Nature of Chromium Ion Species

There are various oxidation states of chromium ion ranging from -2 to +6, but only the trivalent and hexavalent states are the most stable under most natural environments and are more prevalent in aqueous phases. These two stable states, Cr[lII) and Cr(Vl), exhibit very different toxicides and mobilities. It is well known that chromium (III) is relatively insoluble in aqueous systems and exhibits a little toxicity and mobility [74-76]. On the contrary, chromium (VI) occurs as a highly soluble and more toxic species [31, 32]. Hence, the more conventional methods for removing the Cr (VI) from wastewater were based on the chemical reduction of Cr (VI) present using suitable chemical reducing agents. Here, the cited safe, nontoxic, and biodegradable polysaccharides may be used efficiently as reductants for this purpose. 

On the other hand, at higher acidity ([H j 1.0 mol dm ), the formed HCr04 is further protonated to give H2Cr04 as follows  

The formation of such species depends mainly on the pH, ionic strength, and temperature used. 

Some authors have considered H3Cr04 species formation [84-86] (through protonation of H2Cr04), but this suggestion is quite unlikely even at high acid concentration [87, 88]. Thus, in turn, in order to simplify the redox rate equations, the chromium (VI) species are not considered in this review, and the rate equations will be written only in terms of [Cr (VI)]. 

---