Reactive adsorption of pharmaceuticals on tin oxide pillared montmorillonite: Effect of visible light exposure

Abstract

Montmorillonite (Mt) clay was pillared with SnO2 and tested as an adsorbent for the removal pharmaceuticals trimethoprim (TMP) and sulfamethoxazole (SMX) frequently present in wastewaters. Adsorption process was carried out in dynamic conditions and the effects of light exposure were investigated. The initial and modified clays were characterized using XRD, FTIR, SEM, potentiometric titration, TA/MS and adsorption of nitrogen. The XRD analysis revealed an enlargement of the clay’s interlayer space as a consequence of SnO2 particles intercalation into the clay interlayer spaces. A marked increase in the volume of mesopores was also noticed. It indicated the precipitation of tin species on the flake surface. These changes resulted in a five-fold increase in TMP adsorption. Upon exposure to light a 30% increase in TMP adsorption was found in comparison to the performance in the dark. The good performance of the modified clay was linked to specific acid–base and electrostatic interactions of TMP with the Lewis acid sites of SnO2. Since exposure to solar light resulted in the decomposition of some TMP adsorbed on the surface, those smaller molecules formed were able to relocate to small pores of higher adsorption energy, which were initially inaccessible for TMP. SMX was not adsorbed on the surface of the modified clay owing mainly to its acidic character.