Formation of cinnabar--estimation of favourable conditions in a proposed Swedish repository.
A deep repository for permanent storage of mercury will be designed and built in Sweden. The preferred chemical state for mercury in such a repository would be the sulphide HgS (cinnabar), which is a highly insoluble and the dominating natural mercury mineral. The possible formation of HgS from HgO or Hg(0) by reaction with a sulphur source (S, FeS or FeS(2)) is discussed from thermodynamic considerations, and pe-pH-diagrams are constructed by using the computer code MEDUSA to illustrate under which conditions HgS would dominate. Calculations of the speciation (PHREEQE) under varying conditions (S/Hg-ratios, presence of chloride) are given. Long-term laboratory experiments are performed, where the formation of HgS from the basic components is demonstrated (after mixing under various conditions and storage at room temperature for up to 3 years). The feasibility of HgS-formation with time in a geologic repository under conditions representative of deep granitic bedrock (calcium-bicarbonate buffered to pH 7-8.5) is discussed, as well as effects of alkaline conditions (concrete environment, pH 10.5-12.5). Formation of soluble polysulphides is not expected as long as the S/Hg mole ratio is within 1-1.3 and pH is below 10.5-11. Concrete should be used with caution. Suitable ballast materials could be introduced that would reduce porewater-pH that otherwise would be above 12.
Svensson
Margareta
M
Man-Technology-Environment Research Centre, Orebro University, SE-701 82 Orebro, Sweden. Margareta.Svensson@sakab.se
Düker
Anders
A
Allard
Bert
B
D006256:Hazardous Waste; D006285:Health Planning; D006863:Hydrogen-Ion Concentration; D017669:Mercury Compounds / Q000737:chemistry*; D008855:Microscopy, Electron, Scanning; D010084:Oxidation-Reduction; D013455:Sulfur / Q000737:chemistry; D013548:Sweden; D013816:Thermodynamics; D014961:X-Ray Diffraction
D006256:Hazardous Waste; D017669:Mercury Compounds; D013455:Sulfur; C034211:cinnabar