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ISSUES RELATED TO SOLUTION CHEMISTRY IN MERCURY SAMPLING IMPINGERS
Citation:
Linak*, W P., J V. Ryan*, S. B. Ghorishi, AND J. L. Wendt. ISSUES RELATED TO SOLUTION CHEMISTRY IN MERCURY SAMPLING IMPINGERS. JOURNAL OF AIR & WASTE MANAGEMENT ASSOCIATION 51(5):688-698, (2001).
Description:
Analysis of mercury (Hg) speciation in combustion flue gases is often accomplished in standardized sampling trains in which the sample is passed sequentially through a series of aqueous solutions to capture and separate oxidized Hg (Hg2+) and elemental Hg (Hgo). Such methods include the Ontario Hydro (OH) and the Alkaline Mercury Speciation (AMS) methods, which were investigated in the laboratory to determine whether the presence of chlorine gas (Cl2) and other common flue gas species can bias the partitioning of Hgo to front impingers intended to isolate Hg2+ species. Using only a single impinger to represent the front three impingers for each method, it was found that as little as 1 PPM Cl2 in a simulated flue gas mixture led to a bias of approximately 10 to 20% of Hgo rnisreported as Hg2+ for both the OH and the AMS methods. Experiments using 100 ppm Cl2 led to a similar bias in the OH method, but to a 30 to 60% bias in the AMS method. These false readings are shown to be due to liquid-phase chemistry in the impinger solutions, and not necessarily to the gas-phase reactions between Cl2 and Hg that have been previously proposed. The pertinent solution chemistry causing the interference involves the hypochlorite ion (OCI-), which oxidizes Hgo to soluble Hg2+. Addition of sodium thiosulfate (Na2S203)to the front impinger solutions eliminates this false positive measurement of Hg2+ by selectively reacting with the OCI- ion. In general, the presence of sulfur dioxide (SO2) also mitigates this interference in the same way, and so this bias is not likely to be a factor for Hg speciation measurements from actual coal combustion flue gases. It might, however, be a problem for those few combustor flue gas measurements and research studies where Cl2 is present without appreciable amounts of SO2.