Nanomaterials are an important development in nanoscience and technology, and can be defined as any organic, inorganic or organometallic material that is in at least one dimension of 100 nm in three dimensions and exhibits chemical, physical and electrical properties that vary with the size and shape of the material [1]. Nanomaterials have four fundamental effects: bulk effect, surface effect, quantum size effect and macroscopic quantum tunneling effect [2]. Thus, they have some properties that are not found in conventional materials, which makes nanomaterials gain a wide range of application prospects. Recently, nanomaterials have been widely used in environmental fields ^[3][4][3,4], biomedicine [5], chemical engineering [6] and electronic technology [7].

Nanoscale zero-valent iron (nZVI) refers to zero-valent iron particles with a particle size of 1–100 nm, which have a large specific surface area, strong reduction, strong adsorption, and a unique oxide shell compared to bulk materials [8]. These properties have made nZVI the most widely used nanomaterial for soil and groundwater contaminated site remediation. It has been shown that nZVI can adsorb Pb²⁺, Cd²⁺, Cu²⁺ and Ni²⁺ from soil, and with the increase of nZVI concentration, the adsorption of all metals increased, and the adsorption of Pb²⁺ and Ni²⁺ reached almost 100% [9]. In addition to this, nZVI can also convert Cr⁶⁺ in soil into Cr³⁺, thereby reducing the toxicity to the soil [10]. However, the widespread application of nZVI can lead to the release of a large number of nZVI particles into the environment through different pathways, posing a potential risk to the local ecological environment ^[11][12][13][11,12,13]. At present, it has been shown that nanomaterials possess the ability to be ingested by organisms or inhaled into their bodies ^[14][15][14,15]. Microorganisms, which are key players in many basic ecosystem processes, will be the first to be exposed to the nZVI particle environment. When Yoon et al. [16] studied the toxicity of modified nZVI, they found that high concentrations of Bi-nZVI and CMC-nZVI (greater than 100 mg/L) had strong toxic effects on Escherichia coli and Bacillus subtilis in soil. Chaithawiwat et al. [17] studied the toxicity of E. coli after nZVI staining at 10–70 nm particle size and 1000 mg/L concentration. The results showed that the toxicity of nZVI was more serious during the exponential growth phase and decay phase of E. coli, and the toxic effects were strain-dependent. In general, the toxicity of nZVI was positively correlated with its concentration. However, nZVI has also been shown to exhibit stronger toxicity to Agrobacterium sp. PH-08 at low concentrations after 2 h of contact. [18]. Therefore, in order to solve the problems caused by the long-term coexistence of nZVI particles with ecosystems, it is necessary to focus on the effects of nZVI on indigenous microorganisms ^[19][20][19,20].

Although some studies have described the toxicological effects of nZVI on plant cells [21], animal cells [22] and microbial cells [23], few studies have been reported on the toxicological effects of nZVI on bacteria and fungi in soil. Therefore, rethisearches paper firstly outlines the achievements of nZVI in practical applications, summarizes the progress of research on the toxic effects of nZVI on microorganisms, then summarizes the mechanisms of toxicity and influencing factors in the past 10 years at home and abroad, and finally points out the problems that still exist in current nZVI toxicity research, aiming to make some references to subsequent soil microbial toxicology research on nZVI.

With the development of urban industrialization and intensive agriculture in various countries around the world, organic and inorganic contaminants (organochlorine substitutes, heavy metals, nitrates, etc.) are spread into the soil without treatment or improper treatment. Soil pollution is insidious and long-term, and toxic and harmful substances will have a serious impact on the ecosystem after infiltrating into the soil through the material cycle [24]. A large number of studies have shown that nZVI materials, especially modified nZVI materials, have been widely used in remediation of contaminated soil due to their strong adsorption capacity, strong reducibility, low price and no secondary pollution.

Halogenated organic pollutants mainly include fluorinated, chlorinated and brominated pollutants, which are highly persistent and difficult to biodegrade, and also have toxic characteristics and can be a serious ecological hazard if they enter the environment. Stabilized nZVI has been commonly studied for the reductive dechlorination of chlorinated organic pollutants. Polychlorinated biphenyls (PCBs) are widely used in industrial production. Due to their high hydrophobicity and volatility, the natural attenuation of PCBs in soil is usually very slow and tends to accumulate in organic matter-rich soils ^[25][26][25,26]. Sun et al. [27] prepared montmorillonite-loaded nZVI (MMT-nZVI) and proved that it could induce a non-homogeneous Fenton reaction to degrade 2,3,4,5-tetrachlorobiphenyl (PCB67) in soil. Up to 76.38% degradation of PCB67 was achieved in an 80 min reaction with 45.99 g/kg H₂O₂, 29.88 g/kg MMT-nZVI and an initial pH of 3.5. DBDPE is the main material for the production of electronic products, plastics and textiles, and its unique degradability and thermal stability make DBDPE more likely to accumulate in soil [28]. Lu et al. [29] synthesized biochar-loaded nZVI (BC/nZVI) particles from bagasse at 600 °C under nitrogen protection and used them for the removal of DBDPE from soil systems. The results showed that the removal efficiency of BC/nZVI was up to 86.91% at a mass ratio of 2:1 (BC:nZVI) for 24 h of treatment. Nitrate pollution is one of the typical soil pollutants. Excessive use of nitrogen fertilizers in agriculture has led to an increase in nitrate nitrogen content in the soil, causing excessive enrichment of vegetables, and the excess nitrate entering the human body is converted to nitrite, thus increasing the risk of cancer. Zeng et al. [30] loaded nZVI on NaY zeolite and achieved nearly 100% nitrate removal after 6 h of reaction with nitrate at a dosing rate of 4 g/L, and the composite also removed more than 94% of nitrate at pH values up to 9.0. In addition, nZVI can not only degrade NO³⁻ by chemical reduction in soil remediation, but can also act as an electron donor in combination with microorganisms or coupled with bioelectrochemical system to degrade nitrate by the denitrification process [31]. The reaction process of nZVI degradation and transformation of organic pollutants and nitrates such as organic halides is shown in Figure 1. Novel contaminants in soil are not explicitly regulated by laws and regulations in terms of concentration, but are a real and present risk of environmental contamination. Chloramphenicol (CAP), a chlorinated nitroaromatic antibiotic, is an effective antibacterial drug that is widely used in a large number of developing countries due to its availability and low cost. They can pose a threat to ecosystems and human health through the production of resistant bacteria [32]. Liu et al. [33] degraded chlorinated nitroaromatic antibiotic chloramphenicol (CAP) with nZVI and showed that the degradation process can be divided into two stages: a rapid reduction of oxygen atoms in the NO₂ group, followed by a dechlorination reaction. The degradation rate of 0.3 mM CAP in the first stage (1.8 mM of nZVI, pH 3, 6 min) was up to 97%, and the reaction resulted in a great reduction in CAP concentration.

Soil is an interface matrix between different substances (e.g., gas, solid, water, organic/inorganic components) and organisms, with multiple layers and complexity. The unique reaction characteristics of nZVI particles with pollutants in soil ensure the advantages of nZVI in situ remediation of contaminated soil. Figure 2 shows the schematic diagram of nZVI in situ remediation of contaminated soil. It shows the mechanism of nZVI from preparation to delivery to remediation of contaminated soil and the process of final conversion to iron oxide or hydroxide. With the wide application of nano-zero-valent iron and its composite materials in remediation of contaminated soil, more and more scholars have started to pay attention to the environmental safety and toxicological research of nano-zero-valent iron materials [34]. Soil microbial properties are also one of the indicators of soil quality impact. In practice, the toxicity of nZVI to soil microorganisms is influenced by several factors (such as the nature of nZVI, microbial tolerance, soil characteristics, etc.). Saccà et al. [35] compared the effects of nZVI on the community structure of two different types of soil and reported that in fertile soil, the number of cello flavin bacteria and Firmicutes decreased and actinomycetes increased, while the higher the sand content in the soil, the lower the number of α-Proteobacteria and β-Proteobacteria. Fajardo et al. [36] found that the most significant changes in microbial community structure were observed in nZVI-treated lead-contaminated soil. The percentage of cells belonging to the β-Proteobacteria subclass increased to 21.8%, while in the ε-Proteobacteria subclass, the percentage decreased to 6.1%. Vanzetto et al. [37] evaluated the potential toxic effects of nZVI on the growth of Bacillus cereus and Pseudomonas aeruginosa colony-forming units (CFU) in hexavalent chromium (Cr⁶⁺) and pentachlorophenol (PCP) nanoremediation contaminated soil. The results showed that nZVI in Cr⁶⁺ and PCP-contaminated soil had no toxic effect on the population of native soil bacteria. At 90 days after nZVI injection, the mean value of CFU was statistically equal, with the lowest coefficient of variation and the highest level of CFU. This is because the strains of B. cereus and P. aeruginosa are resistant to the concentrations of nZVI, Cr⁶⁺ and PCP, and did not cause significant disturbances in temperature, conductivity, pH and humidity over time. Furthermore, nZVI is not solely toxic to microorganisms in soil aquifers. It has been shown that nZVI inhibits the physiological activity of some microbial groups (e.g., β- and γ-proteobacteria) but promotes the growth of others (e.g., archaea, LGC Gram-positive bacteria and α-proteobacteria) ^[38][39][38,39]. The reason for this is not yet known to the academic community, but a generally accepted view is that the hydrogen produced by nZVI corrosion promotes the growth of bacteria that use hydrogen as a respiratory electron donor. Interestingly, the rapid injection of nZVI into the contaminated soil is able to promote the growth of microorganisms in the original soil to a certain extent for a period of time, and one explanation says that this is because nZVI restores some of the physicochemical properties of the contaminated soil rather than the reaction of microorganisms with nZVI ^[38][40][41][38,40,41].