The mobility of Cr(VI) on the ferrihydrite-Cr(VI) co-precipitates: The effect of co-existing tartaric acid and Cu(II)

Ferrihydrite is a crucial scavenger for hexavalent chromium (Cr(VI)) via coprecipitation processes. Foreign impurities widely exist in soils and sediments, which influence mineral recrystallization and transformation processes. Under the circumstances, the speciation and migration behavior of Cr(VI) will change accordingly. In this research, we explored the impact of tartaric acid and Cu(II) on ferrihydrite transformation, as well as the sequestration of Cr(VI) on iron minerals. Our results manifested that tartaric acid affected the valence of Cr on iron oxides. The adsorbed Cr(VI) was reduced to Cr(III) by tartaric acid on the surface of iron oxides and not released into the aqueous phase, which greatly reduced its toxicity. It was conducive to the conversion of ferrihydrite into hematite when tartaric acid concentration was 1.0 mM, because tartaric acid could facilitate the hematite nucleation to induce the generation of hematite from ferrihydrite. Moreover, Cu(II) promoted the formation of hematite and played a synergistic role in Cr(VI) fixation. Therefore, tartaric acid and Cu(II) are important factors decreasing the toxicity of Cr(VI) when considering Cr(VI) retention on iron minerals in complicated environments. This work has a vital implication for understanding the impact of the co-existent metals and organic substances on Cr(VI) retention in complex environments.