‘Light responsive’ technology turns seawater into clean drinking water in less than 30 minutes
- New technology purifies water using sunlight using a metal-organic framework
- These so-called MOFs separate the salty solute found in brackish and seawater
- The concept could provide potable water for millions of people across the world
Scientists have developed new technology that can turn seawater into clean drinking water in less than 30 minutes.
Australia-based researchers used a metal-organic framework (MOF), a type of lattice-like crystal, to desalinate water.
The hollow framework of pores separates the salty solute within the brackish water or even saltier seawater, in a process known as molecular sieving.
Under dark conditions, the MOF adsorbs salts and other impurities in the water in 30 minutes. The MOF itself is then regenerated for reuse in just four minutes, using sunlight to remove the adsorbed salts.
The light-responsive MOF was used to filter harmful particles from water and generate 139.5 litres of clean water per kilogram of MOF per day.
Scientists say their technology is more energy-efficient manner than current desalination practices, including reverse osmosis, and could provide potable water for millions globally.
An illustration of the proposed single-column setup for desalination in the dark and regeneration under light. Under dark conditions, the technology adsorbs ions from water in 30 minutes, and with sunlight illumination, it rapidly releases these adsorbed salts in four minutes
Water scarcity is one of the largest global risks in the upcoming years, according to the World Economic Forum (WEF).
ermal desalination processes by evaporation using solar energy are widely used to produce fresh water, but can be highly energy intensive.
‘Sunlight is the most abundant and renewable source of energy on Earth,’ said Professor Huanting Wang from the Department of Chemical Engineering at Monash University in Australia.
‘Our development of a new adsorbent-based desalination process through the use of sunlight for regeneration provides an energy-efficient and environmentally-sustainable solution for desalination.’
Metal-organic frameworks are a class of compounds consisting of metal ions that form a crystalline material with the largest surface area of any material known
WHAT ARE MOFs?
Metal organic frameworks (MOFs) are synthetic materials made up of carbon and metal compounds, which form a porous honeycomb structure.
The materials can form sheets or 3D shapes and scientists are investigating ways to apply the technology.
Scientists believe areas which could benefit from their use include hydrogen storage and photovoltaic technology – used in solar power.
But MOFs could also provide advance other areas, including removing toxic molecules from the atmosphere.
Desalination – the process of turning undrinkable saline or salty water into drinkable water – has been used to address escalating water shortages globally.
‘Due to the availability of brackish water and seawater, and because desalination processes are reliable, treated water can be integrated within existing aquatic systems with minimal health risks,’ Professor Wang said.
The World Health Organisation suggests good quality drinking water should have a total dissolved solid (TDS) of less than 600 parts per million (ppm).
Monash researchers were able to achieve a TDS of less than 500 ppm in just 30 minutes and regenerate the MOF for reuse in four minutes under sunlight by rapidly releasing the adsorbed salts.
MOFs were discovered in the 1990s and could have potential for use in hydrogen storage, solar energy and even mopping up toxic gasses from the air.
They are a class of compounds consisting of metal ions that form a crystalline material with the largest surface area of any material known.
They are so porous that they can fit the entire surface of a football field in a teaspoon.
The research team created a dedicated MOF called PSP-MIL-53, which is photoreversible, meaning its functions can be changed from one to the other by light.
PSP-MIL-53 was synthesised by introducing poly(spiropyran acrylate) (PSP) into the pores of MIL-53 – a specialised MOF well-known for its breathing effects and transitions upon the adsorption of molecules such as water and carbon dioxide.
PSP-MIL-53 was able to yield 139.5 litres of fresh water per kilogram of MOF per day, with a low energy consumption, sourced from a river, lake or aquifer.
‘This study has successfully demonstrated that the photoresponsive MOFs are a promising, energy-efficient, and sustainable adsorbent for desalination,’ Professor Wang said.
Spectrum of spectra of SP and MC activated under different pH levels in 10,000 ppm sodium chloride solution
‘Our work provides an exciting new route for the design of functional materials for using solar energy to reduce the energy demand and improve the sustainability of water desalination.
‘These sunlight-responsive MOFs can potentially be further functionalised for low-energy and environmentally-friendly means of extracting minerals for sustainable mining and other related applications.’
Thermal desalination processes by evaporation are usually energy-intensive, while other technologies like reverse osmosis have some drawbacks.
Brackish water has more salinity than freshwater, but not as much as seawater. Desalination has been increasingly used to address the escalating water shortage, due to the wide availability of brackish water and seawater
In reverse osmosis, water, containing dissolved salt molecules, is forced through a semi-permiable membrane filter.
Larger salt molecules do not get through the membrane holes, but the smaller water molecules do.
Reverse osmosis is an effective means to desalinate saline water, but it is more expensive than other methods and has high energy consumption.
The research has been published in the journal Nature Sustainability.
TURNING SALINE WATER INTO FRESHWATER
Water that is saline contains significant concentrations of dissolved salts.
In this case, the concentration is the amount (by weight) of salt in water, as expressed in “parts per million” (ppm).
If water has a concentration of 10,000 ppm of dissolved salts, then one percent of the weight of the water comes from dissolved salts.
Ocean water contains about 35,000 ppm of salt.
The following are the US government’s parameters for saline water:
– Freshwater – Less than 1,000 ppm
– Slightly saline water – From 1,000 ppm to 3,000 ppm
– Moderately saline water – From 3,000 ppm to 10,000 ppm
– Highly saline water – From 10,000 ppm to 35,000 ppm
The scarcity of freshwater resources and the need for additional water supplies is already critical in many arid regions of the world.
Many arid areas simply do not have freshwater resources in the form of surface water such as rivers and lakes.
They may have only limited underground water resources, some that are becoming more brackish as extraction of water from the aquifers continues.
Solar desalination evaporation is used by nature to produce rain, which is the main source of freshwater on earth.
Source: US government
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