High Level Extraction of Recyclable Nanocatalysts by using

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High Level Extraction of Recyclable Nanocatalysts by using Polyphosphazenes Microparticles Yuan Dong, Shuangshuang Chen, Xuemin Lu, and Qinghua Lu Langmuir, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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High Level Extraction of Recyclable Nanocatalysts by using Polyphosphazenes Microparticles Yuan Dong a, Shuangshuang Chen b*, Xuemin Lu a, and Qinghua Lu a* a School of Chemistry and Chemical Engineering, The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China b School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

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ABSTRACT: Improper disposal of metal nanoparticles has caused serious environmental and pathological problems due to their active nanotoxicity. Therefore, there is an urgent need to develop a strategy for efficiently removing redundant metal nanoparticles from water, while also permitting restoration of their catalytic activities to those of pristine particles for reapplication. Herein, we present intrinsically nitrogenrich crosslinked polyphosphazene microparticles to capture silver nanoparticles (AgNPs) from aqueous media by a simple one-step method. The described microparticles exhibit an outstanding adsorption capacity for AgNPs of approximately 59.35 mg/g, exceeding those of other adsorbents. The adsorption kinetics of AgNPs on these microparticles obeyed a pseudo-second-order kinetic model. More importantly, the recovered AgNPs maintained good catalytic activity in the reduction of methylene blue by sodium borohydride. Based on their simple preparation, high adsorption efficiency, and non-destructive effect on the catalytic activity of the recovered AgNPs, the described polyphosphazene microparticles display promising potential for the removal and recovery of AgNPs from water.

KEYWORDS: Polyphosphazene, Microparticles, Silver nanoparticles, Adsorption, water treatment, Catalytic activity

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INTRODUCTION Metal nanoparticles have been widely applied in diverse fields, such as electronics,1 energy,2 cosmetics,3, 4 and medicine.5, 6 Among them, silver nanoparticles (AgNPs)7, 8 have become the most commonly used, for example in antimicrobial materials,9 conductive inks,10, 11 catalysts,12, 13 and flexible touch screens. However, due to the rapid growth in commercial industry and the lack of appropriate disposal protocols and treatments, AgNPs pose a daunting threat to the global environment.14 Exposure to AgNPs in vivo also raises concerns about biological safety. It has been clarified that the toxicity of AgNPs originates from their degradation in lysosome with the release of Ag+ ions, which can induce the formation of reactive oxygen species, trigger oxidation stress, damage DNA, and cause apoptosis of cells.15-17 Therefore, recovery of AgNPs has been proposed and has attracted increasing attention with regard to sustainable development. To date, many purification methods have been developed for the removal of contaminants, such as dyes18 or heavy metal ions19-21 from water, but only a few reports have dealt with nanoparticles. Membrane filtration,22-24 as one of the most widely applied methods, has been hindered by the effort required to fabricate membranes of appropriate pore size and fouling problems. Centrifugation and ultracentrifugation25, 26 methods have also been used to recycle metal nanoparticles, but were obstructed by agglomeration/aggregation of nanoparticles, also leading to a loss of their catalytic activity. Recently, adsorption treatments have attracted ever more attention because of 3


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their facile operation and high efficiency, while maintaining the original morphology and chemical activity of the target NPs. Nanofiber networks,27 functionalized carbon microspheres,28 and dopamine derivatives29 have been designed to address the issue of pollution by metal nanoparticles. It is notable that the majority of previously reported adsorbents required functionalization with amino/catechol groups to improve their adsorption capacities. Adsorbents that are capable of providing abundant adsorbing sites without further functionalization, while maintaining the activity of AgNPs, are urgently required, but to the best of our knowledge have not yet been reported. Polyphosphazene belongs to a new generation of inorganic–organic hybrid polymers and has alternating phosphorus and nitrogen atoms (–P=N–) in its backbone. Due to its flexible molecular design and outstanding biocompatibility, polyphosphazene has been widely applied as a bio-scaffold30, 31, a bio-hydrogel, and an elastomer32, and has also been used in fire retardants33,

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and drug delivery.35 Cyclomatrix polyphosphazene

(PZS) particles can readily be prepared by one-step precipitation polymerization of hexachlorocyclotriphosphazene with di/multi-amine or di/multi-hydroxyl monomers under ambient conditions. The intrinsically nitrogen-rich PZS shows promising potential as an adsorbent for AgNPs. In this work, polyphosphazene particles with surface amine groups have been prepared using the diamine 4,4'-oxybisbenzenamine as a monomer. The obtained crosslinked polyphosphazene particles are uniform microspheres with abundant amine groups, resulting in a high capacity for the adsorption of AgNPs. Furthermore, the 4


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adsorbed and recovered AgNPs were shown to maintain high catalytic activity for methylene blue reduction, indicating that their original morphologies and active sites were undamaged. Compared with other adsorbents, the prepared intrinsically nitrogenrich crosslinked polyphosphazene nanoparticles exhibit superior features, including simple operation without any complicated functionalization, high adsorption capacity, and better catalytic activity of the recovered AgNPs.

EXPERIMENTAL SECTION Materials Hexachlorocyclotriphosphazene (HCCP, 98%) and 4,4'-oxybisbenzenamine (ODA, 98%) were purchased from Aldrich. Triethylamine (TEA), silver nitrate (AgNO3), sodium borohydride (NaBH4), and sodium citrate (Na3Ct) were purchased from Shanghai Chemical Reagent Corporation (Shanghai，China). Organic solvents, such as acetonitrile and anhydrous ethanol, were of analytical grade. All chemicals were used as received. Water was purified using a Milli-Q system (Millipore, Bedford, USA).

Characterization The morphology of the PZS particles was observed using a field-emission scanning electron microscope (SEM; Nova NanoSEM 450, FEI Co., USA) operated at an acceleration voltage of 5 kV. The sizes and distributions of all prepared polyphosphazene nanoparticles were determined from SEM micrographs using ImageJ 5


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software (V1.41, NIH, USA) for image analysis. The morphology of the AgNPs particles was observed by a transmission electron microscope (TEM; Tecnai G2 Spirit Biotwin, FEI Co., USA). Fourier-transform infrared (FTIR) spectra were recorded on a Paragon 1000 spectrometer (Perkin-Elmer, USA). Samples were dried overnight at 45°C in vacuo and thoroughly mixed and crushed with KBr to fabricate KBr pellets. The concentrations of AgNPs and methylene blue (MB) were quantified by comparing absorptions at 390 nm and 664 nm, respectively, on an ultraviolet/visible spectrophotometer (UV/Vis spectrophotometer; Perkin-Elmer Lambda 750S, USA). Xray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS Ultra DLD electron spectrometer (Shimadzu, Japan).

Synthesis of intrinsically nitrogen-rich crosslinked polyphosphazene The PZS particles were prepared by a one-pot precipitation polymerization method.36 Typically, HCCP (0.40 g, 1.15 mmol) and ODA (0.70 g, 3.45 mmol) were placed in a 250-mL round-bottomed flask and dissolved in acetonitrile (100 mL). The reaction was then initiated by adding TEA (4 mL) to the flask, and the mixture was agitated in an ultrasonic water bath (200 W, 40 kHz) for 5 h at approximately 50 °C. When the reaction was completed, the solid product was isolated by centrifugation and washed three times with anhydrous ethanol and deionized water. The final PZS particles were obtained after drying in vacuum for 24 h at 35 °C. The preparation route of the PZS particles is depicted in Scheme 1a. 6


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Synthesis of citrate-capped AgNPs Citrate-capped AgNPs (cit-AgNPs) were prepared according to the previously reported method28,

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with a slight modification. Briefly, 0.02 M aqueous AgNO3

solution (6.25 mL) was added to a solution of Na3Ct (50 mg) in water (180 mL). After stirring the mixture for 30 min, a freshly prepared solution of NaBH4 (10 mg) in water (2 mL) was added dropwise. The resulting mixture was stirred for 24 h and then diluted to 200 mL with distilled water. The prepared cit-AgNPs solution was kept at 4 °C in the dark before use. Adsorption of AgNPs on polyphosphazene microparticles Adsorption experiments were carried out in 100 mL flasks. PZS (15 mg) was added to a flask containing 75 mL of AgNPs solution. The mixture was then stirred at 180 rpm at room temperature (RT). After adsorption, the solid adsorbent (AgNPs@PZS) was separated from the solution by centrifugation. The concentration of the residual AgNPs suspension was determined by UV/Vis spectrophotometry, referring to a standard curve. The equilibrium adsorption capacity was determined using the following equation: 𝑞𝑒 =

𝐶0 ― 𝐶𝑒 𝑚

𝑉

(1)

where C0 is the initial concentration of AgNPs in solution (mg/L) and Ce is their equilibrium concentration (mg/L), qe is the equilibrium adsorption capacity (mg/g), m is the mass of adsorbent (g), and V is the volume of solution (L). 7


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The adsorption kinetics was studied by recording the adsorption of AgNPs at different initial concentrations (3.37–13.46 mg/L) in aqueous solution (75 mL) on PZS particles (15 mg) at 25 °C. The solutions were stirred throughout the process, and 2.5 mL aliquots were withdrawn at appropriate intervals and tested. The adsorption capacity was calculated by the following equation: 𝑞𝑡 =

𝐶0 ― 𝐶𝑡 𝑚

𝑉

(2)

where qt is the adsorption capacity at time t (mg/g) and Ct is the AgNPs concentration at time t (mg/L). Catalytic activity of adsorbed AgNPs for methylene blue reduction MB reduction experiments were carried out by adding AgNPs@PZS (5 mg) to 7.5 mg/L aqueous MB solution (25 mL)29, and then adding of 0.5 M freshly prepared aqueous NaBH4 solution (0.1 mL). The mixture was stirred at 180 rpm at RT for 60 min, and then the solid AgNPs@PZS were separated from the solution by centrifugation. The concentration of MB remaining in the solution was determined by measuring the absorbance of the solution at the absorption maximum (λmax = 664 nm).

RESULTS AND DISCUSSION Preparation and characterization of crosslinked polyphosphazene microparticles PZS particles were prepared via a one-pot precipitation condensation polymerization of HCCP and ODA under ultrasonic, where TEA was used as catalyst. The process is depicted in Scheme 1a. The reaction between HCCP and ODA is a nucleophilic 8


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displacement, in which the phosphorus atoms of HCCP are attacked by the amino groups of ODA with the release of hydrogen chloride (HCl), which can be scavenged using TEA as an acid-binding agent. The reaction was performed under relatively mild conditions. The simple preparation process is favorable for the commercialization of PZS.

Scheme 1. (a) Illustration of the preparation route of crosslinked polyphosphazene particles. (b) Schematic depiction of the extraction of AgNPs by PZS particles. (c) Mechanism of the adsorption of AgNPs.

The adsorption performance of the resultant crosslinked polyphosphazene microparticles is closely related to their morphology. An SEM image (Figure 1a) revealed uniform PZS microparticles, a typical morphology of products obtained by condensation polymerization. The magnified SEM image revealed microparticles of diameter about 2.0 μm with a smooth surface (Figure 1a, b and Figure S1). Because of 9


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the abundant amine groups on the surface of the PZS particles, they exhibited relatively good water dispersibility. The inset optical photograph in Figure 1b provides vivid evidence of a water-dispersed PZS suspension that was stable for 12 h, with a zeta potential of −26.0  1.6 mV. Although the particle suspension did not show long-term stability due to sedimentation caused by the relatively large particle diameter, this did not affect its subsequent use. Indeed, the large particle diameter is conducive to the recovery and recycling operation, obviating the need for high-power separation equipment.

Figure 1. (a, b) SEM images of PZS particles; the inset shows an optical photograph of an aqueous solution of PZS without a stabilizer, which remained stable for 12 h; (c) FTIR spectra of HCCP (black), ODA (red), and PZS particles (blue); (d) XPS analysis of PZS particles.

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The abundant amine groups on the prepared PZS play a critical role in adsorbing AgNPs. FTIR spectroscopy was applied to confirm this. For comparison, the FTIR spectra of the monomers (HCCP, ODA) are also provided in Figure 1c. An intense peak at  = 957 cm−1 is clearly apparent in the spectrum of PZS, which can be assigned to the stretching vibration of P–NH–Ar. This emerging peak provided direct evidence for condensation polymerization between HCCP and ODA. In addition, the disappearance of the P–Cl bond at  = 601 cm−1 in the spectrum of PZS indicated that most chlorine atoms of the HCCP units had been replaced. After polymerization, characteristic peaks of ODA at  = 1499/1620 cm−1 and 1201 cm−1, corresponding to aromatic groups and C–O–C bonds respectively, were maintained in the spectrum of the polyphosphazene product. More importantly, the typical N–H vibration at  = 3387 cm−1 was retained to some extent in the spectrum of the PZS particles, and the amine function made it possible to capture AgNPs from water. The elemental composition of the PZS particles was determined by XPS, as shown in Figure 1d, and the results are presented in Table S1, S2, Figure S2. Some residual Cl remained on the PZS particles, implying incomplete substitution of the P–Cl bonds due to steric hindrance. The original N/P ratio in HCCP was 1:1, but this increased to 2.28:1 after polycondensation with ODA, with a high nitrogen content of about 14.18 %. The chemical states of N were quantitatively studied by XPS. In polyphosphazene, N should be as primary amine (–NH2, free), secondary amine (–NH–, crosslinked) and tertiary amine (–P=N–, originated from phosphazene ring). According to their bond energy 11


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values, XPS deconvolution results were listed in the Table S2. 21.81% of N were free amine, indicating affluent amino groups available for AgNPs extraction. The results were fully consistent with successful preparation of the intrinsically nitrogen-rich crosslinked polyphosphazene particles. Contributing to the intrinsic nitrogen-rich chemistry, the targeted AgNPs were proposed to be enriched and recycled from aqueous solution with high activity for catalysis (Scheme 1b, c). Preparation and characterization of AgNPs To investigate adsorption on the PZS particles, water-dispersible AgNPs were prepared according to previous reports using citrate ion as a stabilizer.28, 37 Figure 2a shows a TEM image of the resultant AgNPs. The average diameter of the prepared AgNPs was 7.56 nm with a dispersion index of 0.27 based on a statistical analysis of 100 particles (Figure 2b). The corresponding zeta potential was –34.2  2.1 mV due to the use of citrate as a stabilizer. Because of the well-defined surface plasma effect, the monodispersed AgNPs exhibited characteristic UV/Vis adsorption bands that were dependent on their sizes. Here, UV/Vis spectroscopy (Figure 2c) was applied to verify the formation of nanoparticles. The UV/Vis spectrum featured a single peak at about 390 nm, in accordance with a size–adsorption relationship reported in the literature.7 Furthermore, a standard plot of UV/Vis adsorption over the concentration range from 0.54 to 13.48 mg/mL was recorded, in preparation for the subsequent evaluation of adsorption (Figure 2d). 12


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Figure 2. (a) TEM image of citrate-stabilized AgNPs; (b) statistical distribution of particle sizes and corresponding average result; (c) UV/Vis spectrum of citratestabilized AgNPs solution with a concentration of 7.0 mg/mL at pH 7; (d) standard plot of absorbance vs. AgNPs concentration over the range 0.54–13.48 mg/mL (R2 = 0.9961).

Adsorption of AgNPs on the crosslinked polyphosphazene microparticles As mentioned above, there are abundant N and P atoms and especially active amino groups on the surfaces of PZS particles. The prepared crosslinked polyphosphazene microparticles were then applied to adsorb AgNPs from an aqueous suspension. We observed that the characteristic absorption peak of AgNPs at 390 nm almost disappeared after 6 h, implying that most AgNPs were removed from the suspension (Figure 3a). The inset photograph shows the color change before and after adsorption. Kinetic experiments revealed that adsorption equilibrium was reached within 12 h. 13


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During this process, the adsorption rate of AgNPs on PZS particles was fastest during the first 2 h, then tailed off, and equilibrium was reached after 8 h (Figure 3b). When the adsorption time was extended to 12 h, there was no further obvious increase in loading due to surface binding saturation on the PZS particles. The equilibrium adsorption capacity (qe) was 54.35  4.72 mg/g. Similarly, the prepared intrinsically polyphosphazene also exhibited highest adsorption capacity to gold nanoparticles (AuNPs) (Figure S3).

Figure 3. (a) UV/Vis spectra of the AgNPs solution before and after adsorption; the PZS particles were removed by centrifugation; (b) time-dependent adsorption capacity of PZS particles for AgNPs; (c, d) SEM images of PZS particles after adsorbing AgNPs.

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The PZS particles with adsorbed AgNPs were resubmitted to SEM inspection to investigate the morphology of the AgNPs (Figure 3c, d). It could clearly be seen that the smooth surface of the PZS was partially covered by AgNPs. Meanwhile, the presence of Ag was confirmed by EDS analysis (Figure S4). We also prepared two other kinds of AgNPs using polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) as stabilizers, respectively. These could also be captured by PZS particles from aqueous suspensions. The adsorption capacities of PZS for the three types of AgNPs with different capping ligands could be ranked in decreasing order: PVA-capped AgNPs > cit-capped AgNPs > PVP-capped AgNPs, as shown in Figure S5. From this, it can be inferred that NH2 groups on the PZS surface underwent a competitive substitution reaction with Ag–OH on PVA-capped AgNPs, Ag–COOH on citrate-capped AgNPs surface, and Ag–OH and Ag–NH on the PVP-capped AgNPs. Due to the relatively strong coordination ability of Ag–N, which is about fourfold higher than that of Ag–O40-42, PZS with an intrinsically nitrogen-rich surface could form a more stable system with AgNPs. As a result, PZS was demonstrated to be an outstanding scavenger for AgNPs. Electrostatic interaction may be another factor in the adsorption of AgNPs on PZS. To obtain more detailed information, the solution pH was varied. As mentioned above, the zeta potentials of AgNPs and PZS (Figure S6) at pH 7 were measured as –34.2  2.1 mV and –26.0  1.6 mV, respectively. To form an Ag–N bond, AgNPs have to overcome the electrostatic repulsion barrier between the ligand and PZS, and this 15


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process is controlled by coordination interaction. When lowering the pH to the isoelectric point of NH2, the zeta potential of PZS became positive at 8.1  0.2, while that of the AgNPs remained negative at –4.3  0.8, as listed in Table S3, due to the overcompensating Stern layer.43 In this case, the negatively charged AgNPs directly bonded with the positively charged PZS through ionic interactions, and hence showed an increased affinity capacity (Figure S7). In conclusion, ionic interactions play the main role at lower pH, whereas coordination interactions govern the adsorption kinetics of AgNPs on PZS at higher pH, as illustrated in Figure 4. Subsequent kinetic and thermodynamic studies were conducted at pH 7. Effect of ion strength was examined by changing the concentration of KNO3 in the 0–50 mM range. Minor effect was observed (Figure S8)

Figure 4. Sketch of adsorption processes of AgNPs on a PZS surface through electrostatic interaction and coordination interaction under different pH conditions, respectively.

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Kinetics of AgNPs adsorption on PZS particles To evaluate the kinetics of the adsorption process, pseudo-first-order 44 and pseudosecond-order 45 models were tested. The pseudo-first-order rate of adsorption can be calculated according to the following equation: log (𝑞𝑒 ― 𝑞𝑡) = log𝑞𝑒 ―

𝑘1

𝑡 2.303

(3)

where qe and qt (mg/g) refer to the amounts of adsorbed AgNPs at equilibrium and time t (min), respectively, and k1 is the rate constant of the pseudo-first-order model, which can be calculated from the slope of a linear plot of log (qe – qt) against t. Figure 5(a) shows the linear plot for the pseudo-first-order model, and the corresponding parameters k1, qe, and the correlation coefficient (R2) are listed in Table 1. The pseudo-second-order model can be represented by the following equation: 𝑡 1 𝑡 = + 𝑞𝑡 𝑘2𝑞2𝑒 𝑞𝑒

(4)

A plot of (t/qt) versus t gives a straight line with a slope of (1/qe) and an intercept of (1/k2qe2), where k2 is the rate constant of the pseudo-second-order model, and can be obtained from the intercept. Figure 5(b) shows the linear plot for the pseudo-secondorder model, and the corresponding parameters k2, qe and the correlation coefficient (R2) are listed in Table 1.

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Figure 5. Pseudo-first-order (a) and pseudo-second-order (b) adsorption kinetics of AgNPs on PZS particles.

The correlation coefficient (R2) for the pseudo-first-order model is 0.9277. The calculated adsorption capacity of 39.68 mg/g is much lower than the experimental value, indicating that the adsorption of AgNPs on PZS particles in the initial stage did not fit the pseudo-first-order model. For the pseudo-second-order model, the linear correlation coefficient was higher at 0.9882. Moreover, the adsorption capacity in this case (63.78 mg/g) was close to the experimental value. Therefore, the pseudo-secondorder adsorption model should be applicable for the adsorption of AgNPs on PZS particles.

Table 1. Kinetic parameters for the adsorption of AgNPs on PZS particles. Pseudo-first-order

Pseudo-second-order

qe,exp (mg/g)

k1

qe,cal

(min−1)

(mg/g)

R2

k2

qe,cal

(g/mg·min)

(mg/g)

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R2

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57.68

0.1626

39.6789

0.9277

0.0102

63.7755

0.9882

Isotherms of AgNPs adsorption on PZS particles The Langmuir and Freundlich adsorption isotherms are applied to describe different adsorption models. The former relates to monolayer adsorption on adsorbent particles, whereby adsorbates are homogeneously distributed over an adsorbent surface, whereas the latter relates to adsorption whereby the adsorption layer becomes increasingly thick with increasing concentration of the adsorbate, without surface restriction. Langmuir isotherm adsorption can be expressed by the following equation: 𝐶𝑒 𝑞𝑒

=

𝐶𝑒 1 + 𝐾𝑎𝑞𝑚 𝑞𝑚

(5)

where qe is the mass of adsorbate for a unit of the adsorbent at equilibrium (mg/g), Ce is the equilibrium concentration of adsorbate in solution (mg/L), Ka is a constant related to the energy of adsorption, and qm is the maximum adsorption capacity, which can be obtained by plotting Ce/qe versus Ce.

Figure 6. Langmuir isotherm (a) and Freundlich isotherm (b) plots for the adsorption of AgNPs. 19


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The Freundlich adsorption isotherm can be expressed as: 1 𝑙𝑛𝑞𝑒 = 𝑙𝑛𝐾𝑓 + 𝑙𝑛𝐶𝑒 𝑛

(6)

where Kf and n are two isotherm constants relating to the capacity and the intensity of the adsorption, respectively.

Table 2. Isotherm constants and R2 values for the adsorption of AgNPs. qe,exp (mg/g) 57.68

Langmuir isotherm model

Freundlich isotherm model

Ka

qm (mg/g)

R2

Kf

n

R2

1.3926

59.35

0.9844

5.1915

13.1044

0.2582

Here, the isothermal adsorption of AgNPs on PZS particles was investigated according to the Langmuir and Freundlich models by plotting Ce/qe versus Ce and 𝑙𝑛𝑞𝑒 versus 𝑙𝑛𝐶𝑒, as shown in Figure 6(a) and (b), respectively. The correlation coefficients of the two adsorption isotherms are listed in Table 2. From the perspective of R2 or adsorption amount, the Langmuir model is more consistent with the actual situation of the experiment, and the calculated maximum adsorption capacity for AgNPs was 59.35 mg/g which is higher than those of previously reported adsorbents, as shown in Table 327, 29, 38, 39. Therefore, the adsorption of AgNPs on PZS particles in aqueous solution

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should conform to monolayer adsorption, which will exposure as large efficient surface as possible and successfully maintain the catalytic activity of Ag nanoparticles..

Table 3. Adsorption capacities of cit-capped AgNPs on different adsorbents. Adsorbents

Modifier

Adsorption Capacity

Refs.

(mg/g) PVA nanofibers Fe3O4@polydopamine shell microspheres

gluten

31.84

27

10.82

29

chitosan

13.1

38

thiols and amines

23.83–55.8

39

none

59.35

this work

core–

cellulose-based nanofibers PVA membrane intrinsically nitrogen-rich crosslinked polyphosphazene microparticles

Catalytic activity of adsorbed AgNPs for methylene blue reduction AgNPs have been widely used as a catalyst in many organic reactions.12, 13 To check the catalytic activity of the AgNPs adsorbed on the surfaces of PZS particles, a wellstudied model reaction, the catalytic reduction of MB by NaBH4, was selected. In detail, 0.5 M freshly prepared aqueous NaBH4 solution (0.1 mL) was placed in a vial with aqueous MB solution (7.5 mg/L; 25 mL), and then AgNPs@PZS (5 mg) were added under stirring. The reaction was monitored by UV/Vis spectrophotometry. For comparison, the same mixture without AgNPs@PZS was also tested. We found that MB could not be reduced by NaBH4 in the absence of the catalyst, whereas the color 21


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immediately faded when AgNPs@PZS was added to the system. Figure 7a shows UV/Vis spectra of the reaction mixture after different times. The characteristic peak of MB at λmax = 664 nm had completely disappeared after 60 min, indicating essentially complete reduction. Figure 7b shows optical photographs of the mixture before and after the catalytic reaction. The fact that bare PZS did not show any catalytic activity was also proved, as shown in the SI (Figure S9). For more systematic investigation, extraction of used AgNPs after catalyzation and the evaluation of the catalytic activity of the captured AgNPs were supplied as supporting information. Before extraction, AgNPs had been beforehand immersed in MB solution. After fully catalyzing MB reduction, the AgNPs were extracted by PZS particles (Figure S10a). Subsequently, the collected AgNPs on PZS were reused to catalyze the reduction of MB. Figure S10b provides vivid evidence of good catalytic activity of AgNPs. It demonstrated that the used AgNPs can be extracted from solution and the catalytic activity was well maintained by using the nitrogen-rich crosslinked polyphosphazene microparticles.

Figure 7. (a) UV/Vis spectra of MB solution after different reduction times. (b) Schematic representation of AgNPs@PZS-catalyzed reduction of a dye by NaBH4 and optical photographs of the MB solution before and after reduction. 22


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CONCLUSIONS In summary, we have shown that cross-linked PZS particles, readily fabricated from 4,4'-oxybisbenzenamine and hexachlorocyclotriphosphazene, constitute a promising adsorbent for the extraction of AgNPs from aqueous solution. Kinetic studies have shown that the adsorption process follows a pseudo-second-order model, and adsorption isotherm studies have shown that it is consistent with monolayer adsorption, fitting the Langmuir model well. The adsorption capacity of PZS particles for AgNPs (59.35 mg/g) is the highest value reported to date. Furthermore, the AgNPs adsorbed on the PZS surface maintained their catalytic activity very well and could be re-used. The synthesis and use of PZS thus represents a simple and cost-effective protocol and is potentially useful for water treatment applications.

ASSOCIATED CONTENT Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org/. Size distribution of PZS particles, deconvolution of N1s XPS spectra of PZS particles, time-dependent adsorption capacity of PZS particles for cit-capped AuNPs, EDS spectra of (A) PZS and (B) AgNPs@PZS, adsorption capacities of PZS particles for 23


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AgNPs with different capping ligands, effect of pH on Zeta potential of PZS particles effect of pH on the adsorption of AgNPs, effect of ionic strength on AgNPs adsorption, effect of contact time on MB reduction by bare PZS/NaBH4, the performance of PZS microparticles of removing Ag nanoparticles after the nanoparticles are used to catalyze decomposition of MB and the catalytic activity test of the adsorbed products. XPS atom concentrations

of

intrinsically

nitrogen-rich

crosslinked

polyphosphazene

microparticles, summary of XPS deconvolution results, effect of pH on zeta potential and AgNPs adsorption, recyclability of AgNPs@PZS for the catalytic removal of MB. AUTHOR INFORMATION Corresponding Author * [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for support from the National Natural Science Foundation of China (51573089, 21704076). This work was also supported by the “China Postdoctoral Innovation Talent Project” and the China Postdoctoral Science Foundation.

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High Level Extraction of Recyclable Nanocatalysts by using

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