Efficient photocatalytic degradation of methylene blue using Arundo donax biochar/graphitic carbon nitride composite

• Synthesized ADB/g-C₃N₄ composite for visible-light methylene blue degradation • Composite showed pHpzc 6.7, BET 274 m²/g, band gap 2.55 eV, amorphous structure • Optimized pH, MB concentration, catalyst dosage, and irradiation time via BBD • Achieved 99.23% MB degradation at pH 7, 10 mg/L, 1.5 g/100 mL, 75 min • Pseudo-first-order kinetics; oxalic acid played key role in degradation This study presents the synthesis and application of a novel Arundo donax -based biochar/graphitic carbon nitride (ADB/g-C₃N₄) composite for the photodegradation of methylene blue (MB) under visible light irradiation. Biochar from Arundo donax was produced under limited oxygen, and g-C₃N₄ was synthesized by thermal polymerization of urea. The composite was then prepared through physical mixing and ball milling of the two materials. Characterization of the developed composite material was conducted using pH point of zero charge (pHpzc), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), diffuse reflectance UV–Vis (DRS), and Brunauer–Emmett–Teller (BET) surface area analysis. Process optimization was performed using a Box–Behnken design under response surface methodology with four factors at three levels namely pH (4, 7, and 10), initial MB concentration (10, 25 and 40 mg/L), photocatalyst dosage (0.25, 0.875 and 1.5 g/100 mL), and irradiation time (30, 75 and 120 min). The results showed that the composite exhibited a pHpzc of 6.7; multiple functional groups (FTIR), predominantly amorphous structure (XRD), a BET surface area of 274.12 m²/g, and a band gap of 2.55 eV. A maximum degradation efficiency of 99.23% was achieved under optimal conditions of pH 7, 10 mg/L MB, 1.5 g/100 mL dosage and 75 min irradiation. Kinetic analysis revealed the degradation followed a pseudo-first-order model with an R² of 0.979. Furthermore, scavenging tests indicated oxalic acid had the most significant role in the degradation mechanism. Generally, the results indicate that the ADB/g-C₃N₄ composite is an efficient photocatalyst for the degradation of organic dyes in industrial wastewater.