Geometrical and Electronic Properties of Hydrated Sodium Montmorillonite and Tetracycline Montmorillonite from DFT Calculations

The adsorption of fully protonated tetracycline (TC) using Na montmorillonite (Na-MMT) as an adsorbent is studied by DFT calculations. Geometric, electronic, and magnetic properties are analyzed. To the best of our knowledge, this is the first theoretical DFT research about Na-MMT as an adsorbent of TC cationic species. Two Na-MMT models are considered: dry and hydrated Na-MMT. The incorporation of four water molecules in the interlayer space results in the expansion of the d(001) parameter from 10.13 angstrom (dry) to 12.81 angstrom (hydrated). Finally, when TC molecule is adsorbed in the basal space of MMT, the increment of the d(001) parameter is about 10 angstrom respect to the hydrated model. Dry and hydrated Na-MMT present a nonmagnetic (mu = 0 mu(B)) and semiconductor behavior. Nevertheless, the TC-MMT system becomes conductor, and presents a very important magnetic moment (mu = 10.60 mu(B)). After TC adsorption in MMT a potential valley with a slight depth in the MMT interlayer is generated. For this reason, it is proposed that TC adsorption would provide active sites for the adsorption of new species (like Na ions and water molecules).