Microwave-assisted fabrication of nanoparticulate TiO(2) microspheres for synergistic photocatalytic removal of Cr(VI) and methyl orange

A critical review on the recent trends of photocatalytic, antibacterial, antioxidant and nanohybrid applications of anatase and rutile TiO2 nanoparticles

Titanium dioxide nanoparticles (TiO2 NPs) have become a focal point of research due to their widespread daily use and diverse synthesis methods, including physical, chemical, and environmentally sustainable approaches. These nanoparticles possess unique attributes such as size, shape, and surface functionality, making them particularly intriguing for applications in the biomedical field. The continuous exploration of TiO2 NPs is driven by the quest to enhance their multifunctionality, aiming to create next-generation products with superior performance. Recent research efforts have specifically focused on understanding the anatase and rutile phases of TiO2 NPs and evaluating their potential in various domains, including photocatalytic processes, antibacterial properties, antioxidant effects, and nanohybrid applications. The hypothesis guiding this research is that by exploring different synthesis methods, particularly chemical and environmentally friendly approaches, and incorporating doping and co-doping techniques, the properties of TiO2 NPs can be significantly improved for diverse applications. The study employs a comprehensive approach, investigating the effects of nanoparticle size, shape, dose, and exposure time on performance. The synthesis methods considered encompass both conventional chemical processes and environmentally friendly alternatives, with a focus on how doping and co-doping can enhance the properties of TiO2 NPs. The research unveils valuable insights into the distinct phases of TiO2 NPs and their potential across various applications. It sheds light on the improved properties achieved through doping and co-doping, showcasing advancements in photocatalytic processes, antibacterial efficacy, antioxidant capabilities, and nanohybrid applications. The study concludes by emphasizing regulatory aspects and offering suggestions for product enhancement. It provides recommendations for the reliable application of TiO2 NPs, addressing a comprehensive spectrum of critical aspects in TiO2 NP research and application. Overall, this research contributes to the evolving landscape of TiO2 NP utilization, offering valuable insights for the development of innovative and high-performance products.

A novel cationic covalent organic framework as adsorbent for simultaneous removal of methyl orange and hexavalent chromium

The simultaneous removal of toxic, carcinogenic organic dyes and metal ions from water by one material offers significant advantages when fast, facile, and robust water purification is required. Ionic covalent organic frameworks (ICOFs) have the combined properties of COFs and ion exchange resins and are expected to achieve simultaneous capture of heavy metal ions and organic dyes from water. Herein, a novel guanidinium-based ICOF was synthesized using a solvothermal method. Benefitting from the cationic character, porosity and nanoscale pore size of ICOFs, the adsorbent exhibited high simultaneous adsorption capacities of 290 mg g-1 and 158 mg g-1 for methyl orange (MO) and Cr(vi), respectively, and retained more than 90% adsorption capacity after six adsorption-desorption cycles. In addition, based on dual control of size-exclusion and charge-selection, precisely selective adsorption is achieved towards diverse mixed anionic and cationic pollutants. This strategy offers a practical solution for COFs to confront environmental pollution issues.

Versatile application of BiVO4/TiO2 S-scheme photocatalyst: Photocatalytic CO2 and Cr(VI) reduction

Herein, the synthesis, characterization, and reduction properties of 2D TiO₂ aerogel powder decorated with BiVO₄ (TiO₂/BiVO₄) were investigated for versatile applications. First, 2D TiO₂ was prepared via lyophilization and subsequently modified with BiVO₄ using a wet impregnation method. The morphology, structure, composition, and optical properties were evaluated using transmission electron microscopy (TEM), X-ray diffractometry (XRD), laser-induced breakdown spectroscopy (LIBS), and diffuse reflectance spectroscopy (DRS), respectively. Significantly enhanced photocurrent densities (by 3-15 times) were obtained for TiO₂/BiVO₄ compared to those of pure TiO₂ and BiVO₄. The reduction of toxic Cr(VI) to Cr(III) was assessed, including the effect of pH on overall photocatalytic efficiency. Under acidic conditions (pH ∼ 2), Cr(VI) reduction efficiency reached 100% within 2 h. For photocatalytic CO₂ reduction, the highest yields of CH₄ and CO were obtained using TiO₂/BiVO₄. A higher efficiency for both applications was achieved because of the better separation of the electron-hole pairs in TiO₂/BiVO₄. The excellent stability of TiO₂/BiVO₄ over repeated runs highlights its potential for use in versatile environmental applications. The efficiency of TiO₂/BiVO₄ is due to the interplay of the structure, morphology, composition, and photoelectrochemical properties that favour the material for the presented herein photocatalytic applications.

The composite of bismuth oxyiodide-bismuth/nitrogen-doped carbon for photoreduction and electrochemical/photoelectrochemical dual-model sensing of Cr(VI)

A porous bismuth oxyiodide-metal bismuth/nitrogen-doped carbon (BiOI-Bi/N-C) composite composed of BiOI nanosheets, N-C sheets, and metallic Bi nanoparticles was prepared. BiOI-Bi/N-C exhibited remarkable cathodic photoelectrochemical activity and rapid adsorption capacity for Cr(VI) ions. Interestingly, the photocatalytic process of BiOI-Bi/N-C toward Cr(VI) was pH dependent. Under acidic medium, the synthesized material displayed efficient photocatalysis and achieved 95.0% photoreduction efficacy for Cr(VI) ions to Cr³⁺ within 30 min under visible light irradiation. Under neutral medium, Cr(VI) state showed a different photocatalytic process, and Cr(OH)₃ as a product covered on BiOI-Bi/N-C, which decreased the electrochemical (EC) and photoelectrochemical (PEC) performance of BiOI-Bi/N-C. Based on the findings, BiOI-Bi/N-C was utilized as EC/PEC dual-model sensing interface for the detection of Cr(VI) ions. The presented dual-model sensing method displayed an ultralow limit of detection down to 6.8 pM for EC and 3.2 pM for PEC. It demonstrated the practical application potential for the assay of Cr(VI) in real samples.

Zeolite/ZnAl-layer double hydroxides with different Zn/Al ratios and intercalated anions as the substrate of constructed wetlands: synthesis, characterization and purification effect of Hexavalent chromium

The study aimed to synthesize novel zeolite substrates modified with four types of ZnAl-LDHs including Cl-LDHs(1:1), Cl-LDHs(3:1), CO₃-LDHs(1:1), and CO₃-LDHs(3:1); investigate Cr(VI) removal efficiencies in lab-scale constructed wetlands (CWs); and explore the effect of different Zn/Al ratios and intercalated anions on the removal efficiencies of Cr(VI) by modified zeolite. Different ZnAl-LDHs were prepared by co-precipitation method and coated onto the surface of original zeolite. Field emission scanning electron microscope and energy dispersive spectrometer were used to analyze physicochemical properties of zeolite/ZnAl-LDHs. Obtained results confirmed the successful LDHs-coating modification. The results of both X-ray diffraction and Fourier transform infrared suggested that the typical diffraction peak and functional groups of ZnAl-LDHs were detected in modified zeolites, and the peak of CO₃^2- in CO₃-LDHs at 1362 cm^-1 was stronger and sharper than Cl-LDHs. It could be demonstrated by above results that the synthesis crystallinity and coating effect of CO₃-LDHs was better than Cl-LDHs. Furthermore, it could be found that under the condition of same intercalated anion, LDHs with metal molar ratio of 1:1 had better crystallinity than LDHs with metal molar ratio of 3: 1. Subsequent determination of the removal performance of Cr(VI) by purification experiments revealed that zeolite/Cl-LDHs(3:1) showed the best Cr(VI) removal performance, and the removal rate of Cr(VI) was improved by 32.81% compared with the original zeolite, which suggested that could be an efficient substrate of CWs for Cr(VI) removal. The high crystallinity indicated that the structure of LDHs was stable and it was difficult to remove Cr(VI) by ion exchange. The above explained why the Cr(VI) removal efficiency by zeolite/Cl-LDHs is superior to that by zeolite/CO₃-LDHs under the condition of same metal molar ratio. With the increase of metal molar ratio, the charge density of LDHs layers and intercalated anion increased, thus enhancing the electrostatic attraction of LDHs layers to Cr(VI) and the interlayer anion exchange capacity. However, the effect of charge density on Cr(VI) removal efficiency may be greater than crystallinity on removal efficiency, which could be responsible for the fact that zeolite/ZnAl-LDHs(3:1) had better Cr(VI) removal efficiency than zeolite/ZnAl-LDHs(1:1) under the condition of same intercalated anion.

Applications of Fluorescent Carbon Dots as Photocatalysts: A Review

Carbon dots (CDs) have attracted considerable interest from the scientific community due to their exceptional properties, such as high photoluminescence, broadband absorption, low toxicity, water solubility and (photo)chemical stability. As a result, they have been applied in several fields, such as sensing, bioimaging, artificial lighting and catalysis. In particular, CDs may act as sole photocatalysts or as part of photocatalytic nanocomposites. This study aims to provide a comprehensive review on the use of CDs as sole photocatalysts in the areas of hydrogen production via water splitting, photodegradation of organic pollutants and photoreduction and metal removal from wastewaters. Furthermore, key limitations preventing a wider use of CDs as photocatalysts are pointed out. It is our hope that this review will serve as a basis on which researchers may find useful information to develop sustainable methodologies for the synthesis and use of photocatalytic CDs.

Microwave-Assisted Solvothermal Synthesis of Mo-Doped TiO2 with Exceptional Textural Properties and Superior Adsorption Kinetics

Assigned to their outstanding physicochemical properties, TiO₂-based materials have been studied in various applications. Herein, TiO₂ doped with different Mo contents (Mo-TiO₂) was synthesized via a microwave-assisted solvothermal approach. This was achieved using titanium (IV) butoxide and molybdenum (III) chloride as a precursor and dodecylamine as a surface directing agent. The uniform effective heating delivered by microwave heating reduced the reaction time to less than 30 min, representing several orders of magnitude lower than conventional heating methods. The average particle size ranged between 9.7 and 27.5 nm and it decreased with increasing the Mo content. Furthermore, Mo-TiO₂ revealed mesoporous architectures with a high surface area ranging between 170 and 260 m² g⁻¹, which is superior compared to previously reported Mo-doped TiO₂. The performance of Mo-TiO₂ was evaluated towards the adsorption of Rhodamine B (RhB). In contrast to TiO₂, which revealed negligible adsorption for RhB, Mo-doped samples depicted rapid adsorption for RhB, with a rate that increased with the increase in Mo content. Additionally, Mo-TiO₂ expressed enhanced adsorption kinetics for RhB compared to state-of-the-art adsorbents. The introduced synthesis procedure holds a grand promise for the versatile synthesis of metal-doped TiO₂ nanostructures with outstanding physicochemical properties.

Simultaneous decontamination of multi-pollutants: A promising approach for water remediation

Water remediation techniques have been extensively investigated due to the increasing threats of soluble pollutants posed on the human health, ecology and sustainability. Confronted with the complex composition matrix of wastewater, the simultaneous elimination of coexisting multi-pollutants remains a great challenge due to their different physicochemical properties. By integrating multi-contaminants elimination processes into one unit operation, simultaneous decontamination attracted more and more attention under the consideration of versatile applications and economical benefits. In this review, the state-of-art simultaneous decontamination methods were systematically summarized as chemical precipitation, adsorption, photocatalysis, oxidation-reduction, biological removal and membrane filtration. Their applications, mechanisms, mutual interactions, sustainability and recyclability were outlined and discussed in detail. Finally, the prospects and opportunities for future research were proposed for further development of simultaneous decontamination. This work could provide guidelines for the design and fabrication of well-organized simultaneous decontaminating system.

Construct interesting CuS/TiO2 architectures for effective removal of Cr(VI) in simulated wastewater via the strong synergistic adsorption and photocatalytic process

Most of the reduction processes for Cr (VI) removal tend to be available only at the acidic condition and the capable extent of pH is limited. Here, we developed a facile strategy for constructing CuS/TiO₂ architectures via a facile precipitation process. The as-prepared urchin-like CuS microspheres possessed hierarchical/large porous structure and unique electrical structure, which provided a strong ability to capture the Cr(VI) ions in water. Once CuS microspheres were combined with TiO₂ crystals (P25), a surprised high removal efficiency for Cr(VI) was obtained. With optimal molar ratio of CuS:TiO₂ (0.72:1), 4.4 and 1.3 times in Cr(VI) removal rate were obtained with respect to pure TiO₂ and CuS. The high removal efficiency was induced by the distinct synergistic role of strong adsorption and photocatalytic reduction originated from unique electrical structure in CuS/TiO₂ hetero-structure. Moreover, these novel CuS/TiO₂ architectures possess promising application for Cr⁶⁺ effluents remediation in a wide range of pH and with co-existing anions and cations.

Bifunctional Bi12O17Cl2/MIL-100(Fe) composites toward photocatalytic Cr(VI) sequestration and activation of persulfate for bisphenol A degradation

Bifunctional Bi₁₂O₁₇Cl₂/MIL-100(Fe) composite (BMx) was firstly constructed via facile ball-milling method. The optimal BM200 was highly efficient for Cr(VI) sequestration and activation of persulfate (PS) for bisphenol A (BPA) decomposition under white light illumination, which was much more remarkable than the pristine MIL-100(Fe) and Bi₁₂O₁₇Cl₂, respectively. Furthermore, the photocatalytic reduction efficiency can be significantly improved via the addition of some green small organic acids (SOAs). As well, the BPA degradation can be achieved over an extensive initial pH range of 3.0-11.0. When the PS concentration increased to more than 2.0 mM, the BPA degradation efficiency decreased due to the SO₄^-• self-scavenging effect. It was also found that the co-existence of inorganic anions like H₂PO₄^-, HCO₃^-, SO₄^2-, Cl^- and NO₃^- could decelerate the BPA degradation. The excellent photocatalytic Cr(VI) reduction and persulfate activation performances originated from both MIL-100(Fe) with excellent PS activation ability and Bi₁₂O₁₇Cl₂ with a favorable band position, which not only enabled the efficient separation of charges but also accelerated the formation of SO₄^-• radicals. The BM200 displayed prominent stability and recyclability. More importantly, the credible degradation pathway was proposed based on UHPLC-MS analysis and DFT calculation. This research revealed that the Fe-based MOFs/bismuth-rich bismuth oxyhalides (Bi_xO_yX_z, X = Cl, Br and I) composites possessed great potential in wastewater remediation.

Three-Dimensional Interpenetrating Frameworks Based on {P4Mo6} Tetrameric Clusters and Filled with In Situ Generated Alkyl Viologens

Four novel three-dimensional interpenetrating frameworks based on {P₄Mo₆} units, H[C₁₂H₁₄N₂]₄[TM₄(PO₄)(H₂O)₄Na₆][TM₂(Mo₆O₁₂(HPO₄)₃(PO₄)(OH)₃)₄]·8H₂O (1, 2) and H[C₁₄H₁₈N₂]₄[TM₄(PO₄)(H₂O)₄Na₆][TM₂(Mo₆O₁₂(HPO₄)₃(PO₄)(OH)₃)₄]·8H₂O (3, 4) (TM = Co²⁺ (1, 3), Mn²⁺ (2, 4)) were synthesized and characterized using infrared spectroscopy, elemental analyses, thermogravimetric analysis, and single-crystal X-ray diffraction. In situ generated methyl viologen (compounds 1 and 2) or ethyl viologen (compounds 3 and 4) cations function as templates to induce the generation of 2-fold interpenetrating structures in which the {P₄Mo₆} tetrameric clusters with [TM₄(PO₄)Na₆] (TM = Co²⁺ (1, 3) and Mn²⁺ (2, 4)) as the core were bridged by transition metal ions. Compounds 1-4 possess high thermal stabilities and the decomposition temperature of the inorganic frameworks were all >500 °C. It is worth noting that the four compounds all exhibited the bifunctional catalytic performance that they not only had an excellent photocatalytic activity for the reduction of hexavalent chromium (Cr(VI)) under visible light irradiation but also showed a good electrocatalytic activity for the reduction reaction of hydrogen peroxide.

Two Novel BODIPY-Functional Magnetite Fluorescent Nano-Sensors for Detecting of Cr(VI) Ions in Aqueous Solutions

In this study, we developed two different very sensitive magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors for the selective detection of Cr(VI) ions. The Cr(VI) metal ions sensing is based on the fluorescent quenching of BODIPY functionalized with Fe₃O₄@SiO₂-TPED and Fe₃O₄@SiO₂-TMPTA nanoparticles in the ethanol-water environment. Characterization of the newly synthesized fluorescent BODIPY compound was performed on a ¹H and ¹³C-NMR spectrometer. The morphology, chemical and physical properties of the sensing nano-sensors were studied by transmission thermogravimetric analysis (TGA), X-ray diffraction (XRD), energy dispersive X-ray (EDX), scanning electron microscopy (SEM), FT-IR spectroscopy, and transmission electron microscopy (TEM). UV-visible and fluorescent spectroscopy were used to characterize BODIPY functionalized magnetite fluorescent nano-sensors. Characterization measurements revealed that the mean particle diameter of magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors was 18.5 and 19 nm, respectively. The magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors (0.1 gL^-1 in EtOH/H₂O, v/v (3/7)) showed fluorescence quenching responses towards Cr(VI) ions in the medium at pH:1. The fluorescence quenches of the magnetite fluorescent Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY nano-sensors by Cr(VI) were completed in first 5 and 3 min. Respectively. These features provide potential uses of BODIPY functionalized magnetite fluorescent nano-sensors (Fe₃O₄@SiO₂-TPED-BODIPY and Fe₃O₄@SiO₂-TMPTA-BODIPY) as a new class of non-toxic sensors for environmental applications. Graphical Abstract.

Acceleration of ammonium phosphate hydrolysis using TiO2 microspheres as a catalyst for hydrogen production

Titania microspheres are considered an adequate material with low cost and easily attainable pathways, and can be utilized in photocatalytic H2 production to solve the energy crisis. Spherical porous titanium dioxide materials, with nanostructure composition, were chemically synthesized from titanate nanotubes via a simple hydrothermal technique, then added as a catalyst to accelerate the route of ammonium phosphate hydrolysis for hydrogen production. The mechanism of sphere formation from titanate nanotubes is elucidated in detail through the current study. The prepared materials were applied as a photocatalyst to facilitate the separation and transfer of photoinduced electrons, while preventing the recombination of electron-hole pairs. Experimental results show that the obtained microspheres possess significantly enhanced photocatalytic hydrogen (H2) production performance. The amount of photocatalytic hydrogen product using the microspheres is found to be ∼2.5 fold greater than that of titanate nanotubes. Analytical techniques such as field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HR-TEM), simulated visible solar light and X-ray diffraction (XRD) were used for the evaluation and characterization of the developed products, as well as the elucidation of the route of hydrolysis in the hydrogen production process.

TiO2-based catalysts for photocatalytic reduction of aqueous oxyanions: State-of-the-art and future prospects

Nowadays, an increasing discharge of oxyanions to the natural environment has been attracting worldwide attention. TiO₂-based photocatalysis is regarded as one of the most promising technologies for the conversion of toxic oxyanions (such as chromate, nitrate, nitrite, bromate, perchlorate and selenate) to harmless and/or less toxic substances in contaminated waters. Various types of TiO₂-based catalysts have been developed, and each of them exhibits its own advantages in catalytic reduction of oxyanions. However, the application of these nanostructured TiO₂ in real water bodies remains a challenge, with limitations associated with sunlight harvesting abilities, production costs, reuse stability and exposure risks. Herein, we aim to present a critical review on reported TiO₂-based photocatalytic reduction of aqueous oxyanions, provide a comprehensive understanding of the possible reaction pathways of formed active species, and evaluate the reduction performance of different types of TiO₂-based catalysts. In addition, the impact of operating parameters (such as solution pH, temperature, dissolved oxygen and coexisting substances) on catalytic reduction performance is discussed. Furthermore, the perspectives of TiO₂-based photocatalytic reduction of oxyanions are also proposed.

Crystal facets engineering and rGO hybridizing for synergistic enhancement of photocatalytic activity of nickel disulfide

Crystal facets engineering and graphene hybridizing have been proved to be effective means to improve the photocatalytic activities of semiconductor photocatalysts in recent years. However, most of these efforts are concentrated in metal oxides. In the present study, crystal facets effect on the photocatalytic activity of metal sulfide NiS₂ was studied for the first time. It was found that the {111}-faceted NiS₂ nanocrystals showed improved photocatalytic activity in the degradation of various typical pollutants in water compared with {100}-faceted NiS₂ nanocrystals. Moreover, through hybridizing with rGO nanosheets, the photocatalytic activity of the {111}-faceted NiS₂ nanocrystals can be further improved, resulting in the complete degradation of heavy metal hexavalent chromium and organic dyes. The photocatalytic mechanism was studied in detail through theory calculation and experimental characterization. It was found that both the surface energies of Ni-terminated and S-terminated {111} facets were much higher than that of {100} facets, indicating that {111} facets were more active. Besides, rGO hybridizing can realize the effective separation of photogenerated electrons and holes. The results provide important guidance for the further development of efficient metal sulfide photocatalysts.

Efficient toxicity elimination of aqueous Cr(VI) by positively-charged BiOClxI1-x, BiOBrxI1-x and BiOClxBr1-x solid solution with internal hole-scavenging capacity via the synergy of adsorption and photocatalytic reduction

The ecological toxicity of photocatalysts and toxicity reduction of Cr(VI) have attracted much attention. The development of environmentally-friendly photocatalysts with adsorption and internal hole-scavenging capacity for toxicity reduction of aqueous Cr(VI) via facile preparation method is desirable. In this study, visible-light-active BiOCl_xBr_1-x, BiOCl_xI_1-x and BiOBr_xI_1-x solid solutions were prepared by simple solvothermal strategy. The BiOCl_0.3Br_0.7, BiOCl_0.7I_0.3 and BiOBr_0.7I_0.3 solid solutions exhibited excellent adsorption capacity and photoreduction ability. The removal efficiency of Cr(VI) by BiOCl_0.3Br_0.7 was 97.7% within 60 min. BiOCl_0.7I_0.3 and BiOBr_0.7I_0.3 can remove Cr(VI) almost completely within less than 30 min, which were much higher than those by BiOCl (81.6%) and BiOBr (67.4%) due to joint effect of adsorption and photoreduction. More importantly, the toxicity evaluation confirmed nontoxicity of BiOCl_xBr_1-x, BiOCl_xI_1-x and BiOBr_xI_1-x, and rapid toxicity elimination process of Cr(VI).

The mechanism of promoter-induced zeolite nanosheet crystallization under hydrothermal and microwave irradiation conditions

The mechanism of MFI zeolite nanosheet formation was established under accelerated conditions using perchlorate ions and fresh zeolite seeds as promoters.

Study on the internal electric field in the Cu2O/g-C3N4 p–n heterojunction structure for enhancing visible light photocatalytic activity

A Cu₂O/g-C₃N₄ p–n heterojunction efficiently removes tetracycline in the presence of a built-in electric field.

Surface-passivated, soluble and non-toxic graphene nano-sheets for the selective sensing of toxic Cr(vi) and Hg(ii) metal ions and as a blue fluorescent ink

Non-toxic amine-functionalized soluble graphene nano-sheets (f-GNS) were synthesized by using an old and well-known simple organic procedure. The f-GNS exhibited enhanced optical properties, such as strong blue fluorescence emission with a high value of quantum yield (∼13%). The O,O'-bis-(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol 800 as block polymeric amine (BPA)-passivized surface of f-GNS exhibited high aqueous solubility and excitation-dependent fluorescence emission behavior with a strong photo-stability performance. These f-GNS were tested for the significant selective sensing of toxic metal ions Cr(vi) and Hg(ii) from various tested toxic metal ions. The sensing experiment was supported by cyclic voltammetry analysis. The dual metal ion sensing method based on fluorescence showed the limit of detection (LOD) of ∼56 nM for Cr(vi) and ∼45 nM for Hg(ii) through a fluorescence quenching process. f-GNS were found to be non-toxic when tested over Escherichia coli (E.coli) cells. Additionally, the strong blue emission properties of f-GNS enabled their use as a suitable blue fluorescent ink under UV light illumination.

Removal of chromium(VI) by MnFe2O4 and ferrous ion: synergetic effects and reaction mechanism

MnFe₂O₄ was a magnetic material that can be used to adsorb contaminants in the wastewater. Fe(II) could act as a reductant to transfer Cr(VI) into Cr(III). In this paper, mesoporous MnFe₂O₄ prepared by the coprecipitation method was incorporated with Fe(II) to remove Cr(VI). The samples before and after reaction were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. A total of 98~100% of Cr(VI) in solution was removed by MnFe₂O₄/Fe(II) hybrid system within a wide pH range (pH 3.0-9.0), which is due to the synergetic effects of adsorption from MnFe₂O₄ and reduction from Fe(II). Cr(VI) was reduced to Cr(III) by ≡Mn(II) on MnFe₂O₄ and Fe(II). Cr(III) and Fe(III) produced during reaction formed Cr(III)-Fe(III) hydroxides/oxyhydroxides and deposited on MnFe₂O₄. The inhibiting action of phosphate on the removal of Cr(VI) was greater than that of sulfate. Cr(VI) removal rate retained 94.5% at the fourth recycle test, which showed excellent re-usability of MnFe₂O₄.

Simultaneous reduction of Cr (VI) and degradation of azo dyes by F-Fe-codoped TiO2/SiO2 photocatalysts under visible and solar irradiation

A new TiO2/SiO2 photocatalyst codoped with fluorine and iron was synthesized using the sol–gel process and immobilized onto glass beads by coupling two methods of dip coating and heat attachment. The prepared samples were characterized by XRD, FTIR, DRS-UV–vis, FESEM, EDX, TEM, and N2 adsorption–desorption analysis. The effective parameters of pH, flow rate, and photoreactor vessels’ angle against solar irradiation on degradation performance were optimized. Subsequently, the photocatalytic activity of the prepared nanocomposites was investigated in an innovative fabricated photoreactor using a four-component pollutant mixture of Cr (VI), basic red 29, basic blue 41, and basic yellow 51 under visible and solar irradiation. Furthermore, the effectiveness of photoreactor performance was assessed under various outdoor climate conditions as natural irradiation sources. Consequently, UV–vis spectrophotometry results showed significant efficiency of photocatalytic removal of the pollutant mixture. TOC results of 74.39% and 78.04% for removal of organic content of the sample under visible and solar light, respectively, were also confirmed the great capability of the designed system for the simultaneous removal of some hazardous inorganic and organic contaminants under natural light sources.

Controlled Growth of LDH Films with Enhanced Photocatalytic Activity in a Mixed Wastewater Treatment

Due to multiple charge transport pathways, adjustable layer spacing, compositional flexibility, low manufacturing cost, and absorption of visible light, layered double hydroxides (LDHs) are a promising material for wastewater treatment. In this study, LDH films and Fe-doped LDH films with different metal ions (Ni, Al, Fe) on the surface of conductive cloth were successfully prepared and applied for the photocatalytic degradation of wastewater containing methyl orange and Ag ions under visible-light irradiation. The chemical state of Fe ions and the composition of LDHs on methyl orange photodegradation were investigated. The experimental results showed that LDH films exhibited high photocatalytic activity. The photocatalytic activity of LDH films on methyl orange improved in the mixed wastewater, and the Fe-doped NiAl-LDH films exhibited best visible-light photocatalytic performance. The analysis showed that Ag ions in the mixed wastewater were reduced by the LDH films and subsequently deposited on the surface of the LDH films. The Ag nanoparticles acted as electron traps and promoted the photocatalytic activity of the LDH films on methyl orange. Thus, we have demonstrated that prepared LDH films can be used in the treatment of mixed wastewater and have broad application prospects in environmental remediation and purification processes.

Facile synthesis of hierarchically nanostructured bismuth vanadate: An efficient photocatalyst for degradation and detection of hexavalent chromium

Heterostructured nanomaterials can paid more significant attention in environmental safety for the detection and degradation/removal of hazardous toxic chemicals over a decay. Here, we report the preparation of hierarchically nanostructured shuriken like bismuth vanadate (BiVO₄) as a bifunctional catalyst for photocatalytic degradation and electrochemical detection of highly toxic hexavalent chromium (Cr(VI)) using the green deep eutectic solvent reline, which allows morphology control in one of the less energy-intensive routes. The SEM results showed a good dispersion of BiVO₄ catalyst and the HR-TEM revealed an average particle size of ca. 5-10 nm. As a result, the BiVO₄ exhibited good photocatalytic activity under UV-light about 95% reduction of Cr(VI) to Cr(III) was observed in 160 min. The recyclability of BiVO₄ catalyst exhibited an appreciable reusability and stability of the catalyst towards the photocatalytic reduction of Cr(VI). Also, the BiVO₄-modified screen printed carbon electrode (BiVO₄/SPCE) displayed an excellent electrochemical performance towards the electrochemical detection of Cr(VI). Besides, the BiVO₄/SPCE demonstrated tremendous electrocatalytic activity, lower linear range (0.01-264.5 μM), detection limit (0.0035 μM) and good storage stability towards the detection of Cr(VI). Importantly, the BiVO₄ modified electrode was also found to be a good recovery in water samples for practical applications.

Dual phototransformation of the pollutants methyl orange and Cr (VI) using phthalocyanine-cobalt ferrite based magnetic nanocomposites

Bifunctional nanocomposites based on zinc phthalocyanines and glutathione capped CoFe2O4 magnetic nanoparticles (GSH-CoFe2O4 MNPs) are applied in a binary system wherein simultaneous photooxidation of methyl orange (MO) and photoreduction of Cr (VI) are conducted. The photoactivity of two zinc Pcs with different functional moieties are compared based on their interactions with GSH-CoFe2O4 MNPs. Conjugation of the Pcs to the GSH-CoFe2O4 MNPs not only enhanced their singlet oxygen production but also their photocatalytic activity in both photooxidation and photoreduction experiments. Using electron paramagnetic resonance (EPR) spectroscopy, the Pc-MNP conjugates reported herein were found to exhibit superparamagnetic behaviour, giving the advantage of easy separation using an external magnetic field post application, an attractive attribute for heterogeneous catalysis. The catalysts reported herein are therefore good candidates as catalysts for real life water purification analyses as they facilitate the treatment of both organic and inorganic water pollutants.

Highly selective and sensitive probes for the detection of Cr(vi) in aqueous solutions using diglycolic acid-functionalized Au nanoparticles

In this study, a variety of diglycolic acid-functionalized gold nanoparticle (Au NP) probes are reported, which are highly sensitive for the detection of chromium ions, Cr(vi) ions, at low concentrations in aqueous solutions based on the application of surface plasmon resonance (SPR) theory. Due to its outstanding affinity for Cr(vi) ions, the capped diglycolic acid would induce the aggregation of the NP probes upon encountering them; this was evidenced by the obvious red-shifting of the SPR peak and the enlarged size of the NPs. For the same reason, the selectivity of the probe for Cr(vi) against other heavy metal ions was found to be remarkable. Under optimized conditions, the probe showed the limit of detection (LOD) of 0.32 ppb for Cr(vi) and a linear detection scale ranging from 0.32 ppb to 0.1 ppm. To the best of our knowledge, this is probably the lowest LOD reported for Cr(vi) detection among those of the methods based on SPR.

In this study, we proposed a diglycolic acid-functionalized gold nanoparticle (Au NP) probe, which are highly sensitive for the detection of chromium ions based on the application of surface plasmon resonance (SPR) theory.

Alkali-corrosion synthesis and excellent DSSC performance of novel jujube-like hierarchical TiO2 microspheres

Novel jujube-like hierarchical TiO₂ microspheres (HTMs) were synthesized by an alkali-corrosion process of titanium phosphate (Ti₂O₃(H₂PO₄)₂ · 2H₂O) microspheres. The hierarchical titanium phosphate microsphere (HTPM) intermediates consisting of nanoflakes with a thickness of 20 nm were firstly prepared by a facile hydrothermal method. After reacting with diluted NaOH at low temperature and atmospheric pressure, followed by subsequent acid washing and a calcination process, the HTPM intermediates were transformed to TiO₂ with the microsphere morphology well retained, while the nanoflakes became porous, and some new nanowires were formed between the nanoflakes. Finally, HTMs consisting of porous nanoflakes and nanowires were obtained. The possible growth mechanisms of HTPMs and HTMs are discussed. The HTMs demonstrate high specific surface area and excellent light-scattering ability. The performance of the dye sensitized solar cells (DSSCs) of the HTMs synthesized under different conditions is studied, and a total conversion efficiency of up to 8.93% was obtained. The improved DSSC performance was attributed to the enhanced dye loading, light-scattering, and charge transporting ability of the HTMs with a unique hierarchical nanostructure.

Controllable synthesis of coloured Ag0/AgCl with spectral analysis for photocatalysis

Broad spectrum absorption of semiconductor photocatalysts is an essential requirement to achieve the best values of solar energy utilization. Here, through precise surface state adjustment, coaxial tri-cubic Ag⁰/AgCl materials with distinct apparent colours (blue and fuchsia) were successfully fabricated. The reasons for the different colour generation of the Ag⁰/AgCl materials were investigated by performing corresponding spectrum analysis. It was revealed that Ag⁰/AgCl-blue and Ag⁰/AgCl-fuchsia crystals could efficiently boost the photon energy harvesting, spanning from the UV to near-infrared spectral region (250–800 nm), and achieved 2.6 and 5.4 times the wastewater degradation efficiency of AgCl-white. Simultaneously, these two fresh coloured candidates were demonstrated to have preferable photocatalytic CO₂ photoreduction capability, with yields of ∼3.6 (Ag⁰/AgCl-fuchsia) and 2.6 (Ag⁰/AgCl-blue) times that of AgCl-white. It is expected that this work will provide a beneficial perspective for understanding the solar absorption feature at both the major structure modulation and particular surface state regulation level.

Through surface state modification, distinctly coloured AgCl (blue and fuchsia) materials have been successfully fabricated and exhibit a broader absorption region than normal white-coloured AgCl crystals for photocatalytic reactions.

Enhanced removal of hexavalent chromium from aqueous media using a highly stable and magnetically separable rosin-biochar-coated TiO2@C nanocomposite

Recently, nanosized metal-oxides have been extensively investigated for their ability to remove metal ions from aqueous media. However, the activity and capacity of these nanosized metal-oxides for removing metal ions decrease owing to their agglomeration in aqueous media. Herein, we synthesized a highly stable and magnetically separable rosin-biochar-coated (RBC) TiO₂@C nanocomposite through a facile and environment-friendly wet chemical coating process, followed by a one-step heating route (pyrolysis) for efficient removal of Cr(vi) from aqueous solution. An array of techniques, namely, TEM, HRTEM, TEM-EDS, XRD, FTIR, VSM, BET and TGA, were used to characterize the prepared nanocomposite. The pyrolysis of rosin into biochar and the fabrication of Fe onto the RBC-TiO₂@C nanocomposite were confirmed by FTIR and XRD examination, respectively. Moreover, TEM and HRTEM images and elemental mapping using TEM-EDS showed good dispersion of iron and carbon on the surface of the RBC-TiO₂@C nanocomposite. Sorption of Cr(vi) ions on the surface of the RBC-TiO₂@C nanocomposite was very fast and efficient, having a removal efficiency of ∼95% within the 1^st minute of reaction. Furthermore, thermodynamic analysis showed negative values of Gibb's free energy at all five temperatures, indicating that the adsorption of Cr(vi) ions on the RBC-TiO₂@C nanocomposite was favorable and spontaneous. Conclusively, our results indicate that the RBC-TiO₂@C nanocomposite can be used for efficient removal of Cr(vi) from aqueous media due to its novel synthesis and extraordinary adsorption efficacy during a short time period.

A biochar-coated RBC-TiO₂@C nanocomposite was synthesized using a wet chemical coating followed by a one-step heating route (pyrolysis) for the efficient removal of Cr(vi).

Polyethylenimine-Functionalized Corn Bract, an Agricultural Waste Material, for Efficient Removal and Recovery of Cr(VI) from Aqueous Solution

In this study, polyethylenimine-functionalized corn bract (PEI-CB) was first used to remove aqueous Cr(VI) via the "waste control by waste" concept. The results indicated that PEI-CB had an excellent performance for Cr(VI) removal and the maximum removal capacity was 438 mg/g. The adsorption of Cr(VI) was fitted to the Langmuir model, and kinetics of uptake could be described by a pseudo-second-order rate model well. Amine was proven to be the active center for Cr(VI) adsorption and partial reduction to Cr(III), while removal efficiency was enhanced at a lower pH value and higher temperature. Besides, nanosized Cr₂O₃ with a high purity was obtained by simple calcination of a Cr(VI)-laden adsorbent. Hence, this study provided a novel strategy for Cr(VI) wastewater remediation and pure Cr₂O₃ recovery. Prepared PEI-CB was then a promising alternative of low cost for replacement of the current expensive absorbent of removing Cr(VI) from wastewater from the view of sustainability.