Effect of heating temperature on the sequestration of Cr3 + cations on montmorillonite

• Heating temperature can affect the sequestration efficiency of Cr on Mt.
• Oxidation of Cr(III) could promote the desorption of Cr.
• Collapsed Mt layers and formed Cr2O3 can help in sequestrating Cr.
• Cr3 + could migrate into Mt layers upon heating in nitrogen atmosphere.
• Structural Al3 + can be desorbed by HCl from heated Cr-Mt.

Montmorillonite (Mt) has been considered as low-cost and high efficient adsorbent for heavy metal cations, and thermal treatment can be a facile approach for sequestration of the adsorbed heavy metal cations on Mt. This work was intended to study the effects of heating temperature on the sequestration efficiency of Cr on Mt. Toward this aim, thermal treatment of the spent Mt after adsorption of Cr3 + was carried out at different temperatures, both in air and in nitrogen atmosphere. The heated samples were characterized by X-ray diffraction (XRD) and X-ray photon spectroscopy (XPS), and the sequestration efficiency of Cr on Mt was further examined by ammonium acetate and HCl washing. The results indicated that heating temperature can significantly affect the sequestration efficiency. For the samples heated in air atmosphere, Cr can be effectively sequestered at a relatively low temperature (e.g., 200 °C), as Cr3 + ions can migrate into the Mt layers because of its small ion radius. As the temperature increased to 300 °C, however, the oxidation of Cr3 + into Cr6 + would promote the desorption of Cr. As the temperature further increased to above 400 °C, the collapse of Mt layers in combination with the formation of Cr2O3 then can help in sequestering Cr. Only after heating at 700 °C, efficient sequestration of Cr on Mt could be achieved by the significant damage of the layered structure. When heated at 900 °C, the desorption of Cr was un-detectable due to the formed amorphous silica structure. On the other hand, as Cr3 + could not be oxidized in nitrogen atmosphere, the sequestration efficiency continually increased with rising temperature. The results of this work showed that heating temperature should be cautiously chosen for the effective sequestration of oxidizable heavy metal cations on Mt.

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