Currently, many industries generate a considerable amount of polluted wastewater because of the consumption of large amounts of water and the employment of chemicals during manufacturing and dyeing of their products [1
Dyes have been widely used for thousands of years for textile, paint and other applications because of their favorable characteristics, such as bright color, simple application and water-fastness [2
]. Today, over 100,000 commercially available different dye types exist and more than 7 × 105
tons are generated annually [3
Dyes are non-biodegradable, stable, resistant to light, heat, oxidizing agents and potentially carcinogenic and toxic, even at small concentrations; therefore, their release into the environment poses serious ecological, aesthetical and health problems (skin irritations, dysfunction of the kidney, respiratory problems and increase cancer risk in humans) [4
]. Moreover, their discharge into the environment may negatively affect photosynthetic processes of aquatic plants, reducing oxygen levels in water and, in serious cases, resulting in the suffocation of aquatic fauna and flora [5
Commercial dyes are usually categorized based on their color, functional groups, chemical structure and application. Furthermore, dyes are classified into cationic, anionic and nonionic according to the produced particle charge upon dissolution in an aqueous medium. Due to the toxicity of amine groups in azo dyes, they are considered very dangerous for human health and the environmental life, consequently, dye industries have been pressed to remove dyes from their wastewater effectively to ensure safe discharge into the environment [6
In order to remove dye from wastewater and to improve the quality of treated wastewater discharged into the environment, it is necessary to select a correct treatment method. As a result, different methods have been applied for the effective treatment of dye wastewater [7
]. Several conventional methods such as adsorption on activated carbons, coagulation and flocculation, chemical oxidation, reverse osmosis, bacterial action, activated sludge, ozonation, membrane filtration, ion exchange and electrochemical techniques are the commonly used methods for the removal of dyes from wastewater [8
These methods may be able to reduce the color of the wastewater but they exhibit several limitations such as high cost, low effectiveness and generation of excess sludge so that some are inappropriate for use by small-scale industries [7
]. Among them, adsorption is considered the most economical method [9
]. Adsorption, which is a rapid method and convenient for toxic contaminants, it is one of the easiest and effective physio-chemical treatment processes for dye removal, with low initial costs, producing nontoxic by-products, flexibility and simplicity of design, fast adsorption rate, facile separation and simple and user-friendly. Recently, materials based on natural polymers have been developed [10
Cyclodextrins (CDs) are cyclic oligosaccharides obtained from the enzymatic degradation of starch and they are used widely in separation science because CD-complexation phenomena serve for separation of compounds and extraction processes, and provide a versatile and useful tool for protecting the environment [11
]; however, over the last several decades, biodegradable and eco-friendly polymers have been developed as an alternative to epichlorohydrin polymers demanding less chemical treatment during production [12
Chitosan is used as an alternative adsorbent with a demonstrated adsorption capacity to conventional wastewater treatment processes [10
]. The use of biopolymers such as chitin and chitosan is one of the new adsorption methods for the removal of heavy metal ions and dyes [8
]. Chitin is an important crustacean by-product produced by the shellfish processing industry that reduces the waste and its negative impact on the environment [13
]. Chitosan is a natural biopolymer synthesized from the deacetylation of chitin, which is the second most abundant polysaccharide in nature, consisting mainly of unbranched chains of β-(1-4)-2-acetoamido-2-deoxy-d
-glucose, and it is obtained from crustacean shells such as crabs, crayfish, lobster prawns, fungi, insects and other crustaceans [14
]. Chitosan is a heterogeneous, linear, cationic polysaccharide with a high molecular weight and hydrophobic that possesses multiple properties such as biodegradability, hydrophilicity, non-toxicity, biocompatibility or adsorption properties [8
Since the retention of dyes by polymers is not 100% efficient, unadsorbed amounts of dyes still pose an ecological issue and a complementary method is required in order to minimize the amount of dye that will be eventually disposed to the environment. Advanced oxidation processes (AOPs) can be used for this purpose. AOPs are a group of methods based on the generation of the highly oxidant hydroxyl radicals, which oxidize organic molecules. AOPs based on pulsed light (PL) uses high-intensity short-time pulses of light of wide-spectrum rich in UV wavelengths to generate hydroxyl radicals from hydrogen peroxide. It has been tested for the degradation of textile dyes successfully [15
In the present study, chitosan-NaOH polymer beads and β-CDs-EPI polymers were prepared in order to analyze the adsorption efficiency of both adsorbents using Direct Blue 78 dye (DB78) due to the high amount used in the textile industry and its high aqueous solubility. These properties make DB78 a suitable dye to study their adsorption properties. To achieve this goal, the experimental data were fitted to different models and isotherms to elucidate the adsorption characteristics of each one. To remove the remaining dye in the aqueous solution after treatment, an advanced oxidation process was considered.
3.1. Polymer Characterization
The characterization of both adsorbents involved the measure of the swelling capacity, porosity, density and the particle size distribution of each polymer. The results obtained could be observed in Table 1
. It could be noted that the chitosan polymer has a swelling capacity greater than the β-CDs polymer, however, the β-CDs polymer presented a higher porosity. In the case of the particle size distribution, similar results were observed for both adsorbents.
The control of the adsorption process of an adsorbent, in this case, chitosan-NaOH and β-CDs-EPI polymers, depends on different physical-chemical characteristics. First, the type of polymer prepared, its chemical structure and functional groups and secondly the chemical characteristics of the molecule to be adsorbed, as well as its concentration and finally, the temperature.
3.2. Effect of Contact Time
The adsorption data for the removal of DB78 versus contact time at different concentrations of dye (from 25 to 300 mg/L) for the polymers are shown in Figure 3
. All the experiments were carried out at pH 7.0, constant stir (500 rpm) and fixed amount of polymer (1 g).
a shows that the adsorption capacity increased in each concentration until the equilibrium was reached for the chitosan-NaOH polymer, indicating that the adsorption of dye on the polymer stopped. When this point is achieved, the amount of adsorbed dye inside the polymer was in a dynamic equilibrium with the amount of dye desorbed. The time needed to reach this point is called equilibrium time and the amount of dye removed by the polymer at that time indicates the maximum adsorption capacity of each polymer under these conditions.
Different adsorption phases may be differentiated in the range of concentrations analyzed (25–300 mg/L). In the case of adsorption of DB78 on the chitosan-NaOH polymer, at low concentrations, only 30 min were needed to reach the equilibrium. At the concentration of 100 mg/L, the adsorption was fast but it was slower than at low concentrations, reaching the equilibrium after 30 min of contact time. However, when the concentrations of dye were higher (>100 mg/L), the curves did not show the typical asymptotic form. Conversely, the equilibrium time increased with increasing concentrations of DB78 (from 150 to 300 mg/L).
For β-CDs-EPI polymers, the results obtained were different from those of the chitosan-NaOH polymers (Figure 3
b). The adsorption was fast at the initial period between the dye and polymer, because of the fast linkage between the dye and the polymer surface. The adsorption is continuous until the equilibrium is reached after it remains constant. All curves were asymptotic after 20 min of contact time approximately. The adsorption process could be considered fast due to the high concentration of dye adsorbed in the initial period. When the concentration increased, the polymer incremented the ability to entrap the dye, confirming the strong interaction between DB78 and the β-CDs-EPI polymer.
3.3. Adsorption Kinetics
In order to test the experimental data obtained in the adsorption of DB78 and the β-CDs-EPI and chitosan polymers, the pseudo-first-order, pseudo-second-order and the intraparticle diffusion model were employed with the purpose to determine the different mechanisms implicated in the adsorption process, such as adsorption surface, mass transfer or intraparticle diffusion.
The fitting of experimental data to the pseudo-first-order plots for the adsorption of Direct Blue on different polymers are shown in Figure 4
a and the corresponding parameters in Table 2
. The goodness of fit of this model was expressed using the linear determination coefficient (R2
). The linearity of the model (log (qe
) versus t
) was plotted for 350 min of contact in the case of the chitosan-NaOH polymer and 100 min for the β-CDs-EPI polymer. The R2
values for chitosan-NaOH ranged from 0.934 to 0.986 and between 0.736 and 0.938 for β-CDs-EPI. Theoretical values of qe
were compared with the experimental data. Despite the fact that some values were relatively high, the obtained R2
value revealed the poor fit to the model. Because of these results, it was appropriate to study the pseudo-second-order model with the experimental data obtained.
The plot of t
produced straight lines in all cases during the whole range of measure, as observed in Figure 4
b. In this model, a relatively high value indicates also that the experimental data fit properly to the model. R2
values obtained indicated that better results were obtained with this model, as it is shown in Table 2
. In all cases, the R2
values were 0.99 or higher and the experimental qe
fitted much better to theoretical values of qe
than the pseudo-first-order model. These results suggested that the adsorption process of DB78 is controlled by the pseudo-second-order model and supports that the adsorption is due to chemisorption or chemical adsorption. The adsorption is carried out by surface exchange reactions, DB78 molecules diffuse inside the polymer where inclusion complexes, hydrogen bonds or hydrophobic interactions could take place [22
]. Similar kinetics were also observed in the adsorption of Direct Red dye using CDs-EPI polymers [16
] or adsorption of three different dyes using chitosan [31
The adsorption is a process with different stages that implicates a transport of solute molecules (dye) from the aqueous solution to the surface of solid particles (polymer), followed by the diffusion of dye molecules onto the polymer. The experiments developed, permitted us to study if the intraparticle diffusion is the process that controls the adsorption. This effect was studied by plotting the amount of dye adsorbed versus the square root of time and this is the intraparticle diffusion model. The kinetics results obtained can be used to know if the intraparticle diffusion is the limiting step in the adsorption of the dye inside the polymer [21
shows the amount of adsorbed dye versus the square root of time for the intraparticle transport of DB78 for both studied polymers and different concentrations of dye. The curves presented multi-steps. In the case of β-CDs-EPI (Figure 5
b), the curves presented two zones, the first curved part indicated the effect of the boundary layer and the second linear part is due to the intraparticle diffusion.
However, in the case of a chitosan-NaOH polymer, a single linear portion for all studied concentration was observed, indicating that the adsorption process inside the polymer was controlled by diffusion of such molecules to the polymer surface (Figure 5
(intraparticle diffusion constant) values could be seen in Table 2
. These values increased with increasing concentrations of DB78 in both polymers. Chitosan-NaOH presented better R2
values than β-CDs-EPI, ranging from 0.627 to 0.814 for β-CDs-EPI and from 0.804 to 0.991 for chitosan-NaOH.
However, the results obtained for each curve do not cross the origin, indicating that the intraparticle diffusion is not the sole rate-limiting step although it plays an important role in the adsorption process, other process control adsorption velocity, confirming that the adsorption is a process that involves different stages [22
The C (qe
) values obtained provide information about the thick of the boundary layer; a higher intercept indicates a higher effect of it. The C (qe
) values increased with increasing dye concentrations for β-CDs-EPI polymer but not for chitosan-NaOH (Table 2
], therefore, there is intraparticle diffusion for the β-CDs-EPI polymer but not for the chitosan-NaOH polymer.
Similar kinetics were also observed in the adsorption of Direct Red dye using CDs-EPI polymers [16
]. As it may be observed in this study, the intraparticle diffusion is not the unique mechanism implicated in the process, though it plays an important role in the adsorption process, as with the adsorption of DB78 with a β-CDs-EPI polymer. However, the results are clearly different from that obtained with the chitosan-NaOH polymer.
3.4. Adsorption Equilibrium
The experimental data of adsorption equilibrium obtained with DB78 and CDs-EPI polymers and chitosan were studied using Freundlich, Langmuir and Temkin isotherms. The representation of the Freundlich isotherm was obtained by the plot of ln qe
versus ln Ce
. The linear plot was obtained for both polymers as may be observed in Figure 6
a. These straights lines were used to calculate the parameters KF
. The value of the Freundlich constant (KF
) was 0.87 (L/g) for β-CDs-EPI, while this value was 2.0 (L/g) for chitosan-NaOH (Table 3
). Using the linear plot, the value of Freundlich exponent (nF
) could also be calculated, this value was 1.2 for β-CDs-EPI and 1.88 for chitosan-NaOH (Table 3
). The adsorption process is favorable when the value of nF
is in the range between 1–10 and this is confirmed for both polymers. The values of R2
obtained were 0.954 for β-CDs-EPI and 0.956 for chitosan-NaOH (Table 3
). It was found that the adsorption equilibrium data fit the Freundlich equation due to the high determination coefficient obtained.
The plot of Ce
shows the representation of the Langmuir isotherm, giving a straight line in both polymers, whose slope is aL
, the intercept is 1/KL
is the parameter qmax
which is the maximum adsorption capacity of each polymer (mg/g) (Figure 6
The parameters obtained for this isotherm can be observed in Table 3
. One of the most useful parameters of this model is qmax
. The value obtained for β-CDs-EPI was 23.47 and 12.30 mg/g for chitosan-NaOH, indicating that β-CDs-EPI presented a better adsorption capacity of Direct Blue than chitosan-NaOH. In our previous research [16
], a table comparing the qmax
of different adsorbents and dyes was included (see Table 4
). The results observed in this article are consistent with those previously compared.
Furthermore, the R2
values obtained were 0.516 for β-CDs-EPI and 0.981 for chitosan-NaOH. This value was lower to that obtained with the Freundlich isotherm for β-CDs-EPI, however, it was higher to that obtained with the Freundlich isotherm for chitosan-NaOH. Similar results were also observed in the adsorption of Basic Blue 9 dye using CDs polymers [25
], and Pellicer et al. in the adsorption of Direct Red dye using CDs-EPI polymers [16
]. In both cases, the adsorption process fitted to the Langmuir isotherm. On the other hand, studies carried out by Subramani and Thinakaran showed similar results for the chitosan-NaOH polymer in the adsorption of three different dyes [31
In the Langmuir isotherm, to know if the adsorption process is considered favorable or unfavorable, the value of a dimensionless constant called separation factor (RL
) is taken into account. When the value of RL
is in the range between 0–1, it indicates that the process is favorable and this is observed by the values obtained between 0–1 in both polymers, as observed in Figure 6
c. It is also observed that the highest values of RL
at low concentrations of dye indicate that the adsorption is more favorable at low concentrations.
The experimental data were also analyzed using the Temkin isotherm to study its fit. The parameters calculated with the Temkin isotherm as well as the values of determination coefficients could be observed in Table 3
. The equation of the Temkin isotherm assumes that because of the interactions between polymer and dye, the heat of adsorption of all the molecules in the layer would decrease linearly with coverage of the adsorbent surface, as well as that the adsorption is characterized by uniform distribution of bond energies, until a bond energy maximum [29
]. From the plot of qe
versus ln Ce
d), the linear form of the isotherm was plotted as well as the parameters bT
using the slope and the intercept respectively.
The bT value obtained for the different polymers were 0.733 kJ/mol for β-CDs-EPI and 1.09 kJ/mol for the chitosan-NaOH polymer. Positive values indicated that in the adsorption process were involved physic as well as chemisorption processes. The values of R2 obtained were 0.844 for β-CDs-EPI and 0.965 for chitosan-NaOH.
3.5. Thermodynamic Study
With the objective to study the effect of temperature on the adsorption of DB78 by chitosan-NaOH and β-CDs-EPI polymers, the experiments were carried out at three different temperatures (Figure 7
) at a concentration of 250 mg/L for both polymers. Table 4
presents the values of thermodynamic parameters obtained for both polymers at different temperatures.
The standard free energy (ΔG°) of the adsorption of DB78 for β-CDs-EPI, calculated using Van’t Hoff equation, was −1.22, −0.21 and 1.97 kJ/mol at temperatures of 25, 54 and 71 °C. In the case of a chitosan-NaOH polymer, the values obtained were 2.5, 8.5 and 9.4 kJ/mol at temperatures of 25, 53 and 68 °C respectively. The negative values of ΔG° for β-CDs-EPI polymer confirm the viability of the process and the spontaneous nature of the adsorption process at 25 and 54 °C, but it is not spontaneous at higher temperatures (71 °C). On the other hand, positive values of ΔG° for chitosan-NaOH polymer indicated that the process is not spontaneous at all temperatures studied. The enthalpy change (ΔH°) was negative in both polymers, indicating the exothermic nature of the adsorption process for the dye. Similar results were observed by Subramani and Thinakaran [31
] in the adsorption of three different dyes using chitosan. The results showed that it was an exothermic process. In relation to the standard free energy, the process was spontaneous, in contrast to the ΔG° obtained with DB78. Table 4
shows that the value of ΔG° increased with increasing temperatures in both polymers. In the case of chitosan-NaOH, the value increased from 2.50 to 9.40 kJ/mol and from −1.22 to 1.97 kJ/mol for β-CDs-EPI, which indicates a clear trend in the process. High temperatures were not suitable for the adsorption process, however, this process was favored at low temperatures.
shows the results obtained in the thermodynamic study. As stated before, at temperatures of 53 and 68 °C, there was not an increase in the adsorption properties of chitosan-NaOH (Figure 8
a). In the case of the β-CDs-EPI polymer, the best conditions to adsorb the dye was achieved at 25 °C, decreasing the capability of this polymer to entrap more dye molecules after increasing the temperature (Figure 8
3.6. Advanced Oxidation Process
The highest residual concentrations of dye remaining after adsorption by each polymer were chosen as the worst case scenario for further improvement of the process with the aim of minimizing the amount of dye that eventually would be disposed to the environment. The adsorption of 300 mg/L of the dye solution by chitosan polymer or β-CDs-EPI polymer retained 83.6% and 95.2% of the dye respectively, leaving a residual concentration of dye of 49.1 and 14.4 mg/L respectively. The application of the AOP to the solutions remaining after the adsorption processes reduced their concentration by more than 90% (Figure 9
) in both cases and rose the dye removal rate of the sequential polymer adsorption/AOP system to >99%.