Nonlinear optical studies of the agricultural antibiotic morantel interacting with silica/water interfaces

It is now known that the untreated discharge of pharmaceuticals into the environment can impact human health and development and lead to increased drug resistance in biota. Here, we present the first direct interface-specific studies that address the mobility of the widely used agricultural antibiotic morantel, which is commonly present in farm runoff. Surface-bound morantel was spectroscopically identified using second harmonic generation (SHG) via a two-photon resonance of its n-π* transition and in the C-H stretching region by vibrational sum frequency generation (VSFG). Resonantly enhanced SHG adsorption isotherm measurements carried out at the silica/water interface between 6 × 10^-7 and 5 × 10^-5 M morantel concentration result in a free energy of adsorption of 42(2) kJ/mol at pH 7. Finally, real-time tracking of morantel interaction with the silica/water interface shows that the binding events are fully reversible, consistent with its high mobility in silica-rich soil environments. This work thus indicates that pharmaceuticals discharged into the environment can enter the groundwater supply of municipal water systems, at which point their removal is challenging. In addition, the high mobility of morantel in silica-rich soil environments could lead to developing increased interaction of this antibiotic with target organisms, which could respond by increased drug resistance.