Preparation of lignin surface-functionalized MXene nanosheets and its U(VI)adsorption properties
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摘要: 为了进一步改善MXene纳米材料对模拟放射性废水中U(VI)的吸附性能,利用天然资源酶水解木质素(EHL)作为生物表面活性剂对MXene进行表面功能化处理,采用SEM-EDS、XRD及FTIR对改性前后的材料进行了表征分析,并在吸附实验中探究了pH、温度、反应时间、干扰离子及不同初始U(VI)浓度等因素对除U(VI)效果的影响。结果表明,EHL阻止了MXene纳米片的聚集堆叠,并且引入了大量活性官能团,提高了EHL功能化MXene纳米片的吸附性能。在MXene与EHL的质量比为1∶5、投加量为0.1 g·L−1、pH为5、温度为303 K时,对U(VI)的最大吸附容量为231.95 mg·g−1。此外,吸附动力学和等温线分析表明,拟二级动力学模型和Freundlich等温线模型能很好地拟合此吸附过程,热力学分析表明其吸附过程是自发吸热的。经历5次循环再生后,对U(VI)的去除率仍在80%以上。表征分析结果表明,MX/EHL与U(VI)之间相互作用机制包括离子交换、静电吸引以及与含氧官能团之间的络合作用。基于此研究,MX/EHL作为一种环境友好型吸附材料,对去除废水中的U(VI)具有巨大潜力。Abstract: In order to further improve the adsorption performance of MXene nanomaterials on U(VI) in simulated radioactive wastewater, the surface functionalization of MXene was carried out by using natural resources of enzymatically hydrolyzed lignin (EHL) as a biosurfactant, and the materials before and after the modification were characterized and analyzed by using SEM-EDS, XRD, and FTIR, and the effects of pH, temperature, and the adsorption experiments were explored, reaction time, interfering ions and different initial U(VI) concentrations on the effect of U(VI) removal. The results show that EHL prevents the re-stacking of MXene nanosheets and introduces a large number of active functional groups, which improves the adsorption performance of EHL-functionalized MXene nanosheets. The maximum adsorption capacity for U(VI) is 231.95 mg·g−1 at the mass ratio of MXene to EHL of 1∶5, the dosage of 0.1 g·L−1, pH=5, and the temperature of 303 K. In addition, the adsorption kinetic and isotherm analyses show that the proposed second-order kinetic model and the Freundlich isotherm model fit this adsorption process well, and the thermodynamic analyses indicate that its adsorption process is spontaneous heat absorption. After five cycles of regeneration, the removal rate of U(VI) is still above 80%. Characterization results reveals that the interaction mechanisms between MX/EHL and U(VI) involve ion exchange, electrostatic attraction, and complexation with oxygen-containing functional groups. Based on this study, MX/EHL has great potential as an environmentally friendly adsorbent material for the removal of U(VI) from wastewater.
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Keywords:
- MXene /
- lignin /
- nanomaterials /
- U(VI) /
- adsorption performance
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随着全球经济的发展和化石燃料的消耗剧增,全球石油的需求量在逐年增加,多次采油技术得到了更广泛应用[1],原油采出液的含水率也在逐年增加,正在给中国这一世界最大石油进口国带来严峻的挑战。同时,随着海上油气开发向深海迈进,深海平台的油水分离、采油井井底油水分离等都迫使人们探索新的油水分离技术。
工业生产运输过程中的各类不溶性有机物的泄露和排放导致了各种严重的环境问题[2-5]。例如石油加工和运输过程中的原油和成品泄露导致的水污染。皮革处理、食品加工、冶金、化工等工业生产过程中排放的含油废水。这些含油废水如不经处理直接排放,会对生态系统、农业生产构成严重破坏,危及人类健康,因此需要有效的油水分离技术来解决分离问题。目前主要的油水分离形式包括重力分离、离心、吸附和膜分离等[6-9]。将含油废水进行油水分离后,对废水的后续的处理难度和成本将会大大降低。膜分离技术作为一种先进的水净化技术,由于其经济、节能、易于操作的特点,已经成为一种不可或缺的选择[10-12]。然而,面对愈加复杂的含油污水(轻/重油和水的混合物、水包油乳液和油包水乳液的混合体系)以及不同酸碱环境,单一的除油型、除水型分离材料无法实现按需、高效和可持续的分离。因此,开发先进的智能油水分离材料是解决世界水环境恶化和石油短缺问题的迫切需要。
对某种外界刺激敏感并产生特殊反应的现象称为智能响应。具有可转换润湿行为的人工智能响应界面材料引起了人们越来越多的兴趣[13-15]。研究表明,可转换润湿行为通常可通过调节pH值[16],改变光源[17]、温度[18]、磁场强度[19]等方式实现。pH响应材料可定义为在其结构中包括弱酸性或碱性基团的聚电解质,酸性或碱性基团如羧基、吡啶、磺酸、磷酸盐、叔胺等通常被称为pH响应基团,这些基团响应于pH的变化。基团会随着环境pH变化而接受或释放质子,导致结构和性质的变化,从而实现材料的pH响应。例如,Liu等[20]合成了一种基于分子印迹聚合物的pH响应型纳米药物,该纳米药物在的模拟肿瘤微环境中表现出良好的 pH 响应性,可通过特定的分子印迹位点选择性地从前列腺肿瘤中螯合睾酮。Surapaneni等[21]合成了一种具有温度和 pH 双重刺激响应的聚N-乙烯基己内酰胺和聚赖氨酸的嵌段共聚物,该共聚物可在两种刺激单独或共同作用下增加细胞对聚合物囊泡的渗透性,用于增强细胞内化和溶酶体靶向药物运送。
光作为一种低成本、绿色环保的刺激方法,使含有光响应基团的材料具有非接触式的遥控特性。光响应性能可以通过引入光响应基团来实现,该基团在特定波长的照射下发生结构变化,从而导致材料的性能发生变化[22-24]。目前常用的光响应基团是偶氮苯及其衍生物。偶氮化合物有两种异构体,一种是稳定态的反式结构,另一种是亚稳态的顺式结构。偶氮苯官能团经紫外线照射后,反式的非极性异构体可以转化为顺式的极性异构体,并且这一过程具有可逆性。由于偶氮苯反式构象的极性较弱,顺式构象的极性较强,在顺反异构体转变的过程中会导致材料的极性发生变化,其亲疏水性也随之变化,可以利用这一特性制备光响应智能转换油水分离材料,因为偶氮苯顺反异构体转换的可逆性,所以亲疏水性能的变化过程也是可逆的[25-28]。Yang等[29]利用偶氮苯聚合物作为衬底,制备一种可光切换的超疏水表面,该表面可受紫外和可见光的影响,使聚合物膜发生亲水到疏水的可逆改变。Du等[30]制备了一种功能化的偶氮苯聚合物,制备的偶氮苯溶液在紫外和可见光交替暴露下表现出偶氮苯的光异构化转变以及亲水到亲油的可逆变化。偶氮苯衍生物因其良好的化学稳定性和多功能性在光响应材料的制备和应用中具有广阔的发展前景[31]。
本文采用可逆加成-断裂链转移(RAFT)一步聚合法,制备得到了具有pH/光刺激响应性能的三元无规共聚物,并将其与无纺布相结合,得到了具有智能化可逆响应性能复合织物的油水分离膜。这种油水分离膜在油水分离、工业复杂废水处理等方面具有巨大的应用潜力。
1. 实验材料及方法
1.1 原材料
四氢呋喃(THF)、盐酸(35%)、甲醇,西陇科学公司;二硫代苯甲酸异丙酯(CDB),阿拉丁生物技术有限公司;无纺布,深圳瑞都净化公司;偶氮二异丁腈(AIBN)、甲基丙烯酸羟乙酯(HEMA)、甲基丙烯酸二甲氨乙酯(DMAEMA)、对氨基苯甲酸、苯酚、氢氧化钠、亚硝酸钠、三乙胺、丙烯酰氯,麦克林公司。上述试剂均是分析纯,实验用水为去离子水。
1.2 样品表征
傅里叶红外光谱仪(VERTEX 70 Bruker)、 超导核磁共振波谱仪(BRUKER 500 MHz AVANCE NEO)、紫外可见分光光度计(UV-3600 Plus,日本岛津)、扫描电子显微镜(ZEISS GeminiSEM 500,德国卡尔蔡司) 、视频光学接触角测试仪(OCA25 Eastern-Dataphy)。
1.3 智能响应油水分离材料的合成
合成主要分为4个步骤:(1)重氮化-偶联反应;(2)酯化反应;(3)酯水解反应;(4)聚合反应。合成路线见图1。
1.3.1 4-(4'-羟基)苯偶氮基苯甲酸的合成
通过重氮化-偶联反应制备4-(4'-羟基)苯偶氮基苯甲酸。将35%的盐酸12 mL与等体积的蒸馏水混合,缓慢滴加到装有对氨基苯甲酸(4.10 g,30 mmol)的烧瓶中搅拌均匀后,将30 mL的NaNO2水溶液(1 mol/L)缓慢滴入烧瓶反应30 min后加300 mL冰水稀释。然后滴加20 mL苯酚(2.94 g,31 mmol)和NaOH (1.72 g,31 mmol)水溶液。0~5℃下反应2 h,用NaOH调节pH至5~6。过滤收集固体,蒸馏水洗涤,得到4-(4'-羟基)苯偶氮基苯甲酸(中间体a)的橘黄色固体6.97 g,产率85%。
1.3.2 丙烯酸-4-((4-(丙烯氧基)苯基)二氮基)苯甲酸酐的合成
将1.3.1制得的偶氮苯(2.42 g,10 mmol)与三乙胺(1.47 mL,10 mmol)溶于25 mL THF并滴加丙烯酰氯(0.8 mL,10 mmol),室温下反应24 h。滤去生成的盐,浓缩滤液并滴入甲醇中沉淀,得到丙烯酸-4-((4-(丙烯氧基)苯基)二氮基)苯甲酸酐黄色固体1.67 g (中间体b),产率57%。
1.3.3 4-((4-(丙烯氧基)苯基)二氮基)苯甲酸的合成
将1.3.2中制备的偶氮苯1.51 g溶解在20 mL THF中,用10%的氢氧化钠水溶液调节pH至7~8,室温反应24 h,后用5%的盐酸水溶液调节pH至2~4,过滤并用蒸馏水洗涤,得到橙色固体1.24 g (单体c),产率83%。
1.3.4 智能双响应聚合物的制备
采用可逆加成-断裂链转移聚合(RAFT),一步反应制得智能响应聚合物。这种聚合也被称为“活性”/可控自由基聚合。由于其反应条件温和、单体选择范围广、分子设计能力强等优点,已发展成为最通用、最强大的聚合技术之一。
将1.3.3中的产物(1.55 g,5 mmol)与 DMAEMA(0.339 g,2.16 mmol)、HEMA (0.280 g,2.16 mmol)溶于THF后通氮排氧,温度升至70℃后加入AIBN (0.08 g,0.5 mmol)、CDB (0.2 g,0.2 mmol)反应48 h,旋蒸去除溶剂,甲醇洗涤后,干燥,得到橘红色聚合物1.84 g (聚合物d),产率85%。
1.4 智能响应涂层的制备
将1.3.4中得到的聚合物均匀地涂膜在载玻片上,60℃下真空干燥,用于测量接触角。
1.5 智能油水分离膜的制备
将直径5 cm的无纺布用无水乙醇浸泡并超声清洁表面灰尘和油脂后,浸入未干燥的聚合物中,使聚合物均匀地涂在无纺布表面,60℃下真空干燥48 h,得到智能双响应油水分离膜。
2. 结果与讨论
2.1 表征与形貌
合成的各中间体、单体和聚合物的红外谱图如图2所示。
3452 cm−1处的吸收峰为偶氮苯中苯环上不饱和C—H键的伸缩振动,1606 cm−1为苯环骨架的C=C伸缩振动,807 cm−1为苯环上的C—H面外弯曲振动峰。a、c、d 在2942 cm−1处的吸收峰为羧基的O—H伸缩振动峰;b、c在1721 cm−1处存在C=C的伸缩振动;b在1786 cm−1处存在酸酐的C=O伸缩振动;聚合物d在3250 cm−1左右处存在DMAEMA中叔胺甲基上C—H伸缩振动峰和HEMA中的O—H伸缩振动峰。图3为偶氮苯单体及聚合物的1H NMR图,图3(a)中,8.13×10−6、8.01×10−6、7.45×10−6和5.88×10−6处的峰对应单体c结构中为A、B、C和D的氢。图3(b)中,1.13×10−6和1.26×10−6处的氢对应于聚合物d中F和G相应位置的氢,A、B、C和D的氢与单体结构中的峰位置基本一致。
将偶氮苯溶于DMF (0.15 g·L−1)中,用365 nm紫外灯(20 W)照射2 h,使溶液中的单体转化为顺式结构后,分别检测不同照射时间的UV-Vis吸收光谱,吸收光谱随时间变化的关系如图4所示。其中,图4(a)为用LED灯(445 nm,20 W)对溶液进行照射,直至吸光度不再变化的吸收光谱。图4(b)为继续用365 nm紫外灯对溶液进行照射后的吸收光谱。偶氮苯特征吸收峰主要是330~380 nm处的π-π*吸收峰和420~500 nm的n-π*吸收峰。从图4中单体在365 nm和445 nm光照射下吸收光谱的变化情况可以看出,365 nm和445 nm的光照下,随照射时间的增加,单体在溶液中π-π*吸收峰的位置几乎不发生移动,而吸光度会逐渐变化。n-π*特征峰处的吸光度和吸收峰的位置均变化很小。吸光度的变化归因于紫外-可见光照射引起的偶氮苯从顺式到反式的异构化。对比图4(a)与图4(b)中吸收光谱变化,可以看到偶氮苯的顺反异构化是一个可逆的过程。
未经处理的无纺布(图5(a))纤维表面相对光滑。经过涂膜的无纺布(油水分离膜,图5(b)),可以观察到纤维表面及相邻纤维间粘附的聚合物,作为基底的无纺布具有多孔结构和交错的纤维,有利于聚合物的粘附并为油水分离提供了必要的空间。无纺布的柔性还使油水分离膜具有良好的柔韧性,可以承受多次折叠而不损坏。
2.2 智能响应及油水分离测试
将聚合物均匀地涂抹在载玻片上烘干后,分别在自然条件、pH=3的缓冲溶液浸泡、紫外-可见光照射、缓冲溶液浸泡后继续用紫外-可见光照射等情况下进行接触角测量实验,每次滴液的量为5 μL,接触角变化如图6所示。
图 6 聚合物d的载玻片涂层在不同条件下的接触角变化:(a)自然条件下;(b)经pH=3缓冲溶液浸泡后;(c)经pH=10缓冲溶液浸泡后;(d)经365 nm光照射后;(e)经445 nm光照射后;(f)经pH=3缓冲溶液浸泡并用365 nm紫外光照射后Figure 6. Contact angle variation of polymer d slide coatings under different conditions: (a) The natural state; (b) After immersion with pH=3 buffer solution; (c) After immersion in pH=10 buffer solution; (d) After exposure to 365 nm light; (e) After exposure to 445 nm light; (f) After immersion in pH=3 buffer solution and irradiation with 365 nm ultraviolet light图6显示了聚合物涂层在不同条件下的接触角的变化情况,该变化能够反映涂层材料的智能响应特性。在自然条件下的接触角143.7°,见图6(a)。将其在pH=3的缓冲溶液中浸泡1 h后烘干,接触角变化至109.9°,见图6(b)。该过程的接触角是由聚合物中DMAEMA单元和偶氮苯上羧基质子化共同作用导致的。随后将涂层置于pH=10的缓冲溶液中浸泡1 h后烘干,其接触角恢复至142.2°,见图6(c),将烘干后的涂层在365 nm (20 W)紫外灯下照射5 min,接触角变化至48.4°,见图6(d)。用LED灯照射15 min后,其接触角恢复至139.3°,如图6(e)所示。该过程接触角的变化是由于聚合物中偶氮苯单元经过紫外-可见光照射后发生构型转化使涂层的润湿性发生转变。最后将涂层置于pH=3的缓冲溶液中浸泡1 h后烘干,然后将其置于365 nm (20 W)紫外灯下照射15 min,该过程由pH响应单元与光响应单元共同作用,其接触角由139.3°变化至19.0°,见图6(f)。
上述过程证明了涂层具有良好的刺激响应能力。以相同的方法重复进行上述实验,接触角的变化如图7所示。实验结果表明,经过多次刺激响应,聚合物涂层发生亲水和疏水的多次转换,且接触角仍可恢复到初始状态,证明了涂层的转换润湿能力具有良好的可逆性。其中,光在转化过程中具有更加显著的效果,这也在随后的油水分离试验中得到证实,切换时间也与偶氮苯单体吸收光谱的转化时间基本对应。由于光照可以在没有物理接触的情况下实现转化,这增强了材料的实际应用的便利性。
为了测试光/pH双响应油水分离膜的分离效果,将柠檬黄染色的水100 mL与石油醚以2∶1的体积比混合后倒入滤杯中进行油水分离实验。为加快油/水的分离速度,在0.005 MPa的压力下用抽滤的方法模拟油水分离系统 (图8)。图8(a)中,抽滤30 s后,无色的石油醚被抽滤下来,不再有液体从上方流下,染色的水保留在上方滤杯中。在图8(b)中,用365 nm光照射分离膜12 h,以相同的方法进行油水分离,抽滤25 s后,染色的水被抽滤下来,石油醚被保留在上方。
分别测量分离后上方滤杯中的油或水的体积,计算得到分离效率分别为96.3% (图8(a))和95.8%(图8(b))。实验结果证明了分离膜具有良好的智能转换油水分离能力。
3. 结 论
(1)以合成的偶氮苯、甲基丙烯酸羟乙酯(HEMA)和甲基丙烯酸二甲氨乙酯(DMAEMA)为原料,采用可逆加成-断裂链转移(RAFT)法一步聚合,制得光和pH智能双响应三元无规共聚物。该聚合物以偶氮苯作为光响应单元,DMAEMA作为pH响应单元,HEMA使聚合物具有良好的柔性附着性能,FTIR和1HNMR谱图说明了聚合物的成功合成。
(2)通过在不同条件下的润湿性实验,验证了聚合物涂层的可转换润湿性。紫外线的照射导致偶氮苯单元的分子构型发生转变,结构上的变化导致聚合物的润湿性和渗透性的变化。在仅有pH=3的缓冲溶液浸泡后其最大接触角转变达到33.8°,仅有紫外光照射下其最大接触角转变达到93.8°,而在光和pH的共同作用下,其最大接触角变化可达到120.3°。说明了涂层在光照和pH的刺激下具有优异的刺激响应性。经过亲水与疏水的多次转换实验后,接触角仍可恢复到初始状态,证明了涂层对水的润湿能力切换具有可逆性。
(3)偶氮苯的引入使聚合物对光具有良好的响应性,DMAEMA上含有的叔胺基团和偶氮苯上的羧基共同增强了涂层的pH响应性和抗酸性,可以使涂层应对一些复杂情况下的油水分离,油水分离实验证明涂层在光照下的极性转变,单次分离效率分别达96.3%和95.8%。
综上,通过一步RAFT聚合制备得到的光/pH双响应的聚合物材料,可用来制备智能光/pH双响应油水分离膜,该膜具有优异的可逆刺激响应性能。
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图 9 MX/EHL吸附U(VI)的Langmuir (a)、Freundlich (b)和Dubinin-Radushkevich (c)等温吸附模型拟合曲线;(d) lnK0与1/T的线性拟合
Figure 9. Fitting curve of Langmuir (a), Freundlich (b) and Dubinin-Radushkevich (c) isothermal adsorption model of U(VI) adsorption by MX/EHL; (d) Linear fit of lnK0 versus 1/T
Ce—U(VI) concentration at adsorption equilibrium; R—Universal gas constant; T—Temperature (K); K0—Equilibrium constant at different temperatures
表 1 MX和MX/EHL的孔隙结构参数
Table 1 Pore structure parameters of MX and MX/EHL
Material Surface area/
(m2·g−1)Pore volume/
(cm3·g−1)Pore diameter/
nmMX 3.8297 0.0100 10.4628 MX/EHL 8.7751 0.0455 20.7320 表 2 MX/EHL对U(VI)的吸附动力学参数
Table 2 The adsorption kinetic parameters of MX/EHL on U(VI)
Name of
samplePseudo-first-order Pseudo-second-order Intraparticle diffusion qe,exp/
(mg·g−1)k1/
min−1qe,cal/
(mg·g−1)R2 k2/
min−1qe,cal/
(mg·g−1)R2 kp1/
(mg·(g·
min0.5)−1)C1 R21 kp2/
(mg·(g·
min0.5)−1)C2 R22 kp3/
(mg·(g·
min0.5)−1)C3 R23 MX 35.22 0.017 3.487 0.882 0.021 35.51 0.999 0.688 29.782 0.973 0.406 31.266 0.989 0.015 35.029 0.804 MX/EHL (1:4) 46.92 0.017 2.737 0.949 0.027 47.13 0.999 0.282 43.819 0.958 0.324 43.568 0.986 0.058 46.175 0.653 MX/EHL (1:5) 48.24 0.018 2.502 0.930 0.030 48.43 0.999 0.338 45.002 0.981 0.303 45.243 0.994 0.035 47.794 0.615 Notes: qe,exp—Actual adsorption capacity at adsorption equilibrium; qe,cal—Calculated adsorption capacity at adsorption equilibrium; k1 and k2—Adsorption rate constants of the pseudo-first and pseudo-second, respectively; R2—Correlation coefficient; kp1, kp2, kp3—Adsorption rate constants of intraparticle diffusion; C1, C2, C3—Adsorption constants of intraparticle diffusion. 表 3 Langmuir、Freundlich和Dubinin‒Radushkevich吸附等温线模型的相关参数
Table 3 Parameters associated with Langmuir, Freundlich and Dubinin-Radushkevich adsorption isotherm models
T/K Langmuir Freundlich Dubinin‒Radushkevich qmax/(mg·g−1) KL/(L∙mg−1) R2 KF 1/n R2 qDR E/(kJ·mol−1) R2 293 205.493 0.164 0.890 48.175 0.399 0.982 115.99 1.879 0.554 298 217.057 0.221 0.925 58.802 0.378 0.989 129.09 2.077 0.607 303 231.947 0.251 0.924 65.565 0.379 0.997 138.33 2.337 0.627 Notes: qmax—Maximum adsorption capacity; KL—Langmuir adsorption equilibrium constant; KF and n—Constants that are related to the adsorption capacity and the adsorption intensity, respectively; qDR—Theoretical isotherm saturation capacity; E—Average free energy of adsorption. 表 4 不同吸附剂对U(VI)的吸附去除效果对比
Table 4 Comparison of adsorption and removal effects of different adsorbents on U(VI)
Adsorbent pH T/K qmax/(mg·g−1) Ref. C-TC 5 308 165.43 [20] MXene/SA 4 298 126.82 [34] C-TC-CS 6 313 141.96 [36] PANI/Ti3C2Tx 5 298 102.80 [37] PAO/Ti3C2Tx 4 298 98.04 [38] Ti3C2-AO-PA 8.3 298 81.10 [41] MX/EHL 5 303 231.95 This work Notes: C-TC—Chloroacetic acid modified-Ti3C2Tx; MXene/SA—MXene composite sodium alginate gel microsphere; C-TC-CS—Chloroacetic acid-modified MXene-CS gel microspheres; PANI/Ti3C2Tx—Polyaniline modified MXene composites; PAO/Ti3C2Tx—Polyamidoxime functionalized MXene composite; Ti3C2-AO-PA—Polyamide enhanced amidoxime-functionalized Ti3C2 nanosheet. 表 5 MX/EHL吸附U(VI)的热力学参数
Table 5 Thermodynamic parameters of MX/EHL adsorption of U(VI)
T/K lnK0 ΔG0/(kJ·mol−1) ΔH0/(kJ·mol−1) ΔS0/(J·(mol·K)−1) 293 4.69 −11.43 38.89 175.26 298 5.00 −12.39 303 5.23 −13.18 Notes: ΔH0—Standard enthalpy change; ΔG0—Standard free energy change; ΔS0—Standard entropy change. -
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目的
新型纳米材料MXene在吸附重金属离子过程中存在稳定性较弱,容易被氧化等不足,此外,在范德华力的影响下,MXene纳米片往往会发生严重的聚集堆叠现象,导致其表面大量的活性位点无法得到有效利用,严重限制了其吸附性能。为了进一步改善MXene纳米材料对模拟放射性废水中U(Ⅵ)的吸附性能,利用天然资源酶水解木质素(EHL)作为生物表面活性剂对MXene进行表面功能化处理,探究酶水解木质素的引入对于MXene纳米材料的作用以及改性前后材料对U(Ⅵ)的吸附性能和作用机制。
方法通过溶液共混法将具有抗氧化性和生物活性的工业副产物酶水解木质素(EHL)引入到MXene纳米材料的表面和层间,制备一种有机-无机杂化复合材料(MX/EHL)。采用现代表征分析技术SEM-EDS、XRD及FTIR等对改性前后的材料进行了表征分析,探究改性前后MXene材料微观结构发生的变化。通过批量静态吸附实验探究pH、温度、反应时间、干扰离子、吸附剂投加量及不同初始U(Ⅵ)浓度等影响因素对吸附剂除U(Ⅵ)效果的影响。通过吸附解吸实验探究吸附剂的循环再生能力。根据吸附实验结果以及表征分析结果探究吸附剂对U(Ⅵ)的作用机制。
结果(1)SEM-EDS、XRD、FTIR等表征分析结果表明酶水解木质素(EHL)成功引入到MXene纳米材料上,并且EHL的引入不仅没有破坏MXene材料的多层结构,而且附着在MXene纳米片层间的EHL颗粒还阻止了纳米片的聚集堆叠,增大了层间距,使得更多的活性吸附位点能够得到利用。此外,EHL表面丰富的含氧基团赋予了其极佳的亲水性和化学反应活性,使其能够与MXene表面的-OH和-F基团产生良好的相互作用,极大地克服了MXene材料在吸附过程中易于氧化的缺点。(2)批量静态吸附实验结果表明,MX/EHL在弱酸性条件下能够有效吸附铀酰离子。在MXene与EHL的质量比为1:5、投加量为0.1 g·L、pH为5、温度为303 K、反应时间为2 h时,对U(VI)的最大吸附容量达到231.95 mg·g,去除率达到96.6%,具有超快的吸附动力学(5 min,去除率>90%),优于同类型的其他吸附剂。吸附动力学和等温线分析表明,吸附过程符合拟二级动力学模型(=0.999),表明化学吸附占主导作用,吸附等温线符合Freundlich模型,说明MX/EHL对U(VI)的吸附以多层吸附为主,U(VI)在MX/EHL非均相表面发生吸附过程。(3)经历5次循环再生后,对U(VI)的去除率仍在80%以上,表明该吸附材料具有良好的循环再生性能。
结论将酶水解木质素(EHL)引入到MXene纳米材料中可以改善纯MXene纳米材料在吸附重金属离子过程中所出现的易聚集堆叠和易氧化的缺点,所合成的MX/EHL复合材料对U(VI)的吸附性能显著提高,在多种干扰离子存在的情况下仍能实现对U(VI)的选择性吸附,并且具有良好的循环再生性能。