CO2-Activated Adsorption: A New Approach to Dye Removal by Chitosan Hydrogel.

This study focuses on development of a new adsorption technique by CO2-activated chitosan. Carbon dioxide was utilized to form the functional chemical groups of chitosan on the adsorptions of anionic dyes, Brilliant Blue FCF and Congo Red, in the aqueous solution. CO2-activated chitosan results in the dye adsorption significantly faster than that of chitosan in pure water. The adsorption capacities and removal efficiencies of the dye are increased by CO2-activated chitosan. Furthermore, the dye adsorptions on CO2-activated chitosan were investigated at various temperatures and initial dye concentrations in the aqueous solution. Interestingly, the high temperature adsorption provides the enhancement of adsorption capacities and removal efficiencies of the dye by the carbamate cross-linking of chitosan with CO2. CO2-activated chitosan was further characterized by Fourier transform infrared spectra, amino group ratio, zeta potential, and thermal gravimetric analysis. These characterizations can be used for understanding the unique adsorption of the dye on CO2-activated chitosan. Carbon dioxide-activated chitosan in this work will provide an effective operation and a clean process of dye adsorption in wastewater treatment.

Introduction

Increment of CO2 concentration in the atmosphere has become a recent concern because of the major cause of global warming.[1] As the main product of fuel combustion, a huge amount of CO2 is being released through industrial activities. Although soil, plants, oceans, and rivers in the natural carbon cycle can all absorb CO2, there is an urgent need to develop effective and sustainable techniques to reduce and reuse CO2 in other applications.[1,2] Supercritical CO2, for example, can be applied in many potential applications such as fabrication process of porous materials,[2,3] drug delivery systems,[4−7] and nanosuspensions.[8,9] Carbon dioxide can be used for switching polarity, hydrophilicity, or ionic strength of a solvent.[10−13] Activated polymer systems by CO2 have been reported by some research groups.[13−17] Nagai et al.[14] investigated the reactions between CO2 and amines to form reversible cross-linked carbamate polymers. Han et al.[15,16] have also presented the modification of lower critical solution temperature of polymers activated by CO2. Carbamate polymers by CO2 were also applied for separation of the complexes.[17] The separation system using CO2-activated polymer is expected to develop the simple separation process and provide CO2 utilization techniques.

Wastewater treatment is one of the most important separation techniques for human life. Synthetic dyes are used for many industrial fields; however they usually come with some drawbacks resulting in environmental and human body damages. The dye removal from the wastewater stream has been demanded strongly because the dyes are difficult to separate and are often carcinogenic.[18] Adsorption by chitosan has become an attractive separation process because of the material’s abundance in the nature.[18−26] Additionally, chitosan in the acidic aqueous solution can be a cationic polymer, which possesses excellent interaction with the anionic pollutant in the wastewater and increases the removal efficiencies of the dye.[25,26] However, there are still challenges of chitosan in the acidic solution on the actual wastewater stream because of the dissolution into the acidic solution[23,27−29] and the toxicity and equipment damage by conventional acids, such as HCl or H2SO4. Although the cross-linked chitosan has been applied for the wastewater treatment,[30,31] the adsorption capacities of the dye would be lower than that without cross-linking.

This work gives a potential separation process using CO2-activated chitosan applied for the dye adsorption in the aqueous solution. The adsorption technique proposed in this work is expected to provide an effective dye removal and “greener” separation process in wastewater treatment and develop a new CO2 utilization method. The pollutant dye adsorptions on CO2-activated chitosan were investigated on the adsorption capacity, dye removal efficiency, and the time required to reach adsorption equilibrium. The stabilities of CO2-activated chitosan are discussed by dissolution into the aqueous solution at various temperatures. The chemical changes on chitosan activated by CO2 were characterized by infrared (IR) spectra, NH2 group ratio, thermal stability, and the surface charge in zeta potential.

Results and Discussion

Stability of Chitosan in CO2-Activated System

The stability of CO2-activated chitosan can be important on dye adsorption because of being dissolved into the low pH aqueous solution. Figure gives the results of the weight of CO2-activated chitosan treated at 25–55 °C, Wc in grams to that of the dried chitosan W0 in grams. Chitosan activated by CO2 at 25 and 35 °C was dissolved in the aqueous solution by pH reduction. Interestingly, CO2-activated chitosan treated at 45 and 55 °C were not dissolved into the solution and stable with the swollen state in the aqueous solution. This high stability of CO2-activated chitosan at 45 and 55 °C could be resulted from cross-linking of chitosan as mentioned at Section .

Figure 1

Stability of the CO2-activated chitosan hydrogel in the aqueous solution at 25 °C (red —●—), 35 °C (red —■—), 45 °C (red —▲—), and 55 °C (red —◆—) and chitosan in pure water at 25 °C (black --◆--) and 45 °C (black --■--).

Adsorption of Brilliant Blue FCF on CO2-Activated Chitosan

The adsorption on CO2-activated chitosan is evaluated in a removal efficiency E and adsorption capacity of dye Qe is defined as followswhere C0 and Ce in mg L–1 are initial and equilibrium concentrations of the dye in the aqueous solution, respectively. V is the volume of the aqueous solution in liters and Wc is the weight of the dried chitosan hydrogel in grams. As given in Figure , the weight of CO2-activated chitosan at 25 °C is reduced by dissolution during adsorption. The value of Wc at the adsorption time over 30 min was corrected by the following equation

The derivation of eq can be found in the Supporting Information.

Figure shows the results of Brilliant Blue FCF (BBF) removal efficiencies and the adsorption capacities of CO2-activated chitosan at 25 °C in the case of the initial dye concentration of 5.0 mg L–1. The results in the case of CO2-activated chitosan are compared with those in the HCl aqueous solution (pH 3.0) and without CO2 (pH 7.0). In Figure a, the removal of BBF by CO2-activated chitosan can reach the saturation state in only 40 min, whereas the removal of BBF by chitosan in pure water requires too long a time over 480 min for the saturation state. The results of CO2-activated chitosan are similar to that in the HCl aqueous solution. CO2-activated chitosan can provide strong interaction with the sulfonic group SO3– of the BBF molecule in the aqueous solution because of the protonation of the amino group NH3+ as given in Figure a. The results of BBF adsorption capacities on CO2-activated chitosan at 25 °C are shown in Figure b. The adsorption capacities of BBF by CO2-activated chitosan are higher than those by chitosan in pure water because of the interaction between BBF and protonated amino groups of chitosan. The rapid increase of the adsorption capacity of CO2-activated chitosan could be by the dissolution of chitosan into the aqueous solution. This result suggest that the benefits of CO2-activated chitosan could be fully maximized by the improvement of the chitosan stability in the aqueous solution.

Figure 2

Results of BBF adsorption on CO2-activated chitosan at 25 °C and C0 = 5.0 mg L–1. (a) Dye removal efficiency and (b) dye adsorption capacity: CO2-activated chitosan (blue ●), chitosan in pure water (red ▲) in the HCl solution (black ■).

Figure 3

Adsorption of (a) BBF and (b) CR on CO2-activated chitosan. (c) Chemical structure of chitosan, BBF, and CR molecules.

Figure gives the effect of temperature on BBF removal efficiencies by CO2-activated chitosan in the initial concentration 5.0 mg L–1. The higher removal efficiencies of BBF are provided at the higher temperature adsorption on CO2-activated chitosan. The removal efficiencies by CO2-activated chitosan at the highest temperature of 55 °C reach over 90% in the adsorption for 180 min. Chitosan in pure water results in the reduction of the BBF removal efficiencies by increasing temperatures from 25 to 45 °C as presented in the Supporting Information. This means an exothermic adsorption by chitosan in pure water as reported well in other studies.[21,32] Reddy and Lee also reported that most of the magnetic chitosan composites adsorbed a reactive dye in exothermic nature.[33] The temperature effect on the BBF adsorption by CO2-activated chitosan tends to be opposite to those of chitosan in pure water. Also, the adsorption on CO2-activated chitosan at high temperatures increases the stabilities of chitosan in the aqueous solution as given in Figure . To investigate these unique results, the pH value in the dye aqueous solution during adsorption on CO2-activated chitosan was measured. The pH value in the solution during adsorption would be increased by dye adsorption on chitosan as reported in other studies.[20,32,34] It was found that the initial pH value did not vary significantly by increasing the temperatures as given in the Supporting Information. This means the stability of chitosan at high temperatures are not affected by CO2 solubility and the pH value in the aqueous solution. This unique phenomenon of CO2-activated chitosan could be from the internal modification of the chitosan network. As shown in Figure , CO2 could form a carbamate cross-linking in the chitosan network, which is reported in the system of CO2 and amine.[14,17,35,36] The high temperature could reduce the activation energy for the carbamate formation by CO2 and amine,[37] consequently the chitosan stability in CO2-activated system is enhanced. The formation of the carbamate cross-linking of chitosan is investigated further by the chitosan characterization at Section . These synergistic effects on the dye removal efficiency and chitosan stability at higher temperatures suggest the possibilities of the dye adsorption by CO2-activated chitosan in wastewater for a longer period.

Figure 4

Effects of temperature on removal efficiency of BBF by CO2-activated chitosan in C0 = 5.0 mg L–1 at 25 °C (black ■), 35 °C (red ◆), 45 °C (blue ▲), and 55 °C (green ●).

Figure 5

CO2-activated chitosan hydrogel after BBF adsorption for 2 h: (a) original chitosan hydrogel, (b) 25, (c) 45, and (d) 55 °C.

Figure 6

Mechanism of chitosan cross-linking by CO2 at high temperatures.

The BBF adsorption on CO2-activated chitosan is also investigated at various initial BBF concentrations 5.0–2000.0 mg L–1 in the aqueous solution at 25 °C as given in Figure . In Figure a, the adsorption capacities of CO2-activated chitosan at high initial BBF concentration are much higher than those of chitosan in pure water because of the interaction between BBF and the protonated chitosan. The adsorption capacity of CO2-activated chitosan was around 3121 mg g–1 at initial 600 mg L–1. The removal efficiencies of BBF increase and then decrease gradually with increasing the initial concentration as given in Figure b. This could be because of the saturation of the protonated amino groups in chitosan by the adsorption of BBF. It can also be seen that at very low initial BBF concentration (less than 10 mg L–1), the dye adsorption capacity of chitosan in pure water is higher than CO2-activated chitosan. This is because at low initial BBF concentration, the SO3– groups on dye molecules can be attracted to both protonated NH3+ groups on chitosan molecules or H+ ions in the aqueous solution. When initial BBF concentration is higher than 10 mg L–1, the attraction with H+ ions becomes negligible, whereas the strong electrostatic attraction between protonated NH3+ groups on chitosan molecules and the SO3– groups on BBF molecules becomes dominant. Therefore, at initial BBF concentration higher than 10 mg L–1, the adsorption capacity of CO2-activated chitosan was found to be significantly higher than chitosan in pure water. The time of reaching the adsorption equilibrium was also found to be affected by the initial BBF concentration. An increase of the initial concentration results in increasing in the time of reaching adsorption equilibrium (see the Supporting Information).

Figure 7

Effects of initial BBF concentration on (a) adsorption capacity and (b) removal efficiency at 25 °C: CO2-activated chitosan (blue ●) and chitosan in pure water (red ▲).

Adsorption of Congo Red on CO2-Activated Chitosan

Figure shows the effect of the temperature on the removal efficiencies of Congo Red (CR) by CO2-activated chitosan. The initial CR concentration in the aqueous solution was 10.0 mg L–1. At temperatures 25–55 °C, the CR removal efficiencies increase rapidly and then decrease gradually because of the chitosan dissolution into the aqueous solution. The removal efficiencies of BBF at 25 °C as given in Figure a are not reduced although chitosan was dissolved. This might be because the interaction of CR with the protonated chitosan is weaker than that of BBF. The protonated chitosan could be exposed to water molecules faster than bonding with CR, consequently leading to the dissolution faster than that in the case of BBF. The removal efficiencies of CR on CO2-activated chitosan are lower than those of BBF. The molecular structure of CR includes an amino group which is protonated by CO2. The protonated amino group of CR could be repulsive with chitosan protonated by CO2 as given in Figure b. The adsorption amount of CR on CO2-activated chitosan is lower than that of BBF. At 65 °C, the removal efficiency of CR is over 40%.

Figure 8

Effects of temperature on CR removal efficiency by CO2-activated chitosan in C0 = 10.0 mg L–1 at 25 °C (black ■), 35 °C (red ◆), 55 °C (green ●), and 65 °C (yellow ▲).

Characterization of CO2-Activated Chitosan

IR Spectra

Figure shows the IR spectra of chitosan after BBF adsorption for 3 h in the initial concentration 5.0 mg L–1 in the CO2-activated system (a), pure water at 25 (b) and 55 °C (c), and the IR spectra of pure chitosan (d). Typical peaks include the overlapping of OH stretching and NH stretching vibrations at around 3348 cm–1, NH bending vibration at 1657 cm–1, which have also been reported in other literature studies.[27,38] The CO2-activated chitosan for adsorption at 55 °C shows a new peak at the wavenumber 1730 cm–1, although no peak is detected for chitosan treated at 25 and 55 °C. This new peak is for C=O stretching vibration, which could be the result of the carbamate cross-linking in the chitosan network as mentioned at Section .

Figure 9

IR spectra of CO2-activated chitosan after BBF adsorption at (a) 55 °C, chitosan in pure water at (b) 55 and (c) 25 °C, and (d) original chitosan.

Ratio of NH2 Group in Chitosan

The amino group NH2 in chitosan plays an important role to both removal efficiency and stability of the chitosan hydrogel because NH2 groups in chitosan work as the dye adsorption site and the cross-linking part by carbamate formation. Table lists the results of the NH2 ratio in CO2-activated chitosan treated for 1.5 h at 65 °C. CO2-activated chitosan results in a NH2 ratio lower that of the pure chitosan hydrogel, which is very similar to the NH2 ratio in chitosan cross-linked by tri-polyphosphate (TPP) 0.0082 M in the solution. Moreover, the BBF adsorption capacity of CO2-activated chitosan and chitosan cross-linked by TPP 0.0082 M was found to be almost the same (see Supporting Information). This result indicates that CO2-activated chitosan at high temperatures can form the cross-linking similar to that by TPP.

Table 1

Amino Group Amount in CO2-Activated Chitosan and Chitosan Cross-Linked by TPP

sample	NH2 amount in chitosan (mmol g–1)
pure chitosan hydrogel	0.124
chitosan-CO2 activated	0.074
chitosan-TPP (0.136 M)	0.010
chitosan-TPP (0.0136 M)	0.011
chitosan-TPP (0.0082 M)	0.070

Zeta Potential

The results of the zeta potentials for CO2-activated chitosan at 45, 55, and 65 °C are given in Table . CO2-activated chitosan has the positive surface charge by the protonation of amino groups. McCann et al.[39,40] have reported that chitosan and CO2 can form three functional groups, the protonated amino group (NH3+), carbamate, and bicarbonate. The reaction of chitosan with CO2 was also investigated on the effect of amine types on the formation of functional groups, NH3+, carbamate, and bicarbonate.[41−45] It is reported that the primary and secondary amines lead to the production of the stable carbamate. Chitosan could produce mainly carbamate because of having the primary amino group in its molecular structure. The carbamate formation with CO2 is exothermic and not favorable at high temperatures. It is thought that the protonated amino group NH3+ could be more dominant than carbamate as the available adsorption site for the dye at high temperatures. This is also explained from the results of the zeta potential in Table as the zeta potential increases with temperature. The results of zeta potential are used for understanding the high removal efficiencies of BBF on CO2-activated chitosan at high temperatures as given in Figure .

Table 2

Zeta Potential of CO2-Activated Chitosan in Aqueous Solution

temperature (°C)	zeta-potential (mV)
45	33.9
55	39.0
65	49.0

Thermal Gravimetric Analysis

The results of thermal gravimetric analysis of CO2-activated chitosan treated for 1.5 h at 65 °C are given in Figure and compared with those of pure chitosan and chitosan cross-linked by TPP 0.136 M. Chitosan activated by CO2 gives the decomposition temperature lower than that of pure chitosan. According to Neto et al.,[46] the reduction of the decomposition temperature can be found in cross-linking of the chitosan network due to the disruption of hydrogen bonding inside the chitosan network. For the chitosan hydrogel cross-linked by TPP, the concentration of the TPP solution was high (0.136 M), which means a lot of amino groups of chitosan are cross-linked. The decomposition temperature of chitosan cross-linked by TPP is much lower than that of pure chitosan as given in Figure . At higher temperatures over than 350 °C, CO2-activated chitosan results in weight reduction same as that of pure chitosan because the carbamate cross-linking can be easily decomposed by heating.[14,47]

Figure 10

Thermal gravimetric analysis results of CO2-activated chitosan treated at 65 °C for 1.5 h (red - - -), chitosan cross-linked by TPP 0.136 M (blue – - −), and original chitosan (—).

Conclusions

Dye adsorption on carbon dioxide-activated chitosan in the aqueous solution was investigated. CO2-activated chitosan possesses dye adsorption more rapidly than that in pure water. The dye adsorption capacities and stability of CO2-activated chitosan are increased at high temperatures, in which the carbamate cross-linking of chitosan is formed with CO2. The dye initial concentration in the aqueous solution affects the adsorption capacities on CO2-activated chitosan, dye removal efficiencies, and the time of reaching the adsorption equilibrium. The dye removal efficiencies by CO2-activated chitosan decrease at the high initial concentration in the aqueous solution. It is found that the BBF dye results in the adsorption capacities of CO2-activated chitosan higher than those in the case of CR with the repulsive amino group to chitosan. The characterizations of CO2-activated chitosan suggest the formation of carbamate cross-linking in the chitosan network.

The dye adsorption using CO2-activated chitosan is expected to achieve a simple, low cost, and short operation in wastewater treatment. Furthermore, the CO2 utilization proposed in this work will provide an environmentally benign and no corrosive nature on dye adsorption in wastewater.

Experimental Section

Materials

Chitosan, acetic acid, sodium hydroxide (NaOH), hydrochloric acid (HCl), sodium TPP, disodium 2-[[4-[ethyl-[(3-sulfonatophenyl)methyl]amino]phenyl]-[4-[ethyl-[(3-sulfonatophenyl)methyl]azaniumylidene]cyclohexa-2,5-dien-1-ylidene]methyl]benzenesulfonate (BBF), and disodium; 4-amino-3-[[4-[4-[(1-amino-4-sulfonatonaphthalen-2-yl)diazenyl]phenyl]phenyl]diazenyl]naphthalene-1-sulfonate (CR) were all purchased from Wako Pure Chemical Ind. Ltd., Japan. The purities of acetic acid, NaOH, HCl, TPP, and BBF were higher than 99.9, 97.0, 35.0–37.0, 85.0, and 85.0 by mass, respectively. Chitosan had a degree of deacetylation of more than 80%. Ultrapure water was produced by the Direct-Q UV3 water purification system (EMD Millipore Co.), and the resistivity was 18.2 MΩ cm.

Fabrication of Chitosan Hydrogel

Two different types of chitosan hydrogel were fabricated; those with and without cross-linking by TPP. Chitosan was firstly dissolved into a 2.0 wt % acetic acid aqueous solution with the chitosan composition 2.5 wt % in the solution. The viscous chitosan aqueous solution was vigorously stirred for 5 h, sealed, and left overnight to remove any bubbles remaining in the solution. After stirring, the chitosan solution was dropped from a syringe (TERUMO SS-10SZ) into the coagulation mixture, TPP aqueous solution, and 10.0 wt % NaOH aqueous solution in the case of those with and without cross-linking. Various concentrations of TPP solutions, 0.0082, 0.0136, and 0.136 M, were used. After coagulation, the chitosan hydrogel formed was left overnight before being washed extensively by ultrapure water. All fabricated chitosan hydrogel samples were spherical in shape with an average diameter of 4.1 mm and were stored in ultrapure water.

Dye Adsorption on CO2-Activated Chitosan

Chitosan hydrogels fabricated at Section without TPP cross-linking were activated by CO2 bubbling in the aqueous solution and used for dye adsorptions. A known amount of the dye powder, BBF, or CR was firstly dissolved into ultrapure water for preparation of the aqueous solution. The setup of the adsorption system with CO2-activated chitosan is illustrated in Figure . Carbon dioxide from a gas cylinder was installed into the adsorption vessel. The flow rate of CO2 was controlled by a gas flow controller (Kofloc RK-1200) as 200 mL min–1. The temperature of the adsorption system was kept in a water thermostatic bath (Yamato Thermomate BF 200). Chitosan 1.0 g was loaded into the dye aqueous solution of 200 mL. The initial pH of the dye aqueous solution before the adsorption was 3.9 because of the dissolution of CO2. The pH of the solution was measured by Waterproof pHTestr 20 (Oaklon).

Figure 11

Schematic diagram of the CO2-activated system for dye adsorption on the chitosan hydrogel: (1) CO2 gas cylinder, (2) pressure regulator, (3) gas flow rate controller, (4) CO2 inlet pipe, (5) CO2 outlet pipe, (6) adsorption vessel, (7) water thermostatic bath, and (8) temperature controller unit.

The dye concentration in the aqueous solution during the adsorption experiment was measured by a UV–vis spectrometer (JASCO V730). The absorbances at the wavelength 629 and 498 nm were used for the determination of BBF and CR, respectively. In the case of CR, as the dye color changes in the low pH solution which leads to a λmax shift, the peak area of absorbance at the wavenumbers 418–578 nm was used for the determination of the concentration.

Characterization of Chitosan

The molecular vibrations of chitosan treated in the adsorption were investigated by a Fourier transform IR spectrometer. The IR spectra of chitosan were measured at the wavenumbers 600–4000 cm–1 using JASCO FT-IR 4100 after drying at 60 °C for 12 h.

The ratio of the NH2 group in chitosan was used for knowing the cross-linking in the CO2-activated adsorption system. The titration method was applied for the determination of the NH2 ratio in chitosan. Chitosan (0.5 g) was dispersed in 100 mL of ultrapure water, and then an aqueous HCl solution with pH 1.9 was slowly dropped at the rate of 0.1 mL min–1. Because of the increased amount of H+ in the solution, pH would decrease and then increase again by the protonation of NH2 groups in chitosan. Therefore, the ratio of the NH2 group in chitosan in the unit of mmol g–1 can be given from the pH change of the solution.

The surface charge of the chitosan hydrogel in the CO2-activated adsorption system was studied by the measurement of zeta potential. The chitosan treated in the CO2-activated adsorption system at 45, 55, and 65 °C for 2 h were dried at 60 °C for 12 h. The dried chitosan was ground into its fine powder and dispersed into ultrapure water in 1.0 mg mL–1. The suspension of chitosan was sonicated for 30 min, vigorously stirred at 500 rpm for 2 h, and allowed to settle for 1 h. The supernatant phase of the suspension was taken for zeta potential measurements by nano partica SZ-100-Z, HORIBA Ltd.

The thermal gravimetric analysis of chitosan was also conducted to investigate the effect of CO2-activated adsorption system on thermal stability. The chitosan activated by CO2 at 65 °C for 1.5 h and that cross-linked by TPP 0.136 M was dried at 60 °C for 12 h and analyzed by using Shimadzu TGA-50.

The weight of CO2-activated chitosan treated in the aqueous solution was measured to know the stability by the dissolution at low pH.