A critical evaluation of the three-stage BCR sequential extraction procedure to assess the potential mobility and toxicity of heavy metals in industrially-contaminated land

Cadmium, chromium, copper, lead, manganese, nickel, vanadium and zinc have been determined in samples of made-up ground from layers of a trial pit excavated on a recently derelict, industrially contaminated site. The pseudototal metal content of the layers was determined following a microwave-assisted digestion with aqua regia. Operational speciation was performed using the BCR three-step sequential extraction procedure. Analyses were carried out by flame or electrothermal atomic absorption spectrometry (FAAS or ETAAS). A suppressive interference (∼30%) was observed in the determination of cadmium in aqua regia digests by ETAAS, but not in 0.11 mol l−1 acetic acid, 0.1 mol l−1 hydroxylammonium chloride or 1.0 mol l−1 ammonium acetate extracts. Agreement between duplicate samples was acceptable (i.e. within 10%) for most elements in most layers, but some large discrepancies were apparent, especially for lead. The amount of metal extracted in the sequential procedure (i.e. Step 1+Step 2+Step 3+residual) did not generally agree well with pseudototal digestion. Various layers of the trail pit contained significant levels of contaminant metals, but these were not always in easily mobilized forms. For example, less than 0.2% of the lead (∼4000 μg g−1) at 65–85 cm depth was present as exchangeable or acid-soluble species. The study illustrates the importance of considering metal speciation when assessing the mobility of potentially toxic elements in industrially-contaminated land.