Mercury(II) ion adsorption behavior in thiol-functionalized mesoporous silica microspheres

Microspherical mercury ion adsorbents with uniform mesopore channels were prepared by fluoride-catalyzed surfactant-directed co-condensation of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) using mildly acidic nonionic surfactant solutions. Using batch adsorption methods, mercury(II) adsorption isotherms and kinetic uptake profiles for these adsorbents were obtained. The mercury(II) isotherms of the microspheres with low thiol group loadings did not fit the Langmuir expression and were indicative of weak interactions between the mercury(II) ions and the adsorbents. With increasing thiol group loading, the isotherms approached type-l in appearance, indicating improved binding energetics of the ions with the adsorbents. The adsorption kinetics of the adsorbents suggested that the uptake of mercury ions by the microspheres was rather slow, with diffusion coefficients ranging between 10(-14) and 10(-15) m(2) s(-1). The coefficients were found to increase as a function of the thiol group density of the adsorbents. The diffusion coefficients of the mercury(II) adsorption process were also found to increase as a function of time, indicating synergistic acceleration of the uptake rate with increasing mercury ion loading in the materials. On the basis of these observed trends, an ion permeation and displacement mechanism is proposed and described.