Insights into the adsorption performance and mechanism of Cr(VI) onto porous nanocomposite prepared from gossans and modified coal interface: Steric, energetic, and thermodynamic parameters interpretations

Herein, iron oxide/hydroxides deposits (gossans) were utilized, for the first time, in the fabrication of magnetite nanoparticles (MNPs) to load modified coal (MC). The as-synthesized MNPs@MC composite was characterized via different techniques and utilized for the Cr(VI) remediation. Experimental studies supported by theoretical treatment were applied to offer a new overview of the Cr(VI) adsorption geometry and mechanism at 25–45 °C. Experimental results suggested that the Cr(VI) uptake was mainly governed by adsorption–reduction coupled mechanism. The Langmuir model fitted well the Cr(VI) adsorption data with maximum adsorption capacities extended from 115.24 to 129.63 mg·g−1. Theoretical calculations indicated that Cr(VI) ions were adsorbed on the MNPs@MC following the theory of the advanced monolayer statistical model. The number of ions removed per site ranged from 1.88 to 1.23 suggesting the involvement of vertical geometry and multi-ionic mechanism at all temperatures. The increment of the active sites density and the adsorption capacity at saturation with improving temperature reflected an endothermic process. Energetically, the Cr(VI) adsorption was controlled by physical forces as the adsorption energies were less than 40 kJ·mol−1. The calculated free enthalpy, entropy, and internal energy explained the spontaneous nature and the viability of Cr(VI) adsorption on the MNPs@MC adsorbent. These results offer a new approach in utilizing the iron-rich deposits as gossans in the preparation of magnetic and low-cost adsorbents for wastewater remediation.