Comments on “Reduction of chromate by granular iron in the presence of dissolved CaCO 1 2 3 4 3” by Gui et al. [Appl. Geochem. 24 (2009) 677–686] C. Noubactep Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax. +49 551 399379 In a recent study, Gui et al. (2009) conducted column experiments to determine the effects of dissolved CaCO3 on the reactivity of metallic iron (Fe0) towards CrVI reduction and hydraulic conductivity. For their discussion, the authors considered CrVI reduction by Fe0 as a surface- mediated process. The thermodynamic instability of Fe0 in water favours the oxidation of Fe0 to FeII/FeIII species and the precipitation of iron species on the Fe0 surface (oxide film or oxide layer). The oxide film will severely impede the electron transfer from Fe0 to CrVI, decreasing the transformation rate of CrVI to CrIII. On the other hand, in situ generated solid iron species will certainly reduce the permeability of the Fe0 wall (loss of hydraulic conductivity). The consideration of Gui et al. (2009) is in conformity with the state-of-the-art knowledge on the mechanism of reducible contaminants in the presence of Fe0 (e.g. in Fe0/H2O systems). However, there are strong evidences that “CrVI reduction at the surface of Fe0” may not be consistent. First, quantitative CrVI reduction to CrIII by (i) FeII-bearing commercial chemical reductants, e.g. FeSO4.7H2O (Park et al., 2008), (ii) various FeII-bearing natural minerals including green rusts (Eary and Rai, 1988; White and Peterson, 1996), and (iii) electrically generated FeII (Zongo et al., 2009) has been reported. These reports suggest that CrVI may be reduced in the aqueous phase by FeII from Fe0 oxidation (primary corrosion products). CrVI may also be reduced within the oxide film on Fe0 by green rusts which are always available in (anoxic) Fe0/H2O systems (Chaves, 2005). Therefore, considering CrVI reduction to CrIII in a Fe0/H2O system a priori as a surface-mediated process may not be acceptable. Moreover, it is not 1 certain whether the Fe0 surface do play any direct role in the process of CrVI reduction, since diffusion of Cr 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 VI to Fe0 is necessarily the rate-determining step. Second, CrVI reduction to CrIII by FeII adsorbed at the surface of minerals (so called structural FeII) has been reported (Fendorf and Li, 1996; White and Peterson, 1996). In particular, White and Peterson (1996) reported that the electrode potential for the redox couple FeIII(s)/FeII(s) ranges from -0.34 to -0.65 V. This makes structural FeII a stronger reducing agent than Fe0 (table 1) under some circumstances (E < -0.44 V). Clearly, CrVI reduction to CrIII can be mediated by metallic iron (Fe0), structural FeII, molecular H2, and dissolved FeII. Given that at pH value > 5 the Fe0 surface is always shielded by an oxide film, there is no reason for quantitative CrVI reduction at the Fe0 surface. Table 1: Relevant half-reactions for the discussion of the mechanism of CrVI removal in Fe0/H2O systems and their relevant standard reduction potentials (E0). E0 values are arranged in increasing order. The lower the E° value, the stronger the reducing capacity for CrVI (e.g. CrO42-). All reducing agents (Fe0, FeII(s), H2(g) and FeII(aq)) can reduce CrO42- to Cr3+. Equation Half-reaction E0(V) (1) Fe2+ + 2e- ⇔ Fe0 -0.44 (2) Fe3+(s) + e-⇔ Fe2+(s) -0.34 to -0.65 (3) 2 H+(aq) + 2 e- ⇔ H2(g) 0.00 (4) Fe3+(aq) + e- ⇔ Fe2+(aq) 0.77 (5) CrO42- + 8 H+ + 3e- ⇔ Cr3+ + 4 H2O 1.51 43 44 45 Third, a Fe0 bed can be considered as filter removing CrVI by simple size exclusion processes. Since the in situ generated iron oxides occupy the pore spaces (loss of hydraulic conductivity) 2 they do increase the filtration capacity/efficiency. Therefore, with increasing service life a Fe0 bed may remove Cr 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 VI without any chemical transformation nor strong adsorptive interactions. The superiority of Fe0 versus FeII-bearing materials for the reductive precipitation of CrIV is well documented (Gould, 1982; Park et al., 2008; Gui et al., 2009). The theoretical facts above demonstrate this is because of the continuous production of reactive FeII (FeII(aq) and FeII(s)) and the diversity of reducing agents in the Fe0/H2O system (Gould, 1982). Therefore, Fe0 could be considered as a source for the in-situ production of oxide film and reducing agents for CrVI removal (Noubactep, 2007; Noubactep, 2008). Clearly, CrIV reduction mostly occurs within the oxide film. This conclusion corroborates the results of iron electrocoagulation. In fact, CrVI removal in electrocoagulation using iron electrodes (Fe0 EC) was shown to occur as follows: (i) CrVI is reduced by electrochemically generated FeII species (yielding CrVI and FeIII), (ii) resulted CrIII is coagulated by freshly formed FeIII agent. The efficiency of Fe0 EC relies on both the reduction of CrVI and the promoted oxidation of FeII (Zongo et al., 2009). Fe0 EC should be considered as an electrochemically driven accelerated corrosion process. Therefore, the processes are rigorously the same with the sole and subtle difference that corrosion is electrically accelerated in Fe0 EC. CrVI removal in column studies (and Fe0 field reactive barriers) combines the size exclusion effect of the Fe0 bed and the chemical reactivity of Fe0. Because adsorbents are in situ generated and transformed (aged), iron oxides of various adsorbing capacity are certainly available in any Fe0/H2O system and are good adsorbents for CrVI (and CrIII). Oxide films continually grow into the metal at the Fe0/film interface while being simultaneously restructured at the film/H2O interface. Accordingly, at the interface Fe0/film, a continual generation and annihilation of point defects occurs and this interface can be considered as “living” (Sikora and Macdonald, 2000). Aged iron oxides at the film/H2O interface should be regarded as “dead” or simple “coatings” with limited CrVI adsorption capacity. However, it is not likely that inflowing CrVI, which is first adsorbed onto “dead” oxides will readily further 3 migrate to the “living” layer which is necessarily richer in FeII species in order to be reduced to Cr 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 III. Therefore, despite spectroscopic evidences of CrIII in Fe0/H2O systems (e.g. Raman spectra), CrVI reduction may not be quantitative in the long term. A time scale investigation of Cr mass balance accompanied by a speciation of total Cr is necessary to addressed this important issue. In conclusion, a close analysis of the Fe0/H2O system shows in agreement with electrocoagulation using iron electrodes that Fe0 is a source of reactive species (oxide film, molecular hydrogen and FeII species) for CrVI adsorptive or reductive removal. Although the reductive removal may be quantitative, its long-term effectiveness should be closely investigated. In the light of the discussion above, the influence of CaCO3 on the process of CrVI removal in Fe0/H2O systems should be reconsidered. It seems that the major question to answer in this effort is: what is the impact of CaCO3 on the formation and transformation of oxide film on Fe0? To this end, a third column without CrVI inflow could serve as reference system to evidence the impact of CrVI on Fe0 corrosion as measure by decrease of hydraulic conductivity. Whether removed Cr is chemically transformed nor not, the most important issue is its long-term stability within the Fe0/H2O system. References Chaves, L.H.G., 2005. The role of green rust in the environment: a review. Rev. Bras. Eng. Agríc. Ambient.9, 284–288. Eary, L.E., Rai, D., 1988. Chromate removal from aqueous wastes by reduction with ferrous ion. Environ. Sci. Technol. 22, 972–977. Fendorf, S.E., Li, G. 1996. Kinetics of chromate reduction by ferrous iron. Environ. Sci. Technol. 30, 1614–1617. Gould, J.P, 1982. The kinetics of hexavalent chromium reduction by metallic iron. Water Res. 16, 871–877. 4 Gui, L., Yang, Y., Jeen, S.-W., Gillham, R.W., Blowes, D.W., 2009. Reduction of chromate by granular iron in the presence of dissolved CaCO 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 3. 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