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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 MnO2-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 (1O2) 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 1O2 was unveiled by radical scavenging tests, electron paramagnetic resonance (EPR), and solvent-exchange experiment (from H2O to D2O). Moreover, superoxide radical (O2•−) generated from the reaction between PMS and metastable manganese intermediate was identified as the precursor of 1O2. O2•− 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 1O2. 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 O2•− adsorption–desorption process was uncovered.

To meet the current challenges of waste water, an efficient, environment friendly and economical treatment process is always desirable for the degradation of toxic organic compounds. Herein, we reported a stable and active bimetallic CuOx@Co-LDH catalyst for the degradation of phenol using persulfate (PS) under mild conditions. The system of PS/CuOx@Co-LDH completely degraded 0.1 mM of phenol in less than 40 min while at the same time, all the other tested catalysts including catalyst precursors remained sluggish. In view of the practical application, CuOx@Co-LDH/PS system demonstrated consistent performance over a broad pH range (pH 5.0–12.0), during which minimum leaching could be detected. Moreover, good resistance to various inorganic anions was demonstrated in CuOx@Co-LDH/PS system. The consistent performance of CuOx@Co-LDH/PS system may originate from the constant buffer pH of catalyst (9.5 ± 0.8), minimum leaching and the generation of diverse free radicals ( SO4−, OH or − O2) under different pH conditions. Additionally, LDH structure offered strong synergetic interactions among active components which might also help in minimum leaching and good reactivity of CuOx@Co-LDH catalyst. The results of radical scavengers, EPR and XPS studies collectively proved the mechanism of diverse free radicals, which was accounted to the redox cyclic reactions of active sites (Cu(III)–Cu(II)–Cu(III) and Co(III)–Co(II)–Co(III)) during degradation of phenol.

Over the past, extensive works executed to treat anionic and cationic heavy metal ions from wastewater. Although very few adsorbents were developed to adsorb anionic and cationic heavy metal ions from the same system. In this study, we explored the pH tunable complete reduction plus adsorption mechanism of Pb2+ and Cr2O72− by dithiocarbamate-based composite. The pH study recommended that; adsorption of Pb2+ occurred at pH 5.5, besides Cr2O72− reduction coupled adsorption executed at pH 3. Furthermore, quick adsorption occurred within 60 min with adsorption capacities of 228.69 and 219.75 mg g−1 for Pb2+ and Cr2O72−, respectively. Adsorption isotherm well described by the Redlich-Peterson model, also adsorption processes were fitted by pseudo-second-order kinetics model. Interestingly, characteristic techniques, a physical monolayer model and thermodynamic models were applied to the experimental results to further understand the adsorption mechanisms. The results indicated that the physical model with two types of interaction energies was more appropriate to interpret the adsorption mechanism. Based on the physical model, three thermodynamic functions were calculated and interpreted to attribute new macroscopic interpretations at the molecular level of the adsorption systems. Inspiringly, the metal-composite system highly effective for 5 regeneration cycles and fixed-bed column could decontaminate 17.2 L of ~10 mg L−1 Pb2+ and 7.3 L of ~10 mg L−1 Cr2O72− to the EPA’s wastewater discharge limit by 5 g of CSC@SDBC with the yielding of 400 and 346 mL of eluent, respectively. Theoretically, a liter of CSC@SDBC column may efficiently adsorb 145,646 mg of Pb2+ and 57,518 mg of Cr2O72− from industrial effluent, these were calculated from the Bohart-Adams model. These collective batch and fixed-bed systems were cost-effective and environmentally tolerable techniques to remove anionic and cationic heavy metal ions from industrial wastewater.