Long-Term Exposure and Effects of rGO–nZVI Nanohybrids and Their Parent Nanomaterials on Wastewater-Nitrifying Microbial Communities

Single nanomaterials and nanohybrids (NHs) can inhibit microbial processes in wastewater treatment, especially nitrification. While existing studies focus on short-term and acute exposures of single nanomaterials on wastewater microbial community growth and function, long-term, low-exposure, and emerging NHs need to be examined. These NHs have distinctly different physicochemical properties than their parent nanomaterials and, therefore, may exert previously unknown effects onto wastewater microbial communities. This study systematically investigated long-term [∼6 solid residence time [(SRT)] exposure effects of a widely used carbon–metal NH (rGO–nZVI = 1:2 and 1:0.2, mass ratio) and compared these effects to their single-parent nanomaterials (i.e., rGO and nZVI) in nitrifying sequencing batch reactors. nZVI and NH-dosed reactors showed relatively unaffected microbial communities compared to control, whereas rGO showed a significantly different (p = 0.022) and less diverse community. nZVI promoted a diverse community and significantly higher (p < 0.05) biomass growth under steady-state conditions. While long-term chronic exposure (10 mg·L–1) of single nanomaterials and NHs had limited impact on long-term nutrient recovery, functionally, the reactors dosed with higher iron content, that is, nZVI and rGO–nZVI (1:2), promoted faster NH4+-N removal due to higher biomass growth and upregulation of amoA genes at the transcript level, respectively. The transmission electron microscopy images and scanning electron microscopyenergy-dispersive X-ray spectroscopy analysis revealed high incorporation of iron in nZVI-dosed biomass, which promoted higher cellular growth and a diverse community. Overall, this study shows that NHs have unique effects on microbial community growth and function that cannot be predicted from parent materials alone

Modeling the Transport of the “New-Horizon” Reduced Graphene OxideMetal Oxide Nanohybrids in Water-Saturated Porous Media

Little is known about the fate and transport of the “new-horizon” multifunctional nanohybrids in the environment. Saturated sand-packed column experiments (n = 66) were therefore performed to investigate the transport and retention of reduced graphene oxide (RGO)metal oxide (Fe3O4, TiO2, and ZnO) nanohybrids under environmentally relevant conditions (mono- and divalent electrolytes and natural organic matter). Classical colloid science principles (Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and colloid filtration theory (CFT)) and mathematical models based on the one-dimensional convection-dispersion equation were employed to describe and predict the mobility of RGO-Fe3O4, RGO-TiO2, and RGO-ZnO nanohybrids in porous media. Results indicate that the mobility of the three nanohybrids under varying experimental conditions is overall explainable by DLVO theory and CFT. Numerical simulations suggest that the one-site kinetic retention model (OSKRM) considering both time- and depth-dependent retention accurately approximated the breakthrough curves (BTCs) and retention profiles (RPs) of the nanohybrids concurrently; whereas, others (e.g., two-site retention model) failed to capture the BTCs and/or RPs. This is primarily because blocking BTCs and exponential/hyperexponential/uniform RPs occurred, which is within the framework of OSKRM featuring time- (for kinetic Langmuirian blocking) and depth-dependent (for exponential/hyperexponential/uniform) retention kinetics. Employing fitted parameters (maximum solid-phase retention capacity: Smax = 0.0406–3.06 cm3/g; and first-order attachment rate coefficient: ka = 0.133–20.6 min–1) extracted from the OSKRM and environmentally representative physical variables (flow velocity (0.00441–4.41 cm/min), porosity (0.24–0.54), and grain size (210–810 μm)) as initial input conditions, the long-distance transport scenarios (in 500 cm long sand columns) of the three nanohybrids were predicted via forward simulation. Our findings address the existing knowledge gap regarding the impact of physicochemical factors on the transport of the next-generation, multifunctional RGOmetal oxide nanohybrids in the subsurface

Aggregation Kinetics of Higher-Order Fullerene Clusters in Aquatic Systems

The aggregation kinetics of <i>n</i>C₆₀ and higher-order fullerene (HOF) clusters, i.e., <i>n</i>C₇₀, <i>n</i>C₇₆, and <i>n</i>C₈₄, was systematically studied under a wide range of mono- (NaCl) and divalent (CaCl₂) electrolytes and using time-resolved dynamic light scattering. Suwanee River Humic Acid (SRHA) was also used to determine the effect of natural macromolecules on <i>n</i>HOF aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged fullerene clusters, and the <i>n</i>C₆₀s and <i>n</i>HOFs alike displayed classical Derjaguin–Landau–Verwey–Overbeek (DLVO) type interaction. The critical coagulation concentration (CCC) displayed a strong negative correlation with the carbon number in fullerenes and was estimated as 220, 150, 100, and 70 mM NaCl and 10, 12, 6, and 7.5 mM CaCl₂ for <i>n</i>C₆₀, <i>n</i>C₇₀, <i>n</i>C₇₆, and <i>n</i>C₈₄, respectively. The aggregation mechanism (i.e., van der Waals interaction domination) was enumerated via molecular dynamics simulation and modified DLVO model. The presence of SRHA (2.5 mg TOC/L) profoundly influenced the aggregation behavior by stabilizing all fullerene clusters, even at a 100 mM NaCl concentration. The results from this study can be utilized to predict aggregation kinetics of <i>n</i>HOF clusters other than the ones studied here. The scaling factor for van der Waals interaction can also be used to model <i>n</i>HOF cluster interaction

Effects of Chloride and Ionic Strength on Physical Morphology, Dissolution, and Bacterial Toxicity of Silver Nanoparticles

In this study, we comprehensively evaluate chloride- and ionic-strength-mediated changes in the physical morphology, dissolution, and bacterial toxicity of silver nanoparticles (AgNPs), which are one of the most-used nanomaterials. The findings isolate the impact of ionic strength from that of chloride concentration. As ionic strength increases, AgNP aggregation likewise increases (such that the hydrodynamic radius [HR] increases), fractal dimension (D_f) strongly decreases (providing increased available surface relative to suspensions with higher <i>D</i>_f), and the release of Ag_(aq) increases. With increased Ag⁺ in solution, <i>Escherichia coli</i> demonstrates reduced tolerance to AgNP exposure (i.e., toxicity increases) under higher ionic strength conditions. As chloride concentration increases, aggregates are formed (HR increases) but are dominated by AgCl⁰_(s) bridging of AgNPs; relatedly, <i>D</i>_f increases. Furthermore, AgNP dissolution strongly increases under increased chloride conditions, but the dominant, theoretical, equilibrium aqueous silver species shift to negatively charged AgCl_<i>x</i>^{(<i>x</i>–1)–} species, which appear to be less toxic to <i>E. coli</i>. Thus, <i>E. coli</i> demonstrates increased tolerance to AgNP exposure under higher chloride conditions (i.e., toxicity decreases). Expression measurements of <i>katE</i>, a gene involved in catalase production to alleviate oxidative stress, support oxidative stress in <i>E. coli</i> as a result of Ag⁺ exposure. Overall, our work indicates that the environmental impacts of AgNPs must be evaluated under relevant water chemistry conditions