Potential environmental implications of nanoscale zero-valent iron particles for environmental remediation
Article information
Abstract
Objectives
Nanoscale zero-valent iron (nZVI) particles are widely used in the field of various environmental contaminant remediation. Although the potential benefits of nZVI are considerable, there is a distinct need to identify any potential risks after environmental exposure. In this respect, we review recent studies on the environmental applications and implications of nZVI, highlighting research gaps and suggesting future research directions.
Methods
Environmental application of nZVI is briefly summarized, focusing on its unique properties. Ecotoxicity of nZVI is reviewed according to type of organism, including bacteria, terrestrial organisms, and aquatic organisms. The environmental fate and transport of nZVI are also summarized with regards to exposure scenarios. Finally, the current limitations of risk determination are thoroughly provided.
Results
The ecotoxicity of nZVI depends on the composition, concentration, size and surface properties of the nanoparticles and the experimental method used, including the species investigated. In addition, the environmental fate and transport of nZVI appear to be complex and depend on the exposure duration and the exposure conditions. To date, field-scale data are limited and only short-term studies using simple exposure methods have been conducted.
Conclusions
In this regard, the primary focus of future study should be on 1) the development of an appropriate and valid testing method of the environmental fate and ecotoxicity of reactive nanoparticles used in environmental applications and 2) assessing their potential environmental risks using in situ field scale applications.
Introduction
Nanomaterials have attracted considerable attention due to their excellent electrical, optical, magnetic, and catalytic properties [1]. In particular, efforts to use nanotechnology in environmental applications have been grown steadily. Nanoscale zero-valent iron (nZVI) particles are one of the most widely used nanoparticles for environmental remediation because of their ability to degrade a wide range of contaminants [2-4]. Such an increasingly widespread application of nZVI will lead to its release into the environment, and this release is likely to bring about unexpected hazards in various organisms [5]. A variety of nZVI toxicity mechanisms toward organisms have been investigated and the results indicate that toxicity might be caused by 1) direct nZVI association with biological components [6], 2) oxidative stress compounds generated by nZVI in the aqueous phase [7] or 3) ferrous ion released from nZVI followed by the fenton reaction [8]. Despite the numerous studies that have been conducted on nZVI toxicity, there is a critical gap in our knowledge on the environmental risks of nZVI, including ecotoxicity as well as the fate and transport of nZVI in natural and engineered environments. For this reason, we present an overview of current research findings related to the environmental implications of nZVI particles. We also provide insight into current knowledge gaps and give pointers for future research directions.
Environmental Applications of Nanoscale Zero-valent Iron Particles
Macro-scale zero-valent iron (ZVI) has been recognized as a good electron donor with a property to release electrons in aquatic environments [2]. ZVI has been used as a reactive material in subsurface permeable reactive barriers to degrade groundwater pollutants since the early 1990s [3]. ZVI is very active in transforming of halogenated compounds, polychlorinated hydrocarbon pesticides and dyes [4].
The nZVI has significantly increased available reactive surface areas compared to larger sized iron particles, which consequently enhances contaminant degradation reactions [9,10]. Moreover, one benefit of nZVI is the ability to inject it directly into a contaminated aquifer [11]. Therefore, the use of nZVI to remediate soil and groundwater has increased within the last decade. Figure 1 illustrates a schematic diagram of the applications of nZVI for site remediation.
Figure 2 presents a schematic structure of nZVI and the major reaction mechanisms with environmental contaminants. In aqueous solutions, nZVI can react with dissolved oxygen and water to form an outer iron oxide/hydroxide layer, which results in a typical core-shell structure [4,6]. The high reactivity and surface area of nZVI particles lead to strong reducing capacity and bring about the degradation of chlorinated compounds, heavy metals, radionuclides, organic dyes, pesticides, inorganic anions, polychlorinated biphenyls and pharmaceutical products [4,12-15]. The most common process identified in reductive dehalogenation on the iron surface are hydrogenolysis and reductive elimination (α or β) [6]. The reaction of nZVI with inorganic contaminants involves reduction and adsorption/precipitation [6]. Recently, surface modification and reuse of nZVI to increase durability, reactivity and mobility were also investigated [11,15]. Table 1 shows the representative results of the remediation of organic and inorganic materials by nZVI.

Schematic structure of nanoscale zero-valent iron and the main reaction mechanisms with environmental contaminants. Fe, iron; R-Cl, chloroalkane; R-H, hydrocarbon; Men+, metal ions.
Environmental Implications of Nanoscale Zero-valent Iron Particles
Ecotoxicity
Despite the increasing use of nZVI particles and concerns for their potential toxic effects on both water and soil organisms, only a limited number of studies have investigated the ecotoxicity of nZVI. In addition, convincing evidence of the ecotoxicity of nZVI has not yet been obtained, because this likely depends on the organism species, composition, concentration, size and surface properties of the nanoparticles and the experimental method used. Table 2 summarizes the currently published ecotoxicological studies on nZVI.
Bacteria
Compared with other organisms, the potential toxic impacts of nZVI on microorganisms have been considerably studied. Several studies have shown that nZVI is toxic to microorganisms, such as Bacillus cereus, Pseudomonas stutzeri, and Escherichia coli [16-18]. The specific mode of action of nZVI appears to be through oxidative stress from reactive oxygen species (ROS) generation that causes oxidative damage to cells via lipid peroxidation and oxidation of thiol groups on proteins and DNA. The toxicity of nZVI under oxic conditions is significantly lower than under anoxic conditions, which is related to the formation of an iron oxide layer resulted from the surface oxidation. This is supported by recent studies [19,20] which showed decreased bactericidal effects through the oxidation of nZVI under aerobic conditions.
Xiu et al. [21] investigated the potential influence of nZVI on the community of indigenous microorganisms that participate in the remediation of trichloroethylene-contaminated sites. They observed that nZVI initially inhibited dechlorinating bacteria, however the populations were able to recover after a lag time. In addition, the authors concluded that nZVI caused the stimulation of H2 production, which can be used as an electron donor by methanogens and dechlorinating bacteria. These findings are also supported by other studies in which the addition of nZVI created significantly more reduced conditions, and stimulated remediation efficiency of microbes [22,23].
The effects of surface coatings were studied by Li et al. [19] in which surface coated nZVI (e.g., polystyrene sulfonate, polyasparatate, and natural organic matter [NOM]) significantly mitigated the adverse effect to E. coli as a result of reduced interaction between particles and organisms. Carboxymethyl cellulose (CMC) stabilized nZVI exerted a minimal oxidative stress response and slowed disruption of cell membrane integrity, resulting in mitigated cytotoxicity towards bacteria Agrobacterium sp. PH-08, as compared with the uncoated nZVI [24]. Furthermore, Chen et al. [13] also investigated the role of NOM in the microbial toxicity of nZVI, concluding that surface modification of nZVI might alter physicochemical interactions with organisms and influence toxicity and bioavailability, likely due to electrosteric hindrance effect.
In the same study, the effect of nZVI on different microbial species was investigated. The authors concluded that B. subtilis (Gram positive) was more tolerant to nZVI than E. coli (Gram negative) due to the thicker Gram positive cell wall. Similarly, Němeček et al. [25] also observed a significant increase in the density of Gram positive bacteria after in situ application of 2 kg/ton of nZVI. In summary, the effect of nZVI on bacteria is variable and depends on not only the species but also on the physicochemical properties of nZVI.
Terrestrial Organisms
Soil organisms including earthworms and plants also have a chance to be exposed to nZVI, at least around injection areas. However, there are only a very limited number of studies which have investigated the effects of nZVI on terrestrial organisms (i.e., earthworms, invertebrates, and plants) (Table 2). El-Temsah et al. [32] evaluated the ecotoxicological effects of nZVI on Eisenia fetida and Lumbricus rubellus. This work demonstrates a negative impact of nZVI on both earthworm species, affecting avoidance, weight changes and mortality (>500 mg/kg) and leading to a reduced reproduction rate (>100 mg/kg). In the other published study which investigated the nZVI effects on earthworms, however, the exposure of C. elegans (strain N2, first CMC, lethal concentration 50, ROS juvenile stage) to 17 mg/g nZVI did not cause any mortality after 96 hr of exposure [33].
Several negative effects of nZVI on soil invertebrates including ostracods (Heterocypris incongruens) and collembola (Folsomnia candida) were observed after 7 days of incubation [34]. These few studies indicate that nZVI can significantly affect terrestrial organisms and can even lead to their death. Considering that the use of nZVIs for environmental remediation is mainly focused on contaminated soils and groundwater, these results are very important and reinforce the need for more detailed and structured studies.
Aquatic Organisms
Although aquatic organisms have little chance to directly contact with nZVI during environmental remediation, nZVI could be flowed in surface water via groundwater discharge, resulting in exposure to aquatic organisms. In an aquatic system, Chen et al. [36,37] investigated the effects of nZVI on acute lethality and oxidative stress in medaka fish (Oryzias latipes). The results indicated that nZVI caused a disruption in the oxidative defense system in embryos and larvae, as well as acute lethality in embryos at concentrations above 100 mg/L. Keller et al. [39] investigated the effects of Daphnia magna and concluded that D. magna survival was dramatically impacted by both bare and surface- coated nZVI, depending on the concentration of nZVI. Great variability in the effects of nZVI on marine and freshwater phytoplankton was observed by Keller et al. [39] and Kadar et al. [40]. For example, the population gowth of the marine phytoplankton species Isochrysis galbana was significantly reduced by organic surface coated nZVI (Nanofer 25S supplied by NANO IRON s.r.o., Rajhrad, Czech Republic) and not affected by inorganic surface coated nZVI (Nanofer STAR supplied by NANO IRON s.r.o). In contrast, the growth of freshwater phytoplankton species Pseudokircheneriella subcapitata was not significantly affected by organic surface coated nZVI and affected by inorganic surface coated nZVI. These studies indicate that there is a possible impact of nZVI on aquatic organisms that is dose- and species-dependent. Therefore, further detailed studies into the effects of nZVI on aquatic organisms should be made before environmental application.
Environmental Fate and Transport
In this review, an overview of the ecotoxicological effects at various concentrations of nZVI in laboratory studies has been provided, but the concentrations and physicochemical properties of nZVI during laboratory studies may not be in accordance with the properties of nZVI under real environmental conditions. Because the environmental fate and transport of nZVI is not yet fully understood, it is difficult to determine the environmental risk of nZVI injected into the subsurface.
The mobility of nZVI has been considerably studied for the purpose of their effective applications. The effect of coating materials to enhance the particle mobility has been predominantly investigated through column transport tests at laboratory scale [41-47]. The enhanced mobility of surface coated nZVI is likely caused by the electrosteric stabilization of polymer molecules which prevent the formation of large aggregates and attachment to the surface soil grains. Tiraferri and Sethi [42] studied the effect of surface coating on the transport of nZVI in a column packed with sand, comparing the mobility of bare nZVI and that of surface modified nZVI with guar gum. They found that bare nZVI was basically immobile in sandy porous media. However the particle mobility was significantly enhanced with guar gum at the tested conditions, regardless of the chemistry of the solutions (ionic strength and ionic composition). The enhanced transport of nZVI in saturated porous media was also observed with polyacrylic acid [43], xanthan gum [44], non-ionic surfactants [45], organic matter [46], CMC [47], and other substances.
Transformation (aging) of nZVI has been observed over a prolonged period, i.e., from a few days to years [48,49]. The rate and degree of oxidation and the type of transformation products are dependent on the environmental conditions to which the particles are exposed. Reinsch et al. [50] investigated the transformations of nZVI over 6 months in simulated groundwater and suggested that dissolved oxygen rapidly oxidizes nZVI, causing the formation of both maghemite and magnetite within the oxide layer. In addition, the presence of common groundwater anions (i.e., SO42-, HCO3-, HPO42-, and Cl-) does not prevent the oxidation of nZVI in the long term, except for nitrate, which passivates the surface, thereby preventing the oxidation of nZVI. The degree of oxidation and chemical composition could further affect overall the fate, transport, reactivity, and potential toxicity of nZVI. For instance, the oxidation of nZVI can decrease the particle-particle interaction and thus increase the transport, which indicates the increase in the potential for unwanted exposure. However, the adverse effects of nZVI on environmental organisms can decrease with increasing oxidation of the particle. Therefore, understanding the transformation of nZVI upon release to the environment is critical to evaluating the potential efficiency as well as risk of nZVI used for environmental remediation
Conclusion
While nZVI has substantial promise for many remediation applications, the environmental implications are still poorly understood. In particular, standard methods for studying the environmental fate, ecotoxicity, and transport of nanomaterials have not been developed yet. The physical-chemical properties of nanomaterials are substantially different from traditional organic and/or inorganic chemicals. In addition, the properties could be changed by sample preparation, choice of media, dispersant use, presence of environmental ligands, and other factors, and hence these factors easily affect nanoparticle toxicity and behavior. Importantly, the existing test methods (i.e., Organization for Economic Co-operation and Development test guidelines) have been developed based upon general chemical properties and thus do not consider the specific properties of nanomaterials. In this respect, the development of new test methods to assess the environmental fate and ecotoxicity of reactive nanomaterials is critical to conclusively determine the risks associated with nZVI.
The fate, transport, and toxicity of nZVI particles appear to be complex and are dependent on their physicochemical properties (e.g., size, chemical composition, surface charge, and coating) and environmental conditions (e.g., oxygen level, pH, ionic strength, and organic content). However, the fate and toxicity of nZVI in real site conditions has not yet been studied in detail and current toxicological data are mainly based on short-term tests using simple exposure methods. Therefore, the long-term evaluation of nZVI in pilot- and full-scale systems under realistic operation conditions needs to be addressed in order to prevent any potential environmental risks
Acknowledgements
This work was supported by “The GAIA Project” by the Korea Ministry of Environment (RE 201402059) and “Environmental Risk Assessment of Manufactured Nanomaterial” Project by the Korea Institute of Toxicology (KK-1403-02).
Notes
The authors have no conflicts of interest with material prsented in this paper.