Quantifying the effects of photoreactive dissolved organic matter on methylmercury photodemethylation rates in freshwaters

Abstract

The present study examined potential effects of seasonal variations in photoreactive dissolved organic matter (DOM) on methylmercury (MeHg) photodemethylation rates in freshwaters. A series of controlled experiments was carried out using natural and photochemically preconditioned DOM in water collected from 1 lake in June, August, and October. Natural DOM concentrations doubled between June and August (10.2-21.2 mg C L ) and then remained stable into October (19.4 mg C L ). Correspondingly, MeHg concentrations peaked in August (0.42 ng L ), along with absorbances at 350 nm and 254 nm. Up to 70% of MeHg was photodemethylated in the short 48-h irradiation experiments, with June having significantly higher rates than the other sampling months (p < 0.001). Photodemethylation rate constants were not affected by photoreactive DOM, nor were they affected by initial MeHg concentrations (p > 0.10). However, MeHg photodemethylation efficiencies (quantified in moles MeHg lost/moles photon absorbed) were higher in treatments with less photoreactive DOM. Congruently, MeHg photodemethylation efficiencies also decreased over summer by up to 10 times across treatments in association with increased photoreactive DOM, and were negatively correlated with DOM concentration. These results suggest that an important driver of MeHg photodemethylation is the interplay between MeHg and DOM, with greater potential for photodemethylation in freshwaters with more photobleached DOM and lower DOM content. Environ Toxicol Chem 2017;36:1493-1502. © 2016 SETAC.

Challenges and opportunities for managing aquatic mercury pollution in altered landscapes

“…2). Augmented NOM transport from watersheds as a function of harvesting, in addition to being a potential vector for Hg transport, may also impact photochemical transformations and thus the pools of inorganic Hg (O’Driscoll et al 2004 ) and MeHg (Klapstein et al 2017 ) in lake waters.…”

Accumulation of Methylmercury in the High-Altitude Lake Uru Uru (3686 m a.s.l, Bolivia) Controlled by Sediment Efflux and Photodegradation

“…One cannot exclude higher activation of demethylating bacteria at higher temperatures [62]. In addition, a positive correlation was found between DOC concentrations and percentages MMHg/THg (R = 0.66, p < 0.01) during the dry season, supporting the notion that rising DOC concentrations decreased the efficiency of MMHg photodegradation [63,64]. Although DOC concentration was lower during the wet season, the absence of a relationship between MMHg and SW temperature or DOC for the filtered fraction suggests that photodemethylation was not a primary sink for MMHg during this season.…”

Photo-oxidation of Micro- and Nanoplastics: Physical, Chemical, and Biological Effects in Environments

“…The 3 MNP* can transfer energy to dissolved O 2 and water molecules to produce other ROS such as 1 O 2 and O 2 •– , accelerating photo-oxidation of aromatic MNPs. ,, However, in the real environments, MNPs do not undergo photo-oxidation in isolation but rather in the presence of various environmental components, such as inorganic ions, natural colloids, and dissolved organic matter (DOM). Under UV irradiation, these coexisting components can also undergo photochemical processes, absorbing photons and either consuming or producing ROS. , For example, the abundant chromophores in DOM can absorb UV energy, which forms higher energy excited states ( 3 DOM*) that can initiate reactions with dissolved oxygen and water molecules, resulting in the production of ROS through energy transfer. , Mediating by these photochemical processes, the coexisting components can either accelerate or inhibit the photo-oxidation process of MNPs, as illustrated in Table S3 (highlighting critical factors influencing the photo-oxidation of MNPs) and Figure .…”

“… 25 , 113 For example, the abundant chromophores in DOM can absorb UV energy, which forms higher energy excited states ( 3 DOM*) that can initiate reactions with dissolved oxygen and water molecules, resulting in the production of ROS through energy transfer. 114 , 115 Mediating by these photochemical processes, the coexisting components can either accelerate or inhibit the photo-oxidation process of MNPs, as illustrated in Table S3 (highlighting critical factors influencing the photo-oxidation of MNPs) and Figure 2 .…”