Uptake and hydration of sulfur dioxide on dry and wet hydroxylated silica surfaces: a computational study

• Published in: Physical chemistry chemical physics : PCCP Vol. 24; no. 1; pp. 172 - 179
• Main Authors: Gladich, Ivan, Lin, Chen, Sinopoli, Alessandro, Francisco, Joseph S
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 22.12.2021
• Subjects: Adsorbates; Adsorption; Dust; First principles; Hydration; Hydrogen bonds; Hydroxyl groups; Hydroxylation; Molecular dynamics; Oxidizing agents; Photochemistry; Silicon dioxide; Solar radiation; Sulfur; Sulfur dioxide; Surface chemistry; Water chemistry
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/d1cp04747g

We present a first-principles molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) α-quartz, which are a proxy for suspended silica dust in the atmosphere. The average adsorption energy for SO 2 is about −10 kcal mol −1 on both dry and wet surfaces. The adsorption is driven by hydrogen bond formation between SO 2 and the interfacial hydroxyl groups (on dry silica), or with water molecules (in the wet case). In the dry system, we report an additional electrostatic interaction between the interfacial hydroxyl oxygen and the sulfur atom, which further stabilizes the adsorbate. On dry silica, the interfacial hydroxyl group coordinates to SO 2 yielding a surface bound bisulfite (Si-SO 3 H) complex. On the wet surface, SO 2 reacts with water forming bisulfite (HSO 3 − ), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur dioxide is 1 kcal mol −1 and 3 kcal mol −1 on dry and wet silica, respectively, while for the backward reaction ( i.e. , bisulfite to SO 2 ) the barrier is 6 kcal mol −1 on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO 2 uptake on silica. These results underline the importance of the surface hydroxylation and/or adsorbed water layers for the SO 2 uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO 2 photochemistry and be an additional source of sulfate; this is especially relevant in atmospheres subject to a high level of suspended mineral dust, intense solar radiation and atmospheric oxidizers. SO 2 uptake and rapid hydration to bisulfite on dry and wet hydroxylated silica-based dust aerosols may prevent direct SO 2 photochemistry, and be an additional source of sulfate in the atmosphere of desert and industrialized areas.