Synthesis of Porous Magnetic Ni0.6Fe2.4O4 Nanorods for Highly

Article Cite This: J. Chem. Eng. Data 2018, 63, 1810−1820

pubs.acs.org/jced

Synthesis of Porous Magnetic Ni0.6Fe2.4O4 Nanorods for Highly Efficient Adsorption of U(VI) Zehua Zhang,†,# Shengxia Duan,†,# Haiying Chen,‡,§ Fengsong Zhang,§ Tasawar Hayat,∥ Ahmed Alsaedi,∥ and Jiaxing Li*,†,∥ †

Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, P.R. China Nuclear and Radiation Safety Center, Ministry of Environment Protection of the People’s Republic of China, Beijing 100082, China § Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 100101 Beijing, China ∥ Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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‡

ABSTRACT: In this study, porous magnetic Ni0.6Fe2.4O4 nanorods (MNs) were successfully synthesized just by calcining the Ni0.6Fe2.4C2O4·2H2O precursor. MNs exhibited porous structure with disorderly nanorods based on the characterizations. Experimental results showed that the removal of U(VI) by MNs can reach equilibrium within 150 min at 293 K with a maximal adsorption capacity of 57.7 mg/g. Besides, the adsorption performance was highly dependent on solution pH values, while ion strength and ion species had little influence. The U(VI) adsorption onto the MNs was fitted better by the pseudo-second-order kinetic model, and the isothermal data can be better described by the Langmuir model, suggesting a monolayer adsorption process. Thermodynamic studies indicated the adsorption process to be spontaneous and endothermic. In addition, this magnetic material can achieve convenient separation using an external magnet. Hence, the MNs can be utilized as an effective candidate to enrich and separate U(VI) from aqueous solutions.

1. INTRODUCTION In today’s world, uranium has been widely used by nuclear power plants. During the production of uranium mining and metallurgy, a lot of waste products, such as some waste ores and mill tailing with radioactive elements, are produced.1 Wastewater containing U(VI) can threaten the human body through the food chains while polluting the environment, mainly from radiation and chemical toxicity. In chemical toxicity terms, the toxicity of U(VI) can cause irreversible structure changes of proteins and affect the function of tissue cells.2 Natural uranium, on the other hand, consists of three radioactive isotopes, 238U, 235U, and 234U, which can destroy the tissue cells of both plants and humans, causing great damage to the central nervous system, immune system, and reproductive system.3 From the perspectives of energy security and environmental protection, it is of great practical significance to enrich and remove uranium irons from polluted water. So far, several enrichment technologies have been investigated to remove U(VI) from an aquatic environment, such as solvent extraction,4 foam flotation,5 liquid membrane enrichment,6,7 evaporation concentrate,8 precipitation method,9,10 adsorption method,11−14 and so on. The adsorption method normally exhibiting high efficiency, easy operation, no byproducts, low cost, and other excellent features has attracted great attention for U(VI) removal from wastewater in previous investigations. The key point of adsorption research is to develop new © 2018 American Chemical Society

adsorbents with high adsorption capacity, fast adsorption rate, and stable mechanical and chemical properties.11−14 However, commonly used adsorbents, such as phosphorus, activated carbon, microorganisms, etc., are difficult to meet these requirements at the same time. Hence, those limits call for the application of new U(VI) adsorbent in large volume wastewater treatment. Recently, magnetic porous materials have been applied to the adsorption of U(VI) because of its easy synthesis and remarkable adsorption performance.15−18 Many magnetic nanomaterials have attracted great attention for U(VI) enrichment and separation, like functionalized Fe3O4 nanoparticles, which can be easily separated from the adsorption system.15−18 However, a relatively complex synthetic method and low adsorption rate can still be observed in the actual application. Herein, the porous Ni0.6Fe2.4O4 nanorods (MNs) with magnetic property were successfully obtained using polyvinylpyrrolidine (PVP) as reactant and calcining the Ni0.6Fe2.4C2O4·2H2O precursor. After that, the inner structure and property of MNs were typically studied by scanning electron microscopy (SEM), transmission electron microscopy Received: February 8, 2018 Accepted: April 20, 2018 Published: April 27, 2018 1810

DOI: 10.1021/acs.jced.8b00127 J. Chem. Eng. Data 2018, 63, 1810−1820

Journal of Chemical & Engineering Data

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JEOL 2010 TEM with 200 kV accelerating voltage. The crystallinity of MNs was studied by using XRD recorded on a Philips X’Pert Pro Super diffractometer equipped with Cu Kα radiation (λ = 1.54178 Å). Moreover, the FT-IR measurement was carried out using a Nicolet-5700 spectrophotometer with KBr pellets. Magnetism of the MNs was studied with SquidVSM (superconducting quantum interference device) from −20 000 to 20 000 Oe at 25 °C. Zeta potential was detected by a Zeatsizer Nano, and the concentration of U(VI) after reaching adsorption equilibrium was measured by Shimadzu UV-18.000 (spectrophotometer) with quartz cells. 2.4. Adsorption Experiments. Adsorption experiments were performed in a centrifuge tube. The different desired concentrations were prepared by diluting 60.0 mg·L−1 of U(VI) stock solution. The solution pH was adjusted to 5.0 by adding a negligible amount of HCl or NaOH solution. The reaction system was oscillated for 24 h on the thermostat oscillator to achieve complete adsorption equilibrium. Kinetic experiments were carried out at 293 K with U(VI) concentration of 18.0 mg· L−1 at initial time. The isotherm experiments were performed at different initial U(VI) concentrations with 0.2 g·L−1 of MNs at 293, 313, and 333 K, respectively. U(VI)-loaded MNs were separated by using an external magnet, and the remaining U(VI) concentration after adsorption was measured by UV spectrophotometry, with Azo chlorine phosphorus used as colorant. The adsorption percentage, distribution coefficient (Kd), and adsorption quantity (qe) can be calculated from the following equations

(TEM), X-ray diffraction (XRD), Fourier-infrared absorption spectrum (FT-IR), and magnetization versus magnetic field (M−H) characterizations. Besides, the effects of pH, ion strength, temperature, and contact time on adsorption performance were also investigated by batch adsorption experiments. Various kinetic and isotherm models were applied for further adsorption mechanism study, and the thermodynamic experiment was carried out to propose impossible adsorption mechanisms. Moreover, the superior features can be concluded by comparing with other adsorbents.

2. EXPERIMENTAL SECTION 2.1. Materials. The ferrous sulfate heptahydrate (FeSO4· 7H2O), oxalate dihydrate (H2C2O4·2H2O), nickel chloride hexahydrate (NiCl2·6H2O), and ethylene glycol (EG) solutions were purchased from Sinopharm Chemical Reagent Co., Ltd. in China. All reagents were of analytical grade and used as received. An amount of 0.01 mol/L HNO3 solution was used to prepare 1.0 mmol/L UO22+ stock solution. Deionized water was prepared for the following experiments. 2.2. Synthesis of MNs. A schematic process for the fabrication of MNs is depicted in Scheme 1. Two parallel Scheme 1. Schematic Illustration on the Synthesis of MNs

Sorption (%) =

C0 − Ce × 100% C0

(1)

Kd =

C0 − Ce V × C0 m

(2)

qe =

(C0 − Ce) × V m

(3)

where C0 and Ce (mg·L−1) are concentrations of U(VI) at the initial and equilibrium time, respectively. Besides, the adsorption capacity (qe) was further studied with changing of temperature, time, pH, ion strength, and ion type. All experimental data recorded were the average value after repeating measurements with no more than 5% relative errors.

3. RESULTS AND DISCUSSION 3.1. Characterization of Magnetic MNs. The surface morphology of MNs was characterized by SEM, as shown in Figure 1a and b. It can be seen that MNs are disorganized nanorods with different lengths and disordered arrangements, setting aside a lot of space among the nanorods for U(VI) adsorption. Figure 1b displays a clearer appearance of one nanorod in which nanoparticles and aggregates were observed on the surface of the nanorod with hollow structure, providing more adsorption sites for U(VI). The structure of MNs was further characterized by TEM characterization. Figure 1c shows uniformity of the product, and Figure 1d and e give fluffy morphology and porous structure, which may indicate the relatively large specific surface area.19 Hence, MNs may have an advantage for U(VI) adsorption based on the SEM and TEM characterizations. XRD characterization of MNs was performed, as shown in Figure 2. The diffraction peaks of 2θ are 18.0.44°, 30.40°,

solutions were obtained by dissolving 1.500 g of PVP in a solution consisting of 10.0 mL of EG and 4.0 mL of water and stirred for ∼30 min. Amounts of 0.220 g of FeSO4·7H2O and 0.048 g of NiCl2·6H2O were added into the above solution, while 0.126 g of H2C2O4·2H2O was dissolved into the other parallel solution. Both solutions were then mixed together and stirred at room temperature for 6 h, obtaining the yellowishbrown precipitate Ni0.6Fe2.4C2O4·2H2O precursor, which were further separated by centrifugation, filtration, and rinsing with absolute ethanol and deionized water. The obtained precursor was dried in vacuum for 12 h (T = 60 ◦C) and then calcined for 2 h (T = 400 °C, heating rate: 1 °C/min). After cooling to room temperature, reddish-brown solid powders were obtained for further collection applying in subsequent experiments. 2.3. Characterization. The morphology and characteristics of MNs were observed by using a Hitachi S-4800 SEM and 1811



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Figure 1. (a, b) SEM images of MNs and (c−e) TEM images of MNs.

Figure 2. XRD pattern of the obtained sample MNs. Figure 3. FT-IR patterns of the obtained samples.

35.72°, 43.46°, 53.88°, 57.48°, and 63.08°, corresponding to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) crystal planes, respectively (JCPDS Card, No. 872338), indicating the high purity of MNs. Furthermore, strong and sharp peaks can also be observed in the XRD pattern of MNs, ensuring a high degree of crystallinity of the obtained sample. The functional groups on the MNs were also investigated by FT-IR characterization, which is shown in Figure 3. MNs have a strong peak at 3431 cm−1, which is the characteristic peak of hydroxide. The hydroxyl is partly derived from the hydrogenation of hydrogen-bonded H2O of the surface layer, and the other part is the hydroxide from the deprotonation reaction on the surface. The strong peaks at 588 cm−1 are the characteristic peaks of the Ni−O and the Fe−O bonds. Apart from this, the intensive peak at 1635 cm−1 results from CO bond vibration.20,21 In order to achieve convenient separation by an external magnet, the MNs are expected to be ferromagnetic; herein, the M−H characterization of MNs was conducted. In the external field from −20 000 Oe to 20 000 Oe, the moment of MNs is investigated at normal temperature as shown in Figure 4. The M−H curve shows that magnetization of MNs increases as the

Figure 4. M−H curve of MNs at 300 K in the range of −20 000 to 20 000 Oe.

external field strength (H) rises. The remnant magnetization (Mr) of 0.72 emu/g exists at 300 K without an external 1812



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Figure 5. (a) Effect of pH and the ionic strength on the adsorption of U(VI) on MNs, T = 293 K, CU(VI)initial = 18.0 mg/L. (b) Zeta potential as a function of pH of the MNs, T = 293 K.

magnetic field. Besides, the coercivity (Hc) stays low at 24.9 Oe at 300 K, indicating the ferromagnetic property of MNs, and Ni0.6Fe2.4O4 nanorods of the natural soft magnet have a good response in an applied magnetic field. Thus, the convenient separation can be achieved by just using an external magnet. 3.2. Static Experimental Results. 3.2.1. Impact of pH and Ionic Strength. Previous studies have shown that pH value can have an impact on adsorbent surface chemical properties and the ionic forms in solution, leading to different adsorption behaviors. The impacts of pH and ionic strength have been investigated in the adsorption experiments at three different NaNO3 concentrations shown in Figure 5a, in which the efficiency of adsorption is greatly influenced by pH value. The adsorption capacity increases slowly at pH 2.0−4.0 and rapidly at pH 4.0−9.0, before reaching the maximum adsorption capacity at about pH 9.0. By continually increasing the solution pH values, the adsorption capacity of U(V) began to decrease, which is consistent with the previous reports.22 The adsorption performance of the MNs is based on the properties of adsorbent surface and ion existence form in solution with different pH value. As for MNs, surface charges can make a difference on adsorption efficiency of U(VI), ascribing to hydrogen ion transfer. The pH at zero point of charge (pHPZC) of MNs is measured as 5.8 by a zeta potential test (see Figure 5b). When pH is below 5.8, protonation reaction (−ROH(surf) + H+ ⇌ −ROH+2(surf)) makes the surface of MNs positively charged and results in electrostatic repulsion, making it difficult to adsorb positively charged U(VI) on the surface of MNs. When pH is above 5.8, deprotonation reaction (−ROH(surf) + OH−(aq) ⇌ −RO−(surf) + H2O) makes a negatively charged surface, causing a higher adsorption rate, which can be ascribed to more surface-active sites of MNs after deprotonation with the increase of pH value and strengthening of the interaction between U(VI) and active sites on the surface by electrostatic attraction. Besides, the acidity of solution can affect the degree of protonation and deprotonation and the morphological distribution of U(VI), resulting in the change of adsorption capacity. As shown in Table 1, U(VI) mainly presents in the form of UO22+ when pH < 5.0, while positive ions containing hydroxide like UO2(OH)+ were dominant species when 5.0 < pH < 8.0. When pH is around 8.0, the primary U(VI) ions will turn into negative ions containing hydroxide such as UO2(OH)42−. Based on the analysis of surface charge of MNs and U(VI) existing forms with varying pH, it is necessary to take both

Table 1. Aqueous Complexation Reaction of U(VI) reactions

log K (I = 0.01 M, T = 298 K)

UO22+ + H2O = UO2(OH)+ + H+ UO22+ + 2H2O = UO2(OH)2 + 2H+ UO22+ + 3H2O = UO2(OH)3 + 3H+ UO22+ + 4H2O = UO2(OH)42− + 4H+ 3UO22+ + 5H2O = (UO2)3(OH)5+ + 5H+ 3UO22+ + 7H2O = (UO2)3(OH)7− + 7H+ 4UO22+ + 7H2O = (UO2)4(OH)7+ + 7H+

−5.25 −12.15 −20.25 −32.4 −15.55 −32.20 −21.90

adsorbent surface properties and ionic presence in solution into account when studying the pH effect. Therefore, the surface of the adsorbent is positively charged when pH < 5.8, leading to the electrostatic repulsion between the positively charged surface and UO22+, resulting in poor adsorption performance of the MNs. When the solution pH value increases from 5.8 to 8.0, the negatively charged surface of MNs attracts positively charged U(VI) species by electrostatic attraction, resulting in higher adsorption rate and better adsorption behaviors. When the pH value is in the range of 8.0−9.0, the U(VI) species begin to convert from positively charged to negatively charged, in which removal efficiency gradually reaches the maximum, ascribing to complete adsorption of positively charged U(VI) remaining in the solution and MNs. When the pH is above 9.0, the adsorption efficiency decreases with the increase of pH value in the solution, which may result from the electrostatic repulsion between the increasingly negative U(VI) species and MN status.23 In general, the electrostatic force between ions and MNs with varying pH is responsible for different adsorption performance, and a similar mechanism can be found when using other adsorbents for U(VI) removal, such as amazon kaolinite,24 activated carbon,25 and so on.26−29 Furthermore, the impact of the ionic strength on adsorption was also investigated. It can be seen from Figure 5a that ionic strength had little effect on similar adsorption efficiency at the same pH value with varying NaNO3 concentrations, indicating inner-sphere surface complexation of U(VI) adsorption onto the MNs. 3.2.2. Effect of Ionic Species. The other ions coexisting in solution may affect the behavior of U(VI) or the surface properties of MNs, leading to different adsorption performance. There are two types of interaction between different ions when interacting with MNs: competition and synergism. Ions having the same charge inhibit adsorption of each other and are 1813



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Figure 6. Influence of pH and ionic species on the adsorption of U(VI) on MNs (a) anions and (b) cations, at T = 293 K, CU(VI)initial = 18.0 mg/L.

considered to be competitive. In comparison, ions that carry the opposite charge are considered to be synergistic. It may be synergistic and competitive in the simultaneous addition of cation and anion, even not be effective.30 As shown in Figure 6a, the presence of weakly absorbent anions such as NO3−, ClO4−, and Cl− does not affect U(VI) adsorption. According to the literature on the possible mechanism of anion-promoted cation adsorption, it can be learned from the adsorption data that the presence of anions did not reduce the surface charge of the adsorbent and did not result in the formation of surface precipitates with three-dimensional spatial structure containing cationic or anionic ternary complexes.31 Also, as shown in Figure 6b, the changes in cationic ions have a small impact on U(VI) removal. For anion and cation ions studied in these experiments corresponding to the strong acid and alkali, the degree of hydrolysis in the solution is very small, which can be responsible for the small effect on pH value. Simultaneously, different anion and cation ions with the same charge result in the same electrostatic repulsion gravity and have no impact on surface characteristics of adsorbent-like interface potential, the thickness of the electric double layer, and so on.32 3.3. Adsorption Kinetics. To achieve an economic and practical effect, the adsorption process is desirable to complete in as little time as possible when dealing with large volumes of wastewater. Given that, the adsorption rate of adsorbent is one of the important parameters to measure adsorption capacity. The adsorption equilibrium time and the adsorption capacity of MNs can be obtained by kinetic investigation at 293 K. As shown in Figure 7, a fast sorption process with high efficiency was observed within the first 50 min; i.e., ∼75% U(VI) was absorbed. The adsorption rate decreases gradually with increasing sorption time and stabilized within ∼150 min with a maximum capacity of 57.70 mg/g. As a comparison, the equilibrium within 150 min in MNs is similar to that of nearly 2 h in mesoporous silicas33,34 but much shorter than that of 20 h in MCM-4135 and also 24 h in MWCNT-g-CMC.36 Therefore, MNs may have good adsorption performance with high adsorption efficiency in real adsorption application. In subsequent adsorption experiments, the contact time was set as 24 h, ensuring complete adsorption process. The adsorption process usually consists of three stages: liquid membrane diffusion, microporous surface adsorption, and adsorbent micropore diffusion. The slower rate-controlling two processes were followed by the next higher rate micropore diffusion stage. To investigate the adsorption kinetic mechanism of U(VI) onto the MNs, the kinetics data were further

Figure 7. Adsorption of U(VI) onto MNs as a function of contact time, at T = 293 K, pH = 5.0 ± 0.1, CU(VI)initial = 18.0 mg/L.

analyzed to the following equations (Figure 8). The pseudofirst-order equation is presented in eq 4, and the pseudosecond-order equation is shown in eq 537 log(qe − qt ) = log qe − t 1 1 = + t 2 qt qe 2k 2qe

k1 t 2.303

(4)

(5) −1

where k1 and k2 (min ) are rate constants of two models, respectively, and qe (mg/g) is adsorption capacity at equilibrium. As presented in Table 2, the pseudo-first-order model has smaller R2 (correlation coefficient) value by comparing to that of the pseudo-second-order, indicating a second-order kinetics U(VI) entrapment process, which is also consistent with previous reports.38,39 In addition to the above two models, the intraparticle diffusion model was also utilized to further study adsorption kinetics. The main concern of this model is the migration of the adsorbate from the surface into the pores. It is based on the assumption that the spread rate of U(VI) is a rate controller of the whole adsorption process, and the linear form of this model can be described as eq 640,41 qt = kit 1/2 + C

(6)

where ki is rate constant and C is proportional to the boundary layer. From Figure 8c, it can be seen that the intraparticle 1814



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Figure 8. Linear plots of different kinetic models for adsorption of U(VI) onto MNs: (a) pseudo-first-order, (b) pseudo-second-order, (c) intraparticle diffusion, (d) Boyd, (e) Elovich, and (f) Bangham models at T = 293 K, pH = 5.0 ± 0.1, CU(VI)initial = 18.0 mg/L.

diffusion from the adsorbent surface to its inner porous structure, the third stage starts, in which the adsorption begins to slow down and reaches its equilibrium when the adsorbent is saturated. Moreover, none of the three stages passed through the origin of the coordinate system, indicating that the first two steps play a decisive role in rate-controlling during the whole adsorption process. Besides, the Boyd kinetics model was investigated to further determine which stage is controlling adsorption rate, the model is given by eq 742

Table 2. Parameters for Pseudo-First-Order, PseudoSecond-Order, Elovich, and Bangham Kinetic Models models pseudo-first-order pseudo-secondorder Elovich equation Bangham

parameters K1 (min−1) 0.0222 K2 (g·mg/min) 0.0249 a (mg/g·min) 5.3351 × 105 a 0.4031

qe (mg/g) 52.0355 qe (mg/g) 65.7895 β (mg/g·min) 0.0803 K0 (mL/g·L) 2.26487

R2 0.98381 R2 0.9996 R2 0.6909 R2 0.8834

ln(1 − F ) = − k f t

(7)

where kf is rate constant of Boyd; F is adsorption percentage of solute at t time; and F can be expressed as follows q F= t qe (8)

diffusion shows three different plots, indicating several stages of the U(VI) adsorption onto the MNs. The first stage is the U(VI) transfer from the liquid phase to the solid surface with the highest slope, suggesting the maximum asorption rate of three stages. After adsorbing on the surface of absorbent, U(VI) gradually diffuses during the micropore of MNs, which is considered as the second stage. Subsequently, owing to the decreased number of available active sites and the slower U(VI)

The slope of the Boyd model is a basis for determining the speed-controlling step: when the line (−ln(1 − F) versus t) in coordinate axis could go through the origin, the third process (intraparticle diffusion) will be dominant. Otherwise, the first 1815



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Scheme 2. Schematic Illustration on the Adsorption Mechanism of U(VI) onto the MNs

Figure 9. Linear plots of different isotherm models for U(VI) adsorption onto MNs at three different temperatures: (a) Langmuir model, (b) Freundlich model, (c) D−R model, and (d) Tempkin model.

shown in Figure 8e, and the obtained R2 value is 0.6909 as shown in Table 2, suggesting monolayer adsorption. Definitively, the Bangham model views the pore diffusion as the ratecontrolling step, and the linear form is shown as follows

process (film diffusion) is preferred. It can be learned from Figure 8d that film diffusion dominated the adsorption process, which is consistent with the analysis of the intraparticle diffusion model. Furthermore, the Elovich model hypothesized that the adsorption sites show exponential growth and can determine whether the adsorption process is single or multiple layer. The line form of this model can be expressed as eq 9 qt =

ln(αβ) ln t + β β

⎛ C ⎞ ⎛ km ⎞ 0 ⎟⎟ = log⎜ 0 ⎟ + a log t log log⎜⎜ ⎝ 2.303V ⎠ ⎝ C0 − qt m ⎠

(10)

where both k0 (mL·g−1·L−1) and a ( 0, of which the adsorption is favorable when 0 < RL < 1 and can be seen as a linear process when RL = 1 and unfavorable when RL > 1. The RL value is tested at three temperatures (Figure 9) indicating a favorable adsorption process. Furthermore, the adsorption capacity Q0 increases with higher sorption temperature, indicating an endothermic adsorption process. Different from the Langmuir model, the Freundlich model is a semiempirical equation which is studied by adsorption on heterogeneous surfaces. The model emphasizes the hetero-

qe = Q 0 exp( −βε 2)

n

(14)

−2

In eq 14, β (mol ·KJ ) is the activity coefficient correlated to the average adsorption energy, and ε is the potential of the Polanyi which can be expressed as eq 15 2

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝

(15)

where E (kJ/mol) represents free energy change of unit mol U(VI) transfer. The E can be determined by eq 16 1817


Journal of Chemical & Engineering Data E=

1 2β

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The ΔH0 values at three temperatures are all positive in Table 4, which can be ascribed to the destroyed hydrated sheath around the U(VI) formed in water, in which heat is released before adsorption onto the MNs. Besides, the process in which U(VI) combined with the surface-active site of MNs is endothermic, owing to less released heat than that of absorbed. The positive ΔS0 value indicates addition of degree of freedom during the entire adsorption process and endothermic reaction caused by increased entropy. The similar U(VI) adsorption work has been carried out by Duan et al. They found that porous magnetic Ni0.6Fe2.4O4 nanoparticles (PMNMs) with high adsorption efficiency can be a candidate for U(VI) removal. The adsorption capacity of Ni0.6Fe2.4O4 nanorods (MNs) is lower than that of PMNM at the same temperature but still higher than many other adsorbent materials used to remove U(VI), such as Fe3O4@ SiO2 with 52.4 mg/g adsorption capacity51 and Fe3O4@SiO2quercetin with 12.3 mg/g.52 Besides, compared with PMNM, there are some advantages in U(VI) removal. First, from SEM and TEM characterization in previous reports, two-dimensional sheet superposition of PMNM can be observed, and the gap among the sheets is not conducive to the adsorption of U(VI). In contrast, the SEM and TEM images display a view of rodshaped nanomaterials in which the MNs are made up of a large amount of nanoparticle agglomerates, forming a three-dimensional sponge-like morphology. The nanorods with disordered arrangements have a tendency to form a closed space, which is beneficial for U(VI) entrapment onto the MNs. The result is also in good agreement with the isothermal experimental result that the U(VI) adsorption onto the PMNM is a multilayer process, while that of MNs is a monolayer process. Second, the active sites on the surface of MNs are more identical than PMNM with less uniform distribution of binding energies based on isotherm analysis. Furthermore, the ΔS0 value for the U(VI) removal onto MNs (132. 2372 J/mol·K) is much larger than that onto PMNM (18.63 J/mol·K), and the value of ΔG0 of MNs (−3.6480 kJ/mol) at 293 K is even more negative than the corresponding value of PMNM (−1.6157 kJ/mol) at 318.5 K based on the thermodynamic analysis, demonstrating a more naturally spontaneous and powerful entropy-driven U(VI) adsorption process onto the MNs and also indicating that MNs will start the adsorption reaction first and the amount of U(VI)loaded MNs will be more than that of PMNM after the same reaction time when both of them exist simultaneously. In summary, the three-dimensional sponge structure of MNs can enrich the morphology of nanomaterials used as adsorbent, and the superior features of MNs concluded provide the sustainable design of magnetic nanomaterials, ensuring promising application in wastewater treatment.53

(16)

The type of adsorption process can be determined by the magnitude of E: The main adsorption process is triggered by chemical forces when E > 8 kJ/mol; otherwise, it is dominated by physical forces when E < 8 kJ/mol. From Figure 9c, the E value calculated is 1.6032 kJ/mol when T = 293 K, 1.4216 kJ/ mol when T = 313 K, and 1.3092 kJ/mol at T = 333 K shown in Table 3. The E values at three temperatures are all less than 8.0, indicating a physical adsorption process onto the MNs. Compared to the D−R model, the Langmuir model has smaller Q0 at three temperatures, which may result from different assumptions of two models. Moreover, the Tempkin isotherm model assumed that the adsorbate molecular heat of entrapment for each layer decreases linearly with the increase of surface coverage, which is attributed to the strengthened repulsion between adsorbate molecules. Besides, the adsorption binding energy is homogeneous and can be maximized. The linear form of this model can be expressed as follows qe = B ln K t + B ln Ce

(17)

where Kt (L/g) is binding constant in balance, showing maximal binding energy, and B is a constant which can be expressed by the equation of B = RT/b. The value of B can be calculated from eq 17, and b value can be obtained. The correlation coefficient R2 is relatively large, suggesting that the adsorption data can also be described by the Tempkin isotherm model well (Figure 9d). 3.5. Thermodynamic Analysis. Thermodynamic studies are usually carried out when taking temperature impact into account. The lower solution viscosity and accelerated ion movement rate can be responsible for temperature impact. The thermal kinetic parameters (ΔH0, ΔS0, ΔG0) can be calculated from the adsorption isotherm, and the value of ΔH0 and ΔS0 can be calculated by using the van’t Hoff equation as follows ln K 0 =

−ΔH 0 ΔS 0 + RT R

(18)

The ln K0 in eq 18 can be obtained by making the value of Ce zero in the function of ln Kd versus ln Ce, and the ΔH0 and ΔS0 value can be given by calculating the slope of the ln Kd versus 1/T straight line. Free energy change (ΔG0) can be calculated by eq 1950 ΔG 0 = ΔH 0 − T ΔS 0

(19)

The thermodynamic analysis of adsorption data was carried out to investigate the mechanism of adsorption. In Table 4, the ΔG0 values at three temperatures are all negative, indicating a spontaneous adsorption process. With increasing operating temperature, the value of ΔG0 decreased, suggesting that the higher temperature contributes to higher adsorption efficiency.

4. CONCLUSION By calcining the Ni0.6Fe2.4C2O4·2H2O precursor, porous MNs were fabricated. At 293 K, the adsorption equilibrium can be reached within 150 min with a maximal adsorption capacity of 57.7 mg/g based on isotherm model analysis, and film diffusion stage and intraparticle diffusion stage are rate controllers during the whole adsorption process based on the kinetic models. Besides, the adsorption performance was highly dependent on solution pH values, while ion strength and ion species had little influence. The process of U(VI) capture onto the MNs is endothermic and spontaneous based on thermodynamic analysis of both positive ΔH0 and negative ΔG0 values. The magnetism of MNs makes an easy separation by an external

Table 4. Thermodynamic Data for the Adsorption of U(VI) onto MNs temperature (K)

Kd (L/g)

ΔG0 (kJ/mol)

ΔH0 (kJ/mol)

ΔS0 (J/mol·K)

293 313 333

1.5223 2.4135 3.2550

−3.6480 −6.2927 −8.9375

35.0975

132.2372

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magnet. In summary, simple synthesis, good adsorption performance, and convenient separation make MNs have potential in practical applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-551-65596617. Fax: +86-55165591310. ORCID

Jiaxing Li: 0000-0002-7683-2482 Author Contributions #

Z.Z. and S.D. contributed equally to this work.

Funding

Financial support from the National Natural Science Foundation of China (21677146, 21707043) is acknowledged. Notes

The authors declare no competing financial interest.

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REFERENCES

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Synthesis of Porous Magnetic Ni0.6Fe2.4O4 Nanorods for Highly

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