Photocatalytic and toxicity assessment of alginate reinforced ZnO-g-C₃N₄ photocatalyst for the degradation of methylene blue - A sustainable strategy

This study presents an eco-friendly strategy by developing alginate based photocatalyst, where heterojunction photocatalyst is reinforced in calcium alginate via a facile method called ionotropic gelation. The prepared heterojunction photocatalyst (ZnO-g-C₃N₄) and alginate reinforced heterojunction photocatalyst (Alg/ZnO-g-C₃N₄) were characterized with Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) spectroscopy to confirm their structural, elemental and physicochemical properties. The photodegradation experiments demonstrated Alg/ZnO-g-C₃N₄ showed excellent efficiency in the removal of methylene blue (MB) under both UV-vis light and natural sunlight irradiation. The photocatalytic performance was quantitatively evaluated, achieving degradation efficiencies of 73.46 % under UV-visible light and 78.18 % under sunlight within 60 min. The alginate functions as a porous scaffold, providing structural support and mitigating the leaching of photocatalyst. The alginate hydrogel beads showcased remarkable reusability, maintaining consistent performance over five consecutive cycles. To evaluate environmental safety and practical feasibility of synthesized materials, the assessment of their toxicity was conducted. The toxicity of ZnO-g-C₃N₄, Alg/ZnO-g-C₃N₄, MB, and its degraded product (tMB) were examined using Danio rerio gill (DrG) cell lines. The results demonstrated that both Alg/ZnO-g-C₃N₄ and tMB were non-toxic, whereas MB exhibited significant toxicity at a concentration of 20 μg/mL.