Green synthesis of carbon/TiO2 nanocomposites mediated by Astragalus extract for photodegradation of reactive yellow 161 dye

This study focuses on the development of a sustainable photocatalyst derived from biomass waste, designed to remove polluting dyes from water. Activated carbon obtained from used coffee grounds and stabilized with Astragalus boeticus leaf extract was combined with titanium dioxide, using titanium isopropoxide as a precursor, to produce CAmc/TiO 2 -x nanocomposites, where x represents the volume of titanium isopropoxide (TTIP) used during synthesis (5, 10, and 15 mL). Materials containing three different TiO 2 loadings were synthesized and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray analysis, and diffuse reflectance UV–Vis spectroscopy. The nanocomposites were evaluated for the degradation of the anionic dye Reactive Yellow 161 (RY161) under irradiation from a mercury vapor lamp. The effects of pH, catalyst loading, TiO 2 content, and initial dye concentration were systematically investigated. Among the prepared materials, the nanocomposite containing the highest TiO 2 content exhibited the best performance. Under optimal conditions (pH 4, 1 g/L catalyst, 30 ppm dye, 180 min), approximately 95% discoloration was achieved, with an apparent rate constant of 0.0123 min −1 , consistent with pseudo-first-order kinetics. The enhanced activity is attributed to improved dispersion of TiO 2 within the carbon matrix and the synergistic interaction between adsorption and photocatalytic oxidation. The catalyst also maintained high efficiency over five consecutive reuse cycles, indicating good structural stability. These findings demonstrate that valorizing coffee waste into functional photocatalytic nanocomposites offers a promising, environmentally responsible approach for treating dye-contaminated wastewater.