Abstract
Acid/base reactions and HgCl (2-n)+ n complexation at the goethite-solution interface have been investigated. Equilibrium measurements were performed as potentiometric titrations at 298.2 K, in NaNO 3 and NaCl solutions at a constant ionic strength of 0.100 mol dm -3. These measurements were combined with analysis of soluble mercury (II). The experimental adsorption data were evaluated on the basis of the constant capacitance model, using a previously determined model for the acid-base characteristics in NaNO 3 medium. The mercury(II) complexation within the range 3.0 < -log[H +] < 8.0 has been described by the following equilibria and intrinsic constants valid in 0.1 mol dm -3 NaNO 3 and NaCl medium: ≡ FeOH + Hg 2+ ⇌ ≡ FeOHg + + H +; log β 5 -1,1,1,0( int) = 4.45 ± 0.045≡ FeOH + Hg 2+ + H 2 O ⇌ ≡ FeOHgOH + 2 H +; log β 5 -2,1,1,0( int) = -2.77 ± 0.042≡ FeOH + Hg 2+ + Cl - ⇌ ≡ FeOHgCl + 2 H +; log β 5 -1,1,1,0( int) = 7.43 ± 0.063Formation constants for the aqueous species HgCl +, HgCl 2, Hg(OH) +, Hg(OH) 2, and HgClOH as well as the solubility product for freshly precipitated HgO(s) were also evaluated. The significance of mercury (II) adsorption at the goethite-water interface in different natural water systems is indicated by means of a number of model calculations.
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