Secondary mineral phases associated with a historic arsenic calciner identified using automated scanning electron microscopy; a pilot study from Cornwall, UK

摘要

Historical polymetallic mining in Cornwall, UK has resulted in significant soil contamination. In particular the processing and refinement of arsenic in purpose built calciners has resulted in areas of localised, but extremely high contamination of soils with arsenic. Consequently, there is concern regarding the impact of this historic arsenic contamination on the local population. In this study automated scanning electron microscopy has been carried out to identify the phase distribution of arsenic within soil samples from: (a) adjacent to a calciner chimney, (b) adjacent to the calciner works and (c) for comparison, a sample of residue from the calciner dumps. Whilst the soil samples contain between 200 and 3325 ppm arsenic, the calciner residue contains approximately 12 percent arsenic. Although arsenopyrite is present in all the samples examined it is rare. Instead, the dominant arsenic mineral species present are As-Fe oxides which occur in a range of textural settings including: rims around coal, coke and fly ash particles; secondary phases infilling porosity with coal and coke particles; rims around silicate minerals and as a secondary phase cementing silicate grains and coal fragments together. In addition, in the sub 20 mu m size fraction from the calciner residue samples up to 15 percent of the grains present are As-Fe oxides, and of these many of the grains are less than 10 mu m in size and are potentially inhalable. These new data are significant in our understanding of the long-term dispersal of arsenic in the environment and its potential bioavailability.