Carl W. Isaacson

Fullerene nanoparticles exhibit greater retention in freshwater sediment than in model porous media.

Increasing production and use of fullerene-based nanomaterials underscore the need to determine their mobility in environmental transport pathways and potential ecological exposures. This study investigated the transport of two fullerenes (i.e., aqu/C(60) and water-soluble C(60) pyrrolidine tris-acid [C(60) PTA]) in columns packed with model porous media (Iota quartz and Ottawa sand) and a sediment from Calls creek under saturated and unsaturated steady-state flows. The fullerenes had the least retention in Iota quartz, and the greatest retention in the sediment at near neutral pH, correlating with the degree of grain surface chemical heterogeneity (e.g., amorphous Al hydroxides concentration increasing in the order of Iota quartz<Ottawa sand<sediment). Surface roughness was elucidated as another important factor responsible for the greatest fullerene retention in the sediment. In accordance with the XDLVO energy calculations, C(60) PTA was less retained than aqu/C(60) at near neutral pH, due to its greater hydrophilicity measured by tolune-water partition coefficient, as well as smaller particle sizes revealed by atomic force microscopy. Fullerene retention exhibited a strong dependency on solution pH that could be explained partly by the pH-dependent surface charge of fullerenes and grain surface, and partly by increased hydrophobicity of C(60) PTA when solution pH approaches its isoelectric point (IEP). Finally, fullerene retention was enhanced in unsaturated media, implying that fullerenes may be more attenuated in the vadose zone than in groundwater.

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 Calls 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 Calls 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 Calls 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 Calls Creek treatment.

Temporal changes in Aqu/C60 physical-chemical, deposition, and transport characteristics in aqueous systems.

Little is known about how temporal changes in the physical-chemical properties of C₆₀ aggregates formed in aqueous systems (termed aqu/C₆₀) can impact transport pathways contributing to ecological exposures. In this study three aqu/C₆₀ suspensions of short-term (100 days), intermediate-term (300 days), and long-term (1000 days) water exposure were first characterized for particle size distribution, water/toluene phase distribution, and surface chemistry. Then, aqu/C₆₀ deposition to a model silica surface and transport in porous media were studied by quartz crystal microbalance (QCM) and saturated sand columns. As suspension time increased, aqu/C₆₀ particle size shifted to a larger size range as determined by asymmetric flow field-flow fractionation (AF4) and the aqu/C₆₀ distribution to toluene was reduced, likely due to surface polarization as revealed by nuclear magnetic resonance (NMR) and UV-visible spectroscopy of the aqu/C₆₀ suspensions. Additionally, the deposition to silica surfaces in both QCM and column studies decreased with increased water exposure time. Although a small increase in aqu/C₆₀ aggregate size with time may partially explain the greater transport of the long-term aqu/C₆₀ because of the decreased collector efficiency for larger submicrometer particles, the polarization of the aqu/C₆₀ (thus a more hydrophilic surface) revealed by the toluene/water phase distribution and confirmed by NMR, is considered the determining factor.

Biological control of biofilms on membranes by metazoans.

Traditionally, chemical and physical methods have been used to control biofouling on membranes by inactivating and removing the biofouling layer. Alternatively, the permeability can be increased using biological methods while accepting the presence of the biofouling layer. We have investigated two different types of metazoans for this purpose, the oligochaete Aelosoma hemprichi and the nematode Plectus aquatilis. The addition of these grazing metazoans in biofilm-controlled membrane systems resulted in a flux increase of 50% in presence of the oligochaetes (Aelosoma hemprichi), and a flux increase of 119-164% in presence of the nematodes (Plectus aquatilis) in comparison to the control system operated without metazoans. The change in flux resulted from (1) a change in the biofilm structure, from a homogeneous, cake-like biofilm to a more heterogeneous, porous structure and (2) a significant reduction in the thickness of the basal layer. Pyrosequencing data showed that due to the addition of the predators, also the community composition of the biofilm in terms of protists and bacteria was strongly affected. The results have implications for a range of membrane processes, including ultrafiltration for potable water production, membrane bioreactors and reverse osmosis.

Changes in agglomeration of fullerenes during ingestion and excretion in Thamnocephalus platyurus

The crustacean Thamnocephalus platyurus was exposed to aqueous suspensions of fullerenes C(60) and C(70) . Aqueous fullerene suspensions were formed by stirring C(60) and C(70) as received from a commercial vendor in deionized water (termed aqu/C(60) and aqu/C(70) ) for approximately 100 d. The Z-average (mean hydrodynamic) diameters of aqu/C(60) and aqu/C(70) aggregates as measured by dynamic light scattering were 517 ± 21 nm and 656 ± 39 nm (mean ± 95% confidence limit), respectively. Exposure of T. platyurus to fullerene suspensions resulted in the formation of dark masses in the digestive track visible under a stereo microscope (×40 magnification). Fullerene ingestion over 1 h of exposure was quantitatively determined after extraction and analysis by high-performance liquid chromatography-mass spectrometry (HPLC-MS). One-hour exposures (at 3 mg/L and 6 mg/L) resulted in aqu/C(60) burdens of 2.7 ± 0.4 µg/mg and 6.8 ± 1.5 µg/mg wet weight, respectively. Thin-section transmission electron microscopy (TEM) images of aqu/C(60) -exposed T. platyurus showed the formation in the gut of fullerene agglomerates (5-10 µm) that were an order of magnitude larger than the suspended fullerene agglomerates. Upon excretion, the observed fullerene agglomerates were in the 10- to 70-µm size range and settled to the bottom of the incubation wells. In contrast to the control polystyrene microspheres, which dispersed after depuration, the aqu/C(60) agglomerates (greater than two orders of magnitude larger than the suspended fullerenes) remained agglomerated for up to six months. When exposed to fullerenes, T. platyurus shows the potential to influence agglomerate size and may facilitate movement of these nanoparticles from the water column into sediment.

Interactions of TiO2 nanoparticles and the freshwater nematode Plectus aquatilis: particle properties, kinetic parameters and bioconcentration factors

Principles for determining uptake kinetics and bioconcentration factors for nanoparticles and test organisms have only been cursorily explored. Here we report the derivation of bioconcentration factors (BCFs) and the role of surface functionalization in the interactions between the nematode Plectus aquatilis and titanium dioxide nanoparticles (TiO2 NPs) dispersed in freshwater. Because of the high background concentration of titanium in natural waters, TiO2 NPs irradiated to produce 48V labeled TiO2 NPs, or doped with 1% niobium, were used to determine BCFs taking either an equilibrium partitioning or a kinetic modelling approach. The BCFs based on equilibrium partitioning increased from 71 (±17) L kg−1 at the highest exposure concentrations to 5.1 (±3.2) × 103 L kg−1 at the lowest exposure concentration, indicating that this approach is not valid. A kinetic modeling approach, based on the uptake rate and elimination rates for a two-phase elimination, best reflected the experimental data and was then used to determine BCFs. To further rationalize the kinetic interactions between the TiO2 NPs and the nematode, organisms were exposed to surface-functionalized TiO2 NPs with positively charged, negatively charged, steric stabilizing and environmentally relevant coatings. Correlations between the extent of TiO2 NP association (i.e. attached and internalized) with the nematode, and TiO2 NP properties, were examined. For all parameters considered, association of the surface functionalized TiO2 NPs with the nematode best correlated with the TiO2 NP sedimentation rate. These results indicate that concepts developed for hydrophobic contaminants are not applicable to nanoparticles and concepts specific to nanoparticles will be of greater utility.