Research results
Development and application of mercury vapor measurement system using quartz crystal microbalance method
National Institute for Minamata Disease (○Masumi Marumoto, Koji Marumoto), The National Institute of Advanced Industrial Science and Technology (Kazutoshi Noda), International Mercury Lab (Hirokatsu Akagi)
1. Introduction.
Artisanal and Small Scale Gold Mining (ASGM) is the largest source of anthropogenic mercury emissions on the planet. ASGM is currently practised in more than 70 countries around the world, where more than 10 million workers are estimated to be employed, and as the mercury amalgamation process is often used to refine gold in ASGM, there is concern about the health effects on workers of the vapour mercury generated during the process. Many workplaces do not take pollution prevention measures, such as equipment to remove the mercury vapour generated, and monitoring of vapour mercury concentrations and exposure to workers has not been carried out. Reasons for the lack of easy monitoring of vapour mercury concentrations include the high cost of equipment and the difficulty of measurement methods. To date, a simple measuring device for vapour mercury (QCM-Hg) using the Quartz Crystal Microbalance (QCM) method has been developed for inexpensive and easy monitoring of vapour mercury concentrations. The QCM-Hg has been examined for validity at levels below 1 µg/m3 , the working environment standard specified by WHO. However, the reality is that in actual ASGM, workers often work under mercury concentrations in the air that exceed the standard values. Therefore, in this study, the effectiveness of this QCM-Hg at concentrations of 1 µg/m3 or higher, especially as a passive sampler, was investigated. In addition, the results of measurements at an ASGM site in Brazil will also be presented in this presentation.
2. Chamber experiments
The QCM-Hg, fitted with a quartz crystal (leaded element, 20 MHz), is capable of measuring temperature, humidity and air pressure as well as mercury, and is powered by a button cell battery. Three QCM-Hg units were installed at the bottom and ceiling of the chamber, respectively. A portable mercury measuring device (Nippon Instruments, EMP-2Hi) was connected to the chamber to directly measure the mercury concentration in the chamber. Metallic mercury was used as the source of mercury vapour and the mercury concentration in the chamber was exposed for one hour at an average mercury concentration of 1-600 µg/m3 and a flow rate of 0.8 L/min to accommodate the different environments at ASGM. Mercury adsorbed on the elements was analysed by heat vaporisation-gold amalgam-atomic absorption spectrophotometry (Nippon Instruments, MA-2).
The amount of frequency fluctuation differed depending on the position of the installed QCM-Hg. The amount of oscillation frequency variation correlated with the amount of mercury adsorbed on the element, suggesting that the mercury concentration varied depending on the location, even within the same chamber. The results of the performance evaluation of the QCM-Hg fitted with a quartz crystal unit showed that at mercury concentrations exceeding 200 µg/m3, the oscillation frequency saturates within a few minutes to several tens of minutes. In addition, as the QCM-Hg was used as a passive sampler, higher humidity in the environment tended to increase the oscillation frequency.
3. Measurements at the Brazilian ASGM site
Airborne mercury concentrations were measured using QCM-Hg and EMP2-Gold in a gold shop and surrounding households in Itaituba, Para, Brazil, in October 2018. Although the gold shop was equipped with a draft, the airborne mercury concentration in the shop was a few µg/m3 and when the amalgam started to burn in the draft, the airborne mercury concentration in the shop increased to about 35 µg/m3 The QCM-Hg oscillation frequency varied well corresponding to the airborne mercury concentration at the work site.
4. Summary
The chamber experiments and the results of the measurements at the Brazilian ASGM site showed that the QCM-Hg can be used in environments above 1 µg/m3 and that changes in frequency can be captured over time at air mercury concentrations up to tens of µg/m3. However, at mercury concentrations above 200 µg/m3 , it was found that the oscillation frequency saturated within a few minutes to several tens of minutes. In addition, because it was used as a passive sampler, the vibration frequency tended to increase when the humidity in the environment was high, but compared to active samplers, it was expected to be lightweight and inexpensive to use because it did not use a pump or power supply.
Acknowledgements.
This research was funded by the Comprehensive Environmental Research Promotion Fund of Environmental Restoration and Conservation Agency (No. 5-1704). We would like to thank Mr Iracina, Director of the Environmental Department, Mr Marcelo and his staff at the Instituto Evandro Chagas, Brazil, and Mr Yurizawa, Portuguese interpreter, for their assistance in presenting this study. We also thank Ms Satoko Furukawa, The National Institute of Advanced Industrial Science and Technology; Ms Mao Uchikado and Ms Miwa Chijiiwa, Department of Basic Medical Science, National Institute for Minamata Disease (NIMD), and Ms Shigemi Onizuka, Ms Fumika Hashimoto and Ms Akane Morimoto, Department of Environment and Public Health (NIMD).