Volatilization and sorption of dissolved mercury by metallic iron of different particle sizes: Implications for treatment of mercury contaminated water effluents

doi:10.1016/j.jhazmat.2014.05.054

Journal of Hazardous Materials

Volume 276, 15 July 2014, Pages 408-414

https://doi.org/10.1016/j.jhazmat.2014.05.054 Get rights and content

Highlights

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Volatilization and adsorption of Hg from aqueous solutions by Fe⁰.
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Hg volatilization accounts only for a very small fraction.
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Temporal changes in the SSA of Fe⁰ particles are a function of water chemistry.
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Nano-size Fe⁰ particles show higher rates of Hg removal than bulk Fe⁰ particles.

Abstract

Batch experiments were conducted to investigate the interactions between metallic iron particles and mercury (Hg) dissolved in aqueous solutions. The effect of bulk zero valent iron (ZVI) particles was tested by use of (i) granular iron and (ii) iron particles with diameters in the nano-size range and referred to herein as nZVI. The results show that the interactions between Hgⁿ⁺ and Fe⁰ are dominated by Hg volatilization and Hg adsorption; with Hg adsorption being the main pathway for Hg removal from solution. Hg adsorption kinetic studies using ZVI and nZVI resulted in higher rate constants (k) for nZVI when k values were expressed as a function of mass of iron used (day⁻¹ g⁻¹). In contrast, ZVI showed higher rates of Hg removal from solution when k values were expressed as a function iron particles’ specific surface area (g m⁻² day⁻¹). Overall, nZVI particles had a higher maximum sorption capacity for Hg than ZVI, and appeared to be an efficient adsorbent for Hg dissolved in aqueous solutions.

Graphical abstract

Introduction

Heavy metals are introduced to aquatic systems from both anthropogenic and natural sources, but unlike organic pollutants, metals released to the environment are not biodegradable and can undergo transformations that affect their potentials for bioaccumulation in food chains and toxicity to living organisms. Although much is now known on the biogeochemistry of several trace metals including mercury (Hg), research on the development of cost-effective and environment-friendly remediation techniques remains challenging. The now well-established effects of Hg on aquatic organisms and human health have led US regulatory agencies to contemplate stringent guidelines for Hg levels in waste gaseous and aqueous effluents. For instance, low Hg concentrations ranging from 0.012 to 0.015 μg/L are now being targeted for wastewater effluents [1]. Unfortunately, current methods for remediation of metal-contaminated environmental matrices often fail to meet the action limit levels imposed or targeted by federal or state regulatory agencies. In addition, the commercial use of available techniques remains limited due primarily to their prohibitive costs.

A wide variety of sorbents have been tested in studies focusing on the removal of pollutants from aqueous solutions. One example is metallic iron or zero valent iron (ZVI) particles, which have been in use as an alternative to the pump and treat method for groundwater remediation. ZVI is both readily available and inexpensive. In fact, the current increasing trend in the use of zero-valent iron nanoparticles (referred to thereafter as nZVI) finds its origin in past intensive and still ongoing use of ZVI in the remediation of groundwater contaminated with recalcitrant organic pollutants [2], [3]. The use of granular ZVI in permeable reactive barrier (PRBs) started several years ago [2], [4], [5], and has been found to be effective for the treatment of many organic pollutants such as volatile organic carbons (VOCs) in polluted ground waters [2], [3], [6]. Besides the organic pollutants, ZVI has also been used to treat waters contaminated with toxic metals [7], [8], [9], [10], [11], [12]. In a laboratory study, Wilkin and McNeil used ZVI to remove trace metals from synthetic acid mine solutions and were able to decrease the initial total-Hg (THg) concentration of ∼3100 μg/L to values <70 μg/L [13]. Additionally, the use of ZVI-packed columns to treat a mercury contaminated groundwater reduced the initial influent THg concentration of ∼40 μg/L down to 0.168 μg/L in the effluent [14]. This value is still about one order of magnitude greater than the targeted 0.012–0.015 μg/L [1].

The use of nZVI in remediation can therefore be considered as an extension of the ZVI technology, and could provide several advantages related to physicochemical characteristics specific to nanosize particles. These characteristics include the distinctive catalytic and chemical properties associated with the large surface-to-volume ratio of nano-size materials; which can lead to interesting and surprising surface and quantum effects. nZVI could be used as alternative or supplement to the conventional ZVI-PRB technology. For instance, injections of nZVI slurries targeting heavily contaminated source areas or “hot spots” could add to the efficiency of traditional ZVI-PRBs that function as barriers to contain the dispersion of contaminants [15].

This study focuses on the mechanisms of interactions of Hg and metallic iron as a function of particle size and water chemical composition. In fact, the interaction of metallic iron and dissolved Hg results in Hg removal from treated water through a combination of mechanisms that depend upon key factors such as Hg speciation, pH, oxidation–reduction potentials, the presence and types of binding ligands, as well as the presence of competitive cations. Such mechanisms include but are not limited to (i) loss by volatilization following the reduction of ionic Hg to Hg⁰, (ii) adsorption onto solid (hydr)oxides as ZVI surfaces undergo oxidation, and (iii) removal through formation and precipitation of highly insoluble Hg-sulfide species in sulfide rich anaerobic systems. Based on these Hg removal mechanisms, one could speculate that the efficiency of Hg removal by ZVI particles can be improved by increasing the surface area, and therefore, the reactivity of used particles. In this study, metallic iron of different particle sizes (ZVI and nZVI) are used comparatively in laboratory experiments to investigate their ability to remove Hg from aqueous solutions by (i) volatilization and (ii) sorption as a function of solution chemistries.

Section snippets

Iron particle characterization and chemistry of water used in laboratory experiments

ZVI particles with a diameter size range of 1–2 mm were obtained from Alfa Aesar (PA, USA). The nZVI particles were purchased from Quantum Sphere, Inc. (CA, USA) in powder form, and had a particle size range of 15–25 nm as reported by the vendor. No preservation reagent was used, but nZVI particles were maintained under UHP nitrogen atmosphere. The N₂-BET specific surface areas (SSA) of these particles were determined using a Quanto-Chrome NOVA 1200. The particle size distribution (PSD), zeta

Particle characterization and water chemistry

The measured SSA of particles used in batch experiments were 0.116 m²/g and 30.5 m²/g for ZVI and nZVI, respectively. The PSD of used nZVI particles determined by dynamic light scattering (DLS) ranged from 300 to 600 nm and from 500 to 700 nm when suspended without surfactant in DI-water and WW effluent, respectively. Accordingly, the abbreviation nZVI used in this paper refers to iron particles with diameters ranging between 300 and 700 nm, which are higher than values reported by the manufacturer.

Conclusion

From these studies, the interactions between iron and Hg seem to be driven by two main mechanisms: volatilization of dissolved ionic Hg from aqueous solution and adsorption onto oxidized metallic iron surfaces. In batch studies, measured Hg volatilization rates were faster for DI-water based Hg solutions containing nZVI than those with ZVI. When the chemically more complex WW effluent was used, the amount of Hg volatilized from solution was the same in both ZVI- and nZVI-treated waters.

Acknowledgements

This work was supported by the US National Science Foundation (NSF) through a combination of a Graduate Assistant in Areas of National Need (GANN) fellowship and a South East Alliance for Graduate Education and the Professoriate (SEAGEP) scholarship to Julianne Vernon. We thank Veronica Llaneza, Priya Hrenko, Nicholas Kominankis for help with laboratory experiments.

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Volatilization and sorption of dissolved mercury by metallic iron of different particle sizes: Implications for treatment of mercury contaminated water effluents

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Iron particle characterization and chemistry of water used in laboratory experiments

Particle characterization and water chemistry

Conclusion

Acknowledgements

J. Hazard. Mater.

J. Contam. Hydrol.

Sci. Total Environ.

Chemosphere

Sci. Total Environ.

J. Hazard. Mater.

J. Contam. Hydrol.

Water Res.

Water Res.

Sci. Total Environ.

J. Mol. Catal. A: Chem.

Electrochim. Acta

Atmos. Environ.

J. Contam. Hydrol.

Chemosphere

Chem. Eng. Sci.

Chem. Eng. J.

Sep. Purif. Technol.

Analytical Methods for Mercury in National Pollutant Discharge Elimination Systems (NPDES) Permits

Enhanced degradation of halogenated aliphatics by zero-valent iron

Ground Water

Reductive dehalogenation of chlorinated methanes by iron metal

Environ. Sci. Technol.

Biogeochemical dynamics in zero-valent iron columns: implications for permeable reactive barriers

Environ. Sci. Technol.

Iron sulfide minerals at cement oil-field, Oklahoma – implications for magnetic detection of oil-fields

Geol. Soc. Am. Bull.

Long-term performance of zero-valent iron permeable reactive barriers: a critical review

Environ. Eng. Sci.

Coupled iron corrosion and chromate reduction – mechanisms for subsurface remediation

Environ. Sci. Technol.

In-situ remediation of Cr(VI)-contaminated groundwater using permeable reactive walls: laboratory studies

Environ. Sci. Technol.