Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants: Review

Recently sulfate radical-based advanced oxidation processes (SR-AOPs) attract increasing attention due to their capability and adaptability in decontamination. The couple of cobalt and peroxymonosulfate (PMS) is an efficient way to produce reactive sulfate radicals. This article reviews the state-of-the-art progress on various heterogeneous cobalt-based catalysts for PMS activation, including cobalt oxides, cobalt-ferrite and supported cobalt by diverse substrates. We summarize the intrinsic properties of these catalysts and their fundamental behaviors in PMS activation, as well as synthetic approaches. In addition, influencing factors and synergistic techniques of Co/PMS systems in organic degradation and possible environmental applications are also discussed. Finally, we propose perspectives on challenges related to cobalt-based catalysts, heterogeneous Co/PMS systems and their potential applications in practical environmental cleanup.

Cobalt-catalyzed sulfate radical-based advanced oxidation: A review on heterogeneous catalysts and applications

The Fenton-like process presents one of the most promising strategies to generate reactive oxygen-containing radicals to deal with the ever-growing environmental pollution. However, developing improved catalysts with adequate activity and stability is still a long-term goal for practical application. Herein, we demonstrate single cobalt atoms anchored on porous N-doped graphene with dual reaction sites as highly reactive and stable Fenton-like catalysts for efficient catalytic oxidation of recalcitrant organics via activation of peroxymonosulfate (PMS). Our experiments and density functional theory (DFT) calculations show that the CoN4 site with a single Co atom serves as the active site with optimal binding energy for PMS activation, while the adjacent pyrrolic N site adsorbs organic molecules. The dual reaction sites greatly reduce the migration distance of the active singlet oxygen produced from PMS activation and thus improve the Fenton-like catalytic performance.

Single Cobalt Atoms Anchored on Porous N-Doped Graphene with Dual Reaction Sites for Efficient Fenton-like Catalysis

N-Doped graphene (NG) nanomaterials were synthesized by directly annealing graphene oxide (GO) with a novel nitrogen precursor of melamine. A high N-doping level, 8–11 at. %, was achieved at a moderate temperature. The sample of NG-700, obtained at a calcination temperature of 700 °C, showed the highest efficiency in degradation of phenol solutions by metal-free catalytic activation of peroxymonosulfate (PMS). The catalytic activity of the N-doped rGO (NG-700) was about 80 times higher than that of undoped rGO in phenol degradation. Moreover, the activity of NG-700 was 18.5 times higher than that of the most popular metal-based catalyst of nanocrystalline Co3O4 in PMS activation. Theoretical calculations using spin–unrestricted density functional theory (DFT) were carried out to probe the active sites for PMS activation on N-doped graphene. In addition, experimental detection of generated radicals using electron paramagnetic resonance (EPR) and competitive radical reactions was performed to reveal the PMS...

Nitrogen-Doped Graphene for Generation and Evolution of Reactive Radicals by Metal-Free Catalysis

S ulfur and nitrogen co-doped reduced graphene oxide (rGO) is synthesized by a facile method and demonstrated remarkably enhanced activities in metal-free activation of peroxymonosulfate (PMS) for catalytic oxidation of phenol. Based on fi rst-order kinetic model, S‐N co-doped rGO (SNG) presents an apparent reaction rate constant of 0.043 ± 0.002 min −1 , which is 86.6, 22.8, 19.7, and 4.5-fold as high as that over graphene oxide (GO), rGO, S-doped rGO (S-rGO), and N-doped rGO (N-rGO), respectively. A variety of characterization techniques and density functional theory calculations are employed to investigate the synergistic effect of sulfur and nitrogen co-doping. Co-doping of rGO at an optimal sulfur loading can effectively break the inertness of carbon systems, activate the sp 2 -hybridized carbon lattice and facilitate the electron transfer from covalent graphene sheets for PMS activation. Moreover, both electron paramagnetic resonance (EPR) spectroscopy and classical quenching tests are employed to investigate the generation and evolution of reactive radicals on the SNG sample for phenol catalytic oxidation. This study presents a novel metal-free catalyst for green remediation of organic pollutants in water.

Sulfur and nitrogen co-doped graphene for metal-free catalytic oxidation reactions

Catalytic activation of peroxymonosulfate (PMS) to generate radicals has received considerable and increasing attention in environmental catalysis field. The development of highly efficient and environmentally friendly oxidation processes has been one of the most important and greatly challenging goals in PMS activation field. Herein, an effective and stable catalyst (ACFs-CoPc) was proposed for PMS activation by supporting cobalt phthalocyanine (CoPc) onto activated carbon fibers (ACFs) covalently. With the dye of Acid Red 1 (AR1) as probe compound, ACFs-CoPc could effectively activate PMS to eliminate AR1, and the catalytic activity of ACFs-CoPc was almost not impaired during ten successive runs without detectable cobalt leaching, indicating that ACFs-CoPc/PMS system was a pro-environmental and efficient oxidation system. A hybrid method that combines electron paramagnetic resonance (EPR) technology with different radical scavengers effect on PMS activation was employed for the investigation of active species during the catalytic reaction. It was found that both hydroxyl and sulfate radical (OH, SO4−) were generated; importantly, SO4− was speculated to serve as the dominant active species in the catalytic oxidation of AR1. Based on these results, a possible mechanism was proposed.

Novel green activation processes and mechanism of peroxymonosulfate based on supported cobalt phthalocyanine catalyst

BACKGROUND

The development of effective and green peroxymonosulfate (PMS) activation processes has attracted considerable attention and still remains a great challenge in the environmental field. Herein, the cobalt catalyst was innovatively supported onto activated carbon fibers (ACFs) to construct an outstanding heterogeneous catalyst, Co@ACFs, for PMS activation.



RESULTS

Co@ACFs could effectively activate PMS to decompose dyes with about 100% removal rate in 35 min. Co@ACFs maintained its catalytic ability without detectable Co leaching during repeated batch experiments. More importantly, the catalyst presented an enhanced decomposition performance over most reported heterogeneous Co catalysts due to the significant role of ACFs: (i) high adsorption capacity in favor of the enhanced catalytic activity; (ii) ACFs as an electron donor to accelerate CoII/CoIII cycle thereby speeding up degradation. Combining electron paramagnetic resonance (EPR) with radical scavengers, it was found that both sulfate and hydroxyl radicals (SO4•-, •OH) were responsible for the degradation of dyes. Unexpectedly, •OH was the dominant radical, different from the reported mechanism in typical Co/PMS.



CONCLUSION

This study not only paves the way to further develop highly efficient and green PMS activation processes, but also provides a new practical method for application to wastewater remediation. © 2015 Society of Chemical Industry

Haiwei Bao

Papers

Novel green activation processes and mechanism of peroxymonosulfate based on supported cobalt phthalocyanine catalyst

Key role of activated carbon fibers in enhanced decomposition of pollutants using heterogeneous cobalt/peroxymonosulfate system