As-O Distances Research Articles

XANES/EXAFS and quantum chemical study of the speciation of arsenic in the condensate formed in landfill gas processing: Evidence of the dominance of As-S species

The XANES/EXAFS data and quantum chemical simulations presented in this study demonstrate several features of the chemistry of arsenic compounds found in the condensates and solids generated in landfill gas (LFG) processing carried out for renewable natural gas (RNG) production. The XANES data show the decrease in the position of the absorption edge of As atoms, similar to that characteristic for sulfur-containing As solutes and solids. The EXAFS data show that the As-O and As-S distances in these matrixes are similar to those in thioarsenates. Quantum-chemical calculations demonstrated the close agreement between the experimental and modeled As-S and As-O distances determined for a range of methylated and thiolated arsenic solutes. These calculations also showed that the increase of the number of the As-S bonds in the coordination shell of arsenic is accompanied by a consistent decrease of the charges of As atoms. This decrease is correlated with the number of the As-S bonds, in agreement with the trend observed in the XANES data. These results provide insight into the intrinsic chemistry and reactivity of As species present in LFG matrixes; they may be helpful for the development of treatment methods to control arsenic in these systems.

Arsenic immobilization from aqueous solution by the precipitation of the pseudo-octahedral arsenate-substituted natroalunite solid solutions

A series of the arsenate-substituted natroalunite solid solutions were synthesized by hydrothermal precipitation at 200 °C and pH of 4 and characterized to investigate the AsO4 substitution for SO4 in natroalunite as the base of a possible immobilization method for arsenic. The AsO4 substitution in natroalunite increased when the [AsO4/(AsO4 + SO4)]aq molar ratios of the initial aqueous solutions increased and the maximum substitution reached ~67% molar for the [AsO4/(AsO4 + SO4)]aq = 0.26. The XRD analysis confirmed that all hydrothermal synthetical solids for the [AsO4/(AsO4 + SO4)]aq ≤ 0.26 were characteristic of natroalunite-type phases. The AsO4 substitution in the natroalunites increased the c lattice parameters, owing to the difference between the SO distance and the AsO distance in the crystal structures. For the [AsO4/(AsO4 + SO4)]aq = 0.28 at 200 °C and pH of 4, a mixture of natroalunite, amorphous arsenate phase and Na2SO4 was formed. The crystals of the arsenate-substituted natroalunites were regular ditrigonal scalenohedron (pseudo-octahedron). The Raman spectra were characterized by two bands centered upon 899–917 cm−1 and 981–997 cm−1, which represented the symmetric stretching vibration v1(AsO4) and the antisymmetric stretching vibration v3(AsO4), respectively. The infrared bands around 868–897 cm−1 were assigned to the symmetric stretching vibration v1(AsO4). The thermal decomposition of the arsenate-substituted natroalunites showed three separated endothermic steps, namely the loss of H3O+, the loss of OH− and the loss of SO3 + (As2O3 + O2). The solubility products [Ksp] and the Gibbs free energies of formation [ΔGfo] for the arsenate-substituted natroalunites decreased from 10−81.21 to 10−109.16 and from −4714.49 kJ/mol to −5352.95 kJ/mol with the increase of the AsO4 substitution from 0% to 67%, respectively.

X-ray Single-Crystal Analysis of Na2H[(P0.52As0.48)O4] 7H2O

The crystal structure of Na2H[(P0.52As0.48)O4]·7H2O at room temperature belongs to the P21/n space group and is isomorphous to Na2HAsO4·7H2O and Na2HPO4·7H2O. The structure was solved and refined with 6310 independent reflections with R1 = 0.0518 and Rw2 = 0.0811 by means of Mo Kα radiation. The lattice parameters are: a = 9.3250(7), b = 11.0063(8), c = 10.5602(7)A, β = 95.530(2)° and Z = 4. This crystal structure is built up from discrete (P,As)O43- tetrahedra connected by O-H…O hydrogen bonds and through the electrostatic actions of the Na+ cations. The mean (P,As)-O distance is intermediate between those of P-O, and the As-O distance is not consistent with those of Na2HAsO4·7H2O and Na2HPO4·7H2O.

Structural and Dynamic Aspects of Hydration of HAsO4−2: An ab initio QMCF MD Simulation

An ab initio quantum mechanical charge field simulation has been carried out in order to obtain molecular level insight into the hydration behavior of HAsO4(-2), one of the major biologically active components of As(V) oxoanion in neutral to slightly alkaline aqueous medium. Moreover, a geometrical definition of hydrogen bonding has been used to probe and characterize both solute-solvent and solvent-solvent hydrogen bonding present in the system. The asymmetry of the anion induced by the protonation of one of the oxygens of the arsenate anion causes rather irregular hydration structure. The nonprotonated oxygen atoms preferably form relatively stable hydrogen bonds with two to three water molecules in their vicinity, while the protonated oxygen forms one or two hydrogen bonds, weaker than water-water hydrogen bonds. The two types of As-O distances obtained from the simulation (1.68 and 1.78 A for the protonated and nonprotonated oxygens, respectively) are in good agreement with the experimental data. The two types of As-O stretching frequencies obtained from the simulation (855 and 660 cm(-1) reproduce well the experimental ATR-FTIR results (859 and 680-700 cm(-1)).

Ag13I4(AsO4)3: Synthesis, Crystal Structure and Ionic Conductivity

Ruby-red single crystals of Ag13I4(AsO4)3 were prepared for the first time by reacting a stoichiometric mixture of Ag2O, AgI and As2O3 at elevated oxygen pressure of 120 MPa and at a temperature of 350 ◦C. The polyhedral crystals belong to the monoclinic space group P21/m with a = 9.247(1), b = 7.152(1), c = 17.674(2) Å , β = 91.492(2)◦, and Z = 2. The structure is fully ordered and was solved by Direct Methods, and refined on single crystal diffraction data (10376 observed reflections, R1 = 3.28 %). The crystal structure is built up of two two-dimensional interlocked partial structures, one consisting of silver and iodine ions, the other of silver and arsenate(V) ions. The slabs extend in the ac plane, and the Ag-I framework is linked to the AsO4 3− units by Ag-O contacts. The silver-oxygen interactions perturb the tetrahedron of the arsenate group, resulting in As-O distances ranging from 1.670 to 1.697 Å, and angles varying from 107.3 to 112.1◦. Pure Ag13I4(AsO4)3 is a solid ion conductor with a r. t. conductivity of 6.4×10−6 Ω−1 cm−1 at 30 ◦C. The activation energy for silver ion conduction is 0.41 eV in the temperature range from 30 to 130 ◦C

Mechanism of Arsenate (V) Adsorption on TiO<sub>2</sub> Surfaces

Adsorption of arsenate on TiO, surfaces was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DIFT) calculation. The adsorption process was independent of ion strength, indicating the formation of inner-sphere complexes of arsenate on TiO, surfaces. EXAFS analysis showed that the -AsO4 tetrahedral geometry remained relatively rigid before and after adsorption with an As-O distance (R) of (0.1.69+/-0.001) nm. When -AsO4 approached TiO2 surfaces, the As-O-Ti bond formed through the O atoms in -AsO4 clusters. Furthermore, based on the combined results from EXAFS and DFT, As(V) in the 1, adsorption samples was in at least two metastable equilibrium adsorption (MEA) states: double comer (DQ linkage mode (R-1=(0.321+/-0.002) nm, stronger adsorption) and single comer (SC) linkage mode (R-2=(0.360+/-0.002) nm, weak adsorption). The proportion of coordination number between DC and SC MEA states (CNI/CN,) decreased from 3.3 to 1.6 as the surface coverage increased from 9.79 to 28.0 mg . g(-1). This result indicated that at low Surface loading arsenate was adsorbed in more stable DC MEA state, but As(V) tended to be adsorbed in SC MEA state as surface coverage increased.

Crystal structure and chemistry of conichalcite, CaCu(AsO4)(OH)

The crystal structures of conichalcite [Ca(Cu,Mg)(AsO4)(OH)] samples obtained from Gozaisho mine, Fukushima, Japan, and Higgins mine, Arizona, USA, were refined by single-crystal X-ray diffraction (XRD) using an imaging plate detector. The results revealed that conichalcite is orthorhombic and belongs to the P212121 space group. The positions of the hydrogen atom and the donor and accepter atoms in the structure were determined by difference Fourier and bond-valence sum methods. The crystal comprised three highly distorted coordination polyhedra: an AsO4 tetrahedron, a CuO4(OH)2 octahedron, and a CaO7(OH) square antiprism. The CuO4(OH)2 octahedron was distorted by the Jahn-Teller effect. The observed As-O distances were shorter than those predicted by ionic bonding. The electronic structure of the atoms affected the distortion of the polyhedra that did not have a symmetry center. The CuO4(OH)2 octahedron shared its edges to form linear chains, which were further linked by the vertices of the AsO4 tetrahedron and the CaO7(OH) square antiprism to form a three-dimensional network. The arrangement of these linear CuO4(OH)2 chains was very similar to that of CuO6 chains in CuGeO3, a spin-Peierls material. Comparisons with several isomorphous minerals revealed that the Jahn-Teller distortion effect caused by Cu atoms differed significantly between the minerals in the Ca series and those in the Pb series. Because of hydrogen bonding, the Cu-O(5) and Cu-O(5)* distances tended to become shorter than the other Cu-O distances in all isomorphous minerals.

Structural and electronic evolution of the As(OH)3 molecule in high temperature aqueous solutions: An x-ray absorption investigation

The geometrical and electronic structure of the arsenious acid molecule As(OH)(3) in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P = 250 and 600 bars, T <or= 500 degrees C) using a new optical cell. The analysis of the XAS spectra is consistent with (1) a constant As-O distance, (2) an opening of the O-As-O angles within the C(3V) pyramidal structure in the range 30-200 degrees C. This structural evolution comes along with a small decrease of the partial charges of the atoms in the As(OH)(3) molecule. The explanation invoked for both structural and electronic modifications observed is the weakening of the interactions, through hydrogen bonds, between the As(OH)(3) complex and water molecules. This is a fingerprint of the similar weakening of hydrogen bonding interactions in the solvent itself.

The catena-arsenite chain anion, [AsO2](n)(n-): (H3NCH2CH2NH3)0.5[AsO2] and NaAsO2 (revisited).

The title compounds contain the catena-arsenite [AsO(2)](n)(n-) unit, in which the As(III) atom is pyramidally coordinated to one terminal and two bridging O atoms, resulting in an infinite anionic chain. Ethylenediammonium catena-arsenite, (C(2)H(10)N(2))(0.5)[AsO(2)], is the first example of this anion in the company of an organic cation. The ethylenediammonium species interact with the [AsO(2)](-) chains by way of N-H.O hydrogen bonds. The structure of sodium catena-arsenite, Na[AsO(2)] [Menary (1958). Acta Cryst. 11, 742-743], has been redetermined to yield more reliable geometrical parameters. The As-O distances are normal and the Na(+) cation is seven-coordinate [Na-O = 2.285 (4)-3.063 (4) A] in a distorted capped trigonal prismatic geometry.

Synthesis and Characterization of the Manganese Pyroarsenate Mn2As2O7 · 2H2O

Abstract The manganese pyroarsenate hydrate, Mn2As2O7 · 2 H2O, has been obtained as a single phase product using hydrothermal methods and the structure has been determined by single crystal X-ray diffraction. The title compound crystallizes in the monoclinic space group P21/n with a = 6.6576(4), b = 14.555(1), c = 7.8147(5) Å, β = 94.935(1)°, V = 754.46(8) Å3 and Z = 4. The manganese ions are each coordinated to five oxygen atoms and a water molecule in a distorted octahedral arrangement. Edge-sharing MnO6 octahedra form chains which are connected to a three-dimensional framework by As2O7 4- ions. The pyroarsenate anion, which attains a nearly eclipsed conformation, has a mean As-O distance for the terminal As-O bonds of 1.669(2) Å, while for the bridging oxygen atom a mean value of 1.757(2) Å is observed. Magnetic susceptibility measurements indicate the presence of high-spin Mn2+ ions. Thermogravimetric as well as IR and Raman spectroscopic studies of Mn2As2O7 · 2 H2O are presented.

Description and crystal structure of cabalzarite Ca(Mg,Al,Fe)2(AsO4)2(H2O,OH)2, a new mineral of the tsumcorite group

Cabalzarite, M1 Ca M2 (Mg,Al,Fe 3+ )2( X AsO4)2(H2O,OH)2, is a new mineral of the tsumcorite group occurring in altered Mn ore at the abandoned Falotta mine (Swiss Alps). Together with other arsenates, cabalzarite documents the mobility of As during the retrograde stage of the Tertiary Alpine metamorphism under lowest to sub-greenschist facies conditions. Cabalzarite crystals vary in morphology from hatchet-like to fibrous and tabular. The color is light-brownish to salmon pink or orange brown, and the average refractive index is around 1.7. Cabalzarite is chemically inhomogeneous, with the main variations occurring on the octahedral M2 site occupied by Mg, Al, Fe 3+ , and Mn 3+ . Al and Mg are the dominant M2 cations, with Al/(Al + Mg) ratios varying between 0.31 and 0.59 (92 analyses). Cabalzarite is the first member of the tsumcorite group with octahedral Mg and Al as major constituents. Single-crystal X-ray structure refinements were performed on two different crystals. Cabalzarite is monoclinic, space group C2/m, Z = 2. The cell parameters for the type crystal CABA11.5, of composition (Ca1.00Sr0.02)(Al0.80Mg0.77Fe0.23Mn0.03)Σ1.83(AsO4)2(H2O1.26OH0.74)2, are a = 8.925(2) A, b = 6.143(1) A, c = 7.352(1) A, β = 115.25(3)˚, ρcalc = 3.73 g/cm 3 . The structure of cabalzarite is isomorphic with that of tsumcorite. The site M1 (Ca) is eightfold-coordinated (6 + 2) with average M1-O = 2.549 A; M2 is octahedral with an average M2-O = 2.010 A; and the average As-O distance of the arsenate group is 1.689 A. Charge balance for the simultaneous occupation of M2 by two- and three-valent cations is achieved by H2O as well as OH contributing to the M2 coordination.

ChemInform Abstract: CRYSTAL STRUCTURES OF RAC‐ AND MESO‐O‐PHENYLENEBIS(IODOMETHYLARSINE), THE LATTER AS ITS LATTICE COMPLEX WITH THE METHIODIDE OF 1,3‐DIMETHYL‐1,3‐DIHYDRO‐2,1,3‐BENZOXADIARSOLE

The crystal structures of the title compounds, o-C6H4(AsMeI)2 (1), and o- C6H4(AsMeI)2- [o-C6H4(AsMe)(As+Me2)O] I- (2), have been determined at 295 K by single-crystal X-ray diffraction and refined by least squares to residuals of 0.037 and 0.067 for 2095 and 2913 reflections respectively. Crystals of (1) are triclinic, Pī, a 9.865(5), b 9.837(7), c 7.765(4) Ǻ, α 98.89(5), β 96.71(4), γ 60.72(4)°, Z 2. is 2.585 Ǻ and 1.97 Ǻ. The angles about the arsenic differ only trivially, the mean being 98.4°. Crystals of (2) are monoclinic, P21/n, a 15.315(4), b 21.511(8), c 7.952(2) Ǻ, β 98.19(2)°, Z 4. In the cation As-O distances are unequal [1.75(1) (quaternary As), 1.86(2) Ǻ]; As-O-As is very small being 115.8(8)°. Charge-transfer interactions between iodine species are present in both derivatives; in (2), there is an interaction between the tertiary arsenics of the meso molecule and the iodide anion, As...I being 3.307(3), 3.551(3) Ǻ, with the geometry of the arsenics approaching that of a tetrahedral disposition. Within the cation, the geometry about the quaternary arsenic is typical of arsenic(v), the bond lengths being shorter than those about the ternary arsenic.

Crystal structures of rac- and meso-o-Phenylenebis(iodomethylarsine), the latter as its lattice complex with the methiodide of 1,3-Dimethyl-1,3-dihydro-2,1,3-benzoxadiarsole

The crystal structures of the title compounds, o-C6H4(AsMeI)2 (1), and o- C6H4(AsMeI)2- [o-C6H4(AsMe)(As+Me2)O] I- (2), have been determined at 295 K by single-crystal X-ray diffraction and refined by least squares to residuals of 0.037 and 0.067 for 2095 and 2913 reflections respectively. Crystals of (1) are triclinic, Pī, a 9.865(5), b 9.837(7), c 7.765(4) Ǻ, α 98.89(5), β 96.71(4), γ 60.72(4)�, Z 2. &lt;As- I&gt; is 2.585 Ǻ and &lt;As-C&gt; 1.97 Ǻ. The angles about the arsenic differ only trivially, the mean being 98.4�. Crystals of (2) are monoclinic, P21/n, a 15.315(4), b 21.511(8), c 7.952(2) Ǻ, β 98.19(2)�, Z 4. In the cation As-O distances are unequal [1.75(1) (quaternary As), 1.86(2) Ǻ]; As-O-As is very small being 115.8(8)�. Charge-transfer interactions between iodine species are present in both derivatives; in (2), there is an interaction between the tertiary arsenics of the meso molecule and the iodide anion, As...I being 3.307(3), 3.551(3) Ǻ, with the geometry of the arsenics approaching that of a tetrahedral disposition. Within the cation, the geometry about the quaternary arsenic is typical of arsenic(v), the bond lengths being shorter than those about the ternary arsenic.

The crystal structure of the complex of triphenylarsine oxide with tetrachlorocatechol; an adduct stabilized by chelating hydrogen bonds

The 1/1 triphenylarsine oxide adduct of tetrachlorocatechol crystallizes in the orthorhombic space group Pbca with a=15.07±0.02, b=18.82±0.03, and c=16.53±0.02Å. An observed density of 1.58g/cm 3 and calculated density of 1.62g/cm 3 indicated the presence of 8 molecules per unit cell. 1219 reflections above background were collected with Cu-Kα radiation using a manual diffractometer of Weissenberg geometry. The structure was solved by Patterson and Fourier techniques and refined by least squares methods (As, O, Cl anisotropic; C, H isotropic) to a conventional R of 0.070 and a weighted R of 0.041. The structure consists of triphenylarsine oxide and tetrachlorocatechol moieties hydrogen bonded together to give discrete monomeric adducts. The arsine oxide oxygen (O As) is hydrogen bonded symmetrically to the two OH groups of the catechol with an O-O As-O angle of 65.4(4)° and O As-O distances averaging 2.616(11)Å. The dimensions of the triphenylarsine oxide and tetrachlorocatechol are equivalent to those observed in other similar compounds except that the configuration of the phenyl rings with respect to the As-O bond is quite distorted. This distortion results from the alternate packing of approximately parallel phenyl and catechol rings along the a axis.