Using sunlight to suck salt from seawater

Polymer foam offers potential route to practical solar desalination

Freshwater scarcity is an urgent global issue. In some parts of the world, cities are growing increasingly reliant on seawater desalination plants. Ultimately, the goal of desalination is to take huge quantities of salt-laden seawater, and turn it into safe, drinking-quality water. Right now, there are two main ways to do that. The first is to force high-pressure saltwater through exceptionally-fine membranes. The second involves boiling the water, and collecting only the vapour. Both of these approaches are exorbitantly expensive, and require huge quantities of electricity.

But in recent years, there’s been a growing interest in solar steam generation for seawater desalination. The idea is that through careful material design, a highly-porous ‘sponge’ could soak up seawater while also concentrating ambient sunlight into intense heat, to produce pure water vapour. However, creating and recycling such sponges can require costly materials, complex preparation steps, and high processing temperatures. And when applied in practice, these materials can often be easily damaged or get irreversibly clogged up with salt.

In a new Nano Energy paper [DOI: 10.1016/j.nanoen.2019.03.087], Chinese researchers report on a durable bilayer polymer foam that solves many of the current problems with solar steam desalination systems. They started with melamine foam – a low-cost material that has been used in everything from soundproofing to cleaning. It has low density and low thermal conductivity, it’s hydrophilic and compressible, and can be produced at an industrial scale. But melamine foam can also break very easily, so to improve its fatigue resistance, the authors pre-pressed it at 230 °C. This reduced the pore size and created a dense 3D structure that was elastic – it could fully recover its original shape without permanent deformation. The upper side of the foam was then immersed in a suspension that contained a monomer that polymerised to polypyrrole (PPy). This formed a rough, black coating on the foam skeleton.
Each part of the bilayer foam provides specific functions – 1. The PPy layer was found to absorb ~94 % of light in the range 400–2500 nm, making it a highly-efficient solar receiver, and 2. The pre-pressed melamine foam could transport bulk water up to the PPy-coated layer much faster than pristine foam. In addition, it acted as a thermal insulator, trapping the heat generated by the capture of sunlight. In addition, because of the superelasticity of bilayer foam, any salt residue can be removed by simply dipping the foam in water and squeezing it.

The results for the foam were impressive – under 1 sun illumination (1 kW m−2, or ambient light), it produced fresh water from seawater at an average rate of 1.568 kg m−2 h−1, with a solar conversion efficiency of 90.1 %. The concentration of Na+ in the cleaned seawater was also found to be far below the limits set for drinking water by the World Health Organization. In addition, the bilayer foam was found to be remarkably robust, even in acidic conditions. The estimated cost per square meter is just US$8.25, suggesting that it may be an ultra-low-cost way to desalinate seawater.

This story first appeared in Materials Today on 9 May 2019. You can see it in its original form here: https://www.materialstoday.com/energy/news/using-sunlight-to-suck-salt-from-seawater/

Research paper: Chenwei Li, Degang Jiang, Bingbing Huo, Meichun Ding, Congcong Huang,Dedong Jia, Haoxiang Li, Chen-Yang Liu, Jingquan Liu. Scalable and robust bilayer polymer foams for highly efficient and stable solar desalinationNano Energy 60 (2019) 841–849. DOI: 10.1016/j.nanoen.2019.03.087