Concentration and fractionation of Cu, Cr, Ni and Zn in montmorillonitic and allophanic soils: Comparison of two different extraction methods

Abstract

The content and the chemical forms of chromium, copper, nickel and zinc in allophanic and montmorillonitic soils, one of them irrigated exclusively with untreated sewage waters, were studied. To gain information about the possible impact of these metals on soil cultivars and the involved environmental damage, two different sequential extraction procedures were used and evaluated as to their usefulness for this purpose. These methods were the one proposed by Tessier et al, and the one used by the European Community Bureau of Reference (BCR), for metals in sediments.

Results obtained showed that total metal concentration varied from background levels up to ones well in excess of the maximum accepted limits for soils. Thus in the wastewater irigated soil total Cr ranged from 31 mg kg(-1) in the subsoil to 223 mg kg(-1) in the surface sample; total Cu values were 296 mg kg(-1) and 435 mg kg(-1), and total Zn 247 mg kg(-1) and 461 mg kg(-1) respectively. Nickel varied between 25 and 56 mg kg-l. Values obtained for the other montmorillonitic soil showed concentration that were about half those values. In the allophanic soils values found were in ranges considered normal.

Evaluating the two sequential extraction procedures it was concluded that the method proposed by Tessier et al. extracted higher amounts of metals in all fractions when compared with the BCR procedure and supplied more information about forms present i.e. carbonate ones which could be important in nonacidic soils. The fifth and last extractant used in the Tessier procedure was considered of little interest in this type of studies and could be omitted.