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

Transmission electron microscopy (TEM) has been used to study phosphate association;vith 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) ina 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 colloids; 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. Copyright (C) 1996 Elsevier Science Ltd