Theoretical study on the potential environmental and ecological risk of 4-ethylphenol induced by hydroxyl radical

This study closely examines the environmental fate of 4-ethylphenol (4-EP), a significant byproduct of biomass combustion. We employed quantum chemical calculations to investigate the reaction mechanism, kinetics, and ecotoxicity of 4-EP initiated by OH radicals in various environments (aqueous, atmospheric liquid, atmospheric and inhomogeneous phases). Our findings highlight that solvent effects contribute to a higher OH-addition reaction branching ratio (Γ_add) of 0.68 for 4-EP in an aqueous solution, compared to 0.26 in the gas-phase environment and 0.22 in the inhomogeneous environment at 298 K. We determined the rate constants for the liquid-phase, gas-phase, and nonhomogeneous phase to be 1.14 × 10⁹ s^-1 M^-1, 3.09 × 10⁹ s^-1 M^-1, and 6.19 × 10¹⁴ s^-1 M^-1, respectively. Notably, the adsorption of mineral particles considerably enhances the reaction rate of 4-EP with OH radicals. 4-ethylbenzene-1,2-diol, 4-hydroxycyclohexa-3,5-diene-1,2-dione, 1-ethyl-6-methyl-6H-benzo(c)chromene-4,9-diol, 5-ethyl-6'-(1-hydroxyethyl)-(1,1'-biphenyl)-2,3,3'-triol and 2-ethyl-4,6,9-trimethyl-6H-benzo(c) chromene are major products in both gas-phase and liquid-phase reactions, and (2Z, 4Z)-4-ethyl-6-oxohexa-2,4-dienoic acid is also one of the major products in gas-phase reactions. Toxicological predictions indicate that the ecotoxicity of 4-ethyl-6-methyl-6H-benzo(c)chromene-1,9-diol, 2-ethyl-6-methyl-6H-benzo(c)chromene-3,9-diol, and 2-ethyl-4,6,9-trimethyl-6H-benzo(c) chromene surpassed that of 4-EP. However, the toxicity of the reaction products is reduced in the presence of NO_x. This investigation provides an exhaustive theoretical foundation for comprehending the degradation behavior of 4-EP and underscores the need to consider various environmental factors in assessing the potential risk of biomass combustion by products.