Andrew T. Harris

Toxicity, Uptake, and Translocation of Engineered Nanomaterials in Vascular plants

To exploit the promised benefits of engineered nanomaterials, it is necessary to improve our knowledge of their bioavailability and toxicity. The interactions between engineered nanomaterials and vascular plants are of particular concern, as plants closely interact with soil, water, and the atmosphere, and constitute one of the main routes of exposure for higher species, i.e. accumulation through the food chain. A review of the current literature shows contradictory evidence on the phytotoxicity of engineered nanomaterials. The mechanisms by which engineered nanomaterials penetrate plants are not well understood, and further research on their interactions with vascular plants is required to enable the field of phytotoxicology to keep pace with that of nanotechnology, the rapid evolution of which constantly produces new materials and applications that accelerate the environmental release of nanomaterials.

Facile synthesis, stabilization, and anti-bacterial performance of discrete Ag nanoparticles using Medicago sativa seed exudates

The biogenic synthesis of metal nanomaterials offers an environmentally benign alternative to the traditional chemical synthesis routes. Colloidal silver (Ag) nanoparticles were synthesized by reacting aqueous AgNO(3) with Medicago sativa seed exudates under non-photomediated conditions. Upon contact, rapid reduction of Ag(+) ions was observed in <1 min with Ag nanoparticle formation reaching 90% completion in <50 min. Effect of Ag concentration, quantity of exudate and pH on the particle size and shape were investigated. At [Ag(+)]=0.01 M and 30°C, largely spherical nanoparticles with diameters in the range of 5-51 nm were generated, while flower-like particle clusters (mean size=104 nm) were observed on treatment at higher Ag concentrations. Pre-dilution of the exudate induced the formation of single-crystalline Ag nanoplates, forming hexagonal particles and nanotriangles with edge lengths of 86-108 nm, while pH adjustment to 11 resulted in monodisperse Ag nanoparticles with an average size of 12 nm. Repeated centrifugation and redispersion enhanced the percentage of nanoplates from 10% to 75% in solution. The kinetics of nanoparticle formation were monitored using ultraviolet-visible spectroscopy and the Ag products were characterized using transmission electron microscopy, selected-area electron diffraction, scanning electron microscopy, X-ray powder diffraction, and atomic force microscopy. X-ray photoelectron spectroscopy was used to investigate the elements and chemical environment in the top layers of the as-synthesized Ag nanoparticles, while the metabolites in the exudate were analyzed using gas chromatography-mass spectroscopy. To our knowledge, this is the first account of M. sativa seed exudate assisted synthesis and stabilization of biogenic Ag nanoparticles; the nanoplates are notably smaller and better faceted compared with those synthesized by vascular plant extracts previously reported. Stabilized films of exudate synthesized Ag nanoparticles were effective anti-bacterial agents.

Dichotomous adsorption behaviour of dyes on an amino-functionalised metal–organic framework, amino-MIL-101(Al)

An amino-functionalised metal–organic framework (MOF), aluminium aminoterephthalate (amino-MIL-101(Al)), has been applied to the adsorptive removal of dyes (cationic methylene blue, MB, and anionic methyl orange, MO) from aqueous solutions in order to examine the effect of the amino group on sorption behaviour. Adsorption isotherms and thermodynamic studies indicated the spontaneous adsorption of MB with a maximum adsorption capacity at 30 °C (762 ± 12 mg gMOF−1) higher than those observed for MB on other MOFs and most other materials. In contrast, lower adsorption capacities were observed in the adsorption of the same dye on the analogous non-amino-functionalised framework (MIL-101(Al), 195 mg g−1) and in the adsorption of MO by amino-MIL-101(Al) (188 ± 9 mg g−1), suggesting that an electrostatic interaction between the amino groups of the MOF and the cationic dye MB may have contributed to the high adsorption capacity. The adsorptions of both dyes on amino-MIL-101(Al) were spontaneous, endothermic, and entropy-driven, as is common for dye adsorptions. However, the ΔS value obtained for the adsorption of MB (346 J mol−1 K−1) was extreme. Further analysis demonstrated that after exposure to MB, the ordered amino-MIL-101(Al) structure was absent, ∼30% of the Al3+ was lost to solution, and significant changes occurred in the X-ray photoelectron spectrum of the MOF. On the other hand, the MOF structure was intact following the adsorption of MO. Several groups have exploited electrostatic interactions to improve dye adsorption; however, these proved excessive in the case of MB (but not MO) adsorption on amino-MIL-101(Al).

A review of techno-economic models for the retrofitting of conventional pulverised-coal power plants for post-combustion capture (PCC) of CO2

In this paper, we compare and contrast the four most promising (i.e. commercially viable in the near future) technologies for the post-combustion capture (PCC) of CO2 that can be retrofitted to a conventional pulverised-coal power plant. These are CO2 capture using: (i) chilled ammonia, (ii) alkali-metal carbonates, (iii) membranes and (iv) calcium looping. These four technologies are compared to the benchmark monoethanolamine (MEA) scrubbing process in terms of efficiency penalty and cost indicators. We first review the relevant CO2 capture chemistry and typical process flow schematics, and then discuss energy- and mass-balance considerations. We consider 18 published techno-economic studies on these technologies and highlight the key measures, including net CO2 capture rate, net power output, net plant efficiency, capital costs, cost of electricity and cost of CO2 avoided. Calcium looping technology results in the lowest efficiency penalty (4.6%-points) and cost of PCC (36.3% increase in levelised cost of electricity). In addition, the cost of CO2 avoided by employing calcium looping for PCC can be as low as 29 USD2010 per tCO2. On all three of these criteria, calcium looping performs more than twice as well as the benchmark MEA PCC process.

The prediction of particle cluster properties in the near wall region of a vertical riser (200157)

Abstract Correlations for predicting the properties of clusters of particles travelling near the riser wall are presented. The correlations were developed from experimental data published in the literature on vertical risers ranging from laboratory to industrial scale. Expressions are presented for predicting the size, shape, density, wall film coverage and velocity of particle clusters. These expressions should prove useful in the development of heat transfer and process models for gas–solid riser flow.

Compositional effects of PEDOT-PSS/single walled carbon nanotube films on supercapacitor device performance

Supercapacitors are promising energy storage and power output technologies due to their improved energy density, rapid charge-discharge cycle, high cycle efficiency and long cycle life. Free standing poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate)/single walled nanotube films have been characterised by scanning electron microscopy, Raman spectroscopy and thermo-gravimetric analysis to understand the physical properties of the films. Films with varying compositions of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) and single walled nanotubes were compared by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge/discharge to understand their electrochemical properties. A comparison of the results shows that having single walled nanotubes dispersed throughout the polymer matrix increase the capacitance by 65% and the energy density by a factor of 3 whilst achieving good capacity retention over 1000 cycles.

Multiwalled carbon nanotubes in alfalfa and wheat: toxicology and uptake

Data on the bioavailability and toxicity of carbon nanotubes (CNTs) in the environment, and, in particular, on their interactions with vascular plants, are limited. We investigated the effects of industrial-grade multiwalled CNTs (75 wt% CNTs) and their impurities on alfalfa and wheat. Phytotoxicity assays were performed during both seed germination and seedling growth. The germinations of both species were tolerant of up to 2560 mg l−1 CNTs, and root elongation was enhanced in alfalfa and wheat seedlings exposed to CNTs. Remarkably, catalyst impurities also enhanced root elongation in alfalfa seedlings as well as wheat germination. Thus the impurities, not solely the CNTs, impacted the plants. CNT internalization by plants was investigated using electron microscopy and two-dimensional Raman mapping. The latter showed that CNTs were adsorbed onto the root surfaces of alfalfa and wheat without significant uptake or translocation. Electron microscopy investigations of internalization were inconclusive owing to poor contrast, so Fe3O4-functionalized CNTs were prepared and studied using energy-filter mapping of Fe3O4. CNTs bearing Fe3O4 nanoparticles were detected in the epidermis of one wheat root tip only, suggesting that internalization was possible but unusual. Thus, alfalfa and wheat tolerated high concentrations of industrial-grade multiwalled CNTs, which adsorbed onto their roots but were rarely taken up.

Synthesis of Multiwalled Carbon Nanotubes on Fly Ash Derived Catalysts

Carbon nanotubes (CNTs) are an allotrope of carbon with unique properties that make them potentially useful in a vast range of applications. However, CNTs are predominantly produced using expensive and/or nonrecyclable catalyst supports, e.g., mesoporous silica and alumina. In this work, coal combustion fly ash, a bulk waste product with limited uses, was impregnated with iron nitrate and successfully used as a substrate to produce industrial grade multiwalled carbon nanotubes (MWNTs) by fluidized bed chemical vapor deposition. CNTs were analyzed using thermogravimetric analysis, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The most successful catalyst trialed at 650 degrees C using ethylene as a carbon source was a 5 wt % Fe fly ash catalyst, which produced a CNT yield in respect to metal loading of approximately 82.5%. The MWNTs had outer diameters of between 12 and 20 nm with a reasonable degree of wall graphitization (I(G)/I(D) of 1.17). Advantages of utilizing fly ash as a catalyst support are its availability at low cost at the megaton scale, its high thermal stability, and suitability for use in industrial fluidized bed reactors. Potential applications for the fly ash produced CNTs include use in composite materials.

Particle residence time distributions in circulating fluidised beds

Abstract This paper gives experimental measurements of the particle residence time distribution (RTD) made in the riser of a square cross section, cold model, circulating fluidised bed, using the fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002a) 127). This technique depends upon all particles having phosphorescent properties. A small proportion of the particles become tracers when activated by a flash of light at the riser entry; the concentration of these phosphorescent particles can subsequently be detected by a photomultiplier. The influence of the solids circulation rate and superficial gas velocity on the RTD were investigated. The results presented are novel because (i) the experiments were performed in a system with closed boundaries and hence give the true residence time distribution in the riser and (ii) the measurement of the tracer concentration is exceedingly fast. The majority of previous studies have measured the RTD in risers with open boundaries, giving an erroneous measure of the RTD. Analysis of the results suggests that using pressure measurements in a riser to infer the solids inventory leads to erroneous estimates of the mean residence time. In particular, the results cast doubt on the assumption that friction and acceleration effects can be neglected when inferring the axial solids concentration profile from riser pressure measurements. An assessment of particle RTD models is also given. A stochastic particle RTD model was coupled to a riser hydrodynamic model incorporating the four main hydrodynamic regions observed in a fast-fluidised bed riser namely (i) the entrance region, (ii) a transition region, (iii) a core-annulus region and (iv) an exit region. This model successfully predicts the experimental residence time distributions.

The influence of the riser exit on the particle residence time distribution in a circulating fluidised bed riser

Abstract This paper reports measurements of the influence of riser exit geometry upon the particle residence time distribution in the riser of a square cross section, cold model, circulating fluidised bed. The bed is operated within the fast fluidisation regime. The fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002) 127–142) was used to measure the residence time distribution. The geometry of the riser exit is shown to have a modest but consistent influence upon the particle RTD; the influence of operating conditions, i.e. superficial gas velocity and solids flux is more significant. Increasing the refluxing effect of the riser exit increases the mean, variance and breakthrough time and decreases the coefficient of variation of the residence time distribution. Changes in reflux do not have a systematic effect upon the skewness of the RTD.