Adsorption of heavy metals by agricultural solid waste based hydrogel: A review
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摘要: 随着经济的快速发展,水中重金属离子污染对人类健康以及生态系统造成威胁。水凝胶以其良好的吸附性能、可再生性和低毒性在处理重金属离子方面具有很大的潜力。本文论述了近年来国内外以农业固体废弃物为原料制备水凝胶(纤维素基水凝胶、半纤维素基水凝胶、木质素基水凝胶等)吸附重金属的研究进展。同时,讨论了农业固废基水凝胶的合成,对重金属的吸附效果、吸附机制及分析方法,并列举了工业固废基和其他固废基水凝胶吸附重金属的效果,以期帮助研究者对农业固废基水凝胶吸附重金属的探究有更深刻的理解。Abstract: With the rapid development of economy, the pollution of heavy metal ions in water poses a threat to human health and ecosystem. Hydrogels have great potential in the treatment of heavy metal ions because of their good adsorption properties, renewability and low toxicity. This paper discusses the research progress in recent years on the preparation of hydrogels (cellulose-based hydrogels, hemicellulose-based hydrogels, lignin-based hydrogels, etc.) for the adsorption of heavy metals from agricultural solid wastes at home and abroad. The synthesis of agricultural solid waste based hydrogels, the adsorption effect, adsorption mechanism, and analytical methods for the removal of heavy metals are also discussed, and the effects of heavy metal adsorption by industrial solid waste based and other solid waste based hydrogels are enumerated. In order to help the researchers to have a deeper understanding of the investigation of heavy metal adsorption by agricultural solid waste based hydrogels.
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表面活性剂是同时具有疏水基团和亲水基团的两亲性化合物,具有乳化、破乳、分散、絮凝、润湿、铺展、起泡、消泡、洗涤和杀菌等功能,被广泛应用于科学研究及石油开采、乳液聚合、食品、农业、纺织等工业领域[1-4]。出于合成角度考虑,一般希望得到结构稳定的表面活性剂,然而,在大多数实际应用中,表面活性剂往往只被需要在某一阶段发挥作用,当过程结束后,需要其失去活性或者从体系中轻易取出以减少残留导致的副作用[5-7]。另外,表面活性剂在用完后若直接排放到环境中,会造成巨大的资源浪费,还可能引起严重的环境污染而破坏生态平衡[8]。
解决这类问题的一个比较理想的办法就是采用刺激响应型表面活性剂取代传统的表面活性剂。刺激响应型表面活性剂在受到外界刺激时,其聚集形态、表面活性、溶解性、黏度和乳液稳定性会发生变化。如果这种变化可逆,即存在“开”与“关”两种状态,则该表面活性剂被认为具有开关性能[9-12]。与小分子表面活性剂相比,高聚物表面活性剂具有很好的乳化稳定性、在各种表/界面上有很好的吸附能力、优良的分散性和凝聚力,并且许多高聚物表面活性剂还具有良好的保水性、增稠性、成膜性及粘附性,更重要的是,大多数高分子表面活性剂是低毒的。
目前,开关型高聚物表面活性剂受到了普遍的关注[13-19]。研究结果表明,现有的表面活性剂还存在不少的问题,如pH开关型高分子表面活性剂在使用过程中需加入大量酸或碱,不仅会对环境造成污染,而且多次循环后也会因盐的累积而影响其性能;光开光型表面活性剂的应用受到了透明度的限制,因很多体系不透明,功能基团无法吸收特定波长的光做出响应或转变不彻底;热开光型表面活性剂需要较大的温度差,能耗大。因此,设计与开发成本低廉、工艺简单、性能优异的新型开关型高分子表面活性剂具有重大的意义。
CO2开关型高分子表面活性剂因其环境友好、可循环利用的优势而备受青睐[20-27]。但是,目前的研究发现,CO2开关型高分子表面活性剂存在合成复杂、价格昂贵且响应速度慢等缺点。此外,研究大多集中于探讨表面活性剂水溶液聚集结构的变化,而开关型乳液的研究较少;且多数高分子表面活性剂均采用嵌段等复杂的方式合成。
为了使操作简便,更适于实际应用,本实验采用廉价易得的单体甲基丙烯酸二乙氨基乙酯(DEAEMA)和乙烯基磺酸钠(SVS),通过简单的无规共聚的自由基聚合法制备出一种CO2开关型高分子表面活性剂,用于水/液体石蜡体系的乳化,研究了乳液的CO2开关性能和乳化机制,可为石油开采中利用开关型表面活性剂水溶液以乳液的形式提取石油并轻易破乳而分离得到石油提供借鉴。
1. 实验材料及方法
1.1 原材料
甲基丙烯酸二乙氨基乙酯(DEAEMA),AR,99%,阿拉丁试剂有限公司;乙烯基磺酸钠(SVS),25%水溶液,阿拉丁试剂有限公司;过硫酸铵(APS),AR,阿拉丁试剂有限公司;CO2,>99.999%,法国液化空气(中国)投资有限公司;N2,>99.999%,法国液化空气(中国)投资有限公司;液体石蜡,AR,国药集团试剂有限公司。
1.2 P(DEAEMA-SVS)聚合物粒子的合成
称取 20 mmol DEAEMA和20 mmol SVS置于干燥的100 mL三颈烧瓶中,加入20 mL去离子水,搅拌并升温至80℃,加入2wt%的APS,反应6 h。所得的粗产物用透析袋(MW 3500 Da)透析24 h,每6 h换一次水。将所得的水溶液冷冻干燥,得到最终产物P(DEAEMA-SVS)。改变单体DEAEMA和单体SVS的物料比例进行同样的实验步骤,得到不同投料比(摩尔比为4∶1,3∶1,2∶1,1∶1,1∶2,1∶3和1∶4)的产物作对比研究。
1.3 样品表征
利用JNM-ECA600 核磁共振仪(日本电子株式会社)测定聚合物粒子1H-NMR谱(C3D6O);采用18角激光光散射仪(DAMN HELEOSⅡ,美国Wyatt公司)测定聚合物的凝胶渗透色谱;采用Zetasizer Nano S90高灵敏纳米粒度分析仪(英国马尔文公司)通过动态光散射(DLS)的方法测聚合物粒径;用吊环法在KRUSS K100表面张力仪上测P(DEAEMA-SVS)水溶液的表面张力;用拉环法在KRUSS K100表面张力仪(德国克吕士科学仪器有限公司)上测定P(DEAEMA-SVS)水溶液与液体石蜡之间的界面张力。采用XPV-990E透反射偏光显微镜(上海长方光学仪器有限公司)和LSM780激光共聚焦显微镜(德国Zeiss公司)观察乳液液滴形态。
1.4 P(DEAEMA-SVS)乳液的制备
将聚合物P(DEAEMA-SVS)水溶液(1wt%)与液体石蜡按体积比1∶2两相混合,然后在超声细胞破碎仪(XC-CDS650,宁波市先倡电子科技有限公司)下超声乳化(450 W),其功率设置为60%,工作时间和间隔时间均设置为2 s,工作次数为8次。将制备好的乳液密封在室温下放置3周,通过直接观察法确定乳液的稳定性。通过液滴测试和电导率(DDSJ-308A电导率仪,上海精密科学仪器有限公司)测试确认乳液类型。
1.5 P(DEAEMA-SVS)乳液的开关性能测定
向P(DEAEMA-SVS)乳液中通入CO2(100 mL/min)30 min后静置30 min,观察乳液外观。在60℃下再向该体系通入N2(100 mL/min)30 min,待溶液降温至室温后,再用超声细胞破碎仪超声乳化。重复上述步骤4次,探究乳液的开关性能。同时探究不同P(DEAEMA-SVS)浓度对乳液稳定性及失稳的影响。
2. 结果与讨论
2.1 P(DEAEMA-SVS)的结构与分子量分布
将P(DEAEMA-SVS)聚合物溶解在氘代丙酮中,利用核磁共振仪测得其1H-NMR谱如图1所示:δ=1.8~2.1 ×10−6(1、2、3)处的化学位移为高分子主链上的C—CH2—C和C—CH—C;δ=4.00×10−6(4)处的化学位移为—COOCH2CH2中的O原子所连的亚甲基H;δ=2.75×10−6(5)处的化学位移为—COOCH2CH2中远离氧原子的亚甲基H;δ=2.55×10−6(6)处的化学位移为—N(CH2CH3)2中的亚甲基H;δ=1.1×10−6(7)处的化学位移为—N(CH2CH3)2中的甲基H;δ=0.9×10−6(8)处的化学位移为高分子主链上的甲基H。
P(DEAEMA-SVS)聚合物的凝胶渗透色谱(GPC)测试结果显示合成的该物质为高分子化合物,其数均分子量为5.262×104,均重分子量为3.857×103,Z均分子量为9.466×104。其分子量分布为:40000~45000(29.22%)、45 000~50 000(24.36%)、50 000~60 000(25.70%)、60 000~80 000(12.11%)、100 000~200 000(3.62%)、200 000~400 000(1.62%),如图2所示。
2.2 不同投料比对P(DEAEMA-SVS)乳液稳定性的影响
单体投料比例不同时,所合成的聚合物粒子粒径会有差别,而粒径过大会使形成的乳液不稳定,粒径过小会使所形成的乳液不易破乳,因此选择合适尺寸的聚合物显得尤为重要。利用动态光散射原理,测定频率位移,由Stokes-Einstein方程计算得出颗粒粒径及分布。图3为不同投料比(DEAEMA和SVS单体摩尔比为4∶1,3∶1,2∶1,1∶1,1∶2,1∶3和1∶4)所得产物的尺寸及粒径分布。当单体DEAEMA和单体SVS的投料摩尔比为1∶1时,聚合物粒子的粒径约为113 nm。随着单体SVS投料比的增加(2∶1→1∶4),聚合物粒径在一定的范围内(60~115 nm)稳定且粒径分布(PDI)较窄。
将不同摩尔比单体投料所合成的聚合物分别配置成质量比为1%的水溶液,测得这些物质的表面张力如图4所示。用该仪器测得纯水的表面张力为74 mN/m。可以发现,不同单体配比投料所合成的聚合物均具有表面活性,都能降低水的表面张力。其中单体DEAEMA和单体SVS比例为1∶1和1∶2合成的聚合物活性更强,能够明显地降低水的表面张力至37.279 mN/m 和35.820 mN/m。
除表面张力外,不同单体配比投料所合成的聚合物粒子对油水界面的吸附能力也是有很大的差别,而其在油水界面吸附的能力的大小会直接影响所形成乳液的稳定性。采用液体石蜡作为油相,研究不同单体配比投料所合成的聚合物水溶液与液体石蜡之间的界面张力(图4)。用该仪器测得纯水与液体石蜡直接的界面张力为66.231 mN/m。相对于其他单体配比,单体配比为1∶1和1∶2时,所得聚合物在油水界面上的吸附能力更强,能将界面张力降至5.492 mN/m和4.994 mN/m。这两种单体配比投料所得聚合物的界面张力值大幅下降,说明这两种聚合物具有很强的表面活性和界面活性,为形成稳定的乳液提供了可能,P(DEAEMA-SVS)聚合物能作为唯一乳化剂稳定乳液。
根据以上数据的分析,不同单体比例投料所合成的聚合物无论是粒径大小还是表面/界面活性都有着很大的差异。聚合物表面活性剂中所含亲/疏水单体基团比例的不同会导致HLB值存在差异,进而引起乳化性能的改变。当DEAEMA∶ SVS为2∶1时,所合成的聚合物粒子的表面张力和界面张力均略低于DEAEMA和SVS的投料摩尔比为1∶1时所合成的聚合物粒子,但作为唯一乳化剂制备的乳液在静置3周后会分出水层,而当DEAEMA∶ SVS=1∶1所合成聚合物粒子的乳液却更稳定。因此接下来选择DEAEMA∶ SVS=1∶1时所合成聚合物粒子形成的乳液进行乳化性能和开关性能的研究。
2.3 P(DEAEMA-SVS)的乳化性能
用P(DEAEMA-SVS)聚合物作为唯一乳化剂制备了水/液体石蜡的乳液,所制备的初始乳液的外观均一,没任何分层或分水分油现象。将得到的乳液取一滴滴入清水中,发现乳液滴能在水中迅速分散;并用电导率仪测得该乳液的电导率为120 μS/cm,其值远大于10 μS/cm,证明用P(DEAEMA-SVS)聚合物制备的水/液体石蜡乳液是水包油(O/W)型。将制备好的P(DEAEMA-SVS)乳液在室温下密封静置3周,发现乳液未出现分层现象,如图5所示。图6为乳液的显微镜照片。可以发现放置三周的乳液与原始乳液的乳液液滴大小并没有明显差别。由此说明P(DEAEMA-SVS) 是一种非常有效的高分子表面活性剂。
将不同浓度的P(DEAEMA-SVS)水溶液与液体石蜡混合形成乳液,将乳液静置一段时间后观察乳液的外观变化,如图7所示。当表面活性剂P(DEAEMA-SVS)浓度低于0.2wt%时,虽然也能形成乳液,但其乳液有明显的分层,这是由于浓度较低时形成的乳液液滴粒径较大,而液体石蜡的密度较小,引出液滴在浮力的作用下集中在上层,其水相也不是澄清的,说明仍有P(DEAEMA-SVS)颗粒存在于水相中;而当其浓度大于0.2wt%时,均能形成稳定均一的乳液。
不同的油水比例对乳液稳定性也存在影响。当液体石蜡与水的比例小于1∶1时,均能形成稳定的乳液。但当液体石蜡与水的比例超过1∶1时,虽然也能形成乳液,如图8(b)所示,但该乳液无法一直稳定为均一相,放置一段时间后液滴将上浮分层,下层水相不澄清(图8(a)),说明当液体石蜡的量大于50%时形成的乳液不稳定。
2.4 P(DEAEMA-SVS)乳液的开关性能
在室温下,向聚合物P(DEAEMA-SVS) 水/液体石蜡的乳液体系中通入CO2 10 min后,发现有明显的油层析出,继续通CO2至30 min后乳液完全破乳,溶液回到初始的两相状态,且油水界线清晰。CO2的加入使聚合物侧链上的叔胺基团质子化从而增加了聚合物的水溶性,使聚集缠绕的聚合物链快速分散并向水相中迅速迁移,因而无法吸附在油水界面上。由于P(DEAEMA-SVS)在发生质子化作用后,其水溶性增大使其在破乳后完全存在于水相中,因此不仅不会对油相造成污染而且更有利于回收和再次利用。
破乳后的体系在60℃温度下,通入N2 30 min后,发现油水两层的界面开始模糊,油相变浑浊。待体系降温至室温后,将该体系经细胞粉碎机乳化后又可再次获得稳定的乳液。N2的加入排出了CO2,使叔胺基团发生去质子化反应,分散的聚合物链再次形成聚集缠绕的结构并从水中析出,吸附在油水界面上。由此看出P(DEAEMA-SVS)粒子(1wt%)制备的水/液体石蜡乳液具有非常好的CO2开关性能,并且上述通CO2破乳/通N2再乳化的过程可重复多次,多次循环过程中制备的乳液完全相同,图9为P(DEAEMA-SVS)聚合物粒子(1wt%)制备的水/液体石蜡乳液对CO2/N2的刺激响应及3次破乳/再乳化过程。
图10为液体石蜡与水的比例对聚合物P(DEAEMA-SVS) 水/液体石蜡乳液失稳破乳的影响。由图10(a)可以看出,当油水比小于1∶1时乳液均具有良好的稳定性。向乳液中通入CO2 30 min后,乳液能快速破乳实现油水分离,表明乳液的破乳与液体石蜡与水的比例无关。
图11为在同一油水比例下不同浓度的P(DEAEMA-SVS)水溶液与液体石蜡形成的乳液破乳10 min时的情况。随着P(DEAEMA-SVS) 水溶液浓度的升高,其所含的聚合粒子增多,使乳液对CO2的响应速率及响应能力不同。可知,由不同浓度的P(DEAEMA-SVS)水溶液制备的乳液均能破乳实现油水分离(图11(b)),但破乳的速率不同。当向体系内通入CO2 10 min后,可以看到低浓度(0.025wt%)的乳液已经完全破乳,油水几乎完全分离。而高浓度的乳液需要更长的破乳时间。P(DEAEMA-SVS)浓度大于0.8wt%所形成的乳液需要通CO2 30 min才能完全破乳,并且破乳后水溶液的乳白色也比低浓度的深。实验结果表明,聚合物的浓度一定程度上会减慢乳液的破乳速率,但均可以实现完全破乳。由此说明,该聚合物P(DEAEMA-SVS) 表面活性剂能与水/液体石蜡形成CO2开关型乳液。
2.5 P(DEAEMA-SVS)乳液的乳化和破乳机制
图12为P(DEAEMA-SVS)水/液体石蜡乳液的乳化和破乳机制。聚合物P(DEAEMA-SVS)因其高分子侧链上的叔胺基团的疏水性,聚集缠绕成小球并能吸附在油水界面上,使液体石蜡以液滴的形式均匀地分散在水相中,形成稳定的水包油型(O/W型)乳液。
当向溶液中通入CO2,使水中产生大量的H+与叔胺基团发生质子化反应形成亲水的季铵盐,增大了聚合物的亲水性,改变了物质本身的水油平衡,降低了物质的表面活性,从而使高分子链在静电斥力的作用下分散并将高分子链迁移至水中,油水界面遭到破坏,乳液的液滴相互兼并,最终引起油水两相分离,使乳液破乳。在60℃温度下,通入N2可将CO2去除,这使聚合物侧链上的叔胺基团发生去质子化作用,重新回到原来疏水的状态,并聚集缠绕成球状再次吸附在油水界面上,再次形成稳定的乳液。
3. 结 论
(1) 制备了CO2开关型聚合物P(DEAEMA-SVS)表面活性剂,能有效降低水的表面张力和水/液体石蜡的界面张力。DEAEMA/SVS单体投料比为1∶1 (摩尔比)时,形成的聚合物粒子粒径分布窄,可将水的表面张力降低至37.279 mN/m,将水/液体石蜡的界面张力降低至5.492 mN/m,其表面活性和界面活性均很强,可作为唯一乳化剂稳定乳液。
(2) 聚合物P(DEAEMA-SVS)表面活性剂水溶液可与液体石蜡形成稳定且灵敏的CO2开关型乳液。通入CO2 30 min可使乳白色的P(DEAEMA-SVS) 乳液破乳变为澄清态;在60℃下通入N2能快速地去除CO2,使溶液变回原始状态再乳化。通CO2破乳/通N2再乳化的过程可重复进行。由P(DEAEMA-SVS)粒子(1wt%)制备的水/液体石蜡乳液具有非常好的CO2开关响应性能,且该开关性能是可循环的。
(3) P(DEAEMA-SVS)表面活性剂因其侧链上的叔胺基团的疏水性,可与水/液体石蜡形成水包油型乳液。在CO2的作用下,叔胺基团发生质子化作用形成亲水的季铵盐,乳液的油水界面遭破坏且两相分离而破乳。60℃温度下通入N2可去除CO2,使聚合物侧链上的叔胺基团去质子化重新形成疏水性聚集球体吸附在油水界面上,再次稳定乳液。
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表 1 重金属对人类健康的危害
Table 1 Hazards of heavy metals to human health
Heavy metal Harm to mankind Pb Accumulates in the bones, brain, kidneys and muscles of the body, leading to brain damage, mental
retardation, anemia and cancer[21]Hg Lung, kidney and chest pain, dyspnea injury[22] Cd Cadmium ions pose a threat to many organs such as kidneys, lungs and liver[23] Cu Leading to liver damage, insomnia and Wilson disease[24] Zn Zinc not only irritates the skin, but also causes headache, dry cough, dizziness and other symptoms[25] Cr Destroy human metabolism, stimulate skin and lung cancer[26] As Skin damage and affect the nervous system of the human body[27] Ni Dermatitis, nausea, chronic asthma, cough[27] Mn Excessive manganese can cause dizziness and fatigue, mental retardation, and cause neurotransmitter disorders[28] 表 2 农业固废基水凝胶的制备及特点
Table 2 Synthesis and characterization of agricultural solid waste-based hydrogels
Solid waste-based raw materials and other reagents Method Characteristic Ref. Cellulose base Soybean residue cellulose,
chitosan, polyvinyl alcohol,
nano-Fe3O4Freeze-thaw method The increase of cellulose content is beneficial to improve the mechanical strength and swelling properties of the hydrogel. [44] Licorice residue cellulose,
crosslinker epichlorohydrinSolution polymerization The adsorption capacity, chemical stability, pore size distribution and mechanical properties of the hydrogel are improved by using epichlorohydrin as cross-linking agent. [45] Bagasse cellulose, polyvinyl alcohol, crosslinker glutaraldehyde Microwave assisted irradiation Microwave assisted irradiation technology can save time and energy. [46] Wheat straw cellulose, monomer acrylic acid and acrylamide, crosslinker N, N'-methylene-bis-acrylamide, initiator ammonium persulfate Free radical graft copolymerization Hydrogel has good thermal stability at temperature below 392℃. [47] Poplar wood flour cellulose, 3-mercaptopropionic acid, L-cysteamine hydrochloride, initiator ammonium persulfate Ultraviolet photoinitiation The cellulose ester synthesized by esterification of anhydride with cellulose under homogeneous condition has higher reaction activity. [48] Banana cellulose, chitosan, crosslinker epichlorohydrin Solution polymerization There are a large number of hydrophilic groups of hydroxyl and amino groups in the synthesized hydrogel, and the surface distributes irregular pore structure. [49] Lignin based Wheat straw lignin, montmorillonite, monomer acrylic acid, crosslinker N, N'-methylene-bis-acrylamide, initiator
K2S2O8-Na2SO3Solution polymerization Montmorillonite is introduced into lignin nanocomposites to improve the adsorption and mechanical properties of lignin nanocomposites. [50] Mulberry lignin, monomer acrylic acid, intercalation agent citric acid modified montmorillonite, crosslinker N, N'-methylene-bis-acrylamide, initiator (NH4)S2O8 Graft intercalation method When the amount of lignin is too high, the excess lignin produces too many cross-linking points, which affects the gel strength and hinders the cross-linking reaction. [51] Rice husk lignin, chitosan, polyacrylamide Free radical polymerization Lignin and chitosan are mixed as the first network to provide active functional groups for the removal of heavy metals.
As a malleable second network, polyacrylamide forms a stable structural hydrogel adsorbent.[52] Hemicellulose group Corn kernel hemicellulose, monomers acrylic acid and N-isopropyl acrylamide, crosslinker N, N'-methylene-bis-acrylamide, photoinitiator benzoin dimethyl ether Ultraviolet photoinitiation Pb2+ solution pH in the range of 3.5-4.5 and elevated temperature favored the adsorption of hydrogels. [53] Reed hemicellulose, monomer acrylic acid, crosslinker N, N, N', N'-tetramethylethylenediamine Free radical polymerization After 8 cycles of adsorption-desorption, the hydrogel still has high adsorption efficiency for metal ions. [39] Xylan hemicellulose, carrageenan, initiator ammonium persulfate, polyvinylpyrrolidone Microwave assisted irradiation Compared with cellulose, xylan hemicellulose is amorphous and consists of branched chains of various sugars, which is easier to dissolve in common solvents. [54] Other types of agriculture Citrus peel pectin, crosslinker calcium chloride, metal organic frame (MOF) In-situ method MOFs can interact with other components or adjust the properties of hydrogel matrix. [55] Pomelo peel pectin, monomers acrylic acid and acrylamide, initiator ammonium persulfate, crosslinker N, N'-methylene-bis-acrylamide Graft copolymerization Microwave extraction-alcohol precipitation method for pectin extraction has the advantages of strong selectivity, short operation time, low solvent consumption and excellent quality of extracted pectin. [56] Livestock bone powder, sodium alginate Solution polymerization Adsorption kinetics and adsorption isotherms indicate that adsorption is chemically and physically interactive. [57] 表 3 农业固废基水凝胶吸附重金属的优缺点
Table 3 Advantages and disadvantages of heavy metal adsorption on agricultural solid waste-based hydrogels
Common type Common ground Advantage Disadvantage Cellulose based hydrogel
Advantages: The raw materials are non-toxic, biodegradable and low cost; Make the hydrogel biocompatible, biodegradable and improve its adsorption performance.
Disadvantages: Unhomogeneity of raw materials, structural complexity, and difficulty in obtaining pure substances, and poor mechanical strength of prepared hydrogels.Contains hydroxyl, carboxyl and hydrogen bonding to improve the adsorption and mechanical properties
of hydrogels.Refractory; After several cycles of use, the morphology and mechanical properties of the hydrogel will change considerably, resulting in an unstable adsorption capacity. Hemicellulose based hydrogel Contains functional groups such as hydroxyl, acetyl and carboxyl groups; Good water solubility. The mechanical strength of the prepared hemicellulose-based hydrogels is poor due to the small molecular weight of hemicellulose and low degree of polymerization. Lignin based hydrogel Hydroxyl and carbonyl groups can chelate with metal ions; Active site for chemical reactions. The inhomogeneity and structural complexity of lignin and the difficulty of obtaining pure lignin;And when the amount of lignin is too high, excess lignin produces too many cross-linking points, which affects the gel strength and hinders the cross-linking reaction. Other agricultural solid waste-based hydrogels Pectin contains reactive groups such as carboxyl and hydroxyl groups. Insoluble; Pectins from different sources have different gelling and adsorption capacities due to different sizes and chemical structures, and are less stable. 表 4 农业固废基水凝胶对重金属的吸附性能总结
Table 4 Summary of adsorption properties of agricultural solid waste-based hydrogels for heavy metals
Hydrogel Target
pollutantAdsorption
conditionMaximum adsorption capacity/(mg·g−1) Adsorption kinetic/
adsorption isothermRecycling rate Ref. Cellulose base Soybean residue cellulose magnetic hydrogel Cu(II) pH=5.0 15.94 Pseudo-second-order/
Freundlich4 times
>80%[44] Licorice residue cellulose hydrogel Pb(II)
Cr(III)
Cu(II)pH=5.0(Pb)
t=120 min,
pH=4.0(Cr)(Cu)
t=60 min591.8
458.3
121.4Pseudo-second-order/
Langmuir5 times
>75%[45] Bagasse carboxymethyl cellulose hydrogel Cu(II) pH=5.0 2.3 — — [46] Wheat straw cellulose hydrogel Cu(II)
Mn(II)500 mg/L Co (Cu)
400 mg/L Co (Mn)238.1
176.9Pseudo-second-order — [47] Aspen cellulose methacrylate hydrogel Pb(II)
Hg(II)Hydrogel
dosage 0.2 g148.44
112.55Pseudo-second-order/
Langmuir— [48] Grapefruit peel cellulose-based hydrogel Cu(II)
Cr(VI)
Cd(II)Hydrogel dosage 10 g/L, pH=7.0, T=33℃, t=60 min, 10 mg/L Co Removal rate 96.21%, 98.02%,
95.43%— — [59] Banana fiber-chitosan hydrogel Cu(II)
Cd(II)
Pb(II)Hydrogel dosage 5 g/L, t=40 min Removal rate 98.35%, 79.22%,
77.3%Pseudo-second-order — [49] Lignin base Wheat straw lignin-montmorillonite hydrogel Cu(II) pH=6.5 74.35 Pseudo-second-order/
Freundlich5 times
>80%[50] Mulberry lignin hydrogel Mn(II)
Zn(II)
Pb(II)320 mg/L Co (Mn),
160 mg/L Co (Zn),
1000 mg/L Co (Pb),
t =720 min77.09
73.95
383.90— — [51] Hemicellulose based Reed hemicellulose-based hydrogel Pb(II)
Cd(II)
Zn(II)200 mg/L Co,
pH=5.5(Pb),
pH=6.5(Cd)(Zn)699
521
265Pseudo-second-order/
Langmuir— [39] Yellow bamboo xylan hemicellulose-acrylic hydrogel Pb(II)
Cd(II)
Zn(II)t=60 min,
pH=5.5(Pb),
pH=6.5(Cd)(Zn)859
495
274Pseudo-second-order/
Langmuir5 times
>90%[60] Other
categoriesMetal-organic frame composite citrus peel gum hydrogel Cr(VI)
Pb(II)pH=1(Cr),
pH=5(Pb)825.97
913.88Pseudo-second-order/
Langmuir8 times
>85%[55] Grapefruit peel pectin-based composite hydrogel Cu(II) pH=6.0 80.6 Pseudo-second-order/
Freundlich4 times
>90%[61] Bone hydrogel of abandoned livestock and poultry in rural area Cd(II) pH=4.0, t=720 min 1010.19 Pseudo-second-order and intra-particle diffusion models/
Langmuir and Freundlich3 times
>90%[57] Notes: T—Temperature; t—Time. 表 5 工业固废基水凝胶对重金属的吸附性能总结
Table 5 Summary of adsorption properties of industrial solid waste-based hydrogels for heavy metals
Hydrogel adsorption
materialTarget pollutant Adsorption condition Maximum adsorption
capacity/(mg·g−1)Ref. Red mud-sodium alginate hydrogel Pb(II) pH=6.0, T=25℃, t=900 min, 900 mg/L Co 454.54 [80] Fly ash-hydroxyethyl cellulose-chitosan hydrogel Cu(II) 100 mg/L Co 130.7 [81] Modified fly ash-sodium alginate crosslinked
acrylic hydrogelCu(II) 300 mg/L Co 131.09 [82] Magnetic attapulgite/fly ash/polyacrylic
acid hydrogelPb(II) pH=5.0, 100 mg/L Co, t=24 h 38 [83] Tobe mullite-starch hydrogel Cd(II) t=12 h, pH=5.21, T=25℃, hydrogel
dosage 0.05 g591.36 [84] -
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目的
水凝胶是一种具有三维立体网状结构的多孔亲水聚合物,在处理重金属离子方面具有很大的潜力。随着经济化的发展,我国固体废弃物的产生量急剧增加。利用固废制备水凝胶作为一种新型绿色材料,在去除环境中的重金属方面具有广阔的应用前景。本文主要论述了农业固废基(纤维素、半纤维素、木质素、果胶等)用于水凝胶的制备,以及对重金属离子的吸附情况,以期为更深入地了解水凝胶在处理重金属的应用提供参考。
方法查阅国内外文献,论述了现阶段利用农业固废材料制备水凝胶的情况,主要包括农业固废基水凝胶的合成,吸附重金属的效果、分析方法以及吸附机理。
结果秸秆、甘蔗渣、稻壳等农业废弃生物质,含有大量的纤维素、半纤维素和木质素。木质素、半纤维素、纤维素分子含有羟基和羧基,具有螯合能力,可以与重金属离子发生静电相互作用。将其加入到水凝胶的制备当中,可以提高水凝胶的生物可降解性以及对重金属的吸附能力。农业固废基水凝胶对重金属离子的吸附机理主要通过络合作用、静电相互作用和离子交换作用。吸附等温线与Langmuir和Freundlich模型较为复合,表明吸附是通过重金属与水凝胶发生化学和物理作用进行的。吸附动力学模型与准二级动更为拟合。
结论(1)农业废弃生物质中含有丰富的纤维素、半纤维素、木质素,这些物质不仅具有无毒性、可生物降解等特点,还含有大量的羟基、羧基和氢键等官能团,能与重金属发生螯合作用。将其加入到水凝胶的制备中,可以使水凝胶具有生物相容性、生物可降解性,并增强对重金属的吸附效果。(2)但是这些物质的不均一性、结构复杂性、纯品物质难以获得,以及分子量较小,聚合度较低,制备的水凝胶的力学强度较差等问题还需要进一步探索。(3)目前大多的研究还停留在实验室阶段,未应用到实际工程中。除了以上的不足,还需考虑以下问题。首先,多数实验所用的重金属离子溶液为实验室模拟配置的废水,为了以后更好的实际工程应用,应选用实际废水进行研究。其次,关于将农业固废经过改性或许与其他物质复合形成水凝胶等手段,以及水凝胶的制备过程都涉及较多的化学试剂,并且工艺流程复杂性、成本问题都制约其发展。