This research provided the first direct evidence that strongly bound polymer coatings on nanoparticles are indeed biodegradable. Polymer biodegradation can proceed either by extracellularly secreted enzymes or by processes that require chain reptation through bacterial wall channels. In either case, polymer confinement at high surface density on a solid nanoparticle surface could be expected to sterically impair biodegradation. For this study, a polystyrene-like nanoparticle with a dense brush of poly(ethylene oxide) (PEO) chains covalently attached via non-hydrolyzable covalent bonds, was designed so that the only possible mechanism for coating loss was biodegradation. PEO-degrading bacterial mixed cultures were enriched from a Monongahela River water sample. These bacteria were shown to degrade nanoparticle-bound PEO brushes, when these were the only available carbon source. The carbon was used for baseline metabolism (indicated by CO2 production) and biomass production (indicated by protein synthesis). Nanoparticles that had been colloidally stable aggregated as a result of brush biodegradation. Control experiments verified that aggregation was the direct result of biodegradation. Thus, it was demonstrated for the first time that nanoparticle stabilizer coatings can be a carbon source for microorganisms and bacterial action may redirect nanoparticle fate in the environment. Polymer biodegradability also was the focus of work on the choice of polymer stabilizer used to disperse nanoscale zero valent iron (NZVI) groundwater remediation agents. NZVI with polyaspartate, poly(acrylic acid) or carboxymethylcellulose stabilizers stimulated total eubacterial growth in TCE-contaminated aquifer materials. Biostimulation was synergistic. Macromolecule-stabilized NZVI was significantly more stimulatory than identical macromolecular solutions that lacked NZVI or NZVI suspensions with no macromolecules. Thus, stabilizers can be chosen not only enhance to NZVI efficacy but possibly also to stimulate bioattenuation.
Colloidal stability was shown to directly impact the toxicity of silver nanoparticles to P. fluorescens biofilms. Flocculated nanoparticles exhibited no toxic effect on biofilms beyond the effect that was directly attributable to dissolved silver, but well stabilized particles exhibited a nanoparticle-specific toxic effect that acted in addition to the dissolved silver toxicity. This is attributed to the inability of flocculated particles to diffuse through the biofilm extracellular polymeric substance (EPS). Adsorbed humic acid had complex effects. It decreased, but did not eliminate, the overall toxicity by complexing Ag+, but the nanoparticle-specific toxic effect was retained, even enhanced, in the presence of humic acid, likely by promoting nanoparticle diffusion into the biofilm.