Mechanistic studies of the photocatalytic degradation of methyl green: an investigation of products of the decomposition processes

Nickel-doped magnetic carbon aerogel derived from xanthan gum: a competent catalyst for the degradation of single and binary dye-based water pollutants

Toxic organic dyes (colorants) are one of the main causes of water pollution that releases destructive effluents in the environment. To overcome this issue, a fundamental need to produce a novel, efficient catalyst for the degradation and mineralization of dye mixtures has arisen. The objective of this research is to develop an eminent Ni-doped magnetic carbon aerogel (Ni-MCA) catalyst using graft co-polymerization method having xanthan gum as backbone doped with Ni-magnetic nanoparticles (Ni-MNPs), that do not show agglomeration and easy to separate. The examination revealed that Ni-MCA provided exceptional magnetic characteristics (Ms = 52.75 emu/g) and potent catalytic activity for the degradation of mono- as well as binary-dye solutions of Congo red (CR) and methyl green (MG) dyes. The formation was verified by various characterization techniques such as FTIR, VSM, XRD, XPS, SEM, TEM, and EDX mapping. Interestingly, Ni-MCA shows faster result on anionic dye CR up to 97% with degradation rate of 5.647 × 10-1 min-1, and MG dye shows degradation of 95.7% with the degradation rate of 2.169 × 10-1 min-1, while dye mixture is showing 90% degradation with rate of 2.159 × 10-1 min-1.

Two dimensional S-scheme Bi2WO6-TiO2-Ti3C2 nanocomposites for efficient degradation of organic pollutants under natural sunlight

Two-dimensional (2D) materials have fascinated the researchers to exploit their properties including large surface area, ability to act as a support and to form face-to-face interfacial contact with other 2D materials for fabricating efficient photocatalytic materials. In this work, Bi₂WO₆, TiO₂ and Ti₃C₂ nanosheets have been used synthesizing different series of binary Bi₂WO₆-TiO₂ and ternary Bi₂WO₆-TiO₂-Ti₃C₂ 2D nanocomposites by an electrostatic self-assembly synthesis route. The as-prepared pristine materials and binary and ternary nanocomposites were characterized by different structural, morphological and compositional characterization techniques to confirm their successful synthesis and 2D morphology. It was found that the optimized Bi₂WO₆-TiO₂ (20 wt%) and Bi₂WO₆-TiO₂ (20 wt%)-Ti₃C₂ (5 wt%) nanocomposites showed 97.0% and 98.5% degradation of methyl green in 80 min and 40 min, respectively, which was higher than their pristine counterparts. The enhanced activity was credited to the large surface area offered by 2D nanocomposites, pollutant adsorption and enhanced photogenerated charge separation and transfer facilitated by S-scheme mechanism and face-to-face interfacial contact of different components of these nanocomposites. This work delivers an example of highly efficient 2D nanocomposites and discusses the role of Ti₃C₂ as an electron acceptor in S-scheme photocatalytic system.

Nanostructured Colloidal Solutions of Malachite Green Formulated: Nonlinear Optical Properties and Simulation

Malachite green (MG) is a dye that has been presented to use as photosensitizers for photodynamic therapy (PDT). Nonlinear absorption coefficient (β) and nonlinear refractive index (n₂) of MG formulations with micelles and its encapsulation are studied aiming PDT. Encapsulation of MG was prepared by the mixture of water droplet in the continuous phase of oil, stabilized with surfactant. MG interacts with micelles and reverses micelle (RM), and the results are related to the size and concentration of micelles, RM, surfactant charge types. At low capsule size, the 1/β linearly increases by the increase of volume fraction of water droplet, while, an exponential behavior was observed in the higher capsule size.

Synthesis and characterization of pristine and strontium-doped zinc oxide nanoparticles for methyl green photo-degradation application

Herein we describe an effective route for the degradation of methyl green (MG) dye under visible light illumination by pristine and strontium (Sr)-doped zinc oxide (ZnO) photocatalysts (synthesized by the simple chemical precipitation method). The x-ray diffraction structural analysis has confirmed that both photocatalysts exhibit the hexagonal wurtzite structure; without any additional phase formation in Sr-doped ZnO, in particular. The optical properties of the synthesized photocatalysts have been investigated using UV-vis absorption spectroscopy in the wavelength range of 250-800 nm. Through Tauc's plot, the slight decrease from 3.3 to 3.2 eV in band gap energy has been elucidated (in the case of Sr-doped ZnO), which has been further confirmed by the quenching in the intensity of Photoluminescence (PL) emission spectrum. This may be due to sub-band level formation between valence and conduction band, caused by the impregnation of Sr²⁺ions into ZnO host. The morphological study has also been performed using Field Emission Scanning Electron Microscope, which indicates nanoparticles (NPs) based surface texture for both photocatalysts. During the photocatalytic activity study, after 30 min irradiation of visible light, ∼65.7% and ∼84.8% photocatalytic degradation of MG dye has been achieved for pristine and Sr-doped (2 wt%) ZnO photocatalysts, respectively. The rate of photocatalytic reaction (K) has been observed to be ∼0.06399 min^-1for Sr-doped (2 wt%), whereas nearly half magnitude ∼0.03403 min^-1has been observed for pristine ZnO, respectively. The significantly improved photodegradation activity may be ascribed to the relatively broader optical absorption capability, surface defects and the enhanced charge separation efficiency of the Sr-doped ZnO photocatalyst.

Cluster-induced desorption/ionization mass spectrometry of highlighter ink: unambiguous identification of dyes and degradation processes based on fragmentation-free desorption

Highlighter inks were analyzed by means of soft Desorption/Ionization induced by Neutral SO₂ clusters (DINeC) in combination with mass spectrometry (MS). The dye molecules of the different inks were directly desorbed from dots of ink drawn on arbitrary substrates. Fragmentation free spectra were observed and the dyes used in the dye mixtures of the different highlighter inks were unambiguously identified. The soft nature of cluster-induced desorption was used to investigate the decomposition of the dye molecules induced by either heat or UV-light. The two processes lead to different decomposition products which are clearly distinguished in the DINeC spectra. The two different degradation processes can thus be discriminated using DINeC-MS.

Fabrication porous adsorbents templated from modified sepiolite-stabilized aqueous foams for high-efficient removal of cationic dyes

High internal phase emulsions (HIPEs) as template for fabrication of porous materials has attracted much attention, due to the high porosity and tunable porous structure. But the enormous consumption of organic solvents is still a nightmare for the practical application. In comparison, the aqueous foam without need any organic solvent and hence has greater advantages in the porous materials preparation. In this study, a novel Pickering foam which was stabilized by modified sepiolite (Sep) was prepared and applied as the template for preparation of the porous material via thermal-initiated polymerization. The Pickering foam had excellent ability and stability in the pH of 4-11 and the obtained porous adsorbent possess sufficient and tuned pore structure. The porous materials as adsorbent has favorable performance for adsorption and selective removal of cationic dyes, and the understanding adsorption capacities for Methylene blue (MB) and Methyl green (MG) can be achieved with 1421.18 mg/g and 638.81 mg/g within 60 and 45 min at 25 °C, respectively. This porous material can be as the potential adsorbent for adsorption or separation of organic pollutants.

Evaluation of photocatalytic fuel cell (PFC) for electricity production and simultaneous degradation of methyl green in synthetic and real greywater effluents

Recycling of alternative water sources particularly greywater and recovery of energy from wastewater are gaining momentum due to clean water scarcity and energy crisis. In this study, the photocatalytic fuel cell (PFC) employing ZnO/Zn photoanode and CuO/Cu photocathode was successfully designed for effective greywater recycling as well as energy recovery. The photoelectrodes were analyzed using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and fourier transform infrared (FTIR) spectroscopy. The PFC performance in terms of electricity generation and parallel methyl green (MG) degradation were evaluated under operating parameters such as electrolyte type, initial MG concentration and solution pH. The results showed that the addition of Na₂SO₄ electrolyte, MG concentration of 40 mg L^-1 and solution pH of 5.2 improved the short circuit current density (J_sc) and power density (P_max) in the as-constructed PFC. Such a system also afforded highest MG and chemical oxygen demand (COD) removal efficiencies after 4 h of irradiation. The photoanodes used in this study demonstrated great recyclability after four repetition tests. The COD removal was reduced to some extents when the PFC treatment was tested in the real greywater under optimal conditions. Various greywater quality parameters including ammoniacal nitrogen (NH₃-N), turbidity, pH and biochemical oxygen demand (BOD₅) were also monitored. The phytotoxicity experiments via Vigna radiate seeds indicated a reduction in the phytotoxicity.

Effect of inorganic ions on the ultrasound initiated degradation and product formation of triphenylmethane dyes

Triphenylmethane (TPM) dyes are an important category of dyes with a variety of industrial applications and consequently, these are found in the aquatic environment at relatively higher concentrations. Here, we report the degradation of two important TPM dyes (para rosaniline (PRA) and ethyl violet (EV)) in an aqueous medium by ultrasound which is one among the Advanced Oxidation Processes (AOPs). The main objective of this work is to study the effect of various inorganic ions on the degradation and the product formation of TPM dyes from the sonochemical reactions. Using a typical concentration of 10 ppm dyes and an ultrasonic frequency of 350 kHz and power of 60 W, a complete degradation of EV and PRA was observed with a pseudo first order rate constant of 0.2339 min^-1 and 0.1956 min^-1, respectively. The product analyses using high-resolution mass spectrometry (LC-Q-TOF-MS) revealed the formation of hydroxylated, de-alkylated, and other collapsed conjugated structure destructed products. The evolution of these products in the presence of various inorganic ions (Cl^-, SO₄^2-, NO₃^-, and CO₃^-) showed that only carbonate ions had a significant impact on the product evolution. The carbonate ions facilitated the formation of conjugated structure destructed product for both the dyes. This is attributed to the reactivity of carbonate radical, which facilitated the formation of carbon-centered radicals. This carbon-centered radical further undergoes reaction to cause the destruction of conjugated structures. This is confirmed by the identification of the corresponding product peaks in the mass spectra. The scavenging effect of carbonate ions was also reflected in the product study where there is a reduction in the formation of most of the hydroxylated products. One of the major inorganic species in any wastewater is carbonate ions and therefore the present result is very relevant to the understanding of oxidation based treatment protocol.

Assessment of solar photocatalytic degradation and mineralization of amoxicillin trihydrate (AMT) using slurry and fixed-bed batch reactor: efficacy of parabolic trough collector

Heterogeneous photocatalytic mineralization of amoxicillin trihydrate (AMT) using TiO₂in suspended and supported form has been investigated in the present study.

Degradation of organic dyes via bismuth silver oxide initiated direct oxidation coupled with sodium bismuthate based visible light photocatalysis

Organic dye degradation was achieved via direct oxidation by bismuth silver oxide coupled with visible light photocatalysis by sodium bismuthate. Crystal violet dye decomposition by each reagent proceeded via two distinct pathways, each involving different active oxygen species. A comparison of each treatment method alone and in combination demonstrated that using the combined methods in sequence achieved a higher degree of degradation, and especially mineralization, than that obtained using either method alone. In the combined process direct oxidation acts as a pretreatment to rapidly bleach the dye solution which substantially facilitates subsequent visible light photocatalytic processes. The integrated sequential direct oxidation and visible light photocatalysis are complementary manifesting a > 100% increase in TOC removal, compared to either isolated method. The combined process is proposed as a novel and effective technology based on one primary material, sodium bismuthate, for treating wastewaters contaminated by high concentrations of organic dyes.

Determining the degradation efficiency and mechanisms of ethyl violet using HPLC-PDA-ESI-MS and GC-MS

BACKGROUND

The discharge of wastewater that contains high concentrations of reactive dyes is a well-known problem associated with dyestuff activities. In recent years, semiconductor photocatalysis has become more and more attractive and important since it has a great potential to contribute to such environmental problems. One of the most important aspects of environmental photocatalysis is in the selection of semiconductor materials like ZnO and TiO2, which are close to being two of the ideal photocatalysts in several respects. For example, they are relatively inexpensive, and they provide photo-generated holes with high oxidizing power due to their wide band gap energy. In this work, nanostructural ZnO film on the Zn foil of the Alkaline-Manganese Dioxide-Zinc Cell was fabricated to degrade EV dye. The major innovation of this paper is to obtain the degradation mechanism of ethyl violet dyes resulting from the HPLC-PDA-ESI-MS analyses.

RESULTS

The fabrication of ZnO nanostructures on zinc foils with a simple solution-based corrosion strategy and the synthesis, characterization, application, and implication of Zn would be reported in this study. Other objectives of this research are to identify the reaction intermediates and to understand the detailed degradation mechanism of EV dye, as model compound of triphenylmethane dye, with active Zn metal, by HPLC-ESI-MS and GC-MS.

CONCLUSIONS

ZnO nanostructure/Zn-foils had an excellent potential for future applications on the photocatalytic degradation of the organic dye in the environmental remediation. The intermediates of the degradation process were separated and characterized by the HPLC-PDA-ESI-MS and GC-MS, and twenty-six intermediates were characterized in this study. Based on the variation of the amount of intermediates, possible degradation pathways for the decolorization of dyes are also proposed and discussed.

Mechanistic pathways differences between P25-TiO(2) and Pt-TiO(2) mediated CV photodegradation

The Crystal Violet (CV) dye represented one of the major triphenylmethane dyes used in textile-processing and some other industrial processes. Various metals doped titanium dioxide (TiO(2)) photocatalysts have been studied intensively for the photodegradation of dye in wastewater treatment. In order to understand the mechanistic detail of the metal dosage on the activities enhancement of the TiO(2) based photocatalyst, this study investigated the CV photodegradation reactions under UV light irradiation using a Pt modified TiO(2) photocatalyst. The results showed that Pt-TiO(2) with 5.8% (W/W) Pt dosage yielded optimum photocatalytic activity. Also the effect of pH value on the CV degradation was well assessed for their product distributions. The degradation products and intermediates were separated and characterized by HPLC-ESI-MS and GC-MS techniques. The results indicated that both the N-de-methylation reaction and the oxidative cleavage reaction of conjugated chromophore structure occurred, but with significantly different intermediates distribution implying that Pt doped TiO(2) facilitate different degradation pathways compared to the P25-TiO(2) system.

Fabrication of porous TiO2 film on Ti foil by hydrothermal process and its photocatalytic efficiency and mechanisms with ethyl violet dye

Most of commercial dyes and pigments have rather complicated polyaromatic chemical structures with prolonged lifetime surviving in the Mother Nature. However, TiO(2) has been reported as one of the best photocatalytic candidates for degrading dye pollutants. In this report, TiO(2) film/Ti foil was prepared by hydrothermal reaction in alkali solution, the porous TiO(2) film with microcrystalline structure has been obtained. The porous structure of TiO(2) film was analyzed and characterized by XRD, FE-SEM and XPS. This is the first report that demonstrates that TiO(2) film/Ti foil has an excellent commercial application potential for photocatalytic degradation of Ethyl Violet (EV). Especially, because of refluxing at 100 degrees C, the porous TiO(2) film structure remained undisturbed, and EV decomposed in the period of 20 h. In addition, porous TiO(2)-mediated EV photo degradation mechanism has been proposed, as intermediates are isolated and clearly identified by GC-MS and HPLC-PDA-ESI-MS.

Comparison of photodegradative efficiencies and mechanisms of Victoria Blue R assisted by Nafion-coated and fluorinated TiO2 photocatalysts

The purposes of this research were to study the effects of two modified photocatalysts, Nafion-coated TiO(2) and fluorinated TiO(2), and photocatalytic degradation of Victoria Blue R in aqueous solution. Photocatalytic degradation of Victoria Blue R was accelerated by the modified photocatalysts. Bulk and surface characterizations of the resulting powders were carried out. Attachment of the anions to the TiO(2) surface using the Nafion-coated-TiO(2) possibly results in increased adsorption of the cationic dye, and the degradation rate is larger for the cationic dye. It was found that Victoria Blue R on the two illuminated TiO(2) surfaces underwent very different changes. To obtain a better understanding on the mechanistic details of this modified-TiO(2)-assisted photodegradation of the Victoria Blue R dye with UV irradiation, a large number of intermediates of the process were separated, identified, and characterized by a high-performance liquid chromatography-mass spectrometry technique. Several probable photodegradation pathways were proposed and discussed.

Bis(2-chloroethoxy)methane degradation by TiO2 photocatalysis: parameter and reaction pathway investigations

Haloethers are widely used in industry, and the release of these species into the environment is of great concern because of their toxicity and carcinogenicity. The present study deals with the photocatalytic degradation of the haloether, bis(2-chloroethoxy)methane (BCEXM), in the presence of TiO(2) particles and UV-A (lambda=365 nm) radiation. About 99.5% of BCEXM was degraded after UV irradiation for 16h. Factors such as solution pH, TiO(2) dosage, and the presence of anions were found to influence the degradation rate. To obtain a better understanding of the mechanistic details of this TiO(2)-assisted photodegradation of BCEXM with UV irradiation, the intermediates of the processes were separated, identified, and characterized by the solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) technique. To the best of our knowledge, this is the first report on the degradation pathways of BCEXM. The first step in the destruction of BCEXM is thought to be abstraction of a hydrogen by ()OH to form a carbon-centered radical which then reacts with O(2) to form a peroxyl radical. Peroxyl radicals react with one another and produce an alkoxy radical. The beta-bond fragmentation of the alkoxy radical produces different intermediates.

Titanium dioxide-mediated heterogeneous photocatalytic degradation of terbufos: parameter study and reaction pathways

The photocatalytic degradation of terbufos in aqueous suspensions was investigated by using titanium dioxide (TiO(2)) as a photocatalyst. About 99% of terbufos was degraded after UV irradiation for 90 min. Factors such as pH of the system, TiO(2) dosage, and presence of anions were found to influence the degradation rate. Photodegradation of terbufos by TiO(2)/UV exhibited pseudo-first-order reaction kinetics, and a reaction quantum yield of 0.289. The electrical energy consumption per order of magnitude for photocatalytic degradation of terbufos was calculated and showed that a moderated efficiency (E(EO)=71 kWh/(m(3)order)) was obtained in TiO(2)/UV process. To obtain a better understanding of the mechanistic details of this TiO(2)-assisted photodegradation of terbufos with UV irradiation, the intermediates of the processes were separated, identified, and characterized by the solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) technique. The probable photodegradation pathways were proposed and discussed.

Kinetics of sonophotocatalytic degradation of anionic dyes with Nano-TiO2

The current research work focuses on the combination of photocatalytic and sonocatalytic (sonophotocatalytic) degradation of anionic dyes, viz., Orange G, Remazol Brilliant Blue R, Alizarin Red S, Methyl Blue, and Indigo Carmine, with solution combustion synthesized TiO2 (CS TiO2) and commercial Degussa P-25 TiO2 (DP-25). The rate of sonophotocatalytic degradation of all the dyes and the reduction of total organic carbon was higher compared to the individual photo- and sonocatalytic processes. The effect of dissolved gases and ultrasonic intensity on the sonophotocatalytic degradation of the dyes was evaluated. A dual-pathway network mechanism of sonophotocatalytic degradation was proposed for the first time, and the rate equations were modeled using the network reduction technique. The kinetic rate coefficients of the individual steps were evaluated for all the systems by fitting the model with experimental data.

Occurrence of residues of the veterinary drug malachite green in eels caught downstream from municipal sewage treatment plants

Residues of malachite green (MG), a veterinary drug illegally used for the treatment of aquacultured fish, have been found in wild eels caught from surface waters downstream from the sewers of different municipal sewage treatment plants (STPs). MG and its metabolite leucomalachite green (LMG) were detected with total concentrations up to 0.765 microg kg(-1) fresh weight in the tissues of 25 out of 45 eels caught from different lakes, a river and a canal in Berlin, Germany. In all cases, the occurrence of the residues could directly be linked to the presence of discharges by municipal STPs into the receiving surface waters. MG is a multiple-use compound that is also used to color materials. Thus, it appears to be reasonable that the residues of MG found in the eel samples originate from such uses, e.g. by wash off from clothes or paper towels colored with MG. Additional loads from legal uses of MG as veterinary drug for the treatment of ornamental fish (private aquaria) are possible. The results obtained from this study are the first proof of background contaminations of a veterinary drug found in samples of fish not intentionally treated with such agents. MG and LMG are regarded as potential genotoxic carcinogens making it impossible to establish a tolerable daily intake. A margin of exposure (MOE) approach was applied to evaluate the human health risk associated with the consumption of the eels investigated in this study. With a MOE of 3.4 million the chronic risk was classified as being very low. Nevertheless, due to their potential to act as genotoxic carcinogens, any oral exposure to residues of MG and LMG should be avoided. According to European Union law, zero tolerance applies to all residues of MG and LMG found in food for human consumption as MG is not registered for use as veterinary drug.