Arsenic(V) Immobilization in Fly Ash and Lead-Zinc Mine Tailing-based Geopolymer: Performance and Mechanism Insight

Global mining activities produce thousands of millions of toxic-bearing mine tailing (MT) wastes each year. Storage of the mine tailings not only encroaches upon large areas of cropland but also arouses additional ecological and environmental risks especially when the toxics (e.g., arsenic) are leached out during the stormy weather. Herein we demonstrate that geopolymerization of a mixture of the toxic-bearing mine tailings and the coal fly ash (FA) with a blending of sodium hydroxide (NaOH) and sodium silicate solution can effectively immobilize extra arsenic (As) species derived from arsenic-bearing wastes in the geopolymer specimens, which also possess high compressive strengths (e.g., > 25 MPa for specimens with 54 wt.% FA and activated with 10 M NaOH), allowing them to be further used as low-carbon, cement-free building materials. The geopolymer strength was found to depend clearly upon the NaOH concentration, the fly ash content, the arsenic content, and the curing time, with the maximum being 37.07 MPa for a specimen with 54 wt.% FA, 0.03 wt.% As activated with 10 M NaOH and cured for 28 days. Leaching tests showed that all specimens can achieve an immobilization efficiency as high as 95.4% toward As, and that the leachabilities of both As and other toxic heavy metals are far below the limits of the Chinese leaching standard for hazardous wastes (GB 5085.3 − 2007). Microstructural analyses using X-ray diffractometer (XRD), scanning electron microscopic (SEM), Fourier transform infrared (FTIR) indicate that the As species was physically encapsulated along with the formation of geopolymer gels at first, and then chemically incorporated into the crystalline phases (e.g., calcium silicate, and calcium silicate hydroxide) derived from the geopolymer gels as a result of prolonging the curing time. The developed FA/MT-based geopolymers represent a promising green material for both the remediation of As-bearing lands and the construction industry.