Modeling the adsorption of mercury(II) on (hydr)oxides II: alpha-FeOOH (goethite) and amorphous silica

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 alpha-FeOOH (Bayferrox 910), In the si mulations, one assumes the formation of a monodentate surface complex =S-OH g+ and of ternary surface complexes with OH- surface groups, =S-OHgOH and = S-OHgCl, when chlorides are present in solution. Participation of the compl ex =S-OHgCl has been especially evidenced. The mercury(II) surface complexa tion on oxides can be described by the following equilibria (298.15 K, I = 0): =S-OH + Hg2+ <----> =S-OHg+ + H+, with log K-S-OHg+(int), = 4.9 for goethite; =S-OH + Hg2+ + H2O <----> =S-OHgOH + 2H(+), with log K-S-OHgOH(int) = 3.7 and -2.3 for amorphous silica and goethite, r espectively; =S-OH + Hg2+ + Cl- <----> =S-OHgCl + H+, with log K-S-OHgCl(int) = 5.8 and 8.0 for amorphous silica and goethite, re spectively. 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 chara cterize the surface of the (hydr)oxides prior to adsorption and to observe when possible the mercury surface compounds. (C) 1999 Academic Press.