Effects of carbonaceous nanomaterials on soil-grown soybeans under combined heat and insect stresses

Because carbonaceous nanomaterials (CNMs) are expected to enter soils, the exposure implications to crop plants and plant-microbe interactions should be understood. Most investigations have been under ideal growth conditions, yet crops commonly experience abiotic and biotic stresses. Little is known how co-exposure to these environmental stresses and CNMs would cause combined effects on plants. We investigated the effects of 1000 mg kg^-1 multiwalled carbon nanotubes (CNTs), graphene nanoplatelets (GNPs) and industrial carbon black (CB) on soybeans grown to the bean production stage in soil. Following seed sowing, plants became stressed by heat and infested with an insect (thrips). Consequently, all plants had similarly stunted growth, leaf damage, reduced final biomasses and fewer root nodules compared with healthy control soybeans previously grown without heat and thrips stresses. Thus, CNMs did not significantly influence the growth and yield of stressed soybeans, and the previously reported nodulation inhibition by CNMs was not specifically observed here. However, CNMs did significantly alter two leaf health indicators: the leaf chlorophyll a/b ratio, which was higher in the GNP treatment than in either the control (by 15 %) or CB treatment (by 14 %), and leaf lipid peroxidation, which was elevated in the CNT treatment compared with either the control (by 47 %) or GNP treatment (by 66 %). Overall, these results show that, while severe environmental stresses may impair plant production, CNMs (including CNTs and GNPs) in soil could additionally affect foliar health of an agriculturally important legume.

Modeling framework for simulating concentrations of solute chemicals, nanoparticles, and solids in surface waters and sediments: WASP8 Advanced Toxicant Module

Toxicant concentrations in surface waters are of environmental concern due to their potential impacts to humans and wildlife. Numerical models provide system insight, support management decisions, and provide scenario testing on the impacts of toxicants. The Water Quality Analysis Simulation Program (WASP) is a widely used framework for developing site-specific models for simulating toxicant concentrations in surface waters and sediments over a range of complexities and temporal and spatial scales. WASP8, with the Advanced Toxicant module, has been recently released, incorporating a complete architecture redesign for an increased number of state variables and different state variable types. WASP8 incorporates a new structure for simulating light intensity and photoreactions in the water column, including the distinction of 10 different wavelength bands, and nanoparticle heteroaggregation to solids. We present a hypothetical case study, using the Cape Fear River, North Carolina as a representative example for simulating solute chemicals, nanoparticles, and solids to demonstrate the new and updated capabilities of WASP8.

Agglomeration Determines Effects of Carbonaceous Nanomaterials on Soybean Nodulation, Dinitrogen Fixation Potential, and Growth in Soil

The potential effects of carbonaceous nanomaterials (CNMs) on agricultural plants are of concern. However, little research has been performed using plants cultivated to maturity in soils contaminated with various CNMs at different concentrations. Here, we grew soybean for 39 days to seed production in soil amended with 0.1, 100, or 1000 mg kg-1 of either multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), or carbon black (CB) and studied plant growth, nodulation, and dinitrogen (N2) fixation potential. Plants in all CNM treatments flowered earlier (producing 60% to 372% more flowers when reproduction started) than the unamended controls. The low MWCNT-treated plants were shorter (by 15%) with slower leaf cover expansion (by 26%) and less final leaf area (by 24%) than the controls. Nodulation and N2 fixation potential appeared negatively impacted by CNMs, with stronger effects at lower CNM concentrations. All CNM treatments reduced the whole-plant N2 fixation potential, with the highest reductions (by over 91%) in the low and medium CB and the low MWCNT treatments. CB and GNPs appeared to accumulate inside nodules as observed by transmission electron microscopy. CNM dispersal in aqueous soil extracts was studied to explain the inverse dose-response relationships, showing that CNMs at higher concentrations were more agglomerated (over 90% CNMs settled as agglomerates >3 μm after 12 h) and therefore proportionally less bioavailable. Overall, our findings suggest that lower concentrations of CNMs in soils could be more impactful to leguminous N2 fixation, owing to greater CNM dispersal and therefore increased bioavailability at lower concentrations.

Surfactant-Wrapped Multiwalled Carbon Nanotubes in Aquatic Systems: Surfactant Displacement in the Presence of Humic Acid

Sodium dodecyl sulfate (SDS) facilitates multiwalled carbon nanotube (MWCNT) debundling and enhances nanotube stability in the aqueous environment by adsorbing on the nanotube surfaces, thereby increasing repulsive electrostatic forces and steric effects. The resulting SDS-wrapped MWCNTs are utilized in industrial applications and have been widely employed in environmental studies. In the present study, MWCNTs adsorbed SDS during ultrasonication to form stable MWCNTs suspensions. Desorption of SDS from MWCNTs surfaces was then investigated as a function of Suwannee River Humic Acid (SRHA) and background electrolyte concentrations. Due to hydrophobic effects and π-π interactions, MWCNTs exhibit higher affinity for SRHA than SDS. In the presence of SRHA, SDS adsorbed on MWCNTs was displaced. Cations (Na(+), Ca(2+)) decreased SDS desorption from MWCNTs due to charge screening effects. Interestingly, the presence of the divalent calcium cation facilitated multilayered SRHA adsorption on MWCNTs through bridging effects, while monovalent sodium reduced SRHA adsorption. Results of the present study suggest that properties of MWCNTs wrapped with commercial surfactants will be altered when these materials are released to surface waters and the surfactant coating will be displaced by natural organic matter (NOM). Changes on their surfaces will significantly affect MWCNTs fate in aquatic environments.

Multiwalled carbon nanotube dispersion methods affect their aggregation, deposition, and biomarker response

To systematically evaluate how dispersion methods affect the environmental behaviors of multiwalled carbon nanotubes (MWNTs), MWNTs were dispersed in various solutions (e.g., surfactants, natural organic matter (NOM), and etc.) via ultrasonication (SON) and long-term stirring (LT). The two tested surfactants [anionic sodium dodecyl sulfate (SDS) and nonionic poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEO-PPO-PEO) triblock copolymers (Pluronic)] could only disperse MWNTs via ultrasonication; while stable aqueous SON/MWNT and LT/MWNT suspensions were formed in the presence of the two model NOMs (Suwannee river humic acid and fulvic acid). Due to the inherent stochastic nature for both methods, the formed MWNT suspensions were highly heterogeneous. Their physicochemical properties, including surface charge, size, and morphology, greatly depended upon the dispersant type and concentration but were not very sensitive to the preparation methods. Aggregation and deposition behaviors of the dispersed MWNTs were controlled by van der Waal and electrostatic forces, as well as other non-DLVO forces (e.g., steric, hydrophobic forces, etc.). Unlike the preparation method-independent physicochemical properties, LT/NOM-MWNTs and SON/NOM-MWNTs differed in their fathead minnow epithelial cell metabolomics profiles.

Multiwalled carbon nanotube deposition on model environmental surfaces

Deposition of multiwalled carbon nanotubes (MWNTs) on model environmental surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Deposition behaviors of MWNTs on positively and negatively charged surfaces were in good agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, although hydrophobic interactions dominated MWNTs deposition on a hydrophobic polystyrene surface. Initial deposition rates (rf) and deposition attachment efficiencies (αD) depended on solution ionic strengths (IS) and surface electrostatic properties. Identical rf and αD values at constant IS on similar surfaces suggested that deposition was insensitive to surface morphology (i.e., bare crystal surface vs coated surface). The dissipation unit (D) was used with frequency (f) to investigate nanoparticle deposition: |ΔD/Δf| values varied for deposition on different surfaces, indicating that the nature of MWNT association with surfaces varied despite constant rf and αD values.

Effects of humic acid and sunlight on the generation and aggregation state of aqu/C60 nanoparticles

Aqueous suspensions of nanoscale C(60) aggregates (aqu/C(60)) were produced by stirring in water with Suwanee River Humic Acid (humic acid) and water from Call's Creek, a small stream near Athens, GA. Time course experiments were conducted to determine the effects of sunlight and solution chemistry on the mass of aqu/C(60) suspended, nanoparticle size, and ζ potential. For all treatments, sunlight had the greatest effect on the mass of aqu/C(60) suspended. The sunlight-exposed Call's Creek samples exhibited the greatest increase in mass suspended with aqu/C(60) concentrations 17 times greater than those of the dark controls, followed by the humic acid treatments, 8.1 times, and deionized water, 3.4 times. Asymmetric flow field-flow fractionation indicated that aqu/C(60) nanoparticles in humic acid were the smallest and their mass was evenly distributed in the 120-300 nm hydrodynamic diameter (D(h)) size range, whereas aqu/C(60) nanoparticles in Call's Creek water were the largest and were evenly distributed in the size range of 200-300 nm D(h). Aqu/C(60) in deionized water and humic acid treatments exposed to sunlight exhibited a trend of increasingly negative ζ potentials as suspension time increased; however, this trend was not observed for the Call's Creek treatment.

Cosolvent effects of phenanthrene sorption-desorption on a freshwater sediment

This study evaluated the effects of the water-miscible cosolvent methanol on the sorption-desorption of phenanthrene by the natural organic matter (NOM) of a freshwater sediment. A biphasic pattern was observed in the relationship between the log of the carbon-normalized sorption distribution coefficient (Koc) and the volumetric fraction of water-miscible cosolvent (fc), in this case methanol, that was accounted for with phenanthrene solubility data. Results also indicated that methanol elicited additional effects on phenanthrene sorption beyond the solution phase effects. The level off, was observed to have a significant effect on sorption-desorption hysteresis, with hysteresis being greater for treatments where phenanthrene and sediment were equilibrated with methanol-phenanthrene solutions but desorbed with aqueous solutions. These results are discussed in light of the deformable pore network of the rubbery/glassy NOM polymer construct.

Cosolvent effects on sorption isotherm linearity

Sorption-desorption hysteresis, slow desorption kinetics, and other nonideal phenomena have been attributed to the differing sorptive characteristics of the natural organic polymers associated with soils and sediments. In this study, aqueous and mixed solvent systems were used to investigate the effects of a cosolvent, methanol, on sorption isotherm linearity with natural organic matter (NOM), and to evaluate whether these results support, or weaken, the rubbery/glassy polymer conceptualization of NOM. All of the sorption isotherms displayed some nonlinear character. Our data indicates that all of the phenanthrene and atrazine isotherms were nonlinear up to the highest equilibrium solution concentration to solute solubility in water or cosolvent ratios (Ce/Sw,c) used, approximately 0.018 and 0.070, respectively. Isotherm linearity was also observed to increase with volumetric methanol content (fc). This observation is consistent with the NOM rubbery/glassy polymer conceptualization: the presence of methanol in NOM increased isotherm linearity as do solvents in synthetic polymers, and suggests that methanol is interacting with the NOM, enhancing its homogeneity as a sorptive phase so that sorption is less bimodal as fc increases. When the equilibrium solution concentration was normalized for solute solubility in water or methanol-water solutions, greater relative sorption magnitude was observed for the methanol-water treatments. This observation, in conjunction with the faster sorption kinetics observed in the methanol-water sediment column systems, indicates that the increase in relative sorption magnitude with fc may be attributed to the faster sorption kinetics in the methanol-water systems, and hence, greater relative sorptive uptake for the rubbery polymer fraction of NOM at similar time scales.

Pesticide sorption on geologic material of varying organic carbon content

Sorption of three pesticides on geologic material ranging in organic carbon content from 0.33 to 6.9 g kg-1 was measured in soil columns using a miscible displacement technique. An octanol-water partitioning model was shown to be inappropriate for predicting sorption of the less hydrophobic pesticides on the low organic carbon materials.