Characterization of pH dependence in organic acid absorption with non-reactive and reactive polymers for application in two-phase partitioning bioreactors Eric C. Peterson, David Contreras-López, Jesse Harris, J. Scott Parent ⇑ , Andrew J. Daugulis ⇑ Department of Chemical Engineering, Queen’s University, Kingston, Ontario K7L3N6, Canada highlights  Species distribution clearly described during acid partitioning in an absorptive polymer.  Effect of acid absorption on equilibrium pH well characterized.  High pressure CO 2 (60 bar) can reversibly lower pH to facilitate acid absorption.  Amine-functionalized hydrogels achieve high acid recovery with low polymer fractions. article info Article history: Received 7 August 2015 Received in revised form 19 November 2015 Accepted 21 November 2015 Available online 2 December 2015 Keywords: Two-phase partitioning bioreactor Organic acids Absorptive polymers High pressure CO 2 Polymeric reactive extractants abstract Strategies for extracting organic acids from aqueous solutions using polymeric absorbents are demon- strated and discussed in the context of two-phase partitioning bioreactor (TPPB) design. Experimental data and material balances for the uptake of butyric acid and benzoic acid by a poly(ether-block- amide) copolymer (Pebax 2533) establish the inherent limitations of unreactive absorbents for organic acid bioprocesses that operate at near-neutral pH. Improvements to TPPB performance are achieved by lowering pH temporarily with CO 2 to enhance acid absorption, and removing the solute-rich polymer before restoring pH to fermentative values by releasing the CO 2 pressure. Butyric acid removal by Pebax 2533 improved from 3% to 40% upon acidifying a pH 6 solution with 60 bar of CO 2 , while benzoic acid absorption increased from 1% to 80% using this pressure manipulation technique. A reactive extrac- tion approach involving a newly-synthesized amine functionalized hydrogel is also described wherein acid/base reaction equilibrium governs the extent of solute uptake. Copolymerization of 2- (dimethylamino)ethyl acrylate (DMAEA) and trimethylolpropane triacrylate (TMPTA) yielded a ther- moset material with sufficient basicity to remove 80% of both butyric and benzoic acid from aqueous solution using just 1 wt% polymer relative to the aqueous phase mass. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Two phase partitioning bioreactor (TPPB) technology can improve the productivity of biological processes by sequestering inhibitory fermentation products and/or compounds intended for biodegradation [1]. Polymeric absorbents have attracted recent attention, with considerable effort expended on developing meth- ods for selecting polymers with high thermodynamic affinity for target solutes [2] . This affinity is generally expressed in terms of the partition coefficient (PC), defined as the ratio of the equilibrium concentration of solute in the polymer ([s] pol , mole/g) to that in the aqueous phase ([s] aq , mole/g) (Eq. (1)), PC ¼ ½S pol ½S aq ¼ n pol S =m pol n aq S =m aq ð1Þ http://dx.doi.org/10.1016/j.cej.2015.11.068 1385-8947/Ó 2015 Elsevier B.V. All rights reserved. Abbreviations: TPPB, two-phase partitioning bioreactor; ISPR, in situ product recovery; PC, partition coefficient; D, distribution coefficient; n tot , total moles of solute; n aq S , moles of solute in the aqueous phase; n pol S , moles of solute in the polymer phase; n aq HA , moles acid in aqueous phase; n pol HA , moles acid in polymer phase; n aq A , moles conjugate base in aqueous; m aq , aqueous mass (g); m pol , polymer mass (g). ⇑ Corresponding authors. E-mail addresses: parent@queensu.ca (J. Scott Parent), daugulis@queensu.ca (A.J. Daugulis). Chemical Engineering Journal 287 (2016) 503–510 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej