Synergistic adsorption and photocatalytic removal of hexavalent chromium and direct red 23 dye using porous magnetic hydroxyapatite-metakaolin geopolymer granules doped with TiO₂

This study presents a novel approach to addressing challenges of photocatalysy by TiO₂ powder and the adsorption of anionic pollutants by geopolymers (GP) through the development of granules based on hydroxyapatite-geopolymer (HAP-MK-GP) materials. The synthesized HAP-MK-GP/Fe₃O₄-TiO₂ granules exhibit promising physicochemical properties, including a high porosity of 0.25 cm³ /g, a specific surface area of 106.59 m²/g, notable magnetic behavior (saturation magnetization of 18.89 emu/g), and strong mechanical integrity (compressive strength of 1.63 MPa). The adsorption kinetics and equilibrium data for Direct Red 23 (DR23) dye and hexavalent chromium (Cr(VI)) conformed to the Langmuir isotherm and pseudo-second-order kinetic models, indicating monolayer adsorption on a homogeneous surface. The maximum adsorption capacities were determined as 15.23 mg/g for DR23 and 36.06 mg/g for Cr(VI) under the following conditions: pH of 3, temperature of 21°C, adsorbent dosages of 4 g/L for DR23 and 2 g/L for Cr(VI), and initial pollutant concentrations of 100 mg/L. Under UV irradiation, the granules demonstrated high photocatalytic efficiency for DR23 degradation, achieving removal rates exceeding 95 % under optimized conditions: pH of 3, temperature of 21°C, 4 g/L of dose, and ${\left[\mathrm{DR}23\right]}_0\mathrm{of}50\mathrm{mg}/L$. Electron paramagnetic resonance (EPR) confirmed the generation of reactive radicals (•OH and O₂•⁻), crucial for photocatalytic activity. Chemical oxygen demand (COD) and total organic carbon (TOC) analyses corroborated the significant mineralization of DR23 dye. These findings underscore the potential of HAP-MK-GP/Fe₃O₄-TiO₂ granules as a highly efficient dual-function photocatalyst-adsorbent material for wastewater treatment applications.