The Characterization of a Weakly Basic Chemisorbent Polyacrylamide Hydrogel for its Applicability to Downstream Succinic Acid Recovery
Commercial bioproduction of succinic acid has been stimulated by its use as a precursor in the synthesis of commodity chemicals, and the benefits of bioproduction compared to traditional chemical synthesis. However, inefficiencies remain in the downstream recovery of succinic acid from fermentation medium. The use of chemically reactive sorbents (chemisorbent) can streamline downstream succinic acid recovery, reducing costs. In this study, Poly[N,N-dimethyletheyleneacrylamide-co-Tris(2-aminoethylacrylamide)] (PMTA) was synthesized and characterized for the downstream recovery of succinic acid. PMTA sorbs succinic acid through acid-base reactions, sequestering it as an ammonium succinate salt. At a 0.02 dry weight (DW) phase fraction (PF) PMTA chemisorbed 0.45 g of succinic acid from a 35 g/l solution, the highest capacity known. PMTA was found to be stable in aqueous solution after 10 days, an improvement over polyacrylate type chemisorbents. Complete desorption of succinic acid from 1g DW polymer was achieved with 50 ml of a 3 M KOH solution, at a 0.02 PF. Despite this elevated concentration of KOH, the polymer retained its sorption capacity after 3 cycles of sorption and desorption. PMTA was found to chemisorb all three acids found in succinic acid fermentations, including formic, acetic and succinic acid, preferentially sorbing the most concentrated acid. However, at equal molar concentrations of these acids, formic acid, having the lowest pKa, was preferentially sorbed. PMTA has properties applicable to the downstream recovery of succinic acid including resistance to hydrolysis, reusability, and high succinic acid sorption capacity. PMTA was applied as an alkaline extractant in an in situ product removal (ISPR) system. To simulate bioproduction, succinic acid was fed into a bioreactor; the drop in pH activated a pump directing bioreactor contents through a vessel containing water alkalized by the presence of PMTA, and this water was displaced into the bioreactor increasing the pH. The pH was further modulated by the sorption and sequestration of succinic acid from the bioreactor contents pumped through the ISPR system. However, it was found that the alkalinity of the tertiary amine functional group of PMTA was insufficient to maintain the bioreactor pH at the set point.
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