Absorption characteristics of elemental mercury in mercury chloride solutions

Elemental mercury (Hg⁰) in flue gases can be efficiently captured by mercury chloride (HgCl₂) solution. However, the absorption behaviors and the influencing effects are still poorly understood. The mechanism of Hg⁰ absorption by HgCl₂ and the factors that control the removal were studied in this paper. It was found that when the mole ratio of Cl^- to HgCl₂ is 10:1, the Hg⁰ removal efficiency is the highest. Among the main mercury chloride species, HgCl₃^- is the most efficient ion for Hg⁰ removal in the HgCl₂ absorption system when moderate concentrations of chloride ions exist. The Hg⁰ absorption reactions in the aqueous phase were investigated computationally using Moller-Plesset perturbation theory. The calculated Gibbs free energies and energy barriers are in excellent agreement with the results obtained from experiments. In the presence of SO₃^2- and SO₂, Hg²⁺ reduction occurred and Hg⁰ removal efficiency decreased. The reduced Hg⁰ removal can be controlled through increased chloride concentration to some degree. Low pH value in HgCl₂ solution enhanced the Hg⁰ removal efficiency, and the effect was more significant in dilute HgCl₂ solutions. The presence of SO₄^2- and NO₃^- did not affect Hg⁰ removal by HgCl₂.