A new simple index for characterizing the labile heavy metal concentration in soil by diffusive gradients in thin films technique.

Diffusive gradients in thin films technique (DGT) is recognized as a more reliable method for determining labile heavy metal (HM) concentration in soil than traditional destructive methods. However, the current DGT measurement index, C DGT , theoretically underestimates the true labile concentration (C labile) of HMs in soil and lacks direct comparability with the conventional soil HM content indices due to unit differences. Here, we proposed C DGT-W , a new simple index which is defined as the HM accumulation in the binding layer, normalized to the weight of soil (optimized water content = 100% of the maximum water holding capacity) filled in the open cavity-type DGT device over a specified deployment time (optimized time = 24 h). The procedure for measuring C DGT-W is analogous to that of C DGT but includes precise determination of water content (water/dry soil) and the mass of soil filled in the cavity. We conducted measurements of Cu, Pb, Cr(Ⅵ) and As(V) as C DGT-W , C DGT , solution concentration (C soln), and CaCl 2 extractable concentration (C CaCl2) on three soils with a diverse range of HM concentrations. C DGT-W showed significant linear correlations with all other tested indexes. The ratios of C DGT-W to C CaCl2 varied between 0.30 and 0.98 for all HM–soil combinations with only one exception, a range much greater than C DGT /C soln (typically <0.1) but lower than 1. This suggested that C DGT-W may more accurately reflect C labile than C DGT (theoretically underestimates C liable) and C CaCl2 (likely overestimates C liable). Additionally, C DGT-W measurements for these four HMs exhibited a broad measure concentration range and a low detection limit (mg/kg level). Consequently, C DGT-W may offer a more reliable alternative to C DGT for characterizing C labile in unsaturated soils. [Display omitted] • We proposed a new index C DGT-W to measure labile content of heavy metals in soil (C labile). • C DGT-W linearly correlated to CaCl 2 extractable content (C CaCl2) and C DGT. • C DGT-W may more accurately reflect C labile than C DGT and C CaCl2. • C DGT-W is probably a better measure of C labile than C DGT in unsaturated soil. [ABSTRACT FROM AUTHOR]