CONTROLLED-RELEASE OF FERTILIZER COMPOSITIONS AND USES THEREOF

Abstract

A controlled-release fertilizer composition, methods of making, and uses thereof are described. The controlled-release fertilizer composition includes a composite graphene-carbon nanotube material having a three-dimensional open-celled network of graphene and carbon nanotubes and a fertilizer impregnated in the three-dimensional open-celled network of graphene and carbon nanotubes.

Claims

1. A controlled-release fertilizer composition comprising: (a) a composite graphene-carbon nanotube material having a three-dimensional open-celled network of graphene and carbon nanotubes; and (b) a fertilizer impregnated within the three-dimensional open-celled network of graphene and carbon nanotubes.

2. The controlled-release fertilizer composition of claim 1, wherein the mass ratio of graphene to carbon nanotubes is 0.1:1 to 5:1.

3. The controlled-release fertilizer composition of claim 1, wherein the composite graphene-carbon nanotube material is a monolith network of graphene and carbon nanotubes having an open-celled foam structure.

4. The controlled-release fertilizer composition of claim 1, wherein the controlled-release three-dimensional open-celled network comprises pores and channels.

5. The controlled-release fertilizer composition of claim 4, wherein the diameter of the pores and channels is 1 to 100 microns.

6. The controlled-release fertilizer composition of claim 1, wherein the graphene comprises a plurality of planar graphene sheets.

7. The controlled-release fertilizer composition of claim 1, wherein the carbon nanotubes are single walled carbon nanotubes, multi-walled carbon nanotubes, or a combination thereof, preferably multi-walled carbon nanotubes.

8. The controlled-release fertilizer composition of claim 1, wherein the fertilizer is controllably released from the composite graphene-carbon nanotube material in response to at least temperature.

9. The controlled-release fertilizer composition of claim 8, wherein the release temperature of the fertilizer is 0 C. to 40 C.

10. The controlled-release fertilizer composition of claim 1, wherein the composite graphene-carbon nanotube material has a thermal conductivity of at least 0.2 milliwatts per meter Kelvin (mW/m.Math.K) at a temperature of 20 C. to 80 C.

11. The controlled-release fertilizer composition of claim 1, wherein the fertilizer comprises urea, ammonium nitrate, calcium ammonium nitrate, one or more superphosphates, molybdenum, zinc, copper, boron, cobalt, iron, a binary nitrogen and phosphorous (NP) fertilizer, a binary nitrogen and potassium (NK) fertilizer, a binary phosphorous and potassium (PK) fertilizer, or a ternary nitrogen, phosphorous, and potassium (NPK) fertilizer, or any combination thereof.

12. The controlled-release fertilizer composition of claim 1, wherein the fertilizer composition is comprised in soil.

13. The controlled-release fertilizer composition of claim 1, comprising 10 wt. % to 95 wt. % of fertilizer, based on the total weight of the controlled-release fertilizer composition.

14. A method of fertilizing soil, the method comprising applying the controlled-release fertilizer composition of claim 1 to soil.

15. The method of claim 14, wherein the controlled-release fertilizer composition is applied to the surface of the soil.

16. The method of claim 14, wherein the fertilizer is controllably released from the composite graphene-carbon nanotube material in response to at least temperature.

17. The method of claim 16, wherein the release temperature of the fertilizer is 0 C. to 40 C.

18. A method of making the controlled-release fertilizer composition of claim 1, the method comprising: (a) obtaining a composite graphene-carbon nanotube material having a three-dimensional open-celled network of graphene and carbon nanotubes; (b) combining the composite graphene-carbon nanotube material with an aqueous solution comprising a fertilizer for a sufficient period of time to allow the aqueous solution to infiltrate the three-dimensional open-celled network of graphene and carbon nanotubes; and (c) drying the composite graphene-carbon nanotube material from step (b) to obtain the controlled-release fertilizer composition.

19. The method of claim 18, wherein steps (b) and (c) are each performed at a temperature of 5 C. to less than 100 C.

20. The method of claim 18, wherein the composite graphene-carbon nanotube material from step (a) is obtained by lyophilizing an aqueous mixture of graphene and carbon nanotubes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings.

[0028] FIG. 1 is a non-limiting illustration of a structural representation of a composite graphene-carbon nanotube material.

[0029] FIG. 2 is a scanning electron microscope (SEM) image of a graphene-carbon nanotube three-dimensional open-celled foam of the present invention having a 0.5:1 weight ratio of graphene to carbon nanotubes.

[0030] FIG. 3 illustrates the distribution of urea after being released in soil at 10 C. using the controlled-release fertilizer composition of the present invention.

[0031] FIG. 4 illustrates the distribution of urea after being released in soil at 20 C. using the controlled-release fertilizer composition of the present invention.

[0032] FIG. 5 illustrates the distribution of urea after being released in soil at 30 C. using the controlled-release fertilizer composition of the present invention.

[0033] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention provides a control-release fertilizer composition and methods of using the composition. The composition includes fertilizer impregnated within a composite graphene/carbon nanotubes material that includes a three-dimensional network of interconnected pores and channels formed by the graphene and carbon nanotubes. The material has excellent thermal conductivity, which can promote effective absorption and release of one or more fertilizer(s). Use of the control-release fertilizer composition provides an elegant way for sustainable and efficient agriculture while mitigating and/or eliminating fertilizer pollution or costly repeated applications.

[0035] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections with reference to the figures.

A Controlled-Release Fertilizer Composition and Method of Making

[0036] Embodiments herein describe the controlled-release fertilizer compositions and methods of making the compositions. The controlled-release fertilizer compositions can include a composite graphene-carbon nanotube material having a three-dimensional open-cell network of graphene and carbon nanotubes, and a fertilizer. Impregnation of fertilizer in the three-dimensional open-celled network of graphene and carbon nanotubes provides an elegant way to provide controlled-release of fertilizer in amounts effective to achieve high absorption rates of nutrient salt substrates by plants. Due to the three-dimensional open-cell network of graphene and carbon nanotubes, significant amounts of fertilizer can be impregnated into the composite graphene-carbon nanotube material relative to the total weight of the controlled-release fertilizer composition. The ability to impregnate high doses of fertilizer into the composite that can be controllably released allows for fewer applications of fertilizer to a given crop. The controlled-release fertilizer composition can be packaged for commercial use (e.g., farms, etc.) or for individual consumer use (e.g., yards, etc.). Such packaging includes bags, containers, railcars, hoppers, etc.

[0037] In some embodiments, the fertilizer composition of the present invention can include 10 wt. % to 95 wt. % of fertilizer or at least, equal to, or between any two of 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94 and 95 wt. % of fertilizer, based on the total weight of the controlled-release fertilizer composition. This high loading of fertilizer can be released in soil or water in a temperature-controlled and stable manner.

[0038] In a non-limiting method to produce a controlled-release fertilizer composition, the composite graphene-carbon nanotube material having a three-dimensional open-celled network of graphene and carbon nanotubes can be obtained as described below. Once formed, the composite three-dimensional composite graphene-carbon nanotube material can be combined with an aqueous fertilizer solution for a sufficient period of time to allow the aqueous solution to infiltrate the three-dimensional open-celled network of graphene and carbon nanotubes. The fertilizer impregnated composite graphene-carbon nanotube material can then be dried to obtain the controlled-release fertilizer of the present invention. The impregnation and drying steps can each be performed at a temperature of 5 C. to less than 100 C., preferably 10 C. to 50 C. or at least, equal to, or between any two of 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 and 50 C. In certain aspects, the impregnation of fertilizer into the pores and channels of the composite graphene-carbon nanotube material can be performed at least, equal to, or between any two of 15 C., 20 C., 25 C., and 30 C., or more preferably at 20 C. to 25 C.

[0039] In certain aspects, the fertilizer of the present invention includes one or more nutrients. Nutrients can be in a salt form. Non-limiting examples of nutrient salts include aluminum sulfate, amino acid salt, ammonium chloride, ammonium molybdate, ammonium nitrate, ammonium phosphate, ammonium phosphate-sulfate, ammonium sulfate, borax, boric acid, calcium ammonium nitrate, calcium silicate, calcium chloride, calcium cyanamide, calcium nitrate, copper acetate, copper nitrate, copper oxalate, copper oxide, copper sulfate, diammonium phosphate (DAP), iron-ethylenediamine-N,N-bis (Fe-EDDHA), iron-ethylenediaminetetraacetic acid (Fe-EDTA), elemental sulfur, ferric sulfate, ferrous ammonium phosphate, ferrous ammonium sulfate, ferrous sulfate, gypsum, humic acid, iron ammonium polyphosphate, iron chelates, iron sulfate, lime, magnesium sulfate, manganese chloride, manganese oxide, manganese sulfate, mono-ammonium phosphate (MAP), monopotassium phosphate, polyhalite, potassium bromide, potassium chloride (MOP), potassium nitrate, potassium polyphosphate, potassium sulfate, sodium chloride, sodium metasilicate, sodium molybdate, sodium nitrate, sulfate of potash (SOP), sulfate of potash-magnesia (SOP-M), single superphosphate (SSP), triple superphosphate (TSP), urea, urea formaldehyde, zinc oxide, zinc sulfate, zinc carbonate, zinc phosphate, and zinc chelate. Binary NP, NK, and PK fertilizers include two component fertilizers listed above containing compositions of nitrogen-phosphorus, nitrogen-potassium, and phosphorus-potassium respectively. Ternary NPK fertilizers include nitrogen, phosphorus, and potassium and superphosphates compounds. Super phosphated compounds can include single superphosphate (SSP) and triple superphosphate (TSP) compounds. A mixture of SSP and TSP compounds is referred to as double superphosphate (DSS) compounds. In some aspects, the fertilizer composition can include combinations of these salts and/or non-salt forms of the above-listed nutrients, among others. In preferred aspects, the at least one nutrient salt can include urea, ammonium nitrate, calcium ammonium nitrate, one or more superphosphates, a binary NP fertilizer, a binary NK fertilizer, a binary PK fertilizer, a NPK fertilizer, molybdenum, zinc, copper, boron, cobalt, or iron, or any combination thereof. In a specific aspect, at least one nutrient salt includes urea. Fertilizers are commercially available from many sources. A non-limiting example of a source of urea is Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

B. Composite Graphene-Carbon Nanotube Materials and Preparation Thereof

[0040] The composite graphene-carbon nanotube material of the present invention can have various three-dimensional structural arrangements. FIG. 1 depicts a non-limiting structural representation of a composite graphene-carbon nanotube material. As shown, graphene 10 and carbon nanotube 20 form a randomly orientated composite having three-dimensional structure. Further non-limiting examples of three-dimensional structural arrangements can include a foam, a honeycomb, a mesh, and the like. In a preferred embodiment, the composite graphene-carbon nanotube material has a three-dimensional open cell network. Without being limited by theory, it is believed that the graphene and carbon nanotubes are coupled together in the composite graphene-carbon nanotube material by van der Waal forces. It is also believed that the piling of the graphene and the carbon nanotubes forms a three-dimensional network skeleton, resulting in pores and channels that are in mutual communication that can effectively implement absorption and release of fertilizers.

[0041] The mass (weight) ratio of graphene to carbon nanotubes in the composite graphene-carbon nanotube material can be 0.1:1 to 5:1, preferably 0.5:1 to 2:1 or at least, equal to, or between any two of 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1 and 2:1. By adjusting the mass ratio of the graphene to carbon nanotubes used to make the composite controlled-release material, several properties can be tuned. For instance, the thermal conductivity of the composition can be optimized to 0.8 mW/m.K when the mass ratio of the graphene to carbon nanotubes is adjusted to 1.3:1. The three-dimensional (3-D) open-celled foam structure can include pores and channels. The pore structure of the foam can be uniform, or disordered, and have a variety of pore and channel sizes. In preferred aspects, at least one pore and/or channel of the controlled-release three-dimensional open-celled network can have a diameter from 1 to 100 microns, preferably 2 to 50 microns or at least, equal to, or between any two of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 60, 70, 80, 90, and 100 microns. The pore volume can be from 0.5 to 2.5 cm.sup.3/g or at least, equal to, or between any two of 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, and 2.5 cm.sup.3/g, preferably 1 to 2 cm.sup.3/g, more preferably 1.5 to 1.8 cm.sup.3/g. The specific surface area of the graphene-carbon nanotube material of the present invention can be 50 m.sup.2/g to 300 m.sup.2/g, preferably 200 m.sup.2/g.

[0042] The composite graphene-carbon nanotube material of the present invention advantageously has a thermal conductivity that allows for release of the fertilizer based on temperature. In some embodiments, the thermal conductivity can be least 0.2 mW/m..Math.K at a temperature of 20 C. to 80 C., preferably a thermal conductivity of 0.3 mW/m..Math.K to 0.8 mW/m..Math.K and all thermal conductivities and ranges at least, equal two or between any two of 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79 and 0.80 mW/m..Math.K at a temperature of 25 C. to 60 C. and all temperatures of at least, equal to, or between any two of 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 and 60 C. Thermal conductivities can be measured by a Hot Disk Instruments TPS 2500S (Hot Disk AB, Sweden) via a steady-state method at ambient pressure and at a temperature ranging between 20 C. and 80 C.

[0043] In one instance, preparation of the controlled-release graphene-carbon nanotube composite of the present invention can include dispersion and/or distribution of the carbon nanotubes onto the surface of graphene sheets (e.g., piling).

[0044] Graphene is an ultra-thin and ultra-light layered carbon material forming a two-dimensional honeycomb lattice with high mechanical strength, super conductivity, and high specific surface area. The graphene contained in the composite graphene-carbon nanotube material of the present invention can include a plurality of planar graphene sheets. In another aspect, the graphene is not functionalized. Graphene is commercially available from many sources. A non-limiting example of a source of graphene is Ningbo Morsh Tech. Co., Ltd., (China).

[0045] Carbon nanotubes (CNTs) are nanometer-scale tubular-shaped graphene structures that have high specific surface area, excellent thermal conductivity, electrical conductivity, and excellent mechanical properties. CNTs have also been shown to be highly resistant to fatigue, radiation damage, and heat. Carbon nanotubes (CNTs) can have a variety of structural forms, thereby allowing tuning or designing of the chemical and/or physical properties pertaining to the environment that the fertilizer is to be released. The CNTs contained in the controlled-release fertilizer composition of the present invention can be single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), triple-walled carbon nanotubes (TWNTs), multi-walled carbon nanotubes (MWNTs), graphenated carbon nanotubes (g-CNTs), nitrogen-doped carbon nanotubes (N-CNTs), or combinations thereof. Preferably, the CNTs are multi-walled carbon nanotubes (MWNTs). CNTs are commercially available from many sources. A non-limiting example of a commercial source of MWCNTs is Shandong Dazhan Nanomaterials Co., Ltd., (China).

[0046] In one embodiment, carbon nanotubes can be precipitated from a solution in the presence of graphene, followed by drying. In another embodiment, graphene and carbon nanotubes can be mixed together in solid form, dissolved, or suspended together in a suitable solvent. The solution can be agitated (e.g., stirred and/or sonicated) and the solvent can be removed (e.g., through evaporation). In a preferred aspect, the composite graphene-carbon nanotube material can be obtained by lyophilization (i.e., freeze drying) of an aqueous mixture of graphene and carbon nanotubes. In the lyophilization method, both graphene and carbon nanotubes of a predetermined concentration (e.g., a mass ratio of graphene to carbon nanotubes of 0.5:1, 1:2, or 2:1) can be dispersed in an aqueous medium or solution. The dispersion can be stirred, sonicated, and/or heated to ensure even distribution or homogeneity, and then subjected to freezing conditions (e.g., 200 to 60 C.) to form a frozen material. The frozen material can be dried at less than about 60 C. and a vacuum of about 1.3 to 13 Pa to remove the water and form the open-cell structure (e.g., lyophilized in a conventional freeze-drying apparatus). The resulting graphene-carbon nanotube open-cell structures can be collected.

[0047] In some embodiments, the composite graphene-carbon nanotube materials can be reduced in size (e.g., macronized, micronized or nanosized), using known sizing methods (e.g., granulation or powderification). In any of the above methods, the materials may be mixed together using suitable mixing equipment. Examples of suitable mixing equipment include tumblers, stationary shells or troughs, Muller mixers (for example, batch type or continuous type), impact mixers, and any other generally known mixers, or generally known devices that can suitably provide dispersion of the graphene and the carbon nanotube. For solution chemistries, a mechanical stirrer, or sonification can be used.

C. Use of Controlled-Release Fertilizer Composition

[0048] Methods of using the controlled-release fertilizer composition of the present invention are described. A method can include applying the controlled-release fertilizer to soil (e.g., for renewable agricultural purposes). Preferably, the controlled-release fertilizer composition is applied to the soil at a depth of at least 2 cm from the soil surface, more preferably a depth of 2 cm to 15 cm, or most preferably 5 to 12 cm, and all depths of at least, equal to, or between any two of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 cm from the soil surface. By way of example, the soil can be tilled or cultivated using mechanical agitation (e.g., dug, stirred, overturned, or the like) and the controlled-release fertilizer applied to the tilled soil using a spreader, and then covered by the soil. In other embodiments, the fertilizer can be added at the time of planting of crops or seeding of a field. The fertilizer can be controllably released from applied controlled-release fertilizer composition in response to at least ground temperature and provide nutrients over time without significant leaching of fertilizer or loss of nutrients.

[0049] The impregnated fertilizer contained within the pores, channels, or both, of the open-celled network can be controllably-released. The release temperature of the fertilizer can be 0 C. to 40 C., preferably 10 C. to 30 C. and temperatures of at least, equal to, or between any two of 0, 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, and 40 C. The efficient release of impregnated fertilizer can be due in part, to the high thermal transport or heat transfer within the composite graphene-carbon nanotube material. This thermal transport or heat transfer known as thermal conductivity can be measured quantitatively by processes known by those of ordinary skill in the art. In some aspects, an increase in ambient temperature results in an increase in fertilizer release such the controlled-release fertilizer of the present invention can be used to release fertilizer at a rate that corresponds with temperature dependent agriculture growth cycles. In some embodiments, the controlled-release fertilizer composition having a composite can be used as a renewable fertilizer. By way of example, once the fertilizer is released, the composite graphene-carbon nanotube material can be collected and recharged. In non-limiting aspects, the controlled-release fertilizer can be provided as granules, pellets, nodules, plates, stakes, rods, cubes, chunks, etc., that may be contained within a permeable container for convenient application, storage, and retrieval.

EXAMPLES

[0050] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters, which can be changed or modified to yield essentially the same results.

[0051] Graphene was obtained from Ningbo Morsh Tech. Co., Ltd., China. Multi-walled carbon nanotubes (MWCNT) were obtained from Shandong Dazhan Nanomaterials Co., Ltd., China. Urea (>99%) was obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Urea concentration in the soil was measured using a spectrophotometry method using a chromogenic reagent (i.e., p-dimethylaminobenzaldehyde) after leaching soil with water.

Example 1

Preparation of a Composite Graphene-Nanocarbon Material

[0052] Graphene and carbon nanotubes (0.5:1, 1:1, or 2:1 wt:wt) were added to water (1000 mL) to form a suspension. The suspension was then frozen in liquid nitrogen and placed under vacuum and held at cryogenic temperature (about 60 C.) to sublime off the frozen water until a dry solid composite foam having the above provided ratios of graphene to carbon nanotubes remained. FIG. 2 shows an SEM of the 0.5:1 graphene-carbon nanotube three-dimensional composite foam. The thermal conductivity of the composite foam was 0.5 mW/m.K, pore volume was 1.2 cm.sup.3/g, and specific surface area was 212 m.sup.2/g.

Example 2

Impregnation of the Composite Material with Urea and Urea Release Testing

[0053] Urea (54 g) was added to water (100 g) under stirring to obtain a saturated urea solution. Graphene-carbon nanotube three-dimensional composite foam (10 g, 0.5:1 from Example 1) was immersed in the saturated urea solution overnight at room temperature, removed from the solution, and then dried in air overnight at room temperature. The resultant impregnated foam was buried in soil at a depth of 11 cm from the surface. The soil was maintained at 10 C. and the urea content of the soil was monitored over a 3 day period. FIG. 3 shows the urea content at different depths in the soil. As determined from the data, the urea was gradually released from the graphene-carbon nanotube composite foam into the soil at a total release rate of 9%, which was lower than 15% for the blank control experiment without composite foam.

Example 3

Impregnation of the Composite Material with Urea and Urea Release Testing

[0054] Urea (54 g) was added to water (100 g) under stirring to obtain a saturated urea solution. Graphene-carbon nanotube three-dimensional composite foam (8 g, 1:1 from Example 1) was immersed in the saturated urea solution overnight at room temperature, removed from the solution, and then dried in air overnight at room temperature. The resultant impregnated foam was buried in soil at a depth of 5 cm from the surface. The soil was maintained at 20 C. and the urea content of the soil was monitored over a 3 day period. FIG. 4 shows the urea content at different depths in the soil. As determined from the data, the urea was gradually released from the graphene-carbon nanotube composite foam into the soil at a total release rate of 16%, which was lower than 28% for the blank control experiment without composite foam.

Example 4

Impregnation of the Composite Material with Urea and Urea Release Testing

[0055] Urea (54 g) was added to water (100 g) under stirring to obtain a saturated urea solution. Graphene-carbon nanotube three-dimensional composite foam (5 g. 2:1 from Example 1) was immersed in the saturated urea solution overnight at room temperature, removed from the solution, and then dried in air overnight at room temperature. The resultant impregnated foam was buried in soil 11 cm from the surface. The soil was maintained at 30 C. and the urea content of the soil was monitored over a 3 day period. FIG. 5 shows the urea content at different depths in the soil. As determined from the data, the urea was gradually released from the graphene-carbon nanotube composite foam into the soil at a total release rate of 36%, which was lower than 49% for blank control experiment without composite foam.

[0056] It can be seen from the above examples that the release rate of urea in the graphene-carbon nanotube composite foam was lower than the controls. As the ambient temperature of the soil increases, the release rate increases, indicating that the controlled release of a fertilizer can be achieved by adjusting the ambient temperature of the soil.