Comparative analysis of EBT dye removal using <i>Spartium junceum</i> and derived activated carbon: experimental and DFT insights

Abstract

This study explores the use of Spartium junceum biomass (SJ) to create activated carbon (SJAC) through a one-step activation with phosphoric acid. Characterisation of SJ and SJAC was conducted using FTIR spectroscopy, pHpzc measurement, nitrogen adsorption/desorption isotherms, and SEM to analyse their surface chemical groups, morphology, and texture. Results showed that SJAC has a well-developed mesoporous structure, high pore volume, and a significantly enhanced specific surface area (325.759 m2 g−1) compared to raw SJ (1.647 m2 g−1). The adsorptive performance of SJ and SJAC was tested by removing the anionic dye Eriochrome Black T (EBT). Using central composite design under response surface methodology, batch experiments were optimised for factors like pH, adsorbent dosage, initial EBT concentration, and contact time. Optimal conditions were determined using a desirability function. Adsorption data were analysed with various isothermal models, showing that the Freundlich model best described SJ, while the Langmuir model was more suitable for SJAC. SJAC exhibited an exceptional adsorption capacity of 261.36 mg g−1, about five times higher than SJ. Thermodynamic analysis indicated that the adsorption process for both sorbents was spontaneous and endothermic. Density functional theory was used to investigate the adsorption mechanism of EBT on SJ and SJAC, identifying multiple mechanisms such as electrostatic interactions, pore filling, hydrogen bonding, and π-π stacking, with pore filling being the primary mechanism for SJAC. SJAC maintained over 77% EBT adsorption efficiency across four reuse cycles, highlighting its potential for EBT remediation applications.