| 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 NaNO3 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 NaNO3 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 NaNO3 and NaCl medium: ≡FeOH + Hg2+ ⇌ ≡ FeOHg+ + H+; logβ5-1,1,1,0(int) = 4.45 ± 0.045≡FeOH + Hg2+ + H2O ⇌ ≡ FeOHgOH + 2H+; logβ5-2,1,1,0(int) = -2.77 ± 0.042≡FeOH + Hg2+ + Cl- ⇌ ≡ FeOHgCl + 2H+; logβ5-1,1,1,0(int) = 7.43 ± 0.063Formation constants for the aqueous species HgCl+, HgCl2, 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. |
