Atomistic View of Mercury Cycling in Polar Snowpacks: Probing the Role of Hg2+ Adsorption Using Ab Initio Calculations

Abstract

Photochemical oxidation of atmospheric elemental mercury (Hg0) promotes reactive oxidized Hg (HgII) adsorption on particles and deposition to the polar snowpack. The deposited Hg either returns to the atmosphere via photochemical reduction or remains in the snowpack depending on the strength of adsorption. In this study, we performed ab initio calculations to understand the atomic-level cause of the fate of adsorbed Hg by determining the adsorption affinity for Hg2+, the simplest form of HgII, of barite, halite, muscovite, illite, and ice-Ih as potential adsorbents. The adsorption affinity was estimated by calculating the energy required to dissociate adsorbed Hg2+ from the adsorbents. The results reveal that Hg2+ is stable on the surfaces of the selected adsorbents, except barite, but is prone to photodissociation under solar ultraviolet radiation. This mild adsorption is expected to contribute to the bidirectional exchange of Hg between the atmosphere and the polar snowpack. Thus, this theoretical approach can provide complementary perspectives on polar Hg dynamics beyond the limitations of field and laboratory experiments. Further studies on more complicated and realistic adsorption models with different HgII species and adsorbent surfaces having diverse defective structures are required to better comprehend air–snow Hg cycling in the polar regions.

Reductive Adsorption of Atmospheric Oxidized Mercury on Ice: Insights from Density Functional Calculations

“…Similar trend can be observed in the results of illite, where some of K atoms on the siloxane ring are removed, making them reactive for positively charged Hg atom (around sites 03 and 04) (see Ref. for the uppermost K defects on illite 36 ).…”

Reductive Adsorption of Atmospheric Oxidized Mercury on Ice: Insights from Density Functional Calculations

“…To confirm the adsorption affinity of Hg II on the ice surface, we calculated E B as follows (stable adsorption causes E B < 0) , Here, E Hg‑surface – E Hg‑surface 0 implies the energy difference between the final and initial states, given that E Hg‑surface 0 and E Hg‑surface are the total energy of the selected adsorbent and the Hg species before (far from each other so as not to interact) and after adsorption, respectively. The energy differences of the adsorbate ( E Hg – E Hg 0 ) driven by a change in molecular configuration and dissociation and those of the adsorbent ( E surface – E surface 0 ) associated with its structural change are listed in Table S3 (Section 3 in the Supporting Information).…”

“…Illustrations and further details of the halite, illite, and ice-Ih surface structures are available in Section 1 of the Supporting Information, that is, Figures S1 and S2 and Table S1, and our previous study. 19 The ice-Ih surface is known to have diverse types of structural and ionic defects produced by full-or half-bilayer terminations, molecular disorientation, and the Grotthuss mechanism. 17,20−22 These defects having unpaired dangling H and O atoms are potential adsorption sites, hereafter d(H) and d(O), respectively.…”

“…Furthermore, to probe the reaction kinetics of Hg dihalides on ice surfaces, transition state searching calculations based on the complete linear synchronous transit and quadratic synchronous transit (LST/QST) method were carried out with a 0.1 eV/Å convergence criterion. 54 To confirm the adsorption affinity of Hg II on the ice surface, we calculated E B as follows (stable adsorption causes E B < 0) 19,38…”

“…In addition, halite and illite were explored as potential marine and terrestrial origin adsorbents for atmospheric Hg, respectively. , We used X-ray diffraction data obtained in previous studies to construct cleaved surface structures of these adsorbents, and the surfaces were cleaved along naturally preferred directions: (0001) and (0001̅) for ice-Ih, (001) for halite, and (001) for illite. − Potential adsorption sites were selected from the surfaces in consideration of the atomic arrangements of each adsorbent. Illustrations and further details of the halite, illite, and ice-Ih surface structures are available in Section 1 of the Supporting Information, that is, Figures S1 and S2 and Table S1, and our previous study …”