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.