Elemental metals for environmental remediation: Learning from cementation process 1 2 3 Noubactep C. Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 Abstract The further development of Fe0-based remediation technology depends on the profound understanding of the mechanisms involved in the process of aqueous contaminant removal. The view that adsorption and co-precipitation are the fundamental contaminant removal mechanisms is currently facing a harsh scepticism. Results from electrochemical cementation are used to bring new insights in the process of contaminant removal in Fe0/H2O systems. The common feature of hydrometallurgical cementation and metal-based remediation is the heterogeneous nature of the processes which inevitably occurs in the presence of a surface scale. The major difference between both process is that the surface of remediation metals is covered by layers of own oxide(s) while the surface of the reducing metal in covered by porous layers of the cemented metal. The porous cemented metal is necessarily electronic conductive and favours further dissolution of the reducing metal. For the remediation metal, neither a porous layer nor a conductive layer could be warrant. Therefore, the continuation of the remediation process depends on the long-term porosity of oxide scales on the metal surfaces. These considerations rationalized the superiority of Fe0 as remediation agent compared to thermodynamically more favourable Al0 and Zn0. The validity of the adsorption/co-precipitation concept is corroborated. Key words: Adsorption; Cementation, Co-precipitation; Surface scale; Zerovalent Iron. Capsule: Hydrometallurgy teaches that sustaining oxide scale formation and transformation on Fe0 is the best way to warrant long service life of iron walls. 1 1 Introduction 26 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 The use of metallic iron (Fe0) for environmental remediation is now well established [1-4]. However, the exact mechanism of aqueous contaminant removal in the presence of Fe0 is not fully understood. It is univocally accepted that contaminant removal is due to the process of iron oxidative dissolution (iron corrosion). However, a net discrepancy exists on the role of the oxide scale on Fe0 in the process of contaminant removal. Oxide scale formation on Fe0 at pH > 4.5 is a fundamental characteristic of aqueous iron corrosion [5-8]. The universal oxide scale on Fe0 is either regarded as beneficial (blessing) or inhibitory (curse) for aqueous contaminant removal in the presence of Fe0. The prevailing concept was introduced in the early phase of investigations regarding the mechanism of aqueous contaminant removal by Fe0 [9,10]. This concept considers that contaminant is removed mainly by an heterogeneous chemical reduction, ideally at the surface of Fe0. Accordingly, the oxide scale on Fe0 is a curse as its represents a diffusion barrier slowing down the kinetics of contaminant removal [11,12]. The initial model assuming the local existence of oxide-free Fe0 in the aqueous solution was proven unrealistic by Bonin et al. [13]. A new conceptual model for the reductive transformation was proposed [13,14]. The conceptual model of Bonin et al. [13] indicated that the reductive transformation is controlled by electron transfer through the surface film. Accordingly the film must be electronic conductive. However, no such conductive film is expected in nature [6,15,16]. Moreover, the concept regarding oxide-scale as curse is built on the premise that Fe0 is a strong reducing agent. The concept is strictly applicable only to reducible contaminants. It is important to notice that the reductive transformation concept has never been univocally accepted [17,18]. For example, Warren et al. [18] wrote that “a convincing mechanism for the reductive dehalogenation of haloorganics by zero-valence metals has not yet been proposed. Matheson and Tratnyek [9] maintained that dehalogenation was not mediated by H2(g) or Fe(II) in the bulk aqueous-phase solution, suggesting that observed reactions take place at the 2 metal surface.” Three years later, O'Hannesin and Gillham [1] acknowledged that “there is a broad consensus that the process is an abiotic redox reaction involving reduction of the organic compound and oxidation of the metal”. Despite this “broad consensus”, the reductive transformation concept 52 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 has felt to explain many experimental observations [19-21]. An alternative concept regards the oxide scale on Fe0 as beneficial (a blessing) for the process of aqueous contaminant removal [22-25]. Independent researchers could traceably demonstrate that quantitative contaminant removal is only observed when iron corrosion products are allowed to precipitate in the system [26-31]. Their results suggest that adsorption and co-precipitation are the fundamental (not the dominant or the major) contaminant removal mechanisms. Accordingly, relevant contaminants could be further (quantitatively) chemically transformed (reduced or oxidized). The first merit of this concept it that its explains why a contaminant like zinc which is non reducible by Fe0 (Tab. 1) could be quantitatively removed in the presence of Fe0 [32]. The present communication is motivated by recent publications speaking disparagingly about the concept of adsorption/co-precipitation as fundamental mechanisms of aqueous contaminant removal in the presence of Fe0 [33,34]. The similarities between aqueous contaminant removal by Fe0 and metal iron cementation on elemental metals (mostly Al0, Fe0, Zn0) will be discussed with the aim to present results from the hydrometallurgical process of cementation which could help to understand and further develop the process of aqueous contaminant removal by Fe0. Both processes are heterogeneous and the metal surface is covered by a scale acting as diffusion barrier. For the sake of clarity the diffusion barrier in the Fe0 remediation will first be presented. 2 Aqueous contaminant removal by metallic iron Aqueous iron corrosion on which remediation with metallic iron is based is an heterogeneous electrochemical process. A simplistic mechanism for iron oxidative dissolution involves four major steps: (i) diffusion of the oxidizing agent (H+, O2, contaminant) to the Fe0 surface, (ii) 3 adsorption of the oxidizing agent onto the iron surface, (iii) the reduction of the oxidizing agent, and (iv) diffusion of reaction products (including Fe 78 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 II species) away from the reactive site on Fe0. Because aqueous iron corrosion (at pH > 4.5) is always coupled to the formation of an oxide scale on the Fe0 surface, the rate of the oxidizing agent diffusion to the iron surface is necessarily the limiting step for the corrosion process which is said to be “diffusion controlled” [7,16]. If, the rate of iron corrosion were limited by the adsorption or electron transfer steps, the reaction would be said to be “chemical controlled”, “surface controlled”, or “reaction controlled” (reaction-limited). The presentation above recalled, that iron corrosion at pH > 4.5 is a “diffusion controlled” or mass transfer limited process. Accordingly, there should have been no need to discuss the active form of rate control in the process of contaminant removal in the presence of metallic iron under subsurface conditions. Clearly, attempts to determine whether the process of contaminant removal in the presence of Fe0 in a field reactive wall is mass transfer or reaction-limited [9,17,35] was not necessary as this was well-documented before the event of the iron remediation technology [25]. In batch experiments or fluidised beds, the rate of contaminant removal by Fe0 could be increased by increasing the mass transfer using various mixing operations (e.g. agitation, stirring, vibration) [36,37]. However, one should acknowledge that such mixing operations are not applicable to packed beds and field reactive walls [25,36,38]. As discussed in details elsewhere [25], the use of various mixing systems with the resulting mixing intensities and their impact on the process of contaminant removal in the presence of Fe0 is the main reason why the inconsistent concept of reductive transformation has survived for more than a decade. The example of the usefulness of mixing operations in investigating processes involving iron corrosion reveals that care must be taken while using well-documented results from other branches of science in designing experiments and/or interpreting new experimental data. A further example is the way to experimentally evidence a chemically controlled reaction. To 4 demonstrate the occurrence of a chemical reaction in a system, the temperature of the system should be varied. An increased reaction rate with increasing temperature is a strong proof for chemical reaction [39]. However, increased contaminant removal with increasing temperature is not necessarily coupled to contaminant reduction by Fe 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 0 as water is also an oxidizing agent and resulting corrosion products are contaminant scavengers. In other words, contaminant removal might only indirectly be coupled to proven chemical reactions. The present communication aims at presenting some aspects of the electrochemical cementation process as used in the hydrometallurgy and discuss their usefulness for metallic iron as currently used in environmental remediation. Two particular aspects will be discussed in some details: (i) the differential reactivity and the suitability of aluminium, iron and zinc as removing agent, and (ii) the proper consideration of the surface scale on Fe0. For the sake of clarity the process of cementation will be first presented. 3 Cementation and its use in the hydrometallurgy Cementation is an electrochemical process by which a more noble metal ion (Mn+ - Eq. 1) is precipitated from solution and replaced by a metal higher in the electromotive series (M1m+ - Eq. 2) [39-45]. Cementation, also known as contact reduction or metal displacement, is necessarily a spontaneous heterogeneous reaction (ΔG0 < 0) that takes place through the galvanic cell M10/M1m+ // Mn+/M (Eq. 3). m Mn+ + m.n e- ⇔ m M0 E0 (V) (1) n M10 ⇔ n M1m+ + m.n e- E10 (V) (2) n M10 + m Mn+ ⇔ n M1n+ + m M0 ΔE0 (V) = E0 – E10 (3) ΔG0 = – z.F.ΔE0 (4) z = n.m is the number of electrons exchanged between M1 and M and F the Faraday’s constant. The thermodynamic basis of cementation can be summarized as follows: The standard free energy (ΔG0 - Eq. 4) of the cementation process after Eq. 3 must be negative (spontaneous 5 reaction). This requires that ΔE0 is positive or E10 < E0. In order words, cementation consists in the spontaneous heterogeneous reduction of a metallic ion present in solution (M 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 n+) by a more electropositive sacrificial metal (M10). M1 is the metal higher in the electromotive series (Table 1). It is evident from table 1 that, from a pure thermodynamic perspective, Al should be the most powerful metal for cementation followed by Zn and Fe. However as will be discussed later the stability of the oxide scale on the individual metals is determinant for the progress of their oxidative dissolution. Cementation is one of the most effective and economic techniques for removing valuable metals from industrial effluents [43,44,45,47,48]. The technique is affordable because of its relative simplicity, ease of control, and low energy consumption. A cementation reaction is an heterogeneous processes limited by diffusion through the mass transfer boundary layer. However, unlike many other heterogeneous reaction systems, cementation reactions are unique in that the reaction product usually does not impede the reaction progress but rather frequently enhances the reaction kinetics ([49] and references therein). Discussing the differential impact of diffusion layers on metals in cementation and contaminant removal is the major reason for this communication and will be presented below. The major difference between both processes relies in the intrinsic nature of each process. However both processes are based on the same concept: The electrochemical reduction. Cementation is a technological process for which the experimental conditions could be case specific optimised. Contaminant removal should be operated on a case-specific basis without changing the chemistry of the system. From this difference it arises that the pH value (and thus the nature of the surface scale) and the mixing operation could be regarded as the two key factors for the design of each system. The further presentation will be focussed on Al, Fe and Zn. 4 Cementation using Al, Fe and Zn The control of the pH value is a key task for the cementation process for a variety of reasons including: (i) corrosion damage of reactors, (ii) excess dissolution of the reducing metal (Al, 6 Fe and Zn), and (iii) hydroxide precipitation. Accordingly, the determination of the optimal pH value is an important economical issue for any cementation plant. The impact of pH on the performance of Al, Fe and Zn as reducing metal will be discussed on the basis of the results from Hg 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 2+ cementation by Al0, Fe0 and Zn0 [50]. The experiments were performed for 30 minutes in Erlenmeyer’s, with an initial mercury concentration equal to 500 mg/L and using 10 mol of reducing agent for each mol of mercury. The pH-dependent evolution of the system was recorded (Fig. 1). Figure 1a represents the variation of final pH value as function of the initial pH for three parallel experiments. Figure 1b represents the variation of the molar ratio dissolved reducing metal to the cemented Hg as function of the initial pH. The stoichiometric ratio is 1.00 for Zn and Fe and 0.67 for Al. Figure 1a clearly shows that, pH stabilises at a constant value for Al (4.7) and Fe (3.7) whereas the pH in the system with Zn was still varying (after 30 min). This behaviour is strongly related to the amount of reducing metal dissolved. Accordingly the order of increasing reactivity based on metal dissolution is: Fe < Al < Zn. Remember that the order of increasing reactivity based on the electrode potential was: Fe < Zn < Al. The difference is certainly due to the differential hydrolysis and solubility behaviour of resulted metallic ions (Al3+, Fe2+/Fe3+, Zn2+) and the adherence of resulting metal oxides to basic surface. These issues will not be discussed here. The most important feature from the pH-dependant cementation is to find the optimal pH for the optimal yield which is ideally the pH where the stoichiometry of the reaction approaches the theoretical value (0.67 for Al and 1.00 for Fe and Zn). Figure 1b shows that, the optimal pH regions are 5.0 - 6.0 for Al, 3.0 - 5.0 for Fe, and 4.0 - 7.0 for Zn. It should be further considered that as pH value increases the precipitation of metal hydroxides is progressively significant. Metal hydroxides are known for their adsorptive properties which are disturbing for the cementation process. Based on these considerations, 7 Anacleto and Carvalho [50] performed their Hg2+ cementation reaction under following conditions: aluminium (3.0 - 4.0), iron (3.0), and zinc (4.0 - 6.0). 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 4.1 Nature of the diffusion layer on reducing metals The presentation above clearly shows that cementation is optimally performed under conditions where ions from the reducing metal (here, Al3+, Fe2+/Fe3+, Zn2+) are soluble and do not readily hydrolyse and precipitate. The precipitating elemental metal (e.g. Hg0) is necessarily insoluble. Therefore, precipitating metals accumulate at the surface of the reducing metal (Al0, Fe0 or Zn0). This metallic layer is porous, and dendritic and thus significantly enhances kinetics of the reaction [41,49]. In essence, only a smooth, coherent deposit can inhibit the cementation reaction. According to Power and Ritchie [40], cementation reactions whose constituent half-reactions have electrode potentials which differ by greater than 0.36 V are likely to be diffusion-controlled (Tab. 1). As recalled above the diffusion is favoured by the porous nature of the metallic deposit which is additionally electronic conductive and constitute a path for electron transport. In other words, the cementation process continues despite the metallic scale for two main reasons: (i) metallic ions are soluble and transported through the porous layer to the bulk solution, (ii) the metallic layer (cemented deposit) is electronic conductive. Consequently, for contaminant reduction to be quantitative in a Fe0/H2O system, the oxide scale should be electronic conductive and porous. 5 Diffusion layers on remediation elemental metals Diffusion is a spontaneous process involving mobility of species due to the existence of a concentration gradient in a system. The extend of diffusion depend on (i) the properties of the diffusing species (including their size), and (ii) the structure of the diffusion layer (connectivity, morphology, porosity, pore site distribution or tortuosity). Here the diffusion layer is a precipitated scale (oxide scale). 8 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 Oxide layers on remediation metals are formed at pH > 5.0 which is the pH of natural waters (assuming comparable redox potential). Upon immersion in an aqueous solution, any reactive metal is instantaneously covered by an oxide scale [6]. The initial scale is possibly porous (non-protective film) but may be more or less rapidly transformed to an impervious scale (protective film). The porosity of the oxide scale is very determinant for the progress of metal oxidative dissolution which is coupled to oxide scale formation and contaminant removal. It is well-documented that upon immersion, the surface of aluminium is rapidly covered by a very thin and adherent layer of oxide (protective layer). Accordingly, despite theoretical thermodynamic suitability, Al is a worse remediation metal than Fe and Zn. As seen above (Fig. 1a), Zn is the most efficient cementation agent because of its more rapid dissolution. However, because ZnII is the only soluble Zn species, the progress of the dissolution will yield to a formation of a dense oxide film on Zn0 which will progressively develop to an impervious layer with the time. For Fe0, the existence of two soluble species (FeII and FeIII) and several iron oxides with different crystal structures [24] is a guarantee for the long term non-protectiveness. Accordingly, Fe0 is best remediation agent. The non toxic nature of iron species and the lost-cost of Fe0 materials are further reasons for its intensive use as remediation agent. 6 Concluding remarks The formation of surface scale on immersed elemental metals is a common feature for remediation with metallic elements and electrochemical cementation (Tab. 2). In both cases the surface scale primarily inhibits the metal dissolution and thus the kinetics of the concerned process. The formation of an oxide film on the cementation agent can be prevented (or limited) by a rational selection of the operational conditions (e.g. pH value, amount of cementation agent, and mixing operations). Provided these operational conditions are accurately selected, the cementation process should not be essentially inhibited by the metal deposit which is even beneficial in some cases [43,45,50]. 9 The avoidance of the oxide scale formation on elemental metals under natural conditions is not possible. Therefore, one could only discuss or access their porosity and their electronic conductivity. As a rule an electronic conductive oxide scale can not be expected under environmental conditions. In fact, regardless from the availability and abundance of molecular oxygen (anoxic or oxic conditions), Fe 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 0 is always covered by a multi-layer of oxide and hydroxide mixture of which only magnetite (Fe3O4) and unstable forms (FeO, green rusts) are electronic conductive. Consequently, the reactivity of Fe0 for environmental remediation is mostly due to the porosity of the oxide scale and factors influencing its evolution (e.g. pH value, water salinity, nature of contaminants). To sustain Fe0 reactivity under environmental conditions, appropriate reactive materials should be selected or manufactured. In this regard, porous composites like those used in SONO arsenic filters could be used [51,52]. In conclusion, a careful consideration of the optimal conditions for the hydrometallurgical process of cementation using Al0, Fe0, and Zn0 has enabled the precision of the role of oxide scale in the process of contaminant removal with the same metals. Its appears that considering the oxide scale as a curse for the remediation process was a mistake. The oxide scale is rather beneficial for the process of contaminant removal by Fe0. Moreover, removed contaminants and their potential reaction products are progressively enmeshed in the matrix of ageing corrosion products and are very stable under natural conditions. Accordingly, instead of maintaining an inconsistent concept [33,34,53,54], the scientific community should focus his attention on ways to sustain the corrosion process rather to try to free the Fe0 from spontaneously generated corrosion products. 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Geochem. 24 (2009) 2208–2210. [54] S.-H. Kang, W. Choi, Response to comment on “oxidative degradation of organic compounds using zero-valent iron in the presence of natural organic matter serving as an electron shuttle”, Environ. Sci. Technol. 43 (2009) 3966–3967. 16 Table 1: Standard electrode potential of selected metals relevant for hydrometallurgical cementation. Electrode potentials are arranged in increasing order of E 389 390 391 392 393 394 395 396 0. An electrochemical reaction occurs between an oxidant of higher E0 and a reducing agent of lower E0. In other words, a more noble metal ion is precipitated from solution and replaced in solution by a metal higher in the electromotive series. It is clear that the three most powerful reducing agents are Al, Zn and Fe. E0 values are from ref. [46]. Electrode Reaction E0 Eq. (V) Al3+/Al Al3+ + 3 e- ⇔ Al0 -1.660 (5) Zn2+/Zn Zn2+ + 2 e- ⇔ Zn0 -0.763 (6) Fe2+/Fe Fe2+ + 2 e- ⇔ Fe0 -0.440 (7) Cd2+/Cd Cd2+ + 2 e- ⇔ Cd0 -0.403 (8) Ni2+/Ni Ni2+ + 2 e- ⇔ Ni0 -0.250 (9) Pb2+/Pb Pb2+ + 23 e- ⇔ Pb0 -0.126 (10) H+/H2 2 H+ + 2 e- ⇔ H2 0.000 (11) Cu2+/Cu Cu2+ + 2 e- ⇔ Cu0 0.337 (12) Cu+/Cu Cu+ + e- ⇔ Cu0 0.521 (13) Pb4+/Pb Pb4+ + 4 e- ⇔ Pb0 0.700 (14) Hg22+/Hg Hg22+ + 2 e- ⇔ 2 Hg0 0.789 (15) Ag+/Ag Ag+ + e- ⇔ Ag0 0.799 (16) Hg2+/Hg Hg2+ + 2 e- ⇔ Hg0 0.854 (17) Au3+/Au Au3+ + 3 e- ⇔ Au0 1.290 (18) 397 398 17 Table 2: Characteristic features of the electrochemical processes of cementation and metal- based remediation. The processes further differ by the fate of the surface scale. While cemented metal deposits are recovered, metal oxides are responsible for contaminant removal but also for porosity loss. 398 399 400 401 402 Process Objective pH Surface scale nature porosity conductivity Cementation Metal recovery < 5.0 Metal high high Remediation Decontamination > 6.0 Metal oxide variable low 403 404 405 18 Figure 1 405 406 1 2 3 4 5 6 7 1 2 3 4 5 6 (a) Zn Fe Al fin al p H initial pH 407 1 2 3 4 5 6 7 0,0 0,5 1,0 1,5 2,0 1.00 0.67 (b) Zn Fe Al M 1m + :H g initial pH 408 409 410 411 412 Figure 1: Final pH value (a) and molar ratio dissolved metal to cemented Hg (b) with different initial pH values. The lines are not fitting functions, they simply connect points to facilitate visualization. Data from ref. [50]. 19