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How Chelating Resins Behave How Chelating Resins Behave HowHow ChelatingChelating ResinsResins BehaveBehave By Peter S. Meyers Chelating resins are gaining chemicals, it is extremely impor- by chelating resins, although not as acceptance as the best tant to know something about the completely as the bare metal cation. available technology for the composition of the other ions in the Chloride complexes are not often seen removal of transition and feedwater besides the metal of in plating rinsewaters, because these heavy metal cations from interest. types of complexes do not form until ground water and from plating 2. The process of ion exchange the chloride concentration is quite rinse waters. These specialty represents a dynamic equilibrium high. In attempts to treat plating baths ion exchange resins are between the liquid solution and the with chelating resins, however, the capable of removing metals solid resin. The equilibrium is formation of these complexes is very selectively in the presence of predictable only when the liquid important. Chelants such as EDTA other ions such as calcium, composition is predictable. combine with metallic cations to form magnesium and sodium. This Chelating resins operate well with very stable anionic complexes. Metals paper describes the various a relatively narrow inlet composi- that are EDTA-complexed are not types of chelating resins that tion, with respect to pH. If the available for removal by cation are commercially available, liquid composition changes exchange. In cases where the pH is discusses the conditions under rapidly, the equilibrium between elevated (as following hydroxide which they will and will not the liquid and resin also change precipitation), the metals that remain work, and explores the input rapidly. This may result in poor in solution are dominantly hydroxide require-ments necessary to performance or a complete complexes. These anionic complexes optimize a system design. unloading of metals previously are not available for removal by exchanged onto the resin. cation exchange resins. In these cases, Solution Chemistry it is necessary to reduce the pH to The chemistry of plating rinsewaters Unfortunately, many plating shops insure that the form of the metal is is particularly interesting, because the combine all of their rinsewaters into cationic. potential number of types of solutions one large waste tank and then treat the and the individual chemistry of these waste simultaneously. Although this is General Behavior solutions is very large. not impossible to do, it can compli- Of Chelating Resins Two basic concepts must be firmly cate the situation with respect to the Although we often consider chelating grasped by users of ion exchange, in resin. A chelating resin will always resins to be selective for a particular order to be successful in these types work better when there is good metal, they are actually selective for of applications. control of the composition of the groups of ions. A cation resin has wastewater leading to the ion ex- some selectivity for all cations. This is 1. All chelating resins are cation change column, and when the important, because even an imino- exchangers, at least with respect to conditions of operation are relatively diacetate chelant, which is considered their chelation properties. Chelat- constant. nonselective for calcium, does ing functionality is created by Complexing agents that are exchange for calcium and can be Lewis base activity. Chelating frequently encountered in plating converted to the calcium form. A resin functional groups do not exchange operations include cyanide, chloride, that is extremely selective for an ion, for anions or for anionic com- ammonia, EDTA and other liquid such as copper in the presence of plexes. Because many metals form chelants. Most cyanide complexes can sodium, will usually be less selective complexes with various other be readily destroyed by chlorination, for copper in the presence of calcium. and this process is routinely practiced. Furthermore, the resin will be Any excess chlorine must be removed selective not only for copper, but also ______________ ahead of the ion exchange column to for other divalent transition metals. Editor’s Note: This “Best Paper” prevent oxidative damage to the resin. True chelants depend on ligand award-winner was presented at the Certain cyanide complexes, ferro- bonding between the metal and the AESF/EPA Conference during AESF cyanide in particular, do not oxidize resin. These types of ligand bonds are Week '98, Orlando, FL. The complete readily. Ammonia complexes are “sprung” by high concentrations of text can be found in the conference generally cationic in nature. In many acid. No true chelating resin can proceedings. cases, these complexes are removed operate in a highly acidic environment. 22 PLATING & SURFACE FINISHING A true chelant has one or more Ion Exchange Iminodiacetate Chelant chela, which form a ligand bond with Selectivity SIR-300 the metal. The term ligand refers to Ion exchange selectivity the interaction between the resin (a refers to the equilibrium Lewis base) and the metal cation (a constant between the solid Lewis acid). A ligand, or complex resin phase and the mobile bond, is the result of stabilizing the liquid phase. It is manipu- charge balance between the resin, lated mathematically in the which has an exposed electron pair, same fashion as solubility and the metal, which is in search of products for various salts electrons to provide atomic stability. in solution. However, so- Commercial chelating resins possess called selectivity constants not only chelating functionality, but for ion exchange resins are not true where “K” is the equilibrium constant also ion exchange functionality. They solubility products, and they vary that describes the relationship capture metal cations by a combina- with the dynamic conditions of the between the ionic concentrations in tion of ion exchange and chelation. process. The most significant of these the resin and in the water. Ionic The stability of the chelated complex variables is pH. Although we use ion concentrations are generally described depends on the nature of the ion exchange selectivity as a working tool in molarity. exchange functionality built into the to predict the general behavior of resin, the ionic charge, the hydrated chelating resins, we also must Descriptions of Various Chelants radius of the metal cation and on the understand that the solution variables & Their General Areas of Use influence of other ions in the solution. have a profound effect on the ob- There are three types chelating resins served selectivity. For this reason, we that are commercially available and in Definitions will briefly present the basic equa- general use. They are the imino- Chela—Gr. The pincer-like claw of a tions and then move onto a more diacetate chelant, such as SIR-300, crab, lobster or scorpion. practical description of how chelating the aminophosphonic chelant, such as Ligand—The chemical term for an resins are used. SIR-500, and the picolyamine chelant, electron pair donor (Lewis base) such as Dow 4196. Other chelants, when it forms a bond with a metal What Makes a Chelating Resin such as the pyridine chelant and the cation. Selective? amoxidine chelant, are less commonly Chelating resin—Ion exchange Ionic charge (valence)—Most used. In addition to the true chelants, resins that have ligands that can chelating resins prefer divalent there are several other selective resins bond with metal cations. The ligands ions to monovalent or trivalent available that are sometimes em- may be in addition to or in place of ions, because the chelant group has ployed in metal removal. These conventional ion exchange sites. two “chela” (claws). include weakly basic polyamine Selectivity—The preference of an ion Hydrated ionic radius—When the resins and the natural zeolites. exchange material for a particular size of the hydrated radius of the Conventional ion exchange resins, ion. Selectivity is always relative ion closely fits the space between such as weak acid cation and strong to a particular operating condition. the “chela,” the resin is very cation resins, are also used. selective for that particular ion. Chelating Lobster Ligand bonding—Nitrogen (and Chelating Resins oxygen) can possess an exposed True chelating resins electron pair, making them Lewis • Iminodiacetate (SIR-300) bases (electron pair donors). • Aminophosphonic (SIR-500) Metals that are Lewis acids form • Picolyamine (DOW 4196) ligands with Lewis bases. Other selective resins • Thiol (SIR-200) Relative Selectvities of • Thiouronium (SIR-400) Iminodiacetate Chelants • Exhausted weak base (WBMP & SIR-700) Ion “K” Ion “K” • Natural zeolites (SIR-600, H+ Co++ 7 Because of the relatively bulky greensand & others) Na+ 0.00001 Cd++ 15 exchange groups of chelating resins • Weak acid cation resins (WACG Ca++ 1 Hg++ 2800 and the stability of the chelant bonds, & WACMP) Cu++ 2300 Pb++ 1200 a chelating resin’s rate of exchange is Ni++ 57 Zn++ 17 limited. They cannot operate at high Iminodiacetate Chelant (SIR-300) Fe++ 4 flow rates without a significant loss of This type of chelating resin has a functional group that consists of two capacity and increase of metal Equlibrium Equation carboxylic acid groups, attached to a leakage. Optimum flow rate is around Using the Selectivity Constant 0.5–1.0 gpm/ft3. Flow rates higher nitrogen atom, bonded to the resin’s 2 + 2 than 2.0 gpm/ft3 cause a chelating Cu [RX - Cu] [Ca ] polymer structure. The carboxylic resin’s performance to suffer and will K = acid groups exchange with various Ca [RX - Ca] 2 [Cu+] 2 probably lead to disappointing results. cations in exactly the same manner as October 1998 23 Capacity of SIR-300 Aminophosphonic Chelant Various Metals (at saturation) SIR-500 weak acid cation resins. In addition, increases to the point where the Aminophosphonic Chelant the nitrogen atom provides the Lewis removal of divalent metal cations is (SIR-500) base necessary to form a ligand bond not preferred. This chelant is quite similar in with the metal cation.
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