Re-utilization Of Heterocyclic Aromatics From Wastewater By CuO And Persulfate

Heterocyclic aromatics are recalcitrant contaminants commonly found in organic wastewater.If not appropriately treated,such contaminants will enter the environment and pose serious threats to organisms.It is of the great importance to develop more economical and efficient oxidizing systems for the treatment of heterocyclic aromatic wastewater.In this study,CuO will be used to activate persulfate（PS）to oxidize thiophene.By means of kinetics,solid characterizations and electrochemical analysis,the oxidation products and mechanisms will be investigated.Meanwhile,electrochemical properties of the oxidation products of pyrrole and aniline in the CuO-PS system will be investigated to re-utilize the oxidation products of heterocyclic aromatics.Our results show that the CuO-PS system shows the high oxidizing reactivity toward thiophene.Kinetics show that the observed rate constant(k_obs)in the CuO-PS system is about 10 times of that in the PS-alone system at pH 7.0.The linear relationship between the consumed thiophene and PS with the ratio of 1:1.11 indicates that thiophene has not been completely mineralized into CO₂ and H₂O.Quenching experiments and electron paramagnetic resonance（EPR）analysis suggest that SO₄^·-and·OH are not the primary oxidants in the CuO-PS system at pH 7.0,and the presence of thiophene radical cation indicates that electron abstraction from thiophene is an important pathway in the oxidation process.Linear sweep voltammetry（LSV）and chronoamperometry analyses confirm that electron transfer obviously occurs on the surface of CuO during the oxidation.Analyses of fourier transform infrared spectroscopy（FT-IR）,¹H nuclear magnetic resonance（¹H NMR）,matrix-assisted laser desorption ionization-time of flight-mass spectrum（MALDI-TOF-MS）,gel permeation chromatography（GPC）and ultraviolet visible spectroscopy（UV-vis）further indicate that thiophene is oxidatively polymerized with the formation of polythiophene S,S-dioxide as the primary oxidation product.We therefore propose that PS is activated on the surface of CuO via a non-radical pathway,and then an electron is transferred from thiophene to the activated PS with the formation of thiophene radical cation.The produced thiophene radical cations are polymerized to thiophene dimers after losing of two protons,and then trimers,oligomers,and eventually insoluble polythiophene polymers.Meanwhile,the thiophene units are oxidized by the produced SO₄^·-and/or·OH to polythiophene S,S-dioxides.Under similar experimental conditions,solid oxidation products can be obtained in the CuO-PS-pyrrole and CuO-PS-aniline systems.MALDI-TOF-MS analysis indicates that pyrrole and aniline are also polymerized in the CuO-PS system.Cyclic voltammetry（CV）curves indicate that the oxidation products of both pyrrole and aniline show capacitive behaviors and good reversibility.The galvanostatic charge-discharge（GCD）analysis shows that the oxidation products of pyrrole after HCl-washing of CuO shows higher specific capacitance,which was 151.43 F g^-1 at the current density of 0.1 A g^-1.This study provides new insight for more efficient and cost-effective re-utilization of heterocyclic aromatics from wastewater.