Formation pathways of Zn-spinel nanoparticles in Zn-contaminated soils and sediments

Abstract This study depicts novel formation pathways of low-temperature spinel nanoparticles (NPs). Understanding these pathways is of environmental and technological importance as the spinel structure can incorporate an extensive range of cations, which allows it to sequester metal contaminants in soils, water, and tailings. Using the focused ion beam and ion mill technique in combination with transmission electron microscopy, this study depicts various formation pathways of Zn-bearing spinel NPs in the smelter-impacted soils of the Flin Flon area, Manitoba, Canada. Four pathways were identified in soil organic matter and a clay mineral microaggregate: I. the formation of euhedral franklinite NPs through crystallization of particle and monomer attachment in proximity to dissolving surfaces of high-T micrometer-sized Zn-Fe oxide particles; II. the heterogeneous nucleation of franklinite NPs on the surface of a dissolving wurtzite (ZnS) precipitate, with the latter being facilitated by the sulfidation of Ag ions or Ag NPs on its surface; III. the heterogeneous nucleation of Zn-bearing magnetite at low nucleation rates preferentially along the edges of basal surfaces of illite-smectite minerals with K:Ca ratios ranging from 1:2 to 2:1; IV. aggregation and attachment of franklinite NPs at higher nucleation rates to clusters on basal surfaces or growing surfaces of euhedral franklinite NPs. These pathways are compared with general observed formation pathways of nanomaterials in the critical zone and with previously identified formation pathways of Zn-spinel nanomaterials. Identification of formation pathways of spinel-group minerals under ambient Earth surface conditions provides a better understanding of the sequestration and environmental fate of metals compatible with their structure. This is relevant for regions impacted by smelter activities, as divalent cations such as Ni2+, Cu2+, and Zn2+ are common metal contaminants in their corresponding soils and sediments.