Removal of Azo Dyes from Water by Combined Techniques of

Article pubs.acs.org/IECR

Removal of Azo Dyes from Water by Combined Techniques of Adsorption, Desorption, and Electrolysis Based on a Supramolecular Sorbent Ming Chen, Wenhua Ding, Jing Wang, and Guowang Diao* College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, People’s Republic of China Key Laboratory of Environmental Materials & Environmental Engineering of Jiangsu Province, Yangzhou, Jiangsu, 225002, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, adsorption and desorption of azo dye-Congo red on a supramolecular sorbent (SiO2-CD) were studied. Static adsorption study showed that the adsorption of Congo red onto SiO2-CD obeyed Langmuir’s model. The sorbent may be easily regenerated by using β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) as a desorption agent. The desorption efficiency of Congo red was strongly dependent on the concentration of the desorption agent and the temperature. The electrochemical degradation of dye solutions revealed that the average current efficiency (ACE) was sharply increased with dye concentration. The combination of above three techniques dramatically increased the concentration of the dye by a factor of 30, improved the removal (approaching 80% color removal), and enhanced ACE by over 50%. These findings suggest that an organic combination of adsorption, condensation, and electrochemical degradation techniques can achieve a satisfactory outcome for the degradation of low-concentration dye effluents with large volume. efficiency, fast reaction rate, easy and clean operation.27 Although electrochemical degradation is an attractive means for wastewater treatment, the low current efficiency is still a critical problem. Currently, much attention has been paid to all types of anodes that have high O2 evolution overpotential. Otherwise, a large amount of the current supplied will be wasted to produce oxygen, leading to a low current efficiency. In fact, the electrochemical degradation rate is also dependent on the initial condition of organic pollutant. When the organic concentration is high, the average current efficiency is high.28 Since the lifetime of hydroxyl radicals is short (only a few nanoseconds), they can only react where they are formed.29 Increasing the quantity of organic pollutant per volume unit theoretically enhances the probability of collision between organic matter and oxidizing species, leading to an increase in the degradation rate. Enhancing the organic concentration may be an effective way to improve treatment efficiency. However, in a real environment, the concentration of organic pollutant is far less than that in laboratory experiments. Therefore, organic pollutants in industrial wastewater should be condensed to obtain a large quantity of organic pollutants before carrying out electrochemical degradation. Recently, combined techniques have brought worldwide attention to the researchers’ cause. Combined techniques include photocatalysis coupling electrochemical oxidation,30−32 sonophotochemistry,33,34 photochemistry with microwave radiation,35 solar-assisted photo-Fenton reaction,36,37 and

1. INTRODUCTION Azo dyes are typical pollutants found in many important industries, such as textile, food colorants, printing, and cosmetics manufacturing. The decolorization of dye effluent has become a focus of attention. Dye effluent is usually treated by conventional methods such as adsorption,1−3 coagulation,4,5 electrochemical degradation,6,7 photocatalysis,8−10 etc. Among these methods, adsorption is one of the most extensively application technologies to remove azo dyes from dye wastewater. Recently, much more attention has been paid to chemical separation techniques and the design and synthesis of new extraction reagents for ions and molecules. In this respect, supramolecular chemistry has provided important avenues to prepare new type adsorbents and extraction reagents. This is achieved with the development of macrocyclic receptors, such as crown ethers,11 cyclodextrins,12−20 and calixarenes.21−23 Many researchers are interested in cross-linking cyclodextrins with suitable cross-linkers to form an insoluble resin that has specific adsorption based on inclusion complex formation. However, the adsorption is actually a transfer of pollutants but difficult for regenerating the sorbent, and cannot completely degrade pollutants. Therefore, the pollutants will be thoroughly decomposed with the assistance of other methods, such as electrochemical degradation, photocatalysis, biological methods, and so on. In recent years, there has been increasing interest in the use of electrochemical technique for the treatment of dye effluent.24−28 The pollutants present in the effluent are destroyed by a direct anodic process via the production of oxidants such as hypochloride, hydroxyl radicals, ozone, etc.24−26 Compared with conventional processes, the electrochemical oxidation has major advantages: high oxidation © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2403

April 8, 2012 January 14, 2013 January 21, 2013 January 21, 2013 dx.doi.org/10.1021/ie300916d | Ind. Eng. Chem. Res. 2013, 52, 2403−2411

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic diagram of the preparation of the SiO2-CD sorbent.

ozonation/fungal treatment.38 In this article, there has been an increasing interest in the combination of adsorption, desorption, condensation, and electrochemical degradation techniques for the degradation of low-concentration dye effluents with large volume. Supramolecular sorbent (SiO2CD) is prepared by chemical bonding monotosyl-β-cyclodextrin (Ts-β-CD) on the surface of aminopropylsilica. A schematic diagram of the preparation of SiO2-CD sorbent is shown in Figure 1. To research the adsorption and desorption behavior of SiO2-CD, Congo red, which is a type of azo dye, is chosen as the adsorbate (the chemistry structure is shown in Figure S1A in the Supporting Information). The influential factors of electrolysis, such as current densities, pH of solution, and supporting electrolyte concentration, had been studied. Using the combined techniques, low-concentration dye effluent with large volume was effectively condensed through adsorption and desorption on SiO2-CD. Subsequently, the condensation solution was treated using an electrochemical technique. The combined techniques were estimated from the condensation ratio of the dye solution and the average current efficiency of electrochemical degradation.

the cathode. Na2SO4 was used as the supporting electrolyte. During each run, the wastewater was stirred by magnetic stirrer at suitable speed (150 rpm) to maintain reactions kinetically controlled, and samples were taken from the sampling port for analysis at appropriate intervals. The analysis was determined triplicately with relative error within 3%. 2.3. Adsorption and Desorption of Congo Red. To research adsorption kinetic behavior of Congo red onto SiO2CD, batch adsorption experiments are done by agitating 50 mL of dye solution (60 mg L−1, pH 7.0) with fresh SiO2-CD (0.1 g) in glass bottles, using a laboratory shaker at 180 rpm and room temperature (25 ± 1 °C). At different adsorption times, the dye solution is separated from the sorbent by centrifugation at 6000 rpm for 5 min. The adsorption quantity of Congo red onto SiO2-CD at different adsorption times was estimated by monitoring the residual Congo red in the solution at the wavelength of maximum absorption (496 nm), using a UV-vis spectrophotometer. The equilibrium adsorption quantity of Congo red, qe (mg g−1), was calculated using the following relationship: qe =

2. MATERIALS AND METHODS 2.1. Reagents. β-Cyclodextrin (β-CD, Shanghai Chemical Reagents Company) was purified by recrystallization twice before use. Hydroxypropyl-β-cyclodextrin (HP-β-CD) was purchased from Yiming Chemical Reagents Factory (Jiangsu, PRC). Congo red was purchased from Sinopharm Chemical Reagents Company. Preparation and characterization of SiO2CD was shown in Figure 1. All other chemicals were of analytical grade and were used without further purification. Highly pure water was obtained from a Millipore Milli-Q UV system and used to prepare all solutions. 2.2. Apparatus. The UV-vis spectra are recorded on a Model UV-2550 (Shimadzu, Japan) double-beam spectrophotometer equipped with a stoppered quartz cell with an optical path length of 1.0 cm. Adsorption and desorption experiments are conducted on a laboratory shaker (Model HZ-9211K, Jiangsu, PRC). All electrochemical experiments were carried out with a Model CHI660c electrochemical workstation (Chenghua, Shanghai, PRC). A graphite fabric electrode (12 mm in diameter × 30 mm long) was used as the anode and stainless steel (12 mm in diameter × 30 mm long) was used as

(c0 − ce)V W

(1)

where c0 is the initial concentration of Congo red (mg L−1), ce the equilibrium concentration of dye (mg L−1), V the volume of the solution (L), and W the mass of SiO2-CD (g). The adsorption saturation efficiency (%) of adsorbent was calculated using the following relationship: q ηA (%) = e × 100 qm (2) where qe (mg g−1) is the equilibrium adsorption quantity of Congo red and qm (mg g−1) is the saturation adsorption quantity (qe ≤ qm). When the value of qe is equal to that of qm, the adsorption capacity of adsorbent reaches the saturation. In a desorption experiment, 2 g of fresh SiO2-CD was put in 1 L of 200 mg L−1 Congo red aqueous solution. The solution above was shaken in a laboratory shaker (180 rpm) for 5 h at 25 °C to make an adsorption equilibrium of Congo red onto SiO2-CD, which was filtrated and dried at room temperature for further use. The adsorption quantity was ∼70 mg g−1. Then, 0.1 g of SiO2-CD was added into each of a dozen conical flasks. 2404

dx.doi.org/10.1021/ie300916d | Ind. Eng. Chem. Res. 2013, 52, 2403−2411


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Figure 2. (A) Flow diagram of combined techniques. (B) Schematic of the electrolysis system depicted as “(6)” in panel (A).

packed with the tested SiO2-CD. Glass wool was placed on the base of each column, minimizing the dead-end volume. The column was packed using 2 g of SiO2-CD, which resulted in an adsorption column 6.64 cm in length with a uniform bulk density of 0.38 g cm−3. After packing, the adsorption column was saturated with 10 L of 3.5 mg L−1 Congo red solution (c0) by a peristaltic pump until the concentration of Congo red in the leachate was close to zero. Congo red in dilute solution was almost completely absorbed onto SiO2-CD. Then, the columns were eluted with 250 mL of solutions at different concentrations HP-β-CD by a peristaltic pump. A flow rate of 0.5 mL min−1 was used for all miscible displacement experiments. Leachate was collected and the concentrations of dye in the leachate (cL) were determined by UV-vis. The condensation ratio is calculated by eq 4:

The desorption kinetics of Congo red from SiO2-CD was operated in 50 mL of different concentrations of β-CD and HPβ-CD aqueous solution. At different desorption times, the concentration of Congo red in the extracting agent phase was measured by UV spectra as previously described. The desorption efficiency (%) was calculated using the following relationship: ηD (%) =

ce,DV qads

× 100 (3)

−1

where ce,D (mg L ) is the desorption equilibrium concentration of dye, V (L) the volume of the extracting agent solution, and qads the actual adsorbed amount. In a typical regeneration experiment, 0.1 g of SiO2-CD covered with Congo red was dipped into fresh extracting agent five times. HP-β-CD aqueous solution (20 mM, pH 7.0) was used as an extracting agent, and the amount was 20 mL. 2.4. Concentration of Congo Red Solution from SiO2CD. The flow diagram of combined techniques is shown in Figure 2A. The condensation of Congo red from SiO2-CD, using HP-β-CD as an extracting agent in laboratory column studies, was carried out using 10-mm-diameter glass columns

condensation ratio =

cL c0

(4)

The above condensation solutions of Congo red from the desorption column then were used to perform electrochemical degradation under optimized electrolysis conditions. 2405



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Table 1. Adsorption Constants and Correlation Coefficients for Freundlich and Langmuir Isotherms at Different Temperature Freundlich Isotherm

Langmuir Isotherm

temperature, T (K)

KF ((mg g−1) (L mg−1)1/n)

n

R2

KL (L g−1)

qm (mg g−1)

R2

288 298 308 318 328

6.82 5.99 5.05 4.66 4.26

0.95 0.99 0.78 0.83 1.25

0.921 0.950 0.931 0.925 0.912

2.55 2.19 2.02 1.78 1.45

76.9 70.1 62.9 56.2 48.7

0.996 0.998 0.997 0.997 0.996

constant (mg g−1 min−1) and parameter b is related to the extent of surface coverage and activation energy for chemisorption (g mg−1). In eq 10, ki is the intraparticle diffusion rate constant (mg g−1 min−0.5). Plots of −ln[1 − (qt/ qe)] vs t, t/qt vs t, qt vs ln t, and qt vs t1/2 are shown in Figure S3 in the Supporting Information, respectively. From the correlation coefficients of these lines (listed in Table S2 in the Supporting Information), it is clearly shown that the adsorption of Congo red onto SiO2-CD greatly coincided with the pseudo-second-order adsorption kinetics model. From the slope and intercept of the straight line, the pseudo-secondorder rate constant (k2) at 25 °C was evaluated as 4.3 g mg−1 min−1. Adsorption isotherms of Congo red onto SiO2-CD at different temperatures were interpreted using two adsorption isotherm models: the Freundlich model and the Langmuir model. The Freundlich isotherm is an empirical equation that is employed to describe heterogeneous systems and is expressed by the following linear equation and nonlinear equations, respectively:

2.5. Electrolysis and Analysis. A schematic diagram of the electrolysis system is shown in Figure 2B. The color removal is calculated by the following formula: color removal (%) =

A0 − At × 100 A0

(5)

where A0 is the absorbance of initial dye concentration and At is the remaining dye concentration at given time t. The average current efficiency (ACE) is calculated using the following expression:39 ACE (%) =

⎛ COD0 − CODt ⎞ ⎜ ⎟FV × 100 ⎝ ⎠ 8It

(6)

where COD0 is the initial chemical oxygen demand (mg L−1), CODt the chemical oxygen demand at a given treatment time t (mg L−1), I the current (A), F the Faraday constant (F = 96 487 C mol−1), t the treatment time (s), and V the volume of the solution (L).

3. RESULTS AND DISCUSSION 3.1. Adsorption. The kinetic behavior of adsorption is shown in Figure S2 in the Supporting Information. It was clearly shown that the adsorption quantity of Congo red increased as the time increased during the first 60 min, and an adsorption equilibrium was reached after 120 min. The kinetic parameters are helpful, with regard to predicting the adsorption rate, which gives important information for designing and modeling adsorption processes. In order to investigate the controlling mechanism of adsorption processes such as transfer and chemical reaction, four different linear kinetic models, i.e., pseudo-first-order model (eq 7),40 pseudo-second-order model (eq 8),41 Elovich equation (eq 9),42 and intraparticle diffusion equation (eq 10)43 were applied to model the kinetics of Congo red adsorption onto SiO2-CD. ⎛ q ⎞ −ln⎜⎜1 − t ⎟⎟ = k1t qe ⎠ ⎝

(7)

t 1 t = + 2 qt qe k 2qe

(8)

qt =

⎛1⎞ ⎛1⎞ ⎜ ⎟ln(ab) + ⎜ ⎟ln t ⎝b⎠ ⎝b⎠

qt = kit 1/2

ln qe = ln KF +

⎛1⎞ ⎜ ⎟ln c ⎝n⎠ e

(11)

qe = KFce1/ n

(12)

where qe is the equilibrium dye concentration on the sorbent (mg g−1), ce the equilibrium dye concentration in solution (mg L−1), KF the Freundlich constant, and 1/n the heterogeneity factor. The capacity constant (KF) and the affinity constant (n) are empirical constants that are dependent on several environmental factors. The plot of lnqe vs lnce is shown in Figure S4 in the Supporting Information. Apparently, the plot in Figure S4 in the Supporting Information demonstrates that the equilibrium adsorption data of Congo red was not excellent for the Freundlich isotherm. From the slope and the intercept of the straight line, the values of n and KF can be evaluated. Table 1 lists the values of KF and n at different temperatures. The Langmuir adsorption isotherm was expressed by the following linear and nonlinear equations, respectively: ce c 1 = e + qe qm KLqm (13)

(9)

qe =

(10)

qmKLce 1 + KLce −1

(14) −1

where ce (mg L ) and qe (mg g ) are the liquid-phase concentration and solid-phase concentration of sorbent at equilibrium, respectively; qm (mg g−1) is the saturation adsorption quantity; and KL (L g−1) is the Langmuir isotherm constant. The saturation adsorption quantity (qm) and the Langmuir isotherm constant (KL) were evaluated using eq 13. Figure 3 shows the relationship between ce/qe and ce at different

where qt is the adsorption quantity of dye adsorbed at any time t (mg g−1), qe) the adsorption quantity of dye adsorbed at equilibrium (mg g−1, k1 the adsorption rate constant of pseudofirst-order kinetic model (min−1), and k2 the adsorption rate constant of pseudo-second-order kinetic model (g mg−1 min−1). In eq 9, parameter a is the initial adsorption rate 2406



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sites. The value of the monolayer saturation capacity indicated that SiO2-CD had exhibited interesting adsorption properties toward the dye molecule, and the results had revealed the potential of this material to be an effective sorbent for removing dyes. To elucidate the thermodynamic origins of the adsorption reaction of Congo red onto SiO2-CD, the thermodynamic parameters for the adsorption reaction were determined using Van’t Hoff analysis. From the Van’t Hoff equation (see Figure S6 in the Supporting Information), at standard pressure and 298 K, the values of the molar enthalpy change, the entropy change, and the Gibbs free-energy change were calculated to be −10.4 kJ mol−1, −28.4 J mol−1 K−1, and −2.0 kJ mol−1, respectively. The adsorption reaction of Congo red onto SiO2CD was driven by the favorable enthalpy change, accompanied by a negative entropy change. The negative value of the molar enthalpy change indicated that the adsorption reaction was an exothermic reaction; therefore, a higher temperature was disadvantageous to the adsorption reaction. The negative value of the entropy change was due to the decrease in the freedom of the dye molecule after the adsorption of Congo red onto SiO2-CD. 3.2. Desorption. The reversibility of inclusion reaction was one of the most important properties of supramolecular chemistry. If a high concentration of β-CD was used as a desorption agent, a competition reaction between the free host and the immobilized β-CD on SiO2 will occur. A high concentration of free β-CD may cause a scramble for guest molecules from the surface of SiO2-CD, which will lead to the recovery of the active site on SiO2-CD and a regeneration of the sorbents. Based on the above theory, the desorption of Congo red from SiO2-CD by β-CD and HP-β-CD aqueous solutions was operated by putting the SiO2-CD adsorbed with Congo red in an extracting agent solution. The desorption efficiency was calculated and the result was shown in Figure 4,

Figure 3. Langmuir adsorption isotherms for Congo red on SiO2-CD. Temperatures: (□) 15 °C, (○) 25 °C, (△) 35 °C, (▽) 45 °C, (◇) 55 °C.

temperatures. Well-behaved linear relationships have shown that the adsorption of Congo red onto SiO2-CD obeyed Langmuir’s model. According to the slope and the intercept of the straight line, qm and KL both can be evaluated. Table 1 lists the values of qm and KL at different temperatures. From Table 1, both qm and KL are dependent on the temperature. With the temperature increasing, the values of qm and KL decreased gradually, which indicated that the higher temperature, the more disadvantageous the conditions for the adsorption of Congo red onto SiO2-CD. From the correlation coefficients given in Table 1, the value of R2 was determined to be higher than the Freundlich isotherm value. It was evident that the Langmuir model was better than the Freundlich model in this case. The nonlinear forms of the Langmuir isotherm model (eq 14) and the Freundlich isotherm model (eq 12) have also been used to interpret the experimental isotherm data.43 Figure S5 in the Supporting Information showed the fitting results (qe vs ce) with two isotherm models and further confirmed that this case coincided with the Langmuir model. The Langmuir equation was applicable to homogeneous adsorption, where the adsorption of each molecule onto the surface had equal adsorption activation energy. This suggested that (i) some homogeneity in the surface of the sorbent will play a key role in dye adsorption and (ii) the homogeneous adsorption model will be better for isotherm simulation. A comparison of the monolayer adsorption quantity (qm) for the adsorption of Congo red by nonconventional sorbents (i.e., waste orange peel,44 coir pith,45 activated carbon,46 and β-CD polymer47) at 25 °C was then made; the results are presented in Table 2. Table 2. Comparison of Monolayer Adsorption Quantity (qm) for the Adsorption of Congo Red by Nonconventional Sorbents at 25 °C adsorbent

qm (mg g−1)

ref

waste orange peel coir pith activated carbon (laboratory grade) activated carbon (commercial grade) β-CD polymer SiO2-CD

22.44 6.72 1.875 0.635 36.20 70.1

44 45 46 46 47 this work

Figure 4. Plot of the desorption efficiency at different concentrations of β-CD and HP-β-CD at 25 °C.

which indicated that the desorption efficiency was dependent on the concentration of the free host and the type of host. The desorption efficiency of HP-β-CD was larger than that of β-CD. Even if a saturation amount of β-CD (at 25 °C, 16.2 mM) was used as the desorption agent, the desorption efficiency was only 83.4%, which meant that the active sites on the surface of SiO2CD cannot be resumed completely. However, when the concentration of HP-β-CD was 25 mM, the desorption efficiency was close to 93.2% ± 5%, which indicated that the

Among these different nonconventional sorbents, the saturation adsorption quantity of SiO2-CD was the largest. Especially, compared with β-CD polymer,44 the adsorption quantity of SiO2-CD was double that of the β-CD polymer, which was attributed to much more β-CD being immobilized on the surface of silica, as well as the greater number of adsorption 2407



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Figure 5. Proposed reaction sequence for Congo red degradation by anodic oxidation with graphite fabric electrodes.

experiment, the following parameters were chosen: current density = 50 mA cm−2; pH 7; [Na2SO4] = 15 g L−1. As stated earlier, except electrode capability, the electrochemical degradation current efficiency is also dependent on the initial condition of the organic pollutant. As expected, in Figure 6, the

active sites on the surface of SiO2-CD can be resumed almost completely. The amount of Congo red desorbed from SiO2-CD was dependent on the temperature (shown as Figure S7 in the Supporting Information). The desorption efficiency of Congo red from SiO2-CD increased with temperature for all desorption agents, which demonstrated that higher temperature is advantageous to the desorption reaction. The regeneration was one of the most important characteristics of the sorbent, which may save costs, simplify the manipulation of sewage treatment, and enhance the disposal efficiency of dye wastewater. The sorbents were used five times repeatedly;d the adsorption and desorption efficiency is shown in Figure S8 in the Supporting Information. After five repeated adsorption−desorption cycles, the adsorption and desorption efficiency was decreased gradually. After the fifth time, the adsorption saturation efficiency still remained 85.2% ± 5%. The good regeneration ability of SiO2-CD was attributed to the excellent solubility and the large inclusion capability of HP-βCD. At 25 °C, and an ionic strength of 0.001 mol kg−1, the value of formation constant between HP-β-CD and Congo red is 2.8 × 107 M−2, which is larger than that between β-CD and Congo red (1.5 × 106 M−2). The formation constants between the host and Congo red are calculated via UV-vis spectroscopy. The larger the formation constant between the dye molecule and the desorption agent molecule, the higher the desorption efficiency. Therefore, HP-β-CD may be used as a desorption agent to extract guest molecules. 3.3. Electrolysis. According to the concentration conversion of intermediates during the electrolysis, a pathway of the electrochemical degradation of Congo red could be proposed.48 Figure 5 showed the supposed pathways on graphite fabric electrode anodes. The electrochemical degradation of Congo red could be divided into three stages. In the first stage, •OH/O2 cleaved the −NN− and Congo red was divided into derivatives of naphthalene and biphenyl, which resulted into the color removal of solution. Subsequently, under the attack of •OH/O2, the formation of dihydroxybenzene and p-benzoquinone could be regarded as the second stage. In the third stage, benzoquinone converted to maleic acid, which further converted to oxalic acid and formic acid. And finally, they were thoroughly mineralized. In this case, the first stage might be the main reaction base on the color removal of the solution. The influential factors of electrolysis, such as current density, pH of solution, and supporting electrolyte concentration, have been studied, and the experimental results are shown in Figure S9 in the Supporting Information. In the next electrolysis

Figure 6. Effect of color removal and average current efficiency (ACE) at different dye initial concentrations. Conditions: current density = 50 mA cm−2; pH 7; [Na2SO4] = 15 g L−1.

average current efficiency (ACE) increased as the effluent initial concentration increased, but the color removal gradually decreased. To obtain high color removal, the time of electrochemical degradation would be extended; however, longer electrolysis times would lead to low ACE values. Therefore, in order to resolve the conflict, the initial concentration of dye in the electrolysis experiment should be increased properly. The combined techniques of condensation and electrochemical degradation apparently may address the above problem. However, if the desorption agent participates in the electrochemical reaction, the current efficiency will be decreased sharply and the combined techniques will have no application value. So, first of all, electrolysis experiments (keeping other parameters constant, except for using HP-β-CD instead of a dye pollutant) were used to confirm whether HP-βCD can be degraded. Because HP-β-CD has no UV-vis absorbance, the COD values of different concentrations of HPβ-CD solutions were detected before and after the electrolysis experiment. After electrolysis, a slight decrease in the COD indicated that HP-β-CD cannot be decomposed under conditions identical to those of an electrolysis dye pollutant. 2408



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pollutant in mixing solution. More significantly, after electrochemical degradation, the electrolyte solution, including HP-βCD and Na2SO4, may be used as a desorption agent again and recycled for use. 3.4. Combination of Adsorption, Condensation, and Electrolysis Techniques. First, the adsorption column that was filled with SiO2-CD was saturated with 10 L of 3.5 mg L−1 Congo red solution. The adsorption quantity of Congo red on SiO2-CD was 17.5 mg g−1 (which was less than the saturation adsorption quantity; qm = 70 mg g−1). Then, 250 mL of different concentrations of HP-β-CD as an desorption agent were used in the adsorption column to extract and condense dye pollutants. Treatment without a desorption agent was done as a control for the HP-β-CD-amended experiments. The product of adsorption, condensation, and electrolysis has been shown in Figure 2. The condensation efficiency is shown in Figure 7A. As the concentrations of HP-β-CD increased, the effluent concentrations of Congo red increased. It can be concluded that the magnitude of the enhanced-condensation effect was dependent on the mass of HP-β-CD in solution. On the one hand, when the concentration of HP-β-CD increased to 20 mM, the total effluent concentration of Congo red with 250 mL of HP-β-CD was 105 mg L−1, which was nearly 30 times greater than the initial concentration of Congo red. Simultaneously, the volume of dye solution was decreased from 10 L to 250 mL (reduced by a factor of 40), which was very beneficial to carry out electrochemical degradation in the next step. On the other hand, during the desorption process, ∼72.5% ± 3.2% of the active sites on the surface of SiO2-CD may be resumed and the sorbent may be used to adsorb the new dye effluent. The desorption efficiency (ηD, %) is shown in Figure 7B. As the concentrations of HP-β-CD increased, the desorption efficiency of Congo red was enhanced. Subsequently, the above condensation solutions of Congo red from the desorption column were used to perform electrochemical degradation under optimal electrolysis conditions. The experiment results are shown in Figure S10 in the Supporting Information. With the increasing in condensation concentration of dye pollutant, the ACE was increased from 10.3% ± 2% to 51.2% ± 2.7%, which coincided with the result of Figure 6. The ΔCOD value (date not shown) was directly proportional to the condensed concentration of dye, which further indicated that HP-β-CD did not contribute to the change of COD. The change in the color removal trend was similar to the result of Figure 6. When ACE was 51.2% ± 2.7%,

The mixing solution of Congo red and HP-β-CD was electrolyzed at the given experimental parameters. The concentration of Congo red was changed, and the concentration of HP-β-CD was constant (5 mM). The experimental results are shown in Table 3. The ΔCOD values in Table 3 are Table 3. Chemical Oxygen Demand (COD)a of Mixing Solutions with Different Concentrations of Congo Red and HP-β-CD (5 mM) before and after Electrolysisb Before Electrolysis concentration (mg L−1)

COD (mg L−1)

standard deviation (mg L−1)

40 60 80 100 120

7654 7814 8014 8286 8600

85 90 95 92 97

After Electrolysis COD (mg L−1)


ΔCOD (mg L−1)

7398 7462 7598 7766 7987

78 91 92 95 94

256 352 416 520 613

a

An average of three times of determination was used for each reported COD. bConditions: current density = 50 mA cm−2; pH 7; [Na2SO4] = 15 g L−1; electrolysis time = 2 h.

proportional to the concentration of Congo red. Furthermore, the Congo red solution was electrolyzed at the given experimental parameters, and the change in COD is shown in Table 4. Compared with the values of ΔCOD in Table 3, the Table 4. Variety of CODa for Different Concentrations of Congo Red before and after Electrolysisb Before Electrolysis

After Electrolysis

concentration (mg L−1)

COD (mg L−1)


COD (mg L−1)


ΔCOD (mg L−1)

40 60 80 100 120

240 456 644 898 1192

8 12 18 26 30

125 256 402 594

5 8 11 15

∼240 331 388 496 598

a

An average of three times of determination was used for each reported COD. bConditions: current density = 50 mA cm−2; pH 7; [Na2SO4] = 15 g L−1; electrolysis time = 2 h.

ΔCOD values in Table 4 are close to the values of ΔCOD in Table 3, which further demonstrated that HP-β-CD indeed cannot interfere with the electrochemical degradation of dye

Figure 7. (A) Plot of the condensation ratio of Congo red condensation solution versus the concentration of HP-β-CD. (B) Plot of the desorption efficiency at different concentrations of HP-β-CD. 2409



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the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Natural Science Foundation of Education Committee of Jiangsu Province (No. 12KJB150023). The authors also acknowledge the Testing Center of Yangzhou University for performing the SEM experiment.

the color removal value reached 80%. It was much more important that, after electrochemical degradation, the electrolysis solution may be used as a desorption agent to grab dye pollution from SiO2-CD again. Furthermore, the supporting electrolyte (Na2SO4) may be economically available. Therefore, an organic combination of adsorption, condensation, and electrochemical degradation techniques achieved a satisfactory outcome for the degradation of low-concentration dye effluents with large volume. The combined techniques confirmed the idea of a low carbon cost and the concept of green chemistry in environmental protection even more.

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4. CONCLUSIONS The adsorption of Congo red onto SiO2-CD obeyed Langmuir’s model. The sorbent may be easily regenerated using β-CD and HP-β-CD as a desorption agent to extract dye from SiO2-CD. When the concentration of HP-β-CD was 25 mM, the desorption efficiency was close to 93.2% ± 5%. Lowconcentration dye effluent (

Removal of Azo Dyes from Water by Combined Techniques of

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