1 Introduction

Water contamination due to the discharge of dyes into water bodies has grown drastically over the past few years. Dye waste water contains a large amount of toxic azo dyes. Azo dye molecules in dye waste water are decomposed into aromatic amines and aromatic derivatives. These aromatic compounds have long been identified as carcinogenic and mutagenic and caused grievous injury to aquatic organisms and human life[1-2]. Therefore, it is essential to remove azo dyes from dye waste water before discharging it into water bodies.

要解决目前面临的科研与实际生产存在差距的问题，首先需要强化研究人员对于市场和生产的动态分析，让其更好的了解农户在实际生产过程中面临的困难和需求，从而更好的调整科研的方向，将更多的重心放在可用性更强的农业技术上。通过这样的方式，可以更好的提升科研成果的转化率，提高科研与生产之间的结合度，让农户可以更好的应用先进的科研成果进行生产。

Many physical/chemical methods such as coagulation/flocculation[3], anion exchange[4-5], chemical precipitation[6], electrochemical processes[7], membrane technology[8], and adsorption processes[9] have been reported for removing azo dyes from waste water. However, some of these methods are not efficient for practical applications; for example, the electrochemical and membrane methods are expensive, while the coagulation/flocculation, precipitation, and oxidation processes produce large amounts of toxic sludge,causing secondary pollutant that need further treatment.The adsorption process is one of the most common and effective methods used to remove dyes from dye waste water. Adsorbents play the most important role in the adsorption process. Many materials such as activated carbon[2], chitosan[10], and cellulose[11] can be used as adsorbents via functional modification. Some modified materials possessing functional groups such as carboxylic[3], amine[11], and amidoxime[12] can be used as adsorbents for the removal of harmful ions from aqueous solutions. These functional groups promote both physical and chemical adsorption, and hence improve the adsorption efficiency of adsorbents.Recently, the application of grafted materials for dye adsorption has received considerable attention[1,8-10].Zhou et al[13] synthesized ethylenediamine-modified magnetic chitosan nanoparticles via a complex route for removing AO7 from aqueous solutions.Kiransan et al prepared modified montmorillonite containing quaternary ammonium groups using cetyltrimethylammonium bromide[14]. Noorimotlagh et al[2] prepared active nano-porous carbon from milk vetch by a complex process. This carbon has nano-sized pore structure, and the pore size of 67% of the pores in the carbon is less than 100 nm.Unfortunately, the disadvantage of these adsorbent materials is that their preparation processes are very complicated and expensive. High-cost and stringent preparation conditions encounter significant challenges for widespread application of dye adsorbents. Hence, it is imperative to develop simple and cost-effective method to synthesize adsorbents for dye waste water treatment.

Cellulose Nanocrystal (CNC)is the most commonly used natural polymer material and possesses a large amount of active hydroxyl groups.

Hence, functional monomers can be easily grafted onto it to synthesize multifunctional materials. N,N-diethylaminomethyl methacrylate (DMAEMA) is a tertiary amine-containing acrylic monomer that can be easily polymerized by various reactions such as Atom Transfer Radical Polymerization(ATRP)[15] and Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT)[16]. In this work, DMAEMA was polymerized on the surface of CNC by ATRP. The functionalized CNC was used as an adsorbent for acid orange 7 (AO7) dye. The modified CNC was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis(TGA). Furthermore, the adsorption behavior of the CNC adsorbent was investigated under equilibrium and dynamic conditions.

疼痛调查:用McGi11疼痛问答法(MPQ)把疼痛共分为5级:0级无疼痛;1级:有疼痛感觉,但是不严重;2级:有轻微的疼痛,不舒服;3级:疼痛,很痛苦;4级:疼痛较为剧烈,有恐惧感;5级:剧痛。其中将1级、2级为归为轻度疼痛,3级为中度疼痛,4级、5级为重度疼痛。[1]对两组患者进行疼痛比较。

2 Materials and methods

2.1 Chemicals and instrument

2-Bromoisobutyryl bromide (BIBB), N-methyl-2-pyrrolidone (NMP), CuBr (99%, Energy Chemical),2-(Dimethyl amino) pyridine (DMAP), AO7, N, N, N′,N′′, N′′-pentamethyl diethylenetriamine (PMDETA),and 2-(diethylamino) ethylmethacrylate (DMAEMA,99%) were purchased from Shanghai Aladdin Co.,Ltd., China. N,N-dimethylformamide (DMF) and triethylamine (TEA, AR) were obtained from Tianjin Chemical Company, Tianjin, China. The chemical structure and characteristics of AO7 are shown in Table 1.DMF was dried by molecular sievesbefore use.CuBr was purified by acetic acid and ethanol. CNC was prepared using a method reported by us previously[17].

她不再相信爱情。因为她对前夫的爱情也是被设计出来的。尽管它万般真实，却无比虚假。可是她需要被爱的感觉。哪怕这感觉只是商家对她的服务。于是秦川在一个黄昏，与她“邂逅”。

1.1 实验动物 雄性SPF级Sprague-Dawley(SD)大鼠25只，体质量200～250 g，购自复旦大学上海医学院动物房。动物由复旦大学附属中山医院实验动物中心于SPF级条件下饲养，自由摄水、饮食。

Table 1 General characteristics of AO7

Items Parameters Structure Molecular formula C16H11N2O4SNa C.I. name Acid orange II Chromophore Monoazo Ionization Acidic Molecular weight/(g·mol-1)350.32 λmax/nm 485O N S ONa+OOH

FT-IR (VERTEX 70, Bruker, Germany), XPS (Kratos Axis Ultra DLD, Kratos, England), and TGA (TA Q500, USA) were used to determine the chemical structure and the surface elemental composition of the CNC samples. An ultraviolet-visible (UV-vis)spectrophotometer (G1369C, Agilent, China) was employed to determine the concentration of dye in the dye solution.

若不失一般性,令t=k,由于n-k≥3,n≥5,故≅An-1,k-1.所以在内存在一个哈密顿圈C,C={u,P,v,Q,u},则

Moreover, it was found that AO7 removal efficiency was high under acidic conditions. This is mainly because of the pH-responsive property of PDMAEMA.The tertiary amine groups (—NR1R2) of PDMAEMA on the surface of CNC acted as the binding sites for anions. Under acidic conditions, these amine groups protonated to form —NR1R2H+ groups, and the electric repulsion between the —NR1R2H+ groups restrained the tendency of the polymer chains to collapse or aggregate in solution[19]. However, under alkaline conditions,the tertiary amine groups could not protonate and the uncharged polymer chains collapsed or aggregated.Hence, PDMAEMA grafted on the CNC adsorbent became positively charged and dissolved completely at pH=2.0 and 4.0. On the other hand, at pH=8.5, the PDMAEMA chains remained uncharged and collapsed.And acidic environment increased the number of adsorption sites and the contact area between the adsorbent and azo ions. Therefore, from Fig.4 it can be observed that the AO7 removal efficiency of the CNC-PDMAEMA adsorbent at pH=2.0 and 4.0 was far greater than that at pH=8.5 when the other conditions were the same.

2.2 Preparation of CNC-based adsorbent

Where, k2 is the rate constant for the pseudo-secondorder kinetics.

Scheme 1 Synthesis route (a) and schematic (b) of the CNC-based adsorbent

2.3 Adsorption experiment

Where, k1 is the pseudo-first-order rate constant,Qe and Qt (mg/g) are the amounts of dye adsorbed at equilibrium and time t (min), respectively.

Adsorption kinetics provide essential information regarding the adsorption rate and reactive pathways.Hence, we applied the pseudo first- and second-order models (most widely used kinetic models) to elucidate the adsorption process. The equations are as follows:

Where, Qe is the dye adsorbed at equilibrium, Ce is the equilibrium dye concentration (mg/L), and m and V are the mass of the adsorbent (g) and volume of the dye solution (L), respectively.

3 Results and discussion

3.1 Characterization of CNC-Br and CNC-PDMAEMA

FT-IR and XPS analyses were carried out to examine the chemical structure and surface elements of CNC-Br and CNC-PDMAEMA. BIBB was anchored on the surface of CNC by esterification to prepare the ATRP initiator.As shown in Fig.1(b), unlike CNC, CNC-Br showed a strong FT-IR peak at 1740 cm-1. This peak corresponds to the stretching vibration of C=O from the ester group[19]. A strong peak at 1280 cm-1 corresponding to C—O was observed in the FT-IR spectrum of CNC-Br (Fig.1(b))[20]. The XPS spectrum of CNC-Br(Fig.2) showed a Br3d peak at 68 eV. It can be observed from Table 2 that the Br content in CNC-Br was 3.01%.These results suggest that BIBB was successfully grafted onto the CNC surface (reaction is shown in Scheme 1).

Fig.1 FT-IR spectra of (a) CNC, (b) CNC-Br, and(c) CNC-PDMAEMA

After the polymerization reaction, the intensity of the peak at 1740 cm-1 increased (Fig.1(b) and Fig.1(c))because of the additional carbonyl groups from PDMAEMA. The peaks at 2946, 2822, and 2773 cm-1 can be attributed to the C—H groups of PDMAEMA.The peak at 1460 cm-1 corresponds to the C—C stretching vibration, while that at 1150 cm-1 corresponds to the stretching vibration of C—N from the —N(CH3)2—group. From Fig.2 it can be observed that after the polymerization, new resonance signals were generated attributing to the carbon of CNC-PDMAEMA. After the ATRP reaction of CNC-PDMAEMA, the characteristic peak of N was detected at 396 eV (Fig.2). In addition,the nitrogen content in CNC-PDMAEMA increased sharply (5.03%, N1s), compared with that in CNC-Br(Table 2). These results confirm that PDMAEMA was successfully grafted onto the surface of CNC.

Fig.2 XPS spectra of CNC-Br and CNC-PDMAEMA

Table 2 Elemental surface composition of CNC-Br and CNC-PDMAEMA, as determined by XPS

Samples Composition/%C1s O1s N1s Br3d CNC-Br 53.66 43.22 0.11 3.01 CNC-PDMAEMA 67.73 25.62 5.03 0.49

The TG/differential TG (DTG) curves for CNC,CNC-Br, and CNC-PDMAEMA are shown in Fig.3.For CNC, the initial weight loss was observed at 240℃. This weight loss can be mainly attributed to the carbonization and volatilization of small molecular compounds and water[21]. The thermal decomposition of CNC-Br began at 170℃ (lower than the initial decomposition temperature of CNC) because of the release of HBr formed during the heating. The TG and DTG curves of CNC-PDMAEMA show that the weight loss occurred in three stages. In the first stage(T1=220～290℃), almost 63% weight loss was observed because of the pyrolytic decomposition of PDMAEMA that was grafted onto CNC. In the second stage of decomposition (T2=330℃), the weight loss occurred because of the splitting of the cellulose structure and main-chain scission. A total weight loss of about 95% was observed in the final stage (T3=420℃) attributing to the formation of large amounts of volatile compounds and solid char from the decomposed cellulose units[22].

Fig.3 TG (a) and DTG (b) curves of CNC, CNC-Br, and CNC-PDMAEMA

3.2 Dye adsorption properties

3.2.3 Effect of the contact time and kinetic study

永和十一年，王右军誓墓去会稽郡，归隐浙江剡东，确切的地点于剡东何处？为使后人研究书圣的书法艺术风格形成和发展、人生轨迹的变化，当时社会状况、人文环境、自然环境等，有必要进行探讨。

The dye (AO7) removal efficiency of CNC at various adsorbent dosages (10～80 mg) and dye solution pH values was measured (Fig.4). The dye removal efficiency increased with an increase in the adsorbent dosage. For a fixed initial dye concentration, an increase in the adsorbent dosage increased the adsorption area and the number of adsorption sites[2,23]. It should be noted that at the pH values of 2.0 and 4.0, an increase in the adsorbent dosage beyond 60～80 mg resulted in an insignificant increase in the AO7 removal efficiency.These results are consistent with those reported previously[2,12,24].

Fig.4 Effect of the adsorbent dosage on the AO7 removal efficiency of CNC-PDMAEMA at different dye solution pH values (t=20 min, T=20℃, C0=400 mg/L)

利用对HSrg 色彩空间中视频帧的中值滤波结果，引入模糊理论，计算每一个色彩通道的隶属度信息（uH，uS，ur 和ug）。隶属度函数采用最为常用的高斯函数的变形形式，以满足隶属度最大值为1 的条件。相应地，可以得到每个通道的隶属度：

3.2.2 Effect of ionic strength

Fig.5 Effect of ionic strength (NaCl) on the AO7 removal efficiency of CNC-PDMAEMA

(T=20℃, t=20 min, m0=60 mg, C0=400 mg/L)

The effect of ionic strength on the AO7 adsorption of CNC-PDMAEMA was investigated by carrying out dye degradation at various NaCl concentrations. As can be seen from Fig.5, as the ionic strength increased from 0.001 mol/L to 0.2 mol/L, the AO7 removal efficiency decreased from 73.6% to 26.3%. This can be attributed to the competition of Cl- with azo anions and other negative ions for adsorption sites. The presence of NaCl in high concentrations suppressed the AO7 adsorption[22].

Fig.6 Effect of the contact time on the AO7 removal efficiency of CNC-PDMAEMA at different dye temperatures

(m0=60 mg, pH=2.0, C0=400 mg/L)

3.2.1 Effects of adsorbent dosage and dye solution pH value

The effect of the contact time on the AO7 removal efficiency of the CNC-based adsorbent was investigated at two different temperatures (i.e. 20℃ and 30℃) and the results are shown in Fig.6. As can be seen from Fig.6, at the beginning of the adsorption, the AO7 removal efficiency increased rapidly with time. After 16 min,the adsorption process reached equilibrium. At this equilibrium, the AO7 removal efficiency did not change significantly with time. Initially, the adsorption mainly occurred on the surface of the CNC-based adsorbent.After that, the adsorption occurred on the inner surface of PDMAEMA[25], which was dominated mainly by the electrostatic interactions between azo ions and the amine groups on PDMAEMA.

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Fig.7 Pseudo-first-order adsorption rate (a) and pseudo-second-order adsorption rate (b) for the adsorption of AO7 by CNC-PDMAEMA

Table 3 Pseudo-first- and second-order parameters for the adsorption

/(mg · g-1) T/℃ First-order kinetic model Second-order kinetic model Qe,cal/(mg · g-1) k1 R2 Qe,cal/(mg · g-1) k2 R2 Qe,exp 99.69 20 81.56 0.17 0.88 111.11 2.51×10-3 0.99 114.80 30 89.58 0.27 0.84 125.00 3.23×10-3 0.99

Where, C0 and Ct are the concentrations (mg/L) of dye initially and at time t, respectively. The amount of dye adsorbed per unit mass of the adsorbent at equilibrium, Qe(mg/g), was calculated using Equation (2):

Batch experiments were conducted to investigate the effects of the contact time, dye solution temperature,ionic strength, initial pH value, and adsorbent dosage on the adsorption capacity of the adsorbent. A certain amount of adsorbent was added to the dye solution under stirring at 400 r/min for a predetermined time and was then removed by centrifugation. The residual concentration of AO7 in the dye solution after the adsorption was determined at 485 nm using a UV-vis spectrophotometer. The residual dye concentration was determined from a standard curve derived from a series of dye solutions with a known dye content. The adsorbent dosage in the AO7 dye (400 mg/L) solution was varied from 10 mg to 80 mg and the solution temperature was maintained at 20℃. The effect of the initial pH value on the dye removal efficiency of the adsorbent was investigated by varying the initial pH value (which was adjusted with 0.1 mol/L HCl or NaOH at the beginning) of the solution. In order to investigate the effects of the contact time and ionic strength on the adsorption of AO7 molecules onto the CNC adsorbent, different contact times (4, 8,12, 16, 20, 30, and 40 min) and NaCl concentrations(0.001～0.1 mol/L) were used. To determine the adsorption capacity of the CNC-based adsorbent at different temperatures (i.e. 20℃ and 30℃),the initial concentration of AO7 in the adsorption medium (pH=2.0, t=20 min) was varied from 100 mg/L to 700 mg/L. All the experiments were carried out in duplicate. The dye removal efficiency of the adsorbent was calculated using Equation (1):

区内外高校对“进一步优化完善部门决算表格设计，建立决算工作奖励和约束机制，并完善决算管理专题分析交流活动”等5个提高部门决算报表质量的改进措施主要因素认知统计数据如表5所示。

The CNC-based adsorbent was synthesized according to a previously reported method[18]. The synthesis route,which involved the ATRP of DMAEMA on CNC, is shown in Scheme 1. First, the initiator was fixed on the surface of CNC. CNC (3.24 g), NMP (200 mL), DMAP(0.02 mol), and TEA (0.03 mol) were added in a flask filled with Ar. BIBB (0.12 mol) was added to this flask over 1 h. The reaction system was then placed in an icewater bath for 24 h. The resulting product, CNC-Br,was thoroughly washed and freeze-dried. CNC-Br(0.4 g) was then added to a flask containing DMF(30 mL), DMAEMA (0.05 mol), CuBr (0.5 mmol),and PMDETA (0.5 mmol). The polymerization reaction was carried out at 60℃ for 6 h under stirring after executing three freeze-evacuate-thaw cycles.The white product so obtained was thoroughly washed and extracted with a mixture of water and ethanol (water∶ethanol=1∶1 (V/V)) for 4 h and was finally freeze-dried.

The adsorption kinetics of CNC-PDMAEMA based on the pseudo first- and second-order models are shown in Fig.7, and the estimated kinetic parameters are listed in Table 3. At the tested temperatures, the R2 for the second-order model was ＞0.99, while that for the firstorder model was found to be within the range 0.84～0.88.Moreover, the Qe,cal values(81.56 mg/g at 20℃, 89.58 mg/g at 30℃) for the pseudofirst-order model were much lower than the Qe,exp values(99.69 mg/g at 20℃, 114.80 mg/g at 30℃). On the other hand, the Qe,cal values (111.11 mg/g at 20℃, 125.00 mg/g at 30℃) for the second-order model matched well with the Qe,exp values. This indicates that the adsorption kinetics of CNC-PDMAEMA were better described by the pseudosecond-order model. This suggests that the adsorption of the anionic dye onto the CNC-based adsorbent was mainly controlled by chemisorption[25-26]. The pseudo-second-order kinetics of dye adsorption have also been reported for other biomass-based materials[25, 27].

3.2.4 Effect of the initial dye concentration and adsorption isotherm

Fig.8(a) shows the effects of the initial AO7 concentration and temperature on the adsorption.The adsorption of the dye at different concentrations increased rapidly in the initial stages and decreased gradually as the adsorption progressed until the equilibrium was reached. The initial concentration provided a driving force to overcome all the mass transfer resistances (between the aqueous and solid phases) of the dye. Hence, high initial dye concentrations increased the adsorption capacity[26].Additionally, an increase in temperature (from 20℃ to 30℃) also showed a positive effect on the adsorption.This is because the temperature increase accelerated the dye molecular diffusion rate and promoted the adsorption of AO7.

Adsorption isotherm models are used to describe the interaction between an adsorbate and adsorbent.In this study, the adsorption of AO7 onto the CNC-based adsorbent was evaluated using the Langmuir and Freundlich isotherm models. The Langmuir isotherm model, which assumes monolayer adsorption with uniform energies of adsorption on the surface, can be expressed as Equation (5):

Where, Qe (mg/g) and Ce (mg/L) are the amounts of dye adsorbed per unit weight of biomass and dye equilibrium concentration in solution, respectively. Qm(mg/g) denotes the monolayer adsorption capacity of the adsorbent and KL is the Langmuir constant.

The Freundlich model is based on multilayer adsorption with the adsorption energy decreasing with the surface coverage. It is expressed as Equation (6):

Fig.8 (a) Adsorption isotherms for the adsorption of AO7 dyes on CNC-PDMAEMA at different temperatures; (b) Linear Langmuir plot for adsorption; (c) Linear Freundlich plot for adsorption

Where, KF is the Freundlich constant and n is an empirical parameter relating the adsorption capacity and adsorption intensity, which varies with the heterogeneity of the material.

Table 4 Estimated kinetic parameters of the two adsorption isotherms

/(mg · g-1) T/℃ Langmuir model Freundlich model KL Qe,cal/(mg · g-1) R2 KF n R2 Qe,exp 123.95 20 3.23×10-2 136.43 0.99 16.78 2.70 0.94 152.93 30 5.73×10-2 162.87 0.99 27.09 2.94 0.96

The adsorption isotherms are shown in Fig.8(b)and Fig.8(c) and the model parameters are summarized in Table 4. As we can see, the Langmuir isotherm model fitted the experimental data well (high correlation coefficients, R2=0.99). The theoretical maximum adsorption capacity (Qe,cal=136.43 mg/g at 20℃, Qe,cal=162.87 mg/g at 30℃) matched well with the experimental values (Qe,exp=123.95 mg/g at 20℃, Qe,exp=152.93 mg/g at 30℃). Therefore, it can be stated that the adsorption of AO7 onto CNCPDMAEMA showed a monolayer uniform adsorption behavior[28-29]. Moreover, the Freundlich isotherm model also described the adsorption accurately. The values of n were in the range of 1～10, indicating a favorable adsorption process[4,13,30].

4 Conclusions

In this study, we demonstrated that poly(N,N-diethylam inomethylmethacrylate) (PDMAEMA)-functionalized CNC (via ATRP) can be effectively used for removing AO7 from aqueous solutions. The CNC-PDMAEMA adsorbent was characterized by FT-IR and XPS. The results showed that the adsorbent was successfully prepared. The CNC-based adsorbent exhibited good adsorption properties. The adsorption process depended significantly on the initial pH value of the dye solution.The pH value of 2.0 favored the adsorption process.The adsorption isotherms of the CNC-PDMAEMA adsorbent could be well described by the Langmuir model, and the adsorption capacity was found to be 162.87 mg/g at 30℃. The kinetic study results showed that the adsorption process followed pseudo-secondorder kinetics. Hence, CNC-PDMAEMA is a promising adsorbent for removing anionic dyes from waste water.

Acknowledgments

This work was supported by the Science and Technology Program of Guangzhou (No. 201704020038); the foundation of State Key Laboratory of Pulp and Paper Engineering (No. 2017QN01); and National Natural Science Foundation of China (No. 31570569).

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