Phosphorus retention by granulated apatite: assessing maximum retention capacity, kinetics and retention processes

Natural apatites have previously shown a great capacity for phosphate retention from wastewater. However, its fine particle size distribution may lead to a premature clogging of the filter. Accordingly, a granulated apatite product was developed and manufactured in order to control the particle size distribution of the media. Experiments were conducted on laboratory columns to assess their phosphorus retention capacity, to identify the processes involved in phosphorus retention and to evaluate their kinetic rates. The results showed phosphorus retention capacities of 10.5 and 12.4 g PO₄-P·kg^-1 and kinetic rate coefficients in the range of 0.63 and 0.23 h^-1 involving lower values than those found for natural apatites in previous studies. Scanning Electron Microscopy images showed that apatite particles in the granules were embedded in the binder and were not readily accessible to act as seeds for calcium phosphate precipitation. The retention processes differ depending on the supersaturation of the solution with respect to calcium phosphate phases: at low calcium concentrations (69.8 ± 3.9 mg·L^-1), hydroxyapatite precipitates fill up the porosity of the binder up to a depth of 100-300 μm from the granule surface; at higher calcium concentrations (112.7 ± 7.4 mg·L^-1) precipitation occurs at the granule surface, forming successive layers of hydroxyapatite and carbonated calcium phosphates.