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Ind. Eng. Chem. Res. 2003, 42, 6898-6903
SEPARATIONS Adsorption of Water-Soluble Dyes onto Resin NKZ Ying Yu,*,† Yuan-Yi Zhuang,‡ Zhong-Hua Wang,‡ and Ming-Qiang Qiu§ College of Physical Science and Technology, Central China Normal University, Wuhan 430079, People’s Republic of China, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of China, and Chemistry College, Central China Normal University, Wuhan 430079, People’s Republic of China
A novel resin NKZ, which is a macroporous resin intermediate modified with nitroguanidine, was synthesized and identified. The adsorption behavior of five kinds of water-soluble dyes onto the resin NKZ was investigated thermodynamically in detail. The results showed that the adsorption rate for reactive brilliant blue KN-R is higher than those for the other dyes. The equilibrium data can be well-described by a three-parameter equation. The values of ∆G° demonstrate that the first layer adsorption of the dyes onto NKZ involves both physical adsorption and chemisorption but multilayer adsorption is a physical one. Both the values of ∆H° and ∆S° on the adsorption of reactive brilliant blue KN-R onto NKZ was positive, indicating that the adsorption is endothermic and can occur spontaneously. The apparent activated energy (Ea) of the adsorption process was 16.84 kJ/mol, which shows that there is a great potential barrier for adsorption. Introduction Water-soluble dyes such as acid dyes and reactive dyes are not easily dealt with for their high solubility. Conventional biological treatment processes are no longer able to achieve adequate color removal nor conventional physicochemical coagulation methods.1 Although a number of materials (natural clay, bagasse pith, maize cob, activated carbon, and so on) have been used as adsorbents for dyestuffs and were effective in the decolorization,2-5 the adsorbed dyes cannot be recovered and might lead to the secondary pollution of the environment. The two most common technologies for the separation of organic chemicals are solvent extraciton and ion exchange. The disadvantage of solvent extraction is the finite aqueous solubility of the extractants, solvents, and modifiers.6 This not only adds to the cost of the process through loss of reagents but also contaminates the water with potentially toxic chemicals. The ion exchange method is one of the most common technologies for the separation of organic chemicals; the resin can be regenerated and reused for a continuous process7 so that the method has been extensively used. There is little reported recently on the adsorption of dyes onto synthesized resins. Only novel synthetic adsorbents Ambensorb 572, Ambensorb 1500, and Calgon F-400C were utilized to remediate the dyehouse effluent.8 Further * To whom correspondence should be addressed. Tel.: 86-27-67867947. Fax: 86-27-67861185. E-mail: yythata@ yahoo.com.cn. † College of Physical Science and Technology, Central China Normal University. ‡ College of Environmental Science and Engineering, Nankai University. § Chemistry College, Central China Normal University.
studies are needed as to lower the treatment price, to recycle dye from dye wastewater, and to prevent dye from polluting the environment. We have investigated the interactions between organic flocculant PAN-DCD, which is polyacrylonitrile (PAN) modified by dicyandiamide (DCD), and dye9,10 and found that some groups in PAN-DCD react with sulfonic acid groups in dye molecules, leading to the formation of a weak chemical bond. On the basis of this discovery, we have synthesized a novel resin NKZ by modifying the macroporous resin intermediate (MRI) with nitroguanidine, which has a special group similar to that in the macromolecular PAN-DCD.11 Through the weak chemical bond, the dyes are adsorbed onto the resin and can be recovered from the resin by rinsing reagents. Moreover, the adsorption behavior of watersoluble dyes onto the functionalized resin NKZ will be discussed in detail. To our knowledge, the resin modified with nitroguanidine is generally used to adsorb precious metals.12,13 There is no report on its applications to removal of dyes from wastewater. Experimental Section Materials. Characteristics of the dyes used are listed in Table 1, and the names of reactive violet K-3R, reactive brilliant blue KN-R, reactive brilliant orange K-GN, reactive brilliant red K-2BP, and acid mordant gray BS are abbreviated to K-3R, KN-R, K-GN, K-2BP, and BS, respectively, in this study. The structures of the dyes are shown in Chart 1. The MRI was supplied by Chemical Factory of Nankai University. Its specific surface area, particle size, and chloride content were 311 m2/g, 1-3 mm, and 14.6%, respectively. Before use, MRI was fully extracted in acetone refluxing for 10 h to remove impurities and
10.1021/ie020622o CCC: $25.00 © 2003 American Chemical Society Published on Web 11/20/2003
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6899 Table 1. Characteristics of the Dyes Used in This Work name of dye
classification
reactive brilliant blue KN-R (KN-R) reactive brilliant orange K-GN (K-GN) reactive brilliant red K-2BP (K-2BP) mordant gray BS (BS) reactive violet K-3R (K-3R)
reactive blue 19 reactive orange 5 reactive red 24 mordant black 13 reactive violet 2
Chart 1. Structure of the Dyes
residual solvents. Nitroguanidine, dimethyl formamide (DMFA), potassium dihydrogen phosphate, disodium hydrogen phosphate, and other reagents were all analytical reagent grade. Working solutions were prepared by appropriate dilution with distilled water. A mixing solution of 0.025 mol/dm3 potassium dihydrogen phos-
color index
molecular weight
maximum adsorption wavelength λmax (nm)
61200
627 876 809 611 954
604 478 570 374 552
63615 18157
phate and 0.025 mol/dm3 disodium hydrogen phosphate anhydrous was the neutral buffer solution with pH 6.84. Apparatus. Spectrophotometric measurements were carried out on a 722 visible spectrometer (China) in the 350-620 nm range using 1 cm matched glass cells. The pH measurements were conducted with a PHS-3C pH meter (China) with a combined glass electrode. The batch adsorption experiment was carried out using a thermostated shaker bath (THZ-82, China). Infrared analysis was conducted with a Nicolet 560 E.S.P. Fourier transform infrared (FTIR) spectrometer (U.S.A.) as KBr disks. Preparation of NKZ. The synthesis of NKZ is similar to the procedure reported elsewhere.14,15 Before reaction, MRI had been immersed in solvent DMFA for 24 h. In a reaction vessel of 500 cm3 equipped with a paddle mixer, MRI and nitroguanidine were mixed with 200 cm3 of DMFA. When the temperature rose to 60 °C, NaOH solution was added. The reaction was allowed to proceed for 4 h. After the mixture was cooled, filtered, and washed with distilled water, the functionalized resin NKZ was obtained. The specific surface area, particle size, and chloride content of NKZ were 37 m2/ g, 0.5-2 mm, and 3.47%, respectively. NKZ was extracted in acetone refluxing for 10 h to remove impurities and residual solvents and was dried at 60 °C before use. Characterization of Resin NKZ. The characteristics of NKZ and MRI were measured with an FTIR spectrometer as KBr disks. The IR spectrum of MRI exhibited more intense and sharp bands at 824.1 and 673.8 cm-1, which may be attributed to the chloride group.16 The sharp band at 1264.5 cm-1 is assigned to the methylene group connecting to chloride. After modification with nitroguanidine, one of chloride group bands was weakened and shifted to lower wavenumbers. The other at 673.8 cm-1 disappeared. Moreover, the band of -CH2- connecting to chloride also disappeared, implying that the chloride in MRI was substituted by nitroguanidine. Meanwhile, the characteristic band of N-H appeared at 2701.4 cm-1. All of the above IR data indicated that the reaction takes place according to Scheme 1. IR results also showed that the reaction proceeds thoroughly as evident in the change of chloride content from 14.6% in MRI to 3.47% in NKZ. Adsorption Procedure. To obtain kinetic data and determine the time required for equilibrium for the adsorption of dyes onto NKZ, 25 cm3 of dye solution of initial concentration with 25 cm3 of neutral buffer solution, pH 6.84, was shaken with 0.100 g of resin NKZ in a thermostated shaker bath (THZ-82) at different temperatures and sampled at different time intervals. The pH changes of the solutions after shaking were within the range of (0.01 so that the pH value of the buffer solution was considered as that of the mixing solution. The initial concentrations of the dye K-2BP, KN-R, BS, K-3R, and K-GN were 2.054 × 10-3, 1.015 × 10-3, 2.015 × 10-3, 1.569 × 10-3, and 1.551 × 10-3 mol/ dm3, respectively. Experiments on the temperature
6900 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Scheme 1
effect were carried out at 25, 30, 35, and 40 °C, respectively, for the dye KN-R onto NKZ. The batch equilibrium technique was used to obtain adsorption isotherm data. A fixed amount of 0.01 g of NKZ and 50 cm3 of different concentration dye solutions mixed with different volumes (12-38 cm3) of dye solution and different volumes (38-12 cm3) of buffer solution were placed in 250 cm3 stopped glass flasks. After the flasks were sealed, they were shaken at different temperatures for the time needed for equilibrium for the adsorption of different dyes. The pH errors of the shaken solutions were all within the range of (0.02. Experiments of temperature effect on adsorption isotherm were also conducted at 25, 30, 35, and 40 °C, respectively, for the dye KN-R onto NKZ. All of the adsorption experiments above were carried out at the same agitation speed. Although there was no agitation speed display on the thermostated bath, the knob was put at the same place every time. Dye concentrations were measured spectrophotometrically at the maximum absorbance wavelengths as listed in Table 1. The experiments were duplicated under identical conditions. Results and Discussion Adsorption Dynamics. Figure 1 shows the dynamic curves for the adsorption of water-soluble dyes K-3R, K-GN, K-2BP, KN-R, and BS onto NKZ at 25 °C, pH 6.84. The figure exhibits that the amount of the dyes adsorbed from aqueous solution increased with time, with equilibrium being achieved within 32 h for KN-R and 60 h for the other dyes. The difference of the initial concentration for the dyes was not great, but the amount of KN-R adsorbed by NKZ at equilibrium was larger than those of the other dyes. Thus, NKZ showed superior adsorption capacity for KN-R. The reason might be related with β-hydroxyethyl sulfone sulfate in the dye KN-R. The group had greater bonding capacity
Figure 1. Dynamics curves for the adsorption of water-soluble dyes onto NKZ. Experimetal conditions: temperature ) 25 °C; pH ) 6.84; NKZ ) 2 g/L. Dye concentrations: K-2BP ) 1.027 mmol/ dm3; KN-R ) 0.5075 mmol/dm3; BS ) 1.008 mmol/dm3; K-3R ) 0.7845 mmol/dm3; K-GN ) 0.7755 mmol/dm3.
Table 2. Kinetic Parameter and Correlation Coefficient for the Fit Lines dye
K-3R
K-GN
K-2BP
KN-R
BS
k (× 10-2 h-1) r
5.23 0.996
4.85 0.998
4.67 0.994
9.89 0.994
5.70 0.991
with -NH- or -NH2 in NKZ than -SO3- in the other dyes, leading to the higher amount of KN-R adsorbed by NKZ. To describe the kinetics of the exchange adsorption of ions from aqueous solution by organic zeolites, Boyd et al.17 derived an equation
ln(1 - F) ) -kt
(1)
where F is the fractional attainment of equilibrium, F ) Q/Qe ) the amount of dye adsorbed at any time/the amount of dye adsorbed at equilibrium, and k is the adsorption rate constant (h-1). If a plot of -ln(1 - F) vs t is linear, diffusion will control the adsorption rate.17,18 For the adsorption of dyes on resin NKZ at 25 °C, pH 6.84, the kinetic plots of -ln(1 - F) vs t were drawn. The k values were calculated from the slopes of the plots and are listed with the correlation coefficient (r) in Table 2. The values of the correlation coefficient in Table 2 indicate the applicability of eq 1 to the system studied. It is observed that the adsorption rate constant of KN-R was far larger than that of the other dyes. Therefore, the time needed for equilibrium of KN-R was much shorter than that for the other dyes. Thus, the diffusion of KN-R was expected to be faster than that of K-GN, K-2BP, K-3R, and BS. The reason might be attributed to the β-hydroxyethy1 sulfone sulfate group in KN-R, which is the long chain structure, and could make it easier for KN-R to diffuse to the surface of NKZ from the solution or penetrate through the polymer matrix. However, there is not a long chain group in the other dyes and the ring structure in the other dyes has a great volume. Without the direction of the long chain group, it was difficult for the ring to diffuse or penetrate quickly and smoothly. The fact that the time to equilibrium for KN-R was 32 h and those for other dyes were all nearly 60 h could be explained by the experimental results. Adsorption Isotherm. Figure 2 presents adsorption isotherms; that is, the relationships between the amounts of dyes adsorbed by NKZ (Qe) and their final concentrations in the aqueous phase (Ce). It is found that Qe increases with increasing Ce. Adsorption isotherms are important to describe how adsorbates interact with adsorbent. To explore more insights into the adsorption mechanism, a three-parameter equation was tried in the present study. The equation is as follows
Qe )
Γm K1 Ce (1 - K2 Ce)[1 + (K1 - K2) Ce]
(2)
where K1 is the equilibrium constant for the first layer
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6901
Figure 2. Adsorption isotherms for water-soluble dyes onto NKZ. Experimetal conditions: temperature ) 25 °C; pH ) 6.84; NKZ ) 2 g/L; dye concentration: varying.
Figure 3. Dynamic curves for the adsorption of KN-R onto NKZ. Experimetal conditions: temperature ) varying; pH ) 6.84; NKZ ) 2 g/L; KN-R concentration ) 0.5075 mmol/dm3.
Table 3. Standard Deviation, Constants of the Three Parameter Equation, and ∆G° dye KN-R K-GN K-2BP K-3R BS
Γm ∆G2° K1 K2 ∆G1° S × 10-4 (mmol/g) (L/mmol) (L/mmol) (kJ/mol) (kJ/mol) 8.28 9.43 9.57 5.69 0
0.196 0.236 0.322 0.241 0.321
2384 152 24.6 28.4 3.25
2.93 0.999 0.292 0.421 0.151
-36.4 -29.6 -25.1 -25.4 -20.0
-19.8 -17.1 -14.1 -15.1 -12.4
adsorption and K2 is the equilibrium constant for multilayer adsorption. This equation was introduced by Wang et al.19 to describe the dye adsorption characteristics onto sludge particulates. Through nonlinear regression of the equilibrium data using the software program Origin 5.0, the values of Γm, K1, and K2 were obtained and they are listed in Table 3, together with the standard deviation of the fit. The values of the standard deviation were small, indicating a good nonlinear fit (s E 0.0957) for all of the dyes studied. The monolayer adsorption density of KN-R was far lower than that of the other dyes. The reason might be related to its molecular structure. The anthraquinone group and β-hydroxyethyl sulfone sulfate group in KN-R render it a big molecule with a big volume so that the number of KN-R molecules adsorbed onto the surface of per gram resin was small. The change of free energy of adsorption can be calculated from equilibrium adsorption constants:
∆G° ) -RTlnK
(3)
where ∆G° is the standard free energy change, R is the gas constant, T is the absolute temperature, and K is the equilibrium constant. The values of ∆G1° and ∆G2° for the adsorption process of the dyes onto NKZ are also listed in Table 3. It can be seen that the value of ∆G1° for the first layer adsorption for all the dyes was in the range of -36.38 to -20.04 kJ/mol, which was just in the middle of physical adsorption and chemisorption.20 Therefore, the first layer adsorption could be considered as a physical adsorption enhanced by the chemical force. For the multilayer adsorption, the value of the standard free energy change for all of the dyes was more than -20 kJ/mol and less than zero, suggesting that the multilayer adsorption of these dyes was typically physical adsorption by nature. Effect of Temperature. The effect of temperature on the time dependence of the adsorption process of KN-R onto NKZ was obtained by batch contact time experiments (Figure 3). The time needed for equilibrium was 32 h under the temperature range studied of 40 to
Figure 4. Plot of lnk vs 1/T.
25 °C. The uptake of KN-R increased with increasing temperature at the same adsorption time, indicating that the dye adsorption is favored at higher temperatures. The kinetic plots of -ln(1 - F) vs t at the different temperatures were drawn. The calculated values of k at 25, 30, 35, and 40 °C were 0.0989 (r ) 0.993), 0.1077 (r ) 0.995), 0.1165 (r ) 0.991), and 0.1385 (r ) 0.992) h-1, respectively. Thus, the adsorption rate of KN-R increased with increasing temperature, which was due to the increase of diffusion coefficient with temperature. Figure 4 shows the plot of lnk vs 1/T. The value of apparent activation energy calculated from the slope of the line, according to Arrhenium equation, lnk ) -Ea/ RT + lnA,21 was 16.84 kJ/mol, which indicates that there is a great potential barrier for dye adsorption. Thus, the adsorption equilibrium of KN-R onto NKZ needed 32 h. Through nonlinear regression with the three-parameter equation, the fit curves for the adsorption of KN-R onto NKZ at different temperatures were obtained, and the one for that at 30 °C is presented in Figure 5. The obtained Γm, K1, and K2 with standard deviation of the fits are displayed in Table 4. It can be seen that the experimental data for equilibrium adsorption were found to agree well with the fit curves, demonstrating the good applicability of the three-parameter equation in all cases. The monolayer adsorption density increased with increasing temperature. This might be attributed to the conformation of dyes that changes at higher temperature or the number of active sites on the surface of NKZ that increases with temperature, leading to the greater amount of the dye adsorbed.
6902 Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 Table 4. Standard Deviation, Constants of the Three Parameter Equation, and Thermodynamic Parameters for the Adsorption of KN-R onto NKZ at Different Temperatures T (°C)
S × 10-4
Γm (mmol/g)
K1 (L/mmol)
K2 (L/mmol)
∆G1° (kJ/mol)
∆G2° (kJ/mol)
∆H1° (kJ/mol)
∆H2° (kJ/mol)
∆S1° (J/mol K)
∆S2° (J/mol K)
25 30 35 45
8.28 3.56 9.04 8.37
0.196 0.248 0.268 0.297
2384 1611 1926 5496
2.93 2.57 3.92 6.24
-36.4 -36.0 -37.1 -40.4
-19.8 -19.8 -21.2 -22.7
163 175 185 196
19.4 25.0 30.3 35.5
670 695 722 754
131 148 167 196
multilayer adsorption, which is mainly originated from the interaction between dye molecules, starts. Conclusions
Figure 5. Nonlinear regression curve for the adsorption of KN-R onto NKZ. Experimetal conditions: temperature ) 30 °C; pH ) 6.84; NKZ ) 2 g/L; KN-R concentration: varying.
The changes for the standard free energy, standard enthalpy, and standard entropy of the adsorption process of KN-R onto NKZ were calculated from equilibrium adsorption constants using the following equations, eqs 4 and 5:22
∆H° ) -R[d(lnK)/d(1/T)]
(4)
∆S° ) (∆H° - ∆G°)/T
(5)
The values for ∆G°, ∆H°, and ∆S° are also listed in Table 4. The negative values of ∆G° imply a spontaneous process with a high affinity of the dye toward the surface of NKZ. Furthermore, the positive values of the enthalpy changes and the entropy changes suggest that the adsorption was endothermic and the system exhibited random behavior. From Table 4, it can be seen that within the temperature ranges studied, the value of |∆G1°| was more than 20 kJ/mol and smaller than 80 kJ/mol. Thus, the adsorption of KN-R was physical adsorption together with chemisorption. Moreover, |∆G1°| increased with temperature, indicating that chemisorption played a great role at higher temperature. The value of the standard enthalpy change for the first layer adsorption was more than 163 kJ/mol, suggesting that there was a chemical reaction and thereby chemisorption might be predominant. That for multilayer adsorption was small, illustrating that multilayer adsorption was a physical process. Table 4 also shows that the values of the standard entropy change for the first layer adsorption were far larger than that for multilayer adsorption. Obviously, the first layer adsorption has a higher degree of randomness. In summary, the whole adsorption process is that the KN-R is adsorbed onto NKZ surface mainly through the formation of a weak chemical bond. That is, the amino in amidine of the resin reacts with the sulfonic acid group in the dye molecules.9 Meanwhile, there are a few dye molecules adsorbed through physical adsorption. When the surface is all covered,
The specific group amidine was already grafted onto macroporous adsorbent, and the functionalized resin NKZ was obtained. NKZ exhibits effective adsorption for the water-soluble dyes studied. As compared with the other dyes, KN-R was most favorably adsorbed by NKZ. The three parameter equation provides a good description for the dye adsorptions. The values of ∆G° showed that the first layer of dye adsorption involved both physical adsorption and chemisorption but the multilayer is the physical one. Negative ∆G° values confirm that the adsorption of the dyes can take place spontaneously. Temperature affects the adsorption of the watersoluble dye KN-R onto NKZ. The amount adsorbed increases with temperature and the adsorption proceeds faster as temperature rises. According to the Arrhenius equation, Ea for the adsorption was calculated and a value of 16.84 kJ/mol was obtained. Therefore, there is a great potential barrier for dye adsorption. Although the first layer of KN-R adsorption is chemisorption together with physical adsorption, chemisorption becomes more predominant at elevated temperatures. The values of both ∆H° and ∆S° are positive so that the adsorption of KN-R is endothermic and has random behavior. Notation A ) constant in Arrhenium equation (-) c ) constant in the equation about distribution ratio (-) Ce ) equilibrium dye concentration in the aqueous solution (mol/dm3) C ) dye concentration in the aqueous solution at time t (mol/dm3) Co ) initial dye concentration in the aqueous solution (mol/ dm3) D ) distribution ratio (mm3/g) F ) fractional attainment of equilibrium (-) ∆G° ) standard Gibbs free energy (kJ/mol) ∆H° ) standard enthalpy of adsorption (kJ/mol) k ) adsorption rate constant (h-1) K ) equilibrium constant K1 ) equilibrium constant of first layer adsorption in three parameter equation (dm3/mmol) K2 ) equilibrium constant of multilayer adsorption in three parameter equation (dm3/mmol) Qe ) equilibrium amount adsorbed on NKY (mmol/g) Q ) amount adsorbed on NKY at time t (mmol/g) Γm ) amount adsorbed corresponding complete coverage (mmol/g) r ) correlation coefficient (-) R ) universal gas constant (J/mol K) ∆S° ) standard entropy of adsorption (kJ/mol) s ) standard derivation (-)
Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6903 t ) time (h) T ) absolute temperature (K) V ) dye solution volume (dm3)
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Resubmitted for review April 21, 2003 Revised manuscript received October 9, 2003 Accepted October 10, 2003 IE020622O
