Facile Microwave-Assisted Synthesis of Carbon Nanospheres from Urban Wastewater Sludge for Efficient Cr(VI) Adsorption in Tannery Effluent Treatment

Abstract

This study investigates the adsorption efficiency of carbon nanospheres (CNSs) synthesized from urban wastewater sludge for the removal of Cr(VI) from aqueous solutions. The CNSs were produced via microwave irradiation and characterized using FESEM, TEM, FTIR, Raman, TGA, and XPS analyses. The combined effects of surface oxygen functional groups and porosity significantly enhanced the chromium removal rate. The adsorption process was examined under various operating conditions, including pH, CNS and Cr(VI) concentrations, and contact time. Optimal adsorption efficiency (approximately 99.91%) was achieved with an adsorbent dosage of 700 ppm, a contact time of 50 min, a temperature of 303 K, a pH of 3.5, and a stirring speed of 500 rpm. Adsorption isotherms (Freundlich, Langmuir, and Temkin) and kinetic models (pseudo-first-order and pseudo-second-order) were thoroughly analyzed. Cr(VI) adsorption followed the Langmuir isotherm model with a maximum adsorption capacity of CNSs of 371.7 mg/g, while its kinetics were best described by the pseudo-second-order model. Furthermore, a reusability study demonstrated that CNSs retained approximately 95% of their adsorption efficiency even after multiple cycles, demonstrating their effectiveness as a cost-effective and sustainable adsorbent for Cr(VI) removal.