Influence of chloride and sediment matrix on the extractability of HgS (cinnabar and metacinnabar) by nitric acid

Abstract

The extractability of metacinnabar and cinnabar, alone or in the presence of some sediment components, with various concentrations of HNO₃ (1, 4, 6, and 14 M) was studied. Both forms of HgS (0.2–0.3 mg HgS in 10–20 mL of acid) were insoluble in all HNO₃ concentrations as pure compounds. The presence of FeCl₃ enhanced solubility of both cinnabar and metacinnabar, especially when concentrated HNO₃ was used for the extraction. As the same effect was not obtained in the presence of FeOOH, we concluded that chloride and not Fe³⁺ was responsible for HgS dissolution. In fact, addition of very low chloride concentration to concentrated HNO₃ provoked partial (Cl>10⁻⁴ M) or even total dissolution (Cl>10⁻² M) of HgS. In dilute HNO₃ (4–6 M) cinnabar was much less affected by chloride addition than metacinnabar. Extraction of HgS by concentrated HNO₃ in the presence of sediment of various salinities demonstrated that the amount of dissolved HgS increased with the increase of the sediment salinity (from freshwater to estuarine and marine sediment), confirming that chloride enhances dissolution of HgS. Removal of chloride by washing the sediment with Milli-Q water significantly reduced dissolution of added HgS during extraction by concentrated HNO₃. These results demonstrate that conclusions based on the extraction schemes using concentrated HNO₃ as single extractant or as the first extractant in the sequential extraction procedures can be biased. A verification of artifactual oxidation of HgS, when using more concentrated HNO₃ as extractant, would help to verify reliability of the applied extraction procedure.