FIELD OF THE INVENTION

[0001] The present invention relates to compositions for the retention and time-elapsed release of water. The present application further relates to the use of compositions for the retention and time-elapsed release of water in agricultural applications.

BACKGROUND OF THE INVENTION

[0002] Hydrogels are macromolecular polymer gels constructed of a network of crosslinked polymer chains. Hydrogels are synthesized from hydrophilic monomers by either chain or step growth, along with a functional crosslinker to promote network formation. A net-like structure along with void imperfections enhance hydrogels ability to absorb and desorb large amounts of water or aqueous solutions via hydrogen bonding. Some hydrogels can be made to be sensitive to environmental stimuli. Such "smart gels" or "intelligent gels" have the ability to undergo changes in response to variation of pH, temperature, and/or metabolite concentrations. Hydrogels have found utility in various industries such as agriculture (for example, for use in granules for retaining soil moisture in arid regions), medicine (for example, absorbable sutures and drug delivery vehicles), and absorbent products (for example, disposable diapers or sanitary textiles).

[0003] At present, the preparation of graphene from graphite by chemical methods enables scaling in production and ensures large-scale industrial exploitation. In particular, oxidation and exfoliation of graphite is the most widespread method. The production of graphite oxide via oxidation of graphite with a strong oxidizing agent has been known since the nineteenth century.

Specifically, the famous "Hummers method" uses sodium nitrate, potassium permanganate and concentrated sulfuric acid to convert graphite to graphite oxide (Hummers W.S., Offeman R.E. Preparation of Graphitic Oxide, J. Am. Chem. Soc, 1958, 80(6), 1339-1339). Subsequent exfoliation and delamination processes can be used to convert graphite oxide to graphene oxide. Graphene oxide can then be reduced to form reduced graphene oxide (rGO) or graphene. Graphene (a sheet of sp ² hybridized carbon and monoatomic thickness) has attracted great interest in recent years due to the unique electronic and mechanical properties presented. These make it interesting for many applications such as conversion and storage of energy (solar cells, supercapacitors), electronics (circuits based on graphene), etc. (See, for example, Camblor et al., Microwave frequency tripler based on a microstrip gap with graphene, J. Electromag. Waves Appl, 2011, 25 (14-15), 1921 -1929).

[0004] At present, the preparation of graphene from graphite ore by chemical methods is the one method that provides for scaling in production and is the most promising in terms of large- scale industrial exploitation. In particular, oxidation/exfoliation/reduction of naturally-occurring graphite ore is the most widespread method for producing graphite oxide/graphene oxide/graphene. In this process, the oxidation of three-dimensional graphite material having a lamellar structure yields graphite sheets with oxidized basal planes and borders having an expanded three-dimensional structure. The delamination/exfoliation of graphite oxide using an external force, such as sonication, yields graphene oxide. Finally, reducing the graphene oxide to form unilamellar (single layer) sheets, which can be produced by various methods, results in graphene. Furthermore, in addition to the well-known benefits of graphene, the intermediate products (graphite oxide and graphene oxide) are materials which in and of themselves have much interest and commercial application. See, e.g., Gonzalez Z., Botas C, Alvarez P., Roldan S., Blanco C, Santamaria R., Granda M., Menendez R., "Thermally reduced graphite oxide as positive electrode in vanadium redox flow batteries." Carbon, 2012, 50 (3), 828-834. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a flowchart, of a method for the production of graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres according to various aspects of the present disclosure,

[0006] FIG. 2 is a flowchart of another method for the production of graphite oxide, graphene oxide and/or graphene-contaming hydrogel spheres according to various aspects of the present disclosure;

[0007] FIG. 3 is a flowchart of a method for incorporating of graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres into a crop field according to various aspects of the present disclosure;

[0008] FIG. 4 is an image of graphene oxide-containing calcium alginate hydrogel spheres (left), and calcium alginate only hydrogel spheres (right) according to various aspects of the present disclosure;

[0009] FIG. 5 shows images of dry graphene oxide-containing calcium alginate hydrogel spheres and calcium alginate hydrogel spheres (left), and graphene oxide-containing calcium alginate hydrogel spheres and calcium alginate hydrogel spheres subjected to hydration for one night under ambient conditions (right) according to various aspects of the present disclosure;

[0010] FIG. 6 is an image of graphene oxide-containing alginate hydrogel spheres (A), and calcium alginate hydrogel spheres (B), undergoing hydration at elevated temperature (70°C) according to various aspects of the present disclosure; and

[0011] FKJ. 7 is schematic illustration for the variation experimental parameters to establish the effect of graphite oxide, graphene oxide and/or graphene concentration, drying time, drying temperature, agitation (ultrasonication) time, and adsorption (i.e. hydration) temperature in the performance of graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres ("Concentracion" means concentration, "peso" means weight, "tiempo de secado" means drying time, "T Secado" means drying temperature, "tiempo de sonicacion" means sonication time, "and T Adsorpcion" means adsorption temperature).

DETAILED DESCRIPTION

[0012] The following description of the embodiments is merely exemplary in nature and is in no way intended to limit the subject matter of the present disclosure, their application, or uses.

[0013] As used throughout, ranges are used as shorthand for describing each and every value that is within the range. Any value within the range can be selected as the terminus of the range. Unless otherwise specified, all percentages and amounts expressed herein and elsewhere in the specification should be understood to refer to percentages by weight.

[0014] For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term "about." The use of the term "about" applies to all numeric values, whether or not explicitly indicated. This term generally refers to a range of numbers that one of ordinary skill in the art would consider as a reasonable amount of deviation to the recited numeric values (i.e., having the equivalent function or result). For example, this term can be construed as including a deviation of ±10 percent, alternatively ±5 percent, and alternatively ±1 percent of the given numeric value provided such a deviation does not alter the end function or result of the value. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present invention.

[0015] It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," include plural references unless expressly and unequivocally limited to one referent. As used herein, the term "include" and its grammatical variants are intended to be non- limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items. For example, as used in this specification and the following claims, the terms "comprise" (as well as forms, derivatives, or variations thereof, such as "comprising" and "comprises"), "include" (as well as forms, derivatives, or variations thereof, such as "including" and "includes") and "has" (as well as forms, derivatives, or variations thereof, such as "having" and "have") are inclusive (i.e., open-ended) and do not exclude additional elements or steps. Accordingly, these terms are intended to not only cover the recited element(s) or step(s), but may also include other elements or steps not expressly recited. Furthermore, as used herein, the use of the terms "a" or "an" when used in conjunction with an element may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one." Therefore, an element preceded by "a" or "an" does not, without more constraints, preclude the existence of additional identical elements.

[0016] Various aspects of the present disclosure are directed toward the use of graphite and derivatives thereof in compositions for the retention and time-elapsed release of water. Such compositions can be applied to various sectors such as thermal and electrical insulation, heat recovery ventilation (HRV) systems, energy recovery ventilation (ERV) systems, high-speed cables, super batteries, flexible touch screens, medicinal applications, textiles manufacturing, reinforced plastics, ceramics and metals, water desalination, microelectronics, solar cells, catalysis, transistors, ultrasensitive chemical detectors, air purification, water purification, and polymer additives.

[0017] While various aspects of the present disclosure are directed to the use of graphite derivatives, specifically graphite oxide, graphene oxide and/or graphene, one of ordinary skill in the art will appreciate that other carbonaceous materials, or allotropes of carbon, such as charcoal, activated charcoal, bone char, biochar, soot, coke, coal, carbon black fullerenes, single- or multiwalled carbon nanotubes, carbon nanosheets, amorphous carbon, or other carbonaceous materials can be used without imparting from the scope of the present disclosure.

[0018] Such carbonaceous materials can increase the mechanical strength of a hydrogel, as well as its ability to absorb water and regulate the desorption of water therefrom. By providing support for hydrogel composites with a variety of materials, increasing its mechanical strength and chemical activity, the use of carbonaceous materials such as graphite oxide, graphene oxide and/or graphene to enhance the hydrogel, is described herein. In some instances, graphite oxide, graphene oxide and/or graphene-containing hydrogels can be used as antibacterial agents. Compositions according to the present disclosure can be beneficial in improving biological activity and increasing agricultural production, promoting water recovery in semi-arid or arid areas or abandoned land, and in the growth of less fertile crops.

[0019] Various aspects of the present disclosure are directed toward the use of graphite and derivatives thereof as dopants in hydrogels. In the present application, the production of hydrogels doped with graphite oxide, graphene oxide and/or graphene is described. Such hydrogel compositions can be used in various industries such as the agricultural industry for the retention and time-elapsed release of moisture therefrom. The ability of such hydrogel compositions to absorb large quantities of water to subsequently release water gradually allows for time-elapsed watering of crops while preventing the loss of excess water by, for example, evaporation or runoff, resulting in a considerable saving of water. Carbonaceous materials such as graphite oxide, graphene oxide and/or graphene has been found to improve the capacity for water retention while also providing mechanical support to the hydrogel and increasing chemical reactivity. Such graphite oxide, graphene oxide and/or graphene-containing hydrogels may also exhibit enhanced antibacterial activity over pure hydrogels.

[0020] Various aspects of the present disclosure are also directed toward the use of graphite oxide, graphene oxide and/or graphene-containing hydrogel compositions as a desiccant. In some instances, graphite oxide, graphene oxide and/or graphene-containing hydrogel compositions can be used as a desiccant in energy recovery ventilation (ERV) systems. In some instances, the compositions can be used as a desiccant in thermal wheels (also known as rotary heat exchangers, or rotary air-to-air enthalpy wheels, heat recovery wheels, or desiccant wheels). In some instances, the compositions can be used as desiccants in plate heat exchangers.

[0021] FIG. 1 is a flowchart of a method for the production of graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres. While the method 100 in FIG. 1 illustrates various processes, steps, or procedures, one of ordinary skill in the art can appreciate that additional processes, steps, or procedures can be added, or certain processes, steps, or procedures can be removed, without imparting from the scope of the method 100. The method 100 can start at block 110.

[0022] In block 110, sodium alginate is mixed in an amount of water sufficient to completely or substantially completely dissolve all of the sodium alginate and form an aqueous sodium alginate solution. In some instances, the sodium alginate is mixed in water such that the aqueous sodium alginate solution has a final concentration of about 0.33 grams of sodium alginate per 1 mL of water (0.33 g/mL). In other instances, the aqueous sodium alginate solution can be made to have a final concentration ranging from about 0.05 g/mL to about 1 g mL, alternatively from about 0.1 g/mL to about 0.8 g/mL, alternatively from about 0.15 g/mL to about 0.6 g/mL, and alternatively from about 0.2 g/mL to about 0.4 g/mL. In some instances, the amount of water used is about 300 mL. In other instances, the amount of water used is from about 50 mL to about 1 L, alternatively about 100 mL to about 800 mL, alter atively about 150 mL to about 600 mL, alternatively about 200 mL to about 400 mL, and alternatively about 250 mL to about 350 mL. Mixing can be performed while stirring and at elevated temperatures. The elevated temperature can range from about 40°C to about 95°C, alternatively from about 50°C to about 90°€, alternatively from about 60°C to about 80°C, and alternatively can be about 70°C.

[0023] In block 120, a predetermined amount of one or more of graphite oxide, graphene oxide and graphene is dispersed by agitation in the aqueous sodium alginate solution to form a composite solution. Agitation may be accomplished by any one of sonication, ultrasonication, or ultrasonic mixing in a bath or using a probe, mechanical stirring, magnetic stirring, shaking, or any other suitable agitation technique known to one of ordinary skill in the art. In some instances, the one or more of graphite oxide, graphene oxide and graphene is added to the aqueous sodium alginate solution in the form of a dispersion, in some instances, the dispersion can have a concentration of 3 mg of one or more of graphite oxide, graphene oxide and graphene per ml, of water (3 mg/mL). In other instances, the dispersion of one or more of graphite oxide, graphene oxide and graphene can have a concentration of about 0.05 mg/mL to about 10 mg/mL, alternatively about 0.5 mg/mL to about 8 mg/mL, alternatively about 1 mg/mL to about 8 mg/mL, alternatively about 1.5 mg/mL to about 6 mg/mL, alternatively about 2 mg/mL to about 4 mg/mL, and alternatively about 2.5 mg/mL. to about 4.5 mg/mL. In some instances, about 50 mL of dispersion having one or more of graphite oxide, graphene oxide and graphene can be used. In other instances, about 10 mL to about 100 mL, alternatively about 20 mL to about 90 mL, alternatively about 25 mL to about 80 mL, alternatively about 30 mL. to about 70 mL, alternatively about 40 mL to about 60 mL, and alternatively about 45 mL to about 55 mL of dispersion having one or more of graphite oxide, graphene oxide and graphene can be used.

[0024] In block 130, after agitating, the composite solution is homogenized and filtered to remove any remaining solids. Filtering should be performed using a filter with pore sufficiently large to allow the graphite oxide, graphene oxide and graphene to pass therethrough. The filtered composite solution is then allowed to sit for a period time sufficient for any bubbles formed to be expelled from the filtered composite solution. The composite solution can be allowed to sit for a period time ranging from about 1 about 12 hours, alternatively about 2 to about 8 hours, alternatively about 3 to about 6 hours, and alternatively about 4 hours.

[0025] In block 140, the filtered composite solution is added dropwise to an aqueous CaCh solution. In some instances the CaCb solution can have a concentration of about 0.06 g/mL. In other instances, CaCb solution can have a concentration of about 0.01 g/mL to about 0.25 g/mL, alternatively about 0.02 g/mL to about 0.2 g/mL, about 0.03 g/mL to about 0.15 g/mL, about 0.04 g/mL to about 0.1 g/mL, and about 0.05 g/mL to about 0.07 g/mL. In some instances, the amount of water in the CaCb solution is about 250 mL. In other instances, the amount of water in the CaCb. solution is about 50 mL to about 1 L, alternatively about 100 mL to about 800 mL, alternatively about 150 mL to about 600 mL, alternatively about 200 mL to about 400 mL, and alternatively about 200 mL to about 300 mL. The CaCb. converts the soluble sodium alginate into insoluble calcium alginate, forming calcium alginate hydrogel spheres. The graphite oxide, graphene oxide and/or graphene becomes entrapped and dispersed within the calcium alginate hydrogel spheres to form graphite oxide, graphene oxide and/or graphene-containing alginate hydrogei spheres.

[0026] In block 150, the graphite oxide, graphene oxide and/ or graphene-containing alginate hydrogei spheres are isolated a d dried to yield the final product. In some instances, the spheres can be subjected to one or more washing steps, to remove impurities, by-products or unwa ted ions, prior to drying.

[0027] FIG. 2 is a flowchart of another method for the production of graphite oxide, graphene oxide and/or graphene-containing hydrogei spheres. While the method 200 in FIG. 2 illustrates various processes, steps, or procedures, one of ordinary skill i the art can appreciate that additio al processes, steps, or procedures can be added, or certain processes, steps, or procedures can be removed, without imparting from the scope of the method 200. The method 200 can start at block 210.

[0028] In block 210, predetermined amounts of kappa ( )-carrageenan and sodium alginate are mixed in an amount of water sufficient to completely or substantially completely dissolve all of the K-carrageenan and sodium alginate and for an aqueous solution. In some instances, the ~ carrageenan and sodium alginate is mixed in water such that the aqueous solution has a final concentration of about 0.33 grams of κ-carrageenan/sodium alginate per 1 mL of water (0.33 g/mL). In other instances, the aqueous sodium alginate solution can be made to have a final concentration ranging from about 0.05 g/mL to about 1 g/mL, alternatively from about 0.1 g/mL to about 0.8 g/mL, alternatively from about 0.15 g/mL to about 0.6 g/mL, and alternatively from about 0.2 g/mL to about 0.4 g/mL. In some instances, the amount of water used is about 300 mL. In other instances, the amount of water used is from about 50 mL to about 1 L, alternati vely about 100 mL. to about 800 mL, alternatively about 150 mL. to about 600 mL, alternatively about 200 niL to about 400 mL, and alternatively about 250 L to about 350 mL. Mixing can be performed while stirring and at elevated temperatures. The elevated temperature can range from about 40°C to about 95°C, alternatively from about 50°C to about 90°C, alternatively from about 60°C to about 80°C, and alternatively can be about 70°C.

[0029] In block 220, a predetermined amount of one or more of graphite oxide, graphene oxide and graphene is dispersed by agitation in the aqueous sodium alginate solution to form a composite solution. Agitation may be accomplished by any one of sonication, ultrasonication, or ultrasonic mixing in a bath or using a probe, mechanical stirring, magnetic stirring, shaking, or any other suitable agitation technique known to one of ordinary skill in the art. In some instances, the one or more of graphite oxide, graphene oxide and graphene is added to the aqueous sodium alginate solution in the form of a dispersion. In some instances, the dispersion can have a concentration of 3 nig of one or more of graphite oxide, graphene oxide and graphene per ml, of water (3 ixig/mL). In other instances, the dispersion of one or more of graphite oxide, graphene oxide and graphene can have a concentration of about 0.05 rag/mL to about 10 mg/mL, alternatively about 0.5 mg/rnL to about 8 mg/raL, alternatively about 1 mg raL to about 8 mg/mL, alternatively about 1.5 mg/rnL to about 6 mg/raL, alternatively about 2 mg mL to about 4 mg/rnL, and alternatively about 2.5 mg/mL to about 4.5 mg/mL. In some instances, about 50 mL of dispersion having one or more of graphite oxide, graphene oxide and graphene can be used. In other instances, about 10 mL to about 100 mL, alternatively about 20 mL to about 90 mL, alternatively about 25 mL to about 80 mL, alternatively about 30 mL to about 70 mL, alternatively about 40 mL to about 60 mL, and alternatively about 45 mL to about 55 mL of dispersion having one or more of graphite oxide, graphene oxide and graphene can be used. [0030] In block 140, the composite solution is added dropwise to an aqueous CaCb solution. In some instances the CaCb solution can have a concentration of about 0.06 g/mL. in other instances, CaCb. solution can have a concentration of about 0.01 g/mL to about 0.25 g/mL, alternatively about 0.02 g/mL to about 0.2 g/mL, about 0.03 g/mL to about 0.15 g/mL, about 0.04 g mL to about 0.1 g/mL, and about 0.05 g/mL to about 0.07 g/mL. In some instances, the amount of water in the CaCb. solution is about 250 mL. In other instances, the amount of water in the CaCb. solution is about 50 mL to about 1 L, alternatively about 100 mL to about 800 mL, alternatively about 150 mL to about 600 mL, alternatively about 200 mL to about 400 mL, and alternatively about 200 mL to about 300 mL. In block 250 the solution is then added dropwise to a KCl-NaCl electrolyte solution. The CaCb converts the soluble sodium alginate into insoluble calcium alginate, forming hydrogei spheres. The graphite oxide, grapheme oxide and/or graphene becomes entrapped and dispersed within the hydrogei spheres to form graphite oxide, graphene oxide and/or grapheme- containing hydrogei spheres. The KCl-NaCl electrolyte solution serves to strengthen the spheres. After a period of time, such as, for example, about two hours, the process continues to block 250.

[0031] in block 250, the hydrogei spheres are removed from the solution. After removal, the hydrogei spheres can be subjected to one or more washing steps, to remove impurities, by-products or unwanted ions.

[0032] In block 260, the hydrogei spheres are dried to yield the final product. Drying can be accomplished using, for example, a 250- Watt lamp.

[0033] FIG. 3 is a flowchart of a method for incorporating of graphite oxide, graphene oxide and/or graphene-containing hydrogei spheres into a crop field. While the method 300 in FIG. 3 illustrates various processes, steps, or procedures, one of ordinary skill in the arc can appreciate that additional processes, steps, or procedures can be added, or certain processes, steps, or procedures can be removed, without imparting from the scope of the method 300. The method 300 can start at block 310.

[0034] In block 310, a field is prepared for the application of graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres.

[0035] If the field does not currently have crops or plants growi g thereon, the field can be prepared by removing a layer of soil therefrom. In some instances, the field ca be prepared by plowing or tilling the field. In some instances, such as when the field currently has crops or plants grow thereon, the field can be prepared by aeration.

[0036] In block 320, the graphite oxide, graphene oxide and/or graphene-co taining hydrogel spheres are applied to the prepared field. In some instances, a layer of soil can then be placed on top of the spheres to cover the spheres.

[0037] In block 330, crops or plants are planted in the soil containing the spheres or in soil located above the spheres.

[0038] in block 340, the field is watered. Watering of the field will initially serve to water the freshly planted crop or plants, to hydrate the soil, and to hydrate the graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres.

[0039] in block 350, as the crop or plants require more water, water in the graphite oxide, graphene oxide and/or graphene-containing hydrogel spheres will desorb therefrom, feeding the plants and dehydrating the spheres. After block 350 is completed, the process can cycle back to 340. Blocks 340 and 350 can therefore be repeated in cyclic fashion.

Production of calcium alginate hydrogel spheres [0040] Hydrogels were produced by first preparing a sodium alginate solution. The solution was formed by dissolving 10 grams of sodium alginate in 300 ml of water at 40°C. The solution was then allowed to stand for 4 hours to ensure bubble formation was avoided. An aqueous calcium chloride solution (15 grams of CaCb in 250 ml of water) was then prepared and the sodium alginate solution was added dropwise to the calcium chloride solution at 40°C. The droplets remained in the solution of calcium chloride for five minutes to allow for the formation of insoluble calcium alginate hydrogel spheres.

[0041] FIG. 4 shows a calcium alginate hydrogel sphere (right), formed according to the above procedure, which is off-white in appearance.

Production of graphene oxide-containing calcium alginate hydrogel spheres

[0042] Hydrogels were produced by first preparing a sodium alginate solution. The solution was formed by dissolving 10 grams of sodium alginate in 300 ml of water at 40°C. The solution was then allowed to stand for 4 hours to ensure bubble formation was avoided. 50 ml of a graphene oxide dispersion (3 mg of graphene oxide / ml of water) was then added to the sodium alginate at 40°C and stirred until homogeneous to form a graphene oxide-sodium alginate mixture. An aqueous calcium chloride solution (15 grams of CaCk in 250 ml of water) was then prepared and the graphene oxide-sodium alginate mixture was added dropwise to the calcium chloride solution at 40°C. The droplets remained in the solution of calcium chloride for five minutes to allow for the formation of insoluble calcium alginate hydrogel spheres. The droplets remained in the solution of calcium chloride for five minutes to allow for the formation of insoluble graphene oxide- containing calcium alginate hydrogel spheres.

[0043] FIG. 4 shows a graphene oxide-containing calcium alginate hydrogel sphere (left), formed according to the above procedure, which is black in appearance. Hydration testing of hydrogel spheres under ambient conditions

[0044] In a first experiment, a portion of the formed calcium alginate hydrogel spheres and a portion of the formed graphene oxide-containing calcium alginate hydrogel spheres were allowed to dry for one night at in air at room temperature.

[0045] In second experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel sphere were placed in water at room temperature for one night to hydrate. No significant hydration was observed in either type of hydrogel sphere.

[0046] FIG. 5 shows images of dry graphene oxide-containing calcium algmate hydrogel spheres and calcium alginate hydrogel spheres (left) and graphene oxide-containing calcium alginate hydrogel spheres and calcium alginate hydrogel spheres subjected to hydration for one night under ambient conditions (right).

Hydration testing of hydrogel spheres at elevated temperatures

[0047] In a first experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel sphere were placed in an aqueous KC1 solution at 70°C for about 1.5 hours to hydrate. In a second experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel sphere were placed in an aqueous KC1 solution at 93 °C for about 1.5 hours to hydrate.

[0048] At 70°C, both types of spheres exhibited considerable hydration. As indicated in FIG. 6, however, the graphene oxide-containing calcium alginate hydrogel spheres, however were visually observed to hydrate more readily due to enhanced levitation in the aqueous KC1 solution. At 93°C the spheres were destroyed. Without being bound to any particular theory, it is believed that temperatures around 93 °C result in water vaporization within the hydrogel spheres which compromises the structure of the spheres.

Hydration testing of hydrogel spheres at varied temperature and hydration solution pH

[0049] In the present set of experiments, hydration tests were carried out varying the values of pH and temperature, simulating the conditions found in an agricultural soil and/or crop.

[0050] In a first experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel spheres were placed in an aqueous HC1 solution (pH = 5.5) at 35°C for about 1.5 hours to hydrate. The same was performed in an aqueous HC1 solution (pH = 6) and an aqueous solution (pH = 7). In all three instances (that is, pH = 5.5, 6, and 7), the hydration capacity of each type of hydrogel sphere was negligible.

[0051] In a second experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel spheres were placed in an aqueous HC1 solution (pH = 5.5) at 45 °C for about 1.5 hours to hydrate. The same was performed in an aqueous HC1 solution (pH = 6) and an aqueous solution (pH = 7). In all three instances (that is, pH = 5.5, 6, and 7), the hydration capacity of each type of hydrogel sphere was negligible.

[0052] In a third experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel spheres were placed in an aqueous HC1 solution (pH = 5.5) at 60-70°C for about 1.5 hours to hydrate. The same was performed in an aqueous HC1 solution (pH = 6) and an aqueous solution (pH = 7). Each test was performed in the presence of NaCl (0.21-0.31 M) to also observe the effect of salt concentration. The hydration capacity of the hydrogel spheres under each set of experimental variables is shown in Table 1 , below.

[0053] In a fourth experiment, a portion of dried (that is, dehydrated) calcium alginate hydrogel spheres and a portion of dried (that is, dehydrated) graphene oxide-containing calcium alginate hydrogel spheres were placed in an aqueous HC1 solution (pH = 5.5) at 60-70°C for about 1.5 hours to hydrate. The same was performed in an aqueous HC1 solution (pH = 6) and an aqueous solution (pH = 7). Each test was performed in the presence of NaCl (0.17-0.31 M) to also observe the effect of salt concentration. The hydration capacity of the hydrogel spheres under each set of experimental variables is shown in Table 1 , below. H Hydrogel Temperature M NaCl % Weight abs

5.5 Calcium Alginate 60- "70°C 0.24 1636

5.5 Alginate + GO 60- "70°C 0.24 1330

6.0 Calcium Alginate 60- "70°C 0.21 2354

6.0 Alginate + GO 60- "70°C 0.21 1982

7.0 Calcium Alginate 60- "70°C 0.31 2345

7.0 Alginate + GO 60- "70°C 0.31 2326

5.5 Calcium Alginate 70- "80°C 0.23 1413

5.5 Alginate + GO 70- "80°C 0.23 2017

6.0 Calcium Alginate 70- "80°C 0.17 1829

6.0 Alginate + GO 70- "80°C 0.17 2275

7.0 Calcium Alginate 70- "80°C 0.31 2468 7.0 Alginate + GO 70 ~ 80 °C 0.31 3294

Table 1.

[0054] As shown in Table 1, the hydrogel spheres absorb between 13 and 30 times its weight in water. At temperatures of 60-70°C, a significant difference between the calcium alginate hydrogel spheres and the graphene oxide-containing calcium alginate hydrogel spheres was observed. At temperatures of 70-80°C, significant differences were observed. Specifically, the graphene oxide- containing calcium alginate hydrogel spheres were found to absorb between 25 and 30 percent water by weight more than the calcium alginate hydrogel spheres.

[0055] The above data indicates that generating hydrogel spheres doped with graphene oxide have the ability to absorb water up to thirty times their weight in water which would have significant utility in the controlled irrigation by allowing for retention of water from, and time- elapsed release of water into, cropland.

[0056] In some instances, hydrogels can be used according to various aspects of the present disclosure that do not use a salt in a hydration/dehydration process. In some instances, hydrogels can be used according to various aspects of the present disclosure that hydrate/dehydrate at lower temperature such as, for example 35-45°C.

Procedures of hydration optimization of graphite oxide, graphene oxide and/or graphene- containing hydrogel spheres

[0057] Experimentation on hydrogels can be conducted with different methodologies and reagents. The effect the variables in the performance of hydrogels can then be analyzed. Control factors can be, for example, graphite oxide, graphene oxide and/or graphene (GO) concentration (for example 0, 0.25, 0.5, 1, and 2 wt%) in the hydrogels, sonication time, hydration/dehydration temperature and drying time. The properties measured can be, for example, mechanical strength, rate of adsorption and rate of desorption.

[0058] Hydration time and drying temperature tested can vary from 2 to 10 hours at 20 to 80°C in each case. Ultrasonication time can vary from 30 minutes to 6 hours. Without being bound to any particular theory, it is believed that ultrasonication is of great importance, since in this step is where the homogenization of base materials and the formation of the polymer (hydrogel) is accomplished.

[0059] Mechanical tests can be performed to measure the resistance of the hydrogel spheres (Young's modulus). Absorption and desorption tests of the hydrogel spheres can also be made. For this, the weight of the hydrogel spheres can be monitored at different times during each process (hydration and dehydration) in order to estimate the amount of water absorbed and released at different times to optimize the time required for each process.

[0060] In the case of desorption rate, the hydrogel spheres can be exposed to controlled temperature profiles which simulate the conditions to which they will exposed to in temperate, arid, or other farmlands. Statistical significance of each level of each variable considered can be estimated by an analysis of variance based on the design of experiments shown in FIG. 7.