Effects of “Fenton-like” reactions of ferric oxalate on atmospheric oxidation processes and radiative forcing

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PICTURE: Methacrolein reacts with superoxide radicals produced by a “Fenton-type” reaction. This affects the atmospheric radiative forcing due to the formation of insoluble iron hydroxide. view After

Credit: Yu Wang

The Fenton reaction is a chemical transition involving hydrogen peroxide (H2O2) and the ferrous ion (iron), which acts as a catalyst. This process is used to destroy hazardous contaminants in wastewater through oxidation. In the atmosphere, a similar reaction, or “Fenton-like” reaction, occurs continuously with ferric oxalate ([Fe(III)(C2O4)3]3-) and airborne aerosols. It is the most common chemical reaction that occurs in the atmosphere. The ability of a particle to oxidize is directly related to its phase, whether gaseous or aqueous, which has a significant impact on the formation of secondary organic aerosols (AOS). Therefore, research is needed not only to assess the contribution of this Fenton-like reaction to atmospheric oxidation, but also to improve the consistency of SOA budgets simulated by model and observed in the field.

“It is generally believed that the contribution of the Fenton reaction to atmospheric oxidation comes from the generation of hydroxyl radicals.” said Professor Wenbo Dong from the Department of Environmental Science and Engineering at Fudan University. “Scientists have not often addressed the role of superoxide radicals, which are generally believed to be the source of hydrogen peroxide and hydroxyl radicals.”

Methacrolein (CH2= C (CH3) CHO) is the main oxidation product of isoprene (CH2= C (CH3) CH = CH2), which is the most abundant biological volatile organic compound (VOC) in the atmosphere. It can react directly with superoxide radicals to generate SOA. Although this is a common reaction, this process shows that there are other oxidation pathways for VOCs.

“Previous studies believed that superoxide radicals do not react with most organic compounds.” remarked Professor Dong.

Some VOCs in the atmosphere can react with superoxide radicals just like methacrolein. However, the potential for SOA production from any VOC with superoxide radicals and associated hydroxyl radicals is distinct from the reaction of methacrolein. The researchers focused on the oxidation process of organic pollutants caused by these free radicals. They discovered that the oxidation process is linked to the reaction mechanism of organic matter accompanying these free radicals.

Previous studies have shown that the change in absorbance of aqueous aerosols is attributed to the formation of brown carbon. However, in the case of the photo-oxidation of methacrolein with ferric oxalate, Professor Dong’s research group noticed a substantial increase in the absorbance of aerosols without the formation of brown carbon. Further analysis is provided in their research article titled “Photooxidation of Methacrolein in Fe (III) -Oxalate Aqueous System and Its Atmospheric Implication” published in Advances in atmospheric science.

“When the Fenton-like reaction with a high concentration of iron occurs, the absorbance of the solution will change significantly, the solution turning yellow.” said Professor Dong. “This may not be the only situation with methacrolein, as it can cause the Fenton-like reaction of other organic compounds.”

Prof. Dong continued, “The formation of insoluble or colloidal iron hydroxide increases the absorbance of atmospheric aerosols, affecting radiative forcing, which has long been overlooked.

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