Zareen Abbas

Release of salts from municipal solid waste combustion residues

Residues from fluidized bed combustion of municipal solid waste were investigated with respect to their leaching behavior and possible extraction of salts. The total water extractable amounts of Na, K, Ca, Cl(-), Br(-), F(-) and SO(4)(2-) along with the total dissolved solids of bottom, hopper, cyclone and bag house filter ashes were determined. A simple multistage washing process (using water as the extraction medium) was tested in lab scale experiments. The effect of variations in parameters, such as water to ash weight ratio, contact time, temperature and number of extraction steps was investigated. The leaching behavior of untreated and washed cyclone and bag house filter ashes was evaluated by a two-step batch-leaching test, i.e. the CEN test. The ashes investigated in this study can be arranged according to their decreasing water extractable contents and total dissolved solids as follows: filter ash > cyclone ash > hopper ash > bottom ash. A triple extraction with water at liquid to solid ratio 2 and extraction time 5 min gave the best results for the extraction of Ca, Na, K, Cl(-) and SO(4)(2-) from the cyclone as well as from the filter ashes. The leached amounts of salts in the CEN test performed on the washed cyclone ash were considerably lower than the corresponding amounts released from the unwashed ash. Thus, the washed cyclone ash was made more stable with respect to salt leachability. On the other hand, large amounts of salts were leached from the washed filter ashes as well as from unwashed filter ashes. Therefore, it can be concluded that three stage water extraction is not a suitable stabilization method for this type of filter ashes.

Monte Carlo Simulations of Salt Solutions : Exploring the Validity of Primitive Models

An extensive series of Monte Carlo (MC) simulations were performed in order to explore the validity of simple primitive models of electrolyte solutions and in particular the effect of ion size asymmetry on the bulk thermodynamic properties of real salt solutions. Ionic activity and osmotic coefficients were calculated for 1:1, 2:1, and 3:1 electrolytes by using the unrestricted primitive model (UPM); i.e., ions are considered as charged hard spheres of different sizes dissolved in a dielectric continuum. Mean ionic activity and osmotic coefficients calculated by the MC simulations were fitted simultaneously to the experimental data by adjusting only the cation radius while keeping the anion radius fixed at its crystallographic value. Ionic radii were further optimized by systematically varying the cation and anion radii at a fixed sum of ionic radii. The success of this approach is found to be highly salt specific. For example, experimental data (mean ionic activity and osmotic coefficients) of salts which are usually considered as dissociated such as HCl, HBr, LiCl, LiBr, LiClO(4), and KOH were successfully fitted up to 1.9, 2.5, 1.9, 3, 2.5, and 4.5 M concentrations, respectively. In the case of partially dissociated salts such as NaCl, the successful fits were only obtained in a more restricted concentration range. Consistent sets of the best fitted cation radii were obtained for acids, alkali, and alkaline earth halides. A list of recommended ionic radii is also provided. The reliability of the optimized ionic radii was further tested in simulations of the osmotic coefficients of LiCl-NaCl-KCl salt mixtures. A very good agreement between the simulated and experimental data was obtained up to ionic strength of 4.5 M.

A study of Cr(VI) in ashes from fluidized bed combustion of municipal solid waste: leaching, secondary reactions and the applicability of some speciation methods

The use of the fluidized bed technique for the combustion of municipal solid waste is a rather new concept. This type of combustor produces ash residues with somewhat different properties than the residues generated from the traditional mass burn techniques. Therefore, chemical characterization and the investigation of toxic metals behavior during ash water reactions are necessary for the safe disposal of these residues. In the present work, the total elemental composition, mineralogy and leaching behavior of ashes from the combustion of municipal solid waste in a fluidized bed combustion boiler have been investigated. The cyclone ash and, in particular, the filter ash contained considerable amounts of soluble substances, thus giving leachates with high levels of Cl-, Na+, K+, Ca2 + and Al(IIl). On the other hand, the two ash fractions taken in the boiler, the bottom and hopper ashes, were much more stable with respect to the release of salts and heavy metals. Since Cr(VI) is mobile and toxic its release from combustion residues can pose environmental problem. Even though the total Cr contents were similar in all ashes studied, the bottom ash gave about a thousand times higher levels of Cr(VI) in test leachates than the hopper, cyclone and filter ashes. However, it was found that the leached amount of Cr(VI) from the bottom ash decreased significantly when bottom ash was mixed with the hopper ash. The most probable cause for this decrease is the coupled oxidation of Al(0) to Al(III) and reduction of dissolved Cr(VI) to Cr(III). This finding that the mixing of two ash streams from the same boiler could result in the immobilization of Cr may point at a simple stabilization method. Selective extraction of water soluble, exchangeable and sparingly soluble forms of Cr(VI) was also investigated. Extraction methods were evaluated for their suitability for ash matrixes. It was found that interferences due to the presence of reducing substances in some ash materials may occur.

Effect of Surface Charge Density on the Affinity of Oxide Nanoparticles for the Vapor–Water Interface

Using in-situ X-ray photoelectron spectroscopy at the vapor-water interface, the affinity of nanometer-sized silica colloids to adsorb at the interface is shown to depend on colloid surface charge density. In aqueous suspensions at pH 10 corrected Debye-Hückel theory for surface complexation calculations predict that smaller silica colloids have increased negative surface charge density that originates from enhanced screening of deprotonated silanol groups (≡Si-O(-)) by counterions in the condensed ion layer. The increased negative surface charge density results in an electrostatic repulsion from the vapor-water interface that is seen to a lesser extent for larger particles that have a reduced charge density in the XPS measurements. We compare the results and interpretation of the in-situ XPS and corrected Debye-Hückel theory for surface complexation calculations with traditional surface tension measurements. Our results show that controlling the surface charge density of colloid particles can regulate their adsorption to the interface between two dielectrics.

Surface charge and interfacial potential of titanium dioxide nanoparticles: Experimental and theoretical investigations

Size dependent surface charging and interfacial potential of titanium dioxide (TiO2) nanoparticles are investigated by experimental and theoretical methods. Commercially available TiO2 (P25) nanoparticles were used for surface charge determinations by potentiometric titrations. Anatase particles, 10 and 22 nm in diameter, were synthesized by controlled hydrolysis of TiCl4, and electrophoretic mobilities were determined at a fixed pH but at increasing salt concentrations. Corrected Debye-Hückel theory of surface complexation (CDH-SC) was modified to model the size dependent surface charging behavior of TiO2 nanoparticles. Experimentally determined surface charge densities of rutile and P25 nanoparticles in different electrolytes were accurately modeled by the CDH-SC theory. Stern layer capacitances calculated by the CDH-SC theory were in good agreement with the values found by the classical surface complexation approach, and the interaction of protons with OH groups is found to be less exothermic than for iron oxide surfaces. Moreover, the CDH-SC theory predicts that the surface charge density of TiO2 nanoparticles of diameter <10nm is considerably higher than for larger particles, and pH at the point of zero charge (pHPZC) shifts to higher pH values as the particle size decreases. The importance of including the particle size in calculating the zeta potentials from mobilities is demonstrated. Smoluchowski theory showed that 10nm particles had lower zeta potential than 22 nm particles, whereas a reverse trend was seen when zeta potentials were calculated by Ohshimas theory in which particle size is included. Electrokinetic charge densities calculated from zeta potentials were found to be only one third of the true surface charge densities.

Aggregation of nanosized colloidal silica in the presence of various alkali cations investigated by the electrospray technique.

The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.

Combined Electrospray-SMPS and SR-SAXS Investigation of Colloidal Silica Aggregation. Part I. Influence of Starting Material on Gel Morphology

The slow aggregation of monodisperse, polydisperse, and preaggregated silica nanoparticles was studied with an electrospray-scanning mobility particle sizer (ES-SMPS) and time-resolved synchrotron radiation-small-angle X-ray scattering (SR-SAXS). Aggregation was induced by varying the NaCl concentration to obtain a fixed gelation time of ∼40 min. The combination of these techniques provides a unique tool to monitor and resolve the aggregate development in detail. The monodisperse spherical particles were converted to dimers, trimers, and eventually larger clusters as the aggregation proceeded, while the polydisperse spherical particles formed large clusters at an early stage. The initial particle shape and polydispersity had profound effects on the morphology of the aggregates; spherical primary particles produced compact spherical clusters, whereas the preaggregated dispersions formed open, elongated aggregates. All dispersions produced gels that contained free primary particles well past the point of gelation. The stability of the aggregates and the gel morphology were interpreted by relating to the structure of porous gel layers around the particles.

Combined electrospray-scanning mobility particle sizer (ES-SMPS) and time-resolved synchrotron radiation-small-angle X-ray scattering (SR-SAXS) investigation of colloidal silica aggregation. Part II. Influence of aggregation initiator on gel stability.

The effect of ion specificity on the slow aggregation of silica nanoparticles with various initial morphology was investigated with an electrospray-scanning mobility particle sizer (ES-SMPS) and time-resolved synchrotron radiation-small-angle X-ray scattering (SR-SAXS). This combination provides a unique tool to monitor and resolve the early aggregate development in detail. Aggregation was induced by varying the K(2)CO(3) or KCl concentration to obtain a fixed gelation time of ∼40 min, and the results were compared with those obtained in a previous paper (Johnsson et al. J. Phys. Chem. B 2011, 115, 765-775) for NaCl. All dispersions produced gels that contained free primary particles well past the point of gelation (PoG). The initial aggregate formation and obtained gel morphologies were independent of the aggregation initiator. Nevertheless, ion-specific effects were observed for the rate of the stability increase of the 3-dimensional (3D) gel structure. The formation of a stable structure was fastest in the presence of the strongly hydrated counterions, and a clear anion effect was observed. The obtained gel stabilities were interpreted by relating the rate of formation of covalent siloxane bonds to the polarization of the water molecules surrounding structure-maker ions.

Aqueous processable WO3−x nanocrystals with solution tunable localized surface plasmon resonance

Heavily doped tungsten oxide nanoparticles with localized surface plasmon resonances (LSPRs) were recently highlighted as potential substitutes for noble metals in the field of plasmonic applications. Herein, oxygen deficient, spherically shaped WO3−x nanocrystals (NCs) were synthesized with a pronounced visible LSPR absorbance peak instead of a broadband tail usually observed for WO3−x nanowires. Although the tuning of the plasmon resonances was achieved mainly by changing the nanocrystals composition or solvent refraction index, we demonstrate this via the interfacial charge donation/extraction. In an aqueous NCs dispersion, the LSPR peak was either blue shifted in an acidic solution up to 80 nm or bleached by a basic solution making the NCs appropriate for sensing applications.

TiO2 nanoparticle interactions with supported lipid membranes – an example of removal of membrane patches

There is a need for different levels of model systems for effect studies of engineered nanoparticles and the development of nanoparticle structure–activity relationships in biological systems. Descriptors for nanoparticles based on their interactions in molecular model systems may become useful to predict toxicological responses of the nanoparticles in cells. Towards this end, we report on nanoparticle-induced formation of holes in supported model membranes. Specifically, TiO2 nanoparticle – lipid membrane interactions were studied under low ionic strength, basic conditions (pH 8), using different membrane compositions and several surface-sensitive analytical techniques. It was found that for mixed POPC/POPG (PG fractions ≥ 35%) membranes on silica supports, under conditions where electrostatic repulsion was expected, the addition of TiO2 nanoparticles resulted in transient interaction curves, consistent with the removal of part of the lipid membrane. The formation of holes was inferred from quartz crystal microbalance with dissipation (QCM-D) monitoring, as well as from optical measurements by reflectometry, and also verified by atomic force microscopy (AFM) imaging. The interaction between the TiO2 nanoparticles and the PG-containing membranes was dependent on the presence of Ca2+ ions. A mechanism is suggested where TiO2 nanoparticles act as scavengers of Ca2+ ions associated with the supported membrane, leading to weakening of the interaction between the membrane and the support and subsequent removal of lipid mass as TiO2 nanoparticles spontaneously leave the surface. This mechanism is consistent with the observed formation of holes in the supported lipid membranes.