Study of Calcium/Lead Apatite Structure Type for Stabilising Heavy Metals
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Study of Calcium/Lead Apatite Structure Type for Stabilising Heavy Metals Z.L.Dong, B.Wei and T.J. White Dept. Solid State Characterisation & Materials Processing, Environmental Technology Institute, 18 Nanyang Drive, Singapore 637723, SINGAPORE ABSTRACT (CaxPb10-x)(VO4)6F2 apatites were synthesised and their microstructures were studied using powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy before and after leach testing. X-ray diffraction showed that the apatites were hexagonal with a ≈ 10Å and c ≈ 7Å. During the leach test, Pb was released into the solution more slowly than Ca, which is desirable as the immobilisation of Pb is of importance. The experimental results also showed that V was almost undetectable in the leaching test solutions. In the (Ca7Pb3)(VO4)6F2 pellet, Ca and Pb distributions were not homogenous from one grain to another. Microstructural evidence from scanning electron microscopy revealed that the dissolution via development of etch pits began at grain boundaries and inside grains, and progressed faster in Ca rich regions. These results suggest that apatites of high Pb to Ca ratio are more durable.
INTRODUCTION Solid Waste Incineration (SWI) processes generate bottom ash and fly ash, as well as flue gas. The fly ash normally contains heavy metal oxides, such as PbO and V2O5, and other hazardous compounds [1,2]. One way to reduce the impact of fly ash on the environment is to stabilise the toxic species before disposal. The tailored addition of extra materials can promote the formation of more durable structure-types, eg., olivine, apatite, spinel and anhydrite structures [2-5]. This investigation is focused on the formation of apatite structures containing the heavy metals Pb and V, and the leaching characteristics of the ceramic pellets. The results of the investigation can be applied not only to incinerator ash stabilisation, but also to other industrial solid wastes.
EXPERIMENTAL DETAILS Apatites of composition (CaxPb10-x)(VO4)6F2 were synthesised from AR grade CaO, CaF2, V2O5 and PbO powder mixtures by firing at 800°C for more than 10 hours. The apatites so obtained were pressed into pellets and heat treated again at 800°C. Leach tests were performed to determine the mechanism of dissolution and the leach rate of each element in the pellets. The leach solution used was TCLP prescribed acetic acid of pH 2.88. The concentrations of Pb, V and Ca released into solution were measured using wavelength dispersive X-ray fluorescence spectrometor (XRF) after periods of 3, 5 and 7 days. A Siemens D5005 X-ray diffractometer (XRD), JSM-5310LV scanning electron microscope (SEM) and
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JEM-3010 transmission electron microscope (TEM) were used for structural characterisation prior to and after leach tests.
RESULTS AND DISCUSSION Formation of apatite structure Apatite is a general term for crystalline materials with the composition of A10(BO4)6X2. In this study, (Ca9Pb1)(VO4)6F2, (Ca7Pb3)(VO4)6F2 and (Ca5Pb5)(VO4)6F2 pellets were obtained by firing the
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