COMPLEXATION OF PB(II) AT THE GEOTHITE (ALPHA-FEOOH) WATER INTERFACE - THE INFLUENCE OF CHLORIDE

The complexation of Pb(II) on a hydrous geoethite surface has been investigated. Equilibrium measurements have been performed as potentiometric titrations at 298.2 K, within the range 2.7 < -log [H+] < 8.5. Three different ionic media were used: 0.1 mol dm-3 NaNO3, 0.1 mol dm-3 NaCl, and a mixture (1:1) of both these media. The evaluation of experimental data was made, using the constant capacitance model, within which a previously determined model for the acid-base reactions was included. The formation of the following complexes within the system H+ - =FeOH - Pb2+, was found to describe the surface complexation: =FeOHPb2+ (log beta0,1,1,0S(int) = 8.20 +/- 0.06); =FeOPb+ (log beta-1,1,1,0S(int) = 0.17 +/- 0.04); =FeOPbOH (log beta-2,1,1,0S(int) = -8.85 +/- 0.08). Within the four component system H+ -=FeOH - Pb2+ - Cl-, the following complexes were added to the model: =FeOHPbCl+ (log beta0,1,1,1S(int) = 7.50 +/- 0.08); =FeOPbCl (log beta-1,1,1,1S(int) = -0.35 +/- 0.16); = FeOPbOHCl- (log beta-2,1,1,1S(int) = -8.00 +/- 0.12). In the presence of an excess of Pb(II) ions to surface binding sites, the formation of a polynuclear surface Pb(II) species is postulated. The proposed model has been verified by independent determination of total soluble Pb(II) concentrations. Model calculations, using concentrations typical for natural waters, demonstrate a strong adsorption of Pb(II) even at low concentrations of surface binding sites. The experimental data show fully reversible reactions.