Zero-valent iron has been reported as a successful remediation agent for environmental issues, being extensively used in soil and groundwater remediation. The use of zero-valent nanoparticles have been arisen as a highly effective method due to the high specific surface area of zero-valent nanoparticles. Then, the development of nanosized materials in general, and the improvement of the properties of the nano-iron in particular, has facilitated their application in remediation technologies. As the result, highly efficient and versatile nanomaterials have been obtained. Among the possible nanoparticle systems, the reactivity and availability of zero-valent iron nanoparticles (NZVI) have achieved very interesting and promising results make them particularly attractive for the remediation of subsurface contaminants. In fact, a large number of laboratory and pilot studies have reported the high effectiveness of these NZVI-based technologies for the remediation of groundwater and contaminated soils. Although the results are often based on a limited contaminant target, there is a large gap between the amount of contaminants tested with NZVI at the laboratory level and those remediated at the pilot and field level. In this review, the main zero-valent iron nanoparticles and their remediation capacity are summarized, in addition to the pilot and land scale studies reported until date for each kind of nanomaterials.
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In this work, the recent advances on use of zero-valent iron nanoparticles-based technologies for soil and groundwater remediation are reviewed. The main types of zero-valent iron nanoparticles used in nanoremediation have been described. In addition, an especial attention has been made in the review of those studies carried out at pilot or full scale for all described nanoparticulate system.
However, due to the complex nature of most contaminated soils and the fact that contamination is often caused by the presence of a mixture of contaminants, the application of more than one remediation technique is, in many cases, required to reduce the concentration of contaminants to acceptable levels [ 12 ].
On the other hand, on site method required of the excavation of contaminated soil before its treatment, and placed in adequate container or tanks where the treatment will be carried out. After the treatment, the soil will be replaced to its original site [ 4 , 11 ].
Faced with the unconcern and uncontrolled spills of past times, in recent decades soil pollution has been raised as a serious concern due to the great importance of preserving soil quality for ecosystems and human health. The most widely used remediation methods are usually based on two main methodologies, in situ or ex situ remediation [ 3 , 4 ]. Both in situ and on site remediation could be performed by different methods such as solidification and stabilization, oxidation, soil vapor extraction, bioremediation, or nanoremediation [ 4 , 5 , 6 , 7 ]. On one hand, in situ soil treatment is a method in which the contaminated soil is treated without removing it. This method is especially interesting because it minimizes the alteration of characteristics such as soil structure and integrity [ 8 , 9 ]. However, this method, frequently, presents a lower remediation potential, being often considered time-consuming and presenting many uncertainties during the process. Also, considering the potential risks, environment and/or human health, this technique could not be suitable for its application at certain sites [ 10 ].
Soil pollution is an arising concern worldwide; it could be defined as the presence of contaminants, persistent toxic compounds, and hazardous substances, in soil. These pollutants must be present in the soil in a concentration beyond a threshold limit, being this limit the concentration beyond which can be injurious or harmful for human and animal health and plant growth [ 1 ]. Soil contamination can be caused due to several factors like improper management of urban and industrial waste, chemical spillage, commonly, due to industrial activity, and excessive usage of fertilizers and pesticides in agriculture [ 2 ].
Initially, granular iron was used, mainly as a permeable reactive barrier (PRB) for chlorinated hydrocarbons, metals and metalloids (arsenic, chromium, uranium, etc.) [ 20 , 21 ], nitroaromatics [ 22 ] or perchlorates, among others [ 17 , 23 , 24 , 25 , 26 , 27 ]. However, the greater specific surface area of zero-valent nanoparticles has encouraged their use, as compared to conventional iron powder or iron filings [ 28 , 29 ]. Zero-valent iron has been successfully used for soil and groundwater remediation, being the PRBs developed with ZVI effective systems to limit the migration of contaminants. However, this method present several limitations since it is restricted by construction limitations of PRBs and it is not capable to target contaminant source zone [ 30 ]. In this context, many studies have shown the effectivity of nanoscale zero-valent iron (NZVI) in the last decades.
Zero-valent iron is inexpensive, non-toxic and a moderate reducing reagent (standard reduction potential E 0 = &#;0.44 V). In presence of oxygen dissolved in water, zero-valent iron is capable to oxidize organic pollutants. In a first reaction, ZVI reacts with O 2 to produce H 2 O 2 (Equation (1)). Consequently, formed hydrogen peroxide is reduced to water by ZVI (Equation (2)) or can react Fe 2+ , Fenton reaction, producing (hydroxyl radicals (·OH) (Equation (3)). It is important to notice that this last reaction, Equation (3), could degrade a considerable amount of organic contaminants due to its strong oxidizing capability.
The use of zero-valent metals for environmental applications was first described in [ 13 ]. Years later, the degradation of trichloroethylene (TCE) in the presence of several metals, mainly zero-valent iron (ZVI), was demonstrated. This was considered the starting point of numerous subsequent studies in this area, beginning with the use of zero-valent metals for the remediation of groundwater contaminated with volatile organic chlorides (VOCl) [ 14 , 15 , 16 ]. As an example, degradation of different halogenated aliphatic hydrocarbons with NZVI was carried out [ 17 ] and the degradation mechanism of tetrachloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and trans-dichloroethylene (trans-DCE) was reported by Arnold et al. [ 18 ]. Nitrate concentration is also reduced in presence of bare NZVI [ 19 ].
Zero-valent iron nanoparticles (NZVI) are more effective than macroscale ZVI, iron powder or iron filings, under similar environmental conditions [31,32,33]. Indeed, considering the exponential relationship between the specific surface area and radius of a nanoparticle, the increase on the particle size compared to microparticles increases the surface per gram several orders of magnitude [34]. The properties of the NZVIs that provide them with a great attraction for use in remediation in situ are their great reactivity towards the different families of pollutants. The reactivity of zero-valent iron is based on its ability to oxidize to ferrous or ferric iron that provides electrons available to reduce other compounds that, through the Fenton reaction, produce strong oxidants capable of reacting with contaminants making them harmless [35]. This allows addressing the decontamination in most of heterogeneously contaminated sites. The nanometric size improves the mobility through the porous medium and the low toxicity of NZVI increasing the remediation process while preserves the characteristics of the soil, so that the subsequent application of other processes such as bioremediation that can complement the treatment is not compromised. In addition, it must be noticed that the few full-scale tests have resulted in a successful remediation of main organic pollutants. This remediation technology involves a series of steps for NZVI [30] based in the transport of the nanoparticles to the area (usually in aqueous phase), and reaction with the target contaminant to form less toxic or less mobile products.
In the last years and decades, the development of nanosized materials has facilitated the application of remediation technologies based on highly efficient and versatile nanomaterials [36,37,38,39]. Among the possible nanoparticulate systems successfully used on a laboratory scale for soil decontamination, zero-valent iron nanoparticles (NZVI) have achieved very interesting and promising results ( ).
In fact, many studies have already corroborated the efficiency of NZVI for the remediation of contaminated groundwater and soil [29,66,67]. Moreover, nanoremediation by using zero-valent iron is the most common used method for soil and groundwater remediation both in Europe and in the United States [68]. The enhanced reactivity of the NZVIs and their high mobility allow the performance of in situ treatments through the injection of nanoparticles. These results suggest highly advantageous method for pollution remediation since their application does not specifically involve previous excavation of the soil or pumping of the groundwater [69,70]. Nanoremediation treatment commonly starts with the application of highly concentrated NZVI slurries by injection at or near the contaminated area. NZVI should be applied or attach to soils in the contaminated zone and react with the target contaminants to form less toxic or less mobile products [30].
As an example, degradation of different halogenated aliphatic hydrocarbons with NZVI was carried out [17] and the degradation mechanism of tetrachloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and trans-dichloroethylene (trans-DCE) was reported by Arnold et al. [18]. Nitrate concentration is also reduced in presence of bare NZVI [19]. summarized various possible mechanism for the degradation of chlorinated pollutants and metals.
The strong attractive forces between NZVI, mainly magnetic interactions could induce the agglomeration of the nanoparticles forming micro sized aggregates, this could reduce mobility and therefore the effectiveness of the treatment [71,72]. This low colloidal stability is even worst under environmental condition reducing significantly their applicability [73]. In order to overcome these limitations and enhance their in situ performance new types of NVZI systems have been developed. Nowadays, zero-valent iron nanoparticles used for soil and groundwater remediation can be classified in three main groups ( ): (A) Bimetallic iron-based nanoparticles (BNP), (B) emulsified iron nanoparticles (EZVI) and (C) polymer coated NZVI, in which the polymer increases suspension stability and particle mobility [74].
Another iron nanoparticle-based product to be highlighted for environmental remediation is emulsified zero-valent iron (EZVI) [103]. The aim of this kind of systems is to deliver NZVI in an oil&#;water emulsion, which eases the transportation into the contaminated zones and reduces the NZVI&#;s degradation [104]. EZVI is a surfactant-stabilized, biodegradable emulsion that forms emulsion droplets consisting of an oil&#;liquid membrane surrounding zero-valent iron (ZVI) particles in water. These emulsions are able to degrade chlorinated hydrocarbons [105]. EZVI can be fabricated from ZVI (macroscopic) to microscale or nanoscale, or as a combination of both. The use of formulations in which micro- and nanoparticles are combined reduces the cost of the materials without losing the benefits provided by the nanoscale iron, since the microparticles are less expensive [74]. The outer oil membranes are hydrophobic, making them similar to some common contaminants such as DNAPL (dense, non-aqueous liquid phase) or TCE, so that the EZVI droplets are miscible with these contaminants. When the emulsion drops are in close contact with TCE, they are mixed and then, TCE is diffused inside the droplet where, in contact with the zero-valent iron, is degraded. A concentration gradient is established due to the diffusion of TCE inside the drop and the subsequent migration of the reaction by-products to the surrounding aqueous phase, thus improving the degradation process [105]. In addition, some studies report that the use of vegetal oil for this kind of emulsions can improve biodegradation processes [106,107].
EZVI could be considered an environmental friendlier approach compared to the bare NZVI and BNP. The encapsulation of the NZVI on biodegradable oil improve the mixture of EZVI with DNAPLs reaching the organic pollutants on groundwater or water flows that could be difficult to access with other technologies [108].
EZVI has been used to clean up contaminated soil and groundwater in several locations ( ). In a field experiment performed at Parrick Island (SC, USA), PCE and TCE concentrations were reduced by the application of EZVI using two different delivery methods: pneumatic injection and direct injection. A significant decrease in groundwater PCE (>85%) and TCE (>85%) concentrations was reported. However, the authors expressed their concern about the efficiency of these methods, since they detected uncertainties in the estimations due to a possible mobilization of DNAPL during and after the EZVI injection process [109]. Often, a compromise between the advantages and disadvantages of the remediation technology is required. For example, the excess on the contaminant mobility induced by EZVI could be reduced with the optimization of the emulsion components, usually surfactants, or complementarily pumping out the mobilized DNAPL if the site and the required technology are compatible.
In addition, comparing EZVI technology with the BNP, the first one presents several advantages refereed to the cost and the homogeneity of the reagents. The best of our knowledge the fabrication of the BNPs still very limited due to the cost and synthetic limitations that prevent them of being massively fabricated.
In a soil and groundwater remediation initiative developed at Cape Canaveral Air Force Station (FL, USA), O&#;Hara and co-workers reported and effective contaminant reductions when ENZVI was applied to DNAPL. The concentration in TCE in soil was reduced to more than 80%, whereas TCE concentration in the groundwater was reduced by 60% [105]. In , the concentration contours of TCE in groundwater of shallow wells could be observed in the pre- and post-demonstration carried out on Cape Canaveral Air Force Station, Florida, showing reported reduction. Similarly, in a field test performed in an industrial site at Patrick Air Force Base (FL, USA), an initial TCE concentration of 150,000 μg/L was reduced to μg/L by a treatment with EZVI introduced by high pressure pneumatic injection [110].
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Nanoparticles present a high reactivity due to their large surface area, being this characteristic crucial for the rapid degradation of contaminants, compared to zero-valent iron or microparticles [74]. In spite of their effectiveness as a decontaminant agent, NZVIs present some weaknesses including a lack of stability, their rapid passivation and limited mobility, since the nanoparticles tend to aggregate rapidly in water solution. In addition, zero-valent iron has a high affinity for oxygen. This tendency to oxidize rapidly causes a passivation of the nanoparticles in contact with the air or in aqueous medium [73]. In order to reduce these problems, different polymer coatings have been used as a strategy to protect the nanoparticles against oxidation and promote a greater degree of dispersion. Indeed, polymer-stabilized nanoparticles present a higher stability in aqueous suspension and a better soil transportability than non-coated NZVIs [29,112]. Nanoparticle stabilization increases the remediation capability of the NZVIs.
Since the polymer coating or stabilizer are in charge of enhance the colloidal stability of NZVI, their adequate selection is also an extremely important factor. The intrinsic properties of the polymers must be considered being the biocompatibility and/or biodegradability extremely important in order to do no increase the environmental problems. In addition, the presence of polymers, mainly of biopolymers, could enhance the biodegradation since they serve as an additional nutrient source for microorganism [113]. In recent years, biodegradable coatings have been incorporated to NZVI surfaces in order to improve the dispersion of the nanoparticles, increase their stability, and protect the reactive centers until contact with the target contaminant. A great effort has been made to develop effective polymer coatings to maximize the remediation capability of NZVI. In general, the higher surface reactivity and the strong interparticle interaction of bare NZVIs make their coating difficult since not many polymer could meet the specific requirement to guarantee their good dispersability, compared to other nanoparticles. Various surfactants and polymers have been successfully used as stabilizers of NZVIs and, according to the remediation results, coating with polymers, both natural and synthetics, lead to improved remediation results [29]. Polymer coatings not only prevent the aggregation of the nanoparticles, but also, in some cases, they can also serve as a food or energy source for microorganisms involved in bioremediation processes. NZVI coating has been developed by using synthetic polymers such as poly (N-vinylpyrrolidone) (PVP), polystyrene sulfonate (PSS), polyacrylic acid (PAA) and its derivatives, or carboxymethycellulose (CMC), among others ( ). In addition, some biopolymers have been used as nanoparticle coatings such as starch, Xanthan gum or guar gum [114,115,116]. Polymer-stabilized zero-valent nanoparticles have been extensively studied for the remediation of both organic and inorganic pollutants ( ). Even more, several studies have reported a successful remediation of chlorinated hydrocarbons (TCBs, PCE, TCE, DCE, VC, DCA, and lindane) and inorganic contaminants in soil and groundwater [117,118].
Open in a separate windowMany formulations of synthetic polymers have been used for coating NVZI, being the most of them polyelectrolytes and few neutral polymers. Negatively charged polyelectrolytes are used since they are capable to form a polyelectrolyte layer that induce strong electrostatic repulsions [124,131]. Poly (acrylic acid), polystyrenesulfonate, polyoxyethylene sorbitan monolaurate, polymethacrylic acid, and di-/triblock copolymers have been used as NZVI coatings and tested against different pollutants. TCE [66] and lidane [64] have been degraded with PAA-coated nanoparticles. The use of other anionic polyelectrolytes such as polystyrenesulfonate (PSS) significantly decreases the aggregation degree and, in consequence, improves the diffusion of the particles through the medium [125,132]. Sirk et al. studied the effect of the coating with different block copolymers based on poly (methacrylic acid) (PMMA), poly (methyl methacrylate) or PSS, among others [124]. From their test in a soil model, they concluded that the electrostatic repulsion between the polyelectrolyte-coated NZVI and the negatively charged soil surfaces reduce the adhesion and therefore enhanced the mobility of the nanoparticles through the soil. Similarly, triblock copolymers had been studied as NZVI coatings. Saleh and co-workers analyzed the effect of amphiphilic triblock copolymer coating PMAA-b-PMMA-b-PSS [126]. The polymer layer was physisorbed on the nanoparticles&#; surface and promoted the colloidal stability of the NVZIs. The evaluation of these nanoparticles on a model soil indicated good mobility. Moreover, they can be absorbed in oil&#;water interface improving their capacity to reach chlorinated pollutants in order to degrade them. In another example of triblock copolymers, polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A) copolymer was used as a nontoxic and biodegradable coatings of NZVI [119]. This coating improved several properties such as surface chemistry and particle stability, and therefore NVZI&#;s mobility through the soil. In addition, the study demonstrates an effective removal of TCE. Finally, it is important to notice that no sedimentation of these nanoparticles were observed for over 6 months.
In addition to polyelectrolytes, neutral synthetic polymer have been used for the fabrication of stabilized NZVIs. For example, neutral polyethylen glycol (PEG) and polytetrahydrofuran (PTHF) have been studied for lindane degradation [65]. Cirtiu et al. comparatively studied colloidal stability of CMC, PAA, PSS, and polyacrylamide (PAM) [123]. The stability of CMC and PAA, both polyelectrolytes with carboxylic functionalities, present very similar stability, being the more stable formulations. PSS present similar stability than PAM, neutral synthetic polymer, which at the same time both of them are 13 times more stable than non-coated NZVI. Among the neutral polymers, polyvinylpyrrolidone (PVP) is the most commonly used synthetic polymer. Several studies indicated good colloidal stability and successful decontamination effect of PVP-coated NZVIs on the removal of TCE and tetracycline (TC), being the dechlorination efficiency for TCE around 85% [120,121,133]. However, Sakulchaincharoen and co-workers described a lower performance of PVP-coated nanoparticles compared to CMC-coated NZVI in TCE degradation rate, however, when the ratio is normalized to the surface area PVP-coated NZVI presents a higher rate [122].
Natural polymer used as NZVI coating could be classified according to the driving effect that induces the stabilization as polyelectrolytes and viscosity modifiers. For example, CMC is adsorbed to the nanoparticle forming a negatively charged layer that promotes electrostatic repulsions with the surrounding nanoparticles. These could be used in a porous medium, manipulating their range by modifying the pressure and flow of the injection [134,135]. A comparative study of NZVI coated with PSS, polyaspartate (PAS) and CMC was reported by Kim et al. [125]. All the formulations present high stability, being the coating layer disrupted only after 4 months. It is important to notice that the mobility through sand columns of the stabilized nanoparticles after 4 months remains the same as the freshly prepared ones. Similarly, lindane was treated in solution using NZVI coated with PAS and CMC. A complete elimination of lindane in 72 h was reported for all studied coatings [64,65]. Considering the influence on the stability of the CMC and the nanoparticle size, He and co-workers reported CMC coating capable to adapt their nanoparticle size and dispersability as a function of several synthetic variations [100]. This adaptation could significantly improve the applicability of these CMC-coated nanoparticles since they can be adapted to the diversity of conditions in different soils and/or groundwater. Overall, the synthesized formulations present better stability and a 17 fold higher degradation rate than bare-NZVI.
A part form natural polyelectrolytes, among the natural polymers used for NZVI stabilization there are some of them that they can stabilize NZVI&#;s slurry by increasing the viscosity. The viscosity increase of nanoparticle slurry reduced the aggregation and sedimentation of NZVI trapped on it. Comba et al. reported a xanthan gum (XG) stabilized NZVI that maintains its stability for more than 10 days [130]. XG formulations of this study are stable to ionic strengths variation in a range between 6 × 10&#;3 and 12 mM. Similarly, a good stability was observed by Tiraferri et al. for guar gum (GG) gels, their aggregation and sedimentation were reduced and they were stable to a high ionic strength media [136]. In addition, Xue and co-workers studied this kind of stabilization on zero-valent iron nano- and microparticles by using XG and GG formulations and a mixture of both [116]. In their study, formulations obtained by XG or GG present a good stability against the aggregation and sedimentation for few hour. However, when these two biopolymers are mixed the resulting materials present an improved stability of over a day due to the interactions between them.
Some pilot and full-scale tests have carried out by using stabilized NZVI ( ). In Hamilton Township, New Jersey (USA), a remediation strategy based on this nanotechnology showed positive results. The NZVI were injected in two phases and the duration of the test was 30 days. The results showed a decrease in the concentration of chlorinated contaminants of up to 90 percent [32]. There are a large number of new trials at pilot and field scale; some of the most recent are in progress or the results are not known yet. The contaminants most frequently treated by these methods are chlorinated solvents such as TCE, PCE, TCA, and VC. Most of the pilot and full-scale tests have been carried out in USA: for example, soil remediation through the application of NZVI was conducted in the Naval Air Station of Jacksonville (USA) and the Naval Air Engineering Station of Lakehurst (USA). Both areas presented high levels of TCA, DCE, TCE, and PCE. After the trial, contamination levels decreased by 80&#;90% [98].
In Europe, only few full-scale tests have been carried out. In , NZVIs stabilized with poly(acrylic acid) were tested in Bornheim (Germany) to remediate PCE from the aerospace industry. The contamination was spread over an area of several square kilometers, down to a depth of 20 m, and the efficiency of the remediation process was 90% [137]. In addition, 2 years after the application of NZVI, a further reduction in contaminant concentrations was observed. Another test developed in the EU was carried out in the Czech Republic (Horice and Pisecna). In two contaminated areas (7 and 2 km2), 82 injection wells were constructed and 300 kg of NZVI were injected. The results revealed a contamination reduction of 60&#;75 and 90% for Horice and Pisecna, respectively [137].
It is important to notice that, commonly, more than 100kg are used on the remediation of full-scale area of around 2 km2, considering NZVI power ranged $66 to $88/kg [137], the use of stabilized NZVI that present less passivation being more efficient, is a highly attractive alternative. Zhao et al. [29] estimated that starch-stabilized and CMC-stabilized nanoparticles cost ranges from $100 to 120/kg, so considered their hypothetical cost and the increase of the active nanoparticles reaching the contaminant due to their higher stability, it could be considered that this kind of NZVI as an interesting alternative for the soil remediation. In addition, those nanoparticles coated with natural polymers, do not added any potential risk by-product since they are biodegradable
The nZVI particles are, like other nanoparticles, usually smaller than 100&#;nm in diameter. They are highly reactive in oxic and aqueous environments, such as soils, forming an outer shell formed by Fe oxyhydroxides around the Fe(0) core. In order to produce nZVI material that would be easily manageable and applicable, nZVI producers provide forms stabilized by various coatings, e.g., carboxymethyl cellulose, polyacrylic acid, guar gum, etc., or Fe oxyhydroxides in suspension13,14,15. Fast nZVI oxidation can result in reduced reactivity, and surface passivation is thus essential for a successful application of nZVI in certain conditions16. This can be achieved by synthesizing various composites, e.g., with biochar17, zeolites18, etc.
Additionally, the interactions of nZVI with soil organic matter significantly influence its behavior and efficiency, as the evolution of the iron oxyhydroxide shell can be altered by dissolved organic matter in soils19. The actual composition of the Fe oxyhydroxides in the nZVI shell after it is equilibrated with the soil is mainly dependent on (i) the original nZVI coating; (ii) soil physico-chemical parameters, e.g., Eh, pH; (iii) composition of soil microorganisms communities, (iv) nZVI aging in soil; and include mainly the formation of ferrihydrite, magnetite, goethite and possibly lepidocrocite13,14,20, which can have different geochemical and adsorption properties. It is not possible to generalize which Fe oxyhydroxides will be formed in soils after nZVI application, and this highlights the need for further long-term studies involving soils with contrasting properties and microorganism communities.
The physical-chemical properties of engineered nanoparticles are one of the most critical factors controlling their environmental behavior. In this context, the processes at the soil-root interface play a vital role in the behavior of engineered nanoparticles and contaminants21,22. Different parameters such as soil type, organic matter content, pH, Eh, ionic strength, and aqueous chemistry can change aggregation kinetics, transformation, and subsequent behavior of engineered nanoparticles and their efficiency23. Natural organic matter alters its stability through electrostatic and steric interactions. The transformation process of engineered nanoparticles is controlled by a combination of factors, depending on the particles&#; characteristics and the environmental receptors10,24,25. Additionally, the changes in the hydraulic conditions in soils after nZVI application need to be considered26. However, studies describing the mobility of engineered nanoparticles in general, including nZVI, in the soil environment are often contradictory, showing that this area is still open to discussion.
The processes responsible for immobilizing metal(loid)s on the nZVI surface include mainly adsorption and, to some extent, reduction. The nZVI particles can remove various metal(loid)s simultaneously; however, the type of adsorption reaction involved in the process depends on the redox potential of metal species. While metals with a standard redox potential more negative than Fe(0) are rather adsorbed on the Fe oxyhydroxides formed in the nZVI shell, e.g., Cd, Zn, other metal(loid)s with a standard redox potential much more positive than nZVI can be reduced and precipitated when in contact with the zero-valent core, e.g., Cr, As, U, etc., or the combination of the two mechanisms, e.g., Pb, Ni10. The specific retention mechanisms, including the forms of the Fe oxyhydroxides in the shell and metal(loid) speciation after interactions with nZVI can be identified by various solid state analyses, i.e., X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) techniques, such as X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (e.g., HR-TEM)10,14,20. However, recovering and separating the nZVI particles from the soil after application for further investigations is problematic, and specialized separation techniques, e.g., magnetic separation, must be used27. Sulfidation of nZVI particles can lead to promising results as it can sequester metals as sulfides, which could be resistant to subsequent reoxidation28,29.
Unfortunately, most studies on metal(loid) stabilization in soils using nZVI are only laboratory-based. However, the first obtained results were promising for subsequent field applications at a larger scale30. Arsenic is well known for its high affinity to Fe oxyhydroxides, and nZVI could thus present an attractive and efficient amendment. In one of the first studies, Gil-Díaz et al. evaluated As stabilization in a soil treated with nZVI, and the application significantly decreased As availability determined by various extraction procedures; however, the studied soil was spiked with As, which does not represent natural conditions31, which is a common issue in several similar laboratory investigations. The dominant mechanism seemed to be, in this case, As adsorption followed by possible reduction and diffusion through the thin oxide nZVI layer, resulting in an intermetallic phase with the Fe(0) core after breaking of As&#;O bonds at the particle surface32. Chromium(VI) is another good target metal for remediation by nZVI. Its reducing properties result in the transformation of Cr(VI) into Cr(III), which lowers its mobility and availability in the environment, which has also been shown in contaminated soils33,34,35. The efficiency of the stabilization process depends on the (i) pH of the treated soil. As expected, anionic metal(loid)s (e.g., As, Cr) are preferentially retained in acidic conditions, cationic metals (e.g., Cd, Pb, and Zn) are better immobilized in near-neutral-alkaline soils; and (ii) the presence of several metal(loid)s and their competition for nZVI-sorption sites36. However, when the stabilization efficiency of nZVI is compared with common iron-based materials (micro- or macro-sized)7, it is not possible to justify the costs associated with nZVI when used extensively on large, contaminated sites.
It has been suggested that the generation of Fe2+ and reactive oxygen species by nZVI results in cell membrane disruption, and together with oxidative stress, these are the main mechanisms contributing to nZVI cytotoxicity and altering the taxonomic and functional composition of indigenous microbial communities14,37,15. These unwanted effects can result in severe toxicity for soil biota, especially in soils with low organic matter contents38. These drawbacks can be alleviated by the co-addition of inorganic and especially organic materials, e.g., compost, biosolids, bentonite, etc., which are beneficial for microbial communities and earthworms in nZVI-treated soils39,40,41,42,43.
In general, iron-based nanomaterials have not shown significant toxicity toward bacteria and plants at concentrations lower than 50&#;mg/L and at 5&#;mg/L for earthworms, except for ball-milled nZVI44. Higher nZVI concentrations (500 and &#;mg/kg) inhibited growth and respiration and increased avoidance of earthworms and oxidative stress in E. fetida as a result of nZVI application45. It is evident that the potential nZVI toxicity depends on environmental conditions, physico-chemical characteristics of the soil, concentration, aggregation, and reactivity of nZVI, and contamination types and levels37,46. Generally, it is possible to assume that plants can take up Fe from nZVI into their aboveground parts13,47. While Wojcieszek et al. () indicate that the Fe in the aerial parts is mainly in the form of particles and not originating from dissolved species, this question remains open, and the exact mechanisms need to be clarified using advanced techniques, e.g., synchrotron Mössbauer and isotope techniques, which is currently under investigations. Counteractive effects of arbuscular mycorrhiza and nZVI on plant physiology and metal(loid) uptake in the arbuscular mycorrhiza fungal&#;root&#;nZVI system need to be taken into account, and its presence is usually beneficial for alleviating the potential stress caused by nZVI to plants13.
The nZVI dose, as for any other material, is one of the crucial factors influencing its potential toxicity. For example, nZVI additions higher than 1&#;2% (w/w), which have been commonly studied under laboratory conditions (e.g., refs. 20, 31, 36, 38, 43, 45, 48), would not be economical and could potentially aggravate the associated toxicological risks. As for other engineered nanoparticles, another important point that needs to be taken into account is the potential toxicity and health-related risks of nZVI to humans, e.g., nZVI has the potential to induce cardiovascular disease through oxidative and inflammatory mediators produced from the damaged lung epithelium in chronic lung diseases49 or directly damage DNA50. Nevertheless, risks associated with ingesting the nanoparticles from the soil seem to be minimal.
Few recent studies suggest that nZVI composites with biochar can be beneficial for selective sorption as well as for the stability of the amendments in soils17,51,52. Biochar-supported nZVI improved the stability, mobility, and stabilization efficiency of nZVI in a Cr(VI)-contaminated soil33. Using nZVI-biochar composites also helps limit the nanoparticles&#; aggregation in the soil and thus possibly increases the efficiency and reduces nZVI leaching through the soil profile. Additionally, using such composites could improve the selectivity of the amendments and possibly reduce application costs as more nZVI surfaces will be available for sorption due to limited aggregation and the influence of the biochar as another sorbent. The properties and preparation conditions of the biochar, e.g., pore structure, functional groups, feedstock composition, and pyrogenic temperature, are crucial for the resulting properties of the composite53. Other possible materials improving the efficiency through limiting nZVI particle aggregation include e.g., zeolites18, bentonite42, vermiculite54, etc.
The materials used for synthesizing the composites should be chosen in accordance with the target contaminants and specific soil conditions. When biochar is used, biowaste materials used as feedstock should be cost-efficient and improve the LCA of the final product. For example, the use of pyrolyzed biosolids, e.g., sewage sludge, could be promising as it would somehow reduce the footprint of this waste43,55; however, the presence of various contaminants in the sludge must be carefully considered.
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