Dual-functional activated carbon from coffee grounds for crystal violet dye removal and subsequent application as an electrode material in energy storage devices

Abstract

The conversion of agricultural waste into high-value functional materials offers a sustainable strategy to address environmental concerns. In this study, activated carbon was synthesized from spent coffee grounds (CGAC) via KOH activation and used for crystal violet (CV) dye adsorption. Under optimal conditions (0.5 g dosage, 300 mg·L⁻¹ CV, 30°C, 120 min), CGAC achieved 91.41% removal efficiency, with adsorption behavior fitting pseudo-second-order kinetics and the Freundlich isotherm. Thermo-dynamic parameters confirmed a spontaneous and endothermic process. Moreover, the reusability study revealed that CGAC maintained a high removal efficiency above 88% after seven adsorption–desorption cycles, demonstrating its structural stability and potential for practical wastewater treatment. Notably, the dye-saturated carbon was further processed into electrode materials through five different preparation routes: unmodified CGAC, dye-loaded CGAC (CGAC_CV), hydro-thermally treated (CGAC_CV_H), thermally treated (CGAC_CV_600), and combined hydrothermal-thermal treated (CGAC_CV_H600). Among them, CGAC_CV_H600 exhibited the best electrochemical performance with a specific capacitance of 326.4 F·g-1 at 0.25 A·g-1 in 6 M KOH due to its optimized porosity and nitrogen/oxygen doping. This study is among the few that explore the reuse of dye-laden adsorbents through systematic post-treatment strategies, offering a circular and sustainable pathway for biomass-derived carbon in both environmental remediation and energy storage.