聚乙烯亚胺交联膨润土对水中Cr(Ⅵ)的吸附性能与机制

孙志勇, 张宇辰, 吴喜军

孙志勇, 张宇辰, 吴喜军. 聚乙烯亚胺交联膨润土对水中Cr(Ⅵ)的吸附性能与机制[J]. 复合材料学报, 2025, 42(2): 949-960. DOI: 10.13801/j.cnki.fhclxb.20240428.003
引用本文: 孙志勇, 张宇辰, 吴喜军. 聚乙烯亚胺交联膨润土对水中Cr(Ⅵ)的吸附性能与机制[J]. 复合材料学报, 2025, 42(2): 949-960. DOI: 10.13801/j.cnki.fhclxb.20240428.003
SUN Zhiyong, ZHANG Yuchen, WU Xijun. Adsorption performance and mechanism of polyethyleneimine cross-linked bentonite for Cr(VI) in aqueous solution[J]. Acta Materiae Compositae Sinica, 2025, 42(2): 949-960. DOI: 10.13801/j.cnki.fhclxb.20240428.003
Citation: SUN Zhiyong, ZHANG Yuchen, WU Xijun. Adsorption performance and mechanism of polyethyleneimine cross-linked bentonite for Cr(VI) in aqueous solution[J]. Acta Materiae Compositae Sinica, 2025, 42(2): 949-960. DOI: 10.13801/j.cnki.fhclxb.20240428.003

聚乙烯亚胺交联膨润土对水中Cr(Ⅵ)的吸附性能与机制

基金项目: 国家自然科学基金(52064048);陕西省科技创新团队(2022TD-08)
详细信息
    通讯作者:

    孙志勇,硕士,教授,硕士生导师,研究方向为功能材料、环境污染治理技术 E-mail: sunzy11@126.com

  • 中图分类号: X703;TB332

Adsorption performance and mechanism of polyethyleneimine cross-linked bentonite for Cr(VI) in aqueous solution

Funds: National Natural Science Foundation of China (52064048); Shaanxi Provincial Science and Technology Innovation Team (2022TD-08)
  • 摘要:

    为提高膨润土的吸附容量,通过交联反应将聚乙烯亚胺(PEI)引入3-氨丙基三乙氧基硅烷(APTES)改性膨润土(APTES/Bent)表面制备得到PEI交联膨润土(PEI-APTES/Bent-4),并采用FTIR、XRD和SEM等手段对其进行表征分析。以水中Cr(Ⅵ)为吸附对象,考察了PEI-APTES/Bent-4的吸附性能,探究了吸附机制和回收利用性。结果表明:PEI成功接枝于膨润土表面,其丰富的活性基团极大地促进了六价铬的去除。吸附最佳pH为2,随pH值增加吸附量降低。PEI-APTES/Bent-4对Cr(Ⅵ)的吸附符合Langmuir等温模型和拟二级动力学模型,吸附过程为化学吸附和单层吸附,在313 K时最大理论吸附量达137.50 mg·g−1。热力学研究表明该吸附为自发吸热过程。结合吸附实验、FTIR和XPS分析推测得出PEI-APTES/Bent-4对Cr(Ⅵ)的吸附机制主要为静电作用、还原和螯合。经6次循环后吸附剂仍保持较好的吸附性能。PEI-APTES/Bent-4去除水中Cr(Ⅵ)具有较大的应用前景。

     

    Abstract:

    In order to improve the adsorption capacity of bentonite, polyethyleneimine (PEI) was introduced onto the surface of 3-aminopropyltriethoxysilane (APTES)-modified bentonite (APTES/Bent) by crosslinking reaction to prepare PEI-crosslinked bentonite (PEI-APTES/Bent-4), which was characterised by FTIR, XRD and SEM. Taking Cr(VI) in water as the adsorption target, the adsorption performance of PEI-APTES/Bent-4 was investigated, and its adsorption mechanism and recyclability were explored. The results showed that PEI was successfully grafted onto the surface of bentonite, and the abundant active groups of PEI dramatically promoted the removal of Cr(VI). The optimum pH for adsorption was 2, and the adsorption capacity decreased with increasing pH. The adsorption of Cr(VI) by PEI-APTES/Bent-4 conformed to the Langmuir isotherm model and pseudo-second-order kinetic model, and the adsorption process was chemical adsorption and monolayer adsorption. The maximum theoretical adsorption capacity reached 137.50 mg·g−1 at 313 K. Thermodynamic studies indicated that the adsorption was a spontaneous endothermic process. Based on the adsorption experiments, FTIR and XPS analysis, it is speculated that the adsorption mechanism of PEI-APTES/Bent-4 for Cr(VI) is mainly electrostatic interaction, reduction, and chelation. After six cycles, the adsorbent still maintained good adsorption performance. PEI-APTES/Bent-4 has broad application prospects for the removal of Cr(VI) from water.

     

  • 膜分离技术具有高能效、易操作、环境友好和占地面积小等优点,近年来在气体分离领域受到广泛关注[1-4]。传统气体分离膜多以聚合物膜为主,然而由于聚合物分离膜固有的选择性和渗透性的制约关系(Trade-off效应),使其性能很难再提升[5-6]。研究者发现将多孔材料与聚合物基体共混制成混合基质膜,通过结合两种材料的优点,能够同时提升膜的气体渗透性和选择性,从而突破聚合物膜的Trade-off效应[7-8]。因此,制备混合基质膜是改善膜气体分离性能的一种有效方法。

    对于混合基质膜,填料和聚合物基体材料的选择尤为重要。聚酰亚胺由于其优异的热稳定性、良好的力学性能以及可加工性,已经在气体分离膜领域发展多年,是混合基质膜聚合物基体候选材料之一[9-11]。对于填料材料的选择,共价有机框架材料(COFs)是一种由有机单元通过共价键构成的多孔材料,由于其具有优异的稳定性,易功能化、永久空隙率以及高比表面积等优点,在气体分离领域展现出不俗的潜力[12]。由于COFs全有机的性质,使其能够均匀地分散在聚合物基质中,减少了混合基质膜中由于界面缺陷产生的非选择性孔[13-14]。然而,大部分COFs的孔径很难做到2 nm以下,相对于气体分子动力学直径(N2:0.36 nm;O2:0.35 nm;CO2:0.33 nm)还是较大,难以实现对气体的高效分离,从而降低了气体选择性[15-16]。因此,需要对COFs的孔径大小调控或引入一些功能性吸附位点进行改善。研究表明,引入氟原子能够有效改善COFs的孔径大小且能够提供与气体相互作用的吸附位点。Alahakoon等[17]通过使用含氟单体制备出两种氟化COFs,将氟化COFs与未氟化的相比,发现氟化COFs具有更大的比表面积、更小且更明确的孔径。Gao等[18]报道了3种具有—H、—Me和—F取代基的同构三维共价有机骨架,对比不含氟的COFs,氟化COFs具有更高的CO2亲和力,对CO2/N2有着更高的IAST选择性。Yang等[19]制备了一种氟化CTF,通过氟原子的强静电作用以及C—F键与CO2的偶极-四极作用,使其具有优异的CO2吸附能力。

    基于上述讨论,本文合成出一种具有较小孔径、高比表面积的氟化共价有机框架材料(TpPa-CF3)。随后,以TpPa-CF3为填料,聚酰亚胺(6FDA-ODA)为基体,制备出不同负载量的TpPa-CF3/6FDA-ODA混合基质膜。表征了其结构和表面、截面的微观形貌,探究了其热性能、力学性能以及疏水性能,最后讨论了混合基质膜的气体渗透性以及在烟道气分离(CO2/N2)和空气分离(O2/N2)上的应用前景。

    4,4-二氨基二苯醚(ODA,98%)、4,4-(六氟异丙烯)二酞酸酐(6FDA,98%+)、2,4,6-三甲酰间苯三酚(Tp,98%)、2-三氟甲基-1,4-苯二胺(Pa-CF3,97%)、1,3,5-三甲基苯(99%+)、1,4-二氧六环(99%)均购自上海阿达玛斯试剂有限公司;乙酸(AR)、N,N-二甲基甲酰胺(DMF,AR)均购自西陇科学股份有限公司;间甲酚(m-Cresol,99%)、异喹啉(97%)均购自上海阿拉丁试剂有限公司;丙酮(Acetone,AR),成都市科隆化学品有限公司;工业酒精(95%),弘昊实验设备有限公司。

    将Tp (63.0 mg,0.30 mmol)、Pa-CF3 (79.0 mg,0.45 mmol)、1,3,5-三甲基苯(1.5 mL)、1,4-二氧六环(1.5 mL)依次加入到Pyrex管(25 mL)中。为使混合物均匀分散,超声处理0.5 h,再加入3 mol/L乙酸溶液(0.5 mL)。随后,用液氮将Pyrex管骤冻抽出空气,再在室温下解冻,此操作循环3次。密闭封管,将Pyrex管在120℃下油浴3天。反应完毕后冷却至室温,过滤收集产物,先用DMF溶液搅拌洗涤3次,再通过索氏提取法进行提纯(提纯溶剂采用丙酮)。随后,收集产物,在真空烘箱中120℃下干燥12 h后,得到橘红色粉末样品TpPa-CF3

    在N2氛围下,向装有机械搅拌、冷凝回流的150 mL三口烧瓶内依次加入ODA (2.00 g,9.99 mmol)、间甲酚(28 mL),待ODA完全溶解后再依次加入6FDA (4.44 g,9.99 mmol)、间甲酚(28 mL),随后将温度升到50℃待反应物完全溶解后,滴加5~6滴异喹啉后升温至80℃反应3 h,120℃反应3 h,180℃反应3 h,最后200℃反应12 h。反应结束冷却至室温后,将聚酰亚胺溶液缓慢倒入大量工业酒精中拉丝沉淀,过滤收集产物,在真空烘箱中150℃干燥8 h。随后,使用适量DMF重新溶解并进行二次沉淀以除去聚酰亚胺中残留杂质。

    取一定量的TpPa-CF3粉末分散在DMF (3 mL)中,使用细胞粉碎机,在300 W功率下超声0.5 h后再搅拌6 h,保证TpPa-CF3粉末在DMF溶液中分散均匀。同时,称取0.2 g 6FDA-ODA溶解在DMF (2 mL)中,用针式滤头(0.45 μm,尼龙)过滤除去杂质。随后,将TpPa-CF3的分散液滴加到6FDA-ODA溶液中搅拌12 h,确保TpPa-CF3和6FDA-ODA充分混合。最后,将混合溶液缓慢流延到光滑平整的玻璃板(5 cm×5 cm)上,在80℃下蒸发12 h除去溶剂,待冷却至室温后在温水中脱膜,最后在150℃的真空烘箱中干燥12 h以除去残留的溶剂分子。按以上步骤分别制备含量为0wt%、1wt%、3wt%、5wt%、7wt%的TpPa-CF3/6FDA-ODA混合基质膜。

    X射线衍射(XRD):采用荷兰帕纳科公司的X'Pert Pro型X射线衍射仪对制备的TpPa-CF3粉末与薄膜进行晶型及结构表征,扫描范围在3°~40°,扫描速度为2°/min。

    傅里叶变换红外光谱(FTIR):采用美国尼高力公司的Nicolette 6700-NXR型傅里叶变换红外光谱仪分析TpPa-CF3和薄膜的化学键组成及官能团。对于粉末样品通过溴化钾压片的方式测试,对于薄膜样品通过制成厚度约为20 μm的薄膜直接测试。扫描范围为400~4000 cm−1,扫描次数64次以上。

    固态核磁共振(ssNMR):通过德国布鲁克公司的Avance Neo 400 WB型固体核磁共振波谱仪测试TpPa-CF313C NMR,分析其化学键连接方式。所需样品压实后的体积应多于0.5 cm3

    X射线光电子能谱(XPS):采用美国热电公司的Escalab 250 Xi型X射线光电子能谱仪分析TpPa-CF3的化学元素及化学态。采用粉末压片的方式制样。

    扫描电子显微镜(SEM):通过日本日立公司的SU 4800型扫描电子显微镜表征TpPa-CF3和薄膜表面、截面的微观形貌。对于粉末样品,用牙签将少量样品涂在导电胶上制样;对于薄膜样品,膜表面直接粘在导电胶上,膜断面通过液氮脆断选取平整截面制样。全部样品在测试前通过喷金处理提高样品导电性。

    N2吸附-脱附测试:通过美国康塔公司的Autosorb IQ型比表面及孔隙度分析仪器表征TpPa-CF3的比表面积及孔径分布。采用BET (Brunauer-Emmett-Teller)法计算比表面积,密度泛函理论(DFT)计算孔径分布。

    热重分析(TGA):通过德国耐驰公司的STA 449C型综合热分析仪测试TpPa-CF3和薄膜的热稳定性。在N2氛围下测试,升温速率为10℃/min,测试范围在50~800℃。

    差示扫描量热分析(DSC):通过德国耐驰公司的DSC 214型差示扫描量热仪测试薄膜的玻璃化转变温度,在N2氛围下以10℃/min的速率升温,测试范围在30~350℃,所有结果均采用消除热历史后的二次升温曲线。

    力学性能:通过美国美特斯公司的CMT2103型万能试验机来表征薄膜的力学性能。薄膜样品尺寸为50 mm×10 mm,拉伸载荷为5 kN,拉伸速率为2 mm/min,标距为20 mm。

    水接触角测试:通过中国承德优特仪器有限公司JY-PHb型接触角分析仪测定薄膜的亲疏水性能。薄膜样品尺寸为20 mm×20 mm,测试次数至少3次。

    气体渗透性测试:通过中国济南兰光公司的VAC-V1型气体渗透仪测试薄膜的气体渗透性能。测试方法为恒体积变压法,测试气体为高纯气体(CO2、O2、N2),测试条件为4 bar,35℃。测试过程如下,将厚度均匀的待测薄膜装入膜腔中,测试前将上下腔气压抽至20 Pa以下,随后下腔关闭,上腔通入待测纯气体形成压差。压差推动气体自上腔(高压侧)向下腔(低压侧)渗透,通过系统计算得到膜的气体渗透系数P

    通过粉末X射线衍射(PXRD)对合成的TpPa-CF3的晶体结构进行分析,图1(a)中2θ=4.79°处出现的强峰对应于COF的(100)晶面,其他峰也出现在2θ=7.81°、26.14°处,分别对应于(200)、(001)晶面,其中(001)晶面也是其π-π堆叠峰,通过布拉格方程计算得出其堆叠层间距为0.33 nm。将测试结果与模拟的晶体模型的衍射峰进行对比,结果表明二者衍射峰的位置与强度均匹配良好。TpPa-CF3在Pawley精修后得到的晶胞参数为a=2.290351 nm,b=2.236760 nm,c=0.423813 nm,α=89.56415β=89.73479γ=120.51471。实验结果与精修后的PXRD之间的残差值较小,Rwp=1.24%,Rp=0.91%。以上结果初步说明成功合成出了目标晶体结构,且具有良好的结晶性。

    为进一步说明TpPa-CF3成功合成,通过FTIR对TpPa-CF3以及其构筑单元测试,从图1(b)中可以看到,构筑单元2,4,6-三甲酰间苯三酚(Tp)在2894 cm−1处醛基的CH=O特征峰和构筑单元2-三氟甲基-1,4-苯二胺(Pa-CF3)在3318 cm−13210 cm−1处的—NH2特征峰在产物TpPa-CF3中消失,表明醛胺缩合反应完全。在1282 cm−1处的C—N的特征吸收峰表明烯醇-酮异构的发生。因为框架是以酮的形式存在,结构中有强的分子内氢键及共轭作用,所以在1592 cm−1处的C=C的特征峰和1610 cm−1处的C=O特征峰合并呈肩状[20]。在1128 cm−1处出现了C—F的特征峰。

    13C固体NMR分析如图1(c)所示,图中显示化学位移在184.2×10−6和108.1×10−6处有两个较明显的信号峰,分别对应于烯醇-酮异构反应所形成的C=O键和C—N键上的C原子,123.6×10−6处归属于C—F上的C原子。其余在119.0×10−6、134.1×10−6和146.5×10−6处的信号峰则归属于芳香单元上的C原子。

    图  1  TpPa-CF3的PXRD精修谱图(a)、FTIR谱图(b)、13C固体核磁谱图(c)
    Figure  1.  PXRD refined spectra (a), FTIR spectra (b), 13C solid-state NMR spectrum (c) of TpPa-CF3
    “*”标记的峰为测试仪器产生的旋转边带峰

    FTIR和固体核磁分析结果证实TpPa-CF3的成功合成且以稳定的β-酮胺形式存在。

    通过XPS测量TpPa-CF3的全谱和各个元素的光谱,由图2(a)的全谱可知TpPa-CF3是由C、N、O、F 4种元素组成的。图2(b)是C1s的高分辨率XPS光谱,其能够被卷积为4个峰,分别对应于TpPa-CF3中的C=C/C—C(284.8 eV)、C—N(286.2 eV)、C=O(288.9 eV)和C—F(292.9 eV)键,N1s的高分辨率XPS如图2(c)所示,其被卷积为2个峰,分别归属于N—C(400.2 eV)和N—H(403.9 eV)键,F1s的高分辨率XPS如图2(d)所示,其只有一个卷积峰归属于C—F(688.3 eV)。所有以上结果说明TpPa-CF3形成目标结构,由C1s和N1s证明该结构发生了烯醇-酮异构。

    图  2  TpPa-CF3的XPS图谱:(a)全谱;(b) C1s;(c) N1s;(d) F1s
    Figure  2.  XPS spectra of TpPa-CF3: (a) Survey spectrum; (b) C1s; (c) N1s; (d) F1s

    通过扫描电镜(SEM)观察TpPa-CF3的微观形貌。如图3(a)所示,TpPa-CF3具有均匀的微观形貌,呈现为“米粒”形颗粒堆积形成的团簇,每一颗“米粒”的尺寸在(100±30) nm。

    图  3  TpPa-CF3的微观形貌(a)、N2吸附-脱附曲线(b)、孔径分布图(c)、TGA和DTG曲线(d)
    Figure  3.  Microscopic morphology (a), N2 adsorption-desorption curves (b), pore size distribution (c), TGA and DTG curves (d) of TpPa-CF3
    dV/(dlogD)—; STP—

    为了解TpPa-CF3的多孔性,对其进行N2吸附-脱附测试。图3(b)中N2吸脱附曲线呈现出I型曲线特征,TpPa-CF3在相对压力较低的区域(p/p0<0.1),N2的吸附量快速增加,说明材料中存在丰富的微孔结构。通过计算分析得出TpPa-CF3具有较大的比表面积(791.83 m2·g−1),图3(c)显示TpPa-CF3具有较小的孔径(1.18 nm)。这归因于TpPa-CF3中氟原子的高电负性增强了框架中芳香环之间的相互作用力,这种相互作用力有助于COF形成较大的比表面积以及较小的孔径[17]

    通过热重分析TpPa-CF3的热性能。热重曲线如图3(d)所示,从图中看到热损失分为两个阶段,大约在400℃之前的损失可能为残留在孔道里的高沸点溶剂(DMF)的挥发。400℃后出现明显的质量损失,从DTG曲线上可以看到在416℃质量损失的速度最快,这主要归因于TpPa-CF3框架的分解。以上结果可以看出TpPa-CF3具有较好的热稳定性。

    通过XRD对膜结构表征,评价了填料对聚合物链排列的影响。从图4(a)中可以看到所有曲线在2θ=15°左右均出现典型的聚合物宽峰。通过布拉格方程计算,得到膜的分子链间距。纯6FDA-ODA膜的分子链间距为0.574 nm,随着填料TpPa-CF3负载量的增加,链间距呈现先增大后下降的趋势,7%TpPa-CF3/6FDA-ODA膜的链间距最小(0.566 nm)。分子链间距先增大主要是由于小负载量的掺入破坏了分子链的堆积,随着负载量的增大,填料与聚合物基质的相互作用逐渐增强,限制了分子链的迁移率,链间距减小有利于提高气体的选择性。同时,在TpPa-CF3/6FDA-ODA混合基质膜中没有观察到TpPa-CF3粉末的特征峰,这主要是由于在超声搅拌过程中COF填料的部分剥落[21]

    图  4  6FDA-ODA及TpPa-CF3/6FDA-ODA混合基质膜的XRD (a)和FTIR图谱(b)
    Figure  4.  XRD patterns (a) and FTIR spectra (b) of 6FDA-ODA and TpPa-CF3/6FDA-ODA mixed matrix membranes

    混合基质膜的FTIR图谱如图4(b)所示,所有膜都表现出6FDA-ODA的特征峰,包括C=O的对称(1783 cm−1)和不对称拉伸(1733 cm−1)、C—N的拉伸振动(1378 cm−1)、C—O—C的拉伸振动(1157 cm−1)、C—F键的吸收峰(1110 cm−1),以及酰亚胺环的弯曲振动(721 cm−1),值得注意的是在1597 cm−1处的特征峰,随着填料的增加而增强,这主要归因于TpPa-CF3和6FDA-ODA中芳香环上的C=C的吸收峰重叠[22]。以上结果说明聚酰亚胺基体和填料之间具有良好的相容性,填料的加入并没有破坏聚酰亚胺的结构。

    为分析TpPa-CF3的加入对混合基质膜热稳定性的影响,对6FDA-ODA及TpPa-CF3/6FDA-ODA混合基质膜进行了热重测试。如图5(a)所示,混合基质膜的分解分为两个阶段,第一阶段是400℃左右TpPa-CF3框架的分解,第二阶段是500℃左右6FDA-ODA基体膜的分解,填料的加入对膜的热稳定性影响不大。所有混合基质膜都表现出高达500℃的良好热稳定性,远高于工业中膜的操作温度,表明这些膜具有良好的适用性。DSC曲线用于分析膜的玻璃化转变温度(Tg)。如图5(b)所示,6FDA-ODA膜的Tg出现在297.5℃。随着TpPa-CF3负载量的增加,TpPa-CF3/6FDA-ODA膜的Tg从297.5℃逐渐增加到302.1℃,说明TpPa-CF3与6FDA-ODA之间具有良好的界面相互作用,这有利于提升混合基质膜的气体选择性[23]

    图  5  6FDA-ODA和TpPa-CF3/6FDA-ODA混合基质膜的TGA曲线(a)和DSC曲线(b)
    Figure  5.  TGA curves (a) and DSC curves (b) of 6FDA-ODA and TpPa-CF3/6FDA-ODA mixed matrix membranes

    膜表面、截面的扫描电镜表征能够反映出填料在膜内的分散情况。如图6(a1)~6(e1)所示,与表面光滑平整的纯膜相比,混合基质膜的表面随着填料负载量的增加逐渐变得粗糙,在负载量达到7wt%时可以看到膜表面出现不平整及大颗粒团聚的现象。图6(a2)~6(e2)为纯膜及其混合基质膜的截面扫描电镜图,纯膜的截面表现出均匀、致密的微观结构,在1wt%~5wt%混合基质膜的截面图中能够观察到随着负载量的增加其截面形貌逐渐变粗糙,同时在膜内能够观察到TpPa-CF3颗粒很好地被聚合物包裹且分散均匀。当填料负载量达到7wt%时膜内出现填料与聚合物基质相分离的现象,说明此时负载量已经达到聚合物基质所能承受的上限,5wt%为其最优负载量。

    图  6  膜表面((a1)~(e1))和膜截面((a2)~(e2))的SEM图像((a)~(e)分别代表不同TpPa-CF3的负载量:0wt%、1wt%、3wt%、5wt%、7wt%)
    Figure  6.  SEM images of membrane surface ((a1)-(e1)) and cross-section ((a2)-(e2)) ((a)-(e) represent different TpPa-CF3 loadings: 0wt%, 1wt%, 3wt%, 5wt%, 7wt%, respectively)

    通过接触角测试仪分析纯膜及其混合基质膜的水接触角(θw)。如图7(a)表1所示,6FDA-ODA膜的水接触角为79.9°,TpPa-CF3/6FDA-ODA混合基质膜的水接触角为81.4°~89.1°,呈现逐渐增大的趋势。这主要归因于TpPa-CF3框架中含有—CF3疏水基团,因此随着TpPa-CF3含量的增加相应负载量的混合基质膜水接触角也逐渐增加。提升膜的疏水性能有助于阻止水汽进入,提升其气体传输性能。

    图  7  不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的接触角(a)和应力-应变曲线(b)
    Figure  7.  Contact angle (a) and stress-strain curves (b) of TpPa-CF3/6FDA-ODA mixed matrix membranes at different loadings
    表  1  不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的力学性能
    Table  1.  Mechanical properties of TpPa-CF3/6FDA-ODA mixed matrix membranes at different loadings
    TpPa-CF3 loadings/wt% Tensile strength/MPa Elongation at break/% Young's modulus/GPa θw/(°)
    0 74.1 10.1 1.59 79.9
    1 79.6 9.7 1.63 81.4
    3 82.9 8.6 1.70 83.1
    5 93.0 7.8 1.82 84.1
    7 84.5 7.3 1.76 89.1
    Note: θw—Water contact angle.
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    对混合基质膜进行拉伸实验以此来检验其力学性能。测试结果如图7(b)表1所示。从表中可以看出,TpPa-CF3/6FDA-ODA混合基质膜的抗拉强度和杨氏模量随着TpPa-CF3负载量的增加呈现出先增加后下降的趋势,而断裂伸长率呈现逐渐下降的趋势。这主要归因于,在混合基质膜中TpPa-CF3与6FDA-ODA之间较好的相互作用力使得填料与聚合物之间具有良好的界面相容性,增强了膜的刚性。然而,当负载量达到7wt%时,抗拉强度和杨氏模量略微下降,这主要是过量的TpPa-CF3颗粒之间发生团聚,使得界面出现缺陷导致应力集中,降低了膜的力学性能[24]

    利用3种纯气体(CO2、O2、N2)渗透测试来评估不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的渗透性及CO2/N2和O2/N2的理想选择性。结果如表2所示,每一种膜气体渗透系数的大小均与气体分子动力学直径呈反比,即膜的3种气体渗透系数大小排列为P(CO2)>P(O2)>P(N2),3种气体分子动力学直径大小排列为N2(0.36 nm)>O2(0.35 nm)>CO2(0.33 nm)。同6FDA-ODA膜相比所有混合基质膜的气体渗透性都有所提升。由图8(a)中可得,随着TpPa-CF3含量的增加,膜的气体渗透性呈现出先增大后下降的趋势,其中5%TpPa-CF3/6FDA-ODA膜气体渗透性能最佳,P(CO2)提升了149%,P(O2)提升了138%,P(N2)提升了98%。这主要归因于TpPa-CF3的高孔隙率提高了TpPa-CF3/6FDA-ODA膜的比表面积及固有孔隙率,为气体传输提供了快速通道。TpPa-CF3负载量到7wt%时,气体的渗透性明显下降,但仍然比6FDA-ODA膜高。这主要是由于负载量过大,造成TpPa-CF3在膜内团聚堵塞了气体传输的孔道。

    表  2  6FDA-ODA及TpPa-CF3/6FDA-ODA混合基质膜的气体渗透系数P和理想选择性
    Table  2.  Gas permeability coefficient P and ideal selectivity of 6FDA-ODA and TpPa-CF3/6FDA-ODA mixed matrix membranes
    Membrane Permeability/Barrer Ideal selectivity α
    CO2 O2 N2 α(CO2/N2) α(O2/N2)
    6FDA-ODA 12.47 2.55 0.64 19.5 4.0
    1%TpPa-CF3/6FDA-ODA 16.91 3.76 0.98 17.2 3.8
    3%TpPa-CF3/6FDA-ODA 22.77 4.43 1.03 22.0 4.3
    5%TpPa-CF3/6FDA-ODA 31.08 6.08 1.27 24.5 4.8
    7%TpPa-CF3/6FDA-ODA 18.62 4.16 1.11 16.8 3.8
    Notes: 1 Barrer=10−10 cm3(STP)·cm·cm−2·s−1·cmHg−1; Ideal selectivity α=P(A)/P(B), A and B are two different pure gases.
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    图  8  不同负载量下TpPa-CF3/6FDA-ODA混合基质膜的气体渗透性(a)、气体选择性(b)、72 h连续渗透性测试(c)
    Figure  8.  Gas permeability (a), gas selectivity (b), 72 h continuous permeability performance test (c) of TpPa-CF3/6FDA-ODA mixed matrix membranes at different loadings

    TpPa-CF3/6FDA-ODA膜的气体选择性变化趋势和气体渗透性变化趋势不同,如图8(b)所示,CO2/N2和O2/N2均呈现出先下降后上升再下降的趋势,CO2/N2及O2/N2的理想选择性范围分别在16.8~24.4和3.8~4.8。其中,当TpPa-CF3负载量为5wt%时,混合基质膜的CO2/N2和O2/N2选择性最好,分别是6FDA-ODA膜的125%和119%。理想选择性的提高主要归因于两个方面:一个方面是CO2和O2的分子动力学直径要小于N2的分子动力学直径,从而CO2和O2分子倾向于优先通过。另一个方面,TpPa-CF3中富含大量对CO2具有亲和力的N、O和F等电负性原子,同时框架内还存在能与CO2发生偶极-四极相互作用的强极性C—F键,因此CO2/N2选择性相较于O2/N2的提升更明显[25]。然而,当负载量到7wt%时,混合基质膜的CO2/N2和O2/N2选择性大幅下降,略低于6FDA-ODA膜,这主要归因于当TpPa-CF3的负载量增加一定程度时,其在膜内发生团聚,并和聚合物基质产生部分相分离,产生一些非选择的孔,从而造成CO2/N2和O2/N2理想选择性的大幅下降。

    对TpPa-CF3/6FDA-ODA混合基质膜进行72 h的连续气体渗透性测试,以验证膜的稳定性。如图8(c)所示,该膜在72 h的运行试验中P(CO2)下降了18%,CO2/N2的选择性下降了16%,总体表现出了良好的分离稳定性。

    为了评估混合基质膜的气体分离性能,图9显示了不同负载量的TpPa-CF3/6FDA-ODA混合基质膜的气体分离性能与Robeson上限的对比。当负载量为5wt%时,其气体分离性能更靠近Robeson上限,气体的渗透性与选择性同步提升。说明适量的引入TpPa-CF3能够改善聚合物膜的气体分离性能。此外,表3显示了文献[26-30]中报道的混合基质膜气体分离性能与本工作的对比,TpPa-CF3/6FDA-ODA混合基质膜显示出适中的气体渗透性以及适中的气体选择性,说明TpPa-CF3/6FDA-ODA混合基质膜还有进一步提升的潜力。

    图  9  不同负载量的TpPa-CF3/6FDA-ODA混合基质膜CO2/N2 (a)和O2/N2 (b)的Robeson上限图
    Figure  9.  Robeson upper bound plots of TpPa-CF3/6FDA-ODA mixed matrix membranes with different loadings CO2/N2 (a) and O2/N2 (b)
    表  3  文献中报道的混合基质膜气体分离性能与本工作的对比
    Table  3.  Comparison of gas separation performance of mixed matrix membranes reported in the literature with the present work
    Membrane type P(CO2)/Barrer P(O2)/Barrer α(CO2/N2) α(O2/N2) Ref.
    TpPa-1-nc/Pebax 21 72 [26]
    COFp-PVAm 270 86 [27]
    TpBD@PBI-BuI 14.8 23 [28]
    ZIF-7-I/(BPDA/6FDA-ODA) 2.9 0.19 [29]
    PBI-PI-based carbon 293.5 93.1 8.3 2.6 [30]
    5%TpPa-CF3/6FDA-ODA 31.08 6.08 24.4 4.8 This work
    Notes: TpPa-1-nc—; COFp—; TpBD—; BPDA—; Pebax—Poly(ether-block-amide); PVAm—Polyvinylamine; PBI-BuI—Tert-butylpolybenzimidazole; ZIF-7-I—Wide-pore ZIF-7; PBI—Polybenzimidazoles; PI—Polyimide.
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    (1)采用溶剂热法合成了一种氟化共价有机框架材料(TpPa-CF3),其具有高比表面积(791.83 m2·g−1),较小且均一的孔径(1.18 nm)以及良好的热稳定性。

    (2)采用共混法成功制备TpPa-CF3/聚酰亚胺(6FDA-ODA)混合基质膜。通过表征得出,所得膜具有良好的界面相容性以及较高的热稳定性(热分解温度在500℃左右)。水接触角的范围在81.4°~89.1°,且膜具有良好的力学性能,有利于膜在分离过程中的稳定性。

    (3) TpPa-CF3的掺入提高了混合基质膜的气体渗透性,随着膜中TpPa-CF3负载量的增加,混合基质膜的气体渗透性呈现先减小后增大再减小的趋势。其中,5%TpPa-CF3/6FDA-ODA膜的气体分离性能最好,其CO2和O2的渗透性能分别提高了149%和138%,CO2/N2和O2/N2的分离性能分别是6FDA-ODA基体膜的125%和119%。

  • 图  1   聚乙烯亚胺交联膨润土(PEI-APTES/Bent)的制备过程

    APTES—3-aminopropyltriethoxysilane; NBent—Bentonite; PEI—Polyethyleneimine; GA—Glutaraldehyde

    Figure  1.   Preparation process of polyethyleneimine cross-linked bentonite (PEI-APTES/Bent)

    图  2   Bent、APTES/Bent和PEI-APTES/Bent-4的FTIR图谱(a)、XRD图谱(b)和TGA曲线(c)

    d—Interplanar spacing

    Figure  2.   FTIR spectra (a), XRD patterns (b) and TGA curves (c) of Bent, APTES/Bent and PEI-APTES/Bent-4

    图  3   Bent ((a), (d))、APTES/Bent ((b), (e))和PEI-APTES/Bent-4 ((c), (f))的SEM-EDS图像

    Figure  3.   SEM-EDS images of Bent ((a), (d)), APTES/Bent ((b), (e)) and PEI-APTES/Bent-4 ((c), (f))

    图  4   不同样品对Cr(Ⅵ)的吸附量

    qe—Equilibrium adsorption capacity

    Figure  4.   Adsorption capacity of different samples for Cr(Ⅵ)

    图  5   pH对吸附Cr(Ⅵ)的影响(a)与PEI-APTES/Bent-4的Zeta电位曲线(b)

    Figure  5.   Effect of pH on the adsorption of Cr(VI) (a) and the Zeta potential curve of PEI-APTES/Bent-4 (b)

    图  6   吸附时间对Cr(Ⅵ)吸附量的影响(a)与动力学模型拟合:(b)准一级动力学模型;(c)准二级动力学模型

    qt—Adsorption capacity at time t

    Figure  6.   Effect of adsorption time on the adsorption capacity of Cr(Ⅵ) (a) and kinetics model fitting: (b) Pseudo-first-order kinetic model; (c) Pseudo-second-order kinetic model

    图  7   PEI-APTES/Bent-4吸附Cr(Ⅵ)等温吸附模型拟合

    Ce—Cr(VI) concentration at adsorption equilibrium

    Figure  7.   Isothermal adsorption model fitting of Cr(Ⅵ) adsorption by PEI-APTES/Bent-4

    图  8   PEI-APTES/Bent-4吸附Cr(Ⅵ)前后的红外图谱

    Figure  8.   FTIR spectra of PEI-APTES/Bent-4 before and after adsorption of Cr(Ⅵ)

    图  9   PEI-APTES/Bent-4吸附Cr(Ⅵ)前后的XPS图谱:(a)全图谱;(b) Cr2p图谱

    Figure  9.   XPS spectra of PEI-APTES/Bent-4 before and after adsorption of Cr(Ⅵ): (a) Full spectrum; (b) Cr2p spectrum

    图  10   PEI-APTES/Bent-4吸附Cr(Ⅵ)的机制

    Figure  10.   Mechanism diagram of PEI-APTES/Bent-4 adsorption of Cr(Ⅵ)

    图  11   循环次数对PEI-APTES/Bent-4吸附Cr(Ⅵ)的影响

    Figure  11.   Effect of cycle times on the adsorption of Cr(Ⅵ) by PEI-APTES/Bent-4

    1   PEI-APTES/Bent-4与其他改性膨润土的Cr(VI)吸附量比较

    Adsorbentqm/mg·g-1Ref.
    CTMAB/Bent27.472[37]
    AC-Fe3O4/Bent29.32[38]
    Citric acid/MBent16.67[39]
    polyacrylic acid-Al/Bent3.125[40]
    Fe3O4-PDA-SDBS/Bent103.6[41]
    Chitosan-NaOH/Bent2.72[42]
    Cetylpyridinium chloride/Bent46.03[43]
    Chitosan/Bent16.40[44]
    PEI-APTES/Bent-4137.50This study
    Notes: CTMAB—Cetyltrimethylammonium bromide; MBent—Magnetic Bentonite; AC—Activated Carbon; PDA—Polydopamine; SDBS—Sodium dodecyl benzene sulfonate.
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    表  1   不同吸附剂的PEI掺杂量

    Table  1   PEI doping amount of different adsorbents

    AdsorbentDoping mass of
    APTES/Bent/g
    Doping mass of
    PEI/g
    PEI-APTES/Bent-0.560.5
    PEI-APTES/Bent-161
    PEI-APTES/Bent-262
    PEI-APTES/Bent-363
    PEI-APTES/Bent-464
    PEI-APTES/Bent-565
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    表  2   PEI-APTES/Bent-4对Cr(Ⅵ)的吸附动力学参数

    Table  2   Kinetic model fitting parameters for Cr(Ⅵ) adsorption on PEI-APTES/Bent-4

    Adsorbent Pseudo-first-order Pseudo-second-order
    qe/(mg·g−1) K1/min−1 R2 qe/(mg·g−1) K2/(g·mg−1·min−1) R2
    PEI-APTES/Bent-4 78.39 0.1286 0.9898 131.06 0.0076 0.9997
    Notes: K1—Quasi-first-order kinetic model constant; K2—Quasi-second-order kinetic model constant; R2—Correlation coefficient.
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    表  3   Langmuir和Freundlich模型参数

    Table  3   Langmuir and Freundlich model parameters

    T/K Langmuir Freundlich
    qm/(mg·g−1) KL/(L·mg−1) RL R2 KF/(mg1-(1/n)·L1/n·g−1) n R2
    293 132.02 0.4046 0.0049-0.1099 0.9869 58.98 6.475 0.8479
    303 135.68 0.6558 0.0030-0.0708 0.9627 65.12 6.893 0.8711
    313 137.50 1.2208 0.0016-0.03935 0.9565 71.11 7.468 0.8835
    Notes: qm—Maximum adsorption capacity; KL—Adsorption equilibrium constant of Langmuir model; KF—Adsorption equilibrium constant of Freundlich model; n—Adsorption strength constant in the Freundlich model; RL—Separation constant.
    下载: 导出CSV

    表  4   PEI-APTES/Bent-4与其他改性膨润土Cr(VI)吸附量比较

    Table  4   Comparison of Cr(VI) adsorption capacity between PEI-APTES/Bent-4 and other modified bentonite

    Adsorbent Maximum adsorption capacity/(mg·g−1) Ref.
    CTMAB/Bent 27.472 [37]
    AC-Fe3O4/Bent 29.32 [38]
    Citric acid/MBent 16.67 [39]
    Polyacrylic acid-Al/Bent 3.125 [40]
    Fe3O4-PDA-SDBS/Bent 103.6 [41]
    Chitosan-NaOH/Bent 2.72 [42]
    Cetylpyridinium chloride/Bent 46.03 [43]
    Chitosan/Bent 16.40 [44]
    PEI-APTES/Bent-4 137.50 This study
    Notes: CTMAB—Cetyltrimethylammonium bromide; AC—Activated carbon; PDA—Polydopamine; MBent—Magnetic bentonite; SDBS—Sodium dodecyl benzene sulfonate.
    下载: 导出CSV

    表  5   吸附Cr(Ⅵ)的热力学参数

    Table  5   Thermodynamic parameters for adsorption of Cr(Ⅵ)

    T/K ΔG0/(kJ·mol−1) ΔH0/(kJ·mol−1) ΔS0/(J·mol−1·K−1)
    293 −6.511
    303 −7.439 23.73 103.11
    313 −8.578
    Notes: ∆G0—Gibbs free energy change; ∆H0—Enthalpy change; ∆S0—Entropy change.
    下载: 导出CSV
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  • 目的 

    膨润土是储量丰富的天然粘土矿物,作为吸附材料具有巨大的潜力。但其吸附能力有限,且由于带有负电荷,对六价铬阴离子污染物吸附性能较差。针对水体Cr(Ⅵ)污染问题,为提高膨润土吸附容量,考虑氨基官能团对Cr(Ⅵ)具有优异的去除能力,拟通过在膨润土表面引入大量氨基官能团,增加膨润土吸附位点,从而制备对Cr(Ⅵ)具有高吸附容量的膨润土基吸附材料。

    方法 

    聚乙烯亚胺(PEI)是含有大量伯胺、仲胺和叔胺基团的水溶性聚合物,对Cr(VI)具有很强的亲和力。3-氨基丙基三乙氧基硅烷(APTES)和戊二醛(GA)被用来作为桥连接将PEI嫁接与膨润土表面。首先将APTES接枝与酸改性活化膨润土(Bent)表面制得APTES改性膨润土(APTES/Bent),APTES既可以为Cr(VI)吸附位点,也可为PEI的引入嫁接点;再利用GA,通过席夫碱反应将PEI引入膨润土表面制得PEI交联膨润土(PEI-APTES/Bent-4)。对Bent、APTES/Bent和PEI-APTES/Bent-4进行分析表征,采用FTIR测定材料表面官能团变化,采用XRD测定材料晶体结构变化,采用SEM-EDS进行形貌和元素分析,采用TGA测量材料的质量损失,采用XPS分析材料吸附前后元素组成和价态。开展吸附实验,测量不同PEI负载量对吸附量的影响;测量pH值对PEI-APTES/Bent-4吸附Cr(VI)的影响;测定接触时间对吸附量的影响,利用准一级、准二级动力学模型进行拟合;测定293 K、303 K和313 K下的吸附等温线,利用Langmuir和Freundlich模型拟合数据;进行吸附热力学计算,得出热力学参数吉布斯能量变化(ΔG)、焓变化(ΔH)和熵变化(ΔS)。进行脱附再生吸附实验,分析吸附材料的循环利用性能。

    结果 

    材料的FTIR、XRD、TGA和SEM-EDS分析表征结果表明,发生了席夫碱反应,PEI交联后,PEI-APTES/Bent-4表面引入了更多的氨基,热重损失由10.7%增大为41%,PEI未进入膨润土层间,N元素峰在EDS图谱中更加明显。吸附实验表明,相比APTES/Bent,经PEI交联后,膨润土表面吸附位点增加,对Cr(VI)的吸附容量得到了显著提升。pH值对吸附影响较大,吸附最佳pH值为2,酸性条件下材料Zeta电位为正,在pH<2时,Cr(Ⅵ)主要存在形式包含H2CrO4,不利于静电吸附,pH增大,质子化氨基数量减少,吸附位点减少,吸附量降低。准二级动力学模型相关系数更高,更接近实验吸附结果。Langmuir模型相关系数更高,理论吸附量更接近实验结果,分离系数(0<RL<1),吸附为有利过程。热力学计算得出Δ为负值,Δ为正值。PEI-APTES/Bent-4经6次再生后,对Cr(VI)的吸附量降为原来的74.58%。吸附前后的XPS全扫描图谱证明铬被吸附到PEI-APTES/Bent-4上,Cr2p高分辨率XPS光谱表明吸附后Cr(VI)与Cr(Ⅲ)的存在,吸附涉及吸附和还原过程。

    结论 

    利用APTES和GA可成功将PEI交联于膨润土表面,制得聚乙烯亚胺交联膨润土(PEI-APTES/Bent-4); PEI-APTES/Bent-4有更多的氨基官能团,吸附容量显著增加;吸附最佳pH值为2,吸附主要是化学吸附过程,为单分子层吸附,其理论最大吸附量达137.50 mg·g;吸附过程为自发的吸热过程。PEI-APTES/Bent-4具有良好的循环利用性。吸附机制主要为静电吸引、还原和螯合,Cr(Ⅵ)首先通过静电引力吸附与吸附材料表面,随后部分还原为Cr(Ⅲ),Cr(Ⅲ)通过螯合固定在吸附剂表面。

  • 针对水体中有毒六价铬阴离子,以膨润土为原料,制备高吸附容量的改性膨润土吸附剂。膨润土是典型的天然粘土矿物,储量丰富。由于天然膨润土直接吸附Cr(Ⅵ)效果较差。为提高吸附容量,在前期3-氨丙基三乙氧基硅烷改性膨润土(APTES/Bent)的基础上,利用含有大量氨基的聚乙烯亚胺(PEI)作进一步交联改性。以戊二醛为交联剂,通过席夫碱反应,继续交联PEI,从而制备高吸附容量膨润土吸附剂PEI-APTES/Bent-4。该方法所制备的膨润土吸附材料相比利用其他方法制备的聚乙烯亚胺改性膨润土、APTES/Bent和其他改性膨润土吸附剂具有更高的吸附容量(理论吸附量可达137.50 mg/g)。

    PEI-APTES/Bent-4与其他改性膨润土的Cr(VI)吸附量比较

    Adsorbentqm/mg·g-1Ref.
    Notes: CTMAB—Cetyltrimethylammonium bromide; MBent—Magnetic Bentonite; AC—Activated Carbon; PDA—Polydopamine; SDBS—Sodium dodecyl benzene sulfonate.
    CTMAB/Bent27.472[37]
    AC-Fe3O4/Bent29.32[38]
    Citric acid/MBent16.67[39]
    polyacrylic acid-Al/Bent3.125[40]
    Fe3O4-PDA-SDBS/Bent103.6[41]
    Chitosan-NaOH/Bent2.72[42]
    Cetylpyridinium chloride/Bent46.03[43]
    Chitosan/Bent16.40[44]
    PEI-APTES/Bent-4137.50This study

    不同样品对Cr(Ⅵ)的吸附量

    PEI-APTES/Bent-4对Cr(Ⅵ)的吸附机制主要为静电吸引、还原和螯合,Cr(Ⅵ)首先通过静电吸引吸附在PEI-APTES/Bent-4上,随后部分Cr(Ⅵ)还原为Cr(Ⅲ)螯合在吸附剂表面。

    PEI-APTES/Bent-4吸附Cr(Ⅵ)的机制图

图(11)  /  表(6)
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出版历程
  • 收稿日期:  2024-03-17
  • 修回日期:  2024-04-14
  • 录用日期:  2024-04-14
  • 网络出版日期:  2024-05-22
  • 发布日期:  2024-04-27
  • 刊出日期:  2024-11-26

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