Understanding Cr(VI) sorption in heterogeneous porous media using column experiments and reactive transport modeling

Abstract:

This work examines the largely unexplored role of clay spatial patterns in dictating Cr(VI) sorption using column experiment and numerical simulations. We investigate the impacts of permeability contrasts, correlation lengths, and column lengths.

Experiments were carried out using columns (10 cm) packed with the same total mass of illite and quartz sand however different spatial patterns and permeability contrast (k_ratio). Higher k_ratio with low permeability and flow-parallel illite pattern minimize the water flow through the illite zone, leading to more than one orders of magnitude lower sorption capacity compared to those with well mixed illite and quartz grains. Two-dimensional reactive transport models that explicitly incorporate the mineral distribution were used to reproduce the data and to understand the mechanism. Both k_ratio and transverse dispersivity a_T are essential in determining the mass transport between the illite and quartz zones and therefore the total sorption amount m_st. For flow-parallel patterns where the illite-sand interface lies in parallel to the main flow, larger a_T values lead to higher m_st. For the flow-transverse 2 zones pattern where the main illite-sand interface is perpendicular to the main flow, larger k_ratio leads to lower m_st.

Two sets of spatial patterns, with one high log hydraulic conductivity variances of 7.95 (High-K) and another low of 0.76 (Low-K), were created using sequential gaussian simulation (SGS) under different correlation lengths (l) and column lengths (L). The m_st decreases linearly with connectivity and , until reaches a critical value of 1.1 cm, after which the m_st remains its minimum. With the same l and L values, the m_st of the High-K patterns are lower than that of the Lower-K patterns due to the mass transport limitation. This work provides a mechanistic and quantitative understanding on the role of mineral distribution patterns in determining the reactive transport of contaminants.