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Femtosecond pulses etch hydrophilic surfaces for solar-based water purification

Researchers have demonstrated a method of purifying water using aluminium surfaces that have been etched using an ultrafast laser.

The surfaces could be used to provide clean drinking water in developing countries.

In Nature Sustainability, the researchers from the University of Rochester in New York, USA, have shown that a burst of femtosecond laser pulses can be used to etch the surface of a normal sheet of aluminium in order to turn it into a hydrophilic (water-attracting) material that is also a highly efficient absorber of sunlight.

To purify water, the sheet is placed at an angle facing the sun in a body of water. Due to its hydrophilic properties, a thin film of water is drawn upwards over the etched surface, where sunlight is then absorbed and quickly heats the water to evaporate it, leaving any contaminants behind.

Due to its high efficiency of solar energy absorption, the surface retains nearly 100 per cent of the energy it absorbs. This, combined with the fact that the etched surface simultaneously changes the intermolecular bonds of the water, significantly increases the efficiency of the evaporation process.

The evaporated water can then be collected and subsequently tested to check its suitability for drinking.

Solar-based water purification: Seeking an efficient method

Using sunlight to boil water has long been recognised as a way to eliminate microbial pathogens and reduce deaths from diarrheal infections.

The most common method of solar-based water evaporation is volume heating, in which a large volume of water is heated but only the top layer can evaporate. This is inefficient, however, according to the researchers, as only a small fraction of the heating energy gets used.

A more efficient approach, called interfacial heating, places floating, multilayered absorbing and hydrophilic materials on top of the water, so that only water near the surface needs to be heated. But the available materials all have to float horizontally on top of the water and cannot face the sun directly. Furthermore, the available hydrophilic materials become quickly clogged with contaminants left behind after evaporation, requiring frequent replacement of the materials.

The panel developed by the researchers avoids these inefficiencies by pulling a thin layer of water out of the reservoir and directly onto the solar absorber surface for heating and evaporation. Its angle can then be continuously adjusted to directly face the sun as it rises and then moves across the sky before setting, maximising energy absorption. In addition, because an open-grooved surface has been used, the panel can be easily cleaned simply by spraying it.

Lastly, while boiling water traditionally does not eliminate heavy metals and certain other contaminants, the researchers have demonstrated that their new panel does indeed reduce the presence of all common contaminants, such as detergent, dyes, urine, heavy metals, and glycerin, to safe levels for drinking.

The work was undertaken in the lab of Chunlei Guo, professor of optics at the University of Rochester, who has long envisioned an array of humanitarian applications for an efficient solar-based purification method. 

‘This is a simple, durable, inexpensive way to address the global water crisis, especially in developing nations,’ he said, noting that the new method could help relieve water shortages in drought-stricken areas and be helpful in water desalination projects.

The project was supported by funding from the Bill and Melinda Gates Foundation, the National Science Foundation, and the US Army Research Office.

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