How to Choose Chelating Resin?

About Chelating Resin

Most chelating resins are polymers (more precisely copolymers) with reactive functional groups that chelate metal ions. Variation in chelating resins results from the nature of the chelating agent attached to the polymer backbone. The functional groups connected to the chelating resin include amino phosphonic acid, thiourea, 2-pyridinemethylamine, etc. There are many types of chelating resins, such as carboxylic acid type, thiol type, Schiff base type, and thiourea type. [1]

Chelating resins are commonly used for the preconcentration of metal ions in dilute solutions, as well as in the chlor-alkali industry for desalination of brine, boron removal in drinking water, and recovery of precious metals from solutions.

How Does Chelating Resin Adsorb Heavy Metals?

Chelating resin is different from other ion exchange resins. It contains O N, S, P, As, Se and other atoms that have not formed bonded lone pairs of electrons. They can coordinate with metal ions in water. This reaction and the functional groups are used to react with metal ions in water to form water-insoluble chelates. When it reaches a certain volume, the chelate is insoluble in water and precipitates under the action of gravity, thus achieving the removal effect.

What Are the Characteristics of Chelating Resin?

The difference between chelated ion exchangers and ordinary types of ion exchangers lies in three main characteristics:

• High selectivity

The affinity of a particular metal ion for a certain chelated ion exchanger depends primarily on the chelating group and not on the size of the ion, its charge, or other physical properties that determine priority in the case of common ion exchangers.

• Bond strength

In ordinary ion exchangers, the binding is electrostatic with a strength of 2-3 kcal/mole, while in the resin treated here, the binding energy is of the order 15-25 kcal/mole.

• Kinetics

While in the ordinary type of exchanger the exchange process is faster and controlled only by diffusion, which is itself a function of the mobility and concentration gradient of the entering and leaving ions, the exchange process in the chelating exchanger is much slower and controlled by particle diffusion mechanisms or secondary chemical reactions.

Chelating Resin Selection Guide

Depending on different application requirements or characteristics such as functional groups, you can find the most suitable chelating resin from the table below. The adsorption selectivity and adsorption capacity are two important indicators for evaluating the performance of chelating resin. Among them, the adsorption capacity depends on the specific surface area of the adsorption material and the density of surface functional groups, which is a key factor affecting water treatment efficiency.

Chelating Resin Selection List

What Are the Precautions for Using Chelating Resin?

• Chelating resin is easily oxidized, so direct treatment with brine solutions containing free chlorine should be avoided. Some measures should be taken, such as preliminary reaction with sulfur dioxide, sulfite or pretreatment with activated carbon. The chlorate concentration of the brine solution is usually very high, so the displacement rinse before regeneration must be effective to prevent the chlorate in the brine solution from contacting the regeneration acid to form free chlorine.
• Wasted chelating resin cannot be discarded directly because chelating resin cannot leach complexed heavy metal ions through organic matter, which will cause secondary pollution to the environment.

References

1. Chu S, et al. Industrial & Engineering Chemistry Research, 2022, 61(31), 11309-11328.
2. Schmuckler G. Talanta, 1965, 12(3), 281-290.