Roles of chloride ion in photo-reduction/oxidation of mercury

Transformation of Mercurous [Hg(I)] Species during Laboratory Standard Preparation and Analysis: Implication for Environmental Analysis

Hg(I) may control Hg redox kinetics; however, its metastable nature hinders analysis. Herein, the stability of Hg(I) during standard preparation and analysis was studied. Gravimetric analysis showed that Hg(I) was stable in its stock solution (1000 mg L-1), yet completely disproportionated when its dilute solution (10 μg L-1) was analyzed using liquid chromatography (LC)-ICPMS. The Hg(I) dimer can form through an energetically favorable comproportionation between Hg(0) and Hg(II), as supported by density functional theory calculation and traced by the rapid isotope exchange between 199Hg(0)aq and 202Hg(II). However, the separation of Hg(0) and Hg(II) (e.g., LC process) triggered its further disproportionation. Polypropylene container, increasing headspace, decreasing pH, and increasing dissolved oxygen significantly enhanced the disproportionation or redox transformations of Hg(I). Thus, using a glass container without headspace and maintaining a slightly alkaline solution are recommended for the dilute Hg(I) stabilization. Notably, we detected elevated concentrations of Hg(I) (4.4-6.1 μg L-1) in creek waters from a heavily Hg-polluted area, accounting for 54-70% of total dissolved Hg. We also verified the reductive formation of Hg(I) in Hg(II)-spiked environmental water samples, where Hg(I) can stably exist in aquatic environments for at least 24 h, especially in seawater. These findings provide mechanistic insights into the transformation of Hg(I), which are indicative of its further environmental identification.

Origin and partitioning of mercury in the polluted Scheldt Estuary and adjacent coastal zone

Estuaries and coastal zones are areas with complex biogeochemical and hydrological cycles and are generally facing intense pollution due to anthropogenic activities. An emblematic example is the Scheldt Estuary which ends up in the North Sea and has been historically heavily contaminated by multiple pollutants, including mercury (Hg). We report here Hg species and their levels in surface waters of the Scheldt Estuary and the Belgian Part of North Sea (BPNS) from different sampling campaigns in February-April 2020 and 2021. Along the estuary, Hg concentration on suspended particles ([Hg_SPM]) progressively decreased with increasing salinity and was strongly correlated with organic matter content (%C_org) and origin (identified with δ¹³C_org). While [Hg_SPM] drives total Hg concentration in the estuary (total dissolved Hg, Hg_TD is only 7 ± 6 %), annual and daily variations of total Hg levels were mostly attributed to changes in SPM loads depending on river discharge and tidal regime. In the BPNS, a significant fraction of total Hg occurs as Hg_TD (40 ± 21 %) and the majority of this Hg_TD was reducible (i.e. labile Hg), meaning potentially available for microorganisms. Compared to the '90s, a significant decrease of [Hg_SPM] was observed in the estuary, but this was not the case for [Hg_TD], which can be due to (1) still significant discrete discharges from Antwerp industrial area, and (2) higher Hg partitioning towards the dissolved phase in the water column relative to the '90s. Our results highlight the important contribution of the Scheldt estuary for the Hg budget in North Sea coastal waters, as well as the need for seasonal monitoring of all Hg species.

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

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.

Effects of typical algae species (Aphanizomenon flosaquae and Microcystis aeruginosa) on photoreduction of Hg2+ in water body

Photoreduction characteristics of divalent inorganic mercury (Hg²⁺) in the presence of specific algae species are still not well known. Laboratory experiments were conducted in the present study to identify the effects of different concentrations of living/dead algae species, including Aphanizomenon flosaquae (AF) and Microcystis aeruginosa (MA), on the photoreduction rate of Hg²⁺ under various light conditions. The experimental results showed that percentage reduction of Hg²⁺ was significantly influenced by radiation wavelengths, and dramatically decreased with the presence of algae. The highest percentage reduction of Hg²⁺ was induced by UV-A, followed by UV-B, visible light and dark for both living and dead AF, and the order was dark > UV-A > UV-B > visible light for both living and dead MA. There were two aspects, i.e., energy and attenuation rate of light radiation and excrementitious generated from algae metabolisms, were involved in the processes of Hg²⁺ photoreduction with the presence of algae under different light conditions. The percentage reduction of Hg²⁺ decreased from 15% to 11% when living and dead AF concentrations increased by 10 times (from 10⁶ to 10⁵ cells/mL), and decreased from11% to ~9% in the case of living and dead MA increased. Algae can adsorb Hg²⁺ and decrease the concentration of free Hg²⁺, thus inhibiting Hg²⁺ photoreduction, especially under the conditions with high concentrations of algae. No significant differences were found in percentage reduction of Hg²⁺ between living and dead treatments of algae species. The results are of great importance for understanding the role of algae in Hg²⁺ photoreduction.

Role of Free Radicals/Reactive Oxygen Species in MeHg Photodegradation: Importance of Utilizing Appropriate Scavengers

A variety of free radicals (FR)/reactive oxygen species (ROS) have been proposed to dominate methylmercury (MeHg) photodegradation, primarily based on the results of FR/ROS scavenger addition experiments. However, in addition to eliminating FR/ROS, the added scavengers may also affect the experimental results by altering some water chemical properties, resulting in a misleading assessment of the importance of FR/ROS. In this study, 20 common FR/ROS scavengers were evaluated in terms of their influence on light absorbance, pH, MeHg analysis, MeHg-dissolved organic matter (DOM) complexation, and the scavenger-induced degradation of MeHg. Only nine scavengers were identified to be appropriate for investigating MeHg photodegradation. By utilizing these appropriate scavengers, direct photodegradation of MeHg-DOM complexes was found to be the major pathway of MeHg photodegradation in Laoshan Reservoir water and Stone Old Beach seawater. In contrast, MeHg photodegradation in Ink River water primarily occurs through both ·OH and ³DOM* mediated indirect pathways and direct photodegradation of MeHg-DOM complexes. The diverse pathways of MeHg photodegradation in the tested water may be due to differences in water chemical properties. A severe overestimation of the role of FR/ROS was observed when several improper but commonly used scavengers were adopted, highlighting the necessity of utilizing appropriate scavengers.