An isotopic investigation of some sequential extraction schemes

The use of sequential extraction schemes for fractionation of metals in different substrates has grown exponentially over the past two decades. Despite the popularity of these schemes, they have been criticized for metal redistribution and non-selectivity by the reagents used. The work presented in this dissertation is aimed at addressing these problems by, (i) an isotopic test of the Tessier scheme, (ii) an isotopic test of different modifications of the Tessier scheme, (iii) an isotopic test of the Hall scheme and comparing it with the Tessier scheme, (iv) an isotopic test of different modifications of the Hall scheme, (v) a comparison of the labile metal recovery by the Tessier scheme, the Hall scheme and the modified Hall scheme to that obtained by a single step concentrated nitric acid leach (the EPA method 3050). Three Canadian soil standards, SO-2, TILL-1 and TILL-3 were analyzed in order to make use of the natural variations in the isotopic composition of elements in the Archean progenitor materials that formed these soils.; An isotopic test of the Tessier scheme showed that each reagent extracts a soil portion with a distinct lead and neodymium isotopic composition. Isotopic inhomogeneity implies that little or no metal redistribution takes place when the scheme is used. Meanwhile, an isotopic test of some modifications of the Tessier scheme revealed that hydroxylamine hydrochloride and hydrogen peroxide were non-selective in the case of lead, but selective for samarium and neodymium. A comparison of the Hall and Tessier schemes revealed that the Hall scheme has better metal recovery and that potassium chlorate extracted an isotopically distinct soil portion. A comparison of the labile metal recovery by the Tessier scheme, the Hall scheme and the modified Hall scheme to that obtained by a single step concentrated nitric acid leach (EPA method 3050) showed that the single step EPA method 3050 extracted more lead than the sequential extraction schemes. Meanwhile the modified Hall scheme extracted more samarium and neodymium than the EPA method 3050. It was also found out that there was an increase in the lead isotopic composition as the strength of the reagents increased. As such, a seven step sequential extraction scheme for the extraction of lead was proposed. This scheme was used to analyse three Canadian sediments; HR-1, TH-2 and SUD-2. Five reagents used in the first five steps recover lead with a similar isotopic composition for each sample and it accounts for 78% to 96% of the total lead in the samples. Therefore a rapid, less costly and easy means to access the labile lead pool in contaminated sediments is to use the single step hot concentrated nitric acid leach (EPA method 3050).