Neville G. Pinto

EFFECT OF CHEMICAL SURFACE HETEROGENEITY ON THE ADSORPTION MECHANISM OF DISSOLVED AROMATICS ON ACTIVATED CARBON

Abstract The effects of oxygen-containing groups, particularly carboxylic and carbonyl groups, on the adsorption of dissolved aromatics on ash-free activated carbon have been studied. Adsorption isotherms for phenol, aniline, nitrobenzene, and benzoic acid were generated in both aqueous and cyclohexane media, using carbons with different amounts of surface oxygen groups. It was found that water adsorption, dispersive/repulsive interactions, and hydrogen-bonding were the main mechanisms by which surface oxygen groups influence the adsorption capacity, while donor–acceptor interactions were found not to be significant. The adsorption mechanism was also found to be influenced by the properties of the functional group on the aromatic adsorbate, especially its ability to hydrogen-bond and through its activating/deactivating influence on the aromatic ring.

Effects of surface properties of activated carbons on adsorption behavior of selected aromatics.

Studies were undertaken to determine the role of surface oxygen complexes and metals on activated carbon on adsorption of selected aromatics. Three kinds of activated carbons (F400, MP and Darco G60) were used in the study. The F400 carbon was treated by oxygenation, deoxygenation and HCl-washing processes. Batch adsorption tests were used to evaluate the effects of surface oxygen complexes and metals on adsorption of phenol under oxic and anoxic conditions. The isotherm results showed that the removal of hydrophilic structures (carboxylic acid groups) of activated carbon increased physisorption and surface polymerization of phenol. It was also found that the metal (Fe) itself can not catalyze the surface polymerization of phenol on the carbon surface at room temperature. The findings of this study suggest that the life of a regenerated carbon adsorption bed could be extended by removing the metal content in activated carbon, and mildly oxidizing the surface.

Immobilization of phenol in cement-based solidified/stabilized hazardous wastes using regenerated activated carbon: leaching studies

In this research, we investigated the use of an inexpensive thermally regenerated activated carbon as a pre-adsorbent in the solidification/stabilization of phenol-contaminated sand. Our results show that even the addition of very low amounts of regenerated activated carbon (1%-2% w/w sand) resulted in the rapid adsorption of phenol in the Chemical solidification/stabilization (S/S) matrix, with phenol leaching reduced by as much as 600%. Adsorption studies indicated that the adsorption of phenol on the reactivated carbon was found to be partially irreversible over time in the S/S waste form, indicating possible chemical adsorption. Pore-fluid analyses of the cement paste containing phenol suggested the formation of a calcium-phenol complex, which further reduced the amount of free phenol present in the pores. Studies using several micro-structural techniques, including field emission scanning electron microscopy, X-ray diffraction, fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy, indicated significant morphological changes in the cement matrix upon the addition of phenol and reactivated carbon. The hydration of cement in the presence of phenol was retarded concomitant with formation of amorphous portlandite.

Study of hydrophobic interaction adsorption of bovine serum albumin under overloaded conditions using flow microcalorimetry.

The adsorption behavior of bovine serum albumin (BSA) on a Sepharose based hydrophobic interaction support has been studied. Flow microcalorimetry has been used to determine the heat of adsorption under overloaded chromatographic conditions. These data have been complemented with capacity factor and isotherm measurements to provide insight on the mechanisms of adsorption. The heat of adsorption data have confirmed that the hydrophobic interaction adsorption of BSA under linear isotherm conditions is driven by entropy changes. Under overloaded (non-linear) conditions, however, it has been shown that the changes in enthalpy can drive adsorption; this behavior is not evident from analyses of capacity factor data. It is postulated that for BSA adsorption on the Sepharose derivative of interest, attractive force interactions between adsorbed protein molecules drive the adsorption process under overloaded conditions in a high (NH4)2SO4 environment. It is further postulated that these interactions are due to a change in confirmation of the adsorbed protein under these conditions.

A Study of the Influence of Hydrophobicity of Activated Carbon on the Adsorption Equilibrium of Aromatics in Non-Aqueous Media

The effect of hydrophobicity on the adsorption of aromatics on metal-free activated carbons was studied. Adsorption isotherms for phenol, aniline, benzene, and xylene were generated in cyclohexane and heptane media, using seven carbons with different surface heterogeneity. The hydrophobicity of these carbons was probed using flow microcalorimetry (FMC). Surface polarity and solvent and adsorbate hydrophobicity were found to influence the adsorption capacity. For adsorbates that do not form hydrogen bonds with oxygen on the carbon surface, higher surface acidity lowers adsorption capacity due to increased polarity. In contrast, for adsorbates that can form hydrogen bonds with surface oxygen, the capacity is enhanced at higher surface acidities. A higher solvent hydrophobicity was found to decrease capacity for all the aromatic adsorbates studied, except at high surface polarity, where the effect of the solvent was found to be minimal.

Use of the sodium salt of poly(vinylsulfonic acid) as a low-molecular-weight displacer for protein separations by ion-exchange displacement chromatography

Abstract The sodium salt of poly(vinylsulfonic acid) (PVSNa), molecular weight 2000, a low-molecular-weight polyelectrolyte, has been identified as a suitable displacer for the concentration and purification of protein mixtures. This displacer has been tested on the separation of ovalbumin from conalbumin, and the fractionation of heterogeneous ovalbumin. The displacement characteristics of the polyelectrolyte were a strong function of the carrier pH, and a pH for good displacement development of heterogeneous ovalbumin has been identified. The displacer can be efficiently removed from the exchanger with a mild regeneration protocol. In this regard, the low-molecular-weight polyelectrolyte appears to have a significant advantage over high-molecular-weight ion-exchange displacers used in the past. Solvent requirements for regeneration and re-equilibration are significantly lower with PVSNa, suggesting the use of molecular weight to tailer ion-exchange displacers with desirable characteristics with respect to both column development and regeneration.

Ideal adsorbed phase model for adsorption of phenolic compounds on activated carbon

The applicability of the ideal adsorbed phase (IAP) model to the adsorption of selected phenolic organics on activated carbon has been studied. Experimental adsorption data were obtained as a function of solution pH and temperature for phenol and aniline, and characterized with the model. The characterizations include the effects of solvent adsorption, a factor that has been consistently neglected in the past. A critical analysis of the IAP model shows that a good fit of the surface excess isotherm data does not ensure that the underlying assumptions of the model are applicable. It has been shown for both phenol and aniline that while the surface excess data may be well characterized, there are significant discrepancies between the experimental and predicted values of the free immersion energy. Also, while the surface free immersion energy of phenol is only a function of surface coverage, for aniline it is also a function of pH and temperature. The importance of specifically accounting for the solvent as an adsorbing species in multicomponent systems has also been demonstrated. Predictions of the IAP model are shown to be significantly different in the presence and absence of water adsorption.

Combination of the steric mass action and non-ideal surface solution models for overload protein ion-exchange chromatography.

A model has been developed for protein ion-exchange equilibria under overloaded chromatographic conditions. This model combines the steric mass action model with the non-ideal surface solution model and accounts for steric hindrance and nearest neighbor interactions between protein-protein, protein-salt and salt-salt in a single formalism. Using the model on protein ion-exchange data, the importance of obtaining ion-exchange heat measurements in addition to adsorption isotherms has been demonstrated. It has been shown that small inaccuracies in the heat of ion-exchange data can lead to large differences in predictions of elution behavior. Additionally, it has also been shown that salt-salt interactions on the surface can strongly influence protein adsorption and that in the presence of these interactions, the shape and orientation of the molecule on the surface are important.

Characterization of enthalpic events in overloaded ion-exchange chromatography

Protein adsorption can be either endothermic or exothermic depending upon the protein, the sorbent and process conditions. In the case of protein adsorption onto ion-exchange surfaces exothermic adsorption heats are usually characterized as representing the electrostatic interaction between two oppositely charged surfaces. Endothermic adsorption heats are typically characterized as representing protein reconfiguration and/or repulsive interactions between adsorbed molecules. In certain segments of the literature surface dehydration and solution non-idealities have been suggested as possible sources of endothermic heats of adsorption. Each of these phenomena was investigated during studies concerning the adsorption of bovine serum albumin and ovalbumin onto an anion-exchange sorbent. The results demonstrated that electrostatic repulsive interactions between adsorbed molecules appears to be a larger contributor to endothermic heats of adsorption than surface dehydration or solution non-idealities. The presence of mobile phase cations can reduce the magnitude of endothermic adsorption heats by screening repulsive interactions between adsorbed molecules. Although water release was not found to be a major contributor to endothermic adsorption heats, it is likely to be a contributor to the entropic driving force associated with the adsorption of bovine serum albumin.

Investigation of the mechanism of protein adsorption on ordered mesoporous silica using flow microcalorimetry

The adsorption of bovine serum albumin (BSA) and lysozyme (LYS) on siliceous SBA-15 with 24 nm pores was studied using flow microcalorimetry; this is the first attempt to understand the thermodynamics of protein adsorption on SBA-15 using flow microcalorimetry. The adsorption mechanism is a strong function of protein structure. Exothermic events were observed when protein-surface interactions were attractive. Entropy-driven endothermic events were also observed in some cases, resulting from lateral protein-protein interactions and conformational changes in the adsorbed protein. The magnitudes of the enthalpies of adsorption for primary protein-surface interactions decrease with increased surface coverage, indicating the possibility of increased repulsion between adsorbed protein molecules. Secondary exothermic events were observed for BSA adsorption, presumably due to secondary adsorption made possible by conformational changes in the soft BSA protein. These secondary adsorption events were not observed for lysozyme, which is structurally robust. The results of this study emphasize the influence of solution conditions and protein structure on conformational changes of the adsorbed protein and the value of calorimetry in understanding protein-surface interactions.