Degradation Of Refractory Organic Pollutants Research Articles

Organic pollutant degradation by UV/peroxydisulfate process: Impacts of UV light source and phosphate buffer

In recent years, ultraviolet (UV) based advanced oxidation processes have been extensively studied for degradation of refractory organic pollutants in water and wastewater, and selection of an appropriate UV light source is an important issue. In this study, bench-scale tests were conducted on a mini-fluidic photoreaction system (MFPS) to determine the degradation kinetics of methylene blue (MB) by UV/peroxydisulfate (UV/PDS) process equipped with a low-pressure UV (LPUV), vacuum UV (VUV)/LPUV, or medium-pressure UV (MPUV) mercury vapor lamp. Results indicate that MB degradation by UV/PDS with various light sources all followed the pseudo-first order kinetics, and the photon fluence-based rate constant (kp,λ′) had a descending order of: VUV/LPUV/PDS ≫ MPUV/PDS > LPUV/PDS. Moreover, it is noted that phosphate buffer (PB) notably inhibited MB degradation: the kp,LPUV′, kp,VUV/LPUV′ and kp,MPUV′ decreased by 35.0%, 44.9% and 37.5% with the PB concentration increasing from 0 to 20 mM, respectively. The maximal decrease in kp,VUV/LPUV′ was ascribed to a strong competition of PB for VUV photons. Thereafter, pilot-scale tests were conducted to evaluate the practical performance of UV/PDS in terms of the electrical energy consumption per order (EEO). It was found again that the VUV/LPUV lamp was the optimal light source in UV/PDS for organic pollutant degradation. This study helps optimize the UV/PDS process for its practical application to water and wastewater treatment.

In-situ synthesis of highly dispersed Cu-CuxO nanoparticles on porous carbon for the enhanced persulfate activation for phenol degradation

The study reported a straight route to prepare multivalent Cu and carbon composite catalysts with good Cu dispersion serving as copper-base catalysts for sulfate radical-advanced oxidation processes. The proposed copper-base catalysts exhibit excellent degradation efficiency toward refractory organic pollutants. • Multivalent copper coexisted composites were synthesized through a facile method. • Cu-Cu x O were well dispersed on carbon substrates. • The Cu-Cu x O@C composite presented superior catalytic activity in PS activation. • The maximum removal efficiency of phenol reached approximately 100%. Copper-mediated activation of persulfate (PS) has been widely investigated for the degradation of refractory organic pollutants. However, the dispersion of the active copper sites in the catalysts remains a great challenge. Herein, a series of well-dispersed Cu-Cu x O embedded into carbon matrix composites (Cu-Cu x O@C) were fabricated via the pyrolysis of HKUST-1 precursor. The Cu-Cu x O@C catalysts were fully characterized by SEM, HRTEM, XRD, BET, Raman, FT-IR, and XPS. The results show that the Cu-Cu x O@C calcinated at 700 °C (Cu-Cu x O@C-700) contained multivalent Cu components. Their heterogeneous activation of PS for phenol degradation was also investigated. Cu-Cu x O@C-700 removed completely phenol with a k obs value greater than 0.40 min −1 . Such superior catalytic activity is mainly attributed to the synergistic effect of carbon matrix adsorption and multivalent Cu active site electron transfer. The reusability studies reveal that the performance of the used Cu-Cu x O@C-700 could be reactivated through re-calcination. Moreover, quenching tests and EPR analyses revealed that OH and SO 4 − are involved in phenol degradation. The electron transfer process and resistance of Cu-Cu x O@C-700 were described by electrochemical method. Finally, the catalytic mechanism was explored by XPS, FT-IR and Raman analyses. This study provides new insights into the fabrication of heterogeneous catalysts for sulfate radical activiation.

A novel composite anode via immobilizing of Ce-doped PbO2 on CoTiO3 for efficiently electrocatalytic degradation of dye

The exploitation of efficient electrocatalyst is significantly important for degradation of refractory organic pollutants. Herein, a novel Ti/CoTiO3/Ce-PbO2 composite electrocatalyst (abbreviated as CTO/CP) is successfully constructed via facile consecutive immersion pyrolysis and electro-deposition method and then systematically characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), energy dispersive spectroscopy (EDS) and near infrared chemical imaging (NIR-CI). Importantly, the electrochemical measurements demonstrate that the CTO/CP possesses numerous prominent properties such as lower charge transfer resistance, larger electroactive area, higher oxygen evolution potential than those of the pristine Ti/CoTiO3 (CTO) and Ti/Ce-PbO2 (CP). Thereby, the CTO/CP exhibits an enhanced electrocatalytic degradation performance with the degradation efficiency as high as 90.0% and COD removal rate of 88.3% at 180 min for the optimal CTO/CP (denoted as 10 layers of CTO and 1 h electrodeposition of CP), in which the ·OH is the major reactive species. Additionally, the optimal CTO/CP also shows a higher ICE/ACE together with lower EEC and desirable stability, universal applicability for many different dyes and reusability. Overall, this work offers a promising approach for enhancing the electrocatalytic properties of CTO via introducing CP.

Degradation of perfluorosurfactant in aqueous solution using non-thermal plasma generated by nano-second pulse corona discharge reactor

Perfluorosurfactants (PFS) are global environmental pollutants and are considered as a new anthropogenic type of carcinogenic contaminant that remains untreated by conventional wastewater treatment plants. These organic surfactants are typically refractory, non-biodegradable, and pose great threats to environmental safety and human health. The gradual increase in PFS product consumption by various industries resulted in their abundant occurrence in the ecosystem. Non-thermal plasma-based advanced oxidation processes with powerful oxidation ability appeared to be applicable for the degradation of refractory organic pollutants and enhancing biodegradability. In this work, the decomposition of Perfluorooctanesulfonic acid (PFOS) in water by the application of nano-second pulse corona discharge under oxygen was examined. Working corona discharge under oxygen, argon, and helium has no effects on the decomposition rate. The energy yields at 50% conversion (G 50 ) of about 1 mg L −1 initial PFOS concentration was estimated as 22 mg/kWh under argon or oxygen and 27 mg/kWh under helium. The energy yield G 50 for initial concentrations of 7.5 and 18.5 µg L −1 in helium and oxygen was estimated as 0.20 and 0.33 mg/kWh, respectively. A considerable difference in the generation of short perfluoroalkyl acids by-products and fluoride ions was observed when corona discharge was conducted under oxygen and much more of these by-products are formed. A synergistic effect was observed in the presence of other surface-active compounds. Degradation efficiency was enhanced by 10% after the addition of surface-active sulfobetaine into the medium following the same experimental procedure.

A dual-biomimetic photocatalytic fuel cell for efficient electricity generation from degradation of refractory organic pollutants

Energy recovery from wastewater treatment is attracting tremendous research interests since the environmental and energy issues can be simultaneously addressed. Photocatalytic fuel cell (PFC) provides a sustainable way for water purification and electricity generation, but its performance has been restricted by the poor mass and charge transfer. Herein, we demonstrated an integrated PFC platform by mimicking the metabolic processes in human body. Inspired by the purification of metabolites in the microstructure of liver lobules, flow-through TiO2 photoanode with numerous microchannels was developed. Compared to traditional PFC with planar photoanode, the biomimetic microflow PFC exhibited 3 to 5-fold enhancements in the removal efficiencies of various organic pollutants, including perfluorooctanoic acid, 4-chlorophenol, bisphenol A, methylene blue, and amoxicillin. To mimic the breathing process in alveoli, an ultrathin air cathode was fabricated by sputtering Pt (loading as small as 0.01 mg cm−2) on nanoporous polyethylene film, contributing to fast oxygen diffusion and abundant gas-liquid-solid three-phase reactive sites. The construction of this dual-biomimetic PFC system resulted in an open circuit voltage of 0.92 V and a record-high power output of 500 μW cm−2 under simulated solar light. The series connection of modularized reactors allowed for the successive purification of wastewater by self-generated electricity. Thus, this work opens up new opportunities of biomimetic photocatalytic system for addressing the water-energy nexus.

Catalytic degradation of acetaminophen by Fe and N Co-doped multi-walled carbon nanotubes

An Fe and N co-doped carbon nanotube (CNT) (Fe/N-CNT) was successfully prepared using a simple hydrothermal method. CNT, Fe doped CNTs (Fe-CNT), N doped CNTs (N-CNT), and Fe/N-CNT were characterized using scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and zeta potential analysis. The catalytic activities of the materials were investigated via pharmaceutical (acetaminophen, ACT) degradation using persulfate (PS). The ACT removal rate was in the order: Fe-CNT > N-CNT > Fe-CNT > CNT, for 30 min with 10 mg/L ACT, 0.05 g/L materials, and 0.08 mM PS. The doped N existed as pyridinic-N, pyrrolic-N/N–Fe, graphitic-N, and oxidized-N, while the doped Fe existed as Fe–N, FeO/Fe3O4, and Fe2O3/FeOOH at the edge. The rates of ACT removal and PS decomposition were well correlated with pyrrolic-N/N–Fe. The ACT removal in the Fe/N-CNT + PS system was as high as >98.4% and was not significantly affected by the initial pH of 2.0–8.2 and ten consecutive uses. However, natural organic matter (NOM) inhibited ACT removal by the accumulation on Fe/N-CNT. The results of ACT removal in the presence of radical scavengers, PS decomposition, and cyclic voltammetry showed that the ACT removal was dominantly attributed to a non-radical pathway with the accelerated electron transfer from ACT to PS through the Fe/N-CNT. The results in this study strongly suggest that the Fe/N-CNT + PS system is an excellent process for the degradation of refractory organic pollutants in various water matrices with improved performance and stability attributed by non-radical pathway.

Mechanical agitation accelerated ultrasonication for wastewater treatment: Sustainable production of hydroxyl radicals

Low efficiency in energy conversion has long been the bottleneck in sonochemistry-based water treatment technologies. In this work, we reported a simple and efficient strategy by introducing mechanical agitation into a low powered ultrasonic system to facilitate the production of cavitation bubbles. The coupled system remarkably intensifies the evolution of reactive oxygen species (ROS) for degradation of refractory organic pollutants. We in-situ monitored the generation of hydroxyl radicals (•OH) by selective scavenging tests and chemical trapping experiments. The operational factors such as rotation speed, gas atmosphere, solution temperature and pH were carefully evaluated for their impacts on the degradation of a plastic microcontaminant, diethyl phthalate (DEP). It was found that the degradation efficiency is closely related to the population of cavitation bubbles in the solution, which was collaboratively governed by the aforementioned factors. A high mechanical agitation speed (600 rpm), great solubility of inert gas atmosphere (Argon), and low reaction temperature (15 ºC) are beneficial to the generation of cavitation bubbles and the associated production of ROS. This work shows a facile strategy to intensify the mechanical energy-to-chemical conversion and provides new mechanistic insights into the ultrasound-based advanced oxidation without external chemical inputs.

Heterogeneous electro-Fenton catalysis with self-supporting CFP@MnO2-Fe3O4/C cathode for shale gas fracturing flowback wastewater

Self-supporting electrodes have triggered great interests in improving electro-Fenton (EF) system for degradation of refractory organic pollutants. In this work, a novel self-supporting carbon fiber paper (CFP) electrode modified by transition metals, e.g. Fe and Mn, was fabricated and employed as a heterogeneous EF cathode. The prepared electrode exhibited excellent degradation for a number of typical organic pollutants along with superior stability. Remarkably, a high removal efficiency was achieved in the EF treatment of shale gas fracturing flowback wastewater. Results indicated that 65.2% TOC and 74.8% COD were eliminated after 4 h degradation. The residual COD value of the real wastewater was 80 mg L−1, meeting the emission requirement of the integrated wastewater discharge standard (COD＜100 mg L−1) with a low specific energy consumption of 6.9kWhkg−1COD−1. This work demonstrates a competing alternative for efficient decontamination of real wastewater using an electro-Fenton strategy with a low-cost electrode.

Rapid degradation of refractory organic pollutants by continuous ozonation in a micro-packed bed reactor

Recently microreactor technology attracts attention due to the excellent multiphase mixing and enhanced mass transfer. Herein, a continuous ozonation system based on a micro-packed bed reactor (μPBR) was used to improve the dissolution rate of ozone and achieved a rapid and efficient degradation of refractory organic pollutants. The effects of liquid flow rate, gas flow rate, initial pH, initial O3 concentration and initial phenol concentration on the phenol and chemical oxygen demand (COD) removal efficiencies were also investigated. Experimental results showed that phenol and COD removal efficiencies under optimal conditions achieved 100.0% and 86.4%, respectively. Compared with large-scale reactors, the apparent reaction rate constant in μPBR increased by 1-2 orders of magnitude. In addition, some typical organic pollutants (including phenols, antibiotics and dyes) were treated by ozonation in μPBR. The removal efficiencies of these organic pollutants and COD achieved 100.0% and 70.2%–80.5% within 71 s, respectively. In this continuous treatment system, 100% of the unreacted ozone was converted to oxygen, which promoted the healthy development of aquatic ecosystems. Thus, this continuous system based on μPBR is a promising method in rapid and efficient treating refractory organic pollutants.

Study on cobalt-phosphate (Co-Pi) modified BiVO4/Cu2O photoanode to significantly inhibit photochemical corrosion and improve the photoelectrochemical performance

The serious recombination of photogenerated carriers and the photochemical corrosion of the electrode extremely restricted its photoelectrochemical (PEC) performance, which could not reach the theoretical photocurrent intensity. However, so far, there are few comprehensive reports on the mechanism of corrosion inhibition and stability improvement of electrodes in photocatalytic degradation. In this paper, BiVO4/Cu2O heterojunction modified by Cobalt-phosphate (Co-Pi) co-catalyst was successfully prepared to address the aforementioned issues. Co-Pi completed the holes capture and release process through the change of its own chemical state (Co2+/3+→Co4+→Co2+/3+), and also affected the energy band bending of the BiVO4/Cu2O heterojunction. In addition, the effect mechanism of the Co-Pi film thickness on the PEC performance of the photoanode was systematically discussed in this report. When the deposition amount was 0.05C∙cm−2, the photocurrent density reached 2.22 mA∙cm−2 at 1.23 V (vs. RHE), which was 2.8 times that of bare BiVO4. The LSV curves, EIS and UV–vis absorption spectra reveal the enhanced PEC performance and light stability of the prepared electrode materials. When it is applied to photocatalytic degradation of organic pollutants, BiVO4/Cu2O/0.05Co-Pi shows high activity and potential for MB and CIP degradation, and is expected to be stably applied to the field of photocatalytic degradation of refractory organic pollutants in water.

Activation of peroxymonosulfate by CuCo2O4-GO for efficient degradation of bisphenol A from aqueous environment

A new catalyst, CuCo2O4 supported on graphene oxide (CuCo2O4-GO), was synthesized as a heterogeneous catalyst to activate peroxymonosulfate (PMS) for the degradation of bisphenol A (BPA). In this study, highly efficient degradation of BPA by PMS (0.2 mM) activated with CuCo2O4-GO (100 mg/L) was achieved and the degradation efficiency was 100% within 5 min at initial pH 7.0. The reaction rate constant (2.662 min−1) of BPA by CuCo2O4-GO/PMS was 35.49 times greater than that of CuCo2O4/PMS (0.075 min−1), and the optimal percentage of GO was 20%. The catalytic activity of CuCo2O4-GO as synthesized in this work was much higher than that of CoFe2O4-GO which has been reported previously. In addition, the degradation efficiency increased with the rise of catalyst dose and PMS dosage, it exhibited the best performance at pH 7.0. The detection of radicals by electron paramagnetic resonance (EPR) and quenching experiments indicated that SO4− was the main free radical that responsible for the degradation of BPA while OH also was produced in the CuCo2O4-GO/PMS system. The CuCo2O4-GO maintained its catalytic activity during the repeated batch experiments and the leaching of metal ion was limited. Finally, the toxicity of BPA decreased synchronously during the reaction process and higher content of TOC removal was achieved by CuCo2O4-GO/PMS than that of CuCo2O4/PMS. The performance of CuCo2O4-GO/PMS in actual water indicated that CuCo2O4-GO/PMS system had great potential in practical application in the degradation of refractory organic pollutants in water.

Influence of processing conditions on the synergism between UV irradiation and chlorine toward the degradation of refractory organic pollutants in UV/chlorine advanced oxidation system

The synergy of applying UV/chlorine advanced oxidation process (AOP) for the degradation of organic pollutants was usually reported. However, very limited information is available on the influence of processing conditions on the resulted synergism. In this work, C.I. reactive green 12 (RG12), a refractory textile dye, has been selected as a pollutant model to examine the synergism dependence of operational conditions in UV/chlorine AOP. Initial tests conducted with 500 μM of chlorine and 20 mg L−1 of RG12 have resulted in a high synergy index (SI) of 3. Operating conditions sensitively affect the value of SI. This latter increased with increasing initial chlorine and RG12 concentrations up to certain optimums at 500 μM of chlorine and 20 mg L−1 of RG12 and decreased afterward. The best SI value, i.e. 3, was obtained at pH 5, followed by pH 7 (SI = 2.2) and then pH 9–10.5 (SI ~ 2). On the other hand, the synergistic index decreased importantly from 3 at 25 °C to only 1.2 at 55 °C. Finally, by using different radical scavengers, it was found that among various suspected oxidants, only ●OH and Cl2●− play a key role in the synergistic effect between UV and chlorine toward RG12 degradation.

One-pot in-situ hydrothermal synthesis of CdS/Nb2O5/Nb2C heterojunction for enhanced visible-light-driven photodegradation

In-situ construction of heterojunction structure on substrate material is a feasible way to improve photocatalyst performance. Herein, we employed two-dimensional (2D) Nb2C MXene as the substrate material to construct CdS/Nb2O5/Nb2C ternary heterojunction structure by the one-pot hydrothermal synthesis method. The as-prepared photocatalyst achieved 92% removal efficiency towards carbamazepine (CBZ, model pollutant) under visible light radiation for 4 h, remarkably higher than that of pure CdS (75%), because the CdS/Nb2O5/Nb2C heterojunction structure significantly promoted electron-hole pair generation, separation and transfer. The 2D accordion-like Nb2C substrate prevented the agglomeration of photocatalysts. CdS nanoparticles and Nb2O5 nanorods grew insert Nb2C substrate, thus to strengthen the binding force between catalyst components and ensure that the Cd2+ leakage was obviously lower than the national standard of China. This work provides a feasible strategy of in-situ construction of visible-light-driven heterojunction photocatalysts on 2D MXene substrate for enhanced photocatalytic degradation of refractory organic pollutants in water.

Can Cu2ZnSnS4 nanoparticles be used as heterogeneous catalysts for sulfadiazine degradation?

As a quaternary copper-based semiconductor, Cu2ZnSnS4 (CZTS) is drawing growing attention and is anticipated as a promising photocatalyst, thanks to its large absorption coefficient, exceptional photostability, and theoretical power conversion efficiency. However, CZTS has never been used as an activator of H2O2 for the degradation of refractory organic pollutants. In this study, the synthesis of CZTS nanoparticles obtained with diverse morphologies and crystallinities using solvents of deionized water (CZTS-W) and ethylene glycol (CZTS-EG) was examined in the activation of H2O2 to degrade sulfadiazine (SDZ). The results revealed that CZTS coupled with H2O2 could be an effective system for the degradation of SDZ. Compared to CZTS-EG, CZTS-W presented higher reusability in consecutive cycles with negligible leaching of copper. Reactive oxygen species quenching tests and electron paramagnetic resonance analyses illustrated that •O2−, •OH, and 1O2 contributed to the degradation of SDZ, and 1O2 prevailed over •O2− and •OH. The mechanistic investigation showed that efficient degradation could be associated to the effective recycling of Cu(II)/Cu(I) and low-valent/high-valent sulfur. Also, the degradation pathways of SDZ have been proposed through the detection of intermediate products. This study manifests that CZTS synthesized using deionized water is encouraging for the elimination of organic pollutants.

Glucose enhanced the oxidation performance of iron-manganese binary oxides: Structure and mechanism of removing tetracycline

In this work, iron-manganese binary oxides (FMO) modified with different proportions of glucose addition (FMOCx) by co-precipitation method showed good activity in activating hydrogen peroxide (H2O2) for tetracycline degradation. The structure and surface characterizations of the FMO and FMOCx were measured by XRD, FTIR, TEM, BET and XPS. With increased glucose addition, FMOCx has more surface functional groups such as –OH and –COOH, particle size decreases, surface area gradually increases, and the ratio of high valence iron and manganese also increases. In addition, the glucose might be oxidized by KMnO4 to form amorphous carbon on the catalyst surface. Glucose modified iron-manganese binary oxides FMOC3 (with 0.003 mol glucose added) showed the highest efficiency removal capability for tetracycline up to 85%, which attribute to it has a larger surface area, more surface functional groups and higher surface active Mn(IV) site content. The results also demonstrated that FMOC3 could efficiently activate hydrogen peroxide. This study proves that glucose modified iron-manganese binary oxides (FMOCx) can offered a possibility of degradation of refractory organic pollutants as an environmentally friendly catalyst in the absence of H2O2 or not.

MXene Ti3C2 derived Z–scheme photocatalyst of graphene layers anchored TiO2/g–C3N4 for visible light photocatalytic degradation of refractory organic pollutants

Highly photoactive Z–scheme of graphene layers anchored TiO2/g–C3N4 (GTOCN) photocatalyst was synthesized from MXene Ti3C2 through one–step in–situ calcination, and used for tetracycline (TC) and ciprofloxacin (CIP), bisphenol A (BPA) and rhodamine B (RhB) degradation under visible light irradiation. The crystal phases, chemical compositions, morphological structures and photoelectrochemical properties were characterized. The results demonstrated that hierarchical structured GTOCN photocatalysts exhibited significant improvement in visible light adsorption and photogenerated carriers separation. The GTOCN3 presented the best visible light photocatalytic synergism of degradation rate towards TC (0.02442 min−1), CIP (0.01675 min−1), BPA (0.01935 min−1) and RhB (0.05586 min−1), which was much higher than that of graphene layers anchored TiO2, g–C3N4 and g–C3N4/TiO2(P25), benefitting from solid interactions among graphene, TiO2 and g–C3N4 for separation and accumulation of g–C3N4 electrons with high reduction capability and TiO2 holes with high oxidation capability. Furthermore, the main species of O2− and OH for pollutants decomposition was confirmed by free radical capture and electron spin resonance experiments. Besides, the acceptable reusability and stability indicated the great application potential of GTOCN in pharmaceuticals, endocrine disruptors and colourants contained wastewater disposal and environmental protection.

Integration of •SO4−-based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II

The sulfate radical (•SO4−)-based advanced oxidation processes (AOPs) for the degradation of refractory organic pollutants consume a large amount of persulfate activators and often generate toxic organic by-products. In this study, we proposed a novel iron-cycling process integrating •SO4−-based AOP mediated by reusable iron particles and a sulfidogenic process to degrade and detoxify Orange II completely. The rusted waste iron particles (Fe0@FexOy), which contained FeII/FeIII oxides (FexOy) on the shell and zero-valent iron (Fe0) in the core, efficiently activated persulfate to produce •SO4− and hydroxyl radicals (•OH) to degrade over 95% of Orange II within 120 min. Both •SO4− and •OH destructed Orange II through a sequence of electron transfer, electrophilic addition and hydrogen abstraction reactions to generate several organic by-products (e.g., aromatic amines and phenol), which were more toxic than the untreated Orange II. The AOP-generated organic by-products were further mineralized and detoxified in a sulfidogenic bioreactor with sewage treatment together. In a 170-d trial, the organic carbon removal efficiency was up to 90%. The inhibition of the bioreactor effluents on the growth of Chlorella pyrenoidosa became negligible, due to the complete degradation and mineralization of toxic AOP-generated by-products by aromatic-degrading bacteria (e.g., Clostridium and Dechloromonas) and other bacteria. The sulfidogenic process also well recovered the used Fe0@FexOy particles through the reduction of surface FeIII back into FeII by hydrogen sulfide formed and iron-reducing bacteria (e.g., Sulfurospirillum and Paracoccus). The regenerated Fe0@FexOy particles had more reactive surface FeII sites and exhibited much better reactivity in activating persulfate in at least 20 reuse cycles. The findings demonstrate that the integrated process is a promising solution to the remediation of toxic and refractory organic pollutants because it reduces the chemical cost of persulfate activation and also completely detoxifies the toxic by-products.

Effect of indium doping on the PbO2 electrode for the enhanced electrochemical oxidation of aspirin: An electrode comparative study

A novel indium (In) doped PbO2 electrode featuring high electrochemical activity and electrode stability toward electrochemical degradation of refractory organic pollutants was prepared by electrodeposition technique. Based on the aspirin removal efficiency, the optimized molar ratio of In/Pb in electrodeposition solution was 1%. In contract with the undoped PbO2 electrode, the constructed In-doped PbO2 electrode displayed a more compact structure, a better oriented crystal of smaller size, an increased concentration of active oxygen species, a higher oxygen evolution potential, a lower charge transfer resistance and a higher electrode stability. The In-doped PbO2 electrode was demonstrated to have a superior electrochemical oxidation ability with higher OH generation capacity. For In-doped PbO2 electrode, the removals of aspirin and COD reached 76.45% and 52.09% after 2.5 h of electrolysis, respectively. Meanwhile, the energy saving could be up to 30.56% after In doping. This outcome was in good agreement with the results of surface characterization of PbO2 electrodes. Finally, combination with the safety evaluation, we could conclude that In-doped PbO2 electrode was a promising anode for the treatment of organic pollutants.

Pre-magnetized Fe0 as heterogeneous electro-Fenton catalyst for the degradation of p-nitrophenol at neutral pH

Pre-magnetized Fe0 (Pre-Fe0) was for the first time applied as heterogeneous catalyst to enhance the oxidation efficiency of electro-Fenton (EF) for the degradation of p-nitrophenol (PNP). The parameters including current, initial pH and pre-Fe0 dosage of Pre-Fe0/EF process were optimized and compared with other two processes (conventional Fe0/EF and electro-oxidation) to confirm its advantage. The rate constants of PNP removal were 1.40–3.82 folds of those by Fe0/EF process under various experimental conditions. The application of pre-Fe0 as catalyst could extend the working pH range from 3.0 to neutral conditions for PNP removal and reduce the Fe0 dosage from 2 to 0.5 mM corresponding to Fe0/EF, avoiding the second pollution of iron sludge. The superiority of Pre-Fe0/EF process was also verified to improve the degradation and mineralization of other phenols and antibiotics. Furthermore, a possible pathway of PNP degradation was revealed by the identification of intermediates and organic acids, and the possible mechanism of pre-Fe0 efficiently enhanced the EF efficiency was proposed. This work demonstrated that such a novel heterogeneous EF process using pre-Fe0 catalyst was clean and promising for the degradation of refractory organic pollutants.

Accelerated oxidation of 2,4,6-trichlorophenol in Cu(II)/H2O2/Cl- system: A unique “halotolerant” Fenton-like process?

The roles of chloride in enhanced oxidative degradation of refractory organic pollutants are recently identified in the Cu(II)/H2O2/Cl− system, but the identity of the reactive oxidants and potential conversion of inorganic chloride to organochlorine in such oxidizing environment remain obscure. Here we report that Cu(II)/H2O2/Cl− system is a unique “halotolerant” Fenton-like process that works most efficiently in saline water among the five tested redox-active metals ions (i.e. Cr(VI), Ce(III), Co(II), Mn(II) and Cu(II)). The observed pseudo first-order rate constant for 2,4,6-trichlorophenol (TCP) degradation was linearly correlated with the elevated Cl− content. The TCP degradation rate at [Cl−]0 = 1000 mM by the Cu(II)/H2O2 system was approximately 46-fold higher than that at [Cl−]0 = 5 mM. The optimal mineralization rate of TCP and percentage of absorbable organic halogens (AOX) decrease were 31.6% and 63.8%, respectively, in the tested Cu(II)/H2O2/Cl− system. However, the detection of fused chlorinated byproducts (i.e. chloro-anthracene-pentaol, dioxine, chlorinated dibenzofuran) reminds us of cautiousness in evaluating the applicability of Cu(II)-catalyzed Fenton-like reaction, particularly while it is to be applied to the treatment of wastewater contaminated with chlorophenols. Two independent models (i.e. “Cu(III) model” and “OH model”) were developed to describe the kinetics of Cu(II)/H2O2/Cl− system. The failure of “OH model” to rationalize the observed AOX decay has disproved the “OH model” through reductio ad absurdum. The ability of “Cu(III) model” to adequately explain the experimental data demonstrates that Cu(III)-chloro complexes, rather than OH, is the major product resulting from reactions between Cu(I)-chloro complexes and H2O2 at neutral pH.