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New Environment-Friendly Use of Wheat Straw in Slow-Release Fertilizer Formulations with the Function of Superabsorbent Lihua Xie, Mingzhu Liu,* Boli Ni, and Yanfang Wang Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China ABSTRACT: With the aim of improving fertilizer use efficiency and minimizing the negative impact on the environment, a new double-coated slow-release nitrogen and phosphorus fertilizer with water retention was prepared. Wheat straw was introduced into the formulations as the basic coating material. Specifically, poly(acrylic acid-co-N-hydroxymethyl acrylamide)/wheat straw superabsorbent composite was used as the outer coating, and wheat straw/sodium alginate blends was used as the inner coating. The degradation of the superabsorbent composite in the soil solution was studied. The impact of the content of wheat straw on the extent of degradation in cellulase solution was also examined. The superabsorbent composite synthesized under the optimal conditions showed super water absorbency and excellent degradability. The water-retention property and the nutrient slowrelease behavior of the product were investigated. The results revealed that the product with water-retention and slow-release capacity, being economical, nontoxic in soil, and eco-friendly, could find good application in agriculture and horticulture.

1. INTRODUCTION Slow-release fertilizer provides an effective means of overcoming the high waste, low use efficiency, and environmental problems associated with the use of conventional fertilizers.1,2 Coated fertilizer is the major category of slow-release fertilizer,3 which is prepared by coating granules of conventional fertilizers with various materials that ensure a slow release of nutrients to soil by diffusion through the pores or by erosion and degradation of the coatings.4−7 Recently, superabsorbent materials used as coatings in slow-release fertilizers have attracted more and more attention.8,9 Superabsorbents are loosely cross-linked hydrophilic polymers with network structure, which have the ability to absorb and retain large amounts of aqueous fluids, and the absorbed solution cannot be released even under certain pressure. On the basis of these properties, they have been widely used in agricultural and horticultural fields to reduce irrigation frequency and improve the physical properties of soil.10,11 However, the applications of superabsorbents in these fields have a fair share of problems such as high production cost, weak salt resistance, and poor biodegradability.12−14 In this study, crop residues and N-hydroxymethyl acrylamide were introduced to mitigate these problems. In one aspect, using crop residues as basic material, the product cost will be significantly reduced. In another aspect, N-hydroxymethyl acrylamide could cross-link polysaccharide materials and poly(acrylic acid) and form ether groups.15 The superabsorbent network consisting of ether groups degrade more easily than those cross-linked by N,N′-methylene bisacrylamide in common use. To the best of our knowledge, superabsorbents based on crop residues coupled with N-hydroxymethyl acrylamide have been scarcely reported. Wheat straw, as a byproduct of wheat production, offers advantages such as being cheap, abundantly available, renewable, and biodegradable. In addition, it contains abundant elements, such as K, Ca, Al, Na, Mg, Fe, P, Mn, Cu, and Zn © 2012 American Chemical Society

(see Table 1). Unfortunately, around 45−60% of wheat straw is disposed of by incinerating or discarding, which contributes to Table 1. Characteristics of WS content

amount (mg/kg)

K Ca Al Na Mg Fe Mn Cu Zn

8.4 ± 0.42 2.8 ± 0.14 0.75 ± 0.038 0.72 ± 0.036 0.62 ± 0.031 0.27 ± 0.014 0.031 ± 0.0016 0.011 ± 0.00055 0.0045 ± 0.00023

severe environment pollution.16 The main components of wheat straw are cellulose, hemicellulose, and lignin. There are various functional groups such as hydroxyl, carboxyl, phosphate, ether, and amino groups on them.17 The existence of these functional groups makes a good reason for wheat straw to be employed in the preparation of superabsorbent composite.18,19 In this study, wheat straw was not only used to synthesis superabsorbent composite but also employed as coating material. Furthermore, it acts as a humic acid precursor when embodied into soil. The adoption of wheat straw is expected to provide a new way for comprehensive utilization of straw. More importantly, it can reduce production cost and improve biodegradation property of superabsorbents. Attapulgite is a type of hydrated octahedral layered magnesium aluminum silicate with one-dimensional nanostructures and moderate Received: Revised: Accepted: Published: 3855

July 25, 2011 February 18, 2012 February 22, 2012 March 5, 2012 dx.doi.org/10.1021/ie2016043 | Ind. Eng. Chem. Res. 2012, 51, 3855−3862

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mm in diameter) in batches. The core (NP compound fertilizer granule) was obtained under water atomization and dried in an oven at 40 °C. Subsequently, the core with desired range of particle size was to be treated with coatings. WS/SA blends powder (bellow 110 mesh) as the inner coating was adhered to the core by atomized CaCl2 solution in the rotating disk. P(AAco-NHMAAm)/WS powder (below 110 mesh) as the outer coating was adhered to the surface of the granules and formed a compact coating. Finally, the coated granules were dried and screened to obtain the final product. 2.5. Fourier Transform Infrared (FTIR) Characterization. The samples were analyzed using a FTIR spectroscopy (Nicolet NEXUS 670 FTIR Spectrometer, USA) in the region of 4000−500 cm−1. Prior to the measurement, the samples were dried under vacuum until reaching to a constant weight. The dried samples were pressed into the powder, mixed with KBr powder, and then compressed to make a pellet for FTIR characterization. 2.6. Morphology Observation. The morphologies of the surface and the cross-section of the fertilizer granules were analyzed by scanning electron microscopy (SEM). The surface morphologies of wheat straw and the P(AA-co-NHMAAm)/ WS particles were also examined with SEM. The samples were coated with a layer of gold and observed in a JSM-5600LV SEM. 2.7. Determination of Water Absorbency of P(AA-coNHMAAm)/WS. The procedures of determination of water absorbency are as the following: 0.2 g of superabsorbents powder (40−90 mesh) was immersed into a certain amount of tap water and allowed to become swollen at the room temperature for 60 min. The swollen superabsorbents were filtered through an 80-mesh sieve to remove nonabsorbed water and weighed. The water absorbency (WA) was calculated using the eq 1:

cation exchange capacity, which has been extensively used as an agriculture carrier.20 In this work, we explored an attempt to prepare a slowrelease fertilizer with water-retention capacity, which was coated by double-layer formulations based on wheat straw. The use of wheat straw as basic materials for the slow release of nitrogen and phosphorus was evaluated. The degradation profile of poly(acrylic acid-co-N-hydroxymethyl acrylamide)/ wheat straw superabsorbent composite was investigated in the soil solution. The influence of the content of wheat straw on the extent of degradation in the cellulase solution was also examined.

2. MATERIALS AND METHODS 2.1. Materials. Acrylic acid (AA, chemically pure, Beijing Oriental Chemical Factory, Beijing, China) was distilled at reduced pressure before use. N-Hydroxymethyl acrylamide (NHMAAm, Tianjin chemical Regaent Co., China) was purified by recrystallization in chloroform. Wheat straws (WS, available from commercial sources) were chopped and dried at 105 °C in an oven for 8 h and then sieved through a 150 mesh screen. Cellulase (with an activity of 10 000 IU g−1, obtained in powdered form from the Aladdin chemistry Co. Ltd.) was stored in a refrigerator. Sodium alginate (SA, the viscosity of 2% solution is 3200 mPa·s at 25 °C) was obtained from Qingdao Haiyang Chemical Company. Raw attapulgite powder (APT, provided by Gansu APT Co. Ltd., China) was sieved with a 250 screen. Other agents were all of analytical grade and used directly as received, and all solutions were prepared with distilled water. 2.2. Preparation of P(AA-co-NHMAAm)/WS. A series of samples with different amounts of NHMAAm and WS were prepared by the following procedures. Typically, 2 mL of AA was put in a flask and then neutralized with 3.2 mL of sodium hydroxide aqueous solution (6 M) in an ice bath. After that, 0.021 g of NHMAAm, 0.0084 g of ammonium persulfate (APS), 0.526 g of WS, and 5 mL of distilled water were successively added to the partially neutralized AA solution under vigorous stirring. Then, the flask with reaction solution was sealed and ultrasonically treated at 20 °C for 30 min. Finally, the flask equipped with a stirrer, thermometer, and nitrogen inlet tube was put in a water bath, heated slowly to 60 °C, and maintained at this temperature for 3 h under nitrogen atmosphere. The resultant polymer was sheared, placed on a dish, and dried at 60 °C to a constant weight. The dried product was milled and screened. 2.3. Preparation of WS/SA Blends. Blends of WS and SA were prepared by the solution technique. In brief, the adopted procedure for preparation of the blends can be described as follows. In a typical experiment, 0.45 g of WS (90−150 mesh) was dispersed into 15 mL of 7 wt % NaOH/12 wt % urea/81 wt % water mixture with stirring for 2 h at room temperature. Meanwhile, 0.15 g of SA was added into 7.5 mL of the same mixture with stirring for 2 h at the ambient temperature. The WS and SA solutions were mixed rapidly and stirred for 6 h. The resultant mixtures were placed in a Petri dish and dried at 60 °C to a constant weight. The dried product was milled and sieved through a 110 mesh screen. 2.4. Preparation of Slow-Release Nitrogen and Phosphorus Fertilizer (SRF). The preparation of the SRF involves three steps. First, an amount of calcium biphosphate and APT powder (below 110 mesh) was mixed well. Then, the mixture was fed into a rotating disk with urea granules (1.0−1.3

WA =

M − M0 M0

(1)

where M and M0 denote the weight of the swollen and dried superabsorbents, respectively. 2.8. Degradation Studies of P(AA-co-NHMAAm)/WS. The degradation studies of P(AA-co-NHMAAm/WS superabsorbent composite with different content of WS were performed by incubating preweighed dried slices (9−10 mm in diameter and 0.4−0.6 mm in thickness) of superasorbents in 100 mL of enzyme buffer solution containing 10 IU mL−1 cellulase at constant pH (5.0) and temperature (40 °C). At the same time, a blank test in the buffer solution without the enzyme using the same sample was carried out. After the predetermined time period (4 d), the samples were removed from the solution by filtering through a silk sieve (300 mesh), washed repeatedly with distilled water, and then dried at 40 °C to a constant weight. The extent of degradation was calculated by the eq 2: Degradation(%) =

M 0 − Md × 100 M0

(2)

where M0 and Md are the initial and final weights (before degradation and after degradation, respectively) of the dry superabsorbents. To simulate natural conditions, the degradation of superabsorbents was also monitored in soil solution. Soil solution was obtained by the centrifugation method.21 In a typical 3856

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experiment, 100 g of soil was extracted with 500 mL of distilled water over 24 h at the ambient temperature, and the pH of the extracts was measured. Then, the sample was centrifugated at 10 000 rpm for 3 min. The degradation of superabsorbents in soil solution was conducted by incubating a preweighed dried slice (9−10 mm in diameter and 0.4−0.6 mm in thichness) of superasorbents in 50 mL of soil solution at the ambient temperature. After the predetermined time period, the samples were removed from the solution by filtering through a silk sieve (300 mesh), washed repeatedly with distilled water, and dried at 40 °C to a constant weight. The extent of degradation was calculated by the eq 2. 2.9. Component Analysis of SRF. Content of nitrogen in the SRF was determined by a 501 ammonia-selective electrode. The phosphorus content in the SRF was determined by an inductively coupled plasma (ICP) instrument (American TJA Crop., model IRISER/S). Prior to measurement, the samples were digested in strong sulfuric acid in the presence of a catalyst. 2.10. Slow-Release Behavior of SRF in Soil. To study the slow-release behavior of SRF in soil, the following experiments were carried out: 1 g of SRF granules was embedded into a nonwoven bag. The bags were buried into a container with 200 g of dry soil (below 18 mesh), 5 cm below the surface of soil. Throughout the experiment, water-holding ratio of the soil was maintained at 20% by weighing and adding tap water if necessary periodically. The bags with SRF granules were picked out after each incubated period (day 1, 3, 5, 10, 15, 20, 25, 30) and then dried at room temperature. Then, the fertilizer granules were removed from the bags and analyzed for the contents of nitrogen and phosphorus. 2.11. Measurement of the Largest Water-Holding Ratio of Soil. To study the effect of SRF on water-holding capacity of soil, different amounts of SRF were mixed with 200 g of dry soil (below 26 mesh) and placed in a 4.5 cm diameter PVC tube. The bottom of the tube was sealed with nonwoven fabric and weighed (marked W1). The soil sample was slowly drenched by tap water from the top of the tube until water seeped out from the bottom. When no water seeped at the bottom, the tube was weighed again (marked W2). A control experiment without SRF was also carried out. The largest water-holding ratio (WH %) of soil was calculated from the eq 3: (W2 − W1) × 100 WH% = 200

Scheme 1. Mechanism of the Reaction of P(AA-coNHMAAm)/WS

Figure 1. FTIR spectra of WS (a), P(AA-co-NHMAAm) (b), and P(AA-co-NHMAAm)/WS (c).

(3)

3420 cm−1 (hydroxyl stretching vibration), 1054 and 892 cm−1 (β-1,4-glycosidic bond), and 2921 and 2857 cm−1 (methylene), which were characteristic absorptions in cellulose structure. The band at 1728 cm−1 is ascribed to CO stretching from ketones, aldehydes, or carboxylic groups of lignin. The bands at 1632 and 1507 cm−1 are ascribed to the skeletal CC stretching vibrations in the aromatic rings bands of lignin.19 In the spectrum of P(AA-co-NHMAAm) (Figure 1b), the intense bands at 3429 cm−1 should be attributed to the stretching of N−H and O−H bonds. The bands at 1632 cm−1 are ascribed to the stretching CO bonds in AA and NHMAAm, respectively. The characteristic peaks at 1158−1031 cm−1 arise from the ether group O of P(AA-co-NHMAAm) providing evidence of a cross-linking reaction between two Nhydroxymethyl groups. Compared the FTIR spectrum of P(AA-co-NHMAAm)/WS (Figure 1c) with P(AA-coNHMAAm), additional peaks of WS have been observed. In

3. RESULTS AND DISCUSSION 3.1. Characterization of P(AA-co-NHMAAm)/WS Superabsorbent Composite. The possible mechanism of the reaction of P(AA-co-NHMAAm)/WS is shown in Scheme 1. The reaction includes three aspects: on the one hand, vinyl monomers could graft copolymerize to the cellulose skeleton on the wheat straw.22 On the other hand, N-hydroxymethyl acrylamide is a bifunctional molecule with an ethylene group and a hydroxymethyl group. Hydroxymethyl is a highly reactive group with a tendency to condense with another similar molecule with loss of water.23−25 In addition, hydroxymethyl groups could react with the free hydroxyl groups of the cellulose.23 FTIR spectra of WS, P(AA-co-NHMAAm), and P(AA-coNHMAAm)/WS are shown in Figure 1. According to the spectrum of WS (Figure 1a), the absorptions were observed at 3857

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addition, the absorption at 1158 cm−1 becomes stronger; it indicates that the ether group O formed between Nhydroxymethyl and hydroxyl of cellulose and the graft copolymerizatin of vinyl and cellulose. On the basis of the analyses of FTIR, the conclusion can be drawn that cross-linked copolymer of AA, NHMAAm, and WS was formed. Moreover, the surface morphology of the product was observed. The SEM micrographs of the wheat straw and the P(AA-co-NHMAAm)/WS are shown in Figure 2. The

AA from 0.50 to 3.50 wt %. As we know, the amount of crosslinker determined the cross-linking density. Apparently, higher NHMAAm content resulted in the generation of more crosslinking points, which caused the formation of an additional network and decreased the available free volume within the superabsorbent polymer network. This would make it more difficult for the network to be swollen by water, which was responsible for the decrease of water absorbency. When the weight ratio of NHMAAm to AA was lower than 0.50 wt %, the absorbency of superabsorbent composite was very low, because the lower cross-linking density with a smaller amount of crosslinking points would cause the increase of the soluble material. Similar observation was reported by Chen et al.27 and Li et al.28 and was in conformity with Flory’s network theory. 3.3. Effect of the Weight Ratio of WS to AA on the Water Absorbency of P(AA-co-NHMAAm)/WS. The effect of the weight ratio of WS to AA on the water absorbency was also studied. The results are indicated in Figure 4. There exists

Figure 2. SEM micrographs of the surface of the wheat straw (a) and the P(AA-co-NHMAAm)/WS (b).

comparison of Figure 2a,b revealed that the surface became coarse and porous after the graft copolymerazation reaction. It indicated that, after the reaction, the monomers were grafted on the cellulose skeleton, which resulted in a broad network and increased porous structure on the surface of the product. 3.2. Effect of the Weight Ratio of NHMAAm to AA on the Water Absorbency of P(AA-co-NHMAAm)/WS. According to Flory’s network theory,26 cross-linking density is an important swelling control element. Relatively small changes in cross-linking density can play a major role in modifying the properties of superabsorbents. The water absorbency as a function of the mass ratio of NHMAAm to AA was investigated for P(AA-co-NHMAAm)/WS superabsorbents, as shown in Figure 3. It can be seen that the water absorbency significantly decreased with the increase of the weight ratio of NNMBA to

Figure 4. Effects of the weight ratio of WS to AA on the water absorbency. (Reaction conditions: neutralization degree of AA, 65 mol %; weight ratio of APS and NHMAAm to AA in the feed is 0.4 wt % and 1.0 wt % respectively; reaction time, 3 h; reaction temperature, 65 °C).

a maximum in the dependence of water absorbency on the weight ratio of WS to AA. As the weight ratio increased from 0 to 15 wt %, the water absorbency increased continuously from 160 to 184 g/g. As the weight ratio increased from 15 to 25 wt %, the water absorbency slightly decreased, and when weight ratio further increased, the water absorbency sharply decreased. It was expected that the action of WS in polymer network could be interpreted as the following two aspects.19 On the one hand, the hydroxyl groups in the cellulose may react with initiator and liberate free radicals on which the graft polymerization would take place or the hydroxyl groups may react with the hydroxymethyl of NHMAAm. These actions would result in improvement of the superabsorbent polymer network. Thereby, when the weight ratio of WS to AA was lower than 15 wt %, with an increase in the amount of WS, graft efficiency increased and in turn resulted in the increase of water absorbency. On the other hand, some of the WS would be physically filled in the polymer network. When the weight ratio of WS to AA was more than 15 wt %, WS in network acted as additional network points. The cross-linking points of superabsorbent polymers

Figure 3. Effects of the weight ratio of NHMAAm to AA on the water absorbency. (Reaction conditions: neutralization degree of AA, 65 mol %; weight ratio of APS and WS to AA in the feed is 0.4 wt % and 10 wt %, respectively; reaction time, 3 h; reaction temperature, 65 °C). 3858

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increased with the increase of the content of WS, which resulted in a decrease in water absorbency. 3.4. Composition and Characteristics of SRF. The SRF was a yellow, round granule with a three-layer structure, of which the core (48 wt %) was NP compound fertilizer produced by a pan granulator with urea, calcium biphosphate, and attapulgite, the inner coating (41 wt %) was WS/SA blends, and the outer coating (11 wt %) was P(AA-coNHMAAm)/WS superabsorbent composite, respectively. 501 ammonia-selective electrode results showed that the nitrogen contents of the core, the inner coating, and the SRF were 21.6 ± 1.08, 24.6 ± 1.23, and 20.6 ± 1.03%, respectively. Inductively coupled plasma results showed that the phosphorus contents (shown by P2O5) of the core and SRF were 20.1 ± 1.01 and 9.90 ± 0.0495%, respectively. The average diameters of the core and SRF granules were 1.9 ± 0.2 and 2.8 ± 0.2 mm, respectively. The ingredients of SRF were analyzed using an FTIR spectrophotometer. The spectra are shown in Figure 5. Figure

Figure 6. SEM micrographs of the surface (a) and the cross-section (b) of the SRF.

coating of superabsorbent composite could absorb water to form a swollen hydrogel and release nutrients from it. From the SEM of the cross-section of the SRF (Figure 6b), it could be seen that the SRF was a core−shell structure. The inner and outer coating materials constituted the shell, and the NP compound fertilizer was the core. 3.5. Slow-Release Behavior of SRF in Soil. To evaluate the effect of slow-release of the coatings, we monitored the slow-release behaviors of nitrogen (N) and phosphorus (P2O5) from NP compound fertilizer (the core) and SRF in soil at the ambient temperature, respectively. The results are shown in Figure 7. From it, we can find that the release rates of N and P

Figure 5. FTIR spectra of the core (a), WS/SA blends (b), P(AA-coNHMAAm)/WS (c), and SRF (d).

5a shows the infrared spectrum of the core (NP compound fertilizer) with the characteristic absorption bands of urea at 3464, 3356, 1664, 1627, and 1465 cm−1,29 and the characteristic peaks of phosphate at 1156 and 950 cm−1,30 and the characteristic peaks of APT at 1067 and 534 cm−1.18 Figure 5b shows the infrared spectrum of the inner coating (WS/SA blends); the characteristic absorption bands of urea were also observed, which are due to the urea solution as the solvents of cellulose. Figure 5c shows the infrared spectrum of the outer coating (P(AA-co-NHMAAm)/WS superabsorbent composite) which has been analyzed in the Section 3.1. Figure 5d shows the infrared spectrum of the SRF; from it, we can see that characteristic peaks of the ingredients all appeared. These results prove that the core of the SRF was NP compound fertilizer granule, and the coating materials were WS/SA blends (the inner coating) and the P(AA-co-NHMAAm)/WS superabsorbent composite (the outer coating). The SEM image of the surface of the SRF is presented in Figure 6a, which displays that the surface of the SRF consisting of P(AA-co-NHMAAm)/WS superabsorbent composite particles was coarse and porous, which structurally increased the surface area of the SRF. When SRF was dipped into water, the

Figure 7. Slow-release profiles of nitrogen (A) and phosphorus (B) from the core (a) and the SRF (b) in soil, respectively. 3859

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and phosphorus suggested that the SRF with coatings had excellent slow-release property. The nutrient release mechanism of SRF could be illustrated as the following: (1) The outer coating material, P(AA-coNHMAAm)/WS superabsorbent composite, would be slowly swollen by soil solution and transformed into hydrogel after SRF was embeded into soil. There existed a dynamic exchange between the free water in the hydrogel and the water in soil.31 The nutrients would be slowly diffused out of the granule through the dynamic water exchange. (2) When the free water in the outer coating migrated to the middle layer, namely, WS/ SA blends coating, the soluble urea in the WS/SA blends was dissolved and diffused first into the superabsorbents coating and then released into soil slowly through the above dynamic exchange of free water. (3) Water entered into the core of NP compound fertilizer through the tiny pores. The fertilizer was dissolved, diffused out the WS/SA layer, entered into the superabsorbents layer, and then released into the soil through the dynamic exchange of free water. Therefore, the presence of the outer coating regulated the release behavior of nutrients and minimized the burst release effect of fertilizers. Meanwhile, the use of WS/SA blends in the middle layer made the nutrients release slowly because of the physical barrier of the matrix. 3.6. Water-Holding Capacity of Soil with SRF. Besides its slow-release property, the other one of the most important characters of the SRF was the water-retention capacity. In this regard, the testing of SRF for water-holding capacity of soil was carried out. For the soil samples, (a) soil only, (b) soil mixed with 1 wt % SRF, and (c) soil mixed with 3 wt % SRF, the largest water-holding ratios were 42.5, 48.5, and 63.2%, respectively. This indicated that the addition of SRF to soil could improve the water-holding capacity of the soil. It was due to that the shell material of the SRF was P(AA-co-NHMAAm)/ WS superabsorbent composite, which has the excellent water absorbency capacity, so the soil with the addition of SRF could hold much more water during the irrigation period or raining time than the soil without it and could efficiently reduce irrigation water consumption. 3.7. Degradation Studies of the P(AA-co-NHMAAm)/ WS. The degradation was investigated by detecting weight loss. Figure 9 shows the extent of degradation of P(AA-co-

from SRF are much slower than that from the core, respectively. More than 80% of nitrogen was released from the core within 3 d, and the amount of the released nitrogen had reached 98% within 5 d (as shown in Figure 7Aa). However, the release rate of nitrogen in the SRF was slower compared with that in the core. As shown in Figure 7Ab, the nitrogen in SRF released 30.5, 40.3, and 98.5% within 1, 3, and 30 d, respectively. The phosphorus in the core released 66.2% within 5 d (as shown in Figure 7Ba), which reached maximum release value. The release profile of phosphorus in SRF was gentle and upward (as shown in Figure 7Bb). The phosphorus released 6.3, 18.7, and 42.6% within 1, 3, and 30 d, respectively. Furthermore, to survey the release behavior, we carried out the FTIR analysis of the core and the SRF in the process of the release. Figure 8A presents the FTIR spectrum of the core

Figure 8. FTIR spectra of the core (A) and the SRF (B) before release (a) and release after 5 d (b), 15 d (c), and 30 d (d).

before release (a) and release after 5 d (b), 15 d (c), and 30 d (d), respectively. From it, we can see that the characteristic absorptions of urea disappear after being released 5 d. However, as presented in Figure 8B, the FTIR spectrum of the SRF before release (a) and release after 5 d (b), 15 d (c), and 30 d (d), the characteristic absorptions of urea was still observable after release at 15 d. In summary, the release results of nitrogen

Figure 9. Degradation of P(AA-co-NHMAAm)/WS with different WS content in buffer solution and buffer/cellulase solution, respectively. 3860

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4. CONCLUSIONS In summary, we have prepared a double-coated slow-release nitrogen and phosphorus fertilizer with water retention (SRF). The outer coating was poly(acrylic acid-co-N-hydroxymethyl acrylamide)/wheat straw superabsorbent composite, the inner coating was wheat straw/sodium alginate blends, and the core was composed of urea, calcium biphosphate, and attapulgite (NP compound fertilizer). The contents of nitrogen and phosphorus were 20.6 and 9.9%, respectively. The results of slow-release experiment demonstrated that the product had preferable slow-release property. The addition of SRF could efficiently improve the water-holding capacity of soil. Moreover, the degradation in the soil solution of the superabsorbent composite based on wheat straw and N-hydroxymethyl acrylamide was studied. The impact of the content of wheat straw on the extent of degradation in cellulase solution was also examined. The coating material based on wheat straw imparted the product with high extent of degradation, low production cost, and broad application in general agricultures. The new approach shows promise in utilizing agrowaste and makes the technique quite environmentally friendly, simultaneously.

NHMAAm)/WS with WS content from 0 to 25 wt % after degradation in cellulase buffer solution for 4 d. We can see from it that the extent of degradation of P(AA-co-NHMAAm)/WS superabsorbent composite was dependent on the content of WS in the composition, which was related to the swelling of the superabsorbent composite. With increasing WS content up to 15 wt %, the extent of degradation of P(AA-co-NHMAAm)/ WS increased. It could be attributed to the water absorbency of superabsorbent composite being increased. As the slice of P(AA-co-NHMAAm)/WS was incubated in the buffer/cellulase solution, it was swelled, and the water molecules diffused into the network of the superabsorbent composite. Meanwhile, the degradation occurred, and the ether linkages between −CH2OH and −OH or two −CH2OH broke homogeneously throughout the degradation process. This ongoing break of cross-links within the polymer decreased the cross-linking density of the network. Then, the cellulase molecules diffused into the network when the pore of network enlarged. Under appropriate conditions, the cellulase molecules have the ability to hydrolyze β-(1−4)-bonds in cellulose.32 Thus, the high water absorbency could result in high extent of degradation. When WS content further increased from 15 to 25 wt %, although the water absorbency of superabsorbents (swollen for 60 min) slightly decreased, the extent of degradation increased continuously. This phenomenon, ascribed to that excessive WS, was filled in the network of superabsorbents, which would dissolve out when the hydrolyzation of ether linkage underwent to a certain degree. This may accelerate the degradation of superabsorbents. The extent of degradation of the superabsorbent composite in buffer/cellulase solution was 92.3% within 4 d when the content of WS was 25 wt %. It is remarkable that the superabsorbent composite based on WS is degradable and can be applied in agriculture as a new kind of coating material to alleviate environmental pollution. Although the degradation in aqueous electrolytes has been studied, to our knowledge, the degradation in true soil solution has never been examined in the absence of soil material. Figure 10 represents the profile of the time-dependent degradation for P(AA-co-NHMAAm)/WS with WS content 25 wt % in the soil solution (pH = 7.7) at the ambient temperature. As it describes, the extent of degradation increased with the time prolongation. After degradation in soil solution for 30 d, the extent of degradation of P(AA-co-NHMAAm)/WS reached to 33.8%.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-931-8912387. Fax: +86-931-8912582. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Special Doctorial Program Fund of the Ministry of Education of China (Grant No. 20090211110004) and Gansu Province Project of Science and Technologies (Grant No. 0804WCGA130).

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REFERENCES

(1) Trenkel, M. E. Controlled-release and stabilized fertilizers in agriculture; International Fertilizer Industry Association: Paris, 1997. (2) Liu, C.; Wu, J.; Chang, J. Diffusion characteristics and controlled release of bacterial fertilizers from modified calcium alginate capsules. Bioresour. Technol. 2008, 99, 1904. (3) Liang, R.; Liu, M. Preparation and Properties of a Double-Coated Slow-Release and Water-Retention Urea Fertilizer. J. Agric. Food Chem. 2006, 54, 1392. (4) García, M. C.; Vallejo, A.; García, L.; Cartagena, M. C. Manufacture and Evaluation of Coated Triple Superphosphate Fertilizers. Ind. Eng. Chem. Res. 1997, 36, 869. (5) Ibrahim, A. A.; Jibril, B. Y. Controlled Release of Paraffin Wax/ Rosin-Coated Fertilizers. Ind. Eng. Chem. Res. 2005, 44, 2288. (6) Jarosiewicz, A.; Tomaszewska, M. Controlled-Release NPK Fertilizer Encapsulated by Polymeric Membranes. J. Agric. Food Chem. 2003, 51, 413. (7) Shaviv, A.; Raban, S.; Zaidel, E. Modeling Controlled Nutrient Release from Polymer Coated Fertilizers: Diffusion Release from Single Granules. Environ. Sci. Technol. 2003, 37, 2251. (8) Wu, L.; Liu, M.; Rui, L. Preparation and properties of a doublecoated slow-release NPK compound fertilizer with superabsorbent and water-retention. Bioresour. Technol. 2008, 99, 547. (9) Zheng, T.; Liang, Y.; Ye, S.; He, Z. Superabsorbent hydrogels as carriers for the controlled-release of urea: Experiments and a mathematical model describing the release rate. Biosyst. Eng. 2009, 102, 44.

Figure 10. Degradation profile of P(AA-co-NHMAAm)/WS with WS content 25 wt % in the soil solution. 3861

dx.doi.org/10.1021/ie2016043 | Ind. Eng. Chem. Res. 2012, 51, 3855−3862

Industrial & Engineering Chemistry Research

Article

(10) Abedi-Koupai, J.; Sohrab, F.; Swarbrick, G. Evaluation of hydrogel application on soil water retention characteristics. J. Plant Nutr. 2008, 31, 317. (11) Chu, M.; Zhu, S. Q.; Li, H. M.; Huang, Z. B.; Li, S. Q. Synthesis of poly(acrylic acid)/sodium humate superabsorbent composite for agricultural use. J. Appl. Polym. Sci. 2006, 102, 5137. (12) Farag, S.; Al-Afaleq, E. I. Preparation and characterization of saponified delignified cellulose polyacrylonitrile-graft copolymer. Carbohydr. Polym. 2002, 48, 1. (13) Shogren, R. L.; Willett, J. L.; Biswas, A. HRP-mediated synthesis of starch-polyacrylamide graft copolymers. Carbohydr. Polym. 2009, 75, 189. (14) Zhang, J. P.; Li, A.; Wang, A. Q. Study on superabsorbent composite. V. Synthesis, swelling behaviors and application of poly(acrylic acid-co-acrylamide)/sodium humate/attapulgite superabsorbent composite. Polym. Adv. Technol. 2005, 16, 813. (15) Douglass, E. F.; Sullivan, M. J.; Young, A. H. Process for producing filled or reinforced polymer compositions. U.S. Patent 4,663,388, 1987. (16) Bhatnagar, A.; Sillanpäa,̈ M. Utilization of agro-industrial and municipal waste materials as potential adsorbents for water treatmentA review. Chem. Eng. J. 2010, 157, 277. (17) Ibrahim, S.; Shuy, W. Z.; Ang, H.-M.; Wang, S. Preparation of bioadsorbents for effective adsorption of a reactive dye in aqueous solution. Asia-Pac. J. Chem. Eng. 2010, 5, 563. (18) Xie, L.; Liu, M.; Ni, B.; Zhang, X.; Wang, Y. Slow-release nitrogen and boron fertilizer from a functional superabsorbent formulation based on wheat straw and attapulgite. Chem. Eng. J. 2011, 167, 342. (19) Liang, R.; Yuan, H.; Xi, G.; Zhou, Q. Synthesis of wheat straw-gpoly(acrylic acid) superabsorbent composites and release of urea from it. Carbohydr. Polym. 2009, 77, 181. (20) Ni, B.; Liu, M.; Lü, S.; Xie, L.; Wang, Y. Multifunctional SlowRelease Organic−Inorganic Compound Fertilizer. J. Agric. Food Chem. 2010, 58, 12373. (21) Davies, B. E.; Davies, R. I. A Simple Centrifugation Method for obtaining Small Samples of Soil Solution. Nature 1963, 198, 216. (22) Ma, Z.; Li, Q.; Yue, Q.; Gao, B.; Xu, X.; Zhong, Q. Synthesis and characterization of a novel super-absorbent based on wheat straw. Bioresour. Technol. 2011, 102, 2853. (23) Kumar, R.; Po, P.; Rozman, H. Studies on the synthesis of acrylamidomethyl cellulose ester and its application in UV curable surface coatings induced by free radical photoinitiator. Part 1: Acrylamidomethyl cellulose acetate. Carbohydr. Polym. 2006, 64, 112. (24) Klimchuk, K. A.; Hocking, M. B.; Lowen, S. Water-soluble acrylamide copolymers. IX. Preparation and characterization of the cationic derivatives of poly(acrylamide-co-N,N-dimethylacrylamid), poly(acrylamide-co-methacrylamide), and poly(acrylamide-co-N-t-butylacryl amide). J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2525. (25) Liu, Z.; Rempel, G. Preparation of superabsorbent polymers by crosslinking acrylic acid and acrylamide copolymers. J. Appl. Polym. Sci. 1997, 64, 1345. (26) Flory, P. J. Principles of polymer chemistry; Cornell University Press: Ithaca, NY, 1953. (27) Chen, J.; Zhao, Y. Relationship between water absorbency and reaction conditions in aqueous solution polymerization of polyacrylate superabsorbents. J. Appl. Polym. Sci. 2000, 75, 808. (28) Li, A.; Wang, A.; Chen, J. Studies on poly (acrylic acid)/ attapulgite superabsorbent composite. I. Synthesis and characterization. J. Appl. Polym. Sci. 2004, 92, 1596. (29) Guo, M.; Liu, M.; Zhan, F.; Wu, L. Preparation and Properties of a Slow-Release Membrane-Encapsulated Urea Fertilizer with Superabsorbent and Moisture Preservation. Ind. Eng. Chem. Res. 2005, 44, 4206. (30) Jin, S.; Yue, G.; Feng, L.; Han, Y.; Yu, X.; Zhang, Z. Preparation and Properties of a Coated Slow-Release and Water-Retention Biuret Phosphoramide Fertilizer with Superabsorbent. J. Agric. Food Chem. 2010, 59, 322.

(31) Smyth, G.; Quinn, F. X.; McBrierty, V. J. Water in hydrogels. 2. A study of water in poly (hydroxyethyl methacrylate). Macromolecules 1988, 21, 3198. (32) Ahola, S.; Turon, X.; Osterberg, M.; Laine, J.; Rojas, O. Enzymatic hydrolysis of native cellulose nanofibrils and other cellulose model films: effect of surface structure. Langmuir 2008, 24, 11592.

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