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

Abstract

• Core-shell Z-type heterojunction with high surface area and photoelectric efficiency. • Millimeter-sized hydrogel beads via simple mixing/crosslinking. • Effective adsorption-photocatalysis for diverse pollutants. • Superior recyclability (80 % after 5 cycles) vs. powder catalysts. 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. The three-dimensional porous network of the hydrogel bead facilitates the adsorption of RhB molecules, thereby enhancing their local concentration. Under visible light irradiation, the g-C₃N₄/TiO₂ Z-scheme heterojunction generates electron-hole pairs (e⁻/h⁺). In this mechanism, electrons from g-C₃N₄ migrate to TiO₂ and recombine with holes, effectively suppressing charge carrier recombination and consequently enhancing photocatalytic efficiency. The remaining charge carriers further degrade RhB molecules through direct oxidation and indirect redox reactions involving reactive oxygen species. This synergistic interplay between adsorption and photocatalytic degradation underscores the superior performance of the hydrogel bead in environmental remediation applications.

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