Atomic scale understanding of organic anion separations using ion-exchange resins

https://doi.org/10.1016/j.memsci.2020.118890Get rights and content

Highlights

  • Computational evaluation of ion-exchange resins for organic acid separation from biobroths.

  • Correlation between diffusion and substrate size and binding interaction described.

  • External electric field can be used to design assembly and operation of EDI devices.

Abstract

A combination of ab initio and classical molecular dynamic simulations was used to explore the adsorption/desorption and diffusion characteristics of ion-exchange resins for extraction of organic anions in electrically-driven separation processes. We considered two classes of carboxylate mixtures that are commonly encountered in bioprocessing separations: a short chain fatty acid mixture (acetate/butyrate) and an aromatic mixture (ferulate/coumarate). The suitability of several resin materials including PFC100E, IRC86, PFA444 and IRA67 was interrogated. The decomposition of the interaction energies by the symmetry-adapted perturbation theory and the classical molecular dynamic simulations of organic diffusion together reveal that the geometries of the organic anions and the functional groups of the resins, as well as their Columbic interactions, are the controlling factors in the diffusion process of the organic compounds in these resins. Classical simulations also show that modifying the functionality of resin beads, the magnitude of electric fields, and the ratio of organic mixtures may provide an effective way to control the diffusion rate and obtain selective separation of these organic mixtures. Finally, a general suggestion for favorable separation conditions is summarized.

Introduction

Advanced biorefining has provided an opportunity to reduce the emission of greenhouse gases, and biofuels are also a part of the renewable energy resource strategy for national energy resilience [[1], [2], [3]]. The realization of a successful bio-economy is dependent on the development of biorefinery systems that allow highly efficient and cost-effective processing of biological feedstocks to a range of platform molecules, many of which are organic acid compounds that are further upgraded to fuels and other co-products [[4], [5], [6], [7]]. Such bio-organic compounds can replace many raw chemicals from the petroleum-based platforms; therefore, low-cost separation techniques to capture these organic compounds from the bioprocessing streams are crucial [[8], [9], [10]].

Electrodeionization (EDI) is an electrochemical separations technology that enables the selective separation and recovery of organic acids or their corresponding salts [[11], [12], [13], [14], [15], [16]]. EDI is an industrial hybrid technology that combines electrodialysis (ED) and the ion-exchange (IX) process into a single unit operation. However, unlike the IX process, EDI can run continuously without regeneration. The separation through IX membranes is electrically driven with ions being drawn to their respective electrode as shown in Fig. 1, and the incorporation of IX resin beads enhances ion transport by increasing conductivity in the process water channels. With the addition of IX resins, ion extraction in EDI is governed by ion adsorption, desorption, and transport under electric field. Conductivity enhancement is dominated by the adsorption and the desorption steps; therefore, EDI performance is influenced by the IX resin properties. Ion affinities for the resin influence the kinetics of adsorption/desorption, and the diffusivities of ions into and out of the resin limit the mass transfer for adsorption/desorption and ion transport.

While EDI is used commercially to produce ultrapure water for the semiconductor and pharmaceutical industries, it can be modified into resin wafer EDI (RW-EDI) devices for industrial bioprocesses. RW-EDI utilizes IX resins in the form of a porous RW, which facilitates handling of the IX resins. The porous RW also provides ionic conductivity and enhances mass transfer between solid (resin) and liquid (feed solution) phases to provide greater performance consistency in comparison to a loose resin bed [17]. This technology is especially advantageous for the removal of weakly ionizable organic acids due to in situ water splitting that continuously occurs within the EDI stack under the application of an electrical potential. Once ionized via the IX resin beads, the ionic conductivity of the acids is sufficiently high to facilitate transport across the IX membranes. The electric resistance of organic salts (i.e., ionized organic acids) to pass through IX membranes is less than that associated with the transport of pure organic acids. Therefore, it is generally accepted that the kinetics and mass transfer on resin beads are the rate determining steps in an EDI process. Ion transport in EDI IX resin beds occurs via three possible pathways: bulk solution, IX solid phase, or a combination of the solution-solid routes. Application of an empirical model to characterize the transport patterns for various ions and IX resins has shown that the combined solution-solid pathway is dominant, with the majority (60–95%) of transport in this pathway occurring through the solid IX resin [18]. Therefore, our study here focuses on characterizing organic ion diffusion in a variety of IX resins.

Resin wafer properties and operating conditions have a significant impact on the performance of RW-EDI processes. For example, interaction between the IX functional groups and the targeted organic species greatly influences the behavior of organics in the resin wafer. In addition, the temperature of operation, and the magnitude of the applied electric field can affect the diffusivity of the organic species inside the resin beads. Computational simulations emerge as an additional evaluation tool of novel materials by providing molecular-level information to determine the structure-property relationships, aid the selection of materials with optimal performance characteristics, and guide process operation. For instance, separations of amino acids in glucose isomerase crystal and metal organic frameworks have been explored using molecular dynamics (MD) simulations to reveal their retention mechanisms [19,20]. MD simulations have also helped to obtain information regarding proton transport in polystyrene membranes [21], water in polyamide membranes [[22], [23], [24]], ion transport in electromembranes [25], and hydroxide transport [26]. Furthermore, quantum methods have also been used to study the organic solute/membrane interaction at nanoscale during sugar transfer [27] and hydroxide transport [26]. Therefore, computational simulations are highly promising tools to fill the gap in understanding and predicting how the organic compounds behave in various IX media at the molecular level.

In this work, we employed ab initio calculations and classical MD simulations to explore the extraction process of the organic mixtures using IX resin beads. In particular, we considered the separation of acetate/butyrate and ferulate/coumarate mixtures in several resin systems with different functionalities that are representative of strong acid cation exchange (PFC100E), weak acid cation exchange (IRC86), strong base anion exchange (PFA444), and weak base anion exchange (IRA67) resins. Ab initio calculations were applied to evaluate the interaction energies between the organic species and the functional groups in the resin beads. The symmetry-adapted perturbation theory (SAPT) [28,29] method was used to decompose the interaction energy into electrostatic, exchange, induction, and dispersion terms to understand the controlling factor in these interactions. MD simulations were used to study the influence of the concentration of organic species, the temperature of organic anion-IX resin system, and the applied external electric fields on the diffusion of organic compounds. The focus of this study was to investigate the diffusion of organic mixtures in different IX resin systems subject to various external conditions and to advance fundamental knowledge related to electrochemical separations. This information provides greater understanding of structure‐property relationships of IX media, guides the rational design of future separation materials, and provides a path towards optimal operating conditions.

Section snippets

Computational details

The resin beads and their components used in the experiments are listed in Table 1. Based on this information, we built small representative fragments of the functional groups in resin beads and performed ab initio calculations to evaluate the interaction energy with organics. Then, larger structures were generated to model the resin systems in MD simulations, as provided in Table 2. Properties such as density, were used to evaluate the computational models, which were in line with the

First-principles calculation results

To understand the interaction affinities between the organic molecules and the functional groups in the resin beads, their binding energies using ab initio calculations are provided in Table 3. Energy decomposition into the electrostatic, exchange, induction, and dispersion terms by SAPT theory is also listed for each interaction energy. Note that the interactions reported here do not consider external effects such as the pH of environment or the solvent effect. We also note that the exchange

Classical simulation results

In classical simulations, force field parameters are important because they affect the accuracy of the simulated results; therefore, we first compared the interaction energy calculated by ab initio methods and our MD simulations as shown in Fig. 3. Since the force field parameters we used were not exactly trained for the specific interactions considered in our systems, quantitative agreements were not obtained. However, the MD results have captured the qualitative trends of the interaction

Conclusion

In this work, we applied ab initio calculations and classical simulations to study the adsorption/desorption and diffusion characteristics of acetate/butyrate mixtures and ferulate/coumarate mixtures in several IX resin media including PFC100E, IRC86, PFA444 and IRA67 used in electrochemical separations to extract and separate organic anions relevant for bioprocessing applications. By coupling the results of the decomposition of interaction energy by ab initio calculations and diffusion

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was financially sponsored by the U.S. Department of Energy's (DOE's) Bioenergy Technologies Office (BETO). Computational resources were provided by Research Computing at Pacific Northwest National Laboratory and the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility. The submitted manuscript was created jointly by University of Chicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a

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