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Pan, X., Zhuang, X. W., and Chen, S. W. (2017). "Rice husk char/poly-(acrylic acid-co-acrylamide) superabsorbent hydrogels: Preparation, characterization, and swelling behaviors," BioRes. 12(3), 4795-4809.

Abstract

A series of novel rice husk char/poly-(acrylic acid(AA)-co-acrylamide(AM)) superabsorbent hydrogels were synthesized by graft copolymerization. The effects of the rice husk char (RHC) loading on their miscibility, shapes, and chemical structures were studied, and their swelling behaviors, kinetics, and pH response were evaluated. During the preparation, RHC reacted with acrylic acid. The RHC at lower loads (< mass ratios of RHC and AA of 1%) was scattered well within the polymer matrix, but an excessive load might result in the formation of large agglomerates. The swelling capacity and swelling rate of the hydrogels first were both increased with the rising RHC loading to 1% and then declined with further RHC loading. The superabsorbent hydrogel containing 1% RHC had the highest water absorbency (869 g/g in deionized water and 97 g/g in 0.9% NaCl solution).


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Rice Husk Char/Poly-(Acrylic Acid-co-acrylamide) Superabsorbent Hydrogels: Preparation, Characterization, and Swelling Behaviors

Xin Pan, Xiao-Wei Zhuang,* and Shun-Wei Chen

A series of novel rice husk char/poly-(acrylic acid(AA)-co-acrylamide(AM)) superabsorbent hydrogels were synthesized by graft copolymerization. The effects of the rice husk char (RHC) loading on their miscibility, shapes, and chemical structures were studied, and their swelling behaviors, kinetics, and pH response were evaluated. During the preparation, RHC reacted with acrylic acid. The RHC at lower loads (< mass ratios of RHC and AA of 1%) was scattered well within the polymer matrix, but an excessive load might result in the formation of large agglomerates. The swelling capacity and swelling rate of the hydrogels first were both increased with the rising RHC loading to 1% and then declined with further RHC loading. The superabsorbent hydrogel containing 1% RHC had the highest water absorbency (869 g/g in deionized water and 97 g/g in 0.9% NaCl solution).

Keywords: Rice husk char; Superabsorbent hydrogels; Swelling behavior; pH Sensitivity

Contact information: Zhejiang Provincial Key Laboratory of Biological and Chemical Utilization of Forest Resource, Zhejiang Forestry Academy, Box 399, Hangzhou, Zhejiang 310023 China;

* Corresponding author: zhuangxw@aliyun.com

INTRODUCTION

Superabsorbents are the hydrophilic polymers that physically and/or chemically crosslink into three-dimensional networks (Bao et al. 2011). Generally, a superabsorbent can absorb abundant water or other aqueous solutions. Because of superior properties compared to traditional materials such as sponges, they are widely used in hygiene (Kosemund et al. 2009), agriculture (Guo et al. 2005; Liang et al. 2009), drug delivery (Reddy et al. 2011; Oun et al. 2014), and water purification (Zheng et al. 2010; Wang et al. 2011). The research on the use of superabsorbents as water and fertilizer managing materials for the renewal desert environment (Banedjschafie and Durner 2015) and slow release fertilizers (Li et al.2016; Yamamoto et al. 2016) has attracted great attention, and encouraging results have been observed, as they can reduce irrigation water consumption, lower the death rate of plants, and increase plant growth rate (Parvathy et al. 2014).

Superabsorbents are biocompatible, biodegradable, renewable, and nontoxic (Guilherme et al. 2005; Murthy et al. 2006; Pourjavadi et al. 2006). Many substances have been loaded into the polymer matrix, such as cellulose (Yoshimura et al. 2006; Chang et al. 2010), starch (Pourjavadi et al. 2010; Zou et al. 2012), chitosan (Mahdavinia et al. 2004; Zhang et al. 2007), and complex substances (Perez and Francois 2016; Qi et al. 2016) because of their biodegradability, abundance, low generation costs, and multifunctional character.

Rice husk chars (RHCs) result from the thermal treatment of rice husks. The rice husks are rich in cellulose and lignin, which is very suitable for the production of chars. Thermal processing leads to the pore development within the RHCs, which make RHCs a potential adsorption material (Shen et al. 2014). The RHCs also contain plenty of silica and porous silica, which are also widely applied in adsorption, separation, and catalysis (Witoon et al. 2008; Jullaphan et al. 2009), so RHCs have large specific surface areas (Paethanom and Yoshikawa 2012). RHCs have been added into superabsorbents, where they contributed to strong methylene blue adsorption (Azevedo et al. 2017). Thus, superabsorbents with RHCs can be used for wastewater treatment (Hafida et al. 2015), recovery or containment of marine crude oil spills (Nguyen and Pignatello 2013), and soil amendment (Ekebafe et al. 2013).

The annual production of RHC was very large in China. In this work, RHCs were chosen and used to prepare RHC/poly-(acrylic acid-co-acrylamide) (RHC/P) superabsorbents through graft polymerization. Also, the swelling behaviors were compared. The new preparation route for RHC/P would reduce the production costs and improve the properties (e.g. thermal stability and swelling at varying pHs).

EXPERIMENTAL

Materials

RHC and reagents

The RHCs (average particle size = 74 μm) were obtained from the Zhejiang Provincial Key Laboratory of Biological and Chemical Utilization of Forest Resources (Hangzhou, China).

Other materials used in this study included ammonium pursulfate (APS) and N,N’-methylenebisacrylamide (NMBA), acrylic acid (AA) (distilled under reduced pressure before use), acrylamide (AM) (all from Tianjin Chemical, Tianjin, China), and NaOH (Chengdu Kelong Chemical, Chengdu, China).

All the substances were analytically pure, and all of the solutions were prepared in deionized water.

Preparation of RHC/P superabsorbent hydrogels

A series of RHC/P superabsorbent hydrogels were synthesized (Huang et al. 2012). Specifically, to 5 g of acrylic acid in a 250-mL four-necked flask (with a thermometer, a condenser, a stirrer, and a nitrogen line), an appropriate amount of NaOH solution was added slowly under an ice bath, until reaching 60% molar neutralization. The resulting solution was heated and kept at 60 ºC for 30 min under a nitrogen atmosphere.

Into the above solution, acrylamide (0.83 g), NMBA (1 mg/mL, 1.2 mL), APS (2 mg/mL, 2.9 mL), and deionized water (17.5 mL) were slowly added under continual stirring. The authors designed four loads of RHC (0.0025 g, 0.005 g, 0.01 g, and 0.025 g) in the blends, that corresponded to the RHC and AA mass ratios of 0.5%, 1%, 2%, and 5%, respectively (the same below). The water bath was heated to 70 ºC and kept for 3 h until complete polymerization.

The resulting product RHC/P was cut into pieces, which were then dried at 70 ºC under vacuum to a constant weight. The PAA (AA-co-AM) superabsorbent hydrogels were also prepared in the above way except for the addition of RHC. All of the samples were milled and sieved to 40-mesh to 80-mesh.

Methods

Fourier transform infrared spectroscopy (FTIR)

The samples were spectroscopically analyzed on a Nicolet 6700 FTIR meter (Thermo Scientific, Madison, USA) within 4000 cm-1 to 400 cm-1 over 36 scans at a resolution of 2 cm-1. The test samples were vacuum-dried and prepared by the KBr-disk method.

Thermogravimetric analysis (TGA)

The thermal stability was studied on an STA 409 PC Luxx thermal analyzer (Netzsch-geraetebau GmbH, Schulstr, Germany) with a temperature range = 30 ºC to 900 ºC, a heating rate at 10 ºC/min, flow rate of 40 mL/min, and a dry nitrogen purge.

Surface morphology

All samples were fixed on aluminum stubs and coated with gold. Then micrographs were explored on an S-3400N scanning electron microscope (Hitachi, Tokyo, Japan).

Specific surface area measurements

The Brunauer-Emmett-Teller specific surface area (BET) was determined by N2 physisorption using an ASAP 2020 automated system (Micromeritics, Norcross, USA). Then 0.5 to 0.9 g of sample was degassed in the ASAP 2020 at 160 C for 4 h prior to the analysis, followed by N2 adsorption at -196 C. BET analysis was carried out for a relative vapor pressure of 0.01 to 0.3 at -196 C.

Measurement of water absorption and swelling kinetics

Specifically, a dry superabsorbent hydrogel (ca. 0.10 g) was soaked in 500 mL of deionized water at room temperature for 4 h, until reaching the swelling equilibrium. The swollen sample was then filtered through a 100-mesh screen and separated from the unabsorbed water. The water swelling ratio (W; g/g) of the hydrogel was calculated as follows,

W = (WsWd) / Wd (1)

where Ws and Wd were the weights of the swollen and dry sample (g), respectively. Each sample was tested in triplicate.

The swelling kinetics was measured. The dried samples (each 0.10 g) were soaked in excessive deionized water. The swollen samples were isolated using a mesh screen and weighed at a preset interval. This procedure was repeated until a constant weight was reached. The water absorbency at any time was calculated from Eq. 1.

Swelling at varying pHs and in saline solutions

Any external solution was regulated to the required pH using 1 mol/L NaOH or HCl solution. The same method was used as the measurement of water absorbency in deionized water.

The swelling in salt solution was measured as follows. A sample (0.10 g) was poured into 250 mL of 0.9% (W/V) NaCl solution and then at a certain time interval, the solution swollen ratio was determined by Eq. 1. The determination condition used was the same as for the measurement of balanced water absorbency in deionized water.

RESULTS AND DISCUSSION

FTIR

Figure 1 shows the FTIR spectra of the RHC, RHC/P, and PAA. The RHC shows three peaks at 1604 cm-1, 1100 cm-1, and 798 cm-1, that corresponded to the stretching of COO-, Si-O, and P-O, respectively (Fig. 1a). The PAA showed peaks at 1569 cm-1, 1455 cm-1, 1408 cm-1, and 1325 cm-1, which revealed the presence of COO- (Fig. 1c). Moreover, the grafting of acrylic acid (RHC-g-PAA) made the absorption band of Si-O shift from 1100 cm-1 to 1050 cm-1 and resulted in the appearance of some new peaks of PAA (Fig. 1b). Compared with PAA, the spectrum of RHC-g-PAA showed some new peaks (Fig. 1b), including the Si-O stretching of RHC (1050 cm-1), P-O stretching of RHC (783 cm-1), C=O band (1677 cm-1), and the associated –OH stretching vibration of the hydroxyl groups (3440 cm-1). The FTIR results suggested the occurrence of a reaction between RHC and acrylic acid during the polymerization.

Fig. 1. IR spectra of (a) RHC, (b) RHC/P, and (c) PAA

TGA

Figure 2 depicts the TGA curves of the superabsorbent hydrogels, RHC and RHC/P with varying RHC contents.

Table 1 lists the thermal decomposition temperatures (TDTs) of 10% and 50% weight loss and char yields. The char yield of RHC was 70.31%, so there were no data of TDT of 50% weight loss for RHC. Clearly, the thermal stability of RHC was much more than that of P(AA-co-AM). So, the RHC/P has a higher TDT than that without RHC, which indicated that the thermal stability could be enhanced by the introduction of RHC particles into the polymer networks. These findings were attributed to the heat barrier effect of RHC. Furthermore, the TDTs of hydrogels rose with the increase of RHC content, which was possibly ascribed to the dispersion effect on the thermal stability.

Fig. 2. TGA curves of PAA and RHC/P with various RHC content at a heating rate of 10 C /min

Table 1. TGA Data of Prepared Samples

Morphology

Because the water absorbency of a hydrogel is affected by its porosity (Bao et al. 2011), its microstructural morphology is a key property to be studied. Figure 3 shows the SEM images of dried PAA, RHC, and RHC/P. Clearly, the PAA hydrogel had a very smooth cross-section, which might have been closely related to its crosslinking polymer networks. The RHC/P hydrogels that contained 0.5% and 1% RHC (mass ratios of RHC and AA) both show a very compact cross-section morphology (Figs. 3c and 3d). This indicated the homogeneous dispersion of RHC within the polymer matrix and the presence of RHC-matrix interfacial interaction. In comparison, the RHC-doped sample has an undulant and coarse surface, which facilitates the water penetration into the polymeric network, and then may improve the water absorbency of the resulting superabsorbent (Pourjavadi et al. 2007; Hua and Wang 2009).

When the amounts of RHC increased to 2% and 5%, the hydrogels had very different cross-sections (Figs. 3e and 3f). Clearly, the RHC and matrix micro-phases were separated at a large scale, making the hydrogel structures relatively loose, which might have been because the RHC aggregated in the matrix during the reactions (Huang et al. 2012).

a                                                                     b

c                                                                   d

e                                                                  f

Fig. 3. Scanning electron micrographs of PAA (a), RHC (b), and RHC/P containing 0.5% (c), 1% (d), 2% (e), and 5% (f) RHC

Specific Surface Area

The adsorption/desorption isotherms of the samples obtained under conditions described above are shown in Fig. 4. The RHC exhibited a Type (I) isotherm. Because of the specific surface area of PAA and RHC/P was relatively small, the PAA and RHC/P did not exhibit typical isotherms.

Table 2 shows the specific surface area of the RHC and the RHC/P superabsorbent hydrogels that contained different RHC contents. The specific surface area of PAA was smallest. The specific surface area of the samples increased with the increase of the content of RHC. This facilitated the water penetration into the polymeric network and improved the performance of water absorbency.

Fig. 4. N2 adsorption/desorption isotherms of RHC (a), PAA (b), and RHC/P containing 0.5% (c), 1% (d), 2% (e), and 5% (f) RHC

Table 2. N2-BET Specific Surface Area of Prepared Samples

Swelling Behaviors of Superabsorbent Hydrogels

Figure 5 shows the swelling capacities of the RHC/P superabsorbent hydrogels that contained different RHC contents in deionized water. Clearly, the swelling ratios of the hydrogels rose with the increase of the RHC loading to 1% and then decreased.

The RHC/P that contained 1% RHC had the largest swelling capacity in deionized water compared with PAA (869 g/g vs. 679 g/g), and this noticeable improvement might have been mainly because the monomer was superfluous in the reaction system when the amount of RHC was low. The monomer (AA) turned out to be a homopolymer, which did not benefit the water absorbency. When the crosslinking density is constant, the homopolymer content decreases with the rise of RHC content (Finkenstadt and Willett 2005).

The hydrophilic groups in the polymer networks were intensified by the presence of surface functional groups (e.g. COO- and C=O) in RHC. Moreover, due to the nanostructure of RHC, the homogeneous dispersion of RHC within the matrix might have affected the microstructures of the polymer networks and thereby influenced the swelling capacity. Moreover, the synergetic intermolecular interactions between the RHC and the polymer networks contributed to the absorbance of water or other fluids, which improved the swelling capacity. However, the swelling capacity of RHC/P decreased clearly when the amount of RHC was above 1%, which might have been because the grafting ratio and molecular weight of the grafted PAA chains both decreased (Huang et al. 2012). Another possible reason was that the excessive RHC tended to aggregate and weakened the RHC-network synergetic interactions, as confirmed by the SEM images (Figs. 3e and 3f).

Fig. 5. Swelling capacities of RHC/P superabsorbent hydrogel with various RHC in deionized water

The RHC in the matrix also affected the water diffusion rate during the swelling. Figure 6 shows the swelling rates in deionized water of the RHC/P that contained different RHC amounts. The swelling rates of all of the RHC/P were higher from 0 min to 60 min and the water absorbencies of RHC/P (containing 0%, 0.5%, 1%, 2%, and 5% RHC) were 510 g/g, 618 g/g, 676 g/g, 289 g/g, and 192 g/g, respectively. Then the swelling rates in the deionized water slowly dropped and a swelling equilibrium was achieved after approximately 125 min.

The swelling rates of the RHC/P hydrogels with < 1% RHC were slightly higher than PAA at the initial stage. The swelling rates of the hydrogels were enhanced after the introduction of RHC into the polymer network because the presence of surface COO- and C=O on RHC made the polymeric networks more affinitive towards water molecules. Another reason might have been that the higher porosity of RHC provided more free volume for water penetration into the polymeric network. In contrast, the swelling rates seemingly decreased at the initial stage when the amount of RHC was above 2%. One of the possible explanations was that the aggravation of excessive RHC and the weakened RHC-network synergetic interactions decreased the swelling rates.

Fig. 6. Swelling rates of PAA and RHC/P superabsorbent hydrogels with various RHC in deionized water

Swelling kinetics can be expressed using the Voigt-based viscoelastic model (Kabiri et al. 2003; Irani et al. 2013) according to Eq. 2,

St = P (1 – e – t / r) (2)

where St (g/g) is the swelling capability at time t (min), P (g/g) is a power parameter indicating the theoretical balance of water absorption, and r (min) is the rate parameter (time required to reach 63% of its ultimate swelling). To calculate r, the equation with slight modification and plotted Ln [1 – (St / P)] versus t was used. The slope P reflects lower water adsorption and a smaller r reflects a higher swelling rate (Wang and Wang 2009).

Results show the rate parameters for the superabsorbent hydrogels (0%, 0.5%, 1%, 2%, and 5%) were 51.23 min, 44.71 min, 44.99 min, 55.90 min, and 56.82 min, respectively, and their P-values were 679 g/g, 744 g/g, 869 g/g, 452 g/g, and 277 g/g, respectively. These data suggested that introducing a small amount (< 1%) of RHC into the superabsorbent hydrogels could improve the swelling rate and water absorbency.

Effect of pH and Salt Solution on Water Absorbency

Figure 7 shows the swelling behaviors of RHC/P hydrogels containing different RHC contents at varying pHs. Clearly, the RHC/P hydrogels containing < 1% RHC always had higher water absorbency than the PAA. When the amount of RHC was above 1%, the water absorbency of RHC/P under varying pH was weakened visibly, which was similar to the RHC in deionized water. All samples were kept nearly constant from a pH of 6 to 8, which could be explained as a buffer action of –COOH and -COO- (Lee and Wu 1996). This was very dominant in the superabsorbents containing –COOH and -COO-. However, the buffer action disappeared after the addition of abundant acid or base.

Under abundant acid (pH<4), most of the carboxylate was protonated. The hydrogen-bonding interaction among carboxylate groups was strengthened and the additional physical crosslinking was generated. However, the electrostatic repulsion among carboxylate group was restricted. So, the network tends to shrink and consequently swelling values are decreased.

The reason for the swelling loss in the highly basic solutions (pH>10) was due to the “charge screening effect” of excess Na+ in the swelling media, which shields the carboxylate anions and prevents effective anion-anion repulsion (Bao et al. 2011).

Fig. 7. Swelling rates of PAA and RHC/P superabsorbent hydrogels with various RHC at different pHs

Figure 8 shows the swelling capacities of RHC/P superabsorbent hydrogels that contained different RHC amounts in 0.9% NaCl solution. The swelling ratios of the hydrogels declined sharply compared to the use of deionized water. In fact, this observation is common in the swelling experiments of polyelectrolyte hydrogels (Ostroha et al. 2004; Saunders et al. 2008). This may have been explained as follows: the charge screening effect caused by the counter ions (Na+) in the salt solution could clearly weaken the electrostatic repulsion and thereby decrease the osmotic pressure between the hydrogel networks and the external solution. However, the swelling kinetics of RHC/P in 0.9% NaCl solution were similar to those in deionized water.

Fig. 8. Swelling rates of PAA and RHC/P superabsorbent hydrogels with various RHC in 0.9% NaCl solution

CONCLUSIONS

  1. A series of rice husk char / poly-(acrylic acid-co-acrylamide) (RHC/P) superabsorbent hydrogels were synthesized by using APS as a free radical initiator in the presence of NMBA as a crosslinking agent. Due to the hydrogen bonds and possible covalent bonds between the RHC and polymer chains, a relatively lower content (< 1%) of RHC could be dispersed well within the polymer matrix, and it effectively enhanced the intermolecular interactions between the components.
  2. An excessive amount of RHC might form large agglomerates and weaken the interfacial interactions, resulting in the micro-phase separation between the components. Furthermore, the RHC greatly influenced the swelling behaviors and swelling kinetics of the superabsorbent hydrogel. The swelling capacities and swelling rates of the hydrogels increased with increased RHC content (< 1%), and then decreased with further increase in the RHC loading.
  3. The Voigt-based viscoelastic model was suitable to represent the swelling kinetics of the RHC/P superabsorbent hydrogel composites. The RHC/P that contained 1% RHC showed the highest swelling capacities (869 g/g in deionized water and 97 g/g in 0.9% NaCl solution), which were much higher than the PAA. The water absorbency for this hydrogel showed buffer action in a pH value from 6 to 8 that resulted from the –COO groups on the polymer chains.
  4. An SEM investigation revealed that the superabsorbent incorporated with RHC exhibited a coarse surface. The TGA implied that the introduction of RHC into the polymeric network was helpful for the improvement of thermal stability of corresponding superabsorbents. Therefore, the RHC/P superabsorbent hydrogels that contained suitable RHC content might have potential applications in many areas.

ACKNOWLEDGEMENTS

The authors are thankful for the support from the Scientific Research plan of the Zhejiang province (No. 2014C32039), and the Scientific Research group of the Zhejiang province Key laboratory of Biological, and Chemical Utilization of Forest Resource.

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Article submitted: January 12, 2017; Peer review completed: March 26, 2017; Revised version received: May 8, 2017; Accepted: May 9, 2017; Published: May 16, 2017.

DOI: 10.15376/biores.12.3.4795-4809