Cr(VI) immobilization in mixed (Mg, Al) oxides

In the reaction sequence of thermal decomposition of chromate exchanged hydrotalcite a spinel structure was detected at 800 °C; the main compound was a periclase-like structure. At 1000–1200 °C the only compounds were MgO and the spinel MgAl1.3Cr0.7O4. The particle size and morphology did not change with gamma irradiation or thermal treatments. The composition of the spinels MgAl2−xCrxO4 varies when samples are irradiated and/or thermally treated. Aluminum in octahedral sites is displaced to tetrahedral sites as chromium is incorporated into spinels. A molecular dynamics simulation of the spinels reproduced the experimental behaviour. A mechanism based on thermodynamical data and Wagner reactions is proposed.