Chromium hazard and risk assessment: New insights from a detailed speciation study in a standard test medium

Despite the consensus about the importance of chemical speciation in controlling the bioavailability and ecotoxicity of trace elements, detailed speciation studies during laboratory ecotoxicity testing remain scarce, contributing to uncertainty when extrapolating laboratory findings to real field situations in risk assessment. We characterized the speciation and ecotoxicological effects of chromium (Cr^III and Cr^VI ) in the International Organization for Standardization (ISO) medium for algal ecotoxicity testing. Total and dissolved (< 0.22 μm) Cr concentrations showed little variability in media spiked with Cr^VI , whereas dissolved Cr concentration decreased by as much as 80% over a 72-h time period in medium amended with Cr^III . Analyses by ion chromatography inductively coupled plasma mass spectrometry (IC-ICP-MS) highlighted the absence of redox interconversion between Cr^III or Cr^VI both in the presence and absence of algal cells (Raphidocelis subcapitata). Furthermore, the concentration of ionic Cr^III dropped below detection limits in less than 2 h with the corresponding formation of carbonate complexes and Cr hydroxides. Precipitation of Cr^III in the form of colloidal particles of variable diameters was confirmed by nanoparticle (NP) tracking analysis, single particle ICP-MS, and single particle counting. In terms of time-weighted dissolved (< 0.22 μm) Cr concentration, Cr^III was 4 to 10 times more toxic than Cr^VI . However, Cr^III ecotoxicity could arise from interactions between free ionic Cr^III and algae at the beginning of the test, from the presence of Cr-bearing NPs, or from a combination of the 2. Future ecotoxicological studies must pay more attention to Cr speciation to reliably compare the ecotoxicity of Cr^III and Cr^VI . Environ Toxicol Chem 2018;37:983-992. © 2017 SETAC.