Recent progress in understanding the mechanism of heavy metals retention by iron (oxyhydr)oxides

Heavy metals are widespread toxic environmental pollutants that can generate enormous health and public concern. Iron (oxyhydr)oxides are ubiquitous in both natural and engineered environments and have great retention capacity of heavy metals due to their high surface areas and reactivity. The sequestration of heavy metal by iron (oxyhydr)oxides is one of the most vital geochemical/chemical processes controlling their environmental fate, transport, and bioavailability. In this review, some of the common iron (oxyhydr)oxides are introduced in detail in terms of their formation, occurrence, structure characteristics and interaction with heavy metals. Moreover, the retention mechanisms of metal cations (e.g., Pb, Cu, Cd, Ni, Zn), metal oxyanions (e.g., As, Sb, Cr), and coexisting multiple metals on various iron (oxyhydr)oxides are fully reviewed. Principal mechanisms of surface complexation, surface precipitation and structural incorporation are responsible for heavy metal retention on iron (oxyhydr)oxides, and greatly dependent on mineral species, metal ion species, reacting conditions (i.e., pH, heavy metal concentration, ionic strength, etc.) and chemical process (i.e., adsorption, coprecipitaton and mineral phase transformation process). The retention mechanisms summarized in this review would be helpful for remediating heavy metal contamination and predicting the long-term behavior of heavy metal in natural and engineered environments.