Heterogeneous Chemistry of Mercuric Chloride on Inorganic Salt Surfaces

Abstract

Gaseous oxidized mercury (GOM) is a major chemical form responsible for deposition of atmospheric mercury, but its interaction with environmental surfaces is not well understood. To address this knowledge gap, we investigated the uptake of gaseous HgCl 2 , used as a GOM surrogate, by several inorganic salts representative of marine and urban aerosols. The process was studied in a fast flow reactor coupled to an ion drift−chemical ionization mass spectrometer, where gaseous HgCl 2 was quantitatively detected as HgCl 2 •NO 3 − . Uptake curves showed a common behavior, where upon exposure of the salt surface to HgCl 2 , the gas-phase concentration of the latter dropped rapidly and then recovered gradually. None of the salts produced a full recovery of HgCl 2 , indicating the presence of an irreversible chemical reaction in addition to reversible adsorption, and all salts showed reactive behavior consistent with the presence of surface sites of a high and a low reactivity. On the basis of the decrease in the uptake coefficient with increasing concentration of gaseous HgCl 2 , we conclude that the interaction follows the Langmuir−Hinshelwood mechanism. The reactivity of a deactivated salt surface after uptake could be partially restored by cycling through an elevated relative humidity at atmospheric pressure. The overall surface reactivity decreased in the series Na 2 SO 4 > NaCl > (NH 4 ) 2 SO 4 > NH 4 NO 3 . The uptake on NH 4 NO 3 was nearly fully reversible, with low values of the initial (0.4 × 10 −2 ) and steady-state (3.3 × 10 −4 ) uptake coefficients, whereas Na 2 SO 4 was significantly more reactive (3.1 × 10 −2 and 1.7 × 10 −3 ). Depending on the aerosol loading, the lifetimes of gaseous HgCl 2 on dry urban and marine particles (as pure (NH 4 ) 2 SO 4 and NaCl, respectively) were estimated to range from half an hour to about a day.

Exchange Reactions Alter Molecular Speciation of Gaseous Oxidized Mercury

“…It is instructive to compare this case against an experiment where the exchange reaction was studied between gaseous HgBr 2 and solid HgCl 2 deposited on the surface of a frosted borosilicate (Pyrex) tube, similar as in our recently reported uptake experiments. 57 Not only HgCl 2 is more volatile than HgBr 2 , 71−74 but also its binding with the frosted tube is weaker than with the membrane. Hence, a significant amount of HgCl 2 was continuously desorbed into the helium flow and carried into ID-CIMS, producing a HgCl 2 F − signal approximately 100 times stronger than the signal of HgBr 2 F − (Figure S2).…”

“…56 Although the limit of detection of our ID-CIMS at present does not allow for ambient GOM analysis, it is more than sufficient for laboratory studies of mercury chemistry, such as the gas-surface interactions. 57 Here, we use the approach, pioneered by Mario Molina and his collaborators, where gasphase chemistry is studied by coupling a flow reactor with a chemical ionization mass spectrometer. 58 We employed a similar flow reactor coupled to ID-CIMS (Figure S1), along with several other instruments to investigate exchange reactions that can occur on surfaces and in solutions, including (1) reactions of mercury(II) compounds with each other and with other chemicals in an aqueous phase, (2) reactions of a gaseous mercury(II) compound or other nonmercury compound with a surface-bound mercury(II) compound, and (3) a reaction of two gaseous mercury(II) compounds with each other in the presence of a surface.…”

“…46,59,60 On the contrary, the smooth surface of the borosilicate glass is relatively inert toward HgCl 2 . 57 In our experiments, the glass was deactivated with dimethyldichlorosilane (DMDC, Sigma-Aldrich, ≥98.5%), as described previously. 57 Aqueous solutions of HgCl 2 (140 mM), HgBr 2 (16 mM), Hg(NO 3 ) 2 •H 2 O (16 mM), KCl (33 mM), and NH 4 Cl (1.2 mM) were prepared by dissolving the corresponding solids in distilled water.…”

“…57 In our experiments, the glass was deactivated with dimethyldichlorosilane (DMDC, Sigma-Aldrich, ≥98.5%), as described previously. 57 Aqueous solutions of HgCl 2 (140 mM), HgBr 2 (16 mM), Hg(NO 3 ) 2 •H 2 O (16 mM), KCl (33 mM), and NH 4 Cl (1.2 mM) were prepared by dissolving the corresponding solids in distilled water. The 6% HCl solution was prepared by diluting the concentrated HCl with distilled water.…”

“…Recently, our group has reported the application of a custom-built ion drift-chemical ionization mass spectrometer (ID-CIMS), similar to the one introduced earlier, for studying the molecular composition of gaseous mercury­(II) species directly without the use of preconcentration . Although the limit of detection of our ID-CIMS at present does not allow for ambient GOM analysis, it is more than sufficient for laboratory studies of mercury chemistry, such as the gas-surface interactions . Here, we use the approach, pioneered by Mario Molina and his collaborators, where gas-phase chemistry is studied by coupling a flow reactor with a chemical ionization mass spectrometer .…”