Sorption and diffusion of organic acid ions in anion exchange membranes: Acetate and lactate ions as a case study
Introduction
Organic acids such as acetic acid and lactic acid are used broadly within the food and pharmaceutical industries as preservatives [1], chemical intermediates [2] and buffer media. In addition, due to their biodegradable properties, there is a growing demand for these compounds in the production of polymeric materials [3]. Generally, organic acids are produced commercially either by fermentation or chemical synthesis. The former approach is preferred in the food industry as many food regulations stipulate that acids used in foods must be of biological origin [4]. The development of effective separation steps for organic acid recovery plays an important role in many biochemical industries since the conventional fermentation processes produce a mixture of organic acids as well as calcium, ammonium, or sodium salts [5]. Membrane processes, especially diffusion dialysis (DD), conventional electrodialysis (ED) and electrodialysis with bipolar membranes (EDBM), are widely used in the production and recovery of these organic acids [[4], [5], [6], [7], [8]] as they operate continuously and virtually eliminate the waste produced by other processes. However, commercial applications of these processes are often affected by the ion selectivity, electrical resistance and manufacturing costs of the ion exchange membranes (IEMs). Thus, to obtain a process with high efficiency and low energy consumption, a fundamental understanding of ion sorption and transport in IEMs is necessary. A better knowledge of the influence of polymer structure on transport properties could further facilitate rational development of ion exchange membranes with improved properties.
Ion transport in ion-exchange dense polymer membranes is normally quantified by ion diffusion coefficients. A variety of experimental techniques based on the Nernst-Planck equation have been employed to measure the diffusion coefficients of inorganic ions in these membranes, including a Donnan dialysis method [[9], [10], [11], [12]], radiotracer method [13,14] and conductivity method [[15], [16], [17], [18]]. The conductivity method is the most often used as diffusion coefficients measured in the presence of an external driving force are closer to the real operation of the membrane process.
For one-dimensional ion transport in a membrane, the molar flux of ion through a membrane, , can be described by the Nernst-Plank equation as Eq. (1):where and are the ion diffusion coefficient and concentration in the membrane, respectively; is the valence of the ion; is absolute temperature, R and F are the ideal gas constant and Faraday's constant, respectively. is the electric potential and is the thickness of the membrane.
In typical electric field driven ion transport, the Fickian component of the driving force in Eq. (1) is minimal and the electric potential difference becomes the dominant driving force for ion transportation. Under this electric potential difference, cations migrate toward the cathode and anions move toward the anode. The electric current density (I) is related to the ionic fluxes in the membrane via Eq. (2).
Combining Eq (1) and Eq (2) yields Eq. (3).
The membrane ion conductivity () is defined by Eq. (4):
Combining Eq (3) and Eq (4) then yields
Eq (5) gives the relationship between the individual ion diffusion coefficients ( ) and the ionic conductivity of the membrane but this expression cannot be solved to yield the individual diffusion coefficients. Some researchers assume that the co-ion concentration () approaches zero [9,[18], [19], [20]], as the Donnan potential repels these ions from the ion exchange membrane and this prevents the internal co-ion concentration from rising beyond an equilibrium value [21]. This allows the diffusion coefficients of the counter-ions () to be determined by Eq (5).
The strength of the Donnan potential depends on the counter-ion/co-ion concentration difference between the membrane and the external salt solution. For conventional membrane processes, the counter-ion concentration difference between the membrane and solution decreases with the increase of external solution concentration as the counter-ion concentrations in the membrane itself are relatively constant [22]. This results in a weaker Donnan potential and greater co-ion sorption in the membrane. Thus, for ion exchange membranes equilibrated with concentrated solutions, the co-ions in the membrane must also be considered. To solve this problem, Kamcev et al. (2018) developed a new procedure in which the individual ion diffusion coefficients are obtained from experimental ion sorption and membrane ionic conductivity results [16]. They also modified Manning's counter-ion condensation theory [23,24], originally developed as a ‘‘limiting law’’ for polyelectrolyte solutions, by relaxing the assumption of immobile condensed counter-ions, then used this as a model for the prediction of counter-ion diffusion coefficients in the membrane [16]. The central parameter in the modified Manning model is the dimensionless linear charge density of a polyelectrolyte, , which depends on the average distance between fixed charge groups on the polymer chain. For monovalent ions, condensation occurs if ξ is greater unity (see Supporting Information).
With only one additional adjustable parameter, the modified Manning condensation theory offers excellent agreement with counter-ion/co-ion diffusion coefficients calculated from experimental results in NaCl systems. It is also predicted that this model will be suitable for similar 1:1 inorganic salt systems. However, it is less clear whether it would suitable for organic acid systems as the dissociation equilibria of the organic acid and its ions may affect their transport behaviour.
As there is a growing demand for organic acid production by electro-membrane processes, a fundamental understanding of organic ion transport in IEMs is necessary, especially the influence of polymer structure on the transport properties. To date, most research on ion transport in ion exchange membranes relates to inorganic ions, with very little information about organic acids. These are generally weak acids that at neutral and low pH are only partly dissociated, which makes their sorption equilibria more complex. In this study, the sorption behaviour of acetic acid (pKa 4.76) and lactic acid (pKa 3.86) in anion exchange membranes are experimentally studied and compared with the sodium chloride system. The individual diffusion coefficients for the acetate and lactate ions in the same anion exchange membranes are then measured by a conductivity method and fitted to the modified Manning model proposed by Kamcev et al. [16] to verify whether it is applicable to organic acid systems.
Section snippets
Materials
Commercially available strongly basic anion exchange membranes (AEMs) (AR103-QDP and AR204-SZRA, GE Power and Water) were used in this study. These membranes were chosen as their behaviour with inorganic ions has already been well studied and because their chemical structure is known [22,25,26]. As shown in Kamcev et al. [25,26], both membranes have quaternary ammonium fixed charge groups, but these are attached to styrenic groups in AR 103 and acrylic groups in AR204. Other relevant membrane
Ion sorption
Membrane water uptake is a key membrane property that influences ion transport in water swollen membranes. The water uptake of a membrane can be affected by its polymer properties and the composition of the external solutions. AR204 uptakes more water than AR103, but as noted by Kamcev et al. [25], it is difficult to relate this to the chemical structure as the degree of crosslinking is unknown. When submerged in NaAc/NaLa solutions of increasing concentration, the water uptake of both AR103
Conclusion
Sorption experiments with organic acid salts (NaAc and NaLa) has shown that the dissociation behaviour of the organic acid can differ between the membrane phase and the bulk solution. Specifically, neutral lactic acid persists in significant quantities within the membrane phase at pH 6.5, whereas it is essentially absent from the bulk solution. The concentration of the neutral species increases as the pH is reduced below this value for both systems. In turn, this increasing concentration
CRediT authorship contribution statement
Q. Wang: Conceptualization, Formal analysis, Methodology, Investigation, Data curation, Writing - original draft, Visualization. G.Q. Chen: Conceptualization, Formal analysis, Methodology, Writing - review & editing, Supervision. S.E. Kentish: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no conflicts of interest.
Acknowledgements
Q. Wang acknowledges the University of Melbourne for a Melbourne Research Scholarship.
References (38)
- et al.
Production of monoprotic, diprotic, and triprotic organic acids by using electrodialysis with bipolar membranes: effect of cell configurations
J. Membr. Sci.
(2011) - et al.
Recovery of hydrochloric acid from simulated chemosynthesis aluminum foils wastewater: an integration of diffusion dialysis and conventional electrodialysis
J. Membr. Sci.
(2012) - et al.
Application of electrodialysis to the production of organic acids: state-of-the-art and recent developments
J. Membr. Sci.
(2007) - et al.
Utilization of electrodialysis for galacturonic acid recovery
Desalination
(2009) - et al.
The determination of diffusion coefficients of counter-ions in the ion-exchange membrane by means of batchwise Donnan dialytic experiments
J. Membr. Sci.
(1990) - et al.
Membrane diffusion-controlled kinetics of ionic transport
J. Membr. Sci.
(1993) - et al.
A simple evaluation of microstructure and transport parameters of ion-exchange membranes from conductivity measurements
Separ. Purif. Technol.
(2008) - et al.
Comparison of transport properties of monovalent anions through anion-exchange membranes
J. Membr. Sci.
(1998) - et al.
Self diffusion and conductivity in NafionR membranes in contact with NaCl+ CaCl2 solutions
J. Membr. Sci.
(1996) - et al.
The determination of diffusion coefficients of counter ion in an ion exchange membrane using electrical conductivity measurement
Electrochim. Acta
(2003)
Monovalent and divalent ion sorption in a cation exchange membrane based on cross-linked poly (p-styrene sulfonate-co-divinylbenzene)
J. Membr. Sci.
Effect of ambient carbon dioxide on salt permeability and sorption measurements in ion-exchange membranes
J. Membr. Sci.
Removal of lactic acid from acid whey using electrodialysis
Separ. Purif. Technol.
Swelling/deswelling of anionic copolymer gels
Biomaterials
Nanofiltration of glucose and sodium lactate solutions: variations of retention between single-and mixed-solute solutions
J. Membr. Sci.
Preservatives and their use, United States Patent US5670160
Catalytic amino acid production from biomass-derived intermediates
Proc. Natl. Acad. Sci. Unit. States Am.
Biodegradable synthetic polymers for tissue engineering
Eur. Cell. Mater.
Lactic acid recovery from fermentation broth using one‐stage electrodialysis
J. Chem. Technol. Biotechnol.: International Research in Process, Environmental & Clean Technology
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