Article pubs.acs.org/JAFC
Macro- and Micronutrient Simultaneous Slow Release from Highly Swellable Nanocomposite Hydrogels Adriel Bortolin,†,‡ André R. Serafim,† Fauze A. Aouada,†,§ Luiz H. C. Mattoso,†,‡ and Caue Ribeiro*,†,‡ †
National Nanotechnology Laboratory for Agribusiness, EMBRAPA-CNPDIA − Rua XV de Novembro, 1452, São Carlos, SP 13560-970, Brazil ‡ PPGQ, Department of Chemistry, Federal University of São Carlos − Rodovia, Washington Luís, Km 235, São Carlos, SP 13566-905, Brazil S Supporting Information *
ABSTRACT: Clay-loaded hydrogels have been arousing great interest from researchers and academics due to their unique properties and broad applicability range. Here we developed hydrogel-based nanocomposites intended for slow/controlled release of macro- and micronutrients into independent or concurrent systems. The produced nanocomposites underwent a hydrolysis treatment that improved their physicochemical properties. We obtained materials capable of absorbing water contents 5000 times greater than their weights, an outcome that makes them promising, particularly if compared with commercially available materials. Though swelling degree was affected by the presence of calcium montmorillonite (MMt), MMt has increased nutrient (urea and boron) loading capacity and, as a consequence of its interaction with the studied nutrients, has led to a slower release behavior. By evaluating the simultaneous release behavior, we observed that both the ionic (sodium octaborate) and the nonionic (urea) sources competed for the same active sites within the nanocomposites as suggested by the decreased loading and release values of both nutrients when administrated simultaneously. Because of its great swelling degree, higher than 2000 times in water, the nanocomposites formulated with high MMt contents (approximately 50.0% wt) as well as featuring high loading capacity and individual (approximately 74.2 g of urea g−1 of nanocomposite and 7.29 g of boron g−1 of nanocomposite) and simultaneous release denote interesting materials for agricultural applications (e.g., carriers for nutrient release). KEYWORDS: swelling degree, hydrolysis, nanocomposite, nanoclay, simultaneous release
■
INTRODUCTION Soil nutritional control plays, alongside other parameters, an essential role in assuring the efficiency of crops by affecting their yield and quality. Therefore, specific macro- and micronutrients must be spread in a controlled fashion in order for them to reach an ideal performance. The conventional means of nutrient spreading, however, present several drawbacks mostly related to their low efficiency and tough control, as well as high cost and waste.1 To overcome these hurdles, scientific/technological strategies have been developed, such as slow/controlled release systems. Several materials have been used for this purpose, among which hydrogels stand out as a promising alternative to deliver agricultural nutrients.2 Modified (e.g., clay-added) hydrogels have aroused growing interest because they feature improved key properties.3 It is highly desirable to take advantage of the superior properties of hydrogel-based nanocomposites to manage essential nutrients in a rational manner. Furthermore, the addition of high clay loads may contribute to a cheaper product with increased feasibility for agricultural applications when compared to the commercially available polyacrylamide (PAAm)-based ones.4 The nutrients commonly used in agriculture are classified as macronutrients (i.e., those required in high contents and that are responsible for plant physical structure) and micronutrients (i.e., those used in low contents, which control plant growth stages such as germination, root growth, and leaf force).5 In previous works,4 our group has analyzed hydrogels as delivery systems for the macronutrient nitrogen (from urea). Though © XXXX American Chemical Society
urea is highly soluble in water, it did not form ions in solution. Micronutrients are generally administered as salts,6 which means that they may irreversibly adsorb to hydrogel structure. There is little published data on the controlled release of micronutrients from hydrogels. Although some authors studied hydrogel interactions with boron,7−9 no previous works dealing with hydrogels as boron carriers were found in the literature. This study therefore aimed to produce nanostructured hydrogels as well as to assess the slow/controlled release of boron from 3 basic sources: boric acid, borax, and commercially available sodium octaborate. The novelty of this work is the evaluation of the nanocomposites’ behavior upon the simultaneous desorption of boron and urea, a micro and a macronutrient, respectively. The nanocomposites comprised polyacrylamide (PAAm) and carboxymethylcellulose (CMC). The latter was modified with different calcium montmorillonite clay (MMt) contents in an effort to improve some of the nanocomposites’ properties (e.g., nutrient sorption and desorption capacities, mechanical resistance, and water absorption rate) and, as a consequence, reduce the production cost of the final product.4 Received: January 16, 2016 Revised: March 20, 2016 Accepted: April 4, 2016
A
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
■
In the model corresponding to the second-order kinetics, the solute release rate at a specific moment is directly proportional to the difference between the squares of the released and adsorbed solute concentrations. It is assumed that the diffusion paths are changed as a result of changes in hydrogel form throughout the process. For this, this model takes into account the parameter α = Fmax/(1 − Fmax), which expresses solute physicochemical affinity with hydrogel and solvent. In this model (eq 3), both the Fickian diffusion and the macromolecular controlled relaxation mechanisms are considered according to theoretical models reported in the literature.11−14
MATERIALS AND METHODS
Synthesis of the Nanostructured Hydrogels. Nanocomposites comprising PAAm and the biodegradable polysaccharide CMC (molecular weight = 90 000 g mol−1, 0.7 carboxymethyl groups per anhydroglucose unit, data provided by Sigma-Aldrich) were formulated through chemical polymerization of acrylamide (AAm, Sigma-Aldrich) monomers in an aqueous solution containing CMC and MMt (Ca0.6(Al,Mg)2Si4O10(OH)2·nH2O)). The hydrogels were synthesized at m(hydrogel [AAm + CMC]):m(MMt) ratios (i.e., hydrogel mass per unit mass of clay) of 1:1, 2:1, 3:1, 4:1, and 1:0. In this sense, the hydrogel (1:1) comprised 50.0% of MMt, whereas the hydrogels (2:1), (3:1), and (4:1) consisted of 33.3%, 25.0%, and 20.0% of MMt, respectively. Finally, MMt was absent in the hydrogel (1:0). To synthesize the nanocomposites, the cross-linking agent (N,N′methylenebis(acrylamide)) (MBAAm, Sigma-Aldrich) content was kept constant in relation to AAm. The polymerization/cross-linking reaction was catalyzed by 3.21 μmol mL−1 of an aqueous N,N,N′,N′tetramethylethylenediamine (TEMED, Sigma-Aldrich) solution. Finally, 3.38 μmol mL−1 of sodium persulfate (Na2S2O8, Sigma-Aldrich) was added to trigger the free radical polymerization. The obtained solution was allowed to polymerize for 24 h in two acrylic plates set apart by an elastomeric spacer. Once polymerization was completed, the hydrogels were transferred to containers filled with distilled water and submitted to dialysis by renewing the water every 12 h during at least 5 days. In this process, all the reagents that did not participate in the polymerization reaction were eliminated. Once purified, the hydrogels were shaped into cylinders and kiln-dried at 35 ± 1 °C. To increase their swelling capacities, the hydrogels underwent a hydrolysis process. The dried nanocomposites were soaked in 0.5 mol L−1 of NaOH and heated at 75 °C for 18 h in an oven. Then they were removed from the alkaline solution and washed with water for 24 h. Water was changed six times to remove excess NaOH. Hydrolyzed hydrogels were named likewise, followed by the acronym “Hd.” (e.g., the hydrolyzed hydrogel (1:1) was named hydrogel (1:1) Hd.). Characterization of the Nanocomposites. To evaluate their swelling degrees (Q), hydrogels were soaked in Milli-Q water and/or solutions of different boron sources for previously determined times. Nanocomposite samples were carefully weighed to obtain Q values. The swelling kinetic parameters were determined to investigate the absorption kinetic behavior of water and nutrient solutions by the nanocomposites. Such parameters were determined through kinetic measurements of Q for the different formulations and experimental conditions. For each curve, the diffusion exponent (n) and diffusion constant (k) were calculated using eq 1. Mt = kt n Meq
kR
FR =
(1)
−(
kR )t Fmax )
kR
1 − 2Fmax + e 2( α )t
(3)
The spectroscopic characterization of the nanocomposites with or without adsorbed nutrient was performed through Fourier transform infrared spectroscopy (FTIR) in a PerkinElmer Spectrum spectrometer (model Paragon 1000). The nanocomposites were previously dried, crushed, mixed with potassium bromide (KBr), and pressed at high pressure to form tablets.11,12 FTIR spectra comprising 128 scans were recorded at wavelengths ranging from 400 to 4000 cm−1 using a resolution of 2 cm−1. Thermal analyses aimed at identifying possible changes in the hydrogel components’ interactions as a result of MMt concentration, hydrolysis, and nutrient addition into the nanocomposite structure. Thermogravimetric (TG) curves were obtained in the TGA Q-500 equipment manufactured by TA Instruments. Hydrogel samples (8− 10 mg) were heated from room temperature to 800 °C at 10 °C min−1 in a nitrogen atmosphere flowing at 60 mL min−1. To evaluate the nanocomposites as slow/controlled release systems of boron, we performed kinetic tests in distilled water. These tests allowed us to quantify the boron content incorporated into the hydrogels after 144 h of immersion in 90.0% saturated boron solution at 25 °C. In other words, we quantified the actual boron content incorporated into the nanocomposites during the sorption process. The model used for boron desorption was adapted from the slow release of drugs.12−17 As previously described, boron desorption from three different sources were tested: boric acid, borax, and sodium octaborate. Magnetic stirring and 25 °C were maintained during the entire release process to ensure that the nutrient content measured in the liquid medium was correspondent to its diffusion to the medium instead of the mechanical action of the stirrer. Aliquots were withdrawn at different time intervals until 196 h because no nutrient diffusion was observed thereafter, i.e., equilibrium was achieved. Desorption measurements were performed in triplicates. For comparison purposes, we also tested the nutrient in the absence of the nanocomposites (control) and determined the intermediate boron content in the hydrogels. An analytical method18 was used to quantify the boron content. We used a 0.9% azomethine-H (Sigma-Aldrich) solution in 2.0% ascorbic acid (Sigma-Aldrich) solution as well as a buffer solution comprising ammonium acetate, EDTA (Vetec), and acetic acid. For UV−vis measurements, 2.0 mL of sample, 0.5 mL of azomethine-H solution, and 0.5 mL of buffer solution were used. The resulting mixtures were rested for 30 min in dark chambers to allow total boron complexation. This method was shown to be quite efficient for all boron sources, as indicated by R2 values higher than 0.99 in all standard curves. It is worth mentioning that individual standard curves were created for each measurement series and that all release data were collected in triplicates. To evaluate the applicability of the nanocomposites in simultaneously releasing the micronutrient boron and a soluble compound (urea), as well as to verify the competition among both compounds for the same active sites in the hydrogels, we prepared three solutions by combining different concentrations of sodium octaborate and urea: (A) 50.0 mL of saturated sodium octaborate solution and 50.0 mL of 50.0% saturated urea solution (this means half of urea mass to produce a saturated solution at 25 °C); (B) 50.0 mL of saturated urea solution
where t is time, k is the diffusion constant (dependent on hydrogel type and swelling medium), n is the diffusion exponent (which provides information regarding the transport mechanisms that drive the sorption of a particular solute), and Mt and Meq are hydrogel masses at a swelling time t and at equilibrium, respectively. To observe the release kinetic parameters, the model developed by Reis et al.10 was applied to each curve (FR versus t). This model predicts that the release of a solute from hydrogels, both release and sorption kinetics, may be assessed through changes in their concentrations in solution. Such a model has some advantages over other protocols reported in the literature because the former can be applied in the release curve as a whole. We applied first- and secondorder release models. The first-order mathematical model, which assumes that the release process is led by the simple diffusion of the solute toward either the solution or the hydrogel, is described by eq 2.
FR = Fmax(1 − e
Fmax(e 2( α )t − 1)
(2)
where FR is the percentage of nutrient desorbed at a specific time, Fmax is the maximum content of the released nutrient, kR is the release constant, and t is time. B
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry and 50.0 mL of 50.0% saturated sodium octaborate solution; (C) 50.0 mL of both solutions, 50.0% saturated each. Boron content was determined as previously described, whereas urea quantification was performed in a UV−vis spectrophotometer in accordance with the methodology proposed by With et al.19 The Ehrlich reagent (5 g of dimethylaminobenzaldehyde + 20 mL of concentrated hydrochloric acid, both supplied by Sigma-Aldrich, diluted up to 100 mL) and a 10% trichloroacetic acid (Sigma-Aldrich) solution were prepared. For UV−vis readings, 0.5 mL of the sample was first mixed with 2.0 mL of the acidic solution and then with 0.5 mL of the Ehrlich reagent. Nitrogen content was plotted against absorbance at λ = 420 nm to quantify urea desorption from hydrogels as a function of time. This method has successfully determined urea content, as all standard curves had R2 values higher than 0.99. It is worth pointing out that, because different complexing agents were used to quantify the macro- and the micronutrient, interactions between both measurements were prevented.
■
RESULTS AND DISCUSSION It may be observed that the boron content in solution was higher for sodium octaborate, which must favor its use as source of this micronutrient. However, all sources presented rapid diffusion in water, stabilizing in up to 3 h of immersion, as shown in Figure 1. Additionally, FTIR spectra of hydrolyzed and nonhydrolyzed hydrogels swollen in boric acid solution were recorded (Figure 1). Boron incorporation in the material was indicated by the typical vibrations of [BO3]3− ions in the regions between 630 and 690, 750 and 810, and 1250 and 1310 cm−1. As expected, the absence of peak shifts indicated weak interactions between the micronutrient and the hydrogel. Considering this behavior, one may conclude that the irreversible retention of the borate ion in hydrogel matrix is unlikely to occur. The morphological investigation of the nanocomposites comprising PAAm-CMC and calcium montmorillonite (MMt) was performed by Scanning Electron Microscopy (SEM) (Figure 2). The morphologies of the nanocomposites (1:1) swollen in distilled water presented homogeneous sheet-like structures typical of polysaccharide-based hydrogels (Figure 2a). Sodium octaborate was shown to remarkably decrease pore dimensions, suggesting interactions between the ionic solution and the hydrogel structure which led to the collapse of the open pore structure probably via adsorption (Figure 2b). Additionally, no clay clustering was observed, even in the hydrogel (1:1) in which MMt denoted 50% of the material. This finding indicates a good clay dispersion within the polymer matrix. Figure S1 shows XRD analysis of (1:1), (1:1) Hd., and MMt samples and confirmed an excellent nanodispersion of MMt clay within the polymer matrix indicated by the suppression of the characteristic MMt clay basal peak and that an exfoliated conformation of MMt was achieved. In a previous paper from our group we showed Energy Dispersive X-Ray Spectroscopy (EDS) analysis of these materials,4 and results suggested that the nanoclay was evenly spread throughout the polymer matrix while possible phase separations were not noticed. Though EDS results are semiquantitative, they indicate the MMt presence as well as the production of real nanocomposites. Figure 3 presents TG and DTG curves of the produced hydrogels. Considering the residual content at 800 °C, it may be observed that almost all MMt used in nanocomposite synthesis was incorporated. For instance, the nanocomposite (1:1) generated approximately 50% of residue, which is attributable to the inorganic material.
Figure 1. FTIR spectra of (a) pure boric acid and hydrogel (1:1) either hydrolyzed or not in water or boric acid (b) for the three hydrolyzed hydrogels swollen in boric acid solution.
Figure 2. SEM images of the hydrogel (1:1) Hd. swollen in (a) distilled water or (b) sodium octaborate solution.
According to Lu and Hsieh,20 the intra- and intermolecular, thermally induced imidization of amide groups from PAAm takes place at temperatures higher than 200 °C and the progressive weight loss at about 250 °C is related to the release of imidization products (e.g., H2O, NH3, and CO2). At temperatures higher than 300 °C, the imides decomposed to form nitriles and the main backbone polymer of the formed hydrocarbons. In the thermograms of the hydrogel (1:0), the initial and maximum degradation temperatures of the nitrile C
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
Figure 3. Thermogravimetric (a) TG and (b) derivative TG curves of the produced hydrogels.
compositions were observed at 290 °C and 355 °C, respectively. For the nanocomposites labeled (3:1) and (1:1), however, these temperatures were shifted to 300−302 °C and 369−370 °C, respectively. These shifts indicate interactions between MMt and the hydrogel, as well as increased thermal stability caused by the presence of MMt. Table 1 presents the n and k values of nonhydrolyzed hydrogels. It can be observed that the constant k was lower in
hydrogel (1:0) Hd. has swollen nearly 2.5 times more than the hydrogel (1:1) Hd. in water). In the ionic solutions, though, the hydrogel (1:1) Hd. showed Qeq values slightly higher than the (1:0) Hd. Figure S2 shows the release of boron nutrient sources studied without the nanocomposites, and for all sources the equilibrium time is reached in the first hour after nutrient is immersed in water. Figure 4 illustrates the release kinetics of the different boron sources from the produced nanocomposites. The presence of MMt hindered boron desorption, which is attributable to the interactions between MMt active sites and the ions resulting from the dissociation of boron fertilizers. Sodium octaborate showed the highest efficiency as micronutrient source, releasing up to 7.29 g B g−1 of hydrogel. This observation may be explained by the higher solubility of sodium octaborate when compared with the other sources (22.0 g 100 mL−1 for sodium octaborate as well as 5.7 and 5.8 g 100 mL−1 at 20 °C for boric acid and sodium tetraborate, respectively).21 Still, the best outcome was presented by the hydrogel (1:1), which featured the highest MMt content. This difference was more pronounced for sodium octaborate than for the other sources, as seen in Table 3. Figure 5 presents FR versus t data for micronutrient desorption from the nanocomposites. Despite the different adsorptions among the tested boron sources, the release kinetics was slower, which is in accordance with the MMt content in the nanocomposite. This observation indicates the interaction between MMt and the ionic groups of boron sources, hindering its release. To the best of our knowledge, this effect has never been reported in the literature for a macronutrient, urea in this case. Here almost all nanocomposites desorbed the same nutrient proportion, as highlighted in a previous study from our group.4 Table 4 presents the kinetic parameters and the determination coefficients (R2) considering first- and second-order release behaviors. Despite the proximity of these values, one may observe a slight preference for classifying the release behaviors as second order, for which the chemical interaction between solute and polymer matrix plays a relevant role. When Fmax tended to 0.50, α tended to 1.0. Next to these values, the second-order kinetic model was equivalent to the first-order model. This is because the effect of chain mobility is not observed for low release rates and also explains the proximity of
Table 1. Kinetic Parameters of Nutrient Sorption Milli-Q water −1
hydrogel
k (h ) × 10
(1:1) (3:1) (1:0)
2.7 ± 0.42 1.9 ± 0.11 1.7 ± 0.09
1
boric acid −1
n
k (h ) × 101
n
0.63 ± 0.03 0.64 ± 0.03 0.49 ± 0.02
23.0 ± 0.510 6.0 ± 0.026 4.5 ± 0.046
0.43 ± 0.030 0.47 ± 0.010 0.47 ± 0.010
boric acid solution than in water. The final swelling values at equilibrium were also different when using ionic and nonionic sources, suggesting stronger interactions between the nutrient and the nanocomposite. This finding corroborates the largest reduction of the Q values. A similar behavior was observed for urea sorption in previous study4 carried out by our group. Because urea does not present ionic character, a smaller reduction of the Q value was detected. Nonetheless, the swelling at equilibrium (Qeq) was shown to be strongly dependent on the presence of ions in solution. In all cases, there was a remarkable reduction in swelling capacity (as seen in Table 2) regardless of boron source. The lower Qeq values are probably associated with the greater competition for hydrogel active sites due to the ions. It was also observed that the hydrolyzed hydrogels swollen in nutrient solutions presented reverse trends as to swelling degrees (e.g., the Table 2. Swelling at Equilibrium (Qeq) Values for the Hydrolyzed Hydrogels Swollen in Different Saturated Nutrient Solutions hydrogel
Qeq (water)
Qeq (boric acid)
Qeq (borax)
Qeq (sodium octaborate)
(1:1) (3:1) (1:0)
2188.1 ± 53.4 3382.5 ± 172.5 5403.6 ± 378.9
41.4 ± 1.6 35.8 ± 2.3 42.7 ± 1.9
38.7 ± 0.9 31.8 ± 1.2 32.5 ± 1.1
54.1 ± 2.7 46.9 ± 4.2 49.7 ± 1.3 D
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
Figure 4. Controlled boron desorption from hydrogels swollen in (a) boric acid, (b) borax, and (c) sodium octaborate.
simultaneous sorption/desorption experiments were performed in mixed solutions of urea and sodium octaborate, as shown in Figure 6. Table 5 presents the nutrient sorption resulting from the soaking of the nanocomposites in the swelling solution for 220 h. Remarkable mass changes were not noticed, which is a suggestion of the equilibrium state. Data for desorption at equilibrium (up to 248 h of release) are presented in Table 6. From the results presented in Figure 6 and Tables 5 and 6, the macro- and micronutrients are competing for the active sites in the nanocomposite because both nutrient load values significantly dropped when compared with their sources, in which they appear separately. The desorption curves presented similar trends, another indicator of nutrient competition for the same active sites. When the release rates of the macro- and micronutrients are compared, nevertheless, the ionic nutrient (sodium octaborate) was desorbed in up to 50% of the loaded content, whereas more than 90% of the loaded urea was desorbed. This clearly indicates the higher affinity of the nanocomposites to ionic compounds. For agricultural applications, in which different crops require varying contents of macro- and micronutrients, one may control the loading and release capacities essentially by modifying the macro- and micronutrient contents in the
Table 3. Total Boron Desorbed, at Equilibrium, to the Hydrolyzed Hydrogels Swollen in Different Micronutrient Sources hydrogel (1:1) Hd. (3:1) Hd. (1:0) Hd.
boric acid H3BO3 (g g−1)
borax Na2B4O7· 10H2O (g g−1)
sodium octaborate Na2B8O13·4H2O (g g−1)
1.48 ± 0.04
0.48 ± 0.06
7.29 ± 0.68
1.02 ± 0.14
0.44 ± 0.08
4.68 ± 0.07
0.74 ± 0.10
0.38 ± 0.06
3.96 ± 0.19
the R2 values obtained for both kinetic models. Additionally, kR values decreased when MMt content in hydrogel matrix increased, indicating that clay retards micronutrient release. The kR values of the nanocomposites (1:1) Hd. and (3:1) Hd. for boric acid and sodium octaborate were too close, but for borax the hydrogel (1:1) Hd. presented a release constant nearly 2.5 times lower than that of the hydrogel (3:1) Hd. and about 3.0 times lower than that of the (1:0) Hd. one. To evaluate the behavior of the nanocomposite (1:1) in the presence of an undissociated, soluble compound that might compete with ions in solution for polymer’s active sites, E
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
Figure 5. Percentage of nutrient released over time from hydrolyzed hydrogels swollen in (a) boric acid, (b) borax, and (c) sodium octaborate.
Table 4. Hydrogel α and R2 Values for First- and Second-Order Kinetics as well as Release Constant (kR) for Different Nutrient Sources hydrogel (1:1)
(3:1)
(1:0)
a
nutrient a
urea(pH = 7.0) boric acid borax octaborate urea (pH = 7.0)a boric acid borax octaborate urea (pH = 7.0)a boric acid borax octaborate
α
R2 first order
R2 second order
kR × 10−2
11.3304 0.4539 0.4984 0.7065 13.1043 0.5115 0.8801 0.7156 8.4607 0.5401 0.5808 1.3201
0.9844 0.9758 0.9732 0.9812 0.9875 0.9526 0.9793 0.9845 0.9501 0.9653 0.9842 0.9910
0.9857 0.9763 0.9736 0.9877 0.9958 0.9528 0.9815 0.9845 0.9698 0.9665 0.9846 0.9915
1.586 0.297 0.491 0.954 1.494 0.298 0.745 0.888 4.552 0.552 0.882 2.394
Bortolin et al.4
swelling solution. The fact that nanocomposites have greater interactions with the micronutrient, thus slowing the desorption of the latter, is interesting for their application in soil. This is because plants require small amounts of these micronutrients, which will remain released from the nanocomposites for an extended period of time. The high MMt load
in the polymer matrix of the nanocomposite hydrogels ensures that this material presents high ion exchange capacity, allowing high nutrient contents to be loaded and released. This is not found in commercially available hydrogels. The nanocomposites formulated here presented outstanding loading and release properties for the tested macro- and F
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
Figure 6. Simultaneous desorption of macro- and micronutrients from hydrogel (1:1) Hd. swollen in (a) solution A, (b) solution B, or (c) solution C.
applications. The hydrolysis treatment improved the swelling properties of the hydrogels because of the conversion of amide groups into carboxylic ones. The presence of MMt associated with the hydrolysis of these novel materials remarkably enhanced their capacities of loading nutrient solutions. Furthermore, the clay acted as an effective barrier to control and retard the release of nutrients to the medium as well as played an important role in reducing production costs. Both ionic intensity and medium solubility influenced the loading and release processes, because higher contents of nonionic, soluble groups were loaded by the hydrogels. The ionic groups presented a lower release rate because of the stronger interaction with the active sites of the hydrogels. The ionic and nonionic sources of macro- and micronutrients competed for the same adsorption sites. This was evidenced by the decreased released contents of each nutrient.
Table 5. Equilibrium Nutrient Sorption Values for Macroand Micronutrients sorption solution
urea (g g )
sodium octaborate (g g−1)
A B C
27.9 ± 1.7 49.0 ± 2.7 26.1 ± 2.6
6.1 ± 1.0 3.6 ± 0.4 3.9 ± 0.2
−1
90% saturated urea (g g−1)
90% saturated octaborate (g g−1)
80.7 ± 4.9
14.9 ± 0.9
Table 6. Comparison between Simultaneous Desorption Values of Swollen Hydrogels in Different Solutions Combined to Macro- and Micronutrients as well as of Macro- and Micronutrients Separately sorption solution
urea (g g−1)
sodium octaborate (g g−1)
A B C
20.9 ± 1.7 44.1 ± 2.7 23.7 ± 2.6
3.2 ± 0.7 1.4 ± 0.1 1.6 ± 0.2
90% saturated urea (g g−1)
90% saturated octaborate (g g−1)
74.2 ± 5.6
7.29 ± 0.68
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.6b00190.
micronutrients, properties that make these materials promising to be used as nutrient carrier systems for agricultural G
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Article
Journal of Agricultural and Food Chemistry
■
based hydrogels for the use in colon specific drug delivery. Carbohydr. Polym. 2007, 69, 631−643. (17) Fu, J.; Wang, X.; Xu, L.; Meng, J.; Weng, Y.; Li, G.; He, H.; Tang, X. Preparation and in vitro−in vivo evaluation of double layer coated and matrix sustained release pellet formulations of diclofenac potassium. Int. J. Pharm. 2011, 406, 84−90. (18) Wolf, B. Improvements in the azomethine-H method for determination of Boron Soil. Commun. Soil Sci. Plant Anal. 1974, 5, 39−44. (19) With, T. K.; Petersen, B.; Petersen, T. D. A simple spectrophotometric method for the determination of urea in blood and urine. J. Clin. Pathol. 1961, 14, 202−204. (20) Lu, P.; Hsieh, Y. L. Organic compatible polyacrylamide hydrogel fibers. Polymer 2009, 50, 3670−3679. (21) Oreil, M. J. The Merck Index, 15th ed.; Pharmabooks Editora; Royal Society: Cambridge, UK, 2013.
XRD analysis of (1:1), (1:1) Hd. and MMt samples (Figure S1) and the release of boron nutrient sources studied without the nanocomposites (Figure S2) (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel: +55 16 2107 2800. Fax: +55 16 2107 2902. E-mail: caue.
[email protected]. Present Address §
Department of Physics and Chemistry, FEIS, São Paulo State University − Av. Brasil, 56, Ilha Solteira, SP, 15385-000, Brazil.
Funding
We acknowledge financial support from Brazilian agencies FAPESP, CNPq, CAPES, FINEP, and Embrapa − Rede Agronano. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Li, J.; Zhuang, X.; Font, O.; Moreno, N.; Vallejo, V. R.; Querol, X.; Tobias, A. Synthesis of merlinoite from Chinese coal fly ashes and its potential utilization as slow release K-fertilizer. J. Hazard. Mater. 2014, 265, 242−252. (2) Azeem, B.; Kuzilati, K.; Man, B. Z.; Basit, A.; Thanh, H. T. Review on materials & methods to produce controlled release coated urea fertilizer. J. Controlled Release 2014, 181, 11−21. (3) Ahmed, E. M. Hydrogel: Preparation, characterization, and applications: A review. J. Adv. Res. 2015, 6, 105−121. (4) Bortolin, A.; Aouada, F. A.; Mattoso, L. H. C.; Ribeiro, C. J. Agric. Food Chem. 2013, 61, 7431−7439. (5) Primavesi, A. Manejo Ecológico do solo em regiões tropicais; Nobel: São Paulo, SP (Brazil), 2002; p 541. (6) Gupta, U. C.; Wu, K.; Liang, S. Micronutrients in Soils, Crops, and Livestock. J. Appl. Polym. Sci. 2008, 15, 110−125. (7) Ersan, H. Y.; Pinarbasi, S. Boron removal by glucaminefunctionalized hydrogel beads in batch fashion. J. Appl. Polym. Sci. 2011, 121, 1610−1615. (8) Bursali, E. A.; Coskun, S.; Kizil, M.; Yurdakoc, M. Synthesis characterization and in vitro antimicrobial activities of boron starch polyvinyl alcohol hydrogels. Carbohydr. Polym. 2011, 83, 1377−1383. (9) Coviello, T.; Bertolo, L.; Matricardi, P.; Palleschi, A.; Bocchinfuso, G.; Maras, A.; Alhaique, F. Peculiar behaviour of polysaccharide/borax hydrogel tablets a dynamo-mechanical characterization. Colloid Polym. Sci. 2009, 287, 413−423. (10) Reis, A. V.; Guilherme, M. R.; Rubira, A. F.; Muniz, E. C. Mathematical model for the prediction of the overall profile of in vitro solute release from polymer networks. J. Colloid Interface Sci. 2007, 310, 128−135. (11) Masaro, L.; Zhu, X. X. Physical models of diffusion for polymer solutions, gels and solids. Prog. Polym. Sci. 1999, 24, 731−775. (12) Brazel, C. S.; Peppas, N. A. Modelling of Drug Release from Swellable Polymers. Eur. J. Pharm. Biopharm. 2000, 49, 47−58. (13) Siepmann, J.; Peppas, N. A. Modeling of drug release from delivery systems based on hydroxypropyl methylcellulose (HPMC). Adv. Drug Delivery Rev. 2001, 48, 139−157. (14) Serra, L.; Domenech, J.; Peppas, N. A. Drug transport mechanisms and release kinetics from molecularly designed poly(acrylic acid-g-ethylene glycol) hydrogels. Biomaterials 2006, 27, 5440−5451. (15) Tang, Y.-F.; Du, Y.-M.; Hu, X.-W.; Shi, X.-W.; Kennedy, J. F. Rheological characterisation of a novel thermosensitive chitosan/ poly(vinyl alcohol) blend hydrogel. Carbohydr. Polym. 2007, 67, 491− 499. (16) Singh, B.; Sharma, N.; Chauhan, N. Synthesis, characterization and swelling studies of pH responsive psyllium and methacrylamide H
DOI: 10.1021/acs.jafc.6b00190 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
