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水是人类生存的基础性和战略性物资,而水污染则会直接影响人类的生命健康。酚类污染物是水体中常见的难降解且有毒有害的污染物之一,其中三氯苯酚(trichlorophenol,TCP)因其高毒性和致癌性已被列入中华人民共和国生态环境部、美国环境保护署和欧洲环境署发布的优先控制污染物目录。目前,处理这类污染物的方法主要有光降解法、化学还原法和吸附法等,然而,光降解法操作复杂成本高昂[1],化学还原法和吸附法处理效果较差[2],这些缺点导致它们在实际应用中受到一定程度的限制。
近年来,基于活性氧物种(reactive oxygen species,ROS)的高级氧化技术(advanced oxidation processes,AOPs)因其操作简单、反应速率高、对后续生化处理影响小等优点而受到人们的广泛关注[3-4]。传统的芬顿氧化法是目前研究与应用中最成熟的高级氧化技术之一,主要利用Fe(Ⅱ)与过氧化氢(hydrogen peroxide,H2O2)间的反应生成具有强氧化性的羟基自由基(•OH),从而对各类有机物进行有效降解[5-6]。但在均相体系中,催化剂回收利用难、反应所需pH受限、反应后产生的铁泥易造成二次污染等问题难以解决。针对这些缺陷,研究者逐渐采用含金属的非均相催化剂来替代传统金属离子进行催化反应[7-8]。其中,纳米零价铁(nanoscale zero-valent iron,nZVI)具有还原能力强,成本低,环境友好等优点,是一种理想的非均相催化剂金属[9]。然而,nZVI颗粒因其表面能高、稳定性差而易发生团聚和钝化,这导致其催化性能下降,故有必要对nZVI进行合理的修饰和改性,以提高nZVI的分散性和稳定性[10]。
近期研究表明,nZVI的硫化产物(sulfidized nanoscale zero-valent iron,S-nZVI)能有效提高nZVI的催化活性[11]。该硫化过程是在nZVI表面包裹一层硫化亚铁(FeS),FeS作为屏障保护nZVI,可以减缓nZVI的氧化速率;同时,FeS具有的低电位还可以改善nZVI的电子传输性能[12-13]。然而,在实际应用中,S-nZVI仍然存在易团聚、难回收等缺点。为此,有必要将金属负载在石墨烯、生物碳、膨润土等基体上,进一步提高其分散性和重复使用性。活性炭纤维(activated carbon fibers,ACFs)是一种高微孔碳质载体,与其它载体相比,具有较大的比表面积、丰富的反应性官能团(—COOH、C=C、C—OH和C=O等)以及廉价易回收等性质[14]。在ACFs上负载nZVI能有效提高nZVI的分散性,同时,这种块状的催化材料具有宏观立体结构,回收利用性好,在实际应用中具有独特的优势[15-18]。而协同硫化负载,将S-nZVI负载于ACFs制备得到的活性炭纤维负载硫化纳米零价铁(activated carbon fibers supported sulfidized nanoscale zero-valent iron,ACFs-S-nZVI),则不仅解决了S-nZVI易团聚、难回收的问题,还有望提升S-nZVI的循环使用性能。但迄今为止,还未有关于ACFs-S-nZVI复合材料用于去除水体中有机污染物的报道。
因此,将深入探索ACFs-S-nZVI复合材料的结构与性能之间的关系,并将其用于活化过一硫酸盐(peroxymonosulfate,PMS)降解废水中的TCP。本研究详细探究了硫铁比、反应温度、pH、催化剂和氧化剂投加量等对ACFs-S-nZVI催化性能的影响。同时,通过大量的对比实验和电子顺磁共振波谱(EPR)分析探讨了ACFs-S-nZVI活化PMS降解去除TCP的反应机理。
活性炭纤维负载硫化纳米零价铁用于活化PMS降解三氯苯酚
Degradation of trichlorophenol by activated carbon fibers-supported S-nZVI activating PMS
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摘要: 针对纳米零价铁(nanoscale zero-valent iron,nZVI)在反应过程中易团聚、易钝化和难回收等问题,采用比表面积大、化学稳定性好的活性炭纤维(activated carbon fibers,ACFs)作为载体,以连二亚硫酸钠(Na2S2O4)作为硫化试剂,制备活性炭纤维负载硫化纳米零价铁复合材料(activated carbon fibers supported sulfidized nanoscale zero-valent iron,ACFs-S-nZVI),并考察了复合材料活化过一硫酸盐(peroxymonosulfate,PMS)降解三氯苯酚(trichlorophenol,TCP)的催化性能、反应机理及重复使用性能。材料表征结果表明,硫化不仅可以缓解nZVI的氧化,还可以加快nZVI的电子传输速率,而引入ACFs载体则极大地提高了硫化纳米零价铁(sulfidized nanoscale zero-valent iron,S-nZVI)的分散性和稳定性。通过条件探究实验发现,当反应条件设置为S/Fe比0.3、温度25 ℃、pH=6、催化剂投加量0.5 g·L−1、PMS投加量0.5 mmol·L−1时,ACFs-S-nZVI/PMS体系表现出最佳的催化性能,在30 min内即可去除99.14% 10 μmol·L−1的TCP,显著优于未改性的样品。此外,该体系具有较宽的pH(2~10)适应范围、优异的抗离子干扰能力以及出色的重复使用性能。电子顺磁共振波谱(EPR)和自由基淬灭实验结果表明,羟基自由基(•OH)、硫酸根自由基(SO4•−)和超氧自由基(O2•−)参与了ACFs-S-nZVI/PMS体系的催化反应。综上可知,ACFs-S-nZVI具有催化性能优异、稳定性高、环境适应性强等优点,在催化降解有机污染物方面表现出较好的应用前景。Abstract: In order to cope with the problems of severe agglomeration, easy passivation and difficult recovery of nanoscale zero-valent iron (nZVI) during reaction process, the activated carbon fibers (ACFs) with large specific surface area and excellent chemical stability were taken as carriers, and sodium dithionite (Na2S2O4) was taken as the sulfidation reagent to successfully fabricate activated carbon fibers-supported sulfidized nanoscale zero-valent iron composite (ACFs-S-nZVI). Subsequently, the catalytic performance, reaction mechanism and recycling performance of this composite catalyst on trichlorophenol (TCP) degradation by activating peroxymonosulfate (PMS) were further investigated. The results of material characterization showed that the deposited ferrous sulfide (FeS) during the vulcanization process could not only mitigate the oxidation of nZVI, but also accelerate the electron transfer rate of nZVI, and the introduction of ACFs carrier substantially improved the dispersion and stability of sulfidized nanoscale zero-valent iron (S-nZVI). Through operational condition experiments, it was found that the ACFs-S-nZVI/PMS system exhibited the best catalytic performance when the reaction conditions were set to S/Fe ratio of 0.3, 25 ℃, pH of 6, catalyst dosage of 0.5 g·L−1 and PMS dosage of 0.5 mmol·L−1, and 99.14% of TCP with a concentration of 10 μmol·L−1 could be degraded within 30 min, which was significantly better than that of the unmodified sample. In addition, the system had a wide pH adaptation range (pH=2~10), an excellent anti-ion interference ability and an outstanding recycling performance. The electron paramagnetic resonance (EPR) spectroscopy and radical quenching experiments showed that hydroxyl radical (•OH), sulfate radical (SO4•−) and superoxide radical (O2•−) participated in the catalytic reaction of ACFs-S-nZVI/PMS system. In summary, ACFs-S-nZVI has the advantages of excellent catalytic performance, high stability, and strong environmental adaptability. Therefore, it shows great application prospects in catalytic degradation of organic pollutants.
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