Criteria for phytotoxicity of total and resin-extractable Cr(VI) in soil

Hexavalent chromium (Cr(VI)) has strong toxicity posing serious threats to the ecosystem, however, the phytotoxicity-based soil criteria for Cr(VI) have not been well established because the changes in Cr(VI) toxicity to plants over time and the influence of soil properties on its toxicity have not yet been considered. In the present study, the total or resin (DOWEX M4195)-extractable Cr(VI) criteria in soil are established by integrating the spiked toxicity dataset with 12 plant species, derived transformation models, and the species sensitivity distribution (SSD) approach. It was found that the total or resin-extractable EC50, which is the concentration that causes a 50% growth inhibition, is well correlated with soil pH and organic matter (OM). Therefore, the developed toxicity prediction models for barley and bok choy can decrease the intraspecific coefficients of variation (CV) for 80% of the studied plant species for both total EC50 and resin-extractable EC50, demonstrating the validity of the cross-species extrapolation approach. Consequently, the total or resin-extractable EC10 values of the other 10 non-model plants can be obtained and then used to construct SSD curves under different soil scenarios. The criteria for phytotoxicity of Cr(VI) have been scientifically derived using the added risk approach, and were found to be 0.25, 1.03, 1.17, and 3.03 mg/kg for the total Cr(VI) and 0.20, 0.68, 0.73, and 1.81 mg/kg for the resin-extractable Cr(VI) in acidic, neutral, alkaline non-calcareous, and alkaline calcareous soils, respectively. Furthermore, the continuous or scenario criteria integrated with soil pH are proposed for risk assessment, which can provide reliable support for establishing the environmental quality standards for Cr(VI) and ecological risk assessment of Cr in soil.