[en] Three sequential extraction procedures (SEPs), modified Tessier, modified BCR, and CIEMAT, were compared for mercury fractionation in polluted soils. With satisfactory total mercury recovery, the modified Tessier and modified BCR SEPs were comparable with each other in terms of extraction efficiency in equivalent mercury fractions, whereas both SEPs were not as efficient as the CIEMAT SEP. However, the CIEMAT SEP might underestimate the oxidizable mercury fractions due to the humic and fulvic complexes instead of the organic matter of the other two SEPs. For mercury bioavailability identification, based on Pearson correlation analysis, all fractions in each SEP were significantly correlated with mercury uptake in Ipomoea aquatica, causing difficulty in comparison. Partial correlation analysis indicated that the mobile mercury fractions extracted by the first step in all three SEPs had a positive correlation with mercury uptake by plant, while mercury bound to organic matter extracted by both modified Tessier and modified BCR SEPs presented negative correlation with mercury uptake by plant which was in contrast to CIEMAT SEP. Meanwhile, clearly positive correlations between mercury fractions extracted by the former three steps of CIEMAT SEP and mercury uptake in Ipomoea aquatica were observed, demonstrating that CIEMAT SEP provided more accurate results related to Hg bioavailability than did the other two SEPs.

[en] Highlights: • A new index is hypothesized to assess site dynamic risk evolution. • The possibility factor indicated different properties of site dynamic risk evolution. • Toxic factor applied to soil was modified by a meta-analysis. • Dynamic risk evolution of specific element (e.g. Cd, Hg) should be particularly noticed. In this study, a potential ecological risk index for site dynamic risk evolution was proposed with the development trend of the Hakanson approach. The possibility factor ${\mathit{P}}_{\mathit{i}}$ was attached to the contamination factor ${\mathit{C}}_{\mathit{f}}^{\mathit{i}}$, representing the burst probability factor of dynamic risk arising from the transformation of metal(loid) fractions, ascertained by the delayed geochemical hazard (DGH) methodology. The toxic effects of different elements were explored through a meta-analysis. The effects of soil pH and soil organic matter on the mobile fraction of elements were examined, and the toxic factors (${\mathit{Tr}}^{\mathit{i}}$) of six elements (arsenic, cadmium, chromium, copper, lead, and zinc) were modified for the site application. A total of 16 case studies representing four soil types (agricultural soil, industrial zone, mining area, and soil of river basin) were tested, and the results indicated that the proposed index could provide an early warning of site risk dynamics.

[en] Highlights: • A DGH-based method warning the compound risk development was proposed and verified. • DGH paths from bound to carbonate and organic matter to exchangeable was dominant. • The interaction of Pb and Cd overall accelerated the compound risk development. • Soil acidification and organic matter decline triggered the risk development. Based on the transformation among metal fractions defined by the Tessier sequential extraction procedure and integrated risk information assessed by delayed geochemical hazard (DGH) methodology, including development paths and their burst probabilities, trigger conditions, and the contribution of each metal to risk development, an approach was proposed to provide an early warning on risk development in metal compound-contaminated sites and tested in a lead and cadmium-contaminated site. Risk assessment indicated that the site was at a high to extremely high ecological risk. DGH analysis revealed that the transformation from the fraction bound to carbonate and organic matter to the exchangeable fraction was dominant in the development of either single or combined lead and cadmium risk, which was triggered by soil acidification and the continuous decline of soil organic matter; risk development might have occurred in 6.52–80.4% of the case site with burst probabilities of 6.52–80.4%, 8.70–39.1% and 8.70–80.4% for lead risk, cadmium risk and combined lead-cadmium risk, respectively; with the dominant role of lead, the two metals overall accelerated the development of their compound risk by changing each other’s DGH paths. The proposed DGH-based approach is promising for early warning on risk development in compound contaminated sites.

[en] Highlights: • The application of RDEI to complex polluted sites was explored. • Calculation of DGH burst possibility for complex polluted sites was ascertained. • DGH cross-fitting methods could reflect the interactions of metal(loid)s. • Lead DGH burst was found in the studied case. In this study, a nonferrous metal smelter with complex Cd-Pb contamination was selected for a site risk assessment. By establishing a hypothesis regarding the dynamic behavior (adsorption/desorption) of metals in soil with metal fractionation, the fractional evolution of Cd and Pb at the site was explored using the delayed geochemical hazard (DGH) model, and the risk dynamic evolutionary index (RDEI) for the complex contaminated site was proposed. Using the Tessier method, a Pb DGH burst was found at the site. The proportion of the mobile fractions of Pb increased by nearly 20% at the site compared with 4 years ago, which may be attributed to the decrease in soil organic matter and weakly acidic soil properties at the site. DGH cross-fitting methods were applied to explore in depth Cd-Pb interactions at the site. The average probability of the DGH burst at the site was calculated to be 48% using the DGH equivalent-fitting method. Based on this, the REDI of the site was calculated to be 2229, indicating that the site poses a very high potential ecological dynamic risk (RDEI ≥ 320). In complex contaminated sites, the combination of the DGH model and RDEI can provide a more detailed dynamic risk assessment in terms of metal fractional transformation.