the relatively mobile and toxic anions HCrO4 -and CrO-42 . The reduced form of chromium, Cr(III), is a cation that under slightly acidic to alkaline conditions is fairly insoluble, readily adsorbed, and much less toxic than Cr(VI). Selenium also undergoes important redox transformations. Selenate (Se(VI)) is more mobile and less toxic than selenite (Se(IV))