Pufferfish have inspired a new device that is capable of the highest rate ever of passive solar water purification – according to its creators Xiaohui Xu and colleagues at Princeton University. Based on an advanced hydrogel, the system can rapidly soak up and filter water when cool, and then release clean water when warmed in the Sun. The team hopes their innovation could lead to low-cost and sustainable off-grid purification systems, potentially improving access to clean water for many communities worldwide.

The solar absorber gel purification system in a lake at Princeton University (left) and the clean water produced during its immersion. (Courtesy: Xiaohui Xu)

One in three people worldwide do not have access to safe drinking water so there is a pressing need for inexpensive, environmentally sustainable systems for filtering water. These are best achieved through passive purification technologies, which use solar energy to separate water from contaminants such as heavy metals, oils, and harmful microbes. Today, this is widely done by evaporating water, and condensing it onto a surface – but this is an energy intensive process, leading to slow production rates.

When pufferfish detect predators, they rapidly absorb water to swell their bodies, making themselves appear more threatening. Once danger passes, the water is quickly released. To mimic this behavior, the researchers developed a sponge-like device they dubbed a solar absorber gel, containing three key components: a temperature-sensitive mesh containing both hydrophobic and hydrophilic regions;  surrounding this inner hydrogel is a dark layer, which efficiently converts sunlight into heat, enabling the mesh to reach its phase transition temperature in cooler conditions, while also filtering out heavy metals and organic molecules before they entered the hydrogel, and finally, an outer layer that filters out any microbes, along with any other larger molecules.

At lower temperatures the mesh remains long and flexible. This enables water to flow into the mesh through capillary action, and bond with its hydrophilic regions. At temperatures higher than 33 °C (91 °F), the mesh undergos a phase transition, becoming short and rigid. As a result it loses around 90% of its volume and it becomes hydrophobic – pushing water out of the material.

Xu’s team tested the performance of the material by placing it in a lake on the Princeton campus with a water temperature of 25 °C (77 °F). After soaking the material, they warmed it in sunlight to release its absorbed water. Over a 2 hour cycle of soaking and discharge, the material demonstrated the highest rate of passive solar water purification ever reported. The material is also highly durable, barely diminishing in performance even after 10 collection cycles.

Owing to their simple, water-based manufacturing process, these materials are low-cost and non-toxic, ensuring both their accessibility and sustainability for off-grid water purification. Xu and colleagues believe their material could be transformative, potentially improving quality of life for many communities around the world.

You can read the original article at physicsworld.com


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