Theoretical Study of Ozonation of Methylparaben and Ethylparaben in Aqueous Solution.

Parabens are widely employed in toothpaste, cosmetics, textiles, beverages, and preservatives, causing a serious environmental concern because they are endocrine-disrupting compounds (EDCs). As one of the highly reactive oxidants, ozone has a great effect on EDC removal. To understand the degradation and transformation of parabens in the aquatic environment and their toxicity to aquatic organisms, the degradation reaction of parabens initiated by O3 was studied meticulously using quantum chemical calculations. The degradation process includes multiple initial reaction channels and consequent degradation pathways of the Criegee intermediates. Through thermodynamic data, the rate constants were computed using the transition state theory (TST). At a temperature of 298 K and a pressure of 1 atm, the calculated rate constants were 3.92 and 3.94 M-1 s-1 for methylparaben (MPB) and ethylparaben (EPB), respectively. The rate constants increased as the temperature increased or as the length of the alkyl chain on the benzene ring increased. Through the ecotoxicity assessment procedure, the ecotoxicity of parabens and the products in the degradation process can be assessed. Most degradation byproducts are either less toxic or nontoxic. Some byproducts are still harmful, such as oxalaldehyde (P2) and ethyl 2,3-dioxopropanoate (P10). Furthermore, the ecological toxicity of parabens increased with augmentation of the alkyl chain on the benzene ring. The effect of the alkyl chain length on the benzene ring in the compound cannot be ignored.

Effect of pH on ·OH-induced degradation progress of syringol/syringaldehyde and health effect.

Syringol and syringaldehyde are widely present pollutants in atmosphere and wastewater due to lignin pyrolysis and draining of pulp mill effluents. The hydroxylation degradation mechanisms and kinetics and health effect assessment of them under high and low-NOx regimes in atmosphere and wastewater have been studied theoretically. The effect of pH on reaction mechanisms and rate constants in their ·OH-initiated degradation processes has been fully investigated. Results have suggested that aqueous solution played a positive role in the ·OH-initiated degradation reactions by decreasing the energy barriers of most reactions and changing the reactivity order of initial reactions. For Sy- and Sya- (anionic species of syringol and syringaldehyde), most initial reaction routes were more likely to occur than that of HSy and Hsya (neutral species of syringol and syringaldehyde). As the pH increased from 1 to 14, the overall rate constants (at 298 K) of syringol and syringaldehyde with ·OH in wastewater increased from 5.43 × 1010 to 9.87 × 1010 M-1 s-1 and from 3.70 × 1010 to 1.14 × 1011 M-1 s-1, respectively. In the NOx-rich environment, 4-nitrosyringol was the most favorable product, while ring-opening oxygenated chemicals were the most favorable products in the NOx-poor environment. On the whole, the NOx-poor environment could decrease the toxicities during the hydroxylation processes of syringol and syringaldehyde, which was the opposite in a NOx-rich environment. ·OH played an important role in the methoxyphenols degradation and its conversion into harmless compounds in the NOx-poor environment.