Research Paper
Sonochemical oxidation and stabilization of liquid elemental mercury in water and soil

https://doi.org/10.1016/j.jhazmat.2022.130589Get rights and content

Highlights

  • Sonochemical oxidation and stabilization of liquid elemental mercury [Hg(0)l] are evaluated in water and soil.

  • Sonication promotes the breakup and oxidation of Hg(0)l beads via acoustic cavitation.

  • Sonication coupled with polysulfide prevents Hg(0)l solubilization and volatilization loss.

  • Polysulfide is more effective than sulfide in sonochemical oxidation of Hg(0)l and formation of HgS nanocrystals.

  • Sonochemical treatment offers a potentially effective means for rapidly converting Hg(0)l to HgS in water and soil.

Abstract

Over 3000 mercury (Hg)-contaminated sites worldwide contain liquid metallic Hg [Hg(0)l] representing a continuous source of elemental Hg(0) in the environment through volatilization and solubilization in water. Currently, there are few effective treatment technologies available to remove or sequester Hg(0)l in situ. We investigated sonochemical treatments coupled with complexing agents, polysulfide and sulfide, in oxidizing Hg(0)l and stabilizing Hg in water, soil and quartz sand. Results indicate that sonication is highly effective in breaking up and oxidizing liquid Hg(0)l beads via acoustic cavitation, particularly in the presence of polysulfide. Without complexing agents, sonication caused only minor oxidation of Hg(0)l but increased headspace gaseous Hg(0)g and dissolved Hg(0)aq in water. However, the presence of polysulfide essentially stopped Hg(0) volatilization and solubilization. As a charged polymer, polysulfide was more effective than sulfide in oxidizing Hg(0)l and subsequently stabilizing the precipitated metacinnabar (β-HgS) nanocrystals. Sonochemical treatments with sulfide yielded incomplete oxidation of Hg(0)l, likely resulting from the formation of HgS coatings on the dispersed µm-size Hg(0)l bead surfaces. Sonication with polysulfide also resulted in rapid oxidation of Hg(0)l and precipitation of HgS in quartz sand and in the Hg(0)l-contaminated soil. This research indicates that sonochemical treatment with polysulfide could be an effective means in rapidly converting Hg(0)l to insoluble HgS precipitates in water and sediments, thereby preventing its further emission and release to the environment. We suggest that future studies are performed to confirm its technical feasibility and treatment efficacy for remediation applications.

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

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