Development of environmental management system in China's financial sector

Abstract

The establishment of the environmental management system in the financial sector can be effectively promoted through the introduction of the environmental protection concept and its implementation by the financial sector. The impact of a sustainable development system on the sector is analyzed in this article from three aspects: environmental risk assessment, financing support for environmental protection projects, and financial services to environmentally friendly corporations and individuals. Influential factors on the development of the environmental management system in China’s financial sector are discussed from the perspective of various entities such as financial institutions, financial regulation authorities, environmental protection departments, corporations, and the public. It is pointed out that China’s financial sector is now in the transitional phase from a defensive attitude to a preventive attitude. Strengthening governmental guidance, the supervision of regulators as well as public awareness of environmental protection should be used to enhance the initiative in the development of the environmental management system in the Chinese financial sector.

1 1 Introduction

Soil pollution, exacerbated by the rapid development of industrialization and urbanization (Sun et al., 2018; Wen et al., 2022; Wu et al., 2023a; Lü et al., 2024), poses a significant threat to soil fertility and leads to environmental degradation (Alengebawy et al., 2021; Mahto et al., 2024). Plants, as ecosystem producers, face a variety of challenges when growing in polluted soil. Pollutants inhibit plant growth and decrease crop yields (Singh et al., 2013; Guo et al., 2023; Li et al., 2023a). In addition, the pollutants accumulated in the edible parts of plants can be enriched through the food chain, thereby affecting human health (Johri et al., 2010; Alengebawy et al., 2021; Geng et al., 2022; Sun et al., 2022; Liu et al., 2023a). Therefore, efficient and sustainable remediation technologies are crucial to ensuring food safety and soil health (Ghrair et al., 2010; Shen et al., 2018; Mazarji et al., 2021; Alazaiza et al., 2022; Cao et al., 2024; Hannan et al., 2024; Li et al., 2024).

The application of nanomaterials has been identified as an effective approach for the remediation of contaminated soil. Cao et al. conducted a comprehensive analysis of impacts of varied nanomaterials on crops under the metal(loid) stress conditions, and their findings indicated that the application of nanomaterials lead to enhanced plant biomass (22.8%) and decreased the metal(loid) accumulation (−38.3%) in plants (Cao et al., 2024). Among them, iron-based nanoparticles (Fe-NPs) exhibit promising potential for soil remediation, including farmland remediation (Wang et al., 2019a; Qian et al., 2022; Jin et al., 2023; Zhou et al., 2023b). However, their toxic effects on plants have also been reported (Chen et al., 2024b; Ghouri et al., 2024; Wang et al., 2025). Published reports contain mixed opinions and results regarding the roles of Fe-NPs in soil–plant systems. For example, Anwar et al. (2024) reported that nano zero-valent iron (nZVI) (2.5 mg/kg) application improved the physiological traits of tomato seedlings . Gil-Díaz et al. discovered that the application of nZVI (0.5%–5%) did not significantly alter the biomass of lettuce plants, while the root cell integrity was more affected by nZVI in alkaline soil (Gil-Díaz et al., 2024). Qian et al. (Qian et al., 2023) reported marked negative effects on grain yield under nZVI treatments. Moreover, it has been found applying a low dosage of Fe-NPs can promote plant growth, while the application of high dosages of Fe-NPs inhibits plant growth (Wang et al., 2016a; Tombuloglu et al., 2021). Fe-NPs hold great potential in alleviating the pollutants in soil–plant systems contaminated by heavy metals or organic pollutants (Jiang et al., 2018; Li et al., 2021; Huang et al., 2022b; Xue et al., 2023; Chen et al., 2024a). In addition, numerous studies have demonstrated the beneficial effects of Fe-NPs on plant growth in polluted soil (Huang et al., 2018; Hussain et al., 2019; Adrees et al., 2020; Liu et al., 2021; Wu et al., 2021). This is significantly important for plant health and productivity.

Several studies have reviewed the effects of nZVI on plants and the uptake, transport, and translocation of nZVI in plants (Rai et al., 2022; Zhang et al., 2022b; Qian et al., 2023). For example, Zhang et al. (2022b) concentrated on the toxicity of plants exposed to nZVI, while Rai et al. (2022) focused on the different effects of nano-Fe on plants but did not distinguish between hydroponic and soil-planting conditions . The interactions between Fe-NPs and soil–plant systems remain unclear. Xue et al. reviewed the performance of nZVI in contaminated soils, further examining the toxicity of nZVI to soil organisms, with a particular emphasis on soil bacteria and fungi (Xue et al., 2018). However, determining the effects of Fe-NPs on plants under polluted soil conditions require further investigation. Therefore, there is an urgent need to explore the roles of Fe-NPs in both soil–plant systems and pollutant–soil–plant systems.

Quantifying the effects of Fe-NPs on plant growth in normal and polluted soil systems is an important yet challenging task because previous studies were conducted under varied experimental conditions. Meta-analysis involves the comprehensive collection and quantitative analysis of various independent studies. It is important to investigate the interactions between Fe-NPs and soil–plant (and pollutant–soil–plant) systems based on the results of meta-analysis.

The present study aimed to achieve the following goals: 1) to discuss the interaction between Fe-NPs and the soil matrix in both normal soil and a polluted soil matrix; 2) to quantify the effects of Fe-NPs on plant morphological and physiological characteristics and explore the effects of related factors (Fe-NPs properties, plant species and organs, and initial soil pH) on plants in both normal soil and polluted soil matrixes; and 3) to reveal the potential mechanisms through which Fe-NPs promote plant growth in soil–plant systems and enhance pollutant alleviation and tolerance in plants in pollutant–soil–plant systems.

2 2 Methods

2.1 2.1 Literature selection and data extraction

To collect publications focusing on the effects of Fe-NPs application on plants, a thorough literature search was conducted from 2000 to 2022 on the Web of Science, Google Scholar, and Science Direct. The search keywords included “iron nanoparticles or nZVI or IONPs (iron oxide nanoparticles) or iron-based nanoparticles” AND “crop or plant” AND “soil or soil amendment or contaminated soil remediation or polluted soil”.

Initially, a total of 456 articles were found. After rigorous filtering based on titles, abstracts, and the removal of duplicate papers and review articles, 137 papers were shortlisted. The following criteria were employed to select papers for meta-analysis:

1) The tested plants were cultivated in soil;

2) The added Fe-NPs were mixed into soil, eliminating studies that used seed priming, foliar application, and hydroponic systems;

3) The experimental data included at least one indicator of plant properties (such as plant morphological or physiological characteristics) or soil properties (such as the pH value or available Fe/pollutants);

4) There were control groups (cases without Fe-NPs) and treatment groups (cases with Fe-NPs);

5) The experimental data were expressed as the mean ± standard deviation (SD) or mean ± standard error (SE).

Finally, a total of 57 articles were collected for the meta-analysis. All articles used in the meta-analysis can be found in the Supporting Information Texts. Among them, 23 articles focused on Fe-NPs application in plant–soil systems and 38 articles dealt with pollutant–soil–plant systems. Within the latter category, 11 studies examined the use of Fe-NPs–biochar composites (Fe-NPs@BC).

2.2 2.2 Meta-analysis

In this study, data on the properties of Fe-NPs, plants, and soil were extracted from the selected papers. Information was directly collected from tables when available. Origin 2019b software was employed to extract data from figures. The SE was converted to SD using the following formula (Eq. (1)):

(1)$SD=SE\times \sqrt{n},$

where n is the number of repetitions. To avoid differences in measurement units, the natural log-transformed response ratio (RR) was used as the mean effect size (Eq. (2)):

(2)$RR=\mathit{l}\mathit{n}\left(X_E/X_C\right),$

where X_E and X_C are the mean values of treatments (with Fe-NPs application) and controls (without Fe-NPs application), respectively. The variance of each ln (RR) was calculated using Eq. (3):

(3)${\sigma }_L={SD}_E^2/N_E{\overline{X}}_E^2+{SD}_C^2/N_C{\overline{X}}_C^2,$

where the SD_C and SD_E are the SD values of the treatments and control group, respectively; and N_C and N_E are the replicates of the treatments and control group, respectively.

This study tested the heterogeneity between groups based on the Q statistic test, and the results revealed significant heterogeneity in the test studies (Tables S1 and S2). Therefore, the random effect model was selected in the present study. The data were then classified into different groups to conduct subgroup analysis (Tables S3 and S4). Egger’s test was employed to test the publication bias. A value of p_E < 0.05 according to Egger’s test indicated that a study exhibited significant publication bias. To adjust publication bias, the trim-and-fill method was employed to recalculate the mean size and 95% confidence interval (CI) (Tables S5 and S6). The specific methods are presented in the previous study (Wen et al., 2022).

All meta-analysis results were calculated using the “metafor” package in R studio, and forest plots were used to display the results. All the sample sizes (N) of indicators were showed in Figure caption.

3 3 Fe-NPs in soil–plant systems

In soil–plant systems, Fe-NPs have the potential to exert stress on plants as possible toxic matter, especially at high doses. Conversely, Fe-NPs can also function as Fe sources for plants.

3.1 3.1 Fe-NPs in the soil matrix

3.1.1 3.1.1 Evolution of Fe-NPs in the soil matrix: transport and transformation

Fe-NPs, particularly nZVI, have been widely applied for groundwater and soil remediation over the past decade. The activity and properties of Fe-NPs are prone to changes and the effects of aging in the soil matrix. Therefore, understanding the evolution of Fe-NPs in soil systems is crucial for their effective use (Fig.1).

Fig.1 Evolution of Fe-NPs in soil matrix. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

Aggregation: In soil environments, Fe-NPs tend to aggregate through homoaggregation and/or heteroaggregation. Many studies have found that spherical nZVI tends to form chain-like homoaggregates due to strong magnetic attraction (Wang et al., 2021b). The homoaggregation of Fe-NPs is thermodynamically spontaneous, driven by van der Waals forces and potential magnetic attraction between particles. As the concentration of Fe-NPs increases, a higher number of particle-particle collisions lead to a higher degree of homoaggregation, causing Fe-NPs to precipitate and halting their transport in soil (Zhao et al., 2016; Cheng et al., 2018). Coating is an effective approach to preventing nZVI from homoaggrgation.

Heteroaggregation between Fe-NPs and soil particles is driven by the presence of various inorganic (minerals and ions) and organic components (soil organic matter, SOM) (Zhang et al., 2020b). In soils, nZVI can be oxidized and/or sorbed on clay mineral particles. This process is facilitated by the interfacial interactions between surface functional groups, which vary under different pH conditions (Wang and Lin, 2017; Wang et al., 2019b). Studies have shown that nZVI forms micrometer-sized aggregates on soil when the aging time is up to 60 d (Wang et al., 2021b). Fe₂O₃ nanoparticles (NPs) interact with SOM, resulting in slight agglomeration (Wang et al., 2020), while Fe₃O₄ NPs exhibit lower mobility in soils and form large aggregates (diameters of hundreds of nanometers on the surface of soils) (Ben-Moshe et al., 2013; Hui et al., 2021). Previous studies have found that SOM fractions with moderate and high molecular weights can be adsorbed onto the surface of bare nZVI, enhancing the stability and migration ability of the nZVI (Wang and Lin, 2017; Wang et al., 2020). In contrast, negative effects have also been reported, suggesting that the presence of SOM can cause NPs aggregation via bridging Fe-NPs or organic molecules through organic molecular fragment flocculation (Wang and Lin, 2017; Chen, 2018).

Oxidization/aging: Fe-NPs are susceptible to oxidation or aging in the soil matrix, potentially leading to decreased remediation activity or changes in the properties of Fe-NPs. In aqueous systems, the Fe⁰ core can be oxidized to form Fe oxide through interactions with water and air, increasing the thickness of the shell. Similarly, in soils, nZVI is transformed into FeO, Fe₃O₄, Fe₂O₃, and α-FeOOH (lepidocrocite) (Dong et al., 2020). Wang et al. (2021b) found that nZVI was gradually oxidized to form clay minerals, with crystalline maghemite and magnetite identified as the primary products of oxidation. In addition, some soil studies have reported another aging product, γ-FeOOH, potentially due to the complex components in soils or insufficient aging time compared to other studies (Wu et al., 2019; Hui et al., 2021; Wang et al., 2021b). The size of nZVI also plays a crucial role in its oxidation. Smaller nZVI (20 nm) has a higher surface iron oxide content than larger nZVI (100 nm), as the Fe⁰ core of smaller nZVI can easily come into contact with oxidants in soils (Wang et al., 2021b). Iron oxide nanoparticles (IONPs) are generally considered to be more stable than nZVI due to their resistance to oxidation, as well as their greater dispersibility and stability. Nonetheless, previous studies observed rod-like FeOOH on the surface of aged Fe₃O₄ NPs, whereas γ-Fe₂O₃ NPs did not exhibit oxidation in soil (Hui et al., 2021). This distinction can be attributed to the stability of fully oxidized γ-Fe₂O₃ NPs, and the instability of Fe₃O₄ NPs due to the higher mobility of electrons within the structure and the diffusion of Fe²⁺ (He et al., 2011).

3.1.2 3.1.2 Changes in soil properties

The extensive application of Fe-NPs can greatly affect the physical and chemical properties of soils. However, the overall effects are highly dependent on the dose and type of Fe-NPs as well as the inherent properties of soil. Moreover, the buffering effect of the soil matrix and soil microorganisms may play key roles over the long-term.

Soil pH: Fe-NPs can alter the soil pH after reacting with soil water and oxygen. In aqueous systems, nZVI can react with water to produce OH⁻, increasing the pH (Eqs. (4) and (5)) (Wang et al., 2021b).

(4)$\mathrm{F}\mathrm{e}+{2\mathrm{H}}_2\mathrm{O}\to {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2+2\mathrm{O}{\mathrm{H}}^{-}$

(5)$2\mathrm{F}\mathrm{e}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to {2\mathrm{F}\mathrm{e}}^{2+}+4\mathrm{O}{\mathrm{H}}^{-}$

Similarly, the introduction of nZVI can induce a pH increase in soil–plant systems (Wang et al., 2016a). However, oxygen is present in soil pores, and the released iron ions undergo hydrolysis. The formation of Fe-OH groups in the soil can generate H⁺, which can acidify soil (Eqs. (6)–(9)) (Li et al., 2006; Zhou et al., 2012; Wang and Lin, 2017).

(6)${\mathrm{F}\mathrm{e}}^{2+}+{2\mathrm{H}}_2\mathrm{O}\to \mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_2+{2\mathrm{H}}^{+}$

(7)${6\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{O}}_2+{2\mathrm{H}}_2\mathrm{O}\to {2\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4+{12\mathrm{H}}^{+}$

(8)${4\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{O}}_2+{6\mathrm{H}}_2\mathrm{O}\to 4\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3+{8\mathrm{H}}^{+}$

(9)${\mathrm{F}\mathrm{e}}^{3+}+{2\mathrm{H}}_2\mathrm{O}\to \mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}\mathrm{H}+{3\mathrm{H}}^{+}$

The overall effects of Fe-NPs addition depend on the balance between negative and positive effects. Moreover, with the introduction of plants and soil microorganisms, the scenario may become more complicated. The present study examined the reported initial and final pH values in plant–soil systems supplemented with Fe-NPs (Fig.2) and found that the overall change in pH was not significant. Interestingly, the addition of nZVI (1000 mg/kg) slightly increased the soil pH from 6.96 to 7.13 after 7 d of rice cultivation. With the application of Fe₂O₃ NPs (30 and 60 mg/kg), the soil pH in both non-rhizosphere and rhizosphere zones slightly increased (Yang et al., 2020). Mixed trends have been reported for Fe₃O₄ NPs. Slight soil acidification was observed with doses of 100 mg/kg Fe₃O₄ NPs (the pH declined from 6.35 to 6.14) and 10 mg/kg Fe₃O₄ NPs (the pH decreased from 8.49 to 8.41) (Zhou et al., 2012; Cao et al., 2017). In most cases, the application of Fe-NPs had a negligible impact on the soil pH.

Fig.2 Changes induced by Fe-NPs application on the soil pH in soil–plant systems. The sample sizes NpH = 15. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

SOM: The addition of Fe-NPs can also alter the components of SOM, especially dissolved organic matter (DOM). Research has demonstrated that Fe-NPs show higher affinity toward aromatic and hydrophobic DOM compounds and are complexed to a greater extent than hydrophilic compounds, potentially altering the content and structure of DOM (Kim et al., 2018; Li et al., 2018b; Hui et al., 2021). Zhou et al. (2012) found that the initial application of nZVI (2–6 g/kg) could reduce the DOM content, and increase the DOM content to a stable value later on. In another previous work, the application of Fe₃O₄ NPs (1 g/kg) significantly increased the fraction of humic-like substances in DOM, though the total SOM did not change (Ben-Moshe et al., 2013). Similarly, Zhou et al. (2012) discovered that the soil DOM content increased following the addition of Fe₂O₃ and Fe₃O₄ NPs, and hypothesized that IONPs released iron ions and combined with DOM in the soil to form soluble compounds. As time progressed, the DOM content of soil amended with Fe₂O₃ and Fe₃O₄ NPs decreased, following a similar trend to the control groups.

Soil microorganisms: Microorganisms play an important role in soil systems, and the effects of Fe-NPs on microorganisms have received wide attention. Fe-NPs have been shown to exert both positive and negative effects on microorganisms. For example, He et al. (2011). reported that Fe-NPs (Fe₃O₄ NPs and γ-Fe₂O₃ NPs) increased the invertase and urease activity. Fe-NPs (1 mg/kg) can also stimulate soil microbial metabolic activity and efficiency, promoting C and N cycling (He et al., 2011; He et al., 2016). Moreover, microorganisms such as AMF can enhance soil phosphatase activity, increase the adsorption of phosphorus onto the Fe-NPs in the soil, and promote plant growth (Mokarram-Kashtiban et al., 2019). However, some studies have also reported the cytotoxic effects of Fe-NPs on microorganisms. For example, Gómez-Sagasti et al. (2019) investigated the influence of nZVI (1–20 mg/g) on soil microbial properties in two types of soil (sandy-loam soil and clay-loam soil) and found that nZVI exposure had a negative impact on the bacterial biomass, richness, and diversity in sandy-loam soil. The primary mechanisms can be attributed to cell membrane and reactive oxygen species (ROS)-induced oxidative stress (Xue et al., 2018; Zhang et al., 2022b).

3.2 3.2 Effects of Fe-NPs on plant morphological characteristics

The present study evaluated the findings of 23 previous reports focusing on the effects of Fe-NPs on the morphological characteristics of plants, including the plant length, dry weight (DW), and fresh weight (FW) (Fig.3). Interestingly, Fe-NPs addition caused a significant decrease in the plant length (RR = −0.1281, 95% CI = [−0.2161, −0.0402]) (p = 0.0043). However, both the plant DW (RR = 0.12, 95% CI = [0.0577, 0.1824]) (p = 0.0002) and FW (RR = 0.3176, 95% CI = [0.2198, 0.4154]) (p < 0.0001) significantly increased with the addition of Fe-NPs. To understand the heterogeneity of these results, a subgroup analysis was conducted to examine the soil pH, Fe-NPs type, Fe-NPs dose, and plant properties.

Fig.3 Changes induced by Fe-NPs application on plant morphological characteristics in soil–plant systems, including (A) plant length (the sample size N = 74); (B) plant dry weight (N = 127); and (C) plant fresh weight (N = 99). Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

The subgroup analysis suggested that plant length was notably influenced by the Fe-NPs type and dose. Specifically, the application of nZVI significantly inhibited plant growth (p = 0.0045), whereas treatments with IONPs increased the length of plants (Fe₂O₃ NPs: p = 0.0065; Fe₃O₄ NPs: p = 0.0314). This trend was consistent previous findings that nZVI (250–1000 mg/kg) significantly inhibited the growth of rice (Wang et al., 2016a). Additionally, a considerable number of researchers have employed IONPs as foliar sprays or soil amendments to supplement plants with sufficient iron (Alidoust and Isoda, 2013; Alidoust and Isoda, 2014; Ghaffarzadeh et al., 2021; Iannone et al., 2021). Theoretically, Fe-NPs induced phytotoxicity should be highly dose-dependent. At low doses of Fe-NPs (10–100 mg/kg), Fe-NPs showed positive effects on the plant length, but the length clearly decreased at higher doses (500–1000 mg/kg). Although the plant length may be a sensitive indicator, some researchers have argued that root length only provides transient information on the early growth stages of plants and cannot be used to predict the long-term effects of Fe-NP application. As a result, plant length should not be used as the only measure when performing toxicity evaluations (López-Luna et al., 2016; 2018).

For both DW and FW, the effect size results showed that the addition of Fe-NPs had positive effects. Specifically, both Fe₂O₃ and Fe₃O₄ NPs were effective in enhancing plant FW. Furthermore, Fe-NPs in the dose ranges of 10–100 mg/kg and 100–500 mg/kg promoted both DW and FW. At doses below 1000 mg/kg, the promotional effect size for plant FW decreased as the dose increased, and Fe-NPs application even had an inhibitory effect at a higher dose range (500–1000 mg/kg). The growth of shoots and roots improved after the application of Fe-NPs. The subgroup analysis based on plant type revealed that rice, a type of cereal crop, was negatively impacted by Fe-NPs application in terms of the three morphological characteristics (length, DW, and FW). In contrast, the application of Fe-NPs to herbaceous plants and vegetables demonstrated promoting trends or no significant difference compared to the control group.

In general, Fe-NPs have the potential to enhance plant morphological characteristics. Previous research demonstrated that toxic effects of NPs on plant growth depended on the particle size, type, dose, and frequency of NPs application (Rizwan et al., 2017). According to the results of the present meta-analysis, stable IONPs (Fe₂O₃ NPs) exhibited positive effects overall, while nZVI application resulted in a negative effect or no significant effect on plant morphological characteristics. Moreover, Fe-NPs at doses between 10 mg/kg and 100 mg/kg were beneficial for plant growth. Furthermore, most previous publications have investigated the plant weight of short-term cultures, while few long-term experiments have focused on crop yield or the edible parts of plants. Therefore, further research is required to better understand the underlying mechanisms and long-term effects of Fe-NPs on plant growth and yield.

3.3 3.3 Effects of Fe-NPs on plant physiological characteristics

Plant physiological characteristics refer to the internal processes and functions that are essential for plant growth, development, and survival. It is important for plants to adsorb adequate mineral elements, especially during the early growth stages. Iron, an essential nutrient for plants, plays critical roles in various essential physiologic processes, including photosynthesis, respiration, and nitrogen fixation. However, the effects of Fe-NPs on plant growth and development are not yet fully understood.

3.3.1 3.3.1 Fe content in plants

Iron accumulation in plant tissues was greatly improved with the addition of Fe-NPs to soil–plant systems (Fig.4). Meta-analysis showed that Fe uptake by plants was significantly different between Fe-NPs treated groups and control treatments (RR = 0.396, 95% CI = [0.3507, 0.4413]) (p < 0.0001). Significant increases in Fe contents were observed with the application of various Fe-NPs types and dosage levels (p < 0.0001). Interestingly, the application of FeOOH (p < 0.0001) and Fe₃O₄ NPs (p < 0.0001) resulted in higher plant iron uptake than the application of nZVI (p < 0.0001) and Fe₂O₃ NPs (p < 0.0001). The results did not reflect the expected linear relationship between the Fe-NPs dose and Fe uptake in plants. Instead, the highest uptake effect size was found at a dosage range of 100–500 mg/kg (p < 0.0001). Regarding plant types, Fe-NPs application increased Fe concentrations in maize (p < 0.0001), herbaceous plants (p = 0.0391), leguminous plants (p < 0.0001), and vegetables (p < 0.0001). The main accumulation of Fe was observed in roots and shoots, while Fe accumulation in grains and leaves showed no significant difference between the Fe-NPs and the control groups.

Fig.4 Changes induced by Fe-NPs application on plant Fe uptake in soil–plant systems (N = 180). Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

Transformation and accumulation of Fe-NPs in plants: The addition of Fe-NPs can greatly increase the Fe content in the soil matrix, resulting in higher Fe bioavailability. Consequently, more Fe can be accumulated in plant tissues. Fe enters plant tissues in two main forms: Fe-NPs and iron ions. Various studies have reported the presence of nano-sized Fe-NPs in plant tissues. For example, Li et al. (2018a) employed TEM imaging to observe whether different Fe-NPs could be internalized into root cells, and the findings suggested that γ-Fe₂O₃ (20 nm) could be internalized into root cells via endocytosis, while α-Fe₂O₃NPs (30 nm) could be taken up by root cell vacuoles through a diapirism process. Fe₃O₄ NPs can be accumulated in the intercellular spaces of the root tissue. Similarly, Yuan et al. (2018) observed a few particles in root epidermis cells, and Yang et al. (2022) found that nZVI could infiltrate into the root tip . These studies all provide further evidence that Fe-NPs can enter plants in the form of NPs. However, Fe-NPs were only found in root tissues. Some studies have suggested that Fe-NPs enter plants through the roots and are transported to the stems and leaves in a bioavailable form (Yuan et al., 2018; Tighe-Neira et al., 2022). Thus, the transformation of Fe-NPs is inevitable in soil–plant systems. Fe-NPs in both soil and in plant tissues can serve as an iron ion sink for plants. In the soil matrix, the added crystalline Fe-NPs can be converted into amorphous structures. These amorphous Fe forms are characterized by their metastability and can maintain an elevated level of soluble inorganic Fe for prolonged periods. This process significantly increases the availability of Fe to plants (Wu et al., 2022). In addition, the Fe-NPs in plants can also provide Fe ions. Accordingly, Shankramma et al. found that the Fe²⁺/Fe_total ratio in plants increased with a higher dosage of Fe₂O₃ NPs. This could be due to the internalization and/or biomineralization of Fe₂O₃ NPs in plants. The reduction of Fe³⁺ to Fe²⁺ can occur due to the presence of rich bio-reductants (Shankramma et al., 2016). Hu et al. found that the reduction of Fe³⁺ by ferric reductase resulted in the transport of Fe²⁺ across the plasma membrane in Citrus maxima plants (Hu et al., 2017). Dwivedi et al. (2018) investigated the nZVI transformation products in plants and found that the major products in the root tissue were ferric citrate (41%) and iron (oxyhydr) oxides (59%), while the shoot samples contained a higher proportion of ferric citrate (60%) and a lower proportion of iron (oxyhydr) oxides (40%). This may be because released root exudates (organic acids, protons, and metabolites) improve the formation of stable Fe–organic complexes and their effective translocation within the plant. The toxicity of iron–citrate complexes is lower than that of ferrous ions, but iron–citrate complexes are only stable at low pH values. pH changes in the xylem and leaf cells promote the dissociation of complexes and lead to the precipitation of iron (hydroxide) (Wang et al., 2011; Dwivedi et al., 2018).

Translocation of Fe-NPs in plants: Plant roots are directly exposed to soil amended with Fe-NPs and make the first contact with Fe-NPs. The roots secrete a viscous substance known as mucilage, making Fe-NPs or Fe ions more likely to be sorbed to the root surface. The route of Fe uptake and translocation involves Fe penetrating the root surface, passing through a series of physiological barriers, and finally traversing the xylem via specific pathways to the shoots or other aerial tissues (Mohammad et al., 2013; Xingmao et al., 2013; Lv et al., 2019). There are two primary pathways through which Fe-NPs enter the xylem from the root surface and are translocated to the aboveground parts of the plant: the apoplastic pathway and the symplastic pathway (Mohammad et al., 2013; Lv et al., 2019; Le Wee et al., 2022; Tighe-Neira et al., 2022) (Fig.5). The apoplastic pathway involves the translocation of Fe through the extracellular spaces between cell walls and membranes in the root cortex, followed by the subsequent translocation of Fe along the intercellular spaces to the endodermis, phloem, and xylem. The size of Fe-NPs is a crucial factor in this pathway, as Fe-NPs must pass through a series of chemical and physiological barriers that control the size exclusion limits (Wang et al., 2016b). For example, the thickness of plant cell walls typically ranges from 5 to 20 nm, theoretically allowing only Fe-NPs smaller than 20 nm to penetrate. Previous studies found that larger Fe₃O₄ NPs (> 20 nm) could not enter the roots (Wang et al., 2011; Yan et al., 2020), while smaller Fe-NPs (< 20 nm) could be taken up by the roots (Zhu et al., 2008; Tombuloglu et al., 2020). It is important to note that size exclusion limits vary among different plant species and growth stages. The symplastic pathway involves the entry of Fe through the cell membrane of root hairs or root tip cells, followed by the traversal of Fe through intracellular channels such as the plasmodesmata, ultimately leading to the arrival of Fe in the xylem. In addition to traversing the cell wall, there are two mechanisms by which Fe may be transported through the symplastic pathway. One is the penetration of the cell membrane by NPs and their subsequent entry into the cytoplasm. The second mechanism by which NPs may be transported across the cell wall is through the plasmodesmata (Lv et al., 2019).

Fig.5 Uptake and translocation of Fe-NPs in plants. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

In general, the application of Fe-NPs can greatly enhance Fe availability and uptake in plants. However, Fe uptake and translocation in plants depend on many factors, including the Fe-NPs type, dose, and size, in addition to soil properties and plant species.

3.3.2 3.3.2 Plant pigments, ROS, and antioxidant enzyme activities in soil–plant systems

Pigments, ROS, and antioxidant defense enzymes are sensitive indicators of environmental stress in plants (Fig.6). Under stressful conditions, cellular redox homeostasis is disturbed, leading to ROS accumulation. This may result in pigment bleaching in plants, causing light inhibition and inducing membrane lipid peroxidation (Powles, 1984; Ott et al., 1999). Therefore, the present study investigated the effects of adding Fe-NPs to soil on changes in plant pigments, ROS, and antioxidant defense enzymes.

Fig.6 Changes induced by Fe-NPs application on plant physiological characteristics in soil–plant systems (NPhotosynthesis pigment = 47, NMDA = 40, and NAntioxidant enzyme = 47). Fe-NPs: iron-based nanoparticles; MDA: malondialdehyde; SOD: superoxide dismutase; CAT: catalase; POD: peroxidase.

Full size|PPT slide

During the process of plant photosynthesis, Fe is a necessary element for chlorophyll synthesis. The chlorophyll content is indicative of the capacity of plants to utilize light energy and produce organic matter through photosynthesis (Wang et al., 2021a). This study explored the impacts of Fe-NPs application on plant photosynthetic pigments, and the results indicated that Fe-NPs exerted a negative impact on plant pigments (RR = −0.3689, 95% CI = [−0.5692, −0.1687]) (p = 0.0003), including chlorophyll (p = 0.0467) and carotenoids (p = 0.0455) (Fig.6). Given that chlorophyll is a critical photosynthetic pigment, chlorophyll levels can be regarded as a significant indicator of toxicity for plants. Several studies found that high doses of nZVI (> 500 mg/kg) enhanced the leaf pigment content, suggesting that the increase in leaf pigment could be attributed to the increase in iron content within the leaves (Rui et al., 2016; Kim et al., 2019; Wang et al., 2021a). Researchers also found that Fe-NPs had a positive effect on plant chlorophyll, with a correlation coefficient of chla/chlb > 0.9 (Mohammad et al., 2013; Kim et al., 2019). In a study on Arabidopsis thaliana, nZVI (500 mg/kg) had nearly no influence on chlorophyll content (Yoon et al., 2019). However, a reduction in chlorophyll content was observed following exposure to IONPs (Barhoumi et al., 2015; Wang et al., 2016a). Researchers have proposed that it is possible that Fe³⁺ is not readily reduced to bioavailable Fe²⁺, resulting in less Fe²⁺ uptake by plants. Furthermore, oxidative damage may occur in chloroplasts through interactions with Fe³⁺ or Fe-NPs, ultimately disrupting chlorophyll synthesis or causing the degradation of the pigment itself (Li et al., 2018a).

The addition of Fe-NPs can be considered environmental stress, which may lead to the overproduction of ROS (including H₂O₂, O^2−•, and •OH). Abiotic stress can be indirectly determined based on the formation of malondialdehyde (MDA), which is an indicator of lipid peroxidation. Lipid peroxidation is a representative parameter of cell membrane integrity, and ROS generation has been well established as a primary factor causing cell membrane damage through lipid peroxidation. Excessive ROS production is connected to a significant increase in lipid peroxidation. However, the results of this study suggested that Fe-NPs application had a negative effect on MDA content (RR = −0.1224, 95% CI = [−0.2471, 0.0023]) (p = 0.0544) (Fig.6). This finding was in line with previous studies in which it was suggested that Fe-NPs did not induce ROS formation or that the antioxidant defense systems in plants were generally sufficient to eliminate excess ROS and prevent oxidative damage (Rui et al., 2016; Yan et al., 2020; Wang et al., 2021a). Furthermore, an appropriate amount of ROS has positive effects on plant growth. Some studies reported that nZVI stimulated the release of ROS and caused •OH radical-induced cell wall loosening in roots (Jae-Hwan et al., 2014; Kim et al., 2019). However, other studies found that ROS-mediated oxidative stress and membrane lipid peroxidation served as the underlying basis of the phytotoxicity (Lei et al., 2016; Ghosh et al., 2017). Wang et al. (2016a) demonstrated that ROS generation led to membrane damage in rice after being exposed to nZVI (> 500 mg/kg). Similarly, Ghosh et al. found that a high dose of nZVI caused excessive ROS in cells, resulting in DNA damage and a decrease in the mitotic index (Ghosh et al., 2017). However, ROS can mediate redox signaling and antioxidant defense pathways that foster plant acclimatization against stress. In many cases, ROS generation below certain threshold levels promoted plant growth (Zhao et al., 2022).

Superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) are important enzymes in the antioxidant defense system in plants. Metalloenzyme SOD can facilitate the conversion of O²⁻ radical to H₂O₂, while CAT and POD can catalyze the H₂O₂ breakdown of hydrogen peroxide, thereby detoxifying ROS. Fe-NPs can induce ROS production in plants, triggering an antioxidant defense response. However, the present meta-analysis indicated that changes in these enzymes induced by Fe-NPs were not significant (SOD and CAT) or were negatively impacted (POD). This implied that Fe-NPs did not induce stress responses in plants or that the process of antioxidation had been completed (Yan et al., 2020). It should be noted that the data utilized in the present study are limited, and the mechanism through which Fe-NPs decrease antioxidant enzyme levels remains unclear (Lei et al., 2016).

In previous studies included in this meta-analysis, the chlorophyll content and antioxidant enzyme activity were all determined at a single time point. Future research should focus more on the dynamic changes of the active oxygen–antioxidant mechanism and chlorophyll.

3.4 3.4 Toxicity or fertilizer? Mechanisms of toxicity and plant growth promotion

3.4.1 3.4.1 Negative effects

Plant roots can be considered the primary point of entry for Fe-NPs, especially in rhizosphere soils. Additionally, root tips and hairs are capable of secreting large amounts of mucilage, which adheres to the root surface. Mucilage is a pectic substance that contains highly hydrated polysaccharides, which can promote the adsorption of NPs onto the root surface (Lin and Xing, 2008). Mohammad et al. (2013) found that the majority of applied Fe-NPs were adsorbed/trapped on the root tissue of soybeans. Notably, the adsorption of nZVI onto the root surface was found to trigger damage to root tips, epidermal cells, and root hairs due to colloidal destabilization (Ghosh et al., 2017). Furthermore, Fe-NPs accumulation on the root surface has been observed to decrease the hydraulic conductivity of the roots, reflecting the disruption of their functionality (Hu et al., 2017). Studies have reported that Fe-NPs may enter the plant vascular system, where they compete with water and nutrients. Additionally, evidence suggests that attachment to root surfaces may hinder water transpiration through the leaves (Asli and Neumann, 2009). Moreover, Fe-NPs have been found to aggregate within the vascular system or even cause blockages, leading to limited translocation or electrolyte leakage (EL) into tissues and cells (Wang et al., 2011). This blockage of water and nutrients can cause reduced photosynthesis, eventually exerting negative effects on plant growth and development (Fig.7).

Fig.7 Influencing mechanisms of Fe-NPs on plants in soil–plant systems. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

Cell damage can occur during the uptake and translocation of Fe-NPs in plants, leading to excess ROS, which induces EL and cell death, subsequently causing phototoxicity. The Fe ions released from Fe-NPs under the action of root exudates interact with oxygen and form ROS through the Fenton reaction. The process increases oxidative stress, subsequently stimulating antioxidant enzyme activity (Trujillo-Reyes et al., 2014; Hu et al., 2017).

The Fenton reaction proceeds as follows (Eqs. (10)–(12)):

(10)$\mathrm{F}\mathrm{e}+{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2$

(11)$\mathrm{F}\mathrm{e}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}^{2+}+{2\mathrm{O}\mathrm{H}}^{-}$

(12)${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{O}\mathrm{H}}^{-}+\cdot \mathrm{O}\mathrm{H}$

Although Fe is a vital micronutrient for plants, excessive Fe can lead to severe toxicity in plants (Zahra et al., 2021). Many have studies found that Fe-NPs are more toxic for plants at high doses (Wang et al., 2016a; Hu et al., 2017). Higher doses of Fe-NPs were found to induce more ROS and a greater degree of toxicity symptoms, including reduced germination percentage, membrane damage, nutrition imbalance, and interference with enzymatic activities (Rui et al., 2016; Yuan et al., 2018). Tighe-Neira et al. (2022) reported that the negative effects of Fe-NPs toxicity at the root morphological and photosynthesis levels led to a decline in plant growth. Some studies have compared Fe ions and Fe-NPs. Hu et al. found both Fe ions and γ-Fe₂O₃ NPs triggered adverse effects on plant growth at higher doses (100 mg/L), and Fe ions seemed to be more toxic than Fe-NPs. Due to the high aggregation and accumulation of γ-Fe₂O₃ NPs on the root surface, NPs may hinder the transmission of water and nutritional components, decreasing their chlorophyll content and root activity (Hu et al., 2017). Some other studies showed that nZVI released lower plant-available iron content than Fe-EDTA at the same dose (Mohammad et al., 2013; Kim et al., 2019). In addition, introduced Fe-NPs may act as an iron sink that continuously releases iron ions, potentially causing oxidative stress and inducing an antioxidant defense response in plants (Rui et al., 2016; Hu et al., 2017; Rizwan et al., 2017).

3.4.2 3.4.2 Positive effects

Positive effects of Fe-NPs on plant growth have also been widely reported, suggesting that Fe-NPs improve plant growth in two ways. First, Fe-NPs function as an additional Fe source to provide the necessary Fe for plants. Second, Fe-NPs can induce changes in the physical characteristics of plants, thereby enhancing plant growth. Plant roots can improve Fe bioavailability via acidifying the media through root exudates, which increase the release of Fe ions and form complexes with organic acid (Zhang et al., 2020b). In previous work, the higher expression of the CsHA1 gene was observed under nZVI treatment compared with the control group. This result suggested that roots released more protons with the addition of nZVI, and the subsequent acidification promoted further Fe release (Dwivedi et al., 2018). Yuan et al. (2018) observed that the cell walls of the Fe-NP exposure group were thinner than those of the control group, and further inferred that the changes in cell walls potentially enhanced plant elongation. Another work demonstrated that Fe-NPs application induced an appropriate level of ROS generation and triggered cell wall loosening, leading to increased root elongation (Jae-Hwan et al., 2014). Furthermore, Fe-NPs have been found to increase the chloroplast number, which is important for the light capture process during photosynthesis (Jae-Hwan et al., 2014). Other studies reported that nZVI addition enhanced stomatal opening via inducing the activation of plasma membrane H-ATPase, leading to increased CO₂ uptake (Jae-Hwan et al., 2015; Yoon et al., 2019).

Fe-NPs can influence plants directly through contact with the root tissues or indirectly via altering the root-zone soil matrix. Xu et al. (2017) investigated the influence of Fe₃O₄ NPs on plant growth and soil bacterial community structure. Their study suggested that the application of a low concentration of Fe₃O₄ NPs increased the abundance of Xanthomonadales, which can enhance plant biomass via promoting soil nutrient cycling. Conversely, research has demonstrated that elevated levels of Fe₃O₄ NPs cannot only suppress plant growth, increase the Fe content in plant tissues, and enhance soil electronic conductivity but can also decrease the diversity of bacterial communities. Yang et al. (2022) found that an appropriate concentration of nZVI promoted the plant photosynthetic capacity as well as the colonization and the development of arbuscular mycorrhizal fungi (AMF) in the rhizosphere.

The agricultural application of Fe-NPs can be beneficial due to their potential to improve plant growth and iron nutrient accumulation in crops. However, some adverse effects have been reported, necessitating careful consideration. The use of Fe-NPs should be optimized to balance their potential benefits and drawbacks.

4 4 Fe-NPs in pollutant–soil–plant systems

4.1 4.1 Interactions between Fe-NPs and pollutants in the soil matrix

Soil pollution has emerged as a pressing environmental issue, prompting significant public attention. Nanotechnology-based interventions, particularly engineered nanomaterials, have proven successful in soil remediation efforts. Among these nanomaterials, Fe-NPs have been extensively studied and utilized. A 2020 survey on the ScienceDirect database by Marcon et al. using the keyword “nano-remediation” identified 125 research papers. The analysis of these papers revealed that 40% of the studies were mainly based on nZVI, followed by other Fe-NPs, such as magnetite, ferrite, and hematite (35%). The results suggested the great potential of Fe-NPs for soil remediation (Marcon et al., 2021). Compared to Section 3, Section 4 discusses the introduction of Fe-NPs into pollutant–soil–plant systems to remediate the contaminants.

Organic pollutants: Fe-NPs, especially nZVI, were initially employed for the remediation of chlorinated organic pollutants in groundwater (Wang and Zhang, 1997; Choi et al., 2008; Ganesh-Kumar et al., 2017). The typical mechanism through which Fe-NPs remediate organic pollutants primarily involves adsorption and reductive dichlorinations. In addition, an electrode reaction in Fe-NPs produces Fe ions (Fe²⁺) and hydrogen gas (H₂). Throughout the degradation process, highly reactive reductants (nZVI, Fe²⁺, structural Fe²⁺, and H₂), as electron donors, react with organic pollutants to convert pollutants into harmless molecules (Li et al., 2016; Xie et al., 2017).

Heavy metals: In soil systems, the toxicity of metals mainly depends on their speciation, which encompasses the acid-soluble fraction, reducible fraction, oxidizable fraction, and residual fraction. Heavy metals in non-residual fractions are more prone to mobilization and exhibit higher bioavailability and toxicity. Consequently, effective remediation strategies need to be designed to transform the unstable fractions of heavy metals into stable forms (Xue et al., 2018). Interactions between Fe-NPs and heavy metals have confirmed that Fe-NPs can decrease the mobility and toxicity of heavy metals in soils. For example, lignin hydrogel was utilized to encapsulate FeS-NPs in Cd-polluted soils, resulting in significant reductions in the total, surfactant-soluble, and fixed Cd by 22.4%–49.6%, 13.5%–68.6%, and 16.6%–40.1%, respectively. Immobilization is usually attributed to mechanisms including precipitation, complexation, sorption, and reduction (Liu et al., 2020). Furthermore, Huang et al. (2016) found that the application of sodium alginate-nZVI (0.1%wt) increased the residual fraction of Cd in sediment (the fraction of residual Cd increased from 15.49% to 57.28% after 30 d). The surface complexation that occurred between Cd²⁺ and iron hydroxides on the surface of Fe-NPs made the dominant contribution to Cd immobilization.

Peng et al. (2019) investigated the effects of various Fe-NPs (nZVI, nZVI@BC, FeS-NPs, FeS-NPs@BC, Fe₃O₄NPs, and Fe₃O₄-NPs@BC) on Pb-contaminated soil and found that all Fe-NPs increased the Pb-oxidizable (16.16%–81.24%) and Pb-residual (10.09%–103.45%) fractions compared with the control treatment. They proposed that the primary immobilization mechanism was the formation of amorphous iron plumbites and secondary oxidation minerals through co-precipitation between Pb and Fe³⁺ under alkaline soil conditions. Additionally, they observed that the application of nZVI and Fe₃O₄ NPs increased the soil pH over time. Generally, mineral soil surfaces exhibit a positive charge at low pH due to proton sorption, while the charge becomes negative as the pH increases. Higher soil pH levels have been reported to promote the sorption of metal cations (Gil-Díaz et al., 2017).

For some heavy metals, such as Cr and As, it has been proposed that both sorption and reduction contribute to immobilization, and reduction is sometimes the primary mechanism, especially when nZVI is used. For example, Cr⁶⁺ (more mobile in soil than Cr³⁺) can be easily adsorbed by nZVI (iron hydroxide and oxide shell) due to its high surface area (Peng et al., 2017; Xue et al., 2018; Chen et al., 2020). Cr⁶⁺ can be reduced to Cr³⁺ by nZVI or Fe₃O₄ NPs, eventually forming Fe³⁺-Cr³⁺ coprecipitation. In addition, Fe-NPs can stimulate microbial activities, thereby impacting the microbially mediated biochemical transformation of Cr (Liu et al., 2023b).

In summary, heavy metals such as Cd, Pb, and Zn, whose valence states cannot be easily changed, are usually immobilized by Fe-NPs via adsorption, precipitation, or complexes. For heavy metals such as As and Cr, whose valence states are prone to change, Fe-NPs undergo redox reactions together with adsorption or precipitation to achieve immobilization.

4.2 4.2 Effects of Fe-NPs on plant morphological characteristics in polluted soil

The present work analyzed 38 papers to investigate the impacts of Fe-NPs on the morphological characteristics of plants in polluted soil. Generally, the presence of Fe-NPs was consistently found to enhance plant growth in polluted soil. Compared to the control groups without Fe-NP treatments, the effect sizes of plant length, DW, and FW were 0.1463 (95% CI = [0.0729, 0.2197], p < 0.0001), 0.1715 (95% CI = [0.1275, 0.2154], p < 0.0001), and 0.106 (95% CI = [0.0086, 0.2034], p = 0.0329), respectively (Fig.8).

Fig.8 Changes induced by Fe-NPs application in plant morphological characteristics in polluted soil–plant systems, including (A) plant length (N = 93); (B) plant dry weight (N = 172); and (C) plant fresh weight (N = 64). Fe-NPs: iron-based nanoparticles; IONP: iron oxide nanoparticles.

Full size|PPT slide

The properties of Fe-NPs have varying impacts on the morphological characteristics of plants. IONP application led to significant improvements in all observed morphological characteristics of plants, including length (RR = 0.2184, 95% CI = [0.1701, 0.2666], p < 0.0001), DW (RR = 0.3143, 95% CI = [0.271, 0.3576], p < 0.0001), and FW (RR = 0.3336, 95% CI = [0.2305, 0.4368], p < 0.0001). Conversely, nZVI application demonstrated a positive effect only on plant DW (RR = 0.1234, 95% CI = [0.0691, 0.1776], p < 0.0001). In terms of the Fe-NPs dose, the effect size of plant morphological characteristics showed significant positive effects at Fe-NPs doses ranging from 10 to 100 mg/kg (length: RR = 0.205, 95% CI = [0.1524, 0.2576], p < 0.0001; DW: RR = 0.2628, 95% CI = [0.2214, 0.3042], p < 0.0001; FW: RR = 0.2377, 95% CI = [0.1549, 0.3205], p < 0.0001). However, at a higher dose range (500–1000 mg/kg), all effect sizes of plant morphological characteristics showed no significant or negative effects, while the effect sizes of DW (RR = 0.3053, 95% CI = [0.0884, 0.52221], p = 0.0058) and FW (RR = 0.28, 95% CI = [0.0781, 0.4818], p = 0.0066) showed positive effects. The responses of plants to the Fe-NP remediation of polluted soil vary among plant properties. The collected data showed that the morphological characteristics of food crops such as maize, rice, and wheat responded positively to Fe-NP remediation. In contrast, the effects of Fe-NP remediation on legumes, herbaceous, and vegetables were insignificant.

The observed positive effects in the edible organs, such as grain and fruit, were significant in terms of the DW (RR = 0.3996, 95% CI = [0.2979, 0.5014], p < 0.0001) and FW (RR = 0.3113, 95% CI = [0.1766, 0.446], p < 0.0001). In contrast, the roots, which are directly exposed to pollutants and Fe-NPs soil conditions, showed inconsistent responses to Fe-NPs. Although Fe-NPs remediation had significant positive effects on the root length (RR = 0.1973, 95% CI = [0.0466, 0.348], p = 0.0103) and DW (RR = 0.1439, 95% CI = [0.0579, 0.2299], p = 0.001), Fe-NPs remediation had inhibitory effects on the FW (RR = −0.2574, 95% CI = [−0.5017, −0.013], p = 0.039).

4.3 4.3 Effects of Fe-NPs on plant physiologic characteristics in polluted soil

4.3.1 4.3.1 Fe content in plants

The meta-analysis results revealed a significant increase in the plant uptake of Fe when Fe-NPs were introduced into pollutant–soil–plant systems (N = 112, RR = 0.3543, 95% CI = [0.2837, 0.4248], p < 0.0001) (Fig.9). This finding aligned with the results obtained from soil–plant systems. The type and dosage of Fe-NPs greatly impacted the Fe contents in plants. Specifically, IONPs (N = 30, RR = 0.4207, 95% CI = [0.3411, 0.5004], p < 0.0001) influenced Fe uptake in plants more than nZVI (N = 82, RR = 0.3257, 95% CI = [0.2324, 0.4191], p < 0.0001). Within the dosage range of 0–500 mg/kg, the effect size of Fe uptake in plants significantly increased as the dosage increased. The effect sizes were 0.1104 (0–10 mg/kg) (p = 0.0045), 0.3575 (10–100 mg/kg) (p < 0.0001), and 0.5104 (100–500 mg/kg) (p < 0.0001). However, no linear relationship was found between the Fe-NPs treatments and the control treatments. Increasing the dosage to higher than 500 mg/kg resulted in effect sizes of 0.2883 (500–1000 mg/kg) (p = 0.013) and 0.2656 (1000–5000 mg/kg) (p = 0.3294). Plant species played a significant role in Fe content uptake, with wheat (N = 30, RR = 0.4207, 95% CI = [0.3411, 0.5004], p < 0.0001), herbaceous plants (N = 51, RR = 0.5366, 95% CI = [0.4659, 0.6074], p < 0.0001), and legumes (N = 8, RR = 0.2207, 95% CI = [0.1311, 0.3104], p < 0.0001) showing significant positive effects in polluted soil. Moreover, Fe mainly accumulated in the root (RR = 0.2846, 95% CI = [0.1573, 0.4119], p < 0.0001), shoot (RR = 0.4225, 95% CI = [0.3296, 0.5155], p < 0.0001), and grain (RR = 0.3967, 95% CI = [0.2402, 0.5532], p < 0.0001) tissues. The initial soil pH also had varying effects on plant Fe uptake. Interestingly, at an initial soil pH > 7.5, plant Fe uptake showed a significant increase (N = 65, RR = 0.4524, 95% CI = [0.3993, 0.5056], p < 0.0001).

Fig.9 Changes induced by Fe-NPs application on (A) plant iron (N = 112) and (B) pollutant uptake (N = 423) in polluted soil–plant systems. Fe-NPs: iron-based nanoparticles; IONP: iron oxide nanoparticles.

Full size|PPT slide

4.3.2 4.3.2 Pollutant content in plants

A total of 423 data points were collected, and the summary effect size was − 0.2152 (95% CI = [−0.2681, −0.1623], p < 0.0001). This indicated that the uptake of target pollutants by plants decreased under Fe-NP application treatments compared to control treatments (Fig.9). The application of nZVI (RR = − 0.1553, 95% CI = [−0.2099, −0.1007], p < 0.0001) and IONPs (RR = −0.6646, 95% CI = [−0.9541, −0.3751], p < 0.0001) resulted in lower plant pollutant uptake. Subgroup analysis based on Fe-NPs dosage revealed that the pollutant uptake was not significantly different between the Fe-NPs treatments and the control treatments within the Fe-NPs dosage range of 100–500 mg/kg (p = 0.9798). However, for other dosage ranges, significantly lower pollutant uptake was observed under Fe-NPs application, with effect sizes of −0.2227, −0.4298, −0.3187, and −0.3004 for dosages of 0–10, 10–100, 500–1000, and 1000–5000 mg/kg, respectively. Linear regression analysis demonstrated that the pollutant content in plants gradually decreased with an increase in Fe-NPs (Fig. S1).

To comprehend the remediation effect of Fe-NPs, this study compared plant types and organs (Fig.9). Decreases in pollutant uptake were reported for maize, rice, wheat, and vegetables, while an increase was noted in herbaceous plants (RR = 0.1014, 95% CI = [0.0175, 0.1852], p = 0.0179). This indicated that the application of Fe-NPs promoted the uptake of pollutants by herbaceous plants. At varied pH levels, the pollutant uptake by plants was significantly reduced, with the highest reduction observed for the pH range of 6.5 ≤ pH ≤ 7.5 (RR = − 0.3067, 95% CI = [−0.4918, −0.1216], p = 0.0012), followed by pH < 6.5 (RR = − 0.2541, 95% CI = [−0.2995, −0.2086], p < 0.0001) and pH > 7.5 (RR = −0.108, 95% CI = [−0.2048, −0.0113], p < 0.0001). Moreover, plants exhibited decreased pollutant uptake of Cd (RR = −0.2432, 95% CI = [−0.3316, −0.1549], p < 0.0001), Pb (RR = −0.0942, 95% CI = [−0.1812, −0.0072], p = 0.0338), Cu (RR = −0.2196, 95% CI = [−0.3332, −0.1061], p = 0.0002), Cr (RR = −0.3576, 95% CI = [−0.5932, −0.122], p = 0.0029), polychlorinated biphenyls (PCB) (RR = −0.4735, 95% CI = [−0.6387, −0.3083], p < 0.0001), and pentachlorophenol (PCP) (RR = −1.1749, 95% CI = [−1.578, −0.7718], p < 0.0001) with the addition of Fe-NPs. However, Fe-NPs enhanced the accumulation of Sb (RR = 0.3847, 95% CI = [0.2231, 0.546], p < 0.0001) in plants, while the other heavy metals showed a nonsignificant difference in plant uptake between Fe-NPs treatments and control treatments. Overall, in soils polluted by single or multiple pollutants, Fe-NPs decreased the pollutant content in plants, with better effectiveness observed in multi-pollutant soils.

4.3.3 4.3.3 Plant pigments, ROS, and antioxidant enzyme activities in pollutant–soil–plant systems

The present study examined the effect of Fe-NPs on plant pigments in polluted soil based on a total of 138 data points (Fig.10). The results showed that Fe-NPs had a significant impact on the effect size of pigments (RR = 0.0962, 95% CI = [0.0476, 0.1449], p = 0.0001). Subgroup analysis revealed a significant positive effect of Fe-NPs on carotene (RR = 0.5031, 95% CI = [0.3274, 0.6788], p < 0.0001), despite its limited sample size. Further examination of other plant pigments, including chlorophyll a (RR = 0.0552, 95% CI = [−0.0049, 0.1153], p = 0.0717) and chlorophyll b (RR = 0.0923, 95% CI = [−0.0015, 0.1862], p = 0.0539), yielded positive effect sizes, though these effects were not statistically significant.

Fig.10 Changes induced by Fe-NPs application in plant physiological characteristics in polluted soil–plant systems (NPhotosynthesis pigment = 138, NOxidative stress = 102, and NAntioxidant enzyme = 206). Fe-NPs: iron-based nanoparticles; MDA: malondialdehyde; EL: electrolyte leakage; SOD: superoxide dismutase; CAT: catalase; POD: peroxidase; APX: ascorbate peroxidase.

Full size|PPT slide

Soil pollutants entering plants can lead to the production of excess ROS, resulting in oxidative stress. This oxidative stress damages the cell membrane, causing EL. Therefore, the integrity of the cell membrane can be indirectly assessed by measuring the change in conductivity. The effect size of EL in in the present study was determined to be − 0.2066 (95% CI = [−0.2631, −0.1501], p < 0.0001). In addition, H₂O₂ (RR = − 0.275, 95% CI = [−0.462, − 0.0881], p = 0.0039) and MDA (RR = −0.1632, 95% CI = [−0.2763, −0.0501], p = 0.0047) also demonstrated negative effects. Altogether, these findings suggested that Fe-NPs reduced the oxidative stress of plants and alleviated the damage caused by pollutants.

Under the stress of soil pollution, the expression and activity of antioxidant enzymes usually increase in response to additional oxidative stress. Notably, a significant negative effect (RR = −0.2143, 95% CI = [−0.4459, −0.0927], p = 0.0006) was observed in plant SOD activity. This could be attributed to the completion of antioxidant reactions by plants, resulting in a decrease in SOD. Alternately, the reduction of SOD may have occurred because SOD activity was not triggered or because plants were unable to cope with oxidative stress after the remediation of polluted soil. Conversely, CAT exhibited an opposite trend (RR = 0.2258, 95% CI = [0.0398, 0.4117], p = 0.0173), indicating that plants promoted the breakdown of H₂O₂ into H₂O and O₂, thereby reducing the risk of oxidative damage under pollution-induced stress.

4.4 4.4 Remediation mechanisms

Fe-NPs have been identified as a promising technology for environmental remediation due to their high reactivity and capacity to reduce and degrade pollutants. Fe-NPs can play a crucial role in polluted soil–plant systems, aiding in remediation through a range of processes. Fe-NPs can directly enhance the removal or degradation of pollutants in soil. In addition, the introduced Fe-NPs can be involved in the interactions between the polluted soil and plants, indirectly affecting the overall remediation efficiency.

4.4.1 4.4.1 Direct pollutant removal by Fe-NPs

The findings of the present study show that Fe-NPs can enhance pollutant removal, degradation, and immobilization in contaminated soil. Fe-NPs can decrease the bioavailability of heavy metals in soil through adsorption, complexation, redox reactions, and co-precipitation, thereby reducing the accumulation of pollutants in plants (as shown in Section 3.1). Based on the linear correlation found in the linear regression analysis, the optimal removal efficiency can be achieved at the appropriate Fe-NP dose (Fig. S2). However, excessively high Fe-NPs doses may lead to secondary soil pollution, such as the release of excessive Fe ions and the generation of significant amounts of reduction products, which can be toxic to the soil and ecosystem. In addition, the performance of Fe-NPs in remediation varied for different pollutants. The results showed that the overall degradation efficiency of organochlorides was lower than the immobilization efficiency of heavy metals in soil (Fig.11).

Fig.11 Effects of Fe-NPs application on soil pollutant removal efficiency in polluted soil–plant systems (N = 361) with (A) varied Fe-NPs dose ranges and (B) different pollutant types. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

4.4.2 4.4.2 Indirect pollutant removal by Fe-NPs

Soil chemical properties: The transformation of pollutants in soil is governed by various soil chemical properties, including the pH, redox potential, organic matter, and cation exchange capacity, along with their interactions. Fe-NPs can reduce the mobility, bioavailability, and toxicity of pollutants in soil through altering the soil characteristics. The oxidation of Fe-NPs was found to promote an increase in the local soil pH, facilitating electrostatic adsorption between negatively charged soil particles and positively charged heavy metal ions (Liu et al., 2021) (Fig.12 and S3). Gil-Díaz et al. (2017) found the immobilization of anionic heavy metals (As and Cr) by Fe-NPs was more effective in acidic soils, whereas the retention of cationic heavy metals (Pb, Cd, and Zn) was enhanced in calcareous soil. Their study further revealed that nZVI application could effectively decrease the availability of As, Cr, Pb, and Zn, but not Cd.

Fig.12 Changes induced by Fe-NPs application in the soil pH in polluted soil–plant systems. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

Microorganism activity: Microorganisms play a critical role in soil health and are instrumental in soil remediation. Numerous studies have demonstrated that Fe-NPs working in conjunction with microorganisms can promote the degradation of chlorinated organic pollutants in soil (Jorfi et al., 2017; Barra Caracciolo and Terenzi, 2021; Liu et al., 2021; Wu et al., 2021). Liu et al. (2021) conducted soil extract experiments after sterilization to confirm the crucial role of soil microbes in degrading PCP when combined with nZVI during rice cultivation. According to their results, limited PCP degradation was observed after sterilization, while the residual amount of PCP in the non-sterile extract gradually decreased. Wu et al. also observed that the remediation efficiencies of nZVI in soil contaminated with PCB-28 and PCB-180 under alfalfa planting were significantly elevated from 66.7% to 93.1% and from 38.5% to 52.3%, respectively, as the dose of nZVI rose from 0 to 1000 mg/kg due to the synergistic effect of changes in the microecological environment of the alfalfa rhizosphere (Wu et al., 2021). Fe-NPs can modify enzymatic activity in the soil, enhance microbial carbon source utilization, and influence the microbial community, thereby improving the degradation of chlorinated organic pollutants (Wu et al., 2021). Fe-NPs can promote an increase in the relative content of root exudates and delay the oxidation of iron by rhizosphere microorganisms, leading to long-term pollutant remediation. Wu et al. (2023b) found that plant growth-promoting rhizobacteria could degrade organic pollutants through increasing the yield of extracellular ROS, while nZVI accelerated the process. AMF were demonstrated to enhance plant defense against Fe-NPs and facilitate the degradation of polychlorinated biphenyls via stimulating rhizosphere microbial activities. In addition, AMF have been reported to play a crucial role in plant–fungal symbiosis, facilitating nutrient absorption, especially phosphorus absorption, and reducing NP-induced phytotoxicity through regulating the expression of heavy metal-tolerant genes in both the host plant and the AMF (Wang et al., 2023).

Microbial-mediated biomineralization is another key process affecting the bioavailability of heavy metals. Xue et al. (2022) reported that graphene oxide-supported nZVI could increase enzyme activity, such as the enzyme activity of sulfate-reducing bacteria involved in the dissimilatory sulfate reduction process, thus enhancing Cd immobilization. They hypothesized that sulfate-reducing bacteria utilized the electrons generated by nZVI oxidation. Moreover, they demonstrated that graphene oxide-supported nZVI boosted the relative abundance of Fe³⁺-reducing bacteria, further driving the reduction of Fe³⁺. These bacteria have the capacity to transfer electrons directly to insoluble Fe³⁺ mineral-associated Cd, thereby increasing the stabilization efficiency for Cd (Zhao et al., 2021; Xue et al., 2022).

Iron plaque (IP): IP is a layer of amorphous or crystalline ferric or ferrous compounds on the surface of root. Fe-NPs showed the potential to enhanced IP content. Zhou et al. found that the application of Fe-NPs at 50 mg/L and 200 mg/L increased the IP content by 185% and 267%, respectively. The total amount of IP significantly increased by 190% in the Cd-contaminated environment under Fe-NPs application compared to the control group (Zheng et al., 2023). IP typically consists of a mixture of amorphous or crystalline iron (hydro)oxides, mainly α-FeOOH, γ-FeOOH, and a small amount of FeCO₃ (Tripathi et al., 2014; Xu et al., 2024). Zhang et al. (2022a) indicated that Fe-NPs promoted the development of IP, particularly its amorphous content.

Most studies revealed that the Cd in IP showed a positive correlation with the dose of Fe-NPs (Suanon et al., 2017; Zhou et al., 2023a; Sun et al., 2024; Xu et al., 2024; Zhu et al., 2024). These studies have proposed that IP acts as a barrier for metal uptake in plants owing to the strong affinity of heavy metals to Fe hydroxides via complexation and Fe-NPs could further improve the barrier to lower Cd accumulation in plant shoots (Zhou et al., 2023a). Although it is widely agreed that IP induced by Fe-NPs can adsorb pollutants, some researchers suggest that its capacity to adsorb pollutants is limited. For example, Zhang et al. (2020a) examined the effect of IP and Cd absorption in rice roots and found that the Cd contents in IP increased with the application of BC-Fe up to 0.2% but decreased when BC-Fe application exceeded 0.2%. In addition, they discovered that the formation of IP (Fe content in IP: 15.0–22.5 g/kg) increased Cd transport to rice roots, but Cd transport to rice roots began to decrease when the Fe content in IP exceeded 22.5 g/kg (Zhang et al., 2020a). This observation aligned with previous findings that the Cr content in both IP and rice roots increased at a higher nZVI dose (0.001% versus 0.1%) (Liu et al., 2023b). In summary, Fe-NPs can enhance the capacity or ability of IP for pollutant absorption, while their overall effect is also dose-dependent (Fig.13).

Fig.13 Remediation mechanisms of Fe-NPs in polluted soil–plant systems. Fe-NPs: iron-based nanoparticles.

Full size|PPT slide

Plants: Fe-NPs can improve the resistance of plants to pollutant uptake and translocation. Most studies in this area have found that Fe-NPs can immobilize heavy metals on the root surfaces in soil (Hussain et al., 2019; Adrees et al., 2020; Ahmed et al., 2021). In addition, Fe-NPs can reduce pollutant translocation in plants in various ways. For example, in previous research Fe-NPs significantly reduced As uptake due to the competitive uptake of Fe and As at the root surface (Rizwan et al., 2019; Bidi et al., 2021). Similarly, Gong et al. observed that Cu and Pb were precipitated on the root surface rather than being taken up (Gong et al., 2017), while Zn and Ni were accumulated in the endodermis and cortex of the roots under Fe-NPs treatment (Wu et al., 2018). In contrast, starch-stabilized nZVI was reported to increase the Cd content in plants. The researchers investigated the subcellular distribution of Cd in plants and found that Cd accumulation mainly occurred in the cell wall fraction (48%–66%), followed by the cell-soluble fraction. The high cell-soluble fraction of Cd (31%–46%) could be the vacuolar compartmentation of Cd in the cell-soluble fraction (Gong et al., 2017). Bidi et al. discovered that Fe-NPs promoted the synthesis of glutathione (GSH), a type of metal binding protein, and induced the sequestration and immobilization of As in vacuoles and cell walls (Bidi et al., 2021). Researchers concluded that the mechanism of decreased heavy metal toxicity in plants was mainly due to cell wall deposition and vacuolar sequestration (Gong et al., 2017; Cao et al., 2018). However, the uptake and transport of pollutants in plants mainly depend on the regulation of several transporters/proteins and the expression of related genes. For example, IRT1 and IRT2 are the main Fe-transporter proteins. Cd can also combine with Fe transporter proteins through the Fe metabolism system, as it shares similar properties with Fe (Zhang et al., 2021). The application of Fe-NPs was found to downregulate the expression of Fe/Cd transporter genes (including OsIRT1, OsIRT2, OsHMA3, OsHMA2, and OsLCT1) of rice in Cd-contaminated soil (Guha et al., 2020; Ahmed et al., 2021). FRDL1, YSL2, and YSL13 are also important Fe transporters involved in long-distance Fe transport in rice. Research has also revealed that Fe-NPs increased Fe content in leaves by upregulating the YSL2, YSL13, and FRDL1 genes (Bidi et al., 2021). Huang et al. further investigated the gene expression quantification and found that the presence of Fe-NPs inhibited the uptake of Fe²⁺ and Cd²⁺ via the OsNRAMP5, OsCd1, OsIRT1, and OsIRT2 transporters but facilitated the uptake of Fe³⁺ via the OsYSL15 transporter (Huang et al., 2022a).

Fe-NPs can also alleviate pollutant toxicity via enhancing plant physiology and metabolism. According to the analysis in the present study, Fe-NPs improve plant morphological characteristics (Fig.8) and decrease the pollutant content in plants (Fig.9). Li et al. (2023b) reported that engineered nanomaterials could obstruct Cd uptake, transport, accumulation, and toxicity in plants. Rizwan et al. suggested that the application of Fe-NPs could improve plant growth under heavy metal toxicity through providing Fe nutrients, enhancing chlorophyll biosynthesis, and promoting the plant redox process (Rizwan et al., 2019). Most studies found Fe-NPs could increase photosynthetic parameters, including the chlorophyll contents, photosynthetic rate, and transpiration rate under pollutant stress (Rizwan et al., 2019; Guha et al., 2020; Hussain et al., 2021). In the present study, the effects of Fe-NPs on chlorophyll contents were not found to be significant, which might be influenced by factors including the Fe-NP dosage and plant species (Rahman et al., 2022; Li et al., 2023b). In addition, GSH, phytochelatins (PCs), and proline have been reported to protect plants from heavy metals by forming nontoxic complexes with heavy metals. Significant increases in GSH and PCs in plants were observed in heavy metal-polluted soil with the addition of Fe-NPs, indicating that heavy metals could complex with PCs induced by Fe-NPs in cytoplasmic solution, and the complexes were then translocated to the vesicles (Guha et al., 2020; Bidi et al., 2021; Kandhol et al., 2022; Zhou et al., 2023a).

Generally, both Fe-NPs and pollutants can induce ROS accumulation in plants, and excessive ROS can cause oxidative damage. Plants deploy detoxification mechanisms to counteract this effect. Evidence suggests that Fe-NPs can enhance the antioxidant defense system of plants to fight pollution-induced oxidative damage. The findings of the present study indicated that Fe-NPs reduced ROS levels, thereby mitigating the adverse effects of pollutant toxicity on the plant in the form of EL, H₂O₂, and MDA (Guha et al., 2020; Kandhol et al., 2022). Previous studies suggested that plants could alleviate oxidative stress through enzymatic (SOD, POD, and CAT) or nonenzymatic antioxidant systems (including GSH and proline). Only CAT in plants showed a positive effect under Fe-NPs treatment in polluted soil systems based on the present results. In the study of Bidi et al. (2021), Fe-NPs (25 and 50 mg/L) increased the activities of CAT and SOD, thereby relieving As-induced phytotoxicity. However, nZVI inhibited enzyme activities at high dosages of 500 and 1000 mg/kg, suggesting that a high dose of nZVI induced toxicity. This indicated that the alleviation of oxidative stress in plants by Fe-NPs was highly dose-dependent (Gong et al., 2017). Nonenzymatic components such as GSH can also scavenge ROS in plants by breaking down sulfhydryl groups (Rai et al., 2022). In addition, GSH can act as a buffer in organelles to maintain the redox protein balance and improve plant tolerance in polluted soil (Bidi et al., 2021).

5 5 Conclusions and perspectives

5.1 5.1 Conclusions

This study investigated the effects of Fe-NPs on plant growth in normal or contaminated soil matrixes. The main conclusions are as follows:

In soil–plant systems, Fe-NPs significantly improved the morphological characteristics (FW and DW) and increased the Fe contents of plants. However, significant negative effects were observed on plant photosynthetic pigments and enzymes involved in oxidative stress;

In pollutant–soil–plant systems, Fe-NPs greatly reduced the pollutant content of plants and increased the Fe content of plants. The interactions between Fe-NPs and pollutant–soil–plant systems involve both direct and indirect mechanisms. Directly, Fe-NPs exhibited the ability to immobilize or degrade pollutants in the soil matrix. Indirectly, the influence of Fe-NPs altered soil properties, modulated soil microbial activity, and impacted plant physiologic activities. Fe-NPs effectively reduced the pollutant concentrations in plants through both mechanisms and contributed to ensuring food safety.

5.2 5.2 Perspectives

Further modifications. In this review, studies using supported or modified Fe-NPs were excluded (e.g., BC-supported Fe-NPs). Considering the high reactivity of Fe-NPs, future research should focus on the modification of Fe-NPs and combining Fe-NPs with other remediation materials/technologies to further increase the cost-effectiveness of Fe-NP application for soil remediation.

Controllable Fe-NP delivery methods for large-scale use. To the best of our knowledge, most of the studies on surface soil remediation included in this analysis were conducted at the laboratory scale. How to deliver Fe-NPs into soil–plant systems for large-scale applications is still a major challenge. The typical method of Fe-NP delivery involves mechanically mixing dry Fe-NP powder/suspension into the soil, leading to potential resource waste and, more importantly, higher exposure risk for workers/farmers. A better method for the controlled delivery of Fe-NPs is therefore needed.

Continuous research over the entire plant growth period. Although this work examined plant morphological characteristics, the data utilized in this analysis only included the effects of Fe-NPs during the early developmental stages or final stage of plants. To date, research on the dynamic changes in Fe-NPs together with the changes in plants and soil over the entire plant developmental period is limited. What happened during the growth of the plants during the entire growth cycle, especially when the edible parts grew?

Long-term studies. Currently, there is a lack of research on the evolution of Fe-NPs and pollutants after a long period of time. Will the immobilized pollutants release again? How will the iron plaque change over time? What are the long-term effects of Fe-NPs application on soil microorganisms?

Key indicators and assessment strategies. In the study of plant physiological characteristics, the chlorophyll content and antioxidant enzyme activity in the early growth stages of plants are used as indicators of the phytotoxicity of Fe-NPs. However, physiological indicators such as photosynthetic pigments and antioxidant enzyme activity change over plant growth stages. Future studies should determine the physiological indicators of plants at different growth stages to identify the key indicators for each stage. A more systematic assessment strategy is highly in need.

6 6 Abbreviations list

AMF: arbuscular mycorrhizal fungi

CAT: catalase

DOM: dissolved organic matter

DW: dry weight

EL: electrolyte leakage

Fe-NPs: iron-based nanoparticles

FW: fresh weight

GSH: glutathione

IONPs: iron oxide nanoparticles

IP: iron plaque

MDA: malondialdehyde

NPs: nanoparticles

nZVI: nano zero-valent iron

PC: phytochelatin

PCB: polychlorinated biphenyls

PCP: pentachlorophenol

POD: peroxidase

ROS: reactive oxygen species

SOD: superoxide dismutase

SOM: soil organic matter

References

1. Jeucken Marcel Sustainable Finance and Banking: The Financial Sector and the Futureof the PlanetLondon, United KingdomsEarthsan Publication Ltd 2001
2. Jeucken M H A Jan J B The Changing Environment ofBanks. Greener Management International:The Journal of Corporate Environmental Strategy and Practice (GMI):Theme Issue: Sustainable Banking: The Greening of Finance 1999 (27)2135
3. The People's Bankof China. The Notice on the Implementation of Credit Policy and Strengtheningof Environmental Protection: PBC (1995) 24BeijingThe People's Bank of China 1995 (in Chinese)
4. State EnvironmentalProtection Administration of China. The Notice on the Promotionof Environmental Protection by the Credit Policy: SEPA (2003) 101BeijingStateEnvironmental Protection Administration of China 2003 (in Chinese)
5. Yu Y D Guo P Y The research and practice offinancial sector on the promotion of sustainable developmentIn: Chinese Society for Environmental Sciences, ed.Developing Circular Economy & Realizing the Guidance of the ScientificConcept of Development: 2004 Research Album of Chinese Society forEnvironmental Sciences InstituteBeijingChinese Environmental SciencePress 2004 451455 (in Chinese)
6. Yu Y D Guo P Y Research and practice of financialsector on the promotion of sustainable developmentEnvironmental Protection2003(12)5053 (in Chinese)
7. State Council of China,National Development and Reform Commission. The ComprehensiveProgram for Saving Energy and Reducing dischargesPeople's Daily 2007 (1) (in Chinese)