Thomas F. Speth

Microwave-Assisted Green Synthesis of Silver Nanostructures

Over the past 25 years, microwave (MW) chemistry has moved from a laboratory curiosity to a well-established synthetic technique used in many academic and industrial laboratories around the world. Although the overwhelming number of MW-assisted applications today are still performed on a laboratory (mL) scale, we expect that this enabling technology may be used on a larger, perhaps even production, scale in conjunction with radio frequency or conventional heating. Microwave chemistry is based on two main principles, the dipolar mechanism and the electrical conductor mechanism. The dipolar mechanism occurs when, under a very high frequency electric field, a polar molecule attempts to follow the field in the same alignment. When this happens, the molecules release enough heat to drive the reaction forward. In the second mechanism, the irradiated sample is an electrical conductor and the charge carriers, ions and electrons, move through the material under the influence of the electric field and lead to polarization within the sample. These induced currents and any electrical resistance will heat the sample. This Account summarizes a microwave (MW)-assisted synthetic approach for producing silver nanostructures. MW heating has received considerable attention as a promising new method for the one-pot synthesis of metallic nanostructures in solutions. Researchers have successfully demonstrated the application of this method in the preparation of silver (Ag), gold (Au), platinum (Pt), and gold-palladium (Au-Pd) nanostructures. MW heating conditions allow not only for the preparation of spherical nanoparticles within a few minutes but also for the formation of single crystalline polygonal plates, sheets, rods, wires, tubes, and dendrites. The morphologies and sizes of the nanostructures can be controlled by changing various experimental parameters, such as the concentration of metallic salt precursors, the surfactant polymers, the chain length of the surfactant polymers, the solvents, and the operation reaction temperature. In general, nanostructures with smaller sizes, narrower size distributions, and a higher degree of crystallization have been obtained more consistently via MW heating than by heating with a conventional oil-bath. The use of microwaves to heat samples is a viable avenue for the greener synthesis of nanomaterials and provides several desirable features such as shorter reaction times, reduced energy consumption, and better product yields.

Evaluation of nanofiltration pretreatments for flux loss control

The loss of membrane flux due to fouling is a major impediment to the development of membrane processes for use in drinking water treatment. The objective of this work was to evaluate fouling in nanofiltration (NF) pilot systems fed conventionally-treated (coagulation/sedimentation/filtration) Ohio River water (CT-ORW) with various additional levels of pretreatment. The chosen additional pretreatments were intended to produce waters with varying biological-fouling potential. Five parallel membranes were fed CT-ORW, ozonated CT-ORW, ozonated/biofiltered CT-ORW, CT-ORW reduced to 7°C, and chloraminated CT-ORW. All systems showed significant flux decline indicating that methods beyond those needed for just biogrowth control are required for NF systems treating conventionally-treated surface waters. The NF systems fed ozonated, ozonated/biofiltered, and untreated CT-ORW had the least amount of flux decline over the course of the study; however, they had significant amounts of biological growth. Fouling in these systems was attributed to the deposition of extracellular material (polysaccharides) in the cake layer, either from the biogrowth on the membrane or carryover from the pretreatment. The low-temperature system had greater flux decline, but it had lower biogrowth than the ozonated, and ozonated/biofiltered and untreated CT-ORW systems. Although lower in biogrowth, the deposited organic material in the low-temperature system still showed a strong biological signature (polysaccharides and aminosugars). The chloraminated system had the greatest flux decline, but the least amount of biogrowth. The organic material deposited in the chloraminated system showed a high level of proteinaceous character.

Research Issues Underlying the Four-Lab Study: Integrated Disinfection By-Products Mixtures Research

Chemical disinfection of drinking water is a major public health triumph of the 20th century, resulting in significant decreases in morbidity and mortality from waterborne diseases. Disinfection by-products (DBP) are chemicals formed by the reaction of oxidizing disinfectants with inorganic and organic materials in the source water. To address potential health concerns that cannot be answered directly by toxicological research on individual DBPs or defined DBP mixtures, scientists residing within the various organizations of the U.S. Environmental Protection Agencys Office of Research and Development (the National Health and Environmental Effects Research Laboratory, the National Risk Management Research Laboratory, the National Exposure Research Laboratory, and the National Center for Environmental Assessment) engaged in joint investigation of environmentally realistic complex mixtures of DBP. Research on complex mixtures of DBP is motivated by three factors: (a) DBP exposure is ubiquitous to all segments of the population; (b) some positive epidemiologic studies are suggestive of potential developmental, reproductive, or carcinogenic health effects in humans exposed to DBP; and (c) significant amounts of the material that makes up the total organic halide portion of the DBP have not been identified. The goal of the Integrated Disinfection Byproducts Mixtures Research Project (the 4Lab Study) is provision of sound, defensible, experimental data on environmentally relevant mixtures of DBP and an improved estimation of the potential health risks associated with exposure to the mixtures of DBP formed during disinfection of drinking water. A phased research plan was developed and implemented. The present series of articles provides the results from the first series of experiments.

Component-Based and Whole-Mixture Techniques for Addressing the Toxicity Of Drinking-Water Disinfection By-Product Mixtures

Chemical disinfection of water is of direct public health benefit as it results in decreased water-borne illness. The chemicals used to disinfect water react with naturally occurring organic matter, bromide, and iodide in the source water, resulting in the formation of disinfection by-products (DBPs). Despite the identification of several hundred DBPs, more than 50% of the mass of total organic halide formed during chlorination remains unidentified. The toxic contribution of the DBPs that are formed and present but not yet chemically identified, the unidentified fraction, has been largely unexplored. A better understanding of the potential for adverse human health consequences associated with exposure to the DBPs present in drinking water will be gained by integration of knowledge on the toxicity of individual DBPs; simple, defined DBP mixtures; complex, environmentally realistic DBP mixtures with partial chemical characterization; and the unidentified fraction.

Integrated Disinfection By-Products Mixtures Research: Disinfection of Drinking Waters by Chlorination and Ozonation/Postchlorination Treatment Scenarios

This article describes disinfection of the same source water by two commonly used disinfection treatment scenarios for purposes of subsequent concentration, chemical analysis, and toxicological evaluation. Accompanying articles in this issue of the Journal of Toxicology and Environmental Health describe concentration of these finished waters by reverse osmosis techniques, chemical characterization of the resulting disinfection by-product (DBP) concentrates, in vivo and in vitro toxicological results, and risk assessment methods developed to analyze data from this project. This project, called the “Four Lab Study,” involved participation of scientists from four laboratories/centers of the U.S. Environmental Protection Agency Office of Research and Development as well as extramural collaborators from the water industry and academia. One of the two finished waters was prepared by conventional treatment and disinfected by chlorination. The other finished water was also prepared by conventional treatment and disinfected by ozonation followed by chlorination (ozonation/postchlorination). Chlorination conditions of dose, time and temperature were similar for both treatment scenarios, allowing for a comparison. Both finished waters had acceptably low levels of particulates and bacteria, representative pH and chlorine levels, and contained numerous DBP. Known effects of ozonation were observed in that, relative to the water that was chlorinated only, the ozonated/postchlorinated water had lower concentrations of total organic halogen, trihalomethanes (THM), haloacetic acids (HAA), and higher concentrations of bromate, and aldehydes.

The effect of molecular oxygen on the activated carbon adsorption of natural organic matter in Ohio river water

Abstract Recently published data show that the adsorptive capacity of granular activated carbon for phenols increases significantly in the presence of molecular oxygen (Vidic, Suidan, Traegner and Nakhla, 1990). In this study, the effect of molecular oxygen on the adsorptive capacity of activated carbon for the natural organic matter present in Ohio River water was investigated. This source of natural organic matter was selected because it is a natural surface water and represents Cincinnatis main drinking water supply. Isotherm studies using pulverized activated carbon were conducted during the first week of each month of the year 1992. Adsorption isotherms were conducted on water from two stages of treatment (raw water directly pumped from the Ohio River and settled water from the California Drinking Water Treatment Plant, Cincinnati, Ohio). Investigations were conducted in the presence and absence of molecular oxygen. An appreciable increase in the adsorptive capacity of activated carbon for natural organic matter was noticed when molecular oxygen was present in the test environment. The adsorption isotherms were further described by fractionating the unknown matrix into a non adsorbable fraction and four fictive components of different adsorbabilities.

Integrated disinfection by-products mixtures research: concentration by reverse osmosis membrane techniques of disinfection by-products from water disinfected by chlorination and ozonation/postchlorination.

To conduct the health-effect studies described in subsequent articles in this series, concentrated aqueous mixtures of disinfection by-products were required for the two water treatment trains described in the preceding article (Miltner et al., 2008). To accomplish this, the finished drinking waters from each treatment train were sent through cation-exchange resin columns to remove hardness and free chlorine. Reverse osmosis membranes were then used to concentrate approximately 2400 L of each finished water down to approximately 18 L. The resulting volumetric concentration factors for the chlorinated and ozonated/postchlorinated waters were 136- and 124-fold, respectively. The concentrates were spiked with select disinfection by-products (DBPs) that were lost during the concentration effort. The results, along with the rationale for choosing the method of concentration, are presented. After reintroduction of a select list of lost DBPs, the concentration methodology used herein was able to produce concentrates that retained large percentages of the DBPs that were in the initial finished drinking waters. Further, the distributions of the DBPs in the concentrates matched those found in the finished drinking waters.

Effect of influent oxygen concentration on the GAC adsorption of VOCs in the presence of BOM

Abstract Two rapid small-scale column tests (RSSCTs) were conducted in combination with a large column adsorber to determine if the small columns could predict the performance of a large column. The adsorbate solution consisted of three volatile organic chemicals (VOCs) in the presence of background organic matter (BOM). Landfill leachate was used as the BOM source. Dissolved molecular oxygen was used as a parametric viriable in the column studies to determine its influence on the breakthrough behavior of the adsorbate solution. The RSSCT, designed according to the assumption of no dependency of the intraparticle surface diffusion coefficient on the activated carbon particle size, accurately predicted the performance of the large column under anoxic conditions (absence of molecular oxygen), but failed to predict its performance under ambient (8 mg 1−1 dissolved oxygen) or oxic conditions (30 mg 1−1 dissolved oxygen). The RSSCT, designed according to the assumption of a linear dependency of the intraparticle surface diffusion coefficient on the activated carbon particle size, accurately predicted the performance of the large column under oxic and ambient conditions, but failed to predict its performance under anoxic conditions. The presence of molecular oxygen in the test environment resulted in an increased adsorption of the BOM on the GAC surface, which in turn, decreased the capacity of the VOCs due to competitive effects.

Integrated Disinfection By-Products Research: Assessing Reproductive and Developmental Risks Posed by Complex Disinfection By-Product Mixtures

This article presents a toxicologically-based risk assessment strategy for identifying the individual components or fractions of a complex mixture that are associated with its toxicity. The strategy relies on conventional component-based mixtures risk approaches such as dose addition, response addition, and analyses of interactions. Developmental toxicity data from two drinking-water concentrates containing disinfection by-products (DBP) mixtures were used to illustrate the strategy. The results of this study showed that future studies of DBP concentrates using the Chernoff–Kavlock bioassay need to consider evaluating DBP that are concentrated more than 130-fold and using a rat strain that is more sensitive to chemically-induced pregnancy loss than Sprague-Dawley rats. The results support the planned experimental design of a multigeneration reproductive and developmental study of DBP concentrates. Finally, this article discusses the need for a systematic evaluation of DBP concentrates obtained from multiple source waters and treatment types. The development of such a database could be useful in evaluating whether a specific DBP concentrate is sufficiently similar to tested combinations of source waters and treatment alternatives so that health risks for the former may be estimated using data on the latter.

Factors affecting atrazine concentration and quantitative determination in chlorinated water

Although the herbicide atrazine has been reported to not react measurably with free chlorine during drinking water treatment, this work demonstrates that at contact times consistent with drinking water distribution system residence times, a transformation of atrazine can be observed. Some transformation products detected through the use of high performance liquid chromatography-electrospray mass spectrometry are consistent with the formation of N-chloro atrazine. The effects of applied chlorine, pH, and reaction time on the transformation reaction were studied to help understand the practical implications of the transformation on the accurate determination of atrazine in drinking waters. The errors in the determination of atrazine are a function of the type of dechlorinating agent applied during sample preparation and the analytical instrumentation utilized. When a reductive dechlorinating agent, such as sodium sulfite or ascorbic acid is used, the quantification of the atrazine can be inaccurate, ranging from 2-fold at pH 7.5 to 30-fold at pH 6.0. The results suggest HPLC/UV and ammonium chloride quenching may be best for accurate quantification. Hence, the results also appear to have implications for both compliance monitoring and health effects studies that utilize gas chromatography analysis with sodium sulfite or ascorbic acid as the quenching agent.