Abandoned chemical smelting sites containing toxic substances can seriously threaten and pose a risk to the surrounding ecological environment. Soil samples were collected from different depths (0 to 13 m) and analyzed for metal(loid)s content and fractionation, as well as microbial activities. The potential ecological risk indices for the different soil depths (ordered from high to low) were: 1 m (D-1) > surface (S-0) > 5 m (D-5) > 13 m (D-13) > 9 m (D-9), ranging between 1840.65–13,089.62, and representing extremely high environmental risks, of which Cd (and probably not arsenic) contributed to the highest environmental risk. A modified combined pollution risk index (MCR) combining total content and mobile proportion of metal(loid)s, and relative toxicities, was used to evaluate the degree of contamination and potential environmental risks. For the near-surface samples (S-0 and d-1 layers), the MCR considered that As, Cd, Pb, Sb, and Zn achieved high and alarming degrees of contamination, whereas Fe, Mn, and Ti were negligible or low to moderate pollution degrees. Combined microcalorimetry and enzymatic activity measurements of contaminated soil samples were used to assess the microbial metabolic activity characteristics. Correlation analysis elucidated the relationship between metal(loid)s exchangeable fraction or content and microbial activity characteristics (p < 0.05). The microbial metabolic activity in the d-1 layer was low presumably due to heavy metal stress. Enzyme activity indicators and microcalorimetric growth rate (k) measurements were considered sensitive indicators to reflect the soil microbial activities in abandoned chemical smelting sites.

Purpose As one of the most critical sources of pollution from detrimental elements, smelting slags pose a huge threat to the surrounding environment. A comprehensive environmental risk evaluation of slag is beneficial to propose more rational suggestions on their management and treatment. Therefore, the aim of this study was to assess the potential environmental hazard of toxic elements in non-ferrous metal slags from Guangxi (GX) and Yunnan (YN) by comparing the leaching behavior and geochemical characteristics. Methods The pH stat and semi-dynamic leaching tests were used to study the leaching behavior of metal(loid)s at different pH and simulate local rainfall, respectively. The potential mobility, bioavailability, and bioaccessibility of metal(loid)s were determined through a series of extraction experiments. In addition, a combination of leachability, mobility, bioavailability, and bioaccessibility is used to explore the potential environmental risk implications of slags. Results The release of metal(loid)s showed a strong dependence on pH. Most metal(loid)s release decreased with increasing pH, while some metal(loid)s release increased under alkaline conditions. Variation in metal(loid)s concentrations induced by pH changes is due to the dissolution/precipitation and the adsorption/desorption processes of minerals. The results of the simulated rainfall indicate different trends in metal(loid)s release due to the differences in slag composition, rainfall, and pH between the two regions. The leaching of metal(loid)s decreases with increasing rainfall, which is due to the depletion of the slag surface. The mobility, bioavailability, and bioaccessibility of Cd, Cu, and Zn in GX were higher than in YN, while As and Pb were lower than in YN. These metal(loid)s are more likely to enter the environment and pose a greater threat to plants and humans. The correlation analysis showed that the exchange and carbonate-bound fractions of metal(loid)s have a positive effect on their bioavailability and bioaccessibility. Conclusion The results show that among the elements studied, the potential environmental risks of Cd, Cu, and Zn in GX are greater than those of YN, while As and Pb are less than those of YN. The toxic elements in the slags may pose a potential risk to the surrounding environment.

Large amounts of metal(loid)s in abandoned non-ferrous metal smelting site are released to the surrounding environment, causing serious harm to the environment. A weighted comprehensive pollution risk score (CRS), combining two typical metal(loid)s pollution and risk index evaluation methods (speciation and total content indices), and two microbial activity evaluation methods (microcalorimetry and enzyme activities), is proposed to evaluate the pollution degree and environmental risk of metal(loid)s. In the surface soil (Site SS) of contaminated site, the CRS identified seven metal(loid)s (CRSAs (20) > CRSCd (19) > CRSSb (18) > CRSZn (13) = CRSCu (13) > CRSNi (10) > CRSPb (8)) with higher risks, and six (CRSCr (7) > CRSMn (4) = CRSV (4) > CRSAl (3) = CRSMg (3) > CRSTi (2)) with lower risks. Different areas were sorted by the potential ecological risk index (RI) and CRS index from large to small: the deep soil (Site SD) > Site SS > the surrounding wasteland (Site Y), indicating that smelting site was heavily polluted by metal(loid)s. In Site SS, microbial activities were low confirming the inhibition effect of metal(loid)s on soil microbial community activities. Microbial activity characteristics were correlated to metal(loid)s concentration and mobility, indicating that microbial activity represents a potential indicator of pollution. Particularly, FDA activity and TMA are sensitive indicators to estimate soil metal(loid)s pollution level. Our study provides a comprehensive assessment of the metal(loid)s pollution risk, including microbial activity, in non-ferrous metal smelting sites, useful information for the management and remediation of metal(loid)s polluted sites.