Using isotopic dilution to assess chemical extraction of labile Ni, Cu, Zn, Cd and Pb in soils

Chemical extractants used to measure labile soil metal must ideally select for and solubilise the labile fraction, with minimal solubilisation of non-labile metal. We assessed four extractants (0.43 M HNO3, 0.43 M CH3COOH, 0.05 M Na(2)H(2)EDTA and 1 M CaCl2) against these requirements. For soils contaminated by contrasting sources, we compared isotopically exchangeable Ni, Cu, Zn, Cd and Pb (E-Value, mg kg(-1)), with the concentrations of metal solubilised by the chemical extractants (M-Ext, mg kg(-1)). Crucially, we also determined isotopically exchangeable metal in the soil-extractant systems (E-Ext, mg kg(-1)). Thus 'E-Ext - E-Value' quantifies the concentration of mobilised non-labile metal, while 'E-Ext - M-Ext' represents adsorbed labile metal in the presence of the extractant. Extraction with CaCl2 consistently underestimated E-Value for Ni, Cu, Zn and Pb, while providing a reasonable estimate of E-Value for Cd. In contrast, extraction with HNO3 both consistently mobilised non-labile metal and overestimated the E-Value. Extraction with CH3COOH appeared to provide a good estimate of E-Value for Cd; however, this was the net outcome of incomplete solubilisation of labile metal, and concurrent mobilisation of non-labile metal by the extractant (M-Ext < E-Ext > E-Value). The Na(2)H(2)EDTA extractant mobilised some non-labile metal in three of the four soils, but consistently solubilised the entire labile fraction for all soil-metal combinations (M-Ext approximate to E-Ext). Comparison of E-Value, M-Ext and E-Ext provides a rigorous means of assessing the underlying action of soil chemical extraction methods and could be used to refine long-standing soil extraction methodologies. (C) 2016 Elsevier Ltd. All rights reserved.