Rapid synthesis of a corncob-based semi-interpenetrating polymer network slow-release nitrogen fertilizer by microwave irradiation to control water and nutrient losses

2.1 Materials

CC was obtained from Shihezi, Xinjiang Province (China). The sample was washed and dried at 105 °C in an oven for 8 h and then smashed and passed through 100 mesh sieves for further use. AA was provided by Tianjin Fuyu Fine Chemical Co., Ltd. AA was distilled under reduced pressure before use to remove the polymerization inhibitor and stored in a brown reagent bottle. PVP (M_w = 10,000 g/mol) was obtained from Sigma–Aldrich. Urea, potassium persulfate (KPS) and N,N′-methylenebis acrylamide (MBA) were purchased from Tianjin Fuchen Chemical Reagent Co., Ltd. KPS was recrystallized twice from distilled water to remove the impurities before use. All reagents were of analytical grade and all solutions were prepared with distilled water. The Bent samples were collected from XiaZiJie deposits in Xinjiang of China, and the Bent samples has a composition (%, by mass) of Al₂O₃ 13.06, SiO₂ 64.62, Na₂O 2.66, K₂O 2.43, CaO 1.92, MgO 2.38, Fe₂O₃ 4.93, TiO₂ 0.59, MnO 0.26, P₂O₅ 0.18, and an ignition loss of 6.20 (Sun et al., 2007). The MMT content was 93.0 g based on 100 g Bent, the cation exchange capacity was 98.4 mmol based on 100 g Bent and the swelling index was 89.5 mL/g.

2.2 Synthesis of SRFs

In this procedure, a mixed solution containing 10.0 g of AA (neutralization degree 70%), 2.0 g of Bent, 17.0 g of urea, 0.03 g of MBA, and 2.96 mL of 0.1 mol/L KPS was introduced to a 250-mL round bottom flask containing 0.5 g of CC and 2.5 g of PVP previously dissolved in 30 mL of distilled water. Then the flask was transferred to a condenser-equipped MW reactor (MCR-3, Keli, China) with 2450 MHz frequency and a maximum 800 W output power. Gradient programming was used to perform the polymerization in the MW cavity. The program initially consisted of agitation for 10 min until the mixture was uniform, followed by irradiation of the mixed solution at 320 W for 4.5 min to complete polymerization under constant stirring. All of these procedures were performed in a N₂ atmosphere. The resulting gel products were carefully washed thoroughly for five times with 100 mL of 70% ethanol in order to remove water-soluble oligomer, uncross-linked polymer, and unreacted reagents. Finally, the resulting products were oven dried at 70 °C until a constant weight, milled, sifted, and then stored for future use.

2.3 Characterization

FTIR spectra of samples were recorded in the wave number range of 4000–400 cm⁻¹ using a Nicolet Avatar 360 FTIR spectrometer (USA). X-ray diffractograms of samples in the 2θ range of 5–90° were analyzed with a D8 Advance X-ray diffractometer (Germany) using Cu kα (λ = 1.54056 Å) radiation and operating at 40 kV, 40 mA. The surface morphology of samples was performed on a JSM-6700F scanning electron microscope (Japan) after coating the samples with gold film. TGA studies of samples were carried out using a STA 449F3 thermogravimetric analyzer (Germany) from 25 to 800 °C at a heating rate of 10 °C/min under flowing nitrogen. Uniaxial compression measurements on swollen fertilizer samples with dimensions of 10 mm × 10 mm × 10 mm were performed at 25 °C using an Instron 3366 universal material testing machine (USA) at a constant crosshead speed of 3 mm/min. The compressive modulus was obtained from the initial (straight line) linear slope of the stress versus strain curve, and the results were the average of at least three measurements.

2.4 Swelling measurement

0.1 g of the dried samples was immersed into 250 mL of distilled water, various saline solutions (0.05 mol/L NaCl, KCl, NH₄Cl, MgCl₂, CaCl₂, AlCl₃, FeCl₃, and 0.05–0.25 mol/L NaCl, MgCl₂, AlCl₃) and solutions of various pH values of 2.0 to 12.0 at room temperature, respectively. After the swelling equilibrium was reached, the swollen samples were separated from the unabsorbed water through a 100-mesh stainless screen and weighed. The equilibrium water absorbency Q_eq (g/g) was calculated by Eq. (1).

2.5 Measurement of the largest WH% and WR% of soil with SRFs

The soil samples used for the experiment were sample A, 200 g of dry soil only; sample B, 200 g of dry soil mixed well with 2.0 g of samples; and sample C, 200 g of dry soil mixed well with 4.0 g of samples. Each sample was placed in a 4.5 cm diameter PVC tube. The bottom of the tube was sealed with 200-mesh nylon fabric and weighed (marked W₀). The soil samples were slowly drenched with distilled water to saturate the soil, and then the tube was weighed again (marked W₁). The WH% of the soil was calculated using Eq. (2).

After tested for the largest WH%, these tubes were weighed every three days (W_i), and the experiment was ended after 30 days. The WR% of the soil with SRFs was calculated by Eq. (3).

2.6 N leaching loss and migrate-to-surface performance

The effects of SRFs on N leaching loss and migrate-to-surface performance were investigated and carried out as follows. First, two types of fertilizers (conventional urea and CC-g-PAA/Bent/PVP/urea) with the same N content were mixed well with 120 g of soil in two separate pots. Subsequently, another 30 g of dry soil was placed evenly on top of each pot, and then the pots were placed in separate germination boxes. Each soil pot was gradually sprayed with 200 mL of water every 4 days, and the leachate corresponding to each pot was collected for 30 days, in which the amount of N was measured using a UV–vis spectrophotometer at a wavelength of 430 nm. After 30 days, a layer of top soil (1 cm depth) from the resulting system was transferred into 150 mL of deionized water, and the resulting suspension was stirred (100 rpm) for 30 min. After centrifugation (12,000 rpm), the amount of N in the supernatant was measured using the same method for N leaching loss to obtain the migrate-to-surface amount of N.

2.7 Release behavior and kinetics of SRFs in soil

1 g of SRFs was embedded into a nylon bag and then buried to a depth of 5 cm in 150 g of soil placed in a plastic cup, and incubated for different periods at room temperature. The moisture content of the soil was maintained at 30% by weighing periodically and adding distilled water if necessary throughout the experiment. The bag with remaining SRFs was collected after 1, 3, 5, 7, 10, 15, 20, 25, and 30 days of burial and estimated for the content of residual nitrogen by an elemental analysis instrument.

With the aim of revealing the slow release kinetics and mechanism in soil, Ritger-Peppas model was employed.

2.8 Pot experiments

The effects of SRFs on seed germination and early-stage seedling growth were investigated and compared with those of urea by growing cotton. First, different amounts of SRFs and primitive urea with the same N content were mixed well with 200 g of vermiculite (60% humidity) in separate pots and then placed in separate germination boxes. Subsequently, 10 cotton seeds were sown in each pot, and all replicates were incubated in an artificial climatic box with a 14 h light (30 °C)/10 h dark (25 °C) photoperiod and a light intensity of 400 μmol photons m⁻² s⁻¹ during the day. Seeds that germinated successfully were counted and observed. Plant growth variables, such as plant height, root length, fresh weight, and dry weight were measured after 30 days of seedling growth.

3.1 Mechanism of SRFs formation

The CC-g-PAA/Bent/PVP/urea was prepared via simultaneous graft copolymerization and semi-IPN technology. The possible mechanism was outlined in Scheme 1. Initially, sulfate anion radicals were generated from the decomposition of KPS under MW. These radicals extract hydrogen atoms from the hydroxyl groups of the CC backbone to form the active alkoxy radicals on the CC backbone. Then these macro radicals initiated the AA monomers that were in close vicinity of the active centers, resulting in chain initiation and the generation of a graft copolymer. During this procedure, parts of urea were reacted with AA and produced acrylamides. Furthermore, a crosslinked structure could form as a result of the presence of MBA and Bent. Finally, the linear PVP interpenetrated this network to form a semi-IPN structure and combined with it through hydrogen-bonding interactions.

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Scheme 1 The preparation scheme of CC-g-PAA/Bent/PVP/urea.

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3.2 Characterization

3.2.1

3.2.1 FTIR spectra

Fig. 1 illustrates the FTIR spectra of the samples. As compared with Fig. 1a, the characteristic absorption band of Bent at 3620 cm⁻¹ (stretching vibration of Al—OH and Mg—OH) disappeared, and the band at 1030 cm⁻¹ (stretching vibration of Si—O) and 463 cm⁻¹ (bending vibration of Si—O—Si) weakened in Fig. 1f–h, suggesting the reaction between —OH groups on the surface of Bent and AA occurred during the synthesis (Wen et al., 2016). In Fig. 1e–h, the characteristic absorption band of CC at 3442 cm⁻¹ (stretching vibration of —OH) weakened after the reaction, and the band at 1055 cm⁻¹ (stretching vibration of C—O—C) was still visible, indicating the graft polymerization between —OH on CC and AA took place (Amin et al., 2016). As shown in Fig. 1e–h, new absorption bands were detected at approximately 1715 cm^–1 (asymmetric stretching vibration of —COOH groups), 1558 cm⁻¹ (asymmetric stretching vibration of —COO⁻ groups), and 1449 and 1406 cm⁻¹ (symmetric stretching vibration of —COO⁻ groups), implying the grafting of the PAA chains onto the CC backbone (Tally and Atassi, 2015; Wen et al., 2016).

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Figure 1 The FTIR spectra of Bent (a), CC (b), urea (c), PVP (d), CC-g-PAA (e), CC-g-PAA/Bent (f), CC-g-PAA/Bent/PVP (g) and CC-g-PAA/Bent/PVP/urea (h).

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As shown in Fig. 1d, the absorption bands observed at 1434 cm⁻¹ (—C⚌O stretching bonded to N atom), 1373 cm⁻¹ (—CH deformation of cyclic CH₂ groups), 1284 cm⁻¹ (—C—N stretching vibrations of the PVP chains), and 2956 cm⁻¹ (—CH and —CH₂ stretching vibrations of the PVP chains) were characteristic absorption bands of PVP, and these bands still appeared in in Fig. 1g and h with slight shifts (Yiğitoğlu et al., 2014). Moreover, the C⚌O stretching of PVP at 1659 cm⁻¹ and the C⚌O stretching of —COOH groups at approximately 1715 cm⁻¹ shifted to 1678 cm⁻¹ with the addition of PVP (Fig. 1g and h), signifying that intermolecular hydrogen-bonding interactions were created between —COOH and C⚌O groups during the polymerization process (Tally and Atassi, 2015; Wang et al., 2011). The above analysis of FTIR information concluded that linear PVP polymer chains existed in the semi-IPN network and were combined with the CC-g-PAA network by hydrogen-bonding interactions.

Comparison of Fig. 1h with Fig. 1c revealed that characteristic bands of urea at 3440 and 3343 cm⁻¹ (asymmetric and symmetric stretching vibration of —NH₂ in —CONH₂) and 557 cm⁻¹ (bending vibration of N—CO—N) appeared in Fig. 1h, suggesting the involvement of urea in CC-g-PAA/Bent/PVP/urea (Wen et al., 2016). In addition, the new absorption bands at 1611 cm⁻¹ (stretching vibration of —C⚌O of acrylamide unit) and 1157 cm⁻¹ (bending vibration of —N—H) implied that acrylamides were formed by the condensation reaction between urea and AA during the synthesis (Li et al., 2015, 2016). In brief, some of N in the samples were simply embedded in the composite in their original form, and the others were transformed into amides and bonded to the polymer backbone.

3.2.2

3.2.2 XRD patterns

The XRD patterns of the samples are shown in Fig. 2. In the XRD patterns of CC-g-PAA, two crystalline reflections of CC at 2θ = 20.87° and 26.67° disappeared, and the reflection at 22.10° became broadened, which indicated that the graft polymerization of CC with AA occurred and the reaction weakened the ordered structure of CC (Xie et al., 2012a). In addition, the intense crystalline reflection of Bent at 7.67° disappeared, and the other characteristic reflections were still visible (Fig. 2e–g), demonstrating the Bent was incorporated in the polymer matrix and exfoliated during polymerization process, and then dispersed in the polymer matrix (Bortolin et al., 2013). This exfoliation and dispersion were conducive to the water-holding capacity of the soil samples (Likhitha et al., 2014). Additionally, the two distinct reflections of PVP at 2θ = 12.51° and 19.70° disappeared (Fig. 2f and g) after the reaction, suggesting that linear PVP chains were uniformly dispersed and interpenetrated the polymer matrix without coacervation. The main characteristic reflections of urea at 2θ = 22.41°, 24.75°, 29.50°, 31.79°, 35.67°, and 37.28° appeared in the CC-g-PAA/Bent/PVP/urea diffractogram, implying that some urea was successfully incorporated in the polymer matrix in its original form (Zhou et al., 2015).

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Figure 2 X-ray diffraction patterns of Bent (a), CC (b), PVP (c), CC-g-PAA (d), CC-g-PAA/Bent (e), CC-g-PAA/Bent/PVP (f) and CC-g-PAA/Bent/PVP/urea (g).

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3.2.3

3.2.3 SEM surface morphology

Fig. 3 displays the surface morphology of the samples. The CC showed a smooth and compact surface morphology (Fig. 3a), whereas CC-g-PAA displayed a slightly coarse surface with some wrinkles (Fig. 3b). As shown in Fig. 3c, the surface roughness of CC-g-PAA/Bent increased compared with CC-g-PAA, which was ascribed to the incorporation of Bent that increased the crosslinking density of the sample. CC-g-PAA/Bent/PVP displayed more undulant and coarse surface with the addition of PVP (Fig. 3d). This finding indicated that the inclusion of PVP improved the surface structure of the samples derived from the formation of intermolecular hydrogen bonds as disclosed by FTIR. The above information demonstrated that incorporating Bent and PVP is favorable to improve the surface and network structure of hydrogels, and the improved surface is convenient for the penetration of water into the polymeric network and is favorable to the enhancement of water absorption. As clearly shown in Fig. 3e, parts of the urea crystals were dispersed on the CC-g-PAA/Bent/PVP/urea sample surface. The elemental data from EDS (Fig. 3f) also verified that urea was incorporated into the polymer matrix. Furthermore, the SRFs evidently contained abundant elements, such as Na, Mg, Al, K, and Fe. These elements could also be released into soil and then contributed to improve soil fertility and promote plant growth.

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Figure 3 SEM micrographs of CC (a), CC-g-PAA (b), CC-g-PAA/Bent (c), CC-g-PAA/Bent/PVP (d) CC-g-PAA/Bent/PVP/urea (e), and EDX spectrum of CC-g-PAA/Bent/PVP/urea (f).

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3.2.4

3.2.4 TGA

The TGA diagrams of the samples are depicted in Fig. 4. The thermal decompositions of CC-g-PAA/urea and CC-g-PAA/Bent/PVP/urea both underwent four steps, and the decomposition rate of CC-g-PAA/urea was clearly faster than CC-g-PAA/Bent/PVP/urea. The first step from 25 to 250 °C with a 10.2% mass loss for CC-g-PAA/Bent/PVP/urea and with a 11.84% mass loss below 250 °C for CC-g-PAA/urea corresponded to the evaporation of water, the continuous vaporization and decomposition of urea, the generation of biuret and its decomposition as well as self-condensation (Wen et al., 2016). The second step within 250–380 °C with a 19.3% mass loss for CC-g-PAA/Bent/PVP/urea and with a 28.93% mass loss within 250–380 °C for CC-g-PAA/urea was attributed to the elimination of the water molecules from the two neighboring carboxyl groups of the polymer chains with the formation of anhydride, chain scission eliminating CO and CO₂, and the thermal decomposition of cellulose, hemicellulose, and a few lignin (Li et al., 2015; Sawut et al., 2014; Zhang et al., 2014a; Zhu et al., 2016). The third step between 380 and 550 °C with a 33% mass loss for CC-g-PAA/Bent/PVP/urea and with a 27.18% mass loss within 380–500 °C for CC-g-PAA/urea was ascribed to the disintegration of branches or the side chain groups of the graft copolymer, the main-chain decomposition of the polymer, and the destruction of the crosslinked network structure (Li et al., 2015; Sawut et al., 2014; Zhang et al., 2014a). The fourth step in the range of 550 to 800 °C with a 15.3% mass loss for CC-g-PAA/Bent/PVP/urea and with a 16.36% mass loss above 500 °C for CC-g-PAA/urea was related to the further decomposition of lignin and other residual organic matter (Zhang et al., 2014a; Zhu et al., 2016). In the end, the residual mass of CC-g-PAA/Bent/PVP/urea was about 22.18%, which was higher than that about 15.69% for CC-g-PAA/urea. The higher decomposition temperature range, slower decomposition rate, and lesser total mass loss of CC-g-PAA/Bent/PVP/urea indicated a higher thermal stability than CC-g-PAA/urea, which may be due to the introduction of Bent and the formation of semi-IPN structure. The above information indicated that the addition of Bent and the formation of semi-IPN obviously improved the thermal stability.

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Figure 4 TGA thermogram of CC-g-PAA/urea and CC-g-PAA/Bent/PVP/urea.

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3.2.5

3.2.5 Mechanical properties

The compressive stress-strain curves of swollen CC-g-PAA/urea and CC-g-PAA/Bent/PVP/urea are shown in Fig. 5. Results showed that the elastic region of CC-g-PAA/Bent/PVP/urea was reduced in comparison with CC-g-PAA/urea. Additionally, the compressive stress at fracture of CC-g-PAA/Bent/PVP/urea was greater than that of CC-g-PAA/urea. The compressive stress of CC-g-PAA/urea was 1.26 MPa at 58.8% fracture strain, while that of CC-g-PAA/Bent/PVP/urea increased to 6.18 MPa at 73.8% fracture strain. Moreover, the compressive modulus increased from 0.13 MPa for CC-g-PAA/urea to 0.52 MPa for CC-g-PAA/Bent/PVP/urea. The obvious improvement was ascribed to the addition of Bent formed a dense and rigid network structure, and the formation of the semi-IPN structure (Hu et al., 2014). As a result, the SRFs with high mechanical properties are potentially applicable to modern agriculture and horticulture.

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Figure 5 Compressive stress-strain curves of two fertilizer samples.

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3.3 Effects of environmental parameters on water absorbency of SRFs

3.3.1

3.3.1 Effect of saline solution

The effects of saline solution types and concentrations on the water absorbency of the sample are illustrated in Fig. 6a and b. Evidently, the water absorbency of sample in various saline solutions was lower than that measured in distilled water. This phenomenon was assigned to the decreased osmotic pressure difference that resulted from the charge screening effect of the additional cations, which induced a decline in the anion-anion electrostatic repulsions (El Salmawi and Ibrahim, 2011; Giri et al., 2016). As shown in Fig. 6a, the water absorbency of the sample in 0.05 mol/L saline solutions was in the order of K⁺ > Na⁺ > NH₄⁺ > Mg²⁺ > Ca²⁺ > Al³⁺ > Fe³⁺; namely, the water absorbency in monovalent saline solutions was higher than that in multivalent saline solutions at the same concentration. This finding was attributed to the multivalent cations forming complexes with the carboxylate group and to the multivalent saline solutions having a higher ionic strength than the monovalent saline solutions at the same concentration, which consequently decreased the water absorbency of the sample (Bardajee et al., 2012; Rashidzadeh and Olad, 2014; Tally and Atassi, 2015). In the case of multivalent cations, the water absorbency of the sample decreased when the cation charge increased from divalent (Mg²⁺, Ca²⁺) to trivalent (Al³⁺, Fe³⁺) because of the increase in network crosslink density (Rashidzadeh and Olad, 2014). Moreover, it was clearly shown that the larger the radius of the monovalent metal cations, the larger the water absorbency was (K⁺ > Na⁺), and the less the radius of the multivalent metal cation, the higher the water absorbency was (Mg²⁺ > Ca²⁺ and Al³⁺ > Fe³⁺) (Zhang et al., 2014a). Furthermore, the water absorbency in polyatomic monovalent cationic solution (NH₄⁺) was lower than that measured in single atom monovalent cationic solutions (Na⁺ and K⁺) (Zhang et al., 2014a). From Fig. 6b, the water absorbency decreased with increasing saline solution concentration. The reason for this fact was that the osmotic pressure difference decreased with increasing saline solution concentration, which in turn decreases the water absorbency (Zhou et al., 2013). Moreover, the SRFs developed in this work displayed higher salt resistance than that described in the literature (Li et al., 2015, 2016; Rashidzadeh et al., 2014). The swelling capacity of the SRFs in 0.15 mol/L NaCl was 78.1 g/g, which was higher than that of NaAlg-g-p(AA-co-AAm)/Clin/NPK (approximately 18 g/g) reported by Rashidzadeh et al. (Rashidzadeh et al., 2014). Furthermore, the swelling capacity of the SRFs in 0.10 mol/L NaCl was 94.32 g/g, which was higher than that of WSC-g-PAA/PVA/NP (58.96 g/g) developed by Li et al. (Li et al., 2015). The higher salt resistance of the as-prepared SRFs has significantly superiority over other SRFs.

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Figure 6 Effects of saline solution types (0.05 mol/L) (a), concentrations (b), and pH (c) on water absorbency of CC-g-PAA/Bent/PVP/urea.

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3.4 Largest WH% and WR% of soil with SRFs

The largest WH% and WR% of the soil with SRFs are depicted in Fig. 7. The largest WH% of soil samples A, B, and C was 28.5%, 53.8%, and 76.3%, respectively (Fig. 7a). It can be obviously seen that the largest WH% of the soil significantly improved with the addition of SRFs, and the water content increased with an increasing dosage of the samples in the soil (Lü et al., 2016). The reason for the fact was that the sample had excellent swelling capacities due to the exfoliation and dispersion of Bent and the formation of semi-IPN structure (Likhitha et al., 2014; Xie et al., 2012b). The largest WH% of the soil with the as-prepared SRFs was higher than that reported in the literature (Table 1). Consequently, the soil could hold much water during the irrigation or rainy period when the samples were applied to the soil; as a result, the soil moisture was efficiently improved and water consumption was reduced.

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Figure 7 The largest WH% (a) and WR% (b) of soil samples dosed with different amounts of CC-g-PAA/Bent/PVP/urea.

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According to Fig. 7b, the water evaporation ratio of the soil mixed with the SRFs was obviously slower compared with that of the soil without the SRFs. The water evaporation ratio of soil sample A reached 100% after 12 days, whereas that of soil samples B and C were 41.7% and 32.3%, respectively. Moreover, soil samples B and C still remained 5.6% and 20.3% water content even after 30 days, respectively. These results demonstrated that the addition of SRFs to soil could remarkably decrease water evaporation and enhance the WR% of soil, and the WR% of soil increased with increasing amount of SRFs in the soil. The reason was that the polymer matrix had excellent water absorbency and the absorbed water could be gradually released into soil with the reduction in soil moisture and subsequently absorbed by crops (Lü et al., 2016; Xie et al., 2012a; Yang et al., 2014). Meanwhile, the nutrients incorporated in the SRFs could also be gradually released into the soil with the water (Lü et al., 2016). Therefore, the soil added with the SRFs could simultaneously supply plants with nutrients and water, and consequently efficiently improve the utilization efficiencies of water and fertilizer at the same time. Hence, the SRFs may be potentially applied in agricultural fields.

3.5 N loss control performance of SRFs

The N loss control performance of SRFs is an important indicator for fertilizers used in agricultural fields. In many arid and semiarid areas, fertilizer nutrients in soil could be easily lost due to the high water evaporation effect and leaching caused by irrigation and rainwater (Zhou et al., 2015). This phenomenon not only caused low fertilizer utilization efficiency but also contributed to large economic losses and major environmental concerns. Therefore, controlling N loss is significant to improve the fertilizer utilization efficiency. Herein, the N loss control performance of the SRFs was investigated, and the results are illustrated in Fig. 8. The leaching loss of urea was 44.5% after 30 days, which was greater than that of the SRFs (13.2%). The reduced N leaching loss was related to the slow N release characteristic of the SRFs (Yang et al., 2013). Additionally, the SRFs possessed obviously higher migrate-to-surface loss control abilities (6.8%) compared with urea (20.5%). This result could be explained by the fact that the higher water retention ability of the SRFs reduced the water evaporation rate and consequently reduced the loss of N that migrated to soil surface (Zhou et al., 2015). The preceding analysis obviously demonstrated that the SRFs could greatly reduce both N leaching loss and migrate-to-surface loss compared with urea. Consequently, the application of SRFs could retain more fertilizer nutrients in soil, improve the N utilization efficiency, and promote plant growth.

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Figure 8 N leaching loss and migrate-to-surface performance of urea and CC-g-PAA/Bent/PVP/urea.

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3.6 Slow-release kinetics of SRFs in soil

The release profile of urea was not presented because almost all of the urea will be quickly dissolved after being added into soil and then released into soil. As reported by Xie et al., more than 98% of N in urea was released within 24 h (Xie et al., 2012a). About 10.4%, 23.8%, and 56.6% of the N in the SRFs were released within 1, 3, and 30 days, respectively (Fig. 9a), implying the SRFs had an excellent slow release property, and this property was significantly superior over urea. This result is reasonable on the basis of the three following factors. (1) The SRFs could be slowly swollen by soil solution and gradually converted into hydrogel after the SRFs were added into soil, and the hydrogel would act as a physical barrier to retard the diffusion of nutrients out of the polymer network (Lü et al., 2016). (2) The inclusion of Bent generated a tortuous path that complicated the path of nutrients through the network (Rashidzadeh and Olad, 2014). (3) The SRFs had adsorption and steric effects on the diffusion of nutrients after the SRFs being added into soil, which reduced the release rate of nutrients (Lü et al., 2016; Li et al., 2015). All these resulted in a good release behavior. The initial release rate was fast, while the later was slowed down after 7 days. This phenomenon was assigned to the embedded urea in the composites by their original form would firstly dissolve and release, while the amides would gradually dissolve and release with the increasing buried time through dynamic water exchange when the SRFs applied into soil (Li et al., 2015; Zhang et al., 2014b).

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Figure 9 The release behavior (a) and release kinetic (b) of CC-g-PAA/Bent/PVP/urea in soil.

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The cumulative release rate of the SRFs was 56.6% within 30 days, which was below 60%. So, the Ritger-Peppas model was applicable to explain the diffusional process. The fitting equation for CC-g-PAA/Bent/PVP/urea was divided into two stages (Fig. 9b). In Table 2, 0.43 < n < 0.85 in the first stage, indicating an anomalous transport involving both Fick diffusion and polymer chain relaxation (He et al., 2015; Wu et al., 2014). Furthermore, n < 0.43 in the second stage, suggesting that the release mechanism approaches Fick diffusion (He et al., 2015; Wu et al., 2014).

3.7 Effect of SRFs on seed germination and early stage seedling growth

The effect of SRFs on cotton growth was investigated, and the data are summarized in Table 3. The SRFs treatment exhibited a higher germination rate of 86.67 ± 5.77% as compared with the primitive urea treatment (66.67 ± 5.77%). Additionally, the significant differences in early-stage seeding growth were also observed; namely, the plant height, root length, fresh weight, and dry weight of cotton seedlings exposed to the SRFs increased by 37.76%, 33.48%, 44.07%, and 64.29% relative to the control urea, respectively. The digital photographs for comparison of cotton plants treated with pure urea and SRFs are presented in Fig. 10. This phenomenon was due to the ability of the SRFs to effectively control the loss of water and fertilizer nutrients, thereby allowing the continuous supply of sufficient fertilizer nutrients and water for the cotton growth (Yang et al., 2014; Zhou et al., 2015). Thus, the SRFs may be potentially applied in agriculture and horticulture for soil improvement and increased crop productivity.

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Figure 10 Digital photographs of the cotton plants treated with the different fertilizers: (a) urea and (b) CC-g-PAA/Bent/PVP/urea.

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3.8 Economic evaluation of SRFs

The result of the economic evaluation of SRFs is presented in Table 4. As can be seen the cost for the production of SRFs prepared was about 0.8489 $/kg, about two times higher than that of commercial SRFs, such as urea-formaldehyde (UF). However, several advantages could be observed for our SRFs in comparison with other fertilizers. The addition of SRFs to soil could effectively improve the water-retention behavior of soil, lower the irrigation frequency, reduce the loss of fertilizer nutrients, and improve the retaining ability of nutrients. Hence, the utilization of SRFs would benefit farmers, and is also beneficial to the protection of the environment, which may be enough to outweigh the cost.