离子液体辅助纳米纤维素吸附剂的制备及其吸附性能

纤维素是天然可再生高分子材料，且无毒、可降解和价格低廉 因为化石能源短缺和化石燃料环境污染，开发和应用纤维素可再生材料极为重要 纤维素的结构中含有大量的羟基，具有一定的吸附能力但是吸附容量很低 对纤维素进行化学改性引入新的官能团，可显著提高其吸附性能[1~4] 将纤维素的尺寸降低到纳米级，也称为纳米纤维素(NC)，可增大其长径比和比表面积，使其吸附性能提高[5,6] 对纤维素进行化学或物理处理，可得到纳米纤维素(至少有一维尺寸达到纳米级) 与传统的纤维素相比，纳米纤维素具有高强度、高比表面积、高杨氏模量和长径比大等性能，是目前纤维素科学和技术领域中最具发展潜力的材料之一[7,8] 目前对纤维素吸附剂的研究比较多，但是对纳米纤维素吸附剂的研究比较少[9]

制备纳米纤维素的常用方法，有机械法、酸解法和酶解法 其中机械法能耗大，成本高；酸水解法对设备的要求苛刻，残留物难以回收和后处理；酶解法的效率不高，反应条件尚需优化 因此开发绿色高效的纳米纤维素制备方法，是该领域的研究热点

离子液体是由有机阳离子和无机或有机阴离子构成的液态有机盐类物质，有不易挥发、不可燃、性质稳定、可循环利用及无污染等优点，是一种“绿色”化学溶剂 自2002年Swatloski等[10]发现离子液体可溶解纤维素以来，离子液体凭借其优异的性能在纤维素的资源化转化与利用方面表现出巨大的潜力 使用酸性离子液体可将纤维素水解成纤维素单链用于制备纳米纤维素，备受关注[11] 鉴于此，本文使用离子液体作为溶剂和催化剂，对纤维素进行溶胀和水解制备纳米纤维素；在纤维素水解的同时对其进行接枝共聚改性制备纳米纤维素吸附剂，并研究其吸附性能

1 实验方法1.1 实验用原料

脱脂棉纤维素；离子液体为1-丁基-3-甲基咪唑硫酸氢盐([Bmim]HSO4)(熔点为28℃，纯度99%)；丙烯酸、过硫酸钾、丙酰胺和无水乙醇，纯度99%以上

1.2 纳米纤维素和纳米纤维吸附剂(AA/AM-g-NC)的制备

NC的制备：将1 g脱脂棉纤维和20 g离子液体 [Bmim]HSO4加入250 mL三口烧瓶中，将三口烧瓶置于100℃油浴锅中加热且均匀搅拌以使脱脂棉纤维完全溶胀、分散并水解，反应2 h后将纤维素溶液在温水浴中超声30 min，然后用无水乙醇和蒸馏水反复离心(2000 r/min)洗涤至中性 将洗涤后的纤维素均匀分散在超纯水(浓度在1%左右)中，放入高压细胞均质机中使其进一步超微细化 为了防止高压过程中的温度过高，实验在5℃的冷却水循环机中进行 高压均质后，取上清液分析备用

AA/AM-g-NC的制备：将1 g脱脂棉纤维和适量的离子液体1-丁基-3-甲基咪唑硫酸氢盐([Bmim]HSO4)加入250 mL三口烧瓶中，将三口烧瓶置于100℃油浴锅中加热且均匀搅拌以使脱脂棉纤维完全溶胀并分散；将分散后的纤维素离子液体溶液的温度降至40~50℃，然后在N2保护下按比例加入一定质量的引发剂过硫酸钾，继续通N2并在匀速搅拌下加入一定比例的单体丙烯酰胺(AM)、80%中和度的丙烯酸(AA)和交联剂N，N-亚甲基双丙烯酰胺，继续反应3 h 反应结束后在离心机中用蒸馏水和无水乙醇或丙酮反复离心洗涤，并用丙酮作为溶剂索氏抽提去除均聚物；随后将抽提后的纤维素基接枝共聚物(记为AA/AM-g-Ce)均匀分散在超纯水中，形成均匀的水相分散液(浓度在1%~2%左右) 将分散液放入高压细胞均质机中，利用匀质机内的匀质阀突然失压形成的空穴效应和高速冲击，使悬浮液进一步超微细化 为了防止高压过程中温度过高，实验在5℃的冷却水循环机中进行 高压均质后即可得到纳米纤维素基接枝共聚物胶体溶液，将其冷冻干燥后得到纳米纤维素吸附剂，记为AA/AM-g-NC

1.3 产物的表征

用超高分辨率场发射扫描电镜(XL-30，Philips-FEI公司)和场发射透射电子显微镜(Tecnai G2 F20对S-TWIN 200kV，Philips-FEI公司)分析产物的微观形貌和尺寸；将干燥的Ce、NC、AA/AM-g-NC分别与KBr按一定比例混合，在玛瑙研钵中精细研磨后挤压成圆盘状 在分析前，将其置于70℃烤箱中干燥10 min 用傅里叶变换红外光谱仪(Nicolet is5，赛默飞公司)分析产物的官能团，设定扫描范围为400~4000 cm -1；用元素分析仪(2400系列II型CHNS/O)定量测定NC、AA/AM-g-NC样品中C、H、N和O各元素的含量；使用X射线粉末光衍射仪(X'Pert Pro MPD)对样品进行扫描分析；用X-射线光电子能谱(Thermo Scientific ESCALAB 250 X，Thermo Fisher Scientific 公司)测定样品表面的化学组分

1.4 纳米纤维素吸附剂吸附性能的表征

以亚甲基蓝溶液为吸附质，用静态吸附试验测定AA/AM -g-NC的吸附性能 将50 mL浓度为20 mg/L的亚甲基蓝溶液放入锥形瓶中，投加0.02 g吸附剂AA/AM-g-NC后置于恒温振荡床中在一定温度和转速条件下震荡一段时间 反应结束后取适量的上清液用紫外可见分光光度计测定亚甲基蓝吸光度(λ=664 nm)，用亚甲基蓝的工作曲线计算其浓度并计算吸附剂的吸附容量

去除率为：

η=Ce-C0Ce×100%

(1)

平衡吸附容量为：

Qe=Ce-C0X×V

(2)

式中Ce为吸附前亚甲基蓝的质量浓度(mg/L)，C0为吸附后亚甲基蓝的质量浓度(mg/L)，V为亚甲基蓝溶液的体积(L)，X为AA/AM-g-NC的用量(g)

2 实验结果和分析2.1 纳米纤维素吸附剂的形貌和组成

纳米纤维素(NC)及纳米纤维素吸附剂AA/AM-g-NC的表面形貌(图1) 从图1可以看出，脱脂棉经离子液体[Bmim]HSO4水解和高压均质处理后尺寸大大减小(图1a)，纤维素呈纳米纤丝状且相互交联成网状结构(图1b) 其原因是，纤维素经酸性离子液体水解并辅助高压均质处理后尺寸减小，且在水解过程中进行了接枝共聚和交联改性，表面的羟基及官能团之间的氢键作用使纳米纤丝相互缠绕 10 μm纳米纤维素和吸附剂的透射电镜TEM照片，如图2所示 从图2a可见，纳米纤维素NC大都呈短棒状，部分呈纤丝状；而纳米纤维素吸附剂则由纤丝交联成网状结构，与SEM观察的结果一致

图1



图1NC和AA/AM-g-NC的SEM 照片

Fig.1SEM images of NC (a) and AA/AM-g-NC (b)

图2



图2NC及AA/AM-g-NC的TEM像

Fig.2TEM images of NC (a) and AA/AM-g-NC (b)

脱脂棉纤维素Ce、纳米纤维素NC及其接枝共聚物AA/AM-g-NC的红外光谱，如图3所示 从图3可见，NC及其接枝产物AA/AM-g-NC都保留了纤维素的特征吸收峰，AA/AM-g-NC还在1730 cm-1处出现C=O的伸缩振动峰[12]，且在3300~3400 cm-1范围峰变宽；1630 cm-1处的峰变尖锐，分别是 N-H 键的拉伸和弯曲振动所致[13]；此外，AA/AM-g-NC的O-H吸收峰发生了明显的蓝移现象，由Ce的3354 cm-1变成了3416 cm-1；以上结果表明，单体丙烯酸和丙烯酰胺成功地接枝在纳米纤维素上

图3



图3Ce、NC和AA/AM-g-NC的红外光谱

Fig.3FTIR of Ce, NC and AA/AM-g-NC

纤维素、NC及其吸附剂AA/AM-g-NC的XRD谱图，如图4所示 从图4可以看出，NC和AA/AM-g-NC的晶型结构与纤维素晶型相同，在2θ=14.8°，16.5°，22.6°和34.5°的峰分别对应纤维素I型的4个晶面，表明离子液体处理、改性及后续的高压均质处理并没有破坏纳米纤维素的晶面；其结晶度由纤维素的67.1%提高到75.1%，因为在纤维素水解及改性过程中纤维素的无定形区被破坏并水解，使NC及纳米纤维素吸附剂的结晶度提高[14]

图4



图4NC和AA/AM-g-NC的XRD谱图

Fig.4XRD patterns of NC and AA/AM-g-NC

纳米纤维素和吸附剂表面元素的XPS图谱，如图5和图6所示 图5表明，接枝改性后的纳米纤维素吸附剂表面的元素除C、H、O外，还有N元素 由N1s的高分辨XPS图谱可知，其特征峰的结合能为399.6 eV，表明N以氨基(-NH2)的形式存在

图5



图5纳米纤维素NC和AA/AM -g-NC的XPS图谱以及N1s的高分辨图谱

Fig.5XPS survey spectra of NC (a) and AA/AM-g-NC (b) and XPS survey scan of N1s core level (b inserted)

图6



图6NC和AA/AM-g-NC处理后C1s、N1s和O1s的XPS图谱

Fig.6XPS survey of C1s, N1s and O1s of NC and AA/AM-g-NC

图6给出了 C1s、O1s及N1s的高分辨图谱 NC及吸附剂上的C有三种类型[15]：一种是与C或H连接的C，即C-(C，H)，其电子结合能为284.6 eV；第二种为与O或N连接的C，即C-O或C-N，其电子结合能为286 eV；第三种是连接两个O的C，即O-C-O或C=O，其电子结合能为287.7 eV 在O1s的高分辨图谱中，NC中的O主要来自C-O单键，即HO-C和C-O-C，其结合能为532.6 eV；而吸附剂AA/AM-g-NC中的O有两种类型：1) C-O单键，结合能为532.6 eV；2) C=O双键，结合能为531.2 eV，主要来自丙烯酸及丙烯酰胺的酯基 吸附剂AA/AM-g-NC表面的N元素，主要来源于接枝的单体丙烯酰胺中的氨基(-NH2)，其结合能为399.6 eV XPS结果表明，吸附剂表面有羧基及氨基基团

为了进一步证明吸附剂上官能团的存在，对NC及吸附剂AA/AM-g-NC进行元素分析，结果列于表1 表1中的数据表明，纳米纤维素接枝后元素的含量发生了较大的变化，C、N元素含量的提高及O元素含量的降低证明氨基(-NH3)、羧基(-COOH)已经成功地接枝在纳米纤维素上

Table 1

表1

表1NC和 AA/AM -g-NC元素含量

Table 1Element content of NC and AA/AM-g-NC

Sample	O/%	C/%	H/%	N/%
NC	51.24	42.65	6.02	0.37
AA/AM-g-NC	46.21	44.07	6.09	3.13

表2列出了Ce、NC和AA/AM-g-NC的比表面积、孔容及平均孔径 由表2中数据可知，Ce的比表面积仅有3.39 m2/g，平均孔径为6.047 nm.与水解均质后的NC和接枝改性后的AA/AM-g-NC相比，BET比表面积分别升高至13.04 m2/g和12.95 m2/g，平均孔径分别为4.01 nm和5.834 nm 这表明，在纤维素变成NC及AA/AM-g-NC过程中纤维素的尺寸减小，比表面积增大 较大的比表面积，为吸附污染物提供了较高的吸附潜能

Table 2

表2

表2Ce、NC和AA/AM-g-NC的吸附参数

Table 2Adsorption parameters of Ce, NC and AA/AM-g-NC

Sample	Parameters of pore structure
	Surface area/m2·g-1	Pore volume /cm3·g-1	Average pore size /nm
Ce	3.390	0.0036	6.047
NC	13.04	0.013	4.010
AA/AM-g-NC	12.95	0.019	5.834

2.2 纳米纤维素吸附剂AA/AM -g-NC的吸附性能2.2.1 AA/AM-g-NC对亚甲基蓝的吸附性能

AA/AM-g-NC对亚甲基蓝的吸附容量，如图7所示 从图7可以看出，改性后的纳米纤维素对亚甲基蓝的吸附容量大大增加，从未改性的14.6增加到43.96 mg/g 与Ce、AA/AM-g-Ce和NC相比，AA/AM-g-NC吸附量的大幅度提高归因于其表面的官能团和比表面积的增加 接枝共聚改性赋予纳米纤维素表面许多羧基及氨基官能团，从而增大了对亚甲基蓝的吸附性能；另一方面，纳米纤维素吸附剂比AA/AM-g-Ce大的比表面积能提供更多的吸附位点，有利于吸附剂对亚甲基蓝的吸附

图7



图7Ce、NC、AA/AM-g-Ce和AA/AM-g-NC对亚甲基蓝的去除率和吸附容量对比

Fig.7Comparison of remove rate and adsorption capacity of methylene blue on Ce, NC, AA/AM-g-Ce and AA/AM-g-NC. Adsorption conditions: pH,10; MB initial concentration, 20 mg/L; adsorption time, 2 h; adsorption temperature, room temperature

此外，与文献中的纳米纤维素吸附剂相比(表3)，改性后的AA/AM-g-NC对亚甲基蓝的吸附性能也表现出一定的优势

Table 3

表3

表3与文献中纳米纤维素吸附剂对亚甲基蓝吸附性能的比较

Table 3Comparison of adsorption performance towards methyl blue between AA/AM-g-NC and other nanocellulse adsorbent reported

Adsorbent	Solute	C0/mg·L-1	qe(exp)/mg·L-1	Ref.
NCC(T=35℃)		4.80	2.9	[16]
		9.60	4.9
		14.39	6.6
NCC(T=45℃)	MB	4.80	2.9
		9.60	5.4
		14.39	6.9
NCC(T=55℃)		4.80	2.9
		9.60	5.4
		14.39	6.7
		100	10.41	[17]
		200	17.24
NCC alginate hydrogel beads	MB	400	35.28
		600	50.29
		800	72.84
Cellulose nanowhiskers-based polyurethane foam	MB	50	43.5	[18]

2.2.2 溶液pH值对吸附性能的影响

以接枝改性后的纳米纤维素AA/AM-g-NC为吸附剂，研究吸附剂对20 mg/L亚甲基蓝的吸附性能 图8给出了吸附过程中pH值对吸附剂吸附性能的影响 由图8可见，在碱性条件下纳米纤维素吸附剂具有更优异的吸附性能 其原因是，在碱性条件下溶液中H+浓度的降低降低了与亚甲基蓝阳离子的竞争吸附，从而提高了吸附剂对亚甲基蓝的去除率和吸附容量

图8



图8pH值对AA/AM-g-NC吸附性能的影响

Fig.8Effect of pH on the adsorption properties of AA/AM-g-NC. Adsorption conditions: MB initial con-centration, 20 mg/L; adsorption time, 2 h; adsorption temperature, room temperature

2.2.3 Langmuir和Freundlich吸附等温式 使用Langmuir和Freundlich吸附等温式[19]

Ceqe=1b?Q0+CeQ0

(3)

lgqe=lgK+1nlgCe

(4)

研究了AA/AM-g-NC的吸附行为，拟合结果如图9和表4所示 式中qe为吸附剂的平衡吸附容量(mg/g)，Ce为亚甲基蓝的平衡浓度(mg/L)，Q0为吸附剂的饱和吸附容量(mg/g)，b为Langmuir常数(L/mg)，K和n为Freundlich参数

图9



图9Langmuir及Freundlich方程拟合图

Fig.9Langumuir (a) and Freundlich (b) equation fitting

Table 4

表4

表4拟合的Langmuir和Freundlich参数

Table 4Calculated parameters for the Langmuir and Freundlich model

Temperature/K	Langmuir parameters			Freundlich parameters
	Q0/mg·g-1	b/L·mg-1	R2	K	1/n	R2
303	35.84	0.982	0.996	16.87	0.311	0.981
313	34.36	0.915	0.995	16.15	0.296	0.990
323	32.57	0.829	0.988	15.25	0.284	0.981

可以看出，Langmuir吸附等温模型在三个温度的相关系数R2均高于Freundlich吸附等温模型，且根据Langmuir等温吸附模型计算出的最大吸附量Q0与实验测得的最大吸附容量qe的一致性很好 这表明，纳米纤维素吸附剂对亚甲基蓝的吸附符合Langmuir等温吸附模型，对亚甲基蓝在吸附剂表面发生的是单分子层吸附

2.2.4 吸附热力学 可根据吉布斯自由能(?G)、焓变化(?H)和熵变化(?S)研究吸附剂AA/AM-g-NC对亚甲基蓝的吸附热力学 ?G、?H和?S分别为[20~22]

ΔG=-RTlnb

(5)

lnb=ΔSR-ΔHRT

(6)

ΔS=(ΔH-ΔG)/T

(7)

式中，b为Langmuir参数(L/mol)，ΔH为焓变(kJ/mol)，R为气体参数(8.314 J/(mol·K))，T为吸附温度(K)，ΔG为自由能的变化(kJ/mol)，ΔS为熵变(kJ/(mol·K))

使用准一级动力学方程

lg(qe-qt)=lgqe-k1t

(8)

和准二级动力学方程

tqt=1k2qe2+tqe

(9)

计算出的热力学参数，列于表5 式中t为吸附时间(min)，qe为AA/AM-g-NC的平衡吸附容量(mg/g)，qt为AA/AM-g-NC在t时刻的吸附容量(mg/g)，k1为一级吸附速率常数(min-1)；k2为二级反应速率常数(g·mg-1·min-1) 由表5中的数据可知，?H<0说明AA/AM-g-NC对亚甲基蓝的吸附过程为放热过程，升温不利于促进AA/AM-g-NC对亚甲基蓝的吸附；?G<0说明AA/AM-g-NC对亚甲基蓝的吸附是在自发有利的条件下进行的；而?S<0说明整个吸附过程为熵减少的过程[23]

Table 5

表5

表5热力学参数

Table 5Thermodynamic parameters

Temperature/K	△G/kJ·mol-1	△S/J·(mol·K)-1	△H/kJ·mol-1
303	-0.0459	-0.0225	-6.876
313	-0.232	-0.0212
323	-0.505	-0.0197

2.2.5 吸附动力学

为了研究AA/AM-g-NC对亚甲基蓝的吸附机理，采用准一级、准二级和粒子内反应扩散方程[24~26]分别对303 K下的吸附实验数据进行动力学拟合，拟合公式为式9和扩散方程

qt=KTt1/2+C

(10)

式中，t为吸附时间(min)，qt为AA/AM-g-NC在t时刻的吸附容量(mg/g)，KT为颗粒内扩散速率常数(mg/(g·min1/2))，C为常数 相关的拟合结果如图10、11和表6所示 根据拟合结果分析，AA/AM-g-NC对亚甲基蓝溶液的吸附过程更符合准二级动力学模型(R2值判断)，且准二级动力学模型的理论平衡吸附量也更接近实验值，说明AA/AM-g-NC对亚甲基蓝的吸附主要以化学吸附为主[27] 颗粒内扩程的拟合图(图11)呈三段式分布，且不经过原点，说明AA/AM-g-NC吸附亚甲基蓝的过程不仅由颗粒内扩散控制，还受表面扩散控制

图10



图10准一级动力学和准二级动力学拟合曲线

Fig.10Fitting curves of Pseudo-first-order (a) and Pseudo-second-order (b)

图11



图11颗粒内扩散反应方程曲线

Fig.11Intraparticle diffusion equation curve

Table 6

表6

表6吸附过程的动力学参数

Table 6Kinetic parameters of adsorption process

Pseudo-first-order kinetic model			Pseudo-second-order kinetic model
k1	qe	R2	k2	qe	R2
0.030	56.02	0.935	0.0012	48.92	0.999

3 结论

(1) 以脱脂棉纤维素为原料、以丙烯酸AA和丙烯酰胺AM为接枝单体，在离子液体[Bmim]HSO4中对纤维素进行水解及接枝改性，然后对其进行高压均质处理，可制备出纳米纤维素吸附剂AA/AM-g-NC 对脱脂棉进行离子液体辅助高压均质处理后得到纤丝网状结构的纳米纤维素吸附剂AA/AM-g-NC，其表面接有AM及AA的官能团，纤维素的晶型没有改变，结晶度有所提高

(2) 纳米纤维素吸附剂AA/AM-g-NC对亚甲基蓝的吸附性能受pH值的影响，碱性条件有利于吸附剂的吸附；对亚甲基蓝的吸附符合Langmuir吸附等温式，为单分子层吸附；吸附过程为放热反应，升温不利于AA/AM-g-NC对亚甲基蓝的吸附 吸附在自发有利的条件下进行，吸附过程为固/液相界面的分子运动趋于稳定的熵减少过程；吸附过程符合准二级动力学模型 对亚甲基蓝的吸附，受控于颗粒内扩散和表面扩散

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PMID

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1

2008