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A new way to generate solar steam for cleaning water and reducing waste

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Posted August 22, 2019

Strategies for purifying water abound, but they tend to require lots of energy. Scientists at Argonne are exploring materials that can efficiently use sunlight to concentrate heat right at the water’s surface to produce clean steam for capture.

Strategies for purifying water abound, but they tend to require lots of energy. To make use of the sun’s abundant resource, scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory are exploring materials that can efficiently convert sunlight to heat and concentrate that heat right at the water’s surface, where it is needed for evaporation.

A schematic representation of a POF-coated surface generating steam (Image by Argonne National Laboratory.)

As part of this goal, a team led by Argonne researchers has discovered a novel, yet simple, method for generating solar steam using a recently discovered class of materials called porphyrin organic frameworks, or POFs. Buoyant materials treated with POFs can capture the sun’s energy to evaporate the water into a clean steam for capture while leaving contaminants behind, or to concentrate wastewater to reduce disposal costs. The details were recently published in the journal Advanced Materials Interfaces.

The journal article is one of the first to emerge from Argonne’s Advanced Materials for Energy-Water Systems (AMEWS) Center, a DOE Energy Frontier Research Center. ​Research at AMEWS focuses on solutions to the world’s water crises by examining the interfaces between materials and water.

The POF finding could inform strategies for desalination and for wastewater treatment, particularly in industrial settings. In the oil and gas industry, for example, water used in hydraulic fracturing typically sits in large disposal ponds until the liquid evaporates and the volume of waste becomes more manageable.

There’s a real opportunity to significantly improve operational efficiency by dramatically increasing the water evaporation rate for those ponds,” said Seth Darling, director of AMEWS and of the Center for Molecular Engineering (CME). The research team also included scientists from the Pritzker School of Molecular Engineering at the University of Chicago and China’s Sun Yat-sen University.

POFs are a subset of covalent organic frameworks — lattice-like solids that can grow indefinitely large like crystals. When the researchers applied a POF-based coating to substrates including wood, fabric, and membranes, they found that these carbon-based materials possessed the right set of properties for generating solar steam. POFs capture a wide range of light wavelengths and transform that light into heat.

Other materials have these photothermal capabilities, but the trick is to find ones that can coat porous materials uniformly, remain stable in liquids, and potentially scale up for use in commercial settings. One example is encapsulated Chinese ink, another material Argonne scientists have demonstrated has strong potential for solar steam generation.

Achieving the kind of stable, uniform coating needed for commercial settings often involves multiple steps and, in some cases, expensive processing. With POFs, only a few simple reactants and some gentle warming — what’s known in chemistry as one-pot synthesis — to grow on top of a substrate are required.

It’s literally one step,” Darling said. ​The ease of preparation is the real selling point here.”

In the study — which was conducted partly at the Center for Nanoscale Materials, a DOE Office of Science User Facility — the researchers documented strong results with a range of POF-treated materials including wood, fabric, membrane, and sponge. Wood treated with POFs performed especially well, converting water to steam with 80% overall efficiency.

The discovery is in line with Argonne’s continuing work to pioneer new solutions that ensure more efficient use of water resources, which now face pressure from a growing population, development, pollution, and climate change.

What happens at water-solid interfaces is what drives the performance of all sorts of different aspects of water-energy systems, whether it’s membranes or sorbents or sensors,” Darling said. ​This study is just one example of the power of interface engineering in applications related to water.”

Source: ANL

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