05-05 oily-duck

[Image above] Access to freshwater is scarce—and it is made even more so by pollution, climate change, and increased farming. Could ceramic photocatalysts help us make drinkable water in a cost-effective and clean manner? Credit: Julie Gentry, PublicDomainPictures.net (CC0 1.0)

The 50th anniversary of Earth Day passed recently without much fanfare, due to the need for social distancing.

On the bright side, the slowdown in human activity has given us a glimpse of what could be, with clean air from Los Angeles to Beijing and clear water in the canals of Venice and San Antonio. Unfortunately, these gains are predicted to go away once we return to business as usual.

One of the most daunting aspects of returning to usual is the lack of clean, drinkable water. Of the 1.9 billion cubic kilometers of water, over 96% is saltwater. And, of the total freshwater, over 68% is locked up in ice and glaciers. Fresh surface-water sources, such as rivers and lakes, only constitute about 93,100 cubic kilometers, which is about 1/150th of 1% of total water. Pollution, the effects of global climate change, and increased farming means all degrade surface water, which means even less is available for consumption.

Credit: United States Geological Survey (public domain)

Making water drinkable is not easy. Current technology is energy intensive, or it uses expensive processes, or both. Using fossil fuels to distill water increases atmospheric carbon dioxide and thus intensifies climate change. Using reverse osmosis creates significant amounts of wastewater with higher concentrations of salts and pollutants.

Solar energy is one possible answer for cleaning our water in a cost-effective and clean manner. Solar energy is free, and most of the solar energy goes toward heating planet earth already. Redirecting that heat into useful processes keeps the earth’s energy in balance and reduces the generation of greenhouse gases by reducing the amount of fossil fuels used.

Ceramics can play significant roles in using solar energy to clean our water. In particular, ceramics can be used as photocatalysts in water purification processes.

Photocatalysis is the use of light energy to speed up reactions. For water purification, photocatalysis techniques are used to break down toxic organic chemicals.

I discussed the use of ceramic materials as photocatalysts in previous CTT posts, including this one and this one. Now, two articles published in the current issues of International Journal of Applied Ceramic Technology (ACT) and Journal of the American Ceramic Society (JACerS), respectively, discuss further developments.

Combining copper and tin oxides improves catalyst performance

In the current issue of ACT, researchers from Konya Technical University in Turkey explored improving the photocatalytic properties of tin(IV) oxide (SnO2), a widely used pollution remediation catalyst.

While SnO2 has many favorable properties, it absorbs mostly UV light and does not use visible light effectively. So the researchers of this study looked for ways to improve this material’s abilities.

Copper(II) oxide (CuO) has desirable electrochemical properties and has been explored for photocatalytic water remediation. In addition, combining CuO with SnO2 and other photocatalytic oxides improves the light absorbing effectiveness, and many studies have investigated these combinations for various applications.

This current study advances the field by fabricating high surface area catalysts with controlled morphology and a high number of active catalytic sites using two relatively inexpensive yet reproducible processes: electrospinning of SnO nanofibers and hydrothermal synthesis of plate-like CuO. The two materials were combined by dropping CuO dispersions onto the fibers and then performing a heat treatment.

The researchers thoroughly characterized and tested the materials and found the combined CuO-SnO2 catalysts showed excellent light absorption—nearly 3 times that of the SnO2 alone—and a photocatalytic degradation rate nearly twice that of untreated SnO2.

Absorbance spectra showing disappearance of methylene blue during exposure to simulated sunlight, thus demonstrating the photocatalyst’s ability. Credit: Dursun et al., International Journal of Applied Ceramic Technology

The researchers then continued to investigate the catalytic mechanism and found the mixed material had additional catalytic mechanisms beyond those for the SnO2 alone.

The researchers concluded they developed a heteroscale material with activity similar to all nanoscale photocatalysts and identified the main achievement as being lower amounts of CuO.

I believe the materials have another advantage compared to nanoparticles. The high surface area with larger particles makes the material much easier to work with. Specifically, these particles can be filtered easily and do not have to be coated onto a substrate as a result.

The paper, published in International Journal of Applied Ceramic Technology, is “Visible light active heterostructured photocatalyst system based on CuO plate‐like particles and SnO2 nanofibers” (DOI: 10.1111/ijac.13467).

SnO2 fibers with CuO particles. Credit: Dursun et al., International Journal of Applied Ceramic Technology

Melamine sponge aids in purification by distillation

The rapid communication article published in JACerS by researchers from Ludong University in China provides a new twist on purification by distillation, i.e., the process in which water is separated from both organic and inorganic substances through evaporation and condensation.

I find this study exciting because it used a melamine sponge as the evaporator for a small amount of relatively inexpensive lanthanum hexaboride (LaB6). Melamine sponge is hydrophilic and easily wicks water via capillary actions. It is also chemically resistant and can be used in acids, bases, and neutral solutions. Melamine sponge is sold to consumers as the “magic eraser” product, which clearly shows it is readily available in large quantities.

Temperature change at the surface of water, the dipcoated sponge in water, and the dipcoated sponge in air. Note that water evaporation removes energy from the sponge, lowering its temperature. Credit: Zhang et al., Journal of the American Ceramic Society

The results of the study are very promising. Under 1 W/m2 solar illumination, the evaporation rate of clean water was over 1 kg/hr for every meter squared of coated surface with 60% efficiency (energy of evaporation versus solar energy). This response was stable over 20 cycles of 1-hour exposures.

The results for cleaning up solutions are promising as well. The evaporation rates range from 0.8–0.9 kg/hr/m2 for simulated organic pollution to simulated seawater, respectively, and efficiencies in the 40–50% range.

The evaporation rate for pure water scaled almost linearly as the light intensity increased up to 10 W/m2. Scaling up intensity is very important because solar intensity ranges up to 1000 W/m2. If the system could retain 40% efficiency at full intensity, 1 m2 of this product could evaporate 700 kg (about 200 gallons) of water per hour. To put this value in perspective, a small family uses 200–300 gallons of water per day.

These results are very exciting, and I look forward to seeing a more from these researchers soon.

As a rapid communication article, this paper is a short report on new findings that are judged sufficiently important and interesting to publish quickly while authors continue to explore the fundamental scientific explanations behind the observed phenomena. Keep an eye out for their follow-on article.

The rapid communication paper, published in Journal of the American Ceramic Society, is “Steam generation by LaB6 nanoparticles through photothermal energy conversion” (DOI: 10.1111/jace.17076).