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).