Pb2+ and Cr3+ Immobilisation Efficiency and Mechanism in Red Mud-Based Geopolymer Grouts

Abstract

In this study, a geopolymer composite based on red mud and granulated blast furnace slag was prepared by immobilising Pb 2+ and Cr 3+ to improve the utilisation rate of red mud, which represents a green, low carbon emission, and efficient material for heavy metal solidification and grouting engineering. We investigated the mechanical strength and leaching characteristics of geopolymer grouts, in addition to the immobilisation mechanisms using 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. The results demonstrated that the mechanical strength of red mud-based grout increased first and then decreased with increment in the content of the alkali activator modulus. Moreover, red mud can participate in the geopolymerisation reaction because of its aluminosilicate component. Furthermore, both Pb 2+ and Cr 3+ improved the mechanical strength of red mud-based geopolymer grouts at contents of 0.4 and 0.6 wt.%, respectively; however, the mechanical strength dramatically decreased when the dosages were 1.0 wt.%. The chemical fractions of Pb 2+ and Cr 3+ demonstrated that the soluble and exchangeable fractions could be converted to a carbonate-bound fraction, iron–manganese oxide and an organic fraction; however, the immobilisation rates reached 91.46% and 95.86%, respectively. The characterisation results confirmed that Pb 2+ and Cr 3+ participated in the geopolymerisation process, and Pb 2+ and Cr 3+ were immobilized by the effects of chemical bonding and physical sealing.

Keywords: red mud, geopolymer, Heavy metals, immobilization mechanism, mechanical strength