Roles of Sulfuric Acid in Elemental Mercury Removal by Activated Carbon and Sulfur-Impregnated Activated Carbon

This work addresses the discrepancy in the literature regarding the effects of sulfuric acid (H2SO4) on elemental Hg uptake by activated carbon (AC). H2SO4 in AC substantially increased Hg uptake by absorption particularly in the presence of oxygen. Hg uptake increased with acid amount and temperature exceeding 500 mg-Hg/g-AC after 3 days at 200 °C with AC treated with 20% H2SO4. In the absence of other strong oxidizers, oxygen was able to oxidize Hg. Upon oxidation, Hg was more readily soluble in the acid, greatly enhancing its uptake by acid-treated AC. Without O2, S­(VI) in H2SO4 was able to oxidize Hg, thus making it soluble in H2SO4. Consequently, the presence of a bulk H2SO4 phase within AC pores resulted in an orders of magnitude increase in Hg uptake capacity. However, the bulk H2SO4 phase lowered the AC pore volume and could block the access to the active surface sites and potentially hinder Hg uptake kinetics. AC treated with SO2 at 700 °C exhibited a much faster rate of Hg uptake attributed to sulfur functional groups enhancing adsorption kinetics. SO2-treated carbon maintained its fast uptake kinetics even after impregnation by 20% H2SO4.