Fe isotope fractionation during the precipitation of ferrihydrite and transformation of ferrihydrite to goethite

Abstract

Ferrihydrite and goethite are amongst the most important substrates for the sorption of contaminants in soil and other environmental media. Isotopic studies of the transition elements, particularly those that exhibit more than one oxidation state and show pH- and/or redox-sensitive behaviour at low temperatures, have been shown to be potentially useful present-day and past proxies for redox (or palaeoredox) conditions. We have made preliminary investigations of Fe isotope fractionation that take place during the formation of FeIII (oxy)hydroxides (FeIIIox) from an aqueous FeIII(NO3)3 solution (FeIIIaq) under laboratory conditions. We have attempted to keep the chemical system simple by excluding 'vital effects' and major changes in redox through the maintenance of abiotic conditions and use of FeIIIaq. Isotopic measurements (56Fe/54Fe, 57Fe/54Fe) of the FeIII(NO3)3 stock solution, the original ferrihydrite and the mixed ferrihydrite/goethite-supernatant FeIIIaq 'pairs' were carried out using a double focusing multicollector inductively coupled plasma mass spectrometer. The results reveal an apparent systematic variation indicating larger ΔFeIIIaq—FeIIIox with decrease in the ferrihydrite:goethite ratio, which reflects the time allowed for isotopic exchange. These values range from virtually zero (0.03%) after 24 h to 0.30% after 70 h. In each FeIIIox-FeIIIaq 'pair' the lighter Fe isotope is partitioned into the FeIIIox, leaving the FeIIIaq isotopically heavier. The observed fractionation reflects isotopic exchange of Fe between the FeIIIox and FeIIIaq upon at least a two step transition of ferrihydrite to goethite.

Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge

“…Oxidative precipitation of ferrihydrite is a combination of (2) and (3) because of the instability of Fe(III) aq species in seawater, producing an overall enrichment in the heavy isotope (Bullen et al, 2001). Further transformation of ferrihydrite to goethite has also been observed to cause isotope fractionation over a time scale of 70 h, but the age of the samples analysed in this study are much younger (Clayton et al, 2005). Adsorption and desorption of Fe(II) onto Fe-oxyhydroxide or Fe-sulfide phases is another potential fractionation mechanism in the plume, with isotopically heavy Fe(II) adsorbing to mineral surfaces.…”

A unique Fe-rich carbonate chimney associated with cold seeps in the Northern Okinawa Trough, East China Sea

“…A previous experiment has shown that Fe isotopic fractionation during the transformation of ferrihydrite into goethite increased with decreasing the ferrihydrite to goethite ratios, ultimately leading to an enrichment of the lighter Fe isotopes in goethite (Clayton et al, 2005). As we noted above, Fe (oxyhydr)oxides are the most possible products of the alteration of the dark minerals in the marine sediment rather than the crystallized Fe oxides (Yokoyama and Nakashima, 2005).…”

Metal Stable Isotopes in Paleoceanography

“…In recent years, researchers have also explored the natural variations of Ti (Zhu et al 2002b), Sn (Clayton et al 2005), Ag (Woodland et al 2005), and Te (Fehr et al 2004) isotopes, and although the variations are generally smaller than 0.1‰ amu −1 , these metal isotopes may find future applications in paleoceanography. Larger variations of stable isotope compositions of metalloids such as Se and Ge have been also reported.…”