Study of photo-assisted electrochemical degradation of carbaryl at dimensionally stable anodes (DSA)

A Review of Photoelectrocatalytic Reactors for Water and Wastewater Treatment

The photoexcitation of suitable semiconducting materials in aqueous environments can lead to the production of reactive oxygen species (ROS). ROS can inactivate microorganisms and degrade a range of chemical compounds. In the case of heterogeneous photocatalysis, semiconducting materials may suffer from fast recombination of electron–hole pairs and require post-treatment to separate the photocatalyst when a suspension system is used. To reduce recombination and improve the rate of degradation, an externally applied electrical bias can be used where the semiconducting material is immobilised onto an electrically conducive support and connected to a counter electrode. These electrochemically assisted photocatalytic systems have been termed “photoelectrocatalytic” (PEC). This review will explain the fundamental mechanism of PECs, photoelectrodes, the different types of PEC reactors reported in the literature, the (photo)electrodes used, the contaminants degraded, the key findings and prospects in the research area.

The recovery of sulfuric acid from spent piranha solution over a dimensionally stable anode (DSA) Ti-RuO2 electrode

Piranha solution is a highly acidic mixture of sulfuric acid and hydrogen peroxide. The present study aimed at developing a dimensionally stable anode (DSA), made of titanium metal foil coated with Ruthenium Dioxide (RuO₂), for the electrochemical oxidation of hydrogen peroxide in the presence of strong sulfuric acid under ambient conditions. Results showed that hydrogen peroxide in the piranha solution was fully degraded in 5 h under a constant current of 2 A (or current density of 0.32 A-cm^-2). The oxidation kinetics of hydrogen peroxide followed the Langmuir-Hinshelwood model. The observed rate constant was a function of applied current. The initial current efficiency was 17.5% at 0.5 A (or 0.08 A-cm^-2) and slightly decreased to about 13.5% at applied current between 1.3 and 1.5 A (or current density of 0.208 and 0.24 A-cm^-2). Results showed the capability and feasibility of the electrochemical oxidation process for the recovery of sulfuric acid from the spent piranha solution in semiconductor industrial installations or general laboratories.

Degradation of oxamic acid using dimensionally stable anodes (DSA) based on a mixture of RuO2 and IrO2 nanoparticles

Dimensionally stable anodes (DSA) have been widely used to degrade organic compounds because these surfaces promote the electrogeneration of active chlorine species in the bulk of the solution, as well as in the vicinity of the anode when NaCl is used as supporting electrolyte. In this work, the nanoparticles synthesis of IrO₂ and RuO₂ was performed to obtain two types of DSA electrodes named Class I and II to degrade oxamic acid. For Class I and II DSA, the nanoparticles used were synthesized separately and in the same reaction medium, respectively. Electrolysis were carried out in an open cylindrical cell without division at 25 °C, DSAs were used as anodes and a stainless-steel electrode as cathode, both elements have a geometric area of 2.8 cm² immersed in 0.05 mol L^-1 of NaCl or Na₂SO₄ and a current density of 3 mA cm^-2 was applied for 6 h. Active chlorine species generated in the absence of oxamic acid in NaCl were also detected and quantified through ion chromatography. In Na₂SO₄ there was no degradation of the compound, but in NaCl the oxamic acid concentration reaching 85% with Class I DSA. The same tendency is observed in mineralization, in which Class I DSA allowed reaching a CO₂ transformation close to 73%. The difference in the results occurs because with Class I DSA, more hypochlorite is generated than with Class II and therefore there is a larger amount of oxidizing species in the solution that enables the degradation and mineralization of oxamic acid.

The biodegradation of carbaryl in soil with Rhodopseudomonas capsulata in wastewater treatment effluent

In this study, the effects of Rhodopseudomonas capsulata present in wastewater effluent on the biodegradation of carbaryl in soil and improvement of soil fertility were investigated. Compared to control treatment, carbaryl was removed efficiently and soil fertility was remediated with the addition of effluent containing R. capsulata. Molecular analysis revealed that carbaryl induced carbaryl hydrolase gene expression to synthesize carbaryl hydrolase through activating MAPKKKs, MAPKKs, MAPKs genes in MAPK signal transduction pathway. The induction and secretion of carbaryl hydrolase occur after one day in R. capsulata, which can be attributed to its characteristics as an ancient bacteria, which require acclimatization to carbaryl before gene induction. However, lack of organics in soil and control treatment could not maintain R. capsulata growth for over one day. The residual organics in the effluent provided sufficient carbon source and energy for R. capsulata under four effluent treatments. This new method resulted in the remediation of carbaryl pollution and improvement of soil fertility and soybean processing wastewater treatment simultaneously, as well as the reutilization of wastewater and R. capsulata as sludge. Meanwhile, the high-order non-linear mathematical model about carbaryl removal rate was established.

Photo-assisted electrochemical degradation of sulfamethoxazole using a Ti/Ru0.3Ti0.7O2 anode: Mechanistic and kinetic features of the process

This study examined the photo-assisted electrochemical degradation and mineralization of the antibiotic contaminant sulfamethoxazole (SMX). All the experiments were perform using a flow electrolytic cell, in which the influence of the current density (10-60 mA cm^-2) and sodium chloride (0.02-0.10 mol L^-1) in the supporting electrolyte composition was analyzed. The results showed that the total SMX and 50% TOC removal was achieved in the current density range used. As expected, the degradation kinetics presented a pseudo first order behavior and the rate constant increased from 0.05 min^-1 to 0.50 min^-1 as the current density raised from 10 to 60 mA cm^-1. In addition, the values of the electrical energy per order (E_EO) increased from 0.67 to 1.06 kW/hm^-3 order^-1 as the current density increased from 10 to 60 mAcm^-2 and drop from 8.82 to 0.57 kW/hm^-3 order^-1 at supporting electrolyte concentration of 0.02-0.1 mol L^-1. The reaction intermediates identified by liquid chromatography-mass spectrometry allowed proposing a mechanism for the degradation. The use of photo assistance in the electrochemical process involved simultaneous reactions, for example, aromatic ring substitutions and hydroxylation. These reactions led to aromatic rings opening that generated simpler organic molecules, making possible the mineralization of the SMX molecule. Probable degradation pathways were proposed and discussed. Comparison of the efficiencies of the photocatalytic, electrochemical (EC) and photo-assisted electrochemical (PAEC) techniques revealed that the combined process showed a synergism for TOC removal.

Treatment of actual effluents produced in the manufacturing of atrazine by a photo-electrolytic process

The photo-assisted electrochemical degradation of a real effluent of the atrazine manufacturing process containing atrazine, simazine, hydroxy-triazine and propazine was carried out galvanostatically using a pilot-scale tubular flow reactor prototype containing DSA^® and Ti as cathode. The effluent was mainly characterized by a high amount of NaCl, required in the synthesis route used, and it was used as taken in the factory. The variables for process optimization were the current density (3.0, 6.0, and 9.0 mA cm^-2) and flow rate (300 and 3,000 L h^-1). These later values produces laminar and turbulent flow regimes, with Reynolds numbers of 1,100 and 11,000, respectively. None of the four organics contained in the waste is refractory to the photo-electrochemical treatment and they are depleted with the photo-electrolytic technology using large current densities and appropriate electric charge passed. Both direct electrochemical process and mediated anodic oxidation occur during the treatment. First process occurs at turbulent flow condition and low current densities, while the chemical oxidation process happens at laminar flow condition and high current densities. Atrazine and propazine are efficiently removed at laminar flow conditions, with an almost total depletion for the largest current densities. On the contrary, simazine is efficiently removed in turbulent flow conditions and intermediate current density, with removals higher than 90% for 20 kWh m^-3. These results have great significance because they demonstrate the applicability of the electrochemical technology in the treatment of real industrial wastes with a cell specially designed to attain high efficiency in the removal of pollutants.

Mechanistic proposal for the electrochemical and sonoelectrochemical oxidation of thiram on a boron-doped diamond anode

A comparative study was carried out of sonochemical (SCh), electrochemical (ECh) and sonoelectrochemical (SECh) strategies for the degradation of the fungicide thiram in dilute aqueous solution. The SCh and SECh studies were performed using a sonicator equipped with an 11 mm titanium-alloy probe and operated at 20 kHz with a power intensity of 523 W cm(-2). In the ECh and SECh investigations, galvanostatic electrolyses were implemented using a single compartment electrochemical cell with a boron-doped diamond electrode as anode and applied current densities in the range 10-50 mA cm(-2). For these processes, the decrease in concentration of thiram was monitored by high performance liquid chromatographic (HPLC) analysis and values of current efficiency and energy consumption were determined. The results showed that the rate of degradation of thiram and the amount of energy consumed were directly proportional to the applied current density, while current efficiency was inversely related to current density. The kinetics of thiram degradation followed a pseudo first order model with apparent rate constants in the region of 10(-3)min(-1). Thiram in aqueous solution was subjected to "exhaustive" degradation by ECh and SECh processes for 5h at applied current densities of 35 mA cm(-2) and the intermediates/byproducts so-formed were identified by HPLC-mass spectrometry. Mechanisms of the degradation reactions have been proposed on the basis of the results obtained.

Photocatalytic degradation of malathion in aqueous solution using an Au-Pd-TiO2 nanotube film

The extensive use of pesticides has promoted the agricultural production, but a series of subsequent environmental issues have drawn the concern of governments and people worldwide, such as groundwater and surface water pollutions. In order to remove these pollutants, photocatalysis has emerged as a powerful method. In this paper, the photocatalytic degradation of an organophosphorus pesticide malathion was investigated using an Au-Pd co-modified TiO(2) nanotube film (Au-Pd-TiO(2)). This film was fabricated by simultaneously photo-depositing Au and Pd precursors on a self-organized TiO(2) nanotube film. Its morphology and structures were well characterized by a scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The photocatalytic experiments revealed that the malathion elimination rate increased by 172% when the photocatalyst of the naked TiO(2) nanotube film was replaced by Au-Pd-TiO(2). Additionally, the amount of H(2)O(2) yielded on the Au-Pd-TiO(2) film in 60 min was 2.89 times that on the naked TiO(2). The enhanced photocatalytic performance could be attributed to both the effective separation of photo-generated charge carriers and the higher synthesis rate of H(2)O(2). The possible photocatalytic mechanism was discussed.

SnO(2)-based materials for pesticide degradation

This study presents the results of the degradation of the pesticide atrazine using electrochemical and photo-assisted electrochemical degradation techniques using SnO(2)-containing electrode of nominal composition electrodes of composition Ti/Ru(X)Sn(1-X)O(2) (where X=0.10, 0.15, 0.20, 0.25 and 0.30). The materials were characterized ex situ and in situ in order to correlate the observed atrazine removal rates with electrode morphology/composition. The results obtained demonstrate the effectiveness of the photo-assisted electrochemical degradation. Using purely electrochemical methods the rate of atrazine removal is almost zero at all the electrodes studied. However, the application of photo-assisted degradation results in almost complete atrazine removal in 1h of electrolysis. The efficiency of atrazine degradation does not seem to be greatly affected by the electrode material or by SnO(2) content, but the overall COD removal is dependent on the SnO(2) content. Overall, the SnO(2)-containing electrodes do not reach the level of COD removal (maximum approximately 21%) seen for the Ti/Ru(0.3)Ti(0.7)O(2) electrode. An interesting correlation between the morphology factor (phi) and chemical oxygen demand removal is observed.