Kinetics of adsorption of carboxylic acids onto titanium dioxide.

A FTIR-ATR kinetic study on the adsorption of carboxylic acids (oxalic, citric, malonic, succinic, gallic, EDTA and TTHA acids, where EDTA = ethylenediaminetetraacetate and TTHA = triethylenetetramine-N, N, N', N ", N"', N"'-hexaacetate, in the concentration range 6 x 10(-7) M-2 x 10(-5) M on TiO2 (Degussa P25) by ATR-FT-IR is reported. The influence of carboxylic acid concentration, pH, ionic strength, TiO2 load in the film is presented. The adsorption processes follows pseudo-first-order kinetics at constant ligand concentration, even though several adsorption modes have been reported in the literature. Plots of the pseudo first order constant k(obs) vs. carboxylic acid concentration are linear for all the studied ligands. The slopes of these plots (a) are not very sensitive to the nature of the ligands; a decrease in a is observed as the size increases. The intercept (b) is inversely related to the stability of the surface complexes. We propose that both the rate of adsorption, and the desorption rate are controlled by the diffusion through the pores of the film, although in the case of (b), the desorption rate is modulated by the stability of the surface complex. These results are relevant for oxide dissolution, remediation of water, pollutants removal, sensors design and heterogeneous photocatalysis.