Mercury immobilization : a requirement for permanent disposal of mercury waste in Sweden

Sammanfattning: The Swedish Government states that mercury in the Swedish industry and private use shall be collected and waste containing more than 1% of mercury shall be conditioned for permanent storage in proposed groundwater-saturated repository in deep bedrock. The owners of mercury waste in Sweden are responsible for finding ways to the conversion of the mercury waste to insoluble forms.The aim of this thesis was to investigate different low-cost immobilization methods for mercury. Choice of treatment method depends on the nature of the waste, such as the mercury concentration and matrix constituents. High-contaminated waste would be suitable to transform to chemically insoluble mercury forms, while low-contaminated wastes would be suitable to treat with an encapsulation method. The investigations assessed conditions favourable for the formation of HgS at room temperature from elemental mercury as well as from mercury oxide. Elemental sulphur in an alkaline anaerobic environment gave a yield of close to 100% mercury sulphide within two years.The stabilization/solidification (S/S) method was estimated as a suitable treatment method for low contaminated mercury waste (below 5%w/w). The lowest leaching rate was observed from slag monoliths with elemental sulphur as sulphur source. After one year the accumulated leaching amount was 0.008-0.013%w/w of the total mercury content of the monolith (1% of the monolith mass). This is about 100-150 times more than from monoliths that initially contained black meta-cinnabar.Apparent diffusion coefficients for Hg(II)(aq) were determined in cement monoliths with different constituents (10-14-10-15 m2/s), for a barrier of Standard Portland concrete (10-14 m2/s) and in quartz sand (10-13 m2/s). After some 60 d in the leaching test, the apparent diffusion coefficient was of the order 10-17-10-21 m2/s, which was indicative of a slow release rate of the remaining mercury that could represent the dissolution/transformation of HgS, which gradually was formed during curing of the cement (aged for one year). The apparent diffusion coefficient for Hg(0) could not be measured since the mercury species oxidizes in aerobic systems.