Modified multi-walled carbon nanotubes assisted foam fractionation for effective removal of acid orange 7 from the dyestuff wastewater

Journal of Environmental Management 262 (2020) 110260

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Research article

Modified multi-walled carbon nanotubes assisted foam fractionation for effective removal of acid orange 7 from the dyestuff wastewater Lei Jia a, Wei Liu a, *, Jilin Cao b, **, Zhaoliang Wu a, Chunyan Yang a a

School of Chemical Engineering and Technology, Hebei University of Technology, No.8 Guangrong Road, Dingzi Gu, Hongqiao District, Tianjin, 300130, China State Key Laboratory of Green Chemical Engineering and Efficient Energy Saving, School of Chemical Engineering, Hebei University of Technology, Tianjin, 300130, China

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Azo dyes Foam fractionation Carbon nanotubes Collector Adsorption isotherm

In this study, multi-walled carbon nanotubes (MWCNTs) had been used to strengthen the removal of acid orange 7 (AO7) from the dyestuff wastewater by using foam fractionation. First, the surface modification of MWCNTs was performed by introducing hypochlorite groups (-OCl). The modified MWCNTs were characterized by using SEM, XRD, FTIR and Raman spectroscopy. Subsequently, the potential of modified MWCNTs as a novel collector for AO7 adsorption was examined. The adsorption conditions of modified MWCNTs towards AO7 were optimized by using response surface methodology (RSM) with a central composite design (CCD). The adsorption capacity of modified MWCNTs towards AO7 could reach 47.72 � 0.79 mg⋅g 1 under the optimum conditions. The kinetics and the equilibrium adsorption data were analyzed by using different kinetic and isotherm models. According to the regression results, adsorption kinetics data were well described by pseudo-second order model, whereas adsorption isotherm data were best represented by Langmuir isotherm model. Finally, foam fractionation was performed with a batch mode. Under the suitable conditions of loading liquid volume 300 mL, modified MWCNTs dosage 180 mg, cetyltrimethylammonium bromide (CTAB) concentration 50 mg⋅L 1, AO7 concen­ tration 30 mg⋅L 1, pore diameter of gas distributor 0.125 mm and air flow rate 100 mL⋅min 1, the removal percentage and enrichment ratio of AO7 were 91.23% and 6.17, respectively. The decolourization ratio of so­ lution after foam fractionation was found to be 98.66%.

1. Introduction Textile printing and dyeing is one of the most critical support in­ dustries serving the national economy, and it has also become a considerable source of environmental contamination. In desizing, scouring, bleaching, dyeing, printing and alkali peeling steps, a large volume of dyestuff wastewater is produced with the characteristics of high biochemical oxygen demand, dark chromaticity and difficult biodegradation (Pathak et al., 2015; Xu et al., 2018). Currently, harm­ less treatment of dyestuff wastewater has attracted wide attention of environmentalists and entrepreneurs, especially decolourization (Aljerf, 2018). Synthetic organic dyes have outstanding advantages over the natural dyes in the aspects of color, adhesion and stability, but they easily pose a serious threat to the ecological balance of the water body and the human body’s health (Brillas and Martínez-Huitle, 2015). Azo dyes are the most important class of synthetic organic dyes (more than

70%) used in the textile industry and their chromophore is a conjugation system constructed by the azo group and aromatic nucleus (Singh et al., 2015; Rawat et al., 2016). Many studies have shown that azo dyes (e.g. butter yellow and tony red) could be reduced to various aromatic amines, which led to the changes in desoxyribonucleic acid structure after activation, inducing malignant diseases (e.g. bladder cancer, ure­ thral carcinoma and renal pelvictumor) (Pielesz et al., 2002; Chung, 2016; Golka et al., 2012). Furthermore, azo dyes and their degradation products could cause the unfavorable mutation of beneficial microor­ ganisms in the water body, resulting in ammonia nitrogen imbalance (Navarro et al., 2017; Papadopoulos et al., 2019). The reported techniques for removing a dye from its aqueous so­ lution mainly involve coagulation, reverse osmosis, membrane filtra­ tion, macroporous resin adsorption and foam fractionation (Esteves et al., 2016; Ghosh et al., 2019). Among above techniques, foam fractionation has gained more acceptance owing to the advantages of

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Liu), [email protected] (J. Cao). https://doi.org/10.1016/j.jenvman.2020.110260 Received 22 August 2019; Received in revised form 9 February 2020; Accepted 10 February 2020 Available online 22 February 2020 0301-4797/© 2020 Elsevier Ltd. All rights reserved.

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Journal of Environmental Management 262 (2020) 110260

mild operating conditions, low energy consumption and environmen­ tally friendly (Anipsitakis and Dionysiou, 2004; Fei et al., 2018). Foam fractionation is an adsorption bubble separation method which uses the bubbles as the media to separate a surfactant from the bulk solution based on favorable thermodynamics (Wouters et al., 2017). However, azo dyes are organic compounds without surface activity. In order to achieve the interfacial adsorption of azo dyes, it is necessary to select a suitable surfactant to play a dual role of frother and collector (Valtera et al., 2017; Streubel et al., 2017; Huang et al., 2016). The surfactant should be simultaneously met the following requirements: (1) there is a strong affinity between the surfactant and azo dye; (2) the complex of surfactant and azo dye can be adsorbed on the bubble surface; (3) after foam fractionation, the surfactant can be easily isolated from azo dye; (4) the surfactant is inexpensive and it cannot cause secondary pollu­ tion (Ghoreishi and Behpour, 2009). Rarely available commercial sur­ factants make the large-scale application of foam fractionation for treating the dyestuff wastewater containing azo dyes even more diffi­ cult (Maqbool, 2016). Moreover, the dosage of surfactant is very huge for achieving a high collecting capacity of azo dye molecules and generating a stable foam phase during foam fractionation (Sarachat et al., 2010). Therefore, the key to remove azo dyes from the dyestuff wastewater by using foam fractionation is to relieve the multiple lim­ itations in surfactant selection. In order to entrain the material without surface activity from the bulk solution into the foam phase, researchers had used a surfactant to play an independent role of frother, and to assume the collector effect by solid particles. Hu et al. (2017) had used hydrophobic silica nano­ particles as the collector to remove methylene blue from its aqueous solution by using froth flotation based on their large specific surface area for adsorption. However, the stability and adsorption capacity of silica nanoparticles were very low due to their high reactivity and aggregation (Rahman and Padavettan, 2012). In recent years, carbon nanotubes, a novel nanomaterial constructed with one or more layers of graphene, have been developed as adsorbent due to their excellent mechanics, large specific surface area and huge interface energy (Wang et al., 2019; Mittal et al., 2015). The special stereoscopic structure of carbon nanotubes contributed to achieving a high adsorption capacity of the desired material (Lico et al., 2019). In this work, metallic multi-walled carbon nanotubes (MWCNTs) had been used as the col­ lector and they were modified by introducing hypochlorite groups on – N-) their surface in consideration of the rich electronic azo groups (-N– in azo dyes (Bafana et al., 2011). Furthermore, the introduction of hypochlorite groups was conducive to facilitating the dispersion of MWCNTs in aqueous solution through improving their surface hydro­ philicity (Zarrabi et al., 2016). In the current work, foam fractionation has been used to remove acid orange 7 (AO7) from the dyestuff wastewater. AO7 is a typical azo dye which has high production quantity and usage amount in the global scope (Zhang et al., 2018). In China, Environmental Protection Agency (EPA) has stipulated that the chromaticity limit for industrial effluents containing AO7 to be discharged to surface water is 50 (GB4287-2012) (Li et al., 2018). First, MWCNTs were modified and characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Subsequently, the adsorption conditions of modified MWCNTs towards AO7 were optimized by response surface method (RSM) using statistical software Design-Expert V8.0.6. The adsorption kinetics and adsorption isotherm of modified MWCNTs towards AO7 were evaluated. Finally, batch foam fractionation was performed by using cetyltrimethylammonium bromide (CTAB) as the frother. This work is expected to develop an effective recovery system for treating dyestuff wastewater, and to facilitate the industrialization of foam fractionation.

2. Materials and methods 2.1. Materials and reagents Single-walled carbon nanotubes, SWCNTs (Assay: min. 90%, Outside diameter 1–2 nm, Length: 5–30 μm) and MWCNTs (outside diameter 20–30 nm, length 0.5–2 μm, Purity > 98 wt%) were purchased from Chengdu organic chemicals Co. Ltd., China. MWCNTs were synthesized by floating catalyst chemical vapor deposition (Liu et al., 2008). CTAB and AO7 were purchased from Tianke Chemical Reagent Co. Ltd., Tianjin, China. NaClO aqueous solution (the mass fraction of available chlorine was equal or greater than 10%) was supplied by Wanjia Chemical Reagent Co. Ltd., Beijing, China. All reagents used in this work were analytical reagents. Silica (SiO2) nanoparticles (99.5% metals basis and 200.0 � 10.0 nm in average particle size), titanium dioxide (TiO2) nanoparticles (80% anatase, 20% rutile, 30 nm in average particle size), porous activated carbon (BET area 1800 m2/g, 5–8 μm in average par­ ticle size) were purchased from Aladdin Industrial Co. Ltd., China. Chitosan microspheres (200.0 � 10.0 μm in average particle size) were prepared in our laboratory according to the method reported our pre­ vious work (Liu et al., 2015b). 2.2. Modification of MWCNTs 500 mg MWCNTs were added into 300 mL NaClO aqueous solution and the mixture solution was magnetic stirred for 12 h. Then, the so­ lution was filtered for several times through a microfiltration membrane with a pore size of 0.45 μm and dried in a vacuum oven at 50 � C over­ night. The collected power was the modified MWCNTs with hypochlo­ rite groups. 2.3. Characterization of modified MWCNTs SEM was performed with a field-emission scanning electron micro­ scope (FEI Nova Nano SEM 450, USA). The samples were evenly distributed on SEM specimen stubs with double adhesive tape. The mi­ crographs were obtained with an accelerating potential of 15 kV under low vacuum. For XRD, mono chromatic Cu Kα radiation (wavelength ¼ 1.540 56 Å) was produced by a D8 Advance A-ray diffractometer (Bruker, Ger­ many). The samples were packed tightly in a rectangular aluminum cell prior to exposure to the X-ray beam. The scanning regions of the diffraction angle, 2θ, were 3–80� , and radiation was detected with a proportional detector. FTIR spectra of samples were collected from 400 cm 1 to 4000 cm 1 on a Tensor 27 infrared spectrophotometer (Bruker, Germany) with 256 scans at a resolution of 4 cm 1 by the KBr method. The data were recorded and processed by Opus software (Bruker, Germany) supplied with the instrument. Raman spectroscopy was recorded with a Raman Jobin Yvon Lab HRRaman at a wavelength of λ ¼ 532 nm. The exposure time and power were 20 s and 5%, respectively. 2.4. Measurement of AO7 and modified MWCNTs concentrations The mixture solution of AO7 and modified MWCNTs was treated by filtration after reaching adsorption equilibrium. The concentration of AO7 in filtrate was measured by a 752 N spectrophotometer (Shanghai Precision Scientific instrument Co. Ltd., China) at 485 nm (Shang et al., 2017). The linear fitting equation is A ¼ 21.9781CAO7 - 0.4423, R2 ¼ 0.9993, where CAO7 is the concentration of AO7, mg⋅L 1. In addition, after foam fractionation experiments, the concentrations of modified

2

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MWCNTs in the foamate and bulk liquid solution were also measured by spectrophotometry at 260 nm (Yu et al., 2007). The linear fitting equation is A ¼ 0.0301CM þ0.0376, R2 ¼ 0.9991, where CM is the concentration of modified MWCNTs, mg⋅L 1.

volumetric flow rate of air. At top of the column, foam was discharged into a collector, in which foam was broken by mechanical stirring.

2.5. Static adsorption experiments

The enrichment performance of foam fractionation was evaluated by removal percentage (R, %) and enrichment ratio (E), and they can be calculated by Eq. (2) and Eq. (3).

2.7. Evaluation of foam fractionation performance

Static adsorption experiments were carried out by adding adsorbents (i.e. SWCNTs, MWCNTs, modified MWCNTs, SiO2 nanoparticles, TiO2 nanoparticles, porous activated carbon and chitosan microspheres) into AO7 solutions. Each mixture solution was shaken in a shaker with 120 rpm at room temperature (298 K). Then the mixture solution was filtered for several times and the filtrate was detected by spectrophotometry. The adsorption capacity of adsorbent towards AO7 was evaluated and it is expressed as Eq. (1) (Aljerf, 2018). Qe ¼

C0

Ce m

� V0

Rð%Þ ¼

E¼

C f Vf � 100% C 0 V0

Cf C0

(2) (3)

where Cf and C0 are the concentrations of AO7 or modified MWCNTs in foamate and feeding solution (mg⋅L 1), respectively; Vf and V0 are the volumes of foamate and feeding solution (L), respectively. The decolourization performance of foam fractionation was evalu­ ated by colorimetric method. The decolourization ratio (DR, %) of dye at 485 nm of the sample is calculated by Eq. (4).

(1)

where Qe is adsorption capacity of adsorbent towards AO7, mg⋅g 1; C0 is AO7 concentration in the feeding solution, mg⋅L 1; Ce is AO7 concen­ tration in the filtrate, mg⋅L 1; V0 is the volume of the feeding solution, L; m is the dosage of adsorbent, g.

DRð%Þ ¼

2.6. Foam fractionation

Ai

At Ai

� 100%

(4)

where Ai is the initial absorbance of the feeding solution and At is the absorbance of the residual solution after foam fractionation.

The experimental setup of foam fractionation with a batch mode is illustrated in Fig. 1. Experiments were performed at room temperature. The foam fractionation column was constructed by a transparent glass tube with 400 mm in height and 44 mm in inner diameter. A ruler was pasted on the outside surface of the tube for measuring the height of foam phase. A porous polyethylene membrane with a given pore diameter was mounted at the bottom of the column serving as a gas distributor. An air compressor was used to pump air into the column for generating bubbles and a rotameter was applied to control the

2.8. Statistical analysis Each of the experiments was at least triply repeated. An analysis of variance of the experimental data was performed by using IBM SPSS Statistics 19.0 software (IBM, USA). The t-test with p � 0.05 was used to determine the difference between mean values. Standard deviation was provided for each mean value.

Fig. 1. The experimental setup of batch foam fractionation.

3

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3. Results and discussion

Table 2 Coded and actual levels of independent variables for experimental design.

3.1. Characterization of modified MWCNTs

Test set 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Table 1 The adsorption capacities of seven adsorbents towards AO7. 8.06 11.61 16.29 12.06 15.13 4.63 2.09

1.0 4.8 1.6

1 3.0 15 5

0

1

1.68

6.0 30 10

9.0 45 15

11.0 55.2 18.4

Table 3 Experimental design of five-level, three-variable central composite design.

In static adsorption experiments, the adsorption capacities of seven adsorbents towards AO7 were studied. As shown in Table 1, the adsorption capacities of TiO2 nanoparticles and modified MWCNTs were considerably higher than those of the other four adsorbents. Modified

SWCNTs MWCNTs Modified MWCNTs SiO2 nanoparticles TiO2 nanoparticles Porous activated carbon Chitosan microspheres

pH, X1 Initial AO7 concentration, X2 (mg⋅L 1) Modified MWCNTs dosage, X3 (mg)

MWCNTs possessed the highest AO7 adsorption capacity of 16.29 mg⋅g 1. RSM was used for optimizing and analyzing the adsorption condi­ tions of modified MWCNTs towards AO7. A five-level, three-variable central composite design (CCD) with six replicated at the central point was employed in this regard. The variables and their levels are given in Table 2. The experimental conditions and the results of adsorption ac­ cording to the factorial design are listed in Table 3. The perturbation graph as in Fig. 2 could be used to compare the effect of all independent variables at a particular point in the design space. The response was plotted by changing only one factor over its range while holding the other factors constant. As shown in Fig. 2, with the increase of pH from 3.0 to 9.0, the adsorption capacity of modified MWCNTs towards AO7 first increased and then decreased. Under acidic conditions, massive hydrogen protons (Hþ) were easily attracted by H2O to generate as H3Oþ, which could be attached on the surface of modified MWCNTs. In addition, the electrophilic –OCl group on the surface of modified – N- double bond MWCNTs could intensively attract the electron rich –N– in AO7 due to the favorable chemical environment of Hþ (Fu et al., 2017). When pH was higher than 7.0, the competitive adsorption be­ tween OH and anionic AO7 molecules resulted in the decrease in the adsorption capacity of modified MWCNTs towards AO7. As the initial AO7 concentration increased, the adsorption capacity of modified MWCNTs towards AO7 first increased and then decreased. When the initial AO7 concentration was low, AO7 molecules in the solution was almost completely adsorbed by modified MWCNTs due to the high active site-to-absorbate ratio. With the increase of initial AO7 concen­ tration, AO7 molecules could be sufficient contacted with modified MWCNTs, thereby enhancing adsorption mass transfer. However, when initial AO7 concentration was higher than 60 mg⋅L-1, the maximum adsorption capacity modified MWCNTs towards AO7 reached because the active sites on modified MWCNTs were saturated by excess AO7 molecules. With increasing the modified MWCNTs dosage from 6 mg to 18 mg, the adsorption capacity of modified MWCNTs towards AO7 first

3.2. Evaluation of adsorption capacity of modified MWCNTs towards AO7

Adsorption capacity, Qe (mg⋅g

Coded level of variables 1.68

SEM graphs of MWCNTs before and after modification are shown in Fig. 1S. MWCNTs appeared as long fiber patterns and they were twined around each other for constructing a complex network based on van der Waals force (Cho et al., 2008). Similar network morphology had also been found in the SEM graph of modified MWCNTs. This result sug­ gested that the length of MWCNTs had not been significantly shortened after modification. Fig. 2S shows the XRD patterns of MWCNTs before and after modification. The XRD pattern of MWCNTs showed two sharp peaks at 26.1� and 42.8� corresponding to the crystal faces (002 and 100) of graphite. In the XRD pattern of modified MWCNTs, above two peaks became broader, indicating that the carbon layer on the surface of MWCNTs was partially destroyed during modification operation (Ying et al., 2003). FTIR spectra of MWCNTs before and after modification are presented in Fig. 3S. A new absorption peak at 966 cm 1 was generated in the FTIR spectrum of modified MWCNTs and it was attributed to the stretching vibration of C–O bond in -C-OCl group (Che Man and Mirghani, 2001). However, the existence of chlorine group led to shifting the peak posi­ tion of C–O bond. The absorption peaks at 1365 cm 1 and 1648 cm 1 were the bending vibration of hydroxyl group (-OH) and graphic – C bond, respectively (Dostert et al., 2016). stretching vibration of C– There were two intense specific peaks could be found in the Raman spectra of MWCNTs and modified MWCNTs (as seen in Fig. 4S). The peak at 1345 cm 1 represented the amorphous hybridized (sp2) of car­ bon atoms in carbon nanotubes (i.e., lattice imperfection of carbon atoms) and it was named the D-band (Aljerf and Nadra, 2019). The peak near 1572 cm 1 was named the G-band and it represented the E2g parallelism of interlayer mode. The G-band could be used to reflect the structural integrity of sp2-hybridized of carbon atoms in carbon nano­ tubes (Sendova et al., 2005). The intensity ratio of D-band to G-band (ID/IG) was assigned to characterize the defect degree of carbon mate­ rials (Aljerf and Nadra, 2019). The lower the ID/IG value was, the less the structure defect on the surface of MWCNTs should get. In the current work, the ID/IG values of MWCNTs and modified MWCNTs were 1.11 and 1.35, respectively. Obviously, the ordered structure of graphite crystallites in MWCNTs had been disturbed due to the introduction of –OCl group (Aljerf and Nadra, 2019). It had been reported that the structure defect contributed to creating numerous reactive activation sites on the surface of modified MWCNTs for adsorbing substances. Above results were consistent with the work of Yu et al. (2011).

Adsorbents

Variable

1

)

*Experimental conditions were pH 3.0, AO7 concentration 30 mg⋅L 1. solution volume 15 mL, adsorbent dosage 5 mg and total adsorption time 24 h. 4

X1

1 0 0 1 0 0 1 0

X2, (mg⋅L 1)

1 0 1.68 1 1

1 0 1.68 0 0 1 0 0

0 0

1 1 1.68 1

1 0 1 1 1.68 1 0 0 0 0 1 0 0

X3, (mg)

Adsorption capacity of modified MWCNTs towards AO7, Qe (mg⋅g 1)

1

15.00 � 0.44 1.94 � 0.32 44.28 � 0.69 7.16 � 0.58 0.95 � 0.11 44.83 � 0.82 45.87 � 1.00 27.14 � 0.97 45.47 � 1.21 4.59 � 1.08 33.89 � 1.23 40.59 � 0.89 27.00 � 0.92 24.44 � 1.03 3.15 � 1.07 45.75 � 0.97 45.38 � 0.90 4.05 � 0.34 45.43 � 1.05 45.50 � 0.92

0 0 1 0 0 1 1 0

1

1 1.68

1 1.68 0 0 0 1 0 0

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Table 4 The kinetic parameters and correlation coefficients of three adsorption kinetics models. Kinetic models

qe ¼ 49.98 mg⋅g 1

k2 ¼ 0.017 g⋅mg 1⋅min

1

Intraparticle diffusion

b ¼ 17.83

k3 ¼ 4.410 mg⋅g 1⋅min

0.5

111.51

0.999

9.91 � 10 4

0.701

795.78

(8)

(9)

Qe ¼ KF C1=n e

where Qe and Ce are the same as defined above; KF is the Freundlich constant, mgn 1⋅L-n; 1/n is the Freundlich exponent which represents adsorption intensity. n ¼ 1, n > 1 and n < 1 indicate irreversible, favorable and unfavorable adsorption states, respectively (Aljerf, 2018). The Temkin isotherm is another model that expresses the adsorption interactions between adsorbent and adsorbate and it is expressed as Eq. (10) (Araújo et al., 2018). (10)

Qe ¼ B lnKT þ B lnCe

where Qe and Ce are the same as defined above; KT is the Temkin con­ stant；B is constant, which is determined by the adsorption heat. The adsorption isotherm models parameters and correlation co­ efficients are listed in Table 5. In Table 5, the correlation coefficient of the Langmuir isotherm

(6)

The intraparticle diffusion model can be expressed as Eq. (7) (Aljerf, 2018): (7)

qt ¼ k3 t1=2 þ b

Pseudo-second order

0.958

where Qe is adsorption capacity at equilibrium, mg⋅g 1; Ce is the con­ centration of adsorbate in solution at equilibrium, mg⋅L 1; Qm is the maximum adsorption capacity, mg⋅g 1; KL is the Langmuir constant, L⋅mg 1. The Freundlich isotherm model is expressed as Eq. (9) and it assumes a heterogeneous distribution of adsorption energy (Liu et al., 2013).

(5)

� t qt ¼ 1 k2 q2e þ ð1 = qe Þt

k1 ¼ 1.573 min

RSS

Qe ¼ Qm Ce =ðKL þ Ce Þ

For understanding the adsorption process of modified MWCNTs to­ wards AO7, three adsorption kinetics models were used to fit the experimental data (Fu et al., 2015; Tomul et al., 2019). The results are presented in Fig. 5S. The pseudo-first order equation and pseudo-second order equation are expressed as Eq. (5) and Eq. (6), respectively (Aljerf, 2018).

�

qe ¼ 46.89 mg⋅g 1

R2

The adsorption isotherm of modified MWCNTs towards AO7 was presented in Fig. 6S. Langmuir adsorption isothermal equation is the most widely used the adsorption model for illustrating the monomolecular layer adsorption, and it is expressed as Eq. (8) (Soltani et al., 2019).

3.3. Adsorption kinetics of modified MWCNTs towards AO7

k1 =2:303t

Pseudo-first order

1

3.4. Adsorption isotherms of modified MWCNTs towards AO7

increased and then decreased. The higher the dosage of modified MWCNTs in the solution was, the larger the surface area for the adsorption of AO7 molecules should get. However, as the modified MWCNTs dosage increased, the obvious precipitates were found in the solution due to the occurrence of modified MWCNTs aggregation. The aggregation of modified MWCNTs would lead to covering up the elec­ trophilic hypochlorite group on their surface, resulting in a low adsorption capacity of AO7. Based on the analysis of Design-Expert software, the maximum pre­ dicted adsorption capacity of modified MWCNTs towards AO7 was 49.03 mg⋅g 1, which was obtained at the optimal conditions of pH 7.0, initial AO7 concentration 30 mg⋅L 1 and modified MWCNTs dosage 9 mg. The static adsorption experiment at the optimal conditions was repeated five times for verification of the optimization. The average adsorption capacity of modified MWCNTs towards AO7 was 47.72 � 0.79 mg⋅g 1. The relative standard deviation between experimental and predicted values was 3.18%.

qt Þ ¼ logðqe Þ

Statistical parameters

adsorption capacity equilibrium of modified MWCNTs towards AO7 in the pseudo-second order kinetic model was close to the experimental value. Therefore, the adsorption process of AO7 on modified MWCNTs could be described as three steps: first is rapid adsorption; then tend to rallentando; finally reached adsorption equilibrium. This result was consistent with the variation trend of experimental values.

Fig. 2. Perturbation plot showing the effect of the variables on adsorption capacity of modified MWCNTs towards AO7.

logðqe

Kinetic parameters

Table 5 The adsorption isotherm parameters and correlation coefficients of three models.

1

where qe is the adsorption capacity at equilibrium, mg⋅g ; qt is the adsorption capacity at different adsorption time, mg⋅g 1; k1 and k2 (min 1) are the rate constant of pseudo-first order and pseudo-second order kinetics, min 1 and g⋅mg 1min 1, respectively; k3 is the rate constant of intraparticle diffusion, mg⋅g 1⋅min 0.5; t is adsorption time, min; b is a constant. The kinetic parameters and correlation coefficients are listed in Table 4. It could be found in Table 4 that the correlation coefficient of the pseudo-second order kinetic model reached as high as 0.999, which was much higher than those of other two kinetic models. Meanwhile, the

Isotherm models

5

Isotherm parameters

1

Statistical parameters R2

RSS

Qm ¼ 59.52 mg⋅g 1

0.999

6.25 � 10 8

Langmuir isotherm

KL ¼ 2.98 L⋅mg

Freundlich isotherm

KF ¼ 26.11 mgn 1⋅L-n

n ¼ 4.62

0.929

0.013

Temkin isotherm

KT ¼ 10.26

B ¼ 9.47

0.961

13.56

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model reached 0.999, which was higher than those of other two models. This result indicated that the adsorption of AO7 on the surface of modified MWCNTs was monolayer. In Freundlich isotherm, n was higher than 1. This result suggested adsorption process was easy to be progressed towards positive direction due to a favorable adsorption state (Aljerf, 2018). In Temkin isotherm, the adsorption heat decreased lin­ early, indicating that AO7 molecules were covered on the surface of modified MWCNTs.

was easily be stripped from the bubble surface due to foam drainage, thus decreasing the mass transfer of AO7 into the foamate. Thus, the introduction of modified MWCNTs could remarkably reduce the dosage of surfactant in foam fractionation through collecting AO7 on the solid-liquid interface in advance. With the increase of CTAB concen­ tration, the foam stability would be improved due to the increase of surface excess of CTAB molecules on bubble surface. Then, the inter­ stitial liquid was difficult to be effectively drained back to the bulk so­ lution, resulting in a low enrichment ratio of AO7 due to the large volume of the foamate. According to above experimental results, 50 mg L 1 could be selected as the suitable CTAB concentration for removing AO7 from the solution by foam fractionation.

3.5. Foam fractionation 3.5.1. Effects of CTAB concentration on the removal efficiency of AO7 In foam fractionation, the surfactant dosage could significantly affect foam property and the adsorption mass transfer on bubble surface (Capodici et al., 2015). The feeding solution was prepared by mixing modified MWCNTs into AO7 solution and then shook in a reciprocating shaker bath for 30 min. After achieving adsorption equilibrium, CTAB was added into the mixture solution. In the control group, the feeding solution was the mixture solution of CTAB and AO7. As shown in Fig. 3, as the CTAB concentration increased, the removal percentage of AO7 in the experimental group first increased and then reached the plateau, while its enrichment ratio gradually decreased. The separation efficiency of AO7 in the control group presented a similar variation tendency as that in the experimental group. However, the maximum removal percentages of AO7 in the experimental group and the control group were obtained at the CTAB concentrations of 50 mg⋅L 1 and 90 mg⋅L 1, respectively. Obviously, the CTAB dosage in the experimental group was much less than that in the control group for achieving the equivalent removal percentage of AO7. This result was due to the fact that CTAB was independently played the role of frother in the experimental column and its foam ability was constant. In addition, previous research showed that nanoparticles would be attached in the foam with the rising bubbles and they mainly existed in the plateau borders which were constructed by three adjacent bubbles. The reflux of interstitial liquid in the plateau borders would induce the aggregation of nanoparticles, thereby increased drainage resistance (Hu et al., 2015). The thick liquid film around the bubbles contributed to obtaining a long half-life period of the foam at a low surfactant concentration. In the control group, CTAB had played the dual role of frother and collector. The conjugation of CTAB and AO7 could significantly affect the hydrophilcity of the amino group in CTAB, thereby weakening the foam ability of CTAB and decreasing the liquid holding capacity of bubble surface (Gong et al., 2017). Moreover, the complex of CTAB and AO7

3.5.2. Effects of air flow rate on the removal efficiency of AO7 It has been well proven that air flow rate played a critical role in determining the foam drainage and interfacial adsorption through affecting the floating behaviors of the bubbles and the hydrodynamic properties of the foam in the column (Liu et al., 2017). As shown in Fig. 4, with the increase of air flow rate, the removal percentage of AO7 increased, while its enrichment ratio gradually decreased. It was acknowledged that the smaller the air flow rate was, the slower the rising velocity of the bubbles in the column should get (Liu et al., 2015a). A long residence time of the bubbles in the bulk solution contributed to the sufficient contact between CTAB molecules and gas-liquid interface, thereby improving the attachment of modified MWCNTs on bubble surface. Moreover, the low air flow rate contributed to strengthening foam drainage because the slow rising velocity of the bubbles in the column would lead to weak flow resistance for interstitial liquid and a long drainage time. Thus, the low air flow rate led to a high enrichment ratio of AO7. However, sufficient foam drainage would result in a low removal percentage of AO7 because the drastic bubble coalescence made the continuous discharge of the foam from the top of the column at a low CTAB concentration more difficult. As air flow rate increased, the low reflux velocity of the interstitial liquid and the short residence time of the bubbles in the column contributed to generating high liquid holdup of the foam. Then, wet and stable foam would result in a large volume of the foamate, decreasing the enrichment ratio of AO7. Furthermore, the occurrence of backmixing of the bubbles in the bulk solution at a large air flow rate was not conducive to interfacial adsorption. Considering both the removal percentage and the enrich­ ment ratio of AO7, 100 mL⋅min 1 was selected as the suitable air flow rate for the following experiments.

Fig. 3. Effects of CTAB concentration on R and E of AO7. Experimental con­ ditions were AO7 dosage 30 mg⋅L 1, loading liquid volume 300 mL, modified MWCNTs dosage 180 mg and air flow rate 100 mL⋅min 1. CTAB concentration ranged from 30 mg⋅L 1 to 100 mg⋅L 1.

Fig. 4. Effects of air flow rate on R and E of AO7. Experimental conditions were AO7 concentration 30 mg⋅L 1, loading liquid volume 300 mL, modified MWCNTs dosage 180 mg and CTAB concentration 50 mg⋅L 1. Air flow rate ranged from 50 mL⋅min 1 to 200 mL⋅min 1. 6

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Fig. 6. The adsorption capacity of modified WMCNTs towards AO7 and the decolourization ratio of solution in foam fractionation.

According to experimental results, some conclusions were drew and listed as follows.

Fig. 5. Effects of pore diameter of gas distributor on R and E of AO7. Experi­ mental conditions were AO7 concentration 30 mg⋅L 1, loading liquid volume 300 mL, modified MWCNTs dosage 180 mg, CTAB concentration 50 mg⋅L 1 and air flow rate 100 mL⋅min 1. Four gas distributors had been used in the current work and their pore diameters were 0.090 mm, 0.125 mm, 0.180 mm and 0.425 mm, respectively.

(1) Morphological and structural analysis revealed that MWCNTs had been successfully modified by NaClO. The introduction of –OCl groups led to disturbing the ordered structure of graphite crystallites in MWCNTs. This was conducive to creating numerous reactive activation sites on the surface of modified MWCNTs for enhancing adsorption performance. (2) Under the optimal conditions of pH 7.0, initial AO7 concentration 30 mg⋅L 1 and modified MWCNTs dosage 9 mg, the adsorption capacity of MWCNTs towards AO7 reached as high as 47.72 � 0.79 mg⋅g 1. The adsorption of MWCNTs towards AO7 contrib­ – N- groups. uted to the good affinity between -OCl and –N– (3) Adsorption kinetics data of modified MWCNTs towards AO7 were fitted better with Pseudo-second order model, whereas adsorp­ tion isotherm data were best represented by the Langmuir isotherm model. These results indicated that AO7 molecules could be rapidly adsorbed on the modified MWCNTs, obeying the monolayer adsorption. (4) Foam fractionation was performed by using the modified MWCNTs as the collector. Under the suitable conditions of loading liquid volume 300 mL, modified MWCNTs dosage 180 mg, CTAB concentration 50 mg⋅L 1, AO7 concentration 30 mg⋅L 1, pore diameter of gas distributor 0.125 mm and air flow rate 100 mL⋅min 1, the removal percentage and enrichment ratio of AO7 were 91.23% and 6.17, respectively. Then, the decolou­ rization ratio of solution after foam fractionation was 98.66%.

3.5.3. Effects of pore diameter of gas distributor on the removal efficiency of AO7 At a given air flow rate, the foam structure and the available surface area for the adsorption of surfactant molecules were determined by pore diameter of gas distributor. As shown in Fig. 5, with the increase of pore diameter of gas distributor, the removal percentage of AO7 decreased, while its enrichment ratio increased. Many studies had found that the rising velocity of large bubbles in the bulk solution was much faster than that of small bubbles at a given air flow rate (Hassanzadeh et al., 2016). The short residence time of the large bubbles in the bulk solution would weaken the adsorption of CTAB molecules on the gas-liquid interface. Additionally, the plateau borders inside the foam constructed by large bubbles were much wider and longer than those by small bubbles (Koehler et al., 2004). The modified MWCNTs aggregates could be smoothly traveled in the wide and long plateau borders, resulting in weak drainage resistance. Correspondingly, the modified MWCNTs ag­ gregates were easier to be discharged into the bulk solution by the reflux of interstitial liquid but they were difficult to be attached on the bubble surface again. Thus, the removal percentage of AO7 was small due to the insufficient interfacial adsorption and strong foam drainage when the experiment was performed by the gas distributor with a large pore diameter. Besides, small bubbles were conducive to shorten the period of foam fractionation because the large specific surface area could lead to a high adsorbed mass flux of the complex of AO7 and modified MWCNTs. Based on above analysis, the gas distributor with a pore diameter of 0.125 mm could be used to generate bubbles for removing AO7 by foam fractionation. In foam fractionation, the adsorption capacity of modified MWCNTs towards AO7 and the decolourization ratio of solution are presented in Fig. 6. Under the suitable operating conditions of foam fractionation for the removal of AO7 from solution were CTAB concentration 50 mg⋅L 1, air flow rate 100 mL⋅min 1, loading liquid volume 300 mL, modified MWCNTs dosage 180 mg, and pore diameter of gas distributor 0.125 mm. The removal percentage and enrichment ratio of AO7 could reach 91.2% and 6.1, respectively. The separation length of foam fractionation in the current work was 0.64 L⋅h 1.

CRediT authorship contribution statement Lei Jia: Investigation, Formal analysis, Writing - original draft. Wei Liu: Conceptualization, Supervision, Writing - review & editing. Jilin Cao: Project administration. Zhaoliang Wu: Resources, Funding acquisition. Chunyan Yang: Data curation, Validation. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21908041), Foundation of Hebei Educational Committee, China (No. QN2018079) and Key Basic Research Program of Hebei, China (No. 16964002D). Appendix A. Supplementary data

4. Conclusions

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2020.110260.

In this work, MWCNTs had been modified and used as the collector for removing AO7 from the dyestuff wastewater by foam fractionation. 7

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