Green synthesis of nickel oxide nanoparticles using <i>Calotropis procera</i> leaf extract for visible-light photocatalytic degradation of RBV-5R dye

The increasing release of azo dyes into wastewater presents a significant environmental challenge due to their chemical stability, toxicity, and persistence in aquatic environments. In this study, nickel oxide nanoparticles were green-synthesized using Calotropis procera leaf extract and employed as photocatalysts for the degradation of RBV 5R dye, a model azo dye. The biosynthesized nanoparticles were characterized using SEM, XRD, FT-IR, and TGA, confirming their morphological, structural, and thermal properties. Photocatalytic degradation experiments were conducted under varying conditions to optimize the degradation efficiency, with results showing that mildly acidic conditions (pH 4) and 5 mM hydrogen peroxide yielded the highest degradation. The reaction kinetics followed a pseudo-second order kinetic (R2 = 0.9989), and the catalyst reusability was evaluated over five cycles, showing a slight decrease in efficiency from 99% to 93.5% by the fifth cycle, confirming good operational stability. This study demonstrates that Calotropis procera-mediated nickel oxide nanoparticles offer a cost-effective and environmentally sustainable solution for the photocatalytic degradation of toxic azo dyes, with significant potential for industrial wastewater treatment.