The influence of the counter ion competition and nature of solvent on the adsorption of mercury halides on SH-modified silica gel

The adsorption behavior of thiol-modified silica gel, SiSH, towards mercury (II) chloride and bromide in water, ethanol, methanol and water–methanol solutions has been studied by a batch technique. The influence of the counter-ion competition and mixture of solvents were investigated. The Langmuir expression for adsorption isotherm was applied in order to determine the maximum adsorption capacity to form a monolayer, Nmon, and the constant related to the adsorption intensity, b. In aqueous solutions there was a significant adsorption increase with the temperature and pronounced synergistic effects were observed. Adsorption measurements made in water(w)/methanol(MeOH) mixtures indicated that the synergism effect of mercury adsorption followed the order 75% (w)+25% (MeOH)>50% (w)+50% (MeOH)>25% (w)+75% (MeOH). From temperature effect on adsorption, exothermic enthalpy changes for a monolayer of anchored cations per gram of surface, ΔmonHm, are shown to be consistent with our previous calorimetric results for similar systems. The antagonistic effects for ΔmonHm results showed that the counter-ion competition is not favorable to the mercury/surface interaction strengths.