Assessment of the phase selectivity of the European Community Bureau of Reference (BCR) sequential extraction procedure for metals in sediment

Sequential extractions potentially offer useful information on how metals are bound to sediment. For example, reagents used in such extractions may be expected to selectively release metal bound to carbonates, iron and manganese oxides or organic/sulphidic material. However, various criticisms have been levelled at sequential methods, including that of inaccuracy in releasing metal from specific geochemical phases. This study examines specificity by analysing individual mineral phases previously equilibrated with metal-spiked artificial sea water. Substrates were then sequentially extracted according to the three-step BCR procedure. The distribution of recovered metal between extracts was compared to that expected if reagents were acting on a specific phase. Acetic acid released most of the metal associated with calcium carbonate, kaolinite, potassium felspar and ferrihydrite. Hydroxylamine hydrochloride contained most of the recovered metal from montmorillonite and manganese dioxide, as well as nickel from humic acid. Iron oxides are expected to be attacked by this reducing agent, but the majority of the metal had already been removed by the first extract (acetic acid). This may reflect the high absorption capacity of ferrihydrite. Zinc on humic acid was split between the first two reagents. The third extraction, hydrogen peroxide/ammonium acetate, which might be expected to release metal from organic or sulphidic material, only significantly recovered the added copper from humic acid. Total recoveries of the added metal were high, except from humic acid, felspar and montmorillonite, suggesting strong metal binding by these substrates. Variability between carefully controlled experiments raises some questions about the reproducibility of the extraction procedure which may be exacerbated when applying the method to real sediments.