  A significant drawback of Lagrangian (particle-tracking) reactive transport
models has been their inability to properly simulate interactions between solid
and liquid chemical phases, such as dissolution and precipitation reactions.
This work addresses that problem by implementing a mass-transfer algorithm
between mobile and immobile sets of particles that allows aqueous species of
reactant that are undergoing transport to interact with stationary solid
species. This mass-transfer algorithm is demonstrated to solve the diffusion
equation and thus does not introduce any spurious mixing. The algorithm is
capable of simulating an arbitrarily small level of diffusion, and can be
combined with diffusive random walks to simulate the desired level of diffusion
in a reactive transport system.
