| 题名 | 镧改性吸附材料对水中砷氟去除机理研究 |
| 作者 | 史乾涛 |
| 学位类别 | 博士 |
| 答辩日期 | 2016-05 |
| 授予单位 | 中国科学院研究生院 |
| 授予地点 | 北京 |
| 导师 | 景传勇 |
| 关键词 | 氟，砷，镧，光谱学，量子化学计算 fluoride, arsenic, lanthanum, spectroscopic, quantum chemistry |
| 其他题名 | Molecular-level Interaction Study for Arsenic and Fluoride Removal using Rare Earth Modified Adsorbents |
| 学位专业 | 环境科学 |
| 中文摘要 | 水体中砷氟的污染是当今世界面临的重要环境问题之一。高氟和高砷地下水严重威胁人们身体健康，因此探寻高效除砷除氟的新材料、新方法及新原理已成为环境科学研究领域的前沿热点。目前，复合材料被广泛报道用于去除水中的污染物，在这其中，稀土改性材料尤其是镧改性材料对砷氟均表现出了良好的去除能力。然而，目前对于砷氟在稀土改性材料上的吸附机理，尤其是分子水平的研究尚不明确。本研究旨在分子水平上阐明砷氟在镧改性材料上的吸附过程及机制，为针对性去除水中砷氟的稀土改性材料的设计和应用提供前沿基础知识。 首先，利用浸渍负载的方法制备了载镧活性氧化铝（La-impregnated activated alumina, LAA）。其最佳烧结温度为 300oC，此时载镧率为 4.9%。随着温度进一步升高至 900oC，活性氧化铝(Activated alumina, AA)载镧率虽有升高，但镧氧化物结构从 LaOOH变化为 La2O3，降低了 F的吸附效率。另一方面，最佳循环负载次数为 5次，进一步增加负载次数，比表面积从 192 m2/g下降至 140 m2/g。镧氧化物在活性氧化铝上形成纳米片结构，La-O的距离为 2.56 Å，配位数为 7.6；La-Al的距离为 3.19 Å，配位数为 3.7，形成了 La-O-Al化学键。X射线光电子能谱（X-ray photoelectron spectroscopy，XPS）和拉曼（Raman）实验结果表明，氟主要与 LAA上的镧氧化物上羟基(La-OH)发生置换反应，而铝氧化物上羟基(Al-OH)对吸附 F几乎没有贡献。LAA对氟的吸附容量为 16.9 mg/g，在实际地下水的 pH范围内（6-9）可去除 70.5~77.2%的氟。此材料的高机械强度（60.8 N）保证了其再生利用的可行性，在 5次反洗再生中，F的吸附效率在 84-98%之间。同时，镧的负载改性有效降低了 Al的析出，与 AA相比，La的负载使吸附过程中 Al的析出率从 0.1~5.9 mg/L显著降低至 0.01~2.35 mg/L。该 LAA材料为安全解决饮用水中氟的去除提供了新思路，新方法。 其次，在分子水平上研究了砷在 LAA的界面吸附机理。吸附等温线和X射线吸收光谱X-ray absorption spectroscopy, XAS）结果表明，镧的负载为 As(V)和 As(III)在 LAA上的吸附提供了新的活性位点，As(V)和As(III)在镧改性吸附材料上更倾向于和镧氧化物结合，而不是铝氧化物。对 X射线吸收光谱的扩展边分析结果表明，As(V)和 As(III)均以单齿单核的吸附构型吸附到 LAA上，相比于双齿双核的吸附构型，单齿单核构型仅占据一个O活性位点，因而减少了砷在镧氧化物上所占据的吸附位点，使其吸附容量显著提升。进一步，在线实时红外光谱和量子化学计算结果表明，在 pH 5-9范围内，LAA上并存两种质子化数为1和2的吸附态As(V)，且与 LAA上 As(V)的单齿单核构型相比，AA上As(V)的双齿双核更容易发生去质子化吸附。另外，量子化学计算结果表明，质子化数为1的吸附能（-482.4~-233.1 kJ/mol）显著低于质子化数为 2时的吸附态As(V)吸附能（32.3~36.9 kJ/mol）。另一方面，根据所得到 As(V)在 LAA上的单齿单核吸附构型，构建了 CD-MUSIC的模型对宏观吸附等温实验结果进行模拟，模型结果表明，LAA上吸附态 As(V)质子化数为 1时，其 logK(26.4)大于其质子化数为 2时的 logK(24.3)，这说明 LAA更倾向于吸附质子化数为 1的 As(V)，与量子化学计算结果一致。研究中所得砷与镧的吸附构型以及质子化状态对于相关利用稀土改性材料去除砷的研究具有指导意义。 最后，我们发现镧氧化物虽然不能氧化 As(III)，但氧化反应可以发生在镧氧化物-石墨烯复合材料表面。水化学试验表明，镧氧化物将 As(III)吸附富集到材料表面。基于同步辐射的 X射线荧光光谱与微区X射线吸收近边光谱结果表明镧氧化物-石墨烯复合材料上吸附态 As(III)发生了氧化，且不同区域吸附态As(III)氧化程度与镧氧化物含量并不相关。X射线吸收光谱近边分析结果表明，铝氧化物和钛氧化物与石墨烯的复合材料也会对 As(III)产生氧化吸附。同时，利用原位在线红外光谱发现了在氧化过程中材料内环氧基团的消耗。另一方面，采用X射线吸收光谱近边谱学分析对镧氧化物-石墨烯复合材料As(III)的氧化能力进行测定，并利用 XPS对不同镧氧化物-石墨烯复合材料上环氧基团的含量变化进行了分析，发现了镧氧化物-石墨烯复合材料对 As(III)氧化能力与环氧基团的显著相关性。X射线吸收光谱扩展边分析结果表明，镧氧化物-石墨烯复合材料上吸附态 As(III)和 As(V)的吸附构型分别为双齿双核和单齿单核。我们根据这一结果构筑了量子化学模型，对镧氧化物-石墨烯复合材料吸附-氧化 As(III)的过程进行了过渡态搜寻，结果表明，As(III)吸附的能量壁垒（氧化所需能）为 207.17 kJ/mol，小于吸附过程所释放的能量 215.30 kJ/mol，说明了 As(III)在复合材料上吸附-氧化过程的自发性。以上光谱学实验和理论计算结果说明了As(III)在金属氧化物-石墨烯复合材料上吸附氧化过程中，金属氧化物的锚点作用和环氧基团的氧化功能，而这对于目前大量有关金属氧化物-石墨烯复合材料吸附氧化的工作有着重 要的意义。 |
| 英文摘要 | Arsenic (As) and Fluoride (F) exposure threatens human health. Among various As removal technologies, adsorption is one of the most cost-effective and user-friendly techniques to provide As and F safe drinking water. Recently, the hybrid materials were widely reported to remove hazardous ions in water. Among those hybrid materials, rare earth modified adsorbents (REMAs) showed high adsorption capacity for As and F. However, the molecular-level interactions between the F&As and REMAs surface are poorly understood. The motivation of the present study was,therefore, to explore the molecular-level interactions of As&F and REMAs. The insights gained from this study shed light on the design and application of REMAs in environmental remediation for As&F and their structural analogues. First, we synthesized a new F adsorbent by impregnating commercially available granulated AA with lanthanum oxide (LAA), and to explore its F adsorption mechanism on the molecular scale. Five cycles of lanthanum impregnation on AA followed by calcination at 573 K increased the La content up to 19.1% and the F removal from 18.1% by pristine AA to 92.4% by LAA. The SEM, TEM, XRD,TGA,and EXAFS results demonstrated that the 5-20 nm thin flakes of LaOOH on LAA were in an amorphous form, with 7.6 oxygen atoms around each La. This LaOOH layer was uniformly distributed inside the micropores of the 1-3 mm AA granules.LAA exhibited four fold higher F adsorption capacity than AA in the pH range 3.9 to 9.6, with substantially reduced Al release. The capability of regeneration and reuse of LAA enables it an attractive sustainable application. Multiple complementary spectroscopic analyses demonstrated that ligand exchange between F and surface hydroxyl groups is the mechanism for F adsorption on LAA. Our work improves the understanding of F interaction with metal oxides on the molecular scale and presents an alternative solution for elevated F water treatment. Second, LAA’s As uptake mechanisms were explored using multiple complementary characterization techniques. Our adsorption experiments showed that LAA exhibited 2-3 times higher As adsorption capacity than AA. In contrast to the bidentate configuration formed on most metal oxide surfaces, our EXAFS and DFT results suggest that As(III) and As(V) form monodentate surface complexes on LAA through As-O-La coordinative bonding. In-situ flow cell ATR-FTIR observed a strong dependence of As-O peak positions on pH, which could be interpreted as the change in the fractions of As(V) surface complexes with zero- to double-protonation on LAA,AA, and LaOOH. As(V) on LAA existed as singly- and doubly-protonated surface species, and the pKa of transition from double to single protonation (~5.8) was lower than for its soluble counterpart (6.97). The surface reaction and structural configuration were incorporated in a CD-MUSIC model to satisfactorily predict macroscopic As adsorption behaviors. The insights gained from the molecular-level reactions shed light on the design and application of REMAs in environmental remediation for As and its structural analogues. Finally, while lanthanum oxides has no oxidation ability on As(III), the adsorbed As(III) on lanthanum-coated reduced graphene oxide was partially oxidized. The batch experiments showed that lanthanum hydroxides could adsorb As(III). The micro-X-ray fluorescence spectroscopy and micro-X-ray absorption spectroscopy results showed the oxidation of adsorbed As(III), which also indicated no correlation between La content and oxidized As(III). Meanwhile, the oxidation of adsorbed As(V) on aluminum- and titanium- coated reduced graphene oxide was also evidenced by X-ray absorption near edge spectroscopy results. Furthermore, the consummation of epoxy group in the As(III)-oxidation process was monitored using in-situ ATR-FTIR techniques. The significant correlation between the epoxy group content and the oxidation of As(III) was analyzed using X-ray absorption near edge structure and XPS. In addition, the density functional theory (DFT), of which model was built based on the extend X-ray absorption fine structure (EXAFS), was further employed to confirm the energetic possible for As(III) oxidized to As(V) on rGM. Our study implies the oxidation role of epoxy group on rGM, which extend our knowledge for the study of rGM in adsorption and catalysis field. |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.rcees.ac.cn/handle/311016/36953]  |
| 专题 | 生态环境研究中心_环境化学与生态毒理学国家重点实验室 |
| 推荐引用方式 GB/T 7714 | 史乾涛. 镧改性吸附材料对水中砷氟去除机理研究[D]. 北京. 中国科学院研究生院. 2016. |
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