Preparation and Properties of Coated Nitrogen ... - ACS Publications

Hydrogel/clinoptilolite nanocomposite-coated fertilizer: swelling, water-retention and slow-release fertilizer properties. Azam Rashidzadeh , Ali Olad...
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Preparation and Properties of Coated Nitrogen Fertilizer with Slow Release and Water Retention Rui Liang and Mingzhu Liu* Department of Chemistry and Stage Key Laboratory of Applied Organic Chemistry, Lanzhou UniVersity, Lanzhou 730000, People’s Republic of China

A coated nitrogen fertilizer with slow release and water retention (CNSW) was prepared by cross-linked poly(acrylic acid)/organo-attapulgite (outer coating), urea-formaldehyde (UF) (inner coating), and urea granule (core). The synthesis conditions of inversion suspension polymerization were studied systematically. The water absorbency of CNSW was 80 times its own weight in tap water. Elemental analysis showed that the nitrogen content of the product was 28.3%. Swelling rate, slow release, and water retention properties of CNSW were also investigated, and a possible slow release mechanism was proposed. The results showed that the product had a high initial swelling rate, and the product had not only a good slow release property but also excellent water retention capacity, which could effectively improve the utilization of fertilizer and water resource. The results of the present work indicated that CNSW would find good application in agriculture and horticulture, especially in drought-prone areas. 1. Introduction Slow-release fertilizers are made to release their nutrient contents gradually and, if possible, to coincide with the nutrient requirements of a plant. A number of slow release fertilizers have been developed in past decades. There are three types of these fertilizers: matrix-type formulations constitute the first major category of slow- or controlled-release fertilizers due to their simple fabrication. The active component is dispersed in the matrix and diffuses through the matrix continuum or intergranular openings, that is, through pores or channels in the carrier phase.1 Another way of regulating the release of fertilizer is accomplished by means of chemically controlled releasing products, such as urea-formaldehyde2 and polyphosphates.3 The third major category of such fertilizers is coated fertilizers; i.e., a fertilizer core is coated by inert materials. The release of the fertilizer is controlled by diffusion through the coating. The materials applied most frequently as coatings are inorganic materials, such as sulfur, phosphates, and silicates; organic materials, for example, polyethylene, poly(vinyl chloride), and poly(lactic acid), and wax.4-7 A soluble fertilizer coated by an insoluble fertilizer, such as urea-formaldehyde, would be an ideal slow release formulation. Superabsorbents are three-dimensionally cross-linked hydrophilic polymers capable of swelling and retaining huge volumes of water in the swollen state. Recently, research on the use of superabsorbents as water management materials for agricultural and horticultural applications has attracted great attention, and testing of superabsorbents for agricultural applications has shown encouraging results as they have been observed to help reduce irrigation water consumption, lower the death rate of plants, improve fertilizer retention in soil, and increase plant growth rate.8 However, their applications in this field have met some problems because most of these superabsorbents are based on pure poly(sodium acrylate), and they are too expensive and not suitable for saline-containing water and soils.9 Recently, there have been many reports on introducing inorganic clays, such as kaolin, bentonite, montmorillonite,10 attapulgite,11 and * To whom correspondence should be addressed. Tel.: +86-9318912387. Fax: +86-931-8912582. E-mail: [email protected].

mica12 into pure polymeric superabsorbents to improve swelling properties and hydrogel strengths, and to reduce production costs. On the basis of the above background and our previous studies on superabsorbent polymers13,14 and slow-release fertilizers,15-17 in this work, we prepared a coated nitrogen fertilizer with slow release and water retention (CNSW), which possessed a threelayer structure: the core was urea granule, the inner coating was urea-formaldehyde (UF, a kind of insoluble fertilizer), and the outer coating was a cross-linked poly(acrylic acid)/organoattapulgite composite. CNSW not only had the slow release property, but also could absorb water and preserve soil moisture. These were significant advantages over the normal slow release fertilizers and superabsorbents for agriculture, which general have only a slow release property or water retention function. The present paper reveals the synthesis conditions of inversion suspension polymerization, swelling rate, slow release, and water retention properties of CNSW. 2. Experimental Section 2.1. Materials. Acrylic acid (AA, chemically pure, Beijing Oriental Chemical Factory, Beijing, China) was distilled at reduced pressure before use (boiling point ) 293-294 K at 0.5 mmHg). Ammonium persulfate (APS, analytical grade, Xi’an Chemical Reagent Factory, Xi’an, China) was recrystallized from water. N,N′-Methylene bisacrylamide (NNMBA, chemically pure, Shanghai Chemical Reagent Factory, Shanghai, China), attapulgite (APT, Linze Colloidal Co., Gansu, China), and hexadecyltrimethylammonium bromide (HDTMABr, analytical grade, Beijing Chemical Reagent Factory, Beijing, China) were used directly as received. Urea was industrial grade. 2.2. Preparation of Organo-APT (org-APT). Attapulgite was milled through a 320-mesh screen and treated with 37% hydrochloric acid for 72 h, followed by washing with distilled water until pH 6 was achieved, and then H+-APT was obtained. Thereafter, 4.0 g of H+-APT was suspended in 40 mL of HDTMABr (0.44 g) anhydrous alcohol solution. The suspension was stirred vigorously at 80 °C for 8 h, and then HDTMABrAPT was formed. Finally, the org-APT was washed with large quantity of anhydrous alcohol to remove excess HDTMABr, and then dried in an oven at 70 °C until the weight was constant.

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2.3. Preparation of CNSW. Pellet urea, previously sieved to about 2 mm in diameter, was placed into a rotary drum, and the UF powder (made from urea and 37% formaldehyde aqueous solution as described in the literature18) was stuck on the pellets by means of epoxy dissolved in acetone. The adhesive was applied by spraying at regular time intervals. The process was finished until a compact and homogeneous coating was formed on urea pellets. Then UF-coated urea granules were obtained. The UF-coated urea granules (5 g) were added into a flask equipped with a mechanical stirrer, a condenser, and a drop funnel. A certain amount of carbon tetrachloride, polyethylene glycol octyl phenyl ether (OP), and sorbite anhydride monostearic acid ester (Span-80) were added into the flask. The temperature was raised to 65 °C using a water bath while the mixture was constantly stirred. After that, a certain amount of mixed solution of partially neutralized acrylic acid (by ammonia, 25-28%), N,N′-methylene bisacrylamide (cross-linker, 1 wt % water solution), ammonium persulfate (initiator, 5 wt % water solution), and org-APT was slowly dropped into the flask. After being stirred 2 h at 65 °C, the mixture was filtered to remove the carbon tetrachloride, and dried at 70 °C. The final product, CNSW, was obtained, whose core, inner coating, and outer coating were urea, UF, and poly(acrylic acid)/org-APT (PAA/ org-APT) composite, respectively. 2.4. Measurement of Nitrogen Content of CNSW. The content of nitrogen in the CNSW was measured by an elemental analysis instrument (Germany Elemental Vario EL Corporation, Model 1106). 2.5. Measurement of Coating Percentage. The actual percentages of UF and PAA/org-APT coatings were determined by the weight difference of samples before and after coating. 2.6. Measurement of Water Absorbency. A 1 g sample of CNSW was immersed into a certain amount of tap water and allowed to soak at room temperature for 90 min. The swollen CNSW was filtrated through an 80-mesh sieve to remove nonabsorbed water and weighed. Water absorbency (WA) per gram of dried CNSW was calculated using eq 1:

WA )

M -1 M0

(1)

where M and M0 refer to the weight of water-swollen CNSW and that of dry CNSW, respectively. 2.7. Slow Release Behavior of CNSW in Soil. A 1 g sample of CNSW was well mixed with 180 g of dry soil and kept in a 200 mL plastic beaker properly covered and incubated for different periods at room temperature. Throughout the experiment, the soil was maintained at 30 wt % water-holding capacity by weighing and adding tap water if necessary, periodically. Blank and control experiments, viz., without any fertilizer and with untreated urea (the total content of nitrogen was the same as that of 1 g of CNSW), respectively, were also carried out. The soils were extracted by 0.01 M CaCl2 solution19 after each incubation period (1, 2, 5, 10, 15, 20, 25, and 30 days), and the nitrogen contents was estimated by the Kjeldahl method.20 For eight measurements, eight beakers were prepared at the same time. The amount of nitrogen released was calculated using eq 2.

nitrogen released (%) )

14CiV (1)(28.3%)

× 100

(2)

where Ci refers to the nitrogen content obtained from the Kjeldahl method and V refers to the volume of CaCl2 solution. 2.8. Largest Water-Holding Ratio of the Soil with CNSW. The sandy soil used in this study was representative of the area

Figure 1. Cross-sectional schematic view of a CNSW fertilizer granule. Table 1. Characteristics of CNSW characteristic

value

nitrogen content diameter of dry sample diameter of swollen sample UF coating percentage PAA/org-APT coating percentage urea percentage

28.3% 2.0-2.5 mm 13-15 mm 20.1% 35.2% 44.7%

of Lanzhou, which lies in the northwest of China and is a dry and semidesert region. A 2 g sample of CNSW was well mixed with 200 g of dry soil and placed in a 4.5 cm diameter PVC tube. The bottom of the tube was sealed by nylon fabric (with an aperture of 0.076 mm) and weighed (marked W1). The soil samples were slowly drenched by tap water from the top of the tube until water seeped out from the bottom. After there was no seeping water at the tube bottom, the tube was weighed again (marked W2). Control experiments, i.e., without CNSW, with 1 g of CNSW, and with 4 g of CNSW, were also carried out. The largest water-holding ratio (W %) of the soil was calculated from eq 3.

W%)

(W2 - W1) × 100 W2 - W1 + 200

(3)

2.9. Measurement of the Water Retention of CNSW in Soil. A 2 g sample of CNSW was well-mixed with 200 g of dry soil and kept in a plastic beaker; then 200 g of tap water was slowly added into the beaker and the beaker was weighed (marked W1). A control experiment, i.e., without CNSW, was also carried out. The beakers were maintained at room temperature and were weighed every 3 days (marked Wi) over a period of 30 days. The water evaporation ratio (W %) of soil was calculated from eq 4.

W%)

(W1 - Wi) × 100 200

(4)

2.10. Morphology of CNSW. The granules of CNSW were also subjected to a scanning electron microscopy (SEM) study. They were split into two halves, and the fractions obtained were adhered to sample holders with carbon LIT-C glue. The samples were coated with a layer of gold and observed in a JSM-5600LV SEM manufactured in Japan. 3. Results and Discussion 3.1. Structure and Characteristics of CNSW. Figure 1 shows the cross-sectional schematic view of a CNSW fertilizer granule, whose core is urea, inner coating is UF, and outer coating is PAA/org-APT composite. The characteristics of CNSW are presented in Table 1. 3.2. Fourier Transform Infrared (FTIR) Analysis of PAA/ org-APT (Outer Coating Material). Figure 2 shows the infrared spectra of org-APT, PAA, and PAA/org-APT. The peaks observed at 3300-3500 cm-1 (Figure 2C) could be

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Figure 2. FTIR spectra of org-APT (A), PAA (B), and PAA/org-APT (C).

attributed to -OH groups of the acrylate unit and org-APT. The peaks observed at about 2926 and 2855 cm-1 (Figure 2AC) correspond to the C-H stretching of the acrylate unit and org-APT. The bands near 1554, 1403, and 622 cm-1 (Figure 2B,C), which are absent in the spectra of org-APT, are designated as the characteristic peaks of PAA. Comparing the spectrum of PAA/org-APT and that of PAA, the appearance of the absorption bands in Figure 2C at 1718 cm-1 (CdO stretching of acrylate unit), 1554 cm-1 (COO- asymmetric stretching), and 468 cm-1 (O-Si-O bending of APT) give direct evidence for the interaction between org-APT and PAA. After polymerization, the absorption bands of org-APT at 990 cm-1 (Si-OH deformation) is absent in the spectrum of PAA/ org-APT, which also proves the participation of Si-OH in the polymerization process. This result is in agreement with the literature.11 3.3. Influence of Parameters on Water Absorbency. The relationship between water absorbency and the network structure parameter for the swelling of ionic network was given by Flory,21 usually used as the following two equivalent equations:

Qm5/3 ) [(i/2VuS1/2)2 + [(1/2) - X1]/V1]/(Ve/V0)

(5)

Qm5/3 ) [(i/2VuS1/2)2 + [(1/2) - X1]/V1](Mc/Fp)(1 - 2Mc/Mn)-1 (6) where Qm is the swelling ratio, i.e., water absorbency; i/Vu, the concentration of fixed charge referred to the unswollen network; S, the ionic concentration in the external solution; [(1/2) - X1]/ V1, the affinity of the hydrogel with water; Ve/V0, the crosslinked density, which refers to the number of effectively crosslinked chains in unit volume; Mc, the average molecular weight of the network chains; Fp, the density of the polymer; and Mn, the average molecular weight of the polymer before crosslinking. According to eqs 5 and 6, the water absorbency of superabsorbent polymer depends on the strength of the hydrophilic groups, cross-linking density, polymer network behavior, elasticity of the polymer networks, type of solvent, ionic strength of the external solution, etc. To improve the swelling capacity of the product, various reaction parameters are employed. The details of the influences of the reaction parameters on water absorbency in tap water, such as monomer ratio, cross-linker, initiator, and neutralization degree, are given below. 3.3.1. Effect of org-APT Content on Water Absorbency. The effect of the org-APT content on water absorbency in tap water is shown in Figure 3. It is obvious that the amount of org-APT is an important factor affecting water absorbency. The water absorbency increases with an increase of org-APT until

Figure 3. Effect of org-APT on water absorbency (cross-linker, 0.12 wt %; initiator, 0.4 wt %; neutralization degree, 70%; reaction temperature, 65 °C).

Figure 4. Effect of cross-linker on water absorbency (org-APT, 10 wt %; initiator, 0.4 wt %; neutralization degree, 70%; reaction temperature, 65 °C).

it reaches a maximum at 10 wt %. However, when the orgAPT content is larger than 10 wt %, the water absorbencies decrease with the increase of the amount of org-APT. The behavior may be attributed to the reaction between PAA and org-APT. When the org-APT content is low, the reaction could improve the polymeric network and, as a result, the water absorbency increases. On the other hand, when the org-APT content is high, the additional org-APT results in the generation of more cross-link points that increase the cross-link density of the composite, and then the elasticity of the polymer chains decreases. Moreover, the content of hydrophilic groups is lower at higher org-APT content, and the osmotic pressure difference decreases, which results in the shrinkage of the PAA/org-APT. The results are in conformity with Flory’s network theory,21 and similar observations have been reported by others.22 3.3.2. Effect of Cross-Linker Content on Water Absorbency. The cross-linker plays an important role in the formation of three-dimensional network structures permanently in the polymerization process. This is also a promising factor directly affecting the water absorbency of the product. Figure 4 shows the water absorbency in tap water as a function of the cross-linker content. It can be found that there exists a maximum, and the highest water absorbency occurs at 0.08 wt % cross-linker content. When the cross-linker content is lower than 0.08 wt %, the water absorbencies decrease because of the increase of soluble materials. On the other hand, higher cross-linker content results in the generation of more crosslink points, which in turn cause the formation of an additional network and decrease the space for holding water. The results are in conformity with Flory’s network theory.21

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Figure 7. Swelling rate of CNSW. Figure 5. Effect of initiator on water absorbency (org-APT, 10 wt %; crosslinker, 0.08 wt %; neutralization degree, 70%; reaction temperature, 65 °C).

Figure 6. Effect of neutralization degree on water absorbency (org-APT, 10 wt %; cross-linker, 0.08 wt %; initiator, 0.4 wt %; reaction temperature, 65 °C).

3.3.3. Effect of Initiator Content on Water Absorbency. In free radical polymerization, the initiator has a great influence on the polymerization rate as well as on the molecular weight of the resulting polymer. In the process of cross-linking polymerization reactions also, the initiator affects both the degree of cross-linking and the molecular weight between two crosslinking points. The low concentrations of initiator result in the decrease of cross-linking density as well as conversion. The effect of initiator content on water absorbency in tap water is shown in Figure 5. The water absorbency increases as initiator content rises from 0.1 to 0.4 wt % and decreases when the initiator content is greater than 0.4 wt %. When the initiator content is lower than 0.4 wt %, the polymerization reaction is slower, and the polymer network is less and water absorbency is lower under the same reaction condition. On the other hand, higher initiator content, such as more than 0.4 wt %, means a faster reaction rate and higher cross-linking density, which lead to the decrease of the water absorbency. 3.3.4. Effect of Neutralization Degree on Water Absorbency. The effect of the neutralization degree on water absorbency is shown in Figure 6. Figure 6 shows that there exists a maximum in the dependence of water absorbency on neutralization degree. The water absorbency increases as neutralization degree increases from 40% to 70%, and decreases with further increase in the neutralization degree of AA. When AA is neutralized with ammonia, the negatively charged carboxyl groups attached to the polymer chains set up an electrostatic repulsion that tends to expand the network. In a certain range of neutralization degree, the electrostatic repulsion increases with the increase of neutralization degree, resulting in the increase of water absorbency. However, with further increases in the neutralization degree of AA, water absorbency

decreases due to the increase of the solubility of the superabsorbent composite. Moreover, at higher neutralization degree, the screening effect of the counterion on the polyanion chain will lead to a reduction in expansion of the network. The optimum neutralization degree is 70% under our experimental conditions. 3.4. Swelling rate of CNSW. The time required to reach the maximum swelling capacity of CNSW was studied, and the results are presented in Figure 7. A 1 g sample was immersed in an excess amount of tap water, and the water absorbency was measured every 5 min. The result indicated that the sample had a high initial swelling rate, and reached its maximum swelling capacity after about 30 min. It has been reported that the swelling rate of a superabsorbent is mainly determined by the swelling ability, surface area, particle size, and density of the polymer.23 The high swelling rate for CNSW is attributed to the fact that the introduction of org-APT into the superabsorbent loosens the polymeric network and increases the capillary effect. Furthermore, a high initial swelling rate is one of the most important factors for superabsorbents used in agriculture, for it could absorb more water during raining or irrigation. Photographs of the dry and swollen (in tap water) samples are presented in Figure 8. From Figure 8B, it can be seen that the CNSW granules are capable of taking up water not only in the form of a swollen polyacrylate gel, but also in the form of additional free water between the core and the surrounding polyacrylate gel layer, which could enhance the water absorbency.24 3.5. Slow Release Behavior of CNSW in Soil. One of the most important characteristics of CNSW is its slow release property. Figure 9 represents the nitrogen release behaviors of urea, and urea/superabsorbent mixture, CNSW in soil. More than 98.5 wt % of nitrogen in urea was released within 12 h (as shown in Figure 9A). The nitrogen release rate of the urea/ superabsorbent mixture (as shown in Figure 9B) decreased compared with that of urea, in agreement with the results of Smith,25 but it was still obviously higher than that of CNSW. As shown in Figure 9C, the nitrogen in CNSW possessed excellent slow release properties: the nitrogen in CNSW released 3.9, 7.5, and 75 wt % within 2, 5, and 30 days, respectively. The CNSW nitrogen release did not exceed 15 wt % by the second day, and about 75 wt % of the nitrogen was released after 30 days. These results indicate that the slow release properties of CNSW conform to the standard of slow release fertilizers of the Committee of European Normalization (CEN).26 It is well-known that urea is easily dissolved in water, so it would quickly dissolve in the soil solution after being added to the soil and the nutrient would be quickly exhausted. While the PAA superabsorbent could absorb a lot of water in soil, the

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Figure 8. Photographs of dry (A) and swollen (B) CNSW.

Figure 9. Release behaviors of nitrogen in soil. (A) Untreated urea; (B) mixture of untreated urea and PAA; (C) CNSW.

urea dissolved in soil solution could be absorbed into it and on its surface. The absorbed urea could be released or desorbed slowly through the exchange of free water or minerals between soil solution and PAA. Therefore, the mixture of PAA and urea had a slower release rate than untreated urea. The nutrient release mechanism of CNSW fertilizer can be described by the following steps: (1) the outer PAA/org-APT layer is slowly swollen by the water in soil and then transformed to hydrogel after being added into soil. A dynamic exchange between the free water in the hydrogel and the water in soil will develop.27,28 (2) When the free water in PAA/org-APT layer migrates to the middle layer, i.e., UF coating, the water will dissolve part of UF (UF could dissolve in cold water partially) and form tiny pores and holes in this layer. The dissolved UF can be released through the dynamic exchange of free water. (3) Water enters the urea core through the tiny pores and dissolves it. (4) The dissolved urea diffuses out from the UF layer and enters the PAA/org-APT layer, and then releases into the soil through the dynamic exchange of free water. (5) The undissloved UF is decomposed by the microbes in soil, and then diffused out from the PAA/org-APT layer and into the soil. Therefore, the nitrogen release rate is slow before the pores are formed in the UF layer, fast after that, and finally slow again after all urea releases out. The thickness of the UF layer and the solubility of UF are the factors that control the urea release rate. The thicker the UF layer and the lower the UF solubility, the more difficult it is for the pores to form, and then the slower the nitrogen release rate is. 3.6. Largest Water-Holding Ratio of the Soil with CNSW. Besides its slow release property, the other one of the most important characteristics of CNSW is its water retention capacity

Figure 10. Largest water-holding ratio of soil with CNSW. (A) Soil without CNSW; (B) soil with 0.5 wt % CNSW; (C) soil with 1 wt % CNSW; (D) soil with 2 wt % CNSW.

Figure 11. Water retention behaviors of CNSW. (A) Soil without CNSW; (B) soil with 1 wt % CNSW.

or, in other words, its effective utilization of water in arid and desert regions. It was reported29 that the use of superabsorbents in agriculture could increase the largest water-holding capacity and water retention capacity of soil. Therefore, experiments to test the largest water-holding capacity and water retention behavior of soil with CNSW were performed. Figure 10 shows that the largest water-holding ratios of the soils mixed with CNSW are larger than that of the soil without it. Samples C and D hold more water than sample B because C and D contain more CNSW than B. Sample C is more efficient than sample D considering the smaller content of CNSW without a significant decrease of water-holding capability. The result is in good agreement with observations in the literature,30 which reported an exponential increase in the water-holding capacity of a sandy soil with increasing additions of hydrophilic polymers. Therefore, CNSW could effectively store rainwater or irrigation water, and improve the utilization of water

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Figure 12. SEM of the surface (A) and part of the cross section (B) of CNSW.

resources. Moreover, it was observed that the water flow rate through the soil was slowed when CNSW was added to the soil. Thus, the soil with the addition of CNSW could hold much more water during the irrigation period than the soil without it, and decrease water losses through infiltration and save water during irrigation.29 This is a significant advantage of CNSW over normal slow release fertilizers. 3.7. Water Retention Behavior of Soil with CNSW. Figure 11 shows the water retention behavior of the soil with and without CNSW. From Figure 11, it could be found that the water evaporation ratio of soil with CNSW obviously decreased compared to the soil without it. The water evaporation ratio of soil without CNSW had reached 60.2 wt % on the 15th day, while that of the soil with CNSW was 45.2 wt %. After 30 days, the soil without CNSW had almost given off all the water, while the soil with CNSW still had a 32 wt % water-holding ratio. From this experiment, it could be indicated that CNSW had good water retention capacity in soil, and that with CNSW use water could be saved and managed so that it could be effectively used for the growth of plants. At the beginning of the experiment, the water evaporation rates are fast and practically linear. There are two types of water, i.e., interstitial water and absorbed water, in the soil.29 The early water losses may be mainly attributed to the evaporation of interstitial water, and the evaporation of absorbed water is responsible for the later water losses. The absorbed water in the PAA/org-APT layer could be slowly released with the decrease of the soil moisture, and then used by the plants. Simultaneously, nutrition could also be released slowly with the water. Therefore, the swollen CNSW is just like a microreservoir to retain and supply moisture and nutrition to crops, and thus could increase the utilization efficiency of water and fertilizer at the same time. At the same time, we also observed that the soil without CNSW hardened and cracked after about 25 days at room temperature, whereas the soil with CNSW still kept continuous configuration and formed many granular structures in it. It has been reported in the literature31 that these granular structures contribute to stabilizing the soil structure and improving aeration, permeability, and till ability of the soil, reducing soil packing and cracking, minimizing soil crusting, thus preventing soil from hardening, and provide a favorable environment for crops. Furthermore, the introduction of attapulgite into poly(acrylic

acid) renders the outer coating biodegradable and environmentally friendly;32-34 therefore, the bioaccumulation of the CNSW would be small. Poly(acrylic acid) is not toxic, and the residual monomer content determined by gas chromatography is lower than 500 ppm, so the product is safe for people and soil.35 The study shows that, besides its slow release property, CNSW has good water retention and moisture preservation capacity, which are properties that normal slow release fertilizers do not have. It is especially significant for arid and desert areas. The effects of air humidity on water retention behavior and the largest water-holding ratio of soil with CNSW were also determined. However, it was found that air humidity had no obvious effects on them under general air humidity conditions. The reason may be that CNSW was buried in the soil and could not contact the air directly. 3.8. Morphology of CNSW. The SEM of the surface and SEM of part of the cross section of CNSW are shown in Figure 12. From Figure 12A it can be seen that the surface of the CNSW is rugged, and it seems to be composed of fine particles. Holes exist between these fine particles, so water can be absorbed easily by the product because it has a high specific surface area.36 The characteristics of surface morphology are very significant for water absorbency and swelling rate. Therefore, when CNSW is dipped in water, it can absorb water quickly to form a swollen hydrogel, which is responsible for the water retention property of CNSW. Figure 12B shows the three-layer structure of CNSW. The outer layer is PAA/orgAPT superabsorbent composite, which can absorb a large amount of water; the middle layer is UF, which serves as a physical barrier for mass transfer and reduces the rate of water diffusion into the core and the nitrogen diffusion outside the core, this provides CNSW with a good slow release property. The inner core is a urea granule. In summary, the outer PAA/ org-APT layer enables CNSW to have the water retention property, and the middle UF layer enables CNSW to have the slow release property. 4. Conclusions A coated nitrogen fertilizer with slow release and water retention (CNSW) was prepared, which possessed a three-layer structure: the core was urea granule, the inner coating was urea-formaldehyde, and the outer coating was cross-linked

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poly(acrylic acid)/organo-attapulgite composite. Its water absorbency was 80 times its own weight in tap water. Elemental analysis results showed that the nitrogen content of the product was 28.3%. The product had a high initial swelling rate and reached its maximum swelling capacity after about 30 min. The product had good slow release property: the nitrogen released did not exceed 75 wt % after being incubated in soil for 1 month. The addition of CNSW into soil could enhance the water-holding capacity of the soil significantly, and the water evaporation rate decreased also; i.e., the product had excellent water retention property in soil. The product could effectively improve the utilization of fertilizer and water resource at the same time; consequently, it could reduce the cost of manpower and resources and decrease the bad effect of simple fertilizers on the environment. Moreover, slow release fertilizers and superabsorbents have long been used in agriculture, so CNSW, which possesses slow release and water retention properties, would definitely have good application in the future. Acknowledgment This work was supported by Special Doctorial Program Funds of the Ministry of Education of China (Grant 20030730013). Literature Cited (1) Kakoulides, E. P.; Valkanas, G. N. Modified rosin-paraffin wax resins as controlled delivery systems as fertilizers. Fabrication parameters governing fertilizer release in water. Ind. Eng. Chem. Res. 1994, 33, 1623. (2) Prasad, R.; Rajale, G. B.; Lakhdive, B. A. Nitrification retarders and slow-release nitrogen fertilizers. AdV. Agron. 1971, 23, 337. (3) Ray, S. K.; Varadachari, C.; Ghosh, K. Novel slow-releasing micronutrient fertilizer. 1. Zinc compounds. Ind. Eng. Chem. Res. 1993, 32, 1218. (4) Al-Zahrani, S. M. Utilization of polyethylene and paraffin waxes as controlled delivery systems for different fertilizers. Ind. Eng. Chem. Res. 2000, 39, 367. (5) Garcia, M. C.; Vallejo, A.; Garcia, L.; Cartagena, M. C. Manufacture and evaluation of coated triple superphosphate fertilizers. Ind. Eng. Chem. Res. 1997, 36, 869. (6) Hanafi, M. M.; Eltaib, S. M.; Ahmad, M. B. Physical and chemical characteristics of controlled release compound fertilizer. Eur. Polym. J. 2000, 36, 2081. (7) Jarosiewicz, A.; Tomaszewska, M. Controlled-release NPK fertilizer encapsulated by polymeric membranes. J. Agric. Food Chem. 2003, 51, 413. (8) Bouranis, D. L.; Theodoropoulus, A. G.; Drossopoulus, J. B. Designing synthetic polymers as soil conditioners. Commun. Soil Sci. Plant Anal. 1995, 26, 1455. (9) Kohls, S. J.; Baker, D. D.; Kremer, D. A.; Dawson, J. O. Waterretentive polymers increase nodulation of actinorhizal plants inoculated with Frankia. Plant Soil 1999, 214, 105. (10) Wu, J.; Wei, Y.; Lin, J.; Lin, S. Study on starch-graft-acrylamide/ mineral powder superabsorbent composite. Polymer 2003, 44, 6513. (11) Zhang, J. P.; Chen, H.; Wang, A. Q. Study on superabsorbent composite. IV. Effects of organification degree of attapulgite on swelling behaviors of polyacrylamide/org-attapulgite composite. Eur. Polym. J. 2006, 42, 101. (12) Lee, W. F.; Chen, Y. C. Effect of intercalated reactive mica on water absorbency for poly(sodium acrylate) composite superabsorbents. Eur. Polym. J. 2005, 41, 1605. (13) Chen, Z. B.; Liu, M. Z.; Ma, S. M. Synthesis and modification of salt-resistant superabsorbent polymers. React. Funct. Polym. 2005, 62, 85. (14) Ma, S. M.; Liu, M. Z.; Chen, Z. B. Preparation and properties of a salt-resistant superabsorbent polymer. J. Appl. Polym. Sci. 2004, 93, 2532.

(15) Liang, R.; Liu, M. Z. Preparation and Properties of a Double-Coated Slow-Release and Water-Retention Urea Fertilizer. J. Agric. Food Chem. 2006, 54, 1392. (16) Zhan, F. L.; Liu, M. Z.; Guo, M. Y.; Wu, L. Preparation of superabsorbent polymer with slow-release phosphate fertilizer. J. Appl. Polym. Sci. 2004, 92, 3417. (17) Guo, M. Y.; Liu, M. Z.; Hu, Z.; Zhan, F. L.; Wu, L. Preparation and properties of a slow-release NP Compound fertilizer with superabsorbent and moisture preservation. J. Appl. Polym. Sci. 2005, 96, 2132. (18) Wang, Y.; Wan, T. Synthesis of polyurea formaldehydesan environmental protection and controlled release chemical fertilizer. Chin. J. Soil Sci. 2001, 32, 73. (19) Abraham, J.; Rajasekharan, P. V. N. Membrane-encapsulated controlled-release urea fertilizers based on acrylamide copolymers. J. Appl. Polym. Sci. 1996, 60, 2347. (20) AOAC. Official Methods of Analysis, 15th ed.; Association of Official Analytical Chemists, Inc.: Arlington, VA, 1990. (21) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (22) Lee, W. F.; Yang L. G. Superabsorbent polymeric materials. XII. Effect of montmorillonite on water absorbency for poly (sodium acrylate) and montmorillonite nanocomposite superabsorbents. J. Appl. Polym. Sci. 2004, 92, 3422. (23) Elliott, J. E.; Macdonald, M.; Nie, J.; Bowman, C. N. Structure and swelling of poly(acrylic acid) hydrogels: effect of pH, ionic strength, and dilution on the crosslinked polymer structure. Polymer 2004, 45, 1503. (24) Ruttscheid, A.; Borchard, W. Synthesis and characterization of glass-containing superabsorbent polymers. Eur. Polym. J. 2005, 41, 1927. (25) Smith, J. D.; Harrison, H. C. Evaluation of polymers for controlledrelease fertilizer properties when incorporated with nitrogen fertilizer. Commun. Soil Sci. Plant Anal. 1991, 22, 559. (26) Trenkel, M. E. International fertilizer industry association, Stratospheric Ozone; HMSO: London, 1997. (27) David, S. G. H.; Mark, T. S. L. Water-polymer interactions and critical phenomena of swelling in inhomogeneous poly-(acrylonitrileacrylamide-acrylic acid) gels. Polymer 1994, 35, 4416. (28) Smyth, G.; Francis, X. Q.; Vincent, J. M. Water in hydrogels. 2. A study of water in poly(hydroxyethyl methacrylate). Macromolecules 1998, 21, 3198. (29) Bakass, M.; Mokhlisse, A.; Lallemant, M. Absorption and desorption of liquid water by a superabsorbent polymer: effect of polymer in the drying of the soil and the quality of certain plants. J. Appl. Polym. Sci. 2002, 83, 234. (30) Hu¨ttermann, A.; Zommorodi, M.; Reise, K. Addition of hydrogels to soil for prolonging the survival of pinus halepensis seedlings subjected to drought. Soil Till. Res. 1999, 50, 295. (31) Krysiak, M. D.; Madigan, D. P. Granular fertilizer. U.S. Patent 0,069,032, 2004. (32) Li, A.; Zhang, J. P.; Wang, A. Q. Synthesis, characterization and water absorbency properties of poly(acrylic acid)/sodium humate superabsorbent composite. Polym. AdV. Technol. 2005, 16, 675. (33) Ye, H.; Zhao, J. Q.; Zhang, Y. H. Novel degradable superabsorbent materials of silicate/acrylic-based polymer hybrids. J. Appl. Polym. Sci. 2004, 91, 936. (34) Chen, P.; Zhang, W. A.; Luo, W.; Fang, Y. E. Synthesis of superabsorbent polymers by irradiation and their applications in agriculture. J. Appl. Polym. Sci. 2004, 93, 1748. (35) Jamshidi, A.; Beigi, F. A. K.; Kabiri, K.; Zohuriaan-Mehr, M. J. Optimized HPLC determination of residual monomer in hygienic SAP hydrogels. Polym. Test. 2005, 24, 825. (36) Omidian, H.; Hashemi, S. A.; Sammes, P. G.; Meldrum, I. Modified acrylic-based superabsorbent polymers (dependence on particle size and salinity). Polymer 1999, 40, 1753.

ReceiVed for reView June 3, 2006 ReVised manuscript receiVed September 24, 2006 Accepted September 30, 2006 IE060705V