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The agglomeration of rGO resulted from the morphological irregularities restrict its direct applications as catalyst for pollutants removal using advance oxidation processes (AOPs). In this study, we addressed this problem after incorporating the nanosheets of reduced graphene oxide (rGO) in the layers of hydroxides (LDHs) as Fe-rGO LDH hetero-composite. Based on the first order kinetic model, the fabricated composite showed excellent phenol removal with degradation rate constant of 0.16 min−1, 5–100 times higher than Fe-LDH/rGO or other control catalysts. This enhanced catalytic activity was resulted from the systematic assembly of both monolayered rGO and LDHs as superlattice materials, avoiding the aggregation of rGO during the reaction and thus promote the interfacial electron transfer among the active sites. Moreover, Fe-rGO LDH/PMS system exhibited consistent performance over a wide pH range (3.0 ─ 12.0) and broad spectrum of co-existing water constituents (anions and humic acid). The spectroscopic and analytical results suggested that the special arrangement of monolayered rGO in Fe-LDHs facilitated Fe sites to form a stable Fe intermediates after reacting PMS, offering a suitable centre to accept electron from another PMS to generate superoxides (•O 2 −). Finally, singlet oxygen (1O 2) was accounted as dominant reactive oxygen species (ROS) for pollutants removal. Image 1 [ABSTRACT FROM AUTHOR]

[Display omitted] • A step-by-step oxidation process including electron transfer and 1O 2 was proposed. • Electron transfer and 1O 2 occurred on OMS-2 surface and in solution, respectively. • O 2 •− on the surface of OMS-2 could significantly extended its half-life. • The step-by-step oxidation driven by the adsorption-desorption of O 2 •− on OMS-2. This study proposed a step-by-step oxidation process based on the in-depth analysis of the catalytic mechanism of peroxymonosulfate (PMS) activated by the common MnO 2 -based catalyst-manganese octahedral molecular sieve (OMS-2). In the first stage, OMS-2-mediated electron transfer dominated the oxidation process. When PMS was completely consumed, the reaction entered the second stage, at which singlet oxygen (1O 2) gradually turned into primary oxidation source. OMS-2-mediated electron transfer was proved based on the results of electrochemical analysis, phosphate substitution experiments, and Raman tests. Meanwhile, the oxidation process of 1O 2 was unveiled by radical scavenging tests, electron paramagnetic resonance (EPR), and solvent-exchange experiment (from H 2 O to D 2 O). Moreover, superoxide radical (O 2 •−) generated from the reaction between PMS and metastable manganese intermediate was identified as the precursor of 1O 2. O 2 •− generated in the first stage could significantly extend its half-life by adsorbing on the (2 1 1) plane of OMS-2, and then desorbed in the second stage and contributed to the formation of 1O 2. More importantly, through the transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and DFT calculations, the essence of step-by-step oxidation driven by O 2 •− adsorption–desorption process was uncovered. [ABSTRACT FROM AUTHOR]

• The nonradical mechanisms in PMS-based heterogeneous catalytic system were arranged. • The evolutionary pathways were summarized and analyzed. • Common identification methods were summarized and evaluated. • Nonradical exhibited its unique oxidation advantages. • Nonradical has extensive application prospect in environmental remediation. Recently, various nonradical oxidation mechanisms different from traditional free radicals have been discovered in peroxymonosulfate (PMS)-based heterogeneous catalytic system. These nonradical oxidation mechanisms have given rise to much controversy in many aspects due to their variety and complex evolutionary pathways. In this paper, we systematically summarized and compared these nonradical oxidation mechanisms. Mainly including the evolutionary pathways, identification methods (qualitative and quantitative), and oxidation characteristics. Moreover, the challenges and strategies for the further development of these oxidation mechanisms, as well as the research and application prospects are proposed. Our work can deepen systematic understanding of the nonradical oxidation mechanism among relevant researchers, and provides some enlightenment for using nonradical oxidation mechanism to deal with the environmental crisis. [ABSTRACT FROM AUTHOR]

• Cu-rGO LDH nanohybrid fabricated by intercalating rGO inside the LDH galleries. • Cu-rGO LDH exhibited higher surface area and electrical conductivity. • Cu-rGO LDH/PMS induced nonradical pathway. • 1O 2 generated through SO 5 − owing to electron conduction between PMS and Cu2+. A new strategy was applied by periodic stacking of active sites of Cu and reduced graphene oxide (rGO) in the form of Cu-rGO LDH nanohybrid material. The experimental results revealed that newly prepared Cu-rGO LDH nanohybrid material was extremely reactive in PMS activation as evident from the degradation rate of 0.115 min−1, much higher than Mn-rGO LDH (0.071 min−1), Zn-rGO LDH (0.023 min−1) or other benchmarked material used during the degradation of bisphenol A (BPA). This excellent activity of Cu-rGO LDH nanohybrid was attributed to the better PMS utilization efficiency as compared to the other catalysts. Additionally, the characterization techniques disclosed that the layer by layer arrangement of active sites in the Cu-rGO LDH catalyst promotes interfacial electron mobility owing to the synergistic association between Cu in LDH and interlayered rGO. Based on the in-situ electron paramagnetic resonance spectroscopy (EPR) and chemical scavengers, singlet oxygen (1O 2) was unveiled as dominant reactive species for pollutant removal, resulting from the recombination of superoxides (O 2 −) or reduction of active Cu centers. We believe that this novel Cu-rGO LDH/PMS system will open up a new avenue to design efficient metal-carbon nanohybrid catalysts for the degradation of emerging aquatic pollutants in a real application. [ABSTRACT FROM AUTHOR]

• Synergistic effect of Cu and Co oxides was find on various supports. • Incorporation of Cu changed the activation pathway of PMS by Co catalysts. • Surface hydroxyl groups on catalysis was studied quantitatively. • Electron transfer between Cu and Co improved the stoichiometric efficiency of PMS. Synergistic effect was commonly observed in bimetallic oxide catalysts in sulfate radical-based advanced oxidation processes by forming spinel structures. In this paper, we found the incorporation of Cu demonstrated the most remarkable promotional effect to Co oxides comparing with Fe and Mn in PMS activation, and no spinel structure of mixed metals was found. Moreover, the synergism between oxides of Cu and Co was observed on various supports including MgAl layered double hydroxides, mesoporous MnO 2 and ZnO. Under conditions of 0.2 g L−1 catalysts, 2 mM PMS, and 40 ppm 4-chlorophenol (4-CP), CuCo@ZnO/PMS system demonstrated excellent performance that 96.0% 4-CP could be removed, while the removal efficiency of 4-CP in Cu@ZnO/PMS and Co@ZnO/PMS systems was only 12.1% and 45.4%, respectively. The simultaneous incorporation of Co and Cu bimetallic oxides in CuCo@ZnO led to a higher surface hydroxyl density, and this surface hydroxyl density showed a linear relationship to their first order kinetic constant in 4-CP degradation. Moreover, the results of peroxymonosulfate (PMS) decomposition, quantification of sulfate radical (SO 4 −), EPR, XPS, H 2 -TPR, and CV indicated that the synergism of Cu/Co also arose from the electron transfer between Cu(I) and Co(III), which altered the redox circle of Co(II)/Co(III) and increased the steady concentration of oxidative radicals. Due to this synergism, the degradation percentage of organic contaminants, stability of heterogeneous catalysts and the stoichiometric efficiency of PMS were all improved. These findings were highly suggestive to the further development of bimetallic heterogeneous catalysts in persulfate based advanced oxidation processes. [ABSTRACT FROM AUTHOR]