Formation, reactivity, and aging of ferric oxide particles formed from Fe(II) and Fe(III) sources: Implications for iron bioavailability in the marine environment

Freshly formed amorphous ferric oxides (AFO) in the water column are potentially highly reactive, but with reactivity declining rapidly with age, and have the capacity to partake in reactions with dissolved species and to be a significant source of bioavailable iron. However, the controls on reactivity in aggregated oxides are not well understood. Additionally, the mechanism by which early rapid aging occurs is not clear. Aging is typically considered in terms of changes in crystallinity as the structure of an iron oxide becomes more stable and ordered with time thus leading to declining reactivity. However, there has been recognition of the role that aggregation can play in determining reactivity, although it has received limited attention. Here, we have formed AFO in seawater in the laboratory from either an Fe(II) or Fe(III) source to produce either AFO(II) or AFO(III). The changes in reactivity of these two oxides following formation was measured using both ligand-promoted dissolution (LPD) and reductive dissolution (RD). The structure of the two oxides was examined using light scattering and X-ray adsorption techniques. The dissolution rate of AFO(III) was greater than that of AFO(II), as measured by both dissolution techniques, and could be attributed to both the less ordered molecular structure and smaller primary particle size of AFO(III). From EXAFS analysis shortly (90 min) following formation, AFO(II) and AFO(III) were shown to have the same structure as aged lepidocrocite and ferrihydrite respectively. Both oxides displayed a rapid decrease in dissolution rate over the first hours following formation in a pattern that was very similar when normalised. The early establishment and little subsequent change of crystal structure for both oxides undermined the hypothesis that increasing crystallinity was responsible for early rapid aging. Also, an aging model describing this proposed process could only be fitted to the data with kinetic parameters that were inconsistent with such a mechanism. The similar aging patterns and existence of diffusion limited cluster aggregation (DLCA) suggested that loss of Fe centre accessibility due to aggregation is the likely cause of early rapid aging of AFO. A simple model describing the loss of surface area during the aggregate growth, measured using dynamic light scattering (DLS), produced aging patterns that matched the reactivity loss of AFO(III) measured using RD but not LPD. The difference between the two measures of dissolution rate could not be explained, but indicated that different measures of reactivity respond differentially to various parameters controlling reactivity. Analysis of aggregate structure using aggregation kinetics and static light scattering (SLS) suggested that restructuring during aggregation was occurring at an aggregate level for AFO(III), but only minimally so for AFO(II). While our investigations support the contention that aggregation is responsible for early rapid aging, the role of aggregate structure is remains unclear.