Research PaperSonochemical oxidation and stabilization of liquid elemental mercury in water and soil
Graphical Abstract
Introduction
Mercury (Hg) emissions and releases from contaminated sites contribute significantly to the global Hg budget and continue to spread through atmospheric and hydrological cycles. Over 3000 Hg-contaminated sites exist worldwide due to the use of liquid metallic Hg [Hg(0)l] in industrial processes or the conversion of Hg ore to metallic Hg(0)l in thermal processes (Dickson et al., 2019, Kocman et al., 2013, Miller et al., 2015). Soils in these contaminated sites often contain high amounts of elemental Hg(0)l (Bavec et al., 2014, Dominique et al., 2007, Guedron et al., 2009, Kocman et al., 2013, Miller et al., 2015, Miller et al., 2013), which, if left untreated, could pose a great threat to the health of ecosystems and human beings. It is estimated that an average of about 198 (137−260) Mg·yr−1 Hg(0) is emitted to the atmosphere from these contamination sites (Kocman et al., 2013), resulting in a serious global environmental concern due to long distance transport of gaseous elemental Hg(0)g (Ariya et al., 2015, Demers et al., 2013, Gonzalez-Raymat et al., 2017, Lamborg et al., 2014). Moreover, following atmospheric Hg(0)g oxidation and deposition, the oxidized form of Hg [Hg(II)], as well as the dissolved elemental Hg(0)aq in water, could be transformed to the potent neurotoxin methylmercury (MeHg) (Parks et al., 2013, Hu et al., 2013, Gilmour et al., 2013, Yu et al., 2013), which is known to bioaccumulate and biomagnify in aquatic food webs. Therefore, the United Nations Environment Programme (UNEP) in 2013 enacted the Minamata Convention (UNEP, 2013), which is now ratified by more than 140 countries (Miller et al., 2015), aimed at protecting human health and the environment from anthropogenic emissions and releases of Hg.
To achieve the goal of the Minamata Convention is, however, challenging due to continued emissions and releases of Hg(0) from these legacy Hg contamination sites. For example, at the Oak Ridge Y-12 National Security Complex, historical use of liquid Hg(0)l resulted in large deposits of Hg(0)l in the soils (Miller et al., 2015, Miller et al., 2013). It is estimated that about 11 million kg of Hg(0)l was used at the Y-12 complex between 1950 and 1963, and about 193,000 kg of Hg(0)l was lost to the soils and continue to be a source of Hg to the atmosphere and nearby aquatic systems (Brooks and Southworth, 2011). While conventional remedial technologies, such as soil excavation and stabilization, have been used to minimize Hg(0) emissions and releases, currently there are no effective treatment technologies available to remove or immobilize Hg(0)l in soil due to its high mobility, volatility, and hazardous nature of liquid Hg(0)l.
Sonochemical engineering is a known technology and involves the application of sonic and/or ultrasonic waves to promote chemical reactions and mass transfer. During this process, ultrasound irradiation leads to acoustic cavitation (i.e., the formation, growth, and collapse of bubbles) producing transient intense localized heating and high pressure, which can drive many chemical reactions, such as oxidation, reduction, decomposition, or polymerization (Adewuyi, 2001, Pokhrel et al., 2016, Thompson and Doraiswamy, 1999). Previous studies have explored sonication-assisted synthesis of Hg-thiolate crystals directly from liquid Hg(0)l (Pokroy et al., 2010) or the synthesis of cinnabar (α-HgS) and metacinnabar (β-HgS) nanoparticles from oxidized Hg(II) (Kristl and Drofenik, 2008, Wang and Zhu, 2004). More recently, Yang et al. (2020) studied emulsification of liquid Hg(0)l in biphasic water-Hg(0)l mixtures and observed the formation of grey Hg(OH)2 precipitates upon exposure to intense sonication. Sonochemical treatments of magnetic tea biochar were also found to improve the thermal stability of biochar to capture gaseous Hg(0)g from syngas (Altaf et al., 2021). These previous studies suggest that sonication technology could be useful in breaking up liquid Hg(0)l and promote its oxidation, although no studies so far have investigated the potential application of the technology in removing and stabilizing liquid Hg(0)l in water and soil.
The present study was therefore aimed at investigating and evaluating a sonochemical engineering approach for oxidizing and stabilizing liquid Hg(0)l in water and soil and thus to minimize its environmental impact. Our specific objectives were: (1) Determine the efficacy of sonication for oxidizing and stabilizing Hg(0)l in water in the presence or absence of complexing agents, such as potassium polysulfide and sodium sulfide, (2) evaluate whether or not the sonochemical approach could be applied to oxidize and stabilize Hg(0)l in solids (e.g., quartz sand and soil), and (3) examine sonochemical reaction products and precipitates of Hg via scanning electron microscope (SEM) imaging and Raman spectroscopic analyses. Additionally, we monitored the headspace gaseous Hg(0)g, dissolved Hg(0)aq and the oxidized Hg(II)aq in solution, as well as particulate Hg during sonochemical treatments to provide new insights into sonication-assisted oxidation and sequestration of Hg(0)l.
Section snippets
Chemical reagents, soil samples and preparations
Potassium polysulfide (K2SX) was purchased from Sigma-Aldrich, and sodium sulfide (Na2S) was obtained from Alfa Aesar. All chemicals are certified analytical grade reagents and were used without further purification. An aqueous dissolved elemental Hg(0)aq stock solution was prepared by submerging a silicone tube containing a small droplet of metallic Hg(0)l in deoxygenated deionized (DI) water, as previously described (Zheng et al., 2019, Zheng et al., 2013). The Hg(0)g vapor diffuses through
Sonication effects on liquid Hg(0)l beads in water
The effectiveness of sonochemical oxidation and stabilization of Hg(0)l was assessed by the analyses of headspace gaseous Hg(0)g, dissolved Hg(0)aq and Hg(II)aq in solution, and the particulate Hg in suspension (Fig. 1). In control samples without sulfide or polysulfide, headspace gaseous Hg(0)g in the sonicated samples markedly increased from 0.6 ± 0.01 to 11.6 ± 0.7 µg·L−1 after sonication for 6 h (Fig. 1A). This is in contrast to unsonicated samples, in which the headspace Hg(0)g remained
Mechanistic considerations of Hg(0)l oxidation and stabilization by sonochemical engineering
This research clearly demonstrates the efficacy of sonochemical treatments in breaking up liquid Hg(0)l beads and oxidative precipitation of Hg, particularly in the presence of complexing agents, such as polysulfide. Even in the absence of complexing agents, intense sound fields could break up Hg(0)l beads and cause partial oxidation of Hg(0)l and formation of Hg(OH)2 or HgO precipitates, as previously observed (Harika et al., 2021, Yang et al., 2020). The mechanism of Hg(0)l oxidation upon
Conclusions
The present study offers a potential remediation approach to effectively sequester Hg(0)l in Hg(0)l-contaminated water and soils via sonochemical reactions. Sonication coupled with polysulfide is highly effective in breaking up and oxidizing liquid Hg(0)l beads via acoustic cavitation and subsequently immobilizing it as HgS precipitates. However, in the absence of complexing agents (polysulfide or sulfide), Hg(0)l beads were not completely oxidized or precipitated as Hg(OH)2 or HgO but mostly
Environmental implication
Over 3000 mercury (Hg)-contaminated sites worldwide contain liquid elemental Hg(0)l representing a continuous source of Hg in the environment, but currently there are few effective treatment technologies available to remove or sequester Hg(0)l in situ. This research demonstrates that sonication with polysulfide is among the most effective approach resulting in nearly complete oxidation of Hg(0)l and its precipitation as metacinnabar (β-HgS) crystals. Our study provides novel insights in
CRediT authorship contribution statement
Hongxia Du: Investigation, Data curation, Writing - original draft. Xin Gu: Investigation, Data curation, Writing - review & editing. Baohua Gu: Supervision, Conceptualization, Writing - review & editing. Alexander Johs: Data curation, Methedology. Xiangping Yin: Data curation, Methedology. Tyler Spano: Data curation, Methedology. Dingyong Wang: Methedology, Resources. Eric Pierce: Funding acquisition, Writing - review & editing
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
A portion of this research was supported by the Office of Groundwater and Soil Remediation, Office of Environmental Management, U.S. Department of Energy (DOE) as part of the Applied Field Research Initiative Program at the Oak Ridge National Laboratory (ORNL). Additionally, the research was sponsored by the Office of Biological and Environmental Research within the DOE Office of Science, as part of the Critical Interfaces Science Focus Area project at ORNL. H.D. was supported by the
References (53)
- et al.
History of mercury use and environmental contamination at the Oak Ridge Y-12 Plant
Environ. Pollut.
(2011) - et al.
Silver/quartz nanocomposite as an adsorbent for removal of mercury (II) ions from aqueous solutions
Heliyon
(2019) - et al.
Elemental mercury: Its unique properties affect its behavior and fate in the environment
Environ. Pollut.
(2017) - et al.
Mercury speciation in a tropical soil association; consequence of gold mining on Hg distribution in French Guiana
Geoderma
(2009) - et al.
Sustainable existence of solid mercury (Hg) nanoparticles at room temperature and their applications
Chem. Sci.
(2021) - et al.
Application of multiple stable mercury isotopes to determine the adsorption and desorption dynamics of Hg(II) and MeHg to sediments
Mar. Chem.
(2004) - et al.
Characterization of iron oxide nanoparticle films at the air-water interface in Arctic tundra waters
Sci. Total Environ.
(2018) - et al.
Contribution of contaminated sites to the global mercury budget
Environ. Res
(2013) - et al.
Sonochemical synthesis of nanocrystalline mercury sulfide, selenide and telluride in aqueous solutions
Ultrason Sonochem.
(2008) - et al.
Formation of metacinnabar by milling of liquid mercury and elemental sulfur for long term mercury storage
Sci. Total Environ.
(2010)
The application and potential artifacts of Zeeman cold vapor atomic absorption spectrometry in mercury stable isotope analysis
Environ. Sci. Tech. Let.
Characterization of soils from an industrial complex contaminated with elemental mercury
Environ. Res
Sonochemistry: science and engineering
Ultrason Sonochem.
Diffusion tests of mercury through concrete, bentonite-enhanced sand and sand
J. Hazard Mater.
A sonochemical method for the selective synthesis of alpha-HgS and beta-HgS nanoparticles
Ultrason Sonochem.
Acoustic cavitation generates molecular mercury(ii) hydroxide, Hg(OH)(2), from biphasic water/mercury mixtures
Chem. Sci.
Isotope exchange between mercuric [Hg(II)] chloride and Hg(II) bound to minerals and thiolate ligands: Implications for enriched isotope tracer studies
Geochim. Cosmochim. Ac
Competitive exchange between divalent metal ions [Cu(II), Zn(II), Ca(II)] and Hg(II) bound to thiols and natural organic matter
J. Hazard Mater.
Transport and interactions of kaolinite and mercury in saturated sand media
J. Hazard Mater.
Sonochemistry: environmental science and engineering applications
Ind. Eng. Chem. Res
Effect of sonochemical treatment on thermal stability, elemental mercury (Hg-0) removal, and regenerable performance of magnetic tea biochar
ACS Omega
Mercury physicochemical and biogeochemical transformation in the atmosphere and at atmospheric interfaces: a review and future directions
Chem. Rev.
Thermal behaviour of cinnabar, alpha-HgS, and the kinetics of the beta-HgS (metacinnabar) → alpha-HgS conversion at room temperature
Eur. J. Miner.
Urban sediment contamination in a former Hg mining district, Idrija, Slovenia
Environ. Geochem Health
Sulfur-limonene polysulfide: a material synthesized entirely from industrial by-products and its use in removing toxic metals from water and soil
Angew. Chem. Int Ed. Engl.
Mercury isotopes in a forested ecosystem: implications for air-surface exchange dynamics and the global mercury cycle
Glob. Biogeochem. Cycles
Cited by (1)
Dissolved Elemental Mercury [Hg(0)<inf>aq</inf>] Reactions and Purgeability in the Presence of Organic and Inorganic Particulates
2023, Environmental Science and Technology Letters