Challenges in mercury speciation and fractionation in soil and sediment

This investigation focused on the development, test and validation of methodologies for mercury fractionation and speciation in soil and sediment. After an exhaustive review of the literature, several methods were chosen and tested in well characterised soil and sediment samples. Sequential extraction procedures that divide mercury fractions according to their mobility and potential availability in the environment were investigated. The efficiency of different solvents for fractionation of mercury was evaluated, as well as the adequacy of different analytical instruments for quantification of mercury in the extracts. Kinetic experiments to establish the equilibrium time for mercury release from soil or sediment were also performed. It was found that in the studied areas, only a very small percentage of mercury is present as mobile species and that mobility is associated to higher aluminium and manganese contents, and that high contents of organic matter and sulfur result in mercury tightly bound to the matrix. Sandy soils tend to release mercury faster that clayey soils, and therefore, texture of soil or sediment has a strong influence on the mobility of mercury. It was also understood that analytical techniques for quantification of mercury need to be further developed, with lower quantification limits, particularly for mercury quantification of less concentrated fractions: water-soluble e exchangeable. Although the results provided a better understanding of the distribution of mercury in the sample, the complexity of the procedure limits its applicability and robustness. A proficiency-testing scheme targeting total mercury determination in soil, sediment, fish and human hair was organised in order to evaluate the consistency of results obtained by different laboratories, applying their routine methods to the same test samples. Additionally, single extractions by 1 mol L-1 ammonium acetate solution, 0.1 mol L-1 HCl and 0.1 mol L-1 CaCl2, as well as extraction of the organometallic fraction were proposed for soil; the last was also suggested for sediment and fish. This study was important to update the knowledge on analytical techniques that are being used for mercury quantification, the associated problems and sources of error, and to improve and standardize mercury extraction techniques, as well as to implement effective strategies for quality control in mercury determination. A different, “non chemical-like” method for mercury species identification was developed, optimised and validated, based on the thermo-desorption of the different mercury species. Compared to conventional extraction procedures, this method has advantages: it requires little to no sample treatment; a complete identification of species present is obtained in less than two hours; mercury losses are almost neglectable; can be considered “clean”, as no residues are produced; the worldwide comparison of results obtained is easier and reliable, an important step towards the validation of the method. Therefore, the main deliverables of this PhD thesis are an improved knowledge on analytical procedures for identification and quantification of mercury species in soils and sediments, as well as a better understanding of the factors controlling the behaviour of mercury in these matrices.