Preparation of recyclable g-C3N4/TiO2 heterojunction/alginate hydrogel microbeads and investigation of their adsorption-photocatalytic properties

Abstract

To address the challenges associated with the recovery difficulties and potential secondary pollution of powdered photocatalysts, this study synthesized a Z-scheme heterojunction g-C₃N₄/TiO₂ (denoted as GT) with a core-shell structure via a hydrothermal method. Subsequently, an efficient and recyclable hydrogel bead was fabricated using sodium alginate and GT as raw materials. The results demonstrated that the hydrogel bead exhibited a remarkable adsorption capacity of 48.79 mg/g for Rhodamine B (RhB) when the GT loading was 0.6 g, significantly surpassing the 26.17 mg/g capacity of GT alone, while also displaying exceptional photocatalytic degradation efficiency. Adsorption kinetic analysis revealed that the adsorption process conformed to the pseudo-second-order kinetic model (R² = 0.976), and isotherm fitting to the Langmuir model indicated monolayer adsorption. Furthermore, the adsorption of RhB by the hydrogel bead was identified as an endothermic process. Under visible light irradiation, the GT(0.6)/CA hydrogel bead achieved a photocatalytic degradation rate of 85.4 % within 100 min for a 100 mL RhB solution with an initial concentration of 40 mg/L, with a degradation rate constant of 0.0205 min⁻¹. The hydrogel bead exhibited outstanding catalytic performance under varying GT loadings, pH conditions, light sources, and synergistic effects, and maintained over 80 % degradation efficiency after five consecutive cycles, demonstrating excellent stability and recyclability. Quenching experiments and electron spin resonance (ESR) analysis further elucidated that the primary active species involved in the degradation process were h⁺ and •O₂⁻. This study provides a novel direction for the development of reusable green photocatalysts.