Abstract

The structure of anthracite (An) was exfoliated by H₂O₂ and impregnated with pomegranate peel extract (PP) to synthesize a new multifunctional (An/​PP) composite. This new composite was characterized and applied for Cr(VI) and Mn(VII) adsorption at pH 3.0 and varied temperatures (i.e., 25, 40 and 55 °C). Traditional Langmuir and Freundlich models fitted well the attained data at all adsorption temperatures. The maximum adsorption capacities were 382.19, 361.50 and 339.54 mg/​g for Cr(VI) and 400.49, 417.99 and 447.38 mg/​g for Mn(VII) at 25, 40 and 55 °C, respectively. The steric and energetic parameters of an advanced multilayer model were interpreted to understand the adsorption mechanisms. The receptor sites (n) was <1.0 at 25 and 40 °C and higher than 1.6 at 55 °C for Cr(VI) and Mn(VII) thus revealing multi – docking and multimolecular adsorption mechanisms. The decrease of active sites number (D_M) and the total adsorbed layers (N_t) resulted in decreasing the adsorption capacities (Q_sat) of Cr(VI) from 888.23 to 550.28 mg/​g in the temperature range of 25 – 55 °C. The adsorption energy (ΔE) was identified as the main factor in enhancing the Q_sat of Mn(VII) from 991.87 to 1025.65 mg/​g. Calculated ΔE for the studied ions varied from 21.6 to 32.48 kJ/​mol with negative values for Cr(VI) and a positive one for Mn(VII). Furthermore, the calculations of ΔE confirmed the exothermic and endothermic natures for Cr(VI) and Mn(VII), respectively, where the removal of both ions was related to physical adsorption interactions. Finally, this study provided new experimental and theoretical results for the Cr(VI) and Mn(VII) adsorption onto An/​PP adsorbent.