Sayandev Chatterjee

Semi-Infinite Linear Diffusion Spectroelectrochemistry on an Aqueous Micro-Drop

We report a technique for conducting semi-infinite diffusion spectroelectrochemistry on an aqueous micro-drop as an easy and economic way of investigating spectroelectrochemical behavior of redox active compounds and correlating spectroscopic properties with thermodynamic potentials on a small scale. The chemical systems used to demonstrate the aqueous micro-drop technique were an absorbance based ionic probe [Fe(CN)(6)](3-/4-) and an emission based ionic probe [Re(dmpe)(3)](2+/+). These chemical systems in a micro-drop were evaluated using cyclic voltammetry and UV-visible absorbance and luminescence spectroscopies.

Luminescence-Based Spectroelectrochemical Sensor for [Tc(dmpe)3]2+/+ (dmpe = 1,2-bis(dimethylphosphino)ethane) within a Charge-Selective Polymer Film

A spectroelectrochemical sensor consisting of an indium tin oxide (ITO) optically transparent electrode (OTE) coated with a thin film of partially sulfonated polystyrene-blockpoly(ethylene-ran-butylene)-block-polystyrene (SSEBS) was developed for [Tc(dmpe)(3)](+) (dmpe = 1,2-bis(dimethylphosphino)ethane). [Tc(dmpe)(3)](+) was preconcentrated by ion-exchange into the SSEBS film after a 20 min exposure to aqueous [Tc(dmpe)(3)](+) solution, resulting in a 14-fold increase in cathodic peak current compared to a bare OTE. Colorless [Tc(dmpe)(3)](+) was reversibly oxidized to colored [Tc(dmpe)(3)](2+) by cyclic voltammetry. Detection of [Tc(dmpe)(3)](2+) was accomplished through emission spectroscopy by electrochemically oxidizing the complex from nonemissive [Tc(dmpe)(3)](+) to emissive [Tc(dmpe)(3)](2+). The working principle of the sensor consisted of electrochemically cycling between nonemissive [Tc(dmpe)(3)](+) and emissive [Tc(dmpe)(3)](2+) and monitoring the modulated emission (λ(exc) = 532 nm; λ(em) = 660 nm). The sensor gave a linear response over the concentration range of 0.16-340.0 μM of [Tc(dmpe)(3)](2+/+) in aqueous phase with a detection limit of 24 nM.

Electrochemistry and Spectroelectrochemistry of europium(III) chloride in 3LiCl-2KCl from 643 to 1123 K.

The electrochemical and spectroelectrochemical behavior of europium(III) chloride in a molten salt eutectic, 3LiCl-2KCl, over a temperature range of 643-1123 K using differential pulse voltammetry, cyclic voltammetry, potential step chronoabsorptometry, and thin-layer spectroelectrochemistry is reported. The electrochemical reaction was determined to be the one-electron reduction of Eu(3+) to Eu(2+) at all temperatures. The redox potential of Eu(3+/2+) shifts to more positive potentials, and the diffusion coefficient for Eu(3+) increases as temperature increases. The results for the number of electrons transferred, redox potential, and diffusion coefficient are in good agreement between the electrochemical and spectroelectrochemical techniques. This research extends our ability to develop a spectroelectrochemical sensor for lanthanides and actinides into molten salt media.

Interaction of SbCl52- and Thioether Groups at the Open Coordination Sites of Platinum(II) Diimine Complexes

In the solid-state, the approximately square planar cation in orange crystals of [Pt(NO(2)phen)(ttcn)](PF(6))(2) (NO(2)phen = 5-nitro-1,10-phenanthroline; ttcn = 1,4,7-trithiacyclononane) has a short apical Pt...S(ttcn) distance (2.9415(15) A). In acetonitrile solution, the electronic spectrum shows a long-wavelength absorption band (412 nm; 2200 M(-1) cm(-1)), consistent with the notion that the axial Pt...S(ttcn) interactions stabilize states having metal-to-ligand charge-transfer (MLCT) character. Reaction with the hexachloroantimonate(V) salt of tris(4-bromophenyl)aminium (TBPA(+)) results in complex redox chemistry, involving the platinum complex, SbCl(5)(2-) and TBPA(+). In the case of Pt(bpy)(ttcn)(2+), orange-yellow crystals of [Pt(bpy)(ttcn)](2)(Sb(4)Cl(16)) were isolated from the reaction, whereas the reaction with Pt(NO(2)phen)(ttcn)(2+) consistently yielded red crystals of [Pt(NO(2)phen)(ttcn)](SbCl(5)) x 2 CH(3)CN. In the latter case, the geometry of the cation, including the apical Pt...S(ttcn) distance (2.9390(12) A), is very similar to that of the PF(6)(-) salt. However, the basal plane of each square pyramidal SbCl(5)(2-) opposes the nearly parallel coordination plane of an adjacent Pt(NO(2)phen)(ttcn)(2+) complex, resulting in an unusually short intermolecular Pt...Sb distance of 3.4259(3) A. The longest wavelength maximum in the diffuse reflectance spectrum and the solid-state emission maximum are shifted by approximately 1200 cm(-1) and approximately 700 cm(-1), respectively, to the red of those of the PF(6)(-) salt, consistent with perturbation of the complexs electronic structure because of the Pt...Sb interaction.

Electronic and Molecular Structures of trans-Dioxotechnetium(V) Polypyridyl Complexes in the Solid State

The structures of novel Tc(V) complexes trans-[TcO(2)(py)(4)]Cl·2H(2)O (1a), trans-[TcO(2)(pic)(4)]Cl·2H(2)O (2a), and trans-[TcO(2)(pic)(4)]BPh(4) (2b) were determined by X-ray crystallography, and their spectroscopic characteristics were investigated by emission spectroscopy and atomic scale calculations. The cations adopt a tetragonally distorted octahedral geometry, with a trans orientation of the apical oxo groups. trans-[TcO(2)(pic)(4)]BPh(4) has an inversion center located on technetium; however, for trans-[TcO(2)(py)(4)]Cl·2H(2)O and trans-[TcO(2)(pic)(4)]Cl·2H(2)O, a strong H bond formed by only one of the oxo substituents introduces an asymmetry in the structure, resulting in inequivalent trans Tc-N and Tc═O distances. Upon 415 nm excitation at room temperature, the complexes exhibited broad, structureless luminescences with emission maxima at approximately 710 nm (1a) and 750 nm (2a, 2b). Like the Re(V) analogs, the Tc(V) complexes luminesce from a (3)E(g) excited state. Upon cooling the samples from 278 to 8 K, distinct vibronic features appear in the spectra of the complexes along with increases in emission intensities. The low temperature emission spectra display the characteristic progressions of the symmetric O═Tc═O and the Tc-L stretching modes. Lowest-energy, triplet excited-state distortions calculated using a time-dependent theoretical approach are in good agreement with the experimental spectra. The discovery of luminescence from the trans-dioxotechnetium(V) complexes provides the first opportunity to directly compare fundamental luminescence properties of second- and third-row d(2) metal-oxo congeners.

X-ray and synchrotron diffraction studies of 2-(pyridin-2-yl)-1,10-phenanthroline in the role of ligand for two copper polymorphs or hydrogen bonded with 2,2,6,6-tetramethyl-4-oxopiperidinium hexafluorophosphate.

Different extended packing motifs of dichlorido[2-(pyridin-2-yl)-1,10-phenanthroline]copper(II), [CuCl2(C17H11N3)], are obtained, depending on the crystallization conditions. A triclinic form, (I), is obtained from dimethylformamide-diethyl ether or methanol, whereas crystallization from dimethylformamide-water yields a monoclinic form, (II). In each case, the Cu(II) centre is in a five-coordinate distorted square-pyramidal geometry. The extended packing for both forms can be described as a highly offset π-stacking arrangement, with interlayer distances of 3.674 (3) and 3.679 (3) Å for forms (I) and (II), respectively. The reaction of diprotonated Pt(tmpip2NCN)Cl [tmpip2NCN = 2,6-bis(2,2,6,6-tetramethylpiperidylmethyl)benzyl] with AgPF6 under acidic conditions, followed by the addition of 2-(pyridin-2-yl)-1,10-phenanthroline, results in a hydrogen-bonded cocrystal, 2,2,6,6-tetramethyl-4-oxopiperidinium hexafluorophosphate-2-(pyridin-2-yl)-1,10-phenanthroline (1/1), C9H18NO(+)·PF6(-)·C17H11N3, (III). The extended packing maximizes π-π interactions in a parallel face-to-face arrangement, with an interlayer stacking distance of 3.4960 (14) Å.

RedOx-controlled sorption of iodine anions by hydrotalcite composites

The radioactive contaminant iodine-129 (I-129) is one of the top risk drivers at radiological waste disposal and contaminated groundwater sites where nuclear material fabrication or reprocessing has occurred. However, currently there are very few options available to treat I-129 in the groundwater, partially related to the complex biogeochemical behavior of iodine in the subsurface and occurrence of I-129 in the multiple chemical forms. We hypothesize that layered hydrotalcite materials containing redox active transition metal ions offer a potential solution, benefiting from the simultaneous adsorption of iodate (IO3−), and iodide (I−) anions, which exhibit different electronic and structural properties and therefore may require dissimilar hosts. To test this hypothesis, Cr3+-based materials were selected based on the rationale that Cr3+ readily reduces IO3− in solution. It was combined with either redox-active Co2+ or redox-inactive Ni2+ so that two model materials were prepared by hydrothermal synthesis including Co2+–Cr3+ or Ni2+–Cr3+ (M–Cr). Obtained M–Cr materials composed of Co2+–Cr3+ or Ni2+–Cr3+ layered hydrotalcite and small fractions of Co3O4 spinel or Ni(OH)2 theophrastite phases were structurally characterized before and after uptake of periodate (IO4−), IO3−, and I− anions. It was found that the IO3− uptake is driven by its chemical reduction to I2 and I−. Interestingly, in the Co2+–Cr3+ hydrotalcite, Co2+ and not Cr3+ serves as a reductant while in the Ni2+–Cr3+ hydrotalcite Cr3+ is responsible for the reduction of IO3−. A different uptake mechanism was identified for the IO4− anion. The Co2+–Cr3+ hydrotalcite phase efficiently uptakes IO4− by a diffusion-limited ion exchange mechanism and is not accompanied by the redox process, while Cr3+ in the Ni2+–Cr3+ hydrotalcite reduces IO4− to IO3−, I2 and I−. Iodide exhibited high affinity only to the Co–Cr material. The Co–Cr material performed remarkably well for the removal of IO3−, I− and total iodine from the groundwater collected from the US DOE Hanford site, WA, USA, outperforming non-redox active hydrotalcites (e.g., Mg2+–Al3+) reported previously. This work demonstrates that redox-controlled sorption can be a highly effective method for the treatment of anions based on elements with mobile oxidation states. Further, multiple anions of interest could be simultaneously removed through a combination of approaches.

Inorganic tin aluminophosphate nanocomposite for reductive separation of pertechnetate

Pertechnetate (TcO4−) is the most abundant chemical form of the radioactive contaminant 99Tc present in legacy nuclear waste streams and in the subsurface of nuclear waste storage sites. One proposed remediation approach is reductive separation of TcO4− and sequestration in low-temperature waste forms. The development of relevant technologies has been slow due to the lack of reductive materials that retain their functionality and are otherwise suitable for application in multicomponent and aggressive media such as highly alkaline, brine-like solutions typifying nuclear tank wastes. In this work, a tin-based reductive material was prepared, and its potential utility for the separation of TcO4− from alkaline nuclear wastes was demonstrated. This material consists of Sn(II/IV) phosphate supported by a polycrystalline aluminophosphate matrix. The aluminophosphate matrix is inert to the reaction conditions and offers the benefits of high stability and low solubility in concentrated alkaline solutions. This Sn(II/IV)-based material exhibits a high loading capacity for Tc and selectively removes a major fraction of TcO4− from the tank waste supernatant simulant, which contains 7.8 M total sodium and 2.43 M free hydroxide concentrations. Observed Kd values for Tc are about 13 000 and 2200 mL g−1 for simulant solutions containing no or 33 mM Cr(VI), respectively, positioning Sn(II/IV) aluminophosphate among the best-performing reductive sorbents for TcO4− developed to date. This advanced behaviour is attributed to the synergistic combination of the Sn(II/IV) aluminophosphate functionalities. The presence of Sn(II/IV)-rich fibres facilitates the reduction of TcO4− to Tc(IV), which is embedded along the fibre branches. Importantly, the Sn(IV)-containing inert polycrystalline matrix also incorporates Tc(IV) which triggers its crystallization to cassiterite SnO2 phase and stabilizes Tc(IV) in the polycrystalline matrix.

Three-component spectroelectrochemical sensor module for the detection of pertechnetate (TcO4-)

Abstract This review looks at the advancements in the development of a sensor for technetium (Tc) that is applicable to characterizing and monitoring the vadose zone and associated subsurface water. Subsurface contamination by Tc is of particular concern for two reasons: the long lifetime of its most common isotope 99Tc (half-life=2×105 years) and the fast migration in soils of pertechnetate (TcO4-), which is considered to be the dominant 99Tc species in ground water. TcO4– does not have a characteristic spectral signature which prevents its rapid, sensitive, and economic in situ detection. To address this problem, a novel spectroelectrochemical sensor has been designed, that combines three modes of selectivity (electrochemistry, spectroscopy, and selective partitioning) into a single sensor to substantially improve specificity, which is critical in the specific detection of an analyte in the presence of potential interfering species. The sensor consists of a basic spectroelectrochemical configuration: a waveguide with an optically transparent electrode (OTE) that is coated with a thin chemically-selective film that preconcentrates the analyte. The key to adapting this generic sensor to detect TcO4- and Tc complexes lies in the development of chemically-selective films that preconcentrate the analyte and, when necessary, chemically convert it into a complex with electrochemical and spectroscopic properties appropriate for sensing. The chemically selective films can be combined with ligands which are capable of reacting with TcO4- to form coordination complexes, the spectral properties of which can be used to enhance the sensitivity of detection. The first half of this review describes the general concept of the sensor and the rationale for the selection of its specific components, and the development and characterization of the sensor for the different detection modules. The second half summarizes the synthesis and characterization of complexes relevant for the detection of technetium, and the progress in the utilization of the sensor module for the effective detection of these complexes.

Development of a Chemistry-Based, Predictive Method for Determining the Amount of Non-Pertechnetate Technetium in the Hanford Tanks: FY 2012 Progress Report

This report describes investigations directed toward understanding the extent of the presence of highly alkaline soluble, non-pertechnetate technetium (n-Tc) in the Hanford Tank supernatants. The goals of this report are to: a) present a review of the available literature relevant to the speciation of technetium in the Hanford tank supernatants, b) attempt to establish a chemically logical correlation between available Hanford tank measurements and the presence of supernatant soluble n-Tc, c) use existing measurement data to estimate the amount of n-Tc in the Hanford tank supernatants, and d) report on any likely, process-friendly methods to eventually sequester soluble n-Tc from Hanford tank supernatants.