Reoxidation of photoreduced polyoxotungstate ([PW12O40](4-)) by different oxidants in the presence of a model pollutant. Kinetics and reaction mechanism.

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(•-), SO4(2-)). 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.