模拟核电厂放射性废液中Ag胶体的絮凝行为及机理研究

Flocculation Behavior and Mechanism of Silver Colloid from Simulated Radioactive Liquid at Nuclear Power Plant

  • 摘要: 110mAg是反应堆含银结构材料的中子活化产物之一,传统核级过滤器与离子交换树脂对胶体形态Ag的去除效果有限。为研究银胶体的絮凝行为,采用化学还原法制备了模拟核电厂废液环境中的Ag胶体溶液,系统分析了硼酸、柠檬酸、联氨及初始Ag+浓度对胶体形貌、粒径及稳定性的影响;探究了絮凝剂种类、pH、絮凝剂添加量及硼酸浓度等关键参数对Ag胶体絮凝效果的影响,并结合絮体表征分析了絮凝机理。结果表明,适当浓度的硼酸与柠檬酸会抑制Ag胶体颗粒间的团聚,降低颗粒粒径并提高胶体稳定性,而联氨过量会引发颗粒团聚。聚合氯化铝铁(PAFC)可在弱酸性至弱碱性条件下通过静电吸附和卷扫絮凝机制高效去除溶液中的Ag胶体,当PAFC添加量为20~50 mg/L时,溶液中总Ag(包括胶体态和离子态)去除率高于96%,胶体去除率超过98%。溶液中硼酸抑制PAFC对Ag胶体的絮凝效果,而Fe3+、Co2+和Mn2+等共存离子则可提高Ag胶体去除率。絮体中银主要以Ag0形态存在,絮凝过程以物理吸附卷扫为主。

     

    Abstract: 110mAg is one of the neutron activation products in silver-containing structural materials within the nuclear power plant (NPP) reactor. 110mAg can exist in either ionic or colloidal forms within the NPP liquid radioactive waste. Conventionally, nuclear-grade filters or ion-exchange resins cannot effectively remove the colloidal 110mAg. The elimination of 110mAg colloids from NPP liquid radioactive waste represents a critical challenge for radiation safety management and environmental protection. The stable silver colloids under simulated NPP wastewater conditions were synthesized by chemical reaction method. The regulatory effects of the addition amounts of boric acid H3BO3, citric acid C6H8O7, and hydrazine hydrate N2H4·H2O on the silver colloids characteristics were systematically examined. The ultraviolet-visible spectrophotometry (UV-vis), zeta potential measurement, nanoparticle size distribution and transmission electron microscope (TEM) were combined to characterize the sliver colloidal morphology, particle size distribution, and stability. The sliver colloid solutions prepared under optimal synthesis conditions produce the monodisperse Ag colloids with the average particle size of (30.2±2.1) nm and the zeta potential of (−38.9±1.5) mV. The results of flocculation experiments indicate that polymeric aluminum ferric chloride (PAFC) is more suitable for removing silver colloids from solution. Under neutral-alkaline conditions, the total Ag (including the ionic Ag+ and colloidal Ag) and colloidal Ag removal rates achieve 96.3% and 98.2%, respectively. Notably, the boric acid inhibits the elimination of Ag colloids by flocculation process. The removal rate of silver colloids reduces to 69.8% when the aqueous boric concentrations achieve to 1 000 mg/L. It can be attributed to the enhanced colloid stabilization caused by boric acid. The coexisting transition metal ions, e.g., Fe3+, Co2+, Mn2+, improve the colloidal removal via hydroxide co-precipitation. When the dosage of PAFC is 5 mg/L, the total Ag removal rate in the solution containing transition metal ions is as high as 98.4%, which is significantly higher than the conditions without transition metal ions. The microscopic morphology of the flocs was characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The results indicate that the flocs are sheet-like stacked with a dense structure. The elements, including Ag, Al, Fe and Cl are uniformly distributed on the surface of the flocs, with Al (63%) and Ag (24%) being the main components. The work further investigates the floc characters with XPS analysis, which verifies the preservation of metallic Ag0 in flocs (with the binding energy 368.1 eV belong to Ag 3d5/2 and the Auger parameter value of 726.5 eV). In summary, PAFC exhibits excellent performance in removing Ag colloids from solutions. Mechanistically, the removal of Ag colloids is mainly achieved through electrostatic adsorption, sweep flocculation, and bridging effects. This work establishes fundamental guidelines for optimizing Ag colloid removal in complex radioactive wastewater matrices, and highlights the importance of solution chemistry control in practical NPP wastewater treatment applications, and provides the reference for the subsequent process of liquid radioactive waste.

     

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