Analysis of lanthanides and actinides in sea sediments from the Gulf of Tehuantepec

Seawater contains trace elements in solution, such as lanthanides and uranium, which can act as natural tracers when they flocculate to form the sediments of the seabed. Uranium is a special case because it is a radioactive element with isotopic ratio close to 1.14 (²³⁴U/²³⁸U) in sea water. Uranium is also present in all fractions of sediments, although the isotopic ratio in each fraction takes a specific value depending on its origin and surrounding environment. This work has been conducted in order to analyze lanthanides and uranium in a core of sea sediments. It is focused on separating the fractions forming the layers along the depth profile of the core sample so as to determine the ²³⁴U/²³⁸U activity ratios in fractions of organic matter, aquatic life skeletons and dissolved minerals. The Neutron Activation Analysis showed the presence of U, Lu, Hf, Eu, Ce and Yb ranging up to 50 ppm while alpha spectrometry gave a series of ²³⁴U/²³⁸U activity ratios which are discussed on the basis of statistical analysis.     

Synergistic extraction of hexavalent actinides by HTTA and neutral donors-II

Synergistic extraction studies on Np(VI) and Pu(VI) have been carried out as a part of the programme on the synergistic extraction of hexavalent actinides. Extraction of Np(VI) and Pu(VI) were carried out by mixtures of HTTA and TBP in benzene from aqueous perchlorate and nitrate media. Equilibrium constant values for the various reaction equilibria involved were calculated from the data obtained by using slope-ratio as well as Job’s method. The extraction of Np(VI) by the synergistic mixture from 1M nitric acid indicated that the species NpO₂ (TTA) (NO₃). TBP was not involved in the extraction. The log values of K_A, K_AB and β_AB were −1.5, 2.92 and 4.43, respectively for Np(VI) and −1.63, 2.50 and 4.13 respectively for Pu(VI).     

X-ray absorption LIII and MV edges of hexavalent lower actinides

X-ray absorption spectroscopy is used to probe the electronic and structural features of the actinides. To better understand the effect of the effective charge and polyhedron on the edge structure, we discuss the L_III and M_V edges in AnO₂(NO₃)₂(TBP)₂ (where the actinide is shortly linked to the ‘yl’ oxygens) and in the perovskite Ba₂ZnAnO₆ (where the hexavalent actinide is in the cubic structure as An⁶⁺), An=U, Np, Pu. Using FEFF8 simulations, both features and position of the L_III and M_V edges are explained within a crystal field splitting of the orbitals picture and a multiple scattering picture. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASXANES / uranium / neptunium / plutonium / perovskite     

Atomic absorption spectrometry of traces of tri- and hexavalent chromium

Summary Atomic absorption spectrometry combined with extraction is described for the determination of tri- and hexavalent chromium. The chromium diethyldithiocarbamate chelate was used for chromium (VI), whereas hydroxyquinolate or thenoyltrifluoroacetonate chelate for chromium(III). The method is rapid and precise.

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Zusammenfassung Extraktion und darauffolgende Messung der Atomarabsorption wurden zur Bestimmung von drei- und sechswertigem Chrom verwendet. Die Chelatverbindung mit Diäthyldithiocarbamat wurde für Chrom(VI), der Chelatkomplex mit Hydroxychinolin oder Thenoyltrifluoraceton für Chrom(III) verwendet. Das Verfahren ist rasch und genau.

     

A comparative study of global hexavalent chromium removal by chemical and electrochemical processes

Direct electroreduction of Cr(VI) to Cr(III) has been widely proposed as an alternative for the treatment of effluents polluted with hexavalent chromium, however, no analysis is available on the cost of a global process, that considers both, the energy required to carry out the reduction reaction, and that associated with the operation of the process, to remove Cr(VI) via precipitation of Cr(OH)₃. This paper presents a study of the operation cost, considering raw material and electric power required by the electrochemical process, to remove Cr(VI) from real samples. A comparison between chemical reduction is presented, where both processes are followed by a step of alkaline chemical precipitation. The differences in pH required for each step are determinant in the overall cost of the process. Operating under optimal conditions the cost is almost 7 times higher for direct electroreduction process compared with the chemical method, and power consumption being secondary. The ratio decreases to 1.3 times when the electrochemical method is carried out at pH 2, but operating time is increased threefold, thereby increasing the cost of pumping the solution to be treated.     

A comparative study on the sorption and desorption of Hg,Th and U on clay

The sorption and desorption of uranium, thorium and mercury on a western Anatolian montmorillonite, obtained from the deposit located in Kula, were studied by application of a batch technique. The clay used is a tertiary clay originally found in a rather large geological formation of west Anatolia. It is nearly pure montmorillonite. Its cation exchange capacity (CEC) determination was performed for ammonium acetate by the Mehlich procedure. The mean CEC was found to be 83 meq/100 g, which, taking into account that CEC determinations were carried out on unfractionated material, is in good agreement with previously reported data. The concentration ranges were between 70–1500 ppm for mercury and 100–2000 ppm for thorium and uranium. The relative importance of test parameters, e.g., pH, clay particle size, groundwater composition, contact time and solid/water ratio, which determine the distribution coefficients was studied. The sorption coefficients varied between 2.7–6.4 ml/g for U, 0.22–1.59 ml/g for Th and 152.4–427.2 ml/g for Hg. The differences of distribution coefficients are discussed. The data could be fitted to Freundlich and Langmuir isotherms. The quantities of the sorbed and desorbed Th were much lower than its theoretical CEC's. This attitude was attributed to the blocking of montmorillonite by cation islands sorbed in the interlayer. Hg is sorbed most strongly. The experimental results indicate that the montmorillonites studied should be effective components of the buffer and backfill material and lead to eventual immobilization of these elements, which are environmentally dangerous.     

Studies on fluoride complexing of hexavalent actinides using a fluoride ion selective electrode

Complex formation between actinide(VI) and fluoride ions in aqueous solutions has been investigated using a fluoride ion selective electrode (F-ISE). As fairly high acidity was used to suppress hydrolysis of the actinide(VI) ions, significant liquid junction potentials (E_j) existed in the systems. An iterative procedure was developed for computing free hydrogen ion concentration [H⁺], as it could not be measured directly, using data obtained with F-ISE. E_j values were estimated from known [H⁺] and the stability constants of fluoride complexes of actinide(VI) ions were calculated following KING and GALLAGHER's method using a computer program. The stability constants were found to follow the order U(VI)>Np(VI)>Pu(VI).     

A comparative study of diffusion samplers for the determination of hexavalent chromium by sequential injection spectrophotometry

A method is described for determining hexavalent chromium in high particulate-containing surface waters by sequential injection (SI). The relative performance of two membrane-based methods for sampling is compared. The first membrane approach is based on a commercial design known as the ‘supported capillary membrane sampler (SCMS)’ (Wolcott, D.K., US Pat. 5 317 932 (1995)) that uses tubular membranes; the second approach is based on a conventional parallel-plate dialyzer (PPD) design that uses planar membranes. The membranes are evaluated using the well-known colorimetric method for the determination of hexavalent chromium by complexation with 1,5-diphenylcarbazide (DPC). Thin-walled (∼200 μm) microporous (pore size ∼0.2 μm) polypropylene membranes are equilibrated with DPC during each sampling period. Formation of the DPC-Cr(VI) complex allows for efficient membrane transport; without the membrane, Cr(VI) transport decreases ∼90%. Factors optimized included reagent concentrations, sampling time, flow rate, and spectrophotometric conditions. Optimal conditions were 2.00 mM DPC and 0.100 M nitric acid for the reagent, and 600 and 900 s sampling times for the planar and tubular designs, respectively. The planar (PPD) design increased the sensitivity relative to the tubular (SCMS) design by ∼225%. The PPD-SI method was applied to the determination of dissolved Cr(VI) in high particulate-containing surface water samples. Figures of merit included a detection limit of <20 μg/l, precision of 1.1% R.S.D. at 100 μg/l (n=4), and selectivity for dissolved Cr(VI) in several surface water samples with high levels of particulate matter.     

Tributyltin determination in marine sediments: A comparative study of methods

Two independent speciation methods have been applied to the determination of tributyltin (TBT) in marine sediments: (1) acetic acid leaching/hydride generation/cold trapping/GC/quartz furnace AA; and (2) toluene–tropolone extraction/HPLC/GFAA. TBT determines in a typically moderately polluted sediment (Porto Vecchio Bay, Corsica, France) are in very good agreement, as also are the determination of total recoverable tin by the first method and by a direct one using strong acid leaching followed by GFAA. These cross-verifications lead one to conclude that both speciation methods are convenient and allow for valid determination of TBT in marine sediments, with no loss of TBT. Method-induced modification of tin speciation has not been observed.     

A comparative study of the adsorption of uranium on commercial and natural (Cypriot) sea sand samples

The adsorption of hexavalent uranium on two different types of sea sand [e.g. a local, Cypriot (N_SS) and a commercially available marine sediment (C_SS)] has been investigated as a function of pH, initial metal concentration, ionic strength and contact time under normal atmospheric conditions. Before carrying out the adsorption experiments, the sea sand samples have been characterized by XRD, XRF, N₂-adsorption, acid/base titrations and FTIR spectroscopy. Sample characterization showed clearly that the two sea sand types differ significantly in their composition, particularly in their calcite and FeOOH content. According to experimental data obtained from acid/base titrations and adsorption batch experiments sea sand composition affects the acid/base and the adsorption properties of the adsorbents. The extraordinary high affinity of N_SS for hexavalent uranium in the alkaline pH region can be attributed to the formation of mixed U(VI)–carbonato surface species on the FeOOH crystall phases present in N_SS, which effectively compete the formation of U(VI)–carbonato complexes in solution. On the other hand, data obtained by adsorption experiments carried out in solution of different ionic strengths don’t differ significantly from one another indicating the formation of inner-sphere complexes. Finally, the adsorption on sea sands is a relatively fast two-step process.     

A comparative study on sorption and diffusion of Cs in crushed argillite and granite investigated in batch and through-diffusion experiment

The batch and through-diffusion experiments in this study were conducted and compared in order to investigate the sorption and diffusion of cesium (Cs) for two potential host rocks in Taiwan: argillite from Taitung and granite from Kinmen Island, with the purpose of establishing a reliable safety-performance assessment methodology for the final disposal of low level radioactive waste. The results of Cs mapping by scanning electron microscope equipping by energy dispersive spectrometer (SEM–EDS) showed that the distribution of Cs on argillite and granite were enriched in illite and biotite, respectively. In addition, it showed that higher sorption capacities were found for argillite than granite; due to the clay mineral content (illite) in the argillite. Experiments for diffusion of Cs is agreement to the values estimated for the diffusive results (D _a) of Cs in argillite were revealed to be lower than those of granite.

     

Equilibrium sorption of hexavalent chromium from aqueous solution using synthetic hematite

Hematite sample prepared by transformation of ferrihydrite was used to remove Cr(VI) from aqueous solution. Effects of initial concentration, contact time, pH, ionic strength, and temperature were investigated. The data were fitted to Langmuir, Freundlich, Dubinin-Kaganer-Radushkevich and Temkin models of adsorption. The results showed the best fit to the Langmuir isotherm. The increase of pH as well as that of increasing ionic strength resulted in the decrease of adsoiption of Cr(VI). Two kinetic models namely pseudo-first order and pseudo-second order were used to fit the experimental data. The thermodynamic parameters were found, and the results indicated spontaneous and exothermic nature of the process.     

A study of cetylpyridinium cation sorption on Na-vermiculite

The sorption of cetylpyridinium on Na-vermiculite has been studied by the adsorption, calorimetry, and X-ray diffraction methods. The ion-exchange component of cetylpyridinium sorption on Na-vermiculite has been distinguished and analyzed. Variations in the integral heat of ion exchange of cetylpyridinium cations on Na-vermiculite have been determined as depending on the amount of absorbed cetylpyridinium by the calorimetry method. X-ray diffraction analysis of cetylpyridinium-Na-vermiculite organomineral complexes with different contents of exchange cations has been carried out in parallel. The formation of monolayer (interlayer spacing Δd = 20.25 Å) and bilayer (Δd = 37.27 Å) intercalation structures with a tilt angle of cetylpyridinium ions to the silicate basal surface of ≈57° has been revealed.     

An ENAA and PGAA comparative study of anoxic Black Sea sediments

A comparative study performed by means of both epithermal neutron activation analysis (ENAA) and prompt gamma neutron activation analysis (PGAA) concerning the distribution of 9 elements (Na, Al, Cl, K, Ca, Ti, Mn Fe and Sm) in unconsolidated sediments collected from the Black Sea anoxic region is presented. Principal component analysis (PCA) was used as a supplementary method in interpreting experimental data. The final results have shown that eight of them (Na, Al, Cl, K, Ti, Mn, Fe and Sm) have concentrations which differ by less than 30% from the corresponding concentrations of Upper Continental Crust (UCC), confirming the continental origin of sediments. At the same time, Ca concentration was about six times higher than that of UCC, this significant enrichment being due to the presence of the neighbouring oxygenated zone of considerable deposits of shell debris, characteristic of the Western Black Sea Continental Platform. Excepting Na and Mn, the concentrations of all other elements involved in this intercomparison were coincident within one standard deviation which attests the accuracy of this test. At the same time, PCA revealed the presence of five clusters, which could be put into concordance with the mineralogical composition of sediments, each of them consisting in equal proportions of elements determined by both methods.     

Pu and Am sorption to the Baltic Sea bottom sediments

Sorption of Am and Pu isotopes to bottom sediments of the Baltic Sea has been studied under natural and laboratory conditions. Data obtained from sequential extraction, sorption of Am(III), Pu(IV) and Pu(V) as well as oxidation state distribution experiments have shown that Pu(V) sorption mechanism includes a very fast Pu(V) reduction (reaction rate ≤ 2.33 × 10^?3 s^?1) to Pu(IV) by humic substances and/or by Fe(II) to Pu(IV) and partly to Pu(III). Following reduction Pu isotopes were bound to various components of bottom sediments via ion exchange and surface complexation reactions and a slow incorporation into the crystalline structure of Fe minerals. Kinetics experiments showed that the sorption of Pu(V), Pu(IV) and Am(III) to bottom sediments from natural seawater was controlled by the inert layer diffusion process.     

CO2 sorption on MgO and CaO surfaces: a comparative quantum chemical cluster study

A comparative quantum chemical study of CO2 adsorption on MgO and CaO has been carried out. Theoretical infrared (IR) frequencies are calculated and compared with IR experiments from the literature. The results show that CO2 adsorbs as monodentate on edge sites and bidentate on corner sites on MgO. The former assignment contradicts the common assumption of adsorption of CO2 in a bidentate configuration. On CaO, CO2 adsorbs as monodentate on both edge and corner sites, which is a reinterpretation of earlier experimental work. On terrace (100) sites, none of the adsorption modes on MgO or CaO possess calculated frequencies in agreement with the experimental IR spectra. These experimental bands were tentatively assigned to some slightly perturbed double negatively charged carbonate ions at the surface, rather than the monodentate structure suggested in the literature.     

Quantification of thorium and uranium sorption to contaminated sediments

Desorption tests, using a sequential extraction method, were used to characterize and quantify thorium and uranium sorption to contaminated wetland sediments collected from the Department of Energy's Savannah River Site located in South Carolina. In situ distribution coefficients, or K _d values (K _d = C _solid/C _liquid), were determined. Sequential extraction data were used to assign solid-phase radionuclide concentrations (C _solid) that, by definition, should represent only the reversibly sorbed fraction. A series of selective and sequential extractions was used to determine desorption K _d values. Thorium K _d values ranged from 115 to 2255 ml/g. Uranium K _d values ranged from 170 to 6493 ml/g. Compared to sorption K _d values, these desorption K _d values were appreciably greater because they captured the aging process of the radionuclides with the sediment, making the radionuclide more refractory. Compared to nonsite-specific data, these in situ K _d values improved accuracy, were more defensible, and removed unnecessary conservatism for subsequent transport and risk calculations. Additional tests were conducted to provide geochemical information relevant for selecting appropriate remediation technologies for the contaminated site. Thorium and uranium were associated with labile fractions and were not concentrated with the smaller sediment particles. These findings suggest that phytoremediation or heap leaching, and not soil washing, are viable remediation approaches for this site.     

Kinetics of the sorption of metal ions from sea water on clinoptilolite

Based on a study of the sorption kinetics of metal ions from sea water onto granular clinoptilolite and powdered clinoptilolite incorporated into a grain of a highly permeable inert support it has been shown that the rate of the sorption processes is limited principally by diffusion in the microgranules of zeolite. For the Tedzami clinoptilolite studied the characteristic dimensions of the microgranules are 15–20 m and are commensurate with the dimensions of microcrystals determined independently.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 263–267, February, 1990.We thank E. V. Venetsianov, L. S. Kamenetskii and M. V. Veksler for assistance in the investigation.