Preparation of Carbonaceous Materials from Coffee Parchment and Their Potential Use for Removal of Gadolinium Ions from Aqueous Phases

This investigation demonstrates the potential use of coffee parchment (CP) thermally treated at 500 or 1000°C (denoted as CP500 or CP1000, respectively) in removing gadolinium (Gd) ions. Their physicochemical characteristics were assessed. In addition, the Gd ion adsorption capacity of the samples was evaluated against certain factors: starting concentration, adsorption operating temperature, reaction time, pH, and the presence of interfering cations. Adsorption capacity was in the following order: CP < CP500 < CP1000. Higher initial concentrations and higher pH values, and lower adsorption temperatures, improved the adsorption of Gd ions. Elemental distribution studies were conducted to understand the adsorption mechanism. Based on the proposed adsorption mechanism, the adsorption of Gd ions on CP1000 may be related to the physicochemical characteristics of the CP1000 surface, specifically its specific surface area. Moreover, the effects of the coexistence of interfering cations on the adsorption of Gd ions were evaluated. Some trivalent cations significantly reduced adsorption compared with mono- and/or divalent cations. Next, the desorption of Gd ions was also assessed using distilled water, a hydrochloric acid (HCl) solution, and a sodium hydroxide solution at different concentrations. The results indicated that the optimal condition for the desorption (recovery) of Gd ions was the use of 100 mmol/L of the HCl solution, with three adsorption–desorption cycles producing favorable results. Finally, CP1000 showed potential use for the removal of Gd-based contrast agents from aqueous phases. Overall, this study suggests that the prepared CP1000 can be used for the adsorption–desorption of Gd ions from water solutions.