Adsorption of food dyes from aqueous solution on a sweet potato residue-derived carbonaceous adsorbent: Analytical interpretation of adsorption mechanisms via adsorbent characterization and statistical physics modeling

The adsorption of three food dyes, namely allura red AC, carmine and tartrazine, on a sweet potato residue-derived activated carbon (SPAC) was analyzed via experimental and theoretical tools. In a first step, SPAC was characterized by BET, FTIR and XRD to unravel its textural and surface properties. Adsorption isotherms of the three dyes were obtained at different temperatures ranging from 30 to 50 °C. Classical models were first applied to model the adsorption isotherms, but they were unable to describe the adsorption mechanisms. An alternative approach based on the application of mono- and multi-layer models from statistical physics theory was then used to better investigate the adsorption mechanisms. This alternative approach indicated that carmine and allura red AC adsorption followed a monolayer behavior, whereas the adsorption of tartrazine was a multi-layer process. These dyes were adsorbed via different configurations (horizontal, non-horizontal, horizontal and non-horizontal at the same time), depending on the system and the temperature. Tartrazine dye was removed via an aggregation process at different temperatures, while carmine was aggregated only at high temperatures. This aggregation process was absent for the allura red AC adsorption. Endothermic physisorption processes was observed for dye adsorption under these experimental conditions. The theoretical analysis indicated that SPAC is a promising material for the removal of these dyes, particularly in the case of allura red AC dye molecule. Overall, this study reports a combination of experimental and theoretical results to provide a new perspective at the molecular level of the removal of dye molecules using a low-cost activated carbon.