Species In Groundwater Research Articles

The Effect of Degradation Products of Strong Acidic Cation Exchange Resins on Radionuclide Speciation: A Case Study with Ni2+

Radiolytic degradation experiments with acidic ion-exchange resins revealed oxalate and an unidentified ligand X to be the most strongly complexing ligands of the degradation products. The influence of these ligands on the Ni speciation in groundwater and cement pore water of a repository is assessed.A complete and reliable thermodynamic database is built for this case study. Missing stability constants are estimated by chemical reasoning. Subsequent sensitivity analyses show whether these species are important or not. The backdoor approach used in this study addresses the following question: What concentrations must the ligand have to significantly influence the Ni speciation?In the case of oxalate, the concentration necessary to complex 90% Ni will never be exceeded within the repository or in its environment due to precipitation of Ca-oxalate solids. Thus, a negative effect of oxalate on Ni speciation and sorption need not be considered in safety assessments.In the case of ligand X, calculations demonstrate that Ni speciation is highly dependent on geochemical conditions and is occasionally ambiguous due to uncertainties in estimated stability constants. Hints are given to deal with these ambiguities in future safety assessments, and further experimental investigations are proposed to decrease uncertainties when necessary.

Experimental study of mineral surface and aqueous aluminum complexation with aniline and 2-chloroaniline

The thermodynamic stabilities of the aqueous and surface Al-aniline and Al-2-chloroaniline complexes were investigated using solubility and adsorption experiments. Aqueous complexation was studied by measuring the solubility of gibbsite (Al(OH) 3) as a function of aniline and chloroaniline concentrations. Experiments measuring aniline and chloroaniline adsorption onto corundum (α-Al 2O 3) were conducted as a function of pH, from pH 1.5 to 9.0, at room temperature in 0.1 molal NaCl solutions, with starting concentrations of 10 −3.0 and 10 −3.3molal aniline, 10 −2.7 molal 2-chloroaniline, and 2.72 × 10 −3 molal total surface sites. The experiments place quantitative constraints on the thermodynamic properties of the aqueous and the surface Al-aniline complexes. The solubility data indicate that if Al-aniline or Al-2-chloroaniline aqueous complexes exist, they are not stable enough to significantly affect the solubility of gibbsite under the experimental conditions. Conversely, the adsorption data provide unequivocal evidence for the presence of two distinct surface Al-aniline and Al-chloroaniline complexes according to the following reactions (1) Aniline 0 + ≡ Al(OH) 0 ⇔ ≡ AlOH(Aniline) 0 (2) Aniline 0 + ≡ Al(O) − ⇔ ≡ AlO(Aniline) − (3) Chloroaniline 0 + ≡ Al(OH) 0 ⇔ ≡ AlOH(Chloroaniline) 0 (4) Chloroaniline 0 + ≡ Al(OH 2 ) + ⇔ ≡ AlOH 2 (Chloroaniline) + We use a constant capacitance model to quantify the stability constants for reactions 1–4, and the results yield log equilibrium constant values of 2.09, 2.67, 2.87, and 2.30, respectively. The experimental results indicate that for most contaminated systems, aqueous Al-aniline, and Al-chloroaniline complexation does not have a significant effect on aniline speciation in groundwater, but that mineral surface complexation can significantly affect total aniline and chloroaniline budgets.

Hydrolysis of Ferric Ion in Water and Conformational Equilibrium

Reported here are results of theoretical calculations on Fe(H2O)63+, Fe(H2O)5(OH)2+, three isomers of Fe(H2O)4(OH)2+, and Fe(H2O)3(OH)2+, which investigate the molecular mechanisms of hydrolysis of ferric ion in water. The combination of density functional electronic structure techniques and a dielectric continuum model for electrostatic solvation applied to the Fe(H2O)63+ complex yields an estimate of −1020 kcal/mol (experimental values −1037 to −1019 kcal/mol) for the absolute free energy of the aqueous ferric ion. The predicted free energy change for the first hydrolysis reaction is surprisingly close to the experimental value (2 kcal/mol predicted compared to 3 kcal/mol experimental). For the second hydrolysis reaction, we found an unexpected low-energy isomer of Fe(H2O)4(OH)2+ with five ligands in the inner sphere and one water outside. The hexacoordinate cis and trans isomers are, respectively, slightly lower and higher in energy. Calculations on the pentacoordinate species Fe(H2O)3(OH)2+ suggest that extrusion of the outer-sphere water is nearly thermoneutral. The reaction free energy for the second hydrolysis is predicted in the range 16−18 kcal/mol, higher than the experimental value of 5 kcal/mol. Because the theoretical predictions are higher than experimental values, and novel structures were encountered among products of the second hydrolysis, we argue that conformational entropy is an important omission in this theoretical treatment of net reaction free energies. A fuller cataloging of low-energy hydrolysis products and direct calculations of partition functions of the isolated complexes should help in modeling equilibrium speciation in groundwaters.

The Dynamics of Aluminum Speciation in Ground Waters from Rhode River Watershed

In the Rhode River estuarine/watershed area, increased acidity of precipitation from atmospheric acidic deposition has deleterious effects on the freshwater ecosystems. One of the characteristics of an acidic watershed is the mobilization of aluminum from soils to aquatic environment. Increased concentrations of aluminum in surface waters are toxic to living organisms. Detection of long-term changes of acidity and elevated Al concentrations in surface and soil waters is a central issue. In this present paper, the dynamics ofAl speciation in the shallow ground waters from Rhode River watershed 110 (la) were investigated. This research provides a unique, regional analysis of important controls on the transport of Al speciation through the rural watershed of the Chesapeake Bay.;In the Rhode River estuarine/watershed area, increased acidity of precipitation from atmospheric acidic deposition has deleterious effects on the freshwater ecosystems. One of the characteristics of an acidic watershed is the mobilization of aluminum from soils to aquatic environment. Increased concentrations of aluminum in surface waters are toxic to living organisms. Detection of long-term changes of acidity and elevated Al concentrations in surface and soil waters is a central issue. In this present paper, the dynamics ofAl speciation in the shallow ground waters from Rhode River watershed 110 (la) were investigated. This research provides a unique, regional analysis of important controls on the transport of Al speciation through the rural watershed of the Chesapeake Bay.

Evaluation of the Gas–Liquid Separation Efficiency of a Tubular Membrane and Determination of Arsenic Species in Groundwater by Liquid Chromatography Coupled With Hydride Generation Atomic Absorption Spectrometry

An experimental approach for evaluating the hydride separation efficiency of a tubular microporous poly(tetrafluoroethylene) (PTFE) membrane by flow injection hydride generation atomic absorption spectrometry (HGAAS) is described. A method is also proposed for determining arsenic species in groundwater by coupling ion-pair chromatography (IPC) with HGAAS. The evaluation was undertaken with a combined gas–liquid separator (GLS), consisting of a tubular PTFE membrane and a conventional U-shaped glass device. Two important controlling variables, viz ., the length of the microporous PTFE tubing and the ratio of the tubing id to the wall thickness, were investigated. The results showed that the absolute separation efficiency of the PTFE membrane can be effectively evaluated with the combined GLS. In addition, arsenic speciation in groundwater was also investigated by IPC–HGAAS. The chromatographic separation of arsenic species with this analytical system was examined in terms of the effects of pH and concentration of eluent. The absolute detection limits achieved with a 100 µl injection can be as low as 0.07, 0.11, 0.12 and 0.30 ng for arsenite (As III ), dimethylarsinic acid, monomethylarsonic acid and arsenate (As V ), respectively. A commercially available standard (NIST SRM 1643b Trace Elements in Water) and the spike additions method were used to confirm the analytical reliability of the method. The proposed method has already been routinely applied to the speciation of arsenic in groundwater samples.

Method for Detecting Selenium Speciation in Groundwater

To better understand the potential toxicity of Se, it is necessary to know the concentration of different Se ionic species (e.g., SeO 3 2- and SeO 4 2- ). The hydride generation atomic absorption spectrophotometry (HGAAS) method of Se analysis cannot separate Se into individual ionic species. Ion chromatography (IC) can determine SeO 3 2- and SeO 4 2- concentrations simultaneously ; however, common anions, such as sulfate (S0 4 2- ), in groundwater interferes with SeO 3 2- and SeO 4 2- speciation. The purpose of this study was to measure the concentration of ionic SeO 3 2- and SeO 4 2- species in groundwater, thereby determining the chemical speciation of dissolved Se. Three groundwater samples with high concentrations of Mg 2+ and SO 4 2- were used in this study. The ionic SeO 3 2- and SeO 4 2- species in groundwatersamples were selectively adsorbed onto copper oxide (CuO) particles by lowering the pH to 5.5. These ionic species were desorbed from the surface of CuO particles by increasing the pH to 12.5. Subsequently, the concentrations of SeO 3 2- and SeO 4 2- ionic species in solutions were determined with HGAAS and IC. The effect of divalent cations (e.g., Mg 2+ ) on the concentration of SeO 4 2- in aqueous solutions was also evaluated. The dissolved Se concentration in three groundwater samples ranged from 22 to 151 μg/L. The CuO particles extracted 97% of SeO 3 2- from groundwater samples, suggesting that Se(lV) concentrations were dominated by the SeO 3 2- ion. However, CuO particles extracted 80% of SeO 4 2- from groundwater samples. These results suggest that Se(Vl) concentrations consisted of SeO 4 2- and metal SeO 4 2- solution species. The dissolved Mg 2+ in groundwater samples formed a strong neutral ion pair with SeO 4 2- (MgSeO 4 0 ), which was not adsorbed by the CuO particles. Overall chemical speciation of dissolved Se, extracted with CuO particles, suggests that groundwater samples consisted of SeO 3 2- (6-36%), SeO 4 2- (32-65%), organic Se species (14-23%), and neutral ion pairs (9-16%). An important aspect of the proposed method is that CuO can be used in the field to extract both SeO 3 2- and SeO 4 2- ionic species from groundwater samples, and these species could be desorbed from CuO and measured using HGAAS or IC methods, depending upon the concentrations of these species.

Selenite and Selenate Determination in Surface Coal Mine Backfill Ground Water

A majority of Wyoming`s public water supplies are based on ground water resources. It is necessary, therefore, to protect these resources from contamination by pollutants (e.g., selenium), which can be harmful to human health. Dissolved selenium (Se) in ground water may contain different species, including selenite (SeO{sup 2-}{sub 3}), selenate (SeO{sup 2-}{sub 4}), organic Se, and Se ion pairs (e.g., MgSeO{sup {degrees}}{sub 4}, CaSeO{sup {degrees}}{sub 4}). For an understanding of the toxicity potential of Se, it is necessary to know the concentrations of SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4}, as well as total Se content in ground water. The hydride generation atomic absorption spectrometry (HGAAS) method of Se analysis requires oxidation of organic matter, which destroys the natural distribution of Se. Ion chromatography (IC) can determine SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4} concentrations, but other common anions (e.g., SO{sup 2-}{sub 4}) will interfere with SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4} measurements. The objective of this study was to determine SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4} concentrations to evaluate the chemical speciation of Se in ground water. Both SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4} from ground water samples were adsorbed onto copper oxide (Cu) at pHmore » 5.5 and subsequently desorbed with NaOH solution at pH 12.5. Extracted SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4} were determined using HGAAS and IC. From the SeO{sup 2-}{sub 3} and SeO{sup 2-}{sub 4} measurements, Se speciation in ground water was calculated. Results showed that dissolved Se concentrations in ground water ranged from 22 to 151 {mu}g/L. These results also show that mobility of dissolved Se will depend on the chemical form in which Se is present in the ground water.« less

The Poços de Caldas project: an introduction and summary of its implications for radioactive waste disposal

The Poços de Caldas project was designed to provide information of direct use to those involved in the safety assessment of buried radioactive wastes. This paper describes the four technical objectives of the project and the extent to which they were successfully addressed. The four objectives were: to test geochemical codes describing radionuclide solubility and speciation in groundwaters, to examine the applicability of models describing geochemical processes at mobile oxidation fronts, to evaluate the role of colloids in geochemical transport in groundwaters and to determine U/Th and REE mobility during hydrothermal alteration. Additional conclusions are drawn with regard to developing repository site investigation methodology. The paper does not present original data from the project, since these are provided in the accompanying papers in this volume, but focuses instead on the project's broader implications for radioactive waste management.

Using data from natural environments to improve models of uranium speciation in groundwaters

Abstract It is essential that computer-based models used in the safety assessment of radioactive waste repositories accurately represent the processes occurring in real field systems. Confidence in long-term predictions of radionuclide migration will then depend upon the completeness of data available, particularly those obtained from the disposal site, and correct implementation of the model. The study of natural geochemical systems provides information on the adequacy of the underlying “generic” database and enhances our understanding of the transport mechanisms which form the basis of performance assessment. This paper concentrates on speciation-solubility modelling and describes four natural occurrences of uranium, each of which displays a different facet of uranium migration behaviour. The attributes of each site and the means by which uranium is immobilised are described. Retardation is highly species specific and this is illustrated through the use of site data in equilibrium speciation and coupled chemical transport calculations. Oxidation of U(IV) to U(VI) species promotes leaching of uranium ore at all the locations studied, emphasising the need to ensure that reducing conditions persist in a repository dominated by its actinide inventory.

Field studies on the migration of arsenic and cadmium in a carbonate gravel aquifer near Munich (Germany)

The migration of arsenic and cadmium species in ground water was investigated with the aid of tracer experiments at a test site in a fluvinglacial aquifer. Experimental breakthrought curves, obtained under natural flow conditions, characterize the migration of arsenic and cadmium species in the ground water in relation to a conservative tracer. Transport velocities of arsenic species corresponded to those of the groundwater flow and resulted in a rapid breakthrough in the observation weells. The recovery of the arsenic species, calculated with respect to the conservative tracer, averaged about 50%. Intensity of sorption increased with decreasing concentrations of arsenic species. The migration of cadmium species, however, was controlled by chemical precipitation of otavite and a slow redissolution of this compound. The breakthrough of cadmium in the observation wells, thus, was much slower with respect to the water movement. A recovery of about 40% was observed after more than 1 year of observation. Increased concentrations of chloride or/and sulphate resulted in remobilization of cadmium. Breakthrough curves are described by a commonly used dispersion model. It appears that a retardation factor is not sufficient to describe the asymmetrical breakthrough curves; it must be supplemented by recovery rates and a specific dispersitivity, defined by the time variance of concentration at a given point. Longer flow distances or lower initial concentrations caused reduced recoveries, higher specific dispersivities and retardations both for arsenic and for cadmium species.

Simultaneous determination of trace iron(II) and iron(III) based on kinetic spectrophotometry of the iron(III)-pyrocatechol violet complex in a micellar medium

A kinetic spectrophotometric method is developed for the simultaneous determination of trace iron(II) and iron(III) with a single spectrophotometric reagent. The method is based on the oxidation of iron(II) by Pyrocatechol Violet (PV) in acetate buffer media; the iron(III) formed is complexed with PV in the presence of a cationic surfactant [cetyltrimethylammonium chloride (CTMAC)]. The absorbance-time dependence is measured at 570 nm and the concentrations of iron(III) and iron(II) are obtained from the absorbance values at the start and equilibrium, respectively. The molar absorptivity of iron(III)-PV-CTMAC ternary complex is 4.80 × 104 l mol−1 cm−1 and the calibration graph is linear between 0.05 and 0.7 μg ml−1 iron. The mechanism of iron(II) oxidation and the dual role of the o-dihydroxy aromatic compound are discussed. The method is applied to iron speciation in groundwaters.

Speciation patterns of actinides in natural waters: a laboratory investigation

The results of laboratory investigations of actinide speciation in ground waters of varying compositions are summarized, re-evaluated and further interpreted, with the inclusion of additional data. Actinide speciation (solubility, oxidation state distributions) in these water samples exhibits great variability at 10 −10 to 10 −8 M total actinide concentrations. Americium solubility appears to be controlled by the formation of radiocolloids. neptunium is predictably oxidized and soluble in these water samples, with only a few exceptions. Plutonium exhibits widely varying oxidation state distributions—and hence solubility— despite the presence of 6 mg liter −1 dissolved oxygen in all experiments. Plutonium solubility is enhanced by carbonate and fluoride but is diminished by sulfate in reducing waters. In general, these results are compatible with the limited information available for actinide speciation from previous in-situ investigations but they emphasize the paucity of accurate thermodynamic data to describe these systems.

A Laboratory Study of Electromigration as a Possible Field Technique for the Removal of Contaminants from Ground Water

AbstractA method for removing dissolved contaminants from ground water by emplacing electrodes in the aquifer has been tested using laboratory columns of pure quartz silty sand saturated with solutions of CuSO4 and copper‐contaminated synthetic ground water. In the Soviet Union a similar method is commonly used in searching for hidden deposits of metallic minerals. In the Soviet method, electrodes are emplaced in the ground and ions of the metals being sought are caused to migrate along an imposed voltage gradient; the ions are collected for analysis in acid‐filled ceramic cylinders which surround the cathodes.In this study, quick‐freezing was used to obtain the distribution of ions within the columns as a function of time and space. With voltages up to 2.5 V and cur‐rents of a few tens of microamps, more than 50 percent of the dissolved copper was removed from the interstitial fluid in the porous columns in a period of five days. Current efficiencies ranged from more than 80 percent to less than 5 percent, depending on such factors as length of time of electrolysis, pH, concentration of Cu, and presence of other ions. The efficiency and economics of the technique of electromigration should be evaluated in additional laboratory studies and in the field, but in theory the method should be useful for removing any charged species in ground water, including some organics.

Investigation on technetium species in ground water and migration through clay columns

Speciation of technetium in ground water has been studied for understanding the migration behaviour of this radionuclide in deep geological formations. A combination of free-liquid electromigration, ion exchange, solvent extraction, coprecipitation and dialysis methods has been applied. Both oxic and anoxic conditions have been employed. Systems studied include leaching of sodium borosilicate glass spiked with99Tc and95mTc followed by its passage through glauconitic sand columns, and dialysis of TcO2 with ground water, sodium chloride, and humic acid solutions. Results indicate the presence of the pertechnetate, TcO4−, ion as the dominating species.