Low Carbonate Contaminative and Ultrasmall NiAl LDH Prepared by

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11985−11998

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Low Carbonate Contaminative and Ultrasmall NiAl LDH Prepared by Acid Salt Treatment with High Adsorption Capacity of Methyl Orange Chuan Jing,†,‡ Yuxiang Chen,† Xing Zhang,§ Xiaolong Guo,∥ Xiaoying Liu,*,‡ Biqin Dong,⊥ Fan Dong,# Xianming Zhang,‡ Yunqi Liu,‡ Shaochun Li,∇ and Yuxin Zhang*,†,‡

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†

State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P. R. China ‡ Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, P. R. China § Research Institute of Petroleum Engineering Technology, Shengli Oilfield Company, Sinopec, Dongying 257000, P. R. China ∥ College of Aerospace Engineering, and State Key Laboratory of Mechanical Transmissions, Chongqing University, Chongqing 400044, P. R. China ⊥ Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen University, Shenzhen 518060, P. R. China # Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China ∇ School of Civil Engineering, Qingdao University of Science and Technology, Qingdao 266033, P. R. China S Supporting Information *

ABSTRACT: Layered double hydroxides (LDHs) are attracting intense research interests as methyl orange (MO) adsorbent due to the unique anionic exchange ability. Herein, ultrasmall NiAl LDHs with Cl− intercalation were prepared by a facile method combining hydrothermal method with acid salt treatment. As a result, the as-prepared NiAl LDHs displayed favorable removal performance toward MO from aqueous solution due to the negligible carbonate contamination and ultrasmall nanosheets (about 50 nm). The maximum experimental and theoretical adsorption capacities of 853.19 and 900.14 mg g−1 at pH = 7 and T = 298 K for MO were achieved for Ni4Al1−Cl LDH. The adsorption mechanism is dominated by chemisorption, and the adsorption process is spontaneous and endothermic. Possible “intercalation-split mechanism” for MO on NiAl−Cl LDH is proposed to explain the split of (003) plane. The improvement of adsorption capacity indicates that the acid salt treatment is an effective strategy to construct the rapid and high-effective anionic dyes adsorbents. ion exchange,5 biodegradation,6 photocatalysis,7,8 membrane filtration,9 flocculation,10 and so on. Among these attempts, adsorption has attracted intensive attention due to the merits of high efficiency and low cost.11 For these reasons, large amounts of materials have been developed, such as active graphene,12 clay,13 zeolites,14 and metal oxides.15 Although these attempts promote the development of the dye adsorption field greatly, there is still a need to fabricate novel materials for

1. INTRODUCTION Water is the fountain of life and an important resource for a city to sustain growth of production. However, large quantities of industrial dye wastewater are being discharged and causing serious pollution to the natural environment.1 Among dye contaminants, methyl orange (MO) is an azo dye that is widely used in industrial production due to its merit of admirable dyeing effect.2 Unfortunately, methyl orange is a highly toxic and refractory organic compound with a stable chemical structure.3 Hence, how to reduce the dye concentration in water is the urgent problem for human society at present. Over the past years, several techniques have been explored to remove the methyl orange from water, including adsorption,4 © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11985

March 28, 2019 June 5, 2019 June 11, 2019 June 11, 2019 DOI: 10.1021/acs.iecr.9b01706 Ind. Eng. Chem. Res. 2019, 58, 11985−11998

Article

Industrial & Engineering Chemistry Research

tetramine (C6H12N4, HMT) were purchased from Alfa Aesar. HCl, NaOH, NaCO3, and NaCl were purchased from Chuandong Chemical (Group) Co., Ltd. (Chongqing, China). The anion dye of methyl orange (C14H14N3SO3Na, MO) was also purchased from Alfa Aesar and used as an adsorbate in this work. 2.2. Hydrothermal Synthesis of NiAl−CO3 LDH. Ni(NO3)2·6H2O, Al(NO3)3·9H2O, and HMT were added into a 500 mL beaker with 200 mL of boiled deionized water and then stirred for 20 min. The obtained mixture solution was divided into six equal volumes, then transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 140 °C for 12 h. The as-made green powders were collected by centrifugation, washed several times with boiled distilled water and ethanol, and dried at 60 °C for 10 h. The total feeding mole concentration of Ni2+ and Al3+ was 0.0300 mol. The mole ratios of Ni2+ and Al3+ were 2:1, 3:1, 4:1, respectively, and HMT was 0.0260 mol. The powders of NiAl LDHs were labeled according to their Ni2+, Al3+ mole feeding ratio and the intercalated anions, such as Ni2Al1−CO3 LDH, Ni3Al1−CO3 LDH, and Ni4Al1−CO3 LDH. It was worth mentioning that HMT severed as the alkali source and anion provider under the hydrothermal process. The possible reaction mechanism can be described as follows:26

the removal of MO to meet the complex requirements of contemporary society. Layered double hydroxides (LDHs) are one kind of 2D layered material.16,17 LDHs are common employed to remove the anionic dye, such as methyl orange, due to their unique anionic exchange ability and environmental friendliness. Many researchers are developing the dye adsorption properties of LDHs by anion exchange, exfoliation reassembly and reconstruction. For example, Wang et al. prepared a SiO2@ LDH materials with CO3 2− intercalation by a facile coprecipitation method. The adsorption capacity of MO on SiO2@LDH reached 303 mg g−1.18 Fang et al. prepared NO3−type NiFe LDH materials using one step coprecipitation. As a MO adsorbent, the saturated adsorption capacity is found to be 476.2 mg g−1.19 Zaghouane-Boudiaf et al. prepared calcined MgNiAl LDH nanomaterial to uptake the MO from the aqueous solution using the anion reconstruction and achieved the maximal adsorption capacity to 375 mg g−1.20 Although the above work has made valuable contributions to the development of LDH dye absorbents, there are still many problems. First, the exchangeable capacity of anion in LDH layers is unknown. According to the previous report, the anion exchange equilibrium constants followed the sequence CO32− > SO42− > OH− > F− > Cl− > NO3−.21,22 Hence, the CO32− is very difficult to be deintercalated by a normal anion exchange manner due to the exceptionally high affinity between CO32− and the host layer of LDH. Second, the preparation of LDH by coprecipitation usually involves a considerable amount of CO32−, resulting in irreversible interference for anion exchange. Third, LDO (layered double oxide) prepared by LDH calcination is difficult to fully restore to the original layer structure, resulting in performance degradation. Hence, it is urgent to find a new method to prepare LDH with low CO32− contamination and high dye adsorption capacity. Acid salt treatment is first reported by Iyi et al, who discovered that the use of a large amount of salts and a small amount diluted acid was favorable for the CO32− deintercalated and shape maintenance.23 This method is now commonly used to exploit the host layers of LDH.24 However, this method has not yet been used in the preparation of dye adsorbent until now. Moreover, hydrothermal method is necessary to obtain wellcrystallized nanosheets which are more suitable to practical applications.25 In this study, ultrasmall NiAl−Cl LDHs with different Ni2+/ Al3+ molar ratios were synthesized by a facile method combining hydrothermal method with acid salt treatment. In contrast, the NiAl−Cl LDH was also prepared by a simple coprecipitation method. The physical and chemical characteristics of the obtained NiAl LDHs were carefully investigated by XRD, XPS, FT-IR, BET, SEM, and EDS. In order to gain a deeper insight of the adsorption process, the removal capacities of obtained NiAl LDHs were also evaluated. The possible adsorption mechanism was proposed according to characterization results. It is believed that such easily prepared precursor with low cost and high-effective adsorption will contribute to the actual dye sewage disposal.

(CH 2)6 N4 + 6H 2O → 6CH 2O + 4NH3

(1)

NH3 + H 2O ↔ NH+4 + OH‐

(2)

Ni2 + + Al3 + + OH− + CO32 − → NiAlCO32 −

(3)

2.3. Preparation of NiAl−Cl LDH by Acid-Salt Treatment. The Cl− intercalated NiAl LDH was prepared by typical acid salt treatment.24,25 1.00 g of as-prepared NiAl-CO3 LDHs was added into a 1 L beaker containing 1.0 L of boiled deionized water, 5 mM concentrated HCl, and 1.5 M NaCl. The system was sealed and stirred for 24 h. The as-made Cl− intercalated NiAl LDH was centrifuged, washed with boiled distilled water and ethanol, and dried at 60 °C for 10 h, labeled as Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, and Ni4Al1−Cl LDH. The anion exchange mechanism can be described as follows: NiAl−CO3 LDH + H+ + Cl− → NiAl−HCO3− ·Cl− LDH (4)

NiAl−HCO3− · Cl−

+

−

LDH + H + Cl

→ NiAl−Cl LDH + H 2CO3 H 2CO3 ↔ H 2O + CO2

(5) (6)

2.4. Coprecipitation Synthesis of NiAl−Cl LDH. To evaluate the influence of carbonate contamination in different preparation methods, coprecipitation synthesis of Ni4Al1−Cl LDH was carried out. 0.0225 mol of Ni(NO3)2·6H2O and 0.0075 mol of Al(NO3)3·9H2O were added into 20 mL of boiled deionized water to form solution A. Meanwhile, 0.051 mol of NaOH and 0.0255 mol of NaCl were dissolved in 20 mL of boiled deionized water to form solution B. Solutions A and B were dropwise added simultaneously into a roundbottom flask with 160 mL of boiled deionized water by injection pump (injection rate, 1 mL min−1). The pure N2 was bubbled into the round-bottom flask to reduce the contamination of CO2. The obtained mixture solution was also divided into six equal volumes, then transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at

2. MATERIALS AND METHODS 2.1. Materials. All chemical reagents used in this study were of A.R. grade and used without any further purification. Deionized water (18 MΩ) was boiled 30 min to remove the dissolved carbon dioxide and maintained at 60 °C for use. Ni(NO 3 ) 2 ·6H2 O, Al(NO 3) 3 ·9H 2O, and hexamethylene11986

DOI: 10.1021/acs.iecr.9b01706 Ind. Eng. Chem. Res. 2019, 58, 11985−11998

Article


Figure 1. (a) Full XRD patterns and (b) detailed view of XRD patterns of Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, Ni4Al1−Cl LDH, Ni4Al1−Cl LDH-1, and Ni4Al1−CO3 LDH.

140 °C for 12 h. The as-made Cl− intercalated NiAl LDH was centrifuged, washed several times with boiled distilled water and ethanol, and dried at 60 °C for 10 h, labeled as Ni4Al1−Cl LDH-1. 2.5. Characterization of NiAl LDHs. X-ray diffraction (XRD) analyses of NiAl LDHs were determined by Rigaku D/ max-2500 XRD. Scanning electron micrography (SEM) was recorded by Zeiss Auriga FIB/SEM with an energy dispersive X-ray spectrometer (EDS) (Oxford Instruments Isis 300). Specific surface area and porosity of LDHs were determined by N2 adsorption−desorption isotherms using micromeritics Gemini VII. Fourier transform infrared (FTIR) spectroscopy was conducted by Nicolet iS5 Fourier transfer infrared spectrophotometer (Thermo Fisher, USA). X-ray photoelectron spectra (XPS) of the LDHs before and after adsorption were carried out on a Thermo ESCALAB 250Xi X-ray photoelectron spectrometer. The DFT calculations were conducted by Vienna ab initio simulation package (VASP5.4) to evaluate the formation energy of Ni4Al1−CO3 LDH and Ni4Al1−Cl LDH. 2.6. Dye Adsorption Experiments. All dye adsorption experiments were performed at 298 K unless otherwise stated. Briefly, 20 mg of NiAl LDHs was added into a 250 mL breaker with 100 mL of freshly prepared methyl orange (MO) aqueous solution at different initial concentrations from 20 mg L−1 to 200 mg L−1 with 300 min of stirring at 298 K. The concentrations of MO aqueous solution before and after adsorption were investigated by an ultraviolet−visible light (UV−vis) (UV-2450 Shimadzu, Japan) with an integrated sphere attachment in the wave range of 350−600 nm. To investigate the adsorption kinetics, the 4 mL suspensions were extracted at various time intervals. Samples were filtered by injection syringe with 0.25 μm filter to remove adsorbents. The effect of pH was investigated within the pH range from 3 to 11. 1 M HCl or NaOH was used to adjust the pH. The pH of the solutions was measured by pH meter calibrated with standard pH 4.01, 6.84, and 9.18 buffers. The anion effects (including Cl − , NO 3 − , SO 4 2− , CO 3 2−, PO 4 3− ) were investigated individually. The initial concentration of these anions is set to be the same as that of MO (0.367 mM). The removal efficiency (R, %) and adsorption capacity at equilibrium (qe, mg g−1) and adsorption capacity (qt, mg g−1) at certain time of MO from aqueous solution were calculated by eqs 7−9:

R=

C0 − Ce × 100% C0

(7)

qe =

(C0 − Ce)V m

(8)

qt =

(C0 − Ct )V m

(9)

where C0 (mg L−1) is the initial concentration and C0 and Ce (mg L−1) are the concentrations of MO and MB at equilibrium and at time t (min), respectively. V (L) is the volume of the aqueous solution, and m (g) is the mass of adsorbents.

3. RESULTS AND DISCUSSION 3.1. Characterization. XRD patterns of all NiAl LDHs, i.e., NiAl LDH with various concentration of Ni2+ and Al3+ and NiAl LDH prepared with various preparation methods (acid salt treatment, hydrothermal method, coprecipitation method) are presented in Figure 1a, and a detailed view of the XRD patterns is presented in Figure 1b. All the XRD patterns present standard LDH diffraction peak. For instance, the red line is the XRD pattern of Ni4Al1−CO3 LDH. The reflections at 2θ = 11.59°, 22.55°, 35.03, 39.29°, 47.04°, 61.06°, and 62.04° are assigned to (003), (006), (012), (015), (018), (110), and (113) planes, respectively. The diffraction peaks of (110) and (113) indicate the presence of two metal cations (Ni2+ and Al3+) in the host layer. The sharp and symmetric peak of (003) and (006) indicates the admirable crystallization. The d-spacing of (003) plane from XRD pattern is 7.63 Å. To obtain Cl− intercalated LDH, the acid salt treatment was employed, and the corresponding XRD pattern is the orange line in Figure 1a. The d-spacing of (003) plane of Ni4Al1−Cl LDH increases to 7.87 Å. The phenomenon is consistent with the theoretical expectation that the monovalent Cl− is less attractive to the laminate than the bivalent CO32−, resulting in increased layer spacing. The Ni3Al1−Cl LDH and Ni2Al1−Cl LDH are prepared using the same method, but the d-spacing of (003) plane moved to 7.81 and 7.67 Å, respectively, indicating the decrease of interlayer spacing with the increase feeding content of Al3+. The result suggests that the divalent cations of Ni2+ in the crystal lattice are gradually replaced by the trivalent cation of Al3+ and lead to more attractive interaction. The d-spacing (003) of Ni4Al1−Cl 11987


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LDH. Additionally, two characteristic peaks at 861.8 and 879.5 eV can be assigned the satellite peak of Ni 2p1/2 and 2p3/2 (named “Sat.”). A single peak of Al 2p1/2 at 74.2 eV can be assigned to the characteristic peak of Al3+ in hydroxide form. The peak in O 1s region at 531.4 eV is ascribed to hydroxyl groups in Ni4Al1−Cl LDH.28−30 The Cl 2p can be separated into 2p1/2 at 197.6 eV and 2p3/2 at 199.1 eV. The presence of Cl 2p indicates the successful anion exchange between Cl− and CO32− by acid salt treatment. The region of the C 1s spectra can be separated into three peaks. The peak at 284.8 eV is assigned to the C−C coordination of adventitious aliphatic carbon contamination. The peak at 286.3 eV can be assigned to the C−O from the CO32− contamination. The peak at 288.4 eV can be assigned to the CO32− and CO2.31,32 Compared with the three sample of NiAl LDHs (Table 2), the elements of Ni, Al, and O exist in three samples and all the Ni/Al molar ratios are close to 4:1, indicating the successful preparation. However, an obvious difference also can be found in three samples. As shown in Figure 4, the intensities as well as peak areas of the CO32− in C 1s spectra of Ni4Al1−Cl LDH are lower than Ni4Al1−CO3 LDH and Ni4Al1−Cl LDH-1 due to the minimal carbonate contamination. The still existent peak of CO 3 2− should be ascribed to the secondary carbon contamination by carbonate in deionized water and carbon dioxide in air.33,34 Noteworthy, the Ni4Al1−Cl LDH-1 prepared by coprecipitation presents a large amount of C (22.36 atom %), a small amount of N (3.31 atom %), and negligible Cl (2.23 atom %), indicating that this method is not suitable for preparing LDH with Cl− intercalation. The FT-IR spectra of NiAl LDHs are given in Figure 5a. The broad peak at around 3348 cm−1 and weak peak at around 1611 cm−1 are assigned to the bending vibration of water in the interlayer and stretching vibration peak of −OH. The peak in the range from 480 to 730 cm−1 is assigned to lattice vibration of Ni−OH and Al−OH. The broad peak at about 1358 cm−1 is assigned to CO32− and possible NO3−. All the samples present the characteristic peak of CO32−, indicating that the occurrence of carbonate contamination is inevitable. But the lower peak of Ni4Al1−Cl LDH indicates minimal carbonate contamination as compared with the remaining two samples. The N2 adsorption/desorption isotherms and pore-size distribution of NiAl LDHs are shown in Figure 5b, and corresponding data are shown in Table 3. The isotherms

LDH-1 (prepared by coprecipitation) is 7.83 Å which is lower than Ni4Al1−Cl LDH and higher than Ni4Al1−CO3 LDH. The results should be ascribed to co-intercalation of Cl− and CO32− (carbonate contamination from air and water). The lattice parameters of the NiAl LDHs are shown in Table 1. The value Table 1. XRD Data of the As-Synthesized NiAl LDHsa sample

d(003) (nm)

d(110) (nm)

c (nm)

a (nm)

σ (nm2)

Ni2Al1−Cl LDH Ni3Al1−Cl LDH Ni4Al1−Cl LDH Ni4Al1−Cl LDH-1 Ni4Al1−CO3 LDH

0.767 0.781 0.787 0.783 0.763

0.151 0.151 0.152 0.153 0.151

2.301 2.343 2.361 2.349 2.289

0.302 0.302 0.304 0.306 0.302

0.24 0.32 0.40 0.41 0.39

a = 2d(110), c = 3d(003), σ = (a2 sin 60°)/x, x = Al/(Ni + Al).

a

of surface area per unit charge (σ) also decreases with increase of the feeding content of Al3+ (0.24 nm2 at 33.3 atom %, 0.32 nm2 at 25.0 atom %, and 0.40 nm2 at 20.0 atom %). At the same anion, the higher surface density (σ) leads to stronger adsorption capacity for MO.27 Hence, the Ni4Al1−Cl LDH is favorable to adsorption of MO. The formation energy (ΔE, eV) of Ni4Al1−Cl LDH and Ni4Al1−CO3 LDH is defined as follows: ΔE = E Total − (n1E Ni + n2EAl + n3E H + n4EO + n5EC + n6ECl) n (10)

wherein ENi, EAl, EH, EO, EC, and ECl (eV) represent the energy of Ni, Al, H, O, C, and Cl atoms, respectively. n1, n2, n3, n4, n5, and n6 represent the atom number of Ni, Al, H, O, C, and Cl atoms. ETotal (eV) represents the total energy of LDH. As shown in Figure 2, the formation energy of Ni4Al1−Cl LDH (−5.08 eV/atom) is larger than Ni4Al1−CO3 LDH (−5.34 eV/ atom), indicating that the stability of Ni4Al1−Cl LDH is lower than Ni4Al1−CO3 LDH. In other words, the interlaminar anion exchangeability of Ni4Al1−Cl LDH is better than that of Ni4Al1−CO3 LDH. The representative XPS spectra of Ni4Al1−Cl LDH are shown in Figure 3. The binding energy peaks of Ni 2p3/2 and Ni 2p1/2 are located at 855.8 and 873.3 eV, respectively, indicating the presence of Ni2+ in hydroxide form in Ni4Al1−Cl

Figure 2. Formation energy of (a) Ni4Al1−CO3 LDH and (b) Ni4Al1−Cl LDH. 11988


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Figure 3. (a) Full XPS spectra of Ni4Al1−Cl LDH, Ni4Al1−Cl LDH-1, and Ni4Al1-CO3 LDH. High resolution (b) Ni 2p, (c) Al 2p, (d) O 1s, (e) Cl 2p, and (f) C 1s XPS spectra.

Table 2. Atomic Concentration of NiAl LDHs from XPS Spectra atomic concentration (atom %) sample

Ni

Al

C

O

Cl

N

Ni4Al1−Cl LDH Ni4Al1−Cl LDH-1 Ni4Al1−CO3 LDH

15.46 15.60 15.88

4.05 4.85 4.19

18.63 22.36 30.56

44.72 51.65 49.37

17.14 2.23

3.31

Figure 4. C 1s spectra from XPS: (a) Ni4Al1−Cl LDH; (b) Ni4Al1−CO3 LDH; (c) Ni4Al1−Cl LDH-1.

clearly show a type V adsorption/desorption isotherm with H2 type hysteresis loop. The uniform mesopores for Ni4Al1−CO3 LDH, Ni4Al1−Cl LDH-1, and Ni4Al1−Cl LDH are about 3.77, 7.66, and 3.78 nm, respectively. The BET surface areas of Ni4Al1−CO3 LDH, Ni4Al1−Cl LDH-1, and Ni4Al1−Cl LDH are about 13.00, 95.44, and 30.65 m2·g−1, respectively. Compared with the Ni4Al1−CO3 LDH and Ni4Al1−Cl LDH, the pore size has almost no change before and after acid salt treatment, but the BET surface area and total pore volume of Ni4Al1−Cl LDH increase more than double the Ni4Al1−CO3

LDH. The results suggest that using acid salt treatment to prepare Cl− intercalated NiAl LDH does not change the pore size in LDH but increases the pore number so as to increase the specific surface area. In addition, no matter the pore size or BET surface area, the Ni4Al1−Cl LDH-1 is much larger than Ni4Al1−CO3 LDH and Ni4Al1−Cl LDH. The difference comes from the different preparation methods. The surface morphologies of the prepared NiAl LDHs are investigated by scanning electron microscope (Zeiss Auriga FIB/SEM), as shown in Figure 6. The morphological features 11989


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Figure 5. (a) FT-IR spectra and (b) N2 adsorption−desorption isotherms and pore-size distribution (inset) of NiAl LDHs.

increases gradually in acidic pH range until a maximum is reached at pH = 4.5 and decreases slowly at pH = 4.5−7 and then decreases quickly at the alkaline pH range. The removal efficiency of MO at acidic pH range is greater than that of alkaline pH range, which is ascribed to the competition between OH− and MO and the decrease of electrokinetic potential. Of course, if the concentration of H+ continues to increase, i.e., pH = 2.93, the crystal integrity of LDH will be destroyed due to the reaction between LDH and H+. The result is consistent with the decrease of electrokinetic potential as the pH ranges from 4.0 to 2.0. Figure 8c shows the removal efficiency of MO at different competitive anions, such as Cl−, NO3−, SO42−, CO32−, PO43−. The molar concentration of the added competitive anions is the same as that of MO. Obviously, the competitive effects of these anions can be sorted in the order PO43− > CO32− > SO42− > Cl− > NO3−, which is consistent with the previous report.35 The radii of PO43−, CO32−, SO42−, NO3−, Cl− are 0.204, 0.189, 0.218, 0.200, and 0.168 nm, respectively.36 As monovalent anions, the radius of Cl− is lower than NO3−, making it easily enter the interlayer galleries; thus the competitive effect of Cl− is greater than NO3−. Similarly, as bivalent anions, the radius of CO32− is lower than SO42−, making it easily enter the interlayer galleries; thus the competitive effect of CO32− also is greater than SO42−. The trivalent anion of PO43− has some differences which will dissolve the metal cation in LDH, leading to the decrease of adsorption performance directly.37 In general, although NO3− possesses the minimum competitive effect and maximum anion exchange ability, common preparation methods, such as coprecipitation or secondary anion exchange, will lead to serious carbonate contamination, making it difficult to achieve maximum dye adsorption performance. Hence, on the basis of the current preparation technology, the LDH with Cl− intercalation prepared by acid-salt treatment is the optimal anion dye adsorbent. 3.3. Adsorption Isotherms and Adsorption Kinetics. In order to characterize the adsorption isotherms, the Langmuir (red line) and Freundlich (blue line) models are used to analyze the equilibrium adsorption parameters. The Langmuir isotherm is expressed as follows:

Table 3. N2 Adsorption−Desorption Isotherms and PoreSize Distribution Data of the As-Synthesized NiAl LDHs sample

BET surface area (m2 g−1)

total pore volume (cm3 g−1)

average pore size (nm)

Ni4Al1−Cl LDH Ni4Al1−Cl LDH-1 Ni4Al1−CO3 LDH

30.65 95.44 13.00

0.086 0.229 0.033

3.78 7.66 3.81

of all the LDH samples are quite similar even if the feeding mole ratios of cation and anion are changed. The LDH presents a uniform and inerratic hexagon morphology; the lateral dimensions of the NiAl LDHs sheets are about 50 nm, and the thickness is about 10 nm. The ultrasmall nanosheets are easily dispersed in dye solution and provide more adsorption sites. Energy dispersive spectrum (EDS) indicates the coexistence of Ni, Al, O, C, Cl elements in the Ni4Fe1−Cl LDH. From the distribution diagram, a large amount of C comes from the conductive adhesive and a small amount of C comes from the unremoved CO32− in LDH. The result suggests the reliability of prepared LDH with Cl− intercalation by the acid salt method. The photographic image of Ni4Al1−Cl LDH power is shown in Figure 6h. Because the concentration of Ni2+ is larger than Al3+, the present color is the color of Ni2+ (bright green). 3.2. Effect of pH and Competitive Anions for Adsorption. The adsorption capacity was pre-evaluated to find the optimal experimental conditions. The initial MO concentration is appointed as 120 mg L−1 because rapid adsorption occurs and removal efficiency achieves 100% at initial MO concentration of 80 mg L−1 within 20 min (Figure 7). Figure 8a presents the zeta potential of Ni4Al1−Cl LDH for MO adsorption. The electrokinetic potential (ζ) is related to adsorption process. A ζ value greater than 0 indicates that the adsorption process tends to occur under this condition. With the increase of ζ, the tendency of the adsorption process can be increased. Overall, the value of ζ decreases with the increase of the pH and has a positive value except at pH = 12.0, indicating that the LDH possesses positive charge, which is favorable for the adsorption of anionic MO. The ζ reached a maximum value at pH = 4 or 5, suggesting that Ni4Al1−Cl LDH has a tendency to adsorb more MO under these pH values. Figure 8b shows the removal efficiency of MO at different pH values. As the initial pH increased from 2.93 to 11.11, the removal efficiency of MO on Ni4Al1−Cl LDH

qe =

KLqmCe 1 + KLCe

(11)

wherein qm (mg g−1) corresponds to the maximum adsorption capacity of the adsorbent and KL (L mg−1) is the equilibrium 11990


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Figure 6. SEM images of synthetic NiAl LDHs (a−e): (a) Ni2Al1−Cl LDH, (b) Ni3Al1−Cl LDH, (c) Ni4Al1−Cl LDH, (d) Ni4Al1−CO3 LDH, (e) Ni4Al1−Cl LDH-1. EDS mapping (f) and EDS (g) of Ni4Al1−Cl LDH. (h) Photographic image of the Ni4Al1−Cl LDH. (The element of Pt came from additional metal spraying to improve the conductivity of LDHs.)

constant of the adsorption proces. The Freundlich isotherm model is expressed as follows: qe = KFCe1/ n

Figure 9a shows the adsorption isotherms of MO by the NiAl LDHs at various initial concentrations and the fitted curves by using Langmuir and Freundlich models, respectively. The obtained fitting results are shown in Table 4. The correlation coefficient (R2) value of the Langmuir model is much higher than that of Freundlich model. The results indicate that the adsorption of MO by the adsorbent is fitted by the Langmuir isotherm reasonably better and governed by

(12)

wherein KF (mg1−1/n L1/n g−1) is the Freundlich constant and n is the dimensionless unit. A higher n reflects that the adsorption system is more heterogeneous. 11991


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modified bentonite,47 NiO.48 The adsorption density (Γ, mg m−2) can be calculated by the equation Γ = qm/SBET. When considering the adsorption density, the specific adsorption density of MO on Ni4Al1−Cl LDH (29.39 mg m−2) is also outperformed compared to those materials. Ultrasmall nanosheets make the contribution to the highest maximum adsorption capacity. Figure 9b presents the kinetic curves of MO on NiAl LDHs. It can be found that the MO adsorption capacity (qt) of Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, Ni4Al1−Cl LDH, and Ni4Al1−Cl LDH-1 at time (t) increased quickly in the early stages (0−50 min) of the adsorption process followed by a slow adsorption process until approaching a plateau (50−300 min). However, the as-made Ni4Al1−CO3 LDH displays a different kinetics such that qt increased slowly within whole contact time until reaching the maximum at t ∼ 300 min. The discrepancy in kinetics between Ni4Al1−CO3 LDH and other materials is probably due to the ions’ repulsion between CO32− and MO, resulting in enormous difficulty for MO to enter the interlayer galleries of LDH.11 This can be proved by the dissatisfactory adsorption capacity of Ni4Al1−Cl LDH-1, which also contains an appreciable amount of CO32− in the interlayer. The adsorption mechanism is investigated using the pseudofirst-order and pseudo-second-order kinetic models. The kinetic curves of MO adsorption are expressed as follows:

Figure 7. Removal efficiency and adsorption capacity of Ni4Al1−Cl LDH at initial MO concentration of 80 mg L−1.

monolayer and heterogeneous adsorption. The experimental maximum adsorption capacities (qm,exp) of Ni4Al1−CO3 LDH, Ni4Al1−Cl LDH-1, Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, and Ni4Al1−Cl LDH at 200 mg L−1 MO are 125.77, 214.07, 712.23, 823.19, and 853.19 mg g−1, respectively. The theoretical Langmuir maximum adsorption capacities (qm,cal) of Ni4Al1−CO3 LDH, Ni4Al1−Cl LDH-1, Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, and Ni4Al1−Cl LDH are 137.96, 290.10, 749.40, 829.29, and 900.84 mg g−1, respectively. The Ni4A1− CO3 LDH displayed much lower adsorption capacity than the other NiAl LDH adsorbents. The result suggests that CO32− causes serious performance degradation of LDH. The adsorption performance of Ni4Al1−Cl LDH-1, which is prepared by the direct coprecipitation method, is lower than that of Ni4Al1−Cl LDH prepared by acid salt treatment. The declined performance should be ascribed to the unavoidable carbonate contamination in the preparation process. Compared with Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, and Ni4Al1−Cl LDH, the adsorption ability of MO is improved after increasing the atomic concentration of nickel, which can be ascribed to the increase of interlayer spacing and specific surface area and decrease of surface area per unit charge. The maximum adsorption capacities of MO on other adsorbents are summarized in Table 5. As can be seen, the theoretical Langmuir maximum adsorption capacities (qm,cal) of Ni4Al1−Cl LDH prepared by acid salt treatment are much higher than those of other materials such as RGO-NiCr−CO3 LDH,38 Ni/Al@PAB,39 SiO2@MgAl LDH,18 Fe3O4@polydopamine@MgAl LDH,40 ZnAl LDO,41 Cr-doped ZnO,42 NiFe− NO3 LDH,19 Co4Al1−Cl LDH,43 amine modified PIM-1 fibers,44 GO@NiFe LDH,45 [Bi6O5(OH)3]·(NO3)5·3H2O,46

qt = qe(1 − e−k1t )

qt =

(13)

k 2qe 2t 1 + qek 2t

(14)

wherein k1 (h−1) and k2 (h−1) are the kinetic rate constants of pseudo-first-order and pseudo-second-order adsorption, respectively. The corresponding nonlinear fitted plots for MO adsorption are given in Figure 9b, and the obtained parameters are shown in Table 6. The correlation coefficient (R2) value of the pseudo-second-order model is higher than that of the pseudo-first-order model, which suggests that the pseudosecond-order kinetic model is more suitable for describing the kinetic adsorption of NiAl LDHs. These results suggest that the adsorption process is dominated by chemical adsorption rather than physical interaction. The values of qe calculated from the pseudo-second-order kinetic model are 886.98, 841.81, 726.09, 213.48, and 159.56 mg g−1 for Ni4Al1−CO3 LDH, Ni4Al1−Cl LDH-1, Ni2Al1−Cl LDH, Ni3Al1−Cl LDH, and Ni4Al1−Cl LDH at 200 mg L−1 MO, respectively, which are close to the experiment data.

Figure 8. Effects of (a) zeta potential, (b) initial pH, and (c) competitive anions for MO adsorption on Ni4Al1−Cl LDH. 11992


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Figure 9. (a) Adsorption isotherms and (b) adsorption kinetics of MO on NiAl LDHs.

Table 4. Parameters of Adsorption Isotherms for MO Adsorbed into NiAl LDHs at 298 K for Langmuir Model and Freundlich Model Langmuir model

Freundlich model

sample

qm (mg g−1)

KL (L mg−1)

R2

KF (mg1−1/n L1/n g−1)

n

R2

Ni2Al1−Cl LDH Ni3Al1−Cl LDH Ni4Al1−Cl LDH Ni4Al1−Cl LDH-1 Ni4Al1−CO3 LDH

749.40 829.29 900.84 290.10 137.96

0.2757 0.3018 0.3402 0.1110 0.6382

0.9793 0.9633 0.9919 0.9581 0.9870

407.33 426.20 441.05 79.48 73.44

0.1401 0.1619 0.1780 0.3064 0.2054

0.9679 0.9224 0.9721 0.9084 0.9512

Table 5. Comparison of the Maximum Adsorption Capacity (qm) of MO on NiAl LDHs with Other Adsorbents adsorbent

dosage (g L−1)

initial pH

T (K)

adsorption time (min)

qm,cal (mg g−1)

RGO-NiCr-CO3 LDH Ni/Al@PAB SiO2@MgAl LDH Fe3O4@polydopamine@MgAl LDH ZnAl LDO Cr-doped ZnO NiFe-NO3 LDH Co4Al1−Cl LDH amine modified PIM-1 fibers GO@NiFe LDH [Bi6O5(OH)3]·(NO3)5·3H2O modified bentonite NiO active carbon Ni4Al1−Cl LDH

5.00 2.20 0.30 0.10 0.20 1.00 0.20 0.20 0.25 0.20 0.40 0.50 2.00 0.20 0.20

7.0 4.5 4.5 5.6 7.0 7.0 7.0 7.0 6.0 7.0 7.0 6.5 7.0 7.0 7.0

306 303 298 293 298 298 298 298 298 298 298 297 333 318 298

1440 1440 120 560 720 1440 350 300 1440 350 120 1440 1440 240 300

312.50 412.80 166.1 624.90 490.20 310.60 476.20 827.50 312.50 438.00 730.00 333.33 370.37 934.58 900.84

Γ (mg m−2) 3.18 0.36 8.33 2.41 26.68 83.70 3.00 137.99 4.00 0.32 29.39

ref 38 39 18 40 41 42 19 43 44 45 46 47 48 49 this study

Table 6. Pseudo-First-Order and Pseudo-Second-Order Model Constants and Correlation Coefficients for Adsorption of MO on NiAl LDHs at 298 K pseudo-first-order −1

−1

pseudo-second-order 2

sample

k1 (min )

qe (mg g )

R

Ni2Al1−Cl LDH Ni3Al1−Cl LDH Ni4Al1−Cl LDH Ni4Al1−CO3 LDH Ni4Al1−Cl LDH-1

0.1183 0.0957 0.0521 0.0118 0.1819

690.50 759.15 772.25 123.51 205.63

0.9942 0.9921 0.9605 0.9730 0.9806

3.4. Adsorption Thermodynamics. The removal efficiencies of MO on Ni4Al1−Cl LDH at 298, 308, and 318 K are shown in Figure 10a. The experimental maximum adsorption capacities (qm,exp) at 298, 308, and 318 K are 823.19, 868.48, and 890.45 mg g−1, and the theoretical Langmuir maximum

k2 (10

−4

−1

g mg

3.31 2.22 0.93 0.69 19.63

−1

min )

qe (mg g−1)

R2

726.09 806.1354 848.89 159.56 213.48

0.9976 0.9973 0.9972 0.9858 0.9962

adsorption capacities (qm,cal) are 900.84, 842.17, and 976.63 mg g−1. The results suggest that the removal efficiency of MO at 318 K is the highest, indicating that the high temperature enhanced MO adsorption. The thermodynamic parameters 11993


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Figure 10. (a) Effect of temperature for MO adsorption on Ni4Al1−Cl LDH. (b) Van’t Hoff plot for the adsorption of MO dye on Ni4Al1−Cl LDH.

Table 7. Parameters of Adsorption Thermodynamics of MO on Ni4Al1−Cl LDH at Different Temperatures Langmuir model temp (K)

qm (mg g−1)

KL (L mg−1)

R2

KC

ΔG° (kJ mol−1)

ΔH° (kJ mol−1)

ΔS° (J mol−1)

298 308 318

900.84 942.17 976.63

0.2753 0.3298 0.3875

0.9919 0.9618 0.9768

5001324 5991778 7039822

−38.22 −39.96 −41.69

13.47

173.47

Figure 11. (a) XRD patterns, (b) detailed view of (a) from 5° to 28.3°, (c) XPS spectra, (d) FT-IR spectra before and after adsorption of MO by Ni4Al1−Cl LDH.

such as Gibbs free energy change (ΔG°), standard enthalpy

ΔG° = −RT ln(M w × 55.5 × 1000 × KL)

(ΔH°), standard entropy (ΔS°) were calculated as follows: K C = M w × 55.5 × 1000 × KL

ln(M w × 55.5 × 1000 × KL) =

(15) 11994

−ΔH ° 1 ΔS° + R T R

(16)

(17)


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Figure 12. Schematic illustration of possible “intercalation-split mechanism” for MO on Ni4Al1−Cl LDH.

wherein R (8.314 J mol−1 K−1) is the universal gas constant, T (K) is the absolute temperature, and KC (L mg−1) is the dimensionless equilibrium constant calculated from the multiplicative value of molecular weight of adsorbate, 1000, 55.5, and Langmuir thermodynamics constant (KL). The values of thermodynamic parameters at different temperature (298, 308, and 318 K) were estimated by plotting ln KD as a function of 1/T, where ΔH° can be obtained from the slope and ΔS° from the intercept. The obtained parameters for the adsorption of MO on the Ni4Al1−Cl LDH are shown in Table 7. The negative value of ΔH° indicates that the adsorption process is endothermic. The positive value of ΔS° indicates that the adsorbate of MO in the gallery of LDH becomes unordered. The negative value of ΔG° suggested that the adsorption of MO on Ni4Al1−Cl LDH is spontaneous. The absolute value of ΔG° increases with increasing temperature, which suggests that adsorption occurs easily at high temperatures. 3.5. Adsorption Mechanisms. To evaluate the possible adsorption mechanism for MO uptake on Ni4Al1−Cl LDH, the adsorbents before and after adsorption were collected and characterized by XRD, XPS, and FT-IR, and the results are shown in Figure 11. The peak of (003) at 11.23° is divided into three peaks after adsorption at 7.50°, 11.29°, and 14.41°. The peaks at 11.23° before adsorption and at 11.29° after adsorption with slightly difference are assigned to the same peak. The peak at 7.50° is ascribed to the increased interlayer from 7.82 to11.80 Å by anions exchange between Cl− and MO. According to previous reports,50,51 the long axis of MO is 13.15 Å, which is larger than the interlayer spacing (11.80 Å). The result suggests that the molecules of MO arrange in the interlayer with an angle instead of vertically oriented to the host layer. The angle of MO and host layer is about 63.04° (sin θ = 11.80 Å/13.15 Å, θ = 63.04°). However, the peak at 14.41° (6.06 Å) cannot be explained because the intercalation doses not result in increased interlayer spacing. For this, we provided a possible mechanism (named “intercalation-split mechanism”), as shown in Figure 12. It is possible that MO intercalation decreases adjacent to the 003 crystal plane. The intercalation results in the change of three adjacent interlayer

distance; one of the increased interlayers spacing is due to the MO intercalation, and the remaining two of the decreased interlayers are due to compression from adjacent layers. The sum of three values equals 23.92 Å according to the equation dsum = dmax + 2dmin, which is much closer to the 3 times of 003 d-spacing of Ni4Al1−Cl LDH (23.61 Å) according to the equation dsum = 3d. The negligible difference of 0.31 Å may be ascribed to the experimental error. Similar results can be found from the splitting of the 006 peaks. The sum of three interlayers spacing is 11.82 Å according to the equation dsum = dmax + 2dmin, which is much closer to the 3 times of 003 dspacing of Ni4Al1−Cl LDH (12.23 Å) according to the equation dsum = 3d. Hence, this phenomenon confirms the rationality of the speculation. Similar phenomenon can be found in Co4Al1−Cl LDH before and after adsorption of MO, as shown in Figure S1. The XPS spectra of Ni4Al1−Cl LDH before and after MO adsorption are measured, as shown in Figure 11c. It is noted that, besides the main elements including Ni, Al, O observed in both spectra, the N 1s peaks appeared in MO-adsorbed Ni4Al1−Cl LDH. The decreases in these peaks’ intensity can be ascribed to the adsorbed dye molecules on LDH suppressing the signals of these elements. Figure 11d presents the FT-IR spectra of Ni4Al1−Cl LDH before and after adsorption. The significant peak at 1162 cm−1 is ascribed to the stretching vibration of SO, and the sharp peak at 1116 cm−1 is deemed as the characteristic peak of SO32−.52 The peaks at 1029 and 846 cm−1 are considered as the aromatic out-of-plane bending vibration of C−H from the skeleton of benzene ring. 38 Figure S2 presents the N2 adsorption−desorption isotherms and pore-size distribution (inset) of Ni4Al1−Cl LDH before and after adsorption. The BET surface area of Ni4Al1−Cl LDH after adsorption is 0.918 m2·g−1, which is much lower than that of Ni4Al1−Cl LDH after adsorption (30.65 m2·g−1). The change of BET surface area is attributed to the adsorption of methyl orange on the surface and interlayers of Ni4Al1−Cl LDH. Figure S3 presents the TEM images and corresponding EDS mapping of Ni4Al1−Cl LDH after adsorption. Due to the quite high mas loading of MO on LDH, the apparent morphology cannot be observed clearly. The presence of elements of N and S in EDS mapping 11995


Industrial & Engineering Chemistry Research indicates the occurrence of adsorption for MO by Ni4Al1−Cl LDH. All the above peaks suggest that the anion exchange took place and confirm that the anion exchange plays a key role in MO adsorption on LDH. On the basis of the above results, a mechanism of adsorption MO on LDH can be proposed. The main adsorption mechanism of MO on LDH is dominated by intercalation and anion exchange. The CO32− is the main factor to inhibit the anion exchange in adsorption and then cause the LDH performance degradation. The LDH prepared by the acid salts treatment is the most effective method to reduce the CO32− contamination and achieve the admirable Cl− intercalation, which further enhances the adsorption rate and adsorption efficiency from aqueous solution. Ultrasmall LDH nanosheets also facilitate the efficient and fast removal of MO by providing more adsorption sites.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01706. Additional XRD patterns for Co4Al1−Cl LDH (Figure S1), BET for Ni4Al1−Cl LDH before and after adsorption (Figure S2), TEM for Ni4Al1−Cl LDH after adsorption (Figure S3), and the effect of competitive anions for MO adsorption on Ni4Al1−Cl LDH (Table S1) (PDF)

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The authors gratefully acknowledge the financial support provided by the Graduate Research and Innovation of Chongqing, China (Grant CYB18002), the Postgraduate Research Innovation Projects of Chongqing, China (Grant CYS18002), the National Natural Science Foundation of China (Grant 21576034), the Joint Funds of the National Natural Science Foundation of ChinaGuangdong (Grant U1801254), and the State Education Ministry and Fundamental Research Funds for the Central Universities (Grant 2019CDQYCL042).

4. CONCLUSIONS In this study, the ultrasmall NiAl LDH prepared by facile hydrothermal method followed by simple acid salt treatment has been examined by removal of a certain amount of MO from aqueous solutions. The Ni4Al1−Cl LDH achieved excellent removal performance due to Cl− intercalation, maximal charge density, and ultrasmall nanosheets (about 50 nm). The maximum experimental and theoretical adsorption capacities of Ni4Al1−Cl LDH achieved 853.19 and 900.14 mg g−1 at pH = 7 and T = 298 K for MO. The adsorption mechanism for MO on LDH mainly contributed to the anion exchange between MO and Cl−. CO32− is the main factor to inhibit the anion exchange in adsorption and then cause the LDH performance degradation. Inhibiting the carbon contamination is the key for the further development of LDH adsorbent. Possible “intercalation-split mechanism” for MO on NiAl−Cl LDH is proposed first to explain the spit of (003) plane. In a word, this study provides some new scientific insights in rational design of rapid and effective removal anionic dyes by LDH adsorbents.

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AUTHOR INFORMATION

Corresponding Authors

*X.L.: e-mail, [email protected]. *Y.Z.: tel, +86 23 65104131; fax, +86 23 65104131; e-mail, [email protected]. ORCID

Fan Dong: 0000-0003-2890-9964 Yuxin Zhang: 0000-0003-4698-5645 Notes

The authors declare no competing financial interest. 11996


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