Abstract
Iron(III)-precipitates formed by the oxidation of dissolved Fe(II) are important sorbents for major and trace elements in aquatic and terrestrial systems. Their reductive dissolution in turn may result in the release of associated elements. We examined the reductive dissolution kinetics of an environmentally relevant set of Fe(II)-derived arsenate-containing Fe(III)-precipitates whose structure as function of phosphate (P) and silicate (Si) content varied between poorly-crystalline lepidocrocite, amorphous Fe(III)-phosphate, and Si-containing ferrihydrite. The experiments were performed with 0.2–0.5 mM precipitate-Fe(III) using 10 mM Na-ascorbate as reductant, 5 mM bipyridine as Fe(II)-complexing ligand, and 10 mM MOPS/5 mM NaOH as pH 7.0 buffer. Times required for the dissolution of half of the precipitate (t50%) ranged from 1.5 to 39 h; spanning a factor 25 range. At loadings up to ~ 0.2 P/Fe (molar ratio), phosphate decreased the t50% of Si-free precipitates, probably by reducing the crystallinity of lepidocrocite. The reductive dissolution of Fe(III)-phosphates formed at higher P/Fe ratios was again slower, possibly due to P-inhibited ascorbate binding to precipitate-Fe(III). The slowest reductive dissolution was observed for P-free Si-ferrihydrite with ~ 0.1 Si/Fe, suggesting that silicate binding and polymerization may reduce surface accessibility. The inhibiting effect of Si was reduced by phosphate. Dried-resuspended precipitates dissolved 1.0 to 1.8-times more slowly than precipitates that were kept wet after synthesis, most probably because drying enhanced nanoparticle aggregation. Variations in the reductive dissolution kinetics of Fe(II) oxidation products as reported from this study should be taken into account when addressing the impact of such precipitates on the environmental cycling of co-transformed nutrients and contaminants.
Highlights
The oxidation of dissolved Fe(II) in aqueous solutions leads to the precipitation of amorphous to nanocrystalline Fe(III)-precipitates that critically impact on the cycling of nutrients and contaminants in aquatic and terrestrial systems [1, 2]
Examples include Fe(III)-precipitates forming at the redoxcline in lakes [3, 4], at anoxic groundwater exfiltration sites in streams and rivers [5,6,7] or in the marine environment [8] that control the cycling of phosphate, Fe(III)-precipitates accumulating in the rhizosphere of wetland plants that affect the uptake of As or other trace elements [9,10,11], or Fe(III)-precipitates forming in Fe-based techniques for As removal from
The measured precipitate phosphate/Fe(II) ratio (P/Fe) and silicate/Fe(II) ratio (Si/Fe) ratios match with the results from our previous work on analogously synthesized samples, indicating that precipitate structure can be inferred from our earlier work in which we used X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) for precipitate characterization [20]: Briefly, Precipitate Ca-15-00 formed at (P/Fe)init of 1.5 in the absence of Si is an amorphous Ca–Fe(III)-phosphate
Summary
The oxidation of dissolved Fe(II) in aqueous solutions leads to the precipitation of amorphous to nanocrystalline Fe(III)-precipitates that critically impact on the cycling of nutrients and contaminants in aquatic and terrestrial systems [1, 2]. The structure of Fe(III)-precipitates formed by dissolved Fe(II) oxidation at near-neutral pH in the presence of phosphate, silicate and Ca can be rationalized in terms of mixtures of three main structural endmembers: poorly-crystalline lepidocrocite, silicate-containing ferrihydrite, and amorphous Fe(III)-phosphate [20]. Synthetic 2-line ferrihydrite formed by the forced hydrolysis of a concentrated ferric iron solution may differ from Fe(III)precipitates formed by the oxidation of dissolved Fe(II) at near-neutral pH in the presence of other solutes with respect to structure and reactivity. Results gained on synthetic 2-line ferrihydrite alone do not allow to assess the variability of natural Fe(III)precipitates in terms of structure and reactivity
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