Theory of pH changes in water desalination by capacitive deionization
Graphical abstract
Introduction
During water desalination by methods such as electrodialysis and capacitive deionization, often large changes in pH develop between feed and effluent water. These pH changes can result in precipitation and scaling of membranes and electrodes, and can affect long-term stability of membrane and electrode materials (De Levie, 2014, Liu et al., 2015, Nir et al., 2015, Cohen et al., 2015). Also, these pH changes can lead to a product that has an undesired pH.
In literature, different explanations can be found for these pH changes. Some studies describe how Faradaic reactions result in production or consumption of protons or hydroxyl ions (Bouhadana et al., 2011b, Bouhadana et al., 2011a, Shanbhag et al., 2016, Nativ et al., 2017), such as the reduction of water, the oxidation of carbon, the reduction of oxygen, or the oxidation of chloride ions. In our view another effect should be considered as well, which is the fact that all ions in the water have different mobilities (diffusion coefficients), which results in different ion adsorption rates, and thus in pH changes. We like to quantify these effects of different ion mobilities and Faradaic reactions by microscopic physics-based modeling of a relevant electrochemical water desalination method.
To elucidate this effect, we choose to study pH changes in Membrane Capacitive Deionization (MCDI), a water desalination technology employing porous carbon electrodes and ion-exchange membranes (IEMs)(Suss et al., 2015, Tang et al., 2015, Gao et al., 2015, Qu et al., 2015, Bian et al., 2015, Jeon et al., 2013, Doornbusch et al., 2016, Gendel et al., 2014, Aslan et al., 2016). In MCDI, during the charging step, or adsorption step, a voltage is applied between electrodes, resulting in cation adsorption into the negatively polarized electrode, the cathode, and anion adsorption into the positively polarized electrode, the anode. Consequently, feed water flowing through the cell is desalinated. Ions are adsorbed in the micropores of the electrodes, where electrical double layers (EDLs) are formed (Giera et al., 2015, Prehal et al., 2015, Kastening and Heins, 2005). After the electrodes are saturated with salt, they are discharged and ions desorb. In MCDI, strong pH changes are often observed and feed water and effluent may have large differences in pH, changing over time (Bouhadana et al., 2011b, Bouhadana et al., 2011a, Lee et al., 2010). The IEMs are placed between the flow channel and electrodes, see Fig. 1, and enhance salt adsorption (Lee et al., 2006, Kim and Choi, 2010, Zhao et al., 2013).
To study pH changes in MCDI, we extend existing models (Kim et al., 2015, Dykstra et al., 2016b, Dykstra et al., 2016a) and include besides sodium and chloride ions (or any other pair of salt ions) also protons and hydroxyl ions. We model the transport of these ions across the flow channel and membranes, into the electrodes. Furthermore, we consider the self-ionization reaction of water, H2O H+ + OH−. In the electrodes, we distinguish two types of pores: macropores and micropores. Macropores are large pores serving as transport pathways across the electrode (Mirzadeh et al., 2014, Johnson and Newman, 1971, Rica et al., 2013). Micropores are small pores where ions adsorb and Faradaic reactions occur. To model ion adsorption, we consider the effect of chemical surface charge (Gao et al., 2016, Biesheuvel et al., 2015). Chemical surface charge is present in the form of acidic groups, because of carboxylic structures (Hatzell et al., 2014, Wu et al., 2016), or basic groups, because of protonated structures bound to carbon particles (Montes-Morán et al., 2004, Leon y Leon et al., 1992).
We show that theory including the effect of different ion mobilities and the effect of acidic and basic groups in micropores, predicts differences in pH between feed water and effluent, which change over time. Furthermore, we show that these differences in pH can be much stronger if we also consider Faradaic reactions.
Section snippets
Theory
To model pH in the MCDI cell as function of position (in membrane, flow channel, and electrode) and time, we present a mathematical framework to calculate transport, adsorption and Faradaic reactions of ions. This framework consists of three elements:
- I)
Transport of ions from the flow channel, through the membranes, into the porous carbon electrodes, which is modelled based on the Nernst-Planck equation (Dykstra et al., 2016b, Thompson Brewster et al., 2016). In the present work, we include
Results and discussion
To illustrate pH differences between feed water and effluent for MCDI predicted by theory, we first present results of equilibrium calculations under no-flow conditions, for which the water flow rate through the cell, , is 0. Thereafter, we present dynamic calculations with flow.
Conclusion
We presented theory to calculate pH changes in electrochemical systems for water desalination, which we used to predict pH changes and differences in pH between feed water and effluent observed in Membrane Capacitive Deionization. In our theory, we included three phenomena that occur in MCDI that may explain the observed pH changes: I) different mobilities of various ions; II) chemical surface charge groups in the micropores of the porous carbon electrodes; and III) Faradaic reactions in the
Acknowledgement
This work was performed in the cooperation framework of Wetsus, European Centre of Excellence for Sustainable Water Technology (www.wetsus.eu). Wetsus is co-funded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the Province of Fryslân, and the Northern Netherlands Provinces. The authors like to thank the participants of the research theme Capacitive Deionization for fruitful discussions and financial support.
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