近年来,抗生素已被广泛应用于医疗行业,在人类和动物疾病防治等方面发挥了极大的作用(丁丽丹等,2021).抗生素是一类由细菌、霉菌等微生物代谢产生或人工合成的能够杀灭其他微生物的化学物质(张玮玮等,2013),按化学结构的不同可分为四环素类、多肽类、大环内脂类等(Bu et al.,2016).其中,四环素(tetracycline,简写为TC)是世界上应用很广泛的抗生素之一(Martinez,2009),因其难生物降解性,常常在地表水、土壤和地下水中被检测到(蒋海燕等,2020),对生态环境和人类健康造成严重的威胁(韩爽等,2021).
近年来基于臭氧、Fenton、过硫酸盐等的各种高级氧化技术已逐渐成为水污染控制领域的研究热点.其中,臭氧催化氧化反应快速高效且臭氧分解产物清洁,被视为一种极具潜力的废水处理方法.臭氧是一种非常活泼的气体,与氧气相比,它更容易溶于水.溶解后的臭氧以两种不同的方式与污染物发生反应,即臭氧分子可以直接选择性降解有机物,或通过生成二次氧化剂,如羟基自由基等非选择性降解有机物(Malik et al.,2019).由于臭氧的制造成本较高,其实际应用受到了限制,而引入催化剂的臭氧催化氧化技术大大提高了臭氧利用率,因此被广泛应用于工业水处理领域(任斌等,2020).一般情况下,催化臭氧化过程通常使用金属离子或金属氧化物作为催化剂(Wang et al.,2020).Chen等(2019)制备出的Fe3O4/Co3O4复合材料通过催化臭氧化系统显著提高磺胺甲恶唑的矿化效果,达到了60%的TOC去除率.但是金属氧化物作为催化剂单独使用也会出现活性位点不足,比表面积有限等缺点.将金属或金属氧化物负载于载体上,载体表面提供大量活性位点,从而表现出更高的活性(Yang et al.,2016).金属成分在赋予基体优异催化性能的同时,不可避免会出现金属浸出问题(李凯等,2021),因此,需要制备出一个具有较高催化活性且良好稳定性的催化剂.彭娟等将Mn、Fe、Ce 3种金属负载于活性炭(Activated Charcoal,简写为AC)上,使用该催化剂臭氧催化氧化120min去除了制药废水中80%的CODCr(彭娟等,2019).Li等用Fe-MCM-48催化剂结合O3处理双氯芬酸,60 min时TOC的去除率约为49.9%,是单一臭氧处理的2.0倍(Li et al.,2018).
在负载金属选择上,铁是一种十分常见的催化剂,铁不仅可以在臭氧催化体系中表现出较高的催化活性,还因其在自然界中含量丰富,廉价易得且易于合成(靳志豪等,2021),受到了越来越多的关注.此外,一些铁化合物具有特殊的特性,如Fe3O4具有磁性.近年来,Fe3O4纳米颗粒由于其低毒性、高磁性、高电性和可循环利用等优点(Zhang et al.,2020),作为催化剂受到越来越多的关注.Wu等制备的Fe3O4@SiO2@Mg(OH)2在臭氧催化氧化磺胺噻唑时,10 min内可去除99.0%以上的TOC,60 min内可去除40.1%(Wu et al.,2020).
图3b为Fe 2p的谱图,在728.1和724.6 eV 处的特征峰(Peak4/5)对应于Fe 2p1/2(Li et al.,2020;黎素等,2021),由Fe(II)氧化物和Fe(III)氧化物形成(Shwan et al.,2015);714.7和710.9 eV 处的特征峰(Peak1/2)分别对应Fe2+卫星峰和Fe 2p3/2(Yamashita et al.,2008);Fe 2p谱中的Fe 2p3/2和Fe 2p1/2的结合能峰值分别为710.9和724.6 eV,这两个峰的之间的距离大于13 eV,证明催化剂中存在Fe3+(Ma et al.,2016;Yu et al.,2019);同时在结合能为718.8和732.7 eV处也获得明显卫星峰(Peak3/6),进一步证实材料中有Fe3+(张帆等,2021).因此,复合催化剂中的铁的存在形态是Fe(II)和Fe(III)的氧化物.根据XPS图各电位峰面积统计,在催化剂的铁元素中,Fe(II)和Fe(III)的比例为0.58∶0.42,接近于1∶1.催化剂制备原料之一为Fe3O4,理论上Fe(II)和Fe(III)的比例应为1∶2,说明催化层中Fe的价态在制备过程中发生了变化,有一部分Fe(III)被还原为Fe(II),这是还原性的制备环境所致.催化层在与臭氧分子作用,产生活性自由基过程中,涉及到电子转移,电子越容易转移,生成的自由基越多.推测Fe(II)和Fe(III)接近1∶1的价态比例有利于电子转移和活性自由基的生成.
图3c为Ce 3d的谱图,在882.1~898.1和900.5~916.5 eV处的结合能对应于Ce 3d 5/2和Ce 3d 3/2(Li etal.,2020). 其中,886.9、897.9 和904.4 eV 处特征峰(Peak1/3/5)对应Ce(III)氧化物;888.2、901.2、907.0 和916.6 eV的峰(Peak2/4/6/8)对应于Ce(IV)氧化物相关(He et al.,2018 ;Wu et al.,2018),这可能是由于催化剂在煅烧制备过程中Ce(III)发生了氧化,也可能是在烘干和保存过程中一部分Ce(III)被氧化(Sun et al.,2019).其中,Ce(III)和Ce(IV)比例约为0.49∶0.51.
催化层中存在大量的氧空位和适宜比例的Fe(II)和Fe(III)、Ce(III)和Ce(IV)两对氧化-还原体系,对于催化过程中增加催化活性位点数量和提高电子转移能力,从而促进·OH的生成具有重要意义.氧空位数量和氧化-还原体系是提高催化剂性能的关键影响因素(Wang et al.,2019),富含氧空位并且能够促进电子转移和O3离解是催化剂展示出优异催化性能的主要原因(Guo et al.,2019).关于该催化剂性能有待进一步的深入研究.
据此,可以将Fe3O4-CeOx/AC的催化活性归因于以下因素:首先是催化剂具有较大的比表面积(Chen etal.,2019);其次,具有丰富的氧空位作为与O3作用的催化点位;再次,特殊的制备条件得到了适宜比例的Fe(II)和Fe(III)、Ce(III)和Ce(IV)活性层,增强了电子传递能力,有利于催化促进·OH等强氧化剂的产生(Park et al.,2003).此外,在水溶液中,金属氧化物表面容易通过水分子的解离性化学吸附而发生羟基化(Tamura et al.,1999).这些羟基可以释放质子,充当Brönsted酸位.同时,当吸附的水分子被解吸时,会形成金属阳离子和配位不饱和氧,分别充当路易斯酸和路易斯碱位,金属氧化物的表面羟基和这些位点都可以有效提升催化效率(Wang et al.,2017).
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