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 Na2H2EDTA and 1 M CaCl2) against these requirements. For soils contaminated by contrasting sources, we compared isotopically exchangeable Ni, Cu, Zn, Cd and Pb (EValue, mg kg -1 ), with the concentrations of metal solubilised by the chemical extractants (MExt, mg kg -1 ). Crucially, we also determined isotopically exchangeable metal in the soil–extractant systems (EExt, mg kg -1 ). Thus ‘EExt EValue’ quantifies the concentration of mobilised non-labile metal, while ‘EExt MExt’ represents adsorbed labile metal in the presence of the extractant. Extraction with CaCl2 consistently underestimated EValue for Ni, Cu, Zn and Pb, while providing a reasonable estimate of EValue for Cd. In contrast, extraction with HNO3 both consistently mobilised nonlabile metal and overestimated the EValue. Extraction with CH3COOH appeared to provide a good estimate of EValue for Cd; however, this was the net outcome of incomplete solubilisation of labile metal, and concurrent mobilisation of non-labile metal by the extractant (MExt < EExt > EValue). The Na2H2EDTA extractant mobilised some non-labile metal in three of the four soils, but consistently solubilised the entire labile fraction for all soilmetal combinations (MExt ≈ EExt). Comparison of EValue, MExt and EExt 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. Introduction The solubility and reactivity of metal added to soil may decrease with prolonged contact as it becomes ‘fixed’ in mineral phases due to aging processes (Bruemmer et al., 1988; Buekers et al., 2007; Young, 2013). In this respect, contaminant metal in soils is often found to be more labile than the metal originating from the soil parent material (Gleyzes et al., 2002). The reverse situation may also occur if recalcitrant waste material is added to soil. To improve the assessment of risks associated with soil-borne metals, it may be useful to determine soil metal solubility and lability alongside total soil metal content. This is particularly relevant if metal uptake from soil pore water, by soil organisms, is being considered (Plette et al., 1999; Nolan et al., 2003). There is a clear dependence of soil metal solubility on the labile fraction, as illustrated by Buekers et al. (2008) and Marzouk (2012), who showed that isotopically exchangeable metal, as an estimate of labile metal, is superior to total metal as an input parameter for geochemical models to predict soil metal solubility. Measuring the isotopically exchangeable soil metal pool, the ‘E value’, by isotopic dilution (Hamon et al., 2008), may be the most conceptually sound approach to determining the labile metal pool in soils, because of the mechanistic basis of the method (Buekers et al., 2008; Groenenberg et al., 2010). However, chemical extraction methods are far more widely used to estimate the labile metal fraction in soils, despite being substantially dependent on operational parameters such as the nature and concentration of chemical extractant, soil-tosolution ratio and extraction time (Young et al., 2006). The preference for simple chemical extractions persists, partly because the methods are more familiar, faster, less analytically demanding and considered cheaper than isotopic dilution. In marked contrast to the dilute suspending matrices used for E value assays, e.g. deionised water, 0.1 M CaCl2, 0.01 M Ca(NO3)2, and 0.0005 M EDTA (Young et al., 2000; Atkinson et al., 2011; Huang et al., 2011), chemical extractants are ideally required to select for and solubilise the labile fraction, while not solubilising any non-labile metal through, for example, chemical attack on the soil solid phase. This dual requirement may present an operational contradiction which remains unresolved; see for example Gleyzes et al. (2002), Young et al. (2006) and Peijnenburg et al. (2007) who have reviewed the extensive range of published chemical extraction methods. In order to introduce some standardisation, the Measurements and Testing Programme of the European Commission published collaboratively tested and harmonised extraction methods for 0.05 M EDTA and 0.43 M CH3COOH (Quevauviller, 1998a; b; 2002). Standardisation alone, however, does not validate their use for the measurement of labile metal in soils. To assess the reliability of estimates of labile metal determined by chemical extraction, several studies have compared chemically extracted metal with E values. For example, it has been found that Cd E value correlates well with Cd extracted with 1 M CaCl2 over a wide range of soil types, total metal concentrations and Cd contamination sources (Young et al., 2000; Gray et al., 2003; Gray et al., 2004; Sterckeman et al., 2009) but that Zn extracted by the same method underestimates E value (Young et al., 2000). Extraction of Cd with both 0.05 M and 0.04 M EDTA has been shown to overestimate E value (Nakhone & Young, 1993; Stanhope et al., 2000; Gray et al., 2003) whereas a lower concentration of EDTA (0.025 M) has been reported to provide a good estimate (Gäbler & Bahr, 2001). Gabler et al. (1999) determined extractable Ni, Cu, Zn, Cd and Pb in water, NH4NO3 and buffered 0.025 M EDTA extractants and compared this with E value measured in the extractant. They observed E value to be nearly independent of extractant. A comparison of 0.025 M EDTA extracts with E value for 115 soils with a wide range of properties resulted in good correlations for Ni, Cu, Zn, Cd and Pb. No significant differences were observed for Zn and Cd indicating that both approaches were accessing the same metal pool whereas EDTA extracted more Ni, Cu and Pb than the E value approach. Conversely, Ayoub et al. (2003) reported that, for both Cd and Zn, extraction with 0.05 M EDTA underestimates E value, and this has also been observed for Pb by Tongtavee et al. (2005). There is therefore a need for a more systematic analysis of how well different chemical extractants satisfy the two requirements outlined