Role of electrostatic and π–π interactions in the removal of crystal violet and reduction of Cr(VI) ions from aqueous solutions using spent coffee grounds

Abstract

Spent coffee grounds (SCGs) have been used as an abundant, low-cost, and sustainable adsorbent for pollutant removal. Herein, SCGs are employed as adsorbent for crystal violet (CV) and hexavalent chromium (Cr(VI)) in mono- and bi-component systems. Experiments varying pH, contact time, temperature, and pollutant concentration revealed that both pollutants followed the Langmuir isotherm and pseudo-second-order kinetics, indicating monolayer adsorption and chemisorption. Thermodynamic parameters (ΔG° < 0, ΔH° < 0) confirmed the process is spontaneous and exothermic, with a slight entropy decrease. Mechanistic studies using SEM, EDS, and FTIR showed CV removal via electrostatic attraction, hydrogen bonding, and π–π interactions, while Cr(VI) was reduced to Cr(III) by electron-donating surface groups under acidic conditions. FTIR, desorption studies and Density Functional Theory (DFT) were used to further elucidate the adsorption mechanism of both pollutants. These findings highlight SCGs as an effective adsorbent for cationic dyes and toxic metal anions, thus bridging waste valorization and environmental remediation. The low cost and substantial environmental advantages of SCGs underscore their potential in sustainable water treatment, aligning well with circular economy principles. Such synergy of removal performance and waste reuse offers a scalable, eco-conscious approach to purification.