Elimination of carcinogenic bromate ions from aqueous environment with 4-vinyl pyridine-g-poly(ethylene terephthalate) fibers

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RESEARCH ARTICLE

Elimination of carcinogenic bromate ions from aqueous environment with 4-vinyl pyridine-g-poly(ethylene terephthalate) fibers

Kübra Günay¹_&Metin Arslan²_&Ogün Bozkaya³_&Yaşar Aluç³&Zehra Gün Gök⁴

Received: 13 April 2019 / Accepted: 26 August 2019

# Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract

In this study, poly(ethylene terephthalate) fibers grafted with 4-vinyl pyridine (PET-g-4VP) was synthesized with using a radical polymerization method and its removal capacity for bromate ions in the aqueous solution was explored. The synthesized graft copolymer was structurally characterized by scanning electron microscopy (SEM) and Fourier transformed infrared spectroscopy (FTIR). The effect of some parameters such as pH, grafting rate, processing time, and ion concentration on bromate removal was examined with batch experiments. The sorptions of bromate onto the PET-g-4VP fibers were both verified with FTIR and X-ray fluorescence analysis (XRF) and the remaining amount of bromate after adsorption process was determined with an ion chromatography (Shimadzu). Moreover, kinetic and isotherm studies were also performed for adsorption of bromate with the grafted fibers. The point of zero charge (pHpzc) of the PET-g-4VP fibers was found to be 7.5 and the fibers removed maximum amount of bromate from aqueous solution at pH 3. Equilibrium time of adsorption was determined to be 75 min and the adsorption kinetic was found to be pseudo-second-order model. It was observed that the increase in the amount of grafted 4VP onto the PET fibers increased the bromate removal capacity of the fibers; however, when the grafting yield of 4VP was over 80%, the bromate removal ability of the fibers decreased. The maximum bromate removal capacity of the PET-g-4VP was determined to be 183 mg/g when the initial bromate amount was 800 mg/L, treatment time was 75 min, pH of the solution was 3, and 4VP grafting yield was 80%. When the initial bromate concentration was higher than 800 mg/L, the removal rate of the PET-g-4VP fibers was not changed. In addition, bromate ion adsorption data indicated compliance with the Freundlich isotherm. The adsorbent fibers obtained by this study may be promising candidates for the removal of bromate ions from the aqueous media.

Keywords Poly(ethylene terephthalate) . Copolymerization . 4-Vinyl Pyridine . Bromate . Adsorption . Aqueous solution

Introduction

Ozone oxidation process is extensively being applied in the treatment of drinking water to increase water quality by

destroying microorganisms and organic pollutants (Chen et al.2018; Xu et al.2018). During this treatment, the bromide ions present naturally in ground and surface water at varying amounts between a few μg/L and nearly 800 μg/L (Wisniewski et al. 2011) are oxidized to bromate ions and hypobromite ions (OBr⁻) (Xu et al.2013; Xu et al.2018).

After ozonation, the amount of bromate in drinking water was reported to be between 2 and 293 μg/L. The formed amount of bromate varies depending on the factors such as the ozone dosage and treatment time, pH and temperature of the ozonation process, and the bromide concentration in water (Han et al.2014; Li et al.2016; Xu et al.2018).

Bromate is classified as a group 2B carcinogenic material by the International Agency for Research on Cancer (IRAC) and it is not found naturally and formed by the conversion of bromide ions to bromate in the water during the ozonation process (Chiu et al.2018; Yang et al.2018). It is difficult to remove bromate because of its high stability and mobility in the water (Yang Responsible editor: Angeles Blanco

* Zehra Gün Gök

zzehragungok@gmail.com

1 Institute of Science, Kırıkkale University, Yahsihan, Kirikkale, Turkey

2 Chemistry and Chemical Processing Technologies Department, Kırıkkale Vocational High School, Kırıkkale University, Yahsihan, Kirikkale, Turkey

3 Kırıkkale University Scientific and Technological Research Application and Research Center, Yahsihan, Kırıkkale, Turkey

4 Department of Bioengineering, Kırıkkale University, Yahsihan, Kırıkkale, Turkey

/ Published online: 4 September 2019

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et al.2018). Furthermore, the toxic effects of bromate on human are very strong and the probability of cancer is quite high in people consuming drinking water containing a fewμg/L of bromate (Chen et al.2018). In addition to the carcinogenic effects, bromate has side effects such as abdominal pain, nau- sea, vomiting, diarrhea, and hemolytic anemia (Moore and Chen2006; Naushad et al. 2015; Xu et al. 2018). Relevant institutions in all countries have a strict limitation on the amount of bromate in drinking water. The US Environment Protection Agency (US EPA) has set bromate maximum contaminant level in drinking water as 10 μg/L (EPA 1999;

Bhatnagar et al.2009; Xu et al.2018) and the World Health Organization (WHO) reduced the permissible bromate concentration from 25 to 10μg/L in the drinking water in 2004 (Chen et al.2018; Yang et al.2018; Xu et al.2018).

Bromate, which is an anionic contaminant, is very difficult to remove from drinking water due to its high stability and mobility. For this reason, some researchers have developed a variety of purification technologies for the removal of bromate from drinking water. Many methods such as ultraviolet irradi- ation (Xiao et al.2017), reduction (Liang et al.2010), membrane filtration (Merle et al.2017), and adsorption (Yang et al.

2018) are used to remove bromate present in drinking water.

Interest in adsorption method has increased due to its easy operation, high efficiency, and low cost (Han et al.2018; Xu et al.2018). Many adsorbents such as aluminum (Chiu et al.

2018), activated carbon (Chen et al.2012), and iron-based materials (Wu et al.2013) have been used to remove bromate ions from water. In general, sorption of the bromate ions to the adsorbents is mainly due to the electrostatic interaction between the negatively charged bromate ions and the positively charged adsorbents (Chiu et al.2018).

PET was obtained from petroleum in 1940 and it is widely used in lots of areas such as food packaging, medical ingredi- ents, and clothing (Kevin et al.2014; Fragal et al.2016). PET is the most widely used fibers in the textile field and has high resistance to forceful acids, oxidizing agents, radiation, and microorganisms (Temoçin and Yiğitoğlu2010; Bozkaya et al.

2012). PET fibers have been used by many researchers as adsorbent material to remove various dyes and heavy metals from aqueous solution (Yiğitoğlu and Arslan 2009; Bozkaya et al.

2012; Arslan and Günay2017; Arslan and Günay2018a, Gün Gök et al.2019). Due to the fact that the structure of PET is hydrophobic and there are no active groups on the surface, in these studies, the PET surface has been functionalized by being grafting with various monomers including functional groups such as -NH2, -OH, -COOH, and -CN.

Since the bromate ions are classified as a group 2B carcinogenic material, the importance of removing the bromate ions from drinking water after the ozonation process is an environmental problem. In this study, 4VP monomers were grafted to the PET fibers for synthesis an adsorbent to remove bromate ions form aqueous solution. The constructed grafted

copolymers were characterized by SEM, FTIR, and XRF and the effects of pH, treatment time, grafting yield, and bromate ion concentration on removal were researched.

Materials and methods Materials

The fibers of PET (122 dTex) were supplied from SASA Co.

(Adana, Turkey). Before using, the PET fibers were washed with acetone for 6 h under Soxhlet and dried at 37 °C in a vacuum oven. 4VP was taken from Merck (Germany) and it was refined by vacuum distillation. Methanol, 1,2-dichloroethane, and acetone used in the study were purchased from Merck (Germany). Benzoyl peroxide, potassium bromate (KBrO3), hydrochloric acid (HCI), sodium hydroxide (NaOH), acetic acid, boric acid, and phosphoric acid were supplied from Sigma-Aldrich (Germany).

Production of the adsorbent

The monomers of 4VP were added onto the PET fibers with a radical polymerization process that has been previously reported (Yiğitoğlu and Arslan2005). In order to increase the polymerization efficiency, the PET fibers (0.3 g) were inflated at 90 °C for 2 h in 1,2-dichloroethane before polymerization.

Then, the inflated PET fibers were taken into a 3-neck tube in which the polymerization was to be carried out. Benzoyl peroxide dissolved in 2 mL of acetone and varying amounts of 4VP were added to the polymerization tube and the volume of the mixture was adjusted to 20 mL with water. Under Soxhlet, the polymerization was performed at 65 °C for 2 h. Two hours later, the fibers obtained from the polymerization medium were washed with methanol to remove unreacted monomer residues, the resulting homopolymer, and solvent residues.

The washed fibers were dried at 50 °C in the vacuum oven and the grafting yield (%) of 4VP onto PET fibers was determined by subtracting the initial weight from the final weight of the fibers and dividing the result by the first weight of the fibers and multiplying by 100.

Characterization of the fibers

Morphological structure of original PET fibers and the synthesized graft copolymer by radical polymerization method was investigated by SEM (JEOL Model JSM 5600) analysis.

The chemical construction of ungrafted PET fibers and PET- g-4VP was investigated by FTIR (Bruker Vertex 70 V) analysis. Bromate adsorption to the grafted fibers was verified by X-ray fluorescence (Spectro Xepos XRF Spectrometer) analysis. For determining of the neutral pHpzcof PET-g-4VP fibers, 0.1 g of the adsorbents was placed in 0.1 M KCl

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solutions (the pH of the solutions was adjusted between 2 and 11 with 1 M HCI and 1 M NaOH). After that, the mixtures were incubated at 110 rpm at room temperature as well as the pH of the mixture was stable. At the end of the experiment, the final pH (pHf) of the solutions was measured with a pH meter and ΔpH of every solution was found by the distinctness between initial pH value and pHf. The plot ofΔpH versus initial pH was drawn and in this graph, the point that intersects thex-axis was decided as pHpzc(Coşkun et al.2018).

Removing of bromate with the grafted PET fibers

Batch experiments were conducted to determine the removal capacity of PET-g-4VP fibers for the bromate ions. Briefly, 0.1 g of the grafted fibers was placed to a 50-mL beaker, in which 25 mL of bromate solution (at different concentration and pH) was contained. Experiments were carried out in a water bath at 110 rpm at 25 °C. After reacting for a certain time, the fibers were removed from the beakers, the bromate solution was filtered with a 0.45-μm filter, and the bromate concentration of the supernatant were determined by an ion chromatography (Shimadzu). The adsorption capacity of the grafted fibers for bromate ions was determined by the following equation:

Q ¼ðC0−C_eÞ V

m ð1Þ

whereQ is the amount of bromate adsorbed by 1 g of the adsorbent (mg/g),C0is the concentration of the bromate solution at the beginning (mg/L),Ceis the concentration of the bromate solution at equilibrium after adsorption (mg/L),V is the volume of the bromate solution (L), andm is the weight of PET-g-4VP fibers used as adsorbents (g).

Bromate desorption from the loaded fibers

The desorption process of bromate from the fibers was conducted by putting bromate-loaded fibers (about 0.1 g) in 25 mL of 1 M NaOH solution. The bromate ions were desorbed from the fibers at distinct moments (between 0 and 60 min) and assayed as mentioned above. The efficiency of desorption was found by using the below equation:

Desorption%¼the desorbed amount of bromate mgð Þ

the adsrobed amount of bromate mgð Þ 100 ð2Þ

Results and discussion

The aim of this study was to eliminate the bromate ions with using the constructed fibers in aqueous solution. Firstly,

monomers of 4VP were grafted onto PET by using a radical polymerization technique. Then, the synthesized PET graft copolymers were examined by SEM and FTIR. In order to determine the optimum conditions for the elimination of bromate ions by the synthesized fibers, the effect of pH, processing time, ion concentration and grafting yield of the fibers on the adsorption process was investigated. In addition, the kinetic constants and adsorption isotherms were investigated to understand the adsorption of bromate to the fibers.

Adsorbent characterization

The morphological structure of the original PET and modified PET fibers was investigated by SEM. The SEM images of the fibers are shown in Fig.1. As shown in Fig.1, the surface of ungrafted PET fibers was smooth and the structure of the fibers was homogeneous. However, after the grafting polymerization, 4VP monomers were grafted onto the PET fibers and the surface of the grafted fibers was seen as rough and heterogeneous (Arslan and Günay2018b). This change in the surface of modified PET fibers indicates that 4VP was successfully grafted on the surface of PET fibers.

FTIR spectra of the original PET fibers, modified PET fibers, the bromate, and bromate-loaded PET-g-4VP fibers are given in Fig.2. In the spectrum of original PET fibers, the peaks could be listed as follows: 2960 and 2880 cm⁻¹due to C-H symmetric and asymmetric vibration in CH2groups, 1711 cm⁻¹due to vibration of C=O groups, and 1402 cm⁻¹ due to CH2groups (Bozkaya et al.2012). After the grafting of PET fibers with 4VP, the FTIR spectrum of the fibers became different. The observed peak at 1600 cm⁻¹appertains to the resonance peak of the 4VP monomers onto PET fibers (Arslan and Günay2018b). This change in the FTIR spectrum indicates that 4VP monomers were successfully added onto the PET fibers. When we compared the FTIR spectra of 4-VP grafted PET fibers and bromate-loaded PET-g-4VP fibers, the peaks of the bromate-loaded PET fibers were found to expand in regions with specific peaks of bromate (772 cm⁻¹ and 420 cm⁻¹) (Yang et al.2018). The band observed at 772 cm⁻¹corresponds to symmetric stretching mode of bromate (Alves and Faria2002) and the band observed at 420 cm⁻¹ corresponds to symmetric bending of bromate (Junaid Bushiri et al.2013). This change in the FTIR spectra indicates that bromate was successfully adsorbed by the PET-g-4VP fibers.

XRF analysis of the fibers was performed for showing the adsorption of bromate ions to the grafted PET fibers (Chen et al.2018). It was found that no bromate ions were detected with XRF on PET-g-4VP fibers and after adsorption of the bromate ions (800 mg/L) to the grafted fibers, the adsorbed bromate ions were detected on the PET fibers. As shown in Fig. 3, the representation of the peak area acquired for the fibers was changed according to the adsorbed bromate ions.

For unloaded PET fibers, there was no XRF signal and the

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peak increased for PET-g-4VP fibers after the adsorption of the bromate ions on the surface of the fibers via 4VP.

pH effects on elimination of bromate by PET-g-4VP fibers

For elimination of bromate ions with PET-g-4VP fibers, hy- drogen ion concentrations of the environment are a significant factor since the charge at the surface of the modified fibers depends on pH. For this reason, the pHpzcof the synthesized fibers was found before performing a pH scan for the adsorption studies. Because this value shows how the surface charge of the material changes according to the pH of the solution.

The pHpzc of PET-g-4VP fibers was found as 7.5 (Fig.4a), which shows that the adsorbent can be used to remove anionic species at solutions having pH lower than 7.5 (Coşkun et al.

2018; Mittal and Ray2016; Mittal et al.2018). After finding this result, the effect of pH on the elimination of bromate with the grafted fibers was carried out between pH 2 and 7. For the adsorption studies at different pH values, the pH values of the buffer solutions (Briton Robinson) were adjusted to the

desired values and 0.1 g of the copolymer was added to the solution. After incubation at room temperature for 1 h at 110 rpm agitation rate, the bromate solution was filtered and the amount of bromate remaining in the solutions was determined. The graph showing the removal capacity of bromate by the synthesized copolymer at different pH values is given in Fig. 4b. As shown in Fig.4b, the amount of bromate removed from the environment has changed considerably according to the pH of the solution and the fibers removed the maximum amount of bromate from the solution at pH 3.

Therefore, subsequent studies continued at pH 3.

Various interactions such as chemical or electrostatic can be used to clarify the change in the amount of bromate adsorption onto the grafted fibers observed at different pH values. The removal mechanism of bromate by the constructed fibers was shown in Fig.4c. At distinct pH, the protonation of pyridine groups of the 4VP changed and this status had effect on the surface charge of the PET-g-4VP fibers (Yiğitoğlu and Arslan2009; Bozkaya et al. 2012). At low pH values, the charges at surface of the modified fibers were more positive and the sorption of bromate onto the fibers

Fig. 2 FTIR spectrum of ungrafted PET, PET-g-4VP fibers, bromate, and bromate-loaded PET-g-4VP fibers (right) Fig. 1 SEM images of original

PET fibers (a) and the grafted PET fibers (b)

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increased due to electrostatic interaction between positively charged pyridine groups and negatively charged bromate ions.

At high pH values, the protonation of pyridine groups diminished and the surface charge of the PET fibers has changed.

For this reason, at lower pH, the removal capacity of the fibers increased. The maximum removal capacity of the fibers for the bromate ions was observed at pH 3 rather than pH 2. This result could be due to the association of Cl⁻ions (existing in the solution of pH 2 since the pH of the mixture was adjusted

to 2 by HCI) with the positively charged fibers by inhibiting the adsorption of bromate ions onto the fibers. There are studies in the literature that have similar results that prove our findings (Arslan2011; Bozkaya et al.2012).

Effect of contact time on bromate removal

In adsorption process, the sorption of ions onto adsorbents attains equilibrium after a particular treatment time. The quick

-1.5 -1 -0.5 0 0.5 1

0 2 4 6 8 10 12 14

HpΔ

pH

a b

0 2 4 6 8 10

0 1 2 3 4 5 6 7 8

Q (mg/g)

pH

PET

CH₂

C H

PET-g-4VP

H⁺ N

BrO₃^-

PET

CH₂

C H

Bromate loaded PET-g-4VP

N⁺ BrO₃^-

c

Fig. 4 pHpzcof the PET-g-4VP (a) and the influence of pH on adsorption of bromate by the grafted fibers (b) (adsorbent grafting yield 80%; contact time, 60 min; bromate concentration 40 mg/L) and sorption mechanism of bromate ions by the modified fibers (c)

Fig. 3 XRF analysis of bromate and unloaded and bromate-loaded PET-g-4VP fibers

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elimination of contaminants is a desirable feature for commercial applications. Therefore, the treatment time for the removal of bromate ions by the grafted fibers was researched. The results showing the adsorption capacity (mg/g) values versus time for the bromate ions were given in Fig.5a. In the first minutes of incubation, the modified PET fibers quickly removed bromate present in the medium and after a while, the adsorption reached equilibrium (75 min). The rapidity of adsorption at the first hours could be due to the more accessible charged groups on the surface of the fibers (Arslan and Günay 2017; Yang et al.2018).

The sorption kinetics is an important feature that describes the effectiveness of adsorption. To examine the bromate adsorption kinetic mechanisms of the PET-g-4VP fibers, the pseudo-first-order model (3) and the pseudo-second-order model (4) are investigated to match the adsorption data.

logðQ_e−Q_tÞ ¼ logQ_e− k1

2:303

t ð3Þ

t Q_t ¼ 1

k2Q²_eþ t

Q_e ð4Þ

In these equations,QtandQe are the amount of bromate adsorbed by 1 g of the fibers at anyt time and equilibrium, and k1is the rate constant of pseudo-first-order adsorption whereas k2is rate constant of pseudo-second-order adsorption.

About the adsorption equation, the results of Fig.5acan be changed into the graph of log(Qe− Qt) versus time (Fig.5b).

Thek1andQ values were calculated from the graphical equation and given in Table1. The experimental values ofQedid not match with the calculated ones, indicating that the adsorption of bromate ions by the PET-g-4VP fibers was not a first-order reaction. The plot oft/Qtversus time was given in Fig.5c.Qe

andk2values calculated using the equation of this graph were given in Table1. As shown in the given table, the experimen- tally foundQevalue for the adsorption of bromate is close to the calculatedQtvalue according to pseudo-second-order adsorption and the correlation coefficient in the second-order kinetic mechanism was 0.9986. This ultimately indicates that the adsorption of bromate was a second-order reaction.

Effect of grafting yield and initial bromate concentration

The adsorption capacity (mg/g) versus 4VP grafting yield onto PET fibers for bromate adsorption was shown in Fig.

6a. The bromate removal capacity of the grafted fibers increased with the increase of 4VP grafting amount.

When, the grafting yield increased from 15 to 80%, the bromate removal capacity increased quickly. However, when the grafting yield was higher than 80%, the removal capacity was diminished. An increase in the amount of bromate removed by increasing the amount of 4VP

-1.00 -0.60 -0.20 0.20 0.60 1.00

0 20 40 60 80

log(Qe-Qt)

Time (min)

0 1 2 3 4 5 6 7

0 20 40 60 80

t/Qt

Time (min)

a

c

b

0.0 2.0 4.0 6.0 8.0 10.0

0 20 40 60 80 100 120

)g/gm(Q

Time (min) Fig. 5 The amount of bromate

adsorbed by 1 g of the adsorbent at different time points (a), the plot of log(Qe− Qt) versus time (b), and the plot of t/Qtversus time (c) (pH, 3; adsorbent grafting yield, 80%; bromate

concentration 40 mg/L)

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grafted onto the PET surface is expected, because the PET fibers removed the bromate ions from the environment via the grafted 4VP groups. Due to increasing of the steric barrier, the diffusion of bromate ions to the surface of the PET fibers became difficult and therefore the adsorption capacity for bromate decreased at high grafting yield.

Similar findings to our results have been shown by the other researchers in the literature (Arslan and Günay, Gün Gök et al.2019). Arslan and Günay (2017) synthesized 2- methylpropenoic acid and acrylonitrile grafted PET fibers and they used this material to remove methylene blue from aqueous solution. They have found that with increase in grafting yield up to 156%, the amount of removed dye increased, and after this level, the amount of removed dye was fixed. Gün Gök et al. (2019) grafted HEMA groups onto PET fibers and they used the grafted fibers to eliminate Congo red. They have figured out that the removal capacity of the grafted PET fibers increased with grafting yield up to 110%. In addition, the researchers have showed that when the grafting yield was higher than 110%, the removing ability of fibers decreased due to steric hindrance.

For studying the effect of the initial bromate amount on the removal capacity of the modified fibers, the adsorption experiments were carried out at distinct initial bromate concentration (from 1 to 1000 mg/L) and the results were given in Fig.

6b. The removing capacity of the fibers increased with the increasing of the initial bromate concentration up to 800 mg/

L. When the amount of bromate concentration in the medium exceeds 800 mg/L, the removal capacity of the fibers was not

changed. This is probably due to the full filling of the active groups of the grafted 4VP monomers as in previous studies (Temoçin and Yiğitoğlu2010; Arslan2011).

The PET-g-4VP fibers synthesized in this study can be obtained easily and inexpensively and the surface load of the synthesized material changes according to the pH of the medium. As the surface charge changes, the ions in the aqueous medium interact electrostatically with PET-g-4VP and attach to the surface. In addition, the synthesized fibers retained their integrity in the aqueous solution and were easily removed after adsorption. The maximum removal capacity of the PET-g-4VP fiber was 183 mg/g when the initial bromate amount was 800 mg/L. Consequently, when we compared our results with the literature, we achieved a remarkable adsorption capacity for bromate ions by using the material obtained in this study. In a study conducted by Ji et al. (2017), a Mg-Al layered double hydroxide material was synthesized for bromate adsorption. The maximum bromate removal capacity of the material obtained in this study was determined to be 59.34 mg/g (at pH 7.5, 10 °C). Han and Xia (2018) synthesized metal-organic frameworks (MOFs) named HKUST-1 ([Cu3(BTC)2(H2O)3]n, BTC = benzene-1,3,5-tricarboxylate) and investigated the bromate elimination with this material.

They found that the maximum adsorption capacity of the material was 59.6 mg/g. Xu et al. (2018) produced a FPA90-Cl resin material was magnetized with supported Fe3O4particles using a chemical co-precipitation method and used this material for removal of bromate ions from aqueous solution. They have found that the maximum bromate separation capacity was 132.83 mg/g.

0 2 4 6 8 10

0 20 40 60 80 100

)g/gm(Q

Grafting yield (%)

0 40 80 120 160 200 240

0 200 400 600 800 1000

Q (mg/g)

Concentration (mg/L)

a b

Fig. 6 Adsorption of bromate ions with the synthesized fibers in different grafting yields of 4-VP (a) (pH, 3; contact time, 75 min;

bromate concentration 40 mg/L) and initial bromate amount (b) (pH, 3; contact time, 75 min;

adsorbent grafting yield, 80%)

Table 1 Rate constant of first and second reactions for adsorption of potassium bromate by PET-g-4VP fibers Qe(exp.) (mg/g) First-order rate constants Second-order rate constants

k1(min⁻¹) Qe(theor.) (mg/g) R² k2(g/(mg/min)) Qe(theor.) (mg/g) R²

10.5 0.051 6.19 0.9524 0.0183 11.10 0.9986

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Adsorption isotherms

The adsorption isotherms are significant in an adsorption treatment and they can fairly indicate the way contaminants interact with adsorption materials (Xu et al.2012). In addition, the isotherms may provide some information about surface prop- erties, affinity, and sorption mechanism of the adsorbents (Ding et al.2010). There are two important adsorption isotherm models which are the Langmuir and Freundlich isotherms (Arslan2011). The Langmuir isotherm model is based on homogeneous single-layer adsorption, and according to this model, all regions have an equal affinity to reagents. The Freundlich isotherm model is based on multi-layer adsorption with a non- homogeneous distribution (Foo and Hameed2010).

The isotherm models of Langmuir (5) and Freundlich (6) were adopted to designate the experimental results of the bromate adsorption on PET-g-4VP fibers.

Ce

Q_e¼ 1 Q₀bþCe

Q_e ð5Þ

logQ_e¼ logKf þ1

nCe ð6Þ

In these equations,Ceis the concentration of bromate at equilibrium (mg/L),Q0is the monolayer adsorption capacity of the fibers (mg/g),Qeis the amount adsorbed bromate ions (mg/g) at equilibrium,b is the Langmuir adsorption constant, andKFis the sorption capacity and n is an empirical parameter (Wang et al.2016).

The linearized Langmuir isotherm (a plot ofCe/Qeversus Ce) and Freundlich isotherm (a plot of logQeversus logCe) of bromate adsorption on PET-g-4VP fibers are shown in Fig.7a and b respectively. The parameters of the Langmuir and

Freundlich models were given in Table2. By comparing the correlation coefficients that have found from the plots, the adsorption of bromate by the fibers fit the Freundlich isotherm model. In this model, the correlation coefficient was found to be 0.9933 whereas it was found 0.9356 in Langmuir isotherm model. This result also represented that the heterogeneity of the grafted PET fibers.

Desorption of bromate ions from the modified fibers

Desorption ability of an adsorbent is economically important for using in industrial applications. The bromate ions were desorbed with NaOH from the bromate-loaded PET-g-4VP fibers. The results of desorption studies also confirm the adsorption phenomena of bromate onto the grafted fibers. When the fibers containing bromate ions were taken into NaOH solution, the positively charged surface of the fibers became neutral and therefore, the electrostatic interaction between the bromate ions and the grafted fibers became lost. Figure 8 shows the percentage of bromate desorbed over time from the fibers. The maximum desorption ratio for bromate adsorbed onto the surface of the PET fibers was found to be 40%. The grafted PET fiber desorbed the bromate ions with- out losing their activity and stability. These results show that the synthesized fibers can be used efficiently and economically for the elimination of anionic ions.

Conclusions

Because of the ozonation process, which is one of the drinking water disinfection methods, bromide ions which are found naturally in water are oxidized to bromate ions (Xu et al.

0.00 0.10 0.20 0.30 0.40 0.50 0.60

0.00 20.00 40.00 60.00 80.00 Ce/Qe

C_e(mg/L)

0.00 0.50 1.00 1.50 2.00 2.50

-0.50 0.50 1.50 2.50

Log Qe

Log C_e

a b

Fig. 7 Langmuir (a) and Freundlich (b) plots of the bromate adsorption on PET-g- 4VP fibers

Table 2 Langmuir and Freundlich constants for adsorption of bromate by PET-g- 4VP fibers

Ion Langmuir isotherm constants Freundlich isotherm constants

QO(mg/mg) b (L/mg) R² Kf(mg/g) n R²

Bromate 188.67 0.02 0.9356 4.31 1.21 0.9933

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2012). Many health organizations, such as the World Health Organization (WHO) and the US Environmental Protection Agency (EPA), agreed that the maximum value of bromate in the drinking water should be 10μg/L. In Turkey, according to“Regulation on Water Intended for Human Consumption (published in 2005),” the maximum amount of bromate should be 3μg/L and 10 μg/L in drinking water. The adsorption process for bromate ion removal from water is a more economical and effective method because other methods re- quire expensive and special structures. In this study, the adsorbent prepared by grafting 4VP monomers onto PET fibers was used to eliminate the bromate ions from the aqueous environment in acidic environment. Various conditions such as pH, incubation time, grafting yield, and initial ion concentration were investigated for the adsorption of the bromate ions. The fibers removed maximum amount of bromate at pH 3. The adsorption of the bromate ions was fast and the adsorption reached equilibrium in 75 min. The kinetics of the bromate adsorption onto the PET-g-4VP fibers monitored pseudo-second-order model and a Freundlich type of adsorption was appeared. The amount of bromate removed by PET fibers increased with grafting yield and when the grafting yield of 4VP was higher than 80%, it started to decrease due to steric and diffusion hindrance. The removing capacity of the PET-g-4VP fibers increased up to 800 mg/L bromate concentration and the adsorbed bromate ions were desorbed from the grafted fibers with 40% efficiency by using 1 M NaOH solution. We can conclude that the constructed adsorbent is very successful in the removal of bromate ions in batch experiments and the removal of bromate from drinking water with the constructed copolymers can be practicable.

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