The Solubility of Elemental Mercury in Water

The solubility of elemental mercury (Hg') at temperatures between 30 and 210°C was determined by direct sampling of mercury saturated water contained in a fixed volume stainless steel autoclave. The temperature dependence of the solubility was best represented by the equation log,^^, = -11.879 + 0.01206T where xHg is the mole fraction of dissolved Hgo and T is the temperature in degrees Kelvin. At temperatures less than 100°C the best literature values are in good agreement with these results. At higher temperatures it is difficult to compare our data with previous studies as they show widely divergent results. Comparison of field results for volatile mercury steam/water partitioning showed that up to 10' times more mercury was retained in solution than would have been indicated from the theoretical distribution coefficients calculated from the solubility data. A kinetic limitation to the mercury steadwater distribution in the Webre separator was the most likely explanation although the presence of other nonvolatile mercury species could not be discounted. INTRODUCTION The ability to predict the transport and partitioning behaviour of elemental mercury (Hg') during exploitation of geothermal reservoirs and power production requires reliable data for the vapour/liquid distribution coefficients. The distribution coefficient is derived from Henry's law constant which can be calculated from the solubility of elemental mercury. This study was undertaken because the large differences in Hgo solubility values between the few published experimental studies at temperatures above 100°C made it difficult to interpret field data collected at the Ohaaki geothermal field (Timperley and Mroczek 1989). The importance of elemental mercury is that it appears to be the dominant chemical form of mercury found in chemical surveys of wells at New Zealand geothermal fields. This is in agreement with previous studies (Robertson et al. 1978) which showed that most if not all the mercury present in geothermal effluent is Hg', even at high H,S gas levels. Chemical modelling (Varekamp and Buseck 1984) has also shown that mercury transport occurs largely as Hgo in dilute hydrothermal systems 'with moderate amounts of S (<0.01 mol/litre). PREVIOUS WORK At temperatures less than 100°C there have been numerous experimental solubility studies using a variety of measurement techniques. These studies also show a wide variation in values. The reason for the disagreement is not clear but trace contamination by air oxygen or not enough time allowed to attain the equilibrium solubility are commonly suggested reasons. Clever et al. (1985) reviewed and evaluated all available data and presented smoothed solubility data and smoothed Henry's constant equations over the temperature interval 273.15-393.15 K. The only high temperature study included in their evaluation was that of Sorokin (1973). Sorokin measured the solubility of mercury in water at temperatures of 573, 673 and 773 K at total pressures between 507 and 1013 bar. Samples were extracted from a flexible reaction cell autoclave at constant pressure and temperature. Clever et al. (1985) combined the hypothetical Henry's constant at the higher temperatures extrapolated to 1 bar with the Henry's constants calculated from the lower temperature data (277-346 K) of Clew and Hames (1971) to obtain an equation for Henry's constant over the temperature interval 393-773 K. Varekamp and Buseck (1984) also used the thermodynamic parameters calculated from Clew and Hames's (1971) data as well as Sorokin's (1973) data to calculate the solubility of Hgo to 250°C. Recently Gushchina et al. (1989) determined the solubility of mercury using a spectrophotometric technique at temperatures of 150,200 and 250°C as well as evaluating previous work. Their results as well as those of Sorokin (1973), Sorokin's more recent experimental results and calculations as reported by Gushchina et al., Varekamp and Buseck's (1984) extrapolations, the smoothed literature interpolations of Clever et al. (1985) as well as selected low temperature