Adsorption behavior and mechanism of Fe-Mn binary oxide nanoparticles: Adsorption of methylene blue

Adsorption behavior and mechanism of Fe-Mn binary oxide nanoparticles: Adsorption of methylene blue

Accepted Manuscript Adsorption behavior and mechanism of Fe-Mn binary oxide nanoparticles : Adsorption of methylene blue Kun Lu, Tingting Wang, Li Zha...

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Accepted Manuscript Adsorption behavior and mechanism of Fe-Mn binary oxide nanoparticles : Adsorption of methylene blue Kun Lu, Tingting Wang, Li Zhai, Wei Wu, Shipeng Dong, Shixiang Gao, Liang Mao PII: DOI: Reference:

S0021-9797(18)31535-2 https://doi.org/10.1016/j.jcis.2018.12.094 YJCIS 24471

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

16 October 2018 18 December 2018 26 December 2018

Please cite this article as: K. Lu, T. Wang, L. Zhai, W. Wu, S. Dong, S. Gao, L. Mao, Adsorption behavior and mechanism of Fe-Mn binary oxide nanoparticles : Adsorption of methylene blue, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.12.094

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Adsorption behavior and mechanism of Fe-Mn binary oxide nanoparticles : Adsorption of methylene blue Kun Lu†, Tingting Wang†, Li Zhai†, Wei Wu‡, Shipeng Dong†, Shixiang Gao†, Liang Mao†,* †

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment,

Nanjing University, Nanjing 210093, China ‡

Dragonfly Agri (Jiangsu) Research Corp. LTD, Nanjing 210000, China

Address correspondence to L. Mao, State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, P. R. China. Telephone: (86)25-89680393. Fax: (86)25-89680393. E-mail: [email protected]

Abstract Wastewater containing organic dyes has caused worldwide concern. It is thus imperative to develop materials to remove organic dyes from wastewater. In this study, a nano-structured Fe-Mn binary oxide (nFMBO) was synthesized via a facile coprecipitation approach and used for methylene blue (MB) removal from aqueous solution. Characteristic results indicated that the as-prepared nFMBO had a typical wrinkled structure. The adsorption performance of the nFMBO was then investigated by batch experiments. The adsorption kinetics was well fitted to a pseudo-second-order kinetics model, and the adsorption isotherms agreed well with the Langmuir model with a maximum adsorption capacity of 72.32 mg/g at 25 ℃. Solution pH was a key factor for adsorption and the absorbent exhibited better removal efficiency for MB in solution with high pH. In addition, it was found that the investigated coexisting anions (CO32-, SO42-, PO33-) did not have a significant influence on MB removal. More importantly, the nFMBO could be easily separated from the water and regenerated by acid elution, and the adsorption efficiency of the nFMBO only decreased to 85.1% of the initial capacity after five adsorption-regeneration cycles. These results indicate that the nFMBO can become an alternative adsorbent for the removal of MB from wastewater.

Keywords: nano-structure, Fe-Mn binary oxide, adsorption, methylene blue, reusability

1. Introduction Dyes and pigments released from various industries, such as the textile, printing, and leather industries, into the environment will result in serious water pollution problems [1-3]. These dyes are known as organic materials composed of aromatic compounds and contained phenyl, azo and amino groups, which make them highly toxic and difficult to degrade [4-6]. Methylene blue (MB) is thought to be one of the most common aromatic-cationic dyes, which is widely used in textile industry [7]. Previous studies have demonstrated that exposure to MB could trigger skin damage and burning sensation in the eyes, while its direct ingestion caused increased heart rate, digestive disease, and methemoglobinemia [8,9]. Therefore, it is urgently necessary to remove MB from wastewater for reducing their threats to humans and ecosystems. Various techniques, including photo-catalysis, adsorption, coagulation and membrane treatment, have been adopted to solve the dye pollution problems [10-16]. Among these treatment approaches, adsorption is regarded as a promising and effective technique to remove the dyes from water due to its advantages: a simple treatment process, low cost, high efficiency and extensive applicability[10,11,17,18]. However, conventional adsorbents may have a slow adsorption rate, low adsorption capacity, and poor reusability [19,20]. Therefore, it is necessary to develop novel high-performance adsorbents to overcome the above challenges. Among the currently available adsorbents, iron oxides are a group of emergent nanomaterials that have been studied in depth as adsorbent for the removal of environmental pollutants from aqueous solutions because of their outstanding adsorption performance, geological abundance and environmentally friendly properties [21-28]. Previous studies have demonstrated that, compared to single iron oxides compounds, binary metal oxides composites usually had superior properties, such as improved adsorption capacity [22,29-31]. For example, it had been reported that the introduction of high valence Mn into Fe-containing adsorbents enhanced its adsorption capacity [30]. Zhong et

al. synthesized a urchin-like Fe-Mn binary oxides with higher adsorption capacity for Cd(II) [25]. Most of these studies focused on the removal of heavy metals, with few studies on the removal of MB from the aquatic environments [7,32,33]. It had been demonstrated that the adsorption of heavy metal on the metal oxide nanoparticles was mainly attributed to electrostatic interaction [22,25]. As methylene blue (MB) is a typical cationic dye [7], we speculated that the metal oxides composites could also be used as an efficient adsorbent for the removal of MB. The objective of this study was to explore the adsorption behavior of MB on a nano-structured Fe-Mn binary oxide (nFMBO) adsorbent through a systemic investigation using batch experiments . The influence of solution pH, ionic strength, co-anions and natural organic matter (NOM) on MB adsorption process was also explored. Furthermore, the reusability of the adsorbent was also investigated to evaluate its applicability in real practice. Finally, Fourier transform infrared spectroscopy (FT-IR) analysis was performed to further elucidate the adsorption mechanism. This study provides valuable insights into the development of mixed metal oxide nanoparticles-based adsorbents for environmental remediation. 2. Materials and methods 2.1. Materials All chemical reagents used in this work were analytical grade or better. Potassium permanganate (KMnO4, >99.5 %) and ferrous sulfate heptahydrate (FeSO4∙7H2O, > 99.7%) were purchased from Nanjing Chemical Reagent Co. Ltd., China. Methylene blue (MB) was purchased from Sigma-Aldrich. Suwannee River natural organic matter (NOM) was obtained from the International Humic Substances Society and its elemental composition is provided in Table S1. 2.2. Synthesis of the nFMBO The adsorbent was synthesized according to a modified literature method [25]. In a typical

procedure, KMnO4 dissolved in 100.0 mL deionized water (0.075 mol/L) were heated to 373 K and simultaneously stirred at 400 rpm using a heater/magnetic stirrer, and then 100.0 ml of a 0.225 mol/L FeSO4∙7H2O solution was slowly added into the KMnO4 solution. Fifteen mL of a 5 mol/L NaOH solution was thereafter added drop-wise into the boiling mixture. After that, the mixture was stirred continuously for 6 h without heating, and then the mixture was kept for another 6 h without stirring. The precipitate was obtained by centrifugation, washed several times using de-ionized water to remove the impurities, and then freeze-dried until a constant weight was obtained. 2.3. Characterization of the nFMBO The crystal phases of the nFMBO were analyzed by powder X-ray diffraction (XRD) using a Bruker D8 Advance (Bruker AXS, Germany) with Cu Kα radiation at a setting of 40 kV and 40 mA and angular variation of 10-80. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were carried out with a QUANTA FEG 250 scanning electron microscope to analyze the surface morphology and element composition of the nFMBO. Transmission electron microscopy (TEM) was carried out on a JEM-200CX. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe with a monochromatic Al Ka X-ray source. The specific surface area of the obtained nFMBO was measured using an ASAP 2020 instrument (Micromeritics Instrument Co., Norcross, GA, USA) at 77 K, and then calculated by the Brunauer-Emmett-Teller (BET) method. The surface ζ potential of the nFMBO was determined by the immersion technique using a Malvern Nano ZS instrument [34]. FT-IR spectroscopy analysis of the nFMBO, MB and MB-adsorbed nFMBO were performed by a Vertex 70v spectrometer ( Bruker, Germany) with a resolution of 4 cm-1. 2.4. Evaluation of adsorption performance Sorption experiments were carried out using a batch adsorption approach as reported in our

earlier studies [35,36]. Generally, all MB experimental samples were added at pre-determined concentrations in a 250 mL glass bottle by dilution of the stock solution using a background electrolyte (NaCl at 0.01 mol/L), and then adjusting the pH by the addition of 0.1 mol/L NaOH or HCl in water. The adsorbent concentration used was 1.0 g/L. The mixture were placed into a rotary shaker with a speed of 170 rpm at room temperature. After sorption experiments were completed, samples were removed, centrifuged at 3000 rpm for 5 min, and then the residual MB concentration in the supernatant was determined using a UV/Vis spectrophotometer at 664 nm [33]. Adsorption kinetic experiments were performed with an initial MB concentration of 100 mg/L at 25 ℃ and pH 7.0 to explore the sorption behavior of MB on nFMBO (1.0 g/L) across time. At pre-specified intervals (0-12 h), the concentration of MB was measured. The quantity of MB adsorbed (qt (mg/g)) at time t was calculated according to the concentration difference between initial solution concentration (Ci) and final solution concentration (Cf), as shown in the following equation: Eq. 1 in which V (L) is the volume of the solution and m (g) is the mass of the absorbent. Isothermal adsorption experiments of MB on the nFMBO were conducted by adding a different MB concentrations ranging from 10 to 100 mg/L, qe (mg/g) was calculated by the following equation: Eq. 2 in which Ce is the equilibrium concentration of MB (mg/L). Solution pH, ionic strength, co-ions and natural organic matter (NOM) were varied to evaluate the adsorption capacity of the nFMBO in different aquatic quality conditions. The effect of solution pH on the adsorption of MB by the nFMBO was performed with an initial MB concentration of 50

mg/L and the solution pH ranged from 3.0 to 11.0 at room temperature. The influence of the ionic strength on the adsorption of MB by the nFMBO was carried out by adding NaCl or CaCl2 with concentration ranging from 0.1-1.0 mol/L into 50 mg/L MB solutions at room temperature with a solution pH of 7.0. Moreover, three common co-anions, namely CO32-, SO42- and PO43- were used to investigate the effect of co-ions on the adsorption performance. The presence of NOM on the MB removal was also studied with various concentrations of NOM (0-10 mg/L, TOC). The removal efficiency of MB was calculated according to the following equation: %

%

Eq. 3

where η was the removal efficiency of MB, C0 was the initial MB concentration (mg/L) and C t was the final MB concentration (mg/L) after a certain period of time. 2.5. Reusability of the nFMBO The reusability of the adsorbent is one of the most important factors to assess [37]. Therefore, regeneration experiments were conducted to evaluate the reusability of the nFMBO. 100 mL MB solution (50 mg/L) was prepared in 250 mL glass conical bottles, and then 100 mg nFMBO was added into the solution. The mixture was placed into a rotary shaker at 170 rpm for 12 h. After that, the mixture was filtered using vacuum filtration to separate the adsorbent from the solution. The MB-loaded nFMBO was regenerated by shaking in 0.1 mol/L HNO3 for 30 min, and then by washing for three times using de-ionized water. Finally, the regenerated nFMBO were used again in the succeeding cycles. Moreover, the leaching of Fe and Mn from the nFMBO were measured using ICP-MS (NexION 300, Perking Elmer, USA). 3. Results and discussions 3.1. Characterization of the nFMBO The phase purities and crystallographic structure of the as-prepared nFMBO were investigated

by XRD. As shown in Fig. 1a. the diffraction peaks for the nFMBO in the 2θ range from 10 to 70︒ were indexed to the pure orthorhombic phase of FeOOH (JCPDS 29-713) [38]. However, no obvious diffraction crystalline peaks of manganese oxides were detected, which was mainly attributed to the fact that most of manganese oxides existed in amorphous form in the as-prepared nFMBO [29]. Additionally, SEM analysis and EDS analysis were performed to further identify the successful synthesis of the composite nFMBO. Fig. 1b showed the SEM micrograph of the as-prepared nFMBO, where it was observed that the nFMBO was composed of many uniform three-dimension sphere and the surface of nFMBO was wrinkle and hierarchical, which increased its specific surface area (Fig. S1). The surface of the nFMBO was further observed by TEM, and it was found that the sphere was surrounded by many irregular branching structures (Fig. S2). In order to figure out the size distribution of the nFBMO, we randomly selected about two hundred spheres from the SEM images and analyzed their diameters using a Gaussian distribution. The result was presented in Fig. 1c. It was found that more that 80 % of the total count was in the range of 450-550 nm (average diameter: 496.7 ± 63.2 nm), which demonstrated that the size of the prepared nFMBO was relatively uniform. Furthermore, the EDS line scan spectra and elemental mappings can characterize the surface element composition and the distribution of element of the sample. From Fig. 1d, the EDS spectrum depicted that the prepared nFMBO contained Fe, Mn and O, and the molar ration of Fe/Mn on surface is about 3:1, which matched well with the amount of precursors used in the synthesis process. EDS elemental mappings (Fig. 2a-d) further confirmed that the heterogeneous structure of the nFMBO which is composed of three elements: Fe, Mn and O.

(a)

(b)

(c)

(d)

Fig. 1. Characterization of the nFMBO. (a) XRD patterns of sample, (b) SEM image of sample, (c) Histogram of the size distribution for Fe-Mn binary oxides (n = 200, n is the number of particles ) (d) EDS pattern of the Fe-Mn binary oxides.

(a)

(b)

(c)

(d)

Fig. 2. (a) SEM-EDS elemental mapping of the prepared nFBMO, (b)-(d) the EDS elemental mappings of O, Fe and Mn, respectively. 4500

Fe2p1/2

Fe2p3/2

3500 3000 2500 2000

(b)

Mn2p3/2

1250

Intensity (a.u.)

Intensity (a.u.)

4000

1300

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Mn2p1/2

1200 1150 1100

1500 1000 740 735 730 725 720 715 710 705 700

Binding Energy (ev)

1050 660

655

650

645

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635

Binding Energy (ev)

Fig. 3. The high-resolution XPS spectra of Fe2p (a) and Mn2p (b) in the nFMBO. X-ray photoelectron spectroscopy was used to examined the oxidation states of the iron and manganese in the nFMBO. In the high-resolution Fe2p XPS spectrum, as shown in Fig. 3a, the

peaks of Fe2p1/2 and Fe2p3/2 located at 724.8 and 711.1 ev were the characteristic positions of α-FeOOH, indicating the existence of α-FeOOH in the nFMBO [29]. The binding energies at 653.3 and 641.6 ev were assigned to Mn2p1/2 and Mn2p3/2 (Fig. 3b), which matched well with the characteristic peaks of MnO2 [29]. These results demonstrated that the iron and manganese in the nFMBO were in the oxidation states +Ⅲand +Ⅳ, respectively. The specific surface area of the nFMBO was analyzed by Nitrogen adsorption and desorption. As shown in Fig. 4a, the obtained nitrogen adsorption isotherm demonstrated a typical type IV isotherm with H3 hysteresis loop, indicating a characteristic distribution of slit-shaped mesopores. According to the BET model, the obtained nFMBO had a specific surface area of 67.5 m2/g. Moreover, based on the BJH method, the obtained nFMBO had a pore volume of 0.15 m3/g and a pore diameter of 12.65 nm. The high specific surface area and pore volume suggested that the nFMBO may be a promising candidate for MB adsorption. Furthermore, we determined the variation of its zeta potential with the change of solution pH, and found that the estimated point of

80 60

20

0.010

0.006 0.004 0.002 0.000 0

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Pore Diameter (nm)

40 20

Adsorption Desorption

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Zeta Potential (mV)

100

(a) dV/dD (cm3/g nm)

120

3

Quantity Adsorbed (cm /g STP)

zero charge (PZC) for the nFMBO was around 6.2, as presented in Fig. 4b.

10 5 0 -5 -10 -15 -20 3

4

Relative Pressure (P/Po)

5

6

7

8

9

pH

Fig. 4. (a) Nitrogen adsorption and desorption isotherms of the nFMBO with the corresponding pore size distribution (inset) calculated by the BJH method from the desorption branch. (b) Variation of zeta potential with solution pH for the nFMBO at 25 ℃.

3.2 MB adsorption kinetics 3.2.1. Effect of contact time The adsorption kinetics of MB on the nFBMO across time was studied with an initial MB concentration of 100 mg/L (Fig. 5). It was found that the adsorption rate of MB on the nFMBO increased quickly during the first 1 h, and that the equilibrium were achieved after 2 h. It was noteworthy that the nFMBO exhibited a high adsorption capacity of 80.13 mg/g for the initial MB concentration of 100 mg/L. The adsorbent prepared in this study possesses a wrinkled structure, which is expected to become a promising adsorbent for rapid and effective treatment of the MB-containing wastewater due to its features of fast adsorption and high capacity. 100

qt (mg/g)

80 60 40 20 0 0

2

4

6

8

10

12

Time (h) Fig. 5. Adsorption kinetic of methylene blue on the nFMBO. Experimental conditions: [nFMBO]0 = 1.0 g/L, [MB]0 = 100 mg/L], T = 25 ℃, pH = 7.0. The dotted line is non-linear curve fitting by the pseudo-first-order model; the solid line is non-linear curve fitting by pseudo-second-order model. 3.2.2. Kinetics models In this study, two common kinetic models, namely pseudo-first-order and pseudo-second-order, were used to further explore the mechanism of the adsorption process of MB on the nFBMO [39]. The pseudo-first-order model can be expressed as the following form:

Eq. 4 in which qe (mg/g) and qt (mg/g) were the amount of MB adsorbed on the nFMBO at equilibrium and at the sampling time (t), and k1 was the pseudo-first-order-rate constant. The pseudo-second-order model can be expressed as the following form: Eq. 5 in which qe (mg/g) and qt (mg/g) were the amount of MB adsorbed on the nFMBO at equilibrium and at the sampling time (t), and k2 is the pseudo-second-rate constant. The kinetic parameters and correlation coefficients for the adsorption of MB on the nFMBO were presented in Table 1. The value of the correlation coefficient (R2) for the pseudo-second-order model was 0.99, which was much higher than the one obtained from the pseudo-first-order model. Moreover, the calculated adsorption capacity (qe,cal) obtained from the pseudo-second-order model was also closer to the experimental adsorption capacity (q e,exp). These results demonstrated that the kinetics experimental data were better fitted to the pseudo-second-order model, indicating that the adsorption process was likely to be a chemisorption process. Table 1. Parameters of adsorption kinetics equation Pseudo-first-order

Pseudo-second-order

qe,exp (mg/g)

k1 (1/h)

qe,cal (mg/g)

R2

k2 (1/h)

qe,cal (mg/g)

R2

80.13

10.54

68.99

0.59

12.63

78.76

0.99

3.3. MB adsorption isotherms The adsorption isotherm experiments were carried out with the initial MB concentration ranging from 10 to 100 mg/L to understand the interaction between the MB and the adsorption sites of the nFMBO. The adsorption time was 12 h, which was an adequate time for equilibrium to occur (the equilibrium were achieved after 2 h). As shown in Fig. 6, it was obvious that the adsorption capacity

of the nFMBO increased with the increasing equilibrium concentration of MB and reached saturation progressively. Two equilibrium isotherm models, namely the Langmuir isotherm and the Freundlich isotherm [40], were used to fit the experimental data. The Langmuir isotherm model and the Freundlich isotherm model were presented in Eq. 6 and Eq. 7, respectively. Eq. 6 Eq. 7 where qe (mg/g) was the adsorption capacity of MB at equilibrium. C e (mg/L) was the concentration of MB when the sorption process reached equilibrium. qm (mg/g) represented the maximum adsorption capacity of MB and KL was a constant according to the Langmuir isotherm model. KF and n were respectively the adsorption constant and linearity index according to the Freundlich isotherm model. 100

qe (mg/g)

80 60 40 20 0 0

3

6

9

12

15

Ce(mg/L) Fig. 6. Adsorption isotherm of MB on the nFMBO. Experimental conditions: [nFMBO]0 = 1.0 mg/L, T = 25 ℃, pH = 7.0, and adsorption time = 12 h. The solid line represents non-linear curve fitting by Langmuir model; the dotted line represents non-linear curve fitting by Freundlich model.

Table 2. Parameters of adsorption isotherm model Langmuir Isotherm qe,exp (mg/g) 72.32

Freundlich Isotherm

KL

R2

1/n

KF

R2

3.57

0.96

0.23

45.73

0.94

The parameters from two adsorption isotherm models were listed in Table 2. The value of correlation coefficient (R2) was used to assess the validity of isotherm models. From Table 2, it was observed that the adsorption of MB was better fitted to the Langmuir model than Freundlich model due to the higher correlation coefficient value (R 2 = 0.96), which indicated that the adsorption of MB on the surface of the nFMBO took place in a monolayer adsorption manner. In addition, the maximum sorption capacity calculated according to the Langmuir model was 72.32 mg/g, which was close to the experimental data. Table 3 showed the comparison between the nFMBO prepared in this study and other Fe or Mn oxide nanoparticles previously used for the adsorption of MB from aqueous solutions. It was clearly observed that the adsorption capacity of MB on the nFMBO was higher than many other previously reported adsorbents, which indicated that the nFMBO prepared in this study had great application potential for the removal of MB in practical wastewater treatment.

Table 3. Comparison of maximum MB adsorption capacities of Fe or Mn oxide nanoparticles Adsorbents [email protected] red [email protected](Fe) chitosan/Fe3O4/graphene [email protected] [email protected] [email protected] Fe3O4/PDA/Si-Ca-Mg [email protected] fibers Fe-Mn [email protected] aerogels MnFe2O4 nFMBO a

Experimental Conditions Dosage T (℃) pH (g/L) 0.3 25 11.0 1.0 25 7.0 0.4 25 7.0 0.4 25 7.0 0.4 25 7.0 0.2 25 7.0 0.4 25 7.0 2.0 30 7.0 0.6 30 7.0 0.15 1.0

25 25

7.0 7.0

(mg/L)

Reference

31.44 49.41 30.10 48.06 45.27 65.79 100.23 46.30 9.37

27 60 5.0 30 35 50 20 30

[27] [41] [26] [42] [23] [43] [44] [45] [46]

44.90 72.32

100

[47] this test

(mg/g)

qmax(mg/g): the maximum adsorption capacity of MB; b Ci (mg/L): the initial concentration of MB;

-: the initial concentration of MB was not given in the reference. 3.4. Influence of solution pH on MB sorption Solution pH is one of the important parameters that greatly affect the feasibility of environmental remediation involving to adsorption treatment because it influences the surface charge and binding sites of the adsorbents as well as the ionization of the adsorbates [48]. Therefore, the effect of solution pH on the removal of MB by the nFMBO was studied. From Fig. 7a, it was observed that the removal of MB adsorbed by the nFMBO increased with the solution pH increasing from 3.0 to 7.0 and plateaued in the pH range of 7.0-11.0. For example, only 17.6 % of MB was adsorbed by the nFMBO when the solution pH was 3.0, while at solution pH 7.0-11.0, the MB adsorption rate reached nearly 92 %. The estimated point of zero charge (PZC) for the nFMBO was around 6.2, as presented in Fig. 4b. Therefore, at lower solution pH (pH < PZC), the surface charge of the nFMBO was positive, so stronger electrostatic repulsion would exist between the positively charged the nFMBO and MB (a kind of cationic dye), which inhibited the adsorption of MB on the nFMBO. Additionally, in acidic pH, there existed competitive adsorption between the

excess H+ and MB. With the solution pH increasing, however, the surface charge of the nFMBO was negative and the electrostatic repulsive force became weaker, enhancing the adsorption of MB on the surface of the nFMBO. Based on the above results, it was concluded that adjusting the pH value of wastewater is a desirable method to improve the MB adsorbed on the nFMBO in practical application. 120

(b)

(a)

100

+

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Na 2+ Ca

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%

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NOM 100 90

%

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CO3

2-

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Concentration (mg/L)

Fig. 7. (a) Effect of solution pH on the MB adsorption capacity of the nFMBO; (b) Effect of ionic strength on the adsorption of MB on the nFMBO; (c) Effect of co-anions (CO32-, SO42- and PO43-) with different concentrations (0, 1, 10 and 50 mM) on the adsorption of MB on the nFMBO; (d) Effect of NOM with different concentrations (0-5 mg/L) on the adsorption of MB on the nFMBO. 3.5. Influence of ionic strength, co-anions and NOM on MB sorption

Inorganic salts are always used as additive in the dyes production. For example, in the treatment of printing and dyeing, inorganic salts are usually added as dye-promoting agents and levelling agents [49]. As electrostatic interaction played a crucial role in the adsorption of MB on the nFMBO, these inorganic salts in wastewater may affect the adsorption process by competing for the charged sites on its surface. Therefore, it is necessary to study the influence of inorganic salts on the adsorption performance of the nFMBO. The influence of Na+ and Ca2+ on the adsorption of MB on the nFMBO was presented in Fig. 7b. It was found that, at a background electrolyte of 0.01 mol/L Na+, ~96.5 % of MB was adsorbed by the nFMBO, while as the concentration of Na+ increased, the adsorbed MB decreased from ~96.5 % to 61.3 %. For Ca2+, the adsorbed MB decreased from ~96.5 % to 38.4 %. These results demonstrated that the presence of Na+ and Ca2+ inhibited the adsorption of MB, and the inhibiting effect became stronger with the ionic strength increased. The main reason was that there was a competitive adsorption between salt ions and cationic MB on the surface of the nFMBO, that is, some salt ions occupied a certain amount of adsorption sites, leading to a decrease in the adsorption sites of MB. Similar observations were reported for the adsorption of MB using Zirconium-based metal organic frameworks loaded on polyurethane foam membrane [16]. In addition, compared the Na+ with Ca2+, it was found that the inhibiting effect of Ca2+ was stronger than that of Na+ at the same concentration, which was attributed to the fact that Ca2+ had a higher positive charge than Na+. Once Ca2+ is adsorbed on the surface of the nFMBO, it could produce stronger electrostatic repulsion to MB than Na+. Additionally, the influence of co-anions, such as CO32-, SO42-, PO33- and NOM on the adsorption of MB by the nFMBO was studied. As shown in Fig. 7c. it was found that the presence of CO32-, SO42-, PO33- did not significantly alter the adsorption capacity of the nFMBO. The presence of NOM had little influence on the adsorption of MB by the nFMBO, as shown in Fig. 7d.

For example, when the concentration of NOM was at 4.0 mg/L, the removal of MB could reach 98.1%, which is higher than that without NOM (94.6%). The decreased MB removal may be attributed to the fact that part of NOM bind to the surface of the nFMBO. Although the removal of MB decreased when the concentration of NOM was at 10.0 mg/L, the removal rate remained above 90 %. These results demonstrated that the nFMBO could be an excellent adsorbent for the treatment of cationic dye wastewater in natural conditions. 3.6. Reusability of nFMBO The reusability of adsorbent is an important criterion to evaluate its application in practical wastewater treatment. Recycling usage of adsorbent can not only save the production cost but also reduce secondary pollution. As nFMBO exhibited a poor adsorption capacity at lower pH values, acid treatment may be considered as a suitable approach for the regeneration of the nFMBO. Therefore, HNO3 was selected as the eluent for the regeneration of the nFMBO. Fig. 8a showed the the stability of the nFMBO. It was clearly observed that the adsorption capacity of the regenerated nFMBO decreased by only about 10 % after five cycles, indicating the excellent regeneration performance of nFMBO. Moreover, the percentage of Fe and Mn leached from the nFMBO was measured, as shown in Fig. 8b. It was found that the leaching of Fe and Mn from the nFMBO was less than 2.0 %, which indicated that the obtained nFMBO had a satisfactory stability. In addition, in the experimental process, we found the nFMBO could be easily separated from the aqueous phase by simple sedimentation. Therefore, it is concluded that the nFMBO could be easily regenerated, which guarantees its long term usage in the treatment of actual dye wastewater.

(a)

2.0

Leaching of Fe and Mn (%)

100 80

%

60 40 20 0

(b) Fe Mn

1.6 1.2 0.8 0.4 0.0

0

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Cycle Times

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5

Cycle Times

Fig. 8. (a) MB removal by the nFMBO over five successive adsorption-desorption cycles. (b) The percentage of Fe and Mn leached from the nFMBO during adsorption-desorption cycles. 3.7. Possible adsorption mechanisms The above results showed that the adsorption of MB on the nFMBO is pH-dependent and ionic strength-dependent. For pH, the adsorption capacity of MB on nFMBO increased at pH > PZC, on the contrary, the adsorption capacity decreased at pH < PZC, which suggested that the intermolecular electrostatic attraction should be considered during the adsorption process. Moreover, the salt effect study showed that the adsorption capacity decreased with the ionic strength increasing, which further confirmed the existence of electrostatic interaction between MB and nFMBO. To further gain insight into the interaction between the adsorption sites of nFMBO and MB, the FT-IR spectra of MB, nFMBO, and nFMBO with adsorbed MB were analyzed. According to Fig. 9a, it could be inferred that chemical bonding between nFMBO and MB may take place. Typically, the characteristic peak at 3238 cm-1, assignable to the hydroxyl group stretching vibration, shifted to 3234 cm-1 after adsorption, which indicated that the electrostatic attraction between the cationic MB dye and the hydroxyl group of the nFMBO [44]. Moreover, the new stretching

vibration adsorption bands at 1595 cm-1, 1392 cm-1 and 879 cm-1, attributed to the C-C stretching vibration of aromatic cycle and the aromatic skeletal group from MB, were found on the spectra of nFMBO after MB adsorption [50] .These results reflected the evidence for the strong interaction between MB and nFMBO.

(a) (i)

% Transmittance

3238

(ii) 879

1392

1595

3346

(iii) 3234

4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers (cm-1)

(b)

pH < PZC

pH > PZC

H3C

CH3

N

N

H3C

CH3

N

Fe-Mn

S

N

Fe-Mn

S

N H3C

N

CH3

H3C

Hydrogen bond

H3C

CH3

Electrostatic attraction

CH3

N

N

H3C

CH3 N

Fe-Mn

S

N

Fe-Mn

-

S

N H3C

CH3

N H3C

CH3

Fig. 9. (a) FTIR spectra of (i) nFMBO, (ii) MB and (iii) MB-loaded nFMBO; (b) Schematic illustration of the governing mechanisms of MB adsorption onto the nFMBO. Based on the above discussion, a reasonable MB adsorption mechanism of the nFMBO towards MB was proposed, as shown in Fig. 9b. At lower pH (pH < PZC) values in which case, the surface charge of the nFMBO and MB were both negative, the major adsorption process was controlled by hydrogen bonding, formed by the interaction between the nitrogen atoms from MB and hydroxyl

groups from the surface of the nFMBO. On the other hand, for the higher pH (pH > PZC), the surface charge of the nFMBO was negative, so the cationic MB could be strongly adsorbed on the negative nFMBO through electrostatic attraction. 4. Conclusions Based on the previously reported preparation approaches [29, 48], the present study has demonstrated a simple and facile coprecipitation approach to prepare the nFMBO. The characterization results indicated that the as-prepared nFMBO exhibited wrinkled structure and excellent surface performance, which facilitated the removal of MB. Compared with other previously reported adsorbents [46, 47], the nFMBO showed higher adsorption capacity for MB (72.32 mg/g). The sorption kinetic process was better fitted by a pseudo-second-order kinetic model. The Langmuir isotherm model was confirmed to be more suitable for describing the adsorption behavior of the nFMBO than the Freundlich model. Additionally, the nFMBO exhibited better removal efficiency for MB in solution with high pH, and the co-anions (CO32-, SO42-, PO33-) did not significantly alter the adsorption capacity of the nFMBO. Zeta potential and FT-IR analysis showed that the possible mechanism for MB removal involved the electrostatic attractions and H-binding interactions. Moreover, the nFMBO had a good reusability and the adsorption capacity still kept 85.1 % after five cycles of the adsorption/desorption process, which was better than the adsorbents used in previous studies [43, 44]. Overall, these results from the study demonstrated that the nFMBO as an easy-prepared adsorbent would be a promising material for efficient treatment of wastewater containing MB. Notes The authors declare no competing financial interests. Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (21677074 and 21806076) and the Foundation Research Funds for the Central Universities (021114380082). We thank Elijah J. Petersen for reviewing the manuscript.

Appendix A. Supplementary material Supplementary data to this article can be found online at http://www.journals.elsevier.com.

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Graphical Abstract

mechanism