Cotransport of zinc oxide and titanium dioxide nanoparticle aggregates with bacteria in saturated porous media: A coupled experimental and modeling approach

Abstract

The broad application of engineered nanoparticles in various fields leads to their inevitable release into the natural environment, causing soil and groundwater contamination. Bacteria, ubiquitous in the subsurface, can alter the transport behavior of nanoparticles. Hence, it is imperative to understand the interactions between nanoparticles and bacteria in the subsurface to protect drinking water wells from contamination. This study investigated the cotransport of metal oxide nanoparticle aggregates (zinc oxide, nZnO, and titanium dioxide, nTiO₂) with E. coli in saturated porous media in 1 mM NaCl and pH 8 under various flow velocities (0.26 - 1.02 cm/min) through column experiments and mathematical modeling. The injection concentrations of nanoparticles and E. coli were 15 mg/L and 10⁷ CFU/mL, respectively. We observed enhanced transport of nZnO and nTiO₂ and reduced transport of E. coli during their cotransport compared to nanoparticle-only and E. coli-only transport. The contrasting transport behaviors of nanoparticles and E. coli are due to the formation of nanoparticle-E. coli heteroaggregates, which have different transport properties than free nanoparticles and E. coli, and the preferential attachment of nanoparticles over E. coli to sand surfaces. Further, nZnO transport was enhanced to a greater extent than nTiO₂ transport due to the greater rate of heteroaggregation of nZnO and E. coli in comparison to nTiO₂ and E. coli. The experimental results were successfully simulated using a model that accounted for the kinetics of heteroaggregation of nanoparticles and E. coli, and heteroaggregate retention in sand.

Environmental Implication

Engineered nanoparticles have significant health impacts on humans and ecosystems. Understanding their transport behavior in the subsurface is essential to protect wells from contamination. Bacteria are ubiquitous in soil and interact with nanoparticles. E. coli facilitated the transport of nanoparticles (nZnO and nTiO₂), resulting in greater groundwater contamination with nanoparticles. Moreover, bacteria lead to a greater concentration of nZnO than nTiO₂ in groundwater. However, the effect of bacteria on nanoparticle transport decreases with increasing groundwater velocity. Thus, the effect of co-contaminants, including bacteria, should be accounted for in assessing the risks of groundwater contamination caused by engineered nanoparticles.