New Carbon Nanomaterials Containing Cobalt Iron Oxide Achieve the Degradation of Levofloxacin (Levo)

Cobalt Iron Oxide is a semi-hard ferrite with the chemical formula CoFe₂O₄. The substance can be thought of as somewhere between soft and hard magnetic materials.

Advanced oxidation processes (SR-AOPs) based on sulfate radicals (SO4⋅^-) are considered to be efficient methods for the degradation and mineralization of refractory pollutants. Persulfate (PS), as a source of SO4⋅^-, is commonly activated with powdered nanocatalysts. However, the heterogeneous electroactivation process can be carried out in situ with low energy consumption and avoids secondary pollution caused by nanoparticles in homogeneous SR-AOPs; at the same time, H₂O₂ can be generated at the cathode to achieve the synergistic effect of H₂O₂ and PMS. . Some researchers constructed a new electrochemically enhanced homogeneous-heterogeneous catalytic system by placing the prepared homogeneous catalyst (CoFe₂O₄/NF) in parallel between the anode and cathode for PMS activation. More than 90% of Levo can be degraded within 40 minutes, with an energy consumption of 2.51 kWh/m³.

It has been confirmed in previous studies that electrospun carbon nanofibers loaded with (CoFe₂O₄ have good degradation efficiency for enrofloxacin (ENR) in a heterogeneous electro-Fenton system. However, high energy consumption (3.07 kWh/m³), low double electron transfer (n = 3.27), and strong acidic reaction conditions will hinder its practical application. In this study, the composite material B, S-Fe/Co@C-NCNFs-900, obtained by co-doping and calcining B and S-containing Fe³⁺ and Co²⁺ nanoelectrospun fibers, was used to achieve the degradation of Levo.

From the SEM image, it was found that the surface of the carbonized pure electrospun PVP was smooth and obviously fibrous. However, when electrospun PVP was mixed with metal salts, it developed a macroporous structure, which may be due to the electrostatic repulsion of metal ions. After calcination at 900°C, granular protrusions grew on the surface of the macroporous structure. With the co-doping of B and S, the size of the porous volume decreases and more buds appear in view, which not only increases the specific surface area but also exposes more catalytically active sites. The atomic ratio of Fe:Co in the EDS spectrum is close to 2:1, indicating that the prepared material contains CoFe₂O₄. The lattice fringes and XRD patterns appearing in the TEM image further verified the generation of CoFe₂O₄ in electrospun nanofibers.

The RS diagram results show that with the doping of B and S, the ID/IG value increases sharply, indicating an increase in the degree of defects. In the high-resolution image, the higher Co²⁺ (51.45%) and Fe³⁺ (68.51%) contents also indicate that the metal oxide present in the composite material is CoFe₂O₄. RRDE tests, EIS spectra and Tafel plots all show that the co-doping of B and S helps to regulate the 2e-ORR path of the composite material, making it have smaller EIS and Tafel values.

The degradation of Levo in the system can be divided into three degradation pathways, involving the entire Levo degradation process such as alcoholization, deoxyzinization, demethylation, depiperazinization, decarboxylation, demethylation and ring opening. First, a hydroxyl group is added to the piperazine ring through an alcoholization process to form P1. Subsequently, P1 undergoes demethylation and depiperazinization processes to produce P2, and further removal of dimethyl and deethyl groups yields P3. Therefore, the removal of the piperazine substituent of Levo has been completed. The second pathway is decarboxylation and removal of the piperazine ring. After decarboxylation and depiperazine processes, intermediate products P4 and P5 are produced in sequence. In the second degradation pathway stage, further demethylation and depiperazine processes generate the intermediate product P6. The third pathway mainly involves the destruction of the oxazine group and the removal of the piperazine ring. The oxazine group is attacked to produce P7, which is then partially ring-opened and oxidized to the piperazine substituent to give P8. Subsequently, further participation in decarboxylation, demethylation, and deethylation resulted in P9. In addition, P9 is also produced by decarboxylation of P7 (to form P8) and further deoxynitrification. Eventually, the ring opening and deep oxidation of P3, P6 and P9 compounds will produce three short-chain carboxylic acids (maleic acid, oxalic acid and fumaric acid), which will eventually be mineralized into CO₂ and H₂O.