Synthesis of montmorillonite-supported nano-zero-valent iron via green tea extract: Enhanced transport and application for hexavalent chromium removal from water and soil.

The nano-zero-valent iron composite (nZVI@TP-Mont) was successfully prepared using a low-cost and environmental-friendly green synthesis via tea leaves extract (tea polyphenols, TPs) and the montmorillonite (Mont). The batch and column experiments and characterization were conducted to investigate the transport behavior and Cr(VI) remediation by nZVI@TP-Mont in water/soil. Due to its particular surface characteristics and morphology (i.e., the Fe0 core wrapped by TPs, the doped sulfur, and interlayer structure), the nZVI@TP-Mont composite showed a great removal capacity of Cr(VI) and sufficient mobility under different soil conditions. We opine the increase in the Cr(VI) reduction of nZVI@TP-Mont was attributed to the tethering of Fe2O3 on the surface of Fe0 core by the support of Mont interlayer, especially the TP-coverage around nZVI@TP-Mont surface unwrapped, thereby increasing the regenerated reactive Fe2+ and the exposed reaction sites of Fe0 cores to Cr(VI). The increased transportability of nZVI@TP-Mont slightly depends on the heterogeneous soil properties (i.e., ionic strength, sand/soil ratio, and pH). The two-site kinetic attachment model fitting results suggest Cr(VI)/Cr(III) speciation associated with the agglomerated nZVI@TP-Mont were efficiently immobilized in soil. Therefore, this study would benefit the efficient application of the green-synthesized nZVI@TP-Mont in in-situ remediation of soils contaminated by Cr(VI).

Facilitated transport of nTiO2-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; nTiO2) in the subsurface environments via forming nTiO2-clay aggregates. This research is designed to unravel the interplay of naturally-occurring bacteria (Escherichia coli) and phosphate on the transport and retention of nTiO2-kaolin aggregates in water-saturated porous media. Our results showed that nTiO2-nTiO2 homoaggregates and nTiO2-kaolin heteroaggregates dominated in the nTiO2-kaolin nanoaggregate suspension. Transport of nTiO2-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 nTiO2-kaolin aggregates, and subsequently stabilized the nTiO2-nTiO2 homoaggregates and nTiO2-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 nTiO2-kaolin aggregates with the presence of E. coli. Nonetheless, transport of nTiO2-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 nTiO2 in real environmental settings (soil and sediment aquifer) where clay, bacteria, and phosphate ubiquitously cooccur.

Transport and retention of hydrochar-diatomite nanoaggregates in water-saturated porous sand: Effect of montmorillonite and phosphate at different ionic strengths and solution pH.

Hydrochar, a solid hydrate with a high energy density, is produced by hydrothermal carbonization of lignocellulosic biomass and is widely applied in agriculture as a soil amendment. The fate and transport of hydrochar when applied to soil need to be investigated. The major components of soil, clay and phosphate, likely interact with hydrochar in the subsurface. This study investigated the cotransport behavior of hydrochar and diatomite (D) through water-saturated quartz sand in the presence of montmorillonite (M) and/or phosphate in NaCl (1-50 mM) solutions at pH 6.0 and 9.0. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images and zeta potential (ZP) results showed that hydrochar-D nanoaggregates formed preferentially due to surface charge heterogeneity. M inhibited the transport of hydrochar-D in sand columns regardless of the solution pH mainly because the organo-mineral clusters of hydrochar-D with M were prone to filling the pores of the sand medium. Moreover, fine M particles preferentially attached to sand could decrease the ZP of the sand surface and subsequently decrease the repulsive forces between hydrochar-D and sand. The copresence of M and phosphate slightly facilitated hydrochar-D transport at pH 6.0 due to phosphate adsorption, whereas a negligible effect on transport occurred at pH 9.0. Thus, phosphate played a predominant role in the transport of hydrochar when clays were also present. A two-site kinetic retention model suggested that k1d/k1 and k2 are responsible for hydrochar-clay aggregate deposition in sand. Our findings relate to the potential risks posed by hydrochar in subsurface soils and aquifers where clay and phosphate ubiquitously co-occur.

Nanoaggregates of silica with kaolinite and montmorillonite: Sedimentation and transport.

Due to a wide range of applications in industrial fields, engineered nanomaterials (ENMs) have a high potential to enter the soil. The soil's major component of clay likely dictates the fate and transport of ENMs in the subsurface. Currently, few studies are available on the fate and transport of nanoparticle silica (nSiO2) in the presence of clay particles. Therefore, the sedimentation and transport of nSiO2 with two representative clays (montmorillonite (M) and kaolin (K)) in porous media were investigated in monovalent (Na+) and divalent (Ca2+) ion solutions with multiple characterizations including SEM/TEM-EDX, zeta potentials, particle sizes and colloid transport modeling. It was shown that nSiO2-nSiO2 homoaggregates and nSiO2-K (or M) heteroaggregates dominated in the nSiO2-clay nanoaggregate suspension. A distinct decrease in the stability and transport of nSiO2-M (or K) in NaCl solution and an increase in CaCl2 occurred when M or K was added to the nSiO2 suspension at pH 6.0. This was attributed to the faster settlement of the individual M or K in NaCl vs. the better stability in CaCl2 (compared to nSiO2 alone). Particularly, more negative individual M platelets occurred in the high NaCl solution until extensive flocculated structures built up, which contributed to the faster deposition of nSiO2-M compared to nSiO2-K, even though the nSiO2-M was more negatively charged. Comparably, the effect of M and K on the fate and transport of nSiO2 almost disappeared at pH 9.0. The values of the first-order attachment/detachment rate coefficients (k1/k1d) and first-order straining coefficient (k2) obtained from two-site kinetic attachment model fitting are responsible for the deposition of nSiO2-clay nanoaggregates in sand. This study suggests potential groundwater contamination due to the clay-facilitated transport of ENMs in calcareous soil.

Synergetic effect of hydrochar on the transport of anatase titanium dioxide nanoparticles in the presence of phosphate in saturated quartz sand.

The rapid development of nanomaterials has led to the unavoidable leakage and release of nanoparticles (NPs) into soil and the underlying groundwater. It is possible for chars and phosphate introduced into soil to improve crop soil properties by improving contact with NPs. In this study, the influences of hydrochar and/or phosphate on the anatase nTiO2 transport behaviors were investigated under different conditions. The breakthrough curves (BTCs) and retention profiles were obtained by the saturated sand column experiments. The additional analysis of zeta potentials, sedimentation kinetics, Raman mapping, and the two-site kinetic attachment model (TSKAM) was conducted to explore the possible underlying mechanisms. The simultaneous presence of phosphate and hydrochar acted in a synergetic fashion to enhance the transport of nTiO2 in a sand medium compared to the facilitated effect of single phosphate or hydrochar. The higher levels of hydrochar induce the more nTiO2 in the high IC solution passing through the saturated sand columns in the co-presence of phosphate. It was attributed to the competitive adsorption of hydrochar with nTiO2 to the sand site and the phosphate adsorption on nTiO2 occurred simultaneously through the sand columns. The fitting results of BTCs using TSKAM showed that the value of k2 for nTiO2 (the irreversible attachment coefficient at site 2) was smaller than that of k1d/k1 (the first-order reversible detachment and attachment coefficient at site 1, respectively), suggesting irreversible retention of anatase nTiO2 at site 1. The value of k1d/k1 could be better used to explain the retention of nTiO2 with combined phosphate and hydrochar. This study provides insight into the implications of phosphate and/or hydrochar for nTiO2 transport in crop soil environments. Graphical abstract ᅟ.

Effects of Escherichia coli and phosphate on the transport of titanium dioxide nanoparticles in heterogeneous porous media.

Transport behaviors of titanium dioxide nanoparticles (nTiO2) 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 nTiO2 deposition was less in uncoated than in iron oxide-coated sands. In contrast, an opposite trend that greater deposition of nTiO2 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 nTiO2 transport. At a high phosphate concentration (>1.0 mM), the deposition of nTiO2 with the individual presence of E. coli was stronger than nTiO2 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 nTiO2 with copresence of E. coli and phosphate was greater than nTiO2 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.

Aggregation and transport of rutile titanium dioxide nanoparticles with montmorillonite and diatomite in the presence of phosphate in porous sand.

Crop soil is inevitably contaminated by the excess of phosphate (P) fertilizers. A large amount of nanoparticle titanium dioxide (nTiO2) 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 nTiO2 deposition with the presence of P in crop soils. This study investigated the transport behavior of (1.0 g L-1) rutile nTiO2 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 nTiO2 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 nTiO2 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 nTiO2 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 nTiO2-montmorillonite and diatomite from sand. The two-site kinetic retention model and the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory suggested that the combination of k1/k1d, k2 and secondary minimum energy can be used to accurately describe the attachment of nTiO2-montmorillonite and diatomite to sand in the presence of P.