Quantitative determination of cerussite (lead carbonate) by X-ray powder diffraction and inferences for lead speciation and transport in stream sediments from a former lead mining area in Scotland

Most current investigations of sites contaminated with heavy metals (e.g. P b, Zn, Cu) emphasise the importance of determining the amounts of physical and chemical forms of metals rather than just the total amounts present. Ch emical extraction techniques used for this purpose are inevitably operation ally defined. A more direct approach to the identification of crystalline f orms can be made by mineralogical techniques such as X-ray powder diffracti on (XRPD), but quantitative determination of a particular form is not often attempted. Recent advances in methods of analysis and sample preparation f or XRPD mean that it is now a relatively simple matter to obtain quantitati ve XRPD data. Here, it is applied to the quantitative determination of the forms of Pb in different size-fractions of stream sediment samples from Lea dhills/Wanlockhead, SW Scotland, an historic Pb mining area. Comp arisen of the XRPD analyses with determinations of Pb by atomic absorption spectroph otometry demonstrates that a large proportion of the Pb present in the stre am sediments is in the form of cerussite (PbCO3). Furthermore, the cerussit e tends to be concentrated in the silt fraction and is even a minor compone nt of the clay-size fraction. However, quantitative analysis of fractions < 6 <mu>m indicates that cerussite alone cannot account for all the Pb in th is size range. Indirectly, this result suggests that Pb adsorbed to clay mi nerals, organic matter and/or amorphous Fe and Mn oxides may be proportiona lly more important for the <6 <mu>m materials. Sediment in this size range, however, typically accounts for no more than 1% by weight of the total str eam bed sediment samples collected in the study area. In relation to its si ze distribution, the mobility of Pb within the wider environment is most li kely to occur principally through physical transport of fine particles. (C) 2001 Elsevier Science Ltd. All rights reserved.