The adsorption of lemon yellow dye using cationic cellulose fibers from rice straw as a sustainable biosorbent in aqueous medi

Abstract

A biosorbent was prepared from the cellulose fibers found in rice straw through cationic modification. The effects of the dosage, pH, contact time, and initial concentration of lemon yellow dye were explored. The static adsorption results showed that cationic modification drastically improved the adsorption capacity of straw cellulose fiber. The maximum equilibrium adsorption capacity value was 137.6 mg/g and the highest removal reached 99%. The pseudo-second-order kinetic model was a good fit for the adsorption process, together with the Langmuir isotherm model. The adsorption reaction was spontaneous, and the adsorption process was an exothermic reaction, which was shown by the thermodynamic model. As the adsorption time became longer, the effluent concentration became larger until reaching equilibrium. The time was 420 min. After desorption using a dilute NaOH solution, the maximum adsorption capacity was still 36.1 mg/g and the maximum removal still reached 36.2%. The parameters calculated from the Yoon-Nelson model have a good fit with the experimental data. In short, cationic straw cellulose fiber is an effective and easy to prepare biosorbent. This work offers a new method for dye wastewater purification and solves the effective utilization of rice straw resources.

Full Article

The Adsorption of Lemon Yellow Dye Using Cationic Cellulose Fibers from Rice Straw as a Sustainable Biosorbent in Aqueous Media

Zhixin Gu,^a,b Sijia Zhang,^a Chuanqing Zhu,^b and Lijuan Wang ^a,*

A biosorbent was prepared from the cellulose fibers found in rice straw through cationic modification. The effects of the dosage, pH, contact time, and initial concentration of lemon yellow dye were explored. The static adsorption results showed that cationic modification drastically improved the adsorption capacity of straw cellulose fiber. The maximum equilibrium adsorption capacity value was 137.6 mg/g and the highest removal reached 99%. The pseudo-second-order kinetic model was a good fit for the adsorption process, together with the Langmuir isotherm model. The adsorption reaction was spontaneous, and the adsorption process was an exothermic reaction, which was shown by the thermodynamic model. As the adsorption time became longer, the effluent concentration became larger until reaching equilibrium. The time was 420 min. After desorption using a dilute NaOH solution, the maximum adsorption capacity was still 36.1 mg/g and the maximum removal still reached 36.2%. The parameters calculated from the Yoon-Nelson model have a good fit with the experimental data. In short, cationic straw cellulose fiber is an effective and easy to prepare biosorbent. This work offers a new method for dye wastewater purification and solves the effective utilization of rice straw resources.

Keywords: Rice straw; Cationic; Cellulose fiber; Lemon yellow dye; Adsorption

Contact information: a: Key Laboratory of Bio-based Materials Science and Technology of Ministry of Education, Northeast Forestry University, Harbin 150040 PR China; b: College of Information and Computer Engineering, Northeast Forestry University, Harbin 150040 PR China;

* Corresponding author: donglinwlj@163.com

GRAPHICAL ABSTRACT

INTRODUCTION

Dye is widely used in many fields, but it easily pollutes the environment (Baldikova et al. 2015). The characteristics of dye wastewater has a complex composition, deep color, high toxicity, strong acidity, and high COD content, and has always been a problem in terms of environmental protection (Heibati et al. 2015; Zhang et al. 2017).

There are three common treatment methods for dye-polluted water: physical, chemical, and biosorption (Elumalai et al. 2016; Asfaram et al. 2017). Each method has its own advantages and disadvantages. Electrochemical methods have different decolorization effects on different dyes (De et al. 2018). However, they are highly efficient in avoiding secondary pollution (Alver et al. 2017). The follow-up treatment is simple, but its cost and energy consumption are high. The advantages of the biological method include high purification efficiency, low price, and no secondary damage to water. However, the decolorization rate and COD removal are not high (Sun et al. 2016). The adsorption method is a physical method that is simple to use and yields a strong purification effect, etc. However, the adsorbent has a high cost and often cannot be reused (Patil et al. 2015). Moreover, improper subsequent treatment may cause secondary pollution. To date, the adsorption property of activated carbon is widely used to treat dye wastewater. It has a porous structure, high specific surface area, and high absorptivity (Ge et al. 2016; Ciftci et al. 2018). However, its disadvantages are it is difficult to regenerate, has a high cost, and the possibility of recycling is low (Sudhan et al. 2016). However, biosorbents can make up for the disadvantages of activated carbon adsorption.

It is estimated that China produces 300 million tons of rice straw a year (Zhu et al. 2017). A primary use of rice straw involves its crushing in the field for the preparation coal-based fuel for heating. However, a large part of it is still directly incinerated, which not only wastes resources but also seriously pollutes the environment (Fan et al. 2017; Zazycki et al. 2017). The primary components of rice straw are cellulose (43.2%), hemicellulose (11.5%), and lignin (28.9%). Rice straw is rich in hydroxyl groups, aldehyde groups, and carboxyl groups, as well as other active groups, which can undergo a variety of chemical modification reactions (Chen et al. 2019). Recently, using rice straw as a biosorbent to adsorb dye wastewater has been a popular research topic.

From this perspective, many studies have made a lot of progress. A solid adsorbent used for SO₂ adsorption was prepared from magnesium salt/rice straw via co-precipitation/ calcination (Hosseini et al. 2018). Xia et al. (2013) used rape straw to extract natural cellulose and prepared several adsorbents for organic dye wastewater treatment with good results. Xie et al. (2019) prepared a wheat straw cellulose/GO composite material for the removal of Cd²⁺ and Pb²⁺ from water and the removal reached 93.25%. Halysh et al. (2020) determined the adsorption capacity for methylene blue (68.8 mg/g) using a lignocellulosic sorbent derived from sugar cane straw. Fiber biosorbents have a low-cost and abundant; the problem is their low adsorption capacity and as such, it needs modification to improve its adsorption capacity.

Hu et al. (2016) used cationic flax noil cellulose as an adsorbent to remove ciprofloxacin from an aqueous solution and confirmed that the cationic groups had been successfully bound to the cellulose molecules after functionalization. Li et al. (2019) showed that KMnO₄-modified rice husk can be used as an effective alternative adsorbent for the removal of malachite green (MG) from wastewater, with an adsorption rate greater than 90%. Ghasemi et al. (2020) used modified barley straw to remove MG from industrial wastewater. The removal efficiency of MG increased as the modified barley straw dosage, pH, and contact time were increased (Ghasemi et al. 2020). Hu et al. (2019) developed wheat straw modified with maleated rosin to improve its adsorption capacity for methylene blue (MB) dyes, with an adsorption capacity of 113.3 mg/g. Lima et al. (2018) studied MG adsorption by raw and modified corn straw (MCS) and the adsorbed quantity at saturation ranged from 164.2 mg/g to 528.2 mg/g.

All the above studies focused on the static adsorption characteristics of modified cellulose adsorbents prepared from straw. These adsorbents had a low adsorption capacity. Very few studies have been done on the application of modified straw cellulose in dye treatment, especially in terms of the dynamic adsorption performance, which can describe the practical application more intuitively and provide data for industrial design. In order to further improve the adsorption capacity, rice straw was modified via cationization to prepare cationic straw cellulose fiber (CSCF), which could purify anionic dye solutions. On the basis of studying the static adsorption, this work studied the dynamic adsorption characteristics of lemon yellow (LY) dye using CSCF. It is mainly used for coloring food, beverage, medicine, cosmetics and wool. Lemon yellow wastewater has deep chroma and high COD content, which is one of the difficulties in dye wastewater treatment. The Yoon-Nelson model was used to simulate the dynamic adsorption behavior to provide basic data for industrial applications. This cheap and efficient biomass material will bring new economic and social benefits to dye wastewater treatment.

EXPERIMENTAL

Materials

Caustic soda SCF was prepared in the laboratory. The sodium hypochlorite (NaClO, 120 g/L) and hydrochloric acid (HCl, 36%) was purchased from Shanghai Shenmei Pharmaceutical Technology Co., Ltd. (Shanghai, China). The sodium hydroxide (NaOH), triethylamine (C₆H₁₅N), epichlorohydrin (C₃H₅ClO), anhydrous ethanol (C₂H₅OH), and LY (C₁₆H₉N₄Na₃O₉S₂) were analytical reagent grade purchased from Yongda chemical Reagent Co. Ltd (Tianjin, China).

Preparation of the Cationic Straw Cellulose Fiber (CSCF)

In order to be able to remove anionic substances from solution, the SCF was cationized. The NaClO solution was well mixed with SCF and stirred at a temperature of 35 °C for 1 h at a constant temperature via a constant speed agitator (JJ-1B). The moisture was filtered out, and the material was rinsed to remove the residual NaClO. The SCF before and after bleaching is shown in Figs. 1a and 1b. The SCF with an 80 mesh to 120 mesh size was selected using an electric vibrating screen (8411), as shown in Fig. 1c. The qualified SCF and NaOH solution (20%) were added to a beaker and stirred via a water bath thermostatic oscillator (DSHZ-300A). A triethylamine ethanol solution with a volume fraction of 34% was added into the flask, which was stirred at a temperature of 80 °C for 3 h. The resulting solution was filtered and rinsed to remove the residual triethylamine. Finally, the CSCF was put into a vacuum drying oven (DZ-2AⅡ) at a temperature of 60 °C, as shown in Fig. 1d.

Fig. 1. CSCF preparation process

Characterization of the Cationic Straw Cellulose Fiber (CSCF)

The morphology of samples was examined via a Quanta 200 scanning electron microscope (Philips-FEI Co., AMS, Netherlands), with an accelerating voltage of 20 kV. The morphology of the CSCF was observed using a JEM-2100 transmission electron microscope (TEM).

Adsorption Process

The effects of the adsorbent dosage, pH, contact time, and initial concentration of LY dye on the CSCF adsorption were studied. Different amounts of CSCF adsorbent and contact time were demonstrated at various time intervals (10 min to 180 min). The CSCF adsorbent and LY dye solution were added into a conical flask at various initial LY dye concentrations (20 mg/L, 40 mg/L, 60 mg/L, 80 mg/L, and 100 mg/L). The solution was shaken at temperatures of 30 °C, 40 °C, and 50 °C and at pH values of 2, 4, 6, 8, 10, and 12. After filtration, the absorbance was measured, and the concentration of the residual dye after adsorption (C_e) was calculated from the standard curve. The adsorption capacity (Q) and removal rate (ω) were calculated using Eqs. 1 and 2,

(1)

(2)

where m is the mass of the adsorbent (mg), V_o is the volume of solution (mL), and C_o and C_e are the initial and final equilibrium concentrations (mg/L).

Dynamic Adsorption

The CSCF adsorbent (0.9 g) was put into a column (the column diameter was 3.0 cm and the column height was 11.5 cm) as the stationary phase, and the adsorption bed height was 2.0 cm (Fig. 2a). The flow rate of the LY solution (45 mg/L and 50 mg/L) was 5 mL/min, and the effluent solution was collected. The absorbance of the filtrate was measured for each 5 mL solution (Fig. 2b).

Fig. 2. Dynamic adsorption process

Desorption and Secondary Adsorption

The CSCF after undergoing saturation adsorption (Fig. 3a) and the NaOH solution (0.1 mol/L) were added into conical flasks. Desorption was conducted by shaking at a temperature of 30 °C for 5 min to 50 min. The CSCF after filtering and desorption is shown in Fig. 3b. The absorbance of the filtrates were measured, respectively. The solution concentration after desorption and the desorption rate of the CSCF after saturated adsorption were calculated. The CSCF adsorbent after desorption and the LY dye solution (50 mg/L) were added into the conical flask. After shaking and filtration, the secondary adsorption of LY dye occurred (Fig. 3c) and the C_e, Q, and ω after the desorption occurred were calculated.

Fig. 3. Desorption and secondary adsorption

RESULTS AND DISCUSSION

Scanning Electron Microscopy (SEM) Analysis

The SEM images of the SCF and CSCF are shown in Figs. 4a and 4b. The surface morphology of the SCF and CSCF had obviously changed. The SCF was filamentous, with many large gullies on the surface and a small amount of impurities. The surface of the CSCF had fewer impurities and was fully saturated. There were some tiny gullies on the surface and some relatively small pore structures of different sizes. These changes visible in the SEM images suggest an increase in surface area, which would be favorable for adsorption interactions.

Fourier-Transform Infrared Spectroscopy (FTIR) and X-Ray Diffraction (XRD) Analysis

The FTIR spectra of the SCF and CSCF are shown in Fig. 4c. The absorption peak of SCF at 3330 cm^-1 is the vibration absorption peak of O-H. The telescopic vibration peak of C-H in -CH₂– occurs at 2900 cm^-1. The telescopic vibration peak of C=O occurs at 1743 cm^-1. The lactone and O-H in-plane bending vibration peak occurs at 1316 cm^-1. The peak at 1028 cm^-1 represents the telescopic vibration peak of the cellulose skeleton. The spectrum of CSCF shows that the strength of the telescopic vibration peak of O-H is obviously weakened and deviates towards a high wave number. The strength of the absorption peak of C-H in -CH₂– is slightly enhanced, which showed that the new CH₂^– group was successfully introduced into the SCF molecules. The telescopic vibration peak of C-N is shown at 1470 cm^-1, and the CSCF spectra had less miscellaneous peaks than the SCF spectra. These findings demonstrate that the quaternary ammonium group molecular structure was successfully introduced into the molecular structure of SCF, which indicated that the CSCF was indeed synthesized.

As can be seen from Fig. 4d, the characteristic peaks at 15.52° and 22.32° show that SCF was a typical cellulose I-type structure. There was only one characteristic peak at 20.26°. After cationic modification, the intermolecular and intramolecular hydrogen bonds of SCF were destroyed, which resulted in a change of crystal shape.

Fig. 4. Characterization images of the SCF and CSCF

Purification Performance

As shown in Figs. 5a and 5b, the adsorption capacity for LY dye was able to reach 138 mg/g, and it gradually decreased as the CSCF dosage was increased. The percentage of removal gradually increased until the system reached equilibrium, with a maximum of 99.3%. The optimal CSCF dosage is 0.05 g.

Rapid adsorption occurred within 30 min, whereas the period of 30 to 90 min was characterized by slow adsorption, as shown in Figs. 5c and 5d. After 90 min, the system reached equilibrium adsorption. The optimal adsorption time was judged to be 90 min, the highest adsorption capacity was 84.0 mg/g, and the maximum removal was 84.3%. As the temperature increased, the adsorption capacity and removal rate decreased at the same contact time, which indicated an exothermic reaction.

The maximum adsorption capacity was 94.0 mg/g, as shown in Figs. 5e and 5f. The optimal initial concentration was judged to be 50 mg/L. The removal percentage was 96.4% at its highest. The increase of the initial concentration not only increases the probability of collision between dye molecules and the adsorbent, but it also increases the concentration difference between the surface of the adsorbent and the main body of the solution.

Fig. 5. Purification performance

As shown in Fig. 6, the pH had a strong influence on the adsorption of LY dye solution by CSCF. When the LY dye has an optimal pH of 4, the adsorption capacity and removal rate can reach 88.7 mg/g and 88.7%, respectively.

Fig. 6. Effects of different pH values on the purification performance

Adsorption Kinetics

Adsorption kinetics studies the relationship between the adsorption capacity and the contact time, i.e., the adsorption rate and the dynamic equilibrium of adsorption.

Pseudo-first-order kinetic model

Assuming that the adsorption is mainly affected by diffusion, it can be calculated according to Eq. 3,

(3)

where k₁ is the rate constant (min^-1), q_e and q_t are the adsorption capacity at equilibrium and at time t (mg/g). The conclusion is that the adsorption rate is proportional to q_e minus q_t.

According to the experiment data, the relationship between ln(q_e–q_t) and t at different times is shown in Fig. 7a. The constants and adsorption capacity were calculated using the slope and intercept of the line.

Pseudo-second-order kinetic model

The expression of the pseudo-second-order kinetic model is shown in Eq. 4,

(4)

where k₂ is the rate constant (min^-1).

According to the experiment data, the relationship between t/q_e and t at different times is shown in Fig. 7b. The rate constant and adsorption capacity were calculated using the slope (1/q_e) and intercept (1/k₂q_e²) of the line.

Fig. 7. Adsorption kinetics of adsorption reaction

The adsorption kinetics parameters are shown in Table 1. There was a linear relationship between ln(q_e–q_t) and t (as shown in Fig. 7a and Fig. 7b). The reaction rate was approximately linear with the concentration of LY dye.

Comparing the results of the two models, the pseudo-second-order kinetic model better described the adsorption process. The linear fit of the experimental data presented a more linear form with R² equal to 1. The theoretical adsorption capacity was closer to the actual adsorption capacity. Such fits of data to the pseudo-second-order kinetic model support diffusion as the rate-determining step (Hubbe et al. 2019).

Adsorption Isotherm

An adsorption isotherm refers to the relationship between the amount of adsorbates attached on the surface of an adsorbent and that in the solution at adsorption equilibrium. It can describe the adsorption process and make it possible to infer the interaction between adsorbents and adsorbents.

Langmuir isotherm

The expression of the Langmuir isotherm is shown in Eq. 5,

(5)

where C_e is the solution concentration at equilibrium (mg/L), q_e is the adsorption capacity at equilibrium (mg/g), q_m is the maximum monolayer adsorption capacity (mg/g), and K_L is the Langmuir adsorption constant related to binding (L/mg) (Langmuir 1916).

The relationship between C_e/q_e and C_e at different adsorbent dosages is shown in Fig. 8a. The q_e, K_L, and q_m were calculated using the slope and intercept of the fit line of the Langmuir isotherm equation. The linear fit of the experimental data showed a linear form, and R² was close to 1.0.

Freundlich isotherm

This isotherm, based on an empirical equation, makes it possible to assume that the adsorption occurs on a heterogeneous surface at nonidentical sites with different energies of adsorption that are not always available (Freundlich 1906).

(6)

In Eq. 6, K_f (L/g) and n are the Freundlich constants relative to the adsorption capacity and intensity of adsorption.

According to the experimental data, the relationship between lnq_e and lnC_e at different CSCF dosages was found. The fit line of the Freundlich isotherm equation is shown in Fig. 8b. The K_f and n values were calculated using the slope and intercept of the line, and the parameter data were calculated (Table 2).

Figure 8b shows there was no linear relationship between the logq_e and logC_e of the Freundlich isothermal adsorption model. There was a good linear relationship between the C_e/q_e and C_e. The maximum of R₂ was greater than 0.95. The theoretical adsorption capacity was close to the actual adsorption capacity.

Fig. 8. Isothermal equation fitting

Thermodynamic Model

In order to test the adsorption capacity of the CSCF adsorbent for LY dye, the thermodynamic parameters were introduced, i.e., ΔG⁰, ΔH⁰, and ΔS⁰. They were studied using the experimental data detailing the effects of the temperature.

The expression of the change of Gibbs free energy ΔG⁰ (kJ/mol), the enthalpy change ΔH⁰ (kJ/mol), and entropy change ΔS⁰ (J/mol) are shown in Eqs. 7, 8, and 9, respectively,

(7)

(8)

(9)

where K_d is the distribution coefficient, q_e is the adsorption capacity at equilibrium (mg/g), C_e is the solution concentration at equilibrium (mg/L), and R is the gas constant (J/mol·K) (Chen et al. 2019).

The experimental data of the different desorption times were put into Eq. 8 to obtain the ΔG⁰. The relation line of lnK_d ~1/T was plotted as shown in Fig. 9. Based on the slope and intercept of the line, ΔH⁰ and ΔS⁰ were calculated.

Fig. 9. The relation of lnKd ~1/T

The ΔG⁰ was negative in the adsorption process (as shown in Table 3), which implies that the adsorption reaction was spontaneous. The ΔH⁰ was less than 0, so the adsorption process was an exothermic reaction. An increase in temperature would be unfavorable to the adsorption process.

The Analysis of Dynamic Adsorption

This analysis assumes that the probability of the decreasing adsorption rate is proportional to the probability of the adsorbate adsorption and adsorbent penetration.

The Yoon-Nelson model is simpler than other dynamic models. It does not need detailed data on the properties of the adsorbents, e.g., the adsorbent, adsorbate, and fixed bed properties, so, the calculation process is simplified. This model is shown in Eqs. 10 and 11,

(10)

(11)

where τ is the time taken for 50% of the adsorbate to be adsorbed (min), k is the rate constant (min^-1), C_o is the initial concentration of the adsorbate (mg/L), and C_e is the outflow concentration of the adsorbate (mg/L).

A line can be obtained by plotting t in terms of ln(C_e/(C_o–C_e)). Using the slope and intercept of the line, k and τ were calculated.

The linear form of Yoon-Nelson model is shown as Eqs. 12 and 13,

(12)

(13)

where q_o is the mass of the adsorbate adsorbed per unit mass of adsorbent at a given flow rate (mg/g), m is the mass of the adsorbent in the column (g), and θ is the effluent rate of the adsorbent (L/min) (Enniya et al. 2018).

Fig. 10. Yoon-Nelson model

The Yoon-Nelson model dynamic adsorption parameters of the CSCF adsorbent for LY dye are shown in Table 4. When the concentration of the LY dye solution was 45 mg/L, the calculated theoretical data was a good fit with the experimental data (as shown in Fig. 10). The adsorption rate constants k were 0.01×10^-3 min^-1 and 0.02×10^-3 min^-1. In a practical application, the Yoon-Nelson model is simpler than the other models because it does not consider the flow rate of the adsorbate and the adsorbent dosage. In addition, it can also obtain the time required to adsorb 50% of the adsorbent, so the adsorption efficiency can be compared.

When the initial concentration was 45 mg/L, the contact time and the effluent concentration became larger until an equilibrium was reached. In the early stage of adsorption, when the LY dye passed through the stationary phase, the CSCF had enough adsorption sites to fully absorb the dye molecules, so the effluent concentration in the initial stage was relatively low. However, with the passage of the dye solution, the CSCF adsorbs dye molecules, the adsorption sites gradually decreased, and the adsorption gradually reached saturation. Finally, the stationary phase did not have the ability to adsorb, i.e., the effluent concentration was equal to the initial concentration of dye and reached equilibrium. The equilibrium time of the dynamic adsorption was 420 min.

Analysis of the Desorption and Secondary Adsorption

With the increase in desorption time, the desorption rate increased from 5 min to 35 min (as shown in Fig. 11a). After 35 min, the desorption reaction reached equilibrium and the highest desorption rate reached 98.183%. Therefore, a NaOH solution has a good desorption effect on the CSCF adsorbent after saturated adsorption of LY dye.

Fig. 11. The desorption and secondary adsorption

When the time was between 10 min and 120 min, when the contact time increased, the adsorption capacity and removal rate increased (Fig. 11b). At 120 min, the adsorption reaction gradually reached equilibrium. The highest adsorption capacity was 36.1 mg/g, and the highest removal rate was 36.2%. According to the experimental data, the CSCF after undergoing saturated adsorption for LY dye had a maximum adsorption capacity of 37.7 mg/g and a maximum removal percentage of 37.7%. The experimental results shows that the adsorption capacity and removal extent of the CSCF secondary adsorption after the desorption were similar to those of the first adsorption. Therefore, the saturated CSCF adsorbent for LY dye still showed good adsorption performance after undergoing desorption.

CONCLUSIONS

1. A cationic straw cellulose fiber (CSCF) adsorbent was prepared via cationic modification with SCF. The optimal adsorption conditions of the CSCF for light yellow (LY) dye were as follows: a pH of 4, a temperature of 30 °C, a CSCF dosage of 0.4 g/L, an initial concentration of 50 mg/L, and a contact time between the CSCF and LY dye of 90 min.
2. Results showed that the highest adsorption capacity was 137.6 mg/g and the highest removal percentage was 99.3%. The pseudo-second-order kinetic model described the adsorption process well, and it also conformed to the Langmuir isotherm model. The thermodynamic model described that the adsorption reaction was spontaneous, and the adsorption process was an exothermic reaction.
3. Dynamic adsorption testing showed that the time to reach equilibrium was 420 min when the concentration of the LY dye solution was 45 mg/L. The theoretical data calculated using the Yoon-Nelson model had a good fit with the experimental data. After desorption via a diluted NaOH solution, the maximum adsorption capacity was still 36.1 mg/g and the maximum removal rate still reached 36.2%.
4. This work showed that CSCF was an efficient biosorbent for dye wastewater purification process. Its preparation and use provide a new method for the effective utilization of rice straw resources.

ACKNOWLEDGMENTS

The authors are grateful for the support of the Fundamental Research Funds for the Central Universities, Grant No. 2572017CB08.

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Article submitted: February 21, 2021; Peer review completed: March 28, 2021; Revised version received and accepted: April 7, 2021; Published: April 20, 2021.

DOI: 10.15376/biores.16.2.3991-4006