现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (2) : 63-67. DOI: 10.16606/j.cnki.issn0253-4320.2025.02.013

技术进展

碳基材料负载纳米零价铁对铬废水的处理研究进展

胡继娟 ¹^,² ,
王小治 ² ,
侯建华 ²^,^*

作者信息 +

Research progress in treatment of chromium-containing wastewater by carbon-based materials loaded with nanoscale zerovalent iron

Ji-juan HU ¹^,² ,
Xiao-zhi WANG ² ,
Jian-hua HOU ²^,^*

Author information +

文章历史 +

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摘要

以纳米零价铁(nZVI)负载到碳基材料孔道结构中或者表面上能有效防止nZVI易团聚、易氧化等缺点。重点分析了能够负载nZVI的碳基材料,介绍了制备碳基负载nZVI复合材料的方法,阐述了影响复合材料去除Cr(Ⅵ)的因素。结果表明,不同的碳基材料负载nZVI后都有利于提高nZVI的稳定性、分散性、反应活性;利用液相还原法和碳热法都能有效提高nZVI的分散性,但是碳热法成本低、nZVI同碳材料之间的结合更加牢固。碳基负载nZVI复合材料对Cr(Ⅵ)的去除能力受pH、温度、共存离子等因素的影响较大。

Abstract

Loading nano zero-valent iron (nZVI) into the pore structure of carbon-based materials or on their surface can effectively overcome the shortcomings of nZVI,such as easy aggregation and easy oxidation.The carbon-based materials that can load nZVI are analyzed.The preparation method for carbon-based loaded nZVI composite is introduced,and the factors that affect the performance of the composite in removing Cr(Ⅵ) are described.It is shown that the stability,dispersibility and reactivity of nZVI can be improved by loading nZVI with different carbon-based materials.Both liquid phase reduction method and carbothermal method can be utilized to improve effectively the disperse performance of nZVI,but the carbothermal method leads to low cost,and makes the combination between nZVI and carbon materials stronger.The removal ability of carbon-based nZVI composite to Cr(Ⅵ) is greatly affected by pH,temperature and coexisting ions.

Graphical abstract

关键词

环境工程学 / Cr(Ⅵ) / 制备方法 / 碳基材料 / 纳米零价铁

Key words

environmental engineering / Cr(Ⅵ) / preparation method / carbon-based materials / nano zero-valent iron

Author summay

胡继娟(1987-),女,本科,助理研究员,研究方向为科技查新和环境功能材料,jjhu@yzu.edu.cn。

引用本文

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1 负载nZVI的多孔碳材料

1.1 石墨烯

石墨烯(graphene)是一种新型二维层状结构的碳材料,具有较大的比表面积,特别是氧化石墨烯通过含氧官能团与其他材料结合,吸附能力强和电导率好,可以提高nZVI的分散性,同时低电阻率和高电荷载流子迁移率可进一步提升处理效果^[1]。Song等^[1]通过超声预处理制备石墨烯/纳米零价铁复合材料,并用该复合材料去除废水中的Cr(Ⅵ)。吸附机理如图1所示^[1],由于石墨烯具有良好的转移电子的能力,可以帮助电子穿透钝化层从nZVI到 Cr(Ⅵ)阴离子,同时将钝化层中的Fe³⁺还原为Fe²⁺,消除钝化层,提供nZVI的新电子传输通道。在酸性条件下,nZVI和还原氧化石墨烯可以形成一个原电池,其中nZVI为阳极,氧化石墨烯为阴极^[2]。当超声波处理时,产生冲击波使钝化层部分脱落,在nZVI表面形成一系列微纳缺口,从而可以暴露新鲜Fe⁰,提高nZVI的反应活性,在治理铬废水中具有巨大的潜力。

1.2 生物炭

生物质材料,如秸秆、花生壳、椰子壳等,因为来源广泛、价格低廉常被用来制备生物质炭。生物质炭具有比表面积大、孔隙度高、官能团种类多等优点,因此生物炭可作为nZVI的有效载体,不仅保持其还原性能,而且增强了稳定性,极大地改善了nZVI团聚失活的现象。侯素珍等^[3]将玉米秸秆作为碳前驱体,采用液相还原法制备生物炭,以聚乙烯亚胺为功能单体对生物炭进行修饰,实验表明材料对Cr(Ⅵ)的去除率可达98.8%。经表征改性生物炭呈多孔片状结构,nZVI颗粒主要分布在生物炭表面,部分镶嵌在生物炭孔隙中,有效改善nZVI易团聚的问题。生物炭表面的含氧官能团通过络合等作用将反应中产生的Cr(Ⅲ)、Fe(Ⅲ)氧化物和氢氧化物等固定在生物炭上,从而达到从水溶液中去除6价铬的目的^[4]。原因是nZVI颗粒部分可以嵌入到改性生物炭片状结构的孔隙中,也可以分布在纳米片状的表面,有效改善nZVI易团聚的问题。同时,生物炭表面的功能化含氧官能团通过络合作用等,将6价铬与nZVI化学反应产生的氢氧化物、Fe(Ⅲ)和Cr(Ⅲ)等固定在多孔碳上^[4]。改性nZVI/生物炭复合材料具备高反应活性以及氨基官能团,通过共沉淀作用和丰富官能团的吸附作用,进一步提高了Cr(Ⅵ)的去除率。周春地等^[5]用鸡骨生物炭为载体,制备出生物炭-纳米零价铁材料(Fe-BC),经过与铜改性的生物炭-纳米零价铁对比,Fe-BC吸附容量153.60 mg/g高于Fe-Cu-BC的吸附容量126.47 mg/g,可能是因为铜占用了一部分生物炭表面的吸附位点,使得一部分nZVI发生团聚,降低材料中nZVI的利用率,导致吸附剂对废水中Cr(Ⅵ)的吸附效果变差。而吸附完成后,Fe-BC会生成具有磁性的铁的氧化物,因此在吸附剂回收时,能够利用磁分离技术将吸附剂与水体进行分离,提高吸附剂的利用率,减少对环境的影响。

1.3 介孔碳

介孔碳因具有较大的孔容和有规则的介孔结构,而且比表面积大,广泛应用于吸附处理废水中的重金属且性能优越^[6]。Dai等^[7]通过热转化法,在铁碳比为3∶1的条件下利用蔗糖为前驱体制备出了介孔碳负载nZVI的复合材料(nZVI/OMC-3),nZVI/OMC-3比表面积达715.16 m²/g,在酸性条件下对Cr(Ⅵ)的去除率为99%,并且在第7次循环之后仍然不低于60%。原因是介孔碳的介孔孔道不仅能够为防止nZVI氧化提供保护层,而且可以增强nZVI的稳定性以及反应活性,有利于提高nZVI的分散性,且介孔孔道加快传质速度,提高去除重金属Cr(Ⅵ)速率,nZVI/OMC-3去除Cr(Ⅵ)机理如图2所示^[7]。Sun等^[8]采用“双溶剂”还原技术将纳米零价铁负载到有序介孔碳上,通过对所制备的OMC/nZVIs复合材料中OMC介孔能显著抑制nZVI的聚集,并能有效地减缓nZVI的氧化,复合材料去除重金属Cr(Ⅵ)的机理如图3^[8]。结果表明,nZVIs/OMC复合材料比没有介孔碳负载的nZVI具有更高的反应活性。NZVIs/OMC对Cr(Ⅵ)具有较好的选择性,30~60 min内OMC对Cr(Ⅵ)的去除效率为77.4%,而NZVIs在120 min才能达到这个水平,NZVIs/OMC复合材料在不到15 min的时间内通过吸附和还原的协同作用去除Cr(Ⅵ)效率接近100%。

2 碳材料负载nZVI复合材料的制备方法

将零价铁负载到碳基材料的方法主要包括液相还原法、碳热法、热解法和球磨法等^[9]。其中液相还原法和碳热法是目前为止人们最常用的制备方法。

2.1 液相还原法

采用液相还原法制备nZVI,生物炭、活性炭、介孔碳等碳基材料作为载体,首先将铁的化合物以吸附或沉积方式负载到多孔碳材料载体上,再用硼氢化物(BH₄^-)还原成nZVI,制备nZVI的反应方程式如下:

(1)

4 Fe 3 + + 3 BH 4 − + 9 H 2 O → 4 Fe 0 s + 3 BO 3 − + 12 H + + 6 H 2 g

(2)

2 Fe 2 + + BH 4 − + 3 H 2 O → 2 Fe 0 s + BO 3 − + 4 H + + 3 H 2 g

Wang等^[10]以氧化石墨烯为原材料,先用硫氰酸铵水热180℃使其功能化(ATGO),再通过硼氢化钠液相还原法合成硫氰酸铵功能化石墨烯负载纳米零价铁复合材料(ATGO-nZVI),合成过程如图4(a)^[10]。如图4(b)和4(c)所示,TEM显示纳米零价铁颗粒Fe纳米颗粒负载在粗糙的石墨烯纳米片上,呈核壳结构,表明ATGO作为支撑载体有效抑制了nZVI的聚集,有效提高了nZVI分散性。席冬冬等^[11]选择用杉木屑作为生物炭前驱体,用硼氢化钾还原生物炭制备出纳米零价铁/生物炭复合材料(nZVI-SM),结合XPS和XRD分析,大部分零价铁存在于生物炭的孔道内,部分被铁的氧化物包裹,形成Fe⁰典型的核壳结构。唐宝玲等^[12]以磷酸活化过的柚子皮为负载材料合成了nZVI-BC-P材料,SEM表明形成的包覆型复合材料具有更高的稳定性和分散性。刘剑等^[13]以Fe²⁺溶液为原料、硼氢化钠为还原剂,采用液相还原技术合成了颗粒活性炭负载纳米零价铁复合材料GAC-nZVI,纳米零价铁沉积在颗粒活性炭上,相比nZVI没有明显聚集。

2.2 碳热法

碳热法是利用碳基材料,在还原性气体存在的条件下,高温将铁离子还原为零价铁^[14]。生物炭、活性炭、介孔碳等多孔碳材料可成为纳米零价铁的载体。例如牛红云等^[15]用免液相浸渍的碳热法将焦炭和铁源混合,如图5^[15]所示,在800℃高温碳化2 h,得到零价铁/焦炭复合材料nZVI/coke,具有表面积大、纳米零价铁粒径小、吸附位点多等优点。通过HRTEM图片可以观察到零价铁的直径2~6 nm,均匀分散在石墨化碳纳米片层上。Song等^[16]在N₂气氛下700℃煅烧2 h制备出纳米零价铁复合材料负载多孔活性炭球材料Fe@PACB-700。SEM显示活性炭孔隙中充满了nZVI颗粒,比表面积为1 032.09 m²/g,提高了nZVI的分散性。

相比液相还原法,碳热法具有以下特点:①成本低,不需要硼氢化钾、硼氢化钠等硼氢化物作为昂贵的还原剂;②碳基材料与纳米零价铁结合更好,材料具有更高的总含铁量。但是值得注意是使用氯化铁作为铁源,高温下容易产生有毒氯代产物。

3 环境因素对去除Cr(VI)的影响

3.1 pH

pH是影响Cr(Ⅵ)去除过程中关键因素^[17]。由表1可知,nZVI负载碳基材料在酸性条件下更有利于去除Cr(Ⅵ),pH越高去除率越低。在溶液pH较低时,nZVI的钝化层被腐蚀,促进了nZVI与 Cr(Ⅵ)的接触和反应^[18],此时Cr(Ⅵ)以HCrO₄^-和Cr₂O₇^2-的形式存在,由于较强的静电吸引,有利于复合材料对这些阴离子的吸附^[19]。随着pH升高,HCrO₄^-会逐渐转化为CrO₄^2-,HCrO₄^-比CrO₄^2-拥有更低的吸附自由能,从而更容易被吸附^[20],pH高时复合材料与Cr(Ⅵ)的静电斥力抑制了吸附剂对 Cr(Ⅵ)的去除。

3.2 温度

温度能够影响去除Cr(Ⅵ)的物理化学反应。nZVI/carbon材料对Cr(Ⅵ)去除是一个自然吸热^[23]过程,随着温度的升高,反应体系中各组分的运动加快,加速了Cr(Ⅵ)从溶液向吸附剂表面的扩散速率,促进了Cr(Ⅵ)与复合材料活性位点的结合^[22]。由表2可知,高温有利于碳基负载nZVI复合材料对Cr(Ⅵ)的去除,温度越高,Cr(Ⅵ)离子对复合材料表面的扩散速率越大。

3.3 共存离子

水环境中共存离子直接影响碳基nZVI复合材料对Cr(Ⅵ)的吸附与去除。据研究,不同阴离子SO₄^2-、HCO₃^-、NO₃^-对材料去除Cr(Ⅵ)至关重要。Zhuang等^[27]研究了NO₃^-、HCO₃^-对生物炭负载nZVI去除Cr(Ⅵ)的影响。研究显示,当HCO₃^-存在时,Cr(Ⅵ)去除效果明显降低,表明HCO₃^-离子显示负面影响,可能的原因是nZVI核上的钝化层生成可以抑制电子转移^[28]。同时NO₃^-根离子对Cr(Ⅵ)去除有明显的抑制作用。据报道,NO₃^-容易被nZVI还原成NH₄⁺或NO₂^-离子,因此用于去除Cr(Ⅵ)的碳基nZVI复合材料表面活性位点被NO₃^-竞争。另外,NO₃^-容易被nZVI还原,从而加速了材料表面氧化铁钝化层的产生,这会进一步抑制Cr(Ⅵ)还原。Song等^[1]研究发现SO₄^2-使碳基nZVI复合材料对 Cr(Ⅵ)有轻微抑制作用,这可以归因于硫酸盐离子与铁氧化物通过配合物形成的相互作用^[29]。这种络合作用影响很小,反应过程中,SO₄^2-几乎不影响复合材料对Cr(Ⅵ)的去除。

4 结论与展望

大量研究证明,纳米零价铁nZVI由于自身比表面积大、还原能力强等特点,能够发生吸附、氧化还原、共沉淀等反应,被应用于去除重金属等领域。但是在实际应用中存在限制因素,然而将nZVI负载碳基材料能够解决nZVI稳定性差、易团聚、易氧化的不足,增大了比表面积、分散了反应物,提高对 Cr(Ⅵ)的吸附容量。但是目前碳基负载nZVI复合材料仍然面临一些待解决的问题。

(1)水体环境复杂,影响复合材料对Cr(Ⅵ)的去除效果因素有很多,比如pH、温度、共存离子、溶解氧等。因此需要深入研究环境对复合材料的影响。

(2)制备复合材料的工序较多,因此简化工艺才能够实现大规模工业化生产nZVI负载碳基材料。

(3)实际水体中存在的污染物繁多,但现在大部分nZVI负载碳基材料止步于实验室研究阶段,只使用Cr(Ⅵ)一种污染物,处理效果缺乏真实性。因此未来将碳基nZVI复合材料应用于实际水污染处理的研究至关重要。

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