At present, the filler of EDI membrane stack is generally ion exchange resin, and the research of ion exchange fiber as filler has also been reported. At the same time, the research and development of other types of packaging materials continues. Different filling methods are used for unfilled materials, and the research in this field has also made progress.
filling
In EDI membrane stack, the filling material is the carrier of ion conduction, and its performance directly affects the EDI process. Packaging materials should have the following characteristics: high exchange capacity; Fast speed; Strong electrical conductivity; Small water flow resistance; High strength; Insoluble, etc.
ion exchange resin
Choosing ion exchange resin as filling material can not only meet the requirements of the latter part, but also be used directly without further treatment, which is cheap and easy to obtain. Therefore, granular ion exchange resin has been widely used since Millipore Company launched the first commercial EDI seven years ago.
At present, there are many kinds of granular ion exchange resins on the market, and the classification methods are not the same. Generally, it is classified according to the characteristics of functional groups on ion exchange resins, the types of counterions on functional groups and the morphology of resins.
Principle analysis
Ion exchange resins can be divided into cation exchange resins and anion exchange resins according to the characteristics of functional groups. Those with acidic functional groups are called cation exchange resins; Those with basic functional groups are called anion exchange resins. According to the strength of acid and base on functional groups, it can be roughly divided into strong acid, weak acid or strong base and weak base ion exchange resin. Different types of ion exchange resins have certain differences in performance, so as filling materials, different phenomena will appear in EDI process. Most EDI membrane stacks at home and abroad use strong acid and strong base ion exchange resins. This resin has strong ion exchange ability and is relatively easy to regenerate. The weak acid and weak base resin is easy to be regenerated by H+ and OH-, but regeneration of resin is seldom used because of its weak ion exchange ability. This is mainly determined by the selective adsorption of weak acid and weak base resin.
In neutral aqueous solution, the selective adsorption order of weak acid and weak base resin for various ions is: h+>; & gtFe3+>； Ca2+>； Mg2+>； k+& gt； na+& gt； Li+; Oh-> & gtSO42->； PO43->； NO2- & gt； cl->； Bicarbonate. It can be seen that the selectivity coefficient of weak acid and weak base resin for H+ and OH- is obviously higher than other ions, which makes it difficult for H+ and OH- on regeneration of resin to exchange with other ions in the solution. Therefore, the ion exchange capacity of regeneration of resin becomes weak, and the ion exchange and regeneration process of resin can not be carried out continuously and efficiently, which ultimately affects the desalination rate of membrane stack.