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Article No:a14_393 Article withColor Figures
Ion Exchangers
FRANC¸OISDEDARDEL,RohmandHaas,Paris,France
THOMASV.ARDEN,Cobham,UnitedKingdom
1. Introduction...................... 474 8. IndustrialUseofIonExchange....... 504
2. StructuresofIon-ExchangeResins .... 476 8.1. DescriptionoftheIon-ExchangeCycle . 504
2.1. PolymerMatrices ................. 476 8.2. MethodsforOvercomingEquilibrium
2.2. FunctionalGroups................. 476 Problems........................ 505
2.2.1. Cation-ExchangeResins ............. 476 9. Ion-ExchangeResinCombinations .... 507
2.2.2. Anion-ExchangeResins.............. 477 9.1. Pretreatment..................... 507
2.2.3. OtherTypesofIon-ExchangeResins.... 478 9.2. Softening........................ 507
2.3. AdsorbentResinsandInertPolymers.. 478 9.3. Demineralization(PrimarySystem)..... 507
3. Properties ....................... 479 9.4. Polishing ........................ 509
3.1. DegreeofCross-LinkingandPorosity.. 479 9.5. ChoiceofResin ................... 509
3.2. ExchangeCapacity ................ 480 10. PlantDesign...................... 512
3.3. StabilityandServiceLife............ 481 10.1. GeneralConsiderations............. 512
3.4. Density.......................... 483 10.2. Fixed-BedIon-ExchangeUnits........ 512
3.5. ParticleSize...................... 483 10.2.1. ColumnDiameterandBedDepth....... 512
3.6. MoistureContent.................. 484 10.2.2. Small-ScaleUnits .................. 513
4. Ion-ExchangeReactions............. 484 10.2.3. IndustrialCo-andCounterflowPlants ... 513
4.1. CationExchange.................. 484 10.2.4. MixedBeds....................... 518
4.2. AnionExchange................... 486 10.2.5. OtherIon-ExchangePolishers ......... 521
4.3. CationandAnionExchangeinWater 10.3. ContinuouslyCirculatedIon-Exchange
Treatment ....................... 486 Resins .......................... 521
5. Ion-ExchangeEquilibria............ 486 10.4. ExternalValvesandPipework........ 522
5.1. DissociationandpKValue........... 486 10.5. ControlSystems................... 522
5.2. Mono–MonovalentExchange ....... 488 11. SpecialProcessesinWaterTreatment.. 523
5.3. Mono–DivalentExchange(Water 11.1. RemovalofOrganicMatter.......... 523
Softening)........................ 488 11.2. TreatmentofPotableWater ......... 523
5.4. GeneralCase..................... 491 11.3. TreatmentofBrackishWater........ 526
6. ExchangeKinetics................. 491 11.4. ProcessesInvolvingSeaWater ....... 527
6.1. Principles ....................... 491 11.5. TreatmentofCondensates........... 527
6.2. KineticCurves.................... 493 11.5.1. ConventionalResins ................ 528
6.3. StronglyAcidicorStronglyBasicResins 494 11.5.2. PowderedResins................... 528
6.3.1. FilmDiffusion..................... 494 11.6. WaterTreatmentintheNuclearIndustry 529
6.3.2. ParticleDiffusion .................. 495 11.7. ProductionofUltrapureWater ...... 530
6.4. WeaklyAcidicorWeaklyBasicResins. 495 12. SpecialApplicationsofIonExchange .. 530
7. PracticalConsequencesofIon-Exchange 12.1. ProcessingSteps................... 532
EquilibriumandKinetics............ 495 12.1.1. Purification....................... 532
7.1. OperatingCapacity,Regeneration 12.1.2. IonSubstitution.................... 535
Efficiency,andRegenerantUsage ..... 495 12.1.3. RecoveryandConcentration .......... 536
7.2. PermanentLeakage................ 496 12.1.4. Separation........................ 536
7.3. WaterAnalysis ................... 497 12.1.5. Diffusion......................... 538
7.4. CalculationsintheDesignofIon- 12.1.6. Catalysis ........................ 539
ExchangePlantsforWaterPurification 499 12.1.7. Dehydration ...................... 539
7.5. ExampleofCalculation............. 501 12.1.8. CoalescenceonOleophilicResins ...... 539
7.5.1. Principle......................... 501 12.1.9. LiquidIonExchangers .............. 540
7.5.2. BasicData ....................... 501 12.1.10. Ion-ExchangeMembranes............ 540
7.5.3. DemineralizationUnit............... 502 12.2. TechnicalConsiderations............ 541
7.5.4. PolishingUnit..................... 504 References....................... 543
(cid:1)2012 Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
DOI: 10.1002/14356007.a14_393.pub2
474 IonExchangers Vol.19
Abbreviations andAcronyms: Symbols:
BV: bed volume C : weight capacity, eq/L
P
DG: atmospheric degasser CV: volume capacity, eq/L
DM: drymatter D: diffusion coefficient, mol/cm3
DVB: divinylbenzene J: ionflux,mol s(cid:1)1cm(cid:1)2
EMA: equivalent mineral acidity k: mass-transfer coefficient,cm/s
eq: equivalents K: equilibrium constant
FMA: free mineral acidity KB: selectivitycoeffient between compo-
A
FR: flowrate nents Aand B
IN: inert resin N: fouling factor
MHC: moisture-holding capacity P: weight of resin
R: resin Q: flowrate,m3/h
SAC: stronglyacidic cation exchanger r: radius ofion-exchange bead
SBA: stronglybasicanionexchanger S: salinity, meq/L
SBC: stronglybasiccapacity t: operatingtime,h
TAlk: total alkalinity V: volume ofresin, m3
TDS: total dissolved solids X: mole fraction
TH: total hardness aB: separationfactor between components
A
TOC: total organic carbon Aand B
U.C.: uniformitycoefficient g: activity coefficient
WAC: weakly acidic cation exchanger d: thicknessof Nernst film
WBA: weakly basic anion exchanger
1. Introduction cyanide) can be removed from solution and
replaced bya nontoxic ion.
DefinitionandPrinciples. Inionexchange, 2. Separation. A solution containing a number
ionsofagivencharge(eithercationsoranions)in of different ions passes through a column
a solution are adsorbed on a solid material (the containing beads of an ion-exchange resin.
ion exchanger) and are replaced by equivalent Theionsareseparatedandemergeinorderof
quantities of other ions of the same charge re- their increasing affinityfor the resin.
leased bythe solid. 3. Removal.Byusingacombinationofacation
Theionexchangermaybeasalt,acid,orbase resin(intheHþform)andananionresin(in
in solid form that is insoluble in water but the OH(cid:1) form), all ions are removed and
hydrated. Exchange reactions take place in the replacedbywater(HþOH(cid:1)).Thesolutionis
water retained by the ion exchanger; this is thus demineralized.
generally termed swelling water or gel water.
Thewatercontentoftheapparentlydrymaterial Historical Aspects. The discovery of ion
mayconstitutemorethan50%ofitstotalmass. exchangedatesfromthemiddleofthenineteenth
Figure 1showsthepartialstructureofacation centurywhenTHOMSON[1]andWAY[2]noticed
exchanger; each positive or negative ion is sur- that ammonium sulfate was transformed into
roundedby water molecules. calciumsulfate after percolationthrougha tube
Ionexchangeformsthebasisofalargenum- filled with soil.
berofchemicalprocesseswhichcanbedivided In1905, GANS [3] softenedwaterfor the first
into three main categories: substitution, separa- time by passing it through a column of sodium
tion, andremoval ofions. aluminosilicate that could be regenerated with
sodium chloride solution. In 1935, LIEBKNECHT
1. Substitution.Avaluableion(e.g.,copper)can [4] and SMIT [5] discoveredthat certain types of
berecoveredfromsolutionandreplacedbya coalcouldbesulfonatedtogiveachemicallyand
worthless one. Similarly, a toxic ion (e.g., mechanicallystablecationexchanger.Inaddition,
Vol.19 IonExchangers 475
Figure1. StructureofacationexchangerthatexchangesHþforNaþions
Swellingwaterisrepresentedintheinset.
ADAMSandHOLMES[6]producedthefirstsynthetic predemineralized solution with a demineraliza-
cationandanionexchangersbypolycondensation tion efficiency of99 – 99.99%
of phenol with formaldehyde and a polyamine, Macroporous Resins. Two of the problems
respectively.Demineralizationthenbecamepos- encounteredintheuseofion-exchangeresinsare
sible.Atpresent,aluminosilicatesandphenol – thefoulingoftheresinbynaturalorganicacids
formaldehyderesins are reserved for special ap- present in surface waters and the mechanical
plicationsandsulfonatedcoalhasbeenreplaced stress imposed by plants operating at high flow
bysulfonatedpolystyrene. rates. To cope with these, three manufacturers
Polystyrene Resins. The first polystyrene- [9–11] invented resins with a high degree of
based resin was invented by D’ALELIO in 1944 cross-linkingbutcontainingartificialopenpores
[7].Twoyearslater,MCBURNEYproducedpoly- in the form of channels with diameters up to
styrene anion-exchange resins by chloromethy- 150 nmthatcanadsorblargemolecules.Resins
lationand amination ofthe matrix [8]. inwhichthepolymerisartificiallyexpandedby
Theanionexchangersknownuntilthenwere the addition of a nonpolymerizable compound
weakly basic and took up only strong mineral that is soluble in the monomer are known as
acids.ThenewresinsproducedbytheMcBurney macroporousormacroreticularresins(seeSec-
process were stronger bases and could adsorb tion 3.1). Other naturally porous resins are
weak acids such as carbon dioxide or silica, knownas gel resins.
allowing complete demineralization of water Polyacrylic Anion Exchangers. Between
withapuritypreviouslyobtainableonlybymul- 1970 and 1972, a new type of anion-exchange
tiple distillation in platinum. Even today, ion resin with a polyacrylic matrix appeared on the
exchange is still the only process capable of market.Thispossessesexceptionalresistanceto
producing the water quality needed for high- organic fouling and a very high mechanical
pressureboilers.Reverseosmosisandelectrodi- stability due tothe elasticityofthe polymer.
alysiscandemineralizesolutionswith50 – 90% UniformSizeResins.Inthe1980sand1990s,
efficiency. Only ion exchange can ‘‘polish’’ the severalproducersdevelopednewmanufacturing
476 IonExchangers Vol.19
technologies aimed at producing resins with
particlesof almost identical size.
2. Structures of Ion-Exchange Resins
Anionexchangerconsistsofthepolymermatrix
and the functional groups that interact with the
ions. This article deals only with organic ion
exchangers; inorganic ion exchangers are of
minor importance and are primarily layer sili- Other Types of Matrix. Other types of
cates andzeolites (! Silicates, ! Zeolites). matrix include
1. Phenol – formaldehyde resins (! Phenolic
2.1. Polymer Matrices Resins) which show interesting adsorption
properties
Polystyrene Matrix. (! Polystyrene and 2. Polyalkylamine resins, obtained from poly-
Styrene Copolymers). The polymerization of amines by condensation with epichlorohy-
styrene[100-42-5](vinylbenzene)underthein- drin,whichgivesananionexchangerdirectly
fluence of a catalyst (usually an organic perox- ina single step.
ide)yieldslinearpolystyrene[9003-53-6].Line-
arpolystyreneisaclearmoldableplasticwhichis 2.2. Functional Groups
solubleincertainsolvents(e.g.,styreneortolu-
ene)andhasawell-definedsofteningpoint.Ifa
2.2.1. Cation-ExchangeResins
proportionofdivinylbenzeneismixedwithsty-
rene,theresultantpolymerbecomescross-linked Cation-exchange resins in current use can be
and isthen completelyinsoluble. separated into two classes according to their
active groups:
1. Strongly acidic (sulfonic groups)
2. Weaklyacidic (carboxylic groups)
StronglyAcidicCation-ExchangeResins.
Chemically inert polystyrene beads are treated
withconcentratedsulfuricorchlorosulfonicacid
togivecross-linkedpolystyrene3-sulfonicacid.
This material is the most widely used cation-
exchangeresin and isstrongly acidic.
In the manufacture of ion-exchange resins,
polymerization generally occurs in suspension.
Monomerdropletsareformedinwaterand,upon
completion of the polymerization process, be-
come hard spherical beads ofthe polymer.
Polyacrylic Matrix. Matrices for ion ex-
changers can also be obtained by polymerizing
an acrylate, a methacrylate, or an acrylonitrile,
any of which can be cross-linked with divinyl-
benzene[105-06-6](DVB)(! Polyacrylamides
and Poly(Acrylic Acids)); ! Polyacrylates, Examples: Amberlite IR 120, Dowex HCR,
Section 3.1. Lewatit S100.
Vol.19 IonExchangers 477
Weakly Acidic Carboxylic Cation-Ex- dimethylethanolammoniumgroupsareknownas
change Resins. The weakly acidic resins are type 2 and are slightly lessbasic.
almost always obtained by hydrolysis of poly- Type 1resinsareusedwhentotalremovalof
methylacrylateorpolyacrylonitriletogiveapoly anions, even those of weak acids (including
(acrylic acid) matrix. silica), isessential. Type 2 resins are also basic
enoughtoremoveallanions,buttheyreleasethe
anions more easily during regeneration with
caustic soda; as a result, they have a high ex-
change capacity and a better regeneration effi-
Examples:AmberliteIRC86,LewatitCNP. ciency(seeSection 7.1).Unfortunately,theyare
chemicallylessstableandproducegreatersilica
leakage than type 1 resins.
2.2.2. Anion-Exchange Resins Resins whose active group is an amine are
generallydenotedasweaklybasic,althoughtheir
Polystyrene Materials. Cross-linked poly- basicitymayvaryconsiderably.Tertiaryamines
styrenebeadsaretreatedwithchloromethylmeth- are sometimes called medium-base or interme-
yletherunderanhydrousconditions,witheither diate-base resins, whereas primary amines are
aluminumchlorideortin(IV)chlorideascatalyst. very weaklybasic and are rarely used.
Chloromethylatedpolystyreneisobtained: The most widely used weakly basic resins
contain tertiary amino groups and adsorb any
strongacidspresentinthesolutiontobetreated
butdo notaffect neutral salts or weak acids.
Manufacturersdonotalwaysindicatethechem-
icalstructureoftheirexchangersintheirliterature.
Careshouldthereforebetakennottoassumethat
In a second stage, the chlorine in the chloro- resins are chemically identical merely because
methylatedgroupcanbereplacedbyanamineor theyhavesimilargeneralcharacteristics.
even by ammonia. Depending on the reaction se-
lected,theanionexchangerobtainedmaybestrong- Secondary and Tertiary Cross-Linking.
ly to weakly basic. The degree of basicity can be During chloromethylation, a side reaction may
‘‘madetomeasure’’becauseofthelargenumberof occur in which the chloromethyl group of a
aminesavailable.Theanionexchangerslistedbe- chloromethylated benzene ring reacts with an
lowarearrangedinorderofdecreasingbasicity: unconverted ring, to yield a methylene bridge.
Thesebridgesformadditionalcross-linksinthe
polystyrene matrix:
where Rcan be
(cid:1)CHNþ(CH)Cl(cid:1) e.g.,AmberliteIRA402
2 33
(type1resin)
(cid:1)CHNþ(CH)CHCHOHCl(cid:1) e.g.,AmberliteIRA410
2 32 2 2
(type2resin)
(cid:1)CHN(CH) e.g.,AmberliteIRA96
2 32
The amount of this secondary cross-linking
canbeadjustedbyvaryingtheconditions(quan-
Resinswithquaternaryammoniumgroupsare tity and type of catalyst, temperature) of the
strongly basic. Those with benzyltrimethylam- chloromethylationreaction.Moststronglybasic
moniumgroupsareknownastype 1andarethe and weakly basic polystyrene resins have some
moststronglybasic,whereasthosewithbenzyl- degreeof secondary cross-linking.
478 IonExchangers Vol.19
Furthermore,duringtheaminationofweakly chosenasthestartingmaterialandthepolyamine
basicresins,anothertypeofcross-linkingmaybe usedforactivation.Inpractice,therangeislimited
produced. This is called tertiary cross-linking bytheavailabilityandcostofrawmaterials.
and yields strongly basic quaternary groups in
addition to the weakly basictertiary groups.
2.2.3. OtherTypesofIon-ExchangeResins
Byusingpolymerizationandactivationmethods
analogoustothosedescribedabove,awidevari-
ety of functional groups can be grafted onto a
givenpolymer.Someofthesegroupscanbeused
for selective uptake of ions, principally metals
(Table 1).
The thiol group forms very stable bonds with
certain metals, particularly mercury. The imino-
diacetic, aminophosphonic, and amidoxime
groupsformmetalcomplexeswhosestabilityde-
Polyacrylic Resins. Polyacrylic resins are pendsmainlyonthepHofthesolution.Selective
manufacturedinamanneranalogoustothatused adsorptionofcertainmetalscanthusbeachieved
forpolystyreneresins.Beadsarepreparedfroman by varying the pH. These types of material are
acrylic ester copolymerized with divinylbenzene knownaschelatingorcomplexingresins.
byusingsuspensionpolymerizationandfree-radi- The N-methylglucamino group is used to
calcatalysis.Thepolyacrylateformedisthengiven makeresinsspecificforboricacid,whichistaken
active groups by reaction with a polyfunctional up asa complex.
aminecontainingatleastoneprimaryaminogroup
and one secondary or, more frequently, tertiary
aminogroup.Theprimaryaminogroupreactswith 2.3. Adsorbent Resins and Inert
thepolyestertoforman amide,whereas thesec- Polymers
ondary or tertiary amino group forms the active
groupoftheanionexchanger.Thismethodalways Strictly speaking, adsorbent resins are not ion
yields a weakly basic exchanger, which can be exchangersbutresemblethemveryclosely.They
further treated with chloromethane or dimethyl haveahighporosityandareusedfortheadsorp-
sulfatetogiveaquaternarystronglybasicresin: tion of nonionic or weakly ionized species as a
complement to ion exchange. They may have
cation- or anion-exchange groups or no ion-
Table1.Principalactivegroupsofionexchangersusedforselective
uptakeofmetals
Activegroup Formula* Example
Thiol (cid:1)(cid:1)SH AmbersepGT74
Iminodiaceticacid (cid:1)(cid:1)CHN(CHCOOH) LewatitTP207
2 2 2
Aminophosphonic (cid:1)(cid:1)CHNHCHCHPOH Amberlite
2 2 2 3
acid IRA747
Amidoxime DuoliteES346
N-Methylglucamine Amberlite
IRA743
*Theactivegroupsaresubstituents(R)ofpolystyrenewiththe
followingformula:
In principle, a wide range of anion-exchange
resinscanbeobtainedbyvaryingthetypeofester
Vol.19 IonExchangers 479
exchange groups at all. The latter are ionically dense,ionicmotionissloweddown,thusreducing
inert. In order of decreasing polarity, adsorbent theoperatingcapacityoftheresin.
resinscanbeclassifiedinthefollowingmanner: For sulfonic resins, maximum operating ca-
pacity (Section 3.2) is obtained with approxi-
1. Ionizedadsorbentsarestronglybasicexchan- mately 8%DVB.
gersusedinchlorideformforcolorremoval
fromsugarjuicesoras‘‘organicscavengers’’ Cross-LinkingandAffinity. Thegreaterthe
(see Section 11.1, e.g.,Amberlite IRA958). ionicmobilityintheresin,thepooreristhediffer-
2. Phenolic adsorbents contain weakly basic entiation between the adsorption of ionic species
amine and phenolic groups or phenolic withthesamecharge.Consequently,thedegreeof
groups,only. They areusedtoremove color cross-linkingintheresinmustbeincreasedwhen
bodies(coloredimpurities)fromsolutionsof greaterdifferencesinionicaffinityarerequired.
organicacidsandfood-processingstreams(e. Inwatertreatment,thesulfonatedpolystyrene
g., Duolite A561, XAD761). resins usually have a DVB concentration of ca.
3. Inert adsorbents are macroporous copoly- 8%. Resins with a slightly higher degree of
mers of styrene and divinylbenzene with a cross-linking(10 – 12%)aresometimesusedto
veryhighdegreeofcross-linkingandalarge increasetheretentionofmineralionswhenwater
surface-to-volumeratio.Theseresinsareused of very high purity isbeing produced.
to remove organic, weakly ionized, or non-
ionicsubstances,suchasphenols,chlorinated NonuniformityintheMatrix. Cross-linking
solvents,antibiotics,andcomplexingagents, reduces the retention of water in ion-exchange
fromaqueousororganicsolutions(e.g.,Am- resins(Section 3.6).Thevolumeoccupiedbythis
berlite XAD4,Diaion HP20). waterisameasureoftheresin’sporosity.Cross-
linking is not uniform because the DVB – DVB
Inert polymers without measurable porosity reactionismorerapidthanthatbetweenDVBand
and without active groups can be used either to styrene. Polymerization begins to occur around
separate two resin layers or to keep a resin the catalyst molecules, and polymer growth is
separatefrom a collector system. faster at sites rich in DVB than at those rich in
styrene.Materialwithanaverageof8%DVBmay
containlocalmicroscopicregionswithmorethan
3. Properties 20%DVB,whereasotherregionsmayhaveless
than4%.
3.1. Degree of Cross-Linking and
Porosity Macroporous resins are made by mixing
the monomers with a compound (e.g., heptane,
Anincreaseinthedegreeofcross-linking(i.e.,the saturatedfattyacids,C – C alcoholsorpoly-
4 10
weight percentage of DVB related to the total alcohols,or lowmolecular masslinear polysty-
amount of monomer prior to polymerization) rene) which expands the resin. The substance
produces harder, less elastic resins. Resins with doesnotitselfpolymerizeand,thus,althoughit
higherdegreesofcross-linkingshowmoreresis- actsasasolventforthemonomers,itcausesthe
tancetooxidizingconditionsthattendtode-cross- polymer toprecipitate fromthe liquid.
link the polymer. Above 10 – 12% DVB, how- Channels are formed inside the beads, pro-
ever, the structure becomes too hard and dense. ducing an artificially high porosity. Resins
Activation (i.e., chemical transformation of the containingsuchchannelsaredescribedasmacro-
inert copolymer into an ion-exchange resin) be- porous,whereasotherresinswithnaturalporosi-
comesdifficultbecauseaccesstotheinteriorofthe tyare known asgel resins(Fig. 2).
beadishinderedbythehighdensityofthematrix. Macroporous resins have a higher degree of
Inaddition,osmoticstresscannotbeabsorbedby cross-linking than gel resins to strengthen the
the elasticity of the structure, which causes the matrix and compensate for voids left by the
bead to shatter. Finally, the rate of exchange added solvent. The porosity and mechanical
increasesinproportiontothemobilityoftheions strengthoftheresincanbemodifiedbyvarying
insidethe exchangerbead: ifthe structure istoo the degree of cross-linking or the amount of
480 IonExchangers Vol.19
Table2.Typicalcapacitiesofion-exchangeresins*
Type Amberlite Cp,eq/kg Cv,eq/L
Stronglyacidicgel, IR120 4.5(Na) 2.05(Na)
8%DVB
Weaklyacidicgel IRC86 11.0(H) 4.2(H)
Stronglybasic,type1 IRA402 3.9(Cl) 1.3(Cl)
Stronglybasic,type2 IRA410 3.6(Cl) 1.3(Cl)
Stronglybasicacrylic IRA458 4.2(Cl) 1.3(Cl)
Weaklybasicstyrene IRA96 4.7(freebase) 1.25(freebase)
Figure 2. Arrangement of structural units in gel (A) and Weaklybasicacrylic IRA67 5.6(freebase) 1.6(freebase)
macroporous(B)resins
*Valuesdependtosomeextentontheanalyticalmethodusedfor
capacitydetermination.Macroporousstronglyacidicresinshave
solvent added. Therefore, various macroporous
adryweightcapacityof4.3–4.5eq/kg.Theirvolumecapacity
resins are available, with different moisture- dependsgreatlyontheirporosity,whichcanbeadjustedwithin
holding capacitiesand internal structures. relativelybroadlimits.Similarly,macroporousstrongbase
Theporediameterisca.100 nminamacro- resinshavedryweightcapacitiesclosetotheirgelcounterparts,
butgenerallylowervolumecapacities.
porous resin and ca. 1 nm in a gel resin. The
macropores form a network of channels filled Table 2. Equivalent resins of other brands have
with free water, and large molecules can move similarcapacityvalues(seeTable10).
freelyintheresinintothecenterofabead.Once
inside the resin, ions generally have a much OperatingCapacity. Theoperatingcapacity
shorter distance to travel before they encounter isdefinedastheproportionoftotalcapacityused
an active group: ca. 100 nm in macroporous during the exchange process. It can amount to a
resinsandupto500 mmingelresins.Exchange largeorsmallproportionofthetotalcapacityand
isthus faster in amacroporous resin. dependsonanumberofprocessvariablesincluding
Macroporousresinsarehighlyresistanttophys-
icalstressandgenerallywithstandosmoticshock 1. Concentrationandtypeofionstobeabsorbed
very well. They are therefore used in systems 2. Rate of percolation
where mechanical and osmotic stress would oth- 3. Temperature
erwisecausegelresinstodeterioraterapidly,such 4. Depthofresin bed
as those involving circulation of resin, fluidized 5. Type,concentration,andquantityofregenerant
beds, high flow rates, oxidizing conditions, con-
centratedsolutions,andshortcycles. Inapackedcolumn,reactionbetweentheionsin
Finally, macroporous resins are used when solutionandthoseintheresinoccursoverawell-
reversibleuptakeoflargemoleculesisnecessary, defined region of the resin bed known as the re-
without fouling the resin. The adsorbents de- action zone. When the selectivity of a resin for a
scribedinSection 2.3haveamacroporositythat dissolved ion is high (see Chap. 5), a sharp ex-
allowsselectiveretentionofvariousmolecules. change wavefront is formed which moves toward
the column outlet, making maximum use of the
resin. The depth of the reaction zone depends on
3.2. Exchange Capacity factors such as flow rate (kinetics) and ionic con-
centration,butisindependentofcolumnlength[12].
TotalCapacity. Thetotalexchangecapacity When selectivity is low, a diffuse wavefront
ofaresin,expressedinequivalentsperunitweight develops. The depth of the reaction zone then
(or per unit volume), represents the number of depends on the selectivity coefficient (Sec-
active sites available. In polystyrene exchangers, tion 5.2) and also on the bed depth. The longer
themaximumnumberofactivesitescorresponds the column, the deeper is the reaction zone and
tothe‘‘grafting’’ofoneactivegroupperbenzene thegreateristheoperatingcapacityoftheresin.
ring.Thecapacityisexpressedinequivalents(eq) Figure 3 illustrates this: the top of the column
perkilogramofdryresin(theweightcapacityCp) contains the completely exhausted resins (a),
or equivalents per liter of wet settled resin (the whereas the reaction zone (b) containspartially
volume capacity Cv). Total capacity values for exhausted resin. The time at which the lowest
some of the most common resins are given in point of this zone reaches the bottom of the
Vol.19 IonExchangers 481
oxidants.However,1 mg/kgofchlorineoxidizes
thepolymerataratedependentontemperature;
this breaks down the cross-linking, releases
sulfonated organic compounds and causes the
resintoswelluntilitsoftens,resultinginexces-
siveheadloss(sloughage)[13].Whenoxidizing
agents are present, highly cross-linked resins
with a greater resistance to oxidation, such as
the macroporous resins, shouldbe used.
Degradationproductsfromacation-exchange
resin may foul anion resins [14, 15]. This is
particularlycriticalinprocessesdesignedtopro-
duceultrapurewater(Fig. 4).Oxidantsbreakthe
cross-linkstoproducesoluble,short-chainoligo-
Figure3. Reactionzoneinaresincolumnduringpercolation mers that can be measured as the total organic
a)Exhaustedresin;b)Reactionzone;c)Regeneratedresin carbon(TOC)inthetreatedwater(Fig. 4 A).The
column (i.e., when the adsorbed ions break
throughthebottomofthebed)isgenerallytaken
to be the time at which the service phase is
complete. A proportion of the resin is still not
exhausted at the time of breakthrough.
Inpractice,stronglyacidicandstronglybasic
resinsarenever100%regeneratedatthebegin-
ning of a cycle. The operating capacity thus
represents the difference between the available
capacity at the beginning of a cycle and that
remaining at the endpoint.Themostimportant
factoristheamountofregenerantusedtoconvert
theresintotheregeneratedformrequiredatthe
beginning of the service cycle (c). The calcula-
tion ofcapacity isdescribedinChapter 7.
3.3. Stability and Service Life
Because ion-exchange resins must give several
yearsofservice,theirstabilityoverlongperiods
of time isofprime importance.
Chemical Stability of the Matrix. Indus-
trially available resins have a degree of cross-
linking high enough to make them insoluble. A
newresinmayreleaseminutequantitiesofshort-
chain polymers orothersoluble substances,but
this effect isshort-lived. Figure4. Suitabilityofresinsforproducingultrapurewater
Highly oxidizing conditions (presence of inmixedbeds[14]
Resinsweretestedatambienttemperatureduringfirstchlo-
chlorine or chromic acid) can attack the matrix
rine exposure by using an influent with 0.30mg/L active
anddestroycross-linking.Asulfonatedpolysty-
chlorinedosedasNaOCland0.02–0.03mg/LTOC.
renecation-exchangeresinwith8%DVBcross- Resins: a) Gel cation–porousgel anion; b) Gel cation–
linkingwithstands0.2 mg/kgofchlorineatam- standard gel anion; c) Macroporous cation–porous gel
anion;d)Macroporouscation–standardgelanion;e)Macro-
bient temperature for several years and is also
porouscation–macroporousanion;f)Macroporouscation–
completely stable at 120 (cid:2)C in the absence of
developmentalanion;g)Gelcation–developmentalanion
482 IonExchangers Vol.19
oligomers bear ionized active groups and thus Type 2 resins are more liable to undergo
decrease the resistivity of the treated water Hofmanndegradationbecausethehydroxyethyl
(Fig. 4B).Undernormalconditionsofwatertreat- group weakens the bond:
ment, resins can operate continuously for many
years(sometimesupto20years)withoutdeterio-
rationoftheirphysicalandchemicalproperties.
Thermal Stability of Active Groups. [16]
The sulfonic group of cation-exchange resins is
extremely stable. Anion-exchange resins, on the
otherhand,aretemperature-sensitive.Whenheat-
ed, Hofmann degradation may transform quater-
nary ammonium groups (strongly basic) into ter-
tiary amines (weakly basic) or even destroy the
active group completely. Because this reaction
occursunderalkalineconditions,anionexchangers
aremorestableintheformofasaltthanasabase. At 15 (cid:2)C these resins lose ca. 50% of their
Strongly basic type 1 resins are the most strongly basic groups in five years; the same
stable; the Hofmann degradation reaction be- effectoccursinaboutoneyearat50 (cid:2)C.(Aresin
comessignificantonlyabove50 (cid:2)C(Fig. 5).At inwhich50%ofthestronglybasicgroupshave
ambienttemperature,theseresinscanlastforfive been converted into weak groups, often has an
toseven years ormore. unchangedoperatingcapacitybecausetheweak-
ly basic groups are fully capable of taking up
mineral acids).
Mechanical Stability. Polycondensation-
type resins that are manufactured in bulk and
brokenupintoirregulargrainsarecomparatively
fragileandusedonlyinfixedbeds(Section 10.2).
Polystyreneandpolyacrylicresinsmadebysus-
pension polymerization are perfect spheres and
suffer little damage when used in continuous
moving-bedion-exchangeplants.However,me-
chanicalstrengthcanvaryconsiderablyfromone
producttoanother,andresinbeadswhichareseen
to have many internal cracks under the micro-
scopearemorelikelytobreakundermechanical
stressthancrack-freeproducts.
Gel-type anion resins are generally weaker
than cation materials and are particularly poor
at withstanding compression. However, new
polymerizationtechniquesproduceamoreuni-
formly structured polystyrene matrix. As a re-
sult, gel resins with high physical stability are
now available. To prevent fragile resins from
breaking,itisimportanttokeepthebedcleanby
frequent backwashing because the water pres-
sureonalayeroffinedebriscanbeasmuchas
60 kPa.
Figure5. Half-lifeofAmberliteIRA402asafunctionof Acrylicresinsaremoreelasticthanpolystyrene
temperature
materialsandcannormallywithstandanymechan-
Half-lifeisthecontacttimeofresinwithhotsolutionrequired
toreducetotalcapacityby50%. ical stress encountered in practice. Macroporous