Pb²⁺ and Cr³⁺ immobilization efficiency and mechanism in red-mud-based geopolymer grouts

Herein, a green geopolymer composite was prepared using red mud and granulated blast furnace slag by immobilizing Pb²⁺ and Cr³⁺ to improve the utilization rate of red mud for heavy metal solidification and grouting engineering. The mechanical strength, leaching characteristics, and immobilization mechanisms of geopolymer grouts were investigated via Fourier transform infrared spectroscopy, X-ray diffraction, magic angle spinning-nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy-energy-dispersive spectroscopy. Results demonstrate that the mechanical strength of the red-mud-based grout first increased and then decreased with an increasing alkali activator modulus. Moreover, red mud could participate in the geopolymerization reaction because of its aluminosilicate component. Furthermore, both Pb²⁺ and Cr³⁺ improved the mechanical strength of red-mud-based grouts at contents of 0.4 and 0.6 wt%, respectively. However, the mechanical strength drastically decreased when the dosages for both were 1.0 wt%. These chemical fractions of Pb²⁺ and Cr³⁺ evidence that the soluble and exchangeable fractions could be converted to a carbonate-bound fraction, iron-manganese oxide, and an organic fraction; however, the immobilization rates reached 91.46% and 95.86%, respectively. The characterization results confirm the participation of Pb²⁺ and Cr³⁺ in the geopolymerization, and Pb²⁺ and Cr³⁺ were immobilized by the chemical bonding and physical sealing effects.