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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): 943-954. 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): 943-954. 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): 943-954. 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): 943-954. 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.

     

  • 六价铬阴离子对生物体具有高度毒性、致突变性和致癌性[1-3],水体中Cr(Ⅵ)的允许浓度为0.05 mg/L[4-5]。因此,含Cr(Ⅵ)废水排放前必须进行处理,降低Cr(Ⅵ)阴离子浓度。许多研究报道了一系列处理含Cr(Ⅵ)废水的方法,如吸附[6-7]、离子交换[8]、化学沉淀[9]、膜分离[10-11]、光催化[12-13]等。吸附法由于操作简单、成本效益高和效率高而成为有效的处理技术之一[14-15]

    氧化石墨烯[16-17]、黏土矿物[18]、活性炭[19-20]和金属有机框架[21]等,已被用于去除水溶液中的Cr(Ⅵ)阴离子。膨润土是典型的天然黏土矿物,具有层状结构,是一种有潜力的重金属吸附剂。由于天然膨润土带负表面电荷,直接吸附Cr(Ⅵ)效果较差。

    氨基是被广泛地用作提高Cr(Ⅵ)去除能力的官能团,在去除Cr(Ⅵ)方面显示出较强的能力[22-23]。聚乙烯亚胺(PEI)由于含有大量的氨基,在酸性条件下易质子化而具有很强的正电性,对阴离子化合物表现出较强的吸附能力[24-25]。此外,PEI还具有生物相容性,无二次污染问题[26]。但PEI在水中的高溶解度限制了其作为吸附剂的实际应用[27-28],可考虑将其用于改性吸附材料制备高效吸附剂,特别是用于去除阴离子污染物的吸附剂[29]。显然,如将PEI用于改性膨润土可显著提高膨润土对Cr(VI)的吸附性能。由于PEI直接接枝于膨润土表面较困难,本文拟在前期研究制备的3-氨丙基三乙氧基硅烷(APTES)改性膨润土基础上,为进一步增大膨润土的吸附量,通过席夫碱反应,利用戊二醛将PEI交联于APTES改性膨润土表面,制备PEI交联膨润土,用于去除模拟水体中的Cr(Ⅵ)阴离子,并研究其对Cr(Ⅵ)的吸附行为。

    膨润土(NBent,陕西府谷);聚乙烯亚胺(分子量70000)、3-氨丙基三乙氧基硅烷购自上海麦克林生化试剂有限公司;戊二醛(GA)、NaOH、HCl、重铬酸钾均为分析纯,购自国药集团化学试剂有限公司。

    吸附材料的制备过程如图1所示。按前期制备方法制得酸活化膨润土(Bent)、3-氨丙基三乙氧基硅烷接枝膨润土(APTES/Bent),取6 g的APTES/Bent加入100 mL去离子水中,再加入不同质量聚乙烯亚胺(0.5 g、1 g、2 g、3 g、4 g、5 g),利用电动搅拌器(JJ-1 A,常州市国旺仪器制造有限公司)搅拌均匀,在搅拌状态下滴入戊二醛溶液(10%),直至成糊状,真空箱(DZF-6024、上海科兴仪器有限公司)反复真空干燥、研磨和水洗,制得聚乙烯亚胺交联膨润土PEI-APTES/Bent (按PEI投加量不同分别标记为PEI-APTES/Bent-0.5、PEI-APTES/Bent-1、PEI-APTES/Bent-2、PEI-APTES/Bent-3、PEI-APTES/Bent-4、PEI-APTES/Bent-5),见表1

    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)
    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
    下载: 导出CSV 
    | 显示表格

    通过TENSOR27型红外光谱仪(德国Bruker公司)测定材料表面官能团;采用D8 Advance型X射线衍射仪(德国Bruker公司)测定材料晶体结构,使用K-Alph型X射线光电子能谱(美国Thermo Fisher公司)测定吸附Cr(Ⅵ)前后材料的元素组成及价态;采用Sigma300型扫描电镜(德国Zeiss公司)分析材料形貌并用其EDS能谱分析元素;采用DTG-60型热重分析仪(日本岛津公司)分析材料质量损失;采用Nano ZS90型Zeta电位仪(英国马尔文公司)测量Zeta电位。

    将0.1 g吸附材料加入100 mL质量浓度为200 mg·L−1模拟废水中,温度20℃,磁力搅拌器(DF-101 S、巩义予华仪器设备有限公司)搅拌,浓度为0.1 mol·L−1的NaOH、HCl溶液调节初始pH,改变不同参数研究吸附剂对Cr(Ⅵ)的吸附量。依据二苯碳酰二肼分光光度法,利用紫外-可见分光光度计(UV-2600、日本岛津公司)于540 nm波长测定Cr(Ⅵ)浓度,计算如下式:

    qe=(C0C1)Vm (1)
    qt=(C0Ct)Vm (2)

    式中:qeqt分别为六价铬阴离子在平衡和时间t时的吸附量(mg·g−1);C0C1Ct分别表示Cr(Ⅵ) 的初始、吸附平衡时和吸附时间t 时质量浓度(mg·L−1);m表示吸附剂质量(g);V表示溶液体积(L)。

    在最佳pH值,吸附剂投加量为1 g·L−1,温度分别设定为293 K、303 K和313 K,Cr(Ⅵ)溶液浓度范围为20~500 mg·L−1的条件下进行平衡吸附实验,利用Langmuir和Freundlich模型进行数据拟合,模型方程分别与分离因数计算公式如下式[30-31]

    qe=KLqmCe1+KLCe (3)
    qe=KFC1ne (4)
    RL=11+KLC0 (5)

    式中:qm为吸附材料对Cr(Ⅵ)的最大吸附容量(mg·g−1);Ce为吸附平衡时Cr(Ⅵ)浓度(mg·L−1);KL是Langmuir常数 (L·mg−1);KF是Freundlich常数(mg1-(1/n)·L1/n·g−1); n是与吸附强度有关的常数;RL为分离因数(有利(0<RL<1)、不利(RL>1)、线性(RL =1)或不可逆(RL =0))。

    在吸附温度为20℃,pH值为最佳, Cr(Ⅵ)初始质量浓度为200 mg·L−1,吸附材料投加量为1 g·L−1条件下研究吸附时间的影响,利用准一级动力模型、准二级动力学模型进行拟合,模型方程如下式:

    ln(qeqt)=lnqek1t (6)
    tqt=1k2q2e+tqe (7)

    式中:k1为准一级动力学吸附速率常数(L·min−1);k2为准二级动力学吸附速率常数(g·mg−1·min−1)。

    将0.1 g吸附剂加入Cr(Ⅵ)溶液(质量浓度为200 mg·L−1、100 mL)中,吸附饱和后的吸附剂利用浓度为0.5 mol·L−1的NaOH溶液处理2 h,去离子水洗至中性,干燥后进行再次吸附。

    为了证实PEI交联膨润土的成功制备,通过FTIR测定了材料表面官能团的变化。图2(a)为Bent、APTES/Bent和PEI-APTES/Bent-4的红外光谱图。Bent经APTES改性后,在2932 cm−1处出现—CH2的振动吸收峰,1561 cm−1处出现—NH2吸收峰[32],表明硅烷偶联剂APTES接枝成功。 APTES/Bent进一步交联PEI后,1561 cm−1处的峰强度有所增加,表明膨润土表面引入了更多的氨基,有更多的氨基官能团[33]。在2835 cm−1处出现—CH2的振动吸收峰,1649 cm−1处出现C=N键振动的信号[34]1468 cm−1处C—H对称弯曲峰的进一步增强[35],表明通过席夫碱反应PEI成功交联接枝到膨润土表面。因此,FTIR结果明显证实了通过交联反应成功制备了PEI-APTES/Bent-4。

    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 spectras (a), XRD patterns (b) and TGA curves (c) of Bent, APTES/Bent and PEI-APTES/Bent-4

    为了探测材料的微观结构,用X射线衍射仪测量了Bent、APTES/Bent和PEI-APTES/Bent-4,XRD图谱如图2(b)所示。从图中可以看出,Bent经APTES接枝改性后首峰2θ向更低的衍射角移动,表明层间距增大,有硅烷偶联剂进入膨润土层间[36],其他峰位置没有变化。经PEI进一步交联后,层间距没有明显变化,表明没有PEI进入膨润土层间。

    Bent、APTES/Bent和PEI-APTES/Bent-4的热重曲线如图2(c)所示。Bent的质量损失约为7.2wt%,这主要归因于30~150℃黏土表面吸附水、层间水和羟基水的脱附。APTES/Bent的质量损失约为10.7wt%,相对Bent增加了约3.5wt%,除了黏土表面吸附水、层间水和羟基水损失外,主要归因于300~600℃时,接枝APTES的损失。PEI-APTES/Bent-4的质量损失约为41wt%,主要归因于300~400℃时PEI和戊二醛的热分解和脱羟基,400~900℃的质量损失归因于PEI和戊二醛的降解。根据TGA曲线的结果,表明PEI成功交联接枝于Bent表面。

    Bent、APTES/Bent和PEI-APTES/Bent-4的SEM-EDS图像如图3所示。可以看出3种材料皆可见膨润土的片状结构,与Bent相比,APTES/Bent和PEI-APTES/Bent-4两种改性膨润土表面更光滑,且层间被堵塞;由EDS分析图谱可以看出Bent图谱中未发现N元素,经APTES改性后图谱中出现N元素。进一步交联PEI后,N元素峰在图谱中更加明显,表明在Bent上成功引入了含有大量氨基的PEI。

    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))

    PEI 在膨润土上的接枝量对膨润土的吸附量有重要影响。因此,将0.1 g不同接枝量的吸附剂分散到 100 mL质量浓度为200 mg·L−1的Cr(Ⅵ)溶液中,温度为20℃,调节pH=2,搅拌240 min。对不同PEI接枝量的吸附材料吸附量进行比较,如图4所示。

    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)

    APTES/Bent对Cr(Ⅵ)的吸附量仅为32.21 mg·g−1。PEI对APTES/Bent表面功能化后,PEI-APTES/Bent吸附剂对Cr(Ⅵ)的吸附能力显著增强。这主要是由于PEI-APTES/Bent表面有更丰富的氨基,质子化后使其吸附位点更多,其吸附能力远高于原始APTES/Bent。从图4可以发现,从PEI-APTES/Bent-0.5到PEI-APTES/Bent-4,对Cr(Ⅵ)的吸附容量一直增加,这可以归因于膨润土表面PEI含量的增加。但PEI-APTES/Bent-5的吸附能力随着PEI含量的进一步增加反而降低,这是由于PEI的接枝率过高使吸附剂上的部分吸附位点无法暴露,吸附位点减少,从而导致吸附容量降低。因此,后续实验中将以PEI-APTES/Bent-4为吸附剂进行实验。

    吸附材料表面特性和Cr(Ⅵ)的存在形式与溶液pH值密切相关,从而会对吸附量产生重要影响。在不同初始pH条件下,PEI-APTES/Bent-4对Cr(Ⅵ)吸附量及Zeta电位如图5所示。由图5(a)可以看出,pH值对Cr(Ⅵ)在PEI-APTES/Bent-4上的吸附量有很大的影响。溶液初始pH<2时,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附量随pH增加而增大;在pH=2时,吸附量达最大值128.24 mg·g−1;随着 pH进一步增大,吸附剂对Cr(Ⅵ)的吸附量显著减小。pH产生影响的原因主要是由于其既影响Cr(Ⅵ)的存在形式,也影响PEI-APTES/Bent-4表面官能团的质子化程度,从而影响材料表面电荷。

    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 curves of PEI-APTES/Bent-4 (b)

    图5(b)可以看出,pH值对PEI-APTES/Bent-4的Zeta电位影响较大,酸性条件下材料Zeta电位为正。在pH<2时,Cr(Ⅵ)主要种类为H2CrO4、HCrO4,在2<pH<6时,HCrO4Cr2O27是主要存在种类,pH>6.8时CrO42–为主要存在种类。PEI-APTES/Bent-4在酸性条件下表面带正电,可与HCrO4Cr2O27产生静电引力,吸附量增大;酸性越强,氨基的质子化增加,吸附量增大;但当pH 值从 2.0降低到 1.0 时,吸附量降低,这主要归因于pH=1.0时,Cr(Ⅵ)部分以H2CrO4存在,静电吸附减少,同时,H2CrO4沉积在PEI-APTES/Bent-4表面,影响静电吸附所致。随pH增加,由于质子化氨基数量减少及氢氧根离子的竞争吸附,使吸附量降低。这些结果表明,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附能力极大地依赖于溶液的pH值,在pH=2时,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附量最大。在后续实验中,将初始溶液pH值调整为2.0。

    图6为PEI-APTES/Bent-4对Cr(Ⅵ)的吸附量随吸附时间变化曲线及吸附动力学模型拟合图。由图6(a)可见,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附量随时间逐渐增加,在40 min时达到饱和。在吸附初始阶段,Cr(Ⅵ)吸附量增加较快,主要是由于在该阶段PEI-APTES/Bent-4上的空白吸附位点较多,吸附速率较快;另外,初始阶段Cr(Ⅵ)浓度较高,两相传质驱动力较大,使吸附速率增加。随着时间增加,吸附速率逐渐放缓,这主要是由于PEI-APTES/Bent-4上的空白吸附位点减少, 同时Cr(Ⅵ)浓度降低,使两相传质动力减小所致。吸附达到平衡时,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附量达128.25 mg·g−1

    为进一步研究PEI-APTES/Bent-4与Cr(Ⅵ)的相互作用,探索吸附机制。利用准一级与准二级2种动力学模型线性拟合图6(a)中实验数据,所得拟合结果如图6(b)图6(c)所示,拟合模型参数见表2

    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
    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: qe—Amount of adsorption at equilibrium; K1—Quasi-first-order kinetic model constant; K2—Quasi-second-order kinetic model constant; R2—Correlation coefficient.
    下载: 导出CSV 
    | 显示表格

    由表可知,准二级动力学模型相关系数(R2)更高,得出的理论吸附量qe值更接近实验吸附结果。这表明PEI-APTES/Bent-4对Cr(Ⅵ)的吸附过程主要受化学吸附控制,涉及Cr(Ⅵ)与吸附剂PEI-APTES/Bent-4之间的化学相互作用,吸附过程中可能存在电子交换。

    不同温度下PEI-APTES/Bent-4对Cr(Ⅵ)的吸附等温线如图7所示。吸附量随着初始浓度的升高而增加,这是由于初始浓度较低时,吸附剂空白吸附位点较多,吸附的吸附质较少,而当浓度增加时,吸附质增多,空白吸附位点减少,吸附量增大。同时,高初始浓度也为克服界面处的差异传质提供了动力。图中也可看出,随着温度的升高,Cr(Ⅵ)在PEI-APTES/Bent-4上的吸附能力提高,表明温度升高有利于Cr(Ⅵ)的去除。为了更好地理解吸附行为,通过Langmuir和Freundlich模型拟合各温度下的实验数据。2种吸附等温模型拟合相关参数见表3

    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
    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 
    | 显示表格

    由表可见, Langmuir模型相关系数(R2)更高,且所得理论吸附量更接近实验结果。表明发生的Cr(Ⅵ)阴离子在PEI-APTES/Bent-4表面为单分子层吸附。由表中分离系数(0<RL<1)可知,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附为有利过程。此外,根据Langmuir等温线模型,PEI-APTES/Bent-4在313 K下对Cr(Ⅵ)理论吸附量为137.50 mg·g−1,与先前报道的其他改性膨润土吸附剂吸附量的对比研究如表4所示。PEI-APTES/Bent-4表现出更显著的吸附能力[37-44]

    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 
    | 显示表格

    为了进一步了解吸附机制,分别计算热力学参数吉布斯能量变化(ΔG0)、焓变化(ΔH0)和熵变化(ΔS0),如下式[45-46]

    ΔG0=RTlnKc (8)
    Kc=qeCe×1000 (9)
    ΔG0=ΔH0TΔS0 (10)
    lnKc=ΔH0RT+ΔS0R (11)

    式中:Kc为热力学平衡常数;R为理想气体常数(8.314 J·mol−1·K−1);T为温度(K)。

    表5为PEI-APTES/Bent-4吸附Cr(Ⅵ)的热力学参数。

    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 
    | 显示表格

    由表可见,ΔG0为负值,且绝对值随温度升高而增加。表明PEI-APTES/Bent-4对Cr(Ⅵ)吸附过程是自发的,温度升高可促进该吸附过程。ΔH0为正,表明PEI-APTES/Bent-4对Cr(Ⅵ)吸附过程是吸热的,升温有利于Cr(Ⅵ)的吸附,这支持了等温线模型的结论。此外,ΔS0为正表明PEI-APTES/Bent-4吸附Cr(Ⅵ)过程为熵增加过程,在吸附过程中固/液界面处的随机性增加。

    为了进一步阐明PEI-APTES/Bent-4对Cr(Ⅵ)的吸附过程,对PEI-APTES/Bent-4吸附Cr(Ⅵ)前后的表面进行了FTIR和XPS表征,如图8图9所示。

    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8显示了PEI-APTES/Bent-4在吸附Cr(Ⅵ)阴离子前后的FTIR图谱。在吸附Cr(Ⅵ)阴离子后,1651 cm−11561 cm−1处对应于氨基的谱带移动到1641 cm−1处,表明氨基参与了Cr(Ⅵ)阴离子的去除[47]。PEI-APTES/Bent-4吸附Cr(Ⅵ)前后的XPS全扫描图谱如图9(a)所示,吸附Cr(Ⅵ)后在光谱上出现了一个与Cr2p特征峰相对应的新峰,证明铬被吸附到PEI-APTES/Bent-4上。图9(b)为PEI-APTES/Bent-4吸附Cr(VI)后表面Cr的高分辨率XPS光谱,其可以拟合为4个峰,576.6 eV和586.1 eV处的峰可归因于Cr(Ⅲ),而579.1 eV和588.1 eV处的峰可归因于Cr(Ⅵ) [48-49]。表明PEI-APTES/Bent-4对Cr(Ⅵ)吸附涉及吸附和还原过程。根据上述表征、结合吸附条件影响因素分析,PEI-APTES/Bent-4吸附Cr(Ⅵ)的机制如图10所示:(1) PEI-APTES/Bent-4的氨基在酸性条件下质子化,通过静电吸引将负电性的Cr(Ⅵ)物质(HCrO4Cr2O27)吸附到表面;(2)氨基起到电子给体的作用,并将Cr(Ⅵ)还原为Cr(Ⅲ) [50-52],如下式;(3)生成的Cr(Ⅲ) 通过氨基螯合作用固定于PEI-APTES/Bent-4表面。

    Cr2O27+6e+14H+2Cr3++7H2O (12)
    HCrO4+3e+7H+Cr3++4H2O (13)
    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)

    对PEI-APTES/Bent-4进行6次吸附与再生实验以评价吸附材料的重复利用性,结果见图11。由图可见,再生使PEI-APTES/Bent-4的吸附量降低,且随着再生次数越多,吸附量越小。这主要是由于再生次数的增加会使未脱附的Cr阴离子越多,导致吸附位点不断减少;另外,再生次数越多,PEI-APTES/Bent-4上的氨基被氧化的越多。经过6次再生后,PEI-APTES/Bent-4对Cr(VI)的吸附量降为原来的74.58%,表明其具有良好的重复利用性。

    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) 通过FTIR、EDS和TGA分析表征表明,通过戊二醛可将聚乙烯亚胺(PEI)交联于膨润土表面,成功制得聚乙烯亚胺交联膨润土(PEI-APTES/Bent-4)。

    (2)进一步交联负载PEI后,膨润土对Cr(Ⅵ)的吸附量显著增加。PEI-APTES/Bent-4对Cr(Ⅵ)的吸附过程受pH值影响较大,吸附最佳pH值为2。在模拟废水Cr(Ⅵ)质量浓度为200 mg·L−1,温度为20℃,吸附剂投加量为1 g·L−1时,PEI-APTES/Bent-4对Cr(Ⅵ)的吸附量达128.24 mg·g−1

    (3) PEI-APTES/Bent-4对Cr(Ⅵ)的吸附动力学拟合更符合准二级动力学模型,吸附主要为化学吸附过程;吸附等温线拟合更符合Langmuir模型,为单分子层吸附,其理论最大吸附量达137.50 mg·g−1;热力学分析证明该吸附过程为自发的吸热过程。

    (4) 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)首先通过静电引力吸附在PEI-APTES/Bent-4上,随后部分Cr(Ⅵ)被还原为Cr(Ⅲ),Cr(Ⅲ)螯合在吸附剂表面。此外,PEI-APTES/Bent-4具有良好的循环利用性。

  • 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 spectras (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 curves 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.
    下载: 导出CSV

    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
    下载: 导出CSV

    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: qe—Amount of adsorption at equilibrium; K1—Quasi-first-order kinetic model constant; K2—Quasi-second-order kinetic model constant; R2—Correlation coefficient.
    下载: 导出CSV

    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:  2025-02-14

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