Toxicity of zero-valent iron nanoparticles to soil organisms and the associated defense mechanisms: a review

Phytotoxicity of seven iron-based materials to mung bean seedlings

Due to the widespread application of various iron-based materials in environmental remediation and agricultural production, it is of significance to assess their environmental risks. Here, seven iron-based materials, including ionic FeCl3, micro- and nano-sized magnetite (i.e., mFe3O4 and nFe3O4), micro- and nano-sized zero-valent iron (i.e., mZVI and nZVI), and nZVI modified by starch and activated carbon (i.e., S-nZVI and A-nZVI), were targeted to compare their phytotoxicity in mung bean grown in the soil with different doses (0, 300, 600, and 1000 mg/kg) for 40 days. S-nZVI and A-nZVI severely inhibited plant growth, decreasing shoot and root dry weights by 45.1-79.2 and 26.0-82.3%, respectively, but other materials showed no or minimal effects on plant growth. All the materials decreased chlorophyll content and photosynthesis performance, with more pronounced inhibition from A-nZVI and S-nZVI, especially at 1000 mg/kg. The activities of superoxide dismutase (SOD) and peroxide dismutase (POD) in leaves were stimulated by all the materials, among which S-nZVI enhanced SOD activity by 206.9% at 300 mg/kg and POD activity by 541.1% at 1000 mg/kg. In most cases, Fe accumulation in the roots was increased by all materials, particularly by S-nZVI and A-nZVI. Fe concentrations in roots and shoots exposed to 1000 mg/kg S-nZVI and A-nZVI were 2-3 times higher than those in the control treatments. The disturbance in the homeostasis of minerals (Zn, Mn, Cu, and Mg) was induced by all iron-based materials. To conclude, the phytotoxicity of iron-based materials was dependent on their type and dosage, and stabilization was crucial for the phytotoxicity and bioaccumulation potential of nZVI in plants.

Simultaneous effects of nanoscale zero-valent iron on wastewater decontamination and energy generation: Mechanisms of sulfamethoxazole degradation and methanogenesis

The presence of sulfamethoxazole (SMX) can adversely affect the anaerobic digestion process, reducing the efficiency of wastewater treatment and methane production. In this study, the addition of exogenous nanoscale zero-valent iron (nZVI) enhanced the efficient treatment of SMX and promoted the energy recovery from antibiotic wastewater. The results showed that the removal of SMX in the reactor pairs with 0.5 g/L nZVI increased by 20 %, 35 %, and 27 %, and the methane production increased by 21.6 %, 40.9 %, and 26.6 %, respectively, compared with the control reactor at different SMX influent concentrations (50, 100, and 200 mg/L). The microbial community distribution indicated that the nZVI facilitated efficient cooperation between acid-producing and methanogens by regulating the relative abundance of functional bacteria, such as Anaerolinea and Methanothrix. Meanwhile, nZVI can effectively facilitate the direct interspecies electron transfer (DIET) and enhance electron transport system (ETS) activity by functioning as a conductive particle and increasing the abundance of genes related to cytochrome C (Cyt C) and type IV pili. In addition, nZVI can reduce the risk of antibiotic resistance genes (ARGs) transmission by decreasing the relative abundance of ARGs. In summary, this study could provide new insights and theoretical support for efficient anaerobic bioremediation and energy recovery of antibiotic wastewater containing SMX.

Efficient remediation of different concentrations of Cr-contaminated soils by nano zero-valent iron modified with carboxymethyl cellulose and biochar

Nano zero-valent iron (nZVI) is widely used in soil remediation due to its high reactivity. However, the easy agglomeration, poor antioxidant ability and passivation layer of Fe-Cr coprecipitates of nZVI have limited its application scale in Cr-contaminated soil remediation, especially in high concentration of Cr-contaminated soil. Herein, we found that the carboxymethyl cellulose on nZVI particles could increase the zeta potential value of soil and change the phase of nZVI. Along with the presence of biochar, 97.0% and 96.6% Cr immobilization efficiency through CMC-nZVI/BC were respectively achieved in high and low concentrations of Cr-contaminated soils after 90-days remediation. In addition, the immobilization efficiency of Cr(VI) only decreased by 5.1% through CMC-nZVI/BC treatment after 10 weeks aging in air, attributing to the strong antioxidation ability. As for the surrounding Cr-contaminated groundwater, the Cr(VI) removal capacity of CMC-nZVI/BC was evaluated under different reaction conditions through column experiments and COMSOL Multiphysics. CMC-nZVI/BC could efficiently remove 85% of Cr(VI) in about 400 hr when the initial Cr(VI) concentration was 40 mg/L and the flow rate was 0.5 mL/min. This study demonstrates that uniformly dispersed CMC-nZVI/BC has an excellent remediation effect on different concentrations of Cr-contaminated soils.

Molecular insights into the anti-cancer activity of chitosan-okra mucilage polymeric nanocomposite doped with nano zero-valent iron against multi-drug-resistant oral carcinoma cells

Recent advances in nanotechnology, particularly those utilizing polymeric nanocomposites, have garnered significant attention for their effectiveness and biocompatibility in cancer diagnosis and treatment. In this study, a chitosan-okra mucilage polymeric nanocomposite doped with nano zero-valent iron (CS-OM-nZVI), synthesized using green chemistry principles, was evaluated for its anti-cancer activity against drug-resistant oral carcinoma cells (KBChR). The nanocomposite was created from chitosan, mucilage derived from okra biomass, and nano zerovalent iron particles synthesized through chemical reduction. The resulting nanocomposite exhibited a highly stable, crystalline nanoscale structure with excellent stability. Anti-cancer activity was assessed by measuring cell viability, apoptosis induction, oxidative stress markers, DNA fragmentation, and performing in silico docking studies between the components of the polymeric nanocomposite (CS-OM-nZVI) and key proteins involved in carcinoma pathogenesis. The nanocomposite demonstrated significant anticancer activity, with an IC50 of 600 μg/mL, indicating notable effects on cell viability. It also induced significant morphological changes associated with apoptosis, such as chromatin condensation and nuclear fragmentation. Additionally, the nanocomposite had a marked effect on oxidative stress markers, particularly catalase and superoxide dismutase activity. In silico docking studies revealed that the polymeric composite modulates and enhances both intrinsic and extrinsic apoptotic pathways, confirmed by chitosan's binding to Caspase-3. This study suggests that the prepared nanocomposite is a promising anti-cancer agent against drug-resistant oral carcinoma cells, demonstrating a significant impact on cancer cell viability.

Remediation of Cr(VI) Polluted Groundwater Using Zero-Valent Iron Composites: Preparation, Modification, Mechanisms, and Environmental Implications

The extensive application of chromium (Cr) in many industries has inevitably resulted in the release of Cr(VI) into the groundwater environment, thus posing damage to the ecosystem and human health. Nano zero-valent iron (nZVI) has been widely studied and applied in the remediation of Cr(VI)-contaminated water as an ideal material with high reductive capacity, which enables the transformation of teratogenic and carcinogenic Cr(VI) into less toxic Cr(III). This review comprehensively summarizes the preparation and modification methods of nZVI Cr(VI) removal performance and mechanisms by nZVI and modified nZVI materials. The field applications of nZVI-based materials, such as combining the injection well and the permeable reactive barrier (PRB) to remove Cr(VI) in groundwater, have been reported. Subsequently, the potential toxicity of nZVI-based materials to organisms during environmental application has been highlighted in the current study. Finally, the review outlines potential improvements and explores future directions for the use of nZVI-based materials in groundwater contamination remediation.

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

PbbHLH155 enhances iron deficiency tolerance in pear by directly activating PbFRO2 and PbbHLH38

Iron (Fe) deficiency is a general stress for many horticulture crops, causing leaf chlorosis and stunted growth. The basic-helix-loop-helix (bHLH) transcription factor (TF) was reported to function in Fe absorption; however, the regulatory mechanism of bHLH genes on iron absorption remains largely unclear in pear. In this study, we found that PbbHLH155 was significantly induced by Fe deficiency. Overexpression of PbbHLH155 in Arabidopsis thaliana and pear calli significantly increases resistance to Fe deficiency. The PbbHLH155-overexpressed Arabidopsis lines exhibited greener leaf color, higher Fe content, stronger Fe chelate reductase (FCR) and root acidification activity. The PbbHLH155 knockout pear calli showed lower Fe content and weaker FCR activity. Interestingly, PbbHLH155 inhibited the expressions of PbFRO2 and PbbHLH38, which were positive regulators in Fe-deficiency responses (FDR). Furthermore, yeast one-hybrid (Y1H) and Dual-Luciferase Reporter (DLR) assays revealed that PbbHLH155 directly binds to the promoters of PbFRO2 and PbbHLH38, thus activating their expression. Overall, our results showed that PbbHLH155 directly promote the expression of PbFRO2 and PbbHLH38 to activate FCR activity for iron absorption. This study provided valuable information for pear breeding.

The synergistic potential of biochar and nanoparticles in phytoremediation and enhancing cadmium tolerance in plants

Cadmium (Cd) is classified as a heavy metal (HM) and is found into the environment through both natural processes and intensified anthropogenic activities such as industrial operations, mining, disposal of metal-laden waste like batteries, as well as sludge disposal, excessive fertilizer application, and Cd-related product usage. This rising Cd disposal into the environment carries substantial risks to the food chain and human well-being. Inadequate regulatory measures have led to Cd bio-accumulation in plants, which is increasing in an alarming rate and further jeopardizing higher trophic organisms, including humans. In response, an effective Cd decontamination strategy such as phytoremediation emerges as a potent solution, with innovations in nanotechnology like biochar (BC) and nanoparticles (NPs) further augmenting its effectiveness for Cd phytoremediation. BC, derived from biomass pyrolysis, and a variety of NPs, both natural and less toxic, actively engage in Cd removal during phytoremediation, mitigating plant toxicity and associated hazards. This review scrutinizes the application of BC and NPs in Cd phytoremediation, assessing their synergistic mechanism in influencing plant growth, genetic regulations, structural transformations, and phytohormone dynamics. Additionally, the review also underscores the adoption of this sustainable and environmentally friendly strategies for future research in employing BC-NP microaggregates to ameliorate Cd phytoremediation from soil, thereby curbing ecological damage due to Cd toxicity.

Persulfate activation using leonardite char-supported nano zero-valent iron composites for styrene-contaminated soil and water remediation

Effective in-situ technology to treat carcinogenic compounds in contaminated areas poses a major challenge. Our objective was to load nano-zero-valent iron (nZVI) onto leonardite char (LNDC), an alternative carbon source from industrial waste, for use as a persulfate (PS) activator for styrene treatment in soil and water. By adding a surfactant during synthesis, cetyltrimethylammonium bromide (CTAB) promotes a flower-like morphology and the nZVI formation in smaller sizes. Results showed that nZVI plays a crucial role in PS activation in both homogeneous and heterogeneous reactions to generate reactive oxygen species (ROS), which can remove 98% of styrene within 20 min. Quenching experiments indicated that singlet oxygen (1O2), superoxide radicals (O2•-), and sulfate radicals (SO4•-) were the main species working together to degrade styrene. XPS analysis also revealed a role of surface oxygen-containing groups (i.e., CO, C-OH) in activating PS for SO4•- and 1O2 generation. The possible reaction mechanism of PS activation by LNDC-CTAB-nZVI composite and factors affecting treatment efficiency (i.e., PS concentration, catalyst dosage, pH, and humic acid) were illustrated. The molarity/molality ratio of PS to nZVI should be set greater than 1 for effective styrene removal. GC-MS analysis showed that styrene was degraded to a less toxic benzaldehyde intermediate. However, the excessive use of PS and catalysts can harm plant growth, requiring a combining approach to achieve safer use for real applications. Overall results supported the use of the LNDC-CTAB-nZVI/PS system as an efficient in-situ treatment technology for soil and water remediation.

Effects of the application of nanoscale zero-valent iron on plants: Meta analysis, mechanism, and prospects

In order to determine the ideal conditions for the application of nanoscale zero-valent iron (nZVI) in agricultural production, this review studies the effects of nZVI application on plant physiological parameters, presents its mechanism and prospective outcomes. In this research, it was observed that the application of nZVI had both favorable and unfavorable effects on plant growth, photosynthesis, oxidative stress, and nutrient absorption levels. Specifically, the application of nZVI significantly increased the biomass and length of plants, and greatly reduced the germination rate of seeds. In terms of photosynthesis, there was no significant effect for the application of nZVI on the synthesis of photosynthetic pigments (chlorophyll and carotenoids). In terms of oxidative stress, plants respond by increasing the activity of antioxidant enzyme under mild nZVI stress and trigger oxidative burst under severe stress. In addition, the application of nZVI significantly increased the absorption of nutrients (B, K, P, S, Mg, Zn, and Fe). In summary, the application of nZVI can affect the plant physiological parameters, and the degree of influence varies depending on the concentration, preparation method, application method, particle size, and action time of nZVI. These findings are important for evaluating nZVI-related risks and enhancing nZVI safety in agricultural production.

Study of the Stability, Uptake and Transformations of Zero Valent Iron Nanoparticles in a Model Plant by Means of an Optimised Single Particle ICP-MS/MS Method

In the context of the widespread distribution of zero valent iron nanoparticles (nZVI) in the environment and its possible exposure to many aquatic and terrestrial organisms, this study investigates the effects, uptake, bioaccumulation, localisation and possible transformations of nZVI in two different forms (aqueous dispersion-Nanofer 25S and air-stable powder-Nanofer STAR) in a model plant-Arabidopsis thaliana. Seedlings exposed to Nanofer STAR displayed symptoms of toxicity, including chlorosis and reduced growth. At the tissue and cellular level, the exposure to Nanofer STAR induced a strong accumulation of Fe in the root intercellular spaces and in Fe-rich granules in pollen grains. Nanofer STAR did not undergo any transformations during 7 days of incubation, while in Nanofer 25S, three different behaviours were observed: (i) stability, (ii) partial dissolution and (iii) the agglomeration process. The size distributions obtained by SP-ICP-MS/MS demonstrated that regardless of the type of nZVI used, iron was taken up and accumulated in the plant, mainly in the form of intact nanoparticles. The agglomerates created in the growth medium in the case of Nanofer 25S were not taken up by the plant. Taken together, the results indicate that Arabidopsis plants do take up, transport and accumulate nZVI in all parts of the plants, including the seeds, which will provide a better understanding of the behaviour and transformations of nZVI once released into the environment, a critical issue from the point of view of food safety.

Reductive transformation of Cr(VI) in contaminated soil by polyphenols: The role of gallic and tannic acid

Polyphenols, as an important category of natural organics, are ubiquitous in plants and structurally diverse. Batch experiments were conducted to investigate the role of natural polyphenol, such as gallic acid (GA) and tannic acid (TA), in the biochemical behavior of Cr(VI) in soil media. GA and TA can effectively convert Cr(VI) to Cr(III) under neutral conditions (pH 7.0). However, there are significant differences in the transport, leaching toxicity, and bioavailability of reduced Cr(III) between the two systems. UV-vis spectra, chromium (Cr) mass balance, speciation distribution, and X-ray photoelectron spectroscopy were used to explore the intrinsic mechanisms of Cr(VI) reduction and (im)mobilization in the presence of GA or TA. Results showed that the reduction of Cr(VI) by GA was accompanied by poor immobilization of reduced Cr(III), especially at high GA concentrations (4-10 g/L), which was associated with the formation of soluble Cr(III) complexes. After treatment with 4 g/L GA, 51.49 ± 3.04% of the Cr in GA system was mobilized as complexes into aqueous phase. In contrast, the reduction of Cr(VI) and the subsequent precipitation of reduced Cr(III) was dominant in the TA system. After treatment with 4 g/L TA, 97.24 ± 0.31% of the total Cr in the TA system was immobilized into soil phase and transformed into more stable fractions. Our findings provide new insights into how natural organics shape the fate and transport of Cr in soils, which also have substantial implications for the development of Cr sequestration technology.

An updated review on environmental occurrence, scientific assessment and removal of brominated flame retardants by engineered nanomaterials

Due to the extensive manufacturing and use of brominated flame retardants (BFRs), they are known to be hazardous, bioaccumulative, and recalcitrant pollutants in various environmental matrices. BFRs make flame-resistant items for industrial purposes (textiles, electronics, and plastics equipment) that are disposed of in massive amounts and leak off in various environmental matrices. The consumption of plastic items has expanded tremendously during the COVID-19 pandemic which has resulted into the increasing load of solid waste on land and water. Some BFRs, such as polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCDs), are no longer utilized or manufactured owing to their negative impacts, which promotes the utilization of new BFRs as alternatives. BFRs have been discovered worldwide in soil, sludge, water, and other contamination sources. Various approaches such as photocatalysis-based oxidation/reduction, adsorption, and heat treatment have been found to eradicate BFRs from the environment. Nanomaterials with unique properties are one of the most successful methodologies for removing BFRs via photocatalysis. These methods have been praised for being low-cost, quick, and highly efficient. Engineered nanoparticles degraded BFRs when exposed to light and either convert them into safer metabolites or completely mineralize. Scientific assessment of research taking place in this area during the past five years has been discussed. This review offers comprehensive details on environmental occurrence, toxicity, and removal of BFRs from various sources. Degradation pathways and different removal strategies related to data have also been presented. An attempt has also been made to highlight the research gaps prevailing in the current research area.

Exogenous iron alters uptake and translocation of CuO nanoparticles in soil-rice system: A life cycle study

The abundant iron in farmland soil may affect the environmental fate of metal-based nanoparticles (MNPs). In this study, the effect of FeSO4 and nano-zero-valent iron (nZVI) as exogenous iron on the uptake and translocation of CuO nanoparticles (NPs) in soil-rice system was performed in a life cycle study. The results show that exogenous iron basically elevated the soil pH and electrical conductivity but lowered the redox potential. Moreover, the Cu bioavailability in soil was significantly increased by 86-269% with exogenous iron at the tillering stage, while was reduced by 15-45% with medium and high concentrations of Fe(II) at the maturation stage. Meanwhile, the addition of exogenous iron resolved the unfilling of grains caused by CuO NPs. Notably, except for highest Fe(II) treatment, both Fe(II) and nZVI reduced Cu accumulation from 31% to 84% in roots and leaves due to more iron plaque. Especially, medium Fe(II) level markedly decreased the Cu content in the brown rice. μ-XRF analysis suggests that high intensity of Cu was primarily located in the rice hull and embryo under Fe(II) treatment. The reduction of CuO NPs to Cu2O caused by Fe(II) can explain the positive effect of exogenous iron on controlling the environmental risk of MNPs.