Graphene: A new activator of sodium persulfate for the advanced oxidation of parabens in water Leonidas Bekris a , Zacharias Frontistis a, * , George Trakakis b , Lamprini Sygellou b , Costas Galiotis a, b , Dionissios Mantzavinos a a Department of Chemical Engineering, University of Patras, Caratheodory 1, University Campus, GR-26504 Patras, Greece b Institute of Chemical Engineering Sciences, Foundation for Research and Technology, PO Box 1414, GR-26504 Patras, Greece article info Article history: Received 9 May 2017 Received in revised form 19 August 2017 Accepted 9 September 2017 Available online 13 September 2017 Keywords: Graphene Carbocatalysis AOPs Sulfate radicals Hydroxyl radicals Water abstract Graphene was successfully employed as a catalyst for the activation of sodium persulfate, towards the effective degradation of propylparaben, an emerging micro-pollutant, representative of the parabens family. A novel process is proposed which utilizes a commercial graphene nano-powder as the catalyst and sodium persulfate as the oxidizing agent. It was found that over 95% of micro-pollutant degradation occurs within 15 min of reaction time. The effects of catalyst loading (75 mg/L to 1 g/L), sodium per- sulfate (SPS) concentration (10 mg/L to 1 g/L), initial solution pH (3e9) and initial paraben concentration (0.5 mg/L to 5 mg/L) were examined. Experiments were carried out in different aqueous conditions, including ultrapure water, bottled water and wastewater in order to investigate their effect on the degradation rate. The efficiency of the process was lower at complex water matrices signifying the role of organic matter as scavenger of the oxidant species. The role of radical scavengers was also investigated through the addition of methanol and tert-butanol in several concentrations, which was found to be important only in relatively high values. An experiment in which propylparaben was substituted by methylparaben was conducted and similar results were obtained. The consumption of SPS was found to be high in all pH conditions tested, surpassing 80% in near neutral environment. However, the results indicate that the sulfate radicals formed react with water in alkaline conditions, which are the optimal for the reaction, producing hydroxyl radicals which appear to be the dominant species leading to the rapid degradation of propylparaben. To the best of our knowledge, this is the first time pristine graphene has been implemented as an activator of sodium persulfate for the effective oxidation of micro-pollutants. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Transition and precious metals have been used extensively as heterogeneous catalysts. However, the cost and limited resources of some of these metals, make the production and use of such cata- lysts prohibitive. For the sake of sustainability and economic con- siderations, researchers have shifted their focus to metal-free catalysis, a field in which carbon materials are currently dominant (Chua and Pumera, 2015). In view of that, carbocatalysis, a form of catalysis that uses carbon and carbon-based materials has now emerged. Although this is not a new concept since carbocatalysts derived from biomass have been investigated extensively, the discovery of graphene and graphene-related materials (GRM) have opened up new possibilities in that field (Navalon et al., 2014). Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) hexagonal honeycomb lattice (Novoselov et al., 2004; Geim and Novoselov, 2007). It is the basic building block for graphitic materials of all other dimensionalities such as graphite, charcoal, carbon nanotubes and fullerenes (Geim and Novoselov, 2007). Single-layer graphene is defined as a single two-dimensional hexagonal sheet of carbon atoms. Bi-layer and few-layer graphenes normally have 2 and up to 10 layers of such two-dimensional sheets, respectively (Choi et al., 2010). Graphene structures that are consisted of more than 10 layers cannot be easily distinguished in terms of properties from graphite and are considered graphitic nano-particles or nano-graphites (Geim and Novoselov, 2007; Choi et al., 2010). All these materials, including their oxidized forms, are included in the family of graphene related * Corresponding author. E-mail address: zfrontistis@chemeng.upatras.gr (Z. Frontistis). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres http://dx.doi.org/10.1016/j.watres.2017.09.020 0043-1354/© 2017 Elsevier Ltd. All rights reserved. Water Research 126 (2017) 111e121