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. Metals are structure to the iminodiacetate removed by a combination of ion Iminodiacetate Chelant chelant, with the exception that the exchange and chelation. Most (SIR-300) Operating Limits carboxylic groups are replaced by commercially available imino- Dissolved Solids phosphonate groups. The differences diacetate chelants contain single • Unaffected by sodium and other between the aminophosphonic and carboxylic acid groups in addition to Group IA metals (except hydrogen) iminodiacetate chelant are rather the double carboxylic groups. When • Practically unaffected by calcium subtle. The aminophosphonic chelant an iminodiacetate resin is protonated and other Group 2A metals has a relatively higher selectivity for (meaning that the carboxylic acid • Selectivity is affected by other the alkaline metals than does the groups are attached to hydrogen ions), complexing agents iminodiacetate chelant. The phos- the charge balance of these exchange —EDTA blocks exchange phonate groups do not destabilize, groups is very stable. Under this entirely for many metals therefore, this resin remains more condition, the carboxylic groups are —Ammonia affects exhange selective at elevated pH. In most not available for exchange with any significantly applications, the aminophosphic and cations, and only the limited chelating —Chloride has a minor effect on iminodiacetate resins can be inter- capability of the resin is available. At selectivity changed with each other without any a pH below approximately 2, the pH observable difference in performance. nitrogen atom is stabilized by the • Optimum pH is usually slightly The notable exceptions are operation presence of hydrogen ions, and the acidic at high pH, where the aminophosphic resin has no capacity whatsoever for • Selectivity is lower at high pH resin maintains its high selectivity, any other cations. As the pH in- • Loses selectivity below pH of and the iminodiacetate loses selectiv- creases, the nitrogen is first destabi- approximately 2.0 ity. In high-calcium environments, the lized, then one of the two paired aminophosphonic resin is less carboxylic acid groups is destabilized, The usefulness of the imino- selective for other metals than is the and finally, both of the carboxylic diacetate chelant is in slightly acidic iminodiacetate resin. acid groups are destabilized. solutions for removal of divalent The aminophosphic resin is The strongest chelation occurs transition metals. The resin, under primarily used in brine softening, when one of the paired carboxylic these conditions, is relatively nonse- where its selectivity for alkaline groups and the nitrogen atom enter lective for alkaline metals such as metals, such as calcium, is advanta- into the ion exchange reaction with calcium and magnesium. Divalent geous. It is also useful in plating the metal, such that half of two metal cations such as copper, nickel, wastes, where the pH cannot be exchange groups are used to capture cadmium, and zinc can be removed reduced into the acidic range neces- the metal. very selectively. SIR-300 is useful in sary for an iminodiacetate chelant. A This behavior sets up the following the treatment of plating wastes word of caution is needed here, scenario with respect to pH. following hydroxide precipitation however, because many metals form Below a pH of 2, the resin has no with lime or caustic, if the pH is anionic complexes at high pHs, selectivity for metal cations and is reduced to the proper level. thereby removing them from the regenerated. Between a pH of 2 and exchange equation. approximately 4.5, half of the Iminodiacetate Chelant diacetate group is destabilized, and (SIR-300) Criteria for Selection Aminophosphonic Chelant the resin reaches peak efficiency. • Removal of divalent Group IV Operating Limits Above pH of 4.5, the other half of the (transition) metals Dissolved Solids diacetate group is destabilized, and • High TDS background (>1000 • Unaffected by sodium the resin becomes more selective for ppm) • Moderate selectivity for calcium monovalent ions. Above a pH of • Significant calcium concentration and other Group 2A metals approximately 9, almost all the present (≥ metal concentration) exchange groups are destabilized. The • pH is slightly acidic (optimum pH resin’s selectivity for monovalent ions is approx. 4) * Dow 4196, Dow Chemical Co.

24 PLATING & SURFACE FINISHING Capacity of SIR-500 • Removal of calcium and Picolyamine Chelant Copper & Nickel (at saturation) other Group 2A metals from Operating Limits saturated brine Dissolved Solids —pH is basic (optimum pH • Unaffected by sodium is approx. 11) • Low selectivity for calcium and —Temperature is slightly el- other Group 2A metals evated (140–160 °F) • Selectivity is affected by other complexing agents Picolyamine Chelant —EDTA blocks exchange Unfortunately, the entirely for many metals picolyamine used in making —Ammonia affects exchange this resin is very expensive significantly Capacity of SIR-500 and is available from only a —Chloride has a minor effect on For Calcium few manufacturers.* This selectivity resin is much more expensive pH than competitively priced • Optimum pH is usually signifi- chelants, such as SIR-300 cantly acidic and SIR-500. • Selectivity is lower at alkaline pH The picolyamine chelant is • Loses selectivity above pH of significantly different. It is a approximately 8.0 weakly basic anion resin. In use, it has no ion exchange Thiol & Thiouronium Functional capacity for metals. It works Resins—SIR-200 & SIR-400 • Selectivity is affected by other solely by its chelating properties. The These resins depend on the sulfur complexing agents picolyamine chelant contains two atom built into the functional group of —EDTA blocks exchange nitrogen atoms that can form ligand the resin. The thiol-based resin, SIR- entirely for many metals bonds with metals. It has very strong 200, has an “SH” group. The thiou- —Ammonia affects exhange chelation properties. It also has the ronium has the sulfur atom sand- significantly advantage of being able to operate wiched between two carbon atoms. —Chloride has a minor effect on into a relatively low pH range The thiouronium resin is a weak selectivity (roughly pH of 1). The useful range is acid cation exchanger. The thiol resin pH still limited, however, because at is also very weakly acidic. Both resins, • Optimum pH is usually signifi- extremely low pHs, the exposed pair however, are so weakly acidic that the cantly basic of electrons on the nitrogen atoms question of whether they are actually • Selectivity is lower at low pH become stabilized by hydrogen ions. ion exchangers is controversial and • Loses selectivity below pH of The picolyamine resin has success- problematic. Either the chelation with approximately 4.0 fully been used to treat acid plating the sulfur atom is so strong that it baths. Because of its high cost, it is cannot be broken by conventional Aminophosphonic Chelant seldom used in treating rinsewaters. chemistry, or the metal actually Criteria for Selection precipitates onto the sulfur atom. • Removal of divalent Group IV (transition) metals in the absence of calcium and other Group 2A metals —High TDS background (>1000 ppm) —pH is slightly acidic to significantly basic

Picolyamine Chelant

Free Details: Circle 112 on reader service card.

October 1998 25 Thiol Functionality Resin Natural Zeolites conventional cation exchange resins. SIR-200 The natural zeolites, such as This is because the selectivity for clinoptilolites, SIR-600 and metals, as compared to sodium, is not manganese greensand, have much greater than unity. In order to interesting properties that remove the metals, it is necessary to make them selective for also remove the sodium. This limits certain classes of metals. The the throughput capacity of the resin to clinoptilolite type of zeolite is the point where it is not practical. extremely selective for cesium There are quite a few systems out and also shows significant there, however, that mistakenly selectivity for ammonia. employ this technology and, frankly, Zeolites have crystalline they do not work very well, if at all. structures that possess ion exchange functionality. They TDS Limits of Various Resins also possess holes or tunnels Used for Metals Removal Thiouronium Functionality Resin in their crystalline structure Strong cation resin 500 ppm SIR-400 that allow them to capture ions (hydrogen form) with the proper atomic Strong cation resin 1,000 ppm geometry. Clinoptilolite is (sodium form) exceptionally selective for Weak cation resin 10,000 ppm cesium, because the atomic (sodium form) dimensions of a cesium ion is Chelating cation resin no limit precisely the right size to fit in (sodium form) the crystalline tunnels. Mixed-bed resin 500 ppm Ammonia is also close to the optimum size. Cation exchange can be useful for In either case, regeneration of Clinoptilolite is used for recycling and recovering individual metals off of these resins is, at best, cesium removal from radioactive rinsewaters if the total dissolved solids extremely difficult and most probably wastewaters, as well as for ammonia are not too high. In these cases, the impossible. There is a close parallel removal from water where high levels make-up water to the rinse tanks should between sulfide solubility and selectiv- of sodium are not present. It is not be deionized to prevent the addition ity of these resins for various cations. selective for any divalent cations and of unwanted ions that will have to be Their primary use has been in has no chelating functionality. taken out later, along with the metals. mercury removal from waste streams Weak acid cation resin has been Natural Zeolites and for precious metal removal. In overlooked in many applications, Criteria for Selection mixed wastes, where other metals are because its useful pH is limited to present that also form insoluble • Primary uses greater than 4 and, when in the sulfide precipitants, these metals must —Removal of cesium from hydrogen form, it has very little be removed prior to the use of the radioactive waste capacity for metals. In the sodium sulfur-based chelants. Otherwise, the —Removal of ammonia from form, however, and at neutral-to- resin loads up with the tramp metals water against moderate levels alkaline pH, weak acid cation resin and becomes exhausted prematurely. of sodium has exceptionally high selectivity for Because of their relative inability to • Some Zeolites are used as redox divalent metals of all kinds. In some be regenerated, the sulfur-bearing media for the removal of iron and cases, this selectivity is even higher chelants have very limited usefulness. manganese than that of chelating resins. Unfortu- • Zeolites are generally stable in nately, weak acid resin cannot Thiol & Thioruonium Resins oxidizing environments and at distinguish between alkaline metals Criteria for Selection moderate pH, but dissolve at very such as calcium and transition metals • Primary use is removal of trace lev- high or low pH such as copper. In cases where els of mercury from wastewater calcium can be excluded from the • Resin can also be used to remove Manganese greensand has catalytic wastewater by means of softening the noble metals, as well as other properties that are generally enhanced make-up and where the TDS is not metals that form insoluble sulfide by the addition of a manganese dioxide extremely high, however, the high precipitants coating. This media is primarily used capacity and relatively low cost of • Due to the resins’ relatively for its redox properties, particularly in weak acid cation resin make it an indiscriminant removal of most the precipitation of iron and manganese. attractive alternative to chelating resins. transition and heavy metals and Conventional Ion Exchange Resins The Case for the unlikeliness that regeneration Weak Acid Cation Resin will be successful, the practical We include a brief discussion of applications for these resins are conventional resins because there are • If all water is softened to remove generally limited to removal of certain cases where they can be used. hardness ions, then the waste will mercury Most combined wastewaters have too contain only monovalent cations high a total dissolved solids for along with the metal contami-

26 PLATING & SURFACE FINISHING nants. In this case, sodium form Either the bed completely plugs up, or There are several types of chemical weak acid cation resin will have the water flows through channels in attack that occur. The general effect, far higher capacity for metals than the bed and does not get treated. however, is that the functional ion any of the chelating resins. During the regeneration phase, if exchange groups are destroyed, thereby • pH must be neutral to alkaline and the sponge is not removed, then the lowering the capacity of the resin. The the TDS must be less than chemical used in the regeneration polymer structure of the resin can also approximately 10,000 ppm. process becomes trapped in the be attacked, resulting in swelling, sponge, along with a portion of the softening and eventual dissolving of Operational Considerations metals. During the next service cycle, the resin. The consequence is that the When considering any type of ion the regenerants and metals then leach resin is completely ruined. exchange in a waste application, it is out of the sponge. It is prudent to include dechlorina- important to consider the two types of The plastic bag effect is when the tion and, possibly, carbon filtration in fouling that will prevent any ion resin beads become coated by a front of the ion exchange resin to exchange system from functioning foreign substance, most notably remove oxidants and to adsorb well. We call these the “sponge organic molecules that have limited organic molecules. It is important to effect” and the “plastic bag effect.” solubility, such as oils and polymers. remember that the capacity of carbon The sponge effect comes from These organic molecules are routinely for adsorption is limited, and carbon resin’s relatively good filtration used as fluxes and have the same beds must be changed regularly to characteristics. In chelating beds that general effect on the resin. maintain their effectiveness. remain in operation for very long They coat the resin beads with a periods of time, there is a tendency “plastic bag” that prevents the exchange Chelating Resins’ General for suspended solids to become of ions. This phenomenon affects both Operating Limits trapped by the resin bed and to build the service cycle and the regeneration Part 1 up in the spaces between the resin cycle. Not only do the metal ions not Flow rates & bed depths beads. The solids become a sponge get removed properly during service, • All chelating resins are very flow- and can interfere with the resin’s they also do not get cleaned off the resin sensitive ability to function in two ways. bed properly during regeneration. The —Optimum flow is ~1 gpm/ft3 They can plug up the bed and cause net result is a failure of the ion —Significant loss of capacity channeling, interfering with the physical exchange system to function. occurs above 2 gpm/ft3 flow of liquid through the resin. The All organic-based ion exchange • Deep beds always work better consequences can be very dramatic. resins are susceptible to oxidation. than shallow beds

Free Details: Circle 114 on reader service card. Free Details: Circle 113 on reader service card. October 1998 27 —4 to 6 ft deep beds are ideal First, the resin is stripped of its What is Needed for a Successful —2 ft deep beds are marginal contaminants by use of acid. The Chelating Bed Design? —Surface flow rates should not chelating groups are stabilized by the In order to determine if a chelating exceed about 12 gpm/ft2 hydrogen ions and cannot form ligand resin will be successful, it is necessary —Pressure loss across the bed bonds under this condition. The resin to have relatively detailed information should not exceed 20 psi becomes non-selective for metals, and about the solution to be treated. It is they are carried out of the resin bed surprising how often a system is put Part 2 along with the spent acid. The resins in place without any knowledge of the Sensitivity to inlet contaminants must be neutralized prior to use, inlet metal concentration or the • Chlorine (and other oxidants) because they are not selective in the effluent requirement. It would seem —Poor. All organic ion exchang- acid form. Neutralization is accom- obvious that these two pieces of data ers are sensitive to oxidation plished by passing a basic solution were invariably necessary. Yet, that (including chelating resins). through the resin. Most commonly, the hasn’t always been the case. Inlet chlorine should be < 0.1 base is sodium hydroxide or ammonia. In addition to the relative inlet and ppm (preferably zero ppm). The regeneration sequence consists outlet metal concentrations, it is • Suspended solids of some type of backwash to (hope- imperative that the pH and total —Poor. Because of extremely fully) clean suspended solids out of dissolved solids of the solution be long run lenghts normally the resin bed. This is followed by acid known and controlled. In cases where produced by chelating resins, injection. Hydrochloric acid is most these parameters vary, it is necessary they are susceptible to fouling commonly used, although in some to know the relative range and to keep by suspended solids. Inlet cases sulfuric acid can be employed. it as narrow as possible. It isn’t turbidity should be as low as The amount of acid required must always necessary to be exact, but it is possible (1 ppm is too high). include an excess to insure that the pH a relatively small ballpark. is low enough to spring the chelant Because of the potential for fouling, Part 3 bonds and to regenerate the ion it is necessary to know something about Sensitivity to inlet contaminants exchange groups. In addition, acid can the level of suspended solids, the • Organic molecules help to dissolve precipitants that may presence or absence of oxidants, the —Polar and non-polar solvents have formed in the resin bed. When temperature of the solution and the are of no consequence. They sulfuric is used, there is a potential for presence or absence of organic do not harm chelating resins; precipitation of metal sulfates. In contaminants. It is not usually however, in some cases, practice, the chelants that are regener- necessary to know exactly what solvents can complex with ated with sulfuric acid have far more organic molecule might be present. metals and prevent them from problems caused by precipitation than We can generally group these by type being removed by the resin. those regenerated with hydrochloric and effect on the resin. For instance, —Polymers and other long chain acid. After the acid is rinsed from the solvents such as alcohol, ketones and molecules (particularly if they resin bed, the resin is neutralized with even low molecular weight hydrocar- are only partially water- caustic and then rinsed to quality. bons have no effect on ion exchange soluble) will coat the resin resins. Polymers, partially water- beads and prevent the ex- Typical Regeneration Schedule soluble fluxes and oils of any kind, change of ions. Backwash are generally very bad for resin. • 50% bed expansion for 15–20 min The presence or absence of Part 4 Acid injection complexing ions (such as chloride, Sensitivity to inlet contaminants • 5–8 lb/ft3 of HCl at 5% concentra- ammonia, EDTA and cyanide) need to • pH tion for 40–60 min be known because they change the —All chelating resins are stable Acid rinse nature of the metals in solution, and from pH < 0 to >14; however, • Approx. 40 gal rinsewater at any may put them in a form where removal their useful operating range is convenient flow and time by cation exchange is not possible. much narrower. This useful Caustic neutralization The type of process generating the range is different for each type • 2–4 lb/ft3 of NaOH at 5%, then wastewater is often quite useful, of chelating resin. Some air-mix and soak 30 min because, in many cases, there is chelants (notably, those with Final rinse relevant experience with ion exchange Thiol-based functionality) are • Until conductivity and pH are from previous attempts to use it, both inactivated at high pH. acceptable for process use successful and unsuccessful. If the • Temperature process is well known, the applicabil- —All chelating resins are stable In the case of the iminodiacetate ity of ion exchange is probably well to at least 140°F. In general, chelant, it is important not to neutral- known, and the need to thoroughly capacity increases with ize with too much caustic. At high pH, study the characteristics of the water increasing temperature. this resin does not have good selectiv- is less important.Where no previous ity for metals. Although in most ion knowledge is available, it is helpful to Regeneration exchange applications it is not proper perform either a lab scale column Regeneration of chelating resins is to add chemicals and mix them with study to determine how well ion almost always accomplished in a two- the resin, in the case of the chelating exchange will work, or to do a plant step process. resin, this method works rather well. trial with a small pilot scale system.

28 PLATING & SURFACE FINISHING There is no substitute for trying it out, changers to function, then they are ResinTech Inc., Cherry Hill, NJ. His as all the theory in the world is likely to be far less expensive to use. responsibilities include providing useless if the dynamics of the solution technical assistance to customers, are not completely understood. Ion exchange can be a plater’s best managing laboratory operations and friend—if it is employed properly and overseeing process and production What Do We Need to Know? the conditions are understood for how applications. His 20 years of experi- In order to determine if a chelating it can be used. Ion exchange can also ence ranges from many types of resin is appropriate technology, it is be the plater’s worst enemy if the makeup demineralization, polishers necessary to know the basic details of system is put in with the mistaken and softeners, to process design and the water to be treated. assumption that it will remove any hardware operation. A graduate of metal, from any stream, at any time. MT San Antonio College in Califor- • TDS (or conductivity) with min These systems are certain to fail. P&SF nia, he has written numerous papers and max, if variable on ion exchange and holds several • pH and temperature with min and About the Author patents related to water treatment. max, if variable Peter Meyers is technical manager for • Basic inorganic analysis of ions

(Ca, Mg, Na, Cl, SO4) • Presence or absence of oxidants • Presence or absence of complexing agents • Level and type of organic molecules • Level of suspended solids

Conclusion The use of chelating resins to remove metal cations is a remarkably effec- tive tool that can be used to prevent the discharge of metals into the environment. Chelants can reduce the metal content of wastewater to practi- cally nondetectable levels. Their use, however, may require a readjustment in the way that the wastewater from a plating shop is handled, and a careful segregation of incompatible streams.

General Limits of All Chelants • All chelants are flow-sensitive and cannot be operated at high space velocities (low EBCT) • They have a relatively narrow chemistry range in which they work well • They are expensive compared to con- ventional ion exchange materials • They are susceptible to fouling due to extremely large volume throughputs

For any specific application, it is helpful to consult an ion exchange specialist with previous experience in your particular situation.

Why Not Use a Chelating Resin? • If it is not possible to carefully control inlet chemical composi- tion, particularly pH and TDS, then chelating resins will prob- ably not perform to expectations. • If the TDS (and hardness) are low enough to permit other ion ex-

Free Details: Circle 115 on reader service card. October 1998 29