【作者】 王晓伟；

【导师】 杨开；

【作者基本信息】 武汉大学 ， 市政工程， 2014， 博士

【摘要】 核设施在运行、维修和退役阶段会产生大量含硼放射性废水,此类废水必须经除放射性并分离硼酸后才能向环境排放。目前采用的蒸发处理方法存在系统工艺复杂、能耗高的缺点。本文在在深入调研含硼放射性废水处理方法的基础上,对苦咸水反渗透膜分离硼酸的特性进行全面研究,并提出采用多羟基化合物络合强化的方法改善反渗透膜分离硼酸的性能。采用人工配制的模拟废水和实际放射性废水进行了含硼废水的处理实验,并基于不可逆热力学模型对反渗透分离硼酸的机理及性能进行了分析,为反渗透方法用于处理含硼放射性废水打下了技术基础。不同条件下反渗透膜对硼酸分离特性实验结果表明,影响反渗透除硼率最重要的因素是进水pH,随着进水pH的上升反渗透除硼率不断提高,pH为10.31时CPA2膜和BW30膜对硼酸的截留率分别达到了91%和92.1%。反渗透除硼率随着操作压力的增大和水温的降低而提高,膜元件的回收率和进水含盐量对反渗透除硼率也有一定影响,而进水硼浓度对除硼率几乎没有影响。硼酸络合实验结果表明,多羟基化合物对硼酸络合作用由强到弱的排列顺序为：山梨醇>甘露醇>果糖≈木糖醇>葡萄糖。通过多羟基化合物的络合强化,可以明显提高反渗透过程的除硼效果。在pH为8.5以上时,经过甘露醇按2：1的络合比进行络合强化后,两种反渗透膜的除硼率均分别提高7～10个百分点；按5：1的络合比进行络合强化后,除硼率提高12～18个百分点。山梨醇和甘露醇络合强化反渗透正交实验结果表明,pH对络合强化反渗透除硼效率的影响最大,水温次之,回收率和压力相对来说影响程度较小。络合强化反渗透可以在相对低的pH条件下实现较高的硼酸去除率,反渗透膜在相对低的pH下运行可以降低膜结垢风险,延长使用寿命,这对于含硼废水的处理具有实际意义。模拟含硼放射性废水反渗透处理实验结果显示,在pH为9.13时,经山梨醇按5：1的摩尔比络合后,CPA2和BW30两种膜对硼酸的去除率分别处于96.60%～98.49%和97.32%～99.21%的范围内,比直接反渗透的除硼率有显著提高。在模拟含硼废水处理过程中,脱盐率随着操作压力的提高、回收率的下降而上升。在同样的进水压力和回收率下,脱盐率随着进水pH的提高明显上升。另外,经反渗透膜处理后,透过水中钴浓度降低到检测限以下。对铯的去除效果与pH、操作压力和回收率有关,在pH为9.13时,铯的去除率可达到95%以上。实际含硼放射性废水反渗透处理实验结果表明,在进水压力1.0MPa、回收率约为25%的条件下,直接反渗透对硼酸的去除率仅为51.12%,山梨醇络合强化反渗透(络合摩尔比5：1)对硼酸的去除率为89.68%,比直接反渗透下的去除率提高了约75%。络合强化反渗透对实际含硼废水中硼酸有非常好的截留效果。实际含硼放射性废水的络合强化反渗透浓缩处理实验结果显示,反渗透膜在除硼的同时,还对废水中的盐分和放射性核素保持了比较好的去除效果,对硼酸的去除率保持在86.78%～89.21%之间,脱盐率则在94.0%-94.8%之间变化,对废水中54Mn、60Co和137Cs的去污因子最高可达934。将反渗透过程中硼酸的传质过程分解成H3B03和H2BO3-两种组分进行讨论,并根据实验数据,运用不可逆热力学模型,计算出了CPA2膜和BW30膜对两种组分的反射系数以及两种组分在反渗透膜中的透过系数。并由组分的反射系数和透过系数,计算出了反渗透过程中的硼反射系数和硼透过系数。并根据反射系数和透过系数的变化,合理解释了反渗透过程中除硼率的变化机理。采用不可逆热力学模型,计算预测了25℃时CPA2和BW30两种反渗透膜在不同进水压力和pH条件下的除硼率,预测结果与实验数据相吻合。更多还原

【Abstract】 Large volumes of boron-containing radioactive wastewater will be drained from some nuclear facilities during operation, maintenance, decommissioning periods. The radioactivity and boron concentration must be decreased to below the limits of relative standards before the wastewater is discharged to environment. Evaporating process which is currently adopted to treat boron-containing wastewater has the disatvantages of great complexity and high energy-consuming. Based on the investigating on various of treating processes of boron-containing wastewater, the rejection performance of brackish water reverse osmosis membrane to boric acid was researched. It is prompted that boron rejection ratio could be elevated by polyol complexing enhanced reverse osmosis. Experiments of treating simulated and actual boron-containing radioactive wastewater were carried out on a pilot reverse osmosis equipment. The boron separation mechanism by reverse osmosis was analyzed and boron removing performance was predicted on the base of irreversible thermodynamic model.Experiments of boron separation by reverse osmosis under various conditions were conducted and results showed that feedwater pH was the most important factor that affecting the percentage of boron rejection. The boron rejecton ratio would increased while feedwater pH was elevated. When feedwater pH was10.31, the rejection ratio would reach91%and92.1%for CPA2and BW30reverse osmosis menbrane respectively. The rejection percentage would also be improved if the operating pressure increased and the temperature decreased. Boron rejection percentage was affected by recovery ratio of membrane element and salt contration to some extent, while be independent on boron concentration of feedwater.Boric acid and polyol complexing experiments results showed that the complexing ability sequence was sorbitol, manntiol, fructose, xylitol and glucose. Boron removing performance of reverse osmosis could be improved remarkably by polyol complexing. When feedwater pH was above8.5, boron rejection ratio could increased by7～10percents if complexing ratio between polyol and boric acid was2, and the rejection ratio increment would reach12～18percents if the complexing ratio was5. Orthogonal experiments of sorbitol and manntiol complexing enhanced reverse osmosis also revealed that feedwater pH was the most important factor, temperature was the less important one. Compared to feedwater pH, the effect of recovery and operating pressre was neglectable. Through complexing enhanced reverse osmosis, high rejection ratio could be achieved in terms of low feedwater pH. Reverse osmosis process which running under low pH would be significative for boron-containing wastewater treatment because low pH could reduce the risk of scaling on membrane surface.Experiments of treating simulated boron-containing wastewater were performed and results showed that boron rejection ratio would be in the range of96.60%～98.49%and97.32%～99.21%for CPA2and BW30membrane respectively when the complexing ratio between sobitol and boric acid was5and feedwater pH was9.13. The results also showed that salt rejection would be improved if the operating pressure increased and the element recovery decreased, especially that salt rejection could increase remarkbly when feedwater pH advanced. The concentration of cobalt in reverse osmosis permeate was under detection limit. The removing performance to cesium by reverse osmsos was attributed to feedwater pH, operating pressure and recovery. Cesium rejection ratio would be above95%when feedwater pH was9.13.Experiments of treating actual boron-containing wastewater draind from a nuclear facility were carried out and results showed that if the operating pressre was1.0MPa and recovery was25%, boron rejection ratio was only51.12%obtained by direct reverse osmosis, while the rejection ratio could reach89.68%by means of sobitol enhanced reverse osmosis. Exellent rejection performance to boric acid in boron-containing wastewater could be achieved through complexing enhanced reverse osmosis. Actual wastewater concentrating experiments by complexing enhanced reverse osmosis were also performed and results revealed that salts and radioactive nuclides as well as boric acid in wastewater could be removed effectively. Boron and salt rejection ratio could maintain in the range of86.78%～89.21%and94.0%-94.8%respectively. Decontamination factor of manganese-54, cobalt-60and cesium-137could be as high as934.Boron transfer mechanism in reverse osmosis process was discussed and boric acid and borate were taken into account as two distinctive species. Reflection coefficients and permeability constants of CPA2and BW30menbrane for boric acid and borate were caculated on the basis of irreversible thermodynamic model. The overall boron reflection coefficients and permeability constants were also caculated. According to caculating results, the cause of variational boron rejection ratio under different conditions was reasonably interpreted. The boron rejection ratio of CPA2and BW30menbrane under25℃and different operating pressure and feedwater pH was predicted adopting irreversible thermodynamic model. Prediction result agreed with experiment data.更多还原