Metallic iron for environmental remediation: Back to textbooks 1 2 3 4 5 6 7 Togue-Kamga F.(a), Btatkeu K.B.D.(b), Noubactep C. *(c,d), Woafo P.(a) (a) Laboratory of Modelling and Simulation in Engineering and Biological Physics, Faculty of Science, University of Yaoundé I, Box 812 Yaoundé, Cameroon; (a) ENSAI/University of Ngaoundere, BP 455 Ngaoundéré, Cameroon; (c) Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany (d) Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37073 Göttingen, Germany 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 * corresponding author: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 Abstract The use of metallic iron as environmental remediation medium was based on an incorrect interpretation of experimental observations. Since then, faced with seemingly contradictory data, researchers have substantially revised their models but controversial reports are still current, suggesting that a substantial revision is unavoidable. This communication analyses redox processes in Fe0/H2O systems and demonstrates that the current paradigm even contradicts textbook knowledge on aqueous iron corrosion that was available before the advent of the Fe0 technology. Accordingly, the use of metallic iron for environmental remediation should be regarded as a classical case where scientists are entrenched in a false paradigm. An immediate correction is recommended before a questionable ‘novelty’ is transferred into standard textbooks. Keywords: Chemical Reduction, Electrochemical Reaction, Paradigm shift, Zerovalent iron. 1 1 Introduction 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Controversy is a part of science but is rarely presented in science textbooks [1-3]. In fact, textbooks mostly present final results in form of established hypotheses and models [4]. This approach has been criticized as learners (e.g. undergraduates) do not access the true nature of science [1-4]. Science is universal knowledge that is conventionally gained in a non-linear ‘step-by-step’ accumulation process. As a matter of fact, new findings may emerge from unexpected places, and lead to rapid progress in previously unexpected directions [4-6]. The use of metallic iron (Fe0) for environmental remediation should be regarded as a discovery from an ‘unexpected’ place [2,7]. In fact, according to textbook knowledge available before the introduction of this technology, Fe0 would never have been regarded as reducing agent in water at pH > 4.5 and containing micro-amounts of pollutants (micro- pollutants) [8,9]. In other words, no working (electro)chemist would have had the idea to use iron as reducing agent under environmental conditions [10-15]. However, used as reducing agent, Fe0 has been proven efficient for environmental remediation, wastewater treatment and safe drinking water provision [16-28]. Apart from wastewater treatment, these are typical situations were species of interest are present in trace amounts (micro-pollutants) [29,30] Over the past two decades, an accumulation of data disapproving the virtually universally accepted idea that Fe0 is a reducing agent has been observed [6,15,31]. Although the still currently accepted paradigm was disproved five years ago [32,33], it has been largely ignored by working scientists and other practitioners of the Fe0 technology [34]. This situation suggests ethical issues may not be obligatory in science as giving the state-of-the-art knowledge on any relevant issue should remain a must for any scientific paper [35]. It should be acknowledged that the concept stipulating that Fe0 is the main reducing agent has never been univocally accepted. Three examples for illustration: (i) Lipczynska-Kochany et al. [10] questioned the long-term efficient of reductive reaction at circumneutral pH, (ii) Odziemkowski et al. [11] demonstrated the impossibility of quantitative reduction of n- 2 nitrosodimethylamine by Fe0 under laboratory conditions, and (iii) Farrell et al. [12] demonstrated that under anoxic conditions Fe 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 0 is quantitatively oxidized by water (H+). Disregarding any ethical issues, the present communication aims at demonstrating why Fe0/H2O systems are good remediation system although Fe0 is not a reducing agent. The discussion is based on an analysis of the Fe0/H2O system under anoxic conditions (Tab. 1) using textbook knowledge available before the introduction of the Fe0 remediation technology. The basic Fe0/H2O system is extended to the redox couple CuII/Cu0 and O2/OH- to expand the discussion to bimetallic systems and system operating under oxic conditions. Results corroborates that Fe0 is not likely to serve as a reducing agent. 2 The natural anoxic Fe0/H2O system Natural Fe0/H2O systems are defined as Fe0 in natural waters (6.5 ≤ pH ≤ 9.5). That is water immersed Fe0 at pH values larger than 5.0. In this pH range, Fe0 corrosion is dominated by ‘oxygen adsorption’ type meaning that the oxidative agents must come in contact with Fe0 or a conductive scale at its surface [36-42]. Table 1 summarizes some relevant equations for such a system. Basically, in the absence of oxidizing agents including oxygen (strictly anoxic conditions), there are three inherent redox couples (FeII/Fe0, HI/H0 and FeIII/FeII) to be considered (Eq. 1, 2, 3, 5). The likely reactions on a pure thermodynamic perspective are given in Eq. 7 to 10. Aqueous corrosion of Fe0 materials at pH > 4.5 is an electrochemical process involving the anodic dissolution of iron (Eq. 1) and the cathodic evolution of hydrogen (Eq. 3) [8]. The overall reaction is given by Eq. 7. Eq. 7 alone shows that hydrogen evolution is driven by Fe0 oxidation through water. Accordingly, any attempt to rationalize contaminant reduction by Fe0 using hydrogen evolution is faulty. However, ideally, Eq. 7 is an equilibrium, meaning that, according to Le Chatellier’s principle, if Fe2+ is consumed in a chemical reaction (e.g. O2 reduction – Eq. 13), increased H2 evolution will be observed. This is the fundamental link between ‘H2 evolution’ and ‘contaminant reduction’. Therefore, increased ‘H2 evolution’ in 3 the presence of any contaminant is an indicator for indirect reduction by FeII. The possibility that the contaminant of concern is rather reduced by H 76 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 2 should not be excluded [11]. In this case, recorded H2 evolution is a fraction of total H2. Another important feature from Tab. 1 (Eq. 7 to 10) is the variety of FeII and H2 sources. In particular H2 may derive from three different sources: (i) Fe0 corrosion by water (Eq. 7), (ii) Fe0(ads) oxidation by water (Eq. 8), and the Shikorr reaction (iii) (Eq. 10). The Shikorr reaction is known to occur at temperatures above 80 °C but could be catalyzed by the presence of Fe0 [43]. On the other hand, beside oxidation of Fe0 by water (Eq. 7), FeII could result from Fe0 oxidation by aqueous FeIII (Eq. 9). These additional sources of reducing agents have been largely overseen as contaminant reduction has been mostly attributed to Fe0 [28,44,45]. However, Data from Hydrometallurgy and Synthetic Organic Chemistry have not yet univocally proven the extent of direct reduction (electrons from Fe0) in chemical reduction involving Fe0, even at elevated temperatures [45-51]. For example, Gould [46] reported on a reaction stoichiometry of 1.33 mol of dissolved iron per mol of CrVI reduced. This high efficiency was attributed to generated H2 acting as a reducing agent for CrVI. As discussed here, beside H2, FeII(ads) and FeII(aq) are further sources of reducing agents for CrVI (E0 = 1.53 V). Given the possibility that FeII is ‘recycled’ by generated FeIII (Eq. 9), discussing the actual reaction stoichiometry is a complex task which is over the scope of this communication. In the real world FeII-recycling by microbial activity render the system more complex. Accordingly, whether Fe0 contribute and to which extent to the process of contaminant reduction under anoxic conditions is still unclear. The situation is more complex under oxic conditions. 3 The natural oxic Fe0/H2O system Table 1 shows that all possible reactions under anoxic conditions are possible under oxic conditions as well. In fact, O2 is a more powerful oxidizing agent than water (H+ or H2O). The kinetics of Fe0 oxidation by O2 is more rapid. According to Cohen [52] the reaction is 65 4 times more rapid than under anoxic conditions. However, oxidation with O2 is coupled with the formation of non conductive oxides (e.g. FeOOH, Fe 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 2O3) that will impede any electron transfer from Fe0 (direct reduction). On the other hand, the rapid production of adsorptive species (e.g. Fe(OH)2, FeOOH, Fe2O3) is the rationale for increased contaminant removal under oxic conditions [53,54]. Under oxic conditions the surface of Fe0 is rapidly covered with a multi-layered oxide scale through which any species, including O2, must migrate to reach the Fe0 surface. It has been traceably demonstrated that, under ‘external’ oxic conditions, Fe0 is oxidized by water (Eq. 7) and dissolved O2 is reduced by FeII (Eq. 13) [41,42,55]. This suggests that, as long as the oxide scale is porous enough to enable FeII diffusive transport from the Fe0 surface to sites within the oxide scale where dissolved oxygen (and any other oxidizing species) can diffuse in the opposite direction, chemical reduction could occur. Whether this chemical reaction is quantitative or not depends on several factors including the intrinsic reactivity of used Fe0, the flux of oxidizing agent and the water chemistry [32,33,40-42]. These aspects are not further discussed here. It is sufficient to consider, that the observed electrochemical Fe0 oxidation is not necessarily coupled with species reduction by electrons from Fe0 (direct reduction). The last important feature from Tab. 1 is the presence of the couple CuII/Cu0 which is considered a model alloying element for bimetallic systems. It is clearly seen that water can not oxidize Cu0 (oxidative dissolution to Cu2+). Accordingly Cu0 acts as galvanic cell and facilitated Fe0 oxidative dissolution [9,53,54]. This process accelerates all other processes discussed above. Moreover, under anoxic conditions, FeII recycling in sustained (Eq. 11). Under oxic conditions, Cu0 dissolution is induced and resulted Cu2+ may sustain Fe0 oxidation (cementation). All these predictions are in tune with the observed increased efficiency of bimetallic systems. They also corroborate the view that bimetallic systems sustain an indirect reaction between Fe0 and dissolved species [53,54,57]. 5 4 Discussion 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 Experience in publishing concepts and results on remediation with Fe0 shows that authors are regularly referred to the grey literature [34,57]. In particular, ITRC [34] is the fifth document published since 1999 by the Interstate Technology & Regulatory Council (50 F Street, NW, Suite 350, Washington, DC 20001) to investigate the development of Fe0 permeable reactive barriers as an “emerging remediation technology". However, the fifth edition failed to consider ground-breaking information available in the international literature from 2007 on [32,33,54] and intensively indexed in databases. Even though the content of these ‘handbooks’ are from renowned scientists and practitioners, it is important that these studies be published in peer-reviewed journals, for example in form of “(Bi-)Annual reviews”. This approach will increase the credibility of the contained information and constitute something like an ‘authoritative basis’ to further shape the design, and management of Fe0 treatment systems and minimise any negative impacts. It is important to notice that the Glossary of ref. [34] defines Fe0 as “a strong reducing agent”. While this definition is correct on a pure thermodynamic perspective (E0 = -0.44 V), under natural situations, Fe0 is at best a producer of reducing agents (FeII, green rust, H2) which are all instable species and are further transformed. Contaminants are certainly removed during this dynamic process [15]. The extent of contaminant chemical reduction, however, is difficult to discuss. Moreover, even reduced species should be removed from the aqueous phase [6,15,58,59]. Relevant contaminant removal mechanisms are adsorption, co- precipitation, and adsorptive size-exclusion [58,59]. Adsorptive size-exclusion refers to the increased straining capacity due to porosity loss. Porosity loss is inherent to Fe0 filtration bed because iron corrosion is expansive in nature. In fact, the volume of each corrosion product (e.g. FeO, Fe3O4, Fe2O3, FeOOH, Fe(OH)3) is 2.1 to 6.4 times larger that the volume of a Fe atom [60-64]. 6 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 Irrespective from the availability of handbooks, any researcher starting in a new domain has to find his way in a jungle information. A good path is to start with textbooks, then read review articles (e.g. from databases) and then repeat some relevant experiments to test reproducibility before start own experiments. Testing repeatability includes verifying the correctness of mathematical equations. If this ‘basic’ approach were generally used, a false premise would not have survived for 20 years in an active field of research with actually more than 1500 peer-reviewed articles (Tab. 2). Since 1994 research within the field of "Fe0 technology" has boomed. On April 1st 2012, a search at “ACS publications”, “Science Direct” (Elsevier journals), “Springer journals”, and “Wiley journals” using the key word “zero-valent iron” suggested that up to 1871 peer- reviewed articles may have been published (Tab. 2). This clearly demonstrates the interest within academia for this technology. Accordingly, it is urgent that active research is done on a common basis. The elevated proportion of scientists currently ignoring the state-of-the-art knowledge on remediation with Fe0 is reflected in Tab. 2 (also see ref. [65]). While some 1871 articles may have been published on remediation with Fe0, only some 273 have referenced ‘Noubactep’. Considering only the year 2011 at Elsevier, Noubactep has been referenced 37 times in 84 articles on “zero-valent iron” and “water”, this clearly shows that more than 50 % of all publications ignores the current state-of-the-art knowledge on ‘Fe0 remediation’. Moreover, journal manuscripts and grant proposals will continue to be rejected by established ‘experts’, the sole ‘curses’ of the applicants being to have been: (i) assiduous students trying to realize knowledge from their undergraduate lessons, or (ii) creative graduates willing to experience what should be ‘inherent in research science’: creativity [4]. The history of science is full of examples of primarily rejected ground-breaking ideas [66]. For example, Avogadro's hypothesis that equal volumes of all gases, under the same temperature/pressure conditions, contain equal numbers of molecules was initially rejected. 7 This hypothesis was later proven of key importance in solving many problems in chemical sciences. To date, the view that Fe 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 0 is mostly a generator of reducing agents (H2 and FeII) and Fe oxides has been either severely refuted or just tolerated [67-73]. The tolerance is based on the simplification that, without Fe0, no secondary reducing agents could be available. Accordingly, Fe0 serves as the original source of electron donors (H, H2 and FeII). The present communication has refuted the named simplification and established that quantitative reduction can only result from secondary reducing agents. Accepting this ‘evidence’ is a prerequisite for further technology development. In fact, designing a system in which contaminant reduction should be mediated by a surface reaction is different from designing a system for continuous iron corrosion, sufficient to warrant contaminant removal by adsorption, co-precipitation and size-exclusion. 5 Concluding remarks This communication is an expansion of some earlier ideas summarized in Noubactep [51]. The overall goal is to demonstrate that progress in 'iron for environmental remediation' cannot be achieved if the theory of the system is not established. The way forward is to revise the view that Fe0 is a reducing agent. The alternative view, that Fe0 is a producer of contaminant ‘scavengers’, is in tune with textbook knowledge available before the discovery of this efficient technology. The alternative theory explained better how contaminant removal is achieved. Testing protocols for Fe0 materials [74] and experimental protocols for contaminant removal [75-77] are also provided. When extensively tested these protocols will allow rapid progress of the Fe0 remediation technology. Moreover, this science-based approach will ease the general technology acceptance. The participation of the entire community is required. In closing, it is wished that more attention is paid to theoretical works in Environmental Sciences. This is indeed a major requirement for creativity inherent in research [4,78]. The effectiveness of the current data-based approach is clearly underscored. Data should only be produced to fill gap of knowledge. Actually the current approach has led to the fact that 8 ‘knowledge’ disproving textbooks has been accepted for 20 years. Moreover, proofs are requested from any ‘dissidents’ as if the aqueous iron corrosion was a new discovery. 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Third Edition. - Committee on Science, Engineering, and Public Policy, National Academy of Sciences, National Academy of Engineering, and Institute of Medicine.- The National Academies Press Washington DC, ISBN: 0-309-11971-5, 82 pp., available at: http://www.nap.edu/catalog/12192.html. [79] Mueller N.C., Braun J., Bruns J., Cerník M., Rissing P., Rickerby D. and Nowack B. (2011) Application of nanoscale zero valent iron (NZVI) for groundwater remediation in Europe. Environ. Sci. Pollut. Res. 19, 550 - 558. [80] Singhal R.K., Gangadhar B., Basu H., Manisha V., Naidu G.R.K. and Reddy A.V.R. (2012) Remediation of malathion contaminated soil using zero valent iron nano- particles. Am. J. Anal. Chem. 3, 76 - 82. 17 Table 1: Standard electrode potentials of the Fe0/H2O system and some relevant related reactions. Apart from Fe 418 419 420 421 422 III (ads)/FeII(ads) all electrode potentials are arranged in increasing order of E0. The higher the E0 value, the stronger the reducing capacity of Fe0 for the oxidant of a couple. The couples CuII/Cu0 (Eq. 4) and O0/O-II (Eq. 6) are considered to discuss the cases of bimetallic systems and reactions under oxic conditions respectively. Chemical reaction E0 Eq. (V) Fe0(s) ⇔ Fe2+(aq) + 2 e- -0.44 (1) Fe2+(ads) ⇔ Fe3+(ads) + e- -0.34/-0.65 (2) H+ + e- ⇔ ½ H2(g) 0.00 (3) Cu0(s) ⇔ Cu2+(aq) + 2 e- 0.34 (4) Fe2+(aq) ⇔ Fe3+(aq) + e- 0.77 (5) O2 + 2 H2O + 4 e- ⇔ 4 OH- 0.81 (6) Reactions under anoxic conditions Fe0(s) + 2 H+ ⇒ Fe2+(aq) + H2(g) (7) Fe2+(ads) + H+ ⇒ Fe3+(ads) + ½ H2(g) (8) Fe0(s) + 2 Fe3+(aq) ⇒ 3 Fe2+(aq) (9) 3 Fe(OH)2(s) ⇒ Fe3O4(s) + H2(g) + 2 H2O (10) Cu0(s) + 2 Fe3+(aq) ⇒ Cu2+(aq) + 2 Fe2+(aq) (11) Reactions under oxic conditions Fe0(s) + ½ O2 + H2O ⇒ Fe2+(aq) + 2 HO- (12) 2 Fe2+ + ½ O2 + H2O ⇒ 2 Fe3+(aq) + 2 HO- (13) Cu0(s) + ½ O2 + H2O ⇒ Cu2+(aq) + 2 HO- (14) 423 424 18 Table 2: Results of a web-search for “Zero-valent iron” and “Noubactep” at four relevant publishers demonstrating the current interest within academia for the Fe 424 425 426 427 428 0 technology (search: 01 April 2012). The results for “Noubactep” was corrected to consider only the publication directly dealing with ‘Fe0 remediation’. Publisher ZVI Noubactep ACS publications 491 19 Elsevier Journals 492 190 Springer Journals 380 40 Wiley Journals 508 24 Total 1871 273 429 430 19