The determination of labile Fe in ferrihydrite by ascorbic acid extraction: Methodology, dissolution kinetics and loss of solubility with age and de-watering

An ascorbic acid extraction at pH 7.5 has been examined to assess the influence of reaction conditions (pH, ascorbic acid concentration) on the dissolution of Fe from synthetic 2-line ferrihydrite and from other Fe-bearing minerals. The method was highly selective for Fe in ferrihydrite with only small amounts of Fe extracted from other (oxyhydr)oxides or clays. The labile Fe extracted from the synthetic 2-line ferrihydrite stored as a slurry decreased with time, and high resolution microscopy showed that the older materials formed networked aggregates that slow down the dissolution. The apparent rate constant for the dissolution of fresh 2-line ferrihydrite (~ 10− 3 s− 1) was an order of magnitude larger than that for aged suspensions (~ 10− 4 s− 1). Fresh 2-line ferrihydrite that was filtered, freeze-dried, frozen and freeze-dried, and stored in the dry was even less readily dissolved (apparent rate constants ~ 10− 6 s− 1). These ferrihydrites also contained networked aggregates and, additionally, appear to occur as granular aggregates (visible to the naked eye) during the early stages of dissolution. Storage of filtered, freeze-dried, and frozen and freeze-dried ferrihydrites, whether in water or air, produced similar dissolution behaviour because aggregation caused by de-watering decreases the labile Fe content and is not reversed by re-hydration. The determination of labile Fe in ferrihydrite requires that natural samples should be collected, stored and extracted wet. The most aged samples dissolved by parabolic dissolution kinetics indicate that the rates of dissolution were controlled by the diffusion of reactant into the internal porosity of aggregates.

Research Highlights
► Ascorbic acid at pH 7.5 quantitatively dissolves Fe in fresh ferrihydrite.
► Ascorbic acid extraction has little effect on other Fe oxyhydroxide minerals.
► Ferrihydrite aggregates with age and de-watering and is less readily extracted.
► The dissolution of aggregates is limited by diffusion into interior porosity.
► De-watering must be avoided in measuring labile ferrihydrite Fe in natural samples.