Impact of surface complexation and electrostatic interactions on pH front propagation in silica porous media

The coupled effects of pH and ionic strength impact a variety of geochemical processes in the subsurface. In this study, we investigate the interactions of H+ and major ions at the surface-solution interface of silica porous media under advection-dominated flow-through conditions. A series of 21 column experiments were performed by systematically injecting solutions of different pH and ionic strengths.

Three types of porous media (i.e., two natural sands and quartz beads) were considered in order to explore differences in surface/solution interactions among quartz materials. Multiple lines of evidence were used to characterize the geochemical processes taking place during the flow-through experiments: (i) breakthrough curves of pH and major ions were measured at the column outlets; (ii) the natural sand surfaces were characterized by chemical extractions and scanning electron microscopy and the quartz structure was analyzed by XRD; (iii) reactive transport modeling was performed to quantitatively interpret the experimental results.

We observed strong reactivity of the quartz surface characterized by significant release of protons when interacting with the major ions. The results also show significant differences in the quantity of protons emitted from the surfaces of the three silica porous media, as well as in the shape of the pH breakthrough curves.

Reactive transport modeling was based on a multicomponent ionic transport formulation and on surface complexation description of the solid/solution interactions. The surface complexation model included the individual contributions of quartz and Fe/Al oxides present in the sand coatings with the component additive approach.

For each medium, a single set of surface complexation parameters capable of reproducing the experimental dataset (i.e., 7 columns) was calibrated by parallelizing the simulations of the flow-through experiments. This approach allowed us to capture the pH and ionic species behavior within a large range of ionic strengths (i.e., [0 – 100] mM).

The SCMs quantitatively show that H+ is released from quartz upon adsorption of Na+ and that the protonation of the oxides surfaces retards the pH front. Differences in acidity behavior between the silica surfaces seem to be primarily controlled by differences in surface topology, crystal structure of quartz and/or various presence of aluminosilicates.

This study demonstrates that the reactivity and diversity of quartz surfaces in silica porous media and the complex interplay with oxide surfaces in the coatings control the transport of aqueous charged species in flow-through systems.