Adsorption of C4-dicarboxylic acids at the hematite/water interface

The adsorption at the hematite/water interface of a suite of C4-dicarboxylic acids (maleic, fumaric, and succinic acids) that differ with respect to the orientation of the carboxylic acid groups was studied using in situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and batch adsorption experiments. Results of phthalic acid adsorption under similar experimental conditions from our previous study were compared with those observed for the C4-dicarboxylic acids. The batch adsorption experiments suggest that the orientation of the two carboxylic groups and pK(a) values of the organic acids noticeably influence the adsorption density as well as the position and characteristics of the pH adsorption edges. In particular, the adsorption capacity of fumaric acid was much lower than that observed for the other organic acids. The ATR-FTIR results also suggest that the adsorption modes of the organic acids on hematite depend on the orientation of the two carboxylic functional groups. Succinic acid with higher molecular flexibility, maleic acid with cis-configuration, and phthalic acid with ortho-position of the two carboxylic groups were similarly present as deprotonated bidentate inner- and outer-sphere complexes likely involving both carboxylic groups. The inner-sphere complex was more significant at acidic pH. Fumaric acid, however, with transconfiguration seems to bind to hematite primarily as a deprotonated outer-sphere complex utilizing only one carboxylic group.