Metal Hexacyanoferrate Absorbents for Heavy Metal Removal

Alternative energy sources are currently worldwide under development to contribute to the increasing energy demand. Along with the introduction of new technologies, heavy metals, such as radionuclides from nuclear power plant leaks, might be released into the environment and contaminate waters, air, and soil. Among the investigated methods, the use of adsorbents has been proven the most suitable one, able to extensively remove heavy metals, e.g., radioactive 137Cs+. Prussian blue analogs (PBAs) have been demonstrated to be effective adsorbents toward the sequestration of a variety of heavy metals, including the recovery and valorization of rare earth elements. Here, we point out the structure-property link of PBAs: the large channels and cavities are able to accommodate a variety of ions, ranging from monovalent to multivalent ones, while the ion exchange may be either diffusion-controlled or electrochemically driven. While distribution coefficients are shown to be key parameters in the diffusion driven process, resulting in high affinities of PBAs toward metals such as Cs+, Tl+, Cu2+, and Zn2+, electrochemical ion exchange is considered to be promising due to the effectiveness in the removal of metals and the possibility to reversibly restore the adsorbent to its initial state. Related examples concerning the capture of Cs+ from wastewaters and the recovery of rare earth elements are herein presented and commented.