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Different ammonium nitrate controlled-release (CR) systems based on ethylcellulose ...... Industrial & Engineering Chemistry Research 2010 49 (20), 96...
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APPLIED CHEMISTRY Controlled Release of Ammonium Nitrate from Ethylcellulose Coated Formulations Susana Pe´ rez-Garcı´a, Manuel Ferna´ ndez-Pe´ rez,* Matilde Villafranca-Sa´ nchez, Emilio Gonza´ lez-Pradas, and Francisco Flores-Ce´ spedes Department of Inorganic Chemistry, UniVersity of Almerı´a, La Can˜ ada de San Urbano s/n Almerı´a, 04120 Spain

Different ammonium nitrate controlled-release (CR) systems based on ethylcellulose have been investigated to reduce environmental pollution derived from nitrogen-fertilizer use. Coated ammonium nitrate granules were produced in Wurster-type fluidized-bed equipment using two different amounts of ethylcellulose. The highest one was modified by the addition of two plasticizers, dibutyl sebacate, and dibutyl phthalate. Having researched the encapsulation efficiency and the homogeneity of the coated granules, we carried out the kineticrelease experiments in water and soil. The release rate of the active ingredient was related to the thickness of the coating film, granule size, and type of plasticizer used. Using an empirical equation, the time taken for 50% of the active ingredient to be released into water and soil (T50) was calculated. From the analysis of the T50 values, we can deduce that the release rate of ammonium nitrate can be controlled, mainly changing the thickness of the coating film and using plasticizer as well. In water experiments, T50 values for granules prepared without plasticizers ranged between 7.47 h for 1 mm < d < 2 mm granules coated with 10% of ethylcellulose and 24.06 h for 2 mm < d < 3 mm granules coated with 20% of ethylcellulose. For those prepared with plasticizers, T50 ranged between 22.80 h for 1 mm < d < 2 mm granules containing dibutyl sebacate and 35.74 h for 1 mm < d < 2 mm granules containing dibutyl phthalate. However, in soil experiments T50 values ranged between 10.24 h for 1 mm < d < 2 mm granules coated with 10% of ethylcellulose and 38.80 h for 1 mm < d < 2 mm granules containing dibutyl sebacate. Finally, a linear regression of the T50 values was obtained by the results of the study carried out in water and soil. This allows us to predict the behavior of the formulations in soil. This could be useful in the design of systems which control the nitrogen release. Introduction The effectiveness of soil-applied fertilizers depends mainly on their ability to maintain a sufficient concentration of the nutrients within the plant root zone for a desired period of time. In most commercially available fertilizers, the concentrations of the fertilizer active ingredients rapidly diminish prior to sufficient plant uptake as a result of degradation (e.g., chemical, photochemical, and biological), volatilization, leaching, adsorption, or soil immobilization. To increase the effectiveness of conventional fertilizers, relatively large doses are often applied, thus, increasing the environmental risk.1 Encapsulation of fertilizers in polymeric matrixes is used for saving fertilizer consumption and minimizing environmental pollution.2 The use of controlled-release fertilizers (CRFs) causes an increase in their efficiency and results in the reduction of nutrients loss by leaching.3-5 Moreover, CRF application reduces the eutrophication of natural waters caused by excessive concentration of nitrogen and phosphorus compounds. These fertilizers can be physically prepared from the granules of the soluble fertilizers by coating them with materials which reduce their dissolution rate. In most cases the fertilizer is encapsulated inside an inert material.2 The coating layer acts * To whom correspondence should be addressed.Tel.: 34 950 015961. Fax: 34 950 015008. E-mail: [email protected].

as a physical barrier which controls the nutrient release.6,7 The use of CRFs causes an increase in their efficiency, reduces soil toxicity and nutrient loss, minimizes the potential negative effects associated with overdoses, and reduces the frequency of the application in relation to normal crop requirement.8-10 However, a combination of fertilizer and polymeric coating has a disadvantage; large amounts of polymers are left in soil when the nutrients are released. On the other hand, these materials may improve soil for cultivation with regard to better soil aeration and friability and soil erosion prevention.7 An application of biodegradable polymers for coating formulation is a better solution. Plasticizer is commonly incorporated with the polymeric film, to change flexibility, tensile strength, and adhesion properties of the resulting film.11 Basically, any plasticizer added must first be compatible with the polymeric film to produce homogeneity and desirable physical properties of the finished film.12 Plasticizer may control the active ingredient penetration through the polymeric film.12-14 Thus, plasticizer plays an important role in the polymeric film coating.11 CRFs are commonly produced by the pan, rotary drum, and fluidized-bed coating techniques. The fluidized-bed process results in coatings of good quality and of uniform thickness. For the production of CRFs, the so-called “Wurster” fluidizedbed process is employed.15-17 The main difference between the

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cases, with two different plasticizers. The coating film consists of 10% ethylcellulose (N90Etc10) and 20% (N80Etc20) by weight of the dry granules, and the mixture of ethylcellulose and plasticizer amounts to about 20% by weight of the granules (N80Etc18S1 and N80Etc18P). Finally, another controlled-release (CR) product containing DBS (N80Etc18S2) was obtained by first coating the ammonium nitrate granules with ethylcellulose and then with an ethanolic solution of DBS and maintaining the same conditions of operation previously described. Coated granules were sieved, and two sets of granules of size between 1 and 2 mm and between 2 and 3 mm were taken. Coated Granules Characterization. The actual concentration of N-NO3- and N-NH4+ in the coated granules was determined by dissolving 70 mg of the granules in 10 mL of absolute ethanol followed by extraction into ultrapure water plus ethanol (90 + 10 by volume). The resulting extract was filtered, and the nitrate and ammonium concentrations were determined by capillary electrophoresis (CE) using a Beckman capillary electrophoresis system (P/ACE 5000) equipped with a UV detector and data station. The CE operating conditions to analyze nitrate and ammonium ions were described by Arce et al.18 Electropherograms were recorded with indirect UV detection at 254 nm for nitrate and 214 nm for ammonium ions. Three replicates were carried out for each formulation. The average diameter of coated granules was determined using a Stereoscopic Zoom microscope from Nikon, model SMZ1000, provided with a camera PixelLINK (Megapixel FireWire Camera) model PL-A662. The yield of the coating process was calculated using the following relation: (weight of dry product obtained/total weight of products processed) × 100. The encapsulation efficiency has been calculated applying the next relation: (amount of ammonium nitrate in dry product/amount of ammonium nitrate in formulation processed) × 100. The surface morphology of the coated granules was examined with the aid of a ZEISS SEM model DSM scanning electron microscope. To examine the internal morphology of the polymeric coating, coated granules were carefully cut with the aid of a chisel and their cross sections were photomicrographed. The thickness of the prepared films was measured. Each film sample was measured at 10 different points. Water Release Kinetics. For each sample (two replicates), an accurately weighed quantity of granules containing about 11 mg of N-NO3- or N-NH4+ was added to 100 mL of bidistilled water and placed into stoppered conical flasks. Flasks were maintained in a thermostat bath at 25 ((0.1) °C. At different time intervals, aliquots of 1 mL were removed for determination of N-NO3- and N-NH4+ and 1 mL of fresh water was added to the flasks. In the final stage of the study, granules were removed by filtration from each flask to determine the amount of NNO3- and N-NH4+ that remained incorporated in the granules. To normalize the release profile curves, the total amount of N-NO3- and N-NH4+ released plus the extracted

Table 1. Percentage (by Weight) of Components for the Preparation of Coated CR Granules Containing Ammonium Nitratea plasticizer CR granules

NH4NO3 granules (%)

ethyl cellulose (%)

N90Etc10 N80Etc20 N80Etc18S1 N80Etc18S2 N80Etc18P

90.1 82.0 79.7 79.7 79.7

9.9 18.0 17.6 17.6 17.6

DBS (%)

DBP (%)

2.7 2.7 2.7

a

N90Etc10: ammonium nitrate-ethylcellulose (10%). N80Etc20: ammonium nitrate- ethylcellulose (20%). N80Etc18S: ammonium nitrateethylcellulose (18%; DBS). N80Etc18P: ammonium nitrate-ethylcellulose (18%; DBP).

Wurster process and a conventional fluidized bed is that, in the latter, the particles are moved randomly, while in a Wurster bed, they follow a predetermined flow pattern.1 The main purpose of this study is to encapsulate NH4NO3 using ethylcellulose in fluidized-bed equipment and evaluate the influence of the types of polymer coatings and their morphology on the release rate of ammonium nitrate from CRFs in water and soil. In the present study, coated fertilizer granules were produced in Wurster-type fluidized-bed equipment, using an ethanolic solution of ethylcellulose, at two different polymer levels, with or without plasticizer. The surface characteristics of the coated fertilizer granules were examined with the aid of a scanning electron microscope. Kinetic-release experiments were carried out in water and soil to determine the release characteristics of the coated fertilizer granules. Materials and Methods Chemicals. The fertilizer employed in this study was commercial granulated NH4NO3 (98.5%) obtained from Riedel-de Hae¨n (Seelze, Germany). The film-forming material of polymer coating was ethylcellulose (Ethocel; ethoxy content, 48.049.5%; viscosity, 9-11 cP). This product was supplied by Fluka Chemie AG (Buchs, Switzerland). Plasticizers selected for the study were dibutyl sebacate (DBS; g97%, d ) 0.936 g mL-1) and dibutyl phthalate (DBP; g98%, d ) 1.046 g mL-1), supplied by Fluka Chemie AG (Buchs, Switzerland). Preparation of Coated Granules. A total of 50 g of ammonium nitrate granules, N (size fraction, 1-2 mm), were loaded into a fluidized-bed coater (Strea 1, Aeromatic-Fielder AG, Bubendorf, Switzerland) assembled with a Wurster device. The bed temperature was adjusted at 60 ( 2 °C. The granules were preheated at this temperature for 10 min. The coating solution was delivered by a peristaltic pump (Watson-Marlow, model 1B-1003/R) and sprayed into the fluidized bed via spray nozzle at the atomizing pressure of 1.2 bar. Coating solutions were pumped with a flow rate of 2.5 mL min-1 or 5 mL min-1. The total spraying time was 40 min. The coated granules were then dried in the same apparatus at 70 °C for 10 min. Appropriate quantities of ethylcellulose and plasticizer were combined and dissolved in ethanol (Table 1) to obtain the coating solutions with two different polymer levels and, in some

Table 2. Characteristics of CR Granules (1 mm < d < 2 mm) Containing Ammonium Nitrate CR granules N90Etc10 N80Etc20 N80Etc18S1 N80Etc18S2 N80Etc18P

NH4NO3 (%)

average weight (mg)

average diameter (mm)

mean thickness of film (µm)

yield (%)

encapsulation efficiency (%)

90.10 82.95 77.54 76.54 77.72

4.45 4.69 4.34 4.48 4.59

1.98 1.99 1.97 2.00 1.98

49.26 70.01 57.94 62.36 91.80

93.07 95.14 89.97 91.94 94.63

94.50 97.74 88.84 89.70 93.64

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Figure 1. SEM of shape and external morphology: (a) ammonium nitrate granules, (b) N90Etc10, (c) N80Etc20. The other pictures show the internal morphology: (d) ammonium nitrate granules, (e) N90Etc10, (f) N80Etc20.

N-NO3- and N-NH4+ remaining in the granules was considered to be 100% of the active ingredient initially present in the granules. Soil Release Kinetics. The soil used was a calcareous soil (Calcareous Regosol) with a pH of 8.7, previously described by Flores-Ce´spedes et al.19 which is representative soil from the Almerı´a area (southeastern Spain). Granules containing 40 mg of ammonium nitrate were placed horizontally in the center of a polyethylene pot (6 × 6 cm) containing 100 g of air-dried soil at 50% maximum water retention capacity. The pots were maintained at this moisture level and at the same time keeping a temperature of 25 ( 0.1 °C, in a thermostat incubator Ing. Climas, model EC-350PP. This temperature was chosen according to the average temperature of representative soils of the Almeria region (southeastern

Spain) recorded in the meteorological station of Palmerillas Research Center. There were two replicates for each sampling time. Release studies were carried out for N90Etc10 (1 mm < d < 2 mm), N80Etc20 (1 mm < d < 2 mm), N80Etc20 (2 mm < d < 3 mm), and N80Etc18S1 (1 mm < d < 2 mm) formulations and for noncoated ammonium nitrate granules. On completion of release experiments it was necessary to determine the quantity of ammonium nitrate that was still remaining in the granules. For that reason, all granules were taken from each of the CR systems, dissolved, and analyzed following the methods described above (coated granules characterization). The ammonium nitrate released in soil was determined by the difference between the initial amount of ammonium nitrate in the granules and the amount remaining in the granules once the release experiments finished.

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Figure 2. SEM of shape and external morphology: (a) N80Etc18S1, (b) N80Etc18S2, (c) N80Etc18P. The other pictures show the internal morphology: (d) N80Etc18S1, (e) N80Etc18S2, (f) N80Etc18P.

Results and Discussion Coated Granules Characterization. Characteristics of CR granules (1 mm < d < 2 mm) containing ammonium nitrate are presented in Table 2. The values of encapsulation efficiency ranged between 88.84% for the system N80Etc18S1 and 97.74% for the system N80Etc20. These data highlight the efficacy of the method used for the encapsulation of ammonium nitrate. The average weight of the granules corresponding to the fractions of 1 mm < d < 2 mm size is very similar. It varies between 4.34 mg/granule for the system N80Etc18S1 and 4.69 mg/granule for the system N80Etc20. The average diameter of the granules can be considered the same for all of them, because it varies between 1.97 mm/granule for the system N80Etc18S1 and 2.00 mm/granule for the system N80Etc18S2. The percentages of active ingredient (NH4NO3) oscillate between 76.54% for the system N80Etc18S2 and 90.10% for the system N90Etc10.

Scanning electron microscopy (SEM) pictures (Figures 1 and 2) show a uniform and intact coating of ethylcellulose and plasticizers. The figures show the disappearance of the wrinkled aspect that the external surface of the ammonium nitrate granules possesses and the existence of a fine film in the coated formulations. The cross-section shows the porous core structure with a coherent film at the boundary. The thickness of the coating material, covering fertilizer granule, was estimated using the SEM photograph. The thickness of the coating film (Table 2) varied between 49.26 µm for the system N90Etc10 and 91.80 µm for the system N80Etc18P. The system with lower quantity of ethylcellulose, N90Etc10, has lower thickness, 49.26 µm, compared to 70.01 µm of the system N80Etc20. The formulations containing plasticizers show a sequence of thickness that varies between 57.94 µm (N80Etc18S1) and 91.80 µm (N80Etc18P). A

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Figure 3. Effect of ethylcellulose content on the release of N-NO3- from granules into static water (error bars represent the standard deviation of two replicates).

higher thickness of the film when DBP was used as a plasticizer can be observed. Comparing the internal morphology for the systems N90Etc10 and N80Etc20, shown in Figure 1e,f, we observe a higher homogeneity in the film of coating for the system that contains a higher quantity of polymer. Besides, if we compare the N80Etc20 internal morphology, Figure 1f, with the internal morphology of the system modified with plasticizer, N80Etc18S1, N80Etc18S2, and N80Etc18P, Figure 2d-f, in the latter a higher adherence and homogeneity of the coating is observed. This last observation is corroborated by other researchers, who explain that the plasticizer, incorporated with polymer, modifies the properties of adhesion, flexibility, and tensile strength of the coating film.11,20 Water Release Kinetics. The effects of ethylcellulose percentage on the release behavior of ammonium nitrate are shown in Figure 3. Apart from the percentage in ethylcellulose coating, these formulations produce a decrease in the process of dissolution of the active ingredient in relation to the commercial granulated ammonium nitrate product. The influence of ethylcellulose percentage defined by means of the two CR systems N90Etc10 and N80Etc20 appears clearly from Figure 3. As expected, the highest quantity in the polymer of the N80Etc20 formulation increases the thickness of the coating film and so causes the highest delay in release rate. This behavior has also been observed by other authors.21,22 Besides, the diffusion of water through the membrane is slowed down as membrane thickness increases.23 Because in all of the experiments carried out in this study a similar release profile from N-NO3- and N-NH4+ is exhibited, we will only show the release profiles from N-NO3-. Comparing the granule fractions of size 1 mm < d < 2 mm and 2 mm < d < 3 mm, for the system N80Etc20 (Figure 4), we can observe a slower release rate from the larger granules. This fact can be explained if we take into account the larger distances over which the active ions must diffuse when granules with the highest size and the highest area/volume ratio are used, in the case of the small granules.24,25 Figure 5 shows the effects of different plasticizers in the release rate of ammonium nitrate. A decrease in release rate of the system N80Etc18P compared to the system N80Etc18S1 is observed. If we also compare the formulation N80Etc18P and N80Etc18S2, we observe similar release profiles for both systems.

Figure 4. Effect of granule size on the release of N-NO3- from N80Etc20 granules into static water (error bars represent the standard deviation of two replicates).

Figure 5. Effect of the plasticizers combined with ethylcellulose on the release of N-NO3- from CR granules (error bars represent the standard deviation of two replicates).

The addition of DBS, once the ethylcellulose coating was performed in N80Etc18S2, produced a slight additional film of the plasticizer which could produce a higher homogeneity with a less porous film as it has been observed in SEM pictures of Figure 2. That fact can retard the release rate of ammonium nitrate. This explains why N80Etc18S2 and N80Etc18S1 show different release rates even though they have a similar film thicknesses and besides why N80Etc18S2 and N80Etc18P show similar release rates even though the thickness of the coating film of N80Etc18P was higher than that obtained for N80Etc18S2. In these latter cases the formulations are not only prepared with different methods, but different plasticizers have also been used, so the release rate depends on the two previous factors. This result could be very interesting because it would allow us to select the plasticizer DBP or DBS (with a minor polluting potential for the environment), according to the application which these formulations are going to be targeted. The release data of N-NO3- in water were analyzed by applying the empirical equation proposed by Ritger and Peppas.26

Mt ) Ktn M0

Ind. Eng. Chem. Res., Vol. 46, No. 10, 2007 3309 Table 3. Constants from Fitting the Empirical Equation Mt/M0 ) Ktn to Release Data of N-NO3- into Static Water CR granules

K × 102 (h-n)

n

r

T50 (h)

N90Etc10 (1 mm < d < 2 mm) N80Etc20 (1 mm < d < 2 mm) N80Etc20 (2 mm < d < 3 mm) N80Etc18S1 (1 mm < d < 2 mm) N80Etc18S2 (1 mm < d < 2 mm) N80Etc18P (1 mm < d < 2 mm)

12.62 5.13 2.00 2.15 0.92 1.22

0.68 0.94 1.01 1.01 1.12 1.04

0.991 0.980 0.971 0.974 0.979 0.960

7.47 11.15 24.06 22.80 35.24 35.74

Table 4. Constants from Fitting the Empirical Equation Mt/M0 ) Ktn to Release Data of N-NO3- into Soil CR granules

K × 102 (h-n)

n

r

T50 (h)

N90Etc10 (1 mm < d < 2 mm) N80Etc20 (1 mm < d < 2 mm) N80Etc20 (2 mm < d < 3 mm) N80Etc18S1 (1 mm < d < 2 mm)

7.41 2.49 1.81 2.64

0.82 0.99 1.00 0.80

0.966 0.959 0.952 0.910

10.24 20.52 27.18 38.80

where Mt/M0 is the percentage of active ingredient released at time t, K is a constant that incorporates characteristics of the macromolecular network system and the active ingredient, and n is a diffusional parameter which is indicative of the transport mechanism. The values of K and n obtained from initial 90% N-NO3released in water are presented in Table 3. There was good correlation of the release profiles of CR ammonium nitrate granules with the empirical equation, the correlation coefficient (r) being higher than 0.96. The values of n ranged from 0.68 for N90Etc10 (1 mm < d < 2 mm) granules up to 1.12 for N80Etc18S2 (1 mm < d < 2 mm) granules. The time period for 50% release of N-NO3-, T50, was calculated for the granules using the constants from Table 3. The values ranged from 7.47 h for N90Etc10 (1 mm < d < 2 mm) granules up to 35.74 h for N80Etc18P (1 mm < d < 2 mm) granules. The variation order is N90Etc10 (1 mm < d < 2 mm) < N80Etc20 (1 mm < d < 2 mm) < N80Etc18S1 (1 mm < d < 2 mm) < N80Etc20 (2 mm < d < 3 mm) < N80Etc18S2 (1 mm < d < 2 mm) ≈ N80Etc18P (1 mm < d < 2 mm). This variation order shows that the presence of plasticizers in formulations retards the release of ammonium nitrate in relation to those without plasticizers. This fact could be due to the formation of films without cracks when plasticizers have been added with ethylcellulose that can diminish the permeability as shown by Lecomte et al.14 From the T50 data for systems without plasticizer, N90Etc10 (1 mm < d < 2 mm) and N80Etc20 (1 mm < d < 2 mm), we see that the value of T50 is higher in the system with a greater ethylcellulose percentage (N80Etc20). It is due to the higher coated thickness. Others have found that T50 in the delivery of coated fertilizers increases lineally with the coated thickness.27 If we observe the T50 values from N80Etc20, in the two size-tested fractions (1 mm < d < 2 mm and 2 mm < d < 3 mm), we notice that the T50 value is higher in the higher sized system. This behavior is related to the results of studying imidacloprid release from lignin matrix,28 where the T50 values increase as the diameter of the matrixes grow. The addition of plasticizer in the coating process produces an increase of T50 values, and the DBP produces a slower release rate compared to the DBS. This is probably because of the hydrophobicity caused by the plasticizer29 as well as the smaller thickness of coating for N80Etc18S1 compared to that for N80Etc18P.24,27 In addition, we found similar values of T50 for the systems N80Etc18S2 and N80Etc18P. This last observation shows

Figure 6. Cumulative release of N-NO3- from granules into soil (error bars represent the standard deviation of two replicates).

Figure 7. T50 values of N90Etc10 (1 mm < d < 2 mm), N80Etc20 (1 mm < d < 2 mm), and N80Etc18S1 (1 mm < d < 2 mm) granules obtained from water release kinetics versus T50 values obtained from soil release kinetics.

that the behavior of these two formulations is analogous. They allow us to use a less aggressive plasticizer for environments like DBS as opposed to DBP. In another way, the T50 value of N80Etc18S2 was higher than that obtained for N80Etc18S1. This can be due to the higher homogeneity, less porosity, and more hydrophobicity of the coating film of the former. Others investigated the use of natural and synthetic organic compounds like paraffin wax, rosin, and polyethylene to control the release of nitrogen fertilizers (e.g., ammonium nitrate).10,30 Ibrahim and Jibril30 showed that the release rate of ammonium nitrate in water from granules coated with wax or a mixture of wax and rosin (wax/rosin) decreased in relation to uncoated fertilizer. The CR profiles of uncoated, wax-coated, and wax/rosin-coated NH4NO3 at 25 °C showed that for the uncoated product NH4NO3 was immediately released and for wax-coated and wax/ rosin-coated granules 80% of ammonium nitrate was released in approximately 50 h. Soil release kinetics. In Figure 6 the release profiles for the coated granules and the solubility profile for commercial ammonium nitrate in soil are shown. Here, ammonium nitrate dissolved immediately into soil under the experimental conditions, and all of the CR coated granules tested produce a decrease in the process of dissolution of the active ingredient in relation to the commercial ammonium nitrate product.

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It is observed that in the systems without plasticizer, the formulation which contains more ethylcellulose (N80Etc20) produces a higher retard in the release process. If its size is bigger, this effect is prominent. A slower release rate for the system N80Etc18S1 is obtained compared to other formulations. This fact is due to a lower permeability of the coating. With the aim of comparing the characteristics of N-NO3release from coated granules in soil with those obtained in water, the release data of N-NO3- from granules into soil were fitted to the empirical equation proposed by Ritger and Peppas.26 The values of K and n obtained from initial 90% N-NO3released are presented in Table 4. There was a good correlation of the release profiles of CR ammonium nitrate granules with the empirical equation, with the correlation coefficients (r) being greater than 0.91. The values of n ranged from 0.80 for N80Etc18S1 (1 mm < d < 2 mm) granules up to 1.00 for N80Etc20 (2 mm < d < 3 mm) granules. Time corresponding to 50% release of N-NO3-, T50, ranged from 10.24 h for N90Etc10 (1 mm < d < 2 mm) granules up to 38.80 h for N80Etc18S1 (1 mm < d < 2 mm) granules. The variation order is N90Etc10 (1 mm < d < 2 mm) < N80Etc20 (1 mm < d < 2 mm) < N80Etc20 (2 mm < d < 3 mm) < N80Etc18S1 (1 mm < d < 2 mm). It is observed that the lowest value in T50 belongs to the system with the lowest amount of ethylcellulose, N90Etc10. However, the highest value is observed in the system containing DBS, N80Etc18S1. In all these cases, the value of the parameter T50 is less in the experiences of release in water than those obtained in soil. This could account for the occlusion of the formulation surface by soil particles and slower diffusion within the soil as compared to water exclusively.31 Given the different applications of fertilizers, it would be interesting to find a relationship between the characteristic parameter of the release process in water, T50(water), and the corresponding one to the delivery process in the soil, T50(soil). Figure 7 shows the plot of the T50 values in water versus the T50 values in soil. The analysis indicates that T50 values in water are well-correlated with the T50 values in soil. A linear correlation of the T50 values from CR granules (1 mm < d < 2 mm) was obtained.

T50(soil) ) 1.792T50(water) - 1.555

r ) 0.991 (p ) 0.084)

From the linear correlation obtained, the release of ammonium nitrate in soil could be readily predicted from the release experiments made in water. As it has been indicated above, these values of T50 in soil will be used to predict the kinetic behavior of the prepared systems and, therefore, to design the correct release profile of the active ingredient. Conclusions Control release systems of ammonium nitrate have been obtained by coating a commercial product with a biodegradable polymer like ethylcellulose. Encapsulation efficiency was good. Moreover, the homogeneity of the prepared coated granules has been tested by SEM. Kinetics of release in water have shown that the process of ammonium nitrate release can be controlled, mainly changing the thickness of the coating film and using different plasticizers. A linear regression of the T50 values in both water and soil was obtained. This allows us to predict the behavior of the formulations in soil, once the study of release was obtained in water. Finally, this study may help us to quantify

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ReceiVed for reView November 29, 2006 ReVised manuscript receiVed March 9, 2007 Accepted March 9, 2007 IE061530S