Reactive fronts in chemically heterogeneous porous media: Experimental and modeling investigation of pyrite oxidation

The spatial distribution of reactive minerals in subsurface porous media is an important control for groundwater quality. In this study we investigate pyrite oxidation reactive fronts in chemically heterogeneous porous media by combining laboratory experiments and reactive transport modeling. We performed experiments in different setups including batch, 1-D column, and 2-D flow-through systems.

The flow-through experiments were performed in physically homogenous but chemically heterogeneous domains with embedded reactive pyrite inclusions at different spatial locations and with different concentrations. The setups were initially maintained under anoxic conditions and subsequently flushed with an inflowing oxic solution.

A non-invasive optode technique was used for high-resolution monitoring of oxygen. This allowed us to capture the dynamics of the reactive oxygen fronts in the 1-D columns and in the 2-D flow-through chamber. Water quality analyses of the products of pyrite oxidation, iron (Fe) and sulfur (S), were also carried out in the different setups.

The concentration of these species released in the 1-D columns (up to 6.2 × 10−5 moL/L Fe and 13.7 × 10−5 moL/L S) and in the 2-D setup (up to 3 × 10−6 moL/L Fe and 9 × 10−6 moL/L S) could be quantitatively related to the consumption of oxygen (up to 1.9 × 10−4 moL/L consumed in the 1-D and 2-D setups).

The reaction rates were found to be different between the setups and dependent on the spatial location and concentration of the pyrite inclusions. A modeling approach coupling 1-D and 2-D transport codes with the geochemical simulator PHREEQC is proposed to simulate the spatial and temporal dynamics of oxygen transport, the kinetics of pyrite oxidative dissolution, and the changes in water quality in the chemically heterogeneous flow-through setups.

The model allowed the quantitative interpretation of the experimental results and represents a valuable tool to capture the coupling between multidimensional transport and geochemical reactions both in laboratory and in field scale applications.