In nature, uranium occurs in the form of tetravalent or hexavalent oxide minerals (uraninite or pitchblende). Tetravalent uranium has, however, very low solubility in both acidic or alkaline media. Thus, to achieve economical uranium recovery, it is important to perform oxidation to the much more soluble hexavalent state. Trivalent iron compounds are commonly used to increase uranium's oxidation state. Hydrogen peroxide is applied for regeneration of the active Fe(III) ions in the eluent.