Sunlight-driving clean water production using thermal-responsive hydrogel

Solar-steam generation originated from saline water, industrial and sanitary sewage is thought to be an efficient way to solve water scarcity. Recently, efforts have been denoted to low-cost floating structures that use solar energy to generate steam for sustainable clean water production. However, harvesting the steam can be extremely energy intensive and impractical, which is exacerbated by the relatively low evaporation velocity under natural solar irradiation. More seriously, the velocity of solar steam generation by photothermal materials has approached its limit.
Sunlight-driving clean water production using thermal-responsive hydrogel

Natural plants have already achieved a high efficiency water treatment rely on differences in osmotic pressure, transpiration and guttation to produce tons of clean water powered by sunlight. The cooperation of transpiration and guttation is expected to break through the energy efficiency limit of the current photothermal materials. Inspired by this, we employ sunlight as the energy input to realize high-efficiency water treatment by synchronous evaporation and forward osmosis process. We utilize a 3D network of thermo-responsive polymer hydrogel coated with polymer modified graphene membrane as the draw media for suction of seawater or pollutant water (Figure 1). Purified water in the thermo-responsive hydrogels is reversibly oozed and evaporated by sunlight irradiation and subsequently temperature ascent. Briefly, water collection (through guttation) involves only two steps, i.e. liquid absorption, and regeneration of clean liquid water, concurrently, the regeneration of the absorbents. Simultaneously, solar heating steam could also be collected. Those characteristics significantly lower energy consumptions and promote water collection efficiency. The procedure turns out to be a repeatable procedure with the help of a waterwheel like device for continuous water production. Obviously, this repeated forward osmotic desalination procedure avoids the clog of water transport channel by prohibiting salt precipitation on the surface of photothermal materials. A single engineered hydrogel composite offers a collection of 4.2 kg m−2 h−1 and an ionic rejection of >99% from brine feed via the cooperation of transpiration and guttation from brine water under one sun irradiation.

Figure 1: A sunlight-driving water purifier has been developed for ultrahigh rate production of clean water whereby integration of transpiration and guttation based on thermal-responsive poly(N-isopropylacrylamide (PNIPAm) hydrogel and solar-heating PNIPAm modified graphene filter.

The mechanism for transpiration and guttation is attributed to the photothermal effect and a drastic phase change arises from the hydrophilicity/hydrophobicity switch of the composite hydrogel, respectively. Under sunlight irradiation, the surface of the water purifier increases to an equilibrium temperature of 55oC within 500 s to generate steam. That initiates the volume phase transition of the hydrogel, which drives liquid water to be oozed. Demonstrated by experimental and theoretical results, an introduction of a superhydrophilic melamine skeleton greatly accelerate the water suction procedure, which in turn, promotes the water production performance.
Following the successful laboratory demonstrations, the experiments of the Bohai Sea (average salinity 3.0%) water purification are conducted under natural sunlight from 7:00 to 19:00. After desalination, the salinities of all the brine samples are significantly decreased to below 0.7 wt% in guttation water, and 0.002 wt% in transpiration water. A daily yield of 21.1 kg m−2 is recorded under natural sunlight irradiation.
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