Environ. Sci. Technol. 2010, 44, 6519
Response to “Comment on ’Surface Complexation of Catechol to Metal Oxides: An ATR-FTIR, Adsorption, and Dissolution Study’” In the recent comment on the interaction of catechol with metal oxides (1), Liu (2) raises some interesting points. His principle remarks focus on the lack of a more detailed discussion of catechol oxidation. We appreciate that questions of this nature may arise, however Liu’s comments fail to address what the focus of the paper is about. Our primary objective was to elucidate the structure of surface complexes of catechol on metal oxide systems and evaluate how these relate to the reactivity of the oxides toward dissolution. While it is true that the interaction of polyphenols with metals has been studied for many decades, the structure of catechol surface complexes on metal oxides surfaces has not been evaluated in significant detail, which is what this paper addresses. We would like to clarify some of the questions proposed regarding the dissolution analysis. Our studies evaluated the amount of soluble metal in solution, and extensive efforts (i.e., filtering and analysis of supernatant by atomic force microscopy) have ensured that the metal oxides were separated from the supernatant. Discrepancies between measured metal solubility and solubility products (Ksp) are common and may result from a variety of reasons including the fact that the metal oxides evaluated are a smaller particle size and may exhibit more defects, structure irregularities, and differ in the degree of crystallinity compared to the crystal phases to which Ksp refers. The same may be true using other models such as Visual Minteq. The higher solubility of the metal oxides used in this study is likely due to larger surface areas and defects of the oxide surfaces. Although, high surface area sol-gel metals oxides are widely used as models of minerals in the environment for spectroscopic evaluation of surface complexations, an investigation more accurately modeling the solubility of these metal oxides would be an important and interesting avenue to explore in the future. In regards to Choi’s remarks on MnO2 dissolution, it is important to note that the surface area of the metal oxides evaluated varies. Therefore, an exact correlation of the metal concentrations measured from the dissolution analysis between the different metal oxides evaluated should not be made. The analysis is meant to reflect trends in the dissolution measured as a function of pH and ionic strength and to correlate with ATR-FTIR spectroscopy analysis of the surface complexes. Our spectroscopic evaluation of MnO2/catechol interactions suggests that an outer-sphere complex is evident, which can lead to reductive dissolution of metal oxides (3, 4). Furthermore, a redox process can be a result of surface complex. In our discussion were careful to not over speculate the role of outer-sphere complexation in the dissolution process of MnO2.
10.1021/es102327j
2010 American Chemical Society
Published on Web 07/22/2010
We agree that oxidation is important to consider when evaluating catechol/metal oxide interactions, and a discussion of this was emphasized within the paper. For all of the metal oxides the recovery rate and analysis of oxidation products for catechol was evaluated, as stated in our discussion. Recovery rates for catechol adsorption on TiO2, Cr2O3, and Fe2O3 were around ∼95% of the total ligand concentration. As we clearly noted, although oxidation products were evident for MnO2, they were not detectable with the other metal oxides evaluated. We are not suggesting that o-quinone is the only oxidation product of catechol, but a likely candidate based on previous reported analysis of MnO2 oxidation of catechol. Oxidation of catechol by Fe2O3 is significantly less vigorous than by MnO2 and oxidation products may not have been detectable under the experimental conditions (i.e., scale) evaluated; previous studies have also shown varied reactivity of phenol oxidation with different hematite samples (5). Regardless, this study expands our understanding of catechol surface complexation and its impact on dissolution processes. In summary, the concerns raised by Liu provide for a valid discussion, but after carefully reviewing his comments and the positive feedback from reviewers and other interested readers we do not believe that the issues raised affect the overall conclusions of our study. This work provides the first spectroscopic evaluation of catechol adsorption on TiO2, Cr2O3, Fe2O3, and MnO2 as a function of pH and ionic strength, providing new insights into the role of catecholates in geochemical processes.
Literature Cited (1) Gulley-Stahl, H.; Hogan, P. A., II; Schmidt, W. L.; Wall, S. J.; Buhrlage, A.; Bullen, H. A. Surface complexation of catechol to metal oxides: An ATR-FTIR, adsorption, and dissolution study. Environ. Sci. Technol. 2010, 44, 4116–4121. (2) Liu, R. Comment on “Surface complexation of catechol to metal oxides: An ATR-FTIR, adsorption, and dissolution study”. Environ. Sci. Technol. 2010, 44, doi: 10.1021/es102068s. (3) Sparks, D. L. Environmental Soil Chemistry; Academic Press: San Diego, 2002. (4) Stone, A. T. Reductive Dissolution of Manganese(III/IV) Oxides by Substituted Phenols. Environ. Sci. Technol. 1987, 21, 979– 988. (5) McBride, M. B. Adsorption and oxidation of phenolic compounds by iron and manganese oxides. Soil Sci. Soc. Am. J. 1987, 51, 1466–1472.
Heather Gulley-Stahl, Patrick A. Hogan II, Whitney L. Schmidt, Stephen J. Wall, Andrew Buhrlage, and Heather A. Bullen* Department of Chemistry, Northern Kentucky University, Highland Heights, Kentucky 41099 ES102327J
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