Water is a basic necessity for humans, animals, and vegetation. Without clean drinking water, economic growth or prosperity can be imagined for human-being. According to the UN’s 6th SDG (Sustainable Development Goal), all nations must provide universal and equitable access to safe and affordable drinking water to their population by 2030. Although almost 91% of the global population has access to clean water, 666 million people, mainly from sub-Sahara and South Asia, lack access to potable water. Almost 1000 children die every day due to preventable water 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)
Various techniques can purify water, but one of the oldest and most time-tested techniques is to purify water using solar energy. Solar stills are used in remote areas such as deserts and dense forests where rain, piped, or healthy water is unavailable. 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 vapour as safe drinking water. Existing solar distillation technologies for water purification and desalinating seawater are energy-intensive and need expensive infrastructure.
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, the black bottom still absorbs sunlight and vaporizes the water. The evaporated water vapour condenses on the clear covering, trickles into a collector and leaves the contaminants behind. Operating at their theoretical best, such solar distil can not produce more than 1.6 L/h/m2 output, insufficient for poor households.
How does Solar distillation with h-LAH hydrogel overcome the shortcoming of solar still?
Although water purification by solar distillation is a promising technology for homes or the entire community, solar vapour generation (SVG), the essential process of solar distillation to separate water and contaminants, is energy-intensive and low-water yield under natural sunlight. Adding nanostructured, highly hydratable light-absorbing(h-LAH) hydrogel can increase water evaporation and give higher output of clean water under ambient sunlight.
Mechanism of Solar distillation with hydrogel
The floating h-LAH is made 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. 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. The PP chains exhibit semiconducting properties and generate water vapour using solar energy upon exposure to solar irradiation. It speeds up the water vapour generation process. A condenser captures the water vapour generated from the hydrogel’s surface.
An (h-HAL )hydrogel creates three water phases 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 by bound water molecules and (FW) free water molecules. AS IW molecules share fewer bonds with their neighbours, they evaporate more readily with reduced energy demand than (FW) free water at the bottom of still. Once this intermediate water molecule evaporates, it is 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 is reduced, which increases the water evaporation rate.
FW (Free water)
FW (Free water) in the solar is still not bound by hydrogel and has stronger hydrogen bonding than IW. The presence of highly hydratable functional groups, such as hydroxyl (OH)and amino(NH2) groups in hydrogel exacerbate the durability (strong interaction with water molecules) of the polymer network, increasing the proportion of IW and lowering the energy demand of vapour generation hence facilitating water evaporation.
The molecular level meshes(cross-linking density of polymer) in the hydratable polymer network of the hydrogel regulates the 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 distilled water at the rate of 3.6 L/h/m2 at energy efficiency of ~92%. At such a rate, about 25-30 litres of clean drinking water is produced per day, quickly satisfying small household needs in remote locations and disaster-affected areas.
All three polymers in the hydrogel are both commercially available and cheap, and this can offer a cost-effective solution for low-income families 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.
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 globe’s population, mainly people of Sub-sahara and South Asia who live in remote areas. Piped water is technically not feasible or very costly. It can be a boon for the natural calamity that affects the place where the potable drinking water supply is disrupted. Furthermore, distillation-based solar seawater desalination techniques (h-LAH) can become an option for membrane-based water desalination processes. It can also be used to purify industrial sewage containing multiple ionic contaminants. Future development in molecular engineering will produce photothermal materials (light-absorbing hydrogels and hydratable polymer), which will have higher solar vapour generation and efficient solar water purification. \r\nAlthough solar distillation was in vogue. The 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 considerable 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.