Photodegradation of malachite green under simulated and natural irradiation: kinetics, products, and pathways

Removal of malachite green from water: Comparison of adsorption in a residue-derived AC versus photocatalytic oxidation with TiO2 and study of the adsorption-photocatalysis synergy

The literature rarely compiles studies devoted to the removal of pollutants in aqueous media comparing adsorption and photocatalytic degradation, and does not pay enough attention to the analysis of combined adsorption-photocatalytic oxidation processes. In the present manuscript, the removal of malachite green (MG) from aqueous solutions has been investigated in three different sustainable scenarios: i) adsorption on activated carbon (AC) derived from a residue, luffa cylindrica, ii) photocatalytic oxidation under simulated solar light using titanium dioxide (TP) and iii) combined adsorption-photocatalytic oxidation using TP-AC (70/30 wt./wt.) under simulated solar light. The study has revealed that in the three scenarios and studied conditions, the total removal of this endocrine-disrupting dye from the solution takes place in the assayed time, 2 h, in some cases just in a few minutes. MG adsorption in the AC is a very fast and efficient removal method. MG photocatalytic oxidation with TP also occurs efficiently, although the oxidized MG is not totally mineralized. MG removal using the TP-AC composite under simulated solar light occurs only slightly faster to the MG adsorption in the AC, being adsorption the dominating MG removal mechanism for TP-AC. Thus, more than 90% of the removed MG with TP-AC under simulated solar light is adsorbed in this carbon-containing composite. The obtained results highlight the interest in adsorption, being the selection of the most suitable removal method dependent on several factors (i.e., the cost of the AC regeneration, for adsorption, or the toxicity of the intermediate oxidation species, for photooxidation). Paying attention to MG photooxidation with TiO2, comparison of two working photodegradation schemes shows that the direct photodegradation of MG from solution, avoiding any initial dark equilibrium period, is more efficient from a time perspective. The use of scavengers has proved that MG photodegradation occurs via an oxidation mechanism dominated by superoxide anion radicals.

Symmetry-breaking malachite green as a near-infrared light-activated fluorogenic photosensitizer for RNA proximity labeling

Cellular RNA is asymmetrically distributed in cells and the regulation of RNA localization is crucial for proper cellular functions. However, limited chemical tools are available to capture dynamic RNA localization in complex biological systems with high spatiotemporal resolution. Here, we developed a new method for RNA proximity labeling activated by near-infrared (NIR) light, which holds the potential for deep penetration. Our method, termed FAP-seq, utilizes a genetically encoded fluorogen activating protein (FAP) that selectively binds to a set of substrates known as malachite green (MG). FAP binding restricts the rotation of MG and rapidly activates its fluorescence in a wash-free manner. By introducing a monoiodo modification to MG, we created a photosensitizer (MG-HI) with the highest singlet oxygen generation ability among various MG derivatives, enabling both protein and RNA proximity labeling in live cells. New insights are provided in the transcriptome analysis with FAP-seq, while a deeper understanding of the symmetry-breaking structural arrangement of FAP-MG-HI was obtained through molecular dynamics simulations. Overall, our wash-free and NIR light-inducible RNA proximity labeling method (FAP-seq) offers a powerful and versatile approach for investigating complex mechanisms underlying RNA-related biological processes.

Fluorinated Pharmaceutical and Pesticide Photolysis: Investigating Reactivity and Identifying Fluorinated Products by Combining Computational Chemistry,19F NMR, and Mass Spectrometry

Fluorinated breakdown products from photolysis of pharmaceuticals and pesticides are of environmental concern due to their potential persistence and toxicity. While mass spectrometry workflows have been shown to be useful in identifying products, they fall short for fluorinated products and may miss up to 90% of products. Studies have shown that 19F NMR measurements assist in identifying and quantifying reaction products, but this protocol can be further developed by incorporating computations. Density functional theory was used to compute 19F NMR shifts for parent and product structures in photolysis reactions. Computations predicted NMR spectra of compounds with an R2 of 0.98. Computed shifts for several isolated product structures from LC-HRMS matched the experimental shifts with <0.7 ppm error. Multiple products including products that share the same shift that were not previously reported were identified and quantified using computational shifts, including aliphatic products in the range of -80 to -88 ppm. Thus, photolysis of fluorinated pharmaceuticals and pesticides can result in compounds that are polyfluorinated alkyl substances (PFAS), including aliphatic-CF3 or vinyl-CF2 products derived from heteroaromatic-CF3 groups. C-F bond-breaking enthalpies and electron densities around the fluorine motifs agreed well with the experimentally observed defluorination of CF3 groups. Combining experimental-computational 19F NMR allows quantification of products identified via LC-HRMS without the need for authentic standards. These results have applications for studies of environmental fate and analysis of fluorinated pharmaceuticals and pesticides in development.

Novel Radiochromic Elastomer Dosimeter Based on the Self-Sensitizing Effect of Disulfide Bonds

γ-Irradiation is a kind of high-energy ionizing ray, which has widespread applications in material, food, and medical industries as well as in the environment. Since this irradiation is invisible, quantitatively monitoring its exposure doses is crucial to irradiated targets. As a type of dosimeter, radiochromic dosimeters can detect γ-irradiation by color changing, and its strategy to realize the radiochromic behavior basically relies on active radicals from radiolysis of an external environmental medium. However, the primary problem of this external environment-mediated sensitization strategy is that it complicates the components of dosimeters. Herein, we present a novel type of self-sensitizing radiochromic poly(urethane-urea) elastomers (PUUEs), where disulfide bonds, serving as radiation-responsive and sensitizing units, are introduced. This is the first attempt to utilize radicals generated from radiolysis of weak bonds in a solid polymer matrix to sensitize color change of dye-doped radiochromic dosimeters. Moreover, it is intriguing that the simultaneously introduced aryl hydrazone bond endows dosimeters with excellent color retention and maintains the Δa* value of 72.9% even after 1 month on the basis of the as-irradiated specimen. Besides, the metathesis of disulfide bonds not only endows dosimeters with better self-healing capability, but also accelerates the postcuring behavior and hydrogen bond reconfiguration, resulting in improved mechanical performance.

Efficient malachite green biodegradation by Pseudomonas plecoglossicide MG2: process optimization, application in bioreactors, and degradation pathway

Microbial degradation of synthetic dyes is considered a promising green dye detoxification, cost-effective and eco-friendly approach. A detailed study on the decolorization and degradation of malachite green dye (MG) using a newly isolated Pseudomonas plecoglossicide MG2 was carried out. Optimization of MG biodegradation by the tested organism was investigated by using a UV-Vis spectrophotometer and the resultant degraded products were analyzed by liquid chromatography-mass spectrometry and FTIR. Also, the cytotoxicity of MG degraded products was studied on a human normal retina cell line. The optimum conditions for the significant maximum decolorization of MG dye (90-93%) by the tested organism were pH 6-7, inoculum size 4-6%, and incubation temperature 30-35 °C, under static and aerobic conditions. The performance of Pseudomonas plecoglossicide MG2 grown culture in the bioreactors using simulated wastewater was assessed. MG degradation (99% at 100 and 150 mg MG/l at an optimal pH) and COD removal (95.95%) by using Pseudomonas plecoglossicide MG2 culture were the best in the tested culture bioreactor in comparison with that in activated sludge or tested culture-activated sludge bioreactors.The FTIR spectrum of the biodegraded MG displayed significant spectral changes, especially in the fingerprint region 1500-500 as well as disappearance of some peaks and appearance of new peaks. Twelve degradation intermediates were identified by LC-MS. They were desmalachite green, didesmalachite green, tetradesmalachite green, 4-(diphenylmethyl)aniline, malachite green carbinol, bis[4-(dimethylamino)phenyl]methanone, [4-(dimethylamino)phenyl][4-(methyl-amino)phenyl]methanone, bis[4-(methylamino)phenyl]methanone, (4-amino- phenyl)[4-(methylamino)phenyl]methanone, bis(4-amino phenyl)methanone, (4-amino phenyl)methanone, and 4-(dimathylamino)benzaldehyde. According to LC-MS and FTIR data, two pathways for MG degradation by using Pseudomonas plecoglossicide MG2 were proposed. MG showed cytotoxicity to human normal retina cell line with LC50 of 28.9 µg/ml and LC90 at 79.7 µg/ml. On the other hand, MG bio-degraded products showed no toxicity to the tested cell line. Finally, this study proved that Pseudomonas plecoglossicide MG2 could be used as an efficient, renewable, eco-friendly, sustainable and cost-effective biotechnology tool for the treatment of dye wastewater effluent.

The removal enhancement of organic contaminations and optimization of the photocatalytic efficiency by Box-Behnken design using ZnO-TiO2/porous graphene aerogel

In this study, zinc oxide-titanium dioxide/graphene aerogel (ZnO-TiO2/GA) was successfully synthesized through a simple and cost-effective hydrothermal self-assembly process. Besides, the surface response model and the experimental design according to the Box-Behnken model were selected to determine the optimal removal efficiency for crystal violet (CV) dye and para-nitrophenol (p-NP) phenolic compound. According to the obtained results, the highest degradation efficiency for CV dye of 99.6% was obtained under the following conditions: pH 6.7, CV concentration of 23.0 mg/L, and catalyst dose of 0.30 g/L. For p-NP, the degradation efficiency reached 99.1% under the following conditions: H2O2 volume of 1.25 mL, pH 6.8, and catalyst dose of 0.35 g/L. Therewithal, kinetic models of adsorption-photodegradation, thermodynamic adsorption, and free radical scavenging experiments were also investigated to propose the specific mechanisms involving the removal of CV dye and p-NP. According to the aforementioned results, the study provided a resulting ternary nanocomposite with great removal performance for water pollutants via the synergetic effects of adsorption and photodegradation processes.

Preparation and characterization of low-cost adsorbents for the efficient removal of malachite green using response surface modeling and reusability studies

Malachite green used in textile and dyeing industries is a common persistent pollutant in wastewater and the environment causing major hazards to human health and aquatic organisms. In this study, the response surface methodology was applied to optimize the adsorptive removal of malachite green using nano-bentonite, MgO-impregnated clay, and Mucor sp. composites. The nano materials and Mucor sp. composite were characterized by FTIR, SEM and X-ray diffractometry. According to the obtained results, nano-bentonite exhibits a maximum MG adsorption efficiency of 98.6% at 35 °C, pH 7.0, 60 min contact time, 1.0 g/L adsorbent dosage, and 50 mg/L initial MG concentration. On the other hand, the maximum efficiency for MG adsorption on MgO-impregnated clay of 97.04% is observed at pH 9.0, 60 min contact time, 0.7 g/L adsorbent dosage, and 50 mg/L initial MG concentration. The Malachite green (MG) adsorption isotherm on MgO-impregnated clay corresponded with the Freundlich isotherm, with a correlation coefficient (R²) of 0.982. However, the Langmuir adsorption isotherm was a superior fit for nano-bentonite (R² = 0.992). The adsorption activities of nano-bentonite and MgO-impregnated clay were fitted into a pseudo-second-order kinetic model with R² of 0.996 and 0.995, respectively. Additionally, despite being recycled numerous times, the adsorbent maintained its high structural stability and removal effectiveness for nano-bentonite (94.5-86%) and MgO-impregnated clay (92-83%).

A green approach for the preparation of ZnO@C nanocomposite using agave americana plant extract with enhanced photodegradation

The present study demonstrates the crucial role of agave americana extract in enhancing the optical properties of zinc oxide (ZnO) through thermal treatment method. Various analytical and surface science techniques have been used to identify the morphology, crystalline structure, chemical composition, and optical properties, including scanning electron microscopy, x-ray diffraction, high resolution transmission electron microscopy (HRTEM), x-ray spectroscopy (EDS) and UV-visible spectroscopy techniques. The physical studies revealed the transformation of ZnO nanorods into nanosheets upon addition of an optimized amount of agave americana extract, which induced large amount of amorphous carbon deposited onto ZnO nanostructures as confirmed by HRTEM analysis. The use of increasing amount of americana extract has significantly reduced the average crystallite size of ZnO nanostructures. The resultant hybrid system of C@ZnO has produced a significant effect on the ultraviolet light-assisted photodegradation of malachite green (MG) dye. The photocatalyst dose was fixed at 10 mg for each study whereas the amount of agave americana extract and MG dye concentration are varied. The functionality of hybrid system was greatly enhanced when the amount of agave americana extract increased while dye concentration kept at lower level. Ultimately, almost 100% degradation efficiency was achieved via the prepared hybrid material, revealing combined contribution from synergy, stabilization of ZnO due to excess of carbon together with the high charge separation rate. The obtained results suggest that the driving role of agave americana extract for surface modification of photocatalyst can be considered for other nanostructured photocatalysts.

Zinc-Acetate-Amine Complexes as Precursors to ZnO and the Effect of the Amine on Nanoparticle Morphology, Size, and Photocatalytic Activity

Zinc oxide is an environmentally friendly and readily synthesized semiconductor with many industrial applications. ZnO powders were prepared by alkali precipitation using different [Zn(acetate)₂(amine)_x] compounds to alter the particle size and aspect ratio. Slow precipitations from 95 °C solutions produced micron-scale particles with morphologies of hexagonal plates, rods, and needles, depending on the precursor used. Powders prepared at 65 °C with rapid precipitation yielded particles with minimal morphology differences, but particle size was dependent on the precursor used. The smallest particles were produced using precursors that yielded crystals with low aspect ratios during high-temperature synthesis. Particles produced during rapid synthesis had sizes ranging from 21-45 nm. The materials were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, BET, and diffuse reflectance. The materials prepared using precursors with less-volatile amines were found to retain more organic material than ZnO produced using precursors with more volatile amines. The amount of organic material associated with the nanoparticles influenced the photocatalytic activity of the ZnO, with powders containing less organic material producing faster rate constants for the decolorizing of malachite green solutions under ultraviolet illumination, independent of particle size. [Zn(acetate)₂(hydrazine)₂] produced ZnO with the fastest rate constant and was recycled five times for dye degradation studies that revealed minimal to no reduction in catalytic efficiency.

Malachite Green Dye Decoloration over Au/TiO2-Nanotubes Photocatalyst under Simulate Visible-Light Irradiation

Au nanoparticles were supported on TiO₂ nanotubes by a novel vapor phase impregnation approach (VPI) using gold dimethyl-acetylacetonate as a precursor. This study aimed to evaluate the capacity of these materials in the photodecoloration of malachite green dye, with the vision to correlate the chemical, structural, morphological, and optical properties with its photocatalytic performance. The photocatalysts were characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectronic spectroscopy (XPS), electronic microscopy (HAADF-STEM and HRTEM), and UV-vis spectroscopy. The techniques mentioned above made it possible to detect the presence of small gold nanoparticles (around 3.1 nm), with a high apparent dispersion even at high metal loading for all analyzed systems. According to the XPS results, the Au nanoparticles remain reduced (Au°), and they have a high electronic interaction with TiO₂, which eventually originates an electronic exchange between them and consequently a decrease in the band gap energy. In addition, the surface plasmonic resonance observed through UV-vis spectroscopy of the Au nanoparticles are factors that can be related to the high decoloration observed in these photocatalysts, specifically in the 15 wt% Au material, which achieves maximum photodecoloration of malachite green dye at 93%.

Aurivillius Oxides Nanosheets-Based Photocatalysts for Efficient Oxidation of Malachite Green Dye

Aurivillius oxides ferroelectric layered materials are formed by bismuth oxide and pseu-do-perovskite layers. They have a good ionic conductivity, which is beneficial for various photo-catalyzed reactions. Here, we synthesized ultra-thin nanosheets of two different Aurivillius oxides, Bi₂WO₆ (BWO) and Bi₂MoO₆ (BMO), by using a hard-template process. All materials were characterized through XRD, TEM, FTIR, TGA/DSC, DLS/ELS, DRS, UV-Vis. Band gap material (E_g) and potential of the valence band (E_VB) were calculated for BWO and BMO. In contrast to previous reports on the use of multi composite materials, a new procedure for photocatalytic efficient BMO nanosheets was developed. The procedure, with an additional step only, avoids the use of composite materials, improves crystal structure, and strongly reduces impurities. BWO and BMO were used as photocatalysts for the degradation of the water pollutant dye malachite green (MG). MG removal kinetics was fitted with Langmuir—Hinshelwood model obtaining a kinetic constant k = 7.81 × 10⁻² min⁻¹ for BWO and k = 9.27 × 10⁻² min⁻¹ for BMO. Photocatalytic dye degradation was highly effective, reaching 89% and 91% MG removal for BWO and BMO, respectively. A control experiment, carried out in the absence of light, allowed to quantify the contribution of adsorption to MG removal process. Adsorption contributed to MG removal by a 51% for BWO and only by a 19% for BMO, suggesting a different degradation mechanism for the two photocatalysts. The advanced MG degradation process due to BMO is likely caused by the high crystallinity of the material synthetized with the new procedure. Reuse tests demonstrated that both photocatalysts are highly active and stable reaching a MG removal up to 95% at the 10th reaction cycle. These results demonstrate that BMO nanosheets, synthesized with an easy additional step, achieved the best degradation performance, and can be successfully used for environmental remediation applications.

Review on effect of different type of dyes on advanced oxidation processes (AOPs) for textile color removal

The textile industry is one of the most valuable industries, especially in developing countries, because it employs a large portion of the workforce. However, the development of the textile industry has led to increasing concern about environmental issues. Wastewater from the textile industry has a high COD and an undesirable color. Color is one of the problems with the wastewater due to its toxicity and low biodegradability. Color in textile wastewater mainly originates from the dyestuff used during the dyeing or printing process. Amongst all of available technology for color removal, advanced oxidation processes (AOPs) are considered one of the best and the most potential technology. However, the understanding of AOPs reaction mechanism to degrade dyestuff is still limited. In general, dyes degradation mechanism will vary and mainly depend on the chemical structure of the dyes itself. Some reaction pathway that seems less favorable thermodynamically can still happen during the process. Understanding the mechanism will be beneficial for future dyes improvement, especially on developing the moiety of the aromatic compound in order to produce easily degraded dyes while maintaining the fastness quality.

Multiple cobalt active sites evenly embedded in mesoporous carbon nanospheres derived from a polymer-metal-organic framework: efficient removal and photodegradation of malachite green

The derivative catalysts of polyMOF(Co) effectively degrade malachite green under visible irradiation.

Enhanced malachite green photolysis at the colloidal-aqueous interface

Colloids, such as natural particulate matter and microplastics, can play a significant role in the fate and transport of organic contaminants. Specifically, these small nano-to micron-sized particles provide large surface area; thus, particle-aqueous interfacial chemistry becomes significant. In this work, we present an experimental investigation of interfacial photokinetics of malachite green cation (MG+) adsorbed at the surface of polystyrene carboxyl (PSC) microspheres suspended in aqueous solution. Second harmonic generation (SHG), an interfacial selective laser spectroscopic tool, has been used to probe the buried interface. It is revealed that relative to the bulk, photoinduced degradation of MG+ is accelerated by approximately 10-fold at this noncatalytic particle surface. By measuring the SHG-based surface electronic spectra, we have also demonstrated that N-demethylated intermediates of MG+ remain at the interface until they are further decomposed. MG+ exhibits a bathochromic shift at the interface. Together with strong binding affinity and faster initial rate of photodegradation of MG+ at the interface, this work highlights that adsorption and surface photolysis are important pathways by which organic compounds can be transformed within the aquatic environment. Moreover, this research also stimulates further questions on the enrichment of reactive species at the colloidal-aqueous interface and their influence on facilitating decompositions of organic pollutants.

A high-efficiency mediator-free Z-scheme Bi2MoO6/AgI heterojunction with enhanced photocatalytic performance

A high-efficiency Z-scheme Bi₂MoO₆/AgI heterojunction was designed and fabricated via in situ growth of AgI on Bi₂MoO₆. Its photocatalytic activity was investigated with the degradation of malachite green (MG). After 40 min of visible light irradiation, near complete degradation of MG (20 mg/L) occurred when BA11 (Bi₂MoO₆:AgI = 1:1, 2.0 g/L) was present, while only 29.0% and 49.7% of the MG could be degraded in the presence of Bi₂MoO₆ and AgI, respectively. The excellent photocatalytic activity of BA11 results from strong visible light absorption and the low recombination efficiency of photogenerated electron-hole pairs induced by the formation of heterojunction. Density function theory (DFT) calculations revealed that the formation of built-in electric field at the interface between Bi₂MoO₆ and AgI facilitates the effective separation and transfer of photogenerated charge carriers. Results of reuse experiments indicated that the heterostructured photocatalyst has excellent stability. Radical scavenging experiments and electron spin resonance spectra showed that superoxide radicals (O₂^-) and hydroxyl radicals (OH) were the major reactive oxygen species in the photocatalytic system. The photocatalytic degradation pathway of MG was proposed based on the organic degradation intermediates detected. These findings demonstrate that the mediator-free Z-scheme Bi₂MoO₆/AgI heterojunction could serve as a promising photocatalyst in photocatalytic treatment of organic pollutants.

Mechanochemical synthesis of MAPbBr3/carbon sphere composites for boosting carrier-involved superoxide species

Lead halide perovskites MAPbX₃ (MA = CH₃NH₃ or Cs; X = I, Br, Cl) are well considered to be potential candidates for photocatalytic reaction due to its excellent photoelectrical properties, but they still suffer from the low charge separation efficiency and slow catalytic reaction dynamics. To tackle the drawbacks, herein, MAPbBr₃/carbon sphere (CS) composite photocatalysts using glucose as the carbon source were elaborately designed and fabricated via a dry mechanochemical grinding process. The interfacial interaction Pb-O-C chemical bonds were constructed between MAPbBr₃ and the carbon sphere surface containing organic functional groups. By optimizing the content of CSs, the enhanced photocatalytic degradation kinetic rate of Malachite Green (MG) pollutants (92% within 20 min) for MAPbBr₃/CS_x (x = 17 wt.%) is about 3.6-fold of that for pristine MAPbBr₃, which is attributed to the corporative adsorption and enhanced carrier transportation and separation of MAPbBr₃/CS_x. Furthermore, the possible degradation mechanism was proposed on basis of the electrochemical, mass spectrometry and optical characterization results. Owing to the robust interfacial interaction, effective electron extraction rate (k_et = 4.6 × 10⁷ sec^-1) from MAPbBr₃ to CS can be established, which driven oxygen activation where superoxide radicals (•O₂^-) played an important role in MG degradation. It is expected that mechanochemistry strategy may provide a new route to design efficient lead halide perovskite-carbon or metal oxide or sulfide composite photocatalysts.

Kinetic and Isothermal Investigations of Cost-Effective Sorptive Elimination of Gentian Violet Dye from Water Using Haplophragma adenophyllum Biowaste

A novel biosorbent, Haplophragma adenophyllum (HAB) was employed to explore the biosorption mechanism of Gentian Violet (GV) dye. The novel sorbent was characterized by using FTIR spectra and physiochemical analysis. The effect of different optimizing factors like HAB dosage, GV initial concentration, contact time between sorbent and sorbate, pH of a solution, and the temperature was studied. The optimum removal of GV by HAB was observed at pH 6.0. The equilibrium study was carried out using Langmuir and Freundlich isotherms. Experimental data fitted well in Langmuir isotherm indicating monolayer isotherm with qmax value obtained at optimum process condition of 13.21 mg/g. Kinetics study was carried out and followed by pseudo-second-order model. Thermodynamics studies reveal the endothermic reaction.

Efficient biodegradation of malachite green by an artificial enzyme designed in myoglobin

Synthetic dyes such as malachite green (MG) have a wide range of applications. Meanwhile, they bring great challenges for environmental security and cause potential damages to human health. Compared with traditional approaches, enzymatic catalysis is an emerging technique for wastewater treatment. As alternatives to natural enzymes, artificial enzymes have received much attention for potential applications. In previous studies, we have rationally designed artificial enzymes based on myoglobin (Mb), such as by introducing a distal histidine (F43H mutation) and creating a channel to the heme pocket (H64A mutation). We herein show that the artificial enzyme of F43H/H64A Mb can be successfully applied for efficient biodegradation of MG under weak acid conditions. The degradation efficiency is much higher than those of natural enzymes, such as dye-decolorizing peroxidase and laccase (13-18-fold). The interaction of MG and F43H/H64A Mb was investigated by using both experimental and molecular docking studies, and the biodegradation products of MG were also revealed by UPLC-ESI-MS analysis. Based on these results, we proposed a plausible biodegradation mechanism of MG. With the high-yield of overexpression in E. coli cells, this study suggests that the artificial enzyme has potential applications in the biodegradation of MG in fisheries and textile industries.

Proposal of a controlled release of citrate by solubility equilibrium

Some substances at high concentrations in both the body and the environment can be toxic. The remediation of contaminated environments, for example by metals in toxic concentrations is a fairly current problem. In this way, organic acids of low molecular weight, because they are biodegradable, constitute an alternative potentially for their use in phytoremediation processes. Among these acids, citric acid was chosen to be used in this work due to its suitable binder behaviour, with high stability constants. The purpose of this work was to develop a controlled release of citrate that allows its release to the soil solution to pre-defined concentrations by the system and that this concentration is maintained even if the binder is consumed in the complexation of metals, diluted or percolated. In this way, the system has an additional feature, compared to the conventional controlled release system. The presence of calcium citrate allowed for a slower release of citrate compared to citrate soluble salts or even with calcium citrate directly in solution. The Noyes-Whitney model allowed to explain the effects of pellet sizes and percentages of agar-agar. The pH of the receptor solution and the calcium concentrations used in the preparation of the gels influenced the equilibrium concentration of the citrate, demonstrating the independence of the controlled release process in relation to other variables in the solution. The controlled release system proposed in this work is a promising and very useful system for phytoextraction processes.

Sonophotocatalytic Degradation of Malachite Green by Nanocrystalline Chitosan-Ascorbic Acid@NiFe2O4 Spinel Ferrite

Statistics show that more than 700 thousand tons of dye are produced annually across the globe. Around 10–20% of this is used in industrial processes such as printing and dyeing, while about 50% of the dye produced is discharged into the environment without proper physicochemical treatment. Even trace amounts of dye in water can reduce oxygen solubility and have carcinogenic, mutagenic, and toxic effects on aquatic organisms. Therefore, before dye-containing wastewater is discharged into the environment, it must be properly treated. The present study investigates the green synthesis of nickel ferrite NiFe2O4 (NIFE) spinel magnetic nanoparticles (MNPs) via chemical coprecipitation of a solution of Ni2+/Fe3+ in the presence of a biopolymer blend of chitosan (CT) and ascorbic acid (AS). The magnetic nanomaterial was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy–energy dispersive X-ray analysis (SEM-EDX), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), differential scanning calorimetry (DSC), and vibrating-sample magnetometry (VSM). The material was further explored as a catalyst for the photocatalytic degradation of malachite green (MG) under visible light irradiation coupled with ultrasonic waves. The combination of 90 min of visible solar light irradiation with 6.35 W·mL−1 ultrasonic power at pH 8 resulted in 99% of the photocatalytic efficiency of chitosan-ascorbic acid@NIFE (CTAS@NIFE) catalyst for 70 mg·L−1 MG. The quenching of the photocatalytic efficiency from 98% to 64% in the presence of isopropyl alcohol (IPA) suggested the involvement of hydroxy (•OH) radicals in the mineralization process of MG. The high regression coefficients (R2) of 0.99 for 35, 55, and 70 mg·L−1 MG indicated the sonophotocatalysis of MG by CTAS@NIFE was best defined by a pseudo first-order kinetic model. The mechanism involves the adsorption of MG on the catalyst surface in the first step and thereby mineralization of the MG by the generated hydroxyl radicals (•OH) under the influence of visible radiation coupled with 6.34 W·mL−1 ultrasonic power. In the present study the application of photodegradation process with sonochemistry results in 99% of MG mineralization without effecting the material structure unlike happens in the case adsorption process. So, the secondary pollution (generally happens in case of adsorption) can be avoided by reusing the spent material for another application instead of disposing it. Thus, the ecofriendly synthesis protocol, ease in design of experimentation like use of solar irradiation instead of electric power lamps, reusability and high efficiency of the material suggested the study to be potentially economical for industrial development at pilot scale towards wastewater remediation.

Singlet oxygen production abilities of oxidated aromatic compounds in natural water

Singlet oxygen (¹O₂) is well known to be formed through energy transfer from excited state organic matters to O₂, playing an important role in the transformations of contaminants. However, the contribution of small oxidated aromatic compounds (OACs) to the production of ¹O₂ in surface water is unclear. In this study, 28 OACs were selected to investigate the correlations between their photochemical production abilities of ¹O₂ and molecular structures. Our results showed that the steady-state concentrations and quantum yields of ¹O₂ (Φ_1O2) generated by OACs were in the range of 7.0 × 10^-14-1.4 × 10^-12 M and 2.2 × 10^-4-4.7 × 10^-2, respectively, indicating that the photochemical production abilities of ¹O₂ by OACs varied greatly with types and positions of functional groups on the molecule. More importantly, the observed photochemical production of ¹O₂ was most notable in cases of molecules containing -OCH₃ group and benzoquinone. A good quantitative structure-property relationship model was established between ¹O₂ producing ability, energy of the lowest unoccupied molecular orbitals (E_LUMO) and the most positive net charge of hydrogen atoms (qH⁺) of OACs. In addition, the role of ¹O₂ produced by 2, 6-dimethoxy-1, 4-benzoquinone, the OAC with the highest Φ_1O2, in the photodegradation of organic contaminants was validated by the enhanced degradation of atorvastatin under simulated sunlight, suggesting that OACs ubiquitously existed in surface water may greatly affect the fate and ecological risks of organic contaminants.

Highly efficient degradation of iohexol on a heterostructured graphene-analogue boron nitride coupled Bi2MoO6 photocatalyst under simulated sunlight

Iohexol (IOH), as a typical iodinated X-ray contrast media (ICMs) with potential threat to human health, is difficult to be removed with the conventional wastewater treatment methods. In this work, new boron nitride coupled Bi₂MoO₆ layered microspheres (BN/Bi₂MoO₆) composites were applied to remove IOH from water via photocatalytic degradation. The degradation constant k_app of IOH over 3.5 wt% BN/Bi₂MoO₆ was 0.016 min^-1, which was 3.2 times that of Bi₂MoO₆ (0.005 min^-1). The degradation rate of IOH on 3.5 wt% BN/Bi₂MoO₆ reached 92% in 150 min. The enhanced photocatalytic activity of BN/Bi₂MoO₆ can be attributed to the heterojunction between BN and Bi₂MoO₆. The matched type-I band alignment heterojunction of two semiconductors prominently improved the charge separation. Based on the trapping experiments, holes and superoxide radicals were proved to be the main active species for photocatalytic IOH degradation. Besides, the degradation products of IOH were analyzed by LC-HRMS and the possible degradation mechanism of IOH was also proposed in this work.

Degradation of several polycyclic aromatic hydrocarbons by laccase in reverse micelle system

Remediation of polycyclic aromatic hydrocarbons (PAHs) in oily sludge has become the focus of attention. UV spectrophotometer analysis showed that four types of PAHs were found in sample, which including phenanthrene, anthracene, benzo(a)anthracene and benzo(b)fluoranthene. In order to degrade PAH effectively, the laccase reverse micelles system was proposed. The system protects laccase from being affected by organic phase. Reverse micelles were prepared by using isooctane to simulate oil. The optimum water content W₀ was 10 by measuring the electrical conductivity of the system. Under this condition, the effects of pH, temperature and ionic strength on the degradation rate of PAHs were investigated. Also, compared with that of non-immobilized laccase, the ratio between the secondary structures of laccase under different conditions was studied. The results showed that the highest laccase activity was obtained at pH 4.2 and 30 °C with 60 mmol/L KCl. Meanwhile, the structure of α-helix accounts for the largest proportion, and the ratio of α-helix in the laccase secondary structure in the laccase-reverse micelle system was higher than that of the non-immobilized one under this condition. Finally, predicting the reactive site of the degradation of polycyclic aromatic hydrocarbons was simulated by ORCA (Version 4.2.0). The application in oily sludge was further conducted. This study provides an effective method and basis for the degradation of PAHs in oily sludge.

Methyl chloride produced during UV254 irradiation of saline water

Ultraviolet (UV) irradiation is widely used for water treatment due to its effectiveness against a wide range of waterborne pathogens with minimal production of regulated disinfection byproducts. However, in this study, the formation of methyl chloride (CH₃Cl) from guaiacol and chloride was observed during UV₂₅₄ irradiation. The results indicated that direct photolysis of guaiacol produced an arenium ion, and the reactive methoxy group was further transformed to CH₃Cl in the presence of chloride. O-quinone was detected as the primary product of the degradation of guaiacol resulting from UV₂₅₄ irradiation. Other organic compounds containing methoxy, ethoxy, or methylamino groups with structures that are similar to guaiacol were also demonstrated to generate halocarbons in aqueous chloride or bromide solution under UV₂₅₄ irradiation. Scavenging experiments and removal of oxygen demonstrated that neither oxygen nor chlorine radicals were involved in CH₃Cl formation. In seawater samples, CH₃Cl was also detected in the presence or absence of added organic matter. These results demonstrate that CH₃Cl can be formed during UV₂₅₄ irradiation in saline water and that attention should be paid to this compound and structurally-related compounds in the application of UV₂₅₄ processes.

Electrospun reduced graphene oxide/TiO2/poly(acrylonitrile-co-maleic acid) composite nanofibers for efficient adsorption and photocatalytic removal of malachite green and leucomalachite green

Electrospun reduced graphene oxide/TiO₂/poly(acrylonitrile-co-maleic acid) composite nanofibers (E-spun RGO/TiO₂/PANCMA NFs) were fabricated using electrospinning of the dispersive solution of PANCMA, GO and TiO₂ followed by post-chemical reduction. The obtained composite nanofibers were compressed in a dialyzer and then used to absorb and degrade malachite green (MG) and leucomalachite green (LMG) from aqueous solution. Compared to the E-spun TiO₂/PANCMA and GO/TiO₂/PANCMA NFs, the E-spun RGO/TiO₂/PANCMA NFs exhibited higher adsorption capacity and photocatalytic degradation ability. Under optimized conditions, 90.6% of MG and 93.7% of LMG from 50 mL aqueous sample solution were adsorbed on the RGO/TiO₂/PANMA NFs (3.0 mg fibers) in 2.0 min, and subsequent the 91.4% and 95.2% of MG and LMG adsorbed on the NFs were degradated in 60 min under UV irradiation, respectively. In addition, the E-spun RGO/TiO₂/PANMA NFs exhibited good reusability and could be reused in multiple cycles of operations for adsorption and photocatalytic degradation of MG and LMG. This work demonstrated that the electrospun composite nanofibers are promising materials for removal of pollutants from environmental water samples.

Acetylacetone extends the working life of laccase in enzymatic transformation of malachite green by interfering with a key intermediate

The potential of acetylacetone (AA) as a mediator of laccase has been tested in the enzymatic transformation of malachite green (MG). AA inhibited the laccase-induced transformation of MG at the beginning of incubation but extended the working life of laccase in long runs. To elucidate the underlying mechanisms, the transformation of MG in the laccase-AA system was systematically investigated. The inhibition of AA on the enzymatic transformation of MG conformed to the partial mixed model. The transformation of N,N,N',N'-tetramethyl-1,1'-biphenyl-4,4'-diamine (NTB) was identified as the rate-controlling step in the laccase system. The generated NTB was oxidized to NTB⁺ by laccase, which acted as a redox mediator to accelerate the transformation of MG. The addition of AA to the enzymatic system quenched the NTB⁺ by forming an intermediate complex of AA-NTB. This quenching reaction led to two contrary effects: the acceleration caused by NTB⁺ in the enzymatic transformation of MG was inhibited whereas the formation of AA-NTB complex enhanced the further transformation at the later stage. As a result, less laccase was consumed, which explained the extended working life of laccase in the long runs. The understanding of these mechanisms are helpful for the better use of laccase as a green biocatalyst.

Degradation of malachite green by a pulsed light/H2O2 process

Pulsed light (PL) is a type of photonic technology characterized by intense short light pulses that enhance the speed of photochemical reactions, and which might be useful as light source in advanced oxidation processes. This work aimed to test PL as light source for the degradation of the dye malachite green (MG) by combining PL with H₂O₂. To this end, the effect of dye and H₂O₂ concentrations and pH on the degradation rate of MG was studied and a degradation pathway was proposed. Dye degradation followed a pseudo-first order kinetics; it increased with low initial dye concentration, high H₂O₂ concentration and low pH. Complete decolourization was achieved after 35 light pulses (75 J/cm²), with a degradation rate of 0.0710 cm²/J. The degradation was initiated by the attack of hydroxyl radicals to the central carbon of MG generating 4-(dimethylamino)benzophenone (DLBP) followed by the addition of hydroxyl radicals to the non-amino aromatic ring of DLBP and the demethylation of the amino group. Results indicate that PL technology has potential to be implemented to decrease the environmental impact of dyeing industries.

The design of a sunlight-focusing and solar tracking system: A potential application for the degradation of pharmaceuticals in water

Photolysis is considered one of the most important mechanisms for the degradation of pharmaceuticals. Photodecomposition processes to remove pharmaceuticals in water treatment presently use artificial UV light incorporated in advanced oxidation processes. However, UV lighting devices consume a substantial amount of energy and have high operational costs. To develop low energy treatment systems and make good use of abundant sunlight, a natural energy resource as a green technology is needed. As such, a system that combines sunlight focusing, solar tracking and continuous reaction was designed and constructed in the present study, and its application potential as a pharmaceutical water treatment option was tested. Two representative photolabile pharmaceuticals, ciprofloxacin and sulfamethoxazole, were chosen as the target pollutants. The results indicate that the sunlight-focusing system consisting of a UV-enhancing-coated parabolic receiver can concentrate solar energy effectively and hence result in a more than 40% improvement in the direct photolysis efficiency of easily photoconvertible ciprofloxacin. The sunlight-focusing coupled with a solar tracker (SFST) system can improve the sunlight-focusing efficiency by more than 2-fold, thus leading to the maximization of the efficient use of solar energy. Sulfamethoxazole, which is difficult to photoconvert, was successfully degraded by more than 60% compared to direct photolysis through the designed SFST system in the presence of persulfate. The treatment system exhibited good and consistent performance during clear and cloudy days of summer. It is proven that the UV-enhanced coated SFST system with the addition of persulfate indeed has development potential for application in the degradation of pharmaceuticals in water.

Photolysis of atorvastatin in aquatic environment: Influencing factors, products, and pathways

Atorvastatin (ATV), a second generation cholesterol-lowering drug, is detected frequently in natural water because of its extensive use and incomplete removal from wastewater. In this study, the photochemical behavior of ATV under simulated solar irradiation was systematically investigated in order to assess the potential of photolysis as its transformation pathway in aquatic environment. The quantum yield of ATV direct photolysis was determined to be 0.0041. Among various water components investigated, including pH, Suwannee River Fulvic Acid (SRFA), Fe³⁺, HCO₃^-, SO₄^2- and NO₃^-, the major factors contributing to the indirect photolysis of ATV were SRFA and NO₃^-, and the co-existence of SRFA and NO₃^- showed no interaction in synthetic water containing the above water components. The results were further verified in natural water samples. Singlet oxygen (¹O₂) played dominant role in the indirect photolysis of ATV, and the contributions of ¹O₂ and ·OH to the photolysis of ATV in the solution with optimum combination of water components were calculated to be 67.14% and 0.66%, respectively. Nine phototransformation intermediates were identified by liquid chromatography - time-of-flight - mass spectrometry (LC-TOF-MS), and the degradation pathways were speculated as hydroxyl addition, pyrrole-ring open and debenzamide reactions. In addition, the evolution of products in the degradation process showed that the ring-opened product P416 and hydroxylation product P575 still remained at a certain level after two days of photodegradation, which may accumulate and cause additional ecological risks. This study provides significant information for understanding the risk and fate of ATV in aquatic environment.

A comprehensive study on the electrocatalytic degradation, electrochemical behavior and degradation mechanism of malachite green using electrodeposited nanostructured β-PbO2 electrodes

This work has investigated the electrocatalytic degradation of malachite green (MG) in aqueous solution with G/β-PbO₂, SS316/β-PbO₂, Ti/β-PbO₂ and Pb/β-PbO₂ electrodes. These electrodes show high oxygen evolution over-potential and excellent electrochemical degradation efficiency for organic pollutants. The optimum conditions for the degradation of MG were obtained by studying the effects of different parameters, such as initial current densities and initial MG concentration. The remaining organic compounds concentrations (color) and chemical oxygen demand (COD) removal efficiency were investigated and compared. The results indicate that the efficiency of G/β-PbO₂ electrode for both color and COD removals is more than those of other electrodes. At the optimum conditions, the color and COD removal efficiencies of MG reached up to 100% and 94%, respectively. The observed degradation rate of MG was found to vary in the order G/β-PbO₂> SS316/β-PbO₂> Ti/β-PbO₂> Pb/β-PbO₂. Moreover, in this paper, the electrochemical behavior and adsorption characteristic of MG in aqueous solutions with different pH values were studied in details at glassy carbon electrode using both constant-current coulometry and cyclic voltammetry techniques. This study has led to the proposed mechanism for the oxidation pathway of MG and determine the absorption properties of MG in acidic, neutral and basic solutions. We also proposed the mineralization pathway of MG at β-PbO₂ electrode.

Metagenomics-Based Discovery of Malachite Green-Degradation Gene Families and Enzymes From Mangrove Sediment

Malachite green (MG) is an organic contaminant and the effluents with MG negatively influence the health and balance of the coastal and marine ecosystem. The diverse and abundant microbial communities inhabiting in mangroves participate actively in various ecological processes. Metagenomic sequencing from mangrove sediments was applied to excavate the resources MG-degradation genes (MDGs) and to assess the potential of their corresponding enzymes. A data set of 10 GB was assembled into 33,756 contigs and 44,743 ORFs were predicted. In the data set, 666 bacterial genera and 13 pollutant degradation pathways were found. Proteobacteria and Actinobacteria were the most dominate phyla in taxonomic assignment. A total of 44 putative MDGs were revealed and possibly derived from 30 bacterial genera, most of which belonged to the phyla of Proteobacteria and Bacteroidetes. The MDGs belonged to three gene families, including peroxidase genes (up to 93.54% of total MDGs), laccase (3.40%), and p450 (3.06%). Of the three gene families, three representatives (Mgv-rLACC, Mgv-rPOD, and Mgv-rCYP) which had lower similarities to the closest sequences in GenBank were prokaryotic expressed and their enzymes were characterized. Three recombinant proteins showed different MG-degrading activities. Mgv-rPOD had the strongest activity which decolorized 97.3% of MG (300 mg/L) within 40 min. In addition, Mgv-rPOD showed a more complete process of MG degradation compared with other two recombinant proteins according to the intermediates detected by LC-MS. Furthermore, the high MG-degrading activity was maintained at low temperature (20°C), wider pH range, and the existence of metal ions and chelating agent. Mgv-rLACC and Mgv-rCYP also removed 63.7% and 54.1% of MG (20 mg/L) within 24 h, respectively. The results could provide a broad insight into discovering abundant genetic resources and an effective strategy to access the eco-friendly way for preventing coastal pollution.

Synthesis and Absorption Properties of Hollow-spherical Dy2Cu2O5 via a Coordination Compound Method with [DyCu(3,4-pdc)2(OAc)(H2O)2]•10.5H2O Precursor

Dy2Cu2O5 nanoparticles with perovskite structures were synthesized via a simple solution method (SSM) and a coordination compound method (CCM) using [DyCu(3,4-pdc)2(OAc)(H2O)2]•10.5H2O (pdc = 3,4-pyridinedicarboxylic acid) as precursor. The as-prepared samples were structurally characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS) and standard Brunauer-Emmett-Teller (BET) methods. Compared to the aggregated hexahedral particles prepared by SSM, the Dy2Cu2O5 of CCM showed hollow spherical morphology composed of nanoparticles with average diameters of 100-150 nm and a larger special surface area up to 36.5 m2/g. The maximum adsorption capacity (Q m ) of CCM for malachite green (MG) determined by the adsorption isotherms with different adsorbent dosages of 0.03-0.07 g, reached 5.54 g/g at room temperature. The thermodynamic parameters of adsorption process were estimated by the fittings of the isotherms at 298, 318, and 338 K, and the kinetic parameters were obtained from the time-dependent adsorption isotherms. The results revealed that the adsorption process followed a pseudo-second-order reaction. Finally, the adsorption mechanism was studied using a competitive ion (CI) experiments, and the highly efficient selective adsorption was achieved due to strong O-Cu and O-Dy coordination bonds between Dy2Cu2O5 and MG.

Spectroscopic investigation and direct comparison of the reactivities of iron pyridyl oxidation catalysts

The growing interest in green chemistry has fueled attention to the development and characterization of effective iron complex oxidation catalysts. A number of iron complexes are known to catalyze the oxidation of organic substrates utilizing peroxides as the oxidant. Their development is complicated by a lack of direct comparison of the reactivities of the iron complexes. To begin to correlate reactivity with structural elements, we compare the reactivities of a series of iron pyridyl complexes toward a single dye substrate, malachite green (MG), for which colorless oxidation products are established. Complexes with tetradentate, nitrogen-based ligands with cis open coordination sites were found to be the most reactive. While some complexes reflect sensitivity to different peroxides, others are similarly reactive with either H₂O₂ or tBuOOH, which suggests some mechanistic distinctions. [Fe(S,S-PDP)(CH₃CN)₂](SbF₆)₂ and [Fe(OTf)₂(tpa)] transition under the oxidative reaction conditions to a single intermediate at a rate that exceeds dye degradation (PDP=bis(pyridin-2-ylmethyl) bipyrrolidine; tpa=tris(2-pyridylmethyl)amine). For the less reactive [Fe(OTf)₂(dpa)] (dpa=dipicolylamine), this reaction occurs on a timescale similar to that of MG oxidation. Thus, the spectroscopic method presented herein provides information about the efficiency and mechanism of iron catalyzed oxidation reactions as well as about potential oxidative catalyst decomposition and chemical changes of the catalyst before or during the oxidation reaction.

Degradation and detoxification of the triphenylmethane dye malachite green catalyzed by crude manganese peroxidase from Irpex lacteus F17

Malachite green (MG), a recalcitrant, carcinogenic, and mutagenic triphenylmethane dye, was decolorized and detoxified using crude manganese peroxidase (MnP) prepared from the white rot fungus Irpex lacteus F17. In this study, the key factors (pH, temperature, MG, Mn(2+), H2O2, MnP) in these processes were investigated. Under optimal conditions, 96 % of 200 mg L(-1) of MG was decolorized when 66.32 U L(-1) of MnP was added for 1 h. The K m, V max, and k cat values were 109.9 μmol L(-1), 152.8 μmol L(-1) min(-1), and 44.5 s(-1), respectively. The decolorization of MG by MnP followed first-order reaction kinetics with a kinetic rate constant of 0.0129 h(-1). UV-vis and UPLC analysis revealed degradation of MG. Furthermore, seven different intermediates formed during the MnP treatment of 0.5 h were identified by LC-TOF-MS. These degradation products were generated via two different routes by either N-demethylation of MG or the oxidative cleavage of the C-C double bond in MG. Based on ecotoxicity analyses performed on bacteria and algae, it was confirmed that MG metabolites produced by the MnP-catalyzed system were appreciably less toxic than the parent compound. These studies indicate the potential use of this enzyme system in the clean-up of aquatic and terrestrial environments.

Adsorption and photocatalytic degradation of malachite green by vanadium doped zinc oxide nanoparticles

Herein the degradation of malachite green (MG) dye from aqueous medium by vanadium doped zinc oxide (ZnO:V3%) nanopowder was investigated. The specific surface area and pore volume of the nanopowder was characterized by nitrogen adsorption method. Batch experimental procedures were conducted to investigate the adsorption and photocatalytic degradation of MG dye. Adsorption kinetics investigations were performed by varying the amount of the catalyst and the initial dye concentrations. Adsorption and photocatalytic degradation data were modeled using the Lagergren pseudo-first-order and second-order kinetic equation. The results showed that the ZnO:V3% nanopowder was particularly effective for the removal of MG and data were found to comply with Lagergreen pseudo-first-order kinetic model.