Indrek Külaots

Aerosol synthesis of cargo-filled graphene nanosacks

Water microdroplets containing graphene oxide and a second solute are shown to spontaneously segregate into sack-cargo nanostructures upon drying. Analytical modeling and molecular dynamics suggest the sacks form when slow-diffusing graphene oxide preferentially accumulates and adsorbs at the receding air-water interface, followed by capillary collapse. Cargo-filled graphene nanosacks can be nanomanufactured by a simple, continuous, scalable process and are promising for many applications where nanoscale materials should be isolated from the environment or biological tissue.

Size distribution of unburned carbon in coal fly ash and its implications

Abstract A set of nine coal fly ashes, obtained from various US utilities, were fractionated by standard dry-sieving techniques. The carbon contents of the different size fractions were measured, and the nature of the carbon particles was microscopically examined. Significant differences were found in the distribution of carbon in class F and class C ashes. The ‘foam index’ test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. This test measures adsorption of air entraining admixtures (AEAs) by the carbon in the fly ash. Application of this test to the different ash fractions confirmed that the smallest particle size fractions of ash make the major contribution to AEA adsorption. The carbon from class F ash has a comparable capacity for AEA adsorption as carbon from class C ash, when compared on a surface area basis. What makes the class C carbons apparently ‘worse’ is the fact that they have much higher surface areas than class F carbons (and it is only by virtue of the low carbon mass in most class C ashes that problems with these ashes are not more common). The importance of accessibility of the surface is also clearly seen from these results.

Mechanisms of surfactant adsorption on non-polar, air-oxidized and ozone-treated carbon surfaces

Ozone treatment of fly ash carbon has recently been reported to inhibit the adsorption of commercial surfactants in concrete paste, thus mitigating the known negative effects of carbon on ash utilization. This paper examines the general mechanism of surfactant adsorption on carbon and its suppression by surface oxidation. Experimental results are presented for two carbon types (carbon black, fly ash carbon), both raw and surface oxidized (by ozone and molecular oxygen) and several commercial anionic and non-ionic surfactants (Darex II, SDS, Tergitol). The treated carbon surfaces were characterized with XPS, FT-IR, thermal desorption in N and H / He, surface acidity, hygroscopic behavior, interfacial 22 energy and its components through contact angle measurement involving standard liquid probes. Surface oxidation is found to decrease surfactant adsorption in each of the carbon / oxidant / surfactant systems examined, and its effect correlates with the amount of surface oxides by XPS. The combined results suggest that surfactant adsorption primarily occurs on non-polar carbon surface patches where it is driven by hydrophobic interactions. The main mechanism of oxidative suppression is the destruction of this non-polar surface, though micropore blockage and increased negative surface charge may also contribute for some systems.  2003 Elsevier Science Ltd. All rights reserved.

Mercury Vapor Release from Broken Compact Fluorescent Lamps and In Situ Capture by New Nanomaterial Sorbents

The projected increase in the use of compact fluorescent lamps (CFLs) motivates the development of methods to manage consumer exposure to mercury and its environmental release at the end of lamp life. This work characterizes the time-resolved release of mercury vapor from broken CFLs and from underlying substrates after removal of glass fragments to simulate cleanup. In new lamps, mercury vapor is released gradually in amounts that reach 1.3 mg or 30% of the total lamp inventory after four days. Similar time profiles but smaller amounts are released from spent lamps or from underlying substrates. Nanoscale formulations of S, Se, Cu, Ni, Zn, Ag, and WS2 are evaluated for capture of Hg vapor under these conditions and compared to conventional microscale formulations. Adsorption capacities range over 7 orders of magnitude, from 0.005 (Zn micropowder) to 188 000 μg/g (unstabilized nano-Se), depending on sorbent chemistry and particle size. Nanosynthesis offers clear advantages for most sorbent chemistries. Unstabilized nano-selenium in two forms (dry powder and impregnated cloth) was successfully used in a proof-of-principle test for the in situ, real-time suppression of Hg vapor escape following CFL fracture.

Counting Calories in Drosophila Diet Restriction

The extension of life span by diet restriction in Drosophila has been argued to occur without limiting calories. Here we directly measure the calories assimilated by flies when maintained on full- and restricted-diets. We find that caloric intake is reduced on all diets that extend life span. Flies on low-yeast diet are long-lived and consume about half the calories of flies on high-yeast diets, regardless of the energetic content of the diet itself. Since caloric intake correlates with yeast concentration and thus with the intake of every metabolite in this dietary component, it is premature to conclude for Drosophila that calories do not explain extension of life span.

Encapsulation of Particle Ensembles in Graphene Nanosacks as a New Route to Multifunctional Materials

Hybrid nanoparticles with multiple functions are of great interest in biomedical diagnostics, therapies, and theranostics but typically require complex multistep chemical synthesis. Here we demonstrate a general physical method to create multifunctional hybrid materials through aerosol-phase graphene encapsulation of ensembles of simple unifunctional nanoparticles. We first develop a general theory of the aerosol encapsulation process based on colloidal interactions within drying microdroplets. We demonstrate that a wide range of cargo particle types can be encapsulated, and that high pH is a favorable operating regime that promotes colloidal stability and limits nanoparticle dissolution. The cargo-filled graphene nanosacks are then shown to be open structures that rapidly release soluble salt cargoes when reintroduced into water, but can be partially sealed by addition of a polymeric filler to achieve slow release profiles of interest in controlled release or theranostic applications. Finally, we demonstrate an example of multifunctional material by fabricating graphene/Au/Fe3O4 hybrids that are magnetically responsive and show excellent contrast enhancement as multimodal bioimaging probes in both magnetic resonance imaging and X-ray computed tomography in full-scale clinical instruments.

Antioxidant Deactivation on Graphenic Nanocarbon Surfaces

This article reports a direct chemical pathway for antioxidant deactivation on the surfaces of carbon nanomaterials. In the absence of cells, carbon nanotubes are shown to deplete the key physiological antioxidant glutathione (GSH) in a reaction involving dissolved dioxygen that yields the oxidized dimer, GSSG, as the primary product. In both chemical and electrochemical experiments, oxygen is only consumed at a significant steady-state rate in the presence of both nanotubes and GSH. GSH deactivation occurs for single- and multi-walled nanotubes, graphene oxide, nanohorns, and carbon black at varying rates that are characteristic of the material. The GSH depletion rates can be partially unified by surface area normalization, are accelerated by nitrogen doping, and suppressed by defect annealing or addition of proteins or surfactants. It is proposed that dioxygen reacts with active sites on graphenic carbon surfaces to produce surface-bound oxygen intermediates that react heterogeneously with glutathione to restore the carbon surface and complete a catalytic cycle. The direct catalytic reaction between nanomaterial surfaces and antioxidants may contribute to oxidative stress pathways in nanotoxicity, and the dependence on surface area and structural defects suggest strategies for safe material design.

ADSORPTION OF SURFACTANTS ON UNBURNED CARBON IN FLY ASH AND DEVELOPMENT OF A STANDARDIZED FOAM INDEX TEST

Abstract The “foam index” test is commonly used for quick evaluation of the suitability, with respect to air entrainment, of pozzolanic additives for concrete. Many foam index test procedures are in common use, and it is difficult to compare results between laboratories. The present paper explores the possibility of standardizing the test for use with coal fly ash pozzolans. It does so by establishing that a series of commercial air-entraining admixtures (AEAs) and pure anionic surfactants all behave in a well-correlated manner, when tested using the same protocol, on a suite of 29 fly ash samples (both class F and class C) obtained from utilities throughout the United States. A pure, reagent-grade surfactant can be chosen as the basis for a standardized test; it appears as though dodecyl benzene sulfonate (DBS) is a good candidate material for such testing. The present results also confirm that it is the carbon in coal fly ash that is the main sink for AEA adsorption in concrete mixtures containing significant amounts of ash. The solution chemistry in the testing mixture is important, and use of cement in the test mix is strongly recommended. Careful attention also needs to be paid to the relative amounts of different components in the mixture to be tested.

Ozonation for the chemical modification of carbon surfaces in fly ash

1 Brown University, Providence RI, 2 Electric Power Research Institute, 3 PCI-WedecoKEYWORDS: concrete, beneficiation, LOI, air entrainmentINTRODUCTIONA practical problem with fly ash concrete is the tendency of residual carbon in ash tointerfere with the air entrainment process. Porous carbon adsorbs the chemicalsurfactants (air entraining admixtures, or AEAs) used to generate and stabilize a micro-void system in concrete pastes, as shown by Helmuth (1987), Freeman et al. (1997),and Hill et al. (1997). Without a sufficient network of sub-millimeter air bubbles,concrete fails under internal pressure generated by the freezing and expansion oftrapped residual water. About two-thirds of the concrete in North America is airentrained (Dolch, 1995), and this surfactant adsorption phenomenon is the primarydriving force for national and regional regulations limiting the carbon content of ashused in concrete. Recent work by Freeman et al. (1997), Gao et al. (1998), and Yu etal. (2000) clearly demonstrate great variability in the extent to which field ash samplesadsorb AEAs. This recent work has identified the following four primary factorsgoverning ash adsorptivity:1. the mass fraction carbon2. the total surface area of the carbon3. the accessibility of that surface, as governed by particle size and pore size distribution4. the carbon surface chemistry.The inorganic fraction of ash is found to play a minor role in AEA adsorption.The role of carbon surface chemistry is particularly apparent from the behavior of ashduring thermal oxidation in air. Introduction of surface oxides by exposure to air at 350-450 C has been observed by Hachmann et al (1998) to significantly reduce subsequentAEA adsorption without consuming a measurable amount of carbon. In contrast,treatment in inert gas at temperatures sufficient to drive-off many pre-existing surfaceoxides (900 o C) has been observed to

The effect of solid fuel type and combustion conditions on residual carbon properties and fly ash quality

Research on pulverized fuel burnout mechanisms derives its primary motivation from industrial problems with the utilization of high-carbon fly ash. The severity of these problems depends on residual carbon properties , but combustion research has focused almost exclusively on the degree of burnout, which determines only the amount of residual carbon. This paper presents measurements of three important residual carbon properties: reactivity, surface area, and adsorptivity toward surfactants, the latter property being the most critical for ash quality in many concrete applications. The paper investigates the independent effects of fuel type and combustion conditions by analyzing samples derived from bench, pilot, and commercial scale combustion processes. Residual carbon surface area is observed to be strongly dependent on parent fuel type ranging from about 1 m 2 /g for petroleum coke residues to values up to 400 m 2 /g for low-rank coal residues. Surfactant adsorptivity also varies strongly with parent fuel and shows the best correlation with residual carbon surface area in pores larger than 2 nm, likely because of the large size of surfactant molecules and slow liquid-phase diffusion. The surfactant adsorptivity is also sensitive to the presence of carbon surface oxides introduced by low-temperature oxidation using air or ozone. The data clearly indicate that oxides suppress adsorptivity, and that the underlying mechanism is an increase in carbon surface polarity and hydrophilicity. Controlled pilot-scale experiments show that air staging for NO x control often produces residual carbon with a high adsorptivity, even when adsorptivity is expressed per gram of carbon. Successful staging conditions that lead both to low-NO x and high burnout are observed to produce residual carbons with lower specific surface area and adsorptivity, so poor carbon quality is not an intrinsic feature of all low-NO x flames.