The mechanisms of preconcentration and voltammetric behaviour of uranium(VI) at glassy carbon electrodes modified by trioctylphosphine oxide

Uranium(VI) (?10^?5 M) can be determined at glassy carbon (GC) electrodes modified with a multilayer of trioctylphosphine oxide (TOPO) in a two-step procedure involving preconcentration at the electrode at potentials near O V vs. SCE and stripping in a cathodic scan. Various voltammetric and capacity measurements were made in attempts to elucidate the mechanisms and kinetics of the processes. The uranium(VI) is preconcentrated by reaction with the TOPO in which GC surface groups participate. During the cathodic stripping step, preconcentrated uranium(VI) amplifies the reduction current of GC surface functional groups, so that the electrode reaction can be considered as mediated electron transfer. The rate-determining steps are the diffusion of the analyte in the solutin during preconcentration and charge transfer in the bulk of the multilayer during stripping. The acetate buffer used affects the reaction in a complicated manner which is discussed in detail.     

Preconcentration of uranium(VI) by solid phase extraction onto dicyclohexano-18-crown-6 embedded benzophenone

Summary The preparation of a solid phase extractant (SPE), dicyclohexano-18-crown-6 embedded benzophenone, for preconcentration of uranium(VI) is described. The uranium(VI) can be quantitatively retained from 0.5 l solution on 1% dicyclohexano-18-crown-6 embedded benzophenone in the pH range 6.0-7.0, and then eluted with 5.0 ml of 1M HCl. Uranium(VI) content of the eluent was determined spectrophotometrically by Arsenazo III. Calibration graphs are rectilinear over the uranium(VI) concentration in the range of 0.004-0.4mg^.ml^-1. Five replicate determinations of 40mg of uranium(VI) present in 0.5 l sample gave a mean absorbance of 0.185 with a relative standard deviation of 2.45%. The detection limit corresponding to three times the standard deviation of the blank was found to be 2.0mg^.l^-1. The accuracy of the developed preconcentration procedure was tested by analyzing standard marine sediment reference material. The uranyl ion content of soils and sediments was estimated spectrophotometrically after the preconcentration procedure and compared to the results gained by standard inductively coupled plasma mass spectrometry (ICP-MS).     

Solid phase extractive preconcentration of uranium(VI) onto diarylazobisphenol modified activated carbon

Diarylazobisphenol (DAB) 1 and diarylazobisphenol modified carbon 2 were synthesized and characterised. The latter has been used for solid phase extractive preconcentration and separation of trace amounts of uranium(VI) from other inorganics. In this, a column mode preconcentration of uranium(VI) was carried out in the pH range 4.0-5.0, eluted with 1.0 mol l⁻¹ HCl and determined by an Arsenazo III spectrophotometric procedure. Calibration graphs were rectilinear over the uranium(VI) concentrations in the range 5-200 μg l⁻¹. Five replicate determinations of 25 μg of uranium(VI) present in 1 l solution gave a mean absorbance of 0.032 with a relative standard deviation of 2.52%. The detection limit corresponding to three times the standard deviation of the blank was found to be 5 μg l⁻¹. The accuracy of the developed preconcentration method in conjunction with the Arsenazo III procedure was tested by analysing MESS-3, a marine sediment certified reference material. Further, the above procedure has been successfully employed for analysis of uranium(VI) in soil and sediment samples.     

Adsorptive Stripping Voltammetric Determination of Trace Uranium with a Bismuth‐Film Electrode Based on the U(VI)→U(V) Reduction Step of the Uranium‐Cupferron Complex

This work reports the use of adsorptive stripping voltammetry (AdSV) for the determination of uranium on a preplated rotating‐disk bismuth‐film electrode (BiFE). The principle of the method relied on the complexation of U(VI) ions with cupferron and the subsequent adsorptive accumulation of the complex on the surface of the BiFE. The uranium in the accumulated complex was then reduced by means of a cathodic voltammetric scan while the analytically useful U(VI)→U(V) reduction signal was monitored. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3σ limit of detection for uranium was 0.1 μg L^?1 at a preconcentration time of 480 s and the relative standard deviation was 4.7% at the 5 μg L^?1 level for a preconcentration time of 120 s (n=8). The accuracy of the method was established by analyzing a reference sea water sample.     

Solid phase extraction and preconcentration of uranium(VI) and thorium(IV) on Duolite XAD761 prior to their inductively coupled plasma mass spectrometric determination

A simple and effective method is presented for the separation and preconcentration of thorium(IV) and uranium(VI) by solid phase extraction on Duolite XAD761 adsorption resin. Thorium(IV) and uranium(VI) 9-phenyl-3-fluorone chelates are formed and adsorbed onto the Duolite XAD761. Thorium(IV) and uranium(VI) are quantitatively eluted with 2 mol L⁻¹ HCl and determined by inductively coupled plasma-mass spectrometry (ICP-MS). The influences of analytical parameters including pH, amount of reagents, amount of Duolite XAD761 and sample volume, etc. were investigated on the recovery of analyte ions. The interference of a large number of anions and cations has been studied and the optimized conditions developed have been utilized for the trace determination of uranium and thorium. A preconcentration factor of 30 for uranium and thorium was achieved. The relative standard deviation (N = 10) was 2.3% for uranium and 4.5% for thorium ions for 10 replicate determinations in the solution containing 0.5 μg of uranium and thorium. The three sigma detection limits (N = 15) for thorium(IV) and uranium(VI) ions were found to be 4.5 and 6.3 ng L⁻¹, respectively. The developed solid phase extraction method was successively utilized for the determination of traces thorium(IV) and uranium(VI) in environmental samples by ICP-MS.     

Electroreduction of uranium (VI) at a platinum electrode and its analytical applications

Under normal conditions, the reduction of uranium(VI) at a platinum electrode, in acid solutions, is masked by the reduction of the hydrogen ion. If the working electrode is subjected to hydrogen evolution (at a current density of about 7 A cm (2) for 90-120 min) the H(ads) on the platinum surface, acting as a bridge in the electron transfer, shifts the reduction wave of uranium(VI), in 1M sulphuric acid solutions, to potentials (E(1 2 ) congruent with - 0.03 V) less negative than that of the hydrogen discharge (about -0.25 V). The wave corresponding to the reduction of uranium(VI) to uranium(IV) is well shaped, diffusion-controlled, and can be used for the determination of uranium down to 2 x 10(-5)M or 3 x 10(-6)M if a rotating electrode is used. Interferences arise from those ions with similar E(1 2 ) [i.e., Cu(II) and Bi(III)], or from those such as permanganate and dichromate, which oxidize the H(ads) on the platinum electrode. Because of the time required for the electrode pretreatment, the determination is time-consuming but in some respects it appears a useful improvement over the DME.     

Amberlite XAD-4 functionalized with succinic acid for the solid phase extractive preconcentration and separation of uranium(VI)

Amberlite XAD-4 resin has been functionalized with succinic acid by coupling it with dibromosuccinic acid after acetylation. The resulting resin has been characterized by FT-IR, elemental analysis and TGA and has been used for preconcentrative separation of uranium(VI) from host of other inorganic species prior to its determination by spectrophotometry. The optimum pH value for quantitative sorption of uranium(VI) in both batch and column modes is 4.5-8.0 and desorption can be achieved by using 5.0 ml of 1.0 mol l⁻¹ HCl. The sorption capacity of functionalized resin is 12.3 mg g⁻¹. Calibration graphs were rectilinear over the uranium(VI) concentrations in the range 5-200 μg l⁻¹. Five replicate determinations of 50 μg of uranium(VI) present in 1000 ml of solution gave a mean absorbance of 0.10 with a relative standard deviation of 2.56%. The detection limit corresponding to three times the standard deviation of the blank was found to be 2 μg l⁻¹. Various cationic and anionic species at 200-fold amounts do not interfere during the preconcentration of 5.0 μg of uranium(VI) present in 1000 ml (batch) or 100 ml (column) of sample solution. Further, adsorption kinetic and isotherm studies were also carried out by a batch method to understand the nature of sorption of uranium(VI) with the succinic acid functionalized resin. The accuracy of the developed solid phase extractive preconcentration method in conjunction with Arsenazo III procedure was tested by analyzing marine sediment (MESS-3) and soil (IAEA soil-7) reference material. Further, the above procedure has been successfully employed for the analysis of soil and sediment samples.     

<Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">tert</Emphasis>-Butylcalix[8]arene loaded silica gel for preconcentration of uranium(VI) via solid phase extraction

This paper reports silica gel loaded with p-tert-butylcalix[8]arene as a new solid phase extractor for determination of trace level of uranium. Effective extraction conditions were optimized in column methods prior to determination by spectrophotometry using arsenazo(III). The results showed that U(VI) ions can be sorbed at pH 6 in a mini-column and quantitative recovery of U(VI) (>95–98%) was achieved by stripping 0.4 mol L⁻¹ HCl. The sorption capacity of the functionalized sorbent is 0.072 mmol uranium(VI) g⁻¹ modified silica gel. The relative standard deviation and detection limit were 1.2% (n = 10) for 1 μg uranium(VI) mL⁻¹ solution and 0.038 μg L⁻¹, respectively. The method was employed to the preconcentration of U(VI) ions from spiked ground water samples.     

Bioaccumulation and determination of lead using treated-Pennisetum-modified carbon paste electrode

The present work describes the development and application of a carbon paste electrode modified by treated-Pennisetum setosum for the determination of lead(II) by anodic stripping differential pulse voltammetry. Most experiments were performed using the preconcentration/voltammetry/regeneration scheme. The resulting modified electrode offers a preferential uptake of lead(II) from solutions. Operational conditions, such as percentage treated-Pennisetum loading in the carbon paste, pH of electrolyte solution, ionic strength, preconcentration time, voltammetric waveform and interference are characterized and optimized to allow quantitative determination of lead. The electrode surface can be regenerated by immersing the modified electrode in 0.05 mol l⁻¹ hydrochloric acid for 2 min. For the measurement step, the optimum conditions were acetate buffer pH 5.0 and 0.60 ionic strength with the preconcentration time of 5 min. The modified electrode contained 10% (w/w) treated-Pennisetum. The detection limit (3σ) was 0.01 mg l⁻¹ Pb(II). For 16 preconcentration/measurement/renewal cycles, the responses could be reproduced with a 5.39% relative standard deviation. This method has been be successfully applied to the determination of lead(II) in natural water samples using standard addition method.     

Spectrophotometric determination of trace amounts of uranium(VI) in water samples after mixed micelle-mediated extraction

A cloud point extraction process using mixed micelle of the cationic surfactant CTAB and non-ionic surfactant TritonX-114 to extract uranium(VI) from aqueous solutions was investigated. The method is based on the color reaction of uranium with pyrocatechol violet in the presence of potassium iodide in hexamethylenetetramine buffer media and mixed micelle-mediated extraction of complex. The optimal extraction and reaction conditions (e.g. surfactant concentration, reagent concentration, effect of time) were studied and the analytical characteristics of the method (e.g. limit of detection, linear range, preconcentration, and improvement factors) were obtained. Linearity was obeyed in the range of 0.20-10.00 ng mL⁻¹ of uranium(VI) ion and the detection limit of the method is 0.06 ng mL⁻¹. The interference effect of some anions and cations was also tested. The method was applied to the determination of uranium(VI) in tap water, waste-water and well water samples.     

Coulometric determination of uranium with a platinum working electrode

Experimental conditions have been established which enable uranium to be determined coulometrically by the reduction of uranium(VI) to uranium(IV) at a platinum working electrode, by controlled-potential or controlled-potential-limit techniques. The procedure has been used successfully as a subsidiary method in the routine determination of uranium in pure uranyl nitrate solutions. The platinum electrode has several important practical advantages over the well established mercury-pool electrode for the coulometric determination of uranium. The consecutive determination of iron(III) and uranium(VI), or plutonium(IV) and uranium(VI) can be carried out with the same working electrode in the same solution and the coulometric oxidation of uranium(IV) to uranium(VT) is practicable. The rate of stirring of the cell liquor is much less critical in the case of the platinum electrode. Two main problems had to be overcome before a practical procedure could be achieved; hydrogen evolution during the uranium(VI)-(IV) reduction had to be eliminated so that 100% current efficiency could be obtained for the desired reaction and electrode-surface poisoning phenomena had to be controlled so that reaction times could be kept reasonably short. It was found that selection of a hydrochloric acid base solution containing a small amount of bismuth(III) enabled hydrogen evolution to be avoided: also electrode-surface poisoning with this base solution was not particularly serious and could be maintained at a satisfactorily low level by occasionally anodizing the electrode in dilute sulphuric acid. Bismuth(III) forms a complex with chloride ions and its presence increases the hydrogen overvoltage at the working electrode: no visible deposit of bismuth metal forms on the electrode during the uranium reduction. Samples containing nitrate can be analysed provided sulphamic acid is added to this hydrochoric acid base solution.     

Determination of uranium(VI) in seawater by means of automated flow constant-current cathodic stripping at carbon fibre electrodes

Uranium(VI) is determined in an automated flow system by means of constant-current reductive stripping with a mercury film-coated carbon fibre electrode and catechol as adsorptive reagent at pH 8.6 Interference from iron(III) is eliminated by addition of sulphite. Increased linear range between stripping signal and sample uranium(VI) concentration can be obtained by adding, in the computer, several stripping curves, each obtained after a short period of adsorptive accumulation. It is shown that the hanging mercury drop electrode can be used for the determination of uranium(VI) by means of computerized constant current stripping without the need for inert gas bubbling. The results obtained for uranium(VI) in two reference seawater samples, NASS-1 and CASS-1, were 2.90 and 2.68 μg l^?1 with standard deviations (n = 8) of 0.57 and 0.75 μg l^?1, respectively.     

Simultaneous extraction and preconcentration of uranium and thorium in aqueous samples by new modified mesoporous silica prior to inductively coupled plasma optical emission spectrometry determination

A new synthesized modified mesoporous silica (MCM-41) using 5-nitro-2-furaldehyde (fural) was applied as an effective sorbent for the solid phase extraction of uranium(VI) and thorium(IV) ions from aqueous solution for the measurement by inductively coupled plasma optical emission spectrometry (ICP OES). The influences of some analytical parameters on the quantitative recoveries of the analyte ions were investigated in batch method. Under optimal conditions, the analyte ions were sorbed by the sorbent at pH 5.5 and then eluted with 1.0 mL of 1.0 mol L⁻¹ HNO₃. The preconcentration factor was 100 for a 100 mL sample volume. The limits of detection (LOD) obtained for uranium(VI) and thorium(IV) were 0.3 μg L⁻¹. The maximum sorption capacity of the modified MCM-41 was found to be 47 and 49 mg g⁻¹ for uranium(VI) and thorium(IV), respectively. The sorbent exhibited good stability, reusability, high adsorption capacity and fast rate of equilibrium for sorption/desorption of uranium and thorium ions. The applicability of the synthesized sorbent was examined using CRM and real water samples.     

Determination of Ultra Trace Amounts of Uranium (VI) by Adsorptive Stripping Voltammetry Using L-3-(3, 4-dihydroxy phenyl) Alanine as a Selective Complexing Agent

Abstract

The spectrophotometric behavior of uranium (VI) with L-3-(3, 4-dihydroxy phenyl) alanine (LDOPA) reagent revealed that the uranium can form a ML₂ complex with LDOPA in solution. Thus a highly sensitive adsorptive stripping voltammetric protocol for measuring of trace uranium, in which the preconcentration was achieved by adsorption of the uranium-LDOPA complex at hanging mercury drop electrode (HMDE), is described. Optimal conditions were found to be a 0.02 M ammonium buffer (pH 9.5) containing 2.0 × 10^?5 M (LDOPA), an accumulation potential of ? 0.1 V (versus Ag/AgCl) and an accumulation time of 120 sec.

The peak current and concentration of uranium accorded with linear relationship in the range of 0.5–300 ng ml^?1. The relative standard deviation (at 10 ng ml^?1) is 3.6% and the detection limit is 0.27 ng ml^?1. The interference of some common ions was studied. Applicability to different real samples is illustrated. The attractive behavior of this reagent holds great promise for routine environmental and industrial monitoring of uranium.     

Preparation of carbon coated Fe3O4 nanoparticles for magnetic separation of uranium

Uranium(VI) was removed from aqueous solutions using carbon coated Fe₃O₄ nanoparticles (Fe₃O₄@C). Batch experiments were conducted to study the effects of initial pH, shaking time and temperature on uranium sorption efficiency. It was found that the maximum adsorption capacity of the Fe₃O₄@C toward uranium(VI) was ∼120.20 mg g⁻¹ when the initial uranium(VI) concentration was 100 mg L⁻¹, displaying a high efficiency for the removal of uranium(VI) ions. Kinetics of the uranium(VI) removal is found to follow pseudo-second-order rate equation. In addition, the uranium(VI)-loaded Fe₃O₄@C nanoparticles can be recovered easily from aqueous solution by magnetic separation and regenerated by acid treatment. Present study suggested that magnetic Fe₃O₄@C composite particles can be used as an effective and recyclable adsorbent for the removal of uranium(VI) from aqueous solutions.     

Sorption of uranium using silica gel with benzoylthiourea derivatives

Benzoylthiourea derivatives (N,N-diphenyl-N′-(3-methylbenzoyl)thiourea and diphenyl-N′-(4-methylbenzoyl)thiourea) were impregnated onto silica gel. The preconcentration of uranium(VI) from aqueous solution was investigated. Extraction conditions were optimized in batch method prior to determination by uv–visible absorption spectrometry using arsenazo(III). The optimum pH for quantitative adsorption was found as 3–7. Quantitative recovery of uranium (VI) was achieved by stripping with 0.1 mol L^?1 HCl. Equilibration time was determined as 30 min for 99% sorption of U(VI). Under optimal conditions, dynamic linear range of for U(VI) was found as 0.25–10 μg mL^?1. The relative standard deviation as percentage and detection limit were 5.0% (n = 10) for 10 μg mL^?1 U(VI) solution and 8.7 ng mL^?1, respectively. The method was employed to the preconcentration of U(VI) ions in soil and tap water samples.     

Uranium(VI) separation from aqueous solution by calix[6]arene modified textiles

The separation of uranium(VI) from aqueous solution by textile bound uranophile calix[6]arenes is possible at pH values ≥4. A maximum of 7.6^.10^-7 mol uranium per 1 g of the calixarene modified textile is bound at pH 5. The influence of competing ions on the uranium separation at pH 5 is very small. However, at pH 7 in carbonate rich waters calcium ions have a competing effect. Under environmentally relevant pH conditions the uranium is strongly bound to the calixarene modified textile. Under acidic conditions an almost complete regeneration of the calixarene modified textile is possible. The regenerated material can be utilized for further uranium separation cycles. This revised version was published online in July 2006 with corrections to the Cover Date.     

Removal, preconcentration and spectrophotometric determination of U(VI) from water samples using modified maghemite nanoparticles

Arsenazo III modified maghemite nanoparticles (A-MMNPs) was used for removing and preconcentration of U(VI) from aqueous samples. The effects of contact time, amount of adsorbent, pH and competitive ions was investigated. The experimental results were fitted to the Langmuir adsorption model in the studied concentration range of uranium (1.0 × 10^?4–1.0 × 10^?2 mol L^?1). According to the results obtained by Langmuir equation, the maximum adsorption capacity for the adsorption of U(VI) on A-MMNPs was 285 mg g^?1 at pH 7. The adsorbed uranium on the A-MMNPs was then desorbed by 0.5 mol L^?1 NaOH solution and determined spectrophotometrically. A preconcentration factor of 400 was achieved in this method. The calibration graph was linear in the range 0.04–2.4 ng mL^?1 (1.0 × 10^?10–1.0 × 10^?8 mol L^?1) of U(VI) with a correlation coefficient of 0.997. The detection limit of the method for determination of U(VI) was 0.01 ng mL^?1 and the relative standard deviation (R.S.D.) for the determination of 1.43 and 2.38 ng mL^?1 of U(VI) was 3.62% and 1.17% (n = 5), respectively. The method was applied to the determination of U(VI) in water samples.     

Synthesis of chitosan resin possessing a phenylarsonic acid moiety for collection/concentration of uranium and its determination by ICP-AES

A chitosan resin possessing a phenylarsonic acid moiety (phenylarsonic acid type chitosan resin) was developed for the collection and concentration of trace uranium prior to inductively coupled plasma (ICP) atomic emission spectrometry (AES) measurement. The adsorption behavior of 52 elements was systematically examined by packing it in a minicolumn and measuring the elements in the effluent by ICP mass spectrometry. The resin could adsorb several cationic species by a chelating mechanism, and several oxo acids, such as Ti(IV), V(V), Mo(VI), and W(VI), by an anion-exchange mechanism and/or a chelating mechanism. Especially, U(VI) could be adsorbed almost 100% over a wide pH region from pH 4 to 8. Uranium adsorbed was easily eluted with 1 M nitric acid (10 mL), and the 25-fold preconcentration of uranium was achieved by using a proposed column procedure, which could be applied to the determination of trace uranium in seawater by ICP-AES. The limit of detection was 0.1 ng mL⁻¹ for measurement by ICP-AES coupled with 25-fold column preconcentration.     

Determination of uranium(VI) in natural waters after preconcentration on adsorbent containing m-aminophenol fragments

Sorption and complexing properties of a modified adsorbent based on a maleic anhydride-styrene copolymer towards uranium(VI) are studied and the main quantitative characteristics of the metal ion sorption are determined. An adsorbent containing m-aminophenol fragments is proposed for the selective sorption of uranium(VI) from solutions. The optimal sorption conditions have been found. The recovery of uranium(VI) under the optimal conditions exceeds 95%. A procedure of the sorption photometric determination of uranium(VI) in sea water is developed.