Extractability of metal sulfide minerals in acidic solutions: Application to environmental studies of trace metal contamination within anoxic sediments

Sulfide minerals are an important sink for metals within anoxic sediments. Extraction techniques using cold HCl to measure the concentration of sulfide mineral associated trace metals have gained wide acceptance as a proxy for potential metal bioavailability within the environmental community. However, certain metal sulfide phases that can potentially release metals into a more bioavailable phase do not extract in HCl. Laboratory experiments have shown that covellite (CuS), chalcocite (Cu2S), cinnabar (HgS), millerite (NiS), heazlewoodite (Ni2S3), and vaesite (NiS2) are poorly soluble in HCl; while greenockite (CdS), mackinawite (FeS1-x), pyrrohtite (FeS), galena (PbS), and sphalerite (ZnS) are highly soluble in HCl. Surface area can also affect the apparent solubility of CuS and NiS in HCl. These results indicate that use of HCl-based extraction schemes (e.g., AVS/SEM ratios) to assay sediments for metal contamination could underestimate the potential bioavailability of several metals of general interest in anoxic sediments.