Investigating the microstructure of high-calcium fly ash-based alkali-activated material for aqueous Zn sorption

The performance of adsorbents prepared by alkali activation of high calcium fly ash was investigated for removing aqueous Zn. Two formulations involving the use of NaOH and Na₂SiO₃ activating solutions were used to prepare the adsorbents that feature different microstructural characteristics. The Zn sorption data indicates a sorption process that is controlled by both chemisorption and intra-particle diffusion. The Na₂SiO₃-activated material displayed higher sorption rates compared to the NaOH-activated material. The sorption kinetics show strong dependence on the microstructures of the adsorbents, wherein the Na₂SiO₃-activated material featuring higher contents of amorphous phases (96 %_mass) in the hydrated phase assemblage, with attendant improved porosity and surface area, performed better than the NaOH-activated material (86 %_mass amorphous phases) which showed higher degree of crystallinity and coarse morphology. The Na₂SiO₃-activated material exhibited 100% Zn removal efficiency within the first 5 min in all studied initial adsorbate concentrations(corresponding to sorption capacity of up to 200 mg/g), while the NaOH-activated analogue tends to lag, reaching 99.99% Zn removal efficiency after about 240 min in most cases. The two formulations were also examined with thermodynamic modeling and the results agree with experimental data in indicating that the use of alkali-silicate activating solution is most suitable for converting high calcium fly ash into efficient adsorbent for removing aqueous heavy metals.