Pentadecafluorooctanoic acid (PFOA) is environmentally persistent, bioaccumulative, globally distributed and dangerous to human beings. Thus, the degradation of PFOA with effective method remains further exploration. Here, an Electro-Fenton (EF) system was studied for efficient PFOA degradation, and where a new composite material ferrous hierarchically porous carbon (FHPC) prepared by high temperature activation of MIL-100 (Fe) was applied as the cathode, and 81.4% PFOA (Initial 50 mg/L) elimination was achieved at a low potential of ?0.4 V (pH = 7, 3 h). With the increasing of the activated temperature, the catalytic ability of the materials is decreasing because the reduced surface area reduced and the iron nanoparticles size enlarged. Moreover, the H2O2 and the ?OH were also detected to confirm the dominating contribution of Electro-Fenton mechanism in the PFOA degradation. Thus, this material could be used in efficient heterogeneous EF technology for PFOA elimination.
Recently, PFOA received extensive concern because of its persistence, bioaccumulation, wide use and potential toxicity, such as endocrine disrupting effects, neurotoxicity, and developmental toxicity [
The Electro-Fenton (EF) system is a potential method for the PFOA degradation result from its numerous advantages. Firstly, the rapidly generated hydroxyl radical (•OH), one of the most powerful oxidants (E˚ = 1.9 - 2.7 V) can rapidly degrade the electron-rich organic compounds. Secondly, the in situ production of H2O2 can avoids the risks associated with handling, transport and storage of H2O2. Simultaneously, Fe2+ can be electro reduced at the cathode. The general mechanism of EF can be described as follows:
The heterogeneous catalysis can not only promote the electro production of H2O2 from O2, but also decompose the H2O2 in suit by the fixed iron particles. Nevertheless, the majority of heterogeneous catalysis has a lot of problems like dissolution, agglomeration and shedding [
MIL-100(Fe) was prepared according to the previous reported by Liu Z M et al. [
The EF degradation of PFOA (50 mg/L) was performed in a single-compartment cell with a batch mode. The FHPC (working area of 10.0 cm2) and Pt plate (1 cm × 2 cm) were used as the cathode and anode, respectively. 0.05 M Na2SO4 were added as electrolyte. High -purity oxygen was fed into the cell at 0.45 L/min. The solution pH was 7. A constant potential of −0.4 V was applied on the cathode during the EF degradation of PFOA. As contrast, PFOA removal by electrosorption (ES) and electrocatalysis (EC) was conducted under N2 atmosphere (to inhibit H2O2 production). Before PFOA degradation, the generation of H2O2 and •OH were detected in the same system without PFOA.
The H2O2 concentration was measured by flow-injection chemiluminescence method [
The generated •OH was detected by EPR with 5,5-dimethyl-1-pyrroline-N- oxide (DMPO) as the spin-trapping agent at room temperature. Added 200 ppm DMPO, the solution was sampled by a 50 μL capillary tube. After analysis, the signal of DMPO-OH was recorded.
The concentration of PFOA in the E-Fenton degradation process was analyzed by liquid chromatography-Triple Quadrupole mass spectrometer (Agilent 1100-6410) equipped with a C18 column (2.1 mm × 100 mm × 3.5 μm), and the mobile phase was acetonitrile and 10 mM ammonium acetate with a flow rate of 0.25 mL/min. The gas temperature of the mass spectrometer is 300, while the capillary voltage is 4 kV.
The morphologies of the FHPC were investigated by SEM. Figures 1(A)-(C) shows that the crystal shape of the activated materials are octahedral, the same with the MIL-100 (Fe) precursor. Implying the retained framework with a hierarchically porous structure, and the HRSEM imagines Figures 1(a)-(c) reveal iron nanoparticles emerged in the porous carbon matrix, and the nanoparticle size of the FHPC enlarges with the increasing of the activated temperature.
XRD was applied to distinguish the iron specie in the FHPC. As
peaks located at 1340 cm−1 (D band) and 1570 cm−1 (G band) associated with disordered-induced and graphitic carbon. Abundant disordered-induced has been reported as active site for promoting H2O2 generation from O2 reduction in the FHPC [
Surface area and porosity are two important characteristics of the catalysis, thus the nitrogen adsorption?desorption isotherms were applied. As
Considering the porosity of the FHPC and the anodic oxidation of Pt, the electrosorption (ES) caused by the cathode and electrocatalysis (EC) caused by the anodic efficiency were tested. The removal efficiency is about 28.3% in total for ES and EC (
It is well-known that the concentration of H2O2 produced is a key parameter for evaluating the performance of Electro-Fenton, however, with the present of ferrous species which can quickly decompose H2O2, it is difficult to quantify the real yield of H2O2, thus the accumulative concentrations of electrogenerated H2O2 were measured. As
The generation of •OH in the EF system can also reflect the efficiency of Elec- tro-Fenton. As the EPR spectra shown (
Activated Temperature | |||
---|---|---|---|
600˚C | 700˚C | 800˚C | |
Surface Area (m2/g) | 255.8 | 206.0 | 158.4 |
Pore Volume (cm3/g) | 0.408 | 0.337 | 0.265 |
peared when the system fed with oxygen, but did not appear under N2 atmosphere. These results indicate the reaction mechanism is Electro-Fenton.
This work was supported by National Natural Science Foundation of China (grant no. 21590813), and the Fundamental Research Funds for the Central Universities (grant no. DUT16TD02).
Liu, X.Y. and Quan, X. (2017) Fe-MOF Derived Ferrous Hierarchically Porous Carbon Used as EF Cathode for PFOA Degradation. Journal of Geoscience and Environment Protection, 5, 9-14. https://doi.org/10.4236/gep.2017.56002