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高级氧化技术(AOPs,advanced oxidation processes)是一种以产生具有强氧化能力的羟基自由基(·OH)为特点,将难降解有机物氧化成小分子物质或完全矿化的污染物处理技术[1-2]。近年来,高级氧化水处理技术得到了较快发展,广泛应用于工业废水处理、土壤和地下水修复、污水深度处理等。通过激活氧化剂(如臭氧、过氧化氢、过硫酸盐等)生成活性自由基,可将有机物逐步降解为小分子物质,如二氧化碳、水和无机盐等,甚至完全矿化,达到高效去除有毒有害污染物的目的[3-5]。
芬顿法是高级氧化工艺(AOPs)的一种,其通过亚铁离子和过氧化氢反应,生成羟基自由基。羟基自由基具有极强的氧化性,能有效降解有机污染物[6-7]。传统的芬顿反应工艺优势明显,但仍存在诸多缺陷,如pH响应范围窄(pH 2.8—3.5最佳)、铁泥产量大、H2O2利用率低等[8]。芬顿氧化技术对有机物的降解效率与反应体系的pH密切相关。在氧化还原体系中,H+和OH−离子影响铁的水解形态,进而影响芬顿反应。当pH>3.5时,溶解性Fe2+形成氧化物、氢氧化物等物种,致使催化活性降低;而当pH过低时,H2O2质子化形成H3O2+,无法与铁物种反应产生·OH,这些因素限制了传统芬顿反应的适用范围。在芬顿反应过程中,不可避免地会产生体系pH的波动,因而铁物种沉淀导致产生大量铁泥。除这些缺点外,传统芬顿反应H2O2的利用率低也是限制芬顿技术应用的一大难题。在反应中,Fe3+与H2O2反应产生催化活性很低的HO2·,并会进一步与·OH反应使其淬灭,影响了整个体系的降解能力。
为了克服均相芬顿反应的缺陷,研究人员将目光聚焦于催化剂及激活氧化剂两方面。一方面,研究人员使用其他过渡金属作为均相芬顿反应的催化剂,如:Cu2+,其溶解的pH值范围在3—7[9-10]。另外,研究人员采用固体催化剂活化H2O2,使得整个非均相芬顿体系不产生铁泥,且可适用于较宽的pH范围,克服了传统均相芬顿的缺点[7, 11]。另一方面,过氧二硫酸盐(PDS)和过氧一硫酸盐(PMS)在废水处理中也受到越来越多研究人员的关注。过硫酸盐是强氧化剂,能够直接降解有机污染物。但是过硫酸盐与H2O2类似,其与大多数有机污染物的直接反应很慢,因此,在应用中通常将过硫酸盐进行活化,产生氧化性更强的SO4−·或·OH。SO4·−具有高的氧化还原电势,其自由基寿命比·OH长,可在宽pH(2—12)范围内应用[12]。在过硫酸盐的活化过程中,部分活化剂的表面氧化还原电势驱动单电子转移的链反应,以裂解过氧化物键形成SO4·−和·OH,并通过这些瞬态自由基氧化降解有机污染物[13]。
与活化过氧化氢的芬顿系统相比,过硫酸盐类芬顿系统涉及了更复杂的氧化途径,除了存在自由基氧化途径外,还存在着多种非自由基氧化途径(如单线态氧1O2),其氧化途径取决于水质和活化剂的类型。过硫酸盐被碳基材料[14-15],N掺杂的石墨烯[16-17],硼掺杂有序介孔碳[18],生物炭[19]等活化时,某些氧化路径不仅涉及自由基,还有非自由基1O2,在中性条件下即可选择性氧化有机物。除此之外,还有文献报道碳纳米管活化过硫酸盐的非自由基途径并非单线态氧,而是通过碳形成的络合物介导电子从有机物转移至过硫酸盐,从而使有机物降解[15, 20-23]。
本文综述了类芬顿催化剂的设计、催化活化原理与机制等方面的研究进展,介绍了基于过渡金属及其氧化物的催化剂,及其在水处理中降解有机污染物的机理,并提出了存在的问题及研究展望。
类芬顿反应的催化剂、原理与机制研究进展
Research progress on catalysts, principles and mechanisms of Fenton-like reactions
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摘要: 芬顿氧化法是高级氧化技术的一种,通过产生高活性的自由基降解水中有机污染物,在水处理领域受到广泛关注。近年来,针对芬顿反应的研究主要集中在开发高活性、高稳定性的非均相类芬顿催化剂。本文综述了近年来非均相芬顿催化领域的研究进展,分析了铁基、锰基、铜基、钴基等过渡金属及其氧化物,以及负载型催化剂的构造特点、催化机理和作用机制,探讨了过氧化氢和过硫酸盐的催化活化路径与方式,总结了非均相类芬顿催化体系实现高效降解有机污染物的机理与途径,以期为高效类芬顿催化剂的设计和应用提供科学依据.Abstract: Fenton processes is an advanced oxidation technology, which degrades organic pollutants by generating highly active free radicals, and has gained much attention in the field of wastewater treatment. In recent years, research on Fenton reaction has focused more on the development of heterogeneous Fenton-like catalysts with high activity and high stability. This article reviews the research progress in the field of heterogeneous Fenton catalysis in recent years, and analyzes the structure characteristics and catalytic mechanism of iron-based, manganese-based, copper-based, cobalt-based and other transition metals and their oxides, as well as supported catalysts. Extensive discussion on the catalytic pathways of hydrogen peroxide and persulfate, and the mechanism and pathways of the heterogeneous Fenton-like catalytic system to achieve high-efficiency degradation of organic pollutants were provided along with literature of some relevant studies, in order to provide a scientific basis for the design and application of efficient Fenton-like catalysts.
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Key words:
- advanced oxidation /
- heterogeneous Fenton /
- Fenton-like reaction /
- hydrogen peroxide /
- persulfate
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图 1 (a) 磁铁矿Fe3-xMxO4(M = Co2+,Mn2+,Zn2+,Mg2+,Cr3+和Al3+)耦合Fe2+芬顿氧化降解污染物[31],(b) Cu—Fe3O4/Cu/C活化过氧化氢示意图[32]
Figure 1. (a) The degradation of organic pollutants by Fenton oxidation with magnetite Fe3-xMxO4(M = Co2+, Mn2+, Zn2+, Mg2+, Cr3+ and Al3+) coupled Fe2+ [31], (b) The diagram of Cu-Fe3O4/Cu/C in hydrogen peroxide activation [32]
图 2 (a) MnO2晶体活化PDS生成1O2的反应机理[42],(b) Co—Mn协同活化PMS机理图[23],(c) Fe掺杂的锰氧化物活化PMS机理图[44],(d) Ce掺杂锰氧化物活化PDS机理图[45]
Figure 2. (a) The mechanism of PDS activation by MnO2 to generate 1O2 [42], (b) The mechanism of PDS activation by the synergistic effect of Co and Mn [23], (c) The mechanism of PMS activation by Fe-doped manganese oxide [44], and (d) The mechanism of PDS activation by Fe-doped manganese oxide [45]
图 4 (a) CuVOx催化过氧化氢类芬顿反应原理[57],(b) CuNiFeLa—LDHs催化过氧化氢类芬顿反应原理[58],(c) 双反应中心催化剂OH—CCN / CuCo—Al2O3活化过氧化氢机理图[61],(d) CuOMgO / Fe3O4的非自由基途径芬顿反应机理图[63]
Figure 4. (a) Schematic illustration of Fenton reactions using CuVOx[57], (b) The mechanism of H2O2 activation by CuNiFeLa—LDHs[58], (c) The mechanism of H2O2 activation by OH—CCN / CuCo—Al2O3 with dual reaction centers[61], and (d) The mechanism Fenton reaction with CuOMgO / Fe3O4 through non-radical pathway[63]
图 5 (a) Al0—CNTs / CNTs-Fe-Cu / O2系统的催化机理[68],(b) Fe2O3 @ FCNT—H芬顿氧化降解污染物示意图[70],(c) FeCo—MCM-41活化PMS反应机理图[75]
Figure 5. (a) The catalytic mechanism of Al0—CNTs / CNTs-Fe-Cu / O2 system[68], (b) The illustration of pollutants degradation in Fe2O3 @ FCNT-H Fenton process [70], and (c) The mechanism PMS activation by FeCo—MCM-41[75]
表 1 催化剂活化H2O2的原理与机制
Table 1. Principle and mechanism of catalysts for the activation of H2O2
反应类型
Reaction type实现途径
Pathway活化过程
Activation process催化材料
Catalysts参考文献
ReferencesH2O2转化为∙OH 增加表面活性位点 产生表面羟基≡Fe—OH,促进H2O2的吸附,H2O2更易于转化为∙OH 天然磁铁矿 [26] 高分散的Fe3O4纳米颗粒,最大化暴露活性位 Fe3O4—NP @ CNF、Fe3O4 MCNs [34] 过渡金属元素的同
构取代钴和锰可形成氧化还原对Co2+ / Co3+和Mn2+ / Mn3+,增加电导率,
并且钴和锰可以增加表面活性位点密度Fe3−xMxO4
(M=Fe, Co, Mn等)[28,31] Fe是活性中心,Co2+ / Co3+氧化还原对可提高催化活性 CoCr2-xFexO4(X=0.0—2.0) [51] 形成中间体 Ru3+与H2O2反应生成中间产物Ru(OOH)2+,Ru3+→ Ru2+,产生·OH RuI3 [76] 协同作用 掺杂金属元素形成双金属催化结构:调节活性金属物种的含量和
表面电荷分布加速·OH的生成Ce5%CuOy [61] 双金属协同促进活性物种生成:Cu+向Cu2+的快速转化加速Fe2+的
再生Fe3O4 / CuO [77] 钒周围形成富电中心和表面氧空位,加快H2O2活化 CuVOx [57] Co2+和Cu+活化H2O2产生·OH,且Cu+可以将生成的Co3+还原为Co2+,实现了Co2+的再生 CuxCo10-x / CNC [59] 形成Fe—O—C结构加速Fe3+ / Fe2+的氧化还原循环 rGS/FexOy / NCL [67] 缺电子与富电子双
反应中心H2O2接受富电子铜中心的电子,形成∙OH;H2O的电子转移到缺电子氮中心,转化为∙OH OH—CCN / CuCo—Al2O3 [61] H2O2转化为多种自由基 协同作用 Fe(III) / Fe(II)、Cu(II) / Cu(I)与Cu(I) / Cu(0)在超声作用下协同将H2O2活化为∙OH和O2·− Cu—Fe3O4 / Cu / C [32] Cu为主要催化活性位点,Ni2+将电子转移至Cu2+,生成更多的Cu+;La2O3可以同时提供Lewis碱性位点和Lewis酸性位点,活化H2O2产生·OH、O2·-和1O2 CuNiFeLa—LDHs [58] Al0—CNTs起活化O2的作用,CNTs—Fe—Cu不仅能活化O2,还可将H2O2催化生成·OH和O2- Al0—CNTs / CNTs—Fe—Cu / O2 [68] 单金属变价 Cu2+被还原为Cu+生成O2·- 3D Cu @ CuO纳米线 [55] Cu2+被还原为Cu+生成O2·-,Cu+通过Cu0再生 Cu2O—Cu / C(CCMs) [54] 形成中间体 Co2+络合物活化H2O2为∙OH和O2·−,或形成中间体直接降解有机
污染物Co(II)络合物 [78] 表 2 催化剂活化过硫酸盐的原理与机制
Table 2. Principle and mechanism of catalysts for the activation of persulfate
反应类型
Reaction type实现途径
Pathway活化过程
Activation process催化材料
Catalysts参考文献
References
过硫酸盐生成∙OH和SO4·−单金属变价 通过高价态锰氧化物(Mn4+)还原和低价态锰氧化物(Mn3+)氧化生成SO4-·,SO4-·与H2O的反应生成·OH。 氧化锰八面体分子筛(OMS-2) [40, 79] 超声Cu0转化为Cu+。 nZVC [80- 81] Co2+和Co3+之间单电子转移 。 Co3O4 @ CNT [69] 多金属协同作用 形成≡CoIII / ≡CoII , ≡MnIII / ≡MnII, ≡MnIII / ≡CoII和≡MnIV /≡MnIII等氧化还原电对。 CoxMn3−xO4 [23] Fe—Co协同。 Fe36Co36Si4.8B19.2Nb4 [52] Co—Cu协同。 CoCuAl-LDO [53] Fe—Cu协同。 Fe20Cu80 [82] 形成中间体 亚稳态锰中间体中的超氧离子和自由基氧化或重组产生1O2。 α-MnO2和β-MnO2 [42] 形成表面羟基,与过硫酸根反应生成亚稳态的铜中间体,进而被O2·-氧化生成1O2。 CuOMgO / Fe3O4 [63] 过硫酸盐生成∙OH和SO4·− 强化电子转移 金属掺杂形成氧空位,促进界面电子转移,利于生成∙OH和SO4·−。 HPCMO [45] 过硫酸盐生成∙OH、SO4·−和1O2 多金属协同作用 Co、Mn协同晶格氧和氧空位。 La2CoMnO6−δ [49] Co、Fe协同氧化还原循环活化过硫酸根 FeCo—MCM-41 [75] 形成中间体 Co3O4表面上生成CoOH+,通过氢键与过硫酸根结合,生成SO4∙-。再进一步转化生成·OH和1O2。 纳米立方Co3O4晶体 [50] -
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