Publication Type:Journal Article
Source:The Canadian MineralogistThe Canadian Mineralogist, Volume 54, Number 1, p.285-309 (2017)
Keywords:amorphous silica coatings, confined pore spaces, contaminated soils, copper phosphate complexes, jarosite in acidic soils, mineral surface coatings, remediation of soils, transmission electron microscopy
Contaminated soils in the Greater Sudbury area (Ontario, Canada) exhibit elevated Cu and Ni concentrations whose geochemical behavior appears contradictory, with Cu appearing to be more mobile than Ni due to its enrichment in the exchangeable, Fe-Mn-oxide, and organic fractions. Although the differential release of Cu and Ni from smelter-derived particulate matter is well understood, their uptake and storage by secondary phases in mineral surface coatings of the soils in the Greater Sudbury area is not. Coatings on mineral surfaces play an important role in the retention and release of metal(loid)s in soils, as they have large surface areas which in turn are favorable areas for the promotion of dissolution, precipitation, diffusion, adsorption, and nucleation processes. To assess the relationship of mineral surface coatings with Cu and Ni in soils that were previously acidic and have now been remediated, soil samples to a depth of <5 cm from the Greater Sudbury area (towns of Falconbridge, Copper Cliff, and Coniston) have been collected. The mineral surface coatings were examined using Focused Ion Beam technology, Transmission Electron Microscopy, and Raman Spectroscopy to characterize their mineralogical compositions at the nano- to micrometer scale, complemented by Laser-ablation inductively-coupled plasma mass spectroscopy to determine the metal(loid)s abundance in mineral surface coatings. Chemical distribution maps for Cu and Ni indicate their common occurrence in Fe-(hydr)oxides and minerals of the spinel group, whereas elevated concentrations for Cu also occur in minerals of the jarosite group and P-enriched Fe-hydroxides. A comparison with previous studies on the Sudbury soils shows that the average Cu# [Cu/(Cu + Ni)] in all examined coatings agrees surprisingly well with the average Cu# in the exchangeable and Fe-Mn-oxide fraction in surficial soils from the same areas, indicating that Cu- and Ni-bearing phases in mineral surface coatings belong to the exchangeable and Fe-Mn-oxide fractions of the soils. The occurrences of metal(loid)-rich phosphates and Cu-phosphate adsorption complexes within Fe-(hydr)oxide grains and coatings suggests a reduction in bioavailability of these metal(loid)s through the application of a phosphate-bearing fertilizer during remediation, in agreement with previous studies on the bioavailability of metal(loid)s in contaminated soils of the Greater Sudbury Area.