Article http://pubs.acs.org/journal/aesccq
Characterizing the Mechanisms of Lead Immobilization via Bioapatite and Various Clay Minerals Zhen Li,*,†,‡ Lingyi Tang,† Yangfan Zheng,† Da Tian,† Mu Su,† Fan Zhang,† Shuojia Ma,† and Shuijin Hu*,†,§ †
Jiangsu Key Laboratory for Organic Waste Utilization, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, Jiangsu 210095, People’s Republic of China ‡ State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing, Jiangsu 210046, People’s Republic of China § Department of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27695, United States 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. KEYWORDS: lead immobilization, bioapatite, pyromorphite, clay, montmorillonite possibility of desorption.10 Moreover, some common Pbrelated minerals, e.g., Pb(NO3), PbC2O4, and PbCO3, are partly soluble in the existing soil environment.14,15 The formation of these minerals, hence, cannot promise complete remediation. Apatite has been assumed to be an ideal material in Pb immobilization via the formation of pyromorphite [Pyro, Pb5(PO4)3(OH,F,Cl)], which has a low Ksp value (∼10−70− 10−80).3,16,17 Therefore, apatite has been applied widely in lead remediation.3,18,19 The modified formulas for the apatite dissolution and Pb immobilization can be described as3,18
1. INTRODUCTION Lead (Pb) contamination in water and soils has been the result of increasing human activities, e.g., mining, battery manufacturing, gasoline consumption, and other solid waste disposal. Pb is one of the most common toxic heavy metals. Its remediation is of great environmental concern as a result of its hazardous effect on human health, which usually causes high toxicity in a natural environment.1−3 Lead immobilization via absorption by clay minerals has been widely investigated.4−10 Clay minerals usually have a high surface area (>50 m2/g), e.g., bentonite, illite, montmorillonite (Mon), and palygorskite (Pal).5,8,9,11 The 1:1-type clay kaolinite (Kao) usually has a low surface area (10−15 m2/ g).9 In addition, the morphology of mineral crystals is various; i.e., Mon and Kao have plate-like mineral crystals, whereas Pal has fibrous crystals.9,12 High cation-exchange capability (CEC) of clay minerals also contributes to their absorption of heavy metals.13 For example, high CEC value of Mon enhances the Pb2+ absorption into the interlayer or edge of the mineral crystals.7 Clay minerals had been applied as barriers in landfills to shield transportation of heavy metals to groundwater.4 However, the absorption of Pb onto clay minerals cannot achieve success of in situ remediation as a result of the © 2017 American Chemical Society
Ca5(PO4 )3 X + 6H+ ⇔ 5Ca 2 + + 3H 2PO4 − + X−
(1)
5Pb2 + + 3H 2PO4 − + X− ⇔ Pb5(PO4 )3 X + 6H+
(2)
However, the most common apatite, geological fluorapatite, has extremely low solubility (P release of
