Sulfide supply rate and organic surface coating affect pyrite formation during sulfidization of ferric (oxy)hydroxides

Pyrite formation has been widely investigated because of its abundance and significance in the iron and sulfur cycles in many anoxic environments. The ferric-hydroxide-surface (FHS) pathway is an important pathway for rapid pyrite formation, relying on the generation of surface-bound precursor species >Fe^IIS^2-.^[1] However, ferric (oxy)hydroxides are often microbially produced and thus associated with organic matter (OM). Additionally, in natural environments, sulfide (S(-II)) supply rates are typically regulated by sulfate-reducing bacteria, providing a more continuous flux, in contrast to the single-pulse S(-II) additions commonly used in laboratory experiments.^[2] To our knowledge, the combined effect of surface coating and sulfide supply rates on pyrite formation and secondary iron mineral transformation remains unexplored. In this study, we therefore compared pyrite formation rates and reaction products by exposing 40 mM synthetic ferric (oxy)hydroxides (goethite and ferrihydrite) and biogenic Fe(III) (oxy)hydroxides (BioFe, which includes associated organic matter, cells and phosphate) to sulfide at pH 6. Sulfide was supplied under strictly anoxic conditions either as single-pulsed 10 mM S(-II) pulse or multiple 0.5 mM/d S(-II) pulses over 20 days (final Fe(III):S(-II) = 4:1). Aqueous- and solid-phase S and Fe speciation as well as changes in Fe mineralogy were tracked using wet chemistry techniques, Raman micro-spectroscopy and X-ray diffraction. Our results show that ferrihydrite was transformed mostly into lepidocrocite, goethite and pyrite after single-pulsed S(-II) addition, and to goethite and pyrite in the multiple-pulsed S(-II) treatment. Rietveld quantitative phase analysis via XRD revealed that the multiple-pulsed S(-II) mode delayed pyrite formation. However, no pyrite was identified in the treatment with biogenic Fe(III) (oxy)hydroxides, where the added sulfide was instead converted to zero-valent sulfur, presumably due to occupation of the surface sites by OM and/or phosphate. Notably, phosphate from the bacterial growth medium was sequestered in vivianite. Our findings demonstrate that pyrite formation via the FHS pathway is strongly influenced by the presence of surface-active components (e.g., organic matter or PO₄^3-) and sulfide addition rates. [1] M. Wan et al., 2017, Geochim. Cosmochim. Acta, 217, 334–348. [2] Skyring, G.W., 1987, Geomicrobiol J 5: 295–374.