Impacts of Sediment Particle Grain Size and Mercury Speciation on Mercury Bioavailability Potential

Abstract

Particle-specific properties, including size and chemical speciation, affect the reactivity of mercury (Hg) in natural systems (e.g., dissolution or methylation). Here, terrestrial, river, and marine sediments were size-fractionated and characterized to correlate particle-specific properties of Hg-bearing solids with their bioavailability potential and measured biomethylation. Marine sediments contained ∼20−50% of the total Hg in the <0.5 μm size fraction, compared to only 0.5 and 3.0% in this size fraction for terrestrial and river sediments, respectively. X-ray absorption spectroscopy (XAS) analysis indicated that metacinnabar (β-HgS) was the main mercury species in a marine sediment, whereas organic Hg-thiol (Hg(SR) 2 ) was the main mercury species in a terrestrial sediment. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis of the marine sediment suggests that half of the Hg in the <0.5 μm size fraction existed as individual nanoparticles, which were β-HgS based on XAS analyses. Glutathione-extractable mercury was higher for samples containing Hg(SR) 2 species than β-HgS species and correlated well with the amount of Hg biomethylation. This particle-scale understanding of how Hg speciation and particle size affect mercury bioavailability potential helps explain the heterogeneity in Hg methylation in natural sediments.

Extraction and Quantification of Nanoparticulate Mercury in Natural Soils

“…44 This is critical for a more accurate quantification of particle number concentration and size distribution. Further progress is likely from the novel approaches such as time-of-flight mass spectrometry that could detect multiple elements per particle (Xu et al 1 ).…”

“…7,8 However, evidence for the exposure concentrations of indigenous Hg-NPs in complex soil matrices is limited. Very recently, Xu et al 1 have shown that half of the Hg in the <0.5 μm size fraction of a contaminated marine sediment presents as individual mercury sulfide nanoparticles; yet, the number concentrations of indigenous Hg-NPs in natural soils remain largely unknown. The absence of these data hampers reliable risk assessments of Hg-NPs in environmentally relevant scenarios, including their actual role in MeHg production.…”

“…22−24 An important issue of spICP-MS analysis is the extraction of NPs from soil solid matrices. Indeed, recent studies have employed various extraction methods (e.g., ultrapure water, sodium nitrate, and tetrasodium pyrophosphate) 1,17,19 ; yet, the lack of data validation across different soil types or the lack of comparability for data among different analytical techniques (e.g., transmission electron microscopy or X-ray absorption spectroscopy) makes the measured values challenge for the risk assessment. 18 Therefore, it is beneficial to develop a standardized method for Hg-NP extraction that is suitable for subsequent spICP-MS analysis across soils with a wide range of properties.…”

“…Particulate mercury, one of the most abundant inorganic mercury forms, is ubiquitous in various environmental matrices in oxic and anoxic settings. − Past efforts on particulate mercury have relied on methylation potentials using spiked particles. − These particles offer a wide range of reactivity and availability to methylators in pure cultures of bacteria, soils, and sediments, − due to their differences in particle size, crystal structure, dissolution, sorption, aggregation, and particle–cell interactions. − Especially, particulate mercury smaller than 100 nm, referred to as Hg-NPs, are of great concern due to their relatively high methylation potentials when compared to micro- and bulk Hg. , However, evidence for the exposure concentrations of indigenous Hg-NPs in complex soil matrices is limited. Very recently, Xu et al have shown that half of the Hg in the <0.5 μm size fraction of a contaminated marine sediment presents as individual mercury sulfide nanoparticles; yet, the number concentrations of indigenous Hg-NPs in natural soils remain largely unknown. The absence of these data hampers reliable risk assessments of Hg-NPs in environmentally relevant scenarios, including their actual role in MeHg production.…”

Particle-Bound Hg(II) is Available for Microbial Uptake as Revealed by a Whole-Cell Biosensor

“…In natural waters, a large amount of total Hg (THg) (approximately 20–95%) is bound with suspended particles, including inorganic minerals and particulate organic matter. − Traditionally, particles are considered as an important sink of Hg in aquatic ecosystems, considering the generally believed low bioavailability of particle-bound Hg­(II) (Hg P ). , Recently, however, growing evidence has shown that Hg P species are also available for microbial uptake and methylation, accounting an important source for aqueous MeHg in lakes and marine environments. − For instance, field studies have suggested that the Hg-methylation potential of settling particles are comparable to or higher than those of surface sediments and are probably mediated by anaerobic methylators in the anoxic microzones of particles . Laboratory simulations further confirmed that Hg P species could be taken up by Hg methylators, and methylation rates were significantly higher than Hg sulfide and even dissolved Hg­(II) (Hg D ). , Considering the large storage of Hg P in aquatic environments (e.g., in water column and sediment) and the rapid integration of MeHg generated from Hg P into food webs, clarifying the bioavailability of Hg P is of critical importance for understanding the environmental fate and ecological risks of Hg.…”