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, CDGT, theoretically underestimates the true labile concentration (Clabile) of HMs in soil and lacks direct comparability with the conventional soil HM content indices due to unit differences. Here, we proposed CDGT-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 CDGT-W is analogous to that of CDGT 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 CDGT-W, CDGT, solution concentration (Csoln), and CaCl2 extractable concentration (CCaCl2) on three soils with a diverse range of HM concentrations. CDGT-W showed significant linear correlations with all other tested indexes. The ratios of CDGT-W to CCaCl2 varied between 0.30 and 0.98 for all HM-soil combinations with only one exception, a range much greater than CDGT/Csoln (typically <0.1) but lower than 1. This suggested that CDGT-W may more accurately reflect Clabile than CDGT (theoretically underestimates Cliable) and CCaCl2(likely overestimates Cliable). Additionally, CDGT-W measurements for these four HMs exhibited a broad measure concentration range and a low detection limit (mg/kg level). Consequently, CDGT-W may offer a more reliable alternative to CDGT for characterizing Clabile in unsaturated soils.

Effect of the interaction of fulvic acid with Pb(II) on the distribution of Pb(II) between solid and liquid phases of four minerals.

Lead (Pb) is one of the top metal pollutants worldwide, and its distribution between liquid and solid phases of soils is strongly controlled by its adsorption on minerals, organic matter, and their composites. This paper presented the effect of fulvic acid (FA) coexistence on the distribution of Pb(II) at the solid-liquid interface of four minerals, which provided reference for how to use humic substances to remove toxic Pb(II) in soils. The free Pb2+ of suspensions, measured by Pb ion selective electrode, was used to characterize the complexation of FA with Pb2+ at various pH. The adsorption isotherms of Pb(II) by montmorillonite, kaolinite, goethite, and gibbsite with and without FA were studied with batch experiments. Results indicated that the free Pb2+ decreased and complexed Pb(II) increased with the increase of FA concentration in Pb(II)-FA solutions, whether the initial concentration of Pb(II) was 0.1 or 1 mM. Pb2+ hydrolysis was low and the free Pb2+ concentration in pure lead solution without FA was generally unchanged with increasing solution pH at pH < 6.0. But free Pb2+ decreased with the increase of pH in the presence of FA, suggesting that the complexation ability of FA with Pb2+ increased with the increase of solution pH. The adsorption of Pb(II) by the minerals without FA followed the order: montmorillonite > kaolinite ≈ goethite > gibbsite at pH5.0. The Pb(II) adsorption by montmorillonite and kaolinite significantly enhanced with 1 g/L FA, while significantly inhibited with 3 g/L FA at low initial Pb(II) concentration. However, the effect of FA on Pb(II) adsorption by montmorillonite was greater than that of kaolinite, which was mainly related to the crystal layer structure, adsorption area, and cation exchange capacity of the minerals. The Pb(II) adsorption by goethite and gibbsite was significantly enhanced by the addition of both 1 g/L and 3 g/L FA, and the enhancement was more evident in goethite system. The effect of FA on the distribution of Pb(II) between solid and liquid phases of the minerals was determined by the factors such as the initial concentration ratio of FA to Pb(II), the adsorption capacity of minerals for FA, and the number of soluble complexes of FA with Pb2+. Therefore, the distribution of FA between solid and liquid of four minerals affected the distribution of Pb(II) between solid and liquid phases of the minerals greatly. The results can provide an important reference for understanding the distribution of Pb(II) and the dynamics and mobility of active components in polluted soils.