RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The surface complexation model is used to describe sorption experiments of inorganic mercury(II) in the presence of an amorphous silica, Aerosil 200, or an iron (hydr)oxide, the goethite α-FeOOH (Bayferrox 910). In the simulations, one assumes the formation of a monodentate surface complex ≡S-OHg⁺ and of ternary surface complexes with OH⁻ surface groups, ≡S-OHgOH and ≡S-OHgCl, when chlorides are present in solution. Participation of the complex ≡S-OHgCl has been especially evidenced. The mercury(II) surface complexation on oxides can be described by the following equilibria (298.15 K, I = 0):

≡S-OH + Hg²⁺ ↔ ≡S-OHg⁺ + H⁺,

with logKS-OHg+int = 4.9 for goethite;

≡S-OH + Hg²⁺ + H₂O ↔ ≡S-OHgOH + 2H⁺,

with logK_S-OHgOH^int = −3.7 and −2.3 for amorphous silica and goethite, respectively;

≡S-OH + Hg²⁺ + Cl⁻ ↔ ≡S-OHgCl + H⁺,

with log_S-OHgCl^int = 5.8 and 8.0 for amorphous silica and goethite, respectively. Comparisons with other data from the literature have been made to investigate the influence of the nature of the oxide on the mechanism of mercury(II) adsorption. X-ray photoelectron spectroscopy was used to characterize the surface of the (hydr)oxides prior to adsorption and to observe when possible the mercury surface compounds.