Environmental context The mobility and bioavailability of both contaminants and nutrients in the environment depends, to a large extent, on their diffusion. Because the majority of microorganisms in the environment are embedded in biofilms, it is essential to quantify diffusion in biofilms in order to evaluate the risk of emerging contaminants, including nanomaterials and charged solutes. This study quantifies diffusion, in a model environmental biofilm, for a number of model contaminants of variable size and charge. Abstract The effect of solute and biofilm charge on self-diffusion (Brownian motion) in biofilms is examined. Diffusion coefficients (D) of several model (fluorescent) solutes (rhodamine B; tetramethylrhodamine, methyl ester; Oregon Green 488 carboxylic acid, succinimidyl ester and Oregon Green 488 carboxylic acid) and nanoparticles (functionalised silicon, gold and titanium) were determined using fluorescence correlation spectroscopy (FCS). Somewhat surprisingly, little effect due to charge was observed on the diffusion measurements in the biofilms. Furthermore, the ratio of the diffusion coefficient in the biofilm with respect to that in water (Db/Dw) remained virtually constant across a wide range of ionic strengths (0.1–100mM) for both negatively and positively charged probes. In contrast, the self-diffusion coefficients of nanoparticles with sizes >10nm greatly decreased in the biofilms with respect to those in water. Furthermore, much larger nanoparticles (>66nm) appeared to be completely excluded from the biofilms. The results indicated that for many oligotrophic biofilms in the environment, the diffusion of solutes and nanoparticles will be primarily controlled by obstruction rather than electrostatic interactions. The results also imply that most nanomaterials will become significantly less mobile and less bioavailable (to non-planktonic organisms) as they increase in size beyond ~10nm.
Role of Manganese Oxyhydroxides in the Transport of Rare Earth Elements Along a Groundwater Flow Path