[1] Particle tracking in the time domain has received increasing attention as a technique for robustly simulating transport along one-dimensional subsurface pathways. Using a stochastic Lagrangian perspective, integral representations of transport including the effects of advection, longitudinal dispersion, and a broad class of retention models are derived; Monte Carlo sampling of that integral leads directly to new time domain particle tracking algorithms that represent a wide range of physical phenomena. Retention-time distributions are compiled for key retention models. An extension to accommodate linear transformations such as decay chains is also introduced. Detailed testing using first-order decay chains and four retention models (equilibrium sorption, limited diffusion, unlimited diffusion, and first-order kinetic sorption) demonstrate that the method is highly accurate. Simulations using flow fields produced by large-scale discrete-fracture network simulations, a transport problem that is difficult for conventional algorithms, demonstrate that the new algorithms are robust and highly efficient.

Experimental studies on lithium in concentrated lithium hydroxide aqueous solutions have confirmed the formation of a passive film on the surface that controls the rate of the dissolution process. 1,2 The prevailing consensus is that the layer is an oxide-hydroxide film. However, it has been shown in Part I of this series 3 that, under opencircuit conditions, lithium hydride and lithium hydroxide are stable phases, whereas lithium oxide cannot form even as a metastable phase. Accordingly, the passive film formed under open-circuit conditions is of a bilayer lithium hydride/lithium hydroxide structure.We analyze experimental data for lithium dissolution in terms of a theoretical model postulating a lithium hydride-hydroxide bilayer structure for the film. The model incorporates charge-transfer phenomena, hydrogen evolution, barrier-layer formation and dissolution, and metal dissolution, and allows for a change in the porosity of the outer layer as a function of the electrolyte composition and applied voltage. The model succeeds in explaining a wide panorama of experimental data with minimal variation of the model parameters. Indeed, the variation of only one parameter, related to the porosity of the outer layer, is sufficient to account for all the observed trends in the total current density and in the anodic and cathodic partial current densities with respect to the independent experimental variables (voltage and electrolyte and additive concentrations).Theory Thermodynamic calculations on the Li/H 2 O system indicate that at the open-circuit potential (OCP) of the system (about Ϫ2.8 V SHE 1,4-6 ), the phase in contact with lithium is lithium hydride. 3 The existence of a porous LiOH layer on the metal surface can be inferred from the limited solubility of this compound in water (and especially in the concentrated KOH solution used in this study), and from the dissolution/precipitation mechanism that was postulated by Littauer and co-workers, 1,2 who performed much of the early work on the electrochemistry of the Li/H 2 O system. We postulated that under open-circuit conditions a hydride barrier layer forms next to the metal, whereas the outer layer consists of LiOH.

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Detailed potential/pH stability diagrams for the lithium‐water system at 25°C have been derived as part of a program to describe the electrochemistry of the lithium anode in aqueous alkaline environments. The diagrams include lithium hydride as a stable solid phase at sufficiently negative potentials, and it is postulated that the passive film on Li in contact with concentrated alkaline solutions, under open‐circuit conditions, comprises a LiH barrier layer covered with a hydrated LiOH outer layer. The postulated existence of Li2O as a barrier layer, under open‐circuit conditions, is inconsistent with the thermodynamic data for metallic lithium in contact with concentrated LiOH and LiOH + KOH solutions. © 1999 The Electrochemical Society. All rights reserved.

An impedance model for the electrochemical dissolution of lithium in alkaline solutions is presented. The construction technique of the impedance function depends on calibration to steady-state properties, described in Part II of this series. The model, which is based on the point defect model for the growth and breakdown of passive films, is used to identify effects of various electrolyte solutes on the properties of the lithium film. The high frequency experimental impedance data are explained by the existence of a capacitance that is voltage and frequency dependent, a property that is theoretically rationalized. It is concluded that electrolyte solutes influence the rate of water transport through the outer layer, rate constants, the polarization of the barrier layer/outer layer interface, and the porosity of the outer layer. Based on the shape of predicted impedance signatures, it is suggested that the derived impedance equation may be applicable to other systems.

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