Insights into tetracycline adsorption onto goethite: Experiments and modeling

- pH greatly influences tetracycline adsorption on goethite, while ionic strength does not.
- The adsorption was successfully predicted by a basic Stern model with CD approach.
- The tricarbonylamide group and the phonetic group are involved in adsorption.
- Two inner-sphere surface complexes FeTCH20.5 + and Fe2TC are proposed.

The surface adsorption behavior of tetracycline (TC), a zwitterionic antibiotic, to goethite was investigated as a function of pH, ionic strength and TC concentration using batch adsorption experiments and structural information was derived from attenuated total reflectance Fourier transform infrared spectrum observations. The spectroscopic results suggested that the tricarbonylamide group and the phenolic diketone group of the TC molecule were involved in interacting with the goethite surface depending on the pH level. A charge distribution surface complexation model was developed to describe the macroscopic adsorption trends. Two inner-sphere surface complexation species could successfully describe observed adsorption trends: under acidic condition TC may interact with the surface of goethite, forming a monodentate complex through the tricarbonylamide group, while under alkaline condition forming a more stable bidentate complex via the tricarbonylamide and phenolic diketone groups. The model could well predict the adsorption behavior of TC under a relatively wide range of pH, ionic strength and surface coverage. However, since the model did not fully consider the molecular size of TC, the model might overestimate the adsorption when TC surface coverage is higher than 1.42 μmol m− 2.