Adsorption of norfloxacin on a hexagonal mesoporous silica: isotherms, kinetics and adsorbent reuse

The adsorption of the antibiotic norfloxacin (NFX) on MCM-41 type mesoporous silica has been studied in batch experiments by performing adsorption kinetics and isotherms under different conditions. Regeneration of the adsorbent and reuse studies were also carried out and are discussed. On the one hand, the adsorption is very fast and strongly dependent on pH, increasing from 30.6 µmol g⁻¹ at pH 3.0 to 192.3 µmol g⁻¹ at pH 7.0 and then decreasing up to 29.6 µmol g⁻¹ as pH increases. The adsorption takes place by direct binding of NFX to silica active sites through electrostatic interactions and H-bonds formations, as deduced from adsorption experiments at several ionic strengths and temperatures. The hydrophobic conformation of the antibiotic zwitterion seems to play also a key role on the maximum adsorption at neutral pH. The presence of calcium ions strongly increases the adsorption of NFX at pH > 4.5 due to the formation of ternary NFX-Ca²⁺-MCM-41 complexes by calcium-bridging. After the first cycle of regeneration through washing using several solvents, the studied solid significantly reduces its removal efficiency—up to 60%—but then it remains constant for another three cycles. The analysis of thermodynamic parameters suggests that the adsorption is exothermic (− 28.8 kJ mol⁻¹) and spontaneous in nature. On the other hand, the capacity of MCM-41 to remove a concentration of the antibiotic commonly-found in water environments is still being too low if it compares with other adsorbents. Improving the silica surface reactivity should be the main goal by the researchers in order to use the material as adsorbent of this kind of molecules in the future.