Objective The aim was to investigate the species and bioavailability of heavy metals in the soil of a coking plant.

Method Soil samples were collected from a decommissioned coking plant. A BCR sequential extraction procedure combined with ICP-OES was used to determine the total contents and different forms of seven heavy metals in the soils. The methods of risk assessment coding and ratio of secondary phase and primary phase were used to evaluate their bioavailability.

Result As, Cd, Cr, Cu, Ni, Pb and Zn were enriched in different degrees in the studied soils, with average contents of 3.15, 18.60, 2.70, 1.93, 2.13, 0.96, and 1.87 times of the background values in Jiangxi Province, respectively. All heavy metal contents were moderately to highly variable. Residual form was the highest form for As, Cd, Cr, Cu, Pb, Ni, and Zn, accounting for 79.25%, 41.10%, 87.37%, 59.00%, 67.92%, 51.02%, and 61.10% of the total contents, respectively, followed by the reducible and oxidizable forms. The exchangeable form accounted for the lowest. The effective form of Cd accounted for the highest percentage (up to 58.90%), showing a greater ecological risk as compared to other metals. Soil pH value and content of soil organic matter (SOM) did not show significant correlations with contents of different forms of the metals, which was mainly influenced by their corresponding total contents. Results of risk evaluation using coding method showed that Cd was in moderate-risk, and As, Cu, Ni, Pb, and Zn were in low-risk, and Cr was in zero-risk. Results of risk evaluation using the ratio of secondary phase and primary phase method showed that the soils were heavily contaminated by Cd, and were slightly contaminated by Pb, and were not contaminated by As, Cr, Cu, Ni, and Zn.

Conclusion Among the seven heavy metals, Cd had the highest bioavailability and mobility in the soils of the study area, and was the main risk factor, while the others had low bioavailability and low ecological hazards.