WO2022180504A1

WO2022180504A1 - Method of making a nano-fertilizer composition for sustained release of macronutrients

Info

Publication number: WO2022180504A1
Application number: PCT/IB2022/051529
Authority: WO
Inventors: K.T.H. Piumi Ayesha Lakmini PEIRIS; Manikkuwadu Thilini Uthpala AMARASINGHE; M.T. Imalka Shanika MUNAWEERA; Nanayakkara Liyanage Veranja Vipul KARUNARATNE; Nilwala Svetlana KOTTEGODA
Original assignee: University Of Sri Jayewardenepura
Priority date: 2021-02-24
Filing date: 2022-02-22
Publication date: 2022-09-01

Abstract

A method of sustained release of macronutrients to a plant locus is described. The method includes providing a nano-fertilizer composition comprising phosphate nanoparticles, optionally combined with a nitrogen source, where the nano-fertilizer composition is prepared using a mechanochemical force, followed by applying the nano-fertilizer composition to soil. A method for making the nano-fertilizer composition for sustained release of macronutrients is also disclosed. The nano-fertilizer composition prepared by the method of present disclosure demonstrates slow and sustained release of macronutrients, particularly, nitrogen and phosphorus, to the plant locus, with a reduced dose of fertilizer in comparison to commercially used fertilizer compositions.

Description

METHOD OF MAKING A NANO-FERTILIZER COMPOSITION FOR SUSTAINED

RELEASE OF MACRONUTRIENTS

BACKGROUND

This patent application claims the benefit of priority to NIPO patent application Ser. No. 21626 entitled “A composition and Method to Manufacture Nano-Rock Phosphate and Urea Modified Nano-Rock Phosphate for Fertilizer Applications” fded on Feb. 24, 2021, the subject matter of which is incorporated herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a nano-fertilizer composition, and more particularly, relates to the method of making a nano-fertilizer composition for sustained release of macronutrients.

DESCRIPTION OF RELATED ART

With the increase in global population, demand for food is ever increasing. To meet the growing demand, agricultural crop yields should be dramatically increased. Fertilizer is one of the major factors which contributes to high crop yields. However, a major problem associated with conventionally used nitrogen and phosphorus fertilizers is their poor nutrient use efficiency (NUE), particularly regarding nitrogen (N), phosphorus (P), and potassium (K).

Nitrogen fertilizers are derived from petroleum by-products, and once applied to soil, a significant portion, about 50 to 70 %, is lost due to leaching, volatilization to ammonia, and nitrogen oxide, and long-term incorporation into soil organic matter. Therefore, nitrogen fertilizers significantly contribute towards a high carbon footprint. Further, conventionally, the NUE of the nitrogen fertilizers has been improved using slow and sustained release fertilizers such as polymer coated urea, urea-nanoparticle composites, urea composites, etc.

On the other hand, the poor NUE of the phosphate fertilizers is due to long-term incorporation by cations and low solubility of rock phosphate. Also, certain conventionally used inorganic phosphate fertilizers contain heavy metals that are derived from phosphate sources. Continuous application of fertilizers to the soils could increase the heavy metal contents exceeding their natural abundances in soils, and transfer of these heavy metals to the human food chain. In order to increase the solubility of phosphorus fertilizers, conventionally phosphate bearing minerals such as rock phosphate (RP) is treated with inorganic acids, such as sulfuric acid or phosphoric acid. However, moving forward with these methods may not be sustainable from an industrial standpoint due to high capital costs, production costs, and various environmental issues associated with acid dissolution reactions. Although attempts have been made in recent years to use low molecular weight organic acid dissolution methods, the performance of the fertilizers prepared by such methods has been very poor in comparison to the commercially available fertilizers, such as triple superphosphate (TSP) or single superphosphate (SSP).

Accordingly, there exists a need to develop methods of improving the phosphate and nitrogen availability to plants.

SUMMARY

In an aspect of present disclosure, a method of sustained release of macronutrients to a plant locus is described. The method includes providing a nano-fertilizer composition comprising phosphate nanoparticles, optionally combined with a nitrogen source, where the nano-fertilizer composition is prepared using a mechanochemical force. The method further includes applying the nano-fertilizer composition to the soil.

In another aspect of the present disclosure, a method of making a nano-fertilizer composition for a sustained release of macronutrients (phosphorus) is disclosed. The method includes mixing a phosphorus source with an organic acid to obtain a first wet slurry. The method further includes applying a mechanical force to the first wet slurry to obtain phosphate nanoparticles. The method further includes drying the phosphate nanoparticles to a temperature range of about 50-120 °C to obtain the nano-fertilizer composition as a dried powder.

In another aspect of the present disclosure, a method of making a nano-fertilizer composition for a sustained release of macronutrients (nitrogen and phosphorus) is disclosed. The method includes mixing the phosphate nanoparticles with a nitrogen source to obtain a second wet slurry. The method further includes applying the mechanical force to the second wet slurry to obtain a nanocomposite. Furthermore, the method includes drying the nanocomposite to a temperature range of about 50-70 °C to obtain the nano-fertilizer composition as a dried powder. The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 illustrates a method of sustained release of macronutrients to a plant locus, according to one embodiment of the present disclosure;

FIGS. 2A and 2B illustrate methods for making a sustained release nano-fertilizer containing macronutrients, according to one embodiment of the present disclosure;

FIG. 3A shows an Energy Dispersive X-ray spectrum (EDX) of untreated Eppawala Rock Phosphate (ERP), according to one embodiment of the present disclosure,

FIG. 3B depicts a Powder X-ray Diffraction pattern (PXRD) pattern of ERP, according to one embodiment of the present disclosure;

FIG. 3C shows a Scanning Electron Microscopy (SEM) image of ERP, according to one embodiment of the present disclosure;

FIG 3D shows a Fourier Transform Infra-Red (FT-IR) spectra of ERP, according to one embodiment of the present disclosure;

FIGS. 4A-4B illustrates the SEM image of mechanochemically derived rock phosphate (RP) nanoparticles after grinding the ERP for (a) 1 hour (h), and (b) 3 h, with an organic acid in presence of water, respectively, according to one embodiment of the present disclosure;

FIG. 5 illustrates the PXRD patterns of (a) ERP, and of the mechanochemically derived RP nanoparticles with ERP: citric acid ratios of (b) 10:4 and c) 10:6, respectively, according to one embodiment of the present disclosure;

FIG. 6 is a PXRD pattern of (a) ERP and ERP ball milled with organic acid for different times (b) 1 h, (c) 1.5 h, (d) 2 h, (e) 3 h with ERP: citric acid ratio of 10:6, according to one embodiment of the present disclosure; FIG. 7 illustrates the PXRD of (a) ERP with citric acid ratio of 10:6, RP nanoparticles with ERP: urea ratios of (b) 1:4 (c) 1:6 and (d) 1:8, according to one embodiment of the present disclosure;

FIGS. 8A-8B illustrates the FT-IR spectra for the P-0 stretching region, and O-H stretching region, respectively, of (a) ERP, RP nanoparticles with ERP: citric ratios of (b) 10:4, (c) 10:6, according to one embodiment of the present disclosure;

FIGS. 9A-9B is the FT-IR spectrum for the P-0 stretching region, and O-H stretching region, respectively, of (a) ERP, and ERP treated with different ball milling times of (b) 1 h, (c) 1.5 h, (d) 2 h, (e) 3 h with ERP: citric acid ratio of 10:6, according to one embodiment of the present disclosure;

FIG. 10A is the FT-IR spectrum for N-H stretching region of (a) urea (b) ERP: urea ratio 1:4 (c) ERP: urea ratio 1:6 (d) ERP: urea ratio 1:8, according to one embodiment of the present disclosure;

FIG. 10B is the FT-IR spectrum for N-H bending region of (a) urea (b) ERP: urea ratio 1 :4 (c) ERP: urea ratio 1:6 (d) ERP: urea ratio 1:8, according to one embodiment of the present disclosure;

FIG. IOC is the FT-IR spectrum for C-N stretching region of (a) urea (b) ERP: urea ratio 1:4 (c) ERP: urea ratio 1:6 (d) ERP: urea ratio 1:8, according to one embodiment of the present disclosure;

FIG 11 illustrates a water release behavior of phosphate (a) ERP, RP nanoparticles with ERP: citric ratios of (b) 10:2, (c) 10:4, (d) 10:6 prepared by 1 h ball milling, according to one embodiment of the present disclosure;

FIG. 12 illustrates a soil release behavior of phosphate for (a) ERP, RP nanoparticles with ERP: citric ratios of (b) 10:2, (c) 10:4, (d) 10:6 prepared by 1 h ball milling, according to one embodiment of the present disclosure;

FIG. 13 illustrates water release behavior of nitrogen in (a) urea, RP nanoparticles with ERP: urea ratios (b) 1:4 (c) 1:6 (d) 1:8, respectively, according to one embodiment of the present disclosure;

FIG. 14 illustrates the soil release behavior of nitrogen in (a) urea, RP nanoparticles with ERP: urea ratios (b) 1 :4 (c) 1:6 (d) 1:8, according to one embodiment of the present disclosure; FIG. 15A depicts the effect of the fertilizer on the yield obtained for maize, obtained after three and a half months (after harvesting), according to one embodiment of the present disclosure; and

FIG. 15B depicts the effect of the fertilizer on the corn ear weight of maize, obtained after three and a half months (after harvesting), according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Wherever possible, corresponding or similar reference numbers will be used throughout the drawings to refer to the same or corresponding parts. Moreover, references to various elements described herein are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be construed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claim.

The terminologies and/or phrases used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art. For clarity, the following specific terms have the specified meanings. Other terms are defined in other sections herein.

As used herein, the term “nano-fertilizer” refers to the materials in the nanometer scale, usually in the form of nanoparticles, containing macro and micronutrients that are delivered to plants or crops in a controlled mode.

As used herein, the term “sustained release” refers to a design to release a substance to a plant slowly over an extended period of time.

As used herein, the term “macronutrients” refers to the nutrients that plants need in large amounts for efficient growth. As used herein, the term “nanocomposite” refers to the incorporation of a material/substance/element into a nanomaterial through chemical bonding.

In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words “a,” “an” and the like generally carry a meaning of “one or more,” unless stated otherwise.

Furthermore, the terms “approximately,” “approximate,” “about,” and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

Aspects of the present disclosure are directed towards a nanotechnology based sustainable, scalable, and low-cost, green synthesis method of making the nano-fertilizer compositions, with improved nitrogen and phosphorus availability to plants. The nano-fertilizer composition or “composition” or “nano-fertilizer” prepared via water assisted mechanochemical grinding method can supply available phosphorus to plant with performance similar to the commercially available phosphate fertilizers like, TSP, with up to 50% reduced dose of fertilizer. Although the description herein refers to use of Eppawala rock phosphate (ERP) as a phosphate source, and urea as a nitrogen source to prepare the nano-fertilizer composition, aspects of the present disclosure may be adapted to preparing the nano-fertilizer compositions with other phosphate sources, such as any phosphorite, phosphate rock or rock phosphate which are a non-detntal sedimentary rock sources and nitrogen sources as well, as may be obvious to a person skilled in the art.

In an embodiment, the nano-fertilizer composition includes phosphate nanoparticles of ERP or nano-rock phosphate which release phosphorus to plant in a slow and a sustained manner. Due to the presence of a minimum amount of heavy metals in the ERP, resulting nano-rock phosphate also has a minimum level of heavy metals, thereby overcoming challenges faced with conventionally used phosphate fertilizers such as, heavy metal contamination of soil. The phosphate nanoparticles can be further used as a carrier with a nitrogen source, such as urea, for releasing nitrogen to the plants in a slow and sustainable manner. The resulting nano-fertilizer composition can be used for efficient and sustained release of the phosphorus and the nitrogen, respectively, with increased NUE, thereby overcoming the drawbacks associated with the current nano-fertilizer formulations/applications. In FIG. 1 , a schematic flow diagram of a method of sustained release of macronutrients to a plant locus is illustrated, according to an embodiment of the present disclosure. The plant is selected from a group consisting of maize, rice, tea, rubber, coconut, fruits, and vegetables. The order in which the method 100 is described is not intended to be construed as a limitation, and any number of the described method steps may be combined in any order to implement the method 100. Additionally, individual steps may be removed or skipped from the method 100 without deviating from the fundamental principles and scope of the present disclosure.

At step 102 the method 100 includes providing a nano-fertilizer composition comprising phosphate nanoparticles, optionally combined with a nitrogen source, where the nano-fertilizer composition is prepared using a mechanochemical force. In some embodiments, the phosphorus source is selected from a group consisting of hydroxyapatite, hydroxyapatite nanoparticles or hydroxyapatite nanocomposites, a chlorapatite, a fluorapatite, a rock phosphate, or a combination thereof. In some embodiments, the phosphorus source is the rock phosphate. In a preferred embodiment, the phosphorus source is Eppawala rock phosphate (ERP) found in Sri Lanka. In some embodiments, the organic acid is selected from a group consisting of citric acid, fulvic acid, fumic acid, oxalic acid, humic acid, salts of these acids, and a combination thereof. In an embodiment, the nitrogen source is urea, ammonium nitrate, ammonium sulphate, calcium nitrate, monoammonium phosphate, diammomum phosphate, potassium nitrate, or a combination thereof. In an embodiment, the nitrogen source is urea.

At step 104, the method 100 includes applying the nano-fertilizer composition to a soil. In an embodiment, the pH of the soil is in a range of 4-8.5.

In FIG. 2A, a schematic flow diagram of a method of making the nano-fertilizer composition for the sustained release of macronutrients (phosphorus) is illustrated, according to an embodiment of the present disclosure. The order in which the method 200 is described is not intended to be construed as a limitation, and any number of the described method steps may be combined in any order to implement the method 200. Additionally, individual steps may be removed or skipped from the method 200 without deviating from the fundamental principles and scope of the present disclosure.

At step 202, the method 200 describes the specific mixing protocol of a phosphorus source with an organic acid to obtain a first wet slurry. In an embodiment, the phosphorus source is selected from a group consisting of hydroxyapatite, hydroxyapatite nanoparticles or hydroxyapatite nanocomposites, a chlorapatite, a fluorapatite, a rock phosphate, and a combination thereof. In some embodiments, the phosphorus source is the rock phosphate. In a preferred embodiment, the phosphorus source is ERP found in Sri Lanka. The presence of comparatively minimum levels of heavy metals in ERP, results m yielding the phosphate nanoparticles that is safer, unlike the conventionally used rock phosphates such as TSP and SSP derived from other global ores. In some embodiments, the organic acid is selected from a group consisting of citric acid, fulvic acid, fumic acid, oxalic acid, humic acid, salts of these acids, and the combination thereof. In an embodiment, the organic acid is citric acid. The organic acid may be in a solid form, such as a powder, or in a liquid form. The effectiveness of the organic acid to dissolve the phosphate nanoparticles depends on the nature of the phosphates, number of acidic protons generated, and pKa values of the acids. The principal chemical, and physical factors affecting the acid dissolution of RP are the sizes of the RP particles, organic acid concentration, temperature, solid to liquid phases ratio, agitation method, agitation time, and nature of the RP material. In a preferred embodiment, the organic acid is the citric acid. In an embodiment, the phosphorus source to the organic acid weight/weight (w/w) ratio is in a range of 10: 1 to 1:1 in the first wet slurry. In an embodiment, the first wet slurry was obtained by mixing a combination of ERP and citric acid in a weight ratio of 10:4. In another embodiment, the first wet slurry was obtained by a combination of ERP and citric acid in a weight ratio of 10: 6.

At step 204, the method 200 includes applying a mechanical force to the first wet slurry to obtain the phosphate nanoparticles. The mechanical force may be applied by way of grinding, or ball milling. In an embodiment, the mechanical force can be applied to the first wet slurry by grinding in absence of a solvent. In some embodiments, the grinding may be manual grinding which requires a longer time as compared to the ball milling. In an embodiment, the method 200 includes ball milling the reactants, ERP and citric acid in various weight ratios (10:2, 10:4 and 10:6, respectively), between 100 to 1000 revolutions per minute (rpm), for a period of 1-3 hours, in presence of water as the solvent, to obtain the phosphate nanoparticles.

At step 206, the method 200 includes drying the phosphate nanoparticles to obtain the nano-fertilizer composition. In some embodiments, the method includes drying the phosphate nanoparticles to temperature range of about 50-70 degree centigrade (°C) to obtain the nano- fertilizer composition. In some embodiments, the drying process may include air drying, low temperature oven drying, drying under solar energy or mechanical pressing. In some embodiments, oven drying was used at 60 °C to obtain the nano-fertilizer composition in form of a dried powder. In some embodiments, the nano-fertilizer composition has a particle size in a range of 1-100 nanometers (nm). In an embodiment, the nano-fertilizer composition is in form of a pellet, powder, a tablet, a chip, or any combination thereof.

In FIG. 2B, a schematic flow diagram of a method of making the nano-fertilizer composition for the sustained release of macronutrients (nitrogen and phosphorus) is illustrated, according to an embodiment of the present disclosure. The order in which the method 250 is described is not intended to be construed as a limitation, and any number of the described method steps may be combined in any order to implement the method 250. Additionally, individual steps may be removed or skipped from the method 250 without deviating from the fundamental principles and scope of the present disclosure.

At step 252, the method 250 includes mixing the phosphate nanoparticles with the nitrogen source to obtain a second wet slurry. In an embodiment, the nanocomposite fertilizer can be obtained by mixing the phosphorus source, the organic acid, and the nitrogen source. Such a modification using the nitrogen source such as urea provides a template for sustained release of nitrogen from the nano-fertilizer composition. In an embodiment, the phosphorus source to the nitrogen source w/w ratio is in a range of 1 :4 to 1:8. In some embodiments, the nitrogen loading in the phosphorus and the nitrogen containing nanocomposite is up to 40%. In an embodiment, the nitrogen source is urea, ammonium nitrate, ammonium sulphate, calcium nitrate, monoammonium phosphate, diammonium phosphate, potassium nitrate, or a combination thereof. In a preferred embodiment, the nitrogen source is urea.

At step 254, the method 250 includes applying a mechanical force to the second wet slurry to obtain the nanocomposite. In an embodiment, the mechanical forces to facilitate chemical reactions can be through grinding or ball milling. In another embodiment, the method 100 includes ball milling the second wet slurry including phosphate nanoparticles and urea, between 100 to 1000 revolutions per minute (rpm), for a period of 1-3 hours, in presence of a solvent, to obtain the nanocomposite. Ceramic balls of diameter 10 mm were used in the ball milling process. In some embodiments, the solvent may be a polar inorganic solvent such as water. In some embodiments, the solvent is a polar organic solvent such as ethanol, isopropyl alcohol, and a combination thereof. In some embodiments, the mechanical force, by way of grinding, can be applied to the nanocomposite in absence of a solvent. In some embodiments, the grinding may be manual grinding which requires a longer time as compared to the ball milling.

At step 256, the method 250 includes drying the nanocomposite to obtain the nano-fertilizer composition. In some embodiments, the nanocomposite is dried to a temperature range of about 50-70 degree centigrade (°C) to obtain the nano-fertilizer composition as a solid. In some embodiments, the drying process may include air drying, low temperature oven drying, drying under solar energy or mechanical pressing. In some embodiments, oven drying was used at 60 °C to obtain the nano-fertilizer composition in a form of a dried powder. In some embodiments, the nano-fertilizer composition has a particle size in a range of 1-100 nanometers (nm). In an embodiment, the nano-fertilizer composition is in a form of a pellet, powder, a tablet, a chip, or any combination thereof.

In an embodiment, the phosphate nanoparticles, or the nanocomposite may be modified with plant macronutrients such as potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), or any combination thereof. In another embodiment, the phosphate nanoparticles or the nanocomposite may be further modified with plant micronutrients such as iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), or any combination thereof.

EXAMPLES

The following examples describe and demonstrate a method for preparing the nano fertilizer compositions for efficient and sustained release of phosphorus and nitrogen to plants, with increased NUE, and reduced fertilizer application. The examples are provided solely for the purpose of illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without deviating from the fundamental aspects and scope of the present disclosure.

Example 1 : Eppawala rock phosphate (ERP) characterization

ERP was used as a phosphate source for preparing the phosphate nanoparticles. This is because ERP contains very low amounts of heavy metals which could potentially cause minimum soil damage when applied to the soil. The elemental composition of the ERP was confirmed with X-ray fluorescence spectroscopy (XRF), Table 1A, and Energy Dispersive X-ray spectroscopy (EDX) (FIG. 3A).

Table 1 A shows the elemental composition of the ERP

From the table 1 A, it can be observed that ERP does not indicate the presence of harmful amounts of heavy metals, confirming that the use of ERP in preparing the nano-fertilizer does not lead to soil contamination. Also, iron, silica and magnesium, that are important plant micronutrients essential for plant growth, are present in the ERP, as can be found in Table 1A. Therefore, the use of natural phosphate sources like ERP, for preparing the nano-fertilizer, offers an additional advantage, in that the nano-fertilizer composition thus prepared can impart both macronutrients, like phosphorus, and micronutrients such as iron, the silica and the magnesium to the plant, which are essential for the plant growth.

The crystallographic features of ERP are studied using Powder X-ray Diffraction (FIG. 3B) and presence of hydroxyapatite (ICDD PDF file - 01-073-2657), hematite (ICDD PDF File - 01-073-0603), silica (ICDD PDF File - 01-070-3755) and corundum (ICDD PDF File -01-078- 3877) in ERP were confirmed. Example 2: Synthesis of rock phosphate (RP) nanoparticles

ERP (10.00 g) was mixed with citric acid, 2.00 g, 4.00 g, and 6.00 g, respectively, to prepare various weight ratios of ERP to citric acid (10:2, 10:4, and 10:6), to obtain a first wet slurry. Mechanical forces were applied to the first wet slurry using both ball milling and manual grinding techniques. Ball milling the ERP with the citric acid demonstrated promising results in comparison to the RP nanoparticles prepared using a manual grinding approach. ERP was converted to RP nanoparticles using the ball milling technique within 1 hour (h); on the other hand, the manual grinding method required a longer time in comparison to the ball milling for converting the ERP to the RP nanoparticles. Ball milling the ERP with citric acid was done with different ratios ofERP: citric acid w/w (10:2, 10:4, 10:6), with 5.00 cm³ of water, ball milled in an Autowest ball mill, to obtain the first wet slurry (aqueous dispersion). The first wet slurry was ball milled at a rate of 1000 rpm for one hour (h) using ceramic balls of diameter 10 mm. The product was oven dried at 60 °C to obtain the RP nanoparticles as a dried powder. The resulting RP nanoparticles were characterized using various analytical techniques to confirm their formation. Example 3: Characterization of the RP nanoparticles

The elemental composition of the RP nanoparticles was confirmed by XRF. The results are presented in Table IB.

Table IB

SEM analysis of the RP nanoparticles after a grinding for 1 hour (FIG. 4A) and for 3 h (FIG. 4B) show a contrasting difference compared to the ERP (FIG. 3C). From the FIG. 3C it can be observed that the ERP does not show specific shapes, whereas the RP nanoparticles prepared by mechanochemical approach shows a nano-plate like morphology (FIGS. 4A-4B. The aspect ratio of the RP nanoparticles demonstrates a significant increase, after ball milling with citric acid. Structural morphology of the RP nanoparticles indicates that the mechanical forces break down the naturally aggregated ERP particles, resulting in increasing the surface area of the RP nanoparticles. In addition, the water assisted milling facilitates the surface modification of the RP nanoparticles by citric acid. The smaller particle size and the presence of citric acid facilitates the release of a phosphate from the ERP in a plant available form. After mechanochemical grinding with citric acid no additional crystallographic phases of the citric acid are detected, as can be observed in FIG. 5.

Figure 6 consists of the PXRD patterns recorded for the samples prepared in order to optimize the ball milling time. As per the results, it is evident that the crystallinity has been retained in all the samples. The PXRD patterns recorded for the samples prepared in order to optimize the ERP and urea ratio to be used in the preparation of nanocomposite are given in FIG. 7. The crystallinity of the samples has been retained despite the different amounts of urea incorporated in the samples.

In FIGS. 8A-8B, the FT-IR spectra for the P-0 stretching region, and O-H stretching region, respectively, of (a) ERP, RP nanoparticles with ERP: citric ratios of (b) 10:4, (c) 10:6, are illustrated. The spectra in the FIGS. 8A and 8B confirm existence of hydrogen (H)-bonding interactions between citric acid and the ERP. Significantly, high wavenumber (35 cm ¹) shift in P04³ stretching frequency (Table 2A and FIG. 8A), and a shift of 31 cm ¹ for OH stretching peaks (Table 2B and FIG. 8B), can be observed in citric acid modified ERP samples (RP nanoparticles).

Table 2A

Table 2B

From a combined observations of FIG. 2D, FIGS. 8A-8B, Table 2A, and Table 2B, it can be concluded that the electron density around both phosphate and hydroxyl functional groups are affected after interacting with the citric acid. In the FT-IR spectroscopic analysis, significant peak broadening of both PO4³ and OH are also observed. These broadening of peaks, and the reduction of peak intensity confirm the existence of H-bonding between citric acid and the ERP particles.

In FIGS. 9A-9B, the FT-IR spectrum for the P-0 stretching region, and O-H stretching region, respectively, of (a) ERP, and ERP treated with different ball milling times of (b) 1 h, (c) 1.5 h, (d) 2 h, (e) 3 h with ERP: citric acid ratio of 10:6, is illustrated. The data from the FIGS. 9A and 9B are tabulated in Table 3 A and Table 3B.

Table 3A

Table 3B

Referring to the FIGS. 9A, and 9B, along with the data in the Tables 3A and 3B, it can be concluded that increase in the grinding time did not lead to a significant peak shift in the FT-IR spectra of PO4³ stretching. In comparison, a peak shift up to 24 cm ¹ was observed in the FT-IR spectra of -OH stretching with the increase in the grinding time. Example 4: Modification of RP nanoparticles with urea

RP nanoparticles were further modified with urea to obtain a nanocomposite, to introduce nitrogen macronutrients into the nano-fertilizer composition. Modification of RP nanoparticles with urea provides a template for slow and a sustained release of the nitrogen from the nano fertilizer. For this purpose, three fertilizer compositions, namely, 10: 6: 40 - ERP: citric acid: urea (composition 1), 10: 6: 60 - ERP: citric acid: urea (composition 2), and 10: 6: 80 - ERP: citric acid: urea (composition 3) were prepared. Composition 1 was prepared by mixing 10 g of ERP with 6 g of citric acid and 40 g of urea in a ball mill. Composition 2, and composition 3 were prepared by mixing 10 g of ERP and 6 g of citric acid with 60 g and 80 g of the urea, respectively, in the ball mill. The ball milling was done for 1 h, at a speed of 1000 rpm using a ball size of 10 mm. The product was oven dried at 60 °C to obtain a dried powder. The three compositions including urea modified RP nanoparticles (or the nanocomposite) were characterized using FT-IR, and the results are given in FIGS. 10A-10C, and Table 4. In FIGS. 10A-10C, the FT-IR spectrum for N- H stretching region, N-H bending region, and C-N stretching region, respectively of (a) urea (b) ERP: urea ratio 1:4 (c) ERP: urea ratio 1:6 (d) ERP: urea ratio 1:8, is illustrated, and the results of the FIGS. 1 OA-l 0C are summarized in Table 4.

Table 4

Example 5: Phosphate solubility studies

For this purpose, phosphorus release from different samples of the RP nanoparticles, prepared with varying weight ratios of ERP and citric acid, were measured using the ascorbic acid method, using the ultraviolet-visible (UV-Vis) spectrometer at 718 nanometer (nm) wavelength. To optimize the ball milling time, ERP and citric acid w/w, 10:6 ratio was ball milled for 1 h, 1.5 h, 2 h, and 3 h. The phosphate solubilization of these samples were also done using ascorbic acid method.

Example 6: Water and soil release studies

Four soil samples (300 g each) were taken from the soil composition that was used to cultivate maize plants. One of the four soil samples was mixed with 0.82 g of commercial urea formulation, 0.32 g of TSP and 0.16 g of murate of potash, all of which are recommended for maize. The chemicals were purchased from the Ceylon fertilizer company (CFC), Sri Lanka. Each soil sample was filled into a glass column. Similarly, RP nanoparticles with ERP: urea ratios of 1:4, 1:6, and 1:8, respectively, having nitrogen-phosphorus-potassium (NPK) content similar to those used in the commercial fertilizer compositions, were taken separately and filled into three glass columns containing the soil samples obtained from the soil composition used for the cultivation of the maize plants. Next, 180 millilitres (ml) of water were added to all the four columns until they reached the soil water saturation point. The water content inside the columns was approximately constant throughout the period of study. The water (100 ml) was added at five- day intervals prior to elution. The eluted solutions (50 ml) were analysed for nitrogen and phosphorus contents. N analysis was done by using Kjeldahl method and the P analysis was done using the ascorbic acid methods. Nitrogen release from different samples at different time intervals was conducted using FT-IR spectroscopy. Meantime Kjeldahl method was used to study the nitrogen content in the water collected from soil release study.

Example 7: Plant trials to study the efficacy of the phosphate availability

The plant uptake studies were conducted at Borelasgamuwa, Sri Lanka, which belongs to the wet zone. The soil used for the experiment was a mixture of sand and compost in a ratio of 1 : 1 with the pH value 6.5. The experiment was carried out for a period of three and half months. Pot trials were conducted using the maize (Zea maize L) as the model crop. Each pot was filled with 3 kilograms (kg) of sand, and 3 kg of compost sand and compost in equal ratios. The pot experiments were designed in the Randomized Complete Block Design (RCBD) with seven treatments (T1-T7 as provided in Table 5) of different types of fertilizer prepared by ball milling the ERP with the citric acid. The treatment types for the pot trials are given in Table 5.

Table 5

Each treatment was replicated six times In all the above treatments, other fertilizer factors, example use of the urea, muriate of potash were kept constant. Data collection was performed in two weeks intervals after the application of fertilizers. The plant height, number of leaves, stem girth, leaf area, ear girth, ear length, number of grains per ear, weight of grains per ear, were recorded. The crop yield was measured at the end of three and a half months. The crop yield was measured using the corn yield component and the corn ear weight (shelling percentage) was determined. a) Corn yield component: The number of harvestable ears were counted in 1/1000^th acre. On each ear, the number of kernel rows per ear were counted and the average was determined. On each of these kernel rows, the number of kernels per row was counted and the average was determined.

Yield (bushels per acre) = no. of ears in 1000^th of an acre x average number of rows of kernel x average number of kernels per row/ 89.6 b) Determining corn ear weight (shelling percentage): 10 randomly selected ears were weighed. Ears were shelled, and the grain was weighted.

Shelling percentage was calculated using (gram weight / ear weight) x 100.

Results and Discussion

In FIGS. 11 and 12, water release behavior and soil release behavior of phosphate in the (a) ERP, RP nanoparticles with ERP: citric ratios of (b) 10:2, (c) 10:4, (d) 10:6 prepared by 1 h ball milling, are illustrated. From the FIGS. 11 and 12, it can be observed that the highest release of phosphorus, both in water and in soil, was shown by the RP nanoparticles prepared with ERP: citric acid ratio 10:6. The percentage release of phosphorus was found to be similar with all the tested samples, except for ERP: citric acid ratio 10:6. For this particular sample, the percentage release was found to be slow and steady for a period of 30 days. Almost a 100% increase in phosphate release was observed, both in soil and in water, with ERP: citric acid ratio 10:6 in comparison to the ERP after 30 days, concluding that the RP nanoparticles obtained method of the present disclosure demonstrated slow and sustained release of phosphorus. This implies the suitability of this composition (ERP: citric acid ratio 10:6) for use as a phosphorus releasing nano fertilizer composition.

In FIGS. 13 and 14, the water release behavior and soil release behavior of nitrogen in (a) urea, RP nanoparticles with ERP: urea ratios (b) 1:4 (c) 1:6 (d) 1:8, are illustrated. Sustained release of nitrogen up to a period of a month was observed with the RP nanoparticles having ERP: urea ratio of 1 :4, comparable to that of urea, or RP nanoparticles prepared with other ratios of ERP: urea.

In FIGS. 15A-15B the yield of the maize plants, three and a half months after harvesting showing com yield component (FIG. 15A), and com ear weight (FIG. 15B), are illustrated. According to the results obtained after two months, the average leaf length of the maize plant on treatments with the RP nanoparticles was greater than the plants grown using pure ERP or TSP. The fertilizer ratio of 10: 6 ERP to citric acid showed a 4% higher average leaf length compared to TSP treatments. The corn yield component of fertilizer prepared with ERP: citric acid ratio 10:6 was 13% higher than the treatments in which TSP was applied. The corn weight (shelling percentage) of the fertilizer prepared with ERP: citric acid ratio 10:6 was 8% higher than the treatments in which TSP was applied. According to the crop yield, the highest corn yield and grain weight (shelling percentage) was shown with the fertilizer where the ratio of ERP: citric acid was 10:6. The crop yield was higher than TSP by a percentage of 13% and 8%, respectively.

INDUSTRIAL APPLICABILITY

The present disclosure provides a nanotechnology based sustainable, scalable, and low- cost, green synthesis method of making the nano-fertilizer composition, with improved nitrogen and phosphorus availability to plants, in a slow and sustained manner. The sustained release improves availability of nutrients to plants, thereby reducing the amount of fertilizer to be used, while also improving the crop yields. The nano-fertilizer composition prepared via water assisted mechanochemical grinding method can supply available phosphorus to plant with performance similar to the commercially available phosphate fertilizers like, TSP, with about 50% reduced dose of fertilizer. This process uses minimum amounts of a water and an energy, and also has minimum waste, thereby making the entire process cost-effective. The method of the present disclosure does not use hazardous mineral acids, and does not generate by-product during the manufacturing process, thereby circumventing the need for multiple purification steps. The approach is based on the use of mechanical forces, and uses a minimum amount of chemicals leading to minimal waste generation. Also, the method of the present disclosure is easily scalable, economically viable and environmentally sustainable.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

1. A method of sustained release of macronutrients to a plant locus, the method comprising: providing a nano-fertilizer composition comprising phosphate nanoparticles, optionally combined with a nitrogen source, wherein the nano-fertilizer composition is prepared using a mechanochemical force; and applying the nano-fertilizer composition to soil.

2. The method according to claim 1, wherein a pH of the soil is in a range of 4-8.5.

3. The method according to any one of claim 1 or 2, wherein the plant is selected from a group consisting of maize, rice, tea, rubber, coconut, fruits, and vegetables.

4. A method of making a nano-fertilizer composition for sustained release of macronutrients, the method comprising: mixing a phosphorus source with an organic acid to obtain a first wet slurry; and applying a mechanical force to the first wet slurry to obtain phosphate nanoparticles.

5. The method according to claim 4, further comprising drying the phosphate nanoparticles to a temperature range of about 50-120 °C to obtain the nano-fertilizer composition.

6. The method according to claim 4, further comprising: mixing the phosphate nanoparticles with a nitrogen source to obtain a second wet slurry; and applying the mechanical force to the second wet slurry to obtain a nanocomposite.

7. The method according to claim 6, further comprising drying the nanocomposite to a temperature range of about 50-70 °C to obtain a dried nano-fertilizer composition.

8. Use of the phosphate nanoparticles according to claim 4 as a foliar fertilizer.

9. Use of the nanocomposite according to claim 6 as a foliar fertilizer.

10. The method according to any one of claims 1-7, wherein the nano-fertilizer composition has a particle size in a range of 1-100 nanometers (nm).

1 l.The method according to any one of claims 4-7, wherein the phosphorus source is one or more selected from a group consisting of hydroxyapatite, hydroxyapatite nanoparticles or hydroxyapatite nanocomposites, chlorapatite, fluorapatite, rock phosphate, and a combination thereof, and the organic acid is one or more selected from a group consisting of citric acid, fulvic acid, humic acid, fumic acid, oxalic acid, salts of these organic acids, or any other organic acid and a combination thereof.

12. The method according to any one of claims 1-11, wherein the nitrogen source is one or more of urea, ammonium nitrate, ammonium sulphate, calcium nitrate, monoammonium phosphate, diammonium phosphate and potassium nitrate or a combination thereof.

13. The method according to any one of claim 1-12, wherein the phosphorus source is rock phosphate, the organic acid is citric acid, and the nitrogen source is urea.

14. The method according to any one of claims 4-13, wherein the phosphorus source to the organic acid weight ratio (w/w) is in a range of 10:1 to 1:1, and the phosphorus source to the nitrogen source w/w ratio is in a range of 1 :4 to 1:8.

15. The method according to any one of claims 1-14, wherein the mechanical force comprises ball milling, grinding, or a combination thereof.

16. The method according to claim 15, further comprising applying the mechanical force by ball milling between 100 to 1000 rpm, for a period of 1-3 hours in presence of a solvent.

17. The method according to claim 16, wherein the solvent is water, a polar organic solvent, or the combination thereof.

18. The method according to claim 15, wherein the grinding is done in the absence of any solvent.

19. The method according to any one of claims 4-18, further comprising, adding at least one nutrient selected from a group consisting of potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), and any combination thereof, to the nano-fertilizer composition.

20. The method according to any one of claims 1-19, wherein the nano-fertilizer is in form of a pellet, powder, tablet, chip, or any combination thereof.