Methodology for the extraction of carbonate-bound trace metals from carbonate-rich soils: application to Lakhouat soils, Tunis, Tunisia

This study aims to optimise the method by which carbonate-bound trace metals are extracted during sequential extractions of carbonate-rich contaminated soils. Specifically, we aim to optimise the volume and concentration of sodium acetate that must be added to samples to completely dissolve carbonate, and hence to release into solution all trace metals associated with this phase. We apply a range of volumes and concentrations of sodium acetate to fixed masses of two raw soil samples (H1) and (H2) from the mining region of Lakhouat (Gouvernorat of Siliana Tunisia). The soils contain >80% carbonates, and carbonate-phase trace metals are therefore expected to be a major fraction of the total trace metal contamination. The applied range of volumes and concentrations of sodium acetate typifies those used in existing sequential extraction protocols. By X-ray diffraction analysis of the solid phase before and after extraction, we show that complete carbonate removal is guaranteed by application of 2 M sodium acetate, at 40 mL per gram of soil. Lower concentrations and volumes fail to completely dissolve soil carbonate. Atomic absorption analysis of the extracts shows concentrations of Zn and Pb to be highest in the samples from which the carbonate is completely removed, confirming the association of these metals with the carbonate phase.