engleski

Extractability of HgS (cinnabar and metacinnabar) by hydrochloric acid

In this work we investigated the behaviour of pure HgS during extraction with dilute HCl in order to establish its extractability in 1 and 6 M HCl at the concentration level close to the ones occurring in natural sediments and soils. It was found that neither cinnabar nor metacinnabar were soluble in 1 M HCl, whereas both of them were partially extracted by 6 M HCl. Matacinnabar precipitated in the laboratory was most prone to dissolution in 6 M HCl (up to 90%), followed by crystalline (commercial) metacinnabar (up to 70 %) and cinnabar (up to 15 %). Solubility of HgS in 6 M HCl was found to be dependant on its concentration and an exponential relationship between quantity of HgS added to 20 ml of 6 M HCl (the range of 0.1 - 10 mg was used) and the solubility in 6 M HCl was established. For higher concentrations of HgS (10 mg in 20 ml of acid) similarly low solubility of cinnabar as found in the literature was obtained. A study of dissolution kinetics of HgS in 6 M HCl indicated that it was a fairly slow process. Unexpected oxidation of HgS in water or 1 M HCl was found under conditions when the extraction was made in Teflon vials previously used for the digestion of residual undissolved HgS by aqua regia. It was presumed that the Teflon material could preserve some oxidising gasses (presumably Cl2) developed during digestion with aqua regia which can then oxidise HgS during extraction with water or 1 M HCl. Regarding the extraction of Hg from natural sediments it was concluded that 6 M HCl could not be used to extract reactive Hg and predict bioavailability of mercury in sediments containing HgS and that experiments with model compounds should not be done at the concentration level several orders of magnitude higher than in natural samples.

Extraction; Cinnabar; Hydrochloric acid; Mercury; Sediment

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