Calorimetric Investigation of the Nature of Sulfate and Phosphate Sorption on Amorphous Aluminum Hydroxide

The fate and transport of sulfate and phosphate in the environment are largely mediated by how these anions react with soil colloids, especially metal (hydr)oxides. The overall objective of this study was to use flow calorimetry to compare and better understand sulfate and phosphate sorption and sorption mechanisms on amorphous aluminum hydroxide (AHO). This was performed by measuring the in situ heats of NO3- and Cl- and K+ and Na+ exchange before and after reaction of AHO with sulfate and phosphate, by measuring heats of sulfate and phosphate sorption on AHO, and by determination of the amount of sulfate and phosphate sorbed. The AHO used in our study was synthesized in-house and was amorphous. All solutions used in our experiments had pH of 5.8 and ionic strengths of 50 mmol/L. Sulfate and phosphate treatment of the AHO reduced anion exchange by 61% and 77%, respectively, relative to the measured heats of NO3- and Cl- exchange before SO42- or H2PO4- treatment. Sulfate and phosphate treatment of the AHO did not change measured heats of K+ and Na+ exchange. The molar heats of sorption were 2.9 +/- 1.4 kJ/mol for sulfate and -6.5 +/- 3 kJ/mol for phosphate. The thermodynamic exchange (NO3- and Cl- and K+ and Na+) and sorption (SO42- and H2PO4-) data obtained by the flow calorimeter differentiated between outer-and inner-sphere SO42- and H2PO4 sorption mechanisms in situ and indicated that these ligands irreversibly sorbed on AHO by most likely forming monodentate complexes, bidentate bridging complexes, and Al-hydroxy surface precipitates.