Study on the Desorption Mechanism of Sulfur Dioxide–Modified Activated Carbon Based on Density Functional Theory

Activated carbon injection technology is the primary method used to control the mercury emissions from coal-fired power plants. The preparation of sulfur-loaded carbon-based adsorbents through SO₂ modification of the surface of activated carbon (the primary component) provides an effective solution to enhancing the mercury removal performance of adsorbents. However, there remains a lack of clarity on the adsorption performance and mechanism of mercury on the newly formed active sites of the carbon surface after SO₂ sulfur loading modification. In this study, four potential structures of SO₂ loaded onto the surface of activated carbon were constructed. Quantum chemical calculation methods were applied to calculate the adsorption process of Hg in these four models, with those important characteristics identified such as bonding properties, adsorption energy, electrostatic potential, and molecular orbitals. As indicated by those results, the adsorption bonds of the SO₂-modified activated carbon were mainly C-O-S and C-S-C. After the carbon cluster model adsorbed SO₂ molecules, the sulfur in SO₂ exhibited a strong positive potential that facilitated the loss of electrons from Hg due to the potential difference. Consequently, HgO was firmly adsorbed onto the surface of the carbon cluster. As revealed by the molecular orbital calculations performed after Hg adsorption on the two carbon cluster models, SO₂-modified and elemental sulfur-modified activated carbons, in the SOAC-Arm-1 configuration, there was a clear exchange orbital around the adsorbed Hg atom in the LUMO, with a small HOMO–LUMO energy gap of only 0.01713 eV. At this point, the free electrons on the molecule were prone to orbital transitions, promoting the occurrence of adsorption reactions. The SOAC-Arm-3 conformation exhibited the shortest C-Hg bond length and had an adsorption energy of up to −70.42 kJ/mol, indicating a stronger chemical bonding ability and a higher likelihood of adsorption reactions. These results demonstrate the feasibility of sulfur-loaded modified activated carbon to mitigate Hg pollution through SO₂.