Contrasting mechanisms of metoprolol uptake on kaolinite and talc

Due to their widespread use and limited human metabolism, β-blockers were widely detected in sewage effluents and surface waters. β-blockers are persistent in agricultural soils that have been irrigated with treated wastewater or fertilized with sewage sludge. It is important to study β-blockers interaction with kaolinite and talc because kaolinite is a major component of soil that formed under warm and humid conditions, and talc is frequently used as an excipient for drug formulation. In this study, the interactions between metoprolol (MT), a β-blocker, and kaolinite and talc, clay minerals of 1:1 and 2:1 structural types, were investigated under varying physical and chemical conditions. Uptake of MT on both minerals was instantaneous with capacities close to the cation exchange capacity of the substrates. MT sorption was exothermic and the free energy of sorption was small and negative. These parameters pointed out to physical sorption for the uptake of MT on these minerals. Both solution pH and ionic strength had significant effects on MT uptake on kaolinite, but minimal influence on talc, contrasting different mechanisms of MT uptake on these minerals. The FTIR results showed bands blue shifted for both in-plane and out-of-plane bending of NH₂. Meanwhile, blue shift was also found for the 1246 and 1011cm^-1 bands, attributed to aromatic ether, after MT sorption on kaolinite and talc. For talc, blue shift of the band at 965cm^-1, attributed to aliphatic ether, was also observed. These results suggested that electrostatic interactions or hydrogen bonding played important roles in MT uptake on kaolinite. In contrast, the hydrogen bonding and hydrophobic interaction were likely the main mechanisms of MT uptake on talc.