Tetracycline adsorption on montmorillonite: pH and ionic strength effects

The adsorption of tetracycline (TC) on a Patagonian montmorillonite is studied as a function of pH, ionic strength and TC concentration by using X-ray diffraction, UV–Visible spectroscopy, ATR-FTIR spectroscopy, batch experiments and adsorption modeling. X-ray diffraction reveals that TC species can intercalate into the interlayer space of montmorillonite as deduced from the change in the basal spacing from 12.6 Å to 19.5 Å, in the absence or presence of TC respectively. UV–Visible and ATR-FTIR spectroscopies show that the corresponding spectra of TC change upon adsorption, suggesting important montmorillonite–TC interactions in the adsorbed state. Batch experiments, on the other hand, reveal that there is an important adsorption of TC in a wide range of pH. Since at low pH (pH < 4) the prevailing species of TC is a cation with a net charge of + 1, the adsorption is rather high at this pH, reaching nearly the cation exchange capacity of the mineral. The adsorption then decreases as the pH is increased and eventually becomes negligible at pH 11. Experiments and modeling suggest that not only the cationic species but also the neutral and monoanionic species of TC adsorb on the negatively charged montmorillonite surface. Although there is a significant electrostatic interaction between TC and montmorillonite, a rather strong non-electrostatic interaction also exists. At all pH values TC species compete with sodium ions for surface sites, and thus there is an important effect of ionic strength on the adsorption.