Procedimiento metodológico para la estimación

The current consensus on GEM to GOM oxidation is that it occurs as a two-stage process[12]

Hg + X → HgX (1.10)

HgX + Y → HgXY (1.11)

where X / Y could be any oxidant such as a halogen, NOx, ClO and BrO. An interesting

characteristic of mercury chemistry is that the first Hg-X is significantly weaker than the second XHg-X bond. Consider the reactions

Hg + Cl → HgCl (1.12)

HgCl + Cl → HgCl2 (1.13)

using reference data for Hg (ΔH0f = 14.67) ref [19], Cl (ΔH0f = 28.99) ref [19], HgCl (ΔH0f =

18.75) ref [19], and HgCl2 (ΔH0f = -34.96) ref [19] the heats of reaction for Equation (1.12)

and Equation (1.13) are -24.91 and -82.7 kcal/mol, respectively. Considering -dHrxn as the bond dissociation energy (BDE), then clearly the second bond XHg–X formed is significantly stronger than the first Hg-X bond. This is because the first HgX species formed is a radical whereas the second XHgX species is a closed shell molecule. This also means that the limiting factor to forming a stable (GOM) species is determined by the bond strength of the first Hg–X bond formed. As a rule of thumb, the BDE of the first

Hg-X bond be should be greater than 10 kcal/mol in order for the HgX species to exist long enough to react with another species X or Y. Additional criterion for the first stage oxidant X include availability near mercury emissions sites and regions where GEM is located, and ideally, a low reactivity of X with non-mercury species.

It is generally accepted that atmospheric mercury oxidation is thought to occur from Br or Cl as the first stage oxidant[12]. Hydroxyl (OH) could be another oxidant, but the Hg-OH BDE is on the order of 10 to 14 kcal/mol making HgOH a relatively short- lived species. In coal fueled power plants (CFPP) environments, where the temperatures are upwards of 600 K, F sources could be an important stage oxidant as F forms the strongest Hg–X bond among all the halogens (see Table 3.13 in Chapter 3). Cao et al. studied the impact of halogen addition on GEM oxidation in CFPP environments[34]. In their study, the authors doped the flue gas with either HF, HBr, HCl, or HI as a halogen source. The results of this study are presented in Figure 1.6. Note, every HX dataset in Figure 1.6 is an independent experiment.

Figure 1.6 Effect of HX on the GEM oxidation rate at T = 620K.

Note: Figure is adapted Cao et al. Source: [34].

Between the halogens added, Br is the most efficient at oxidizing GEM on a ppm basis. Relative to Cl, the behavior of F in the oxidation of GEM similar. At the elevated CFPP temperatures, the Cao et al. study shows empirical evidence for the oxidation of GEM by F. The mechanism by which F oxidizes GEM is not yet understood. It is likely that the initial HgF species is formed via heterogeneous pathway.

Another potential GOM formation pathway could be via an insertion reaction, such as

Hg + X2 → XHgX 1.14

where X is any halogen species. Table 1.1 shows the calculated heats of reaction for reactions (1.14) and (1.15).

Table 1.1 Reactions Enthalpies for the Hg + X2 / OX Insertion a

Reaction ΔHrxn (kcal/mol) Reactions with X2 Hg + F2 → HgF2 -84.85 Hg + Cl2 → HgCl2 -49.63 Hg + Br2 → HgBr2 -35.09 Hg + I2 → HgI2 -18.53 Reactions with OX Hg + OF → OHgF -44.35 Hg + OCl → OHgCl -21.34 Hg + OBr → OHgBr -18.48 Hg + OI → OHgI -10.37

a _{Heats of formation for Hg, X2, OCl, OHgX, and HgX2 are in Tables 3.2 and 3.3 in Chapter 3 of this} dissertation.

Based on the thermochemistry, reactions (1.14) and (1.15) are exothermic and could potentially be a plausible GOM formation pathway. Unlike the free radical reactions (1.10) and (1.12), reactions (1.14) and (1.15) have significant free energy barriers. Prior computational studies have shown the insertion of Hg + Br2 / Cl2 /I2 to

have barriers in the range of 40 kcal/mol[35], making these gas-phase reactions highly unlikely. To the best of our knowledge, there are no studies where the (1.15) insertion reaction barriers have been calculated or experimentally determined.

A third possible GOM formation pathway could be a heterogeneous or interfacial reaction pathway between mercury and a surface. The term surface is loosely defined here and could represent an a water droplet, snow, ice, a soot particle, or the walls of a

reactor. Reactions that are endothermic in the gas phase could be catalyzed by a surface or interface. Rigorous theoretical calculations for accurate heterogenous or interfacial reaction predictions, including a transition state prediction, would be best with an ab initio molecular dynamics (AIMD)[36] approach.

The AIMD approach uses a molecular dynamics algorithm to move the molecules in space using Newton’s equations of motion and density functional theory (DFT) to determine the electronic interactions. Even with DFT, these calculations are very computationally expensive for even the smallest of systems. Adding to the complexity is the requirement of including relativistic corrections due to the atomic size (mass) of mercury. There are also very few experimental data sets available for mercury chemistry so even if the calculations can be properly executed, it would be difficult to gauge their accuracy. Although AIMD calculations are cutting edge and could potentially be used for heterogenous mercury oxidation-reduction reactions, it is outside the current scope of this work.