Biomass/adsorbent Interactions During Ion Exchange Expanded Bed Adsorption

Expanded bed adsorption (EBA) is a novel primary recovery technology allowing the adsorption of target proteins directly from unclarified feedstock. Ion exchange adsorbents are most extensively used due to high capacity and reasonable cost. However, it was found that during ion exchange EBA the biomass (cells or cell debris) in the feedstock might adhere to the adsorbent, especially to the anion exchanger, which resulted in bad bed stability and low adsorption capacity. Earlier studies revealed that the biomass/adsorbent interactions could reduce when biomass was treated with appropriate homogenization. In the present work, the impacts of two normal homogenization techniques (sonication and high pressure homogenization with French Press) on the biomass/adsorbent interactions were investigated in details. In order to gain a deeper insight into the mechanisms behind biomass/adsorbent electrostatic interactions, the concept of zeta potential based on colloid science was introduced to find out the proper control parameter for EBA process design.Escherichia coli, Bacillus subtilis and Pichia pastoris were chosen as the model biomass, which represented Gram negative bacteria, Gram positive bacteria and yeast, respectively. Typical anion exchanger, Streamline DEAE, was used as the model EBA adsorbent. The biomass transmission index (BTI) obtained through the biomass pulse response experiment was used to quantitatively evaluate the biomass/adsorbent interaction in expanded bed. The value of BTI is in inverse proportion to the biomass/adsorbent interaction, and a threshold value of 0.9 was defined as the limiting condition for the formation of stable expanded bed in the biomass containing suspension. The results indicated that the BTI values increased obviously with the increase of sonication power or treatment time. Enhancing the homogenization pressure of French Press or increasing the operation cycles could also obtain similar result. All the biomass tested showed the same tendency, which indicated that more exhaustive homogenization resulted in slighter biomass/adsorbent interaction. The size and zeta potentials of homogenate were investigatedduring the homogenization procedure. The mean size of homogenate reduced obviously after the homogenization, and the absolute value of zeta potential of biomass also decreased significantly as the mean size reduced. Based on the analysis of electrostatic interactions between a charged sphere and plane, combining zeta potential of adsorbent (ï¿¡A), zeta potential of biomass (ï¿¡B) and biomass size (de), the parameter (-^A'^B'dB) was used as the reasonable indicator of biomass/adsorbent interactions in expanded beds. A good linear correlation was found between BTI and zeta potential parameter (-C^B'dB) for all biomass and homogenization conditions tested. A threshold value of (-CA*^B*dB) < 120 mV2um was derived for BTI above 0.9, which could be used as control parameter in the homogenization process. It was demonstrated that the proper homogenization control could reduce the biomass/adsorbent interactions and improve the stability of expanded bed. The zeta potential parameter seems to be a very promising tool for simplifying and accelerating the EBA process design.In addition, a preliminary research on weakening the biomass/adsorbent interaction by modificating the anion exchanger surface was carried out. The results demonstrated that polyacrylic acid (PAA) coated Streamline DEAE effectively prevented the binding of biomass without affecting the adsorption of the target biomolecules. However more investigations should be done to keep the stability of coated adsorbents under the harsh regeneration conditions.