Microbial controls on contaminant metal transport in porous media
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Metal contamination in groundwater aquifers poses risks to human health as well as other life forms. Previous laboratory experiments have demonstrated that bacteria found in geologic settings like aquifers are likely to adsorb metal contaminants and attenuate metal migration. However, as bacteria can also migrate through the groundwater aquifer a better understanding of the combined effect of these two processes is required. The aim of this laboratory study was to a) explore the affinity bacteria exhibit towards metals and porous media of varying composition, b) investigate the effect of mineral and solution composition on the bacterial filtration and c) use the combined data to predict the impact of microbes on metal mobility in porous media. Pantoea Agglomerans was used as a model bacterium while column materials consisted of quartz sand and iron-oxide coated sand (IOCS). Bacteria were characterised using potentiometric titrations to identify the type and concentration of sites present on their bacterial wall. Particular attention was paid to the effect of kinetics of proton and metal adsorption due to the variable contact times that solutions have with bacteria in columns. It was found that increasing the contact time between cell surfaces and protons during potentiometric titrations resulted in less reproducible results. This was due to the release of cell exudates under high pH conditions rather than cell death. Exudates were also found to adsorb protons. Moreover, zinc adsorption onto cell surfaces is higher after 60 to 90 minutes of contact time, while there is a decline in adsorption for longer contact times due to release of cell exudates in the solution. Stability constants for the adsorption of zinc onto cell surface sites, quartz and IOCS materials were determined through batch adsorption experiments, providing a mechanistic explanation of the adsorption process. Reactive transport models incorporating kinetics and surface complexation are developed to describe zinc movement through packed columns. Batch kinetic studies showed that significant Zn sorption to IOCS takes place gradually during the first two hours of contact time. Adsorption continues to take place at a slower rate for an additional 10 hours. This kinetic effect is manifested also during flow-through experiments (column dimensions: length 0.12 m, diameter 0.025 m) with a Darcian velocity 6.1·10-3 cm s-1, which is comparable to natural groundwater flow rates through sand porous media. A pseudo-second order kinetic adsorption model is combined with a numerical advection dispersion model for the first time to predict Zn transport. Model output results are of mixed quality as the model cannot successfully describe contaminant arrival time and breakthrough curve shape simultaneously. Moreover, a mechanistic surface complexation reactive transport model is capable of predicting Zn sorption under varying pH conditions demonstrating the versatility of mechanistic models. However, these models do not account for kinetics and therefore they are not intended to fit the dispersion of the contaminant due to kinetic effects of adsorption. Experiments in mixed zinc/cell systems demonstrate that transport through IOCS is dominated by the adsorption to the porous medium. This is consistent with the batch surface complexation predictions for the system. Adsorption to bacteria is reversible and zinc is stripped from the cells and redistributed onto the IOCS. Adsorption onto cells becomes significant and plays a role in mobile metal speciation only once the column is saturated with zinc.