蒙脱石负载纳米零价铁对水溶液中铀的去除研究
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摘要
锕系元素中的铀因其半衰期较长在放射性废物处置方面倍受关注。铀的天然放射性同位素有三种分别是234U,235U和238U,具有较长的半衰期,最高可达4.51×109年。地球上存量最多的同位素是铀238U(99.2742%),其次是可用作核能发电的燃料235U(0.7204%),丰度最少的是234U(0.0054%)。铀元素通过采矿作业、核试验、核燃料、核武器和意外泄露等方式释放到土壤、沉积物和地下水中,造成土壤和地下水中的铀污染,危害人类赖以生存的环境。工业中使用的铀和采矿业造成高浓度的铀污染,对环境和人的身体健康都是很大的威胁。世界卫生组织在2004年建立关于饮用水中铀的标准：最大含量不得超过15μg/L。然而,最新的调查研究表明在全世界范围内饮用水中铀的最大含量应低于10μg/L或者5μg/L更为合理。研究表明在水岩界面,利用地球化学过程包括溶解沉淀反应、氧化还原反应和吸附解吸反应等一系列反应过程,可以控制铀在土壤和地下水系统中的迁移和转化。
本文选择接近于我国铀尾矿“返酸”后渗出的中低浓度含铀水溶液(初始浓度100μg/L)作为处理对象,以羟基铝柱撑蒙脱石为支撑负载材料,用硼氢化钠液相还原法制备蒙脱石负载纳米零价铁材料,并对比研究蒙脱石、纳米零价铁和蒙脱石负载纳米零价铁等不同材料对水溶液中低浓度的铀进行去除;采用批实验方法,研究在不同pH、固液比、温度、离子强度、初始浓度及共存离子(EDTA.HA和FA)等条件下对铀去除率的影响,并明确反应过程探讨反应机理。论文的主要研究内容可以分为以下三个方面：1.利用蒙脱石去除水溶液中低浓度的铀。
通过对比试验发现,pH对蒙脱石去除铀有着显著的影响：在酸性条件下(pH=2-6.5),铀去除率随pH值的升高而增加；碱性条件下(pH=7-9),铀去除率随pH值的升高而降低；实验最佳pH值为6.5,最大去除率48.05%。同时,溶液中离子强度对蒙脱石吸附铀影响也比较显著：在酸性条件下(pH=2-7),随离子强度的增大,吸附去除率反而越低,证明在酸性条件下高离子强度将抑制蒙脱石对铀的吸附；碱性条件下(pH=7-9),离子强度越大,吸附去除率越高,高的离子强度能够促进蒙脱石对铀的吸附。U(Ⅵ)在蒙脱石上的吸附百分数随固液比而增大。铀在蒙脱石上的吸附与Langmuir方程描述的等温吸附模型较为符合。温度越高,越有利于蒙脱石对铀的去除。铀酰离子在蒙脱石上的吸附机理是铀酰离子通过离子交换吸附和表面位点吸附两种方式共同作用。在酸性条件下(pH<7),蒙脱石以离子交换吸附的方式去除铀；在碱性条件下(pH=7-9),蒙脱石吸附铀主要是通过表面络合作用反应形成内层配合物。
2.利用纳米零价铁去除水溶液中低浓度的铀。
纳米零价铁是一个壳核结构,α-Fe0位于中间核部,表面是由一层铁氧化物(FeO)包裹形成的壳部。pH对纳米零价铁去除铀有着显著的影响：在pH=2的时候,对U(Ⅵ)的去除率基本为零；在pH=3-5时,对铀的去除率在73-78%；当pH=6-9,随pH的升高,去除率降低；纳米零价铁去除铀的最佳pH=5,去除率达78%。离子强度对纳米零价铁去除铀的基本无影响。去除率随随着固液比的增加而升高。随着铀初始浓度的升高,铀的去除率一直在下降。铀在纳米零价铁的吸附与Langmuir方程描述的等温吸附模型最为符合。温度对纳米零价铁去除低浓度铀的影响不明显,证明溶液中的U(Ⅵ)与纳米零价铁的反应是
一个化学反应控制的反应,不受温度的影响。初始pH=2时,反应后pH基本没有变化,反应后溶液中铁元素含量高达658mg/L；初始pH=3时,反应后pH变为5.57,反应后溶液中铁元素含量为18.9mg/L；初始pH在4-9时,反应后pH变为9左右；溶液中铁的释放量较少。纳米零价铁去除铀的主要机理为还原沉淀作用,反应过程中纳米零价铁和U(Ⅵ)发生氧化还原反应,电子转移生成四价铀的氧化物UO2和铁氢氧化物。
3.利用蒙脱石负载纳米零价铁去除水溶液中低浓度的铀。
纳米零价铁成功负载在蒙脱石上,有效解决纳米零价铁的团聚问题和易氧化的问题。pH对蒙脱石负载纳米零价铁去除铀的影响显著：在pH=2时,蒙脱石负载纳米零价铁去除铀的效果较差,去除率仅为5.13%；当pH=3时,去除率高达97.8%；蒙脱石负载纳米零价铁去除铀最佳pH范围是3-5,去除率在95.11-97.8%；当pH在6-9时,随pH升高去除率下降。离子强度对蒙脱石负载纳米零价铁去除铀的效果没有影响。去除率随固液比的增加而升高。随着铀初始浓度的升高,铀的平衡吸附量升高。蒙脱石负载纳米零价铁去除的等温吸附模型与Langmuir方程描述的等温吸附模型最为符合。温度不影响蒙脱石负载纳米零价铁对铀的去除,证明此反应受化学反应控制,不受温度的影响控制。反应初始pH=2,反应后的pH基本没有变化,反应后溶液中铁元素含量33.79mg/L；初始pH=3,反应后的pH变为5.15,反应后溶液中铁元素含量为4.67ng/L；pH=4-9,反应后的pH变为8.9左右,铁元素在溶液中的释放量非常少(17-53ug/L)。
通过加入不同的共存离子(EDTA、HA和FA),研究其对铀去除率的影响。实验结果表明EDTA的存在使蒙脱石负载纳米零价铁对铀的去除率降低,原因在于EDTA与铀络合作用形成U(Ⅵ)-EDTA复合物,此复合物抑制了铀在蒙脱石负载纳米零价铁上的去除反应。不同浓度EDTA和EDTA加入顺序的不同对蒙脱石负载纳米零价铁去除铀反应后的pH变化是相同的：相同初始pH下,低浓度(0.001mol/L)比高浓度(0.01mol/L)的EDTA反应后的pH值要高。结合其对铀去除率的结果,低浓度比高浓度的EDTA对铀去除率要高,反应中消耗的氢离子更多,导致反应后pH升高。共存离子HA对蒙脱石负载纳米零价铁去除铀的影响：在铀溶液依次加入HA和蒙脱石负载纳米零价铁,铀与HA先形成U(VI)-HA复合物在酸性条件下(pH2-7)阻碍蒙脱石负载纳米零价铁对铀的去除；在碱性条件下(pH8-9)促进蒙脱石负载纳米零价铁对铀的去除。共存离子FA对蒙脱石负载纳米零价铁去除铀的影响：在酸性条件下(pH2-6)抑制蒙脱石负载纳米零价铁对铀的去除：在碱性条件下(pH8-9)促进蒙脱石负载纳米零价铁对铀的去除。
蒙脱石负载纳米零价铁与铀反应后的产物通过XRD鉴定为FeOOH;扫描电镜可以看见表面纳米零价铁铁颗粒消失,被黑色物质覆盖；EDS结果显示反应后铁含量减少而氧含量增加；XPS对比反应前后发现,零价铁的峰在反应后消失,铁氧化物与铁氢氧化物的含量增大。综上所述,蒙脱石负载纳米零价铁去除铀的反应机理主要是氧化还原沉淀作用：零价铁(Fe0)与U(Ⅵ)发生氧化还原反应,在蒙脱石负载纳米零价铁的表面生成FeOOH。
本文创新点：1、制备羟基铝柱撑蒙脱石负载纳米零价铁材料。2、将羟基铝柱撑蒙脱石负载纳米零价铁材料应用于放射性元素铀的吸附研究。
Actinides are of great interest in terms of radioactive waste disposal because of their longevity. Uranium occurs naturally as U isotopes,238U(99.28%),235U(0.711%),234U(0.006%), and they exist as hexavalent uranyl complex in the natural environment. The use of uranium at industrial and military sites has resulted in very high uranium contamination, and dangerous to human healthy and environmental protection because of its long half-life (such as238Ut1/2=4.51×109years).Uranium, an actinide element, has been released into the environment through mining operations, nuclear testing, nuclear fuel, nuclear weapons production sites and accidental spill, and therefore, it is a major contaminant in soils, sediments, and groundwater. A provisional drinking water maximum contaminant level (MCL) for uranium of15μg/L has been established by the World Health Organization (WHO2004). However, it is discussed worldwide that10or5μg/L would be more reasonable. Geochemical processes occurring naturally, including dissolution/precipitation, redox reactions, and sorption/desorption reactions at the water-rock interface, control the mobility and transport of uranium in the subsurface system, such as aquifer sediments, soils, and groundwater.
Montmorillonite-supported zero-valent iron nanoparticles (M-nZVI) was synthesized by sodium borohydride reduction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM). The interaction of uranium with montmorillonite, zero-valent iron nanoparticles and M-nZVI were studied using batch technique under different experimental conditions such as pH, ionic strength, initial U(Ⅵ) concentration, solid-to-liquid ration (m/V), EDTA, humic acid, fulvic acid and temperature. An effluent solution with a low level of uranium, i.e.,4.2x10-7mol/L(100μg/L) was used in the experiments to avoid precipitation of amorphous uranium-hydroxides. Uranium occurs naturally in low concentrations below100μg/L in soil, rock, as well as in surface and groundwater. The research contents of this thesis are summarized as follows:
1. Removal of uranium from aqueous solution using montmorillonite
The SSA for montmorillonite was10.23m2/g and the layer spacing was1.28nm. The chemical composition was determined by X-Ray Fluorescence spectrometer:Al2O319.0%, SiO259.5%, MgO3.9%, Fe2O31.7%, K2O0.6%, Na2O3.6%and CaO2.3%.The removal efficiency of U(Ⅵ) using montmorillonite was strongly dependent on the pH values. The removal rate increases with the increase of pH value when the pH range was about2-6.5. The removal efficiency of U(Ⅵ) using montmorillonite was48.05%at pH6.5. The removal rate decreases with the increase of pH value when the pH range was about6.5-9. The concentration of NaNO3had significant effect on the U(Ⅵ) adsorption using the montmorillonite. The removal rate decreases with the increase of ions strength when the pH range was about2-7. The removal rate increases with the increase of ions strength when the pH range was about7-9. The removal percentage of U(Ⅵ) from the aqueous solution using montmorillonite increases with increasing solid content. The results show that the adsorption amount of U(Ⅵ) using the montmorillonitel was increased with increasing the initial U(Ⅵ) concentration at C[U(Ⅵ)]initial<100ppb. The isotherm of U(Ⅵ) on montmorillonite were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a significant effect on the reduction of U(Ⅵ) using montmorillonite in aqueous solution. The U(Ⅵ) removal percentage increased when the temperature increased initially. Hence, it is proposed that the mechanism of U(Ⅵ) removal efficiency using montmorillonite should include both ion exchange and surface complexation. When the pH<7, the main mechanism of U(Ⅵ) removal efficiency using montmorillonite is ion exchange adsorption. When the pH>9, the main mechanism of U(Ⅵ) removal efficiency using montmorillonite is inner-sphere surface complexation.
2. Removal of uranium from aqueous solution using zero-valent iron nanoparticles (nZVI)
The SSA for nZVI was26.6m2/g. The particles are oxidation resistant well with iron core-iron oxide shell structure. The core, consisting of zero-valent iron, forms an electron source that might reduce ions possessing higher standard reduction potential than that of iron. Attempts to form shell coating iron core have also been applied to protect the iron core from further oxidation. The removal efficiency of U(Ⅵ) using nZVI was strongly dependent on the pH values. The removal efficiency of U(Ⅵ) using nZVI was strongly dependent on the pH values. Results indicated that when pH is pH2, it produced less removal efficiently of U(Ⅵ). The removal efficiency of U(Ⅵ) using nZVI was73-78%at pH3-5. The results indicated that the optimum removal efficiency of U(Ⅵ) using M-nZVI was78%at pH5.0. The removal rate decreases with the increase of pH value when the pH range was about6-9. The concentration of NaNO3had a insignificant effect on the U(Ⅵ) adsorption using the nZVI. The removal percentage of U(Ⅵ) from the aqueous solution using nZVI increases with increasing solid content at m/v<0.1g/L. The removal efficiency of U(Ⅵ) using0.125g/L of nZVI was99%. The results show that the adsorption amount of U(Ⅵ) using the nZVI was decreased with increasing the initial U(Ⅵ) concentration at C[U(Ⅵ)]initial<100ppb. The isotherm of U(Ⅵ) on montmorillonite were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a insignificant effect on the reduction of U(Ⅵ) using nZVI in aqueous solution. These results are consistent with the hypothesis that the removal of U(Ⅵ) from aqueous solution is not only an adsorption process but also a reduction process in which U(Ⅵ) ions are reduced concomitantly by nZVI. The pH did not change after the reaction at the pH2. The pH was5.57after the reaction at the initial pH3. The pH was about9after the reaction at the initial pH4-8. The pH did not change after the reaction at the pH9. The iron content of the reaction solution was65.8mg/L at pH2. The iron content of the reaction solution was18.9mg/L at pH3. The iron content of the reaction solution was4.22mg/L at pH4. The iron content of the reaction solution was22-37ug/L at pH5-9. The main mechanism of U(Ⅵ) removal efficiency using nZVI is redox mechanisms, namely, the oxidation of iron, adsorption of U(VI) to nZVI, formation of oxide and hydroxide precipitates of U(Ⅳ) and Fe(Ⅲ) that coated the surface of the nZVI.
3. Removal of uranium from aqueous solution using montmorillonite-supported zero-valent iron nanoparticles (M-nZVI)
The SSA for as-synthesized M-nZVI was91.42m2/g. The isoelectric point (IEP) of M-nZVI was at pH5.6. These images clearly demonstrate that the aggregation of nZVI was eliminated and the nZVI was well dispersed on the M surface. The results indicate that the removal efficiency of U(Ⅵ) using M-nZVI was strongly dependent on the pH values. Results indicated that when pH is low (pH2), it produced less removal efficiently of U(Ⅵ). The removal efficiency of U(Ⅵ) was97.8%at pH3.0. The optimum pH values were in the range3.0to5.0. Therefore, pH3.0was selected for subsequent experiments. It can be seen that the pH solution did not significantly influence on the removal of U(Ⅵ) at pH of3-5. The removal rate decreases with the increase of pH value when the pH range was about6-9. The removal efficiency of U(Ⅵ) using M-nZVI was9.5%at pH9. The concentration of NaNO3had insignificant effect on the U(Ⅵ) adsorption using the M-nZVI. The removal percentage of U(Ⅵ) from the aqueous solution using M-nZVI increases with increasing solid content at m/v<0.1g/L. The removal efficiency of U(Ⅵ) using0.1g/L of M-nZVI was97.8%. However, the removal efficiency of U(Ⅵ) reached a steady state above0.1g/L of M-nZVI. The results show that the adsorption amount of U() using the M-nZVI was increased with increasing the initial U(Ⅵ) concentration at C[U(vi)]initiai<100ppb. The isotherm of U(Ⅵ) on M-nZVI were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a insignificant effect on the reduction of U(Ⅵ) using M-nZVI in aqueous solution.
The presence of EDTA decreases U(Ⅵ) sorption at pH2-9. After pre-equilibrium of EDTA sorbed on U(Ⅵ), and subsequent sorption of M-nZVI. The presence of EDTA decreases U(Ⅵ) sorption on M-nZVI at pH2-9. The EDTA-U(Ⅵ) complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-9. After pre-equilibrium of U(Ⅵ) sorbed on M-nZVI, and subsequent sorption of EDTA. The presence of EDTA decreases U(Ⅵ) sorption on M-nZVI at pH2-9. Different concentration of EDTA and EDTA to join order of M-nZVI removal of uranium pH change after the reaction is the same. The pH of0.001mol/L EDTA is higher than0.01mol/L EDTA after reaction at pH2-9. The U(Ⅵ)-HA complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-7and enhances U(Ⅵ) sorption on M-nZVI at pH8-9. After pre-equilibrium of U(Ⅵ) sorbed on M-nZVI, and subsequent sorption of HA.The U(Ⅵ)-FA complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-6and enhances U(Ⅵ) sorption on M-nZVI at pH8-9.
Hence, it is proposed that the mechanism of U(Ⅵ) removal efficiency using M-nZVI was redox mechanisms, namely, the oxidation of iron, adsorption of U(Ⅵ) to M-nZVI, formation of oxide and hydroxide precipitates of U(Ⅳ) and FeOOH that coated the surface of the M-nZVI. It can be concluded that the iron hydroxide were formed as a result of Fe0corrosion reaction, where Fe0first oxidizes to Fe(Ⅱ) and then to Fe(Ⅲ). This resulted in the formation of U(Ⅳ) hydroxide precipitates which gradually coated the surface of the M-nZVI particles. The Fe content in the M-nZVI after reacting with U(VI) decreased from43.13to11.90wt%, while the oxygen content increased from41.18to63.28wt%. This result may be explained on the basis of the corrosion of Fe0to Fe(Ⅱ), Fe(Ⅲ), iron oxide or hydroxide on the surface of M-nZVI, and therefore, a decrease in the Fe content and increase in the oxygen content were observed.
There is one novel point of this theses:(1) Preparation of Al-montmorillonite-supported zero-valent iron nanoparticles (M-nZVI).(2) Development of M-nZVI for uranium removal.
本文选择接近于我国铀尾矿“返酸”后渗出的中低浓度含铀水溶液(初始浓度100μg/L)作为处理对象,以羟基铝柱撑蒙脱石为支撑负载材料,用硼氢化钠液相还原法制备蒙脱石负载纳米零价铁材料,并对比研究蒙脱石、纳米零价铁和蒙脱石负载纳米零价铁等不同材料对水溶液中低浓度的铀进行去除;采用批实验方法,研究在不同pH、固液比、温度、离子强度、初始浓度及共存离子(EDTA.HA和FA)等条件下对铀去除率的影响,并明确反应过程探讨反应机理。论文的主要研究内容可以分为以下三个方面：1.利用蒙脱石去除水溶液中低浓度的铀。
通过对比试验发现,pH对蒙脱石去除铀有着显著的影响：在酸性条件下(pH=2-6.5),铀去除率随pH值的升高而增加；碱性条件下(pH=7-9),铀去除率随pH值的升高而降低；实验最佳pH值为6.5,最大去除率48.05%。同时,溶液中离子强度对蒙脱石吸附铀影响也比较显著：在酸性条件下(pH=2-7),随离子强度的增大,吸附去除率反而越低,证明在酸性条件下高离子强度将抑制蒙脱石对铀的吸附；碱性条件下(pH=7-9),离子强度越大,吸附去除率越高,高的离子强度能够促进蒙脱石对铀的吸附。U(Ⅵ)在蒙脱石上的吸附百分数随固液比而增大。铀在蒙脱石上的吸附与Langmuir方程描述的等温吸附模型较为符合。温度越高,越有利于蒙脱石对铀的去除。铀酰离子在蒙脱石上的吸附机理是铀酰离子通过离子交换吸附和表面位点吸附两种方式共同作用。在酸性条件下(pH<7),蒙脱石以离子交换吸附的方式去除铀；在碱性条件下(pH=7-9),蒙脱石吸附铀主要是通过表面络合作用反应形成内层配合物。
2.利用纳米零价铁去除水溶液中低浓度的铀。
纳米零价铁是一个壳核结构,α-Fe0位于中间核部,表面是由一层铁氧化物(FeO)包裹形成的壳部。pH对纳米零价铁去除铀有着显著的影响：在pH=2的时候,对U(Ⅵ)的去除率基本为零；在pH=3-5时,对铀的去除率在73-78%；当pH=6-9,随pH的升高,去除率降低；纳米零价铁去除铀的最佳pH=5,去除率达78%。离子强度对纳米零价铁去除铀的基本无影响。去除率随随着固液比的增加而升高。随着铀初始浓度的升高,铀的去除率一直在下降。铀在纳米零价铁的吸附与Langmuir方程描述的等温吸附模型最为符合。温度对纳米零价铁去除低浓度铀的影响不明显,证明溶液中的U(Ⅵ)与纳米零价铁的反应是
一个化学反应控制的反应,不受温度的影响。初始pH=2时,反应后pH基本没有变化,反应后溶液中铁元素含量高达658mg/L；初始pH=3时,反应后pH变为5.57,反应后溶液中铁元素含量为18.9mg/L；初始pH在4-9时,反应后pH变为9左右；溶液中铁的释放量较少。纳米零价铁去除铀的主要机理为还原沉淀作用,反应过程中纳米零价铁和U(Ⅵ)发生氧化还原反应,电子转移生成四价铀的氧化物UO2和铁氢氧化物。
3.利用蒙脱石负载纳米零价铁去除水溶液中低浓度的铀。
纳米零价铁成功负载在蒙脱石上,有效解决纳米零价铁的团聚问题和易氧化的问题。pH对蒙脱石负载纳米零价铁去除铀的影响显著：在pH=2时,蒙脱石负载纳米零价铁去除铀的效果较差,去除率仅为5.13%；当pH=3时,去除率高达97.8%；蒙脱石负载纳米零价铁去除铀最佳pH范围是3-5,去除率在95.11-97.8%；当pH在6-9时,随pH升高去除率下降。离子强度对蒙脱石负载纳米零价铁去除铀的效果没有影响。去除率随固液比的增加而升高。随着铀初始浓度的升高,铀的平衡吸附量升高。蒙脱石负载纳米零价铁去除的等温吸附模型与Langmuir方程描述的等温吸附模型最为符合。温度不影响蒙脱石负载纳米零价铁对铀的去除,证明此反应受化学反应控制,不受温度的影响控制。反应初始pH=2,反应后的pH基本没有变化,反应后溶液中铁元素含量33.79mg/L；初始pH=3,反应后的pH变为5.15,反应后溶液中铁元素含量为4.67ng/L；pH=4-9,反应后的pH变为8.9左右,铁元素在溶液中的释放量非常少(17-53ug/L)。
通过加入不同的共存离子(EDTA、HA和FA),研究其对铀去除率的影响。实验结果表明EDTA的存在使蒙脱石负载纳米零价铁对铀的去除率降低,原因在于EDTA与铀络合作用形成U(Ⅵ)-EDTA复合物,此复合物抑制了铀在蒙脱石负载纳米零价铁上的去除反应。不同浓度EDTA和EDTA加入顺序的不同对蒙脱石负载纳米零价铁去除铀反应后的pH变化是相同的：相同初始pH下,低浓度(0.001mol/L)比高浓度(0.01mol/L)的EDTA反应后的pH值要高。结合其对铀去除率的结果,低浓度比高浓度的EDTA对铀去除率要高,反应中消耗的氢离子更多,导致反应后pH升高。共存离子HA对蒙脱石负载纳米零价铁去除铀的影响：在铀溶液依次加入HA和蒙脱石负载纳米零价铁,铀与HA先形成U(VI)-HA复合物在酸性条件下(pH2-7)阻碍蒙脱石负载纳米零价铁对铀的去除；在碱性条件下(pH8-9)促进蒙脱石负载纳米零价铁对铀的去除。共存离子FA对蒙脱石负载纳米零价铁去除铀的影响：在酸性条件下(pH2-6)抑制蒙脱石负载纳米零价铁对铀的去除：在碱性条件下(pH8-9)促进蒙脱石负载纳米零价铁对铀的去除。
蒙脱石负载纳米零价铁与铀反应后的产物通过XRD鉴定为FeOOH;扫描电镜可以看见表面纳米零价铁铁颗粒消失,被黑色物质覆盖；EDS结果显示反应后铁含量减少而氧含量增加；XPS对比反应前后发现,零价铁的峰在反应后消失,铁氧化物与铁氢氧化物的含量增大。综上所述,蒙脱石负载纳米零价铁去除铀的反应机理主要是氧化还原沉淀作用：零价铁(Fe0)与U(Ⅵ)发生氧化还原反应,在蒙脱石负载纳米零价铁的表面生成FeOOH。
本文创新点：1、制备羟基铝柱撑蒙脱石负载纳米零价铁材料。2、将羟基铝柱撑蒙脱石负载纳米零价铁材料应用于放射性元素铀的吸附研究。
Actinides are of great interest in terms of radioactive waste disposal because of their longevity. Uranium occurs naturally as U isotopes,238U(99.28%),235U(0.711%),234U(0.006%), and they exist as hexavalent uranyl complex in the natural environment. The use of uranium at industrial and military sites has resulted in very high uranium contamination, and dangerous to human healthy and environmental protection because of its long half-life (such as238Ut1/2=4.51×109years).Uranium, an actinide element, has been released into the environment through mining operations, nuclear testing, nuclear fuel, nuclear weapons production sites and accidental spill, and therefore, it is a major contaminant in soils, sediments, and groundwater. A provisional drinking water maximum contaminant level (MCL) for uranium of15μg/L has been established by the World Health Organization (WHO2004). However, it is discussed worldwide that10or5μg/L would be more reasonable. Geochemical processes occurring naturally, including dissolution/precipitation, redox reactions, and sorption/desorption reactions at the water-rock interface, control the mobility and transport of uranium in the subsurface system, such as aquifer sediments, soils, and groundwater.
Montmorillonite-supported zero-valent iron nanoparticles (M-nZVI) was synthesized by sodium borohydride reduction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM). The interaction of uranium with montmorillonite, zero-valent iron nanoparticles and M-nZVI were studied using batch technique under different experimental conditions such as pH, ionic strength, initial U(Ⅵ) concentration, solid-to-liquid ration (m/V), EDTA, humic acid, fulvic acid and temperature. An effluent solution with a low level of uranium, i.e.,4.2x10-7mol/L(100μg/L) was used in the experiments to avoid precipitation of amorphous uranium-hydroxides. Uranium occurs naturally in low concentrations below100μg/L in soil, rock, as well as in surface and groundwater. The research contents of this thesis are summarized as follows:
1. Removal of uranium from aqueous solution using montmorillonite
The SSA for montmorillonite was10.23m2/g and the layer spacing was1.28nm. The chemical composition was determined by X-Ray Fluorescence spectrometer:Al2O319.0%, SiO259.5%, MgO3.9%, Fe2O31.7%, K2O0.6%, Na2O3.6%and CaO2.3%.The removal efficiency of U(Ⅵ) using montmorillonite was strongly dependent on the pH values. The removal rate increases with the increase of pH value when the pH range was about2-6.5. The removal efficiency of U(Ⅵ) using montmorillonite was48.05%at pH6.5. The removal rate decreases with the increase of pH value when the pH range was about6.5-9. The concentration of NaNO3had significant effect on the U(Ⅵ) adsorption using the montmorillonite. The removal rate decreases with the increase of ions strength when the pH range was about2-7. The removal rate increases with the increase of ions strength when the pH range was about7-9. The removal percentage of U(Ⅵ) from the aqueous solution using montmorillonite increases with increasing solid content. The results show that the adsorption amount of U(Ⅵ) using the montmorillonitel was increased with increasing the initial U(Ⅵ) concentration at C[U(Ⅵ)]initial<100ppb. The isotherm of U(Ⅵ) on montmorillonite were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a significant effect on the reduction of U(Ⅵ) using montmorillonite in aqueous solution. The U(Ⅵ) removal percentage increased when the temperature increased initially. Hence, it is proposed that the mechanism of U(Ⅵ) removal efficiency using montmorillonite should include both ion exchange and surface complexation. When the pH<7, the main mechanism of U(Ⅵ) removal efficiency using montmorillonite is ion exchange adsorption. When the pH>9, the main mechanism of U(Ⅵ) removal efficiency using montmorillonite is inner-sphere surface complexation.
2. Removal of uranium from aqueous solution using zero-valent iron nanoparticles (nZVI)
The SSA for nZVI was26.6m2/g. The particles are oxidation resistant well with iron core-iron oxide shell structure. The core, consisting of zero-valent iron, forms an electron source that might reduce ions possessing higher standard reduction potential than that of iron. Attempts to form shell coating iron core have also been applied to protect the iron core from further oxidation. The removal efficiency of U(Ⅵ) using nZVI was strongly dependent on the pH values. The removal efficiency of U(Ⅵ) using nZVI was strongly dependent on the pH values. Results indicated that when pH is pH2, it produced less removal efficiently of U(Ⅵ). The removal efficiency of U(Ⅵ) using nZVI was73-78%at pH3-5. The results indicated that the optimum removal efficiency of U(Ⅵ) using M-nZVI was78%at pH5.0. The removal rate decreases with the increase of pH value when the pH range was about6-9. The concentration of NaNO3had a insignificant effect on the U(Ⅵ) adsorption using the nZVI. The removal percentage of U(Ⅵ) from the aqueous solution using nZVI increases with increasing solid content at m/v<0.1g/L. The removal efficiency of U(Ⅵ) using0.125g/L of nZVI was99%. The results show that the adsorption amount of U(Ⅵ) using the nZVI was decreased with increasing the initial U(Ⅵ) concentration at C[U(Ⅵ)]initial<100ppb. The isotherm of U(Ⅵ) on montmorillonite were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a insignificant effect on the reduction of U(Ⅵ) using nZVI in aqueous solution. These results are consistent with the hypothesis that the removal of U(Ⅵ) from aqueous solution is not only an adsorption process but also a reduction process in which U(Ⅵ) ions are reduced concomitantly by nZVI. The pH did not change after the reaction at the pH2. The pH was5.57after the reaction at the initial pH3. The pH was about9after the reaction at the initial pH4-8. The pH did not change after the reaction at the pH9. The iron content of the reaction solution was65.8mg/L at pH2. The iron content of the reaction solution was18.9mg/L at pH3. The iron content of the reaction solution was4.22mg/L at pH4. The iron content of the reaction solution was22-37ug/L at pH5-9. The main mechanism of U(Ⅵ) removal efficiency using nZVI is redox mechanisms, namely, the oxidation of iron, adsorption of U(VI) to nZVI, formation of oxide and hydroxide precipitates of U(Ⅳ) and Fe(Ⅲ) that coated the surface of the nZVI.
3. Removal of uranium from aqueous solution using montmorillonite-supported zero-valent iron nanoparticles (M-nZVI)
The SSA for as-synthesized M-nZVI was91.42m2/g. The isoelectric point (IEP) of M-nZVI was at pH5.6. These images clearly demonstrate that the aggregation of nZVI was eliminated and the nZVI was well dispersed on the M surface. The results indicate that the removal efficiency of U(Ⅵ) using M-nZVI was strongly dependent on the pH values. Results indicated that when pH is low (pH2), it produced less removal efficiently of U(Ⅵ). The removal efficiency of U(Ⅵ) was97.8%at pH3.0. The optimum pH values were in the range3.0to5.0. Therefore, pH3.0was selected for subsequent experiments. It can be seen that the pH solution did not significantly influence on the removal of U(Ⅵ) at pH of3-5. The removal rate decreases with the increase of pH value when the pH range was about6-9. The removal efficiency of U(Ⅵ) using M-nZVI was9.5%at pH9. The concentration of NaNO3had insignificant effect on the U(Ⅵ) adsorption using the M-nZVI. The removal percentage of U(Ⅵ) from the aqueous solution using M-nZVI increases with increasing solid content at m/v<0.1g/L. The removal efficiency of U(Ⅵ) using0.1g/L of M-nZVI was97.8%. However, the removal efficiency of U(Ⅵ) reached a steady state above0.1g/L of M-nZVI. The results show that the adsorption amount of U() using the M-nZVI was increased with increasing the initial U(Ⅵ) concentration at C[U(vi)]initiai<100ppb. The isotherm of U(Ⅵ) on M-nZVI were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a insignificant effect on the reduction of U(Ⅵ) using M-nZVI in aqueous solution.
The presence of EDTA decreases U(Ⅵ) sorption at pH2-9. After pre-equilibrium of EDTA sorbed on U(Ⅵ), and subsequent sorption of M-nZVI. The presence of EDTA decreases U(Ⅵ) sorption on M-nZVI at pH2-9. The EDTA-U(Ⅵ) complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-9. After pre-equilibrium of U(Ⅵ) sorbed on M-nZVI, and subsequent sorption of EDTA. The presence of EDTA decreases U(Ⅵ) sorption on M-nZVI at pH2-9. Different concentration of EDTA and EDTA to join order of M-nZVI removal of uranium pH change after the reaction is the same. The pH of0.001mol/L EDTA is higher than0.01mol/L EDTA after reaction at pH2-9. The U(Ⅵ)-HA complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-7and enhances U(Ⅵ) sorption on M-nZVI at pH8-9. After pre-equilibrium of U(Ⅵ) sorbed on M-nZVI, and subsequent sorption of HA.The U(Ⅵ)-FA complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-6and enhances U(Ⅵ) sorption on M-nZVI at pH8-9.
Hence, it is proposed that the mechanism of U(Ⅵ) removal efficiency using M-nZVI was redox mechanisms, namely, the oxidation of iron, adsorption of U(Ⅵ) to M-nZVI, formation of oxide and hydroxide precipitates of U(Ⅳ) and FeOOH that coated the surface of the M-nZVI. It can be concluded that the iron hydroxide were formed as a result of Fe0corrosion reaction, where Fe0first oxidizes to Fe(Ⅱ) and then to Fe(Ⅲ). This resulted in the formation of U(Ⅳ) hydroxide precipitates which gradually coated the surface of the M-nZVI particles. The Fe content in the M-nZVI after reacting with U(VI) decreased from43.13to11.90wt%, while the oxygen content increased from41.18to63.28wt%. This result may be explained on the basis of the corrosion of Fe0to Fe(Ⅱ), Fe(Ⅲ), iron oxide or hydroxide on the surface of M-nZVI, and therefore, a decrease in the Fe content and increase in the oxygen content were observed.
There is one novel point of this theses:(1) Preparation of Al-montmorillonite-supported zero-valent iron nanoparticles (M-nZVI).(2) Development of M-nZVI for uranium removal.
引文
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