Polyindole–Silver Oxide Composite as a Persulfate‐Enhanced Photocatalyst for Malachite Green Degradation: Kinetic and Mechanistic Insights Under Natural Sunlight and Room‐Light Conditions

ABSTRACT Polyindole@silver oxide (PIN@Ag 2 O or PIN@SO) composites were synthesized via a simple, green‐assisted chemical oxidative route and evaluated as photocatalysts for the rapid degradation of malachite green (MG) dye under natural sunlight and ambient light. The synthesized materials were characterized by Fourier‐transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), scanning electron microscopy coupled with energy‐dispersive X‐ray analysis (SEM‐EDX), and UV–Vis spectroscopy, confirming the formation of semi‐crystalline, porous composites with reduced optical band gaps compared to pristine polyindole. The photocatalytic performance of the PIN@SO composites was investigated in aqueous media in the presence of potassium persulfate (1.6 mM) at neutral and acidic pH. Under natural sunlight, complete degradation of 0.4 mM MG was achieved within approximately 10 min at neutral pH and 7 min under acidic conditions (pH 1) using 0.7 mg of the PIN@SO‐3 catalyst. Kinetic analysis revealed that the overall degradation of MG follows a 2.5‐order reaction, comprising 0.5 order with respect to MG, second order with respect to persulfate, and zero order with respect to the catalyst, under optimized conditions. The temperature dependence of the reaction was evaluated using Arrhenius and Eyring analyses, revealing an endothermic activation process. Degradation intermediates were identified by gas chromatography‐mass spectrometry (GC‐MS), and a plausible degradation pathway was proposed based on the observed intermediates and relevant literature, indicating extensive MG breakdown without confirmed complete mineralization. The composites exhibited good stability over five successive reuse cycles with only a minor decrease in activity. Furthermore, the optimized system was successfully applied to a real textile wastewater sample under sunlight, demonstrating substantial dye degradation and practical applicability. While the catalyst synthesis follows an environmentally benign approach, the requirement for acidic conditions and persulfate addition represents a trade‐off that should be considered when assessing overall process sustainability.