Photocatalytic degradation of Metformin using TiO2-CuO heterojunctions synthesized by green chemistry and immobilized on beach sand granules in fluidized bed annular photoreactor (FBAP)

Abstract

This research is focused on the degradation of metformin by heterogeneous photocatalysis using TiO2-CuO heterojunctions deposited on beach sand granules in a pilot-scale fluidized bed annular photoreactor (FBAP). The precursor metal oxides were prepared by green chemistry using an aqueous extract of Moringa (Moringa Oleifera) and the heterojunctions by ultrasound-assisted wet impregnation for subsequent immobilization through a two-step immersion/calcination methodology. Batch tests showed a synergistic effect of the inorganic substrate due to its inherent adsorption. Besides, TC1 heterojunction (1wt.% of CuO) represents the best option for deposition due to its optical, morphological, and textural properties and favors its photocatalytic activity. Also, an optimal dose of the active phase of 1.0g/L was identified, guaranteeing this load for the granular catalyst with 4wt.% (g of catalyst/g of beach sand).On the other hand, the values for the pHzpc were 6.45 and 7.26 units, allowing to determine optimal operating pH of 7 and 8 units, respectively, which is attributed to the interaction between the surface loads and the metformin-derived species present in the sample. Optimal values of 18.7L/min achieved a maximum efficiency of 74.8% and 3.0wt.% for volumetric flow and the TC1/S mass ratio, respectively; these operation parameters significantly influence the degradation efficiency. Further, the reaction rate was slightly higher for powdered samples against immobilized heterojunctions due to the mass transfer limitations, which generated a difference in performance close to 2.7%. On the other hand, the reaction byproducts identified were MBG and 4,2,1-AIMT, which are harmless to the biological model C. elegans. Despite this, the segregated nanomaterials and the metal ions leached during the metformin removal generated lethal toxicity. These results reveal the proposed reactor's high potential for treating effluents contaminated metformin and other organic compounds using a granular catalyst with stable performance, including a centrifugation step that removes the segregated nanomaterials.