Separation of succinic acid from fermentation broth: dielectric exclusion, Donnan effect and diffusion as the most influential mass transfer mechanisms

Nanofiltration can be used to separate bio-succinic (SA) acid from other organic acids in fermentation broth. However, commercial nanofiltration membranes can be insufficiently selective since some produced organic acids have similar molecular weights. SA production from biomass fermentation entails separation of bio-succinic acid from the other concomitantly produced organic acids such as acetate, formate, pyruvate etc.

In this study, five commercial nanofiltration membranes were tested for succinic acid (SA) rejection and selectivity under dead-end and crossflow modes of SA recovery from selected synthetic solutions and real fermentation broth. SA rejections up to 94.1 ± 1.5 % and 95.5 ± 3.5 % were observed for the synthetic broth and the fermentation broth, respectively.

The SA fractions % obtained were nearly 70% and 60% for synthetic and fermentation broth, respectively. The highest rejections and SA fraction values were achieved with DK and NF270, which also showed the highest permeate flux. Model evaluation using the Donnan-steric pore model with dielectric exclusion (DSPM-DE) showed that 99% of the succinate rejection occurs at the membrane interface and not inside the membrane.

Based on the modelling, the most important mechanism affecting separation was dielectric exclusion at both sides of the membrane surfaces. Moreover, identified sensitive model parameters pore dielectric constant , charge density and pore radius were estimated through model fitting and were found to be highly correlated, albeit with significant variations.

Finally, for the first time, a calibrated model with synthetic broth was successfully used to predict organic acids rejection for a real fermentation broth (with 12% and 3% prediction error for succinate rejection by NF270 and DK, respectively). Such prediction was achieved only by measuring organic acid concentrations in the fermentation broth and using the three sensitive parameters.

Furthermore, the potential contribution of different transport mechanisms of SA was quantified for the first time.