网络首发时间: 2019-05-22 16:20

稀有金属 2020,44(01),87-99 DOI:10.13373/j.cnki.cjrm.xy19010004

吸附法去除水中重金属复合污染物的研究状况

汪振文 王会才 杨继斌 刘霞平 岳瑞瑞

天津工业大学环境与化学工程学院

摘 要：

随着经济、社会的快速发展,重金属污染已经成为世界上最严重的环境问题之一。在水环境中,重金属通常与其他金属或非金属污染物之间形成复合污染物。本文首先介绍了重金属复合污染物之间复杂的相互作用,简要总结了三大相互作用:竞争作用、协同作用和强络合作用对重金属去除的影响。在上述基础之上,然后对吸附法同时去除水体中重金属及其复合污染物的研究情况作了简要的综述,着重评述了各种吸附材料在去除重金属复合污染物的各项优势,包括:改性多功能化复合材料可以提高与复合污染物的作用力,磁性纳米材料易于分离等。最后就吸附法、高性能吸附材料在重金属复合物染物去除领域中的应用前景进行了分析,指出了今后的研究应侧重于重金属复合污染物的去除,并且提出了通过提高吸附剂与复合污染物的作用力从而实现污染物去除的新思路。

关键词：

重金属;水环境;复合污染物;相互作用;复合材料;吸附剂;

中图分类号： X703

作者简介：汪振文(1992-),男,安徽黄山人,硕士研究生,研究方向:水中重金属污染的治理研究;E-mail:464160025@qq.com;;*王会才,教授;电话:022-83955859;E-mail:wanghuicai@tjpu.edu.cn;

收稿日期：2019-01-02

基金：天津市科技计划项目(16ZXIILSF00200)资助;

Removal of Heavy Metal Complex Pollutants in Water by Adsorption

Wang Zhenwen Wang Huicai Yang Jibin Liu Xiaping Yue Ruirui

School of Environmental and Chemistry Engineering,Tianjin Polytechnic University

Abstract：

With the rapid economic and social development,heavy metal pollution has become one of the most serious environmental problems in the world.Heavy metals often form complex pollutants with other metals or non-metallic pollutants in water environment.This article first introduced the complex interactions between heavy metal complex pollutants,include competitive effect,synergistic effect and strong complexation.Then,the research on the simultaneous removal of heavy metals and their composite pollutants in water by adsorption method was briefly reviewed.And the advantages of various adsorbent materials in removing heavy metal composite pollutants ware highlighted.This paper focused on the advantages of various adsorbent materials in removing heavy metal complex pollutants.Finally it also analyzed the application prospects of the adsorption method and high-perfonnance adsorption materials in the removal of heavy metal complex pollutants,and pointed out that the future of research should focus on the removal of heavy metal complex pollutants,and proposed a new idea to achieve pollutants removal:through enhancing the interaction force of adsorbents and complex contamlnants.

Keyword：

heavy metals; water environment; complex pollutants; interactions; composites; adsorbents;

Received： 2019-01-02

随着工业的快速发展,重金属的污染越来越严重。水中重金属的来源广,主要有矿山的开采,金属的冶炼与加工以及农业生产中农药化肥的使用等,并且重金属离子一般都具有毒性且不可生物降解,易造成各种健康危害和生态失衡问题 ^[1,2,3] 。随着研究的深入,人们逐渐意识到水体中重金属的污染不仅仅是单一金属离子的污染。Dutta等 ^[4] 在对印度纳格河的污染物统计分析时发现,河水中除了重金属离子之外,还存在大量的其他无机污染物以及动物粪便等有机污染物。B ing等 ^[5] 对整个三峡水库干流沿岸沉积物进行了调查,他们发现沉积物中有6种重金属(Cd,Cr,Cu,Ni,Pb和Zn)共存。重金属污染具有伴生性和综合性,即不同重金属之间以及重金属与非重金属污染物之间会产生联合作用,形成复合污染。研究表明重金属共存污染物之间存在复杂的交互作用,会改变污染物的理化性质使得重金属的污染变得更为复杂,也更接近于实际情况 ^[6,7,8] 。因此,研究重点必须转向重金属的复合污染,才能更有助于重金属污染的治理。

目前,水中重金属污染的处理方法主要有:化学沉淀,离子交换,反渗透和吸附法等。其中化学沉淀,离子交换和反渗透等方法在去除单一重金属污染的应用中取得了较好的效果,但对治理重金属复合污染仍有不足:化学沉淀法难以去除复合体系中的有机污染物,离子交换和反渗透技术由于成本较高和操作复杂也很难在实际中应用 ^[9,10,11] 。吸附法具有吸附材料设计的灵活,操作简单,可以推广使用等优点,被认为是一种有效去除重金属复合污染物的方法 ^[12,13] 。新型吸附剂被广泛的用于去除工农业生产中的污染物,Chen等 ^[14] 利用聚氨酯/二硫代氨基甲酸钠吸附去除电镀废水中的Cu(Ⅱ),Ni(Ⅱ)和Zn(Ⅱ),Aljerf等 ^[15] 把改性的沸石吸附剂应用于去除制革工业废水中的溴甲酚紫色染料和重金属铬。然而,目前大多数的研究都集中在单一重金属污染的吸附去除,对更接近实际污染的重金属复合污染的研究还在起步阶段 ^[16,17] 。与此同时,吸附材料的设计还存在许多的缺陷,如吸附速度慢、适应性差,特别是只对某种金属离子或是有机污染物的吸附去除能力强等 ^[12] 。因此,要实现水体中重金属复合污染物的绿色、快速、经济、高效去除,应当在认识重金属复合物染污形成、特征及转化机制的基础上,设计、制备相应的吸附材料,从而实现重金属复合污染物的高效吸附去除。

1 重金属复合污染物及其相互作用

在工业生产中产生的印染废水和农业生产中过度施用农药化肥的污水中,存在着多种有机和无机污染物,而且它们大多会发生作用。重金属与其他污染物之间通过相互作用形成的复合污染比单一重金属的污染更加复杂,这是因为重金属复合微污染物中各组分的浓度、组合关系可能产生污染物之间竞争吸附位点,或者因强络合作用形成多元复合物 ^[18,19] ,从而使吸附表现为拮抗或协同效应 ^[20,21] ,其中拮抗效应对重金属复合污染物的同时去除非常不利,大大增加了治理的难度。要实现复合污染物的吸附去除就必须对这种交互作用有正确的认识。国内外的研究表明,复合污染物之间的复杂作用主要包括污染之间的竞争作用、协同作用和强络合作用。

1.1 竞争作用

竞争作用是指在重金属复合污染物的吸附去除过程中各组分竞争吸附位点,降低吸附剂的吸附性能,不利于复合污染物的同时去除 ^[22] 。在巯基功能化聚乙烯醇/介孔二氧化硅基纳米纤维吸附去除双酚A和Cu(Ⅱ)的研究中,由于Cu(Ⅱ)竞争吸附剂中的巯基,大大降低了双酚A的吸附 ^[23] 。研究表明这种竞争作用大多发生在相同类型的污染物之间,对于不同类型的污染物影响较少 ^[24] ,因此多种重金属共存的体系中重金属之间易发生竞争,且不同的重金属之间具有不同的竞争效应。有人利用Two-site Langmuir吸附等温模型研究了泥炭对铜、铅和镉的同时去除,证明了Pb(Ⅱ),Cu(Ⅱ)和Cd(Ⅱ)的竞争能力顺序为Pb (Ⅱ)>Cu(Ⅱ)>Cd(Ⅱ) ^[25,26] 。重金属的竞争吸附必然与吸附材料表面的官能团有关,Wu等 ^[27] 在研究钙基磁性生物炭对砷和镉复合物的吸附去除中发现,砷对镉的竞争主要通过离子交换、羟基络合以及π-π相互作用来实现。此外,Ding等 ^[28] 利用扩展的X射线吸收精细结构(EXAFS)光谱和表面络合模型探索碳纳米纤维吸附重金属之间的相互作用机制,发现这种竞争吸附还与重金属的初始浓度、体系的pH有关。因此,在重金属污染的治理中要防止复合污染物的竞争作用,不仅要选择合适的吸附材料,还应控制吸附体系的pH等参数。

1.2 协同作用

协同作用是指在多种污染物共存的体系中,某种污染的存在有利于其他污染物的去除,如Hg(Ⅱ)的存在下亚甲蓝的吸附率显著提升,接近100% ^[29] 。污染物之间的协同作用主要是指静电引力、π-π相互作用力和氢键的协同作用,经研究发现,这些协同作用力的形成不仅与吸附剂密切相关,还与离子的加入顺序有关。

Wu等 ^[27] 在钙基磁性生物炭(Ca-MBC)吸附Cd(Ⅱ)和As(Ⅲ)的体系中,在先加入As(Ⅲ)时,由于吸附As(Ⅲ)导致负电荷密度增加,同时形成MBC-As-Cd复合物,导致Cd(Ⅱ)的吸附表现出协同作用;而先加入Cd(Ⅱ)时,由于Cd吸附速度较快,且无法形成三元复合物,导致As(Ⅲ)的吸附受到抑制,这说明可以通过控制污染物进入吸附体系中的顺序来提高污染物的去除效果。

同时,相同的污染物在不同的吸附体系中会产生不同的效果。在A-NMP新型复合材料去除镍和钴的二元吸附体系中,吸附过程中镍和钴表现为协同作用,两种污染物的吸附量明显高于单一体系 ^[30] ,然而在γ-Al₂O₃同时吸附镍和钴的体系中,镍的存在抑制了钴的去除 ^[31] 。这些现象说明了不同的吸附剂可能出现不同的吸附机制,因此我们不能抛开吸附剂而谈论污染物的竞争或协同效应。Zou等 ^[32] 在研究甲基橙与Cd (Ⅱ)的协同吸附中,选择氢氧化镁改性斜发沸石作吸附剂,发现甲基橙与Cd(Ⅱ)的二元体系中只存在协同效应,而不存在竞争,这表明选择合适的吸附剂可以有效的控制竞争作用并促进协同作用,从而使污染物高效去除。

1.3 强络合作用

在某些重金属复合污染体系中含有大量给电子基团(如-COOH,-OH和-NH₂)的有机污染物,这些有机物很容易与金属阳离子和无机含氧阴离子结合形络合物,很难被功能单一的吸附剂吸附 ^[33] 。例如重金属与废水中的氨很容易就结合生成二元配合物 ^[34] 。Cr(Ⅲ)易于与腐殖酸,As(As(Ⅲ)和As(V)),Se(Se(Ⅳ)和Se(Ⅵ))的氧阴离子形成三元络合物 ^[33] 。这些通过络合作用形成的二元和三元复合物在水溶液中非常稳定,不仅使得重金属在水体中的溶解度增大,还会使沉降在底泥中的重金属污染物重新溶解到水体中,造成二次污染,给重金属的去除增加了难度。研究发现pH对络合作用的影响很大,重金属与其他污染物的络合一般发生在中性或碱性环境中,在酸性环境中很难形成 ^[35,36] 。实际上被污染的水体大多呈中性,有利于重金属与有机污染物络合,不利于重金属的吸附治理。总之,重金属复合污染非常复杂,我们在设计吸附材料时不仅要考虑水环境因素,更应对复合污染物各组分的形态以及理化性质进行分析,了解它们之间的相互作用,才能对症下药,实现污染物的去除。

2 吸附法去除重金属复合污染物

吸附法具有很大的灵活性,可以根据污染水体类型和污染物种类的不同而合理设计吸附材料。印染废水中常含有大量的有机染料和金属离子,这些污染物之间的相互作用,导致很难被传统的吸附剂吸附。研究发现在材料表面进行官能化,可提高吸附效果,且官能团的密度越高吸附效果越好,Doshi等 ^[37] 将壳聚糖羧甲基化后能有效的去除Cd(Ⅱ),Pb(Ⅱ)以及染料MB;Hu等 ^[38] 用超支化聚胺改性的石墨烯也能成功的应用于甲基蓝和铅等污染物的去除。然而在电镀废水中,污染物主要是Cu(Ⅱ)和Cr(Ⅵ)等多种共存重金属离子,Yue等 ^[39] 发现使用Mg-Al-Cl层状双氢氧化物吸附剂去除这些离子的效果较好。重金属的复合污染物主要分为重金属与其他污染物,多种重金属共存的复合污染物两大类,下面主要总结了吸附法去除这两类污染物的研究现状。

2.1 吸附法去除重金属与其他污染物

在含有重金属以及其他污染物的工业废水中,重金属离子与其他污染物之间会发生螯合作用。针对这种现象,以下总结了几种代表性的吸附剂在这方面的应用。

2.1.1 改性天然复合吸附材料

迄今为止,大多数吸附材料,如氧化石墨烯、介孔碳、壳聚糖等已被用于吸附去除水中的单一污染物 ^[7] ,然而上述材料在去除重金属复合污染物的应用中,因为其表面官能团单一导致的竞争吸附降低了吸附效率 ^[40,41] 。针对这一缺点有人提出了通过引进多种官能团的方法改善吸附性能,Chen等 ^[42] 将4-氨基苯硫酚和3-氨基丙基三乙氧基硅烷用于氧化石墨烯改性,合成了多功能化的石墨烯纳米复合材料,用于吸附亚甲基蓝和铜的复合污染,如表1所示,与其他材料对比该吸附剂吸附量都有了明显的增加。研究表明这种通过引入官能团而制成的石墨烯复合材料,既具有了石墨烯的超大的表面积和孔隙率、疏水性等特征,又含有多种官能团易于重金属发生螯合作用,增加了重金属的去除率 ^[50] 。另一方面,吸附材料表面官能团密度不高是制约复合污染物去除率的关键因素,将聚合物分子链接枝到纳米吸附材料表面可以显著提高官能团密度 ^[51] 。Wang等 ^[23] 将3-巯基丙基三甲氧基硅烷、正硅酸乙酯、聚乙烯醇、造孔剂等在均相反应体系中聚合以增加巯基量,然后电纺成二氧化硅基纤维,这一方法不仅增加了官能团密度,而且与以往将官能团接枝到非均相反应体系中的聚合物纤维上不同,它克服了在非均相体系中因官能团改性而限制了官能化效率的问题。尽管如此,聚合物的引入虽然提高了吸附剂表面的官能团密度,但是由于聚合物分子链之间的缠结,增大了功能化反应的空间位阻,导致反应效率不高 ^[52] 。研究发现超支化聚合物有近似球形结构和许多表面官能团,这些官能团的密度比线性分子聚合物高,另外超支化聚合物中分子链缠结很弱,甚至不存在分子链缠结。因此超支化聚合物更适合用于吸附材料的表面改性,形成含高密度官能团的多功能吸附材料,增强吸附材料与复合污染物之间的多种吸附作用力的耦合,提高吸附性能。Hu等 ^[38] 开发了超支化聚胺-石墨烯复合材料吸附剂,用于吸附甲基蓝和铅。他们发现吸附剂对Pb(Ⅱ)的吸附主要是靠螯合作用和静电吸引力,甲基蓝的吸附是靠π-π作用力和静电吸引力,而多官能超支化聚胺含有丰富的羟基和胺基,可为吸附剂提供大量的静电吸引、螯合作用以及π-π作用吸附位点,这些作用力的协同作用,提高了甲基蓝和Pb(Ⅱ)的去除效率。

表1 几种不同吸附剂对亚甲基蓝(MB)和铜的最大吸附量  下载原图

Table 1 Maximum adsorption capacity of several different adsorbents for methylene blue and copper

2.1.2 磁性复合吸附材料

磁性纳米吸附材料通过外部磁场的作用能快速地从水中分离出来,这样有利于吸附剂的回收利用和重复使用,大大降低了操作成本 ^[24] 。为了提高去除效率,早在几年前Padervand等 ^[53] 通过水热和共沉淀法制备磁芯-沸石壳纳米复合材料用于重金属的吸附去除。随着科技的进步,人们对磁性纳米材料吸附性能的研究由去除单一的污染物向多种污染物的复杂体系转变。Bharath等 ^[54] 报道了一种新的合成路线:低温下将钾原子连续插入石墨烯夹层,使得该石墨烯上产生可控的微米至亚微米尺寸的多孔结构,然后将平均尺寸为30 nm的超顺磁Fe₃O₄纳米颗粒均匀地分散在多孔石墨烯片上,这样有效避免了磁性纳米颗粒的聚集和多孔石墨烯层的重新堆积。这种方法合成的Fe₃O₄/多孔石墨烯纳米复合材料,不仅对重金属(Cu(Ⅱ))和有机染料(结晶紫)等污染物有很高的吸附量,而且克服了某些吸附剂分离不便的缺点,有利于吸附剂的回收和再利用。另外,在磁性材料的表面通过官能团的接枝提供多种附加的吸附活性位点,增加吸附剂与污染物的多种相互作用,如静电引力、氢键和螯合作用等,从而提高吸附性能。Liu等 ^[24] 用尿素作为改性剂和软模板通过简单的一锅溶剂热法制备了功能化的磁性微球NiFe₂O₄,用于去除金属离子(铜、镉、铬和锌离子)和三种氟喹诺酮类(环丙沙星、恩诺沙星和诺氟沙星)共存污染物。由于官能团的引入增加了吸附活性位点,使得60 min内复合体系污染物的去除效率均达到80%以上。Tao等 ^[55] 制备了谷氨酰胺改性的壳聚糖和二氧化硅包覆Fe₃O₄纳米粒子的磁性复合微球吸附剂,明显提高了酸性绿25两性染料和汞离子的吸附效率,主要归因于酸性绿25的仲胺与谷氨酰胺的羧基之间的发生了氢键作用,汞离子与谷氨酰胺的羧基和酰胺基团之间形成的螯合效应。然而,裸露的磁性纳米粒子易于在酸性介质中溶解,还会聚集成较大的纳米粒子使活性基团减少。研究发现SiO₂是一种生物相容性材料并且在酸性条件下较稳定,可有效保护磁性纳米粒子。因此,如图1所示Santhosh等 ^[65] 通过溶剂热法和溶胶-凝胶法制备氧化石墨烯修饰的SiO₂@CoFe₂O₄纳米颗粒,将合成的纳米复合材料用3-氨基丙基三乙氧基硅烷进一步官能化以附着官能团,获得的材料可吸附酸性黑1染料和六价铬离子。通过与其他吸附剂的对比如表2所示,这种SiO₂@CoFe₂O₄纳米复合材料的吸附效果更优于其他吸附剂,去除效率能达到96%以上。

图1 SiO2@CoFe2O4纳米复合材料的合成示意图

Fig.1 Schematic diagram of the synthesis of SiO₂@CoFe₂O₄ ^[55]

表2 不同吸附剂吸附Cr (Ⅵ)与有机污染物的最大吸附容量  下载原图

Table 2 Maximum adsorption capacity of Cr(Ⅵ) and organic pollutants adsorbed by different adsorbents

2.1.3 其他复合吸附材料

纳米零价铁以其比表面积大、反应活性高等特点常被作为去除水中重金属和有机污染物的吸附剂。在污水处理中一般通过高温锻烧或离子交换等方法,将纳米铁负载到载体上,如硅胶、活性炭、沸石等,这样不仅可以有效防止纳米铁的团聚,提高反应活性,并且增强其稳定性 ^[71] 。目前纳米零价铁在重金属与有机物的同时去除方面也有了报道。Kim等 ^[72] 采用沸石负载的纳米级零价铁复合材料同时去除Cr(Ⅵ)和活性黑5,去除效率达到了99%。他们还发现这种负载型的纳米零价铁的作用机理与纳米零价铁的作用机理基本相同,都是把污染物吸附在纳米零价铁的表面,通过纳米零价铁还原降解或还原活性黑5和铬(Ⅵ)以降低污染物的毒性。另外,膜吸附材料在复合污染的去除方面也有研究,将纳米结构吸附剂截留到超滤膜的指状孔中,制备的双功能超滤膜能够同时去除水中的多种污染物,与共混膜相比,这种双功能超滤膜的过滤性能更高,并且增加了有效吸附活性位点。Pan等 ^[73] 将中空多孔的Zr(OH)_x纳米球通过聚多巴胺涂层固定在聚醚砜超滤膜中,可以同时高效率去除胶体金、聚乙二醇和铅等多种污染物,这一设计的最大优点在于超滤膜既能保持良好的超滤特性,还具有了吸附能力。最近Huang等 ^[74] 开发一种比表面积大的高效吸附材料,能同时去除废水中的无机和有机污染物。这是一种将层状双氢氧化物负载在空心碳微球表面的复合材料(LDHs/HCMSs),它利用了层状双氢氧化物较大的层状空间以及碳纳米材料具有丰富的官能团,提高了对目标分子和离子具有高亲和力,从而提高去除率。这项研究还探讨了这种吸附剂对不同体系中Pb(Ⅱ)和腐殖酸(HA)的吸附效率,如图2所示,在不含腐殖酸的废水中,Cu(Ⅱ),Co(Ⅱ),和Ni(Ⅱ)的存在降低了Pb(Ⅱ)的吸附效率,而HA也会影响体系中Pb(Ⅱ)的去除效果。

2.2 吸附法去除多种重金属共存的复合污染物

在多种金属离子共存的体系中,各种重金属之间会相互竞争吸附位点,使得有些在单一体系下吸附效果较好的吸附剂在复合体系中吸附性能急剧下降。因此为了提高吸附性能,研究者也开始对新型吸附材料的探究,以下简单介绍了几种这类型的吸附材料。

2.2.1 改性天然复合吸附材料

在过去的几年中,天然材料因其具有来源广泛,价格便宜且可以重复使用等优点,受到了越来越多的关注。Eloussaief等 ^[25] 用铝柱撑蒙脱石研究Pb)(Ⅱ),Zn(Ⅱ)和Cd (Ⅱ)共存废水的吸附行为。这种对原料蒙脱石进行改进而得到的新材料对含Pb(Ⅱ),Zn(Ⅱ)和Cd(Ⅱ)的去除取得了较好的效果,但是在低能量吸附位点Pb (Ⅱ),Zn (Ⅱ)和Cd (Ⅱ)会因竞争效应导致吸附量降低。这项研究对去除同时含有Pb (Ⅱ),Zn(Ⅱ)和Cd (Ⅱ)等离子的工业废水有很大的帮助。将农业生产过程中产生的废弃物用于环境污染的治理,不仅节约资源,而且还能防治污染。从这个角度出发,Yu等 ^[75] 用改性甘蔗渣填充层析柱分别对Pb (Ⅱ)/Cu (Ⅱ),Pb (Ⅱ)/Zn(Ⅱ)和Cu(Ⅱ)/Cd(Ⅱ)两组分的竞争吸附行为进行研究。Chand等 ^[76] 采用丙烯酸甲酯和硝酸铈铵对原生苹果渣改性制得聚合苹果渣吸附剂,并且在单金属和多金属污水体系中对去除铅和锌进行了研究。各项研究表明,改性的天然材料可以用来去除污水中的重金属离子,在单一组分体系中对某一特定种离子去除效果比在多组分体系中好,在含有两种或两种以上的金属离子体系中,虽然单个金属离子的吸附量在减少,但有更多的金属离子被吸附。

2.2.2 改性纳米复合材料

随着纳米技术的出现,各种纳米材料通过引入功能团或其他方法改善性能用于废水的治理。由于纳米材料的尺寸小于100 nm,并且具有独特的性能,如比表面积大、吸附能力强、超顺磁性、量子限域效应等 ^[77,78] ,使得纳米材料在重金属污染治理中具有高效、多功能等特点。结合磁分离和纳米技术的磁性纳米材料在重金属离子去除方面表现非常出色,而且这些磁性纳米材料在磁分离后具有出色的可重复使用性。截至目前,共沉淀和微乳液等众多技术已应用于制备纳米材料吸附剂,但是裸露的磁铁矿倾向聚集,从而影响吸附效率 ^[79,80,81] 。为了克服这些缺点提高吸附性能,Wang等 ^[82] 制备了氨基功能化Fe₃O₄@SiO₂纳米复合材料,并用于吸附去除Pb(Ⅱ),Cu(Ⅱ)和Cd(Ⅱ)等离子。Badruddoza等 ^[84] 描述了羧甲基-β-环糊精聚合物修饰的Fe₃O₄纳米颗粒的合成及其在去除多种重金属离子中的应用。不幸的是,这些改性方法通常过程较为复杂,并且制备的材料存在磁响应差和比表面积低等缺点。为了寻找简单有效的制备方法,采用多元醇法制备了羧基官能化的磁性纳米材料(CMNPs)用于重金属的吸附。这项研究中报道了材料的制备方法,并研究了Pb(Ⅱ),Cu(Ⅱ)和Cd(Ⅱ)等离子的去除效率。表3为几种不同吸附材料对Pb(Ⅱ),Cu(Ⅱ)和Cd(Ⅱ)的吸附效果比较,不同材料对3种金属离子的去除效果明显不同,其中多元醇法制备的CMNPs吸附剂去除率最高。因此我们在去除重金属污染时,应当先确定不同吸附剂对水中污染物的去除效率,从而选择最佳的吸附材料。

图2 吸附材料LDHs/HCMSs对Pb(Ⅱ),Cu(Ⅱ),Co(Ⅱ)和Ni(Ⅱ)的单一和竞争吸附能力

Fig.2 Inpidual and competitive adsorption capacities of Pb(Ⅱ),Cu(Ⅱ),Co(Ⅱ) and Ni(Ⅱ) on LDHs/HCMSs ^[74] (a) Without HA;(b) with HA

2.2.3 生物吸附材料

生物吸附是一种很有潜力的新兴重金属去除技术,许多生物吸附剂对各种重金属离子表现出优异的结合能力,所以生物技术被认为是治理重金属污染废水的有效方法 ^[93] 。由于藻类生物在世界范围普遍存在,并且生长迅速,所以生物吸附技术常用大型藻类作吸附剂。藻类的吸附能力与其细胞壁结构有关,大部分藻类细胞壁中含有蛋白质,蛋白质与多糖结合形成糖蛋白。这使得细胞壁中包含许多官能团,如氨基、羟基、羧基和硫酸根能够结合重金属 ^[94,95] 。Figueira等 ^[96] 发现了两种大型藻类对Hg的去除率高达96%～99%,对铅的去除效果也达到了86%。但在多种重金属离子共存的体系中,重金属在吸附过程中因竞争而产生拮抗作用,导致吸附的效果明显降低。研究发现多糖物质具有特殊的结构、物理化学性质和化学稳定性,具有较强的重金属吸附能力,可用于吸附去除重金属,这是因为:(1)由于存在葡萄糖单元的羟基使多糖聚合物具有较高亲水性;(2)存在高化学反应性的极性官能团;(3)聚合物链的柔性结构 ^[97] 。Dobrowolski等 ^[98] 利用细菌菌株红球菌和紫红红球菌合成胞外多糖,通过调节pH使得多元体系中的重金属达到快速去除。这项研究中pH不仅影响金属离子竞争胞外多糖聚合物表面的有效吸附位点,而且影响细胞外多糖聚合物质子化的程度,这也可能影响金属结合的行为,所以需要严格控制体系的pH。虽然目前生物吸附技术在去除水中重金属复合污染物方面还存在不足,但生物技术中采用的吸附剂是天然的,具有低成本、可重复使用、环保等诸多优点,对复合污染物的治理具有潜在的价值。

3 总结与展望

近年以来,越来越多的吸附材料用于重金属去除研究,但重金属复合污染物的吸附去除研究仍处于起步阶段,且复合污染更接近于实际污染。因此,今后研究应侧重于重金属复合污染物的去除,重点是在对复合污染物各组分性质及其相互作用分析的基础上,根据不同的吸附作用力匹配相应的功能基团,通过提高吸附剂与复合污染物的作用,来实现重金属污染的治理。与此同时,提高吸附材料表面的官能团密度可增加吸附活性位点,从而提高吸附性能,因此如何提高吸附材料表面的活性位点仍是今后研究追逐的重点。此外,吸附材料的设计制备不仅要实现高效率去除,同时也要考虑制备工艺的难易程度以及成本控制等问题。然而目前的相关报道中,虽然有些材料可实现高效率去除,但由于成本、重复使用性以及回收利用等问题仍无法应用到实际工程中。因此,新型、简便、低成本的吸附材料制备方法及工艺仍将是今后研究的重点。

表3 几种不同吸附剂对Pb(Ⅱ),Cd(Ⅱ)和Cu(Ⅱ)的最大吸附量  下载原图

Table 3 Maximum adsorption capacity of Pb(Ⅱ),Cd(Ⅱ) and Cu(Ⅱ) by several different adsorbents

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