离子液体辅助纳米纤维素吸附剂的制备及其吸附性能

图1 NC和AA/AM-g-NC的SEM 照片

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

图2

图2 NC及AA/AM-g-NC的TEM像

Fig.2 TEM 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3 Ce、NC和AA/AM-g-NC的红外光谱

Fig.3 FTIR 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4 NC和AA/AM-g-NC的XRD谱图

Fig.4 XRD 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.5 XPS survey spectra of NC (a) and AA/AM-g-NC (b) and XPS survey scan of N1s core level (b inserted)

图6

图6 NC和AA/AM-g-NC处理后C1s、N1s和O1s的XPS图谱

Fig.6 XPS 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。

表1 NC和 AA/AM -g-NC元素含量

Table 1 Element 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。

表2 Ce、NC和AA/AM-g-NC的吸附参数

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

Sample	Parameters of pore structure
Sample	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7 Ce、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.7 Comparison 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。

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

Adsorbent	Solute	C₀/mg·L^-1	q_e(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8 pH值对AA/AM-g-NC吸附性能的影响

Fig.8 Effect 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]

\frac{C_{e}}{q_{e}} = \frac{1}{b \cdot Q_{0}} + \frac{C_{e}}{Q_{0}}

(3)

l g q_{e} = l g K + \frac{1}{n} l g C_{e}

(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q_e为吸附剂的平衡吸附容量(mg/g)，C_e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9 Langmuir及Freundlich方程拟合图

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

表4 拟合的Langmuir和Freundlich参数

Table 4 Calculated parameters for the Langmuir and Freundlich model

Temperature/K	Langmuir parameters			Freundlich parameters
Temperature/K	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q_e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]

Δ G = - R T l n b

(5)

l n b = \frac{Δ S}{R} - \frac{Δ H}{R T}

(6)

Δ S = (Δ H - Δ G) / 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

l g (q_{e} - q_{t}) = l g q_{e} - k_{1} t

(8)

和准二级动力学方程

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}

(9)

计算出的热力学参数，列于表5。式中t为吸附时间(min)，q_e为AA/AM-g-NC的平衡吸附容量(mg/g)，q_t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]。

表5 热力学参数

Table 5 Thermodynamic 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

q_{t} = K_{T} t^{1 / 2} + C

(10)

式中，t为吸附时间(min)，q_t为AA/AM-g-NC在t时刻的吸附容量(mg/g)，K_T为颗粒内扩散速率常数(mg/(g·min^1/2))，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.10 Fitting 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.11 Intraparticle diffusion equation curve

表6 吸附过程的动力学参数

Table 6 Kinetic parameters of adsorption process

Pseudo-first-order kinetic model			Pseudo-second-order kinetic model
k₁	q_e	R²	k₂	q_e	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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In this article, the ability of chaff to adsorb heavy metal ions from aqueous solution was investigated in a fixed-bed column. The effect of important parameters, such as the value of pH, the flow rate, the influent concentration of solution and the effect of coexistence ions, was studied. Also the adsorption/desorption recycles of chaff were shown, and the results indicated that chaff could be recycled to remove heavy metal ions. The Thomas model was applied to adsorption of copper and lead at different flow rate and different influent concentration to predict the breakthrough curves and to determine the characteristic parameters of the column useful for process design. The model was found suitable for describing the biosorption process of the dynamic behavior of the chaff column. All the results suggested that chaff as adsorbent to removal heavy metal ions from solution prove efficient, and the rate of biosorption process is speedy. Furthermore, the efficiency of adsorption is high. When the flow rate was 3.6 ml min(-1) and the influent concentration of copper and lead was 14.82 mg l(-1) and 50.12 mg l(-1) respectively, the equilibrium adsorption biomass reached 1.98 mg g(-1) and 6.72 mg g(-1), respectively. The competitive adsorption for lead and copper was studied. Moreover the total adsorbing capability of chaff did not decrease when there were both copper(II) and lead(II) in solution.

[23]

Acemioğlu

Removal of Fe(II) ions from aqueous solution by Calabrian pine bark wastes

[J]. Bioresour. Technol., 2004, 93: 99

The ability of Calabrian pine bark wastes (Pinus brutia Ten) for the removal of Fe(II) ions from aqueous solution at different concentrations and temperatures at a fixed pH was investigated. While the amounts of Fe(II) ions adsorbed onto the bark increased with increasing concentration, it increased slightly with increasing the temperature. Kinetics studies showed that adsorption process followed the first-order kinetic model as well as intra-particle diffusion kinetics. Adsorption isotherm followed both Langmuir and Freundlich models. And it was determined that the adsorption was favorable from a dimensionless factor, R(L). Furthermore, the thermodynamic parameters demonstrated that the removal of Fe(II) by the bark was a physical process.

[24]

Rengaraj

, Kim

, Joo

C K

, et al.

Removal of copper from aqueous solution by aminated and protonated mesoporous aluminas: kinetics and equilibrium

[J]. J. Colloid Interface Sci., 2004, 273: 14

A novel adsorbent, aminated and protonated mesoporous alumina, was prepared and employed for the removal of copper from aqueous solution at concentrations between 5 and 30 mg/l, in batch equilibrium experiments, in order to determine its adsorption properties. The removal of copper by the adsorbents increases with increasing adsorbent dosages. The adsorption mechanism is assumed to be an ion exchange between copper and the hydrogen ions present on the surface of the mesoporous alumina. The adsorbent was characterized by XRD, TEM, SEM, and BET methods. The sorption data have been analyzed and fitted to linearized adsorption isotherm of the Freundlich, Langmuir, and Redlich-Peterson models. The batch sorption kinetics have been tested for first-order, pseudo-first-order, and pseudo-second-order kinetic reaction models. The rate constants of adsorption for all these kinetic models have been calculated. Results also showed that the intraparticle diffusion of Cu(II) on the mesoporous catalyst was the main rate-limiting step.

[25]

Chiron

, Guilet

, Deydier

Adsorption of Cu(II) and Pb(II) onto a grafted silica: isotherms and kinetic model

[J]. Water Res., 2003, 37: 3079

URL PMID

[26]

Hua

, Rocha

, Zhang

, et al.

Transition from bioinert to bioactive material by tailoring the biological cell response to carboxylated nanocellulose

[J]. Biomacromolecules, 2015, 17: 1224

URL PMID [本文引用: 1]

[27]

Vadivelan

, Kumar

K V

Equilibrium, kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk

[J]. J. ColloidInterface Sci., 2005, 286: 90