Influence of microbial processes on fluid flow and nanoparticle transport in porous media

Abstract

Biofilm growth is a significant factor in subsurface processes governing fluid flow and contaminant transport. Biobarriers are known to reduce hydraulic conductivity, as well as to immobilise metals in the matrix of the exopolymeric substances (EPS) produced by bacterial cells. It is therefore necessary to develop understanding of how bioclogging and related mechanisms occur in porous media, and how contaminants interact with biofilms at the laboratory scale, which ultimately can be scaled up to field scenarios. The aims of the laboratory experiments were to a) enable uniform biofilm growth in columns packed with different types of porous media, b) develop methods of quantifying and visualising biofilm distribution in porous media, and c) measure transport of zinc oxide (ZnO) nanoparticles in columns with and without biofilm growing on the porous media. Experiments were conducted with columns and batch tests. Biofilms were grown by inoculating columns with Pseudomonas putida. Biofilm distribution was quantified by biomass extraction and visualised using X-ray computed microtomography (μCT) imaging. Colorimetric methods were used predominantly to quantify protein and polysaccharide content in biofilms. However, these methods possess several major disadvantages, which were highlighted using experimental data from batch tests. X-ray computer microtomography is a non-destructive method of visualising biofilm growth and illustrating flow paths in porous media. Particular components of μCT images (porous media, biofilm, tracer) were subtracted from images based on density contrasts. Reconstructed images of small, bio-clogged columns show that clogging occurs not only as a result of abundant biofilm growth but also air bubbles. Nanoparticle transport in porous media involved the injection of bare and capped ZnO nanoparticle suspensions into columns packed with glass beads, sand and calcite with and without inoculation of bacteria. Results, as well as modelled predictions, showed that ZnO nanoparticles generally possess low mobility, and that biofilm impedes nanoparticle transport. Porous media surface charge, as well as the extent of biofilm growth, play an important role in nanoparticle transport.