Continuous water softening process for industrial applications

Aims and Concept

Conventional methods for water softening are using spezial resins containing ionic functional groups, e.g. sulfonate groups in cation exchangers.
After loading this special resins with cations (and anions), this materials have to be regenerated (removal of bonded cations and anions) under usage of chemicals like concentrated aqueous solutions of acids or salts.
This disadvantageleads to the development of new concepts for water softening techniques without the demand of chemicals for the regeneration process.
The aim of this project was the further development of an already investigated innovative water softening technique based on an electro-chemically switchable ion-exchanger with conducting polymers, e.g. polypyrrole / polystyrenesulfonate [1,2].
This new type of ion-exchangers should be improved to overcome some problems and disadvantages concerning the desired technical application.
Amongst other things the adherence and the thickness of the electroactive polypyrrole layer on the graphite felts had to be increased in order to reach the technical requirements for such a type of ion-exchanger.
During this work, also another concept of electrochemically switchable ion-exchangers was found to be attractive, which is based only on activated carbon electrode materials (without conducting polymers). In this case, the ion-exchanger simply works like an electrochemical double layer capacitor (EDLC) to remove the corresponding ions (e.g. calcium and magnesium ions) from the aqueous solutions.

 Results

Several types of carbon felts with different pretreatments were tested with respect to their ability of reversible binding of cations (e.g. calcium ions) and anions when used as an electrode material.
A prototype of a water softening module was built by the DFI‐workshop.The module consists of to half cells separated by an ion exchanger membrane. Each half cell contains an carbon felt, which is located between the membrane and the graphite plates which are used as current collectors.

Outlook

Stability tests with the prototype, which are still in progress, will show, if this concept of electrochemically switchable ion exchangers will meet the requirements of technical applications (e.g. water softening units for technical purposes).

References:

[1] C. Weidlich, K.-M. Mangold, K. Jüttner, Electrochimica Acta, 50 (2005), 5247-5254.
[2] C. Weidlich, K.-M. Mangold, K. Jüttner, Electrochimica Acta, 47 (2001) 741-745.