Synthesis of zeolite-geopolymer composites with high zeolite content for Pb(II) removal by a simple two-step method using fly ash and metakaolin

Zeolite-geopolymer composites synthesized using industrial solid waste have mechanical strength, high specific surface area and cation exchange ability. This self-supporting adsorbent can replace natural heulandite zeolite to remove metal ions. In this study, a zeolite-geopolymer composite with a high zeolite content was synthesized by a simple two-step process using fly ash and metakaolin as raw materials. The compressive capacity, pore structure and adsorption ability for Pb(II) in the solution of the composite were analyzed and compared with natural heulandite zeolite. Through the two-step synthesis, the dynamic conditions of zeolite crystallization and geopolymer solidification were determined, and the zeolite-geopolymer composite with 57.5% zeolite A content and 6.42 MPa compressive strength was successfully prepared. The composite had hierarchical pore sizes, including microporous, mesoporous and macroporous pores, and its specific surface area was larger than that of natural heulandite zeolite. The Pb(II) adsorption capacity of the composite could reach 446.73 mg/g through ion exchange and hydroxyl reactions in solution, which was higher than the value of 323.37 mg/g for natural heulandite zeolite. Therefore, zeolite-geopolymer composites with high zeolite content synthesized by the simple two-step method can be used as self-supporting adsorbents to replace natural heulandite zeolites for removing metal ions from water.