Investigating the Processes of Contaminant Removal in Fe0/H2O Systems 1 2 3 4 Noubactep Chicgoua Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37005 Göttingen, Germany. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, FAX: +49 551 399379 Journal Name: The Korean Journal of Chemical Engineering Specific Area: Environmental Engineering Paper number: KJ2011-030 Submission date: 2011-01-12 (Accepted: 2011-12-15) Korean Journal of Chemical Engineering 2010 Impact Factor: 0.748 Abstract The instability of the premise of direct quantitative contaminant reduction by elemental iron (Fe0) materials in Fe0/H2O systems is pointed out. Recalled basic knowledge on aqueous iron corrosion shows that Fe0 surface is not available for decontamination in nature. A comparison of the reactivity of Fe0 and Zn0 shows that the effectiveness of Fe0 materials for environmental remediation is due to the formation of a non-adhesive, porous oxide scale on Fe0. Contaminants are enmeshed within the scale and possibly reduced by primary FeII and H/H2. An evaluation of current experimental conditions shows that well-mixed batch systems have disturbed the process of scale formation. Therefore, the majority of published works has operatively created conditions for contaminant reduction that are not likely to occur in nature. Since working under such unrealistic conditions has mediated the above mentioned premise, interactions in Fe0/H2O systems yielding contaminant removal should be revisited. Keywords: Adsorption, Decontamination, Reduction, Remediation, Zerovalent iron. 1 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 The strong oxidation state dependent toxicity of several pollutants has prompted the development of remediation strategies that stimulate redox reactions in wastewaters, contaminated soils, sediments, and groundwater [1-13]. Thereby, hazardous species are transformed to less toxic or less mobile species through a redox reaction. For example, chromium(VI) reduction to chromium(III) substantially decreases this metal’s solubility, mobility, and toxicity [12,14]. The same principle governs the expected reduction of uranium(VI) to uranium(IV) [15-17]. Tested geochemical approaches included injection of reactive solutions [18,19], permeable reactive walls of elemental iron materials [4,20,21] or other reactive minerals such as FeS2 and FeCO3 [22-24], and injection of H2S [25]. Biogeochemical strategies rely on supplying organic carbon (OC) to stimulate direct microbial transformation of pollutants and indirectly microbially mediated reactions [26,27]: The use of Fe0-based alloys (zerovalent iron, Fe0 materials or Fe0) is based on the premise, that quantitative contaminant removal in Fe0/H2O systems is mostly due to contaminant reduction through electrons from the metal body (direct reduction) [28,29]. Because of the electrochemical nature of aqueous iron corrosion [30], this is only possible at the Fe0 surface or at the surface of an overlaying electrically conductive oxide-film (e.g. Fe3O4). However, given that under typical ranges of sub-surface pH (6 ≤ pH ≤ 9) there are several possible contaminant removal mechanisms (adsorption, co-precipitation, precipitation) in Fe0/H2O systems, it is doubtful whether direct reduction may significantly contribute to contaminant removal as it is currently accepted – and repeated in the introduction of many articles/books. For the sake of clarity, it should be stated that contaminant reduction is not a stand-alone removal mechanism as reduction products must be removed from the aqueous phase. The objective of the present communication is to recall the instability of the concept of direct reductive contaminant removal through Fe0 in Fe0/H2O systems which is currently the main prop that holds up the iron reactive barrier technology. The study is based on a literature 2 review on the process of iron corrosion and the reductive properties of FeII species and H/H2. It is shown that quantitative contaminant removal can only occur within the film of corrosion products (oxide-film) at the vicinity of the 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 0 surface. The process of iron corrosion will first be presented, followed by a discussion of the contaminant interactions within the oxide-film and two strong arguments against direct reduction through Fe0 materials. 2 The Process of Aqueous Iron Corrosion: Oxide-film Generation Under aqueous conditions elemental iron (Fe0) may dissolve (solvation) to an oxidized FeII specie (active dissolution or corrosion) according to Eq. (1), or form a second phase film – usually an insoluble 3D surface oxide-film (passivation) – according to Eq. (2): Active dissolution: Fe0 + 6 H2O ⇔ Fe(H2O)62+ + 2 e- (1) Passivation: Fe0 + 2 H2O ⇔ Fe(OH)2 + 2 H+ + 2 e- (2) Active dissolution and passivation are competing reactions. An oxide-film will form whenever the metal solubility is low (circumneutral pH values – Fig. 1). The further active metal dissolution (corrosion) depends on the protectiveness of the generated oxide-film. Whether a film is protective or not primarily depends on the relative unit-cells (packing distances or lattice parameters a, b, c, see Tab. 1) of the metal and its oxides [33,34]. In the case of iron, the unit-cells of the elemental metal and its oxides are not particularly close (Tab. 1); thus there is no tendency for an iron oxide-layer (oxide-film) to adhere to metallic iron (Fe0). Therefore, an oxide-film instantaneously forms but is constantly flaked off and exposes fresh iron surface to the environment [33,34]. Iron corrosion will normally continue until the material is depleted. The formation of an oxide-film on a Fe0-material upon aqueous corrosion is characteristic for pH > 4.5 [35,36] and further depends on several parameters: reactivity of the underlying Fe0- material, temperature, composition, and water flow velocity. These parameters determine the structure, the thickness and the porosity of the generated film. Such films grow by counter migration of FeII species from the Fe0 surface (outward migration) and H+, O2, and other 3 solutes from the flowing water (inward migration). FeIII species are generated, they migrate, precipitate and are possibly reduced within the oxide-film. The driving forces for species transport are mostly electromigration (ionic species) and concentration gradients (all species). 77 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 The reaction scheme for film formation can be divided into four steps [37]: (i) active dissolution (Eq. 1), (ii) transition range, (iii) pre-passive range, and (iv) oxide-film formation. In the transition and pre-passive range the metal becomes progressively covered by Fe(OH)2 and/or Fe(OH)3 adsorbates. These adsorbates increasingly retard the active dissolution. The oxide-film is subsequently formed when the Fe0 is completely covered with Fe(OH)2 / Fe(OH)3 and deprotonization leads to the formation of a layer of iron oxide and oxyhydroxides (iron oxides: FeOOH, Fe2O3, Fe3O4…). It is very important to note that, once formed, the oxide-film should not be considered as a rigid layer, but instead as a system in dynamic equilibrium between film dissolution and film growth [37]. In other words, the oxide-film can adjust its composition and thickness to changing environmental factors (time, groundwater composition, temperature, microbial activity). In particular, the presence of some components (e.g. PO43-) in groundwater may favour the production of insoluble corrosion products, possibly leading to an impervious and tenacious oxide-film. The corrosion reaction becomes self-limiting, as the corrosive medium can no longer diffuse through the oxide-film. Other groundwater components (e.g. Cl-), will disturb the formation of continuous oxide-films or increase their porosity [38]. In this case Fe0 active dissolution will continue until the material is depleted. As already discussed, Fe0 materials own their suitability for groundwater remediation to the fact that the oxide-film continually flakes off and exposes the Fe0 surface to the corrosive aqueous environment. However, new films are suddenly generated such that the bare Fe0 surface is not accessible for contaminants as a rule. In fact, all contaminants (organic and inorganic) interact more or less strongly with the overlaying iron oxides [39]. The next section will discuss some of these interactions. 4 3 Contaminant/Oxide-Film Interactions in Fe0/H2O Systems 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 As shown clearly, in a Fe0/H2O system an interface Fe0/H2O does not exist as a rule [17,35, 40-42], but rather two interfaces: (i) Fe0/oxide-film, and (ii) oxide-film/H2O. Even though the metal surface may be temporally accessible at locations where the oxide-film is discontinued [40], this can not be a rule. With these rare exceptions, to reach the Fe0 surface, any contaminant must found its way across the oxide-film. Therefore, investigations regarding contaminant removal in Fe0/H2O systems should always be conducted under conditions favouring the generation and the transformation of an oxide-film (ideally stagnant conditions, see below). In a Fe0/H2O system, a contaminant may be subject to four types of reactions: adsorption (physical and chemical), co-precipitation, oxidation and reduction. Oxidation reactions are not further considered in this communication. However, it is considered that oxidation products must be removed from the aqueous phase by adsorption, co-precipitation, precipitation or volatilization. Three different reduction pathways were specified [21,43,44]: (i) direct electron transfer from iron metal (direct reduction), (ii) catalyzed reduction by molecular H (or atomic H) from H O reduction; the reaction is catalyzed by the 2 2 Fe0 surface or the Fe oxyhydroxides surface, (iii) catalyzed reduction by structural or dissolved FeII at the Fe oxyhydroxides surface (FeII derived both from iron corrosion and from oxide dissolution). Therefore, potential roles of the oxide-film include [41,45]: (a) serving as an electron and ion transport barrier, hence reduction of solutes may occur primarily at pits or similar defects; (b) behaving as a conductor or semiconductor, which allows charge to pass across the interface with some resistance; (c) functioning as a catalyst, where adsorbed FeII (and H/H2) provides strongly reducing surface sites, and (d) serving as FeII source whereas FeIII oxides are reduced chemically (e.g. by H2) or biologically. The redox reactivity of a Fe0/H2O system is believed to primarily depend on the chemical thermodynamics of the two redox-systems of iron [46]: FeII/Fe0 (E0 = -0.44 V) and FeIII/FeII 5 (E0 = 0.77 V). Therefore, the aim of using Fe0 in groundwater remediation under anoxic conditions has been to exploit the negative potential of the couple Fe 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 II/Fe0 to degrade or immobilize redox-labile contaminants (direct reduction). However, dissolved ferrous iron from the FeIII/FeII redox couple can act as reductant for soil components (e.g. MnO2) and contaminants (indirect reduction 1). Furthermore, it has been shown that adsorbed or structural FeII (structural FeIII/FeII: -0.34 ≤ E0(V)≤ 0.65) can be more powerful in reducing contaminants [47] (indirect reduction 2) than the Fe0 surface (E0 = -0.44 V). On the other side recent results from Strathmann and co-workers [48] demonstrated that, when complexed with organic substances, aqueous FeII (dissolved organic FeIII/FeII: 0.520 ≥ E0(V) ≥ -0.509) is significantly more powerful than aqueous FeII (E0 = 0.77 V) (indirect reduction 3). Therefore, abiotic contaminant reduction in a Fe0/H2O system does not necessarily take place by reduction through electron from Fe0 (direct reduction). It is to keep in mind that this discussion considered molecular (H2) and atomic (H) hydrogen as potential reducing agents without considering further details on the reaction mechanism. Contaminant migration through an oxide-film to the Fe0 surface may be a process which is limited by size exclusion effects [49]. Since contaminants are mostly larger than H2O in their molecular sizes, in extreme cases, inner layers of the film may only be accessible to protons (H+) and water molecules (H2O). Consequently, with decreasing porosity, iron is corroded merely by water (H2O or H+). Even under external (groundwater) oxic conditions oxygen may deplete completely within the oxide-film (e.g. through oxidation of FeII, as discussed above) [50]. In this case, contaminant reduction can only be the result of structural FeII or hydrogen (H or H2) activity in the oxide-film or at the film surface. The reaction products remain adsorbed or are dissolved in groundwater depending on their relative affinity to iron oxides and groundwater chemistry. Besides the molecular size of contaminants, their interactions with iron corrosion products (FeII-species, H/H2) and the body of the transforming oxide-film are important. Since the 6 oxide-film is a good adsorbent for both contaminants and Fe2+ ions (resulting in more reactive structural Fe 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 II), contaminant reduction will certainly occur at the meeting point within the oxide-film, more or less far from the Fe0 surface. In fact, Fe2+ ions from Fe0 oxidation migrate in the direction of increasing pore sizes and contaminants in the opposite direction [51]. In all the cases quantitative contaminant reduction at the surface of Fe0 is not likely to occur. Furthermore, irrespective of any interactions between contaminant and film materials (non- specific mechanism) [52], a contaminant can be entrapped in the matrix of precipitating and growing oxide-films (co-precipitation). A co-precipitated contaminant can not further migrate to the Fe0 surface. Such species will remain adsorbed and can be reduced by diffuse H/H2 or FeII. Even physically adsorbed contaminants may be reduced within the oxide-film because of diffusion hindrances and the thermodynamic favorable catalytic reduction through H/H2 or FeII. This affirmation is valid irrespective from the conductive properties of the oxide-film. Remember that direct reduction within or at the surface of the film is only possible if the oxide-film is electrically conductive. But even electrically conductive magnetite-layers, may passivate Fe0 if they inhibit the migration of Fe2+ away from the surface. This discussion reiterates that the formation of oxide-films at Fe0 surfaces is an inherent process of aqueous iron corrosion under environmental conditions. Depending on the film thickness, its porosity and its interactions with individual contaminants, an oxide-film can act as a diffusion barrier, a reactive barrier and/or as a molecular sieve hindering or lowering contaminant access to Fe0 surfaces [50,53]. Therefore, mechanistic investigations regarding contaminant removal by Fe0 materials should be accomplished under conditions favoring surface oxide-film generation and transformation [51]. On the contrary, the large majority of reported Fe0 mechanistic results from batch experiments were achieved under well-mixed conditions. Under these experimental conditions however, (i) Fe0 dissolution is accelerated, yielding to more corrosion products which may adsorb contaminants or immobilize them during their transformation (e.g., precipitation, recrystallization). Thus, increased generation 7 of corrosion products complicates mechanistic investigations; (ii) corrosion products precipitation (oxide-film formation) in the vicinity of Fe 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 0 surface is avoided or delayed since FeII transport away from the surface is accelerated; and (iii) chemical dissolution and mechanical abrasion of oxide-films is possible. 4 Low Reactivity of Iron Metal under Environmental Conditions Iron-based alloys for environmental remediation have been originally introduced for the reduction of halogenated organic compounds [20,21,54]. Initial encouraging results prompted researchers to extend their applicability to a large array of organic and inorganic compounds [12,15,55-57]: Thereby, key features characterizing the reactivity of Fe0 materials under environmental conditions have been overseen. Three examples for illustration: • Beside catalytic hydrogenation many reductive agents have been successfully used in the synthetic organic chemistry. The most classic and practical reductants are zinc (Zn0), tin (Sn0), and iron (Fe0) [58]. However, the reactions with Fe0 are performed in organic solvents or in aqueous acidic solutions (statement 1). This statement suggests that using Fe0 materials for quantitative contaminant reduction under environmental pH conditions (6 ≤ pH ≤ 9) is questionable. • While using elemental metals in the synthetic organic chemistry, it is well-known that the reactivity of metallic zinc (E0 = -0.763 V) is superior to that of iron (E0 = -0.440 V) [46]. Therefore, to perform the reduction of some compounds (e.g., nitro compounds to amines) with Fe0, the reaction should be carried out at higher temperature [58] (statement 2). Statement 2 suggests that using Fe0 materials for quantitative contaminant reduction under ambient temperatures (T < 30 °C) is questionable. • Because of the poor solubility of most organic compounds in water at ambient temperature, the remarkable properties of water near its critical point (Tc = 374 °C, Pc = 221 bar) have prompted researchers to use it in organic synthesis [58,59]. There are increasing numbers of papers which suggest that near-critical water is an excellent solvent for organic 8 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 reactions because organic reactions in it offer many advantages over those in traditional organic solvents or acidic aqueous solutions [59] (statement 3). Statement 3 corroborates the conclusions of statement 1 and 2. Altogether statement 1 through 3 clearly show that direct quantitative organic contaminant reduction through Fe0 materials is not likely to occur. Additionally, working under critical conditions [58] or in acidic aqueous conditions has not univocally elucidated the real reaction mechanism (hydrogenation by H2 or direct reduction by Fe0). Therefore, before proposing direct “reductive transformation” as the main pathway of organic compounds removal in Fe0/H2O systems (under atmospheric conditions), the pioneers of the iron barrier technology should have brought clarity on the key issues of statements 1 and 2. Instead of that, the voluminous literature on Fe0 in the synthetic organic chemistry has been ignored. Even very well-documented results have been contradicted, one of the most important been without doubt the reported quantitative reduction of aromatic azo compounds by Fe0 materials [44,60,61]. This reaction (Béchamp reduction) has been investigated more than 150 years ago by Antoine J. Béchamp [62] and is known to take place in a Fe0/HCl system [63]. Beside the low Fe0 reactivity under environmental conditions, inappropriate experimental conditions are other error sources in investigations regarding the processes of contaminant removal in Fe0/H2O systems. The next section will present parameters influencing Fe0 reactivity and show how too large Fe0/contaminant molar ratios in combination with mixing operations may have created unrealistic reducing conditions. 5 Inappropriate Experimental Conditions Factors affecting Fe0 reactivity can be divided into three subgroups: (i) material-dependant factors (mostly not directly accessible to researchers), (ii) environment-dependant factors (investigable at individual relevant sites), and (iii) operational experimental parameters (should be designated to mimic environment-dependant factors). 9 Material-dependant factors include: manufacturing history of Fe0 materials, elemental composition of 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 materials (alloying elements: C, Cr, Mn, Ni, P, S, Si…), Fe0 particle size (nm, μm, mm), Fe0 surface area, and the oxidation state of the Fe0 surface [11,12]. Centuries of investigations on iron corrosion have not clarified the relative importance of the individual factors [3, 34,64,65]. Among environment-dependant factors, the following can be enumerated: water flow velocity, character of the generated oxide-film on Fe0 (composition, porosity, thickness), the ambient temperature, the water composition (major ions, co-contaminants), the availability of molecular oxygen (oxic vs. anoxic conditions), the nature of the contaminant. Environment- dependant factors are site specific and may significantly vary periodically (e.g. daily, seasonally). Their profound knowledge is indispensable for the rational choice of operational experimental parameters. For example the hydrodynamic of the system should reflect the groundwater flow velocity. This condition is partly considered in designing column experiments [4], but is almost totally ignored in the design of batch experiments as discussed above [17,51]. Experimental operational parameters are fully determined by individual investigators. Ideally, their choice should always be rationalized by real situations or by the objective of individual experiments. These factors include [3,66-68]: Fe0 preparation (e.g., acetone or acid wash), buffer application, the molar ratio of Fe0 to contaminant (Fe0 mass loading and initial contaminant concentration), volume of the bottles used in the experiment, volume of model solution added, mixing operations (bubbling, shaking, stirring), geometry of the reaction vessel, experimental duration or reaction time. Due to the lack of a unified procedure for conducting contaminant removal tests in Fe0/H2O systems, different mechanisms for the same contaminant can be found in the literature [69,70]. From isolated sets of experiments, a huge number of variables (partly enumerated above) have been shown to affect aqueous Fe0 reactivity. This suggests that it is practically 10 impossible to correlate data obtained with different natural waters (or synthetic solutions) on different Fe 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 0 samples. Fortunately, however, while each of these factors is undoubtedly important under some particular set of conditions, there are wide ranges of conditions within which comparatively few variables have any large effect on Fe0 reactivity [36]. In particular, the main factors in the aqueous iron corrosion in natural waters are: (i) the protectiveness of films of corrosion products, and (ii) the rate of oxidant diffusion (including H2O, O2, H+). Accordingly, any investigation on processes in Fe0/H2O systems should have been conducted under conditions favouring the formation and the transformation of oxide-films (e.g. in the mass transfer controlled regime). Ideally, this is obtained under stagnant conditions [51,71]. The discussion above reveals that the failure to consider Fe0/H2O systems as a dynamic system consisted of the Fe0 material underlying a layer of corrosion products (oxide-film) is one of the major reasons why, despite two decades of intensive laboratory investigations, several aspects of contaminant removal from aqueous solutions in Fe0/H2O systems are not really understood. Another important point is the Fe0/contaminant molar ratios. In the synthetic organic chemistry, used Fe0/educt molar ratios are rarely larger than 10 [29]: For example, Wang et al. [58] found that a ratio of nanosized iron metal to p-nitrotoluene of 3:1 was sufficient for satisfactory results (stirring conditions, 210 °C and 2 h). In the reactive barrier literature Fe0/contaminant ratios up to 20,000 [29 and ref. therein] are used without justification and the solutions are mixed more or less strongly for hours or days. While using inappropriate mixing operations and too large Fe0/contaminant ratios, reducing conditions may have been generated and investigated by means of sophisticated experimental designs. These conditions are, however, irrelevant for natural systems. Moreover, geochemical models have been developed for contaminant removal in Fe0/H2O systems while assuming reductive transformation at the Fe0 surface [72] 6 Discussions 11 It has been considered for almost two decades that the use of Fe0 materials for groundwater remediation is based on scientific principles [73-76]. Thereafter, the suitability of Fe 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 0 materials for contaminant reductive removal is based on the low redox potential of the couple FeII/Fe0 making Fe0 a powerful reducing agent for many soluble contaminants [21,77]. But these considerations are proved instable because the pioneer works on contaminant reductive removal by Fe0 materials did not test and prove their theories by any scientific criteria [75, 76]. In particular the thermodynamics of hydroxide precipitation or Fe solubility were not properly considered (Fig. 1) [73]. Fortunately, contaminants are successfully removed in Fe0/H2O systems by other processes [73,74]. The rational investigation of these processes and their optimisation is a challenge for the scientific community [78,79]. The instable premise of direct contaminant reduction by Fe0 materials upon which the iron reactive barrier technology is based, has dragged a part of the environmental science community deeper into confusion during the past decade. For example, in their efforts to rationalize the removal of uranium (VI) from the aqueous phase in the presence of Fe0, Gu et al. [80] worked with clearly over-saturated U(VI) solutions (up to 10,000 mg/L or 0.042 M at near-neutral pH values) and reported that “results from the batch adsorption and desorption and from spectroscopic studies indicate that reductive precipitation of U on Fe0 is the major reaction pathway”. Under their experimental conditions however, U(VI) precipitation is thermodynamically and kinetically more favourable than the discussed mechanisms (adsorption and reduction). Clearly, all has been done to support the questionable premise. Here are three further examples: (i) very well-established scientific results have been contradicted (e.g. Béchamp reaction) [29]; (ii) contaminant co-precipitation with FeII and FeIII species which is the principle of flotation [81-86] has been ignored in order ‘to force’ direct reduction of metallic species to occur; (iii) the well-known “physical barrier” function of oxide-films has been transformed to that of “mediator of electron transfer” (electron-shuttle) from underlying Fe0 to overlying contaminants (semiconductor and/or coordinating surface) 12 in order to rationalize the hypothetic electron transfer from Fe0 surface to dissolved oxidants [41]. 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 The present work and related works [29,51,71,87] have presented a more simple and realistic model, acknowledging that quantitative contaminant removal primary occurs via adsorption or co-precipitation within the oxide-film. Depending on the availability of reductants and/or micro-organisms the contaminant may be further reduced, irrespective from the conductive properties of the oxide-film. This model is supported by the known adsorptive properties of iron oxides [39,88,89], and the fact that biological agents (viruses) [90], arsenic [91], and electrochemically non-reducible contaminants like Zn [92] are successfully removed from aqueous solution by Fe0 materials (e.g. in Fe0/H2O systems). 7 Concluding Remarks The further development of Fe0 bed filtration in general and the reactive wall technology in particular requires the use of appropriate experimental methodologies (e.g. no vigorously batch experiments). Available scientific results from two research areas must be properly exploited: (i) The results of investigations regarding passive film formation on Fe0 in neutral and alkaline aqueous solutions. These investigations aimed at developing a detailed understanding of the film growth mechanisms, the film structure and composition, and the kinetics and mechanisms of reduction of iron oxides that form on the Fe0 surface [93 and ref. therein], and (ii) The results of investigations regarding the polymerization of iron via hydrolysis yielding non-crystalline particles of key importance in the adsorption and transport of organic or inorganic pollutants [38,81]. Finally, the most important feature from the present communication is the solution of the existing vagueness for the design of Fe0 beds [94]. In fact, determining an amount of Fe0 and a bed thickness is no more dependant on a reductive reaction [2,95] but is solely a characteristic of water flow, water chemistry and Fe0 intrinsic reactivity [96-98]. 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Substance Structure Density lattice parameters (in Å) (g/cm3) a b c Fe0 cubic 7.86 2.866 2.866 2.866 Fe(OH)2 trigonal 3.40 3.27 3.27 4.62 FeOOH (gel) cubic 8.37 8.37 8.37 Fe3O4 (magnetite) cubic 5.175 8.396 8.396 8.396 FeOOH (goethite) orthorhombic 4.28 4.60 10.01 3.04 FeOOH (akageneite) tetragonal 3.55 10.52 10.52 3.028 FeOOH (lepidocrocite) orthorhombic 4.09 3.65 12.50 3.07 Fe2O3 (hematite) trigonal 5.035 13.72 5.26 587 588 24 Figure 1: pH dependence of metal hydroxide solubilities at pH values of natural waters. The trend for iron precipitation under oxic conditions (Fe 588 589 590 591 592 II) is clear and delineates the suitability of Fe0 for water treatment by adorption and co-precipitation. Even under anoxic condition Fe(OH)2 polymerize and readily precipitate. Data from ref. [32] 5 6 7 8 9 -8 -6 -4 -2 0 Fe(OH)2 Fe(OH)3 Zn(OH)2 co nc en tr at io n / [ M ] pH value / [-] 593 25