Correspondence/Rebuttal pubs.acs.org/est
Comment on “Inhibitory Effect of Dissolved Silica on H2O2 Decomposition by Iron(III) and Manganese(IV) Oxides: Implications for H2O2−Based In Situ Chemical Oxidation”
I
read this paper1 with great interest and would like to raise a few scientific and practical questions on the interpretation of the experimental data. Interaction of hydrogen peroxide with metal oxides and in particular iron oxyhydroxides is not a new topic and has been studied for many decades. These interactions have attracted a great deal of attention not only because of the complicated chemical processes in both homogeneous and heterogeneous systems but also because the reaction has significant environmental implications. In Fenton heterogeneous system where iron solid mineral is used instead soluble iron, interactions of H2O2 with iron surface sites is a prerequisite to initiate the Fenton reaction according to the classical Haber−Weiss mechanism.2 The surface interactions of oxidant, pollutant, and ligand with iron surface sites involving both sorption and decomposition reactions are largely studied (refs 3−8 and references cited within). In fact, Fenton heterogeneous oxidation is mainly controlled by surface mechanism reaction where each species compete with each other for adsorption on a fixed number of surface active sites. Consequently, any ligand capable of adsorbing on the surface of mineral oxide will certainly decrease the reactivity of metal oxide toward H2O2. This behavior is previously explained in mineral-catalyzed Fenton-like studies (refs 2−6 and references cited within) and recently observed for fluoride,7 and for some organic ligands.8 In this work,1 the same behavior was observed by using one another strongly sorbed inorganic ligand (i.e., silicate). Although the sorption of silicate on iron oxides has been extensively studied at all molecular, microscopic and macroscopic levels (17 references cited in ref 1 plus Eick’s works,9 Swedlen’s works,10 Rusch et al.,11 etc.), a substantial portion of this manuscript1 is dedicated to this part. In this work,1 the authors found that the presence of dissolved silica decreases the reactivity of iron minerals toward H2O2, because silica adsorbs onto the surface of iron minerals and alters catalytic sites. This conclusion which is previously documented in literature is valid. But, it is incomplete in the case of this study. Unfortunately, the authors neglected important effects of dissolved silica on the stabilization of H2O2 molecule in aqueous solution. As hydrogen peroxide is inherently unstable in aqueous solutions, anionic ligands (phosphate, silicate, polymers, etc.) capable of H-bonding formation were used for hydrogen peroxide stabilization and particularly at alkaline pH.12−14 Indeed, owing to the presence of lone electron pairs in the molecule and its specific geometry, hydrogen peroxide can form peroxohydrates with a variety of hydrophilic compounds. In aqueous solution, the role of stabilizers such as sodium silicate is to reduce the rate of hydrogen peroxide decomposition by forming complex with hydrogen peroxide, with an intermediary compound, and/or with metal ions in the solution.12−19 Recently, authors reported that hydrogen peroxide may form © 2012 American Chemical Society
relatively strong hydrogen bond to oxygen of the siloxane bridge, Si−O−Si, of disilicic acid.20 On the other hand, Kinrade et al.21 have suggested that silicate form labile complexes with radical H2O2-decomposition products, possibly O2·−. All these observations suggest that the molecular interactions of silicate with H2O2 molecule cannot be neglected. I will attempt to explain the contribution of these interactions in the interpretation of the data of this work:1
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Fe OXIDES and OXYHYDROXIDES The surface speciation of silicate on the Fe oxides and oxyhydroxides is strongly dependent on silicate concentration. A lower Si concentration, it is present as a loosely bound surface complex. At higher concentration, polymerization occurs and silica polymers form surface clusters that cause only minimal changes in the quantity of reactive surface sites when initial silica concentrations increases.9−11 Therefore, the decrease of H2O2 decomposition rate observed in this paper1 (Figure 3) when Si varies from 0 to 1.5 mM cannot be only due to the decrease of overall site density of goethite, but also to the increase of residual silicate concentration that could stabilize H2O2 molecule. This paper1 did not include control and blanks experiments in homogeneous systems to provide better insights about the chemistry of H2O2 and dissolved silicate. To test rapidly the impact of silicate on H2O2 decomposition in homogeneous system, two tests (i) {H2O2 + dissolved FeIII} and (ii){H2O2 + dissolved FeIII + silicate} were done at pH 6.9 ± 0.1 in our laboratory. The results showed that when silicate was added to the test (ii) the residual peroxide was found to be higher than the control (i), indicating that silicate is able to decrease the iron-induced peroxide decomposition. The stabilizing effect of silicate may be probably due to the formation of aqueous complexes between silicate and iron leading to deactivation of catalyst role, and also with H2O2 enhancing its stabilization. On the other hand, due to the formation of H2O2-silica complex and/or competition between H2O2 and silicate for surface adsorption (own conclusion of the authors (1)), addition of H2O2 will certainly affect the sorption of silicate on iron oxide surface, and cause a release of silicate from surface, as previously reported.6−8 This paper1 did not include measurements of residual silicate concentration or silicate sorbed amount upon addition of H 2 O 2 . For an appropriate interpretation of the experimental data in Figure 3b, kobs should be plotted against SiO2 sorbed in the presence of H2O2 (or versus residual silicate concentration), and not against SiO2 sorbed in the absence of H2O2. Published: February 13, 2012 3591
dx.doi.org/10.1021/es3002103 | Environ. Sci. Technol. 2012, 46, 3591−3592
Environmental Science & Technology
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Correspondence/Rebuttal
(6) Kwan, W. P.; Voelker, B. M. Influence of electrostatics on the oxidation rates of organic compounds in heterogeneous fenton systems. Environ. Sci. Technol. 2004, 38 (12), 3425−3431. (7) Xue, X.; Hanna, K.; Abdelmoula, M.; Deng, N. Adsorption and oxidation of PCP on the surface of magnetite: Kinetic experiments and spectroscopic investigations. Appl. Catal., B 2009, 89, 432−440 and references cited therein. (8) Xue, X.; Hanna, K.; Despas, C.; Wu, F.; Deng, N. Effect of chelating agent on the oxidation rate of PCP in the magnetite/H2O2 system at neutral pH. J. Mol. Catal. A: Chem. 2009, 311, 29−35. (9) Eick, M. J.; Luxton, T. P.; Welsh, H. A. Effect of silica polymerization on the oxalate-promoted dissolution of goethite. Clays Clay Miner 2009, 57, 578−585 and references cited therein. (10) Swedlund, P. J.; Sivaloganathan, S.; Miskelly, G. M.; Waterhouse, G. I. N. Assessing the role of silicate polymerization on metal oxyhydroxide surfaces using X-ray photoelectron spectroscopy. Chem. Geol. 2011, 285, 62−69 and references cited therein. (11) Rusch, B.; Hanna, K.; Humbert, B. Coating of quartz silica with iron oxides: Characterization and surface reactivity of iron coating phases. Colloid. Surf., A 2010, 353, 172−180 and references cited therein. (12) Colodette, J. L.; Dence, C. W. Factors affecting hydrogen peroxide stability in the brightening of mechanical and chemimechanical pulps, part IV: The effect of transition metals in Norway spruce TMP on hydrogen peroxide stability. J. Pulp Pap. Sci. 1989, 15, 79−83. (13) Panarin, E.; Kalninsh, K.; Azanova, V. IR spectra and structure of poly(vinylamide) complexes with hydrogen peroxide. Polym. Sci. Seri. A 2007, 49 (3), 275−283. (14) Bambrick, D. R. Effect of DTPA on reducing peroxide decomposition. Tappi J. 1985, 68 (6), 96−100. (15) Wekesa, M.; Ni, Y. Stabilization of peroxide systems by silicate and calcium carbonate and its application to bleaching of recycled fibers. Pulp Pap. Canada 2003, 104 (12), 320−322. (16) Jaakko, R. Hydrogen peroxide-metals-chelating agents; interactions and analytical techniques. Academic Dissertation, Faculty of Technology, University of Oulu-Finland, 2003. (17) Wekesa, M.; Habtewold, A.; Mirdaniali, J. Stabilization of manganese (Mn)-induced peroxide decomposition. Afr. J. Pure Appl. Chem. 2011, 5 (7), 176−180. (18) Hämäläinen, H.; Aksela, R.; Rautiainen, J.; Sankari, M.; Renvall, I.; Paquet, R. Silicate-free peroxide bleaching of mechanical pulps: Efficiency of polymeric stabilizers. Proceedings TAPPI of International Mechanical Pulping Conference, 2007; pp 215−236. (19) Kakabadse, G.; Dewsnap, J. W. The sodium silicate - hydrogen peroxide system. Nature 1960, 185, 4715−4761. (20) Zeglinski, J.; Piotrowski, G. P.; Piekos, R. A study of interaction between hydrogen peroxide and silica gel by FTIR spectroscopy and quantum chemistry. J. Mol. Struct. 2006, 794, 83−91 and references cited therein. (21) Kinrade, S. D.; Holah, D. G.; Hill, G. S.; Menuz, K. E.; Smith, C. R. The peroxysilicate question. 29Si-NMR evidence for the role of silicates in alkaline peroxide brightening of mechanical pulp. J. Wood Chem. Technol. 1995, 15 (2), 203−222. (22) Kakarla, P. K. C.; Watts, R. J. Depth of Fenton-like oxidation remediation of surface soil. J. Environ. Eng. 1997, 123 (1), 11−17.
MnO2 According to the own data of the authors,1 SiO2 adsorption on MnO2 surface is negligible. Despite this very low sorption, the effect of SiO2 concentration (ranging from 0 to 1.5 mM) on the H2O2 decomposition rate is relatively significant (Figure 51). This inhibitory effect cannot be explained by the sorption of silicate because no sorption was detected in the same experimental setup used for both adsorption and oxidation tests. The most plausible hypothesis is that the decrease in H2O2 decomposition rate may result from the interactions of H2O2 with silicate in aqueous phase. In addition, the decrease of H2O2 decomposition rate is positively (perhaps linearly?) correlated with the increasing of dissolved silicate concentration (Figure 51). Plotting H2O2 decomposition rate vs residual or initial silicate concentration would give better insights. This data (Figure 5 in ref 1) provides strong evidence to support that molecular interaction with dissolved silicate protect the hydrogen peroxide against Mn-induced decomposition. On the other hand, the groundwater contains a lot of organic and inorganic ligands that can strongly compete with silicate for sorption, and can interfere with the radical or nonradical H2O2 decomposition. Because there are many side reactions and propagation reactions in hydrogen peroxide-soil systems, the H2O2 decomposition in natural systems are multiple and complex. Thus, the proposition1 for injecting of Si in ISCO technologies for extending the longevity of hydrogen peroxide is, in my view, irrelevant. Finally, it is worthy to note that the effect of several stabilizing agents including silicate on H2O2 decomposition in real soil matrices has been previously tested, and silicic acid was found ineffective in stabilizing H2O2 in real soils.22 Only K2HPO4 at high dose provided satisfactory results.22 K. Hanna* Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France Université Européenne de Bretagne
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AUTHOR INFORMATION
Corresponding Author
*Phone: 00 33 2 23 23 80 27; fax: 00 33 2 23 23 81 20; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Pham, A. L.-T.; Doyle, F. M.; Sedlak, D. L. Inhibitory effect of dissolved silica on H2O2 decomposition by Iron(III) and manganese(IV) oxides: Implications for H2O2-based in situ chemical oxidation. Environ. Sci. Technol. 2012, 46 (2), 1055−1062. (2) Kwan, W. P., Ph.D. Thesis, Massachusetts Institute of Technology: Cambridge, MA, 2003; and references cited therein. (3) Valentine, R. L.; Ann Wang, H. C. Iron oxide surface catalyzed oxidation of quinoline by hydrogen peroxide. J. Environ. Eng. 1998, 124 (1), 31−38. (4) Watts, R. J.; Bottenberg, B. C.; Hess, T. F.; Jensen, M. D.; Teel, A. L. Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified Fenton’s reactions. Environ. Sci. Technol. 1999, 33 (19), 3432−3437. (5) Huang, H.-H.; Lu, M.-C.; Chen, J.-N. Catalytic decomposition of hydrogen peroxide and 2-chlorophenol with iron oxides. Water Res. 2001, 35 (9), 2291−2299. 3592
dx.doi.org/10.1021/es3002103 | Environ. Sci. Technol. 2012, 46, 3591−3592
