Changes of mineralogical-chemical composition, cation exchange capacity, and phosphate immobilization capacity during the hydrothermal conversion process of coal fly ash into zeolite

In the search for a technique to augment the nutrient removal capacity of zeolite synthesized from fly ash (ZFA), the present study investigated the changes of mineralogical-chemical composition, cation exchange capacity (CEC), and phosphate immobilization capacity (PIC) during the synthesis process. The ZFAs were obtained as a function of temperature (40-120 ^oC), liquid/solid ratio (1-18 ml/g), NaOH concentration (0.5-4 mol/L) and reaction time (2-72 h). The formation of low-silica zeolites (P1, hydroxysodalite, and chabazite) and the stability of mullite were observed, causing a marked decrease in SiO₂ content but roughly no change in Al₂O₃ content during the synthesis process. The decrease in K₂O, MgO content and the insignificant change in Fe₂O₃ and TiO₂ content were related to the solubility of the oxides while the increase in Na₂O and CaO was due to the increase in CEC. A high CEC was achieved under a high temperature, a high liquid/solid ratio, a long reaction time, and an appropriate NaOH concentration (2 mol/L), while a maximum PIC was achieved under relatively mild synthesis conditions instead (e.g., a reasonably short reaction time 10 h). This discrepancy was explained by the fact that different controlling factors/components in ZFA are responsible for CEC (content and kind of zeolite) and PIC (Ca component, specific surface area, and dissociated Fe₂O₃ and Al₂O₃).