Influence of iron and cobalt on oxalic acid degradation during underwater non-thermal plasma treatment of simulated chemical decontamination effluents

When a nuclear power plant is retired, its metal components must be scrubbed clean of radioactive dust and corrosion before they can be recycled or disposed of safely. To do this, workers often use a strong cleaning solution based on oxalic acid, a common chemical found in rust removers and even in some foods. This acid is excellent at dissolving the stubborn oxide films that hold radioactive particles in place. However, once the cleaning is done, the liquid waste is left with a new problem: it is full of leftover acid and dissolved metals like iron and cobalt. If this mixture is not treated properly, the remaining acid can interfere with future waste processing steps. Scientists have been exploring a high-tech way to destroy this leftover acid using underwater plasma, a form of electrical discharge that creates a burst of reactive chemicals in the water. The goal is to break the acid molecules apart so they no longer pose a chemical threat, but the presence of dissolved metals might change how well this process works.

A team of researchers at the Korea Atomic Energy Research Institute and the Korea Institute of Fusion Energy set out to see how different metal mixtures affect this cleanup process. They created four separate batches of simulated waste water, each containing the same amount of oxalic acid but with different combinations of iron and cobalt added to them. One batch had no metals, one had only cobalt, one had only iron, and the last had a mix of both. They then subjected each batch to the underwater plasma treatment for up to sixty minutes, taking samples every ten minutes to measure how much acid remained. To track the acid, they used a precise method called ion chromatography, which separates and counts the acid molecules, and they also watched how the liquid absorbed light, looking for changes in color or clarity that might indicate chemical shifts.

The results revealed a striking difference depending on which metals were present. In the batches without iron, or with only cobalt, the plasma worked quickly and efficiently. Within twenty to thirty minutes, the amount of detectable acid dropped to such low levels that the instruments could no longer find it. The liquid became chemically clean of the target acid in a short time. However, the batch containing only iron behaved very differently. After sixty minutes of treatment, a significant amount of acid remained in the water. The instruments still detected 42.2 milligrams per liter, which was nearly eighty percent of the amount present at the very start. While the other solutions had been cleared, this iron-rich mixture held onto its acid stubbornly.

The researchers also looked at the liquid with a spectrometer, an instrument that measures how much light the solution absorbs at different colors. They found that the iron-containing liquid kept a persistent glow in the near-ultraviolet range, a type of light invisible to the human eye but detectable by machines. This optical signal stayed strong even as the treatment continued, suggesting that the iron and the acid were forming a stable partnership, or complex, that the plasma struggled to break apart. In contrast, the other batches lost this light-absorbing signal as the acid disappeared. To understand what was happening to the iron itself, the scientists added a special dye called ferrozine to small samples of the liquid. This dye turns purple when it binds to iron in a specific chemical state. The samples turned a deep purple, indicating that the iron was changing its chemical form, shifting between different states during the treatment. This suggests that the iron was not just sitting there; it was actively participating in a cycle of chemical reactions that may have protected the acid from being destroyed.

The study suggests that the presence of iron creates a unique chemical environment where the acid is shielded from the plasma's cleaning power. The iron likely forms a tight bond with the acid molecules, making them harder to break down. Furthermore, the iron might be consuming the very reactive chemicals that the plasma generates to do the cleaning, effectively fighting against the process. The researchers noted that the amount of chloride, a component of the salt used to make the iron solution, was lower in the iron-only batch compared to the others, but the primary factor seemed to be the iron itself. While the cobalt-containing batches cleaned up quickly, the iron-only batch showed that the specific type of metal matters immensely. The team concluded that when assessing how well a plasma treatment works on nuclear waste, one cannot simply look at the acid levels; one must also consider the specific metals present and watch for lingering optical signals that hint at hidden chemical interactions.

This work highlights a crucial nuance in cleaning up nuclear waste. It is not enough to apply a powerful treatment and assume it will work the same way on every type of liquid. The chemical makeup of the waste, particularly the presence of iron, can fundamentally alter the outcome. The researchers found that while the plasma was highly effective at destroying oxalic acid in most scenarios, the iron-rich solution resisted the process, retaining a large portion of the acid even after an hour of treatment. This does not mean the plasma failed entirely, but rather that the iron created a barrier that slowed the reaction down significantly. The study serves as a reminder that in complex chemical mixtures, the behavior of one component can dictate the success of the entire cleanup effort. For future decontamination projects, this means that engineers may need to adjust their strategies or add extra steps if iron is a major part of the waste stream, ensuring that the acid is fully neutralized before the water is processed further.