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Covalent triazine polymer-Fe3O4 nanocomposite for strontium ion removal from seawater Arunkumar Rengaraj, Yuvaraj Haldorai, Pillaiyar Puthiaraj, Seung-Kyu Hwang, Taegong Ryu, Junho Shin, Young-Kyu Han, Wha-Seung Ahn, and Yun Suk Huh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00052 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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Industrial & Engineering Chemistry Research

Covalent triazine polymer-Fe3O4 nanocomposite for strontium ion removal from seawater Arunkumar Rengaraj,aǂ Yuvaraj Haldorai,bǂ Pillaiyar Puthiaraj,cǂ Seung Kyu Hwang,a Taegong Ryu,d Junho Shin,d Young-Kyu Han,b* Wha-Seung Ahn,c* and Yun Suk Huha*

a

Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha

University, Incheon 402-751, Republic of Korea. b

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715,

Republic of Korea. c

Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751,

Republic of Korea. d

Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources,

Daejeon 305-350, Republic of Korea. ǂ

These authors contributed equally to this work.

Fax: +82-2-2268-8550; Email: [email protected] (Y.-K. H) Fax: +82-32-872-4046; Email: [email protected] (Y. S. H.) Fax: +82-32-872-4046. E-mail: [email protected] (W.-S.A)

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Abstract A microporous covalent triazine polymer (CTP) is synthesized via a Friedel-Crafts reaction and used as a solid support to immobilize magnetite Fe3O4 nanoparticles. Thermogravimetric analysis shows that approximately 60 wt% Fe3O4 is loaded onto the composite and transmission electron microscopy analysis illustrates that the Fe3O4 nanoparticles are uniformly impregnated into the CTP surface. The CTP-Fe3O4 nanocomposite is an efficient adsorbent for the removal of strontium ion (Sr2+) from seawater. Response surface methodology, employed to optimize the removal of Sr2+, confirms that the optimal conditions for this removal are 0.55 mg, pH 7, 40 oC, and 250 min. The experimental results illustrate that the adsorption process fits well with the Freundlich isotherm, with a correlation coefficient of 0.976 and a maximum adsorption capacity of 128 mg g-1. The kinetic study demonstrates that the adsorption behavior follows pseudosecond-order kinetics. The adsorbent is easily recovered from seawater using an external magnetic field, thereby offering facile and economic separation of the adsorbent.

Keywords: Covalent triazine polymer, iron oxide, composite strontium recovery, Box–Behnken model

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1. Introduction The demand for inorganics like trace metals and rare earth metals has increased sharply in this decade due to the development of the electronics industry and global weaponization policies. The result has been shortages and arbitrary price increases throughout the world, mainly in industrially integrated countries.1 Strontium and its isotopes have much importance uses such as Positron Emission Tomography (PET),2 and producing ferromagnets3. Its salts were utilized to produce colors in flames, flares.4 Strontium titanate used in optical due to its high refractive index and an optical dispersion.5 Strontium naturally occurs in minerals such as celestite and strontianite,6 which have traditionally been recovered from the earth as sediments. However, the increased demand has created a need to identify new methods for recovery of these minerals.7 Seawater is a very important source of strontium ion (Sr2+), which occurs in large amounts at trace (8 ppm) concentrations.8 However, the recovery of Sr2+ from extremely diluted sources is neither easy nor economical. Accordingly, new processes are needed that will enable economic and environmentally friendly removal of Sr2+. Over the decades, various methods have been investigated for the removal of Sr2+, including chemical precipitation, chemical oxidation or reduction, electrochemical treatment, ion-exchange, reverse osmosis, filtration, evaporation, engineered multilayer membrane and electrocoagulation.9,10 Of these methods, adsorption-based processes are among the more efficient techniques for the removal of Sr2+.11 Metal oxides show potential for use in adsorbing metal ions like Sr2+ because of their high surface area and low production cost.12 Among the metal oxides, magnetite (Fe3O4) has attracted increasing interest because of its wide range of applications in catalysis,13 drug delivery,14 adsorption processes,15 and environmental remediation.16 Moreover, Fe3O4 can be easily

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recovered after an adsorption process simply by using an external magnetic field. However, bare Fe3O4 can easily form large aggregates, which may alter its magnetic property and adsorption capacity. To retain the required nano size, Fe3O4 is usually composited with polymers for use as an as adsorbent for the recovery of metal ions. A wide range of polymers is currently available that have different metal oxide loadings and solvent compatibilities.17 One class of porous organic polymers that can be synthesized using inexpensive organic starting materials is covalent triazine polymer (CTP), which is known for its high surface area, thermal and chemical stability, and nitrogen content.18 The free lone pair of electrons present in the nitrogen provides high electron density in the network. These free electrons can easily interact with guest molecules, thereby facilitating the formation of quadrupole and ionic interactions.19 The loading capacity of guest molecules depends on the number of nucleation sites per unit weight of the polymer, which can directly influence the level of metal oxide loading.20 Considering the exceptional properties of Fe3O4 and CTP, a combination of these might show enhanced adsorption properties. Several researchers have investigated strontium removal from water bodies using different adsorbents,

including

hydrous

ferric

oxide,21

green

almond

hulls,22

ammonium

molybdophosphate polyacrylonitrile,23 pyrochlore Ta-doped antimony oxide,24 and dolomite.25 However, the low recovery of these adsorbents is a bottleneck in these regeneration systems. The present study examined the synthesis of a CTP-Fe3O4 nanocomposite and its use as an adsorbent for the removal of Sr2+ from seawater. The removal efficiency of Sr2+ from seawater using this nanocomposite was demonstrated and further optimized by a Box–Behnken model (BBM) experimental design in response surface methodology (RSM).26-28 The BBM is one of the most

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widely used responses surface designs, requiring fewer experimental runs to provide sufficient information. We investigated the effect of various parameters, such as the concentration of adsorbate, adsorbent dosage, pH, reaction time, and temperature, on the removal of Sr2+ from seawater. We also investigated the selectivity and recyclability of this nanocomposite in terms of Sr2+ removal.

2. Experimental section 2.1. Materials Anhydrous aluminum chloride, cyanuric chloride, dichloromethane, biphenyl, ferric chloride hexahydrate, and ferrous chloride tetrahydrate were purchased from Sigma–Aldrich. All other reagents were obtained from commercial suppliers and used without further purification. 2.2. Synthesis of CTP-Fe3O4 nanocomposite A typical experiment involved the dispersion of 1 g of CTP in 100 mL of 0.02 M ferric chloride hexahydrate and 0.01 M ferrous chloride tetrahydrate solution. The solution was stirred for 60 min, and then 1 M NaOH was added. The reaction mixture was heated to 60 °C for 3 h, cooled to room temperature, and the precipitate recovered using a magnet. The precipitate was then washed with water several times, followed by acetone, and dried at 60 °C for 24 h in an oven. In this study, bare CTP was prepared according to our previous report.29 2.3. Design of experiments The interaction between variables during the adsorption of Sr2+ was investigated using the Box Berman Design, an important tool in RSM. The design expert software (Stat-Ease, Inc., Version 9, USA) was employed to simulate the experimental runs over the pH (x1), temperature (x2), 5



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adsorbent dosage (x3), adsorbate concentration (x4), and reaction time (x5) on the removal of Sr2+. Over 46 experimental runs were created, based on the following equation:

Xi =

( )

, i = 1, 2, 3, ……. k --------------------------------- (1)

where xi and Xi are the uncoded and coded values of the ith variable, respectively, ∆xi is the step change value and xoi is the uncoded value of the ith variable at the middle point. The influences of each parameter in this study are listed in Table S1. The removal of Sr2+ was calculated using the second-order polynomial equation: Y = β0 + ∑ + ∑ + ∑ , () + ε

------------------ (2)

where Y is the response variable, β0, βi, βii, and βij are the regression coefficients for the intercept, linear and quadratic effects, and double interaction, respectively, ε is a random error, and xi and xj are the independent variables. Analysis of variance (ANOVA) and RSM was evaluated using the statistical analysis system STATISTICA 5.1 (StatSoft, Inc., Tulsa, OK, USA), which produced the 3D response surface plots. 2.4. Batch adsorption experiment All experiments were performed using 50 mL seawater containing varying initial concentrations of Sr2+ (50–200 ppm) at different pH levels (3–11), temperatures (20–60 °C), adsorbent amounts (0.1–2 mg/50 mL), and reaction times (50–250 min), to investigate the effects of these factors on the removal of Sr2+. Upon completion of the adsorption process, the composite was recovered with a magnet and the solution was analyzed by ICP-MS to quantify the amount of Sr2+ adsorbed. The amount adsorbed (mg g-1) at a given time was then determined using the following equation: qt =

( )

------------------------------------------------- (3) 6


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where C0 is the initial concentration of Sr2+, Ct is the concentration Sr2+ at time t (mg L-1), V is the volume of Sr2+ (L), and m is the weight of the adsorbent (g). The percentage removal of Sr2+ was determined from the following equation: Removal (%) = {(C0 – Ct)/C0} ×100 --------------------------------- (4) 2.5. Selectivity and stability of the composite Selectivity experiments were carried out using 100 mL of the strontium (75 ppm) spiked seawater containing 10 mg of the adsorbent. After 250 min of agitation, the adsorbent was recovered with a magnet and the solution was analyzed by ICP-MS to quantify the initial and residual concentrations of Sr2+. After the end of each cycle, the Sr2+ was extracted from the composite by adding 5 mL of 0.1 M nitric acid and the recovered composite was washed with excess water. 2.6. Characterization FT-IR analysis was performed using a Bruker VERTEX 80V spectrometer. X-ray diffraction (XRD) analysis was performed using Cu Kα radiation and an RIGAKU, DMAX 2500 system. The surface area of the sample was determined using a BELsorp-max instrument (BEL, Japan) with liquid nitrogen. Thermogravimetric analysis (TGA) was conducted using a STA 409 PC/NETZSCH thermal analyzer (25–1000 °C at a rate of 5 °C min-1, with argon gas as the carrier). The morphology of the composite was examined with a HITACHI S-4300 scanning electron microscope equipped with an energy dispersive X-ray detector (EDS). The concentration of Sr2+ was measured with an inductive coupled plasma-mass spectrometer (ICPMS, ELAN6100/Perkin Elmer). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Scientific, K–Alpha electron spectrometer with an Al X-ray source.

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3. Results and discussion 3.1. Characterization of the CTP-Fe3O4 nanocomposite The CTP composite impregnated with Fe3O4 nanoparticles was synthesized by a co-precipitation method and used as an effective adsorbent for the removal Sr2+ from seawater. A schematic representation of the synthesis of the CTP-Fe3O4 nanocomposite is presented in scheme 1. The synthetic methodology and characterization data of bare CTP have been discussed in detail in our previous report.29 Fig. 1a shows the XRD data for CTP and the CTP-Fe3O4 nanocomposite. The amorphous nature of CTP was confirmed by the presence of a broad peak at around 23.0°. The CTP-Fe3O4 composite showed diffraction peaks at 30.1°, 35.5°, 43.3°, 53.7°, 57.2°, and 62.9°, which could be assigned to the (220), (311), (400), (422), (511), and (440) reflections, respectively, of the crystalline cubic spinel phase (JCPDS No. 65-3107) of Fe3O4. In addition, the CTP peak was diminished because of the crystalline nature of Fe3O4. Fig. 1b presents the textural property of the composite obtained from its nitrogen adsorption/desorption isotherm. The BET surface area of the composite was calculated as 473 m2 g-1, which was evaluated from the 0.01 < P/P0 < 0.05 region of the adsorption curve. The BJH pore size distribution (Fig. 1c) suggested that the material had both a microporous and a mesoporous nature, with an average pore size of 1.42 and 3.95 nm, respectively. The microporosity was observed for the CTP, while the mesoporous nature arose from the Fe3O4 nanoparticles. Fig. 1d demonstrates the survey spectrum of the composite, which exhibited C 1s (284.9 eV), O 1s (529.8 eV), and Fe 2p peaks (710 –730 eV), which all confirmed the presence of these elements in the composite. The high-resolution spectrum Fe 2p (Fig. 1e) showed two

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peaks at 711.1 and 724.6 eV, which were consistent with the binding energies of Fe 2p3/2 and Fe 2p1/2, respectively.30 These results supported the formation of a CTP-Fe3O4 composite. The thermal stabilities of the bare CTP and CTP-Fe3O4 composite were examined by TGA from 25–1000 °C under an N2 atmosphere, as illustrated in Fig. 1f. The two samples followed a similar decomposition trend. The as-prepared polymer exhibited a two-step weight loss. The first very small weight loss (10%) step in the TGA curve below 120 °C was attributed to the loss of water. The second rapid major mass loss step at around 300 °C was assigned to the dehydration of the polymer. However, the thermal stability of composite was higher than that for the bare CTP, which was obviously related to the presence of thermally stable Fe3O4. The residual mass of the composite remaining after heating at 1000 °C was calculated as 60%. The morphological analyses of the bare CTP and CTP-Fe3O4 are shown in Fig. 2. The SEM images (Fig. 2a) showed that the polymer particles were spherical in shape and slightly aggregated. In the case of the composite (Fig. 2b), the Fe3O4 nanoparticles were uniformly impregnated into the surface of the polymer. An image from a close SEM inspection of the composite is shown in Fig. 2c. TEM images were used to further clarify the physical nature of Fe3O4 in the composite. The TEM image of CTP (Fig. 2d) confirmed the porous nature of the material, and Fe3O4 particles were clearly observable after formation of the composite (Fig. 2e). The Fe3O4 nanoparticles (quasi-spherical in shape) were well dispersed and impregnated into the polymer matrix, with a mean particle size of around 10 nm. However, aggregated nanoparticles were observed in certain areas. Fig. 2f shows an image of the corresponding close inspection. The SAED patterns (Fig. 2f inset) revealing the crystalline nature of Fe3O4 were consistent with the XRD analysis.

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3.2. Linear model for the Sr2+ adsorption Based on the BBM design, the amount of Sr2+ adsorbed was calculated experimentally and the values were fitted to the following second-order polynomial equation: Adsorption of Sr2+ = 25.81 * x1 + 16.55 * x2 + 19.12 * x3 +12.42 * x4 + 8.18* x5 +58.41… (5) This equation indicates the influence of independent variables on the adsorption of Sr2+ by the CTP-Fe3O4 composite. Based on this equation, nearly 100% of Sr2+ was adsorbed onto the CTPFe3O4 composite. A reliable model for this system was determined using the experimental curve fitting method; the generated values are listed in Table 1. An F-test and probability values (pvalues) were calculated for each model, and the one with the larger F and lower p-values was chosen. The linear vs mean model was recommended for this experimental design because of its higher F-value (13.71) and lower p-value (0.0001) relative to other related models. The cubic model was found to be aliased. The significance of the variables was analyzed using ANOVA (Table S2) and confirmed that the linear vs. mean model was significant, based on the P and F values. The influence of the variables over the adsorption process was as follows: pH > temperature > adsorbent dosage > concentration of Sr2+ > reaction time. The sum of squares (SS), degrees of freedom (DF), and mean squares (MS) were also calculated. The F-value was obtained by dividing the MS by the DF, whereas the MS value was obtained by dividing the SS by the DF.31 The predicted versus actual graph for the adsorption of Sr2+, shown in Fig. 3a indicated that the distribution of actual values was relatively close to a straight line. This signified that the linear model was appropriate for predicting the efficient adsorption of Sr2+ under the experimental parameters studied. The plot of residuals versus run number was tested and is displayed in Fig. 3b. The residuals were

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scattered randomly around ± 3.50. This indicated that the experimental data fit well with the model. 3.3. Effect of process variables on the adsorption of Sr2+ The interaction between variables was investigated and the reaction conditions were optimized by constructing 3D plots (Fig. 4). Overall, the pH of the solution had a greater influence when compared to the other variables. The adsorption of Sr2+ was sharply increased with increases in the solution pH. At an alkaline pH (over pH 7), all other parameters influenced the adsorption process. The amount of Sr2+ adsorbed at pH 8 at room temperature (25°C) was around 75%, but an increase in temperature to 60 °C raised the adsorption to 98% (Fig. 4a). The pH had a strong effect on the reaction time, as increases in pH sharply reduced the reaction time (Fig. 4b). At pH 9, the adsorption process required about 200 min to reach approximately 4%, while at pH 11, 98% recovery was achieved within 250 min. Under alkaline conditions, the adsorption process was very rapid and required only a small amount of adsorbent to reach the maximum Sr2+ adsorption (Fig. 4c and 4d). These results demonstrated that the CTP-Fe3O4 composite could effectively adsorb Sr2+ from seawater at pH 7–8 and 27–30 °C. 3.4. Adsorption isotherm The adsorption isotherms were investigated based on batch experiments. The initial Sr2+ concentration in the spiked seawater was varied from 50 –200 ppm. A total of 10 mg of the adsorbent was added to 100 mL of the Sr2+ spiked seawater. The seawater pH was around 8, and we conducted the experiment at room temperature. The flask was shaken at 500 rpm on a rotary shaker for 250 min. The adsorbent was then separated with a magnet, and the residual Sr2+ concentration was analyzed using ICP-MS. The adsorption of Sr2+ using CTP-Fe3O4 composite

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was evaluated by fitting the data into Langmuir and Freundlich isotherm models. The Langmuir model32 explains properties such as physisorption and monolayer adsorption, and all the active sites are equivalent and independent. The linear and nonlinear forms of the equation are written as:

=

!"# = !$% +

+ ……………………………… (6)

'

!(# ……………………………. (7)

where n is the heterogeneity coefficient and KF is the Freundlich constant related to multilayer adsorption capacity.33 As shown in Fig. 5a, the Langmuir isotherm plot of Ce vs Ce/qe gave a straight line with an R2 value of 0.879. The KL and qe values were calculated as 0.002 and 128 mg g-1, respectively. In Freundlich isotherm (Fig. 5b), the slope n (2.111) and intercept KF (3.017), which fitted with an R2 value of 0.976, for the plot of log Ce vs log qe. The n and KF values confirmed the easy recovery of Sr2+ from the spiked seawater and indicated favorable adsorption. The straight line plot suggested that the adsorption of Sr2+ onto the CTP-Fe3O4 composite followed the Freundlich isotherm. The value of n = 2.111 indicated that the adsorption process had sufficient physisorption for the removal of Sr2+. The maximum adsorption capacity (qmax) of Sr2+ from spiked seawater was calculated as 128 mg g-1, which was higher than the previously reported values for materials such as carboxymethylated cellulose (108.70 mg g-1),34 zeolite A (99.11 mg g-1),35 tantalum-doped hexagonal tungsten oxide (28.19 mg g-1),36 NH2MCM-48 (67.30 mg g-1),37 carboxymethylated chitosan (99.00 mg g-1),38 and hydroxyapatite nanoparticles (50.47 mg g-1).39 These results indicate that the developed CTP-Fe3O4 composite could be an excellent adsorbent for the removal of Sr2+. The adsorption performance of CTP12


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Fe3O4 nanocomposite is derived from the following three factors: (i) Micro and mesoporous properties, including a large surface area and an enormous pore volume; (ii) the electrostatic non-covalent interaction between nitrogen atoms and Sr2+. The nitrogen atoms in the polymer network can function as donor groups, attracting Sr2+, because of their high electronegativity; and (iii) the π-conjugation present in the aromatic rings, forming a π-electron cloud, which facilitates a non-covalent interaction between the π-electron cloud and Sr2+. 3.5. Adsorption kinetics For the kinetic studies, 10 mg of the adsorbent was added to a flask containing 100 mL of Sr2+spiked seawater (100 ppm). The flask was then shaken at 500 rpm on a rotary shaker for 10–250 min. The adsorbent was then separated using a magnet, and the residual Sr2+ concentration was analyzed by ICP-MS. The widely used Lagergren pseudo-first-order and pseudo-second-order kinetic models were applied to determine the rate of adsorption at different times.40 The adsorption mechanism and rate controlling steps, such as chemical reaction and mass transport, can be explained by first-order kinetics. The pseudo-first-order rate equation can be expressed as: 0

1 56 log("# − "- ) = log("# ) − /.343

------------------------- (8)

where qe and qt are the adsorption capacities (mg g-1) at equilibrium and at time t, respectively, and k1 is the pseudo first-order rate constant (g mg-1 min-1). Upon integration and after applying the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, a simplified linear form of the rate equation can be obtained, A plot of log (qe - qt) versus t (Fig. 5c) showed a straight line with an R2 value of 0.881. The slope and intercept of the straight line gave qe and k1 values of 6.18 g mg-1 and 0.0009 g mg-1 min-1, respectively. The second-order model is based on the assumption that 13



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the rate-limiting step may be chemisorption involving valence forces through the sharing of electrons between the adsorbent and the adsorbate as covalent forces. The second-order rate equation can be described as follows: -

=

07 7

+

-

------------------------- (9)

The qe and k2 (second-order rate constant, g/mg min) values were calculated from the linear plot of t/qt - qt) versus t (Fig. 5d) as 7.83 mg g-1 and 0.002 g mg-1 min-1, respectively. These results revealed that the second-order model provided a better fit, with an R2 value of 0.9861, implying that the adsorption rate of the composite depends on the active sites rather than on the concentration of Sr2+ in the solution. 3.6. Selectivity The selectivity of the CTP-Fe3O4 composite for Sr2+ (Fig. 6b) was evaluated by measuring the efficiency of removal of Sr2+ in the presence of competing cations such as Na+ (10,030 ppm), and Mg2+ (1195 ppm) in seawater. Adsorbent (10 mg) was added to a flask containing 50 mL of the Sr2+ (75 ppm) spiked seawater. The flask was then shaken at 500 rpm using a rotary shaker for 50 min. The adsorbent was separated using a magnet, and the residual Sr2+ concentration was analyzed using ICP-MS. The composite showed excellent selectivity toward Sr2+ because the concentration of Sr2+ in any environment is significantly lower than the concentrations of coexisting cations, such as Na+ and Mg2+. The metal ions adsorbed onto the adsorbent were extracted using 0.1 M nitric acid and the adsorption process was repeated for three cycles. The recoveries of Sr2+, Na+, and Mg2+ from seawater were 89.86%, 0.85%, and 0.39%, respectively, 14


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in the first cycle. No appreciable changes in the recovery of Sr2+ were observed in the second and third cycles. The amounts of Na+ and Mg2+ were 0.66% and 0.82% in the second cycle and 2.0% and 1.28% in the third cycle, respectively. The results demonstrated that the Sr2+adsorption was not affected by the presence of coexisting cations. Fig. 6b displays a SEM image of the adsorbent after three cycles of reuse. Before the acid treatment, no specific morphology was observed (Fig. 2b). The composite showed a globular cluster of CTP and Fe3O4. After the acid treatment, the apparent physical nature of the composite changed due to degradation. The microporous structure of the CTP is believed to influence the Sr2+ uptake because the diffusion of seawater inside the adsorbent is closely related to the number pores in the CTP. The corresponding EDS analysis is shown in Fig. 6c.

4. Conclusions A CTP-Fe3O4 composite with a high surface area of 473 m2 g-1 was prepared using a coprecipitation method for effective removal of Sr2+ adsorption from seawater. The BBM design was utilized to stimulate the interaction between variables, such as pH, temperature, the amount of adsorbent, and adsorption time. Adsorption isotherm models showed that the composite had excellent adsorption and a high metal loading capacity. ICP-MS analysis showed that the composite could selectively adsorb Sr2+ from seawater relative to competing ions such as sodium and magnesium. The CTP-Fe3O4 composite was successfully utilized for the adsorption of Sr2+ from seawater, and it showed relatively high capacity for Sr2+ recovery of 128 mg g-1. Analysis of its magnetic properties and stability showed that this material could be easily recovered after

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the adsorption process and reutilize, maintaining high adsorption and good stability for three more runs.

Acknowledgements Wha-Seung Ahn gratefully acknowledges financial support from the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: 2015R1A4A1042434) for Basic Science Research Program. This work was also supported by the National Research Foundation of Korea grant funded by the Ministry of Science, ICT and Future Planning of Korea (2014R1A5A1009799 and 2016R1A2B4013374).

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critical

behavior

at

paramagnetic

to

ferromagnetic

phase

transition

in

La0.57Nd0.2Sr0.33MnO3. Solid State Sci., 2013, 21, 19-25. (4) Martín-Alberca, C.; García-Ruiz, C. Analytical techniques for the analysis of consumer fireworks. Trends Anal. Chem.; TrAC, 2014, 56, 27-36. (5) Yadav, A.K.; Gautam, C.R. Synthesis, structural and optical studies of barium strontium titanate borosilicate glasses doped with ferric oxide. Spectrosc. Lett., 2015, 48, 514-520. (6) Andersson, B.-A. Materials availability for large‐scale thin‐film photovoltaics.Prog. Photovoltaics, 2000, 8, 61-76. (7) Berish, V. Southwestern Energy Production Co, F. Supp. 2d, Dist. Court, MD Pennsylvania 2011, 702. (8) Mero, J.-L. The mineral resources of the sea, Elsevier, 1965. (9) Esalah, J.O.; Weber, M.E.; Vera, J.H. Removal of lead, cadmium and zinc from aqueous solutions by precipitation with sodium Di‐(n‐octyl) phosphinate. Can. J. Che., 2000, 78, 948-954. (10) Amer, M.W.; Khalili, F.I.; Awwad, A.M. ◌Adsorption of lead, zinc and cadmium ions on ِ polyphosphate-modified kaolinite clay. JECET, 2010, 2, 1-8.

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Table 1. Model prediction for investigating the interaction between variables

F Source

SS

DF

p-value

MS Value

Mean vs Total

1.569E+005

1

1.569E+005

Linear vs Mean

24,422.25

5

4884.45

13.71

< 0.0001

2FI vs Linear

2607.75

10

260.78

0.67

0.7411

Quadratic vs 2FI

2952.33

5

590.47

1.70

0.1718

6496.19

15

433.08

1.97

0.1402

2195.28

10

219.53

Suggested

Cubic vs Quadratic

Residual

22


Aliased

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Scheme 1. Schematic representation of the synthesis of the CTP-Fe3O4 nanocomposite for Sr2+ adsorption from seawater.

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Fig. 1. (a) XRD patterns, (b) BET, (c) pore size distribution, (d) survey XPS, (e) individual Fe 2p spectrum, and (f) TGA of the CTP-Fe3O4 composite.

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Fig. 2. SEM images of the (a) CTP, and (b, c) CTP-Fe3O4 composite; TEM images of the (d) CTP, and (e, f) CTP-Fe3O4 composite. Inset in Fig. 2f shows the SAED patterns of Fe3O4 nanoparticles.

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Fig. 3. (a) Comparison between actual and predicted values of RSM model for Sr2+ adsorption and (b) plot of studentized residuals versus the experimental run number.

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Fig. 4. 3D model of the interaction of pH with other parameters.

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Fig. 5. (a) Langmuir isotherm (b) Freundlich isotherm, (c) pseudo-first-order, and (d) pseudosecond-order kinetics of the CTP-Fe3O4 composite.

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Fig. 6. (a) Removal efficiency of Sr2+ compared to competitor cations, such as Na+ and Mg2+, in seawater (initially spiked with 75 ppm Sr2+) for three repeated cycles, (b) SEM image of the composite after Sr2+ adsorption (inset photograph shows the recovery of the composite with a magnet), and (c) corresponding EDS analysis.

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Graphical Abstract Covalent triazine polymer-Fe3O4 nanocomposite for strontium ion recovery from sea water

Covalent triazine polymer supported magnetite Fe3O4 nanoparticles is synthesized via a FriedelCrafts reaction and used as an efficient adsorbent for the removal of Sr2+ ion from seawater.

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Covalent Triazine PolymerâFe3O4 Nanocomposite ... - ACS Publications

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Covalent Triazine PolymerâFe3O4 Nanocomposite ... - ACS Publications