Mercury photochemistry and speciation in the Jijuktu'kwejk (Cornwallis River) estuary of the Minas Basin, Nova Scotia
LE3 .A278 2022
Master of Science
Earth and Environmental Science
Earth & Environmental Sciences
The photochemistry and speciation of mercury are important factors in the retention of mercury in estuarine ecosystems. Net photoreduction of divalent mercury (Hg(II)) and volatilization of photoreduction products (i.e., elemental mercury (Hg(0))/dissolved gaseous mercury (DGM) can be a mechanism by which mercury burdens in ecosystems are decreased and thereby bioaccumulation and biomagnification could be decreased. The interaction of salinity and dissolved organic matter (DOM) and the potential influence on mercury photoreactions is not well understood. Anions may compete with DOM for binding sites on Hg(II), and cations may compete with Hg(II) for binding sites on DOM. The effects of salinity were investigated while controlling the concentration of high molecular weight (>1 kDa) DOM using natural surface water from the tidal Jijuktu’kwejk (Cornwallis River) using a tangential ultrafiltration-dilution technique. Pseudo first-order rate constants in estuarine water dilutions ranged between 0.22 h-1 and 0.73 h-1. The amount of mercury available for photoreduction in the dilutions (Hg(II)RED) ranged between 67.25 pg and 265.91 pg. Pseudo first-order rate constant was found to decrease with increasing salinity, with lower salinity solutions (<13.5 g L-1) incurring the most drastic change in rate constant. However, Hg(II)RED increased with increasing salinity, possibly due to effects of the experimental design. In lower salinity dilutions (≤ 13.5 g L-1), DOM concentration decreased after irradiation (>57% loss), suggesting a loss of DOM reactivity. In a separate field experiment, mercury photoreactions were quantified in a tidal river by measuring dissolved and particulate mercury species concurrently with incoming solar radiation intensity, TSS, DOM, and salinity every 0.5 hours during daylight hours. Hg(0)aq concentration was not correlated with incoming radiation intensity, but was negatively correlated with TSS, suggesting radiation penetration is limited by TSS. Overall, photoproduction of Hg(0)aq appears to be highly suppressed by TSS limiting transmission of UV radiation through surface waters. This is demonstrated by the large amount of Hg(0) production in controlled lab experiments with filtered surface water in comparison to field measurements using unfiltered samples, Hg(0)aq concentrations in unfiltered surface water were largely nondetectable (0–12 pg L-1) in the presence of high ERM concentrations in filtered surface water (mean=147 pg L-1, SD=104 pg L-1). This thesis highlights the importance of salinity and DOM interactions in estuarine surface water and their effects on mercury photochemistry. It also demonstrates the importance of complementary laboratory and field measurements in mercury photochemistry research.
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