Enhanced solute transport in porous media due to pH-dependent adsorption and transverse dispersion

Abstract

The process of pH-dependent adsorption/desorption controls the transport of solutes in reactive porous media, the greater the adsorption the slower the solute migration. Earlier works show that in the presence of longitudinal hydrodynamic dispersion and pH-dependent adsorption, a fast solute transport phenomenon may arise. However, the effect of transverse dispersion on this fast transport phenomenon has not been investigated, yet.In this paper, we report an experimental and modeling work on the fast transport of strontium (Sr2+) through a reactive porous medium made of sand and hydrous ferric oxide (HFO) coated sand, where both longitudinal and transverse dispersion were important. A reactive transport model for an incompressible fluid was developed combining surface complexation with mass conservation equations for a solute and the acidity (difference between the concentration of total proton and hydroxide ions). The model was used to design and describe experiments run in a column-flood system and in a two-dimensional (2D) bead-pack. Results show that when an alkaline solution containing Sr2+ is injected into a 2D bead-pack containing HFO-coated sand and equilibrated with an acidic solution, a Sr2+ plume forms which comprises: (i) a retarded front due to the adsorption of Sr2+ onto the porous medium; (ii) a fast wave (or pulse) ahead of the retarded front due to longitudinal dispersion and traveling at the average fluid velocity; and (iii) a continuous leakage from the retarded front due to transverse dispersion.The results of this work confirm that pH-dependent adsorption/desorption process in conjunction with longitudinal dispersion causes fast wave, i.e., a solute migration which is faster than expected if the interplay between pH and dispersion is neglected. Furthermore, transverse dispersion in the presence of a pH gradient gives rise to a plume composed of a leakage from the retarded front connected to the fast wave.