The Effects of Dissolved Organic Matter on Mercury-Sulfide Interactions in Aqueous Systems

Gerbig, Chase A.

Graduate Thesis Or Dissertation

The Effects of Dissolved Organic Matter on Mercury-Sulfide Interactions in Aqueous Systems Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/bg257f267

Abstract

Mercury speciation with sulfide and dissolved organic matter was characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy under a variety of conditions, including a range of Hg:DOM ratios, sulfide concentrations, and equilibration times. Five different DOM isolates were used to elucidate the effects of DOM characteristics on mercury speciation in sulfidic environments. In the absence of sulfide, the mercury-DOM complex was characterized by thiol-like binding with 2.4 sulfur atoms at a distance of 2.35 Å from the mercury atom. In sulfidic systems, mercury existed as a very small (<10 >nm) and/or poorly crystallized metacinnabar-like particle stabilized by DOM. Typically, the mercury-sulfur bond distance was representative of metacinnabar (2.53 Å), but changes in the number of sulfur atoms coordinating mercury were representative of changes in particle size and/or crystallinity. At high Hg:DOM ratios (> 4 nmol mg^-1) the nanocolloidal metacinnabar-like nanocolloids were significantly larger or more ordered than at low Hg:DOM ratios (< 4 nmol mg^-1) that are more representative of natural environments. The size and/or degree of crystallinity decreased as the specific UV absorbance (SUVA) of the DOM stabilizing the nanocolloidal metacinnabar like particle increased. As the sulfide concentration decreased, at a fixed Hg:DOM ratio, the mercury-sulfide particle size/crystallinity decreased such that the smallest nanoparticles were modeled as less than 1 nm in diameter. The reactivity of the nanoparticles was tracked with a tin-reducible mercury assay. Sulfide-free systems indicated that kinetic limitations drive the fraction of reducible mercury and that mercury bound to thiol groups in DOM is substantially less reactive than mercury bound to weaker carboxyl groups. The fraction of reducible mercury in sulfidic systems was substantially higher when the particles were formed with high SUVA DOM compared to low SUVA DOM. The fraction of reducible mercury decreased rapidly with equilibration time, up to about 2 h, indicating that metacinnabar-like particles form rapidly. Poorly crystallized and/or nanocolloidal metacinnabar-like particle that are stabilized by DOM are relatively reactive and may be important mercury species in natural systems.

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