Metal speciation in soil: Extraction of exchangeable cations from a calcareous soil with a magnesium nitrate solution

The exchangeable fraction of metals in a calcareous soil was studied with a magnesium nitrate solution and its extraction procedure was optimised. Par ameters like extraction time, pH and concentration of the extractant soluti on, number of extractions or soil/solution ratio were examined. Experiments showed that after a 1-hour extraction, an equilibrium was established betw een extracted and re-sorbed metal. Moreover, carrying-out successive extrac tions re-sorbed quantities and maximum extracted ones could be estimated. I nitial pH was fixed at 7 to avoid carbonates dissolution or hydroxides prec ipitation. A concentration of 0.5 mol/L and a soil/solution ratio df 1/20 w ere chosen. Leachate analyses showed a good selectivity of the extraction t oward other soil fractions.