SELECTIVE LEACHES REVISITED, WITH EMPHASIS ON THE AMORPHOUS FE OXYHYDROXIDE PHASE EXTRACTION

A sequential extraction scheme for application to soils, tills and sur ficial sediments is described that elucidates the form in which an ele ment is held and provides information as to its provenance. The phases selected for extraction have been categorised as: adsorbed/exchangeab le/carbonate using CH3COONa as extractant); amorphous Fe oxyhydroxide (0.25 M NH2OH . HCl); crystalline Fe oxide (1 M NH2OH . HCl); sulphide s and organics (KClO3/HCl); and residual, mainly silicates (HF-HClO4-H NO3-HCl). Particular attention has been paid to the specificity of the reagent used for extraction of the amorphous Fe oxyhydroxide phase, w ell recognised for its scavenging properties of trace metals in the su rficial environment. Atomic absorption spectrometry (AAS) was the main analytical technique employed to determine the elements Zn, Cu, Pb, N i, Co, Mn and Fe. Inductively coupled plasma emission spectrometry (IC P-ES) and ICP mass spectrometry were used to extend this element suite . Results from standard reference materials, TILL 1-4 and LKSD-4, indi cate long term relative standard deviations (RSD) for Zn, Cu, Pb, Ni, Co, Mn and Fe extracted from the five phases of 5-12%, at concentratio ns a decade above detection limits. Comparison of results for the TILL series with those obtained by direct digestion in HF-HClO4-HNO3-HCl s hows that losses of element through the various manipulations of seque ntial extraction are minimal (< 10%). Replicate analyses of 1-g subsam ples of soil, till and humus show excellent precision, at 3-10% RSD. A second extraction with fresh solution in each of the first three leac hes was examined. An additional recovery of 35-40% for the CH3COONa ex traction, 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 di fferently: a less consistent pattern between the two leaches is shown by the 221 samples rested; and the second application generally dissol ves proportionately less than is the case for the other elements. Omis sion 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 signif icant dissolution of sphalerite and galena. Less than 1% of these sulp hides is dissolved if the acid strength is reduced to 0.05 M. However, at either HCl concentration, this attack was shown to dissolve a cons iderable 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.