Activation of Peroxydisulfate on Carbon Nanotubes: Electron-Transfer Mechanism

Degradation of sulfamethoxazole with persulfate using spent coffee grounds biochar as activator

In the present study, biochar from spent coffee grounds was synthesized via pyrolysis at 850 °C for 1 h, characterized and employed as catalyst for the degradation of sulfamethoxazole (SMX) by persulfate activation. A variety of techniques, such as physisorption of N₂, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and potentiometric mass titration, were employed for biochar characterization. The biochar has a surface area of 492 m²/g, its point of zero charge is 6.9, while mineral deposits are limited. SMX degradation experiments were performed mainly in ultrapure water (UPW) at persulfate concentrations between 100 and 1000 mg/L, biochar concentrations between 50 and 200 mg/L, SMX concentrations between 500 and 2000 μg/L and initial solution pH between 3 and 10. Real matrices, besides UPW, were also tested, namely bottled water (BW) and treated wastewater (WW), while synthetic solutions were prepared spiking UPW with bicarbonate, chloride, humic acid or alcohols. Almost complete removal of SMX can be achieved using 200 mg/L biochar and 1000 mg/L sodium persulfate (SPS) within 75 min. The presence of biochar is important for the degradation process, while the activity of the biochar increases linearly with SPS concentration. Degradation follows a pseudo-order kinetic model and the rate increases with increasing biochar concentration and decreasing SMX concentration. Although SMX adsorption onto the biochar surface is favored at acidic conditions, degradation proceeds equally fast regardless of the initial solution pH. Reactions in either real matrix are slower, resulting in 55% SMX removal in 60 min for WW. Bicarbonate causes severe inhibition as only 45% of SMX can be removed within 75 min in UPW. The addition of alcohol slightly inhibits degradation suggesting that the reaction pathway is either under electron transfer control or due to the generation of surface oxygen radicals with higher oxidation potential than the homogeneously produced radicals.

Potential Difference Driving Electron Transfer via Defective Carbon Nanotubes toward Selective Oxidation of Organic Micropollutants

Nanocarbon-based persulfate oxidation emerges as a promising technology for the elimination of organic micropollutants (OMPs). However, the nature of the active site and its working mechanism remain elusive, impeding developments of high-performance oxidative technology for water treatment practice. Here, we report that defect-rich carbon nanotubes (CNTs) exhibit a superior activity in the activation of peroxymonosulfate (PMS) for OMP oxidation. Quantitative structure-activity relationship studies combined with theoretical calculations unveil that the double-vacancy defect on CNTs may be the intrinsic active site, which works as a conductive bridge to facilitate the potential difference-dominated electron transfer from the highest occupied molecular orbital of OMPs to the lowest unoccupied molecular orbital of PMS. Based on this unique mechanism, the established CNTs@PMS oxidative system achieves outstanding selectivity and realizes the target-oriented elimination of specific OMPs in a complicated aquatic environment. This work sheds new light on the mechanism of carbocatalysis for selective oxidation and develops an innovative technology toward remediation of practical wastewater.

Efficient Fenton-like Process for Pollutant Removal in Electron-Rich/Poor Reaction Sites Induced by Surface Oxygen Vacancy over Cobalt-Zinc Oxides

To achieve high efficiency and low consumption for water treatment in the Fenton reaction, we use the surface oxygen vacancies (OVs) as the electron temporary residences to construct a dual-reaction-center (RDC) Fenton-like catalyst with abundant surface electron-rich/poor areas consisting of OV-rich Co-ZnO microparticles (OV-CoZnO MPs). The lattice-doping of Co into ZnO wurtzite results in the formation of OVs with unpaired electrons (electron-rich OVs) and electron-deficient Co³⁺ sites according to the structural and electronic characterizations. Both experimental and theoretical calculations prove that the electron-rich OVs are responsible for the capture and reduction of H₂O₂ to generate hydroxyl radicals, which quickly degrades pollutants, while a large amount of pollutants are adsorbed at the electron-deficient Co³⁺ sites and act as electron donors for the system, accompanied by their own oxidative degradation. The electrons obtained from the pollutants in the electron-deficient sites are transferred to the OVs through the internal bond bridge to achieve the balance of electron gain/loss. Through this process, pollutants are efficiently converted and degraded by multiple pathways in a wide range of pH (4.5-9.5). The reaction rate of the OV-CoZnO MPs/H₂O₂ system is increased by ∼17 times compared with the non-DRC system. This discovery provides a sustainable strategy for pollutant utilization, which shows new implications for solving the troublesome issues of the Fenton reaction and for developing novel environmental remediation technologies.

Sustainable Catalytic Processes Driven by Graphene-Based Materials

In the recent two decades, graphene-based materials have achieved great successes in catalytic processes towards sustainable production of chemicals, fuels and protection of the environment. In graphene, the carbon atoms are packed into a well-defined sp2-hybridized honeycomb lattice, and can be further constructed into other dimensional allotropes such as fullerene, carbon nanotubes, and aerogels. Graphene-based materials possess appealing optical, thermal, and electronic properties, and the graphitic structure is resistant to extreme conditions. Therefore, the green nature and robust framework make the graphene-based materials highly favourable for chemical reactions. More importantly, the open structure of graphene affords a platform to host a diversity of functional groups, dopants, and structural defects, which have been demonstrated to play crucial roles in catalytic processes. In this perspective, we introduced the potential active sites of graphene in green catalysis and showcased the marriage of metal-free carbon materials in chemical synthesis, catalytic oxidation, and environmental remediation. Future research directions are also highlighted in mechanistic investigation and applications of graphene-based materials in other promising catalytic systems.

The Intrinsic Nature of Persulfate Activation and N-Doping in Carbocatalysis

Persulfates activation by carbon nanotubes (CNT) has been evidenced as nonradical systems for oxidation of organic pollutants. Peroxymonosulfate (PMS) and peroxydisulfate (PDS) possess discrepant atomic structures and redox potentials, while the nature of their distinct behaviors in carbocatalytic activation has not been investigated. Herein, we illustrated that the roles of nitrogen species in CNT-based persulfate systems are intrinsically different. In PMS activation mediated by nitrogen-doped CNT (N-CNT), surface chemical modification (N-doping) can profoundly promote the adsorption quantity of PMS, consequently elevate potential of derived nonradical N-CNT-PMS* complexes, and boost organic oxidation efficiency via an electron-transfer regime. In contrast, PDS adsorption was not enhanced upon incorporating N into CNT due to the limited equilibrium adsorption quantity of PDS, leading to a relatively lower oxidative potential of PDS/N-CNT system and a mediocre degradation rate. However, with equivalent persulfate adsorption on N-CNT at a low quantity, PDS/N-CNT exhibited a stronger oxidizing capacity than PMS/N-CNT because of the intrinsic higher redox potential of PDS than PMS. The oxidation rates of the two systems were in great linearity with the potentials of carbon-persulfate* complexes, suggesting N-CNT activation of PMS and PDS shared the similar electron-transfer oxidation mechanism. Therefore, this study provides new insights into the intrinsic roles of heteroatom doping in nanocarbons for persulfates activation and unveils the principles for a rational design of reaction-oriented carbocatalysts for persulfate-based advanced oxidation processes.

The key structural features governing the free radicals and catalytic activity of graphite/graphene oxide

Progressive oxidation modulates the radical content of graphite/graphene oxide.

Removal of contaminants by activating peroxymonosulfate (PMS) using zero valent iron (ZVI)-based bimetallic particles (ZVI/Cu, ZVI/Co, ZVI/Ni, and ZVI/Ag)

In this study, four different ZVI/M-PMS systems (e.g., ZVI/Cu, ZVI/Co, ZVI/Ni and ZVI/Ag) were fabricated to investigate the removal of contaminants (Rhodamine B, 2,4-dichlorophenol, bisphenol A, bisphenol F, levofloxacin, and chloramphenicol).