Transport coupling of ions: Influence of ion pairing and pH gradient — Application to the study of diagenetic fluxes

Abstract

Experiments have been performed with the closed-capillary technique to investigate diffusion coupling processes in multicomponent aqueous electrolytes: 1. (1) the influence of a supposed ion-pair formation has been studied: a typical experiment consists in inducing the migration of sulfate ion by the migration of a salt (LiCl) and looking at the influence of the addition of a definite quantity of Mg 2+ ion. The same kind of experiment has been done with the Ca 2+ ion. The results are compared with theoretical predictions based on solutions to the diffusion equations. It is observed that the simple treatment in which the aqueous solution is considered as ideal (no departure from ideality, besides the association phenomenon) can be satisfactory if the equilibrium constant of the ion-pairing reaction is replaced by an effective constant. 2. (2) A study of the influence of a gradient of pH on the transport of ions is presented. This study has been approached on the example of sulfate and phosphate ions. A complete theoretical description of the phenomenon, based on the transport equations for ideal solutions, is presented. It is shown that some approximate interesting results can be obtained. Experiments on sulfate and phosphate, using the closed capillary, are also presented. Both experimental and theoretical approaches show that the pH-gradient effect is generally small compared to self-diffusion and to the electric field effect. Nevertheless experimental results on phosphate, confirmed by numerical simulations, give an example of a situation in which a noticeable effect can be obtained. Moreover, considerations on the orders of magnitude of fluxes lead to the conclusion that a large pH-gradient effect may be observed for the carbonate system at a lacustrine water-sediment interface.