CONVECTIVE-TRANSPORT OF ACIDS AND BASES IN POROUS-MEDIA

We present a convective description of acid-base transport in porous m edia which is based on classical one-component nonlinear chromatograph ic theory applied to the acidity of the system. In the mobile phase th e 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 sigma, which is commonly presented as a function of pH of the solution. The response of a chromatographic column to a step pH chang e at column inlet results in a pH breakthrough curve which consists of a combined chromatographic front. This combined front begins with a d iffuse subfront and ends with a self-sharpening subfront. The present chromatographic theory is used to predict experimentally observed pH b reakthrough curves for columns filled with materials of known pH-depen dent charging behavior. Good agreement between theoretical predictions and experimental results is observed. Essentially, the same chromatog raphic theory is used to explain pH breakthrough curves when the salin ity of the input solution is changed. This situation leads usually to an additional nonretarded front where a change in pH occurs.