Expanded and packed bed adsorption of serum albumin on mixed-mode fluoride modified zirconia: Experiments and modeling

The utility of dense (2.8 g/cc), mixed mode fluoride modified zirconia particles (FmZr) (38--75 mum) for the expanded bed adsorption of proteins from dense cell suspensions was investigated using human serum albumin (HSA) and Saccharomyces cerevisiae as a model protein-microbial cell system. It was feasible to adsorb HSA (4 mg/mL) from 100 g dry cell weight (DCW)/L of rehydrated yeast suspension in a 3x expanded bed. The expanded bed adsorption of a protein from a suspension containing >50 g DCW/L cells has not been reported. The binding capacity at 5% breakthrough decreases from 22 mg HSA/mL settled bed volume for 20 g DCW/L yeast to a constant value of 15 mg/mL for 40--100 g DCW/L, yeast. The zirconia particles were cleaned with 1500--2000 column volumes of 0.25 M NaOH at room temperature without any significant effect on the chromtographic integrity. In order to predict the behavior of large expanded bed adsorbers, BSA adsorption on a packed bed of FmZr as a function of flow rate (1.5--9 mL/min) and operating temperature (4 and 21°C), was modeled using a general rate model of chromatography. This model considers axial dispersion, film mass transfer, pore diffusion and adsorption-desorption kinetics. HSA adsorption in expanded beds was also modeled using a general rate model. The effect of axial dispersion, degree of bed expansion, ratio of the settled bed height to the column diameter and the solute-adsorbent dissociation constant on the expanded bed performance was investigated. The modeling results show that for serum albumin adsorption on FmZr the adsorption-desorption kinetics are 2 to 3 fold slower than pore diffusion at 21°C and are rate limiting at 4°C. Therefore, the use of highest possible operating temperature will be advantageous due to faster pore diffusion and adsorption-desorption kinetics. The high dissociation constant (4.9 mg/mL) for the HSA-FmZr interaction limits the available protein binding capacity for feed concentrations of interest (<8 mg/mL). The use of other surface modifications of zirconia, with a lower dissociation constant for HSA, may result in an appreciable increase in the available binding capacity.