Metallic Iron for Environmental Remediation: Learning from the Becher Process 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Noubactep C. Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. Tel. +49 551 39 3191, Fax: +49 551 399379 e-mail: cnoubac@gwdg.de Abstract Metallic iron (Fe0) is a moderately reducing agent that has been reported to be capable of reducing many environmental contaminants. Reduction by Fe0 used for environmental remediation is a well-known process to organic chemists, corrosion scientists and hydrometallurgists. However, considering Fe0 as a reducing agent for contaminants has faced considerable scepticism because of the universal role of oxide layers on Fe0 in the process of electron transfer at the Fe0/oxide/water interface. This communication shows how progress achieved in developing the Becher process in hydrometallurgy could accelerate the comprehension of processes in Fe0/H2O systems for environmental remediation. The Becher process is an industrial process for the manufacture of synthetic rutile (TiO2) by selectively removing metallic iron (Fe0) from reduced ilmenite (RI). This process involves an aqueous oxygen leaching step at near neutral pH. Oxygen leaching suffers from serious limitations imposed by limited mass transport rates of dissolved oxygen across the matrix of iron oxides from initial Fe0 oxidation. In a Fe0/H2O system pre-formed oxide layers similarly act as physical barrier limiting the transport of dissolved species (including contaminants and O2) to the Fe0/H2O interface. Instead of this universal role of oxide layers on Fe0, improper conceptual models have been developed to rationalize electron transfer mechanisms at the Fe0/oxide/water interface. Keywords: Becher process, Leaching, Oxide layers, Remediation, Zerovalent Iron. Capsule: Results from Becher process are valuable for mass transfer in Fe0/H2O systems. 1 Introduction 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Permeable reactive barriers of elemental iron (Fe0 walls or remediation Fe0/H2O systems) are a valuable technological application that has been shown to be both environmentally friendly and cost-effective in the removal of various substances from contaminated waters [1-7]. Since its development in 1990 by Canadian hydrogeologists numerous papers have been written on the topic and approximately 120 iron walls installed worldwide [4]. Even though the importance of the mechanism of contaminant removal was recognised in the early stage of technology development [1,8], there is still discussion about the mechanism of contaminant removal in Fe0/H2O systems [9-12]. The main reason cited for reported discrepancies was the improper consideration of the significance of the oxide-film on the process of contaminant removal [11,12]. In fact, after considerable initial scepticism [2], it was accepted that contaminants are removed in Fe0/H2O systems by an abiotic reductive transformation. Based on this consensus, “the role of oxides in reduction reactions” were discussed [13]. Accordingly, the oxide layers may act either as: (i) a semiconductor to relay electron transfer from Fe0 to the contaminants (direct reduction) or (ii) FeII adsorbent to yield very reactive adsorbed FeII (so-called structural FeII for indirect reduction). Additionally, electron transfer from the Fe0 body exposed by pitting of the oxide layer to contaminants is likely to occur. The objective of this paper is to show how the consideration of progress achieved in developing the Becher process in the hydrometallurgy, could accelerate the comprehension of processes in Fe0 walls by carefully surveying the impact of the oxide-layer on contaminant mass transfer to the Fe0/H2O interface. For the sake of clarity the Becher process is presented first. The Becher process The Becher process is an environmentally friendly, cost-effective extraction method for upgrading ilmenite from 55 % TiO2 to about 94 % TiO2 by reduction with charcoal [14-22]. A typical feed made up of 55 % ilmenite (FeTiO3), 38 % pseudorutile (Fe2Ti3O9) and 7 % rutile (TiO2) is heated in air at about 1000 °C to oxidise the FeII in the ilmenite to FeIII resulting in a 2 phase known as pseudobrookite (Fe2TiO5 = Fe2O3 + TiO2). Pseudobrookite is then reduced with coal (C/CO) in an iron reduction kiln at 1100 °C. This carbothermic reaction reduces the iron in the pseudobrookite to the metallic (Fe 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 0). The reduction product (reduced rutile - RI) consists mainly of grains of Fe0 embedded in a matrix of titanium dioxide [19,22]. After cooling RI (approximately 2 Fe + TiO2), embedded Fe0 should be leached from (or “rusted out” of) the matrix (about 58 % Fe) at near neutral pH. Leaching Fe0 from RI is achieved by oxidation with dissolved oxygen from air (so-called “aerial oxidation”) mostly with ammonium chloride (NH4Cl) acting as a catalyst for rusting. Since the solution is not buffered, the pH will rise resulting in the formation of various iron oxides. Magnetite (Fe3O4) is the preferred product since it can be magnetically separated from TiO2. Ideally, RI should be upgraded by dissolved O2 in the presence of NH4Cl from 42 % to approximately 94 % TiO2. However, the oxygen leaching of Fe0 suffers from a serious limitations imposed by the sluggish mass transport rates of low soluble oxygen (≤ 8 mg/L) to the interface Fe0/H2O. This physical barrier of iron corrosion products (oxide-layer or oxide-film) characterizes the similarity between the leaching process in the Becher process and contaminant removal in Fe0/H2O systems. Mechanism of iron removal in the Becher process Aqueous iron oxidation (corrosion) by dissolved oxygen (O2) has been extensively described in the literature, including the aeration step in the Becher process for the manufacture of synthetic rutile [14,15,18,19,22]. The overall reaction can be represented as: 2 Fe0 + 3/2 O2 + x H2O ⇒ Fe2O3.xH2O [1] The primary process is essentially an electrochemical reaction made up of (i) an anodic Fe0 oxidation (Eq. 2), and a cathodic O2 reduction (Eq. 3). Fe2+ ions from Eq. 2 are (at least partially) further oxidized by dissolved O2 to Fe3+ ions (Eq. 4), which are subsequently hydrolysed and precipitated as Fe2O3.(x-6)H2O (Eq. 5). 2 Fe0 ⇒ 2 Fe2+ + 4 e- [2] 3 O2 + 2 H2O + 4 e- ⇒ 4 HO- [3] 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 4 Fe2+ + O2 + 4 H+ ⇒ 4 Fe3+ + 2 H2O [4] 2 Fe3+ + x H2O ⇒ Fe2O3.(x-6)H2O + 6 H+ [5] It was established that the iron oxide products varied with time, yielding an iron oxide mixture of unknown composition (FeOOH, Fe2O3, Fe3O4) [15,17]. A survey of the reactions above reveals three major features: (i) dissolved oxygen is consumed by both heterogeneous (electrochemical Fe0 dissolution) and homogenous (Fe2+ oxidation) reactions; (ii) additional solid phases are formed (FeOOH, Fe2O3, Fe3O4) during the process leading to a “three”-phase (Fe0-oxides-H2O) reaction system; (iii) in-situ formed solid phases influence the rates of O2 mass transfer to the Fe0 surface. Clearly, iron oxides as inert solid phase influence O2 mass transfer and thus the kinetics of heterogeneous reactions at the interface Fe0/H2O. The impact of iron oxide will increase with increasing reaction time since the amount of oxides is increasing. Accordingly, the heterogeneous reactions at the Fe0 surface can be divided into two phases: (i) an initial constant rate phase, followed by (ii) a second phase, in which a steep decrease in rate is expected and was observed [15]. Becher process versus contaminant removal in Fe0/H2O systems In the Becher process iron removal from a matrix of reduced ilmenite (RI - 58 % Fe0 and 42 % TiO2) by oxidation with dissolved O2 is the goal. The oxidation is performed in the presence of NH4Cl to limit iron oxide precipitation in the vicinity of RI (see below). Nevertheless, iron oxides precipitate and the goal of a matrix with 94 % TiO2 is sometimes difficult to achieve [19,22]. Generated iron oxides are regarded as inert micro-particles influencing mass transfer between RI and dissolved O2. To accelerate mass transfer and thus maximize O2 leaching in the Becher process, RI might be ground to small particle sizes and/or intensive solution mixing (stirring, agitating) yielding particle suspension might be applied. 4 In permeable reactive barriers, an elemental iron material (usually > 80 % Fe) is used; Fe0 oxidation is ideally induced by the oxidized form of a contaminant. No external addition of chemical to limit iron oxide precipitation is suitable. The aquifer might contain chloride ions (Cl 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 -) known for their destabilizing effects on the formation of oxide layers on Fe0. With regards to the objective (contaminant removal), Fe0 oxidation by dissolved O2 should be considered a disturbing factor yielding iron oxides that should act as physical barrier for contaminant transport to the Fe0/H2O interface (as in the Becher process). This is the fundamental role of an oxide-layer (oxide-film), which should have been considered as the rule. The exceptions of conducting oxide-layers or lattice FeII surface sites [13,23,24] could be treated separately. This has not been the case, showing that lessons from the Becher process are very helpful for the development of the Fe0 technology. Contrary to the Becher process, acceleration of mass transfer of contaminant to the Fe0/H2O interface should be limited to conditions of mass-transfer control [11,12]. Under these conditions material suspension should not occur and the formation of the oxide layer on Fe0 should not be significantly disturbed. The most important features common to the Becher process and contaminant removal in remediation Fe0/H2O systems are: (i) iron dissolution and iron precipitation yielding a complex oxide mixture of unknown composition, (ii) Fe0 and FeII are potential reducing agents, and (iii) the weight fraction of iron oxide particles would increase from zero at the beginning to higher proportions depending on the reaction progress. The net result for both processes is a multi-solid reaction system involving two types of solids of widely differing size (RI/Fe0 and iron oxides). The large changes in the solid composition during the reaction can be expected to influence the mass transfer of species to the Fe0/H2O interface and thereby play a significant role in the determination of reaction rates. The major difference between both processes is that iron oxides can be regarded as inert in the Becher process but are contaminant adsorbents in Fe0/H2O systems. Furthermore, during their 5 polymerisation and precipitation, iron (hydr)oxides may sequester contaminants in their matrix (so-called co-precipitation) [11,12]. Therefore, contaminants can be immobilized by oxide-layers on Fe 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 0 regardless of any chemical or electrochemical redox reaction, possibly involving Fe0. Finally, oxides in Fe0/H2O systems are good adsorbents for FeII from Fe0 oxidation yielding adsorbed FeII (structural FeII). Structural FeII is a very strong reducing agent for inorganics and organics [25]. Table 1 summarises some relevant processes involved in remediation Fe0/H2O systems and in the Becher process. It should be acknowledged that increasing attention has been paid to the role of oxide layer on Fe0 in the mechanisms of contaminant reductive transformation in the literature [8,10,13,24,26-29,35]. However, these experiments were mostly performed to gain information on the nature of surface coatings and their influence on kinetics and reaction pathways of the reductive transformation of water pollutants. While these experiments show the role of oxide layers (iron oxides/hydroxides) on Fe0 as possible electron mediators, the Becher process indicates that oxide layers fundamentally act as physical barriers. Recent works [11,12] demonstrated that contaminants are primarily removed by adsorption and co- precipitation within the oxide layers and are stable as long as the iron oxide is not dissolved. Lessons from the Becher process: Modelling is now a fundamental step in science, since it allows a better understanding of investigated systems. Modelling can be used to support experimental design, operation control, experiment planning, experiment optimization, experiment scaling, and system or results analysis [30]. Upon adequately representing a system, one might be able to scale-up, design, optimize, or control a system. To be efficient, each model needs (i) relevant parameters, (ii) relevant data, and (iii) expertise of the modeller in terms of adequate computational methods. Models for the adequate understanding of remediation Fe0/H2O systems could benefit from expertise from the Becher process if they considered the above- mentioned differences (see also table 1). In particular, the interplay between mass transfer and 6 kinetics encountered in the Becher process will help to better plan experiments as well as improve the interpretation of results from extensive studies on the kinetics of contaminant removal in Fe 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 0/H2O systems. Similar to the Becher process, this involves both Fe0 and FeII oxidation by relevant dissolved species (including contaminants and oxygen). Several different types of models have been developed with regards to the Becher process [17,18]. These models consider various aspects of the operation such as O2 mass transfer and the presence of solid, liquid and gaseous phases. For example, the model developed by Geetha and Surender [18] considered the Becher process as a multi-phase system involving a complex interplay between mass transfer and chemical reactions. This model can be further developed to meet the specifics of remediation Fe0/H2O systems. In particular the following aspects need to be considered: (i) the fact that iron dissolution should be coupled to contaminant removal, (ii) the limited availability of oxygen, which is a disturbing factor (concurrent for Fe0 oxidation), (iii) the fact that iron oxides are not inert particles on mass transfer rates but effective contaminant removing agents (adsorption and co-precipitation), and (iv) the specific interactions of individual contaminants with the oxide layer on Fe0. It can be emphasized that the overall process of contaminant removal under field conditions will be controlled by an internal diffusion through the oxide layer, since almost all contaminants are larger in size than molecular oxygen (the oxidant of the Becher process). Another important result from the Becher process is highlighted by the work of Farrow et al. [21] who compared the role of sodium chloride (NaCl) and ammonium chloride (NH4Cl) on the process of selective iron leaching. It is well-known that the chloride ion plays an important role in the breakdown of oxide layers and is therefore a desirable constituent in any electrolyte solution used for the promotion of iron corrosion [31,32]. To elucidate the role of ammonium ions in the leaching reaction, parallel experiments with 0.1 M of each chloride were carried out at 70°C. In both cases the dissolved iron concentration and the bulk solution pH were monitored during the course of the reaction. The results showed that the ammonium 7 chloride plays three distinct and important roles in the selective leaching [21]: (i) NH4+ acts as a buffer for hydroxyl ions (OH 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 -) and prevents excessively high local pH values, which might otherwise cause precipitation of iron (II) hydroxide before the iron (II) ions could diffuse from the rutile matrix; (ii) the ammonia (NH3) formed as a result of this buffering reaction complexes iron (II) ions until they also have moved away from the regions of high pH, thus preventing the precipitation of iron (II) hydroxide in the pores of the rutile; (iii) the chloride ion helps to break down any passive films which might form during aeration. The large difference in the leaching rates for NaCl and NH4Cl could be explained using pH data, which indicate that most of the reactions in NH4Cl takes place at a pH of about 4, and most of the reactions in NaCl takes place at a pH between 9 and 10. These results show clearly that in investigating process relevant for remediation Fe0/H2O systems while avoiding iron oxide precipitation, it will be more advantageous to work with NH4Cl than with strong chelating agents like ethylenediaminetetraacetic acid [1,33] of larger molecular size and little buffering capacity. A further relevant result from the Becher process is obtained by Bruckard et al. [22] on the optimisation of selective iron leaching using the anthraquinone-based redox catalysts while guaranteeing the formation of the preferred magnetite as the reaction product. This result could be very important for enhancing long-term reactivity of Fe0/H2O systems. The redox catalysts are effective for three main reasons [22, 34]: (i) their electrochemical reactions are fast and reversible, (ii) their redox potentials are located between those of the FeII/Fe0 and H2O/O2 couples such that their oxidised forms (the anthraquinones) oxidise metallic iron and their reduced forms (the semi-quinones or hydroquinones) are oxidised by oxygen, and (iii) both the reduced and oxidised forms of the catalysts are stable in solution. A modest enhancement of contaminant reduction by Fe0 in the presence of several quinonoid model compounds (e.g. juglone, lawsone, and anthraquinone disulfonate) was also reported by Tratneyk et al. [24,35] in remediation Fe0/H2O systems. However, the observed effects were 8 not expected to be significant in field applications. Yet, if quinonoid model compounds effectively favour the formation of electronic conductive magnetite (Fe 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 3O4) as reported by Bruckard et al. [22], their effects for long-term reactivity of Fe0 walls should be significant. More research under conditions pertinent to remediation Fe0/H2O systems is needed to clarify this issue. Concluding remarks While the complexity of mass-transfer in remediation Fe0/H2O systems is yet to be properly assessed, the Becher process for the manufacture of synthetic rutile has already dealt with the prediction of solid-liquid mass transfer in systems in with particles of various densities and sizes are available [15,17-19]. The results achieved in developing the Becher process could be very useful to investigations regarding processes in Fe0 walls. In particular, the interplay between mass transfer and kinetics reported in the Becher process could help in improving the interpretation of data from extensive investigations on the kinetics of contaminant removal in Fe0/H2O systems. For this purpose, specifics of the Fe0/H2O system (limited O2 availability, reactivity of the oxide layer, dynamic nature of the film formation and transformation) should to be properly considered and data should be obtained under relevant experimental conditions [11,12]. Furthermore, electron mediators have been unambiguously described in the Becher process [22]. Electron mediators facilitate electron transport from Fe0 to reducible species (oxygen, contaminants), they can undergo many successive reduction/oxidation cycles in the process and act as catalysts. Catalytic effects of quinonoid model compounds in remediation Fe0/H2O systems should be further investigated. 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Irrespective of the cathodic reactants (O2, contaminants), the rate of aqueous iron corrosion is proportional to the total amount of reducible species transferred to the Fe0 surface. In the Becher Process, enhanced O2 mass-transfer rate can be achieved by various mixing operations and the extent of Fe0 dissolution increased for example by using ammonium chloride. In groundwater remediation species transport to Fe0 depends on natural hydrodynamic conditions. Process Groundwater treatment Becher process Objective Contaminant removal Upgrading of reduced ilmenite (42 % TiO2) to synthethic rutile (94 % TiO2) Used material Fe-based material (up to 99.0 % Fe) Reduced ilmenite (approx. 58 % Fe) Method Oxidation of Fe0 by contaminants Selective oxidation of Fe0 by O2 Role of FeII suitable contaminant reducing agent within the oxide layer O2 scavenger within the oxide layer (non-relevant process – side reaction) Nature Environmental process (controlled exclusively by environmental conditions) Industrial process (O2 leaching process can be intensified e.g., by the use of ferrous chloride) Conditions 6.0 < pH < 9.5 pH > 6.0 Temperature lower than 20 °C Temperature: ambient to 130 °C limited O2 availabiliy (< 2 mg/L) O2 available (ideally 8 mg/L) no iron oxyhydroxides at t = 0 no iron oxyhydroxides at t = 0 various iron oxyhydroxides at t > 0 various iron oxyhydroxides at t > 0 Limitations (passivation) Fe-oxides limit mass transport rates of species (incl. O2) to the Fe0 Fe-oxides limit mass transport rates of oxygen to the Fe0 surface Remedy to passivation Use of bimetallics and nanoparticles. Use of ultrasound. use of ammonium chloride as a buffering and complexing agent (initial pH = 4.5) 332 14