•Hydrogels for atmospheric water harvesting, seawater desalination, and wastewater treatment are reviewed.•Water absorption and desorption mechanism of hydrogels are discussed.•Strategies to improve ...hydrogel performance are presented.•Challenges and outlooks of hydrogel for water purification are given.
Over the past few years, with the development of global industry and the increase of population, the shortage of freshwater resources has prompted people to conduct in-depth research on advanced water treatment and water harvesting technologies. As a polymer material with a three-dimensional network structure, hydrogel has been widely used in water resources harvesting and treatment due to its excellent water absorption, water retention, and adsorption properties. In this review, we first introduced several synthetic routes of hydrogel materials, including chemical cross-linking, physical cross-linking and dual cross-linking, and discussed the advantages and disadvantages of several synthetic methods. Then, we emphasized the water absorption and desorption mechanism of hydrogels and related influencing factors. Next, we described the latest research progress of hydrogel materials in detail as a platform for atmospheric water harvesting, seawater desalination and wastewater treatment in recent years. Finally, we discussed the shortcomings and future orientation of water purification and water treatment systems based on hydrogel materials.
Atmospheric water harvesting (AWH) has received tremendous interest because of population growth, limited freshwater resources, and water pollution. However, key challenges remain in developing ...efficient, flexible, and lightweight AWH materials with scalability. Here, we demonstrated a radiative cooling fabric for AWH via its hierarchically structured cellulose network and hybrid sorption-dewing mechanisms. With 8.3% solar absorption and ∼0.9 infrared (IR) emissivity, the material can drop up to 7.5 °C below ambient temperature without energy consumption via radiative cooling. Water adsorption onto the hydrophilic functional groups of cellulose is dominated by sorption at low relative humidity (RH) and dewing at high RH. The cellulose network provides desirable mechanical properties with entangled high-aspect-ratio fibers over tens of adsorption-extraction cycles. In the field test, the cellulose sample exhibited water uptake of 1.29 kg/kg at 80% RH during the night. The profusion of radiative cooling fabric features desirable cost effectiveness and allows fast deployment into large-scale AWH applications.
Freshwater scarcity is a global challenge posing threats to the lives and daily activities of humankind such that two-thirds of the global population currently experience water shortages. Atmospheric ...water, irrespective of geographical location, is considered as an alternative water source. Sorption-based atmospheric water harvesting (SAWH) has recently emerged as an efficient strategy for decentralized water production. SAWH thus opens up a self-sustaining source of freshwater that can potentially support the global population for various applications. In this review, the state-of-the-art of SAWH, considering its operation principle, thermodynamic analysis, energy assessment, materials, components, different designs, productivity improvement, scale-up, and application for drinking water, is first extensively explored. Thereafter, the practical integration and potential application of SAWH, beyond drinking water, for wide range of utilities in agriculture, fuel/electricity production, thermal management in building services, electronic devices, and textile are comprehensively discussed. The various strategies to reduce human reliance on natural water resources by integrating SAWH into existing technologies, particularly in underdeveloped countries, in order to satisfy the interconnected needs for food, energy, and water are also examined. This study further highlights the urgent need and future research directions to intensify the design and development of hybrid-SAWH systems for sustainability and diverse applications.
Biopolymer Hygroscopic Materials
The effective management of atmospheric water will create huge value. Biopolymers synthesized by organisms have become a choice for developing novel hygroscopic ...materials. In article number 2209479, Swee Ching Tan, Wenshuai Chen, and co‐workers review the use of biopolymers for hygroscopic material development, and discuss the applications of biopolymer‐derived hygroscopic materials for dehumidification, atmospheric water harvesting, and power generation.
•The prepared aerogels can simultaneously realize highly efficient solar desalination and atmospheric water harvesting.•The aerogel achieves an evaporation rate of up to 2.4 kg m-2h-1 in 5 wt% salt ...solution.•The aerogel achieves an atmospheric water harvesting capacity of up to 3.29 g g-1 at 90 % relative humidity.•The aerogel has excellent salt resistance and the ability to inhibit leakage of adsorbed water.•The compressive strength of the aerogel is up to 110.9 kPa.
Solar desalination and atmospheric water harvesting are economically friendly techniques for obtaining freshwater. However, developing aerogels capable of simultaneously achieving long-term stable operation of efficient solar desalination and atmospheric water harvesting remains challenging. The aramid fiber (AF)-reinforced vertically aligned sheet-like structural aerogel incorporated with expanded graphite was prepared through a sequential process involving freeze-drying, carbonization, and tannin surface modification. The aerogel can serve directly as a seawater desalination evaporator and function as an atmospheric water harvester through LiCl loading. The vertical sheet-like structure of the aerogel enables the evaporator to exhibit a high evaporation rate, reaching 2.4 kg m-2h-1 under 1 kW m-2 simulated solar radiation. The high specific surface area allows the aerogel to load a large amount of LiCl, resulting in a water absorption capacity as high as 3.29 g g-1 (90 % RH), and it can release 80 % of the adsorbed water within 120 min under only 0.5 kW m-2 simulated solar radiation. The laminar void structure of EG can restrict the movement of LiCl, preventing its leakage and ensuring stable water harvesting performance. The water evaporator and harvester both demonstrated excellent stability during cycling tests. Additionally, due to the reinforcing effect of AF, the compression strength of the aerogel reaches as high as 110.9 kPa. The prepared aerogel has promising prospects for freshwater acquisition.
Harvesting water from air in sorption‐based devices is a promising solution to decentralized water production, aiming for providing potable water anywhere, anytime. This technology involves a series ...of coupled processes occurring at distinct length scales, ranging from nanometer to meter and even larger, including water sorption/desorption at the nanoscale, condensation at the mesoscale, device development at the macroscale and water scarcity assessment at the global scale. Comprehensive understanding and bespoke designs at every scale are thus needed to improve the water‐harvesting performance. For this purpose, a brief introduction of the global water crisis and its key characteristics is provided to clarify the impact potential and design criteria of water harvesters. Next the latest molecular‐level optimizations of sorbents for efficient moisture capture and release are discussed. Then, novel microstructuring of surfaces to enhance dropwise condensation, which is favorable for atmospheric water generation, is shown. After that, system‐level optimizations of sorbent‐assisted water harvesters to achieve high‐yield, energy‐efficient, and low‐cost water harvesting are highlighted. Finally, future directions toward practical sorption‐based atmospheric water harvesting are outlined.
The potential applications for atmospheric water harvesting are tantalizing. This review provides an up‐to‐date summary of the most recent developments and challenges for developing practical sorption‐based atmospheric‐water harvesters. The sorbent design at the nanoscale, condensation enhancement at the mesoscale, device development at the macroscale, and water scarcity assessments at the global scale are discussed in detail.
Atmospheric water harvesting (AWH) is regarded as one of the promising strategies for freshwater production desirable to provide sustainable water for landlocked and arid regions. Hygroscopic ...materials have attracted widespread attention because of their water harvesting performance. However, the introduction of many inorganic salts often leads to aggregation and leakage issues in practical use. Here, polyzwitterionic hydrogels are developed as an effective AWH material platform. Via anti‐polyelectrolyte effects, the hygroscopic salt coordinated with polymer chains could capture moisture and enhance the swelling property, leading to a strong moisture sorption capacity. The hydrogel shows superior AWH performance (0.62 g g−1, 120 minutes for equilibrium at 30 % relative humidity) and produces 5.87 L kg−1 freshwater per day. It is anticipated that the polyzwitterionic hydrogels with unique salt‐responsive properties could provide new insights into the design and synthesis of next‐generation AWH materials.
A novel polyzwitterionic hydrogel with efficient atmospheric water harvesting is developed based on anti‐polyelectrolyte effects. The as‐prepared hydrogel could achieve 0.62 g g−1 water vapor sorption within 120 minutes at 30 % relative humidity and produce 5.87 L kg−1 freshwater per day. This tactful designed polyzwitterionic hydrogel provides new insight on how to fabricate efficient atmospheric water harvesting materials.
Sorption‐based atmospheric water harvesting (SAWH) is a promising technology to alleviate freshwater scarcity. Recently, hygroscopic salt‐hydrogel composites (HSHCs) have emerged as attractive ...candidates with their high water uptake, versatile designability, and scale‐up fabrication. However, achieving high‐performance SAWH applications for HSHCs has been challenging because of their sluggish kinetics, attributed to their limited mass transport properties. Herein, a universal network engineering of hydrogels using a cryogelation method is presented, significantly improving the SAWH kinetics of HSHCs. As a result of the entangled mesh confinements formed during cryogelation, a stable macroporous topology is attained and maintained within the obtained entangled‐mesh hydrogels (EMHs), leading to significantly enhanced mass transport properties compared to conventional dense hydrogels (CDHs). With it, corresponding hygroscopic EMHs (HEMHs) simultaneously exhibit faster moisture sorption and solar‐driven water desorption. Consequently, a rapid‐cycling HEMHs‐based harvester delivers a practical freshwater production of 2.85 Lwater kgsorbents−1 day−1 via continuous eight sorption/desorption cycles, outperforming other state‐of‐the‐art hydrogel‐based sorbents. Significantly, the generalizability of this strategy is validated by extending it to other hydrogels used in HSHCs. Overall, this work offers a new approach to efficiently address long‐standing challenges of sluggish kinetics in current HSHCs, promoting them toward the next‐generation SAWH applications.
A universal network engineering is proposed to develop the entangled‐mesh hydrogels (EMHs) with a distinct aerogel‐like macroporous topology, exhibiting a significantly improved mass transport property compared with the conventional dense hydrogels by regular polymerization. As such, the hygroscopic EMHs deliver remarkably faster moisture sorption and solar‐driven water desorption kinetics, achieving high‐performance sorption‐based atmospheric water harvesting.
•A device that harvests clean drinking water from air was designed built and tested.•Harvesting water naturally from air using adsorption materials.•The prototype produced water more than 2 times ...compared to reference sorbents.•Effects of relative humidity, solar irradiation and operating parameters are investigated.
Drinking water resources have always been limited in the gulf region of the Middle East and other desert regions around the world. In attempt to provide viable supplement, a device that harvests clean drinking water from air is designed, built and tested. The operation of the device is based on harvesting water naturally from air using adsorption materials. The prototype of this device consists of sorbent (silica gel is used in this study) exposed to radiant flux, water sorbent unit, condenser and reflector. Experimental studies of production of fresh water from air in controlled indoor environment have been carried out using the prototype. Several experimental tests were conducted under the conditions of 22 °C ambient temperature, a range of relative humidity (RH) from 30 to 60%, a range of silica gel thickness from 25 to 35 mm, surface area to volume ratio from 0.29 to 0.4 and radiant heat flux range from 509 to 556 W/m2. The prototype was able to produce up to 159 g of water per 1 kg of silica gel in a 12 h cycle when exposed to 556 W/m2 radiant flux. In terms of per one day (24 h), the harvester can produce 800 mL of water with an overall efficiency of 50% for 25 mm silica layer thickness. Increasing the relative humidity speeds up the adsorption cycle and increases the water capture, release and collection rates. The system can be improved by adding multiple layers of sorbent stacked on top of each other and by using sorbents with improved adsorption and desorption properties.
Even if people live in an arid desert, they know that plenty of water exists in the air they breathe. However, the reality tells us the atmospheric water cannot help to slake the world's thirst. Thus ...an important question occurs: what are the fundamental limits of atmospheric water harvesting that can be achieved in typical arid and semi-arid areas? Here, through a thorough review on the present advances of atmospheric water-harvesting technologies, we identify the achievements that have been acquired and evaluate the challenges and barriers that retard their applications. Lastly, we clarify our perspectives on how to search for a simple, scalable, yet cost-effective way to produce atmospheric water for the community and forecast the application of atmospheric water harvesting in evaporative cooling, such as electronic cooling, power plant cooling, and passive building cooling.
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Airborne moisture is a potential source of a plentiful amount of freshwater that is accessible everywhere and can be easily co-operated with a renewable energy source (solar energy). This paper presents a comprehensive and critical review of state-of-the-art research on atmospheric water harvesting. From the viewpoint of applications, we are concerned most about whether an atmospheric water harvester can produce sufficient freshwater under a wide range of weather conditions in an energy-efficient way. Therefore, a variety of harvesting methods, including radiative cooling, solar distilling, and sorption-based water collecting, are reviewed and discussed based on their capture materials, system designs, and thermodynamic cycles. The study also presents a systematic performance comparison of recently proposed atmospheric water harvesters. Furthermore, we discuss four key problems that limit the cost-effectiveness and provide some solutions as perspectives.
Atmospheric water-harvesting technology has experienced significant progress in the past 20 years. However, little research on atmospheric water harvesters is conducted with broad horizons, and system integrations have been poorly examined. More research is expected to deal with these issues to facilitate the efforts of turning decades of research on atmospheric water harvesting into tangible benefits in our daily life.
Even if people live in an arid desert, they know that plenty of water exists in the air they breathe. However, the reality tells us the atmospheric water cannot help to slake the world's thirst. Thus an important question occurs: what are the fundamental limits of atmospheric water harvesting that can be achieved in typical arid and semi-arid areas? Here, through a thorough review on the present advances of atmospheric water-harvesting technologies, we identify the achievements that have been acquired and evaluate the challenges and barriers that retard their applications. Lastly, we clarify our perspectives on how to search a simple, scalable, yet cost-effective way to produce atmospheric water for the community and forecast the application of atmospheric water harvesting in evaporative cooling, such as electronic cooling, power plant cooling, and passive building cooling.
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