New insights in the use of a strong cationic resin in dye adsorption

Water pollution is one of the largest and most important problems of our time. There are a variety of pollutants in aqueous environments that can produce serious health effects in plants, animals and humans. These contaminants can be of organic (e.g., dyes, pharmaceuticals, pesticides, etc.) or inorganic (e.g., heavy metals) origin (Vakili et al. 2014; Saxena et al. 2020).

Dyes are difficult to treat because of their synthetic origin and complex molecular structure, which makes them more stable and difficult to be biodegraded (Forgacs et al. 2004; Rai et al. 2005).

Some dyes are toxic and mutagenic, and they have the potential to release carcinogenic species. Because of their toxic properties, dyes can also contribute to the failure of biological processes in wastewater treatment plants (Zahrim et al. 2010). The presence of very small amounts of dyes in water, even less than 1 mg L⁻¹ for some dyes, is highly visible and undesirable (Crini 2006).

In general, synthetic dyes are widely used in many industries, e.g., textile, paper, leather tanning, food processing, plastics, cosmetics, rubber, printing and dye manufacturing (Sokolowska-Gajda et al. 1996; Kabdaşli et al. 1999; Wróbel et al. 2001; Bensalah et al. 2009; Aravind et al. 2016). Several classes of synthetic dyes (over 7 × 10⁵ metric tons) are produced worldwide every year for industrial purposes, and approximately 5–10% of this quantity is released into the ecosystem along with wastewater. Because of increasingly stringent restrictions on the organic content of industrial effluents, it is necessary to eliminate dyes from wastewater before discharge. One type of dye, methyl orange (MO), which is anionic, has applications in different fields, including coloring paper, temporary hair coloring, and dyeing cottons and wools, among others (Robati et al. 2016). Various treatment processes, such as coagulation/flocculation, ozonation, membrane filtration, adsorption, chemical oxidation, solvent extraction, and ion exchange have been widely used to eliminate dyes from wastewater (Foo & Hameed 2010; Karadağ & Üzüm 2012; Moradi et al. 2014; Allouche et al. 2015). Among these proposed methods, adsorption technology is considered to be one of the most effective methods because of its simplicity, high efficiency, flexibility and insensitivity to toxic substances (Qin et al. 2016). Various types of sorbents, including activated carbon, silica gel, clay minerals, fly ash and agricultural solid wastes, have been employed as adsorbents for the removal of dyes from wastewater (Mohan et al. 2002; Yan et al. 2006; Gómez et al. 2007; Wang & Peng 2010; Salleh et al. 2011; Chen et al. 2012). In the literature, some studies on the removal of dyes using synthetic-based polymer materials have been reported. These materials are for example an ammonium-functionalized hollow polymer (Qin et al. 2016) a polyaniline-based nanotube (Ayad & El- Nasr 2010), a polymer-loaded bentonite (Li et al. 2010), an ionic-liquid-based cross-linked polymer (Gao et al. 2013), quaternary ammonium polyethylenimine (Liu et al. 2013), and poly(acrylic acid–acrylamide) hydrogels (Li et al. 2011) among a few others. However, the literature is limited to some few polymers and dyes. More research should be done to evaluate well the performance.

Considering the scarce information about the use of commercial resins in dye removal, the aim of this work is to contribute with new information about the use of this kind of resin in the adsorption of sulfonated azo dyes. In this study, we use a commercial cationic resin Amberlite Chloride (IRA 402), a strong cationic resin, to remove methyl orange (MO) dye using the batch method under different conditions such as: amount of resin, MO concentration, and kinetic studies at different pH.

Sorption performances of Amberlite Chloride (IRA 402) (Sigma-Aldrich) were examined using the batch adsorption method. MO was purchased from Sigma-Aldrich (85% dye content) and a standard solution of 1,000 mg L⁻¹ was prepared and used. For batch adsorption tests, various amounts of resin (15, 30, 50, 75 mg) were contacted with 40 mL of MO solution (20 mg L⁻¹ to 200 mg L⁻¹) and stirred to 175 rpm for different times. After that, the resin–MO solution was filtrated and the filtrate was measured in triplicate by UV–vis (Biobase BK560) at 460 nm and the average values of the results were plotted. In the study on the effect of pH, the pH was adjusted using 0.1 mol L⁻¹ HCl (Merck) and 0.1 mol L⁻¹ NaOH (Merck). For the kinetic study, the interaction times were between 30 and 300 min at 20, 35 and 55 °C in separate experiments. Figure 1 shows the molecular structure of the Amberlite IRA 402 resin adsorbent and MO dye.

A batch test was performed in order to determine the ideal resin amount needed to remove MO. Therefore, different resin loads in a solution of 150 mg L⁻¹ of MO at 20 °C and 300 min of contact time were used as initial conditions. The amount of resin was changed from 0.125 to 1.75 g-resin L⁻¹-MO solution. Figure 2 shows that the maximum adsorption of MO was obtained at 119.8 mg MO/g-resin, reaching 99.8% of MO removal. The removal capacity increased when the amount of resin increased until reaching a saturation. This removal can be explained by the attractive interaction between quaternary ammonium groups of polymer and sulfonate groups of MO.

In order to determine the influence of pH on the adsorption capacity of the resin, a study at different pH was performed. The pH studied was 3.00, 7.64 (pH of the MO solution without adjustments) and 10.00. In Figure 3 it is possible to observe that the maximum adsorption is obtained at pH 7.64 under the studied conditions (MO = 150 mg L⁻¹, resin amount of 1.25 g-resin L⁻¹-solution, 120 min of contact time at 20 °C). Thus, this pH was chosen as a working condition in next experiments. The pH of the MO solution affects the adsorption capacity of the resin. The quaternary ammonium groups can interact with MO dye in a wide range of pH due to its low pKa (pKa = 3.4), however at pH 3.00, the removal capacity of the polymeric resin is lower. This is probably due to the protonation of the sulfonate group of the MO, which hinders the attractive interaction between polymer and dye.

Subsequently, a study of the influence of the initial concentration of MO was performed. Resin amount was kept constant while the solution concentration was changed from 50 to 200 mg L⁻¹. Figure 4 shows the adsorption capacity of the Amberlite IRA 402 against each MO concentration. As it is possible to observe, the best adsorption results were obtained at 100 mg L⁻¹, therefore this concentration was used in further experiments. The resin showed a good adsorption performance at higher MO concentrations (above 100 mg L⁻¹). It can be considered an advantage for dye removal in industrial wastewater treatment due to the wide range of dye concentrations.

In order to study the adsorption performance of the resin, the effect of the temperature as a function of time was analyzed (see Figure 5). As it is possible to observe, the adsorption capacity increases with the increase of the contact time. Under the studied conditions, after 180 min there is no further increase of the sorption capacity, which remains practically constant. Additionally, it is possible to observe that the increase of temperature increases the adsorption capacity, however there are no significant differences in removal capacity at 55 °C and 75 °C.

Based on the experimental data it is possible to establish the best experimental conditions, which are established in Table 1.

Using the best operational conditions, a maximum adsorption capacity of 161.3 mg MO/g-resin was achieved. This result is highly superior to the previous results reported in the literature. Leszczyńska & Hubicki (2009) used Amberlite IRA 402 to remove another sulphonated azo dye (brilliant yellow), obtaining a maximum adsorption capacity of 27 mg g⁻¹. On the other hand, Behera et al. (2017) used Amberlite IRA 400 to remove MO, obtaining a maximum adsorption capacity of 74.4 mg g⁻¹.

In this study, the adsorption of MO was evaluated in the presence of anionic species (chloride and sulfate) that compete with the dye, interfering with the adsorption capacity of the resin. The study was performed at different anion/MO molar ratios (see Figure 6). The results show a gradual decrease of the adsorption capacity as the concentration of chloride and sulfate increases when the initial concentration of MO is 100 mg L⁻¹ and at a pH of 7.64. The chloride and sulfate anions interact with the resin and this interferes with the level of MO adsorption as the concentration of interfering anions increases, hindering the purpose of the adsorbent and decreasing the adsorption capacity of the resin for the dye. This may be because of the competitive interaction of the MO and the interfering anions with the quaternary ammonium of the resin. In the present study, the influence of interfering anions on MO retention was sulfate > chloride when Amberlite IRA 402 resin was used as adsorbent. However, the resin decreased the removal percentage less than 20% in all the experiments with interfering anions.

Similar results have been obtained in previous research related to chromate anion sorption using quaternary ammonium polymer as sorbent and in the presence of chloride and sulfate as interfering anions. The results indicate that when interfering anions were added to the solution, the chromate retention decreased. In general, the concentration, charge, nature of interfering anions, and nature of the functional polymers have effects on its retention capacity. The divalent anions (sulfate) produce a greater reduction than the monovalent anions (chloride) (Sánchez et al. 2017).

Figure 7 shows the plots for the (a) pseudo-first-order model and (b) pseudo-second-order model at different working temperatures.

Table 2 shows the kinetic parameters calculated for each temperature. In all the cases the kinetic data fit well with the pseudo-first-order model. As expected, the kinetic constants increase with the increase of the temperature.

To determine the rate-determining steps in the adsorption process, diffusion and reaction models, such as the infinite solution volume (ISV) model and unreacted core model (UCM) equations, were also used. These kinetic models are developed for spherical particles (Yilmaz et al. 2007). Table 3 summarizes the ISV and UCM models and Table 4 shows the correlation coefficients for each model at different temperatures.

The ISV model shows that the rate is controlled by the film diffusion process. On the other hand, the chemical reaction is the rate-determining step when the UCM model is applied.

In order to understand the mechanism of the adsorption process, the Langmuir isotherm was used. Owing to some parameters being undetermined for the Freundlich model, a linear behavior was assumed.

It seems that the Langmuir model fitted better than the linear model due to the higher correlation coefficient. This result shows that MO adsorption is determined for the homogeneous surface of the adsorbent with sites energetically equivalent.

The influence of temperature on the adsorption capacity of MO was investigated in order to determine the spontaneity and thermal properties of the adsorption process.

According to the experimental data, activation energy value E_a= 13.92 kJ mol⁻¹ and correlation coefficient R² = 0.9532 were obtained. These values could suggest that MO adsorption onto the resin is a physisorption process.

Thermodynamic parameters calculated from the plot are summarized in Table 6. The negative values of Gibbs free energy indicate that the MO sorption is of spontaneous nature. It is possible to observe also that the ΔG° value is more negative with increasing temperature, which suggests that higher temperature facilitates the adsorption process. The positive value of ΔH° indicates that the process is endothermic and the positive value of ΔS° indicates a greater stability of the adsorption process with no structural changes at the solid–liquid interface, confirming the spontaneity of the process.

The present work studied the adsorbent properties of Amberlite IRA 402 on MO removal. It was demonstrated that Amberlite IRA 402 can efficiently remove MO dye. The adsorption capacity is affected by different parameters such as initial concentration, contact time, and temperature, among others. The maximum adsorption capacity of the resin was 161.3 mg g⁻¹ at 20 °C, better than the results obtained in previous work for similar studies. Kinetic studies showed that the optimum contact time was 300 min with an optimum temperature of 55 °C. The sorption process follows a pseudo-first-order kinetic model. The rate is controlled by a film diffusion process according to the ISV model and by the chemical reaction, according to the UCM model.

The thermodynamic parameters were also studied. Equilibrium data presented a better fit to the Langmuir isotherm, indicating monolayer sorption. The positive value of activation energy (13.92 kJ mol⁻¹) suggested that the adsorption process was endothermic, and the mechanism was physical adsorption. The values of thermodynamic parameters (ΔG°, ΔH°, ΔS°) were calculated and showed that the adsorption process was endothermic and spontaneous. This resin seems to be a good solution for the removal of this type of dye in wastewater.

The authors thank the FONDECYT Project no. 1191336.