In situ ATR‐FTIR spectroscopic study of metformin adsorption on gibbsite and Loring silt loam

Metformin is an anti‐diabetic drug that has received increased environmental attention due to probable toxic effects on aquatic ecosystems. Although several studies reported metformin adsorption on soils and minerals, spectroscopic evidence of adsorption mechanisms is limited. Thus, we evaluated metformin adsorption mechanisms on gibbsite and a Tennessee soil (Loring silt loam) using in situ attenuated total reflectance Fourier transform infrared (FTIR) spectroscopy. FTIR results (wavenumber shifts at 1638, 1570, and 1499 cm −1 ) suggested that 250 µM metformin interacts with soil and gibbsite through the lone pair of electrons on N atoms (N 5 , N 6 , or N 7 moieties) and delocalized electrons in N 3 ‐N 5 ‐N 6 systems. For soil, metformin may additionally bind via cation exchange in the inter‐layer spaces of permanent charge minerals. Direct spectroscopic evidence corroborates earlier studies and provides focus for future investigations to elucidate the fate of metformin at the soil–water interface, depending on different soil physicochemical properties and mineralogy. Core Ideas Metformin showed adsorption affinity for Tennessee soil and model mineral gibbsite at natural pH of 6.5. Changes in infrared bands of aqueous metformin spectra due to pH variation are consistent with reported pK a values. Spectroscopic results indicated adsorption of metformin on soil via combinations of cation exchange and electrostatic attractions. For gibbsite, the metformin adsorption was mainly by electrostatic attraction.