Transmission electron microscopy of a phosphate effect on the colloid structure of iron hydroxide

Abstract

Transmission electron microscopy (TEM) has been used to study phosphate association with iron hydroxide colloid and consequent alteration of aggregated colloid structure. Three complementary sample preparation procedures were tested and images derived from them were correlated. A Nanoplast preservation and embedding procedure was found to minimize particle aggregation artifacts. With Nanoplast, primary particle size down to 1 nm was detected. Solvent dehydration followed by epoxy resin embedding produced similar results to Nanoplast embedding, although the use of epoxy resin resulted in a more time consuming procedure and image resolution was not as good. The TEM results show that aggregated iron hydroxide colloid has a ramified chain structure, extending in three dimensions to produce a highly porous, sponge-like precipitate. The size of the primary particles visualized was in the 1–4 nm range. Submicron scale elemental analysis for Fe and P in ultrathin sections of embedded colloid was done using energy dispersive X-ray spectroscopy (EDX) in a scanning transmission electron microscope (STEM). An increased P Fe ratio in the starting solution for producing iron-rich colloids led to an increased P Fe ratio in the resulting coloids; it also caused an alteration of the chain structure of iron hydroxide colloid, leading to large agglomerates. This provides evidence of a phosphate effect on the aggregation behaviour of iron hydroxide. A study of field samples from a wastewater treatment plant suggests that phosphate incorporation by iron hydroxide colloid might be the main mechanism for chemical phosphorus removal in a municipal wastewater treatment process using iron salts.