A new solar-powered desalination method could produce drinking water from seawater without chemical additives, according to a promising study. With millions of people lacking access to safe drinking water, this provides great promise. Typically, water desalination is extremely costly and energy-intensive, prompting researchers to seek alternative methods for producing clean, fresh water. If successful, the new technique could provide a more sustainable desalination method as well as a less invasive means of extracting critical minerals.
Approximately 2.2 billion people worldwide lack access to safe, clean drinking water. In certain parts of the world, there are vast quantities of readily available water that cannot be consumed because of the salt it contains. Drought-ridden regions surrounded by seas and oceans often battle with this issue, with many investing in massive desalination projects to convert seawater into fresh water.
Conventional desalination techniques include reverse osmosis and thermal distillation. However, these methods are energy-intensive, require pre- and post-treatment of water, and leave behind concentrated saltwater waste known as brine. This brine is extremely damaging to marine life when released back into the ocean, as it increases salinity and reduces oxygen.
The desalination of 1 cubic meter of water can produce about 12.6 kg of carbon dioxide, which is significant, given that the average person uses about 382 litres of water a day. Water usage is even higher in the agriculture industry, with 3,000 litres of water needed to produce enough food for one person for one day. Conventional desalination also uses chemicals, such as anti-foaming agents and chlorine, which typically get pumped back into the sea and can harm marine life.
In May, researchers at the University of Rochester in New York announced that they had developed a novel solar-thermal desalination process, which they claim can produce fresh water in an energy-efficient way. Moreover, the researchers from the Institute of Optics say the process does not produce waste brine and requires no chemical additives to pre-treat the water. It also produces between 0.6 and 6.7 kg of carbon dioxide per cubic meter of water. The team, led by Professor of Optics and of Physics Chunlei Guo, published their findings in a recent paper.
The technology uses solar panels made of black metal etched with femtosecond lasers to make the surface super light-absorbing and superwicking—extremely water-attracting, according to the team. The panels are equipped with a laser-treated active region that attracts a thin layer of water to the surface, which absorbs almost all solar radiation, distils the water, and deposits the leftover salts and minerals into the panel’s untreated sides or “passive” region so that the salt does not clog the active region and disrupt continuous desalination.
While other scientists have previously established solar-thermal desalination techniques that work under laboratory conditions, these techniques have typically been tested on fresh water mixed with sodium chloride rather than on ocean water. In many of these experiments, researchers have found that the sodium chloride crystallises, leaving a grainy residue that allows water to pass through and dissolve the salt. The solar panels can then be easily cleaned.
However, as seawater has a more complex composition, many of these methods have failed in real-world conditions. Water from the ocean contains other minerals, such as magnesium and calcium, which crystallise differently and can clog the surface of the solar panel, restricting the flow of water, just as it does to shower heads when limescale accumulates.
Guo’s team used a different approach to try and counter some of the commonly seen challenges. The Rochester researchers precisely etched grooves into the black metal so seawater salts and minerals can run off without hindering the filtration process. They also introduced the “coffee ring effect” to the experiment. “If you drop coffee on a surface, eventually the water evaporates, and there’s a ring left at the outer edge that is the concentrated coffee particles,” explained Guo. “We use that same principle to advance the salts to the passive region.”
The team tested their solar-thermal desalination method using water samples from the Pacific, Atlantic, and Indian Oceans to assess its effectiveness. As the team’s technique extracts the salts and minerals in solid form, it leaves behind no polluting salty brine. In addition to transforming saltwater into fresh water, it could be used to produce table salt and extract critical minerals, such as lithium, needed to support a global green transition.
The researchers say they can use the same superwicking solar panels to separate lithium from the other salts during desalination by incorporating hydrogen titanate nanoparticles in the panel’s grooves. In a test of water samples from the Great Salt Lake, the team extracted about 50 percent of the lithium from the waste salt particles.
Having tested the technique extensively on ocean water, the team at Rochester believes the technology is scalable and is far more sustainable than current desalination methods. The bonus of critical mineral extraction could make it highly popular among governments and companies seeking to invest in more sustainable mineral supply chains that do not rely heavily on environmentally destructive mining operations.
By Felicity Bradstock for Oilprice.com