Labile Cd and Pb in vegetable-growing soils estimated with isotope dilution and chemical extractants

E value, based on isotope dilution method, is considered as an unambiguous index to assess the labile heavy metals in soils compared with the methods of chemical extraction. In the present study, the E values of Cd and Pb in polluted soils (QZDH and PTHJ soils) and non-polluted vegetable-growing soils (XMTA soils) were determined using stable isotope dilution method, and the compartments of E1 min, E1 min–24 h and E> 24 h values were calculated based on the two models of isotopic exchange kinetics (IEK) including an iterative nonlinear regression and a log–log linear regression. In addition, the pools of labile Cd and Pb in soils were also estimated with four single chemical extractants and a modified sequential extraction procedure of BCR approach. Results showed that the amounts of Cd in Eavailable including E1 min and E1 min–24 h pools were generally higher than those extracted with EDTA, and were also higher than the amounts in ∑BCR fractions (including acid-extractable, reducible and oxidizable fractions) except for QZDH soils. But for Pb, Eavailable value was only higher than that extracted with EDTA in polluted soils (QZDH), but lower than ∑BCR fractions in the three soils. Compared with the extractant of EDTA, Eavailable pools of Cd and Pb, which are usually used to rapidly assess the metals' bioavailability and impact on plants in soils, might be overestimated due to the possible fixation of the spiked isotopes in soil suspensions and the incompletely extraction of isotopically exchanged metals in E measurement.

Research Highlights► E value is considered as one of the unambiguous indexes to assess the labile metal in soils. ► Two IEK models are found reasonably fitting the dynamic changes of isotope ratios of Cd and Pb. ► An obvious plateau of dynamic changes of isotope ratios is observed for Cd but not for Pb. ► Eavailable pools of Cd and Pb might be overestimated due to the possible fixation of the spiked isotopes in soil suspensions and the incompletely extraction of isotopically exchanged metals in E measurement.