Anna Gagor

Adsorption of divalent metal ions from aqueous solutions using graphene oxide

The adsorptive properties of graphene oxide (GO) towards divalent metal ions (copper, zinc, cadmium and lead) were investigated. GO prepared through the oxidation of graphite using potassium dichromate was characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FT-IR). The results of batch experiments and measurements by flame atomic absorption spectrometry (F-AAS) indicate that maximum adsorption can be achieved in broad pH ranges: 3-7 for Cu(II), 5-8 for Zn(II), 4-8 for Cd(II), 3-7 for Pb(II). The maximum adsorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) on GO at pH = 5 are 294, 345, 530, 1119 mg g(-1), respectively. The competitive adsorption experiments showed the affinity in the order of Pb(II) > Cu(II) ≫ Cd(II) > Zn(II). Adsorption isotherms and kinetic studies suggest that sorption of metal ions on GO nanosheets is monolayer coverage and adsorption is controlled by chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of GO. Chemisorption was confirmed by XPS (binding energy and shape of O1s and C1s peaks) of GO with adsorbed metal ions. The adsorption experiments show that the dispersibility of GO in water changes remarkably after complexation of metal ions. After adsorption, the tendency to agglomerate and precipitate is observed. Excellent dispersibility of GO and strong tendency of GO-Me(II) to precipitate open the path to removal of heavy metals from water solution. Potential application of GO in analytical chemistry as a solid sorbent for preconcentration of trace elements and in heavy metal ion pollution cleanup results from its maximum adsorption capacities that are much higher than those of any of the currently reported sorbents.

Suspended aminosilanized graphene oxide nanosheets for selective preconcentration of lead ions and ultrasensitive determination by electrothermal atomic absorption spectrometry.

The aminosilanized graphene oxide (GO-NH2) was prepared for selective adsorption of Pb(II) ions. Graphene oxide (GO) and GO-NH2 prepared through the amino-silanization of GO with 3-aminopropyltriethoxysilane were characterized by scanning electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The batch experiments show that GO-NH2 is characterized by high selectivity toward Pb(II) ions. Adsorption isotherms suggest that sorption of Pb(II) on GO-NH2 nanosheets is monolayer coverage, and adsorption is controlled by a chemical process involving the surface complexation of Pb(II) ions with the nitrogen-containing groups on the surface of GO-NH2. Pb(II) ions can be quantitatively adsorbed at pH 6 with maximum adsorption capacity of 96 mg g(-1). The GO-NH2 was used for selective and sensitive determination of Pb(II) ions by electrothermal atomic absorption spectrometry (ET-AAS). The preconcentration of Pb(II) ions is based on dispersive micro solid-phase extraction in which the suspended GO-NH2 is rapidly injected into analyzed water sample. Such features of GO-NH2 nanosheets as wrinkled structure, softness, flexibility, and excellent dispersibility in water allow achieving very good contact with analyzed solution, and adsorption of Pb(II) ions is very fast. The experiment shows that after separation of the solid phase, the suspension of GO-NH2 with adsorbed Pb(II) ions can be directly injected into the graphite tube and analyzed by ET-AAS. The GO-NH2 is characterized by high selectivity toward Pb(II) ions and can be successfully used for analysis of various water samples with excellent enrichment factors of 100 and detection limits of 9.4 ng L(-1).

Graphene oxide/cellulose membranes in adsorption of divalent metal ions

In this paper, graphene oxide/cellulose membranes were prepared in order to perform effective adsorption of heavy metal ions: cobalt, nickel, copper, zinc, cadmium and lead. Two types of membranes were fabricated, i.e. pressed and non-pressed membranes. The experiment showed that the pressed membranes are highly durable at different pH values, even in basic solutions, and they can be applied in separation/removal of heavy metal ions during vigorous shaking in the aqueous solution. The non-pressed membranes were proved to be less stable, however, they can be successfully applied in the filtration process at the high flow-rates. The results of the batch experiments and the measurements by the inductively coupled plasma atomic emission spectroscopy (ICP-OES) indicated that the maximum adsorption can be achieved at pH 4–8. Adsorption isotherms and kinetic studies indicated that the sorption of the metal ions on the membranes occurs in a monolayer coverage, hence it is controlled by the chemical adsorption involving the strong surface complexation of metal ions with the oxygen-containing groups on the surface of graphene oxide. The maximum adsorption capacity values of Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) on the graphene oxide/cellulose membranes at the pH of 4.5 are 15.5, 14.3, 26.6, 16.7, 26.8, 107.9 mg g−1, respectively. The competitive adsorption experiments showed the affinities of prepared membranes for the metal ions in the order of Pb > Cu > Cd > Zn ≥ Ni ≥ Co. The affinity order agrees with the first stability constant of the associated metal hydroxide and acetate. The adsorption properties of the graphene oxide/cellulose membranes, their reusability (more than 10 cycles) and durability in the aqueous solutions open the path to removal of heavy metals from water solution. The membranes can be also used in the field of analytical chemistry for the preconcentration and/or separation of trace and ultratrace metal ions.

Structural phase transitions in tetra(isopropylammonium) decachlorotricadmate(II), [(CH3)2CHNH3]4Cd3Cl10, crystal with a two‐dimensional cadmium(II) halide network

Single crystals of tetra(isopropylammonium) decachlorotricadmate(II) as a rare example of a two-dimensional cadmium(II) halide network of [Cd(3)Cl(10)](n)(4-) have been synthesized and characterized by means of calorimetry and X-ray diffraction. The crystals exhibit polymorphism in a relatively narrow temperature range (three phase transitions at 353, 294 and 259 K). Our main focus was to establish the mechanism of these successive transformations. The crystal structure was solved and refined in the space group Cmce at 375 K (Phase I), Pbca at 320 K (Phase II) and P2(1)2(1)2(1) (Phase III) at 275 K in the same unit-cell metric. The structure is composed of face-sharing polyanionic [Cd(3)Cl(10)](4-) units which are interconnected at the bridging Cl atom into four-membered rings forming a unique two-dimensional network of [Cd(3)Cl(10)](n)(4-). The interstitial voids within the network are large enough to accommodate isopropylammonium cations and permit thermally activated rotations. While in Phase I isopropylammonium tetrahedra rotate almost freely about the C-N bond, the low-temperature phases are the playground of competition between the thermally activated disorder of isopropylammonium cations and stabilizing N-H···Cl hydrogen-bond interactions. The transition from Phase I to II is dominated by a displacive mechanism that leads to significant rearrangement of the polyanionic units. Cation order-disorder phenomena become prominent at lower temperatures.

Graphene Oxide Decorated with Cerium(IV) Oxide in Determination of Ultratrace Metal Ions and Speciation of Selenium

Graphene oxide decorated with cerium(IV) oxide (GO/CeO2) was synthesized and applied in adsorption of several metal ions such as As(III), As(V), Se(IV), Cu(II), and Pb(II) from aqueous samples. The important feature of GO/CeO2 nanocomposite is also its selectivity toward selenite in the presence of selenate. The structure of GO/CeO2 has been proven by microscopic and spectroscopic techniques. The maximum adsorption capacities of GO/CeO2 calculated by Langmuir model toward arsenic, selenium, copper, and lead ions are between 6 and 30 mg g-1. An interesting feature of this adsorbent is its excellent dispersibility in water. Thus, GO/CeO2 nanocomposite is ideal for fast and simple determination of heavy metal ions using dispersive microsolid phase extraction (DMSPE). Moreover, coupling DMSPE with energy-dispersive X-ray fluorescence spectrometry (EDXRF) is extremely beneficial because it allows direct analysis of adsorbent. Thus, the analyte elution step, as needed in many analytical techniques, was obviated. The influence of sample volume and the sorption time as well as the influence of foreign ions and humic acid on the recovery of determined elements are discussed in the paper. The results showed that developed methodology provided low limits of detection (0.07-0.17 μg/L) and good precision (RSD < 4%). The GO/CeO2 nanocomposite was applied to analysis of real water samples and certified reference materials (CRM) groundwater (BCR-610) and pig kidney (ERM-BB186).

Highly selective determination of ultratrace inorganic arsenic species using novel functionalized miniaturized membranes

A simple method for highly selective determination of trace and ultratrace arsenic ions, i.e. arsenite and arsenate, was developed. The method is based on new miniaturized membranes, 5 mm diameter and 4.4 mg weight, which are prepared by synthesis of amorphous silica coating on cellulose fibers followed by the modification with (3-mercaptopropyl)-trimethoxysilane. The batch adsorption experiments show that membranes have high selectivity toward arsenite in the presence of heavy metals and anions that usually exist in natural water. Arsenite can be quantitatively adsorbed at pH 1 from 50 mL sample within 60 min using the miniaturized membrane with maximum adsorption capacity of 60 mg g-1. The excellent adsorptive properties of membranes open the path to simple and selective determination of trace and ultratrace arsenite in water. Moreover, the membranes can be applied in the arsenic speciation due to their selectivity toward arsenite in the presence of arsenate. After adsorption, the arsenite retained onto the membrane is directly measured by energy-dispersive X-ray fluorescence spectrometry, avoiding elution step usually required in other spectroscopy techniques. The method is characterized by excellent enrichment factor of 972, detection limit of 0.045 ng mL-1 and can be successfully applied in analysis of high salinity water, which is difficult to analyze by other spectroscopy techniques. The proposed method is a solvent-free approach based on the use of miniaturized membranes as sorbent followed by the direct measurement using a low-power X-ray spectrometer without either elution step or gas consumption during measurement. It can be considered as environmentally friendly and meets the standards of green analytical chemistry principles.

Penta­potassium europium(III) dilithium deca­fluoride, K5EuLi2F10

The title compound, K5EuLi2F10, belongs to so-called self-activated materials containing lanthanoid ions within the matrix. A common feature of these systems is a large separation between the closest lanthanoid ions, which is one of the crucial factors governing the self-quenching of luminescence. The crystal structure of K5EuLi2F10 is isotypic with other K5 RELi2F10 compounds (RE = Nd, Pr). As expected from the lanthanoid contraction, the unit-cell volume for crystal with Eu3+ ions is the smallest of the three structures. Accordingly, the corresponding interatomic RE—RE distances are shorter. In the structure, distorted EuF8 dodecahedra and two different LiF4 tetrahedra, all with m symmetry, are present, forming sheets parallel to (100). The isolated EuF8 dodecahedra exhibit a mean Eu—F distance of 2.356 Å. The K+ cations are located within and between the sheets, leading to highly irregular KFx polyhedra (x = 8–9) around the alkali metal cations.

Penta­potassium praseodymium(III) dilithium deca­fluoride, K5PrLi2F10

The crystal structure of K5PrLi2F10 is isotypic with those of other K5 RELi2F10 compounds (RE = Eu, Nd). The lanthanoid ions are isolated in K5PrLi2F10, with a mean separation between the Pr ions of 7.356 Å. It classifies this crystal as a so-called self-activated material containing lanthanoid ions within the matrix. Except for two K+ and two F− ions, all atoms are located on sites with m symmetry. In the structure, distorted PrF8 dodecahedra and two different LiF4 tetrahedra share F atoms, forming sheets parallel to (100). The isolated PrF8 dodecahedra exhibit a mean Pr—F distance of 2.406 Å. The K+ cations are located within and between these sheets, leading to highly irregular KFx polyhedra with coordination numbers of eight and nine for the alkali metal cations.