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Adsorption and heat-energy-aid desorption of cationic dye on a new thermo-sensitive adsorbent: methyl cellulose/calcium alginate beads.

INTRODUCTION

Cationic dyes known as basic dyes are extensively used in dyeing of textile fibers such as cotton and polyester [1]. The complex aromatic molecular structures and xenobiotic properties of cationic dyes make them difficult to be degraded in nature [2]. Hence, these chemical materials often pose certain health hazards and environmental pollution when cationic dyes were discharged into natural streams and rivers. Nowadays, a number of promising processes have been used to remove dyes from the wastewater. Biological treatment, coagulation, flotation, electrochemical techniques, advanced oxidation processes, and adsorption are the most widely used techniques for treating cationic dyes in industrial effluents [3, 4].

Adsorption usually is an easy and environmentally friendly process to remove the pollutant from wastewater. Various materials including zeolites, activated carbons, clays, agricultural wastes, biomass, and synthetic polymers have been employed as adsorbents in adsorption process [5-7]. However, many of these adsorbents have some disadvantages such as high cost, poor adsorption capacity and so on. It is well known that biopolymers which are abundant, biodegradable and renewable resources have a high capacity to bind with a variety of molecules or ions through chemical and physical interactions [8]. Among these biopolymers, polysaccharide type biopolymers, such as cellulose, chitosan, cyclodextrin, and sodium alginate (SA), have been received more attention. SA is a naturally occurring polysaccharide composed of two types of monomer units, i.e., [beta]-D-mannuronic acid and [alpha]-L-guluronic acid [9]. SA can be extracted from abundant natural resources, including brown seaweeds. Because of its excellent hydrophilicity, binding ability, low cost, biocompatibility and renewability, SA has been used as adsorbent for the removal of pollutants from wastewater [10, 11]. Even with good performance for adsorption of dye when SA was directly used as colloid adsorbent [12], it is difficult to remove SA colloid from treatment system after adsorption, and guarantee the safety level of SA colloid in treated water. Thus, SA is widely used as a gelling agent to prepare bead adsorbents due to its ability to form gels under mild conditions with divalent cations, such as calcium cation [13, 14]. Calcium alginate beads, one kind of alginate gels, is a simple solution to overcome the above-mentioned problem caused by SA adsorbent. The ionotropic gelation of sodium alginate with calcium cations is conventionally described by the "egg-box" model [15]. And in this model, calcium cations interact with guluronic acid monomers in the cavities which are formed by pairing up of the G sequences of alginate molecular chains [16]. Calcium alginate beads are commonly used as the support material in bioscience to immobilize enzymes and living cells. In the environmental field, alginate beads have been used to remove dyes and metal ions from water. Pandey et al. used calcium alginate beads with and without humic acid as adsorbents to remove various metals [17]. Aravindhan et al. also studied the removal of dyes from commercial tannery effluents by calcium alginate beads [18]. Nasr et al. reported the adsorption of basic dyes by activated carbon fiber and modified alginate [19]. However, the regeneration of calcium alginate beads has not been studied in most literatures. Even a few reports on the regeneration of calcium alginate beads in some literatures, the methods were only using chemical reagents such as acids and salts. The thermo-reversible gelation of methyl cellulose (MC) from the aqueous solution was first investigated by Heymann [20]. MC gels are semisolid materials, which behave like solid materials in the respect of dynamic mechanical property, and generally exhibit high storage modulus [21]. Water-soluble MC solutions can form thermo-reversible hydrogels in the hot water mainly due to an increase of the hydrophobic association between methyl cellulose chains and [H.sub.2]O at temperatures between 50 and 70[degrees]C [22, 23]. In our present work, the calcium alginate beads containing MC were prepared by a simple process: the MC to be encapsulated in beads were mixed with a SA solution, and then the MC/SA solution was dropped into a calcium solution, resulting in an instantaneous formation of methyl cellulose/calcium alginate beads (MC/CABs). Based on the proprieties of SA and MC, the prepared MC/CABs might have a high capacity for adsorption of cationic dyes from wastewater. In theory, adding MC to SA could boost the difference of adsorption capacity between low temperature and high temperature.

The aim of this article was to provide the fundamental knowledge of adsorption and heat-energy-aid desorption of cationic dye on MC/CABs. Methylene blue (MB), a typical cationic dyestuff, is often founded in dyestuff wastewater for its widespread use in dyeing [24]. Hence, to correctly understand the adsorption and heat-energy-aid desorption property of MC/CABs for remove of cationic dye from water, MB was selected as the model compound in this study to investigate the adsorption and heat-energy-aid desorption of cationic dye on MC/CABs. The structure and characteristics of MB are presented in Table 1.

EXPERIMENTAL

Chemicals

SA was purchased from Xilong Chemical (Guangdong, China). MC and anhydrous Ca[Cl.sub.2] were purchased from Shanghai Chemical Reagent (Shanghai, China). Acetic acid was purchased from Chengdu Kelong Chemical (Chengdu, China). MB was purchased from Tianjin Damao Chemical Reagents (Tianjin, China). All chemicals used in the study were of analytical reagent grade.

Preparation of MC/CABs

A certain amount of MC was added into 10 mL of deionized water. After the dissolution of MC in the water, 0.1 g of SA was added into the MC solution. The formed MC/SA solution was added dropwise into 40 mL of Ca[Cl.sub.2] solution (5 wt%) under stirring, and then the drops of MC/SA solution were calcified in the Ca[Cl.sub.2] solution. The calcified reaction was allowed to proceed for 12 h to form MC/CABs. The MC/CABs were separated from the solution and washed for three times with deionized water, and then the MC/CABs were kept in deionized water for maintaining their structure.

Adsorption and Desorption Procedures

About 200 mL of MB solution with a certain concentration was added into a 250-mL flask. The MB solution in the flask was kept at a desire temperature by controlling of water bath temperature. Prepared adsorbent was added into the flask at a magnetic stirring speed of 200 rpm. Solution samples were withdrawn from the flask at predetermined time intervals and were measured at the MB maximum absorbance wavelength of 664 nm using a TU-1900 UV-vis spectrophotometer. Adsorption capacity ([q.sub.t], mg [g.sup.-1]) and adsorption ratio of MB ([[eta].sub.a], %) were calculated using Eqs. 1 and 2, respectively.

[q.sub.t] = ([c.sub.0] - [c.sub.t]) x V/m (1)

[[eta].sub.a] = [c.sub.0] - [c.sub.t]/[c.sub.0] x 100 (2)

where [c.sub.0] (mg [L.sup.-1]) is the initial MB concentration in adsorption experiment, [c.sub.t] (mg [L.sup.-1]) is the MB concentration at time t in adsorption experiment, m (g) is the mass of adsorbent, and V (L) is the volume of MB solution.

The adsorbents after the adsorption of MB were desorbed with 200 mL of 0.5 mol [L.sup.-1] acetic acid aqueous solution in the flask under different temperatures. After desorption equilibrium, the final concentration of MB in the equilibrium solution was determined. Desorption mass of MB ([m.sub.d], mg) and desorption ratio of MB ([[eta].sub.d], %) were calculated using Eqs. 3 and 4, respectively.

[m.sub.d] = [c.sub.d] x V (3)

[[eta].sub.d] = [c.sub.d]/(c.sub.0] - [c.sub.e]) x 100 (4)

where [c.sub.e] (mg [L.sup.-1]) is the equilibrium MB concentration in adsorption experiment, [c.sub.d] (mg [L.sup.-1]) is the equilibrium MB concentration in desorption experiment.

Characterization of Adsorbent

The adsorbents before and after adsorption were characterized through by Fourier-transform infrared spectroscopy (FTIR). FTIR analysis was collected on a 1752X FTIR spectroscopy using KBr pellet in the range of 4000-500 [cm.sup.-1] with 4 [cm.sup.-1] resolution.

RESULTS AND DISCUSSION

Effect of [m.sub.MC]/[m.sub.SA] on Adsorption of MB

To understand the effect of MC dosage on the adsorption ability of MC/CABs under different temperatures, the adsorption tests were investigated at 20[degrees]C and 60[degrees]C. The MC dosage is measured in the mass ratio of MC to SA ([m.sub.MC]/[m.sub.SA]). As showed in Fig. 1a, at [m.sub.MC]/[m.sub.SA] of 0:1, 1:1, 2:1, and 3:1, the adsorption ratios of MB were 75.7, 71.7, 70.2, and 69.8% at 20[degrees]C, respectively, and at 60[degrees]C the adsorption ratios of MB were 20.2, 17.3, 7.3, and 7.0%, respectively. The differences in adsorption capability ([DELTA]q, mg [g.sup.-1]) of MC/CABs between 20[degrees]C and 60[degrees]C at different [m.sub.MC]/[m.sub.SA] are presented in Fig. 1a. As shown in Fig. la, at [m.sub.MC]/[m.sub.SA] of 0:1, 1:1, 2:1, and 3:1, the differences were 18.06, 17.70, [20.48.sup.-], and 20.43 mg [g.sup.-1] respectively. The MB adsorption ability of MC/CABs decreased with the increasing MC dosage. The adsorption ratio of MB by MC/CABs at 20[degrees]C declined slightly, but the adsorption ratio of MB at 60[degrees]C had an obvious reduction when the [m.sub.MC]/[m.sub.SA] was over 2:1, which resulted in the [DELTA]q increase. The results indicated that the prepared methyl cellulose/calcium alginate was a good thermosensitive material, and the addition of MC increased the thermosensitivity of material, i.e., boosting the difference of adsorption capacity of MC/CABs between low and high temperatures. As we want to prepare an adsorbent with good property of thermal desorption for its regeneration, the [m.sub.MC]/[m.sub.SA] of 2:1 was used to prepare the MC/CABs in following experiments.

Effects of Temperature and Time on Adsorption of MB

The effects of temperature and time on MB adsorption by MC/CABs are showed in Fig. 1b. The adsorption ratio of MB decreased with the increase of temperature, indicating that the adsorption of MB by MC/CABs was an exothermic process. With time increase, the adsorption ratio of MB by MC/CABs under different temperature increased rapidly in the first 25 min. Compared with the adsorption at high temperature, the adsorption at low temperature needed a longer time to reach the adsorption equilibrium. As shown in Fig. lb, the adsorptions at 40, 50, and 60[degrees]C all reached their equilibrium at time of 25 min. However, the adsorptions at 30 and 20[degrees]C needed 64 and 85 min, respectively, to reach their equilibrium, the experimental [q.sub.e] values at 20, 30, 40, 50, and 60[degrees]C were 22.88, 16.41, 8.32, [5.26.sup.-], and 2.39 mg [g.sup.-1], respectively.

Effect of Initial MB Concentration on Adsorption

The effect of initial MB concentration of solution on adsorption is shown in Fig. 1c. With the increase of initial MB concentration, the adsorption ratio decreased, and the MB adsorption amount of MC/CABs increased until the initial concentration reached 1,000 mg [L.sup.-1]. The increase of adsorption amount of MC/CABs was ascribed to the change of concentration gradient. The concentration gradient at higher initial concentration enhanced the probability of collision between MB molecules and MC/CABs. With the MB intial concentration of 1,200 mg [L.sup.-1], the equilibrium adsorption amount of MC/CABs reached 288.50 mg [g.sup.-1].

Kinetic Studies

To understand the adsorption mechanism and the determining factors that affect the adsorption ratio, three kinds of kinetic models, i.e., pseudo-first-order kinetics (Eq. 5) [25], pseudo-second-order kinetics (Eq. 6) [26] and intra-particle diffusion model (Eq. 7) [27] were applied to treat the MB adsorption data.

ln [q.sub.e]/[q.sub.e] - [q.sub.t] = [k.sub.1]t (5)

t/[q.sub.t] = 1/[k.sub.2][q.sup.2.sub.e] + t/[q.sub.e] (6)

where [q.sub.e] (mg [g.sup.-1]) is the adsorption amount of MB on MC/CABs at equilibrium (values of calculated [q.sub.e] were obtained from Eq. 6), [q.sub.t] (mg [g.sup.-1]) is the adsorption amount of MB on MC/CABs at time t, [k.sub.1] ([min.sup.-1]) is the ratio constant of pseudo-first-order mode, [k.sub.2] (g [mg.sup.-1] [min.sup.-1]) is the ratio constant of pseudo-second-order model, and [k.sub.p] (mg [g.sup.-1] [min.sup.1/2]) is the ratio constant of intra-particle diffusion, and C (mg [g.sup.-1]) is a constant.

The kinetic plots of the above three models are shown in Fig. 2a-c, and the adsorption kinetic constants corresponding to the models under different temperatures are presented in Table 2. As shown in Fig. 2a and Table 2, even though the adsorption data of MB on MC/CABs were well fitted at low temperatures ([less than or equal to] 40[degrees]C) by the pseudo-first-order kinetic model with high value of correlation coefficients ([R.sup.2]), the data were not well fitted by the pseudo-first-order kinetic model at high temperatures (>40[degrees]C). The correlation coefficients at 50 and 60[degrees]C were 0.6795 and 0.6857, respectively. The same was happened in the fitting results of intra-particle diffusion kinetic model at high temperatures, so the MB adsorption on MC/CABs did not follow the intra-particle diffusion kinetic model. However, the adsorption data showed good agreement with the pseudo-second-order kinetic model in the full temperature range from 20 to 60[degrees]C ([R.sup.2] > 0.94). Besides, the calculated [q.sub.e] values obtained theoretically from the pseudo-second-order kinetic model agreed satisfactorily with the experimental [q.sub.e] values which were given in the subsection "Effects of temperature and time on adsorption of MB" of this study. This suggested that the MB adsorption on MC/CABs followed the pseudo-second-order kinetic model, based on the assumption that the ratio-limiting step may be chemisorption.

Adsorption Isotherms

Adsorption isotherm can describe the relation between adsorption capacity of adsorbent and the concentration of adsorbate when the adsorption process reaches equilibrium, and it is a fundamental tool elucidating the relationship between adsorbate molecules and the adsorbent surface. In the present work, the adsorption of MB on MC/CABs at 20[degrees]C was simulated using the nonlinear Langmuir (Eq. 8) [28], Freundlich (Eq. 10) [29] and Temkin isotherm models (Eq. 10) [30].

[c.sub.e]/[q.sub.e] = 1/[q.sub.m][K.sub.1] + [c.sub.e]/[q.sub.m] (8)

log [q.sub.e] = log [K.sub.f] + 1/n log [c.sub.e] (9)

[q.sub.e] = B ln([K.sub.t]) + B ln([c.sub.e]) (10)

where [q.sub.m] (mg [g.sup.-1]) is the maximum adsorption amount of MB on adsorbent, [K.sub.1] (L [mg.sup.-1]) is Langmuir constant related to the affinity of binding sites, [K.sub.f] (mg [g.sup.-1] x [[l/mg].sup.1/n]) and n are Freundlich constants related to adsorption capacity and adsorption intensity of the adsorbent, respectively, B is Temkin constant corresponding to adsorption heat, and [K.sub.t] (L [mg.sup.-1]) is Temkin constant related to maximum binding energy.

The plots of Langmuir, Freundlich, and Temkin adsorption isotherms for the MB adsorption on MC/CABs are shown in Fig. 3a-c, respectively. The detailed parameters of these different forms of isotherm equations are listed in Table 3. As shown in Table 3, with the highest correlation coefficient among the three models, the equilibrium data were well fitted by Langmuir isotherm model. The applicability of Langmuir isotherm model suggested that the adsorption of MB on MC/CABs was monolayer adsorption. Moreover, as shown in Table 3, the maximum adsorption capacity of adsorption isotherm for MB on MC/ CABs was 336.70 mg [g.sup.-1], indicating that MC/CABs was a good adsorbent with high adsorption capability.

Thermodynamic Analyses

Thermodynamics is a critical aspect predicting the stability of the solid-liquid phase equilibrium, and it is a basic requirement for the characterization of an adsorption system. The values of Gibbs free energy of adsorption ([DELTA]G) at different temperatures were calculated by using the Eqs. 11 and 12 [31, 32]. The adsorption enthalpy ([DELTA]H) and adsorption entropy ([DELTA]S) were obtained from the slope and intercept of In Ka vs. 1 IT plots via Eq. 13 [33].

[K.sub.d] = [c.sub.0 - [c.sub.e]/[c.sub.e] (11)

[DELTA]G = -RT ln [K.sub.d] (12)

ln [K.sub.d] = -[DELTA]H/RT + [DELTA]S/R (13)

where [K.sub.d] (L [g.sup.-1]) is the distribution coefficient of adsorption equilibrium, R (8.314 J [mol.sup.-1] [K.sup.-1]) and T (K) are universal gas constant and absolute temperature, respectively.

The fitting plot is shown in Fig. 4, and the calculated thermodynamic parameters are listed in Table 4. As seen from Table 4, the [DELTA]G value changed from negative value to positive value with the increase of temperature, which indicated that in low temperature the adsorption of MB molecules on MC/CABs was a spontaneous process, while in higher temperature it was a nonspontaneous process. The negative value of [DELTA]H showed the adsorption of MB on MC/CABs was an exothermic process. The absolute value of [DELTA]H was larger than the common physical adsorption heat (40 kJ [mol.sup.-1]) and smaller than the common chemical adsorption heat (100 kJ [mol.sup.-1]), indicating that the adsorption of MB on MC/CABs was not merely physical adsorption or chemical adsorption but was a comprehensive adsorption (physical and chemical adsorption). The negative value of [DELTA]S illustrated MB adsorbed on MC/CABs was more order than that in the solution before adsorption.

Heat-energy-Aid Desorption of MB From Used MC/CABs

The heat-energy-aid desorption experiments were carried out under different temperatures. To explore the desorption superiority of MC/CABs with the addition of MC, calcium alginate beads (CABs), without MC, was prepared according to the preparation condition of MC/CABs, and then were used as contrast adsorbent in the desorption experiments. The mass of used CABs was same as that of used MC/CABs in desorption experiments, which was about 3.07 g. The desorption mass of dye and the desorption ratio of dye for MC/CABs and CABs are presented in Table 5.

As shown in Table 5, when temperature changed from 20 to 60[degrees]C, the desorption mass of dye from CABs increased 1.30 mg and the corresponding increase of desorption ratio of dye was 17.3%, while the desorption mass of dye from MC/CABs increased 3.56 mg and the corresponding increase of desorption ratio of dye was 50.8%. These results indicated that the addition of MC boosted the adsorption thermo-sensitivity of MC/CABs, and the prepared MC/CABs held a desorption superiority with heat-energy-aid. With the aid of heat energy, the thermosensitivity of MC/CABs after adsorption can be easily regenerated with low consumption of chemical regenerants, which is an environment-friendly method for regeneration of used MC/ CABs. In industry, for using the waste heat energy in the regeneration of used adsorbent, MC/CABs can have a wide prospect in the adsorption application.

Adsorption Mechanism and FTIR Characterization of Samples

Cationic dye with its planar molecule can be easily adsorbed on the cellulose-based bio-adsorbents by van der Waals force [34]. Hence, cationic dye MB can be adsorbed on MC/CABs, which are composed of methyl cellulose and calcium alginate, under the action of van der Waals force between MB molecules and MC/CABs molecules. The FTIR spectra of MC/CABs before and after adsorption of MB are shown in Fig. 5. For the FTIR spectrum of MC/CABs, the band at 3,431 [cm.sup.-1] was attributed to the stretching of O--H group, and the band 2,925 [cm.sup.-1] was related to the C--H stretching. The band at 1,427 [cm.sup.-1] was attributed to stretching vibration of CO[O.sup.-]. After adsorption of MB, significant changes in the FTIR spectrum were observed. Some new little peaks appeared in the FTIR spectrum of used MC/CABs at 1,355, 1,250, [813.sup.-], and 667 [cm.sup.-1], which were attribute to the adsorbed MB on the surface of MC/CABs. And the peak corresponding to O--H, as shown in the FTIR spectrum of used MC/CABs, shifted from 3,431 to 3,421 [cm.sup.-1], indicating that the adsorption of MB on MC/CABs involved the H-bond interaction which from hydrogen atoms of O--H in MC/CABs molecules. After adsorption of MB, the peak corresponding to [COO.sup.-] also shifted from 1,427 to 1,396 [cm.sup.-1], suggesting MB was adsorbed on MC/CABs through specific electrostatic interaction between the functional groups of MB molecules and the CO[O.sup.-] of MC/CABs molecules. Moreover, after adsorption of MB, the peaks of O--H group at 3,421 [cm.sup.-1] and COO at 1,396 [cm.sup.-1] also became sharp and strong. Thus, it was safely concluded that H-bond interaction and electrostatic interaction were main interactions for adsorption of MB on MC/CABs. These possible interactions between MB molecules and MC/CABs adsorbent are shown in Fig. 6. MC, as a polymer of nonionic cellulose with excellent thermo-sensitive property, plays an important role for MC/ CABs' desorption superiority with heat-energy-aid. At high temperature, the hydration layer around the methyl of MC, which was formed by water molecules via hydrogen-bond forces, was broken, and then the hydrophobic combination occurred by the interactions of exposed methyl of MC. The microstructure change of MC molecules at high temperature might effectively weaken the adsorption interactions between MB molecules and MC/CABs molecules. Thus, the addition of MC increased the desorption property of MC/CABs at high temperature, and for its excellent thermo-sensitive property, the used MC/CABs adsorbent could be easily regenerated with heat-energy-aid.

CONCLUSIONS

The addition of methyl cellulose increased the thermosensitivity of MC/CABs, and boosted the difference of adsorption capacity of MC/CABs between low temperature and high temperature. As for adsorption of MB by MC/CABs, the difference of adsorption capacity of MC/CABs between 20 and 60[degrees]C reached 20.48 mg [g.sup.-1] at [m.sub.MC]/[m.sub.SA] of 2:1. The adsorption ratio of MB decreased with the increase of temperature. MB adsorption on MC/CABs followed the pseudo-second-order kinetic model, and the ratio-limiting step was chemisorption. The applicability of Langmuir isotherm model suggested that MB adsorption on MC/CABs was monolayer. The maximum adsorption capacity of MB (336.70 mg [g.sup.-1]) exhibited MC/CABs had a high adsorption capacity. According to [DELTA]H value, physical and chemical adsorption took place in the MB adsorption process. The increasing of [DELTA]G with temperature indicated high temperature was not favorable to the MB adsorption on MC/CABs. MC/ CABs had a desorption superiority with heat-energy-aid. Based on the FTIR characterization, the possible mechanisms involving in adsorption and heat-energy-aid desorption of MB on the thermo-sensitive MC/CABs were presented.

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Zhongmin Li, (1,2) Yi Yao, (1) Guangtao Wei, (2) Wenyan Jiang, (1) Yizhi Wang, (1) Linye Zhang (1,3)

(1) Department of Energy Chemical Engineering, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

(2) Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Nanning 530004, China

(3) Department of Chemical Engineering, University of Waterloo, Waterloo, Canada

Correspondence to: G.T. Wei; e-mail: gtwei@hotmail.com or L.Y. Zhang; e-mail: yezi@gxu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 21366003; contract grant sponsor: Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology; contract grant number: 2012K11; contract grant sponsor: Department of Science and Technology of Guangxi Zhuang Autonomous Region; contract grant number: 2015GXNSFCA139017, 2015GXNSFBA139031.

DOI 10.1002/pen.24373

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Properties of MB.

Index                  Methylene blue

Molecular structure    [FORMULA NOT REPRODUCIBLE IN ASCII]

Molecular formula      [C.sub.16][H.sub.18][Cl[N.sub.3]S
Molecular weight       319.86
Hazard class           22-36/37/38-11
Hazard code            Xn
Safety statements      26-36-24/25-16-7

TABLE 2. Parameters of adsorption kinetic models at
different temperatures.

                                                Value at different
Kinetic                                            temperatures

model          Parameter                   20[degrees]C   30[degrees]C

Pseudo-        [q.sub.e] (mg [g.sup.-1])   20.51          25.29
  first-       [k.sub.1] ([min.sup.-1])    0.053          0.076
  order        [R.sup.2]                   0.9783         0.9751
Pseudo-        [q.sub.e] (mg [g.sup.-1])   27.88          20.70
  second-      [k.sub.2] (g [mg.sup.-1]    0.0021         0.0025
  order          [min.sup.-1])
               [R.sup.2]                   0.9897         0.9681
Intra-         [k.sub.p1], (mg [g.sup.-1   3.55           3.29
  particle       [min.sup.-1/2])
  diffusion    [R.sup.2.sub.1]             0.9409         0.9362
               [k.sub.p2] (mg [g.sup.-1]   0.98           0.80
                 [min.sup.-1/2])
               [R.sup.2.sub.2]             0.9732         0.9214

                                                Value at different
Kinetic                                            temperatures

model          Parameter                   40[degrees]C   50[degrees]C

Pseudo-        [q.sub.e] (mg [g.sup.-1])   7.70           3.41
  first-       [k.sub.1] ([min.sup.-1])    0.085          0.023
  order        [R.sup.2]                   0.9354         0.6795
Pseudo-        [q.sub.e] (mg [g.sup.-1])   8.91           5.71
  second-      [k.sub.2] (g [mg.sup.-1]    0.022          0.013
  order          [min.sup.-1])
               [R.sup.2]                   0.9956         0.9495
Intra-         [k.sub.p1], (mg [g.sup.-1   1.63           0.73
  particle       [min.sup.-1/2])
  diffusion    [R.sup.2.sub.1]             0.8178         0.7372
               [k.sub.p2] (mg [g.sup.-1]   0.10           0.36
                 [min.sup.-1/2])
               [R.sup.2.sub.2]             0.9592         0.9039

                                           Value at different
Kinetic                                      temperatures

model          Parameter                   60[degrees]C

Pseudo-        [q.sub.e] (mg [g.sup.-1])   1.37
  first-       [k.sub.1] ([min.sup.-1])    0.041
  order        [R.sup.2]                   0.6857
Pseudo-        [q.sub.e] (mg [g.sup.-1])   2.52
  second-      [k.sub.2] (g [mg.sup.-1]    0.066
  order          [min.sup.-1])
               [R.sup.2]                   0.9964
Intra-         [k.sub.p1], (mg [g.sup.-1   0.44
  particle       [min.sup.-1/2])
  diffusion    [R.sup.2.sub.1]             0.6660
               [k.sub.p2] (mg [g.sup.-1]   0.05
                 [min.sup.-1/2])
               [R.sup.2.sub.2]             0.7941

TABLE 3. Isotherm parameters for adsorption of
MB on MC/CABs at 20[degrees]C.

Isotherm
model         Parameter                                      Value

Langmuir      [q.sub.m] (mg [g.sup.-1])                      336.70
              [K.sub.1] (L [mg.sup.-1])                        0.0089
              [R.sup.2]                                        0.9620
Freundlich    [K.sub.f] (mg [g.sup.-1] x [(1/mg).sup.1/n])    11.153
              n                                                1.898
              [R.sup.2]                                        0.6921
Temkin        [K.sub.t] (L [mg.sup.-1])                        0.1715
              B                                               62.348
              [R.sup.2]                                        0.9290

TABLE 4. Thermodynamic parameters for MB adsorption on MC/CABs.

[DELTA]G (kJ
[mol.sup.-1])
293.15 K         303.15 K   313.15 K   323.15 K   333.15 K

-2.100            -0.041     2.782      4.422      7.026

[DELTA]G (kJ                             [DELTA]S
[mol.sup.-1])        [DELTA]H        (kJ [mol.sup.-1])
293.15 K         (kJ [mol.sup.-1])      (K.sup.-1])

-2.100                -68.731             -0.227

TABLE 5. Desorption masses and desorption rates under
different temperatures.

                         [m.sub.d] (mg)

Adsorbent    20[degrees]C   60[degrees]C
MC/CABs          3.28           6.84
CABs             2.60           3.90

                         [[??].sub.d] (%)

Adsorbent    20[degrees]C     60[degrees]C
MC/CABs         47.4%            98.2%
CABs            34.0%            51.3%


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Author:Li, Zhongmin; Yao, Yi; Wei, Guangtao; Jiang, Wenyan; Wang, Yizhi; Zhang, Linye
Publication:Polymer Engineering and Science
Article Type:Report
Date:Dec 1, 2016
Words:5319
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