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Chromium (Cr) is a well-known heavy metal contaminant with toxicity highly dependent on its oxidation state. Hexavalent Cr (Cr­(VI)) is a known carcinogen while trivalent chromium (Cr­(III)) is significantly less toxic. The reduction of Cr­(VI) and oxidation of Cr­(III) in different compartments of the environment occur through intermediate species pentavalent Cr (Cr­(V)) and tetravalent Cr (Cr­(IV)), which are highly reactive. The environmental literature generally lacks information on Cr­(V) and Cr­(IV) species in various redox processes. This Perspective presents the aquatic chemistry of Cr­(V) and Cr­(IV), which includes their spectroscopic characterizations and kinetic behaviors under different environmental conditions. Examples are presented to demonstrate the possible existence of the intermediate Cr species in different systems of environmental importance such as the reduction of Cr­(VI) by iron­(II) (Fe­(II)), molecules of natural organic matter (e.g., fulvic acids and carboxylic acids), and oxidation of Cr­(III) by hydrogen peroxide, hypochlorite, and manganese oxides (MnO x ). The oxidation of organic pollutants by the Cr­(VI)–S­(IV) system is also discussed. This Perspective suggests in-depth investigations on the redox reactions of Cr relevant to environmental processes to shed light on the mechanisms of the generation of Cr­(V) and Cr­(IV) species and their roles in water decontamination.

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