| 题名 | 共存离子对砷在纳米二氧化钛上吸附影响的微观机制研究 |
| 作者 | 胡珊 |
| 学位类别 | 博士 |
| 答辩日期 | 2015-05 |
| 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 | arsenic, TiO2, adsorption mechanisms, co-existing ions, molecular-level, in situ analysis |
| 其他题名 | Molecular-level Interactions of Arsenic and Co-existing ions on TiO2 |
| 学位专业 | 环境科学 |
| 中文摘要 | 水体砷污染是当今国际社会面临的重要环境问题之一。高砷地下水及工业废水严重威胁人们身体健康，因此探寻高效除砷的新材料、新方法及新原理已成为环境科学研究领域的前沿热点。新型高效纳米 TiO2吸附材料由于其优越的吸附性能和良好的稳定性已受到广泛关注。然而，由于环境水体中共存离子的复杂性和多样性，共存离子对砷吸附及迁移转化的影响机制仍不明确，限制了 TiO2吸附材料的广泛应用。本研究旨在分子水平上阐明复杂体系中共存离子影响砷在TiO2上吸附的过程及机制，为 TiO2吸附材料在工业废水与地下水处理中的应用提供理论基础和实际范例。 首先，通过宏观吸附试验并结合多种光谱手段，在分子水平上研究了As(III)与 Cd在TiO2上的共吸附机理。吸附试验和Zeta电位结果表明As(III)与 Cd在TiO2上形成了三元表面络合结构从而相互促进吸附。另一方面，脱附试验结果证明这种三元表面络合结构能够有效减少 As(III)和 Cd的溶出，有利于吸附态As(III)和 Cd的长期稳定。扩展 X射线吸收精细结构（EXAFS）和原位衰减全反射傅里叶变换红外光谱（ATR-FTIR）结果表明，与Cd相比，As(III)与 TiO2表面有较强的亲和力，因此形成了以As(III)为桥联分子的三元表面络合结构，即Cd-As(III)-TiO2，且 Cd与 As(III)的先后吸附顺序不会影响此表面络合物的分子结构。基于量子化学的密度泛函理论（DFT）计算所得 As-O、As-Ti、As-Cd的键长与 EXAFS试验结果吻合，且计算所得的红外特征振动频率与ART-FTIR结果匹配，确证了 EXAFS及 ATR-FTIR推断得出的表面络合结构。研究结果不仅发现了As(III)、Cd共去除的分子机理，同时为三元表面络合吸附态 As(III)与Cd的稳定性提供了有力证据。 其次，研究了 As(V)与 Cd在 TiO2上的共去除机理。吸附试验与X射线衍射(XRD）结果表明，与 As(III)不同，As(V)与 Cd在TiO2上的共去除主要归结于Cd-As(V)表面沉淀的形成，且表面沉淀形态依赖于 pH及As(V)/Cd的摩尔比。在不同条件下，TiO2表面生成Cd3(AsO4)2、Cd5H2(AsO4)∙4H2O以及非晶态的沉淀。以上不同形态表面沉淀及其变化规律的结论也通过 ATR-FTIR特征频率的偏移得到了验证。吸附试验结果显示，同时生成 Cd3(AsO4)2和Cd5H2(AsO4)∙4H2O两种表面沉淀时，As(V)与 Cd去除量较高。同样，解吸附试验结果显示表面沉淀的形成可有效减少 As(V)与 Cd的溶出量及溶解速率，有利于 As(V)与 Cd的固定化。以上研究结果一方面拓宽了人们对阴阳离子在矿物表面共去除机理的认识，另一方面为优化工业废水的处理条件提供了理论基础。 最后，研究了地下水共存离子影响砷吸附去除的过程机理，并将此微观吸附机理用于预测砷的吸附容量与穿透曲线。地下水过滤试验结果表明，砷浓度为542 µg/L的地下水，在 TiO2滤柱出水浓度小于10µg/L时，可过滤2955柱体积，此时颗粒TiO2上吸附量为1.53 mg-As/g-TiO2；TiO2再生后，可再次处理2563柱体积的地下水，对应的砷吸附量为 1.36 mg-As/g-TiO2。通过比较地下水过滤前后主要离子浓度，表明本研究地下水中只有Si、Ca和 HCO3会影响砷的吸附。EXAFS结果证明 Si不会影响 As(V)在 TiO2上的吸附构型，但会与 As(V)竞争吸附位点。原位流动池 ATR-FTIR结果表明 Si在 TiO2上形成单体、二聚体、低聚体和多聚体四种络合结构，其中 Si的低聚体及高聚体随反应时间增加而增多，且与TiO2的结合强于 As(V)，会抑制 As(V)的吸附并阻碍吸附材料的再生。Ca能够与As(V)形成 Ca-As(V)-TiO2三元表面络合结构从而促进As(V)的吸附。考虑所有离子对砷吸附的复合作用，利用批试验中所得砷的吸附常数，建立的 PHREEQC模型能够成功拟合地下水中 As、Si、Ca和 HCO3的穿透曲线。当地居民饮用除砷水后，尿中总砷浓度在 15-33天内从 972-2080 µg/L降低至 31.7-73.3 µg/L，且符合拟一级动力学方程。研究结果为TiO2吸附材料在砷污染地区的应用提供了理论基础与实际处理范例。 |
| 英文摘要 | Arsenic (As) exposure threatens human health. Among various As removal technologies, adsorption is one of the most cost-effective and user-friendly techniques to provide As safe drinking water. Recently, a TiO2-based adsorbent has demonstrated its advantage in As removal due to its chemical stability and high specific affinity to As. However, the molecular-level interactions between the co-exsisting ions and As on the TiO2 surface are poorly understood. The motivation of the present study was, therefore, to explore the molecular-level interactions of As and co-exsisting ions on TiO2. The insights gained from this study should improve our knowledge in predicting the fate and transport of As in the presence of multiple co-existing ions. First, the molecular-level interactions of neutrally charged As(III) and cationic Cd on TiO2 were investigated using multiple complimentary techniques. The results of adsorption edge, Zeta potential, and surface complexation modeling suggest that coexistence of As(III) and Cd enhanced their synergistic adsorption on TiO2, and consequently resulted in the formation of a ternary surface complex. This ternary surface complex, in turn, inhibited the metal release into the aqueous phase, and therefore facilitated the immobilization of the heavy metals. Our in situ ATR-FTIR and EXAFS evidence showed that, regardless of the order of contact, As(III) was preferentially adsorbed on TiO2 rather than Cd. In agreement with our spectroscopic analysis, quantum chemistry calculations also illustrated that the Cd-As(III)-TiO2 ternary surface complex should be formed with the adsorbed As(III) as the bridging molecule. At high As(III) concentrations, the formation of the Cd-As(III)-TiO2 complex is responsible for the Cd removal. The simultaneous removal mechanisms will further our understanding of the removal of multiple pollutants in industrial wastewaters. Second, the molecular-level interactions of oxyanion As(V) and cationic Cd on TiO2 were studied. The results of adsorption edge and XRD suggest the co-existence of As(V) and Cd enhanced their synergistic removal on TiO2 by the formation of surface precipitates. The speciation of surface precipitate is a function of pH and As(V)/Cd molar ratios. The ATR-FTIR analysis fits well with the XRD results, which confirmed the formation of surface precipitates. The formation of surface precipitates was beneficial to the immobilization of toxic heavy metals. The conclusions of the present study extend our knowledge of synergistic removal of oxyanions and metals on TiO2. Finally, the As adsorption mechanisms and capacities were integrated to predict effluent As from granular TiO2 columns in the field as well as its health impacts. Approximately 2,955 bed volumes of groundwater with an average of 542 µg/L As were filtered before the effluent As concentration exceeded 10 µg/L, corresponding to an adsorption capacity of 1.53 mg As/g TiO2. After regeneration, the TiO2 column could treat 2,563 bed volumes of groundwater, resulting in an As load of 1.36 mg/g TiO2. Column filtration and EXAFS results showed that among coexisting ions present in groundwater, only Ca, Si, and HCO3 would interfere with As adsorption. The compound effects of co-existing ions and molecular level structural information were incorporated in the PHREEQC program to satisfactorily predict the As breakthrough curves. The total urinary As concentration from four volunteers of local residences, ranging from 972 to 2,080 µg/L before groundwater treatment, decreased to the range 31.7-73.3 µg/L at the end of the experimental cycle (15-33 days). Such molecular level structural information was incorporated in the PHREEQC program to predict the As adsorptive filtration breakthrough curves. The change in urinary As concentration and speciation were monitored in local residents as an indicator of health impact. |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.rcees.ac.cn/handle/311016/34331]  |
| 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 | 胡珊. 共存离子对砷在纳米二氧化钛上吸附影响的微观机制研究[D]. 北京. 中国科学院研究生院. 2015. |
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