A Comparative Study on the Adsorption of Acid and Reactive Dyes on

Apr 25, 2012 - Modern industrial processes and households produce a lot of wastewater ... recycle of wastewater provide a solution to water shortage i...
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A Comparative Study on the Adsorption of Acid and Reactive Dyes on Multiwall Carbon Nanotubes in Single and Binary Dye Systems Shaobin Wang,*,† Choon Wei Ng,† Wentai Wang,† Qin Li,†,‡ and Liqing Li§ †

Department of Chemical Engineering, Curtin University, G.P.O. Box U1987, Perth, WA 6845, Australia Environmental Engineering & Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan Campus, Queensland 4111, Australia § School of Energy Science and Engineering, Central South University, Changsha 410011, P.R. China ‡

ABSTRACT: The adsorption of reactive blue 4 (RB4) and acid red 183 (AR183) on a multiwall carbon nanotube (MWCNT) was investigated in single and binary dye systems. The MWCNT presented a higher adsorption of RB4 than AR183 in single and binary dye systems, due to stronger interactions. In single dye solutions, adsorption capacities of the MWCNT for RB4 and AR183 at 25 °C are (69 and 45) mg·g−1, respectively. The higher temperature resulted in lower adsorption of the dyes on the MWCNT. In binary dye solutions, RB4 and AR183 showed competitive adsorption, resulting in RB4 adsorption but desorption of AR183 on the MWCNT. RB4 adsorption capacity in binary dye systems was also reduced to 56 mg·g−1 at 25 °C. The interaction of the dyes with MWCNT was found to be dominated by electrostatic attraction.

1. INTRODUCTION Modern industrial processes and households produce a lot of wastewater each year. Treatment and recycle of wastewater provide a solution to water shortage in the world. There are many different types of contaminants presented in wastewater. Dyes are usually present in the effluent water from textile, leather, paper, printing, and cosmetics industries. The removal of the dyes from water can be achieved via several techniques such as physical, chemical, and biochemical processes. The adsorption technique has proven to be an effective and attractive process for the treatment of these dye-bearing wastewaters.1−3 Currently, many adsorbents at low cost have been tested for dye removal from wastewater.4−9 Carbon nanotubes (CNTs) are new carbon materials and have demonstrated many good applications. Due to their large specific surface area, small size, and hollow and layered structures, carbon nanotubes have been proven to possess a great potential as superior adsorbents for removing many kinds of organic and inorganic contaminants from water.10−12 For dye removal, however, few investigations have been reported. Wu investigated adsorption equilibrium, kinetics, and thermodynamics of a reactive dye, Procion Red MX-5B, on carbon nanotubes at various pH and temperatures.13 The adsorption increased with the CNTs dosage with the maximum dosage at 0.25 g·L−1. Kuo et al.14 examined the adsorption of two direct dyes, C.I. Direct Yellow 86 and C.I. Direct Red 224, on carbon nanotubes and evaluated the effects of dye concentration, CNT dosage, ionic strength, and temperature on adsorption of the direct dyes by CNTs. The adsorption © 2012 American Chemical Society

percentage of the direct dyes increased as CNT dosage, NaCl addition, and temperature increased. Conversely, the adsorption percentage decreased as dye concentration increased. Gong et al.15 reported the removal of cationic dyes, methylene blue, neutral red, and brilliant cresyl blue, from aqueous solution using a magnetic multiwall carbon nanotube nanocomposite. They investigated adsorption kinetics, adsorption capacity of the adsorbent, and the effects of adsorption dosage and solution pH on the removal of cationic dyes. Yao et al.16 also investigated the effect of temperature on the equilibrium adsorption of methylene blue dye from aqueous solution using carbon nanotubes. Mishra et al.17 prepared functionalized multiwalled carbon nanotubes (f-MWNTs) for the adsorption of three different azoic dyes. Ghaedi et al.18 tested a multiwalled carbon nanotube for the removal of Alizarin Red S (ARS) and morin from wastewater. In those works, dye adsorption on carbon nanotubes was all carried out in single dye system, and no investigation has been reported on the adsorption of dyes in multiple dye systems. In real wastewater, generally a variety of dyes would be in presence and these dyes will show different adsorption behavior. Dye adsorption could be in competitive adsorption or other modes depending on dye and adsorbent. In this work, we report an investigation of reactive and acid dye adsorption on a MWCNT in single and binary dye systems. The interaction of dye-MWCNT and the effect of second dye on Received: February 7, 2012 Accepted: April 18, 2012 Published: April 25, 2012 1563

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primary dye adsorption on MWCNT will be comprehensively investigated.

Table 1. Elemental Compositions of MWCNT before and after Dye Adsorption elemental content (%)

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. A multiwall carbon nanotube (MWCNT) was obtained from Chengdu Organic Chemicals Co. Ltd., China. The MWCNT is about 30 μm long with an outer diameter of (10 to 20) nm. The Brunauer− Emmett−Teller (BET) surface area determined by N 2 adsorption (Quantachrome AS1, USA) was found to be 217 m2·g−1. Acid red 183 (AR183, C.I. 18800, C16H11ClN4Na2O8S2) and reactive blue 4 (RB4, C.I. 61205, C23H14Cl2N6O8S2) were purchased from the Sigma-Aldrich Chemicals. 2.2. Characterization of MWCNT. FT-IR spectra were collected on a Perkin-Elmer Spectrum 100 (USA) with a resolution of 4 cm−1 in a transmission mode at room temperature. Thermogravimetric analysis (TGA) was performed by heating the samples in an air flow at a rate of 25 mL·min−1 using a Perkin-Elmer Diamond TG/DTA thermal analyzer (USA) with a heating rate of 10 °C·min−1. Zeta potential was determined on a Malvern Instrument Zetasizer Nano-ZS (USA) at room temperature. The sample was dispersed in Milli-Q water and sonicated for 15 min before measurement. Elemental analysis was conducted on a PerkinElmer Series II Analyzer 2400, CHNS mode (USA) was selected with a combustion temperature of 968 °C and a reduction temperature of 498 °C. 2.3. Adsorption Tests. Adsorption tests of single and binary dye systems were carried out in a batch mode. For single dye adsorption, 100 mL dye solutions at varying dye concentrations were prepared, and 0.02 g of carbon nanotube was mixed with the solutions. The solutions were controlled at (25 or 35) °C under constant stirring at a rate of 700 rpm. In addition, solution pH in all of the tests was maintained at pH = 6.0. At varying time, a 5 mL solution was withdrawn and put in a centrifuge to separate carbon from the solution at 4700 rpm for one min. The dye concentration in the clean solution was measured by a METERTECH UV−vis spectrophotometer (USA). The wavelengths at the maximum absorption for acid red 183 and reactive blue 4 are (485 and 600) nm, respectively. For binary dye systems, 100 mL solutions were prepared with one dye at fixed concentration and the other dye at varying concentrations. Carbon nanotube at 0.02 g was mixed with the dye solutions. The absorbance was measured at two wavelengths of (485 and 600) nm. The dye concentration was determined by the following equations.6 CA =

CB =

sample

C

H

N

MWCNT AR183/MWCNT RB4/MWCNT AR183-RB4/MWCNT

98.33 94.41 91.85 93.02

0.12 0.10 0.06 0.12

0.06 0.47 0.94 0.88

S

O

0.03 0.25 0.26

1.5 4.99 6.9 5.72

and O with a carbon content of more than 98 %. The oxygen species is 1.5 %, attributing to the presence of some oxygencontaining groups. The synthesis of MWCNTs is usually involved in acid washing, which will bring in surface carbon oxidation. Zeta potential measurement showed a positive value of 11.5 mV of MWCNT at pH 6, suggesting the positive charge of the MWCNT surface. Figure 1 shows FTIR spectrum of the MWCNT, and some peaks can be observed. The weaker bands at (1400 to 1700) cm−1 were assigned to the CO stretch mode of carboxylic acid and aliphatic ketones. The bands at (3500 to 3700) cm−1 were due to hydroxyl groups.

k B2A1 − k B1A 2 k A1k B2 − k A 2k B1

(1)

k A1A 2 − k A 2A1 k A1k B2 − k A 2k B1

(2)

where kA1, kB1, kA2, and kB2 are the calibration constants for components A and B at wavelength λ1,max and λ2,max, respectively. A1 and A2 are the absorbance at wavelength λ1,max and λ2,max, respectively.

3. RESULTS AND DISCUSSION 3.1. Dye Adsorption in Single-Dye Solutions. Elemental analysis (Table 1) indicates that MWCNT contains C, H, N,

Figure 1. FTIR spectra of MWCNT before and after dye adsorption. 1564

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Figure 2 shows the differential thermogravimetric/thermal analysis (DTG-DTA) profiles of MWCNT in air. The weight

Figure 3. Kinetics of dye adsorption on MWCNT in a single dye system at 25 °C. ○, AR183; □, RB4; , second-order kinetics.

the sorption capacity at equilibrium (mg·g−1). Table 2 presents the parameters from the kinetic model. As shown, the model fit Table 2. Parameters of the Pseudosecond-Order Kinetics of Dye Adsorption on MWCNT

loss curve of MWCNT showed that the MWCNT presented a strong weight loss at (500 to 700) °C, which is attributed to carbon combustion. Meanwhile, DTA showed two strong peaks at (550 to 700) °C, confirming the exothermic reaction of carbon oxidation. Figure 3 displays the dye adsorption on MWCNT at varying time in single dye system. For RB4 and AR183, dye adsorption showed a fast rate, and equilibrium could be reached in 30 min. The MWCNT showed a higher capacity of RB4 adsorption than that of AR183. Table 1 displayed elemental analysis of the MWCNT after dye adsorption. One can see that N, S, and O contents on the MWCNT were all enhanced after dye adsorption, confirming the loading of the dyes on the MWCNT. For RB4/MWCNT, S, N, and O contents are much greater than those on AR183/MWCNT, corroborating the higher adsorption of RB4 on the MWCNT. The variation of dye adsorption against time was evaluated using a pseudosecond-order kinetics as listed in eq 3. qe 2k 2t 1 + qek 2t

k2

qe,cal

mg·g−1

g·mg−1·min−1

mg·g−1

R2

RB4 AR183

58.8 45.3

0.013 0.032

57.9 46.7

0.993 0.990

well to the experimental data with regression coefficients higher than 0.990. The calculated values of RB4 and AR183 equilibrium adsorption are close to those values from experiments. AR183 adsorption showed a faster rate constant. Figure 1 also shows FTIR spectra of MWCNT after RB4 or AR183 adsorption. For AR183-MWCNT, the bands at (1054, 1142, and 1400) cm−1 were observed, which are attributed to the sulfonate group of the adsorbed dye. A strong band at 1505 cm−1 corresponds to the −NN− group. For RB4/MWCNT, similar peaks were observed. The band at (1054, 1145, and 1404) cm−1 were assigned to the sulfonate group of RB4. A strong peak at 1518 cm−1 is referred to the −NN− group. Therefore, FTIR confirms the adsorption of the two dyes on the MWCNT. In comparison with MWCNT, carboxylic groups ((1404 and 1495) cm−1) on AR183/MWCNT ((1404 and 1495) cm−1) showed no change, while a slight shift was found on RB4/MWCNT ((1403 and 1517) cm−1). These suggest that the functional groups on MWCNT may not be involved in the dye adsorption. Figure 2 further displays DTG-DTA profiles of dye-adsorbed MWCNT samples in air. For RB4/MWCNT, combustion was occurring at lower temperatures with the TG peak centered at 630 °C. DTA profile presents two peaks at (580 and 670) °C, respectively. AR183/MWCNT showed an even lower combustion with TG peak centered at 600 °C and also presented two peaks in the DTA profile, which were further shifted to (530 and 675) °C, respectively. Therefore, adsorption of the dyes on MWCNT introduced functional groups, resulting in a reduced stability of MWCNT. The combustion at lower temperatures for AR183/MWCNT may

Figure 2. DTG-DTA profiles of MWCNT before and after dye adsorption in air atmosphere.

qt =

qe,exp dye

(3) −1

where qt is the amount of dye adsorbed at time t (mg·g ), k2 is the pseudosecond-order rate constant (g·mg−1·min−1), and qe is 1565

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where qe represents equilibrium dye concentration on an adsorbent (mg·g−1), Ce represents equilibrium dye concentration in solutions (mg·L−1), qmax is the monolayer capacity of the adsorption (mg·g−1), and KL represents the Langmuir adsorption constant. The Freundlich equation is:

suggest low stability of the composite and less interaction between AR183 and MWCNT. For dye adsorption on MWCNT, few investigations have been reported, and the interaction of dye with MWCNT is not well-elucidated. In this investigation, the zeta potential showed a positive value of MWCNT at pH 6, which suggests a positive charge on the MWCNT surface. The two dyes, RB4 and AR183, are both anionic dyes, which show a negative charge in solution. Due to electrostatic attraction, both dyes present strong adsorption on MWCNT. The FTIR spectrum of MWCNT showed the presence of carboxylic and hydroxyl groups. However, these functional groups did not provide strong chemical binding with dyes. Therefore, it is deduced that electrostatic attraction will be the key driving force for dye adsorption on the MWCNT. Figure 4 displays RB4 and AR183 adsorption isotherms on MWCNT at varying temperatures. As shown, the MWCNT

qe = KFCe1/ n

where qe represents equilibrium dye concentration on an adsorbent (mg·g−1), Ce represents equilibrium dye concentration in solutions (mg·L−1), and KF and n are the Freundlich constants, characteristics of the system. Table 3 gives the isotherm parameters from the two models from dye adsorption isotherms. As can be seen, the both isotherm models produced good results. The Langmuir isotherm presented better fit to the experiments in terms of regression coefficients. Some investigations have been reported on AR183 and RB4 adsorption on various materials. Aydin et al.19 used walnut shells as an adsorbent for the removal of AR183 at (35 to 45) °C and found the capacity to be (37 to 45) mg·g−1. Genc and Oguz20 investigated AR183 adsorption on granulated blast furnace slag (GBFS) and furnace bottom ash (FBA) and found that AR183 adsorption capacity on FBA to be 0.8 mg·g−1. Atar et al.6 studied the adsorption of AR183 and RB4 on boron industry wastes and reported the adsorption capacities of AR183 and RB4 in single dye solutions as (53.3 and 46.7) mg·g−1, respectively. Ibrahim et al.21,22 investigated different surfactant-modified barley straws for RB4 adsorption. The maximum adsorption capacity was (29.2 to 31.5) mg·g−1 at 25 °C. Therefore, it is seen that the MWCNT presents a better adsorption capacity than those low-cost adsorbents reported. Thermodynamic parameters, that is, free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes were also calculated using eqs 6 to 84,23 and are given in Table 4.

Figure 4. Adsorption isotherms of dye adsorption on MWCNT in single dye system at (25 and 35) °C. ○, AR183, 25 °C; ●, AR183, 35 °C; □, RB4, 25 °C; ■, RB4, 25 °C; solid black line, Langmuir isotherm; dashed red line, Freundlich isotherm.

Table 4. Thermodynamic Parameters for the Adsorption of RB4 and AR183 Adsorption on MWCNT ΔG°/(kJ·mol−1)

exhibited higher RB4 adsorption than AR183. For both dyes, the temperature significantly influenced adsorption capacity of dyes. Higher temperature resulted in lower adsorption for both dyes. The adsorption capacities of AR183 at (25 and 35) °C are approaching to (45 and 37) mg·g−1, respectively, while the adsorption capacities of RB4 at (25 and 35) °C are (62 and 50) mg·g−1, respectively. The adsorption isotherms of RB4 and AR183 were fitted using two isotherm models, Langmuir and Freundlich isotherms. The well-known expression of the Langmuir model is: qe =

qmax KLCe 1 + KLCe

(5)

(4)

ΔH° −1

dye

25 °C

35 °C

kJ·mol

AR183 RB4

−14.3 −11.7

−15.7 −17.5

−27.3 −161.7

ΔS° J·mol−1·K−1 −40.5 −485.3

ΔG 0 = −RT ln K1

(6)

⎛ TT ⎞ K ΔH 0 = −R ⎜ 2 1 ⎟ln 2 ⎝ T2 − T1 ⎠ K1

(7)

Table 3. Isotherm Parameters of RB4 and AR183 Adsorption on MWCNT in a Single Dye System at Different Temperatures Langmuir isotherm

Freundlich isotherm

dye

T/°C

qmax/(mg·g−1)

KL/(L·mg−1)

R2

KF

1/n

R2

AR183

25 35 25 35

45.2 37.4 68.2 53.0

0.616 0.881 0.185 1.925

0.992 0.993 0.976 0.949

27.6 26.0 37.0 51.9

0.129 0.0958 0.123 5.77·10−19

0.982 0.990 0.953 0.947

RB4

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ΔH 0 − ΔG 0 T

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profile as RB4/MWCNT. TGA-DTA profiles (Figure 2) show that the combustion temperature of RB4-AR183/MWCNT is similar to RB4/MWCNT with a slight difference. The maximum heat flow temperature is closer to that of RB4/ MWCNT but much higher than that of AR183/MWCNT. These results suggested that the MWCNT showed dominant adsorption of RB4, not AR183 in RB4-AR183 binary systems. Thermodynamic studies of single dye system have indicated stronger interactions of RB4 with MWCNT, due to chemical adsorption and thus resulting in RB4 dominant adsorption in RB4-AR183 binary systems. Figure 6 shows a RB4 adsorption isotherm on MWCNT at a fixed concentration of AR183 in binary dye systems. At the

(8)

where K1 and K2 are the Langmuir constants at T1 = 25 °C and T2 = 35 °C. The negative values of ΔG° indicate the feasibility and spontaneous nature of two dye adsorption on MWCNT. The change in enthalpy (ΔH°) for both dyes was found to be negative (Table 4). The negative values confirm the exothermic nature of adsorption. The negative values of the entropy change show the decreased affinity of the MWCNT toward the dyes. In general, adsorption can be ascribed to physical or chemical adsorption. ΔH° for physical adsorption ranges from (−4 to −40) kJ·mol−1, compared to that of chemical adsorption ranging from (−40 to −800) kJ·mol−1. The values in Table 4 may suggest that RB4 adsorption on MWCNT is much stronger, possibly due to chemical binding. TGA-DTA profiles (Figure 2) show a higher temperature for RB4/MWCNT combustion than AR183/MWCNT, confirming the strong interaction of RB4 with MWCNT and stability of RB4/ MWCNT. Kuo et al.14 studied thermodynamics of adsorption of direct dyes, Direct Yellow 86 and Direct Red 224, on a MWCNT and reported positive values of ΔH° and ΔS°. Yao et al.16 also found positive values of ΔH° and ΔS° for methylene blue adsorption on a MWCNT. The difference between our results and the previous reports may be due to the nature of dyes and different MWCNT samples. 3.2. Dye Adsorption in Binary-Dye Solutions. Figure 5 displays dye adsorption on MWCNT in binary dye systems at

Figure 6. Adsorption isotherm of RB4 on MWCNT in AR183-RB4 binary system at fixed AR183 concentration. , Langmuir isotherm; ---, Freundlich isotherm.

fixed concentration of AR183, RB4 adsorption increased with increasing RB4 concentration and reached a maximum of 54 mg·g−1. Using the Langmuir and Freundlich isotherms for data fitting as shown in Table 5, a maximum adsorption capacity was Table 5. Parameters of Langmuir and Freundlich Isotherms for RB4 Adsorption on MWCNT at 20 ppm AR183 at 25 °C Langmuir isotherm

Figure 5. Kinetics of dye adsorption on MWCNT in AR183-RB4 binary system at 25 °C. ○, RB4 at 20 ppm RB4/10 ppm AR183; ●, AR183 at 20 ppm RB4/10 ppm AR183; △, RB4 at 20 ppm RB4/40 ppm AR183; ▲, AR183 at 20 ppm RB4/10 ppm AR183.

dye

t/°C

qmax/ (mg·g−1)

RB4

25

55.9

Freundlich isotherm

KL/ (L·mg−1)

R2

KF

1/n

R2

1.50

0.973

41.1

0.0802

0.944

obtained at 55.9 mg·g−1. Compared to RB4 adsorption in a single dye system, the maximum adsorption of RB4 in the binary dye systems was reduced. As stated before, RB4 and AR183 are both anionic dyes with negative surface charges in solution. Adsorption of both dyes on MWCNT in binary dye solutions would still be dominated by electrostatic attraction, showing a competitive adsorption. From single dye adsorption, it has been shown that RB4 presented a stronger adsorption and higher affinity to MWCNT, which was induced by the different structures of RB4 and AR183. Liu et al.24 have suggested that molecules with planar structures and high charge load are favored for the adsorption. RB4 and AR183 have the same charge, but AR183 shows a nonplanar structure, which prevents it to be favored for adsorption. Therefore, RB4 has the preference for adsorption in

varying concentrations. As shown, dye adsorption behaved quite differently in the binary dye systems. The MWCNT presented initial AR183 adsorption, but the adsorption decreased very fast to zero. Meanwhile, RB4 adsorption increased with the time and reached equilibrium. At higher AR183 concentrations, RB4 showed a slightly reduced adsorption. Elemental analysis (Table 1) showed that N and S contents in RB4-AR183/MWCNT are similar to the values of RB4/ MWCNT. The FTIR spectrum of RB4-AR183/MWCNT shown in Figure 1 also presented a similar functional band 1567

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Scheme 1. Dye Adsorption on MWCNT with Different DyeMWCNT Systems

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

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the RB4-AR183 system. The adsorption mechanism of the dyes on MWCNT can be illustrated in Scheme 1.

4. CONCLUSION MWCNT exhibited different adsorption behavior in single dye and binary acid-reactive dye systems. The MWCNT displayed stronger adsorption of RB4 than AR183. In a single dye system, RB4 and AR183 adsorption followed a pseudosecond-order kinetics, and the isotherm was fitted by the Langmuir isotherm. Solution temperature affected dye adsorption and higher temperature brought reduced adsorption for both dyes. In binary dye systems, the MWCNT could adsorb RB4 stronger and did not show AR183 adsorption. The presence of AR183 would result in lower adsorption of RB4 compared with the single dye system. From the viewpoint of structure and properties of the dyes and MWCNT, dye adsorption was found to be attributed to strong electrostatic attraction. The competitive behavior of AR183 and RB4 is due to the competition for the same electrophilic sites and MWCNT’s preference to adsorbing planar molecules. 1568

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