FE ISOTOPE FRACTIONATION DURING REDUCTION OF FE(III) TO FE(II)

Iron is an important nutrient for life in the ocean, where low Fe concentrations often limit the growth of marine phytoplankton. In order for these phytoplankton to acquire Fe from seawater, Fe(III) is most often reduced to Fe(II) prior to uptake, either at the cell surface or in the water column. Here, the natural reduction of Fe(III) to Fe(II) in the oceans is simulated in the lab with three very different sorts of experiments. Each of these three Fe reduction pathways has a very different isotope effect, leading to Fe(II) with dramatically different isotopic ratios.

First, exposing Fe(III) bound to an organic ligand to direct sunlight leads to photochemical reduction of Fe(III) to Fe(II) by ligand-to-metal charge transfer. Positive Δδ⁵⁶Fe values as high as +1.53 ‰ result from this method of reduction indicating a positive isotope effect.

Secondly, iron can be chemically reduced from Fe(III) to Fe(II) using a using a reducing agent such as hydroxylamine hydrochloride. Chemical reduction of Fe(III)-EDTA with hydroxylamine hydrochloride has an isotope effect of Δδ⁵⁶Fe = -3.61 ‰.

Thirdly, iron can be reduced from Fe(III) to Fe(II) electrochemically by applying a voltage to a rotating disk electrode. By controlling the effects of electric potential (voltage) and mass transport (diffusion), both a negative and a positive isotope effect can be simulated.

Iron reduction is an important process that impacts the bioavailability of iron for phytoplankton. Understanding how iron isotopes fractionate when Fe(III) is reduced will lead to a greater understanding of iron cycling in the ocean.