A number of geodisposal concepts for intermediate level radioactive waste involve geological emplacement within cementitious repositories. Such facilities, once rehydrated with groundwater, will ...create high pH environments due to aqueous phase reaction of the cements. This work focuses on the interactions of several important long-lived radionuclide cations with dissolved organic matter (DOM) constituents (humic and fulvic acids) under high pH conditions. We also sought to test the comprehensive speciation model WHAM/Humic Ion Binding Model VII for these specific conditions. Results for Th demonstrate high fractions present as organic complexes at all pH values. Binding of neptunyl to DOM shows a maximum over the pH range expected within an evolving repository. Uranyl exhibits decreasing binding with pH, however, the majority of metal in solution is present as organic complexes under the lower pH conditions investigated (10–10.5). We have updated the WHAM/Model VII binding values for UO₂ ²⁺, and have for the first time added NpO₂ ⁺ values to the database. These updates now allow application of the model for more complex mixtures across the entire repository pH range. Calculations for three simulated cement interstitial waters (representing different degradation phases) suggest U(VI) and Np(V) are not likely to be significantly bound to DOM under these conditions.
The DGT Chelex binding phase has not been tested for binding efficiency over the extreme high pH range (i.e., 10 to 13). Here, we examined the uptake efficiency of the gel-encapsulated Chelex cation ...exchange resin binding phase when in direct contact with solutions of radionuclides of different oxidation states over the circumneutral to high pH range (∼7 to 13). Results show that the Chelex binding gel is suitable for Eu(3+) for circumneutral pH, for UO2(2+) up to at least pH 10.7 and for NpO2(+) up to at least pH 11.7. Application may be appropriate at higher pH values but testing of complete solution deployment units will be required. This work provides the framework to use DGT as a tool for the study of high pH radionuclide systems.
The WHAM-FTOX model quantifies the combined toxic effects of protons and metal cations towards aquatic organisms through the toxicity function (FTOX), a linear combination of the products of ...organism-bound cation and a toxic potency coefficient for each cation. We describe the application of the model to predict an observable ecological field variable, species richness of pelagic lake crustacean zooplankton, studied with respect to either acidification or the impacts of metals from smelters. The fitted results give toxic potencies increasing in the order H+ < Al < Cu < Zn < Ni. In general, observed species richness is lower than predicted, but in some instances agreement is close, and is rarely higher than predictions. The model predicts recovery in agreement with observations for three regions, namely Sudbury (Canada), Bohemian Forest (Czech Republic) and a subset of lakes across Norway, but fails to predict observed recovery from acidification in Adirondack lakes (USA).
•We used the model WHAM-FTOX to relate combined toxic effects of protons and metal cations towards lake crustacean zooplankton.•The fitted results give toxic potencies increasing in the order H+ < Al < Cu < Zn < Ni.•Due to non-toxicity factors, observed species richness is mostly either close to or lower than the toxicity-predicted value.
The WHAM-FTOX model has been applied to field data for pelagic lake crustacean zooplankton. The fitted results give metal toxic potencies increasing in the order H+ < Al < Cu < Zn < Ni.
The WHAM-FTOX model uses chemical speciation to describe the bioavailability and toxicity of proton and metal mixtures (including Al) to aquatic organisms. Here, we apply the previously parameterised ...model to 45 UK and Norwegian upland surface waters recovering from acidification, to compare its predictions of the maximum species richness of the macroinvertebrate Orders Ephemeroptera, Plecoptera and Trichoptera (SR-EPT) with time-series observations. This work uses data from two national scale survey programmes, the Acid Waters Monitoring Network in the UK and a lakes survey in Norway. We also investigate data from a long-studied catchment, Llyn Brianne in Wales. For the national surveys, model results relate well with actual trends, with Regional Kendall analysis indicating biological recovery rates for both actual and predicted species richness that are generally consistent (1.2–2.0 species per decade). However, actual recovery rates in AWMN lakes were less than in the rivers (0.6 vs. 2.0 species per decade), whilst predicted rates were similar (1.7 vs. 2.0). Several sites give a very good fit between model predictions and observations; at these sites chemistry is apparently the principal factor controlling limits of species richness. At other sites where there is poorer agreement between model predictions and observations, chemistry can still explain some of the reduction in species richness. However, for these sites, additional (un-modelled) factors further suppress species richness. The model gives a good indication of the extent of these un-modelled factors and the degree to which chemistry may suppress species richness at a given site.
This thesis consists of several studies relating to small-scale heterogeneity in sediments. The principal aim was to further our understanding of processes occurring at microniches. The individual ...studies consist of: 1) a critical review of previous studies of microniches that used probes with high spatial resolution and modelling approaches; 2) an experimental study of analysis of oxyanions in sediment at high resolution that applied a newly developed preparation method for a combined AgI/FeOOH binding phase, to investigating processes occurring at a sulphidic microniche within a freshwater sediment; 3) analysis of the relationship between trace metal (cobalt) and iron and manganese in a marine sediment using DGT, although this is not directly related to microniches, these data are useful in modelling the release of microniche trace metal from authigenic oxides; 4) the development and application of a three-dimensional diagenetic model to investigate conceptually the geochemical behaviour of microniches under different conditions, and to interpret modelled observations in terms of data from the literature and known trace element geochemistry. The key results/conclusions from both the laboratory and modelling studies are: 1) for a freshwater sediment, depletions in anions (of P, V, As) at a microniche of elevated sulphide were observed and the behaviour of phosphate at this niche was attributed to uptake associated with elevated activity of sulphate reducing bacteria; 2) modelled scenarios, with varying microniche properties, were shown to be relevant to experimental observations reported in the literature. The preferential deposition of FeS at the edge of microniches (with lifetimes of 2.5-5 days), forming ‘crustal’ deposits was demonstrated. The modelled data indicated that microniches may be significant in terms of the formation of some trace element sulphides. This thesis also contains an assessment of the significance of microniche processes and a discussion of priorities for future work.
This thesis consists of several studies relating to small-scale heterogeneity in sediments. The principal aim was to further our understanding of processes occurring at microniches. The individual ...studies consist of: 1) a critical review of previous studies of microniches that used probes with high spatial resolution and modelling approaches; 2) an experimental study of analysis of oxyanions in sediment at high resolution that applied a newly developed preparation method for a combined AgI/FeOOH binding phase, to investigating processes occurring at a sulphidic microniche within a freshwater sediment; 3) analysis of the relationship between trace metal (cobalt) and iron and manganese in a marine sediment using DGT, although this is not directly related to microniches, these data are useful in modelling the release of microniche trace metal from authigenic oxides; 4) the development and application of a three-dimensional diagenetic model to investigate conceptually the geochemical behaviour of microniches under different conditions, and to interpret modelled observations in terms of data from the literature and known trace element geochemistry. The key results/conclusions from both the laboratory and modelling studies are: 1) for a freshwater sediment, depletions in anions (of P, V, As) at a microniche of elevated sulphide were observed and the behaviour of phosphate at this niche was attributed to uptake associated with elevated activity of sulphate reducing bacteria; 2) modelled scenarios, with varying microniche properties, were shown to be relevant to experimental observations reported in the literature. The preferential deposition of FeS at the edge of microniches (with lifetimes of 2.5-5 days), forming ‘crustal’ deposits was demonstrated. The modelled data indicated that microniches may be significant in terms of the formation of some trace element sulphides. This thesis also contains an assessment of the significance of microniche processes and a discussion of priorities for future work.
The desire to measure low metal concentrations in natural waters stems principally from the aim to understand the effects of trace metals on aquatic biota and to understand the fate of metal ...pollutants. Trace metals rarely exist in the free ion form, with distribution of the different species being dependent upon pH, Eh, and the types of organic/inorganic ligands and colloidal surfaces. Use of cation exchange resins, particularly Chelex-100, has been developed over the past few decades and enables preconcentration to yield, upon elution, measurable concentrations of trace metals. These resins can also yield data on the speciation (by measuring lability) of a metal in solution by adjusting the contact time of the sample with the resin. Several speciation schemes have been developed where separation techniques (such as dialysis) are coupled with Chelex-column methods (short contact time) and Chelex-batch methods (long contact time). With the exception of recent equilibrium based studies Chelex is always used in an environment where the number of binding sites is in large excess compared to the number of competing cations, meaning competition effect are not significant, although kinetic factors are important is Chelex lability studies. Nitric acid elution generally yield good recovery rates (>90%) and stepwise elution has been shown to be the most effective for both metal recovery and minimising the eluent volume. The development of DGT speciation studies may bring about a technique that is less complex but easier to undertake and yields simple yet useful complexation data. The potential for IDA-resins to yield free-ion concentrations based on equilibrium-based sorption is a beneficial development that is currently being investigated. This review investigates the use of cation exchange resins by assessing the resin properties, the techniques developed and the optimum sorption conditions.
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