Journal of Molecular Liquids 211 (2015) 1074–1081
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Adsorption of acid dyes from aqueous solution by CTMAB modiﬁed bentonite: Kinetic and isotherm modeling Liang-guo Yan ⁎, Li-lu Qin, Hai-qin Yu, Shuang Li, Ran-ran Shan, Bin Du School of Resources and Environment, University of Jinan, Shandong Provincial Engineering Technology Research Center for Groundwater Numerical Simulation and Contamination Control, Jinan 250022, PR China
a r t i c l e
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Article history: Received 25 June 2015 Received in revised form 9 August 2015 Accepted 11 August 2015 Available online xxxx Keywords: Organobentonite Cetyltrimethyl ammonium bromide Adsorption kinetic Adsorption isotherm Nonlinear regression
a b s t r a c t Adsorption of four acid dyes onto cetyltrimethyl ammonium bromide (CTMAB) modiﬁed organobentonite from aqueous solution was studied, and effects of the parameters like adsorbent dosage, solution pH, contact time and dye concentration were investigated. The adsorption onto CTMAB-Bent attained equilibrium quickly within 10 min for Acid Blue 93 (AB93), Acid Turquoise Blue A (ATBA), Acid Golden Yellow G (AGYG) and 50 min for Acid Blue 25 (AB25). The adsorption capacity decreased slightly along with the increasing solution pH of 7.0. Four adsorption kinetics equations (pseudo-ﬁrst-order, pseudo-second-order, intraparticle diffusion and Elovich) were employed to evaluate experimental kinetics data. The rates of the acid dyes adsorption conformed to the pseudo-second-order equation well. The adsorption equilibrium data were analyzed with six models. Both linear and nonlinear regression methods were implemented to calculate isotherm parameters, and the nonlinear regression was more favorable. Regarding to the results of nonlinear analysis, the most applicable adsorption isotherm models were found to be Freundlich, Langmuir, Radke–Prausnitz, Fritz–Schlunder, and Sips equations. The removal ratios of the acid dyes by CTMAB-Bent were all above 88% at the experimental conditions, suggesting that CTMAB-Bent was an excellent adsorbent for acid dyes removal from aqueous solution. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Organic dyes are familiar pollutants in wastewater, which could generate a great deal of toxic substances. Their source can be tracked down from food, paper-making, leather, paint, plastics, cosmetics and textile industries, resulting in high organic pollutant content, deep color, and serious impact on water quality inevitably [1,2]. Dyes could be divided into natural and synthetic dyestuffs, and the synthetic dyestuff can be further classiﬁed into three categories: anionic dyes (direct, acid and reactive dyes), cationic dyes (basic dyes) and non-ionic dyes (disperse and vat dyes) . Acid dyes often contain carboxyl, hydroxyl and sulfonic groups, which are soluble in water. According to the different chemical structures of acid dyes, there are usually anthraquinonic, triphenylmethane and azo types, which were reported to be carcinogenic to human health and could exert negative inﬂuence to the environment and ecosystem as well [3–6]. Thus, it is a task of top priority to ﬁnd an effective process for treating dye wastewater. Several technologies have been developed for dye removal including adsorption, chemical and electrochemical oxidation, super ﬁlter ﬁlm, membrane separation, and coagulation [7,8]. Because of the cheap adsorbents in adsorption technique, adsorption has gradually become an economical and feasible method for dye wastewater decontamination ⁎ Corresponding author. E-mail addresses: [email protected]
, [email protected]
[3,9,10]. Some low-cost or easy-accessibility materials such as zeolite , layered double hydroxides [12,13], palygorskite , palm ash , blast furnace slag , oxide tailings  and ferric hydroxide  have been investigated in many research studies. The application of raw mineral adsorbents for wastewater treatment saw rapid growth in recent years. Bentonite, as a representative clay mineral, is primarily composed of montmorillonite, which consists of layers of two tetrahedral silica sheets sandwiching one octahedral alumina sheet [18–20]. Several properties are presented such as the large surface area, high swelling capacity and high cation exchange capacity . Bentonite has a permanent negative charge, caused by the isomorphous substitution of Al3+ for Si4 + in the tetrahedral layer and Mg2+ for Al3 + in the octahedral layer. The negative charge is balanced by the presence of exchangeable cations (Na+, Ca2+, etc.) in the lattice structure, which ensures its good performance in adsorbing cationic contaminants by cationic exchange . These inorganic cations can be substituted for cationic surfactant or hydroxy-metal, producing materials such as organobentonite and pillared bentonite . Organobentonite can be generated by adding quaternary ammonium cationic surfactants onto the bentonite external surface, and sheet-like inner layers as well . The organic intercalated modiﬁed bentonite, which changes the surface properties from hydrophilic to hydrophobic, proves to be an effective adsorbent for organic pollutants, such as dyes [25–28], phenol , benzoic acids , and other environmental
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pollutants [31,32]. Ma et al. revealed that the removal of Orange II and Orange G by four kinds of organobentonite was in high efﬁciency, and the adsorption capacity was inﬂuenced by the surfactant alkyl chain length . The longer alkyl chain surfactant was modiﬁed, the higher adsorption capacity organobentonite had. The simultaneous adsorption of − RB5 and Pb2+ by organobentonite exhibited synergic effect, and the adsorption capacity for both −RB5 and Pb2+ was higher in bicomponent than in single-component solutions . In this study, the frequently-used surfactant cetyltrimethyl ammonium bromide (CTMAB) was chosen to be an organic intercalation modiﬁer of bentonite for the acid dyes removal. Four acid dyes, Acid Blue 25 (AB25), Acid Blue 93 (AB93), Acid Turquoise Blue A (ATBA) and Acid Golden Yellow G (AGYG) were selected as the representative dye in wastewater. They are a group of synthetic colorants extensively used in textile and dye industries. Then, we aimed to evaluate adsorption properties of AB25, AB93, ATBA, AGYG onto CTMAB-Bent. Effects of adsorbent dosage, solution pH, contact time and adsorption isotherms were investigated in detail in batch experiments. Another objective of this study was to test various kinetics and equilibrium isotherm models because an exact mathematical equation represented the relation between equilibrium adsorption capacity and adsorbate concentration. 2. Materials and methods
66.89% SiO2, 20.91% Al2O3, 4.18% MgO, 2.88% CaO, 1.97% Fe2O3, 2.21% Na2O, 0.73% K2O and 0.23% others. 2.2. Preparation of CTMAB-Bent Firstly, the raw bentonite was converted to sodium bentonite, following the method described by Yu et al. . The CTMAB-Bent was prepared by adding 10 g of sodium bentonite to CTMAB solution in amounts equivalent to the CEC of sodium bentonite (0.882 mmol/g), and stirring for 24 h at room temperature. The suspensions were then separated by centrifuge, washed with deionized water several times until no Br− was detected in the supernatant. The results were ovendried at 80 °C, activated 1 h at 105 °C, then grounded to 100 mesh. This product was referred to as CTMAB-Bent. 2.3. Characterization methods The mineralogy, functional groups and surface properties were characterized using XRD, FTIR and BET respectively. The XRD data of the adsorbents were collected with a D/MAX 2200 diffractometer using Cu Kα radiation (Rigaku, Japan). The FTIR spectra were recorded with a Nicolet 400D spectrophotometer (Thermo, United States). Surface area measurements were performed on ASAP 2020 surface area and porosity analyzer (Micromeritics, United States).
2.1. Materials and chemicals 2.4. Adsorption experiments The dyes used for this study were purchased from Tianjin Damao Reagent Factory (Tianjin, China). The chemical structures and properties of the four acid dyes were depicted in Table 1. CTMAB, NaOH, HCl and AgNO3 were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd, China. All reagents were analytical grade and used as received. Natural raw bentonite used in this study was purchased from Fangzi Bentonite Plant (Weifang, China). According to the supplier, the raw bentonite contained about 91% montmorillonite, 5.5% quartz, 2.5% feldspar and 1.0% other minerals. Its chemical composition was as follows:
Dyes (AB25, AB93, ATBA, AGYG) stock solution of 100 mg/L were prepared in deionized water. The subsequent adsorption experiments were carried out in a series of glass centrifuge tubes, which contained 20 mL of dye solutions with a ﬁxed amount of CTMAB-Bent. For the experiments of effects of adsorbent dosage, contact time and solution pH, the dye concentrations were 100 mg/L for AB25, ATBA, AGYG and 150 mg/L for AB93, respectively. For the experiments of effects of contact time, solution pH and isotherm study, the CTMAB-Bent dosages
Table 1 Structures and properties of selected dyes. Molar mass (F.M.)
Acid Blue 25 (AB25)
Acid Blue 93 (AB93)
Acid Turquoise Blue A (ATBA)
Acid Golden Yellow G (AGYG)
L. Yan et al. / Journal of Molecular Liquids 211 (2015) 1074–1081
were 0.05 g for AB25, 0.06 g for AGYG, 0.10 g for AB93 and ATBA, respectively. These centrifuge tubes were capped and placed on an orbital shaker at 170 rpm to ensure equilibrium, and then centrifuged at 2500 rpm for 20 min. The residual concentration of dyes AB25, AB93, ATBA and AGYG in the supernatant was determined by a UV2450 spectrophotometer (Shimadzu, Japan) at wavelength of 560 nm, 663 nm, 637 nm and 440 nm, respectively. Each experiment was duplicated under uniform condition. The adsorbed dyes were measured by the differences between the initial and ﬁnal dye concentrations in the equilibrium solutions. Removal efﬁciency R (%) (Eq. (1)) and adsorption capacity q (mg/g) (Eq. (2)) were calculated using the following formulas: R¼
c0 −ct 100% c0
ðc0 −ct Þ V m
where c0 and ct (mg/L) were the initial and equilibrium concentration of each dye, V (mL) was the volume of dye solution and m (g) was the mass of CTMAB-Bent. 3. Results and discussion 3.1. Characterization of CTMAB-Bent The X-ray diffraction patterns of raw bentonite and CTMAB-Bent were shown in Fig. 1, exhibiting bentonite typical diffraction peak at 2θ of about 5°. The 2θ values of raw bentonite and CTMAB-Bent were 6.04 and 4.66, respectively. The peak shifts from the right side to the left side, indicating the expansion of d001 spacing and the completeness of crystal structure. The calculated d001 distance of 15.4 Å conformed to the main compensating cations of calcium and magnesium. After being CTMAB modiﬁed, the d001 value increased to 18.97 Å, suggesting that the surfactant CTMA+ ions entered into the interlayer space of the bentonite by ion exchange. The FTIR spectra of raw bentonite and CTMAB-Bent, in the range of 4000 to 400 cm− 1, were given in Fig. 2. The peaks at 1039, 520 and 470 cm−1 were due to stretching vibration of Si–O, bending vibration of Si–O–Al and Si–O–Si, respectively, exhibiting the initial structure of raw bentonite. The bands at 2920 and 2851 cm−1 were only observed for CTMAB-Bent. They could be assigned to the anti-symmetric and symmetric stretching vibration modes of –CH3 and –CH2. In addition, the banding vibration of –CH3 was observed at 1469 cm−1 in organic
Fig. 1. XRD patterns of raw bentonite and CTMAB-Bent.
Fig. 2. FTIR spectra of raw bentonite and CTMAB-Bent.
intercalator. The relative wide peaks at 1640, 3440 and 3632 cm−1, observed in both raw bentonite and CTMAB-Bent, were attributed to –OH deformation of water, but the peak intensity of CTMAB-Bent was lower than that of raw bentonite. This may be the evidence for the increased hydrophobic nature of bentonite surface due to the CTMAB addition [6,36,37]. From the results of BET surface area measurement, the speciﬁc surface area of CTMAB-Bent (4.42 m2/g) was smaller than that of raw bentonite (10.2 m2/g), indicating that organic molecules entered into the interlayers of bentonite and overlapped its surface, thus blocking the channel between the layers and reducing the speciﬁc surface area . The pore volume and pore size diameter of CTMAB-Bent and raw bentonite were 0.0179 cm3/g, 0.0307 cm3/g, 3.12 nm and 3.52 nm, respectively. 3.2. Optimization of experimental variables 3.2.1. Effect of adsorbent dosage Effects of raw bentonite and CTMAB-Bent dosage on the four acid dyes adsorption were listed in Fig. 3. The adsorption amount of the acid dyes by CTMAB-Bent increased with the increasing amount of adsorbent. For AB25 and AB93, the adsorption ratio could reach up to 80% when the adsorption dosage was only 0.02 g. For ATBA and AGYG, the adsorption ratio rose quickly, and the equilibrium state was achieved at 0.05 g. At the ﬁxed concentration of AB25 (100 mg/L), AB93 (150 mg/L), ATBA (100 mg/L) and AGYG (100 mg/L) with CTMAB-Bent dosages of 0.10 g, the maximum remove ratios were 88.71%, 93.30%, 97.62% and 94.42%, respectively, while the adsorption ratios were all below 30% for raw bentonite. These results indicated that the capacity of CTMAB-Bent to the four acid dyes was strongly reinforced after being surfactant modiﬁed, since the surface properties changed from hydrophilic to hydrophobic. 3.2.2. Effect of initial solution pH Generally, solution pH is recognized as an important parameter that dominates the adsorption process at water–solid interfaces. As listed in Fig. 4, the adsorption capacity kept decreasing with the increasing of solution pH. For AB25, the removal efﬁciency decreased sharply at the pH of 2–3, then changed little with further increase of pH to 8. For AB93, ATBA and AGYG, the adsorption capacity remained stable at the pH of 2–7. All the four acid dyes adsorption capacity began to decrease significantly above pH 8. The similar trend of acidity effect can be found in the adsorption on Acid Red 151, using Na-Bent and CTMAB-Bent as adsorbent . The pH of natural wastewater is usually in the range of 6–9,
L. Yan et al. / Journal of Molecular Liquids 211 (2015) 1074–1081
Fig. 3. Effect of raw bentonite and CTMAB-Bent dosage on the adsorption of AB25 (a), AB93 (b), ATBA (c) and AGYG (d).
so it does not need to adjust the solution pH in view of practical application. 3.2.3. Effect of contact time Fig. 5 shows the effect of contact time on the adsorption of acid dyes onto CTMAB-Bent. It was clear the initial adsorption
process of AB93, ATBA and AGYG was relatively fast, and the adsorption reached equilibrium in less than 10 min. However, the curve of AB25 by CTMAB-Bent indicated that the adsorption was slow growth up to equilibrium at 50 min. Then in the following experiments, 60 min was selected to ensure adsorption equilibrium.
Fig. 4. Effect of initial solution pH on adsorption of the acid dyes onto CTMAB-Bent.
Fig. 5. Effect of contact time on the acid dyes adsorption by CTMAB-Bent.
L. Yan et al. / Journal of Molecular Liquids 211 (2015) 1074–1081
As shown above, the best experimental conditions achieved for 100 mg/L AB25, 150 mg/L AB93, 100 mg/L ATBA and 100 mg/L AGYG were: CTMAB-Bent dosages of 0.05 g, 0.10 g, 0.10 g and 0.06 g, respectively, contact time of 60 min and natural dye solution pH. 3.3. Adsorption kinetics In order to ﬁnd the best-ﬁt kinetics model of adsorption of acid dyes onto CTMAB-Bent, the pseudo-ﬁrst-order, pseudo-second-order, intraparticle diffusion and Elovich kinetics model (Table 2) were applied to test the experimental data. The pseudo-ﬁrst-order kinetics equation can describe the initial phase in adsorption process and with the progressing of adsorption, the adsorption data may deviate the ﬁtted curve. The pseudo-second-order equation agrees with chemisorption as the rate-control mechanism . According to intraparticle diffusion model, several mechanisms are involved and the adsorption process can be characterized into three steps: external surface adsorption, intraparticle diffusion which is the rate-limiting step, and the ﬁnal equilibrium which is very fast . Elovich equation was used to describe the rate of adsorption that decreased exponentially with an increase in the amount of adsorbate but did not provide any mechanistic information about adsorption process. The values of these parameters obtained from the linear plots were comprised in Table 2. As shown in Fig. 6, the adsorption kinetics data ﬁt the pseudo-second-order equation very well. The correlation coefﬁcients R2 were all above 0.99. The adsorption of AB25, AB93 and ATBA could be ﬁt by the pseudo-ﬁrst-order kinetic model, and the R2 were 0.9387, 0.9885 and 0.9283. Intraparticle diffusion and Elovich model can describe the adsorption process of AB25 and AB93 by CTMABBent. This indicated that the adsorption rate of AB25, AB93, ATBA and AGYG onto CTMAB-Bent was dominated by the combined effect of surface adsorption, liquid diffusion process and internal surface adsorption. The results share similarity with that presented in the literature by Tabak et al. , in which the kinetics of Reactive Red 120 adsorption onto cetylpyridinium-bentonite was explained with the pseudosecond order equation. As shown in Table 2, values of b, the intercept of the second linear section, were proportional to the boundary layer thickness, and slope of the linear portion indicated the rate of the adsorption. Considering the two parameters, AB25 and AB93 exhibited good performance of the intraparticle diffusion process, while for ATBA and AGYG the intraparticle diffusion was not the only step controlled rate, boundary layer diffusion also controlled the adsorption to some extent . 3.4. Adsorption isotherms Equilibrium isotherms are frequently applied to elucidate the experimental data of equilibrium adsorption. To acquire the best-ﬁt isotherm
Fig. 6. Linear ﬁt of the pseudo-second-order kinetics equation for the acid dyes adsorption on CTMAB-Bent.
and conﬁrm the adsorption mechanism, several isotherm models were used to analyze the adsorption data. Apart from such regularly applied models as Langmuir  and Freundlich , some less-used models (Temkin , Radke–Prausnitz , Sips  and Fritz–Schlunder ) were cited as well (Table 3). The calculated parameters by linear ﬁtting of Langmuir, Freundlich and Temkin models were summarized in Table 4. It was found that the best ﬁtting was obtained by the Freundlich model (R2 N 0.93) and Langmuir-II model (R2 N 0.86). This was in agreement with other works on the adsorption of acid dyes by clays and organoclays [15,28, 36,49]. The Freundlich equation is often applied for nonideal adsorption on heterogeneous surfaces and multilayer adsorption . It assumes that adsorbent surface sites have a spectrum of different binding energies. The constant n is also a measure of the deviation from linearity of adsorption. With n ≈ 1, the adsorption is practically considered linear. This means that the adsorption sites are homogeneous in energy and no interaction occurs between the adsorbed species; when n b 1, the isotherm is concave downward and implies that added adsorbates are bound with weaker and weaker free energies; when n N 1, the isotherm is convex upward and infers that more adsorbate presence in the adsorbent enhances the free energies of further sorption . The linear ﬁt of adsorption isotherms (Fig. 7) and n values (Table 4) indicated that partition was apparently the dominant mechanism for acid dye adsorption by CTMAB-Bent. Here, the high degree of adsorption into the CTMAB
Table 2 Adsorption kinetics equation and linear ﬁt parameters for the adsorption of acid dyes on CTMAB-Bent. Model
k1 lgðqe −qt Þ ¼ lgqe − 2:303 t
qe k1 R2 qe k2 R2 b kp R2 a b1 R2
10.21 0.05283 0.9387 20.63 0.007373 0.9982 9.431 1.277 0.9035 15.51 0.2955 0.9484
5.497 0.04816 0.9885 55.28 0.02353 0.9999 49.37 0.7417 0.9235 5.01 · 1011 0.5548 0.9682
9.853 0.05057 0.9283 98.04 0.01858 0.9998 87.73 1.349 0.8278 3.57 · 1013 0.3509 0.8789
31.41 0.07010 0.8826 166.9 0.03470 0.9990 111.7 7.512 0.5602 2.50 · 103 0.0532 0.6378
¼ k 1q2 þ qt 2 e
1 qt ¼ kp t =2 þ b
qt ¼ b11 lnðab1 Þ þ b11 lnt
qt (mg/g) is the amount of dye adsorbed at time t (min), qe (mg/g) is the equilibrium adsorption capacity, k1 (1/min) is the rate constant of the pseudo-ﬁrst-order equation, k2 (g/(mg · min)) is the constant rate of the pseudo-second-order equation, kp (mg/(g·min1/2)) and b are the rate constant and intercept of intraparticle diffusion equation, a (mg/(g·min)) and b1 (g/ mg) are the initial adsorption rate and parameter related to the extent of surface coverage and activation energy of Elovich equation.
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Table 3 Adsorption isotherm models and their linear forms. Isotherm
bce qm 1þbc e
Langmuir-II Langmuir-III Freundlich
qe ¼ k f ce n qe = B1 ln(A1ce)
Linear form of equation
ce qe 1 qe qe ce
¼ qce þ bq1 m
¼ bq1 c1e þ q1 m
¼ bqm −bqe
lnqe ¼ lnk f þ 1n lnce
qe = B1 ln A1 + B1 ln ce
qm K RP ce qe ¼ 1þK m RP c e ms
qs K s c e qe ¼ 1þK cms
s e Aceα 1þBcβe
qe (mg/g) is the amount adsorbed, and ce (mg/L) is the equilibrium concentration of adsorbate in aqueous phases, qm (mg/g) is the maximum adsorption capacity, b (L/mg) is the Langmuir constant related to adsorption energy, kf (L/g) is the Freundlich adsorption constant and n is the Freundlich constant which is correlated with adsorption capacity and adsorption intensity, B1 is the constant related to heat of sorption and A1 (L/mg) is the Temkin equilibrium isotherm constant, KRP (1/mg) is the Radke–Prausnitz isotherm model constant and m is the relevant exponent, qs (mg/g) is the Sips maximum adsorption capacity and Ks ((L/mg)ms) is the Sips equilibrium constant and ms is the Sips model exponent, A ((mg/g)/(mg/L)α) and B ((mg/g)/(mg/L)β) are the Fritz–Schlunder constants, α and β (both of them ≤ 1) are the Fritz–Schlunder equation exponents.
phase substantially changed the composition of the phase. This caused the solvency (i.e., the solubilization capability) of CTMAB phase for dye to increase, so that the isotherms became convex to the abscissa. A similar phenomenon was observed in the partition effect of the acid dyes exerted on surfactant organic phase of CTMAB-Bent . The Langmuir equation describes the adsorption process based on the assumption that the maximum adsorption corresponds to the saturated monolayer adsorption by adsorbate on speciﬁc homogeneous sites with a constant energy . In this part, the acid dye adsorption isotherm data were analyzed by three linear forms of Langmuir equation. Much literature [45,52,53] pointed out that the Langmuir-I was the best ﬁtted curve, but it turned out that this was not always the case. Langmuir-II model exhibited higher correlation than Langmuir-I form except AGYG. While the R2 values for Langmuir-III ﬁtting were lower than that of Langmuir I and Langmuir II. For most the case, the application of linear regression analysis alone is not a convenient way for decision on the best ﬁtting models, thus the nonlinear regressions of six isotherm models were applied to analyze the acid dyes equilibrium data as well. The obtained parameters were listed in Table 5. The nonlinear ﬁt of Langmuir and Freundlich in the same condition was presented in Fig. 8. Compared with the linear and nonlinear form of the same isotherm equation (Tables 4 and 5), it could be seen that the nonlinear form had a better ﬁtting curve. The parameters obtained by linear and nonlinear method were very close to each other. Moreover, as seen from the Langmuir equation ﬁtting data,
Table 4 Calculated parameters of adsorption isotherm equations by linear ﬁtting for the acid dyes onto CTMAB-Bent. Model
qm b R2 qm b R2 qm b R2 kf n R2 B1 A1 R2
195.7 0.01125 0.5359 105.2 0.0246 0.9774 185.9 0.01233 0.2961 2.680 1.179 0.9682 20.57 0.3933 0.8332
250.6 0.01728 0.9301 833.3 0.004651 0.9975 268.2 0.01596 0.7835 4.809 1.157 0.9841 30.76 0.4360 0.9077
490.2 0.07275 0.8792 298.5 0.1384 0.9931 526.2 0.06776 0.5565 35.01 1.318 0.9769 74.08 1.551 0.8456
324.7 0.09818 0.9956 296.7 0.1921 0.8694 307.4 0.1682 0.8105 160.7 4.744 0.9333 44.98 5.429 0.9660
Fig. 7. Linear ﬁt of Freundlich isotherm model for the acid dyes on CTMAB-Bent.
the maximum absorption capacities (qm) of CTMAB-Bent for AB25, AB93, ATAB and AGYG were 360.8 mg/g, 226.0 mg/g, 487.2 mg/g and 304.7 mg/g, respectively. The reported adsorption capacities of some other adsorbents for acid dyes were presented in Table 6 for comparison. It was clear that CTMAB-Bent showed adsorption capacities superior to the other sorbent materials. This indicated that the prepared organobentonite was an effective adsorbent for acid dyes. From Table 5, it was obvious that the isotherm equations with three or four parameters (Radke–Prausnitz, Sips and Fritz–Schlunder) exerted better performance than two-parameter isotherm models (Temkin), which was in accordance with previous studies . Radke–Prausnitz isotherm is an empirical equation and was used initially to ﬁt organic solvents over a broad concentration range . The R2 values of Radke–Prausnitz isotherm were calculated as 0.987, 0.998, 0.965, and 0.961, respectively. Sips isotherm is a hybrid of Langmuir and Freundlich isotherm models and is based on the theory of a homogenous–heterogeneous adsorbent surface. At low adsorbate concentrations it turns into Freundlich isotherm, while at high adsorbate concentrations it reduces a monolayer adsorption capacity characteristic of Langmuir isotherm . For the four acid dyes, the R2 values obtained for the Table 5 Calculated parameters of adsorption isotherm equations by nonlinear ﬁtting for the acid dyes onto CTMAB-Bent. Model
qm b R2 kf n R2 B1 A1 R2 qm KRP m R2 qs Ks ms R2 A α B β R2
360.8 0.005110 0.9863 2.606 1.175 0.9920 20.57 0.3933 0.8332 12.39 0.2128 0.3527 0.9871 237.4 0.008420 0.9994 0.9959 62.37 3.867 23.91 3.016 0.9880
226.0 0.02015 0.9983 7.134 1.384 0.9911 30.76 0.4360 0.9077 213.6 0.02143 0.9865 0.9980 216.4 0.0203 1.019 0.9980 3.498 1.627 0.2294 1.095 0.9984
487.2 0.07561 0.9609 44.76 1.568 0.9354 74.08 1.551 0.8456 690.8 0.05877 1.068 0.9650 349.1 0.06598 1.430 0.9587 23.07 1.214 0.009080 1.815 0.953
304.7 0.1684 0.9190 112.5 5.061 0.9511 44.98 5.429 0.9660 161.3 0.7135 0.8724 0.9612 421.6 0.2613 0.4505 0.9582 67.45 1.515 0.4688 1.368 0.9524
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adsorbent for the acid dyes wastewater treatment because of its convenience preparation and low cost.
Acknowledgments This work was funded by the Science & Technology Development Project of Jinan (201303081), the Natural Science Foundation of Shandong Province (ZR2014BL033), the Natural Science Foundation of China (21377046) and the Special Project for Independent Innovation and Achievements Transformation of Shandong Province (2014ZZCX05101).
Fig. 8. Nonlinear ﬁt of Freundlich and Langmuir isotherm models for the acid dyes on CTMAB-Bent.
Sips isotherms were all above 0.95. As for the four-parameter equations Fritz-Schlunder model, which is another Langmuir–Freundlich type developed empirically by Fritz and Schlunder , the results demonstrated favorable work as well. 4. Conclusions In this study, the adsorption of acid dyes AB25, AB93, ATBA and AGYG onto CTMAB modiﬁed bentonite was performed. Such effects of the experimental conditions as adsorbent dosage, contact time, initial solution pH were examined. The adsorption reached equilibrium after contacting with CTMAB-Bent of 10 min for AB93, ATBA, AGYG and 50 min for AB25. With the increasing of initial solution pH to about 7, the adsorption capacities kept decreasing slightly. The experimental data of adsorption kinetics and isotherms were modeled by four kinetics and six isotherm equations, respectively. The kinetics data of the acid dyes adsorption were very well represented with the pseudo-second order kinetic model. The equilibrium isotherms data of AB25, AB93, ATBA adsorption onto CTMAB-Bent best ﬁt to the Freundlich and Langmuir-II equations, while AGYG adsorption data ﬁt to the Freundlich and Langmuir-I equations for regarding to the linear analysis. The nonlinear regression results indicated that the two parameters equations of Langmuir and Freundlich, three or four parameters equations of Radke– Prausnitz, Sips and Fritz–Schlunder were all ﬁt the adsorption isotherms data well. As a result, the nonlinear method was the more convenient way than the linear method to determine the best-ﬁtting equilibrium isotherm models. The adsorption capacity of the four dyes followed the order of ATAB N AGYG N AB93 N AB25. The maximum removal ratios were above 88%, 93%, 97% and 94% for 100 mg/L AB25, 150 mg/L AB93, 100 mg/L ATBA and 100 mg/L AGYG with CTMAB-Bent dosages of 0.10 g, respectively. Thus, CTMAB-Bent can be preferred as a promising
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Table 6 Reported sorption capacities of adsorbent for acid dyes. Adsorbent
Adsorption capacity (mg/g)
Monoamine modiﬁed silica particles
Acid Orange 10 Acid Orange 12 Acid Red 88 Acid Red 1 Acid Green 25 Acid Turquoise Blue A Acid Scarlet GR Acid Dark Blue 2G
36.28 14.29 111.1 108 123.4 2.639 47 63
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