Complexation of Pb(II) at the goethite ( -FeOOH)/water interface: The influence of chloride

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The complexation of Pb(II) on a hydrous goethite 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 NaNO 3 , 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 - Pb 2+ , was found to describe the surface complexation: FeOHPb 2+ ( log 0,1,1,0 s ( int ) = 8.20 ± 0.06); FeOPb + ( log -1,1,1,0 s ( int ) = 0.17 ± 0.04); FeOPbOH ( log -2,1,1,0 s ( int ) = -8.85 ± 0.08). Within the four component system H + - FeOH -- Pb 2+ -- Cl - , the following complexes were added to the model: FeOHPbCl + ( log 0,1,1,1 s ( int ) = 7.50 ± 0.08); FeOPbCl ( log -1,1,1,1 s ( int ) = -0.35 ± 0.16); FeOPbOHCl - ( log -2,1,1,1 s ( 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.

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