Surfactant–modified serpentine for fluoride and Cr(VI) adsorption in single and binary systems: Experimental studies and theoretical modeling

Single-compound and binary adsorption isotherms of hexavalent chromium (Cr (VI)) and fluoride (F) ions onto surfactant–modified serpentine (MSP) are investigated at different temperatures (298, 308 and 318 K) and constant pH (2.0). For each temperature, F is adsorbed more than Cr(VI), in both single-compound and binary systems, mainly due to both higher electronegativity and number of active receptor sites (i.e., hydroxyl groups) on adsorbent surface. Moreover, in binary systems, both the adsorbates showed a reduction in their adsorption capacities, testifying the occurrence of competitive adsorption mechanisms. Classical models fit the experimental results but are not able to provide important information about the adsorption mechanisms. In order to understand the mechanism of Cr(VI) and F adsorption, the parameters of two advanced statistical physics models are determined and interpreted. For Cr(VI), more than one ion is adsorbed on a same active site (multimolecular mechanism), likely by an ion exchange mechanism between chromium anions/cations and the protonated/non-protonated single bondOH groups of the MSP surface. For F, a multi-docking mechanism occurs (i.e. more than one active site is needed to bond one F ion), likely associated with electrostatic interactions between MSP active sites and the adsorbed ions. Sterically, the receptor sites density (NM) is found to be the main parameter controlling the adsorption capacities of the adsorbed ions and their difference, confirming the predominance of F ions. The adsorption capacity ratio between single-compound and binary values (i.e. Qb/Qs) indicates that Cr(VI) adsorption slightly decreases in the presence of F, allowing to quantify the competition between ions. The estimated adsorption energies for the investigated systems well represent the endothermic adsorption of both Cr(VI) and F, having a physical reversible character.