Study on the solidification/stabilization of Cu(II) and Cd(II)-contaminated soil by fly ash-red mud based geopolymer

Abstract

The preparation of geopolymer for solidification/stabilization of heavy metal contaminated soils using industrial solid waste was a sustainable method. In this study, a binary geopolymer curing agent was synthesized from red mud and fly ash for the treatment of copper- and cadmium- contaminated soils. The changes in the properties of the cured soil were investigated by analyzing compressive strength, permeability coefficient, pH value, toxicity leaching, and the chemical forms of heavy metals. These parameters were examined under varying amounts of curing agent and curing time. The solidification mechanism of contaminated soil was revealed by microscopic experiments such as X-ray diffraction (XRD), infrared spectroscopy (FTIR), scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS). The results showed that geopolymer could significantly improved the mechanical properties and environmental safety of contaminated soil. Compressive strengths of Cu and Cd contaminated soils after 28d of curing with 30 % geopolymer were 1.27 and 1.44 MPa, the permeability coefficients were 4.2 and 3.8⁻⁶cm/s, and toxic leaching amounts of Cu²⁺ and Cd²⁺ were 4.8 and 0.21 mg/L, and pH values were 10.9 and 10.6, respectively. Geopolymer gel structures not only filled the voids between soil particles but also physically encapsulated, chemically bonded, precipitated and ion-exchanged to achieve solidification/stabilization of contaminated soils. This research provided a new technology for the management of heavy metal contaminated soil and promoted the sustainable use of industrial solid waste.