Quaternized triethanolamine-sebacoyl moieties in highly branched polymer architecture as a host for the entrapment of acid dyes in aqueous solutions

This paper reports the synthesis of a hyperbranched polymer by a cost-effective one-step copolymerizationofA3 and B2 monomers, namely, triethanolamine and sebacoyl chloride,respectively, followedbymethylationoftertiaryaminegroups. Thestructureofthe hyperbranchedpolymer QTEAS as an ef ﬁ cient material for the removal of acid dyes was demonstrated by Fourier transform infrared spectroscopy (FTIR), cross polarization magic angle spinning (CPMAS) 13 C NMR, thermogravimetric analysis (TGA), powder X-ray diffraction (DRX) and scanning electron microscopy (SEM). The removal of indigocarmine(IC)andEvansblue(EB)wasexpectedtobedrivenbytheelectrostaticattractionbetween positively charged quaternary ammonium groups within the hyperbranched polymer and the negatively charged dyes. The removal process was found to be closely connected to the total number of sulfonate groups on the surface of the dyes. Nonetheless, the ionic strength does not affect the dyes ’ removal ef ﬁ ciency by the hyperbranched polymer. The sorption capacities at saturation of the monolayer q max were determined to be 213.22 mg g (cid:1) 1 and 214.13 mg g (cid:1) 1 , for IC and EB, respectively, thus showing the greater af ﬁ nity of QTEAS sorbent for both dyes. Despite its extended molecular structure, EB is removed with the same effectiveness as IC. Finally, the great ef ﬁ ciency of the highly branched polymer for dye removal from colored wastewater was clearly demonstrated.


INTRODUCTION
Dendrimers are a class of highly branched three-dimensional polymers characterized by a compact shape that have numerous reactive functional end groups and room between branches for taking up guest molecules (Tomalia & Fréchet ). Dendrimers have been shown to be effective for dyeing One of the major factors that influences the performance of polymeric sorbents is the nature of the functional groups available on their surface for interactions with contaminants. These functional groups determine the removal capacity, stability, and reusability of the sorbent material.
According to the literature (Wawrzkiewicz & Hubicki a, b, c), the use of commercially available anion-exchanger resins to remove acid dyes from water has been thoroughly investigated. These resins showed high potential for adsorption of anionic dyes and excellent adsorption capacity due to their high content of positively charged functional groups. Moreover, complete regeneration without loss of their sorption capacity can be achieved in alkaline media (Karcher et al. , ).
The current study was set to prepare a hyperbranched polymer as an efficient material for solid-liquid extraction of acid dyes. The hyperbranched polymer QTEAS was synthesized by a cost-effective one-step copolymerization of multifunctional A3 and B2 monomers, namely, triethanolamine and sebacoyl chloride, followed by methylation of amine groups.
Quaternization of tertiary amine groups was necessary to lead to a material with plenty of positively charged sites on its surface, and this was also intended for rightly avoiding pH adjustment in our sorption studies. Sebacoyl monomer was chosen for its chain length (8 sp 3 carbons) in order to get ample room between the branches in the QTEAS hyperbranched material, thus facilitating the encapsulation of guest molecules. Although its structure is not as perfect as that of dendrimers, QTEAS will still have many similar characteristics and properties of dendrimers, such as the three-dimensional globular architecture and abundant quaternary amine groups required for the removal of targeted dyes by electrostatic interactions. Two negatively charged dyes were chosen for the study: indigo carmine (IC), a divalent anion that is a real concern in textile wastewaters and a diazo dye with extended molecular structure; Evans blue (EB), a tetravalent anion (see Table 1). Furthermore, the structure of the prepared macromolecule is demonstrated by Fourier transform infrared spectroscopy (FTIR), cross polarization magic angle spinning (CPMAS) 13 C NMR, thermogravimetric analysis (TGA), powder X-ray diffraction (DRX), and scanning electron microscopy (SEM). Sorption kinetics, isotherms, effect of pH, ionic strength, and effect of temperature have been investigated to identify a sorption mechanism of the dyes. Moreover, the data were analyzed using different well-known adsorption isotherms and kinetics models. The high performance of the QTEAS material after regeneration cycles has been carefully examined to ascertain its stability and reusability. Finally, the great efficiency of the hyperbranched polymer for dye removal from colored wastewater was clearly demonstrated.
flask equipped with a condenser and a mechanical stirrer.
This solution was kept at 0 W C, and neat sebacoyl chloride (12 mL, 56.25 mmol) was slowly added. During the whole addition (30 min), the reaction temperature was kept at 0 W C and was then allowed to gradually rise to 10 W C. After stirring at 10 W C for 2 h, a dark brown chunk was formed and stuck to the stirring rod, allowing no further efficient stirring. Then, excess iodomethane (20 mL) was added and where q e is the amount (mg g À1 ) of dye sorbed, C i and C e are the initial and equilibrium dye concentrations (mg L À1 ) in solution, respectively, V is the adsorbate volume (L) and w is the sorbent weight (g). All experiments described below were undertaken in either duplicate or triplicate. respectively, and then the dye concentration was analyzed.

Desorption and reusability
To assess the feasibility for consecutive reuse of QTEAS, sorption-desorption studies of IC and EB on the hyperbranched polymer were carried out at room temperature using 50 mg of material and 50 mL of dye solution at a concentration of 50 mg L À1 . Initially, the sorbent material was loaded with the dye following the general sorption procedure described above. The recovered material was washed with distilled water, air dried, and then suspended in 50 mL of aqueous NaOH solution (0.1 M) for desorption of the dye. The obtained suspensions were stirred for 15 min, then centrifuged and the regenerated material was thoroughly washed with distilled water and subsequently suspended in dye solutions under the same conditions as above. The sorptiondesorption cycles were repeated three times.

Characterization of QTEAS
The prepared hyperbranched polymer is absolutely insoluble in a wide range of solvents including high boiling polar aprotic solvents such as DMF and DMAc. Owing to experimental difficulty, we were not able to characterize the material by using established methods such as MALDI-TOF mass spectrometry for the determination of the degree of branching, purity, and structural integrity of the polymer.
Nevertheless, the morphology of QTEAS was determined by SEM, and Figure  The thermal stability of the synthesized QTEAS was studied by TGA (Figure 4(b)). The first derivative of the TG curve is proportional to the rate of decomposition, and represents the temperature corresponding to the maximum rate of weight loss (T max ). As shown in Figure 4(    In order to examine the controlling mechanism of the sorption process, pseudo-first order model and pseudosecond order kinetic models were used to analyze the experimental data.

Effect of contact time
The pseudo-first order model can be expressed in its linear form by Equation (1): The pseudo-second order model can be expressed in its linear form by Equation (2): where k 1 (min À1 ), k 2 (g mg À1 min À1 ) are the rate constants of the pseudo-first order and the pseudo-second order for the sorption process, respectively, q e and q t (mg g -1 ) are the amounts of dye sorbed at equilibrium and at time t (min), respectively. respectively. The equilibrium sorption capacity (q e ), the rate constants (k 1 , k 2 ), and the coefficient (R 2 ) values were calculated from the linear plots. The parameters obtained for the two models are presented in Table 2. Perfect correlation is however observed between experimental data and the pseudo-second order kinetic model with excellent correlation coefficients. Additionally, the calculated q e values from the model were totally in agreement with experimental sorption capacities, emphasizing therefore the efficiency of the model. In the literature, many studies have shown that  Although the surface of the QTEAS adsorbent is tidy, with positively charged quaternary ammonium groups which make it a powerful reactant toward anionic dyes, the presence of alcohol/carboxylic acid termini groups within the polymer could however participate in the removal process via covalent, coulombic, hydrogen bonding or weak van der Waals forces with the dyes' functional groups (ÀOH, ÀNH 2 , ÀSO 3 Na, ÀN ¼ NÀ). To shed light on the adsorption mechanism of the anionic dyes on the QTEAS material, a selective adsorption experiment was carried out using a mixture of cationic and anionic dyes in aqueous solutions. Figure 6(b) (photo in the left corner) shows that preliminary tests revealed that QTEAS was not able to remove the cationic dye Rhodamine B (RhB) at a concentration of 50 mg L À1 from aqueous solutions after 16 h of contact time. Figure 6(b) shows also the UV-vis spectra of aqueous solutions of RhB/IC (3/2) (cationic dye RhB (100 mg L À1 ); anionic dye IC (100 mg L À1 )) mixture before and after the adsorption process. As shown in Figure   6(b), after 16 h of contact time, IC was almost completely removed from the mixture solution by the QTEAS adsorbent, whereas RhB remained intact in the solution. These results suggest that the removal process of anionic dyes by QTEAS could be an ion-exchange mechanism and also confirm that the QTEAS adsorbent has a selective adsorption property for anionic dyes and could have potential for application in purification of cationic dyes from anionic dyes.

Sorption isotherms
The sorption isotherms of anionic dyes IC and EB onto QTEAS are reported in Figure 7(a). The isotherms are superimposed and characterized by a regular shape with a steep initial slope, and are concave to the concentration axis. The great affinity of QTEAS material for the dyes is suggested by the quantitative removal at low dye concentrations. The adsorption of IC and EB onto QTEAS increases with increasing initial dye concentration to reach equilibrium.
The experimental data were analyzed by using Langmuir and Freundlich isotherm models.
The linear form of Langmuir's isotherm model is given by the following equation: The Freundlich isotherm equation can be written in the linear form as given below: where q e (mg g À1 ) is the amount of dye adsorbed per gram of sorbent, q max (mg g À1 ) is the maximum sorption capacity per gram of adsorbent, C e (mg L À1 ) is the equilibrium concentration of dye in solution, and b (L mg À1 ) is the Langmuir constant related to the energy of adsorption, K F (L g À1 ) is the relative adsorption capacity constant of the adsorbent and 1/n is the intensity of the adsorption constant.
As shown in Figure 7 Table 3). These results clearly indicate that both models adequately fitted the experimental data of the dyes' removal by QTEAS material.    Table 4).

Thermodynamic parameters
The removal of IC and EB by the QTEAS sorbent was studied at three temperatures to determine the thermodynamic parameters, and the results are summarized in Table 5. The dyes' uptake decreased with an increase in temperature, indicating an exothermic process.
The thermodynamic parameters ΔG, ΔH, and ΔS were calculated by using the following equation (Zhu et al. ): where T is the temperature (K), R is the ideal gas constant (8.314 J mol -1 K -1 ), and K L is the Langmuir equilibrium constant (L mg -1 ).
The vant'Hoff plot of log K L versus 1/T gave straight lines. The calculated slope and intercept from the plot were used to determine ΔH and ΔS, respectively ( Table 5).
The negative value of ΔG (À31.75 to À26.11 kJ·mol À1 ) at each temperature implies a favorable and spontaneous adsorption process and confirms the affinity of the QTEAS material for IC and EB dyes. In general, the change in free energy for physisorption is between À20 and 0 kJ·mol À1 , whereas chemisorption is in the range of À80 to 400 kJ·mol À1 (

Desorption and reusability
The desorption studies contribute to elucidating the nature of the adsorption process and allow the recovery of the dyes as well as the regeneration of the adsorbent to make the treatment process economical. The QTEAS hyperbranched material can be readily regenerated with a 0.1M NaOH aqueous solution. Therefore, quantitative desorption rates were achieved for both dyes over three repeated sorption-desorption cycles. IC and EB dyes were instantly released as soon as the dye-loaded sorbent made contact with the alkaline solution. This was illustrated by instantaneous coloration of the solution. Nonetheless, the sorbent material was left in contact with the alkaline solution for 15 min of contact time, a period considered as sufficient for the dyes' desorption to be achieved, and on the other hand to prevent alkaline hydrolysis of the hyperbranched polymer. As shown in Figure 9, the third repeated use of QTEAS material for the removal of IC and EB dyes showed neither signs of deterioration nor any decrease in its capacity for the dyes' sorption. Finally, the high performance of the QTEAS ion-exchanger indicates that its reusability is quite feasible.

CONCLUSIONS
In the present study, the sorption of two acid dyes onto QTEAS hyperbranched polymer was investigated. The results demonstrated that the hyperbranched polymer performs efficiently in a wide pH range of dye solutions.
From the kinetic studies, it was found that the sorption process followed the pseudo-second order model. The sorption isotherms were adequately fitted by the Langmuir isotherm model and the removal process was found to take place by the electrostatic attraction between the positively charged hyperbranched polymer and the negatively charged dyes.
Nevertheless, the ionic strength does not affect the dyes' removal efficiency by the hyperbranched polymer. Moreover, the removal capacity is closely connected to the total sulfonate groups on the surface of the dyes. The calculated thermodynamic parameters indicated the exothermic and spontaneous nature of the removal process. Finally, the third repeated use of QTEAS material for the dyes' removal showed no decrease of its capacity for the dyes' sorption.