How water affects mercury-halogen interaction in the atmosphere

Area of Science:

• Environmental Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Mercury and halogen ions play crucial roles in atmospheric mercury depletion reactions.
• Understanding their solvation in water is key to modeling these environmental processes.

Purpose of the Study:

• To investigate the solvation structures and energetics of mercury-halide ion pairs in water.
• To elucidate the reaction mechanisms of mercury-halogen recombination in aqueous environments.

Main Methods:

• Application of transition state theory for recombination reactions involving Hg²⁺, Hal^-, and water as a third body.
• Molecular dynamics simulations using semiempirical tight-binding and density functional theory for mercuric halide-water complexes ([HgHal(H₂O)_n]⁺).

Main Results:

• Hydrated [Hg-Hal]⁺ ion pairs adopt clathrate-like structures with a central [Hg²⁺(H₂O)₆] motif and external halogen ions.
• Contact ion pairs are favored at lower hydration levels (up to 50 water molecules), while solvent-separated pairs dominate with increased hydration.
• The transition from contact to solvent-separated ion pairs is specific to the ion pair and sensitive to temperature.

Conclusions:

• The solvation behavior of mercury-halide ions is governed by a balance of water-water, Hg²⁺-water, and Hal^--water interactions.
• Explicit water molecules are critical for localizing reaction barriers in mercury-halogen recombination.
• The findings provide insights into mercury cycling and speciation in aquatic environments.