Investigations of Redox-Dependent Processes Involving Mercury, Sulfur, and Dissolved Organic Matter
Mercury is a potent neurotoxin that poses threats to humans and the environment. The behavior of mercury in terrestrial and aquatic environments is intimately linked to the distribution of sulfur between inorganic sulfide and reduced sulfur groups in dissolved organic matter (DOM). Redox-dynamic environments (e.g., wetlands, riparian zones), in particular, play a central role in the transport of divalent mercury (HgII) and transformation of HgII to bioavailable methylmercury (MeHg) and volatile dissolved gaseous mercury (Hg0(aq)). This thesis explores redox-dependent processes in wetlands and riparian soils involving mercury, DOM, and reduced sulfur in organic and inorganic forms.
Chapter 2 investigates the long-term fate of mercury in saturated riparian soils under transient redox conditions. Pore water mercury dynamics identify two modes of mercury mobilization: (1) rapid release of HgII from surficial soil by co-mobilization with DOM and (2) postponed release of Hg0(aq) from soil at greater depth in the soil profile. The formation of Hg0(aq) in soil that contains mercury predominantly as mercuric sulfide (β-HgS) challenge the notion that authigenic β-HgS is exclusively an environmental sink for mercury.
Chapter 3 explores the sulfurization of DOM across wetlands in the Florida Everglades that vary in sulfate enrichment. Vertical and lateral spatial relationships are observed in the concentration and relative abundance of sulfur functionalities in DOM. Reduced sulfur in DOM is formed in sulfidic sediments and is subsequently subject to oxidation in oxic surface waters. Results provide the first comprehensive field evidence of DOM sulfurization and identify sulfate inputs to wetlands as a primary determinant on DOM sulfur chemistry.
Chapter 4 examines the biogeochemical controls on mercury speciation in Alaskan wetlands that differ in sulfate levels and nutrient status. The abundance of mercury and reduction of HgII to Hg0(aq) were controlled by pore water DOM concentration. Mercury methylation was limited by the availability of HgII and correlated with sulfide, a byproduct of sulfate reduction. Insights on the environmental controls on mercury speciation in Alaskan wetlands assist in predicting mercury fate under future hydrologic and climatic conditions in northern environments.