Insights into dynamic adsorption of lead by nano-hydroxyapatite prepared with two-stage ultrasound

Nano-hydroxyapatite (nHAP) has an excellent effect on the remediation of Pb contaminated water and soil. In this study, an efficient modified nHAP was prepared assisted with two-stage ultrasonic irradiation. The effects of ultrasound modification on the nHAP were tested using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform InfraRed spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and combined batch Pb uptake experiments. The nHAP with ultrasound has a fine structure with the width and length of around 9 nm and 40 nm respectively. The ultrasound parameter of 1s/36s in stage 1 and 16s/18s in stage 2 was verified as the optimum under which the nHAP prepared performed the best with the maximum adsorption capacity of 1300.93 mg/g. The results of XRD and SEM indicated that the sorbent after uptake of Pb²⁺ was mainly Pb₁₀(PO₄)₆OH₂ (HPY) with insignificant Ca₁₀Pb_10-x(PO₄)₆OH₂. Compared the results of Pb/Ca, pH and XRD with the metal fraction of Pb in adsorbents during the dynamic sorption process, this research proved that the effects of complexation, cation exchange and dissolution and precipitation coexisted in the initial stage, while the dissolution and precipitation gradually dominated the adsorption mechanism with contact time. The processes of Pb²⁺ uptake by nHAP sorbents prepared under different ultrasound parameter presented almost the same dynamic mechanism with a little difference in time node. The research of dynamic mechanism of Pb²⁺ uptake by a superior nHAP is essential for both contaminated water and soil remediation.