Abstract

Capacitive deionization (CDI) was developed for desalination of brackish water. In contrast to ion exchange, CDI is based on the accumulation of ions within the electrical double layer formed at the surface of conductive materials when an electrical potential is applied. Ion adsorption and desorption can therefore be controlled by switching between a potential of approx. 1.2 V and zero. In this work we demonstrate that the principle of CDI is also suitable for concentrating dilute solutions of crotonic acid resulting from processes such as biotechnological production via fermentation. For this we built a CDI cell using rapid prototyping and fabricated capacitive electrodes based on activated carbon. Applying voltages of 1.2 V and adjusting the feed solution to pH = 7 caused the electrodes to reach a salt adsorption capacity of approx. 0.1 mmol crotonic acid (in the form of sodium crotonate) per g of electrode material. If an aqueous feed solution of pH = 5, which is in the range of the pK_s of crotonic acid is used, the electrodes only captured around 0.02 mmol∙g⁻¹. This shows that pH dependent dissociation of weak electrolytes has a strong impact onto potential applications of CDI, and must be taken into account when designing a process combining production and separation steps. Despite the relatively low adsorption capacities, a process is suggested which accumulates sodium crotonate in a CDI system followed by a release of the accumulated ions with a concentration factor up to two. In the discussion we present suggestions how to improve these numbers and a first estimate of the energy consumption of such a system, which can be assumed to be around 1 kWh for a feed of 1 m³·h⁻¹ and 20 mM crotonic acid. In the case of dilute ionic feed streams, concentration using capacitive deionization could be an interesting alternative for biotechnological applications.

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