Synthesis of iron oxyhydroxides of different crystal forms and their roles in adsorption and removal of Cr(VI) from aqueous solutions

Iron oxyhydroxides (FeOOH),as a mineral material and a group of α-,β-,γ-FeOOH polymorphs,are commonly existent in some soils,sediments,and acid mine drainage environments.They can remove heavy metals from contaminated environments by such means as coprecipitation,ions exchange and adsorption.Thus,iron oxyhydroxides are usually applied in adsorption and removal of heavy metals from contaminated water because they have stable chemical properties,relatively large specific surface areas and fine particle structures.In this paper,the type,structure,composition,and mineral phase transformation of iron oxyhydroxides are discussed in detail.This paper gives a review on the synthetic methods for iron oxyhydroxides,which include ferric hydrolysis and neutralization,ferrous oxidation by air,ferrous biooxidation and biomineralization and gel-network precipitation means.Some factors affecting the formation of oxyhydroxides are also dealt with.The roles of various crystalline iron oxyhydroxides in the chromium removal from contaminated environments are summarized in the end of this paper.