Mercury photoreduction and photooxidation kinetics in estuarine water: Effects of salinity and dissolved organic matter

• Published in: Chemosphere (Oxford) 2023-01, Vol.312, p.137279, Article 137279
• Main Authors: Clarke, Rachel G., Klapstein, Sara J., Keenan, Robert, O'Driscoll, Nelson J.
• Format: Article
• Language: English
• Subjects: Dilution; Estuary; Mercury photooxidation; Mercury photoreduction; Salinity gradient; Tangential ultrafiltration
• ISSN: 0045-6535; 1879-1298
• DOI: 10.1016/j.chemosphere.2022.137279

Net photoreduction of divalent mercury (Hg(II)) and volatilization of photoreduction products (i.e., elemental mercury (Hg(0))/dissolved gaseous mercury (DGM)) is a mechanism by which mercury burdens in ecosystems are lessened. The effects of salinity on mercury photoreactions were investigated while controlling the concentration of DOM (>1 kDa) using natural surface water from the tidal Jijuktu'kwejk (Cornwallis River) and processed with a tangential ultrafiltration-dilution technique. Pseudo first-order rate constants in estuarine water salinity dilutions ranged between 0.22 h−1 and 0.73 h−1. The amount of mercury available for photoreduction (Hg(II)RED) ranged between 67.2 and 265.9 pg. Pseudo first-order rate constants decreased with increasing salinity treatments (0–13.5 g L−1), with minimal change in rate constants occurring in higher salinity treatments (e.g. 20.3 or 26.8 g L−1), while Hg(II)RED increased with salinity. In lower salinity treatments, DOM was more photoactive. Taken together, results suggest changes in the mercury photoreduction mechanism from DOM-bound electron transfer to photochemically produced secondary reduction products with increasing salinity. Experiments examining photooxidation showed decreases in Hg (0) with longer exposure time, suggesting transformation of Hg(II)RED into a non-reducible form. This research highlights the importance of salinity and DOM interactions in estuarine surface water and their effects on mercury photochemistry. [Display omitted] •Photoreduction of divalent mercury (Hg(II)) and can lessen both mercury concentrations and methylation potential in ecosystems.•Photoreduction rates ranged between 0.22 h−1 and 0.73 h−1 and photoreducible mercury (Hg(II)RED) between 67.2 and 265.9 pg.•Photoreduction rates decreased with increasing salinity (0–13.5 g L−1), with minimal change at higher salinities, while Hg(II)RED increased with salinity. The influence of salinity on mercury photoreactions in estuaries is not well studied. This study reports interactions between salinity and DOM, potentially inhibiting Hg(II) photoreduction in estuaries.