Photocatalysis in TiO2 aqueous suspension: Effects of mono- or di-hydroxyl substitution of butanedioic acid on the disappearance and mineralisation rates

The photocatalytic degradation of butanedioic acid (BDA), hydroxybutanedioic acid (OH-BDA) and 2,3-dihydroxybutanedioic acid (diOH-BDA) (succinic acid, malic acid and tartaric acid, respectively) in a TiO2 aqueous suspension at pH 3 was investigated to determine the effects of hydroxyl substitution. Temporal variations in the concentrations of the main intermediate products are reported. The use of a set of reactions involving decarboxylation and formation of alkylperoxy and alkoxy radicals has allowed us to account for the occurrence of these products, and to suggest the preferential decarboxylation of the COOH/COO− group adjacent to the OH group in OH-BDA. The adsorbed amounts in the dark increased with the number of OH groups, while the irradiation time necessary for total disappearance of the initial diacid, decreased. However, the evolution rate of CO2, which was initially the same for the three acids, was then lower for the hydroxylated diacids for a significant period of irradiation time. This difference is tentatively attributed predominantly to the effect of hydroxyl substitution on the adsorption mode. One of the carboxyl groups would remain away from the surface (and hence not be available for direct electron transfer) due to the restricted mobility of the adsorbed diacid caused by hydrogen-bonding of TiO2 with the alcohol group adjacent to the other carboxyl group. This hypothesis is also effective to qualitatively interpret how the zeta potential varied in the course of the degradation of each diacid. The present study further illustrates the essential role of distinct adsorption modes in photocatalytic events.

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► OH substitution led to different diacid adsorption mode on the TiO2 surface.
► The amount of diacid adsorbed in dark conditions increased with OH group number.
► Hydroxylation accelerated parent compound disappearance from the suspension.
► Diacid hydroxylation altered intermediates formed and/or their accumulations.