I would also like to thank the Sally McDonnell Barksdale Honors College for giving me the opportunity and the push to participate in a research project during my undergraduate career. In addition, the funding from the Honors College has been very helpful in conducting my research, and I am extremely grateful for that. This research focuses on studies developing methods to measure the dry deposition of mercury (Hg) using an ion exchange membrane (IX) to capture gaseous mercury species in the air.
While routine methods have been developed to study wet deposition of Hg, measuring dry deposition of Hg is more problematic and often overlooked. We tested several different analytical methods to measure the Hg on the membranes, including atomic absorption spectrometry, atomic fluorescence spectrometry, and mass spectrometry. The aim was to estimate the relative rate of dry deposition of Hg in order to investigate differences in the levels of Hg found in fish from these lakes.
We hypothesized that point sources near Lake Grenada, including a coal-fired power plant, may result in higher Hg deposition rates, which may account for the higher Hg levels observed in fish from Lake Grenada compared to the other lakes. However, the results show that Sardis Lake had the highest dry deposition rates, followed by Lakes Enid and Grenada.
The Biogeochemical Cycle of Mercury
GOM and PBM are removed from the atmosphere via dry deposition much faster than GEM due to their surface/reactive properties (Zhang et al., 2012). Methylmercury is a neurotoxin that biomagnifies in the food chain and is easily absorbed in the gastrointestinal tract. Consumption of fish with high MeHg levels is harmful to humans and other wildlife (Costley et al., 2000).
Measuring Dry Deposition of Mercury
Another approach to measuring dry mercury deposition is to measure airborne mercury species (GEM, GOM, and PPE) using a commercially available automated system from Tekran Corporation. The system determines GEM at approximately 5 minute intervals, and GOM and PBM at approximately 2-3 hour intervals, reflecting the different levels of these species in the air. However, unlike the IX method, the system is expensive and has logistical challenges (e.g., power requirements, maintenance, etc.) that make it difficult to compare deposition rates at multiple locations.
Prior research and purpose of this study
The objectives of this study were to (1) improve field and laboratory (analytical) methods for estimating dry deposition of mercury using a cation exchange membrane and (2) apply the methods to compare dry deposition of mercury at Sardis, Enid, and Grenada Lakes to test the above hypothesis.
Sampling Sites
Deploying Cation-Exchange Membranes for Capturing Airborne Mercury
Experimental set-up for setting up cation exchange membranes in the field (above) and the set-up for early experiments atop Anderson Hall (below). Membrane strips hang inside bottles that have holes in the bottom to allow gas exchange.
Background on the Analytical Instrumentation Used
Elemental mercury vapor is then fed to a tube containing gold-coated sand and forms an amalgam with gold. The trap is heated and mercury is then pulsed through a single beam spectrometer. The mercury concentration is then calculated using the Beer-Lambert law by the absorbance measured at 253.7 nm.
Compared to AAS, AFS is more sensitive and has lower detection limits because it has a lower background signal. The gold traps are heated to release the trapped mercury, which is then delivered to the fluorescent cell by a stream of ultra-high purity argon. The mercury is excited with a mercury lamp and off-axis fluorescence is detected with a photomultiplier tube.
Both DMA and Tekran 2600 use mercury's unique properties (high vapor pressure, tendency to amalgamate with gold) to measure mercury. One of the advantages of ICP-MS over other instruments is that it can be used to determine Hg isotopes. Ions and plasma gas pass through the core of the sampler, where the plasma is centered, into the vacuum pumping region.
Most of the argon gas is pumped away from the area and the remaining ions pass through the simulator cone into the mass. Ion lenses are used to introduce ions into a mass spectrometer, where the ions are counted and a plot of the intensity versus m/z ratio can be generated. Mass spectra are relatively simple and elements can be easily identified from mass and isotopic ratios.
Analytical Methods
- Analysis of cation exchange membranes using the DMA
- Analysis of cation exchange membranes using DMA-ICPMS
- Analysis of Dry Deposition of Mercury via Leaching of CEM
- Analysis of Dry Deposition of Mercury Using Tekran 2700
- Methods to lower blank levels on the membranes prior to deployment
Another set of samples was deployed in the same manner as before: samples were suspended in the polycarbonate bottles with plastic clips for bidirectional capture of gaseous oxidized mercury species. To reduce blank concentrations, blanks were sealed in Ziploc bags and placed in polycarbonate containers. Membranes were placed in 15 ml tubes and leached with 10 ml of 1% HCl and 0.2 M BrCl solutions.
The vials were then sealed and Hg was injected from the vial through a needle into the CVAFS-based instrument.
Preliminary Experiment
Membranes deployed at the lakes and analyzed by the Direct Mercury Analyzer
However, the smaller sections gave inconsistent results, perhaps due to the smaller absolute concentrations of Hg. Interestingly, regardless of membrane size, a general spatial trend emerged with samples from Sardis Lake yielding the highest concentration of Hg per membrane range, Grenada the lowest and Enid somewhere in between.
DMA-ICPMS Analysis of Membranes and time-series experiments
Either the isotope data was collected under the wrong mass value or there was an isobaric interference for the lighter isotope arising from the sample membranes (matrix effect) that contributed to its signal. Although data for 202Hg appear to be unaffected, this phenomenon still deserves attention and may be considered for future work. Also, sometimes during DMA analysis of the membrane there was a popping sound indicating small explosions as the membrane burned, which could be a safety concern.
Analysis of Dry Deposition of Mercury by Leaching of Membranes
Analysis of Dry Deposition of Mercury with Tekran 2700
34;Determination of total mercury in crude oil by combustion cold vapor atomic absorption spectrometry (CVAAS).". Measurements of atmospheric mercury species (TGM, TPM) and Hg deposition in the Silesian region, Poland – concept of the mercury deposition coefficient." . An experiment was done to see if heating the membranes could drive away the mercury on the membrane.
Another experiment was done to see if washing the cation exchange membrane with hydrochloric acid could get rid of some of the mercury on the membrane. The membrane was passed through the DMA at low temperatures to drive off any retained Hg in an attempt to lower blank levels. The results showed that this approach was effective in lowering the Hg on the membrane, but if the membrane was placed too close to the hot catalyst, it became carbonized.
The membrane was tested with argon gas flowing through the DMA instead of oxygen because it was believed that argon prevented combustion with the membrane. The membrane also contained smaller amounts of mercury each time it passed through the DMA when using argon. A UV lamp was used to determine whether gaseous elements could be expelled by exposure to UV light.
For the experiment, the cation exchange membrane was suspended in a test tube with a drop of liquid mercury for about 90 minutes. After exposure to liquid mercury, the membrane was exposed to UV light for one hour. Although exposure to UV light was thought to leach elemental mercury in the gaseous state, the results show that the mercury content of the membrane actually increased after exposure to UV light.
The UV light experiment was performed again to confirm the results of the first UV light experiment. In the second experiment with UV light, one membrane was used as a blank and three were exposed to gaseous elemental mercury vapor. One group was analyzed immediately, while the other was then exposed to UV light.
Unlike the first experiment where the lamp was placed over the folded membrane, the second experiment had the UV lamp parallel to the membrane and each side of the membrane was exposed to UV light for one hour. The results from the second experiment agreed with the results of the first experiment: exposure to UV light actually increased the amount of mercury in the membranes.
