We present a convective description of acid‐base transport in porous media which is based on classical one‐component nonlinear Chromatographic theory applied to the acidity of the system. In the mobile phase the solution acidity is given by c = [H+]t ‐ [OH−]t, where [H+]t and [OH−]t are the total solution concentrations of H+ and OH−, respectively. In the stationary phase the surface acidity corresponds to the charge density σ, which is commonly presented as a function of pH of the solution. The response of a Chromatographic column to a step pH change at column inlet results in a pH breakthrough curve which consists of a combined Chromatographic front. This combined front begins with a diffuse subfront and ends with a self‐sharpening subfront. The present Chromatographic theory is used to predict experimentally observed pH breakthrough curves for columns filled with materials of known pH‐dependent charging behavior. Good agreement between theoretical predictions and experimental results is observed. Essentially, the same Chromatographic theory is used to explain pH breakthrough curves when the salinity of the input solution is changed. This situation leads usually to an additional nonretarded front where a change in pH occurs.

EDTA and NTA were widely used in the processing of nuclear materials and in the production of nuclear energy. Their in-plant chemical transformations and post-disposal chemical and biological transformations generated many ad ditional amino carboxylic acid compounds. Toxic and radioactive metals are frequent co-contaminants of these chelating agents. Some of the resulting metal ion-chelating agent complexes, most notably 60CoIIIEDTA, resist decomposi tion and adsorb poorly onto subsurface minerals, thereby facilitating migration. This study sought to separate and detect free amino carboxylic acids and their Co(II) and Co(III) complexes in a single run without prior pretreatment using capillary electrophoresis (CE). The electrolyte filling the CE capillary contained 25 mM phosphate buffer (pH 7). Equilibrium calculations performed for this electrolyte identified predominant cobalt species and helped explain the CE findings. Co(III) complexes are thermodynamically (and kinetically) more stable than corresponding Co(II) complexes and are readily separated and detected. Co(III) complexes of EDTA, NTA, and IDA were synthesized and analyzed. Two different diastereomers of CoIIIIDA2 were found and could be separated. Co(II)−amino carboxylate complexes range from ones that are not thermodynamically stable in the capillary and therefore not detectable to those that are highly stable in the capillary and yield good linearity.

We propose a general method for the description of multi-component reactive transport in porous media. The approach is based on the local equilibrium assumption and consists of four steps: (i) measurement of the adsorbed amounts of all components of interest by means of laboratory batch experiments, (ii) construction of a simple, empirical binding model consistent with the experimental data by means of constrained, regularized least-squares, (iii) calculations of multi-component breakthrough curves based on numerical solutions of the transport equations coupled to the empirical binding model, (iv) verification of these model predictions with laboratory column experiments. We demonstrate the applicability of the procedure by investigating simultaneous transport of cadmium and zinc at constant pH in a silica sand column.

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