Günther Meinrath

Testing the Suitability of Zerovalent Iron Materials for Reactive Walls

Environmental Context. Groundwater remediation is generally a costly, long-term process. In situ remediation using permeable reactive barriers, through which the groundwaters pass, is a potential solution. For redox-sensitive contaminants in groundwater, a metallic iron barrier (zerovalent iron, ZVI) can immobilize or degrade these dissolved pollutants. Scrap iron materials are a low-cost ZVI material but, because of the wide variation of scrap metal compositions, testing methods for characterizing the corrosion behaviour need to be developed. Abstract. Zerovalent iron (ZVI) has been proposed as reactive material in permeable in situ walls for contaminated groundwater. An economically feasible ZVI-based reactive wall requires cheap but efficient iron materials. From an uranium treatability study and results of iron dissolution in 0.002 M EDTA by five selected ZVI materials, it is shown that current research and field implementation is not based on a rational selection of application-specific iron metal sources. An experimental procedure is proposed which could enable a better material characterization. This procedure consists of mixing ZVI materials and reactive additives, including contaminant releasing materials (CRMs), in long-term batch experiments and characterizing the contaminant concentration over the time.

Aqueous Solutions of Uranium(VI) as Studied by Time-Resolved Emission Spectroscopy: A Round-Robin Test

Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.

Assessment of uncertainty in parameter evaluation and prediction

Like in all experimental science, chemical data is affected by the limited precision of the measurement process. Quality control and traceability of experimental data require suitable approaches to express properly the degree of uncertainty. Noise and bias are nuisance effects reducing the information extractable from experimental data. However, because of the complexity of the numerical data evaluation in many chemical fields, often mean values from data analysis, e.g. multi-parametric curve fitting, are reported only. Relevant information on the interpretation limits, e.g. standard deviations or confidence limits, are either omitted or estimated. Modern techniques for handling of uncertainty in both parameter evaluation and prediction are strongly based on the calculation power of computers. Advantageously, computer-intensive methods like Monte Carlo resampling and Latin Hypercube sampling do not require sophisticated and often unavailable mathematical treatment. The statistical concepts are introduced. Applications of some computer-intensive statistical techniques to chemical problems are demonstrated.

Database uncertainty as a limiting factor in reactive transport prognosis.

The effect of uncertainties in thermodynamic databases on prediction performances of reactive transport modeling of uranium (VI) is investigated with a Monte Carlo approach using the transport code TReaC. TReaC couples the transport model to the speciation code PHREEQC by a particle tracking method. A speciation example is given to illustrate the effect of uncertainty in thermodynamic data on the predicted solution composition. The transport calculations consequently show the prediction uncertainty resulting from uncertainty in thermodynamic data. A conceptually simple scenario of elution of uranium from a sand column is used as an illustrating example. Two different cases are investigated: a carbonate-enriched drinking water and an acid mine water associated with uranium mine remediation problems. Due to the uncertainty in the relative amount of positively charged and neutral solution species, the uncertainty in the thermodynamic data also infers uncertainty in the retardation behavior. The carbonated water system shows the largest uncertainties in speciation calculation. Therefore, the model predictions of total uranium solubility have a broad range. The effect of data uncertainty in transport prediction is further illustrated by a prediction of the time when eluted uranium from the column exceeds a threshold value. All of these Monte Carlo transport calculations consume large amounts of computing time.

Investigating the Mechanism of Uranium Removal by Zerovalent Iron

Environmental Context.Groundwater is the water that fills the spaces between sand, soil, and rock below the water table. It discharges into ecologically sensitive wetlands and is used as drinking water or in agriculture and industry. Inappropriate waste disposal and poor land management can contaminate groundwater and may minimize its use for decades. The common method for pumping contaminated groundwater to the surface for treatment is costly and labour intensive. Zerovalent iron is a new, more cost-effective method of groundwater remediation. Abstract. Zerovalent iron (ZVI) has been proposed as a reactive material in permeable in situ walls for groundwater contaminated by metal pollutants. For such pollutants that interact with corrosion products, the determination of the actual mechanism of their removal is very important to predict their stability in the long term. From a study of the effects of pyrite (FeS2) and manganese nodules (MnO2) on the uranium removal potential of a selected ZVI material, a test methodology (FeS2–MnO2 method) is suggested to follow the pathway of contaminant removal by ZVI materials. An interpretation of the removal potential of ZVI for uranium in the presence of both additives corroborates coprecipitation with iron corrosion products as the initial removal mechanism for uranium.

LJUNGSKILE: a program for assessing uncertainties in speciation calculations.

Speciation calculations are often the base upon which further and more important conclusions are drawn, e.g., solubilities and sorption estimates used for retention of hazardous materials. Since speciation calculations are based on experimentally determined stability constants of the relevant chemical reactions, the measurement and experimental uncertainty in these constants will affect the reliability of the simulation output. The present knowledge of the thermodynamic data relevant for predicting the behaviour of a complex chemical system is quite heterogeneous. In order to predict the impact of these uncertainties on the reliability of a simulation output requires sophisticated modelling codes. In this paper, we will present a computer program, LJUNGSKILE, which utilises the thermodynamic equilibrium code PHREEQC to statistically calculate uncertainties in speciation based on uncertainties in stability constants. A short example is included.

Computer-intensive methods for uncertainty estimation in complex situations

Abstract International regulations require the specification of an uncertainty estimate related to experimental data. In chemistry, the situation that a straightforward statistical machinery is not available for assessing the uncertainty of a datum extracted from complex systems often occurs. Non-linearity, non-normality, correlation and other nuisance factors add to the complications. Monte Carlo resampling algorithms, in combination with abundant fast computing power, have made techniques feasible that do not require profound mathematical insight, but, nevertheless, are fairly general. Assessment of confidence limits at different levels of correctness is discussed using standard and bootstrap methods. Inferiority of standard normal approaches becomes evident even in mildly non-linear situations.

Threshold bootstrap target factor analysis study of neodymium with pyridine 2,4 dicarboxylic acid N-oxide—an investigation of traceability

Threshold bootstrap computer-assisted target factor analysis (TB CAT) has been applied to the evaluation of UV-Vis spectra of Nd(III) in aqueous solutions containing varying amounts of pyridine 2,4-dicarboxylic acid N-oxide (dipyr NO). Using the total uncertainty budget concept, probability distributions of the conditional formation constant(s) of the respective Nd(III) dipyr NO species have been evaluated. The experimental data are compatible with two different systems: a single Nd(III) dipyr NO species with logbeta(11)=2.8+/-0.11or, alternatively, a two species system with logbeta(11)=3.7+/-0.35and logbeta(12)=5.2+/-0.8. The results seem to favor the single species interpretation. However, a discussion of the influence of spectral correlation indicates that the situation should be considered with caution. The results are compared to a previous TB CAT study on Nd(III) picolinic acid N-oxide interaction illustrating the advantage of arguing in terms of probability density functions. The pyridine N-oxides with substituted carboxylic acids are structurally poorly investigated. Hence, this study calls for more structural information on rare earth pyridine N-oxide derivates.

A retraceable method to assess uncertainties in solubility estimations exemplified by a few americium solids

Abstract An important safety function in ensuring nuclear waste safety is the possible formation of solubility limiting phases that incorporate actinides and other critical nuclides. For a credible demonstration of such a safety function, it is essential that uncertainties related to the choice of models, calculation cases, and parameter values are evaluated. In this paper, a method is outlined to evaluate thermodynamic data uncertainty and to analyse situations in the presence of uncertainty. Americium(III) solubility in an Aspo groundwater is taken as an illustrative example. Methods include a sensitivity analysis to single out critical parameters and a Monte Carlo analysis to provide an empirical probability distribution of americium solubility taking into account relevant uncertainty contributions. The analysis shows that the solubility products of 241Am(III) phases are the most sensitive parameters. Aqueous 241Am(III) species, on the contrary, are of less importance while the stability of several other complexes incorporating major element ions have considerable impact.

Mechanism of Uranium Fixation by Zero Valent Iron: The Importance of Co-precipitation

The co-precipitation of U (VI) with iron corrosion products from aqueous solutions by zero valent iron was investigated. The evidence of co-precipitation was demonstrated by conducting experiments with well characterized scrap iron, pyrite and a mixture of both materials with experimental durations of up to four months. Results indicate that under anoxic conditions only less than one tenth of the immobilized U(VI) was associated with the surface of scrap iron, whereas the remaining amount is entrapped in aging corrosion products.