Phase Allocation | Methylation | Mercury Home
Mercury can assume many forms and, through interactions with the environment, can be transformed into a variety of structures. The most commonly known forms of mercury include: Elemental Mercury, divalent mercury (mercuric chloride) and methyl mercury.
It is thought that stack emissions of mercury commonly consist of both elemental mercury and divalent mercury (U.S. EPA, HHRAP, 2001). Divalent mercury further evolves through a process called methylation when it comes into contact with the surrounding environment. The following sections are derived from the U.S. EPA-OSW, Human Health Risk Assessment Protocol.
Stack emissions are thought to include both vapor and particle-bound forms; and speciated as both divalent and elemental mercury. Based on review of mercury emissions data presented for combustion sources in U.S. EPA (1997d) and published literature (Peterson et al. 1995), estimates for the percentage of vapor and particle-bound mercury emissions range widely from 20 to 80 percent. Therefore, at this time U.S. EPA OSW recommends a conservative approach that assumes phase allocation of mercury emissions from hazardous waste combustion of 80 percent of the total mercury in the vapor phase and 20 percent of total mercury in the particle-bound phase. This allocation is:
- Consistent with mercury emissions speciation data for hazardous waste combustion sources reported in literature (Peterson et al. 1995); and
- Believed to be reasonably conservative, since it results in the highest percentage of total mercury being deposited in proximity to the source, and therefore, indicative of the maximum indirect risk.
As indicated in the global cycle mass percentages in Figure 1, mercury exits the stack in both the elemental and divalent vapor forms. Based on U.S. EPA (1997d), a vast majority of mercury exiting the stack does not readily deposit and is transported outside of the U.S. or vertically diffused to the free atmosphere to become part of the global cycle (see Figure 1). The divalent form emitted, either in the vapor phase or particle?bound, are thought to be subject to much faster atmospheric removal than elemental mercury (Lindberg et al. 1992; Peterson et al. 1995; Shannon and Voldner 1994). In addition, vapor phase divalent mercury is thought to be more rapidly and effectively removed by both dry and wet deposition than particle-bound divalent mercury, as a result of the reactivity and water solubility of vapor divalent mercury (Lindberg et al. 1992; Peterson et al. 1995; Shannon and Voldner 1994).
Vapor Phase Mercury
As illustrated in Figure 1, of the 80 percent total mercury in the vapor phase, 20 percent of the total mercury is in the elemental vapor form and 60 percent of the total mercury is in the divalent vapor form (Peterson et al. 1995). A vast majority (assumed to be 99 percent) of the 20 percent vapor phase elemental mercury does not readily deposit and is transported outside of the U.S. or is vertically diffused to the free atmosphere to become part of the global cycle (U.S. EPA 1997d). Only a small fraction (assumed to be one percent) of vapor-phase elemental mercury either is adsorbed to particulates in the air and is deposited or converted to the divalent form to be deposited (assumed to be deposited as elemental mercury, see Figure 1). Of the 60 percent vapor phase divalent mercury, about 68 percent is deposited and about 32 percent is transported outside of the U.S. or is vertically diffused to the free atmosphere to become part of the global cycle (U.S. EPA 1997d).
Particle-bound Mercury
Of the 20 percent of the total mercury that is particle-bound, 99 percent (assumed to be 100 percent in Figure 1) is in the divalent form. U.S. EPA (1997d) indicates that only 36 percent of the particle-bound divalent mercury is deposited, and the rest is either transported outside of the U.S. or is vertically diffused to the free atmosphere to become part of the global cycle.
Deposition and Modeling of Mercury
Consistent with U.S. EPA (1997d) and as shown in Figure 1, it is assumed that deposition to the various environmental media is entirely divalent mercury in either the vapor or particle-bound form. Without consideration of the global cycle, mercury speciations will result in 80 percent of the total mercury emitted being deposited as divalent mercury and the remaining 20 percent being deposited as elemental mercury.
Figure 1 - Phase Allocation and Speciation
Overview
U.S. EPA recommends utilizing the percentages provided in U.S. EPA (1997d) to account for the global cycle, the percentage of total mercury deposited is reduced to a total of 48.2 percent (40.8 percent as divalent vapor, 7.2 percent as divalent particle-bound, and 0.2 percent as elemental vapor). As discussed in Appendix A?3, these speciation splits result in fraction in vapor phase (Fv) values of 0.85 (40.8/48.2) for divalent mercury, and 1.0 (0.2/0.2) for elemental mercury. Also, to account for the remaining 51.8 percent of the total mercury mass that is not deposited, the deposition and media concentration equations (presented in Appendix B), multiply the compound-specific emission rate (Q) for elemental mercury by a default value of 0.002; and divalent mercury by a default value of 0.48. Media equations assume pseudo steady-state conditions.
Consistent with U.S. EPA (1997d) and as shown in Figure 1, it is assumed that deposition to the various environmental media is entirely divalent mercury in either the vapor or particle-bound form. Therefore, divalent mercury is considered for both the indirect exposure and inhalation pathways. A small fraction (assumed to be one percent) of elemental mercury is in the vapor phase and is assumed to be deposited in its original form. Therefore, elemental mercury will only be considered in the inhalation pathway and not the indirect pathways of the risk assessment. Based on these assumptions, human exposure to (1) elemental mercury occurs only through direct inhalation of the vapor phase elemental form, and (2) divalent mercury occurs through both indirect exposure and direct inhalation of the vapor and particle-bound mercuric chloride.
Inhalation of elemental mercury should be assessed using the reference concentration (RfC) for elemental mercury. Exposure to divalent mercury should be assessed using the RfD for mercuric chloride (divalent mercury). Inhalation of divalent mercury should be assessed using the RfC for elemental mercury due to lack of available toxicity data. Appendix A?3 provides the parameter values specific to the various forms of mercury, and Appendix B provides media concentration equations for modeling mercury through the exposure pathways assuming steady-state conditions.
Note: For references please see the U.S. EPA-OSW Human Health Risk Assessment Protocol.
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