Potassium Release Kinetics & Water Retention of Controlled-Release Fertilizers PDF

J Polym Environ (2010) 18:413–421 DOI 10.1007/s10924-010-0228-6 ORIGINAL PAPER Potassium Release Kinetics and Water Retention of Controlled-Release Fertilizers Based on Chitosan Hydrogels Tongsai Jamnongkan Supranee Kaewpirom Published online: 13 July 2010 Ó Springer Science+Business Media, LL...

J Polym Environ (2010) 18:413–421 DOI 10.1007/s10924-010-0228-6 ORIGINAL PAPER Potassium Release Kinetics and Water Retention of Controlled-Release Fertilizers Based on Chitosan Hydrogels Tongsai Jamnongkan Supranee Kaewpirom Published online: 13 July 2010 Ó Springer Science+Business Media, LLC 2010 Abstract Controlled release fertilizer (CRF) hydrogels Introduction were prepared from poly(vinyl alcohol), poly(vinyl alco- hol)/chitosan and chitosan using glutaraldehyde as a Chitosan (CS) is a natural polysaccharide, produced by crosslinker. Intermolecular interactions of the CRF deacetylation of chitin, and has a repeating structure unit of hydrogels were elucidated using FTIR. Water absorbency b-(1-4)-2-amino-deoxy-b-D-glucan. This biopolymer is characteristics of the CRF hydrogels were also studied. It synthesized by an enormous number of living organisms and was found that the CRF hydrogels exhibited the equilib- is the most abundant polymer after cellulose. Chitosan is rium swelling ratio (SR) in the range 70–300%. The water biocompatible, biodegradable and nontoxic. It has been retention of soil containing the CRF hydrogels was also widely used as materials for controlled release which have examined. It was found that the CRF hydrogels increased been applied in food, drug, biochemical and agricultural the water retention of the soil. After 30 days, soil con- areas [2, 3]. Controlled release systems made from chitosan taining the PVA-, PVA/CS- and CS-hydrogels showed the are often applied to fertilizer controlled release studies. water retention capacities of 25%, 10% and 4%, respec- Recently, it was reported by Lan and Mingzhu that the tively. While the soil without the CRF hydrogel had release behavior of a controlled release fertilizer coated with already given off most of the water. The release behavior chitosan depends on water absorbency of the chitosan. of potassium from the CRF hydrogels, both in deionized Although chitosan has been shown to have excellent biode- water and in soil, was investigated. In soil, the potassium gradability [5–7], it has a lower swelling ability when it release mechanism from the PVA- and PVA/CS-hydrogels forms hydrogel due to the slower relaxation rate of polymer were non-Fickian diffusion. On the other hand, the CS chains. Therefore, blending chitosan with other hydro- hydrogel showed, n value that was close to 1.0 corre- philic polymers help to improve its water absorbency at gel sponding to case II transport. In deionized water, all the state. Poly(vinyl alcohol) (PVA) is interested due to its CRF hydrogels showed small values of release exponent biocompatibility and excellent hydrophilicity [9, 10]. PVA is (n \ 0.5) indicating a quasi-Fickian diffusion mechanism. a water soluble synthetic polymer and is prepared by hydrolysis of polyvinyl acetate. PVA is an excellent film- Keywords Controlled release  Hydrogel  forming material and swells easily by absorbing water. Some Release kinetics  Swelling ratio  Water retention commercial grade PVA has shown volume expansion up to 500% at 37 °C , and therefore has been blended with other biopolymers in order to improve the water absorbency of the blends. However, PVA is difficult to be degraded in T. Jamnongkan  S. Kaewpirom (&) Department of Chemistry, Faculty of Science, natural environment. Thus, blending PVA with chitosan will Burapha University, Chonburi 20131, Thailand help to improve its biodegradation ability. Recently, it has e-mail: [email protected] been studied by many researchers that blending or copoly- merizing chitosan with PVA produces biodegradable poly- T. Jamnongkan  S. Kaewpirom Center of Excellence for Innovation in Chemistry, mer blend hydrogels that can be used in controlled release Burapha University, Chonburi 20131, Thailand applications [12, 13]. 123 414 J Polym Environ (2010) 18:413–421 Polymer hydrogels can be defined as three-dimensional Houwink. Detailed methodology is as follows. High polymeric networks that can retain a significant amount of molecular weight chitosan was used to prepare 1% (w/v) water within their structures and swell without dissolving in chitosan solution in solvent A (0.1 M acetic acid in water [14, 15]. Polymer hydrogels have been used in agri- 0.2 M NaCl). After maintaining it overnight at room culture as controlled release fertilizers. The use of temperature it was filtered through glass wool to remove controlled release fertilizers causes an increase in their insoluble part. Four dilutions were made by mixing 5, efficiency, reduces soil toxicity, minimizes the potential 10, 15, and 20 mL of the chitosan solution to 25 mL negative effects associated with overdosage and reduces the with solvent A. The relative viscosities were calculated frequency of the application. Moreover, these materials from the depletion times of the chitosan solutions and help to improve soil for cultivation with regard to better soil solvent A using an Ostwald–Fensky viscometer and a aeration. They also help to reduce irrigation water con- stopwatch. Experiments were conducted in triplicate sumption, lower the death rate of plants, improve fertilizer at room temperature. retention in soil, and increase plant growth rate [14, 18]. This paper presents the synthesis and characterization of new chitosan-based CRF hydrogels. Unlike the system Preparation of CRF Hydrogels reported by Rui et al. , in our systems, potassium fertil- izer compound was dispersed in the hydrogel matrix. The A PVA solution with a concentration of 10% (w/v) was CRF hydrogels, in forms of circular pads with *2 mm in prepared by dissolving PVA powder in deionized water thickness and *120 mm in diameter were prepared from at 90 °C under stirring for 2 h. A chitosan solution with poly(vinyl alcohol), chitosan and the blend of these two a concentration of 1% (w/v) was prepared by dissolving polymers, by interpenetrating polymer network technique. It chitosan flakes in 2% (w/v) acetic acid aqueous solution has been known that both PVA and chitosan are biodegrad- under stirring. The fertilizer solution was prepared by able polymers and their degradation products are environ- dissolving 2.0010 g of ammonium nitrate, 3.3015 g of mental friendly. In our study, PVA was blended with dihydrogen ammonium phosphate and 2.5275 g of chitosan in order to improve the water absorbency of chito- potassium nitrate to make a 100.00 mL aqueous solution. san as well as its mechanical properties at the gel state. Water To prepare the CRF CS hydrogel, 10 mL of chitosan absorbency and potassium release behavior of such the CRF solution was mixed with 2.5 mL of fertilizer solution. Then hydrogels in deionized water and in soil were investigated. 0.5 mL of 1.20% (w/v) glutaraldehyde was added into the The release kinetics and mechanism of potassium fertilizer mixture under constant stirring. After well mixing, the from each CRF hydrogels were also studied. mixture was poured into a Petri disk and the gel formed within 30 min. The CRF CS hydrogel obtained was dried at 40 °C in a vacuum oven overnight. Experimental The CRF PVA hydrogel was prepared by adding 2.5 mL of fertilizer solution into 10 mL of PVA solution Materials under constant stirring. Afterwards, 2.8 mL of crosslink solution (50% w/v methanol (the quencher), 10% w/v w High molecular weight chitosan and poly(vinyl alcohol), M acetic acid (the pH controller), 1.20% (w/v) of glutar- 85,000–124,000, were purchased from Sigma–Aldrich aldehyde and 10% (w/v) sulfuric acid (the catalyst), Company, Germany. Glutaraldehyde (GA, 1.2%, w/v) was make up a 3:2:1:1 weight ratio solution) was added into from Fluka Company, Switzerland. Ammonium nitrate the mixture under constant stirring. After well mixing, (NH4NO3) was analytical grade from John D. Bolton Com- the mixture was poured into a Petri disk and the gel pany, England and was dried under reduced pressure before formed within 15 min. The prepared CRF PVA hydrogel use. Potassium nitrate (KNO3) was analytical grade from was dried at 40 °C in a vacuum oven overnight. Merck Company, Germany. Dihydrogen ammonium phos- The CRF PVA/CS hydrogel was prepared by the phate ((NH4)2HPO4) was analytical grade from Fluka following method. PVA solution was mixed with chito- Company, Switzerland, and was used as received without san solution at a ratio 1:1 by weight. The mixture was further purification. stirred constantly until uniform and the appropriate amount of fertilizer solution and the crosslinking agent were added into the mixture under constant stirring. Molecular Weight of Chitosan After well mixing, the mixture was poured into a Petri disk and the gel formed within 30 min. The CRF PVA/ The viscosity average molecular weight (Mv ) of chitosan CS hydrogel product was dried at 40 °C in a vacuum was determined using the viscometric method of Mark and oven overnight. 123 J Polym Environ (2010) 18:413–421 415 Characterization of Hydrogels by FTIR Release Behavior in Water The synthesized hydrogels were characterized by a To study release behavior of potassium from the CRF Fourier-transform infrared (FTIR) spectrophotometer hydrogels, at room temperature, a dry CRF hydrogel (American Nicolet Corp., model 170-SX). sample was added into a beaker containing 75 mL of dis- tilled water. At certain time intervals (every 24 h until Water Absorbency of CRF Hydrogels 30 days), 30 mL solution was sampled for K determina- tion, and an additional 30 mL of distilled water was added A preweighed dry hydrogel sample was immersed into a into the beaker to maintain a constant amount of solvent. certain amount of deionized water. At certain time intervals The K content was obtained by Atomic absorption spec- (every 10 min) the hydrogel was taken out of the water. trophotometry. Excessive surface water of the swollen hydrogel was removed with a filter paper, and the weight of the swollen sample was measured. Swelling ratio (%SR) of the Release Behavior in Soil hydrogel was calculated using the equation: Ws  Wd To study the release behavior of potassium from the CRF %SR ¼  100 ð1Þ hydrogels in soil with different pH values (pH 4.1 and 7.3), Wd a dry sample of the CRF hydrogel was buried in 75 g of dry where Ws and Wd refer to the weight of swollen and dry sandy soil which was placed in a 200 mL plastic beaker. hydrogels, respectively. Then 22 mL of deionized water was added and the beaker was covered by a plastic film and was kept at room Water Retention Behavior of Soil Containing CRF temperature. At certain time intervals (every 24 h) during Hydrogels a period of 30 days, the soil was sampled for K determination. To study water retention of soil containing the CRF hydrogels, a dry sample of CRF hydrogel was buried in 100 g of dry sandy soil, which was placed in a plastic cup Modeling and Release Kinetics (A). The other 100 g of dry sandy soil, without CRF hydrogel, was placed in an identical plastic cup (B), then Mechanisms of potassium release from the CRF hydrogels each cup was weighed (W) for the precise weight. 50 mL of were characterized under the same physiological condi- deionized water was added into each cup and the cup was tions. In this present study, the mechanisms of potassium reweighed (Wo). Both cups were kept under identical release from the CRF hydrogels were investigated using a conditions at room temperature and were weighed every semi-empirical model, known as the power law or the day (Wt) over a period of 30 days. Water retention (%WR) Korsmeyer–Peppas model [22, 23]: of soil was calculated using the equation: Mt Wt  W ¼ ktn ð4Þ %WR ¼  100 ð2Þ M1 Wo  W where Mt and M? represent the amount of nutrient released Encapsulation Efficiency Analysis at a time t and at equilibrium, respectively, k is a constant characteristic of fertilizer–polymer system, and n is the To study encapsulation efficiency of fertilizer in the CRF diffusion exponent characteristic of the release mechanism. hydrogels, a CRF hydrogel sample was immersed into a For quasi-Fickian diffusion the value of n \ 0.5, Fickian certain amount of deionized water for 1 min and then kept diffusion n = 0.5, non-Fickian or anomalous transport aliquot solution was sampled for K determination, assayed n = 0.5–1.0 and Case II transport n = 1.0. Initial diffusion to determine the concentration of the unencapsulated fer- coefficient (D) can be calculated from the following tilizer. Encapsulation efficiency (%) was calculated by the equation : following formula :  0:5 Mt Dt %Encapsulation efficiency ¼4 ð5Þ   M1 pl2 Unencapsulated fertilizer ð3Þ ¼ 1  100 where l is the thickness of the hydrogel sample. Total fertilizer 123 416 J Polym Environ (2010) 18:413–421 Results and Discussion Molecular Weight of Chitosan Molecular weight is one of the key indices governing the functional properties of chitosan. Viscometry is the sim- plest and the most effective method for determining vis- cosity average molecular weight, M  v , of polymers. For a linear polymer, the relationship between intrinsic viscosity, [g], and M v is shown by the Mark–Houwink equation: v ½ g ¼ k M a ð6Þ where, k and a are constants independent of M v over a wide range. They are affected by solvent conditions such as temperature, pH and ionic strength. However, values of k and a depend on kind of solvent, polymer and temperature. For chitosan in solvent A, k and a values have been Fig. 1 The FTIR spectra of (a) chitosan and (b) chitosan hydrogel reported as 1.81 9 10-3 g/mL and 0.93 at 25 °C, respec- tively. In the present study, the intrinsic viscosity of Water Absorbency of CRF Hydrogels chitosan was experimentally determined using the intercept of reduced viscosity versus concentration plot, which was The effect of chemical structure of the hydrogels on their 735.55 mL/g, and M  v of the chitosan was estimated as water absorbency is shown in Fig. 5. Each data point 1.8 9 106 g/mL through the Mark–Houwink equation. represented in the figure is the average value of three measurements with standard deviation of 4%. In Fig. 5, all Characterization of Hydrogels by FTIR the hydrogels exhibit high initial swelling rates and then the rates become constant after 20 min. It can also be seen The IR spectra of chitosan and chitosan hydrogel (Fig. 1a, b) from the figure that, at equilibrium, PVA hydrogel shows show similar peaks as following. Broad peaks at 3367 cm-1 the highest water absorbency (*300%) among the three and at 2943 cm-1 relate to –OH and C–H stretching in hydrogels. PVA/CS- and CS-hydrogels show equilibrium chitosan, respectively. A peak found around 1562 cm-1 water absorbency around 225 and 70%, respectively. indicates the N–H group. A peak at 1408 cm-1 is assigned to Hydrophilic groups are responsible for such high water the –CH3 symmetrical deformation mode. Another two peaks absorbency. PVA is more hydrophilic than CS, thus, the found around 1154 and 1075 cm-1 indicate the C–O stretch- higher the PVA content within the CRF hydrogel structure, ing vibration in chitosan. A characteristic peak of chitosan the higher the water absorbency is. hydrogel is found at 1651 cm-1 (Fig. 1b). This corresponds to the formation of imine bond (C=N) via Schiff’s base structure Water Retention Behavior of Soil Containing CRF by the reactions between amino groups of chitosan and alde- Hydrogels hyde groups of glutaraldehyde (Fig. 2a). The IR spectra of PVA and PVA hydrogel (Fig. 3a, b) Figure 6 shows the water retention behavior of soil with show a broad band around 3309 cm-1 relating to the addition of the CRF hydrogels and soil without the CRF stretching O–H from the intermolecular and intramolecular hydrogels. Each data point represented in the figure is the hydrogen bonds. Peaks at 2943 cm-1 and at 1430 cm-1 average value of three measurements with standard deviation refer to the C–H stretching and –CH2– bending from alkyl of 3%. It was found that the CRF hydrogels increase the groups, respectively. A peak at 1094 cm-1 corresponds to water retention of the soil. Without CRF hydrogels, the water C–O stretching. For the PVA hydrogel, an observed retention capacity of soil remains only 2% after 20 days, vibration peak at 1214 cm-1 refers to C–O–C groups while that of the soil containing PVA-, PVA/CS- and (Fig. 3b), indicating a formation of acetal bridges, as pre- CS-hydrogels are 26%, 12% and 7%, respectively. After sented in Fig. 2b. 30 days, the water retention capacities of the soil containing The IR spectrum of the PVA/CS hydrogel film (Fig. 4c) PVA-, PVA/CS- and CS-hydrogels are 25%, 10% and 4%, shows all the characteristic peaks of both chitosan hydrogel respectively, while the soil without the CRF hydrogels had and PVA hydrogel. Those include the peak around already given off most of the water. Hence, all the CRF 1651 cm-1, relating to the imine bond (C=N), and a peak hydrogels show high water retention capacity in soil, and around 1214 cm-1, relating to C–O–C groups. with their use, water can be saved and managed. All the 123 J Polym Environ (2010) 18:413–421 417 Fig. 2 Formation of a the Schiff’s base (C=N) structure via the reactions between amino groups of chitosan and aldehyde groups of glutaraldehyde, b poly(vinyl alcohol) hydrogel using glutaraldehyde as a crosslinker Fig. 3 The FTIR spectra of (a) PVA and (b) PVA hydrogel Fig. 4 The FTIR spectra of (a) PVA hydrogel, (b) CS hydrogel and synthesized CRF hydrogels in this study, therefore, can be (c) PVA/CS hydrogel used for agricultural applications. possess different chemical structures, they exhibited very Encapsulation Efficiency Analysis high fertilizer encapsulation efficiency. It was found that the PVA-, PVA/CS- and CS-hydrogels Release Behavior in Water show encapsulation efficiency values of 99.98%, 99.96% and 99.69%, respectively. It can be concluded from the Figure 7 represents the potassium release behavior of the results that although all the prepared CRF hydrogels CRF hydrogels in deionized water at room temperature. 123 418 J Polym Environ (2010) 18:413–421 Fig. 5 Swelling ratio of (a) PVA hydrogel, (b) PVA/CS hydrogel and Fig. 7 Release behavior of potassium in water from (a) PVA (c) CS hydrogel hydrogel, (b) PVA/CS hydrogel and (c) CS hydrogel dissolves potassium nitrate leading to concentration dif- ference between the inside structure of the hydrogel and the outer solution. Consequently, potassium is released from the hydrogel. It can be concluded from the results that the chemical structure of the CRF hydrogel is one of the factors affecting the release behavior of potassium from the CRF hydrogels. In the other words, the release behavior of potassium can be controlled by varying the chemical structure of the CRF hydrogels. Release Behavior in Soil According to Jarosiewicz and Tomaszewska , the nutrient release from CRF hydrogels into soil takes place Fig. 6 Water retention of soil with (a) PVA hydrogel, (b) PVA/CS when the CRF hydrogels are swollen by soil solution. The hydrogel, (c) CS hydrogel and (d) soil without hydrogel solution, then, dissolves the soluble part of fertilizer, and the nutrients slowly diffuse through the CRF hydrogel The release rates of all the CRF hydrogels are high initially structure and release into the soil. and become constant after 3–6 days. PVA-, PVA/CS- and The release behavior of potassium, from all the CRF CS-hydrogels exhibit percent potassium cumulative release hydrogels, into soil (pH 4.1) is presented in Fig. 8a. The of 80%, 71% and 53%, within 6, 4 and 3 days, respec- results show the burst release behavior at the beginning and tively. Afterward, the release rates of all the CRF hydrogels the slow release after 2–6 days. PVA-, PVA/CS- and CS- become constant. At the beginning of the release period, hydrogels show the values of percent cumulative release of the potassium release rate is fast due to the high concen- potassium of 9%, 18% and 24%, within 5, 7 and 8 days, tration difference between the inside structure of the CRF respectively. Then the release rates become slower. The hydrogels and the outer solution. Then, the potassium values of total percent cumulative release of potassium release rate decreases as the concentration difference from PVA-, PVA/CS- and CS-hydrogels are 17%, 24% and decreases. The results are in good agreement with the 34%, respectively. The release behavior of potassium into results reported by Rui et al.. Among the three pre- soil with different pH value was also investigated. Fig- pared CRF hydrogels, PVA-hydrogel shows the highest ure 8b represents the potassium release behavior of the percent cumulative release of potassium, both initially and CRF hydrogels in soil with pH 7.3 at room temperature. It at equilibrium. The hydrophilicity of the PVA hydrogel is was found that the cumulative release of potassium into responsible for making potassium nitrate solution, since it soil increases with time. The results show the burst release can adsorb water within its structure. The water, then, behavior at the beginning and the slow release after 123 J Polym Environ (2010) 18:413–421 419 3–6 days. PVA-, PVA/CS- and CS-hydrogels show the values of percent cumulative release of potassium of 19%, 39% and 40%, within 4, 6 and 3 days, respectively. The values of total percent cumulative release of potassium from PVA-, PVA/CS- and CS-hydrogels are 34%, 46% and 63%, respectively. It can be seen that the fertilizer release rates and the total cumulative fertilizer release values are dependent upon the pH value. In aqueous media, it was reported by El-Sherbiny et al. that the % equi- librium swelling values of chitosan-based hydrogels were higher at low pH than at neutral pH because of the pro- tonation of the unreacted NH2 groups of chitosan at acidic pH, leading to dissociation of the hydrogen bonding involving the amino groups, and consequent facilitation of the entrance of solvent into the hydrogels and the release rate is higher at lower pH value. On the other hand, the increase of pH weakens the protonation of unreacted NH2 groups, and results in the smaller swelling ratio and the lower release rate in neutral media. However, in this study we found that at soil pH 4.1, the fertilizer release rates and the total cumulative fertilizer release values are lower than that at soil pH 7.3. Degradation of the hydrogels in soil is believed to be the reason for such results. It can also be seen from Fig. 8 that CS hydrogel exhibits the highest percent cumulative release of potassium among the three prepared hydrogels. In contrast with the release behavior in water, CS- and PVA-hydrogel display the highest and the lowest percent cumulative release, respectively, regardless of time. This is due to the fact that chitosan has reactive amino and hydroxyl groups, which provide the possibility of degradation [5–7, 27, 28]. In addition, the existence of many kinds of ions and micro- organisms in soil solution possibly increases the degrada- tion rate of the CRF hydrogels. These contribute to Fig. 8 Release behavior of potassium from (a) PVA hydrogel, (b) PVA/CS hydrogel and (c) CS hydrogel into soil a pH 4.1 and b pH 7.3 such high percent cumulative release of nutrients from the CS-hydrogel. On the other hand, the low percent cumulative release of the PVA hydrogel caused by its fitted to Eq. 4, the Korsmeyer–Peppas exponential equa- swelling ratio in soil solution is less than that in deionized tion. Figure 9a and b show plots of log(Mt =M1 ) versus log water. Thus, the diffusion of soluble fertilizers from the time for potassium release in soil and in deionized water, PVA hydrogel into soil is more difficult than that in respectively. From these plots, the release exponent, n, deionized water. correlation coefficient, R2 and release factor, k of each CRF In conclusion, for all the synthesized CRF hydrogels, the hydrogel were obtained. To further calculate the initial swelling ratio in soil solution is less than that in deionized diffusion coefficient (D), Eq. 5 has been employed. Results water. Thus the diffusion of soluble fertilizers from the of diffusion exponent, release factor and initial diffusion CRF hydrogels into soil is more difficult. Therefore, the coefficient for the swellable CRF hydrogels are listed in release of potassium from the CRF hydrogels in soil is Table 1. slower than that in deionized water. The PVA-, PVA/CS- and CS-hydrogels have n values, in soil, of 0.52, 0.95 and 1.05, respectively. The values of n Release Kinetics and Mechanisms for PVA- and PVA/CS-hydrogels are both greater than 0.5 and smaller than 1.0, indicating that the release of potas- In the present study the release patterns of potassium in soil sium from PVA- and PVA/CS-hydrogels are non-Fickian (pH = 4.1) and in deionized water have been studied. In diffusion or anomalous transport. This mechanism indi- order to investigate the release mechanism, the data were cates that the potassium is released by the combined 123 420 J Polym Environ (2010) 18:413–421 chitosan. However, it was found that in deionized water, all the CRF hydrogels show small values of release exponent (n \ 0.5). This indicates a quasi-Fickian diffu- sion mechanism , in which potassium diffuses partially through a swollen matrix and water filled pores in the CRF hydrogels. The values of the initial diffusion coeffi- cient for the release of potassium, both in deionized water and in soil, show that the diffusion coefficient is affected by chemical structure of the CRF hydrogel. PVA hydrogel shows higher initial diffusion coefficient than that of PVA/ CS-hydrogel, and PVA/CS-hydrogel shows higher initial diffusion coefficient than that of CS-hydrogel. This is due to the fact that PVA hydrogel possesses the highest initial swelling ratio among the three hydrogels. Therefore, the potassium can be released fastest from the PVA hydrogels. Hence, it can be concluded that chemical structure of the CRF hydrogels affects the swelling ratio of the hydrogels which is directly related to the release rate of potassium from the CRF hydrogels. Conclusions Controlled release fertilizer (CRF) hydrogels were pre- pared in forms of PVA, PVA/CS and CS hydrogels. PVA hydrogel showed the highest equilibrium water absorbency (*300%) among the three hydrogels while PVA/CS- and CS-hydrogels showed equilibrium water absorbency around 225 and 70%, respectively. This is one of the most important properties of the CRF hydrogels for their appli- cations in agriculture, for the water absorption during Fig. 9 Plots of log release fractions of potassium against log time raining or irrigating. The water retention of soil containing from (a) PVA-, (b) PVA/CS- and (c) CS-hydrogel a in soil and b in the CRF hydrogels was also examined. It was found that deionized water the CRF hydrogels increased the water retention of the soil. After 30 days, the water retentions of soil containing mechanisms of pure diffusion controlled and swelling PVA-, PVA/CS- and CS-hydrogels were 25, 10 and 4%, controlled release. On the other hand, the CS hydrogel respectively, while the soil without CRF hydrogels had shows, n value that is close to 1.0, which corresponds to already given off most of the water. The release behavior case II transport, and is controlled by a combination of of potassium from the CRF hydrogels, both in deionized diffusion of fertilizer from hydrogel and degradation rate of water and in soil, was also investigated. In deionized water, Table 1 Released exponent CRF hydrogels Release Correlation Release Diffusion (n), correlation coefficient (R2), exponent, n coefficient, R2 factor, k coefficient, D (cm2/s) release factor (k) and diffusion coefficient (D) of potassium In soil release in deionized water and in soil for the swellable CRF PVA 0.52 0.99 0.4091 8.10 9 10-8 hydrogels PVA/CS 0.95 0.98 0.1685 7.74 9 10-8 CS 1.05 0.99 0.1506 4.65 9 10-9 In deionized water PVA 0.27 0.98 0.6622 3.41 9 10-8 PVA/CS 0.33 0.98 0.6578 2.95 9 10-8 CS 0.28 0.99 0.7238 1.92 9 10-9 123 J Polym Environ (2010) 18:413–421 421 the PVA hydrogel showed highest percent cumulative 5. Kathuria N, Tripathi A, Kar KK, Kumar A (2009) Acta Biomater release among the three prepared hydrogels. The potassium 5:406 6. Martino AD, Sittinger M, Risbud MV (2005) Biomaterials release behavior also related to degree of swelling of the 26:5983 CRF hydrogels. The higher the swelling ratio, the more the 7. 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AOAC (1990) Official methods of analysis, 15th edn. Association Acknowledgements Financial support from the Center of Excel- of Analytical Chemistry, Washington, DC lence for Innovation in Chemistry (PERCH-CIC), Commission on 22. Siepmann J, Peppas NA (2001) Adv Drug Deliver Rev 48:139 Higher Education, Ministry of Education is gratefully acknowledged. 23. Singh B (2007) Int J Pharm 334:1 Acknowledgements also go to Faculty of Science, Burapha University 24. Bettini R, Colombo P, Peppas NA (1995) J Control Release and Rajamangala University of Technology Tawan-Ok Bangpra 37:105 Campus. The authors also thank Dr. Siridech Boonsang for his 25. El-Sherbiny IM, Lins RJ, Abdel-Bary EM, Harding DRK (2005) priceless advisory. Eur Polym J 41:2584 26. Shu XZ, Zhu KJ, Song W (2001) Int J Pharm 212:19 27. Gorochovceva N, Makuska R (2004) Eur Polym J 40:685 28. Jayakumar R, Prabaharan M, Reis RL, Mano JF (2005) Carbohyd References Polym 62:142 29. 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