PVA magnetic nanocomposites for removal of methyl orange from aqueous solution

Applied Clay Science 174 (2019) 127–137

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Research Paper

An effective, low-cost and recyclable bio-adsorbent having amino acid intercalated LDH@Fe3O4/PVA magnetic nanocomposites for removal of methyl orange from aqueous solution

T

Shadpour Mallakpoura,b, , Masoud Hatamia ⁎

a b

Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran

ARTICLE INFO

ABSTRACT

Keywords: Layered double hydroxide Environmentally friendly Polymeric magnetic nanocomposite Ultrasonic irradiation Methyl orange Adsorption

At first, layered double hydroxides (LDH) was intercalated with eco-friendly molecule using the coprecipitation method, which this act was confirmed using X-ray diffraction showing the increase of interlayer space to 1.36 nm. In the second step, we hybridized the intercalated LDH material with the pre-synthesized Fe3O4 nanoparticles via ultrasound waves as a simple technique. Then, different percentages of the LDH hybrid were applied for the fabrication of crosslinked poly(vinyl alcohol) (PVA) nanocomposites (NCs), and their physiochemical properties were examined. Hence, thermogravimetry analysis showed the great thermal stability of obtained NCs. Also, NCs had a mesoporous structure with high specific surface area. Finally, the influence of crosslinked PVA on the adsorption ability of LDH hybrid as an efficient magnetic adsorbent was studied to remove methyl orange (MO) from water test solution. The maximum adsorption efficiency of the NC toward MO was obtained at pH = 6 using 100 mg of the adsorbent. Consequently, we used various error functions to find the suitable adsorption isotherm among linear and non-linear forms of isotherm models. The results showed that linear Freundlich model was the better model of the adsorption process. Also, we did the same calculations for the linear and non-linear forms of kinetic models and outcomes were match better with the non-linear form of Pseudo-second-order model. Subsequently, the observed results of the adsorption thermodynamic revealed the spontaneous and exothermic nature of the adsorption process. At the end, the multi-cycling study indicated the stable and remarkable adsorption efficiency of NC 6 wt% after 3 cycles.

1. Introduction The improvements in the present technology are very significant, thus, the protection of drinking water from the poisonous metallic ion contaminants and organic dyes is very critical in recent decades. Although, organic dyes are extensively used in the modern industry such as textile, cosmetics, leather, and food, most of them are poisonous and may destruct the life of human and other organisms (Ciesielczyk et al., 2017; Zhang et al., 2018). However, different methods and technologies were employed to uptake toxic and poisonous dyes from the aqueous solution including coagulating, electrochemical, oxidation, adsorption, magnetic separation, and so on (Akbour et al., 2018; Wang et al., 2018). Recently, the adsorption technique is used as an important approach to eliminate the poisonous dyes from wastewater. For this aim, several materials are used which among them layered double hydroxide (LDH) showed itself as promising material.

LDH is an important group of two-dimensional nano-clays with inorganic nature with a formula of [M1-x2+Mx3+(OH)2]x + (An-)x/ 2+ and M3+ are metallic cations n·mH2O. In the mentioned formula M with charge of 2 and 3, respectively (Chhetri et al., 2018; Zubair et al., 2017), An- is anions which exist between the layers and x is the molar ratio that mainly is in the range of 0.20 to 0.33 for pure LDH (Mohapatra and Parida, 2016). These compounds are bio-safe, low-cost, and environmentally friendly, but their high charge density and their hydrophilic nature restricted use of them in many industrial fields such as polymeric NCs (Mallakpour and Hatami, 2019). Therefore, modification of LDH materials with bio-safe and organic molecules is a significant way to improve their interactions with the polymeric matrixes (Mallakpour and Hatami, 2017b). Nowadays, the use of LDHs as a safe adsorbent for the elimination of poisonous dyes from aqueous solution is a very interesting field for researchers. For example, Lupa et al. (2018) functionalized LDH with

⁎ Corresponding author at: Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran. E-mail address: [email protected] (S. Mallakpour).

https://doi.org/10.1016/j.clay.2019.03.026 Received 10 September 2018; Received in revised form 3 March 2019; Accepted 25 March 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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S. Mallakpour and M. Hatami

Aliguat 336 through ultrasonic irradiations and the obtained adsorbent was employed for the phenol separation from the wastewater. Zhu et al. (2018) eliminated congo red poisonous dye from the aqueous solution using Cu-AL-LDH/Laccase hybrid films. Novillo et al. (2014) studied elimination of phosphate form water solution using LDH nano-sheet and the obtained data indicated the maximum adsorption efficiency of 71.2 mg/g for the adsorbent. Monash and Pugazhenthi (2014) applied the calcinated Ni-Al-LDH to eliminate methyl orange (MO) from the water solution at 30, 40 and 50 °C and the maximum adsorption efficiency for the adsorbent was obtained about 5.7 × 10−4 mol/g at 50 °C. One of the disadvantage of LDHs as an important adsorbent comes from the platelet-like structure of these materials, which causes easy dispersion of them in the aqueous solution. As a result, this would lead to a difficulty in easy separation of them from the water solution (Chen et al., 2012b). Thus, for the simple separation, these materials are combined with magnetite (Fe3O4) to create a sorbent, which can be isolated from solution using an external magnet (Li et al., 2017). In recent years, combined LDHs with magnetite nanoparticles (NPs) have a great interest in the separation of noxious cations, degradation of dyes, drug delivery, and also in order to develop the mechano-chemical properties of polymeric nanocomposites (NCs) (Li et al., 2014; Zhang et al., 2013). For example, Koilraj and Sasaki (2016) applied Fe3O4/ MgAl-NO3-LDH as a separable adsorbent in order to uptake phosphate from the water test solution. Zhang et al. (2013) separated the uranium (VI) as a radioactive pollutant through a highly effective and magnetic Fe3O4@C@LDH adsorbent. Hu et al. (2016) intercalated the LDH layers with sulfated β-cyclodextrin (SCD), and then prepared a composite containing SCD-LDH and Fe3O4 NPs. The obtained composite was used to degrade methylene blue from the aqueous solution. Shao et al. (2012) used Fe3O4@SiO2@-LDH as an important magnetic Core-Shell for the separation of proteins. The observed results exhibited the great capability and selectivity of Core-Shell materials to remove His-tagged protein with numerous separation cycles. A widely used polymer for creation of NCs is poly(vinyl alcohol) (PVA), which is biodegradable, non-expensive, non-toxic, and water soluble (Halima, 2016; Song et al., 2018). This polymer has a chemical and mechanical resistance, but its high solubility and its low thermal stability restricted its applications (Halima, 2016). Therefore, only a low percentage of LHDs can improve the thermal, mechanical, optical, and other features of PVA compounds (Mallakpour et al., 2015b). Hence, this would be great advantage of using these materials in the PVA. Nevertheless, up to now, no one used LDH-Fe3O4 in the PVA matrix for the remediation of dyes in the water solution. Herein, we wish to report bio-safe sonication method for the intercalation of LDH, hybridization of it with Fe3O4 (LDH@Fe3O4), and finally synthesis of PVA NCs (LDH@Fe3O4/PVA NCs). The obtained samples in this process were fully characterized with Fourier transform infrared (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), energy-dispersive X-ray spectroscopy (EDX), N2 adsorption-desorption and thermogravimetry analysis (TGA) techniques. Furthermore, the behavior of LDH@Fe3O4/PVA NC 6 wt% was studied as a notable adsorbent to eliminate MO dye from the aqueous solution. The remaining concentration of the MO dye was recorded using ultraviolet visible (UV–Vis) analysis.

compounds. L-phenylalanine (Lp) and also additional solvents were employed without more purification in this work.

2. Materials and methods

The LDH@Fe3O4 hybrid was prepared using the ultrasonic tool, which leads to the easy incorporation of Fe3O4 NPs into the LDH layers. For this purpose, at first, 1 g of LpPMDA-LDH was dispersed in the beaker having 10 mL of DI water via ultrasonic irradiation for 15 min. On the other hand, a suspension of Fe3O4 particles (0.5 g) was prepared in another beaker containing DI water (10 mL) using ultrasonic for 15 min. Then, two suspensions were combined and stirred at 35 °C for 120 min. Last of all, the achieved hybrid was sonicated for 40 min, filtered, and dried at RT for 48 h.

2.2. Characterization FT-IR spectra of solid products were given by Jasco-680 model (Japan) using KBr floppy tablets as a non-absorption cell at the wavenumber range of 4000–400 cm−1. XRD patterns of solid products were obtained using a Philips Xʼpert MPD, at the range of 2θ = 2–80° (λ = 1.5418 A° through voltage of 40 kV). Low angel XRD was examined in the range of 2θ = 0.5-10°. The surface morphology of nanostructured samples were observed with images of FE-SEM (HITACHI, S-4160 model, Japan) and TEM (a Philips CM 120 operated, Netherlands) with an accelerating voltage of 100 kV. For FE-SEM, a very thin cover of Au coated on the surface of compounds to make their surface conductive and find a better contrast before taking images. Magnetization behavior of materials was performed with VSM (VSM/ AGFM, Iran) on a 7410 vibrating sample. Thermal features of solid products were obtained in the range of 25–800 °C by TGA instrument (STA503 TA model, with Argon gas through heating rate of 20 °C min−1). Ultrasonic apparatus (TOPSONICS, Tehran, Iran) with the power of 100 W and frequency of 20 ± 1 kHz was applied for the synthesis and intercalation of LDH and also the synthesis of LDH@ Fe3O4 and LDH@Fe3O4/PVA NCs. The obtained data related to EDX was assigned with SERON Al52300 spectrometer. BELSORP MINI II (Japan) device at 77 K was applied in order to examine the specific surface area, pore volume and pore mean diameter. For this purpose, at first, 0.1 g of samples was degassed at 75 °C for 30 min. 2.3. Synthesis of organic diacid (LpPMDA) and Fe3O4 NPs Bio-safe LpPMDA was synthesized through the reaction of Lp and PMDA according to the earlier research (Supporting information, Section 1.1) (Mallakpour and Hatami, 2017a). Also, for the synthesis of Fe3O4 NPs the earlier procedure was examined (Supporting information, Section 1.2) (Mallakpour et al., 2015a). 2.4. Intercalation of Mg/Al-LDH with LpPMDA using ultrasonic irradiation In this paper, LDH was intercalated with LpPMDA through coprecipitation manner using the ultrasonic technique, similar to the previous procedure (Mallakpour and Hatami, 2017a). For this purpose, at first a mixture of 1 g (2 mol) of Mg(NO3)2.6H2O and 0.74 g (1 mol) of Al (NO3)3.9H2O was dissolved in 25 mL of deionized (DI) water at room temperature (RT). Then, a mixture of 1.02 g (1.5 mol) of LpPMDA and 0.24 g (3 mol) of NaOH was dissolved in the 25 mL of DI water. Next, the first solution was slowly added into the second solution whereas the obtained blend was stirring at RT. Afterward, its pH was adjusted to 10 using a solution of NaOH 1 mol/L. After stirring the obtained suspension under nitrogen atmosphere at 35 °C for 3 h, it was sonicated for 1 h. Lastly, the formed sample was filtered using Whatman paper, washed with DI water 3 times, and dried at RT. Likewise, for the comparison, pure Mg/Al-LDH having carbonated inter-layer anions was fabricated with the similar manner (Supporting information, section 1.3). 2.5. Preparation of LDH@Fe3O4 hybrid

2.1. Materials Al(NO3)3. 9H2O, FeCl3. 6H2O, Mg(NO3)2. 6H2O and MO anionic dye [Sigma Aldrich Chemical Company (Germany)], FeCl2. 4H2O [Daejung Chemical Company (South Korea)], pyromellitic dianhydride (PMDA) and PVA with Mw = 145,000 g/mol [Merck Chemical Company (Germany)] were employed as important reagents for the synthesis of 128

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2.6. Preparation of LDH@Fe3O4/crosslinked PVA NCs The LDH@Fe3O4/crosslinked PVA NCs were prepared using the sonication way, which can improve the homogeneity of them. For this purpose, diverse suitable percentages (3, 6, and 9 wt%, respect to the polymer weight) of fabricated hybrid were distributed in 10 mL of DI water via ultrasonic irradiation for 15 min. Then, an appropriate value of PVA (0.1 g) was added to 10 mL DI water and stirred at 95 °C until PVA was completely dissolved. Next, the LDH@Fe3O4 suspension was added to the PVA solution and the mixture was sonicated for 45 min. Finally, 0.15 mL of glutaraldehyde and 1 mL of H2SO4 were added to the mixture and stirred for 24 h. The films were prepared by vaporization of solvent at RT. 2.7. Adsorption experiments Fresh MO working solution (100 mg/L) was used for all adsorption steps. The efficiency of adsorption (E, %) and also the adsorption capacity of LDH adsorbent (Q, mg/g) were measured via below equations (Eqs. (1) and (2)) (Mallakpour and Hatami, 2018; Zhu et al., 2016):

E = (Co–Ce)/Co × 100

(1)

Q = (Co–Ce)V/m

(2)

where, Co (mg/L), Ce (mg/L), and V (L) are the initial concentration, final concentration, and volume of MO solution, respectively, and m (g) if LDH adsorbent mass. To investigate the influence of solution pH on the adsorption capacity of adsorbent, 50 mg of LDH@Fe3O4/PVA NC 6 wt% was added into the 10 mL of MO solution (100 mg/L) with different pHs (2, 4, 6, 7, 8 and 10). Then, it was shaken with an electrical shaker at 200 rpm for 5 h at RT (The pH value of MO solution was adjusted with HCl (1 mol/L) and NaOH (1 mol/L) solutions). After the separation of NC 6 wt% with an external magnet, the remaining MO solution was recorded with UV–Vis spectrophotometer. For the examination of effect of adsorbent mass on the adsorption efficiency of the adsorbent, 20, 40, 60, 80, 100 and 120 mg of LDH@Fe3O4/PVA NC 6 wt % were added into 10 mL of MO solution (100 mg/L) at the optimal pH and it was shaken at RT for 5 h. Subsequently, after the separation of the adsorbent, the concentration of remained dye was monitored. To examine the effect of MO contact time on the adsorption process, an optimal mass of NC 6 wt% was placed in 10 mL of MO solution (100 mg/L) at optimal pH, then it was shaken at RT for different times (60, 120, 180, 240, 300, 360, 420, 480 and 540 min). After removing LDH@Fe3O4/PVA NC 6 wt% from the test solution, the concentration of remaining MO solution was evaluated. To examine the effect of MO concentration on the adsorption process, an optimal mass of NC 6 wt% was incorporated into the 10 mL of MO solution having different MO concentrations (50, 100, 150, 200, 250, 300 and 350 mg/L). After shaking the solution at RT for 6 h, finally the remaining concentrations of MO solution were measured after removing the adsorbent. The reusability of NC 6 wt% was examined with adsorption-desorption cycles to evaluate the efficacy of the adsorbent. For this purpose, in each cycle, the MO was adsorbed onto the NC 6 wt% (under optimal conditions) and then the adsorbent was placed in 20 mL of ethanol. The solution was shaken for 24 h (25 °C) and the adsorbent was separated through an external magnet. After three times washing with distilled water, it was dried for 3 days at RT. This procedure was repeated for four cycles and in each step, the removal efficiency of the adsorbate was examined.

Scheme 1. Schematic intercalation of LpPMDA molecule in the LDH structure and the calculated size of d-spacing.

hydroxide-like chemistry and crystalline structure of Mg2+ and Al3+ metals, these cationic metals can raise the crystalline structure of LDH materials (Mallakpour et al., 2014). Scheme 1 indicates the possible intercalation of LpPMDA molecule with LDH layers. According to Scheme 1, the hydroxyl groups on the surface of LDH nano-materials enhanced the interactions of them with LpPMDA molecule. Likewise, due to the cationic nature of layers and anionic charge of LpPMDA, the electrostatic interactions were increased in the structure. Also, environmentally friendly and low-cost feature of ultrasonic way was worth nothing to mention for the dispersion of LDH@Fe3O4 in the matrix of polymer. The probable mechanism of ultrasonic irradiation was based on acoustic cavitation which obtained during the ultrasonic waves (Mallakpour and Darvishzadeh, 2018). The importance of crosslinking agent in this paper was due to development of physiochemical properties of polymeric NCs. 3.2. Structural characterization and morphological behavior The FT-IR characterization for determination of functional groups of each compound was done and is designated in the Fig. S1. As reported by Fig. S1(a) part A, the FT-IR spectrum of pure LDH indicated an absorption band at 3417 cm−1 related to the stretching vibrations of hydroxyl groups, a sharp band at 1386 cm−1 associated to the presence of carbonate groups between the LDH layers, and also showed the bands of metal-oxide (MgeO and AleO) at 800–400 cm−1. As was shown in the Fig. S1(b) part A, the FT-IR spectrum confirmed the functional groups of LpPMDA molecule (Mallakpour and Hatami, 2017a). According to the Fig. S1(c) part A, although the FT-IR spectrum showed the absorption bands associated to the LDH layers, the observed bands at 3100–2930 cm−1, 1776 cm−1 and 1726 cm−1 were also related to the CeH aromatic-aliphatic functional groups, and presence of imide carbonyl spacer, respectively. Similarly, the absorption band at 1401 cm−1 may be due to the formed carbonate from CO2 of air or due to the nitrate anions. The FT-IR spectrum of the pure Fe3O4 NPs, (Fig. S1(d) part A) was reported in our previous research work and was used for the comparison (Mallakpour et al., 2015a). The FT-IR spectrum of LDH@Fe3O4 hybrid indicates the absorption bands associated to both

3. Results and discussion 3.1. Preparation of Mg/Al-LDH intercalated with bio-safe LpPMDA molecule Based on the previous research, LDHs with high crystalline structure were provided with M2+/M3+ = 2 and at basic pH in the range of 9 to 11 (El Hassani et al., 2017; Mallakpour and Hatami, 2017b). Because of 129

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Fig. 1. (A) XRD patterns of (a) LDH-CO32– (b) LpPMDA-LDH (c) Fe3O4 NPs and (d) LDH@Fe3O4 hybrid, and (B) the low angle XRD of LDH@Fe3O4 (C) XRD patterns of (a) pristine PVA and LDH@Fe3O4/PVA NCs of (b) 3 wt%, (c) 6 wt% and (d) 9 wt%.

LDH and Fe3O4 functional groups. Fig. S1 part B indicates the FT-IR vibration bands of the neat PVA and its NCs. Fig. S1(a) part B, shows absorption bands of the pure PVA at 3489–3134 cm−1 and 1421 cm−1, which are related to the stretching and bending vibrations of hydroxyl groups, respectively. The observed band at 2914 cm−1 is corresponded to the stretching vibration of CeH aliphatic group. Also, the observed band at 1720 cm−1 is attributed to the residual acetate C]O group in the structure of PVA. The FT-IR spectra of LDH@Fe3O4/PVA NCs (Fig. S1(b, c and d) part B) showed that the intensity of some of the absorption bands was changed, which it might be due to the hydrogen bonding interactions and also owing to the formation of acetal groups from crosslinking PVA chains. Also, the assignments at the range of 750–480 cm−1 are ascribed to the presence of metal-oxide; MgeO, AleO and FeeO, which can prove the occurrence of the LDH@Fe3O4 in the composite structure. The chemical composition as well as the dispersion of elements of LDH@Fe3O4 hybrid was explored with EDX spectrum (Fig. S2). Clearly, Mg and Al as metallic cations of LDH layers, Fe as basic element of Fe3O4 NPs, and also the presence of C, O and N as basic elements of LpPMDA molecule were observed in this spectrum. Additionally, the presence of N element may show nitrate in the interlayer space. Moreover, the homogeneous dispersion of various elements in the structure of LDH@Fe3O4 hybrid was observed through mapping images. Thus, this spectrum confirmed the stability of LDH@Fe3O4 hybrid, which it can be because of the positive interactions including hydrogen bonding and electronic interaction in this hybrid structure (Mardani, 2017). One important technique for the characterization of the crystal structure of LDHs as well as the measurement of their inter-layer spacing is XRD analysis. The basal spacing (d) can be calculated based on Bragg's law (Eq. (3)).

n =2dsin

2θ = 11.8° and the calculated d001 basal spacing was equal to 0.76 nm. Therefore, considering the thickness of 0.48 nm for each layer (Miyata, 1975), the interlayer distance was equal to 0.27 nm, which approved the existence of carbonate between the LDH layers (Mallakpour et al., 2015b). After intercalation of LDH with LpPMDA (Fig. 1(b) part A) the XRD reflection peak shifted to lower 2θ and also d001 basal spacing was obtained 1.36 nm, which based on the size of LpPMDA (obtained with Gauss View software) it can be said that there is an horizontal intercalation (Scheme 1). Also, the observed weak peak at 11.8° may show the presence of carbonate or nitrate in the interlayer space. Based on Fig. 1(c) part A, the results of XRD presented the reflection peaks at 2θ equal to 18.7°, 29.9°, 34.9°, 42.9°, 54.3°, 59.1° and 64.1° which approved the cubic spinel structure of Fe3O4 (Mallakpour et al., 2015a; Shen et al., 2009). According to the Scherrerʼs equation (Eq. (4)), Fe3O4 NPs indicated the average crystalline size about 10 nm which was calculated based on full width at half maximum of the (311) reflection at 2θ = 35.0°.

D = K / cos

(4)

where D (nm), β (radian), λ (nm) and θ are crystalline size, full width at half maximum, wavelength, and Bragg's angle, respectively. Also, K is a constant which is equal to 0.9. For LDH@Fe3O4 hybrid, the XRD pattern showed two different reflections (Fig. 1(d) part A); the characteristic peaks of both LpPMDALDH layers and Fe3O4 NPs were observed which indicated the crystal structure of product. Fig. 1B shows the results of low angle XRD for LDH@Fe3O4. The observed (003) peak at 2θ = 6.1° is related to the LpPMDA-LDH. Also, Fig. 1C shows the XRD results for net PVA and LDH@Fe3O4/PVA NCs. Based on Fig. 1(a) part C, PVA showed a semicrystalline structure with observed peaks at 2θ 19.3° (101), 20.9° (200) and 39.1° (111), which approved the monoclinic structure of it (Ricciardi et al., 2004). For LDH@Fe3O4/PVA NCs (Fig. 1(b, c and d) part C), the crystal structure of PVA was changed. Also, the novel reflection peaks with low intensities in the XRD patterns confirmed the LDH@Fe3O4 hybrids in the structure of PVA. The low intensity of reflection peaks and the lack of some peaks in the polymer matrix may be

(3)

where, n, λ and θ are reflection coefficient, wavelength and angle in a common Bragg-Brentano geometry, respectively. As was shown in Fig. 1(a) part A, pure LDH indicated the characteristic peak at 130

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Fig. 2. TEM images of (a) LDH-CO32– (b) LpPMDA-LDH (c) Fe3O4 NPs and (d) LDH@Fe3O4 and (e and f) LDH@Fe3O4/PVA NC 6 wt%.

due to the low concentration of LDH@Fe3O4 and/or exfoliated structure of LDH in the NCs. To investigate the morphological properties of produced samples, the FE-SEM and TEM analyses were studied. Fig. S3 displays the FE-SEM photographs of prepared samples. For LpPMDA-LDH (Fig. S3(b)), it showed that the bio-organic molecule affected the morphology and shape of sample compared with the pure LDH (Fig. S3(a)). The presence of LpPMDA caused more hydrophobic nature, separated the LDH layers, and reduced the agglomeration of them. The FE-SEM image of pure Fe3O4 NPs indicated more homogeneous and uniform structure of NPs (Fig. S3(c)). For LDH@Fe3O4 hybrid (Fig. S3(d)), two kinds of nano-metric structures can be seen; the plate-like layers related to LDH materials and the sphere shaped particles due to the presence of Fe3O4 NPs. Comparing FE-SEM images of crosslinked PVA NCs [Fig. S3(e-j)] with LDH@Fe3O4 hybrids indicate that, they are well dispersed into the PVA matrix. TEM analysis reveals the further morphology of the obtained compounds (Fig. 2). As was shown in Fig. 2a, LDH disclosed the plate-like layer with nano-metric thickness. Although the plate-like structure was preserved for LpPMDA-LDH, however, its shape was changed (Fig. 2b). Fe3O4 indicated spherical particles (in the size of 5 to 13 nm) with homogeneous dispersion. For LDH@Fe3O4 hybrid (Fig. 2d), TEM image represented the attached spherical Fe3O4 NPs on the surface of platelike layers of LDH. Based on TEM images of LDH@Fe3O4/PVA NC 6 wt % (Fig. 2e and f), LpPMDA-LDHs and Fe3O4 NPs were homogeneously distributed in the matrix of PVA. Moreover, LpPMDA-LDHs showed both exfoliation (transparent layers) and intercalation (black-like layers) structure. To observe the thermal properties of produced samples, the TGA was examined (Fig. 3A). For LDH, it revealed two steps of weight losses,

similar to the previous study (Mallakpour and Hatami, 2017a). The TGA thermogram was changed for LpPMDA-LDH, which it was due to the molecules of LpPMDA. The theoretical calculation showed the presence of both carbonate (about 4%) and LpPMDA (about 31%) in the structure of LDH (Supporting information). According to Fig. 3B, for LDH@Fe3O4 hybrid, the thermal stability was decreased compared to the pure Fe3O4 NPs. Fig. 3C displays the TGA thermograms of PVA and its NCs. The thermal property of produced NCs was studied with different parameters such as temperature with weight loss equal to 50% (T50) and char yield (CR; remained weight loss at 800 °C). Likewise, to examine the self-extinguishing behavior of NCs, the limiting oxygen index (LOI) was calculated using Eq. (5) (Mallakpour et al., 2014) and the related data are shown in Table S1.

LOI = 17.5 + 0.4 CR

(5)

As was shown in Table S1, the obtained value of CR for NCs was enhanced with increasing the concentration of LDH@Fe3O4. An important aspect which should be considered in this paper was related to the comparable values of CR for NCs 6 and 9 wt% (CR for NC 9 wt% was partially increased in compared to NC 6 wt%), which it can be due to the more homogeneous nature of NC 6 wt% than NC 9 wt%. Also, due to the high values of LOI, these NCs can be considered as self-extinguishing compounds. One of the proper advantages of this research is using low-cost and environmentally friendly LDH@Fe3O4 nano-materials, which increased the thermal properties of PVA in comparison to the earlier nanofiller (Mallakpour et al., 2015a). Consequently, the obtained products due to biocompability, biodegradability, and lowcost as well as high thermal stability have great potential in different fields such as transpiration, food packaging, water remediation, and so 131

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Fig. 3. (A) TGA curves of LDH-CO32−, pure LpPMDA and LpPMDA-LDH, and (B) Fe3O4 NPs and LDH@Fe3O4 and (C) TGA diagrams of (a) pristine PVA and LDH@ Fe3O4/PVA NCs of (b) 3 wt%, (c) 6 wt% and (d) 9 wt%.

on. Therefore, in this paper, due to the great morphological and thermal properties, NC 6 wt% was selected to uptake MO anionic dye from the aqueous solution. The observed diagrams of N2 adsorption-desorption for LDH-CO32−, LpPMDA-LDH, LDH@Fe3O4, and LDH@Fe3O4/PVA NC 6 wt% are shown in Fig. S4 and the obtained data from them are summarized in Table 1. All samples indicated the mesoporous structure with IV-type based on IUPAC classification (Faisal et al., 2018; Zhang et al., 2019). According to Table 1, the specific surface area and pore mean diameter for LpPMDA-LDH were higher than those of LDH-CO32−. It may be owing to the LpPMDA segments, which increased the hydrophobic nature of nano-layers and reduced the agglomeration of them. LDH@ Fe3O4 showed higher specific surface area than that of LDH and LpPMDA-LDH, which it was due to the presence of Fe3O4 NPs hybridized with plate-like layers. Also, the highest surface area was observed for LDH@Fe3O4/PVA NC 6 wt%, due to the presence of crosslinked PVA molecules. The ultrasonic irradiation with high power and energy increased the homogeneity and uniform dispersion of LDH@ Fe3O4 in the polymer matrix, which caused an increase in the specific surface area. The results of magnetization curves for pure Fe3O4, LDH@Fe3O4, and LDH@Fe3O4/PVA NC 6 wt% are shown in Fig. S5. The magnetization curves show the non-linear diagrams without hysteresis loop and remnant magnetization for all samples. Also, they showed the superparamagnetic properties for the samples (Shou et al., 2015; Yan et al.,

2015). Because of high superparamagnetic features of Fe, Fe3O4 NPs showed the highest saturation magnetization value (about 7.30 emu/g), while the saturation magnetization value was decreased for LDH@ Fe3O4 hybrid (about 5.30 emu/g). It was due to the presence of LpPMDA-LDH layers with nonmagnetic feature. LDH@Fe3O4/PVA NC 6 wt% indicated a low value of saturation magnetization. Also, the magnetic features of Fe3O4 and LDH@Fe3O4 were tested in water by employing an external magnet near the glass (Fig. S5), indicating the magnetic behavior of the samples. 3.3. Adsorption studies In this study, crosslinked LDH@Fe3O4/PVA NC 6 wt% was employed as an affordable and green adsorbent to eliminate MO from the aqueous solution. The LDHs can easily interact with MO molecule and remove it from the water solution due to its anion exchange behavior. On the other hand, Fe3O4 has a key role because it caused the simple separation of adsorbent. Also, the role of PVA as a polymeric matrix was very significant; it not only can increase the resistance of LDH@Fe3O4 hybrids against external agents (e.g. acidic pH), but also due to having many polar functional groups on their surface, can increase the adsorption capacity of the adsorbent for the MO removal. Furthermore, the crosslinked PVA NC caused insolubility of the adsorbent in the MO aqueous solution. The correlation between the pH of solution and removal efficiency of NC 6 wt% is shown in Fig. 4a. According to Fig. 4a, the maximum value of Qe for NC 6 wt% was obtained at pH 6 (18.71 mg/g), whereas in the acidic and basic conditions, the values of Qe for that NC were decreased. Decreasing the Qe value in the acidic condition may be due to degradation of the crystalline structure of the adsorbent. While, in the basic condition the high values of OH– can compete with anionic MO dyes and decrease the removal efficiency of MO. Hence, pH = 6 was used as an optimal value for further adsorption studies. Fig. 4b showed that the higher value of adsorption efficiency of NC 6 wt% was obtained with higher mass of this adsorbent. It may be

Table 1 The obtained parameter from N2 adsorption-desorption of fabricated materials. Samples

Specific surface area (m2/g)

Pore diameter (nm)

Total pore volume (cm3/g)

LDH LpPMDA-LDH LDH@Fe3O4 LDH@Fe3O4/PVA NC 6 wt%

2 5 72 87

12 39 39 42

0.01 0.01 0.02 0.39

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Fig. 4. (a) The obtained value of Qe for LDH@Fe3O4/PVA NC 6 wt% at different pHs [Adsorbent mass = 50 mg, MO concentration = 100 mg/L, room temperature and 5 h], (b) the removal efficiency of LDH@Fe3O4/PVA NC 6 wt% versus adsorbent mass [pH = 6, MO concentration = 100 mg/L, room temperature and 5 h].

related to the LDH nano-layers with high anionic properties, which can trap the higher values of anionic dyes among their layers. Additionally, the high concentration of the adsorbent provided many functional groups and more active sites, which led to an improvement in the MO removal. Moreover, increasing the adsorbent mass may increase the agglomeration of plate-like structures and limit the accessibility of MO dyes to active sites, which cause a decrease in the amount of MO removal. In order to examine the dye concentration effect on the adsorption efficiency of NC 6 wt%, the adsorption process was studied at different concentrations of MO. Fig. 5a displayed that the maximum adsorption efficiency of NC 6 wt% was obtained with 100 mg/L of MO; about 97%, although its value was decreased with higher MO concentration. This reduction in the adsorption efficiency of NC 6 wt% may be due to the high saturated sites of the NC, which it restricted the ability of the adsorbent for the uptake of MO dye. Moreover, we used both well-

known linear and non-linear Langmuir (Eqs. (6) and (7), respectively) and Freundlich (Eqs. (8) and (9), respectively) isotherms in order to evaluate the experimental data (Foo and Hameed, 2010; Hudcová et al., 2018).

Ce C 1 = + e Qe KLQm Qm

Qe =

QmKLCe 1 + KLCe

ln Qe = ln KF + Q e = KFCe

1

nF

(6) (7)

1 ln Ce nF

(8) (9)

where, Ce (mg/L) denotes equilibrium MO concentration, Qe (mg/g)

Fig. 5. (a) The adsorption efficiency of NC 6 wt% versus the concentration of MO [pH = 6, adsorbent mass = 100 mg, room temperature and 6 h], (b) non-linear isotherms, (c) linear isotherm of Langmuir and (d) linear isotherm of Freundlich. 133

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and Qm (mg/g) mean the adsorption capacity and maximum adsorption capacity of NC 6 wt%, respectively. KL (L/mg) is a constant related to the heat of adsorption and n and KF are constant parameters related to the adsorption intensity and adsorption capacity in the Freundlich isotherm. According to the Langmuir isotherm and dimensionless RL, the favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1), and irreversible (RL = 0) nature of adsorption process was studied through Eq. (10) (Yang et al., 2016).

1 RL = 1 + KLC0

NC 6 wt% was increased rapidly at the early times and the equilibrium was obtained after 420 min (Fig. 6a). Also, in order to evaluate the adsorption mechanism of MO onto NC 6 wt%, the non-linear and linear pseudo-first-order (Eqs. (17) and (18), respectively) and pseudo-secondorder (Eqs. (19) and (20), respectively) models were considered and their outcomes are observed in Fig. 6 (Foo and Hameed, 2010; Hudcová et al., 2018).

ln(Q e

Qt = Q e(1

(10)

2

n data exp i=1 ( 2 data cal) +

n data exp i= 1 (

n

= i=1

(data exp data cal) data cal

1 n

RMSE =

n

(data exp

data exp

data cal

n data exp i=1 (

datacal)2

2

(12)

datacal)2

100 n

HYBRID=

n

data exp

100 n p

n i=1

(20)

(21)

RT lnK d

(22)

K d = Qe /Ce × 1000 ln K d = H°/RT

S° /R

(23)

wherein, Kd means thermodynamic equilibrium constant, T (K) denotes temperature of the system, and R is the gas constant (8.314 J/mol.K). According to Eq. (23), ΔS° and ΔH° were obtained from the intercept and slope of the linear plot of lnKd versus 1/T (Fig. S6). Table 4 summarizes the values of each thermodynamic parameters. The negative amounts of ΔG° showed the spontaneous nature of adsorption process and it can be seen that the 0 < ΔG° < 40, which it confirmed the physical adsorption behavior (Meili et al., 2019; Mourid et al., 2019). Similarly, the exothermic behavior of the adsorption process was proved with the negative value of ΔH°. For further observation, the adsorption efficiency of produced

2

(14)

data cal

(data exp data cal) data exp

(19)

K 2Q 2et 1 + K 2Q et

G° =

(13)

data exp

i= 1

(18)

exp( K1t))

where, Qe and Qt are the removal capacity of NC 6 wt% at equilibrium and reaction time, respectively. k1 (min−1) is kinetic constant related to the pseudo-first-order model, k2 (g/mg. min) is rate constant related to the second-order model. For the comparison of adsorption mechanism, the kinetic parameters from the linear and non-linear regressions fitting with the Igor Pro software for both models were characterized and their outcomes are displayed in Table 3. Also, Table 3 shows the importance of calculated data from the error functions (RMSE, χ2, and SSE) on the adsorption process. As can be observed, the higher value of R2 and the lower values of error functions confirmed that the non-linear form of pseudo-second-order was the best model for the investigation of adsorption process. The adsorption efficiency of NC 6 wt% was evaluated at the temperature range of 298 K to 358 K. Its adsorption ability was decreased with increasing in temperature and the maximum efficiency was obtained at 298 K. Furthermore, the thermodynamic data of elimination procedure were examined in order to investigate the further adsorption mechanism of MO. For this purpose, the changes of thermodynamic parameters such as free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated according to well-known Eqs. (21)–(23):

(11)

i=1

ARE =

Qt =

2

i=1

n

SSE =

data cal)

(17)

K1t

t 1 1 = + t Qt Qe K 2Q e2

Fig. 5b–d shows the observed data from the non-linear and linear regressions fitting with an Igor Pro software for both isotherms and the obtained results are summarized in the Table 2. Also, the mechanism of the adsorption process was evaluated through different parameters such as correlation coefficient (R2) (Eq. (11)) and some error functions such as Chi-Square (χ2) (Eq. (12)), Root Mean Square Error (RMSE) (Eq. (13)), Sum Squares Error (SSE) (Eq. (14)), Average Relative Error (ARE) (Eq. (15)), and Hybrid Functional Error Function (HYBRID) (Eq. (16)) (Foo and Hameed, 2010). As was shown in Table 2, the higher value of R2 and lower values of error functions for linear isotherms confirmed that these models were better than nonlinear isotherms. Moreover, comparing linear forms of both models, the lower error functions of χ2, SSE, ARE and HYBRID were obtained for the Freundlich model. It approved that this model is a better one for expression of the adsorption process. Thus, it can be said that the NC 6 wt% separated the MO molecules mainly through multi-layer adsorption mechanism. Also, the higher R2 and lower RMSE for linear form of Langmuir showed that the mono-layer adsorption may occur. Furthermore, the obtained values of RL for NC 6 wt% were in the range of 0 to 1 (0.24 < RL < 0.53) which indicated the favorable adsorption process.

R2 =

Qt) = ln Q e

(15) 2

(16)

The results of effect of contact time on the adsorption efficiency of NC 6 wt% are revealed in Fig. 6. As shown, the adsorption efficiency of

Table 2 The obtained results from the linear and non-linear equations of Langmuir and Freundlich models as well as related error functions. Isotherm

R2

Parameters

Langmuir Linear Non-linear

Qm (mg/g) 19.591 19.703

KL (L/mg) 0.239

Freundlich Linear Non-linear

Kf (L/g) 8.330 9.225

n 5.436 6.328

Error function χ2

RMSE

SSE

ARE

HYBRID

0.992 0.900

0.050 0.643

0.148 1.487

0.088 8.849

5.125 6.532

0.525 0.209

0.876 0.839

0.020 0.946

0.166 1.953

0.055 15.257

3.159 9.416

0.206 2.487

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Fig. 6. (a) The adsorption efficiency of NC 6 wt% versus the contact time of reaction [pH = 6, adsorbent mass = 100 mg, MO concentration = 100 mg/L, and room temperature], (b) non-linear models, linear models of (c) pseudo-first-order and (d) pseudo-second-order.

anionic dye exchange easily between the LDH layers and remove from the wastewater. LDH materials also have nano-metric structures with great surface area, which it can improve the uptake of other poisonous molecules on their surface. Moreover, due to the high hydrophilic and hydrophobic groups, these materials can easily interact with anionic organic dyes through both hydrophobic and hydrophilic interactions such as hydrogen bonding, electrostatic interactions, and Van der Waals forces. Similarly, the presence of PVA matrix not only increased the stability of LDH, but also raised the accessible sites of the adsorbent to interact with MO molecules. Table 5 shows the values of Qm for different studied adsorbent materials for the removal of MO from wastewater. The obtained Qm of LDH@Fe3O4/PVA NC 6 wt% was comparable with the other reported studies. Thus, the non-expensive and benign behavior of LDH@Fe3O4/ PVA NC 6 wt% can candidate it in remediation technology. One important aspect of LDH@Fe3O4/PVA NC 6 wt% as a proper adsorbent originated from the superparamagnetic property of Fe3O4 NPs, which caused its easy separation with an external magnet. At the final stage, due to the superparamagnetic property and the easy separation of LDH@Fe3O4/PVA NC 6 wt%, the importance of this material as a reusable adsorbent was accomplished four times. According to Fig. 7b, the adsorption efficiency of NC 6 wt% decreased from 97% down to 81% after three cycles, but in the fourth step it decreased down to 58%. These results are notable in compared to the obtained data from previous studies (Chen et al., 2012a; Chen et al., 2012b).

Table 3 The obtained parameters from the linear and non-linear equations of kinetic models, as well as related error functions. Model

R2

Parameters Qe (cal)

Qe (exp)

Error function χ2

RMSE

SSE

Pseudo-first-order Linear 2.449 Non-linear 9.788

9.215 9.215

0.917 0.963

7.509 0.277

0.112 0.439

0.075 1.350

Pseudo-first-order Linear 12.819 Non-linear 13.050

9.215 9.215

0.980 0.975

0.913 0.165

1.970 0.358

27.171 0.899

Table 4 The calculated thermodynamic data of MO uptake with LDH@Fe3O4/PVA NC 6 wt%. T (K)

ΔG° (kJ/mol)

ΔH° (kJ/mol)

ΔS° (J/mol.K)

298 313 328 343 358

−18.15 −15.85 −15.22 −14.73 −12.81

−37.86

−0.07

materials was compared in the same optimal conditions. Fig. 7a indicates the adsorption efficiencies of LDH-CO32−, LpPMDA-LDH, LDH@Fe3O4 hybrid, and LDH@Fe3O4/PVA NC 6 wt% toward MO under the same conditions. It displayed that the removal efficiency of LpPMDA-LDH was higher (about 4%) than LDH-CO32−, although the adsorption efficiency of LDH@Fe3O4 was decreased. Moreover, the highest value of the adsorption efficiency (about 97%) was obtained for LDH@Fe3O4/PVA NC 6 wt%. Generally, the mechanism of the pollutant adsorption on LDHs can be described according to many factors exist in the LDH structure and adsorbate. One of the important and unique factors is the high anion-exchange feature of LDH, which causes the MO

4. Conclusions For the first time, the bio-organic LpPMDA modified LDH was hybridized with the pre-synthesized Fe3O4 NPs through ultrasonic irradiation as a simple, non-expensive, and benign manner. The existence of Fe3O4 NPs in the matrix of LpPMDA-LDH was evidenced via EDX analysis. Also, FE-SEM and TEM images indicated the homogeneous distribution of Fe3O4 in the matrix of LpPMDA-LDH. Next, different 135

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Fig. 7. (a) A comparison of adsorption efficiency of synthesized samples to uptake MO under optimal conditions [pH = 6, T = 298 K, adsorbent mass = 100 mg, time = 420 min and MO concentration = 100 mg/L], and (b) the recycling test of LDH@Fe3O4/PVA NC 6 wt% for adsorption of MO after four steps. Table 5 A comparison of obtained Qm from the linear regression of Langmuir isotherm for different adsorbents to remove MO. Adsorbent

Conditions

Qm (mg/g)

Ref.

Organic matters-rich clays Amidoxime Magnetic lignin-based carbon nanoparticles CNTs/Fe@C Hybrid Nanoporous carbon Graphene oxide Ferric oxide-biochar NCs LDH@Fe3O4/PVA NC 6 wt%

T = 298 K, pH = 2.0, Time = 60 min, CMO = 100 mg/L pH = 2.7, Time = 48 h, CMO = 50 mg/L T = 293 K, pH = 5, CMO = 120 mg/L pH = 5, CMO = 120 mg/L, Time = 40 min T = 273 K, Time = 20 min pH = 3.0, T = 273 K, Time = 100 min, dosage = 20 mg pH = 8, Time = 30 min T = 298 K, pH = 6.0, Time = 420 min, CMO = 30 mg/L, mass = 0.10 g

41.67 142.85 113.0 16.53 18.80 16.83 20.53 19.59

(Zayed et al., 2018) (Rahman et al., 2018) (Ma et al., 2018b) (Ma et al., 2018a) (Kundu et al., 2017) (Robati et al., 2016) (Chaukura et al., 2017) This work

values of LDH@Fe3O4 were dispersed in the matrix of crosslinked PVA NC in the presence of glutaraldehyde. The prepared LDH@Fe3O4/PVA NC represented the extraordinary physiochemical features. LDH@ Fe3O4/PVA NC 6 wt% showed the suitable adsorption ability toward MO poisonous dye from the water solution at pH 6. The calculated amounts of R2 and some error functions including RMSE, SEE and χ2 confirmed that the adsorption process fitted with linear Langmuir and Freundlich models. Also, study of kinetic models revealed that the nonlinear form of pseudo-second-order model was the better one in compared to others. In addition, the calculated results of thermodynamic observations showed that the adsorption of MO onto the NC have spontaneous and exothermic nature. Moreover, the multi-cycling study indicated the stable and remarkable adsorption efficiency of NC 6 wt% after 3 cycles.

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