Early warning on risk development in compound lead and cadmium contaminated sites.

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.

Risk dynamic evolution index based on fraction transformation and its application to site risk assessment.

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 Pi was attached to the contamination factor Cfi, 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 (Tri) 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.

Comparison of different sequential extraction procedures for mercury fractionation in polluted soils.

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.

Mercury accumulation plant Cyrtomium macrophyllum and its potential for phytoremediation of mercury polluted sites.

Cyrtomium macrophyllum naturally grown in 225.73 mg kg-1 of soil mercury in mining area was found to be a potential mercury accumulator plant with the translocation factor of 2.62 and the high mercury concentration of 36.44 mg kg-1 accumulated in its aerial parts. Pot experiments indicated that Cyrtomium macrophyllum could even grow in 500 mg kg-1 of soil mercury with observed inhibition on growth but no obvious toxic effects, and showed excellent mercury accumulation and translocation abilities with both translocation and bioconcentration factors greater than 1 when exposed to 200 mg kg-1 and lower soil mercury, indicating that it could be considered as a great mercury accumulating species. Furthermore, the leaf tissue of Cyrtomium macrophyllum showed high resistance to mercury stress because of both the increased superoxide dismutase activity and the accumulation of glutathione and proline induced by mercury stress, which favorited mercury translocation from the roots to the aerial parts, revealing the possible reason for Cyrtomium macrophyllum to tolerate high concentration of soil mercury. In sum, due to its excellent mercury accumulation and translocation abilities as well as its high resistance to mercury stress, the use of Cyrtomium macrophyllum should be a promising approach to remediating mercury polluted soils.

Comparison of three sequential extraction procedures for arsenic fractionation in highly polluted sites.

Three sequential extraction procedures (SEPs) including Tessier, Rauret, and Shiowatana SEPs, were compared for arsenic fractionation using highly polluted soils. In the definition context of exchangeable, reducible, oxidizable and residual fractions, with similar arsenic recovery and reproducibility, Tessier and Rauret SEPs were comparable to each other, whereas Shiowatana SEP showed higher extraction efficiency in all the first three arsenic fractions, although it might overestimate the reducible arsenic. Pot experiment indicated three SEPs all could provide an estimation of the most bioavailable arsenic fraction, and the application of Shiowatana SEP should be preferred. Accordingly, a case study with Shiowatana SEP for a site near a realgar mine area is conducted. The results show that although arsenic in this area presents predominantly in the stable fractions, the sum of most bioavailable fractions was accounted around 11% of total arsenic, and moreover, about another 10% of the total arsenic, the fourth fraction in Shiowatana SEP is likely to be transferred into bioavailable species under suitable conditions, such as strong acid impact, revealing a real major risk source being formed. The study indicated that Shiowatana should be more suitable for arsenic fractionation to provide valuable information in the framework of risk assessment.

Risk assessment for the mercury polluted site near a pesticide plant in Changsha, Hunan, China.

The distribution characteristics of mercury fractions at the site near a pesticide plant was investigated, with the total mercury concentrations ranging from 0.0250 to 44.3 mg kg-1. The mercury bound to organic matter and residual mercury were the main fractions, and the most mobile fractions accounted for only 5.9%-9.7%, indicating a relatively low degree of potential risk. The relationships between mercury fractions and soil physicochemical properties were analysed. The results demonstrated that organic matter was one of the most important factors in soil fraction distribution, and both OM and soil pH appeared to have a significant influence on the Fe/Mn oxides of mercury. Together with the methodology of partial correlation analysis, the concept and model of delayed geochemical hazard (DGH) was introduced to reveal the potential transformation paths and chain reactions among different mercury fractions and therefore to have a better understanding of risk development. The results showed that the site may be classified as a low-risk site of mercury DGH with a probability of 10.5%, but it had an easy trend in mercury DGH development due to low critical points of DGH burst. In summary, this study provides a methodology for site risk assessment in terms of static risk and risk development.