Interactions of ciprofloxacin (CIP), titanium dioxide (TiO2) nanoparticles and natural organic matter (NOM) in aqueous suspensions

Abstract : The aim of the present study was to investigate interactions of the antibiotic ciprofloxacin (CIP), titanium dioxide nanoparticles (TiO2 NP) and natural organic matter (NOM) in aqueous suspensions. The mean hydrodynamic diameter of particles of TiO2 NP and NOM in the suspensions ranged from 113 to 255 nm. During batch experiments the radioactivity resulting from 14CIP was determined in the filtrate (filter pore size 100 nm) by scintillation measurements. Up to 72 h, no significant sorption of NOM to TiO2 NP was observed at a TiO2 NP concentration of 5 mg/L. When the concentration of TiO2 NP was increased to 500 mg/L, a small amount of NOM of 9.5% ± 0.6% was sorbed at 72 h. The low sorption affinity of NOM on TiO2 NP surfaces could be explained by the negative charge of both components in alkaline media or by the low hydrophobicity of the NOM contents. At a TiO2 NP concentration of 5 mg L− 1, the sorption of CIP on TiO2 NP was insignificant (TiO2 NP/CIP ratio: 10). When the TiO2 NP/CIP ratio was increased to 1000, a significant amount of 53.6% ± 7.2% of CIP was sorbed on TiO2 NP under equilibrium conditions at 64 h. In alkaline media, CIP is present mainly as zwitterions which have an affinity to sorb on negatively charged TiO2 NP surfaces. The sorption of CIP on TiO2 NP in the range of TiO2 NP concentrations currently estimated for municipal wastewater treatment plants is estimated to be rather low. The Freundlich sorption coefficients (KF) in the presence of NOM of 2167 Ln mg mg− n kg− 1 was about 10 times lower than in the absence of NOM. This is an indication that the particle fraction of NOM < 100 nm could play a role as a carrier for ionic organic micro-pollutants as CIP.