Focus on Mercury - Environmental Analysis typical step extractant description


compounds removed


F1 DI water water soluble F2 pH 2 HCl/acetic acid “stomach acid” F3 F4 F5 1 N KOH 12 N HNO3 Aqua regia organo complexed strong complexed


mercury sulphides


HgCl2 HgO, HgSO4


Hg humics, Hg2Cl2 CH3Hg


mineral lattice, Hg2Cl2


HgS, HgSe


Table 1: Sequential chemical extraction for determining Hg speciation (Bloom et al. 2003).


Sequential extractions are of limited value in determining organic mercury speciation and combinations of extraction, separation and detection instrumentation is usually employed for this purpose. Figure 2 shows the different combinations of Hg extraction/concentration, separation and detection components that can be coupled to provide the sensitivity needed to quantify mercury species.


Total mercury determinations, while useful, provides limited information of the form, and therefore potential toxicity of mercury present in soil and water. Coupling of separation techniques, such as high performance liquid chromatography (HPLC) with ICP-MS or AFS is widely used to separate methyl mercury species from inorganic mercury species and to quantify the amount present.


21


Figure 3: Map of the Kuskokwim River showing Red Devil Case study : Red Devil Mine, Alaska


The challenges of screening a contaminated site are best illustrated by examining a case study. The state of Alaska has a well- characterised mercury mineralisation belt that was mined for most of the 20th century (Gray et al., 1998), of which the most productive mine was Red Devil in the Kuskokwim region of southwest Alaska (Figure 3), which operated from 1937 until 1971, after which it was abandoned. Over 1,224 tonnes of Hg were produced at the site during its operational life. Despite successive remediation efforts, the Red Devil mine site remains heavily contaminated by waste tailings from mining and ore processing and is believed to be a point source for mercury entering the Kuskokwim River.


extracted and measured to assess possible pathways through consumption of vegetation by game animals.


Despite the considerable body of analytical data collected at Red Devil it is extremely difficult to model mercury fluxes from the site, in part because there is no baseline data from before mining in the region Cinnabar and even liquid mercury was noted by early gold prospectors panning streams in the region, so there clearly was a natural Hg flux from the mineralisation. The main transportation mechanism by which mercury is transported into the Kuskokwim River is believed to be erosion of the tailings, with subsequent methylation by bacteria in the organic-rich river sediments. Consequently, analytical investigations have focused on measurement of methyl mercury in fish tissue sampled at different points on the river. However extraction of methyl mercury from tissue is more difficult than from soils and yields must be quantified using certified reference materials.


Figure 2: Hyphenated analytical methods used to separate and measure mercury species


In many studies, the determination of methyl mercury, ethyl mercury and inorganic mercury is adequate to access the bioavailability of mercury at a contaminated site and can be accomplished by selective extraction. Extraction of methyl mercury and ethyl mercury by dichloromethane (DCM) solvent has been successfully used in combination with HPLC-CV-AFS to measure methyl mercury in Hg-contaminated sediments (Chen, 2012) with detection limits around 200 ngL-1


for MeHg reported. Soil Screening


Soils metal levels are normally determined by extraction using acid followed by ICP-MS or ICP-AES and compared to published soil guideline values. Elemental mercury, which may be present at many sites, is highly volatile, so care must be taken to preserve soil samples such as freezing. Soil extractions are a time consuming and expensive method and are also limited by the detection levels of the instruments. Field portable X-ray fluorescence (FPXRF) is being increasingly utilised for in-situ screening of soil, with hand- held instruments available from Bruker GmbH, Thermo-scientific (Niton) and Oxford Instruments, among others. As the analysis time using FPXRF is a few minutes or less, hundreds of measurements can be made during a site investigation. Analysis takes less than a minute, with result immediately available to investigators, allowing the sampling plan to be modified to target anomalous zones. Although measurements can be made with little or no sample preparation, repeatability is greatly improved if the samples are sieved, dried and homogenised. Levels of detection are typically around 5 – 30 ppm for intermediate atomic weight metals and as low as 2 ppm for total mercury. However, it should be noted that at Minimata Bay, mercury levels in sediment rarely exceeded 1 ppm (Kudo & Miyahara, 1991), so FPXRF cannot be used as the sole soil screening method.


In summary, a range of sampling and analytical methods can be used to assess mercury contamination at former industrial and mining sites. Analysis of mercury in soils and water at such sites is complicated by sub-ppb detection levels in instrumentation needed to quantify mercury species and the requirement to properly preserve samples to avoid loss of volatile Hg0


. The choice Figure 4: Drilling a monitoring well at Red Devil. (Photo credit Keith Torrance)


Hg is present at Red Devil as the mineral cinnabar, which is found in quartz veins within a Cretaceous greywacke sequence, in


association with the mineral stibnite (Sb2S3). Liquid mercury was extracted from cinnabar ore at the mine site using a simple furnace/retort to roast the ore and condense Hg vapour sublimated from the ore; at least three generations of retorts operated at Red Devil. Other metals were not recovered from the ore, so waste calcines from the furnace have elevated levels of Hg, As and Sb, with the potential to pollute the environment.


As part of a remedial investigation in 2011, soil samples were collected and water sampled at Red Devil by a contractor working on behalf of the Bureau of Land Management (BLM). The development of a detailed sampling plan is important if the goals of the site investigation are to be met and the possibility of undiscovered contamination excluded. As the site is remote with no road connections, equipment and personnel had to be flown in and samples preserved for weeks before they could be sent to the laboratory for analysis. Water samples were collected from a series of newly installed monitoring wells and Red Devil Creek, which bisects the site, and analysed by ICP-MS for elements of concern and AFS for total mercury.


FPXRF was used to screen soil inaccessible areas of the site that may have been used to dump tailings, delineate sluiceways and locate any ‘hot spots’ for future investigation. Sub-surface soil samples were collected from the drill cuttings and by split spoon recovery and selectively extracted to measure methyl mercury and total mercury (Ecology & Environmental, Inc., 2012). Samples of spruce, alder and blueberry were also collected and methyl mercury


of analytical techniques should reflect expected Hg levels at the site and include speciation to allow a proper assessment of potential toxicity to be made. However, for results to be meaningful, robust quality assurance protocols must be followed and verified by concurrent extraction and analysis of certified reference materials


References


Bacon, J. R. & Davidson, C. M. (2008), 'Is there a future for Sequential Chemical Extraction?', The Analyst 133, 25 - 46.


Chen, B. (2012), 'Speciation of Mercury Species using HPLC-CV-AFS', P S Analytical Ltd.


Cole, S. & Jeffries, J. (2009), 'Using Soil Guideline Values. Science Report SC050021/SGV', Technical Report, Environmental Agency.


Ecology & Environmental, Inc. (2012), 'Draft Remedial Investigation Report. Red Devil Mine, Alaska.', Technical report, Bureau of Land Management., Draft version.


Gray, J. E.; Gent, C. A. & Snee, L. W. (1998), 'The Southwestern Alaska Mercury Belt and its relationship to the circum-Pacific Metallogenic Mercury Province.', Polarforshung 68, 187 - 196.


Kudo, A. & Miyahara, S. (1991), 'A Case History: Minimata Mercury Pollution in Japan - From Loss of Human Lives to Decontamination.', Water Science and Technology. 23(1/3), 283 - 290.


Leopold, K.; Foulkes, M. & Worsfold, P. (2010), 'Methods for the Determination and Speciation of Mercury in Natural Waters - A Review.', Analytica Chimica Acta 668, 127 - 138.


USEPA (1994), 'Method 245.1 Determination of Mercury in Water by Cold Vapor Atomic Absorption Spectroscopy, Revision 3.0.', Technical Report, United States Environmental Protection Agency.


www.envirotech-online.com IET November / December 2012


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