海绵铁三金属降解对硝基苯酚的影响因素及催化机理

李方芳,鞠勇明,邓东阳,贾文超,丁紫荣,雷国元

海绵铁三金属降解对硝基苯酚的影响因素及催化机理

李方芳1,2,鞠勇明2,3,邓东阳2,贾文超2,丁紫荣2,雷国元1*

(1.武汉科技大学资源与环境工程学院,湖北 武汉 430000；2.生态环境部华南环境科学研究所,广东 广州 510655；3.生态环境部南京环境科学研究所,江苏 南京 210042)

通过超声置换反应制备钯铜共修饰海绵铁三金属催化剂(Pd-(Cu-s-Fe0)),研究了三金属负载顺序、金属负载量、材料投加量以及重复利用对材料降解对硝基苯酚(PNP)的影响,并利用扫描电子显微镜(SEM)和X射线光电子能谱(XPS)表征材料表面结构特征.结果表明,Pd-(Cu-s-Fe0)催化活性高于Cu-(Pd-s-Fe0)和(Cu-Pd)-s-Fe0;Cu和Pd的最佳负载量分别为5%和0.025%.在100mL初始浓度为100mg/L的PNP溶液中投加3g Pd-(Cu-s-Fe0)并超声反应30min,PNP的降解率超过80%,降解反应基本符合一级动力学方程;Pd-(Cu-s-Fe0)材料循环利用4次表现出良好的循环利用性能.此外,PNP的主要催化还原产物是对氨基苯酚(PAP),主要的反应路径是催化还原反应.

海绵铁;Pd-(Cu-s-Fe0)三金属；对硝基苯酚(PNP)；降解机理

对硝基苯酚(PNP)被广泛用作染料、农药和防腐剂的原料或中间体[1],具有溶解度高和结构稳定等特点,在自然条件下很难被降解[2].作为一种环境内分泌干扰物,PNP具有显著的高毒性和致癌性[3-5],长期接触含PNP废水能造成机体内分泌系统功能紊乱[3].目前PNP的降解方法主要有微生物法[1-2]、化学氧化还原法[2-8]、微波辅助催化氧化[6]和电化学氧化[7]等.然而,微生物降解周期长,微波和电化学氧化对设备要求高且能耗大.零价铁具有无毒、含量丰富、强还原性,在含PNP废水处理的应用成为研究热点.纳米零价铁比表面积大、还原性能强,但在应用过程中容易团聚而降低活性,且纳米零价铁的价格昂贵,成为限制其实际应用的因素.因此探索高效、经济的降解材料处理PNP具有重要意义.

海绵铁(s-Fe0)作为一种新型零价铁材料,具有不易团聚、价格低廉等优点[8],已成功应用于含卤代有机物、芳香族硝基化合物等各类废水的处理. 通过在s-Fe0表面负载贵金属形成双金属和三金属催化剂能够显著提高材料的催化活性[9].已有研究表明,零价铁粉(ZVI)、铁铜双金属(Fe/Cu)和铁铜银三金属(Fe-Cu-Ag)能有效催化还原降解PNP[8-11].然而,零价铁粉(ZVI)的腐蚀产物容易沉积在零价铁表面上形成钝化膜并显著降低零价铁表面催化降解PNP的活性,在£200W的超声功率下,零价铁粉(ZVI)几乎无法降解PNP[8].铁铜双金属(Fe/Cu)[10]和铁铜银三金属(Fe-Cu-Ag)[11]降解PNP的反应活性与铁基材料上铜银金属负载量、负载顺序和Na2SO4溶剂浓度显著相关.例如:Fe/Cu双金属在Cu负载量为6%时,几乎不能降解PNP. Fe-Cu-Ag三金属降解PNP需要0.75%的Ag负载量,材料成本显著增加.贵金属钯(Pd)作为一种优良的产氢材料,其催化产氢性能远高于Cu和Ag等金属.目前,暂未检索到利用Cu-Pd双金属修饰零价铁进行催化降解PNP的研究.

本研究通过超声辅助还原法制备微量Cu、Pd负载的铁铜双金属和Fe-Cu-Pd三金属材料.考察制备过程中调控不同参数对三金属催化活性的影响,研究三金属在超声条件下(200W)催化降解PNP的主要影响因素,探索了三金属的催化反应机理,为海绵铁材料在去除酚类有机物的应用提供理论基础.

1 材料与方法

1.1 试剂和仪器

试剂:对硝基苯酚(PNP;天津大茂化学试剂厂),硝酸铜(天津市大茂化学试剂厂),氯钯酸钾(上海麦克林生化科技有限公司),海绵铁(河南西尔环保科技有限公司),实验用水为超纯水(Milli-Q 超纯水系统).

仪器:紫外可见分光光度计(UV-2450,日本岛津公司);扫描电子显微镜(SEM;S-3400N,日本HITACHI);X射线光电子能谱(XPS; ESCALAB250Xi,美国Thermo公司);超声波清洗机(KQ5200E,昆山市超声清洗有限公司);高速离心机(SIGMA,4K15),pH计(FE20,上海梅特勒托利多有限公司),便携式溶解氧测定仪(雷磁JPB-607A,上海仪电科学仪器股份有限公司).

1.2 实验方法

1.2.1 材料制备 Cu-s-Fe0和Pd-s-Fe0双金属材料的制备:双金属的制备参照之前的制备方法[12-13],将s-Fe0投加到装有适量2M Cu(NO3)2溶液中进行置换反应,溶液由蓝色变为黄色后结束反应,用过量超纯水反复冲洗,得到Cu-s-Fe0双金属颗粒;将s-Fe0投加到装有适量浓度为500mg/L的氯钯酸钾溶液中进行置换反应,溶液由橘黄色变为无色后结束反应,用过量超纯水反复冲洗,得到Pd-s-Fe0双金属颗粒.

Pd-(Cu-s-Fe0)三金属材料的制备:将制备好的Cu-s-Fe0双金属投加到装有适量浓度为500mg/L的氯钯酸钾溶液中进行置换反应,溶液由橘黄色变为无色后结束反应,用过量超纯水反复冲洗,得到Pd-(Cu-s-Fe0)三金属颗粒.

Cu-(Pd-s-Fe0)三金属材料的制备:将制备好的Pd-s-Fe0投加到装有适量2mol/L Cu(NO3)2溶液中进行置换反应,溶液由蓝色变为黄色后结束反应,用过量超纯水反复冲洗,得到Cu-(Pd-s-Fe0)三金属颗粒.

(Cu-Pd)-s-Fe0三金属材料的制备:将活化后的s-Fe0投加到同时装有适量500mg/L的氯钯酸钾和2mol/L Cu(NO3)2溶液中进行置换反应,当溶液变为黄色后结束反应,用过量超纯水反复冲洗,得到(Cu- Pd)-s-Fe0三金属颗粒.置换反应方程式如下:

1.2.2 PNP降解实验 以250mL烧杯为反应器,每个反应器中加入一定浓度的PNP溶液100mL和一定量的海绵铁材料,置于超声反应器中进行降解反应,每隔一定时间取样离心后,再取上清液上机测量.

1.3 双金属和三金属催化剂的表征

利用扫描电子显微镜(SEM)和X射线光电子能谱(XPS)对5%-Cu-s-Fe0双金属和Pd负载量为0.025%的三金属颗粒的表面形貌结构进行分析.

1.4 PNP浓度检测

PNP采用紫外分光光度计检测,检测波长为316nm.本实验采用相对浓度/0表示不同影响因素对PNP的去除效果,表示时刻溶液中PNP的剩余浓度,0则表示溶液的初始浓度.采用一级动力学模拟PNP的降解,方程式为:

2 结果与讨论

2.1 双金属和三金属颗粒的表征

不同负载顺序三金属的SEM图如图1所示.由Cu-s-Fe0双金属的Cu元素XPS分峰处理(图1(a)插图)可知,超声置换法成功将Cu沉积到s-Fe0表面,但Cu沉积到s-Fe0表面过程中生成大量CuO和Cu2O,这是由于s-Fe0、Cu-s-Fe0能有效还原溶液中的NO3-[14],从而导致双金属表面上负载新生成的Cu单质被氧化.根据图1所示的SEM可知,相对于Cu-s-Fe0双金属材料,三金属表面疏松,并且出现一定量的微小颗粒,有利于增大材料比表面积,促进材料与污染物接触.而Cu-(Pd-s-Fe0)和(Cu-Pd)-s-Fe0相对于Pd-(Cu-s-Fe0)表面则出现部分团聚,因此Pd-(Cu-s-Fe0)表面结构有利于和污染物的接触,理论上具有更高的催化反应活性.

图1 不同材料扫描电镜(SEM)图片(´2000)

图2 3种海绵铁三金属XPS表征

图2展示了不同负载顺序三金属的XPS数据.由图2(a)可知,3种三金属表面Fe主要为Fe的氧化态[15];由图2(b)可知,3种三金属表面Cu的结合能约为932.58和952.38eV,分别代表Cu(0)和Cu(Ⅰ)[16], (Cu-Pd)-s-Fe0和Pd-(Cu-s-Fe0)均在943.78和962.28eV处的特征峰代表Cu(Ⅱ)[17].由图2(c)可知,(Cu-Pd)-s-Fe0和Pd-(Cu-s-Fe0)表面Pd的结合能主要约为335.4和340.8eV,为Pd(0)[18].Cu-(Pd- s-Fe0)由于Cu将Pd覆盖,并未有效检测出Pd.

2.2 PNP降解的影响因素

2.2.1 不同材料对PNP降解的影响 如图3(a)所示,当反应30min后,s-Fe0体系中PNP的去除率为62.4%,而Cu-s-Fe0、Pd-s-Fe0和Pd-(Cu-s-Fe0)对PNP降解率分别为26.9%、63.5%和78.8%.s-Fe0负载Cu材料对PNP的降解效率明显下降,负载Pd材料对PNP降解效率无明显提高.这是由于本实验选用Cu(NO3)2作为铜源,Cu-s-Fe0可有效还原硝酸根[19],从而生成大量铜氧化物使材料钝化(详见图1(a)插图).而Pd作为一种高效产氢材料[20],能产生大量活性氢原子[H]abs并还原铜氧化物,从而使Pd- (Cu-s-Fe0)三金属材料具有更高催化还原活性.因此,结果推测Pd-(Cu-s-Fe0)降解PNP的催化位点主要在铜上.

2.2.2 金属负载顺序对PNP降解的影响 如图3(b)所示,反应30min后,(Cu-Pd)-s-Fe0、Cu-(Pd-s-Fe0)和Pd-(Cu-s-Fe0)对PNP的降解率分别为34.9%、26.8%和78.8%.Pd-(Cu-s-Fe0)具有最高活性,与已有的三金属活性研究一致[21].这是由于Fe/Pd氧化还原电位(θ(Fe/Pd) =1.398V)大于Fe/Cu氧化还原电位(θ(Fe/Cu) =0.7889V)[11].当先负载Cu后再负载Pd,Pd产生活性氢原子[H]abs还原材料表面的各种氧化物,形成Fe/Pd最大电位差,调整负载顺序则无法有效形成原电池催化体系,降低了催化反应效率.由表1可知,三金属降解PNP的过程遵循一级动力学,且动力学常数Pd-(Cu-s-Fe0)>(Cu-Pd)-s-Fe0>Cu-(Pd-s-Fe0).

2.2.3 Cu负载量对PNP降解的影响 如图3(c)所示,当Pd负载量为0.025%时,Cu负载量从1%增加到5%,PNP去除率由67.5%增加至78.8%;当Cu负载量由5%增加至10%,PNP去除率降低为63.0%,这与此前铁铜双金属的规律基本一致[12].由表1可知,一级动力学常数obs也随着Cu负载量增加呈现先增后减的趋势.这是由于Cu在s-Fe0表面形成原电池促进s-Fe0腐蚀失去电子;而继续增加Cu负载能在s-Fe0表面形成致密的Cu层,抑制内部s-Fe0和溶液接触,从而抑制s-Fe0的腐蚀、降低材料活性[10].

表1 不同降解条件下的拟一级动力学参数变化规律

表2 水溶液中PNP:O2:H+的物质的量比

2.2.4 Pd负载量对PNP降解的影响 如图3(d)和表1所示,随着Pd负载量不断增加,材料催化活性和obs也呈现出先增加后减少的规律.Pd负载量为0.015%、0.025%、0.5%、0.075%和0.1%的三金属对PNP的降解率分别为47.5%、78.8%、77.5%、69.3%和67.8%.材料表面负载Pd后能显著提高材料产氢能力并提高催化效率,而当Pd负载超过0.025%时,进一步提高负载量会导致材料产氢过快并在材料表面形成一层氢气膜[22]阻止目标物和材料表面接触,从而抑制PNP的降解.

2.2.5 材料投加量对PNP降解的影响 如图3(e)所示,随着三金属投加量从10g/L增加到30g/L, 100mg/L PNP反应30min后降解率从32.1%显著增加到83.2%;当投加量从30g/L继续增加到50g/L, PNP降解率仅增加0.7%.这表明当PNP浓度一定时,增加三金属投加量能显著增加有效活性位点和体系中原电池数量;当三金属投加量达到30g/L后,体系中腐蚀电池数量已经接近饱和状态,继续提高投加量无法显著提高PNP的降解效率.因此,三金属催化剂投加量选择为30g/L.

图3 不同单因素对材料降解PNP的影响

2.2.6 重复利用对PNP降解的影响 如图3(f)所示.当Pd-(Cu-s-Fe0)三金属材料在最优反应参数下持续循环4个周期,PNP去除率分别为83.2%、82.0%、79.9%和77.2%.结果表明,在4个循环利用周期内,三金属对PNP的去除率保持在75%以上,表明材料具有良好的循环利用稳定性.与纳米零价铁nZVI相比(8000元/kg),s-Fe0的价格低廉(5000元/t)[23],并且在制备三金属过程中不需要氮气的保护.因此,海绵铁催化剂具有相对较高且稳定的活性,在含PNP废水处理中具有良好的应用前景.

2.3 三金属催化机理

零价铁催化降解PNP主要包括3条路线:(1)由三金属表面转移的电子和生成的活性氢原子[H]abs将PNP中的硝基(-NO2)还原为氨基(-NH2)[24];(2)溶液中溶解氧得到电子生成羟基自由基(·OH),将PNP氧化降解为NO3-和小分子酸[25-26];(3)先由活性氢原子[H]abs将硝基(-NO2)还原为氨基(-NH2),再由羟基自由基(·OH)将氨基(-NH2)氧化为NO3-和小分子酸[27].图4(a)插图呈现了降解反应过程中DO、pH值和铁离子浓度的变化.反应30min后,溶液中DO由7.8mg/L降至0.5mg/L,pH值无明显变化,铁离子在前6min迅速增加为12.89mg/L,随后略微下降,这是由于材料在降解过程中不断释放Fe2+,极易被氧化为Fe3+随后生成Fe(OH)3沉淀,从而维持溶液pH值和铁离子浓度基本不变.如表2所示,溶液中PNP : O2: H+由884.56 : 299.88 : 1降低86.35 : 8.01 : 1,这表明, DO能够有效竞争Pd-(Cu-s-Fe0)转移电子并引发一系列降解反应.

紫外-可见光吸收波谱表明,316nm处吸收峰主要是由苯环和硝基(-NO2)的共轭引起[10],227nm处吸收峰是由于单环芳香烃苯环的π-π*跃迁引起[28].图4(a)记录了PNP降解过程中的全波扫描波谱变化.在30min降解过程中,316nm处峰的强度随反应时间增加而逐渐下降,并蓝移至297nm.如图4(b)可知,对氨基苯酚(PAP)的浓度随降解反应时间逐渐增加,反应30min后PAP浓度为79.75mg/L,这表明Pd-(Cu-s-Fe0)能有效催化还原-NO2为-NH2,与已有研究一致[29].其次,由图4(a)可知,227nm处吸收峰强度略有下降,这表明少量苯环被羟基自由基(·OH)氧化破坏[30],这与图4(b)中TOC数据略有下降相符.因此,Pd-(Cu-s-Fe0)材料催化降解PNP是还原为主,氧化为辅的过程,催化降解机理如图5.

图4 30g/L Pd-(Cu-s-Fe0)降解100mL 100mg/L PNP过程中各因素随时间的变化

图5 Pd-(Cu-s-Fe0)催化降解PNP机理

3 结论

3.1 Pd-(Cu-s-Fe0)比Cu-s-Fe0和Pd-s-Fe0对PNP具有更好的降解效果.相同条件下,Pd-(Cu-s-Fe0)比Cu-s-Fe0和Pd-s-Fe0的降解效率分别高51.9%和15.3%,且循环利用4次后, 其降解效率仅下降6%.

3.2 三金属材料的催化活性与金属负载顺序有关,最优三金属为Pd-(Cu-s-Fe0),且最佳负载量为5%Cu和0.025%Pd,投加量为30g/L.

3.3 Pd-(Cu-s-Fe0)催化降解PNP的反应过程遵循一级动力学规律,PNP的主要降解途径为还原催化降解,主要降解产物为PAP.

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Study on influencing factors and catalytic mechanism of p-nitrophenol degradation with sponge iron-based tri-metals.

LI Fang-fang1,2, JU Yong-ming2,3, DENG Dong-yang2, JIA Wen-chao2, DING Zi-rong2, LEI Guo-yuan1*

(1.Department of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430000, China；2.South China Institute of Environmental Science, Ministry of Ecology and Environment of the People’s Republic of China, Guangzhou 510655, China；3.Nanjing Institute of Environmental Science, Ministry of Ecology and Environment of the People’s Republic of China, Nanjing 210042, China)., 2021,41(10)：4670～4676

Pd-(Cu-s-Fe0) trimetals were synthesized adopting with displacement reactions under ultrasonic conditions, and the surface structure of the aforementioned materials was further characterized with scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Moreover, the effects of noble metal loading sequence, the loading amount, input dosage and recycling reuse for the degradation of p-nitrophenol (PNP) was studied in detail. The experimental results show that the catalytic activity of Pd-(Cu-s-Fe0) was higher than that of Cu-(Pd-s-Fe0) and (Cu-Pd)-s-Fe0. The loading amounts of Cu and Pd were optimized as 5% and 0.025%, respectively. Under the optimized conditions including 30g/L of Pd-(Cu-s-Fe0), the removal content of PNP (100mL, initial concentration of 100mg/L) reached more than 80% after 30min of ultrasonic reactions, and the degradation reactions conformed to a pseudo-first-order kinetics equation. Furthermore, after 4times of recycling tests, Pd-(Cu-s-Fe0) showed good recycling performance. Based on the UV-visible spectral variations and high-performance liquid chromatography, we proposed the degradation mechanism mainly via catalytic reductions of PNP into p-aminophenol (PAP).

sponge iron；Pd-(Cu-s-Fe0) trimetal；p-nitrophenol (PNP)；degradation mechanism

X703.5

A

1000-6923(2021)10-4670-07

李方芳(1997-),女,湖北荆州人,武汉科技大学硕士研究生,主要从事海绵铁材料降解机理研究.发表论文1篇.

2021-02-04

国家重点研发计划(2019YFE0111100);广东省国际合作项目(2018A050506045);广东省基础与应用基础研究基金资助项目(2020A1515010969);公益性科研院所专项项目(GYZX210301)

* 责任作者, 教授, leiguoyuanhit@126.com