催化湿式氧化/过氧化法处理难降解有机物的研究进展

郭俊江, 王紫嫙, 刘帅, 李彬, 张宇威, 杨迪. 催化湿式氧化/过氧化法处理难降解有机物的研究进展[J]. 环境化学, 2023, 42(8): 2790-2801. doi: 10.7524/j.issn.0254-6108.2022030406
引用本文: 郭俊江, 王紫嫙, 刘帅, 李彬, 张宇威, 杨迪. 催化湿式氧化/过氧化法处理难降解有机物的研究进展[J]. 环境化学, 2023, 42(8): 2790-2801. doi: 10.7524/j.issn.0254-6108.2022030406
GUO Junjiang, WANG Zixuan, LIU Shuai, LI Bin, ZHANG Yuwei, YANG Di. Research progress of catalytic wet oxidation/peroxidation treatment of refractory organics[J]. Environmental Chemistry, 2023, 42(8): 2790-2801. doi: 10.7524/j.issn.0254-6108.2022030406
Citation: GUO Junjiang, WANG Zixuan, LIU Shuai, LI Bin, ZHANG Yuwei, YANG Di. Research progress of catalytic wet oxidation/peroxidation treatment of refractory organics[J]. Environmental Chemistry, 2023, 42(8): 2790-2801. doi: 10.7524/j.issn.0254-6108.2022030406

催化湿式氧化/过氧化法处理难降解有机物的研究进展

    通讯作者: E-mail:libin@kust.edu.cn; 
  • 基金项目:
    云南省自然科学基金(202001AT07070088)和国家自然科学基金(52060010)资助

Research progress of catalytic wet oxidation/peroxidation treatment of refractory organics

    Corresponding author: LI Bin, libin@kust.edu.cn
  • Fund Project: National Science Foundation of Yunnan Province (202001AT07070088) and Natural Science Foundation of China (52060010)
  • 摘要: 常规的水处理工艺成熟,运行成本低,但其对难降解有机物的处理效果差,难以满足日益严格的排放标准. 本文将催化湿式氧化法(CWAO)与催化湿式过氧化氢氧化法(CWPO)合称为催化湿式氧化/过氧化法,两者都具有效率高、占地少的显著特征,可以直接把难降解有机物分解为二氧化碳和水,已成为新的研究热点. 本文综述了催化湿式氧化/过氧化法降解有机物的原理和进展,分析了催化剂对常规湿式氧化/过氧化反应过程的加速和降解效率的影响,讨论了催化湿式氧化/过氧化技术存在的主要制约瓶颈,提出了有机物的定向调控转化和资源化是今后减污降碳的主要方向.
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  • 图 1  不同制作方法的催化剂对喹啉和TOC转化率的影响[41]

    Figure 1.  Effect of catalysts with different preparation methods on quinoline and TOC conversion[41]

    图 2  双酚A转换随时间的函数[57]

    Figure 2.  Bisphenol A conversion as a function of time

    表 1  常温水和超临界水的性质比较

    Table 1.  Comparison of properties between constant temperature water and supercritical water

    性质

    Property
    常温水

    Constant temperature water
    超临界水

    Supercritical water
    温度/℃25450
    压力/MPa0.127
    密度/(g·cm−30.9980.128
    介电常数78.51.8
    黏度/cp0.8900.0298
    扩散系数/(cm2·s−17.74×10−67.76×10−4
    离子积10-1410-22
    性质

    Property
    常温水

    Constant temperature water
    超临界水

    Supercritical water
    温度/℃25450
    压力/MPa0.127
    密度/(g·cm−30.9980.128
    介电常数78.51.8
    黏度/cp0.8900.0298
    扩散系数/(cm2·s−17.74×10−67.76×10−4
    离子积10-1410-22
    下载: 导出CSV

    表 2  铁基催化剂用于降解有机污染物研究

    Table 2.  Research on iron-based catalyst for degradation of organic pollutants

    难降解有机物
    Refractory
    organic
    matter
    催化剂
    Catalyst
    制备方法
    Preparation
    比表面积/
    (m2∙g−1
    Specific
    surface
    area
    氧化剂
    Oxidizer
    COD
    去除率/%
    COD
    removal
    rate
    TOC
    去除率/%
    TOD
    removal
    rate
    铁浸出浓度/
    (mg ∙L−1
    Iron leaching
    concentration
    底物浓度/
    (mg ∙L−1
    Substrate
    concentration
    参考文献
    References
    苯酚Fe-ZSM-5湿浸渍法225.5H2O299.277.71.91000[15]
    间甲苯酚Fe2O3-ZSM-5气相沉积法251.7H2O29980.51.11000[16]
    苯甲酸Fe3O4@CeO2溶剂热法104.9H2O280484.250[17]
    苯酚3D Fe/SiC3D打印法23.5H2O2100608.31000[18]
    苯酚Al/Fe-PILCs超声浸渍法200O2100800.71000[19]
    2,4,6-三氯酚铁基碳氧凝胶浸渍法510O274.4924.310.0911600[20]
    草甘膦Fe-SBA(20)凝胶法705O280N/AN/A15[21]
    香草酸Fe/TS-1湿浸渍法N/AH2O2100N/AN/A10000[22]
    苯酚Fe3C@NCNT/PSSF气相沉积法15H2O29241N/A1000[23]
    甲酚Fe/ZSM浸渍法574.9H2O290.7240.12100[24]
    甲硝唑Fe/Al2O3微波浸渍法0.11H2O273N/A0.10.1[25]
    磺胺甲恶唑Fe/SiC微波浸渍法0.23H2O283N/A0.270.1[25]
    卡马西平Fe/ZrO2微波浸渍法0.35H2O290N/A0.350.1[25]
    P-4B染料Al/Fe-PILCs气相沉积法201H2O299.2458.130.24100[26]
    扑热息痛铁碳干凝胶(RFFeC)熔融-凝胶法263H2O299600.250[27]
    苯酚Al-Ce-FeNaOH激活+
    热处理
    121.8H2O2100540.251000[28]
    乙二胺四乙酸Fe-MCM-41浸渍法937H2O2100500.25845[29]
      N/A,无法获得的. N/A,Not available.
    难降解有机物
    Refractory
    organic
    matter
    催化剂
    Catalyst
    制备方法
    Preparation
    比表面积/
    (m2∙g−1
    Specific
    surface
    area
    氧化剂
    Oxidizer
    COD
    去除率/%
    COD
    removal
    rate
    TOC
    去除率/%
    TOD
    removal
    rate
    铁浸出浓度/
    (mg ∙L−1
    Iron leaching
    concentration
    底物浓度/
    (mg ∙L−1
    Substrate
    concentration
    参考文献
    References
    苯酚Fe-ZSM-5湿浸渍法225.5H2O299.277.71.91000[15]
    间甲苯酚Fe2O3-ZSM-5气相沉积法251.7H2O29980.51.11000[16]
    苯甲酸Fe3O4@CeO2溶剂热法104.9H2O280484.250[17]
    苯酚3D Fe/SiC3D打印法23.5H2O2100608.31000[18]
    苯酚Al/Fe-PILCs超声浸渍法200O2100800.71000[19]
    2,4,6-三氯酚铁基碳氧凝胶浸渍法510O274.4924.310.0911600[20]
    草甘膦Fe-SBA(20)凝胶法705O280N/AN/A15[21]
    香草酸Fe/TS-1湿浸渍法N/AH2O2100N/AN/A10000[22]
    苯酚Fe3C@NCNT/PSSF气相沉积法15H2O29241N/A1000[23]
    甲酚Fe/ZSM浸渍法574.9H2O290.7240.12100[24]
    甲硝唑Fe/Al2O3微波浸渍法0.11H2O273N/A0.10.1[25]
    磺胺甲恶唑Fe/SiC微波浸渍法0.23H2O283N/A0.270.1[25]
    卡马西平Fe/ZrO2微波浸渍法0.35H2O290N/A0.350.1[25]
    P-4B染料Al/Fe-PILCs气相沉积法201H2O299.2458.130.24100[26]
    扑热息痛铁碳干凝胶(RFFeC)熔融-凝胶法263H2O299600.250[27]
    苯酚Al-Ce-FeNaOH激活+
    热处理
    121.8H2O2100540.251000[28]
    乙二胺四乙酸Fe-MCM-41浸渍法937H2O2100500.25845[29]
      N/A,无法获得的. N/A,Not available.
    下载: 导出CSV

    表 3  铜基催化剂用于降解有机污染物研究

    Table 3.  study on copper based catalyst for degradation of organic pollutants

    难降解有机物
    Refractory
    organic
    matter
    催化剂
    Catalyst
    制备方法
    Preparation
    BET比表面积/
    (m2∙g−1
    Specific
    surface
    area
    铜负载率/%
    Copper
    load rate
    氧化剂
    Oxidizer
    COD
    去除率/%
    COD
    removal
    rate
    TOC
    去除率/%
    TOD
    removal
    rate
    铜浸出浓度/
    (mg ∙L−1
    Copper
    leaching
    concentration
    底物浓度/
    (mg ∙L−1
    Substrate
    concentration
    参考文献
    References
    喹啉Cu/沸石Y水离子交换法、湿浸渍法9095.03H2O210065.461100[41]
    制药污泥Cu/Ce共沉淀法N/AN/AO280N/A518000[42]
    ETBE和TAMECuO/γ-Al2O3溶胶凝胶法44916.9O2100740N/A[43]
    苯酚Cu3-Al-500共沉淀法22.8O299N/A10.32100[44]
    氯酚CeCu湿浸渍法63.44.46H2O299.5822.4650[45]
    苯酚Cu-ZSM-5离子交换法1651.91H2O298787.21000[46]
    4-氯苯酚Zn-CNTs-Cu渗透熔融化学置换法N/AN/AH2O210068N/A1000[47]
    咖啡因CuNi-YC湿浸渍法57.81N/AH2O286.1668.852.0540[48]
    N/A,无法获得的. N/A,Not available.
    难降解有机物
    Refractory
    organic
    matter
    催化剂
    Catalyst
    制备方法
    Preparation
    BET比表面积/
    (m2∙g−1
    Specific
    surface
    area
    铜负载率/%
    Copper
    load rate
    氧化剂
    Oxidizer
    COD
    去除率/%
    COD
    removal
    rate
    TOC
    去除率/%
    TOD
    removal
    rate
    铜浸出浓度/
    (mg ∙L−1
    Copper
    leaching
    concentration
    底物浓度/
    (mg ∙L−1
    Substrate
    concentration
    参考文献
    References
    喹啉Cu/沸石Y水离子交换法、湿浸渍法9095.03H2O210065.461100[41]
    制药污泥Cu/Ce共沉淀法N/AN/AO280N/A518000[42]
    ETBE和TAMECuO/γ-Al2O3溶胶凝胶法44916.9O2100740N/A[43]
    苯酚Cu3-Al-500共沉淀法22.8O299N/A10.32100[44]
    氯酚CeCu湿浸渍法63.44.46H2O299.5822.4650[45]
    苯酚Cu-ZSM-5离子交换法1651.91H2O298787.21000[46]
    4-氯苯酚Zn-CNTs-Cu渗透熔融化学置换法N/AN/AH2O210068N/A1000[47]
    咖啡因CuNi-YC湿浸渍法57.81N/AH2O286.1668.852.0540[48]
    N/A,无法获得的. N/A,Not available.
    下载: 导出CSV

    表 4  稀有金属催化剂用于降解有机污染物研究

    Table 4.  Research on rare metal catalysts for degradation of organic pollutants

    难降解
    有机物
    Refractory
    organic matter
    催化剂
    Catalyst
    制备方法
    Preparation
    比表面积/
    (m2∙g−1
    Specific
    surface
    area
    金属
    负载率/%
    Metal
    loading rate
    氧化剂
    Oxidizer
    COD去除率/%
    COD removal
    rate
    TOC去除率/%
    TOD removal
    rate
    底物浓度/
    (mg ∙L−1
    Substrate
    concentration
    参考文献
    References
    活性黑5LaNiO3溶胶-凝胶柠檬酸法N/A1O265.433100[55]
    腐殖质NiCo2O4溶剂热法66.88Ni/Co=1:2.06O21009025000[56]
    双酚ARu/ZrO2溶胶-凝胶和浸渍法803O297N/A10[57]
    腐殖酸Mo/Al2O3原位水热法、煅烧法N/A1.39O21006020[58]
    罗丹明6GLa/CoFe2O4溶胶自燃法3.381.15H2O291.64010[59]
    Gd/CoFe2O42.691.0892.82710
    Dy/CoFe2O42.511.0791.73510
    苯酚Mn/Ce共沉淀法160Mn/Ce=6/4O2100941000[60]
    偶氮染料Ce2O3-Fe2O3/-Al2O3浸渍法193.60.39H2O288.7781.44500[61]
      N/A,无法获得的. N/A,Not available.
    难降解
    有机物
    Refractory
    organic matter
    催化剂
    Catalyst
    制备方法
    Preparation
    比表面积/
    (m2∙g−1
    Specific
    surface
    area
    金属
    负载率/%
    Metal
    loading rate
    氧化剂
    Oxidizer
    COD去除率/%
    COD removal
    rate
    TOC去除率/%
    TOD removal
    rate
    底物浓度/
    (mg ∙L−1
    Substrate
    concentration
    参考文献
    References
    活性黑5LaNiO3溶胶-凝胶柠檬酸法N/A1O265.433100[55]
    腐殖质NiCo2O4溶剂热法66.88Ni/Co=1:2.06O21009025000[56]
    双酚ARu/ZrO2溶胶-凝胶和浸渍法803O297N/A10[57]
    腐殖酸Mo/Al2O3原位水热法、煅烧法N/A1.39O21006020[58]
    罗丹明6GLa/CoFe2O4溶胶自燃法3.381.15H2O291.64010[59]
    Gd/CoFe2O42.691.0892.82710
    Dy/CoFe2O42.511.0791.73510
    苯酚Mn/Ce共沉淀法160Mn/Ce=6/4O2100941000[60]
    偶氮染料Ce2O3-Fe2O3/-Al2O3浸渍法193.60.39H2O288.7781.44500[61]
      N/A,无法获得的. N/A,Not available.
    下载: 导出CSV

    表 5  Rh6G染料CWPO降解的颜色/TOC去除数据和动力学参数[59]

    Table 5.  Color/TOC removal data and kinetic parameters of Rh6G dye CWPO degradation[59]

    催化剂

    Catalyst
    颜色去除率/%

    Color removal rate
    TOC去除率/%

    TOC removal rate
    C35.717
    C-La91.640
    C-Gd92.827
    C-Dy91.735
    催化剂

    Catalyst
    颜色去除率/%

    Color removal rate
    TOC去除率/%

    TOC removal rate
    C35.717
    C-La91.640
    C-Gd92.827
    C-Dy91.735
    下载: 导出CSV

    表 6  非金属催化剂用于降解有机污染物的研究

    Table 6.  research on non-metallic catalysts for degradation of organic pollutants

    难降解有机物
    Refractory
    organic
    matter
    催化剂
    Catalyst
    制备方法
    Preparation
    比表面积/
    (m2∙g−1
    Specific
    surface
    area
    催化剂
    含量/%
    Catalyst
    content
    氧化剂
    Oxidizer
    COD
    去除率/%
    COD
    removal
    rate
    TOC
    去除率/%
    TOD
    removal
    rate
    底物浓度/
    (mg∙L−1
    Substrate
    concentration
    参考文献
    References
    布洛芬石墨粉商用1297.2H2O21005320 μmol∙L−1[72]
    双氯芬酸1007420 μmol∙L−1
    苯酚单层石墨烯薄膜化学气相沉积法0.6685N/AH2O2100911000[73]
    氮掺杂炭高温氨化130587.1O2100N/A1000[74]
    活性炭商用101989.3H2O297705000[75]
    扑热息痛椰子壳活性炭炭化法118092.3O295602000
    [76]
    木材活性炭186071.189412000
    木麻黄活性炭123086.598622000
      N/A,无法获得的. N/A,Not available.
    难降解有机物
    Refractory
    organic
    matter
    催化剂
    Catalyst
    制备方法
    Preparation
    比表面积/
    (m2∙g−1
    Specific
    surface
    area
    催化剂
    含量/%
    Catalyst
    content
    氧化剂
    Oxidizer
    COD
    去除率/%
    COD
    removal
    rate
    TOC
    去除率/%
    TOD
    removal
    rate
    底物浓度/
    (mg∙L−1
    Substrate
    concentration
    参考文献
    References
    布洛芬石墨粉商用1297.2H2O21005320 μmol∙L−1[72]
    双氯芬酸1007420 μmol∙L−1
    苯酚单层石墨烯薄膜化学气相沉积法0.6685N/AH2O2100911000[73]
    氮掺杂炭高温氨化130587.1O2100N/A1000[74]
    活性炭商用101989.3H2O297705000[75]
    扑热息痛椰子壳活性炭炭化法118092.3O295602000
    [76]
    木材活性炭186071.189412000
    木麻黄活性炭123086.598622000
      N/A,无法获得的. N/A,Not available.
    下载: 导出CSV
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  • 收稿日期:  2022-03-04
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  • 刊出日期:  2023-08-27

催化湿式氧化/过氧化法处理难降解有机物的研究进展

    通讯作者: E-mail:libin@kust.edu.cn; 
  • 昆明理工大学环境科学与工程学院,昆明,650093
基金项目:
云南省自然科学基金(202001AT07070088)和国家自然科学基金(52060010)资助

摘要: 常规的水处理工艺成熟,运行成本低,但其对难降解有机物的处理效果差,难以满足日益严格的排放标准. 本文将催化湿式氧化法(CWAO)与催化湿式过氧化氢氧化法(CWPO)合称为催化湿式氧化/过氧化法,两者都具有效率高、占地少的显著特征,可以直接把难降解有机物分解为二氧化碳和水,已成为新的研究热点. 本文综述了催化湿式氧化/过氧化法降解有机物的原理和进展,分析了催化剂对常规湿式氧化/过氧化反应过程的加速和降解效率的影响,讨论了催化湿式氧化/过氧化技术存在的主要制约瓶颈,提出了有机物的定向调控转化和资源化是今后减污降碳的主要方向.

English Abstract

  • 难降解有机物是废水中难以在自然条件下被生物逐级降解的有机污染物. 由于其难降解性,在水环境中的大量滞留,大部分难降解有机物具有高毒性,不仅对水环境造成了严重的危害,甚至威胁到了人类的健康. 随着工业化和经济的迅速增长,近年来难降解有机物在废水中的浓度越来越高,高浓度有机废水中难降解有机物的浓度甚至已经达到了2000—20000 mg·L−1[1]. 我国难降解有机物主要来源于工业废水,如印染废水、造纸废水、食品行业废水、农药废水和制药工业废水[2]. CWAO和CWPO被认为是目前处理难降解有机污染物最有效的方法. 本文综述了CWAO和CWPO降解有机物的原理和进展,分析了催化剂对常规湿式氧化反应过程的加速和降解效率的影响,并提出了未来难降解有机物可能的发展方向.

    • 催化湿式氧化/过氧化法是指液相中的有机物在高温高压以及催化剂存在的环境下被氧化处理的方法,是目前处理难降解有机物运用最广泛的方法之一. 高于临界温度和临界压力的状态是超临界状态[3],而催化湿式氧化/过氧化法的温度和压力低于超临界状态,被称为亚临界状态. 亚临界状态的性质介于超临界水和常温水之间. 如表1所示常温水和超临界水的性质差距很大.

      催化湿式氧化/过氧化法分为CWAO和CWPO,这两种方法工作条件和氧化效果几乎相同,但有各自不同的适用环境,CWAO适用于治理焦化、染料、农药、印染、石化、皮革等工业中含高化学需氧量(COD)的(如氨氮、多环芳烃、致癌物质BPA等)工业有机废水;CWPO适用于各种工业废水和医疗废水,尤其适用于难降解的复杂的有机污染物. 这两者的主要区别在于CWAO的氧化剂为O2或空气,而CWPO的氧化剂通常为H2O2,所以导致两者的氧化机理略有不同.

      当氧化剂为O2时:

      当氧化剂为H2O2时:

      CWAO和CWPO均可在氧化有机物过程中分为三个阶段:热分解阶段、局部氧化阶段和完全氧化阶段[4].

      在热分解阶段,大分子量的有机物在液相中溶解和水解(但没有被氧化),其溶解和水解速度随温度升高而加快,有机物在液相中溶解和水解越完全,氧化就会越彻底[5];在局部氧化阶段,大分子量的有机物未被完全氧化,氧化分解成为分子量较低的中间产物,如甲酸、乙酸、甲醛等[6];在最后的完全氧化阶段,中间有机产物进一步氧化成二氧化碳和水[7].

    • CWAO在一定的温度、压力和催化剂的作用下,经空气氧化,使污水中的有机物及氨分别氧化分解成CO2、H2O及N2等无害物质,达到降解有机物的目的[8]. CWAO具有以下特点[9]

      (1) 在传统的湿式氧化处理体系中加入催化剂,降低反应的活化能,从而在不降低处理效果的情况下,降低反应的温度和压力,提高氧化分解的能力,缩短反应的时间,提高反应效率,并减少了设备的腐蚀,降低了成本;

      (2) 具有净化效率高、无二次污染、流程简单和占地面积小等优点;

      (3) 催化剂有选择性,并且污水中含有许多种类和结构不同的有机物,需要对催化剂进行筛选.

    • CWPO是在一定的温度、压力和催化剂的作用下,在液相中加入过氧化氢,作为氧化剂,使污水中的有机物及氨分别氧化分解成CO2、H2O及N2等无害物质,从而达到降解的效果[10]. CWPO的特点如下[11]

      (1) 与催化湿式氧化法相比,以过氧化氢代替空气或氧气,过氧化氢的氧化性高于氧气,可使有机物氧化更加彻底,但对设备质量的要求更高,提高了运行的成本和操作难度;

      (2) 具有净化效率高、无二次污染、流程简单、占地面积小等优点;

      (3) 过氧化氢无毒无害,且分解生成水和氧气,可安全排放.

    • 用于催化湿式氧化/过氧化反应的催化剂众多,如非稀有金属催化剂、稀有金属催化剂和非金属催化剂等,本文主要综述了非稀有金属催化剂、稀有金属催化剂和非金属催化剂的3种催化剂在催化湿式氧化/过氧化领域的研究进展.

    • 铁基催化剂是在催化剂中负载金属铁、铁的氧化物或者以铁和铁的氧化物负载其他金属,以此来增强催化剂的活性和催化效果[12]. 铁基催化剂多用于Fenton反应中[13],Fenton法反应机理如下 [14]

      如今大多数的铁基催化剂为非均相催化剂,在催化湿式氧化/过氧化反应中展现出了优异的活性和催化效果. Yan等制备了Fe-ZSM-5催化剂,其在固定床反应器中对苯酚的降解效率高达99.2%[15];Yang等利用金属有机化学气相沉积(MOCVD)法制备的Fe2O3-ZSM-5催化剂,间甲苯酚转化率在0.5—2.5 h内可达到99%,在60 ℃、400 r·min−1条件下,3 h后TOC去除率可高达80.5%[16];Qin等采用简单的溶剂热法制备了磁性核壳结构Fe3O4@CeO2催化剂,苯甲酸的去除率可达到80%[17]. 表2对比了近年来铁基催化剂处理难降解有机污染物的研究,可以看出铁基催化剂在降解有机物这一方面,效果好,降解效率高,具有广阔前景.

      铁基催化剂在催化湿式氧化/过氧化反应中会浸出一部分铁离子,对反应会造成一定的影响[30]. 有研究发现反应溶液中的铁离子浓度随着过氧化氢浓度的增加而增加,这是因为过氧化氢可以电离出H+,加大过氧化氢的浓度相当于加大了H+的浓度,从而促进了铁离子的浸出[26]. 同时中间产物有机酸如苹果酸、草酸、琥珀酸、甲酸和醋酸等,这些小分子量羧酸的形成会降低溶液的pH,也会促进铁离子的浸出[18, 31]. 除了pH和过氧化氢效应外,中间体与铁离子的络合反应也会促进铁离子的浸出[32].

      铁离子的浸出是使用铁基催化剂的一大难题,许多学者在降低铁浸出做了大量工作,如:有学者通过增大催化剂与水的接触角,降低催化剂表面湿润性,使铁不易与水相互作用;或者降低pHPZC值,增加催化剂内部的吸附能力,可以将铁在催化剂表面固定化,从而阻止铁的浸出[25, 33],相比通过3D打印技术制造出的铁催化剂[18],铁浸出率要低得多. 但铁离子的浸出在催化湿式氧化/过氧化反应中,在一定程度上可以起到促进有机物降解的作用,因为浸出的铁离子会与溶液中的氧气或过氧化氢发生反应,生成具有强氧化性的羟基自由基,羟基自由基可以快速氧化有机物,增加有机物的矿化程度. 所以合理利用铁离子浸出的同时有效控制溶液中铁离子的含量,或许是铁基催化剂应用在催化湿式氧化/过氧化领域中一个重要的方向.

    • 铜基催化剂多以氧化铜为活性组分,采用不同的助剂、不同载体以及不同的制备工艺,制得的复合型催化剂来满足催化湿式氧化/过氧化法的需要[34]. 近年来,铜被认为是比铁还优异的催化剂,这是由于铜基催化剂拥有较宽的pH值以及稳定的结构和催化性能,在催化湿式氧化/过氧化反应中,铜基催化剂不会与铁离子一样与溶液中的有机酸发生络合反应生成配合物,因此,它不会阻断羟基自由基的生成,从而可以提供高矿化作用[35]. 在反应过程中,铜基催化剂会浸出一部分铜离子,铜离子会与溶液中的过氧化氢反应生成具有强氧化性的羟基自由基,从而增强有机物的降解能力,反应机理如下[36]

      金属铜适用于许多催化剂的合成,如铜铁催化剂[37]、铜镍催化剂[38]、铜铈催化剂[39]和铜活性炭催化剂[40]等,表3对比了近年来铜基催化剂处理难降解有机污染物的研究,铜基催化剂表现出突出的催化性能. 不同催化剂,其结构和性能会有很大的差异,相同的催化剂,如果制作方法不同,其结构也是有较大差异,有学者研究了水离子交换法(CuYAIE)、湿浸渍法(CuYIMP)和沉淀法(CuYPI)的3种方法制备的催化剂进行了比较. 如图1所示,CuYPI对过氧化氢转化的活性最高,但对污染物的降解活性最低,表明过氧化氢转化率并非与污染物去除率呈现正相关性. 过氧化氢与活性组分相结合的速率是更重要的参数. CuYPI催化剂的比表面积大于CuYAIE和CuYIMP催化剂,但CuYAIE和CuYIMP催化剂比CuYPI催化剂更有活性. 说明高表面积并不是CWPO获得高催化活性的必要参数,与前人研究结果一致[29],活性相的位置和电子状态比结构性质起着更重要的作用.

      铜基催化剂通常会与其他活性物质协同催化降解有机物,如氧化铈[42]和氧化铝[43]等. 有学者研究发现,氧化铜和氧化铈之间的强相互作用会削弱Cu—O和Ce—O的化学键,从而促进化学键在适当的反应条件下分裂形成活性氧化物. 同时氧化铜和氧化铈之间的强相互作用会增加Cu2+/Cu+与Ce4+/Ce3+之间电子转移速率,从而提高过氧化氢与活性组分的反应速率,加快反应进程[45].

      铜的负载率是影响催化剂活性的显著性能之一,有研究表明有机物的去除效率随着铜含量的增加而提高,但当铜增加到25%时,催化剂活性开始下降[47]. 这是由于铜具有显著的催化活性,活性组分含量越高,对有机物的降解效率越强. 但将过量的铜负载在催化剂上,可能会占据催化剂的活性位点,从而减弱了催化剂的活性,降低催化剂的降解效果. 催化剂的活性是铜基催化剂最关键的问题,取决于金属负载量、活性铜、酸性位点、晶格氧和铜离子交换度. 较高分散的氧化铜物种和Cu+使催化剂具有更好的氧化还原性;低离子交换浓度有利于提高催化剂的活性,离子交换浓度越高,溶液中的低分子量有机酸使铜浸出越严重[46];催化剂的酸位点含量越高,越有利于有机污染物的吸附和活化,有机物就越容易降解[44].

    • 非稀有金属除了铁和铜以外,还有锌、铝和锰等. 这几种金属及其氧化物很少单独作为催化剂出现,大多数以双金属或多金属用于CWAO和CWPO中,且催化剂能表现出较好的催化性能. 有学者通过原位合成法和浸渍法制备了CuO/ZnO-A12O3水滑石衍生催化材料,当苯酚初始浓度500 mg ∙L−1时,COD去除率能达到95.3%[49];杨韶平通过浸渍法制备的MnO2/A12O3催化剂,降解糖蜜酒精废水,处理水均能达到《污水综合排放标准GB9878—1996》的三级排放标准[50];许银采用浸渍法制备的Mo-Zn-Al-O催化剂对阳离子红GTL的脱色率和TOC去除率高达90.9%和65.8%[51];Sanabria使用Al-Ce-Fe膨润土催化剂处理咖啡加工废水,总酚类化合物矿化率可达70%,化学需氧量(COD)降低了66%[52].

      本文将膨润土、沸石等矿物制备的催化剂归为了其他非稀有金属催化剂一类,此类催化剂通常具有较大的比表面积、活性点位多以及简单易得等优点,得到广大学者的喜爱,目前用此类催化剂用于降解有机物的研究在逐渐增多. Balci通过硫酸活化膨润土制得降解苯酚的催化剂,苯酚去除率约为96%[53];Ramírez通过水热法将粉煤灰制成沸石催化剂,可将酸性橙完全降解,矿化率达到45%[32]. 这类催化剂虽然能达到较好的催化效果,但会浸出较多的金属如铁、铝和钠等,可能会对水环境造成二次污染. 减少金属离子的浸出或对浸出的金属离子进行处理与回收,是目前这类催化剂的研究重点.

    • 稀有金属是在地壳中含量较少、分布稀散或难以从原料中提取的金属,因稀有金属催化剂多活性组分之间的协同作用,有利于催化剂酸性位点增加、氧化物流动性增加以及催化活性增强,因而表现出较好的催化性能[54]. 表4列举了稀有金属催化剂降解有机污染物的研究.

      湿润性是催化剂的表面性质之一,对催化剂在催化湿式氧化/过氧化反应途径具有主导影响,当使用钌时,RuO2能从水相中吸附苯酚和O2这两种反应物;然而,当使用铂时,Pt0活性位点可以直接从水相吸附苯酚和从气相吸附O2,因此在载体的活性位点或周围具有大量的疏水性和局部的亲水性会更有效[62]. 有研究发现,CeO2不仅可以增加催化剂的储氧能力,而且还可以对Pt/SiC或Ru/Al2O3之间的界面上形成固溶体[63]. 这种固溶体将在金属支撑界面上提供亲水性和额外的吸附位点,从而提高有机物从液相到Pt或Ru表面的传质速率. 催化剂的湿润性或成为未来的研究重点.

      协同效应是影响稀有金属催化剂催化效果的主要因素之一,有学者通过溶胶-凝胶柠檬酸法制备的LaNiO3催化剂表现出了优异的催化活性,在La和Ni的协同作用下,活性黑5的降解效率和脱色效率分别可达到65.4%和89.6%[55]. 虽然稀有金属之间存在着协同效应,但协同作用效果皆有所不同. 有研究将稀有金属Ru、Pb和Pt分别负载在TiO2和ZrO2上降解有机物,如图2所示,催化效果都有不同程度的提升,其中Ru/ZrO2是最活跃的催化剂,催化效果最好,这是因为Ru在ZrO2中良好的分散,产生了强烈的Ru-O-Zr相互作用.

      不仅稀有金属之间存在协同效应,稀有金属与铁和铜这种非稀有金属之间也存在协同效应. 有研究表明氧化铜纳米棒(CuONRs)可负载单金属或双金属金和钯纳米颗粒,制备的4种催化剂CuONRs、CuONRs@Au6NPs、CuONRs@Pd6NPs和CuONRs@Au3Pd3NPs,其中CuONRs@Au3Pd3NPs表现出更高的催化强度,其次是CuONRs@Pd6NPs,这表明金和钯与铜之间的协同效应,以及金与钯之间的协同效应是可以叠加的[64],这为催化剂的多元化研究提供了更多的可能性.

      同属一系的稀有金属具有相似的性质,但催化性能有一定的差别. 有学者通过制备掺杂镧系金属La、Gd和Dy的钴铁酸体探究了罗丹胺6G-(Rh6G)的降解效果,研究发现通过掺杂稀土阳离子,均显著提高了钴铁氧体在CWPO中的催化性能和Rh6G的矿化作用. 如表5所示,同属一系的金属催化性能也有所差别,这是因为其不同的晶粒尺寸与各异的晶径比和尖晶石相纯度相互作用,使得掺杂钴铁氧体的催化剂表现出不同的催化性能.

      稀有金属催化剂由于特异的结构性质,其降解机理与其他金属催化剂有所不同. 稀有金属催化剂氧化有机物的降解机理,本文总结了以下四步[56]:(1)有机物通过静电吸附和化学吸附吸附在催化剂表面;(2)第一步形成的羟基自由基,即活性氧[65],直接氧化有机物,导致表面氧种类减少;(3)晶格氧转化为表面氧,维持催化剂表面电荷的平衡,形成氧空位;(4)外部O2或H2O2填充氧空位,将电子转移到还原的催化剂上.

    • 在催化湿式氧化/过氧化反应的研究中,非金属催化剂主要为活性炭和石墨烯,因为两者都是碳结构,具有良好的催化性能,被众多学者广泛用于降解有机物的研究[66-67].

      活性炭具有微晶结构,微晶排列不规则,晶体中有微孔(半径10—20 nm)、中孔(半径20—1000 nm)、大孔(半径1000—100000 nm),使它具有很大的内表面,比表面积为500—1700 m2∙g−1,这决定了活性炭具有良好的吸附性[68-69]. 石墨烯是一种以 sp² 杂化连接的碳原子紧密堆积成单层二维蜂窝状晶格结构的新材料. 石墨烯具有优异的光学、电学、力学特性,在材料学、催化、微纳加工、能源、生物医学和药物传递等方面具有重要的应用前景,被认为是一种未来具有革命性的材料[70-71]. 表6列举了近些年非金属催化剂降解有机污染物的研究.

      活性炭是一种内表面积很大的催化材料,在其内表面上存在许多活性位点,这一点既促进了反应的进行,也抑制了有机物的降解. 这是因为活性炭的活性位点大多在内表面上,而活性炭的孔径大多为微中孔,在短时间内,氧化剂难以进入活性炭的内表面,就很难与内表面的活性位点相结合产生强氧化性的羟基自由基. 在多数情况下,氧化剂和有机物会同时竞争活性炭表面上的活性位点. 如果有机物浓度过高,会对活性位点产生阻碍,氧化剂的分解速率将减慢[75]. 增加氧化剂的量,可以提高氧化剂在活性位点上的竞争力,但添加过量的过氧化氢会形成比羟基自由基氧化性弱的过羟基自由基,导致催化效率下降. 因此,增强过氧化氢分解成羟基和氢过氧自由基的转化率,并使其更有效地消耗有机物是研究的重点.

      众所周知高浓度的盐会抑制废水中过氧化氢分子的活性,温度的升高可以降低盐的抑制作用,加速过氧化氢的氧化速率[77-78]. 这是因为高盐和高浓度的有机物会阻塞活性炭内孔,升高温度可以打开活性炭的孔径,促进过氧化氢和有机物扩散到AC的内孔中,使内表面的活性位点得到利用,增加过氧化氢的转化率以及有机物的降解率[79].

      温度的升高可以增强活性炭的活性,但同时也增加了能耗,为避免这一问题,有学者通过增加活性炭表面的活性位点,以此增强活性炭的活性[80]. 有研究表明将铁负载在活性炭上制作的铁碳复合催化剂,大大提高了活性炭的表面催化活性,铁作为异相芬顿反应的高效催化剂,催化活性较高,极大提高了活性炭的催化效率[81]. 后续的研究发现单活性组分材料已经很难满足现有的有机物降解,所以出现了多种活性组分共同负载在活性炭上,如在活性炭表面掺入Fe-S作为活性位点,可以促进氧化剂和相界面氧化铁之间的电子转移[82, 83],或是将N引入到Fe/C的表面,导致了Fe-NX配位活性位点的形成,从而形成Fe3C与Fe3N之间的协同效应[84]. 有研究表明N、S、铁三掺杂碳催化剂(NSFe-Cs)与唯一的铁掺杂催化剂(Fe/AC)相比较催化效果更好,这是因为S的掺入,与铁形成了二硫化铁,二硫化铁为催化剂表面提供了大量的Fe2+,同时N的加入,使催化剂表面更具亲水性[85]. 活性炭由于其巨大的比表面积、优异的内孔结构和优秀的催化性能,成为了最受欢迎的载体之一.

      石墨烯是一种高效稳定的催化剂,Yoo等制备了石墨烯薄膜来降解苯酚,石墨烯表现出较强的催化效率(苯酚转化率高达92%)[86]. Liu等采用化学气相沉积(CVD)技术在三维网结构的纸状烧结不锈钢纤维(PSSF)上合成了单层石墨烯薄膜(Gr)催化剂,结果表明,在最佳条件下连续运行72 h后,苯酚完全氧化,TOC显著去除(值在80.7%—91.0%之间)[73]. 石墨烯优异的催化性能,证明了石墨烯在催化湿式氧化/过氧化领域的巨大潜力.

    • 目前催化湿式氧化/过氧化技术发展迅速,能非常有效地处理难降解有机物,但依旧存在一些问题,如金属离子浸出、非金属催化剂失活以及有机物降解的资源化利用. 根据对现状的分析,提出如下催化湿式氧化/过氧化研究的建议:

      (1)金属与非金属催化剂普遍拥有较理想的催化活性,但金属催化剂在反应过程中会浸出部分金属离子,不利于重复利用;而活性炭易被有机物或其他杂质堵塞孔径,导致内表面的活性组分不能被充分利用而导致催化剂逐渐失活,这是催化剂领域的两大难题. 石墨烯作为一种高效稳定的催化剂,有利于活性组分分散和降低反应壁垒,将成为催化湿式氧化/过氧化领域潜力最大的催化剂之一.

      (2)目前的研究多停留在氧化降解有机物,降解过程中可能形成新的污染物,所以精准调控降解程度和目标产物,实现资源化回收,将成为未来处理难降解有机物的方向之一.

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