Atmospheric cycling of mercury and other pollutants has become a major concern as industrialized countries have eliminated point discharges, sometimes by relocating the industries to underdeveloped and developing countries where point sources have become problems. Such circumventions may be to no avail in the long run as pollution continues to elevate levels of methylmercury in fish in waterways that are remote from direct contamination as well as where the source can be readily identified. Much has been learned about the cycling of mercury in the environment since human disabilities and deaths at Minamata, Japan, initially drew attention to the problem of methylmercury poisoning from the consumption of contaminated seafood in the 1950s. In that instance, methylmercury produced industrially concentrated to toxic levels in fish. As this manufacturing process was not used outside Japan, concern did not become immediate in other developed nations until the 1960s when it was established that mercury was not only biomethylated by microorganisms but also biomagnified through the food chain. Point sources to the waterways may have been eliminated too late to return the levels in fish to background because of the geochemical cycling of mercury through the environment. Despite decreases in domestic, industrial and agricultural point source releases over the last two decades, large quantities from non-point sources such as fossil fuel combustion, smelters, and incinerators are still being released. Much of this mercury is transported from the atmosphere to aquatic ecosystems and stored in sediments until it is again released to the water and atmosphere.
Figure 10. Fraction of sulfur dioxide removed vs. moles of ozone generated during the same residence time Solid line represents A(SOz) = (03); dashed line is least-squares fit. Residence times = 2.5 min, Al 1-92 min, 1-25 min, A1 0.63 min, ; 0.42 min, •; 0.32 min, The nucleation process was not studied in this work, but since air humidified by a bubbling process contains water nuclei and since the metal wire cathode is continuously ejecting metal particles, the presence of condensation nuclei may enhance the overall rate of droplet formation and ultimate size of the deposited droplets. This is not expected to be a ratelimiting step, however, since S03 is strongly self-nucleating in the presence of water vapor, and proceeds at rates far greater than the S02 oxidation step.
ConclusionsThe rate of oxidation of sulfur dioxide in humid air in the presence of a corona discharge is zero order with respect to S02 in the range 500-3000 ppm. The reaction was strongly dependent on oxygen and the oxidation rate diminished for Oz concentrations below 14%. By measuring the rate of ozone formation in the absence and presence of SO», it appears that the rate-controlling mechanism for the SO» oxidation lies in the breakdown of the oxygen molecule by the corona discharge to form atomic oxygen. Apparently the S02-0 reaction is several orders of magnitude faster than the 02-0 reaction. About 75 % of the S02 removed precipitated in the form of sulfuric acid mist on the wall of the reactor. Studies of the precipitated droplets indicate a mean size of 6.36 µ before deposition. Deposition patterns were consistent for the entire length of the reactor suggesting that aerosol movement was almost exclusively in the radial direction. Oxides of nitrogen at 50 ppm showed no measurable effect on the sulfur dioxide reaction rate. Literature Cited American Society for Testing and Materials, "ASTM Standards, Part 23. Industrial Waters; Atmospheric Analysis," pp 824-6, March 1964.
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