Evaluation of cation exchange membrane performance under exposure to high Hg⁰ and HgBr₂ concentrations

• Main Authors: M. B. Miller, S. M. Dunham-Cheatham, M. S. Gustin, G. C. Edwards
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-02-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://www.atmos-meas-tech.net/12/1207/2019/amt-12-1207-2019.pdf
• ISSN: 1867-1381; 1867-8548

<p>Reactive mercury (RM), the sum of both gaseous oxidized Hg and particulate bound Hg, is an important component of the global atmospheric mercury cycle, but measurement currently depends on uncalibrated operationally defined methods with large uncertainty and demonstrated interferences and artifacts. Cation exchange membranes (CEMs) provide a promising alternative methodology for quantification of RM, but method validation and improvements are ongoing. For the CEM material to be reliable, uptake of gaseous elemental mercury (GEM) must be negligible under all conditions and RM compounds must be captured and retained with high efficiency. In this study, the performance of CEM material under exposure to high concentrations of GEM (<span class="inline-formula">1.43×10⁶</span> to <span class="inline-formula">1.85×10⁶</span>&thinsp;pg&thinsp;m<span class="inline-formula">⁻³</span>) and reactive gaseous mercury bromide (<span class="inline-formula">HgBr₂</span> <span class="inline-formula">∼5000</span>&thinsp;pg&thinsp;m<span class="inline-formula">⁻³</span>) was explored using a custom-built mercury vapor permeation system. Quantification of total permeated Hg was measured via pyrolysis at 600&thinsp;<span class="inline-formula">^∘</span>C and detection using a Tekran^® 2537A. Permeation tests were conducted over 24 to 72&thinsp;h in clean laboratory air, with absolute humidity levels ranging from 0.1 to 10&thinsp;g&thinsp;m<span class="inline-formula">⁻³</span> water vapor. GEM uptake by the CEM material averaged no more than 0.004&thinsp;% of total exposure for all test conditions, which equates to a non-detectable GEM artifact for typical ambient air sample concentrations. Recovery of <span class="inline-formula">HgBr₂</span> on CEM filters was on average 127&thinsp;% compared to calculated total permeated <span class="inline-formula">HgBr₂</span> based on the downstream Tekran^® 2537A data. The low <span class="inline-formula">HgBr₂</span> breakthrough on the downstream CEMs (&lt;&thinsp;1&thinsp;%) suggests that the elevated recoveries are more likely related to suboptimal pyrolyzer conditions or inefficient collection on the Tekran^® 2537A gold traps.</p>