COMPOSITION AND STABILITY OF CD(II)-CHLORO AND CD(II)-HYDROXO COMPLEXES AT THE GOETHITE (ALPHA-FEOOH) WATER INTERFACE

The surface complexation of Cd(II) to goethite (alpha-FeOOH) in the absence and presence of chloride has been investigated. Equilibrium measurements have been performed as potentiometric titrations at 298.2 K, within the range 2.7 < -log[H+] < 9.0, in 0.1 mol dm-3 NaNO3, NaCl, and a mixture (1:1) of both these media. The data were evaluated according to the electrostatic constant capacitance model, including a previously determined model for the acid-base reactions of the goethite surface. The following model is presented (errors +/- 3sigma): =FeOH + Cd2+ half arrow right over half arrow left =FeOHCd2+; log beta0,1,1,0s(int) = 6.43 +/- 0.05; =FeOH + Cd2+ half arrow right over half arrow left =FeOCd+ + H+; log beta-1,1,1,0s(int) = -2.22 +/- 0.05; =FeOH + Cd2+ + H2O half arrow right over half arrow left =FeOCdOH + 2H+; log beta-2,1,1,0s(int) = -12.01 +/- 0.08; =FeOH + Cd2+ + Cl- half arrow right over half arrow left =FeOHCdCl+; log beta0,1,1,1s(int) = 6.85 +/- 0.02, =FeOH + Cd2+ + Cl- half arrow right over half arrow left =FeOCdCl + H+; log beta-1,1,1,1s(int) = -2.38 +/- 0.04. In the presence of excess Cd(II) ions to surface hydroxyl groups, it was found that the results could be explained by including bulk precipitation of Cd(OH)2. The calculated formation constant for Cd(OH)2 (s) was within an order of magnitude from the literature value for the stable phase in solution, beta-Cd(OH)2. Modeling of natural water conditions showed that relatively high concentrations of goethite have to be present to influence the concentrations of Cd(II). Furthermore, the reactions were found to be nearly fully reversible, which means that adsorbed Cd(II) can be released to the environment by a relatively moderate lowering of pH. (C) 1994 Academic Press, Inc.