Adsorptive removal of ciprofloxacin with different dissociated species onto titanate nanotubes

Abstract Exploring the specific characteristics of pharmaceuticals and personal care products (PPCPs) via adsorption and degradation are scientific and practical significance to control their release to water matrix. In this work, a good adsorbent and ion-exchange material, i.e., titanate nanotubes (TNTs), was employed for adsorption of ciprofloxacin (CIP, a model PPCPs). The adsorption behaviors and mechanisms of CIP with different dissociated species by TNTs were studied through both experimental and theoretical calculations. The multilayered TNTs with high BET surface area (272.3 m2/g) and large pore volume (1.26 cm3/g) exhibited good adsorption property for CIP. The CIP species (i.e., CIP+, CIP±, CIP−) at various pH exhibited significantly different adsorption favorability. Adsorption kinetics and isotherms data revealed that TNTs offered the high uptake for CIP+ (Qmax = 464.47 μmol/g or 153.90 mg/g at pH 5) than CIP± and CIP−. Characterizations indicated the formation of Ti−O−N linkage between CIP molecules and TNTs after adsorption, suggesting the chemical interaction between CIP and TNTs. Density functional theory (DFT) calculations reveal variation on pH affects the protonation/deprotonation state of CIP, and then changes the distribution of molecular orbitals and the electrostatic potential (ESP) energy of CIP. ESP follows the trend as: CIP+ (180.57 kcal/mol) > CIP± (146.78 kcal/mol) > CIP− (12.30 kcal/mol), indicating the side of piperazine ring in CIP oriented to TNTs dominates the CIP adsorption. The integrated experimental and theoretical results, for the first time, suggest that ESP energy can serve as the indicator and predictor of adsorption ability for the PPCPs molecules with various speciation, and can help to deeply describe the adsorption mechanism of PPCPs. In addition, TNTs have great application for the removal of PPCPs through adsorption in practical wastewater treatment area.