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 (Ca-5(PO4)(3)OH, 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 (Pb-5(PO4)(3)Cl) 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.