Sequestration of Subsurface Elemental Mercury

Background:

Mercury pollution is a growing concern due to its neurological health effects and vast prevalence in the environment.  Mercury has sparked much interest in the past decade, resulting in an increase of research and articles.  It is listed as one of EPA’s priority persistent bio-accumulative toxins (PBTs) and is among the top five metals that most frequently exceeds ecological screening criteria list (Salatas, Lowney et al. 2004).  In order to protect our natural environment and human health, mercury contamination must be controlled and eliminated.

Mercury can undergo complex physical and chemical transformations, which determine its reactivity, mobility, and bioaccumulation.  Methyl mercury (MeHg) is a highly toxic mercury species and can bioaccumulate, making it a primary environmental concern.  Soluble inorganic mercury species, such as mercuric chloride (HgCl₂), are easily mobilized and are the most common forms of mercury that become methylated.  Both methyl mercury and soluble inorganic mercury contribute to the major portion of mercury contamination in water, soil, and living organisms.  Elemental mercury is a less toxic species because it is less mobile, relatively unreactive, and has little tendency to dissolve in water.  The least toxic mercury species is mercuric sulfide (HgS) because it is kinetically stable in the soil.  Two types of HgS(s) exist including metacinnabar, the black form, and cinnabar, the red form; both of which have very low solubility product constants (Morel, Kraepiel et al. 1998; Han, Kingston et al. 2003). 

Mercury contamination originates from both natural and man-made sources.  Natural sources of mercury are mainly found in mineral deposits, while man-made contamination results primarily from coal-fired power plants and chlor-alkali plants   In an effort to remediate the problem, contaminated soils have been excavated and incinerated.  However, this is often not economically feasible and can result in mercury air emissions (Piao and Bishop 2006).  Furthermore, disturbing the soils often causes the contamination to spread. An alternative solution is in situ immobilization of mercury using sulfide minerals.

Sulfide minerals are utilized for several reasons: mercury has a high affinity for sulfide, sulfides are common and economical minerals to extract, and sulfide minerals do not pose an environmental threat (Brown, Bancroft et al. 1979).  Furthermore, immobilization of mercury by sulfides is currently among the most widely used techniques for removal of inorganic mercury from wastewater (Piao and Bishop 2006). 

Immobilization of mercury can occur by both precipitation and sorption (Morel and Hering 1993).  When precipitation occurs, the result is a substitution reaction in which mercury reacts with the metal sulfide, replacing the metal, to form mercuric sulfide, HgS(s).  Due to the exceptionally low solubility of HgS(s), the reaction of mercury with a metal sulfide mineral is thermodynamically favorable and would immobilize any dissolved Hg.  Additionally, HgS(s) is less volatile than other forms of mercury, and thus potentially less harmful.  The example below shows the reaction of mercury with FeS(s) to form HgS(s):

Hg(OH)₂° + FeS(s) + 2 H⁺ ↔ HgS(s) + Fe²⁺ + H₂O      K = 10⁴⁰

The weathering of HgS(s) by simple (non-oxidative) dissolution is thermodynamically restricted due to its strong insolubility:

HgS(s) + 2H2O ↔ Hg(OH)2° + H2S    K = 10^-38

However, HgS(s) is thermodynamically unstable in the presence of dissolved oxygen and other oxidants found in nature because of the existence of reduced sulfur, which results in dissolution:

HgS(s) + 2O2(aq) + 2H2O ↔ Hg(OH)2° + SO4^2-  + 2H⁺    K = 10⁹³

To the degree HgS(s) can kinetically withstand oxidation, it may perpetuate in the environment after it is formed, even under oxidizing conditions (Barnett, Turner et al. 2001).

Mercury sorption occurs when the Hg sorbs onto the surface of the sulfide mineral to form surface complexes, removing the mercury from an aqueous solution.  Sorption takes place because mercury is a soft Lewis acid and thus has a strong affinity for ligands containing sulfur (Ehrhardt, Behra et al. 2000; Behra, Bonnissel-Gissinger et al. 2001).  When the mercury is sorbed to the mineral, it is highly insoluble and stable over a wide range of pH values and oxidizing conditions  (Porter, Scheckel et al. 2004). The most common explanation for Hg sorption onto sulfide minerals is by the formation of HgS.  X-ray photoelectron spectroscopy (XPS) studies have also shown that several other weakly and strongly bound species are formed including Hg-chloro and Hg-sulfhydryl complexes  (Hyland, Jean et al. 1990; Behra, Bonnissel-Gissinger et al. 2001).