Remediation of mercury polluted soil

Abstract: Mercury in contaminated soil constitutes a wide spectrum of species with highly different properties due to the chemical complexity of the element. Assessments of risk, ecological effects and removal potential require process and mobility studies. Identification of various chemical states of mercury in soil systems can be achieved by selective extraction procedures that would demonstrate the mobility and potential reactivity. In this thesis, a selective extraction method for mercury is presented. The method is mainly based on a parallel extraction procedure. The method was used to investigate the mercury mobility in two soils collected at two different chlor-alkali industrial sites; Bohus and Bengtfors. Both soils are heavily contaminated with mercury, 80 and 180 mg Hg/kg, respectively. In Bohus soil mercury was associated to organic matter with low water solubility and to a very resistant phase, strongly indicated to be dominated by mercury sulphides. In Bengtfors soil some mercury appeared to be weakly adsorbed, bound to secondary precipitates and associated to soluble humic matter. These result indicates a higher mobility of mercury in Bengtfors compared to Bohus soil. Both soils are fine grained with a clay dominated matrix. The main difference is the sulphur content. 0.4% and 0.03% for Bohus and Bengtfors soil, respectively. The preference of mercury for sulphides and reduced sulphur groups in organic matter is well established. The corresponding phases are in general very insoluble. This indicates that sulphur contents in soil to some extent reflects geochemical speciation as well as mobility of mercury. Further, the possibility to mobilise insoluble mercury phases in soils using soil fungi as well as halogenides were investigated. This was made in an attempt to investigate possible in-situ technologies for remediation of mercury polluted soils. Some of the fungi studied were able to dissolve mercury sulphides. It was also indicated that the same fungi responded with volatilsation, accumulation and possible also precipitation of mercury, as soon as toxic levels of soluble mercury were reached. These latter responses can be of advantage or disadvantage depending on remediation design. In batch experiments made with acidic iodide solutions and Bohus soil, a very fast and extensively dissolution of mercury was shown. Mobilisation of the mercury sulphide fraction present, could not be achieved solely by competitive complexation. The mechanism suggested was formation of tri-iodide at low pH. The combination of complextion and oxidation in the same molecule, breaks the bond between inorganic mercury and the sulphide. Extensively and fast mobilisation was also seen in a bench experiment, where iodide in combination with electroremediation simulated a possible in-situ remediation of Bohus soil.

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