Metal sulfide nanoparticles (NPs) such as iron (Fe) sulfides are commonly found in nature, often in association with other transition metal sulfides (e.g., CuS, CoS, NiS, ZnS). Their (trans)formation is involved in key environmental processes on the modern and ancient Earth with huge impacts on the fate of Fe, sulfur, carbon, oxygen, nutrients and trace metals. Their small sizes, colloidal behavior and sensitivity to oxygen make metal sulfide NPs particularly challenging to investigate. Here, we present several examples that utilized an interdisciplinary approach to investigate the role of metal sulfide NPs in environmental processes. A synthesis-based approach is first described in which mixed-metal sulfides (Cu/Co/Ni mixed with Fe) were precipitated by the activity of the sulfate-reducing bacteria <i>Desulfovibrio vulgaris</i>. Transmission electron microscopy (TEM) shed light on the identity of the primary metal-bearing solid phases that formed at different metal-to-Fe ratios – many of these phases were found to be comparable to metal sulfide NPs that have been observed in natural sediments. A state-of-the-art single particle ICP-MS (spICP-MS) technique was then applied to study metal release from these NPs by simultaneously measuring their dissolution and aggregation in solution. The application of spICP-MS was subsequently extended to the characterization of complex environmental NPs including organic-associated minerals. We finish with a description of recent work to synthesize pyrite (FeS<sub>2</sub>) of defined sizes and shapes with high yield, which is of large interest from both an environmental (e.g., biosignature) and technological (e.g., semiconductor) application. High temperature synthesis (80°C) coupled with scanning electron microscopy (SEM) showed clear evidence for colloid-sized pyrite that formed via the particle attachment pathway from precursor NPs. However, attempts to synthesize pyrite via a biological route at ambient temperature have so far failed. Pyrite is formed everyday in ocean sediments – our synthesis failure likely highlights the need for designing growth medium that better reflects natural conditions, which is a critical consideration for any lab experiment.

Metal sulfides in the environment: Dynamic (bio)synthesis, particle aggregation and dissolution investigated through an interdisciplinary approach

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ACS Fall 2022

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