Adsorptive Removal of Thiophenic Compounds from Oils by Activated Carbon Modified with Concentrated Nitric Acid

Adsorptive removal of thiophenic compounds from oils by commercial coconut-based activated carbon (AC) and modified AC samples was studied systematically in a batch-type adsorption system. The modified AC samples were obtained by treating the commercial AC sample using 65 wt % concentrated nitric acid (HNO3) at different temperatures (30-120 degrees C). The effects of the modification temperature on morphology, pore structure, and surface chemistry of the AC samples were analyzed and compared. It has been found that oxidation with concentrated HNO3 at ambient conditions removes inorganic components or ashes of ca. 50% in the AC sample, produces carboxyl functional groups on the AC surface, and introduces high volume micropores with sizes around 0.54 nm. The effects of pore structure and surface features on adsorptive capability for the thiophenic compounds were investigated in detail. The results show that the as-received AC sample is able to adsorb the bigger size sulfur compounds. The adsorptive removal efficiency for the sulfur compounds decreases in the order of 4,6-dimethyldibenzothiophene > dibenzothiophene > benzothiophene > thiophene. The modified AC samples can adsorb more thiophene and benzothiophene molecules, but this is not the case for dibenzothiophene and 4,6-dimethyldibenzothiophene molecules. On the basis of the results obtained, it was proposed that the pore structure and surface chemistry of the AC as well as frontier orbital energies of the thiophenic compounds and the AC samples govern the adsorption of these species on the surface of AC.