SURFACE COMPLEXATION AND PRECIPITATION AT THE H-ORTHOPHOSPHATE-AGED GAMMA-AL2O3()WATER INTERFACE/

Surface complexation of orthophosphate ions at the water-suspended-and -aged gamma-Al2O3/water interface has been studied by means of a serie s of batch experiments in 0.1 M Na(Cl) medium at 25.0 degrees C in the range 4.8 < -log [H+] < 9.6. The ratio between phosphate concentratio n and concentration of surface active groups (=AlOH) was varied betwee n 0.15 and 1.50. The suspensions were equilibrated for 5 h, and experi mental data consisted of measured -log [H+] and nonbound phosphate ana lyses. The orthophosphate ions were found to bind to the surface with high affinity at -log [H+] < 7.5. In the data evaluation, contribution s from electrostatic forces were accounted for by using the constant-c apacitance model. The acid/base properties of the hydroxylated alumina surface (=AlOH) have been investigated earlier and are described by t wo intrinsic equilibrium constants, log beta(110) = 7.51 and log beta( -110) = -8.87 and with a specific capacitance of 1.40 F/m(2). The mode l describing the phosphate complexation to the alumina surface is give n by the following equilibria: =AlOH + H2PO4- + H+ reversible arrow =A lPO4H2 + H2O (log beta(111(int)) = 11.49 +/- 0.08); =AlOH + H2PO4- rev ersible arrow =AlPO4H- + H2O (log beta(011(int)) = 5.14 +/- 0.07); =Al OH + H2PO4- reversible arrow =AlPO42- + H+ + H2O (log beta(-111(int)) = -1.82 +/- 0.04). The uncertainties reported correspond to 3 sigma(lo g beta). In the presence of excess phosphate and at extended equilibra tion periods, a slow continuing decrease in nonbound phosphate concent ration was observed. By means of diffuse reflectance FTIR measurements , this phenomenon was shown to be caused by a slow transformation into an aluminum phosphate solid phase. The surface complexation reactions evaluated in this work should therefore be regarded as a metastable s tate, strictly valid only in freshly prepared suspensions. However, FT IR data collected at deficit phosphate conditions indicate that this p hase transformation is hardly noticeable unless an excess of ligand wa s introduced to the system. This implies that the presented semiequili brium model is likely to provide a thermodynamic description of the eq uilibria in the system for [H2PO4-](tot)/[=AlOH](tot) < 1.