A Comparison of Extractants for Heavy Metals in Contaminated Soils From Spain

Many soils in industrialized countries are affected by acid deposition, mine waste disposal and organic refuse including sewage sludge that are sources of pollutants and especially heavy metals. In Spain, the measurement of heavy metals in soils has encountered problems with the standardization of extraction procedures due to the wide diversity of soils, the low concentrations of these elements, and the lack of appropriate analytical instruments. In this research, the extractability of Cd, Ni, Pb, Co and Cr from eight contaminated soils from Spain was evaluated using ammonium chloride, calcium chloride, strontium chloride and DTPA extractants. Earlier workers who assessed plant available metals have used these procedures for soils from other countries. Each extraction procedure was performed in triplicate in 50 mL polycarbonate centrifuge tubes. All the extracts were centrifuged for 10 min at 3000 rpm, and the supernatant filtered into volumetric flasks containing 10 μg In/Rh L−1. All the extracts and standard solutions were acidified to 1% HNO3. The amounts of metal extracted were related to total metal contents, determined following microwave digestion using HNO3. Quantification of dissolved metals was by ICP-MS using matrix-matched standards. Results showed a high variability of metal extraction depending on extraction procedure, source of pollution, and nature of the soil. We found that the extractability for calcareous soils was best determined by DTPA. In a comparison of chloride salts, we found a higher efficiency of extraction with ammonium chloride for almost all the soils and heavy metals analyzed.