surfactant

Journal of Molecular Liquids 197 (2014) 353–367

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Process optimization, kinetics and equilibrium of orange G and acid orange 7 adsorptions onto chitosan/surfactant Lei Zhang a,b,⁎, Zhengjun Cheng a,c,⁎⁎, Xiao Guo b, Xiaohui Jiang c, Rong Liu c a b c

School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, Sichuan, China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, China Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637002, China

a r t i c l e

i n f o

Article history: Received 7 May 2014 Received in revised form 2 June 2014 Accepted 4 June 2014 Available online 20 June 2014 Keywords: Response surface methodology Chitosan/surfactant Orange G Acid orange 7 Adsorption kinetics Isotherms

a b s t r a c t We report an investigation of the title process. The corresponding process variables such as the concentration of surfactant, temperature, initial concentration of dye have been optimized by the response surface methodology (RSM). A 23 full factorial design was employed to analyze the individual and interaction effects of the process variables. The experimental data have been well-fitted to second order polynomial equations and the models were also examined using the analysis of variance and t test statistics. The obtained optimal conditions are as following: Cs = 30.92 μM, T = 20 °C and COG = 320 mg/L for the orange G, and Cs = 34.10 μM, T = 50 °C and COA = 500 mg/L for the acid orange 7. The measured experimental maximum adsorption capacities for the orange G and acid orange 7 systems under above optimization conditions were 1452.07 and 2352.99 mg/g, which were in good agreement with their corresponding predicted values (1427.13 and 2357.31 mg/g), with small relative errors of 1.72% and −0.18%, respectively. Moreover, many aspects of the adsorption kinetics and isotherms for the two dyes are presented. The results indicated that the two adsorption equilibriums were achieved within 210 and 240 min for the acid orange 7 and orange G, the pseudo-second-order model was followed, and Langmuir isotherm model could be well described their adsorption data. In addition, the adsorption mechanism of two dyes onto chitosan/surfactant was discussed further by FTIR and X-ray diffraction (XRD) methods. © 2014 Elsevier B.V. All rights reserved.

1. Introduction At present, many synthetic dyes have been extensively used in many industries (such as, textile, leather, cosmetic, paper and pulp, inkjet printing, plastic, food, and pharmaceutical), which would induce that the aquatic ecosystem is polluted by the discharge of dye effluents with more than 7 × 105 tons produced annually because most synthetic dyes are either toxic or mutagenic and even carcinogenic [1]. Moreover, the synthetic dyes usually have a complex aromatic molecular structure (for example, benzene, toluene, xylene, naphthalene, anthracene, etc.), making them more stable and more difficult to biodegrade [2]. In the synthetic dyes, acid dyes are often applied to dye a variety of materials (such as food, cosmetics, detergents, nylon, aluminum, silk, and wool) because they are popular water-soluble dyes. Orange G and acid orange 7 (Scheme 1) belong to the acidic dye classes. In the past, the acid orange 7 was often used for tanneries, paper manufacturing and textile

⁎ Correspondence to: L. Zhang, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, Sichuan, China. ⁎⁎ Correspondence to: Z. Cheng, Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637002, China. E-mail addresses: [email protected] (L. Zhang), [email protected] (Z. Cheng).

http://dx.doi.org/10.1016/j.molliq.2014.06.007 0167-7322/© 2014 Elsevier B.V. All rights reserved.

industry. Unfortunately, it was discovered having high toxicity which included causing nausea, chronic toxicity, carcinogen, dermatitis, skin, mucous membrane, eye, methemoglobinemia, and upper respiratory tract irritation [3]. And orange G was used as a drug and cosmetic colorant until October 1966 in the USA, which has obvious genotoxicity for Swiss Albino mice and latent danger for humans as well [4]. In the past few decades, some technologies [5] have been developed to treat the contaminants from wastewater, such as biological treatment [6,7], microwave irradiation [8], ion exchange [9], electrochemistry [10], heterocatalytic Fenton oxidation [11], and photochemical degradation [12,13]. Based on the comparison among these methods, adsorption is often employed to remove the contaminants from aqueous solutions because it is a low cost and easy to implement method [1,14]. However, it is still a key issue to research high effective adsorbents for the adsorptions of pollutants in wastewater. Therefore, a number of research laboratories have prepared more economical and efficient adsorbents using agricultural wastes [15–21], industrial wastes [22–27], natural materials [28–30], or composite materials [4,14,31–37], which were used for the adsorptions of contaminants. In these adsorbents, chitosan is an abundant biopolymer obtained from alkaline N-deacetylation of chitin and the second most natural biopolymer, and it has higher adsorption capacities for anionic dyes than some conventional adsorbents due to the presence of amounts of

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2. Materials and methods 2.1. Materials

Scheme 1. Molecular structures of orange G (a) and acid orange 7 (b).

reactive hydroxyl (−OH) and amino (−NH2) groups that have a capacity to bind with the contaminants. In the last few years, some researchers have applied chitosan or its composite to study the adsorptions of metal ions (such as Cr6+, Pb2+, Cu2+, Cd2+, Ni2+, and Pt4 +) [38–43], organic acids [44], phenolic compounds [45], toluene [46], organophosphorous pesticide [47], or dyes [48–50]. Lately, Jiang et al. [51] reported that chitosan (CTS) was modified by graft copolymerization with Poly (methyl methacrylate) and evaluated its adsorption capacity for C.I. Reactive Blue 19, indicating that there was a strong affinity between the new adsorbent and C.I. Reactive Blue 19. Recently, a promising adsorbent (CS/CNT) [52] has been synthesized to remove the acid red 18 from aqueous solution. The results suggested that the adsorption of acid red 18 onto CS/CNT could achieve equilibrium quickly and its maximum adsorption capacity was 809.9 mg · g−1 at 323.15 K. Conclusions as a result, many research groups have developed highly efficient absorbents to enhance their absorption capacities for the contaminants. In addition, a few researchers applied RSM method to optimize the process variables of contaminants adsorptions by different adsorbents so that the removal efficiency of contaminants in the stimulated wastewater could be increased. For example, Dotto et al. [53] optimized the adsorption process of food dyes (acid blue 9 and food yellow 3) onto chitosan, i.e. a full factorial design was used for investigating the effects of contact time, stirring rate, and the solution pH for the adsorptions of two food dyes. Later, Yin et al. [54] utilized HEDP-BH to remove the heavy metals from stimulated wastewater. Moreover, they discussed the combined effects of 20-HEDP-BH dosage, pH, and initial concentration of Au (III) using RSM technique. A 23 factorial design was employed to optimize the preparation condition of activated carbon, and two responses (the yield and iodine number) were simultaneously optimized by the central composite design to analyze the optimal process conditions [55]. Recently, Shakeel et al. [56] used the Box–Behnken design for the optimization of nanoemulsion compositions that could induce the maximum removal of Eriochrome black T (EBT) from aqueous solution by liquid–liquid adsorption. As stated above, the removals of contaminants (such as dyes, metal ions, etc.) by different adsorbents were commonly studied based on traditional methods, but the adsorptions of orange G and acid orange 7 onto chitosan/surfactant have not been investigated. Moreover, full factorial design and Box–Behnken design are rarely used for the LRB and AO10 removals from aqueous solutions. Then in the present study, the removals of two dyes by chitosan/surfactant were studied in batch systems. The main effects and interaction effects between various parameters on the adsorptions of two dyes were analyzed by a 23 full factorial design. And their maximum adsorption capacities have been optimized using RSM method. In addition, the adsorption kinetics and isotherms for the orange G and acid orange 7 systems were also discussed.

Orange G (96% purity) and acid orange 7 (higher than 85% purity) acquired from Aladdin Chemistry Co., Ltd. (Shanghai, China), were used without further purification. They were both dissolved in the double distilled water to form solutions of 400 and 500 mg L−1, respectively. And the solution pH was adjusted using hydrochloric acid (HCl) of 0.5 mol L−1 or sodium hydroxide (NaOH) of 0.2 mol L−1 solutions. Chitosan (extracted from snow crab shell, degree of deacetylaion: 95.0%, 120 mesh size) was purchased from Jinhu Crust Product Co., Ltd. (Qingdao, China). Spirulina powder was purchased from WUDI LV QI Bioengineering Co., Ltd. (Shangdong, China). Bentonit and activated clay were purchased from Gongyi city yuanheng water purification materials Co., Ltd. (Henan, China), and water must be removed before they are used. Activated carbon (It is prepared from wood sawdust; particle size: more than 150 mesh size; specific surface-area: 800–900 m2/g) was purchased from Beijing kecheng guanghua new technology Co., Ltd. (Beijing, China). Octadecyl Trimethyl Ammonium Chloride (OTAC, 98% purity), Dioctadecyl Dimethyl Ammonium Chloride (DDAC, 97% purity), and Benzyl Hexadecyl Dimethyl Ammonium Chloride (BHDAC, 95% purity) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Dodecyl Trimethyl Ammonium Chloride (DTAC, 99% purity) was purchased from Shanghai CIVI chemical technology Co., Ltd. (Shanghai, China). Other materials were of analytical reagent grade, and doubly distilled water was used throughout the experiment. The sample masses were accurately weighted on an electronic analytical balance ESJ180-4 (Shenyang Longteng Electronic Co., Ltd, China) with a resolution of 0.1 mg. 2.2. Adsorption experiments The batch adsorption tests have been carried out in the laboratory by contacting a certain volume of dyes aqueous solution (pH = 3.0) with different adsorbents (such as chitosan, spirulina powder, bentonit, activated clay, and activated carbon) at 200 rpm (Changzhou Guohua THZ82 digital thermostatic oscillator, China) and 293.15 K (for the orange G) or 323.15 K (for the acid orange 7) for 5 h (It is ascertained by the preliminary kinetic investigations) to ensure apparent equilibrium. The suspensions were filtered when the equilibrium was obtained, and the absorbance of supernatant solutions was determined using a UV-3600 spectrophotometer (Shimadzu, Japan) at their maximum wavelengths (478.00 nm for the orange G, 483.00 nm for the acid orange 7), respectively. Their percent removals are calculated based on the following equation: Adsorptionð%Þ ¼


 C 0 −C e  100 C0

ð1Þ

where C0 and Ce are the liquid-phase concentrations of dyes at initial and equilibrium, respectively. To investigate the adsorption capacities of composite adsorbents (chitosan/different type surfactants) for the two dyes, a certain volume of the orange G and acid orange 7 solutions (pH 3.0) with the initial concentrations of 400 mg/L and 500 mg/L was prepared in a series of 150 ml Erlenmeyer flasks. 20 mg of chitosan and a certain volume of surfactant (such as OTAC, DDAC, BHDAC, or DTAC) were dripped into each Erlenmeyer flask covered with rubber plugs, respectively. Then the flasks were placed in an isothermal water bath shaker at 293.15 K (for the orange G) or 323.15 K (for the acid orange 7) with rotation speed of 200 rpm; the aqueous sample was taken at present time interval. The effects of solution pH were discussed by adjusting their values in the range of 2–10 for the two dye systems. The solution pH was adjusted by 0.5 M HCl or 0.2 M NaOH solutions and measured by a pH meter (Instrument model: PHS-3C, Jiangsu jiangfen electroanalytical instrument

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

Co., Ltd., China). The initial concentrations of orange G and acid orange 7 were fixed at 320 mg/L and 500 mg/L with the chitosan dosage of 20 mg/100 mL and 34.10 μM DTAC, and their solution temperature was kept at 293.15 K and 323.15 K, respectively. For the batch kinetic studies, the procedure was same with those of investigating the adsorption capacities of composite adsorbents for the two dyes. Their uptake at any time, Qt (mg/g) is calculated as follows: Qt ¼

ðC 0 −C t ÞV W

Qe ¼

ðC 0 −C e ÞV W

ð3Þ

where C0, Ce , V and W are the same as in Eqs. (1) and (2). 2.3. Factorial design (FA) In order to investigate the individual and their interaction effects for the orange G and acid orange 7 removals onto chitosan/DTAC, a 23 full factorial experimental design was utilized. That is to say, three factors, namely the concentration of DTAC, temperature, and initial concentration of dye were changed at two levels as shown in Table 1. The experiments were carried on duplicates and two central points were added. 2.4. Box–Behnken design (BBD) A Box–Behnken design (3-factor and 3-level) was applied to locate the maximum adsorption capacities of the orange G and acid orange 7 onto chitosan/DTAC and their corresponding optimum conditions. And the preliminary range of independent variables was ascertained by single factor test. Their range and levels were shown in Table 1. The adsorption capacity of dye (y) was selected as the response for the combination of independent variables, which is fitted by a second order polynomial model as follows: y ¼ b0 þ

3 X

bi xi þ

i¼1

Table 2 Adsorption parameters for the orange G and acid orange 7 onto different sorbents. Name

3 X

2

bii xi þ

i¼1

2 X 3 X

ð4Þ

bij xi x j

i¼1 j¼1

where y is the predicted response associated with each factor level combination; b0 is the constant coefficient; and bi, bii, and bij are linear, quadratic, and 2-way linear by linear interaction coefficients, respectively;

Orange G

Activated carbon Bentonit Activated clay Spirulina powder Chitosan

ð2Þ

where V denotes the volume of solution, W denotes the mass of adsorbent used, and Ct is the liquid-phase concentration of dye at any time, t (min). Batch equilibrium experiments have been performed for the orange G and acid orange 7 adsorptions onto chitosan/DTAC at different temperatures (293.15, 308.15, and 323.15 K). The same procedure was followed, but their initial concentrations were changed from 200 to 320 mg/L (with equal interval, 30 mg/L for the orange G) and from 300 to 500 mg/L (with equal interval, 50 mg/L for the acid orange 7). Their amounts adsorbed at equilibrium (Qe) are calculated by Eq. (3):

355

Acid orange 7

Qe (mg/g)

Adsorption (%)

Qe (mg/g)

Adsorption (%)

555.34 210.06 128.61 344.76 1270.71

37.02 14.00 8.57 22.98 84.71

493.49 64.37 94.13 143.27 2145.14

19.74 2.57 3.77 5.73 85.81

xi and xj are the coded values of independent variables. Analysis of variance (ANOVA) was carried out to identify the adequacy of developed model and the statistical significance of the regression coefficients. In addition, six additional experiments were also done to verify the validity of two models obtained, respectively. 2.5. XRD and FT-IR spectra analysis To explain further the adsorption mechanism of two dyes onto chitosan/DTAC, FT-IR and XRD analysis was employed. Chitosan samples before and after the adsorption dye (orange G or acid orange 7) were dried in 373.15 K until constant weight. Then Nicolet-6700 FT-IR spectrometer (Nicolet, USA) was applied to characterize the samples. Powder X-ray diffraction (XRD) patterns of different samples were recorded at different diffraction angles (2θ) by Rigaku D/max-rA powder diffractometer (Ultima, IV, Japan) with Cu Kα radiation. 3. Results and discussion 3.1. Selection of sorbents for the orange G and acid orange 7 systems Five sorbents (Table 2) were chosen to investigate their adsorbability for the orange G and acid orange 7 systems. As could be seen from Table 2, the adsorption capacities (1270.71 and 2145.14 mg/g) and removal efficiency (84.71 and 85.81%) of orange G and acid orange 7 onto chitosan were much higher than those of the two dyes onto other four sorbents. Generally, the adsorption abilities of sorbents rely on their characteristics (such as surface functional groups, surface area, pore distributions, and mineral matter content). In the acidic conditions (pH 3.0), the adsorption reactions between the two dyes and chitosan were easier by the electrostatic interaction forces than the two dyes with other sorbents. Therefore, chitosan was selected as a sorbent for the orange G and acid orange 7 removals in the experiment. In order to enhance the adsorption capacity of chitosan for the orange G and acid orange 7 systems, compound adsorbent, i.e., different types of surfactants and chitosan in a one-step process have been investigated for the two dyes adsorptions (Fig. 1S, see support information). It could be seen from Fig. 1S that chitosan/DTAC showed higher adsorption efficiency for the orange G and acid orange 7 than the other three surfactants, and the adsorption efficiency for two dyes onto chitosan/ surfactant followed the order: DTAC N DDAC N OTAC N BHDAC. Moreover, the adsorption efficiency of compound adsorbent (chitosan/

Table 1 Factors, levels and coded values of the factorial design (FA) and Box–Behnken design (BBD) for the orange G and acid orange 7 systems. Orange G

FA BBD

Acid orange 7

Factor levels

Cs(μM) (x1)

Coded value

T(°C) (x2)

Coded value

C(mg L−1)(x3)

Coded value

Cs(μM) (x1)

Coded value

T(°C) (x2)

Coded value

C(mg L−1) (x3)

Coded value

1 2 1 2 3

17.05 34.10 17.05 25.58 34.10

1 −1 1 0 −1

20 50 20 35 50

1 −1 1 0 −1

200 320 200 260 320

1 −1 1 0 −1

17.05 34.10 17.05 25.58 34.10

1 −1 1 0 −1

20 50 20 35 50

1 −1 1 0 −1

300 500 300 400 500

1 −1 1 0 −1

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DTAC) for the orange G and acid orange 7 increased to 91.49% and 93.46% compared with those of only chitosan for the orange G (83.75%) and acid orange 7 (85.81%), respectively. Thus, chitosan/ DTAC was used as a compound adsorbent for the two dyes adsorptions in the study. 3.2. Effect of solution pH on adsorption efficiency for the orange G and acid orange 7 systems In the adsorption process, the solution pH can affect the degree of ionization, the surface charge of the sorbent, and speciation of adsorbate, so the orange G and acid orange 7 adsorptions have been investigated at different pH (from 2.0 to 10.0) (Fig. 2S, see support information). As could be seen in Fig. 2S, the orange G and acid orange 7 removals decreased with increasing the pH of solution (from 3.0 to 10.0). Moreover, the maximum adsorption efficiency of two dyes was observed at pH 3.0 (the removal efficiency of 90.85% and 93.58% for orange G and acid orange 7). This behavior could be explained due to the interactions between two dyes and chitosan/DTAC in the acid conditions [57]. In the acidic conditions, the amine groups (− NH2) of chitosan could be protonated by H+ in the solution; in addition, the orange G and acid orange 7 were dissolved and their sulfonate groups were dissociated. Then the two adsorption processes can proceed by the electrostatic interactions between protonated amino groups of chitosan and the anionic sulfonic groups of two dyes. Adsorption sites increase with decreasing the solution pH because of the protonation of more chitosan amino groups, which would induce that the adsorption capacities of orange G and acid orange 7 by chitosan/DTAC increased. The results were consistent with the reported acid dyes adsorptions onto nanochitosan [58]. However, at lower pH, the removal efficiency of two dyes decreased, which may be attributed to reducing the dissociation of two dyes, then it will result in a lower anionic dye concentration available to interact with DTAC+. At high pH, the protonated amine groups’ amount decreases and more OH− ions are available to compete with the anionic sulfonic groups, so the removal of two dyes will decrease at high pH [31]. As suggested above, pH 3.0 was selected for further studies. 3.3. Adsorption kinetics for the orange G and acid orange 7 systems Fig. 3S (see support information) shows the change of adsorption capacity versus adsorption time at different initial concentrations of dye. From Fig. 3S, the adsorption capacities of orange G and acid orange 7 increased until their equilibriums were attained around 240 and 210 min, respectively. In the present study, to evaluate the adsorption mechanisms of two dyes based on chitosan/DTAC, five kinetic models (the pseudo-firstorder, pseudo-second-order, Elovich, intraparticle diffusion, and Boyd kinetic models) were utilized.

3.3.1. Adsorption kinetics models The pseudo-first-order and pseudo-second-order kinetic models [59] are described as follows: ln ðQ e −Q t Þ ¼ ln ðQ e Þ−k1 t

ð5Þ

t 1 1 ¼ tþ Qt Qe k2 Q 2e

ð6Þ

where Qe (mg · g−1) is the equilibrium adsorption capacity; Qt (mg · g−1) is the adsorption capacity at time t; k1(min− 1) is the rate constant of the first-order adsorption; and k2 (g · mg− 1 · min− 1) is the rate constant of the second-order adsorption. The Elovich equation assumes that the actual solid surfaces are energetically heterogeneous, and it can be expressed as follows [60]: Qt ¼


.  1

β

ln ðαβÞ þ


.  1

β

ð7Þ

ln t

where α (mg · g−1 · min− 1) is the initial adsorption rate and the parameter β (g · mg−1) is related to the extent of surface coverage and activation energy. The kinetic parameter values of three kinetic models were calculated and listed in Table 3. As shown in Table 3, the correlation coefficient values based on the pseudo-first-order kinetic model for the two systems were low, except for the acid orange 7 at 500 mg/L. Moreover, the equilibrium adsorption capacity (Qe) obtained from the experimental and calculated values has a large difference, suggesting the experimental data of two systems couldn’t be fitted well by the pseudo-firstorder equation. However, the pseudo-second-order model could fit the experimental data very well for the two systems because their correlation coefficient values were very high (R2 N 0.998). Moreover, the experimental data and their corresponding calculated Qe values based on Eq. (6) were very similar, indicating that the adsorption data for the two systems were well represented by pseudo-second-order kinetics model. Abdelkader et al. [4] and Kousha et al. [29] reported similar results with the orange G adsorption onto LDH and CLDH, and the acid orange 7 adsorption onto chemically modified brown macroalga Stoechospermum marginatum, respectively. It could be observed from Table 3 that the k2 values for the two systems decreased with increasing their initial concentrations. The reason for this phenomenon could be attributed to the higher competition of dye molecules for the surface active sites with increasing the initial concentrations of dye. In contrast, at lower dye concentrations, the competition for the surface active sites is low, so higher adsorption rates would be obtained. In addition, the experimental data for the two systems were also analyzed by Elovich equation. The results indicated that the correlation coefficient values obtained based on Eq. (7) were lower than those of the pseudosecond-order equation. But in the higher initial concentrations of dye,

Table 3 Kinetic parameters of three models at two concentration levels. Orange G

Acid orange 7

−1

C0/mg·L−1

C0/mg·L Model

Parameter

200

320

300

500

pseudo first order

Qe/mg·g−1 k1/min−1 R2 Qe,fit/mg·g−1 Qe,exp./mg·g−1 k2/g·(mg·min)−1 R2 α/mg·(g·min)−1 β/g·mg−1 R2

206.21 0.0193 0.9812 1000.00 971.11 0.0002 0.9998 2.55E+07 0.0163 0.9697

679.05 0.0197 0.9535 1515.15 1459.55 0.0001 0.9993 3318.04 0.00559 0.9930

142.07 0.0285 0.9022 1492.54 1489.34 0.0005 0.9999 4.80E+08 0.0137 0.8662

1197.75 0.0198 0.9954 2439.02 2336.45 0.000034 0.9987 1731.26 0.00291 0.9944

pseudo second order

Elovich

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

Fig. 1. Intraparticle diffusion (A) and Boyd kinetic (B) models for the adsorptions of orange G and acid orange 7 onto chitosan/DTAC at different initial concentrations of dyes (T = 20 °Cfor the orange G system; T = 50 °Cfor the acid orange 7 system).

the Elovich equation could be applied to predict the adsorption capacities of orange G and acid orange 7 onto chitosan/DTAC possibly because chitosan possesses heterogeneous surface active sites. 3.3.2. Intraparticle diffusion and Boyd kinetic model As stated above, the experimental data for the orange G and acid orange 7 systems could be fitted very well based on the pseudo-secondorder model, but the diffusion mechanism of two adsorption reactions is still blurry. In order to explore their adsorption mechanisms, intraparticle diffusion and Boyd kinetic models were employed for the two systems. For a solid–liquid adsorption process, adsorbate transfer is usually governed by either liquid-phase mass transport step or the intraparticle diffusion step or both. The intraparticle diffusion model is expressed as [61]: Q t ¼ kdi t

0:5

þB

ð8Þ

357

where B is the intercept and kdi (mg · g−1 · min−0.5) is the intraparticle diffusion rate constant. The kdi values for the two systems can be evaluated from the slope of the linear plot of Qt versus t0.5 (Fig. 1A). It could be observed from Fig. 1A that the plots of Qt vs. t0.5 were not linear in the whole time range, but they could be separated into two linear regions, which indicated that the multistage adsorptions should happen for the two adsorption processes. The first linear portion of the plot (Fig. 1A) represented external mass transfer, i.e. the dye molecules could be transported to the external surface of chitosan by the film diffusion. At this stage, the instantaneous adsorption with a high rate should happen because there were strong electrostatic attraction forces between the dye molecule and the external surface of chitosan. The second linear portion of the plot denoted intraparticle diffusion, i.e. the dye molecules entered the interior of chitosan. The stage of gradual adsorption rate was controlled based on the intraparticle diffusion, and then when the adsorption reactions were slowly close to equilibrium, their intraparticle diffusion rate started to slow and become stagnant because all the active sites of chitosan were occupied by the dye molecules. These steps indicated that both external mass transfer and intraparticle diffusion might occur simultaneously. At the same time, the experimental data of the two systems could be the best fitted by the pseudo-second-order model, reconfirming that two or more steps were involved in the two processes. However, the linear plots at each concentration did not pass through the origin (Table 4), indicating that intraparticle diffusion was not the only sole rate-controlling step for the adsorptions of orange G and acid orange 7 and the external mass transfer was also significant in the rate-controlling step due to the large intercepts of the second linear portion of the plot [62]. Hameed et al. [63] and Ahmad et al. [64] reported similar trends for the Reactive Red 120 and Remazol Brilliant Orange 3R adsorptions by activated oil palm ash and coffee husk-based activated carbon, respectively. In addition, the kdi values of orange G and acid orange 7 increased with increasing their initial concentrations (Table 4), suggesting that the increased driving force at high initial concentrations of dye could enhance the intraparticle diffusion of two dyes onto chitosan/DTAC. In order to identify the slowest step in the adsorption process, Boyd kinetic equation was used, and it can be calculated as [65]: F ¼ 1−

6 expð−Bt Þ π2

ð9Þ

F ¼ Qt=Qe

ð10Þ

where Qt and Qe are the same as in Eq. (6), and Bt is a mathematical function of F. Eq. (9) can be represented as: Bt ¼ −0:4977−lnð1− F Þ

ð11Þ

The plots of Bt versus t for the orange G and acid orange 7 systems were shown in Fig. 1B. If the plots are linear and pass through the origin, then the slowest step in the adsorption process is governed by the intraparticle diffusion mechanism; otherwise it is governed based on the external mass transport (the film diffusion). The plots for the two

Table 4 Intraparticle diffusion model parameters at two concentration levels for the orange G and acid orange 7 systems. System

C0/(mg·L−1)

kd1/mg·(g·min0.5)−1

kd2/mg·(g·min0.5)−1

B1

B2

R1

R2

Orange G

200 320 300 500

33.24 84.31 48.16 151.74

8.90 25.88 2.17 57.67

681.17 656.96 1140.46 865.97

844.46 1085.90 1459.74 1557.59

0.9791 0.9927 0.9862 0.9816

0.9759 0.9941 0.9784 0.9763

Acid orange 7

358

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

systems (Fig. 1B) were not only nonlinear, but also they did not pass through the origin, indicating that external mass transport mainly governed the adsorption rate-limiting process of orange G and acid orange 7 onto chitosan/DTAC. The similar results have been reported by Sharma et al. [65] and Mittal et al. [66], respectively.

3.4. Adsorption isotherms for the orange G and acid orange 7 systems Adsorption isotherm studies are important to describe adsorption behavior between liquid and solid phases at equilibrium. Different isotherm models were applied to determine the adsorption efficiency of dye onto adsorbent. In the study, three adsorption isotherm models (Langmuir, Freundlich, and Temkin isotherms) were used for the orange G and acid orange 7 adsorptions onto chitosan/DTAC. In order to find out a better adsorption isotherm model for the two systems, the correlation coefficients of the isotherm models were compared, which were helpful to optimize the design of adsorption process for the orange G and acid orange 7 systems.

The linear form of Langmuir isotherm equation is given as: Ce C 1 ¼ e þ Q e Q m KLQ m

ð12Þ

where Ce (mg · L−1) is the equilibrium concentration of dye in solution, Qe (mg · g−1) is the amount of dye molecules adsorbed at equilibrium, Qm (mg · g−1) gives the maximum theoretical monolayer adsorption capacity, and KL (L · mg−1) is the Langmuir constant related to adsorption energy, respectively. The plots of Ce/Qe versus Ce yielded 1/(KLQm) as the intercept, and 1/Qm as the slope (Fig. 2 (A, a) and (B, a)). Moreover, the essential characteristics of Langmiur isotherm can be expressed by a dimensionless equilibrium parameter, RL, which is calculated using Eq. (13) [67]: RL ¼ 1=ð1 þ K L C 0 Þ

ð13Þ

where C0 (mg · L− 1) is the highest initial concentration of solute, and KL (L · mg− 1) is the Langmuir constant. The plots of RL versus C0 were

Fig. 2. Adsorption isotherms of the orange G (A) and acid orange 7 (B) adsorptions onto chitosan/DTAC by Langmuir (a), Freundlich (b) and Temkin (c) models at different temperatures (t = 240 and 210 min for the orange G and acid orange 7 systems, respectively; agitation speed, 200 rpm).

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

359

Table 5 Isotherm parameters for the orange G and acid orange 7 onto chitosan/DTAC. Orange G

Acid orange 7

T/K

T/K

Model

Parameter

293.15

308.15

323.15

293.15

308.15

323.15

Langmuir

Qmax/mg·g−1 KL/ L·mg−1 R2 KF /mg·g−1 1/n R2 KT/ L·mg−1 bT/ kJ·mol−1 R2

1538.46 0.25 0.9907 692.08 0.20 0.9777 11.19 0.0105 0.9592

1440.92 0.19 0.9944 681.50 0.17 0.9684 20.21 0.0136 0.9532

1344.09 0.16 0.9935 655.50 0.15 0.8920 28.89 0.0166 0.8842

1265.82 0.46 0.9921 1517.77 −0.03 0.2872 2.83×10−16 −0.0582 0.2983

1886.79 0.22 0.9958 1290.78 0.07 0.9006 75,021.76 0.0231 0.8908

2415.46 0.21 0.9928 1174.97 0.16 0.9844 33.11 0.0091 0.9663

Freundlich

Temkin

shown in Fig. 4S (see support information). It could be seen from Fig. 4S that all RL values have been found less than unity at the different initial concentrations and temperatures used, respectively, indicating there were favorable processes for two dye adsorptions [68]. Moreover, the values of RL decreased with increasing their initial concentrations, suggesting that the two adsorption reactions were more favorable at higher initial concentrations. The Freundlich isotherm is an empirical equation, which is often applied to describe heterogeneous systems. The Freundlich model is given by the relation lnQ e ¼ ln K F þ

1 ln C e n

ð14Þ

where Q e and Ce are the same as in Eq. (12). KF (mg · g−1) and 1/n are the Freundlich constants as measures of the adsorption capacity of adsorbent and adsorption intensity of the dye-adsorbent, respectively. The plots of lnQe versus lnCe were given in Fig. 2 (A, b) and (B, b) and the model parameters were calculated and listed in Table 5. As could be seen in Table 5, the 1/n values obtained for the two systems were less than 1, moreover they were close to zero at different temperatures, except for the acid orange 7 adsorbed at 293.15 K because it couldn’t be fitted well by Freundlich model (the value of R2 was small), indicating that the orange G and acid orange 7 adsorptions onto chitosan/DTAC were nonlinear and have high affinity [41,52]. Temkin isotherm model can be used for investigating the adsorption heat of all the molecules in the layer and adsorbate–adsorbent

interaction on the adsorbent surface. The linear form of the model is described as follows: Qe ¼

RT RT lnK T þ lnC e bT bT

ð15Þ

where bT (kJ · mol−1) and KT (L · mg−1) are the Temkin constant related to the adsorption heat and the equilibrium binding constant, respectively. The plots of Qe versus lnCe were shown in Fig. 2 (A, c) and (B, c) for the two systems, and their corresponding parameters were calculated from the slope and intercept of linear plots of Qe versus ln Ce and showed in Table 5. If the bonding energy is less than −40 kJ · mol−1, the adsorption reaction is dominated by physiosorption, and ionexchange mechanism is reported because of its bonding energy in the range of 8–16 kJ · mol−1 [69]. In the study, the bT values indicated the two adsorption reactions involved chemisorption. The parameters of three isotherm models and their corresponding R2 values were summarized in Table 5. As shown in Table 5, the experimental data could be fitted better (R2 N 0.99) by the Langmuir isotherm model than those by the Freundlich and Temkin models, indicating the surfaces of chitosan for the orange G and acid orange 7 adsorptions were made up of homogeneous adsorption patches [64]. The results agreed with the works reported by previous researchers for the pollutants adsorptions (such as heavy metal ions, toluene, dyes, etc.) onto chitosan [38,39,45], modified chitosan [42,49], or bottom ash and de-oiled soya [70,71]. For the orange G adsorption, monolayer adsorption capacity decreased (from 1538.46 to 1344.09 mg · g− 1) with increasing the

Table 6 Comparison of the maximum adsorption capacities for the removal of orange G and acid orange 7 using different sorbents. Orange G

Acid orange 7 −1

Refs

Adsorbent

Qe (mg g−1)

Refs

76.4; 378.8 48.98, 61.33 9.129 922.9 101.42 53.38 1017.8

4 14 16 17 28 30 31 32

13.25, 9.59 25.06 71.05 110.70 1215.6 1368 192 ± 1 99 ± 1 1940

3 15 29 30 31 33 72

290.9;665.9; 470.5 4.57 1497 54.31 ± 1.573 1012.62 923.48 139.12 1452.07

Bottom ash, de-oiled soya Canola stalks C3H9N treated S. marginatum DTCSCu EMCN Amberlite IRA-958 GAC MAMS Cross-linked chitosan beads TWNC Waste brewery’s yeast Oxihumolite Amberlite IRA-900 Amberlite IRA-910 Amberlyst A-21 Chitosan/DTAC

312.5 3.561 50 1289.65 1097.55 235.05 2352.99

74 75 77 78

Adsorbent

Qe (mg g

LDH; CLDH MAMPS, MAMMS Activated carbon Chitosan HDTMAB DTCSCu EMCN MgAl500; MgAlHT500; MgAlM500 [email protected]/2-Gel SA/METAC Nano-zirconia Amberlite IRA-900 Amberlite IRA-910 Amberlyst A-21 Chitosan/DTAC

)

34 35 76 78

present study

73

present study

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Table 7 Factorial design experimental data. Run

Adsorption capacity (mg g−1)

Factor

Orange G

1 2 3 4 5 6 7 8 9 10

Acid orange 7

x1

x2

x3

Exp. 1

Exp. 2

Average

Exp. 1

Exp. 2

Average

0 1 1 −1 −1 1 1 0 −1 −1

0 −1 1 −1 −1 −1 1 0 1 1

0 1 −1 −1 1 −1 1 0 1 −1

1130.14 1462.69 922.55 949.18 1392.73 956.32 1245.67 1131.36 1294.05 893.67

1113.61 1459.21 912.29 948.31 1391.68 960.15 1243.06 1120.92 1294.74 892.10

1121.88 1460.95 917.42 948.75 1392.20 958.23 1244.36 1126.14 1294.40 892.88

1541.25 1255.54 1489.06 1206.81 1014.67 1335.96 2335.96 1547.90 2189.23 1475.50

1550.39 1250.00 1489.62 1214.98 1021.59 1338.73 2336.93 1550.39 2211.93 1478.13

1545.82 1252.77 1489.34 1210.89 1018.13 1337.35 2336.45 1549.14 2200.58 1476.81

of model are calculated based on dividing the net effects by two. The T-values are obtained based on dividing the regression coefficients by standard error. By substituting the coefficients a i in Eq. (16) with their values from Table 8, two model equations relating the level of parameters for the orange G (Eq. (17)) and acid orange 7 (Eq. (18)) systems are shown as follows: y ¼ 1135:72 þ 6:59  x1 −51:38  x2 þ 209:93  x3 −12:97 ð17Þ  x1 x2 −1:91  x1 x3 −27:21  x2 x3 −16:73  x1 x2 x3

y ¼ 1541:73 þ 63:69  x1 þ 335:50  x2 þ 161:69  x3 −26:59  x1 x2 þ 28:94  x1 x3 þ 231:03  x2 x3 þ 1:89  x1 x2 x3 ð18Þ

temperature (from 293.15 to 323.15 K). However, for the acid orange 7 adsorption, monolayer adsorption capacity increased (from 1265.82 to 2415.46 mg · g−1) with rise in the temperature. The results indicated that the two adsorption processes were exothermic and endothermic reactions for the orange G and acid orange 7 systems, respectively. In addition, we have compared the adsorption capacities of the two dyes based on other adsorbents (Table 6). It could be seen from Table 6 that the adsorption capacities of orange G and acid orange 7 by chitosan/DTAC were higher than those of most other adsorbents reported. Therefore, the method was worth considering for the removals of two dyes from printing and dyeing sewage. 3.5. Statistical analysis of factorial design experiments The design matrix of coded values for factors and response in terms of adsorption capacity for the orange G and acid orange 7 systems were shown in Table 7. The experimental data were analyzed using Minitab 15 program for Windows and the main effects and interaction between the factors have been also investigated. In this study, the adsorption capacities of orange G and acid orange 7 are defined as the change in response, which can occur as a result of a change in the level of a factor from a lower to higher level. The codified model employed for a 23 factorial design is given as: y ¼ a0 þ a1 x1 þ a2 x2 þ a3 x3 þ a4 x1 x2 þ a5 x1 x3 þ a6 x2 x3 þ a7 x1 x2 x3

ð16Þ

where y (mg/g) is the adsorption capacity of orange G or acid orange 7, a0 is the global mean, ai (i = 1, 2,…,7) represents the regression coefficients of the main factor effects and interactions, and x1, x2, and x3 indicate the concentration of DTAC, temperature, and the initial concentration of dye, respectively. The effects, regression coefficients, standard errors of coefficient, T-value and P-value were listed in Table 8. The regression coefficients

Analysis of variance (ANOVA) was carried out to determine the significant main effects and interactions (2-way and 3-way) of the factors influencing their adsorption capacities. The sum of squares, mean squares, F-value, and P-value were shown in Table 9. If the P-value of a factor is closer to zero, the factor has greater significance than other factors. However, for a 95% confidence level, a factor can be considered statistically significant when its P-value is less than or equal to 0.05 [79]. According to the P-value obtained (Table 9), for the orange G system, the main effects were found to be statistically significant, but for the acid orange 7 system, the main effects and 2-way interactions were statistically significant. In addition, the normal probability plot of standardized effects was given in Fig. 3. The significant effects will stay farther from the line; on the other hand, the insignificant effects will fall along a line. According to Fig. 3, the main effects (x2 and x3) and the interaction x2x3 were judged to be statistically significant at the 5% level for the orange G system; and the main effects (x1, x2, and x3) and the interactions (x1x2, x1x3, and x2x3) were judged to be statistically significant at the 5% level for the acid orange 7 system. The initial concentrations of orange G (x3) and temperature (x2) have the largest effects for the orange G and acid orange 7 adsorptions because they lie the farthest from the straight line, respectively. The Pareto charts of standardized effects were shown in Fig. 4, which can illustrate both the magnitude and the importance of the individual and interaction effects. The vertical line in Fig. 4 implies the minimum statistically significant effect magnitude with a 95% confidence level and nine degrees of freedom. The bars for x1, x1x2, x1x3, and x1x2x3 remained inside the vertical line in the Pareto chart (Fig. 4A), indicating that these terms contributed the least to the prediction of orange G removal efficiency; likewise, 3-way interaction x1x2x3 contributed the least to the prediction of acid orange 7 removal efficiency (Fig. 4B). The results confirmed previous graphical analysis of the normal probability plots for the two systems. According to the analysis above, the effect of concentration of DTAC (x1) and several interaction effects (such as x1x2, x1x3, and x1x2x3) for the

Table 8 Statistical parameters for factorial design. Term

Orange G Effects

Constant x1 x2 x3 x1 x2 x1 x3 x2 x3 x1 x2 x3 S R-Sq R-Sq (adj)

13.18 −102.77 418.66 −25.93 −3.83 −54.43 −33.46 13.2674 0.9991 0.9959

Acid orange 7 Coefficient

Standard error of coefficient

T-value

P-value

1135.72 6.59 −51.38 209.33 −12.97 −1.91 −27.21 −16.73

4.196 4.691 4.691 4.691 4.691 4.691 4.691 4.691

270.70 1.41 −10.95 44.63 −2.76 −0.41 −5.80 −3.57

0.000 0.295 0.008 0.001 0.110 0.723 0.028 0.070

Effects 127.37 671.01 323.38 −53.17 57.88 462.05 3.79 6.6416 0.9999 0.9997

Coefficient

Standard error of coefficient

T-value

P-value

1541.73 63.69 335.50 161.69 −26.59 28.94 231.03 1.89

2.100 2.348 2.348 2.348 2.348 2.348 2.348 2.348

734.06 27.12 142.88 68.86 −11.32 12.32 98.39 0.81

0.000 0.001 0.000 0.000 0.008 0.007 0.000 0.504

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

361

Table 9 Analysis of variance (ANOVA) of the fitted quadratic polynomial models for the orange G and acid orange 7 adsorption systems by factorial design. Source Orange G Main effects 2-way interactions 3-way interactions Residual error Pure error Total Acid orange 7 Main effects 2-way interactions 3-way interactions Residual error Pure error Total

Degrees of freedom

Sum of squares

Adj. Sum of squares

Adj. Mean squares

F-value

P-value

3 3 1 2 1 9

372,016 7299 2239 352 9 381,906

372,016 7299 2239 352 9

124,005 2433 2239 176 9

704.48 13.82 12.72

0.001 0.068 0.070

3 3 1 2 1 9

1,142,109 439,342 29 88 6 1,581,568

1,142,109 439,342 29 88 6

380,703 146,447 29 44 6

8630.50 3319.95 0.65

0.000 0.000 0.504

orange G system were statistically insignificant comparing with other effects, and then they were discarded, leading to Eq. (19); similarly, 3-way interaction x1x2x3 for the acid orange 7 system was discarded, leading to Eq. (20): y ¼ 1135:72−51:38  x2 þ 209:93  x3 −27:21  x2 x3

ð19Þ

y ¼ 1541:73 þ 63:69  x1 þ 335:50  x2 þ 161:69

ð20Þ

 x3 −26:59  x1 x2 þ 28:94  x1 x3 þ 231:03  x2 x3

Fig. 3. Normal probability plot of the standardized effects for the orange G (A) and acid orange 7 (B) adsorptions at P = 0.05.

In order to ascertain which factors may most affect the response value, the main effect plots (Fig. 5) were analyzed. For the acid orange 7 adsorption system, high levels of x2 and x3 resulted in higher mean response values comparing with their low levels; but for the orange G adsorption system, low level of x2 and high level of x3 induced higher mean response values comparing with their corresponding levels in reverse. Additionally, the factor x2 and x3 have greater effects on the response as indicated by a steep slope for the acid orange 7 and orange G adsorptions, respectively. Since the slope of x1 for the orange G

Fig. 4. The Pareto charts of standardized effects for the orange G (A) and acid orange 7 (B) adsorptions by chitosan/DTAC.

362

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

Fig. 6. Interaction effect plots for the orange G (A) and acid orange 7 (B) adsorption systems.

Fig. 5. Main effect plots for the orange G (A) and acid orange 7 (B) adsorption systems.

adsorption is close to zero, the effect of the factor is insignificant. That is to say, increasing the concentration of DTAC (from 17.05 to 34.10 μM) could not bring a remarkable effect on the orange G removal efficiency. The interaction effects plots were given in Fig. 6. The non-parallel lines in Fig. 6 indicated there was the interaction between the two factors. Fig. 6(A) and the coefficients of Eq. (19) showed the negative interaction between temperature and the initial concentration of orange G (x2 × x3). However, for the acid orange 7 system, Fig. 6(B) and the coefficients of Eq. (20) suggested the positive interactions of the concentration of DTAC and temperature with the initial concentration of acid orange 7 (x1 × x3 and x2 × x3), respectively; and a negative interaction effect was also observed between the concentration of DTAC and temperature (x1 × x2). Moreover, the interaction effect (x2 × x3) for the acid orange 7 adsorption was more significant than its main factors (x1 and x3) because it has bigger regression coefficient than those of x1 and x3. 3.6. Optimization of orange G and acid orange 7 adsorptions by RSM approach 3.6.1. Statistical analysis of BBD Response surface methodology (RSM) is more advantageous than the traditional single parameter optimization because it can save time, space and raw material. In the experimental design, BBD (Box–Behnken design) is a type of RSM, and it helps to optimize the factors, the square of factors and interaction of various factors for obtaining the best response values. Moreover, BBD is slightly more efficient for the quadratic model compared to central composite design (CCD), and in particular it is favorable in avoiding treatment combinations which

are extreme [80]. Then BBD is applied to optimize the important operating parameters of the orange G and acid orange 7 adsorptions by chitosan/DTAC. The codified values of three important factors (x1, x2 and x3) together with their corresponding response and predicted values were shown in Table 10. Runs 7, 11 and 12 at the center point were employed to calculate the pure error and the variance. The quadratic equations in terms of coded factors for the orange G (Eq. (21)) and acid orange 7 (Eq. (22)) systems were obtained as: y ¼ 1128:46 þ 6:04  x1 −45:72  x2 þ 208:44  x3 −1:61 2

2

2

 x1 þ 9:36  x2 −0:15  x3 þ 4:02  x1 x2 −34:68  x2 x3

ð21Þ

y ¼ 1581:37 þ 26:02  x1 þ 318:70  x2 þ 182:42  x3 2

2

2

þ 62:30  x1 −35:79  x2 þ 2:21  x3 −8:74  x1 x2 ð22Þ þ2:37  x1 x3 þ 226:43  x2 x3 As could be seen in Fig. 7, the predicted values by the two models were in good agreement with their corresponding experimental values. And their coefficients of determination (R2) were 0.9953 and 0.9974, suggesting that only 0.47% and 0.26% of the total variations couldn’t be explained based on Eqs. (21) and (22) for the orange G and acid orange 7 systems, respectively. Moreover, their adjusted determination coefficient values (R2adj = 0.9868 and 0.9928) were close to 1.0, indicating that the two models have high reliability for predicting the experimental data. Moreover, we have utilized analysis of variance (ANOVA) method to evaluate further the significance and accuracy of the two models and identify the complex relationship between variables and responses. The statistic results reported for Box–Behnken design were summarized in Tables 11 and 12. The ANOVA results (Table 11) indicated that the two equations were highly significant, as evident from the Fisher’s

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

363

Table 10 Box–Behnken design experimental data. Run

Adsorption capacity (mg g−1)

Factor

Orange G

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Acid orange 7

x1

x2

x3

Exp. 1

Exp. 2

Average

Cal.

Exp. 1

Exp. 2

Average

Cal.

0 −1 1 1 0 −1 0 0 −1 1 0 0 −1 1 0

−1 −1 −1 0 1 0 0 1 0 0 0 0 1 1 −1

1 0 0 1 1 −1 0 −1 1 −1 0 0 0 0 −1

1443.37 1165.82 1174.87 1346.43 1255.76 915.25 1125.44 900.97 1333.21 920.64 1137.45 1131.53 1099.34 1117.44 954.58

1436.58 1162.51 1167.21 1334.77 1245.32 910.37 1120.05 908.46 1329.55 923.42 1133.80 1122.49 1087.33 1115.18 956.32

1439.98 1164.17 1171.04 1340.60 1250.54 912.81 1122.75 904.72 1331.38 922.03 1135.62 1127.01 1093.33 1116.31 955.45

1426.530 1179.936 1183.961 1341.209 1265.751 912.245 1128.484 918.212 1329.136 924.318 1128.484 1128.484 1080.456 1100.578 940.286

1162.79 1249.72 1349.39 1853.54 2278.52 1438.82 1575.72 1480.76 1820.32 1467.19 1592.33 1586.10 1884.97 1939.65 1268.69

1160.02 1266.33 1346.62 1859.08 2280.18 1441.58 1561.18 1481.87 1819.63 1466.92 1581.95 1590.95 1885.52 1940.89 1269.52

1161.41 1258.03 1348.01 1856.31 2279.35 1440.20 1568.45 1481.31 1819.98 1467.05 1587.14 1588.52 1885.24 1940.27 1269.10

1180.557 1251.549 1321.229 1852.578 2271.441 1438.464 1578.844 1456.536 1795.592 1485.902 1578.844 1578.844 1906.911 1941.655 1271.653

F test (Fmodel = 117.41 and 214.50) with a very low probability value (pmodel = 0.000) for the orange G and acid orange 7 systems. Moreover, the lack of fit of the two models was nonsignificant because their F and p values were 12.88 and 0.073, and 8.33 and 0.109, respectively. Therefore, the two equations (Eqs (21) and (22)) could fit well the experimental data of the orange G and acid orange 7 systems. In the orange G system studied, the linear coefficients (x2 and x3) and interaction coefficient (x2x3) were significant because they have very small p-values (p b 0.05) (Table 12), and its other term coefficients were insignificant (p N 0.05). But for the acid orange 7 system, the linear coefficients (x1, x2 and x3), two quadratic term coefficients (x21 and x22), and interaction coefficient (x2x3) have significance (Table 12) and antagonistic effects on the response value.

within the pores, the adsorption capacity of acid orange 7 increased with increasing the temperature. This phenomenon can be explained based on the thermodynamic parameters of adsorption reaction (such as, ΔHo, ΔSo and ΔGo). The Gibbs free energy changes (ΔGo) of the adsorption process can be calculated by Eq. (23) based on the prior determining equilibrium constant (Eq. (24)) [81], and their enthalpy changes (ΔHo) and entropy changes (ΔSo) can be determined by the van’t Hoff equation (Eq. (25)): o

ΔG ¼ −RT lnK c

Kc ¼ 3.6.2. Effects of process variables In order to understand further the effects of independent variables, 3D response surface plots based on the predictive quadratic models (Eqs. (21) and (22)) for the orange G and acid orange 7 adsorptions onto chitosan/DTAC were shown in Figs.8a–c and 9a–c, respectively. From Figs. 8(a, c) and 9(a, c), the effects of temperature were remarkable for the orange G and acid orange 7 systems. At optimum C = 320 (for the orange G) and 500 mg/L (for the acid orange 7), their adsorption capacities based on the prepared adsorbents decreased from 1417.17 to 1270.58 mg g−1, and increased from 1280.07 to 2353.49 mg g−1 at the range of 20–50 °C, respectively. In general, an endothermic process depends upon diffusion. Since higher temperatures induce larger diffusion rate of the adsorbate molecule across the external boundary layer and

C Ae Ce

ln K c ¼ −

ð23Þ

ð24Þ

ΔH o ΔSo þ RT R

ð25Þ

where R (J mol−1 K−1) is the gas constant, T (K) is the experimental temperature, CAe (mg g−1) and Ce (mg g−1) indicate the orange G or acid orange 7 amounts on adsorbent phase and adsorbate phase, respectively. The values of ΔHo, ΔSo and ΔGo for the two systems were calculated and summarized in Table 13. The negative ΔGo values suggested that the two adsorption reactions were spontaneous processes. Moreover, the ΔG° values for the acid orange 7 system shifted to more negative

Fig. 7. Predicted vs. experimental values for the orange G (a) and acid orange 7 (b) adsorptions.

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L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

Table 11 Analysis of variance (ANOVA) of the fitted quadratic polynomial models for the orange G and acid orange 7 adsorptions onto chitosan/DTAC by Box–Behnken design. Source

Degrees of freedom

Orange G Regression Linear Square Interaction Lack of fit Residual error Pure error Total Acid orange 7 Regression Linear Square Interaction Lack of fit Residual error Pure error Total

Sum of squares

Adj. Sum of squares

Adj. Mean squares

F-value

p-value (F N F0.05)

9 3 3 3 3 5 2 14

369,797 364,577 345 4875 1664 1750 86 371,547

369,797 364,577 345 4875 1664 1750 86

41,089 121,526 115 1625 555 350 43

117.41 347.26 0.33 4.64 12.88

0.000 0.000 0.806 0.066 0.073

9 3 3 3 3 5 2 14

1,310,078 1,084,213 20,450 205,415 3142 3393 251 1,313,471

1,310,078 1,084,213 20,450 205,415 3142 3393 251

145,564 361,404 6817 68,472 1047 679 126

214.50 532.55 10.04 100.90 8.33

0.000 0.000 0.015 0.000 0.109

with rise in the temperature, which implied that the removal of acid orange 7 by chitosan/DTAC was more favorable at high temperature than that at low temperature. The high ΔHo value (+67.79 kJ mol−1) indicated that the acid orange 7 adsorption process was endothermic and the chemisorption might be responsible for the adsorption of acid orange 7 onto chitosan/DTAC [62,81]. However, for the orange G adsorption system, its ΔHo value was −23.65 kJ mol−1, implying that the adsorption of orange G by chitosan/DTAC was exothermic and the adsorption might be dominated based on the physisorption [62,82]. In addition, when the solution temperature increased, the mobility of orange G was promoted and it escaped from the solid phase to the liquid phase. The results indicated that the adsorption capacity of orange G decreased with increase in the temperature. The adsorptions of orange G (Fig. 8(b, c)) and acid orange 7 (Fig. 9(b, c)) onto chitosan/DTAC were carried out with different initial concentrations ranges of 200–320 mg L−1 and 300–500 mg L−1, respectively. As shown in Figs. 8(b, c) and 9(b, c), the adsorbed amounts of two dyes were smaller at lower initial concentrations and greater at higher initial concentrations; it was also observed that increase in their initial concentrations induced increase in the two dyes' uptake. That is to say, the orange G uptake increased from 940.7 to 1417.2 mg g−1 with an increase in its initial concentrations from 200 to 320 mg L−1 and optimum conditions, but their corresponding removal rates decreased from 94.07% to 88.58%; in the same way, the adsorption capacities and removal rates of acid orange 7 increased (from 1498.8 to 2353.5 mg g−1) and decreased (from 99.92% to 94.14%) with optimum conditions at the range between 300 and 500 mg L−1, respectively. During adsorption, the main driving force can overcome mass transfer resistances between the dye and adsorbent phase, which can be provided by the initial concentration of dye, adsorption sites, and available adsorption surface. Then, for the constant adsorbent dosage, the lower removal rates were obtained for the two

dyes due to less available adsorption sites of chitosan at higher initial concentrations of dye, and the two dye adsorptions became rely on their initial concentrations. The adsorption capacities of orange G and acid orange 7 increased with increasing their initial concentrations because of enhancing the interactions between adsorbent and dyes. The effects of increasing DTAC concentrations have also been presented in Figs. 8 (a, b) and 9 (a, b). It showed that the acid orange 7 adsorption capacity increase with increasing the concentration of DTAC (from 17.05 to 34.10 μM) was more remarkable than that of the orange G. According to the results of RSM analysis, linear and quadratic coefficients of DTAC (Eq. (18)) are positive for the acid orange 7 system; moreover, the two coefficients are bigger than those of the orange G system. The results indicated that the adsorption capacity of the acid orange 7 was stronger than that of the orange G at higher DTAC concentration. 3.6.3. Process optimization In the present study, we have applied the numerical optimization function based on the D-optimality index in the Minitab software to locate the values of independent variables which can bring a maximum response value. A desirability function is utilized in the approach. For all the factors, the D-optimality index varies between zero (completely undesirable) and one (completely desirable). Combining the individual desirability function for each variable is employed to obtain the composite desirability of a multi-response system, which can determine the optimal operating conditions [83]. The optimality plots for the response variable were given in Fig. 10. The profiles showed the composite desirability of the two systems and the orange G and acid orange 7 adsorption capacities as a function of each factor (Fig. 10(A) and (B)), respectively. The D-optimality values of the two systems were computed for all levels of the three experimental factors (the concentration of

Table 12 Statistical parameters for Box–Behnken design. Term

Constant x1 x2 x3 x21 x22 x23 x1 x2 x1 x3 x2 x3

Orange G

Acid orange 7

Coefficient

Standard error of coefficient

T-value

p-value

Coefficient

Standard error of coefficient

T-value

p-value

1128.46 6.04 −45.72 208.44 −1.61 9.36 −0.15 4.02 0 −34.68

10.801 6.614 6.614 6.614 9.735 9.735 9.735 9.354 9.354 9.354

104.482 0.913 −6.912 31.515 −0.165 0.961 −0.015 0.430 0.000 −3.707

0.000 0.403 0.001 0.000 0.875 0.381 0.988 0.685 1.000 0.014

1581.37 26.02 318.70 182.42 62.30 −35.79 2.21 −8.74 2.37 226.43

15.040 9.210 9.210 9.210 13.557 13.557 13.557 13.025 13.025 13.025

105.142 2.826 34.603 19.806 4.596 −2.640 0.163 −0.671 0.182 17.384

0.000 0.037 0.000 0.000 0.006 0.046 0.877 0.532 0.863 0.000

L. Zhang et al. / Journal of Molecular Liquids 197 (2014) 353–367

Fig. 8. Response surface plots for the combined effects on the orange G adsorption capacities: (a) the concentration of DTAC (Cs) and temperature, C = 320 mg/L; (b) Cs and the concentration of orange G (C), T = 20 °C; (c) Temperature and C, Cs = 25.58 μM.

DTAC, temperature, and the initial concentration of dye). For the orange G system, the composite desirability is relatively constant for the change of DTAC concentrations (from 17.05 to 34.10 μM). However, for the temperature or initial concentration of the orange G, its composite desirability values were gradually decreased or increased and subsequently reached a peak value. Then the adsorption capacity of orange G attained a maximum value at an elevated DTAC level, low temperature and high initial concentration of orange G. Similarly, the maximum adsorption value of acid orange 7 was obtained at high DTAC level, high temperature and high initial concentration of acid orange 7. In summary, a D-optimality of 0.9933 with a maximum response value (the adsorption capacity of orange G) of 1427.13 mg/g was obtained

365

Fig. 9. Response surface plots for the combined effects on the acid orange 7 adsorption capacities: (a) the concentration of DTAC (Cs) and temperature, C = 500 mg/L; (b) Cs and the concentration of acid orange 7 (C), T = 50 °C; (c) Temperature and C, Cs = 34.10 μM.

at Cs = 30.92 μM, T = 20 °C and C = 320 mg/L, and a D-optimality of 0.9925 with a maximum response value (the adsorption capacity of acid orange 7) of 2357.31 mg/g was gained at Cs = 34.10 μM, T = 50 °C and C = 500 mg/L. In addition, to confirm that the predicted values were not biased toward their corresponding experimental results, six additional experiments for the two dyes' adsorptions have been performed under the aforementioned optimum conditions, respectively. For the orange G and acid orange 7 adsorptions, their average experimental values were 1452.07 mg/g and 2352.99 mg/g, which were in good agreement with their corresponding predicted values based on the two regression models established, with small relative errors of 1.72% and −0.18%, respectively. The results validated that the response models (Eqs. (21)–(22)) were adequate for reflecting the expected optimization for the two systems.

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Table 13 Thermodynamic parameters for the adsorptions of orange G and acid orange 7 onto chitosan/DTAC. System

Orange G Acid orange 7

Ci (mg L−1)

320 500

ΔH0 (kJ mol−1)

−23.65 67.79

ΔS0 (J mol−1 K−1)

7.85 296.31

3.7. XRD and FT-IR spectra analysis The XRD patterns of chitosan, chitosan/DTAC sorbing the orange G, and chitosan/DTAC sorbing the acid orange 7 were shown in Fig. 5S (see support information). The chitosan has a strong peak at 20.20°, which is attributed to the anhydrous crystal (Forms II). However, after the chitosan sorbed the orange G and acid orange 7, their complexes exhibited one broad peak at around 2θ = 22.46° and 24.36°, respectively. Moreover, their diffraction peaks intensities decreased. The results suggested that the adsorption reactions of the two dyes onto chitosan were initiated. The FT-IR experiments were used for attaining information on the possible interactions between the functional groups of chitosan and the two dyes. The FT-IR spectra of the chitosan, dyes, DTAC, and dyes treatment by chitosan/DTAC were shown in Fig. 6S (see support information). For the chitosan, its characteristic absorption peaks at 3441 cm− 1 (− OH groups stretching and − NH groups stretching), 2876 cm−1 (C−H stretching), 1641 cm−1 (N−H bending and C = O stretching), 1384 cm−1 (asymmetric C−H bending) and other characteristic peaks of the polysaccharide at 1156 cm−1 and 1094 cm−1 [84]

R

0.9964 0.9923

ΔG0 (kJ mol−1) 293.15 K

308.15 K

323.15 K

−26.00 −19.28

−25.96 −23.03

−26.25 −28.22

are observed in the spectra of chitosan. FT-IR spectra of chitosan/DTAC sorbing the two dyes showed significant changes in some of the peaks or appearing new peaks, which suggested that functional groups present on the chitosan were involved in the interactions with the two dyes. In addition, after the two adsorption processes happened in the acid conditions, it was possible to observe the bands of (1222 and 1208) cm−1 for the orange G and acid orange 7 corresponding to the stretching of sulfonated groupings presented in the two dyes' structure, respectively. The results indicated the two adsorption reactions could occur by the electrostatic interactions between the anionic sulfonic groups of two dyes (R-SO− 3 ) and protonated amino groups of chitosan (R′-NH+ 2 ). 4. Conclusion In the research, according to the results analyzed by full factorial design, the main factor effects (x2 and x3) and the interaction x2x3 have greater effects than other factors for removing the orange G from aqueous solutions; however, the entire main factors and their interaction effects are important to get rid of the acid orange 7 from aqueous solutions. Moreover, the initial concentration of orange G (x3) and temperature (x2) have the largest effect for the adsorptions of orange G and acid orange 7, respectively. The process variables of acidic dyes adsorptions by chitosan/DTAC have been optimized using RSM method. The experimental data were analyzed by the numerical optimization function based on the D-optimality index in the Minitab software to search the values of independent variables which could result in the maximum response values. The two maximum response values (1427.13 and 2357.31 mg/g) were discovered at Cs = 30.92 μM, T = 20 °C and COG = 320 mg/L, and Cs = 34.10 μM, T = 50 °C and COA = 500 mg/L for the orange G and acid orange 7 systems, which were well matched with their corresponding experimental values determined in the optimal conditions (1452.07 and 2352.99 mg/g), respectively. The adsorptions of two dyes were evaluated with the aspect of thermodynamics and kinetic. The adsorption of acid orange 7 has high ΔHo value (+67.79 kJ mol−1), suggesting that the adsorption reaction was endothermic and the chemisorption might be responsible for the adsorption reaction. However, for the orange G adsorption, its ΔHo value was −23.65 kJ mol−1, indicating that the adsorption of orange G onto chitosan/DTAC was exothermic and the adsorption might be dominated based on the physisorption. Kinetic studies suggested that the two adsorption equilibriums were achieved within 210 and 240 min for the acid orange 7 and orange G, and the pseudo-second-order model was followed. From the Boyd plot, the two dyes' absorptions onto chitosan/DTAC occurred by the film diffusion mechanism. In addition, the equilibrium data could be fitted well based on the Langmuir adsorption isotherm equation with the maximum adsorption capacity of 2415.46 mg g−1 (323.15 K) and 1538.46 mg g− 1 (293.15 K) for the acid orange 7 and orange G systems, respectively. Acknowledgements

Fig. 10. Optimization plots for the orange G (A) and acid orange 7 (B) adsorptions onto chitosan/DTAC.

Financial support from Sichuan Provincial Science & Technology Fund for Distinguished Young Scholars (2012JQ0058), the NSFC (20873104) and SKLOGRGE (PLN-ZL002, SWPU) is gratefully acknowledged.

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