Synthesis, inhibition behavior and recycling of Fe3O4@ZnAl-MoO4 LDH nanocomposite inhibitor

Journal of Alloys and Compounds 801 (2019) 489e501

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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Synthesis, inhibition behavior and recycling of Fe3O4@ZnAl-MoO4 LDH nanocomposite inhibitor Xin Liu a, b, Jihui Wang a, b, *, Wenbin Hu b a b

State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin, 300072, PR China Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 March 2019 Received in revised form 7 June 2019 Accepted 10 June 2019 Available online 11 June 2019

The nanocomposite corrosion inhibitor with MoO2
4 intercalated ZnAl-LDH coating on the Fe3O4 surface was synthesized by ion exchange methods. The morphology, structure and magnetic properties of the composite were characterized by transmission electron microscope, X-ray diffraction, Fourier transform infrared spectra and vibrating sample magnetometer. The release behavior of MoO2
4 and corrosion inhibition of the sample under “NO MF” (without external magnetic field) and “MF ON” modes (under external magnetic field) were measured by UVevis spectroscopy, electrochemical tests and scanning electron microscopy. Calcination the composite after release test and redispersing the sintered products in Na2MoO4 solution, the composite was regenerated. The results show that the superparamagnetic nanocomposite inhibitor Fe3O4@MoO4-LDH presents well-defined core-shell structure. The release test revealed that for “NO MF” mode the release of MoO2
4 from Fe3O4@MoO4-LDH displayed smaller t0.5 (Time to reach half release equilibrium) of 4.25min and shorter time (3 h) to reach equilibrium with high release amount of 71.95% for 0.5 g/L nanocomposite. Under “MF ON” mode the release rate is slower with longer t0.5 (from 26.40min to 1.60 h for 0.5 g/L to 4.0 g/L nanocomposite, respectively), thus realizing sustained release up to 12 h. At the same time, the release amount decreases and the release time has a long-term excellent prolongs with the concentration increasing. Correspondingly, the MoO2
4 corrosion inhibition effect over 71.96% at 24 h on Q235 steel. For “NO MF”, the release route mainly contains the interlayer intraparticle diffusion between the LDH layers and interparticle diffusion among the outer LDH. While for “MF ON” mode, the diffusion between magnetic particles of corrosion inhibitor becomes important. The reconstructed inhibitor still maintains excellent superparamagnetic properties and high corrosion inhibition efficiency, up to 64.84% after five cycles. © 2019 Elsevier B.V. All rights reserved.

Keywords: Magnetic Layered double hydroxide Inhibition Core-shell Molybdate Ion exchange

1. Introduction Seawater circulating cooling system has aroused great interest because of its many advantages, such as abundant resources, reducing environmental pollution and saving freshwater resources [1,2]. While the corrosion problem of metal used as the application equipment in seawater is serious and assignable since it usually brings huge economic losses and hidden dangers [3,4]. Among many methods to resist corrosion, addition of green and environmentally friendly corrosion inhibitors is an effective way to slow down corrosion rate and prolong the service life of metal

* Corresponding author. State Key Laboratory of Hydraulic Engineering Simulation and Safety, Tianjin University, Tianjin, 300072, PR China. E-mail address: [email protected] (J. Wang). https://doi.org/10.1016/j.jallcom.2019.06.118 0925-8388/© 2019 Elsevier B.V. All rights reserved.

equipment [5,6]. However, there are still problems of short action time and environmental pollution [7]. In recent years, the controlled release of inhibitor by loading inhibitor into nanocontainer has gained much attention. Common nanocontainers include polymer microcapsules [8], mesoporous silica [9], layered double hydroxides (LDH) [10], halloysite nanotubes (HNTs) [11], etc. Among them, LDH is a kind of compound which is assembled by non-covalent bond interaction between positively charged host lamellar and interlayer anions, with general formula of [M2þ13þ xþ (An
)x/n, mH2O (M2þ and M3þ are divalent and xM x(OH)2] trivalent lamellar metal cations, respectively, An
is interlayer anion) [12,13]. LDH has been widely applied in catalysis, pollutant adsorption, and drug delivery on account of their characteristics of adjustable modification of host lamellar metal ions, exchangeability of interlayer anions, adsorption, thermal stability and memory effect [14e16]. Its special lamellar structure make it is

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extensively used as nanocontainer for loading corrosion inhibitors, thus achieving controlled release of corrosion inhibitor. Wang et al. [17] synthesized benzoate anion intercalated Zn-Al LDH and studied the release kinetics of the inhibitor and its anticorrosion capabilities toward Q235 carbon steel. Molybdate (MoO2
4 ) inhibitor intercalated Zn-Al-Ce LDH was prepared by co-precipitation method, and its inhibitory effect on carbon steel in simulated seawater medium was studied [4]. Recently, in order to achieve targeted positioning, further controlled release and recycling properties, combination of magnetic nanoparticles with the LDH has attracted increasing attention. Among them, Fe3O4 is the most commonly used nanoscale magnetic materials with superparamagnetic properties [18e20]. CO2
3 intercalated MgAl LDH composite with core-shell structure where the LDH as shell and Fe3O4 as core was fabricated. By controlling the ratio of methanol to water in the solvent during the process of synthesis, LDH was grown vertically on the surface of Fe3O4 nanoparticles, resulting in a large specific surface area of the prepared sample, achieving good adsorption performance for anionic dyes in wastewater and good recycle ability [21]. The ibuprofen (drug) [22], salicylic acid (drug) [23] intercalated magnetic LDH with core-shell structure were synthesized by one-step co-precipitation method and the effects of magnetic core content and applied magnetic field (MF) on drug release rate were also investigated, simultaneously the targeted release of drug was realized. However, it should be pointed out that there are few studies on LDH-based magnetic nanocomposite corrosion inhibitor. In this study, the magnetic Fe3O4 with large particle size was prepared firstly. The composite material with magnetic Fe3O4 as core and NO3-LDH as shell was synthesized by co-precipitation method. Subsequently the MoO2
4 inhibitor was inserted into the LDH layer via ion exchange method. The morphology, structure and magnetic property of the composite were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM). The MoO2
release curves were measured by 4 UVevis spectrophotometer. Polarization curves, electrochemical impedance spectroscopy and scanning electron microscope (SEM) were used to investigate the corrosion inhibition of the composite on Q235 steel. Furthermore, the release mechanism and corrosion resistance were discussed. Meanwhile the recycling and reusing properties of the composite were also studied.

for 8 h. Cooling to room temperature, the precipitated black product was collected by permanent magnet, repeated washed with water and ethanol and then dried at 60  C for 24 h to obtain Fe3O4 nanoparticles. 2.2.2. Synthesis of Fe3O4@NO3-LDH Fe3O4@NO3-LDH was synthesized by coprecipitation method. Briefly, 0.463 g as-prepared Fe3O4 NPs was dispersed in 100 ml of methanol and treated by ultrasonic for 15min to obtain a uniform suspension. Then the suspension was transferred into a 500 ml four-necked flask and stirred in water bath at 60  C. An alkaline solution (methanol solution of 0.5 mol/L NaOH) was added dropwise to the above suspension to make pH reach 8.5 and maintained for 5min. Subsequently, the salt solution including with Zn(NO3)2$6H2O (1.785 g), Al(NO3)3$9H2O and Ce(NO3)3$9H2O (M2þ/M3þ molar ratio ¼ 3.0/1.0, Al 3þ/Ce 3þ molar ratio ¼ 6.0/1.0) in 60 ml of methanol was dropwise added to the above suspension, synchronously keeping the pH at 8.5 by using the above alkaline solution. N2 is used to remove CO2 throughout the titration process. After dripping, the reactant was aged at 60  C for 24 h. Separating by a magnet, the obtained product Fe3O4@NO3-LDH was washed by water and ethanol for several times until pH is neutral and finally dried at 60  C for 24 h. 2.2.3. Synthesis of Fe3O4@MoO4-LDH Ion exchange method was used to prepare the compound Fe3O4@MoO4-LDH. Specifically, the above synthesized Fe3O4@NO3LDH was dispersed in 200 ml of methanol aqueous solution (Vmethanol/Vwater ¼ 1:1) containing Na2MoO4,2H2O (0.2 mol/L) with high speed agitation for 24 h under N2 atmosphere and room temperature. The products after ion exchange were separated by magnet, washed with ethanol and dried at 60  C for 24 h to acquire the nanocomposite inhibitor Fe3O4@MoO4-LDH. 2.3. Characterization

2. Experimental

The morphology and component of nanocomposite inhibitor were observed and determined by transmission electron microscope (TEM, jem-2100f, Hitachi, Japan) and Fourier transform infrared (FTIR, FTS3000, U.S.). The structure of as-prepared sample was measured by X-ray diffraction (XRD, D8 advanced, Bruker, Germany) in the 2q range of 5e70 at a scan speed of 5 /min. The magnetic properties was investigated by the vibrating sample magnetometer (VSM, squid-vsm, U.S.).

2.1. Materials

2.4. Release tests

FeCl3,6H2O, ethylene glycol, Zn(NO3)2$6H2O are bought by Tianjin kwangfu Fine Chemical Industry Research Institute. Polyethylene glycol (M ¼ 200), Methanol, NaAC,3H2O and Na2MoO4,2H2O are purchased by Tianjin Yuanli Chemical Company. Al(NO3)3$9H2O, Ce(NO3)3$9H2O and NaOH are provided by Shanghai Aladdin Industrial Corporation. All chemical reagents are analytical grade.

Different amounts of Fe3O4@MoO4-LDH powders were added to a beaker containing 100 ml of 3.5% NaCl solution with slowly stirred. In order to simulate the release under magnetic field (“MF ON” mode), a magnet was placed just beside the beaker, remaining the same position during the whole release test. After a certain time interval, three sets of 0.5 ml solution sample which was replaced by another same volume fresh 3.5% NaCl solution was taken out and filtered for determining the MoO2
4 concentration released into the solution. The content of MoO2
was detected by thiocyanate 4 spectrophotometric method using the UVevis spectrophotometer (UV, UV-2700, Shimadzu, Japan) at a maximum absorption of 460 nm. A linear equation of Abs lmax¼460nm ¼ 0.06229 (MoO2
4 , mg/L)-0.02854 was obtained with R2 of 0.99297 in the range of 2 mg/L~100 mg/L to calculate the MoO2
4 concentration. To explore the effect of MF on the release process of inhibitor, a release experiment without additional magnet (“NO MF” mode) was also performed, where the concentration of composite inhibitor is 0.5 g/L.

2.2. Preparation of Fe3O4@MoO4-LDH 2.2.1. Synthesis of Fe3O4 nanoparticles Fe3O4 nanoparticles were synthesized by the solvothermal method. 2.700 g FeCl3,6H2O and 7.200 g NaAC,3H2O were dissolved in 60 ml of ethylene glycol, and the mixture was mechanically stirred for 30 min to form a uniform yellow solution. And then 1 ml polyethylene glycol was added slowly to the above solution. After stirring for another several minutes, the missed solution was transferred to the stainless steel Teflon reactor, keeping at 180  C

X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501

The release behavior of the composite inhibitor with different concentrations in 3.5% NaCl solution was fitted by several kinetic modes [23e26]. (1)The first-order mode: ln CC0t ¼
k1 * t; (2)The C

parabolic diffusion mode:

1
C t 0 t

¼ kp t
0:5 þ b; (3)The Elovich

mode: 1
CC0t ¼ ke lnt þ b; (4)The modified Freundlich mode:   log 1
CC0t ¼ logkf þ alogt; (5)The Bhaskar mode: log CC0t ¼ logkb * t 0:65 . In these modes, Ct and C0 represent the inhibitor content in LDH matrix at the release time of t and 0, respectively, k is the release rate constant, and a and b are constants. The C0 is measured by ultrasonic treatment of composite material for sufficient long time to make completely release of the inhibitor MoO2
4 .

2.5. Inhibitor behavior

(1)

where h1 is the inhibition efficiency; I0 is the corrosion current density of Q235 steel in 3.5% NaCl solution. Icorr is the corrosion current density of Q235 steel in 3.5% NaClþ2 g/L Fe3O4@MoO4-LDH solution at different release time. EIS measurements were conducted at the open-circuit potential with an amplitude signal of 10 mV and the scanning frequency ranged from 100 kHz to 0.01 Hz. The obtained datas of EIS were fitted by ZSimpWin software. The inhibition efficiency of Fe3O4@MoO4-LDH for Q235 steel was expressed as follows:

h2 ¼

Rtotal
Rtotal0 Rtotal

For comparison, the corrosion morphology of Q235 steel immersed in 3.5% NaCl solution was also observed. 2.6. Recycling tests After release test, centrifugal washing and drying of this residue to get the sample (Fe3O4@LDH-re). The Fe3O4@LDH-re was sintered at 450  C for 2 h to obtain sample Fe3O4@LDO. Re-dispersing the pre-sintered Fe3O4@LDO in methanol aqueous solution of Na2MoO4,2H2O (0.2 mol/L) with stirring for 24 h under N2 atmosphere (this process is referred as hydration) to obtain the reconstructed sample (Fe3O4@LDH-hy). The reconstructed samples Fe3O4@LDH-hy were added into 3.5% NaCl solution to conduct the experiment in section 2.5. Repeating the above experimental process, the inhibition efficiency and magnetic properties of the composite corrosion inhibitor during each circle are measured. 3. Results

0.2 g prepared inhibitos was dispersed in 100 ml of 3.5% NaCl solution (2 g/L) with continuous mixing under “NO MF” mode and “MF ON” mode. After a certain release time, the suspension was centrifuged and the Q235 carbon steel was immersed in the acquired supernatant for 30 min to measure the corrosion inhibition of the compound inhibitor for the carbon steel in 3.5% NaCl solution. The corrosion inhibition performance of Fe3O4@MoO4-LDH was measured using polarization curve and electrochemical impedance spectroscopy (EIS) by the electrochemical workstation (Autolab 302 N, Metrohm, Switzerland). A conventional threeelectrode cell with the Q235 acting as the work electrode, saturated calomel electrode (SCE) serving as reference electrode and a platinum plate electrode serving as counter electrode was employed. The Q235 steel plate (wt. %, 0.1C, 0.4Mn, 0.12Si, 0.02S, 0.05P, and Fe balance) used in this experiment were firstly ground with 400#, 800#, 1200# and 2000# abrasive paper, cleaned with ethanol, dried in the air and then encapsulated in epoxy resin to expose an area of 1 cm2. The polarization experiments were performed from
0.5 V to 0.5 V versus open circuit potential at scan rate of 1.667 mV/s. The inhibition efficiency was calculated by the following equations:

I
I h1 ¼ 0 corr I0

491

(2)

where h2 is the inhibition efficiency calculated by EIS fitting; Rtotal0 ð ¼ Rct Þ is the charge transfer resistance of Q235 steel immersed in 3.5% NaCl solution. Rtotal ð ¼ Rfilm þ Rct Þ is the total resistance of Q235 steel in 3.5% NaClþ2 g/L Fe3O4@MoO4-LDH solution released in different time. After different release time, the Q235 steel was immersed in the above acquired supernatant for 4 h and the surface morphology of Q235 steel was observed by Hatchi JSM-7800 scanning electron microscopy equipped with energy dispersive spectrometer (EDS).

3.1. Surface morphology Fig. 1 is the TEM images of Fe3O4, Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH. Fe3O4 is a monodisperse spherical particle with a diameter of 370 nm (Fig. 1a). In Fig. 1b, after coating with NO3LDH, the composite presents a well-defined core-shell structure with black Fe3O4 core and a slightly gray LDH shell completely attached closely to the surface of Fe3O4 core. The LDH sheets stacked arbitrarily with ab-face perpendicular/inclined/parallel to the surface of Fe3O4 [23] with thickness about 20 nm. After ion exchange with MoO2
4 , the core-shell structure of the sample is well maintained (Fig. 1c). Fig. 2 is the EDX mapping of Fe3O4@MoO4-LDH. As can be seen from the figure, the Fe element is located in the center of the composite, while the elements of Zn, Al, Ce and Mo are uniformly distributed throughout the material, fully proving that the core is Fe3O4 and the shell is MoO2
4 intercalated ZnAlCe-LDH. Namely, nano-scale corrosion inhibitor with core-shell structure was successfully synthesized. 3.2. Structure Fig. 3a is the XRD pattern of Fe3O4, Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH. In the XRD pattern of Fe3O4, the peaks at 18.285 , 30.076 , 35.426 , 43.053 , 53.406 , 56.935 and 62.520 corresponded to (111), (220), (311), (400), (422), (511) and (440) are indexed to a typical fcc Fe3O4 phase (JCPDS 79e0418) [27,28]. After generating NO3-LDH on the surface of Fe3O4, in addition to the above diffraction peaks of Fe3O4, there are characteristic diffraction peaks at 11.461, 23.029 , 34.290 and 61.323 , which are indexed to (003), (006), (012) and (110) planes, of hcp LDH phase [29], indicating the NO3-LDH was adhered to the Fe3O4. The basal spacing of LDH in Fe3O4@NO3-LDH calculated by (003) plane is 0.772 nm according to Bragg Equation [4]. After ion exchange with  MoO2
4 , the peak of (003) at 11.276 , shift slightly to the left. Correspondingly, the basal spacing of LDH in Fe3O4@MoO4-LDH (0.799 nm) calculated by (003) plane is a little bigger than that of Fe3O4@NO3-LDH, implying the MoO2
has successfully inserted 4 into the interlayer of LDH via ion exchange. Fig. 3b shows the FTIR pattern of Fe3O4, Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH. In the spectrum of Fe3O4, the peaks at 583 cm
1 is attributed to the vibrational peak of Fe-O [30], suggesting the synthesis of Fe3O4. Coating with NO3-LDH, the peak at 581 cm
1 is assigned to the Fe-O vibration peak of Fe3O4. The strong broad peak at 3455 cm
1 is belong to the symmetrical stretching of hydroxyl OH of LDH layer and LDH interlayer water [31]. The band at

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X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501

Fig. 1. TEM of (a) Fe3O4, (b) Fe3O4@NO3-LDH, (c) Fe3O4@MoO4-LDH.

Fig. 2. EDX mapping of the Fe3O4@MoO4-LDH.

Fig. 3. Xrd (a) and FTIR (b) of Fe3O4, Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH.

1360 cm
1 can be ascribed to the stretching vibration of nitrate [32], indicating the formation of LDH intercalated with NO
3 . The peaks at 772 cm
1 and 430 cm
1 are attributed to M
O (metaloxygen) and M-O-H vibrations [33,34]. For FTIR of Fe3O4@MoO4LDH, peaks at 3431 cm
1 and 1626 cm
1 are designated to stretching vibration of hydroxyl O-H and bending vibration of interlayer water, respectively. Meanwhile the emerging peak at 785 cm
1 which is resulting from the antisymmetric stretching vibration band of Mo-O-Mo [4] and the disappearing of peaks at
1360 cm
1 suggest the successful exchange of MoO2
4 with NO3 from the interlayer of LDH. In addition, the peak appearing at 580 cm
1 coming from Fe3O4 demonstrating that the core-shell structural compound combining MoO4-LDH with Fe3O4 have been synthesized. 3.3. Magnetic properties Fig. 4 is the magnetization curve of Fe3O4, Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH. All the samples show the superparamagnetic properties with rather small coercive force and remanence [23,35]. For Fe3O4 nanoparticles, the magnetic saturation (Ms) is up to 81.1 emu,g
1, showing quite high superparamagnetic properties. While

Fig. 4. Magnetization curve of Fe3O4, Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH (inset: separation/redispersion of Fe3O4@MoO4-LDH with a magnet).

X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501

493

for the composites of Fe3O4@NO3-LDH and Fe3O4@MoO4-LDH, a downward phenomenon of Ms is observed decreasing to 35.0 emu,g
1 and 26.7 emu,g
1, respectively, which is caused by the LDH shell coating on the surface of Fe3O4, shielding slightly from superparamagnetic property of Fe3O4 [23,36]. From the inset chart, it's clearly that the uniformly dispersed sample Fe3O4@MoO4-LDH can be easily separated by the magnet in the 15s. Conversely, after removing the magnetic field, the mixture is uniform immediately after shaking. The good superparamagnetic potential may be used to realize the properties of magnetic controlled release, targeting, and recycling, simultaneously reducing the environmental pollution of the corrosion inhibitor. 3.4. Release behavior Fig. 5 exhibits the release curves of MoO2
4 from Fe3O4@MoO4LDH in 3.5% NaCl solution under “MF ON” and “NO MF” modes. For 0.5 g/L Fe3O4@MoO4-LDH under “NO MF” mode, the release curve presented a rapid release rate in early release stage, the equilibrium release percentage reached 71.95% in 3 h. The t0.5 (Time to reach half release equilibrium) [23] by calculation is 5.22min. After applying MF, the release rate is obviously smaller in early time with an equilibrium release of 49.01% at release time of 7 h. The t0.5 extended a bit to 26.40min. Increasing the concentration of corrosion inhibitor under MF, it can be found that the release curve of MoO2
4 shows a gradually decreasing release rate. The t0.5 of 0.5 g/L, 1 g/L, 2 g/L and 4 g/L are 26.40min, 1.04 h, 1.33 h and 1.60 h respectively, approximately prolonging with the increase of concentration. Notably, with higher inhibitor content the equilibrium released amount also has a downward trend, at the same time, the time for reaching release equilibrium was significantly increased. For 4 g/L Fe3O4@MoO4-LDH, the final equilibrium released amount is only 17.98%, however, it achieves 12 h0 s sustained release. In order to confirm the regulation and control properties of MF on the release of composite inhibitor, the release behavior of inhibitor under periodic MF was measured, as Fig. 6 shows. Evidently, in the early release stage when there is no MF employing, the release rate is fairly fast, reaching 27.25%/h. But the release rate deduces to 4.42%/h as the MF is employed. Remove the magnetic field again, and the release rate increases to 9.43%/h. Similarly, the release rate is much lower as far as the magnetic field is applied one more. With the increase of release time, the release rate of corrosion inhibitor decreases, which is resulting from the decrease of loading capacity. The phenomenon observed implies that the

Fig. 5. The release curve of MoO2
4 from Fe3O4@MoO4-LDH in 3.5% NaCl solution (“NO MF”: under no external MF, “MF ON”: with external MF).

Fig. 6. Released behavior of 2 g/L MoO2
4 from Fe3O4@MoO4-LDH in 3.5% NaCl solution under periodic MF (a) the variation of released amount and release rate as time, (b) corresponding variation of applied external periodic MF as time (on the “Y” axis, “NO MF” represents under no external MF and “MF ON” represents with external MF).

performance of controlled release of this composite can indeed be achieved by applying external MF. 3.5. Electrochemical tests The polarization curves of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH after different release time was measured as Fig. 7 shows. The fitted corrosion potential (Ecorr) and corrosion current density (Icorr) were exhibited in Table 1 and Table 2. For mode of “NO MF” the Ecorr moves positively, the Icorr decreases and the h1 increases (Fig. 7a and Table 1) evidently with the prolongation of release time, showing the characteristics of anode corrosion inhibitor [37,38]. While for “MF ON” mode, the same trend was observed with the Ecorr moving positively, the Icorr and h2 decreasing over release time (Fig. 7b and Table 2). Nevertheless, the inhibition efficiency for “MF ON” mode is obviously lower than that of “NO MF” mode at the same release time, which accounts to the release rate of MoO2
4 was slowed down by external MF, thus leading to the long term favorable inhibition effect. The EIS results of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH under MF and without MF are shown in Fig. 8. For Q235 steel in 3.5% NaCl, there is only one capacitive arc and the radius of capacitive arc is 1012.76 U cm2. For 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH without external MF, the Nyquist plot displays two capacitive arcs, indicating the two constant times. The constant time at higher frequency is resulting from the coating capacitance Qf and coating resistance of inhibitor film Rf, and the constant time at low frequency is owing to double layer capacitance Qdl and charge transfer resistance Rct. Furthermore, the radius of capacitive arc increases with time, which can due to the MoO2
4 amount in the NaCl solution increasing with release time, leading to gradually increasing protective effect. As the inhibitor is putted in and a MF is applied, similarly, two capacitive arcs appear and the radius of capacitive arc increases with time. These are also ascribed to the increasing MoO2
4 amount released to NaCl solution with release time as Fig. 5.

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Fig. 7. Polarization curves of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH after different release time (a) “NO MF” mode, (b) “MF ON” mode.

Table 1 Ecorr, Icorr and inhibition efficiency h1 of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH after different release time under “NO MF” mode. Solution

Ecorr (VSCE)

Icorr (mA,cm2)

h1 (%)

3.5% NaCl 10 min 30 min 1h 2h 4h


0.693
0.653
0.541
0.500
0.495
0.474

37.402 23.222 13.788 9.959 8.364 7.883

e 37.91 63.14 73.37 77.64 78.92

Table 2 Ecorr, Icorr and inhibition efficiency h1 of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH after different release time under “MF ON” mode. Solution

Ecorr (VSCE)

Icorr (mA,cm2)

h1 (%)

3.5% NaCl 2h 4h 8h 12 h 24 h


0.693
0.618
0.560
0.520
0.476
0.473

37.402 22.646 16.707 13.521 10.387 9.135

e 39.45 55.33 63.85 72.23 75.58

The equivalent circuits in Fig. 9 were applied to fit the above measured EIS results and the fitted electrochemical parameters fitted and calculated inhibition efficiency h2 are listed in Table 3. The variation of Rtotal of Q235 steel in 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH under “MF ON” and “NO MF” modes extracted from Table 3 were plotted as Fig. 10. In this Figure, the Rtotal were

gradually increasing with release time for “MF ON” mode. This phenomenon can be explained by release curve of MoO2
4 from the samples in Fig. 5, where the release amount of MoO2
4 increases with the release time, leading to better protective effect in inhibiting corrosion over release time. For “NO MF” mode, the Rtotal also has the same varying tendency. Moreover, it can be observed that the Rtotal under “MF ON” is lower than that of “NO MF” mode. The difference is because of the reduction of MoO2
concentration 4 released to solution under “MF ON” mode at the same release time comparing to “NO MF” mode. When releasing for 24 h, the Rtotal under “MF ON” mode measured is up to 4215.44 U cm2, close to the measured one after 2 h0 s release under “NO MF” mode. That is to say, compared to “NO MF”, the MoO2
4 released from Fe3O4@MoO4LDH under “MF ON” can play a more durable corrosion inhibition role. The inhibition efficiency extracted from Tables 1e3 are plotted with release time, as shown in Fig. 11. In this figure, the inhibition efficiency measured by polarization curves and EIS, for both cases of “MF ON” and “NO MF”, show similar trends. After the large increase in the early stage, the increase of corrosion inhibition efficiency slows down in the later stage. This is consistent with the growth trend of MoO2
4 amount as release time in NaCl solution in Fig. 5, meaning that the higher the MoO2
4 concentration is, the better the corrosion inhibition effect is. In addition, it can be seen the value of inhibition efficiency under MF is lower than those without external MF at the same release time. After releasing for 4 h in the absence of MF, the corrosion inhibition efficiency is more than 74.0%. However, in the presence of MF, the corrosion inhibition efficiency is about 72% after the duration of continuous release is

Fig. 8. The EIS results of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH after different release time (a) “NO MF” mode, (b) “MF ON” mode.

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495

Fig. 9. Equivalent circuit used for Q235 in (a) 3.5% NaCl solution and (b) 3.5% NaClþ2 g/L Fe3O4@ MoO4-LDH, in which Rs is the solution resistance, Qf is the coating capacitance of inhibition film formed on the steel, Rf is the coating resistance of inhibition film formed on the steel, Qdl is the double layer capacitance of substrate steel, Rct is the charge transfer resistance of steel substrate.

Table 3 EIS fitting parameters and inhibition efficiency h2 of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH after different release time under “NO MF” and “MF ON” mode. t

Rs

Qf
2

nf

Rf

Qdl
2

nct

Rct

h2

Rtotal

(U$cm )

(F$cm

e

(U$cm )

(F$cm

e

(U$cm )

(U$cm )

(%)

NO MF

3.5% NaCl 10 min 30 min 1h 2h 4h

5.515 5.849 8.224 9.379 6.685 8.14

e 0.0003784 0.0006315 0.0002642 0.0006225 0.0007117

e 0.7742 0.7111 0.7967 0.7123 0.7772

e 6.963 7.654 8.704 10.26 13.11

0.001236 0.0005111 0.0002417 0.0004185 0.0005631 0.0001781

0.8075 0.8461 0.8852 0.8137 0.8384 0.8794

1182 1863 3112 3824 4222 4427

1182 1863.963 3119.654 3832.704 4232.26 4440.11

e 36.79 62.11 69.16 72.07 74.05

MF ON

3.5% NaCl 2h 4h 8h 12 h 24 h

5.515 6.642 4.958 4.928 4.915 4.899

e 0.001044 0.001068 0.001076 0.001211 0.001360

e 0.7248 0.6294 0.6189 0.6104 0.6036

e 9.87 11.64 12.70 14.99 17.44

0.001236 0.0002765 0.0002538 0.0002638 0.0002005 0.0001573

0.8075 0.8652 0.8589 0.8316 0.8634 0.8858

1182 1919 2757 3210 3803 4198

1182 1928.87 2768.64 3222.7 3817.99 4215.44

e 38.72 57.30 63.32 69.04 71.96

2

)

Fig. 10. The Rtotal variation of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 Fe3O4@MoO4-LDH as release time under NO MF mode and MF ON mode.

2

)

2

2

g/L

24 h. That is, the composite corrosion inhibitor works longer under “MF ON” mode than in the absence of MF. 3.6. Corrosion morphology Fig. 12 is the morphology of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH (release for 2 h and 8 h under “MF ON” mode). For Q235 steel immersed in 3.5% NaCl for 4 h, the steel surface got rough and corroded severely, with many corrosion products on the surface (Fig. 12a). From the EDS of corrosion product mainly containing the Fe and O elements (Fig. 12b), it

Fig. 11. The inhibition efficiency variation of Q235 steel in 3.5% NaCl and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH as release time under “NO MF” and “MF ON” modes (solid: calculated by polarization curves; hollow: calculated by EIS).

means that the corrosion product may be the iron oxides [39]. The Na element is coming from the NaCl leaved on the surface of steel. For Q235 steel immersed in the corrosion solution with Fe3O4@MoO4-LDH nanocomposite inhibitor adding after 2 h of release, the surface was still smooth and not corroded (Fig. 12c). From its enlarged picture in Fig. 12d, an inhibition film is formed on the surface of Q235 steel. With longer release time of 8 h, it can be found that the inhibitor film is denser (Fig. 12e). Combining with the EDS result of emerging elements of Mo (Fig. 12f), it can be inferred that an inhibitor film in ferrous or iron molybdate formed

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X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501

Fig. 12. Morphology and EDS of Q235 steel in 3.5% NaCl (a) SEM200  , (b) EDS, and 3.5% NaCl þ 2 g/L Fe3O4@MoO4-LDH for different release time (c) SEM200  for release 2 h, (d) SEM10000  for release 2 h, (e) SEM10000  for release 8 h, (f) EDS.

X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501

on the Q235 steel surface as reported earlier [4]. Conforming to the release rule of MoO2
4 under “MF ON” mode, the longer the release time, the higher the MoO24 content in the electrolyte solution reacting with Fe3þ, forming denser corrosion inhibition film. The elements of “Zn” and “Al” may be attributed to the hydroxide of Zn and Al in the inhibition film [4]. 3.7. Recycling performance Fig. 13a is the XRD of Fe3O4@LDH-re, Fe3O4@LDO and Fe3O4@LDH-hy within first cycle. For the pattern of Fe3O4@LDH-re, the peaks at 11.523 , 23.029 , 34.412 , 38.987 and 46.124 corresponding to (003), (006), (012), (015) and (018), is belong to characteristic diffraction peaks of LDH. The interplanar spacing calculated is 0.781 nm, meaning that Cl
entering the interlamellar space through ion exchange with MoO2
4 [40]. Upon sintering, the LDH loses the interlayer water and anionic Cl
[40] with the evidence of the disappearance of representative diffraction peaks (003), (006), (012), (015) and (018) of the layered structure. Moreover, the peaks at 31.808 , 34.392 and 36.423 to (100), (002) and (101) planes are assigned to ZnO phase (JCPDS 36e1451) [40,41]. The layered structure of shell was recovered after Fe3O4@LDO-1 was immersed in MoO2
4 methanol aqueous solution according to the reappearance of peaks (003), (006), (012) and (110) in XRD diagram. This is in accord with the shape memory effect of LDH [42]. The basal spacing is 0.771 nm, suggesting that the MoO2
4 inhibitor inserted into the interlayer of LDH once again. For all the XRD spectra, there are diffraction peaks of Fe3O4 phase, indicating that all samples always maintain superparamagnetic property. Fig. 13b is the magnetization curves of Fe3O4@MoO4-LDH during five cycles. In the cyclic test, the composite inhibitor still keeps high

497

Ms and rather small coercive force, showing that the recovered corrosion inhibitor still possesses excellent superparamagnetic property. The EIS for Q235 steel immersed in 3.5% NaCl adding recovered corrosion inhibitor and the corrosion inhibition measured by EIS after 12 h’ release test in five cycles are shown as Fig. 13c and d. With the increase of cycle times the capacitance arc radius (Fig. 13c) and corrosion inhibition efficiency (Fig. 13d) gradually decreases. After five cycles, the corrosion inhibition efficiency measured by EIS is as high as 64.84% (Fig. 13d). From a series of characterization results, a conclusion is drawn that the composite corrosion inhibitor can be indeed used for recycling, and it is a promising corrosion inhibitor in circulating cooling water. 4. Discussion From the above experimental results, it can be found that the nanocomposite corrosion inhibitor not only owns good corrosion inhibition performance, but also has excellent magnetic field control release behavior. In order to reveal the regulation mechanism of magnetic field, the release behavior of corrosion inhibition with different inhibitor concentrations under “NO MF” and “MF ON” modes was studied, and the corrosion resistance mechanism was discussed on this basis. 4.1. Release mechanism For release behavior in Fig. 5, five release kinetics equations [23e26] were used to fit the measured release data, as Fig. 14 shows. The fitted parameters are listed in Table 4. For concentration of 0.5 g/L under “NO MF” mode, the fitted R2 for five equations are 0.72848, 0.95751, 0.96114, 0.95561 and

Fig. 13. Recycling performance of Fe3O4@MoO4-LDH composite inhibitors (a) XRD of Fe3O4@LDH-re, Fe3O4@LDO and Fe3O4@LDH-hy within first cycle, (b) magnetization curves of Fe3O4@MoO4-LDH during five cycles, (c) EIS for Q235 steel immersed in 3.5% NaCl adding recovered corrosion inhibitor, (d) corrosion inhibition efficiency measured by EIS after 12 h0 release test after five cycles.

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Fig. 14. Plot of kinetic modes for the MoO2
4 release from Fe3O4@MoO4-LDH in 3.5% NaCl solution (a) first-order mode, (b) Parabolic diffusion mode, (c) Elovich mode, (d) Modified Freundlich mode, (e) Bhaskar mode.

0.83315, respectively, which illustrates that the release behavior of inhibitor conforms to Parabolic diffusion, Elovich and the Modified Freundlich modes, strongly implying the MoO2
release from 4 Fe3O4@MoO4-LDH consists of the interlayer intraparticle diffusion between the LDH layers and interparticle diffusion among the outer LDH shell. While applying external MF, the R2 for 0.5 g/L are 0.76792, 0.98652, 0.92673, 0.94507, 0.86773, the above three modes still fit better, suggesting the MoO2
4 under “MF ON” mode presents same release mechanism. Differently, it is noted that the Bhaskar equation under “MF ON” mode fits better than that under “NO MF” mode, demonstrating an increase of diffusion occurs

between magnetic nanoparticles under external MF. With the increase of inhibitor concentration from 0.5 g/L to 4 g/ L under “MF ON” mode, the Parabolic diffusion and Modified Freundlich mode are in good agreement with measured release data (R2 in 0.945e0.991). That is to say, the interlayer intraparticle diffusion between the LDH layers and interparticle diffusion among the outer LDH are both involved in the inhibitor diffusion. Moreover, apparently the Bhaskar equation fitting is better and better with increasing concentration from 0.5 g/L to 4 g/L (R2 from 0.867 to 0.955). Based on the above results, a schematic diagram of the release

X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501

499

Table 4 Fitting parameters for several kinetic modes of the measured release curve. Kinetic modes

parameters

0.5 g/L

0.5 g/L

1 g/L

2 g/L

4 g/L

NO MF

MF ON

MF ON

MF ON

MF ON

First-order

-k1 R2


0.07345 0.72848


0.03660 0.76792


0.02118 0.85065


0.01600 0.85358


0.01368 0.87534

Parabolic diffusion

kp b R2

1.54235
0.67971 0.95751

0.51429
0.14552 0.98652

0.23551
0.06614 0.96937

0.16732
0.04486 0.96984

0.12433
0.03064 0.97482

Elovich

ke b R2

0.04838 0.64692 0.96114

0.07333 0.31987 0.92673

0.04932 0.16145 0.90290

0.03938 0.12079 0.91384

0.03393 0.09730 0.89707

Modified Freundlich

kf a R2

0.64340 0.07582 0.95561

0.28649 0.23787 0.94507

0.13690 0.31006 0.98407

0.10097 0.32827 0.98877

0.07932 0.35371 0.99121

Bhaskar

kb R2


0.06822 0.83315


0.04154 0.86773


0.02377 0.93705


0.01796 0.94119


0.01528 0.95504

process of the magnetic composite inhibitors is given, as shown in Fig. 15. In the absence of MF, magnetic nanoparticles exhibit good monodispersity (Fig. 14a), when the dominate release mechanism of MoO2
4 releasing from the compound is from stacked LDH shell to the solution. In general, intralayer diffusion between LDH layers and interparticle diffusion between LDH particles are the main diffusion modes. Whereas, in the presence of MF, the magnetic particles are closely gathered (Fig. 15b), which makes the diffusion path of corrosion inhibitor from the complex to the corrosion medium longer and tortuous by increasing difficulty in diffusion between magnetic nanoparticles. Therefore, in addition to the intralayer diffusion and the diffusion between LDH particles, the diffusion between magnetic particles of corrosion inhibitor becomes reasonably important. With concentration increasing from 0.5 g/L to 4 g/L, closer aggregation occurs on magnetic nanoparticles due to magnetic enhancement with increasing concentration. Thereafter, the diffusion between magnetic particles will be of great difficulty, endowing this path becoming the controlling step for inhibitor from LDH interlayer to release medium. Naturally the R2 of Bhaskar equation is gradually increasing with concentration. Closer aggregation of particles under “MF” mode can commendably interpret that the higher the inhibitor concentration, the smaller the release equilibrium amount. Combining Fig. 5, the change of MF does effectively control the release of corrosion inhibitors in the composite.

4.2. Inhibition mechanism and inhibition efficiency As the composite inhibitor was added to NaCl solution, MoO2
4 in the LDH shell can reduce the chloride ion concentration in the solution by ion exchange, while MoO2
4 released into the solution can form passivation film with dissolved iron ions covering on the whole carbon steel surface, which results in positively shifting of corrosion potential and decreasing of corrosion current density. On the other hand, zinc ion in the shell LDH layer can also be released into the solution and formed zinc hydroxide deposition film on the surface of carbon steel to further improve the corrosion resistance (Figs. 7, 8 and 12). As core of the fabricated composite corrosion inhibitor, Fe3O4 nanoparticles have distinct characteristics of regulation and longterm release after applying external magnetic field due to its superparamagnetic property. When an external MF is imposed, the aggregation occurs between the magnetic nanocomposites as described above, leading to the slow release of MoO2
4 inhibitor (Fig. 5). Therefore, the release time was significantly prolonged (Fig. 5), making long-lasting protective effect of composite inhibitor under “MF ON” mode (Fig. 11). This was also proved in Fig. 12 where with the MoO2
4 content increasing with release time, the inhibition film gets denser and more protective. As shown in Fig. 6, the release process can be further regulated by MF “switch” at any time. Briefly, adjusting the MF “switch” is able to control the release rate of corrosion inhibitor, further control the corrosion.

Fig. 15. The schematic diagram of the release process of the magnetic composite inhibitors (a) “NO MF” mode, (b) “MF ON” mode, (c) high concentration nanocomposites inhibitors under “MF ON” mode.

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Therefore, the magnetic nano-scale corrosion inhibitors with well-defined core-shell structure not only have good corrosion inhibition performance, but also have excellent magnetic field control characteristics. It is a corrosion inhibitor with prospects and application value. 5. Conclusion (1) The superparamagnetic nanocomposite inhibitor Fe3O4@MoO4-LDH has been successfully synthesized by ion exchange. The obtained composite material possesses well defined core-shell structure with MoO2
4 inhibitor intercalated LDH shell, with approximately thickness of 20 nm, uniformly covering on the spherical Fe3O4 nanoparticle surface. (2) The composite corrosion inhibitor has good corrosion inhibition performance over 71.00%. Compared with the nonmagnetic field, the magnetic field application reduces the release rate of the inhibitor with increasing t0.5 from 4.25min to 1.60 h, and accordingly reduces the corrosion inhibition efficiency, but prolongs the protection time up to 24 h. (3) The Fe3O4@MoO4-LDH owns excellent magnetic field control characteristics. Under “NO MF” mode, the release mechanism mainly involves the interlayer intraparticle diffusion between the LDH layers and interparticle diffusion among the outer LDH particles. While under “MF ON” mode, in addition to the interlayer intraparticle diffusion between the LDH and interparticle diffusion among the LDH particles, the diffusion between magnetic nanoparticles is also included. (4) The fabricated Fe3O4@MoO4-LDH can be recycled, still owning superparamagnetic property with magnetic saturation of 21.3 emu,g
1 and good corrosion inhibition efficiency as high as 64.84% after five cycles. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51771133 and No. 51471117) and National Basic Research Program of China (2014CB046801). References [1] Y.H. Gao, L.H. Fan, L. Ward, Z.F. Liu, Synthesis of polyaspartic acid derivative and evaluation of its corrosion and scale inhibition performance in seawater utilization, Desalination 365 (2015) 220e226. https://doi.org/10.1016/j.desal. 2015.03.006. [2] M. Al-Bloushi, J. Saththasivam, S. Jeong, G.L. Amy, T. Leiknes, Effect of organic on chemical oxidation for biofouling control in pilot-scale seawater cooling towers, J. Water Process Eng. 20 (2017) 1e7. https://doi.org/10.1016/j.jwpe. 2017.09.002. [3] X.N. Qi, Y.Q. Liu, Q.J. Guo, J. Yu, S.S. Yu, Performance prediction of seawater shower cooling towers, Energy 97 (2016) 435e443. https://doi.org/10.1016/j. energy.2015.12.125. [4] H.J. Yan, J.H. Wang, W.B. Hu, Preparation and inhibition properties of molybdate intercalated ZnAlCe layered double hydroxide, J. Alloy. Comp. 678 (2016) 171e178. https://doi.org/10.1016/j.jallcom.2016.03.281. [5] C.J. He, Z.P. Tian, B.R. Zhang, Y. Lin, X. Chen, M.J. Wang, F.T. Li, Inhibition effect of environment-friendly inhibitors on the corrosion of carbon steel in recirculating cooling water, Ind. Eng. Chem. Res. 54 (2015) 1971e1981. https:// pubs.acs.org.ccindex.cn/doi/10.1021/ie504616z. [6] F. Liu, X.H. Liu, W. Yang, J.J. Lu, H.Y. Zhong, X. Chang, C.C. Zhao, Optimizations of inhibitors compounding and applied conditions in simulated circulating cooling water system, Desalination 313 (2013) 18e27. https://doi.org/10. 1016/j.desal.2012.11.028. [7] W.H. Li, A. Liu, H.W. Tian, D.P. Wang, Controlled release of nitrate and molybdate intercalated in Zn-Al-layered double hydroxide nanocontainers towards marine anticorrosion applications, Colloid Interface Sci. Commun. 24 (2018) 18e23. https://doi.org/10.1016/j.colcom.2018.03.003. [8] D.A. Leal, I.C. Riegel-Vidotti, M.G.S. Ferreira, C.E.B. Marinoa, Smart coating based on double stimuli-responsive microcapsules containing linseed oil and benzotriazole for active corrosion protection, Corros. Sci. 130 (2017) 56e63. https://doi.org/10.1016/j.corsci.2017.10.009.

[9] J.B. Xu, Y.Q. Cao, L. Fang, J.M. Hu, A One-Step preparation of inhibitor-loaded silica nanocontainers for self-healing coating, Corros. Sci. 140 (2018) 349e362. https://doi.org/10.1016/j.corsci.2018.05.030. [10] T.T.X. Tang, T.A. Truc, N.T. Duong, T. Hoang, Preparation and characterization of nanocontainers of corrosion inhibitor based on layered double hydroxides, Appl. Clay Sci. 67e68 (2012) 18e25. https://doi.org/10.1016/j.clay.2012.07. 004. [11] X.T. Xing, J.H. Wang, W.B. Hu, Inhibition behavior of Cu-benzoltriazolecalcium alginate gel beads by piercing and solidification, Mater. Des. 126 (2017) 322e330. https://doi.org/10.1016/j.matdes.2017.04.024. [12] Z.L. Meng, Y.H. Zhang, Q. Zhang, X. Chen, L.P. Liu, S. Komarneni, F.Z. Lv, Novel synthesis of layered double hydroxides (LDHs) from zinc hydroxide, Appl. Surf. Sci. 396 (2017) 799e803. https://doi.org/10.1016/j.apsusc.2016.11.032. [13] S.J. Yu, X.X. Wang, Z.S. Chen, J. Wang, S.H. Wang, T. Hayat, X.K. Wang, Layered double hydroxide intercalated with aromatic acid anions for the efficient capture of aniline from aqueous solution, J. Hazard Mater. 321 (2017) 111e120. https://doi.org/10.1016/j.jhazmat.2016.09.009. [14] K. Yan, Y.Q. Liu, Y.R. Lu, J.J. Chai, L.P. Sun, Catalytic application of layered double hydroxide-derived catalysts for the conversion of biomass-derived molecules, Catal. Sci. Technol. 7 (2017) 1622e1645. https://pubs.rsc.org/en/ Content/ArticleLanding/2017/CY/C7CY00274B#!divAbstract. [15] W. Yao, S.J. Yu, J. Wang, Y.D. Zou, S.S. Lu, Y.J. Ai, N.S. Alharbi, A. Alsaedi, T. Hayat, X.K. Wang, Enhanced removal of methyl orange on calcined glycerolmodified nanocrystallined Mg/Al layered double hydroxides, Chem. Eng. J. 307 (2017) 476e486. https://doi.org/10.1016/j.cej.2016.08.117. [16] S. Senapati, R. Thakur, S.P. Verma, S. Duggal, D.P. Mishra, P. Das, T. Shripathi, M. Kumar, D. Rana, P. Maiti, Layered double hydroxides as effective carrier for anticancer drugs and tailoring of release rate through interlayer anions, J. Control. Release 224 (2016) 186e198. https://doi.org/10.1016/j.jconrel. 2016.01.016. [17] Y. Wang, D. Zhang, Synthesis, characterization, and controlled release anticorrosion behavior of benzoate intercalated Zn-Al layered double hydroxides, Mater. Res. Bull. 47 (2012) 3185e3194. https://doi.org/10.1016/j.materresbull. 2011.07.021. [18] X.L. Zhao, S.L. Liu, P.F. Wang, Z. Tang, H.Y. Niu, Y.Q. Cai, F.C. Wu, H. Wang, W. Meng, J.P. Giesy, Surfactant-modified flowerlike layered double hydroxidecoated magnetic nanoparticles for preconcentration of phthalate esters from environmental water samples, J. Chromatogr. A 1414 (2015) 22e30. https:// doi.org/10.1016/j.chroma.2015.07.105. [19] Y. Li, H.Y. Bi, H. Li, X.M. Mao, Y.Q. Liang, Synthesis, characterization, and sustained release property of Fe3O4@(enrofloxacin-layered double hydroxides) nanocomposite, Mater. Sci. Eng. C 78 (2017) 886e891. https://doi.org/10. 1016/j.msec.2017.04.104. [20] R.R. Shan, L.G. Yan, K. Yang, Y.F. Hao, B. Du, Adsorption of Cd(II) by Mg-Al-CO3and magnetic Fe3O4/Mg-Al-CO3-layered double hydroxides: kinetic, isothermal, thermodynamic and mechanistic studies, J. Hazard Mater. 29 (2015) 42e49. https://doi.org/10.1016/j.jhazmat.2015.06.003. [21] Q.J. Yan, Z. Zhang, Y.L. Zhang, A. Umar, Z.H. Guo, D. O'Hare, Q. Wang, Hierarchical Fe3O4 core-shell layered double hydroxide composites as magnetic adsorbents for anionic dye removal from wastewater, Eur. J. Inorg. Chem. 25 (2015) 4182e4191. https://doi.org/10.1002/ejic.201500650. [22] H. Zhang, D.K. Pan, X. Duan, Synthesis, characterization, and magnetically controlled release behavior of novel core-Shell structural magnetic ibuprofenintercalated LDH nanohybrids, J. Phys. Chem. C 113 (2009) 12140e12148. https://pubs.acs.org.ccindex.cn/doi/10.1021/jp901060v. [23] X. Bi, T. Fan, H. Zhang, Novel morphology-controlled hierarchical core@shell structural organo-layered double hydroxides magnetic nanovehicles for drug release, ACS Appl. Mater. Interfaces 6 (2014) 20498e20509. https://pubs.acs. org.ccindex.cn/doi/10.1021/am506113s. [24] X.G. Kong, L. Jin, M. Wei, X. Duan, Antioxidant drugs intercalated into layered double hydroxide: structure and in vitro release, Appl. Clay Sci. 49 (2010) 324e329. [25] M. Yasaei, M. Khakbiz, E. Ghasemi, A. Zamanian, Synthesis and characterization of ZnAl-NO3(-CO3) layered double hydroxide: a novel structure for intercalation and release of simvastatin, Appl. Surf. Sci. 467e468 (2019) 782e791. https://doi.org/10.1016/j.apsusc.2018.10.202. [26] Z. Gu, A.C. Thomas, Z.P. Xu, J.H. Campbell, G.Q. Lu, In vitro sustained release of LMWH from mgal-layered double hydroxide nanohybrids, Chem. Mater. 20 (2008) 3715e3722. https://pubs.acs.org.ccindex.cn/doi/10.1021/cm703602t. [27] R.R. Shan, L.G. Yan, K. Yang, S.J. Yu, Y.F. Hao, H.Q. Yu, B. Du, Magnetic Fe3O4/ MgAl-LDH composite for effective removal of three red dyes from aqueous solution, Chem. Eng. J. 252 (2014) 38e46. https://doi.org/10.1016/j.cej.2014. 04.105. [28] P. Kumar, K. Gill, S. Kumar, S.K. Ganguly, S.L. Jain, Magnetic Fe3O4@MgAl-LDH composite grafted with cobalt phthalocyanine as an efficient heterogeneous catalyst for the oxidation of mercaptans, J. Mol. Catal. A Chem. 401 (2015) 48e54. https://doi.org/10.1016/j.molcata.2015.03.001. [29] H. Zhang, G.Y. Zhang, X. Bi, X.T. Chen, Facile assembly of a hierarchical core@ shell Fe3O4@CuMgAl-LDH (layered double hydroxide) magnetic nanocatalyst for the hydroxylation of phenol, J. Mater. Chem. A 1 (2013) 5934e5942. https://pubs.rsc.org/en/Content/ArticleLanding/2013/TA/c3ta10349h#! divAbstract. [30] T. Fan, D.K. Pan, H. Zhang, Study on formation mechanism by monitoring the morphology and structure evolution of nearly monodispersed Fe3O4 submicroparticles with controlled particle sizes, Ind. Eng. Chem. Res. 50 (2011)

X. Liu et al. / Journal of Alloys and Compounds 801 (2019) 489e501 9009e9018. https://pubs.acs.org.ccindex.cn/doi/10.1021/ie200970j. [31] L. Li, Y.J. Feng, Y.S. Li, W.R. Zhao, J.L. Shi, Fe3O4 core/layered double hydroxide shell nanocomposite: versatile magnetic matrix for anionic functional materials, Angew. Chem. Int. Ed. 48 (2009) 5888e5892. https://doi.org/10.1002/ anie.200901730. [32] L. Mohapatra, K. Parida, M. Satpathy, Molybdate/tungstate intercalated oxobridged Zn/Y LDH for solar light induced photodegradation of organic pollutants, J. Phys. Chem. C 116 (2012) 13063e13070. https://pubs.acs.org. ccindex.cn/doi/10.1021/jp300066g. [33] L. Deng, W.P. Jia, W. Zheng, H. Liu, D.G. Jiang, Z.M. Li, Y. Tian, W.L. Zhang, J.Q. Liu, Hierarchically magnetic Ni-Al binary layered double hydroxides: towards tunable dual electro/magneto-stimuli performances, J. Ind. Eng. Chem. 58 (2018) 163e171. https://doi.org/10.1016/j.jiec.2017.09.021. [34] J. Ni, J.J. Xue, L.F. Xie, J. Shen, G.Y. He, H.Q. Chen, Construction of magnetically separable NiAl LDH/Fe3O4-RGO nanocomposites with enhanced photocatalytic performance under visible light, Phys. Chem. Chem. Phys. 20 (2018) 414e421. https://pubs.rsc.org/en/Content/ArticleLanding/2018/CP/ C7CP06682A#!divAbstract. [35] D.X. Yang, X.X. Wang, N. Wang, G.X. Zhao, G. Song, D.Y. Chen, Y. Liang, T. Wen, H.Q. Wang, T. Hayat, A. Alsaedi, X.K. Wang, S.H. Wang, In-situ growth of hierarchical layered double hydroxide on polydopamine-encapsulated hollow Fe3O4 microspheres for efficient removal and recovery of U(VI), J. Clean. Prod. 172 (2018) 2033e2044. https://doi.org/10.1016/j.jclepro.2017.11.219. [36] X.T. Chen, F. Mi, H. Zhang, H.Q. Zhang, Facile synthesis of a novel magnetic core-shell hierarchical composite submicrospheres Fe3O4@CuNiAl-LDH under

[37]

[38]

[39]

[40]

[41]

[42]

501

ambient conditions, Mater. Lett. 69 (2012) 48e51. https://doi.org/10.1016/j. matlet.2011.11.052. F.B. Ravari, S. Mohammadi, A. Dadgarinezhad, Corrosion inhibition of mild steel in stimulated cooling water by blends of molybdate, nitrite and picrate as new anodic inhibitor, Anti-corrosion Methods & Mater. 59 (2012) 182e189. https://doi.org/10.1108/00035591211242014. K. Aramak, An attempt to prepare nonchromate, self-healing protective films containing molybdate on iron, Corrosion 55 (1999) 1020e1030. https://doi. org/10.5006/1.3283939. M.Y. Miao, J.H. Wang, W.B. Hu, Synthesis, characterization and inhibition properties of ZnAlCe layered double hydroxide intercalated with 1hydroxyethylidene-1,1-diphosphonic acid, Colloids Surf., A 543 (2018) 144e154. https://doi.org/10.1016/j.colsurfa.2018.01.056. A.A.A. Ahmed, Z.A. Talib, M.Z. Hussein, Y.A.A. Al-Magdashi, New DC conductivity spectra of Zn-Al layered double hydroxide (Zn-Al-NO3-LDH) and its calcined product of ZnO phase, AIMS Mater. Sci. 4 (2017) 670e679. https:// doi.org/10.3934/matersci.2017.3.670. X.Y. Yuan, Q.Y. Jing, J.T. Chen, L. Li, Photocatalytic Cr(VI) reduction by mixed metal oxide derived from ZnAl layered double hydroxide, Appl. Clay Sci. 143 (2017) 168e174. https://doi.org/10.1016/j.clay.2017.03.034. S. Tang, H.C. Guo, H.K. Lee, Magnetic core-shell iron(II,III) oxide@layered double oxide microspheres for removal of 2,5-dihydroxybenzoic acid from aqueous solutions, J. Colloid Interface Sci. 437 (2015) 316e323. https://doi. org/10.1016/j.jcis.2014.09.038.