Applied Catalysis B: Environmental 185 (2016) 22–30 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Insights into the mechanism of heterogeneous activation of persulfate with a clay/iron-based catalyst under visible LED light irradiation Yaowen Gao a,b , Zhuoyue Zhang a , Simiao Li a , Jin Liu a , Linyu Yao a , Yixi Li a , Hui Zhang a,b,∗ a Department of Environmental Engineering, Wuhan University, Wuhan 430079, China b Shenzhen Research Institute of Wuhan University, Shenzhen 518057, China a r t i c l e i n f o Article history: Received 20 September 2015 Received in revised form 22 November 2015 Accepted 1 December 2015 Available online 2 December 2015 Keywords: Kaolinite Visible LED light Persulfate Photocatalytic degradation Near-neutral pH a b s t r a c t Activation of persulfate (PS) by ultraviolet (UV) light or metal catalysts has been extensively studied, however, little is known about the activation of persulfate by clay-based catalysts in the presence of visible light-emitting diode (LED) irradiation. Herein, a novel kaolinite-supported iron oxide (K-Fe)/PS/Vis process for the degradation of Rhodamine B (RhB) from aqueous solution is reported. It was found that although persulfate can degrade RhB via a non-radical reaction, the excited RhB molecule (RhB * ) and the Fe(II) species formed on the catalyst surface can effectively activate persulfate to generate radicals which degrade RhB under visible light irradiation. On the basis of quenching experiments and electron paramagnetic resonance (EPR) studies, it is suggested that the free radicals produced from persulfate coupled with the surface-adsorbed radicals formed on the catalyst were responsible for the degradation of the dye via RhB * . Moreover, the K-Fe catalyst showed excellent reusability and stability with a low level of iron leaching. The findings of this work demonstrate a new pathway for activation of persulfate, which could effectively degrade organic pollutants and also provide some new insights into persulfate remediation of contaminated water. © 2015 Elsevier B.V. All rights reserved. 1. Introduction In recent years, advanced oxidation processes (AOPs) have been the subject of increasing attention, as shown by the large body of fundamental and applied research work [1–3]. Indeed, AOPs consti- tute promising, efficient and environmentally-friendly methods to treat water contaminated by recalcitrant organic pollutants involv- ing the generation of highly reactive radicals. Activated persulfate (PS, S 2 O 8 2− ) oxidation is regarded as an emerging technology which is gaining importance in water treatment applications, due to PS being relatively cheap, highly soluble in water and stable at ambient temperatures [4–10]. In previous work, the sulfate radical (SO 4 •− ), a strong oxidant with an oxidation–reduction potential of 2.5–3.1 V, was generated through activation of PS by ultraviolet (UV) radia- tion, heat, quinone, or transition metal catalysis, and the produced SO 4 •− can rapidly react with the organic compounds with second- order rate constants in the range of 10 7 –10 9 M −1 s −1 [10]. Although PS can be activated by UV radiation and heat to gener- ate SO 4 •− , these techniques are restricted in practical applications ∗ Corresponding author at: Department of Environmental Engineering, Wuhan University, Wuhan 430079, China. Fax: +86 27 68778893. E-mail address: eeng@whu.edu.cn (H. Zhang). owing to the requirement of high energy input [11,12]. Recently, transition metals have been utilized to activate PS for the degra- dation of pollutants in aqueous media [13]. Among the various transition metals, iron (often as dissolved Fe 2+ ) is the most com- monly used metallic element for the activation of PS because of its non-toxicity, abundance and effectiveness [5,14–16]. A drawback to the homogeneous reaction involving dissolved ferrous iron is the problem of rapid oxidation and precipitation as ferric iron, which inactivates ferrous iron and incurs additional operation costs for the subsequent removal of iron sludge from the treated water after the reaction. To address or minimize this problem caused by the presence of residual iron ions in solution and to reduce the recovery cost, the use of heterogeneous catalysis is a promising alternative that could allow conventional operation without the need for soluble iron salts and, hopefully, at near-neutral pH and at ambient tem- peratures [17]. Various iron-bearing catalysts, such as bulk catalysts containing iron (goethite, hematite and magnetite [18]), can be employed for this process but another approach is the incorpora- tion of iron into various supports and mesoporous silica, zeolite and carbon have been proposed [19–21], but clays offer an interesting alternative source of the required iron. Using more or less simple techniques, including cationic exchange, pillaring and impregnation processes, the active iron http://dx.doi.org/10.1016/j.apcatb.2015.12.002 0926-3373/© 2015 Elsevier B.V. All rights reserved.