Analysis of factors affecting visible and UV enhanced oxidation of dibenzothiophenes on sulfur-doped activated carbons

Commercial wood-based activated carbon was treated with hydrogen sulfide at 650 and 800 °C to introduce sulfur to the carbon matrix. On the initial and S-doped samples adsorption of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) from model diesel fuel with equal molar concentrations of these species was measured at room temperature. The amount adsorbed increased with an increase in the content of sulfur and in the volume of pores similar in sizes to the adsorbates molecules. This increase was attributed to the dispersive interactions in small pores that are enhanced by the presence of sulfur. Then the carbons with adsorbed DBT and DMDBT were exposed to UV, visible light and stored in dark. On the surface of sulfur-doped carbons an extensive oxidation of both DBT and DMDBT species was detected upon exposure either in UV or visible light. The photoactivity was linked to the effect of sulfur functionalities. That sulfur, besides decreasing the energy gap, increases the ability of carbons to adsorb oxygen and its reduction to active superoxygen ion. Another important factor is the affinity of carbons to retain water, which is the source of active radicals formed upon absorption of photons.