Transport of industrial PVP-stabilized silver nanoparticles in saturated quartz sand coated with Pseudomonas aeruginosa PAO1 biofilm of variable age.

Understanding the environmental fate and transport of engineered nanoparticles (ENPs) is of paramount importance for the formation and validation of regulatory guidelines regarding these new and increasingly prevalent materials. The present study assessed the transport of an industrial formulation of poly(vinylpyrrolidone)-stabilized silver nanoparticle (PVP-nAg) in columns packed with water-saturated quartz sand and the same sand coated with Pseudomonas aeruginosa PAO1 biofilm of variable age (i.e., growth period). Physicochemical characterization studies indicate that the PVP-nAg is stable in suspension and exhibits little change in size or electrophoretic mobility with changing ionic strength (IS) in either NaNO3 or Ca(NO3)2. The collector surface had a relatively homogeneous biofilm coating, as determined by CLSM, and a near uniform distribution of biomass and biofilm thickness following column equilibration. Transport experiments in clean sand revealed changes in the particle deposition behavior only at and above 10 mM IS Ca(NO3)2 and showed no discernible change in PVP-nAg transport behavior in the presence of 1 to 100 mM NaNO3. Transport experiments in P. aeruginosa-coated sand indicated significantly reduced retention of PVP-nAg at low IS compared to clean sand, irrespective of biofilm age. Nanoparticle retention was also generally reduced in the biofilm-coated sand at the higher IS, but to a lesser extent. The decreased retention of PVP-nAg in biofilm-coated sand compared to clean sand is likely due to repulsive electrosteric forces between the PVP coatings and extracellular polymeric substances (EPS) of the biofilm. Additionally, the slope of the rising portion of the PVP-nAg breakthrough curve was noticeably steeper in biofilm conditions than in clean sand. More mature biofilm coating also resulted in earlier breakthrough of PVP-nAg compared to younger biofilm coatings, or to the clean sand, which may be an indication of the effect of repulsive surface forces combined with selective pore size exclusion from the pores of denser, more developed biofilm. These results, when considered with other literature, indicate the importance in considering the flow dynamics, pore network and structure, the effective particle size, and particle permeability with regard to the biofilm matrix when considering the possible influence of biofilms on ENP transport.