Effective removal of metformin from water using an iron-biochar composite: Mechanistic studies and performance optimisation

Metformin (MF) is widely used in the prevention and treatment of Type 2 diabetes and gestational diabetes. Following its medical administration, it may pass through the body and be introduced into the wastewater treatment system. However, conventional treatment plants are ineffective in removal of MF from wastewater resulting in its introduction into the aquatic environment. Novel and effective methods of MF removal are therefore required to prevent contamination of natural waters. In this work, a magnetised iron-biochar composite derived from brewery waste (magnetic brewery spent grain; MBSG) was generated and evaluated for its ability to remove MF from aqueous media. Response surface methodology (RSM) was applied to identify optimal adsorption parameters. Extensive instrumental characterisation including XPS, FTIR, EPR, Raman spectroscopy and SQUID were used in conjunction with kinetic, isotherm and thermodynamic studies to determine that H-bonding, electrostatic attraction and so-called π-π (electron donor-acceptor) EDA interactions were the mechanisms involved in the adsorption processes. Modelling showed that the MF adsorption was best described by both the Langmuir and the Freundlich models, with the value of qmax = 15.2 mg/g governed by the pseudo-second order kinetic model; and it was a spontaneous (∆G = −17.7 kJ/mol) and endothermic (∆H =38.5 kJ/mol) process. Desorption studies were conducted to evaluate the potential for regeneration and reuse of the material. These demonstrated citric acid (0.2 M) to have significant potential as a regenerative reagent, but that further optimisation is required. This work revealed that MBSG could be a sustainable and effective adsorbent for the removal of aqueous MF in water treatment.