Experimental and theoretical studies into the hydroxyl radical mediated transformation of propylparaben to methylparaben in the presence of dissolved organic matter surrogate

Abstract

Dissolved humic and biogenic substances might be present in the aphotic zone and contribute to the fate of parabens in natural aquatic ecosystem under the fluctuation of water multi-parameters. Through the combination of batch tests with quantum chemical calculation, hydroxyl radical (•OH) mediated degradation of propylparaben (PP) to methylparaben (MP) has been confirmed in the present study. The interaction of dissolved oxygen with environmental relevant concentration of humic acid (HA), algal and bacterial cell lysis leads to a slow production of •OH. Aqueous PP undergoes a mild removal process with the pseudo-first order rate constant (10−7, s−1) higher at 7.43 in HA than at 3.30-4.89 in biogenic cell lysis. PP removal is correlated with the aromaticity of DOM surrogate and the produced •OH concentration, which could be enhanced by the increase of light intensity and DO other than HA. The •OH mediated process on PP removal has been confirmed by the linearly inhibited effect of tert-butanol while totally inhibited effects of higher concentration of sodium azide and co-existent chemical (17β-estradiol). Based on the detection of byproduct MP, two possible reaction pathways, •OH attacking at β-carbon (path-β) and terminal γ-carbon (path-γ) of the propyl side chain of PP, are proposed. Through the analysis of thermal and kinetics parameters, the •OH initiated H-abstraction and the resulting C-C bond cleavage leading to the formation of MP and acetaldehyde in path-β is confirmed to be the dominant reaction mechanism. Considering the universal occurrence of parabens and these DOM surrogates, this mild removal process has special implications for the self-purification of organic pollutants in natural aquatic ecosystems, especially in DOM-rich matrices in the aphotic zone.