Application of CO-loaded geopolymer in Zn removal from water: A multi-win strategy for coal fly ash disposal, CO emission reduction, and heavy metal-contaminated water treatment.

Coal fly ash accumulation, global warming, and heavy metal-contaminated water environments are three primary environmental concerns. Porous geopolymers are economical porous adsorbents that can be produced using coal fly ash as a raw material and employed for heavy metal removal from water. However, residual alkalis on the geopolymer can lead to extreme increases in pH and cause environmental stresses, which limits the large-scale production and application of geopolymers in industries and environments. A green approach to alleviating the high basicity of geopolymers through CO exposure is proposed, with CO adsorption experiments as well as Zn removal batch and column experiments conducted to evaluate the practicality of the synergistic strategy. CO adsorption experiments show the CO capture capacity of fresh geopolymer (F@PG) is 0.80 mmol g, greater than that of the conventionally washed geopolymer (W@PG, 0.26 mmol g), with the pH of the geopolymer decreasing after both washing and CO exposure. Batch experiments suggest neither washing nor CO exposure cause a significant change in the Zn adsorption capacity of the geopolymer; column experiments show the CO-exposed geopolymer (C@PG) has a pH < 9.5 and a satisfactory Zn removal performance similar to W@PG, but F@PG with a pH ∼12 results in a conversion of Zn to anionic forms and a decrease in Zn removal efficiency. These results indicate CO exposure is a practical method to decrease the pH of geopolymers for applications related to heavy metal-contaminated water treatment and provide a large-scale industrial option for coal fly ash consumption and CO emission reduction.