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 microscopy─energy-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.

Aggregation Behavior of Inorganic 2D Nanomaterials Beyond Graphene: Insights from Molecular Modeling and Modified DLVO Theory.

We report the comparative aggregation behavior of three emerging inorganic 2D nanomaterials (NMs): MoS2, WS2, and h-BN in aquatic media. Their aqueous dispersions were subjected to aggregation under varying concentrations of monovalent (NaCl) and divalent (CaCl2) electrolytes. Moreover, Suwanee River Natural Organic Matter (SRNOM) has been used to analyze the effect of natural macromolecules on 2D NM aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged 2D NMs, thus displaying classical Derjaguin-Landau-Verwey-Overbeek (DLVO)-type interaction. The critical coagulation concentrations (CCC) have been estimated as 37, 60, and 19 mM NaCl and 3, 7.2, and 1.3 mM CaCl2 for MoS2, WS2, and h-BN, respectively. Theoretical predictions of CCC by modified DLVO theory have been found comparable to the experimental values when dimensionality of the materials is taken into account and a molecular modeling approach was used for calculating molecular level interaction energies between individual 2D NM nanosheets. Electrostatic repulsion has been found to govern colloidal stability of MoS2 and WS2 while the van der Waals attraction has been found to govern that of h-BN. SRNOM stabilizes the 2D NMs significantly possibly by electrosteric repulsion. The presence of SRNOM completely stabilized MoS2 and WS2 at both low and high ionic strengths. While h-BN still showed appreciable aggregation in the presence of SRNOM, the aggregation rates were decreased by 2.6- and 3.7-fold at low and high ionic strengths, respectively. Overall, h-BN nanosheets will have higher aggregation potential and thus limited mobility in the natural aquatic environment when compared to MoS2 and WS2. These results can also be used to mechanistically explain fate, transport, transformation, organismal uptake, and toxicity of inorganic 2D NMs in the natural ecosystems.

Phenol and Cr(VI) removal using materials derived from harmful algal bloom biomass: Characterization and performance assessment for a biosorbent, a porous carbon, and Fe/C composites.

Lake Erie experiences annual harmful algal blooms (HAB), but generated HAB biomass may provide a waste-based precursor for environmental remediation materials. Three classes of materials (i.e., algal powder biosorbent, porous carbon, and iron/carbon (Fe/C) composite) are prepared from HAB biomass. Algal powder is nonporous with diverse functional groups. Porous carbon, prepared via one-pot carbonization and activation, has surface area up to 430 m2/g. Fe/Cs are prepared by cultivating HAB biomass in iron-rich media, followed by one-pot pyrolysis. Fe/Cs have over 6 wt% iron (Fe0 and Fe3O4) and nitrogen doping (up to 4 wt%). Materials were applied in phenol and Cr(VI) removal tests to identify preferred products for use in water treatment applications. In deionized water, porous carbon removes the most phenol (52 mg/g), followed by algal powder (38 mg/g) and Fe/C (33 mg/g). Micropore volume and functional groups improve phenol removal. Cr(VI) removal follows: Fe/C (43 mg/g) > porous carbon (28 mg/g) > algal powder (17 mg/g), with synergistic adsorption and reduction elevating Fe/C's performance. Cr(VI) and phenol removal studies were completed with variable pH, ionic strength, and water composition to highlight application potential. This work proposes HAB biomass reuse for pollution control, investigating interaction mechanisms between materials and contaminants.

Magnetic graphene oxide-nano zero valent iron (GO-nZVI) nanohybrids synthesized using biocompatible cross-linkers for methylene blue removal.

GO and nZVI have been used for removing different contaminants from aqueous solution; however, difficulty in the separation of GO, and the aggregation propensity of nZVI particles prevent them from having efficient practical applications. In this study, a green synthesis method was performed to prepare nanohybrids of GO and nZVI to provide an adsorbent with high adsorption efficiency that can be removed from aqueous solution easily by magnetic separation. GO-nZVI nanohybrids were synthesized by using biocompatible cross linkers named 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The effect of the nZVI ratio in the synthesized nanohybrids was studied at three different ratios of GO : nZVI, 1 : 1, 1 : 5 and 1 : 10. SEM/EDS, HRTEM, STEM/EDS, XRD, Raman, FTIR, and TGA analyses were conducted to provide physical and chemical properties of the adsorbents. The performance of nZVI and GO-nZVI nanohybrids as an adsorbent have been studied for methylene blue (MB) removal from an aqueous solution with an initial concentration of 12 mg L-1 at adsorbent dosages of 0.1, 0.3, 0.5, and 1 mg mL-1. Results indicated that GO-nZVI (1 : 5) provided the highest MB removal (99.1%) by using 10 mL of the 1 mg mL-1 adsorbent. After regeneration of the GO-nZVI (1 : 5) nanohybrids with ethanol, 84.3%, 67.2%, and 63.0% of MB removal were achieved in the first to third regeneration cycle. Results also showed that the GO-nZVI nanohybrids were not affected by aggregation compared to nZVI.

Carboxymethylcellulose Mediates the Transport of Carbon Nanotube-Magnetite Nanohybrid Aggregates in Water-Saturated Porous Media.

Carbon-metal oxide nanohybrids (NHs) are increasingly recognized as the next-generation, promising group of nanomaterials for solving emerging environmental issues and challenges. This research, for the first time, systematically explored the transport and retention of carbon nanotube-magnetite (CNT-Fe3O4) NH aggregates in water-saturated porous media under environmentally relevant conditions. A macromolecule modifier, carboxymethylcellulose (CMC), was employed to stabilize the NHs. Our results show that transport of the magnetic CNT-Fe3O4 NHs was lower than that of nonmagnetic CNT due to larger hydrodynamic sizes of NHs (induced by magnetic attraction) and size-dependent retention in porous media. Classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory can explain the mobility of NHs under varying experimental conditions. However, in contrast with colloid filtration theory, a novel transport feature-an initial lower and a following sharp-higher peaks occurred frequently in the NHs' breakthrough curves. The magnitude and location of both transport peaks varied with different experimental conditions, due to the interplay between variability of fluid viscosity and size-selective retention of the NHs. Promisingly, the estimated maximum transport distance of NHs ranged between ∼0.38 and 46 m, supporting the feasibility of employing the magnetically recyclable CNT-Fe3O4 NHs for in situ nanoremediation of contaminated soil, aquifer, and groundwater.