Metal Ions In Aqueous Solution Research Articles

On-Line pH Measurement Cation Exchange Kinetics of Y3+-Exchanged Alginic Acid for Y2O3 Nanoparticles Synthesis.

A new sol-gel method that employs cation exchange from an aqueous metal ion solution with H+ ions of granulated alginic acid was developed for synthesizing high-purity Y2O3 nanoparticles. In this study, the cation exchange kinetics of H+~Y3+ in aqueous solution were analyzed using on-line pH technology and off-line inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. Pseudo 2nd-order models were utilized to evaluate the parameters of the kinetics, suggesting that the concentration of H+~Y3+ involved in the cation exchange reaction was 1:1.733. Further, a comprehensive understanding of the Y-ALG calcination process was developed using thermo-gravimetric analysis, along with results obtained from differential scanning calorimetry (TGA/DSC). A detailed analysis of the XRD Rietveld refinement plots revealed that the crystallite sizes of Y2O3 nanoparticles were about 4 nm (500 °C) and 15 nm (800 °C), respectively. Differential pulse voltammetry (DPV) was employed to investigate the electrochemical oxidation of catechol. The oxidation peak currents of catechol at Y2O3 (500 °C)/GCE and Y2O3 (800 °C)/GCE showed two stages linear function of concentration (2.0~20.0 × 10-6 mol/L, 20.0~60.0 × 10-6 mol/L). The results indicated that the detection limits were equal to 2.4 × 10-7 mol/L (Y2O3 (500 °C)/GCE) and 7.8 × 10-7 mol/L (Y2O3 (800 °C)/GCE). The study not only provided a method to synthesize metal oxide, but also proposed a promising on-line pH model to study cation exchange kinetics.

Wood dimensional stability enhancement by multivalent metal-cation-induced lignocellulosic microfibrils crosslinking

Wood is a hygroscopic material that responds to the moisture changes of the surrounding environment through swelling and shrinkage, making it dimensionally unstable. Here, we introduce a facile metal-ion-modification (MIM) approach to enhance the dimensional stability of wood. The MIM process involved swelling the wood samples with aqueous metal ion solutions and drying. The high valent metal cations, such as Fe3+, Al3+, and Zr4+, interacted with the hydrophilic groups (e.g., OH, COOH) present in the wood fibers, limiting their access to water and moisture, thereby enhancing the wood's hydrophobicity and dimensional stability. Evaluation of three wood species, southern yellow pine, poplar, and red oak, revealed water contact angles of 120–130° after MIM, indicative of enhanced surface hydrophobicity. Fe3+ treatment decreased southern yellow pine's swelling ratio from 6 % to 4 %. Fe3+-treated wood exhibited tangential anti-swelling efficiencies ranging from 39.83 % to 57.14 % and radial anti-swelling efficiencies from 34.74 % to 48.33 %, varying across wood species. The enhancement of wood dimensional stability can be attributed to the formation of irreversible coordination bonds between metal cations and lignocellulosic microfibrils in the wood cell wall. These bonds prevent the microfibrils from slipping in response to moisture absorption and desorption.

Deep eutectic solvent assisted electrodialysis towards selective resource recovery from model spent batteries effluents

Critical minerals will remain major active ingredients to most battery technologies for decades and innovation to improve extractive operations during spent battery effluent recycling must be carried out. Although the recovery of metal-ions in aqueous solutions has been demonstrated with membrane systems, including electrodialysis, process intensification to increase the diffusion rate at the membrane/liquid interface must be achieved to improve recovery efficiency. This manuscript presents a unique approach to increase the rate of recovery and transfer of Li+ and Co2+ ions from a membrane bulk phase to an organic liquid phase by applying deep eutectic solvent (DES) instead of an aqueous solution as the permeate stream during electrodialysis. The DES, based on choline chloride and urea at a molar ratio of 1:2, is considered green and relatively non-toxic valuable for solvent extraction purpose. Here, electrodialysis tests were conducted with this solvent on the permeate side using commercial Neosepta and laboratory-made cobalt selective membranes. The DES significantly affected cobalt transport across the membranes and separation factor which increased from 247 to 516, leading to cobalt recovery as high as 66 %. Furthermore, the use of DES on the permeate side greatly improved the energy efficiency increasing the amount of recovered cobalt per Joule of energy by over 20 %.

Machine Learning Assisted Metal Oxide‐Bismuth Oxy Halide Nanocomposite for Electrochemical Sensing of Heavy Metals in Aqueous Media

AbstractHeavy metal in excess quantity is one of the major inorganic pollutants in water. It causes several hazards to human life and ecosystem. It exists in traces in most of the commonly available drinking water sources from lakes, ponds, wells, etc., However, their presence in treated water is relatively significant. As the treated water is primarily used for agricultural purposes, it is necessary to monitor and measure their concentration. This requires sensing of metals in aqueous medium with good sensitivity and stability. Recently, nanosensors coupled with electrochemical transducer is preferred for analyzing heavy metal in aqueous solutions. In this work, Silver oxide‐bismuth oxy bromide coated with nafion is proposed as an electrochemical sensor for detection of heavy metal ions in aqueous solution. Cyclic voltammetry (CV) behavior of the proposed electrode is observed in different electrolytes. Further, Differential Pulse Voltammetry (DPV) study shows that current increases with trace nickel and copper metal ions of different concentration. Further, machine learning (ML) algorithms such as Naïve Bayes, ANN, SVM and decision trees are employed for nickel ions to train the cyclic voltammetry data and evaluate its performance. Naïve Bayes algorithm provides the best accuracy of 93.2% among all the models.

Thermodynamics of Metal-Acetate Interactions.

Metal ions play crucial roles in protein- and ligand-mediated interactions. They not only act as catalysts to facilitate biological processes but are also important as protein structural elements. Accurately predicting metal ion interactions in computational studies has always been a challenge, and various methods have been suggested to improve these interactions. One such method is the 12-6-4 Lennard-Jones (LJ)-type nonbonded model. Using this model, it has been possible to successfully reproduce the experimental properties of metal ions in aqueous solution. The model includes induced dipole interactions typically ignored in the standard 12-6 LJ nonbonded model. In this we expand the applicability of this model to metal ion-carboxylate interactions. Using 12-6-4 parameters that reproduce the solvation free energies of the metal ions leads to an overestimation of metal ion-acetate interactions, thus, prompting us to fine-tune the model to specifically handle the latter. We also show that the standard 12-6 LJ model significantly falls short in reproducing the experimental binding free energy between acetate and 11 metal ions (Ni(II), Mg(II), Cu(II), Zn(II), Co(II), Cu(I), Fe(II), Mn(II), Cd(II), Ca(II), and Ag(I)). In this study, we describe optimized C4 parameters for the 12-6-4 LJ nonbonded model to be used with three widely employed water models (Transferable Intermolecular Potential with 3 Points (TIP3P), Simple Point Charge Extended (SPC/E), and Optimal Point Charge (OPC) water models). These parameters can accurately match the experimental binding free energy between 11 metal ions and acetate. These parameters can be applied to the study of metalloproteins and transition metal ion channels and transporters, as acetate serves as a representative of the negatively charged amino acid side chains from aspartate and glutamate.

A bifunctional fluorescent sensor based-on Pillar[5]arene for efficient detection of MnO4− as an oxidant and Hg2+ ion in aqueous media: Bio-imaging in living cells

The high amounts of oxidants and heavy metal ions caused undesirable health results and the detection of these ions gained increasing importance with new techniques. This paper focuses on a fluorescent macrocyclic sensor based on Pillar[5]arene and Bodipy (Bodipy-P[5]arene) for the recognition of oxidants and heavy metal ions in aqueous solution. After the characterization of the macrocyclic ligand, the spectroscopic measurements were carried out by Uv–vis and fluorescence spectroscopies. The bifunctional fluorescent macrocycle, Bodipy-P[5]arene, presents to a dual effect with fluorescence quenching response for fluorometrically recognizing MnO4- and Hg2+ ions. Bodipy-P[5]arene has low detection limits of MnO4- and Hg2+ such as 1.15 × 10-7 M and 8.12 × 10-7 M, respectively. Moreover, both Bodipy-P[5]arene can respectively detect the target ions in acidic and basic mediums and the macrocyclic ligand has short response times. In addition, the Bodipy-P[5]arene probe demonstrates low cellular toxicity and exceptional membrane permeability, making it a successful tool for fluorescently imaging Hg2+ and MnO4- ions within living HeLa cells.

KMnO4-activated spinach waste biochar: An efficient adsorbent for adsorption of heavy metal ions in aqueous solution

Developing advanced carbon materials by utilizing biomass waste has attracted much attention. Herein, a novel carbon adsorbent derived from Spinach waste was prepared by using pyrolysis procedure with KMnO4 as the activator. The preparation condition such as activator mass ratio (0–0.16) and activation temperature (200–800 °C) were comprehensively studied. The optimal sample (SBC-0.08–500) showed a 7.38-fold increase in specific surface area compared to the original biochar (SBC-500). Adsorption experiments revealed that SBC-0.08–500 had efficient removal capabilities for Cd(II), Pb(II), Cu(II), and Zn(II) ions. The maximum adsorption capacities for Cd(II), Pb(II), Cu(II), and Zn(II) ions at 298.15 K were 1.66, 2.88, 1.31, and 2.09 mmol∙g−1, respectively. Thermodynamic results indicated that the adsorption of Cd(II), Pb(II), and Zn(II) ions on SBC-0.08–500 was a spontaneous endothermic process. Additionally, SBC-0.08–500 had a high affinity for Pb(II) ions, making it a potential selective adsorbent for the removal of mixed heavy metal ions. The mechanism underlying the adsorption process was attributed to multiple interactions, including ion exchange, electrostatic interaction, and surface complexation. The as-prepared biochar by oxidative activation pyrolysis effectively solves the issues of subsequent chemical recycling and secondary pollution, thus providing greater potential for large-scale use.

Selective detection of ionic liquid fluorescence probes for visual colorimetry of different metal ions

At present, the fast distinction of different metal ions in pure water media is not only a great challenge, but also drives the protection of water quality in environmental water bodies. In this paper, a novel ionic liquid fluorescent probe Glycolic Acid-L-Arginine (GA-L-Arg) was rationally created and designed through an in-depth study of ionic liquids. It is also used as an innovative multi-ion fluorescent probe for colorimetric detection and separate identification of Fe3+ and Co2+ in aqueous solutions of various metal ions. GA-L-Arg has excellent water solubility due to the strong hydrophilicity of Glycolic Acid and L-Arginine. The probe showed high sensitivity, extremely significant selectivity, and great pH stability for Fe3+ and Co2+ in pure water. The GA-L-Arg structure and the mechanism of Fe3+ and Co2+ detection were analyzed by infrared spectroscopic characterization and quantum chemical calculations. More importantly, the distinct colorimetric partitioning of Fe3+ and Co2+ was performed by the unique extraction of Fe3+ in the presence of the fluorescent probe and buffer solution.

A new eco-friendly fluorescent microcapsules for effective and selective detection of Hg(II)&Cu(II)

The detection of heavy metals in wastewater with eco-friendly materials and sustainable processes is extremely important for the environment and human health. In this study, sporopollenin, which is a natural polymer material, was derivatized with an innovative macrocyclic compound, and then fluorescence property was acquired. Bodipy and Pillar[5]arene were used to prepare fluorescent microcapsules (Sp-P[5]-bodipy) based on organic compounds. The compounds used in the preparation of fluorescent microcapsules were synthesized by conventional procedures and characterized by several methods such as NMR, FT-R, mass spectroscopy, elemental analysis, and melting point. The characterization of the obtained surfaces was carried out with TGA, FT-IR and SEM device in the determination of the surface morphology and the functional groups. The target fluorescent microcapsules, Sp-P[5]-bodipy, were performed using fluorescence spectroscopy to detect metal ions in aqueous solutions that the modified surfaces encapsulated Hg(II) and Cu(II) ions, respectively. Moreover, some parameters such as pH, temperature, response time, limit of detection (LOD) and binding stoichiometry were examined for practical chemo-sensor applications that Sp-P[5]-bodipy have a low LOD for both Cu(II) (6.16 µM) and Hg(II) ions (3.42 µM).

Construction of double cross‐linked networks in calcium alginate/poly‐N‐isopropylacrylamide hydrogel with photothermal responsiveness for absorbing heavy metal ions

AbstractHydrogels have been widely used for adsorbing heavy metal ions in wastewater treatment. Herein, a calcium alginate/poly‐N‐isopropylacrylamide double‐network hydrogel with photothermal responsiveness was prepared for adsorbing heavy metal ions in aqueous solution. The chemical composition, micromorphology, swelling properties, photothermal responsiveness, adsorption properties, and adsorption mechanism of gel microspheres were studied, respectively. The gel microspheres had an interpenetrating three‐dimensional porous structure, and their high porosity morphology and high specific surface area could ensure the adequate exposure of active adsorption sites. The gel microspheres showed excellent selective adsorption of Cr(III), and the theoretical maximum adsorption capacity of 146.16 mg/g was calculated according to the Langmuir model. Besides, its excellent photothermal conversion ability enhanced the resistance to external interference after the adsorption. Our work has provided an innovative approach to solar water purification while optimizing the preparation process.

Preparation of a Lignin-Based Adsorbent from Xylitol Residue for the Capture of Metal Ion in Aqueous Solutions

Preparation of a Lignin-Based Adsorbent from Xylitol Residue for the Capture of Metal Ion in Aqueous Solutions

Chemometric analysis of voltammetric-based sensing of heavy metal ions in aqueous solution for lead (Pb+2) ions

In the present work, we have developed a sensitive sensing scheme to identify lead ions in the aqueous water system at ultra-trace levels. The electrochemical technique, anodic stripping voltammetry (ASV) was used to quantify lead ion (Pb2+) in aqueous samples within the range of 10–50 μg/L concentration. The linear ASV signals were found in the 10–50 μg/L concentration range from the experimental work with the regression coefficient R2 0.975. Based on the standard deviation (0.14), the detection limit and quantification limit of the sensor was found to be 1.49 μg/L (S/N = 3) and 4.92 μg/L (S/N = 10), which is a smaller order of magnitude below the 10 μg/L thresholds for drinking water imposed by the World Health Organization.Further, Chemometric modelling was imposed to analyse the data appropriately. Principal component analysis (PCA) was used to conduct a quantitative study and multiple response analysis of the sensor's behaviour to lead metal ions, demonstrating the proposed sensing scheme's excellent performance and stability. The PLSR model performance measures obtained include- R-square (0.99) and RMSE (0.90), indicating the reliability and robustness of the prediction model for testing unknown samples. Therefore, the proposed sensing scheme's effectiveness can be used for various environmental applications, making it a potential platform.

Polyamidoamine Dendrimers Functionalized Water-Stable Metal-Organic Frameworks for Sensitive Fluorescent Detection of Heavy Metal Ions in Aqueous Solution.

In this work, polyamidoamine (PAMAM)-functionalized water-stable Al-based metal-organic frameworks (MIL-53(Al)-NH2) were proposed with enhanced fluorescence intensity, and used for the sensitive detection of heavy metal ions in aqueous solution. The size and morphology of MIL-53(Al)-NH2 were effectively optimized by regulating the component of the reaction solvents. PAMAM dendrimers were subsequently grafted onto the surface with glutaraldehyde as a cross-linking agent. It was found that the size and morphology of MIL-53(Al)-NH2 have great influence on their fluorescence properties, and PAMAM grafting could distinctly further improve their fluorescence intensity. With higher fluorescence intensity, the PAMAM-grafted MIL-53(Al)-NH2 showed good linearity (R2 = 0.9925-0.9990) and satisfactory sensitivity (LOD = 1.1-8.6 μmol) in heavy metal ions determination. Fluorescence enhancement and heavy metal ions detection mechanisms were discussed following the experimental results. Furthermore, analogous water-stable Materials of Institute Lavoisier (MIL) metal-organic frameworks such as MIL-53(Fe)-NH2 were also proved to have similar fluorescence enhancement performance after PAMAM modification, which demonstrates the universality of the method and the great application prospects in the design of PAMAM-functionalized high-sensitivity fluorescence sensors.

Amidoxime-functionalized aerogels for efficient adsorption of low-concentration metal pollutants

The treatment methods for heavy metal contaminants at low-concentrations (<10 mg/L) are attracting extensive attention due to the difficulty of degrading and the tendency of causing serious environmental hazards. As a result of the strong chelating ability to various metals, amidoxime structure is applicable in the adsorption of metal ions. Herein, it has been successfully synthesized amidoxime-functionalized acrylamide/amidoxime-chitosan (AM/AO-CS) aerogels as efficient adsorbents for low-concentration metal ions in aqueous solutions. The porous characteristics of chitosan carrier and abundant amidoxime groups make AM/AO-CS aerogels environmentally friendly and well-performing. AM/AO-CS aerogels displayed 97.4 % adsorption ratio of Cu2+ at a concentration of 10 mg/L. The adsorption isotherms of Cu2+ could be fit well by the Langmuir model. Moreover, AM/AO-CS aerogels adsorbed with Cu2+ were further applied as the catalysts in the hydrogenation of p-nitrophenol.

Hydrothermal synthesis and ionic sensing properties of graphitic carbon nitride quantum dots

BackgroundThis work adopts an efficient method including thermal calcination, surface hydroxylation, and hydrothermal cutting to synthesize graphitic C3N4 quantum dots (CNQDs) for detecting metal ions in aqueous solutions. MethodsFunctionalized CNQDs, ranged from 2 to 6 nm in particle size, were of layered crystalline and contain a large amount of surface functionalities such as carbonyl, carboxylic, and amino groups. The CNQD suspension exhibited a maximal fluorescence intensity at 420 nm with high quantum yield of 13.6% under UV irradiation. Significant findingsThe CNQD suspension demonstrated superior sensitivity and selectivity toward iron ions with ppm-level detection limit in liquid phase. The Stern-Volmer analysis revealed that CNQD suspension was appropriate for the detection of Fe3+ and Fe2+ ions with LOD of 6.5 and 7.7 ppm, respectively. The fluorescence quenching mechanism of CNQD suspension mainly originates from a strong interaction between the ions and the surface functionalities followed by the photoinduced energy transfer effect, thus inducing the fluorescence extraction. Moreover, the CNQD suspension also offers a satisfactory biocompatibility toward 3T3 cell after 24 h. Accordingly, the robust design of CNQDs developed in this work paves the way for developing highly sensitive and selective probes for detecting target species in aqueous solutions.

Structural analysis of lignite-derived humic acid and its microscopic interactions with heavy metal ions in aqueous solution

Understanding heavy metal environmental behavior with humic acid (HA) is critical. There is currently a lack of information on the control of its structure organization on its reactivity to metals. The difference in HA structures under non-homogeneous conditions is critical for revealing its micro-interaction with heavy metals. The heterogeneity of HA was reduced using the fractionation method in this study, the chemical properties of HA fractions were analyzed using py-GC/MS, and the structural units of HA were proposed. Pb2+ was used as a probe to investigate the difference in the adsorption capacity of HA fractions. The microscopic interaction of structures with heavy metal was investigated and validated by structural units. The results show that as molecular weight increased, the oxygen content and the number of aliphatic chains decreased, but the opposite was true for aromatic and heterocyclic rings. The adsorption capacity for Pb2+ was as follows: HA-1 > HA-2 > HA-3. According to the linear analysis of the influencing factors of maximum adsorption capacity and possibility factors, the adsorption capacity was positively correlated with the contents of acid groups, carboxyl groups, phenolic hydroxyl groups, and the number of aliphatic chains. The phenolic hydroxyl group and the aliphatic-chain structure have the greatest impact. Therefore, structural differences and the number of active sites play an important role in adsorption. The binding energy of HA structural units to Pb2+ was calculated. It was found that the chain structure is easier to bind to heavy metals than aromatic rings, and the affinity of–COOH to Pb2+ is greater than that of –OH. These findings can help improve the adsorbent design.

Natural mollusk shells as a potential dental material

The mollusk shells in Nature have evolved ingenious architectures and achieved outstanding mechanical properties, thereby demonstrating a potential for tissue engineering applications; nevertheless, despite a broad investigation of them for gaining inspiration towards guiding the design of man-made materials, the mollusk shells have rarely been explored for directly using as a biomedical material. This study offers a preliminary evaluation about the potential and feasibility of natural mollusk shells for using as dental replacements by systematically examining their aesthetic performance, machinability, as well as mechanical and biological properties. The shells exhibit a variety of colors with natural lusters, and can be processed into desired shapes and dimensions with good surface finish. The Strombus latissimus shell has a comparable flexural strength as human teeth, and exhibits higher fracture toughness than most of commercial dental materials along with stable crack extension featured by rising R-curve behavior owing to the effective toughening effects of crossed-lamellar architecture. Additionally, the shell displays a good cytocompatibility and certain antifungal function because of its hydrophilic and negative charged surfaces with low roughness. A simple and viable approach was further exploited for enhancing the hardness of natural mollusk shells by immersing them in aqueous solutions of metal ions. The unique combination of properties makes the natural mollusk shells promising for dental applications which may represent a potentially new protocol for tissue engineering.

Determination of Metals by Portable Millisecond Pulsed Solution Cathode Glow Discharge (SCGD) Atomic Emission Spectroscopy (AES)

In order to determine metal ions in aqueous solution, a millisecond pulsed solution cathode glow discharge (SCGD) source coupled with a portable spectrometer was developed for atomic emission spectrometry (AES). The millisecond pulsed SCGD source was operated with two high-voltage DC power supplies and a high-voltage pulse switch, specifically boosting the analyte excitation and sustain the glow discharge. The method to excite the plasma discharge contrasts with traditional approaches that employ a single power source. The influence of operating parameters, including pulsed voltage, pulsed frequency, duty ratio, and sample flow rate, were optimized to construct a system having good stability and improved excitation of metal ions. The optimum detection limits for Na, K, Li, Mg, Ag, Cs, Rb, and Sr were 0.17, 0.32, 0.38, 0.50, 2.65, 1.30, 0.22, and 74.02 µg·L−1, respectively. Furthermore, mountain spring, Yangtze River, and reservoir water were spiked with Ag, Rb, and Cs; the recoveries ranged from 105% to 106%, 106% to 107%, and 107% to 108% indicating that the developed system has favorable analytical performance.

Use of Chemically Modified Chitosan for the Adsorptive Removal of Toxic Metal Ions in Aqueous Solutions

Chemical modification of chitosan was successfully carried out using three derivatives namely: 3 hydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, and 3,5-di-tert-butyl-2-hydroxybenzaldehyde by a condensation reaction. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area, thermal gravimetric analysis (TGA), and X-ray diffraction (XRD) methods were performed for characterization of the chitosan-derived Schiff base adsorbent materials. The efficiency of the prepared adsorbents in removing cobalt and nickel ions from aqueous solution was explored, and experimental data were analyzed using isothermal and kinetic models. The BET surface area of chemically modified chitosan was greatly enhanced 125.83 m2 g-1 with mesoporous characteristics. The maximum uptake was recorded at pH 5-6, while the maximum removal capacity was 243.90 mg g-1 for cobalt ions whereas 166.67 mg g 1 was achieved for nickel ions. The kinetic data were better fitted using pseudo-second-order.

Removal of Pb2+ in aqueous solutions using Na-type zeolite synthesized from coal gasification slag in a fluidized bed：Hydrodynamic and adsorption

As the heavy metal ion in aqueous solutions, Pb2+ has certain harm to the surrounding natural and ecological environment. To eliminate the pollution of Pb2+ in aqueous solution, Na-type zeolite was prepared from the coal gasification slag by alkaline activation technology, and then the hydrodynamic of fluidized bed with Na-type zeolite granules was studied to achieve fluidization stability. Finally, the adsorption process and mechanism of Pb2+ in aqueous solutions using Na-type zeolite were studied by multiple adsorption experiments in stable fluidized bed. The results showed that: The removal process of Pb2+ using Na-type zeolite is a single-layer adsorption dominated by chemical adsorption, companying with surface electrostatic interaction and ionic exchange processes in the interlayer. Fluidized adsorption process of Pb2+ by Na-type zeolite is mainly affected by rising-water velocity, original concentration of Pb2+, and bed material quantity in fluidized bed. The order of influence significance was as below: Original concentration (co) > Bed material quantity (mo) > Rising-water velocity (vt). Under optimal operating conditions (co=200 mg/L, vt=0.053 m/s, mo =72 g), the fluidized bed have higher adsorption saturation (qs=0.649 g/g) than that of fixed bed (qs=0.501 g/g), promoting the adsorption process of Pb2+ by Na-type zeolite. This research provided a new method for the efficient removal of Pb2+ from aqueous solution.