*2.1.2.3 Chromium*

In its compounds, chromium exists in several oxidation states: from bivalent to hexavalent. In solutions, chromium can occur in trivalent and hexavalent forms. Hexavalent chromium is usually present in the compounds as chromate (CrO4) <sup>2</sup><sup>−</sup> or dichromate (Cr2O7) <sup>2</sup><sup>−</sup> ion. Cr(VI) is toxic due to its high degree of oxidation and easily enters the biological membranes. Therefore, this form of chromium is considered carcinogenic. Because chromium(VI) is toxic, carcinogenic, and mutagenic to living organisms, damages the liver, and causes lung congestion, skin irritation, and the formation of ulcer, it needs to be removed from the wastewater before their release into natural recipients. On the other hand, trivalent chromium, Cr(III), is 300 times less toxic than chromium(VI). Chromium is a vital nutrient for many animal and plant species, but it can also cause allergic reactions on the skin and can be carcinogenic [13].
