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The adsorption kinetics exhibited by selected charge mutants of T4 lysozyme at silanized silica surfaces were monitored with in situ ellipsometry. Mutant lysozymes were produced by substitution of lysine (Lys) with glutamic acid (Glu). Each substitution resulted in a decrease in the net charge of the protein by 2 units. The wild type lysozyme of net charge +9, and two mutants of net charge +7 and +5 were obtained from E. coli strain RR1 . Adsorption kinetics recorded at hydrophilic and hydrophobic interfaces were compared to the kinetic behavior predicted by two simple models for protein adsorption. One was a three-rate-constant model allowing for reversible adsorption followed by conversion to an irreversibly adsorbed form, and was analyzed under three different conditions. The first condition allowed the adsorption rate (k₁) and the desorption rate (k₋₁) to be variable while the surface-induced conversion rate (s₁) was assumed constant. The second condition assumed k₁ and k₋₁ constant instead of S₁, and the third allowed all kinetic rate constants to be variable. The second model allowed for irreversible adsorption into one of two states directly from solution. Both models suggested that substitution of Lys with Glu in the backbone of T4 lysozyme facilitates the adsorption of the protein at these interfaces. Proteins apparently adsorbed at the interfaces more tightly and occupied a greater interfacial area with substitution of Lys with Glu, and these effects were related to the location of the substitutions relative to other charged residues of the protein, and not to net charge. / Graduation date: 1995

Thesis (PhD)--University of South Australia, 1997

Thesis (PhD)--University of South Australia, 2000

The objective of the work described in this thesis was to improve the scientific basis for modelling the migration of U in the sub-surface environment. The project involved: ?? studying the sorption of U on model minerals (Georgia kaolinite and ferrihydrite) in laboratory experiments ?? carrying out experimental studies of U sorption on complex natural substrates ?? studying the mechanisms influencing U retardation in the natural environment, including transformation processes of iron oxides ?? identifying chemical factors which control U sorption on model and natural substrates ?? developing a mechanistic model for U sorption on ferrihydrite and kaolinite using the surface complexation adsorption model , and ?? assessing and modelling the effect of complexing ligands on uranyl adsorption. Uranium (VI) sorption on geological materials is influenced by a large number of factors including: pH, ionic strength, partial pressure of CO2, adsorbent loading, total amount of U present, and the presence of inorganic and organic ligands. The sorption of UO22+ typically increases with increasing pH (the 'low pH sorption edge') up to about pH 7. In systems equilibrated with air, there is a sharp decrease in sorption above this pH value (the 'high pH edge'), due to strong complexation between uranyl and carbonate. The adsorption model being used for ferrihydrite is a surface complexation model with a diffuse double layer, and both strong and weak sites for U sorption. Based on the analysis of EXAFS data, the U surface complexes were modelled as mononuclear bidentate surface complexes of the form (&gtFeO2)UO20. Ternary surface complexes involving carbonate with the form (&gtFeO2)UO2CO32- were also required for the best simulation of U sorption data. There was a slight decrease in U sorption on ferrihydrite in systems that contained sulfate. It was necessary to consider competition between UO22+ and SO42- for surface sites, as well as complexation between UO22+ and SO42- to model the data. The presence of citrate considerably reduced U sorption and caused dissolution of ferrihydrite. Complexation of citrate with both uranyl and ferric ions was taken into account in modelling this system. The model required the optimisation of the formation constant for a postulated mixed metal (U/Fe/citrate) aqueous complex. Humic acid increased U uptake at pH values below 7, with little effect at higher pH values. In terms of the amount of U sorbed per gram of adsorbent, U uptake on kaolinites KGa-1 and KGa-1B was much weaker than U uptake on ferrihydrite under similar experimental conditions. Electron microscope examination showed that titanium-rich impurity phases played a major role in the sorption of U by these standard kaolinites. A relatively simple model for uranyl sorption on the model kaolinites was able to account for U sorption under a wide range of experimental conditions. The model involved only three surface reactions on two sites (&gtTiOH and &gtAlOH), with a non-electrostatic surface complexation model. The relative amounts of the sites were estimated from AEM results. Precipitation was taken into account in modelling the experimental data obtained with high U concentrations. The effects of sulfate and citrate on U sorption by kaolinite were also assessed and modelled. Sulfate had a small effect on U sorption, which may be explained by aqueous complexation. Citrate had a greater effect, and this was not wholly explained by the formation of aqueous U-citrate complexes. The most likely explanation would also involve competition for surface sites between U and citrate. Uranyl uptake on ferrihydrite was greatly increased by the presence of phosphate. This was not due to precipitation, and was attributed to the formation of a ternary surface complex with a proposed structure of (&gtFeO2)UO2PO43-. The log K value for the formation of this complex was optimised using FITEQL. Phosphate also increased uptake of uranyl on kaolinite, and this was also attributed to the formation of ternary uranyl phosphate surface species. Uranium sorption on weathered schist samples from the vicinity of the Koongarra U deposit in northern Australia was generally similar to the model minerals (in terms of the effects of pH, ionic strength, total U, etc). Many experiments with the natural materials were spiked with an artificial U isotope (236U), which allowed adsorption (of 236U) and desorption (of 238U) to be distinguished, and provided a means of estimating the 'labile' or 'accessible' portion of the natural U content. A significant advantage of this method is that (unlike chemical extractions) it does not rely on the assumptions about the phases extracted by 'selective' reagents. Uranium sorption experiments were also carried out with Koongarra samples which had been treated with citrate / dithionite / bicarbonate (CDB) reagent to remove iron oxides. Uranium sorption was greatly decreased by the CDB extraction, which reduced the surface area of the samples by about 30-40%. To further elucidate the impact of iron minerals on U mobility in the natural environment, the transformation of synthetic ferrihydrite containing adsorbed natural uranium was studied. In these experiments, the ferrihydrite was partially converted to crystalline forms such as hematite and goethite. The uptake of an artificial uranium isotope (236U) and the leaching of 238U from the samples were then studied in adsorption / desorption experiments. The transformation of ferrihydrite to crystalline minerals substantially reduced the ability of the samples to adsorb 236U from solution. Some of the previously adsorbed 238U was irreversibly incorporated within the mineral structure during the transformation process. Therefore, transformation of iron minerals from amorphous to crystalline forms provides a possible mechanism for uranium immobilisation in the groundwater environment. In considering the overall effect on U migration, this must be balanced against the reduced ability of the transformed iron oxide to adsorb U. The experiments with the model and natural substrates demonstrated that trace impurities (such as Ti-oxides) and mineral coatings (such as ferrihydrite) can play a dominant role in U adsorption in both environmental and model systems. Although the various substrates had different affinities for adsorbing U, the effects of chemical factors, including pH, ionic strength, and carbonate complexation were similar for the different materials. This suggests that a mechanistic model for U sorption on model minerals may eventually be incorporated in geochemical transport models, and used to describe U sorption in the natural environment.

Typewritten / Includes bibliographical references. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Agricultural Chemistry, 1960

Research Doctorate - Doctor of Philosophy (PhD) / The adsorption characteristics of pH responsive tertiary amine methacrylate-based diblock copolymers have been investigated. The main focus of this work is on poly(2-(dimethylamino)ethyl methacrylate-b-poly(2-(diethylamino)ethyl methacrylate (PDMA-b-PDEA) adsorption to the silica/aqueous solution interface at pH 9. Differing degrees of polymerisation and quaternisation were investigated with some attention given to variation of the block hydrophobicity utilising poly(2-(dimethylamino)ethyl methacrylate-b-poly(2-(diisopropylamino)ethyl methacrylate (PDMA-b-PDPA). Principally, optical reflectometry (OR) and atomic force microscopy (AFM) have been employed to monitor the adsorption in terms of adsorbed mass and layer morphology. A variety of other techniques have been utilised to provide ancillary information, including quartz crystal microbalance, zeta potential, dynamic light scattering and contact angle measurements. The combined results have provided a comprehensive understanding of the adsorption characteristics for the copolymers studied. Micelles of the tertiary amine methacrylate-based copolymers adsorbed readily to silica from aqueous solution at pH 9. The adsorption isotherms were determined, exhibiting a high affinity Langmuirian shape where the CMC did not appear to impact on the adsorbed mass. The adsorption was rationalised by the interaction between the cationic PDMA corona of the micelles with the negatively charged substrate. The more hydrophobic PDEA core block increased the level of adsorption above that observed for the PDMA homopolymer. It was shown that the adsorbed layers were robust to rinsing with electrolyte at high pH, although reduction of the pH to 4 yielded significant desorption. The adsorbed layer morphology observed by in situ AFM exhibited distinct micellar structures. The combined adsorbed mass and AFM images showed a significantly higher surface aggregation number than the measured solution aggregation number, indicating a more complex adsorption process than simple direct micelle adsorption. The adsorption kinetics were studied to elucidate the adsorption mechanism and revealed complex dynamic processes. Particular focus was given to the adsorption of 0q PDMA93-b-PDEA24 where the impact of concentration was evident and three mechanistic regimes could be defined; below the CMC, just above the CMC and far above the CMC. Interestingly, the adsorption process just above the CMC indicates a surface aggregation mechanism, while well above the CMC, the adsorption proceeds via a process that includes both direct micelle and unimer adsorption. On longer timescales, the adsorption at higher concentrations revealed an additional induction period of micelle relaxation on the surface that allowed for further adsorption. Increasing the PDMA quaternisation was found to reduce post adsorption rearrangement and as result equilibrium was reached more quickly for the highly quaternised analogues. The response of the adsorbed PDMA-b-PDEA copolymer films to multiple changes in solution pH (9 and 4) was monitored. After the initial desorption of copolymer with rinsing at pH 9 and then at pH 4, the adsorbed mass of copolymer was found to be constant with multiple cycles of pH. The remaining robust adsorbed layers, then exhibited reversible uptake and release of water with multiple pH cycles as measured by QCM. This observation was readily rationalised by the observed changes in copolymer charge (and hence hydrophobicity) affecting the interaction of the copolymer chains with the surrounding solution. While these characteristics were found to be reversible with pH cycling it was found that the initial micelle structure of the adsorbed film was lost upon the first rinse to pH 4. Finally, the first low Tg micelle-micelle multilayers of up to six layers were constructed using alternating layers of cationic and anionic tertiary amine methacrylate-based copolymers at pH 9. The existence of true micellar structures within each layer was proven using in situ AFM imaging where the alternating layer characteristics were supported by measured force curves. The construction of the individual micelle layers was also monitored by OR, where clear evidence of layer build-up was shown. In addition, each layer was robust to rinsing with electrolyte at the adsorbing pH, although, the stability of the formed multilayer was found to be limited to six layers. Upon reduction of the pH, almost all the adsorbed material was instantaneously removed from the surface. The stimulus-responsive nature of such multilayer films augurs well for potential controlled uptake/release applications. These findings should greatly encourage a larger research focus on micelle-micelle multilayers.

Thesis (M.S. - Hydrology and Water Resources)--University of Arizona, 1984. / Includes bibliographical references (leaves 72-74).