Photo-assisted Electrochemical Process Research Articles

Photo-assisted electrochemical degradation of ciprofloxacin using DSA® anode with NaCl electrolyte and simultaneous chlorine photolysis

This study aimed to investigate and improve the performance of the photo-assisted electrochemical process (PAEC) for the degradation of ciprofloxacin (CIP) and the production of active chlorine species (ACS), using a DSA® (Dimensionally stable anodes) electrode, NaCl as the supporting electrolyte and UV radiation provided by a high-pressure mercury lamp (UVC). The performance of the system was assessed by monitoring the CIP concentration, ACS, chemical oxygen demand (COD), total organic carbon (TOC), and phytotoxicity tests. The effect of operational parameters, such as current density, NaCl concentration, volumetric flow, and initial pH of the solution were evaluated. Under operating conditions of j = 60.7 mA/cm2, [NaCl] = 0.3 mol/L, qv = 10 mL/min, and pH = 5, complete removal of CIP was achieved in 5 min and, with only 1 h of electrolysis, removal of approximately 60% TOC and 65% COD was obtained. The PAEC process promoted greater mineralization of the solution in relation to the isolated techniques and the phytotoxicity tests indicated that, despite the high salinity of the effluent (0.3 mol/L NaCl), the PAEC process (UVC 250 W) did not generate an increase in toxicity of the treated solution. The elucidated CIP degradation mechanism indicates that the piperazine group was the most attacked site of the molecule. Furthermore, the identified intermediates show that CIP was gradually destroyed, generating compounds with a lower risk of increasing bacterial resistance proliferation.

Employing speciation of metals to assess photo-assisted electrochemical efficiency for improving rainwater quality in Tehran, Iran

Photo-assisted electrochemical process is a promising technology for a clean ecosystem. It is largely utilized for recovering metals and oxidizing recalcitrant organic compounds. This paper aims to discuss the performance of the photo-oxidation and reduction by applying semiconductor photo-catalysis as the anode and graphite as the cathode for treating rainwater in Tehran, Iran. Organic pollutants and different metals were investigated to study the probable roles of the anodic potential, the gap between electrodes, and the concentration of electrolytes in the possible mechanisms. The laboratory experiments which lasted for 120 min exhibited that the best performance for organic, iron, manganese, and lead removal was at the anodic potential of 10 V and gap of 6 cm. But, for zinc and cadmium, the best performance occurred at the anodic potential of 5.5 V and sodium chloride of 0.65 g/L. The presented reactions and mechanisms demonstrated incidents using the treatment process. They were identical to the presented dendrogram and Pourbaix diagrams. The results of the diagrams showed iron, manganese, and lead were removed by oxidation at the photo-anode. Zinc and cadmium were mainly removed by precipitation. Chemical oxygen demand, iron, manganese, and lead were significantly related and behaved similarly in the treatment process. Their removal was controlled by all the analyzed parameters. This paper may present a cost-effective approach for removing organic and metallic pollutants by photo-assisted electrochemical process. It can be suggested for future evolution of photo-assisted electrochemical on the treatment applications of rainwater.

The influence of pH and current density on an UV254 photo-assisted electrochemical process generating active chlorine and radicals for efficient and rapid ciprofloxacin mineralization compared to individual techniques

A photo-assisted electrochemical process is herein evaluated with the aim of improving and accelerating the degradation and mineralization of Ciprofloxacin (CIP) in aqueous media, in comparison with single electrochemical or photochemical methods. Active chlorine species are initially electro-generated on a synthesized (Pechini method) Ti/TiO2-RuO2 DSA, and subsequently fragmented homolitically using UVC irradiation under very low fluency rate (3300 μW cm−2, λ = 254 nm) to produce radicals with higher oxidation potentials compared to the original active chlorine. A faster CIP degradation (5 min) and higher mineralization is obtained in the coupled system against the individual techniques. Variations of pH and current density in the UV-assisted electrochemical process reveal that a similar degradation rate is obtained during 60 min for pH 6 and 9 due to the co-existence of HOCl and OCl− species in this range of pH, leading to the possible production of highly reactive radicals (e.g. HOaq•, Claq•, Oaq-•) after chlorine species fragmentation. The highest total organic Carbon (TOC) removal was obtained at pH = 6 (70 % in 60 min), where the highest HOCl concentration is observed, thus, enhancing the HOaq• production. The contaminant degradation and mineralization strongly depend of the radicals formed from the irradiation of chlorine species, which on its turn rely on the redox couples formed as a function of pH: 3 (HOCl + Cl2), 6 and 9 (HOCl + OCl−). Mechanisms are proposed for these homolysis reactions, while the degradation pathway of CIP is rationalized using HPLC analyses during 60 min of electrolysis.

Effects of a photo-assisted electrochemical etching process removing dry-etching damage in GaN

We investigated the ability of a photo-assisted electrochemical (PEC) etching process to remove the damage that dry etching causes in the near-surface region of GaN samples. The process consists of anodic oxidation of the GaN surface and subsequent dissolution of the oxide with a chemical treatment, and the extent of the PEC reactions depends on the total charge density transferred in them. The PEC process was conducted for samples prepared with various dry-etching conditions followed by fabrication of Schottky barrier diodes (SBDs) and metal–insulator–semiconductor (MIS) capacitors. The PEC process greatly improved the barrier height, ideality factor, and reverse leakage current of SBDs. Capacitance–voltage measurements of MIS capacitors revealed that the densities of interface states and discrete traps were both reduced by the PEC process. The results obtained here show that the PEC process can remove dry-etching damage from the GaN surface.

(Invited) Electrochemical Formation and Application of Porous Gallium Nitride

This article provides an overview of our recent work on the formation and application of porous GaN. The photo-assisted electrochemical process in back-side-illumination mode, compared with front-side-illumination mode, has been demonstrated to form deeper pores where illumination with photon energy below the bandgap plays an important role. Electrochemical etching by using the avalanche effect in the dark was very effective in controlling the pore depth with an anodization time. Precise control of pore diameter was achieved by subsequent wet chemical etching after the formation of porous GaN. Photoelectrochemical measurements revealed the importance of precise structural controlling of porous GaN for application to energy-conversion systems.

Treatment of Wastewater Containing Sulfa Drug by Photo Active Anode (Ti/Ru0.3Ti0.7O2) in Photo-assisted Electrochemical Process

Drug residues are unregulated pollutants and are frequently found in the aquatic environment. Due to their extensive use, presence in the aquatic environment and their potential for impacts on wildlife and humans, it is becoming an important environmental issue. The present study evaluates the photo-assisted electrochemical treatment of sulfamethoxazole (SMX), an antibiotic used in the prevention and treatment of human and animal diseases. The experiments were carried out in a photo illuminated flow cell reactor and the effects of current density and concentration of sodium chloride in the supporting electrolyte were determined. The Drug could be removed completely at all current densities while TOC abatement was appreciable during the process. The process followed pseudo-first order kinetics and values of electrical energy-per-order were dependent on current density and sodium chloride concentration. Different reaction intermediates were separated and identified by liquid chromatography coupled with mass spectrometry (LC-MS), thereby providing an insight into the mechanistic details of the degradation pathway. The photo-assisted electrochemical degradation proceeded through competitive reactions such as chlorination, hydroxylation and substitution on the aromatic ring, while further ring opening gave rise to more simple organic compounds and, ultimately, the mineralization of SMX. The possible degradation route were also proposed and discussed. Figure 1

Development of kinetic models for photoassisted electrochemical process using Ti/RuO2 anode and carbon nanotube-based O2-diffusion cathode

A coupled photoassisted electrochemical system was utilized for degradation of C.I. Basic Yellow 28 (BY28) as a cationic azomethine dye under recirculation mode. Experiments were carried out by utilizing active titanium/ruthenium oxide (Ti/RuO2) anode and O2-diffusion cathode with carbon nanotubes (CNTs). Transmission electron microscopy (TEM) image of the CNTs demonstrated that CNTs had approximately an inner and outer diameter of 5nm and 19nm, respectively. Then, the dye degradation kinetics was experimentally examined under various operational parameters including BY28 initial concentration (mg/L), current density (mA/cm2), flow rate (L/h) and pH. Based on the generally accepted intrinsic elementary reactions for photoassisted electrochemical process (PEP), a novel kinetic model was proposed and validated for predicting the kapp. The developed kinetic model explicitly describes the dependency of the kapp on BY28 initial concentration and current density. A good agreement was obtained between the predicted values of kapp and experimental results (correlation coefficient (R2)=0.996, mean squared error (MSE)=2.10×10−4 and mean absolute error (MAE)=1.10×10−2). Finally, in order to profoundly evaluate and compare the accuracy of the suggested intrinsic kinetic model, an empirical kinetic model was also developed as a function of main operational parameters, and an artificial neural network model (ANN) by 3-layer feed-forward back propagation network with topology of 5:9:1. The performance of the mentioned models was compared based on the error functions and analysis of variance (ANOVA). A comparison according to the errors function demonstrated that the experimental data were fitted appropriately by all of the proposed models with an adequate accuracy. Moreover, ANOVA results showed that there is no significant discrepancy among the predicted values of the three proposed models.

Bias-dependent Photoabsorption Properties of GaN Porous Structures under Back-side Illumination

The high-density formation of semiconductor nano- and micro-structures—targeting applications such as optoelectronic devices and high-efficiency energy-conversion devices—has been intensely researched. Among these structures, porous structures formed by an electrochemical process are one of the most promising due to their unique features. The purpose of this study is to clarify the photoabsorption properties of GaN porous structures formed by the photo-assisted electrochemical process. GaN is one of the most attractive materials because of its chemical stability and its potential to achieve direct photoelectrolysis by solar power without consuming electric power. Fundamental photocurrent on porous structures was first measured and compared with that on GaN planar substrates. The mechanism of the observed photoabsorption properties of GaN porous structures was then investigated from both experimental and theoretical aspects. A custom-made cell equipped with a crystal window and indium-tin-oxide (ITO) plate was used for both the porous formation and spectroscopic measurements. The electrochemical cell has three electrodes: an n-type GaN electrode as a working electrode (W.E.), a platinum (Pt) counter electrode (C.E.), and Ag/AgCl reference electrode (R.E.). GaN-porous structures were formed using a photo-assisted electrochemical process in BSI mode [1]. As a UV light source for the photo-assisted electrochemical process, a monochromatic light with a wavelength of 370 nm was irradiated using a xenon (Xe) lamp through an optical band-path filter. The pore diameter and density of the GaN-porous samples were estimated to be about 25 nm and 6.5×1010 cm-2, respectively, which remain almost the same throughout the porous layer. It was previously demonstrated that deeper pores were obtained in BSI mode, as compared with those obtained in the FSI mode, because the pores are only etched at the pore tips due to the supply of holes. Photocurrents on the planar and porous electrodes measured in the NaCl electrolyte under the light irradiation with an intensity of 100 µW/cm2. For the planar electrode, large photocurrents were observed under light with wavelength of 360 nm. As the wavelength of the monochromatic light increased, the photocurrents drastically decreased. This result indicates that the band-edge absorption around 3.4 eV, corresponding to photon energy with a wavelength of 365 nm, plays an important role in the generation of photocarriers on the planar electrode. As expectedly, larger photocurrents were observed on the porous electrode, reflecting the large surface area of the electrode and its low photo-reflectance properties due to the high-density array of pores [2]. It is also noted that the photocurrents were even observed under illumination with wavelength of 380 nm, corresponding to a photon energy of 3.26 eV, which is smaller than the bandgap energy of the GaN bulk (i.e., 3.4 eV). The photocurrents obtained on the porous electrode are 11 times larger under the light with a wavelength of 370 nm and 380 nm. These results clearly indicate that light with photon energy, hν, below the bandgap contributes to the generation of photocarriers on GaN-porous structures. The Franz-Keldysh effect is one possible phenomenon to explain the present results, in which high electric field causes a redshift of the photoabsorption edge, leading to the presence of absorption below the bulk bandgap. Generally, a high electric field is applied to the GaN surface or interface due to Fermi-level, E F, pinning at about 1.0 eV below the conduction band minimum, E C. For example, when doping density is as high as 1018 cm-3, the internal electric field in a thin depletion layer with width of several dozen nanometers reaches 5×105V/cm. In such a situation, the wave function of an electron in the conduction band and a hole in the valence band are characterized by tails in the forbidden bandgap, resulting in band-to-band transition at an energy lower than the bandgap. To clarify the possibility that the Franz-Keldysh effect can explain that observation, the potential distribution of the GaN porous structures was calculated by a computer program. In this calculation, no external voltage was applied, but the potential distribution near the pore tips was strongly modified, where the contour intervals of the potential became narrow. These results suggest that the observed redshift of the photoabsorption edge arises from the high-electric field concentrated at the pore tips and that it is consistently explained by the Franz-Keldysh effect. [1] A. Watanabe, Y. Kumazaki, Z. Yatabe and T. Sato, ECS Electrochem. Lett. 4 (2015) H11. [2] Y. Kumazaki, A. Watanabe, Z. Yatabe and T. Sato, J. Electrochem. Soc. 161 (2014) H705. Figure 1

Formation of GaN-Porous Structures Using Photo-Assisted Electrochemical Process in Back-Side Illumination Mode

We investigated the structural features of gallium-nitride-porous structures formed using the photo-assisted electrochemical process in the back-side illumination (BSI) mode. The pore diameter and depth were strongly affected by the direction of illumination, where higher controllability was achieved compared with front-side illumination. The spectroscopic measurements revealed that illumination with photon energy below the bulk bandgap plays an important role in pore formation. We propose a formation model by considering the Franz-Keldysh effect that can consistently explain the obtained experimental data in which anodic etching occurs only at the pore tips under the high electric field induced in the depletion region.

Degrading a mixture of three textile dyes using photo-assisted electrochemical process with BDD anode and O₂-diffusion cathode.

In this paper, degradation of a mixture of three azo dyes was studied by the photo-assisted electrochemical process using an O₂-diffusion cathode containing carbon nanotubes and boron-doped diamond (BDD) anode. The concentration of three textile dyes (C.I. Acid Orange 8 (AO8), C.I. Acid Orange 10 (AO10), and C.I. Acid Orange 12 (AO12)) was determined simultaneously despite the severe overlap of their spectra. For this purpose, partial least square (PLS), as a multivariate calibration method, was utilized based on recording UV-Vis spectra during the decolorization process. Moreover, the central composite design was used for the modeling of photo-assisted electrochemical decolorization of the aqueous solutions containing three dyes. The investigated parameters were the initial concentration of three dyes, applied current and reaction time. Analysis of variance (ANOVA) revealed that the obtained regression models match the experimental results well with R (Khataee et al. 2010, Clean-Soil Air Water 38 (1):96-103, 2010) of 0.972, 0.971, and 0.957 for AO8, AO10, and AO12, respectively. Three-dimensional surface and contour plots were applied to describe the relation between experimental conditions and the observed response. The results of TOC analysis confirmed good ability of proposed photo-assisted electrochemical process for degradation and mineralization of textile industry wastewater.

Structural and Optical Characterization of GaN Porous Structures Formed By Photo-Assisted Electrochemical Process

not Available.

Degradation of Acid Blue 40 dye solution and dye house wastewater from textile industry by photo-assisted electrochemical process

In this paper, electrochemical and photo-assisted electrochemical processes are used for color, total organic carbon (TOC) and chemical oxygen demand (COD) degradation of one of the most abundant and strongly colored industrial wastewaters, which results from the dyeing of fibers and fabrics in the textile industry. The experiments were carried out in an 18 L pilot-scale tubular flow reactor with 70%TiO2/30%RuO2 DSA. A synthetic acid blue 40 solution and real dye house wastewater, containing the same dye, were used for the experiments. By using current density of 80 mA cm−2 electrochemical process has the capability to remove 80% of color, 46% of TOC and 69% of COD. When used the photochemical process with 4.6 mW cm−2 of 254 nm UV-C radiation to assist the electrolysis, has been obtained 90% of color, 64% of TOC and 60% of COD removal in 90 minutes of processing; furthermore, 70% of initial color was degraded within the first 15 minutes. Experimental runs using dye house wastewater resulted in 78% of color, 26% of TOC and 49% of COD in electrolysis at 80 mA cm−2 and 90 min; additionally, when photo-assisted, electrolysis resulted in removals of 85% of color, 42% of TOC and 58% of COD. For the operational conditions used in this study, color, TOC and COD showed pseudo–first-order decaying profiles. Apparent rate constants for degradation of TOC and COD were improved by one order of magnitude when the photo-electrochemical process was used.