Interaction between palladium-doped zerovalent iron nanoparticles and biofilm in granular porous media: characterization, transport and viability

Abstract

Palladium-doped nanoscale zerovalent iron (Pd-NZVI) can induce rapid transformations of a variety of pollutants during in situ remediation of contaminated aquifers. Pd-NZVI stabilized with polymeric surface modifiers has shown substantially improved stability and transport compared to bare Pd-NZVI in model subsurface granular media such as clean quartz sand. The natural subsurface environment is, however, much more complex. For example, biofilms may coat aquifer grain surfaces and thus alter the transport of these reactive nanoparticles. Herein, we compare the transport behavior of Pd-NZVI coated with carboxymethylcellulose or rhamnolipid as surface modifiers, in clean and biofilm-coated sand packed columns. Transport studies suggest that, for both types of nanoparticles tested, the particle attachment efficiency to the collector surface generally increases (up to 6 fold in NaCl and 26 fold in CaCl₂) in the presence of biofilm. This result indicates the potential for reduced Pd-NZVI transport in a natural groundwater system. The retentive behavior of biofilm-coated media increases with particle–particle aggregation (e.g., C/C₀ ~ 0.15 for a markedly aggregated CMC-coated Pd-NZVI in divalent salt), implicating physical straining as an important retention mechanism. Retained Pd-NZVI on biofilm-laden matrices was characterized using enhanced darkfield hyperspectral imaging. Assessments of biofilm viability imply that the retained surface-modified Pd-NZVI is non-toxic to the cells within the biofilm matrices (viability >95%). Moreover, the coating molecules do not negatively impact the viability of the biofilm bacteria.