Adsorption of tylosin and sulfamethazine by carbon nanotubes and titanium dioxide nanoparticles: pH-dependent mechanisms

The extensive application of antibiotics in livestock industry inescapably leads to their release to the aquatic environment. The occurrence of multiple engineered nanomaterials (ENMs) in various water systems also has been widely observed. The interaction between the coexisting antibiotics and ENMs will affect their transport and fate in water. In this study, the adsorption of two representative veterinary antibiotics-tylosin (TYL) and sulfamethazine (SMT) by binary nano-adsorbents-carbon nanotubes (CNTs) and titanium dioxide nanoparticles (nTiO(2)) was investigated. Results exhibited that the adsorption behavior and mechanisms of TYL/SMT by the binary nano-adsorbents were highly pH-dependent. Under near-neutral condition (pH unadjusted), nTiO2 inhibited the adsorption of TYL/SMT by CNTs, mainly because of adsorption sites competition, weaker EDA interaction and less negatively charged CNT-nTiO(2) clusters formation. Under acidic condition (pH 5.0), nTiO(2) heteroaggregated with the oppositely charged CNTs via electrostatic attraction and generated large agglomerates, thus significantly decreased the possible adsorption sites of the adsorbents. Under alkaline condition (pH 8.0), nTiO(2) inhibited the adsorption of TYL, but slightly promoted the adsorption of SMT, which resulted from the weaker electrostatic repulsion between the anionic SMT- and the less negatively charged nTiO(2). The adsorption behavior and mechanisms were also confirmed by the dynamic light scattering (DLS) and electrophoretic mobility (EPM) measurement, the transmission electron microscopy (TEM) images and the Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation. The observations of this study highlighted the mechanisms controlling the adsorption affinity between antibiotics and binary nano-adsorbents, and also opened up new horizons for the interaction and fate of nanomaterials and organic contaminants in the aquatic environment.