Impact of chemical, textural and swelling properties of bentonite on the adsorption of trimethoprim and chlorphenamine from water

Abstract

The influence of textural, chemical, and swelling properties of bentonite toward trimethoprim (TMP) and chlorphenamine (CPA) adsorption was investigated. Samples of bentonite and bentonite saturated with different uptakes of these pharmaceuticals were characterized by N2 physisorption, Zeta potential, SEM, and XRD, and the uptake of cations exchanged was assessed. TMP and CPA were adsorbed within bentonite mesopores, causing pore blockage. Adsorption modified the surface charge, making it less negative as TMP and CPA uptake increased. XRD patterns showed that the d001-value expanded from 1.50 to 1.70 nm as TMP uptake increased, while no significant shift was observed for CPA. These values were consistent with the molecular sizes of both compounds, suggesting intercalation within the interlayer space of bentonite in different orientations. Cation exchange occurred with an approximate 1:1 ratio of cations exchanged. Adsorption capacities were strongly influenced by pH. Maximum uptake adsorbed was attained at pH 5 for TMP (186.6 mg/g) and pH 9 for CPA (159.8 mg/g), diminishing at higher pH. Ionic strength emphasized the role of electrostatic attractions in the adsorption mechanisms, while solution temperature had negligible effects. Reversibility studies revealed that adsorption was governed by physical and chemical interactions and demonstrated that bentonite could be reused for at least 5 cycles with adsorption percentages remaining above 85%. Experiments in wastewater treatment plant effluent confirmed effective adsorption under realistic conditions, with TMP removal being limited by speciation at pH > 7.16 and CPA efficiently adsorbed, also improving general water quality parameters.