Metallic iron for water treatment: A critical revie1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 w Chicgoua Noubactep(a,b) (a) Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. (b) Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37005 Göttingen, Germany. Corresponding address: Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany e-mail: cnoubac@gwdg.de Tel. +49 551 39 3191; Fax: +49 551 399379 Abstract Water treatment with metallic iron (Fe0) is still based on the premise that Fe0 is a reducing agent. An alternative concept stipulates that contaminants are removed by adsorption, co-precipitation and size-exclusion in a reactive filtration process. This article underlines the universal validity of the alternative concept. It is shown that admixing non-expansive material to Fe0 as a pre-requisite for sustainable Fe0-based filtration systems. Fe0-based filters are demonstrated an affordable, appropriate and efficient decentralized water treatment technology. Keywords: Media filtration, Size-exclusion, Water treatment, Zerovalent iron. 1 1 Introduction 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Fe-based alloys (elemental iron, Fe0 materials or zerovalent iron) have been found to be effective for removing a wide range of compounds from water. Studies on the successful removal of organic and inorganic chemicals [1-16] and pathogens [17-21] have been widely published and reviewed [22-30]. However, reports on the mechanism of contaminant removal have not been univocal. For more than a decade reductive transformations (degradation of organics and precipitation of inorganics) have been regarded as the fundamental mechanism of contaminant removal in Fe0/H2O systems [3,31-34]. But the literature contains many contradictory findings regarding the processes of aqueous contaminant removal in the presence of Fe0. Reported discrepancies include the nature of reaction products [35], the extent of contaminant reduction [36,37], the actual reducing agents (Fe0, FeII or H/H2) [12,38] and the relative importance of adsorption and reduction [37]. These conflicting findings suggest that reductive transformations may not be as important as currently considered. Nevertheless, a ‘broad consensus’ on reductive transformations persists in the literature despite parallel acknowledgment that the real mechanisms of contaminant removal have not yet been completely elucidated [6,24,32-34,39-41]. Recently, a new concept was introduced stating that contaminants are fundamentally adsorbed onto and co-precipitated with insoluble Fe0 oxides and hydroxides [42,43]. As for any subject on which there is a difference of opinion, it is pertinent to compare the concepts with the hope of finding the truth in the matter. 2 Reduction or adsorption/co-precipitation? If contaminants are mostly reduced in Fe0/H2O systems (concept 1), then one should consistently explain why this is possible at the long-term despite the oxide film formation (layer insoluble Fe0 oxides and hydroxides) and transformation at the surface of Fe0. It is important to notice in this regard, that the model for oxide film formation, that was compatible with progressive contaminant reduction [23] was proven unrealistic [44]. 2 If contaminants are primarily adsorbed and co-precipitated (concept 2) within the oxide film, then one must simply sustain iron corrosion to ensure contaminant removal. 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 Concept 1 gave birth to the iron reactive barrier technology as only reducible species (mostly chlorinated compounds) were considered [1,3,6,31]. However, reaction products for many chlorinated hydrocarbons have not been clearly identified [35,45,46]. Moreover, Fe0 consumption, oxide film formation on the clean Fe0 surface, and rise of pH inevitably accompany the removal processes [47-49]. Due to these inherent properties of Fe0-mediated reactions, the contaminant removal rate should necessarily decrease with increasing Fe0 consumption yielding contaminant breakthrough. As this was not observed as the rule, concept 2 is more close to the reality. Therefore, future researchers should follow concept 2. That is working on ways and means to sustain Fe0 reactivity which automatically yields contaminant adsorption and co-precipitation [48,50-54]. It is essential to recall that reality (to be found out) is the action of nature under relevant conditions. Accordingly calling a technology ‘passive’ is not related to ‘no action’ but no external input of energy. In other words, active technologies need external energy input to initiate/support the action of nature (the reality). The challenge is to find out, how the nature works. Finding out how nature works is knowledge acquisition. The question here is “how are contaminants removed in Fe0/H2O systems?” 2.1 Reduction is not a removal mechanism In water treatment, chemical reactions are used to facilitate contaminant elimination by one or several removal mechanisms [55,56]. Relevant removal mechanisms are: (i) adsorption, (ii) co- precipitation, (iii) precipitation, (iv) size-exclusion and (v) volatilisation. All water treatment methods are based on these five mechanisms. For water treatment at a specific site, it is important to identify the treatment method that is the most suitable: efficient, affordable and applicable. The treatment system that is best for a particular situation depends mostly on the nature and the concentration of contaminants and the 3 operational requirements of the system. As a rule, a combination of treatment methods is more effective. A practical example is the chemical reduction of Cr 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 VI to CrIII at pH value < 4, followed by an increase of the pH to values > 6.0 for which CrIII precipitates as Cr(OH)3. The example of chromium (atomic number: 24) is very illustrative as chromium and iron (atomic number: 26) are two heavy metals. In other words, aqueous CrIII is only quantitatively removed when precipitation is favourable, e.g. at pH > 6.0 [57]. 2.2 Iron solubility and contaminant removal Iron is a potential contaminant for water. Its maximum contaminant level (MCL) is 0.3 mg/L or 5.4 μM [58]. Accordingly, while using Fe-based reactive material (e.g. Fe0, FeS, FeS2) in water treatment systems, care must be taken for the residual iron concentration to remain below 0.3 mg/L. This requirement delineates the importance of pH dependant Fe solubility of the process of contaminant removal (Fig. 1). In Fig. 1, literature experimental data for the solubility curve of Fe(OH)3 and FeS are represented together with the line for the MCL for Fe (5.4 μM) [59,60]. It is seen that FeII is by far more soluble than FeIII at all pH values. For pH > 5.5, FeII concentration is at most comparable to MCL suggesting that Fe0 is only applicable at pH > 5.5. Clearly, if pH ≤ 4.5 (e.g. acid mine drainage), an pH enhancement should precede Fe0 application. It should be noticed that Fe0 corrosion is always coupled with a pH increased: H+ consumption (Eq. 1) or OH- production (Eq. 2). Thus for systems at pH 4.0-4.5 the technology can be tested [61]. Fe0 + 2 H+ ⇒ Fe2+ + H2 (1) Fe0 + 2 H2O ⇒ Fe2+ + 2 HO- + H2 (2) Because Fe0 is oxidized by water (Eq. 2) and water is a solvent a Fe0/H2O should be regarded as a zone of precipitating iron oxides and hydroxides [62]. During this precipitation, available foreign species (including contaminants) are inevitably enmeshed in the mass of precipitates (co- precipitation). Resulting precipitates are in turn potential adsorbents for biological and chemicals contaminants. Therefore, adsorption and co-precipitation are definitively the fundamental 4 processes of aqueous contaminant removal in the presence of reactive Fe0. In a Fe0 bed, size- exclusion is the third important process (‘reactive filtration’) [62-65]. A porous bed of granular materials primarily removes dissolved molecules (e.g. size exclusion chromatography) and suspended components (e.g. sand filtration) based on their molecular sizes and shapes. The molecular sieve properties of used (porous) materials is exploited. In all cases volatilisation may occur (if applicable) but chemical precipitation is not likely to occur given that contaminants are usually present in trace amounts [66,67]. 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 2.3 What went wrong? 2.3.1 The aqueous Fe0 reactivity The major problem with the introduction of the Fe0 remediation technology is that no critical survey of the data available on iron corrosion was done [30,63]. Ideally, the results of such a survey should have been linked for all possible hypotheses. Then well-designed experiments under strictly controlled conditions should have been performed to uncover the observed process of contaminant removal. On the other hand, reactive Fe0 was tested under the same experimental conditions as inert adsorbents and conflicting results were reported [68-70]. Tested adsorbents included activated carbon, coal, hematite, goethite, lignite, lime, magnetite, peat, sawdust. For example, based on their previous works on the efficiency of several industrial materials for uranium removal [68], Morrison et al. [69] tested Fe0 and concluded that Fe0 was the most efficient material for their purpose. In contrast, Indelicato [70] compared Fe0 and granular activated carbon (GAC) for the removal of chlorinated compounds from groundwater and concluded that GAC was superior to Fe0. It is obvious, that these articles have overseen the key aspect that the efficiency of Fe0 primarily depends on the (long-term) kinetics of iron corrosion under the experimental conditions. It should be recalled that Fe0 reactive barriers were primarily designed for large volumes of low contaminated water (micro-pollutants) flowing slowly through Fe0 beds. Under such conditions, the kinetics of iron corrosion and the residence time of water within the beds may be sufficient to 5 generate enough corrosion products for quantitative contaminant removal. In contrast, the adsorptive capacity of adsorbents is maximal at the beginning of the experiment. Provided that the water flow velocity is satisfactory, adsorptive filtration (e.g. GAC) could be efficient where reactive filtration with Fe 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 0 is not efficient. The key issue is not the relative efficiency but the appropriateness of each class of materials. If this issue is properly addressed, then an appropriate design could be achieved. The results of Miyajima [71] have recently clarified the relationship between ‘intrinsic reactivity’ and ‘removal efficiency’. Summarized, the intrinsic reactivity is an invariable characteristic of a material that does not depend on its amount or the operational conditions. The efficient of a material characterizes the extent to which the material can remove a given contaminant under defined operational conditions. In other words, ‘efficiency’ and ‘reactivity’ should never be randomly interchanged. 2.3.2 The origin of the mistake A careful look on the first 4 peer-reviewed articles on Fe0 [1-4] (Table 1) suggests that a systematic investigation of all observed phenomena would have avoided the mistake of considering Fe0 as a reducing agent. For example, the observed time lag for contaminant removal reported by Schreier and Reinhard [4] is consistent with the view that (at pH > 4.5) contaminants are removed by adsorption and co-precipitation (concept 2). This time lag thus corresponds to the time necessary for the in-situ production of removing agents (FeII/FeIII hydroxides and oxides). On the other hand, the necessity to sustain chemical reduction by an addition of pyrite as reported by Lipczynska-Kochany et al. [2] depicted a clear concern that there would be a problem with chemical reduction (e.g. reductive degradation) at pH values relevant for natural waters. It is important to notice that reactive pyrite is added by Lipczynska-Kochany et al. [2] as a long-term pH shifting agent and not as an own-reducing agent as successfully tested for example by Kriegman-King and Reinhard [72,73]. Both these articles [2,4] were almost ignored (less than 70 citations each as referred to Table 1) and the idea presented above were not further investigated. 6 The article of Matheson and Tratnyek [3], one of the favoured (Table 1), has recently been granted as 2011 Outstanding Publication Award from the Association of Environmental Engineering and Science Professors (AEESP). The AEESP is made up of professors in academic programs throughout the world who provide education in the sciences and technologies of environmental protection. In fact, this article [3] is currently among the most cited articles published at Environmental Science & Technology. However, the theory propagated by this article is based on a wrong interpretation of good experimental observations as discussed in section 2.1. In other words, the current paradigm for the rationalization of the operating mode of Fe 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 0/H2O remediation systems is unstable. Paradigm refers to all knowledge about which there is agreement in science. 2.3.3 The propagation of the mistake Having met an agreement on a false premise, researchers have been reporting on findings (i) disagreeing 150 years intensive research on aqueous iron corrosion [74-76], (ii) disagreeing good results of synthetic organic chemistry [30], (iii) neglecting the voluminous work available from the hydrometallurgy, and the petroleum industry [75-79], and (iv) not able to explain why non- reducible contaminants are quantitatively removed in Fe0/H2O systems [30,42,43]. It is important in this regard to notice that Fe0 is also used for oxidative conversion of aqueous contaminants [13]. However, contaminant oxidation is also not a removal mechanism (section 2.1). Beside the improper consideration of available results from other branches of science two other key factors have contributed to maintain confusion on the mechanism of contaminant removal in Fe0/H2O systems: (i) the use of inappropriate experimental conditions and (ii) the failure to use sequential extraction while making mass balances. 2.3.4 Inappropriate experimental conditions There is actually no standard experimental protocol for the investigation of processes in Fe0/H2O systems. Available results are not really comparable [80,81]. In particular the used mixing operations (agitating, shaking, stirring) have disturbed the process of oxide film formation yielding 7 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 possibly reproducible results under well-designed laboratory conditions. These conditions are however difficult to reproduce in the subsurface [30]. 2.3.5 Non-conclusive mass balance No convincing carbon balances between reactants and products have ever been successfully done for many chlorinated hydrocarbons [45]. This means that organic contaminants that have disappeared from the aqueous phase are mostly considered chemically reduced. The situation is similar for inorganic contaminants for which speciation experiments have been mostly made without efforts to reductively dissolve iron corrosion products [82]. In other words, available results from geochemistry have equally not been properly considered. In fact, reductive dissolution of iron (and manganese) oxides is integral part of all sequential extraction schemes [83-85]. For example, Ma and Rate [84] used ammonium oxalate for amorphous iron/manganese oxides and hydroxylamine hydrochloride for crystalline iron/manganese oxides. As far as the author could ascertain, only Kishimoto et al. [86] have chemically reduced iron corrosion products for mechanistic demonstration. Previous research articles have used reducing agents to demonstrate the stability of removed contaminants [87,88]. 3 Discussion The presentation above has acknowledged that the concept of contaminant reductive transformation as removal mechanism in Fe0/H2O is clearly inadequate for explaining many experimental and field observations. Furthermore, irreversible contaminant removal which could result from contaminant co-precipitation with iron corrosion products has been mistakenly regarded as contaminant reductive transformation. However, the actual reactive wall design (e.g. wall sizing) is based on this concept [39,89-91]. Therefore, it is urgent to reconsider available data and models [92,93]. Moreover, further research work should be performed under adequate experimental conditions, including non-disturbed conditions of slow mixing regimes in batch experiments [30,81]. More research under relevant conditions is needed before the concept of 8 197 198 199 200 201 202 203 204 205 contaminant co-precipitation can be fully understood and predicted. Considering its nature, this is a challenge which can only be properly addressed by several research groups. Finally, it must be explicitly said that the concept of contaminant co-precipitation is not a contradiction but an extension of the reductive transformation concept. The new concept explains better why various contaminants are continuously removed in Fe0/H2O systems despite “passivation” of the Fe0 surface [52,94]. Adsorbed and co-precipitated contaminants can be further reduced [42,43]. Based on this knowledge, Fe0-based filtration systems (including reactive walls) can be better designed [71,93]. The next section presents Fe0 as a universal material for safe drinking water provision at small scale. 4 Metallic iron for safe drinking water provisio206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 n The conventional approach for safe drinking water provision is to treat natural water in a treatment plant and distribute through a pipeline network to the population [95,96]. One of the most severe shortcomings of this approach is that any sudden interruption (e.g. disasters: floods, droughts, quakes, tsunamis, hurricanes) could leave thousands of people without drinking water supply for some days or weeks. There is a current trend for decentralized solutions for safe drinking water supply [56,64,95-98]. 4.1 Basic requirements for decentralized water supply solutions Centralized waterworks are sophisticated systems with high demand of energy, skilled operation personnel and chemicals [55,96,99]. To be applicable worldwide, a water supply system must be (i) efficient, (ii) affordable and (iii) applicable in small and secluded remote areas (including islands) without electricity grid and possibly without (enough) skilled personnel [94,98,100-104]. Presently only chlorination, coagulation, filtration, solar disinfection, ceramic filters and biosand filters fulfil these basic criteria [56,103]. However, chlorination and coagulation need skilled personnel and should never be performed by illiterates (e.g. in developing countries). Solar disinfection can not address chemical contamination and the efficiency for both ceramic and biosand filters for virus removal was shown non satisfactory [97]. In other words, there is 9 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 presently no simple, efficient and affordable technology for water supply in low-income remote communities. One exception is the recently developed “WaterBackpack” at the University of Kassel (Germany) [97]. The “WaterBackpack” is a “small, transportable and easy to use dead-end membrane filtration unit for basic water supply” for small communities in the range of 200 up to 500 people [97]. The “WaterBackpack” is suitable for critical situations like natural disasters (e.g. earthquakes, tsunami) or wars (refugee camps). The need for sustainable, affordable safe drinking water technologies for low-income communities persists. Some of these communities have only so few inhabitants (down to less than 10 persons) for who the current version of “WaterBackpack” is not appropriate even though it could be affordable (around Euro 700). 4.2 Concept of Fe0 for safe drinking water provision The suitability of metallic iron for decentralized drinking water provision arises from two main reasons: (i) metallic iron is widely available; iron filings can be produced locally even in poor localities (so-called ‘indigenous iron’) at low-cost or no money expense and (ii) water corrodes Fe0 to strongly adsorbing iron hydroxides and oxides [49,105-114]. As demonstrated above these iron precipitates should be regarded as collectors in a sand filter [115,116]. Accordingly, biosand filters should be amended with Fe0 to yield efficient gravity filters. The design of Fe0 filters has been discussed in several recent articles [52,116] and will not be repeated here. The heart of the Fe0 filter is a suitable reactive Fe0 which should be mixed to an inert material (e.g. anthracite, gravel, pumice, sand) or a reactive but not expansive material (e.g. MnO2, TiO2) in a reactive zone [53]. The reactive zone should be sandwiched between two biosand filters (BSFs) (Fig. 2) [117]. The first BSF scavenges O2 and removes pathogens and the second removes dissolved iron from the reactive zone. It can be anticipated that to each Fe0 (intrinsic reactivity) will correspond a thickness (Hrz) of the reactive zone and a thickness (HBSF2) of the second fine sand layer. The concept presented here can be realized at any corner of this world at several scales. With regard on the developing world, this is not a technology transfer in the conventional sense 10 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 [102], but a ‘knowledge sharing’ that could enable research institutions in the developing world to solve a long lasting problem by local initiatives. The concept is affordable and applicable because: (i) no chemicals is needed, (ii) no energy is needed (gravity filtration), (iii) no (skilled) operation personnel is needed, and (iv) no intensive maintenance is needed. The sole need is a concept for recycling iron for new filters. 5 Further applications of Fe0 in environmental remediation The knowledge, that Fe0 is used to produce reactive species is not unique to Fe0 filters. For example, Gould [118] used Fe0 as reducing agent for CrVI, but their results demonstrated that more CrVI is reduced than predicted by the stoichiometry of the reaction between Fe0 and CrVI. In this case, FeII and H2 are efficient reducing agents for CrVI (Table 2). In another example, Bafghi et al. [119] used powdered Fe0 as FeII generator for the reductive dissolution of MnO2. Their results showed that Fe0 is superior to FeII-bearing materials “as far as dissolution rate and efficiency were concerned”. In a third example, Chen et al. [120] positively investigated the potential of nano-Fe0 for hydrogen generation. In a fourth example, Biswas and Bose [121] successfully tested Fe0 as source of H2 for autotrophic denitrification. In a fifth example, Fe0- and FeII-bearing materials are used as cost-effective oxygen scavengers to protect oxygen-sensitive foods from oxidation [122- 124]. The idea is to eliminate or reduce the levels of oxygen inside packs. The working mechanism is the reaction of iron (Fe0, FeII) with oxygen in the container to form FeIII oxides [124]. These five examples show clearly that Fe0 can efficiently be used as parent material to produce useful species for different purposes. In Fe0/sand filters, Fe0 is used as generators for colloids which adsorb and/or enmesh contaminants during their precipitation. In other words, Fe0 is used to improve filtration which is basically a size-exclusion process [99]. Accordingly, the removal of very small particles (e.g. viruses) is not guaranteed by small pore sizes like in membrane filtration [97,103] but by the dynamic process of aqueous iron corrosion [47,64,65]. The geochemistry of iron in general and the behaviour of iron minerals in soils with regards to contaminant removal [125,126] suggest that Fe0 can be used as progressive source for slow release 11 of Fe (hydr)oxides in several remediation scenarios [127] Relevant applications include: (i) remediation of contaminated groundwater, (ii) production of safe drinking water, (iii) treatment of industrial and agricultural wastewater, (iv) treatment of hospital effluents, (v) improvement of water quality in aquifers, (vi) improvement of river bank filtration, (vii) treatment of contaminated soils, and (viii) optimisation of artificial aquifer recharge. The material to be used in each application depends on the suitable operational conditions. However, it can be anticipated that a large array of materials with different reactivity should be available. Therefore, a standard protocol for the characterization of the intrinsic reactivity of Fe 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 0 materials is urgently needed. Depending on their intrinsic reactivity, materials could be classified with respect to their suitable application; e.g. Fe0 for safe drinking water, Fe0 for soil treatment, Fe0 for irrigation water, Fe0 for drainage water, Fe0 for river bank filtration… 6 Concluding remarks The universality of the view that Fe0 is not a reducing agent is delineated. Regarding Fe0 as a generator of ‘contaminant collectors’ [116] has enabled the conceptual design of Fe0-amended slow sand filters (Fe0 SSFs) which are yet to be realized. A Fe0 SSF has a large potential for application to small-scale systems, in particular in low-income communities worldwide: (i) it is totally chemistry free, (ii) it is simple to design, (iii) it is easy in operation and maintenance, (iv) it is cost effective and (v) it is reliable upon proper design. Innovative designs of the reactive zones (e.g. use of Fe0-composites, Fe0/MnO2, Fe0/pumice) will increase the sustainability of Fe0 filtration beds [54,128]. Intensive research with column and pilot studies are necessary to verify and optimise the presented concept. In this effort, the proper consideration of the volumetric expansive nature of iron corrosion should be carefully considered. In particular, lowering the concentration of dissolved O2 at the inlet of the filter is a key issue (Table 3). The proper disposal of spent media as well as the recycling of used materials should be considered during the testing stage. 12 The knowledge that Fe0 is not a reducing agent is also essential for the further development of the iron wall technology for groundwater remediation. In fact, considering the volumetric expansive nature of iron corrosion [74,76,129,130], the question as whether mixing Fe 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 0 and inert materials (e.g. gravel, sand) is beneficial or not [40,71, 131-133] is now definitively solved. Mixing Fe0 and non-expansive materials is even a pre-requisite for system sustainability [134]. Accordingly, a reactive wall containing a zone with 100 % Fe0 is not viable. Consequently, the rationale for the sustainability of reactive walls with a pure Fe0 layer [24,27,66] is yet to be elucidated. A plausible explanation is that used materials were not very reactive. In such a constellation the reactivity of the wall could be sustained by an array of abiotic and biotic reductive reaction recycling FeIII to FeII [135]. The impact of chemical reaction within the barrier is the progressive generation of colloids for contaminant ‘collection’. Without recycling, FeII/FeIII colloids are irreversibly transformed to less/non reactive crystalline forms [134,136-138]. As regarding the failure cases [66,70,139,140], it can be anticipated that used materials were very reactive under site specific conditions. Verifying this hypothesis is a challenge for the scientific community and an opportunity to further develop the already established remediation technology. Acknowledgments The original manuscript was improved by the insightful comments from Florence Tsagué Assopgoum (University of Siegen/Germany). References [1] R.W. Gillham, S.F. O’Hannesin, Enhanced degradation of halogenated aliphatics by zero- valent iron, Ground Water 1994, 32, 958–967. [2] E. Lipczynska-Kochany, S. Harms, R. Milburn, G. Sprah, N. Nadarajah, Degradation of carbon tetrachloride in the presence of iron and sulphur containing compounds, Chemosphere 1994, 29, 1477–1489. [3] L.J. Matheson, P.G. Tratnyek, Reductive dehalogenation of chlorinated methanes by iron metal, Environ. Sci. Technol. 1994, 28, 2045–2053. 13 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 [4] C.G. Schreier, M. Reinhard, Transformation of chlorinated organic compounds by iron and manganese powders in buffered water and in landfill leachate, Chemosphere 1994, 29, 1743–1753. [5] K.J. Cantrell, D.I. Kaplan, T.W. Wietsma, Zero-valent iron for the in situ remediation of selected metals in groundwater, J. Hazard. Mater. 1995, 42, 201–212. [6] S.F. O´Hannesin, R.W. Gillham, Long-term performance of an in situ "iron wall" for remediation of VOCs, Ground Water 1998, 36, 164–170. [7] D.W. Blowes, C.J. Ptacek, S.G. Benner, W.T. Mcrae Che, T.A. Bennett, R.W. Puls, Treatment of inorganic contaminants using permeable reactive barriers, J. Contam. Hydrol. 2000, 45, 123–137. [8] S.J. Morrison, D.R. Metzler, B.P. Dwyer, Removal of As, Mn, Mo, Se, U, V and Zn from groundwater by zero-valent iron in a passive treatment cell: reaction progress modelling, J. Contam. Hydrol. 2002, 56, 99–116. [9] C.-H. Liao, C. Wantawin, M.-C. Lu, C.-I. Huang, Fe0-based system as innovative technology for degrading trichloromethane: Redox removal characteristics, Environ. Sci. Pollut. Res. 2004, 11, 254–259. [10] D.I. Song, Y.H. Kim, W.S. Shin, A simple mathematical analysis on the effect of sand in Cr(VI) reduction using zero valent iron, Korean J. Chem. Eng., 2005, 22, 67–69. [11] J.E. Yang, J.S. Kim, Y.S. Ok, S.-J. Kim, K.-Y. Yoo, Capacity of Cr(VI) reduction in an aqueous solution using different sources of zerovalent irons. Korean J. Chem. Eng. 2006, 23, 935–939. [12] Y. Jiao, C. Qiu, L. Huang, K. Wu, H. Ma, S. Chen, L. Ma, L. Wu, Reductive dechlorination of carbon tetrachloride by zero-valent iron and related iron corrosion, Appl. Catal. B: Environ. 2009, 91, 434–440. [13] D.h. Kim, J. Kim, W. Choi, Effect of magnetic field on the zero valent iron induced oxidation reaction, J. Hazard. Mater. 2011, 192, 928–931. 14 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 [14] C. Wanner, U. Eggenberger, U. Mäder, Reactive transport modeling of Cr(VI) treatment under fast flow conditions, Appl. Geochem. 2011, 26, 1513–1523. [15] A. Shimizu, M. Tokumura, K. Nakajima, Y. Kawase, Phenol removal using zero-valent iron powder in the presence of dissolved oxygen: Roles of decomposition by the Fenton reaction and adsorption/precipitation, J. Hazard. Mater. 2012, 201–202, 60–67. [16] Y. Wang, D. Zhou, Y. Wang, L. Wang, L. Cang, Automatic pH control system enhances the dechlorination of 2,4,4'-trichlorobiphenyl and extracted PCBs from contaminated soil by nanoscale Fe0 and Pd/Fe0, Environ. Sci. Pollut. Res., 2012, 19, 448–457. [17] Y. You, J. Han, P.C. Chiu, Y. Jin, Removal and inactivation of waterborne viruses using zerovalent iron, Environ. Sci. Technol. 2005, 39, 9263–9269. [18] C. Lee, J.Y. Kim, W.I. Lee, K.L. Nelson, J. Yoon, D.L. Sedlak, Bactericidal effect of zero valent iron nanoparticles on Escherichia coli, Environ. Sci. Technol. 2008, 42, 4927–4933. [19] M. Diao, M. Yao, Use of zero-valent iron nanoparticles in inactivating microbes, Water Res. 2009, 43, 5243–5251. [20] L.G. Cullen, E.L. Tilston, G.R. Mitchell, C.D. Collins, L.J. Shaw, Assessing the impact of nano- and micro-scale zerovalent iron particles on soil microbial activities: Particle reactivity interferes with assay conditions and interpretation of genuine microbial effects, Chemosphere 2011, 82, 1675–1682. [21] C. Shi, J. Wei, Y. Jin, K.E. Kniel, P.C. Chiu, Removal of viruses and bacteriophages from drinking water using zero-valent iron, Sep. Purif. Technol. 2012, 84, 72–78. [22] T. Bigg, S.J. Judd, Zero-valent iron for water treatment, Environ. Technol. 2000, 21, 661– 670. [23] M.M. Scherer, S. Richter, R.L. Valentine, P.J.J. Alvarez, Chemistry and microbiology of permeable reactive barriers for in situ groundwater clean up. Rev. Environ. Sci. Technol. 2000, 30, 363–411. 15 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 [24] A.D. Henderson, A.H. Demond, Long-term performance of zero-valent iron permeable reactive barriers: a critical review. Environ. Eng. Sci. 2007, 24, 401–423. [25] A.B. Cundy, L. Hopkinson, R.L.D. Whitby, Use of iron-based technologies in contaminated land and groundwater remediation: A review. Sci. Tot. Environ. 2008, 400, 42–51. [26] R. Thiruvenkatachari, S. Vigneswaran, R. Naidu, Permeable reactive barrier for groundwater remediation. J. Ind. Eng. Chem. 2008, 14, 145–156. [27] S. Comba, A. Di Molfetta, R. Sethi, A Comparison between field applications of nano-, micro-, and millimetric zero-valent iron for the remediation of contaminated aquifers. Water Air Soil Pollut. 2011, 215, 595–607. [28] M. Gheju, Hexavalent chromium reduction with zero-valent iron (ZVI) in aquatic systems. Water Air Soil Pollut. 2011, 222, 103–148. [29] N.C. Mueller, J. Braun, J. Bruns, M. Cerník, P. Rissing, D. Rickerby, B. Nowack, Application of nanoscale zero valent iron (NZVI) for groundwater remediation in Europe, Environ. Sci. Pollut. Res. 2011, 19, 550–558. [30] C. Noubactep, Investigating the processes of contaminant removal in Fe0/H2O systems. Korean J. Chem. Eng. 2012, 29, 1050–1056. [31] E.J. Weber, Iron-mediated reductive transformations: investigation of reaction mechanism, Environ. Sci. Technol. 1996, 30, 716–719. [32] J.A. Mielczarski, G.M. Atenas, E. Mielczarski, Role of iron surface oxidation layers in decomposition of azo-dye water pollutants in weak acidic solutions, Appl. Catal. B: Environ. 2005, 56, 289–303. [33] J. Chen, X. Qiu, Z. Fang, M. Yang, T. Pokeung, F. Gu, W. Cheng, B. Lan, Removal mechanism of antibiotic metronidazole from aquatic solutions by using nanoscale zero-valent iron particles, Chem. Eng. J. 2012, 181–182, 113–119. [34] F. Sun, K.A. Osseo-Asare, Y. Chen, B.A. Dempsey, Reduction of As(V) to As(III) by commercial ZVI or As(0) with acid-treated ZVI, J. Hazard. Mater. 2011, 196, 311–317. 16 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 [35] W.A. Arnold, A.L. Roberts, Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles, Environ. Sci. Technol. 2000, 34, 1794–1805. [36] R. Mantha, K.E. Taylor, N. Biswas, J.K. Bewtra, A continuous system for Fe0 reduction of nitrobenzene in synthetic wastewater, Environ. Sci. Technol. 2001, 35, 3231–3236. [37] Y. Furukawa, J.-W. Kim, J. Watkins, R.T. Wilkin, Formation of ferrihydrite and associated iron corrosion products in permeable reactive barriers of zero-valent iron, Environ. Sci. Technol. 2002, 36, 5469–5475. [38] B.K. Lavine, G. Auslander, J. Ritter, Polarographic studies of zero valent iron as a reductant for remediation of nitroaromatics in the environment, Microchem. J. 2001, 70, 69–83. [39] S.-W. Jeen, R.W. Gillham, A. Przepiora, Predictions of long-term performance of granular iron permeable reactive barriers: Field-scale evaluation, J. Contam. Hydrol. 2011, 123, 50–64. [40] L. Chen, S. Jin, P.H. Fallgren, N.G. Swoboda-Colberg, F. Liu, P.J.S. Colberg, Electrochemical depassivation of zero-valent iron for trichloroethene reduction. J. Hazard. Mater. 2012, http://dx.doi.org/10.1016/j.jhazmat.2012.08.074. [41] C. Zhao, E.J. Reardon, H2 gas charging of zero-valent iron and TCE degradation. J. Environ. Protect. 2012, 3, 272–279. [42] C. Noubactep, Processes of contaminant removal in “Fe0–H2O” systems revisited. The importance of co-precipitation, Open Environ. J. 2007, 1, 9–13. [43] C. Noubactep, A critical review on the mechanism of contaminant removal in Fe0–H2O systems, Environ. Technol. 2008, 29, 909–920. [44] M. Odziemkowski, Spectroscopic studies and reactions of corrosion products at surfaces and electrodes, Spectrosc. Prop. Inorg. Organomet. Compd. 2009, 40, 385–450. [45] G. Lee, S. Rho, D. Jahng, Design considerations for groundwater remediation using reduced metals, Korean J. Chem. Eng. 2004, 21, 621–628. [46] D.M. Cwiertny, A.L. Roberts, On the nonlinear relationship between kobs and reductant mass loading in iron batch systems, Environ. Sci. Technol. 2005, 39, 8948–8957. 17 [47] C. Noubactep, The suitability of metallic iron for environmental remediation, Environ. Progr. Sust. En. 2010, 29, 286–291. 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 [48] A. Ghauch, H. Abou Assi, H. Baydoun, A.M. Tuqan, A. Bejjani, Fe0-based trimetallic systems for the removal of aqueous diclofenac: Mechanism and kinetics, Chem. Eng. J. 2011, 172, 1033–1044. [49] D.E. Giles, M. Mohapatra, T.B. Issa, S. Anand, P. Singh, Iron and aluminium based adsorption strategies for removing arsenic from water, J. Environ. Manage. 2011, 92, 3011– 3022. [50] D. Burghardt, A. Kassahun, Development of a reactive zone technology for simultaneous in situ immobilisation of radium and uranium, Environ. Geol. 2005, 49, 314–320. [51] C. Noubactep, G. Meinrath, J.B. Merkel, Investigating the mechanism of uranium removal by zerovalent iron materials, Environ. Chem. 2005, 2, 235–242. [52] A. Ghauch, H. Abou Assi, A. Tuqan, Investigating the mechanism of clofibric acid removal in Fe0/H2O systems, J. Hazard. Mater. 2010, 176, 48–55. [53] C. Noubactep, B.D. Btatkeu K., J.B. , Impact of MnO2 on the efficiency of metallic iron for the removal of dissolved metal, Chem. Eng. J. 2011, 178, 78–84. [54] C. Noubactep, S. Caré, K.B.D. Btatkeu, C.P. Nanseu-Njiki, Enhancing the sustainability of household Fe0/sand filters by using bimetallics and MnO2. Clean – Soil, Air, Water 2012, 40, 100–109. [55] E. Worch, Wasser und Wasserinhaltsstoffe, Eine Einführung in die Hydrochemie. Teubner- Reihe Umwelt. B.G. Teubner Verlagsgesellschaft, Stuttgart, Leipzig. ISBN 3-8154-3525-0, 1997. [56] S.I. Ali, Alternatives for safe water provision in urban and peri-urban slums. J. Water Health 2010, 8, 720–734. [57] L.-Y. Chang, Chromate reduction in wastewater at different pH levels using thin iron wires - A laboratory study, Environ. Prog. 2005, 24, 305–316. 18 [58] US EPA, National Primary Drinking Water Regulations. Retrieved December 11, 2011, from 454 http://www.epa.gov/safewater/consumer/pdf/mcl.pdf (2009). 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 [59] X. Liu, F.J. Millero, The solubility of iron hydroxide in sodium chloride solutions, Geochim. Cosmochim. Acta 1999, 63, 3487–3497. [60] D. Rickard, The solubility of FeS, Geochim. Cosmochim. Acta 2006, 70, 5779–5789. [61] M. Gheju, I. Balcu, Removal of chromium from Cr(VI) polluted wastewaters by reduction with scrap iron and subsequent precipitation of resulted cations, J. Hazard. Mater. 2011, 196, 131–138. [62] C. Noubactep, An analysis of the evolution of reactive species in Fe0/H2O systems, J Hazard. Mater. 2009, 168, 1626–1631. [63] C. Noubactep, The fundamental mechanism of aqueous contaminant removal by metallic iron, Water SA 2010, 36, 663–670. [64] C. Noubactep, Metallic iron for safe drinking water worldwide, Chem. Eng. J. 2010, 165, 740–749. [65] C. Noubactep, Metallic iron for safe drinking water production. Freiberg Online Geol. 2011, 27, 38 pp. [66] A.D. Henderson, A.H. Demond, Impact of solids formation and gas production on the permeability of ZVI PRBs, J. Environ. Eng. 2011, 137, 689–696. [67] K. Kümmerer, Emerging contaminants versus micro-pollutants, Clean – Soil, Air, Water 2011, 39, 889–890. [68] S.J. Morrison, R.R. Spangler, Extraction of U and Mo from aqueous solutions: A survay of industrial materials for use in chemical barriers for uranium taillings remediation. Environ. Sci. Technol. 1992, 26, 1922–1932. [69] S.J. Morrison, Comparison of oxidized to reduced forms of iron for in situ remediation of uranium-contaminated groundwater, Fry Canyon, Utah. 1997 GSA Annual Meeting, Salt Lake City, October 20–23, 1997. p.A–157. 19 [70] B.M. Indelicato, Comparision of zero-valent iron and activated carbon for treating chlorinated contaminants in groundwater. Master thesis, Massachusetts Institute of Technology, 1998, 82 pp. 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 [71] K. Miyajima, Optimizing the design of metallic iron filters for water treatment. Master thesis, University of Göttingen, 2012, 50 pp. [72] M.R. Kriegman-King, M. Reinhard, Transformation of carbon tetrachloride in the presence of sulfide, biotite, and vermiculite, Environ. Sci. Technol. 1992, 26, 2198–2206. [73] M.R. Kriegman-King, M. Reinhard, Transformation of carbon tetrachloride by pyrite in aqueous solution, Environ. Sci. Technol. 1994, 28:692–700. [74] N.B. Pilling, R.E. Bedworth, The oxidation of metals at high temperatures, J. Inst. Metals 1923, 29, 529–591. [75] S. Nesic, Key issues related to modelling of internal corrosion of oil and gas pipelines – A review, Corros. Sci. 2007, 49, 4308–4338. [76] J. Ožbolt, G. Balabanic, M. Kušter, 3D Numerical modelling of steel corrosion in concrete structures, Corros. Sci. 2011, 53, 4166–4177. [77] E.O. Obanijesu, V. Pareek, R. Gubner, M.O. Tade, Corrosion education as a tool for the survival of natural gas industry, Nafta Sci. J. 2010, 61, 541–554. [78] C. Noubactep, Elemental metals for environmental remediation: Learning from cementation process, J. Hazard. Mater. 2010, 181, 1170–1174. [79] R. Crane, C. Noubactep, Elemental metals for environmental remediation: learning from hydrometallurgy, Fresenius Environ. Bull. 2012, 21, 1192–1196. [80] C. Noubactep, Characterizing the discoloration of methylene blue in Fe0/H2O systems, J. Hazard. Mater. 2009, 166, 79–87. [81] C. Noubactep, T. Licha, T.B. Scott, M. Fall, M. Sauter, Exploring the influence of operational parameters on the reactivity of elemental iron materials. J. Hazard. Mater. 2009, 172, 943– 951. 20 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 [82] B. Gu, L. Liang, M.J. Dickey, X. Yin, S. Dai, Reductive precipitation of uranium (VI) by zero-valent iron, Environ. Sci. Technol. 1998, 32, 3366–3373. [83] A. Tessier, P.G.C. Campbell, M. Bisson, Sequential extraction procedure for the speciation of particulate trace metals, Anal. Chem. 1979, 51, 844–851. [84] Y. Ma, A.W. Rate, Metals adsorbed to charcoal are not identifiable by sequential extraction, Environ. Chem. 2007, 4, 26–34. [85] F. Madrid, R. Reinoso, M.C. Florido, E. Díaz Barrientos, F. Ajmone-Marsan, C.M. Davidson, L. Madrid, Estimating the extractability of potentially toxic metals in urban soils: A comparison of several extracting solutions, Environ. Pollut. 2007, 147, 713–722. [86] N. Kishimoto, S. Iwano, Y. Narazaki, Mechanistic consideration of zinc ion removal by zero- valent iron, Water Air Soil Pollut. 2011, 221, 83–189. [87] C. Noubactep, Effects of selected ligands on U(VI) immobilization by zerovalent iron, J. Radioanal. Nucl. Chem. 2006, 267, 13–19. [88] C. Noubactep, A. Schöner, H. Dienemann, M. Sauter, Investigating the release of co- precipitated uranium from iron oxides, J. Radioanal. Nucl. Chem. 2006, 267, 21–27. [89] K.U. Mayer, D.W. Blowes, E.O. Frind, Reactive transport modeling of an in situ reactive barrier for the treatment of hexavalent chromium and trichloroethylene in groundwater, Water Resour. Res. 2001, 37, 3091–3103. [90] L. Li, C.H. Benson, E.M. Lawson, Modeling porosity reductions caused by mineral fouling in continuous-wall permeable reactive barriers, J. Contam. Hydrol. 2006, 83, 89–121 [91] F. Karlicky, M. Otyepka, First step in the reaction of zerovalent iron with water, J. Chem. Theory Comput. 2011, 7, 2876–2885. [92] C. Noubactep, S. Caré, Dimensioning metallic iron beds for efficient contaminant removal, Chem. Eng. J. 2010, 163, 454–460. [93] C. Noubactep, S. Caré, Designing laboratory metallic iron columns for better result comparability, J. Hazard. Mater. 2011, 189, 809–813. 21 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 [94] A. Ghauch, H. Abou Assi, S. Bdeir, Aqueous removal of diclofenac by plated elemental iron: Bimetallic systems, J. Hazard. Mater. 2010, 182, 64–74. [95] S. Slaughter, Improving the sustainability of water treatment systems: Opportunities for innovation, Solutions 2010, 1, 42–49. [96] M. Moglia, A. Sharma, K. Alexander, A. Mankad, Perceived performance of decentralised water systems: a survey approach, Water Sci. Technol. Water Supply 2011, 11, 516–526. [97] F.-B. Frechen, H. Exler, J. Romaker, W. Schier, Long-term behaviour of a gravity-driven dead end membrane filtration unit for potable water supply in cases of disasters, Water Sci. Technol. Water Supply 2011, 11, 39–44. [98] D.T. Ingram, M.T. Callahan, S. Ferguson, D.G. Hoover, D.R. Shelton, P.D. Millner, M.J. Camp, J.R. Patel, K.iE. Kniel, M. Sharma, Use of zero-valent iron biosand filters to reduce E. coli O157:H12 in irrigation water applied to spinach plants in a field setting, J. Appl. Microbiol. 2012, 112, 551–560. [99] G.S. Simate, S.E. Iyuke, S. Ndlovu, M. Heydenrych, L.F. Walubita, Human health effects of residual carbon nanotubes and traditional water treatment chemicals in drinking water, Environ. Int. 2012, 39, 38–49. [100] M.D. Sobsey, C.E. Stauber, L.M. Casanova, J.M. Brown, M.A. Elliott, Point of use household drinking water filtration: A practical, effective solution for providing sustained access to safe drinking water in the developing world, Environ. Sci. Technol. 2008, 42, 4261– 4267. [101] P.R. Hunter, Household water treatment in developing countries: Comparing different intervention types using meta-regression, Environ. Sci. Technol. 2009, 43, 8991–8997. [102] P. Byars, B. Antizar-Ladislao, Water treatment and supply: intermediate education in Sub- Saharan Africa, Water Sci. Technol. Water Supply 2011, 11, 578–585. [103] B. Michen, Virus removal in ceramic depth filters: The electrostatic enhanced adsorption approach. PhD Dissertation, TU Bergakademie Freiberg, 2011. 22 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 [104] B. Michen, F. Meder, A. Rust, J. Fritsch, C. Aneziris, T. Graule, Virus removal in ceramic depth filters based on diatomaceous earth, Environ. Sci. Technol. 2012, 46, 1170 –1177. [105] A.H. Khan, S.B. Rasul, A.K.M. Munir, M. Habibuddowla, M. Alauddin, S.S. Newaz, A. Hussam, Appraisal of a simple arsenic removal method for groundwater of Bangladesh. J. Environ. Sci. Health A 2000, 35, 1021–1041. [106] O.X. Leupin, S.J. Hug, Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron, Water Res. 2005, 39, 1729–740. [107] O.X. Leupin, S.J. Hug, A.B.M. Badruzzaman, Arsenic removal from Bangladesh tube well water with filter columns containing zerovalent iron filings and sand, Environ. Sci. Technol. 2005, 39, 8032–8037. [108] A. Hussam, A.K.M. Munir, A simple and effective arsenic filter based on composite iron matrix: Development and deployment studies for groundwater of Bangladesh. J. Environ. Sci. Health A 2007, 42, 1869–1878. [109] T.K.K. Ngai, R.R. Shrestha, B. Dangol, M. Maharjan, S.E. Murcott, Design for sustainable development – Household drinking water filter for arsenic and pathogen treatment in Nepal, J. Environ. Sci. Health A, 2007, 42, 1879–1888. [110] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes,Science and technology for water purification in the coming decades. Nature 2008, 452, 301–310. [111] A. Hussam, Contending with a Development Disaster: SONO Filters Remove Arsenic from well water in Bangladesh, Innovations 2009, 4, 89–102. [112] D. Pokhrel, B.S. Bhandari, T. Viraraghavan, Arsenic contamination of groundwater in the Terai region of Nepal: An overview of health concerns and treatment options, Environ. Int. 2009, 35, 157–161. [113] J. Bundschuh, M. Litter, V.S.T. Ciminelli, M.E. Morgada, L. Cornejo, S.G. Hoyos, J. Hoinkis, M.T. Alarcón-Herrera, M.A. Armienta, P. Bhattacharya, Emerging mitigation needs 23 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 and sustainable options for solving the arsenic problems of rural and isolated urban areas in Latin America – A critical analysis, Water Res. 2010, 44, 5828–5845. [114] M.I. Litter, M.E. Morgada, J. Bundschuh, Possible treatments for arsenic removal in Latin American waters for human consumption, Environ. Pollut. 2010, 158, 1105–1118. [115] J.T. Cookson, Removal of submicron particles in packed beds, Environ. Sci. Technol. 1970, 4, 128–134. [116] K.-M. Yao, M.T. Habibian, C.R. O’melia, Water and waste water filtration: concepts and applications. Environ. Sci. Technol. 1971, 5, 1105–1112. [117] C. Noubactep, E. Temgoua, M.A. Rahman, Designing iron-amended biosand filters for decentralized safe drinking water provision, Clean: Soil, Air, Water 2012, 40, 798–807. [118] J.P. Gould, The kinetics of hexavalent chromium reduction by metallic iron, Water Res. 1982, 16, 871–877. [119] M.Sh. Bafghi, A. Zakeri, Z. Ghasemi, M. Adeli, Reductive dissolution of manganese ore in sulfuric acid in the presence of iron metal, Hydrometallurgy 2008, 90, 207–212. [120] K.-F. Chen, S. Li, W.-x. Zhang, Renewable hydrogen generation by bimetallic zero valent iron nanoparticles, Chem. Eng. J. 2011, 170, 562–567. [121] S. Biswas, P. Bose, Zero-valent iron-assisted autotrophic denitrification, J. Environ. Eng. 2005, 131, 1212–1220. [122] G. Tewari, D.S. Jayas, L.E. Jeremiah, R.A. Holley, Absorption kinetics of oxygen scavengers. Int. J. Food Sci. Technol. 2002, 37, 209–217. [123] V.A. Polyakov, J. Miltz, Modeling of the humidity effects on the oxygen absorption by iron-based scavengers. J. Food Sci. 2010, 75, 91–99. [124] H. Mu, H. Gao, H. Chen, F. Tao, X. Fang, L. Ge, A nanosised oxygen scavenger: Preparation and antioxidant application to roasted sunflower seeds and walnuts. Food Chem. 2013, 136, 245–250. 24 [125] Y.N. Vodyanitskii, Iron hydroxides in soils: A review of publications, Eurasian Soil Sci. 2010, 43, 1244–1254. 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 [126] Y.N. Vodyanitskii, Iron minerals in urban soils, Eurasian Soil Sci. 2010, 43, 1410–1417. [127] D.D.J. Antia, Sustainable zero-valent metal (ZVM) water treatment associated with diffusion, infiltration, abstraction and recirculation, Sustainability 2010, 2, 2988–3073. [128] A.Lj. Bojic, D. Bojic, T. Andjelkovic, Removal of Cu2+ and Zn2+ from model wastewaters by spontaneous reduction–coagulation process in flow conditions, J. Hazard. Mater. 2009, 168, 813–819. [129] S. Caré, Q.T. Nguyen, V. L'Hostis, Y. Berthaud, Mechanical properties of the rust layer induced by impressed current method in reinforced mortar. Cement Concrete Res. 2008, 38, 1079–1091. [130] Y. Zhao, H. Ren, H. Dai, W. Jin, Composition and expansion coefficient of rust based on X- ray diffraction and thermal analysis. Corros. Sci. 2011, 53, 1646–1658. [131] E. Bi, J.F. Devlin, B. Huang, Effects of mixing granular iron with sand on the kinetics of trichloroethylene reduction. Ground Water Monit. Remed. 2009, 29, 56–62. [122] S. Ulsamer, A model to characterize the kinetics of dechlorination of tetrachloroethylene and trichloroethylene by a zero valent iron permeable reactive barrier. Master thesis, Worcester Polytechnic Institute, 2011, 73 pp. [133] K. Miyajima, C. Noubactep, Effects of mixing granular iron with sand on the efficiency of methylene blue discoloration. Chem. Eng. J. 2012, 200–202, 433–438. [134] C. Noubactep, A. Schöner, P. Woafo, Metallic iron filters for universal access to safe drinking water, Clean: Soil, Air, Water 2009, 37, 930–937. [135] G. Senanayake, J. Childs, B.D. Akerstrom, D. Pugaev, Reductive acid leaching of laterite and metal oxides - A review with new data for Fe(Ni,Co)OOH and a limonitic ore, Hydrometallurgy 2011, 110, 13–32. 25 634 635 636 637 638 639 640 641 642 643 644 645 [136] E. Sikora, D.D. Macdonald, The passivity of iron in the presence of ethylenediaminetetraacetic acid I. General electrochemical behavior, J. Electrochem. Soc. 2000, 147, 4087–4092. [137] C. Noubactep, Aqueous contaminant removal by metallic iron: Is the paradigm shifting? Water SA 2011, 37, 419–426. [138] C. Noubactep, Metallic iron for water treatment: A knowledge system challenges mainstream science, Fresenius Environ. Bull. 2011, 20, 2632–2637. [139] K.C.K. Lai, I.M.C. Lo, V. Birkelund, P. Kjeldsen, Field monitoring of a permeable reactive barrier for removal of chlorinated organics, J. Environ. Eng. 2006, 132, 199–210. [140] S.J. Morrison, P.S. Mushovic, P.L. Niesen, Early breakthrough of molybdenum and uranium in a permeable reactive barrier, Environ. Sci. Technol. 2006, 40, 2018–2024. 26 Table 1: Overview on important results of the four first published peer-reviewed articles on the Fe 645 646 647 648 0/H2O system in 1994 and the number of their citations in Scopus (2012/09/10). [X stands for contaminant; RCl is a chlorinated hydrocarbon]. Reference Systems X Findings Citations Matheson et al. [2] Fe0/H2O CHxCly Degradation mostly by Fe0 682 Gillham et al. [1] Fe0/H2O RCl Enhanced degradation 650 Schreier et al. [4] Fe0/H2O C2Cl4 Partial degradation with lag time 70 Lipczynska-Kochany [3] Fe0/FeS2/H2O CCl4 FeS2 sustains degradation 56 649 650 651 27 Table 2. Some relevant reactions involved in CrVI removal in the system Fe0/H2O. It can be seen that Fe 651 652 653 654 655 656 0 and its both secondary reaction products (Fe2+, H2) can reduce CrVI. Fe0 is oxidized by water (H+), Fe3+, dissolved O2 and CrVI. Ternary reaction products (FeOOH, Fe3O4, Fe2O3) are involved in the process of Cr removal (adsorption). Whether reduced or not Cr is enmeshed in the mass of Fe precipitates or adsorbed at their surface. Reaction equation E0 Eq. (V) Fe0 ⇔ Fe2+ + 2 e- -0.44 (1) 2 H+ + 2 e- ⇔ H2 0.00 (2) Fe3+ + e- ⇔ Fe2+ 0.77 (3) O2(aq) + 2 H2O + 4 e- ⇔ 4 OH- 0.81 (4) CrO42- + 8 H+ + 3 e- ⇔ Cr3+ + 4 H2O 1.51 (5) Fe(OH)3 ⇒ α-, β-FeOOH, Fe3O4, Fe2O3 (6)* FeOOH + CrVI(aq) ⇒ FeOOH–CrVI(adsorbed) (10) *non stoichiometric 657 658 28 Table 3: Coefficient of volumetric expansion (η) of relevant iron species. The reference (η = 1) is Fe 658 659 660 661 662 0 with a molar volume (Vm) of 7.6 cm3/mol. xFe is the stoichiometry of Fe in the solid phase. It is seen that the largest volumetric expansion occurs under oxic conditions (η = 4.53 for ferric hydroxide). Vm values are adopted from Henderson and Demond [66]. Solid phase Name Vm xFe η η (cm3/mol) (-) (-) (%) Fe0 Iron metal 7.6 1.0 1.00 0 FeOOH Goethite 20.3 1.0 2.67 167 Fe(OH)2 Ferrous hydroxide 26.4 1.0 3.47 247 Fe2O3 Maghemite 29.1 2.0 1.91 91 FeCO3 Siderite 29.3 1.0 3.86 286 Fe2O3 Hematite 30.1 2.0 1.98 98 Fe(OH)3 Ferric hydroxide 34.4 1.0 4.53 353 Fe3O4 Magnetite 45.0 3.0 1.97 97 FeII4FeIII2(OH)12CO3.2H2O Carbonate green rust 176.3 6.0 3.87 287 663 664 665 666 29 Figure Caption 666 667 668 669 670 671 672 673 674 675 676 677 Figure 1: Comparison of the solubility limit of iron with the EPA maximum contaminant level (0.3 mg/L or 5.4 μM). Data for FeIII solubility (0.01 M NaCl at 25°C) are from Liu and Millero [59] while data for FeII solubility are from Rickard [60]. Although the experiments are performed under different conditions, it can be seen that iron solubility is minimal between pH 5.5 and 10. This is necessarily the pH range of water treatment using Fe0 and other Fe-bearing materials. Figure 2: Schematic diagram of a three compartments Fe0-amended biosand filter (BSF). The first and the third columns are conventional BSF. The thickness of column 2 (reactive zone) depends on the intrinsic reactivity of used Fe0 (after ref. [117]) 30