Quantifying and normalizing the combined phytotoxicity of mixed-valence chromium in soil integrated by toxicity equivalence

When Cr(VI) and Cr(III) coexist, the reasonable assessment of the combined toxicity of chromium in soil and its ecological risk is still not well resolved. In the present study, exogenous mixed concentration combinations were set up to determine the interaction and combined toxicity of Cr(VI) and Cr(III), which were quantified as measured total and resin extractable forms for dose-response experiments with barley root elongation. The concept of toxicity equivalence "α" (the ratio of toxicity intensity coefficient between Cr(VI) and Cr(III), which can be expressed as the relative toxic strength of Cr(VI) to Cr(III)) was proposed for the toxicity assessment of mixed-valence chromium in soil. The results showed that the dose-response relationship was determined more precisely by the extended independent action model (e-IA) than traditional models (e.g., concentration addition model), and the mutual antagonism for resin extractable form (Resin-Cr) was stronger than the measured total form (T-Cr). The values of toxicity equivalence (α) between coexisting Cr(VI) and Cr(III) as Resin-Cr and T-Cr were 0.74 and 160, respectively, which indicated Resin-Cr(III) had relatively stronger toxicity than Resin-Cr(VI), while T-Cr(III) was much less than T-Cr(VI). The α values between Cr(VI) and Cr(III) decreased with their more active forms (decreased to about 0.5% of the original), even as total concentration and activity in solutions, making a dialectical view of the toxicity of both in different forms necessary. Finally, the log-logistic models were developed, enabling mixed-valence Cr toxicity to be assessed from a unilateral perspective using the Cr(III) equivalence concentration (Cr(III)-eq). This work provided innovative ideas for ecological threshold studies for mixed-valence metals in soils.