Makram T. Suidan

IMPACT OF ENVIRONMENTAL CONDITIONS (PH, IONIC STRENGTH, AND ELECTROLYTE TYPE) ON THE SURFACE CHARGE AND AGGREGATION OF SILVER NANOPARTICLES SUSPENSIONS

The impact of capping agents and environmental conditions (pH, ionic strength, and background electrolytes) on surface charge and aggregation potential of silver nanoparticles (AgNPs) suspensions were investigated. Capping agents are chemicals used in the synthesis of nanoparticles to prevent aggregation. The AgNPs examined in the study were as follows: (a) uncoated AgNPs (H(2)-AgNPs), (b) electrostatically stabilized (citrate and NaBH(4)-AgNPs), (c) sterically stabilized (polyvinylpyrrolidone (PVP)-AgNPs), and (d) electrosterically stabilized (branched polyethyleneimine (BPEI)-AgNPs)). The uncoated (H(2)-AgNPs), the citrate, and NaBH(4)-coated AgNPs aggregated at higher ionic strengths (100 mM NaNO(3)) and/or acidic pH (3.0). For these three nanomaterials, chloride (Cl(-), 10 mM), as a background electrolyte, resulted in a minimal change in the hydrodynamic diameter even at low pH (3.0). This was limited by the presence of residual silver ions, which resulted in the formation of stable negatively charged AgCl colloids. Furthermore, the presence of Ca(2+) (10 mM) resulted in aggregation of the three previously identified AgNPs regardless of the pH. As for PVP coated AgNPs, the ionic strength, pH and electrolyte type had no impact on the aggregation of the sterically stabilized AgNPs. The surface charge and aggregation of the BPEI coated AgNPs varied according to the solution pH.

An evidence-based environmental perspective of manufactured silver nanoparticle in syntheses and applications: a systematic review and critical appraisal of peer-reviewed scientific papers.

BACKGROUND Most recently, renewed interest has arisen in manufactured silver nanomaterials because of their unusually enhanced physicochemical properties and biological activities compared to the bulk parent materials. A wide range of applications has emerged in consumer products ranging from disinfecting medical devices and home appliances to water treatment. Because the hypothesized mechanisms that govern the fate and transport of bulk materials may not directly apply to materials at the nanoscale, there are great concerns in the regulatory and research communities about potential environmental impacts associated with the use of silver nanoparticles. In particular, the unlimited combinations of properties emerging from the syntheses and applications of silver nanoparticles are presenting an urgent need to document the predominant salt precursors, reducing agents and stabilizing agents utilized in the synthesis processes of silver nanoparticles to guide the massive efforts required for environmental risk assessment and management. OBJECTIVES The primary objective of this study is to present an evidence-based environmental perspective of silver nanoparticle properties in syntheses and applications. The following specific aims are designed to achieve the study objective: Aim 1--to document the salt precursors and agents utilized in synthesizing silver nanoparticles; Aim 2--to determine the characteristics of silver nanoparticles currently in use in the scientific literature when integrated in polymer matrices to form nanocomposites and combined with other metal nanoparticles to form bimetallic nanoparticles; Aim 3--to provide a summary of the morphology of silver nanoparticles; and (4) Aim 4--to provide an environmental perspective of the evidence presented in Aims 1 to 3. METHODS A comprehensive electronic search of scientific databases was conducted in support of the study objectives. Specific inclusion criteria were applied to gather the most pertinent research articles. Data and information extraction relied on the type of synthesis methods, that is, synthesized silver nanoparticles in general and specific applications, nanocomposites, and bimetallic techniques. The following items were gathered for: type of silver salt, solvent, reducing agent, stabilizing agent, size, and type of application/nanocomposite/bimetallic, and template (for nanocomposites). The description of evidence was presented in tabular format. The critical appraisal was analyzed in graphical format and discussed. RESULTS An analysis of the scientific literature suggests that most synthesis processes produce spherical silver nanoparticles with less than 20nm diameter. Silver nanoparticles are often synthesized via reduction of AgNO(3), dissolution in water, and utilization of reductants also acting as capping or stabilizing agents for the control of particle size to ensure a relatively stable suspension. Two of the most commonly used reductants and stabilizing agents are NaBH(4) and citrate which yield particles with a negative surface charge over the environmental pH range (3-10). The environmental perspectives of these parameters are discussed. CONCLUDING REMARKS It is expected that the antibacterial property of bulk silver is carried over and perhaps enhanced, to silver nanoparticles. Therefore, when one examines the environmental issues associated with the manufacture and use of silver nanoparticle-based products, the antibacterial effects should always be taken into account particularly at the different stages of the product lifecycle. Currently, there are two arguments in the scientific literature about the mechanisms of antimicrobial properties of silver nanoparticles as they relate to colloidal silver particles and inonic silver. Methodologies of risk assessment and control have to account for both arguments.

Gas treatment in trickle-bed biofilters: Biomass, how much is enough?

The objective of this article is to define and validate a mathematical model that desribes the physical and biological processes occurring in a trickle-bed air biofilter for waste gas treatment. This model considers a two-phase system, quasi-steady-state processes, uniform bacterial population, and one limiting substrate. The variation of the specific surface area with bacterial growth is included in the model, and its effect on the biofilter performance is analyzed. This analysis leads to the conclusion that excessive accumulation of biomass in the reactor has a negative effect on contaminant removal efficiency. To solve this problem, excess biomass is removed via full media fluidization and backwashing of the biofilter. The backwashing technique is also incorporated in the model as a process variable. Experimental data from the biodegradation of toluene in a pilot system with four packed-bed reactors are used to validate the model. Once the model is calibrated with the estimation of the unknown parameters of the system, it is used to simulate the biofilter performance for different operating conditions. Model predictions are found to be in agreement with experimental data. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 583-594, 1997.

Evaluation of Trickle Bed Biofilter Media for Toluene Removal

Abstract In this research, pilot-scale trickle bed biofilter systems have been analyzed to determine their effectiveness in controlling toluene in waste gas streams. These studies evaluated two synthetic microbial attachment media—a monolithic channelized medium and a pelletized ceramic medium. Operational parameters considered included toluene loading, empty bed residence time (EBRT), temperature, and long-term operation. The channelized medium provided 99% removal efficiency for a toluene loading of 0.725 kg COD/m3-day during the initial stages. However, continuous operation resulted in reduced and erratic efficiencies, due to air channeling caused by random plugging. After biomass accumulated within the channels and was subsequently removed by hosing, performance of the channelized medium never regained the previous levels. Similarly, the pelletized medium exhibited consistently good performance until the accumulation of excess biomass in the medium interstices also caused overall performance to deteri...

Kinetic modeling of U.V. disinfection of water

The response of pure cultures of Escherichia coli, Candida parapsilosis, and bacterial virus f2 to ultraviolet light radiation was studied in a batch reactor and in a completely mixed, flow-through annular reactor. Two kinetic models were tested as to their ability to scale between batch results and flow-through reactor results. Using the respective best-fit kinetic parameters for each model from batch data, the response of the organisms in the flow-through reactor could be predicted by using either multi-target kinetics or series-event kinetics. The series-event model was judged to be superior to multi-target kinetics because it better represents the known mechanism of u.v. inactivation than does multi-target kinetics. Since two different models can be used to describe the data, the simple agreement between experimental data and model predictions does not necessarily prove that either model is mechanistically correct.

Titania powder modified sol-gel process for photocatalytic applications

Thick films of TiO2 were prepared on glass and stainless steel substrates using an alkoxide sol-gel process modified by addition of Degussa P-25 powder. The prepared films were characterized by SEM, EDS, XRD and other methods. The TiO2 films obtained from the powder modified sol were compared to films obtained from the alkoxide sol-gel without modification. The films obtained from the modified sol-gel were about ten times thicker for a single dip coating/heat treatment cycle than the films obtained from the sol without powder addition. The prepared thick films were smooth and free of macrocracking, fracture or flaking. The grain size of these films was uniform and in the range 100–150 nm and the films were a mixture of anatase and rutile TiO2. The films obtained from the powder modified sol on the stainless steel substrate were also much harder compared to the films obtained from sols without modification and displayed excellent adhesion to the substrate.

The impact of stabilization mechanism on the aggregation kinetics of silver nanoparticles.

The use of silver nanoparticles (AgNPs) for various applications is growing drastically. The increase in use will eventually lead to their release into the environment. The tendency of AgNPs to aggregate and the kinetics of aggregation are major factors that govern their fate in the environment. Dynamic light scattering (DLS) was utilized to investigate the electrolyte-induced aggregation kinetics (NaNO₃, NaCl and Ca(NO₃)₂) of coated and uncoated AgNPs which are electrostatically (H₂-AgNPs and Citrate-AgNPs), sterically (polyvinylpyrrolidone (PVP)-AgNPs) and electrosterically (branched polyethyleneimine (BPEI)-AgNPs) stabilized. The aggregation kinetics of the electrostatically stabilized AgNPs was in agreement with the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and the AgNPs exhibited both reaction-limited and diffusion-limited regimes. The H₂-AgNPs had critical coagulation concentrations (CCC) of 25, 30 and 3mM in the presence of NaNO₃, NaCl and Ca(NO₃)₂ salts, respectively. The Citrate-AgNPs had CCC of 70, 70 and 5 mM in the presence of NaNO₃, NaCl and Ca(NO₃)₂ salts, respectively. The values of the Hamaker constant for the electrostatically stabilized AgNPs were also determined and the values were in agreement with the reported values for metallic particles. The aggregation kinetics for both the sterically and electrosterically stabilized AgNPs (PVP-AgNPs and BPEI-AgNPs) was not in agreement with the DLVO theory and the particles were resistant to aggregation even at high ionic strength and electrolyte valence. The PVP-AgNPs and the BPEI-AgNPs had no critical aggregation concentration value at the investigated ionic strength values.

A numerical model for density-and-viscosity-dependent flows in two-dimensional variably saturated porous media

We present a formulation for water flow and solute transport in two-dimensional variably saturated media that accounts for the effects of the solute on water density and viscosity. The governing equations are cast in a dimensionless form that depends on six dimensionless groups of parameters. These equations are discretized in space using the Galerkin finite element formulation and integrated in time using the backward Euler scheme with mass lumping. The modified Picard method is used to linearize the water flow equation. The resulting numerical model, the MARUN model, is verified by comparison to published numerical results. It is then used to investigate beach hydraulics at seawater concentration (about 30 g l−1) in the context of nutrients delivery for bioremediation of oil spills on beaches. Numerical simulations that we conducted in a rectangular section of a hypothetical beach revealed that buoyancy in the unsaturated zone is significant in soils that are fine textured, with low anisotropy ratio, and/or exhibiting low physical dispersion. In such situations, application of dissolved nutrients to a contaminated beach in a freshwater solution is superior to their application in a seawater solution. Concentration-engendered viscosity effects were negligible with respect to concentration-engendered density effects for the cases that we considered.

ROTATING DISK PHOTOCATALYTIC REACTOR: DEVELOPMENT, CHARACTERIZATION, AND EVALUATION FOR THE DESTRUCTION OF ORGANIC POLLUTANTS IN WATER

This work focuses on the development, characterization and evaluation of the TiO2 Rotating Disk Photocatalytic Reactor (RDPR) for the treatment of organic pollutants in water. A commercial TiO2-based catalyst in the form of composite ceramic balls was used as the immobilized photocatalyst on the rotating disk. LiCl tracer studies conducted at different disk angular velocities, ranging from 20 to 5 rpm, proved that the mixing in the RDPR is close to that of an ideal CSTR. Two different techniques, an overflow method and the potassium ferrioxalate actinometry method were employed for determining the liquid holdup and turnover time of the rotating disk with glass beads substituted for TiO2 coated beads. The two methods were in agreement and only the overflow method was employed for determining the same parameters for the rotating disk when loaded with TiO2 composite ceramic balls. Power law correlations were obtained in all cases. TiO2-assisted photocatalytic degradation of 4-chlorobenzoic acid (4-CBA) was investigated in the RDPR at a specified angular velocity of the rotating disk (4 rpm), initial pH=3.0, and room temperature using near-UV radiation.

Electrolytic denitrification: Long term performance and effect of current intensity

The long-term performance of the biofilm-electrode reactor is demonstrated for denitrification. The total nitrate and nitrite concentration, oxidation-reduction potential (ORP), total organic carbon (TOC), color, and viable cell count in the effluent, were monitored to evaluate the performance of the reactor. Initially, the applied electric current intensity was varied from 0 to 100 mA. A nitrate removal efficiency exceeding 98% was achieved at a current of 20 mA when phosphate was used as a buffer. Current intensities between 20 to 25 mA resulted in the lowest effluent concentrations of nitrate, whereas, lower current intensities affected proportionately less nitrate reduction. For higher current intensities, hydrogen inhibition and charge induced repulsion caused decreased reduction efficiencies. ORP was governed by both the concentrations of hydrogen and effluent nitrate. A total nitrate removal efficiency of 85% was achieved at a current level of 25 mA in the absence of any nutrient. No appreciable change occurred in viable cell count in the effluent with time.