Porous tungsten oxide photocatalyst gel enables efficient water purification in high-salinity conditions

Chemical plants and similar facilities generate large amounts of wastewater. When salts are present, the consumption of reagents and processing costs increase. A joint research group comprising Assistant Principal Researcher Tsuyoshi Sugita, Assistant Principal Researcher Yuki Ueda, and Researcher Rintaro Nakabe from the Materials Sciences Research Center, Nuclear Science Research Institute, Japan Atomic Energy Agency (JAEA); Assistant Principal Researcher Takuya Nankawa and Principal Researcher Yurina Sekine from the Research Co-ordination and Promotion Office of the same institute; and Professor Masanobu Mori from the Faculty of Science and Technology, Kochi University have demonstrated that by forming fine particles of tungsten oxide photocatalyst into a porous gel, contaminants can be decomposed 4 to 13 times more efficiently than with conventional materials, even when salts are present. "Going forward, we aim to expand applications to industrial wastewater and seawater, examine other photocatalysts, and conduct experiments to improve durability, advancing our efforts toward practical implementation," said Sugita.

Chemical plants and desalination facilities treat water containing high concentrations of NaCl, sulfates, and other salts using reverse osmosis membranes and similar methods. When organic matter is present in salt-rich water, membranes become clogged and require cleaning or replacement. Chemical oxidizers and ozone decomposition are used to remove organic matter from salt-rich water, but efficiency tends to decrease compared with non-saline water, and high chemical consumption results in environmental risks and elevated treatment costs. These costs are ultimately reflected in product prices and water utility rates, creating demand for low-cost water treatment technologies.

Photocatalytic water treatment technology has attracted attention as an environmentally friendly technology because it can decompose organic matter into water and carbon dioxide simply by exposure to light. However, when salt and ion concentrations in water are high, powdered photocatalyst particles aggregate, or ions inhibit the reaction between the photocatalyst and organic matter, significantly reducing performance. When photocatalysts are immobilized on flat surfaces to prevent aggregation, the surface area of particles available for reaction decreases, causing a substantial drop in reaction efficiency.

The research group synthesized a photocatalyst gel by mixing tungsten oxide, a photocatalyst that functions even under sunlight, with carboxymethyl cellulose nanofiber (CMCF). Then, they utilized a freeze-crosslinking method involving freezing followed by immersion in citric acid solution and thawing.

The hydrogel produced through this original method develops complex water channels about 100-200 microns in diameter (equivalent to the thickness of 1-2 human hairs) during the fabrication process; it has the property of adsorbing organic matter onto the gel surface even in salt-rich water. By dispersing and immobilizing photocatalyst microparticles within the gel, particle aggregation is suppressed, and efficient contact between the photocatalyst and organic matter is maintained. This allows organic matter in salt-rich water to be more readily adsorbed onto the gel surface, leading to improved decomposition efficiency.

Decomposition experiments using indigo carmine (a blue dye) as a model organic pollutant showed that in salt-free water, the photocatalyst gel demonstrated superior decomposition efficiency compared with powder suspensions (powder form) and immobilized glass materials (thin film form). Even at relatively high salt concentrations (100 millimolar) of sodium nitrate, NaCl, sodium dihydrogen phosphate, and Na₂SO₄, the gel showed 4 to 13 times higher decomposition efficiency than powder and glass-immobilized materials. While salts generally reduce decomposition efficiency, the researchers also discovered an exceptional property of the photocatalyst gel: when sodium nitrate and sodium dihydrogen phosphate coexist, decomposition efficiency actually improves.

The newly developed photocatalyst gel has demonstrated its potential as a next-generation water purification material for actual industrial wastewater and seawater applications, providing new guidelines for "photocatalyst design in salt-rich water." This material can be produced relatively easily in large areas and various shapes through the freeze-crosslinking method, and a notable feature is its scalability from laboratory to practical levels.

Journal Information
Publication: Journal of Photochemistry and Photobiology A: Chemistry
Title: Highly water-permeable WO₃-containing porous hydrogel via freeze-crosslinking for efficiency and salt-robust dye Decolorization
DOI: 10.1016/j.jphotochem.2025.116773