Sotira Yiacoumi

Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) is employed to study chemical interactions between three metal ions — Cu(II), Mo(VI), and Cr(VI) — and chitosan, a natural biopolymer extracted from crab shells. Three forms of chitosan are used — flakes, beads, and modified beads obtained by glutaraldehyde cross-linking. XPS provides identification of the sorption sites involved in the accumulation of metals, as well as the forms of species sorbed on the biopolymer. It is found that sorption occurs on amine functional groups for all the three metals. With copper, the sorption step is not followed by reduction of the metal. More complex phenomena are involved in molybdate removal. A partial reduction (about 20–25% of the total molybdenum content) occurs with chitosan beads and flakes. The distribution of reduced Mo(V) on the surface of the sorbent differs from that in the bulk of the sorbent for raw chitosan beads, while the glutaraldehyde cross-linking allows uniform distribution of reduced Mo(V) throughout the sorbent. The difference between these two forms of chitosan can be related to a complementary photoreduction step occurring on the surface of the biopolymer. For chromium, a similar trend with molybdenum is followed but to a greater extent; with cross-linked sorbents all chromate previously sorbed is reduced to Cr(III), while with raw chitosan beads Cr(VI) reduction does not exceed 60%.

Mesoporous Carbon for Capacitive Deionization of Saline Water

Self-assembled mesoporous carbon (MC) materials have been synthesized and tested for application in capacitive deionization (CDI) of saline water. MC was prepared by self-assembly of a triblock copolymer with hydrogen-bonded chains via a phenolic resin, such as resorcinol or phloroglucinol in acidic conditions, followed by carbonization and, in some cases, activation by KOH. Carbon synthesized in this way was ground into powder, from which activated MC sheets were produced. In a variation of this process, after the reaction of triblock copolymer with resorcinol or phloroglucinol, the gel that was formed was used to coat a graphite plate and then carbonized. The coated graphite plate in this case was not activated and was tested to serve as current collector during the CDI process. The performance of these MC materials was compared to that of carbon aerogel for salt concentrations ranging between 1000 ppm and 35,000 ppm. Resorcinol-based MC removed up to 15.2 mg salt per gram of carbon, while carbon aerogel removed 5.8 mg salt per gram of carbon. Phloroglucinol-based MC-coated graphite exhibited the highest ion removal capacity at 21 mg of salt per gram of carbon for 35,000 ppm salt concentration.

Microbubble generation for environmental and industrial separations

Small gas bubbles are used in many environmental and industrial processes for solid-liquid separations or to facilitate heat and mass transfer between phases. Typically, smaller bubbles are preferred in treatment techniques due to both their high surface area-to-volume ratio and their increased bubble density at a fixed flow rate. This study examines some of the factors that affect the size of bubbles produced in the processes of electroflotation, dissolved air flotation, and a relatively new method known as electrostatic spraying. The effect of voltage, current and ionic strength was studied in electroflotation, the effect of pressure was studied in dissolved air flotation and the effect of voltage, capillary dimensions and flow rate was studied in electrostatic spraying. In electroflotation, the flow rate of gas produced increased as a function of voltage and current. Flow rate also increased as the ionic strength of the aqueous medium was increased. However, no clear trends in bubble size as a function of these parameters were evident. The bubbles produced in dissolved air flotation showed a decrease in size as saturation pressure was increased; however, the differences were insignificant at high pressures. Bubble size in electrostatic spraying decreased as voltage was increased. Finally, this study compares the three methods of bubble production in terms of average bubble diameter, bubble size distribution and power consumed during production. Dissolved air flotation produced the largest average bubble diameters, while electroflotation produced the smallest average bubble diameters. In terms of bubble size distribution, dissolved air flotation produced the most narrow distribution, electrostatic spraying produced the widest distribution, and electroflotation produced an intermediate distribution. In terms of power consumption, the pilot-scale dissolved air flotation system maximized surface area production, electroflotation produced an intermediate value, and electrostatic spraying of air produced the least surface area as a function of power consumed.

Electrosorption capacitance of nanostructured carbon aerogel obtained by cyclic voltammetry

Abstract Cyclic voltammetry experiments at various electrolyte solution concentrations (0.001–0.1 M) and scan rates (1 to 5 mV s −1 ) have been performed to study the electrical double layer (edl) formation in nanostructured carbon aerogel. The results show that carbon aerogel is a good edl capacitor and can be further divided into mesoporous and microporous capacitors. According to the experiments, the mesoporous capacitor shows a fast charging/discharging response and is only minimally affected by the electrolyte concentration and scan rate. Therefore, the specific capacitance of the mesoporous capacitor is found to be constant over a wide range of applied electrical potentials. On the other hand, the microporous capacitor shows a slow charging/discharging response and its capacitance strongly depends on the electrolyte concentration and potential. Unlike previous experiments, in which only a flat minimum was observed at the point of zero charge (pzc), in the current study, a deep minimum is observed near the pzc at low electrolyte concentration if a slow scan rate is used. This unique feature is a result of edl overlapping in the micropores and is consistent with the predictions by the Gouy–Chapman model employed in this study. Based on this behavior, a new approach is suggested for pzc measurements of solid porous materials for which a large portion of the surface area is in the micropore region.

Effects of chemical and physical properties of influent on copper sorption onto activated carbon fixed-bed columns

Abstract Experimental studies were carried out to determine the effects of influent pH, ionic strength, metal concentration, and empty-bed contact time (EBCT) on copper sorption in the activated carbon fixed-bed columns. As the influent pH was increased, the column performance significantly improved. An overshoot of the effluent concentration was observed at pH 3.0 due to desorption of copper from the surface–metal complexes. The uptake enhanced as the ionic strength was increased; however, this role was much weaker than that of pH. An increase in metal concentration resulted in a decrease in the bed volumes at the breakthrough. In the beginning of all the experiments, the effluent pH increased dramatically and then dropped and approached lower values. Less sorption was observed when prewashed columns were used, in which the pH in the columns was kept constant. Modelling of the metal sorption in the prewashed columns was performed using a metal ion transport model hydrogeochem and a fixed-bed model.

Biosorption of metal ions from aqueous solutions

Abstract Copper biosorption from aqueous solutions by calcium alginate is reported in this paper. The experimental section includes potentiometric titrations of biosorbents, batch equilibrium and kinetic studies of copper biosorption, as well as fixed-bed biosorption experiments. The potentiometric titration results show that the surface charge increases with decreasing pH. The biosorption of copper strongly depends on solution pH; the metal ion binding increases from 0 to 90 percent in pH ranging from 1.5 to 5.0. In addition, a decrease in ionic strength results in an increase of copper ion removal. Kinetic studies indicate that mass transfer plays an important role in the biosorption rate. Furthermore, a fixed-bed biosorption experiment shows that calcium alginate has a significant capacity for copper ion removal. The two-pK Basic Stern model successfully represents the surface charge and equilibrium biosorption experimental data. The calculation results demonstrate that the copper removal may result fr...

High-gradient magnetically seeded filtration

Abstract A two-step magnetically seeded filtration process that includes heterogeneous flocculation (shear-flow and Brownian) and magnetic filtration is examined experimentally. The effects of various parameters — magnetic-field strength, size of particles, flow rate, seeding concentration, and solution pH — on the removal efficiency are investigated. A breakthrough model — which combines trajectory analysis, a particle buildup model, and a bivariate population-balance model applicable for Brownian flocculation — is developed to predict particle breakthrough in a magnetic filter. Experiments show that the removal efficiency increases as magnetic-field strength and particle size are increased and flow rate is decreased. A maximum in the removal efficiency is observed at a certain seeding concentration and at the lower pH values, which is explained from competing effects that take place with respect to magnetic susceptibility and size of aggregates as the seeding concentration and solution pH are increased. Modeling results of the trajectory analysis show that the effect of hydrodynamic resistance becomes important as Reynolds number and particle size are increased or the magnetic-field strength is decreased. Similarly to experimental observations, the modeling results predict that the removal efficiency increases with increasing magnetic-field strength and particle size indicating that the relative importance of magnetic and drag forces and the aggregation rate in the flocculation step play an important role in the magnetically seeded process. The breakthrough model developed in this study provides a good description of the experimental breakthrough data obtained from magnetic filtration of paramagnetic particles and magnetically seeded filtration with Brownian-flocculation.

Kinetics of Metal Ion Adsorption from Aqueous Solutions

Preface. 1. Introduction. 2. Adsorption of Ionic Solutes at Solid-Solution Interfaces. 3. Adsorption of Metal Ions from Aqueous Solutions: Model Development. 4. Adsorption of Metal Ions from Aqueous Solutions: Algorithm Development. 5. Adsorption of Metal Ions from Aqueous Solutions: Model Application. 6. Summary and Future Development. Computer Programs. Index.

Equilibrium and kinetic studies of copper adsorption by activated carbon

Copper adsorption by granular activated carbon is reported in this paper. The experimental section includes titrations of activated carbon, as well as equilibrium and kinetic studies of copper adsorption. The potentiometric titration results show that the point of zero charge is 9.5, and that the surface charge increases with decreasing pH. The adsorption of copper strongly depends on solution pH and increases from 10 to 95% at pH ranging from 2.3 to 8. A dramatic increase in pH and emission of small gas bubbles are observed during the experiments, which may result from adsorption of hydrogen ion and/or reduction-oxidation reactions. The two-pK triple-layer model is employed to describe copper adsorption. KINEQL, an adsorption kinetics algorithm, is used to represent the experimental data, and it is found that the model can describe reasonably well the experimental measurements of surface charge, adsorption equilibrium, and adsorption kinetics. Calculations show that formation of the surface-metal complexes SO−Cu2+ and SO−CuOH+ (a hydrolysis product of SO−Cu 2+) in the outer layer around the surface of carbon results in removal of copper ion. It is also found that mass transfer controls the adsorption rate, and that adsorption occurs in the micropore region where both external mass transfer and diffusion are important.

Monte Carlo simulations of electrical double-layer formation in nanopores

The formation of the electrical double layer (EDL) in an aqueous solution in contact with the charged solid surfaces of a slit-type nanopore has been simulated by grand canonical Monte Carlo (GCMC) and canonical Monte Carlo (CMC) methods. In the GCMC simulations, a primitive EDL model in which water is considered as a continuum and the ions are considered as hard spheres is used. The results are found to be slightly different from those predicted by the Gouy–Chapman model at low electrolyte concentration and low surface charge density. The GCMC results were then used as an initial condition for the CMC simulations of a nonprimitive model in which the EDL is composed of molecular water (four-point transferable intermolecular potential, together with fluctuating charge model), cations (Na+), and anions (Cl−). The nonprimitive model provides a very different view of the EDL at the atomic level. For example, a single layer of water molecules, instead of counterions, is strongly adsorbed on negatively charged ...