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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 principle of unreducible metals regulating oxidation mechanism was revealed. • ROS was altered from radical to non-radical after doping unreducible metals. • The reducibility of Cu3+ are dependent on different unreducible metals. • O v raised pH pzc of CuM@Fe 3 O 4 and promoted the adsorption and activation of PMS. In this paper, we surprisingly found that the incorporation of unreducible metal oxides M x O y (M = Mg, Zn, Ca, Ba, Al) onto CuO hybrid magnetic nano ferric oxide (Cu@Fe 3 O 4) may alter the reaction pathway in persulfate activation, and increase the reaction rate constant. The activation of peroxymonosulfate (PMS) by Cu@Fe 3 O 4 led to a classic sulfate radical based oxidation process (SR-AOP) with an acetaminophen (ACE) degradation rate constant of 0.004 min−1, while 1O 2 -dominated nonradical oxidation process was disclosed in CuM@Fe 3 O 4 with wildly fluctuated reaction rate constants from 0.003 to 0.242 min−1. Mechanism studies indicated that singlet oxygen (1O 2) derived from the direct oxidation of superoxide anions radicals (O 2 −) or the recombination of O 2 − was the main reactive oxygen species (ROS) in CuM@Fe 3 O 4 /PMS system. A series of characterization experiments (pH pzc tests, XPS, H 2 -TPR, et al.) and DFT calculation disclosed that the addition of an unreducible metal M yielded many positive effects: (1) the formation of surface oxygen vacancies (O V) raised the zero point charge (pH pzc) of CuM@Fe 3 O 4 , thus enhanced the adsorption and activation of PMS; (2) promoting the generation of a new Cu species (Cu3+) on the surface of CuM@Fe 3 O 4 , which then participated in the generation of 1O 2. The different reducibility of Cu3+ led to differences in the catalytic properties of CuM@Fe 3 O 4. In addition, the effects of various water matrix species and the results of reusability experiment, mineralization experiment, and ecotoxicity test exhibited that CuM@Fe 3 O 4 /PMS system possessed excellent practical application value. [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]