Adsorption Behavior of Chiral Pharmaceuticals onto Montmorillonite Clay: Evaluating Removal Efficiency and Stereoselectivity

Chiral pharmaceuticals (CPs) have gained growing attention in environmental studies regarding the differential behavior of individual enantiomers in racemic mixtures. This study investigates the stereoselectivity and efficiency of montmorillonite (MMT), a natural and low-cost adsorbent, for the removal of a wide group chiral pharmaceuticals and metabolites (atenolol, propranolol, metoprolol, fluoxetine, venlafaxine, norfluoxetine, and O-desmethylvenlafaxine). The effects of adsorption conditions including initial CP concentration, contact time, adsorbent dose, solution pH, and humic acid content were evaluated. In most adsorption experiments, no significant stereoselective behavior was observed, except for the case where a low adsorbent dose was applied. Interestingly, as the solution humic acid content increased (up to 40 mg/L), the adsorption capacity was increased for most of the target CPs. Isotherm studies revealed that the Freundlich model described the experimental data well and the process was favorable. Adsorption mechanism was interpreted by material characterization before and after adsorption. High removal efficiencies (88.0 to 99.8%) and the non-enantioselective behavior of MMT indicate that it can be used effectively for the simultaneous removal of both enantiomeric forms of various chiral pharmaceuticals from aqueous matrices.