PhD Thesis Defence by Jingjing Wang
Title: Multirate mass transfer and biofilm growth modeling in porous media.
Thesis Comittee: Jaime Gómez Hernández, Marco Dentz, Verónica Morales, Jean-Raynald de Dreuzy and Paula Felicidad Rodríguez Escales.
Reactive transport modeling is a methodological tool to study the coupled physical, chemical and biological processes in Earth system. It is complex not only because of the nature of the equations, but also because of the effects of the porous medium heterogeneity on reactive transport. This thesis aims to deepen the understanding of reactive transport processes in order to explain the biochemical degradation process in porous media, with special emphasis on the role of biofilm and its growth.First, we propose a general and efficient numerical solution of reactive transport in multicontinuum media using Multirate Mass Transfer (MRMT) approach. To overcome the non-linearity of the problem, induced by non-linear kinetics, we use the Newton-Raphson method to get the global solution. We solve the system of equations in block form, which allow us to reduce the unknowns to those of mobile zones and to, thus improving efficiency. The solution is validated by comparison with analytical solution for linear kinetics. The code is developed in Object-oriented way, which enables the code reusability and data polymorphism. Second, we investigate the conditions for chemical localization (i.e., the occurrence of reactions that would not be possible in single continuum media). To this end, we write the multicontinuum transport equations in dimensionless form to find that reactive transport in multicontinuum media is governed by three characteristic times: the distribution of residence times in immobile zones, and the characteristic reaction and transport times. To study the interplay between these three characteristic times, we simulate three chemical systems: conservative, single reaction and sequential reaction. Results demonstrate that reactions driven by species that result from previous reactions will localize in immobile zones whose residence time is comparable to reaction times. Furthermore, immobile zones with residence times much smaller than those for transport can be lumped together (assuming that very fast reactions are assumed in equilibrium), which greatly reduces computations.Third, we perform simulations of reactive transport incorporating biochemical reactions that not only oxidize organic carbon, but also produce biomass, thus causing biofilm growth. Biofilm growth is known to cause clogging (i.e., reduction of permeability), which has concentrated most research on the topic. But it also causes a significant change in the pore space geometry and connectivity, which leads to not only an overall increase in mean residence time in immobile regions, but also on its distribution. As discussed above, this is critical to (bio)chemical localization, especially considering that microbial mediated reactions tend to concentrate in biofilms. We propose a model for the evolution of residence time distribution in immobile zones in response to biofilm growth. We test this model by comparison with laboratory experiments extracted from the literature, where tracer tests have been performed at various stages of growth. Results show that the dynamic MRMT model is capable of reproducing the salient features of these experiments.