Walter J. Weber

Surface Chemistry of Active Carbon: Specific Adsorption of Phenols

The characteristics of the uptake of phenol and nitrophenols by active carbon have been investigated. The measurements of solution equilibrium parameters as well as surface structural characteristics determined by infrared internal reflection spectroscopy are presented. A charge-transfer interaction is postulated which explains the observed adsorption and spectroscopic characteristics on the basis of the electron densities in the sorbate molecule, as well as the types and concentrations of surface functional groups present on the carbon.

Sorption of hydrophobic compounds by sediments, soils and suspended solids—I. Theory and background

Abstract This is the first of a two-part series describing the adsorption of hydrophobic pollutants by sediments, soils, and suspended solids. Many pollutants, including a large number of hazardous compounds, are hydrophobic and their environmental behavior varies markedly between sorbed and dissolved states. It is thus necessary to account for sorption reactions in analysis and prediction of the environmental transport and fate of such pollutants. In this first part the energetics of sorption reactions are discussed and models for description of equilibrium relationships are summarized. Factors which affect the sorption of pollutants in natural environmental systems are then considered and evaluated.

Multifunctional Dendrimer-Modified Multiwalled Carbon Nanotubes: Synthesis, Characterization, and In Vitro Cancer Cell Targeting and Imaging

Carbon nanotubes hold great promise for their use as a platform in nanomedicine, especially in drug delivery, medical imaging, and cancer targeting and therapeutics. Herein, we present a facile approach to modifying carbon nanotubes with multifunctional poly(amidoamine) (PAMAM) dendrimers for cancer cell targeting and imaging. In this approach, fluorescein isothiocyanate (FI)- and folic acid (FA)-modified amine-terminated generation 5 (G5) PAMAM dendrimers (G5·NH(2)-FI-FA) were covalently linked to acid-treated multiwalled carbon nanotubes (MWCNTs), followed by acetylation of the remaining primary amine groups of the dendrimers. The resulting MWCNT/G5.NHAc-FI-FA composites are water-dispersible, stable, and biocompatible. In vitro flow cytometry and confocal microscopy data show that the formed MWCNT/G5·NHAc-FI-FA composites can specifically target to cancer cells overexpressing high-affinity folic acid receptors. The results of this study suggest that, through modification with multifunctional dendrimers, complex carbon nanotube-based materials can be fabricated, thereby providing many possibilities for various applications in biomedical sensing, diagnosis, and therapeutics.

Sorption of hydrophobic compounds by sediments, soils and suspended solids—II. Sorbent evaluation studies

Abstract Concepts underlying the sorption of hydrophobic compounds and models for representation of observed equilibrium relationships were presented in Part I of this series. The earlier paper also summarized and evaluated major factors which affect the sorption of pollutants in natural environmental systems. This second part of the series presents a detailed summary and evaluation of the sorption of a particular class of hydrophobic pollutants, polychlorinated biphenyls (PCB), on a variety of different types of sediments, soils, suspended solids, and microorganisms. Equilibrium models described in Part I are used here to describe and analyze the PCB sorption data.

DETERMINATION OF MASS TRANSPORT PARAMETERS FOR FIXED-BED ADSORBERS

An innovative technique has been developed for determination of external (“film”) and internal (“intraparticle”) mass transport parameters associated with uptake of solutes by porous solids in fixed-bed adsorbers. These parameters are required for predictive modeling of the dynamics of such systems as activated carbon columns and ion exchange beds. Current methods for evaluation of film transfer coefficients and effective intraparticle diffusivities involve correlation procedures and separate batch-reactor measurements of adsorption rates, respectively. These methods frequently introduce and compound significant errors in subsequent predictive modeling Tor full-scale process design. The present work develops a method by which, on the basis of the characteristics of breakthrough curves measured in specially designed micro-columns, more accurate and reliable simultaneous determinations of both film transfer coefficients and effective surface diffusivities can be done. Verification of the procedure is demons...

MODELING THE SORPTION OF HYDROPHOBIC CONTAMINANTS BY AQUIFER MATERIALS--I RATES AND EQUILIBRIA

This is the first of a two-part series describing experimental studies and numerical modeling of the sorption of hydrophobic contaminants by aquifer materials. The work focuses on the evaluation of predictive modeling methods for simulating sorption processes in groundwater systems. Equilibrium behavior and rates of approach to equilibrium were investigated for two hydrophobic solutes and three aquifer materials utilizing different reactor configurations. This paper discusses investigations conducted in completely mixed batch reactors. These investigations illustrate that sorption equilibria are nonlinear for the systems studied and that sorption rates involve an initial rapid step followed by a slower continuing uptake that can persist for several days. Alternative models for description of these equilibria and rate conditions are presented and compared. The second paper evaluates the use of sorption model coefficients determined from batch-reactor systems to model column-reactor systems.

The determinants of divalent/monovalent selectivity in anion exchangers

Abstract This research demonstrates that it is the distance of fixed-charge separation in the resin which is the primary determinant of divalent/monovalent selectivity. Anion resins, particularly acrylics and epoxies, with closely spaced N-containing (amine) functional groups are inherently divalent-ion selective. This is because the uptake of a divalent anion, e.g. sulfate, requires the presence of two closely-spaced positive charges. Results from this study of 30 commercially-available, strong- and weak-base, anion resins indicate that in order to bring positive charges into close proximity within a resin one can: (1) incorporate the amine functional groups into the polymer chains as opposed to having them pendant on the chains; (2) minimize the size and number of organic (“R”) groups attached to the N atom, i.e., minimize the size of the amine; and (3) maximize the resin flexibility, i.e., its ability to reorient to satisfy divalent counterions, by minimizing the degree of crosslinking. The distance-of-charge-separation theory is not restricted to divalent anion exchangers but also applies to cation exchangers and to polyvalent ions in general.

Correlation of humic substance trihalomethane formation potential and adsorption behavior to molecular weight distribution in raw and chemically treated waters

The molecular weight distributions (MWDs) of several commercially prepared humic and fulvic acids and organic matter present in natural waters were analyzed by gel chromatography. The responses of these substances to treatment by alum coagulation, lime softening, and activated carbon adsorption were also analyzed, as were their trihalomethane formation potentials before and after each treatment. The treatability characteristics and trihalomethane formation properties of the different organic sub- stances were then related to their respective MWDs. Differences in the MWDs were found to effect differences in the behavior of lumped parameter measures of organic matter (such as TOC) with respect to the several treatment operations. The treatments in turn were observed to alter the MWDs of the organic substances as well as their phenomenological behavior in subsequent process operations.

Surface oxides of activated carbon: Internal reflectance spectroscopic examination of activated sugar carbons

Abstract The nature of the activated carbon surface with respect to surface functional groups has been examined using infrared internal reflectance spectroscopy (IRS). By employing this recently developed technique, the surface character of laboratory-activated sugar carbons has been examined as a function of activation temperature and atmosphere. The IRS spectra obtained for series of O 2 and CO 2 + O 2 activated carbons indicate the presence of dicarboxylic acids and quinone carbonyl groups. The adsorption isotherms of p -nitrophenol on these carbons has been studied and has been shown to conform with the previously suggested adsorption mechanism involving the formation of a donor-acceptor complex with surface carbonyl oxygen.

Modeling the sorption of hydrophobic contaminants by aquifer materials—II. Column reactor systems

This is the second of a two-part series describing laboratory investigations and mathematical modeling of the sorption of hydrophobic solutes by aquifer materials. An evaluation is made of several rate and equilibrium models for description of solute sorption as a rational basis for predicting sorption processes in groundwater systems. The first paper presents and compares sorption in completely mixed batch-reactor systems while this paper addresses column-reactor systems. The results show that accurate representation of the sorption process can be obtained with either a dual-resistancc diffusion model or an equilibrium/first-order sorption rate model. Changes in velocity of the fluid phase affect the magnitude of fitted model parameters, but changes in concentration have negligible influence on these parameters.