Enhanced decolorization of aqueous dye solutions by a high quality copolymer flocculant

Abstract

A high quality copolymer (CATCS) based on starch and chitosan was synthesized and applied for the enhanced decolorization of aqueous dye solutions. The flocculant was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The effects of the initial pH, dosage of CATCS, initial dye concentration, salt concentration and reaction time on the decolorization ability of CATCS were studied. An ultrahigh decolorization efficiency (97.1–99.7%) for the dye solutions was observed for CATCS. It was effective in a wide pH range (2 to 10) and could withstand a high salt concentration. The CATCS flocculant was ten times more efficient than commercial activated carbon in the treatment of real dye wastewater. A decolorization efficiency of 98.3% for the real dye wastewater was reached using 0.34 g L⁻¹ CATCS. The thermodynamics and kinetics of the decolorization process were investigated in detail and the decolorization mechanism with CATCS was determined. The enthalpy change was over 41 kJ mol⁻¹ for the decolorization process of dye solutions by CATCS. The positive entropy change value for the decolorization corresponded to an increase in the degree of freedom of the adsorbed species. The decolorization process was endothermic and the interaction was controlled with chemisorption.

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1 Introduction

Dye wastewater discharged from industries, such as dyestuff, textile, paper printing, cosmetic and leather industries would be harmful to the environment and aquatic life if improperly disposed of. Dyes in wastewater, even if the amounts of which are small, are highly visible and undesirable.^1,2 If these highly colored and undesirable components were discharged into bodies of water, the reoxygenation capacity of the water would be damaged and sunlight would be cut off. Thus, the biological activity in the aquatic environment would be destroyed.^3,4 Moreover, some of these dyes are toxic and carcinogenic in nature and result in serious health problems.⁵ However, with the increasing diversity of industrial products, the constituents of dye wastewater have become more and more intricate in recent years, which makes color removal one of the most difficult tasks.^6,7

Methods for processing dye wastewater involve physical, chemical and biological approaches.^8–11 Techniques such as adsorption, coagulation, chemical oxidation, electrolysis, and biological treatments, have been extensively applied in dye wastewater treatment. Among these techniques, the adsorption process is regarded as an attractive alternative treatment, especially if the adsorbent is inexpensive and readily available.^12,13 Many adsorbents such as activated carbon, peat, clay, silica, algae, fungi, etc., have been investigated to lower dye concentrations in aqueous solutions.^14–16 Nevertheless, most of them possess limitations such as hazardous by-products, high cost or intensive energy requirements. Moreover, it is difficult to remove the dyes from the effluent, because dyes are not easily degradable and are generally not removed from wastewater by conventional wastewater systems. Biological processes can overcome some of these disadvantages because they are of low cost and environmentally friendly, but they require long hydraulic retention times and will form large amounts of solid residues.^17,18 Therefore, color removal was extensively studied with physico-chemical methods such as coagulation, ultra-filtration, electro-chemical adsorption and photo-oxidation.⁴ In terms of adsorption processes, flocculants have been widely used to remove suspended particles and colored materials.^19,20 However, some dyes such as reactive dyes cannot be easily removed by the conventional flocculation and sedimentation processes due to the poor biodegradability of the reactive dye. Moreover, the application of a conventional flocculant may generate a significant amount of sludge or may easily cause secondary pollution due to excessive chemical usage.^7,21 To improve the efficiency of the adsorption and sedimentation processes, it is essential to develop more effective and cheaper flocculants with higher adsorption capacities.

In the present study, we prepared a novel cationic co-polymer flocculant (CATCS) and applied it in the decolorization of aqueous dye solutions. There are several reasons for carrying out this study. First, cationic polymeric flocculants can adsorb colored matter through charge neutralization and subsequent settling. Second, the novel co-polymer flocculant (CATCS) was highly positively charged and the unique strong bridging function of the flocculant CATCS enabled the formation of lots of dye–polymer–dye bridges, which is favourable for the adsorption and sedimentation processes of dyes. Third, the co-polymer CATCS was first used for decolorization and presented ultrahigh capacities for dye removal. The decolorization of four dye solutions and a real dye wastewater were investigated with CATCS. The effects of the initial pH, dosage of CATCS, initial dye concentration, concentration of the NaCl electrolyte and adsorption time on the decolorization ability of CATCS were studied. The CATCS flocculant was ten times more efficient than commercial activated carbon in the treatment of real dye wastewater. The thermodynamics and kinetics of the decolorization process were investigated in detail and the decolorization mechanism with CATCS was determined.

2 Experimental section

2.1 Materials

Corn starch was obtained from Wenxing Starch Co., Ltd, China; chitosan with a molecular weight of 5.2 × 10⁵ and a deacetylation degree of 95% was purchased from Yuhuan Ocean Biochemical Ltd, China. Double distilled water was used throughout the experiments. Sodium hydroxide, potassium dichromate, ammonium ferrous sulfate and acetic acid were purchased from Shanghai Chemical Reagent Co., China. C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 were obtained from Taixing Dyestuff Co., Ltd, China and their chemical formulae are shown in ESI Fig. S1.†

2.2 Preparation of CATCS flocculant

The adsorbent CATCS was prepared by a microwave-assisted heating method as described in our previous work²² using starch and chitosan as raw materials. First, Cat St (cationic starch) was prepared by reacting starch with 2,3-epoxypropyl trimethylammonium chloride in the presence of sodium hydroxide under microwave irradiation conditions. The Cat St obtained was washed with an 80% ethanol aqueous solution and then dried in a vacuum oven at 50 °C for 4 h. Then, 2 g Cat St was dispersed in the distilled water and 0.4 g chitosan was dissolved in a 1% acetic acid solution. Both were mixed together in a reactor, then 1.5 mL epichlorohydrin was added as a crosslinker. This mixture was reacted at 70 °C for 1.5 h and the flocculant was dispersed. A schematic representation for the synthesis of the co-polymer is represented in ESI Scheme 1.†

2.3 Characterization

Scanning electron microscopy (SEM) was performed using a scanning electron microscope (JSM-6390). The scanning electron microscope was operated at an accelerating voltage of 20 kV. Samples were dispersed at an appropriate concentration and cast onto glass sheets at room temperature and sputter-coated with gold. Fourier transform infrared (FT-IR) spectra were obtained on a NEXUS-870 FT-IR spectrometer. The spectra widths are typically over the range of 400–4000 cm⁻¹. All the dried samples were mixed with KBr and then compressed into thin pellets.

2.4 Decolorization experiments for dye solution

Dye solutions were prepared by dissolving the dye in deionized water to a concentration of 30 mg L⁻¹, 50 mg L⁻¹, 80 mg L⁻¹, 100 mg L⁻¹, 120 mg L⁻¹, 140 mg L⁻¹, 160 mg L⁻¹ and 180 mg L⁻¹. Each dye solution was scanned from 300 to 800 nm and its maximum absorbency wavelength was determined (the maximum absorbency wavelengths of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 are 401 nm, 552 nm, 510 nm and 570 nm, respectively). 50 mL of dye solution was introduced into an Erlenmeyer flask, then the flocculant CATCS was added. Decolorization experiments were carried out in a constant temperature shaker. The concentration of dye in solution was measured by a spectrophotometer²³ at the maximum absorbency wavelength. Dye solution without CATCS served as the control. The decolorization efficiency or removal rate of the dye was calculated using the following equation:
where C_o refers to the initial dye concentration and C_b refers to the dye concentration after adsorption.

The effects of the initial pH, dosage of CATCS, initial dye concentration, concentration of NaCl and adsorption time on the decolorization ability of CATCS were studied. The pH in these experiments was the original pH of the dye solutions unless mentioned specifically.

2.5 Decolorization experiments for real dye wastewater

Decolorization of real dye wastewater was carried out following the procedure described in Section 2.4 and the chemical oxygen demand (COD) was measured by an oxidation–reduction titration method using K₂Cr₂O₇ as the standard solution (an international standard method, reference number ISO 6060).²⁴ Because the real dye wastewater was a complex system and the amount of dyes was hard to determine, the decolorization efficiency was calculated using the following equation:²⁵
where A₀ is the absorbance of the original dye waste water and A_b is the absorbance of the dye wastewater after decolorization.

3 Results and discussion

3.1 Characterization of CATCS

The morphology of the flocculant CATCS was characterized by scanning electron microscopy (SEM), shown in Fig. 1. It was obvious that the specific surface of CATCS was very rough. The macromolecule chains of components in CATCS assembled together and had a network structure, which could promote the adsorption and bridging performance of the flocculant for dye solution decolorization.

Fig. 1 SEM photos of the CATCS flocculant.

The FT-IR spectrum of CATCS in the range of 4000–400 cm⁻¹ was obtained to confirm the presence of functional groups. The broad band (Fig. 2) at 3411 cm⁻¹ was due to the stretching mode of the O–H groups; that at 2927 cm⁻¹ was due to the C–H stretching; and an intense band at 1646 cm⁻¹ was assigned to the first overtone of the O–H bending vibration. The peaks at 1489 cm⁻¹ were due to the scanty amount of O=C–NH– groups. The bands at 1157 cm⁻¹ were assigned to the C–O stretching vibration. Two bands at 1082 cm⁻¹ and 1018 cm⁻¹ were attributed to CH₂–O–CH₂ stretching vibrations. The presence of an additional band at 1417 cm⁻¹ assignable to the C–N stretching vibration, was clear proof of the successful crosslinking reaction of the macromolecular chain of chitosan and cationic starch. The adsorption of C–N–O (706 cm⁻¹) and C–N–N (572 cm⁻¹ and 562 cm⁻¹) both resulting from the crosslinking reaction were in the fingerprint area, and were difficult to distinguish one by one; because of the combination of functional groups, some peaks were overlapped or enhanced.

Fig. 2 FT-IR spectrum of the CATCS flocculant.

3.2 Effect of CATCS dosage on decolorization efficiency

Solutions of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 were investigated in this section. The decolorization of the four dye solutions by different dosages of CATCS was carried out. The results (Fig. 3) show that the dosage of CATCS had a significant effect on the color removal. The decolorization efficiency of the dye solutions increased sharply as the dosage of CATCS increased, then remained level after reaching the peak value. The peak values of the decolorization efficiencies for the four dye solutions were 99.7%, 97.1%, 97.3% and 99.2% for C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71, respectively, and the corresponding dosages of CATCS were 0.36, 0.32, 0.35 and 0.10 g L⁻¹, respectively.

Fig. 3 Effect of the dosage of CATCS on the decolorization efficiency (dye concentration 100 mg L⁻¹, temperature 25 °C, reaction time 90 min).

3.3 Effect of reaction time on decolorization efficiency

The results described in Fig. 4 illustrate the effect of the reaction time on the decolorization of the dye solutions by CATCS. More than 90% of the color was removed within 60 min for all dye solutions and the decolorization reaction reached equilibrium in 70 min. The difference in the time to reach equilibrium may be explained by the difference in the molecular weight and chemical structure. The molecular weight order of the four dyes is C.I. Acid Red 18 (604.47) < C.I. Reactive Yellow 1 (714.29) < C.I. Reactive Violet 2 (953.6) < C.I. Direct Blue 71 (1029.86). Generally, a lower molecular weight results in a faster rate of diffusion of the adsorbate molecules due to the smaller resistance force when they cross the external boundary layer and the internal gap of the adsorbent. Meanwhile, the four dyes have different structures and various active sites for adsorption onto the adsorbent, and thus the decolorization rate was C.I. Acid Red 18 > C.I. Reactive Yellow 1 > C.I. Direct Blue 71 > C.I. Reactive Violet 2.

Fig. 4 Effect of reaction time on the decolorization efficiency (dye concentration 100 mg L⁻¹, temperature 25 °C).

3.4 Effect of initial pH on decolorization efficiency

The pH has a significant influence on both the surface binding sites of adsorbents and aqueous chemistry.²⁶ Fig. 5 depicts the effect of the pH on the decolorization. It indicated that acidic and low alkaline pH was favorable for the removal of the dyes by CATCS. The color removal of the dye solutions was affected slightly by the pH ranging from 2 to 10. However, when the pH was over 10, the decolorization efficiency dramatically decreased as the pH increased. It affected the decolorization efficiency significantly, as well as the zeta potential. The decline of the decolorization efficiency may be due to the decrease of the zeta potential of CATCS (see Fig. S2†). According to Chiou and Yoshida,^27,28 a reduction of electrostatic attractions between negatively charged dye anions and positively charged adsorption sites may cause a decrease in dye adsorption. This explanation agrees with our data on the pH effect. In low pH conditions, a positive charge dominates the surface of CATCS and a strong electrostatic attraction exists between the positively charged surface of CATCS and the negatively charged dyes. As the pH increased, the positive charge on CATCS reduced, which made the electrostatic attraction force decline, and the existence of excess OH⁻ ions competed with dye anions for the adsorption sites,²⁹ therefore preventing the adsorption of dye anions on the adsorbent surface.

Fig. 5 Effect of initial pH on the decolorization efficiency (dye concentration 100 mg L⁻¹, temperature 25 °C, adsorption time 90 min).

Though the pH dependence showed evidence of electrostatic interactions, the complex effect of the pH on the dissociation of dyes and the protonation of functional groups such as –OH, –SO₃, –NH₂, –NH, and –CO– in the dye molecules and CATCS should be noted. The complex and unpredictable effect of the pH on the color removal of C.I. Acid Red 18 by CATCS may be explained by this, because these functional groups were not only important for electrostatic interactions, but also played an indispensable role in forming dye–polymer–dye bridges.

3.5 Effect of initial dye concentration

The adsorption capacity profiles of CATCS versus the initial dye concentration are presented in Fig. 6. It is observed that an increase in the initial dye concentration led to an increase in the amount of dye adsorbed per unit weight of CATCS. A higher concentration resulted in a higher driving force of the concentration gradient. This driving force accelerated the diffusion of dyes from the solution into the adsorbent and decreased the resistance to the uptake of solute from the dye solution.²⁶ As the initial concentration increased from 30 to 180 mg L⁻¹, the amounts of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 adsorbed on the adsorbent increased from 354.2 to 873.9 mg g⁻¹, 193.7 to 903.6 mg g⁻¹, 384.3 to 1295.8 mg g⁻¹ and 721.9 to 3005.6 mg g⁻¹, respectively.

Fig. 6 Effect of the initial dye concentration (temperature 25 °C, adsorption time 90 min, dosages of CATCS were 0.33, 0.23, 0.06 and 0.04 g L⁻¹ for C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71, respectively).

3.6 Effect of salt (NaCl) concentration on decolorization efficiency

The effect of the salt (NaCl) is shown in Fig. 7. The NaCl concentration used was in the typical range for the salt tested.³⁰ Fig. 7 shows that the increase of NaCl resulted in an increase in the decolorization efficiency when the concentration of NaCl ranged from 0–0.02 mol L⁻¹, but a decrease when the concentration increased from 0.02 to 0.2 mol L⁻¹. The increase in the decolorization efficiency after NaCl addition may be attributed to an increase in the dimerization of reactive dyes in solution because intermolecular forces such as van der Waals forces, ion–dipole forces and dipole–dipole forces occurring between the adsorbent and the adsorbate increased upon the addition of the salt to the dye solution.^31–33 The decrease of color removal upon NaCl addition may be explained by the effect of NaCl on the zeta potential of the CATCS flocculant (see Fig. S3†). The zeta potential declined with the increase of the NaCl concentration, which reduced the electrostatic interactions between the adsorbent and the dyes and therefore led to the decrease of the decolorization efficiency. Moreover, Cl⁻ will compete with negative groups such as –SO³⁻ for the electropositive adsorption sites and a large amount of Na⁺ may hinder the ionization of –SO₃Na; thus, the adsorptive force between the dyes and CATCS declined and the decolorization efficiency decreased.

Fig. 7 Effect of NaCl concentration on the decolorization efficiency (dye concentration 100 mg L⁻¹, temperature 25 °C, adsorption time 90 min).

3.7 Decolorization of real dye wastewater

The properties of the real dye wastewater are shown in Table 1. The decolorization of the real dye wastewater by CATCS was studied. Commercial activated carbon, which has been widely used and investigated by a number of researchers as a relatively efficient adsorbent for color removal,³¹ was studied for comparison.

Table 1 Properties of the real dye wastewater

pH	COD (mg L^−1)	Maximum absorption wavelength (nm)	Absorbance	Color
6.89	3307.58	612	0.385	Atrovirens

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According to Fig. 8(a), the decolorization efficiency of the real dye wastewater increased as the dosage of CATCS increased until it reached a peak value of 98.3%. When the dosage of CATCS was 0.15 g L⁻¹, the decolorization efficiency of the dye wastewater was over 93%. The removal rate of the COD increased as the amount of CATCS increased from 0.10 g L⁻¹ to 0.34 g L⁻¹ and the peak value was 87.6%. When the amount of CATCS was over 0.42 g L⁻¹, the removal rate of the COD declined with the increase in the dosage of CATCS. This may be attributed to excess CATCS dissolved in the solution, which contributed to the increase of the COD. The experimental results for the commercial activated carbon are shown in Fig. 8(b). The removal rate of the COD increased with the increase of the dosage of commercial activated carbon until it reached a peak value of 97.1% at the concentration of 11.5 g L⁻¹. To get a decolorization efficiency of 90%, over 2 g L⁻¹ commercial activated carbon is needed, which is ten times the amount of CATCS required. Thus, the flocculant CATCS has an ultrahigh efficiency for the decolorization of the real dye wastewater.

Fig. 8 Decolorization efficiency of real dye wastewater by (a) CATCS flocculant and (b) commercial activated carbon (temperature 30 °C, decolorization time 90 min).

3.8 Adsorption isotherms

The equilibrium adsorption isotherm is one of the most important sources of data for understanding the mechanism of the adsorption systems. The adsorption isotherms of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 by CATCS at various temperatures are shown in Fig. 9(a), (b), (c) and (d) respectively.

Fig. 9 Adsorption isotherms of dyes at various temperatures by CATCS. (a) C.I. Reactive Yellow 1, (b) C.I. Reactive Violet 2, (c) C.I. Acid Red 18 and (d) C.I. Direct Blue 71.

Isotherm models including the Langmuir, Freundlich, Dubinin–Radushkevich, Temkin and Redlich–Peterson isotherm models were tested for the equilibrium description.³⁴ The Langmuir adsorption isotherm assumes that adsorption takes place at specific homogeneous sites within the adsorbent and has found successful application in many adsorption processes of monolayer adsorption.³⁵ The linear form of the Langmuir isotherm equation is represented by the following equation:

where q_e (mg g⁻¹) is the amount of the dyes adsorbed per unit weight of CATCS, C_e (mg L⁻¹) is the equilibrium concentration of the dyes, Q_L (mg L⁻¹) is the maximum monolayer adsorption capability and K_L (L mg⁻¹) is a constant related to the free energy of adsorption.

The well-known Freundlich isotherm, which is often used for heterogeneous surface energy systems, can also be used to describe the equilibrium data. It states that the ratio of the amount of solute adsorbed onto a given mass of adsorbent to the concentration of the solute in the solution is not constant at different concentrations.³⁶ A linear form of the Freundlich equation is

where K_F (L mg⁻¹) and n (dimensionless) are constants related to adsorption capacity and intensity. The values of K_F and n can be obtained from the intercept and slope, respectively, of the linear plot of the experimental data of ln q_e versus ln C_e. The larger the n value is, the more heterogeneous the system is.

The Dubinin–Radushkevich (D–R) isotherm is a more general isotherm because it does not assume a homogeneous surface or constant adsorption potential.³⁷ It was applied to distinguish between the physical and chemical adsorptions of the dye. The linear form of the D–R isotherm equation is

ln q_e = ln Q_D − βε² (3)

where β (mg² kJ⁻²) is a constant related to the mean free energy of adsorption per mole of the adsorbate, Q_D (mg g⁻¹) is the theoretical saturation capacity and ε is the Polanyi potential, which is equal to ε = −RT ln(1 + 1/C_e), where R (R = 8.314 J mol⁻¹ K⁻¹) is the gas constant and T (K) is the absolute temperature. Thus, by plotting ln q_e versus ε², the value of Q_D is obtained from the intercept and the value of β from the slope.

The Temkin isotherm⁴ considers the effects of some indirect adsorbate–adsorbate interactions on adsorption isotherms. This isotherm assumes that: (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbate–adsorbate interactions, and (ii) adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The Temkin isotherm is represented by the following equation:

It can be expressed in its linear form as:

q_e = B₁ ln K_t + B₁ ln C_e (6)

where

The adsorption data can be analysed according to eqn (6). A plot of q_e versus ln C_e enables the determination of the isotherm constants K_t and B₁. K_t is the equilibrium binding constant (L mg⁻¹) corresponding to the maximum binding energy and the constant B₁ is related to the heat of adsorption.

The Redlich–Peterson (R–P for short) isotherm equation is expressed as:³⁸

where K_R is an R–P isotherm constant (L mg⁻¹), b_R is an R–P isotherm constant (L mg⁻¹) and α is the exponent which lies between 1 and 0. Plotting the left-hand side of eqn (7) against ln C_e to obtain the isotherm constants is not applicable because of the three unknowns, K_R, α and b_R. Therefore, a minimization procedure is adopted to solve eqn (7) by maximizing the correlation coefficient between the theoretical data for q_e predicted from eqn (7) and experimental data. Therefore, the parameters of the equations were determined by minimizing the distance between the experimental data points and the theoretical model predictions with the solver add-in function of Microsoft Excel.

In order to compare the validity of the isotherm equations, a normalized standard deviation Δq_e (%) is calculated:

where N is the number of data, q_e (mg g⁻¹) is the experimental value and q_e,cat (mg g⁻¹) is the calculated value by the isotherm equations.

The isotherm parameters of the adsorption of the dyes onto CATCS under different temperature conditions, calculated from plots of these linear isotherm equations, are given in Tables 2–5 and in ESI Tables S1–S4.† Meanwhile, the experimental data have been analyzed by a non-linear fitting of theses isotherm equations. The results are shown in Tables S5–S8 in the ESI.† It is evident from these results that the surface of CATCS was made up of homogeneous and heterogeneous adsorption patches. For C.I. Reactive Yellow 1, the Langmuir isotherm model fit best when the R² values were compared (Table 2). The value of Q_L was almost equal to the experimental equilibrium adsorption capacity (Fig. 9(a)) and it increased as the temperature increased. The linear correlation coefficients of the Langmuir model were in the range of 0.981–0.998. Meanwhile, in comparison to the non-linear R² values (Table S5 in the ESI†), the Langmuir isotherm (the R² value was in the range of 0.978–0.990) also represented a better fit with the experimental data than the other isotherm models. Moreover, the Δq_e (%) values (Table 6) by the Langmuir model were the lowest and were narrowly distributed (3.99–9.44%), which also showed that the Langmuir isotherm model fit best. This model indicated that the adsorption took place at specific homogeneous sites within the adsorbent. It also showed that as the adsorption proceeded, the surface of the adsorbent became crowded with adsorbed molecules of dye attached to its active spots and as a result it became increasingly difficult for free dye molecules in solution to find a vacant site. Moreover, the Langmuir model implied that the adsorbed dye molecules were not vertically oriented or that there was not strong competition from the solvent.³⁹ The results showed that the adsorption of C.I. Reactive Violet 2 onto CATCS fitted best to the D–R isotherm model, as indicated by the R² and adsorption capacity values in Table 3. Further, Δq_e (%) values (Table 6) were much lower than those from other isotherm models. The increasing value of the parameter β suggested that the higher temperature benefitted the removal of C.I. Reactive Violet 2 by the flocculant CATCS. The results by the non-linear method (Table S6 in the ESI†) showed the R² values of the D–R isotherm model were over 0.980, indicating that the D–R isotherm model described the decolorization process of C.I. Reactive Violet 2 very well. This paralleled the above results of the linear method. Comparing the results of Tables 4–6, suggests that the experimental data collected for C.I. Acid Red 18 and C.I. Direct Blue 71 both fitted very well to the Freundlich isotherm, which exhibited the highest coefficient at various temperatures (R² in the range of 0.97–0.99) and the lowest and narrow Δq_e (%) values (5.22–12.62% for C.I. Acid Red 18 and 2.14–4.57% for C.I. Direct Blue 71). To confirm the results, the experimental data have been analyzed by a non-linear fitting of these isotherm equations. According to Tables S7 and S8,† C.I. Acid Red 18 and C.I. Direct Blue 71 both followed the Freundlich isotherm best (the non-linear R² value was in the range of 0.970–0.998). This is the same as that predicted by the above linear analysis. The application of the Freundlich model confirmed that the dye adsorption capacity by CATCS increased with the temperature, because the value of K_F increased as the temperature was raised. The value of n indicates the type of isotherm to be favorable (n > 1), unfavorable (n < 1) or linear (n = 1). The high values of n (n > 2) at equilibrium for all the temperature conditions indicated that the biosorption of the dyes onto the CATCA flocculant was a favorable process. The Temkin isotherm, the R–P isotherm constants and the correlation coefficients are listed in Tables S1–S4 (in the ESI†). It was observed that the Temkin isotherm (0.89 < R² < 0.93) and the R–P isotherm (0.88 < R² < 0.93) did not adequately fit the experimental values. The low correlation coefficients indicated that the decolorization data of the dyes did not fit well to both the Temkin isotherm and the R–P isotherm. The high and widely distributed Δq_e (%) values in Table 6 also showed that both the Temkin isotherm and the R–P isotherm did not fit the decolorization progress of the dyes.

Table 2 Linear fitting parameters of isotherms of C.I. Reactive Yellow 1 by CATCS at various temperatures

T (K)	Langmuir			Freundlich			D–R isotherm
	QL (mg g^−1)	KL (mg^−1)	R^2	KF (L mg^−1)	n	R^2	QD (mg g^−1)	β (mg^2 kJ^−2)	R^2
293.15	500	0.426	0.998	155.21	3.35	0.923	434.89	0.82	0.901
298.15	555	0.429	0.997	166.09	3.07	0.940	460.77	0.81	0.903
303.15	588	0.623	0.987	202.86	3.59	0.936	503.16	1.03	0.922
308.15	588	0.739	0.992	219.14	3.62	0.936	533.68	0.36	0.844
313.15	625	0.842	0.981	239.85	3.70	0.903	544.46	0.26	0.838

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Table 3 Linear fitting parameters of isotherms of C.I. Reactive Violet 2 by CATCS at various temperatures

T (K)	Langmuir			Freundlich			D–R isotherm
	QL (mg g^−1)	KL (L mg^−1)	R^2	KF (L mg^−1)	n	R^2	QD (mg g^−1)	β (mg^2 kJ^−2)	R^2
293.15	1250	0.081	0.909	189.43	2.39	0.813	778.68	2.28	0.985
298.15	1428	0.073	0.920	186.90	2.22	0.831	810.70	2.08	0.981
308.15	2000	0.067	0.890	235.31	2.34	0.778	930.94	1.44	0.990
313.15	2500	0.054	0.871	230.95	2.13	0.788	985.84	1.37	0.990

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Table 4 Linear fitting parameters of isotherms of C.I. Acid Red 18 by CATCS at various temperatures

T (K)	Langmuir			Freundlich			D–R isotherm
	QL (mg g^−1)	KL (L mg^−1)	R^2	KF (L mg^−1)	n	R^2	QD (mg g^−1)	β (mg^2 kJ^−2)	R^2
293.15	1250	0.24	0.963	172.14	2.15	0.985	979.1	5.09	0.738
298.15	1250	0.14	0.952	249.49	2.58	0.987	1017.5	2.20	0.728
303.15	1250	0.24	0.939	349.60	2.92	0.977	1157.9	1.17	0.726
308.15	1429	0.24	0.931	427.82	3.41	0.99	1233.9	1.42	0.737
313.15	1429	0.54	0.969	482.27	2.92	0.986	1204.2	0.22	0.782

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Table 5 Linear fitting parameters of isotherms of C.I. Direct Blue 71 by CATCS at various temperatures

T (K)	Langmuir			Freundlich			D–R isotherm
	QL (mg g^−1)	KL (L mg^−1)	R^2	KF (L mg^−1)	n	R^2	QD (mg g^−1)	β (mg^2 kJ^−2)	R^2
293.15	1825	0.80	0.894	764.79	4.12	0.995	1757.5	0.22	0.704
298.15	2500	0.19	0.969	778.06	3.53	0.974	2164.6	2.12	0.794
303.15	2242	1.24	0.920	804.40	3.78	0.988	2155.5	0.10	0.708
308.15	2500	4	0.898	1163.98	4.18	0.998	2296.8	0.045	0.723
313.15	2500	2	0.907	1223.54	3.97	0.995	2375.8	0.045	0.731

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Table 6 Normalized standard deviation Δq_e (%) of the linear isotherm equations

Dyes	T (K)	Langmuir Δqe (%)	Freundlich Δqe (%)	D–R isotherm Δqe (%)	Temkin Δqe (%)	R–P isotherm Δqe (%)
C.I. Reactive Yellow 1	293.15	9.23	12.89	14.46	26.94	19.74
	298.15	9.39	13.42	14.18	15.72	18.63
	303.15	9.44	10.22	28.17	17.23	18.27
	308.15	7.91	13.86	20.27	20.99	18.06
	313.15	3.99	15.61	21.63	18.80	18.96
C.I. Reactive Violet 2	288.15	19.12	25.03	6.54	21.5	23.96
	293.15	18.31	24.69	7.71	19.75	23.42
	298.15	26.84	30.17	7.78	27.17	30.53
	308.15	26.57	30.57	6.34	26.82	34.38
C.I. Acid Red 18	288.15	29.38	9.16	41.84	17.29	19.04
	293.15	13.16	5.22	25.76	28.57	13.77
	298.15	40.20	12.62	45.35	23.05	35.8
	308.15	29.38	9.16	31.85	20.49	25.05
	318.15	11.97	5.89	25.5	14.33	37.9
C.I. Direct Blue 71	288.15	16.35	2.65	49.23	7.29	19.04
	298.15	5.55	4.57	31.51	28.57	13.77
	303.15	17.14	2.49	50.72	23.05	35.84
	308.15	21.7	2.14	43.46	20.49	25.05
	313.15	21.05	3.7	42.85	14.33	9.04

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3.9 Thermodynamic parameters

The thermodynamic equilibrium constant K_a for the sorption reaction describes thermodynamic properties. Its dependence on the temperature can be used to estimate thermodynamic parameters, such as free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS). These parameters were determined by using the following equations (eqn (9) and (10)).

ΔG = −RT ln K_a (9)

The constant K_a was determined by plotting ln(q_e/C_e) versus q_e and extrapolating zero q_e.⁴⁰ The plot of ln K_a as a function of 1/T yields a straight line from which ΔH and ΔS were calculated from the slope and intercept, respectively. R in eqn (10) is the universal gas constant, 8.314 (J mol⁻¹ K⁻¹). The results are given in Table 7.

Table 7 Thermodynamic parameters for the adsorption of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 by CATCS

Dyes	T (K)	Ka (L g^−1)	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)
C.I. Reactive Yellow 1	293.15	308.68	−13.97	52.17	224.48
	298.15	380.51	−14.73
	303.15	732.60	−16.63
	308.15	980.24	−17.65
	313.15	1057.12	−18.13
C.I. Reactive Violet 2	293.15	2530.57	−19.10	41.34	206.55
	298.15	3743.96	−20.39
	308.15	6043.27	−22.31
	313.15	7700.96	−23.10
C.I. Acid Red 18	293.15	110.73	−11.47	90.18	347.27
	298.15	222.87	−13.40
	303.15	469.84	−15.51
	308.15	612.96	−16.44
	313.15	1277.30	−18.63
C.I. Direct Blue 71	293.15	1596.55	−17.98	89.35	365.03
	298.15	1752.33	−18.51
	303.15	5864.66	−21.87
	308.15	10 231.23	−23.66
	313.15	12 298.1	−24.52

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The overall free energy change (ΔG) during the adsorption process was negative (Table 7) for the experimental range of temperatures, corresponding to a spontaneous process not requiring a large activation energy. The positive value (over 40 kJ mol⁻¹) of ΔH indicated that the decolorization process by CATCS was endothermic. Generally, an adsorption process is considered as physical if the absolute magnitude of ΔH < 25 kJ mol⁻¹ and as chemical when ΔH > 40 kJ mol⁻¹.⁴¹ Therefore, a chemical reaction was involved in the decolorization process. The entropy changes (ΔS) in the adsorption processes of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71, were 224.48 J mol⁻¹ K⁻¹, 206.55 J mol⁻¹ K⁻¹, 347.27 J mol⁻¹ K⁻¹ and 365.03 J mol⁻¹ K⁻¹, respectively. The positive entropy change (ΔS) value for the adsorption corresponded to an increase in the degree of freedom of the adsorbed species.

3.10 Adsorption kinetics

The kinetics of adsorption is one of the most important characteristics in defining the efficiency of adsorption. It describes the rate of uptake of the dye onto the CATCS flocculent and the rate controls the equilibrium time. Fig. 10 illustrates plots of the amount of dye adsorption versus the reaction time for different temperatures. The reaction in 60 min was more rapid at a higher temperature and all the curves finally reached the equilibrium in 80 min. In order to determine the kinetics of the adsorption of the dyes onto the flocculant, a kinetic investigation was carried out with the pseudo-first-order equation, pseudo-second-order equation and the Elovich equation. The conformity between the experimental data and the predicted values from the equations was expressed by the correlation coefficient (R²) as shown in Table 7 and in the ESI Table S5.†

Fig. 10 Kinetic curves of the dyes flocculated by CATCS, (a) C.I. Reactive Yellow 1, (b) C.I. Reactive Violet 2, (c) C.I. Acid Red 18 and (d) C.I. Direct Blue 7.

The pseudo-first-order rate model of Lagergren³⁵ is based on the solid capacity as described by eqn (11):

ln(q_e − q_t) = ln q_e − k₁t (11)

where q_e is the amount of solute adsorbed at equilibrium per unit weight of adsorbent (mg g⁻¹), q_t is the amount of solute adsorbed at any time (mg g⁻¹) and k₁ is the adsorption constant.

The pseudo-second-order equation can be represented as eqn (12):

where k₂ (g mg⁻¹ min⁻¹) is the second order rate constant and q_t is the amount adsorbed at time t. The constants k₁ and k₂ and the corresponding correlation coefficients (R²) have been calculated and are summarized in Table 8.

Table 8 Kinetic parameters for the adsorption of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 by CATCS

Dyes	Pseudo-first-order equation				Pseudo-second-order equation
	T K	k1 min^−1	qe mg g^−1	R^2	k2 (×10^3) g (mg^−1 min^−1)	qe mg g^−1	R^2
C.I. Reactive Yellow 1	293.15	0.037	333.97	0.8632	1.91	333.3	0.9998
	298.15	0.042	399.77	0.8961	1.30	400	0.9998
	303.15	0.073	478.79	0.5671	1.34	476.2	0.9995
	308.15	0.055	503.69	0.9404	0.77	526.3	0.9992
	313.15	0.10	540.01	0.6204	2.16	555.6	1.0000
C.I. Reactive Violet 2	288.15	0.044	345.92	0.9039	0.26	384.6	0.9924
	293.15	0.037	381.97	0.9246	0.59	384.6	0.9977
	298.15	0.057	443.00	0.8248	1.38	454.6	0.9999
	308.15	0.053	500.10	0.9086	1.08	500	0.9999
C.I. Acid Red 18	288.15	0.039	455.56	0.8326	1.49	460.2	0.9998
	293.15	0.037	603.67	0.5674	1.13	608.6	0.9996
	298.15	0.048	626.66	0.7738	1.11	635.7	0.9998
	308.15	0.049	680.6	0.5335	1.05	690.1	0.9995
	318.15	0.048	727.29	0.7534	1.06	736.9	0.9997
C.I. Direct Blue 71	288.15	0.057	1339.5	0.9338	0.072	1428.6	0.9966
	298.15	0.034	1721.5	0.9703	0.082	1666.7	0.995
	303.15	0.056	1973.4	0.9583	0.056	2000	0.9974
	308.15	0.043	2010.7	0.959	0.074	2000	0.9998
	313.15	0.049	2100.2	0.9899	0.044	2500	0.9993

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The Elovich equation is generally expressed as:¹⁵

Simplifying the Elovich equation, with αβ > t and by applying the boundary conditions q_t = 0 at t = 0 and q_t = q_t at t = t, eqn (13) becomes

The plot of q_t versus ln t should yield a linear relationship with a slope 1/β and an intercept of 1/β ln(αβ). The α initial adsorption rate (mg g⁻¹ min⁻¹) and β desorption constant (g mg⁻¹) values and the correlation coefficients R² are given in the ESI Table S9.†

The R² values showed that the adsorption of dyes onto CATCS didn’t follow pseudo-first-order kinetics (0.56 < R² < 0.95).

Meanwhile, it was observed that the Elovich kinetic model did not adequately fit the experimental values (0.7691 < R² < 0.9244). However, the pseudo-second-order kinetic model described all the adsorption processes very well according to the correlation coefficients, which are generally above 0.990. The calculated q_e value agreed very well with the experimental value (Fig. 10) also confirming that the adsorption phenomena followed pseudo-second-order kinetics. The good fitting of the pseudo-second-order equation suggested that the adsorption process was interaction controlled with chemisorption, which involves valance forces through sharing or exchange of electrons between the adsorbent and the dyes.^42,43

Physical and chemical are the two main types of adsorption. Insight of the activation energy may give an idea about the type of adsorption. Generally, the activation energy in a physical adsorption is below 4.2 kJ mol⁻¹ and chemical adsorption involves forces much stronger than those of physical adsorption.⁴⁴ The activation energies of the dyes adsorbed onto CATCS were determined using the Arrhenius equation (eqn (15)):⁴⁵

where E_a is the activation energy, R the gas constant (8.314 J mol⁻¹ K⁻¹), T is the temperature in Kelvin, k is the adsorption rate constant and A is the frequency factor. The value of E_a can be determined from the slope of the ln k versus 1/T plot.

The adsorption rate constants according to the pseudo-second-order kinetic model at various temperatures were used to calculate the activation energy value. The activation energies (E_a) of the adsorption processes of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 onto CATCS were calculated as −27.74 kJ mol⁻¹, −51.71 kJ mol⁻¹, −19.3 kJ mol⁻¹ and −14.31 kJ mol⁻¹, respectively, which also indicated the existence of a chemisorption mechanism.

4 Conclusion

A high quality copolymer flocculant CATCS was synthesized and applied in the enhanced decolorization of four dye solutions and a real dye wastewater. The flocculant was characterized by SEM and Fourier transform infrared spectrum. The decolorization efficiency was found to depend on the initial pH, dosage of CATCS, initial dye concentration, salt (NaCl) concentration and reaction time. The flocculant CATCS had a very high capacity for dye removal. An ultrahigh decolorization efficiency (97.1–99.7%) for the dye solutions was observed for CATCS. It was effective in a wide pH range (2 to 10) and could withstand a high salt concentration. A decolorization efficiency of 98.3% for the real dye wastewater was reached with 0.34 g L⁻¹ CATCS. To remove 90% of the dyes in real dye wastewater, the required amount of CATCS was less than one tenth of that of commercial activated carbon. The thermodynamics and kinetics of the decolorization process were investigated in detail and the decolorization mechanism was determined. The thermodynamics study showed that the Langmuir isotherm described the decolorization process of C.I. Reactive Yellow 1 well and the D–R isotherm model fitted that of C.I. Reactive Violet 2 very well. C.I. Acid Red 18 and C.I. Direct Blue 71 both followed the Freundlich isotherm (the R² value was in the range of 0.97–0.99). The overall negative free energy change (ΔG) during the adsorption process indicated that it was a spontaneous process. The enthalpy change was over 41 kJ mol⁻¹ in the decolorization process of the dye solutions by CATCS indicating that the dye adsorption onto the flocculant CATCS is an endothermic reaction and involves a chemical reaction. The entropy changes for the adsorption processes of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71, were 224.48 J mol⁻¹ K⁻¹, 206.55 J mol⁻¹ K⁻¹, 347.27 J mol⁻¹ K⁻¹ and 365.03 J mol⁻¹ K⁻¹, respectively. The removal of the dyes by CATCS followed pseudo-second-order kinetics. This suggested that the decolorization process was interaction controlled with chemisorption, which involves valance forces through sharing or exchange of electrons between the adsorbent and the dyes. The activation energies (E_a) of the adsorption processes of C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 onto CATCS were −27.74 kJ mol⁻¹, −51.71 kJ mol⁻¹, −19.3 kJ mol⁻¹ and −14.31 kJ mol⁻¹, respectively. A charge neutralization mechanism and strong adsorption dye–CATCS–dye bridging interaction contributed to the high efficiency of decolorization by the flocculant CATCS.

Acknowledgements

This work was supported by the Start Scientific Research Fund of Fuzhou University (Grant No. XRC-1462).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Scheme 1: schematic representation for the synthesis of CATCS; Fig. S1: chemical formula of dyes used in this study; Fig. S2: pH effect on zeta potential of CATCS; Fig. S3: effect of NaCl on zeta potential of CATCS; Table S1: linear fitting parameters of isotherms of C.I. Reactive Yellow 1 by CATCS at various temperatures; Table S2: linear fitting parameters of isotherms of C.I. Reactive Violet 2 by CATCS at various temperatures; Table S3: linear fitting parameters of isotherms of C.I. Acid Red 18 by CATCS at various temperatures; Table S4: linear fitting parameters of isotherms of C.I. Direct Blue 71 by CATCS at various temperatures; Table S5: non-linear fitting parameters of isotherms of C.I. Reactive Yellow 1 by CATCS at various temperatures. Table S6: non-linear fitting parameters of isotherms of C.I. Reactive Violet 2 by CATCS at various temperatures. Table S7: non-linear fitting parameters of isotherms of C.I. Acid Red 18 by CATCS at various temperatures. Table S8: non-linear fitting parameters of isotherms of C.I. Direct Blue 71 by CATCS at various temperatures; Table S9: kinetic parameters for C.I. Reactive Yellow 1, C.I. Reactive Violet 2, C.I. Acid Red 18 and C.I. Direct Blue 71 by CATCS. See DOI: 10.1039/c5ra07662e

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