Arsenic(V) immobilization in fly ash and mine tailing-based geopolymers: 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. Herein we demonstrate that geopolymerization of a mixture of the toxic-bearing mine tailings and the coal fly ash (FA) can effectively immobilize exogenous arsenic (As) species in addition to inherent As from the raw materials. The geopolymers also possess high compressive strengths (e.g., >25 MPa for specimens with 54 wt% FA and activated with 10 M sodium hydroxide (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 FA 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 achieved an immobilization efficiency as high as 95.4% toward As, and that both the short-term and long-term leachabilities of all toxic elements are far below the standard maximum contaminant levels. Microstructural analyses indicate that calcite, calcium silicate, and calcium silicate hydroxide are likely to play a crucial role in immobilizing As species and heavy metals of concern in the geopolymer matrixes. Given the superior mechanical strengths and long-term stabilities, the FA/MT-based geopolymers demonstrate a promising low-carbon material for both the remediation of As-bearing lands and the construction industry.