Generation of carbon dioxide anion radical by UV/small molecular monocarboxylic acid system for reductive dechlorination of chlorinated alkanes

Abstract

The persistence of chlorinated alkanes in aquatic environments poses significant health risks due to its biotoxicity and high volatility, which contributes to both water and air pollution. This study investigates the efficacy of carbon dioxide radical anion (CO₂•⁻) mediated advanced reduction processes (ARPs) for the reductive dechlorination of chlorinated alkanes using small molecular monocarboxylic acids (SMAs) under UV irradiation. The study focused on formic acid (HCOOH), acetic acid (CH₃COOH), and propionic acid (CH₃CH₂COOH) to generate CO₂•⁻, revealing that UV/HCOOH system exhibits a notably high chloroform (CF) degradation efficiency of 97.8 % in 90 min. Kinetic studies indicated a linear relationship between the HCOOH concentrations and the observed reaction rate constants (k_obs), demonstrating that CO₂•⁻ production is crucial for CF degradation. Electron paramagnetic resonance spectroscopy identified CO₂•⁻ and hydroxyl radicals (HO•) as the active species, with the former playing a predominant role in CF degradation. The study also explored the influence of carbon chain length in SMAs on CF degradation, finding that longer chains decrease the degradation efficiency, potentially due to reduced UV activation. A higher reaction rate constant (k_obs) under acidic conditions, with a marked decrease in efficiency as the pH exceeds 3.7, where HCOO⁻ becomes predominant. This study enhances our understanding of CO₂•⁻ mediated ARPs and explores potential applications in environmental remediation, providing insights into the pathways and mechanisms of CF degradation. The UV/SMAs systems offer advantages for practical applications, such as milder reaction conditions and higher efficiency compared to traditional methods.