Development of alkali-activated adsorbents from co-combustion ashes

The aim of this thesis was to investigate the adsorption capacity of alkali-activated fly ash and bottom ash granules for the removal of phosphate and ammonium from wastewater. In the first phase of the study, the ashes underwent pre-treatment, and granules were synthesized using different precursor materials (fly ash, bottom ash and metakaolin as a reference) and alkali activators (3 M sodium hydroxide (NaOH) and sodium silicate). The granule preparation process was optimized, and the effect of different activators on granule formation and stability was examined.

The second phase focused on evaluating the adsorption performance of the prepared granules. Key operational parameters, including contact time and granule volume, were optimized. The granules were thoroughly characterized before and after each adsorption cycle to assess their mechanical durability, while pH fluctuations were monitored to understand the system behavior. Based on the results, bottom ash granules activated with 3 M NaOH exhibited the highest removal efficiency for both phosphate and ammonium and were thus selected for further testing.

Subsequent adsorption experiments were conducted using both synthetic and municipal wastewater over three consecutive adsorption cycles, with regeneration steps between each cycle. Different regeneration chemicals were compared, and 0.3 M nitric acid (HNO3) demonstrated the most effective desorption performance for both nutrients. Notably, regeneration experiments revealed that phosphate was not only desorbed from the granules but also leached from the internal matrix, indicating the release of intrinsic phosphorus. This suggests that bottom ash-based granules may function dually as adsorbents and secondary phosphate sources, offering potential for fertilizer applications.
Keywords: adsorption, alkali-activated materials, nutrient removal, wastewater treatment