Phosphate immobilization from aqueous solution by fly ashes in relation to their composition

Phosphate sorption capacities of 15 Chinese fly ashes were determined and related to their composition. The data of P sorption were best fitted to Langmuir equation, and the calculated sorption maxima of phosphate (Q_m) ranged from 5.51 to 42.55 mg/g. The Q_m value showed a significantly positive correlation with total Ca content (r = 0.9836^**) and total Fe content (r = 0.8049^**), but negative correlation with total Si and total Al content. Correlation coefficients of CaO (r = 0.9647^**) and CaSO₄ (r = 0.9399^**) were much greater than that of CaCO₃ (r = 0.6361^*). Correlation coefficients of Q_m with Fe₂O_3d and Al₂O_3d were much higher than those of total Fe and total Al contents, respectively. Fractionation of P sorbed by fly ash revealed that loosely bound P fraction and/or Ca + Mg-P fraction were the dominant form of immobilized phosphate. Ca content was strongly correlated with the Ca + Mg-P fraction instead of Mg content, whereas Fe content was highly correlated with Fe-Al-P fraction compared with Al content. The loosely bound P was correlated well with both Ca and Fe content. The greatest removal of phosphate occurred at alkaline conditions for high calcium fly ash, at neutral pH levels for medium calcium fly ash, while low calcium fly ash immobilized little phosphate at all pH values. This behavior was explained by the reaction of phosphate with Ca and Fe related components. It was concluded that P immobilization by fly ash was governed by Ca ingredient (especially CaO and CaSO₄) and Fe ingredient (especially Fe₂O_3d).