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, theoretically underestimates the true labile concentration (C) of HMs in soil and lacks direct comparability with the conventional soil HM content indices due to unit differences. Here, we proposed C, 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 is analogous to that of C 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, C, solution concentration (C), and CaCl extractable concentration (C) on three soils with a diverse range of HM concentrations. C showed significant linear correlations with all other tested indexes. The ratios of C to C varied between 0.30 and 0.98 for all HM-soil combinations with only one exception, a range much greater than C/C (typically <0.1) but lower than 1. This suggested that C may more accurately reflect C than C (theoretically underestimates C) and C(likely overestimates C). Additionally, C measurements for these four HMs exhibited a broad measure concentration range and a low detection limit (mg/kg level). Consequently, C may offer a more reliable alternative to C for characterizing C in unsaturated soils.