Patent Grant
8696784
This invention relates to a composition for and a method of providing sustained release of agricultural nutrients. More particularly this invention relates to nitrogen containing macronutrient adsorbed hydroxyapatite phosphate (HAP) nanoparticles and a method of using nitrogen containing macronutrient adsorbed hydroxyapatite phosphate nanoparticles as a slow-release fertilizer.
Commercial fertilizers contain macronutrients and micronutrients that are essential for plant growth and macronutrients are used by plants in relatively large amounts. As defined herein primary macronutrients are nitrogen (N), phosphorous (P) and potassium (K) while calcium (Ca), magnesium (Mg) and sulfur (S) are secondary macronutrients. All six nutrients are important for plant growth.
As defined herein, micronutrients required in small amounts for plant growth are boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo) and selenium (Se).
Nitrogen, phosphorus and potassium (NPK), which are required in large amounts for plants, are not always adequately available in natural soils to support the sustained growth of plants. Therefore, these macronutrients (NPK) are often needed to be applied externally through fertilizer. Water soluble conventional fertilizers typically result in a large amount of macronutrients being lost by leaching and evaporation. Thus, there is an increased interest in developing slow release fertilizers that release macronutrients to plants over time.
Advantages of slow release fertilizers are improved efficiency and quality as the fertilizer is released over time, thus providing sufficient quantities of macronutrients as required for higher crop yields. In addition, slow release fertilizers result in reduced environmental damage from leaching of macronutrients into water and emissions as gasses, compared to conventional water soluble fertilizers.
Macronutrients in fertilizers can be applied to the soil as a solid in the form of a powder or pellets or as a spray. The uptake of macronutrients by the plant needs to be compensated by their external application to the soil periodically. Nitrogen is a key macronutrient source in agriculture particularly for economic crops such as tea. For example, large amount of fertilizer is applied to the soil of the tea plant to improve the quality and the yield of the leaves produced. A study in Japan (Yamada et al., Journal of Water and Environmental Technology, 7, 4, 331-340, 2009) reported that of the large amount of nitrogen fertilizer applied to tea, only 12% of the nitrogen input was taken up by the plant and the rest discharged to the environment. Therefore, one of the unsolved problems of fertilizer application is, in relation to the amounts of nitrogen applied to soil, the low Nitrogen Use Efficiency (NUE) by crops. This is because an excessive amount of nitrogen, up to 70%, is lost when using conventional fertilizers due to leaching, emissions and long-term incorporation by soil microorganisms. As such, supplying nitrogen macronutrient is critical in preventing the decline of productivity and profitability due to degradation and aging of tea plants (Kamau et al., Field Crops Research 1, 108, 60-70, 2008). Attempts to increase the NUE have so far has met with little success.
U.S. Pat. No. 6,261,997 B1 to Rubin et al. discloses slow release of pesticides adsorbed on organically modified clay to prevent leaching in underground and surface water. U.S. Pat. No. 4,219,349 to Bardsley discloses compositions of calcined clay granules and solution or suspension containing micronutrients (Fe, Zn, Mn, Cu, B, Mo, Cl and S). U.S. Pat. No. 4,849,006 to Milburn et al. discloses a controlled release composition comprising of an organic, biologically active material absorbed on an organically modified clay. U.S. Pat. No. 6,821,928 B2 to Ruskin discloses a method to reduce the rate of diffusion of slow release materials through polymers and a process for making drip irrigation devices with long term control of root growth. It further, discloses bioactive material such as herbicide that is intercalated into nanoclays to protect against root intrusion in drip irrigation applications. U.S. Pat. No. 3,902,886 to Banin et al. discloses clay attached micronutrients to provide micronutrients to plants. US2009/0169524 A1 to Kalpana et al. discloses biopolymer based nanocomposites of chitosan, montmorillonite (MMT) and hydroxyapatite for bone growth in medical applications.
Solutions are needed to provide slow release macronutrient formulations for plant growth applications.
A nitrogen containing macronutrient is adsorbed on HAP nanoparticles and used as a fertilizer. The macronutrient adsorbed HAP nanoparticles disclosed herein, when applied to aqueous and terrestrial environments, slowly release the macronutrient to the soil. The soil medium acts as a conduit for providing the transport of the macronutrients such as urea to the roots of the plant.
As defined herein, slow release of macronutrients provides the plant with nutrients gradually over an extended period of time. As described herein in further detail, such an extended period of time can be up to three months. Soils applied with slow release fertilizer that contain macronutrients will require fewer applications of such fertilizer. Use of a slow release fertilizer leads to higher efficiency of macronutrient release compared to conventional fast release fertilizers.
Adsorption, as defined herein, refers to any means that forms a complex between the nitrogen containing macronutrient compound and the hydroxyapatite phosphate (“HAP” or “HA”) nanoparticles. These include covalent bonds, electrostatic bonds, Van der Waals bonds and hydrogen bonds.
Any other nitrogen containing substance which can deliver nitrate or nitrite to the plant can be used as the macronutrient for adsorption onto the HAP nanoparticles. Examples of such nitrogen containing substances include, but are not limited to, urea, thiourea, amides, polyamines, ammonia and alginates.
Overview of Manufacture and Morphology of HAP Nanoparticle—Nitrogen Containing Macronutrient Composite
HAP nanoparticles can be chemically synthesized using calcium hydroxide suspension and phosphoric acid (Mateus et al., Key Engineering Materials, 330-332, 243-246, 2007). A more detailed description of the synthesis of HAP nanoparticles is described herein.
Structural morphology of the HAP-nanoparticles described herein indicates an initial formation of bead-like HAP nanoparticles that grow into a bead-chain-like structures. This growth pattern suggests that one face of the bead-like HAP nanoparticle is possibly crystallized with a hexagonal unit cell and is highly energetic thus leading to a directional growth along one orientation. This directional growth may occur through the PO42− terminating plane. (See
Methods for adsorption of nitrogen containing macronutrient compounds such as urea on the HAP nanoparticles are also described herein.
SEM imaging indictes particle size of less than 30 nm for a preferred embodiment of macronutrient adsorbed HAP nanoparticles. According to TEM imaging (see
According to the methods described herein, prior to drying, HAP-nitrogen containing macronutrient nanoparticles can be obtained as a stable aqueous dispersion. After drying, the HAP-nitrogen containing macronutrient nanoparticles can be obtained as a white solid chips or granules. Furthermore, these chips or granules can be ground to provide a powder. This grinding preferably takes place using a roll mill or a ball mill. The aqueous dispersions, chips or granules can be used as slow release macronutrient formulations.
Release Behavior in Soils
The macronutrient-adsorbed HAP nanoparticles disclosed herein can be used for supplying macronutrients for crops such as tea; rubber; coconut; soybeans; cotton; tobacco; sugar cane; cereals such as rice, corn (maize), wheat, sorghum and wheat; fruits such as apples, oranges, tomatoes; vegetables; ornamental plants; and other short term cash crops that grow in a range of pH soils. Nitrogen-containing fertilizer is needed because production of crops removes nitrogen, which is essential for plant growth, from the soil. For example, the production of 1000 kg of tea leaves (dry weight) removes up to 100 kg of nitrogen from soil. This nitrogen has to be replenished by external application of fertilizer.
The nitrogen containing macronutrient adsorbed HAP nanoparticle composition described herein can be applied to the soil in the form of a powder, pellets, chips, a spray, or as an aqueous dispersion encapsulated within a biodegradable coating. In certain embodiments of the present invention, a slow release of nitrogen over a period up to three months is observed. During the fertilizing of tea plants, for example, the frequency of application can be attenuated depending on the fertilizer requirement of a given tea plantation. This can be done by starting a second round of application at a suitable period prior to reaching the end of the viability of the first application of the macronutrient adsorbed HAP nanoparticles. In another embodiment, multiple applications of the macronutrient-adsorbed HAP nanoparticles are distributed on soils within three months.
As a person skilled in the art may recognize, soil pH plays a role in the release behavior of the macronutrients from the macronutrient-adsorbed HAP nanoparticles to the soil. Further, soil pH is important in the growth of economic plants (Rice, Tea and Rubber) and ornamental plants (Ferns and Orchids). Generally, tea plants thrive in acidic soils in the pH range between about 4.2 to 5.7. However, rice is more tolerant of slightly higher pH the ideal range being between about 5.0-6.0. It is believed that high organic matter content in soil could lead to lowering of pH of the soil. Elevation may play a role in the effect. In general, higher elevations contain more organic matter compared to lower elevations such as sea level. Organic matter content of soil between 1600 to 4000 feet elevation can range from 2 to 3%.
It is believed that, while not bound by theory, protonation of the macronutrient adsorbed HAP nanoparticles leads to the release of the adsorbed macronutrient. Here, urea, due to its basicity, can be readily protonated. This may aid the release process.
In an embodiment of the slow release method, soil having a pH of 5 found at about 1600 feet from tea plantations in Kandy, Sri Lanka, can be used with macronutrient adsorbed HAP nanoparticles to release the macronutrient in a slow and sustained manner. In another embodiment, pot trials carried out with rice at the Rice Research and Development Institute, Sri Lanka (pH 5.5-6.0) can be used with macronutrient adsorbed HAP nanoparticles to slowly release the macronutrient. Even in sandy soils found at sea level (pH 7), for example in Colombo, Sri Lanka, where the organic content is lower than 2%, the slow and sustained release may be achieved. To summarize, while slow release of macronutrient compound will occur in soils having a pH range of 3.5 to 7.00, soils having acidic pH values in the range between about 4.2-6.5 are most preferred.
Release Behavior Through Foliage
In another embodiment, the aqueous dispersion obtained directly after the synthesis of the HAP-nitrogen containing macronutrient nanoparticles can be used to slowly release the macronutrient in foliar applications. Since leaf surface chemistry generally has a pH range between 5 and 7, such an aqueous formulation can release macronutrient as a foliar application in a local setting through manual application or on a wider scale by aerial spraying. These applications can be made multiple times during the life cycle of a plant as necessary.
Ca(OH)2 solution (19.29 g, 250 ml) was prepared and stirred for half an hour. Urea (31.28 g) was added to the solution and stirred further for one hour. Then, H3PO4 (0.6 M, 250.0 ml) was added drop wise while stirring at 750 rpm. The solution was further stirred for 10 minutes after completion of the addition of phosphoric acid to yield a stable dispersion containing HAP-urea nanoparticles (HAP:Urea ratio 1:1). In a similar manner, HAP-urea nanoparticles were prepared with HAP:Urea ratios of 1:3, 1:4, 1:5 and 1:6 resulting in a foliage fertilizer formulation.
The dispersions that resulted can be dried in an oven at 60° C. or preferably flash dried to get the solid fertilizer compositions as chips or granules. These resulting chips or granules can be ground into a powder.
As seen in
A similar bead-chain-like structure was observed at lower urea encapsulation while the particles appeared to be more spherical in shape when the loading was increased. However even after a loading of 1:6 ratio of HAP:Urea, the composite displayed particle sizes below 100 nm.
H3PO4 (0.6 M, 250 ml) was added drop-wise into a suspension of calcium hydroxide (19.29 g Ca(OH)2 in 250 ml water), while stirring vigorously under mechanical agitation (1000 rpm). HAP-nanoparticle dispersion is created. The reaction takes place according to the following equation.
6H3PO4+10Ca(OH)2→Ca10(PO4)6(OH)2+18H2O
HAP nanoparticle synthesis was repeated at different experimental conditions varying the following parameters:
Surface modification of HAP nanoparticles with urea was carried out as described below. Urea solution (1 M, 250 ml) was added drop wise into the above-prepared HAP nanoparticle dispersion. In a preferred embodiment 25 g of solid urea is added to the HAP dispersion. Using solid urea reduces the amount of water in the mixture, improving the drying process. The resulting solution was allowed to age further for 2 hrs at room temperature to yield a stable dispersion, which can be used for foliar applications or encapsulated within a biodegradable coating. Afterwards, the dispersion was dried at 60° C. overnight by use of oven drying or flash drying.
As can be seen with reference to the TEM image of
b) is a TEM image of HAP-urea nanocomposites. These HAP-urea nanocomposites were created by the Sol-Gel method described in Example 2 above.
Using the Sol-Gel method described in Example 2 above, bead-like nanoparticles (diameter 10-20 nm with uniform size) formed initially and quick directional growth leading to a bead-chain like structure (10-15 nm diameter, 30-150 nm length) was observed.
(i) Effect of Addition Rate
Bead-like HAP nanoparticles with a diameter of approximately 10-35 nm were initially formed. Quick directional growth leading to bead-chain-like structures with 10-35 nm diameter and 150 nm length occurred with faster addition rates of phosphoric acid. Bead-chain-like morphology was observed for addition rate of 20 ml min−1. Bead-chain-like morphology with a particle diameter of 10-60 nm, was observed for the slower rates 6 ml min−1. Bead-chain-like nanostructures were longer in length with the decrease in the addition rate of phosphoric acid suggesting that the longer time duration allows more efficient directional growth of bead-like nanoparticles. It is most preferable for the fertilizer application disclosed herein to use the addition rate of 70 ml min−1.
(ii) Effect of Solution pH
As shown in
(iii) Effect of Addition Method
As can be seen in
(iv) Effect of Stirring Speed
Synthesis of HAP nanoparticles was carried out under different stirring speeds (100, 200, 400, 600 and 800 rpm) in a semi pilot plant reactor. Phase pure bead-chain-like nanoparticles (diameter 10-60 nm, length 50-100 nm) were observed under all tested stirring speeds. Particle diameter (ca. 60 nm) was observed for the lowest rate speed (100 rpm) and small particles with an average diameter ranging from 10-20 nm were observed for the highest, and most preferred, stirring speed of 800 rpm.
(v) Reaction Temperature
As shown with reference to
(vi) Concentration of Phosphoric Acid
With reference to
PXRD Characterization
PXRD studies on the HA nanoparticles synthesized were in close agreement with the reported results in the powder diffraction file for HA in the ICDD, (PDF No. 09-0432) with lattice parameters of a=9.42 Å and c=6.90 Å based on a hexagonal unit cell. No characteristic peaks of impurities such as Ca(OH)2 and Ca3(PO4)2 were observed suggesting the formation of phase pure HA prepared according to example 2. Polycrystalline nature of both HAP nanoparticles and Urea-HAP nanoparticle were confirmed by the diffraction data obtained by electron diffraction methods. The crystallite size as calculated according to Schrerrs formula is about 18 nm which may possibly corresponding to the nanospheres and further suggest that the directional growth occurs along c-axis.
Elemental Analysis
Energy Dispersive X-ray (EDX) analysis confirmed the presence of Ca (17.24%) and P (10.16%) which is in agreement with the expected Ca:P ratio of 1.67. Kjeldhal analysis confirmed the presence of 22%±3% nitrogen in the nanocomposite.
FTIR Characterization
The nature of the interactions between HA nanoparticles and urea molecules were studied by FTIR characterization. Specifically, the peak shifts in the FTIR spectrum of HA nanoparticles particularly broadening and to a lower wavenumber shift in the O—H stretching frequency predicts that the interactions have occurred through the O—H bond of the HA nanoparticles. Urea, the N—H stretching frequency appeared as a doublet at 3430 cm−1 and 3340 cm−1 which in urea bonded to HA nanoparticles had led to a noticeable peak broadening. Further, the change in the carbonyl stretching frequency of pure urea from 1682 cm−1 to 1669 cm−1 in urea adsorbed HA nanoparticles indicated that, as is expected, the C═O electron density was being affected by interaction between urea molecules and HA nanoparticles. This observation was lent further credence by a noticeable peak shift of the N—C—N stretching frequency (1460 cm−1) of urea to a lower frequency in urea modified HA nanoparticles (1446 cm−1).
Raman Spectroscopy
Raman spectroscopy analysis (as shown in
BET Analysis
BET surface area, pore size and average pore volumes of HAP nanoparticles and U-HA nanocomposites are summarized in Table 1.
As evidenced by the BET analysis in Table 1, the HAP nanoparticles synthesized as described in this study have a significantly high surface area compared to literature values. The directional growth of bead-like nanoparticles into a bead-chain-like nanostructure may have introduced the observed unique features. The number of layers of urea molecules around one HAP nanoparticles calculated referring to the BET was 53, suggesting the presence of a nanocomposite where HAP nanoparticles are surrounded by urea molecules which are H-bonded to each other in an extended fashion representing a poly urea molecule.
Release Properties of Nitrogen
(a) In Water
Method:
After adjusting the nitrogen content in each of the following samples to 20% of dry weight, they were used for evaluating the release of urea (N) in water:
(1) Urea
(2) Urea—HAP nanoparticles (powdered and in chip form)
(3) Urea and HA (200-800 microns)
Urea adsorbed HAP-nanoparticles (5 g, powder form), Urea adsorbed HAP-nanoparticles (5 g, chips), Urea—HAP macroparticles (5 g, powder form), urea (2.15 g), all equalized to 20% of nitrogen of dry weight were used for the study of the release behavior in water.
Each of above samples was placed in a vessel partitioned with a semi-permeable membrane. The samples were allowed to equilibrate with water (25 ml) in one side of the membrane and diffuse through the semi-permeable membrane to the other side; samples were withdrawn at 1 hr intervals. The samples were analyzed using FTIR and for the appearance of the urea peak in each sample: The peaks were normalized with respect to O—H stretching frequency peak of water which did not shift and area under the peak was analyzed for N—C—N stretching frequency peak of urea.
A similar procedure was followed to study the release behavior of aqueous solutions of HAP-Urea nanoparticle composites prepared with different HAP: Urea ratios (1:1-1:6).
With reference to
A similar release behavior in water was observed for the HAP-urea nanocomposite prepared by the template synthesis method.
With reference to
(b) In Acidic Soil (pH 5.0)
Soil sample (400 g each of soil found at an elevation of 1600 feet in a tea plantation; pH 5.0) was mixed with 1.8 g of commercial urea fertilizer. The soil sample containing urea fertilizer was filled into a glass column. Similarly, three equal amounts of Urea-HAP nanoparticle composite (N—15.5%) having a N content equal to urea, were taken separately and filled into three glass columns containing three soil samples (three replicates). Next, 180 ml water was added to all four soil columns until they reached the soil water saturation point, and maintained the water content approximately constant throughout the period of study. Water (100 ml) was added at five day intervals prior to elution. The eluted solutions (50 ml) were collected for nitrogen analysis. Nitrogen analysis was done by the Kjeldhal (N) method.
At soil pH 5, sustained release behavior of nitrogen was shown by the nanocomposite based on Urea-HAP nanoparticle and gradual release behavior can be clearly identified with the increasing of cumulative nitrogen content up to 80th day in a slow manner, which is in agreement with a typical slow release profile presented by cumulative release vs. time in the literature. However, urea composition had released almost 50% of N within 25 days and release of nitrogen had leveled off at 70% after the 50th day. Only 60% of the urea was released even after the 80th day and the results were highly reproducible.
Bio-Availability Test—Using Rice as the Crop Plant
1—No fertilizer
2—Urea as recommended by the Department of Agriculture—Sri Lanka
3—½ the amount of urea as recommended by the Department of Agriculture—Sri Lanka
4—½ the amount of Urea-HAP nanoparticles composite containing N as recommended by the Department of Agriculture—Sri Lanka
Pot trials conducted at the Rice Research and Development Institute, Sri Lanka, using rice as the model crop (see
This application is a continuation-in-part of U.S. application Ser. No. 12/794,741 filed Jun. 5, 2010 now U.S. Pat. No. 8,361,185, which is hereby incorporated herein in its entirety by reference.
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| Number | Date | Country | |
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 12794741 | Jun 2010 | US |
| Child | 13707985 | US |
