ADVANCES AND TRENDS IN ADVANCED OXIDATION PROCESSES Degradation of ethyl paraben by heat-activated persulfate oxidation: statistical evaluation of operating factors and transformation pathways Zacharias Frontistis 1 & Maria Antonopoulou 2 & Ioannis Konstantinou 3 & Dionissios Mantzavinos 1 Received: 24 March 2016 /Accepted: 24 May 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract 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 persul- fate oxidation. All individual effects, except the solution pH, were statistically significant and so were the second-order inter- actions 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 detect- ed as TBPs of EP. Degradation occurred through (i) hydroxyl- ation, (ii) dealkylation, and (iii) oligomerization reactions leading to TBPs with ether and biphenyl structures. Oligomerization re- actions 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 Parabens . Factorial design . Sulfate radicals . By-products . Mechanism . Thermal activation Introduction In recent years, the occurrence, toxicity, and removal of person- al care products and pharmaceuticals, which are considered as contaminants of emerging concern in the water cycle, have been extensively studied (Belgiorno et al. 2007). In most cases, conventional biological treatment is not able to completely eliminate these compounds (Luo et al. 2014). Consequently, micro-pollutants can accumulate in the environment, where they can cause adverse effects due to their inherent biological potency toward organisms even at extremely low concentra- tions (Belgiorno et al. 2007). Among other xenobiotics, parabens, the alkyl esters of p- hydroxybenzoic acid, have spurred the interest of the scientific community. Parabens are preservatives mainly used in food and personal care products including, among others, products for children (Boberg et al. 2010). In a recent study (Larsson et al. 2014), early-morning urine samples from 79 mother- child pairs living in either a rural or an urban area were ana- lyzed; ethyl paraben (EP) was detected in levels above the detection limit (0.4 μg/L) in 95 % of the samples taken from mothers and 77 % of the samples taken from children. Advanced oxidation processes (AOPs) are a group of pro- cesses based on the in situ production of very reactive species (radicals), and they are considered as a promising alternative for micro-pollutant degradation in wastewater treatment. Among other AOPs, the sulfate radical AOP has been discussed in the literature as an efficient and affordable Responsible editor: Vítor Pais Vilar * Zacharias Frontistis zfrontistis@chemeng.upatras.gr 1 Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, 26504 Patras, Greece 2 Department of Environmental and Natural Resources Management, University of Patras, 2 Seferi St., 30100 Agrinio, Greece 3 Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece Environ Sci Pollut Res DOI 10.1007/s11356-016-6974-9