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 JAMP  Vol.9 No.7 , July 2021
Facile Preparation of Novel Manganese Dioxide Modified Nanofiber and Its Uranium Adsorption Performance
Abstract: In this study a novel manganese dioxide modified nanofiber was facile prepared using the electrospinning technique. The as-prepared manganese dioxide/poly(vinyl alcohol)/poly (acrylic acid) (briefly as MnO2-PVA/PAA) was firstly characterized by SEM, FT-IR, XRD, stress-strain test and secondly tested as an adsorbent to remove uranium from aqueous solution. Effect of pH, ionic strength, initial uranium concentration, mixing time, temperature on the adsorption, reusability and adsorption mechanism were illustrated. The theoretical adsorption amount of MnO2-PVA/PAA calculated as 398.85 mg/g was competitive compared with the reported values. The study proved MnO2-PVA/PAA is promising in the uranium removal from aqueous medium.

1. Introduction

Global fossil fuels such as coal, petroleum and natural gas are being intensively consumed and expectedly depleted within two hundred years in terms of currently annual consumption [1]. Nuclear energy is an extremely important candidate owning to high energy density (3.9 × 106 MJ/Kg uranium) [2] and zero greenhouse gas emissions [3]. Nevertheless, there is a tricky problem associating with nuclear energy. Uranium is an element of non-degradable nature, strong fluidity and long-lasting pollution of chemical and biological toxicity. By aqueous organisms uranium can migrate with the food chain readily, finally into human body. Once over the limited range many fatal and irreversible injuries occur in lungs, kidneys or other organisms. The World Health Organization prescribes uranium limit as 14.4 µg/L [4]. Overall remediation of waters contaminated by uranium is of great significance.

At present many remediation approaches includes chemical precipitation [5], solvent extraction [6], ion exchange [7], particle flotation [8], membrane separation [9], catalytic [10] and adsorption [11] [12], in which the adsorption attains more attention due to high feasibility, low energy cost and less second waste. Besides adsorption craft parameters the adsorbent plays a key role.

Metal oxide is explored as an effective kind of adsorbent for waste water remedy. Manganese dioxide (MnO2) presents excellent potentiality in this respect owing to acidity stability, negatively charged surface, low cost and high affinity [13]. The challenges in utilization of manganese dioxide as an adsorbent are as following: 1) Uranium uptakes of the raw manganese dioxide were demonstrated as commonly less than 60 mg/g [14]; 2) The fine particle size and aggregation of manganese dioxide lead to the difficulty in solid-liquid separation as well as the low efficiency in removal of uranium. It is such reason that many modification of manganese dioxide were conducted. For example, Zhu et al. [15] used poly(N-hydroxymethylacrylamide/2-hydroxyethylacrylate)hydrogel immobilized manganese dioxide for removal of Pb2+, Cu2+, Cd2+ and Ni2+. Kim et al. [16] fabricated magnetic manganese dioxide nanocomposites (Fe3O4/MnO2), which could be recovered by an external magnet.

Electrospinning is one of the most compatible techniques to produce continuous nanofibers of inorganic, organic, metals and composites [17]. The as-produced nanofibers often with large specific surface area and abundant porosity are promising in the fields of catalysis [18], batteries [19], medicine [20] and separation [21] [22]. To separate metal ions many electrospinning nanofibers were reported, for example, polyacrylonitrile/SiO2 for Th4+, U6+, Cd2+ and Ni2+ [23], polyvinylpyrrolidone/CeO2/(3-mercaptopropyl)trimethoxysilane for Pb2+ and Cu2+ [24], poly(vinyl alcohol)/tetraethoxysilane/(3-aminopropyl) triethoxysilane for Cd2+ [25], poly(vinyl alcohol)/titanium oxide/zinc oxide for Th4+ [26], polyvinylpyrrolidone/iron acetylacetonate for Hg2+ [27], poly(vinyl alcohol)/titanium oxide for Th4+ and uranyl [28], chitosan/multi-walled carbon nanotubes for Cr6+ [29], chitosan/hydroxyapatite for Pb2+, Co2+ and Ni2+ [30].

In the study of MnO2-PVA/PAA nanofiber was prepared by electrospinning and further in-situ coating. Advantages of the consequent MnO2-PVA/PAA are as following: 1) high availability; 2) better uranium separation performance including high uranium uptake capacity, reusability and mechanical strength. MnO2-PVA/PAA was tested for uranium separation effectivity at different pH, ionic strength, adsorption time, initial uranium concentration and temperature. The kinetics, isotherms, thermodynamics, reusability and membrane performance were discussed. The results herein demonstrated the high potential of MnO2-PVA/PAA as a candidate in membrane separation for uranium.

2. Experimental Section

2.1. Materials and Instruments

All chemicals of reagent-grade purity were attained from commercial companies and directly utilized as received. The surface morphology of electrospinning nanofiber was measured by SEM (Nova Nano 450, Netherlands FEI Company). Chemical groups in nanofiber were observed by FT-IR (Nicolet 380, Thermo Nicolet Corporation). The crystal structure was determined by XRD (D8. Advance, Shuyun instruments (Shanghai) Co., Ltd). XPS measurement was performed with Escalab 250 Xi (Thermo Fisher). The hydrophily was analyzed by measuring contact angle (JC2000C1, Shanghai Zhongchen Digital Co., Ltd.). The mechanical property was measured using Universal mechanical testing machine (WDT-5, Shenzhen Kaiqiangli Experimental Instrument Company).

2.2. Preparation of PVA/PAA and MnO2-PVA/PAA Nanofiber

The solution composed of 1.6 g poly (vinyl alcohol) and 10 mL deionized water was magnetically stirred to homogeneous phase at 85˚C, followed by addition of 0.3 g poly (acrylic acid). After cooling down to room temperature the resultant solution stirred for well-mix was stored as the electrospinning precursor solution.

The electrospinning operation for PVA/PAA nanofiber was described as the following. The above precursor solution was transferred in a syringe equipped with a stainless-steel needle of 22 G that serving as the anode. A rotation drum connected with the cathode was used as a collector. The anode-cathode voltage and distance were set as 23.5 kV and 15 cm, respectively. The flow rate was fixed as 0.5 mL/h. The collected nanofiber was thermo-treated for 3 h for cross-linking esterification reaction between PVA and PAA. The final mat was poly(vinyl alcohol)/poly(acrylic acid) nanofiber briefly as PVA/PAA.

In-situ coating of MnO2 onto PVA/PAA was completed via a redox chemical reaction. Specifically, the mixture containing 1.0 g PVA/PAA and 100 mL potassium permanganate solution (2.0 g/L) was agitated for 2 h at 35˚C at pH = 2.0, followed by dropwise addition of 5.0 mL absolute ethanol. The resultant solution was continuously agitated for 30 min until clarification. The nanofiber was taken out from the solution, washed with deionized water for three times, and dried at 85˚C for 5 h. The as-prepared product was named as MnO2-PVA/PAA nanofiber.

2.3. Uranium Adsorption Performance Test

All batchwise experiments were conducted by controlling MnO2-PVA/PAA mass as 10 mg and solution volume as 50 mL. The solution pH was kept at 6.0 except the study on pH influence where pH was from 2.5 to 6.0 with interval value of 0.5. The NaClO4 concentration was zero except the study on effect of ionic strength, in which NaClO4 concentration ranged from 0.1 mol/L to 0.6 mol/L. The uranium concentration in feed solution was fixed at 50 mg/L except the isotherms study. The duration was set as 2 h except the kinetics research where it varied from 5 min to 150 min. The environmental temperature was controlled at 298.15 K except in thermodynamics experiment, in which it was in the range of 288.15 K to 318.15 K with an interval of 5 K. After adsorption, the nanofiber was taken out and the residue uranium concentration in solution was detected with the standard arsenazo III method. The uranium uptake amount (qe (mg/g)) was calculated out using Equation (1):

q e = ( [ UO 2 2 + ] 0 [ UO 2 2 + ] e ) V m (1)

where [UO22+]0 is uranium concentration in the feed solution (mg/L); [UO22+]e is uranium concentration (mg/L) before solid-liquid separation; V (L) is the volume of uranium-containing feed solution; m (g) is the mass of MnO2-PVA/PAA or PVA/PAA.

The strip experiment followed the adsorption experiment were carried out by washing the spent nanofiber with eluate solution (50 mL, 1.0 mol/L) such as HCl, H2SO4, HNO3, Na2CO3, EDTA. The strip efficiency was figured out using Equation (2):

D % = C D × V D q e × 100 (2)

where CD is uranium concentration in eluate (mg/L). VD is the volume of the collected eluate (L). qe is the uptake amount before strip (mg/g).

For the recycling experiments, the recovered MnO2-PVA/PAA or PVA/PAA regenerated using HCl (1.0 mol/L) was applied to adsorb uranium repeatedly.

3. Results and Discussion

3.1. Characterization

The microscopic morphologies of PVA/PAA and MnO2-PVA/PAA are shown in Figure 1(a), Figure 1(b). One can see that both nanofibers were non-directional homogeneous fiber beams without beads. The average diameter were 270 nm for PVA/PAA and 320 nm for MnO2-PVA/PAA. The particles on MnO2-PVA/PAA surface were MnO2. The FT-IR spectrum shown in Figure 1(c) displayed the characteristic bands of PVA/PAA at 3340 cm−1, 2942 cm−1, 1710 cm−1, 1090 cm−1 and 850 cm−1 were resulted from vibrations of hydroxyl, C-H asymmetric, C=O in carboxyl and ester groups, C-O-C from the crosslinking reaction of PVA and PAA stretching and C-H bending. The FT-IR spectrum of MnO2-PVA/PAA retained the characteristic bands of PVA/PAA and meanwhile highlighted characteristic bands of MnO2 at 1370 cm−1 and 559 cm−1 assigned as stretching vibration of Mn-OH and Mn-O. XRD patterns shown in Figure 1(d) indicated MnO2 indeed existed in MnO2-PVA/PAA according to PDF#44-0092. The stress-strain curve of PVA/PAA and MnO2-PVA/PAA are shown in Figure 1(e), Figure 1(f). As can be seen, as the immersion duration increased, stress-strain of PVA/PAA and MnO2-PVA/PAA decreased. The reason was attributed to the corrosion from the aqueous solution.

Figure 1. SEM images of (a) PVA/PAA, (b) MnO2-PVA/PAA; (c) FT-IR spectra of PVA/PAA and MnO2-PVA/PAA; (d) XRD patterns of PVA/PAA and MnO2-PVA/PAA; Stress?strain curves of PVA/PAA (e) and MnO2-PVA/PAA (f).

3.2. Effect of pH and Ionic Strength on Uranium Adsorption

The uranium uptake amount varying with pH is shown in Figure 2(a). It can be seen that pH increasing from 2.5 to 6.0 was beneficial for uranium adsorption. The further increase in pH would count against the adsorption. The explanation for the results was as following. Firstly positive charged species such as UO22+, UO2(OH)+, (UO2)2(OH)22+, (UO2)3(OH)42+, (UO2)3(OH)5+ and (UO2)4(OH)7+ were dominant at pH < 6.0, which showed high affinity toward both nanofibers. Secondly more hydrogen ion at lower pH could weaken the adsorption ability through protonating the nanofibers and occupying the adsorption sites.

Effect of NaClO4 concentration in the range of 0 mol/L to 0.6 mol/L on the adsorption is conducted to illustrate the ionic strength influence and the result is shown in Figure 2(b). It can be seen that as NaClO4 concentration increased, the uranium adsorption on both fibers varied slightly. The result proved that the adsorption of uranium on both fibers werein the way of the inner-sphere surface complex [31]. The finding agreed with other uranium adsorption phenomenon reported in the literature [32].

3.3. Adsorption Isotherms

Effect of initial uranium concentration on the adsorption onto PVA/PAA and MnO2-PVA/PAA are shown in Figure 3. It can be seen that qe increased with the increase in Ce. The reasonable explanation was that higher concentration gradient promoted the utilization in adsorption sites. To explain the adsorption

Figure 2. (a) Effect of pH on uranium adsorption onto PVA/PAA and MnO2-PVA/PAA (C0 = 50 mg/L, V = 50 mL, m = 10 mg, mixing time = 2 h, T = 298.15 K); (b) Effect of ionic strength on uranium adsorption (C0 = 50 mg/L, m = 10 mg, V = 50 mL, pH = 6.0, mixing time = 2 h, T = 298.15 K).

Figure 3. Effect of initial U (VI) concentration on adsorption capacity. (m = 10 mg, V = 50 mL, pH = 6.0, mixing time = 2 h, T = 298.15 K).

mechanism, the data were further analyzed by fitting Langmuir and Freundlich isotherm models.

The Langmuir model is associated with identical active sites and surface coverage in the way of single layer. Its mathematical description is shown in Equation (3):

q e = K L q m C e 1 + K L C e (3)

where Ce (mg/L) and qe (mg/g) are uranium concentration and uptake amount at equilibrium state; qmax (mg/g) is the theoretically saturation uptake amount; b (L/mol) is the Langmuir constant.

The Freundlich model fits well with the adsorption on hetergeneous adsorption sites. Its empirically mathematical equation is presented as Equation (4).

q e = K F C e 1 / n (4)

where n (mg/g) and KF (mol1−n∙Ln/g) are used to measure adsorption intensity and uptake amount, respectively.

As can be seen from Table 1, R2 values of the Langmuir model were closer to 1.0 than the values of Freundlich. It could be concluded that the adsorption processes were in agreement with the assumption of Langmuir model. Both adsorption processes took the monolayer way. The modification using MnO2 significantly improved the adsorption amount from 58.51 mg/g to 398.85 mg/g.

The theoretical qe of MnO2-PVA/PAA achieved to 398.85 mg/g, a very competitive value compared with other adsorbent materials (shown in Table 2). The competitive qe of MnO2-PVA/PAA might originated from high disperse of MnO2 on PVA/PAA surface proved in Figure 1(b).

3.4. Adsorption Kinetics

Figure 4 shows the effect of mixing time on adsorption of U (VI) with PVA/PAA and MnO2-PVA/PAA. It can be found that the variation in qe presented three phases including: 1) the rapid boost stage within 0 - 20 min resulted from abundant unoccupied sites; 2) the slow increase stage between 20 min - 120 min due to insufficient adsorption sites; 3) the steady stage after 120 min attributed to the dynamic state equilibrium.

Figure 4. Effect of mixing time on adsorption capacity. (C0 = 50 mg/L, m = 10 mg, V = 50 mL, pH = 6.0, T = 298.15 K).

Table 1. The parameters of Langmuir and Freundlich isotherm models for uranium adsorption on PVA/PAA and MnO2-PVA/PAA.

Table 2. The maximal adsorption capacity of various sorbents for uranium.

To illuminate kinetics process in detail, two frequently used models, pseudo-first-order Equation (5) and pseudo-second-order Equation (6) kinetics equations, were adopted to fit the data.

q t = q e ( 1 e k 1 t ) (5)

q t = q e 2 k 2 t 1 + q e k 2 t (6)

where qt (mg∙g−1) and qe (mg∙g−1) are the uptake amount at time t (min) and equilibrium, respectively; k1 (min−1) and k2 (g∙mg−1∙min−1) represent adsorption rate constants, respectively.

The fitting values are listed in Table 3. It can be clearly seen that: 1) the correlation coefficients values derived from the pseudo-second-order kinetics (0.997 for PVA/PAA and 0.987 for MnO2-PVA/PAA) were closer to 1.0 than the pseudo-first-order kinetics (0.987 for PVA/PAA and 0.957 for MnO2-PVA/PAA); 2) the experimentally maximal uptake amounts (38.52 mg/g for PVA/PAA and 227.87 mg/g for MnO2-PVA/PAA) were more approximate to the theoretical values (42.16 mg/g for PVA/PAA and 242.56 mg/g for MnO2-PVA/PAA) from the pseudo-second-order kinetics rather than the theoretical values (33.04 mg/g for PVA/PAA and 193.29 mg/g for MnO2-PVA/PAA) from the pseudo-first-order kinetics. The consequent parameters proved both adsorption processes were coincide with the assumption of the pseudo-second-order kinetics. The results indicated both adsorption processes are mainly chemical.

3.5. Adsorption Thermodynamics

The effect of temperature (288.15 K - 318.15 K) on the adsorption of uranium

Table 3. The calculated parameters of pseudo-first-order and pseudo-second-order models fitting the adsorption of uranium on PVA/PAA and MnO2-PVA/PAA.

with PVA/PAA and MnO2-PVA/PAA were studied. The results are shown in Figure 5(a). It can be seen that both qe slightly increased with the increase in temperature. It is revealed that a higher temperature favored the adsorption.

To study the thermodynamic nature of the adsorption, the thermodynamic parameters such as ΔH˚ and ΔS˚ were calculated out by Equation (7) from the slope and intercept of the linear plot between lnKd vs. 1/T. The change in Gibbs free energy ΔG˚ values was acquired using Equation (8).

ln K d = Δ S ˚ R Δ H ˚ R T (7)

Δ G ˚ = Δ H ˚ T Δ S ˚ (8)

where Kd (mL∙g−1) is the distribution coefficient at the equilibrium state; R (8.314 J∙K−1∙mol−1) is the ideal gas constant; T (K) is thermodynamic Kelvin temperature.

lnKd as a function of 1/T is shown in Figure 5(b). The obtained values are listed in Table 4. ΔH˚ and ΔS˚ of positivity indicated both adsorption were endothermic processes with entropy increase. The negative ΔG˚ values proved that both adsorptions were spontaneous. A higher temperature leading to a higher absolute value of ΔG˚ illustrated that increasing temperature promoted the adsorption process. In addition, the absolute value of ΔG˚ for MnO2-PVA/PAA was greater than that of PVA/PAA, indicating that MnO2-PVA/PAA was more likeable for U(VI) than PVA/PAA.

3.6. Desorption, Reusability

The reusability is an important indicator for the practical application of a material. The desorption behavior of PVA/PAA and MnO2-PVA/PAA were studied using stripping reagents such as HCl, H2SO4, HNO3, Na2CO3 and ethylenediaminetetraacetic acid (EDTA) of 1.0 mol・L−1. The desorption rate D% are shown in Figure 6(a). It is found that HCl solution had the most efficient strip ability. The reason maybe that Cl with stronger polarity combined with UO 2 2 + to form UO 2 Cl 3 readily. HCl solution was thus used for the next adsorption-desorption cycle experiments.

The adsorption-desorption cycle experiments was conducted to assess the reusability of MnO2-PVA/PAA. HCl solution of 1.0 mol・L−1 was used as the desorption reagent. The result is shown in Figure 6(b). It can be seen that the uptake amount (qe) decreased as the cycle time increased. The decrease was resulted from the decrease in effective adsorption sites. Firstly HCl solution could

Figure 5. (a) Effect of temperature on adsorption capacity; (b) linear plots of lnKd vs. 1/T (C0 = 50 mg/L, m = 10 mg, V = 50 mL, pH = 6.0, mixing time = 2 h).

Figure 6. (a) Desorption efficiency of many eluents; (b) Reusing performance of MnO2-PVA/PAA. (C0 = 50 mg/L, m = 10 mg, pH = 6.0, mixing time = 2 h, T = 298.15 K.)

Table 4. Thermodynamic parameters of adsorption of uranium onto PVA/PAA and MnO2-PVA/PAA.

not completely desorb all the uranium adsorbed on MnO2-PVA/PAA; Secondly the adsorption in the chemical way of the inner-sphere surface complex was not fully reversible. Even so qe decreased from the initial 208.87 mg/g to the final 172.65 mg/g after five cycles.

3.7. Mechanism Explored with XPS

To explore the interaction of uranyl with MnO2-PVA/PAA in the molecular level, XPS was used to reveal the binding energy of MnO2-PVA/PAA and MnO2-PVA/PAA-U(VI). C, O and Mn, the major composition of MnO2-PVA/PAA, were verified by C 1s, O 1s and Mn 2p peaks in Figure 7. Due to the characteristic doublet of U 4f (shown in Figure 7(a), Figure 7(e)), it was no doubt that uranium had been adsorbed on the near-surface of MnO2-PVA/PAA. As can be seen in Figure 7(e), U 4f was splitted into U 4f5/2 and U 4f7/2, of which the former corresponded to U-O in uranyl and the latter was attributed to uranium atom binding with the oxygen atom in manganese dioxide.

The C 1s signals (shown in Figure 7(d)) was deconvoluted into three peaks at 288.52 eV, 285.65 eV and 284.53 eV assigned to carbon in C=O, C-O and C-C bonds, in which the former chemical bonds slightly shifted to 288.57 eV and 285.71 eV owing to the participation of oxygen in interaction with uranium.

The Mn 2p signal shown in Figure 7(c) presented as Mn 2p1/2 and Mn 2p3/2 corresponding to the Mn-O bond. The slight change in binding energy proved that manganese did not directly interact with uranium.

Obvious distinct are observed in the spectrum of O 1s (shown in Figure 7(b)). Indetail, the O 1s signal was splitted into four peaks originally positioned at 532.89 eV, 531.73 eV, 531.27 eV and 529.39 eV assigned as C-O, C=O, Mn-O-H and Mn-O-Mn. After adsorption, the binding energy changed obviously and the relative intensity of four peaks varied significantly. The obvious distinct proved oxygen atom was the major donor atom. The adsorption mechanism of uranium onto MnO2-PVA/PAA is thus proposed as Figure 7(f). Namely the fixed uranyl

Figure 7. XPS spectra (a) full spectrum, (b) O 1s, (c) Mn 2P, (d) C 1s, and (e) U 4f of MnO2-PVA/PAA and MnO2-PVA/PAA-U(VI); (f) The proposal mechanism of uranium interacting with MnO2-PVA/PAA.

with MnO2-PVA/PAA was anchored with two oxygen atoms in manganese dioxide.

4. Conclusion

In the study a novel manganese dioxide modified nanofiber, MnO2-PVA/PAA, was facile prepared using the electrospinning technique. MnO2-PVA/PAA was proved by SEM, FT-IR, XRD, stress-strain test. Effect of pH, ionic strength, initial uranium concentration, mixing time, temperature on the adsorption, reusability and adsorption mechanism were illustrated. The optimal pH was determined as 6.0. The adsorption took the way of inner-sphere surface complex. The theoretical adsorption amount of MnO2-PVA/PAA calculated as 398.85 mg/g. The pseudo-second-order kinetics well fitted the adsorption process, illustrating chemical mode. The thermodynamic parameter ΔH˚ and ΔS˚ were 13.55 kJ・mol−1 and 106.04 J・K−1・mol−1. The absolute value of ΔG˚ increased as the temperature increased. XPS revealed that uranyl complexed MnO2-PVA/PAA through the U-O-Mn bond. The results herein demonstrated high potential of MnO2-PVA/PAA as a candidate in separation for uranium.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22066001).

Cite this paper: Tao, W. , Lv, R. and Tao, Q. (2021) Facile Preparation of Novel Manganese Dioxide Modified Nanofiber and Its Uranium Adsorption Performance. Journal of Applied Mathematics and Physics, 9, 1837-1852. doi: 10.4236/jamp.2021.97118.
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