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Sodium alginate-g-poly(acrylic acid-co-2-hydroxyethyl methacrylate)/montmorillonite superabsorbent composite: Preparation, swelling investigation and its application as a slow-release fertilizer
⁎Corresponding author. mmkazaam@yahoo.com (Mohamed M. Azaam) Mohmed.azam@science.tanta.edu.eg (Mohamed M. Azaam)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Sodium alginate-g-poly(acrylic acid-co-2-hydroxyethyl methacrylate)/montmorillonite superabsorbent composites (SACs) were prepared by graft copolymerization of acrylic acid (AA) and 2-hydroxyethyl methacrylate (HEMA) onto sodium alginate (Na-Alg) in the presence of montmorillonite (MMT) using N,N′-methylenebisacrylamide (MBA) as a crosslinker and potassium persulfate (KPS) as an initiator in aqueous solution. The composite structures were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Thermal Gravimetric Analysis (TGA) and Scanning Electron Microscope (SEM). The effect of % initiator, crosslinker amount and clay ratio on the swelling capacity was investigated. The results indicated that the highest swelling capacity of the composites in distilled water was 752 g/g by using 1.25% KPS and 0.06% MBA, 75% AA, 6.25% HEMA and 12.5% Na-Alg. Different samples were loaded with urea to evaluate their release potentials, and the release was studied by measuring the conductivity. The amount of urea release increased with increasing MMT amount.
Keywords
Superabsorbent
Sodium alginate
Swelling
Composites
Montmorillonite
Slow-release
Agrochemical
1 Introduction
Superabsorbent composites (SACs) are three-dimensional network structure. They can absorb huge amount of water, saline solution up to thousand folds of the composites dry weight without losing their structure. This is due to the presence of ionic groups in these classes of materials (Sadeghi et al., 2012a; Mirdarikvande et al., 2014; Shi et al., 2011; Kaith et al., 2010; Tang et al., 2014). The high swelling properties of these materials make them ideal for use in various applications. Drug-delivery systems, agriculture, water treatment and healthcare are common examples for those applications (Gils et al., 2009; Xie and Wang, 2010; Rashidzadeh et al., 2014; Olad et al., 2017; Tang et al., 2013; Zhu et al., 2014).
In spite of high advantages of the superabsorbent hydrogels, there is a limitation of their production due to high cost. To defeat this limitation, inorganic fillers were introduced to lower their cost (Marandi et al., 2011; Fu et al., 2016). The formulation containing inorganic fillers are called superabsorbent composites. Montmorillonite (MMT) is one of the most appropriate clay minerals for production the superabsorbent composites due to the small particle dimension and high intercalation properties. Montmorillonite is a layered aluminosilicate containing reactive hydroxyl groups on the surface. The superabsorbent composites are produced by the reaction of MMT, monomer and natural polymers (Fu et al., 2016; Pourjavadi et al., 2007a).
Much attention has been made towards superabsorbent composites containing natural biodegradable polymers such as alginate to increase the degradability of the composite. Generally, this is usually achieved by graft copolymerization of vinyl monomers to natural polymers (Xie and Wang, 2010; Mirdarikvande et al., 2014; Gharekhania et al., 2017). Sodium alginate is a natural and biodegradable polymer from renewable sources which has high capacity of gelatinization (Sadeghi and Yarahmadi, 2011a).
Many SACs based on alginate were prepared previously to attain highly absorbance capacity. For example, the copolymerization of Na-Alg, sodium acrylate, sodium p-styrene sulfonate and attapulgite yielded absorbance capacity of 500 g/g in distilled water (Wang et al., 2013a). In addition, Na-Alg-g-Poly(Na-acrylate)/Kaolin superabsorbent polymer composite showed swelling ability 308 g/g (Pourjavadi et al., 2007b).
The renaissance of arid and desert surroundings, minimizing irrigation water, decreasing the death time of plants, and advancement plant enlargement are examples for the significance of superabsorbent polymers in agriculture (Maziad et al., 2016).
Also, SACs were used as slow-release fertilizers to improve fertilizer preservation in soil (Ganguly and Das, 2017; Sukriti et al., 2017; Maziad et al., 2016; Zhou et al., 2018). The release of fertilizers is due to their electric charges and the presence of pores in the 3D gel network. Superabsorbent composites modify themselves into water-loaded gel after the contact with water and then they engrossed urea which could be slowly released out with exchange of free water between gel and solution (Xie et al., 2013).
In the current work, SACs were synthesized using sodium alginate (Na-Alg), 2-hydroxyethyl methacrylate (HEMA), acrylic acid (AA) and montmorillonite clay (MMT). The influence of some parameters on swelling capacity of the composite was performed using SPSS 13 software for analysis of variance (ANOVA). Descriptive statistics such as mean and standard error were calculated for optimized parameters. The release of urea from the produced composites was also investigated.
2 Experimental
2.1 Materials
Sodium alginate (Na-Alg) (chemical grade, MW 14,000–132,000) was purchased from Fisher Scientific, UK. Acrylic acid (AA), 2-hydroxyethyl methacrylate (HEMA) and N,N′-methylenebisacrylamide (MBA) were obtained from across organics company, New Jersey (USA). Potassium persulfate (KPS), Urea, NaOH and HCl were obtained from El-Nasr Pharmaceutical Co. for Chemicals, Egypt. Montmorillonite was purchased from southern clay company, USA.
2.2 Characterization techniques
FT-IR spectra were recorded by Bruker, TENSOR 27–Series, KBr Pellet, (400–4000 cm−1). XRD spectra were recorded by GNR APD-2000 at room temperature using Cu-K radiation (λ = 0.1541 nm) (generator setting of 40 kV and 30 mA) (2θ = 2–15°). TGA was obtained by Shimadzu’s TGA-50 (with temperature rate 10.0 degree/minute). The morphology of the samples was determined using scanning electron microscopy JSM6510 JEOL 30 Kv, with coating the composites with a thin layer of gold under reduced pressure. Conductivity meter was obtained from HANNA instruments, woonsoket RI USA, made in ROMANIA.
2.3 Synthesis of superabsorbent composites
A series of composites using different quantities of sodium alginate, crosslinker, initiator, monomers and MMT were prepared by the subsequent method:
Na-Alg (0.5–2) g was dissolved in distilled H2O (20 mL) in a four-necked flask set with a mechanical stirrer. The reaction was carried out under nitrogen atmosphere and kept at a water bath (70 °C). Suitable amounts of MMT (0–2 g) were added with stirring to form a solution. After stirring 10 min, AA (60% neutralized) (6 g), certain amount of HEMA (0–2 g), MBA (0.005–0.02 g) and KPS (0.08–0.2 g, dissolved in 5 mL of water) were added. The reaction was incessantly stirred for 2 h. Excess ethanol (200 mL) was added to dewater and then removed by decantation. The product was scissored to small pieces. Once more, ethanol (100 mL) was added and kept for 24 h. Lastly, the composite was filtered, dried in oven at 80 °C for 10 h, grinded, and stored away from moisture, heat and light. This reaction was carried out by varying composition quantity as illustrated in Table 1.
Sample codes
Na-Alg (wt%)
MMT (wt%)
AA (wt%)
HEMA (wt%)
KPS (wt%)
MBA (wt%)
A1
12.5
0
75
0
1.25
0.06
A2
12.5
0
75
6.25
1.25
0.06
A3
12.5
0
75
11.76
1.25
0.06
A4
12.5
0
75
16.67
1.25
0.06
A5
12.5
0
75
21.05
1.25
0.06
B1
12.5
5
75
6.25
1.25
0.06
B2
12.5
10
75
6.25
1.25
0.06
B3
12.5
15
75
6.25
1.25
0.06
B4
12.5
20
75
6.25
1.25
0.06
C1
12.5
5
75
6.25
1.25
0.1
C2
12.5
5
75
6.25
1.25
0.125
C3
12.5
5
75
6.25
1.25
0.18
C4
12.5
5
75
6.25
1.25
0.25
D1
6.67
5
75
6.25
1.25
0.06
D2
17.65
5
75
6.25
1.25
0.06
D3
22.22
5
75
6.25
1.25
0.06
D4
26
5
75
6.25
1.25
0.06
G1
12.5
5
75
6.25
1
0.06
G2
12.5
5
75
6.25
1.5
0.06
G3
12.5
5
75
6.25
1.75
0.06
G4
12.5
5
75
6.25
2
0.06
2.4 Water absorbency measurement
The dry composite (0.1 g) was soaked in distilled H2O (500 mL) at room temperature for 24 h until the swelling equilibrium was reached. The unabsorbed H2O was removed from the swollen composite by filtering through a 70-mesh screen. The water absorbency (Qeq) was evaluated three replicate as follows: where m1 is the dry weight and m2 is the swollen weight.
2.5 Study variables affecting the swelling behavior
Different reaction variables as initiator, MMT clay, MBA, Na-Alg, HEMA amounts and pH of medium were studied by varying composition of each formula to obtain the best formulation as shown in Table 1.
2.6 Loading of urea onto Na-Alg-g-P(AA-co-HEMA)/MMT superabsorbent composite
This procedure was performed by inserting 0.1 g of series of dried samples [B1, B2, B3 and B4] into aqueous solution of urea (0.5 M) and left them to swell for 24 h. The swollen gels were dried at 60 °C.
2.7 Study of urea release from loaded superabsorbent composite in distilled water
To study the release of urea, the loaded gel with known weight (0.1 g, mesh ≤ 70) was placed in 100 mL of distilled water (the release medium) in enclosed bottle at room temperature. At different time, the released amount of urea was determined by measuring the conductivity of the release medium using a conductivity meter. The concentrations of urea in the solutions were determined by using previously built calibration curve.
3 Results and discussion
SACs derived from Na-Alg, HEMA, AA and MMT were prepared by graft copolymerization using MBA and KPS. Macroinitiators were existed from hydrogen removal from —OH groups of the Na-Alg. backbones using sulfate anion radicals that produced through decomposition of persulfate under heating. These macroradicals initiate the copolymerization of AA and HEMA onto MMT clay leading to the corresponding composite. A possible mechanism of the copolymerization was shown in Scheme 1. The structure of the composites was characterized by FT-IR, XRD, TGA and SEM. The effect of various parameters such as Na-Alg, HEMA, MBA, KPS, MMT amounts and pH of the medium on swelling ability was studied. Error bars shown in figures (6–10) were presented according to standard error using ANOVA Statistical analysis.
Preparation of superabsorbent composites (SACs).
3.1 Characterization of the prepared superabsorbent composites
The structures of the synthesized composites were investigated by FT-IR. Fig. 1 showed the FT-IR spectra of MMT (a), Na-Alg (b), Na-Alg-g-(AA-co-HEMA) (c), and the prepared composites (Na-Alg-g-P(AA-co-HEMA)/MMT) (d). The absorption bands at 1089 and 1046 cm−1 were due to the stretching vibration of C—OH groups of Na-Alg. These bands were noticeably weak after the formation of the composite (Fig. 1c, d). The new bands at 1734 and 1572 cm−1 were attributed to asymmetric stretching vibration of —COOH groups and —COO− groups that were confirmed by appearance of another peaks at 1448 and 1414 cm−1 (symmetric stretching vibration of —COO− groups). These peaks were observed in the spectrum of Na-Alg-g-(AA-co-HEMA) and Na-Alg-g-(AA-co-HEMA)/MMT (Fig. 1c, d), which confirmed that PAA and PHEMA chains were grafted onto the Na-Alg chains. Similar results were reported early (Zhang et al., 2007; Pourjavadi et al., 2007b; Zhu et al., 2015; Sadeghi, 2012b; Zhu et al., 2012; Hosseinzadeh et al., 2011).
FT-IR spectrum of (a) MMT, (b) Na-alginate, (c) Na-Alg-g-(AA-co-HEMA) (A2), (d) Na-Alg-g-(AA-co-HEMA)/MMT (B1).
In Fig. 1a, the distinguishing vibration bands of MMT (—OH stretch from Si—OH, —OH stretch from free H2O adsorbed on surface, —OH bending and Si—O stretch) were shown at 3631, 3449, 1638 and 1046 cm−1, respectively which disappeared after the reaction. This might be due to a strong chemical interaction between the Si—O and —OH groups of the clay and functional groups of AA and HEMA monomers during the copolymerization reaction (Marandi et al., 2011).
The structure of clay and Na-Alg-g-(AA-co-HEMA)/MMT composites containing 5, 10, 15, 20 wt% of clay was investigated by XRD (Fig. 2) which showed a strong peak at 2θ = 7.16° that corresponding to the distance of clay sheets with d spacing 12.33 nm (Fig. 2a). Fig. 2b–d showed that no diffraction peaks in composite containing 5, 10, 15 wt% of clay. This might be due to the complete exfoliation dispersion of the clay layers with organic network. At 20 wt% clay content (Fig. 2e), the peak corresponding to MMT was shifted to 6.78° due to the intercalation of MMT with organic network (Marandi et al., 2011).
XRD spectrum of (a) MMT, (b) Na-Alg-g-(AA-co-HEMA)/MMT composite with 5% MMT (B1) (c) composite with 10% MMT (B2), (d) composite with 15% MMT (B3), (e) composite with 20% MMT (B4).
The TGA of Na-Alg-g-P(AA-co-HEMA) and Na-Alg-g-P(AA-co-HEMA)/MMT superabsorbent composite of 5 wt% MMT was shown in Fig. 3. At 220 °C, the weight loss of Na-Alg-g-P(AA-co-HEMA) and Na-Alg-g-P(AA-co-HEMA)/MMT were related to the evaporation of water present in the samples, dehydration of saccharide rings and breaking of C—O—C bonds in the chain of Na-Alg. The weight losses at 220 and 550 °C were due to the decomposition of the carboxyl groups of PAA and PHEMA. It was obvious that the thermal stability of the superabsorbent composites was enhanced due to the introduction and the generation of chemical bonds between of MMT clay and the Na-Alg-g-P(AA-co-HEMA) polymeric network (Sadeghi, 2011b; Zhu et al., 2015; Sadeghi, 2012b).
TGA curves of (a) Na-Alg-g-P(AA-co-HEMA) (A2), (b) Na-Alg-g-P(AA-co-HEMA)/MMT superabsorbent composite, containing 5 wt% of MMT (B1).
The surface appearance of Na-Alg-g-P (AA-co-HEMA) and Na-Alg-g-P(AA-co-HEMA)/MMT superabsorbent composite containing 5 wt% of MMT was investigated using scanning electron microscopy (SEM) (Fig. 4). The morphology of the surfaces indicated the porous structure of composite. These pores permit water to be absorbed and interacted with hydrophilic groups on structure. As a result, the swelling ability was increased.
SEM images with different magnification of (a), (b) Na-Alg-g-P(AA-co-HEMA)/MMT (B1) and (c), (d) (Na-Alg-g-P (AA-co-HEMA) (A1).
3.2 Effect of variables on swelling behavior
3.2.1 Effect of HEMA content on the swelling behavior
It was observed that the swelling was increased by increasing HEMA content, afterwards absorbency was found to be decreased (Fig. 5). On increasing the amount of HEMA to 6.25 wt%, the swelling was increased to 752 g/g. However, further increase in HEMA content, the swelling was decreased. For example, at 11.76 wt% of HEMA content, the swelling decreased to 291 g/g. Also, at 21.05 wt% HEMA, the swelling decreased to 194 g/g. This might be due to the hydroxyl group (a weak hydrophilic group) which forms a hydrogen bond between two neighboring chains that causes shrinkage of the network. Similar observation was reported previously (Gils et al., 2009).
Effect of HEMA content on the swelling behavior.
3.2.2 Effect of clay content on the swelling behavior
The effect of montmorillonite content on the swelling properties was studied (Fig. 6). The results showed that the swelling was decreased on increasing the amount of MMT. For example, when the MMT amount was zero wt.%, the swelling was 752 g/g. On increasing MMT content to 5 wt%, the swelling was decreased to 263 g/g. Further increment in MMT to 20 wt%, the swelling was decreased to 100 g/g. This might be due to the interference of clay with the graft polymerization by acting as additional crosslinking agent or by preventing the growth of the polymer chains (Zhu et al., 2015; Zhu et al., 2012).
Effect of MMT content on the swelling behavior.
3.2.3 Effect of crosslinker content on the swelling behavior
A Crosslinker plays an important role in the synthesis of a superabsorbent composite due to prevention the dissolution of the hydrophilic polymer chains in an aqueous medium. It was found that when the amount of MBA was increased, the water absorbency was decreased (Fig. 7). For example, when the crosslinker ratio was 0.06 wt%, the swelling was 263 g/g. When the crosslinker ratio was increased to 0.25 wt%, the swelling decreased to 150 g/g. On increasing the crosslinker amount, the cavities between polymer chains in the composite were reduced, consequently, the amount of water retained in the composite cavities was decreased. This explanation is consistent with the previous work reported in the literature (Zhang et al., 2007; Zhu et al., 2015; Zhu et al., 2012; Hosseinzadeh et al., 2011; Wang and Wang, 2009; Soliman, 2016).
Effect of crosslinker content on the swelling behavior.
3.2.4 Effect of sodium alginate content on the swelling behavior
Fig. 8 showed that the water absorbency of the superabsorbent composite increased with increasing the amount of Na-Alg from 6.67 to 12.5 wt% and decreased with further increase in Na-Alg amounts. The highest swelling (263 g/g) was obtained with a Na-Alg content of 12.5 wt%. When the amount of Na-Alg was low, the monomer was excess in the reaction system. The excess AA, HEMA turned to be a homopolymer, which cannot contribute to the water swelling. The swelling was increased as homopolymer content was decreased and Na-Alg amount was increased at fixed crosslinking density. On the other hand, when of Na-Alg amount was above 12.5 wt%, the viscosity of Na-Alg solution was increased, the initiation efficiency was decreased, and the reactive sites cannot be sufficiently formed. As a result, grafting ratio was decreased resulting in a decrease of the water swelling. The explanation was reported by the other (Wang et al., 2013b; Wang and Wang, 2010; Hua and Wang, 2009; Zadeha, 2010).
Effect of Na-Alg content on the swelling behavior.
3.2.5 Effect of initiator content on the swelling behavior
The swelling was increased by increasing initiator amount then was decreased (Fig. 9). On increasing the amount of initiator from 1 wt% to 1.25 wt%, the swelling was increased from 174 g/g to 263 g/g. Further increase in the amount of initiator to 1.75 wt%, the swelling was decreased to 141 g/g. This increase might be attributed to the attack of the sulfate anion-radical to the Alginate chains. Then, the swelling was decreased due to the increase in terminating step reaction and increasing polymerization products which sequentially, enhancing crosslinking density and preventing expanding the network. Additionally, the absorbance of water was decreased due to an increase in the number of radical centers which leads to the production of low molecular weight polymers (Ghasemzadeh and Ghanaat, 2014; Xie et al., 2011a; El Hefnawy and Bader, 2013; Sadeghi et al., 2012c; Wang et al., 2017; Fu et al., 2016).
Effect of initiator content on the swelling behavior.
3.2.6 Effect of pH on the swelling behavior
The effect of pH of the aqueous medium on swelling capacity was investigated (sample A2) in medium ranging from pH 2.0–10.0 at room temperature. pH solutions were prepared by dilution of NaOH solution (pH 13.0) and HCl (pH 1.0) using distilled H2O to reach the desired basic and acidic pHs. No additional counter ions (cations) were existed by adding buffer solution because swelling of anionic superabsorbent is affected by ionic strength.
Fig. 10 showed that the swelling was increased by increasing pH from 2.0 to 4.0 and then was slightly decreased when pH is increased to 6.0. Further increasing pH, the swelling was sharply decreased. When the medium was acidic, pH 4, the swelling was increased because the carboxylate anions were protonated, repulsion forces were removed and H-bonding interaction between COOH groups occurred. At pH 6, part of the carboxylate groups were ionized and consequently the repulsion between COO− caused decrease in swelling. At pH >6, excess of Na+ was existed in swelling medium which shielded the carboxylate anions and decreased swelling.
Effect of pH of medium on the swelling behavior (A2). Error bars shown in figures (6–10) are presented according to standard error. Mean values with different superscript letters are significantly different at (p ≤ .05). a, b, c, d and e indicated the difference or similarity between experimental groups.
3.2.7 The swelling capacity of composites over time
The relation between the swelling capacity over time for the composite B1 (the highest swelling capacity after adding 5 wt% MMT) was studied to calculate the maximum equilibrium time (Fig. 11). The results showed that the maximum equilibrium time was attained after 45 min.
The relation between the swelling capacity of the composite B1 and time.
3.3 Release of urea from Na-Alg-g-P(AA-co-HEMA)/MMT
The release of agrochemical entrapped into network was happened after the penetration of water and the swelling of SACs, after that the diffusion alongside the aqueous pathways to the surface. The release performance was investigated in distilled water as shown in Fig. 12. The results indicated that the increase in release rate by increasing MMT content and decreasing swelling rate. This might be due a long time to exchange free water between network and those in external solution (Xie et al., 2011a,b). The release of urea was studied at pH 4, 10 (prepared from 1 N HCl and 1 N NaOH). The results showed that the fast release initially for 16 days followed by constant release. The initial fast release might be attributed to the release of urea loaded near the surface of SAC (Pourjavadi et al., 2009). Fig. 13 showed that there is slight difference in the release behavior in basic and in acidic medium.
The release behavior urea with different MMT content in distilled water.
The release behavior urea with 5% MMT at pH 4 and 10.
4 Conclusion
A novel series of superabsorbent composite was synthesized by graft copolymerization based on Na-Alg, HEMA, AA and MMT clay. The effect of reaction variables on the swelling capacity was investigated. The composite A2 with composition of 1.25% KPS and 0.06% MBA, 75% AA, 6.25% HEMA and 12.5% Na-Alg showed the highest swelling capacity (752 g/g). The release performance of urea was investigated in distilled water. The results showed that the fast release initially for 16 days followed by constant release and there is slight difference in the release behavior in basic and in acidic medium. These results encouraged us to test the produced SACs in agricultural application as water saving materials and nutrient retention and the results will be reported in a separate paper.
Acknowledgement
The authors extend their appreciation to the Science and Technology Development Fund (STDF) Egypt for funding this work through research project entitled “Superabsorbent polymer composite for agricultural applications” (Project ID: 5842).
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