Calorimetric Investigation of Sulfate and Phosphate Sorption On Amorphous Aluminum Hydroxide.

The fate and transport of sulfate and phosphate in the environment is largely mediated by how these anions react with soil colloids, especially metal (hydr)oxides.  Moreover, their mechanisms of sorption as well as the extent to which they react with metal (hydr)oxides vary and are not completely understood.  The overall objective of this study was to use flow calorimetry to determine and compare sulfate and phosphate sorption and sorption mechanisms on amorphous aluminum hydroxide (AHO).  This was performed by measuring heats of NO₃^- and Cl^- and K⁺ and Na⁺ exchange prior to and post reaction of AHO with sulfate and phosphate, by measuring heats of sulfate and phosphate sorption on AHO, as well as 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 pHs of 5.8 and ionic strengths of 50 mM.  Sulfate and phosphate addition to the AHO reduced anion exchange by 61% and 77%, respectively relative to the measured heats of NO₃^- and Cl^- exchange prior to ligand addition.  Addition of these ligands to the AHO did not change measured heats of K⁺ and Na⁺ exchange.  The molar heats of sorption were 2.9±1.4 kJ mol^-1 for sulfate and -6.5±3 kJ mol^-1 for phosphate.  The thermodynamic exchange (NO₃^- and Cl^- and K⁺ and Na⁺) and sorption (SO4^2- and H₂PO₄^-) data indicated these ligands sorbed on AHO by most likely forming bidentate bridging complexes and Al-hydroxy surface precipitates.