Adsorption of anionic surfactants from aqueous solution by high content of primary amino crosslinked chitosan microspheres

Adsorption of anionic surfactants from aqueous solution by high content of primary amino crosslinked chitosan microspheres

International Journal of Biological Macromolecules 97 (2017) 635–641 Contents lists available at ScienceDirect International Journal of Biological M...

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International Journal of Biological Macromolecules 97 (2017) 635–641

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac

Adsorption of anionic surfactants from aqueous solution by high content of primary amino crosslinked chitosan microspheres Caihong Zhang, Haifeng Wen ∗ , Yingying Huang, Wenjian Shi School of Environment and Architecture, University of Shanghai for Science and Technology, 516 Jungong Road, 200093 Shanghai, China

a r t i c l e

i n f o

Article history: Received 14 October 2016 Received in revised form 21 December 2016 Accepted 19 January 2017 Available online 21 January 2017 Keywords: Crosslinked chitosan microspheres Anionic surfactants Adsorption

a b s t r a c t High content of primary amino crosslinked chitosan microspheres (ACCMs) were synthesized and characterized with IR, XRD and SEM technologies. Subsequently, ACCMs were adopted to adsorb three common anionic surfactants from aqueous solution: sodium dodecyl benzene sulfonate (SDBS), sodium lauryl sulfate (SLS), and sodium dodecyl sulfonate (SDS). The adsorption performances were evaluated based on different variables such as the pH, contact time, temperature and initial concentration of the anionic surfactants. Moreover, the adsorption were investigated with kinetic models, equilibrium isotherms and thermodynamic models. The experimental results indicated that the adsorption processes were fitted very well with a pseudo-second-order model. The adsorption isotherms could be better described by Langmuir model rather than Freundlich model. The adsorption of SDBS was a spontaneous, exothermic process. While the adsorption of SLS and SDS were spontaneous, endothermic. The adsorption processes were complex physical-chemistry adsorption models, which are dominated by physisorption. Furthermore, this study found that the material had strong absorption abilities for anionic surfactants, the saturation adsorption capacity of ACCMs were 1220 mg/g for SDBS, 888 mg/g for SLS, and 825 mg/g for SDS at pH 3.0 and 298 K, respectively. The adsorption capacity was reduced only 5.7% after 8 cycles of the adsorption-desorption processes. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Many industries, including textile industry, catering industry, tanneries, chemical factories, and our daily life release a great amount of wastewater containing anionic surfactants. Due to the hydrophilic and hydrophobic groups on the molecule, anionic surfactant molecules could accumulate and generate an isolation layer under the water surface. This would limit the reoxygenation process and reduce the dissolved oxygen in water. Besides, the anionic surfactants containing effluents could apparently increase the solubility of hydrophobic persistent organic pollutants, such as PCBs, lindane, and PAHs [1,2], and result in acute and chronic toxicity [3]. Therefore, the overloaded anionic surfactants are regarded as a serious threat to the water ecological balance. Multiple techniques have been applied to water treatment in terms of anionic surfactants containing effluents, such as adsorption [4], microbial degradation [5], electrochemical degradation [6], photoeletrocatalytic oxidation [7] and so on [8]. Among these methods, adsorption

∗ Corresponding author. E-mail address: [email protected] (H. Wen). http://dx.doi.org/10.1016/j.ijbiomac.2017.01.088 0141-8130/© 2017 Elsevier B.V. All rights reserved.

has its own advantage based on its high efficiency of treatment, simple operation procedure and commercial available adsorbent. Chitin is one of the most abundant natural polymer in the terrestrial biosphere. And it is a kind of renewable and low-cost material. Chitosan was prepared after de-acetylation of chitin. Previous researches found that chitosan had enormous potential to adsorb pollutions, such as heavy metal [9] and dyes [10]. Furthermore, under acid conditions, chitosan had high adsorption capacity to anionic surfactant due to the electrostatic attraction between anions and the protonated amino groups [4]. However, chitosan was soluble in acid and had low mechanical strength and small surface area, limiting its adsorption performance. To overcome these drawbacks, chitosan microspheres were synthesized with self-crosslinking reactions between the chitosan molecules. Besides, Schiff-base was composited [11] to protect the active sites of adsorption. There are a large number of researches focused on the adsorption of heavy metal and dye onto chitosan [9,10,12–14]. But studies of anionic surfactants are very scarce and are limited to SDBS [4,13]. No information is available about different anionic surfactants. This paper firstly evaluated the adsorption of chitosan for a class of anionic surfactants: sodium dodecyl benzene sulfonate (SDBS), sodium lauryl sulfate (SLS), and sodium dodecyl sulfonate (SDS).

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Fig. 1. SEM images of chitosan (a) and ACCMs (b and c).

With studying the adsorption equilibrium, kinetics and thermodynamics processes, the adsorption mechanism of anionic surfactants on ACCMs was elucidated. Moreover, the performance of adsorbent was evaluated in terms of the saturated adsorption capacity and reusability. 2. Experimental 2.1. Chemicals Chitosan in powder form (deacetylation degree of 85%–95%) and methylene blue (indicator grade) are commercially available from Shanghai Chemical Reagent Co., China. Basic brilliant blue BO (C.I. 42595) can be purchased from Shanghai Santai Dyestuff Chemical Factory. Liquid paraffin, Span 80 and Triton X-100 were chemical grade. Other reagents (SDBS, SLS, SDS, formaldehyde, epichlorohydrin, HCl, H2 SO4 , acetic acid, NaOH, etc.) were analytical grade. These chemicals were purchased from Shanghai Chemical Reagent Co., China. 2.2. Preparation of high amino ACCMs The amino radical ACCMs were synthesized by the inverse suspension method [15]. The synthetic process could be described by Fig.s1 briefly. Firstly, Formaldehyde, precrosslinker, reacted with amino groups of chitosan to generate the Schiff-base to shield the active adsorb sites of adsorbent. Then, the chitosan molecules were connected with each other by epichlorohydrin (ECH). Finally, the Schiff-base was removed by HCl solution, and the primary amine of chitosan molecules was generated. Chitosan (5.0 g) was dissolved in 100 mL of 4.0% (v/v) acetic acid solution; and then 100 mL liquid paraffin was added. After the mixture was stirred for 10 min, three drops of emulsifier span-80 was added into it. The suspension was stirred and emulsified for 10 min at 50 ◦ C. After that, 6.0 mL formaldehyde was added, and the mixture was stirred for 1 h at 60 ◦ C. The temperature was increased up to 70 ◦ C, the pH value of the mixture was adjusted to 10 with addition of 5.0% (w/v) NaOH solution. Extra 3.0 mL 37% (w/v) ECH was added into the chitosan suspension. Right after that, the NaOH solution was added dropwise continually to maintain the pH value of the mixture to be around 10. Meanwhile the mixture was stirred for 3 h. The prepared chitosan derivative was filtered and purified for 24 h by extraction with the petroleum ether in a Soxhlet apparatus. The derivative was then soaked in 0.1 mol/L HCl solution for 9 h. It was filtered and rinsed by NaOH solution and deionzed water. Finally, the chitosan particles was dried in the air oven at 40 ◦ C.

2.3. Characterization ACCMs and chitosan were characterized on an X-ray diffractometer (XRD) (D8 ADVANCE, Bruker Corporation, the Germany) with Cu K␣ radiation (␭ = 1.5419 Å). The scanning electron microscopy (SEM) images were taken with a scanning electron microscopy (SEM, Tescan vega3,operated at 10 kV). The Fourier transform infrared (FT-IR) spectra of ACCMs and chitosan were tested with a Nicolet 6700 spectrometer within the range of 400–4000 cm−1 . 2.4. The methods of measuring the concentration of the anion surfactant solution The concentration of SDBS and SLS solution were measured with alkaline brilliant blue BO spectrophotometry [16]. 5.0 mL 0.2 mmol/L alkaline brilliant blue BO solution, 5.0 mL pH buffer solution (0.6 mol/L H3 PO4 -0.6 mol/L NaH2 PO4 ), distilled water, a certain volume of test solution, and 1.0 mL 0.2% (w/v) emulsifier OP solution were one by one in order added into a 50 mL volumetric flask and diluted with water to the exact volume. The absorbance of the solution was measured in 1.00 cm quartz cells at 570 nm after 5 min by 732N type visible spectrophotometer (Shanghai Jingke Industrial Co., China). The concentration of the SDS solution was measured by methylene blue spectrophotometry [17]. 2.5. Determination of saturation adsorption capacity of ACCMs It started with distribution of 100 mL of the anionic surfactants solutions with different initial concentrations (10, 20, 50,100, 200, 300, 400, 500, 1000, 2000 mg/L) into different 250 mL closed Erlenmeyer flasks. The pH of the solution was adjusted to a value of 3.0 ± 0.2 with 0.5 mol/L H2 SO4 , and then 50 mg ACCMs was added. The flasks were placed into a water bath and shaken at 120 rmp and under 25 ◦ C for 24 h. From the above methods, the equilibrium concentration of anionic surfactants can be determined. The adsorption capacity, qe (mg/g) of the adsorbent for anionic surfactants at equilibrium status was calculated with following Eq. (1): qe =

(c0 − ce ) V m

(1)

Where c0 and ce (mg/L) represent the concentration of the anionic surfactants at initial and equilibrium status, respectively. V (L) is the volume of anionic surfactants solution, and m (g) is the mass of adsorbent.

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Fig. 3. XRD of the chitosan (a) and ACCMs (b). Fig. 2. IR spectra of chitosan (a) and ACCMs (b).

3. Results and discussion 3.1. ACCMs characterstics 3.1.1. Sem The images of chitosan and ACCMs are showed in Fig. 1. The raw chitosan was a thick platy large particle in Fig. 1(a). It was observed in Fig. 1(b) and (c) that ACCMs was spherical particles with diameter of 50 ␮m. Therefore, ACCMs provided a high surface area and a large number of adsorption sites. 3.1.2. IR spectra Fig. 2a and b exhibit the FTIR spectra of the dried chitosan and ACCMs. In Fig. 2a, a broad band with strong intensity in 3437 cm−1 is assigned to O H bond stretching as well as N H bond stretching [18]. The peak near 1631 cm−1 refers to the amide I band and the vibration mode of N H bending. Three bands at 1160 cm−1 , 1073 cm−1 and 1034 cm−1 are corresponded to C O C stretching vibration, C O stretching vibration of secondary hydroxyl on chitosan C3 and primary hydroxyl on chitosan C6 [19]. From Fig. 2b, the overlapping peak of the amide I band and primary amine in 1631 cm−1 became shaper and weaker, along with the “red shift” to longer wavelength. The absorption peak of secondary amine appeared in 1465 cm−1 . Moreover, the intensity was reduced of the absorption peak of hydroxyl in 1073 cm−1 and 1034 cm−1 . These changes indicated that the primary amine on chitosan C2 had reacted with ECH and generated secondary amine [12], and the hydroxyl group on chitosan had involved in the crosslinking reaction as well.

ratios. 50 mg ACCMs was added into the solution. These flasks were placed in a water bath at 35 ◦ C and shaken for 48 h at 120 rmp. The experimental results are plotted in Fig. 4. It was notable that ACCMs had higher adsorption capacity for the three adsorbates in acidic solution than in neutral and alkaline solution. The adsorption capacity was largest at pH value of 3.0. Therefore, the pH 3.0 ± 0.2 was selected as the optimistic environment for the entire research. The acidity of solution had a significant influence on the adsorption of the anionic surfactants onto ACCMs, where the amino groups of chitosan are protonated and positively charged; And sulfonate group and sulfate group are typical strong acid group, therefor even under the acidic condition which would not easily hydrolyze with water, and surfactants will be fully in their anionic form. Due to the electrostatic interactions between the amino cation and reactive sulfonate groups of anionic surfactants, anionic surfactants were adsorbed on ACCMs. ACCMs had the best performance of adsorption on the anionic surfactants at the pH of 3.0. With the decrease of the pH value, the electrostatic forces between amino cation and reactive sulfonate groups became stronger and the amount of removal of anionic surfactants increased [21]. However, the adsorption capacity decreased with further decrease of pH. Since there were a great number of H+ in the acidic solution, and the solubility of sulfonate anion decreased. Moreover, the active group (−NH3 + on ACCMs) was surrounded by such anions as SO4 2− , the shielding effect reduced the adsorption of anionic surfactants.

3.1.3. XRD The XRD of the dried chitosan and ACCMs are illustrated in Fig. 3. The XRD of chitosan (Fig. 3a) had two characteristic peaks, located at 12◦ and 20◦ . The peaks demonstrated that the raw chitosan was semicrystalline [20]. The diffraction peak (2␪ = 20◦ ) of ACCMs is broadened and the intensity is reduced. The crystallinity of ACCMs were decreased since the crosslinking reaction had breakdown the order of chitosan molecular chain. 3.2. Effect of pH Three groups of anionic surfactants (SDBS, SLS, SDS) solution with 200 mg/L concentration were prepared. And 100 mL these solution were add into different 250 mL closed Erlenmeyer flasks separately. The pH value of the solutions were adjusted around (1.0–9.0) with 1.0 mol/L NaOH and 0.5 mol/L H2 SO4 at different

Fig. 4. Adsorption of different anionic surfactants at different pH.

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Fig. 5. Adsorption kinetic curve of SDBS.

Fig. 6. Adsorption kinetic curve of SLS.

3.3. Effect of initial concentration The effect of initial concentration on the equilibrium adsorption was studied and followed by the experimental steps in “Determination of saturation adsorption capacity of ACCMs”. The adsorption isotherms of ACCMs (Fig.s2) showed that the equilibrium concentration increased along with the increase of adsorbate’s initial concentration under water phase. However, after the initial concentration increased to a certain value, the adsorption did not increase anymore. When initial concentration of SDBS, SLS and SDS were higher than 1.00, 0.500, 0.500 g/L, respectively, the adsorption reached saturation. The corresponding adsorption capacity were 1220, 888, 825 mg/g (or 3.50, 3.08 and 3.03 mmol/g), respectively. In previous studies [4,22], the saturated adsorption capacities of some chitosan based materials to SDBS were ranged from 150 mg/g to 610 mg/g. Compared with their materials, ACCMs produced in this work had big adsorption capacity. Therefore, ACCMs was proved to be an excellent adsorbent to remove anionic surfactants from aqueous solutions. 3.4. Kinetics analyses 3.4.1. Adsorption dynamics curves Several samples (100 mL of 400 mg/L SDBS, 200 mL 200 mg/L SLS, 100 mL 200 mg/L SDS) were added to the 250 mL closed Erlenmeyer flasks. The pH was adjusted to 3.0 ± 0.2. And then 50 mg ACCMs added. The flasks were placed in a water bath and shaken at 120 rmp. In order to fit the environment temperature, the adsorption temperature was set around 278 K–308 K. The experimental data are shown in Figs. 5–7. When the temperature increased from 278 K to 308 K, the adsorption of SDBS decreased slightly. However the adsorption to SLS and SDS increased to a small amount. The effect of contact time on the adsorption of anionic surfactants by chitosan was evaluated at a time period of 6 h. All these adsorption processes were rapid during the initial 60 min, and then gradually slowed with adsorption proceeded. The rapid increase of the adsorption capacity in the initial stages is because of the existence of enormous vacant active sites in the adsorbent surface. Along with the time passes, these active sites were filled gradually by the adsorbate, which finally led to a saturated adsorbent surface [23]. As shown in Figs. 5–7, the adsorption processes of three anionic surfactants reached 50% in first 5–10 min; while the adsorption processes reached the equilibrium around 240 min. 3.4.2. Adsorption kinetics models Two kinetic models, i.e. the Lagergren pseudo-first order model (Eq. (2)) and the pseudo-second order model (Eq. (3)), were adopted

Fig. 7. Adsorption kinetic curve of SDS.

to evaluate the experimental adsorption procedure of anionic surfactants onto ACCMs. ln (qe − qt ) = lnqe − k1 t

(2)

t 1 t = + qt qe k2 × q2e

(3)

Where qe (mg/g) and qt (mg/g) are the amounts of anionic surfactants adsorbed onto ACCMs at equilibrium and at various times t (min), k1 (1/min) and k2 (g/(mg min)) are the rate constant for the pseudo-first and −second order kinetic equation, respectively. The slope (-k1 ) and intercept (lnqe ) of plot (ln(qe −qt )) versus t were used to determine the pseudo-first order constant k1 and the equilibrium adsorption capacity qe,cal . The slope (1/qe ) and intercept (1/(k2 · q2e )) of plot (t/qe ) versus t were used to calculate the parameters of k2 and qe,cal (Figs. 5–7). The generated kinetic constants are listed in Table 1. Similar to previous work [4,24] the kinetic simulations of the three surfactant fitted well with pseudo-second order models. The correlation coefficients varied in the range of 0.442–0.974 for pseudo-first order model. Under four different temperatures from 278 K to 308 K, all of the correlation coefficients (R2 ) of three anionic surfactants exceed 0.994 for pseudo-second order model. Moreover, the values (qe,cal ) estimated from the pseudo-second order model were very close to the experimental values(qe,exp ). Therefore, the adsorption processes could be approximately regarded as the two stage reaction process. The adsorption rate were affected by the concentration of anionic surfactants and the number of vacant adsorption sites.

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Table 1 Pseudo-first-order and pseudo-second-order kinetic model parameters of three anionic surfactants onto ACCMs. anionic surfactant

T/K

Pseudo-first-order model

Pseudo-second-order model

qe,exp

k1

qe,cal

R2

k2

qe,cal

R2

SDBS

278 288 298 308

0.0196 0.0168 0.0176 0.0147

460 297 219 160

0.974 0.939 0.927 0.803

1.29 × 10−4 1.73 × 10−4 2.55 × 10−4 3.12 × 10−4

820 800 794 787

0.999 0.999 1.000 1.000

793 786 782 780

SLS

278 288 298

0.0163 0.0147 0.0164

545 415 582

0.928 0.875 0.954

7.00 × 10−5 8.91 × 10−5 6.70 × 10−5

769 781 820

0.998 0.998 0.996

744 755 782

SDS

278 288 298 308

0.0156 0.0138 0.0165 0.0153

126 99.5 97.5 65.4

0.934 0.739 0.933 0.442

2.93 × 10−4 2.96 × 10−4 5.24 × 10−4 7.31 × 10−4

330 336 342 345

0.999 0.994 0.999 0.999

318 327 338 341

3.4.3. Adsorption apparent activation energy The Arrhenius equation: lnk = −

Ea + lnA RT

(4)

where k (1/s) is the pseudo-second order kinetic equation rate constant. Ea (kJ/mol) is adsorption apparent activation energy; T (K) is thermodynamic temperature, the gas constant R = 8.3145 (J/(mol K)) and A refers to the former factors. The values of Ea was calculated from the slop of the plot between lnK versus 1/T by using the adsorption rate constant (k2 ) in Table 1. The adsorption apparent activation energy of SDBS, SLS, SDS were calculated to be 21.7 kJ/mol, 8.23 kJ/mol, 30.8 kJ/mol, respectively. The Ea were within the range of 5–40 kJ/mol for SDBS, SLS and SDS in the experimental temperature (5–35 ◦ C). These low activation energies are characteristic for diffusion controlled processes and suggest physisorption [25,26]. 3.5. Adsorption isotherms In order to understand the performance and mechanism of adsorption, both Langmuir and Freundlich isothermal models were adopted to describe the adsorption isotherms of ACCMs (Fig.s2). The adsorption parameters were calculated from the models, which were presented in Table 2. Langmuir isothermal equation: ce 1 ce = + qe qm b × qm

(5)

Freundlich isothermal equation: lnqe =

1 lnce + lnkF n

(6)

Where qm is the saturated adsorption of anionic surfactants adsorbed onto ACCMs (mg/g), b (L/mg) and kF are the paremeter for the Langmuir and Freundlich isothermal equation, respectively. n is the constant for the Freundlich model, which is affected by adsorption strength. As shown in Table 2, the Langmuir model was a more suitable model with describing the adsorption behavior of the anionic surfactants onto ACCMs because the regression coefficients are big (R2  0.958). The adsorption behaviors of SDBS, SLS and SDS on ACCMs could be approximately considered as monolayer adsorption within the range of the experimental concentrations [27]. Compare the value of b for three anionic surfactants (SDBS > SLS > SDS), it can be claimed that the attraction force of ACCMs to SDBS was strongest but SDS was weakest. Which is agrees well with experimental maximum adsorption capacity. Beside Freundlich parameters (n > 1) show that the adsorption processes for SDBS, SLS and SDS were easy to carry out [24].

The maximum adsorption capacityqm of ACCMs which is estimated from the Langmuir model was 1230 mg/g for SDBS, 909 mg/g for SLS, and 884 mg/g for SDS, respectively. These values were very close to the experimental values. Huang et al. [4] applied quaternary chitosan salt membranes to adsorb SDBS and found that the saturated adsorption capacities was 285.7 mg/g under 298 K. The qm of this paper was 4.3 times larger than above materials. 3.6. Thermodynamics analyses Thermodynamic equation: S  H  − R RT

(7)

G = H − TS

(8)

lnD =

The adsorption distribution coefficients (D = qe /ce ) is tightly related to the anionic surfactants adsorption-desorption of ACCMs under equilibrium state; where R is the gas constant (8.3145 J/(mol K)) and T is temperature (K). The data of “Effect of initial concentration” were adopted to calculate the adsorption distribution coefficient (D) under different temperatures, from 278 K to 308 K. Both H␪ and S␪ were determined from the slope and intercept of the plots of lnD versus 1/T. The thermodynamic parameters of H␪ , S␪ and G␪ are calculated and listed in Table 3. The negative values of G␪ suggest that the adsorption of anionic surfactants onto ACCMs were feasible and spontaneous over the tested temperatures. With increase of the operational temperature (278–308 K), the absolute values of G␪ for the adsorption of SDBS reduced, but that of SLS and SDS increased. The adsorption of SDBS onto ACCMs was more favorable at lower temperature, while the adsorption of SLS and SDS were on the opposite. The negative H␪ indicates that adsorption was exothermic of SDBS onto ACCMs. Furthermore, the negative S␪ indicates that disorder was reduced at the solid-liquid interface during adsorption of SDBS onto the adsorbent. The positive H␪ and S␪ values show that the adsorption of SLS and SDS were endothermic and the chaos increased. Compare with the others, SDBS has a large plane functional group, benzene ring. The special molecular structure increases the molecular size and weight of SDBS, further enhances the van der Waals force and physisorption for SDBS; moreover, the large flat structure hinders SDBS close to the adsorbent and restrains the formation of chemical bonds between them, thereby inhibits chemical adsorption. So the enthalpy change of SDBS is different from the others. The H␪ of SDBS (H␪ < 0) indicated that the adsorption was dominated by physisorption. But the H␪ of SLS and SDS (H␪ > 0) just demonstrated that the system state parameter enthalpy

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Table 2 Adsorption isotherm parameters of three anionic surfactants onto ACCMs. anionic surfactant

SDBS SLS SDS

Langmuir

Freundlich

qm

b

R2

n

kF

R2

1230 909 884

0.0674 0.0343 0.0116

0.999 0.998 0.958

2.20 1.97 1.66

102 50.6 26.2

0.711 0.710 0.572

Table 3 Thermodynamic parameters for the adsorption of three anionic surfactants onto ACCMs. anionic surfactant

H (kJ/mol)

S (J/(mol K))

SDBS SLS SDS

−24.2 42.4 9.68

−43.2 184 52.0

increased and that chemisorption played an important role (not major role) in the process. At certain conditions, the physisorption and chemisorption can be classified by the magnitude of H␪ , G␪ . Bonding strengths of <84 kJ/mol are typically considered as those of physisorption interaction. The bond strengths of chemisorption can be 84–420 kJ/mol [28]. Generally, G␪ for physisorption is less than that for chemisorption. The G of physisorption is between 0 and −20 kJ/mol and that of chemisortion is between −80 and −400 kJ/mol [29]. Therefore, both of the values of H␪ and G␪ suggest that adsorption were dominated by physisorption of anionic surfactants onto ACCMs. 3.7. Interaction ACCMs-anionic surfactants The solution acidity had great influence on the adsorption under the experimental conditions. Within a certain pH range 3.0–9.0, the adsorption capacities increased with increase of the acid in the solvent. These results proved that the strong interaction of adsorbates with adsorbent can be explained by the presence of the cationic charges, −NH3 + , in acid solution. ACCMs adsorb anionic surfactants mainly based on electrostatic interactions. Because of the polarization of amino cation and the deformability of anionic surfactants, the affinity might translate to covalent bond possibly. Therefore, the adsorption rate should be related to the concentrations of cationic amino and anionic sulfonate groups. One amino cation can only associate with one anion. Hence the adsorption would fit the second order reaction and conform to a monolayer adsorption, which is in agreement with the results of kinetic and isothermal model validation. In addition, van der Waals force also was involved in the adsorption. Since benzene ring is a the large plane functional group, the van der Waals force between the adsorbent and SDBS is stronger than the force between adsorbent and linear aliphatic anionic surfactants (SLS, SDS). This might be the reasons that the adsorption capacity of ACCMs for SDBS was much higher than others, and that the adsorption of SDBS was exothermic but the adsorption of SLS and SDS was endothermic thermodynamically.

G (kJ/(mol K)) 278 K

288 K

298 K

308 K

−12.2 −8.84 −4.76

−11.7 −10.7 −5.28

−11.3 −12.5 −5.80

−10.9 −14.4 −6.32

retained sorption efficiency of ACCMs for SDBS adsorption at various cycles was as shown in the Fig.s3. The study found that the adsorption capacity was still above 94.3% after 8 cycles of reuse. The reuse experiments confirmed that ACCMs can be recycled and reused repeatedly. 4. Conclusions The prepared ACCMs had a high content of primary amine and provided a high surface area. Under the experimental conditions, the acidity of solution has significant influence on the adsorption of anionic surfactants. Meanwhile, the temperature had slight effect on the adsorption. At 298 K, the saturated adsorption capacities of ACCMs for SDBS, SLS, SDS were 1220, 888 and 825 mg/g, respectively. In addition, the adsorption processes fit well with the pseudo-second-order kinetic model, and the adsorption isotherm are descried by Largmuir isothermal properly. Moreover, the adsorption processes of the three anionic surfactants were spontaneous. The adsorption of SDBS was an exothermic process with decrease of entropy, but the adsorption of SLS and SDS were endothermic processes with increase of entropy. Furthermore, the adsorption of these three anionic surfactants were complex processes with both physisorption and chemisorption. ACCMs adsorb anionic surfactants mainly by physisorption with electrostatic interactions between amino cation and anionic sulfonate groups. After 8 cycles of recycling and reuse, the adsorption capacity of ACCMs could retain to 94.3%. This study confirmed that the prepared ACCMs are promising adsorbents to remove anionic surfactants from effluents. Acknowledgements This work was supported by the National Natural Science Foundation of China (No.51208299) for financial support. Authors are very grateful to the University of Shanghai for Science and Technology for the laboratory apparatus. Appendix A. Supplementary data

3.8. Regeneration and reusability of ACCMs The regeneration was studied of ACCMs adsorbed by anionic surfactant solution as well. Firstly, 50 mg adsorbent was added into the 100 mL solution with initial concentration of 2.0 mg/L SDBS, and the solution had been shaken continuously for 24 h. Secondly, ACCMs saturated with SDBS was regenerated by 100 mL of 0.1 mol/L sodium hydroxide solution. After repeating the adsorption-desorption process, the adsorption capacity of ACCMs was measured and the regeneration rate was calculated. The

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