【作者】 刘瑜；

【导师】 于宏兵；

【作者基本信息】 南开大学 ， 环境工程， 2012， 博士

【摘要】 随着全球经济和社会的发展，环境污染日益严重。目前，水污染已被称作“世界头号杀手”，引发了一系列负面效应。据报道，有机污染物是水体污染中最主要的污染物。染料废水是目前导致水体污染的最重要的污染源之一，其不仅引发视觉污染，还直接导致有机生物体中毒、影响水生生物生存，而且可经过生物积聚作用进入人体、最终影响人类健康。五氯酚（PCP）作为一种典型的有毒、难降解有机化合物，既是优先污染物，也是持久性有机污染物。因此，对这两种典型的水体有机污染物进行有效的处理，对于保护环境、维持生态平衡、实现可持续发展具有非常重大的意义。近几年的研究表明，处理水体中有机污染物的研究方法重点围绕着吸附法和光催化氧化技术，这两种方法分别作为物理脱除法和化学降解法而得到广泛的关注和研究。然而，这两种方法均存在一些问题需要解决。吸附法目前面临的一些挑战主要有：吸附剂分散性差、吸附容量较小、回收困难，甚至造成二次污染；对吸附脱除水中污染物的吸附机理尚缺系统研究；关于吸附法脱除水中重金属污染的研究比较多，而针对于水中有机污染物的处理则开展较少。因此，围绕着分散性好、吸附速率快、吸附效果好、且回收容易的吸附剂的研究和开发是目前吸附研究领域的重要方向。另一方面，光催化氧化技术也存在很多发展空间，例如：目前被广泛使用的光催化剂如TiO₂等只能吸收紫外线，且大多数光催化剂都是以纳米颗粒组成的粉体为主，在使用过程中回收困难。因此，开发具有可见光响应型的新型金属氧化物纳米催化剂非常必要；通过改变催化剂的形貌或物理性质使得其能被有利于回收也值得思考。针对以上问题，本课题在总结现有文献的基础上，制备了几种新型金属氧化物纳米材料，表征了各纳米材料的物理、化学性质，并应用物理吸附法和光催化氧化法脱除水中典型的有机污染物——亚甲基蓝（MB）染料和五氯酚（PCP）。本论文以实验研究为主，结合理论探讨，主要开展了以下几个方面的研究工作：1.采用溶胶-凝胶法结合静电纺丝技术合成了具有层状结构的赤铁矿（α-Fe₂O₃）纳米结构纤维，表征测试了其物理、化学结构，分析了层状结构的可能形成机理，考察了其在紫外光下催化降解亚甲基蓝（MB）水溶液的催化性能。对于初始浓度为10mg/L、初始pH为5.3的MB溶液，加入50mg/L的α-Fe₂O₃纤维后可在紫外光下催化降解MB高达99.2%，比直接光照的降解效率提高了40.2%，且明显优于自制的α-Fe₂O₃纳米粉体（降解效率为88.2%）及商业Fe₂O₃粉末（降解效率为80.5%）。同时，考察了各种实验条件（如MB溶液的初始pH、MB溶液的初始浓度、无机阴离子、光照强度等）对降解效果的影响，从UV-vis图谱和α-Fe₂O₃能带结构两个角度分析了α-Fe₂O₃纤维降解MB的机理。2.采用溶胶-凝胶结合静电纺丝技术制备了具有三维蠕虫状介孔结构的Ce掺杂TiO₂/SiO₂复合纳米结构纤维，采用TG、FT-IR、XRD、SEM、TEM及UV-VisDRS等手段对其进行了表征，探讨了其在模拟太阳光下催化降解MB水溶液的光催化性能。在最佳催化条件下，即Ce掺杂量为0.2%、纤维投加量为100mg/L、pH=11时，初始浓度为2.5mg/L的MB溶液在模拟太阳光下照射120min后，其光催化降解效率可达92.6%；无机阴离子NO3-、SO42-、Cl-的存在对MB的光催化降解效率均具有抑制作用。重复利用实验表明该催化剂具有良好的光催化稳定性。通过H₂O₂和叔丁醇的影响研究，结合掺杂理论及相关参考文献，分析和探讨了Ce-TiO₂/SiO₂纤维光催化降解MB的机理。3.利用溶剂热法制备了壳聚糖修饰的四氧化三铁（Fe₃O₄/CS）单分散磁性纳米绒球，且对其进行了结构和性能测试。比较了Fe₃O₄/CS与Fe₃O₄分别作为吸附剂、吸附脱除水中五氯酚（PCP）的效果，表明Fe₃O₄/CS吸附脱除PCP的优越性：吸附过程在30min内迅速实现了吸附平衡，PCP的脱除量高达91.5%。影响因素研究指出：Fe₃O₄/CS的最佳投加量为2.8g/L；当pH=6.5时PCP的吸附量最高；盐度对PCP的脱除影响不大。吸附等温线、吸附热力学和吸附动力学研究表明：Fe₃O₄/CS吸附脱除PCP的吸附过程是自发放热过程，遵循Langmuir吸附模型和Lagergren二级动力学方程。机理研究表明：Fe₃O₄/CS吸附脱除PCP的吸附过程很复杂，其吸附机理源于静电力、氢键和π-π键的共同作用，且这些作用力的相对强弱取决于实验条件。4.采用高效、节能的微波-水热法合成了Ag₂CrO₄纳米结构粉体。通过XRD、SEM及UV-Vis DRS等手段表征了样品的形貌和性质。通过比较不同水热pH条件下制备的Ag₂CrO₄对PCP的光催化降解效果，得出：于原始pH（即pH=9.5）条件下制备的Ag₂CrO₄在可见光下对PCP具有卓越的光催化降解活性，反应60min后PCP已经发生分解（直接光照对PCP几乎没有产生任何影响，P25在相同时间内对PCP的光催化降解效率仅为17.7%）。同时，在反应60min后TOC的降解效率达68.6%；当催化反应进行到300min的时候，TOC的降解效率高达98.2%，说明此时PCP分子已经基本实现完全矿化。优化实验条件结果表明：Ag₂CrO₄的最佳投加量为0.75g/L；PCP的降解效率随PCP初始浓度的增加而下降，随溶液初始pH的增加而降低。催化剂的重复利用实验表明Ag₂CrO₄具有良好的光催化稳定性。机理研究指出： OH和O₂是参与光催化反应的主要活性物种，Ag₂CrO₄特有的晶体结构和电子结构有利于其吸收更多可见光，从而有利于发挥高效的光催化反应活性。更多还原

【Abstract】 With the development of the global society and economy, the environmentalpollution becomes more and more serious. Today, water pollution has become theNO.1killer to the world, causing many negative effects. According to the previousreports, the organic pollutants are the most important pollutants in the waterpollution. As a typical organic contaminant, textile dyes and other industrialdyestuffs are not only aesthetically unpleasant, but are also toxic to some organisms.And it also affects the human health by bio-accumulation. Pentachlorophenol （PCP）and sodium pentachlorophenate （PCP-Na）, the most toxic representatives amongchlorophenols, have attracted great attention worldwide and have been listed aspriority pollutants by the U.S. Environmental Protection Agency owning to theirtoxicity, mutagenicity, carcinogenicity, bioaccumulation and endocrine disturbingeffect. Thus, it is urgent to to remove the dye and PCP from water, and it is of greatsignificance for protecting the environment, maintaining the ecological balance, andkeeping the sustainable development.Recently, for treating the organic pollutants in water, many studies have focusedon the adsorption and photocatalytic technology. The adsorption and photocatalysisare the main routes for the removal of such compounds, which have attracted muchattention. However, there are some problems to be solved.For the adsorption method, the dispersion and the adsorption capacity of theadsorbents are challenging. The recovery difficulty and long time required foradsorption equilibriums limit their practical applications. And the use of relativelyexpensive adsorbents such as activated carbons may pose a serious problem from theeconomic standpoint. Moreover, the adsorption mechanism is not yet available forremoving contaminants from water system. Furthermore, the adsorption method ismostly used to treat the heavy metal pollution, rather than the organic pollutants inwater.On the other hand, for the photocatalytic technology, titanium dioxide （TiO₂） has been extensively studied as an effective photocatalyst for widespread application.However, there are several disadvantages in using TiO₂to purify wastewater. Firstly,the wide band of TiO₂（3.2eV） inhibits the utilization of solar energy in thephotocatalytic process. Secondly, the fixation and recovery of nanometer-sizedphotocatalysts are very difficult and tortuous. Therefore, novel materials with narrowband gap should be studied extensively to develop efficient visible-light-activephotocatalysts. For help recovering, it is also worthy to modify the morphologies orphysical properties of the photocatalysts.In our study, several nanometer-sized metal oxides were prepared and werecharacterized by various ways. Many experiments were carried out to remove themethylene blue （MB） and PCP in water. This dissertation mainly contains foursections as follow.1. The a-Fe₂O₃fibers have been prepared by electrospinning the correspondingsol-gel precursor. The characteristics of the fibers were investigated, andexperiments were conducted to study the formation mechanism of hierarchicalstructures. Photocatalytic degradation of MB in water was carried out underultraviolet （UV） light, showing that the fibers had better efficiency for removing MBthan other catalysts. Only59%of the degradation efficiency was observed after60min of continuous UV light irradiation without catalyst. However, in the presence ofa-Fe₂O₃fibers with the same UV light irradiation,99.2%of the dye was degraded,showing the excellent photocatalytic activity of a-Fe₂O₃fibers under UV irradiation.For comparison, the degradation efficiencies of the self-prepared a-Fe₂O₃nanoparticles and commercial Fe₂O₃powder were88.2%and80.5%respectively.Moreover, several process parameters have also been studied, which showed that theremoval of MB was influenced by the process parameters, such as the initial dyeconcentration, catalyst amounts, inorganic anions, UV intensity and so on. Finally,the mechanism of the photocatalysis process was analysed based on the UV-visspectra of MB and the energy band of a-Fe₂O₃.2. Cerium-doped SiO₂/TiO₂nanostructured fibers were prepared by a facilesol-gel and electrospinning technology. The fibers, with worm-like mesoporousstructure, were as long as several centimeters with diameters of0.4-1.0μm. The fibers were evaluated as efficient photocatalysts to degrade MB aqueous solutionunder simulated sunlight irradiation. In the absence of photocatalysts, thephotodegradation efficiency of MB was38.04%. The removed MB with P25, pureTiO₂/SiO₂fibers,0.2%Ce-doped TiO₂fibers and0.2%Ce-doped TiO₂/SiO₂fibersunder irradiation for2h were62.18%,64.44%,38.06%and80.16%respectively.The0.2%Ce doping is proved to be the optimal concentration for the doping ofTiO₂/SiO₂, the highest photodegradation efficiency of which is92.6%. Severaloperative conditions were studied for their further practical application. Thephotodegradation efficiency decreased with the increase in dye concentration. Theoptimal solution pH was found to be equal to11. Negative ions （NO3-, SO42-and Cl-）showed inhibiting effects in MB degradation. The fibers can be easily recycled andtheir photocatalytic activity had good stability. Furthermore, electron acceptors andradical scavengers were used to study the mechanism of the photocatalytic process.We expect the as-prepared fibers to be utilized as promising photocatalysts in the dyeeffluents treatment and other wastewater treatment.3. Novel monodisperse magnetic pompon-like magnetite/chitosan （Fe₃O₄/CS）composite nanoparticles were successfully synthesized by a solvent-thermal method.The prepared Fe₃O₄/CS was used to remove PCP from aqueous solution. Themagnetic adsorbents can be well dispersed in the aqueous solution and be easilyseparated from the solution with a magnet after adsorption. The adsorptionequilibrium was achieved quite rapidly （within30min） and the prominent removal ofPCP （91.5%） was obtained at25℃and pH6.5. The PCP removal was stronglypH-dependent, and low concentration of NaCl hardly affected the adsorption. Thenegative values of ΔG and ΔH showed that the adsorption was a spontaneous andexothermic process. The adsorption process follows Langmuir isotherm andpseudo-second-order kinetics model. The adsorption mechanism can be summarizedas complex with electrostatic attraction, hydrogen bonding and π-π interactions. It isexpected that this new monodisperse magnetic Fe₃O₄/CS has great potential forremoval of contaminants from aqueous media.4. Novel visible-light-induced Ag₂CrO₄photocatalysts were successfullysynthesized via a facile and energy-efficient microwave-hydrothermal （MH） method. The photocatalysts prepared at different pH display diverse crystal structures andmorphologies. The Ag₂CrO₄samples prepared at natural conditions （pH=9.5） had thestrongest absorption in the visible light region and had obviously enhancedphotocatalytic activity in PCP-Na degradation. Moreover, the TOC reductiondemonstrated the mineralization of PCP-Na over Ag₂CrO₄photocatalysts. Severalprocess parameters have also been studied, showing that the removal of PCP wasinfluenced by the process parameters, such as the initial concentration, the initial pH,catalyst amounts, and so on. In addition, the possible photocatalytic mechanism wasproposed based on the energy band positions of the Ag₂CrO₄and the effects ofradical scavengers.更多还原