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Greener Methods for Cleaner Water

Water, water everywhere…but is it clean enough to drink? For more than one-third of the world’s population, the answer is no.1 Access to clean drinking water is currently one of the most challenging global issues, exacerbated by climate change, increasing water scarcity, population growth, demographic changes, and continued urbanization. But scientists are now harnessing the power of the sun to effectively and sustainably turn salty ocean water into a clean, drinkable resource. 

Advances in Solar-Powered Desalination Technology

A Paper-Based Answer to Salt Accumulation

While the majority of the earth’s surface is covered by water, more than 97% is found in the oceans and cannot be consumed due to its high salinity. But chemists have been working to address the global water crisis by developing more efficient and environmentally sustainable seawater desalination techniques.

Solar-powered desalination is steadily becoming a leading force in battling global water scarcity, and there is a strong drive to advance solar desalination methods for more widespread applications in sustainable clean water production. Most traditional solar evaporation systems operate using thermal conduction, but the biggest challenge for these evaporators is excessive salt accumulation on the absorption layer, which hinders evaporation efficiency and makes the devices difficult to clean and maintain.

However, research recently published in ACS Applied Materials & Interfaces demonstrates a novel solution in the form of a paper-based thermal radiation-enabled evaporation system (TREES).2 This system uses a contactless configuration consisting of a vertical evaporation wall made of filter paper, which surrounds a thermally insulated bottom solar absorber constructed from surface-inked wood and polystyrene foam.

The evaporation wall can efficiently capture thermal energy from the solar absorber while also gaining energy from the warmer environment, enhancing the evaporation process. The wall is also unique in its ability to efficiently collect the salt on its exterior and, through energy down-conversion, enable water to serve as its own absorber and create a dynamic evaporation front from the accumulated salt layer. Furthermore, since the TREES system is contactless, the salt layer does not accumulate on the bottom absorber surface.

After testing the TREES system outdoors for eight consecutive days, the researchers reported that it can enhance evaporation by more than 1000% compared to traditional systems. By overcoming the salt accumulation challenge and improving the evaporation process, TREES exhibits tremendous potential as a driver of next-generation desalination technology. Watch a video of TREES in action.

Doing Double Duty with Hydrogel

Another desalination approach published in ACS ES&T Water uses a hydrogel platform to produce fresh water from both the ocean and the atmosphere.3 Salt accumulation presents a challenge here as well—hydrogel-based solar steam generators currently used for seawater desalination are easily clogged and dirtied by excess salt deposits.

Despite their obstructive nature, researchers observed that these salts could be quite useful for absorbing water from the atmosphere, and they worked to develop a versatile solar-thermal hydrogel (TA-Fe@PAM) that could integrate oceanic desalination and atmospheric water collection within a singular device.

The team constructed the TA-Fe@PAM hydrogel by embedding a tannin-iron (TA-Fe) photothermal complex into a polyacrylamide (PAM) hydrogel system. The hydrogel’s porous nature allowed for efficient photothermal conversion and water transport while effectively trapping large amounts of deliquescent salts during rapid solar desalination. The hydrogel containing the incorporated salts (DS-TA-Fe@PAM) was then dried and tested for atmospheric water collection performance. The DS-TA-Fe@PAM hydrogel was able to successfully capture atmospheric water vapor and then release almost all of the water it had collected.

Finally, the team tested DS-TA-Fe@PAM within a device made from cheap, easy-to-assemble household materials, and it again demonstrated efficient water harvesting and release. This is especially promising for use in developing countries and low-resource settings where it is difficult to regularly access clean water.

Taken together, these new findings provide novel insights into the design of next-generation salt-harvesting solar evaporators and take a step further to advance their applications in sustainable desalination.

Explore Related Research on Desalination from ACS Journals

  1. Zhang, C. et al. Dual-Layer Multichannel Hydrogel Evaporator with High Salt Resistance and a Hemispherical Structure toward Water Desalination and Purification. ACS Appl. Mater. Interfaces 2022, 14, 22, 26303–26313
  2. Aleid, S. et al. Salting-in Effect of Zwitterionic Polymer Hydrogel Facilitates Atmospheric Water Harvesting. ACS Materials Lett. 2022, 4, 3, 511–520
  3. Pan, Y. et al. Simple Design of a Porous Solar Evaporator for Salt-Free Desalination and Rapid Evaporation. Sci. Technol. 2022, 56, 16, 11818–11826
  4. Chu, A. et al. Sustainable Self-Cleaning Evaporators for Highly Efficient Solar Desalination Using a Highly Elastic Sponge-like Hydrogel. ACS Appl. Mater. Interfaces 2022, 14, 31, 36116–36131
  5. Wilson, H. et al. Highly Efficient and Salt-Rejecting Poly(vinyl alcohol) Hydrogels with Excellent Mechanical Strength for Solar Desalination. ACS Appl. Mater. Interfaces 2022, 14, 42, 47800–47809

References

  1. Patel, P. Improving the efficiency of solar desalination. C&EN Global Enterprise 2019, 97, 26, 8-8
  2. Bian, Y. et al. Enhanced Contactless Salt-Collecting Solar Desalination. ACS Appl. Mater. Interfaces 2022, 14, 29, 34151–34158
  3. Li, X. et al. Multipurpose Solar-Thermal Hydrogel Platform for Desalination of Seawater and Subsequent Collection of Atmospheric Water. ACS EST Water 2022, Article ASAP

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