Photocatalytic Degradation of Methylene Blue by Titanium Dioxide: Experimental and Modeling Study

Designing Photocatalytic Nanostructured Antibacterial Surfaces: Why Is Black Silica Better than Black Silicon?

The efficiency of photocatalytic antibacterial surfaces is limited by the absorption of light in it. Light absorption in photocatalytic surfaces can be enhanced by structuring it, leading to increased generation of reactive oxygen species (ROS) and hence improved bactericidal efficacy. A second, more passive methodology to kill bacteria involves the use of sharp nanostructures that mechanically disrupt the bacterial membrane. Recently, these two mechanisms were combined to form photoactive nanostructured surfaces with better antibacterial efficacy. However, the design rules for fabricating the optimal photoactive nanostructured surfaces have not been articulated. Here we show that for optimal performance it is very important to account for optoelectrical properties and geometry of the photoactive coating and the underlying pillar. We show that TiO₂-coated nanopillars arrays made of SiO₂, a material with a low extinction coefficient, have 73% higher bactericidal efficacies than those made of Si, a material with a high extinction coefficient. The finite element method (FEM) shows that despite the higher absorption in higher aspect ratio nanopillars, their performance is not always better. The concentration of bulk ROS saturates around 5 μm. For taller pillars, the improvement in surface ROS concentration is minimal due to the diffusion bottleneck. Simulation results corroborate with the experimentally observed methylene blue degradation and bacterial count measurements and provide an explanation of the observed phenomenon. The guidelines for designing these optically activated photocatalyst nanopillars can be extended to other photocatalytic material after adjusting for their respective properties.

Novel Bacteria-Immobilized Cellulose Acetate/Poly(ethylene oxide) Nanofibrous Membrane for Wastewater Treatment

In this study, electrospun cellulose acetate - poly(ethylene oxide) nanofibrous membrane was found to be unique in immobilizing bacterial cells. Here, removal of methylene blue in aqueous media was achieved by using isolated species of bacteria (Bacillus paramycoides) from industrial wastewater and immobilized on cellulose acetate- poly(ethylene oxide) nanofibers using DMSO as a solvent. The decolorization time was varied from 0 to 72 h, different dye concentrations from 20 to 200 mg/L and bacterial cells count was investigated to achieve the maximum MB removal by bacteria-immobilized CA/PEO nanofibrous membrane. The effective dye decolorization was achieved within 48 h and MB removal % was around 93%. Furthermore, reusability of the bacteria-immobilized CA/PEO nanofibrous membrane was tested. It was found that after the 4^th usage, 44% of the dye decolorization capacity still could be achieved. These results are promising and suggest that bacteria-immobilized CA/PEO nanofibrous membrane could be economically feasible and eco-friendly when used in MB removal from industrial wastewater. Combination of both adsorption and biodegradation methods was found to be effective in MB removal from aqueous media.

Preparation of Manganese Dioxide Nanoparticles on Laterite for Methylene Blue Degradation

The laterite-coating manganese dioxide nanoparticle material (M2) prepared by the immersion method was used for the efficient removal of methylene blue (MB) from aqueous solution. The adsorption and heterogeneous Fenton catalytic oxidation experiments of M2 were investigated by changing the effective factors such as time, pH, amount of M2, and concentration of MB. The adsorption data of M2 showed good fitting with the Langmuir isotherm, suggesting that the adsorption of MB on the surface of M2 is a heterogeneous and physical adsorption process. Degradation of MB was also carried out to evaluate the heterogeneous Fenton catalytic oxidation characterization of a new catalytic oxidation material (M2). The results show that the M2 material has both adsorption and heterogeneous Fenton catalytic oxidation. However, the heterogeneous Fenton catalytic oxidation of the M2 material is the main performance. Hence, our groups have investigated the ability of the catalytic column treatment with high efficiency of 98–100% and the degradation efficiency after the sample running through the column almost does not change much. This proves that heterogeneous Fenton catalytic activity of the catalytic column is completely unaffected and reused many times after oxidizing MB. Specifically, even if the M2 material is reused for five times, the degradation efficiency still reaches 98.86%.

Enhancing photocatalytic degradation of methyl orange by crystallinity transformation of titanium dioxide: A kinetic study

This work aimed to enhance the photocatalytic degradation of methyl orange (MO) by crystallinity transformation of titanium dioxide (TiO₂ ). In addition, the kinetic degradation of MO was determined. To transform its crystallinity, TiO₂ was synthesized using a sol-gel method and calcined at between 200°C to 600°C. Calcination at a temperature of 250°C resulted in TiO₂ that showed the best performance, corresponding to MO removal of 87% ± 7%. MO removal by TiO₂ calcined between 250°C to 400°C was higher than for commercial TiO₂ powder (Sigma-aldrich) (62% ± 4%). TiO₂ with a small crystallite size and high anatase fraction enhanced the photocatalytic degradation of MO, while the specific surface area and surface roughness seemed to play a minor role. The photocatalytic degradation of MO was NaCl-independent, while the photocatalytic activity increased with decreased pH. Reused TiO₂ showed similar photocatalytic degradation of MO compared with pristine TiO₂ , at 84 ± 2%. The oxidation kinetics of TiO₂  calcined at 250°C were fitted to the Langmuir-Hinshelwood model (R²  = 0.9134). The k_r and K_s values were 0.027 mg L^-1  min^-1  and 0.621 L/mg, respectively. Crystallinity transformation was a major factor in the enhancement of photocatalytic degradation of MO. PRACTITIONER POINTS: Photocatalytic activity of TiO₂ depends on calcination temperature, pH, and a number of UVC lamps. TiO₂ with a small crystallite size and high anatase fraction enhanced the photocatalytic degradation of MO.

Sequential photocatalysis and biological treatment for the enhanced degradation of the persistent azo dye methyl red

A combination of photocatalysis and biodegradation is a promising approach for the removal of xenobiotic organic compounds from wastewater, since photocatalysis cleaves the molecules into simpler intermediates that are later mineralized by microorganisms. Sequential photocatalytic and biological treatment (SPABT) consisting of ZnO as a photocatalyst and a microbial consortium (Galactomyces geotrichum and Brevibaccilus laterosporus) enhanced the degradation of a model textile dye, methyl red (MR). SPABT completely decolorized 500 mg MR/L within 4 h. Biotreatment alone required 6 h for 100% decolorization. A maximum of 70% decolorization was achieved with the photocatalytic treatment but reductions in COD and toxicity were not adequate. Significant elevated activities of enzymes, including azo reductase, laccase and veratryl alcohol oxidase, were observed in the microbial consortium after exposure of MR. The degradation pathway and products of MR varied with treatment applied. The persistent azo bond was cleaved by following photocatalytic treatment with the microbial biotreatment. Tests with Sorghum vulgare and Phaseolus mungo indicated the products obtained by SPABT were non-phytotoxic.

Microscale flower-like magnesium oxide for highly efficient photocatalytic degradation of organic dyes in aqueous solution

Flower-like MgO microparticles with excellent photocatalytic performance in degradation of various organic dyes (e.g., methylene blue, Congo red, thymol blue, bromothymol blue, eriochrome black T, and their mixture) were synthesized by a facile precipitation method via the reaction between Mg2+ and CO3 2- at 70 °C. The reaction time was found to be crucial in determining the final morphology of flower-like MgO. After studying the particles from time-dependent experiments, scanning electron microscope observation, Fourier transform infrared spectra and thermogravimetric analyses demonstrated that the formation of flower-like particles involved a complex process, in which agglomerates or rod-like particles with a formula of xMgCO3·yH2O (x = 0.75-0.77 and y = 1.87-1.96) were favorably formed after the initial mixture of the reactants. Owing to the chemical instability, they would turn into flower-like particles, which had a composition of xMgCO3·yMg(OH)2·zH2O (x = 0.84-0.86, y = 0.13-0.23, and z = 0.77-1.15). After calcination, the generated product not only possessed a superior photocatalytic performance in degradation of organic dyes (100 mg L-1) under UV light irradiation in contrast to other morphologies of MgO and other related state-of-the-art photocatalysts (e.g., N-doped TiO2, Degussa P25 TiO2, ZnO, WO3, α-Fe2O3, BiVO4, and g-C3N4), but also could be used for five cycles, maintaining its efficiency above 92.2%. These capacities made the flower-like MgO a potential candidate for polluted water treatment. Also, the underlying photocatalysis mechanism of MgO was proposed through radical trapping experiments.

Bio-Removal of Methylene Blue from Aqueous Solution by Galactomyces geotrichum KL20A

The conventional treatments used to remove dyes produced as a result of different industrial activities are not completely effective. At times, some toxic by-products are generated, affecting aquatic ecosystems. In this article, an efficient use of microorganisms is presented as a biodegradation technique that is a safe environmental alternative for the benefit of aquatic life. A strain of the yeast Galactomyces geotrichum KL20A isolated from Kumis (a Colombian natural fermented milk) was used for Methylene Blue (MB) bioremoval. Two parameters of the bioremediation process were studied at three different levels: initial dye concentration and growth temperature. The maximum time of MB exposure to the yeast was 48 h. Finally, a pseudo-first-order model was used to simulate the kinetics of the process. The removal percentages of MB, by action of G. geotrichum KL20A were greater than 70% under the best operating conditions and in addition, the kinetic simulation of the experimental results indicated that the constant rate of the process was 2.2 × 10-2 h−1 with a half time for biotransformation of 31.2 h. The cytotoxicity test based on the hemolytic reaction indicated that by-products obtained after the bioremoval process reached a much lower percentage of hemolysis (22%) compared to the hemolytic activity of the negative control (100%). All of these results suggest that the strain has the capacity to remove significant amounts of MB from wastewater effluents.

Comparative toxicity of a food additive TiO2, a bulk TiO2, and a nano-sized P25 to a model organism the nematode C. elegans

To help fill the knowledge gap regarding the potential human health impacts of food pigment TiO₂, a comparative toxicity study was performed on a food-grade TiO₂ (f-TiO₂), a bulk TiO₂ (b-TiO₂), and a nano-sized TiO₂ (Degussa P25), and in the nematode Caenorhabditis elegans. Acute phototoxicity and chronic toxicity effects including reproduction, lifespan, and vulval integrity were evaluated. The f-TiO₂, b-TiO₂, and P25 had a primary particle size (size range) of 149 (53-308) nm, 129 (64-259) nm, and 26 (11-52) nm, respectively. P25 showed the greatest phototoxicity with a 24-h LC50 of 6.0 mg/L (95% CI 5.95, 6.3), followed by the f-TiO₂ (LC50 = 6.55 mg/L (95% CI 6.35, 6.75)), and b-TiO₂ was the least toxic. All three TiO₂ (1-10 mg/L) induced concentration-dependent effects on the worm's reproduction, with a reduction in brood size by 8.5 to 34%. They all caused a reduction of worm lifespan, accompanied by an increased frequency of age-associated vulval integrity defects (Avid). The impact on lifespan and Avid phenotype was more notable for P25 than the f-TiO₂ or b-TiO₂. Ingestion and accumulation of TiO₂ particles in the worm intestine was observed for all three materials by light microscopy. These findings demonstrate that the food pigment TiO₂ induces toxicity effects in the worm and further studies are needed to elucidate the human health implication of such toxicities.

Composition-Tunable Optical Properties of Zn x Cd(1 - x)S Quantum Dot-Carboxymethylcellulose Conjugates: Towards One-Pot Green Synthesis of Multifunctional Nanoplatforms for Biomedical and Environmental Applications

Quantum dots (QDs) are colloidal semiconductor nanocrystals with unique properties that can be engineered by controlling the nanoparticle size and chemical composition by doping and alloying strategies. However, due to their potential toxicity, augmenting their biocompatibility is yet a challenge for expanding to several biomedical and environmentally friendly applications. Thus, the main goal of this study was to develop composition-tunable and biocompatible Zn x Cd1 - x S QDs using carboxymethylcellulose polysaccharide as direct capping ligand via green colloidal aqueous route at neutral pH and at room temperature for potential biomedical and environmental applications. The ternary alloyed QDs were extensively characterized using UV-vis spectroscopy, photoluminescence spectroscopy (PL), transmission electron microscopy (TEM), X-ray diffraction (XRD), electron energy loss spectroscopy (EELS), and X-ray photoelectrons spectroscopy (XPS). The results indicated that Zn x Cd(1 - x)S QDs were surface stabilized by carboxymethylcellulose biopolymer with spherical morphology for all composition of alloys and narrow sizes distributions ranging from 4 to 5 nm. The XRD results indicated that monophasic ternary alloyed Zn x Cd1 - x S nanocrystals were produced with homogenous composition of the core as evidenced by EELS and XPS analyses. In addition, the absorption and emission optical properties of Zn x Cd1 - x S QDs were red shifted with increasing the amount of Cd2+ in the alloyed nanocrystals, which have also increased the quantum yield compared to pure CdS and ZnS nanoparticles. These properties of alloyed nanomaterials were interpreted based on empirical model of Vegard's law and chemical bond model (CBM). As a proof of concept, these alloyed-QD conjugates were tested for biomedical and environmental applications. The results demonstrated that they were non-toxic and effective fluorophores for bioimaging live HEK293T cells (human embryonic kidney cells) using confocal laser scanning fluorescence microscopy. Moreover, these conjugates presented photocatalytic activity for photodegradation of methylene blue used as model organic industrial pollutant in water. Hence, composition-tunable optical properties of ternary Zn x Cd1 - x S (x = 0-1) fluorescent alloyed QDs was achieved using a facile eco-friendly aqueous processing route, which can offer promising alternatives for developing innovative nanomaterials for applications in nanomedicine and environmental science and technology.

Photosensitization of TiO2 nanofibers by Ag2S with the synergistic effect of excess surface Ti3+ states for enhanced photocatalytic activity under simulated sunlight

TiO₂ nanofibers, with mean diameter ~200 nm, were fabricated by electrospinning and successfully photosensitized with low bandgap Ag₂S nanoparticles of 11, 17, 23 and 40 nm mean sizes, with corresponding loading of 4, 10, 18 and 29 wt.% Ag₂S, respectively. 17 nm Ag₂S@TiO₂ nanofibers exhibited optimal activity in the photodegradation of methylene blue under simulated sunlight with pseudo-first order rate constant of 0.019 min⁻¹ compared to 0.009 min⁻¹ for pure TiO₂ nanofibers. In spite of greater visible-light absorption and reduced bandgap, larger than 17 nm Ag₂S nanoparticles exhibited sluggish photodegradation kinetics probably due to less photo-induced carriers generation in TiO₂ and reduced electron injection rates from the larger sized Ag₂S into TiO₂. Furthermore, a UV-O₃ surface treatment induced excess Ti³⁺ surface states and oxygen vacancies which synergistically enhanced the photodegradation rate constant to 0.030 min⁻¹ for 17 nm Ag₂S@TiO₂ sample which is ~70% better than the previously reported for Ag₂S/TiO₂ hierarchical spheres. This was attributed to the efficient charge separation and transfer driven by increased visible-light absorption, bandgap narrowing and reduced electron-hole recombination rates. The present study demonstrate the potential utilization of Ag₂S@TiO₂ nanofibers in filtration membranes for removal of organic pollutants from wastewater.

Heterostructured TiO2@OC core@shell photocatalysts for highly efficient waste water treatment

Facile, scalable and cost-effective organic layer coating of TiO₂ particles greatly enhances the photocatalytic degradation of dye molecules.

One-pot synthesis of MnO2-chitin hybrids for effective removal of methylene blue

Manganese dioxide (MnO₂)-chitin-hybrid material was prepared by a facile "one-pot" synthesis method. MnO₂-chitin hybrid was used for the effective removal of methylene blue (MB) from liquid solution as model for wastewater treatment. The hybrid obtained was characterized by field emission scanning electron microscopy and energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction and thermogravimetric analysis. The effect of pH and temperature were studied. MnO₂-chitin hybrid showed high performance for oxidative decolorization and removal of MB. Typically, 25mL of MB (20mg/L) can be completely decolorized in 2.5min with 8.5mg of the MnO₂-chitin hybrid. The hybrid material exhibited excellent recyclability and durability with the degradation value of 99% for MB after ten consecutive cycles.

A simple UV-ozone surface treatment to enhance photocatalytic performance of TiO2 loaded polymer nanofiber membranes

UV-ozone treated electrospun nanofiber membranes for increased photocatalytic activity.

Structural band-gap tuning in g-C3N4

Experimental and theoretical results uncover an almost perfectly linear relationship between the band gap and structural aspects of g-C₃N₄, allowing the tuning of the frequency at which g-C₃N₄ absorbs light.