Rates of geochemical reactions in pore networks can become transport limited if solute concentrations equilibrate with reacting phases faster than fluids can transport solutes away. Pore networks can often be separated into a well-connected backbone, where fluid flow is advective and solutes are advected out rapidly, and a poorly connected secondary network, where solute transport is dominated by diffusion. In these cases, reaction rates can be tranpsort limited in the secondary network, but not the backbone. We employed microfluidic techniques to evaluate the distribution of kinetic and transport limitation conditions and averaging of anorthite dissolution rates across pore networks of variable connectivity. The effect of pore network connectivity on anorthite dissolution rates was evaluated at pH 3 and pH 5 in microfluidic devices machined in anorthite substrate. The microfluidic devices were created by laser ablating pore networks into the top of a polished anorthite wafer using a femtosecond laser, which limited energy transfer into the mineral and associated heat damage to the crystal structure. Pore networks were created with and without dead-end pores by disconnecting some of the pore channels. Images of fluorescing beads and dye in the devices showed that diffusion dominated transport in dead-end pores in an otherwise advection dominated system. Anorthite dissolution rates calculated from effluent Ca2+ fluxes out of the microdevices and normalized to total surface area ranged from 2.3 to 4.3 x 10-9 mol m-2 s-1 for pH 3 experiments and 1.2 to 2.4 x 10-9 mol m-2 s-1 for pH 5 experiments and decreased with decreasing pore network connectivity. We evaluated the contribution of transport-limited reaction in dead-end pores to the average dissolution rate by normalizing to both the surface area of the total network and the surface area of just the connected backbone and found that at a pore connectivity of &gt; ~50% the contributions of dead-end pores were negligible to the overall reaction rate. However, at pore connectivity of &lt; ~50% transport-limited reactions in the dead-end pores caused an overall decrease in the average dissolution rate across the device. These results illustrate that surface area and pore network connectivity exert an important control on mineral dissolution rates at pore-scales, especially in highly heterogeneous systems where a poorly connected secondary network is more than half of the volume of the total pore network.

Using microfluidics to evaluate the role of transport on mineral dissolution at the pore scale

ACS Spring 2022

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