Structural alteration within fly ash-based geopolymers governing the adsorption of Cu2+ from aqueous environment: Effect of alkali activation

Fly-ash based geopolymers have been considered as a low-cost yet effective adsorbent for the removal of heavy metal cations, including Cu²⁺, from the aqueous environment. In the synthesis of geopolymers, the fly-ash needs to be alkali activated using several systems rich in either Na⁺ or K⁺. Herein, we investigate the effect of alkali activation on the structural alteration and its consequence on the adsorption capacity. Based on the series of detailed characterizations, the geopolymers formed in Na⁺-based alkali system is found to have more organized structure compared to that formed in K⁺-based alkali system. Moreover, the incorporation of additional silicate creates ancillary structure which positively contributes to the organization of the overall structure. All the samples, fly-ash and geopolymers, exhibits Cu²⁺ adsorption based on Langmuir isotherm and pseudo-second order kinetic. The geopolymers with more organized structure display higher Cu²⁺ adsorption capacity, which reaches 40 mg g^-1 higher in comparison to 7 mg g^-1 for fly ash sample. The structural alteration induces the formation of open-framework structure with more accessible sites which can accommodate more Cu²⁺. Our study provides a fundamental understanding for the design and fabrication of geopolymers as an effective adsorbent for the removal of heavy metal cations.