Biofilms and extracellular polymeric substances mediate the transport of graphene oxide nanoparticles in saturated porous media

Abstract

Understanding the fate and transport of graphene oxide nanoparticles (GONPs) in the subsurface environments is of crucial importance since they may pose potential risks to the environment and human health. However, little is known about the significance of biofilm on mobility of GONPs in the subsurface. Here we investigated the transport of GONPs in saturated sand coated with Bacillus subtilis (Gram-positive) and Pseudomonas putida (Gram-negative) biofilms, and their secreted extracellular polymeric substances (EPS) under environmentally relevant ionic strengths (1–50mM NaCl) at pH 7.2. Our results showed that irrespective of bacteria type, greater retention of GONPs occurred in biofilm-coated sand compared to clean sand, likely attributed to the increased surface roughness and physical straining. However, EPS showed negligible influence on GONPs transport, which was inconsistent with the findings in the presence of biofilms, while they exhibited comparable ζ-potentials. The different retention phenotype of GONPs in the presence of EPS was induced by hydration effect and steric repulsion. A two-site kinetic retention model well-described the transport of GONPs in porous media covered with different surface coatings, which proves the applicability of mathematical model in predicting nanoparticles’ mobility in the subsurface environments, when considering the potential effects of biofilm and EPS.