1391 Research Article Received: 23 March 2015 Revised: 22 April 2015 Accepted article published: 6 May 2015 Published online in Wiley Online Library: 29 May 2015 (wileyonlinelibrary.com) DOI 10.1002/jctb.4720 Mass transfer considerations in solid–liquid two-phase partitioning bioreactors: a polymer selection guide Margaret J Pittman, Michael W Bodley and Andrew J Daugulis * ABSTRACT BACKGROUND: Selection of polymers for two-phase partitioning bioreactors (TPPBs) has been focused primarily on predicting a polymer’s affinity for the target molecule. Although the extent to which a polymer absorbs a solute is important, the rate of uptake/release must be sufficiently rapid such that a TPPB is not mass transfer limited. This work focused on developing a guide to identify combinations of polymer diffusivities and diffusional path lengths that will ensure a TPPB is not limited by substrate delivery. RESULTS: TPPB systems limited by substrate delivery yielded linear growth, while biologically limited systems exhibited exponential growth. Release rates of phenol from various polymer phases increased as polymer diffusivity increased, or as diffusional path length (polymer bead size) decreased. A polymer selection guide was developed identifying combinations of polymer diffusivity and bead size that will ensure a TPPB is not mass transfer limited, for a desired maximum substrate consumption rate. CONCLUSION: In selecting polymers for TPPB applications, solute affinity (extent of uptake) has been relatively well character- ized using first principles methods, and the present work has now ‘completed the picture’ by providing a description of polymer transport properties (diffusivity and diffusional path length) to be able to generate a guide for selecting polymers. © 2015 Society of Chemical Industry Keywords: Bioreactors; Diffusion; Kinetics; Mass Transfer INTRODUCTION Conventional bioremediation strategies for the degradation of toxic contaminants can be limited by substrate toxicity. 1 Two-phase partitioning bioreactors (TPPBs) overcome this lim- itation by utilizing a second immiscible phase that selectively partitions compounds to/from the cell containing aqueous phase. Substrates with low water solubility will partition into the sequestering phase at much higher concentrations than the cell-containing aqueous phase, thus maintaining the aqueous concentrations below inhibitory levels. The partitioning phase then acts as a reservoir which continually delivers the substrate to the aqueous phase at a rate that maintains a balance between thermodynamic equilibrium of the system and the metabolic demand of the cells. 2 Immiscible organic solvents have proven to be effective par- titioning phases for the degradation of phenol, 3 benzene, 4 toluene, 5 and polycyclic aromatic hydrocarbons. 6,7 However organic solvents must meet several stringent requirements including biocompatibility, nonbioavailability, low volatility, and low cost. 8 Furthermore, the nonbioavailability requirement often limits immiscible solvent TPPB applications to pure cultures, as mixed cultures are often capable of degrading a wide spectrum of organic molecules, including the selected solvent itself. 9 Polymers have shown to be a promising alternative as the par- titioning phase in TPPBs that can overcome these limitations. Commercial polymers are readily available in a wide range of homo-and-copolymer chemistries, can be formed into a variety of shapes with varying sizes, are non-biodegradable, non-toxic, non-flammable, can be easily handled, recovered, and reused, 10 and are generally much less expensive than organic solvents. Although polymers have been successfully used to sequester a wide range of toxic organic substrates, further success in iden- tifying effective polymers requires that a rational approach be taken in selecting them, presumably on the basis of sound scien- tific principles. One absolute property that an effective sequester- ing phase must possess is a high affinity for the target molecule, usually characterized by the partition coefficient of the solute in the polymer. Using rigorous first principles thermodynamics, we have generated frameworks for selecting high affinity polymers by consideration of Hildebrand and Hansen Solubility Parame- ters, Flory–Huggins solution theory, and UNIFAC activity coeffi- cient based models. 11 – 13 The TPPB community can now use these tools in selecting polymers on the basis of predicted solute affinity. Although thermodynamic affinity considers the extent to which a polymer will sorb a solute, implementation of the TPPB ∗ Correspondence to: A. J. Daugulis, Department of Chemical Engi- neering, Queen’s University, Kingston Ontario, Canada K7L 3 N6. Email: daugulis@queensu.ca Department of Chemical Engineering, Queen’s University, Kingston Ontario, Canada K7L 3 N6 J Chem Technol Biotechnol 2015; 90: 1391–1399 www.soci.org © 2015 Society of Chemical Industry