Physico-chemical characteristics of a colloidal iron phosphate species formed at the oxic-anoxic interface of a eutrophic lake

Abstract

Natural iron oxyhydroxide colloidal particles have been isolated from the redox transition boundary layer in a eutrophic lake. They have been physically and chemically characterized on a particle specific basis using new technology from the biomedical sciences which enables minimization of artefacts of particle sizing and optimization of the microscopical parameters for chemically quantifying their elemental composition by STEM/EDS. The Fe-rich particles have also been microscopically analysed using classical techniques. The Fe particle size range lies between 0.4 and 0.04 μm, the latter limit being determined by experimental conditions. Particles larger than 0.4 μm have also been observed but are loose aggregates of smaller Fe-rich particles. Chemically, the predominant particle type contains Fe, P and Ca exclusively, although occasional associations of the latter elements with Si or Si/Al occur. The former three elements are strongly correlated, and the mean molar ratios in the particles are 0.25, 0.19 and 1.34 for P/Fe, Ca/Fe and P/Ca, respectively. The Fe is approximately 50% Fe(II) and the P is present as phosphate. Although a good correlation exists between Al and Si, neither is significantly correlated to Fe, P or Ca. The chemical composition is relatively constant spatially seasonally and yearly. These results indicate that a well-defined chemical entity is formed at the redox interface in the water column, composed mostly of Fe(III), Fe(II), PO 4 and Ca (and probably OH −).