Improving the performance of activated carbon towards dibenzothiophene adsorption by functionalization and sulfur-metal interactions

In this paper, we developed and evaluated the potential synergistic effect of a hybrid two-step sequential modification of a coconut shell activated carbon (AC) by oxidative treatment with HNO3 (ACO), followed by aluminum impregnation (ACO-Al) by incipient wetness method (Al(NO3)3.9 H2O - 2% of Al) aiming to improve the adsorbent properties (i.e diffusivity, adsorption capacity and adsorbent-adsorbate affinity) for desulfurization of tiophenic compounds. Characterization of the adsorbent (TPD, FTIR, XPS) confirmed acidic oxygenated functional groups (carboxylic acids, anhydrides, lactones, phenols, and quinones) and aluminum compounds (AlO(OH) and Al2O3) on the adsorbent's surface, which promoted enhanced interactions between adsorptive and adsorbent (i.e., sulfur-metal (S-M) interactions) that improved the performance of the adsorbent on dibenzothiophene (DBT) adsorption, besides π-π stacking interactions with the aromatic structures of the DBT molecule. Textural properties were slightly affected, indicating that the modifications provided surface modifications Aiming to support the understanding of the phenomena involved in the process, a phenomenological modeling approach was used, wherein adsorption equilibrium data was described by Langmuir isotherm and kinetic data by applying mass transfer kinetic models. The equilibrium and kinetic experiments showed an increment of 59% in the maximum adsorption capacity and 23% in the adsorption rate (controlled by the internal diffusion), respectively, for ACO-Al compared with the ACO adsorbent. Furthermore, the ACO-Al adsorbent showed a remarkable potential to remove different thiophenic compounds in binary systems (for benzothiophene (BT), dimethyldibenzothiophene (DMDBT), besides the DBT), and also to operate in adsorption-desorption cycles. Overall, the proposed hybrid adsorbent modification proved to be efficient in boosting the performance of activated carbon performance towards desulfurization given the specific oxygen groups and sulfur-metal interactions provided.