Colloidal iron oxyhydroxy-phosphate: The sizing and morphology of an amorphous species in relation to partitioning phenomena

Abstract

An iron-rich fraction was isolated from a small eutrophic lake and studied on a particle-specific basis by (i) conventional transmission electron microscopy (TEM) and (ii) scanning transmission electron microscopy in conjunction with energy-dispersive spectroscopy (STEM/EDS). Additionally, the fraction per se was characterized by classical wet chemistry. A morphologically distinctive particulate was discovered to be rich in iron (in an X-ray amorphous form) and phosphorus (as phosphate), and it also contained some calcium. Particle sizes determined by TEM fell entirely into the colloidal range, with a great majority of those trapped by a cutoff filter of 0.45 μm showing a mean diameter < 0.13 μm. Several morphotypes could be identified at 3 nm resolution in ultrathin sections of plastic-embedded colloid fractions. New technology transferred from the biomedical sciences was used to obtain ca. 1 nm resolution, and revealed (i) irregular arrays of subunits with a least diameter approaching the resolution limit and (ii) many varieties of the principal morphotypes. A range of 130 nm down towards the molecular range was found to be more representative of the true particle size distribution than one derived solely from filtration data based on filter pore sizes. Size distributions established by cascade filtration assessments are shown to be related to colloid aggregation phenomena, which may affect phosphate partitioning between the operationally-defined solid and soluble phases.