•A novel Ti/Sn-Sb/SnO2-F-Sb anode is prepared for degradation of recalcitrant PFOS.•Sn-Sb interlayer and F/Sb co-doping obviously improve Ti/SnO2 anode property.•Ti/Sn-Sb/SnO2-F-Sb anode possesses high OEP and electrooxidation performance.•Mechanistic detail of PFOS degradation on the novel electrode is proposed.The F and Sb co-doped Ti/SnO2 electrode containing Sn-Sb interlayer was innovatively synthesized for electro-oxidation (EO) removal of perfluorooctane sulfonate (PFOS) in water. Stronger oxidation capability and remarkably longer lifetime were performed for Ti/Sn-Sb/SnO2-F-Sb electrode than these similar electrodes in this study without Sn-Sb interlayer and F/Sb doping, which could removed more than 99% of PFOS (C0 = 100 mg L−1) after 120-min electrolysis. The attribute of Ti/Sn-Sb/SnO2-F-Sb anode and its EO mechanism were surveyed by XRD, XPS, SEM, and LSV. F and Sb co-doping could improve the stability and EO capability of the electrode, which resulted from that the smooth surface was formed conveniently to produce physically adsorbed hydroxyl radical and lead to relatively high OEP. The operating parameters of EO process were also investigated including current density, pH value, and stirring speed, which revealed the effect of these parameters on PFOS decomposition. Additionally, the mineralization parameters, TOC and F− concentration, as well as short-chain perfluorocarboxylic acids were analyzed in solution during the electrolysis. Consequently, the EO pathway of PFOS was proposed through the detection of intermediates. These results manifest that Ti/Sn-Sb/SnO2-F-Sb anode is prospective to effectively decompose PFOS in wastewater.

•The ZVC/air system was effective in degradation of ACT.•The Cu+ was responsible for activation of O2 to produce H2O2.•The O2− was formed via the decomposition of H2O2 in the ZVC/air system.•It was proved that the O2− mediated the copper cycling by reduction of Cu2+ to Cu+.In this study, the commercial zero-valent copper (ZVC) was investigated to activate the molecular oxygen (O2) for the degradation of acetaminophen (ACT). 50 mg/L ACT could be completely decomposed within 4 h in the ZVC/air system at initial pH 3.0. The H2O2, hydroxyl radical (OH) and superoxide anion radical (O2−) were identified as the main reactive oxygen species (ROSs) generated in the above reaction; however, only OH caused the decomposition and mineralization of ACT in the copper-catalyzed O2 activation process. In addition, the in-situ generated Cu+ from ZVC dissolution not only activated O2 to produce H2O2, but also initiated the decomposition of H2O2 to generate OH. Meanwhile, the H2O2 could also be partly decomposed into O2−, which served as a mediator for copper cycling by reduction of Cu2+ to Cu+ in the ZVC/air system. Therefore, OH could be continuously generated; and then ACT was effectively degraded. Additionally, the effect of solution pH and the dosage of ZVC were also investigated. As a result, this study indicated the key behavior of the O2− during Cu–catalyzed activation of O2, which further improved the understanding of O2 activation mechanism by zero-valent metals.Download high-res image (146KB)Download full-size image

•The CuO surface has high sorption affinity for the ferrous ion in solution.•The sorbed Fe(II) on CuO surface processed higher reactivity than solution Fe(II).•The formed Cu(I) can act as the electron mediator between the sorbed iron and O2.•It was proposed for the scheme of the ROSs generation in the Fe(II)/CuO/O2 system.A synergistic effect of Fe(II) and copper oxide (CuO) was observed on the degradation of acetaminophen (ACT) in the presence of O2. The results showed that 89% of ACT (50 mg/L) was degraded within 6 h by 10 mM Fe2+ with 5 g/L CuO powder at pH 3.0, among which about 30% of ACT was mineralized. The sorbed Fe(II) on the CuO surface which is more reactive than the dissolved Fe(II) was capable of reducing both Cu(II) and O2. The resulting Cu(I) significantly accelerated the destruction of ACT by serving as an electron-mediator between the sorbed Fe(II) and O2. In addition, the hydrogen peroxide (H2O2), hydroxyl radical (OH) and superoxide anion radical (O2−) were identified as the main reactive oxygen species (ROSs) generated in Fe(II)/CuO suspension by Electron paramagnetic resonance (EPR) technique and quenching studies; however, only OH was found to have caused the decomposition and mineralization of ACT. First O2− was generated by reduction of O2 through a one-electron transfer pathway. Then the generated O2− mediated the production of H2O2, which further decomposed into OH by Cu(I)- and Fe(II)-catalyzed Fenton reactions. The effects of Fe(II) concentration, CuO dose, solution pH and the degradation pathway were investigated, respectively.Download high-res image (108KB)Download full-size image

•Ti/TiO2-NTs/Ag2O/PbO2 anode has the higher oxygen evolution potential of 2.12 V (vs. saturated calomel electrode, SCE) compared with Ti/PbO2 anode (1.33 V) and Ti/TiO2-NTs/PbO2 (1.83 V) anode.•A removal ratio of 74.87% for perfluorooctane sulfonate (PFOS) can be achieved at Ti/TiO2-NTs/Ag2O/PbO2 anode after 180 min of electrolysis, with a pseudo first-order kinetic constant of 0.0165 min−1 and a half-life of 43.18 min−1.•The heavy water tracer experiments show that the oxygen in the intermediate products came from electrolyte, and the intermediate products of PFOS decomposition were short-chain perfluorocarboxyl anions (i.e., C3F7COO−, C4F9COO−, C5F11COO−, C6F13COO−, and C7F15COO−).Ti/TiO2-NTs/Ag2O/PbO2 anode was prepared by electrodeposition technique and characterized by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), linear sweep voltammetry (LSV), and accelerated life test. Electrochemical oxidation of persistent organic pollutant wastewater perfluorooctane sulfonate (PFOS) was carried out with the novel PbO2 anode. Ti/TiO2-NTs/Ag2O/PbO2 anode exhibited a pyramid structure, which was the typical PbO2 electrodes prepared using electrochemical deposition method. XRD spectra indicated that diffraction peaks of PbO2 coating conformed to that of JCPDS (Joint Committee on Powder Diffraction Standards) card for β-PbO2. Ti/TiO2-NTs/Ag2O/PbO2 anode showed high oxygen evolution potential, and longer life service, compared with Ti/PbO2 and Ti/TiO2-NTs/PbO2 anodes. The degradation ratio of PFOS (90 mL of 0.0929 mmol L−1) was 74.87%, with a pseudo first-order kinetic constant of 0.0165 min−1 and a half-life of 43.18 min−1 at a constant current density of 30 mA cm−2 after 180 min of electrolysis. PFOS oxidation yielded sulfate, fluoride, and perfluorocarboxyl anions (i.e., C3F7COO−, C4F9COO−, C5F11COO−, C6F13COO−, and C7F15COO−). The electrospray ionization (ESI) mass spectrum confirmed that oxygen in the intermediate products originated from a heavy-oxygen water electrolyte, and the degradation of PFOS was initiated by the dissociation of a sulfonic group. A possible mechanism was revealed; that was, PFOS was desulfated at the anode to form C8F17· and then transformed into C8F17OH, followed by intramolecular rearrangement and hydrolysis reactions to form C7F15COO−. Kolbe decarboxylation occurred in C7F15COO− at the anode to generate C7F15·, which evolved into C6F13COO− in a similar way and the CF2 unit fell off from C7F15COO−. PFOS was gradually degraded into short-chain perfluorocarboxyl anions by repeating the CF2 unzipping cycle.Download high-res image (211KB)Download full-size image

Electrocoagulation (EC) technique was used to investigate the removal performance of aqueous perfluorooctanoic acid (PFOA) with relatively high concentration as simulating the wastewater from organic fluorine industry. A comparison was done with the similar amount of coagulant between EC and chemical coagulation process. PFOA removal obtained was higher with EC process, especially for Fe anode. Several factors were studied to optimize the EC process. At the optimal operating parameters including 37.5 mA/cm2 of current density, initial pH 3.77, and 180 rpm of mixing speed, 93% of PFOA could be removed with 100 mg/L of initial concentration after 90-min electrolysis. Furthermore, the remove efficiency could be obviously improved by H2O2 intermittent addition, which removed more than 99% of PFOA within 40-min EC. It could be attributed to that H2O2 facilitated the oxidative transformation from ferrous to ferric ion. In addition, the adsorptive removal of aqueous PFOA on Fe flocs during EC was also verified by fourier transform infrared spectra.