Solar photocatalytic degradation of metformin by TiO₂ synthesized using Calotropis gigantea leaf extract

A novel TiO₂ nanoparticle was prepared through green synthesis using Calotropis gigantea (CG) leaf extract. Morphological analysis showed dispersed spherical CG-TiO₂ nanoparticles with an average size of 42 nm. The prepared catalyst was used for the degradation of metformin (a widely used diabetic medicine) by solar photocatalysis. A three-factor central composite design (CCD) was used to explore the effect of independent variables, i.e., pH 3-7, metformin concentration 1-10 mg/L, and catalyst (CG-TiO₂) dosage 0.5-2.0 g/L. A maximum metformin degradation of 96.7% was observed under optimum conditions i.e., pH = 9.7, initial metformin concentration = 9.7 mg/L and catalyst dosage = 0.7 g/L, with ∼86% mineralization efficiency. A quadratic model with an error <±5% was developed to predict the metformin degradation and the rate of degradation under the optimum conditions followed pseudo-first-order kinetics (k = 0.014/min). CG-TiO₂ exhibited higher metformin degradation efficiency (96.7%) compared to P-25 (23.9%) at optimum conditions. The recyclability study indicated effective reuse of the catalyst for up to three cycles. The proposed metformin degradation route is hydroxyl radical (^•OH) generation on the CG-TiO₂ surface, transfer of ^•OH to the aqueous phase from CG-TiO₂ and subsequent oxidation of metformin in the aqueous phase.