Characterizing the Mechanisms of Lead Immobilization via Bioapatite and Various Clay Minerals

Abstract

Immobilizing lead (Pb) in contaminated water and soils via mineralization is an emerging field of interest in environmental remediation. This study investigated the feasibility of applying bioapatite and typical clay minerals (kaolinite, palygorskite, and montmorillonite) to immobilize Pb2+ cations in water. The mechanisms of lead immobilization were studied by inductively coupled plasma optical emission spectrometry (ICP–OES), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). Montmorillonite shows the highest efficiency in Pb remediation (reduced from ∼2000 to 30 ppm) with the addition of bioapatite. The XRD and HRTEM results demonstrated that aqueous Pb removal efficiency is facilitated by bioapatite via reacting with Pb to form pyromorphite mineral [Pb5(PO4)3(F,Cl,OH)]. The high surface area and cation-exchange capability of montmorillonite allow its abundant absorption of Pb2+ and, hence, cause the enriched formation of pyromorphite on its surface. In contrast, the low surface area of kaolinite does not allow substantial absorption of Pb, and pyromorphite was primarily formed in the solution rather than on its surface. In addition, some Pb2+ cations were trapped within the mineral fibrous aggregates of palygorskite, which limits the lead immobilization via the formation of pyromorphite on its surface or within the fibrous aggregates. This study sheds light on the bright future of the application of bioapatite and montmorillonite in Pb-contaminated water and soils.