POTENTIOMETRIC AND SPECTROSCOPIC STUDIES OF SULFATE COMPLEXATION AT THE GOETHITE-WATER INTERFACE

The complexation of sulfate ions at the goethite-water interface has b een studied by equilibrium measurements at 298.2 +/- 0.5 K and by infr ared spectroscopy. A unified set of data from potentiometric titration s and batch adsorption experiments were evaluated according to an exte nded Constant Capacitance Model. The obtained equilibrium model consis ts of two outer-sphere complexes: = FeOH2+, SO42- (log beta(1,1,1(intr )(out)) = 8.3 +/- 0.03) and drop FeOH2+, HSO4- (log beta(2,1,1(intr)(o ut)) = 13.5 +/- 0.07) and an inner capacitance, C-1, of 4.5 and an out er capacitance, C-2, of 1.8. The outer-sphere model was confirmed by t he infrared data and adsorption experiments at different ionic strengt hs. Furthermore, the infrared measurements provided direct evidence fo r protonation of the sulfate ions at the interface.