Water is a basic necessity for all: humans, animals, and vegetation. Without clean drinking water, no economic growth or prosperity can be imagined for human-being. According to the UN(united nations)’s 6th SDG (Sustainable Development Goal), all the nations have to provide universal and equitable access to safe and affordable drinking water to their population by 2030. Although today almost 91% of the global population has access to clean water, 666 million people, particularly from sub-Sahara and South Asia, are lacking access to potable water and almost 1000 children are dying every day due to preventable water and sanitation-related diarrhoeal diseases. it is estimated that about 76 million people in India have no access to a safe water supply.
Solar water distillation without hydrogel(h-LAH)
Water can be purified by myriad techniques but one of the oldest and time- tested technique is to purify water using solar energy. Solar stills are used in remote areas such as deserts and dense forests where rain, piped, or well water is not available. In case of a natural disaster when the potable water supply is disrupted due to power outages Solar still can be handy to provide potable water to the affected population. Solar stills are also used in ships to desalinate seawater. Akin to rain formation, solar stills use solar energy to evaporate muddy or salty water and collect condensed vapor as safe drinking water. Existing solar distillation technologies for water purification and desalinating seawater are energy-intensive and need expensive infrastructure. (optical instruments to concentrate sunlight)
Mechanism of Solar Still
The traditional still consists of a black-bottomed vessel and clear glass on top. When solar rays fall on solar water still, the black bottom of still absorbs sunlight and vaporised the water. The evaporated water vapor condenses on the clear covering, trickles into a collector and leaves the contaminants behind. Operating at their theoretical best, such solar distill can not produce more than 1.6 L/h/m2.output which is not sufficient for the poor household.
How solar distillation with h-LAH hydrogel overcome the shortcoming of solar still
Although, water purification by solar distillation is a promising technology for home or the entire community, solar vapor generation (SVG), the essential process of solar distillation to separate water and contaminants is energy-intensive and low-water yield under natural sunlight. Adding hierarchically nanostructured highly hydratable light-absorbing(h-LAH) hydrogel can increase water evaporation and give higher output clean water at ambient sunlight.
Mechanism of Solar distillation with hydrogel
The floating highly hydratable light-absorbing hydrogel (h-LAH) is made up by inserting light absorbent hydrogel polypropylene(pp) into highly hydratable (water-absorbent) polymer networks consisting of cross-linked hydrophilic polymer polyvinyl alcohol (PVA) and chitosan. This 3D(dimension) porous, water-absorbent network can efficiently convert solar irradiation to heat, increase water vaporization and speed up solar water purification. PVA can play the role of surfactant. Used as an additive in the sand water filtration process, Chitosan a hydrophilic linear polysaccharide strongly attracts water and removes up to 99% of turbidity, heavy minerals, dyes, and oils from the water. The (h-LAH) hydrogel floating on top of the water is placed under direct sunlight. Upon exposure to solar irradiation, the PPy chains exhibit semiconducting property and generate water vapor using solar energy. It speeds up the water vapor generation process. The water vapor generated from the hydrogel’s surface is captured by a condenser.
A (h-HAL )hydrogel creates three water phase in the solar still.
Due to the hydration effect, the polymer chains( PVA and chitosan) in hydrogels capture nearby water molecules through hydrogen bonding to form bound water.
IW- Intermediate water (swell water)
Exist between bound water and free water molecules, intermediate water(IW) molecules are loosely bounded with bound water molecules and (FW) free water molecules. AS IW molecules share fewer bonds with their neighbors, they evaporate more readily with reduced energy demand than (FW) free water at the bottom of still. Once this intermediate water molecule evaporates, it immediately replaced by other water molecules from FW in the still. As the volume of IW(swell water) in the solar still increase overall energy demand for water evaporation reduced which increases the water evaporation rate.
FW (Free water)
FW (Free water) in the solar still is not bound by hydrogel and have stronger hydrogen bonding compare to IW. The presence of highly hydratable functional groups, such as hydroxyl (OH)and amino(NH2) groups in hydrogel exacerbate hydrability (strong interaction with water molecules) of the polymer network, increase the proportion of IW and lower the energy demand of vapor generation hence facilitating water evaporation. The molecular level meshes(cross-linking density of polymer) in the hydratable polymer network of the hydrogel regulates water state, influences the water diffusion, and serve as water pathways when the water diffuses to the evaporating surface. A solar still of one square meter in size could purify any polluted or salty water and produce distill water at the rate of 3.6 L/h/m2 at the energy efficiency of ~92%. At such a rate about 25-30 liters of clean drinking water is produced per day which can easily satisfy small household needs in a remote location and in disaster-affected areas. All three polymers in the hydrogel are both commercially available and cheap, this can offer a cost-effective solution for the poor family located in remote areas. Due to long-term durability and antifouling functionality toward complex ionic contaminants, the h-HAL-based solar still can produce clean, safe drinking water from the oceans or contaminated supplies. As hydrogels can easily be retrofitted in most existing solar desalination systems, it obviates the need for a complete overhaul of existing solar desalination systems.
Due to recent technological breakthroughs in solar technology and the latest hydrogel technology, it is now possible to provide potable drinking water to all the population of the globe, particularly people of Sub-sahara and South Asia who are living in remote areas where piped water is technically not feasible or very costly. It can be a boon for the natural calamity affected the area where potable drinking water supply is disrupted. Furthermore, distillation-based solar seawater desalination techniques using ( h-LAH) can become an option to membrane-based water desalination processes. It can also use for purification of industrial sewage containing multiple ionic contaminants. Future development in molecular engineering will produce photothermal materials (light-absorbing hydrogels and hydratable polymer)which will produce higher solar vapor generation and can produce efficient solar water purification. \r\nAlthough solar distillation was in vogue, latest hydrogel technology may pave the way for solar distillation application at a massive level in remote areas where establishing critical infrastructure for piped water is technically challenging and with a huge cost. Solar distillation using hydrogel (h-LAH) can help all the nations to achieve universal and equitable access to safe and affordable drinking water for all, by 2030, as enunciated in SDG 6.