Phosphate removal by mineral-based sorbents used in filters for small-scale wastewater treatment

The mineral-based sorbents Filtra P, Polonite ®, natural wollastonite and water-cooled blast furnace slag (WCBFS) were studied in terms of their PO 4 removal performance. Results from a long-term column experiment showed that both Filtra P and Polonite ® removed >95% of PO 4 from the applied synthetic solution, and that the used filter materials had accumulated several (1.9–19) g kg −1 P. Phosphorus was removed also by natural wollastonite and WCBFS, but these materials were less efficient. Batch experiments on the used materials showed that the solubility PO 4 was considerably larger than the one expected for crystalline Ca phosphates such as hydroxyapatite, and results from investigations with attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR) on the Filtra P material showed that the formed P phase was not crystalline. These evidence suggest that a soluble amorphous tricalcium phosphate (ATCP) was formed in the mineral-based sorbents; the apparent solubility constant on dissolution was estimated to log K s=−27.94 (±0.31) at 21 °C. However, since only up to 18% of the accumulated PO 4 was readily dissolved in the experiments, it cannot be excluded that part of the phosphorus had crystallized to slightly less soluble phases. In conclusion, Filtra P and Polonite are two promising mineral-based sorbents for phosphorus removal, and at least part of the accumulated phosphorus is present in a soluble form, readily available to plants.