Water treatment: Catching steroid hormones with nanotubes

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Steroid hormones are among the most widespread aquatic micropollutants. They are harmful to human health, and they cause ecological imbalances in aquatic environments.

At the Karlsruhe Institute of Technology (KIT), researchers have investigated how steroid hormones are degraded in an electrochemical membrane reactor with carbon nanotube membranes. They found that adsorption of steroid hormones on the carbon nanotubes did not limit the hormones' subsequent degradation.

Supplying clean water to people around the world is one of the great challenges of today and tomorrow. Various micropollutants (organic and inorganic substances) are present in low concentrations in wastewater but can still be harmful to humans and the environment.

Considerable risks are posed by endocrine-disrupting substances, such as steroid hormones, which can affect the hormonal system. Such substances are present in pharmaceuticals, contraceptives and other products. Though difficult to detect in water, they can seriously harm human health and disrupt the ecological equilibrium of aquatic environments.

Oxidation facilitates micropollutant degradation

Steroid hormones can be neither detected nor removed with conventional water treatment methods. Electrochemical oxidation (EO) is gaining recognition as a promising approach for their removal; EO systems consist of an anode and a cathode connected to an external power source.
The electrical energy at the electrodes is varied (modulated), leading to the oxidation and degradation of pollutants at the anode's surface.

EO can be exploited more effectively with electrochemical membrane reactors (EMR), in which a conductive membrane serves as a flow-through electrode, improving mass transfer and making active sites more accessible for the reacting molecules.

Unique physical and chemical properties of carbon nanotubes

In collaboration with scientists from the University of California, Los Angeles, and Hebrew University of Jerusalem, researchers at KIT's Institute for Advanced Membrane Technology (IAMT) have announced progress in understanding the mysterious mechanisms at work in EMR.

For a paper in "Water Treatment and Harvesting," a special issue of Nature Communications, they investigated the degradation of steroid hormone micropollutants in an EMR with carbon nanotube membranes. With diameters in the nanometer range, carbon nanotubes (CNT) have unique physical and chemical properties.

"Their high electrical conductivity enables efficient electron transfer," said Andrea Iris Schäfer, Professor of Water Process Engineering and head of the IAMT at KIT.

"Thanks to their nanostructure, CNTs have an extremely large surface area, which gives them huge potential for adsorbing various organic compounds. That makes subsequent electrochemical reactions easier."

In their research, the scientists used state-of-the-art analytical methods to investigate the complex interactions of adsorption and desorption, electrochemical reactions, and byproduct formation in an EMR.

"We found that pre-adsorption of steroid hormones, meaning their enrichment on the surface of the CNT, did not limit the later degradation of the hormones," said Dr. Siqi Liu, an IAMT postdoc. "We attribute this to rapid adsorption and effective mass transfer."

The study's analytical approach facilitates the identification of factors limiting hormone degradation under varying conditions.

"Our analysis explains some of the underlying mechanisms in electrochemical membrane reactors and provides valuable insights for the improvement of electrochemical strategies for eliminating micropollutants from water," concluded Schäfer.

More information: Siqi Liu et al, Differentiation of adsorption and degradation in steroid hormone micropollutants removal using electrochemical carbon nanotube membrane, Nature Communications (2024). DOI: 10.1038/s41467-024-52730-7

Journal information: Nature Communications

Provided by Karlsruhe Institute of Technology