A factorial design methodology was implemented to evaluate the importance of ethyl paraben (EP) concentration (500-1500 μg/L), sodium persulfate concentration (400-500 mg/L), temperature (40-60 °C), reaction time (2-30 min), water matrix (pure water or secondary treated wastewater), and initial solution pH (3-9) on EP removal by heat-activated persulfate oxidation. All individual effects, except the solution pH, were statistically significant and so were the second-order interactions of ethyl paraben concentration with temperature or the reaction time. The influence of the water matrix was crucial, and the efficiency of the process was lower in secondary treated wastewater due to the presence of natural organic matter and inorganic salts that compete with ethyl paraben for the reactive oxygen species. Liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) was employed to identify major transformation by-products (TBPs); 13 compounds were detected as TBPs of EP. Degradation occurred through (i) hydroxylation, (ii) dealkylation, and (iii) oligomerization reactions leading to TBPs with ether and biphenyl structures. Oligomerization reactions were found to be the dominant pathway during the first steps of the reaction. The toxicity of 500 μg/L EP in secondary treated wastewater was tested against marine bacteria Vibrio fischeri; toxicity increased during the first minutes due to the production of several TBPs, but it consistently decreased thereafter.
Keywords: By-products; Factorial design; Mechanism; Parabens; Sulfate radicals; Thermal activation.