Characterisation and source identification of selected pollutants in house dust

House dust is a heterogeneous matrix, which contains a number of biological materials and particulate matter gathered from several sources. It is the accumulation of a number of semi-volatile and non-volatile contaminants. The contaminants are trapped and preserved. Therefore, house dust can be viewed as an archive of both the indoor and outdoor air pollution. There is evidence to show that on average, people tend to stay indoors most of the time and this increases exposure to house dust. The aims of this investigation were to: " assess the levels of Polycyclic Aromatic Hydrocarbons (PAHs), elements and pesticides in the indoor environment of the Brisbane area; " identify and characterise the possible sources of elemental constituents (inorganic elements), PAHs and pesticides by means of Positive Matrix Factorisation (PMF); and " establish the correlations between the levels of indoor air pollutants (PAHs, elements and pesticides) with the external and internal characteristics or attributes of the buildings and indoor activities by means of multivariate data analysis techniques. The dust samples were collected during the period of 2005-2007 from homes located in different suburbs of Brisbane, Ipswich and Toowoomba, in South East Queensland, Australia. A vacuum cleaner fitted with a paper bag was used as a sampler for collecting the house dust. A survey questionnaire was filled by the house residents which contained information about the indoor and outdoor characteristics of their residences. House dust samples were analysed for three different pollutants: Pesticides, Elements and PAHs. The analyses were carried-out for samples of particle size less than 250 µm. The chemical analyses for both pesticides and PAHs were performed using a Gas Chromatography Mass Spectrometry (GC-MS), while elemental analysis was carried-out by using Inductively-Coupled Plasma-Mass Spectroscopy (ICP-MS). The data was subjected to multivariate data analysis techniques such as multi-criteria decision-making procedures, Preference Ranking Organisation Method for Enrichment Evaluations (PROMETHEE), coupled with Geometrical Analysis for Interactive Aid (GAIA) in order to rank the samples and to examine data display. This study showed that compared to the results from previous works, which were carried-out in Australia and overseas, the concentrations of pollutants in house dusts in Brisbane and the surrounding areas were relatively very high. The results of this work also showed significant correlations between some of the physical parameters (types of building material, floor level, distance from industrial areas and major road, and smoking) and the concentrations of pollutants. Types of building materials and the age of houses were found to be two of the primary factors that affect the concentrations of pesticides and elements in house dust. The concentrations of these two types of pollutant appear to be higher in old houses (timber houses) than in the brick ones. In contrast, the concentrations of PAHs were noticed to be higher in brick houses than in the timber ones. Other factors such as floor level, and distance from the main street and industrial area, also affected the concentrations of pollutants in the house dust samples. To apportion the sources and to understand mechanisms of pollutants, Positive Matrix Factorisation (PMF) receptor model was applied. The results showed that there were significant correlations between the degree of concentration of contaminants in house dust and the physical characteristics of houses, such as the age and the type of the house, the distance from the main road and industrial areas, and smoking. Sources of pollutants were identified. For PAHs, the sources were cooking activities, vehicle emissions, smoking, oil fumes, natural gas combustion and traces of diesel exhaust emissions; for pesticides the sources were application of pesticides for controlling termites in buildings and fences, treating indoor furniture and in gardens for controlling pests attacking horticultural and ornamental plants; for elements the sources were soil, cooking, smoking, paints, pesticides, combustion of motor fuels, residual fuel oil, motor vehicle emissions, wearing down of brake linings and industrial activities.

Discriminating stromatolite formation modes using rare earth element geochemistry: Trapping and binding versus in situ precipitation of stromatolites from the Neoproterozoic Bitter Springs Formation, Northern Territory, Australia

Stromatolites consist primarily of trapped and bound ambient sediment and/or authigenic mineral precipitates, but discrimination of the two constituents is difficult where stromatolites have a fine texture. We used laser ablation-inductively coupled plasma-mass spectrometry to measure trace element (rare earth element – REE, Y and Th) concentrations in both stromatolites (domical and branched) and closely associated particulate carbonate sediment in interspaces (spaces between columns or branches) from bioherms within the Neoproterozoic Bitter Springs Formation, central Australia. Our high resolution sampling allows discrimination of shale-normalised REE patterns between carbonate in stromatolites and immediately adjacent, fine-grained ambient particulate carbonate sediment from interspaces. Whereas all samples show similar negative La and Ce anomalies, positive Gd anomalies and chondritic Y/Ho ratios, the stromatolites and non-stromatolite sediment are distinguishable on the basis of consistently elevated light REEs (LREEs) in the stromatolitic laminae and relatively depleted LREEs in the particulate sediment samples. Additionally, concentrations of the lithophile element Th are higher in ambient sediment samples than in stromatolites, consistent with accumulation of some fine siliciclastic detrital material in the ambient sediment but a near absence in the stromatolites. These findings are consistent with the stromatolites consisting dominantly of in situ carbonate precipitates rather than trapped and bound ambient sediment. Hence, high resolution trace element (REE + Y, Th) geochemistry can discriminate fine-grained carbonates in these stromatolites from coeval non-stromatolitic carbonate sediment and demonstrates that the sampled stromatolites formed primarily from in situ precipitation, presumably within microbial mats/biofilms, rather than by trapping and binding of ambient sediment. Identification of the source of fine carbonate in stromatolites is significant, because if it is not too heavily contaminated by trapped ambient sediment, it may contain geochemical biosignatures and/or direct evidence of the local water chemistry in which the precipitates formed.

Manufacture of specific BSCCO powder compositions by co-precipitation

A co-precipitation process for large-scale manufacture of bismuth-based HTSC powders has been demonstrated. Powders manufactured by this process have a high phase purity and precisely reproducible stoichiometry. Controlled time and temperature variations are used to convert precursors to HTSC compounds and to obtain specific particle-size distributions. The process has been demonstrated for a variety of compositions in the BSCCO system. Electron microscopy X-ray diffraction, inductively coupled plasma spectroscopy and magnetic-susceptibility measurements are used to characterize the powders.

Manufacture of fine-grained Y2BaCuO5 powder by coprecipitation

Quantities of Y2BaCuO5 powder greater than 500g have been manufactured by a co-precipitation process. By suitable heat treatments, the particle size of these powders can be varied from 5µm to less than 500nm. Sub-micrometer size powders may, under some conditions, have a duller green colour which is attributed to <2% unreacted material. However, after re-grinding and re-firing of this powder, high-purity powders can be achieved without significant grain growth. Inductively coupled plasma (ICP) spectroscopy is used to measure the stoichiometry of the powders and X-ray diffraction is used to determine phase purity. In both cases, the bulk composition is consistent with Y2BaCuO5 and phase purity is considered better than 95%.

Trace-element modeling of the magmatic evolution of rare-earth-rich carbonatite from the Miaoya deposit, Central China

Carbonatites are known to contain the highest concentrations of rare-earth elements (REE) among all igneous rocks. The REE distribution of carbonatites is commonly believed to be controlled by that of the rock forming Ca minerals (i.e., calcite, dolomite, and ankerite) and apatite because of their high modal content and tolerance for the substitution of Ca by light REE (LREE). Contrary to this conjecture, calcite from the Miaoya carbonatite (China), analyzed in situ by laser-ablation inductively-coupled-plasma mass-spectrometry, is characterized by low REE contents (100–260 ppm) and relatively !at chondrite-normalized REE distribution patterns [average (La/Yb)CN=1.6]. The carbonatite contains abundant REE-rich minerals, including monazite and !uorapatite, both precipitated earlier than the REE-poor calcite, and REE-fluorocarbonates that postdated the calcite. Hydrothermal REE-bearing !uorite and barite veins are not observed at Miaoya. The textural and analytical evidence indicates that the initially high concentrations of REE and P in the carbonatitic magma facilitated early precipitation of REE-rich phosphates. Subsequent crystallization of REE-poor calcite led to enrichment of the residual liquid in REE, particularly LREE. This implies that REE are generally incompatible with respect to calcite and the calcite/melt partition coefficients for heavy REE (HREE) are significantly greater than those for LREE. Precipitation of REE-fluorocarbonates late in the evolutionary history resulted in depletion of the residual liquid in LREE, as manifested by the development of HREE-enriched late-stage calcite [(La/Yb)CN=0.7] in syenites associated with the carbonatite. The observed variations of REE distribution between calcite and whole rocks are interpreted to arise from multistage fractional crystallization (phosphates!calcite!REE-!uorocarbonates) from an initially REE-rich carbonatitic liquid.

Bauxite residue neutralisation precipitate stability in acidic environments

This investigation used a combination of techniques, such as X-ray diffraction, inductively coupled plasma optical emission spectroscopy and infrared spectroscopy, to determine the dissolution mechanisms of the Bayer precipitate and the associated rate of dissolution in acetic, citric and oxalic acid environments. The Bayer precipitate is a mixture of hydrotalcite, calcium carbonate and sodium chloride that forms during the seawater neutralisation of Bayer liquors (waste residue of the alumina industry). The dissolution rate of a Bayer precipitate is found to be dependent on (1) the strength of the organic acid and (2) the number of donating H+ ions. The dissolution mechanism for a Bayer precipitate consists of several steps involving: (1) the dissolution of CaCO3, (2) formation of whewellite (calcium oxalate) when oxalic acid is used and (3) multiple dissolution steps for hydrotalcite that are highly dependent on the pH of solution. The decomposition of the Al–OH hydrotalcite layers resulted in the immediate formation of Al(OH)3, which is stable until the pH decreases below 5.5. This investigation has found that the Bayer precipitate is stable across a wide pH range in the presence of common organic acids found in the rhizosphere, and that initial decomposition steps are likely to be beneficial in supporting plant growth through the release of nutrients such as Ca2þ and Mg2þ.

Minimization of bauxite residue neutralization products using nanofiltered seawater

Currently, open circuit Bayer refineries pump seawater directly into their operations to neutralize the caustic fraction of the Bayer residue. The resulting supernatant has a reduced pH and is pumped back to the marine environment. This investigation has assessed modified seawater sources generated from nanofiltration processes to compare their relative capacities to neutralize bauxite residues. An assessment of the chemical stability of the neutralization products, neutralization efficiency, discharge water quality, bauxite residue composition, and associated economic benefits have been considered to determine the most preferable seawater filtration process based on implementation costs, savings to operations and environmental benefits. The mechanism of neutralization for each technology was determined to be predominately due to the formation of Bayer hydrotalcite and calcium carbonate, however variations in neutralization capacity and efficiencies have been observed. The neutralization efficiency of each feed source has been found to be dependent on the concentration of magnesium, aluminium, calcium and carbonate. Nanofiltered seawater with approximately double the amount of magnesium and calcium required half the volume of seawater to achieve the same degree of neutralization. These studies have revealed that multiple neutralization steps occur throughout the process using characterization techniques such as X-ray diffraction (XRD), infrared (IR) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Preventing industrial scale using bauxite refinery residues

An Australian green power (AGP) company produces energy from burning biomass from the sugar industry and recycled wood waste, however alkali in biomass is released into a recirculating stream that forms a scale as it becomes more concentrated. This investigation has shown that the addition of Bayer liquor (alumina waste residue) successfully removes scale-forming species from the recirculating stream and thus has the potential to reduce the rate of scaling. Characterisation of the scale and Bayer precipitates has been performed using X-ray diffraction (XRD), infrared spectroscopy (IR) and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Current control in the magnetron systems for nanofabrication : a comparison

A cylindrical magnetron system and a hybrid inductively coupled plasma-assisted magnetron deposition system were examined experimentally in light of their discharge characteristics with a view to stress the enhanced controllability of the hybrid system. The comparative study has shown that the hybrid magnetron + the inductively coupled plasma system is a flexible, powerful, and convenient tool that has certain advantages as compared with the cylindrical dc magnetrons. In particular, the hybrid system features more linear current-voltage characteristics and the possibility of a bias-independent control of the discharge current.

Single-step, rapid low-temperature synthesis of Si quantum dots embedded in an amorphous SiC matrix in high-density reactive plasmas

A simple, effective and innovative approach based on low-pressure, thermally nonequilibrium, high-density inductively coupled plasmas is proposed to rapidly synthesize Si quantum dots (QDs) embedded in an amorphous SiC (a-SiC) matrix at a low substrate temperature and without any commonly used hydrogen dilution. The experimental results clearly demonstrate that uniform crystalline Si QDs with a size of 3-4 nm embedded in the silicon-rich (carbon content up to 10.7at.%) a-SiC matrix can be formed from the reactive mixture of silane and methane gases, with high growth rates of ∼1.27-2.34 nm s-1 and at a low substrate temperature of 200 °C. The achievement of the high-rate growth of Si QDs embedded in the a-SiC without any commonly used hydrogen dilution is discussed based on the unique properties of the inductively coupled plasma-based process. This work is particularly important for the development of the all-Si tandem cell-based third generation photovoltaic solar cells.

Effect of elemental composition and size on electron confinement in self-assembled SiC quantum dots : a combinatorial approach

A high level of control over quantum dot (QD) properties such as size and composition during fabrication is required to precisely tune the eventual electronic properties of the QD. Nanoscale synthesis efforts and theoretical studies of electronic properties are traditionally treated quite separately. In this paper, a combinatorial approach has been taken to relate the process synthesis parameters and the electron confinement properties of the QDs. First, hybrid numerical calculations with different influx parameters for Si1-x Cx QDs were carried out to simulate the changes in carbon content x and size. Second, the ionization energy theory was applied to understand the electronic properties of Si1-x Cx QDs. Third, stoichiometric (x=0.5) silicon carbide QDs were grown by means of inductively coupled plasma-assisted rf magnetron sputtering. Finally, the effect of QD size and elemental composition were then incorporated in the ionization energy theory to explain the evolution of the Si1-x Cx photoluminescence spectra. These results are important for the development of deterministic synthesis approaches of self-assembled nanoscale quantum confinement structures.

Aluminum-assisted crystallization and p-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH 4+Ar+H2 gas mixture at a low substrate temperature of 300∈°C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si-Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×1018 cm-3. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300∈°C is proposed.

Rapid, low-temperature synthesis of nc-Si in high-density, non-equilibrium plasmas : enabling nanocrystallinity at very low hydrogen dilution

Nanocrystalline silicon thin films were deposited on single-crystal silicon and glass substrates simultaneously by inductively coupled plasma-assisted chemical vapor deposition from the reactive silane reactant gas diluted with hydrogen at a substrate temperature of 200 °C. The effect of hydrogen dilution ratio X (X is defined as the flow rate ratio of hydrogen to silane gas), ranging from 1 to 20, on the structural and optical properties of the deposited films, is extensively investigated by Raman spectroscopy, X-ray diffraction, Fourier transform infrared absorption spectroscopy, UV/VIS spectroscopy, and scanning electron microscopy. Our experimental results reveal that, with the increase of the hydrogen dilution ratio X, the deposition rate Rd and hydrogen content CH are reduced while the crystalline fraction Fc, mean grain size δ and optical bandgap ETauc are increased. In comparison with other plasma enhanced chemical vapor deposition methods of nanocrystalline silicon films where a very high hydrogen dilution ratio X is routinely required (e.g. X > 16), we have achieved nanocrystalline silicon films at a very low hydrogen dilution ratio of 1, featuring a high deposition rate of 1.57 nm/s, a high crystalline fraction of 67.1%, a very low hydrogen content of 4.4 at.%, an optical bandgap of 1.89 eV, and an almost vertically aligned columnar structure with a mean grain size of approximately 19 nm. We have also shown that a sufficient amount of atomic hydrogen on the growth surface essential for the formation of nanocrystalline silicon is obtained through highly-effective dissociation of silane and hydrogen molecules in the high-density inductively coupled plasmas. © 2009 The Royal Society of Chemistry.

Structural evolution of nanocrystalline silicon thin films synthesized in high-density, low-temperature reactive plasmas

Silicon thin films with a variable content of nanocrystalline phase were deposited on single-crystal silicon and glass substrates by inductively coupled plasma-assisted chemical vapor deposition using a silane precursor without any hydrogen dilution in the low substrate temperature range from 100 to 300 °C. The structural and optical properties of the deposited films are systematically investigated by Raman spectroscopy, x-ray diffraction, Fourier transform infrared absorption spectroscopy, UV/vis spectroscopy, scanning electron microscopy and high-resolution transmission electron microscopy. It is shown that the structure of the silicon thin films evolves from the purely amorphous phase to the nanocrystalline phase when the substrate temperature is increased from 100 to 150 °C. It is found that the variations of the crystalline fraction fc, bonded hydrogen content CH, optical bandgap ETauc, film microstructure and growth rate Rd are closely related to the substrate temperature. In particular, at a substrate temperature of 300 °C, the nanocrystalline Si thin films of our interest feature a high growth rate of 1.63nms-1, a low hydrogen content of 4.0at.%, a high crystalline fraction of 69.1%, a low optical bandgap of 1.55eV and an almost vertically aligned columnar structure with a mean grain size of approximately 10nm. It is also shown that the low-temperature synthesis of nanocrystalline Si thin films without any hydrogen dilution is attributed to the outstanding dissociation ability of the high-density inductively coupled plasmas and effective plasma-surface interactions during the growth process. Our results offer a highly effective yet simple and environmentally friendly technique to synthesize high-quality nanocrystalline Si films, vitally needed for the development of new-generation solar cells and other emerging nanotechnologies.

Low-temperature growth of large area, vertically aligned carbon nanotubes for field emission applications

Large area, highly uniform vertically aligned carbon nanotips (VACNTP) and other nanostructures have been grown on silicon (100) substrates with Ni catalyst in the low-temperature, low-frequency, high-density inductively coupled plasmas (ICP) of methane-hydrogen-argon gas mixtures. The control strategies for the morphology, crystalline structure and chemical states of the resulting nanostructures by varying the growth conditions are proposed. XRD and Roman analyses confirm that the nanotips are well graphitized, which is favorable for the field emission applications.