Synthesis and solubility of brompyromorphite Pb5(PO4)3Br

Urszula Janicka,Maciej Manecki,Tomasz Bajda

doi:10.2478/v10002-012-0004-4

Abstract

AbstractThe bromide analogue of pyromorphite Pb5(PO4)3Br was synthesized and characterized by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. The solubility of the brompyromorphite was measured at 25°C and pH values of 2.0, 2.6 and 3.2. For the 3 pH measurements, the average solubility product, log KSP, for the reaction Pb5(PO4)3Br ⇔ 5Pb2+ + 3PO- 3- + Br- at 25ºC is -77.38 ± 0.70. The free energy of formation, ΔG°f,298, calculated from this measured solubility product is -3724.7 ± 4.3 kJ mol−1. These results confirm that brompyromorphite is more soluble than pyromorphite.

Highlights

Because of numerous possible substitutions, over 40 different minerals belong to the apatite supergroup (Pan, Fleet 2002; Pasero et al 2010)
The white precipitate produced in the synthesis was identified as the bromide analogue of pyromorphite Pb5(PO4)3Br by comparing peak positions with those of Br-pyromorphite reported in Joint Committee on Powder Diffraction Standards (JCPDS) cards 32-0533 and 06-0365
Calculated unit cell parameters are: a = 10.074(1) Å and c = 7.373(1) Å. These values agree well with the values of a = 10.145 Å and c = 7.347 Å reported for synthetic lead bromide phosphate (JCPDS card 32-0533) and with the values of a = 10.1 Å and c = 7.38 Å reported for the lead bromide phosphate (JCPDS card 06-0365)

Summary

Introduction

Because of numerous possible substitutions, over 40 different minerals belong to the apatite supergroup (Pan, Fleet 2002; Pasero et al 2010) This fact has inspired many studies of synthetic solid solutions based on the apatite structure. Pyromorphite is one of the most stable lead compounds in earth-surface environments and its precipitation is one of the effective methods used to remove lead ions from contaminated soils and solutions (Cotter-Howells 1996; Shevade et al 2001; Krik et al 2002) Other pyromorphite variants such as brompyromorphite (BrPY) are less well known (Nriagu 1973). The aims of this study are the characterization of the synthetic bromide analogue of pyromorphite and the determination of its solubility product based on the results of dissolution experiments

Objectives

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Results

Conclusion