 indicate that chemically synthesized S-nZVI achieves high contaminant removal efficiencies, making it a viable method for practical applications, though potential contamination risks during production remain a consideration.

Synergistic effects significantly enhance the performance of materials by combining different components to achieve superior properties compared to individual elements (monometallic). These effects can lead to improved catalytic activity, increased stability, and greater efficiency in various applications. By leveraging the interactions between different substances, synergistic effects facilitate faster reaction rates, higher selectivity, and more effective treatment of complex pollutants. This approach optimizes material performance and supports the development of more sustainable and versatile technologies for diverse industrial and environmental applications.

3.2.2. Surface modifications. In most cases, biological agents function as reducing agents and as capping and stabilizing agents for NPs, thereby preventing agglomeration and enhancing stability in solution.¹⁷¹ Moreover, functional groups present in biological agents, such as proteins, polyphenols, or polysaccharides, can interact with the nanoparticle surface, forming natural coatings that modify surface properties and improve particle interaction with target pollutants. Therefore, the biosynthesis method itself can be considered an advanced technique for surface modification of NPs, offering numerous advantages in environmental and biomedical applications. However, beyond this, recent studies have shown that combining and modifying bio-NPs with polymers can enhance the optical, barrier, thermal, antimicrobial, and mechanical properties of the resulting composite materials. Jafarzadeh et al.¹⁷² evaluated the potential of combining biopolymer films with bio (green)-synthesized nanomaterials and their effectiveness in reducing the negative environmental impacts of synthetic packaging (Fig. 10). Although this is a promising research direction with positive environmental benefits, the study of bio-nano-based Fe⁰ modification requires careful attention, as elucidating specific mechanisms remains a highly complex challenge.

Fig. 10 The potential of integrating polymers with green-synthesized nanomaterials in addressing environmental challenges.¹⁷²

4. Challenges and future prospective

Although laboratory-scale studies have demonstrated potential, scaling up the biosynthesis of Fe⁰ NPs for real-world applications presents numerous challenges. Consistently controlling the size, shape, and reactivity of NPs at a larger scale remains difficult. The biosynthesis process also tends to vary depending on the biological agent used, which can lead to changes in the properties of the final product. Despite advancements, the precise mechanisms underlying interactions between NPs and chlorine compounds are not yet fully understood. Identifying the key factors influencing catalytic efficiency, especially within complex environmental matrices, is crucial for optimizing their performance. Consequently, there are still limited studies worldwide on practical applications and large-scale trials using biosynthesized Fe⁰ NPs to treat chlorinated organic compounds in the environment.

Future research should focus on strategies to enhance the stability and longevity of Fe⁰-based NPs, refine biological agents to achieve desired synthesis efficiencies, and reduce the costs associated with nanoparticle synthesis and the treatment of chlorinated organic compounds and other pollutants. The reaction mechanisms, degradation pathways, and environmental factor impacts need further investigation through advanced modeling techniques to optimize catalytic performance under various conditions. Translating laboratory results to field applications and large-scale pilot studies is crucial to demonstrate the feasibility of biosynthesized Fe⁰ NPs. These studies will provide valuable insights into practical challenges such as particle dispersion, reactivity, and recovery in different environmental contexts. Developing a standardized technological process for applying biosynthesized Fe⁰-based NPs will also be a significant future prospect. Establishing a clear and scalable protocol for synthesizing, stabilizing, applying, and recovering these NPs under real-world conditions could bridge the gap between laboratory research and practical environmental solutions.

5. Conclusion

This review has highlighted the significant progress and emerging trends in developing and applying biologically synthesized Fe⁰-based NPs for treating chlorinated organic compounds in soil and water. The various synthesis methods, ranging from microbial, plant, and biological waste sources to industrial and agricultural by-products demonstrate the versatility and sustainability of biologically synthesized Fe⁰-based NPs. Recent advancements have shown promising results in enhancing the efficiency of these NPs in environmental remediation, emphasizing their potential for large-scale applications. In conclusion, biologically synthesized Fe⁰-based NPs represent a valuable tool in mitigating the environmental and health impacts of chlorinated organic compounds. By addressing the current gaps and pursuing targeted research, these NPs have the potential to contribute significantly to sustainable and effective environmental remediation strategies.

Data availability

No new experimental data were generated or analyzed in this review article. All data discussed in this manuscript are derived from previously published sources, which are appropriately cited in the reference section. For any inquiries regarding data access or additional information, the corresponding author can be contacted.

Author contributions

The authors collectively contributed to the conceptualization, data analysis, and writing of this manuscript. Specific roles included statistics, document collection, data collection, and manuscript revision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the scientific and technical assistance facilities at the Institute of New Technology/Vietnam.

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