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Abstract
The dissolution of ferrihydrite induced by low molar mass (LMM) organics is an important process that provides bioavailable iron for organisms. Here, ATR-FTIR analysis was combined with characterization of ferrihydrite nanoparticles and kinetic modeling to investigate the roles of different oxalate surface complex species in the dissolution of ferrihydrite aggregates. ATR-FTIR results revealed that at least four different species were present at or near the ferrihydrite surface in the process of ferrihydrite aggregate dissolution. At a relatively low addition of oxalate (oxalate/Fe < 0.1), oxalate was dominantly present as binuclear bidentate surface complexes and aqueous species. The binuclear bidentate complexes mainly caused electrostatic repulsion between particles, resulting in the disaggregation of large ferrihydrite aggregates into colloidal particles with hydrodynamic diameters of 116–174 nm. Kinetic modeling showed that these colloidal particles were stable at the oxalate/Fe ratio of 0.1. With increasing addition of oxalate (oxalate/Fe ≥ 0.1), mononuclear bidentate oxalate complexes and hydrogen-bonded surface complex replaced the binuclear bidentate complexes and aqueous species. The aggregates or larger colloidal particles were further disaggregated into smaller colloidal particles with hydrodynamic diameters of 35–64 nm. Additionally, the mononuclear bidentate oxalate complexes promoted the dissolution of ferrihydrite colloids into dissolved Fe.
Highlights
The dissolution of ironoxides is an important process providing bioavailable iron for plants and microorganisms and affecting the fate of contaminants in the environment[1,2,3,4]
Detailed investigations showed that the effects of organic ligands on the kinetics and extent of dissolution are closely related to their surface coordination modes of the low molar mass (LMM) organic ions[22,23,24,25,26,27,28,29,30]
Some attention has been paid to the influence of outer-sphere complexes of LMM organics on metaloxide dissolution
Summary
The dissolution of iron (hydr)oxides is an important process providing bioavailable iron for plants and microorganisms and affecting the fate of contaminants in the environment[1,2,3,4]. Several variables, which link macro-scale dissolution with molecular-scale complexation, have been well documented, including solution pH, types of organic ligands, and particle size of iron oxides[22,23,24,25,26,27,28,29,30] Another important factor that remains to be fully explored is the influence of the inherent mineralogical properties of iron (hydr) oxides on the dissolution processes induced by LMM organics. Similar experiments have been performed to quantify the influence of surface speciation of oxalate on boehmite dissolution[28,29] Results from these ATR-FTIR studies were combined with the results from various characterization methods to provide molecular-level insights into the impacts of different oxalate surface complex species on the dissolution of ferrihydrite aggregates. These different methods of characterization include X-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), and mathematical modeling of the dissolution kinetics
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