Effect of a typical antibiotic (tetracycline) on the aggregation of TiO nanoparticles in an aquatic environment.

The effect of tetracycline (TC) on the behavior of TiO nanoparticles (NPs) in solution is investigated. The results illustrate that TC molecules do not exhibit an obvious effect on the aggregation of TiO NPs under circumneutral conditions (pH of approximately 5.5). However, the TC molecules exhibit a remarkable effect on the behavior of TiO NPs when the environmental factors (such as pH, ionic strength, and humic acid; HA) are adjusted. In the pH range from 3.0 to 10.0, the zeta potentials of NPs become more positively charged after TC adsorption. The point of zero charge of the TiO NPs increases from 6.5 to 7.5. The stability of the TiO NPs is improved after TC adsorption in different salt solutions. The critical coagulation concentration (CCC) increases from 10.57mmol/L to 17.21mmol/L, 4.95mmol/L to 9.73mmol/L, and 0.23-1.67mmol/L in the presence of NaCl, CaCl, and NaSO (where Cl and SO are the counter-ions), respectively. Under different HA concentrations, the C (the HA concentration that causes the zeta potential of NPs to be zero) increases from 0.5mg/L to 1.3mg/L after TC adsorption. The sizes of the TiO NP aggregates are larger than those without TC adsorption. All of these results are due to an increase in the surface charge of the TiO NPs after TC adsorption. The adsorption of H during TC adsorption causes protonation of the dimethylamine group on the TC molecules.