Kinetic study of the stabilization and reduction of the aqueous mercuric ion by sulfite: Implications for atmospheric deposition.

Kinetic study of the stabilization and reduction of the aqueous mercuric ion by sulfite: Implications for atmospheric deposition.

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Title: Kinetic study of the stabilization and reduction of the aqueous mercuric ion by sulfite: Implications for atmospheric deposition.
Author: Van Loon, Lisa L.
Abstract: In aqueous solution, Hg2+ forms both 1:1 and 1:2 complexes with the sulfite anion. Hg(SO3)22- is redox-stable, but when one SO32- ligand dissociates, the redox-unstable HgSO3 is generated. With Hg 2+(aq) in excess, HgSO3 was observed for the first time. This complex undergoes a highly temperature sensitive intramolecular redox reaction which is kinetically first-order. The activation parameters are consistent with a unimolecular bond cleavage. In the presence of excess sulfite, Hg(SO3)2 2- is formed. Under these conditions, the rate of reduction of Hg2+(aq) to Hg0 depends inversely on the concentration of uncoordinated sulfite, yet remains unaffected by the sulfite liberated through dissociation. Using the kinetics of decomposition of Hg(SO3)22-, we have re-evaluated the sequential binding constants for sulfite to the mercuric ion and find a value for K1 that is four orders of magnitude greater than K 2, which differs from previous estimates in which they were similar in magnitude. The new values greatly alter speciation predictions for atmospheric Hg in global models. Rather than the major mercury species in clouds being stable Hg(SO3)22, we predict that HgSO 3 should predominate in both unpolluted and polluted (by SO2) air. At 25°C and pH 3, HgSO3 decomposes to Hg0 in 5 minutes. If this is in fact the major reduction route for mercury, as has been proposed, very little mercury should be present in rainwater. We find evidence for the existence of a highly soluble Hg0·SO 2 complex. According to our results, the presence of sulfite causes the solubility of Hg(0) to increase by at least three orders of magnitude due to its formation of an SO2 adduct. This has serious implications for atmospheric deposition, since it may be that the major mercury species in precipitation is the elemental form.
Date: 2001
URI: http://hdl.handle.net/10393/6316

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