Electrochemical degradation of refractory pollutant using a novel microstructured TiO2 nanotubes/ Sb-doped SnO2 electrode

A promising electrode material modified by Nb-doped TiO2 nanotubes for electrochemical degradation of AR 73

A distinctive SnO₂Sb electrode with highly ordered Nb doped TiO₂ nanotubes sheet as a new substrate, obtained by NbTi alloy anodization, is prepared by pulse electrochemical deposition for the first time as electrocatalytic oxidation anode for wastewater treatment. The novel electrode has a larger surface area and smaller crystallite particles than conventional SnO₂Sb electrodes as obtained from the analysis of scanning electron microscopy and X-ray diffraction. Compared with Ti/SnO₂Sb and Ti/TiO₂-NTs/SnO₂Sb prepared by pulse electrochemical deposition, the electrode modified by NbTiO₂-NTs has the higher oxygen evolution potential of 2.29 V (vs. Ag/AgCl), and the lower charge transfer resistance, which decreased by 65% and 79%. The service lifetime of NbTi/NbTiO₂-NTs/SnO₂Sb is 4.9 times longer than that of Ti/SnO₂Sb and 1.9 times longer than that of Ti/TiO₂-NTs/SnO₂Sb. The new electrode is proved to have an excellent electrochemical oxidation and degradation ability using Acid Red 73 as a target organic pollutant. The AR 73 removal, chemical oxygen demand removal and kinetic rate constant are increased obviously due to the introduction of NbTiO₂-NTs. Besides, the energy consumption reduces 37.2% and 31.4% in contrast with Ti/SnO₂Sb and Ti/TiO₂-NTs/SnO₂Sb. Hence, the Ti/SnO₂Sb modified by NbTiO₂-NTs is a very promising anode material for the electrochemical treatment of dye wastewater.

Titanium dioxide nanotube membranes for solar energy conversion: effect of deep and shallow dopants

Here we show the effect of shallow and deep dopants on titanium dioxide (TiO₂) nanotube membranes, for applications in photocatalytic, photoelectrochemical, photovoltaic, and other photosensitized devices for converting light into chemical feedstocks or electricity.

Preparation and characterization of a TiO2-NT/SnO2–Sb tubular porous electrode with long service lifetime for wastewater treatment process

We have studied the formation process of a novel TiO₂-NTs/SnO₂–Sb tubular porous electrode with a long service lifetime for the wastewater treatment process.

Synthesis and Stabilization of Blue-Black TiO2 Nanotube Arrays for Electrochemical Oxidant Generation and Wastewater Treatment

Efficient, inexpensive, and stable electrode materials are key components of commercially viable electrochemical wastewater treatment system. In this study, blue-black TiO₂ nanotube array (BNTA) electrodes are prepared by electrochemical self-doping. The 1-D structure, donor state density, and Fermi energy level position are critical for maintaining the semimetallic functionality of the BNTA. The structural strength of the BNTA is enhanced by surface crack minimization, reinforcement of the BNTA-Ti metal interface, and stabilized by a protective overcoating with nanoparticulate TiO₂ (Ti/EBNTA). Ti/EBNTA electrodes are employed as both anodes and cathodes with polarity switching at a set frequency. Oxidants are generated at the anode, while the doping levels are regenerated along with byproduct reduction at the cathode. The estimated maximum electrode lifetime is 16 895 h. Ti/EBNTA has comparable hydroxyl radical production activity (6.6 × 10^-14 M) with boron-doped diamond (BDD, 7.4 × 10^-14 M) electrodes. The chlorine production rate follows a trend with respective to electrode type of Ti/EBNTA > BDD > IrO₂. Ti/EBNTA electrodes operated in a bipolar mode have a minimum energy consumption of 62 kWh/kg COD, reduced foam formation due to less gas bubble production, minimum scale formation, and lower chlorate production levels (6 mM vs 18 mM for BDD) during electrolytic wastewater treatment.

Electrocatalytic oxidation of phenol from wastewater using Ti/SnO2-Sb2O4 electrode: chemical reaction pathway study

In this study, a titanium plate was impregnated with SnO2 and Sb (Ti/SnO2-Sb2O4) for the electrocatalytic removal of phenol from wastewater, and the chemical degradation pathway was presented. The effects of various parameters such as pH, current density, supporting electrolyte, and initial phenol concentration were studied. At optimum conditions, it was found that phenol was quickly oxidized into benzoquinone because of the formation of various strong radicals during electrolysis by the Ti/SnO2-Sb2O4 anode from 100 to <1 mg/L over 1 h. The results of GC/MS analysis showed the presence of some esters of organic acid such as oxalic acid and formic acid. HPLC analysis showed only trace amounts of benzoquinone remaining in the solution. The efficiency of TOC removal at the Ti/SnO2-Sb2O4 anode surface showed a degradation rate of 49 % over 2 h. Results showed that the molecular oxygen potential at the electrode was 1.7 V. The phenol removal mechanism at the surface of the Ti/SnO2-Sb2O4 anode was influenced by the pH. Under acidic conditions, the mechanism of electron transfer occurred directly, whereas under alkaline conditions, the mechanism can be indirect. This research shows that the proposed electrolyte can significantly influence the efficiency of phenol removal. It can be concluded that the treatment using an appropriate Ti/SnO2-Sb2O4 electrode surface can result in the rapid oxidation of organic pollutants.

Electrochemical degradation of perfluorooctanoic acid (PFOA) by Yb-doped Ti/SnO2–Sb/PbO2anodes and determination of the optimal conditions

Model aqueous solutions of perfluorooctanoic acid (PFOA, 100 mg L⁻¹) were electro-oxidized in a homemade container.

Enhanced electrochemical oxidation of phenol using a hydrophobic TiO2-NTs/SnO2-Sb-PTFE electrode prepared by pulse electrodeposition

Novel TiO₂-NTs/SnO₂-Sb-PTFE electrodes were fabricated by pulse electrodeposition with higher oxygen evolution potential, improved surface hydrophobicity and enhanced electrocatalytic activity.

Electrochemical degradation of refractory pollutants using TiO2 single crystals exposed by high-energy {001} facets

Anodic material plays a vital role in electrochemical water treatment. Titanium dioxide (TiO2) has been widely recognized as an excellent semiconductor photocatalyst, rather than an efficient electrocatalyst, due to its relatively low electric conductivity and poor electrochemical activity. In this work, it is found that TiO2 can actually become a superior electrocatalyst when its crystal shape and exposed facet are finely tuned. The shape-engineered TiO2 single crystals with {001} facets exhibit an excellent electro-catalytic activity and stability for degrading typical organic pollutants such as rhodamine B and bisphenol A, and treating complex landfill leachate. Its electro-catalytic superiority is mainly attributed to the single-crystalline structure and exposed polar {001} facet. Our findings could provide new possibility of utilizing TiO2 for efficient electrochemical water treatment because of its high activity, great stability, low cost and no toxicity.

Enhancing electrocatalytic performance of Sb-doped SnO ₂ electrode by compositing nitrogen-doped graphene nanosheets

An efficient Ti/Sb-SnO2 electrode modified with nitrogen-doped graphene nanosheets (NGNS) was successfully fabricated by the sol-gel and dip coating method. Compared with Ti/Sb-SnO2 electrode, the NGNS-modified electrode possesses smaller unite crystalline volume (71.11Å(3) vs. 71.32Å(3)), smaller electrical resistivity (13Ωm vs. 34Ωm), and lower charge transfer resistance (10.91Ω vs. 21.01Ω). The accelerated lifetime of Ti/Sb-SnO2-NGNS electrode is prolonged significantly, which is 4.45 times as long as that of Ti/Sb-SnO2 electrode. The results of X-ray photoelectron spectroscopy measurement and voltammetric charge analysis indicate that introducing NGNS into the active coating can increase more reaction active sites to enhance the electrocatalytic efficiency. The electrochemical dye decolorization analysis demonstrates that Ti/Sb-SnO2-NGNS presents efficient electrocatalytic performance for methylene blue and orange II decolorization. And its pseudo-first order kinetic rate constants for methylene blue and orange II decolorization are 36.6 and 44.0 min(-1), respectively, which are 6.0 and 7.1 times as efficient as those of Ti/Sb-SnO2, respectively. Considering the significant electrocatalytic activity and low resistivity of Ti/Sb-SnO2-NGNS electrode, the cost of wastewater treatment can be expected to be reduced obviously and the application prospect is broad.

Transformation and removal of arsenic in groundwater by sequential anodic oxidation and electrocoagulation

Oxidation of As(III) to As(V) is generally essential for the efficient remediation of As(III)-contaminated groundwater. The performance and mechanisms of As(III) oxidation by an as-synthesized active anode, SnO2 loaded onto Ti-based TiO2 nanotubes (Ti/TiO2NTs/Sb-SnO2), were investigated. The subsequent removal of total arsenic by electrocoagulation (EC) was further tested. The Ti/TiO2NTs/Sb-SnO2 anode showed a high and lasting electrochemical activity for As(III) oxidation. 6.67μM As(III) in synthetic groundwater was completely oxidized to As(V) within 60min at 50mA. Direct electron transfer was mainly responsible at the current below 30mA, while hydroxyl radicals contributed increasingly with the increase in the current above 30mA. As(III) oxidation was moderately inhibited by the presence of bicarbonate (20mM), while was dramatically increased with increasing the concentration of chloride (0-10mM). After the complete oxidation of As(III) to As(V), total arsenic was efficiently removed by EC in the same reactor by reversing electrode polarity. The removal efficiency increased with increasing the current but decreased by the presence of phosphate and silica. Anodic oxidation represents an effective pretreatment approach to increasing EC removal of As(III) in groundwater under O2-limited conditions.

Electrochemical oxidation of 1H,1H,2H,2H-perfluorooctane sulfonic acid (6:2 FTS) on DSA electrode: operating parameters and mechanism

The 6:2 FTS was the substitute for perfluorooctane sulfonate (PFOS) in the chrome plating industry in Japan. Electrochemical oxidation of 6:2 FTS was investigated in this study. The degradabilities of PFOS and 6:2 FTS were tested on the Ti/SnO₂-Sb₂O₅-Bi₂O₃ anode. The effects of current density, potential, and supporting electrolyte on the degradation of 6:2 FTS were evaluated. Experimental results showed that 6:2 FTS was more easily degraded than PFOS on the Ti/SnO₂-Sb₂O₅-Bi₂O₃ anode. At a low current density of 1.42 mA/cm², 6:2 FTS was not degraded on Ti/SnO₂-Sb₂O₅-Bi₂O₃, while the degradation ratio increased when the current density ranged from 4.25 to 6.80 mA/cm². The degradation of 6:2 FTS at current density of 6.80 mA/cm² followed pseudo first-order kinetics with the rate constant of 0.074 hr⁻¹. The anodic potential played an important role in the degradation of 6:2 FTS, and the pseudo first-order rate constants increased with the potential. The surface of Ti/SnO₂-Sb₂O₅-Bi₂O₃ was contaminated after electrolysis at constant potential of 3V, while the fouling phenomenon was not observed at 5V. The fouled anode could be regenerated by incinerating at 600°C. The intermediates detected by ultra-performance liquid chromatography coupled with a triple-stage quadrupole mass spectrometer (UPLC-MS/MS) were shorter chain perfluorocarboxylic acids. The 6:2 FTS was first attacked by hydroxyl radical, and then formed perfluorinated carboxylates, which decarboxylated and removed CF2 units to yield shorter-chain perfluorocarboxylic acids.

Shift of the reactive species in the Sb-SnO2-electrocatalyzed inactivation of e. coli and degradation of phenol: effects of nickel doping and electrolytes

The electrocatalytic behavior and anodic performance of Sb-SnO2 and nickel-doped Sb-SnO2 (Ni-Sb-SnO2) in sodium sulfate and sodium chloride electrolytes were compared. Nickel-doping increased the service lifetime by a factor of 9 and decreased the charge transfer resistance of the Sb-SnO2 electrodes by 65%. More importantly, Ni doping improved the electrocatalytic performance of Sb-SnO2 for the remediation of aqueous phenol and the inactivation of E. coli by a factor of more than 600% and ∼20%, respectively. In the sulfate electrolyte, the primary reactive oxygen species (ROS) identified were OH radicals (Faradaic efficiency η = 2.4%) with trace levels of ozone and hydrogen peroxide (η < 0.01%) at Sb-SnO2. In contrast, the primary ROS at Ni-Sb-SnO2 was ozone (η = 9.3%) followed by OH radicals (η = 3.7%). In the chloride electrolyte, the production of hypochlorite (OCl(-)) was higher (η = 0.73%) than that of ozone (η = 0.13%) at Sb-SnO2, whereas the level of ozone (η = 13.6%) was much higher than that of hypochlorite (η = 0.24%) at Ni-Sb-SnO2. Based on the shift of the reactive species, the primary effect of Ni doping is to catalyze the six-electron oxidation of water to ozone and inhibit the competing one or two-electron oxidation of water (generation of OH radicals, hydrogen peroxides, and hypochlorites). A range of electrochemical and surface analyses were performed, and a detailed mechanism was proposed.

Enhanced electrocatalytic activity of nano-TiN composited Ti/Sb–SnO2 electrode fabricated by pulse electrodeposition for methylene blue decolorization

The Sb-doped SnO₂ electrode is modified by TiN nanoparticles and has higher stability and significantly enhanced electrochemical decolorization activity.

Bismuth-doped tin oxide-coated carbon nanotube network: improved anode stability and efficiency for flow-through organic electrooxidation

In this study, a binder-free, porous, and conductive 3D carbon-nanotube (CNT) network uniformly coated with bismuth-doped tin oxide (BTO) nanoparticles was prepared via a simple electrosorption-hydrothermal method and utilized for the electrooxidative filtration of organics. The BTO-CNT nanocomposite was characterized by scanning electron microscopy, thermogravimetric analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, linear sweep voltammetry, and Tafel analysis. The submonolayer BTO coating is composed of 3.9±1.5 nm diameter nanoparticles (NPs). The oxygen-evolution potential of the BTO-CNT nanocomposite was determined to be 1.71 V (vs Ag/AgCl), which is 440 mV higher than an uncoated CNT anode. Anodic stability, characterized by CNT oxidative corrosion to form dissolved species, indicated that the BTO-CNT incurred negligible corrosion up to Vanode=2.2 V, whereas the uncoated CNT was compromised at Vanode≥1.4 V. The effect of metal oxide-nanoparticle coating on anodic performance was initially studied by oxalate oxidation followed by total organic carbon (TOC) and chemical oxygen demand (COD) analysis. The BTO-CNT displayed the best performance, with ∼98% oxalate oxidation (1.2 s filter residence time) and current efficiencies in the range of 32 to >99%. The BTO-CNT anode energy consumption was 25.7 kW h kgCOD(-1) at ∼93% TOC removal and 8.6 kW h kgCOD(-1) at ∼50% TOC removal, comparable to state-of-the-art oxalate oxidation processes (22.5-81.7 kW h kgCOD(-1)). The improved reactivity, current efficiency, and energy consumption are attributed to the increased conductivity, oxygen-evolution potential, and stability of the BTO-CNT anode. The effectiveness and efficiency of the BTO-CNT anode as compared to the uncoated CNT was further investigated by the electrooxidative filtration of ethanol, methanol, formaldehyde, and formate, and it was determined to have TOC removals 2 to 8 times greater, mineralization current efficiencies 1.5 to 3.5 times greater, and energy consumption 4 to 5 times less than the uncoated CNT anode. Electrooxidation and anode passivation mechanisms are discussed.

Fabrication of bidirectionally doped β-Bi2O3/TiO2-NTs with enhanced photocatalysis under visible light irradiation

Stable β-Bi2O3/TiO2-NTs photocatalyst with excellent visible-light-activity is successfully prepared by bidirectional doping. Stake structure of the TiO2-NTs provides a larger specific surface area and makes the contact area between the TiO2-NTs and β-Bi2O3 much larger; The stake structure of TiO2-NTs not only leads to a firmer combination of TiO2-NTs and β-Bi2O3, but also makes them dope one another deeply. The modification of Bi species into TiO2-NTs can form Bi-O-Ti chemical absorption bonds, then a localized impurity level is generated within the band gap. Electrons can be excited and transferred from the Bi(3+) impurity level to the conduction band (CB) of TiO2, similar to narrowing the band-gap of TiO2-NTs, resulting in a red shift of the absorption edge and an enhancement in visible-light activity. During annealing, Bi atoms are partially replaced by Ti atoms. The lattice of β-Bi2O3 is compressed around the Ti impurity, making the lattice dislocate and distort. This dislocation and distortion leads to an increase in the β-Bi2O3 valance band (VB), from 2.02 to 2.28 eV. Accordingly, the weak oxidability of β-Bi2O3 is improved, and its photocatalytic ability is further enhanced. Moreover, this lattice dislocation and distortion changes the Bi-O distances, thus remarkably improving the stability of the β-Bi2O3/TiO2-NTs.

Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for TCE reduction in groundwater

A novel cathode, Pd loaded Ti/TiO2 nanotubes (Pd-Ti/TiO2NTs), is synthesized for the electrocatalytic reduction of trichloroethylene (TCE) in groundwater. Pd nanoparticles are successfully loaded on TiO2 nanotubes which grow on Ti plate via anodization. Using Pd-Ti/TiO2NTs as the cathode in an undivided electrolytic cell, TCE is efficiently and quantitatively transformed to ethane. Under conditions of 100 mA and pH 7, the removal efficiency of TCE (21 mg/L) is up to 91% within 120 min, following pseudo-first-order kinetics with the rate constant of 0.019 min(-1). Reduction rates increase from 0.007 to 0.019 min(-1) with increasing the current from 20 to 100 mA, slightly decrease in the presence of 10 mM chloride or bicarbonate, and decline with increasing the concentrations of sulfite or sulfide. O2 generated at the anode slightly influences TCE reduction. At low currents, TCE is mainly reduced by direct electron transfer on the Pd-Ti/TiO2NT cathode. However, the contribution of Pd-catalytic hydrodechlorination, an indirect reduction mechanism, becomes significant with increasing the current. Compared with other common cathodes, i.e., Ti-based mixed metal oxides, graphite and Pd/Ti, Pd-Ti/TiO2NTs cathode shows superior performance for TCE reduction.

Photoelectrocatalytic properties of a vertically aligned Ti-W alloy oxide nanotubes array and its applications in dye wastewater degradation

A highly ordered and vertically oriented array of nanotubes (NTs) of mixed oxide was prepared in situ by Ti-W alloy anodization. Compared with the traditional TiO2 NTs, the photoelectrocatalytic activity of the resulting Ti-W-O NTs was greatly enhanced. Results indicated a narrowing of the band gap from 3.2 eV for pristine TiO2 to 2.7 eV for Ti-W-O NTs. Under irradiation with 254 and 365 nm UV lights, Ti-W-O NTs showed much higher photoelectroconversion efficiency (eta) than TiO2 NTs and TiO2-WO3 coating. The eta254 and eta365 on Ti-W-O NTs reached as high as 51.8% and 57.0% respectively, four to five times those on TiO2 NTs and TiO2-WO3 coating. As a result of its narrow band gap energy and fast electron-hole separation, Ti-W-O NTs presented outstanding photoelectrocatalytic features. The electrochemically assisted photocatalytic degradation of highly concentrated Rhodamine 6G wastewaters was studied. The results showed that the rates of colour and TOC removal were much higher on Ti-W-O NTs than on TiO2 NTs and TiO2-WO3 coating. The photocatalytic material obtained by alloy anodization is of significance in the advanced oxidation of environmental pollutants.

Efficient electrochemical oxidation of perfluorooctanoate using a Ti/SnO2-Sb-Bi anode

The electrochemical decomposition of persistent perfluorooctanoate (PFOA) with a Ti/SnO2-Sb-Bi electrode was demonstrated in this study. After 2 h electrolysis, over 99% of PFOA (25 mL of 50 mg·L(-1)) was degraded with a first-order kinetic constant of 1.93 h(-1). The intermediate products including short-chain perfluorocarboxyl anions (CF3COO-, C2F5COO-, C3F7COO-, C4F9COO-, C5F11COO-, and C6F13COO-) and F- were detected in the aqueous solution. The electrochemical oxidation mechanism was revealed, that PFOA decomposition first occurred through a direct one electron transfer from the carboxyl group in PFOA to the anode at the potential of 3.37 V (vs saturated calomel electrode, SCE). After that, the PFOA radical was decarboxylated to form perfluoroheptyl radical which allowed a defluorination reaction between perfluoroheptyl radical and hydroxyl radical/O2. Electrospray ionization (ESI) mass spectrum further confirmed that the oxidation of PFOA on the Ti/SnO2-Sb-Bi electrode proceeded from the carboxyl group in PFOA rather than C-C cleavage, and the decomposition processes followed the CF2 unzipping cycle. The electrochemical technique with the Ti/SnO2-Sb-Bi electrode provided a potential method for PFOA degradation in the aqueous solution.

A novel method for photodegradation of high-chroma dye wastewater via electrochemical pre-oxidation

A new two-step process involving the electrocatalytic (EC) pre-oxidation and the following photoelectrocatalytic synergistic (PEC) oxidation is proposed to treat the high concentration and high-chroma methyl orange dye wastewater, which cannot be degraded by photocatalytic oxidation (PC) directly. The SnO(2)/TiO(2)-NTs/Ti electrode simultaneously possessing the outstanding PC oxidation properties of TiO(2)-NTs and the excellent EC oxidation abilities of the Sb doped SnO(2) was synthesized by impregnating Sb doped SnO(2) nanoparticles into TiO(2)-NTs. In the pre-oxidation process as the first stage, the high-color dye wastewater is decolorized with electrochemical method to some extent. Then, the wastewater becomes a light transmission system. It provides a suitable condition for PC oxidation reaction in the second stage. The synergistic effects of PC and EC oxidation led to the high PEC efficiency and the complete mineralization of dye wastewater is achieved. This two-step process is fast and efficient, which is worthy to study and explore in the practical environmental treatment.

Fabrication and electrochemical treatment application of a novel lead dioxide anode with superhydrophobic surfaces, high oxygen evolution potential, and oxidation capability

A novel PbO(2) electrode with a high oxygen evolution potential (OEP) and excellent electrochemical oxidation performance is prepared to improve the traditional PbO(2) electrode, which is modified by changing the microstructure and wetting ability. A middle layer of TiO(2) nanotubes (NTs) with a large surface area is introduced on Ti substrate, and a small amount of Cu is predeposited at the bottom of TiO(2)-NTs. The modification will improve the electrochemical performance by enhancing the loading capacity of PbO(2) and the combination between PbO(2) and Ti substrate. The hydrophilic surface becomes highly hydrophobic by adding fluorine resin. The improved PbO(2) electrode exhibits a similar morphology, surface wetting ability, high OEP, and electrochemical performance with boron-doped diamond film (BDD) electrode. However, the physical resistance of the PbO(2) electrode is much lower than that of BDD, exhibiting higher conductivity. The hydroxyl radical utilization is significantly enhanced, resulting in a higher oxidation rate and higher removal for 2,4-dichlorophenoxyacetic acid.