The Usage and Separation of Mercury Species

Mercury (Hg) has been discovered and used by mankind for over 2000 years and has been used as a drug, preservative and fungicide [17]. Mercury mineral Cinnabar (Hg(II) sulphide) was used for decorating temples and palaces for years because of its bright red/orange colour pigment (figure 1) [18]. Little did they know at that time about the severe toxicity of this element! Mercury is a heavy metal that is found in all major departments of our environment, namely air, water, and soil. It exists in three forms, that is, elemental (or metallic) mercury, inorganic mercury compounds, and organic mercury compounds, that have different properties, usage, and toxicity [19]. Specialists on wild and human health are concerned about Mercury, because of its pervasiveness and adverse effects of wildlife and human health, declaring it as the most toxic of all elements (tested on E. coli) [12, 20]. The spread of the anthropogenic sources of Mercury to lakes and lagoons initiated the extensive surveys of Mercury speciation, distribution, and concentrations in water and soil [21-23]. Mercury is crucial also to environmentalists and food specialists. However, not all mercury species have same toxicity level. The bioavailability, toxicology, solubility, and ecological effects largely depend on the speciation of the element and the way each species interact with soil or sediment ingredients [24]. Other surveys validate that the organic matter content and the pH of the soil have a great influence on Mercury species [10, 23]. The most unwanted scenario is the methylation of inorganic mercury.

Sediment is a challenging sample for chemists to extract mercury species due to its complex and never constant matrix. However, the devotion of these scientists made it possible to have a handful methods which can be applied to extract mercury species. Some methods are good for determining total mercury, others focus more on individual species. There quite a lot of instrumental combination to analyse and quantify the mercury species accurately and precisely. For instance, direct mercury analyzer (DMA) [8, 12], atomic fluorescence spectroscopy (AFS) [25-27], atomic absorption spectrometry (AAS) [10, 16], inductively coupled plasma mass spectrometry (ICP-MS) [8, 27-30] are frequently used to analyse total mercury. Cold vapour (CV), high-performance liquid chromatography (HPLC), gas chromatography (GC), are some of the most popular devices coupled with the aforementioned instruments.

In case of analysing individual species, it is often required to separate them prior to analysis. There are several separation methods in practice as well. Starting with the Westöö technique from the 1960s [31], there is hot alkaline digestion [14], solvent extraction [1], steam distillation [32, 33], acid leaching method [14], and other new or modified extraction methods reported in recent years. However, limitations have been also reported for some of the extraction methods. For instance, aqueous distillation has been reported to perform interconversion of inorganic Hg2+ to CH3Hg+ which leads to a positive bias for CH3Hg+ [14, 29]. On the other hand, these methods require much solvent, labour, and time [8]. Microwave-assisted extraction is popular for its less solvent- and less time-consuming properties. Therefore, many researchers are applying this method for sample preparation. This paper deals with microwave-assisted extraction procedure published in EPA 3200 method. The results are compared with a typical acid-leaching/solvent extraction process presented by Bloom, Colman and Barber [14], termed as Bloom’s method in this paper.