石墨烯负载铁镍复合材料去除水中的2,4-二氯酚

黄雪征,张永祥,张大胜,朱新锋,李厚运

石墨烯负载铁镍复合材料去除水中的2,4-二氯酚

黄雪征1,2,3*,张永祥2,张大胜4,朱新锋1,李厚运1

(1.河南城建学院,河南省水体污染防治与修复重点实验室,河南 平顶山 467000；2.北京工业大学城市建设学部,北京 100124；3.南阳理工学院土木工程学院,河南 南阳 473000；4.河北水利科学院研究院,河北 石家庄 050051)

通过液相还原法成功制备了石墨烯负载纳米铁镍复合材料,该材料可高效快速地吸附水中的2,4-二氯酚(2,4-DCP)并对其进行脱氯.微观形貌分析结果表明,粒径为80～150nm的球形 Fe/Ni纳米颗粒成功插入石墨烯片层,并主要分布在石墨烯片层边缘和褶皱处,Fe/Ni颗粒团聚现象明显减少,更多活性位点暴露出来.XRD分析和FTIR分析表明,纳米零价铁(nZVI)通过Fe-O键成功嵌入石墨烯(rGO)中,且Fe/Ni纳米颗粒结晶度较差,外围包覆有无定形的铁氧化物沉淀.探讨了不同制备条件如碳铁比、镍化率、氧化石墨烯(GO)还原程度对材料去除2,4-二氯酚(2,4-DCP)性能的影响.综合考虑材料制备成本及对2,4-DCP的吸附脱氯性能,Fe/Ni@rGO复合材料的最优制备条件为:石墨烯与Fe质量比1:2,镍负载率5%,硼氢化钠与铁盐的物质的量比为5:1.研究表明5种材料对2,4-DCP的去除率遵循如下顺序:Fe/Ni@rGO复合材料>Fe/Ni>Fe @rGO复合材料>石墨烯> nZVI.储存稳定性试验和循环试验表明,与Fe/Ni双金属相比,Fe/Ni@rGO材料具有稳定的反应活性和较高的重复利用价值.研究结果表明Fe/Ni@rGO复合材料对2,4-DCP的去除为吸附和脱氯协同作用的结果.

石墨烯；2,4-二氯酚；纳米铁；脱氯；吸附

氯酚类化合物(CPs) 广泛用于木材、纺织和纸张等工业的防腐,并用做杀虫剂,杀菌剂等,在世界范围内已经使用数十年[1-2].氯酚类物质的大量生产和广泛应用,使得氯酚类化合物通过吸附、渗透、淋滤等作用由地表进入地下水,从而给地下水及土壤造成了一定的污染,特别在一些造纸厂、化工厂的土壤及地下水中氯酚污染严重.由于氯酚类化合物很强的毒性、持久性、致突变性和致癌性,美国环保署在1977年颁布的"清洁水法"修正案中将11种氯酚化合物列为环境优先污染物名单.氯酚化合物被列入"中国环境有限污染物黑名单",成为需要优先处理的污染物[3].

为了降低氯酚废水对地下水体的污染,众多研究者分别采用活性炭吸附法、挥发法、溶剂萃取法、化学氧化法和好氧/厌氧生物降解法、化学还原法等多种处理技术对氯酚废水进行了研究[4-8].吸附法投资少、操作成本低、操作简便,为去除有机污染物,特别是氯酚类化合物有效可行的成熟技术[9].但是污染物仅仅是实现了相转移,并没有从根本上对氯酚污染物去除,且存在吸附剂重复利用困难、仅适用于低浓度废水等缺点,污染物脱附后容易造成二次污染.氧化法处理成本高,且难以有效处理作为电子受体的含氯有机污染物[10].化学还原法采用活泼金属,通过还原脱氯的方式,逐级脱氯,降低氯酚污染物的毒性,进一步开环,生成低毒易降解的物质,所采用的还原剂金属有镁、铝、铁,镁、铝反应活性虽强,但表面易形成钝化层,不利于反应的进一步进行,且铝对环境的污染毒性较大.1994年加拿大滑铁卢大学Gillham教授首次采用金属铁屑实地修复地下水[11].随着研究的进一步深入,人们发现零价铁粒度的减小会导致铁颗粒比表面积增大,反应活性增强.1997年Wang等[12]首次采用液相还原法合成了纳米零价铁颗粒,并将纳米零价铁浆液直接注入到污染含水层用以处理三氯乙烯,开创了纳米零价铁在环境修复领域的应用研究.纳米零价铁由于其环境友好、还原能力强和成本低廉等受到了人们的广泛关注,被广泛应用于氯代烃、重金属、多氯联苯、有机染料、硝酸盐等的去除研究中[13-16].但纳米零价铁在应用中存在易团聚、易钝化、迁移能力较差等缺陷.为提高纳米铁微粒的稳定性和迁移能力,有必要对纳米铁进行改性.纳米铁改性材料来源广泛,涵盖无机物、矿石、天然或合成聚合物等[17-22].

本研究通过引入过渡金属镍作为脱氯加氢催化剂,与纳米零价铁组合形成纳米双金属体系,同时以还原氧化石墨烯作为载体,将纳米铁镍双金属颗粒负载在石墨烯片层上,研制出具有协同效应的石墨烯负载纳米双金属体系,比较分析材料与nZVI、石墨烯、石墨烯负载铁、Fe/Ni纳米双金属的吸附-脱氯性能.通过调整制备过程中C:Fe质量比、镍负载率(Ni与Fe的质量比)、氧化石墨烯还原度等因素制备系列石墨烯负载纳米铁镍双金属材料,通过对2,4-DCP的去除效果优选最佳制备工艺条件,并考察最优工艺下制备的石墨烯负载双金属型纳米铁复合材料的储藏稳定性能以及重复利用性能,揭示材料去除2,4-DCP的反应机理.

1 材料与方法

1.1 试剂与仪器

试剂:六水硫酸镍,盐酸,2, 4-二氯酚,2-氯酚,4-氯酚,苯酚,硼氢化钠,七水硫酸亚铁等(以上试剂均为分析纯).氧化石墨烯购自深圳穗衡石墨烯科技公司,厚度2nm左右,片层直径0.4 ～10mm,纯度>99%.

仪器:JJ-1型精密定时电动搅拌器(北京中兴伟业仪器有限公司),HY-6型双层调速多用振荡器(江苏省金坛市荣华仪器制造有限公司), SHIMADZU LC-2030C 3D型高效液相色谱(岛津企业管理(中国)有限公司),JEM2100F型透射电子显微镜(日本电子株式会社),BT100-1L型流量型蠕动泵(保定兰格恒流泵有限公司仪旧本理学公司), D8advance bruker型X射线衍射仪(布鲁克(北京)科技有限公司), Autosorb iQ型吸附脱附仪(美国康塔仪器公司), FD-1A-50型冷冻干燥机(上海比朗仪器制造有限公司)

1.2 材料制备

取0.28g氧化石墨烯粉末加入100mL去离子水中,超声20min,制得分散的氧化石墨烯悬浮液并转移至三口烧瓶中,充N2鼓泡15min后将一定量的FeSO4×7H2O加入到溶液中继续搅拌300min,然后将适量浓度的NaBH4溶液通过蠕动泵匀速缓慢滴加到溶液中,滴加完毕后继续搅拌反应60min,再向三口烧瓶中加入一定量的硫酸镍水溶液.机械搅拌下持续反应30min.再用无水乙醇快速真空抽滤洗涤3次,冷冻干燥24h后密封保存.通过调整氧化石墨烯和铁的质量、镍与铁的质量比、氧化石墨烯的还原程度等制备一系列石墨烯负载双金属型纳米铁复合材料.以上操作均在氮气环境下进行.

1.3 批试验

在室温下,在250mL的具塞锥形瓶中加入245mL调节好pH值的去离子水,然后通过移液枪加入5mL 1000mg/L 2,4-DCP储备液,即可配制初始浓度为20mg/L的2,4-DCP溶液,在通氮气条件下,加入一定量的制备好的石墨烯负载双金属型纳米铁复合材料,将其置于台式恒温水浴振荡器中振荡,调节振荡速率为200r/min,设置温度为25℃,使材料与溶液充分接触.间隔一定时间(5,10,20,45,60,90,120, 150,180,240,300min)用一次性注射器取样2mL左右,并用孔径0.45µm的滤膜过滤,滤液存放于高效液相自动进样器瓶中,然后用高效液相色谱仪测出剩余2,4-DCP及产物的峰面积,通过外标法计算出2,4-DCP及产物的浓度.2,4-DCP及其降解产物邻氯苯酚(2-CP)、对氯苯酚(4-CP)和苯酚的测定均采用高效液相色谱仪(岛津LC2030C)测定.采用反相色谱柱SHIMADZU ODS-SP Column(250´4.6mm)分离,L-2420型紫外检测器测定2,4-DCP溶液的浓度,流动相采用(CH3OH):(H2O)=60:40,流动相流速1.0mL/min;进样量为20µL,柱温40℃,检测波长均为280nm.

2,4-DCP的去除率按如下公式计算:

式(1)中:2,4-DCP,0是2,4-DCP初始时刻的浓度,mg/L;2,4-DCP,t是反应进行至时刻2,4-DCP的浓度,mg/L;是2,4-DCP的去除率,%.

2 结果与讨论

2.1 材料的表面形貌

石墨烯负载纳米铁镍复合材料与2,4-DCP反应前后的电镜扫描谱图如图1所示.从图中可以看出反应前球形的纳米铁镍颗粒主要分布在石墨烯的片层边缘或镶嵌于褶皱处,并被石墨烯片层包裹,颗粒团聚较少.这是因为氧化石墨烯羧基官能团数量主要分布在片层边缘,而Fe2+与氧化石墨烯的络合主要是通过羧基络合的,所以所产生的纳米铁镍金属颗粒主要分布在片层边缘.反应后分散在石墨烯上的球形纳米双金属铁镍颗粒消失,出现针状晶体,颗粒表面呈团簇状聚集体.根据反应推测,该针状晶体为铁的氧化物γ-羟基氧化铁[23].

图1 不同反应阶段石墨烯负载纳米铁镍复合材料的SEM图像

从图2a可以看出,由于自身磁性和纳米尺度效应,纯纳米Fe/Ni颗粒具有明显的链状聚集体结构,甚至聚集为片状,颗粒大部分呈球形,单个颗粒不能清晰分辨出来.图2b为石墨烯负载纳米铁镍复合材料的TEM图.图中的黑色球形颗粒即为纳米铁镍金属颗粒,片状的透明层状物质即为石墨烯,呈现为具有皱褶和折叠边缘的二维薄层.纳米铁镍颗粒均匀分散在石墨烯表面,无明显团聚现象,相较于纳米Fe/Ni材料,纳米Fe/Ni金属颗粒团聚现象明显降低.从图2c可以看出,负载在石墨烯上的纳米铁镍颗粒呈明显的核壳构型,外层表面比较疏松,呈非晶态.这种松散的结构可能是Fe3O4,其导电能力较强,有利于内核铁原子的电子转移,表面可观察到一些致密的小颗粒,结合图3分析可知,这些小颗粒为Ni纳米颗粒,沉积在纳米铁表面形成小突起,从而抑制了纳米铁核与空气中氧气的直接接触,从而减少了nZVI的氧化和消耗.图2d的高分辨TEM照片表明内部颜色较深部分即为Fe核,尺寸约为10nm左右,外部被非晶态的铁氧化物覆盖.可以在纳米颗粒上观察到清晰的晶格条纹,说明Fe/Ni纳米颗粒的核结晶性较好.其晶格间距约为0.248nm,这一结果与Fe (JCPDS 06-0696)的(110)晶面十分吻合[24].

图2 材料的TEM及HRTEM图像

图3 石墨烯负载铁镍材料的EDS面扫图

2.2 石墨烯负载纳米铁镍复合材料反应前后的XRD谱图

图4 氧化石墨烯,纳米铁镍,反应前后石墨烯负载纳米铁镍的X射线衍射图

从图4中可以看出,氧化石墨烯GO的衍射峰在2=11.6°处有一个最强特征峰(002)[25].通过布拉格方程计算其层间距为0.761nm,高于普通石墨粉的层间距0.337nm,这是因为石墨粉经过氧化之后引入了大量的含氧官能团,含氧官能团的插入增加了石墨烯层间距,为金属纳米颗粒的负载提供了空间.在纳米Fe/Ni双金属 XRD谱图中,在2=44.7°处出现了一个明显的宽钝峰,对应于α-Fe(JCPDS. 06-0696)的(110)衍射面[26].峰型较宽,强度较弱,这是由于纳米铁外壳层的氧化,呈无定形态,所以没有观察到氧化铁峰的存在,说明纳米铁结晶度较低,晶粒尺寸较小,这和纳米铁镍的TEM图观察相符.由于镍金属与铁金属的衍射峰十分接近[27],而且镍金属的含量相对铁而言很少,所以未检测到明显的镍的衍射峰.石墨烯负载纳米铁镍复合材料反应前后在2=12.5°处均有很弱的衍射峰,说明氧化石墨烯特征峰消失,氧化石墨烯基本还原,未出现明显的石墨烯特征峰(2=25.1°和2=43.0°)[28],这可能是由于复合材料中石墨烯的无序堆积和团聚较少所致,这在石墨烯负载纳米铁镍复合材料的SEM图中得到了证实.反应前的石墨烯负载纳米铁镍复合材料XRD谱图在2=44.7°处出现了一个较弱的宽峰,说明纳米铁已经成功插入石墨烯片层结构上,在63.1°处的较小的峰对应于Fe(200)的特征衍射峰,而纳米铁镍的XRD谱图中没有观察到此峰,说明纳米铁颗粒负载到石墨烯上后结晶度有所提高[29].由于Fe/Ni@rGO复合材料中Ni的含量较低,没有观察到Ni的衍射峰[30].反应前的石墨烯负载纳米铁镍复合材料在2=30.7°和2=35.0°处可以观察到磁赤铁矿Fe3O4/-Fe2O3的较弱的衍射峰,纳米铁的氧化峰强度普遍都很弱,说明了纳米铁外层的氧化层呈无定形状态[31].与反应前的Fe/Ni@rGO复合材料相比,反应后的Fe/Ni@rGO复合材料XRD谱图2=44.7°处宽峰消失,在2=30.7°和2=35.0°处磁赤铁矿峰强增大,并在2=53.8°和2=62.8°处出现纤铁矿-羟基氧化铁特征峰[32],表明零价纳米铁经过反应后得以氧化,生成Fe3O4/-Fe2O3和-羟基氧化铁.

图5 氧化石墨烯和石墨烯负载铁镍复合材料的红外谱图

2.3 FTIR分析

从图5中可以看出,在3445cm-1附近氧化石墨烯有一个较宽、较强的吸收峰,这归属于-OH的伸缩振动峰, 1650cm-1处的峰对应于C=O基团的伸缩振动,1411cm-1处的峰对应于羧基O=C-O基团的伸缩振动[33],1222和1065cm-1的峰分别对应于环氧基C-O-C和烷氧基C-O的振动吸收峰[34-35],说明GO至少存在-OH,-COOH, C-O-C,C=O四种类型含氧官能团,说明GO高度亲水.对于Fe/Ni@rGO复合材料红外谱图, -OH、C=O、O-C=O的峰强相对于GO峰强出现了较大程度的降低,环氧基C-O-C和烷氧基C-O峰消失,表明材料中含氧亲水极性官能团减少,GO被成功还原成石墨烯rGO,这也解释了Fe/Ni@rGO材料表面高度疏水的原因[36],复合材料的强疏水性能有利于对有机物2,4-DCP分子的吸附.同时可以看到Fe/Ni@rGO材料在1121cm-1出现Ni-O峰[37],在587cm-1出现Fe-O峰[38],表明石墨烯与nZVI的结合主要通过Fe-O键来完成,表明nZVI颗粒己成功嵌入石墨烯中.

2.4 碳铁比对Fe/Ni@rGO复合材料性能的影响

图6 不同碳铁比对去除2,4-DCP的影响

镍化率5%,反应温度30 °C,[2,4-DCP]0=20mg/L,初始 pH=5.0

2.5 镍负载率对Fe/Ni@rGO复合材料性能的影响

Fe/Ni双金属体系已被证明可以有效去除含氯有机物,在加氢脱氯过程中,2,4-DCP吸附在Ni催化剂的活性中心上会生成Ni-Cl键,Ni表面的原子氢通过加氢脱氯反应取代氯原子,参与C-Cl键的断裂,从而得到脱氯产物.大量的研究[42-44]也充分证实了过渡金属镍的负载量对双金属体系中氯代烃脱氯效率的影响.

图7 镍负载率对2,4-DCP的去除率和苯酚产率的影响

C:Fe=1:2,反应温度30°C,[2,4-DCP]0=20mg/L,pH=5.0

从图7可以看出,不同镍含量复合材料对2,4- DCP的去除率排序为 5%>14%>9%> 3%,苯酚的产率排序为 9% >5% >14% >3%.2,4-DCP的加氢脱氯作用主要是由双金属催化剂上吸附的活性原子氢(H*)脱氯引起的,氢原子和镍以类氢化物的形式存在于镍的表面[45].当镍化率小于9%时,2,4-DCP的脱氯效率随着镍化率的上升而增大.这是由于镍能够使纳米铁腐蚀所产生的氢气分解成活性氢原子并附着在镍的表面,活性氢原子的强还原能力使得2,4-DCP的脱氯效率增大,镍化率越高,材料的脱氯效率就越高,此时镍化率是材料脱氯反应的控制因素.当镍化率大于9%时,双金属中镍的含量过高,较高含量的镍覆盖在纳米铁颗粒的表面,不利于纳米铁的电子转移,最终降低双金属催化剂的脱氯效率.从图中可以明显看出5%的镍含量对2,4-DCP的去除效果最优.

2.6 氧化石墨烯还原度对Fe/Ni@rGO材料的影响

图8 硼氢化钠投加量对2,4-二氯酚去除率和苯酚产率的影响曲线图

C:Fe=1:2,反应温度30°C,[2,4-DCP]0=20mg/L,pH=5.0

Shin等[46]通过调整NaBH4溶液的投加量制备了不同还原程度的rGO,结果表明硼氢化钠与亚铁离子的物质的量比越大,氧化石墨烯的还原就越彻底,高的C/O比意味着rGO具有更多被修复的共扼结构和相对少的含氧官能团,具有相对高的导电性.由此可通过调整NaBH4物质的量来改变生成的氧化石墨烯的还原程度,从而对材料的吸附性能和导电性能产生较大影响[47],并由此影响复合材料对2,4-DCP的吸附及脱氯效果.从图8a可以看出,当投加的NaBH4物质的量是亚铁离子2倍时,此时材料对2,4- DCP的去除能力强于其他两种还原程度的材料,这是因为虽然此时材料的还原程度最小,脱氯还原能力较弱,但负载材料上存在的羧基、羟基、羰基等含氧官能团较多,可与酚类化合物形成氢键,吸附能力较强.但此投加量仅仅还原了部分亚铁离子,且对氧化石墨烯的还原程度较低,且低还原度的氧化石墨烯导电能力很弱,不利于材料的脱氯反应,这从图8(b)可以得到证实.综合考虑材料的吸附脱氯性能以及制备成本,选用NaBH4与亚铁离子物质的量比值为5:1的反应条件制备材料为宜.

2.7 五种材料对2,4-DCP去除效果比较研究

从图9可以看出,石墨烯、Fe@rGO复合材料和Fe/Ni@rGO复合材料对2,4-DCP的去除效果在反应最初的20min远优于Fe/Ni和nZVI,随着反应的进行,nZVI、石墨烯和Fe@rGO复合材料对2,4-DCP的去除率趋于平缓.Fe/Ni和Fe/Ni@rGO复合材料对2,4-DCP的去除率稳步上升,最终在反应进行至 240min时达到95%以上的去除率.在反应初始阶段,材料对2,4-DCP的去除以吸附为主.Fe@rGO复合材料和Fe/Ni@rGO复合材料两者均以石墨烯为载体,具有多孔结构,比表面积大,其特有的大p环可通过p-p作用力吸引2,4-DCP分子,所以吸附能力远强于nZVI和Fe/Ni对2,4-DCP分子的吸附能力.石墨烯材料比表面积大,由于其表面没有纳米金属颗粒的干扰,参与形成大p键的碳原子数目更多,其形成的共轭体系比Fe@rGO复合材料要大,所以在吸附达到平衡时其对2,4-DCP的去除率高于Fe@rGO复合材料[48].由于纯纳米铁易团聚,外层形成致密的氧化铁外壳,阻碍内层铁核的电子转移,再加上2,4-DCP为芳香性氯代物,和苯环相连的C-Cl键难以断裂,脱氯反应活化能高达138.91kJ/mol[49],单纯的nZVI还原能力一般,很难发生脱氯反应,所以nZVI和Fe@rGO复合材料主要通过吸附作用去除2,4-DCP.在Fe/Ni和Fe/Ni@rGO体系中,反应20min后2,4-DCP去除率能够持续增大,这是因为发生了脱氯反应.随着脱氯反应的进行,对Fe/Ni体系来说,固相中2,4-DCP逐渐脱氯生成苯酚,其占据的吸附位点得到释放,其空出的吸附位点对液相中的2,4-DCP分子产生新的吸引力,所以整个反应过程中2,4-DCP去除速率相对比较稳定;而对Fe/ Ni@rGO体系来说,经过最初的20min的吸附,液相中的大部分2,4-DCP分子被吸附至材料表面,并通过孔道扩散至Fe/Ni颗粒活性位点上发生脱氯反应,生成产物苯酚释放至溶液中,但后续2,4-DCP浓度越来越低,故去除速率逐渐趋缓.

图9 不同材料去除2,4-DCP的浓度变化曲线图

C:Fe=1:2,反应温度30°C,[2,4-DCP]0=20mg/L,初始pH=5.0

2.8 储存稳定性能及重复利用性能

从图10a可以看出,在空气中暴露90d后,Fe/ Ni@rGO复合材料对2,4-DCP的去除率仍达到92.9%,而纳米Fe/Ni材料空气中暴露90d后,其对2,4-DCP的去除率降至52.8%.这说明Fe/Ni@rGO复合材料具有比n-Fe/Ni更高的抗氧化性能和催化活性,Fe/Ni@rGO复合材料不仅能阻止nZVI的聚集,而且有助于复合材料保持较高的反应活性.由此可见,Fe/Ni@rGO复合材料抗氧化性能强,可在开放环境中存放三个月时间而保持反应活性基本不变.

从图10b可以看出,2,4-DCP在5个循环试验中的去除率分别为100%,98.5%,96.8%,85.1%,80.7%,从第4次循环试验开始,2,4-DCP去除率明显下降,这可能是由于随着2,4-DCP降解反应的进行,nZVI在还原降解反应过程中不断发生腐蚀,生成的Fe3O4和Fe2O3等氧化物沉淀覆盖了材料表面的活性位点,导致降解速率降低.与Fe/Ni@rGO复合材料相对应,纳米Fe/Ni双金属作为还原剂在2,4-DCP第一轮循环试验中性能很好,但到第五轮降解试验中只能去除40.2%的2,4-DCP,其持久活性较差,说明其随着反应的进行,容易被氧化失活.综上所述,Fe/Ni@rGO复合材料反应活性持久,可循环性强.

图10 材料储存稳定性及重复利用性能试验分析

[2,4-DCP]=20mg/L,投加量,1.0g/L,温度30°C,pH=5.0,反应时间300min

2.9 Fe/Ni@rGO复合材料去除2,4-DCP反应机理分析

Fe/Ni@rGO复合材料去除2,4-DCP是一个吸附及脱氯协同作用的结果.在反应前,Fe/Ni@rGO 复合材料表面有大量空置的吸附位点,因而具有较大的表面吉布斯自由能.当投加入2,4-DCP溶液中,复合材料的大π键与2,4-DCP分子的苯环形成p-p堆积作用,石墨烯表面也含有部分含氧官能团,其和2,4-DCP分子之间也存在氢键作用,从而对2,4-DCP分子产生较强的吸附作用,如图11所示.同时溶液不断震荡下形成的紊流扩散加剧了2,4-DCP分子从溶液本体向复合材料表面的扩散.2,4-DCP分子被吸附至材料表面后,通过材料介孔扩散至Fe/Ni@rGO复合材料的吸附活性位点及反应活性位点Fe/Ni颗粒附近,与Fe/Ni颗粒发生加氢还原脱氯反应.

图11 石墨烯和2,4-DCP分子之间π-π作用力与氢键示意

2,4-DCP分子在 Fe/Ni双金属纳米颗粒表面发生加氢脱氯反应,具体反应路径如下:

Fe/Ni双金属纳米颗粒表面脱氯反应示意图如图12 所示.在反应过程中,石墨烯作为载体,有效分散了纳米Fe/Ni双金属颗粒,降低了纳米颗粒的团聚,同时由于其强吸附作用和强导电能力,通过吸附和脱氯协同效应增强对2,4-DCP的反应活性.Fe作为还原剂释放出电子,生成氢气,同时提供电子给石墨烯,由于单层石墨烯π电子的自由移动特性,电子迅速传导给石墨烯表面吸附的2,4-DCP分子,参与到2,4-DCP的加氢脱氯还原反应中.在Fe/Ni双金属体系中,Ni与Fe可形成原电池效应,加速Fe的腐蚀,导致析氢速度加快,抑制nZVI颗粒氧化层的形成.Ni作为过渡金属,具有空轨道,可与2,4-DCP中的氯原子提供的孤对电子成键,形成过渡络合物Ni…Cl…R(式2),削弱C-Cl键,降低脱氯反应活化能[50].同时镍作为储氢金属,可吸附H2,并在嵌入的晶格中形成强还原性物质 Ni×2H*(式3),并释放出活性氢原子 H*(式4),与吸附在Ni上的2,4-DCP发生加氢脱氯反应(式5).金属镍的加入改变了 2,4-DCP的反应途径,2,4-DCP不再直接通过接受零价铁与Fe2+释放的电子来实现脱氯还原,而是和活性氢原子发生催化加氢脱氯还原反应,从而大大提高了脱氯效率[51].

图12 Fe/Ni双金属纳米颗粒表面脱氯反应示意

3 结论

3.1 成功研制了石墨烯负载纳米铁镍复合材料.综合考虑材料制备成本及对2,4-DCP的吸附脱氯性能,Fe/Ni@rGO复合材料的最优制备条件为:石墨烯与Fe质量比1:2,镍负载率5%,硼氢化钠与铁盐的物质的量比为5:1.

3.2 研究表明4种材料对2,4-DCP的去除效率遵循如下顺序:Fe/Ni@rGO复合材料>Fe/Ni>Fe @rGO复合材料>nZVI.储存稳定性试验和循环试验表明,与Fe/Ni双金属相比,Fe/Ni@rGO复合材料对2,4-DCP的去除效率均有显著提高,这表明Fe/Ni@rGO材料具有稳定的反应活性和较高的重复利用价值.

3.3 揭示了Fe/Ni@rGO复合材料去除2,4-DCP的反应机理.Fe/Ni@rGO复合材料对2,4-DCP的去除为吸附和脱氯协同作用的结果,反应初期主要为物理吸附作用,复合材料通过p-p作用力将2,4-DCP分子从溶液主体吸附至材料表面,随后吸附在材料上的2,4-DCP分子通过孔道扩散到达反应活性位点,与Ni催化剂形成过渡态络合物Ni…Cl…R,同时Ni吸附氢气并分解成活性氢原子,发生加氢脱氯反应,产生最终脱氯产物苯酚,并逐渐释放至溶液中.

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Reduced graphene oxide supported Fe/Ni nanocomposites for 2,4-dichlorophenol removal.

HUANG Xue-zheng1,2,3*, ZHANG Yong-xiang2, ZHANG Da-sheng4, ZHU Xin-feng1, LI Hou-yun1

(1.Henan Province Key Laboratory of Water Pollution Control and Rehabilitation Technology, Henan University of Urban Construction, Pingdingshan 467000, China；2.Department of Urban Construction, Beijing University of Technology, Beijing 100124, China；3.School of Civil Engineering, Nanyang Institute of Technology, Nanyang 473000, China；4.Hebei Institute of Water Science, Shijiazhuang 050051, China)., 2023,43(12)：6352～6362

Reduced graphene oxide supported Fe/Ni nanocomposites were prepared for the rapid and effective adsorption and dechlorination of 2,4-dichlorophenol (2,4-DCP) by using liquid phase reduction method. The morphological characterization showed that the spherical Fe/Ni bimetallic nanoparticles with the size of 80～150nm were successfully inserted into the graphene sheets and mainly distributed at the edges and folds of the graphene sheets. The agglomeration of Fe/Ni nanoparticles decreased significantly. XRD patterns and FTIR analysis showed nZVI nanoparticles were successfully embedded into graphene through Fe-O bond, Fe/Ni bimetallic nanoparticles had poor crystallinity and amorphous iron oxide which covered the outer layer of nanoparticles. The effects of different preparation conditions such as carbon iron ratio, nickel loading and reduction degree of graphene oxide on the removal of 2,4-DCP were discussed. The optimum preparation conditions of the Fe/Ni@rGO composites are as follows: the mass ratio of graphene to Fe is 1:2, the Ni loading is 5%, and the molar ratio of NaBH4to Fe2+is 5:1. The adsorption and dechlorination perfermance of 2,4-DCP by nZVI, Fe/Ni, Fe@rGO composites and Fe/Ni@rGO composites were compared and analyzed. The results showed that the removal efficiency of 2,4-DCP by five materials followed the sequence: Fe/Ni@rGOcomposites>Fe/Ni>rGO>Fe@rGOcomposites>nZVI. However, the cycle test and storage stability test showed: compared with Fe/Ni bimetallic, Fe/Ni@rGO composites had stable reactivity activity and high reruse value. The results demonstrated the removal mechanism of 2,4-DCP by Fe/Ni@rGO composites was the synergistic effect of adsorption and dechlorination.

graphene；2,4-dichlorophenol；nanoscale zerovalent iron；dechlorination；adsorption

X52

A

1000-6923(2023)12-6352-11

黄雪征,张永祥,张大胜,等.石墨烯负载铁镍复合材料去除水中的2,4-二氯酚 [J]. 中国环境科学, 2023,43(12):6352-6362.

Huang X Z, Zhang Y X, Zhang D S, et al. Reduced graphene oxide supported Fe/Ni nanocomposites for 2,4-dichlorophenol removal [J]. China Environmental Science, 2023,43(12):6352-6362.

2023-04-28

河南省科技攻关项目(202102310609);国家重点研发计划子课题(2016YFC040140402);河南省高等学校重点科研项目(21A610009)

* 责任作者, 副教授, 58626472@qq.com

黄雪征(1978-),男,河南南阳人,副教授,博士,主要从事环境功能材料研究,场地污染治理修复.发表论文30余篇.58626472@qq.com.