Role of bacterial biofilms and extracellular polymeric substances in the colloidal stability and transport of hematite nanoparticles in synthetic porous media The heterogeneous physicochemical properties of biofilm and extracellular polymeric substances (EPS) play an important role in deciding the fate of transport of nanoparticles in subsurface. However contrasting results have been observed for the mobility of different nanoparticle transport in porous media coated with biofilms. Also no comparative studies ave been done linking the effect of biofilms on colloidal stability and mobility of colloids to date. Laboratory scale experiments were conducted to obtain insights into different ways bacterial biofilms and EPS can influence the transport of nanoparticles in porous media. Using complementary experimental approaches we investigated (1) the effect of microbial EPS on the colloidal stability of nanoparticles (2a.) the transport of uncoated colloidal nanoparticles in biofilm and EPS coated porous media (2b.) the transport of EPS coated colloidal nanoparticles in uncoated porous media (2c) the combined transport of EPS coated colloids in biofilm and EPS coated porous media and finally (3) the remobilization of deposited colloidal nanoparticles by EPS. Laboratory synthesized hematite nanoparticles (HNP) and Bacillus subtilis 168 (DSM 402) were used as nanoparticles and biofilm forming bacterium respectively. To study the effect of EPS on the colloidal stability HNP were amended with three increasing concentrations of EPS (20 200 and 500 mg l carbon) resulting in carbon:iron ratios 1:5 2:1 and 5:1 respectively. The zeta potential (ZP) the shift in isoelectric point and the change in the critical coagulation concentration (CCC) for NaCl and CaCl2 showed formation of colloidally stable organo mineral colloids at higher concentrations of EPS (200 mg l and 500 mg l of carbon HNP2:1 HNP 5:1). However lower concentrations of EPS (20 mg l carbon HNP 1:5) resulted into aggregation of HNP. The transport of uncoated and EPS coated HNP colloids at environmental pH was studied with closed flow columns experiments in three different synthetic porous media i.e. uncoated biofilm coated and EPS coated glass beads. No drastic change was observed in the pH and the electrical conductivity of the system during the transport experiments. However a constant amount of organic matter and bacterial cells were continuously released from the porous media coatings. In the first scenario positively charged uncoated HNP were immobile and retained in uncoated glass bead porous media. Contrary a mobile fraction of negatively charged HNP was observed in biofilms and EPS coated porous media. The charge reversal of HNP during transport is due to adsorption of organic matter released from the coatings forming organomineral associations of HNP and components of the biofilm or EPS.