Migration of phosphate into aggregated particles of ferrihydrite

Abstract

SUMMARY The slow reaction of phosphate with aggregated particles of ferrihydrite, after initial rapid phosphate sorption, was investigated by measuring the changes, with time and temperature, in the amount of phosphate sorbed, and the extractability of the sorbed phosphate. The ferrihydrite was, subsequently, recovered and examined by infra–red spectrometry (IR) and electron probe micro–analysis. Phosphate continued to react with ferrihydrite for at least 90 d at 25°C, but was completely recovered by extraction with 0.1 m NaOH. The IR spectra of sorbed phosphate was insensitive to the temperature and duration of the reaction. Electron probe micro–analysis of the aggregates showed that phosphate migrated to surface sorption sites within the aggregated particles of ferrihydrite. There was no evidence for the formation of surface coatings of ferric phosphate, for changes in the type of bonding, or for penetration of phosphate into the crystal lattice. The slow reaction was attributed to the migration of phosphate to surface sorption sites of decreasing accessibility within aggregates.