Phosphorus recovery using porous calcium silicate hydrate as seed crystal in form of hydroxyapatite

A phosphorus crystallisation process for recovering phosphate was developed using porous calcium silicate hydrate (PCSH) as seed crystal. Phosphorus can be removed by producing hydroxyapatite (HAP) on the surface of seed crystal, which can be applied to the recovery of phosphates from wastewater. Influence factors were investigated. Results indicated that increasing the initial concentration of phosphorus, intensifying the stirring speed and decreasing the sample dosage were beneficial for the increase in phosphorus content in the resulting samples. The phosphorus content of samples after phosphorus recovery could reach 19·05%. On the basis of Ca2+ and OH− releasing from PCSH and characterisation results from X-ray diffraction, field emission scanning electron microscopy and Brunauer–Emmett–Teller, the mechanism of phosphorus recovery by PCSH as seed crystal in the weak alkaline condition was revealed. A large specific surface area and porous structure of PCSH provided local conditions to form a proper concentration for Ca2+ and OH− releasing. This condition could maintain the pH value at 8·0–9·0 in order to improve the production of HAP on the surface of PCSH.