Ultrasound-assisted adsorption of congo red dye using green synthesized silver nanoparticles coated with chitosan

Abstract Congo Red (CR), an anionic azo dye widely discharged by paper and textile industries, persists in aquatic systems and poses carcinogenic and ecological risks. This study developed a green, ultrasound-intensified adsorption system for CR removal using chitosan-coated silver nanoparticles (Chi-AgNPs) synthesized via Moringa oleifera seed pod extract. Antioxidant screening confirmed the extract’s reducing capability, and physicochemical characterization verified nanoparticle formation and chitosan encapsulation. Dynamic light scattering indicated a mean particle size of 218.9 nm and moderate polydispersity, while the positive zeta potential (+ 31.45 mV) reflected colloidal stability and favorable electrostatic interaction with anionic dye molecules. Two-way ANOVA demonstrated significant and synergistic effects of adsorbent dosage and sonication time on adsorption performance, consistent with ultrasound-enhanced mass transfer and active-site accessibility. Under optimal conditions (0.025 g; 5 min sonication), CR removal reached 90.14% with an adsorption capacity of 18.028 mg g⁻ 1 . Kinetic data conformed to a pseudo-second-order model (R 2 = 0.9948), indicating chemisorption-dominated uptake, while the Langmuir isotherm provided the best equilibrium fit (R 2 = 0.9446; RL = 0.3598) with a maximum capacity of 103.7851 mg g⁻ 1 . The results show that green-synthesized Chi-AgNPs, when coupled with ultrasound, deliver rapid and monolayer-dominant adsorption, positioning the system as a viable intensified remediation route for dye-laden wastewater.