Preparation and Properties of Novel Slow-Release NPK Agrochemical

Ind. Eng. Chem. Res. 2009, 48, 6527–6534

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Preparation and Properties of Novel Slow-Release NPK Agrochemical Formulations Based on Poly(acrylic acid) Hydrogels and Liquid Fertilizers Mircea Teodorescu,* Anamaria Lungu, Paul O. Stanescu, and Constantin Neamt¸u Department of Polymer Science and Engineering, Polytechnic UniVersity, 149 Calea Victoriei, 010072 Bucharest, Romania and National Institute of Research and DeVelopment for Chemistry and Petrochemistry-ICECHIM, 202 Spl. Independentei, 06002, Bucharest, Romania

New slow-release agrochemical formulations based on cross-linked poly(acrylic acid) hydrogels and liquid fertilizers (LF) were prepared by free radical copolymerization of acrylic acid (AA) and N,N′-methylenebisacrylamide directly in the LF solution. Two NPK liquid fertilizer compositions containing urea and potassium and ammonium phosphates were employed. For comparison, AA was also polymerized under identical conditions in distilled water. The resulting products were characterized by FTIR spectroscopy and scanning electron microscopy, and their water absorption and slow-release properties were determined. The results showed that the swelling degree (SD) of the hydrogels synthesized depended on the overall concentration of reactants (monomers and initiator), LF composition and cross-linking agent, and initiator concentrations. By appropriately combining these reaction parameters, superabsorbent hydrogels with SDs in distilled water ranging from a few hundred to 1000 g of water/g of xerogel can be obtained. The fertilizer-containing hydrogels displayed slow-release properties in still distilled water at room temperature. These slow-release formulations will be tested in the future for their effect on corn and sunflower crops. 1. Introduction Hydrogels are polymeric networks made up of hydrophilic polymers and able to incorporate and retain large amounts of water.1,2 A large variety of homo- and copolymers, both naturally occurring, for example, chitosan, dextran, agarose, collagen, etc., and synthetic, for instance, poly(vinyl alcohol), poly(N-vinyl pyrrolidone), poly(2-hydroxyethyl methacrylate), poly(acrylic acid), polyacrylamide, poly(N-isopropylacrylamideco-acrylic acid), and poly(ethylene oxide), have been employed for the preparation of hydrogels.3 Their ability to absorb water is due to the presence of hydrophilic groups like hydroxyl, amide, carboxyl, ether, sulfonate, ammonium, etc.4 The hydrogel field has continuously expanded within the last decades, as hydrogels have found numerous and important applications in medicine and pharmacy,3,5-7 as well as agriculture,8-10 and many other fields. The main biomedical applications of hydrogels are in the area of biosensors, controlled drug delivery, and tissue regeneration and repair, while the agricultural ones can be divided into two main categories: (i) soil conditioners for improving water retention in soils and the water supply of plants and (ii) controlled release of agrochemicals (AGC), both pesticides and fertilizers. Slow-release AGC formulations have been designed to correct some of the drawbacks of the conventional application of AGC, such as low efficiency and groundwater contamination. It was determined that up to 90% of the applied amount of pesticides and fertilizers has not reach the plant to induce the expected biological effects, but it is washed off by rain and irrigation water, ending up in both surface and underground waters.11,12 Therefore, besides the substantial economic losses due to a very low efficiency use of pesticides and fertilizers, environmental pollution becomes a very serious issue. The slow/controlled release of the AGC in smaller amounts over a longer period of time effectively reduces losses and almost entirely solves the problem. This effect is obtained * To whom correspondence should be addressed. E-mail: mirceat@ tsocm.pub.ro.

through encapsulating the pesticides and fertilizers into polymer structure, which allows the active component to slowly diffuse toward the exterior, thus making it available in the field for a longer period of time. The lower AGC discharge rate brings many advantages to slow-release formulations over conventional ones, such as (i) a decrease of environmental pollution, (ii) sustained supply of AGC for a longer period of time, (iii) increased proportion of the AGC being used by plants, (iv) reduced toxicity, etc.13,14 Among other materials employed to slow down the release of the AGC in the field, hydrogels have been intensely researched within the last two decades, as they are able to both control the release of the active component and improve the water retention in soil due to their ability to absorb water.11,14-19 Especially useful from this point of view are the so-called superabsorbent hydrogels, which are able to absorb water several hundred times their weight in dry state.12,13,20-26 To obtain such controlled release devices, the hydrogel can be loaded with the active component (fertilizer or pesticide) by two methods: (i) absorption of the aqueous solution of the

Figure 1. FTIR spectra of nonloaded (a) and LF-A- (b) and LF-B- (c) loaded xerogels. Synthesis conditions: [AA] ) 1.52 mol/L, [MBA]/[AA] ) 0.01, [APS]/[AA] ) 0.01, 50 °C, polymerization time ) 3 h.

10.1021/ie900254b CCC: $40.75  2009 American Chemical Society Published on Web 06/02/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009

Figure 2. SEM micrographs of the surface of nonloaded (a) and LF-A- (b) and LF-B- (c) loaded xerogels. Synthesis conditions: [AA] ) 1.52 mol/L, [MBA]/[AA] ) 0.01, [APS]/[AA] ) 0.01, 50 °C, polymerization time ) 3 h.

Figure 3. Influence of the concentration of reactants on SD: [MBA]/[AA] ) 0.01, [PSA]/[AA] ) 0.01, 50 °C.

fertilizer by the dry gel (xerogel) until the equilibrium swelling is reached, followed by removal of water from the hydrogel11,18 and (ii) the hydrogel is prepared directly into the fertilizer aqueous solution, followed by drying up of the resulting material.13,14,21,25 Both methods have advantages as well as disadvantages. Thus, the first technique allows a better control of hydrogel formation by avoiding the side reactions with the active component, which can also be affected during the hydrogel synthesis reaction, for example, when irradiation is employed to cross-link the polymer.14,18,24 In exchange, in many cases, the method is more disadvantageous than the second one from

the industrial application point of view as it requires a larger number of steps: hydrogel synthesis in aqueous solution, drying, absorption of fertilizer, and again drying. The advantage of the second method is mainly technological as it requires only one drying step. However, the polymerization reaction has to be carried out to completion, especially in the case of hazardous monomers, as it may be very difficult to remove the unreacted monomer. The present paper investigates the preparation conditions and properties of some slow-release NPK fertilizer formulations based on poly(acrylic acid) (PAA) hydrogels and liquid fertilizers (LF), which are also able to improve the water retention in soil. LF are fertilizers that are delivered by the manufacturer as aqueous solutions. Their use has increased steadily over the last 25 years, as they are generally easier to transport than the solid ones and cause fewer work problems in handling and application.27 We chose LF as the loading medium for the hydrogel for two reasons: (i) in many cases LF provide nutrients not generally available in solid fertilizers, especially micronutrients, and (ii) LF have a technological advantage over the solid fertilizers, as they are already aqueous solutions, and therefore, it is not necessary to dissolve the fertilizer into water. To the best of our knowledge, the employment of LF to the preparation of slow-release agrochemical formulations has not been reported in the literature yet. Because of the above-mentioned technological advantages,. the preparation of the poly(acrylic acid) hydrogel was carried out by polymerizing the monomer directly into the LF. Two typical NPK liquid fertilizer compositions containing urea and potassium and ammonium phosphates were employed as polymerization media, and the influence of the synthesis conditions (concentration of reactants, reaction time,


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Scheme 1. Influence of Reactant Concentration on Hydrogel Structure

Figure 5. Dependence of SD on urea and K2HPO4 concentration in the polymerization medium: [AA] ) 1.52 mol/L, [MBA]/[AA] ) 0.01, [PSA]/ [AA] ) 0.01, 50 °C.

LF composition) onto hydrogel water absorption and fertilizer release rate was investigated. 2. Experimental Section 2.1. Materials. Acrylic acid (AA, Fluka, 99%) was distilled under vacuum to remove the polymerization inhibitor and stored in the freezer. Ammonium persulfate (APS, Loba Chemie, 98%), N,N′ -methylenebisacrylamide (MBA, Merck, 98%), H3PO4 (Riedel de Hae¨n, 85%), urea (Riedel de Hae¨n, 99%), KOH (Riedel de Hae¨n, 85%), ammonia solution 25% (Merck), and all other reagents were employed without further purification. The two LF compositions employed have been prepared similar to an industrial procedure28 as follows. LF-A composition (NPK 411-type fertilizer) contained the following: (1) mono- and dipotassium phosphates obtained by mixing H3PO4 (37% aqueous solution), 168 g/L, and KOH (40% aqueous solution), 134 g /L, resulting a solution with pH ) 7; (2) urea ) 388 g/L; (3) demineralized water up to 1 L total volume. The concentration of solids of the resulting liquid mixture was determined to be 486 g/L, while the composition was N ) 179 g/L, P2O5 ) 45 g/L, and K2O ) 48 g/L, which agreed very well with the NPK 411 fertilizer formula, where the numbers indicate the approximate N/P2O5/K2O weight ratio, i.e., 4/1/1. LF-B composition (NPK 231 - type fertilizer) contained the following: (1) mixture of mono- and dibasic potassium and

Figure 4. Dependence of SD on polymerization medium: [AA] ) 1.52 mol/L, [MBA]/[AA] ) 0.01, [PSA]/[AA] ) 0.01, 50 °C.

ammonium phosphates obtained by mixing H3PO4 (37% aqueous solution) ) 505 g/L with KOH (40% aqueous solution) ) 161 g/L, and then adding ammonia 25% solution ) 165 g/L until a neutral pH of the resulting solution was obtained, (2) urea ) 121 g/L; (3) demineralized water up to 1 L total volume. The concentration of solids of the resulting liquid mixture was determined to be 387 g/L, while the composition was N ) 90 g/L, P2O5 ) 136 g/L, K2O ) 54 g/L, which agreed well with the NPK 231 fertilizer formula. 2.2. Preparation of FertilizersLoaded and Nonloaded Hydrogels. A stock solution was prepared in a 20 mL glass vial from the appropriate amounts of MBA and APS dissolved in 8 mL of distilled water or liquid fertilizer and the calculated amount of AA. In the case of polymerizations carried out in LF-B, a small amount of a white precipitate formed inside the vial after the AA addition, which dissolved easily by slightly warming the mixture. Samples of 2 mL each were transferred by means of a syringe from the stock solution to four 10 mm diameter glass tubes and bubbled with nitrogen gas for 5 min to remove any dissolved oxygen. The tubes were then sealed by rubber septa and placed in an oil bath at 50 °C. After appropriate time intervals, tubes were removed from the heating bath, cooled in an ice-water bath to stop the reaction, and broken, and the resulting hydrogel rods were cut into small diskshaped pieces about 3 mm thick. Part of the hydrogel pieces obtained were then placed into an excess of distilled water for 5 days at room temperature. The water was changed daily in order to remove the salts and unreacted monomer. After this purification step, the swelled hydrogels were placed into an oven kept at 30 °C, for 16 h, in order to determine the swelling degree. At the end of the purification-swelling period, the hydrogels were recovered by vacuum filtration using a G1 filtering crucible, weighed (Wh) and dried to constant weight (Wx) at 50 °C in an oven at atmospheric pressure. The unpurified pieces of the hydrogels were dried as well in the oven at 50 °C to constant weight. The fertilizer content of the loaded hydrogels was practically equal to that calculated from the amount of reactants employed, as demonstrated by the results of the fertilizer release experiments (see below). 2.3. Determination of the Swelling Degree. The swelling degree (SD) was calculated as the ratio between the amount of water absorbed by the hydrogel during the purification-swelling period and the amount of dry polymer (xerogel) according to eq 1. SD(g of water/g of dry polymer) )

Wh-Wx Wx

(1)

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Figure 6. FTIR spectra of purified xerogels prepared in distilled water (a), LF-A (b), and LF-B (c): [AA] ) 1.52 mol/L, [MBA]/[AA] ) 0.01, [APS]/ [AA] ) 0.01, 50 °C, polymerization time ) 3 h.

2.4. Determination of the Fertilizer Release. To study the release of the fertilizer, 0.5 g of crushed fertilizer-loaded xerogel of known composition was placed into 100 mL of distilled water under unstirred condition at room temperature. A 2 mL amount of solution was collected from the liquid surface at different time intervals, and the amount of fertilizer released was determined gravimetrically by completely removing the water in an oven at 60 °C until a constant weight was obtained. The percentage of the fertilizer released was determined according to eq 2: % fertilizer released ) n-1

(∆W)n × [100 - (n - 1) × 2]/2 +

∑ (∆W)

i

i)1

W0

(2)

where (∆W)i is the amount of fertilizer released found in the ith 2 mL sample and W0 is the initial amount of fertilizer contained by the xerogel sample analyzed. 2.5. Characterization. The IR spectra of the sample were recorded on a Shimadzu 8900 FTIR instrument from KBr pellets. Scanning electron microscopy (SEM) analysis of the surface of the fertilizer-loaded and nonloaded xerogels was carried out on a Hitachi S2600N instrument. 3. Results and Discussion The encapsulation of fertilizers in PAA hydrogels was carried out through APS-initiated cross-linking radical copolymerization of AA and MBA in LF at 50 °C. Two LF compositions, NPK 411 and NPK 231, containing urea and mono- and dibasic potassium and ammonium phosphates were employed as polymerization solvents in order to investigate the influence of the fertilizer components on the hydrogel properties. As this work was meant to be an applied (technological) study, the AA concentrations employed were chosen based on economic consideration, i.e., a smaller concentration of PAA and a larger one of solid fertilizer (SF) in the final product. Thus, the AA/ LF-A ratios were calculated so that final products with PAA/ SF-A ratios of 1/2, 1/3, and 1/4 were obtained. These ratios correspond to an AA concentration in the LF-A polymerization medium of 2.74, 1.94, and 1.52 mol/L, respectively. For comparison, AA was polymerized under identical conditions in LF-B and in distilled water. The hydrogels prepared were characterized, and the influence of the synthesis conditions (concentration of reactants, polymerization time, and composition of the reaction medium) upon

their SD/water absorption was investigated. Also, the release of the fertilizer by the hydrogels was studied as a function of the preparation conditions. 3.1. Characterization of Hydrogels. The hydrogels prepared were characterized by both FTIR spectroscopy and scanning electron microscopy (SEM). 3.1.1. FTIR Spectral Analysis. The FTIR spectra of the nonloaded and LF-loaded xerogels prepared are shown in Figure 1. The FTIR spectrum of the nonloaded xerogel (Figure 1a), which is made up mainly of PAA, displays the characteristic carbonyl peak at 1712 cm-1 in addition to the broad band at 2500-3500 cm-1 ascribed to the carboxylic OH group. The spectrum of the LF-A-loaded xerogels (Figure 1b) shows mainly peaks characteristic for urea at 3445, 3344, 1682, 1628, 1465, and 1153 cm-1 due to its bigger concentration as compared to the other constituents of the material, i.e., PAA and potassium phosphates. However, the presence of PAA in the material is confirmed by the shoulder at 3252 cm-1, ascribed to the carboxylic OH group. In the case of LF-B-loaded xerogels (Figure 1c), where the salts were in a much larger amount than urea, the characteristic peaks of ammonium dihydrogen phosphate at 2426, 1292, 1103, and 914 cm-1 can be seen in the IR spectrum along with those indicative for the presence of urea. The new peak at 1404 cm-1 may be ascribed to both ammonium dihydrogen phosphate and diammonium hydrogen phosphate, while the peak at 3252 cm-1 due to the PAA network was more prominent than that observed previously. No peaks characteristic for potassium phosphates (1465 and 1377 cm-1) can be clearly seen, very likely due to both their low intensity and superposition to the peaks of the other components. 3.1.2. Scanning Electron Microscopy Study. Scanning electron micrographs of the surface of the nonloaded and LF-loaded disk-shaped xerogels prepared are shown in Figure 2 The analysis clearly indicates the presence of the fertilizer at the xerogel surface, whose morphology depended on the fertilizer composition. Thus, the surface of the LF-A-loaded, urea-rich, xerogel was covered by small urea crystals around 10 µm in size (Figure 2b), while in the case of LF-B-loaded xerogel (Figure 2c) the surface displayed a multilayered morphology, more compact, very likely made up of a mixture of ammonium and potassium phosphates, which are the main components of the LF-B fertilizer. Only a small amount of well-defined crystals could be seen on the surface of LF-B-loaded xerogel. The micrographs also reveal the presence of large cracks on the xerogel surface in all cases, which formed very likely during the drying process. 3.2. Influence of the Overall Concentration of Reactants on Water Absorption. As previously mentioned, we employed three AA concentrations in order to obtain final products with various SF/PAA ratios. To check the influence of the overall concentration of reactants on the SD of the resulting hydrogels, the AA concentration was modified while keeping constant the reactant ratios, i.e., MBA/AA and APS/AA. The results displayed in Figure 3 show that lower reactant concentrations led to larger water absorptions, the behavior being valid for polymerizations carried out in both LF and distilled water. One can also notice that the reaction time had a very small influence upon the SD, especially for larger reactant concentrations, which indicates that the monomer conversion changed very little within the investigated time interval under the experimental conditions employed, being very large from the beginning of the polymerization. As a consequence, we may consider that the SD increasing with dilution was due to the formation of a lower


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Figure 7. Influence of initiator concentration on the SD of the hydrogels prepared in distilled water: [MBA]/[AA] ) 0.01, 50 °C.

Scheme 2. Influence of Initiator Concentration on Hydrogel Structure

cross-linking density hydrogel, as dilution favors the formation of loops instead of cross-linking points (Scheme 1), thus diminishing the number of elastically effective network chains. 3.3. Influence of the Polymerization Medium on Water Absorption. Figure 3 and even more clearly Figure 4 show that the SD of the forming network was strongly affected by the polymerization medium in the sense that the polymerizations carried out in LF led to hydrogels with higher water absorption than the syntheses performed in distilled water. The composition of the LF counted also because LF-B as the polymerization medium determined higher SDs than LF-A. This dependency of the hydrogel water absorption on the composition of the polymerization medium may be rationalized through the equilibrium reaction taking place between AA and the phosphates contained by the LFs, leading to the formation of AA salts (eq 3). Insertion of these ionic acrylates into the hydrogel network promotes water absorption through both expanding the polymer lattice due to the electrostatic repulsion between identical charges and the charge imbalance produced between the interior and the exterior of the hydrogel (Donnan equilibrium effect).4 CH2dCHsCOOH + K2HPO4 h CH2dCHsCOO-K+ + KH2PO4 (3)

Figure 8. Influence of initiator concentration on the SD of the hydrogels prepared in LF-A: [MBA]/[AA] ) 0.01, 50 °C.

To support this explanation, AA polymerizations were carried out in the presence of increasing concentrations of urea and dipotassium phosphate, two of the main components of both LF compositions (Figure 5). Indeed, practically no effect of urea concentration on the SD of the resulting hydrogels was observed, while the SD strongly increased with the K2HPO4 concentration. Additional support for our hypothesis was brought by the FTIR analysis of the purified hydrogels prepared in both distilled water and LF solutions (Figure 6). The formation of acrylate unit salt was revealed by the presence of the peak at 1557 cm-1 (indicated with an arrow on the figure) in the IR spectra of the hydrogels prepared in LF, which is characteristic for the carbonyl group belonging to a carboxylate (-COO-) group. The spectra in Figure 6 also show that the peak at 1557 cm-1 (-COO- group) increased while the one at 1712 cm-1 (-COOH group) decreased, as the concentration of salt in the polymerization medium increased (distilled water < LF-A < LF-B). Therefore, the concentration of carboxylate groups (ionic charges) was the highest in the hydrogel obtained in LF-B, leading to the largest water absorption (Figure 4).

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Figure 9. Influence of cross-linking agent concentration on SD of the hydrogels prepared: [APS]/[AA] ) 0.01, 50 °C, polymerization time ) 3 h.

Figure 10. Comparison between the influence of the concentrations of MBA and APS onto the SD of the hydrogels prepared: [AA] ) 1.94 mol/L, 50 °C, polymerization time ) 3 h.

Figure 11. Slow-release behavior of the LF-loaded hydrogels prepared under various conditions. Hydrogel preparation: [AA] ) 1.52 mol/L, [APS]/[AA] ) 0.01.

Additional experimental evidence for the presence of acrylate salts in the LF-AA mixture is the formation of a small amount of a white precipitate after addition of AA to the LF-B solution (see the Experimental Section). This precipitate was very likely made of acrylate salts whose solubility in the reaction mixture was diminished by the presence of the inorganic salts. Polymerization of partially neutralized acrylic acid solutions was already reported in the literature as a method for the preparation of slow-release fertilizer hydrogel formulations with superabsorbent properties.13,25 In our case, there was no need for an AA partial neutralization step as the LF successfully fulfilled this task and superabsorbent hydrogels (SD > 200 g of water/g of xerogel, Figure 4) resulted directly.

3.4. Influence of the Initiator Concentration on Water Absorption. Initiator concentration is an important parameter of the free radical polymerization, affecting polymerization rate, molecular weight of the resulting polymer, and monomer conversion. However, the way it influences these polymerization features depends on several parameters, including reaction temperature, viscosity of the reaction mixture, etc. In the case of hydrogels prepared by free radical polymerization, it was already shown that initiator concentration may affect the parameters of the forming polymer network and thus the swelling degree.13,17,22,25 We investigated the effect of the initiator concentration on the water absorption for our polymerization systems as well, and we noticed a different influence depending on the composition of the reaction medium, i.e., distilled water vs LF-A. Thus, when the polymerization was carried out in distilled water, increasing the concentration of initiator led to an increase of the SD (Figure 7a and 7b). This dependency can be explained through a fast decomposition of APS at the reaction temperature with the generation of a relatively large amount of primary radicals that react with the growing macroradicals (primary radicals termination), leading to the formation of grafts instead of cross-linking points and, consequently, to a decrease of the cross-linking degree (Scheme 2).29 Therefore, when distilled water was the polymerization medium, the larger the initiator concentration was, the lower the cross-linking degree and the higher the SD. In addition, we noticed that in more diluted solutions ([AA] ) 1.52 mol/L vs [AA] ) 2.74 mol/L) SD slightly decreased with time at lower initiator concentration, indicative of an increase of conversion on the time interval investigated. In these cases, the polymerization rate was lower due to the smaller concentrations of both monomer and initiator. In the case of polymerizations carried out in LF-A, an opposite influence of the initiator amount was noticed on the concentration interval investigated, i.e., SD decreased with increasing concentration of APS (Figure 8). A possible explanation is based on the higher viscosity of LF-A. Because of the higher viscosity of the polymerization medium, the lifetime of the primary radicals in the solvent cage increased, leading to a larger proportion of radical-radical reactions and, therefore, to a smaller number of radicals escaping the solvent cage (the cage effect30). The number of primary radicals participating in the polymerization process was thus decreased, and therefore, the graft formation phenomenon diminished, simultaneously with an increase of the cross-linking degree, leading to lower SD. In this case, larger initiator concentrations resulted in higher


monomer double bond conversions and, consequently, higher cross-linking networks with lower water absorptions. 3.5. Influence of the Cross-Linking Agent Concentration on Water Absorption. The cross-linking agent concentration is one of the most important tools available to control the cross-linking density of a network and therefore its ability to swell in a certain solvent. Indeed, our experiments showed that by changing MBA concentration the SD of the resulting hydrogel was strongly affected (Figure 9a and 9b). Thus, by increasing MBA concentration a steady decrease of the hydrogel water absorption was noticed on the interval investigated due to the formation of networks with increasing cross-linking degrees, irrespective of the polymerization medium. It is worth noting that in the case of the polymerization carried out in LFB, hydrogels with remarkably high SDs (>500 g of water/g of xerogel) formed (Figure 9b), which may be employed as superabsorbent materials able to improve the water retention in soil. By comparing the influence of the last two factors discussed (Figure 10), one can see that the modification of the cross-linking agent concentration had a much stronger effect on the SD of the resulting hydrogel than the initiator concentration. Therefore, to modify the hydrogel water absorption is much more efficient to adjust the concentration of the cross-linking agent instead of that of the initiator. 3.6. Release Rate of the Fertilizer. Some of the fertilizercontaining xerogels prepared under different conditions were tested for their slow-release properties in still distilled water at room temperature. The results displayed in Figure 11 showed a reduced rate of fertilizer release from the hydrogel in all cases. The release rate of the fertilizer was smaller for higher MBA/ AA ratios. This is in agreement with what was expected, as a larger concentration of cross-linking agent leads to a more dense network, with a lower SD, which will discharge the fertilizer in a slower way.11,21 A lower fertilizer release rate was also displayed by the LF-B-loaded hydrogel by comparison with the LF-A-loaded one, although the SD was higher in the first case (Figure 9b). In this case the difference may be ascribed to both different composition of the fertilizers contained by hydrogels and different ionic group content of the hydrogels. The LF-Bloaded hydrogel has a more ionic network, and its fertilizer loading possesses a higher salt content. The interactions among the salt ions and the carboxylate groups located onto the hydrogel networks slowed down the release of the salts from the hydrogel.31 4. Conclusions Novel slow-release fertilizer formulations based on crosslinked PAA hydrogels and two LF compositions containing urea and potassium and ammonium phosphates were prepared by APS-initiated free radical copolymerization of AA and MBA, in various ratios, in the LF solution. The resulting products were characterized from the SD and slow-release properties point of view. The results showed that the water absorption of the hydrogels synthesized strongly depended on (i) overall concentration of reactants, because lower concentrations favor the formation of loops instead of crosslinking points within the hydrogel structure, leading to higher SDs, (ii) LF composition, as the acrylate salts resulting from the equilibrium reaction between AA and the phosphates contained by the LFs lead to the formation of an ionic hydrogel network, whose water absorption is larger for higher ionic units content, (iii) concentration of cross-linking agent, lower MBA concentrations leading to higher SDs. Initiator

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concentration also affected the water absorption of the hydrogels by favoring the formation of either grafts or crosslinking points, depending on the viscosity of the reaction medium, which is different for distilled water and LFs, respectively. However, initiator influence was relatively small as compared to the previously mentioned parameters. An appropriate combination of these factors, i.e., higher dilution, lower MBA concentration, and LF-B as the polymerization medium, allowed us to obtain superabsorbent hydrogels with a SD of about 1000 g of water/g of xerogel in distilled water. The fertilizer-containing hydrogels prepared displayed slowrelease properties in still distilled water at room temperature. The release rate was smaller for higher cross-linking density hydrogels, because of the lower SD, and LF-B-loaded hydrogels, due to the larger concentration of ionic groups within the network, which interact with the phosphate salts, slowing down their discharge into water. The formulations developed within this work display several technological advantages over the slow-release fertilizer compositions already described in the literature. Thus, they are synthesized starting from fertilizers that are commercially available as liquids, i.e., liquid fertilizers, instead of from the solid ones. This avoids the dissolution step, in addition to the storage of a solid fertilizer that in some cases, for example, ammonium nitrate, requires special precautions. In the case of our formulations we also showed that it is possible to obtain superabsorbent hydrogels from poly(acrylic acid) directly, without partially neutralizing the acrylic acid before polymerization,13,19,25 which is another technological simplification of the synthesis process. We consider that these technological simplifications may make our procedure more economical to run. The slow-release formulations prepared in this work will be tested in the future for their effect on corn and sunflower crops. Acknowledgment The financial support of the National Authority for Scientific Research through the PN-II Grant No. 51-080/2007-ECOMICROFERT is gratefully acknowledged. Literature Cited (1) Peppas, N. A. Hydrogels. In Biomaterials Science: An Introduction to Materials in Medicine, 2nd ed.; Ratner, B. D., Hoffman A. S., Schoen, F. J., Lemons, J. E., Eds.; Academic Press: New York, 2004; p 100. (2) Hennink, W. E.; van Nostrum, C. F. Novel Crosslinking Methods to Design Hydrogels. AdV. Drug DeliV. ReV. 2002, 54, 13. (3) Hoffman, A. S. Hydrogels for Biomedical Applications. AdV. Drug DeliV. ReV. 2002, 54, 3. (4) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (5) Langer, R.; Peppas, N. A. Advances in Biomaterials, Drug Delivery and Bionanotechnology. AIChE J. 2003, 49, 2990. (6) Kashyap, N.; Kumar, N.; Ravi Kumar, M. N. V. Hydrogels for Pharmaceutical and Biomedical Applications. Crit. ReV. Ther. Drug Carrier Syst. 2005, 22, 107. (7) Ulijin, R. V.; Bibi, N.; Jayawarna, V.; Thornton, P. D.; Todd, S. J.; Mart, R. J.; Smith, A. M.; Gough, J. E. Bioresponsive Hydrogels. Mater. Today 2007, 10 (4), 40. (8) Kazanskii, K. S.; Dubrovskii, S. A. Chemistry and Physics of “Agricultural” Hydrogels. AdV. Polym. Sci. 1992, 104, 97. (9) Shaviv, A. Advances in Controlled Release Fertilizers. AdV. Agron. 2001, 71, 1. (10) Rudzinski, W. E.; Dave, A. M.; Vaishnav, U. H.; Kumbar, S. G.; Kulkarni, A. R.; Aminabhavi, T. M. Hydrogels as Controlled Release Devices in Agriculture. Des. Monomers Polym. 2002, 5, 39. (11) Bajpai, A. K.; Giri, A. Swelling Dynamics of a Macromolecular Hydrophilic Network and Evaluation of its Potential for Controlled Release of Agrochemicals. React. Funct. Polym. 2002, 53, 125.

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ReceiVed for reView February 16, 2009 ReVised manuscript receiVed May 13, 2009 Accepted May 22, 2009 IE900254B

Preparation and Properties of Novel Slow-Release NPK Agrochemical

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