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

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 CaCl₂ 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.