Abstract
The amendment of the subsurface with nanoscale metallic iron particles (nano-Fe0) has been discussed in the literature as an efficient in situ technology for groundwater remediation. However, the introduction of this technology was controversial and its efficiency has never been univocally established. This unsatisfying situation has motivated this communication whose objective was a comprehensive discussion of the intrinsic reactivity of nano-Fe0 based on the contemporary knowledge on the mechanism of contaminant removal by Fe0 and a mathematical model. It is showed that due to limitations of the mass transfer of nano-Fe0 to contaminants, available concepts cannot explain the success of nano-Fe0 injection for in situ groundwater remediation. It is recommended to test the possibility of introducing nano-Fe0 to initiate the formation of roll-fronts which propagation would induce the reductive transformation of both dissolved and adsorbed contaminants. Within a roll-front, FeII from nano-Fe0 is the reducing agent for contaminants. FeII is recycled by biotic or abiotic FeIII reduction. While the roll-front concept could explain the success of already implemented reaction zones, more research is needed for a science-based recommendation of nano-Fe0 for subsurface treatment by roll-fronts.
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1 Introduction
The development of new methods and materials for environmental remediation is a real challenge for the scientific community. Such technologies will only be adopted by industry if they can exhibit marked improvements in efficiency, affordability or eco-compatibility compared with conventional techniques. The use of metallic iron (Fe0) in subsurface reactive permeable barriers has been identified as such a technology (Gillham and O’Hannesin 1991, 1994; Matheson and Tratnyek 1994; Blowes et al. 1997; O’Hannesin and Gillham 1998). Since this discovery almost 20 years ago, extensive research of the Fe0/H2O system has been performed in an attempt to understand the controlling mechanisms behind the remediation of redox-amenable contaminant species using Fe0-based materials (Scherer et al. 2000; Henderson and Demond 2007; Bartzas and Komnitsas 2010; Gillham 2010; Li and Benson 2010; Noubactep 2010a, b; Noubactep and Caré 2010a; Kim et al. 2010; Comba et al. 2011; Courcelles et al. 2011; Gheju 2011; Jeen et al. 2011; Noubactep 2011a). Two different tools are commonly used to optimise the efficiency of Fe0 for aqueous contaminant removal: (1) reducing the particle size of Fe0 down to the nanoscale (nano-Fe0) (Wang and Zhang 1997; Ponder et al. 2000) and (2) using bimetallic systems (Muftikian et al. 1995; Korte et al. 2000). In recent years, there has been considerable interest into combining the two methods (Schrick et al. 2002; Karn et al. 2009; Nagpal et al. 2010; Chen et al. 2011; Mossa et al. 2011).
Since the original proof of concept study into the application of nano-Fe0 for water treatment at Lehigh University, USA (Wang and Zhang 1997), research within this field has boomed. On August 20th 2011, a search at “ACS publications”, “Science Direct” (Elsevier journals), and “Springer journals” using key words “nanoscale” and “zerovalent iron” yielded 740 peer-reviewed articles (Table 1). As shown in Table 1, 234 articles have already been published in the first 7 months of 2011 by the three selected publishers. This clearly demonstrates the interest within academia for this technology.
In recent years, several review articles and critical views on nano-Fe0 for environmental remediation have been published (Zhang et al. 1998; Gillham 2003; Zhang 2003; Li et al. 2006; Macé 2006; Macé et al. 2006; Tratnyek and Johnson 2006; Karn et al. 2009; Pradeep and Anshup 2009; Agarwal and Joshi 2010; Gillham 2010; Müller and Nowack 2010; Noubactep and Caré 2010b; Cullen et al. 2011; Noubactep 2011b; Peralta-Videa et al. 2011; Shi et al. 2011; Truex et al. 2011a, b). However, the original discussion on the suitability of nano-Fe0 for in situ field applications (Gillham 2003, 2010) has not been satisfactorily addressed (Macé 2006; Noubactep 2011b). Moreover, a recent comparison between field applications of Fe0 of different particle sizes (nanometres, micrometres, and millimetres) for field applications has clearly demonstrated the superiority of mm-Fe0 (average efficacy 97%) (Comba et al. 2011). The decreasing order of reactivity was mm-Fe0 (97%) > μm-Fe0 (91%) > nano-Fe0 (65%). Expectably, the lower efficiency of nano-Fe0 is due to their high reactivity (Gillham 2003, 2010; Noubactep and Caré 2010b). Therefore, the question arises on the fundamental necessity to further increase the reactivity of nano-Fe0 by using a noble metal combination.
1.1 The Problem
Nano-Fe0 technology for environmental remediation was introduced as an alternative to the conventional Fe0 walls mostly for inaccessible aquifers (Masciangioli and Zhang 2003; Li et al. 2006). The very small particle size of nano-Fe0 (1–100 nm) would allow the material to penetrate deep into soil networks (Ghauch et al. 2009; Karn et al. 2009; Comba et al. 2011; Gheju 2011; Truex et al. 2011a, b).
Due to the exponential relationship between specific surface area (SSA) and radius (R = d/2) of a perfectly spherical object (SSA = 4πR 2), as a rule, a decrease in Fe0 particle size increases the surface area per gram by up to three orders of magnitude (Macé et al. 2006; Chen et al. 2011). In other words, the inverse relationship between Fe0 particle size and reactivity is due to a greater density of reactive sites on the particle surface at smaller scale. The following three claims have been made with regard to the use of nano-Fe0 for aqueous contaminant removal (Comba et al. 2011): (1) some aqueous contaminant species that have been proven as unsuccessful for remediation using μm-Fe0 and mm-Fe0 can be effectively removed using nano-Fe0, (2) nano-Fe0 can be used for more rapid degradation of contaminants, and (3) the formation of some undesirable by-products during remediation using μm- and mm-Fe0 can be avoided by using nano-Fe0. Such processes whilst correct are all linked to the greater reactivity that nano-Fe0 possesses due to its size (reactive surface area). When performed in conditions without a large nano-Fe0 stoichiometric excess, e.g. a system analogous to the environment, it may prove that such claims will be unfounded (Nagpal et al. 2010; Noubactep and Caré 2010b; Noubactep 2011b; Sakulchaicharoen et al. 2010; Nagpal et al. 2011).
An undisputed drawback with regards to the use of nano-Fe0 for environmental applications is their strong tendency to aggregate and adhere to solid surfaces (Li et al. 2006; Tratnyek and Johnson 2006; Karn et al. 2009; Sakulchaicharoen et al. 2010; Comba et al. 2011; Gheju 2011; Truex et al. 2011a, b). Karn et al. (2009) listed some parameters that influence nano-Fe0 adsorption onto soil and aquifer materials: (1) the surface chemistry of soil and Fe0 particles, (2) the groundwater chemistry (e.g. ionic strength, pH and presence of natural organic matter), and (3) the hydrodynamic conditions (pore size, porosity, flow velocity and degree of mixing or turbulence). Several methods have been developed for the stabilization of nano-Fe0 particles over the past decade and proven efficient to sustain the reactivity of nano-Fe0 (Karn et al. 2009; Sakulchaicharoen et al. 2010; Comba et al. 2011; Gheju 2011). One factor that has been overlooked, however, is the impact volumetric expansion has on the mobility of (1) residual Fe0, (2) primary corrosion products (FeII and H2) and contaminants. The volume of any corrosion product (Fe hydroxide or oxide) is higher than that of the original metal (Fe0). The ratio between the volume of expansive corrosion product and the volume of iron consumed in the corrosion process is called “rust expansion coefficient” (η) (Anstice et al. 1993; Caré et al. 2008; Zhao et al. 2011). Volumetric corrosion products are likely to: (1) contribute to porosity loss, (2) impact the retention of contaminants and transformation products, and (3) increase the particle agglomeration.
Another area of heightened research is with regard to the determining the toxicity of nano-Fe0, with mixed results reported (Grieger et al. 2010; Gheju 2011). For example, Barnes et al. (2010) reported minimal change to the structure of a river water community due to the addition of nano-Fe0 while Diao and Yao (2009) reported nano-Fe0 particles as highly cytotoxic towards both gram-positive and gram-negative bacteria species.
While taking into account all known influencing parameters, the following seven features have to be systematically studied in order to optimise the general applicability of this technique (Karn et al. 2009; Tervonen et al. 2009; Gheju 2011): (1) mobility changes due to nano-Fe0 volumetric expansion during corrosion, (2) the bioavailability of Fe0 and corrosion products (FeII/FeIII species, H/H2), (3) the ecotoxicity of Fe0 and its corrosion products, (4) the bioaccumulation of Fe0 and its corrosion products, (5) the translocation potential of nano-Fe0, (6) the long-term reactivity of nano-Fe0 particles and (7) the speciation, persistence and fate of contaminants and their transformation products. A major contributing factor to the latter point is that little is known (compared to permeable reactive barrier technologies) about the extent contaminants are removed via size exclusion using nano-Fe0.
Only when all seven “operational drivers” have been determined can the global community have full faith in the technology.
1.2 Objectives of the Study
The present communication is focused on the “field persistence” or reactive “life span” of nano-Fe0 particles. For in situ applications, a keen understanding of nano-Fe0 reactive fate is essential for effective and prudent site clean-up. The knowledge of which is likely to largely underpin decisions as to (1) the choice of material selected, (2) the mechanism of application and (3) the strategy (if any) for repeated treatments.
In the current work, a multidisciplinary approach is used to analyse the relationship between nano-Fe0 reactivity and its performance for in situ field applications. The discussion is based on the contemporary knowledge of the mechanism of aqueous contaminant removal by Fe0 (Noubactep 2007, 2008, 2010c, 2011d, e). Much of the impetus for this work has come from the work of Noubactep and Caré (2010b) who have challenged the concept that nano-Fe0 is a strong reducing agent for contaminant reductive transformation.
2 Nanoscale Metallic Iron for Environmental Remediation
To date, nano-Fe0 particles have been reported as largely successful for water and soil treatment (Pradeep and Anshup 2009; Agarwal and Joshi 2010; Litter et al. 2010; Comba et al. 2011; Crane et al. 2011). A wide variety of redox-amenable organic and inorganic species and non-reducible species (e.g. Cd and Zn) have been efficiently treated. Similar to μm- and mm-Fe0, adsorption is considered important only for non-reducible species (Celebi et al. 2007; Li and Zhang 2007; Noubactep 2007, 2008, 2010c; Boparai et al. 2011; Noubactep 2011c, d; Xiao et al. 2011). For example, Boparai et al. (2011) reported that heavy metals are either reduced (e.g. Cu2+ and Ag2+) at, or directly adsorbed (e.g. Zn2+ and Cd2+) onto the Fe0 surface. They further argued that “the controlling mechanism is a function of the standard redox potential of the contaminant”. Recent work has however challenged this concept (Noubactep 2010c, 2011b, d, e), which is explained below.
2.1 Contaminant Reduction by Nano-Fe0
The chemical reaction between Fe0 and redox-amenable aqueous species is considered to involve three steps: (1) direct electron transfer from Fe0 at the metal surface or through a conductive oxide film on Fe0 (direct reduction), (2) catalyzed hydrogenolysis by the H/H2 (indirect reduction mechanism 1) and (3) reduction by FeII species resulting from Fe0 corrosion (indirect reduction mechanism 2). In this constellation, H2 is supposed to result from H2O reduction during anoxic iron corrosion (Reardon et al. 2008; Chen et al. 2011). However, evidence exists in the literature, e.g. Stratmann and Müller (1994), that even under external oxic conditions, Fe0 is oxidized by H2O (or more precisely by H+) and O2 by FeII (Table 2). Despite the significant reaction rate exhibited by nano-Fe0 due to its high surface area, such processes are considered to occur (discounting any quantum size effects) independent of particle size.
Table 2 summarizes the half reactions for the aqueous oxidation of Fe0 under both anoxic and oxic conditions. Thermodynamically, the major cathodic reaction depends on the availability of molecular O2 (E 0 = 0.81 V). In the absence of O2, Fe0 is oxidized by H+ (E 0 = 0.00 V). It can therefore be stated that the rate of Fe0 oxidation is dictated by the concentration of dissolved O2, H+ and H2O in proximity to Fe0 surfaces. Le Chatelier’s principle also states that the consumption of FeII (via oxidation to FeIII) will also result in an increase in Fe0 oxidation.
The electrode potential of the redox couple FeII/Fe0 is −0.44 V, a value which is independent of the particle size (nanometre, micrometre or millimetre). The value −0.44 V is considered largely unchanged due to the presence of alloying materials (e.g. low alloy steel and bimetallic systems).
As a consequence, statements including “nano-Fe 0 are more reactive than μm-Fe 0 and mm-Fe 0” are misleading; as the reactivity of Fe0 (discounting quantum size effects), is independent of the particle size. Any enhanced reactivity reported is likely to be due to the significantly high surface area of nano-Fe0 compared with other forms. A second statement “bimetallic nano-Fe 0 is more reactive that monometallic nano-Fe 0” is also a qualitative statement, as the reactivity of the materials depends on numerous factors associated with the materials synthesis route and varies depending on the chemistry of the chosen alloying metal. Ideally, comparisons should be made versus standard reference materials using established standard experimental protocols (Noubactep et al. 2009), which once established, will significantly improve the design of future field applications.
2.2 Limitations of the Nano-Fe0 Technology
The efficiency of nano-Fe0 for aqueous contaminant reduction faces some key issues for in situ applications in porous media. These challenges include: (1) the strong tendency of aggregation/agglomeration, (2) the rapid settlement on subsurface solid phases and (3) the porosity and permeability loss of porous media (Behrens et al. 2000; Dickinson and Scott 2010; Cullen et al. 2011; Mossa et al. 2011). Aggregation and settlement limit nano-Fe0 transport through porous media. Porosity and permeability loss limit nano-Fe0 transport to target contaminants. It was demonstrated that nano-Fe0 may travel only a few centimetres in porous media from the injection position under typical groundwater conditions (Tratnyek and Johnson 2006; Johnson et al. 2008; Comba et al. 2011; Gheju 2011). Accordingly, recent efforts have been made to (1) increase the porosity of porous media, (2) mechanically increase the distribution of nano-Fe0 and/or (3) chemically modify nano-Fe0 for improved aqueous mobility in porous networks.
2.3 Improving the Efficiency of the Nano-Fe0 Systems
2.3.1 Dispersion Agents
Methods to improve the aqueous mobility of nano-Fe0 have received the greatest research interest. It has been determined that the key to improving particle mobility is found in modifying their surface properties such that the nano-Fe0 have significantly improved colloidal stability and a commensurate reduction in the likelihood of adherence to mineral surfaces. Several synthetic methods are now available to produce more mobile nano-Fe0. Efficiently tested dispersants include anionic surface chargers (e.g. polyacrylic acid), non-ionic surfactants, starch and oil (Wu et al. 2009; Fang et al. 2011; Jiang et al. 2011; Mossa et al. 2011; Tong et al. 2011).
2.3.2 Bimetallic Combinations
In recent years, noble metals have been used to increase the reactivity of monometallic nano-Fe0 (Nagpal et al. 2010; Yuan et al. 2010; Zhu et al. 2010; Kadu et al. 2011; Mossa et al. 2011). As mentioned above, this appears counterintuitive as nano-Fe0 is already highly reactive due to its size (Wang and Zhang 1997; Zhang 2003; Macé et al. 2006; Ghauch et al. 2009) and is unstable during synthesis, storage and application (Jiang et al. 2011). This chemical instability has been documented as a key reason for the observed lower efficiency exhibited by nano-Fe0 systems compared with μm- and mm-Fe0 (Comba et al. 2011). Accordingly, it is questionable whether further enhancing the reactivity of nano-Fe0, e.g. by plating with more noble elements, may be of any benefit. The reactivity of nano-Fe0 will be discussed in the next section on the basis of mathematical modelling.
3 Significance of Increased Reactivity
3.1 The Problem
The increased Fe0 reactivity from mm to nm size should be better characterized. The relative reactivity of four different materials is discussed on the basis of 1 kg Fe0: 1 nm-Fe0 (d 0 = 25 nm), 1 μm-Fe0 (d 0 = 25 μm) and 2 mm-Fe0 (d 0 = 250 and 1,000 μm). Calculations for the number particles (N) in 1 kg of each material and the number of layers (N′) in each particle are made after the Eqs. 1 and 2 presented in details elsewhere (Noubactep and Caré 2010a; Noubactep et al. 2010).
where M is the mass of Fe0 (here 1 kg), ρ Fe is the specific weight of Fe (7,800 kg/m3), R 0 is the initial radius of the Fe particle (d = 2*R 0) and a the lattice parameter (a = 2.866 Å).
The results are summarized in Table 3. It can be seen that the number of layers of Fe0 in individual particles varies from 87.2 for nano-Fe0 to more than 3*106 for mm-Fe0 (d = 1 mm). In the meantime, the number of particles in 1 kg decreased from 1.96*1018 for nano-Fe0 to only 3.1*104 for mm-Fe0. The ratio of the number of Fe0 layers in each particle to the number of Fe0 layers in nano-Fe0 varies from 1 to 4*104. This ratio corresponds to the relative time (τ) as defined later (section 3.2). On the other hand, the ratio of the number of particles in 1 kg of nano-Fe0 to the number of particles in the same mass of each other materials varies from 1 to 6.4*1013. These results are summarized in Fig. 1. Instead of the mass of Fe0, the number of electrons released by the conversion of Fe0 to FeII is used to assess the kinetics of Fe0 consumption. This is discussed in the next section.
3.2 Relative Corrosion Kinetics of Fe0 Materials
For the discussion in this section, uniform corrosion for spherical particles is assumed. Individual particles corrode independently until material depletion. It is further assumed for simplicity that individual layers corrode with the same kinetics independent of particle size. The latter assumption is conservative as larger particles react slower than smaller (Macé et al. 2006; Reardon et al. 2008; Nagpal et al. 2010). With these assumptions, a relative time (τ) can be defined while taking the time for the corrosion of the smallest particle (here 87.2 layers of nano-Fe0) or t ∞, nano as unit.
Accordingly, one unit of time corresponds to the time to nano-Fe0 depletion. Remember that all 1.96*1018 particles in the 1 kg of nano-Fe0 simultaneously corrode with the same kinetics. The results of the calculations are presented in Fig. 2. From Fig. 2a and Table 3, it can be seen that after nano-Fe0 depletion, the material with 1,000-μm (or 1 mm) diameter will still react for more than 3*104 times longer than the time necessary for nano-Fe0 depletion (τ = 4*104, see Table 3). Figure 2b shows that the mm-Fe0 with 250-μm diameter is depleted after about 104*t ∞, nano.
Based on the assumptions above, the service life of a nano-Fe0 particle can be estimated. Table 4 summarizes the results of such estimations while varying the service life of a 1 mm Fe0 particle from 5 to 40 years. This assumption is based on the fact that conventional Fe0 walls are supposed to function for several decades (here up to 4 decades). Results show (Table 4) that the maximum life span of a nano-Fe0 is about 8.8 h (less than 1 day). In other words, following approximately 9 h from subsurface deployment it is suggested that all nano-Fe0 would be reactively exhausted. The success of this is dependent on three key factors: (1) the hydrodynamic conditions: pore size, porosity, flow velocity and degree of mixing or turbulence, (2) the water chemistry and the affinity of nano-Fe0 and its transformation products to the soil materials and (3) the reactivity of Fe0.
It is certain that the dynamic process of transformation of concentric layers of Fe0 atoms to concentric layers of iron (hydr)oxides cannot be linear (Noubactep and Caré 2010b). In fact, effects similar to “case hardening” for food and wood drying will lead to “surface hardened layers” (Tarvainen et al. 2006; Fernando et al. 2008) leading to differential kinetics/extents of Fe0 passivation for different particle size ranges. In other words, the extent of restricted corrosion rates through resulting surface hardened layers will be different for nm-, μm- and mm-Fe0. Bearing this in mind, the very short relative life span of a nano-Fe0 estimated above will be used for the discussion in this work. It is certain that “case-hardening”-like effects will prolong this hypothetical life span to some days or weeks.
3.3 Extent of Iron Corrosion from Fe0 Materials
A discussion as to the extent of Fe0 consumption is limited in the present section to τ = 1 or t ∞, nano. It is considered for simplification that the sole iron corrosion product is Fe3O4. The corresponding coefficient of volumetric expansion is η Fe3O4 = 2.08 (Eq. 4) (Caré et al. 2008). Using ρ = M/V, the volume of Fe corresponding to 1 kg Fe0 is calculated as 127.0 mL (V 0). This is the initial volume of Fe0 (V 0). Following corrosion, this volume is partly or totally consumed. The volume (ΔV) corresponding to the volume of pores occupied by the volumetric expansion of corrosion products can be estimated.
Assuming that the coefficient of volumetric expansion (η) (“rust expansion coefficient” or “specific volume”) (Anstice et al. 1993; Caré et al. 2008; Zhao et al. 2011) of the reaction products is:
where V oxide is the volume of the reaction product and V Fe the volume of the parent Fe0.
The volume ΔV characterizing the extent of porosity loss due to volumetric expansion is given by Eq. 5:
Where V consumed Fe is the volume of consumed Fe0 at time t ∞, V 0 is the volume occupied by the initial Fe0 particles and ν (ν ≤ 1) is the fraction of the initial amount of Fe0 (1 kg) which has reacted at t ∞, nano. V ∞ is the total volume occupied by residual Fe0 and in situ formed corrosion products. t ∞ corresponds to nano-Fe0 depletion (25 nm in this section). In the discussion on the reactivity at nanoscale, t ∞ corresponds to the depletion of the material with 10-nm diameter (Section 4).
The volumetric expansion (ΔV, Eq. 5) can be characterized as percent of the initial volume (V 0) using Eq. 7:
Table 5 summarizes the results. It is shown that at τ = 1 (nano-Fe0 depletion), a volume augmentation of 108% has occurred in the nano-Fe0 system, with volume augmentations in all other systems lower than 0.5%. This clearly shows that the porosity of the subsurface will be significantly influenced by nano-Fe0 at t ∞. Remember that 100% reactive exhaustion of nano-Fe0 is predicted to occur by approximately 9 h time. During this same period the porosity loss due to expansive iron corrosion is likely to be negligible for all other Fe0 particle size fractions. Calculations for Akageneite β-FeOOH (η FeOOH = 3.48) as sole corrosion products shows that V ∞,nano = 448.7 mL, ΔV = 320.5 mL or 250.4%. The examples of Fe3O4 (anoxic) and FeOOH (oxic) demonstrate the crucial importance of the nature of formed corrosion products for the discussion of the extent of porosity loss.
Another important aspect of Fe0 consumption is given by the number of moles of Fe0 that have been oxidized (Table 5). Assuming contaminant reduction, Table 5 shows that after τ = 1, 35.71 moles of electrons have been released in the nano-Fe0 system but less than 0.11 moles in all other systems. In other words, up to 35.71 moles of electrons are available for contaminant reduction per kg nano-Fe0 within a few hours of reaction (<9 h). But what proportion of the electrons produced would reach the contaminant within this period? That is the major question to be answered for the further development of the nano-Fe0 technology for in situ applications.
4 Reactivity of Nano-Fe0 Materials
The presentation until now has discussed the reactivity of nano-Fe0 in comparison to larger scale Fe0. Section 4 will focus only on the nanoscale size fraction (d ≤ 100 nm). Equations 1–7 will be used and the particle size will vary from 10 to 100 nm. As stated above t ∞ is for a nano-Fe0 of 10 nm in diameter and the reaction proceeding until 100% reactive exhaustion has been achieved.
4.1 Fe0 Reactivity at Nanoscale
Table 6 summarizes the results of calculations for the number of Fe0 particles and number of layers of Fe0 in each nano-Fe0. It is shown that 1 kg of the material with d = 10 nm contains 1,000 times more particles than a material of d = 100 nm.
Table 6 also shows that the maximum value of the relative time (τ) is 10 (or 101). This is more practical for graphical representations than situations where nano-Fe0 are compared with larger particles (t ≤ 104). The physical significance of τ is more important, it means that if a nano-Fe0 with a diameter of 10 nm depletes after 2 days, the material with a diameter 100 nm will deplete after 20 days. For field applications, the selection of the particle size to be used should be dictated by site specific characteristics. Which diameter could quantitatively reach the contaminants before depletion? And what fraction of the material will have already oxidized on the path? What is the impact of this oxidation on the transport of nano-Fe0 in the porous aquifer? These are some key questions to be answered in order to give this possibly very efficient technology a scientific basis.
Figure 3 summarizes the evolution of the volumetric expansion in all five nano-Fe0 systems. It can be seen from Fig. 3a that the smallest material (d = 10 nm) experiences the 108% volumetric expansion within a short time (τ = 1) while the larger materials (d = 100 nm) needs ten more time for the same expansion. Accordingly, beside the question whether the material will reach the contaminant under site specific conditions, the question has to be answered how the volumetric expansion will impact the aquifer porosity (and permeability).
Figure 3b compares the variation of the volumetric expansion for two different iron corrosion products, Fe3O4 and FeOOH, which are considered the most likely products in anoxic and oxic aquifers respectively. It also shows that when designing a nano-Fe0 injection strategy, however, the availability of oxidizing species (e.g. MnO2 and O2) must also to be taken into account. Figure 3a shows that under both conditions the trend of porosity loss is similar but the extent is proportional to the coefficient of volumetric expansion (η). In particular, at τ = 1, the system with the material d = 10 nm experiences 250% volumetric expansion under oxic conditions and only 110% under anoxic conditions. As a result the kinetics of more rapid Fe0 corrosion in an oxygen-rich environment must also be considered for an effective treatment strategy.
4.2 Fe0 Reactivity of Nano-bimetallics
The reactivity of monometallic nano-Fe0 can be improved by combining it with a noble metal. Assuming α (α > 1) the coefficient of reactivity enhancement, the relation between the relative time of a bimetallic system (τ Fe/M) and that of a non-plated metal (τ Fe) is given by Eq. 6:
To characterize the impact of plating on nano-Fe0, the material with the largest size (d = 100 nm) will be plated by three hypothetical metals (M 01 , M 02 and M 03 ) to yield a reactivity factor of 2.5 (for Fe0/M 01 ), 5 (for Fe0/M 02 ) and 10 (for Fe0/M 03 ). The considered α values of (α ≤ 10) are realistic and even conservative. In fact, reported reactivity enhancement is essentially larger (Chen et al. 2011; Zhuang et al. 2011). For example, Zhuang et al. (2011) reported that palladized nano-Fe0 promoted the dehalogenation kinetics for polybrominated diphenyl ethers by orders of magnitude equal to 2, 3 and 4 (α ≥ 100). The results of the calculations for the four systems (d = 10 nm) are summarized in Fig. 4. The system with d = 10 nm is represented for comparison. It can be seen that system Fe0/M 03 (d = 100 nm) is as reactive as Fe0 (d = 10 nm). Given that the reactivity of nano-Fe0 (d = 100 nm) could already significantly been too high in some situations, the results from Fig. 4 strongly question the suitability of plating at nanoscale. Accordingly, while the application of bimetallic Fe0 is definitively useful at μm and mm scale, it usefulness at nanoscale is likely inappropriate. It can also be noted that by increasing the reactivity of the material the rate at which volumetric pore clogging also increases. As a consequence it should be acknowledged that there exists a conceptual play-off between increased reaction rate and increased porosity loss, the impact of which will vary depending on the physiochemical conditions of each contaminated site.
4.3 Characterizing the Process of Reactivity Loss
To better characterize the process of porosity loss due to the volumetric expansion of nano-Fe0, the evolution of the porosity of a sand column filled with nano-Fe0 will be discussed as volumetric expansion proceeds. A laboratory column with a height h (h = 75.0 cm) and diameter D (D = 5.0 cm) is composed of spherical sand particles (d = 5.0 mm). The compactness of the column is ideally C = 0.64 (Noubactep and Caré 2010a, Noubactep et al. 2010). The pore volume is given by Eq. 9:
where V is the apparent volume of the sand column (V = h*π*D 2/4).
It is supposed that the nano-Fe0 particles fill the inter-granular porosity of the sand column V pore without modifying the compactness C and the apparent volume V of the sand column. The residual porosity of the sand column (V′pore) is given by Eq. 10:
where V 0 is the volume of the Fe particles.
The evolution of the residual porosity (V′pore) as nano-Fe0 particles undergo volumetric expansive corrosion is considered by introducing the specific volume (η) of the reaction products according to Eq. 11:
where V consumed Fe is the volume of nano-Fe0 particles which is consumed at a given time.
Equations 9 through 11 are very useful to design reactive zone. However, they are limited to describe the initial (V pore) and a final conditions (V′pore) regardless on the nature of iron corrosion products and the kinetics of the process.
Using a sand column comparable to one of those used by Moraci and Calabrò (2010) and 1 kg of nano-Fe0, the process of pore filling (porosity loss) can be better characterized. For simplicity, nano-Fe0 considered as transported by a biodegradable dispersant which does not significantly contribute to porosity loss. As shown in Section 3.3, 1 kg of nano-Fe0 occupies a volume of 127 mL. The initial pore volume of the sand column calculated after Eq. 9 is 530.36 mL (100% porosity), i.e. a capacity for approximately 4.17 kg of nano-Fe0. Filling the initial pore volume of the sand column (530.36 mL) with 1 kg of nano-Fe0 (127.00 mL) yields a 23.9% porosity loss (Table 7). This, however, does not take into account the expansive nature of iron during oxidative corrosion.
Using the eight possible iron corrosion products documented by Caré et al. (2008) and their respective coefficient of volumetric expansion (2.08 ≤ η ≤ 6.40), the extent of porosity loss is calculated and summarized in Table 7. The results show that the residual volume of pores (V′pore) decreases with increasing η values and is zero for Fe(OH)3 and Fe(OH)3∙3H2O (100% porosity loss). Ferrihydrite (Fe(OH)3∙3H2O) is the largest known iron corrosion products. In other words, depending on environmental conditions as little as 1 kg of nano-Fe0 could clog the tested column. Although this discussion considers the nature of the corrosion products, there are other important factors which must be considered. The negative values (−3.04 and −282.4 mL) corresponds to the mass of Fe0 which will not oxidized because of lack of space for expansion (Noubactep and Caré 2010a; Noubactep et al. 2010).
The extent of porosity loss (ΔV in %) given in Table 7 assumes uniform distribution of nano-Fe0 in the whole column. This is, however, not a very good field representation. For example, if 1 kg of nano-Fe0 (V 0 = 127 mL) is uniformly distributed only in the first third of the column (V 1/3pore = 176.8 mL), with Fe3O4 as the primary corrosion product (ΔV = 137.16 mL) a 78% porosity loss can be expected. For all other oxide phases it is calculated that complete porosity loss (100%) will precede nano-Fe0 reactive exhaustion. However, in the practice a system with 78% porosity loss is considered as clogged. One possibility to avoid the clogging of the entrance zone of a porous system is to intermittently inject calculated amounts of nano-Fe0. The volume to be injected at each event and the time scale between two injections are necessarily determined by site specific characteristics (e.g. porosity of the aquifer and water flow rate).
5 Discussion
A primary reason behind the interest into the use of nano-Fe0 particles over μm-Fe0 and mm-Fe0 particles for water treatment is ascribed to a significant increase the materials efficiency (Comba et al. 2011; Gheju 2011). For example, as reported by Vodyanitskii (2010), Kanel et al. (2006) reported near-total remediation of a 1 mg L−1 AsV solution within only 10 min by a nano-Fe0 with a specific surface of 24 m2/g, whereas the same goal was achieved by mm-Fe0 (1–2 m2/g) after only 4 days or 5,760 min (ratio of time, 570; average ratio of surface, 16). However, this experimental evidence is highly qualitative as neither the number of atoms directly accessible at the surface nor the intrinsic reactivity of individual materials are considered in both cases (Noubactep and Caré 2010b). For a better comparative result, the following three key conditions must be considered: (1) the intrinsic Fe0 reactivity should be characterized, (2) the amount of used materials should be based on the reaction stoichiometry and (3) the experimental conditions should be relevant for field applications. In particular, the driving force for the transport of contaminants and Fe species should be relevant for field situations: (1) mixing operation (type and intensity) in batch studies and (2) flow rate and column dimensions in column studies (Noubactep et al. 2009).
5.1 Transport of Nano-Fe0 to the Contaminants
The efficiency of nano-Fe0 for the in situ treatment of a contaminated aquifer body is intrinsically linked to the extent of physical contact between Fe0 and any aqueous contaminant species present. In some circumstances, contaminants could diffuse to the suspended Fe0 particles and be degraded in the aqueous phase. However, typically the suspended Fe0 particles must migrate to the contaminants. As Fe0 particles are transported from the injection zone to the target contaminant plume by natural groundwater, diffusion experiments under relevant groundwater velocity, using site specific aquifer materials are essential in order to effectively assess the suitability of nano-Fe0 for in situ applications (Antia 2011).
Contaminants are typically partitioned between sediment and water phases in a “pseudo-equilibrium” state. Therefore, it is likely that Fe0 particles whilst acting to reduce any soluble contaminants are also likely to promote the dissolution of a range of adsorbed chemical species (Le Chatelier’s principle). However, as water is also a redox-amenable species the specific mechanism for nano-Fe0 reactivity in a range of conditions is difficult to resolve (Tratnyek and Johnson 2006). In other words, nano-Fe0 is readily oxidized by H2O during subsurface migration to the target contaminant plume and also competes with any other redox-amenable (including contaminants) present in the groundwater. Additionally, expansive iron corrosion will yield voluminous iron (hydr)oxides (Table 7) with limited aqueous mobility due to (1) an increased size and weight and (2) a possible increased affinity to aquifer material.
This discussion has intentionally neglected the segregation between parent compounds, the reaction products and their relative affinity to Fe0 and Fe (hydro)oxides. The fact that the core Fe0 is always covered by oxide layers is also neglected for simplification. The process of preferential flow which is crucial in predicting mass transfer in the subsurface is also not considered (Flury and Flühler 1994; Fryar and Schwartz 1998; Simunek et al. 2003; Clothier et al. 2008; Allaire et al. 2009). However, it is clearly shown, that due the acute redox sensitivity of nano-Fe0 and the subsequent significant formation of highly voluminous oxidative corrosion products it is likely that for environmentally relevant distances (metre), a significant proportion of the originally injected nano-Fe0 will remain “clogged” in pore spaces.
Beside the transport of nano-Fe0 to contaminants, the possibility of quantitative contaminant desorption and their subsequent transformation by suspended Fe0 could be considered. However, it is not likely that concentration-gradient-driven mass transfer could be quantitative at considered distances (metre). It should be recalled that the slow kinetics of contaminant desorption form aquifer materials is the major cause of the ineffectiveness of the pump-and-treat technology for groundwater remediation (McMurty and Elton 1985; Mackay and Cherry 1989; Starr and Cherry 1994).
This section has shown that it is likely that the success of nano-Fe0 for in situ remediation is seriously limited by the intrinsic formation of voluminous iron corrosion products (Tratnyek and Johnson 2006; Comba et al. 2011; Gheju 2011; Truex et al. 2011a, b). Bearing this in mind, the next section suggests an alternative nano-Fe0 subsurface deployment mechanism that more effectively takes into account the aforementioned nano-Fe0 hydraulic mobility issues than conventional injection processes: the formation of a nano-Fe0 “redox front” injection array system for progressive contaminant reduction. The geochemical process of redox front migration is a well-documented one (Fryar and Schwartz 1998; Min et al. 2005; Sidborn and Neretnieks 2007).
5.2 Nano-Fe0 as Source of FeII for a Redox Front?
5.2.1 The Concept
The progressive consumption of mm-Fe0 (Fig. 1; Table 3) is the guarantee for the long-term efficiency of reactive barriers (Comba et al. 2011). In fact, continuously generated small amount of high reactive iron minerals (Gu et al. 1999; Su and Puls 2001; Furukawa et al. 2002; Kohn et al. 2005; Noubactep 2007, 2008, 2010c, 2011b, c, d) are sufficient for the removal of contaminants which are present in trace amounts (Palmer and Wittbrodt 1991). As discussed above, for nano-Fe0 however, (1) Fe0 reactive exhaustion typically occurs in a relatively short time scale (<9 h) and (2) it is likely that nano-Fe0 subsurface mobility is significantly retarded or even prevented due to the volumetric expansive nature of iron corrosion (Zhao et al. 2011). As a consequence an alternative method of subsurface deployment is suggested in the current work: the deployment of a linear nano-Fe0 injection array orientated perpendicular to the flow direction of the contaminant plume. The injected nano-Fe0 can effectively form a redox front (roll-front) which migrates through the contaminated zone and transforms the contaminants during its migration as illustrated in Fig. 5. The FeII/FeIII roll-front travels across the contaminated zone with all possible mechanisms (e.g. diffusion, dispersion, convection and preferential flow) and the contaminants are transformed and immobilized during the cycle FeII⇔FeIII. In other words, it is a plume of FeII/FeIII formed from injected nano-Fe0 which migrates through the contaminated zone and “sweeps” the contaminants. As a consequence this method considers all nano-Fe0 mobility issues.
Schematic diagram of the flow process of the U-shaped redox front through a contaminated zone. Despite the relative importance of preferential flow paths, contaminants are “swept” by the roll-front. The red circle is the contaminated zone. This zone is transformed to blue the circle upon treatment by the FeII/FeIII front
5.2.2 Nano-Fe0 as FeII Generator
Nano-Fe0 in the aqueous phase is certainly a FeII/FeIII producer. FeII species are the main reducing agents for contaminants under both anoxic and oxic conditions (Stratmann and Müller 1994; Nesic 2007; Kiser and Manning 2010; Noubactep 2010c, 2011c, d; Zhuang et al. 2011). Microbial activity could regenerate FeII (bio-corrosion) for more contaminant reduction (Vodyanitskii 2010). In this case, more contaminant is reduced than can be predicted from the reaction stoichiometry. In order words, the operating mode of nano-Fe0 for contaminant reduction can be summarized as follows: (1) Fe0 is oxidized to produce FeII, (2) FeII reduces the contaminant and is oxidized to FeIII and (3) a proportion of FeII is regenerated by the biological reduction of FeIII. Accordingly, before Fe0 depletion, there are three sources of FeII: (1) the Fe0-mediated abiotic oxidation by H2O, (2) the Fe0-mediated abiotic oxidation by FeIII and (3) the biological reduction of FeIII. After Fe0 depletion, the only remaining source of FeII is the biological reduction of FeIII. Provided that the appropriate micro-organism species are present in the subsurface, this process, however, could conceptually proceed for a significantly long-time period (Cullen et al. 2011).
Evidence suggests that such micro-organism colonies can be sustained by a consistent supply of FeII, FeIII and molecular hydrogen (H/H2). Another further process that is worth noting is the generation of atomic or molecular hydrogen (H/H2) by Fe0-mediated hydrolysis reactions, which is likely to aid and the aforementioned biotic processes (Cullen et al. 2011).
The abiotic conversion of FeIII to FeII has been successfully utilised in the hydrometallurgy industry, for example Lottering et al. (2008) reported on the sustainable use of MnO2 for the abiotic regeneration of FeIII for UIV oxidation.
The fate of contaminant reduction products is discussed in the next section.
5.3 Mechanism of Contaminant Removal by Injected Nano-Fe0
The successful application of nano-Fe0 injection technology for in situ remediation is highly dependent on a comprehensive understanding of the fundamental processes governing the processes of contaminant removal. The hitherto discussion has focused on reductive transformations by nano-Fe0. However, contaminant reductive transformation is not a guarantee for contaminant removal (Noubactep 2010c, 2011c). Additionally, certain reaction products are more toxic than their parent compounds (Jiao et al. 2009). Accordingly, efforts have to be focused on the specific mechanism of aqueous contaminant removal. Relevant removal processes include: (1) adsorption, (2) chemical precipitation, (3) co-precipitation, (4) size exclusion or straining and (5) volatilization (Crawford et al. 1993a, b; Noubactep 2007, 2008, 2010c; Vodyanitskii 2010; Eusterhues et al. 2011; Johnson et al. 2011; Noubactep 2011d, e). Chemical precipitation is a characteristic of inorganic compounds when the solubility limit is exceeded (Crawford et al. 1993a, b; Kalin et al. 2005). Volatilization is subsequent to chemical transformation yielding gaseous species like AsH3, CH4, CO2, H2 and N2. In Fe0 reactive barrier systems, contaminants are efficiently removed by the combination of adsorption, co-precipitation and size exclusion within the engineered barrier (Noubactep 2010d; Noubactep and Schöner 2010; Noubactep 2011a). As a result the current discussion concentrates on such processes.
With Fe0 (<1 m2/g) first transformed to voluminous hydroxides species (>100 m2/g) and subsequently transformed to oxides (<40 m2/g), contaminant size exclusion (straining) is driven by the dynamic cycle of expansion/compression accompanying the corrosion process (Pokhrel and Viraraghavan 2008; Vodyanitskii 2010; Noubactep 2011a). During these cycles contaminants are enmeshed and sequestrated in a “matrix” of iron corrosion products.
For conventional nano-Fe0 injection arrays, size exclusion may play an important role (1) in proximity for Fe0 particles and (2) by reducing the pore space during expansive corrosion of the materials. However, if roll-fronts are formed as discussed above, the extent of permeability loss in aquifer will be limited. The roll-front could act as a colloidal reactive barrier for the removal of parent contaminants and reaction products. Species are removed or immobilized by colloids and not because they are reduced. More research is needed to test this hypothesis.
5.3.1 FeII/FeIII Redox Front as a Colloidal Reactive Barrier
Aqueous contaminants have been reported to be quantitatively removed both during abiotic and biotic (1) oxidation of FeII and (2) reduction of FeIII (Pokhrel and Viraraghavan 2008; 2009; Pokhrel et al. 2009; Vodyanitskii 2010). On the other hand, injection of FeIII salts for adsorptive contaminant removal has been reported (Morrison and Sprangler 1993; Morrison et al. 1996). Accordingly, the migration of the FeII/FeIII redox front may be coupled to quantitative contaminant removal by adsorption and co-precipitation.
The primary reason for contaminant removal during these redox reactions is the colloidal nature of in situ generated Fe species (Fe(OH)2 and Fe(OH)3) (Hanna and Boily 2010), which necessarily experience volumetric contraction to form oxides (of FeII or FeIII). Contaminants are first adsorbed by highly reactive colloids and are co-precipitated during transformation to amorphous and crystalline oxides (Ghauch et al. 2010; Noubactep 2011c, d).
6 Concluding Remarks
Constructed geochemical barriers of metallic iron (Fe0) have been used for groundwater remediation since 1996 (O’Hannesin and Gillham 1998; Scherer et al. 2000; Henderson and Demond 2007; Gillham 2010, Comba et al. 2011; Gheju 2011). In recent years, however, nano-Fe0 has received proclaim as a new tool for water treatment due to (1) improvements in reactivity and associated aqueous contaminant removal performance compared with conventional materials and (2) the option of subsurface deployment via injection for targeted in situ treatment of contaminant plumes (Comba et al. 2011; Gheju 2011).
Considering reactivity first, the current work has highlighted the need for prudent use of terminology. Discounting any quantum size effects, which are only prevalent for Fe0 less than approximately 10 nm in diameter, the reactivity of nano-Fe0 as a function of surface area is no more reactive than larger forms. Nano-Fe0 only exhibits such high reactivity due to it significantly high surface area as a function of mass/volume. Despite this, a recent trend in research has been the development of bimetallic nano-Fe0 wherein the combination of a noble metal acts to further increase the reactivity of nano-Fe0. It is argued in the current work that as reactive exhaustion is already achieved by monometallic nano-Fe0 in the order of minutes this seems counterintuitive for the majority of environmental applications.
Considering the nano-Fe0 subsurface injection procedure, in the current work it has been highlighted that the hydraulic mobility of the particles is likely to be significantly retarded by voluminous expansion due to particle corrosion. An alternative nano-Fe0 injection procedure has been suggested herein. The injected nano-Fe0 effectively forms an in situ migrating front which possibly reductively transforms contaminant and removes reduced species by adsorption and co-precipitation.
It is also outlined in the current work that a number of studies with experiments “proclaimed” as analogous to environmental systems are largely overlooked a range of operational drivers including changes in nano-Fe0 (1) reactivity and (2) voluminous as a function of time. It is hoped that the huge literature on redox front migration (Reynolds and Goldhaber 1978; Posey-Dowty et al. 1987; Romero et al. 1992; Read et al. 1993) and the cycle of iron in the hydrosphere (Vodyanitskii 2010) will now be used for the further development of nano-Fe0 injection technology.
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Acknowledgements
Mohammad A. Rahman (Angewandte Geologie-Universität Göttingen) is acknowledged for technical support.
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Noubactep, C., Caré, S. & Crane, R. Nanoscale Metallic Iron for Environmental Remediation: Prospects and Limitations. Water Air Soil Pollut 223, 1363–1382 (2012). https://doi.org/10.1007/s11270-011-0951-1
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DOI: https://doi.org/10.1007/s11270-011-0951-1