By Pradeep Divvela, Project Chemist, Kelso, WA

Sequential extraction is an analytical process that chemically leaches metals out of soil, sediment and sludge samples. The purpose of sequential “selective” extraction is to mimic the release of the selective metals into solution under various environmental conditions.

One commonly used sequential extraction procedure is designed to partition different trace metals based on their chemical nature.

The sequential extraction process is typically accomplished in four (4) steps using:

1. Acetic acid to extract all exchangeable, acid and water soluble metals
2. Hydroxyammonium chloride to extract all reducible metals
3. Hydrogen peroxide to extract all oxidizable metals
4. Aqua regia to extract all remaining, non-silica bound metals

In each of the steps, calculated concentrations of chemicals and buffers are added and the sample is shaken on an end-over-end shaker. The leachate from each step is then digested and analyzed on an inductively coupled plasma atomic emission spectrometer (ICP/AES) or an ICP/mass spectrometer (ICP/MS), depending on the project requirement.

This multi-step procedure assures that all the metals of concern are completely extracted from the sample. The results from all the different steps are calculated and used to determine the accurate concentrations under different conditions. Factors such as pH of the acid used for adjustment, temperature and duration of extraction are the critical factors that control the concentration of metal extracted from the sample.

Sequential extraction procedure has been extensively studied in the analysis of trace metals including Cd, Co, Cr, Cu, Fe, Mn Ni, Pb and Zn in both river sediments and marine sediments.

Use of total metals concentration as a criteria to assess the potential effects of sediment contamination implies that all forms of metals have equal impact on the environment, which is clearly wrong. Although the total concentration of trace elements in soil gives some indication of the level of contamination, it provides no insight into element bioavailability or mobility. Elements in soils are present in various physicochemical forms, which in turn influence availability. Sequential chemical extraction techniques have been widely used to examine these physicochemical forms, and thus to better understand the processes that influence element availability.

References:

1. Jennifer M. Jones, Sequential extraction method: a review and evaluation, Environmental Geochemistry and Health, Volume 15, Numbers 2-3, September, 1993
2. Z.S. Ahnstrom and D.R. Parker, Development and Assessment of a Sequential Extraction Procedure for the Fractionation of Soil Cadmium, Soil Science Society of America Journal 63:1650-1658 (1999)
3. Shuman, L.M. 1991. Chemical forms of micronutrients in soils. p. 113–144. In J.J. Mortvedt et al. (ed.). Micronutrients in agriculture. 2nd ed. SSSA Book Ser. 4. SSSA, Madison, WI.

Tags: acetic acid, Aqua Regia, cadmium, Cd, chromium, Co, cobalt, copper, Cr, Cu, Fe, Hydrogen peroxide, Hydroxyammonium Chloride, Hydroxylamine hydrochloride, ICP/AES, ICP/mass spectrometer, ICP/MS, inductively coupled plasma atomic emission spectrometer, iron, lead, manganese, marine sediments, Metals, micronutrients, Mn, Ni, nickel, Pb, river sediments, Sequential extraction, soil, trace metals, zink, Zn

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