Coupled interactions between metals and bacterial biofilms in porous media: Implications for biofilm stability, fluid flow and metal transport

The ability to predict the impact of metal–microbe interactions on biofilm stability, hydrodynamics and contaminant transport is a key goal in hydrogeology, membrane bioreactors and geomicrobiology. Yet, we have not been able to find experimental studies reporting the coupling of these three elements in a systematic way. In this study sand column experiments were carried out to investigate the coupled effects of aqueous zinc (Zn2 +) on biofilm stability and of biofilm on Zn2 + transport in saturated sand columns. A series of aqueous Zn concentrations ranging from 0 to 100 ppm were pumped through biofilm-colonised columns and a set of related parameters (hydraulic conductivity, cell numbers, EPS, metal breakthrough curve and metal distribution) was measured. Our results show that biofilm formation prior to introduction of Zn resulted in a significant decrease of hydraulic conductivities. After 10 days of metal exposure, Zn showed concentration-dependent toxicity on bacterial cells in the biofilm. However different Zn concentrations produced distinct non-linear effects on EPS production, which resulted in either recovery or further decrease of hydraulic conductivity in the porous matrix. This non-linear response of biofilm to metal concentration could lead to different metal transport patterns in the long term. The concentration of metal contaminants plays a critical role in regulating the effect of metal–biofilm interaction. Our phenomenological study establishes linkages between chemical, microbial and physical processes of metal–biofilm interaction, and is an essential precursor to the development of models for this complex system. Specifically, these interactions are shown to be unpredictable, suggesting that more work needs to be done to constrain flow and transport parameters in biofilm-colonised porous media.