Urea Inhibits the Formation of Environmentally Persistent Free Radicals as well as Accelerates Their Decay

Environmentally persistent free radicals (EPFRs) have emerged as newly identified air pollutants, contributing to various environmental and health concerns. Consequently, the mitigation process, involving the prevention of their formation or acceleration of their decay, is pivotal in reducing the environmental risks associated with EPFRs. This study focuses on the mitigating effect of urea on the generation of EPFRs from 2-monochlorophenol (MCP) at 230 °C thermal remediation under laboratory conditions, abbreviated as MCP230 EPFRs. A strong inhibition of MCP230 EPFR formation was observed, increasing with the addition of urea to 10 wt % relative to the mass of 5% CuO/SiO₂ particles prior to exposure. Loss of activity of Cu²⁺ toward EPFR formation in the presence of urea is likely due to the complexation of Cu²⁺ by urea or by ammonia, one of the major decomposition products of urea. Formation of this complex prevents the adsorption of EPFR precursor molecules on the surface-bound OH groups. It has also been observed that adding solid urea or a urea water solution accelerates the degradation of pre-existing EPFRs within the CuO/SiO₂ solid matrix. In other words, a 1/e lifetime of MCP230 EPFRs from standard exposure was 42 h in a vacuum, while it decreased to 26 and 3 h when 5 and 10% urea were added, respectively, indicating that urea may reduce the stability of EPFRs. This hypothesis is also supported by characterization with X-ray spectroscopies. Our results suggest that urea possesses enormous potential for use as a mitigation agent for the prevention of EPFR formation and the acceleration of decay of existing EPFRs.