Immobilization of Pb in Alkaline Soil Amend with Nano-hydroxyapatite: Speciation, Bioavailability, and Bioaccessibility.

With the development of nanotechnology, nano-hydroxyapatite (nHAP) is gradually used for the immobilization and remediation of heavy metals in soil. However, there are few studies on the immobilization of Pb in alkaline agriculture soil amend with nHAP. This study applied 0.5% and 1% nHAP as remediation materials to two Pb contaminated alkaline soils (A and B) from the surrounding of substation in northern China. Comprehensive evaluations of Pb mobility were conducted using the Tessier sequence extraction method, DTPA extraction, and in vitro gastrointestinal simulation (SBRC and PBET methods). The Tessier sequence extraction results revealed that following nHAP addition, there was a shift of Pb towards the residual fraction, with decreases in exchangeable, carbonate, and Fe/Mn oxide fraction. The risk assessment code (RAC) decreased from 27.7 to 36.0% to 21.9-35.1%, and the mobility factor (MF) dropped from 78.8 to 85.9% to 65.6-75.0%. DTPA extraction indicated contrasting effects: the bioavailable Pb extracted by DTPA assay in soil A increased from 36.6% to 43.0-58.3%, while the bioavailable Pb in soil B decreased from 68.3% to 48.4-52.7%. Similar to DTPA results, the Pb bioaccessibility of soil A increased from 63.9% to 41.9% to 70.0-72.6% and 47.6-63.8% in SBRC and PBET gastric phase after 0.5% and 1.0% nHAP treatments 90 days. However, the Pb bioaccessibility of soil B decreased from 76.6% to 56.5% to 54.9-56.5%, 35.0-35.6% by SBRC and PBET, respectively. The study highlights the potential of nHAP in alkaline soil, but the immobilization efficiency varied with soil characteristics.