Effect of hydroxyapatite particle size on the formation of chloropyromorphite in anglesite–hydroxyapatite suspensions

Anglesite (PbSO4), a widely available form of Pb whose bioavailability has been the focus of a few studies, was selected in the present study and reacted with synthetic hydroxyapatites (Ca5(PO4)3OH, HAPs) of different sizes (approximately 20.08 nm, 60.24 nm, and micron-sized) at various P : Pb molar ratios and under various constant pH conditions. Particle size was discovered to strongly affect the immobilisation of Pb and the transformation from anglesite to chloropyromorphite (Pb5(PO4)3Cl) by the HAPs, in addition to the dissolution properties of HAP. The complete transformation from anglesite to chloropyromorphite was achieved at a pH of 4–5 for 20.08 nm-sized HAPs, whereas incomplete transformations of the 60.24 nm and micro-HAPs were obtained at these pH. At pHs of ≥6 (HAPs of size 20.08 nm) and ≥5 (60.24 nm), the smaller-particle HAPs were incompletely dissolved, and their surfaces were coated by newly formed chloropyromorphite, which was in contrast to the micro-HAPs, which were at pH ≥ 3. These results demonstrated that the surface coating rate decreased when the HAP particle size was decreased; thus, smaller-particle HAPs completely immobilised anglesite by forming chloropyromorphite at higher pH, particularly when the P : Pb molar ratio was high. This study demonstrated that nano-HAPs have potential for use in the situ immobilisation of Pb in low-acidity soil and waste without requiring the P solubility enhancement through the use of acidic conditions or an increase in the number of HAPs.