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Coacervate of Polyacrylamide and Cationic Gemini Surfactant for the Extraction of Methyl Orange from Aqueous Solution Weiwei Zhao, Yaxun Fan, Hua Wang, and Yilin Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01421 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017

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Coacervate of Polyacrylamide and Cationic Gemini Surfactant for the Extraction of Methyl Orange from Aqueous Solution Weiwei Zhao,†,‡ Yaxun Fan,†,‡ Hua Wang,†,‡and Yilin Wang*,†,‡ †

Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular

Sciences (BNLMS), CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

ABSTRACT: Coacervation in aqueous solution of the mixture of cationic ammonium surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) and 10% hydrolyzed polyacrylamide (PAM) has been investigated. It was found that the 12-6-12/PAM mixture forms coacervate with a large network structure over a wide concentration range of surfactant and polyelectrolyte, and shows great efficiency in the extraction of methyl orange (MO) from water owing to the cooperation of hydrophobic, electrostatic and π-cation interactions. Meanwhile the dye joins the coacervate and strengths the network structure of the coacervate. In particular, benefiting from partial excess of 12-6-12 molecules, the coacervate phase presents selective adsorption behavior toward anionic dye MO in the presence of cationic dye methylene blue (MB). Furthermore, the coacervate phase is utilized to modify quartz sand and melamine foam, and the coacervate-treated adsorbents can adsorb MO

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efficiently. Moreover, the MO-loaded adsorbents are easily regenerated with hydrochloric acid, making this an inexpensive and environment benign process. These findings offer a simple and effective alternative for the treatment of dye contaminated water and the recovery of dyes.

INTRODUCTION Dyes are a family of compounds that are extensively used in many industrial and scientific aspects, including textile industry,1 paper production,2 food technology3,4 and microscopy staining.5 It is estimated that the amount of dyes produced in the world per year is over 700000 tons,6 with 1-2% discharged in production and 1-10% lost in coloration.7 As most of the commercially available dyes are hazardous and even carcinogenic, the release of considerable amount of dyes causes serious environmental and public problems. During the past four decades, a wide range of methods have been developed and attempted for the removal of pollutants from effluents to weaken the impact on the environment, involving adsorption by inorganic or organic supports, separation by membrane process or ion-exchange, decolorization by oxidation or biodegradation process, etc.6, 8-10 However, so far effective, lowcost and convenient techniques for the recovery of dyes in wastewater are still very scarce. Surfactant-mediated liquid-liquid phase separation, i.e. coacervation, refers to a process during which a colloidal dispersion separates into two immiscible liquid phases: coacervate phase rich in surfactant and a diluted phase containing few surfactant molecules.11 It has been demonstrated that coacervate phase is able to preconcentrate a variety of substances including metal ions and organic compounds with tunable solubilization capability and high preconcentration factors.12-16 Watanabe et al.17 applied the liquid-liquid phase separation of


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nonionic surfactant upon elevating temperature in the removal of Zn(II), and this method has also been applied in the extraction of organic compounds.18-23 Changing the ethylene oxide groups or hydrophobe size of nonionic surfactants can lead to different solubilization capability to organic molecules.24 Utilizing nonionic surfactants as separate agents is primarily driven by hydrophobic interaction between organic solutes and coacervate phase. Compared with the nonionic surfactant coacervation above, ionic surfactant-based coacervation has enlarged the scope of coacervation in extraction applications. It was found that the coacervation of nonionic-ionic surfactant mixtures improve the preconcentration factor of charged organic compounds compared to nonionic surfactants.25,26 The coacervation of anionic surfactant induced by the addition of acid can concentrate thermally labile compounds with very short extraction time.27-29 Moreover the coacervate of cationic/anionic surfactant mixtures exhibit superior properties in extraction applications due to their much lower CMC and more compacted aggregates resulted from the reduction of electrostatic repulsion between the headgroups.30-32 In particular, the coacervates formed by ionic surfactants and polyelectrolytes have also been utilized to remove dye molecules. The mixtures of oppositely charged chitosan and alkylethoxy carboxylate form compacted core-corona superstructures at high pH and large charge ratio, and can selectively remove hydrophobic dye Sudan Red,33,34 while the coacervate formed by poly(diallyldimethylammonium chloride) (PDADMAC) and mixed micelles of SDS/Triton-X 100 are immobilized onto controlled pore glass and quartz sand, and shows great interception capability for Orange OT at higher ionic strength.35 In brief, ionic surfactantbased coacervation is controlled by hydrophobic, electrostatic, hydrogen bonding, π-cation interactions and so on, and thus provides more binding sites for pollutants. Meanwhile,


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polyelectrolytes can significantly reduce the concentration of surfactants required for coacervation and make it attractive to actual applications. Although gemini surfactants have been greatly developed due to their strong self-assembling and various unique properties, so far there are only several publications about gemini surfactant/polymer complex coacervation as summarized in our recent review.36 Each gemini surfactant molecule contains two polar headgroups and two hydrophobic chains, thus both electrostatic interaction and hydrophobic interaction between gemini surfactant molecules and polyelectrolytes are strengthened compared with traditional single-chain surfactants. Multiple electrostatic binding sites and multiple hydrophobic associated tails of gemini surfactants enhance their chance in generating coacervation at an extremely low surfactant concentration and cover a larger concentration range. These properties greatly improve the possibility of the coacervation in practical applications. Herein, coacervation formed by a cationic ammonium surfactant hexamethylene-1,6bis(dodecyldimethylammonium bromide) (12-6-12) and 10% hydrolyzed polyacrylamide (PAM) is proposed to be used in the extraction of a dye, methyl orange (MO). The molecular structures of 12-6-12, PAM and MO are shown in Scheme 1. Previously, we found that the aqueous mixture of 12-6-12 and PAM separates into two liquid phases in a neutral medium.37 Without any other additives, the coacervation occurs at very low surfactant concentration and exhibits spongelike morphology with tunable pore diameter. This means that the balance of electrostatic, hydrophobic and solvent interaction required by the coacervation is achieved by the well matched molecular structures of these two components. The present work systematically studied the phase boundaries of the 12-6-12/PAM mixture and indicates that the cross-linked network structure in coacervate phase is able to trap MO of a considerable load


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amount. Moreover the adsorption and desorption processes of MO are realized on coacervatetreated quartz sand and melamine foam, allowing for the easy recycle of solid adsorbents and MO. This approach is a faster, safer and more efficient technique for removing anionic dyes from wastewater.

12-6-12

PAM

MO

Scheme 1. Molecular structures of 12-6-12, PAM and MO.

EXPERIMENTAL SECTION Materials.

Cationic

ammonium

gemini

surfactant

hexamethylene-1,6-

bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (12-6-12) was synthesized and purified as reported in literature.38 Polyacrylamide (PAM), 10% hydrolyzed, was purchased from Sigma and used without further purification. The average molecular weight of PAM is approximately 200000. The pKa value of acid groups of PAM is about 4.60. In this work, pH 7.0 was chosen at which all the acrylic acid groups were deprotonated except that pH 3.0 was chosen while releasing dyes from coacervate. Methyl orange (MO), methylene blue (MB) and congo red (CR) were purchased from the TCI company with a purity higher than 98%. Acid red 1 (AR1) was also purchased from TCI. Acid orange 7 (AO7 > 99%) and brilliant crocein (BC > 98%) were purchased from Adamas-beta. Quartz sand with grain size of 15 ~ 42.5 µm was purchased from Acros company and used as


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received. Melamine foam, purchased from local supermarket, was made into small pieces, washed with water and acetone, and dried in oven. Deionized water of 18.2 MΩ·cm from Milli-Q equipment was used in all experiments. Turbidity Measurements. The turbidity of the 12-6-12/PAM solution and the MO/12-612/PAM solution, reported as 100 − % T, was measured at 670 nm using a Brinkman PC920 probe colorimeter thermostatted at 25.0 ± 0.1 °C. Turbidimetric titration was carried out by adding equal volume of 12-6-12 solution and 1 wt % PAM solution into a stirred solution of 0.50 wt % PAM or 0.50 wt % PAM/MO to bring the solutions to different 12-6-12 concentrations while keeping the PAM concentration constant. All measured values were corrected by taking the turbidity of 0.50 wt % PAM solution as zero. Coacervate Phase Extraction Measurements. 1 mL aqueous solution of 12-6-12, PAM and dye was prepared in a centrifuge tube. The aqueous suspension was stand for 12 h and then centrifuged at 8000 rpm for 7 min to separate coacervate phase and dilute phase. UV-vis absorption measurements were used to study the amount of dyes extracted by coacervate phase and carried out with a Shimadau UV 2800 spectrometer. The concentration of dye in the dilute phase, i.e., supernatant was determined by UV spectrometer. Then the amount of dye extracted by coacervate phase was calculated from the relationship, Efficiency = (C0 − Ce)/C0, where C0 is the initial concentration of dye, and Ce is the equilibrium concentration of dye (mM) in the supernatant. Solid Phase Adsorption Measurements. Quartz sand and melamine foam were treated in a 0.50 wt % PAM/3.50 mM 12-6-12 solution at room temperature for 12h and then dried in an oven at 70 °C overnight. 0.50 wt % PAM/3.50 mM 12-6-12 solution is chosen owing to the formation of high volume fraction for coacervate phase. Then the adsorption experiments were


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performed at 25 ± 1°C in batches by mixing the 12-6-12/PAM treated quartz sand or melamine foam with the MO aqueous solution and stirring at 700 rpm for a fixed time. After that, the residual MO in aqueous solution was measured by UV-visible absorption spectroscopy. The amounts of MO adsorbed by the 12-6-12/PAM treated quartz sand or melamine foam were determined from the initial and final concentrations in the aqueous solution. Adsorption Kinetic Measurements. A serious of MO solutions with a fixed initial concentration were stirred with 2 g quartz sand or 80 mg melamine foam for different contact times. The change of MO uptake per unit mass of modified adsorbents with time was determined from the difference between the initial and final concentrations of MO. The adsorption kinetics of MO onto quartz sand and melamine foam was simulated by pseudo-firstorder and pseudo-second-order kinetic equations. The pseudo-first-order process kinetic equation can be expressed as follows,39 ln (qe − qt) = ln qe − k1t where qe and qt refer to the amounts of MO adsorbed on adsorbent (mg/g) at final equilibrium time and at intermediate time t, respectively, and k1 (g mg−1 min−1) is the pseudo-first-order rate constant of adsorption. The pseudo-second-order kinetic equation40 is given as, t qt

=

1 k 2 qe

2

+

t qe

where k2 (g mg−1 min−1) is the pseudo-second-order rate constant of adsorption. The rate constants can be obtained by fitting the experiment data with the two models. Adsorption Isotherm Measurements. Adsorption isotherm provides a relationship of the uptake amount of MO per unit mass of adsorbents with the concentration of adsorbates when


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the adsorption is at equilibrium. Adsorption isotherm experiments were conducted at 25 ± 1°C in a mixture of 2 g quartz sand or 80 mg melamine foam with 5 mL or 25 mL of solution containing different concentrations of MO for 15mins and 60 mins, respectively. The final solutions were used to determine the equilibrium MO concentration with UV-visible absorption spectroscopy. The amount of MO adsorbed onto modified quartz sand or melamine foam was calculated by the difference of initial and residual amount of MO in solution. The Langmuir41 and the Freundlich42 isotherms are the most commonly used models to predict the adsorption process. The Langmuir isotherm assumes that the adsorption process occurs at a homogeneous surface and the surface sites have the same activation energy. Thus, this model predicts the monolayer adsorption on the surface of adsorbents. The Langmuir isotherm is expressed as follows, qm qe

=

1 kLCe

+1

where qm and Ce represent the maximum adsorption capacity (mg/g) and equilibrium concentration of MO (mM/L) remaining in the solution; qe is the adsorption capacity at equilibrium conditions; kL is the Langmuir constant indicative of the adsorption energy (L/mM). The Freundlich isotherm assumes that the adsorption process occurs at a heterogeneous surface with different adsorption energies, which provides the possibility of multilayer adsorption with an increase in the MO concentration. The Freundlich isotherm is expressed as follows, qe = kF Ce 1/n


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where kF and n are the Freundlich constants related to the adsorption capacity and adsorption intensity of the adsorbent. The n value being larger than 1 manifests that the adsorption is a favorable process. Larger n value means higher adsorption strength. Scanning Electron Microscopy (SEM). The morphology of the coacervate formed by 5.00 mM 12-6-12 and 0.50 wt % PAM with and without 0.10 mM MO were imaged with a fieldemission scanning electron microscope (Hitachi S-4800). The samples were prepared by freezing a small drop of the coacervate on a clean silica wafer with liquid nitrogen, and the microstructures of the mixtures were well retained. Immediately afterward, the frozen sample was lyophilized under vacuum at about −50 °C. Finally, a 1−2 nm Pt coating completed the sample preparation. 1

H NMR Spectroscopy. 2D NOESY spectra were performed on a Bruker Avance 600-

NMR spectrometer equipped with a 5 mm BBI probe operating at 600 MHz. Experiments were carried out at 25 °C by using standard pulse sequences pulprog noesyphpr in the phase sensitive mode and states-time-proportional phase incremention (States-TPPI).

RESULTS AND DISCUSSION Phase Boundaries of 12-6-12/PAM Mixture. In order to measure the phase boundaries of the 12-6-12/PAM mixtures, turbidity measurements were carried out as a function of the surfactant concentration (C12-6-12) at constant PAM concentrations (CPAM).


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100

a 0.50% PAM 0.30% PAM 0.10% PAM 0.05% PAM 0.01% PAM

100-%T

80 60 40 20 0

0.01

0.1

1

10

100

C12-6-12 (mM) 100

C1

b

C2

C3

100-%T

60 100 40 20 0.50% PAM turbidity Differential turbidity 10 100

0 0.1

1

d(100-%T)/dC

200 80

0

C12-6-12 (mM) 0.5 0.4

c C1

ion rva t Co ace

Redi ssolu tion

0.1

Co mp lex atio n

0.2

C3

ind i

0.3

ng

C2 Mo nom er b

CPAM (g/L)

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0.0 0.1

1

C12-6-12 (mM)

10

Figure 1. (a) Turbidity curves of concentrated 12-6-12 solution being titrated into PAM solutions of 0.01, 0.05, 0.10, 0.30 and 0.50 wt %; (b) Representative turbidity and differential turbidity curves of 12-6-12 with 0.50 wt % PAM; (c) Phase boundaries of the 12-6-12/PAM system with 0.01, 0.05, 0.10, 0.30 and 0.50 wt % PAM.


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Figure 1a shows the turbidity titration curves of 12-6-12 into 0.01, 0.05, 0.10, 0.30 and 0.50 wt % PAM at 25 °C. All the turbidity curves experience a process of increasing and then decreasing. Obviously the maximum turbidity value for the aqueous solution of 12-6-12/PAM at 0.01 wt % is much lower than those at other higher PAM concentrations. Moreover, coacervation was not observed at 0.01 wt % PAM. Differently the mixtures form coacervation over a wide concentration range for the cases of 0.05-0.50 wt % PAM. In the following text, the situation at CPAM = 0.50 wt % is chosen for discussing the phase transition in detail. Three critical concentrations C1, C2, and C3 are determined from the turbidity curve and the differential turbidity curve as demonstrated in Figure 1b. Similar to the situation in our previous work,37 four ranges are formed in this process as a function of C12-6-12 and the schematic representation for the interaction process of 12-6-12 and PAM is illustrated in Figure S1. The first critical concentration C1 is far below the CMC of 12-6-12 (1 mM).43 When C12-6-12 is below C1, the turbidity is very low, which indicates that the 12-6-12/PAM mixture does not form aggregates, where the surfactant monomers may bind with the polymer chain through electrostatic interaction. When C12-6-12 is beyond C1, the turbidity starts to increase, indicating that more surfactant molecules are bound on the chain of PAM and in turn the 12-6-12/PAM complexes assemble into larger aggregates. Herein, the critical concentration C1 refers to the normally mentioned critical aggregation concentration (cac) in surfactant/polyelectrolyte systems, which corresponds to the total concentration of surfactant at the onset of the formation of surfactant aggregates in the presence of polyelectrolytes. With further increasing C12-6-12 to C2, the turbidity sharply increases to very large value and coacervation was observed, which means that the association of the 12-6-12/PAM aggregates leads to the liquid-liquid phase separation. When C12-6-12 is larger than C3, the turbidity becomes smaller again, which means


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that the coacervates are dissolved into soluble complexes with excess surfactant molecules. So coacervation takes place between C2 and C3, which represent the starting point and ending point of coacervation, respectively. The phase boundaries are summarized in Figure 1c and the figure indicates that the C1, C2 and C3 values shift to larger values when CPAM increases from 0.05 to 0.50 wt %, similar to the variation tendency of the cac values in dodecyltrimethylammonium bromide and poly(styrenesulfonate) mixtures.44 This can be interpreted that the coacervation region is around the charge neutralization point between cationic 12-6-12 and anionic PAM, and PAM of higher concentration needs more 12-6-12 molecules to form neutralized complexes and dissolve the coacervate phase. The contribution of electrostatic interaction between ammonium and carboxylate groups and hydrophobic interaction between surfactant alkyl chains lead to the increasing aggregate size of the 12-612/PAM complex, which resembles the situations of cationic monomeric surfactant and gemini surfactant with anionic polyelectrolyte in literatures.45-50 But the difference is that the present system can form coacervate. As shown in Figure 1a and 1c, coacervation only occurs when the PAM concentration is above 0.01 wt % and covers a broader 12-6-12 concentration range at higher PAM concentration. Extraction Efficiency and Selectivity of 12-6-12/PAM Coacervate to Dyes. The 12-612/PAM mixture generates coacervation over a wide surfactant concentration range and thus this system is applied to extract dyes. The C12-6-12-dependent retention efficiency of the 12-6-12/PAM coacervates for 0.03 mM MO is presented in Figure 2a. In the PAM concentration range from 0.05 to 0.50 wt %, the extraction efficiency increases linearly at lower 12-6-12 concentration, and reaches a maximum of more than 95% when the calculated charge ratio of 12-6-12 to PAM is close to


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1:1 in the coacervation range. While further increasing the 12-6-12 concentration, the extraction efficiency decreases, corresponding to the range where the coacervate phase is gradually dissolved. This suggests that the coacervate phase is more efficient in extracting MO than other phases.

100

100

a Extraction Efficiency (%)

Extraction Efficiency (%)

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80 60 40 20 0

0.50% PAM 0.30% PAM 0.10% PAM 0.05% PAM 1

10

b

90 80 70 60

0.50% PAM/3.50 mM 12-6-12 0.30% PAM/2.10 mM 12-6-12 0.10% PAM/0.70 mM 12-6-12 0.05% PAM/0.35 mM 12-6-12

50

C12-6-12 (mM)

0.1

1

CMO (mM)

Figure 2. (a) Extraction efficiency of 0.03 mM MO by the 12-6-12/PAM solution as a function of the 12-6-12 concentration (C12-6-12); (b) Extraction efficiency of MO by the 12-6-12/PAM solution at the calculated 12-6-12/PAM charge ratio of 1:1 in the coacervation range as a function of the MO concentration (CMO).

Subsequently, the 12-6-12/PAM mixtures of different concentrations at the calculated 12-612/PAM charge ratio of 1:1 in the coacervation range are utilized to study the extraction efficiency to MO at different MO concentrations. As shown in Figure 2b, all the curves exhibit a similar tendency. Taking the mixture of 0.50 wt % PAM and 3.5 mM 12-6-12 as an example, when CMO is lower than 0.30 mM, the extraction efficiency is approximately 95%; while CMO is larger than 0.30 mM, more dye molecules are detected in dilute phase with the increase of CMO, and the extraction efficiency decreases, which means that the 12-6-12/PAM coacervate has been saturated by MO. Clearly, increasing the 12-6-12/PAM concentration enhances the


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corresponding saturation concentration of MO solubilized in the coacervate phase. In the extraction of MO by the 12-6-12/PAM coacervate, MO should be mainly solubilized in the hydrophobic microdomain formed by the hydrophobic chains of 12-6-12. Since the 12-6-12/PAM coacervate can efficiently extract anionic dye MO from its own aqueous solution, the selective removal of MO from its aqueous solution in the presence of cationic dye MB is investigated. Figure 3a present the UV-vis spectra of the supernatant phase of the 12-6-12/PAM solution mixed with 0.03 mM MO and 0.03 mM MB at 0.05 wt % PAM and various 12-6-12 concentrations. At the PAM and 12-6-12 concentrations used, the mixed solutions form coacervate. The intensity of the characteristic absorption peak of MO in the supernatant at 460 nm is extremely lower than that of MO in aqueous solution without PAM and 12-6-12, and the peak decreases with the increase of the 12-6-12 concentration. This means that almost all MO (more than 95%) is extracted by the 12-6-12/PAM coacervate phase at higher 12-6-12 concentration. Oppositely, the characteristic absorption band of MB around 665 nm almost does not change with the increase of the surfactant concentration and the intensity is the same as those in the solution only containing 0.03 mM MB with and without the 12-6-12/PAM mixture in the absence of MO (Figure 3b). This indicates that the 12-612/PAM coacervate cannot extract cationic dye MB. Thus, the 12-6-12/PAM coavervate is of selectivity in extracting anionic dye MO. Similarly, the coacervates can also be used to extract other anionic dyes, Acid Red 1 (AR1), Acid Orange 7 (AO7), Congo Red (CR) and Brilliant Crocein (BC) from aqueous solution, and the extraction efficiency is also more than 95% (Figure S2 in Supporting Information). In brief, the 12-6-12/PAM coacervate can selectively and efficiently extract anionic dyes.


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0.7 Abs

0.6

0.7

0.01 mM MO

0.6

0.6

0.4 0.2

0.5

0.30 mM 0.32 mM 0.33 mM 0.35 mM 0.38 mM 0.40 mM

0.2 0.1

500 600 λ (nm)

0.0

700

400

0.4

Abs

400

0.3

0.4 0.2

0.5

0.0

0.4

0.01 mM MB

0.6

b Abs

a

Abs

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0.30 mM 0.32 mM 0.33 mM 0.35 mM 0.38 mM 0.40 mM

0.3 0.2 0.1

0.0

500 600 λ (nm)

700

0.0 350

400

450

500

550

600

650

700

750

350

400

450

λ (nm)

500

550

600

650

700

750

λ (nm)

Figure 3. UV-vis spectra of the supernatant phase (diluted by three times) of the 12-6-12/PAM solution mixed with (a) 0.03 mM MO and 0.03 mM MB, and (b) 0.03 mM MB at 0.05 wt % PAM and various 12-6-12 concentrations marked in the plots. At the PAM and 12-6-12 concentrations used, the mixed solutions form coacervate. The UV-vis spectra inserted are from the aqueous solution of 0.01 mM MO and 0.01 mM MB, respectively.

The selectivity of the 12-6-12/PAM coacervate to anionic dyes is mainly resulted from the charge property of the coacervate. Because the coacervate is viscous, the zeta potential of the coacervate cannot be precisely measured. Thus we can only discuss the charge situation of the coacervate approximately. As shown in Figure 1a, coacervation always occurs when the calculated 12-6-12/PAM charge ratio close to 1:1 or larger than 1. Then the cationic charge number from 12-6-12 in the coacervates is equal or more than that of PAM. Owing to the electrostatic repulsion and micelle geometry, some of the surfactant micelles cannot be locally neutralized by PAM. Therefore the coacervate aggregates carry some cationic charges and in turn the anionic dyes can be extracted in the coacervate phase. Phase Behavior and Dye Extraction Mechanism. To further understand the interaction of anionic dyes with the 12-6-12/PAM mixture, the phase behavior, morphology and molecular packing situation of the 12-6-12/PAM mixture are studied in the presence of MO.


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Figure 4a shows the turbidity variations by titrating concentrated 12-6-12 solution into 0.50 wt % PAM solutions in the presence of 0.05, 0.10, or 1.00 mM MO. The turbidity curve of 126-12 solution titrated into 0.50 wt % PAM in the absence of MO is also included for comparison. Without MO, the 12-6-12/PAM mixture forms coacervate when the 12-6-12 concentration is between 1 and 70 mM. When the MO concentration is 0.05 and 0.10 mM, with the increase of C12-6-12, the turbidity initially keeps a constant and then starts to increase at about 0.01 mM 12-6-12 and reach a maximum at about 0.10 mM 12-6-12, where yellow solid precipitate deposits in the bottom of vials. Beyond the maximum, the precipitate is dissolved, leading to a decrease in turbidity. The mixed solution becomes clear again and the turbidity is approximately a constant close to zero. Afterwards the turbidity starts to increase again with the increase of C12-6-12 and generates a platform between 2 mM and 70 mM of 12-6-12, where coacervation takes place. However, when the MO concentration is 1.0 mM, a larger amount of precipitate is formed. The precipitation region and the following coacervation region are fused. This variation can be ascribed to the fact that cationic 12-6-12 strongly binds with anionic MO molecules at low 12-6-12 concentration and form precipitates immediately upon the addition of MO. Then, the MO molecules are incorporated into the precipitate at the maximum of the turbidity curve, and the precipitate is gradually solubilized by excess 12-6-12 molecules. More 12-6-12 molecules are needed to neutralize the anionic charges of both MO and PAM at higher MO concentration. Normally dyes are at a concentration of 10-50 mg/L in dyestuff effluent,5153

so the present 12-6-12/PAM coacervate shows great potential in effectively extracting

anionic dyes in practical dyestuff effluent.


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Figure 4. (a) Turbidity curves of concentrated 12-6-12 solution titrated into the mixed

solutions of 0.50 wt % PAM and MO of 0, 0.05, 0.10 and 1.00 mM; (b) The SEM image of the coacervate formed by 5.00 mM 12-6-12 and 0.50 wt % PAM in the presence of 0.10 mM MO (bar = 2 µm); (c) The SEM image of the coacervate formed by 5.00 mM 12-6-12 and 0.50 wt % PAM without any dyes (bar = 1 µm).

Figure 4b and 4c shows the morphologies of coacervates formed by the mixtures of 5.00 mM 12-6-12 and 0.50 wt % PAM with and without MO. Both the SEM images exhibit connected network-like structures, while 0.50 wt % PAM only form spherical aggregates without 12-6-12 as shown in Figure S3. In particular, the images indicate that the network-like structure of coacervate is reinforced by MO (Figure 4b) compared with the 12-6-12/PAM coacervate without MO (Figure 4c). This suggests that MO joins the formation of the coacervate while it is extracted. In order to characterize the molecular packing situation of MO and 12-6-12 in coacervate, NOESY experiment is carried out. Figure 5 shows the NOESY spectrum of the MO/12-612/PAM mixtures at CMO = 0.10 mM, C12-6-12 = 2.50 mM, and CPAM = 0.50 wt %, located in the coacervation region. Upon the addition of 0.10 mM MO, the cross peaks appear between the protons of MO (H2, H3, H4, H5) and those of 12-6-12 (Hbh, Hdf, He). The cross peaks of Hbh-H2 and Hbh-H345 indicate a close spatial relationship between the benzene ring of MO and the


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hydrophobic tails of 12-6-12, while the cross peaks of He-H2, He-H345 and Hdf-H345 imply that the protons at the benzene ring of MO molecules are close to the protons in the headgroups and the spacer groups of 12-6-12. These NOESY results show that some of the MO molecules are solubilized in the coacervate with the benzene ring inserting into the hydrophobic region of the surfactant micelle core, whereas other MO molecules are located at the surface of the micelles, as shown in the cartoon of Figure 5. So the possible mechanism is that the 12-6-12 micelles form complexes with PAM and the association of the complexes results in the network-like structure of the coacervate, while the MO molecules are solubilized into the surfactantpolyelectrolyte complexes possibly through hydrophobic interaction, electrostatic interaction and π-cation interaction and in turn strengthen the network-like structure of the coacervate. Adsorption and Desorption of MO on Modified Quartz Sand and Melamine Foam by 12-6-12/PAM Coacervate. To facilitate the application of the 12-6-12/PAM coacervate,

quartz sand and melamine foam are selected to be modified with coacervates for adsorbing MO from aqueous medium. Figure 6 shows the adsorption and desorption process of MO on the quartz sand modified by the coacervate generated in the aqueous solution of 3.50 mM 12-6-12 and 0.50 wt % PAM. In a typical experiment, quartz sand was added to the 12-6-12/PAM coacervate at nearly 1:1 volume ratio and the mixture was stirred for 12h. After that, the modified quartz sand was dried in an oven. The loading amount of 12-6-12/PAM coacervate by quartz sand is 4.78 mg/g. Then 7 mL, 0.10 mM MO aqueous solution was added to the modified quartz sand of 18 g and the mixture was stirred for a few minutes. The color change of the MO solution and the sand indicates that MO is effectively adsorbed on the modified quartz sand from the solution. For a comparison purpose, unmodified quartz sand and the quartz sand modified by 3.50 mM 12-6-


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12 or 0.50% PAM were also used to extract MO. Obviously these different quartz sands display very different adsorbing ability to MO. As shown in the UV-vis spectra, the residual amount of MO in the aqueous solution is the least after adsorption by the quartz sand modified by the 12-6-12/PAM coacervate. Although the quartz sand modified by 3.50 mM 12-6-12 can also adsorb MO, the residual amount of MO is about three times higher than that by the quartz sand modified by the 12-6-12/PAM coacevate. As to the unmodified quartz sand and the modified quartz sand by 0.50 wt % PAM, the residual amounts of MO are almost the same as that of the original MO solution. Therefore the 12-6-12/PAM modified quartz sand shows the strongest ability to adsorb MO from its solution. In this process, the 12-6-12/PAM complexes with the coacervate structure are adsorbed on the negatively charged surface of quartz sand, and the MO molecules are adsorbed on the modified sands through binding with the 12-612/PAM complexes by means of the cooperation of electrostatic interaction, hydrophobic interaction and π-cation interaction. These results further confirm that the large network structure of coacervate benefits the adsorption of MO.

Figure 5. NOESY spectrum of the MO/12-6-12/PAM mixtures at 0.50 wt % PAM, CMO = 0.10

mM, and C12-6-12 = 2.50 mM in D2O and the possible molecule packing in the coacervate.


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Figure 6. Top: Photographs of adsorption process of 0.10 mM MO aqueous solution being

added in the quartz sand modified by the coacervate of 3.50 mM 12-6-12 and 0.50 wt % PAM, and the subsequent desorption process by adjusting the pH to 3.0 with HCl. Bottom: The UVvis spectra of the aqueous solution (diluted by three times) after adsorption by the unmodified quartz sand and the modified quartz sands with 0.50 wt % PAM, 3.50 mM 12-6-12, or the mixture of 3.50 mM 12-6-12 and 0.50 wt % PAM, respectively, of which the situations after adsorption process are shown in the insets from right to left. The UV-vis spectrum of 0.10 mM MO solution diluted by three times is also included.

In order to recycle the dye and reuse the quartz sand, the adsorbed MO molecules are released from the quartz sand by changing the pH from 7.0 to 3.0. The image on the top of Figure 6 shows that MO can be released from the quartz sand at pH 3.0, where the color of the solution becomes dark orange while the color of the sand becomes light. The main reason is that the 12-6-12/PAM system cannot form coacervate at pH 3.0 as shown in Figure S4 of


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Supporting Information. At acidic condition, the carboxylic groups of PAM and the sulfonic group of MO are neutralized due to the protonation, thus the electrostatic interactions of PAM and MO with 12-6-12 are weakened, suppressing the 12-6-12/PAM coacervation and the MO extraction in the 12-6-12/PAM complexes. Thereby both the dye and the quartz sand can be recycled. The kinetics of adsorption is one of the important characteristics depicting the uptake rate of solute and represents the efficiency of adsorbent. Figure 7a shows the adsorbed amount of MO from its aqueous solution by per gram coacervate-modified quartz sand as a function of contact time. In order to elucidate the rate of MO adsorption on the coacervate-treated quartz sand, the pseudo-first-order and pseudo-second-order kinetic models are applied to simulate the experimental data. The adsorption rate constants k1 and k2, the equilibrium adsorption capacity qe,calc as well as the correlation coefficients R2 are summarized in Table 1. As shown in Figure 7a, the adsorbed amount of MO increases rapidly in the initial time and the process achieves equilibrium in 2.5 mins. By contrasting the parameters of the two models, the correlation coefficient is larger for the pseudo-second-order model and the qe,cal value shows a good compliance with the experimental data. So the pseudo-second-order kinetic model displays a better correlation with the adsorption of MO, implying that the adsorption process is chemisorption, herein mainly involving electrostatic interaction. Figure 7b present the isotherm relationship between the equilibrium adsorption capacity qe on the modified quartz sand and the equilibrium concentration Ce of MO. The equilibrium adsorption capacity qe increases sharply at low MO concentration, and the maximum adsorption capacity qm is achieved with the increase of concentration. Both Langmuir model and Freundlich model are employed to describe the adsorption isotherm curve and the equation


21

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constants derived are illustrated in Table 1. Obviously, the Langmuir model is more fitting the experimental results than the Freundlich model, indicating that the modified quartz sand owns homogeneous surface. The maximum adsorption constant qm of MO on the coacervatemodified quartz sand is 1.64 mg/g. As the loading amount of the 12-6-12/PAM coacervate onto quartz sand is 4.78 mg/g, the maximum adsorption constant qm can be converted to the adsorption ability of the coacervate phase to MO with a value of 344.5 mg/g, i.e, per gram coacervate adsorbed on quartz sand can adsorb 344.5 mg MO. The coacervate phase plays an important role in the adsorption process.

a

0.08

b 1.6

0.06 Experimental data Pseudo-first-order model Pseudo-second-order model

0.04

0.02

0.00

qe (mg/g)

1.2

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

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Experimental data Langmuir model Freundlich model

0.8

0.4

0

5

10

15

20

25

30

0.0 0.0

0.5

t (min)

1.0

1.5

2.0

2.5

Ce (mM)

Figure 7. (a) Effect of time on the amount of MO adsorbed by per gram quartz sand modified

by 3.50 mM 12-6-12/0.50 wt % PAM (CMO = 0.10 mM). Data points are experimentally obtained. Line and dash line correspond to the fitting curves by the pseudo-first-order kinetic equation and pseudo-second-order kinetic equation. (b) Adsorption isotherms of MO onto quartz sand modified by 3.50 mM 12-6-12/0.50 wt % PAM. The solid points represent experimental results and the line and dash line represent the curves simulated by Langmuir model and Freundlich model.


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Table 1. Kinetic parameters and isotherm constants for the MO adsorption on the quartz sand

modified by the coacervate formed with 3.50 mM 12-6-12 and 0.50 wt % PAM. Pseudo-second-order

Pseudo-first-order Kinetic Model

qe,calc (mg·g-1)

k1 (g·mg-1·min-1)

R2

qe,calc (mg·g-1)

k2 (g·mg-1·min-1)

R2

0.0776

3.568

0.809

0.0780

389.4

0.936

Langmuir Isotherm Model

Freundlich

qm (mg·g-1)

kL (L·mM-1)

R2

kF

n

R2

1.640

67.660

0.860

1.538

5.231

0.692

Melamine foam is used as another solid phase to replace quartz sand in order to extract and recycle MO more conveniently. Melamine foam, made up of formaldehyde-melamine-sodium bisulfite copolymer, is of high-porosity, low density, three-dimensional open-cell structure, good elasticity and excellent water and oil sorption capacity. In this study, melamine foam is modified by the coacervate phase so as to adsorb MO. The melamine foam was cut into small pieces and immersed in nearly equal volume coacervate formed by 3.50 mM 12-6-12 and 0.50 wt % PAM for 12h. Subsequently, the melamine foams were dried and the loading amount of 12-6-12/PAM coacervate by melamine foam is 826 mg/g. The CHN analysis results of the 126-12/PAM modified melamine foam as well as the unmodified one are depicted in Table 2. The results manifest that the value of N composition is reduced by 3.94% under the treatment of coacervates, while C and H compositions are enhanced by 3.19% and 0.58% in the modified melamine foam, respectively. Thus, the changes of elemental contents provide direct evidence for the adsorption of the 12-6-12/PAM complexes, thereby demonstrating the validity of the treating process.


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Table 2. Elemental analysis of melamine foam unmodified or modified by the coacervate

formed by 3.50 mM 12-6-12 and 0.50 wt % PAM. Elemental Analysis

Unmodified Melamine Foam

Melamine Foam modified by 0.50% PAM/3.50 mM 12-6-12

N (%)

45.38

41.44

C (%)

34.65

37.84

H (%)

4.74

5.32

The top pictures in Figure 8 show the adsorption and desorption of MO on the melamine foam modified by the 12-6-12/PAM coacervate. By adding 0.05 mM MO at pH 7.0, the MO was sufficiently adsorbed onto the modified melamine foam in a few minutes and the adsorbent exhibits yellow color. Squeezing melamine foam almost does not change the yellow color of the melamine foam with the nearly clean solution deposited in the bottom of conical flask. The corresponding UV-vis spectrum of the MO solution after the squeezing process is shown in Figure 8. In order to compare the effects of the coacervate on the adsorption capacity to MO, the unmodified melamine foam and the melamine foam modified by 3.50 mM 12-6-12 or 0.50 wt % PAM are also used to extract MO and the corresponding UV-vis spectra of the solutions after the squeezing process are also presented in Figure 8. Similar to the results of quartz sand, the lowest characteristic adsorption of MO indicates that the melamine foam modified by the 12-6-12/PAM coacervate shows the best adsorption capacity for MO, of which the absorbance peak is much smaller than those of the MO solutions after treated with the unmodified melamine foam and the melamine foam modified by 12-6-12 or PAM. So the modification with the 12-6-12/PAM coacervate is the crucial reason for the outstanding ability in adsorbing MO. Afterwards, by adjusting the pH to acidic condition with adding HCl, MO is removed from the melamine foam. Thus the melamine foam and the dye can also be resumed.


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Figure 8. Top: Photographs of adsorption process of 0.05 mM MO onto the melamine foam

modified by the coacervate formed with 3.50 mM 12-6-12 and 0.50 wt % PAM at pH 7.0, and the subsequent desorption process by changing the pH to 3.0. Bottom: UV-vis spectra of the MO aqueous solution after adsorption by the unmodified melamine foam and the modified melamine foams by the 12-6-12/PAM coacervate formed with 0.50 wt % PAM and 3.50 mM 12-6-12, 0.50 wt % PAM, or 3.50 mM 12-6-12, respectively.

The adsorption kinetic and isotherm are also investigated for the melamine foam treated by the 12-6-12/PAM coacervate as shown in Figure 9. By fitting the results to the kinetic models and isotherm models, the parameters obtained are summarized in Table 3. Figure 9a indicates that the adsorption of MO onto the modified melamine foam increases rapidly with time and reaches equilibrium after 60 mins. On the basis of the correlation coefficients R2, the experimental kinetic data are much better fitted by the pseudo-second-order model as quartz sand does. Figure 9b shows that the adsorption isotherm curve fits the Langmuir model. The maximum adsorption constant qm is 146.8 mg/g, i.e, per gram coacervate-modified melamine foam can adsorb 146.8 mg dye. Considering that the amount of coacervate loaded on melamine


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foam is 826 mg/g, the adsorption ability of the coacervate phase to MO is 324.5 mg/g, per gram coacervate adsorbing 324.5 mg MO. This suggests that the coacervate modified melamine foam is a very strong absorbent to anionic dyes. 1.2

a

150

b

0.8

Experimental data Pseudo-first-order Pseudo-second-order

qe (mg/g)

125

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

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Experimental data Langmuir model Freundlich model

100

0.4

75 50 25

0.0

0

20

40

60

80

100

0

120

0.0

0.2

0.4

0.6

t (min)

0.8

1.0

1.2

1.4

1.6

Ce (mM)

Figure 9. (a) Effect of time on the amount of MO adsorbed by per gram melamine foam

treated by the coacervate formed with 3.50 mM 12-6-12 and 0.50 wt % PAM (CMO = 0.02 mM). Data points are experimentally obtained. Line and dash line correspond to the fitting curves by the pseudo-first-order kinetic equation and pseudo-second-order kinetic equation. (b) Adsorption isotherms of MO onto melamine foam treated by the 12-6-12/PAM coecervate. The solid points represent experimental data, and the line and dash line represent the curves simulated by Langmuir model and Freundlich model.

Table 3. Kinetic parameters and isotherm constants of the MO adsorption on the melamine

foam modified by the coacervate formed with 3.50 mM 12-6-12 and 0.50 wt % PAM. Pseudo-first-order Kinetic Model

-1

-1

Pseudo-second-order -1

2

-1

qe,calc (mg·g )

k1 (g·mg ·min )

R

qe,calc (mg·g )

k2 (g·mg-1·min-1)

R2

1.057

0.227

0.941

1.144

0.274

0.978

Langmuir Isotherm Model

-1

Freundlich -1

2

qm (mg·g )

kL (L·mM )

R

kF

n

R2

146.8

221.9

0.876

146.2

4.978

0.737

In brief, the high adsorption efficiency endows the coacervate-modified quartz sand and melamine foam with great potential for removing anionic dye MO from wastewater with the


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maximum equilibrium adsorption capacity (qm) of 1.64 and 146.8 mg/g, respectively. By counting the coacervate as the valid adsorbent, the maximum adsorption capacities of the coacervates to MO are 344.5 mg/g and 324.5 mg/g for the coacervate on quartz sand and melamine foam, respectively. That is to say, the coacervates on the two solid supports show almost the same adsorption capacity to MO. But the two solid supports show differences in the convenience while adsorbing and recycling the dye. Anyway, either the adsorbents or the dye can be recycled in acidic condition. The easy-handle and cost-effective adsorption and desorption procedures provide a feasible method for the wastewater remediation.

CONCLUSIONS

This work has studied the coacervation generated in the mixed aqueous solution of cationic ammonium gemini surfactant 12-6-12 and 10% hydrolyzed PAM as well as its application in the extraction of dyes. The 12-6-12/PAM coacervate with a large network structure is formed over a wide concentration range of surfactant and polyelectrolyte, and shows great efficiency in extracting anionic dyes from water due to the cooperation of hydrophobic, electrostatic and π-cation interactions. The anionic azo dye MO molecules in the coacervate phase are solubilized in the hydrophobic domain of the 12-6-12/PAM complexes and at the surface of the micelles in the complexes. In addition, the dye joins the coacervate and strengths the network structure of the coacervate. Due to the presence of excess surfactant molecules, the coacervate phase turns out to be selective for removing anionic MO coexisting with a cationic dye MB. Furthermore, the quartz sand and melamine foam modified by the coacervate can serve as ideal adsorbents for the removal of MO from aqueous medium. The adsorption kinetic data are well


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explained by the pseudo-second-order adsorption model, and the adsorption isotherm follows the Langmuir model. The derived maximum equilibrium adsorption capacity (qm) values are 1.64 and 146.8 mg/g for modified quartz sand and melamine foam, respectively. The maximum adsorption capacities of the coacervates to MO are 344.5 mg/g and 324.5 mg/g for the coacervates on quartz sand and melamine foam, respectively. Moreover, the dye can be easily released from the dye-adsorbed quartz sand and melamine foam by adding acid, and thus both the solid adsorbents and the dye can be recycled. The whole procedure is simple and costeffective, and can be used to extract all anionic dyes from water in principle. Thus the approach has a great potential in practical treatment of dye-contaminated wastewater. ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. Schematic illustration for the 12-6-12/PAM interaction; Extraction efficiency of other anionic dyes (AR1, AO7, CR and BC); SEM image of PAM aggregates; Turbidity curves of the 12-6-12 solution being titrated into the PAM solution at different pH (PDF). AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Y.W.). Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT

This work was supported by National Natural Science Foundation of China (21633002) and Beijing National Laboratory for Molecular Sciences (BNLMS).

REFERENCES

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(9) Crini, G. Non-Conventional Low-Cost Adsorbents for Dye Removal: A Review. Bioresour. Technol. 2006, 97, 1061-1085. (10) Pearce, C. I.; Lloyd, J. R.; Guthrie, J. T. The Removal of Colour from Textile Wastewater Using Whole Bacterial Cells: A Review. Dyes Pigments 2003, 58, 179-196. (11) Wang, M.; Wang, Y. L. Development of Surfactant Coacervation in Aqueous Solution. Soft Matter 2014, 10, 7909-7919. (12) Armstrong, D. W. Micelles in Separations: Practical and Theoretical Review. Sep. Purif. Methods 1985, 14, 213-304. (13) Ballesteros-Gómez, A.; Sicilia, M. D.; Rubio, S. Supramolecular Solvents in the Extraction of Organic Compounds. A Review. Anal. Chim. Acta 2010, 677, 108-130. (14) Hinze, W. L.; Pramauro, E. A Critical Review of Surfactant-Mediated Phase Separations (Cloud-Point Extractions): Theory and Applications. Crit. Rev. Anal. Chem. 1993, 24, 133177. (15) Rubio, S.; Pérez-Bendito, D. Supramolecular Assemblies for Extracting Organic Compounds. Trends Anal. Chem. 2003, 22, 470-485. (16) Melnyk, A.; Wolska, L.; Namieśnik, J. Coacervative Extraction as a Green Technique for Sample Preparation for the Analysis of Organic Compounds. J. Chromatogr. A 2014, 1339, 112. (17) Watanabe, H.; Tanaka, H. A Non-Ionic Surfactant as a New Solvent for Liquid-Liquid Extraction of Zinc(II) with 1-(2-Pyridylazo)-2-Naphthol. Talanta 1978, 25, 585-589. (18) Tatara, E.; Materna, K.; Schaadt, A.; Bart, H.-J.; Szymanowski, J. Cloud Point Extraction of Direct Yellow. Environ. Sci. Technol. 2005, 39, 3110-3115.


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Table of Contents Graphic


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