Kinetics and equilibrium of surface complexation reactions: the adsorption of polycarboxylic acids on titanium dioxide

A Fourier transform infrared attenuated total reflectance (FT-IR-ATR) study on the adsorption equilibria of several polycarboxylic acids, citric acid, ethylenediaminetetraacetic acid (EDTA), and triethylenetetramine-N, N, N ', N ", N "', N "'-hexaacetic acid (TTHA), on titanium dioxide (TiO2) in the concentration range from 3 x 10(-7) to 6 x 10(-4) mol, is reported. The phenomenon involved is the heterogenous complexation of surface Ti ions by the adsorbing ligand. The adsorption of citric acid, EDTA, and TTHA is compared with that of acetic and oxalic acids. The IR spectra were analyzed by singular value decomposition, showing that the only one basis spectrum contributed significantly to the experimental spectra for all the cases. Adsorption isotherms were fitted to two Langmuirian equations. Langmuir constants (K-L) were interpreted in terms of the molecular structure and a number of carboxylic groups. The highest KL characterizes the citric acid adsorption (7.1 +/- 0.3 x 10(6) mol(-1)), followed by oxalic acid and TTHA (2.1 +/- 0.1 x 10(6) mol(-1)). Surprisingly, EDTA has the smallest Langmuir constant of the group (except for acetic acid that adsorbs negligibly), 4.06 +/- 0.7 x 10(5) mol(-1). Structures for the most important surface complexes of each of the studied ligands are proposed based on the previous studies on related systems. We propose that the most stable adsorption mode corresponds to fully deprotonated ligands interacting with Ti ions through all the carboxylates.