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Reoxidation of Photoreduced Polyoxotungstate ([PW12O40]4–) by Different Oxidants in the Presence of a Model Pollutant. Kinetics and Reaction Mechanism

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College of Resources and Environment, Northwest A&F University, P.O. Box 59, NO. 3 Taicheng Road, Yangling, Xianyang, Shaanxi 712100, China
Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, 123 Huntington Street, P.O. Box 1106, New Haven, Connecticut 06504-1106, United States
§ Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520, United States
Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States
*J. Pignatello. E-mail: [email protected]
*D. Qu. E-mail: [email protected]
Cite this: J. Phys. Chem. A 2015, 119, 6, 1055–1065
Publication Date (Web):January 28, 2015
https://doi.org/10.1021/jp510036x
Copyright © 2015 American Chemical Society
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Abstract

Polyoxometalates (POMs) are attractive photocatalysts for water purification. Reoxidation of the photoreduced form of POM by a bulk oxidant is an important step in the cycle yet has received little attention. Photoreduced phosphotungstate ([PW12O40]4–; “POM”) was reacted with bulk oxidants, XOOX, including hydrogen peroxide (HP), peroxyacetic acid (PAA), peroxymonosulfate (MS), peroxydisulfate (DS), and dioxygen (O2), in the presence of the model pollutant 2-propanol under various conditions, and the stoichiometries and rate laws were established. A unified chain reaction is proposed in which the rate-limiting step is outer-sphere one-electron transfer to XOOX yielding OX (OH, SO4 or CH3CO2). This step is found to be proton-assisted when the leaving group OX is a strong base (OH), but independent of [H+] when the leaving group is a weak base (O2, SO42–). The rate of this step follows the order PAA > MS > O2 > HP > DS at pH 1.3, but O2 > PAA > MS > HP > DS at pH 4.1. The chain includes a number of POM-regenerating steps that, with some bulk oxidants (especially MS and DS), leads to further consumption of bulk oxidant and transformation of pollutant. These steps were identified through effects of conditions on reaction stoichiometry, order with respect to [POM], and suppression by POM. Chloride ion “short-circuits” the chain by reducing OX and forming Cl2, which scavenges POM. The results provide insight into POM-catalyzed redox reactions in water purification and selective redox applications.

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Detailed descriptions of materials, methods, and intermediate steps in the derivation of equations as well as figures of spectra, kinetic simulation, reaction stoichiometry, rate constants, and effects of [Na+] on kobs and a table of indicators calculated from plots in Figure S14 and eq S30. This material is available free of charge via the Internet at http://pubs.acs.org.

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