Correspondence/Rebuttal pubs.acs.org/est
Comment on “Mechanochemically Enhanced Degradation of Pyrene and Phenanthrene Loaded on Magnetite”
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 and the proposed pathway of PAH degradation in “magnetite” and “soil” systems, and I hope that helps toward a better understanding of the main mechanisms. First, the authors found that grinding with the low particle sized samples (called magnetite b and magnetite c) resulted in little degradation, whereas that of “magnetite a” (size ≤500 μm) resulted in high degradation of PAH. The authors thought that this experiment corroborated their hypothesis, that is, large particles are cleaved into small f ragments leading to the formation of chemically active sites, with the ultimate conclusion that the smaller the particle size, the lower the PAH degradation rate. I will explain in the following lines why this experiment cannot support the authors’ conclusive statement. From a chemical point of view, correlation for variations in particle size with variations in surface reactivity is valid only if the investigated particle size fractions are derived from the same material. In this study,1 however, three different samples were tested (one natural (I presume?) and two commercials) of the so-called “magnetite”. Magnetites of differential composition having different crystal habits, morphologies and surface properties may exist depending on the source or the synthesis method.2−6 The importance of size cannot override the importance of particle composition. While natural magnetite differs from synthetic one in being of unhydrated bigger particles, commercial nanoparticles (like sample c ≤ 50 nm) generally consist of a complex mixture of magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) phases on the surface.1−6 The latter is commonly referred to as nonstoichiometric or partially oxidized magnetite.7 The oxidation process of magnetite depends on the particle size and on the degree of lattice order,8−11 whereas the degree of stoichiometry strongly influences its reactivity including sorption capacity and reduction potential.12 Unfortunately, this paper1 did not include neither surface nor bulk characterizations of the tested solid samples. There is no doubt that the authors demonstrated a significant degradation of PAH with “magnetite a”, but characterizations of solid samples are essential for a full understanding of the underlying mechanisms. On the other hand, mechanical treatments of some minerals as ball milling in air or dry grinding may lead to changes in the physicochemical properties and/or phase transformations, depending on the particle size and source of mineral (natural or synthetic).13,14 I am not saying that the short grinding or milling investigated here1 may transform the bulk or surface of “magnetites”, but strong doubts persist in the absence of basic characterization of the starting and final solid samples. Second, the mechanism suggested in this paper1 to explain the transformation of PAH molecules at the surface of magnetite (sorption, electron transfer, formation of cation radical, etc.) was largely documented in literature for the oxidation of organic compounds by MnVI−MnIII-oxides and FeIII-oxides.15−17 This conclusion may be valid, but in my view © 2014 American Chemical Society
it is incomplete in the case of magnetite ((FeIII)tet(FeIIIFeII)octO4). It was reported that the reactivity of crushed solids may be mainly dominated by the properties of new artificial surfaces (e.g., microcracks) within a mechanically disturbed zone at the particle edge.18,19 As bulk magnetite contains FeII but the air-exposed surface is often oxidized to FeIII,2−4 grinding may lead to the formation of fresh FeII sites on the surface of grinded particles. It is possible that these new FeII sites may react with atmospheric O2 and/or neighboring organic compounds. It is also possible that the “magnetite” grinding may generate new FeIII-sites, which would be responsible for PAH oxidation as claimed by the authors.1 If the latter was the sole mechanism, similar results should be obtained with FeIII-oxides like maghemite or hematite. Without this type of data, the specific contribution of magnetite as a FeII−FeIII mixed valence oxide in the observed oxidation of PAH cannot be evidenced. It is worthwhile to note that the mechanochemical degradation of pyrene was observed here1 only with divalentcation bearing minerals (i.e., Cu-mont and magnetite), and not with the MnVI−MnIII-oxide (i.e., birnessite) that is usually shown to induce this type of degradation, as reported in numerous works cited by the authors.1 Third, the authors found that the water addition to the PAHspiked soil/magnetite mixture followed by drying before milling resulted in 10−16% of PAH degradation. This has been attributed by the authors to desorption of PAH from soil components, following by a diffusion of molecules to reach the added magnetite surface. However, the low aqueous solubility and the strong sorption of the two tested hydrophobic PAH to soil components such as clay and soil organic matter, let us suppose that the proportion susceptible to diffuse/migrate to reach polar surfaces (e.g., iron oxides) should be insignificant.20,21 In addition, the highest extent of degradation was observed for the less soluble PAH (the most hydrophobic one) according to the data of the authors (16% for pyrene vs 10% for phenanthrene).1 The lack of data on the composition of the investigated soil1 makes difficult a relevant interpretation of the observed behavior. It is, however, reasonable to suppose that adding distilled water (and especially the sudden wetting), following by air drying for 72h and milling may disturb the soil structure and/or cause a rearrangement of soil aggregates (as reported in ref 22). To highlight the real contribution of magnetite in the observed degradation extent (≤16%), the effect of water addition/drying/milling in PAH-spiked soil without magnetite should be investigated. Unfortunately, I did not find in this paper1 such a control test, and thus the data reported here do not permit to prove the specific role of magnetite in PAHcontaminated soil remediation.
K. Hanna*
Published: July 3, 2014 8928
dx.doi.org/10.1021/es502738q | Environ. Sci. Technol. 2014, 48, 8928−8929
Environmental Science & Technology
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Correspondence/Rebuttal
(19) Dubois, I.; Holgersson, S.; Allard, S.; Malmströ m, M. Dependency of BET surface area on particle size for some granitic minerals. Proc. Radiochim. Acta 2011, 1s, 75−82. (20) Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Sorption of polynuclear aromatic hydrocarbons by sediments and soils. Environ. . Sci. Technol. 1980, 14, 1524−1528. (21) Chiou, C. T.; McGroddy, S. E.; Kile, D. E. Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments. Environ. Sci. Technol. 1998, 32, 264−269. (22) Chesworth, W. Encyclopedia of Soil Science, Series: Encyclopedia of Earth Sciences Series; Springer: Berlin, 2008.
Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11, Allée de Beaulieu CS 50837, 35708 Rennes Cedex 7, France
AUTHOR INFORMATION
Corresponding Author
*Phone: 00 33223238027; fax: 00 33223238120; e-mail: khalil.
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Joseph-Ezra, H.; Nasser, A.; Ben-Ari, J.; Mingelgrin, U. Mechanochemically enhanced degradation of pyrene and phenanthrene loaded on magnetite. Environ. Sci. Technol. 2014, 48, 5876− 5882. (2) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrence, and Uses; VCH: New York, 2003. (3) Tronc, E.; Belleville, P.; Jolivet, J. P.; Livage, J. Transformation of ferric hydroxide into spinel by Fe(II) adsorption. Langmuir 1992, 8, 313−319. (4) Vikesland, P. J.; Heathcock, A. M.; Rebodos, R. L.; Makus, K. E. Particle size and aggregation effects on magnetite reactivity toward carbon tetrachloride. Environ. Sci. Technol. 2007, 41, 5277−5283. (5) Zegeye, A.; Abdelmoula, M.; Usman, M.; Hanna, K.; Ruby, C. In situ monitoring of lepidocrocite bioreduction and magnetite formation by reflection Mossbauer spectroscopy. Am. Mineral. 2011, 96, 1410− 1413. (6) Usman, M.; Abdelmoula, M.; Hanna, K.; Grégoire, B.; Faure, P.; Ruby, C. FeII induced mineralogical transformations of ferric oxyhydroxides into magnetite of variable stoichiometry and morphology. J. Solid State Chem. 2012, 194, 328−335. (7) Gorski, C. A.; Scherer, M. M. Determination of nanoparticulate magnetite stoichiometry by Mössbauer spectroscopy, acidic dissolution, and powder X-ray diffraction: A critical review. Am. Mineral. 2010, 95, 1017−1026. (8) Gallagher, K. J.; Feitknecht, W.; Mannweler, U. Mechanism of oxidation of magnetite to γ-Fe2O3. Nature 1968, 217, 1118−1121. (9) Colombo, U.; Fagherazzi, G.; Gazzarrini, F.; Lanzavecchia, G.; Sironi, G. Mechanism of low temperature oxidation of magnetites. Nature 1968, 219, 1036−1037. (10) Murad, E.; Schwertmann, U. Temporal stability of a fine-grained magnetite. Clays Clay Miner. 1993, 41, 111−113. (11) Tang, J.; Myers, M.; Bosnick, K. A.; Brus, L. E. Magnetite Fe3O4 nanocrystals: Spectroscopic observation of aqueous oxidation kinetics. J. Phys. Chem. B 2003, 107, 7501−7506. (12) Gorski, C. A.; Scherer, M. M. Influence of magnetite stoichiometry on FeII uptake and nitrobenzene reduction. Environ. Sci. Technol. 2009, 43, 3675−3680. (13) Elder, T. Particle-size effect in oxidation of natural magnetite. J. Appl. Phys. 1965, 36, 1012−1016. (14) Linderoth, S. L.; Jiang, J. Z.; Morup, S. Reversible α-Fe2O3 to Fe3O4 transformation during ball milling. Mater. Sci. Forum 1997, 235, 205−210. (15) McBride, M. B. Adsorption and oxidation of phenolic compounds by iron and manganese oxides. Soil Sci. Soc. Am. J. 1987, 51, 1466−1472. (16) Stone, A. T.; Morgan, J. J. Reduction and dissolution of manganese (III) and manganese (IV) oxides by organics. 1. Reaction with hydroquinone. Environ. Sci. Technol. 1984, 18, 450−456. (17) Feitosa-Felizzola, J.; Hanna, K.; Chiron, S. Adsorption and transformation of selected human-used macrolide antibacterial agents with iron (III) and manganese (IV) oxides. Environ. Pollut. 2009, 157, 1317−1322. (18) André, M.; Malmström, M. E.; Neretnieks, I. Specific surface area measurements on intact drillcores and evaluation of extrapolation methods for rock matrix surfaces. J. Contam. Hydrol. 2009, 110, 1−8. 8929
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