Synthesis of Metal-Organic Framework ZnO x -MOF@MnO2 Composites for Selective Removal of Strontium Ions from Aqueous Solutions.

A Zn(II)-based metal-organic framework (MOF) compound and MnO2 were used to prepare ZnO x -MOF@MnO2 composites for selective Sr2+ removal in aqueous solutions. The ZnO x -MOF@MnO2 composites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, and Brunauer-Emmett-Teller surface area analysis. The functional groups, morphologies, thermal stabilities, and specific surface areas of the composites were suitable for Sr2+ adsorption. A maximum adsorption capacity of 147.094 mg g-1 was observed in batch adsorption experiments, and the sorption isotherms were fit well by the Freundlich model of multilayer adsorption. Adsorption reached equilibrium rapidly in kinetic experiments and followed the pseudo-second-order kinetic model. The adsorption capacity of the ZnO x -MOF@MnO2 composite with the highest MnO2 content was high over a wide pH range, and the composite was highly selective toward Sr2+ in solutions containing coexisting competing ions. Also, it has a good reusability for removing Sr2+.

Introduction

The strontium isotopes 89Sr and 90Sr are fission products found in wastes from nuclear power plants and nuclear fuel reprocessing.[1]90Sr is a particularly hazardous radioactive contaminant because it emits high-energy 0.5460 MeV beta (β) particles and has a long half-life of 29 years.[2] Because it is chemically similar to calcium, 90Sr in the human body can cause osteosarcoma and leukemia.[3] Therefore, the removal of Sr2+ from aqueous media is essential to prevent harm to both the environment and human health. Numerous methods are available for removing Sr2+ from aqueous solutions, including chemical precipitation, solvent extraction, and adsorption.[4,5] Among these techniques, adsorption is simple and well-suited to wastewater treatment.

The removal of Sr2+ using manganese(IV) oxide (MnO2) has been reported by several researchers.[6−8] MnO2 particles have a large Sr2+ sorption capacity, and they are selective for Sr2+ in the presence of competing ions such as Na+, K+, Ca2+, and Mg2+.[6,9] In the present paper, the adsorption of Sr2+ onto MnO2 is highlighted. However, MnO2 nanoparticles have drawbacks, such as a tendency to agglomerate because of van der Waals forces and other interactions. The nanoparticles are also difficult to separate from treated wastewater after adsorption.[10] Nanoparticulate MnO2 must therefore be immobilized onto a support to enhance its Sr2+ adsorption efficiency and reusability.[11] Potentially suitable solid supports for immobilized MnO2 nanoparticles include carbon nanotubes,[12] mesoporous silica,[11] graphene oxide,[13] and metal–organic frameworks (MOFs).[14] Among these candidates, MOFs are particularly attractive because of their high removal efficiency.[15,16] In particular, MOFs have been used as adsorbents for removal of alkaline-earth-metal ions in several studies.[17−19] MOFs are porous materials composed of metal ions and organic ligands linked through coordinate bonds.[20] MOFs have several unique advantages. Depending on their metal constituents and organic ligands, MOFs can exhibit various morphologies. MOFs also feature large specific surface areas, and their pore sizes can be controlled.[21,22] Highly porous MOFs with a carefully designed pore space have been shown to capture metal ions and various pollutants.[23−25] The Zn(II)-based ZnO-MOF has not been widely studied for Sr2+ adsorption. The advantages of the ZnO-MOF include a facile synthesis from inexpensive starting materials.[26,27] ZnO-MOF has a three-dimensional (3D) crystalline structure with cavities for metal ions.[14,28]

The aim of the present study was to synthesize ZnO-MOF/MnO2 (ZnO-MOF@MnO2) composites for the adsorption of Sr2+ in aqueous media. A ZnO-MOF was impregnated with MnO2 particles via alkali precipitation. To improve the Sr2+ adsorption capacity of the ZnO-MOF, we prepared ZnO-MOF@MnO2 composites with different MnO2 contents. The ZnO-MOF@MnO2 composites were then characterized using several analytical techniques. Finally, Sr2+ adsorption by the composites was tested under various conditions. Several isotherm and kinetic models were applied to the experimental data to analyze the Sr2+ adsorption behavior of the ZnO-MOF@MnO2 composites.

Results and Discussion

Characterization of ZnO-MOF@MnO2 Composites

The FT-IR spectra of the ZnO-MOF and the ZnO-MOF@MnO2 composites are shown in Figure . The spectrum of the ZnO-MOF includes the various peaks reported by Kabir et al.[27] Specifically, the ZnO-MOF spectrum shows a broad peak from 3350 to 3570 cm–1, which is attributed to O–H groups participating in hydrogen bonding with cations. The broad peak from 1005 to 1120 cm–1 is ascribed to O–H groups participating in hydrogen bonding with SO42– anions. C–O–C symmetric stretching vibrations are observed at 982, 1147, and 2988 cm–1 in the spectrum of the ZnO-MOF.[29] The peaks at 1055 and 1202 cm–1 are attributed to a SO3– vibrational band. The peaks from 2800 to 2300 cm–1 are attributed to C–H stretching vibrations in the ZnO-MOF. Some peaks disappeared after the addition of MnO2, and the spectra of the ZnOx-MOF@MnO2 were nearly identical to each other irrespective of the MnO2 concentration. The peak at 538 cm–1 was attributed to Mn–O bonds in the MnO2 structure. The O–H vibrations in the spectra of the ZnO-MOF@MnO2 composites were the same as those observed from 3200 to 3600 cm–1 in the ZnO-MOF spectrum. The vibrations of O–H groups interacting with adsorbed Mn2+ were observed at 1166, 1172, and 1357 cm–1,[30] whereas hydrogen bonding was indicated by the band at 2081 cm–1. In the spectra of both ZnO-MOF and ZnO-MOF@MnO2, sharp peaks ascribed to O–H vibrations were observed at 1727 cm–1.

Figure 1

FT-IR spectra of the ZnO-MOF and the ZnO-MOF@MnO2 composites.

The powder X-ray diffraction (PXRD) patterns of the ZnO-MOF and the ZnO-MOF@MnO2 composites are shown in Figure . In the ZnO-MOF, 3D hydrogen bonding appeared, indicating interaction between the dimethylammonium cations and sulfate anions.[26] As indicated by the peaks in the pattern of ZnO-MOF, the ZnO-MOF composite was confirmed to be highly crystalline. The PXRD patterns of composites ZnO-MOF@MnO2 (1), (2), and (3) showed broad, low-intensity peaks; however, a peak attributed to MnO2 (ramsdellite) was observed at 22 ≤ 2θ ≤ 25°.[31] Obviously, the strong XRD intensity in ZnO-MOF significantly lowered the XRD peak through impregnated MnO2 particles. Therefore, it is considered that some frameworks have the potential to be destroyed during the MnO2 precipitation process.

Figure 2

PXRD patterns of the ZnO-MOF and the ZnO-MOF@MnO2 composites.

The morphologies of the ZnO-MOF and the ZnO-MOF@MnO2 composites were investigated via SEM (Figure ). The micrographs of the ZnO-MOF show cubic structures and a rough surface. The specimen had a large surface area and pore size, and its morphology was similar to that of a reference sample of ZnO-MOF.[27] The ZnO-MOF@MnO2 composites exhibited the same spherical morphology irrespective of their MnO2 content. The magnified image of the ZnO-MOF@MnO2 composites in Figure reveals nanowire-like structures that joined to form spherical shapes around the ZnO-MOF composite particles.[32,33]

Figure 3

SEM images of the ZnO-MOF and the ZnO-MOF@MnO2 composites.

The contents of MnO2 in the ZnO-MOF@MnO2 (3) composites were confirmed by SEM-EDS (Figure ). EDS analysis analyzes the content according to the sensitivity of each element. Elements such as O and N have low sensitivity, which reduces the accuracy.[34] However, Zn and Mn have high sensitivity and reliable results. Therefore, ZnO-MOF has Zn and S contents of 5.10 and 5.08 at. %, respectively, and in addition, it was confirmed that the ZnO-MOF was well synthesized with high contents of C, N, and O. This may support the results of the FT-IR (Figure ). In addition, ZnO-MOF@MnO2 (3) was found to have 2.55 at. % K content using KMnO4, and the MnO2 content of ZnO-MOF@MnO2 (3) was about 28.4%. Accordingly, ZnO-MOF@MnO2 (1), ZnO-MOF@MnO2 (2), and ZnO-MOF@MnO2 (3), which were synthesized from MnSO4 and KMnO4 by ratio, were approximately 5.68, 11.36, and 28.4% in ZnO-MOF@MnO2, respectively.

Figure 4

Results of SEM-EDS of the ZnO-MOF and the ZnO-MOF@MnO2 (3) composites.

The thermal stabilities of the ZnO-MOF and the ZnO-MOF@MnO2 composites were compared on the basis of their mass losses during thermal decomposition (Figure ). The ZnO-MOF composite was stable to 210 °C, and its mass did not change. However, it lost mass rapidly at higher temperatures. The ZnO-MOF@MnO2 components lost between 10 and 20% of their mass when heated to 1000 °C. Among the three composites, ZnO-MOF@MnO2 (3), which had the highest MnO2 content, was the most thermally stable.

Figure 5

TGA curves of the ZnO-MOF and the ZnO-MOF@MnO2 composites.

The surface areas, pore sizes, and pore volumes of the samples were determined via Brunauer–Emmett–Teller (BET) analysis. The N2 adsorption–desorption isotherms are shown in Figure , and Table summarizes the physical properties of the ZnO-MOF and the ZnO-MOF@MnO2 composites. The MOF structure of the ZnO-MOF exhibited a small surface area of 64.11 m2 g–1, which was not sufficient for Sr2+ adsorption. The ZnO-MOF@MnO2 composites, in which MnO2 was combined with the ZnO-MOF, exhibited larger surface areas than the ZnO-MOF. The surface area of each of the ZnO-MOF@MnO2 composites exceeded 100 m2 g–1, which indicates that the adsorbents had suitable physical properties. The pore sizes and volumes of the ZnO-MOF@MnO2 composites were also suitable for Sr2+ adsorption and were slightly larger than those of the ZnO-MOF.

Figure 6

N2 adsorption–desorption isotherms of the ZnO-MOF and the ZnO-MOF@MnO2 composites.

Table 1

Physical Properties of the ZnO-MOF and the ZnO-MOF@MnO2 Composites

composite	BET surface area (m2 g–1)	pore size (Å)	pore volume (cm3 g–1)
ZnOx-MOF	64.11	33.92	0.085
ZnOx-MOF@MnO2 (1)	162.73	38.39	0.427
ZnOx-MOF@MnO2 (2)	137.78	37.34	0.124
ZnOx-MOF@MnO2 (3)	122.18	37.26	0.091

Equilibrium Adsorption Isotherms

The experimental adsorption isotherms were fitted using the Langmuir,[35] Freundlich,[36] and Temkin[37] equilibrium isotherm models. The equations for the Langmuir, Freundlich, and Temkin models are expressed in eqs –3, respectivelywhere qm is the maximum adsorption capacity of the adsorbent (mg g–1). In eq , b is the Langmuir adsorption constant related to the free energy of adsorption (L mg–1). Kf [(mg g–1)(L mg–1)1/] and n in the Freundlich model are constants. KT (L g–1) and B (L mg–1) in eq are isotherm constants, and R (8.314 J mol–1 K–1) is the universal gas constant.

The error was evaluated by analyzing the correlation coefficient (r2) and chi-squared (χ2) values.[38] The χ2 values were calculated using eq where qe,calc and qe,exp are the calculated and experimental qe values, respectively.

The Sr2+ adsorption isotherms of the MnO2, ZnO-MOF, ZnO-MOF@MnO2 (1), ZnO-MOF@MnO2 (2), and ZnO-MOF@MnO2 (3) are shown in Figure . The experimental data were subjected to regression analysis after fitting with the Langmuir, Freundlich, and Temkin isotherm models. The parameters of each isotherm model are listed for MnO2, ZnO-MOF, and each of the composites in Table . Although MnO2 and ZnO-MOF themselves exhibited good Sr2+ adsorption, the ZnO-MOF@MnO2 composites exhibited higher adsorption capacities. On the basis of the Langmuir model, ZnO-MOF@MnO2 (3) exhibited a maximum adsorption capacity of 147.094 mg g–1. This result confirms that the number of Sr2+ adsorption sites depended on the MnO2 concentration. Thus, increasing the MnO2 concentration resulted in a higher adsorption capacity. The Langmuir model afforded the best fit for the ZnO-MOF adsorption isotherm data (r2 = 0.961), indicating monolayer adsorption of Sr2+ onto the ZnO-MOF composite. By contrast, the best fits of the ZnO-MOF@MnO2 composite isotherm data were obtained with the Freundlich model (r2 > 0.98), suggesting multilayer adsorption. Moreover, Sr2+ adsorption on MnO2 particles was also most suitable for Freundlich. The Temkin isotherm was well-suited for Sr2+ adsorption onto the ZnO-MOF; however, the Temkin r2 values of MnO2 and ZnO-MOF@MnO2 composites were low. The ZnO-MOF@MnO2 (3) composite exhibited an adsorption capacity (qm) for Sr greater than those of previously reported adsorbents (Table ). Among the previously reported adsorbents, MOF/KNiFC, MOF/Fe3O4/KNiFC, and Nd-BTC MOF are MOF-based adsorbents.

Figure 7

Sr2+ adsorption isotherms of MnO2, ZnO-MOF, and the ZnO-MOF@MnO2 composites.

Table 2

Langmuir, Freundlich, and Temkin Isotherm Model Parameters

model	parameter	MnO2	ZnOx-MOF	ZnOx-MOF@MnO2 (1)	ZnOx-MOF@MnO2 (2)	ZnOx-MOF@MnO2 (3)
Langmuir	qm (mg g–1)	95.512	101.985	112.820	135.049	147.094
	b (L mg–1)	0.002	0.006	0.008	0.028	0.048
	r2	0.994	0.961	0.954	0.915	0.937
	χ2	1.380	43.162	53.985	176.608	183.616
Freundlich	Kf [(mg g–1)(L mg–1)1/n]	0.635	1.582	2.007	3.202	3.358
	n	1.382	2.393	5.615	21.760	28.256
	r2	0.996	0.917	0.981	0.982	0.992
	χ2	0.868	92.514	22.836	36.764	22.378
Temkin	KT (L g–1)	0.110	0.087	0.812	12.328	14.661
	B (L mg–1)	9.861	24.483	14.436	21.723	21.844
	r2	0.859	0.945	0.790	0.843	0.877
	χ2	34.129	61.367	246.412	325.058	357.946

Table 3

Comparison of Maximum Adsorption Capacities Reported in Various Studies

adsorbent	maximum adsorption capacity (qm)	ref
ZnOx-MOF@MnO2 (3)	147.094 mg g–1 (1.689 mmol g–1)	this study
MOF/KNiFC	110 mg g–1	(39)
MOF/Fe3O4/KNiFC	90 mg g–1	(39)
Nd-BTC MOF	58 mg g–1	(40)
MnO2–alginate beads	102.0 mg g–1	(7)
bacterial cellulose membrane (BCM)	44.86 mg g–1	(41)
hydroxyapatite (HAP)	27 μmol g–1	(42)
CTS-g-AMPS-PANI	88.89 mg g–1	(43)
TNTs@DCH18C6	48.97 mg g–1	(44)
alginate microspheres	110 mg g–1	(45)
Fe3O4@titanate fibers	37.1 mg g–1	(46)

The adsorption is greatly dependent on the cation content of solution and binding sites of the adsorbent.[47,48] To investigate the affinity of the adsorbent for the Sr2+ adsorption, distribution coefficient (Kd) was calculated byHerein, Co and Ce are the initial and effluent concentrations (mg L–1) of Sr2+ in the solution, respectively. V is solution volume (mL), and m represents the mass of the adsorbent (g).

Table shows the distribution coefficients of the ZnO-MOF and the ZnO-MOF@MnO2 composites in experimental ranges. The experiments were conducted in 10–400 ppm Sr2+. It is shown that the Kd value increased as the MnO2 concentration increased. ZnO-MOF showed the highest Kd value at 100 ppm, and ZnO-MOF@MnO2 composites all showed the highest Kd value at 10 ppm. This means that, in the case of ZnO-MOF@MnO2, ZnO-MOF@MnO2 can be efficiently adsorbed even at a low concentration of Sr2+. Normally, the adsorption process is not efficient at low concentrations because it is hardly adsorbed at low concentrations even if the qe value is very high at high concentrations. However, it was confirmed that the ZnOMOF@MnO2 synthesized in this study has a high partition coefficient in a wide range from low concentration to high concentration and is effective in various ranges.

Table 4

Distribution Coefficient (Kd) Values of the ZnO-MOF and the ZnO-MOF@MnO2 Composites

parameter	concentration	ZnOx-MOF	ZnOx-MOF@MnO2 (1)	ZnOx-MOF@MnO2 (2)	ZnOx-MOF@MnO2 (3)
Kd (mL g–1)	10 ppm	206.89	9724.12	136630.8	205006.6
	20 ppm	292.99	3473.08	173978.4	182673.5
	50 ppm	428.37	936.95	3911.89	6344.85
	100 ppm	697.06	791.09	1742.48	3300.43
	200 ppm	472.26	472.76	792.50	1050.19
	300 ppm	345.58	419.58	619.46	786.61
	400 ppm	266.99	322.96	461.91	593.67

Adsorption Kinetics

The adsorption behaviors of the composites depended on the duration of contact with the solution. The data was fit using the Lagergren pseudo-first-order[49] and pseudo-second-order[50] modelsandwhere q and qe are the Sr2+ concentrations at time t and at equilibrium, respectively (mg g–1), k1 is the pseudo-first-order constant (min–1), and k2 is the pseudo-second-order constant (g mg–1 min–1).

Nonlinear fits of the experimental concentration vs contact time data obtained with the pseudo-first-order and pseudo-second-order models are shown in Figure . Sr2+ adsorption proceeded rapidly, and 50% of the maximum adsorption capacity at each concentration was attained within 5 min. The higher the concentration of Sr2+, the faster the equilibrium is achieved. The calculated pseudo-first-order and pseudo-second-order parameters are shown in Table . The kinetic behaviors of the composites were best fit with the pseudo-second-order model. The time required to reach equilibrium decreased with increasing Sr2+ concentration, and the corresponding qe values were higher. The results indicate that the adsorption contact time in the isotherm experiments was sufficient.

Figure 8

Nonlinear Sr2+ adsorption kinetics on the ZnO-MOF@MnO2 (3) composite at initial Sr2+ concentrations of 50, 100, and 200 ppm.

Table 5

Pseudo-first-order and Pseudo-second-order Kinetic Model Parameters for Sr2+ Adsorption at Initial Concentrations of 50, 100, and 200 ppm Using ZnO-MOF@MnO2 (3)

	pseudo-first-order model				pseudo-second-order model
[Sr2+] (ppm)	qe (mg g–1)	k1 (min–1)	r2	χ2	qe (mg g–1)	k2 (g mg–1 min–1)	r2	χ2
50	47.055	0.337	0.975	0.0024	48.808	0.015	0.994	0.0006
100	71.201	0.193	0.942	0.0034	76.121	0.004	0.981	0.0011
200	88.816	0.183	0.922	0.0028	94.956	0.003	0.968	0.0007

pH Effect of Sr2+ Adsorption

The pH of a solution strongly influences Sr2+ adsorption.[51] The effect of pH on Sr2+ adsorption by ZnO-MOF@MnO2 (3) was investigated in the pH range of 1 to 11 (Figure ). At low pH (pH 1), the adsorption capacity (qe) was low; Qe values were higher in the range of pH 5 to 11, as determined on the basis of the point-of-zero charge (pHpzc). pHpzc is the point at which the net total charge on the particle is zero. A pHpzc value of 6.4 was obtained from the plot of initial pH vs the difference between the initial and final pH levels (Figure ). The ZnO-MOF@MnO2 (3) composite was positively charged above pH 6.4. Adsorption of Sr2+ onto the composite was inhibited in acidic conditions because of interference by H+ ions.[52]

Figure 9

Effect of pH on Sr2+ adsorption by the ZnO-MOF@MnO2 (3) composite.

Figure 10

Point-of-zero charge (pHpzc) of the ZnO-MOF@MnO2 (3) composite.

Sr2+ Selectivity

Figure shows the impact of competing cations on the Sr2+ adsorption capacity of ZnO-MOF@MnO2 (3). The concentrations of Na+, K+, Ca2+, and Mg2+ ranged from 50 to 400 ppm in a 200 ppm Sr2+ solution. In a pure 200 ppm Sr2+ solution, the qe of ZnO-MOF@MnO2 (3) for Sr2+ was 108.51 mg g–1. However, the concentrations of coexisting cations affected the adsorption capacity of the composite. The divalent Ca2+ and Mg2+ cations, in particular, reduced Sr2+ adsorption. Ca2+ interfered most with Sr2+ adsorption because Ca2+ has an ionic radius very similar to Sr2+.[7] However, seawater contains a higher concentration of Na+, which does not substantially affect the removal of Sr2+.

Figure 11

Effect of competing cations on Sr2+ adsorption by the ZnO-MOF@MnO2 (3) composite.

Reusability

The results of three consecutive adsorption–desorption experiments are shown in Figure . When 0.1 N HNO3 solution, a desorption solution, was used, a desorption rate of 88.28% was obtained from the adsorbent adsorbed with Sr2+ once. The adsorption and desorption of the desorbed adsorbents were conducted two and three times, respectively. The adsorption rates decreased by 6.86 and 14.91%, respectively, and the desorption rates decreased by 86.29 and 78.01%. However, Sr2+ was sufficiently desorbed with 0.1 N HNO3, and the ZnO-MOF@MnO2 (3) adsorbent was judged to be reusable. Additionally, it is confirmed that 0.5 N HNO3 or more was required to complete desorption of Sr2+ and that no leaching of the adsorbent metal was observed.

Figure 12

Reusability assessment of the ZnO-MOF@MnO2 (3) composite by sequential adsorption–desorption.

Conclusions

A Zn(II)-based ZnO-MOF was successfully combined with MnO2 particles to afford ZnO-MOF@MnO2 composites for Sr2+ adsorption in aqueous solutions. The adsorption isotherms of the composites confirmed that the adsorption capacity increased with increasing MnO2 content. The ZnO-MOF@MnO2 (3) composite exhibited a maximum Sr2+ adsorption capacity of 147.094 mg g–1 on the basis of the Langmuir isotherm model. Its goodness of fit was highest with the Freundlich model of multilayer adsorption. Evaluation of the effects of pH, contact time, and coexisting ions confirmed that adsorbent sites were available within a wide pH range (5–11). Adsorption equilibrium was reached rapidly, and the selectivities of the composites for Sr2+ were high. Therefore, the ZnO-MOF@MnO2 composites can be used for the selective removal of Sr2+ in actual radioactive liquid waste.

Materials and Methods

Materials

Zinc sulfate heptahydrate (ZnSO4·7H2O, 99%, MW 287.56 g mol–1), potassium sulfate (K2SO4, 99%, MW 174.24 g mol–1), sodium chloride (NaCl, 99%, MW 58.44 g mol–1), anhydrous magnesium sulfate (MgSO4, 99%, MW 120.37 g mol–1), and ethyl alcohol (C2H5OH, 94%) were purchased from Duksan Pure Chemicals (South Korea). N,N-Dimethylformamide (C3H7NO, 99.5%, MW 73.1 g mol–1), oxalic acid dihydrate (H2C2O4·2H2O, 99.5–100.2%, MW 126.07 g mol–1), 5 N sodium hydroxide standard solution, potassium permanganate (KMnO4, 99.3%, MW 158.03 g mol–1), and manganese(II) sulfate pentahydrate (MnSO4·5H2O, 98%, MW 241.08 g mol–1) were purchased from Daejung Chemicals & Metals (South Korea). Calcium sulfate dihydrate (CaSO4·2H2O, 95%, MW 172.17 g mol–1) was purchased from Oriental Chemical Industries (South Korea). Strontium nitrate (Sr(NO3)2, 99%, MW 211.63 g mol–1) was purchased from Sigma-Aldrich (USA) and used to prepare simulant solutions. All solutions were prepared with deionized (D.I.) water (EXL 5 A16, EXL Service, USA).

Synthesis of ZnO-MOF

ZnO-MOF was synthesized using the method reported by Kabir et al.[27] ZnSO4·7H2O (1 mmol) and H2C2O4·2H2O (2.50 mmol) were dissolved in 10 mL of N,N-dimethylformamide. The solution was stirred at 200 rpm for 30 min and then reacted in a Teflon-lined autoclave at 160 °C for 4 days. The obtained white product was washed several times with ethyl alcohol and dried for 24 h at 60 °C in a vacuum oven.

Synthesis of ZnO-MOF@MnO2 Composites

Dark-violet solutions were obtained by mixing KMnO4 and MnSO4·5H2O. The prepared ZnO-MOF powder (1 g) was added to each solution, and the solutions were stirred for 1 h. The solutions had initial pH values of 1–2, which were adjusted to between 8 and 9.5 using 5 N NaOH. The pH remained stable, and the color of the solutions changed from purple to brown. The brown ZnO-MOF@MnO2 powders were allowed to settle and then collected via filtration using 0.2 μm filters. The powders were washed several times with D.I. water to remove unreacted chemicals. The final products were dried in a 60 °C oven. The Sr2+ adsorption capacity was enhanced with increasing MnO2 content of the composites. ZnO-MOF@MnO2 (1), ZnO-MOF@MnO2 (2), and ZnO-MOF@MnO2 (3) were prepared using 0.02, 0.04, and 0.1 M KMnO4 along with 0.028, 0.056, and 0.14 M MnSO4·5H2O, respectively.

Characterization of ZnO-MOF@MnO2 Composites

FT-IR analysis was performed on a Frontier spectrometer (PerkinElmer, USA). The morphologies of the ZnO-MOF@MnO2 composites were analyzed using an SU8220 field-emission scanning electron microscope (FE-SEM, Hitachi, Japan). The thermal properties of the composites were characterized using a Q600 thermogravimetric analyzer (TA Instruments, Japan). The specific surface areas, pore volumes, and pore sizes of the ZnO-MOF@MnO2 composites were determined via BET analysis using Autosorb-iQ and Quadrasorb SI gas sorption analyzers (Quantachrome Instruments, USA).

Adsorption Experiments

All experiments were performed in duplicate in a batch system using 15 mL polyethylene conical tubes (SPL, South Korea). Simulant solutions with Sr2+ concentrations ranging from 10 to 400 mg L–1 were prepared for the adsorption experiments. Each of the synthesized composites (10 mg) was contacted with a Sr2+ solution (10 mL) at 250 rpm for 24 h at room temperature. For the kinetic experiments, the samples were suspended in 100, 200, and 300 mg L–1 Sr2+ solutions and contacted for various time intervals. Solutions containing 200 ppm Sr2+ were prepared over a range of pH values to evaluate the effect of pH on Sr2+ adsorption. The effect of coexisting ions (i.e., Na+, K+, Ca2+, and Mg2+) on Sr2+ adsorption was evaluated using 200 ppm Sr2+ solutions. To investigate the reusability of adsorbent, three consecutive adsorption–desorption experiments were conducted using 0.1 N HNO3 as the desorption solution. The ZnO-MOF@MnO2 (3) adsorbent was used for the kinetics, pH, competitive adsorption, and reusability experiments. All of the samples were centrifuged at 3500 rpm for 10 min. The solids were removed via filtration using 0.20 μm nitrocellulose membrane filters (Whatman, USA). The Sr2+ concentrations in the filtered solutions were determined using an Optima 2100DV inductively coupled plasma optical emission spectrometer (PerkinElmer, USA).

The equilibrium adsorption capacity (qe, mg g–1) was calculated using eq where Co and Ce are the initial and equilibrium Sr2+ concentrations, respectively, in mg L–1, V is the volume of the Sr2+ simulant solution (mL), and W is the weight of the adsorbent (g).