Determination of mercury and its compounds in water, Sediment, soil and biological samples

Abstract

Mercury occurs naturally in the Earth's crust principally as the ore, cinnabar, HgS. Mercury is quite different from other metals in several respects: (i) it is the only metal that is liquid at room temperature; (ii) it is the only metal that boils below 650°C; (iii) it is quite inert chemically, having a higher ionization potential than any other electropositive element with the sole exception of hydrogen; (iv) it exists in oxidation states of zero (Hg) and 1 (Hg2+) in addition to the expected state of 2 (Hg 2+). Mercury forms alloys ("amalgams") with many metals. Mercury and its chemical derivatives are extremely hazardous. Since the early 1960s, the growing awareness of environmental mercury pollution (e.g. the Minamata tragedy resulting from methyl-mercury poisoning) has stimulated the development of more accurate, precise and efficient methods of determining mercury and its compounds in a wide variety of matrices. Many of the environmental aspects of mercury and its compounds have been reviewed (UNEP 2002; Drasch et al, 2004; Pirrone et al, 2002). In recent years, new analytical techniques have become available and have been used in environmental studies and consequently the understanding of mercury chemistry in natural systems has improved significantly. Mercury can exist in a large number of different physical and chemical forms with a wide range of properties. Conversion between these different forms provides the basis for mercury's complex distribution pattern in local and global cycles and for its biological enrichment and effects. The most important chemical forms are: elemental mercury (Hg), divalent inorganic mercury (Hg2+), methylmercury (CH3Hg +), and dimethylmercury (CH3)2Hg). There is a general biogeochemical cycle by which monomethyl and mercury (II) compounds, dimethylmercury and mercury (0) may interchange in the atmospheric, aquatic, and terrestrial environments. Mercury vapour is released into the atmosphere from a number of natural sources and through anthropogenic emissions (mainly from combustion of fossil fuels). A small portion of Hg is converted into water soluble species (probably Hg2+) which can, in part, be re-emitted to the atmosphere as Hg by deposition on land or exchange at the air/water boundary. The atmospheric cycle entails retention of Hg in the atmosphere for long periods and consequently it is transported over very long distances. The bottom sediment of oceans is thought to be ultimate sink where mercury is deposited in the form of highly insoluble HgS. Changes in speciation from inorganic to methylated forms is the first step in aquatic bioaccumulation processes. These processes are considered to occur in both the water column and sediments. The mechanism of synthesis of methylmercury is not very well understood. Although methylmercury is the dominant form of mercury in higher organisms, it represents only a very small amount of the total mercury in aquatic ecosystems and in the atmosphere. Methylation-demethylation reactions are assumed to be widespread in the environment and each ecosystem attains its own steady state equilibrium with respect to the individual species of mercury. However, owing to the bioaccumulation of methylmercury, methylation is more prevalent in the aquatic environment than demethylation. Once methylmercury is formed, it enters the food chain by rapid diffusion and tight binding to proteins in aquatic biota and attains its highest concentrations in the tissues of fish at the top of the aquatic food chain due to biomagnification through the trophic levels. The main factors that affect the levels of methylmercury in fish are the diet/trophic level of the species, age of the fish, microbial activity and mercury concentration in the upper layer of the local sediment, dissolved organic carbon content, salinity, pH, and redox potential. Speciation is therefore a term frequently used and in the case of mercury, (i) it rules the way Hg is transported from its sources to the local environment of man and wildlife, (ii) it rules how mercury is bound in the environment and it therefore more or less available to cause adverse effects, and (iii) it rules one of the most important mercury transformation and buildup of MeHg in fish and other aquatic and terrestrial foods. The term "speciation" and "fractionation" in analytical chemistry were addressed by the International Union for Pure and Applied Chemistry (IUPAC) which published guidelines (Templeton et al., 2000) or recommendations for the definition of speciation analysis: Speciation analysis is the analytical activity of identifying and/or measuring the quantities of one or more individual chemical species in a sample. The chemical species are specific forms of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure. The speciation of an element is the distribution of an element amongst defined chemical species in a system. In case that it is not possible to determine the concentration of the different individual chemical species that sum up the total concentration of an element in a given matrix, that means it is impossible to determine the speciation, it is a useful practice to do fractionation instead. Fractionation is the process of classification of an analyte or a group of analytes from a certain sample according to physical (e.g. size, solubility) or chemical (e.g. bonding, reactivity) properties During recent years new analytical techniques have become available that have contributed significantly to the understanding of mercury chemistry in natural systems. In particular, these include ultra sensitive and specific analytical equipment and contamination-free methodologies. These improvements eventually allow for the determination of total and major species of mercury to be made in air, water, sediments, and biota. Analytical methods are selected depending on the nature of the sample and, in particular, the concentration levels of mercury. The present review is an updated and extended version of a review published in 1996 (Horvat, 1996) in order to meet the opbjectives of the present book.