Review
Nanoscale zero-valent iron: Future prospects for an emerging water treatment technology

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Abstract

For the past 15 years, nanoscale metallic iron (nZVI) has been investigated as a new tool for the treatment of contaminated water and soil. The technology has reached commercial status in many countries worldwide, however is yet to gain universal acceptance. This review summarises our contemporary knowledge of nZVI aqueous corrosion, manufacture and deployment, along with methods to enhance particle reactivity, stability and subsurface mobility. Reasons for a lack of universal acceptance are also explored. Key factors include: concerns over the long-term fate, transformation and ecotoxicity of nZVI in environmental systems and, a lack of comparable studies for different nZVI materials and deployment strategies. It is highlighted that few investigations to date have examined systems directly analogous to the chemistry, biology and architecture of the terrestrial environment. Such emerging studies have highlighted new concerns, including the prospect for remobilisation of heavy metals and radionuclides over extended periods. The fundamental importance of being able to accurately predict the long-term physical, chemical and biological fate of contaminated sites following nZVI treatment is emphasised and, as part of this, a universal empirical testing framework for nZVI is suggested.

Highlights

► Contemporary knowledge of nano-Fe0 synthesis, modification and toxicity is reviewed. ► Discrepancies in the literature are highlighted with regard to testing parameters. ► Compared to “real-systems” nano-Fe0 performance has been largely overestimated. ► A universal testing procedure and reactivity scale are suggested to effectively compare future nano-Fe0 materials.

Introduction

By virtue of their size, nanomaterials have been shown to possess distinctive chemical, catalytic, electronic, magnetic, mechanical and optical properties [1]. In a little over 15 years, the multidisciplinary nano ‘boom’ has led to the development of a wide array of novel technologies for both domestic and industrial applications; ranging from enhanced drug delivery to new methods for the treatment of contaminated water.

As particle size decreases, the proportion of atoms located at the surface increases, which raises its tendency to adsorb, interact and react with other atoms, molecules and complexes to achieve charge stabilisation. Additionally, their miniscule size allows nanoparticles to be incorporated within aqueous suspensions and behave as a colloid. Such unique properties have been shown as beneficial for a wide range of applications. For example, macroscale silver is considered chemically inert, but at the nanoscale, silver is used for a wide range of applications including antimicrobial sterilisation, solar energy absorption and as a catalyst for numerous chemical reactions [2]. Today, engineered nanomaterials are in many of the products that we use on a daily basis. The Woodrow Wilson nanotechnology consumer product inventory lists over 1000 everyday items [3]. For example, carbon nanotubes are used to enhance the strength of sports equipment and construction materials, titanium dioxide nanoparticles are used for “self cleaning” surfaces and enhanced UV protection, whilst nano-silicon is used to increase the speed and efficiency of computer processors [3].

As just one aspect of the global nano-revolution, the potential use of engineered nanomaterials for the treatment of polluted waters has sparked a great deal of interest. Compared to conventional macroscale materials, nanomaterials exhibit significant improvements in surface area as a function of mass. By using a smaller mass of material to achieve the same objective, both raw materials and energy can be theoretically conserved [4] with significant associated cost savings. Additionally (and significantly) their colloidal size allows subsurface deployment via injection with the rapid treatment of aqueous contaminants at almost any location and depth in terrestrial groundwater systems.

Conceptually, the key properties required for the use of any engineered nanoparticle for in situ remediation of polluted groundwater are: (i) high reactivity for contaminant removal; (ii) sufficient mobility within porous media; (iii) sufficient reactive longevity; and (iv) low toxicity. These properties are operational drivers but at the same time the material must be manufactured and deployed at a cost that is competitive with other existing technologies. Not many engineered nanoparticles fulfil the above mentioned requirements. For example, silver nanoparticles are well recognised for their reactivity with aqueous contaminants and also their stability as colloidal suspensions [2]. However, for groundwater remediation, the material's expense for kilogram quantities, in addition to the well recognised environmental toxicity issues, precludes their use [5]. Due to its cheap cost, environmental compatibility and high reactivity, the most widely studied nanomaterial for water treatment is metallic iron (hereafter referred to as nZVI) [6].

From a simple perspective, corrosion is the degradation of a material caused by the environment in which it resides. The manufacture of all metals from oxide requires an input of energy and as a result the material has a strong thermodynamic driving force to return to its native low-energy state. This process of reversion is most commonly referred to as corrosion; an inevitable process but one which may be controlled using various methods. Metallic iron (Fe0), also referred to as zero-valent iron, is well recognised as being highly susceptible to corrosion in aqueous media. Its corrosion is considered to occur primarily through an electrochemical process, with anodic and cathodic components. The anodic reaction involves the dissolution of Fe0 (forming soluble ionic products or insoluble oxide/hydroxide) and is coupled with reduction of redox amenable species at the cathode. In natural waters, the primary components available for corrosion reactions are dissolved oxygen (DO) and water, with the former being thermodynamically favoured (Eqs. (1), (2)).2Fe0(s) + 4H+(aq) + O2(aq)  2Fe2+ + 2H2O(l) E0 = +1.67 V2Fe0(s) + 2H2O(l)  2Fe2+ + H2(g) + 2OH(aq) E0 = −0.39 V

Ferrous iron (Fe2+) is a primary product from these reactions and, in turn, can undergo further oxidative transformation (Eqs. (3), (4)).2Fe2+(s) + 2H+(aq) + ½O2(aq)  2Fe3+ + H2O(l) E0 = +0.46 V2Fe2+(s) + 2H2O(l)  2Fe3+ + H2(g) + 2OH(aq) E0 = −1.60 V

Implicit in the above reactions is an increase of solution pH as either protons are consumed or hydroxyl ions are produced. This is commonly observed during the early stages of laboratory-scale nZVI aqueous reaction experiments. Similarly, due to the high reactive surface area of nZVI (up to 100 m2 g−1), when a significant mass of material is added to an aqueous system, chemically reducing conditions can be very rapidly achieved through the production of H2 (Eqs. (2), (4)); inducing local conditions far from equilibrium and favourable for contaminant removal.

During aqueous corrosion both Fe0 and Fe2+ are an active source of Fe3+(aq), H2 and various precipitates such as Fe(OH)2, Fe(OH)3, Fe3O4, Fe2O3, FeOOH, Fe5HO8·4H2O and green rusts. It is these corrosion reactions and the product(s) thereof which are responsible for the reductive transformation and/or physical removal (sorption or enmeshment) of exposed chemical species. As surface precipitated iron oxide/hydroxide is initially porous, the material can develop a “core–shell” structure during the early stages of reaction with both sorption (at the oxide/hydroxide) and chemical reduction (at the metallic iron oxide/hydroxide interface) able to occur simultaneously. However, as the reaction progresses, increasing quantities of corrosion product(s) and a commensurate lowering in the material's porosity can significantly limit direct Fe0–H2O/O2 and Fe0–contaminant interactions [7]. It should also be noted at this stage that because the kinetics of the initial stages of Fe0 oxidation are so rapid, corrosion will proceed even in extremely controlled conditions. Consequently, Fe0 that is introduced to an environmental system (whether as granular or nZVI) will already have a film of surface oxide acquired directly after synthesis. Therefore, from the outset, it should be understood that although this nanomaterial is referred to as being metallic, each particle exists in natural conditions with a thin but encapsulating layer of surface oxide [8], [9].

Building on original work by Glavee et al. [10], nZVI was first tested for contaminated water treatment by Wang and Zhang at Lehigh University, USA, [11]. Since then nZVI has been proven as highly effective for the removal/degradation of a wide range of chemical pollutants, including: β-lactam and nitroimidazole-based antibiotics [12], [13]; azo dyes [14], [15]; chlorinated solvents [11], [16], [17], [18]; chlorinated pesticides [19], [20], [21], [22]; organophosphates [23]; nitroamines [24]; nitroaromatics [17], [25], [26]; p-chlorophenol [27]; polybrominated diphenyl ethers [28], [29]; polychlorinated biphenyls [11], [30]; inorganic anions, including nitrate [31], [32], and perchlorate [33]; alkaline earth metals, including barium [34] and beryllium [35]; transition metals, including chromium [35], [36], [37], [38], cobalt [39], copper [35], [38], [40], [41], lead [37], [40], molybdenum [38], nickel [35], [40], silver [40], technetium [42] and vanadium [35]; post-transition metals, including zinc [35], [40] and cadmium [35], [40]; metalloids, including arsenic [35], [43], [44], [45], [46], selenium [47]; and actinides, including uranium [8], [35], [37], [48], [49], [50] and plutonium [51]. Due to the significant variation in contaminant chemistry, numerous possible contaminant removal pathways have been determined, including sorption, complexation, (co)precipitation and surface mediated chemical reduction (Table 1) [52].

As suggested by Li and Zhang [40], for metal ions such as Cd2+ and Zn2+ which have standard electrode potentials (E0) for reduction to a metallic state that are very close to, or more negative than, Fe0 (−0.41 V), the removal mechanism by nZVI is occurs predominantly via sorption/surface complexation. In comparison, with metal ions such as Hg2+ and Cu2+, which have E0 much more positive than Fe0, removal occurs predominantly via surface mediated reductive precipitation. Where metal cations are only slightly more electropositive than iron, such as Ni2+ and Pb2+, sorption, with partial chemical reduction, has been shown to occur. It is recognised, however, that aqueous nZVI treatment systems do not exist at standard conditions and consequently the above comparison of potentials is somewhat invalidated. Instead the Nernst equation may be used to relate standard cell potentials to those actually existing. As displayed in Table 1, the presence of a significant molar excess of aqueous Fe2+ derived from the rapid anodic dissolution of the nZVI can significantly alter the cell potential from the standard value (due to alteration of the thermodynamic reaction quotient), making contaminant reduction reactions more favourable.

The most recognised mechanism by which Fe0 and Fe2+ solid materials remove contaminants from groundwater is via chemical reduction, and typically requires the contaminant to be adsorbed or in close proximity (electronic range) of the iron surface. For the treatment of organic contaminants, such as chlorinated organics and polychlorinated biphenyls, removal generally occurs via the reductive degradation of the chemical, i.e. the contaminant is physically destroyed. In comparison, for the treatment of many heavy metals and radionuclides removal typically occurs via immobilisation. Contaminants are merely removed from the water and trapped in an immobile state without physical destruction. For in situ remediation of heavy metals and radionuclides it is therefore significant to note that, as recovery of the injected nanomaterials (with associated contaminants) is unfeasible, contaminants are neither destroyed nor extracted from the system. This presents the prospect for future contaminant remobilisation should geochemical conditions change. Considering the significant geochemical perturbation caused by nZVI injection, subsurface treatment zones are often highly metastable, and even a gradual reversion in groundwater conditions toward a pre-injection state may be enough for significant remobilisation to occur. This is a key issue which may limit the development of the technology and is discussed in Section 3.

Section snippets

NZVI synthesis

To date, numerous methods have been developed for the manufacture of metallic nanoparticles, including chemical vapour deposition, inert gas condensation, pulsed laser ablation, spark discharge generation, sputtering gas-aggregation, thermal decomposition, thermal reduction of oxide compounds, hydrogenation of metallic complexes and the aqueous reduction of iron salts. These manufacturing methods can be considered as either “bottom up” or “top down” approaches. The former involves physical or

Is nZVI as good as some studies suggest?

With nZVI proven as highly effective for the removal of a wide range of contaminant species from simple synthetic solutions, subsequent work has focussed on determining the material's performance for the treatment of chemically complex and/or “real” solutions. It has been outlined that some previous studies using ‘simple solution’ tests, lacking complexing agents and/or competitive chemical reactions, have largely overestimated nZVI performance [8], [48], [50], [66]. This is also evidenced by

Operational drivers for water treatment

The structure, geochemistry and hydrogeology of each polluted site is unique. Resultantly the strategy adopted for nZVI deployment must take into account various operational parameters. Variables that may be altered to optimise nZVI performance include the particle size range (Section 4.1), mobility (Section 4.2), reactivity and longevity (Sections 4.3 Improving reactivity; bimetallic nanoparticles, 4.4 Thermal treatments), injection strategy (Section 4.5) and the material's ecotoxicity

UK legislation

The use of nZVI for in situ environmental applications has reached regulated status in many countries, including the USA, Canada, the Czech Republic, Germany, Italy and Slovakia [140]. Other countries, such as the UK, are yet to establish a legislative framework for commercial application. Since the Royal Society and Royal Academy of Engineering ‘Nanoscience and Nanotechnologies: Opportunities and Uncertainties’ report in 2004 [141] it has been advocated that a “precautionary approach” should

A universal nZVI testing framework

Due to the extremely wide array of different nZVI materials available, the selection of an appropriate material for site remediation can be bewildering for even the most capable geo-environmental engineer. Moreover, due to the highly reactive nature of nZVI it is a perishable product, meaning that one can never assure that the material will maintain its reactivity after its manufacture/purchase date; even under extremely controlled storage conditions. It therefore makes logical sense to

Conclusions

Nanotechnology is one of the most rapidly growing sectors of the global economy. Over a thousand products using nanomaterials are currently available for a diverse range of applications within the private and public sector. For the treatment of contaminated water and soils, a growing body of theoretical and empirical evidence has proven nZVI as both highly effective and versatile. In recent years there have been significant innovations in terms of manufacture techniques, physicochemical

Acknowledgements

We would like to thank Mr. Jonathan Jones from the School of Chemistry, University of Bristol for performing TEM analysis.

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