Ammonium recovery from wastewater by ion exchange using porous metakaolin geopolymer granules over extended adsorption/regeneration cycles

Ion exchange on porous geopolymer granules represents a potentially promising unit process for ammonium (NH 4 + ) recovery from wastewater. However, the previous studies have not systematically optimized the regeneration nor evaluated the robustness of the ammonium recovery over extended adsorption/regeneration cycles. In this study, NaCl, Na 2 SO 4 , NaNO 3 , KNO 3 , K 2 SO 4 , CH 3 COOH, and combined NaOH/NaCl at concentrations of 0.3–0.9 M and thermal regeneration at 500 °C were compared for the regeneration of NH 4 + -saturated geopolymer granules. 0.9 M KNO 3 exhibited the best performance. The adsorption/regeneration cycles were repeated 30 times in synthetic wastewater and 5 times in tertiary municipal wastewater (directly or after filtration with 0.45 µm membrane). The porous geopolymer granules (size 2–4 mm) had an average NH 4 + ion exchange capacity of 9.0 mg/g and NH 4 + recovery of 51–98 % over the 30 adsorption/regeneration cycles in synthetic wastewater (initial concentration 500 mg/L). After 20 cycles, the ion exchange capacity of the granules started to decrease. The characterization of the granules after 30 cycles indicated that a minor amount of µm-sized particles were disintegrated from their surface and 70 % of the Al sites were no longer converted into K-form during regeneration. In municipal wastewater, the NH 4 + uptake was 3.9 mg/g on average (initial concentration of 67 mg/L). The NH 4 + recovery was 68 % and 74 % during the 5th cycle in raw tertiary wastewater and filtrated wastewater, respectively. Wastewater caused pore blocking and accumulation of solids on the surface which were reduced by the filtration. This study highlights the importance of the pretreatment before applying geopolymers for NH 4 + recovery in realistic conditions and that the porous geopolymer granules have a lifetime of 20 adsorption/regeneration cycles before their ion exchange capacity begins to decline.