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Fe3O4 magnetic nanocomposite
Hydrometallurgy 191 (2020) 105224
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Recovery of strontium (Sr2+) from seawater using a hierarchically structured MnO2/C/Fe3O4 magnetic nanocomposite Jungho Ryua, Jeongsik Hongb, In-Su Parkc, Taegong Ryuc, Hye-Jin Hongc,
T
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a
Geologic Environment Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, South Korea R&D Team, Ecopro Innovation Co.,LTD, Chungbuk 28116, South Korea c Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Strontium Seawater Recovery Magnetic separation Enrichment
The recovery of strontium ions (Sr2+) from seawater has attracted much attention as an approach to securing Sr resources to meet increasing industrial demand. In this study, we synthesized a magnetic MnO2 nanocomposite (MnO2/C/Fe3O4) using a simple redox reaction under ambient conditions and applied this nanocomposite to the extraction of Sr2+ from natural seawater. The synthesized nanocomposite exhibited a hierarchical structure of MnO2, carbon, and Fe3O4 with a saturation magnetization of 25 emu/g that enabled effective separation under an external magnetic force. Regardless of the initial Sr2+ concentration, the adsorption of Sr2+ onto MnO2/C/ Fe3O4 proceeded rapidly (within < 10 min) following a pseudo-second-order kinetic model and agreed well with the Langmuir isotherm model, indicating a maximum adsorption capacity of 42 mg/g. Among the competitive ions, Mg2+ and Ca2+ significantly hindered Sr2+ adsorption onto MnO2/C/Fe3O4, whereas Na+ and K+ had little effect on Sr2+ adsorption. A detailed study of the distribution coefficient (Kd) revealed 11.2-fold and 1.8fold higher selectivities toward Sr2+ than toward Mg2+ and Ca2+, respectively. The Sr2+ ions adsorbed onto MnO2/C/Fe3O4 were completely recovered through desorption in 0.1 M HCl eluent. Finally, a Sr2+-enriched solution with a concentration of 501 mg/L could be obtained via seawater adsorption and subsequent acid desorption over 8 iterative cycles, demonstrating its effectiveness for practical applications of Sr2+ recovery from seawater.
1. Introduction
concentrations of these useful resources in seawater. Among the various methods tested for selectively separating low-concentration elements from seawater, adsorption techniques are reported to be the most effective and feasible options based on the preparation of adsorbent materials with high efficiencies and high selectivities (Bezhin and Dovhyi, 2015; Dhandole et al., 2016, 2018; Hong et al., 2016b, 2017; Ryu et al., 2016). Conventional adsorbents are prepared in a powder form, and solid–liquid separation methods, such as centrifuging and filtering or the introduction of powder-immobilized media, are required for any practical recovery process. Conventional solid–liquid separation processes, such as filtration or centrifugal separation, have drawbacks in that they are complicated, incur substantial adsorbent loss, and require long process times. When the adsorbent particles are adsorbed onto a substrate rather than in powder form, their exposed surface areas are significantly reduced, thus diminishing their adsorption capacity. In addition, immobilized adsorbents require support materials with a high chemical stability that can withstand repeated chemical treatments and long-term exposure to seawater. Thus, the use of free rather than
Strontium (Sr) is a rare metal that is conventionally used in the ceramic and glass manufacturing industries. Industrial demand for Sr is increasing, a trend that will only continue because several industries, including the pharmaceutical industry, are expected to increase their consumption of this rare metal (Ober, 2015). Accordingly, it has become critical to secure Sr resources that can satisfy current and future industrial demand. One approach to overcoming the limitations on available land resources involves the recovery of useful resources from seawater or brine water (Bardi, 2010; Quist-Jensen et al., 2016). Seawater could potentially provide copious reserves, but the selective recovery of Sr from seawater made difficult by the presence of complexed matrix ions and the relatively low concentration of Sr2+ in seawater. Virtually unlimited seawater could potentially provide copious reserves, but the technology required for selective separation faces several challenges due to the presence of complexed matrix ions and the relatively low
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Corresponding author. E-mail address: [email protected] (H.-J. Hong).
https://doi.org/10.1016/j.hydromet.2019.105224 Received 21 August 2019; Received in revised form 11 November 2019; Accepted 29 November 2019 Available online 30 November 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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3020 Surface Area analyzer, and the pore volume was determined using a BJH Porosity Analyzer. The morphologies of the composites were examined using analytical SEM (Topcon sm-300) and FE-TEM (JEM2100F HR) with energy dispersive X-ray spectrometry (EDS) to obtain the distribution of elements. Zeta potentials were measured using an electrophoretic light scattering spectrophotometer (Zetasizer, Malvern).
immobilized adsorbent particles give substantially higher adsorption capacity for a given quantity of adsorbent. However, there remains an unmet need for processes that can achieve facile solid–liquid separation while maintaining adsorption performance by directly applying particulate adsorbents. Recently, magnetic nanocomposites have been actively investigated in research fields such as water treatment and catalytic processes because they can be readily separated from water by application of an external magnetic force, thereby enabling high-performance particletype adsorbents (Ghaeni et al., 2019; Hong et al., 2016a; Kim et al., 2013; Rengaraj et al., 2017). The use of magnetic composite materials for the adsorptive recovery of mineral resources from aqueous media overcomes problems associated with the solid–liquid separation steps required during unit processes (e.g., adsorption–washing–desorption–washing). The resulting resource recovery systems are highly efficient and cost-effective. The physicochemical stabilities of magnetic composites and the separation efficiencies under a magnetic force must be tested ahead of any recovery process application. This study reports the facile fabrication of hierarchically structured manganese oxide-magnetite nanocomposites and their application to the adsorptive recovery of Sr2+ from seawater. MnO2 is a promising adsorbent of Sr2+, but has not been extensively used in Sr recovery from seawater due to problems with adsorbent loss (Hong et al., 2018). Rather than applying the hydrothermal synthesis methods that have been used to fabricate MnO2/Fe3O4 core shell structures (Kim et al., 2013), we employed a redox reaction of MnO4− ions to form an outer MnO2 layer on a carbon-coated magnetite core at room temperature under normal pressure. This process was simple and facilitated mass production. The carbon layer coated onto the Fe3O4 periphery was essential for MnO2 formation and served as an electron donor and support while simultaneously ensuring the chemical stability of the magnetite under acidic conditions. The physical and chemical properties of the magnetic nanocomposite were systematically characterized using TEM, XRD, BET, TGA, and FT-IR. The Sr2+ adsorption performance of the nanocomposite was comprehensively evaluated in terms of the adsorption isotherms, adsorption kinetics, and the effect of the solution chemistry, such as the solution pH and presence of co-existing ions. Finally, a test was conducted to assess the feasibility of recovering Sr2+ from natural seawater using the magnetic nanocomposite adsorbent.
2.3. Adsorption experiments The synthesized MnO2/C/Fe3O4 particles were dispersed in distilled water at 1 g/L (0.02 g / 0.02 L) and sonicated for 30 s. A desired amount of Sr2+ and different metal cation stock solutions were added to the suspension, and the solution pH was adjusted using HCl or NaOH standard solutions, followed by stirring for 30 min to permit equilibrium adsorption. The sample aliquots were intermittently withdrawn during the 30 min adsorption reaction after temporary separation using a permanent magnet. The concentrations of Sr2+ and other metal cations were measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin and Elmer). The performances of the MnO2/C/Fe3O4 samples were evaluated in terms of the uptake of Sr2+. The adsorption isotherm and kinetic models were used to analyze the Sr2+ adsorption behavior on the MnO2/C/Fe3O4 (see Supporting Information). The effects of co-present ions were tested by introducing solutions containing Na+, K+, Mg2+, or Ca2+. The concentrations of these ions in seawater are much higher than the concentration of Sr2+. The initial concentration of Sr2+ was fixed to 10 mg/L, and the concentrations of the co-present ions ranged from 10 mg/L to their concentrations in seawater. Competitive adsorption studies between Sr2+ and Mg2+/Ca2+ were performed using different molar ratios of Sr2+ to Ca2+ and equivalently increasing concentrations of Sr2+ and Mg2+/ Ca2+. The selectivity of MnO2/C/Fe3O4 toward Sr2+ was analyzed by calculating the adsorption-desorption distribution coefficient (Kd) and separation factor (αionsSr) (see Supporting Information). MnO2/C/Fe3O4 sample regeneration was tested by immersing the samples in a 0.1 M HCl solution for 30 min and washing with DI water to neutralize the samples. CaCl2 solutions of different concentrations were used as alternative regenerating reagents. 2.4. Sr2+ recovery from natural seawater
2. Experimental section Sr2+ recovery from natural seawater was tested by dispersing MnO2/C/Fe3O4 particles in seawater at 1 g/L (1.5 g/1.5 L) for 3 days. After adsorption, the particles were rinsed with distilled water repeatedly in combination with magnetic separation using an electromagnet. Sr2+ desorption was conducted at a solid/liquid ratio of 37.5 g/L (1.5 g in 0.04 L of 0.5 M HCl solution) over 30 min. Repeated desorption cycles produced a Sr2+ concentrate, and the pH of the eluent was monitored during each run to maintain optimal desorption conditions. When the pH increased to 1.5 or more over the course of the repeated desorption tests, HCl was added to maximize the desorption efficiency.
2.1. Fabrication of the composite adsorbent The MnO2/C/Fe3O4 magnetic composites were synthesized in two stages. In the first stage, a carbon layer was deposited onto magnetite particles (50–100 nm particles Fe3O4 purchased from Sigma–Aldrich and used as received). The carbon-layered magnetite (C/Fe3O4) was prepared by adding 1 g magnetite and 2 g glucose to 30 mL distilled water and sonicating with stirring for 10–20 min. The suspension was then placed in a 50 mL Teflon-lined autoclave and incubated at 200 °C for 12 h. After cooling to room temperature, C/Fe3O4 was collected using a magnet, rinsed with distilled water and ethanol, and dried in an oven at 60 °C. The second step involved forming MnO2 on C/Fe3O4. A 0.1 g sample of C/Fe3O4 was added to 30 mL KMnO4 (Sigma–Aldrich) with a concentration between 0.01 and 0.1 M. This suspension was stirred using a rotary shaker at 25 °C under atmospheric pressure over 6 days. After magnetic separation, the collected magnetic composite (MnO2/C/Fe3O4) was washed with distilled water and ethanol and dried in an oven at 60 °C.
3. Results and discussion 3.1. Synthesis and characterization Fig. 1 shows SEM and TEM images of the prepared MnO2/C/Fe3O4. The MnO2 formed a core-shell structure that completely surrounded Fe3O4 (Fig. 1b and c). This hierarchical structure was confirmed by EDS analysis (Fig. 1d–f). The MnO2/C/Fe3O4 structures were synthesized by depositing a carbon layer onto the Fe3O4 cores. Carbon layers can be formed on Fe3O4 surfaces under a variety of reaction conditions, depending on the carbon precursor used. In this study, glucose was selected as the carbon precursor, and carbon-layered magnetite (C/Fe3O4) was prepared using a simple hydrothermal reaction. TEM images of the
2.2. Characterization The crystalline phase of the samples was identified by powder X-ray diffraction (XRD) using CuKα radiation (D/Max-2200, Rigaku). The surface area was determined using the BET method with a Tristar II 2
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Fig. 1. (a) SEM and (b), (c) TEM images of MnO2/C/Fe3O4. (d), (e), and (f) represent corresponding EDS mapping of Fe, Mn, and O for TEM specimen, respectively.
both the bare Fe3O4 and MnO2/C/Fe3O4 particles no hysteresis was observed during magnetization, and the coercivity and remanence remained zero, suggesting typical superparamagnetic behavior.(Deng et al., 2008) The saturation magnetization of MnO2/C/Fe3O4 was found to be 25 emu/g, much less than that of pure Fe3O4 (75 emu/g), as shown in Fig. 2a. Nevertheless, the MnO2/C/Fe3O4 particles could be separated from the reaction medium quickly and easily in a magnetic field (Fig. 2b). Moreover, when the external magnetic field was switched off, the MnO2/C/Fe3O4 particles returned to a well dispersed configuration. The N2 adsorption-desorption isotherm of MnO2/C/ Fe3O4 indicated type IV and H3 hysteresis (see Fig. S4), typical of mesoporous materials in which slit-shaped nanoplates form aggregates. (Sing, 1985) Finally, the specific surface area of the MnO2/C/Fe3O4 sample with 28 wt% MnO2 was measured to be 9.24 m2/g. The crystal structure within the synthesized MnO2/C/Fe3O4 was identified using XRD analysis. The diffraction pattern of MnO2/C/Fe3O4 was similar to that of pristine Fe3O4, suggesting that MnO2 was present in an amorphous phase (see Fig. S5). FT-IR spectroscopy was used to examine the chemical structures of the MnO2/C/Fe3O4 samples. The IR spectra (see Fig. S6) showed an absorption peak at 576 cm−1, which can be assigned to the stretching vibration of the FeeO bond in Fe3O4. Comparison of the IR spectra of pure Fe3O4 and MnO2/C/Fe3O4 revealed a remarkable change at 506 cm−1 attributed to the stretching vibration of the MneO bond.(Wang et al., 2015)
bare Fe3O4 and C/Fe3O4 revealed that a thin carbon layer of thickness approximately 10 nm uniformly covered the Fe3O4 core (Fig. S1 inset figures). TG analysis confirmed the carbon content of C/Fe3O4 to be 7.36 wt% (see Fig. S1). MnO2 was synthesized onto the C/Fe3O4 by adding a manganese precursor (KMnO4) to the suspension and aging the solution under ambient conditions in the absence of additional heat or pH adjustment. This approach is distinct from the general hydrothermal synthesis and precipitation procedure using pH control. The concentration of KMnO4 was varied from 0.01 M to 0.1 M, and the corresponding MnO2 content of the composite increased linearly with increasing KMnO4 concentration. At the maximum KMnO4 concentration of 0.1 M, the MnO2 content of the composite was 28 wt%, as determined by ICP analysis (see Table S2). The synthetic mechanism for this MnO2 relies on the presence of a carbon layer with a large number of π electrons because the reaction proceeds under electron-rich conditions (Eq. 1): (Jin et al., 2007; Ma et al., 2007)
MnO4− + 4H+ + 3e− → MnO2 + 2H2 O
(1)
The role of the carbon layer in MnO2 formation was explored by preparing SiO2-coated Fe3O4 (SiO2/Fe3O4) as a reference and comparing its behavior with that of C/Fe3O4 (see Figs. S2 and S3). SiO2 has been widely used to synthesize core-shell structured nanocomposites. (Deng et al., 2008; Yi et al., 2006) MnO2 formation on SiO2/Fe3O4 particles was characterized under the same conditions as used above for C/Fe3O4. ICP data revealed that the amounts of MnO2 loaded onto bare magnetite (Fe3O4) and SiO2-coated magnetite (SiO2/Fe3O4) were only 1.7 and 2.4 wt%, respectively, indicating that the carbon layer was clearly essential to the synthesis of MnO2. The adsorption capacity of the MnO2/C/Fe3O4 particles increased with increasing amount of MnO2; however, larger quantities of MnO2 covering the Fe3O4 core in a thicker layer weakened the magnetic properties of the particles relative to those of pure Fe3O4 particles, thereby reducing the separation efficiency. Fig. 2 shows the magnetic hysteresis loop of bare Fe3O4 and MnO2/C/Fe3O4, for a sample prepared in the presence of the largest amount of MnO2 (28 wt%). Fore
3.2. Sr adsorption behavior The performances of the synthesized MnO2/C/Fe3O4 samples were evaluated by measuring the Sr2+ adsorption kinetics and isotherms. Fig. 3 shows the time profiles of Sr2+ adsorption onto MnO2/C/Fe3O4 as a function of the initial concentration of Sr2+. Adsorption proceeded quickly and reached a plateau within 10 min, regardless of the initial Sr2+ concentration. This rapid adsorption indicated the absence of diffusion resistance inside the adsorbent, and the adsorption reaction took place on the surface of the adsorbent. The adsorption kinetics 3
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Fig. 2. Magnetic hysteresis curves (b) and picture of magnetic separation (b) of composite materials. Table 1 Isotherm model constants for Sr2+ adsorption onto MnO2/C/Fe3O4. Langmuir model
Freundlich model 2
[KMnO4] (M)
Qm (mg/g)
KL (L/mg)
R
Kf
n
R2
0.025 0.05 0.1
44.24 41.66 42.19
0.17 0.39 0.71
0.9806 0.9901 0.9964
12.02 41.03 28.77
3.46 8.83 12.67
0.9999 0.8965 0.9487
different MnO2/C/Fe3O4 samples increased with increasing initial Sr2+ concentrations ranging from 10 mg/L to 100 mg/L. Samples with thicker MnO2 layers (i.e., those synthesized using higher KMnO4 concentrations) (Table S2) showed higher adsorption capacities. The experimental Sr2+ adsorption profiles were fitted to Langmuir isotherm and Freundlich models, and the resultant equilibrium parameters are listed in Table 1. The Sr2+ adsorption behaviors onto MnO2/C/Fe3O4 confirmed to the Langmuir isotherm model (R2 = 0.98–0.99), indicating that the adsorption of Sr2+ proceeded in a homogeneous monolayer, and that each adsorption active site acted independently. The MnO2/C/Fe3O4 particles synthesized using 0.025 M KMnO4 displayed a higher linear regression coefficient using the Freundlich isotherm model compared with the Langmuir model. However, under these synthesis conditions, insufficient MnO2 was synthesized to fully cover the surface of the C/ Fe3O4 composite. As a result, the MnO2 Sr2+ adsorption sites were not homogeneously distributed, resulting in heterogeneous adsorption of Sr2+ onto the composite. The theoretical maximum Sr2+ adsorption capacity (Qm) of the MnO2/C/Fe3O4 particles with the thickest MnO2 layer (i.e., those prepared using [KMnO4] = 0.1 M) was calculated to be 42 mg/g using the Langmuir model fit, in good agreement with the experimental results. Although all MnO2/C/Fe3O4 samples showed similar Sr2+ adsorption capacities, 41–45 mg/g, the MnO2/C/Fe3O4 sample synthesized in the presence of 0.1 M KMnO4 showed the highest Sr2+ adsorption capacity at a low Sr2+ equilibrium concentration. Because the Sr2+ concentration in seawater is only 7–8 mg/L, the precursor concentration used for MnO2/C/Fe3O4 synthesis was chosen to be 0.1 M and was used in further experiments.
Fig. 3. Time-dependent Sr2+ adsorption profiles on MnO2/C/Fe3O4 samples as a function of initial concentration of Sr2+. ([MnO2/C/Fe3O4] = 1 g/L and pHi = 8).
Fig. 4. Sr2+ adsorption isotherms of MnO2/C/Fe3O4 samples with different loading amount of MnO2. ([MnO2/C/Fe3O4] = 1 g/L, pHi = 8, and contact time = 30 min).
agreed better with a pseudo-second-order model than with the intraparticle diffusion model, indicating correlation coefficient (R2) values exceeding 0.99 (see Table S3) values exceeding 0.99 (see Table S3). The overall adsorption kinetics were, therefore, determined by a chemisorption process rather than by mass transfer. Information on the adsorption behavior of the MnO2/C/Fe3O4 materials was obtained from Sr2+ adsorption isotherms. As shown in Fig. 4, Sr2+ adsorption onto
3.3. Effect of the solution chemistry on Sr adsorption Because the presence of matrix ions such as Mg2+ and Ca2+ in seawater may affect the adsorption of Sr2+ ions onto the MnO2/C/ Fe3O4 particles, it was crucial to thoroughly examine the effects of different solution chemistries on the Sr2+ adsorption capacity prior to testing any practical applications in seawater. The effects of solution pH 4
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Fe3O4, as indicated by the uptake profiles of co-present Mg2+ and Ca2+ over Sr2+ in proportion to their concentrations (see Fig. S7). During competitive adsorption, higher ionic charges and lower hydration energies yield better adsorption profiles. Sr2+ and Ca2+ have very similar chemical characteristics, such as valence, ionic radius, and hardness,(Li et al., 2012) which facilitates competition against Sr2+ adsorption onto MnO2/C/Fe3O4. The selectivities of Sr2+ against Mg2+ or Ca2+ were further investigated by evaluating the uptake ratio of each metal ion as a function of the metal ion concentration. The molar concentration of Sr2+ to metal ion (Mg2+ or Ca2+) was set to 1, to highlight any differences. As shown in Fig. 7a and b, the uptake ratio was linearly proportional to the molar ratio in both cases, indicating that the concentrations of co-present cations determined the selectivity of adsorption. The distribution coefficient (Kd) was calculated to assess the adsorption selectivity. Fig. 7c and d show the correlations between the Kd values of Sr2+ vs. Mg2+ and Sr2+ vs. Ca2+ at equimolar concentrations of these cations. The slopes of the linear regression fits, which corresponded to the separation factors (αMetalSr, see SI) were determined to be 11.17 and 1.75 for Sr2+ vs. Mg2+ and Sr2+ vs. Ca2+, respectively. MnO2/C/Fe3O4, therefore, had an 11-fold higher selectivity for Sr2+ than for Mg2+ and an approximately 2-fold higher selectivity compared to Ca2+. Considering that the seawater concentrations of Mg2+ (1200 mg/L) and Ca2+ (400 mg/L) are 150-fold and 50-fold higher, respectively, than the seawater concentration of Sr2+ (8 mg/L), the presence of two coexisting ions may present a major obstacle to recovering Sr2+ from seawater. Desorption must be considered in the context of adsorbed Sr2+ recovery and adsorbent regeneration. Given that MnO2/C/Fe3O4 was found to be positively charged at low pH (pH < 3, Fig. 5a) and protons, as the smallest cation, readily participate in ion exchange, Sr2+ desorption was conducted using acid treatment. As shown in Fig. 8, the amount of Sr2+ released in a 0.1 M HCl solution was identical to the amount adsorbed, as measured from the adsorption isotherms (see Fig. 4), indicating that the adsorbed Sr2+ was completely desorbed during acid treatment. It should be noted that release of iron ions was observed during acid treatment of MnO2/C/Fe3O4 particles with lower MnO2 loadings, but this loss of iron was blocked by thicker MnO2 coatings, indicating that thicker MnO2 outer shells offered more rigid protection of the magnetite core against acid treatment. Thus, the MnO2 layer not only provides active absorption sites for Sr2+, it also acts as a barrier to chemical degradation of the magnetic core. During acid treatment, Mn2+ was released from MnO2/C/Fe3O4. The amount of Mn2+ released increased slightly with the MnO2 content of MnO2/C/ Fe3O4. However, MnO2/C/Fe3O4 particles with higher total MnO2 contents were more stable in acid in terms of Mn2+ release during repeated Sr2+ adsorption-desorption tests. As shown in Fig. S8, the adsorption efficiency of Sr2+ on MnO2/C/Fe3O4 dramatically decreased to ca. 40% during the 2nd cycle, mainly because MnO2 is amorphous and vulnerable to acid degradation. To overcome this limitation, the acid concentration was diluted in an eluent or the eluent was replaced with a neutral solution, such as a CaCl2 solution. The utility of CaCl2 as an eluent was tested by conducting desorption experiments using a Ca2+ solution in place of HCl. Because Ca2+ competed most strongly with Sr2+ adsorption in seawater, desorption was postulated to proceed efficiently in the presence of a highly concentrated Ca2+ solution. Fig. S9 shows the Sr2+ desorption performance as a function of the Ca2+ concentration, revealing desorption efficiencies of 40% in the presence of 0.01 M Ca2+ and 50% in the presence of > 0.1 M Ca2+. Although Ca2+ solutions are less effective than HCl in inducing desorption, they may be desirable during repeated MnO2/C/Fe3O4 regeneration cycles in that they may preserve the integrity of the adsorbent.
Fig. 5. (a) Zeta potential of and (b) Sr2+ adsorption efficiency as a function of the solution pH. ([MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 10 mg/L, and contact time = 30 min).
and co-present ions on the adsorption of Sr2+ were therefore investigated. Solution pH can significantly influence the surface charge of a MnO2/C/Fe3O4 surface and thus affect the electrostatic interactions between adsorbate species and the surface, thereby affecting the adsorption behavior Fig. 5a shows the surface potential changes observed in solutions containing pure Fe3O4 or MnO2/C/Fe3O4 as a function of the solution pH. The pHzpc of pure Fe3O4 at which the surface charge reached zero was approximately 7, whereas that of MnO2/C/Fe3O4 was found to be 3. These results suggested that the surfaces of the MnO2/C/ Fe3O4 at pH values below 3 assumed a positive potential, suggesting that adsorption would be inhibited due to electrostatic repulsion because Sr2+ is present in seawater as a divalent cation. As shown in Fig. 5b, the profile of Sr2+ adsorption as a function of the solution pH agreed well with the pH-dependent changes in the surface charge. The Sr2+ adsorption efficiency remained constant at pH values above 4, decreased at pH 3, and markedly decreased to approximately 10% of its value at pH 2, a drop that can be ascribed to electrostatic hindrance. The major cations in seawater are Na+, Mg2+, Ca2+, and K+, in order of increasing concentration (Table S1). The effects of these ions on Sr2+ adsorption were measured by measuring Sr2+ adsorption in a solution containing 10 mg/L Sr2+ and various concentrations of the major cations in seawater. As shown in Fig. 6, the presence of Na+ did not significantly affect Sr2+ adsorption below 1000 mg/L, and the adsorption efficiency decreased by 40% at the seawater level of 10,000 mg/L. K+ did not hinder Sr2+ adsorption across the range of K+ concentrations tested; however, the presence of Mg2+ and Ca2+ significantly interfered with Sr2+ adsorption. Mg2+ inhibited Sr2+ adsorption by 40% at 600 mg/L and continued to show this level of inhibition up to the Mg2+ seawater level of 1200 mg/L. The presence of Ca2+ remarkably hindered the Sr2+ adsorption efficiency by 60% at 100 mg/L and 80% at 400 mg/L (the Ca2+ concentration in seawater). The inhibition effects of Mg2+ and Ca2+ were ascribed to competitive adsorption processes because Mg2+ and Ca2+ adsorbed onto MnO2/C/
3.4. Recovery of Sr2+ from natural seawater Sr2+ adsorption from natural seawater was tested to obtain a Sr2+ concentrate for the practical recovery of Sr2+ from seawater. Sr2+ 5
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Fig. 6. Effects of co-existing ions (a) Na+, (b) K+, (c) Mg2+, and (d) Ca2+ on Sr2+ adsorption efficiency. ([MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 10 mg/L, pHi = 8, and contact time = 30 min).
Fig. 7. Uptake ratio of (a) Mg2+ and (b) Ca2+ to Sr2+ as a function of the molar ratio of Mg2+ and Ca2+ to Sr2+ added, (c) and (d) represent the correlation of the distribution coefficient for Sr2+ vs Mg2+ and Sr2+ vs Ca2+ at an equimolar concentration of Sr2+, Mg2+, and Ca2+ ranging from 0.25 mM to 2 mM, respectively. ([MnO2/C/Fe3O4] = 1 g/L, pHi = 8, and contact time = 30 min).
6
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Fig. 8. Fe release after acid treatment of MnO2/C/Fe3O4 for the regeneration of adsorbents. (Adsorption: [MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 25 mg/L, pHi = 8, contact time = 30 min; Desorption: [adsorbent] = 1 g/L, [HCl] = 0.1 M, contact time = 30 min). Table 2 The composition and concentration factor of Sr2+ concentrate extracted from seawater. Element
Sr
Ca
Mg
Na
K
Mn
Fe
Concentration (mg/L) Concentration factor (CF, L/g)
501.4 1.714
5424.2 0.363
1421.6 0.0292
1592.1 0.004
834.4 0.057
2464.3 6571.5
3.4 –
efficiency under the given desorption conditions. The concentration of Sr2+ increased linearly as the desorption cycles proceeded (Fig. 9), reaching 500 mg/L after the 8th cycle. These results indicated that desorption was effective, even over repeated desorption cycles, and iterative desorption in the presence of HCl extracted concentrated Sr2+. The solution pH was monitored over each desorption cycle. The solution pH during the first desorption cycle was below 0.5, and the pH increased in proportion to the desorption cycle due to proton consumption. After the 5th cycle, the pH was 1.3, corresponding to 0.05 M acid, suggesting that proton concentrations lower than 0.1 M did not retard Sr2+ desorption. A desirable amount of HCl equivalent to the protons consumed was added to encourage desorption over subsequent cycles, and the pH level of the Sr2+ concentrate was maintained below pH 1. The composition of the Sr2+ concentrate extracted from natural seawater during the 8th desorption cycle is listed in Table 2. The major solution components were found to be Ca2+, Mn4+, Na+, Mg2+, and K+, in order of decreasing concentration. Due to their high initial concentrations, Ca2+, Mg2+, Na+ and K+ were more concentrated than Sr2+ in the extract. The competitive ions, Ca2+ and Mg2+, were highly concentrated in the eluent. The concentration factor (CF, see Table 2) indicated that Sr2+ was the only metal concentrated in the eluent, except for Mn4+. The concentration of Sr2+ in seawater is very low (≈
Fig. 9. Concentration profile of Sr2+ and pH variation in the concentrate as a function of repeated desorption. (Adsorption: [MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 7.8 mg/L (seawater), contact time = 3 day; Desorption: [MnO2/C/ Fe3O4] = 37.5 g/L, [HCl] = 0.5 M, contact time = 30 min).
adsorption from seawater was tested under conditions of 1.5 g MnO2/ C/Fe3O4 in 1.5 L natural seawater over 3 days in a batch system. Confirming that the experimental conditions, such as the solid-to-liquid ratio and contact time, were suitable for the extraction of Sr2+ from seawater. After the 3-days adsorption period, the Sr2+-adsorbed MnO2/ C/Fe3O4 particles were collected using a magnetic field and dried to remove moisture. Desorption was tested to determine whether the Sr2+ solution could be further concentrated. All desorption conditions were identical to those used during the first desorption study, and 8 desorption cycles were tested. Fig. 9 shows the profile of the Sr2+ concentration and pH during the repeated desorption cycles. The concentration of Sr2+ obtained from the first desorption cycle was measured to be 71.5 mg/L, corresponding to a 95% desorption 7
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8 mg/L) compared with the concentrations of other ions, but Sr2+ was selectively concentrated (≈ 500 mg/L) in the eluent. The high concentration of Mn4+ in the concentrate should be considered in relation to the durability of the MnO2 magnetic composite, given that MnO2 is vulnerable to acid treatment. The observation that the concentration of Fe3+ was very low (3.4 mg/L) in the final concentrate suggested the operational durability of the hierarchical structure of MnO2, carbon, and Fe3O4 under the acidic conditions tested.
funded by the Ministry of Science and ICT.
4. Conclusions
Bardi, U., 2010. Extracting minerals from seawater: an energy analysis. Sustainability 2 (4), 980–992. Bezhin, N., Dovhyi, I., 2015. Sorbents based on crown ethers: preparation and application for the sorption of strontium. Russ. Chem. Rev. 84 (12), 1279. Deng, Y., Qi, D., Deng, C., Zhang, X., Zhao, D., 2008. Superparamagnetic high-magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins. J. Am. Chem. Soc. 130 (1), 28–29. Dhandole, L.K., et al., 2016. Hydrothermal synthesis of titanate nanotubes from TiO2 nanorods prepared via a molten salt flux method as an effective adsorbent for strontium ion recovery. RSC Adv. 6 (100), 98449–98456. Dhandole, L.K., Chung, H.-S., Ryu, J., Jang, J.S., 2018. Vertically aligned Titanate nanotubes hydrothermally synthesized from anodized TiO2 nanotube arrays: an efficient adsorbent for the repeatable recovery of Sr ions. ACS Sustain. Chem. Eng. 6 (12), 16139–16150. Ghaeni, N., Taleshi, M.S., Elmi, F., 2019. Removal and recovery of strontium (Sr (II)) from seawater by Fe3O4/MnO2/fulvic acid nanocomposite. Mar. Chem. 213, 33–39. Hong, H.-J., et al., 2016a. Highly stable and magnetically separable alginate/Fe3O4 composite for the removal of strontium (Sr) from seawater. Chemosphere 165, 231–238. Hong, H.-J., et al., 2016b. Investigation of the strontium (Sr (II)) adsorption of an alginate microsphere as a low-cost adsorbent for removal and recovery from seawater. J. Environ. Manag. 165, 263–270. Hong, H.-J., et al., 2017. Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism. Chem. Eng. J. 319, 163–169. Hong, H.-J., Park, I.-S., Ryu, T., Jeong, H.S., Ryu, J., 2018. Demonstration of seawater strontium (Sr (II)) extraction and enrichment by a biosorption technique through continuous column operation. Ind. Eng. Chem. Res. 57 (38), 12909–12915. Jin, X., Zhou, W., Zhang, S., Chen, G.Z., 2007. Nanoscale microelectrochemical cells on carbon nanotubes. Small 3 (9), 1513–1517. Kim, E.-J., Lee, C.-S., Chang, Y.-Y., Chang, Y.-S., 2013. Hierarchically structured manganese oxide-coated magnetic nanocomposites for the efficient removal of heavy metal ions from aqueous systems. ACS Appl. Mater. Interfaces 5 (19), 9628–9634. Li, N., et al., 2012. Highly efficient, irreversible and selective ion exchange property of layered titanate nanostructures. Adv. Funct. Mater. 22 (4), 835–841. Ma, S.-B., Ahn, K.-Y., Lee, E.-S., Oh, K.-H., Kim, K.-B., 2007. Synthesis and characterization of manganese dioxide spontaneously coated on carbon nanotubes. Carbon 45 (2), 375–382. Ober, J.A., 2015. Strontium. In: United States Geological Survey Mineral Commodity Summaries. U.S. Geological Survey, Reston, VA. Quist-Jensen, C., Macedonio, F., Drioli, E., 2016. Integrated membrane desalination systems with membrane crystallization units for resource recovery: a new approach for mining from the sea. Crystals 6 (4), 36. Rengaraj, A., et al., 2017. Covalent triazine polymer–Fe3O4 nanocomposite for strontium ion removal from seawater. Ind. Eng. Chem. Res. 56 (17), 4984–4992. Ryu, J., et al., 2016. Strontium ion (Sr2+) separation from seawater by hydrothermally structured titanate nanotubes: removal vs. recovery. Chem. Eng. J. 304, 503–510. Sing, K.S., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (recommendations 1984). Pure Appl. Chem. 57 (4), 603–619. Wang, J.-W., Chen, Y., Chen, B.-Z., 2015. A synthesis method of MnO2/activated carbon composite for electrochemical supercapacitors. J. Electrochem. Soc. 162 (8), A1654–A1661. Yi, D.K., Lee, S.S., Papaefthymiou, G.C., Ying, J.Y., 2006. Nanoparticle architectures templated by SiO2/Fe2O3 nanocomposites. Chem. Mater. 18 (3), 614–619.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.hydromet.2019.105224. References
To date, studies of adsorbent materials have been limited to the removal of heavy metals and radionuclides from aqueous media. This study focused on developing a method for recovering Sr resources from seawater. We prepared a magnetic nanocomposite for use in facile separation from water using magnets, and we evaluated the adsorption of Sr2+ in seawater onto the nanocomposite under various reaction conditions. A hierarchical MnO2/C/Fe3O4 structure was obtained by forming MnO2 on a carbon layer coated onto magnetite via oxidationreduction of MnO4−, under ambient conditions without pH control. The core–shell structure reduced the saturation magnetization to ca. 33% of the value obtained from the pristine Fe3O4 core; nevertheless, the prepared magnetic nanoparticles could be effectively separated using an external magnetic force. Among the major ions in seawater that competed with Sr2+, the divalent cations Mg2+ and Ca2+ significantly hindered Sr2+ adsorption. This competition was attributed to the similar chemical properties of these cations, such as the valence, hardness, hydration energy, and ionic radius, which are crucial factors determining the selectivity of certain ions for adsorption. Although the selectivity of MnO2/C/Fe3O4 for Sr2+ adsorption was found to be approximately 11-fold and 2-fold higher than the selectivities for Mg2+ and Ca2+, respectively, Sr2+ adsorption from seawater is not expected to be favorable because the concentrations of Mg2+ and Ca2+ in seawater are much higher than the concentration of Sr2+. The MnO2/C/ Fe3O4 particles were used to extract a Sr2+ concentrate ([Sr2+] = 501 mg/L) from seawater during the recovery of adsorbed Sr2+ via washing and Sr2+ desorption in an acid solution. Magnetic separation was used during each step. These results demonstrated the effectiveness of this seawater Sr2+ recovery process utilizing magnetic adsorbents. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (project 19-3413)
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Recovery of strontium (Sr2+) from seawater using a hierarchically structured MnO2/C/Fe3O4 magnetic nanocomposite Jungho Ryua, Jeongsik Hongb, In-Su Parkc, Taegong Ryuc, Hye-Jin Hongc,
T
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a
Geologic Environment Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, South Korea R&D Team, Ecopro Innovation Co.,LTD, Chungbuk 28116, South Korea c Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, South Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: Strontium Seawater Recovery Magnetic separation Enrichment
The recovery of strontium ions (Sr2+) from seawater has attracted much attention as an approach to securing Sr resources to meet increasing industrial demand. In this study, we synthesized a magnetic MnO2 nanocomposite (MnO2/C/Fe3O4) using a simple redox reaction under ambient conditions and applied this nanocomposite to the extraction of Sr2+ from natural seawater. The synthesized nanocomposite exhibited a hierarchical structure of MnO2, carbon, and Fe3O4 with a saturation magnetization of 25 emu/g that enabled effective separation under an external magnetic force. Regardless of the initial Sr2+ concentration, the adsorption of Sr2+ onto MnO2/C/ Fe3O4 proceeded rapidly (within < 10 min) following a pseudo-second-order kinetic model and agreed well with the Langmuir isotherm model, indicating a maximum adsorption capacity of 42 mg/g. Among the competitive ions, Mg2+ and Ca2+ significantly hindered Sr2+ adsorption onto MnO2/C/Fe3O4, whereas Na+ and K+ had little effect on Sr2+ adsorption. A detailed study of the distribution coefficient (Kd) revealed 11.2-fold and 1.8fold higher selectivities toward Sr2+ than toward Mg2+ and Ca2+, respectively. The Sr2+ ions adsorbed onto MnO2/C/Fe3O4 were completely recovered through desorption in 0.1 M HCl eluent. Finally, a Sr2+-enriched solution with a concentration of 501 mg/L could be obtained via seawater adsorption and subsequent acid desorption over 8 iterative cycles, demonstrating its effectiveness for practical applications of Sr2+ recovery from seawater.
1. Introduction
concentrations of these useful resources in seawater. Among the various methods tested for selectively separating low-concentration elements from seawater, adsorption techniques are reported to be the most effective and feasible options based on the preparation of adsorbent materials with high efficiencies and high selectivities (Bezhin and Dovhyi, 2015; Dhandole et al., 2016, 2018; Hong et al., 2016b, 2017; Ryu et al., 2016). Conventional adsorbents are prepared in a powder form, and solid–liquid separation methods, such as centrifuging and filtering or the introduction of powder-immobilized media, are required for any practical recovery process. Conventional solid–liquid separation processes, such as filtration or centrifugal separation, have drawbacks in that they are complicated, incur substantial adsorbent loss, and require long process times. When the adsorbent particles are adsorbed onto a substrate rather than in powder form, their exposed surface areas are significantly reduced, thus diminishing their adsorption capacity. In addition, immobilized adsorbents require support materials with a high chemical stability that can withstand repeated chemical treatments and long-term exposure to seawater. Thus, the use of free rather than
Strontium (Sr) is a rare metal that is conventionally used in the ceramic and glass manufacturing industries. Industrial demand for Sr is increasing, a trend that will only continue because several industries, including the pharmaceutical industry, are expected to increase their consumption of this rare metal (Ober, 2015). Accordingly, it has become critical to secure Sr resources that can satisfy current and future industrial demand. One approach to overcoming the limitations on available land resources involves the recovery of useful resources from seawater or brine water (Bardi, 2010; Quist-Jensen et al., 2016). Seawater could potentially provide copious reserves, but the selective recovery of Sr from seawater made difficult by the presence of complexed matrix ions and the relatively low concentration of Sr2+ in seawater. Virtually unlimited seawater could potentially provide copious reserves, but the technology required for selective separation faces several challenges due to the presence of complexed matrix ions and the relatively low
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Corresponding author. E-mail address: [email protected] (H.-J. Hong).
https://doi.org/10.1016/j.hydromet.2019.105224 Received 21 August 2019; Received in revised form 11 November 2019; Accepted 29 November 2019 Available online 30 November 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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3020 Surface Area analyzer, and the pore volume was determined using a BJH Porosity Analyzer. The morphologies of the composites were examined using analytical SEM (Topcon sm-300) and FE-TEM (JEM2100F HR) with energy dispersive X-ray spectrometry (EDS) to obtain the distribution of elements. Zeta potentials were measured using an electrophoretic light scattering spectrophotometer (Zetasizer, Malvern).
immobilized adsorbent particles give substantially higher adsorption capacity for a given quantity of adsorbent. However, there remains an unmet need for processes that can achieve facile solid–liquid separation while maintaining adsorption performance by directly applying particulate adsorbents. Recently, magnetic nanocomposites have been actively investigated in research fields such as water treatment and catalytic processes because they can be readily separated from water by application of an external magnetic force, thereby enabling high-performance particletype adsorbents (Ghaeni et al., 2019; Hong et al., 2016a; Kim et al., 2013; Rengaraj et al., 2017). The use of magnetic composite materials for the adsorptive recovery of mineral resources from aqueous media overcomes problems associated with the solid–liquid separation steps required during unit processes (e.g., adsorption–washing–desorption–washing). The resulting resource recovery systems are highly efficient and cost-effective. The physicochemical stabilities of magnetic composites and the separation efficiencies under a magnetic force must be tested ahead of any recovery process application. This study reports the facile fabrication of hierarchically structured manganese oxide-magnetite nanocomposites and their application to the adsorptive recovery of Sr2+ from seawater. MnO2 is a promising adsorbent of Sr2+, but has not been extensively used in Sr recovery from seawater due to problems with adsorbent loss (Hong et al., 2018). Rather than applying the hydrothermal synthesis methods that have been used to fabricate MnO2/Fe3O4 core shell structures (Kim et al., 2013), we employed a redox reaction of MnO4− ions to form an outer MnO2 layer on a carbon-coated magnetite core at room temperature under normal pressure. This process was simple and facilitated mass production. The carbon layer coated onto the Fe3O4 periphery was essential for MnO2 formation and served as an electron donor and support while simultaneously ensuring the chemical stability of the magnetite under acidic conditions. The physical and chemical properties of the magnetic nanocomposite were systematically characterized using TEM, XRD, BET, TGA, and FT-IR. The Sr2+ adsorption performance of the nanocomposite was comprehensively evaluated in terms of the adsorption isotherms, adsorption kinetics, and the effect of the solution chemistry, such as the solution pH and presence of co-existing ions. Finally, a test was conducted to assess the feasibility of recovering Sr2+ from natural seawater using the magnetic nanocomposite adsorbent.
2.3. Adsorption experiments The synthesized MnO2/C/Fe3O4 particles were dispersed in distilled water at 1 g/L (0.02 g / 0.02 L) and sonicated for 30 s. A desired amount of Sr2+ and different metal cation stock solutions were added to the suspension, and the solution pH was adjusted using HCl or NaOH standard solutions, followed by stirring for 30 min to permit equilibrium adsorption. The sample aliquots were intermittently withdrawn during the 30 min adsorption reaction after temporary separation using a permanent magnet. The concentrations of Sr2+ and other metal cations were measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin and Elmer). The performances of the MnO2/C/Fe3O4 samples were evaluated in terms of the uptake of Sr2+. The adsorption isotherm and kinetic models were used to analyze the Sr2+ adsorption behavior on the MnO2/C/Fe3O4 (see Supporting Information). The effects of co-present ions were tested by introducing solutions containing Na+, K+, Mg2+, or Ca2+. The concentrations of these ions in seawater are much higher than the concentration of Sr2+. The initial concentration of Sr2+ was fixed to 10 mg/L, and the concentrations of the co-present ions ranged from 10 mg/L to their concentrations in seawater. Competitive adsorption studies between Sr2+ and Mg2+/Ca2+ were performed using different molar ratios of Sr2+ to Ca2+ and equivalently increasing concentrations of Sr2+ and Mg2+/ Ca2+. The selectivity of MnO2/C/Fe3O4 toward Sr2+ was analyzed by calculating the adsorption-desorption distribution coefficient (Kd) and separation factor (αionsSr) (see Supporting Information). MnO2/C/Fe3O4 sample regeneration was tested by immersing the samples in a 0.1 M HCl solution for 30 min and washing with DI water to neutralize the samples. CaCl2 solutions of different concentrations were used as alternative regenerating reagents. 2.4. Sr2+ recovery from natural seawater
2. Experimental section Sr2+ recovery from natural seawater was tested by dispersing MnO2/C/Fe3O4 particles in seawater at 1 g/L (1.5 g/1.5 L) for 3 days. After adsorption, the particles were rinsed with distilled water repeatedly in combination with magnetic separation using an electromagnet. Sr2+ desorption was conducted at a solid/liquid ratio of 37.5 g/L (1.5 g in 0.04 L of 0.5 M HCl solution) over 30 min. Repeated desorption cycles produced a Sr2+ concentrate, and the pH of the eluent was monitored during each run to maintain optimal desorption conditions. When the pH increased to 1.5 or more over the course of the repeated desorption tests, HCl was added to maximize the desorption efficiency.
2.1. Fabrication of the composite adsorbent The MnO2/C/Fe3O4 magnetic composites were synthesized in two stages. In the first stage, a carbon layer was deposited onto magnetite particles (50–100 nm particles Fe3O4 purchased from Sigma–Aldrich and used as received). The carbon-layered magnetite (C/Fe3O4) was prepared by adding 1 g magnetite and 2 g glucose to 30 mL distilled water and sonicating with stirring for 10–20 min. The suspension was then placed in a 50 mL Teflon-lined autoclave and incubated at 200 °C for 12 h. After cooling to room temperature, C/Fe3O4 was collected using a magnet, rinsed with distilled water and ethanol, and dried in an oven at 60 °C. The second step involved forming MnO2 on C/Fe3O4. A 0.1 g sample of C/Fe3O4 was added to 30 mL KMnO4 (Sigma–Aldrich) with a concentration between 0.01 and 0.1 M. This suspension was stirred using a rotary shaker at 25 °C under atmospheric pressure over 6 days. After magnetic separation, the collected magnetic composite (MnO2/C/Fe3O4) was washed with distilled water and ethanol and dried in an oven at 60 °C.
3. Results and discussion 3.1. Synthesis and characterization Fig. 1 shows SEM and TEM images of the prepared MnO2/C/Fe3O4. The MnO2 formed a core-shell structure that completely surrounded Fe3O4 (Fig. 1b and c). This hierarchical structure was confirmed by EDS analysis (Fig. 1d–f). The MnO2/C/Fe3O4 structures were synthesized by depositing a carbon layer onto the Fe3O4 cores. Carbon layers can be formed on Fe3O4 surfaces under a variety of reaction conditions, depending on the carbon precursor used. In this study, glucose was selected as the carbon precursor, and carbon-layered magnetite (C/Fe3O4) was prepared using a simple hydrothermal reaction. TEM images of the
2.2. Characterization The crystalline phase of the samples was identified by powder X-ray diffraction (XRD) using CuKα radiation (D/Max-2200, Rigaku). The surface area was determined using the BET method with a Tristar II 2
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Fig. 1. (a) SEM and (b), (c) TEM images of MnO2/C/Fe3O4. (d), (e), and (f) represent corresponding EDS mapping of Fe, Mn, and O for TEM specimen, respectively.
both the bare Fe3O4 and MnO2/C/Fe3O4 particles no hysteresis was observed during magnetization, and the coercivity and remanence remained zero, suggesting typical superparamagnetic behavior.(Deng et al., 2008) The saturation magnetization of MnO2/C/Fe3O4 was found to be 25 emu/g, much less than that of pure Fe3O4 (75 emu/g), as shown in Fig. 2a. Nevertheless, the MnO2/C/Fe3O4 particles could be separated from the reaction medium quickly and easily in a magnetic field (Fig. 2b). Moreover, when the external magnetic field was switched off, the MnO2/C/Fe3O4 particles returned to a well dispersed configuration. The N2 adsorption-desorption isotherm of MnO2/C/ Fe3O4 indicated type IV and H3 hysteresis (see Fig. S4), typical of mesoporous materials in which slit-shaped nanoplates form aggregates. (Sing, 1985) Finally, the specific surface area of the MnO2/C/Fe3O4 sample with 28 wt% MnO2 was measured to be 9.24 m2/g. The crystal structure within the synthesized MnO2/C/Fe3O4 was identified using XRD analysis. The diffraction pattern of MnO2/C/Fe3O4 was similar to that of pristine Fe3O4, suggesting that MnO2 was present in an amorphous phase (see Fig. S5). FT-IR spectroscopy was used to examine the chemical structures of the MnO2/C/Fe3O4 samples. The IR spectra (see Fig. S6) showed an absorption peak at 576 cm−1, which can be assigned to the stretching vibration of the FeeO bond in Fe3O4. Comparison of the IR spectra of pure Fe3O4 and MnO2/C/Fe3O4 revealed a remarkable change at 506 cm−1 attributed to the stretching vibration of the MneO bond.(Wang et al., 2015)
bare Fe3O4 and C/Fe3O4 revealed that a thin carbon layer of thickness approximately 10 nm uniformly covered the Fe3O4 core (Fig. S1 inset figures). TG analysis confirmed the carbon content of C/Fe3O4 to be 7.36 wt% (see Fig. S1). MnO2 was synthesized onto the C/Fe3O4 by adding a manganese precursor (KMnO4) to the suspension and aging the solution under ambient conditions in the absence of additional heat or pH adjustment. This approach is distinct from the general hydrothermal synthesis and precipitation procedure using pH control. The concentration of KMnO4 was varied from 0.01 M to 0.1 M, and the corresponding MnO2 content of the composite increased linearly with increasing KMnO4 concentration. At the maximum KMnO4 concentration of 0.1 M, the MnO2 content of the composite was 28 wt%, as determined by ICP analysis (see Table S2). The synthetic mechanism for this MnO2 relies on the presence of a carbon layer with a large number of π electrons because the reaction proceeds under electron-rich conditions (Eq. 1): (Jin et al., 2007; Ma et al., 2007)
MnO4− + 4H+ + 3e− → MnO2 + 2H2 O
(1)
The role of the carbon layer in MnO2 formation was explored by preparing SiO2-coated Fe3O4 (SiO2/Fe3O4) as a reference and comparing its behavior with that of C/Fe3O4 (see Figs. S2 and S3). SiO2 has been widely used to synthesize core-shell structured nanocomposites. (Deng et al., 2008; Yi et al., 2006) MnO2 formation on SiO2/Fe3O4 particles was characterized under the same conditions as used above for C/Fe3O4. ICP data revealed that the amounts of MnO2 loaded onto bare magnetite (Fe3O4) and SiO2-coated magnetite (SiO2/Fe3O4) were only 1.7 and 2.4 wt%, respectively, indicating that the carbon layer was clearly essential to the synthesis of MnO2. The adsorption capacity of the MnO2/C/Fe3O4 particles increased with increasing amount of MnO2; however, larger quantities of MnO2 covering the Fe3O4 core in a thicker layer weakened the magnetic properties of the particles relative to those of pure Fe3O4 particles, thereby reducing the separation efficiency. Fig. 2 shows the magnetic hysteresis loop of bare Fe3O4 and MnO2/C/Fe3O4, for a sample prepared in the presence of the largest amount of MnO2 (28 wt%). Fore
3.2. Sr adsorption behavior The performances of the synthesized MnO2/C/Fe3O4 samples were evaluated by measuring the Sr2+ adsorption kinetics and isotherms. Fig. 3 shows the time profiles of Sr2+ adsorption onto MnO2/C/Fe3O4 as a function of the initial concentration of Sr2+. Adsorption proceeded quickly and reached a plateau within 10 min, regardless of the initial Sr2+ concentration. This rapid adsorption indicated the absence of diffusion resistance inside the adsorbent, and the adsorption reaction took place on the surface of the adsorbent. The adsorption kinetics 3
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Fig. 2. Magnetic hysteresis curves (b) and picture of magnetic separation (b) of composite materials. Table 1 Isotherm model constants for Sr2+ adsorption onto MnO2/C/Fe3O4. Langmuir model
Freundlich model 2
[KMnO4] (M)
Qm (mg/g)
KL (L/mg)
R
Kf
n
R2
0.025 0.05 0.1
44.24 41.66 42.19
0.17 0.39 0.71
0.9806 0.9901 0.9964
12.02 41.03 28.77
3.46 8.83 12.67
0.9999 0.8965 0.9487
different MnO2/C/Fe3O4 samples increased with increasing initial Sr2+ concentrations ranging from 10 mg/L to 100 mg/L. Samples with thicker MnO2 layers (i.e., those synthesized using higher KMnO4 concentrations) (Table S2) showed higher adsorption capacities. The experimental Sr2+ adsorption profiles were fitted to Langmuir isotherm and Freundlich models, and the resultant equilibrium parameters are listed in Table 1. The Sr2+ adsorption behaviors onto MnO2/C/Fe3O4 confirmed to the Langmuir isotherm model (R2 = 0.98–0.99), indicating that the adsorption of Sr2+ proceeded in a homogeneous monolayer, and that each adsorption active site acted independently. The MnO2/C/Fe3O4 particles synthesized using 0.025 M KMnO4 displayed a higher linear regression coefficient using the Freundlich isotherm model compared with the Langmuir model. However, under these synthesis conditions, insufficient MnO2 was synthesized to fully cover the surface of the C/ Fe3O4 composite. As a result, the MnO2 Sr2+ adsorption sites were not homogeneously distributed, resulting in heterogeneous adsorption of Sr2+ onto the composite. The theoretical maximum Sr2+ adsorption capacity (Qm) of the MnO2/C/Fe3O4 particles with the thickest MnO2 layer (i.e., those prepared using [KMnO4] = 0.1 M) was calculated to be 42 mg/g using the Langmuir model fit, in good agreement with the experimental results. Although all MnO2/C/Fe3O4 samples showed similar Sr2+ adsorption capacities, 41–45 mg/g, the MnO2/C/Fe3O4 sample synthesized in the presence of 0.1 M KMnO4 showed the highest Sr2+ adsorption capacity at a low Sr2+ equilibrium concentration. Because the Sr2+ concentration in seawater is only 7–8 mg/L, the precursor concentration used for MnO2/C/Fe3O4 synthesis was chosen to be 0.1 M and was used in further experiments.
Fig. 3. Time-dependent Sr2+ adsorption profiles on MnO2/C/Fe3O4 samples as a function of initial concentration of Sr2+. ([MnO2/C/Fe3O4] = 1 g/L and pHi = 8).
Fig. 4. Sr2+ adsorption isotherms of MnO2/C/Fe3O4 samples with different loading amount of MnO2. ([MnO2/C/Fe3O4] = 1 g/L, pHi = 8, and contact time = 30 min).
agreed better with a pseudo-second-order model than with the intraparticle diffusion model, indicating correlation coefficient (R2) values exceeding 0.99 (see Table S3) values exceeding 0.99 (see Table S3). The overall adsorption kinetics were, therefore, determined by a chemisorption process rather than by mass transfer. Information on the adsorption behavior of the MnO2/C/Fe3O4 materials was obtained from Sr2+ adsorption isotherms. As shown in Fig. 4, Sr2+ adsorption onto
3.3. Effect of the solution chemistry on Sr adsorption Because the presence of matrix ions such as Mg2+ and Ca2+ in seawater may affect the adsorption of Sr2+ ions onto the MnO2/C/ Fe3O4 particles, it was crucial to thoroughly examine the effects of different solution chemistries on the Sr2+ adsorption capacity prior to testing any practical applications in seawater. The effects of solution pH 4
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Fe3O4, as indicated by the uptake profiles of co-present Mg2+ and Ca2+ over Sr2+ in proportion to their concentrations (see Fig. S7). During competitive adsorption, higher ionic charges and lower hydration energies yield better adsorption profiles. Sr2+ and Ca2+ have very similar chemical characteristics, such as valence, ionic radius, and hardness,(Li et al., 2012) which facilitates competition against Sr2+ adsorption onto MnO2/C/Fe3O4. The selectivities of Sr2+ against Mg2+ or Ca2+ were further investigated by evaluating the uptake ratio of each metal ion as a function of the metal ion concentration. The molar concentration of Sr2+ to metal ion (Mg2+ or Ca2+) was set to 1, to highlight any differences. As shown in Fig. 7a and b, the uptake ratio was linearly proportional to the molar ratio in both cases, indicating that the concentrations of co-present cations determined the selectivity of adsorption. The distribution coefficient (Kd) was calculated to assess the adsorption selectivity. Fig. 7c and d show the correlations between the Kd values of Sr2+ vs. Mg2+ and Sr2+ vs. Ca2+ at equimolar concentrations of these cations. The slopes of the linear regression fits, which corresponded to the separation factors (αMetalSr, see SI) were determined to be 11.17 and 1.75 for Sr2+ vs. Mg2+ and Sr2+ vs. Ca2+, respectively. MnO2/C/Fe3O4, therefore, had an 11-fold higher selectivity for Sr2+ than for Mg2+ and an approximately 2-fold higher selectivity compared to Ca2+. Considering that the seawater concentrations of Mg2+ (1200 mg/L) and Ca2+ (400 mg/L) are 150-fold and 50-fold higher, respectively, than the seawater concentration of Sr2+ (8 mg/L), the presence of two coexisting ions may present a major obstacle to recovering Sr2+ from seawater. Desorption must be considered in the context of adsorbed Sr2+ recovery and adsorbent regeneration. Given that MnO2/C/Fe3O4 was found to be positively charged at low pH (pH < 3, Fig. 5a) and protons, as the smallest cation, readily participate in ion exchange, Sr2+ desorption was conducted using acid treatment. As shown in Fig. 8, the amount of Sr2+ released in a 0.1 M HCl solution was identical to the amount adsorbed, as measured from the adsorption isotherms (see Fig. 4), indicating that the adsorbed Sr2+ was completely desorbed during acid treatment. It should be noted that release of iron ions was observed during acid treatment of MnO2/C/Fe3O4 particles with lower MnO2 loadings, but this loss of iron was blocked by thicker MnO2 coatings, indicating that thicker MnO2 outer shells offered more rigid protection of the magnetite core against acid treatment. Thus, the MnO2 layer not only provides active absorption sites for Sr2+, it also acts as a barrier to chemical degradation of the magnetic core. During acid treatment, Mn2+ was released from MnO2/C/Fe3O4. The amount of Mn2+ released increased slightly with the MnO2 content of MnO2/C/ Fe3O4. However, MnO2/C/Fe3O4 particles with higher total MnO2 contents were more stable in acid in terms of Mn2+ release during repeated Sr2+ adsorption-desorption tests. As shown in Fig. S8, the adsorption efficiency of Sr2+ on MnO2/C/Fe3O4 dramatically decreased to ca. 40% during the 2nd cycle, mainly because MnO2 is amorphous and vulnerable to acid degradation. To overcome this limitation, the acid concentration was diluted in an eluent or the eluent was replaced with a neutral solution, such as a CaCl2 solution. The utility of CaCl2 as an eluent was tested by conducting desorption experiments using a Ca2+ solution in place of HCl. Because Ca2+ competed most strongly with Sr2+ adsorption in seawater, desorption was postulated to proceed efficiently in the presence of a highly concentrated Ca2+ solution. Fig. S9 shows the Sr2+ desorption performance as a function of the Ca2+ concentration, revealing desorption efficiencies of 40% in the presence of 0.01 M Ca2+ and 50% in the presence of > 0.1 M Ca2+. Although Ca2+ solutions are less effective than HCl in inducing desorption, they may be desirable during repeated MnO2/C/Fe3O4 regeneration cycles in that they may preserve the integrity of the adsorbent.
Fig. 5. (a) Zeta potential of and (b) Sr2+ adsorption efficiency as a function of the solution pH. ([MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 10 mg/L, and contact time = 30 min).
and co-present ions on the adsorption of Sr2+ were therefore investigated. Solution pH can significantly influence the surface charge of a MnO2/C/Fe3O4 surface and thus affect the electrostatic interactions between adsorbate species and the surface, thereby affecting the adsorption behavior Fig. 5a shows the surface potential changes observed in solutions containing pure Fe3O4 or MnO2/C/Fe3O4 as a function of the solution pH. The pHzpc of pure Fe3O4 at which the surface charge reached zero was approximately 7, whereas that of MnO2/C/Fe3O4 was found to be 3. These results suggested that the surfaces of the MnO2/C/ Fe3O4 at pH values below 3 assumed a positive potential, suggesting that adsorption would be inhibited due to electrostatic repulsion because Sr2+ is present in seawater as a divalent cation. As shown in Fig. 5b, the profile of Sr2+ adsorption as a function of the solution pH agreed well with the pH-dependent changes in the surface charge. The Sr2+ adsorption efficiency remained constant at pH values above 4, decreased at pH 3, and markedly decreased to approximately 10% of its value at pH 2, a drop that can be ascribed to electrostatic hindrance. The major cations in seawater are Na+, Mg2+, Ca2+, and K+, in order of increasing concentration (Table S1). The effects of these ions on Sr2+ adsorption were measured by measuring Sr2+ adsorption in a solution containing 10 mg/L Sr2+ and various concentrations of the major cations in seawater. As shown in Fig. 6, the presence of Na+ did not significantly affect Sr2+ adsorption below 1000 mg/L, and the adsorption efficiency decreased by 40% at the seawater level of 10,000 mg/L. K+ did not hinder Sr2+ adsorption across the range of K+ concentrations tested; however, the presence of Mg2+ and Ca2+ significantly interfered with Sr2+ adsorption. Mg2+ inhibited Sr2+ adsorption by 40% at 600 mg/L and continued to show this level of inhibition up to the Mg2+ seawater level of 1200 mg/L. The presence of Ca2+ remarkably hindered the Sr2+ adsorption efficiency by 60% at 100 mg/L and 80% at 400 mg/L (the Ca2+ concentration in seawater). The inhibition effects of Mg2+ and Ca2+ were ascribed to competitive adsorption processes because Mg2+ and Ca2+ adsorbed onto MnO2/C/
3.4. Recovery of Sr2+ from natural seawater Sr2+ adsorption from natural seawater was tested to obtain a Sr2+ concentrate for the practical recovery of Sr2+ from seawater. Sr2+ 5
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Fig. 6. Effects of co-existing ions (a) Na+, (b) K+, (c) Mg2+, and (d) Ca2+ on Sr2+ adsorption efficiency. ([MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 10 mg/L, pHi = 8, and contact time = 30 min).
Fig. 7. Uptake ratio of (a) Mg2+ and (b) Ca2+ to Sr2+ as a function of the molar ratio of Mg2+ and Ca2+ to Sr2+ added, (c) and (d) represent the correlation of the distribution coefficient for Sr2+ vs Mg2+ and Sr2+ vs Ca2+ at an equimolar concentration of Sr2+, Mg2+, and Ca2+ ranging from 0.25 mM to 2 mM, respectively. ([MnO2/C/Fe3O4] = 1 g/L, pHi = 8, and contact time = 30 min).
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Fig. 8. Fe release after acid treatment of MnO2/C/Fe3O4 for the regeneration of adsorbents. (Adsorption: [MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 25 mg/L, pHi = 8, contact time = 30 min; Desorption: [adsorbent] = 1 g/L, [HCl] = 0.1 M, contact time = 30 min). Table 2 The composition and concentration factor of Sr2+ concentrate extracted from seawater. Element
Sr
Ca
Mg
Na
K
Mn
Fe
Concentration (mg/L) Concentration factor (CF, L/g)
501.4 1.714
5424.2 0.363
1421.6 0.0292
1592.1 0.004
834.4 0.057
2464.3 6571.5
3.4 –
efficiency under the given desorption conditions. The concentration of Sr2+ increased linearly as the desorption cycles proceeded (Fig. 9), reaching 500 mg/L after the 8th cycle. These results indicated that desorption was effective, even over repeated desorption cycles, and iterative desorption in the presence of HCl extracted concentrated Sr2+. The solution pH was monitored over each desorption cycle. The solution pH during the first desorption cycle was below 0.5, and the pH increased in proportion to the desorption cycle due to proton consumption. After the 5th cycle, the pH was 1.3, corresponding to 0.05 M acid, suggesting that proton concentrations lower than 0.1 M did not retard Sr2+ desorption. A desirable amount of HCl equivalent to the protons consumed was added to encourage desorption over subsequent cycles, and the pH level of the Sr2+ concentrate was maintained below pH 1. The composition of the Sr2+ concentrate extracted from natural seawater during the 8th desorption cycle is listed in Table 2. The major solution components were found to be Ca2+, Mn4+, Na+, Mg2+, and K+, in order of decreasing concentration. Due to their high initial concentrations, Ca2+, Mg2+, Na+ and K+ were more concentrated than Sr2+ in the extract. The competitive ions, Ca2+ and Mg2+, were highly concentrated in the eluent. The concentration factor (CF, see Table 2) indicated that Sr2+ was the only metal concentrated in the eluent, except for Mn4+. The concentration of Sr2+ in seawater is very low (≈
Fig. 9. Concentration profile of Sr2+ and pH variation in the concentrate as a function of repeated desorption. (Adsorption: [MnO2/C/Fe3O4] = 1 g/L, [Sr2+]0 = 7.8 mg/L (seawater), contact time = 3 day; Desorption: [MnO2/C/ Fe3O4] = 37.5 g/L, [HCl] = 0.5 M, contact time = 30 min).
adsorption from seawater was tested under conditions of 1.5 g MnO2/ C/Fe3O4 in 1.5 L natural seawater over 3 days in a batch system. Confirming that the experimental conditions, such as the solid-to-liquid ratio and contact time, were suitable for the extraction of Sr2+ from seawater. After the 3-days adsorption period, the Sr2+-adsorbed MnO2/ C/Fe3O4 particles were collected using a magnetic field and dried to remove moisture. Desorption was tested to determine whether the Sr2+ solution could be further concentrated. All desorption conditions were identical to those used during the first desorption study, and 8 desorption cycles were tested. Fig. 9 shows the profile of the Sr2+ concentration and pH during the repeated desorption cycles. The concentration of Sr2+ obtained from the first desorption cycle was measured to be 71.5 mg/L, corresponding to a 95% desorption 7
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8 mg/L) compared with the concentrations of other ions, but Sr2+ was selectively concentrated (≈ 500 mg/L) in the eluent. The high concentration of Mn4+ in the concentrate should be considered in relation to the durability of the MnO2 magnetic composite, given that MnO2 is vulnerable to acid treatment. The observation that the concentration of Fe3+ was very low (3.4 mg/L) in the final concentrate suggested the operational durability of the hierarchical structure of MnO2, carbon, and Fe3O4 under the acidic conditions tested.
funded by the Ministry of Science and ICT.
4. Conclusions
Bardi, U., 2010. Extracting minerals from seawater: an energy analysis. Sustainability 2 (4), 980–992. Bezhin, N., Dovhyi, I., 2015. Sorbents based on crown ethers: preparation and application for the sorption of strontium. Russ. Chem. Rev. 84 (12), 1279. Deng, Y., Qi, D., Deng, C., Zhang, X., Zhao, D., 2008. Superparamagnetic high-magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins. J. Am. Chem. Soc. 130 (1), 28–29. Dhandole, L.K., et al., 2016. Hydrothermal synthesis of titanate nanotubes from TiO2 nanorods prepared via a molten salt flux method as an effective adsorbent for strontium ion recovery. RSC Adv. 6 (100), 98449–98456. Dhandole, L.K., Chung, H.-S., Ryu, J., Jang, J.S., 2018. Vertically aligned Titanate nanotubes hydrothermally synthesized from anodized TiO2 nanotube arrays: an efficient adsorbent for the repeatable recovery of Sr ions. ACS Sustain. Chem. Eng. 6 (12), 16139–16150. Ghaeni, N., Taleshi, M.S., Elmi, F., 2019. Removal and recovery of strontium (Sr (II)) from seawater by Fe3O4/MnO2/fulvic acid nanocomposite. Mar. Chem. 213, 33–39. Hong, H.-J., et al., 2016a. Highly stable and magnetically separable alginate/Fe3O4 composite for the removal of strontium (Sr) from seawater. Chemosphere 165, 231–238. Hong, H.-J., et al., 2016b. Investigation of the strontium (Sr (II)) adsorption of an alginate microsphere as a low-cost adsorbent for removal and recovery from seawater. J. Environ. Manag. 165, 263–270. Hong, H.-J., et al., 2017. Enhanced Sr adsorption performance of MnO2-alginate beads in seawater and evaluation of its mechanism. Chem. Eng. J. 319, 163–169. Hong, H.-J., Park, I.-S., Ryu, T., Jeong, H.S., Ryu, J., 2018. Demonstration of seawater strontium (Sr (II)) extraction and enrichment by a biosorption technique through continuous column operation. Ind. Eng. Chem. Res. 57 (38), 12909–12915. Jin, X., Zhou, W., Zhang, S., Chen, G.Z., 2007. Nanoscale microelectrochemical cells on carbon nanotubes. Small 3 (9), 1513–1517. Kim, E.-J., Lee, C.-S., Chang, Y.-Y., Chang, Y.-S., 2013. Hierarchically structured manganese oxide-coated magnetic nanocomposites for the efficient removal of heavy metal ions from aqueous systems. ACS Appl. Mater. Interfaces 5 (19), 9628–9634. Li, N., et al., 2012. Highly efficient, irreversible and selective ion exchange property of layered titanate nanostructures. Adv. Funct. Mater. 22 (4), 835–841. Ma, S.-B., Ahn, K.-Y., Lee, E.-S., Oh, K.-H., Kim, K.-B., 2007. Synthesis and characterization of manganese dioxide spontaneously coated on carbon nanotubes. Carbon 45 (2), 375–382. Ober, J.A., 2015. Strontium. In: United States Geological Survey Mineral Commodity Summaries. U.S. Geological Survey, Reston, VA. Quist-Jensen, C., Macedonio, F., Drioli, E., 2016. Integrated membrane desalination systems with membrane crystallization units for resource recovery: a new approach for mining from the sea. Crystals 6 (4), 36. Rengaraj, A., et al., 2017. Covalent triazine polymer–Fe3O4 nanocomposite for strontium ion removal from seawater. Ind. Eng. Chem. Res. 56 (17), 4984–4992. Ryu, J., et al., 2016. Strontium ion (Sr2+) separation from seawater by hydrothermally structured titanate nanotubes: removal vs. recovery. Chem. Eng. J. 304, 503–510. Sing, K.S., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (recommendations 1984). Pure Appl. Chem. 57 (4), 603–619. Wang, J.-W., Chen, Y., Chen, B.-Z., 2015. A synthesis method of MnO2/activated carbon composite for electrochemical supercapacitors. J. Electrochem. Soc. 162 (8), A1654–A1661. Yi, D.K., Lee, S.S., Papaefthymiou, G.C., Ying, J.Y., 2006. Nanoparticle architectures templated by SiO2/Fe2O3 nanocomposites. Chem. Mater. 18 (3), 614–619.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.hydromet.2019.105224. References
To date, studies of adsorbent materials have been limited to the removal of heavy metals and radionuclides from aqueous media. This study focused on developing a method for recovering Sr resources from seawater. We prepared a magnetic nanocomposite for use in facile separation from water using magnets, and we evaluated the adsorption of Sr2+ in seawater onto the nanocomposite under various reaction conditions. A hierarchical MnO2/C/Fe3O4 structure was obtained by forming MnO2 on a carbon layer coated onto magnetite via oxidationreduction of MnO4−, under ambient conditions without pH control. The core–shell structure reduced the saturation magnetization to ca. 33% of the value obtained from the pristine Fe3O4 core; nevertheless, the prepared magnetic nanoparticles could be effectively separated using an external magnetic force. Among the major ions in seawater that competed with Sr2+, the divalent cations Mg2+ and Ca2+ significantly hindered Sr2+ adsorption. This competition was attributed to the similar chemical properties of these cations, such as the valence, hardness, hydration energy, and ionic radius, which are crucial factors determining the selectivity of certain ions for adsorption. Although the selectivity of MnO2/C/Fe3O4 for Sr2+ adsorption was found to be approximately 11-fold and 2-fold higher than the selectivities for Mg2+ and Ca2+, respectively, Sr2+ adsorption from seawater is not expected to be favorable because the concentrations of Mg2+ and Ca2+ in seawater are much higher than the concentration of Sr2+. The MnO2/C/ Fe3O4 particles were used to extract a Sr2+ concentrate ([Sr2+] = 501 mg/L) from seawater during the recovery of adsorbed Sr2+ via washing and Sr2+ desorption in an acid solution. Magnetic separation was used during each step. These results demonstrated the effectiveness of this seawater Sr2+ recovery process utilizing magnetic adsorbents. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (project 19-3413)
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