Facilitated transport of nTiO-kaolin aggregates by bacteria and phosphate in water-saturated quartz sand.

The soil major component of clay plays an important role in governing the fate and transport of engineered nanomaterials (e.g., the most commonly used titanium dioxide nanoparticles; nTiO) in the subsurface environments via forming nTiO-clay aggregates. This research is designed to unravel the interplay of naturally-occurring bacteria (Escherichia coli) and phosphate on the transport and retention of nTiO-kaolin aggregates in water-saturated porous media. Our results showed that nTiO-nTiO homoaggregates and nTiO-kaolin heteroaggregates dominated in the nTiO-kaolin nanoaggregate suspension. Transport of nTiO-kaolin aggregates was enhanced with the copresence of E. coli and phosphate, particularly at the low pH of 6.0. This effect is due to the greater adsorption of phosphate and thus the greater enhancement in repulsive interaction energies between aggregates and sand grains at pH 6.0 (vs. pH 9.0). The charged "soft layer" of E. coli cell surfaces changed the aggregation state and the heterogeneous distribution of nTiO-kaolin aggregates, and subsequently stabilized the nTiO-nTiO homoaggregates and nTiO-kaolin heteroaggregates via TEM-EDX measurements and promoted the physical segregation between the aggregates (separation distance = 0.486 vs. 0.614 μm without vs. with the presence of E. coli) via 2D/3D AFM identifications, both of which caused greater mobility of nTiO-kaolin aggregates with the presence of E. coli. Nonetheless, transport of nTiO-kaolin aggregates was lower with the copresence of E. coli and phosphate vs. the singular presence of phosphate due to the competitive adsorption of less negatively charged E. coli (vs. phosphate) onto the aggregates. Taken altogether, our findings furnish new insights into better understanding the fate, transport, and potential risks of nTiO in real environmental settings (soil and sediment aquifer) where clay, bacteria, and phosphate ubiquitously cooccur.