Structure and exchange principle of ion exchange resin

I. Structure of Ion Exchange Resin

        The internal structure of ion exchange resin is shown in the figure below. It consists of three parts:

        1 Polymer matrix. Composed of cross-linked polymer;

        2 Ion exchange groups. It is connected to the polymer backbone, with ionic functional groups carrying exchangeable ions (called counterions) or polar non-ionic functional groups;

        3 Pores. These are pores (gel pores) in the polymer structure that exist in both dry and wet ion exchange resins, and pores (capillaries) between the polymer structures.

        Many exchange groups are chemically bonded to the cross-linked polymer matrix (backbone). These exchange groups also consist of two parts: a fixed part and a mobile part.

        The fixed part of the exchange group is bound to the polymer matrix and cannot move freely, so it is called a fixed ion; the mobile part of the exchange group is an ion with the opposite sign to the fixed ion, bound to the fixed ion by an ionic bond, called a counterion or exchangeable ion. Counterions can dissociate into freely moving ions in solution, and under certain conditions, they can undergo exchange reactions with other counterions of the same sign.

II. Basic Principles of Ion Exchange

        1 Definition of ion exchange selectivity:

        The property of an ion exchanger showing preferential activity for certain ions. Ion exchange resins have different abilities to adsorb various ions. Some ions are easily adsorbed by the exchange resin, but it is difficult to displace them after adsorption; while other ions are difficult to adsorb, but they are easy to displace. This property is called the selectivity of ion exchange. The selectivity of ion exchange resins for different ions in water is related to factors such as the degree of cross-linking of the resin, the exchange group, the properties of the exchangeable ions, the concentration of ions in water, and the temperature of water.

        Ion exchange is the exchange between exchangeable ions in solution and exchangeable ions on exchange groups. Generally speaking, ion exchange resins have greater selectivity for ions with higher valence. For ions of the same valence, the selectivity for ions with smaller ionic radius is greater. Among the metal ions of the same family and valence, the ion with a larger atomic number has a smaller hydrated radius, and the cation exchange resin has greater selectivity for it. For strongly acidic cation exchange resins, the selectivity order for some ions is: Fe3+>A13+>Ca2+>Mg2+>K+>Na+>H+ Ion exchange reactions are reversible reactions, but this reversible reaction does not take place in a homogeneous solution, but at the interface between the solid resin and the solution. The reversibility of this reaction allows ion exchange resins to be reused repeatedly.

        2 For example, 001×7 strongly acidic cation exchange resin:

001×7 Strongly acidic cation exchange resin is a gel-type ion exchange resin. There are countless channels in its internal network structure, and the channels are filled with water molecules. Exchangeable ions are distributed at certain positions in the channels. When Ca2+ and Mg2+ and other cations in the raw water diffuse into the resin channels, because the resin has a stronger selectivity for these cations than for H+, therefore, these cations will undergo a rapid exchange reaction with the cations entering the resin channels, and these cations will be fixed to the resin exchange groups, and the exchanged H+ will diffuse into the resin channels and finally diffuse into the water. Ca2+ ， Mg2+ cations - diffuse into the resin channels, because the resin's selectivity for Ca2+ ， Mg2+ cations is stronger than for H+ so H+ will undergo a rapid exchange reaction with Ca2+ ， Mg2+ cations entering the resin channels, Ca2+ ， Mg2+ cations are fixed to the resin exchange groups, and the exchanged H+ diffuses into the resin channels - and finally diffuses into the water.

（ 1) Diffusion in the boundary water film   

        The Ca2+ ， Mg2+ cations in water migrate to the surface of the resin particles and diffuse through the boundary water film layer on the surface of the resin to reach the surface of the resin;

（ 2) Diffusion in the cross-linked network pores (or pore diffusion)

Ca2+ ， Mg2+ cations enter the cross-linked network pores inside the resin particles and diffuse to the exchange point;

（ 3) Ion exchange

Ca2+ ， Mg2+ cations undergo an exchange reaction with the exchangeable H+ on the resin group;

（ 4) Diffusion in the cross-linked network pores

          The exchanged H+ diffuses from the inside of the resin to the surface through the cross-linked network pores.

（ 5) Diffusion in the boundary water film

        Finally diffuses into the water.

III. Regeneration of Ion Exchange Resin

        Given the reversibility of the ion exchange resin reaction, the reacted resin is treated to be converted back into the original ion exchange resin, which can then enter the next cycle. The number of cycles depends on the type of resin used.