Adsorption of short chain carboxylic acids from aqueous solution by swellable organically modified silica materials

Abstract

The adsorption of C2–C6 short chain carboxylic acids was measured for swellable organically modified silicas (SOMS), hydrophobic polysilsesquioxane based materials prepared via the sol–gel process. Adsorption was measured for SOMS materials that were either hydrophobic or incorporated variable amounts of quaternary amine groups to serve as ion exchange sites. Batch equilibrium experiments were performed for carboxylic acids and alcohols to standard hydrophobic SOMS. Results show that SOMS has partition coefficients for short chain alcohols that are approximately 3× greater than liquid–liquid extraction into octanol, and 10× greater for the extraction of short chain carboxylic acids. Affinity for valeric acid was similar to commercially available Amberlite XAD-4 resins, however the organosilica derived sorbent had a 3× higher total adsorption capacity which was attributed to the ability of the pores to expand. Column experiments using SOMS were used to measure breakthrough curves for valeric acid across a wide range of concentrations. The isotherm for dynamic adsorption was linear indicating that capacity to adsorb valeric acid was controlled by equilibrium and not limited by set pore volume capacity. Addition of quaternary amine sites to SOMS improved adsorption affinity of valeric acid at low concentrations, but those modifications reduced the swelling of the matrix and lowered capacity at higher concentrations. Overall, SOMS was found to be poorly suited to extract organic acids at concentrations < 10 mM, but has high capacity for adsorption at higher concentrations (up to 700 mg/g). Adsorbed valeric acid could be recovered by solvent rinse.