Regularities of Chromium(VI) Ions Sorption by Magnetite (Review)

The present review is devoted to the sorption properties of magnetite towards hexavalent chromium ions and the possibility of its use as a sorbent for removing these toxic ions from contaminated aqueous solutions. The behavior of magnetite nanoparticles in aqueous solutions has been also investigated. It has been shown that two processes occur simultaneously during the sorption of chromium(VI) ions by magnetite: ordinary adsorption and chemisorption (redox reaction between hexavalent chromium ions and magnetite). The latter is accompanied by the oxidation of iron(II) in magnetite to iron(III) and the reduction of chromium(VI) ions to chromium(III) with the formation of maghemite and a number of other compounds, including mixed chromium(III) compounds with iron, in the surface layer of magnetite (or onto its surface). It has been demonstrated that the kinetics of the redox process between chromium(VI) and magnetite is described by the first-order reaction equation with respect to the concentration of chromium(VI) ions in a solution. As a result of chemisorption, unlike the case of traditional sorbents, chromium(VI) is irreversibly bound by magnetite, which eliminates the reentry of its ions into the environment. Here, the sorption capacity of magnetite irreversibly decreases in the course of its saturation with chromium. In addition, the sorption capacity of magnetite towards chromium(VI) ions also decreases along with increasing pH of the purified solution, and at pH > 11 it becomes almost zero.