Measuring surface chemical properties using flow adsorption calorimetry: The case of amorphous aluminum hydroxides and arsenic (V)

Flow adsorption calorimetry provides a direct, quantitative measure of the heat evolved in a reaction and is ideally suited for measuring reactions occurring at the liquid/solid interface. This study centers on demonstrating the application of flow adsorption calorimetry as a powerful technique in probing chemical surfaces, thus obtaining information not readily accessible by other methods. Methodologies developed were applied to the study of arsenate sorption onto amorphous aluminum hydroxides (AHO). The AHO surface was chemically characterized using flow calorimetry. Heats of exchange determined for Cl/NO 3 were 3.6 to 5.8 kJ mol-1 and those for K/Na 1.2 to 1.6 kJ mol-1. A new technique to measure the zero point of charge (ZPC) calorimetrically was developed and used. Measured ZPC values were consistent with literature values and were around 8.5 and 9.5 for the AHO. The charging of the AHO surfaces supported a 2 pK mechanism as calorimetric data indicated that at pHs around the ZPC, the surface of AHO is neutrally charged: i.e., neither positive nor negative charges exist. The reaction of arsenate with AHO was exothermic (40 to 60 kJ mole-1 ) and decreased with increasing surface coverage. Most of the arsenate was sorbed on positively charged sites and involved no increase in surface negative charge. Arsenate sorption exhibited an initial, rapid uptake phase, thought to be ligand exchange with aquo- and hydroxo-groups, followed by a slower secondary reaction resulting from arsenate gaining access to the less accessible reactive sites that are dispersed throughout the structure. With increasing amounts of arsenate sorbed, arsenate reacted with the bridging of groups, creating negative sites for cation exchange. This bond-breaking mechanism occurred at a slower rate and was energy consuming. Sorbed arsenate caused the ZPC to shift by around 1 pH unit. The shift in ZPC resulted from a change in the pK of reactive hydroxyl groups. The role of polydentate arsenate complexes and deprotonation of sorbed arsenate was also discussed. The results of this study show that flow adsorption calorimetry is a uniquely informative yet rapid experimental tool that can be applied to numerous applications in surface chemistry studies.