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
Diminished mercury emission from waters with duckweed cover
[1] Duckweeds (Lemnaceae) are a widely distributed type of floating vegetation in freshwater systems. Under suitable conditions, duckweeds form a dense vegetative mat on the water surface, which reduces light penetration into the water column and limits gas exchange at the water-air interface by decreasing the area of open water surface. Experiments were conducted to determine whether duckweed decreases mercury emission by limiting gas diffusion across the water-air interface and attenuating light, or, conversely, enhances emission via transpiration of mercury vapor. Microcosm flux chamber experiments indicate that duckweed decreases mercury emission from the water surface compared to open water controls. Fluxes under duckweed were 17–67% lower than in controls, with lower fluxes occurring at higher percent cover. The decrease in mercury emission suggests that duckweed may limit emission through one of several mechanisms, including limited gas transport across the air-water interface, decreased photoreactions due to light attenuation, and plant-mercury interactions. The results of this experiment were applied to a model lake system to illustrate the magnitude of potential effects on mercury cycling. The mercury retained in the lake as a result of hindered emission may increase bioaccumulation potential in lakes with duckweed cover.
A rapid method for estimating mercury in undigested biological samples
Magos, L., and Cernik, A. A. (1969).Brit. J. industr. Med.,26, 144-149. A rapid method for estimating mercury in undigested biological samples. A method has been developed for estimating mercury in biological samples using an ultraviolet mercury detector, after liberating the mercury from the urine and other biological samples by a simple chemical method. The overall time for one estimation is less than two minutes, and no special skills are required to operate the technique. The principle of the method is that stannous chloride at high pH liberates mercury from sulphhydryl bonds and converts the liberated mercury into volatile metallic mercury which is subsequently estimated by an ultraviolet photometer. Stannous chloride is added to the acidified sample, and the presence of extra cysteine ensures that no appreciable amount of mercury is lost before the reaction is triggered by the addition of excess sodium hydroxide. The standard solution of mercury developed is stable for months and behaves in the reaction like urine, and, because of its stability, the instrument can be standardized with it frequently and conveniently.