Investigation of the adsorption mechanism of methylene blue (MB) on Cortaderia selloana flower spikes (FSs) and on Cortaderia selloana flower spikes derived carbon fibers (CFs)

In this article, a theoretical study was performed in order to elucidate the adsorption mechanism of methylene blue (MB) on Cortaderia selloana flower spikes (FSs) and on Cortaderia selloana flower spikes derived carbon fibers (CFs). For this objective, the Hill model developed using grand canonical ensemble was successfully applied. This model represents the general case of Langmuir model assuming that the receptor sites of CFs and FSs accepts a variable number of MB molecules and is defined by the number of molecules captured per adsorbent site (n). Based on the model parameters, the MB adsorption geometry was described at different temperatures indicating that the MB molecules were fixed by an angled position on the investigated adsorbents. The elucidation of the adsorption capacity showed that CFs was more effective for removing MB from wastewater than FSs. This behavior was detailed and corroborated by an energetic investigation. Moreover, the adsorption energy varies from 22.187 to 25.378 kJ mol−1 for FSs and from 24.503 to 27.412 kJ mol−1 for the CFs reflecting a physisorption process. Interestingly, and based on this energetic investigation, it was deduced that the adsorption mechanism of MB was endothermic. Excellent correlation between the number of molecules captured by the receptor site, the adsorption capacity, and the adsorption energy was observed.