Characterization of phase evolution during lead immobilization by synthetic hydroxyapatite

https://doi.org/10.1016/j.matchar.2004.08.002Get rights and content

Abstract

Immobilization of toxic metals by calcium phosphates is a promising technology for treating contaminated soil, water and wastes. A detailed study on the mechanisms of lead immobilization by hydroxyapatite has been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). For this, synthetic hydroxyapatite powder were submitted to a sorption process through exposure to an aqueous solution containing 917 mg L−1 of lead for times that varied from 3 min to 54 h. The results obtained reinforce the hypothesis that hydroxypyromorphite formation is the end of a kinetic process in which the hydroxyapatite crystals are continuously dissolved and recrystallized in order to form more stable structures with higher lead content. Consequently, the use of calcium phosphates to immobilize lead ions seems to be technically viable.

Introduction

Currently, several remediation technologies are available to reduce and remove heavy metals from soil, wastes and underground waters [1]. These technologies, which are based on physical, chemical or biological treatments, are either too costly or only partially effective. For example, the technique of soil removal can effectively remove harmful elements but this is an expensive procedure. Immobilization in situ, like geochemical reactive barriers, is a promising technology that can remove and/or stabilize metals in soil, wastes and water, and frequently results in a substantial reduction of costs.

Among heavy metals, lead is considered one of the most toxic, even in low concentrations. Despite the fact that its use in paints and gasoline has been restricted in the last decade, high lead concentrations have been observed in several kinds of soil. A soil is considered contaminated when the lead concentration exceeds 300 mg kg−1, and a clean up step is required when concentration is higher than 400 mg kg−1 [2].

Immobilization of lead and other toxic metals by synthetic hydroxyapatite [HA] (Ca10(PO4)6(OH)2) is a promising remediation technology for contaminated soil and wastes [3], [4], [5]. The high efficiency of uptake by apatite of metals such as lead, cadmium, zinc, magnesium, copper, iron and strontium has encouraged extensive fundamental research concerning the mechanisms involved in metal immobilization. Several mechanisms, such as ion exchange at the hydroxyapatite, surface complexation, hydroxyapatite dissolution followed by metal phosphate precipitation and co-precipitation, have been proposed in the literature [4], [6], [7], [8], [9], [10]. In a recent X-ray diffraction (XRD) study, Mavropoulos et al. [10] proposed that the lead immobilization by HA was controlled by the hydroxyapatite dissolution and the formation of a new lead and calcium phosphate solid solution, Pb(10−x)Cax(PO4)6(OH)2 (PbCaHA). The HA crystallites were continuously dissolved and recrystallized in order to form structures with lower calcium content, with Pb2+ ions mostly occupying Ca(II) sites [10]. It was also observed by these authors that the solid solution mean crystal size along the hydroxyapatite (300) and (002) directions increased during the lead immobilization. However, up to now, no other technique besides XRD has been used to confirm these results.

The aim of this work is to investigate the modification of crystal morphology and composition during lead uptake by synthetic hydroxyapatite by using electron microscopy in order to confirm previous results concerning the mechanisms of lead immobilization by this material.

Section snippets

Experimental procedures

Hydroxyapatite powder, with a calcium-to-phosphorus molar ratio of 1.68 and specific surface area BET of 53 m2/g, were submitted to a sorption experiment [10]. For this, powder samples (0.100 g) were added to 40 ml of aqueous solutions containing 917 mg Pb2+ L−1 using Pb(NO3)2·4H2O. The solutions were kept mechanically shaken for 54 h at 25 °C. The solid fraction was separated from the solution by centrifugation after 3 min, 2, 6, 24 and 54 h, then washed and dried. The lead content into

Results and discussion

Fig. 1 shows the TEM image of HA crystals before the sorption experiment. These crystals exhibit the needle-like morphology commonly observed on HA synthesized by the co-precipitation method [12]. The EDS spectrum (data not shown) indicated that the calcium-to-phosphorus relative peaks intensity corresponds to a calcium-deficient hydroxyapatite. Fig. 2 presents the concentration of lead immobilized by HA (Pb2+ initial concentration of 917 mg L−1). It can be observed that the Pb2+ uptake by HA

Conclusions

From the electron microscopy analysis we have obtained direct evidence of the immobilization of lead by hydroxyapatite, leading to the formation of Pb(10−x)Cax(PO4)6(OH)2 (PbCaHA) and hydroxypyromorphite (PbHA). The crystals of the PbHA phase exhibit a plate-like morphology and size bigger than the original hydroxyapatite crystals. Moreover, the use of the TEM permitted the confirmation of the existence of PbCaHA as an intermediate phase, confirming the hypothesis of the XRD results obtained

Acknowledgements

The authors thank several Brazilian agencies (CNPq, CAPES, FAPERJ and FUJB) for their financial support. They would also like to thank the Military Institute of Engineering for the use of TEM facilities.

References (13)

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