Preparation and dye removal capacities of porous silver nanoparticle-containing composite hydrogels via poly(acrylic acid) and silver ions

Abstract

In this work, hierarchical porous composite hydrogels of poly(acrylic acid)–silver/silver nanoparticles (PAA–Ag/AgNPs) were successfully prepared by an in situ coordination and self-assembly process. The obtained composite PAA–Ag/AgNPs hydrogels were achieved through different coordination times between PAA and silver ions in the self-assembly process. These as-formed hydrogels were characterized by various morphological and spectroscopy techniques. Interestingly, unique 3D porous network structures in the nanocomposite hydrogels were demonstrated, due to which the as-obtained composite hydrogels demonstrate good removal capacities in accordance with both a pseudo-first-order model and a pseudo-second-order model for three model dyes. Thus, the present obtained composite hydrogels provide new alternatives for developing hybrid hydrogels and nanomaterials for removal of organic dye pollutants from aqueous environments in wastewater treatment applications.

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1. Introduction

In recent years, water pollution has become a focus of attention all over the world; harmful organic components are a major source of water pollution, especially commercially used dyes which are drained with wastewater into the local environment by some factories without any treatment. So removing organic dye components from wastewater is an urgent and challenging issue faced by scientists.^1–7 On the other hand, porous hydrogel materials have been a central topic studied by scientists in recent years.^8–11 The unique 3D porous net-like structure of these hydrogel materials endows them with novel characteristics, such as strong mechanical strength, large surface area, ultralow density, good compressibility, and so on. These new properties make the hydrogels well able to be applied for removing organic dye from aqueous environments. For instance, Banerjee's group reported the preparation of different hydrogels that could be utilized in wastewater treatment as excellent adsorbent agents.^12,13 Interestingly, these hydrogel materials were highly friendly towards the environment and could be easily separated from an aqueous environment after reaction because of their high mechanical strength. Therefore, the design and successful scalable application of suitable hydrogels for removal of organic dyes are promising. For example, Zhu et al. have reported the preparation of palygorskite/poly(acrylic acid) nanocomposite hydrogels for adsorbing cationic basic dye.¹⁴ Mabrouk and coworkers have designed a novel crosslinked HEC-PAA porous hydrogel composite as drug carrier.¹⁵ In addition, Huang and coworkers have done some good research about supramolecular gels originated from host–guest macrocyclic molecular recognition.^16–19 In our previously reported work, the gelation properties of some supramolecular composite hydrogels were investigated.^20–23 Therein, some graphene oxide–polymers (polyethylenimine or chitosan) and gold nanoparticle–collagen hybrid hydrogels were successfully prepared and their applications for adsorption, catalyst, and photothermal/photodynamic therapy were demonstrated.

In this study, we report the preparation of composite hydrogel materials of poly(acrylic acid)–silver/silver nanoparticles (PAA–Ag/AgNPs) by self-assembly through coordinate bonding and electrostatic interactions. Owing to the 3D porous network nanostructures formed, the as-prepared composite hydrogels demonstrated good catalytic capacity towards three model dyes, namely Congo red (CR), rhodamine B (RhB), and methylene blue (MB). The obtained dye removal behavior of the present hydrogels demonstrated good removal capacities and accorded with both a pseudo-first-order model and a pseudo-second-order model. More importantly, the present obtained hydrogel materials can be prepared in an eco-friendly way, which could be applied on a large scale for removing organic dyes in the process of disposing of sewage.

2. Materials and methods

2.1 Materials

Silver nitrate and poly(acrylic acid) (PAA, M.W. ∼ 2000) were obtained from Aladdin Chemicals (Shanghai, China) and Alfa Aesar Chemicals (Tianjin, China), respectively. Congo red (CR), rhodamine B (RhB), and methylene blue (MB) were obtained from Tianjin Kaitong Chemicals. The deionized (DI) water used in all steps was obtained via a Milli-Q water purification system with 18.2 MΩ cm⁻¹ resistivity.

2.2 Preparation of composite hydrogels

Firstly, aqueous PAA solutions with different concentrations (1.0, 0.5, and 0.25 g mL⁻¹) were prepared and kept under magnetic stirring at room temperature for several hours until the PAA had fully dissolved in deionized water. Aqueous silver nitrate solutions with different concentrations (0.125, 0.100, and 0.075 g mL⁻¹) were also prepared and completely dissolved by magnetic stirring. Then, the AgNO₃ solutions were mixed with the PAA solutions at a volume ratio of 4 : 3, respectively. After magnetic stirring for several minutes, only two mixed systems, i.e. PAA solutions with 1.0 and 0.5 g mL⁻¹ and AgNO₃ solution with 0.125 g mL⁻¹, demonstrated gel states after 5 minutes. So the minimum gelation conditions were obtained and chosen to investigate in the next steps. In addition, the color of the as-prepared hydrogels was observed to become gradually blue-black from light grey with increasing time. The as-prepared hydrogels at different preparation times (10 min, 30 min, 2 h, 5 h, 10 h, and 24 h) were freeze-dried at −50 °C for 2–3 days.

2.3 Catalytic properties tests

The catalytic properties of the present composite hydrogels were investigated via absorption spectroscopy. Catalytic experiments were conducted at room temperature. Typically, during adsorption experiments, about 1 mL of fresh PAA–Ag/AgNPs composite hydrogels was added to 100 mL of dye solutions with different concentrations (RhB, 4 mg L⁻¹; MB, 10 mg L⁻¹; CR, 30 mg L⁻¹). After dispersion under magnetic stirring in the dark for 30 min, the mixed system was irradiated by UV light with 15 cm distance from light source (high pressure mercury lamp; 365 nm, 100 W) to liquid surface. The concentrations of dyes were analyzed at different time intervals. The supernatant solutions after centrifugation were measured by a 752-type UV-vis spectrometer (Sunny Hengping, Shanghai, China) at the absorbance wavelengths of 497 nm (CR), 664 nm (MB), and 554 nm (RhB) using pre-established calibration curves.

2.4 Characterization

The measured composite hydrogels were treated by lyophilizing at −50 °C via an FD-1C-50 type lyophilizer (Beijing Boyikang Experimental Instrument Co., Ltd, China) to remove the water component over 2–3 days. The as-obtained dried samples were attached to different solid substrates for spectral and structural measurement, respectively. The nanostructures of all hydrogels were investigated by field-emission scanning electron microscopy (FE-SEM, S-4800II, Hitachi, Japan) with the accelerating voltage of 1.5 kV and transmission electron microscopy (TEM, HT7700, Hitachi High-Technologies Corporation, Japan). Energy-dispersive X-ray spectroscopy (EDXS) was used to characterize the chemical compositions of samples via an Oxford Link-ISIS X-ray EDXS microanalysis system attached to a transmission electron microscope. FTIR spectra were obtained by Fourier infrared spectroscopy (Thermo Nicolet Corporation) via the KBr tablet method. X-ray diffraction (XRD) was performed on an X-ray diffractometer equipped with a Cu Kα X-ray radiation source and a Bragg diffraction setup (SMART LAB, Rigaku, Japan). Thermogravimetry and differential scanning calorimetry (TG-DSC) were done with a NETZSCH STA 409 PC Luxxsi multaneous thermal analyzer (Netzsch Instruments Manufacturing Co., Germany) in argon gas. The Brunauer–Emmett–Teller (BET) surface area and pore size distributions were obtained by N₂ adsorption and desorption using an ASAP 2010 system at 77 K. X-ray photoelectron spectroscopy (XPS) was done via a Thermo Scientific ESCALAB 250Xi using 200 W monochromated Al Kα radiation. Both survey scan and individual high-resolution scan peaks were characterized.

3. Results and discussion

Firstly, the PAA–Ag/AgNPs composite hydrogels were obtained by coordination reaction between PAA molecules and Ag(I) ions as well as a self-assembly process with different preparation times, which determine the nature of supramolecular gels after chemical reaction. The fabrication process of the present composite hydrogels was moderate and eco-friendly with utilization of pure water as solvent. Photographs of the as-obtained composite hydrogels with different preparation times are presented in Fig. 1. It was clearly demonstrated that the color of the as-obtained hydrogels finally became dark blue from a milky state with different preparation times varying in the range 10 min to 24 h. This experimental phenomenon suggested that more silver nanoparticles appeared with increasing time. At the same time, the gel state became more stable owing to the complete coordination of carboxyl groups in PAA with Ag(I) ions to enhance the mechanical properties of the composite hydrogels.

Fig. 1 Photographs of as-obtained PAA–Ag/AgNPs composite hydrogels with different preparation times: (a) 10 min; (b) 30 min; (c) 2 h; (d) 5 h; (e) 10 h; (f) 24 h.

Next, the obtained composite hydrogels were characterized by morphological and spectral methods. The morphologies of the as-prepared hydrogels were investigated by SEM. As shown in Fig. 2a–f, the SEM pictures of the obtained hydrogels presented porous 3D network structures formed by coordinate bonding between carboxyl groups in PAA and silver ions. And it is clearly observed that the sizes of holes in the nanostructures become larger with increasing reaction time. TEM pictures (Fig. 2g–h) demonstrated the appearance in the composite gels of homogeneously and uniformly deposited silver nanoparticles with diameters mainly in the ranges of 10–15 nm and 25–35 nm with preparation times of 30 min and 24 h, respectively. Moreover, the chemical components in the obtained composite hydrogels were elucidated by EDXS (Fig. 2i). The chemical signatures of Ag, Cu, and C elements were observed. It should be noted that the Cu peaks originated from the TEM grid substrate. From the EDXS data it can be concluded that silver nanostructures were obtained. At the same time, the as-formed composite hydrogels with porous structure and large surface area provide the important potential for the removal of organic dyes owing to the special nanostructures.²⁰

Fig. 2 SEM images of as-obtained PAA–Ag/AgNPs composite hydrogels with different preparation times: (a) 10 min; (b) 30 min; (c) 2 h; (d) 5 h; (e) 10 h; (f) 24 h. (g and h) TEM images of the composite hydrogels with preparation times of 30 min and 24 h. (i) Typical EDXS of composite hydrogels prepared at 24 h.

Fig. 3 demonstrates the nitrogen adsorption–desorption behaviours and pore size distribution data via the Barrett–Joyner–Halenda (BJH) method. Both curves exhibit typical isotherms over most of the relative pressure P/P₀ range, indicating the existence of numerous micro- and mesopores in the composite gels.^24–26 The specific surface area for all sample materials was calculated by the BET method. The relative physical properties obtained from N₂ adsorption of the as-prepared composite materials are shown in Table 1. The specific surface areas of the as-obtained PAA–Ag/AgNPs composite hydrogels with preparation times of 30 min and 24 h were 109.13 and 106.37 m² g⁻¹, respectively. The large specific surface areas of both samples could be considered to be the result of porous nanostructures in the obtained PAA–Ag 3D composites, which can effectively accommodate the objective dye molecules and increase the contact probability of dye molecules with catalyst material. BJH investigation for microporous distribution was also undertaken. The average pore sizes of both composite gels are within the range of 2–8 nm, respectively.

Fig. 3 The nitrogen isotherms (a) and pore size distributions (b) of the as-obtained PAA–Ag/AgNPs composite hydrogels with different preparation times calculated by the BJH method.

Table 1 Physical properties deduced from N₂ adsorption at 77 K of as-prepared PAA–Ag/AgNPs composite hydrogels

Sample	BET surface area (m^2 g^−1)	Average pore diameter (nm)	Average pore volume (cm^3 g^−1)
Gel-24 h	106.37	2.362	0.123
Gel-30 min	109.13	2.126	0.129

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In addition, Fig. 4 shows the comparative FT-IR spectra of pure PAA and all the as-obtained composite hydrogels with different preparation times. The pure PAA exhibits the major characteristic peak at 1721 cm⁻¹ assigned to the carboxyl group.^27,28 In all the composite hydrogels with different reaction times, one obvious change is the appearance of a broad peak centered at 3472 cm⁻¹. Another change is the increase in the size of the peak at 1622 cm⁻¹ and a new peak at 1529 cm⁻¹. These changes indicated the PAA molecules could react with silver ions to form coordinated structures. Moreover, it is well known that the structural changes and identification of the obtained organized nanostructures in composites are usually studied with the XRD technique. For the present system, the XRD patterns of pure PAA and the composite hydrogels with different preparation times were obtained, as shown in Fig. 5. The pure PAA curve showed two broad diffraction peaks centered at 19.1° and 37.4°, which are due to the main chain and side chain in PAA molecules.¹² However, for the as-obtained composite hydrogels, the shifted peaks appeared at positions of 21.4° and 34.3°. The obvious changes obtained indicated the coordination process between carboxyl group in PAA molecules and silver ions, which regulated the steric configuration of PAA and self-assembly into more organized nanostructures. At the same time, the composite hydrogel with reaction time of 24 h showed obvious characteristic diffraction peaks at 2θ of 38.12°, 44.28°, and 64.43°, which are consistent with those for the Ag standard card (JCPDS card no. 04-0783, space group Fm3̄m(225)) and correspond to the characteristic (111), (200), and (220) Miller indices of Ag, respectively.

Fig. 4 FT-IR spectra of pure PAA and as-obtained PAA–Ag/AgNPs composite hydrogels with different preparation times.

Fig. 5 XRD patterns of pure PAA and as-obtained PAA–Ag/AgNPs composite hydrogels with different preparation times.

Fig. 6 shows the thermograms (TGs) of pure PAA and the as-obtained composite hydrogel with preparation time of 24 h conducted under argon atmosphere. The TG curve of pure PAA shows that the mass loss started in the temperature range 180–500 °C because of the decomposition of organics.¹⁴ In addition, the data of the PAA–Ag/AgNPs hydrogel with preparation time of 24 h indicate that the mass loss started in the temperature range 75–165 °C for the loss of hydrated water molecules. The existence of hydrated water molecules suggests that the as-prepared hydrogel was not dried completely. In the next region, the organic components in the obtained hydrogel gradually decomposed. At temperatures higher than 560 °C, the masses of pure PAA and the composite hydrogel remained essentially constant. At temperatures above 560 °C, the quality retention ratios of the pure PAA and the hydrogel were 16.46% and 42.8%, respectively. Compared with the data for pure PAA, the mass value of the composite hydrogel was obviously higher, which can be attributed to the existence of silver nanoparticles and silver ions in the hydrogel due to the coordination process.

Fig. 6 TG curves of pure PAA and as-obtained PAA–Ag/AgNPs composite hydrogel with preparation time of 24 h.

The surface composition of the as-obtained PAA–Ag/AgNPs hydrogel with preparation time of 24 h was characterized by XPS, as shown in Fig. 7. First, the survey XPS spectrum showed three main characteristic peaks, namely C(1s), O(1s), and Ag(3d). Then, the detailed deconvolution of XPS peaks was investigated. For the Ag(3d) spectrum (Fig. 7b), the Ag(3d_5/2) and Ag(3d_3/2) peaks appeared at 368.1 and 374.1 eV, respectively. It should be noted that the characteristic splitting of the 3d doublet of Ag is 6.0 eV, suggesting the successful synthesis of metallic silver components in the present composite hydrogel.^21,22,29 Moreover, the deconvolution of the Ag(3d) peak was also investigated. The Ag(3d_5/2) peak could be deconvoluted to two Gaussian component peaks after a Shirley backline correction. Two deconvoluted peaks of Ag(3d_5/2) appeared at 368.4 and 367.6 eV for Ag and AgO, respectively. The deconvolution results indicated that 63% of the silver was in the Ag⁰ chemical state, with 37% of the silver in the Ag²⁺ chemical state. This also confirmed the formation of silver nanoparticles in the composite hydrogel. In addition, the O(1s) peak (Fig. 7c) can be deconvoluted into two Gaussian component peaks after a Shirley backline correction. One component peak at 532.0 eV could be assigned to the C–O band and oxygen of surface-bound OH⁻ in the obtained composite material. Another deconvoluted O(1s) peak at 533.0 eV was mainly due to the oxygen in water molecules in the composite and/or adsorbed on the hydrogel surface. The present results indicated that the obtained composite hydrogel was still porous, which was helpful to remove organic dye from aqueous environments. Finally, Fig. 7d demonstrates deconvolution of the C(1s) peak of the obtained composite hydrogel. The component peak at 284.0 eV was mainly assigned to the contribution of C–C and C–H bonds.

Fig. 7 Survey XPS spectra of as-obtained PAA–Ag/AgNPs composite hydrogel with preparation time of 24 h (a). Deconvolution of XPS peaks: (b) Ag(3d); (c) O(1s); (d) C(1s).

The catalytic capacity of the as-prepared composite hydrogel materials was also demonstrated for the application of photocatalytic degradation of dye solutions (CR, MB, and RhB, respectively). The degradation behavior was measured by putting the as-obtained PAA–Ag/AgNPs hydrogels in different dye solutions. Fig. 8 shows the obtained dye degradation rate versus time plots for the three dye solutions used with the present obtained composite hydrogel-24 h as catalyst. The dye degradation rates seem to reach an equilibrium state for MB within 80 min, and at approximately 50 min for the other two dyes, indicating the high efficiency of the present as-obtained PAA–Ag/AgNPs hydrogels as dye removal catalysts. On the other hand, without UV light irradiation, the degradation behaviors of the obtained hydrogels were significantly reduced. In addition, catalytic kinetic experiments of the as-prepared PAA–Ag/AgNPs hydrogel-24 h were performed, and the results are shown in Fig. 9. Different absorbance wavelengths (CR, 497 nm; MB, 662 nm; RhB, 554 nm) were chosen to measure the residual dye concentrations at various time intervals. The composite hydrogel demonstrated a rapid removal process with equilibrium times of approximately 80 min for MB and 50 min for CR and RhB. The obtained equilibrium time seems sufficient for efficient applications in photocatalytic fields. The achieved kinetic behaviors can be mainly attributed to the organized 3D porous network structures formed by coordinate bonding and electrostatic interactions as well as highly dispersed Ag nanoparticles as active photocatalytic sites.

Fig. 8 Photocatalytic properties of the PAA/Ag hydrogel-24 h for degradation of different dyes: (a) MB; (b) RhB; (c) CR.

Fig. 9 Catalytic kinetics curves of the PAA/Ag hydrogel-24 h for degradation of different dyes: (a, c and e) pseudo-first-order kinetics; (b, d and f) pseudo-second-order kinetics.

In addition, classical kinetic models were demonstrated to show the obtained degradation results as the following formulas:

The pseudo-first-order model:

The pseudo-second-order model:

where q_e and q_t are the amount of dye degraded (mg g⁻¹) at equilibrium and time t, respectively, and the k₁ and k₂ values represent the kinetic rate constants.²⁶ From the kinetic data in Table 2, it can be concluded that the catalytic process in the present research systems is in good accordance with both the pseudo-first-order model (R² > 0.9789) and the pseudo-second-order model (R² > 0.9855) with good correlation coefficients for the three dyes. In consideration of the above described experimental data, it is interesting to note that during the photocatalysis process, the dye molecules were transferred from the solution to the active interface in the obtained composite catalysts and anchored with offset face-to-face orientation via π–π conjugation and hydrogen bonding between the dyes and the silver nanoparticles.³⁰ When UV irradiation was applied to the surface of composite hydrogels, the photo-excited electrons quickly moved to the AgNPs' surfaces and next reacted with adsorbed O₂ molecules to produce active ˙O₂ radicals.³¹ Thus, the obtained composite hydrogels could generate more useful electrons and holes, and generate more superoxide anions and/or peroxide species.³² As a result, dyes are fragmented into H₂O, CO₂ and other species. Owing to the process of electron transfer, charge recombination is suppressed in the present obtained PAA–Ag/AgNPs composite and hence largely enhances the efficiency of photocatalytic behaviors.

Table 2 Kinetic parameters for the removal of three dyes by the composite hydrogel-24 h at 298 K (experimental data obtained from Fig. 9)

	Pseudo-first-order model			Pseudo-second-order model
	qe (mg g^−1)	R^2	K1 (min^−1)	qe (mg g^−1)	R^2	K2 (g mg^−1 min^−1)
CR
Light avoidance	10.21	0.9971	6.77 × 10^−2	12.70	0.9926	5.29 × 10^−3
UV light	13.00	0.9985	1.10 × 10^−3	15.17	0.9971	7.66 × 10^−3
RhB
Light avoidance	1.96	0.9976	7.14 × 10^−2	2.50	0.9966	2.68 × 10^−2
UV light	3.41	0.9921	5.37 × 10^−2	4.52	0.9855	1.04 × 10^−2
MB
Light avoidance	7.65	0.9789	5.27 × 10^−2	9.34	0.9978	6.90 × 10^−3
UV light	11.39	0.9906	5.04 × 10^−2	14.08	0.9951	3.58 × 10^−3

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Considering the above described results and the self-assembly nature of the present obtained hydrogels, a reasonable explanation for the formation of PAA–Ag/AgNPs hydrogels is proposed, as shown schematically in Fig. 10. The silver component in the present obtained hydrogels has two important roles. One role is that a small quantity of silver ions are coordinated with carboxyl groups in PAA molecules, causing self-assembly to form hydrogels. Another role is that the in situ formed AgNPs act as a good photocatalyst for degrading organic dye pollutants. First, after AgNO₃ was added to the PAA solution, at short reaction times only a small quantity of silver ions were coordinated with carboxyl groups and the organized self-assembly process formed hydrogels with many free carboxyl groups in the PAA molecules. The hydrogel at this stage is a so-called composite hydrogel with partial coordination. With increasing time, more silver ions reacted with carboxyl groups to enhance the mechanical properties of hydrogels and make the gels “harder”. Meanwhile, the color changed to dark blue from a milky state with preparation times in the range 10 min to 24 h. This indicated the formation of more silver nanoparticles with increasing time mainly due to the oxidation by air and decomposition of silver nitrate itself. This stage can be called a “composite hydrogel with complete coordination”. Here, the as-obtained composite hydrogel at 24 h has better mechanical properties through coordinate bonding and electrostatic interactions. So the as-prepared composite hydrogels show new clues for the investigation of silver nanoparticle-containing composite hydrogels as catalyst materials for dye degradation in wastewater treatment.

Fig. 10 Schematic illustration of the formation of the present PAA–Ag/AgNPs hydrogels.

4. Conclusions

In summary, we investigated the preparation and self-assembly of PAA–Ag/AgNPs composite hydrogel materials and measured their catalytic capacity for removing three model dyes. Morphological and spectroscopy data demonstrate that the coordination between PAA and silver ions plays an important role in forming organized porous network nanostructures in hydrogels. At the same time, the in situ formation of silver nanoparticles in composite hydrogels is crucial to dye degradation for wastewater treatment. In addition, the good eco-friendly gelation process and organized 3D nanostructures of the as-prepared composite hydrogel materials show they have potential application as highly efficient catalysts in the field of wastewater treatment.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21473153), the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (No. YQ2013026), the Support Program for the Top Young Talents of Hebei Province, the Post-graduate's Innovation Fund Project of Hebei Province (No. 2016SJBS009 and 2015XJSS054), the China Postdoctoral Science Foundation (No. 2015M580214), the Scientific and Technological Research and Development Program of Qinhuangdao City (No. 201502A006), and the Open Funding Project of the National Key Laboratory of Biochemical Engineering (No. 2013KF-02).

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Footnote

† Caili Hou and Kai Ma contributed equally to this work.

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