Spectroscopic Characterization of Azo Dyes Aggregation Induced by

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Spectroscopic Characterization of Azo Dyes Aggregation Induced by DABCO-Based Ionene Polymers and Dye Removal Efficiency as a Function of Ionene Structure Ecaterina Stela Dragan, Judith Mayr, Marleen Häring, Ana Irina Cocarta, and David Diaz Diaz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09853 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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ACS Applied Materials & Interfaces

Spectroscopic Characterization of Azo Dyes Aggregation Induced by DABCO-Based Ionene Polymers and Dye Removal Efficiency as a Function of Ionene Structure Ecaterina Stela Dragan,a* Judith Mayr,b Marleen Häring,b Ana Irina Cocarta,a David Díaz Díazb,c* a

“Petru Poni” Institute of Macromolecular Chemistry, Grigore Ghica Voda Alley 41 A, Iasi

700487, Romania b

Institut für Organische Chemie, Universität Regensburg, Universitätsstr. 31, 93053 Regensburg,

Germany c

IQAC-CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain

CORRESPONDING AUTHORS FOOTNOTE. Ecaterina Stela Dragan; Telephone number: +40.232217454; Fax number: +40.232211299, e-mail address: [email protected]; David Díaz Díaz; Telephone number: + (0) 941 943-4373; e-mail address: [email protected]

ACS Paragon Plus Environment

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KEYWORDS. ionene; metachromasy; methyl orange; ponceau SS; direct blue 1; sorption.

ABSTRACT. The aggregation mode of three azo dyes, methyl orange (MO), ponceau SS (PSS) and direct blue 1 (DB1) induced by three 1,4-diazabicyclo[2.2.2]octane (DABCO)-based ionene polymers having different topologies (i.e. 1,2-ionene, 1,3-ionene and 1,4-ionene) was investigated in this work. Metachromatic behavior of the dyes in the presence of ionenes, and the stability of the ionene/dye complex were discussed as a function of ionene structure. It was demonstrated that the association of the dye molecules with the ionenes and the metachromasy were strongly influenced by both the dye structure and the ionene topology. Thus, MO, having one -SO3Na group per molecule, was almost stoichiometrically bound to all ionenes regardless their topology, showing also a metachromatic effect. In sharp contrast, the interaction of PSS and DB1 molecules with ionenes was strongly dependent on the polymer topology. It was found that PSS having two -SO3Na groups per molecule was preferentially bound onto both 1,2-ionene and 1,3-ionene, but DB1, having four -SO3Na groups per molecule and a more complex structure, was efficiently bound only onto 1,2-ionene. The dye removal efficiency with each ionene was evaluated in batch mode taking into account the affinity of ionenes for azo dyes. The experimental isotherms of the dye sorption were fitted with four isotherm models, i.e. Langmuir, Freundlich, Sips and Dubinin-Radushkevich. It was found that the best fitting of the experimental data was given by the Langmuir, Sips and Dubinin-Radushkevich isotherm models. The maximum equilibrium sorption capacity, qm, evaluated by the Langmuir model, at 35 oC, was as follows: 985.71 mg MO/g 1,3-ionene, 483.71 mg PSS/g 1,3-ionene, 1010.49 mg PSS/g 1,2-ionene, and 976.7 mg DB1/g 1,2-ionene. Kinetic study of the dye removal indicated chemisorption as the main mechanism of sorption.


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INTRODUCTION Spectral changes of dyes in the presence of ionic polymers have been exploited as a valuable source of information on the structural parameters of polyion, including charge distance and chain conformation, among other properties.1-5 Within this context, cationic dyes of phenothiazine type such as Azure B, methylene blue, and toluidine blue O have been used to get information about the chain regularity, interfunctional distance and copolymer composition.1-3 Moreover, spectral changes of anionic dyes such as methyl orange (MO) in the presence of different polycations have been deeply investigated.4,5 The color changes induced by the aggregation of the dyes caused by polyions have been attributed to the metachromasy phenomenon, characterized by new peaks (i.e. metachromic bands) arising in the UV-Vis spectra of the dyes, similar to the peaks found when the dye molecules aggregate due to the increase of dye concentration.6 In the absorption spectrum of a metachromatically adsorbed dye, the bands characteristic to the dye monomer and dimer are almost annihilated, and a new band called metachromatic or µ–band, at a longer wavelength is visible, which is usually diffuse as in the case of toluidine blue O (540 nm),6 and Azure B (570 nm),2 or even sharp as in the case of MO in the presence of polycations. The interaction between ionic polymers and dyes could be the result of three types of interactions: (i) electrostatic interactions between the ionic dyes and the oppositely charged polyions, (ii) non-specific interactions, hydrophobic interactions between dye molecules and nonpolar parts of the ionic polymer being the most representative, and (iii) interactions between π-electrons on the adjacent adsorbed dye molecules. The spectral changes and the metachromatic behavior of the adsorbed dye molecules are mainly induced by the last type of interactions.1 In dilute aqueous solution, polyion chains are extended because of the repulsion between the adjacent groups similarly charged. In the presence of oppositely charged


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dyes, the effective charge of the chain diminishes and, therefore, the polymer chains coil up and bring closer the adsorbed dye molecules. It is known that synthetic dyes are extremely toxic and must be removed from waste waters where their presence is visible even at very low concentrations, being very dangerous for the photosynthetic activity in aquatic life.7,8 Among them, azo dyes are recognized to be very dangerous because, under reduction conditions, they generate aromatic amines suspected to be carcinogenic.8 Inorganic sorbents or organic-inorganic composites have recently been reported as very efficient for the removal of dyes.9-11 The influence of tautomerization, and/or dye molecule aggregations on the photocatalytic degradation of polyazo dyes (Direct Red 80) in aqueous solutions containing TiO2 as photocatalyst has been investigated.12 In addition, polysaccharides, such as chitosan,7,13-15 and modified starch16,17 have intensively been used for dye removal. The main advantages of polymers derived from renewable resources are their biocompatibility and biodegradability. However, sometimes the biodegradability of natural polymers could be a drawback because it can reduce their storage life. On the other hand, synthetic polymers, known for their high biological and chemical stability, can be designed with a controlled and reproducible chemical structure and molar mass. Water soluble polymers demonstrated high efficiency in the removal of dyes, alone or in specialized systems.18-23 Among the synthetic polymers used for dye removal, those bearing ionic groups, either soluble or cross-linked, are of large interest, since their charge density can be easily controlled by the synthesis strategy.24-27 Within this context, ionenes are synthetic polycations bearing a regular distribution of quaternary ammonium groups along the backbone. Importantly, a large variety of structures is achievable through a judicious choice of reactants.28-30


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The aim of the present work was twofold. The first objective was to report on the spectral changes of three important azo dyes, i.e. MO, Ponceau SS (PSS), and Direct blue 1 (DB1), having one, two and four -SO3Na groups, respectively, in the presence of three 1,4diazabicyclo[2.2.2]octane (DABCO)-based ionene polymers recently reported by our group,31 as a function of the dye and ionene structure. The second goal of the investigation was to evaluate the dye binding efficiency of the ionene polymers and the sorption of dyes at equilibrium. The information obtained from these studies was expected to support the possible applications of the ionenes in the removal of these dyes from aqueous solutions. Usually, ionic polymers are used either as aqueous solutions17,19,23,25 or as cross-linked systems.8,13,15-17 To the best of our knowledge, the spectral changes of azo dyes, which could arise in the presence of ionene polymers as aqueous dispersions, and the removal of dyes by DABCO-based ionenes have not yet been reported.

MATERIALS AND METHODS Ionenes. DABCO-containing 1,2-, 1,3- and 1,4-ionene polymers were synthesized as previously reported.31 In order to achieve adequate solubility and mobility of the polymers for GPC measurements, counteranion exchange of chloride by bis(trifluoromethanesulfonyl)amide (TFSA) anions was carried out as reported.31 These polymers are characterized by low degree of polymerizations and dispersity values (Đ = Mw/Mn) ranging from 2.1 to 2.9: 1,2-ionene·TFSA: Mw = 8.1 × 103 Da; Mn = 3.9 × 103 Da; ĐM = 2.1; n = 7; 1,3-ionene·TFSA: Mw = 1.2 × 104 Da; Mn = 5.0 × 103 Da; ĐM = 2.4; n = 7; 1,4-ionene·TFSA: Mw = 1.7 × 104 Da; Mn = 5.9 × 103 Da; ĐM = 2.9; n =10.


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Azo Dyes. MO was used after three purifications by recrystallization from a 8 wt % solution in a water:methanol (1:1 v/v) mixture to remove inorganic salts. PSS was used after purification by recrystallization from an aqueous methanol solution (methanol/water 70/30, v/v). The purification of DB1 was performed twice as previously shown.44 The main characteristics of the azo dyes are presented in Table 1. Table 1. Main Characteristics of Azo Dyes. Azo dye

Color index name

Molar mass, g/mol

λmax, nm

Methyl orange (MO)

Acid Orange 52

327.33

462

Ponceau SS (PSS)

Acid Red 150

556.48

514

Chicago Sky Blue 6B Direct Blue 1 (DB1)

992.82

620

Methods and Apparatus Investigation of the Interaction between Dyes and Ionenes. Dispersions of each ionene were prepared as follows. The desired polymer (0.2 g) was first milled and sieved. Only the fraction with 32 µm < φ < 50 µm was selected for further studies. Subsequently, the ionene (0.1 g) was mixed with distilled water (100 mL). A homogeneous dispersion was obtained using a Sonics (SUA) 750 W Model (Ultrasonic Processor) with a power of 4700 W (100000 Joules/min). To prevent the heating of the dispersion, the glass was kept in an ice bath during the process. The dispersion was then kept under magnetic stirring for 8 h. All dye solutions were prepared using pure dyes and Milli pore water at a concentration of 2 × 10–4 M. Increasing volumes of ionene dispersion were dropped under magnetic stirring on the dye solution (5 mL) and the final volume was adjusted to 10 mL with pure water. The stirring was maintained for 30 min and the mixture was kept in the dark overnight. Each mixture (2 × 2 mL) was centrifuged for


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30 min at 10000 rpm. After this time, dye concentration in supernatant was measured using a SPECORD 200Plus Spectrophotometer (AnalytikJena) according to a premade calibration curve. UV-Vis spectra were registered to show the type of interaction between dye molecules and ionenes and quantify the excess of dye molecules. FT-IR spectra were recorded with a Bruker Vertex FT-IR spectrometer as previously presented.37 Sorption of Dyes onto Ionenes. Dye retention onto ionene microparticles was estimated by a batch method, the dye removal efficiency as a function of the amount of ionene being investigated first at 25 oC. Increasing amounts of ionene as powder were placed into a flask and then the desired dye solution (10 mL, c = 2 × 10–4 M) was added under vigorous magnetic stirring (i.e. 45 min at 750 rpm followed by15 min at 180 rpm). After 3 h of decantation, 2 × 2 mL were withdrawn from the supernatant and centrifuged at 10000 rpm for 30 min, the dye concentration being measured by UV-Vis spectroscopy at the characteristic wavelength of each dye using the calibration curves previously generated. The amount of the dye bound onto the ionene powder at equilibrium, qe, was calculated using Eq. 1 and expressed in mg dye/g ionene.

qe =

(C0 − Ce )V W

(1)

where: Co and Ce represent the dye concentration (mg/L) before and after the interaction with ionene, respectively; V - the volume of aqueous phase (L); W - the mass of ionene powder (g). Color removal efficiency, CRE (%), was evaluated using Eq. (2): CRE (%) = (1 - Ac/Ai) x 100

(2)

where Ai and Ac are the absorbance of the dyes before and after the adsorption process, respectively.


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To generate the experimental isotherms of adsorption, the initial concentration of the dye ranged between 5 × 10–6 – 10–3 M. Thus, 2 mg of ionene as dispersion with a concentration of 1 mg/mL were added under stirring to 10 mL aqueous solution of the dye. All isotherms were generated at 35 ºC, at pH 6, under magnetic stirring for 6 h. Isotherm Models. To design the sorption of ionic species for large-scale applications, various isotherm models are fitted on the experimental data.32,33 Four isotherm models, namely Langmuir, Freundlich, Sips, and Dubinin-Radushkevich isotherms, were employed in this work to depict the correlation between the dye amount adsorbed onto ionene particles and the dye in solution, at equilibrium. Langmuir isotherm is applicable to homogeneous adsorption, its nonlinear form being described by Eq. 3:33,34 q K C q = m L e e 1+ K C L e

(3)

where: qe represents the dye adsorbed at equilibrium onto ionene particles (mg/g), Ce is the equilibrium concentration of dye in solution (mg/L), qm is the theoretical limit of adsorption when the monolayer surface is fully covered with dye molecules (mg/g), and KL is the Langmuir constant (L/mg). The constant separation factor, RL, defined by Eq. 4, indicates the feasibility of adsorption in a certain concentration range, as follows: unfavorable if RL > 1, linear when RL = 1, favorable when 0 < RL < 1, and irreversible for RL = 0.35

RL =

1 1 + K LCi

(4)

where: KL is the Langmuir adsorption constant (L/mg), and Ci is the maximum initial concentration of the dye (mg/L).


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Freundlich isotherm assumes heterogeneous surface with a non-uniform distribution of heat of adsorption, and is expressed by Eq. 5:36

qe = K F CeN

(5)

where: KF is Freundlich constant, and indicates the amount of dye sorbed at equilibrium per gram of ionene (mg/g); the meaning of qe and Ce is that given for Eq. 3. N is a measure of the nature and strength of the adsorption process and of the distribution of active sites.36,37 Sips isotherm is a combination of the Langmuir and Freundlich isotherm models, being equivalent with the Freundlich isotherm at low sorbate concentrations, while at high sorbate concentrations predicts a monolayer adsorption characteristic to the Langmuir isotherm.33 The nonlinear form of the Sips isotherm is expressed by Eq. 6:

qe =

q m a S C eN 1 + a S C eN

(6)

where: qe and Ce have the same meaning as in Eqs. 3 and 5, qm is the monolayer adsorption capacity (mg/g), and aS represents the Sips constant. The nonlinear form of the Dubinin-Radushkevich isotherm model is given by Eq. 7:32,33,37

q e = q DR exp{− β [ RT ln(1 +

1 2 )] } Ce

(7)

where: qDR is the maximum adsorption capacity of the dye (mg/g) and β is the D-R isotherm constant (mol2/kJ2). D-R isotherm constant, β, is related to the mean free energy of adsorption, E (kJ/mol), and was evaluated with the following equation: E = 1/(2β)1/2

(8)


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The type of adsorption can be assumed based on the values of E, as follows: E < 8 kJ/mol indicates physical sorption, while values of E in the range 8 -16 kJ/mol indicate that the adsorption process occurs mainly by ion exchange. Values of E > 40 kJ/mol characterize chemisorption.37-39 Kinetic Study For the kinetic study, 2 mL of ionene dispersion with a concentration of 1 mg/mL were added quickly, under magnetic stirring, to 8 mL of dye solution with a concentration in the range 50 – 250 mg/L, at pH 6; 2 × 2 mL were withdrawn from each sample and centrifuged at 10000 rpm for 30 min. The residual concentration of each dye was measured by UV-Vis spectroscopy at the characteristic wavelength. Eq. 1 was used to calculate the amount of the dye adsorbed as a function of contact time. Error Analysis. The fitness of isotherm and kinetic models was evaluated by the correlation coefficient of determination (R2), and by the non-linear Chi-square (χ2) test, defined by Eq. 9:32,36

χ =∑ 2

( q e,exp − q e,cal ) 2

(9)

q e,cal

χ2 is a small number when the data from a model are similar with the experimental data, and a big number if they differ in a high proportion.32

RESULTS AND DISCUSSION Chemical structures and names of the ionenes and azo dyes used in this work are presented in Figure 1.


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O

O NH

Cl + N

HN

O

O Cl + N

N H

N H

Cl + N

Cl + N

n

Cl+ N

H N

O

O

N H

Cl + N

n

n

1,2-Ionene

1,3-Ionene SO3Na

N

SO3Na

N

N

NH2 OH NaO3S

N

H3C N

1,4-Ionene

N N

N

H3CO

N

2

CH3

HO

Methyl Orange

SO3Na

Ponceau SS

SO3Na

Direct Blue 1

Figure 1. Chemical structures and names of polycations and dyes used in this work. As can be seen in Figure 1, the three ionenes have the same molar mass of the repeating unit and the same type of cationic charges (1,4-diazabicyclo[2,2,2]-octane, DABCO), being different by their topology, i.e. the geometry of the repeat unit, which allows to control the effective distance between the positive charges as well as on the molecular weight (see Materials and Methods) and their self-assembly properties.31 It was anticipated that the variation in structure of the ionene employed would result in differential binding, retention and consequently absorption of dyes.

Spectroscopic Characterization of Azo Dyes Aggregation as a Function of Ionene Structure UV-Vis Spectra The ratio between the ionenes as dispersion and the azo dyes was considered as molar ratio in all cases, with the approximation that the concentration of polycation in the aqueous dispersion


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was identical to that in solution. Preliminary experiments regarding the spectral changes of the azo dyes in the presence of 1,2-ionene as aqueous dispersion and as aqueous solution (at 50 oC this being the only water soluble ionene) were first performed and are compared in Figure 1S (Supporting Information). The concentration of dyes for these experiments was 2 × 10–4 M, and therefore 5 mL of dye solution contain 10–3 mM dye. The numbers attached to each spectrum show the molar ratio between polycation and azo dye (P/D). The mass of the repeat unit of all ionenes used in this work is Mr.u. = 545 g/mol, and therefore, at a concentration of 1 mg ionene/mL, 1 mL of polymer dispersion will contain 1.835 × 10–3 mM, and P/D = VPC × 1.835, the only variable being the volume of ionene dispersion, VPC. It is known that, the aggregation of dye molecules is a consequence of their flat geometry, but the aggregation mode is depending on both the dye structure and the polymer characteristics. A spectral blue shift occurs when the face-to-face aggregation is possible (H-aggregation), where the interaction between the π-electrons of the bound dye molecules is very strong.40-43 The spectral red shifts characterize the so-called J-aggregates where the dye molecules are arranged mainly side-by-side.44 In the discussion of the influence of the ionene structure on the spectral changes of the azo dyes, the number of positive charges of the ionene repeating unit, which is 2 in all cases, was correlated with the number of -SO3Na groups of each dye, which is 1, 2, and 4, for MO, PSS and DB1, respectively (see Figure 1). Spectral changes of MO after the interaction with each ionene as aqueous dispersion are presented in Figure 2. A blue shift of the characteristic maximum from 462 nm to about 380 nm arose after the interaction of 1,2-ionene with MO molecules. The position of the maximum was not re-established even at P/D = 4.77, demonstrating a very strong interaction between ionene and MO molecules. The strong blue shifts are attributed to the increasing number of interacting


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chromophores formed when dye molecules are bound to polycations.4,5,25 The interaction between 1,3-ionene and MO molecules was also very strong, the complex stoichiometry (the absence of dye molecules in the supernatant) being located at P/D about 0.5, which corresponds to two MO molecules per repeating unit of ionene. Excess of 1,3-ionene particles up to P/D = 6.6 had no influence on the concentration of MO in supernatant, indicating a high stability of the formed complex.

Absorbance, a.u.

0.5 P/D = 0

0.4

P/D = 0.37

P/D = 0.73

0.3

P/D = 2.2

P/D = 1.47

0.2

P/D = 2.75

P/D = 4.77

0.1 0.0 300

350

400

450

500

550

600

λ, nm

1,2-Ionene 0.5 P/D = 0

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3

P/D = 6.6 P/D = 0.18

P/D = 4.8

0.2

P/D = 0.37

P/D = 3.7

P/D = 0.64

0.1

P/D = 0.92

0.0 300

350

400

450

500

550

600

λ, nm

1,3-Ionene


13


0.5 P/D = 0

Absorbance, a.u.

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0.4

P/D = 0.09

P/D = 0.92 P/D = 0.64

P/D = 0.15

0.3

P/D = 0.2 P/D = 0.26

0.2 P/D = 0.37

0.1 0.0 300

350

400

450

500

P/D = 5.5 P/D = 4.4 P/D = 2.9

550

600

λ, nm

1,4-Ionene Figure 2. UV-Vis spectra of MO in the presence of increasing P/D molar ratios of ionenes as aqueous dispersions. This result could be attributed to the possibility of the MO molecules to form tight face-to-face aggregates as recently reported also for poly(allylamine hydrochloride) in the presence of Allura Red AC.45 The high efficiency of 1,4-ionene in binding MO molecules is supported by the spectra presented in Figure 2. A diffuse blue shift of the maximum is visible even at P/D = 0.15, the MO molecules being almost completely removed at a P/D around 0.5, i.e. close to the theoretical ratio between the positive charges of ionene and -SO3Na groups of MO, which corresponds to the stoichiometric compensation of charges. The excess of 1,4-ionene after the stoichiometric ratio had a small influence on the concentration of MO in supernatant (at least up to P/D = 5.5). This result also demonstrates that the MO molecules were tightly bound on the ionene microparticles. It is obvious that the binding of MO to 1,4-ionene was comparable with that found in the case of 1,3-ionene, the stability of the ionene/dye complex being high for both ionenes. From the spectra presented in Figure 2, it is clear that all ionenes were favorable for a face-to-face aggregation of


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the bound MO molecules, the 1,3-ionene/MO and 1,4-ionene/MO complexes being more stable than 1,2-ionene/MO complex. Azo dyes have been recently incorporated into dye-sensitized solar cells.46 The azo dye pmethyl red has been used as a model to probe the effect of organic dye aggregation at the TiO2 surface, because it was demonstrated that para-substituted azo dyes function better than the ortho- or meta-substituted ones.46 It has been proposed that the H-aggregation mode onto the TiO2 surface dominates also for this dye. The spectral changes of PSS after its interaction with the three ionenes are presented in Figure 3. In the case of 1,2-ionene, the concentration of PSS was about zero at P/D of 1.1, i.e. very close to the theoretical ratio between positive charges of ionene and -SO3Na groups of PSS, and after the stoichiometric ratio the absorbance only slowly increased. The shape of spectra was almost the same when 1,3-ionene was used as polycation. In this case, the P/D value corresponding to the stoichiometry was about 2, the dye molecules being detected in supernatant at higher concentration of 1,3-ionene dispersion than in the case of 1,2-ionene. Because no metachromatic effect was visible in the case of PSS in the presence of these two ionenes, when the initial concentration of the dye was 2 x 10-4 M, further UV-Vis spectra were recorded in more diluted solutions of PSS at much higher P/D ratios; in this case the amount of ionene was kept constant, while the concentration of PSS steadily increased (Figure 3, left, down).


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0.8

0.8

P/D = 0

0.5 0.3

P/D = 1.84 P/D = 1.47

P/D = 0.73 P/D = 0.92

Absorbance, a.u.

P/D = 0.37

0.6 0.4

P/D = 0

0.7 0.6 P/D = 0.47

0.5

P/D = 4

0.4 0.3

P/D = 0.73

P/D = 3.3

0.2

P/D = 2

0.1

0.1

P/D = 2.6

0.0 300 350 400 450 500 550 600 650 λ, nm

0.0 300 350 400 450 500 550 600 650 λ, nm

1,2-Ionene

1,3-Ionene

0.2

P/D = 1.1

2.0 P/D = 44.3 1.8 P/D = 17.5 P/D = 8.54 1.6 P/D = 5.82 1.4 P/D = 4.22 1.2 P/D = 2.11 1.0 0.8 0.6 0.4 0.2 0.0 300 350 400 450 500 550 600 650 λ, nm

0.8 0.7 Absorbance, a.u.

Absorbance, a.u.

0.7

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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P/D = 0

0.6 0.5

P/D = 0.37

P/D = 5

0.4

P/D = 0.64

P/D = 4.2

P/D = 0.92 P/D = 1.3

0.3 0.2

P/D = 2.94

P/D = 1.56

0.1 0.0 400

P/D = 1.84

450

500

550

600

650

λ, nm

1,4-Ionene

1,2-Ionene

Figure 3. UV-Vis spectra of PSS in the presence of ionenes as aqueous dispersions; UV-Vis spectra at left, down, were recorded at decreasing P/D ratios, from bottom to up. A bathochromic effect can be observed in Figure 3 (left, down) when 1,2-ionene was in a high excess, such red shifts supporting a side-by-side interaction between the dye molecules in the presence of 1,2-ionene.23,42 Similar changes of the UV-Vis spectra were observed for PSS in the presence of 1,3-ionene, in dilute solutions and at high ratios (spectra not shown here). In the case of the interaction between 1,4-ionene and PSS, the concentration of PSS was about zero at P/D = 1.84, but the dye molecules were detected very fast in supernatant at a small excess of ionene (P/D = 2.94). The bathochromic effect that the excess of 1,4-ionene caused on the PSS


16

Page 17 of 42

spectrum is visible at P/D = 4.2 and 5. The different behavior of PSS compared with MO molecules (Figure 2) in the presence of 1,4-ionene particles could be attributed to the differences in the topology of the ionene repeating unit, which in the case of 1,4-ionene is not favorable for the formation of compact complexes. The side-by-side aggregation of the PSS molecules (Jaggregation) on the surface of the ionene particles, can be easily disturbed by the ionene in excess, having as a consequence the redistribution of the same number of dye molecules between an increasing number of ionene particles. The UV-Vis spectra of DB1 in the presence of ionenes, as a function of P/D and ionene structure, are presented in Figure 4. A very strong interaction was observed between 1,2-ionene as dispersion, and DB1 molecules at P/D molar ratio around 2.4 that is close to the stoichiometry of the ionene/dye complex. Increasing the concentration of 1,2-ionene did not influence the stability of the complex, while in the case of 1,3-ionene, the dye molecules were still detectable in the supernatant at P/D = 4.04. This difference could be attributed to the geometry of the DB1 molecule, which seems to prefer a shorter distance between the positive charges of the ionene to compensate its negative charges, the 1,2-ionene topomers being able to satisfy this requirement.

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 450

P/D = 0 P/D = 0.73

P/D = 1.47 P/D = 2.02 P/D = 4.04 P/D = 2.94 P/D = 2.4

500

550

600 650 λ, nm

700

750

1,2-Ionene


17

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 450

Page 18 of 42

P/D = 0 P/D = 0.73 P/D = 2.02 P/D = 2.4 P/D = 2.94 P/D = 4.04

500

550

600 650 λ, nm

700

750

1,3-Ionene

Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorbance, a.u.


1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 450

P/D = 0 P/D = 2.02 P/D = 2.94 P/D = 4.95 P/D = 6.6

500

550

600

650

700

750

λ, nm

1,4-Ionene Figure 4. UV-Vis spectra of DB1 in the presence of increasing P/D molar ratios of ionenes as aqueous dispersions. The interaction between ionene and DB1 was even more difficult in the case of 1,4-ionene, where the dye was still detectable in the supernatant at P/D molar ratio of 6.6. In this case, the distance between the positive charges, which belong to two different repeating units, hinder DB1 molecules to compensate their charges. FT-IR Spectra To get further information about the interaction between azo dyes and the ionenes under study, FT-IR spectra of two ionenes before and after the binding of the dyes were compared (Figure 5). Figure 5a supports the strong interaction between 1,2-ionene and PSS by the shifts and/or


18

Page 19 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decrease in intensity of some characteristic peaks of 1,2-ionene, as well as by the appearance of new peaks attributed to the dye structure. Specifically, the band at 1660 cm–1, assigned to the vibrations of the C=O bond in amide group (amide I), was blue shifted at 1654 cm–1; whereas the peak at 1599 cm–1, assigned to the N-H bending vibration, was red shifted at 1611 cm–1. The peaks at 1506 cm–1 and 1476 cm–1 in the spectrum of 1,2-ionene can be seen at 1499 cm–1 and 1475 cm–1 in the complex with PSS (these bands being also present in the spectrum of pure dye), being attributed to the C=C bonds in the aromatic ring; the band at 1390 cm–1, assigned to the CN stretching vibration, decreased in intensity and blue shifted at 1387 cm–1. The strong band at 1309 cm–1, assigned to the amide III mode of vibration, dramatically diminished in intensity; the band characteristic for -SO3Na group, found in the spectrum of pure PSS at 1190 cm–1, was located at 1182 cm–1 in the spectrum of the 1,2-ionene/PSS complex. From the strong bands at 854 cm–1, 769 cm–1 and 712 cm–1, present in the spectrum of pure 1,2-ionene and assigned to the C-H out-of-plane bending of aromatic ring, only two of those bands (i.e. 850 cm–1, 767 cm–1) were observed in the ionene/PSS complex and showed a dramatic decrease in intensity. Moreover, the characteristic band of C-S bond was identified at 704 cm–1 in the ionene/dye complex. The interaction between 1,3-ionene and PSS is supported by the spectral changes (shifts) that can be seen in Figure 5b. The bands located at 1659 cm–1, 1607 cm–1, and 1542 cm–1, assigned to the vibrations of the C=O bond in amide group (amide I), the N-H bending vibration, and vibration of C=C bonds in aromatic ring, are present in the spectrum of the 1,3-ionene/PSS complex at 1658 cm–1, 1608 cm–1, and 1544 cm–1, respectively, increasing in intensity from the first to the last. The band observed at 1482 cm–1 in the spectrum of ionene, was much more intense in the spectrum of its complex with PSS, being located at 1480 cm–1. The strong bands at


19


854 cm–1, 791cm–1 and 690 cm–1, present in the spectrum of 1,3-ionene, assigned to the substituted aromatic ring, are visible at 853, 789, and 683 cm–1, respectively, after the complexation with PSS. In addition, two new peaks can be seen in the spectrum of 1,3ionene/PSS complex at 1187 cm–1 and 1032 cm–1, assigned to -SO3Na group and C-H stretching vibration of aromatic ring, respectively.

1,2 - ionene

a

902 1020 854 573

704 767 850

2000 -1 Wavenumber Wavenumber (cm(cm-1) )

1031

1182

3000

769

991 1101

1310 1387 1445 1475 1499 1529 1611 1654

2923

3432

4000

663 712

1,2 - ionene + PSS

583

1059 1092 1309 1446 1390 1476 1506 1529 1660 1599

3013

3413

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 42

1000


20

Page 21 of 42

b 1,3 - ionene

662 690

1059

791

578

1020 1196

854

1,3 - ionene + PSS

1091 1253 1306 1422 1482 1542 1607 1659

3012

3402

789 853

1277

1421

1181

786

1187

1481 1542 1659

1032

1480 1544 1608 1658

1,3 - ionene + MO

574 683

993 1103

3017

3413

848 1004

2000 -1 Wavenumber (cm-1) Wavenumber (cm )

1028 1113

3000

1363

1602

4000

571 619 692

942

2921 3022

3420

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

Figure 5. FT-IR spectra of 1,2-ionene and of the complex with PSS (a) and of the 1,3-ionene and its complexes with MO and PSS (b).


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 42

The strong interaction between 1,3-ionene and MO is supported by the dramatic changes observed in the spectrum after complexation. The peaks located at 1659 cm–1 and 1542 cm–1 diminished while the peak at 1607 cm–1 was blue shifted at 1602 cm–1 and strongly increased in intensity. The new peaks visible in the spectrum of 1,3-ionene/MO complex at 1181 cm–1 and 1028 cm–1 were attributed to -SO3Na group and to the stretching vibration of C-H bond in the aromatic ring, respectively. Dramatic changes occurred also in the region where the peaks characteristic of substituted aromatic ring are located. Thus, the peaks at 854 cm–1 and 791 cm–1 were blue shifted at 848 cm–1, and 786 cm–1, respectively, the last one being very small. The peak at 690 cm–1 increased in intensity and was located at 692 cm–1 in the complex with MO.

Removal Efficiency of Azo Dyes as a Function of Ionene Structure The amount of dye sorbed at equilibrium onto the ionenes, qe, calculated with Eq. 1, and the CRE values, calculated with Eq. 2, for all dyes, are plotted as a function of the mass of ionene in Figure 6. As can be observed in Figure 6A, MO molecules were bound to all ionenes, the amount of the dye bound onto ionene particles increasing with the decrease of ionene dose. The amount of dye bound per weight unit of ionene increased by the decrease of sorbent dose, due to the increase of the number of dye molecules in contact with the ionene weight unit. The values of CRE abruptly increased up to about 100% in the case of 1,3-ionene, and up to about 95% in the case of 1,4-ionene, and did not decrease when the ionenes were in excess. These results are supported by the UV-Vis spectra presented in Figure 2. Figure 6B shows that PSS molecules have been tightly bound onto the microparticles of 1,2- and 1,3-ionenes, the stability of the complex being less influenced by the ionene in excess, while the complex formed with 1,4-


22

Page 23 of 42

ionene was not stable, being destroyed by the ionene in excess after the formation of the stoichiometric complex (P/D about 2). A 160 140 120 100 80 60 40 20 0

100 90

qe, mg/g

80 1,2-ionene 1,3-ionene 1,4-ionene

60 50 40

0

B

70

1

2 3 4 ionene dose, mg

5

6

1800 1400

CRE, %

qe, mg/g

1200 1000 800 600 400

1,2-ionene 1,3-ionene 1,4-ionene

200 0

0.0

0.5

1.0 1.5 2.0 2.5 ionene dose, mg

3.0

800 700

500

100 90 80 70 60 50 40 30 20 10 0

CRE, %

qe, mg/g

600

400 300 200

1,2-ionene 1,3-ionene 1,4-ionene

100 0

30

100 90 80 70 60 50 40 30 20 10 0

1600

C

CRE, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1

2

3 4 5 6 7 8 ionene dose, mg

9 10

Figure 6. Effect of the ionene dose on the sorption capacity, qe, (left Y axis, full symbols) and the color removal efficiency, CRE, (empty symbols, right Y axis) of MO (A), PSS (B), and DB1


23


(C); constant parameters: temperature 25 ºC; the initial concentration of the dye, 2 × 10–4 M; magnetic stirring 30 min at ~ 750 rpm, and 30 min at ~ 200 rpm. The results are represented as means ± standard deviation (n = 3). This behavior was also in agreement with the UV-Vis spectra presented in Figure 3. The structure of DB1 is much more complex than that of the first two azo dyes, and therefore the tight binding of its molecules onto the ionene particles was more difficult. The amount of polycation necessary to remove all dye molecules in solution was much larger than in the case of MO and PSS (Figure 6C). The topology of 1,2-ionene seems to be the most suitable to form stable aggregates with DB1 molecules, in accordance with the UV-Vis spectra showed in Figure 4, while 1,4-ionene hardly bound DB1 molecules. This results prompted us to test the possibility to separate two azo dyes, i.e. MO and DB1 based on the affinity of 1,4-ionene for MO molecules. Figure 7 shows that 1,4-ionene added to a 1:1 mixture of MO and DB1 completely removed MO molecules, while DB1 molecules are still present in a high amount. 1.6 1.4 Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 42

1.2 1.0 0.8 MO + DB1, 1:1 Before After 1,4-ionene

0.6 0.4 0.2

MO + DB1, 1:1; before (green) and after the addition of 1,4-ionene(blue)

0.0 300

400

500

600

700

800

λ, nm

Figure 7. Sorption of MO onto 1,4-ionene in the presence of DB1. Based on the sorption results presented in Figure 6, the following ionene/dye pairs were selected for the investigation of the dye sorption at equilibrium: 1,3-ionene/MO; 1,3-ionene/PSS, 1,2-ionene/PSS, and 1,2-ionene/DB1. The values of the dye sorbed at equilibrium onto the


24

Page 25 of 42

ionene microparticles, qe, were plotted as a function of the concentration of the dye at equilibrium in Figure 8. A

400

100 0

0

C

500 1,2-ionene + PSS Langmuir Freundlich Sips D-R

0 100 200 300 400 500 600 700 800 Ce, mg/L

1,3-ionene + PSS Langmuir Freundlich Sips D-R

200 100

50 100 150 200 250 300 350 400 Ce, mg/L

600

300

1000 900 800 700 600 500 400 300 200 100 0

0 100 200 300 400 500 600 700 800 Ce, mg/L

D

qe, mg/g

0

700

200

qe, mg/g

1,3-Ionene + MO Langmuir Freundlich Sips D-R

800

300

B

500 400

qe, mg/g

1000 900 800 700 600 500 400 300 200 100 0

qe, mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1,2-ionene + DB1 Langmuir Freundlich Sips D-R

0

200

400 600 800 Ce, mg/L

1000

Figure 8. Equilibrium adsorption isotherms of MO onto 1,3-ionene (A), PSS onto 1,3-ionene (B), PSS onto 1,2-ionene (C), and DB1 onto 1,2-ionene (D), fitted with four isotherm models; the results are represented as average of three replicates, the error ± 5% (error bars were not included for clarity reasons). Figure 8 shows that the profile of the experimental isotherms was strongly influenced by the structure of ionene and dye, which finally determine the dominant type of interactions. The shape of the isotherm found for the sorption of MO onto 1,3-ionene (Figure 8A) indicates an isotherm of type ‘‘H’’, characterized by high initial slope, which indicates a very high affinity of the


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 42

ionene for MO molecules.15,47-50 The shape of the isotherm which describes the sorption of PSS onto 1,3-ionene (Figure 8B) is almost similar with that presented in Figure 8A, while the experimental isotherm which describes the sorption of PSS molecules onto 1,2-ionene is closer to a “S” isotherm, which shows that the dye molecules are better adsorbed after the adsorption of some molecules; this isotherm has an inflection point which supports a “cooperative adsorption”.47 The adsorption of DB1 onto the 1,2-ionene is described by an “L” isotherm because the ratio between the concentration of the dye in solution and that adsorbed onto the ionene particle is a concave curve. The experimental data were fitted by four isotherm models, namely Langmuir isotherm, Freundlich isotherm, Sips isotherm, and Dubinin-Radushkevich isotherm, described by the equations 3, 5, 6, and 7, respectively. The isotherm parameters for all ionene/azo dye pairs are presented in Table 2. Table 2. Isotherm parameters of Langmuir, Freundlich, Dubinin-Radushkevich (D-R), and Sips models obtained by non-linear regression method for the sorption of azo dyes onto the ionenes at 35 oC. Isotherm

Dye/Ionene

qe,exp, mg/g

Langmuir

MO/1,3-ionene

PSS/1,3ionene

PSS/1,2ionene

DB1/1,2ionene

954

450.76

768

869.71

qm, mg/g

985.71

483.71

1010.49

976.7

KL, L/mg

0.1675

0.0389

0.00749

0.01028

RL

0.0089

0.0313

0.1678

0.0808

R2

0.9036

0.9807

0.9367

0.9708

χ2

16542

658.6

7096.18

3461

KF, mg/g

245.51

77.68

44.88

67.48


26

Page 27 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Freundlich

D-R

Sips

1/n

0.2578

0.2974

0.4652

0.3918

R2

0.7439

0.8416

0.8332

0.9228

χ2

43911

55405.9

18698

9171

qDR. mg/g

925.49

431.58

767.8

892.01

β, mol2/kJ2

1.3248x10-6

1.4446 x 10-5

5.995 x 10-4

0.00209

E, kJ/mol

614.34

186

28.88

15.47

R2

0.979

0.9849

0.9755

0.8261

χ2

3169

514.88

2746.5

20658

qm, mg/g

895.97

451.24

771.56

1065.9

aS

0.0144

0.0195

3.259 x 10-5

0.01515

1/n

4.183

1.344

2.3664

0.8614

R2

0.9862

0.9856

0.9888

0.9707

χ2

2087

490.54

1259.44

3556

As can be seen in Table 2, the theoretical qm values estimated by the Langmuir model are close to the experimental values, except the adsorption of PSS onto 1,2-ionene. However, the correlation coefficient of determination R2 are the highest for the sorption of PSS onto 1,3-ionene and of DB1 onto 1,2-ionene, when R2 = 0.9807 and 0.9708, respectively. These results show that the sorption process of the dyes in these systems could be described by the Langmuir isotherm model. The low values of RL, calculated by Eq. 4 for the highest initial concentration of the dye for all ionene/dye pairs, show that the sorption was favorable for all the azo dyes. The low values of R2 and the high values of Chi-square test (χ2) in the case of the Freundlich isotherm indicate a very low degree of fitness on the experimental data. However, the feasibility of the dye sorption onto 1,2- and 1,3-ionenes is supported by the values of 1/n < 1, which show that the bond energies increase with the surface density.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 42

As can be seen in Table 2, the D-R isotherm described well the experimental data in the case of MO/1,3-ionene, PSS/1,2-ionene, and PSS/1,3-ionene. The low value of R2 and the high value of

χ2 for the sorption of DB1 onto 1,2-ionene show that the D-R isotherm is not suitable to describe the sorption process in this case. The values of the mean free energy of adsorption, E, of 614.34 kJ/mol, and 186 kJ/mol for the sorption of MO and PSS, respectively, onto 1,3-ionene support chemisorption as the most probable mechanism of sorption, while the E values of 28.88 kJ/mol , and 15.47 kJ/mol found in the case of the sorption of PSS and DB1 onto the 1,2-ionene, respectively, would support another type of adsorption to be more important such as ion exchange (electrostatic interaction). Table 2 shows also that the values of the monolayer sorption capacity, qm, given by the Sips model are very close to the experimental sorption capacity values, qe,exp , for all ionene-dye pairs. The high values of R2 and low values of χ2 show the Sips isotherm model to be the best fitting model for all ionene-dye pairs selected for the investigation of the dye sorption at equilibrium. The results presented in Table 2 show that the ionene topology and the dye structure play a crucial role in the outstanding adsorption efficiency of the selected azo dyes. The optical images presented in Figure 2S also support the specific interaction between the ionenes and dyes. Various materials have been used for the removal of MO from the aqueous solutions.9,13,51-55 The maximum adsorption capacity, qm, obtained in the present work when MO was adsorbed onto 1,3-ionene is compared in Table 3 with the values of MO adsorption onto other sorbents. Table 3. Maximum Equilibrium Sorption Capacity of MO onto Different Sorbents. Sorbent

T, oC

Sorbent dose,

Initial

qm, mg/g Ref.


28

Page 29 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


g/L

pH

Activated carbon nanotubes (CNTs-A) 25

0.75

7

149

9

Core-Shell structured Graphene oxide45 chitosan beads

0.5

7

353

13

De-Oiled Soya

30

-

3

16.66

52

Activated clay

30

5

7

13.63

53

Chitosan/Alumina composite

35

8

7

30.33 ± 5.9

54

Chemically modified straw

25

1

7

300

55

1,3-ionene

35

0.167

6.0

985.71

This study

As can be seen in Table 3, the maximum sorption capacity of MO onto the DABCO-based ionenes was higher than that of other reported sorbents, and this suggests that these ionic polymers hold great potential for removal of MO from aqueous solutions. For the removal of PSS and DB1 other techniques have been reported, including coagulation/flocculation,22,24 and photodegradation processes.56 That is the reason the results obtained by sorption of PSS and DB1onto ionenes have been not discussed in comparison with other sorbents. Kinetic Study The sorption capacity of 1,2-ionene for PSS and DB1, and of 1,3-ionene for MO and PSS as a function of contact time is presented in Figure 9, the dye concentrations ranging from 11 to 130 mg/L. As can be observed, in all cases the equilibrium of sorption was reached very quickly.


29


A

600

B

500

500

400 qt, mg/g

qt, mg/g

400

300

300 200

Ci = 124.92 mg/L Ci = 63.11 mg/L

100

PFO model PSO model

0

0

20

40

60 80 Time, min

100

Ci = 116.66 mg/L Ci = 72.28 mg/L

100

Ci = 11.14 mg/L PFO model PSO model

0

C

0

20

40 60 Time, min

80

100

D

500

400

400 Ci = 125.2 mg/L Ci = 60.01 mg/L

qt, mg/g

300

300

Ci = 116 mg/L Ci = 11.6 mg/L

200

PFO model PSO model

200

PFO model PSO model

100 0

200

120

500

qt, mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 42

100 0

20

40

60 80 100 120 140 Time, min

0

0

20

40 60 80 Time, min

100

120

Figure 9. Nonlinear fitting of PFO and PSO kinetic models on the azo dye retention data onto the DABCO-based ionenes: (A) sorption of MO onto 1,3-ionene; (B) sorption of PSS onto 1,3ionene; (C) sorption of PSS onto 1,2-ionene; (D) sorption of DB1 onto 1,2-ionene; (sorbent dose 0.002 g; volume 10 mL; temperature 30 oC; magnetic stirring at about 250 rpm). The results are represented as means ± standard deviation (n = 3). The kinetic data in Figure 9 were analyzed by two kinetic models, pseudo-first-order (PFO) kinetic model,57 and pseudo-second-order (PSO) kinetic model.58 The non-linear forms of the PFO and PSO kinetic models are described by equations 10 and 11, respectively:

qt = qe (1- e-k1t)

(10)


30

Page 31 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

k q t qt = 2 e 1 + k 2 qe t

(11)

where: qe and qt - the amount of dye sorbed at equilibrium (mg/g) and at time t, respectively; k1 the rate constant of the PFO kinetic model (min-1); k2 - the rate constant of PSO kinetic model (g mg-1 min-1). According to the data presented in Table 4, the PSO kinetic model fits more accurately the experimental data than the PFO kinetic model, the R2 values being higher, and χ2 being lower for the first model, in the case of the 1,3-ionene/MO, 1,3-ionene/PSS and 1,2-ionene/PSS pairs, at least. Table 4. Kinetic Model Parameters for the Adsorption of Azo Dyes onto DABCO-Based Ionenes. Ci, mg/L

qexp, mg/g

PFO qcalc, mg/g

k1 , min-1

R2

PSO χ2

qcalc, mg/g

k1 , g/mg.min

R2

χ2

MO + 1,3-ionene 124.92

506

501.36 0.3355 0.9982

54.36

519.82 0.00158

0.9991

29.65

63.11

296

290.56 0.2854 0.9944

57.39

305

0.9977

24.07

0.00193

PSS + 1,3-ionene 116.66

385

377.23 0.1854 0.9877

192.1

405.59 0.000771

0.9982

27.8

72.28

260

253.33 0.2576 0.9922

60.38

268.1

0.00182

0.9985

11.52

11.14

23

22.75

0.3756 0.9981

0.14

23.85

0.039

0.9991

0.062

443

0.4098 0.9943

122.61

454

0.00263

0.9992

17.6

PSS + 1,2-ionene 116.66

450


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11.14

21.8

21.68

0.362

0.9948

0.29

22.37

0.0389

Page 32 of 42

0.9981

0.12

DB1 + 1,2-ionene 125.2

433

425.16 0.3231 0.9954

101

442.76 0.00167

0.9997

5.86

60.01

141

139.71 0.1512 0.9977

5.56

153.95 0.00143

0.9891

27.16

These results suggest that the sorption rate of the dyes onto the 1,2- and 1,3-ionenes depends on the availability of sorption sites and that the rate limiting step could be chemical adsorption, supported also by the values of mean free energy of adsorption (Table 2). Thus, the sorption kinetics would support the chemisorption as the controlling mechanism of adsorption, at least when 1,2- and 1,3- ionenes were used as sorbents.

CONCLUSIONS In conclusion, DABCO-based ionene polymers having different topologies governed by the substitution pattern (ortho-, meta-, para-) of their aromatic core induced aggregation of three azo dyes, namely methyl orange (MO), ponceau SS (PSS) and direct blue 1 (DB1). The results demonstrated that the association of the dye molecules with these ionenes and the induced metachromasy are strongly influenced by both the dye structure and the ionene topology. Specifically, MO having one -SO3Na group was almost stoichiometrically bound to all ionenes regardless their topology and showed a significant metachromatic effect. In sharp contrast, PSS having two -SO3Na groups was preferentially bound onto both 1,2-ionene and 1,3-ionene, but DB1, having four -SO3Na groups and a more complex structure was efficiently bound only onto 1,2-ionene. The experimental isotherms of the dye sorption for each ionene were best fitted by the Langmuir, Sips and Dubinin-Radushkevich isotherm models. The sorption kinetics, well


32

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described by the PSO kinetic model, would indicate the chemisorption as the controlling mechanism of adsorption. The remarkable influence of the ionene topology on their dye sorption abilities may serve as an inspiration for the design of new polyelectrolytes with higher affinity for specific toxic dyes. SUPPORTING INFORMATION UV-Vis spectra of dyes after the interaction with 1,2-ionene as aqueous solution and aqueous dispersion; optical images of the dye solutions with increasing concentrations after the contact with ionene microparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCSIS – UEFISCDI (PN-II-ID-PCE-2011-3-0300), Universität Regensburg and Deutsche Forschungsgemeinschaft (DFG, 9209720). D.D.D. thanks DFG for the Heisenberg Professorship Award. REFERENCES (1) Kugel, R. Metachromasy: The Interactions between Dyes and Polyelectrolytes in Aqueous Solutions, Adv. Chem. Sci. 1993, 236, 507–533. (2) Simionescu, B. C.; Smets, G. J. Polymer Induced Aggregation of Dye Molecules, Makromol. Chem. Suppl. 1975, 1, 246–261.


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Table of Contents Graphic


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Spectroscopic Characterization of Azo Dyes Aggregation Induced by

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