Combining kinetic investigation with surface spectroscopic examination to study the role of aromatic carboxyl groups in NOM adsorption by aluminum hydroxide

The adsorption of a series of aromatic carboxylic acids, with different numbers and positions of carboxyl groups in the phenyl ring, on aluminum hydroxide was investigated with ATR-FTIR and kinetic analyses to verify the role of aromatic carboxyl groups in the surface complexation of NOM with aluminum hydroxide. It was revealed that the formation of outer-sphere complexes dominated the adsorption of most of the aromatic carboxylates over the pH range examined in this study. Inner-sphere complexes were only detected at some pH levels for some aromatic carboxylates adsorption. The aromatic carboxylates were most likely to be adsorbed onto the first surface layer of hydroxyl groups and water molecules without forming coordinative bonds with the aluminum hydroxide surfaces, but strong hydrogen bonds were formed in this process. The adsorption data fitted the pseudo-second-order kinetic model very well. The activation energies of adsorption calculated from the rate constants of pseudo-second-order kinetics agreed with the ATR-FTIR analysis that the aromatic carboxylates, except pyromellitate, were adsorbed predominantly as outer-sphere complexes on aluminum hydroxide at pH 7. This study revealed that phenolic groups may be more significant than carboxylic groups for the chemical adsorption of NOM onto minerals. The presence of additional carboxylic groups enhanced the adsorption considerably, while the enhancement was limited when there were three or four carboxylic groups on the phenyl ring. The adsorption of aromatic carboxylates was affected by the positions and pK_a values of the donor groups and the solution pH.