Maria Izquierdo

Coal fly ash-slag-based geopolymers: microstructure and metal leaching.

This study deals with the use of fly ash as a starting material for geopolymeric matrices. The leachable concentrations of geopolymers were compared with those of the starting fly ash to evaluate the retention of potentially harmful elements within the geopolymer matrix. Geopolymer matrices give rise to a leaching scenario characterised by a highly alkaline environment, which inhibits the leaching of heavy metals but may enhance the mobilization of certain oxyanionic species. Thus, fly ash-based geopolymers were found to immobilize a number of trace pollutants such as Be, Bi, Cd, Co, Cr, Cu, Nb, Ni, Pb, Sn, Th, U, Y, Zr and rare earth elements. However, the leachable levels of elements occurring in their oxyanionic form such as As, B, Mo, Se, V and W were increased after geopolymerization. This suggests that an optimal dosage, synthesis and curing conditions are essential in order to obtain a long-term stable final product that ensures an efficient physical encapsulation.

Leaching of potential hazardous elements of coal cleaning rejects

The geochemical characteristics of coal cleaning rejects (CCR) in Santa Catarina State, Brazil, were investigated. Around 3.5 million ton/ year of coal waste are dumped in Santa Catarina State. Coal beneficiation by froth flotation results in large amounts of CCR composed of coaly and mineral matter, the latter characterised by the occurrence of sulphide minerals and a broad array of leachable elements. The total and leachable contents of more than 60 elements were analysed. Atmospheric exposure promotes sulphide oxidation that releases substantial sulphate loads as well as Ca2 + , K + , Mg2 + , Cl −  and Al3 + . The metals with the most severe discharges were Zn, Cu, Mn, Co, Ni and Cd. Most trace pollutants in the CCR displayed a marked pH-dependent solubility, being immobile in near-neutral samples. The results highlight the complex interactions among mineral matter solubility, pH and the leaching of potentially hazardous elements.

Geochemistry of ultra-fine and nano-compounds in coal gasification ashes: A synoptic view

The nano-mineralogy, petrology, and chemistry of coal gasification products have not been studied as extensively as the products of the more widely used pulverized-coal combustion. The solid residues from the gasification of a low- to medium-sulfur, inertinite-rich, volatile A bituminous coal, and a high sulfur, vitrinite-rich, volatile C bituminous coal were investigated. Multifaceted chemical characterization by XRD, Raman spectroscopy, petrology, FE-SEM/EDS, and HR-TEM/SEAD/FFT/EDS provided an in-depth understanding of coal gasification ash-forming processes. The petrology of the residues generally reflected the rank and maceral composition of the feed coals, with the higher rank, high-inertinite coal having anisotropic carbons and inertinite in the residue, and the lower rank coal-derived residue containing isotropic carbons. The feed coal chemistry determines the mineralogy of the non-glass, non-carbon portions of the residues, with the proportions of CaCO₃ versus Al₂O₃ determining the tendency towards the neoformation of anorthite versus mullite, respectively. Electron beam studies showed the presence of a number of potentially hazardous elements in nanoparticles. Some of the neoformed ultra-fine/nano-minerals found in the coal ashes are the same as those commonly associated with oxidation/transformation of sulfides and sulfates.

The role of open and closed curing conditions on the leaching properties of fly ash-slag-based geopolymers.

This study deals with the synthesis of geopolymers from co-fired fly ash and blast furnace slags. Geopolymer bodies were simultaneously synthesized in open and closed curing conditions in order to elucidate the role of this parameter on their resultant properties. Open curing conditions produce solid bodies characterized by high porosity, low compressive strength and exacerbated leaching of certain oxyanionic metalloids. By contrast, protected curing promotes the binder development, giving rise to higher strength and less porous systems. This imposes physical restrictions to leaching which decreases and/or retards releases of oxyanionic metalloids in comparison to open curing conditions. Fly ash-slag-based geopolymers may immobilize a number of trace pollutants such as Be, Bi, Cd, Co, Cr, Cu, Nb, Ni, Pb, REE, Sn, Th, U, Y and Zr, regardless of the curing conditions. Due to geopolymers displaying weak assimilation capacity for oxyanionic species, their successful regarding oxyanionic retention is strongly dependent on porosity and therefore on curing conditions applied.

Low cost adsorbents for low temperature cleaning of flue gases

The adsorption capacity with regard to SO2 of chars prepared from a low-grade coal has been evaluated at typical conditions of flue gas (presence of oxygen and water vapour and linear gas velocity of 0.12 m s−1) at an inlet concentration of 2 g SO2 m−3. The flue gases have been simulated, varying the water vapour and oxygen contents as well as the adsorption temperature which was varied between 80°C and 150°C. The study has focused on the influence of several process variables as well as the intrinsic features of the chars, such as their porous texture, on the SO2 adsorption capacity of the chars. Under the conditions studied, the positive effect of water vapour in the absence of oxygen has been observed. This effect can be explained by the intervention of oxygen functionalities in chars. Hero two effects overlap: the oxygen present in chars and their surface chemistry as well as the porous structure available. Temperature is also an important parameter in the SO2 adsorption efficiency. A minimum surface area is required for the temperature effect to be observed; otherwise the catalytic oxidation of SO2 to SO3 and subsequent storage of H2SO4 takes place to a very small extent and the negative effect of temperature increase to 150°C is not appreciable.

Mineralogy and Leaching Characteristics of Coal Ash from a Major Brazilian Power Plant

The feed coals, fly ashes and bottom ashes collected from seven different units in a major Brazilian PF power plant have been subjected to comprehensive mineralogical, geochemical, and petrographic studies, to investigate the links between feed coal and ash characteristics. Ashes from two of the units were collected while the coal was being co-fired with oil as part of the boiler start-up procedure, allowing the impact of oil co-firing on ash characteristics also to be evaluated. High proportions of unburnt carbon and high proportions of retained sulphur were found in the fly ashes produced during oil co-firing, probably reflecting less efficient combustion and associated lower combustion temperatures. Higher concentrations of a number of relatively volatile trace elements were also noted in these fly ashes, compared to the fly ashes collected from units under normal operating conditions. The fly ashes produced during oil co-firing gave rise to acid pH conditions in water-based leaching tests, in contrast to the alkaline pH associated with fly ashes produced during normal operations. This probably reflects higher SO3 contents relative to total CaO + MgO for the co-fired ash samples. Many trace elements that are typically mobilised as cations were also more abundant in leachates from the co-fired fly ashes. This is due, most likely, to the more acid pH conditions involved. Despite similar or even higher total concentrations, however, elements that are typically released from coal ash as oxy-anions were less mobile from the co-fired fly ashes than from the normally-fired fly ash materials. f 2010 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 28 July 2010; Received in revised form 17 August 2010; Accepted 23 August 2010

Fly ash from a Mexican mineral coal. II. Source of W zeolite and its effectiveness in arsenic (V) adsorption

Coal-fired plants in Coahuila (Mexico) produce highly reactive fly ash (MFA), which is used in a one-step process as a raw material in producing zeolite. We explored two routes in the synthesis of zeolite: (a) direct MFA zeolitization, which resulted in the formation of W zeolite with KOH and analcime with NaOH and (b) a MFA fusion route, which resulted in the formation of zeolite W or chabazite with KOH and zeolite X or P with NaOH. No residual crystalline phases were present. When LiOH was employed, ABW zeolite with quartz and mullite were obtained. For both zeolitization routes, the nature of the alkali (KOH, NaOH, LiOH), the alkali/MFA ratio (0.23-1.46), and the crystallization temperature and time (90-175 degrees C; 8-24 h) were evaluated. Additionally, the effect of temperature and time on MFA fusion was studied. W zeolite was obtained by both zeolitization methods. The direct route is preferred because it is a straightforward method using soft reaction conditions that results in a high yield of low cost zeolites with large crystal agglomerates. It was demonstrated that aluminum modified W zeolite has the ability to remove 99% of the arsenic (V) from an aqueous solution of Na(2)HAsO(4).7H(2)O originally containing 740 ppb.

Complex nanominerals and ultrafine particles assemblages in phosphogypsum of the fertilizer industry and implications on human exposure

Phosphogypsum (CaSO(4).2H(2)O), a by-product of phosphate-rock processing, contains high amounts of impurities such P(2)O(5), F, radioactive elements, organic substances, secondary nanominerals, and ultrafine particles (UFP) enriched in metals and metalloids. In this study, we examine phosphogypsum (PG) collected from abandoned fertilizer industry facility in south Brazil (Santa Catarina state). The fragile nature of nanominerals and UFP assemblages from fertilizer industry systems required novel techniques and experimental approaches. The investigation of the geochemistry of complex nanominerals and UFP assemblages was a prerequisite to accurately assess the environmental and human health risks of contaminants and cost-effective chemical and biogeological remediation strategies. Particular emphasis was placed on the study and characterization of the complex mixed nanominerals and UFP containing potentially toxic elements. Nanometer-sized phases in PG were characterized using energy-dispersive X-ray spectrometer (EDS), field-emission scanning electron microscope (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM) images. The chemical composition and possible correlations with morphology of nanominerals and UFP, as well as aspects of nanominerals and UFP, are discussed in the context of human health exposure, as well as in relation to management of the nanominerals and UFP in PG environments.

Influence of temperature and regeneration cycles on Hg capture and efficiency by structured Au/C regenerable sorbents.

The objective of this work is to evaluate a novel regenerable sorbent for mercury capture based on gold nanoparticles supported on a honeycomb structured carbon monolith. A new methodology for gold nanoparticles deposition onto carbon monolith support has been developed to obtain an Au sorbent based on the direct reduction of a gold salt onto the carbon material. For comparison purposes, colloidal gold method was also used to obtain Au/C sorbents. Both types of sorbents were characterized by different techniques in order to obtain the bulk gold content, the particle size distribution and the chemical states of gold after deposition. The mercury capture capacity and mercury capture efficiency of sorbents were tested in a bench scale facility at different experimental conditions. The regenerability of the sorbents was tested along several cycles of Hg capture-regeneration. High retention efficiencies are found for both types of sorbents comparing their gold content. Moreover, the high retention efficiency is maintained along several cycles of Hg capture-regeneration. The study of the fresh sorbent, the sorbent after Hg exposition and after regeneration by XPS and XRD gives insight to explain those results.

Procedural uncertainties of Proctor compaction tests applied on MSWI bottom ash

MSWI bottom ash is a well-graded highly compactable material that can be used as a road material in unbound pavements. Achieving the compactness assumed in the design of the pavement is of primary concern to ensure long term structural stability. Regulations on road construction in a number of EU countries rely on standard tests originally developed for natural aggregates, which may not be appropriate to accurately assess MSWI bottom ash. This study is intended to assist in consistently assessing MSWI bottom ash compaction by means of the Proctor method. This test is routinely applied to address unbound road materials and suggests two methods. Compaction parameters show a marked procedural dependency due to the particle morphology and weak particle strength of ash. Re-compacting a single batch sample to determine Proctor curves is a common practise that turns out to overvalue optimum moisture contents and maximum dry densities. This could result in wet-side compactions not meeting stiffness requirements. Inaccurate moisture content measurements during testing may also induce erroneous determinations of compaction parameters. The role of a number of physical properties of MSWI bottom ash in compaction is also investigated.