Preparation of NiFe-LDHs Adsorbent and Its Adsorption to Methyl Orange

The Ni-Fe-LDHs adsorbent with different molar ratio of Ni/Fe was synthesized with Ni(NO3)2 and Fe(NO3)3 serving as the resource of metal ions and the ammonia water serving as the precipitant. The methyl orange (MO) was chosen to simulate the dye wastewater to study the adsorption ability of different adsorbent to MO. And XRD, FT-IR and BET were employed to characterize the prepared adsorbents. The experimental results show that LDHs with well crystalline structure can be obtained when the mole ratio of Ni/Fe of 2.3~4. The Ni4Fe adsorbent with the Ni/Fe molar ratio of 4 shows the largest adsorbing capacity of 168.4 mg/g to MO. The XRD results of the spent adsorbent show that the LDHs structure was not destroyed. And the BET result of the fresh Ni4Fe adsorbent proves that it is possessed with microstructure with the surface area of 125.325 m2/g.


Introduction
With the rapid development of chemical industry, the problem of water pollution is becoming more serious. The pollutants in water including toxic organic matter, heavy metals, ammonia nitrogen, and so on, can harm the organisms in the water, accumulate in the organisms, transmit through the food chain, and endanger human health [1]. Dye wastewater with large amount has high content of organic pollutants, which is one of the most difficult industrial waste to be treated. The common methods used to treat the dye sewage include chemical oxidation method, photocatalytic method, membrane separation, biodegradation process, and so on [2,3]. However, these methods usually require a complex reaction system with high cost. Adsorption is an effective technology because of its low cost and insensitivity to toxic substances [4].
Hydrotalcite compounds (LDHs) are bimetallic hydroxides with layered structure, which have been widely studied in catalysis, adsorption, electrochemistry and other fields [5,6]. LDHs are usually composed of two different valence cations (divalent and trivalent) and interlayer anions. The general formula of LDHs is [M 1-x 2+ Mx 3+ (OH)2] x+ (A n-)x/n·mH2O. It is reported that under alkaline conditions, LDHs compounds can be formed when the solution contains divalent metal ions and trivalent metal ions with similar radius [7]. Due to the adjustability of the divalent and trivalent metal cations, as well as the intercalation anions, LDHs can realize a variety of molecular assembly, applying to different fields. Wang [8]. In this paper, NiFe-LDHs was prepared by coprecipitation method, and the effect of Ni/Fe molar ratio on its structure and adsorption performance was investigated.

Reagents and instruments
Ni(NO3)2•6H2O, Fe(NO3)3•9H2O and methyl orange (MO) used in this experiment are analytically pure, and the deionized water is self-made in the laboratory. X-ray powder diffractometer (XRD, ultima IV, Japan Science Company), infrared spectrometer (FT-IR, ALHPA, Germany Brooke company), physical adsorption instrument (BET, Autosorb iQ, USA conta instrument company), ultraviolet visible spectrophotometer (UV-5800P, Shanghai Yuanxi Instrument Co., Ltd) are employed to character the properties and adsorption ability of the sample.

Preparation of adsorbent
The preparation of NiFe-LDHs adsorbent is as follows: firstly, a certain amount of Ni(NO3)2•6H2O and Fe(NO3)3•9H2O is dissolved in 20 mL deionized water (the total concentration of metal cations in the solution is 1 mol/L, and the molar ratio of Ni and Fe is 1:1, 2.3:1, 3:1, 4:1, respectively). Then, 100 mL, 0.5 mol/L ammonia solution is quickly added, followed by aging at 65 ℃ for 20 h under stirring condition. After that, the precipitates were centrifuged, and washed several times to neutral with deionized water and anhydrous ethanol. The obtained sample was dried at 80 ℃ for 24 h to prepare NiFe-LDHs adsorbents. According to the different molar ratio of Ni and Fe, the prepared adsorbents were labeled as Ni1Fe, Ni2.3Fe, Ni3Fe and Ni4Fe, respectively.

Adsorption capacity test
In order to eliminate the interference of temperature and light conditions on the adsorption properties of the samples, the adsorption experiments were carried out under the conditions of avoiding light and 30 ℃ water bath. 50 mL of MO solution (100 mg/L) was used to simulate the waste, and 100 mg of adsorbent is used. After stirring the above suspension at 200 r/min of stirring speed for a period of time, 5 mL of sample was taken and measured by UV-Vis spectrophotometer to obtain its concentration. The adsorption rate (removal rate) of MO on the adsorbent was calculated by the following formula: (1) Where, R% is the removal rate; C0 is the initial MO concentration, mg/L; Ct is the concentration of MO at different time, mg/L.

Results and discussion
The MO removal rate of different adsorbents is shown in Figure 1. It can be seen that it increases with the extension of time. In the first 30 min, the MO removal rate increases rapidly, and then slows down. After 180 min, it approaches the adsorption equilibrium. At this time, the MO removal rates of Ni1Fe, Ni2.3Fe, Ni3Fe and Ni4Fe adsorbents are 80.3%, 71.6%, 76.9% and 84.2% respectively, corresponding to the adsorption capacity to MO are 160.6 mg/g, 143.2 mg/g, 153.8 mg/g and 168.4 mg/g respectively.
The XRD spectra of the fresh adsorbent are shown in Figure 2a. It can be seen that the characteristic peaks of the hydrotalcite structure (JCPDS No. 40-0215) at 2θ = 11.5°(003), 22.9° (006), 34.8° (012) and 60.7° (110) [9] are obtained on Ni2.3Fe, Ni3Fe and Ni4Fe samples, indicating that hydrotalcite structure can be formed when the molar ratio of Ni/Fe is 2.3 ~ 4. And the low peak intensity of Ni1Fe compound indicates the low crystalinity. In addition, the peak intensity increases with the increase of Ni/Fe molar ratio, suggesting that hydrotalcite compounds with higher crystallinity, better regularity and symmetry can be formed when Ni/Fe molar ratio is higher. It is pointed out that hydrotalcite materials have good adsorption capacity to the hydrolytic electronegative pollutants, and its adsorption capacity is related to the charge density and van der waals force of the laminate. Increasing the charge density on the laminate, the distance between adsorbed MO molecules decreases and the density increases, leading to the increasing van der Waals force. Hence, it is difficult for MO molecules to further adsorb on the 3 laminate of hydrotalcite materials. Therefore, in this work, although the charge density of Ni4Fe adsorbent is small and the adsorption active sites are fewer, fewer Mo molecules are adsorbed. As a result, the van der Waals force between the molecules is smaller, which is conducive to the further adsorption of Mo molecules on the hydrotalcite laminate. For Ni1Fe sample, its adsorption capacity is higher than that of Ni2.3Fe and Ni3Fe, which should be due to the larger charge density and more active sites. The FT-IR results of fresh adsorbent are shown in Figure 2b. It can be seen that the FT-IR spectra of all adsorbents are similar. The absorption peak at 3447 cm -1 is the expansion vibration peak of O-H on the laminate, and the absorption peak at 1640 cm -1 is the bending vibration peak of O-H on the laminate. The absorption peak at 1384 cm -1 is the asymmetric expansion vibration peak of NO3 -. In addition, the absorption peak below 800 cm -1 is mainly caused by Ni-O, Ni-O-Ni, Fe-O, Fe-O-Fe and Ni-O-Fe on the laminated plates [10]. It can be inferred that NO3deriving from metal nitrate is the interlayer anion of the prepared adsorbent. Ammonia provides the alkaline environment and interlayer OHion. The molecular formula of the prepared sample in this work is [Ni1-xFex(OH)2] x+ (NO3)x, where X is 0.5, 0.3, 0.25 and 0.2, respectively, corresponding to the Ni/Fe molar ratio of 1/1, 2.3/1, 3/1 and 4/1.  The N2 isothermal adsorption-desorption curve of the fresh Ni4Fe adsorbent is shown in Figure. 3. It is the type IV isotherm in IUPAC classification, which is the mesoporous structure with a hysteresis loop caused by capillary condensation in the region of low relative pressure. The porous structure may be formed by the accumulation of lamellar structure of Ni4Fe hydrotalcite. And the specific surface area, average pore size and pore volume of the prepared Ni4Fe adsorbent are 125.325 m 2 /g, 3.4 nm and 0.029 cc/g, respectively. So it has better adsorption performance. The XRD spectra of the spent adsorbent are shown in Figure. 4. It can be seen that, consistent with the fresh adsorbent, Ni2.3Fe, Ni3Fe and Ni4Fe adsorbents have obvious hydrotalcite structural with characteristic peaks at 2θ = 11.5 o (003), 22.9 o (006), 34.8 o (012) and 60.7 o (110). It is worth noting that the diffraction peak intensity of fresh Ni4Fe adsorbent is significantly higher than that of Ni3Fe, and they are almost the same after MO adsorption, which may be due to MO replacing part of the interlayer anions, affecting the regularity and symmetry of its structure.

Conclusion
NiFe-LDHs with nitrate as interlayer anion were prepared with Ni(NO3)2 and Fe(NO3)3 as divalent and trivalent metal cation sources and ammonia as precipitant. The experimental results show that the hydrotalcite structure could be formed when the Ni/Fe ratio is 2.3 ~ 4, and the crystallinity increase with the increase of Ni/Fe ratio. When the Ni/Fe ratio was 1, the diffraction peak intensity is low and broad, which might be the amorphous structure. And there is a close relationship between the adsorption capacity to MO and the charge density, as well as the van der Waals force in the NiFe-LDHs. The charge density of Ni4Fe is smaller, and the van der Waals force between adsorbed MO molecules is also smaller, which is conducive to the further adsorption of MO on the laminate. Therefore, Ni4Fe has the highest adsorption performance for MO in this work.