1767 Korean J. Chem. Eng., 36(11), 1767-1779 (2019) DOI: 10.1007/s11814-019-0398-4 REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: susinny82@korea.ac.kr Copyright by The Korean Institute of Chemical Engineers. Heterogeneous metals and metal-free carbon materials for oxidative degradation through persulfate activation: A review of heterogeneous catalytic activation of persulfate related to oxidation mechanism Yong-Yoon Ahn and EunTae Yun † School of Civil, Environmental, and Architectural Engineering, Korea University, Seoul 02841, Korea (Received 6 August 2019 • accepted 1 October 2019) Abstract-Activated persulfate has gained substantial interest owing to its potential removal efficiency toward recalci- trant organic pollutants, including pharmaceuticals, pesticide products, and industrial dyes. Various investigations into the activation of persulfate have been conducted to achieve a higher oxidation efficiency. Specifically, heterogeneous catalysts have been extensively applied to the activation of persulfate, enabling a more simplistic and practical method compared to other approaches. Therefore, the present review concentrates closely on various types of heterogeneous activators for the activation of persulfate and its proposed oxidation mechanisms. Keywords: Heterogeneous Catalysts, Activation of Persulfate, Carbonaceous Catalysts, Heterogeneous Transition Metals INTRODUCTION As an increasingly widespread environmental problem, water pollution has become the most dangerous threat jeopardizing the future of humanity. In fact, urbanization and industrialization have resulted in the discharge of massive amounts of contaminants into the environment. For a remediation of contaminated water, physi- cochemical water treatment, such as separation and oxidation (in- cluding biotechnology), has been introduced to remove such waste materials [1]. Oxidation technology, in particular, has become a promising water treatment method, chemically transforming organic pollutants into minerals such as water and carbon dioxide. Various oxidants have been used for the oxidation of water pol- lutants. Persulfate (collectively indicated as peroxymonosulfate (PMS, HSO 5 - ) and peroxydisulfate (PDS, S 2 O 8 2- )) has been intensively in- vestigated for the purification of contaminated water for several rea- sons (including easy control, non-dependence on the pH, and a strong redox potential (E 0 (HSO 5 - /SO 4 2- )=1.75 V NHE [2] and E 0 (S 2 O 8 2- / SO 4 2- )=1.96 V NHE [3])). Furthermore, the sulfate radical (SO 4 • - , E 0 (SO 4 •- /SO 4 2- )=2.43 V NHE [4]), a strong and effective oxidant, can be generated from the activation of persulfate. In fact, PDS has effec- tively remediated soil and groundwater through in situ chemical oxidation [5-8]. In addition, PMS has shown a high oxidation effi- ciency toward organic substances such as pesticides [9], pharmaceu- ticals [10], and dyes [11]. Based on many different studies, SO 4 •- is believed to be responsible for the oxidation mechanism because the hemolytic or heterolytic cleavage of persulfate bonds through an energy or electron transfer causes the production of SO 4 •- . The breaking of a peroxide bond has been carried out using vari- ous activation processes. Thermolysis [12,13], sonolysis [14], and pho- tolysis [15] are representative methods for an energy transfer activation. The catalysis through an electron transfer activation method has attracted the interest of researchers owing to the lack of an external energy requirement, a fast reaction rate, and its effectiveness. Spe- cifically, a heterogeneous catalyst has been intensively studied be- cause it can be separated from a water environment after use. In addition, a heterogeneous catalyst can be modified in various ways to improve the catalytic activity. Metal species and carbon materi- als have mainly been utilized in the development of a heterogeneous catalyst for persulfate activation. Oh et al. systematically summa- rized the synthesis and preparation methods of different heteroge- neous catalysts [16]. An SO 4 •- based advanced oxidation process (SR-AOP) was recently studied as an alternative technology to a conventional hydroxyl radi- cal ( • OH) based oxidation process (HR-AOP) owing to the unique properties of SO 4 •- . Many metal- and carbon-based heterogeneous catalysts have been developed and tested to activate a persulfate for SR-AOP. However, oxidation, which does not rely on the reactivity of SO 4 •- , has been reported in different research articles, contrasted with the scheme of SR-AOP. Such aspects are based on the follow- ing observations: (i) no radical scavenging effect and (ii) no spe- cific electron paramagnetic resonance (EPR) spectrum detection using 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Although the cor- rect role of a heterogeneous catalyst (such as the electron donor behavior of metal species in a Fenton-like reaction) has yet to be fully understood, two possible explanations regarding the non-radi- cal aspect have been proposed according to the experimental evi- dence. First, electron transfer mediation was suggested because oxi- dation occurs only if the three components (electron acceptor (per- sulfate), electron transfer mediator (heterogeneous catalyst), and elec- tron donor (pollutant)) coexist. Second, a singlet oxygen generation was suggested owing to the observations of the quenching effect using L-histidine and azide ions and the identifiable specific EPR signal of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). Such sug-