Research On Performance And Mechanism Of Removing Boric Acid From Boron Containing Radioactive Wastewater By Reverse Osmosis

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.