Mercury cycling

Mercury provides an excellent example of the importance of metal speciation in understanding biogeochemical cycling and the impact of human activities on these cycles. Mercury exists in solid, aqueous, and gaseous phases, and is transported among reservoirs in all these forms. It undergoes precipitation-dissolution, volatilization, complexation, sorption, and biological reactions, all of which alter its mobility and its effect on exposed populations. The effect of all... 

During depressurization, pores which commenced filling at point D begin to empty at point F and at lower pressures pores that filled between points B and C begin to empty at G. At point H on the extrusion curve the cycle is terminated. The intrusion-extrusion cycle does not close when the initial pressure is reached indicating that some mercury has been permanently entrapped by the sample. Often, at the completion of an intrusion-extrusion cycle mercury will slowly continue to extrude, sometimes for hours. 

Mason RP. Mercury emissions from natural processes and their importance in the global mercury cycle. Mercury Fate and Transport in the Global Atmosphere. 2009, pp. 173-191. 

Kaufmann J., R. Loser and A. Leemann (2009). Analysis of cement-bonded materials by multi-cycle mercury intrusion and nitrogen sorption . Journal of Colloid and Interface Science 336 730-737. 

A typical example, from the extensive study by Kamakin on an alumina-silica gel, is shown in Fig. 3.32. When the mercury pressure was reduced to 1 atm at the end of the first cycle, 27 per cent of the intruded mercury was retained by the sample a second intrusion run followed a different path from the first, whereas the second extrusion curve agreed closely with the first. Change in f re structure of the kind described above could perhaps account for the difference between the two intrusion curves, but could not explain the reproducibility of the remainder of the loop. There is no doubt that hysteresis can exist in the absence of structural change. 

Volatilization is also a dominant transport mode for mercury, which is the most volatile metal in its elemental state. As with lead, a key reaction that can increase the volatility of mercury is formation of an organometallic compound. In this case, the reactions take place in water and are primarily biological, being mediated by bacteria commonly found in the upper levels of sediments. These reactions and their importance in the global mercury cycle are discussed in some detail later in the chapter. 

Fig. 15-8 The mercury cycle, demonstrating the bioaccumulation of mercury in fish and shellfish. Reprinted with permission from An Assessment of Mercury in the Environment" (1978) by the National Academy of Sciences, National Academy Press, Washington, DC.

In this final section, the global cycles of two metals, mercury and copper, are reviewed. These metals were chosen because their geochemical cycles have been studied extensively, and their chemical reactions exemplify the full gamut of reactions described earlier. In addition, the chemical forms of the two metals are sufficiently different from one another that they behave differently with respect to dominant... 

Fig. 15-16 The (a) present day and (b) pre-industrial global cycles for mercury. Units are 10 g Hg (burdens) and 10 g Hg/yr (fluxes). Hgp refers to mercury in particles. Redrawn from Mason et al, 1994.

Mason, R. P., Fitzgerald, W. F. and Morel, F. M. M. (1994). The biogeochemical cycling of elemental mercury Anthropogeruc influences. Geochim. Cosmochim. Acta 58, 3191-3198. 

FIGURE 1.1 Conceptual diagram of mercury cycling and bioaccumulation in the environment. 

Policy makers would benefit from a combination of strong field evidence of trends and well-established models to draw upon when assessing the benefits of past or future policy decisions. Models of mercury cycling and bioaccumulation are not yet adequately predictive across a range of conditions and landscapes. Results from a national mercury monitoring program, if carefully designed, offer the potential to... 

Mercury in soil is not only likely to have a different potential for evasion and methylation than Hg in runoff, but soil Hg may be perturbed by land disturbance. Land disturbances that are particularly relevant to Hg cycling include the formation of wetlands and flooding of reservoirs (Rudd 1995 see Chapter 3). Disturbances such as clear-cutting can also result in marked increases in the release of THg and MeHg from soils (Munthe and Hultbeig 2003 Porvari et al. 2003). Fire can result in large Hg losses by volatilization (Grigal 2002). 

Hudson RJM, Gherini SA, Watras CJ, Porcella DB. 1994. Modeling the biogeochemical cycle of mercury in lakes the Mercury Cycling Model (MCM) and its application to the MTL study lakes. In Watras CJ, Huckabee JW, editors. Mercury pollution integration and synthesis. Boca Raton (FL) Lewis Publishers, CRC Press Inc., p. 473-523. 

Johnson DW, Lindberg SE. 1995. Sources, sinks, and cycling of mercury in forested ecosystems. Water Air Soil Pollut 80 1069-1077. 

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