Efficiency And Mechanism Of In-situ Electrocatalytic Degradation Of Organic Pollutants By Iron-based Bifunctional Cathode Based On Confinement Effect

The pollutants in organic wastewater cannot be completely degraded by traditional water treatment technology,resulting in more and more serious problems of environmental pollution and antibiotic resistance.Electrochemical advanced oxidation technologies（EAOPs）based on bifunctional cathode could simultaneously achieve in-situ generation and conversion of H₂O₂ to active species,and easy to recovery and separation,which have been applied for the degradation of various organic pollutants.However,the current bifunctional cathodes face some problems,such as metal from cathode easy to leach and loss,resuling in inactivation of cathode,and narrow p H range and high electric energy consumption（EEC）.In this work,a series of nano-confined iron-based bifunctional cathodes with high activity were prepared,which simultaneously realized generation and conversion of H₂O₂ to efficiently degrade pollutants.First,based on carbon nanotube（CNTs）cavity encapsulated Fe as bifunctional cathode,the effect of confinement effect on electron transfer process,H₂O₂generation and pollutant degradation in electrocatalytic processes were investigated,and the structure-activity relationship between the confinement effect and phenols was established to clarify the mechanism of selective oxidation.Second,N element or B and N elements were further introduced into CNTs encapusalted Fe to study the production and mechanism of selective regulation of active species,which broadened the p H applicable range to achieve selective and efficient degradation of pollutants.Finally,the flow-through system was used to improve the removal efficiency of pollutants on the surface of the bifunctional cathode,and the feasibility of treatment of real wastewater was verified to provide theoretical and technical support for the efficient treatment of organic wastewater.The main research contents and conclusions of this work are as follows:（1）Bifunctional cathodes（Fe-in/out-CNTs and Fe⁰-in/out-CNTs）with confined Fe inside and outside the cavity of CNTs were prepared.The resulted showed that the cathode of Fe encapsulated in the cavity showed higher electron transfer and less corrosion rate.Due to the protective isolation of CNTs cavity,Fe⁰ from Fe⁰-in-CNTs cathode was more stable,which exhibited 77%of H₂O₂ selectivity,however,Fe⁰ from Fe⁰-out-CNTs was slowly oxidized to Fe₃O₄.The pseudo-first-order rate constant（k）value for 50 mg L^-1 of phenol degradation by Fe⁰-in-CNTs cathode at p H 3 was 0.02min^-1,which was 21.44-folds and 9.68-folds of Fe-in-CNTs(9.30×10^-4 min^-1)and Fe⁰-out-CNTs(0.0021 min^-1)cathode,respectively.The leaching total iron from Fe⁰-in-CNTs cathode was only 0.15 mg L^-1 at p H 3,which was lower than that of Fe-in-CNTs cathode(1.40 mg L^-1)and Fe⁰-out-CNTs(0.87 mg L^-1).The main active species for phenol degradation by Fe⁰-in-CNTs cathode was generated^·OH on the cathode surface,rather than homogeneous reaction.The phenol degradation by Fe⁰-in-CNTs cathode decreased from 91.12%to 79.32%after five cycles,mainly due to the decrease in the cycle efficiency of≡Fe³⁺/≡Fe²⁺caused by the continuous consumption of Fe⁰.This work achieved stabilization of iron components in bifunctional cathodes through nanoconfinement to reduce losses,and enhanced electron transfer to improve pollutants removal.（2）Fe@CNTs（x）（x referred to 3 nm,5 nm,7 nm,10 nm and 30 nm）cathodes were prepared,using CNTs with different inner diameters to encapsulate Fe nanoparticles.There was a strong interaction and electron transfer performance between Fe particles and CNTs in Fe@CNTs（3）-（7）cathodes.Due to the larger nano-space,weakened electron transfer was exhibited by Fe@CNTs（10）and Fe@CNTs（30）cathodes.Fe@CNTs（3）-（7）cathodes exhibited little difference in the degradation of phenol and sulfamethazine（SMT）,and Fe@CNTs（5）cathode had more advantages.Larger more homogeneous effect from Fe@CNTs（10）cathode was observed in the role of higher leaching iron.Weaker degradation of phenol by Fe@CNTs（30）cathode was exhibited due to weak adsorption.In addition,the rate constants order of Fe@CNTs（x）cathode for four phenols degradations was:p-nitrophenol（p-NP）<p-chlorophenol（p-CP）<p-diphenol（p-DP）<p-aminophenol（p-AP）.Since the degradation of phenols was highly dependent on the electron transfer process determined by the potential difference,the Fe@CNTs（3）-（7）cathode was more favorable to degrade pollutants with electron-donating groups（p-DP and p-AP）.Derived from electrons in the highest occupied molecular orbital（HOMO）of p-AP migrated to the lowest unoccupied molecular orbital（LUMO）of Fe@CNTs（3）-（7）cathodes and further migrated to the HOMO of Fe@CNTs（x）,which could further attack H₂O₂ to generate^·OH.This work revealed the stable generation and enrichment of^·OH in the CNTs cavity as microreactor with inner diameter of 3～7 nm to efficient degrade pollutants,which could effectively oxidation pollutants with electron-donating groups.（3）N-doped CNTs encapsulated transition metal as bifunctional cathodes（M@N-C,where M is Fe,Co,Ni,and Cu）with similar structural and physicochemical properties were prepared.It was observed that the activity trend of M@N-C cathode for H₂O₂ generation and SMT degradation at p H=3 and 7 was Co@N-C>Fe@N-C>Ni@N-C>Cu@N-C.The k value of per-metal site at p H=7 of Co@N-C and Fe@N-C cathode was 1.21×10³ min^-1 mol^-1 and 0.90×10³ min^-1 mol^-1,respectively,which are2～30-folds higher than those of cathodes in the literature,and the EEC for SMT degradation by M@N-C cathode was lower than 0.5 k Wh log^-1 m^-3.The M@N-C cathode could simultaneously generate atoms H*,^·OH,¹O₂ and O₂^·-using density functional theory（DFT）calculations and in-situ Fourier transformed infrared spectroscopy（FTIR）.Under the role of pyridinic N,the Fe@N-C cathode was more likely to generate atomic H*.The Fe@N-C cathode extended the p H application range to 9 in the role of atomic H*and ¹O₂.The Fe@N-C cathode was more efficient for p-CP degradation due to the generated atomic H*could more preferentially act on Cl group of p-CP.The Fe@N-C cathode only lost 10%for SMT degradation after ten cycles with the both 0.12 mg L^-1 of leaching Fe at every cycle,however,the SMT degradation by Co@N-C cathode decreased from 95%to 61%after five cycles.At the same time,the SMT degradation in the real wastewater（reverse osmosis concentrate,lake water of Mati lake and pharmaceutical wastewater）by Fe@N-C cathode showed similar treatment results with the simulated wastewater.This work demonstrated that the efficient degradation of SMT by the Fe@N-C cathode was achieved at the neutal and alkaline environment,and the generation mechanism of different active species by M@N-C cathode disclosed to provide theoretical evidence for the selective degradation of pollutants.（4）B,N co-doped CNTs encapsulated iron bifunctional cathode（Fe@BN-C）was further prepared,and the H₂O₂ selectivity of Fe@BN-C with pyrolysis temperature of800°C and 1 mmol iron content close to 100%at the wider range of 0.15～-0.25 V（vs RHE）.The h-BN structure in the Fe@BN-C played decisive role for H₂O₂ generation,while the nanoconfinement of Fe⁰ overcome the rate-limiting step of H₂O₂ generation to further promote the H₂O₂ generation.The k value of SMT degradation by Fe@BN-C₈¹ ₀₀ cathode was 0.018 min^-1 at neutral environment with 0.16 mg L^-1 of leaching iron concentration and 0.064 k Wh log^-1 m^-3 of EEC,which was only one tenth of the traditional electro-Fenton.The generated reactive oxygen species（ROS）during the electrocatalytic process by Fe@BN-C cathode mainly was ¹O₂,which was determined by h-BN-C.It was proved through DFT calculation,electron paramagnetic resonance（EPR）measurement and in-situ FTIR that Fe under nano-confinement could convert electrochemically generated H₂O₂ into^·OOH/O₂^·-,which was further activated by the generated surface^·OH to form ¹O₂.The Fe@BN-C₈¹ ₀₀ cathode in electrocatalytic process extended the p H range to 9,and exhibited excellent stability with more than 1-folds of Fe²⁺regeneration rate in catalysts and effective degradation for various pollutants（phenol,tetracycline,rhodamine B,methylene blue and carbamazepine）.Due to the generation of ¹O₂,Fe@BN-C₈¹ ₀₀ cathode in electrocatalytic process undergone less interference from component of wastewater.The SMT degradation in real wastewater（reverse osmosis concentrate,lake water of Mati lake and pharmaceutical wastewater）showed similar treatment with simulation wastewater.This work provided new insights into the development of bifunctional cathodes to improve the ability of anti-interference of the cathode to degradae pollutants with cost-effective,which was different from generated conventional ROS（^·OH）in electro-Fenton.（5）Due to the excellent performance of Fe@BN-C cathode,it was supported on graphite felt(Fe@BN-C₈¹ ₀₀/GF)as cathode in flow-through system to further improve the removal efficiency of pollutant on the cathode surface.The study obtained the optimal parameters of Fe@BN-C₈¹ ₀₀/GF cathode in flow-through system:50 m A of current,100 m L min^-1 of aeration,10.5 m L min^-1 of influent flow rate,150 mg of Fe@BN-C₈¹ ₀₀ loading.At this time,the SMT degradation reached 93.85%at p H=7and the k value was 0.022 min^-1,and the leaching iron was 0.19 mg L^-1.The pollutants and oxygen molecules have faster electron transfer rate on Fe@BN-C₈¹ ₀₀/GF surface in flow-through system,which could stably generate O₂^·-and convert to ¹O₂ to accelerate the SMT degradation.The SMT degradation and TOC removal in the flow-through system were 2.6～3.3-folds and 1.4～2.4 folds that of the batch system,especially in the p H range of 7～11.The SMT degradation by Fe@BN-C₈¹ ₀₀/GF cathode was well maintained above 90%using 0.005～0.2 M Na₂SO₄ electrolyte,which exhibited degradation prospect for wastewater with low-salt.The SMT degradation by Fe@BN-C₈¹ ₀₀/GF cathode in flow-through system had no effect in the presence of HCO₃^-,NO₃^-,Cl^-,K⁺,Ca²⁺and humic acid,and the addition of Cl^-promoted the generation of^·OH to increase the SMT degradation.The removal of total phosphorus（TP）,ammonia nitrogen（NH₃-N）and total organic carbon（TOC）in reverse osmosis concentrate by Fe@BN-C₈¹ ₀₀/GF was 100%,98.68%and 94.51%after 6 h treatment,respectively.The TP,NH₃-N and TOC removal in secondary wastewater effluent by Fe@BN-C₈¹ ₀₀/GF was 100%,96.47%and 87.18%after 6 h treatment,respectively.The concentrations of TP and NH₃-N in the reverse osmosis concentrate and secondary wastewater effluent after treatment met the discharge standard of“Pollutant Discharge Standards for Urban Sewage Treatment Plants”（GB 18918-2002）.The treatment of organic wastewater with low EEC by Fe@BN-C₈¹ ₀₀/GF cathode was realized,and the EEC(0.015 k Wh g^-1_TOC for industrial wastewater and 0.12 k Wh g^-1_TOC for secondary wastewater effluent)was only18%～43%that of conventional electro-Fenton.This work improved the mineralization of pollutants using flow-through system,providing theoretical and technical support for efficient and low-consumption treatment of real wastewater.