Abstract:

The treatment of ammonia nitrogen wastewater by struvite (MAP) crystallization method has the characteristics of good removal effect and fast reaction speed. However, the slow growth processes and small grain sizes of MAP limit the recovery rate of MAP by-products and reduce their economic value. The effects of different factors (temperature, stirring rate, pH, ion molar ratio) on the growth rate of MAP crystals were studied by using the chemical potential gradient model based on the adhesive-type growth mechanism and the Debye-Hückel limiting law to calculate the activity coefficient. The results showed that the increase of temperature, stirring speed and molar ratio led to the enhancement of growth rate constant k_t, which led to the increase of struvite growth rate. The higher pH enhanced the rate constant k_t and thermodynamic driving force ?\mu at the beginning of the reaction. At the later stage of the reaction, the growth rate constant k_t was mainly affected. Finally, the growth rate of MAP increased during the whole reaction process. The average growth rate of MAP crystals were calculated, and it was found that the growth rates were between 10^-9 and 10^-8 m/s. When pH=9.5, the growth rate increased to 1.02×10^-8 m/s, as the ionic molar ratio [Mg²⁺]∶[\mathrm{P}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{3}-}]∶[NH₄⁺] =1∶1∶1 increased to 1.2∶1.2∶1, the highest growth rate of MAP reached 1.29×10^-8 m/s. In conclusion, with the increase of temperature, stirring speed, pH and ion-molar ratio, the growth rates of MAP crystals increased to varying degrees. Among them, the influence of ion-molar ratio was the largest, followed by pH, and the influence of temperature and stirring speed was small. Therefore, changes in pH and ion molar ratio are expected to regulate the crystallization process of MAP in practical industry.

Key words: struvite, ammonia nitrogen wastewater, growth rate, rate constant, driving force

摘要：

鸟粪石（MAP）结晶法处理氨氮废水具有去除效果好、反应速度快等特点，但MAP生长过程缓慢，晶粒细小，限制了MAP副产品的回收率，经济价值降低。利用基于黏附型生长机制建立的化学势梯度模型结合德拜-休克尔极限公式计算活度系数，研究了不同因素（温度、搅拌速度、pH、离子摩尔比）对MAP晶体生长速率的影响。结果表明，温度、搅拌速度和摩尔比的提升导致动力学生长速率常数k_t提高，从而导致鸟粪石生长速率上升，pH的升高使得反应初期的生长速率常数k_t和热力学推动力?\mu都得到了增强，到了反应后期，主要影响生长速率常数k_t，最终导致整个反应过程中MAP生长速率上升。对MAP晶体的平均生长速率计算发现，其生长速率分布在10^-9～10^-8 m/s之间；在pH=9.5时，生长速率提高至1.02×10^-8 m/s；离子摩尔比[Mg²⁺]∶[\mathrm{P}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{3}-}]∶[\mathrm{N}{\mathrm{H}}_{\mathrm{4}}^{+}]=1∶1∶1提高至1.2∶1.2∶1时，MAP生长速率最高达到了1.29×10^-8 m/s。综上分析表明，温度、搅拌速度、pH和离子摩尔比提升，MAP晶体的生长速率均有不同程度的提高，其中，离子摩尔比影响最大，pH次之，温度和搅拌速度影响较小。因此，在实际工业中，pH和离子摩尔比的改变有望调控MAP的结晶过程。

关键词: 鸟粪石, 氨氮废水, 生长速率, 速率常数, 推动力