Selective leaches revisited, with emphasis on the amorphous Fe oxyhydroxide phase extraction

A sequential extraction scheme for application to soils, tills and surficial sediments is described that elucidates the form in which an element is held and provides information as to its provenance. The phases selected for extraction have been categorised as: adsorbed/exchangeable/carbonate using CH3COONa as extractant); amorphous Fe oxyhydroxide (0.25 M NH2OH . HCl); crystalline Fe oxide (1 M NH2OH . HCl); sulphides and organics (KClO3/HCl); and residual, mainly silicates (HF-HClO4-HNO3-HCl). Particular attention has been paid to the specificity of the reagent used for extraction of the amorphous Fe oxyhydroxide phase, well recognised for its scavenging properties of trace metals in the surficial environment. Atomic absorption spectrometry (AAS) was the main analytical technique employed to determine the elements Zn, Cu, Pb, Ni, Co, Mn and Fe. Inductively coupled plasma emission spectrometry (ICP-ES) and ICP mass spectrometry were used to extend this element suite. Results from standard reference materials, TILL 1-4 and LKSD-4, indicate long term relative standard deviations (RSD) for Zn, Cu, Pb, Ni, Co, Mn and Fe extracted from the five phases of 5-12%, at concentrations a decade above detection limits. Comparison of results for the TILL series with those obtained by direct digestion in HF-HClO4-HNO3-HCl shows that losses of element through the various manipulations of sequential extraction are minimal (< 10%). Replicate analyses of 1-g subsamples of soil, till and humus show excellent precision, at 3-10% RSD. A second extraction with fresh solution in each of the first three leaches was examined. An additional recovery of 35-40% for the CH3COONa extraction, 20-25% for 0.25 M NH2OH . HCl, and 15-20% for the 1 M NH2OH . HCl reagent was seen for Zn, Cu, Pb, Ni and Co. Manganese behaves differently: a less consistent pattern between the two leaches is shown by the 221 samples rested; and the second application generally dissolves proportionately less than is the case for the other elements. Omission of this second step could lead to erroneous interpretation of the results. The relatively high concentration of HCl, at 0.25 M, used in the extraction of the amorphous Fe oxyhydroxide phase leads to significant dissolution of sphalerite and galena. Less than 1% of these sulphides is dissolved if the acid strength is reduced to 0.05 M. However, at either HCl concentration, this attack was shown to dissolve a considerable amount of ''soluble'' organic component. As much as 15000 ppm C was dissolved from the SRM LKSD-4 by this leach, an amount equal to 63% of that extracted by 0.1 M sodium pyrophosphate which is designed to dissolve metals bound to humate and fulvate components of soil.