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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 2) in the subsurface environments via forming nTiO 2 -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 2 -kaolin aggregates in water-saturated porous media. Our results showed that nTiO 2 -nTiO 2 homoaggregates and nTiO 2 -kaolin heteroaggregates dominated in the nTiO 2 -kaolin nanoaggregate suspension. Transport of nTiO 2 -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 2 -kaolin aggregates, and subsequently stabilized the nTiO 2 -nTiO 2 homoaggregates and nTiO 2 -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 2 -kaolin aggregates with the presence of E. coli. Nonetheless, transport of nTiO 2 -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 2 in real environmental settings (soil and sediment aquifer) where clay, bacteria, and phosphate ubiquitously cooccur. Unlabelled Image • nTiO 2 -kaolin aggregates were preferentially formed in the influent suspension. • Transport of nTiO 2 -kaolin aggregates was increased due to phosphate adsorption. • E. coli stabilized nTiO 2 -kaolin aggregates by promoting their physical segregation. • nTiO 2 -kaolin transport was relatively lower with both of E. coli and phosphate. [ABSTRACT FROM AUTHOR]

Abstract Transport behaviors of titanium dioxide nanoparticles (nTiO 2) were examined in the individual- and co-presence Escherichia (E.) coli and phosphate in heterogeneous sand (uncoated and iron oxyhydroxide-coated sand) columns. The results showed that for the individual presence of phosphate, the degree of nTiO 2 deposition was less in uncoated than in iron oxide-coated sands. In contrast, an opposite trend that greater deposition of nTiO 2 in uncoated than in coated sands occurred in the individual presence of E. coli. These observations are due to the phosphate adsorption changing the charge of NPs and iron oxyhydroxide-coated sand, or the preferential adhesion of bacterial to coated sand. In the copresence of E. coli and phosphate, interestingly, the phosphate level plays an important role in influencing nTiO 2 transport. At a high phosphate concentration (>1.0 mM), the deposition of nTiO 2 with the individual presence of E. coli was stronger than nTiO 2 in the copresence of both E. coli and phosphate, regardless of sand type. The potential mechanism was that phosphate adsorption led to the formation of more negatively charged NPs-bacteria complexes that have higher mobility in sand columns. At a low phosphate level (≤0.1 mM), a similar observation occurred in uncoated sand. Nevertheless, the deposition of nTiO 2 with copresence of E. coli and phosphate was greater than nTiO 2 with E. coli in oxyhydroxide-coated sand. It was attributed to the formation of large NPs-bacteria-phosphate clusters (less mobile) and the preferential adhesion of E. coli cells to iron oxyhydroxide coating simultaneously. Taken together, our findings provide crucial knowledge for better understanding the fate, transport, and potential risks of engineered nanoparticles in complicated environmental settings where bacteria and phosphate are ubiquitous. Graphical abstract Image 1 Highlights • Transport of nTiO 2 w. P was different in heterogeneous sand from that w. E. coli. • Deposition of nTiO 2 with E. coli was higher in uncoated than in coated sand. • Adhesion affinity of E. coli or P on coated sand accounted for different transport. • P level plays important role in influencing transport of nTiO 2 w. E. coli + P. [ABSTRACT FROM AUTHOR]

Crop soil is inevitably contaminated by the excess of phosphate (P) fertilizers. A large amount of nanoparticle titanium dioxide (nTiO 2 ) entered soils as well due to the wide use of engineered nanomaterials. It is of great urgency and a high priority to investigate the mechanisms of nTiO 2 deposition with the presence of P in crop soils. This study investigated the transport behavior of (1.0 g L −1 ) rutile nTiO 2 with two representative clay particles (montmorillonite or diatomite) in the presence of P through the saturated quartz sand. In 10 mM NaCl electrolyte solution at pH 6.0, the recovery percentage of nTiO 2 was 36.3% from sand column. Nevertheless, it was reduced to 18.6% and 11.1% while montmorillonite and diatomite present in suspensions, respectively. Obviously, the improvement of nTiO 2 retention in sand was more pronounced by diatomite than montmorillonite. The likely mechanism for this result was that large aggregates were formed due to the attachment of nTiO 2 to montmorillonite and diatomite. Moreover, the surface of diatomite with the larger hydrodynamic radius was less negatively charged by comparison with montmorillonite. However, this phenomenon disappeared with the addition of P. P adsorption increases the repulsive force between particles and sand and the fast release of attached nTiO 2 -montmorillonite and diatomite from sand. The two-site kinetic retention model and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory suggested that the combination of k 1/ k 1d , k 2 and secondary minimum energy can be used to accurately describe the attachment of nTiO 2 -montmorillonite and diatomite to sand in the presence of P. [ABSTRACT FROM AUTHOR]

Due to the potential negative impact of nano silica (nSiO 2 ) on human's health and living environments, it is important to investigate their transport in soil environments. Hydrochars has been widely used in agricultural soil and phosphate (P) is an important nutrient, thus the aggregation and transport of nSiO 2 in saturated sand columns were investigated in single and binary systems of hydrochars and P. The experimental results showed that the nSiO 2 aggregates can be restablized by the adsorption of P or the attachment of hydrochars at high IS (>100 mM) and low pH (<7.0). Accordingly, the transport of nSiO 2 in sand columns is enhanced due to the smaller particle size. However, the nSiO 2 presents the distinct surface characteristics at pH > 7.0 from that at pH < 7.0. Thus, nSiO 2 has a better dispersivity in 300 mM NaCl solution at high pH (9.0). Nevertheless, their deposition to sands becomes pronounced in the presence of hydrochars and/or P. In particular, the formation of nSiO 2 -hydrochar-Phosphate clusters associated with the larger size mainly contributes to the enhancement of nSiO 2 retention in sand columns during the wide pH range, when hydrochars and P coexist in suspensions. The two-site dynamic model fitting results showed that the reversible retention is related to k 2 (First-order straining coefficient on site 2). The results in this study will provide the theoretical basis for assessing the retention of nSiO 2 in soil environment with the presence of hydrochars and phosphate. [ABSTRACT FROM AUTHOR]