Size distribution of ferrihydrite aggregate and its implication for metal adsorption and transport

Adsorption experiments of Zn onto ferrihydrite were carried out at different initial Fe concentrations and pH, and at room temperature. Zinc-adsorbed ferrihydrite was fractionated to examine Zn adsorption among different-size aggregates of ferrihydrite. Primary ferrihydrite particles were as small as 5-7 nm in diameter and they formed aggregates of <0.25 to >32 μm in size. The dominant size of the aggregates decreased with a decrease in the initial Fe concentrations and pH. Zinc was adsorbed on the surface of individual ferrihydrite nano-particles but not on that of ferrihydrite aggregates, indicating that ferrihydrite nano-particles were so loosely attached to one another in aggregates that individual nano-particles could adsorb Zn. The size distribution of ferrihydrite aggregates and the Zn adsorption on individual ferrihydrite nano-particles can change the sedimentation rate of ferrihydrite aggregates and the behavior of Zn during the transformation of ferrihydrite to goethite and hematite. Therefore, our results will give a deeper understanding of metal transport by ferrihydrite.