9.02.5.1 Release from Primary Minerals
The arsenic in many natural waters is likely to have been derived naturally from the dissolution of a mineral phase. The most important primary sources are sulfide minerals, particularly arsenic- rich pyrite, which can contain up to 10% arsenic and FeAsS. In one study, the greatest concen¬trations of arsenic were found in fine-grained (<2 mm) pyrite formed at relatively low tempera-tures (120-200 °C) (Simon et al., 1999). A variety of other sulfide minerals such as orpiment As2S3, and realgar As2S2 also occur in association with gold and base-metal deposits. Arsenic is a component of some complex copper sulfides such as enargite (Cu3As4) and tennantite ((Cu,Fe)12As4Si3). Rarer arsenides are also found in mineralized areas. All of these minerals oxidize rapidly on exposure to the atmosphere releasing the arsenic for partitioning between water and various secondary minerals, particularly iron oxides. Both microbially mediated redox reactions (Section 9.02.4.5) and abiotic processes are involved. The microbial oxidation of arsenic minerals such as FeAsS, Cu3As4, and As2S3 has been discussed by Ehrlich (2002).
đang được dịch, vui lòng đợi..
