Model Organic Contaminant Research Articles

Photocatalytic activities of TiO2 layers immobilized on glass substrates by dip-coating technique toward the decolorization of methyl orange as a model organic pollutant

The aim of this article is to evaluate the photocatalytic activities of the immobilized catalyst layers of titanium dioxide for the decolorization of methyl orange aqueous solution (MO) as a model organic contaminant under UV irradiation. Three stable layers of TiO2 powder were coated on glass substrate by facile dip-coating technique. XRD analysis showed anatase crystalline structure of catalyst films. The films crystallinity increased with the layers number. SEM analysis showed porous TiO2 films. The multi-coating increased the coverage surface of glass support. The catalyst films showed well reproducibility and good adhesion to the support after tests. The effects of operating parameters and the number of catalyst layers on the MO decolorization were investigated. The kinetic study of the MO photodecolorization showed a pseudo-first-order reaction. The MO color removal by the use of three coated layers of catalyst in the optimum reactor was more effective and 5 times faster than that with the use of one coated layer of catalyst at normal conditions. The immobilized layers of TiO2 powder was found photocatalytic active for complete MO decolorization in the optimum reactor. The immobilized system may replace suspension mode and eliminate the costly separation process.

Introducing saccharic acid as an efficient iron chelate to enhance photo-Fenton degradation of organic contaminants

The identification of iron chelates that can enhance photo-Fenton degradation is of great interest in the field of advanced oxidation process. Saccharic acid (SA) is a polyhydroxy carboxylic acid and completely non-toxic. Importantly, it can effectively bind Fe(III) as well as induce photoreduction of Fe(III). Despite having these interesting properties, the effect of SA on photo-Fenton degradation has not been studied. Herein, we demonstrate the first assessment of SA as an iron chelate in photo-Fenton process using methylene blue (MB) as a model organic contaminant. Our results demonstrate that SA has the ability to (i) enhance the photo-Fenton degradation of MB by about 11 times at pH 4.5 (ii) intensify photochemical reduction of Fe(III) to Fe(II) by about 17 times and (iii) accelerate the rate of consumption of H2O2 in photo-Fenton process by about 5 times (iv) increase the TOC reduction by about 2 times and (v) improve the photo-Fenton degradation of MB in the presence of a variety of common inorganic ions and organic matter. The influential properties of SA on photo-Fenton degradation is attributed to the efficient photochemical reduction of Fe(III) via LMCT (ligand to metal charge transfer reaction) to Fe(II), which then activated H2O2 to generate OH and accelerated photo-Fenton degradation efficiency. Moreover, the effect of operational parameters such as oxidant: contaminant (H2O2: MB) ratio, catalyst: contaminant (Fe(III)SA: MB) ratio, Fe(III): SA stoichiometry and pH on the degradation of MB by photo-Fenton in the presence of SA is demonstrated. Importantly, SA assisted photo-Fenton caused effective degradation of MB and 4-Chlorophenol under natural sunlight irradiation in natural water matrix. The findings strongly support SA as a deserving iron chelate to enhance photo-Fenton degradation.

Single step peroxidase extraction and oxidation of highly concentrated ethanol and phenol aqueous solutions using supercritical carbon dioxide

Extraction of peroxidase from raw vegetables using supercritical carbon dioxide (scCO2) and its simultaneous application for oxidation of organics in wastewater in the presence of hydrogen peroxide has been demonstrated. Ethanol and phenol were used as the model organic contaminants and potato and cabbage were employed as the sources of peroxidase. The extractions were carried out at 10MPa for 2h over a temperature range of 25°C to 60°C. Oxidation of ethanol increased by 330% when the treatment was carried out in the scCO2 medium compared to conventional aqueous medium when cabbage was employed, while use of potato as the enzyme source resulted in an increase of 85%. In the case of phenol, scCO2 exhibited a huge increase of 400% in presence of cabbage and 140% in presence of potato. Comparison of vegetable extracts with commercial horseradish peroxidase indicated the feasibility, importance and advantage of employing vegetables as a peroxidase source. This work presents the simultaneous extraction of enzymes and its application for oxidation of organics and opens up new opportunities for recovery of resources from unwanted/discarded plant materials and their use for environmental remediation in a single step.

Application of Engineered Si Nanoparticles in Light-Induced Advanced Oxidation Remediation of a Water-Borne Model Contaminant.

Surface-engineered amphiphilic polymer-coated silicon nanoparticles (SiNPs) were employed as photocatalysts to capture and degrade a model organic contaminant (methanol) in water. This study represents the first time SiNPs have been employed in the initiation of advanced oxidation processes that are commonly used to degrade organic constituents in industrial wastewaters. The quantum yield of photocatalytic methanol oxidation and the corresponding yield factor for the generation of active OH radicals are reported. The size and surface defect dependent photocatalytic activity of SiNPs was investigated. The yield factors (η) decreased with increasing particle size and reached impressive values that exceeded that of equivalent TiO2 nanoparticle systems by 3-4 times and are comparable to the robust UV/Cl2 and UV/H2O2 systems. The higher photocatalytic efficiency of SiNPs is attributed to the combined effects of quantum confinement, effective band gap, and surface states, among which surface states play a dominant role. SiNPs provide a potentially tunable, biologically inert, and robust nanoparticle system for photocatalytic oxidation of wastewater contaminants.

Removal of phenol from water by catalytic wet air oxidation using carbon bead – supported iron nanoparticle – containing carbon nanofibers in an especially configured reactor

Catalytic wet air oxidation (CWAO) is extensively used for the destruction of organic contaminants in wastewater. The present study describes the removal of phenol from water, used as a model organic contaminant, by CWAO with iron (Fe) metal nanoparticles (NPs)-doped carbon microbeads (∼0.6mm) as the catalyst. The Fe-carbon composite was prepared by the carbonization and activation of the phenolic precursor-based polymeric beads in which the Fe NPs were in-situ added during the polymerization stage. Carbon nanofibers (CNFs) were grown on the carbon microbead substrate by catalytic chemical vapor deposition with acetylene as the carbon source. Oxidation reactions were carried out under different operating conditions in a high pressure-stirred reactor, viz., temperature, catalyst loading, and speed of the stirrer. The reactor was fitted with an especially configured impeller cum catalyst basket which held the prepared CNF-decorated Fe-doped carbon beads. The data showed an efficient remediation of the phenol-laden water, indicating the potential scale-up of the proposed CWAO catalyst and impellor cum catalyst holder-assembly in this study.

Formation of environmentally persistent free radical (EPFR) in iron(III) cation-exchanged smectite clay.

Environmentally persistent free radicals (EPFRs) have been found at a number of Superfund sites, with EPFRs being formed via a proposed redox process at ambient environmental conditions. The possibility of such a redox process taking place at ambient environmental conditions is studied utilizing a surrogate soil system of phenol and iron(III)-exchanged calcium montmorillonite clay, Fe(III)CaM. Sorption of phenol by the Fe(III)CaM is demonstrated by Fourier-transformed infra-red (FT-IR) spectroscopy, as evidenced by the peaks between 1345 cm(-1) and 1595 cm(-1), and at lower frequencies between 694 cm(-1) and 806 cm(-1), as well as X-ray diffraction (XRD) spectroscopy, as shown by an increase in interlayer spacing within Fe(III)CaM. The formation and characterization of the EPFRs is determined by electron paramagnetic resonance (EPR) spectroscopy, showing phenoxyl-type radical with a g-factor of 2.0034 and ΔHP-P of 6.1 G at an average concentration of 7.5 × 10(17) spins per g. EPFRs lifetime data are indicative of oxygen and water molecules being responsible for EPFR decay. The change in the oxidation state of the iron redox center is studied by X-ray absorption near-edge structure (XANES) spectroscopy, showing that 23% of the Fe(III) is reduced to Fe(II). X-ray photoemission spectroscopy (XPS) results confirm the XANES results. These findings, when combined with the EPFR concentration data, demonstrate that the stoichiometry of the EPFR formation under the conditions of this study is 1.5 × 10(-2) spins per Fe(II) atom.

Environmental reduction of carbon nanomaterials affects their capabilities to accumulate aromatic compounds

Carbon nanomaterials (e.g., graphene oxides and carbon nanotubes) may undergo chemical transformation in the environment, resulting in significant changes in surface chemistry properties. This will affect their capabilities to accumulate toxic contaminants and therefore, may alter their risk. Herein, we demonstrate that ferrous iron (i.e., Fe(II), an environmentally abundant, mild reductant) can significantly affect the capabilities of four common carbon nanomaterials—including graphene oxide (GO), reduced graphene oxide (RGO), hydroxylated multi-walled carbon nanotubes (OH-MWCNT), and carboxylated multi-walled carbon nanotubes (COOH-MWCNT)—to accumulate two model organic contaminants, phenanthrene and 1-naphthol. Reduction of carbon nanomaterials enhances adsorption of phenanthrene consistently, by increasing the surface hydrophobicity of carbon nanomaterials and by increasing π–π interactions, through partial restoration of the carbon graphitic structures, even though the extent of adsorption enhancement varied among different carbon nanomaterials. Interestingly, reduction of nanomaterials can either enhance or weaken adsorption of 1-naphthol, depending on the type and distribution of surface O-functionalities of a given carbon nanomaterial. The net effects depend on the overall contribution of enhanced hydrophobic effect and π–π interactions, as well as enhanced or weakened polar interactions between 1-naphthol and surface O-functional groups (e.g., H-bonding and cation-bridging by Fe2+/Fe3+). The findings underline the reactive nature of carbon nanomaterials and may have important implications for their risk assessment.

Ultrasonic-assisted degradation of phenazopyridine with a combination of Sm-doped ZnO nanoparticles and inorganic oxidants

Pure and samarium doped ZnO nanoparticles were synthesized by a sonochemical method and characterized by TEM, SEM, EDX, XRD, Pl, and DRS techniques. The average crystallite size of pure and Sm-doped ZnO nanoparticles was about 20nm. The sonocatalytic activity of pure and Sm-doped ZnO nanoparticles was considered toward degradation of phenazopyridine as a model organic contaminant. The Sm-doped ZnO nanoparticles with Sm concentration of 0.4mol% indicated a higher sonocatalytic activity (59%) than the pure ZnO (51%) and other Sm-doped ZnO nanoparticles. It was believed that Sm3+ ion with optimal concentration (0.4mol%) can act as superficial trapping for electrons in the conduction band of ZnO and delayed the recombination of charge carriers. The influence of the nature and concentration of various oxidants, including periodate, hydrogen peroxide, peroxymonosulfate, and peroxydisulfate on the sonocatalytic activity of Sm-doped ZnO nanoparticles was studied. The influence of the oxidants concentration (0.2–1.4gL−1) on the degradation rate was established by the 3D response surface and the 2D contour plots. The results demonstrated that the utilizing of oxidants in combination with Sm-doped ZnO resulting in rapid removal of contaminant, which can be referable to a dual role of oxidants; (i) scavenging the generated electrons in the conduction band of ZnO and (ii) creating highly reactive radical species under ultrasonic irradiation. It was found that the Sm-doped ZnO and periodate combination is the most efficient catalytic system under ultrasonic irradiation.

Photocatalytic Elimination of Cr(VI) in Aqueous Solution by Using ZSM-5 Zeolite as Catalyst and Urea as Coexisting Organic Contaminants

The photocatalytic reduction of Cr ( VI ) to Cr ( III ) in aqueous solutions without or with urea (a model organic contaminant) using ZSM-5 zeolite as catalyst under UV illumination was studied. The used ZSM-5 zeolite has the characteristics of pure ZSM-5 zeolite crystalline structure, with the point of zero charge (pHPZC) value of pH = 3.7–3.9. The effects of illumination time, mass content of ZSM-5 zeolite, urea concentrations, Cr ( VI ) initial concentrations and ionic strength on the photocatalytic reduction of Cr ( VI ) to Cr ( III ) were investigated. The results indicated that both the increase of urea concentration and the mass content of ZSM-5 zeolite can improve the photocatalytic reduction of Cr ( VI ) to Cr ( III ) under UV illumination. The results are important to understand the photocatalytic reduction of Cr ( VI ) to Cr ( III ) in natural environment with organic contaminant.

Photocatalytic degradation of organic compounds by Au-TiO2/sepiolite composites as the highly efficient catalysts

Photocatalytic oxidation of methyl orange (MO) and Congo red (CR) as typical model organic contaminants was investigated in aqueous solution within a cooperating Au/TiO2/sepiolite heterostructure system under UV light irradiation. The Au/TiO2/sepiolite composites with a single-crystalline (anatase) framework was synthesized by a facile sol-gel method using titanium tetrachloride as a TiO2 precursor and depositing metal Au on the surface of TiO2 nanostructures via a facile chemical reduction process. The crystal structure, surface area, light adsorption and the photoinduced charge separation rate of the photocatalyst prepared were characterized in detail. As compared with the pristine TiO2, the Au/TiO2/sepiolite hybrid material exhibited good photocatalytic efficiency (90%) for the UV-light photooxidation of methyl orange, which is four-fold of that of reference TiO2. In addition, Au/TiO2/sepiolite hybrid material also shows a good photodegradation performance toward Congo red removal. The highly efficient photocatalytic activity is associated with the strong adsorption ability of sepiolite for aromatic dye molecules, fast photogenerated charge separation due to the formation of Schottky junction between TiO2 and metallic Au. This work suggests that the combination of the excellent adsorption properties of sepiolite and the efficient separation effect of noble metallic nanoparticles provides a versatile strategy for the synthesis of novel and highly efficient photocatalysts.

Evaluation of gold-decorated halloysite nanotubes as plasmonic photocatalysts

Herein we report on the synthesis of gold-decorated halloysite nanotubes (HNTs) and their evaluation as photocatalysts in the degradation of a model organic contaminant. The combination of photo-thermally-induced local heating and charge transfer mechanisms between the nanostructured gold surface and the organic molecule results in the observed dye decomposition. The use of an inexpensive natural clay support with suitable surface chemistry and adsorption properties represents an eco-friendly method for light-induced oxidation reactions carried out at ambient temperatures.

Effect of preparation temperature on the ability of bone char to remove fluoride ion and organic contaminants

The excessive intake of fluoride ion (F) from drinking water causes dental and skeletal disorders and thus methods for the removal of F from water are desired. A method for removing organic contaminants from water is also desired. Bone char is a composite material composed of char and hydroxyapatite (HA). Bone char can remove F because of the HA in the bone char. It is expected that bone char containing small grain size HAwill remove F effectively because of its high surface area. Additionally, the char in bone char can remove organic matter from contaminated water, and controlling the amount of char in bone char is also important. Bone char is fabricated by heating porcine bone at 200­600°C for 1 h under a limited oxygen supply. The grain size of HA in these samples increases with an increase in heating temperature. The amount of organic matter and char in these samples decreases with an increase in heating temperature. Bone char with a controlled HA grain size and the amount of char was fabricated by changing the heating temperature. Their ability to remove F and methylene blue (MB), as a model organic contaminant, was evaluated by immersing the samples in F-containing and MB-containing solutions. The sample prepared at 400°C was able to remove most of the F and less than 1.5mg·dm13 F remained, which is within the World Health Organization recommended level for drinking water. Additionally, the sample prepared at 400°C was also able to remove most of the MB. Bone char can thus be used to remove F and organic contaminants.

A critical review of the reactivity of manganese oxides with organic contaminants.

Naturally occurring manganese (Mn(iii/iv)) oxides are ubiquitous in a wide range of environmental settings and play a key role in numerous biogeochemical cycles. In addition, Mn(iii/iv) oxides are powerful oxidants that are capable of oxidizing a wide range of compounds. This review critically assesses the reactivity of Mn oxides with organic contaminants. Initial work with organic reductants employed high concentrations of model compounds (e.g., substituted phenols and anilines) and emphasized the reductive dissolution of the Mn oxides. Studies with lower concentrations of organic contaminants demonstrate that Mn oxides are capable of oxidizing a wide range of compounds (e.g., antibacterial agents, endocrine disruptors, and pesticides). Both model compounds and organic contaminants undergo similar reaction mechanisms on the oxide surface. The oxidation rates of organic compounds by manganese oxides are dependent upon solution conditions, such as pH and the presence of cations, anions, or dissolved organic matter. Similarly, physicochemical properties of the minerals used affect the rates of organic compound oxidation, which increase with the average oxidation state, redox potential, and specific surface area of the Mn oxides. Due to their reactivity with contaminants under environmentally relevant conditions, Mn oxides may oxidize contaminants in soils and/or be applied in water treatment applications.

펜톤 및 펜톤 유사반응에서 말론산을 이용한 과산화수소의 안정화

Hydrogen peroxide takes much of the cost for Fenton reaction applied for treatment of organic contaminants. Therefore, the effective use of hydrogen peroxide makes the technology more cost effective. The effective use of hydrogen peroxide is especially needed in the soil and groundwater remediation where complete mixing is not possible and it takes a long time for reactive species to transport to the fixed target compounds. Stabilization ability for hydrogen peroxide of malonic acid was evaluated in Fenton and Fenton-like reactions in this study. Malonic acid contributes on the stabilization of hydrogen peroxide by weak interaction between iron and the stabilizer and inhibiting the catalytic role of iron. The stabilization effect increased as the solution pH decrease below the <TEX>$pK_{a1}$</TEX>. The stabilization effect increased as the concentration of malonic acid increased and the effect was maximized at the malonic acid concentration of about ten times higher than the iron concentration. The model organic contaminant was successfully oxidized in the presence of the stabilizer but the degradation rate was slower than the system without the stabilizer. The stabilization effect was also proved in a Fenton-like reaction where magnetite and hematite were used instead of soluble iron species.

Cu-containing MFI zeolites as catalysts for wet peroxide oxidation of formic acid as model organic contaminant

The catalytic behavior of different Cu(Fe)/zeolite catalysts with the MFI morphology in wet peroxide oxidation of formic acid as a model organic substrate was thoroughly examined. The influence of the method for Cu2+ ions introduction into the zeolite matrix, Cu loading and the electron state of Cu on catalytic properties of zeolites, including their stability to leaching of Cu was studied. Cu-substituted ZSM-5 zeolites with the atomic ratio Si/Al=30 and Cu content of 0.5–1.5, which were prepared by ion exchange, were shown most promising for wet peroxide oxidation. The impact of temperature, initial pH and concentrations of reagents on the catalytic performance of the catalyst with the optimal composition also was studied. The characterization of fresh and spent catalysts using UV–vis DR and ESR spectroscopic techniques led to suppose that the high efficiency of Cu-ZSM-5 to redox reactions in aqueous media is provided by nanostructured square-planar copper oxide clusters localized in the zeolite channels.

Combined processes of glycerol polymerization/carbonization/activation to produce efficient adsorbents for organic contaminants

AbstractBACKGROUND: Glycerol was used to produce efficient adsorbents with a high surface area for organic contaminants by a combined process based on polymerization, carbonization and activation.RESULTS: Glycerol and sulfuric acid catalyst at concentrations of 0, 0.5, 1, 2 and 5 mol% were heated to 150 °C to form polyglycerol, which was then decomposed at 580 °C and activated with CO2 at 850 °C. The resulting activated carbons had a high specific area (1630 m2g−1) and high adsorption capacity of methylene blue used as a model organic contaminant. This process was also used to produce a special composite adsorbent based on expanded vermiculite (EV) coated with activated carbon. These composites were produced by impregnation of EV with glycerol followed by polymerization, thermal decomposition and activation with CO2 to produce up to 25 wt% carbon and a surface area of 835 m2g−1.CONCLUSIONS: The carbon layer present in the EV composite/activated carbon (GVE4CA2) produces a remarkable increase in the methylene blue adsorption capacity of the expanded vermiculite and strongly decreases undesirable water absorption. Copyright © 2012 Society of Chemical Industry

Assisted activated carbon‐microwave degradation of the sodium dodecyl benzene sulfonate by nano‐ or micro‐Fe3O4 and comparison of their catalytic activity

An improved method of the integration of activated carbon‐microwave (AC/MW) system with magnetic material such as nano‐ or micro‐Fe3O4 is proposed. A series of experiments to study the Fe3O4 assisted AC/MW (Fe3O4/AC/MW) degradation of sodium dodecyl benzene sulfonate (SDBS), as a model organic contaminant, was performed. The catalytic activities of nano‐Fe3O4/AC and micro‐Fe3O4/AC under MW irradiation were also compared. Some influence factors like added amount of Fe3O4, MW irradiation time, initial SDBS concentration, solution acidity, and Fe3O4/AC dose on the degradation were assessed in detail. The results showed that the addition of Fe3O4 significantly promoted the AC/MW degradation and nano‐Fe3O4/AC displayed higher catalytic activity under MW irradiation. By comparison, the nano‐Fe3O4/AC/MW process exhibited many advantages including high degradation rate, short irradiation time, low cost, no residual intermediates, and no secondary pollution in application. Thus, it shows to be a promising technology for the destruction of organic contaminants in wastewater treatment applications. © 2011 American Institute of Chemical Engineers Environ Prog, 32: 181‐186, 2013

Decolorization and degradation of acid dye with immobilized titania nanoparticles

This paper addresses the decolorization and degradation of acid dye by a heterogeneous photocatalytic process using immobilized nano-sized TiO 2 particles as the photocatalyst. Sackcloth fiber was used as a support to immobilize the nano-sized TiO 2 photocatalyst. The structural properties of the immobilized photocatalyst were characterized by XRD, SEM and EDX. UV–Vis absorption spectroscopy and the measurement of the chemical oxygen demand (COD) were also used for the process performance studies. The XRD results did not show significant changes in the structure of P25 as a consequence of the immobilization procedure. The formation of titania crystallites in the sackcloth fiber was confirmed by SEM/EDX. The photocatalytic activities of TiO 2-coated sackcloth fiber catalyst were evaluated using Acid Black 26 as a model organic contaminant and using UV-A radiation. Experimental results showed that after 60 min, the degradation of Acid Black 26 with the immobilized TiO 2 particles was higher than that with plain TiO 2. Based on the COD results, after 3 h, the TiO 2-coated sackcloth fiber effectively decomposed all of the organic compounds present in dye solution under the studied experimental conditions. The effects of the oxidant H 2O 2, initial dye concentration and pH on the photocatalytic degradation were also investigated. The presence of CO 3 2− as a dissolved inorganic anion had the highest inhibitory effect on the decolorization of the dye, when compared with the other anions investigated. Kinetics analysis indicates that the photocatalytic decolorization rate of Acid Black 26 can be described by a pseudo-first-order model.

Reactive nanostructured membranes for water purification.

Many current treatments for the reclamation of contaminated water sources are chemical-intensive, energy-intensive, and/or require posttreatment due to unwanted by-product formation. We demonstrate that through the integration of nanostructured materials, enzymatic catalysis, and iron-catalyzed free radical reactions within pore-functionalized synthetic membrane platforms, we are able to conduct environmentally important oxidative reactions for toxic organic degradation and detoxification from water without the addition of expensive or harmful chemicals. In contrast to conventional, passive membrane technologies, our approach utilizes two independently controlled, nanostructured membranes in a stacked configuration for the generation of the necessary oxidants. These include biocatalytic and organic/inorganic (polymer/iron) nanocomposite membranes. The bioactive (top) membrane contains an electrostatically immobilized enzyme for the catalytic production of one of the main reactants, hydrogen peroxide (H(2)O(2)), from glucose. The bottom membrane contains either immobilized iron ions or ferrihydrite/iron oxide nanoparticles for the decomposition of hydrogen peroxide to form powerful free radical oxidants. By permeating (at low pressure) a solution containing a model organic contaminant, such as trichlorophenol, with glucose in oxygen-saturated water through the membrane stack, significant contaminant degradation was realized. To illustrate the effectiveness of this membrane platform in real-world applications, membrane-immobilized ferrihydrite/iron oxide nanoparticles were reacted with hydrogen peroxide to form free radicals for the degradation of a chlorinated organic contaminant in actual groundwater. Although we establish the development of these nanostructured materials for environmental applications, the practical and methodological advances demonstrated here permit the extension of their use to applications including disinfection and/or virus inactivation.

Biosynthesis of Iron and Silver Nanoparticles at Room Temperature Using Aqueous Sorghum Bran Extracts

Iron and silver nanoparticles were synthesized using a rapid, single step, and completely green biosynthetic method employing aqueous sorghum extracts as both the reducing and capping agent. Silver ions were rapidly reduced by the aqueous sorghum bran extracts, leading to the formation of highly crystalline silver nanoparticles with an average diameter of 10 nm. The diffraction peaks were indexed to the face-centered cubic (fcc) phase of silver. The absorption spectra of colloidal silver nanoparticles showed a surface plasmon resonance (SPR) peak centered at a wavelength of 390 nm. Amorphous iron nanoparticles with an average diameter of 50 nm were formed instantaneously under ambient conditions. The reactivity of iron nanoparticles was tested by the H(2)O(2)-catalyzed degradation of bromothymol blue as a model organic contaminant.