Photoelectrocatalytic production of active chlorine on nanocrystalline titanium dioxide thin-film electrodes

Bias-free driven ion assisted photoelectrochemical system for sustainable wastewater treatment

Photoelectrochemical (PEC) systems have emerged as a prominent renewable energy-based technology for wastewater treatment, offering sustainable advantages such as eliminating dependence on fossil fuels or grid electricity compared to traditional electrochemical treatment methods. However, previous PEC systems often overlook the potential of ions present in wastewater as an alternative to externally applied bias voltage for enhancing carrier separation efficiency. Here we report a bias-free driven ion assisted photoelectrochemical (IAPEC) system by integration of an electron-ion acceptor cathode, which leverages its fast ion-electron coupling capability to significantly enhance the separation of electrons and holes at the photoanode. We demonstrate that Prussian blue analogues (PBAs) can serve as robust and reversible electron-ion acceptors that provide reaction sites for photoelectron coupling cations, thus driving the hole oxidation to produce strong oxidant free radicals at photoanode. Our IAPEC system exhibits superior degradation performance in wastewater containing chloride medium. This indicates that, in addition to the cations (e.g., Na+) accelerating the electron transfer rate, the presence of Cl- ions further enhance efficient and sustainable wastewater treatment. This work highlights the potential of utilizing abundant sodium chloride in seawater as a cost-effective additive for wastewater treatment, offering crucial insights into the use of local materials for effective, low-carbon, and sustainable treatment processes.

Photoelectrocatalytic Surfactant Pollutant Degradation and Simultaneous Green Hydrogen Generation

For the first time, we demonstrate a photoelectrocatalysis technique for simultaneous surfactant pollutant degradation and green hydrogen generation using mesoporous WO3/BiVO4 photoanode under simulated sunlight irradiation. The materials properties such as morphology, crystallite structure, chemical environment, optical absorbance, and bandgap energy of the WO3/BiVO4 films are examined and discussed. We have tested the anionic type (sodium 2-naphthalenesulfonate (S2NS)) and cationic type surfactants (benzyl alkyl dimethylammonium compounds (BAC-C12)) as model pollutants. A complete removal of S2NS and BAC-C12 surfactants at 60 and 90 min, respectively, by applying 1.75 V applied potential vs RHE to the circuit, under 1 sun was achieved. An interesting competitive phenomenon for photohole utilization was observed between surfactants and adsorbed water. This led to the formation of H2O2 from water alongside surfactant degradation (anode) and hydrogen evolution (cathode). No byproducts were observed after the direct photohole mediated degradation of surfactants, implying its advantage over other AOPs and biological processes. In the cathode compartment, 82.51 μmol/cm2 and 71.81 μmol/cm2 of hydrogen gas were generated during the BAC-C12 and S2NS surfactant degradation process, respectively, at 1.75 V RHE applied potential.

Electrochemical Disinfection of Simulated Ballast Water Using RuO2-TiO2/Ti Electrode

The present work investigated the treatment of ballast water via electrochemical disinfection using a RuO₂-TiO₂/Ti electrode. Batch tests were conducted with simulated ballast water containing Escherichia coli as an indicator organism. The effect of varying NaCl concentrations (1%, 2%, and 3%; w/v) and current densities (0.3, 1.0, 2.0, and 3.0 mA/cm²) on the inactivation of E. coli was examined. Results showed higher disinfection efficiency of E. coli was obtained at higher NaCl concentration and current density. Complete inactivation of E. coli was attained within 2 and 1 min at 0.3 and 1 mA/cm², respectively, under 3% NaCl concentration. Meanwhile, complete disinfection at 1 and 2% NaCl concentrations was observed in 6 and 2 min, respectively, using a current density of 0.3 mA/cm². The 100% inactivation of E. coli was achieved with an energy consumption in the range of 2.8 to 2.9 Wh/m³ under the NaCl concentrations at 1 mA/cm² and 1 min of electrolysis time. The complete disinfection attained within 1 min meets the D-2 standard (<250 CFU E. coli/100 mL) of ballast water under the International Maritime Organization. The values of energy consumption of the present work are lower than previous reports on the inactivation of E. coli from simulated ballast water.

Heterogeneous Photocatalytic Chlorination of Methylene Blue Using a Newly Synthesized TiO2-SiO2 Photocatalyst

The titanium dioxide-silicon dioxide (TiO2-SiO2) nanocomposite used for the study was synthesized using a sol-gel method followed by UV-treatment. The physicochemical properties of the synthesized catalyst, TiO2-SiO2 were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL). The photocatalytic degradation of methylene blue (MB) dye was evaluated in the presence of TiO2-SiO2 and reactive chlorine species (RCS) under experimental conditions. By comparing the important reaction processes in the study, including photocatalysis, chlorination and photocatalytic chlorination, it was found out that the process of photocatalytic chlorination had the highest photodegradation efficiency (95% at 60 min) of the MB under optimum reaction conditions (MB = 6 mg L−1, catalyst = 0.1 g and pH = 4). The enhanced removal of MB from the aqueous medium was identified because of the synergy between chlorination and photocatalysis activated in the presence of TiO2-SiO2. The mechanism of the photocatalytic chlorination process was scrutinized in the presence of various RCS and reactive oxygen species (ROS) scavengers. Based on the experimental data attained, Na2S2O3 exhibited the highest inhibitory effect on the degradation efficiency of MB, indicating that the RCS is the main contributor to visible light-induced photodegradation of MB.

Effect of Potential and Chlorides on Photoelectrochemical Removal of Diethyl Phthalate from Water

Removal of persistent pollutants from water by photoelectrocatalysis has emerged as a promising powerful process. Applied potential plays a key role in the photocatalytic activity of the semi-conductor as well as the possible presence of chloride ions in the solution. This work aims to investigate these effects on the photoelectrocatalytic oxidation of diethyl phthalate (DEP) by using TiO2 nanotubular anodes under solar light irradiation. PEC tests were performed at constant potentials under different concentration of NaCl. The process is able to remove DEP following a pseudo-first order kinetics: values of kapp of 1.25 × 10−3 min−1 and 1.56 × 10−4 min−1 have been obtained at applied potentials of 1.8 and 0.2 V, respectively. Results showed that, depending on the applied potential, the presence of chloride ions in the solution affects the degradation rate resulting in a negative effect: the presence of 500 mM of Cl− reduces the value of kapp by 50 and 80% at 0.2 and 1.8 V respectively.

Photo-Chlorine Production with Hydrothermally Grown and Vacuum-Annealed Nanocrystalline Rutile

Photo-generated high-energy surface states can help to produce chlorine in aqueous environments. Here, aligned rutile (TiO2) nanocrystal arrays are grown onto fluorine-doped tin oxide (FTO) substrates and activated either by hydrothermal Sr/Ba surface doping and/or by vacuum-annealing. With vacuum-annealing, highly photoactive films are obtained with photocurrents of typically 8 mA cm−2 at 1.0 V vs. SCE in 1 M KCl (LED illumination with λ = 385 nm and approx. 100 mW cm−2). Photoelectrochemical chlorine production is demonstrated at proof-of-concept scale in 4 M NaCl and suggested to be linked mainly to the production of Ti(III) surface species by vacuum-annealing, as detected by post-catalysis XPS, rather than to Sr/Ba doping at the rutile surface. The vacuum-annealing treatment is proposed to beneficially affect (i) bulk semiconductor TiO2 nanocrystal properties and electron harvesting, (ii) surface TiO2 reactivity towards chloride adsorption and oxidation, and (iii) FTO substrate performance.

Photometric Sensing of Active Chlorine, Total Chlorine, and pH on a Microfluidic Chip for Online Swimming Pool Monitoring

A microfluidic sensor was studied for the photometric detection of active chlorine, total chlorine, and pH in swimming pool samples. The sensor consisted of a four-layer borosilicate glass chip, containing a microchannel network and a 2.2 mm path length, 1.7 mL optical cell. The chip was optimised to measure the bleaching of methyl orange and spectral changes in phenol red for quantitative chlorine (active and total) and pH measurements that were suited to swimming pool monitoring. Reagent consumption (60 mL per measurement) was minimised to allow for maintenance-free operation over a nominal summer season (3 months) with minimal waste. The chip was tested using samples from 12 domestic, public, and commercial swimming pools (indoor and outdoor), with results that compare favourably with commercial products (test strips and the N,N'-diethyl-p-phenylenediamine (DPD) method), precision pH electrodes, and iodometric titration.

In Situ Photoelectrochemical Chloride Activation Using a WO3 Electrode for Oxidative Treatment with Simultaneous H2 Evolution under Visible Light

Reactive chlorine species (RCS) such as HOCl and chlorine radical species is a strong oxidant and has been widely used for water disinfection. This study investigated a photoelectrochemical (PEC) method of RCS production from ubiquitous chloride ions using a WO₃ film electrode and visible light. The degradation of organic substrates coupled with H₂ evolution using a WO₃ electrode was compared among electrochemical (EC), photocatalytic (PC), and PEC conditions (potential bias: +0.5 V vs Ag/AgCl; λ > 420 nm). The degradation of 4-chlorophenol, bisphenol A, acetaminophen, carbamazepine, humic acid, and fulvic acid and the inactivation of E. coli were remarkably enhanced by in situ RCS generated in PEC conditions, whereas the activities of the PC and EC processes were negligible. The activities of the WO₃ film were limited by rapid charge recombination in the PC condition, and the potential bias of +0.5 V did not induce any significant reactions in the EC condition. The PEC activities of WO₃ were limited in the absence of Cl^- but significantly enhanced in the presence of Cl^-, which confirmed the essential role of RCS in this PEC system. The PEC mineralization of organic compounds was also markedly enhanced in the presence of Cl^- where dark chemical chlorination by NaOCl addition induced a negligible mineralization. The H₂ generation was observed only at the PEC condition and was negligible at PC and EC conditions. On the other hand, the oxidation of chloride on a WO₃ photoanode produced chlorate (ClO₃^-) as a toxic byproduct under UV irradiation, but the visible light-irradiated PEC system generated no chlorate.

Comparative Analysis of Photocatalytic and Electrochemical Degradation of 4-Ethylphenol in Saline Conditions

We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD) technologies for saline water purification, with a focus on rate comparison and formation and degradation of chlorinated aromatic intermediates using the same non-chlorinated parent compound, 4-ethylphenol (4EP). At 15 mA·cm^-2, and in the absence of chloride (0.6 mol·L^-1 NaNO₃ was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of 30 μmol L^-1·h^-1, whereas rates of ∼300 μmol L^-1·h^-1 and ∼3750 μmol L^-1·h^-1 were computed for low (0.03 mol·L^-1) and high (0.6 mol·L^-1) NaCl concentration, respectively. For PCD, initial rates of ∼330 μmol L^-1·h^-1 and 205 μmol L^-1·h^-1 were found for low and high NaCl concentrations, at a photocatalyst (TiO₂) concentration of 0.5 g·L^-1, and illumination at λ_max ≈ 375 nm, with an intensity ∼0.32 mW·cm^-2. In the chlorine mediated ECD approach, significant quantities of free chlorine (hypochlorite, Cl₂) and chlorinated hydrocarbons were formed in solution, while photocatalytic degradation did not show the formation of free chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated compounds in photocatalytic degradation is discussed based on photoelectrochemical results and existing literature, and explained by a chloride-mediated surface-charge recombination mechanism.

Electrochemical disinfection of bacteria-laden water using antimony-doped tin-tungsten-oxide electrodes

Electrochemical disinfection has been shown to be an efficient method with a shortrequired contact time for treatment of drinking water supplies, industrial raw water supplies, liquid foodstuffs, and wastewater effluents. In the present work, the electrochemical disinfection of saline water contaminated with bacteria was investigated in chloride-containing solutions using Sb-doped Sn_80%-W_20%-oxide anodes. The influence of current density, bacterial load, initial chloride concentration, solution pH, and the type of bacteria (E. coli D21, E. coli O157:H7, and E. faecalis) on disinfection efficacy was systematically examined. The impact of natural organic matter and a radical scavenger on the disinfection process was also examined. The electrochemical system was highly effective in bacterial inactivation for a 0.1 M NaCl solution contaminated with ∼10⁷ CFU/mL bacteria by applying a current density ≥1 mA/cm² through the cell.100% inactivation of E. coli D21 was achieved with a contact time of less than 60 s and power consumption of 48 Wh/m³, by applying a current density of 6 mA/cm² in a 0.1 M NaCl solution contaminated with ∼10⁷ CFU/mL. Reactive chlorine species as well as reactive oxygen species (e.g. hydroxyl radicals) generated in situ during the electrochemical process were determined to be responsible for inactivation of bacteria.

Phenols, flame retardants and phthalates in water and wastewater - a global problem

Organic pollutants in water and wastewater have been causing serious environmental problems. The arbitrary discharge of wastewater by industries, and handling, use, and disposal constitute a means by which phenols, flame retardants (FRs), phthalates (PAEs) and other toxic organic pollutants enter the ecosystem. Moreover, these organic pollutants are not completely removed during treatment processes and might be degraded into highly toxic derivatives, which has led to their occurrence in the environment. Phenols, FRs and PAEs are thus highly toxic, carcinogenic and mutagenic, and are capable of disrupting the endocrine system. Therefore, investigation to understand the sources, pathways, behavior, toxicity and exposure to phenols, FRs and PAEs in the environment is necessary. Formation of different by-products makes it difficult to compare the efficacy of the treatment processes, most especially when other organic matters are present. Hence, high levels of phenols, FRs and PAEs removal could be attained with in-line combined treatment processes.

The electrochemistry of nanostructured titanium dioxide electrodes

Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.

Improvement of gaseous pollutant photocatalysis with WO3/TiO2 heterojunctional-electrical layered system

Since the photogenerated holes play a much more important role than electrons in gas-phase photocatalysis, it is better to enrich the holes in the surface of a material system. Here, a novel [interdigital electrode/WO(3)/TiO(2)] heterojunctional-electrical layered (HEL) system is proposed to realize this attempt. The HEL system consists of interdigital electrode, WO(3) layer and TiO(2) layer, and they are orderly printed onto the alumina substrate from bottom to top using the technology of screen printing. It is surprise that the synergistic effect of layered heterojunction and external low bias can strengthen the separation of electron-hole pairs in both TiO(2) and WO(3), and enrich the TiO(2) surface layer with photogenerated holes to degrade the gaseous pollutants. In comparison with the pure TiO(2) film, a 6-fold enhancement in photocatalytic activity was observed using the HEL system by applying a very low bias of 0.2V. Furthermore, the results also showed that the remarkable improvement could not be obtained when either the WO(3) layer or the low external bias was absent.

Disinfection and formation of disinfection by-products in a photoelectrocatalytic system

In this study, disinfection and formation of disinfection by-products (DBPs) were studied in a photoelectrocatalytic (PEC) treatment system. Disinfection performance of titanium dioxide (TiO(2)) in the PEC system was determined through Escherichia coli (E. coli) inactivation. Humic acid (HA) was used as a model organic compound and its removal was monitored by total organic carbon (TOC) measurements using 410 nm (color) and 254 nm (UV(254)) wavelengths. Trihalomethanes (THMs) were measured for the evaluation of DBPs formation during PEC treatment of chloride and HA mixture. It was found that unlike photocatalytic treatment, THMs might form in the PEC system. To investigate the effects of anions on the PEC treatment, chloride (Cl(-)), sulfate (SO(4)(2-)), phosphoric acid (H(2)PO(4)(-))/hydrogen phosphate (HPO(4)(2-)) and bicarbonate (HCO(3)(-)) ions were added separately to the HA and bacterial suspensions. Presence of H(2)PO(4)(-)/HPO(4)(2-) and HCO(3)(-) ions resulted in inhibitory effects on both HA degradation and E. coli inactivation, which were also examined in the photoanode. It was observed that the presence of HA had a strong inhibitory effect on the disinfection of E. coli.

Electrosorption-promoted photodegradation of opaque wastewater on a novel TiO(2)/carbon aerogel electrode

A novel electrosorption-photocatalysis synergistic electrode of TiO(2)/carbon aerogel (TiO(2)/CA) is prepared. The thermal stability and dispersion of the anatase TiO(2) particles are well facilitated by the porous and discontinuous microstructure of CA. The degradation experiments show that the TiO(2)/CA material is not only a good photocatalyst but also an excellent electrosorptive electrode. The TiO(2)/CA is easily molded to an agglomerate electrode. The opaque wastewater with dyestuff is degraded effectively by the electrosorption-promoted photocatalytic process on this electrode. For the simulated methylene blue (MB) wastewater (150 mg L(-1)), the rate constant of MB degradation in the electro-assisted photocatalytic process with the conventional ITO-supported TiO(2) (TiO(2)/ITO) is 0.55 x 10(-3) min(-1) and that the electrosorption-promoted photocatalysis with TiO(2)/CA is 10.27 x 10(-3) min(-1), which is 18 times the former. In the electrosorption-promoted photocatalytic process with TiO(2)/CA, the energy consumption removing per unit TOC is only 15% of that in the electro-assisted photocatalysis with TiO(2)/ITO, because the electrosorption is a nonfaradic process irrelative to any electron transfer and requires very low consumption. This study provides a new method for exploring highly efficient electrosorption-promoted photocatalytics technology in the treatment of opaque wastewater.

Cu-TiO2/Ti dual rotating disk photocatalytic (PC) reactor: dual electrode degradation facilitated by spontaneous electron transfer

A Cu-TiO2/Ti dual rotating disk photocatalytic (PC) reactor has been developed based on our single rotating disk photoelectrocatalytic (PEC) reactor (Y. Xu, et al, Environ. Sci. Technol. 2008, 42, 2612-2617), and successfully applied to the treatment of laboratory and industrial dye wastewater. Round TiO2/Ti and Cu disks of the same size are connected by a Cu wire and fixed parallel on an axis continually rotating at 90 rpm. High treatment efficiency is obtained due to direct photooxidation on the TiO2/Ti photoanode as well as additional degradation on the Cu cathode, which is speculated via indirect hydrogen peroxide (H2O2) oxidation and direct electro-reduction of dye on cathode. The mechanism of the Cu-TiO2/Ti dual rotating disk PC reactor was investigated. In a 20 mg L(-1) Rhodamine B (RB) solution, approximately 100 mV of potential and 10 microA of current were measured between the Cu and TiO2/Ti electrode during PC treatment. Such phenomenon was explained by spontaneous electron transfer based on the same principle of establishing a Schottky barrier. On the Cu electrode surface, the photoelectrons either reduced dye molecules directly or reacted with dissolved oxygen (DO) to form H2O2. Rotation of electrodes out of the solution enhanced the mass transfer of target compound and kept the aqueous film fresh. The Cu-TiO2/ Ti dual rotating disk PC reactor is a simple and effective device for the treatment of RB dye wastewater.

Photocatalytic and photoelectrocatalytic humic acid removal and selectivity of TiO(2) coated photoanode

In this study, photocatalytic (PC) and photoelectrocatalytic (PEC) treatment methods were comparatively investigated as a possible means of removing humic acid (HA) following absorbance at 254 nm (UV(254)) and total organic carbon (TOC) analysis. The enhanced HA degradation rates were obtained in the PEC system over the conventional PC process under acidic, neutral and alkaline conditions. Preliminary and binary experiments were performed to determine the selectivity of the photoanode in terms of HA and chloride oxidation. TOC, chlorine and photocurrent parameters proved that HA was selectively removed before chlorine generation. The inhibitory effect of carbonate ions on the performance of photoanode was also studied under different pH values.

Photoelectrocatalytic degradation of pentachlorophenol in aqueous solution using a TiO2 nanotube film electrode

Titanium dioxide (TiO2) nanotube film electrodes are fabricated by the anodic oxidation method. Scanning electron microscopy (SEM) showed that these tubes were well aligned and organized into high-density uniform arrays. XRD analysis showed the TiO2 nanotubes to be in the anatase crystal form. The TiO2 nanotube film electrode exhibited increased photoelectrocatalytic (PEC) capability compared to a traditional TiO2 film electrode fabricated using the anodizing method for pentachlorophenol (PCP) degradation in aqueous solution. The bias potential, pH value, and electrolyte concentration were shown to be important factors influencing the degradation of PCP by the PEC method using the TiO2 nanotube film electrode as the working electrode.