Metakaolin geopolymer characterization and application for ammonium removal from model solutions and landfill leachate

Geopolymers are aluminosilicate compounds that are amorphous analogues of zeolites, and thus possess similar cation-exchange properties. Geopolymers have been successfully applied to remove toxic metals and organic dyes from aqueous solutions. In this study, geopolymer was synthesized from metakaolin and applied to remove ammonium (NH 4 + ) from model solutions and landfill leachate. Geopolymerization increased the ammonium removal capacity, surface area and average pore width and changed the chemical structure of metakaolin. The maximum NH 4 + removal capacity of the geopolymer was 21.07 mg g − 1 which was 46% higher than the capacity of the reference clinoptilolite–heulandite zeolite (14.42 mg g − 1 ). The adsorption data of the geopolymer and zeolite fitted best to the Langmuir–Freundlich isotherm and the removal kinetics followed the pseudo-second order kinetic equation. The spent geopolymer adsorbent was regenerated efficiently using 0.2 M NaCl and 0.1 M NaOH as a regenerant. A small-scale continuous field experiment with landfill leachate was performed and further indicated that the metakaolin-based geopolymer could be a feasible NH 4 + removal adsorbent.