Atom probe tomography and transmission electron microscopy: a powerful combination to characterize the speciation and distribution of Cu in organic matter.

The large surface areas in porous organic matter (OM) and on the surface of altered minerals control the sequestration of metal(loid)s in contaminated soils and sediments. This study explores the sequestration of Cu by OM in surficial forest soil in close proximity to the Horne smelter, Rouyn-Noranda, Quebec, Canada. The organic-rich soils have elevated concentrations of Cu (Cu = 〈0.75〉 wt%) but lack associations between organic matter (OM) and Cu-sulfides, commonly observed in organic-rich Cu-contaminated soils. This provides a unique opportunity to study the sequestration of Cu by OM in a sulfur-depleted environment using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT). In two examined OM particles, Cu is predominantly sequestered as (I) nano- to micrometer-size Cu-bearing spinels, (II) as cuprite (Cu2O) nanoparticles or (III) finely dispersed Cu in association with clusters of magnetite (Fe3O4) nanoparticles embedded in amorphous silica-rich pockets and (IV) in the OM matrix. The occurrence of euhedral crystals and nanoparticles in the single-digit range within the OM matrix indicate that the majority of the nanoparticles formed in situ within the OM particles. A model is developed which proposes that the sequestration of Cu in OM is promoted by (I) the partial mineralization of the OM matrix by amorphous silica; (II) the nucleation of magnetite nanoparticles on highly reactive silanol groups; (III) the diffusion of Cu within mineralized and altered areas of the OM; (IV) the availability of Cu-bearing species, which in turn is controlled by the hydrodynamic properties of the pore channels; (V) the formation of precursors and nucleation of Cu-bearing nanoparticles. This study shows that the combination of SEM, TEM and APT provides new insights into the sequestration of metal contaminants by OM at various scales ranging from the single-digit nano- to micrometer scale.

The role of organic colloids in the sequestration and mobilization of copper in smelter-impacted soils.

This study shows that Cu occurs predominantly as Cu-sulfides and Cu-bearing phosphates and -carbonates in organic matter (OM) colloids within smelter-impacted soils in which they are often associated with other inorganic components. Major emitters of Cu are smelters and coal-power plants, which cause severe damage to the health of soils and aquatic systems as elevated Cu concentrations are toxic for terrestrial and aquatic organisms. Toxic effects and the long-term environmental fate of Cu depend among many other factors on its speciation in soil and water bodies. This study explores the role of OM colloids (defined as particles with diameters in the range of 100 to 1000 nm and with a larger proportion of organic than inorganic material) in the sequestration of Cu in contaminated soils around the Horne smelter, Rouyn Noranda, Quebec, Canada, focusing on a thin soil overlying bedrock (bedrock soil) and forest soil. The sequestration and mobilization of Cu by OM colloids are studied using a combination of column leaching experiments, ultra-centrifugation and transmission electron microscopy (TEM). TEM analysis indicates that Cu occurs as nano-sized CuSx phases in OM colloids of the bedrock soil, and as Cu-bearing Ca-Mg-phosphates and Ca-carbonates in OM colloids of the forest soil. The nano-sized CuSx phases occur along the rim of OM colloids or are attached to silica inclusions located within OM colloids, suggesting that their in situ formation is strongly controlled by the presence of polar groups within or on the surface of OM colloids. The proportion of Cu-bearing colloids in the soil leachates ranges from 20 to 40% of the total colloidal fraction, suggesting that OM colloids can play a significant role in the sequestration of Cu in surficial soil environments.