Browsing by Author "Brown, PL"

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    Guidelines for thermodynamic sorption modelling in the context of radioactive waste disposal
    (Elsevier, 2013-01-01) Payne, TE; Brendler, V; Ochs, M; Baeyens, B; Brown, PL; Davis, JA; Ekberg, C; Kulik, DA; Lützenkirchen, J; Missana, T; Tachi, Y; Van Loon, LR; Altmann, S
    Thermodynamic sorption models (TSMs) offer the potential to improve the incorporation of sorption in environmental modelling of contaminant migration. One specific application is safety cases for radioactive waste repositories, in which radionuclide sorption on mineral surfaces is usually described using distribution coefficients (K-d values). TSMs can be utilised to provide a scientific basis for the range of K-d values included in the repository safety case, and for assessing the response of K-d to changes in chemical conditions. The development of a TSM involves a series of decisions on model features such as numbers and types of surface sites, sorption reactions and electrostatic correction factors. There has been a lack of consensus on the best ways to develop such models, and on the methods of determination of associated parameter values. The present paper therefore presents recommendations on a number of aspects of model development, which are applicable both to radioactive waste disposal and broader environmental applications. The TSM should be calibrated using a comprehensive sorption data set for the contaminant of interest, showing the impact of major geochemical parameters including pH, ionic strength, contaminant concentration, the effect of ligands, and major competing ions. Complex natural materials should be thoroughly characterised in terms of mineralogy, surface area, cation exchange capacity, and presence of impurities. During the application of numerical optimisation programs to simulate sorption data, it is often preferable that the TSM should be fitted to the experimentally determined K-d parameter, rather than to the frequently used percentage sorbed. Two different modelling approaches, the component additivity and generalised composite, can be used for modelling sorption data for complex materials such as soils. Both approaches may be coupled to the same critically reviewed aqueous thermodynamic data sets, and may incorporate the same, or similar, surface reactions and surface species. The quality of the final sorption model can be assessed against the following characteristics: an appropriate level of complexity, documented and traceable decisions, internal consistency, limitations on the number of adjustable parameter values, an adequate fit to a comprehensive calibration data set, and capability of simulating independent data sets. Key recommendations for the process of TSM development include: definition of modelling objectives, identification of major decision points, a clear decision-making rationale with reference to experimental or theoretical evidence, utilisation of a suitable consultative and iterative model development process, testing to the maximum practicable extent, and thorough documentation of key decisions. These recommendations are consistent with international benchmarks for environmental modelling. Copyright © 2013, Elsevier
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    The kinetics of chlorite dissolution
    (Elsevier, 2007-03-15) Lowson, RT; Brown, PL; Comarmond, MJ; Rajaratnam, G
    A model for the dissolution of chlorite has been developed based on fast ligand assisted proton attack of the alumina tetrahedra within the alumina-silica lattice followed by slower dissolution of the remnant silica lattice. While the rate determining step is within the silica dissolution regime, the rate is a function of the H+ and Al3+ concentrations and the dominant aqueous Al species. Individual rates may be described by a generic rate equation applicable across the spectrum of Al species: where rn is the rate subscripted for the nth Al species, k is the rate constant of the rate controlling step, K is the surface exchange constant, β is the solution stability constant subscripted for the Al species, a is the species activity subscripted for species and raised to the power of the stoichiometry, p and q are stoichiometric coefficients, z is the ligand charge and τ is the fractional coefficient for the precursor of the rate defining step. The observed rate is the sum of the individual rates. When the observed rate is in a domain of dominance for a single aluminium species and in the absence of strong complexing agents such as oxalate, the observed rate is proportional to (a3H+/aAl3+)τn. The model is supported by experimental data for the dissolution of chlorite over a pH range of 3–10 and temperature range 25–95°C. The results have hydrometallurgical application. © 2007, Elsevier Ltd.
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    Management and quantification of the impact of acid rock drainage
    (Royal Australian Chemical Institute & Australasian Society of Ecotoxicology, 2002-07-21) Brown, PL
    Acid rock drainage from sulfidic waste piles is recognised as one of the most significant problems facing the global mining industry, costing the industry billions of dollars annually. Acid drainage affects all sectors of the industry including coal, precious metals, base metals, iron ore and uranium. A number of government and industry initiatives have been formed to combat the problem. These initiatives have focused on the development of scientifically-based technologies for the management of wastes, resulting in a reduction (or minimisation) of the potential for ecological / environmental risk. This presentation will outline some of the best practice tools and techniques developed to quantify and manage acid rock drainage. It will provide an overview of the quantification of the effectiveness of control measures, determination of the level of acceptance ecological impact and appropriate means of measuring this in the field, what is required for prediction of impact and effluent quality and over what time frame realistic predictions can be made. It will also examine the implementation of management strategies through the mining life cycle. © The Authors.
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    Thermochemical data (log K) for environmentally-relevant elements : 1. H, Na, K, Ca, Mg, Fe, Mn, U, A1, Pb, Zn, Cu and Cd with model fulvic acid (Aspartate, citrate, malonate, salicylate and tricarballyate)
    (Australian Nuclear Science and Technology Organisation, 1999-12) Markich, SJ; Brown, PL
    This study provides an extensive stability constant (log K) database suitable for calculating the speciation of selected environmentally-relevant elements (H Na K Ca Mg Fe Mn U Al Pb Zn Cu and Cd) in an aqueous system where a model fulvic acid (comprising aspartic citric malonic salicylic and tricarballylic acids) is used to simulate metal binding by dissolved organic material Stability constants for inorganic metal complexes and minerals were selected primarily from critical literature complications and/or reviews. In contrast few critically evaluated data were available for metal complexes with aspartic citric, malonic, salicylic and tricarballylic acids. Consequently, data from original research articles were carefully evaluated and compiled as part of the study, following defined selection criteria. To meet the objective of compiling a comprehensive and reliable database of stability constants, all relevant equilibria and species, ranging from simple binary metal complexes to more complex ternary, and even quaternary, metal complexes were included where possible. In addition to the selection of stability constants from empirical sources, estimates of stability constants were performed when this could be done reliably, based on the unified theory of metal ion complexation and/or linear free energy relationships. The stability constants are given as common logarithms (log10) in the form required by the HARPHRQ geochemical code and refer to the standard state, i.e. 298.15 k (25—C), 106 Pa (1 atm) and, for all species, infinite dilution (ionic strength = 0 moI L-1). In addition to the compilation of stability constant data, an overview is given of geochemical speciation modeling in aqueous systems and available conceptual models of metal binding by humic substances.
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    Unified theory of metal ion complex formation constants
    (Australian Atomic Energy Commission, 1987-04) Brown, PL; Sylva, RN
    A new concept in solution chemistry, electronicity, has been derived and developed. Electronicity describes quantitatively the ability of a ligand to complex a metal ion which, in turn, allows the prediction of the formation constants of metal ion complexes. The principle of electronicity quantifies onto one scale the earlier concepts of hard and soft acids and bases.
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    Uranium and other contaminant migration in groundwater at a tropical Australian Uranium Mine
    (Elsevier, 1998-12-15) Brown, PL; Guerin, M; Hankin, SI; Lowson, RT
    Hydrogeochemical modelling (utilising the modelling tools MODFLOW, MT3D and HARPHRQ) has been used in conjunction with laboratory-based experiments and a field monitoring program to investigate the fate of uranium and other contaminants in excess water sprayed on a 33 ha region of the Ranger Uranium Mine (RUM), northern Australia. The results indicate that uranium is retained in the surficial layer of the lateritic soils of the area. Conservative contaminants are not retained by the soils and are transported into the groundwater. Subsequently, they migrate relative to the groundwater flow rate towards the river system down hydraulic gradient of the irrigation area. © 1998 Elsevier Science B.V.