Synthesis and characterization of poly(ethylene terephthalate) fibers grafted with N-(hydroxymethyl) acrylamide by free radical: its application in elimination of Congo red

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ORIGINAL PAPER

Synthesis and characterization of poly(ethylene terephthalate) fibers grafted with N‑(hydroxymethyl) acrylamide by free radical: its application in elimination of Congo red

Metin Arslan¹ · Kübra Günay² · Zehra Gün Gök^3,4 · Mustafa Yiğitoğlu³

Received: 29 July 2019 / Revised: 28 February 2020 / Accepted: 10 March 2020

Abstract

In this study, firstly, N-(hydroxymethyl) acrylamide (HMAAm) was grafted on poly(ethylene terephthalate) fibers (PET) by utilizing benzoyl peroxide (Bz₂O₂) as an initiator. The alteration in graft percentage with the polymerization time, temperature, concentration of initiator and monomers were investigated. For grafting experiments, the optimum temperature, duration for grafting and initiator concentration were found be 85 °C, 4 h and 0.008 M, respectively, and the maximum grafting yield was determined to be 25% at these conditions. After grafting, the morphological and chemical changes on the grafted fibers were examined scanning electron microscopy and Fourier-transform infrared spectroscopy. The thermal properties of the obtained PET fibers were examined with differential scanning calorimeter analysis. In addition, the water absorption capacities of the grafted fibers obtained at optimal conditions were investigated and it was found the maximum percentage of water retention reached 56%. Removal of Congo red (CR) with the grafted fibers was performed using a batch process. The effects of different parameters such as pH, grafting yield of HMAAm, adsorption time and CR concentration on the sorption capacity of the grafted fibers were investigated. The optimal pH for adsorption of CR was found to be 2, and the adsorption process reached equilibrium in 4 h. The amount of adsorbed CR molecules increased with the increment in the grafting yield up to 17%, and the graft copolymers adsorbed CR with 5.62 mg/g efficiency when the initial CR concentration was 80 ppm. The PET-g-HMAAm fibers synthesized in this study can be obtained easily and inexpensively, and the constructed fibers could be used to remove ionic substance from aqueous solution.

Keywords Poly(ethylene terephthalate) fibers · Grafting copolymerization · N-(Hydroxymethyl) acrylamide · Removal

* Metin Arslan [email protected]

Extended author information available on the last page of the article

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Introduction

In the last fifty years, uncontrolled use of technology that developed very rapidly has caused a great increase in environmental pollution due to wastewater drained by many industries such as textile, paint, paper, rubber and leather using various dyes in their processes [1, 2]. The used dyes are highly toxic to the environment and living organisms and cannot be biodegraded as organic pollutants. Therefore, it is necessary to remove the dyes from wastewater [1, 3]. However, developing countries are still confronted with these problems due to lack of technical knowledge, poor implementation of environmental policies and limited research budgets.

Many processes such as chemical oxidation [4], reverse osmosis [5] and adsorption [6] are used to remove the dyes from industrial wastewater. One of the methods used to remove dyes is the adsorption process. Other methods have some disadvantages, such as the need for expensive and special structures. The adsorption process is a very economical and effective method for removing dyes from wastewater [1]. Activated carbon [7], sporopollenin [8], chitosan [9], modified polymer [10], cellulose [11] and fibers [12, 13] have been used to remove dyes from aqueous environment.

PET is an important commercial polyester and widely used in many different applications such as clothing and packaging material as different physical forms.

PET fibers with a wide surface area can be used as an adsorbent due to its high mechanical properties and inexpensiveness. In addition to being resistant to bac- teria and insects, PET fibers do not undergo degradation due to light and are resistant to acids, bleaching agents and detergents even at boiling temperatures.

PET fibers have been used as adsorbents in the separation of toxic metal ions and textile dyes, and enrichment of trace elements [12–18].

Functional groups such as pyridine, COOH, NH₂ and OH on the surface of the adsorbent are very effective in the removal of ionic dyes from aqueous solutions.

Various methods are used to improve the properties of PET fibers by imparting appropriate functional groups [12, 13]. One of them is the graft copolymerization method. In the graft copolymerization, one kind of monomer can be grafted or a mixture of two or more kinds of monomers can be grafted at the same time on a polymer. By grafting the monomers onto polymers, the aim is to bring the properties of these monomers which join the structure of the fibers in a copolymeric structure and having different functional groups to the fibers simultaneously. Depending on the number of monomer species present in the main chain and side chains, it is pos- sible to obtain graft copolymers in a wide variety of structures. A random copolymer formed by the copolymerization of two different monomers generally exhibits a behavior between the properties of their homopolymers, while a graft copolymer can combine the superior properties of the polymers that form it. The properties of graft copolymer change according to grafting yield of chains onto the main polymer chain. For this reason, in the graft copolymerization studies, it is also important to identify the graft yield and the factors that influence the conditions of grafting, as well as the new properties of the fibers grafted with any monomer since the obtained polymers properties change according to grafted polymers amount [19–24].

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The aim of this work was to produce an adsorbent based on PET for removing CR molecules in the aqueous solution. For this purpose, PET and HMAAm were selected as the polymer substrate and grafting monomer, respectively, since the grafted PET fibers were produced easily and inexpensively, used easily as an adsorbent without losing their integrity, and the surface load of the synthesized material changes according to the pH of the medium due to OH group of HMAAm-grafted chains. In the first part of the study, HMAAm vinyl monomers were grafted onto PET fibers with using benzoyl peroxide as initiator, and the parameters affecting grafting yield of HMAAm were determined. The morphological structure of the fibers was analyzed by SEM, the chemical structure of the fibers was analyzed by FTIR, and the thermal properties of fibers investigated by DSC analysis and the water-holding properties of the fibers were determined. In the second part of the study, PET fibers grafted with HMAAm were used as an adsorbent for the removal of CR from the aqueous solutions by batch method. The effects of different parameters such as pH, grafting yield, treatment time and dye concentration on the adsorption capacity of the reactive fibers were investigated.

Experimental method Materials

The used PET fibers (middle drawing, 122 dTex) were products of SASA Company (Turkey). The original fibers were cleared by washing with acetone for six hours and dried in an oven at ambient temperature. HMAAm monomers were provided by TCI (Tokyo Chemical Industry) with 98% purity. Bz₂O₂ (Merck) was recrystallized from chloroform in methanol and dried before use. 1,2-Dichloroethane (DCE), CR, ace- tic acid (CH₃COOH), boric acid (H₃BO₃) and phosphoric acid (H₃PO₄), potassium hydroxide (KOH), sodium hydroxide (NaOH), hydrochloric acid (HCI) and ammo- nium (NH₃) were supplied from Merck (Analytical) and used as received.

Graft copolymerization method

The fibers weighed at 0.3 ± 0.01 g were swelled in DCE solvent at 90 °C for 2 h to increase polymerization efficiency. At the end of the swelling process, the excess solvent on the fibers was removed with a filter paper and the fibers were immediately put into the graft copolymerization medium. After putting the PET fibers into a 100-mL polymerization tank, the appropriate amount of HMAAm monomer (in the range of 0.1–1 M) was placed on it. Then, 2 mL Bz₂O₂ dissolved in acetone was added and the mixture was completed to 20 mL with water and immediately immersed in a water bath which was controlled at a temperature between 55 and 95 ± 0.1 °C. After a certain period, the fibers samples from the polymerization mixture were rinsed with water to remove homopolymers. It was extracted from the homopolymer by extraction with boiling water for 48 h and dried in vacuum at 55 °C. The graft yield (GY) was calculated gravimetrically by the following equation from the original and grafted fibers masses.

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In this equation, g_a is the dry weight of the PET-g-HMAAm fibers and g_o is the dry weight of the original fibers.

Determination of water‑holding capacities of grafted PET fibers

The PET fibers with different grafting ratios were kept in pure water at 20 ± 0.1 °C for 24 h. The fibers taken from pure water were dried with filter paper and weighed.

The water-holding capacities of the fibers were calculated in % with the following equation.

In this equation, w_a is the mass of the fibers kept in aqueous medium and w_o is the dry mass of the fibers.

Characterization of the PET fibers

After the surface of the original and (PET-g-HMAAm) fibers was coated with gold, the photographs were taken with an SEM (JOEL Model JSM 5600). Infrared spectra of the PET fibers samples were taken by preparing KBr pellet using Bruker Vertex 70 V model spectrometer. The thermal properties of the fibers were examined with a PerkinElmer Sapphire model DSC device. The analysis of fibers was performed by heating the samples by increasing the temperature by 10 °C/min between 30 and 300 °C.

Adsorption study

The adsorption studies were carried out by the batch system in 50-mL conical flasks.

The solutions at the desired concentration were prepared using different buffer solutions, and HMAAm-grafted PET fibers that will be used as adsorbents were weighed 0.1 ± 0.01 g and placed in the flask which contains 25 mL dye solution. Then, the flasks were placed in an agitator water bath and agitated at 110 rpm at room temperature. Samples taken at certain times were filtered through a 0.45-μm pore filter paper and the concentration of dye in the filtrates was determined by a spectropho- tometer (PerkinElmer Lambda 25) at 497 nm after the pH was adjusted to 6.8. The amount of adsorbed dye was calculated using the following equation.

q the amount of CR adsorbed by 1 g (PET-g-HMAAm) fibers (mg/g), C_o initial concentration of CR solution (mg/L), C equilibrium concentration of CR solution (mg/L), V the volume of the CR solution (L), g the amount of (PET-g-HMAAm) fib- ers used as adsorbent (g).

(1) GY(%) =[(g_a− g_o)∕g_o] × 100

(2) Water Uptake Capacity(%) =[(w_a−w_o)∕w_o] × 100

(3) q=(C_o− C)V∕g

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Desorption study

The dye-loaded fibers were placed in 30 mL of KOH solution (1 M) and NH₃ solution (2 M), and the desorbed amount of CR at different times was analyzed. The desorption result was calculated according to the following formula.

Results and discussion Grafting mechanism

After the active centers were formed on the polymer chains of PET by the radical initiator, the modified PET fibers were formed, as shown in Fig. 1, by binding these active centers to polymer chains of HMAAm in side branches.

The effects of temperature and polymerization time on grafting yield

The effect of temperature on grafting HMAAm vinyl monomer on PET fibers was investigated for the range 55–95 °C; the finding is shown in Fig. 2a. It was clearly seen that increment in the temperature from 55 to 85 °C increased the grafting yield. The increase in the temperature enhances the polymerization of monomers, the propagation reaction and the chain transfer reaction. Also, the increment in the polymerization temperature accelerated the degradation rate of Bz₂O₂ and the interaction between the PET fibers and the radical homopolymer chains. At the same time, the increase in temperature increased grafting yield since it increased the concentration of radicals in the medium, the mobility of the monomer molecules, blow- ing capacity of PET fibers and accelerated the diffusion of the monomers into the fibers. The maximum grafting yield occurred at 85 °C. This is close to the glass transition temperature (80 °C) of the maximum grafting. On glass transition temperature

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% Desorption =[amount of dyes passed into desorption environment (mg)∕

amount of loaded dyes(mg)] × 100

PET

C C

C

H H

H

O

N C OH H

N-(Hydroxymethyl)acrylamide (HMAAm) Fig. 1 The chemical structure of the PET-g-HMAAm fibers

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and above, molecules gained mobility and facilitated the diffusion of molecule types that will join the graft copolymerization. The decrease in the graft yield when the temperature is above 85 °C can be associated with the more dominant reactions of the termination with the increase in temperature and to the termination of the initiator radicals by binding among themselves. A similar result has been reported in the grafting of the acrylamide monomer [25] onto the PET fibers.

The grafting experiments were performed at a different time interval, by keeping the concentrations of monomer and initiator constant at 85 °C, and the results are shown in Fig. 2b. As the time of the reaction increased, the grafting yield rapidly increased and reached the equilibrium in 240 min (25%). No increase was observed in the grafting yield after this time. By examining the figure, it was seen that, in the first step, grafting yield increased due to the increase in chain growth grafted on PET fibers and the formation of new chains of homopolymers of HMAAm monomers onto the PET fibers. In the later steps, grafting yield did not substantially change due to the increase in viscosity and the formation of diffusion barrier due to homopolymer of HMAAm formation in the polymerization environment. Similar findings have been reported in the grafting of acrylamide [26], N-vinyl-2-pyrrolidone [27], 2-methyl-5-vinyl pyridine [28] and 4-VP [29]

vinyl monomers onto PET fibers.

The effects of concentration of initiator and monomer on grafting yield

On the grafting yield, the effect of Bz₂O₂ concentration was investigated and the results are shown in Fig. 3a. The important increase was observed in the grafting yield when the initiator concentration was increased up to 8.0 × 10⁻³ M. In the further increases in the concentration of Bz₂O₂, the grafting yield started to decrease.

The increment in the grafting yield with the increase in initiator amount was caused by the increase in the number of radicals due to the initiator’s decomposition by

0 5 10 15 20 25 30

50 60 70 80 90 100

Graftingyield(%)

Temperature (°C)

0 5 10 15 20 25 30

0 60 120 180 240 300 360

Grafting yield (%)

Time (min)

a b

Fig. 2 Effect of temperature on grafting percentage (a) (the monomer concentration was 1 M, initiator concentration was 8.0 × 10⁻³ M, and incubation time was 6 h) and the effect of polymerization time on grafting percentage (b) (the monomer concentration was 1 M, initiator concentration was 8.0 × 10⁻³ M, and incubation temperature was 85 °C)

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giving C₆H₅COO^· and C₆H₅^·. With further increase in Bz₂O₂ concentration, the number of free radicals increased, resulting in an increase in homopolymeric radical chains. The increase in radical polymer chains in the polymerization medium has increased the rate of chain transfer reactions between PET fibers and these radicals.

The variation in the grafting yield with HMAAm concentration is shown in Fig. 3b. The variation in the grafting yield was studied by increasing the mixture concentration between 0.1 and 1.0 M. The graft yield increased linearly (reached 25% with the increase in monomer concentration up to 0.6 M) and reached equilibrium at higher concentrations. While the HMAAm concentration increased, the monomers of HMAAm molecules diffused into the PET fibers and active homop- oly-HMAAm chains in the polymerization environment increased the grafting yield.

These active chains break the hydrogen with PET fibers chain transfer reactions and raise the number of active centers on the PET fibers. This facilitated the addition of monomer molecules to newly formed active centers and actively grafted side chains on PET fibers [29].

Water uptake study

Because of the hydrophobic nature of the PET fibers, their chemical structure and their high crystalline structure, their chemical reactivity, and water absorption properties are very low. The aim of this study was to increase the water absorption capacity by grafting of HMAAm vinyl monomers on the PET fibers. The water uptake ability of PET-g-HMAAm fibers was found with the increase in fibers mass, and the findings are shown in Fig. 4. As clearly seen from Fig. 4, the percentage of water absorption of PET-g-HMAAm fibers was increased as the percentage of grafting increased, and the percentage of water retention at 25% grafting yield reached 56%. The increase in the amount of water retention is due to the incorporation of

0 5 10 15 20 25 30

0 2 4 6 8 10 12 14

Graftingyield(%)

Amount of initaitor (10^-³ M)

0 5 10 15 20 25 30

0.0 0.2 0.4 0.6 0.8 1.0

Graftingyield(%)

Amount of monomer (M)

a b

Fig. 3 The effect of initiator concentration on grafting yield (a) (the monomer concentration was 1 M, incubation time was 4 h, and incubation temperature was 85 °C) and the effect of monomer concentration on grafting yield (b) (the initiator concentration was 8.0 × 10⁻³ M, incubation time was 4 h, and incubation temperature was 85 °C)

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hydrophilic groups such as –NH–, –OH into the PET fibers structure resulting from HMAAm grafting.

Characterization of the adsorbents

SEM photographs of un-grafted and HMAAm-grafted (25%) fibers are given in Fig. 5. Examining the SEM photographs, it was seen that the surface of the un- grafted PET fibers (Fig. 5a) was flat, smooth and homogeneous. At 25% grafting, it was seen that the PET fibers were coated with the grafted chains of HMAAm

0 10 20 30 40 50 60

0 5 10 15 20 25

Water Absorption (%)

Grafting Yield (%)

Fig. 4 Effect of grafting yield on water uptake abilities of the PET-g-HMAAm fibers) (the initiator con- centration was 8.0 × 10⁻³ M, incubation time was 4 h, and incubation temperature was 85 °C)

Fig. 5 SEM images of a original and b PET-g-HMAAm fibers (with 25% grafting yield)

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(Fig. 5b) and had a heterogeneous surface. SEM photographs are another proof of grafting.

The FTIR spectra of un-grafted and HMAAm-grafted (with 25% grafting yield) PET fibers are given in Fig. 6. In the FTIR spectrum of the original PET fibers, the peak at 2963 cm⁻¹ shows the absorbance of aromatic C–H bands. The peak at 1712 cm⁻¹ shows the C=O stretching in ester and carboxylic acid groups of PET. The absorptions around 1505 and 1407 cm⁻¹ represent aromatic C=C stretching. The absorption near to 1371 cm⁻¹ shows OH in plane bending of carboxylic acid groups. The peaks around 1239, 1092 and 1016 cm⁻¹ represent C–H in plane bending. The absorption around at 970 cm⁻¹ is due to C–OH deforma- tion of carboxylic acid groups [30, 31]. Comparing the spectrum of PET with PET-g-HMAAm fibers, the broad peak at 3301 cm⁻¹ comes from the OH of the HMAAm group. The presence of monomer is also confirmed by characteristic absorption bonds at 1538 cm⁻¹ and 2965 cm⁻¹ due to C–N stretching vibration and N–H stretching vibration of the secondary amide of grafted monomers, respectively [26]. Therefore, the change from the FTIR spectrum can be demon- strated as evidence that the monomers are covalently bound to the PET surface by radical polymerization (because original PET fibers do not have C–N bonds in their structure), since there is no unreacted monomers left on the surface after the washing process. In the literature, there are many studies proving grafting with the change in FTIR analysis [30, 31].

To prove grafting and examine the thermal properties of the obtained graft copolymers, DSC studies were carried out. DSC analysis was performed with original PET fibers, HMAAm monomers, the obtained graft copolymers and PET-HMAAm mixture. To obtaining the physical mixture of PET-HMAAm monomers, we melted the HMAAm monomers to 80 °C and put the original PET fibers into it. After wait- ing for a while, we removed the PET fibers from the melt and allowed to dry. After drying, we used the PET fibers obtained by physical treatment for DSC analysis.

The DSC thermograms of original PET, HMAAm monomers, PET-g-HMAAm fibers and PET-HMAAm mixture are shown in Fig. 7. Original PET fibers had a glass transition at 119.54 °C and an endothermic melting peak at 255.53 °C. These values are consistent with the literature. The HMAAm powders showed an endothermic melting peak at 76.26 °C and a decomposition at 122.41 °C (this was supported with TGA analysis, it was not given). The thermal properties of PET-g-HMAAm fibers were changed according to original PET fibers due to grafting of HMAAm monomers. The glass transition of the grafted fibers was reduced to 71.75 °C since the structural symmetry of original PET fibers was broken down with grafting. In addition, in graft copolymers, the thermal decomposition of HMAAm at 141.26 °C was seen in DSC thermograms. In the DSC analysis of the mixture (not chemically modified), the melting and decomposition peaks (the melting temperature of HMAAm monomers in pure form and in the mixture was 76.26 and 75.58 °C, respectively) of HMAAm monomers were also observed since there was no change in the chemical structure of HMAAm. Since there is no change in the chemical structure of PET, the melting point is almost the same as that of the original PET fiber (255.53 and 253.34). With the results of DSC, we realized that chemical change is different from physical attachment and we proved the grafting process.

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1712.47 1504.61 1407.88 1338.72 1239.62 1092.75 1016.21 970.05 871.78846.06 792.15 721.91 501.57 430.91

500 1000

1500 2000

2500 3000

3500 Wavenumber cm-1

707580859095100Transmittance [%] 3391.83 3282.70 1664.401625.14 1534.58 1456.851415.771396.48 1267.301229.62 1080.451058.11 1006.24979.76 811.99 713.36 661.55 615.03 505.20 417.33

500 1000

1500 2000

2500 3000

3500

Wavenumber cm-1

5060708090100Transmittance [%] 3301.06 2965.68 1712.53 1538.26 1451.561408.761371.581340.49 1241.65 1093.78 1039.851017.03 972.14 871.44846.46 791.98 722.07 631.66

500 1000

1500 2000

2500 3000

3500

Wavenumber cm-1

60708090100Transmittance [%]

Original PET

HMAAm

PET-g-HMAAm

Fig. 6 The FTIR spectra of original PET fibers, HMAAm powders and PET-g-HMAAm fibers (having 25% grafting yield)

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pH and treatment time effects on removing of CR

The pH of a solution is one of the most significant factors affecting the adsorption of dyes on adsorbents. The effect of pH on the elimination of CR was investigated in the range of pH 2–7. The results in Fig. 8a show that the pH values had an effect on the elimination of CR and as the pH values of aqueous solutions of CR solution increased, there was a decrease in adsorption rate. The grafted PET fibers have reached the maximum adsorption capacity at pH 2 for CR.

Figure 8b shows the effect of treatment time on the removing of CR onto the modified PET fibers. Examining the results, the adsorption rate of CR was very fast at the beginning, then slowed down and reached equilibrium. The removal of CR reached equilibrium in 240 min.

The adsorption of CR at various pH values on HMAAm monomer-grafted PET fibers can be explained by a mechanism involving steps such as electrostatic attraction, ion exchange and chemical reactions (Fig. 9). In solutions with low pH, almost all –OH groups of HMAAm on the PET fibers are protonated; hence, the adsorbent is active for the adsorption of negatively charged CR. At low pH, high adsorption values were observed because the high H⁺ concentration in the interface adsorbed electrostatically negatively charged CR molecules.

119.54°C(I)112.06°C 128.97°C

255.53°C -1.0

-0.8 -0.6 -0.4 -0.2 0.0

Heat Flow (W/g)

0 50 100 150 200 250 300 350 400

Temperature (°C)

Exo Up Universal V4.5A TA Instruments

76.26°C 122.41°C

315.09°C

-4 -2 0 2 4 6

Heat Flow (W/g)

0 50 100 150 200 250 300 350 400

Temperature (°C)

141.26°C

247.04°C 82.06°C(I)

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Heat Flow (W/g)

0 50 100 150 200 250 300 350 400

Temperature (°C)

Original PET

75.58°C 111.62°C

253.34°C 327.29°C

-2 -1 0 1 2

Heat Flow (W/g)

0 50 100 150 200 250 300 350 400

Temperature (°C)

HMAAm monomers

PET-g-HMAAm PET-HMAAm

mixture

Fig. 7 DSC thermograms of original PET, HMAAm monomer, PET-g-HMAAm fibers (with 25% graft- ing yield) and the mixture of original PET and HMAAm monomer

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The effect of grafting yield and initial CR concentration on adsorption

The influence of grafting yield on the amount of removed CR was investigated by keeping the other parameters constant, and the findings are shown in Fig. 10a.

The amount of adsorbed ionic substance increased with the increment in the grafting yield up to 17%. Since the non-grafted fibers do not have convenient functional

0.0 0.5 1.0 1.5 2.0 2.5 3.0

1 2 3 4 5 6 7

q (mg /g)

pH

0 1 2 3 4 5

0 60 120 180 240 300

q (mg/g)

Time (min)

a b

Fig. 8 The effect of pH on elimination of CR (a) (CR concentration was 20 ppm, incubation time was 2 h, and the grafting yield was 17%) and the effect of treatment time on adsorption (b) (CR concentration was 20 ppm, pH was at 2, and the grafting yield was 17%)

Fig. 9 Sorption mechanism of CR molecules on the adsorbent

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groups, the adsorption of ionic dyes is almost zero with the original fibers. With the increase in grafting yield, the adsorbed amounts of dyes also increased as the number of functional groups of HMAAm entering the PET fibers structure as side polymeric chains increased. Thus, as the amount of grafting increased compared to the original fibers, the adsorption capacity of the fibers increased. Although the adsorption of CR on PET-g-HMAAm fibers is through the HMAAm groups grafted onto the PET surface, branching increased with increasing amount of HMAAm in the main polymer chain. This affects the adsorption process that starts from the surface of the polymer but goes into the inner regions. The increase in the amount of HMAAm grafted on the PET surface affects the adsorption capacity after a value for reasons such as steric hindrance and diffusion difficulty. There are many studies confirming our findings in the literature [13, 20, 22, 32].

The effect of initial dye concentration (10–100 mg L⁻¹) on the adsorption of CR in PET-g-HMAAm fibers was systematically investigated. Figure 10b shows that the dye initial concentration at the optimum pH is a function of the amount of adsorbed dye. As clearly seen in Fig. 10b, when the initial concentration of CR is increased, the amount of adsorbed material increased rapidly, and then it got fixed by reaching the equilibrium adsorption value. The maximum adsorption values of CR at 80 ppm were 5.62 mg g⁻¹. Although the grafting yield of HMAAm monomers was low, the adsorption capacity of HMAAm-grafted PET fibers is high because the –NH and –OH groups provide the interaction between the adsorbent and CR molecules.

Desorption results for CR

The reuptake of CR was examined, and the results are given in Fig. 11. CR adsorbed at room temperature was desorbed by 1 M KOH and 2 M NH₃ solution with 34

0 1 2 3 4 5

0 10 20 30

q (mg/g)

Grafting yield (%)

0 2 4 6

0 20 40 60 80 100

q (mg/g)

C (ppm)

a b

Fig. 10 The removal of CR by PET fibers with different grafting yields (a) (CR concentration was 20 ppm, pH was at 2, and incubation time was 4 h) and removing of CR with PET-g-HMAAm fibers at different initial CR concentrations: (pH was at 2, incubation time was 4 h, and the grafting yield was 17%)

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and 53% desorption ratio in 60 min, respectively. The desorption ratio supported the proposed adsorption mechanism. By putting dye-loaded fibers into the desorption medium, the OH₂⁺ groups in the HMAAm polymer structure are converted to OH groups, thereby reducing the interaction with negatively charged CR molecules. As a result, dye molecules attached to the fibers by electrostatic interactions pass into the desorption medium. This desorption ratio indicates that HMAAm monomers grafted PET fibers may be used in industrial applications as an effective adsorbent for removing anionic dyes from wastewater.

Conclusions

HMAAm monomer was grafted on PET fibers by utilizing Bz₂O₂ as an initiator in the aqueous medium. The highest (25%) percent of grafting in HMAAm monomer on PET fibers was reached at under [Bz₂O₂] = 8 × 10⁻³ M, [HMAAm] = 0.6 M, t = 240 min and T = 85 °C conditions. It was observed that water-retention capac- ity values increased depending on the grafting percentage. The modified PET fibers were employed to eliminate CR from aqueous solution at different conditions. It was found that different factors such as grafting yield, pH, adsorption time and initial CR concentration were effective on the adsorption rate of CR on the grafted fibers.

The modified PET fibers removed CR with 5.62 mg/g efficiency when the initial CR concentration was 80 ppm. Accordingly, it was found that PET-g-HMAAm fibers had an adsorption capacity for anionic dyes and it was thought that PET-g-HMAAm fibers could be used as alternative and economical industrial adsorbents.

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Affiliations

Metin Arslan¹ · Kübra Günay² · Zehra Gün Gök^3,4 · Mustafa Yiğitoğlu³

1 Department of Chemistry and Chemical Processing Technologies, Kırıkkale Vocational High School, Kırıkkale University, 71450 Yahsihan, Kirikkale, Turkey

2 Graduate School of Natural and Applied Sciences, Kırıkkale University, 71450 Yahsihan, Kirikkale, Turkey

3 Department of Bioengineering, Kırıkkale University, 71450 Yahsihan, Kirikkale, Turkey

4 Department of Bioengineering, Institute of Science, Hacettepe University, Ankara, Turkey