Hierarchically Porous Melamine-Formaldehyde Resin Microspheres

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Hierarchically porous melamine-formaldehyde resin microspheres for the removal of nanoparticles and simultaneously as the nanoparticle immobilized carrier for catalysis Qinying Li, Jun-Jun Liu, xiao sun, and Li Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04490 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Sustainable Chemistry & Engineering

Hierarchically porous melamine-formaldehyde resin microspheres for the removal of nanoparticles and simultaneously as the nanoparticle immobilized carrier for catalysis

Qinying Li, Jun-jun Liu, Xiao Sun and Li Xu* Tongji School of Pharmacy, Huazhong University of Science and Technology, Wuhan 430030, China * Corresponding author. Tel: +86 27 83692735 (L. Xu);

Abstract Hierarchically porous melamine-formaldehyde resin microspheres (MFRM) with mesopores/macrospores (10-90 nm) and flow-through pores (~1000 nm) were firstly prepared by sacrificing template approach. MFRM had satisfactory theoretically maximum adsorption capacity based on Langmuir model for gold (179.2 mg g-1) and silver (132.5 mg g-1) nanoparticles (AuNPs and AgNPs), which were one kind of high concern emerging environmental contaminants. Based on molecular docking study, the interaction mechanism was ascribed as electrostatic interaction, metal ligand and regium-π bonds. In addition, the hierarchically porous structure of MFRM was beneficial to the adsorption. The abundant mesopores provided large surface area and thus high adsorption capacity; the flow-through pores and macropores ensured full access of the relatively large adsorbate, herein nanoparticles, to the adsorbent, and

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facilitated fast mass transfer during adsorption. After adsorption, the hybrid materials, i.e. MFRM adsorbing AuNPs or AgNPs, succeeded as heterogeneous catalysts in reducing 4-nitrophenol to 4-aminophenol with reasonable catalytic activity and recyclability, which was the second usage of noble metallic nanoparticles. Finally, MFRM were regenerated by acidic thiourea, and meanwhile leaching of precious noble metal was possible. The prepared hierarchically porous MFRM was a potential green adsorbent for the removal of AuNPs/AgNPs with the second usage of the nanoparticles as catalysts, recycling of precious noble metal and reuse of the adsorbent, suggesting the sustainability of the present approach.

Keywords: Nanoparticles, Adsorption, Melamine-formaldehyde resin microspheres, Hierarchically porous structure, Water remediation, Sacrificing template approach.


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Introduction With the explosive growth of nanotechnology, the engineered nanoparticles, e.g. gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs), gained increasing applications in various fields. For instance, AuNPs are widely applied in biosensing, imaging, cancer theranostics and catalysis for their appreciated optical properties and physicochemical activities. Similarly, AgNPs with excellent antimicrobial activities are commercially incorporated in cosmetics, textiles, household items, detergents, food packaging and processing. As a result, their release to the environment was inevitably accelerated.1 Recently, the residual AgNPs concentrations in Malaysia river waters and effluent water from sewage treatment plants were reported up to 0.13-10.16 mg L-1 and 0.13-20.02 mg L-1, respectively.2 In Netherlands, AuNPs concentrations reached up to 6 μg L-1 in effluent water from sewage treatment plants and 0.25 μg L-1 in drinking water.3 These nanoparticles posed adverse effects on the ecosystem, living organisms and human health, and was a potential pollution on the environment.4 As evidence, they were claimed to have cytotoxicity and genotoxicity on marine microalgae, fish and crustaceans, mammals and human cells.5 Therefore, it is urgent to develop the efficient method to remove nanoparticles from the environment for environment remediation.

Hitherto, different technologies such as aggregation, membrane filtration, coagulation and adsorption are implemented to remove nanoparticles from water.6, 7 Among them, adsorption technology is popular for its low cost, high efficiency, easy and safe operation. The adsorbents are the kernel to achieve the goal. Many efforts are devoted



to developing various adsorbents for enhanced removal efficiency of nanoparticles, including silica, carbon and polymer of different morphologies like nanofiber, magnetic nanoparticle and microsphere.8-12 These adsorbents for nanoparticles removal usually possessed single-sized porous structure, i.e. the mesopores from 2-50 nm, which would provide large surface area, and thus high adsorption capacity; or macropores larger than 50 nm and particularly flow-through pores, which would facilitate mass transfer, and help to accommodate the “large molecules” like nanoparticles. Anyway, the absence of either pores in the adsorbents may cause some deficiency during adsorption. However, when preparing the adsorbents, the problem aroused from the difficulty in compromising the mesopores and macropores/flow-through pores; another problem lied in the tedious preparation steps to generate functional groups aiming to interact with the target nanoparticles. Hence, it is crucial to develop the adsorbents with appropriate porous structures and desirable functionalities to achieve efficient adsorption of nanoparticles.

As our continuous efforts on water remediation,13-15 herein we proposed hierarchically porous melamine-formaldehyde resin microspheres (MFRM) as the adsorbent for the removal of nanoparticles. The melamine-formaldehyde resin was well applicable to adsorb various small molecules and ions.16-19 However, to the best of our knowledge, there was no report about the removal of nanoparticles by this material. MFRM had plentiful imino/amino groups and triazine rings to endow the material with suitable anion-exchange ability in the acid, neutral and weak basic environment with excellent


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physical and chemical stability. Hence, owing to the nitrogen-rich property, the MFRM without post modification were expected to interact with the AuNPs and AgNPs via electrostatic interaction and specific affinity between the nitrogen and the nanoparticles based on hard and soft acid and base (HSAB) principle besides the regium-π bonds disclosed by the molecular docking study in our experiment. Additionally, as discussed above and reported previously,20-22 the porous structure of the adsorbent is crucial to enhance the adsorption ability towards the large molecules, particularly nanoparticles. Herein, the hierarchically porous structure of the MFRM was delicately designed and obtained via sacrificing template approach, i.e. the mesopores contributing to the surface area, and the macropores and flow-through pores making the adsorbent available to the nanoparticles and providing fast mass transfer. Thus-obtained MFRM were used to remove AuNPs and AgNPs from water with high adsorption capacities. Moreover, after adsorption, the hybrid materials, i.e. MFRM adsorbing AuNPs or AgNPs (MF-AuNPs or MF-AgNPs), showed good catalytic activity and recyclability towards 4-nitrophenol reduction by sodium borohydride (NaBH4), indicating that the contaminants of noble metallic nanoparticles could be recycled as heterogeneous catalysts for the consideration of green chemistry.23 At last, the adsorbent could be regenerated suggesting potential reusability of MFRM, and simultaneously recycling of precious noble metal was possible.

Experimental Chemicals



The flow-through silica (2-5 m diameter, 2 mL g-1 pore volume) was prepared as previously reported.24 AuNPs (1000 mg L-1) were purchased from Hu Zheng nano technology Co., Ltd (Shanghai, China) and AgNPs (1000 mg L-1) were from DK nano technology Co., Ltd (Beijing, China), both of which contained capping agent citrate. Melamine, formaldehyde (37%, wt%), NaOH, HCl, 4-nitrophenol, NaBH4 and thiourea were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water was produced with a Heal Fore NW system (Shanghai, China).

Preparation of MFRM MFRM were prepared by sacrificing template method. Typically, pre-polymer solution containing the mixture of melamine and formaldehyde (1 g: 2 mL, mass/volume) was heated to 80 °C with stirring until the solution became transparent, followed by purging with N2 to remove the air bubble. Afterwards, the template, i.e. the flow-through silica (1 g), was added to the pre-polymer solution (1.8 mL) with sonication to make the solution infiltrate the template. The mixture was then sealed to polymerize statically at 80 °C for 24 h. In this case, the volume of the pre-polymer solution was slightly lower than the pore volume of the flow-through silica (2 mL g-1) to ensure the polymerization taking place in the pores of silica. After polymerization, the resulting silica-polymer complex was immersed in 2 mol L-1 NaOH solution overnight to dissolve the silica skeleton, resulting in the MFRM. The MFRM were washed with abundant water and vacuum dried at 120 °C overnight for further use.


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Characterization of materials Transmission electron microscopic (TEM) images were acquired on a JEM-2100 Plus microscope (JEOL, Japan). The mesoporous property of MFRM was characterized by a TriStar II surface area analyzer (Micromeritics, USA). The macroporous structure was determined by mercury intrusion porosimeter (AutoPore IV 9500, Micromeritics, USA). Thermal gravimetric analysis (TGA) was performed on a thermal analyzer (TG 209 F1 Libra, Netzsch, Germany). An AVATAR 360 Fourier transform infrared spectrometer (FT-IR) (Thermo, USA) was used to characterize the functional groups. Elemental analysis was conducted by a Vario Micro cube elemental analyzer (Elementar, Germany). X-ray diffraction (XRD) patterns were recorded by an XRD6100 X-ray diffractometer (Shimadzu, Japan). The surface charge of the materials was measured as zeta potential using a ZetaPALS Zeta Potential Analyzer (Brookhaven, USA).

Batch adsorption experiments The adsorption studies were carried out in a batch system as below. Typically, MFRM (5 mg) were added to AuNPs or AgNPs aqueous solution (40 mL) as specified pH and concentrations with shaking in a rotary shaker (QB-210, Haimen, China) at constant temperature (25 °C) and shaking speed (60 rpm) for 24 h. After centrifugation, the supernatant was collected for quantification of the residual nanoparticles at 529 nm for AuNPs and 412 nm for AgNPs by a UV-3600 UV-vis spectrophotometer (Shimadzu, Japan) at room temperature.



The adsorption kinetics/isotherm tests were conducted at pH values of 6 for AuNPs and 8 for AgNPs. In the kinetics experiments, the adsorption time was investigated ranging from 15 min to 24 h at the nanoparticles concentration of 15 mg L-1. As for isotherm experiments, the initial concentrations of AuNPs solution (7.5-100 mg L-1) and AgNPs solution (2.5-50 mg L-1) were studied for 24 h adsorption.

The adsorption capacity is calculated according to the Eq. (1): 𝑄𝑡 = 10 ―3(𝐶0 ― 𝐶𝑡)𝑉 𝑚

(1)

where 𝐶0 and 𝐶𝑡 (mg L-1) are the initial concentration and the concentration at time t (h) of AuNPs or AgNPs in solutions, respectively; V (mL) is the volume of the nanoparticles solution; m (g) is the mass of MFRM; 𝑄𝑡 (mg g-1) is the adsorption capacity of MFRM at time t.

Method of molecular docking The AuNP was built by using the nanoparticleBuilder program in OpenMD package.25 The lattice constant of gold was set to 4.08 Å. The AutoDockTools (The Scripps Research Institute) was used to prepare the ligands for citrate and MFRM monomer units for docking simulation. Both ligands were added with Gasteiger partial charge. The grid box was set to 30 × 30 × 30 Å. The docking simulations were performed in two steps by using Autodock vina program.26 In the first step, the AuNP was treated as receptor and citrate was treated as ligand. The top ranked binding pose was then


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selected to build receptor for the second step, in which the receptor contained both AuNP and citrate. Ten units segment of MFRM were treated as ligand in the second step to dock with the citrate-AuNP complex. The first unit was set protonated, resulting in a positive charged MFMR model. The best binding pose was then selected according to Autodock vina’s docking score.

Catalytic performances of MF-AuNPs and MF-AgNPs To investigate the catalytic performance of MF-AuNPs and MF-AgNPs, the reduction of 4-nitrophenol by NaBH4 was chosen as a model reaction. For MF-AuNPs as the catalyst, once 5 mg MF-AuNPs were added to the mixture of 30 mL freshly prepared NaBH4 (60 mmol L-1) and 6 mL 4-nitrophenol (0.12 mmol L-1) aqueous solution, the UV absorption spectra of the catalytic solution were monitored at specific time intervals and the catalytic kinetics was calculated by the absorbance reduction of ionized 4nitrophenol at the wavelength of 400 nm. The MF-AgNPs catalytic experiment was similar to MF-AuNPs but with the addition of 11 mg MF-AgNPs to 15 mL NaBH4 (60 mmol L-1) and 6 mL 4-nitrophenol (0.12 mmol L-1). After a single catalytic test, MFAuNPs or MF-AgNPs were washed with copious water and used for the next catalysis cycle; this procedure was repeated five times to evaluate catalytic reusability of the hybrid materials.

Regeneration of MFRM and leaching of gold Acidic thiourea solution (0.25 mol L-1 thiourea in 1 mol L-1 HCl with 0.5% H2O2 as the



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oxidant) played dual roles as the regenerative and leaching solvent. Taking MF-AuNPs as an example, MF-AuNPs were added to the acidic thiourea solution with shaking for 1 h to desorb gold from MF-AuNPs. By centrifugation, the MFRM were collected and washed with copious water. Meanwhile, the gold was obtained in the leaching solution. After drying, the regenerative MFRM were used for the next cycle of adsorbing AuNPs.

Results and discussion Preparation and characterization of MFRM There are different approaches to prepare MFRM, like dispersed polycondensation,27 hydrothermal

method,28

polymerization,18,

29

sol-gel

method

and

dilute

solution

precipitation

etc. However, it was difficult to obtain hierarchically porous

structure by these approaches. Herein, sacrifice template method was adopted to generate hierarchically porous MFRM. The flow-through silica which possessed plentiful flow-through pores and mesopores was employed as the template, and the MFRM were prepared in its pores. After removing the template, the MFRM mirroring the porous structure of the template were generated.

As illustrated in Figure 1, three steps are mainly involved in the preparation of MFRM. Firstly, the pre-polymer solution was obtained by simply mixing melamine and formaldehyde at specific ratio without additional dispersion medium. The prepolymer solution turned transparent gradually under heating, during which the insoluble melamine converted to soluble hydroxymethyl derivatives in


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formaldehyde medium.29 The template silica was added to the hot pre-polymer solution with slow stirring to facilitate the solution to permeate the silica. With continuous heating under static condition, the polymerization took place in the pores and interlaced skeleton of the silica to form silica-polymer complex. Benefit from the template, the present method saved catalyst, surfactant or stabilizer which were commonly and critically controlled in other methods to obtain porous microspheres,18 thus enormously simplifying the preparation process and providing high spheres yield without agglomeration as shown in Figure 2A. At last, by sacrificing the template entirely in alkaline condition, the porous structure of the template silica, i.e. mesopores/macropores and flowthrough pores, was mirrored in the MFRM. The highly porous structure of MFRM was intuitively observed in Figure 2B.

Figure 1. Schematic illustration of the preparation process of MFRM by sacrificing silica.


60

40 1003 1159 812

3415

20

1334 1557 1481

4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers (cm-1)

18 16 14 12 10 8 6 4 2 0

dV/dw Pore Volume (cm3/g·nm)

Transmittance (%)

C

D 0.012 0.010 0.008 0.006 0.004 0.002 0.000 0

10

20

0.15 0.10

1.0

0.05

50

60

70

0.2

0.4

0.6

0.8

Relative pressure (p/p0)

1.0

F

80

0.20

1.5

40

Adsorption Desorption

100

0.25

2.0

30

Pore Width (nm)

E 0.30

2.5

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0.014

0.0

TG (%)

dV/dlogD pore volume (mL g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Quantity adsorbed (mmol g-1)


20

40

60

80 100 120 140 160 180 200

0.5

60 40 20 0

0.0 1

10

100

1000

Pore diameter (nm)

10000

0

100 200 300 400 500 600 700 800

Temperature (C)

Figure 2. TEM images with different magnification (A and B), FT-IR spectrum (C), BET nitrogen adsorption-desorption isotherm and mesopore size distribution (inset) (D), the pore size distribution determined by mercury intrusion porosimetry (E) and TGA curve (F) of MFRM.


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As shown in Figure 2C, MFRM possessed characteristic FT-IR absorption as reported:30 3415 cm-1 owing to amino (-NH2) and imino (-NH-) stretching vibration, 1557 cm-1 and 812 cm-1 typical of 1,3,5-s-triaze ring, 1481 cm-1 and 1159 cm-1 representing methylene (-CH2) stretching, 1334 cm-1 belonging to CN vibration, and 1003 cm-1 from hydroxymethyl groups (-CH2-OH). Besides, by elemental analysis, nitrogen content up to 47.6% was obtained demonstrating the nitrogen-rich property of MFRM, based on which the anion-exchange capacity of MFRM was estimated to be 17 mmol g-1.

Figure 2D depicts the nitrogen adsorption-desorption isotherm and pore size distribution curve of MFRM. Typical Langmuir IV hysteresis curve was observed. From the inset, MFRM had wide mesopore distribution from 10 nm to 50 nm. The surface area of MFRM was calculated to be 110 m2 g-1, and the total mesopore volume was 0.61 cm3 g-1. As shown in Figure 2E, the macropores of MFRM ranged from 60 nm to 90 nm, and the flow-through pores centered at ~1000 nm; the porosity was high up to 49%. Obviously, hierarchically porous structure including mesopores/macropores and flow-through pores were present in the prepared MFRM.

The thermal stability of MFRM was evaluated from the TGA curve of MFRM (Figure 2F). The 5.88% mass loss under 180 °C was attributed to the release of water from the material, and the slight mass loss of 9.45% from 180 °C to 350



°C was due to elimination of formaldehyde from the ether bridge to form methylene bridge while the crosslinking structure of MFRM remained. The rapid mass loss of 41.45% occurring at 350 °C to 450 °C was attributed to the breakdown of methylene bridge, indicating the stability of it up to 350 °C. With further heating up above 450 °C, thermal degradation of the stable triazine rings occurred.31 Based on TGA analysis, the MFRM were stable around the room temperature and suitable for routine use. Furthermore, the almost 100% mass loss of MFRM from 25 °C to 800 °C also demonstrated that the silica template was entirely removed.

From the above characterizations, nitrogen-rich MFRM which had large anionexchange capacity of 17 mmol g-1, possessed mesopores/macropores (10-90 nm) and flow-through pores (~1000 nm) and exhibited eminent thermal stability were successfully prepared by sacrificing flow-through silica as the template method.

Adsorption of AuNPs and AgNPs by MFRM According to the properties as discussed above, the MFRM were attempted as the adsorbent to adsorb AuNPs and AgNPs. This hypothesis was demonstrated by comparing TEM images, TGA and XRD analysis of the MFRM before and after adsorption of the nanoparticles in our preliminary experiments. After adsorption, nanoparticles were obviously distributed on both the surface and pores of MFRM, as reflected in Figure 3A and B. Besides, the mass loss of MF-


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AuNPs (95.05%) and MF-AgNPs (98.41%) from the TGA analysis were slightly lower than MFRM (~100%) from 25 °C to 800 °C, ascribing to the adsorbed AuNPs and AgNPs. From the XRD pattern in Figure 3C, only a big broad peak was observed for MFRM, indicating its amorphous nature. However, the characteristic diffraction peaks of AuNPs [38.18° (111), 44.39° (200), 64.58° (220), 77.55° (311)] and AgNPs [38.12° (111), 44.28° (200), 64.43° (220), 77.47° (311)] obviously appeared after MFRM adsorbed the respective nanoparticles,12,

32

also illustrating the successful adsorption of AuNPs and

AgNPs onto MFRM.

C MFRM MF-AuNPs MF-AgNPs

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(111) (200)

(111) (200)

10

20

30

40

50

(220)

(311)

(220)

(311)

60

70

80

2 Theta () Figure 3. TEM images of MF-AuNPs (inset: AuNPs) (A) and MF-AgNPs (inset: AgNPs) (B), and XRD pattern of MFRM, MF-AuNPs and MF-AgNPs (C).



In our previous work,16 mesoporous MFRM was prepared via suspension polymerization for the removal of perfluoroalkyl and polyfluoroalkyl substances. However, this mesoporous MFRM showed much less adsorption capacity than the present proposed one for the AuNPs and AgNPs, despite both materials possessed the same functionalities. This phenomenon indicates that the large porosity of the adsorbent may be the extra merit for efficient adsorption of the nanoparticles in addition to the fast mass transfer.33 The macropores and flowthrough pores in adsorbents would make the functionalities fully accessible to the adsorbates thus to enhance the adsorption efficiency of the nanoparticles, and facilitate the mass transfer during adsorption.

To investigate the interaction mechanism, molecular docking was studied using citrate-AuNP as model nanoparticle. As observed from Figure 4, there are three major interactions between MFRM and the citrate-AuNP complex, i.e. (i) the electrostatic interaction, (ii) metal-ligand interaction between gold and nitrogen atom of MFRM, and (iii) the regium-π interaction between AuNP and the electron-rich triazine ring of MFRM. Owing to the presence of the amino and imino groups, MFRM were expected to adsorb AuNPs and AgNPs which were capped with negatively charged citrate by electrostatic interaction. On the other hand, in the light of HSAB principle, nitrogen as a borderline donating atom which was widely present in MFRM was supposed to have strong affinity for


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AuNPs and AgNPs for the metal-ligand interaction.34 In addition, the apparent regium-π interaction between AuNP and π systems of MFRM, which was extensively observed in noble metal nanoparticle systems, significantly strengthened the affinity of MFRM for AuNPs and AgNPs.35 Thereby, the MFRM could be the suitable candidate adsorbent for adsorbing the AuNPs and AgNPs.

Figure 4. Side view (A) and top view (B) of molecular docking study between the monomer units of MFRM and citrate-AuNP.

To maximize the adsorption capacity of the MFRM, the pH value of adsorption system was investigated as it is crucial for the electrostatic interactions between adsorbent and adsorbate. Since AuNPs and AgNPs solutions were unstable under extreme pH conditions, the pH values of 6-10 were studied. As shown in Figure 5A, the adsorption capacity for both AuNPs and AgNPs was almost the highest at pH 6-8, and gradually decreased with the increasing pH. The zeta potentials of MFRM and nanoparticles at different pHs are depicted in Figure 5B. It is conceivable that MFRM are positive charged at pH 6-9, and the protonation of them would be enhanced under lower pH value. Besides, the opposite zeta



potentials are observed for AuNPs and AgNPs, and the differences of zeta potential between MFRM with nanoparticles at pH 6-8 were relatively larger than other conditions indicating the stronger electrostatic attraction, which may result in higher adsorption capacity as depicted in Figure 5A. As the natural environment pH was usually 6-8, pH 6 for AuNPs and pH 8 for AgNPs in

A

100

AuNPs AgNPs

80 60 40 20 0

6

7

8

pH

9

10

Zeta potential (mV)

subsequent adsorption experiments were randomly chosen.

Qe (mg g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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B

15

MFRM AuNPs AgNPs

10 5 0 -5

6

-10

7

8

9

10

pH

-15 -20

Figure 5. The effect of pH on adsorption capacity of MFRM towards AuNPs and AgNPs (A) and the zeta potential of MFRM, AuNPs and AgNPs (B).

Adsorption kinetics The adsorption kinetic studies for AuNPs and AgNPs onto MFRM were carried out from 15 min to 24 h, as depicted in Figure 6A. It can be seen that the adsorption capacities for AuNPs and AgNPs were all rapidly increased within 6 h and reached more than 80% of equilibrium adsorption capacities. To better evaluate the adsorption process of MFRM, pseudo-first-order (Eq. (2)) and pseudo-second-order (Eq. (3)) rate laws were adopted to model the kinetics as below:


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ln (Qe - Qt) = ln Qe - k1t

(2)

t Q = 1 (k Q 2) +t/Q t 2 e e

(3)

Qe and Qt represent the adsorption capacity (mg g-1) on equilibrium time and time t (h). k1 (h-1) and k2 (g mg-1 h-1) are the pseudo-first-order and pseudosecond-order kinetic rate constants, respectively.

The fitting parameters are listed in Table 1 and the linear fitting plots are depicted in Figure 6B and C. From the results, the better fit with pseudo-second-order kinetics for AuNPs (R2=0.9884) is obtained. Although the small difference of R2 for AgNPs fitting by these two kinetics models, the conspicuous deviation of the calculated Qe value from the pseudo-first-order kinetics to the experimental one indicated that the pseudo-second-order kinetics could be better to fit the adsorption process of AgNPs. In addition, the calculated adsorption capacities of MFRM for AuNPs (49.6 mg g-1) and AgNPs (80.0 mg g-1) by pseudo-secondorder kinetic model are highly matched with the experimental values from Figure 6A, i.e. equilibrium adsorption capacity at 24 h for AuNPs (49.4 mg g-1) and AgNPs (76.5 mg g-1).



90

B

4.0

70

3.5

60

3.0

Ln(Qe-Qt)

Qe (mg g-1)

A

AuNPs AgNPs

80

50 40 30

AuNPs AgNPs

2.5 2.0 1.5 1.0

20 0

5

10

20

25

0.5

C

0.2 0.1 0.0 5

10

15

Time (h)

20

25

2

4

6

8

10

Time (h)

AuNPs AgNPs

70

0.3

0

0

80

Qt (mg g-1)

0.4

15

Time (h)

AuNPs AgNPs

0.5

t/Qt (h g mg-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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D

12

II

60 I

50 40 30 20

0

1

2

3

t1/2 (h1/2)

4

5

Figure 6. Adsorption kinetics (A), the fitting of pseudo-first-order (B), pseudosecond-order kinetics (C) and intraparticle diffusion model (D) for the adsorption of AuNPs and AgNPs by MFRM.

It is well accepted that adsorption process can be described as four consecutive kinetic steps,36 i.e. transport in the bulk solution, external mass transfer (boundary layer diffusion), intraparticle diffusion and adsorption at a site on the surface (internal or external) which is often assumed to be extremely rapid. To interpret the actual rate-limiting step involved in the adsorption of nanoparticles to MFRM, an intraparticle diffusion model proposed by Weber and Morris was used (Eq. (4)):37 Qt = kit

12

+S

(4)

where ki is the intraparticle diffusion rate constant, the values of S depict the


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boundary thickness of adsorbent and Qt is the adsorption capacity at time t. By plotting Qt versus t

12

(Figure 6D), the adsorption of AgNPs displayed two

distinct regions, implying that more than one process affected the adsorption. The first sharper portion (I) was attributed to the boundary layer diffusion and the second portion (II) described the gradual adsorption stage where intraparticle diffusion was the rate-limiting step. However, the plot did not pass through the origin so that the boundary layer diffusion controlled the adsorption to some degree.38 As for AuNPs, only one linear plot without passing through the origin over the whole time range was observed, which indicated that intraparticle diffusion was not the only rate-limiting step.39

Table 1. Kinetic parameters of pseudo-first-order and pseudo-second-order equation for AuNPs and AgNPs adsorption on MFRM.

Qe, exp (mg g-1) AuNPs AgNPs

49.4 76.5

Pseudo-first-order kinetics Qe k1 (h-1) R2 (mg g-1) 22.2 0.12 0.8827 53.1 0.27 0.9828

Pseudo-second-order kinetics Qe k2 R2 -1 (mg g ) (g mg-1 h-1) 49.6 1.81×10-2 0.9884 80.0 9.06×10-3 0.9886

Qe, exp: the adsorption capacity of experimental result.

Adsorption isotherms The adsorption isotherm studies were conducted to describe the interaction of AuNPs and AgNPs onto MFRM as well as the adsorbent’s property. Varying with the increasing concentrations of nanoparticles, the equilibrium adsorption



capacities increased gradually and then reached plateaus (Figure 7A). The commonly-used Langmuir (Eq. (5)) and Freundlich (Eq. (6)) models were employed to analyze the adsorption data:37 𝐶𝑒 𝑄 = 1/(𝑄 𝐾 ) + 𝐶𝑒 𝑄 𝑒 𝑚𝑎𝑥 𝑚𝑎𝑥 𝑙 ln 𝑄𝑒 = ln 𝐾𝑓 + (1 𝑛)ln 𝐶𝑒

(5)

(6)

𝐶𝑒 (mg L-1) and 𝑄𝑒 (mg g-1) are the concentration and adsorption capacity at equilibrium. 𝐾𝑙 (L mg-1) and 𝐾𝑓 are the Langmuir and Freundlich constants, respectively. 𝑄𝑚𝑎𝑥 (mg g-1) is the theoretical maximum adsorption amount. When the value of n is in the range of 1-10, the adsorption is generally considered favorable. The fitting linear plots of two isotherm models are shown in Figure 7B and C, and the specific modeling parameters are presented in Table 2. Compared to the Freundlich isotherm, the Langmuir isotherm reveals higher fitting degree for the higher R2 for AuNPs (R2=0.9900) and AgNPs (R2=0.9915), which is valid for monolayer adsorption onto a perfectly smooth and homogeneous surface with a finite number of identical sites.

Table 2. Isotherm parameters of Langmuir model and Freundlich model for AuNPs and AgNPs adsorption on MFRM.

AuNPs AgNPs

Langmuir model Qmax (mg g-1) Kl (L mg-1) 179.2 0.051 132.5 0.640

R2 0.9900 0.9915


Freundlich model Kf n R2 17.143 1.924 0.9457 70.545 5.582 0.9300

Page 22 of 36

Page 23 of 36

A

160

Qe (mg g-1)

140 120 100 80 AuNPs AgNPs

60 40 20 0

0.6

AuNPs AgNPs

0.5

20

40

B

0.4 0.3 0.2 0.1 0.0 0

20

40

Ce (mg L-1)

60

60

80

Ce (mg L-1)

Ln Qe

Ce/Qe (g L-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 -2

C

AuNPs AgNPs

-1

0

1

2

Ln Ce

3

4

5

Figure 7. Adsorption isotherms (A), the fitting of Langmuir (B) and Freundlich model (C) for the adsorption of AuNPs and AgNPs by MFRM.

The theoretically maximum adsorption capacities based on Langmuir isotherm of AuNPs and AgNPs onto MFRM are 179.2 mg g-1 and 132.5 mg g-1, respectively, which were higher or comparable to other reported adsorbents as listed in Table 3. This could be highly related to the chemical properties and porous structures of the proposed MFRM.



Page 24 of 36

Table 3. Comparison of the removal of AuNPs and AgNPs with various adsorbents.

Adsorbent

NPs

Adsorption capacity* (mg g-1)

Dopamine-polyethylenimine co-coated micrometer-sized carbon fiber aerogels Polyethylenimine founctionalized carbon spheres

AuNPs AgNPs AuNPs AgNPs AuNPs AgNPs

31.2 41.8 83 116 50-109 99-117

AgNPs

114.9

11

AgNPs AuNPs AgNPs AuNPs AgNPs AuNPs AgNPs AuNPs AgNPs AuNPs AgNPs

30-62 79-84 24-56 3.4-77 5-55 13.1 17.9 36.5 31.8 179.2 132.5

40

Amine-functionalized block copolymers Mesoporous silica modified with 3mercaptopropyltrimethoxysilane Activated carbon Surface modified electrospun polyvinyl alcohol nanofibers Biomimetic metal oxides Cellulosic nanofibers Polyvinyl alcohol/Gluten hybrid nanofibers Hierarchically porous MFRM

Ref. 7 8 10

41 42 43 44 This work

* Theoretically maximum adsorption capacities based on the Langmuir isotherm.

Catalytic performances of MF-AuNPs and MF-AgNPs From a green and sustainable perspective, the contaminant noble metallic nanoparticles after being adsorbed were herein recycled as the heterogeneous catalyst. Noble metal nanoparticles are reported to be used in a variety of catalytic reactions, such as reduction, oxidation, hydrogenation, etc.45 Herein, the reduction of 4-nitrophenol (Amax=317 nm), one of organic pollutant,46 to 4aminophenol (Amax=300 nm) was studied. In the experiment, 4-nitrophenol was transformed to its ionic form (Amax=400 nm) in alkaline condition. In the absence


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of the catalyst, i.e. MF-AuNPs or MF-AgNPs, no obvious reaction occurred during the test time (90 min) (Figure 8A). However, as evidenced by the change of the UV absorption peak in Figure 8B and C, i.e. gradually weakened peak strength at 400 nm attributed to the ionized 4-nitrophenol and appearance of strengthened new peak at 300 nm associated with 4-aminophenol,47 MF-AuNPs and MF-AgNPs played the role as the catalyst. Once the catalytic reaction took place, both the donor BH4- ions and the acceptor 4-nitrophenol molecules were adsorbed onto the catalytic centres of nanoparticles, leading to electron transfer from BH4- to 4-nitrophenol thus generating reduction product 4-aminophenol.48 Because of the high concentration of NaBH4 in the catalytic system, the pseudofirst-order kinetics was applied to model the catalytic kinetics (Eq. (7)):21 ln (𝐶𝑡 𝐶0) = ―𝑘𝑡

(7)

𝐶𝑡 and 𝐶0 are the concentration of 4-nitrophenol at time t (min) and initial time, and k (min-1) is the reaction rate constant. As shown in the insets of Figure 8B and C, the catalytic reaction processes were well fitted with the pseudo-firstorder kinetics for both MF-AuNPs (R2=0.9902, k=0.053 min-1) and MF-AgNPs (R2=0.9606, k=0.027 min-1) as catalysts. Furthermore, in the catalyst recyclability experiments, the elimination ratio of 4-nitrophenol was used to represent catalytic activity of the catalyst. For both MF-AuNPs and MF-AgNPs, above 94% of catalytic activities for transforming 4-nitrophenol into 4aminophenol were obtained even after five-time cycles repetitive uses (Figure 8D). Hence, MF-AuNPs and MF-AgNPs can serve as catalysts that possess



satisfactory catalytic reactivity and reusability, which is second use of nanoparticles for green chemistry.

0.8

4-nitrophenol+NaBH4 90 min

0.6 0.4 0.2

0.3 0.2

0.4 0.3 0.2

450

Wavelength (nm) 0 min 10 min 20 min 30 min 40 min 50 min 60 min 70 min 80 min 90 min

C

0.0

-0.5

5

10 15 20 25 30 Ln(Ct/C0)=-0.053t-0.078

-0.4

2

R =0.9902

-1.2 -1.6

-2.0

300

350

Ln(Ct/C0)=-0.027t+0.2364 2

R =0.9606

-2.5

400

300

350

450

Wavelength (nm)

500

550

400

450

500

Wavelength (nm)

Time (min)

0.1 0.0 250

Time (min) 0

-0.8

D

0 10 20 30 40 50 60 70 80 90

-1.0 -1.5

0.0 250

550

Catalytic activity (%)

0.5

350

Ln(Ct/C0)

0.6

0.0

0.1

0.0

0.7

0 min 2 min 4 min 8 min 12 min 16 min 20 min 24 min 28 min

0.4

Absorbance

Absorbance

1.0

4-nitrophenol 4-nitrophenol+NaBH4 0 min

250

B

0.5

Ln(Ct/C0)

A

1.2

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

550

MF-AuNPs MF-AgNPs

100 80 60 40 20 0

1

2

3

4

5

Cycles

Figure 8. UV-vis absorption spectra of 4-nitrophenol and the mixture of 4nitrophenol and NaBH4 (A), the time-dependent reduction of 4-nitrophenol in the presence of MF-AuNPs (B) and MF- AgNPs (C), ln(C/C0) versus reaction time for the reduction of 4-nitrophenol (insets of B and C), catalytic recyclability of MF-AuNPs and MF-AgNPs (D).

Regeneration of MFRM and leaching of gold It is turning waste into wealth by leaching of gold from waste material. Enlightened by this idea, we fulfilled the reuse of MFRM and leaching of gold by thiourea [CS(NH2)2], for its low toxicity and high leaching rate. The


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mechanism for regeneration of MF-AuNPs as well as leaching of gold was interpreted as below:49 2Au + 4CS(NH2)2 + H2O2 + 2H+→ 2Au[CS(NH2)2]2+ + 2H2O The AuNPs were oxidized to be dissolved in acidic thiourea media, and H2O2 served as the oxidant to facilitate the dissolution process. The regeneration percentage, which was calculated as the ratio of the adsorption amount of AuNPs on the regenerative adsorbent to that on the same adsorbent for the first time usage, was above 90% in three cycles. Meanwhile, the AuNPs were dissolved in thiourea solution, and thus achieved the goal of leaching of precious gold and had the potential for further usage.

Conclusions The hierarchically porous melamine-formaldehyde resin microspheres (MFRM) were prepared via sacrificing the flow-through silica as the template method. The prepared MFRM were used to efficiently remove AuNPs and AgNPs from the aqueous based on electrostatic interaction, metal ligand and regium-π bonds. The abundant mesopores endowed MFRM large surface area and thus high adsorption capacity; the macropores and flow-through pores were beneficial to the nanoparticles adsorption and fast mass transfer. For adsorption of both AuNPs and AgNPs by MFRM, the adsorption obeyed the pseudo-second order kinetics and Langmuir isotherms. The MFRM showed high adsorption capacities for AuNPs (179.2 mg g-1) and AgNPs (132.5 mg g-1) based on Langmuir model. For the purpose of second use, the MF-AuNPs and MF-AgNPs



exhibited excellent catalytic performance for reduction of 4-nitrophenol, and retained above 94% catalytic activity even after five-time recycles. At last, taking the MFAuNPs as an example, the adsorbent could be regenerated for repetitive usages, and simultaneously precious noble metal could be recycled. In summary, the hierarchically porous MFRM were a promising adsorbent for AuNPs and AgNPs removal owing to its special meso/macropores and flow-through pores and excellent physicochemical property. This study paved the way to design the adsorbents for nanoparticles’ removal, second usage and recycling the noble metal from the “green” and sustainable respect.

Acknowledgements The authors gratefully acknowledge the financial support of this research by the Fundamental Research Funds for the Central Universities (No. 2017 KFYXJJ149). The authors also thank the Analytical and Testing Center of HUST for instrumental support.

References 1

Markiewicz, M.; Kumirska, J.; Lynch, I.; Matzke, M.; Köser, J.; Bemowsky, S.;

Docter, D.; Stauber, R.; Westmeier, D.; Stolte, S. Changing environments and biomolecule coronas: consequences and challenges for the design of environmentally acceptable engineered nanoparticles. Green Chem. 2018, 20 (18), 4133-4168. 2

Syafiuddin, Achmad.; Salmiati, S.; Hadibarata, T.; Kueh, A. B. H.; Salim, M. R.;

Zaini, M. A. A. Silver Nanoparticles in the Water Environment in Malaysia: Inspection, characterization, removal, modeling, and future perspective. Sci. Rep. 2018, 8 (1), 986-


Page 28 of 36

Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000. 3

Bäuerlein, P. S.; Emke, E.; Tromp, P.; Hofman, J. A. M. H.; Carboni, A.;

Schooneman, F.; Voogt, P. d.; Wezel, A. P. v. Is there evidence for man-made nanoparticles in the Dutch environment? Sci. Total Environ. 2017, 576, 273-283. 4

Navarro, E.; Baun, A.; Behra, R.; Hartmann, N. B.; Filser, J.; Miao, A.-J.; Quigg,

A.; Santschi, P. H.; Sigg, L. Environmental behavior and ecotoxicity of engineered nanoparticles to algae, plants, and fungi. Ecotoxicology 2008, 17 (5), 372-386. 5

Sajid, M.; Ilyas, M.; Basheer, C.; Tariq, M.; Daud, M.; Baig, N.; Shehzad, F.

Impact of nanoparticles on human and environment: review of toxicity factors, exposures, control strategies, and future prospects. Environ. Sci. Pollut. Res. 2015, 22 (6), 4122-4143. 6

Zhou, X.-x.; Li, Y.-j.; Liu, J.-f. Highly efficient removal of silver-containing

nanoparticles in waters by aged iron oxide magnetic particles. ACS Sustainable Chem. Eng. 2017, 5 (6), 5468−5476. 7

Liu, R.-L.; Mao, S.; Wang, Y.; Wang, L.; Ge, Y.-H.; Xu, X.-Y.; Fu, Q. A mussel-

inspired hybrid copolymer adhered to chitosan-coated micro-sized carbon fiber aerogels for highly efficient nanoparticle scavenging. Environ. Sci. Nano 2017, 4 (11), 2164-2174. 8

Kumar, J.; Mallampati, R.; Adin, A.; Valiyaveettil, S. Functionalized carbon

spheres for extraction of nanoparticles and catalyst support in water. ACS Sustainable Chem. Eng. 2014, 2 (12), 2675-2682. 9

Padil, V. V. T.; Černík, M. Poly (vinyl alcohol)/gum karaya electrospun plasma



treated membrane for the removal of nanoparticles (Au, Ag, Pt, CuO and Fe3O4) from aqueous solutions. J. Hazard. Mater. 2015, 287, 102-110. 10 Qureshi, Z. S.; Dsouza, R.; Mallampati, R.; Valiyaveettil, S. Synthesis of aminefunctionalized block copolymers for nanopollutant removal from water. J. Appl. Polym. Sci. 2014, 131 (20), 1366-1373. 11 Pongkitdachoti, U.; Unob, F. Simultaneous adsorption of silver nanoparticles and silver ions on large pore mesoporous silica. J. Environ. Chem. Eng. 2018, 6 (1), 596603. 12 Zhang, X.; Zhang, Y.; Zhang, X.; Li, S.; Huang, Y. Nitrogen rich core-shell magnetic mesoporous silica as an effective adsorbent for removal of silver nanoparticles from water. J. Hazard. Mater. 2017, 337, 1-9. 13 Li, J.; Xu, L.; Shi, Z.-g.; Hu, M. A novel two-dimensional liquid chromatographic system for the online toxicity prediction of pharmaceuticals and related substances. J. Hazard. Mater. 2015, 293, 15-20. 14 Xue, S.-W.; Tang, M.-Q.; Xu, L.; Shi, Z.-g. Magnetic nanoparticles with hydrophobicity and hydrophilicity for solid-phase extraction of polycyclic aromatic hydrocarbons from environmental water samples. J. Chromatogr. A 2015, 1411, 9-16. 15 Li, J.; Ma L.-y.; Xu, L. Transformation of benzophenone-type UV filters by chlorine: Kinetics, products identification and toxicity assessments. J. Hazard. Mater. 2016, 311, 263-272. 16 Li, J.; Li, Q.; Li L.-s.; Xu, L. Removal of perfluorooctanoic acid from water with economical mesoporous melamine-formaldehyde resin microsphere. Chem. Eng. J.


Page 30 of 36

Page 31 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2017, 320, 501-509. 17 Tan, M. X.; Sum, Y. N.; Ying, J. Y.; Zhang, Y. A mesoporous poly-melamineformaldehyde polymer as a solid sorbent for toxic metal removal. Energy Environ. Sci. 2013, 6 (11), 3254-3259. 18 Ming, G.; Duan, H.; Meng, X.; Sun, G.; Sun, W.; Liu, Y.; Lucia, L. A novel fabrication of monodisperse melamine-formaldehyde resin microspheres to adsorb lead (II). Chem. Eng. J. 2016, 288, 745-757. 19 Drage, T. C.; Arenillas, A.; Smith, K. M.; Pevida, C.; Piippo, S.; Snape, C. E. Preparation of carbon dioxide adsorbents from the chemical activation of ureaformaldehyde and melamine-formaldehyde resins. Fuel 2007, 86 (1), 22-31. 20 Ma, L.; Li, J.; Zhao, J.; Liao, H.; Xu, L.; Shi, Z.-g. Penetrable silica microspheres for immobilization of bovine serum albumin and their application to the study of the interaction between imatinib mesylate and protein by frontal affinity chromatography. Anal. Bioanal. Chem. 2016, 408 (3), 805-814. 21 Qiao, T.; Ma, L.-y.; Wang, X.; Shi, Z.-G. The flow-through silica as the matrix to immobilize gold nanoparticles for HPLC applications. J. Liq. Chromatogr. Relat. Technol. 2018, 41 (3), 107-113. 22 Zhao, X.-B.; Ha, W.; Jiang, K.; Chen, J.; Yang, J.-L.; Shi, Y.-P. Efficient synthesis of camptothecin propargylamine derivatives in water catalyzed by macroporous adsorption resin-supported gold nanoparticles. Green Chem. 2017, 19 (5), 1399-1406. 23 Dai, J.; Zou, H.; Wang, R.; Wang, Y.; Shi, Z.; Qiu, S. Yolk-shell Fe3O4@SiO2@PMO: amphiphilic magnetic nanocomposites as an adsorbent and a



catalyst with high efficiency and recyclability. Green Chem. 2017, 19 (5), 1336-1344. 24 Wei, J.-X.; Shi, Z.-G.; Chen, F.; Feng, Y.-Q.; Guo, Q.-Z. Synthesis of penetrable macroporous silica spheres for high-performance liquid chromatography. J. Chromatogr. A 2009, 1216 (44), 7388-7393. 25 Stocker, K. Building and equilibrating a gold nanoparticle in OpenMD. http://openmd.org/category/examples/ (accessed August 2018). 26 Trott, O.; Olson, A. J. AutoDock Vina: improving the speed and accuracy of docking with a new scoring function, efficient optimization, and multithreading. J. Comput. Chem. 2010, 31 (2), 455-461. 27 Cheong, I. W.; Shin, J. S.; Kim, J. H. Preparation of monodisperse melamineformaldehyde microspheres via dispersed polycondensation. Macromol. Res. 2004, 12 (2), 225-232. 28 Zhou, H.; Xu, S.; Su, H.; Wang, M.; Qiao, W.; Ling, L.; Long, D. Facile preparation and ultra-microporous structure of melamine-resorcinol-formaldehyde polymeric microspheres. Chem. Commun. 2013, 49 (36), 3763-3765. 29 Wu, Y.; Li, Y.; Qin, L.; Yang, F.; Wu, D. Monodispersed or narrow-dispersed melamine-formaldehyde resin polymer colloidal spheres: preparation, size-control, modification, bioconjugation and particle formation mechanism. J. Mater. Chem. B 2013, 1 (2), 204-212. 30 Song, Y.; Ma, R.; Jiao, C.; Hao, L.; Wang, C.; Wu, Q.; Wang, Z. Magnetic mesoporous polymelamine-formaldehyde resin as an adsorbent for endocrine disrupting chemicals. Microchim. Acta 2018, 185 (1), 19-26.


Page 32 of 36

Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31 Merline, D. J.; Vukusic, S.; Abdala, A. A. Melamine formaldehyde: curing studies and reaction mechanism. Polym. J. 2013, 45 (4), 413-419. 32 Dwivedi, A. D.; Dubey, S. P.; Hokkanen, S.; Fallah, R. N.; Sillanpää, M. Recovery of gold from aqueous solutions by taurine modified cellulose: An adsorptive-reduction pathway. Chem. Eng. J. 2014, 255 (7), 97-106. 33 Sun, Z.; Deng, Y.; Wei, J.; Gu, D.; Tu, B.; Zhao, D. Hierarchically ordered macro/mesoporous silica monolith: Tuning macropore entrance size for size-selective adsorption of proteins. Chem. Mater. 2011, 23 (8), 2176-2184. 34 Nath, S.; Ghosh, S. K.; Kundu, S.; Praharaj, S.; Panigrahi, S.; Pal, T. Is gold really softer than silver? HSAB principle revisited. J. Nanopart. Res. 2006, 8 (1), 111-116. 35 Frontera, A.; Bauz á , A. Regium- π bonds: An unexplored link between noble metalnanoparticles and aromatic surfaces. Chem. Eur. J. 2018, 24, 7228-7234. 36 Rudzinski, W.; Plazinski, W. Theoretical description of the kinetics of solute adsorption at heterogeneous solid/solution interfaces on the possibility of distinguishing between the diffusional and the surface reaction kinetics models. Appl. Surf. Sci. 2007, 253, 5827-5840. 37 Liu, Y.; Luo, C.; Sun, J.; Li, H.; Sun, Z.; Yan, S. Enhanced adsorption removal of methyl orange from aqueous solution by nanostructured proton-containing -MnO2. J. Mater. Chem. A 2015, 3 (10), 5674-5682. 38 Cheung, W. H.; Szeto, Y. S.; McKay, G. Intraparticle diffusion processes during acid dye adsorption onto chitosan. Bioresour. Technol. 2007, 98 (15), 2897-2904. 39 Senthil Kumar, P.; Ramalingam, S.; Senthamarai, C.; Niranjanaa, M.;



Vijayalakshmi, P.; Sivanesan, S. Adsorption of dye from aqueous solution by cashew nut shell: Studies on equilibrium isotherm, kinetics and thermodynamics of interactions. Desalination 2010, 261 (1), 52-60. 40 Gicheva, G.; Yordanov, G. Removal of citrate-coated silver nanoparticles from aqueous dispersions by using activated carbon. Colloid Surf. A: Physicochem. Eng. Asp. 2013, 431 (33), 51-59. 41 Mahanta, N.; Valiyaveettil, S. Surface modified electrospun poly(vinyl alcohol) membranes for extracting nanoparticles from water. Nanoscale 2011, 3 (11), 46254631. 42 Mallampati, R.; Valiyaveettil, S. Biomimetic metal oxides for the extraction of nanoparticles from water. Nanoscale 2013, 5 (8), 3395-3399. 43 Mahanta, N.; Leong, W. Y.; Valiyaveettil, S. Isolation and characterization of cellulose-based nanofibers for nanoparticle extraction from an aqueous environment. J. Mater. Chem. 2012, 22 (5), 1985-1993. 44 Dhandayuthapani, B.; Mallampati, R.; Sriramulu, D.; Dsouza, R. F.; Valiyaveettil, S. PVA/gluten hybrid nanofibers for removal of nanoparticles from water. ACS Sustainable Chem. Eng. 2014, 2 (4), 1014-1021. 45 Islam, M. T.; Saenz-Arana, R.; Wang, H.; Bernal, R.; Noveron, J. C. Green synthesis of gold, silver, platinum, and palladium nanoparticles reduced and stabilized by sodium rhodizonate and their catalytic reduction of 4-nitrophenol and methyl orange. New J. Chem. 2018, 42 (8), 6472-6478. 46 Kohantorabi, M.; Gholami, M. R. MxNi100-x (M = Ag, and Co) nanoparticles


Page 34 of 36

Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


supported on CeO2 nanorods derived from Ce-metal organic frameworks as an effective catalyst for reduction of organic pollutants: Langmuir-Hinshelwood kinetics and mechanism. New J. Chem. 2017, 41 (19), 10948-10958. 47 Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. Kinetic analysis of catalytic reduction of 4-nitrophenol by metallic nanoparticles immobilized in spherical polyelectrolyte brushes. J. Phys. Chem. C 2010, 114 (19), 8814-8820. 48 Shao, Y.; Zhou, L.; Bao, C.; Wu, Q.; Wu, W.; Liu, M. Facile preparation of tiny gold nanoparticle loaded magnetic yolk-shell carbon nanoreactors for confined catalytic reactions. New J. Chem. 2016, 40 (1-11), 9684-9693. 49 Deschênes, G.; Ghali, E. Leaching of gold from a chalcopyrite concentrate by thiourea. Hydrometallurgy 1988, 20 (2), 179-202.



Abstract graphic

Synopsis Hierarchically porous MFRM was used to efficiently adsorb noble metal nanoparticles and simultaneously as the sustainable catalyst meanwhile recycling noble metal.


Page 36 of 36

Hierarchically Porous Melamine-Formaldehyde Resin Microspheres

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