Functionalized silica xerogels for adsorption of heavy metals from groundwater and soils

Heavy metals occur naturally but are released into the environment in large amounts by human activity, reaching dangerous concentrations. Both in aqueous media and soils, these can be found as dissolved cations. Adsorption was shown to be an effective mechanism to remove heavy metals presented in this form. Two new adsorbents, mercapto and amine-mercapto-functionalized silica xerogels (M and AM), have been synthesized using sol–gel chemistry to decontaminate the above mentioned polluted media. Their adsorption capacity has been tested in batch equilibrium and kinetic adsorption tests with single-cation aqueous solutions of copper and lead. It was found that both adsorbents remove the cations from solution but it is the amine-mercapto xerogel, the one with the highest uptake for the metals, reaching a capacity of 125 mg/g for Pb and 140 mg/g for Cu when the initial concentration of the cations is 500 ppm. Kinetic studies showed that the removal process is fast, with most of the cation being removed in the first 20 min of the process (60% for lead and 80% for Cu). The combination of equilibrium and kinetic results evinces that the adsorption of both copper and lead by mercapto and amine-mercapto xerogels happens predominantly as monolayer chemisorption.