Effect of Additives on Immobilization of Se(Ⅵ) and Cr(Ⅵ) in Geopolymer

Geopolymers have a low immobilization efficiency to oxyanions, such as Se(Ⅵ) and Cr(Ⅵ). Effects of Ca, Mg and Fe(Ⅱ) additives on the immobilization of Se(Ⅵ) and Cr(Ⅵ) in geopolymer were thus investigated. The results indicate that the leaching amount of Se or Cr ions is greater than 60% for the geopolymers without any additives. CaO or Fe(Ⅱ) has a limited depressive effect on Se(Ⅵ) or Cr(Ⅵ) leaching from geopolymers, and the corresponding leaching amount of Se(Ⅵ) or Cr(Ⅵ) is still greater than 40%. However, the leaching amount of Se(Ⅵ) or Cr(Ⅵ) is lower than 10% for the geopolymers with the addition of MgO. According to the LCF fitting results, Cr(Ⅵ) can also exist in the geopolymer network through electrostatic interaction as Se(Ⅵ). Besides, MgO or CaO can promote Fe(Ⅱ) to release its reducibility, thereby reducing approximately 15% of Cr(Ⅵ) to Cr(Ⅲ). Se(Ⅵ) can have a bonding interaction with Ca. Fe(Ⅱ) does not present its reducibility during the solidification of Se(Ⅵ). To some extent, MgO, CaO and Fe(Ⅱ) have a synergistic effect on the leaching of Se(Ⅵ) and Cr(Ⅵ) from geopolymers. In addition, the mechanical strength of geopolymers with or without these additives can satisfy a requirement for industrial landfill.