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Adsorption of selenite ions onto poly(1,8-diaminonaphthalene) synthesized by using ammoniumpersulfateSeda Fındıka, Mustafa Gülfena & Ali Osman Aydına

a Department of Chemistry, Faculty of Arts & Sciences, Sakarya University, TR54187-SakaryaTURKEYAccepted author version posted online: 26 Aug 2014.

To cite this article: Seda Fındık, Mustafa Gülfen & Ali Osman Aydın (2014): Adsorption of selenite ions ontopoly(1,8-diaminonaphthalene) synthesized by using ammonium persulfate, Separation Science and Technology, DOI:10.1080/01496395.2014.946144

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Adsorption of selenite ions onto poly(1,8-diaminonaphthalene) synthesized by using ammonium persulfate Seda Fındık, Mustafa Gülfen* and Ali Osman Aydın

Department of Chemistry, Faculty of Arts & Sciences, Sakarya University, TR54187-Sakarya TURKEY

*Corresponding author: Dr. Mustafa Gülfen

Department of Chemistry

Faculty of Arts & Sciences

Sakarya University

TR-54187, Sakarya, TURKEY

mgulfen@sakarya.edu.tr

Tel: +90 264 2956051

Fax: +90 264 2955950

ABSTRACT

In the present work, poly(1,8-diaminonaphthalene) (poly(1,8-DAN)) was synthesized by

chemical oxidation of 1,8-diaminonaphthalene (1,8-DAN) monomer with ammonium persulfate

(APS) oxidant and it was used in the adsorption of selenite ions. The polymer samples

synthesized at various molar ratios of 1,8-DAN to APS were characterized by FT-IR analysis

and they were used in the adsorption experiments. The effects of initial acidity, the molar ratio

and initial selenium concentration on the adsorption behavior of poly(1,8-DAN) were examined

by using batch adsorption technique. It was found that selenite was adsorbed onto poly(1,8-

DAN) in strong acidic conditions (3-8 M HCl). The experimental adsorption data were applied to

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the Langmuir and Freundlich isotherms. The adsorption capacities of the polymer samples at the

molar ratios of 1/0.5 and 1/2 were calculated as 75.19 and 45.05 mg Se/g, respectively. It was

estimated that the adsorption mechanisms of selenite ions onto poly(1,8-DAN) were governed by

electrostatic interaction (>NH2+SeOCl3

-) and piazoselenol type binding (-N=Se=N-).

Keywords: Selenium; Selenite; Poly(1,8-diaminonaphthalene); Adsorption

INTRODUCTION

Selenium (Se) is an important trace element or micronutrient for human and animal health, but it

is a toxic element above daily recommendation [1-3]. Selenium separation or recovery from

industrial intermediate solutions, wastewater, foods, leaching solutions and drinking water is

important for later usages. Several techniques may be used to reduce the level of selenium from

aqueous media: Anion exchange, reverse osmosis, distillation, and adsorption onto activated

carbon, alumina/oxides and chelating polymer [2,4]. Amberlite IRA-67 and duolite A7 with

primary and secondary amines [5], eporasu K-6 with polyamine [6], activated carbon [7,8],

aluminium/iron oxides [9,10], silica gel modified with 3-mercaptopropyl [11], ion exchange

resin with bismuthiol-II and 2,3-diaminonaphthalene [12], amberlite-XAD-4 resin with 2,3-

diaminonapthalene [13], poly(o-phylenediamine) [14] and chitosan resin with 3,4-diamino

benzoic acid [15] are examples of selenium adsorbents studied by different researchers.

Selenium and aromatic diamines such as o-phenylenediamine, 2,3-diaminonaphthalene and 1,8-

diaminaphthalene can form piazoselenol compounds in solution. Aromatic diamines are used in

the solvent extraction of selenium [16]. Therefore, the polymers synthesized from aromatic

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diamines may have selenium adsorption capability. Poly(1,8-DAN) is an aromatic diamine

polymer and it can be synthesized by the methods of chemical (FeCl3, (NH4)2S2O8, H2O2),

electrochemical, enzyme and photo oxidation [17-20]. This polymer is a conductive polymer and

it has got the functionality of primary and secondary aromatic diamines. It can adsorb metal ions

by binding to the N donor atoms of the amine groups [16,17]. It may also absorb selenium via

piazoselenol type binding (-N=Se=N-) or electrostatic interaction between protonated amines

and selenium anions (>NH2+SeOCl3-). In addition, poly(1,8-DAN) can be synthesized easily in

short time by chemical oxidation method at room temperature. This is an advantage according to

many chelating polymer. Li et al. [17] stated that the synthesis of poly(1,8-DAN) by ammonium

persulfate resulted in more primary amine group in the structure of the polymer.

Poly(1,8-diaminanaphthalene) polymer (poly(1,8-DAN)) functionalized as aromatic diamine has

not been studied in the adsorption of selenium species. In the present work, poly(1,8-

diaminanaphthalene) polymer was synthesized by the chemical oxidation polymerization of 1,8-

diaminonaftalen with ammonium persulfate. The adsorption of selenite ions on poly(1,8-DAN)

was examined. The effects of the molar ratio of 1,8-DAN to the persulfate, acidity and initial

selenium concentration on the adsorption process were studied by batch adsorption technique.

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EXPERIMENTAL PROCEDURE

Materials

In the experimental studies, 1,8-diaminonaphthalene (1,8-DAN) (Merck), ammonium persulfate

(Sigma-Aldrich), sodium selenite (Sigma-Aldrich) and acetonitrile (Merck) were in analytical

grade and they were used without further treatment. Selenite stock solutions were prepared by

dissolving Na2SeO3 in ultra pure water. The synthesized polymer samples were characterized by

Perkin Elmer Spectrum Two model FT-IR spectrometer with equipped ATR. FTIR

measurements were performed with a resolution of 2 cm-1 in the range of 500-4000 cm-1.

Selenium concentrations in aqueous phases before and after the adsorption studies were

measured with Spectro Arcos model inductively coupled plasma- optic emission spectrometer

(ICP-OES) at the wavelength of 296.09 nm and argon gas flowing of 13.5 L/min. The calibration

range in the ICP-OES measurements was 1.42-24000 µg/L. The all solutions used in the

experimental studies were prepared with ultra pure water from Millipore Milli-Q system (18.2

MΩ.cm resistivity at 25 °C ultra pure water).

Poly(1,8-DAN) synthesis

Poly(1,8-DAN) can be synthesized by different oxidation methods. In this study, poly(1,8-DAN)

was synthesized by chemical oxidation method using (NH4)2S2O8 at different initial molar ratios.

In the synthesis of poly(1,8-DAN), 3.955 g (25 mmol) of 1,8-DAN was dissolved in 50 mL of

acetonitrile (CH3CN) in a 200 mL-glass beaker at room temperature. To provide 1/0.25; 1/0.5;

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1/1; 1/2 and 1/4 molar ratios of 1,8-DAN to the persulfate, 1.415 g (6.25 mmol), 2.85 g (12.50

mmol), 5,70 g (25.00 mmol), 11.4 g (50.00 mmol) and 22.80 g (100.00 mmol) ammonium

persulfate were dissolved in 50 mL of ultra pure water. The prepared five solutions having 25

mmol 1,8-DAN were stirred by adding dropwise (one drop every 3 s) to each one of the

persulfate solutions having the different molar quantities. The obtained polymer precipitates

were filtered and washed thoroughly with ultra pure water and were left to dry at the temperature

of 40 °C for 2 days [17-20]. The percent ratios of poly(1,8-DAN) to 1,8-DAN were calculated

according to the initial monomer/oxidant molar ratios of 1/0.25; 1/0.5; 1/1; 1/2 and 1/4. The

polymer samples were characterized by FT-IR analysis and used in the adsorption experiments.

Selenium adsorption studies

The poly(1,8-DAN) samples synthesized at the different molar ratios were used in the adsorption

of selenium and the effects of acidity, monomer/oxidant ratio and initial selenium concentration

were examined.

Effects of acidity and monomer/oxidant molar ratio

To examine the molar ratio of 1,8-DAN to ammonium persulfate in the experimental studies, the

polymer samples synthesized at the molar ratios of 1/0.25; 1/0.50; 1/1; 1/2 and 1/4 were stirred in

100 mL of 50 mg Se/L sodium selenite solution for 48 hours at the room temperature. A 0.1-g

amount of each polymer sample was used in the adsorption experiments. While sodium selenite

solutions were being prepared, at the same time the acidities of the selenite solutions were

adjusted to 10-5; 0.1; 1; 3; 4; 6 and 8 M HCl concentrations. The final selenium concentrations in

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the aqueous phase were determined by ICP-OES spectrometer. The selenium quantities adsorbed

on the surface of the polymer were calculated according to Eq.1;

qe= Vm

CC e .)( 0 − (1)

where qe is the amount of selenium adsorbed on the surface of the polymer (mg/g), C0 and Ce are

the initial and equilibrium concentrations (mg Se/L) of selenium, respectively, V is the volume

(L), and m is the mass (g) of the polymer.

Effect of initial concentration

Initial concentration is one of the effective factors on adsorption efficiency. The adsorption

studies were carried out by shaking 0.1 g of the polymer samples at the molar ratios of 1/0.5 and

1/2 as the optimum ratios, in a 100-mL volume of each selenium solution at 10, 25, 50, 75, 100,

150 and 200 mg Se/L concentrations, containing 6 M HCl as the optimum acid concentration.

Then, the initial and equilibrium selenium concentrations in the aqueous phases were measured

by ICP-OES. The selenium amounts adsorbed on the polymer were calculated using Eq-1.

Adsorption isotherms

The experimental data obtained from the adsorption studies with the different initial selenium

concentrations were applied to the Langmuir (Eq.-2) and the Freundlich (Eq.-4) isotherm

equations. Their linearized equations are given as follows:

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maxmax

1QC

bQqC e

e

e += (2)

L

eL

Le

e

KCa

KqC

+=1 (3)

log qe= log KF + n1

log Ce (4)

where qe is the amount of selenium adsorbed on the surface of the polymer (mg/g), Ce is the

equilibrium selenium concentration in the solution (mg/L), Qmax is the maximum capacity at

monolayer coverage (mg/g), b is the Langmuir adsorption constant (L/mg), KL is the Langmuir

constant (L/g), aL is the Langmuir constant related to the adsorption energy (L/mg), KF is the

Freundlich constant (mg/g) which indicates the adsorption capacity and represents the strength of

the adsorptive bond and 1/n is the heterogeneity factor which represents the bond distribution

[21-28].

RESULTS AND DISCUSSION

Poly(1,8-DAN) synthesis

Many researchers have synthesized poly(1,8-DAN) by chemical or electrochemical oxidation

[17-19]. In the present work, poly(1,8-DAN) was synthesized by chemical oxidation method

with ammonium persulfate, examining the different molar ratios of 1,8-DAN (monomer) to

ammonium persulfate (oxidant). The synthesis with the persulfate results in more primary amine

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in the polymer [17]. The molar ratio of 1,8-DAN to the persulfate is also an important factor in

the synthesis of poly(1,8-DAN). The polymer samples synthesized at the different molar ratios

will have the different selenium adsorption cavity or capability. Therefore, the effect of the molar

ratio in the synthesis both to the polymer yield and the adsorption of selenite was examined. The

polymer yield efficiencies obtained for the molar ratios of 1,8-DAN to the persulfate of 1/0.25;

1/0.5; 1/1; 1/2 and 1/4 were given in Table-1. It was found that high efficiencies were achieved

at the molar ratios of 1/2 and 1/4. In other words, the lower persulfate molar amounts then 1,8-

DAN were not enough to synthesize the poly(1,8-DAN). The efficiencies at the molar ratios of

1/2 and 1/4 show that poly(1,8-DAN) sulfate salt as the polymer yield formed at the same time.

It was found that the molar ratio of 1,8-DAN to the persulfate was effective in the amounts of the

obtained polymer. Moreover the possible oxidation reactions of 1,8-DAN with ammonium

persulfate were given in Fig-1 [17-20, 29].

FT-IR spectra

The FT-IR spectra of 1,8-DAN, poly(1,8-DAN) synthesized at the molar ratios of 1/0.25; 1/0.5;

1/1; 1/2 and 1/4, and selenium adsorbed poly(1,8-DAN) are given in Fig-2. FT-IR spectra

analysis led us to some important conclusions. N-H peaks for primary amine at 3356-3447 cm-1

changed to secondary amine and protonated imino peaks by increasing persulfate quantity. At the

same time, C-H peaks for aromatic substitution at 768, 825, 1026 and 1075 cm-1 also changed to

different frequency and low intensity. By increasing persulfate quantity, new peaks at 581, 577

and 1041 and 1047 cm-1 were assigned for HSO4- group because of poly(1,8-DAN) sulfate salt.

In addition, the shifting of the C-N peak at 1605 cm-1 to the peak at 1574 cm-1 in the FT-IR

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spectrum of 1/2 polymer after the selenium adsorption shows that that selenium was bound

covalently to N atoms as piazoselenol type. It was also seen that the peaks at 581 and 1047 cm-1

were disappeared since selenium exchanged with sulfate ions during the adsorption

[17,19,29,30].

Selenium adsorption studies

Effects of acidity and monomer/oxidant molar ratio

In the selenium adsorption studies, the effects of acidity and the molar ratio of 1,8-DAN

(monomer) to persulfate (oxidant) were examined. The polymer samples synthesized at the

monomer/oxidant molar ratios of 1/0.25; 1/0.5; 1/1; 1/2 and 1/4 were stirred in 100 mL 50 mg

Se/L sodium selenite solutions containing 10-5; 0.1; 1; 3; 4; 6 and 8 M HCl concentrations. The

equilibrium adsorption capacities of the polymer samples, qe (mg Se/g) were calculated using

Eq.1 and the obtained results were given in Fig-3. It was found that the polymer samples

adsorbed high selenium quantities at the molar ratios of 1/0.25 and 1/0.50. It adsorbed

moderately at 1/2 and 1/4 and but the lowest adsorption was observed at 1/1. It was seen that the

molar ratio of the monomer to the oxidant was effective in the selenium adsorption. Considering

the efficiency of the polymer in the synthesis and selenium adsorption capacities, the later

adsorption studies were performed with the polymer samples at the molar ratios of 1/0.5 and 1/2.

On the other hand, it was seen that HCl concentration was also effective in the selenium

adsorption. Selenite (SeO32-) ions in the solutions at different HCl concentrations may be found

in the forms of SeO2Cl-, SeOCl3-, SeCl5

- ve SeCl62- anionic species according to pH and chloride

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concentration of the solution. Selenium forms the chloro complexes (SeCl5- and SeCl6

2-) at high

HCl concentrations. However, it forms selenite or selenium oxychloride complexes in the

solutions at low HCl or chloride concentrations [31]. Poly(1,8-DAN), a secondary amine

polymer, with >NH2+ group at high HCl concentrations can adsorb anionic selenium species such

as SeCl5- and SeCl62-. So, the selenium adsorption can be governed by electrostatic interaction

mechanism as >NH2+SeCl5

- and -NH3+SeCl62-. In case of selenate (SeO4

2-), similar results can be

obtained, but the optimum acidity and adsorption capacity may be slightly different because of

the different equilibrium constants of selenite and selenate.

In addition, selenium can form piazoselenol compounds with 1,8-DAN or similar aromatic

diamines (Fig-4). The fact that the C-N vibration peak at 1605 cm-1 shifted to 1574 cm-1 in the

FT-IR spectrum of selenium adsorbed polymer (Fig-2) confirms that selenium was bound

covalently to the amine group via piazoselenol type binding (-N=Se=N-), (Fig-4). A metal ion

adsorption mechanism may be governed by coordinative covalent binding, but this adsorption is

a covalent binding between selenium and N atoms [32].

According to the experimental results examining the effects of HCl acidity and the molar ratio on

the selenium adsorption, it was determined that the optimum HCl concentration was about 6 M

and the molar ratios were 1/0.5 and 1/2. The results show that poly(1,8-DAN) can be used to

separate the selenium in the leaching solutions including high HCl concentrations (3-8 M).

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Effect of initial selenium concentration

The adsorption studies examining the effect of initial selenium concentration were carried out by

mixing a 0.1-g amount of each polymer sample at the molar ratios of 1/0.5 and 1/2, in a 100-mL

volume of each selenium solution at 10, 25, 50, 75, 100, 150 and 200 mg Se/L including 6 M

HCl concentration. After the mixtures were stirred for 48 hours, the equilibrium selenium

concentrations in aqueous phase were measured experimentally by ICP-OES. The adsorption

isotherms were drawn as qe vs Ce and the results were given in Fig-5. It was obtained that the

polymer sample synthesized at the molar ratio of 1/2 had more saturated adsorption capacity then

those prepared at 1/0.5.

Adsorption isotherms

The data obtained from the selenium adsorption experiments with different initial selenium

concentrations were applied to the Langmuir (Eq.-2) and Freundlich (Eq.-4) isotherms [21, 26-

28, 33]. The Langmuir isotherm model depends on the assumption that intermolecular forces

decrease rapidly with distance and predicts the existence of monolayer coverage of the adsorbate

on the surface of the adsorbent. Moreover, the Langmuir equation is based on the assumption of

a structurally homogeneous adsorbent where all sorption sites are identical and energetically

equivalent. Theoretically, the sorbent has a finite adsorption capacity. On the other hand, the

Freundlich isotherm model is an empirical equation employed to describe heterogeneous

systems, in which it is characterized by the heterogeneity factor, 1/n. It is not restricted to the

formation of the monolayer adsorption [21, 33].

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The plots of the Langmuir and Freundlich isotherms are given in Figs-6 and 7, respectively. The

Langmuir and Freundlich constants are given separately in Table-2. The selenium adsorption

data with the sample at the molar ratio of 1/0.5 fitted more to the Freundlich isotherm (R2:

0.9672) and the selenium adsorption with 1/2, fitted more to the Langmuir isotherm (R2: 0.9844),

based on the regression coefficient (R2) values. The Langmuir isotherm describes the

homogeneity of any adsorbent and the Freundlich does the heterogeneity. Therefore, it can be

concluded that the polymer sample at the molar ratio of 1/0.5 has more heterogeneous sites as

primary and secondary amines. The polymer sample at 1/0.5 had higher selenium adsorption

capacity than the polymer at 1/2. At the same time, 1/n values obtained from the Freundlich

equation confirm the heterogeneity of the polymer surface at the molar ratio of 1/0.5. In addition,

the selenium adsorption capacities of the polymers at the molar ratios of 1/0.5 and 1/2 were

calculated as 75.19 and 45.04 mg Se/g, respectively.

One of the essential characteristics of the Langmuir isotherm model could be expressed by

dimensionless constant called equilibrium parameter, RL. The value of RL indicates the shape of

the isotherms to be either unfavorable (RL>1), linear (RL =1), favorable (0 < RL < 1) or

irreversible (RL = 0). RL could be calculated using following equation;

011

CKR

LL +=

(5)

where C0 is the initial concentration (mg/L) and KL is the Langmuir constant related to the

energy of adsorption (L/mg) [34]. The KL values were calculated by using the linear graphs of

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Eq. 3. Then, the RL values by using Eq. 5 were found in the range of 0.0360 – 0.6357 (0<RL<1).

The RL results show that the selenium adsorption is a favorable process.

A literature search about the adsorbents for selenium was given in Table 3. Many adsorbents

have been studied in the adsorption of selenium. Among these adsorbents, metal oxide type

adsorbents have got low adsorption capacities. The hydroxides are valuable adsorption values.

However, the oxides and hydroxides are inconvenient adsorbents for high acidic conditions. As

the other adsorbents, the biometarials may be also inconvenient for high acidic media. Poly(1,8-

DAN) polymer is a possible adsorbent for the adsorption of selenium in strong acidic condition

(3-8 M HCl) with valuable adsorption capacity. This polymer may have an advantage in the

adsorption of selenium in high acidic solution as leach solutions.

CONCLUSIONS

Poly(1,8-diaminonaphthalene) was synthesized by the chemical oxidation of 1,8-

diaminonaphthalene monomer with ammonium persulfate oxidant and it was used in the

adsorption of selenium. The experimental attempts led to the following conclusions: The molar

ratio of 1,8-DAN to the persulfate in the synthesis was effective in the polymer yield and also in

the adsorption of selenium. The poly(1,8-DAN) showed higher affinity to selenium at 3-8 M HCl

concentrations. Therefore, poly(1,8-DAN) can be used as a potential adsorbent to separate

selenium from the leaching solutions at high acid concentrations. The adsorption capacities of

the polymer samples at the molar ratios of 1/0.5 and 1/2 were found as 75.19 and 45.05 mg Se/g,

respectively. The Langmuir and Freundlich isotherms showed that the synthesis at the different

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molar ratios resulted in the different heterogeneity on the polymer surface. It was estimated that

the adsorption mechanism of selenite ions onto poly(1,8-DAN) was governed by electrostatic

interaction (>NH2+SeOCl3

-) and piazoselenol type binding (-N=Se=N-).

ACKNOWLEDGMENTS

The authors acknowledge the research grant provided by Sakarya University Scientific Research

Commission (Project No: BAP 2010-02-04-011).

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Fig-1. Poly(1,8-DAN) synthesis reactions [17,29].

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Fig-2. FT-IR spectra of 1,8-DAN, poly(1,8-DAN) at the different molar ratios and the selenium adsorbed polymer.

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Fig-3. Effects of acidity and molar ratio on the adsorption (0.1 g polymer; 100 mL 50 mg/L selenite solution; 48 hours; room temperature; Error bars:RSD%).

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Fig-4. Piazoselenol compounds [32].

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Fig-5. Adsorption isotherms at the molar ratios of 1/0.5 ve 1/2. (0.1 g polymer; 100 mL selenite solution; 48 hours; room temperature; Error bars:RSD%).

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Fig-6. The Langmuir isotherms for the molar ratios of 1/0.5 and 1/2.

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Fig-7. The Freundlich isotherms for the molar ratios of 1/0.5 and 1/2.

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TABLE 1 Efficiencies of poly(1,8-DAN) synthesis

1,8-DAN

g,(mmol)

(NH4)2S2O8 ,

g (mmol)

1,8-

DAN/persulfate

molar ratio

Poly(1,8-

DAN) g

Poly(1,8-DAN) /

1,8-DAN

%

3.955 (25) 1.425 (6.25) 1/0.25 1.02 25.80

3.955 (25) 2.85 (12.50) 1/0.50 2.50 63.21

3.955 (25) 5.70 (25.00) 1/1 1.90 48.04

3.955 (25) 11.4 (50.00) 1/2 5.90 149.2

3.955 (25) 22.8 (100.00) 1/4 7.10 180.0

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TABLE 2 The Langmuir and Freundlich isotherm parameters

Molar ratio

Langmuir isotherm Freundlich isotherm

Qmax

(mg/g) b (L/mg) R2 KF (mg/g) n R2

1/0.5 75.19

0.07074 0.9219 13.025 2.79 0.9672

1/2 45.05 0.2280 0.9844 12.59 3.21 0.9369

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TABLE 3 Some selenium adsorbents

Adsorbent Sorption capacity

(mg/g) pH/CHCl Ref.

Chelating resin

(Diethylenetriamine-polyacetic acid)

30.02 4.0 [35]

Green algae

(Cladophora hutchinsiae)

74.9 5.0 [36]

Iron and Aluminium oxides 2.4 -32.7 5.0 [37]

Rice husk (wet-dry sorbent) 40.9-32.1 1.5 [38]

Double hydroxides

(Zn/Mg–Al)

125.0-152.0 5–10 [39]

Aluminum oxide coated sand 1.08 4.8 [40]

Iron oxide coated sand 1.34 4.5–6 [41]

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Ion-exchange resin

(Polyamine-type weakly basic)

134.2 3–12 [42]

Ganoderma lucidum 126.99 5 [43]

Thiourea-formaldehyde resin 833.3-526.3 3-5 M HCl [2]

Poly(1,8-diaminonapthalene) 75.19 3-8 M HCl This study

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