1,720,967 research outputs found
Use of iron-based technologies in contaminated land and groundwater remediation: a review
No full textReactions involving iron play a major role in the environmental cycling of a wide range of important organic, inorganic and radioactive contaminants. Consequently, a range of environmental clean-up technologies have been proposed or developed which utilise iron chemistry to remediate contaminated land and surface and subsurface waters, e.g. the use of injected zero zero-valent iron nanoparticles to remediate organic contaminant plumes; the generation of iron oxyhydroxide-based substrates for arsenic removal from contaminated waters; etc. This paper reviews some of the latest iron-based technologies in contaminated land and groundwater remediation, their current state of development, and their potential applications and limitations
Effect of undensified silica fume on the dispersion of carbon nanotubes within a cementitious composite
No full textThe synergistic effect of multi-walled carbon nanotubes (MWCNTs) and Undensified Silica Fume (USF) on the microstructure of cementitious composites has been studied. In the current work, USF was used to enhance the dispersion of nanotubes throughout the composite and prevent the re-agglomeration of nanotubes by providing a physical barrier of particles of small size. Ultrasonication was employed to disperse MWCNTs in water in the presence of polycarboxylate-based superplasticizer (PCE) as a dispersion agent. The results indicate that incorporation of USF considerably improves the dispersion of nanotubes in the composites, with subsequent enhancement of composite packing density. This enhancement can be attributed to the synergistic effect of MWCNTs and USF in reducing the volume of pores through the cementitious composites
Creation of spherical carbon nanoparticles and clusters from carbon dioxide via UV dissociation at the critical point
No full textCarbon nanomaterials have become increasingly important for many applications, including sensors, electronics, biomedical materials and functional composites. Currently their production is based on hydrocarbons or graphite and requires very high temperatures. Here we present a method for the synthesis of carbon nanomaterials from carbon dioxide. Unlike previously described methods, our synthesis method works near room temperature. Carbon dioxide is irradiated at its critical point, producing spherical carbon nanoparticles even without the use of a catalyst. We examine the influence of irradiation parameters and different metals and catalysts on the nanocarbon production. Together with analysis of the fluid phase, this allows us to draw some conclusions on the carbon dioxide dissociation mechanism
Creation of 3-dimensional carbon nanostructures from UV irradiation of carbon dioxide at room temperature
No full textA method is presented for the production of carbon nanomaterials from carbon dioxide in a low temperature process. In this method, carbon dioxide is irradiated with an ultraviolet laser at the conditions of critical opalescence where light is scattered and absorbed. Spherical carbon nanoparticles are obtained under these conditions on metal substrates without any additional catalyst near room temperature. The particles are of approximately uniform shape and size of around 100 nm. Some of the particles form clusters. The method is reproducible on different substrates. Quantum chemical calculations have been employed in order to elucidate the role of critical opalescence and of the substrate. The calculations show that the presence of molecular clusters at the critical point is essential in decreasing the excitation energy. The dissociation reaction most likely occurs on the surface of the substrate, where the excitation energy is decreased even further
Effect of high-intensity sonication on the dispersion of carbon-based nanofilaments in cementitious composites, and its impact on mechanical performance
Full text linkCarbon-based nanofilaments are promising materials for improving the mechanical performance of cementitious composites. To date, the main challenge in their effective use has been controlling the dispersion of these additives in water and in the resulting mixed composites due to their strong van der Waals self-attraction and hydrophobic surfaces. This study uses high-intensity sonication to disperse different nanofilament types in water, and assesses their resulting reinforcing efficiency in cementitious composites. The proportion of nanofilaments used (in this case, multiwall carbon nanotubes MWCNTs, functionalized multiwall carbon nanotubes F-MWCNTs, and carbon nanofibres CNFs) was 0.025% by weight of cement. Aqueous dispersions were examined using transmission electron microscopy (TEM) and optical microscopy, and ultraviolet-visible (UV–vis) spectroscopy. Compressive, flexural and splitting tensile strengths tests, and porosity and density measurements, were used to evaluate the mechanical properties of the composites. High-intensity sonication over short durations significantly improved the dispersion, and reinforcing and filling effects, of carbon-based nanofilaments in cementitious composites, with increases in compressive strength of 24–32%, splitting tensile strength of 45–50%, and flexural toughness factor of 30–40%, observed after 28 days curing. A 17–26% reduction in the porosity of the composite materials was also recorded
Phenolic carbon tailored for the removal of polar organic contaminants from water : a solution to the metaldehyde problem?
Full text linkCurrent water treatment technologies are inefficient at treating water contaminated with metaldehyde, an 8-member cyclic tetramer of acetaldehyde widely used as a molluscicide in large-scale agriculture and in gardens, and which has been frequently observed to breach European regulatory limits in the UK due to its high solubility and frequent use. Here, we examine the controls on metaldehyde adsorption onto activated phenolic carbon, namely the influence of activation degree, pore size distribution, particle size, point of zero charge and surface functionalisation, by synthesising “tailored” carbons from phenolic resin. Metaldehyde adsorption has been found to be independent of specific surface area (SBET), which is highly unusual for an adsorption process, and is favoured in carbons with (a) high microporosity with narrow pore size distribution, (b) presence of mesopores which allow efficient diffusive transport, and (c) an absence of negatively charged functional groups. The maximum adsorption capacity of the phenolic resin-derived carbons, tested at an elevated (i.e. exceeding environmental levels) water concentration of 64 mg metaldehyde/L, was 76 mg metaldehyde/g carbon compared with 13 mg metaldehyde/g carbon in industrial granular activated carbon (GAC). The phenolic resin-derived carbons and GAC showed similar adsorption kinetics with maximum metaldehyde uptake occurring within 30 min under batch adsorption conditions, although adsorption isotherms indicate much stronger adsorption of metaldehyde on the phenolic resin-derived carbons. Adsorption efficiency for metaldehyde was maintained even in the presence of high background concentrations of organic matter and inorganic salts, indicating the potential utility of these “designer” carbons in waste and/or drinking water treatment
High efficiency removal of dissolved As(III) using iron nanoparticle-embedded macroporous polymer composites
No full textNovel nanocomposite materials where iron nanoparticles are embedded into the walls of a macroporous polymer were produced and their efficiency for the removal of As(III) from aqueous media was studied. Nanocomposite gels containing ?-Fe2O3 and Fe3O4 nanoparticles were prepared by cryopolymerisation resulting in a monolithic structure with large interconnected pores up to 100 ?m in diameter and possessing a high permeability (ca. 3 × 10?3 m s?1). The nanocomposite devices showed excellent capability for the removal of trace concentrations of As(III) from solution, with a total capacity of up to 3 mg As/g of nanoparticles. The leaching of iron was minimal and the device could operate in a pH range 3–9 without diminishing removal efficiency. The effect of competing ions such as SO42? and PO43? was negligible. The macroporous composites can be easily configured into a variety of shapes and structures and the polymer matrix can be selected from a variety of monomers, offering high potential as flexible metal cation remediation devices
Morphological and chemical features of nano and macroscale carbons affecting hydrogen peroxide decomposition in aqueous media
No full textChemical and structural factors of carbon materials affect their activity in adsorption and surface reactions in aqueous media. Decomposition of hydrogen peroxide studied is a probe reaction for exploring parameters of carbons that might be involved, such as specific surface area, nitrogen and oxygen doping and conformational changes. To date, a detailed comparison of the behavior of carbon nanoscale (Carbon Nanotubes, CNT, Single Layer Graphene Oxide, SLGO) with macroscale (Activated carbons, AC) materials in this reaction has not been forthcoming. Herein, we demonstrate that on their first cycle, ACs in doped and undoped forms outperform all nanoscale carbons tested in the H2O2 decomposition. Among the nanocarbons, nitrogen-doped CNT exhibited the highest activity in this reaction. However, subsequent recycling of each carbon, without chemical regeneration between uses, reveals SLGO exhibits greater reaction rate stability over an extended number of cycles (n > 8) than other carbons including nitrogen-doped CNT and ACs. The effects of pH, temperature and concentration on the reaction were analyzed. Quantum-chemical modeling and reaction kinetics analysis reveal key processes likely involved in hydrogen peroxide decomposition and show evidence that the reaction rate is linked to active sites with N-and O-containing functionalities
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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