Effect of adsorbed carbonate on surface charge characteristics and physical properties of aluminum hydroxide gel

Preclinical Toxicology of Vaccines1

Immunity to targeted infectious diseases may be conferred or enhanced by vaccines, which are manufactured from recombinant forms as well as inactivated or attenuated organisms. These vaccines have to meet requirements for safety, quality, and efficacy. In addition to antigenic components, various adjuvants may be included in vaccines to evoke an effective immune response. To ensure the safety of new vaccines, preclinical toxicology studies are conducted prior to the initiation of, and concurrently with, clinical studies. There are five different types of preclinical toxicology study in the evaluation of vaccine safety: single and/or repeat dose, reproductive and developmental, mutagenicity, carcinogenicity, and safety pharmacology. If any adverse effects are observed in the course of these studies, they should be fully evaluated and a final safety decision made accordingly. Successful preclinical toxicology studies depend on multiple factors including using the appropriate study designs, using the right animal model, and evoking an effective immune response. Additional in vivo and in vitro assays that establish the identity, purity, safety, and potency of the vaccine play a significant role in assessing critical characteristics of vaccine safety.

Effects of dissolved carbonate on arsenate adsorption and surface speciation at the hematite--water interface

Effects of dissolved carbonate on arsenate [As(V)] reactivity and surface speciation at the hematite-water interface were studied as a function of pH and two different partial pressures of carbon dioxide gas [P(CO2) = 10(-3.5) atm and approximately 0; CO2-free argon (Ar)] using adsorption kinetics, pseudo-equilibrium adsorption/titration experiments, extended X-ray absorption fine structure spectroscopic (EXAFS) analyses, and surface complexation modeling. Different adsorbed carbonate concentrations, due to the two different atmospheric systems, resulted in an enhanced and/or suppressed extent of As(V) adsorption. As(V) adsorption kinetics [4 g L(-1), [As(V)]0 = 1.5 mM and I = 0.01 M NaCl] showed carbonate-enhanced As(V) uptake in the air-equilibrated systems at pH 4 and 6 and at pH 8 after 3 h of reaction. Suppressed As(V) adsorption was observed in the air-equilibrated system in the early stages of the reaction at pH 8. In the pseudo-equilibrium adsorption experiments [1 g L(-1), [As(V)]0 = 0.5 mM and I = 0.01 M NaCI], in which each pH value was held constant by a pH-stat apparatus, effects of dissolved carbonate on As(V) uptake were almost negligible at equilibrium, but titrant (0.1 M HCl) consumption was greater in the air-equilibrated systems (P(CO2) = 10(-3.5) atm) than in the CO2-free argon system at pH 4-7.75. The EXAFS analyses indicated that As(V) tetrahedral molecules were coordinated on iron octahedral via bidentate mononuclear ( 2.8 A) and bidentate binuclear (approximately equal to 3.3 A) bonding at pH 4.5-8 and loading levels of 0.46-3.10 microM m(-2). Using the results of the pseudo-equilibrium adsorption data and the XAS analyses, the pH-dependent As(V) adsorption under the P(CO2) = 10(-3.5) atm and the CO2-free argon system was modeled using surface complexation modeling, and the results are consistent with the formation of nonprotonated bidentate surface species at the hematite surfaces. The results also suggest that the acid titrant consumption was strongly affected by changes to electrical double-layer potentials caused by the adsorption of carbonate in the air-equilibrated system. Overall results suggest that the effects of dissolved carbonate on As(V) adsorption were influenced by the reaction conditions [e.g., available surface sites, initial As(V) concentrations, and reaction times]. Quantifying the effects of adsorbed carbonate may be important in predicting As(V) transport processes in groundwater, where iron oxide-coated aquifer materials are exposed to seasonally fluctuating partial pressures of CO2(g).

Stability of aluminium-containing adjuvants to autoclaving

Aluminium phosphate adjuvant remained amorphous when autoclaved for 30 or 60 min at 121 degrees C. However, deprotonation and dehydration reactions occurred as evidenced by a decrease in the pH. The protein adsorption capacity, rate of acid neutralization at pH 2.5 and point of zero charge also decreased indicating that the deprotonation/dehydration reactions resulted in a decreased surface area. Autoclaving aluminium hydroxide adjuvant increased the degree of crystallinity as measured by the width at half height of the major band in the X-ray diffractogram. The pH decreased during autoclaving suggesting that the same deprotonation/dehydration reactions which reduced the surface area of aluminium phosphate adjuvant were responsible for the increased degree of crystallinity. These reactions also resulted in a reduced surface area as both the protein adsorption capacity and viscosity decreased following autoclaving.

The importance of surface charge in the optimization of antigen-adjuvant interactions

The adsorptive behavior of the recombinant malarial antigens R32tet32, R32NS181 and NS181V20 to aluminum hydroxide and aluminum phosphate gels was studied as a function of pH and buffer ions. The Plasmodium falciparum antigen, R32NS181, and the P. vivax antigen, NS181V20, with isoelectric points (pI) of 5.9 and 5.5, respectively, adsorbed readily to the positively charged boehmite form of aluminum hydroxide gel. These two antigens displayed reversible, linear adsorption behavior in the pH range 5-9, with maximal adsorption observed at the lowest pH studied. The addition of acetate buffer ions had little effect on adsorption, while the presence of phosphate decreased adsorption for R32NS181 and NS181V20 by 25 and 40% respectively. The adsorptive behavior of these two antigens with the negatively charged adjuvant, aluminum phosphate, was markedly decreased. The converse situation was observed with the R32tet32 antigen, whose pI is estimated to be 12.8. There was minimal interaction of this antigen with aluminum hydroxide gel except in the presence of phosphate counter ions and significant, nonreversible adsorption with aluminum phosphate gel. Enhanced adsorption of R32tet32 to aluminum hydroxide gel in the presence of phosphate is suggested to be the result of a covalent bond between a surface aluminum and a phosphate anion that modifies the surface charge of the aluminum hydroxide gel. These results indicate that the role of complementary surface charges, both for the ionization state of the protein and for the aluminum adjuvants, is the key in optimizing conditions for significant antigen-adjuvant interactions.

Desorption of porcine parvovirus from aluminum hydroxide adjuvant with subsequent viral immunoassay or hemagglutination assay

Cell culture fluids containing porcine parvovirus were adjuvanted with varying concentrations of aluminum hydroxide gel. Adsorption of virus and total protein to adjuvant was proportional to adjuvant concentration. Desorption of virus and protein from the adjuvant in substantial, reproducible quantities was achieved by washing adjuvanted preparations with 1.2 M potassium phosphate, followed by dialysis and concentration of wash fluids. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of unadjuvanted viral fluids, supernatants of adjuvanted mixtures, and desorptive washings of the corresponding adjuvant pellets revealed no qualitative differences in protein banding patterns. Desorbed virus was quantitated by a hemagglutination assay, or by an enzyme-linked immunoassay employing a monoclonal anti-porcine parvovirus antibody. Virus desorption and subsequent assays permitted in vitro estimation of virus content in adjuvanted porcine parvovirus preparations. This approach may be useful in estimating the antigen content of inactivated aluminum hydroxide adjuvanted veterinary vaccines and reducing the extent of required in vivo testing.

Effect of shear on the apparent viscosity of amorphous carbonate ion containing aluminum hydroxide suspensions

The application of shear to carbonate ion containing aluminum hydroxide suspensions caused a change in the apparent viscosity by two possible mechanisms: change in the surface charge because of desorption of specifically adsorbed carbonate ion, and aggregate dispersal and formation of more extensive particle networks. The desorption of specifically adsorbed carbonate ion is related to the expansion of the air-liquid interface during shear. Shear-inducing processing equipment which generates a minimal amount of new air-liquid interface was found to produce the least change in pH and, consequently, in surface charge. However, viscosity increases caused by aggregate dispersal and formation of more extensive particle networks may occur without a shear-induced change in surface charge.

Point of zero charge of amorphous aluminum hydroxide as a function of adsorbed carbonate

The point of zero charge of 36 carbonate-containing aluminum hydroxide gels decreased with increased carbonate-to-aluminum molar ratio. Theoretical analysis supported this observation and showed that the point of zero charge was sensitive to low fractional coverage of surface sites by carbonate and that divalent carbonate is the predominant form of carbonate on the surface. The implications of the pH-point of zero charge relationship of aluminum hydroxide on the viscosity, adsorptive properties, rate of filtration, and removal of sodium are discussed.

Microstructure of aluminum hydroxides and the formation of aluminum dye lakes

The effect of processing variables on the type of aluminum hydroxide and the dye content of the resultant lakes were investigated. The solid-state structure of aluminum lakes in conjunction with the associated aluminum hydroxide was studied by electron microscopy, X-ray diffraction, and IR spectroscopy. The conditions under which the aluminum hydroxide was prepared were shown to govern the amorphous/crystalline character of the product, which was reflected in changes in particle size and surface area. A hypothesis for the interaction of the dye and aluminum hydroxide is formulated in which the amount of dye adsorbed on the aluminum hydroxide is dependent on the surface area of the microstructures present.

Adsorption of pepsin by aluminum hydroxide I: Adsorption mechanism

Adsorption of pepsin by gibbsite and boehmite, non-acid-reactive forms of aluminum hydroxide, was observed and related to the surface area of the adsorbent. Adsorption was pH dependent, with maximum adsorption occurring between pH 2.7-3.3 for gibbsite and pH 2.7-4.3 for boehmite. Electrostatic attraction was an important adsorption mechanism at the pH conditions encountered in the GI tract; the isoelectric point of pepsin was approximately 1, giving it a negative charge, and the point of zero charge for the adsorbents was greater than 9, giving them a positive charge. However, the pH-adsorption profile can not be fully explained by electrostatic considerations. Desorption studies indicate the importance of specific adsorption because pepsin was not desorbed by washing with acidified water, but was partly desorbed by exchange with phosphate. The IR spectrum of adsorbed pepsin also suggested that specific adsorption of pepsin occurred through anionic ligand exchange involving carboxylate groups of pepsin and surface aluminum ions.

Adsorption of phosphate by aluminum hydroxycarbonate

Phosphate is specifically adsorbed by aluminum hydroxycarbonate by anion ligand exchange. IR analysis indicated that phosphate exchanged with specifically adsorbed carbonate. Adsorption is favored by low pH and is inversely related to particle size. Adsorption of phosphate decreases the rate of acid neutralization of aluminum hydroxycarbonate. The results are applied to the treatment of hyperphosphatemia and hypophosphatemia.

Exchange of sodium by magnesium in aluminum hydroxycarbonate gel

Approximately 90% of the sodium present in a washed aluminum hydroxycarbonate gel was removed by exchange with magnesium. This behavior supports recent structural studies which have suggested that cations such as sodium serve as counterions in aluminum hydroxycarbonate gel. However, sodium could not be removed from dihydroxyaluminum sodium carbonate by exchange with magnesium because sodium is part of the crystal structure. It is hypothesized that aluminum hydroxycarbonate gels which resist removal of sodium are actually mixtures containing dihydroxyaluminum sodium carbonate in addition to aluminum hydroxycarbonate.

Properties of carbonate-containing aluminum hydroxide produced by precipitation at constant pH

Carbonate-containing aluminum hydroxide was precipitated at constant pH at intervals of 0.5 pH units from pH 6 to 10 by pumping 0.5 M AlCl3 into the reaction vessel at a constant rate of 2 mL/min and infusing 2 M Na2CO3 at a rate necessary to maintain the desired pH. The pH of precipitation affected both the particle size and the composition of the precipitate. The particle size of the precipitate decreased as the pH of precipitation was increased from 6 to 10. The precipitates formed between pH 7.5 and 9.5 contained crystalline sodium aluminum hydroxycarbonate (dawsonite) in addition to amorphous carbonate-containing aluminum hydroxide.

Desorption of carbonate from aluminum hydroxycarbonate gel by nitrogen purging

Carbonate was completely desorbed from amorphous aluminum hydroxycarbonate gel by purging with nitrogen. The reversibility of carbonate adsorption suggests that aluminum hydroxycarbonate particles are composed of planes of aluminum hydroxide with carbonate adsorbed at edge aluminum sites. A slow-reacting phase which was identified as a precursor of gibbsite, formed when the carbonate/aluminum molar ratio was less than 0.20.

Mechanism of freeze-thaw instability of aluminum hydroxycarbonate and magnesium hydroxide gels

The effect of freeze-thaw cycles on the physical stability of aluminum hydroxycarbonate and magnesium hydroxide gels was studied. Coagulation following a freeze-thaw cycle, leading to the formation of visible aggregates, affected the content uniformity of both gels. The freeze-thaw cycles did not affect the crystal form or surface characteristics of the gels as determined by X-ray powder diffraction and point of zero charge, but caused a slight reduction in the rate of acid neutralization and a large increase in the rate of sedimentation. The greatest effect was observed after the first freeze-thaw cycle. While the duration of freezing was not a factor, the rate of freezing was important and was inversely related to the aggregate size. The aggregates which formed following a freeze-thaw cycle were not redispersed by shaking, but were reversed by ultrasonic treatment or homogenization. The adsorption of polymers or surface-active agents prior to freezing reduced and, in some cases, prevented the formation of aggregates. The physical instability produced by a freeze-thaw cycle was explained by the modified DLVO theory. The force exerted on the particles by the growing ice crystals forced the particles into the primary minimum, producing strong interparticle attraction. On thawing, simple agitation did not provide enough force to overcome the attractive force of the primary minimum. Adsorption of polymers or surface-active agents increased the steric repulsive force and prevented the particles from reaching the primary minimum.

Effect of the pH-zero point of charge relationship on the interaction of ionic compounds and polyols with aluminum hydroxide gel

The adsorption of magnesium nitrate, docusate sodium, and mannitol by chloride-containing aluminum hydroxide gel or aluminum hydroxycarbonate gel can be directly related to the surface charge characteristics of the aluminum hydroxide gel as determined by the pH-zero point of charge (ZPC) relationship. Magnesium cation is completely adsorbed under pH conditions where the gel has a negative surface charge, i.e. when the pH is above the ZPC. Docusate sodium is more strongly adsorbed when the pH-ZPC relationship causes the surface charge of aluminum when the pH-ZPC relationship causes the surface charge of aluminum hydroxycarbonate gel to be positive indicating adsorption of the docusate anion. However, adsorption also occurred when the pH was above the ZPC suggesting that adsorption of the hydrophobic portion of docusate anion by van der Waals forces also contributes to the overall adsorption mechanism. Mannitol is adsorbed under all pH conditions. However, greater adsorption occurs when the pH is above the ZPC. Maximum hydrogen bonding is believed to occur when mannitol acts as the proton donor and the negative aluminum hydroxycarbonate gel surface serves as the proton acceptor.

Effect of surface charge and particle size on gel structure of aluminum hydroxycarbonate gel

The effect of surface charge and particle size on the gel structure of aluminum hydroxycarbonate gel was studied through the use of a specially designed tension cell. Surface charge has a major effect on the coefficient of bulk compressibility. The charged state is more compressible at lower tensions while the neutral gel is more compressible at higher tensions. In addition, physical properties of gels having a small particle size are more profoundly influenced by interparticle forces than are gels consisting of larger particles. The effect of surface charge and particle size on gel structure is applied to physical properties such as viscosity and dewatering.

Investigation of gel structure of aluminum hydroxycarbonate gel using a tension cell

A tension cell was designed to measure the gel structure of aluminum hydroxycarbonate gel. Equilibrium measurements of the total water outflow versus applied tension given insight into the rigidity of the gel structure. Dynamic measurements of the rate of outflow of water at various applied tensions yield the capillary conductivity of the gel, which is related to the apparent surface area, particle size, and porosity. An aluminum hydroxycarbonate gel with a high apparent viscosity exhibited an increase in viscosity upon aging at 25 degrees, which was accompanied by an increase in the equilibrium tension of the water phase, an increase in delta (-)GH2O, and a decrease in capillary conductivity. In contrast, the apparent viscosity of a low viscosity aluminum hydroxycarbonate gel, which was precipitated from the same reagents but with different precipitation temperature, pH, and stirring speed, exhibited a decrease in equilibrium water tension, a decrease in delta (-)GH2O, and an increase in capillary conductivity during aging at 25 degrees.