Efficient Removal of Methyl Orange and Alizarin Red S from pH

Research Article pubs.acs.org/journal/ascecg

Efficient Removal of Methyl Orange and Alizarin Red S from pHUnregulated Aqueous Solution by the Catechol−Amine Resin Composite Using Hydrocellulose as Precursor Qiang Liu,† Qinze Liu,*,† Zhuotong Wu,† Yang Wu,‡ Tingting Gao,§ and Jinshui Yao† School of Materials Science and Engineering and §School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, PR China ‡ State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Downloaded via UNIV OF CAMBRIDGE on July 8, 2018 at 20:20:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: We report a novel composite absorbent prepared by the simple method that catechol-amine resin coats the hydrocellulose based on the adhesion property like polydopamine. The composite which contains many chelating groups on its surface was characterized by scanning electron microscopy (SEM), infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), etc. The obtained adsorbents were investigated to remove Methyl Orange (MO) and Alizarin Red S (AR) from pH-unregulated aqueous system by batch experiments, including the affected factors of adsorbent dosage, contact time, initial concentration, and temperature. Results showed the adsorption processes belonged to the chemisorption and exhibited a spontaneous and endothermic nature. Besides, the removal performances fitted with the Langmuir isotherm model and pseudo-second order kinetic model very well. The maximum adsorption amounts of MO and AR were 189.39 and 284.09 mg/g at 303 K, respectively. The difference about adsorption amounts may be caused by the strong effect of π−π conjugation and hydrogen bonding between adsorbent and AR. Furthermore, the adsorption processes exhibited a spontaneous and endothermic nature. The recycling test indicated that the adsorbent stayed stable for the removal of both dyes by desorbed three times. Accordingly, the adsorbent with high adsorption capacity and rapid removal rate should be a promising material for the removal of anionic dyes from sewage. KEYWORDS: Adhesion, Adsorption, Alizarin Red S, Catechol−amine resin, Composite, Methyl Orange

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INTRODUCTION

into our lives, their widespread applications are also potential threats for aquatic species living in polluted waters.6 Although water-soluble dyes have low optical stability and oxidation performance, they are not easy to degrade in nature.7 Therefore, the uptake of hazardous dyes before inflowing to the receiving water bodies is becoming an urgent issue for concern.8 So far, several conventional approaches have been developed to remove such kinds of pollutants from dyecontaining effluents, including biological treatment,9 coagulation/flocculation,10 chemical oxidation,11 membrane filtration,12 photocatalysis,13 and adsorption.14 Most of above methods exhibit distinct and effective results, but require a substantial financial investment and/or high energy.15 However, the adsorption process recognized as the most promising approach has recently attracted great attention because of its economic, environmental, and readily available function.16 The

Increasing industrial pollutants containing toxic dyes have been greatly noticed due to their hazardous nature. These dyes in wastewater have been spread into the environment from many factories using dyes to color their products, such as textiles, pulp and paper, dyestuffs, and plastics industries.1 Most synthetic dyes with nonbiodegradable and carcinogenic properties will destruct some organisms directly in aquaeous environments.2 Thus, with the increase of excess dyes accumulating in the environment, hazardous influences can not be ignored. For example, the aesthetic quality of water is hampered by the presence of dyes, and the penetration of sunlight and oxygen can be reduced in the aquatic ecosystem.3 Among dyes in this area, Methyl Orange (MO) is a watersoluble azo dye, which has been widely applied in the field of textiles, printing, paper, food and pharmaceutical industries, and research laboratories.4 As a kind of anthraquinone dye, Alizarin Red S (AR) is very durable and also widely used in many aspects.5 While these color products have been incorporated © 2016 American Chemical Society

Received: October 27, 2016 Revised: December 11, 2016 Published: December 30, 2016 1871

DOI: 10.1021/acssuschemeng.6b02593 ACS Sustainable Chem. Eng. 2017, 5, 1871−1880

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Diagram of Preparation of Hydrocellulose and PCEC-C Composite

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key to expand the application of this adsorption method is the development of low cost and highly efficient adsorbents, which determine the separation speed and efficiency of target pollutants.17,18 Recently, polymeric adsorbents have emerged as representatives of high capability materials due to their performance for deep uptake of trace pollutants from wastewater.19 Especially, chelating polymers which have abundant functional groups on their molecular chain structure, which could create complexes with adsorbates in liquid medium.20 For example, polydopamine and its composites have good abilities for the removal of heavy metals and organic dyes; as reported by Lee, polydopamine can adhere to the surface of almost all materials and lend their properties to the hose material.21,22 Like polydopamine, catechol−amine resin is a novel polymeric material which was synthesized by catechol and diamine.23 The cost of materials for catechol−amine resin is low, and the structures can be controlled by using different kinds of amines according to practical requirements.24 Meanwhile, it contains numerous groups of phenolic hydroxy and amine for forming surface-adherent films onto a wide range of materials and further modification.25,26 To utilize the strong surface-adherent ability of catechol−amine resin, hydrocellulose, which can be a good skeleton template because of its porous structure, high surface area, and recognized adsorption nature for organic pollutants, is used as a matrix for preparing the composite.27 In this work, the polymerization of catechol and tetraethylenepentamine took place on hydrocellulose which is prepared by the template of cellulose in sulfuric acid and sodium hydroxide. In order to improve the hydrophobic effect of the polymeric composite, a small amount of cyanuric chloride was incorporated into the mixture when the reaction reached a certain process. This composite can be directly used for efficient removal of the MO and AR from pH-unregulated aqueous solution. And, many factors, including adsorbent dosage, contact time, initial concentration, and environmental temperature, have been studied. Moreover, kinetic, introparticle diffusion, isothermal, and thermodynamic analyses have also been conducted comprehensively.

EXPERIMENTAL SECTION

Materials. Catechol and cyanuric chloride (reagent grade) were obtained from Aladdin, and tetraethylenepentamine was produced by Tianjin Damao Chemical Reagent Factory. Poly(ethylene glycol) 20 000 was supplied by Tianjin Hongyan Reagent Factory. MO and AR were obtained from Sinopharm Chemical Reagent Co., Ltd. The degreasing cotton is a common medical product. Hydrochloric acid, isopropanol, ethanol, sulfuric acid, and sodium hydroxide were all of analytical grade. The ultrapure water (18.25 MΩ. cm) was employed in all procedures. The SHA-C constant-temperature shaker (Guohua Electric Appliance co. Ltd., Changzhou) was shaking all flasks with the speed of 150 rpm. Preparation of Hydrocellulose. The hydrocellulose was prepared through hydrolysis process by sulfuric acid and sodium hydroxide. At first, the weighted degreasing cotton (5 g) was added to sodium hydroxide solution (10 wt %, 200 mL) and stirred for 1 h. Then, to squeeze the product, and put it into sulfuric acid solution (60 wt %, 200 mL) with stirring for 1 h in a constant temperature of 90 °C. Finally, the hydrocellulose was obtained after the suspension alternatively washed and centrifuged with water and saved in aqueous solution. Preparation of the Composite. The hydrocellulose (0.022 g), polyethylene glycol (PEG, 0.20 g), catechol (0.22 g), tetraethylenepentamine (0.378 g) were added to a conical flask with water (50 mL). The mixture was reacted with mechanical agitation at room temperature for 2 days. The suspension was centrifuged, and the obtained solids were mixed with cyanuric chloride (0.0184 g) and water (50 mL). After 1 day, the product was washed with water and isopropanol, respectively. Finally, the poly(catechol−tetraethylenepentamine−cyanuric chloride)@hydrocellulose (PCEC-C) composite is obtained after freeze-drying for about 24 h. The schematic diagram for the preparation of the PCEC-C composite is displayed in Scheme 1. Characterization. Energy dispersive spectrometer (EDS, JEOL Ltd., Japan) was used to identify the elementary composition of sample. Thermogravimetric analysis (TGA, Mettler Toledo, Switzerland) was performed to test the thermal property of samples from 45 to 800 °C under N2 flow (20 mL/min). Scanning electron microscope (SEM, FSM-5600LV, Japan) was selected to observe the morphologies of samples at 5 kV. An ESCALAB 250Xi spectrometer with XPS spectra (Thermon Scientific, USA) was analyzed the chemical components in the samples using Al Kα radiation. A TENSOR 27 instrument (Bruker, Germany) with FT-IR spectra was recorded chemical bonds of samples. A BDX3300 X-ray diffractometer (Bruker, 1872


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ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of hydrocellulose (a). Low- (b) and high-magnification (c) images of the PCEC-C composite.

Figure 2. FT-IR spectra (a) and thermogravimetric curves (b) of the PCEC-C, polymer, and hydrocellulose. Desorption of Dyes. The reusability of the PCEC-C adsorbent was checked after adsorption. The composite (0.025 g) loaded dye molecules was involved to a 150 mL glass jar with 0.1 mol/L HCl solution (100 mL). After being shaken for 1 h, the solid was separated from liquid by centrifuging the mixture solution. The obtained product can be used in the next adsorption.

Germany) equipped with a multichannel detector was conducted to XRD analysis within a 2θ range of 5−75° at a scan rate of 1°/min and using a Cu Kα1 (λ = 0.15406 nm) monochromatic X-ray beam. UV2550 spectrophotometer (Shimadzu, Japan) was selected to measure the concentration of target solutions before and after adsorption. Brunauer−Emmett−Teller adsorption (BET) and Barrett−Joyner− Halenda (BJH) were used to estimate the BET specific surface area (SBET) and pore size distribution, respectively. Adsorption Experiments. The adsorption of dye MO and AR from pH-unregulated aqueous solution onto PCEC-C composite was evaluated in a batch system. At the beginning of each adsorption experiments, 100 mL target solution with certain concentration was put into a 150 mL glass jar placed in a thermostatic water bath shaker. Subsequently, the resulting mixture was shaken for a definite time with an agitation speed of 150 rpm when the certain amount of adsorbent was added the above solution. The supernatant liquid was separated from the mixture by centrifuging after adsorption finished, and whose concentration was measured by a UV-spectrophotometer. Moreover, their character peaks in UV spectra were 460 nm for dye MO and 422 nm for AR, respectively. All adsorption experiments were carried out in solution medium and performed in triplicates, and the results were averaged. Effects of adsorbent dosage (50−200 mg), contact time (0− 720 min), initial dye concentration (0−220 mg/L), and environment temperature (20, 30, 40 °C) on adsorption of dyes were investigated successively when the adsorption process reached equilibrium. Then, the adsorbed amounts (qt, mg/g) of dyes onto the PCEC-C composite at any time (t, min) was calculated as in the following equations:28

qt = (CO − Ct )V /m

(1)

removal (%) = 100%(CO − Ce)/CO

(2)

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RESULTS AND DISCUSSION Characterization. As is presented in Figure 1a, the morphology of original hydrocellulose is similar to the irregular rod-like structure, and its surface relatively smooth. In Figure 1b and c, the morphology of the composite is made up by some flowerlike clusters and dendriticlike structures appear on the hydrocellulose surface. These complicated structures have larger specific surface area, which will promote the adsorption sites to expose. Figure 2a shows the FT-IR spectra of the PCEC-C, hydrocellulose, and polymer. It appears that peaks in the spectrum of PCEC-C consist of bands from two materials. As one of original material, the specific peaks of hydrocellulose existed in the composite around 1000−1200 cm−1. For instance, the bands appear at 1160 and 1318 cm−1 in the PCEC-C could be ascribed to the antisymmetric bridge stretching of C−O−C groups and CH2− wagging vibrations in cellulose and hemicellulose, respectively. A strong peak at 3340 cm−1 may be attributed to the −OH and −NH2 stretching vibrations in the PCEC-C, which groups supplied by the other components. The broad peaks between 1600 and 1350 cm−1 were assigned to the C−H band from the polymer. Among this, the peak at about 1520 cm−1 owing to the existence of benzene ring. Comparisons of the FT-IR spectra of the PCEC-C composite before and after adsorption dyes are displayed in Figure S1. Some peaks distributed below 1000 cm−1 in the pure

where CO and Ct (mg/L) are concentration of dye at initial and any time, V (ml) is the volume of solution, m (mg) is the weight of adsorbent. The adsorption capacity of dyes at equilibrium (qe, mg/g) is also calculated with final concentration of dyes (Ce, mg/L) by eq 1. 1873


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Figure 3. XPS survey spectra (a). High resolution XPS spectra for Cl 2p in the PCEC-C (b). S 2p after adsorption (c).

Figure 4. Effect of (a) adsorbent dosage (dos = 50−200 mg, C0[MO] = 100 mg/L, V = 100 mL, T = 30 °C, t = 6 h), (b) contact time (t = 0 to 720 min, C0[MO] = 100 mg/L, C0[AR] = 150 mg/L, V = 100 mL, T = 30 °C, dos = 50 mg), (c) initial concentration (C0 = 0−220 mg/L, V = 100 mL, T = 30 °C, t = 6 h, dos = 50 mg), (d) temperature environment (T = 20−40 °C, C0[MO] = 100 mg/L, C0[AR] = 150 mg/L, V = 100 mL, t = 6 h, dos = 50 mg) on the adsorption amount of dyes onto the PCEC-C composite.

Figure 2b. With temperature rising, the PCEC-C has the same thermal decomposition process with the single polymer until they are heated up to 330 °C. This difference is not clear that the decrease of the composite is lower than the polymer, because it contained a few hydrocellulose in which thermal stability is relatively bad. Obviously, the PCEC-C is formed by the polymer adhere to hydrocellulose from the change of weight loss. In addition, the XRD diffractogram is fully verified

absorbent are different with the dye-absorbed composite. These new peaks also emerge in the spectra of two dyes. In conclusion, compared with the spectra of peaks in the single materials, the evidence confirmed the characteristic groups making up the composite body and dyes absorbing in the adsorbent. The results of thermogravimetric analysis between the prepared composite and original materials are depicted in 1874


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Figure 5. (a) Kinetic models, (b) introparticle diffusion model, (c) isotherm models, and (d) thermodynamics analysis of the MO and AR removal from pH-unregulated solution.

the composite contained hydrocellulose with crystallinity by comparing the other materials. As is shown in Figure S2, an obvious peak at 2θ = 22.5° representing the cellulose-I structure has been observed in the PCEC-C composite, which does not appear on the pure polymer.29 The isotherms data for the N2 adsorption/desorption analysis of the PCEC-C and polymer are presented in Figure S3. The composite has an increase of the specific surface area, which compared to the pure catechol-amine resin. Its specific surface area calculated by the standard method is 13.13 m2/g. The pore volume and average pore diameter are 10.02 m3/g and 8.59 nm, respectively. What’s more, mesoporous and macroporous are also observed to simultaneous presence. As the III type of pore, it indicates the weak interactions among materials. The elementary composition of the PCEC-C adsorbent before and after dyes removal is illustrated in EDS spectra (Figure S4). The peak of Cl element is easily observed in spectrum of the composite before adsorption. A further peak of S is emerged clearly after dyes retained in the adsorbent, which was supplied by the MO and AR. Except for conventional elements in the material, other peaks of impurity are not identified. Additionally, a detailed characterization by employing XPS is performed to evaluate the element of the PCEC-C before and after adsorption. Figure 3a displays the difference between the composite and dyes adsorbed. Except for the conventional elements in the PCEC-C, an obvious feature is the existence of Cl 2p which comes from cyanuric chloride, also shown in Figure 3b. After the processes of MO-absorbing or ARabsorbing, depicted in Figure 3c, S 2p as a special element of two dyes can be clearly observed and its intensity in ARabsorbing is higher than that in MO-absorbing. This result

confirms that dyes have been absorbed on the surface of the PCEC-C body. Effect of Adsorbent Dosage. Effect of the composite amount involved in adsorption process is investigated by varying dosage with MO removal as the representative. As is shown in Figure 4a, the adsorption capacity is decreasing from 163 to 47 mg/g with the increase of dosage for the certain dye solutions. And this change gradually tends to be a slowing down that no longer subjects to the restriction of increased dosage. However, the percentage removal shows an opposite trend with adsorption capacity when adsorbent dosage increased. In other words, it is difficult to improve the removal efficiency even increasing the amount of adsorbent. At this time the removal rate reached 94.52%. The reason is that the active sites in the composite remain unsaturated for a small amount of adsorbent in the target solution. Purification effect is better due to the fact that adsorption sites will be sufficient or excessive with the increase of dosage. But, the utilization rate of adsorbent is gradually reduced. Therefore, the dosage of 50 mg is selected for the following procedures, whose adsorption capacity and removal rate at this time are 163 mg/g and 84%, respectively. Effect of Contact Time. Contact time is a factor that shows the process of adsorption to reach the equilibrium.30 Figure 4b displays the relationship between the adsorption capacities of the MO and AR with contact time. At first, it can be seen that the removal amounts rise up rapidly within a very short time after adsorption beginning. The adsorption capacities of the MO and AR reached 150.22 and 223.31 mg/g, respectively. Thereafter, these changes begin to level off gradually, and the capacities are unchanged. When the adsorption time was more than 240 min, the amounts of the two dyes uptake kept around 152.40 and 261.34 mg/g, respectively. The rapid growth of adsorption amounts at the beginning is attributed to the active 1875


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Table 1. Parameters of Kinetic Models for the MO and AR Removal from Solution by the PCEC-C Composite pseudo-first-order model

pseudo-second-order model

adsorbate

k1 (1/min)/(mg/L)1/n

qe (mg/g)

R2

k2 (g/mg·min)

qe (mg/g)

R2

loaded-MO loaded-AR

0.007 0.012

37.23 127.71

0.5775 0.9529

0.0007 0.0003

159.24 270.27

0.9997 0.9998

correlation coefficients (R2) for the second-order kinetic model are more satisfactory than that of first-order kinetics. Furthermore, the qe values of second-order also agrees with the experimental data very well. So, the pseudo-second-order equation with higher R2 > 0.99 might be sufficient to describe the interactions between the adsorbent and the MO or AR molecules. This suggests the dyes removal processes are involved in the chemisorption rate-controlling mechanism.37 Introparticle Diffusion Study. The variation in the removal amount with time can be selected to evaluate the role of diffusion during the adsorption period. The intraparticle diffusion rate constant (kid, mg/g·min0.5) is calculated using the equation:38

sites on the surface starting to capture the target molecules in the solution. When molecules have occupied the most of binding groups, the process turns slow until the equilibrium state of adsorption and desorption is achieved. Hence, the removal of AR is in a state of equilibrium after 240 min, while only 60 min for MO. Effect of Initial Concentration. Varying the initial concentration of solutions had an obvious effect on dyes removal capacity.31 In Figure 4c, all the adsorption performance required to reach equilibrium depends on the initial dye concentration. The adsorption capacity of MO or AR increased rapidly with increasing dye concentration at low level. At this time, target molecules never meet the binding need of functional groups in adsorbent. But at higher concentrations, more dye molecules stay in liquid phase, which ascribe to the saturation of chelating groups in the composite. Even the concentration of solution to be rising, the adsorbent could not provide more active sites for dye molecules to occupy. Then, the removal capacity of the MO is lower than the AR. Effect of Temperature. The temperature variation affects adsorption performance based on the environment change.32 As is illustrated in Figure 4d, it can be observed that the removal abilities of two dyes are all enhanced with temperature increasing from 20 to 40 °C. Accordingly, the amounts for the MO and AR removal reach 177.14 and 235.43 mg/g, respectively. The increased capacities for dyes uptake are attributed to the thermal energy, which accelerates the movement of target molecules. In other words, the collision process between the active sites and molecules on the adsorbent surface turns to be intense. More energy leads to increase the amount of dyes retained on the composite. Adsorption Kinetic Study. Adsorption kinetics is significant in the adsorption efficiency improvement as it provides valuable insights about the adsorption mechanism.33 In order to define the controlling rate of removing dyes, the pseudo-first order and pseudo-second order kinetic models are cited to investigate the mechanism. Specifically, the pseudo-first order model assumes that one dye molecule is absorbed onto one adsorption site in solid−liquid systems, and the hypothesis of pseudo-second order model is that one molecule occupies two sites on the composite surface.34 All parameters were calculated by the liner equations as follows. Pseudo-first-order model:35 log(qe /qt ) = log qe − k1t /2.303

qt = k idt 0.5 + C

where C is the intercept. A multilinearity presented by such plots in Figure 5b, implies that two or more steps had happened. Above all, two portions are observed in the AR-absorbing. In the first part, a drastic interaction occurred rapidly which ascribe the adsorbate to spread on the external surface of adsorbent or the boundary layer of solute molecules diffusing into solution. With the decrease of adsorption sites, the process gets into the gradual second stage, which suggests the diffusion is rate limiting.39 The final balance between the action of adsorbent and adsorbate led to the appearance of the third portion because the concentration of target molecules is not enough. As the amount of AR retained in adsorbent is relatively high, its introparticle diffusion is affected by more than one process than MO. All rate parameters together with R2 are also given in Table S1. Adsorption Isotherm Study. Two adsorption isotherm models are used to simulate the degree of the removal of dyes. The Langmuir isotherms represent a monolayer adsorption (chemisorption) hypothesis, which states that over the homogeneous surface heat is uniform and the adsorbent surface consists of identical sites which have equal energies for adsorbate equally available.40 Meanwhile, the Langmuir model also follows Henry’s law that effectively reduces the linear isotherm at low adsorbate concentrations. Similarly, Freundlich isotherms assume that multilayer adsorption (physisorption) without a uniform distribution of heat has taken place over the heterogeneous surface.41 But the Freundlich model agrees with the Langmuir model over a wide range of concentrations, which does decrease to Henry’s law at low adsorbate concentrations.42 The linearized equations of the above models are given as follows. Langmuir isotherm:43

(3)

Pseudo-second-order model:36 t /qt = 1/(k 2qe 2) + t /qe

(5)

(4)

Ce/qe = 1/(KLqm) + Ce/qm

where k1 (1/min) and k2 (g/mg·min) are the pseudo-first-order and pseudo-second-order rate constants, respectively. Figure 5a shows the slopes and intercepts of plots of log(qe − qt) or t/qt versus t, which were applied to determine the k1 or k2. Among them, the plots of t/qt versus t are good straight lines for two dyes uptake. A comparison of the computed results obtained from the above equations is given in Table 1. All the

(6)

Freundlich isotherm:44 log qe = log KF + 1/n log Ce

(7)

where KL (L/mg) and qm (mg/g) are constants related to the maximum removal amount and maximum removal energy, 1876


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ACS Sustainable Chemistry & Engineering Table 2. Parameters of Isotherm Models for MO and AR Removal from Solution by the PCEC-C Composite Langmuir isotherm model

Freundlich isotherm model

adsorbate

KL (L/mg)

qm (mg/g)

RL

R2

KF (mg/g)/(mg/L)1/n

1/n

R2

loaded-MO loaded-AR

0.30 0.15

189.39 284.09

0.018−0.066 0.028−0.074

0.9941 0.9977

6.90 7.63

0.18 0.21

0.9009 0.9521

Figure 6. (a) Comparison of the adsorption capacity of dyes by PCEC-C and hydrocellulose (C0[MO] = 100 mg/L, C0[AR] = 150 mg/L, V = 100 mL, T = 30 °C, t = 6 h, dos = 50 mg). (b) Recyclability of PCEC-C adsorbent for dye removal (C0[MO] = 100 mg/L, C0[AR] = 150 mg/L, V = 100 mL, T = 30 °C, t = 6 h, dos = 50 mg).

Scheme 2. Illustration of the Interactions with the MO and AR Dyes on the Polymer Layer Surface

respectively. KF (mg/g)/(mg/L)1/n and 1/n are constants related to the removal amount for dyes per unit of concentration at equilibrium and the adsorption intensity, respectively. If 1/n = 1, the adsorption is linear, but if 1/n < 1, it is chemical; if 1/n > 1, it is a favorable physical process, and 1/n = 0 reveals the more heterogeneous surface. The separation factor (RL) in the Langmuir model can be defined as45 RL = 1/(1 + KLC0)

unfavorable if RL > 1, liner while RL = 1, and irreversible if RL = 0. Figure 5c displays the adsorption isotherms of the MO and AR uptake by the PCEC-C composite. And, the corresponding parameters and statistical fits of equilibrium data to the models are given in Table 2. Obviously, the Langmuir model effectively describes the adsorption proceedings of dyes onto the adsorbent with R2 > 0.99. The calculated values of qm for the MO and AR are 189.39 and 284.09 mg/g, respectively. The plots of RL confirm the fact that adsorption of dyes in each equilibrium concentration is favorable. From the fitting in the Freundlich model, the obtained 1/n values support the removal

(8)

The value of RL relates the suitability of isotherm model for dye uptake; in particular, it is favorable if 0 < RL < 1, 1877


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the hydrogen bonding existing in where AR contacted with phenolic hydroxyl group on polymer layer may also increase the adsorption amount.52 As mentioned above, these results reveal the absorbent is more suitable to capture the AR molecule, which leads to the difference of maximum adsorption capacities of two dyes by the composite. Comparison of Adsorption Capacities with Other Adsorbents. The maximum removal capacities of various adsorbents from previous works are given in Table 3. It is clear to observe that the PCEC-C composite has an efficient performance of dyes uptake, when compared with other absorbents reported in the literature.

dyes processes are chemical because they below 1. Besides, the R2 values, which are not close enough to 1, demonstrate that the Freundlich model is unable to predict the adsorption data. Adsorption Thermodynamic Study. Some thermodynamic parameters, namely, standard free energy (ΔG°, kJ/mol), standard enthalpy (ΔH°, kJ/mol) and standard entropy (ΔS°, J/mol·K), are considered to determine the process that transfers solute of unit mole from solution to the solid−liquid interface. Their relations are expressed as46 ΔG° = −RT ln K C ln K C = ΔS°/R − ΔH °/RT

(9) (10)

Table 3. Comparison of Adsorption Capacities with Other Adsorbents from Previous Studies

where, KC is the adsorption equilibrium constant which was obtained by the ratio of the concentration of dyes in the solid and liquid phases, and R (8.314 J/mol·K) is universal gas constant. In Figure 5d, all thermodynamic data can be calculated at various temperatures from the plot of ln K vs 1/T. Thermodynamic parameters for the MO and AR removal are listed in Table S2. While the positive values of ΔH° suggest the endothermic nature belonging to the adsorption process.47 Furthermore, the values of ΔS° is also positive, which presents good affinities of two dyes to adsorbents and the increased randomness among the interface of solid and solution.48 Finally, the negative values of ΔG° confirm that the adsorption spontaneity and feasibility at all temperatures are clear.49 And, the decreased values of ΔG° with an increase of temperature illustrate that the removal processes are more favorable at high temperature. Adsorption Capacity of Hydrocellulose. Generally, hydrocellulose also has an excellent performance for anionic dyes removal due to the existence of a great deal of −OH groups, which had been reported in many works.50,51 In order to verify the advantage of the prepared composite on the removal of dyes, the comparison of adsorption amount between the PCEC-C and hydrocellulose prepared by this way is illustrated in Figure 6a. Only 48.32 mg/g of MO and 17.71 mg/ g of AR are removed by hydrocellulose itself from pHunregulated dye solutions, far less than the PCEC-C. Reusability of Adsorbent. The recycling test of the PCEC-C composite for the dyes uptake was conducted three cycles. In Figure 6b, the first removal amounts are slightly higher than those in other processes. Moreover, the capacity decreases with the increase of desorption cycles. However, the PCEC-C composite still maintains a favorable performance for the removal of the MO and AR. Presumed Adsorption Mechanism. From the above results, the adsorption mechanism of the MO and AR, two typical anionic dyes, by the PCEC-C composite may be attributed to the following reasons. As is shown in Scheme 2, the positively charged surface on the adsorbent can provide active sites for electrostatic interaction with dye molecules, based on the existence of abundant groups of amine. There is a π−π stacking interaction between adsorbate and adsorbent during the adsorption period because the aromatic backbone in the ideally planar molecule of dye affects with aromatic rings in the polymer layer of composite.21 However, the effect of π−π conjugation in AR molecule is stronger than it in MO due to the difference of molecule structures of two dyes. That means the chelation is weak when the composite is absorbing the MO. The molecule of MO may be easily separated from the adsorbent body after adsorption reached equilibrium. Besides,

absorbate MO (mg/g)

absorbent magnetic chitosan (MIMC) multiwalled carbon nanotubes (MWCNTs) calcined MgNiAl carboxymethyl cellulose (CMC) magnetic cellulose beads-activated carbon (MCB-AC) PCEC-C composite

AR (mg/g) 53 54 55 56 57

284.09

this work

375.40 100 2.12 189.39

ref

40.12 161.29

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CONCLUSION In summary, the poly(catechol-tetraethylenepentamine-cyanuric chloride)@hydrocellulose (PCEC-C) composite was successfully fabricated via the deposition of the catecholamine resin onto the hydrocellulose. The composite was applied to efficiently remove the anionic dyes, like the MO and AR, from pH-unregulated aqueous solution. It is found that the adsorption equilibrium is only 60 min for MO and 240 min for AR. The pseudo-second-order equation with higher R2 = 0.999 might be sufficient to fit the adsorption processes, and the introparticle diffusion study implied the processes are involved in the chemisorption rate-controlling mechanism. Additionally, the removals of both dyes were appropriately represented by Langmuir isotherm model which illustrated that chemisorption existed in the purification process of water with two dyes. The maximum adsorption capacities of MO and AR are 189.39 and 284.09 mg/g at 303 K in simulated wastewater, respectively, which is higher than that of hydrocellulose made by this work. The adsorption processes fit the Langmuir isotherm model and the pseudo-second-order equation well. The difference of adsorption capacities for MO and AR may be owed to the effect of electrostatic interaction, hydrogen bonding, and π−π stacking interaction existing in different degrees on the surface of the polymer layer and hydrocellulose body. As much, the composite prepared by catechol−amine resin using hydrocellulose as precursor is promising for further tests to remove contaminants rapidly and effectively from wastewater.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02593. 1878


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Additional component analyses, including FT-IR, XRD, BET, and EDS and parameters of adsorption models (PDF)

AUTHOR INFORMATION

Corresponding Author

*Mailing address: Daxue Road, Western University Science Park, Jinan 250353, Shandong, People’s Republic of China. Tel.: +008613806410075. E-mail address: [email protected]. ORCID

Qinze Liu: 0000-0002-2240-9320 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51303086) and Natural Science Foundation of Shandong Province (Grant No. ZR2013EMQ005). This study also was supported by the National Natural Science Foundation of China (Grant No. 51503108).

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Efficient Removal of Methyl Orange and Alizarin Red S from pH

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