Alain Clément

Modeling mineral/solution interactions: the thermodynamic and kinetic code KINDISP

Abstract The kinetic and thermodynamic geochemical model KINDISP (KINetics of DISsolution and Precipitation) describes the interactions between minerals and aqueous solutions, taking into account the irreversible dissolution of the reactants and the reversible precipitation of secondary products. The general laws included in the model are based on the theory of the Thermodynamics of Irreversible Processes. The water/rock interactions at low temperature are interpreted in a classical manner with the help of the Theory of the State of Transition and the chemistry of surface coordination. The mechanism which limits the rate of mineral dissolution or precipitation, the slowest one in successive irreversible reactions, is represented either by the aqueous molecular diffusion of an elementary entity (atom, molecule, etc.) or by the surface reaction in a broad sense. At each step of the calculation, KINDISP computes the reaction rates for each mineral reacting in the system and selects the slowest rate to represent the dissolution or precipitation law in this phase. The growth of secondary minerals is simulated in the domain of oversaturation (in nonequilibrium) after a nucleation step. The KINDISP model already has been used to simulate natural or induced water/rock interactions, not only at low temperatures, for example to study the effects of acid rain on surface weathering of a granite formation, estimate the formation time of a laterite layer, and the effects of pollution on the environment, but also at higher temperatures, for example to describe and account for diagenetic reactions in sedimentary basins for the purpose of exploiting the reservoirs as well as to study a system of hydrothermal reactions caused by heat storage or disposal of nuclear waste packages.

A computer program for the simulation of evaporation of natural waters to high concentration

Abstract EQL/EVP is a FORTRAN 90 program that simulates the evaporation of dilute waters as well as concentrated brines. The code calculates stepwise the composition of the evaporated solution and the amounts of precipitated minerals. Activity coefficients are based on the Pitzers interaction model which allows calculation to high ionic strength. The Newton–Raphson method is used to solve a set of linear mass-balance and non-linear mass-action equations. The simulation may be carried out in equilibrium mode where minerals are allowed to redissolve into the solution or in fractional crystallization mode where minerals are removed as they precipitate. Temperature dependence of various parameters and mineral solubility products are tentatively included between 0 and 50°C. The code processes all invariant points: those where the activity of water is constrained by several minerals and the invariant end-points. The code may be a useful tool for understanding hydrological and geochemical processes in arid regions.

Numerical validation of a Eulerian hydrochemical code using a 1D multisolute mass transport system involving heterogeneous kinetically controlled reactions

It is demonstrated that at steady state, the 1D thermo-kinetic hydrochemical Eulerian mass balance equations in pure advective mode are indeed identical to the governing mass balance equations of a single reaction path (or geochemical) code in open system mode. Thus, both calculated reaction paths should be theoretically identical whatever the chemical complexity of the water–rock system (i.e., multicomponent, multireaction zones kinetically and equilibrium-controlled). We propose to use this property to numerically test the thermo-kinetic hydrochemical Eulerian codes and we employ it to verify the algorithm of the 1D finite difference code KIRMAT. Compared to the other methods to perform such numerical tests (i.e., comparisons with analytical, semi-analytical solutions, between two Eulerian hydrochemical codes), the advantage of this new method is the absence of constraints on the chemical complexity of the modelled water–rock systems. Moreover, the same thermo-kinetic databases and geochemical functions can be easily and mechanically used in both calculations, when the numerical reference comes from the Eulerian code with no transport terms (u and D=0) and modify to be consistent with the definition of the open system mode in geochemical modelling. The ability of KIRMAT to treat multicomponent pure advective transport, subjected to several kinetically equilibrium-controlled dissolution and precipitation reactions, and to track their boundaries has been successfully verified with the property of interest. The required numerical validation of the reference calculations is bypassed in developing the Eulerian code from an already checked single reaction path code. A forward time-upstream weighting scheme (a mixing cell scheme) is used in this study. An appropriate choice of grid spacing allows to calculate within the grid size uncertainty the correct mineral reaction zone boundaries, despite the presence of numerical dispersion. Its correction enables us to improve the convergence and to extend the numerical test to mixed advective–dispersive mass transport. However, the skewness factor involves numerical oscillations that prevent to compute different grid spacing. The use of a different chemically controlled time step constraint in both calculations induces some inconsistencies into the validation tests. This numerical validation method may be applied as well as to check a thermo-kinetic hydrochemical finite element based code, from a 1D heterogeneous systems, and 2D–3D systems provided that they are designed so as to be 1D equivalent. A one-step algorithm and the use of a numerical reference coming from the Eulerian code to be tested ensure the potential success (accuracy) of the numerical validation method.

Optimizing the Power Take Off of a Wave Energy Converter With Regard to the Wave Climate

Considered as a source of renewable energy, wave is a resource featuring high variability at all time scales. Furthermore wave climate also changes significantly from place to place. Wave energy converters are very often tuned to suit the more frequent significant wave period at the project site. In this paper we show that optimizing the device necessitates accounting for all possible wave conditions weighted by their annual occurrence frequency, as generally given by the classical wave climate scatter diagrams. A generic and very simple wave energy converter is considered here. It is shown how the optimal parameters can be different considering whether all wave conditions are accounted for or not, whether the device is controlled or not, whether the productive motion is limited or not. We also show how they depend on the area where the device is to be deployed, by applying the same method to three sites with very different wave climate.

Sorption/Desorption Processes of Uranium in Clayey Samples of the Bangombe Natural Reactor Zone, Gabon

Experimental studies have been undertaken in order to provide new insights into the relative efficiency of the different mineral phases and sorption processes for the control of U retention in the weathered zones surrounding the natural nuclear reactor at Bangombe (Oklo, Gabon). Clayey and Fe-oxihydroxides rieh samples from the oxidizing weathered zones located above the reactor were examined. An experimental study of uranium adsorption/desorption processes in these samples was carried out using a uranium isotope exchange technique in order to estimate the proportion of uranium adsorbed on mineral surfaces. A sequential extraction technique was used to identify the major U-containing minerals in the samples. In the U-rich iron crust rocks dose to the reactor, the fraction of total uranium adsorbed at mineral surfaces is small. Extraction experiments reveal that a large part of uranium is associated to Fe-oxihydroxides, to minor P-rich phases, and presumably to Mn-oxihydroxides. A possible mechanism for U retention is an incorporation into the structure of iron oxihydroxides and/or of ferric phosphates occurring as surface precipitates on Fe-oxihydroxides. Traces of autunite-like mineral are also present in the zone. For the clayey samples in the weathering profile, it may be inferred that several processes and minerals contribute significantly to U retention: adsorption processes occurring mainly at clay surfaces, association with traces of Mn-containing carbonates and iron oxihydroxides. A significant proportion of total U is adsorbed at mineral surfaces and is thereby easily accessible to weathering solutions.

Adsorption of Neptunium(V) on Hydrargilite

The sorption of tracer concentrations of neptunium(V) on hydrargilite was studied both under C02-free conditions and with aqueous solutions with a low total concentration of carbonate (CIO 4 mol · L ). The fractional uptake of Np(V) by hydrargilite was measured as a function of pH (4—9), using various conditions of ionic strength (0.1, 0.01 and 0.001 mol L NaC104) and two solid concentrations (10 and 5 0 g L _ 1 ) . At nearly neutral and high pH values, hydrargilite acts as an efficient trap for Np(V) at trace levels. The pH is the main solution parameter that governs the Np(V) sorption, whereas major changes in ionic strength induce no changes in Np(V) uptake. The presence of small amounts of carbonate in solution has no measurable effect on Np(V) sorption. The experimental results were modeled using the non-electrostatic surface complexation model (NEM), assuming that only monodentate surface species (SOH) and the neptunyl ion NpOJ participate in the adsorption reaction. To account for effects of the acid/base behaviour of aluminol groups at the hydrargilite surface, the protonation and dissociation constants of the surface groups were determined for the NEM from C02-free Potentiometrie titrations. Application of the non-electrostatic model provided an accurate description of Np(V) sorption at trace levels on hydrargilite.

General implications of aluminium speciation-dependent kinetic dissolution rate law in water–rock modelling

Recent experimental and theoretical work has demonstrated that the dissolution rates of many aluminosilicate minerals are inversely proportional to the activity of aqueous Al3+. The consequences of these observations on the rates of natural geochemical processes have been calculated by the KIRMAT hydrochemical code. Comparisons are performed at the steady state limit of the pure advective transport through a homogeneous semi-infinite isothermal porous media at 25 and 150°C. K-feldspar, albite, and muscovite dissolution kinetics are studied over a broad range of initial pH (2–10) and aluminium concentration (from 1×10−9 to 1×10−3 molal) matching most the natural conditions. Regardless of the mineral, the characteristic distance requires to reach equilibrium (leq) is two and three orders of magnitude larger and lower than predicted using the standard Transition State Theory (TST) law, respectively. The maximum decrease in muscovite and alkali-feldspar dissolution rates due to aqueous aluminium at 25°C is found at near to neutral pH, and at 150°C it is found at basic pH. The maximum dissolution rate increase at 25°C at acid pH, but at 150°C it is found at basic pH. These results demonstrate that consideration of the effect of the aluminium speciation on aluminosilicate dissolution rates is required to improve the accuracy in water–rock interaction modelling.

Effect of pH and Carbonate Concentration in Solution on the Sorption of Neptunium(V) by Hydrargilite: Application of the Non-Electrostatic Model

Batch experiments were performed in order to investigate the influence of pH, neptunium concentration, and concentration of carbonate in Solution on the Sorption of Np(V) on a synthetic hydrargilite. The concentration of Np{V) at hydrargilite surface was found to be proportional to the concentration of dissolved Np(V) at equilibrium, within the Np(V) concentration ränge =10 to 10 M, in C02-free 0.1 M NaC104 solutions at pH = 7.45±0.05. The sorption of tracer concentration of Np(V) on hydrargilite was strongly pH-dependent. Increasing the total concentration of carbonate from about 0 to 10^ M in the aqueous phase resulted in a net decrease of the extent of Np(V) uptake in the alkaline pH region. Surface complexation modeling suggested that Np(V) sorption behavior in carbonate solutions resulted mainly from a competition between surface complexation and aqueous carbonate complexation of NpOJ. The surface complexation reactions and conditional constants proposed in this study for the non-electrostatic model allowed an accurate description of the sorption behavior of Np(V) on hydrargilite over a wide ränge of pH and carbonate concentration of the aqueous phase.

Calcite formation by hydrothermal carbonation of portlandite: complementary insights from experiment and simulation

The present study complements experimental results of calcite nanoparticle formation by hydrothermal carbonation of calcium hydroxide by a simulation strategy, in which both the chemical evolution of the aqueous solution and the solid phases – dissolution of portlandite and nucleation and growth of secondary calcite particles – are considered. The simulation is performed with the help of the NANOKIN code. It includes a full treatment of speciation processes in the aqueous solution, a rate equation for the dissolution of primary minerals, and a full account of nucleation and growth processes during the formation of new particles. This strategy has allowed us to decipher the various steps in the mineral transformation and solution evolution. The comparison between experiment and simulation puts strong constraints on simulation parameters, while modeling can give information on in situ conditions, currently not often available experimentally.

Precipitation mechanism of amorphous silica nanoparticles: A simulation approach

HYPOTHESISnDespite its importance in numerous industrial and natural processes, many unsolved questions remain regarding the mechanism of silica precipitation in aqueous solutions: order of the reaction, role of silica oligomers, existence of an induction time and characteristics of the particle population. This may be traced back, in past models, to the lack of account of the first stages of nucleation, size dependence of the growth law, and full particle population. COMPUTATIONAL METHOD: A microscopic description of the nucleation and growth of amorphous silica nanoparticles is achieved which reproduces a large set of experimental measurements, under various thermodynamic conditions. The time evolution of the solution supersaturation and of the precipitate characteristics is established.nnnFINDINGSnA growth law of order 6 allows reproducing experimental results, without being correlated to the presence of silica oligomers in the aqueous solution. The saturation plateaus are shown not to be due to an induction period. The characteristics of the particle population are more complex than assumed by simple precipitation models (Johnson-Mehl-Avrami-Kolmogorov or Chronomal models) and strongly depend on how supersaturation is reached. Such a microscopic approach thus proves to be well suited to elucidate the mechanism of nanoparticle formation in natural and industrial contexts.