Mechanisms of removal of heavy metals and arsenic from water by TiO2-heterogeneous photocatalysis

M. I. LITTER

Congreso; Fourth Latin-American Congress of Photocatalysis, Photoelectrochemistry and Photobiology - LACP3 2016; 2016

The photocatalytic reduction of Cr(VI) in water has been widely reported in the literature, both as an environmentally friendly process for the treatment of this pollutant as well as a simple system to evaluate the photocatalytic activity of various semiconductors [1-3]. This photocatalytic system is unique: Cr(VI) is the only metal species whose reductive TiO2-photocatalytic removal is not influenced by the presence of oxygen, at least at acid pH, in contrast with most metals; this has been explained by the fast capture of electrons caused by a very strong association between Cr(VI) and TiO2 through the formation of a charge-transfer complex. However, several aspects of this system, where Cr(III) is the final stable product, are still under discussion, as the influence of the physical properties of the photocatalyst samples used in the reaction and the progressive inactivation that the photocatalyst undergoes during the reaction, especially in the absence of electron donors. Time Resolved Microwave Conductivity (TRMC) is a powerful tool for analyzing charge carrier lifetimes in TiO2 [4-6]. The technique can be used also to understand the effect of species adsorbed or deposited on the surface of a semiconductor, like porphyrins, oxalic acid, platinum, etc. In this work, the photocatalytic efficiency for Cr(VI) transformation in the presence of etilendiaminetetraacetic acid (EDTA) at pH 2 over various commercial TiO2 samples (Evonik P25, Cristal Global PC50, PC100 and PC500, Hombikat UV100, Fluka and Kemira S230) was evaluated. The decay was adjusted to a pseudo-first order kinetics in all cases and the photocatalytic activity of each sample was estimated from the values of the kinetic constants. The order of the photocatalytic activity was PC500  UV100 > P25  PC100 > S230  PC50 >> Fluka and it strongly depends on the nature of the TiO2 samples, especially on the surface area, except for S230 and at a less extent for P25. The formation of the Cr(III)-EDTA complex in solution as a product of the Cr(VI) photocatalytic reduction was confirmed by visible spectrophotometry, and the amount of Cr on the photocatalysts at the end of the run was quantified. In addition, it was demonstrated that the final products of the photocatalytic reduction of Cr(VI) in the presence of EDTA are Cr(III)-EDTA and Cr on TiO2, and that their speciation is almost independent of the nature of the photocatalyst used; besides, the photocatalytic reduction of Cr(VI) and the formation of Cr(III)-EDTA are clearly related. The results were related to the TRMC measurements obtained on the different powders before and after the photocatalytic experiments.