摘要:

Organic coatings on mineral surfaces are ubiquitous organic–liquid interfaces in soil and sediment systems, and their functional groups have been reported to control heterogeneous iron (Fe) (oxyhydr)oxide precipitate amounts and sizes. However, the role of these organic–liquid interfaces as controls on Fe oxyhydroxide transformation kinetics and pathways remain poorly constrained, particularly regarding the role of individual functional groups during this process. Here, self-assembled organosilane films terminated with carboxyl, thiol or amine functional groups on fused silica (SiO2) substrates were used as idealised organic-functionalised mineral surfaces to mechanistically study the phases of homogeneously and heterogeneously precipitated Fe (oxyhydr)oxides and their Fe(II)-catalysed transformation at short and long timescales (4 and 79 days, pH 6.0). Transformation was analysed using synchrotron scattering techniques, transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and stable Fe isotope (57Fe) tracer measurements. Ferrihydrite was the initial phase for all precipitates, but the subsequent kinetics and pathways of heterogeneous precipitate transformation varied substantially with organic functional group identity and were distinct from those of the homogeneous precipitates. Precipitate-aqueous Fe isotopic exchange, and hence the reductive dissolution of ferrihydrite, was complete within four days of transformation for homogeneous precipitates but incomplete for heterogeneous precipitates, contributing to slower transformation kinetics for the latter. Transformation pathways were additionally controlled by surface functional group interactions with early-stage transformation products. After four days, strong carboxylate-Fe binding hindered crystallisation, forming only trace lepidocrocite at a lower surface carboxyl density. Comparatively weaker interactions with SiO2 and amine groups resulted in trace amounts of lepidocrocite, goethite and magnetite. However, much weaker interactions with thiol groups resulted in large amounts of lepidocrocite and goethite, similar to transformation of the homogeneous precipitates. After 79 days, large amounts of lepidocrocite, goethite and magnetite formed for both the homogeneous precipitates and heterogeneous precipitates on SiO2. In comparison, carboxylate-Fe binding almost entirely limited crystalline phase formation. Interactions with amine groups limited ferrihydrite/lepidocrocite transformation into goethite/magnetite, whilst the oxidation of thiol groups led to interactions with Fe species that limited magnetite formation. These results emphasise the importance of organic coatings as microenvironments for controlling the kinetics and pathways of ferrihydrite transformation.

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