Emulsion liquid membrane separation of As(III) and As(V)

Emulsion liquid membranes (ELM) consisting of L113A (surfactant), liquid paraffin (stabilizer) and kerosene (solvent), with HCl solution acting as the external phase and KOH solution acting as the internal phase, were applied to the prior separation of arsenic(III) and arsenic(V) with subsequent spectrophotometric determination by AgDDTC. The effect of various parameters on the recovery of arsenic(III) were investigated. 8 mol/L HCl was required for 95% As(III) recovery. After reduction of As(V) to As(III) with sufficient KI, total arsenic could be determined. The RSD of As(III) and As(total) were both less than 3%. The procedure was applied to aqueous samples with a recovery of 93.5%–101%. Received: 22 March 1998 / Revised: 12 September 1998 / Accepted: 17 September 1998     

The successive polarographic determination of As(III) and As(V)

A method has been described for the successive polarographic determination of As(III) and As(V)in a sulphuric acid solution.A directly recorded polarogram shows a limiting current corresponding to the As(III) concentration, another polarogram, recorded with a second sample after reduction of As(V) to As(III) by hydrazine sulphate, gives a limiting current corresponding to the concentration As(II) + As(V).     

Emulsion liquid membrane separation of As(III) and As(V)

Emulsion liquid membranes (ELM) consisting of L113A (surfactant), liquid paraffin (stabilizer) and kerosene (solvent), with HCl solution acting as the external phase and KOH solution acting as the internal phase, were applied to the prior separation of arsenic(III) and arsenic(V) with subsequent spectrophotometric determination by AgDDTC. The effect of various parameters on the recovery of arsenic(III) were investigated. 8 mol/L HCl was required for 95% As(III) recovery. After reduction of As(V) to As(III) with sufficient KI, total arsenic could be determined. The RSD of As(III) and As(total) were both less than 3%. The procedure was applied to aqueous samples with a recovery of 93.5%–101%. Received: 22 March 1998 / Revised: 12 September 1998 / Accepted: 17 September 1998     

Polarographic studies of catalytic reduction of hydrogen in Cr(VI)−As(III) and Cr(III)—As(III) systems

In the presence of traces of Cr(VI) or Cr(III) ions in ammonia or borate buffers containing the As(III) ions a catalytic hydrogen wave arises in the dc polarogram. It was established that the complex Cr(H₂AsO₃)_n^+3?n is formed in the solution, and that its reduced form adsorbed at DME is of catalytic activity. The wave can be employed for the determination of low concentrations (2×10^?8×10^?7M) of Cr(VI) and Cr(III) ions.     

Determination of As(III) and As(V) ions by chemiluminescence method

A new chemiluminescence (CL) method for the selective determination of As(III) and As(V) ions in aqueous solution has been studied using a FIA system. The method is based on the increased CL intensity with the addition of As(V) ion into a solution of lucigenin and hydrogen peroxide. The addition of As(III) ion into the solution did not change the CL intensity. Total concentration of As ions was determined after pre-oxidation of As(III) to As(V) with hydrogen peroxide in basic solution. The As(III) content was estimated by subtracting the content of As(V) ion from total As concentration. The effects of concentrations of KOH and H₂O₂, and flow rates of reagents on CL intensity have been investigated. The calibration curve for As(V) ion was linear over the range from 1.0×10^-2 to 10 μg/g, the coefficient of correlation was 0.997 and the detection limit was 5.0×10^-3 μg/g under the optimal experimental conditions.     

Preservation of As(III) and As(V) in some water samples

This paper reports on the behavior of arsenite [As(III)] and arsenate [As(V)] in some water samples at storage under several conditions (pH=2/natural pH, 4°C/20°C). The investigation was carried out using⁷³As as a radiotracer for both forms and with the aid of earlier developed simple speciation methods for differentiation between arsenite and arsenate. Although arsenate is the thermodynamically stable arsenic form, it was observed that arsenate in deionized water is completely converted to the trivalent state; this phenomenon took place in about one week. By monitoring the radioactive As(III) and As(V) over a period of one month in two natural water samples, a fresh water and a sea water sample, it could be concluded that no adsorption occurs on the surface of polyethylene containers, independent of storage conditions. During that period, storage at natural pH values results for both water samples in a gradual oxidation of As(III); the oxidation rate is higher for storage at 20°C. At pH=2 As(III) is fairly stable in fresh water at both storage temperatures. However, in sea water a fast oxidation of As(III) is observed (complete oxidation within 3 d at both temperatures). As(V) is stable at all storage conditions studied.     

Thermal decomposition of halo-bis(piperidyldithio-carbamates) of As(III), Sb(III) and Bi(III)

Simultaneous TG/DTG/DTA studies under non-isothermal conditions have been carried out in air and nitrogen on some halo-dithiocarbamates of the general formula XM[S₂CN(CH₂)₅]₂ (X=Cl, Br and I; andM=As, Sb and Bi).E* values for the 1st stage of decomposition were determined by graphical methods and the TTN temperatures were calculated from the TG profiles. A possible mechanism of the decomposition reaction is suggested, based on the thermoanalytical and pyrolysis results and the mass spectral data. The kinetic analysis data on five of the above dithiocarbamates and nine complexes of the general formula M[S₂CN<]₃ (M=As, Sb and Bi; and N<=NEt₂, N(CH₂)₅ and N(CH₂)₄O) were studied by the QIA (quasi-isothermal analysis) technique in air atmosphere. An example of kinetic parameter (k andn) estimation for the first decomposition stage is given for Bi[S₂CN(CH₂)₅]₃, with the assumption of different kinetic equations.

---

Zusammenfassung Einige helo-Dithiocarbamate der allgemeinen Formal XM[S₂CN(CH₂)₅]₂ (X=Cl, Br und I;M=As, Sb und Bi) wurden mittels simultaner TG/DTG/DTA unter nichtisothermen Bedingungen in Luft und Stickstoff untersucht. Für den ersten Zersetzungsschritt wurden E^*-Werte durch graphische Methoden bestimmt und die TIN-Temperaturen aus den TG-Profilen berechnet. Ein auf den Ergebnissen der Thermogravimetrie und Pyrolyse sowie auf massenspektroskopischen Daten beruhender möglicher Mechanismus wird vorgeschlagen. Fünf der angeführten Dithiocarbamate und 9 Komplexe der allgemeinen Formel M[S₂CN<]₃] (M=As, Sb und Bi;N=NEt₂, N(CH₂)₅ und N(CH₂)₄O) wurden mittels QIA (quasi-isotherme Analyse) in Luft untersucht. Als Beispiel ist die Bestimmung der kinetischen Parameter (k undn) für den ersten Schritt der Zersetzung von Bi[S₂CN(CH₂)₅]₃ unter Annahme verschiedener kinetischer Gleichungen angegeben.

---

, - XM[S₂CN(CH₂)₅]₂, =Cl, r l, a M=As, Sb Bi. ^a , -. - . M[S₂CN<]₃, M=As, Sb Bi, a N<=NEt₂, N(CH₂)₅ N(CH₂)₄O. k ³[S₂N(₂)₅]₃.

     

Thermal decomposition of tris(piperidyldithiocarbamates) of As(III), Sb(III) and Bi(III)

Thermal studies by TG, DTG and DTA of tris(piperidyldithiocarbamates) of arsenic(III), antimony(III) and bismuth(III) of the general type M[S₂CN(CH₂)₅]₃ (M=As, Sb and Bi) have been carried out in nitrogen and air, as well as under vacuum, to determine their modes of decomposition. The apparent activation energies were determined by graphical methods and the TIN temperatures were calculated from the TG profiles. A possible mechanism of the decomposition reaction is suggested on the basis of the results of their pyrolysis and their mass spectral data. The intermediates obtained at the end of various decomposition steps were identified via their elemental analysis, i.r. spectral data and X-ray diffraction studies. A dimeric structure of type M₂[S₂CN(CH₂)₅]₄ (M=As, Sb) is proposed.     

Photometric measurement of trace As(III) and As(V) in drinking water

A simple, fast and sensitive light-emitting diode (LED)-based photometric method for the differential determination of ppb-ppm levels of As(III) and As(V) in potable water in the presence of ppm levels of phosphate was developed. The detection chemistry is based on the well-known formation of arsenomolybdate, followed by reduction to heteropoly blue. The front-end of the measurement system is configured to selectively retain P(V) and As(V), based on the considerable difference of the pK(a) of the corresponding acids relative to As(III). Thus, it is As(III) that is injected into the medium, oxidized in-line with KBrO(3) to As(V) and forms Mo-blue that is detected by an LED-based detector. Only As(III) is measured if the sample is injected as such; if all As in the sample is prereduced to As(III) (by the addition of cysteine, in a provided in-line arrangement), the system measures As(V)+As(III). In the present form, limit of detection (LOD) (S/N=3) is less than 8 mug l(-1) As, and the linear range extends to 2.4 mg l(-1). Potential interference from dissolved silica and Fe(III) is eliminated by the addition of NaF to the sample.     

Removal of aqueous As(III) and As(V) by hydrous titanium dioxide

Dissolved arsenic in drinking water is a global concern as it causes serious health problems. The purpose of this research was to study the applicability of an industrial intermediate product, a mixture of titanium hydroxide and titanium dioxide for removing aqueous arsenic. The material is common, inexpensive, and non-toxic, making it an attractive choice for drinking water purification. The kinetics and equilibrium of removing both primary inorganic arsenic forms, As(III) and As(V), were studied by separate batch experiments. The tested material functioned well in removing both of these arsenic forms. The apparent values for Langmuir monolayer sorption capacities were 31.8 mg/g for As(III) and 33.4 mg/g for As(V) at pH 4. The studied TiO(2) performed the best in acidic conditions, but also reasonably well in other pH conditions.     

Detection of As(III) via oxidation to As(V) using platinum nanoparticle modified glassy carbon electrodes: arsenic detection without interference from copper

The electrochemical detection of As(III) was investigated on a platinum nanoparticle modified glassy carbon electrode in 1 M aqueous HClO4. Platinum nanoparticle modified glassy carbon electrodes were prepared by potential cycling in 0.1 M aqueous KCl containing 1 mM K2PtCl6. In each potential cycle, the potential was held at + 0.5 V for 0.01 s and at -0.7 V for 10 s. 25 cycles were optimally used to prepare the electrodes. The resulting electrode surfaces were characterized with AFM. The response to arsenic(III) on the modified electrode was examined using cyclic voltammetry and linear sweep voltammetry. By using the As(III) oxidation peak for the analytical determination, there is no interference from Cu(II) if present in contrast to the other metal surfaces (especially gold) typically used for the detection of arsenic; Cu(II) precludes the use of the As(0) to As(III) peak for quantitative anodic stripping voltammetry measurements due to the formation of Cu3As2 and an overlapping interference peak from the stripping of Cu(0). After optimization, a LOD of 2.1 +/- 0.05 ppb was obtained using the direct oxidation of As(III) to As(V), while the World Health Organization's guideline value of arsenic for drinking water is 10 ppb, suggesting the method may have practical utility.     

Tetrachloroaurate(III) oxidation of thallium(I)

Summary The complementary reaction between Tl^I and AuCl _inf4 ^sup– was studied in a 3.0 mol dm^–3 HCl medium. The active species of oxidant and reductant were found to be HAuCl₄ and TlCl _inf2 ^sup– , respectively. A mechanism involving these reactive species is proposed, in agreement with the results obtained.     

Surface speciation of As(III) and As(V) in relation to charge distribution

The adsorption of As(III) and As(V) on goethite has been studied as a function of pH and loading. The data can be successfully described with the charge distribution (CD) model (extended Stern layer option) using realistic species observed by EXAFS. The CD values have been derived theoretically. Therefore, the Brown bond valence approach has been applied to MO/DFT optimized geometries of a series of hydrated complexes of As(III) and As(V) with Fe(III) (hydr)oxide. The calculated ionic CD values have been corrected for the effect of dipole orientation of interfacial water, resulting in overall interfacial CD coefficients that can be used to describe the surface speciation as a function of pH and loading. For As(III), the main surface species is a bidentate complex and a minor contribution of a monodentate species is found, which is in agreement with EXAFS. The CD values have also been fitted. Such an analysis of the adsorption data resulted in the same surface species. The fitted CD values for the bidentate complex points to the presence of strong AsO bonds with the surface and a weaker AsOH bond with the free OH ligand. This agrees quantitatively with the MO/DFT optimized geometry. Interpretation of free fitted CD values for As(V) binding suggests that the main surface species is a non-protonated bidentate complex (B) with a contribution of a singly protonated surface complex (MH) at sub-neutral pH and high loading. In addition, a protonated bidentate surface complex (BH) may be present. The same species are found if the theoretical CD values are used in the data analysis. The pH dependency of surface speciation is strongly influenced by the charge attribution of adsorbed species to the electrostatic surface plane while the effect of loading is primarily controlled by the amount of charge attributed to the 1-plane, illustrating the different action of the CD value. The MO/DFT geometry optimizations furthermore suggest that for As(V) the B, MH and BH surface complexes may have very similar AsFe distances which may complicate the interpretation of EXAFS data.     

Removal of As(III) and As(V) by natural and synthetic metal oxides

The removal properties of As(III) and As(V) by the several metal oxides having different mineral type and content of metals were investigated in batch and column reactors. The used metal oxides were Fe-oxide loaded sand (ILS), Mn-oxide loaded sand (MLS), activated alumina (AA), sericite (SC) and iron sand (IS). From the pH-edge adsorption experiments with AA and ILS, maximum As(III) adsorption was observed around neutral pH while As(V) adsorption was followed an anionic-type behavior. Among five metal oxides, AA showed the greatest removal capacity for both As(III) and As(V) through adsoption process but it has little oxidation capacity for As(III). Eventhough IS had much greater content of Fe-oxides than ILS, it showed a relatively lower removal capacity for both As(III) and As(V). This result suggests that adsorption of arsenic onto metal oxides is controlled by not only the contents of Fe-oxides but also mineral type of Fe-oxides. Column tests were performed at different combinations of metal oxides in a column reactor to find the best column system, which effectively treat both As(III) and As(V) at the same time. Among several combinations, the column reactors packed with MLS-AA and MLS-ILS showed a near complete oxidation of As(III) by MLS for a long time and the greatest adsorption of total arsenic compared to the column reactor packed with MLS-IS.     

Electroreduction of As(III) in Acid Environment

The main reduction products of aqueous sulfuric acid solutions of As(III) are arsine and elemental arsenic, whose yields are substantially dependent on the electrolysis conditions. At a current density of 500 A m^–2and an As(III) concentration of 0.25 M, the following current efficiencies (CE) for arsenic and arsine are obtained: Cd, 28.2 and 26.2; Pb, 29.3 and 22.9; Ti, 16.1 and 22.4; steel 3, 21.0 and 21.0; Cu, 20.3 and 19.9; and stainless steel, 24.9 and 16.9. For arsine, CE on a Pb cathode is virtually temperature-independent at 15–80°C. Multiple use of the same Pb cathode barely impacts its activity. The arsine CE considerably increases at 2500–3000 A m^–2and reaches 75–78% and 50–53% on Pb and Cd, respectively.     

Bemerkenswerte Wirt-Gast-Beziehungen in Tetraphenylphosphoniumsalzen vontris-Trithiocarbonato-Komplexen des As(III), Bi(III) und Fe(III)

Summary The structure and packing of the lattice of the compounds (PPh₄)₃[As(CS₃)₃] (1), (PPh₄)₃[Bi(CS₃)₃] (2), and (PPh₄)₃[Fe(CS₃)₃]·H₂O (3) is discussed in terms of hostguest chemistry in a soft matrix of large cations. The hitherto unknown Fe-complex was characterized by elemental analysis and single crystal structure analysis. Here CS ₃ ^2– acts as a classical bidentate ligand, whereas in the case of the arsenic and bismut complexes, the coordination of this ligand is essentially unidentate due to an interaction with the lone pair on the central atom. For the last two complexes, there seems to be an influence of the packing on the coordination geometry.

Herrn Prof. Dr.E. Hengge zum 65. Geburtstag gewidmet     

Rhodium(III) catalyzed oxidation of cyclic ketones by alkaline hexacyanoferrate(III)

The kinetics of the oxidation of 2-methyl cyclohexanone and cycloheptanone with Fe(CN)₆ ³⁻ catalyzed by RhCl₃ in alkaline medium was investigated at four temperatures. The rate follows direct proportionality with respect to lower concentrations of hexacyanoferrate(III) ion, but tends to become zero order at higher concentrations of the oxidant, while the reaction shows first-order kinetics with respect to hydroxide ion and cyclic ketone concentrations. The rate shows a peculiar nature with respect to RhCl₃ concentrations in that it increases with increase in catalyst at low catalyst concentrations but after reaching a maximum, further increase in concentration retards the rate. An increase in the ionic strength of the medium increases the rate, while increase in the Fe(CN)₆ ⁴⁻ concentration decreases the rate.     

Effects of As(III) binding on beta-hairpin structure

While arsenic(III) compounds can exert profound toxicological and pharmacological effects, their modes of action and, in particular, the structural consequences of their binding to cysteinyl side chains in proteins, remain poorly understood. To gain an understanding of how arsenic binding influences beta-structure, pairs of cysteines were introduced into a model monomeric beta-hairpin to yield a family of peptides such that coordination occurs either across the strands or within the same strand of the beta-hairpin. Circular dichroism, NMR, UV-vis spectroscopy, and rapid-reaction studies were used to characterize the binding of monomethylarsonous acid or p-succinylamidephenyl arsenoxide (PSAO) to these peptides. Placement of cysteines at non-hydrogen bond (NHB) positions across the beta-hairpin, such that they occupy the same face of the sheet, was found to enhance the structure as assessed by CD. Cross-strand cysteine residues that project on opposite faces close to the termini of the hairpin can still bind arsenic tightly and show modestly increased beta-sheet content. NMR and modeling studies suggest that arsenic can be accommodated at this locus without disrupting the core interactions stabilizing the turn. However, As(III) binding to nonopposed cysteines, or to cysteines at HB and NHB positions along one strand of the hairpin, caused loss of structure. UV-vis titrations show that all these hairpin peptides bind PSAO stoichiometrically with K(d) values from 13 to 106 nM. Further, binding is moderately rapid, with second-order rate constants for association of 10,000-22,000 M(-1) s(-)1 irrespective of the placement of the cysteines within the hairpin and the consequent extent of structural reorganization required as a result of binding. These studies complement recent work with alpha-helices and further demonstrate that capture of a pair of thiols by As(III) may result in significant changes in local secondary structure in the protein targets of these potent bioactive agents.