In recent years, new advanced oxidation processes based on the electrochemical technology, the so-called electrochemical advanced oxidation processes (EAOPs), have been developed for the prevention and remediation of environmental pollution, especially focusing on water streams. These methods are based on the electrochemical generation of a very powerful oxidizing agent, such as the hydroxyl radical (•OH) in solution, which is then able to destroy organics up to their mineralization. EAOPs include heterogeneous processes like anodic oxidation and photoelectrocatalysis methods, in which •OH are generated at the anode surface either electrochemically or photochemically, and homogeneous processes like electro-Fenton, photoelectro-Fenton, and sonoelectrolysis, in which •OH are produced in the bulk solution. This paper presents a general overview of the application of EAOPs on the removal of aqueous organic pollutants, first reviewing the most recent works and then looking to the future. A global perspective on the fundamentals and experimental setups is offered, and laboratory-scale and pilot-scale experiments are examined and discussed.

Electrochemical advanced oxidation processes: today and tomorrow. A review

Titanium dioxide (TiO2) has been the most intensively investigated binary transition metal oxide in the past four decades as indicated by Figure S1. Furthermore, the annual number of papers published on TiO2 has seen a continuous increase, particularly since the beginning of this century (Figure S2). This is understandable when one considers the wide range of applications of TiO2 from the conventional areas (i.e., pigment, cosmetic, toothpaste, and paint) to the later developed functional areas such as photoelectrochemical cell,(1-3) dye-sensitized solar cells (DSSCs),(4-11) photocatalysis,(12-24) catalysis,(25-31) photovoltaic cell,(32-34) lithium ion batteries,(35-41) sensors,(42-46) electron field emission,(47-51) microwave absorbing material, biomimetic growth, and biomedical treatments.(52-57) Nearly all these functional applications of TiO2 fall in the scope of energy, environment, and health, which are definitely the three most important and challenging themes facing the Human race that need to be addressed in this century. Besides the apparent merits including nontoxicity, elemental abundance, good chemical stability, and easy synthesis, TiO2 has attracted strong research interest worldwide due to its physicochemical properties for realizing various functions.(15, 58, 59) Especially, very encouraging progresses in photocatalysis and DSSCs with the involvement of TiO2 have greatly stimulated the rapid development of TiO2 crystals with controllable phase, size, shape, defect, and heteroatom.(58, 60-68)

Titanium dioxide crystals with tailored facets

Remediation of water pollution caused by pharmaceutical residues based on electrochemical separation and degradation technologies: a review.

Titanium Dioxide Nanomaterials for Sensor Applications

Modification of bio-char derived from fast pyrolysis of biomass and its application in removal of tetracycline from aqueous solution.

The principal impetus for the fabrication of functional nanotube materials comes from the promise of discovering unique structure-dependant properties and superior performance that are derived from their intrinsic nanotubular architecture. 1D TiO2 nanotube arrays prepared by the electrochemical anodization of self-organized porous structures on Ti foil have attracted great research interest in recent years owing to their peculiar architecture, remarkable properties, and potential for wide-ranging applications. Uniform TiO2 nanotubes are quite remarkably different in structure from other forms of TiO2, and are highly ordered, high-aspect-ratio structures with nanocrystalline walls perpendicular to electrically conductive Ti substrates, thereby naturally forming a Schottky-type contact. Moreover, these structures can be directly used as electrodes for photoelectric applications since the size of the nanotubes is very precisely controllable. The technological applications of TiO2 nanotube arrays are still at an early stage, but these remarkable structures have already been shown to be very promising for applications in sensing, catalysis, photovoltaics, photoelectrolysis, and nanotemplating. The electrical resistance of the TiO2 nanotubes changes by almost 7 orders of magnitude upon exposure to 1000 ppm H2, [13] the largest ever reported sensitivity of a material to a gas. Furthermore, the H2 evolution rate of TiO2 nanotube arrays has been reported to be 76 mL hw , which is the highest reported H2 generation rate for any oxide system upon photoelectrolysis. TiO2 nanotube arrays have also attracted great interest for enhancing the photocatalytic degradation of various organics, which makes them promising materials for the detection of pollutants. Given the increasing quantities of pollutants that are being dumped into water bodies, environmental monitoring and control have become issues of global concern. Chemical oxygen demand (COD) is one of the most widely used metrics in the field of water-quality analysis in many countries, and is frequently used as an important index for controlling the operation of wastewater treatment plants, wastewater effluent monitoring, and taxation of wastewater pollution. Up till now, several methods have been developed for the determination of accurate COD values, but the development of a rapid, accurate, and environmentally friendly method still remains a formidable challenge. The standard method for COD determination, the K2Cr2O7 method, requires reflux over a long period of time to achieve adequate oxidation and also results in the consumption of several expensive (Ag2SO4), corrosive (H2SO4), and toxic (Hg , Cr2O7 ) chemicals. In an attempt to shorten the time required for analysis, modified K2Cr2O7 methods have been developed based on microwaveor ultrasound-assisted oxidation. Other alternative assays have also been developed such as electrocatalytic determination using PbO2 or Cu sensors in thin-cell reactors, and photocatalytic and photoelectrocatalytic methods based on TiO2 nanomaterial sensors. [18,19] However, all these modified K2Cr2O7 methods are still plagued by the secondary pollution caused by highly toxic Cr(VI) ions, and moreover, the PbO2 sensors pose the risk of the potential release of hazardous Pb during the preparation and disposal of the active material of the sensors. As compared to traditional analytical methods, photoelectrocatalytic approaches are more promising because of the superior oxidative abilities of illuminated TiO2. Furthermore, TiO2 nanomaterials are typically non-toxic, inexpensive, photostable, and environmentally benign. The main drawback of TiO2-based methods in practical applications is the easy recombination of the photogenerated electron/hole pairs in discrete TiO2 nanoparticles and coated nanofilms, which results in low photocatalytic activity. For the determination of COD, this implies a narrow dynamic working range and relatively poor reproducibility. To tackle these problems, here we have combined TiO2 nanotube array sensors that exhibit structure-dependant enhanced photoelectrocatalytic activity with thin-cell reactors that have the advantage of small volume and rapid diffusion rate. A photoelectrocatalytic assay has been developed using these two components to establish a rapid, accurate, and environmentally friendly method for COD determination. The preparation, characterization, and photoelectrocatalytic properties of the TiO2 nanotube array sensors and their application in COD determination are described in detail below. C O M M U N IC A TI O N

Self‐Organized TiO2 Nanotube Array Sensor for the Determination of Chemical Oxygen Demand

We synthesized highly ordered TiO 2 nanopore arrays (TNPs) with controllable pore size and good uniformity by anodization at low temperature in fluorinated dimethyl sulfoxide (DMSO) solution with a post-sonication treatment. TNPs possess excellent separation and transport properties of photo-generated electron/hole pair and hence reveal enhanced photocurrent response and photochemical properties for water splitting and organic compound degradation. The TNPs present maximum photo-conversion efficiency for water splitting of 0.28% under AM1.5 irradiation. This value compares favorably with a maximum photo-conversion efficiency of 0.21% for TiO 2 nanotube arrays (TNAs) under the same conditions. When illuminated with 1.0 mW cm −2 UV light, the maximum photo-conversion efficiency can be increased to 22% for TNPs, which is 5% higher than TNAs. The kinetic constant of photoelectrocatalytic (PEC) degradation of methyl orange (MO) for TNPs is found to be 1.27 times as high as that for TNAs when biased at 0.5 V. The visible light response of TNPs might be further improved by relevant surface modification technology.

Efficient photochemical water splitting and organic pollutant degradation by highly ordered TiO2 nanopore arrays

Photoelectrocatalytic COD determination method using highly ordered TiO2 nanotube array

Self-organized and highly ordered titania nanotube arrays (TNAs) were prepared through electrochemical anodic oxidization on a titanium foil in 0.5 wt.% hydrofluoric acid (HF) electrolyte. The current density and morphology images during the formation process of TNAs were studied. Results show that the formation of TNAs includes the following processes. Initially, dense oxide of titania was rapidly formed on the titanium surface, followed by small pore formation. The adjacent small pores were then integrated and become larger pores. At the same time, small tubes were transformed. These small tubes were further integrated into larger tubes until the primary tube formation. Finally, the tubular structure was gradually optimized and eventually developed into the highly ordered TNAs. A model was proposed to explain the formation mechanism of TNAs fabricated on a titanium foil in HF acid electrolyte.

The formation mechanism of titania nanotube arrays in hydrofluoric acid electrolyte

A novel thin-layer photoelectrocatalytic (PEC) reactor has been developed and successfully applied to the degradation of tetracycline, a typical pharmaceutical and personal care products (PPCPs), in bulk wastewater treatment. The reactor is designed with double thin-layer in order to increase the A/V ratio (ratio of electrode area and solution volume) and improve the mass transfer as well as the photonic efficiency of the system. The efficiency of organic degradation by the new reactor is much higher in comparison with conventional PEC process under similar treatment conditions. Using 20–120 mg L−1 model tetracycline wastewater as the target organics, the PEC removal rate of tetracycline in the reactor reached 96.4–54.8% with a single-side illumination, much higher than that of conventional PEC reactor (about 80.4–14.6%) within 1 h. When the reactor was illuminated from double sides, the treatment efficiency of thin-layer reactor was doubled without increasing new electrodes. This novel reactor always keeps high quantum efficiency regardless of the different concentration levels of the organic compounds, especially superior for the treatment of high concentration solutions. Repeated experiments in the treatment of 400 mg L−1 tetracycline solution demonstrated excellent stability and reliability of the TNAs electrode.

Qing Zheng

Papers

Self‐Organized TiO2 Nanotube Array Sensor for the Determination of Chemical Oxygen Demand

Efficient photochemical water splitting and organic pollutant degradation by highly ordered TiO2 nanopore arrays

Photoelectrocatalytic COD determination method using highly ordered TiO2 nanotube array

The formation mechanism of titania nanotube arrays in hydrofluoric acid electrolyte

A novel thin-layer photoelectrocatalytic (PEC) reactor with double-faced titania nanotube arrays electrode for effective degradation of tetracycline