Porous Covalent Organic Polymers Comprising a Phosphite Skeleton

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Porous Covalent Organic Polymers Comprised of Phosphite Skeleton for Aqueous Nd(III) Capture Seenu Ravi, Pillaiyar Puthiaraj, Kwangsun Yu, and Wha-Seung Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00546 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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ACS Applied Materials & Interfaces

Porous Covalent Organic Polymers Comprised of Phosphite Skeleton for Aqueous Nd(III) Capture Seenu Ravi+, Pillaiyar Puthiaraj+, Kwangsun Yu, and Wha-Seung Ahn* Department of Chemistry and Chemical Engineering, Inha University, Incheon, Republic of Korea +These

authors made equal contributions.

*Corresponding author: [email protected]

ABSTRACT In order to meet the ever-increasing industrial demand for rare earth elements (REEs), it is desirable to separate and recycle them at low concentrations from various sources including industrial and urban wastes. Here, we introduced phosphorus binding sites on the hydrophobic surface of a robust and high-surface-area porous polymer backbone for environmentally benign and selective recovery of REEs via adsorption. For this purpose, two porous covalent organic polymer (COP) materials incorporated with in-built phosphite functionality (P-COP-1 and PCOP-2) were synthesized and applied for the adsorptive separation of Nd(III) ions from aqueous solution. A strategy to develop a series of P-COPs via a simple Friedel–Crafts reaction was introduced, and their application to the selective adsorption of REEs was explored for the first time. The newly synthesized P-COPs were amorphous and/or weakly crystalline and showed excellent chemical stability and large specific surface area with sufficient mesoporosity for enhanced diffusion of REE ions. P-COP-1 exhibited an exceptionally high Nd(III) adsorption capacity of 321.0 mg/g corresponding to the stoichiometric ratio of P:Nd(III) = 1:0.7 and high selectivity of >86% over other competing transition and alkaline earth metal ions, whereas P-COP-2 gave a Nd(III) adsorption capacity of 175.6 mg/g at 25 °C and pH 5. Moreover, P-COP-1 showed a distribution coefficient value of 5.45×105 mL/g, which is superior to other benchmark adsorbent materials reported so far. Finally, the P-COPs were 1 ACS Paragon Plus Environment

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reusable for a minimum of 10 cycles without deterioration in adsorption capacities. KEYWORDS: Covalent organic polymers, Rare earth elements, Adsorption, Phosphite, Neodymium ion.

1. INTRODUCTION The last decade has seen exponential growth in the utilization of rare earth elements (REEs) in magnets, electronics/communication devices, sensors, photonics, energy storage, and catalysis, owing to their distinctive physicochemical properties.1-2 Nd(III), for example, is a major component in high-performance magnets and laser generators.3-4 However, due to the limited mineral deposit and the diminishing supply from the major producer China, securing an uninterrupted supply of REEs has become difficult. Therefore, it is attractive to separate and recycle REEs from various minor sources including industrial and urban wastes.5 Since these sources often contain impurities and show lower REE concentrations than the respective ores, more efficient and economic separation schemes for REE recovery are desirable. The most frequently used method to separate REEs is the solvent extraction process, which depends on the coordination ability of an extracting solvent with REEs.6-7 However, harmful chemical substances such as 2-ethyl-hexyl-2-ethyl-hexyl phosphonic acid6 and/or bis(2-ethylhexyl)phosphoric acid7 are employed, and the disposal of excessive solvent also involves a large amount of nitric and phosphoric acids. Other existing REEs separation techniques include chemical precipitation,8 electrochemical extractions,9 hydrometallurgy,10 ion-exchange,11 and adsorption.12-15 However, many of these processes require costly and environmentally hazardous chemicals and a large amount of energy for operation.8, 10-11 Among these techniques, adsorption is considered to be a mature and eco-friendly process for the 2 ACS Paragon Plus Environment

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recovery of REEs, with the advantages of easy handling, low cost, relatively minimum waste, and reusability.12-15 To date, a large number of adsorbent materials have been reported for the recovery of REEs including functionalized silicas,5,

16-18

graphene oxide-based nanomaterials,19 metal

oxide composites,20-21 biopolymers,22-23 and metal-organic frameworks.24 For efficient REE recovery, the adsorbents should have high structural stability with the appropriate textural properties in aqueous acidic condition. Despite the reported adsorbents for REEs with high adsorption capacity and reasonable selectivity, their physicochemical stability and sustained reusability remain challenging.16,

20, 22, 24

In this regard, porous covalent organic polymers

(COPs), which have high structural stability in both aqueous acidic and basic conditions owing to the strong covalent bond linkages formed by various organic reactions,25-30 can be a promising adsorbent material for REEs. Their structural stability and excellent textural properties have already led to various applications in catalysis,31-32 gas storage,33-34 sensors3537,

optoelectronics,38 drug delivery,39 and energy storage.40-41 Diverse functionalities can also

be imparted to the COPs: pyrene,42 porphyrin,43 bipyridyl,32 phthalocyanine,44 silane,45 hydroxyl,46 triazine,27 and imines31 have been introduced into their mostly microporous frameworks so far. To improve upon the low recovery capacity of traditional adsorbents and achieve high selectivity for REEs, adsorbents with phosphorous functional groups that resemble the phosphoric acids used in solvent extraction are ideal candidates. However, the introduction of chelating functional groups to a polymer backbone through a post-synthetic protocol is generally difficult due to the low reactivity of ligands. A common practice has been using phospho/amide/acid-functionalized porous materials for REE recovery. Only two COPs with carboxylic functionalities have been reported for recovering REEs so far. Long et al. prepared 3 ACS Paragon Plus Environment


a carboxylic acid-functionalized porous aromatic framework, BPP-7 via a Pd-catalyzed Suzuki-coupling reaction at 60 °C for 72 h, followed by deprotection of carboxylic acid at 150 °C for 24 h. The product achieved an adsorption capacity of ca. 360 mg/g for Nd(III) ions from aqueous solution.47 However, this synthesis methodology was difficult and required an expensive Pd catalyst and long reaction time. Qiuyu et al. synthesized a 3D covalent organic framework via a Schiff-base reaction and functionalized it post-synthetically with succinic anhydride to incorporate carboxylic acid groups.48 However, despite the required multiple synthesis steps, it fell short in the adsorption capacity (ca. 102 mg/g for Nd(III)). Therefore, an alternative synthetic protocol should be considered by incorporating functional groups such as the phosphorous moiety, in order to exploit COPs as candidates for REE recovery and other potential environmental applications. Herein, we report a general strategy to synthesize phosphite-incorporated COPs (PCOPs) through a one-step Friedel–Craft synthesis. To the best of our knowledge, this is the first time COPs with in-built phosphite functional groups are directly synthesized and used for the adsorption of aqueous Nd(III) ions.

2. EXPERIMENTAL SECTION 2.1.

Materials 1,3,5-Triphenylbenzene (TPB), dibenzyl phosphite (DBP), formaldehyde dimethyl

acetal, ferric chloride, N,N’-dimethyl formamide, 1,2-dichloroethane, methanol, neodymium nitrate hexahydrate, ferric nitrate, aluminum nitrate nonahydrate, copper nitrate, magnesium nitrate, sodium hydroxide, and nitric acid were purchased from Sigma-Aldrich (USA) and used without further purification. 4 ACS Paragon Plus Environment

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2.2.

Synthesis of Porous Covalent Organic Polymers

2.2.1. Synthesis of P-COP-1 DBP (1.0 g) and anhydrous FeCl3 (1.3 g) were dispersed in 100 mL of 1,2dichloroethane and purged with N2 for 10 min. Formaldehyde dimethyl acetal (0.99 g) was then added into the solution and the reaction mixture was refluxed overnight. The polymerized product was cooled, filtered, and washed sequentially using methanol, DMF, and excess water. The material was then purified using a Soxhlet extraction unit in methanol and DMF mixture to remove the metal impurities from the polymer product. Finally, the material was dried in a vacuum oven for 5 h at 150 ℃ to obtain solvent-free P-COP-1. 2.2.2. Synthesis of P-COP-2 DBP (1.0 g), TPA (1.0 g), and anhydrous FeCl3 (2.6 g) were dispersed in 100 mL of 1,2-dichloroethane and purged with N2 for 10 min. Formaldehyde dimethyl acetal (1.4 g) was then added and the reaction mixture was refluxed overnight. The rest of the synthesis steps are identical to P-COP-1. 2.3.

Characterization Physicochemical properties of the synthesized adsorbent materials were evaluated

using various instrumentation, and the detailed procedures are given in Supplementary Information. 2.4.

Adsorption Experiments

2.4.1. Batch Adsorption Tests Solutions of Nd(III) ions in HNO3 (pH = 5) were prepared at different concentrations ranging from 10 to 500 ppm. The PCOP adsorbents (10 mg) were charged into 10 mL of 5 ACS Paragon Plus Environment


adsorbate solutions, and the mixture was stirred in a rotatory orbital shaker for 0–3 h. Upon completion of the run, the solution was filtered through a 0.2 µm syringe filter, and the concentration of the mother liquor was analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, ELAN 6100). 2.4.2. Distribution Coefficient A solution of Nd(III) ions in HNO3 (pH = 5) was prepared at 10 ppm concentration. The adsorbent sample (5 mg) in 100 mL of adsorbate solution was stirred in a rotatory orbital shaker for 2 h. Upon completion of the run, the solution was filtered through a 0.2 µm syringe filter, and the concentration of the mother liquor was analyzed by ICP-MS. 2.4.3. Selectivity Test Solutions of Nd(III) ions with competing ions of Fe(III), Al(III), Cu(II), and Mg(II) and also among the lanthanide ions such as Eu(III), Ce(III), and La(III) in HNO3 (pH = 5) were prepared at 100 ppm concentrations for each metal cation. To determine the competitive adsorption of Nd(III) among the transition and alkaline earth metal ions, the adsorption was carried out using a vial containing 30 ml of adsorbate solution (100 ppm of each of metal ions such as Nd(III), Fe(III), Al(III), Cu(II), and Mg(II)) was treated with 10 mg of adsorbent. The same experiment was carried out similarly to determine the competitive adsorption of Nd(III) among the lanthanide ions. The solutions were stirred in a rotatory orbital shaker for 2 h, and the solution was filtered through a 0.2 µm syringe filter upon completion. The initial and final concentrations of Nd(III) in the solution were determined by ICP-MS. All the adsorption experiments were conducted in triplicates, and the mean values were used for the relevant plots in this work.

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3. RESULTS AND DISCUSSIONS 3.1.

Material Synthesis and Characterization The phosphite-containing porous COPs were produced through a simple Friedel–

Crafts alkylation of DBP or (DBP + TPB) and formaldehyde dimethyl acetal in the presence of anhydrous FeCl3, and their schematic synthesis steps are illustrated in Scheme 1. The networks were insoluble in organic solvents (such as acetone, DMF, DCM, THF, and toluene), boiling water, and 6 M acidic and basic solutions (Figure S1-S3). Therefore, the formed networks are highly stable.

Scheme 1. Schematic representation for the synthesis of phosphite-containing COPs. The formation of polymer networks and their chemical functionalities were investigated using Fourier-transform infrared and X-ray photoelectron spectroscopy. Fouriertransform infrared spectra of the monomers (DBP and TPB) and porous polymer products (PCOP-1 & P-COP-2) are shown in Figure 1. The polymeric networks exhibited aliphatic C-H, aromatic C=C, and C-C stretching bands at 2923, 1613, and 1440 cm-1, respectively, which were slightly shifted from those in the monomeric DBP and TPB (2956, 1603, and 1456 cm7 ACS Paragon Plus Environment


1).49

In addition, P-H and P=O stretching bands of DBP were detected at 2430 and 1259 cm-1,

respectively, whereas the polymeric networks showed the same stretching bands with small shifts (2455 and 1176 cm−1, respectively).50

Figure 1. Fourier-transform infrared spectra of DBP, TPB, P-COP-1, and P-COP-2. Figure 2(a) displays the de-convoluted peaks of C1s X-ray photoelectron spectra from P-COP-1/P-COP-2 with the binding energies of 283.7/283.4, 284.8/284.5, 286.0/286.4, and 289.8/289.5 eV corresponding to the C-C, C-H, C-O bonds, and π-π* transition, respectively.51 Similarly, Figure 2(b) shows the O1s spectra from P-COP-1/P-COP-2 with binding energies of 531.3/531.2 and 533.2/533.1 eV corresponding to the phosphorus P=O and bridging oxygen C-O-P, respectively, in phosphite units.52 Figure 2(c) displays the two de-convoluted peaks of P2p spectra from P-COP-1/P-COP-2 with the respective binding energies of 132.7/132.9 and 134.2/134.5 corresponding to the P2p3/2 and P2p1/2 regions.53 These Fourier-transform infrared 8 ACS Paragon Plus Environment

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and X-ray photoelectron spectral results confirm the successful formation of the porous polymer materials and the presence of phosphite functionality in them.

Figure 2. X-ray photoelectron spectra of (a) C1s, (b) O1s, and (c) P2p of P-COP-1 and P-COP2. 9 ACS Paragon Plus Environment


Figure 3. Powder X-ray diffraction patterns of P-COP-1 and P-COP-2. Figure 3 shows the powder X-ray diffraction patterns of P-COP-1 and P-COP-2. The broad peaks at 2θ = 10 and 22° for P-COP-1 are due to the amorphous nature of the polymer matrix; whereas P-COP-2 also shows a set of peaks in the range of 2θ = 10 to 60°, indicating that the polymer has partial crystallinity with some order.31 As illustrated in Figure 4(a-d), PCOP-1 and P-COP-2 exhibited different morphologies: P-COP-1 was of uniform capsule type with a mean particle size of ca. 40 nm, whereas P-COP-2 was of a unique skeletal type with the network particle size larger than 2 µm. This morphology difference must be caused by the TPB used for the synthesis of P-COP-2 (see Scheme 1), which is expected to display high pore accessibility due to its open pore structure. Transmission electron microscope (Figure S4(a-d)) images at low and high magnification showed the micro- and mesoporous structure of the materials.


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Figure 4. Field emission-scanning electron microscope images of (a and b) P-COP-1 and (c and d) P-COP-2. The textural properties of P-COP-1 and P-COP-2 were measured by N2 adsorptiondesorption isotherms at 77 K, and the corresponding textural parameters are listed in Table 1. P-COP-1 (Figure 5(a)) showed a steep nitrogen gas uptakes at the relative pressures of 0-0.1 bar and the desorption isotherm showed a small type IV hysteresis at P/P0 = 0.4–0.6 bar, indicating that P-COP-1 is microporous in nature but with a small number of mesopores. PCOP-2 also showed steep nitrogen gas uptakes at relative pressures of 0–0.1 bar followed by a constant increase in adsorption from 0.1 to 1 bar. The desorption isotherm showed a large type IV hysteresis loop at P/P0 between 0.4–0.6 bar, indicating that the P-COP-2 network has a fair amount of mesoporosity. The Brunauer-Emmett-Teller surface area of P-COP-1 and P-COP-2 were 695 and 1112 m2/g, respectively. In addition, the respective pore size distribution was estimated using non-local density functional theory (Figure 5(b)), which confirmed that PCOP-2 has significant mesopores. The percentage of mesopores was estimated by using 11 ACS Paragon Plus Environment


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equation (1),54 which showed that P-COP-2 has significantly higher mesoporosity than P-COP1.

[ (( ) )]

𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑚𝑒𝑠𝑜𝑝𝑜𝑟𝑜𝑠𝑖𝑡𝑦 = 100 ―

𝑉0.1

𝑉𝑡𝑜𝑡

× 100 ……………(1)

Figure 5. (a) N2 adsorption-desorption isotherms; and (b) pore size distribution curves measured at 77 K for P-COP-1 and P-COP-2. Phosphorous contents of the COP samples were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 7300DV, USA), and the results are listed in Table 1. As expected, P-COP-2 showed significantly lower P content per unit mass than P-COP-1 because of the TBP with a high molecular weight incorporated into the structure. Table 1. Textural Properties of P-COP-1 and P-COP-2 Pore width P contentd (nm) (mmol/g) 1.33, 1.7, 2.8, 3.6, P-COP-1 695 0.432 0.265 38.7 3.04 4.3, 5.6 1.8, 3.5, 4.5, 6.1, P-COP-2 1112 1.090 0.396 63.7 1.31 12.6, 26 aS b c BET = Brunauer-Emmett-Teller surface area; Vtot = pore volume at 0.99 bar; V0.1 = pore d volume at 0.1 bar; measured by ICP-OES. Materials

SBETa (m2/g)

Vtotb V0.1c (cm3/g) (cm3/g)

% of mesoporosity


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3.2.

Nd(III) Adsorption P-COP-1 and P-COP-2 were then tested for adsorptive removal of Nd(III) from

aqueous solutions. The effect of pH on the Nd(III) adsorption over P-COP-1 was examined first to identify the suitable pH for adsorption measurement. According to the result shown in Figure 6(a), Nd(III) adsorption capacity remained almost constant at pH >5 and decreased sharply below pH 5 even in the presence of Na+ ions. Adsorption isotherms for Nd(III) solutions with different initial concentrations are shown in Figure 6(b). P-COP-1 and P-COP-2 were introduced at a ratio of 0.1 mg/mL with a contact time of 3 h at room temperature. The obtained experimental data were fitted to Langmuir and Freundlich isotherm models. The Langmuir model is based on the assumption of homogeneous monolayer adsorption of Nd(III) independent of binding sites on the adsorbent surface. It is expressed by equation (2),



𝑄𝑒 =

𝐾𝐿𝑄𝑚𝐶𝑒

……………(2)

(1 + 𝐾𝐿𝐶𝑒)

where Qe (mg/g) is the amount of adsorbed Nd(III) at equilibrium, Qm represents the maximum adsorption capacity, Ce is the Nd(III) concentration at equilibrium (mg/L), and KL represents the Langmuir affinity constant (L/mg). The non-linear Freundlich model, on the other hand, assumes reversible multilayer adsorption of Nd(III) on the heterogeneous surface, and is given by equation (3), 𝑄𝑒 = 𝐾𝐹𝐶𝑒1/𝑛……………(3) where KF is the Freundlich constant representing the Nd(III) ions adsorption capacity, and n defines the adsorption intensity. The values of all estimated parameters are listed in Table 2, and the correlation coefficients (R2) indicated that the Langmuir adsorption isotherm fitted the data better than the Freundlich adsorption model. According to the Langmuir model, the maximum Nd(III) adsorption capacity for P-COP-1 and P-COP-2 was estimated to be 321.0 and 175.6 mg/g, respectively. The significantly higher adsorption capacity of P-COP-1 must be owing to the presence of more binding phosphite groups in the network. Remarkable, these values were significantly higher than those of previously reported representative adsorbents, such as N-octyl-N-tolyl-1,10-phenanthroline-2-carboxamide-functionalized silica monolith (162 mg/g),16 chitosan-silica-DTPA (106 mg/g),22 ethylenediaminetetraacetic acid-grafted KIT-6 (110 mg/g),57 SBA-15-BTATPA (130 mg/g),58 phosphorus functionalized sol-gel adsorbent (160 mg/g),59 Cys@Fe3O4 (85.5 mg/g),60 magnetic alginate (P507) microcapsules (149 mg/g),61 BPP-7 (360 mg/g),47 and carboxylic acid-functionalized COF (102 mg/g).48 Only BPP-7 showed a somewhat higher equilibrium adsorption capacity than P-COP-1 of this work. However, the synthesis of BPP-7 required costly Pd catalyst, long reaction time, and postsynthetic deprotection of carboxylic acid. Further, multiple steps are required to prepare the 14 ACS Paragon Plus Environment

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precursors using a hazardous chemical (Br2) under harsh reaction condition. In addition, the adsorption efficiency of Nd(III) ions for BPP-7 decreased to ⁓87% after 5 runs, when the carboxylic acid was deactivated due to network decomposition under the desorption treatment at extreme pH. Table 2. Parameters of Nd(III) ions adsorption isotherm models for P-COP-1 and P-COP2 Sorbents

Langmuir isotherm equation

Freundlich isotherm equation

Qm (mg/g)

KL (L/mg)

R2

KF (mg/g)

n

R2

P-COP-1

321.0

1.191

0.910

179.0

9.0

0.636

P-COP-2

175.6

6.399

0.997

128.5

14.7

0.803

Figure 6. (a) pH effect on the Nd(III) ions adsorption over P-COP-1 (Initial Nd(III) ion concentration = 300 ppm, time = 3 h, and temperature = 25 °C); (b) Nd(III) adsorption 15 ACS Paragon Plus Environment


isotherms over P-COP-1 and P-COP-2; (c) adsorption kinetic profiles; and (d) comparison of distribution coefficient values with previously reported materials.

3.3.

Kinetic Study The adsorption kinetics for Nd(III) on P-COP-1 and P-COP-2 were measured using a

solution with 300 and 150 ppm Nd(III) at pH 5, respectively. The Nd(III) ion concentrations were predetermined by considering the phosphorous density on the respective COP sample and the maximum adsorption capacity determined by the Langmuir model. During the measurements, solution samples were collected at different time intervals of 5, 10, 15, 20, 25, 30, 35, and 40 min. Figure 6(c) shows the plot of removed Nd(III) ions as a function of contact time. More than 73% of Nd(III) ions were adsorbed within 10 min of contact with P-COP-1, whereas the P-COP-2 captured >84% during the same time interval. The faster adsorption rate to reach equilibrium is most probably due to the high mesoporosity of P-COP-2. The observed kinetic data were fitted to the pseudo-first-order and pseudo-second-order kinetic models (Figure S7), and the corresponding values for all parameters are listed in Table 3. The correlation coefficients for the pseudo-second-order model were somewhat higher than those for the pseudo-first-order model. The pseudo-second-order model generally assumes a chemisorption process driven by covalent forces or ion exchange interaction.62 The rate constant (k2) of P-COP-2 was 2.440 ×10-3 g mg-1 min-1, which is considerably higher than that of P-COP-1 (Table 3), in line with the enhanced adsorption due to the large pore sizes of the P-COP-2 polymer matrix.

Table 3. Nd(III) ion Adsorption Kinetics and Distribution Coefficients over P-COP-1 and P-COP-2


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Sorbents

Pseudo-first-order

Pseudo-second-order

Distribution coefficient

k1 (min-1)

R2

k2 (g mg-1 min-1)

R2

(mL/g)

P-COP-1

0.097

0.964

0.691×10-3

0.999

5.45×105

P-COP-2

0.186

0.956

2.440×10-3

0.999

2.01×105

The distribution coefficient (Kd) represents the affinity towards a particular adsorbate by a given adsorbent material. It can be calculated by the following equation (4),

𝐾𝑑 =

[

𝐶𝑖 ― 𝐶𝑒 𝐶𝑒

]

×

𝑉 ……………(4) 𝑀

where Ci is the initial concentration of Nd(III) ions (mg/L), Ce the concentration of Nd(III) ions at equilibrium (mg/L), V the volume of solution (mL), and M is the mass of the adsorbent (g). The Kd values were determined for P-COP-1 and P-COP-2 after treatment with 10 ppm Nd(III) aqueous solutions. The P-COPs captured >95% of the Nd(III) ions within the first 15 min of contact, which surpassed the adsorption rates reported so far, and a treatment duration of 60 min with the P-COPs reduced the Nd(III) concentration to 86% of Nd(III) from the solution, with only a small amount of adsorbed trivalent metal ions. Selectivity among the lanthanides such as La(III), Ce(III), Nd(III), and Eu(III) ions over P-COP-1 was also examined. As shown in Figure 10(b), the adsorption selectivity increased in the order of Eu(III)>Nd(III)>Ce(III)>La(III) with decreasing ionic radius among the REEs which resulted in stronger coordination.22 According to the hard and soft acid/ base theory, the 20 ACS Paragon Plus Environment

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hard Lewis base oxygen atoms in phosphite group tend to adsorb the hard Lewis acid smaller ionic radius REE ions more through formation of coordination complex.

Figure 10. (a) Selectivity for Nd(III) against Fe(III), Al(III), Cu(II), and Mg(II) ions and (b) Selectivity among various REE ions.

Figure 11. Reusability test of P-COP-1 and P-COP-2 for Nd(III) recovery. The recyclability of P-COP-1 and P-COP-2 was further investigated using the initial Nd(III) ion concentrations of 300 and 150 ppm, respectively. The P-COPs could be easily regenerated by treatment with 2 M HNO3 after each adsorption cycle and retained their initial Nd(III) adsorption capacity for 10 consecutive cycles as shown in Figure 11. Furthermore, the Fourier-transform infrared spectra of the reused adsorbents after 10 cycles (Figure S1) did not 21 ACS Paragon Plus Environment


show any changes in the aromatic and phosphite stretching peaks compared with the fresh ones indicating that no loss of phosphite units or structural damage was detected in the recovered PCOP materials, which proved that these materials are highly stable under the adsorption and desorption conditions.

4. CONCLUSIONS In summary, we have developed a novel type of porous COPs with built-in phosphite functional groups through a simple one-step synthesis of the Friedel–Crafts reaction with commercially available chemicals. The produced COPs showed high performance for Nd(III) ions adsorption from aqueous solutions. The Nd(III) capture was satisfactorily fitted to the Langmuir model. The Nd(III) ion capture capacities were very high for both P-COP-1 (321.0 mg/g) and P-COP-2 (175.6 mg/g), and >73% and >84% of Nd(III) ions were adsorbed within 10 min of contact, respectively. The Kd value of P-COP-1 for Nd(III) ion is 5.45×105 mL/g owing to the presence of more sites with binding affinity (phosphite units) on the network, and it is among the highest Kd values reported for benchmark materials. Moreover, both P-COPs exhibited >86% selectivity over the common competing ions of Fe(III), Al(III), Cu(II), and Mg(II). The high density of phosphite chelating sites on the easily accessible polymer surface, combined with their outstanding affinity for Nd(III) ions, is responsible for the outstanding performance of these adsorbents.

Supporting Information Methods for material characterization; Kinetics study equations; Fourier-transform infrared, Powder X-ray diffraction patterns, and N2-isotherm of fresh and acid/base-treated P22 ACS Paragon Plus Environment

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COPs; Nd(III) ion adsorption kinetic models.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Pillaiyar Puthiaraj: 0000-0002-9728-4753 Wha-Seung Ahn: 0000-0002-1655-7685

ACKNOWLEDGMENTS This

work

was

supported

by

the

Basic

Science

Research

Program

(2015R1A4A1042434) and the National Strategic Project-Carbon Upcycling Project (2017M3D8A2086050) through the National Research Foundation of Korea (NRF). PP acknowledges the support by the Korea Research Fellowship program funded by the Ministry of

Science

and

ICT

through

the

National

Research

Foundation

of

Korea

(2017H1D3A1A02013620).

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Porous Covalent Organic Polymers Comprising a Phosphite Skeleton

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