Arsenic is one of the world’s most widespread inorganic contaminants. When As5+ reaches soils or sediments, it can be immobilized by adsorption to the surfaces of iron oxide minerals or it can be incorporated into iron oxides. The second possibility has not been investigated much, so far.
Ralph M. Bolanz, Friedrich-Schiller-University Jena, Germany, and colleagues from Germany and Slovakia investigated the As5+ incorporation into the crystal structure of hematite, one of the most common iron oxides. They used transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to demonstrate the presence of up to 1.9 wt % of As within the hematite crystals. A number of iron arsenates were used as reference compounds for the XAS work and additional structural models were constructed.
The researchers concluded that the incorporated As5+ displays a short-range order similar to angelellite-like clusters, epitaxially intergrown with hematite. Angelellite (Fe4As2O11) is a triclinic iron arsenate with structural relations to hematite.
This structural composite of hematite and angelellite-like clusters represents a new immobilization mechanism and potentially long-lasting storage facility for As5+ by iron oxides.