Chelating resins

Characteristics of chelating resins

In addition to cation exchange resins and anion exchange resins, there are chelating resins that have functional groups introduced to form chelates with metal ions.

Chelating resins form very strong complexes with specific metal ions, making them widely used, especially in environmental applications. In particular, chelating resins are used for treating wastewater containing heavy metals such as cadmium, lead, copper, and zinc, which are regulated by discharge standards, and their characteristics are as follows.

1. It has selective adsorption for metal ions and can selectively remove and recover specific metal ions.
2. It is used for purifying brine and plating solutions, and for removing harmful heavy metals (such as mercury, copper, zinc, and cadmium).
3. It is also used for recovering precious metals and valuable metals such as gold, platinum, and palladium.

Functional groups that form chelates contain two or more electron-donating elements such as N, S, O, and P, and include N-O, S-N, N-N, O-O, and P-N types. Representative chelating resins include those with iminodiacetic acid groups, among others.

Functional Group	Structure	Functional Group	Structure
Iminodiacetic acid		Thiol
Polyamine		Sulfonic acid
Primary amine		Phosphonic acid
Methylglucamine		Aminophosphonic acid (phosphonate group)
Amidoxime		Bispicolylamine
Isothiouronium (thiourea)		Semithiocarbamic acid

Functional Group	Structure
Iminodiacetic acid
Thiol
Polyamine
Sulfonic acid
Primary amine
Phosphonic acid
Methylglucamine
Aminophosphonic acid (phosphonate group)
Amidoxime
Bispicolylamine
Isothiouronium (thiourea)
Semithiocarbamic acid

High selectivity of chelating resin

Chelate resins have a much higher selectivity for specific metal ions compared to cation or anion resins. For example, even in aqueous solutions such as saturated sodium chloride solutions or highly concentrated acidic solutions, they can form complexes with and adsorb heavy metals (copper, mercury, lead, etc.) and alkaline earth metals (calcium, magnesium, strontium, etc.), and they hardly release them again unless the pH changes.
Since the selectivity for metal ions varies depending on the type of chelate resin, it is necessary to select the appropriate chelate resin based on the target metal species. An example is shown below.

Iminodiacetic Acid

Hg²⁺ > Cu²⁺ > Pb²⁺ > Ni²⁺ > Zn²⁺ > Cd²⁺ > Co²⁺ > Fe²⁺ > Be²⁺ > Mn²⁺ > Ca²⁺ > Mg²⁺ > Sr²⁺ >> Na⁺, K⁺

Isothiouronium

High selectivity for precious metals such as mercury, gold, platinum, and palladium

Methylglucamine

High selectivity for boric acid

Precautions When Using Chelate Resins

Metal ions adsorbed onto chelating resins can be eluted, recovered, and reused using strong acids such as hydrochloric acid or sulfuric acid, just like with cation exchange resins. However, it is necessary to determine the optimal type and concentration of chemicals depending on the chelating resin used and the species of metal ions to be recovered. In many cases, these settings are largely based on years of practical experience.

• The ease with which metals are adsorbed varies depending on the type of chelate.
• The order of ease of adsorption varies depending on the environment, such as the liquid composition and pH.
• It is necessary to select the type and concentration of chemicals, such as acids, used for regeneration. In particular, for metal species that are highly prone to adsorption, desorption may be difficult.
• The adsorption of metal ions can cause significant volume changes (swelling).
• When eluting adsorbed metal ions, there may be cases where metal oxide deposition occurs.