Occurrence and formation of incidental metallic Cu and CuS nanoparticles in organic-rich contaminated surface soils in Timmins, Ontario
This study investigates the fate of Cu in organic-rich soils contaminated by mining related activities at the Timmins Kidd Creek metallurgical site, Ontario, Canada. The surficial soil layers (top 5 cm) contain on average 13 000 ppm Cu, predominantly in the form of Cu–Fe-sulfide particulate matter (PM) attached to organic residues. The sequestration of Cu by organic material (OM) is investigated with a combination of focused ion beam (FIB) technology and transmission electron microscopy (TEM). FIB sections are extracted from the interior of OM as well as along the interface between OM and a Cu–Fe sulfide grain. Nanoscale analyses show that Cu occurs as incidental metallic Cu and covellite (CuS) nanoparticles (NPs) within the OM. These NPs can occur along the interface towards attached Cu–Fe sulfide grains (e.g. mooihoekite Cu9Fe9S16, talnakhite Cu9(Fe,Ni)8S16), in remote pore spaces in the interior of the OM, and in association with magnetite precipitates within the OM. These occurrences indicate that Cu is sequestered by OM through various redox and precipitation mechanisms, induced by ferrous iron or humic substances. This sequestration process is thus similar to the formation of supergene Cu-ore deposits where the dissolution of primary ore minerals (e.g. chalcopyrite CuFeS2) and the subsequent precipitation of secondary sulfides (chalcocite Cu2S, covellite) result in an enrichment of Cu. Furthermore, the extraction of the colloidal (mobile) fraction of the upper 1 cm of the studied soil (through column experiments and ultra-centrifugation) indicate that Cu-bearing colloids are predominantly composed of incidental covellite NPs embedded within colloidal OM. The speciation of Cu within organic residues and colloids indicates that Cu is recycled in surficial organic-rich soil through repetition of (a) sequestration processes within OM, (b) decomposition of OM resulting in the release of Cu-bearing incidental NPs, and (c) re-adsorption to newly formed OM as solutes or incidental NPs.