Research On The Mechanism Of Phosphate Immobilization And The Intensified Phosophate Immobilization Technology Of Zeolite Synthesized From Coal Fly Ash

The research objective of this dissertation is to develop a novel adsorption material of nitrogen and phosphorus to intensify the advanced treatment ability of ecological recovery process for the urban landscape river with low-concentration pollutants. The nitrogen and phosphate sorption capacities of natural zeolite, fly ash and zeolite synthesized from fly ash were compared and the correlation of the amount of immobilized phosphate with the chemical composition was analyzed from the angle the statistics. The factors for the phosphate immobilization were also discussed from the aspect of chemical reaction in solution. On the basis of the phosphate immobilization mechanism, the synthesis conditions were optimized in terms of the cation exchange capacity(CEC) and phosphorus adsorption coefficient(PAC) instead of the CEC alone. Metallic cation saturation and surface acid treatment were operated on the zeolite synthesized from fly ash to further intensify the phosphate adsorption capacity. The purification ability of the intensified zeolite was investigated by using the domestic wastewater in the dynamic filter column of zeolite for the project application.The comparison experiment of nitrogen and phosphate adsorption capacities of natural zeolite, fly ash and zeolite synthesized from fly ash showed that the CEC of 15 synthesized zeolites ranged from 103.3 cmol/Kg to 213.0cmol/Kg, 2～3 times higher than that of natural zeolite. The PAC of 15 synthesized zeolites ranged from 11.79mg/g to 47.17mg/g, 2～5 times higher than that of fly ash. Like the natural zeolite, the ammonium removal rate of synthesized zeolite decreased with the increase in pH under the alkaline condition. Though the ammonium removal rate of synthesized zeolite was lower than that of natural zeolite at the neutral and acidic pH level, the pH range of synthesized zeolite for phosphate removal increased remarkably. Within the temperature range of 5～42℃, the phosphate adsorption amount increased with the increase of temperature, but a decrease was observed for the ammonium adsorption amount with the increase of temperature.According to the data of correlation analysis between the amount of sorbed phosphate and the chemical composition, calcium and iron components were mainly involved in phosphate immobilization. The Mg content was low on the whole, so the phosphate immobilized amount was negligible. Furthermore, the elements of Si and Al contributed little to the phosphate removal. The loosely bound P and/or Ca+Mg-P were the dominant phosphate fraction. The Fe+Al-P in synthesized zeolite increased markedly, 1.4～142.0 times higher than that in fly ash, indicating that Fe and Al began to play an increasing role for phosphate removal. Free CaO and dissociated Fe₂O₃(Fe₂O₃ d) were the specific components of phosphate immobilization. CaSO₄ and dissociated Al₂O₃(Al₂O₃ d) were also conductive to the phosphate adsorption. There was no significant change of Fe, Al, Ca and Mg content following the conversion of fly ash to zeolite, but the increase in Fe2O3 d and specific surface area probably accounted for the enhancement in PAC and Fe+Al-P of synthesized zeolites compared with corresponding fly ashes. The specific surface area of synthesized zeolite was 26～89 greater than that of fly ash, which is favorable for phosphate adsorption reaction. Ca²⁺ played a crucial role in phosphate removal. Since high calcium zeolite released large amounts of Ca²⁺ ions at the alkaline condition, it is not surprising that high phosphate removal occurred within this pH range. Zeolites synthesized with medium and low Ca content released very little Ca²⁺ ion within this pH range, leading to the low phosphate removal efficiency. At the neutral to weakly acidic conditions, the phosphate removal capacities of low and medium calcium zeolites improved because of the increase in Ca²⁺ ions. The sorption through ligand exchange with Fe-related component appeared to be the main mechanism under the acidic condition for zeolites with different calcium content.It was demonstrated from the optimization experiment of zeolite synthesis that the optimum temperature, synthesis time, alkaline concentration and liquid/solid ratio were 95℃, 8h, 1mol/L and 5:1, respectively. The phosphate immobilization capacity could be further enhanced notably by metallic cation saturation and surface acid treatment, and Ca-saturated zeolite was more practical for the project application. NH4+ and PO43- in the wastewater inter-restrained the adsorption on the zeolite, but, on the whole, the change of NH4+ and PO43-concentration put a little effect on the simultaneous removal of nitrogen and phosphate by the synthesized zeolite just owing to its large adsorption capacities. The increase in CEC and PAC was weak after the conversion of fly ash to zeolite, so the high calcium fly ash could be applied directly for the wastewater purification with the phosphate removal as a main target. For the wastewater with the ammonium removal as a goal. The low calcium fly ash should be used to synthesize zeolite through lengthening the synthesis time and synthesis temperature to improve the CEC. Since both CEC and PAC increased remarkably following the conversion of fly ash to zeolite for medium calcium fly ash, it is suitable to choose medium calcium fly ash to synthesize zeolite for the simultaneous removal of nitrogen and phosphate. The successful application of the nubby functional filling made by fly ash in the pilot-scale experiment indicated that the zeolite synthesized from fly ash could be made into granular or nubby fillings for the advanced treatment of urban landscape water bodies just because of their similar mechanism of phosphate immobilization.In this dissertation, the phosphate immobilization of zeolite synthesized from fly ash was clarified, and the theory of simultaneous removal of ammonium and phosphorus was enriched. The selection criteria of fly ash for zeolite synthesis were put forward based on the statistics study on the a lot of fly ashes, which was directive for enhancing the adding value of fly ash use. The research on the optimization of synthesis conditions was meaningful to further improve the simultaneous removal capacities of ammonium and phosphate by synthesized zeolite. The application of intensified zeolite as the filling would stand out the technical advantage of the ecological gravel bed process.