Gry Lyngsie, PhD student working at the University of Copenhagen, is busy searching for and testing filter materials that have a high affinity for phosphate, but at the same time have a particle size that does not violate a satisfactory hydraulic conductivity in the filters that should be filled with the filter material.
Very few materials have a satisfactory binding at relevant and low phosphate concentrations in drainage water. Some materials even desorb phosphate, and some materials trap phosphate in forms that dissolve during desorption or during shifts in water chemistry, e.g. shift to a lower pH. Future work will focus on Fe and Al oxide based sorbents, including engineered and modified sorbents, as these sorbents show higher phosphate sorption affinity and less desorption than Ca oxide (hydroxide or carbonate) based systems.
The figure gives an example of two phosphate sorption isotherms for filter materials, which have the same particle size, but which consist of different sorbents. The Fe-oxide rich sorbent is much more effective than the Ca-rich sorbent.