Dugan Hayes and Cali Antolini, using a technique involving laser pulses fired at intervals on the order of quadrillionths of a second, can investigate chemical reactions driven by light. Credit: URI Photo/Michael Salerno
Ferrate is a material that releases the powerful oxidant Fe(V) – or iron-5+ – when excited by light, making it a promising tool for next-generation water purification systems. Compared to chlorine and ozone treatment systems, ferrate produces fewer toxic byproducts and is potentially cheaper and easier to deploy, but the exact mechanisms of Fe(V) generation from ferrate are not fully understood. In order to gain insights that can be used to optimize ferrate-based treatment systems, researchers from the University of Rhode Island (URI) used ultra-fast laser and X-ray pulses to analyze every step of the chemical reaction that arises when ferrate is excited by UV and visible light.
The team utilized a technique known as transient absorption spectroscopy. An initial pulse of light is used to kick off the reaction that produces Fe(V), while subsequent ultra-fast laser pulses probe the reaction steps as they play out. The femtosecond laser pulses enable a detailed record to be obtained of even the short-lived reaction products throughout the process. The X-ray experiments were performed at Argonne National Laboratory’s Advanced Photon Source while experiments using UV and visible light were conducted at URI facilities.
The experiments revealed that the conversion rate from ferrate to Fe(V) was about 15% – similar to that of radical production in ozone purification systems. Additionally, the team found that Fe(V) could be produced using a wide range of wavelengths from UV to visible. Because visible light requires less energy to produce and undergoes less scatter in cloudy water, visible wavelengths could be used to induce ferrate excitation more efficiently and in a wider variety of water conditions than UV light. Revealing these details about ferrate photochemistry can aid in the optimization and deployment of less expensive and more environmentally-friendly ferrate-based treatment systems in the future. This research was published in the Journal of the American Chemical Society.
“The formation of powerful oxidants from ferrate has been difficult to understand mechanistically, and this has blocked process optimization and full-scale implementation in water treatment applications,” explained study coauthor Joseph Goodwill. “The results presented in this paper improve our fundamental understanding of the ferrate system, which opens doors for applications.”
Ferrate-based systems are especially promising for smaller systems – such as those in small rural areas – where costly and elaborate ozone-based systems are impractical. Additionally, ferrate could reduce reliance on harsh chemicals like chlorine, and can potentially remove more stubborn contaminants that chlorine cannot, such as per- and polyfluoroalkyl substances (PFAS). Additional research into ferrate chemistry will be needed to harness its full potential as a powerful oxidant.