TITLE OF THE INVENTION

MATERIAL AND METHOD FOR SUSTAINABLE AND AFFORDABLE ATMOSPHERIC WATER HARVESTING

FIELD OF THE INVENTION

The present invention relates to water harvesting from atmosphere, more particularly relates to a sustainable water harvesting system for harvesting humidity from surrounding air using a composite material.

BACKGROUND OF THE INVENTION

Providing safe drinking water for all at an affordable cost has been a global challenge. Water resources are declining day by day. A robust water management system is needed to alleviate water scarcity. Methods such as wastewater purification and seawater desalination for providing clean water are limited due to high energy consumption, and high water transportation costs for landlocked areas. Therefore, there is a need to focus on decentralizing clean water solutions [Lord J et al., Nature. 2021;598(7882):611-617].

There’s abundant freshwater present in the air we breathe. Nature holds nearly 1.42 x 10 ¹⁹ litres of freshwater in its atmosphere, or more than 1.8 billion litres of water per person. Therefore, atmospheric water harvesting has a huge potential to address the water scarcity faced by our planetfNagar A et al., ACS Nano. 2020;14(6):6420-6435]. Water in the atmosphere exists in the form of clouds in the sky, fog in the hilly areas, and water vapor (or humidity) close to the land. Humidity harvesting is possible primarily in two ways: condensation-based and sorptionbased. Condensation-based technologies require high energy input to cool a surface below the dew point of the surrounding air. In comparison, sorption-based harvesting is a viable cost and energy efficient alternative where a sorbent readily adsorbs moisture onto its porous surface and subsequently releases the vapors upon nominal heating. The released vapors can be trapped and converted to liquid water by placing the sorbent in a closed chamber such that vapor condenses to liquid on the chamber wallsfEjeian M et al., Joule. 2021;5(7): 1678-1703; LaPotin A et al., Acc Chem Res. 2019;52(6): 1588-1597]. Next generation sorbent materials, such as, zeolites, metal organic frameworks (MOFs) and their derivatives, polymeric gels and hygroscopic salt-based composites harvest substantial humidity in the range of 0.1-6 g/g[Yilmaz G et al., Sci Adv. 2020;6(42): l-9; Kim S et al., Chem Eng J. 2021;425:131601; Zhao F et al., Adv Mater. 2019;31(10): l-7].These sorbents have high adsorption capacity, high specific surface area, robustness, and chemical stability. However, slow sorption kinetics, higher regeneration temperature, complex and expensive fabrication process and consequently higher cost of the materials used, and possible toxicity of the released water, due to dissolved components, limit their prospects of scalability.

In the present work, we have used a biocompatible and inexpensive polymeric material, namely pectin, that can quickly adsorb and desorb moisture with minimum energy requirements as one of the components for humidity harvesting. Pectin is an anionic polysaccharide found in the cell walls of several plant species. Because of its excellent gelling character, pectin is widely used in pharmaceutical and food industries. Due to its biocompatibility and biodegradability properties, pectin is also employed in tissue engineering, biomimetic constructs, and skin-care products [Markov PA et al., J Biomed Mater Res - Part A. 2017;105(9):2572-2581].Pectin-based hydrogels have been shown to graft easily with vinyl monomers such as acrylamide and acrylic acidfJing G et al., Colloids Surfaces A Physicochem Eng Asp. 2013;416(l):86-94; Kowalski G et al., Polymers (Basel). 2019;l 1(1): 1-16].

OBJECT OF THE INVENTION

An object of the invention relates to a composite material and method for water harvesting from atmosphere.

Another object of the present invention relates to biocompatible and inexpensive polysaccharide material composite with a hydrophilic polymer can quickly adsorb and desorb moisture with minimum energy requirements as one of the components for humidity harvesting.

Yet another object of the present invention relates to a method of harvesting clean water from atmosphere using a device that comprises a highly porous, polysaccharide -based composite.

SUMMARY OF THE INVENTION The present invention relates to a material and method for water harvesting using a a highly porous, polysaccharide-based composite, where the polysaccharide can be easily and effectively extracted from agricultural by-products such as citrus peels, apple pomace, etc., making the overall extraction of water environmentally sustainable and affordable.

In one embodiment, the present invention relates to the preparation of composite material for water harvesting. The pectin-poly( acrylic acid) composite (pectin-PAA) was prepared by first mixing acrylic acid and deionized water. The pH of the solution was adjusted to 11 by dissolving NaOH pellets into the solution. Thereafter, pectin gel in DI water was added to the solution under constant stirring at room temperature. In order to perform free-radical polymerization, an inert atmosphere of N?_ gas was created in the reaction container. Further, a cross-linker N,N'- methylenebisacrylamide was added. The temperature of the container was then raised to 70 'C and potassium persulfate was added to initiate the free-radical polymerization. The temperature was gradually increased from 70' C to 90 'C over the next 2 h. The resulting composite was then washed with a meth anol: water mixture (80:20) and freeze dried. The obtained powdered material was highly porous. The composites showed maximum moisture uptake and fastest desorption.

In other embodiment, the present invention illustrates the method of water harvesting from atmosphere using the prepared composite material. The method comprises of a chamber transparent to sunlight, a green composite for atmospheric water harvesting, and the lid of the chamber affixed with the composite. The prepared composite powder is packed on the lid of a closed chamber transparent to sunlight. The lid is kept open at night to expose the material to the surrounding humidity. Upon overnight absorption, the lid was closed in the morning so that sunlight could enter the chamber and irradiate the composite. The irradiation causes the adsorbed moisture to desorb from the composite and subsequently saturate the closed chamber with water vapor, resulting in condensation of droplets on the inner walls of the chamber. The condensed droplets roll down and collect under the effect of gravity, thus providing clean water. The transparent chamber can be as small as a teapot to obtain a glass of water, or as big as to spread over a few-m area to fulfil water requirements of multiple people. Also, in sunlight-deprived regions, the absorbed moisture can be released in a closed space by irradiating the composite with photons, or by using a simple mechanical ‘piston’ to squeeze the water out of the material.

Other aspects of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learnt by the practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 An illustration showing working of the water harvesting device, wherein the material 1 is allowed to adsorb humidity at night (left panel), and desorb the humidity in a closed, transparent container 3 in the morning.

Figure 2 shows SEM images depicting the morphology of the pectin-PAA composite at different magnifications.

Figure 3 shows the kinetics of a. adsorption under 99% relative humidity, and b. Subsequent desorption characteristics of the saturated materials under One Sun irradiation.

Figure 4 shows FESEM images depicting surface morphologies of pectin and its composites, each sample at two magnifications. Data for 7 wt% is presented in Figure 2 as well.

Figure 5 Cyclic stability of the 7 wt% composite.

Figure 6 Images of dry composite, saturated composite, and desorption, a-dry composite; b- saturated composite, c& d-water oozing.

Referring to the drawings, the embodiments of the present invention are further described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated or simplified for illustrative purposes only. One of ordinary skill in the art may appreciate the many possible applications and variations of the present invention based on the following examples of possible embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

The present invention relates to a composite and device for harvesting humidity from surrounding air, composed of a green composite comprising a hygroscopic, biocompatible and biodegradable polysaccharide, like pectin, chitosan, cellulose or its derivatives, etc., grafted into a polymeric hydrogel matrix by means of a cross-linker. Embedding the composite in a device to absorb humidity from air, primarily during night and releasing the absorbed humidity in a closed chamber upon absorption of sunlight and condensing it as water on the walls of the device. The porous polymeric hydrogel matrix is comprised of highly polar functionalities like carboxyl, sulfonate, amide, etc., and therefore, is hydrophilic in nature. It comprises of at least one of the following polymers including poly( acrylic acid), PAA, poly(vinyl alcohol), PVA, hydroxyethyl cellulose, HEC, poly(acryl amide), PAM, poly(N-isopropylacrylamide), PNIPAM, or poly(ethylene oxide), PEO, polyethylene glycol (PEG).

The composite of the present invention is heated using a solar energy conversion device, like solar cells or solar concentrators, or a solar absorbing paint, to concentrate solar energy onto the composite for humidity desorption and it takes energy from waste or biomass for moisture desorption. The composite is packed into a porous metal frame. The frame is then combined to the lid of a closed chamber transparent to sunlight. The composite is packed into an opaque enclosure and irradiated with photons from an independent source other than sunlight to desorb moisture and collect water, thereby, eliminating the dependency on sunlight. The composite is retrofitted with a mechanical device like a fan, to force circulation across the composite for efficient moisture adsorption. The composite is retrofitted with a mechanical device like a piston, to squeeze the adsorbed water out of the composite. The composite is made more hygroscopic by the addition of salts like CaCE, LiCl, etc. The composite is made more light absorbing by inclusion of a photothermal agent comprising at least one of the following: titanium oxide; iron oxide; metal oxide; metal nanomaterial; a two-dimensional metal carbide; a two-dimensional metal nitride; a polymer; a carbon material; phosphorous.

A sustainable water harvesting method for harvesting humidity from surrounding air is proposed, comprises of a chamber transparent to sunlight, a green composite for atmospheric water harvesting, and the lid of the chamber affixed with the composite (Figure 1).A prototype is assembled such that the prepared composite powder is packed on the lid of a closed chamber transparent to sunlight. The lid is kept open at night to expose the material to the surrounding humidity. Upon overnight absorption, the lid was closed in the morning so that sunlight could enter the chamber and irradiate the composite. The irradiation causes the adsorbed moisture to desorb from the composite and subsequently saturate the closed chamber with water vapor, resulting in condensation of droplets on the inner walls of the chamber. The condensed droplets roll down and collect under the effect of gravity, thus providing clean water. The transparent chamber can be as small as a teapot to obtain a glass of water, or as big as to spread over a few- m area to fulfil water requirements of multiple people. Also, in sunlight-deprived regions, the absorbed moisture can be released in a closed space by irradiating the composite with photons, or by using a simple mechanical ‘piston’ to squeeze the water out of the material. The device can be used for multiple cycles for humidity absorption and release by repeating the process. The device can work in conjunction with another device to enhance the quantity of humidity being made available to the composite for collection.

Example 1

The pectin-poly (acrylic acid) composite (pectin-PAA) was prepared by first mixing acrylic acid and deionized water. The pH of the solution was adjusted to 11 by dissolving NaOH pellets in to the solution. Thereafter, pectin gel in DI water was added to the solution under constant stirring at room temperature. In order to perform free-radical polymerization, an inert atmosphere of N2 gas was created in the reaction container. Further, a cross-linker N,N'- methylenebisacrylamide was added. The temperature of the container was then raised to 70°C and potassium persulfate was added to initiate the free -radical polymerization. The temperature was gradually increased from 70°C to 90°C over the next 2 h. The resulting composite was then washed with a methanol: water mixture (80:20) and freeze dried. The obtained powdered material was highly porous, as evident from the FESEM images (Figure 2). Composites with different weight percentages of pectin were tested for water uptake for a period of 72 h. It was observed that the 2 wt% and 7 wt% composites showed maximum moisture uptake. Further, all composites saturated with water vapor were individually exposed to one sun irradiation to observe loss in moisture content as a function of time. The 7 wt% composite was found to undergo fastest desorption in comparison to others. All these data are summarized in Figure 3. The faster desorption kinetics can be attributed to the microporous morphology in case of 7 wt% composite (Figure 4). Other composites as well as pure pectin are comparatively less porous in nature. As higher porosity implies a higher surface area available for vapor adsorption and abundant pore channels to offer subsequent desorption pathways, therefore, 7 wt% has the highest water uptake and fastest desorption.

Example 2 The cyclic stability of the composite is observed by conducting adsorption-desorption cycles. The first cycle began with humidity uptake for 1 hour at 99% RH, followed by a one- hour exposure to solar irradiation (1 Sun) to release the adsorbed water. From second cycle and onwards, the uptake period was limited to 30 minutes, followed by a 45- minute desorption period. The cyclic performance of the material remained stable after seven continuous cycles. During all cycles (except the first), the material desorbed 100% of the adsorbed humidity due to the hygroscopic nature of the composite. This hygroscopicity prevented thorough release of water vapor during the desorption period. Figure 5 shows cyclic stability of the 7 wt% composite. Figure 6 shows the 7 wt% composite adsorbing humidity in ambient conditions of 25°C and 99% RH for 10 minutes. This was followed by covering the material with another petri dish and heating it to 50 °C for desorption. As a result, the material release vapors which condensed on the top petri dish as macro water drops which can be collected subsequently. However, extended exposure to solar irradiation led to the eventual complete moisture desorption.

It may be appreciated by those skilled in the art that the foregoing drawings, examples and experimental evidences are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.