Ion Exchange and Solvent Extraction A Series of Advances Volume 16 edited by Arup K. SenGupta Lehigh University Bethlehem, Pennsylvania, U.S. A. Yizhak Marcus The Hebrew University of Jerusalem Jerusalem, Israel Jacob A. Marinsky Founding Editor M A R C E L MARCEL DEKKER, INC. D E K K E R NEW YORK BASEL Although great care has been taken to provide accurate and current information, neither theauthor(s) nor the publisher, nor anyone else associated with this publication, shall be liablefor any loss, damage, or liability directly or indirectly caused or alleged to be caused by thisbook. The material contained herein is not intended to provide specic advice or recom-mendations for any specic situation.Trademark notice: Product or corporate names may be trademarks or registered trademarksand are used only for identication and explanation without intent to infringe.Library of Congress Cataloging-in-Publication DataA catalog record for this book is available from the Library of Congress.ISBN: 0-8247-5489-1This book is printed on acid-free paper.HeadquartersMarcel Dekker, Inc., 270 Madison Avenue, New York, NY 10016, U.S.A.tel: 212-696-9000; fax: 212-685-4540Distribution and Customer ServiceMarcel Dekker, Inc., Cimarron Road, Monticello, New York 12701, U.S.A.tel: 800-228-1160; fax: 845-796-1772Eastern Hemisphere DistributionMarcel Dekker AG, Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerlandtel: 41-61-260-6300; fax: 41-61-260-6333World Wide Webhttp:==www.dekker.comThe publisher offers discounts on this book when ordered in bulk quantities. For more infor-mation, write to Special Sales=Professional Marketing at the headquarters address above.Copyright # 2004 by Marcel Dekker, Inc. All Rights Reserved.Neither this book nor any part may be reproduced or transmitted in any form or by anymeans, electronic or mechanical, including photocopying, microlming, and recording, orby any information storage and retrieval system, without permission in writing from thepublisher.Current printing (last digit):10 9 8 7 6 5 4 3 2 1PRINTED IN THE UNITED STATES OF AMERICAPrefaceIn July 2003, one of us (A.S.) attended the Third International Conference on IonExchange (ICIE 03) at the Kanazawa Institute of Technology in Japan. During thecourse of the conference, about 75 oral and 80 poster presentations from research-ers around the globe were made on diverse aspects of ion exchange. These presenta-tions covered a wide range of areas, namely, drug delivery, real-time sensing,analytical chromatography, catalysis, polymer and inorganic syntheses, bioseparation,biomembranes, and, obviously, water and wastewater treatment. The breadth ofand synergy among many emerging areas with ion exchange as the common linkwere indeed mind boggling. This conference and many other ongoing activitiesin seemingly disjointed elds clearly demonstrate the continuing advances in theeld of ion exchange. These observations truly bolstered our earlier decision tocontinue with the series of advances in ion exchange and solvent extraction. Afterpublication of volume 14, A.S. collected informal feedback about the size andcontent of each volume. The general consensus was that about six or seven com-prehensive chapters covering subject areas that include both fundamentals andpotentials for future applications are highly desirable. This volume contains sixchapters encompassing a wide gamut of topics; they truly reect the diversity inthe eld of ion exchange.It is true that synthetic polymer-based materials constitute the majority of theion-exchange market and this trend is unlikely to change in the near future. Never-theless, many naturally occurring, biorenewable inexpensive materials exhibit ion-exchange properties resulting from the presence of a variety of chemical functionalgroups. Chapter 1 provides comprehensive coverage of how activated carbons andother carbonaceous materials can be engineered to remove metal ions and organicmicropollutants from water. Engineered activated carbons are quite effective iniiireducing trace toxic metals such as lead and mercury in water as well as syntheticorganic compounds such as PCBs and MTBEs. New technologies are underwaythat will help convert biomass and other carbonaceous wastes into activated car-bon. In this regard, the possibility of controlling the carbon pore structure, whichin turn will make it possible to control the selectivity and sorption capacity of targetcontaminants, offers challenging application opportunities. Engineered activatedcarbons and carbonaceous materials are also likely to be able to remove traceamounts of newly emerging organic pollutants, such as antibiotics and other drugsfound in our surface waters.In general, ion-exchange type favorable sorption processes tend to beexothermic; i.e., the overall enthalpy change for the reaction is negative. Widelyavailable ion-exchange sorption data attest to this premise. Ion-exchange behaviorsof hydrophobic ionizable organic compounds, or HIOCs, are, however, somewhatcounterintuitive because they tend to be endothermic. Many industrially signicantsynthetic organic compounds, such as pentachlorophenate, benzenesulfonates,naphthalene sulfonates, and quaternary ammonium compounds fall into this cate-gory of HIOCs. Chapter 2 presents the favorable sorption behaviors of severalenvironmentally signicant aromatic anions, e.g., pentachlorophenate, chlorophe-nate, and benzene and naphthalenesulfonates, onto polymeric anion exchangers.Such favorable sorption equilibria are distinctively unique because they are allendothermic processes and are accompanied by highly positive entropy changes.Experimental data validate that the sorption of HIOCs onto ion-exchangers followsan ion-exchange stoichiometry; i.e., sorption of an aromatic anion is always accom-panied by the desorption of an equivalent amount of other anions from the ion-exchanger phase. However, the ion-exchange selectivity is determined by concurrenthydrophobic interactions, which are further inuenced by the cosolvent polarityand hydrophobicity of the ion-exchanger matrix. The chapter provides a series ofexperimentally determined values of enthalpic and entropic changes for variousHIOCs and different anion exchangers in support of the proposedsorption=desorption mechanism. In addition, the chapter includes an efcientregeneration methodology for desorption of HIOCs with high sorption afnities.During an ion-exchange process, be it during the sorption step or in theregeneration cycle, the ion of interest can interact with the counter-ion of the initialionic form of the resin or with the coion of the displacing agent. In both cases thisinteraction may result in the formation of low-solubility substances whoseconcentration exceeds their solubility at a given temperature. Moreover, this super-saturated solution (SS) may remain stable within the column interstitial space for along period of time; this phenomenon is referred to as ion-exchange isothermalsupersaturation (IXISS). The phenomenon of IXISS can be exploited to overcomethe apparent shortcomings of many ion-exchange separation processes in the areasof product purication and improved efciency of regeneration. Chapter 3provides a comprehensive treatment covering all aspects of IXISS, namely, theiv Prefacethermodynamics and kinetics of supersaturation in the presence of ion-exchangerbeads, and a host of application opportunities for bioseparation, desalination,and inorganic salt syntheses.Selective removal of heavy or toxic metals from contaminated water andwastewater streams has been the subject of innumerable studies around the world.Ion-exchange resins with chelating or appropriate selective functional groups havecome a long way in attaining this goal. However, current existing ion-exchangetechnologies are not capable of separating individual heavy metals leading to theirpurication and reuse. Such separation processes are truly challenging and para-metric pumping attempts to offer a rational approach toward attaining such a goal.Simply put, parametric pumping is a technique in which a process parameter isintentionally adjusted for the desirable separation. The majority of the previousinvestigations dwelled on using temperature as the adjustable process parameter.Chapter 4 elaborately discusses the potential of a pH-driven parametric pumpingwith judicious combination of water-soluble ligands and commercially availableion-exchange resins. Included in the chapter is a process ow schematic with alaboratory-based experimental set-up, a mathematical framework based on stage-wise equilibrium, and the results for separation of copper and nickel and other toxicmetal cations.Industrialization and use of synthetic chemicals have on many occasionsbrought unpleasant environmental surprises to communities, especially in deve-loped countries. The nding of perchlorate in hundreds of groundwater wells inwestern states of the United States is a serious environmental concern that needsto be dealt with immediately. Although hot and concentrated perchloric acid isan extremely strong oxidizing solution, in dilute concentrations the perchlorateion is stable and extremely nonreactive. These properties greatly facilitate its trans-port in groundwater. For drinking-water production from contaminated wells,ion-exchange has been identied as the best potential treatment process. Chapter5 provides a detailed account of the effects of all important process variables onperchlorate removal. Specically, the chapter includes the following: (1) effectsof matrix, functional group, and crosslinking of anion-exchange resins on perchlo-rate removal and the efciency of regeneration, (2) the effect of temperature onboth the equilibrium and the kinetics of perchlorate uptake, and (3) validation ofequilibrium multicomponent theory in predicting perchlorate breakthroughs andchloride regeneration using experimental data.Ultrapure water containing water with no dissolved solutes is an essentialingredient for electrical utilities and microelectronic industries. The allowableelectrolyte concentration is often less than 20 parts per billion NaCl equivalent.The production of ultrapure water is a complex multistep process in which the lastunit operation invariably involves an ion exchange system containing a mixed bedof cation- and anion-exchange resins. Both the equilibrium and the kinetics of themixed-bed systems are distinctively unique compared to the single cation or anionPreface vexchange units with high electrolyte concentration in the feed. Chapter 6 providescomprehensive coverage of the kinetics of the mixed-bed ion-exchange processes.Since the electrolyte concentrations are extremely low, liquid lm diffusion andwater dissociation greatly inuence the overall mass transfer coefcient. Thechapter discusses the development of theoretical models using transportcorrelations and stresses the importance of validating individual models with actualphysical performance of the mixed-bed systems.Arup K. SenGuptaYizhak Marcusvi PrefaceContributors to Volume 16Christian Bartosch Lurgi Oel Gas Chemie, Frankfurt, GermanyDennis A. Clifford Department of Civil and Environmental Engineering,University of Houston, Houston, Texas, U.S.A.Gary L. Foutch School of Chemical Engineering, Oklahoma State University,Stillwater, Oklahoma, U.S.A.Wolfgang H. Holl Institute of Technical Chemistry, ForschungszentrumKarlsruhe, Karlsruhe, GermanyDennis F. Hussey iSagacity, Inc., Half Moon Bay, California, U.S.A.Ruslan Khamizov Vernadsky Institute of Geochemistry and AnalyticalChemistry, Moscow, RussiaRandolf Kiefer Krupp-Uhde GmbH, Dortmund, GermanyPing Li Department of Civil and Environmental Engineering, Lehigh University,Bethlehem, Pennsylvania, U.S.A.Danish J. Malik Department of Chemical Engineering, LoughboroughUniversity, Loughborough, Leicestershire, United KingdomviiDmitri N. Muraviev Department of Chemistry, Autonomous University ofBarcelona, Barcelona, SpainBasudeb Saha Department of Chemical Engineering, Loughborough University,Loughborough, Leicestershire, United KingdomArup K. SenGupta Department of Civil and Environmental Engineering, LehighUniversity, Bethlehem, Pennsylvania, U.S.A.Cornelia Stohr Sartorius AG, Gottingen, GermanyMichael Streat Department of Chemical Engineering, LoughboroughUniversity, Loughborough, Leicestershire, United KingdomAnthony R. Tripp Department of Civil and Environmental Engineering,University of Houston, Houston, Texas, U.S.A.viii Contributors to Volume 16ContentsPreface iiiContributors to Volume 16 viiContents of Other Volumes xiii1. Adsorption and Ion-Exchange Properties of EngineeredActivated Carbons and Carbonaceous Materials 1Michael Streat, Danish J. Malik, and Basudeb SahaI. Introduction 1II. Preparation and Properties of Activated Carbon 4III. Characterization of the Surface Chemical Groups inEngineered Carbons 20IV. Activated Carbon Fibers and Woven Cloths 32V. Sorption of Trace Metals onto Activated Carbon 35VI. Summary of Metal Sorption 56VII. Sorption of Herbicides on Activated Carbon andHypercross-Linked Polymers 57VIII. Summary of Herbicide Sorption on Activated Carbonsand Hypercross-Linked Polymers 77IX. Future Trends and Concluding Remarks 78References 812. Entropy-Driven Selective Ion Exchange for HydrophobicIonizable Organic Compounds (HIOCs) 85Ping Li and Arup K. SenGuptaixI. Introduction 85II. Nature of SoluteSorbent and SoluteSolvent Interactions 87III. Experimental Section 91IV. Results and Discussion 95V. Conclusions 116References 1173. Ion-Exchange Isothermal Supersaturation: Concept, Problems,and Applications 119Dmitri N. Muraviev and Ruslan KhamizovI. Introduction 119II. Main Features of the IXISS Technique 120III. Areas of Potential Application of the IXISSEffectWasteless Ion-Exchange Processes 122IV. IXISS of Zwitterlyte Solutions 125V. Aminecarboxylate Interaction of Zwitterlyte Molecules 138VI. Application of IXISS of Amino Acids 160VII. IXISS of Inorganic Substances 165VIII. IXISS-Based Green Ion Exchange Technologies 179IX. Concluding Remarks 203References 2054. Metal Separation by pH-Driven Parametric Pumping 211Wolfgang H. Holl, Randolf Kiefer, Cornelia Stohr, and Christian BartoschI. Introduction 211II. Basic Principles of pH-Induced Parametric Pumping 212III. Development of pH-Induced Parametric Pumping 216IV. Experimental Work 222V. Application of Strongly Acidic Cation Exchangers 225VI. Separation with Weakly Basic Anion Exchangers 244VII. Summary and Outlook 261List of Symbols 263References 2635. Selectivity Considerations in Modeling the Treatment ofPerchlorate Using Ion-Exchange Processes 267Anthony R. Tripp and Dennis A. CliffordI. Introduction 267II. Results and Discussion 274x ContentsIII. Computer Modeling 302IV. Process Recommendations 331V. Summary and Conclusions 333References 3366. Ion-Exchange Kinetics for Ultrapure Water 339Dennis F. Hussey and Gary L. FoutchI. Introduction 339II. Mass Transfer CoefcientA Method to EvaluateResin Effectiveness 344III. Modeling Approach to UPW Kinetics 355IV. Concluding Comments 369Nomenclature 370References 372Index 375Contents xiContents of Other VolumesVolumes 14, 6 out of printVolume 5NEW INORGANIC ION EXCHANGERS A. Cleareld, G. H. Nancollas,and R. H. BlessingAPPLICATION OF ION EXCHANGE TO ELEMENT SEPARATIONAND ANALYSIS F. W. E. StrelowPELLICULAR ION EXCHANGE RESINS IN CHROMATOGRAPHYCsaba HorvathVolume 7INTERPHASE MASS TRANSFER RATES OF CHEMICAL REACTIONSWITH CROSSLINKED POLYSTYRENE Gabriella Schmuckler andShimon GoldsteinINFLUENCE OF POLYMERIC MATRIX STRUCTURE ONPERFORMANCE OF ION-EXCHANGE RESINS V. A. Davankov,S. V. Rogozhin, and M. P. TsyurupaxiiiSPECTROSCOPIC STUDIES OF ION EXCHANGERS Carla Heitner-WirguinION-EXCHANGE MATERIALS IN NATURAL WATER SYSTEMSMichael M. ReddyTHE THERMAL REGENERATION OF ION-EXCHANGE RESINSB. A. Bolto and D. E. WeissVolume 8METAL EXTRACTION WITH HYDROXYOXIMES Richard J. Whewelland Carl HansonELECTRICAL PHENOMENA IN SOLVENT EXTRACTIONGiancarlo Scibona, Pier Roberto Dansei, and Claudio FabianiEXTRACTION WITH SOLVENT-IMPREGNATED RESINSAbraham WarshawskySOLVENT EXTRACTION OF ELEMENTS OF THE PLATINUM GROUPLev M. GindinSOLVENT EXTRACTION FROM AQUEOUS-ORGANIC MEDIAJiri HalaVolume 9ION-EXCHANGE PROCESSES USED IN THE PRODUCTION OFULTRAPURE WATER REQUIRED IN FOSSIL FUEL POWERPLANTS Calvin CalmonA SYSTEMATIC APPROACH TO REACTIVE ION EXCHANGEGilbert E. Janauer, Robert E. Gibbons, Jr., and William E. BernierION-EXCHANGE KINETICS IN SELECTIVE SYSTEMS Lorenzo Libertiand Roberto PassinoSORPTION AND CHROMATOGRAPHY OF ORGANIC IONSG. V. Samsonov and G. E. Elkinxiv Contents of Other VolumesTHERMODYNAMICS OF WATER SORPTION OF DOWEX 1OF DIFFERENT CROSSLINKING AND IONIC FORM Zoya I. Sosinovich,Larissa V. Novitskaya, Vladimir S. Soldatov, and Erik HogfeldtDOUBLE-LAYER IONIC ADSORPTION AND EXCHANGE ON POROUSPOLYMERS Frederick F. CantwellHUMIC-TRACE METAL ION EQUILIBRIA IN NATURAL WATERSDonald S. Gamble, Jacob A. Marinsky, and Cooper H. LangfordVolume 10SOLVENT EXTRACTION OF INDUSTRIAL ORGANIC SUBSTANCESFROM AQUEOUS STREAMS C. Judson King and John J. SenetarLIQUID MEMBRANES Richard D. Noble, J. Douglas Way, and Annett L. BungeMIXED SOLVENTS IN GAS EXTRACTION AND RELATEDPROCESSES Gerd BrunnerINTERFACIAL PHENOMENA IN SOLVENT EXTRACTIONValery V. Tarasov and Gennady A. YagodinSYNERGIC EXTRACTIONS OF ZIRCONIUM (IV) AND HAFNIUM (IV)Jiri HalaVolume 11CHEMICAL THERMODYNAMICS OF CATION EXCHANGEREACTIONS: THEORETICAL AND PRACTICAL CONSIDERATIONSSteven A. Grant and Philip FletcherA THREE-PARAMETER MODEL FOR SUMMARIZING DATA IN IONEXCHANGE Erik HogfeldtDESCRIPTION OF ION-EXCHANGE EQUILIBRIA BY MEANS OF THESURFACE COMPLEXATION THEORY Wolfgang H. Holl, Matthias Franzreb,Jurgen Horst, and Siefried H. EberleContents of Other Volumes xvSURFACE COMPLEXATION OF METALS BY NATURALCOLLOIDS Garrison SpositoA GIBBS-DONNAN-BASED ANALYSIS OF ION-EXCHANGE ANDRELATED PHENOMENA Jacob A. MarinskyINFLUENCE OF HUMIC SUBSTANCES ON THE UPTAKE OF METALIONS BY NATURALLY OCCURRING MATERIALS James H. Ephraim andBert AllardVolume 12HIGH-PRESSURE ION-EXCHANGE SEPARATION IN RAREEARTHS Liquan Chen, Wenda Xin, Changfa Dong, Wangsuo Wu, and Sujun YueION EXCHANGE IN COUNTERCURRENT COLUMNS Vladimir I. GorshkovRECOVERY OF VALUABLE MINERAL COMPONENTS FROMSEAWATER BY ION-EXCHANGE AND SORPTION METHODSRuslan Khamizov, Dmitri N. Muraviev, and Abraham WarshawskyINVESTIGATION OF INTRAPARTICLE ION-EXCHANGE KINETICSIN SELECTIVE SYSTEMS A. I. KalinitchevEQUILIBRIUM ANALYSIS OF COMPLEXATION IN IONEXCHANGERS USING SPECTROSCOPIC AND DISTRIBUTIONMETHODS Hirohiko WakiION-EXCHANGE KINETICS IN HETEROGENEOUS SYSTEMSK. BunzlEVALUATION OF THE ELECTROSTATIC EFFECT ON METALION-BINDING EQUILIBRIA IN NEGATIVELY CHARGED POLYIONSYSTEMS Tohru MiyajimaION-EXCHANGE EQUILIBRIA OF AMINO ACIDS Zuyi TaoION-EXCHANGE SELECTIVITIES OF INORGANIC IONEXCHANGERS Mitsuo Abexvi Contents of Other VolumesVolume 13EXTRACTION OF SALTS BY MIXED LIQUID ION EXCHANGERSGabriella Schmuckler and Gideon HarelACID EXTRACTION BY ACID-BASE-COUPLED EXTRACTANTSAharon M. EyalHOST-GUEST COMPLEXATION AS A TOOL FOR SOLVENTEXTRACTION AND MEMBRANE TRANSPORT OF (BIO)ORGANICCOMPOUNDS Igor V. Pletnev and Yuri A. ZolotovNEW TECHNOLOGIES FOR METAL ION SEPARATIONS:POLYETHYLENE GLYCOL BASED-AQUEOUS BIPHASIC SYSTEMSAND AQUEOUS BIPHASIC EXTRACTION CHROMATOGRAPHYRobin D. Rogers and Jianhua ZhangDEVELOPMENTS IN SOLID-LIQUID EXTRACTION BY SOLVENT-IMPREGNATED RESINS Jose Luis Cortina and Abraham WarshawskyPRINCIPLES OF SOLVENT EXTRACTION OF ALKALI METAL IONS:UNDERSTANDING FACTORS LEADING TO CESIUM SELECTIVITYIN EXTRACTION BY SOLVATION Bruce A. Moyer and Yunfu SunVolume 14POLYMER-SUPPORTED REAGENTS: THE ROLE OFBIFUNCTIONALITY IN THE DESIGN OF ION-SELECTIVECOMPLEXANTS Spiro D. AlexandratosRECOVERY OF VALUABLE SPECIES FROM DISSOLVING SOLIDSUSING ION EXCHANGE Jannie S. J. van Deventer, P. G. R. de Villiers, andL. LorenzenPOLYMERIC LIGAND-BASED FUNCTIONALIZED MATERIALS ANDMEMBRANES FOR ION EXCHANGE Stephen M. C. Ritchie andDibakar BhattacharyyaBIOSORPTION OF METAL CATIONS AND ANIONS Bohumil Volesky,Jinbai Yang, and Hui NiuContents of Other Volumes xviiSYNTHESIS AND APPLICATION OF FUNCTIONALIZEDORGANO-CERAMIC SELECTIVE ADSORBENTS Lawrence L. Tavlarides andJ. S. LeeENVIRONMENTAL SEPARATION THROUGH POLYMERIC LIGANDEXCHANGE Arup K. SenGuptaIMPRINTED METAL-SELECTIVE ION EXCHANGER Masahiro GotoSYNTHESIS AND CHARACTERIZATION OF A NEW CLASS OF HYBRIDINORGANIC SORBENTS FOR HEAVY METALS REMOVALArthur D. Kney and Arup K. SenGuptaVolume 15AN INTEGRATED METHOD FOR DEVELOPMENT AND SCALING UPOF EXTRACTION PROCESSES Baruch GrinbaumDESIGN OF PULSED EXTRACTION COLUMNS Alfons Vogelpohl andHartmut HaverlandPURIFICATION OF NICKEL BY SOLVENT EXTRACTIONKathryn C. Sole and Peter M. ColeTREATMENT OF SOILS AND SLUDGES BY SOLVENT EXTRACTIONIN THE UNITED STATES Richard J. Ayen and James D. NavratilTHE DESIGN OF SOLVENTS FOR LIQUIDLIQUID EXTRACTIONBraam van Dyk and Izak NieuwoudtEXTRACTION TECHNOLOGY FOR THE SEPARATION OF OPTICALISOMERS Andre B. de Haan and Bela SimandiREGULARITIES OF EXTRACTION IN SYSTEMS ON THE BASIS OFPOLAR ORGANIC SOLVENTS AND USE OF SUCH SYSTEMS FORSEPARATION OF IMPORTANT HYDROPHOBIC SUBSTANCESSergey M. LeschevDEVELOPMENTS IN DISPERSION-FREE MEMBRANE-BASEDEXTRACTIONSEPARATION PROCESSES Anil Kumar Pabby andAna-Maria Sastrexviii Contents of Other Volumes1Adsorption and Ion-ExchangeProperties of EngineeredActivated Carbons andCarbonaceous MaterialsMichael Streat, Danish J. Malik, and Basudeb SahaLoughborough University, Loughborough, Leicestershire, United KingdomI. INTRODUCTIONThe importance of environmental pollution control has increased signicantly inrecent years. Environmentalists are primarily concerned with the presence of heavymetals, pesticides, herbicides, chlorinated hydrocarbons, and radionuclides ingroundwater, surface water, drinking water, and aqueous efuents due to their hightoxicity and impact on human and aquatic life.Several techniques have been developed and used to remove and=or recover awide range of micropollutants from water and a variety of industrial efuents.Adsorption using activated carbon is well established for the removal of organicmolecules from aqueous solution but to a much lesser extent for the removal oftoxic heavy metals. Of course, polymeric ion-exchange resins are applied for watertreatment and for trace metal removal under extreme conditions, ranging fromhighly acidic to highly alkaline solutions. The high sorption capacity of thesematerials, usually greater than that of carbonaceous adsorbents, and good selectivitytoward metal ions render them attractive candidates for a wide range of appli-cations. However, the use of ion-exchange resins involves signicant capital andoperating costs. Activated carbons are generally a cheaper alternative because theyare derived from renewable natural materials although the operating costs remaina signicant factor.1The removal of toxic metal ions from dilute or concentrated solutions hasreceived considerable attention in the last few decades. In recent years, stringentstatutory regulations were introduced to reduce the discharge of toxic metals tolow levels at the source, particularly from plating shops and other metal processingindustries. The Environmental Protection Agency (EPA) and the EuropeanCommunity (Directive 98=78=EC [1]) have highlighted the most common heavymetals arising in residual water, and the maximum admissible concentrations aregiven in Table 1. Cadmium and mercury are two of the most toxic metals presentin the aqueous environment; hence, their maximum permissible concentrations indrinking water have been set at 5 and 1 mg=L (ppb), respectively, and this presents aparticularly challenging problem. The threshold limits given in Table 1 are achievedpartially by minimization and recycling of existing resources. With the increasingdemand for cleaner water, attention has been focused on improvements to existingtreatments and the development of new techniques and materials.The removal of metal contaminants from efuent streams has the advantageof reducing the cost of waste disposal. In most cases, the treatment of wastewatergives rise to secondary efuents. Efciency of such processes can be improved byrecycling treated water and=or by metal recovery. General methods applied tothe removal of metals include ion exchange, precipitation, coagulation, occulation,evaporation, and membrane processes. By using ion exchange or adsorption, mostTable 1 Maximum Admissible Concentrations of Undesirableand Toxic Metals in WaterSubstanceMaximum admissibleconcentration (mg=L) CommentsArsenic 10Antimony 10Cadmium 5Chromium 50Copper (3,000) Guide level after standing12 h at point of consumptionCyanides 50Iron 50Lead 50 In running waterMercury 1Nickel 50Selenium 10Zinc (5,000) Guide level after standing12 h at point of consumption2 Streat et al.of the water can be recycled without the need for further treatment. In some cases,the metal can also be recovered in a useful form.A variety of materials have been investigated for the removal of metals frommetallurgical efuents. Conventional activated carbons are used extensively in watertreatment for removal of color, odor, and organic contaminants [2,3]. Thesecarbonaceous materials possess the potential for removal of inorganic species fromefuent streams. Activated carbons have high porosities and high surface areas andare prepared from readily available carbonaceous precursors such as coal, wood,coconut shells, and agricultural wastes. These precursors are normally exposed toa number of different activation methods in an effort to achieve an activated carbonwith the most favorable properties for a particular application. The texture ofactivated carbons can be adapted to suit the situation by adequate choice of theactivation procedure.Removal of metals by conventional activated carbons has been studied by anumber of authors (46). In general, ordinary activated carbons possess a largesurface area but have a relatively low capacity for metal ions. Modied activatedcarbons have been examined as alternatives to conventional polymeric ion-exchangeresins. By far the most widely developed large-scale application of activated carbonin hydrometallurgy is the recovery of gold from dilute cyanide leach solutions. Anextensive review of this process is given by Bailey [7]. Tai and Streat [8] reported theion-exchange reactivity of oxidized carbon for the removal of copper, zinc, andnickel from solution.In this chapter, we discuss the preparation, properties, and metal sorptionperformance of a range of as-received and oxidized samples of granular and brousactivated carbon that were either prepared or modied in our laboratory. Sampleswere evaluated for the removal of trace toxic metal ions from aqueous solutions.Batch and column experiments were performed to elucidate the relationshipbetween sorptive performance and the physical and chemical structure of thesematerials.The EU and UK national standard for any individual pesticide in drinkingwater at the point of supply is 0.1 mg=L (0.1 ppb), with a maximum of 0.5 mg=Lfor all detected compounds. In 1990, the most frequently detected pesticides inUK drinking water supplies were atrazine, simazine, isoproturon, diuron, chloroto-luron, and mecoprop. Atrazine is one of the most difcult herbicides to removefrom potable water supplies and as a consequence was prohibited from nonagricul-tural uses in England and Wales in 1993. Since that time there has been an increasein the use of alternative herbicides for both agricultural and nonagriculturalpurposes. Imazapyr and triclopyr are two of a group of four herbicides (benazolin,bentazone, imazapyr, and triclopyr) that have been identied as alternatives. Theseherbicides are more soluble in water than atrazine and therefore also constitute apotential pollution hazard. The average concentration of the pesticides in sourcewaters is generally below the legal limit of 0.1 mg=L. However, seasonal variationsActivated Carbons and Carbonaceous Materials 3in the use of pesticides results in concentrations that signicantly exceed the limit,so that the source cannot be used for drinking water production. Data provided bythe UK Environment Agency show the maximum concentration of each herbicideunder investigation during sampling in 1995, 1996, and 1997 (see Table 2).The adsorption of herbicides onto activated carbon is also discussed in detailin this chapter, and the ndings are compared with data for a set of novel hyper-cross-linked polymer phases that offer an alternative approach for the treatmentof potable waters.II. PREPARATION AND PROPERTIES OFACTIVATED CARBONThe extraordinary ability of carbon to combine with itself and other chemicalelements in different ways is the basis of organic chemistry and life itself [10]. Asa consequence, there is a rich diversity of structural forms of solid carbon becauseit can exist as any of several allotropes. It is found abundantly in nature as coal oras natural graphite and also in much less abundant form as diamond. Engineeredcarbons can take many forms, e.g., coke, graphite, carbon and graphite ber, carbonbrecarbon composite, carbon monoliths, glassy carbon, carbon black, carbonlm, and diamond-like lm.The principal reasons that engineered carbons nd extensive use as adsorbentsare their porous and highly developed internal surface area and the complex natureof their surface chemical structure. In this chapter, we review the synthesis, struc-ture, and adsorption properties of engineered carbons with reference to the bodyof work carried out in our research laboratory. We also present techniques thatwe have employed to modify the surface properties of engineered carbons byintroducing heteroatoms such as oxygen, sulfur, nitrogen, and phosphorus. ActiveTable 2 Maximum Concentration of Herbicides in UK Water during 1995, 1996,and 19971995 1996 1997Maximumconc. (mg=L)Maximumconc. (mg=L)Maximumconc. (mg=L)Atrazine 1.37 1.8 5.42Benazolin 0.093 0.525 0.331Bentazone 1.12 0.423 1.84Imazapyr 0.058 0.074 bentazone. However,the order changes depending upon pH, as presented in Figs. 45 and 46. At pH3, the selectivity order is triclopyr >bentazone >benazolin >imazapyr, whereasat pH 10 it is imazapyr >benazolin >triclopyr >bentazone, suggesting that themechanism of adsorption on MN-200 is dependent on pH. Triclopyr and bena-zolin are smaller than bentazone and imazapyr, which is thought to be the reasonfor the comparatively better adsorption of these molecules in the multicomponentsystem than in the single-component system.Tables 25 and 26 show the Freundlich coefcients for adsorption at pH 3 andpH 10, respectively, on F400 and MN-200. The herbicides adsorb to a greaterextent on F400 with decreasing pH, which suggests that surface charge has a signif-icant role in the adsorption of these particular herbicides. The same trend is gener-ally observed with MN-200, but to a lesser extent, with the selectivity of adsorptionalso affected by pH.Figure 45 Multicomponent adsorption isotherms for MN-200 at pH 3. (From Ref. 62,with permission from The Institution of Chemical Engineers.)66 Streat et al.At pH 3, the surface of MN-200 is positively charged, whereas the adsorbatesare neutral or partially dissociated. This will promote adsorption. With increasingpH, the surface of MN-200 becomes negatively charged and the functional groupson the adsorbates will be almost completely dissociated, giving rise to a repulsiveeffect and thus diminished adsorption. In addition to carboxylic acid functionality,triclopyr and benazolin also contain chlorine groups that enhance the negativecharge of the molecules. As a result, the adsorption capacity of these two moleculesFigure 46 Multicomponent adsorption isotherms for MN-200 at pH 10. (FromRef. 62, with permission from The Institution of Chemical Engineers.)Table 25 Freundlich Coefcients for Multicomponent Adsorption of Benazolin,Bentazone, Imazapyr, and Triclopyr at pH 3Benazolin Bentazone Imazapyr TriclopyrK 1=n K 1=n K 1=n K 1=nF400 295.6 0.198 178.1 0.157 161.1 0.190 214.8 0.147MN-200 8941.3 1.140 93718.0 1.455 392.6 0.667 81617.1 1.247Source : Ref. 62, with permission from The Institution of Chemical Engineers.Activated Carbons and Carbonaceous Materials 67shows the greatest decline with increasing pH. This explains the changes in theorder of selectivity with pH of solution. Therefore, electrostatic interactions suchas dipoledipole or hydrogen bonding are likely to play a signicant role in theadsorption of benazolin, bentazone, imazapyr, and triclopyr onto MN-200.The Freundlich 1=n coefcient, derived from the isotherm data, is indicativeof the strength of adsorption. No clear trends can be observed from the data,although it is clear that the values for F400 are consistently lower than those forMN-200, indicating stronger binding to F400.Table 26 Freundlich Coefcients for Multicomponent Adsorption of Benazolin,Bentazone, Imazapyr, and Triclopyr at pH 10Benazolin Bentazone Imazapyr TriclopyrK 1=n K 1=n K 1=n K 1=nF400 184.3 0.319 233.8 0.581 289.6 0.924 90.2 0.165MN-200 40.80 0.749 47.6 1.019 104.1 0.845 30.8 0.704Constants based on units of qe (mmol=g) and units of Ce (mmol=L).Source : Ref. 62, with permission from The Institution of Chemical Engineers.Figure 47 Multicomponent adsorption isotherms for F400 in the presence of 20 mg=Lfulvic acid. (From Ref. 62, with permission from The Institution of Chemical Engineers.)68 Streat et al.The inuence of a high concentration of fulvic acid on the adsorption of tracelevels of the herbicides for F400 and MN-200 is presented in Figs. 47 and 48,respectively. Fulvic acid reduces the capacity of the adsorbents for all the herbicides,although the isotherms cannot be modeled by the standard Freundlich (or anyother) equation. The isotherms are of Type II, according to the classical denition.There are few data in the literature that can be used for comparison. TheFreundlich coefcients presented by Speth and Miltner [56] for atrazine adsorptionon pulverized F400 (1=n = 0.291 and K = 858) do not appear to compare to thosepresented in Table 23. However, the value of K is strongly dependent upon the1=n coefcient, making comparisons doubtful. Table 27 compares the equilibriumadsorption capacities at solution concentrations of 5 104mmol=L (approxi-mately 0.1 mg=L) and 0.01 mmol=L (approximately 2.2 mg=L). The data showreasonable comparison for F400. The differences are probably due to the differentparticle size ranges used in the two studies as well as batch variances in the carbon.The capacity of MN-200 is about one-seventh that of F400 at an equilibriumolution concentration of 0.1 mg=L.The only other relevant adsorption capacity data located in the literature werepresented by Hopman et al. [59] for bentazone adsorption on ROW 0.8S carbon.The carbon showed a capacity of 47 mg=g at a solution concentration of 1 mg=L.The relative molecular mass of bentazone is 240.3, which gives a comparablecapacity of 41.3 mg=g for bentazone adsorption on F400 at a solution concentra-tion of 1 mg=L. However, the capacity of MN-200 is just 0.197 mg=g at thissolution concentration.Rapid small-scale column tests are recommended by the American WaterWorks Association as a protocol for the selection and evaluation of granular acti-vated carbon [60]. Crittenden et al.[61] adpted this technique to provide quantita-tive adsorption data. The experimental rig for minicolumn experiments is illustratedin a simplied ow diagram in Fig. 49, and full experimental details are given byStreat and Horner [62].Minicolumn experiments were performed using an empty-bed contact time(EBCT) of approximately 4.3 s for both F400 and MN-200. The capacity ofF400 is far superior to that of MN-200 because no breakthrough occurred onthe carbon column after 28 days of service and nearly 200 L of water treated,whereas the MN-200 column showed instant breakthrough, indicating that theEBCT was too low.The EBCT of the carbon column was reduced and that of the polymericcolumn increased. The breakthrough curves obtained from the second experimentare shown in Figs. 50 and 51 The experiment was stopped after 35 days, with eachcolumn having processed 214 L of water. Breakthrough of the carbon column to theEU legal limit (Fig. 52) occurs between 89,000 and 160,000 BV, in the sequenceimazapyr, bentazone, benazolin, and nally triclopyr. The selectivity sequence forbenazolin and triclopyr is reversed compared to the results obtained in the batchActivated Carbons and Carbonaceous Materials 69isotherm experiments. This may be due to kinetic effects encountered duringthe batch equilibrium of 7 days. During the long duration of the columnexperiment, triclopyr may have been able to diffuse into the pores to a greater extentthan benazolin. The adsorption capacity for benazolin, bentazone, imazapyr,and triclopyr is 34.20, 22.84, 15.22, 38.01 mg=g, respectively. Chromatographicelution is observed for the herbicides, which caused the imazapyr and bentazoneconcentrations to reach 25 and 23 mg=L, respectively. The MN-200 adsorptionbed showed breakthrough almost instantly, which is reected in the low adsorptionFigure 48 Multicomponent adsorption isotherms for MN-200 in the presence of20 mg=L fulvic acid. (From Ref. 62, with permission from The Institution of ChemicalEngineers.)Table 27 Adsorption Capacities for Atrazine Adsorption on F400Adsorbent qe at ce = 5 104mmol=L qe at Ce = 0.01 mmol=LF400 61.9 265.4MN-200 8.8 76.0Speth and Miltner [56] 94.0 224.6Source : Ref. 62, with permission from The Institution of Chemical Engineers.70 Streat et al.Figure 50 Minicolumn breakthrough curves for F400 sorbing benazolin, bentazone,imazapyr, and triclopyr (EBCT 2.3 s). (From Ref. 62, with permission from The Institu-tion of Chemical Engineers.)Figure 49 Simplied ow diagram of column apparatus. (From Ref. 62, with permis-sion from The Institution of Chemical Engineers.)Activated Carbons and Carbonaceous Materials 71Figure 51 Minicolumn breakthrough curves for MN-200 sorbing benazolin, bentazone,imazapyr, and triclopyr (EBCT 8.4 s). (From Ref. 62, with permission from The Institu-tion of Chemical Engineers.)Figure 52 Minicolumn breakthrough curves for F400 sorbing benazolin, bentazone,imazapyr, and triclopyr (EBCT 2.3 s)expanded view. (From Ref. 62, with permissionfrom The Institution of Chemical Engineers.)72 Streat et al.capacities of 0.48, 0.42, 0.26, 1.28 mg=g for benazolin, bentazone, imazapyr, andtriclopyr, respectively.The breakthrough curves are shallow, suggesting that the ow rate throughthe column was too high, thus spreading the mass transfer zone. A slower ow rateand increased EBCT would probably result in a greater lifetime of the columns.In large-scale practice, an EBCT of 15 min is standard. The concentration usedfor the breakthrough curves was also exceptionally high, approximately 20 timesgreater than that found in surface waters. However, the large capacity of theadsorbents and the limited time for experiments necessitated the use of this feedconcentration.The minicolumn breakthrough curves in the presence of fulvic acid arepresented in Figs. 53 and 54. The introduction of fulvic acid into the herbicidemixture caused instant breakthrough on the F400 column. MN-200 also showedinstant breakthrough, although the reduction in capacity is not as pronounced.Fulvic acid adsorption isotherms presented by Streat et al. [57] show thatF400 has a much higher capacity for fulvic acid than MN-200, which is attributedto the mesoporous nature of the carbon [57]. It is thought that the fulvic acidFigure 53 Minicolumn breakthrough curves for F400 in the presence of fulvic acid(EBCT 2.3 s). (From Ref. 62, with permission from The Institution of ChemicalEngineers.)Activated Carbons and Carbonaceous Materials 73molecules adsorb in the mesopores, thus preventing diffusion of the herbicidesinto the micropore structure of the carbon.Table 28 shows the regeneration efciencies of herbicides from minicolumnsusing ethanol as eluent. Figures 55 and 56 show the elution curves for F400 andMN-200 columns, respectively. The regeneration of MN-200 is virtually completewithin 10 BV, because 99.2% of the total mass of herbicide is removed. The regen-eration efciencies are 100% for all herbicides except imazapyr, for which only79.5% was recovered. A signicantly greater volume of regenerant is required forF400; in all, 200 bed volumes was passed. The recovery of bentazone and imazapyrFigure 54 Minicolumn breakthrough curves for MN-200 in the presence of fulvicacid (EBCT 8.4 s). (From Ref. 62, with permission from The Institution of ChemicalEngineers.)Table 28 Regeneration Efciencies (%) for F400 and MN-200 Adsorption ColumnsBenazolin Bentazone Imazapyr TriclopyrF400 102.8 53.3 50.4 103.9MN-200 98.9 100.3 79.5 101.0Source : Ref. 62, with permission from The Institution of Chemical Engineers.74 Streat et al.Figure 55 Elution curves for F400 at 25C used in minicolumn runs. (From Ref. 62,with permission from The Institution of Chemical Engineers.)Figure 56 Elution curves for MN-200 at 25C used in minicolumn runs. (FromRef. 62, with permission from The Institution of Chemical Engineers.)Activated Carbons and Carbonaceous Materials 75was approximately 50% for each herbicide. Benazolin and imazapyr exhibited twoapparent elution maxima, the early peak being attributed to bed equilibration timeat the start of the experiment. HPLC chromatograms show a large number of peaksin the early stages of elution that are probably due to impurities in the herbicidesand the organic content of the ultrapure water. The total organic content of theultrapure water was continually monitored and measured around 2 mg=L.The adsorption cycle was repeated for the F400 column to assess the regen-eration recovery efciency. Figure 57 shows the breakthrough curve in the secondcycle. Herbicides start to break through the column between 30,000 and60,000 BV, which is lower than for the virgin carbon. Because regenerationremoves only about 50% of bentazone and imazapyr, it is not surprising thatadsorption capacity is reduced. A second regeneration of the column was performedby passing 200 BV of eluent at 50C. Subsequently, 1 mL of the efuent wasevaporated and reconstituted in 1 mL of 15% acetonitrile, 85% 10 mmol KH2PO4buffer at pH 3.0 for HPLC analysis. The recovery efciencies for the second adsorp-tion cycle are presented in Table 29. The gures presented for bentazone andimazapyr represent the recovery efciencies based on the total amount adsorbedafter the two cycles. The gures in parentheses show the recovery efciencies basedon the amount adsorbed in the second adsorption cycle only. It can be seen thatFigure 57 Minicolumn breakthrough curves for regenerated F400 column (EBCT 2.3 s). (From Ref. 62, with permission from The Institution of Chemical Engineers.)76 Streat et al.almost all of the bentazone and imazapyr adsorbed in the second cycle is recoveredin the second regeneration cycle, so the bed life will stabilize with repeated cycles.Using a slightly larger volume (or mass) of adsorbent, so that the capacity reductionis offset, could compensate for the loss of capacity.VIII. SUMMARY OF HERBICIDE SORPTION ONACTIVATED CARBONS ANDHYPERCROSS-LINKED POLYMERSOur work conrmed that activated carbons are effective adsorbents for traceherbicides in water. In particular, we found that conventional activated carbon canbe employed for the removal of heterocyclic aromatic herbicides such as atrazineand also for more water-soluble pesticides that contain hydrophilic carboxylic acidand amino functional groups. We have presented a rational approach to the repre-sentation of the adsorption isotherms for these species by applying the conventionalLangmuir and Freundlich equations. This does not provide a precise description ofthe adsorption mechanism, which is extremely complex, but does provide us withan adequate basis for the design and development of conventional process equip-ment. We have found that there is selectivity among the selected herbicides and thatit depends on the surface characteristics of the adsorbent material. The underlyingprinciples of adsorption of organic molecules on activated carbon are still thesubject of considerable research effort, as can be seen in the comprehensive reviewof the subject by Radovic et al. [25]. The majority of published work has focused onthe sorption of phenol and substituted phenols, and we appear to have performedthe most extensive experimental study of herbicides, pesticides, fungicides, etc.Further work is necessary to fully understand the precise mechanisms of adsorptionof complex aromatic molecules onto carbon, and this forms the basis of our contin-ued work. From a practical point of view, the regeneration and reactivation ofcarbon for cyclic use is of equal importance. Here, we nd that the binding energyof aromatics is so strong that arduous regeneration and reactivation techniquesare required, i.e., high-temperature furnaces are required to remove organics boundto the surface of activated carbon. This represents a severe operational and energyTable 29 Regeneration Efciencies (%) for Second Regeneration of F400 Usedin Minicolumn RunsBenazolin Bentazone Imazapyr TriclopyrF400 100.7 47.4 (89.3) 41.6 (94.1) 104.2Source : Ref. 62, with permission from The Institution of Chemical Engineers.Activated Carbons and Carbonaceous Materials 77cost consideration in the industrial application of these materials in the treatment ofwater and efuents. To overcome some of these problems, we have embarkedon a study of the adsorption of pesticides onto hypercross-linked polymer phases.Our results are most encouraging for atrazine and similar triazine herbicides. Wehave shown effective adsorption and regeneration of unfunctionalized hypercross-linked hydrocarbon polymers for this case study. Atrazine adsorption in minicol-umns parallels the performance of activated carbon, and moreover we have shownthat the binding energies are sufciently low to enable efcient solvent strippingat ambient temperature. This has already proved an attractive alternative processfor the treatment of atrazine-contaminated waters. The adsorption of morehighly soluble herbicides with unfunctionalized hydrocarbon hypercross-linkedpolymers is less favorable, and this has persuaded us to consider tailored polymersfor this case study. Further work is in progress to modify the structure ofhypercross-linked polymers to remove soluble herbicides without sacricing thefavorable low-temperature solvent-stripping regeneration stage.The environment is challenged by other micropollutants: chlorinated hydro-carbons, aliphatic intermediates arising from the chemical industry, and, of course,endocrine disrupters. Our study of activated carbon and hypercross-linked polymerscontinues to explore for potential solutions to these important problems.IX. FUTURE TRENDS AND CONCLUDING REMARKSIn this chapter we have highlighted the preparation, properties, and character-ization of engineered activated carbon and carbonaceous materials for the removalof metal ions and organic micropollutants from water. Water pollution arises frommany sources. Surface water is contaminated by agricultural runoff, communitylandlls, polluted runoff, and hazardous waste produced as by-products of manu-facturing. Groundwater is contaminated by leaks of pollutants such as gasolineand methyl tert-butyl ether (MTBE) from underground storage tanks and the injec-tion of hazardous waste into deep wells. The water treatment industry is thereforeunder pressure to produce a pure product that is free of potentially harmfulcontaminants.Activated carbon is used primarily for water purication and is essential inwater treatment facilities. The worldwide market for activated carbon exceeds $1billion. For example, U.S. demand alone for activated carbon is rising steadily,and production has risen to about 430 million pounds (about 195,000 metric tons)in 2002. The market for activated carbon remains closely linked to environmentallegislation, which has been a primary factor driving growth in key applications forseveral decades. In particular, legislation has been highly inuential in the choice oftreatment procedures used in municipal drinking water and industrial wastewaterapplications. Activated carbon will continue to nd widespread use in various78 Streat et al.industrial purication processes, most prominently in the food and beverage andpharmaceutical industries.Activated carbons are especially effective in reducing trace toxic metals such aslead and mercury in water as well as organic compounds such as PCBs and MTBE.They are also likely to reduce contaminants such as antibiotics and other drugs thatare now found in drinking water supplies. New technology is being developed thatconverts biomass and other carbonaceous wastes and by-products into activatedcarbon and combustible gas.Bottled water suppliers are being asked to cap off existing multibarrierprocesses with activated carbon treatment in order to meet the U.S. Food and DrugAdministrations (FDA) revised quality standards. FDA revised the existing allow-able levels in bottled water for three residual disinfectantschloramine, chlorine,and chlorine dioxideand disinfectant by-products (DBPs), including haloaceticacids (HAAs) and trihalomethanes (THMs). Concurrently, the FDA introducedprotocols for testing and enforcement of both source water and nished bottledwater products. In essence, the amendment ensures that the minimum quality ofbottled water remains comparable with the quality of public drinking water thatmeets U.S. Environmental Protection Agency (EPA) standards. This representsan interesting opportunity for the large-scale application of engineered activatedcarbons.There are many ongoing studies that implicate a variety of other drinkingwater contaminants as possible causes of problems with pregnancy or the develop-ing fetus. We are already studying activated carbons for the specic removal ofestrogens and estrogen-type compounds at very low (ng=L) concentration fromwater.Engineered activated carbon and advanced formulations could lead to theselective removal of antidepressants and other drugs in poisoning cases. The admin-istration of activated charcoal (AC) preparations in acute poisoning is rmly estab-lished as a standard medical treatment because of their ability to adsorb poisons andtoxins from the gastrointestinal tract, thereby reducing absorption into the blood-stream of the patient. In commercial preparations, the pore structure and surfacechemistry of these carbons have not been tailored to enhance the adsorption ofspecic drugs. Our future work is directed toward the development, formulation,and evaluation of novel AC products prepared using synthetic polymer precursorsfor the adsorption of commonly ingested antidepressants. We propose to make atailored nonspecic adsorbent with a surface area containing predominantly meso-pores. The faster kinetics and improved accessibility to the internal surfaces of theAC should result in more efcient use of the adsorbent phase, reducing the dosageamount of carbon that must be controlled and providing signicant benet to thepatient.Work is in progress on the manufacture of tailored activated carbonseffective in the removal of middle molecular weight and other toxins from blood.Activated Carbons and Carbonaceous Materials 79Present work could lead to effective and novel adsorbents for the treatment ofacute and chronic renal failure and could demonstrate hemo- and biocompa-tibility of uncoated medical adsorbents in the design and manufacture ofhemoperfusion columns suitable for augmenting the treatment of renal dialysispatients.There are two main features of activated carbons that invite their use asbiomaterials: the possibility to control, to a large extent, carbon pore structure (andthus to control the selectivity and sorption capacity with respect to molecules of dif-ferent sizes) and compliance with strict requirements for materials intended formedical use. To ensure that the nal biomaterial grade adsorbent possesses all therequirements for materials used in medicine for detoxication, it is essential thatno toxic substances be liberated into blood or any other contacting liquidplasma,lymph, cerebrospinal uid, etc. The adsorbent must not destroy blood cells or alterthe physicochemical properties of perfused solutions; i.e., the adsorbent must bechemically inert. In addition, any biomaterial coming into contact with blood mustbe mechanically robust and must not liberate into the human body or contactingliquids any substance that would cause allergic or pyrogenic reactions. We aim todevelop novel nitrogen-containing polymerderived carbons and carbon bersfor example, those prepared using polymer precursors including polyacrylonitrile,vinylpyridine, etc.to prepare adsorbents possessing high strength, chemical stabil-ity, and ion-exchange capacity for medical applications. Their mesopore structurewill be optimized to ensure the sorption of high molecular weight substances frombiological uids. The biocompatibility of carbon sorbents, i.e., prevention ofdamage to blood cells, is crucial to the development and commercial applicationof these materials in a clinical setting.Water is often referred to as the universal solvent because it dissolves so manysubstances. Water also contains many materials in suspension and is not particularlyselective in what compounds are dissolved or suspended. The water that dissolvesour coffee or tea and sugar in the morning or that we use to reconstitute orangejuice or an infants formula might have low concentrations of lead from the distri-bution pipes in the home dissolved in it. If the water is chlorinated it almost cer-tainly contains a few micrograms of chloroform (a by-product of the disinfectionprocess). Therefore, the question that needs to be asked is not simply, Does thetap water contain contaminants? The real questions are, What are the contaminantsin the water, What are their concentration levels, and Do they pose short- or long-term health risks at those levels? Finding answers to all these questions is a continu-ing challenge.This chapter has provided some answers to these questions insofar as itrelates to processes involving engineered activated carbons and carbonaceousmaterials. It is our hope that the information presented here will prove helpfulto practitioners and moreover stimulate research into the potential of tailoredactivated carbon for water treatment and environmental remediation.80 Streat et al.ACKNOWLEDGMENTSWe wish to acknowledge the work of many postgraduate and postdoctoral research-ers and academic collaborators who have contributed to studies in the Loughbor-ough research group over a period of many years. 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Strelko, Jr. V.; Malik, D.J.; Streat, M. The inuence of active carbon oxidation on thepreferential removal of heavy metals. Sep. Sci. Technol., 2001, 36 (15), 33673383.84 Streat et al.2Entropy-Driven Selective IonExchange for HydrophobicIonizable Organic Compounds(HIOCs)Ping Li and Arup K. SenGuptaLehigh University, Bethlehem, Pennsylvania, U.S.A.I. INTRODUCTIONDuring a sorption process, solute molecules or ions are essentially transferredfrom the solvent phase to the sorbent phase. As the binding of a solute takes placeat the sorption site, the rotational and translational freedom of the solute arereduced. Hence, the entropy change (DS ) during sorption is negative. In orderfor the sorption to be favorable, Gibbs free energy change (DG) must be negative,which in turn requires the enthalpy change (DH) to be negative becauseDG=DHTDS. In general, all favorable sorption processes (including ionexchange) conform to this stipulation, i.e., they are exothermic and accompaniedby an overall decrease in entropy. Figure 1 illustrates such enthalpy-drivensorption processes.Many synthetic aromatic compounds exhibit acidic characteristics due tothe presence of carboxylic, phenolic, and sulfonic acid moieties, and their aciditiesare often strengthened because of the electron-withdrawing effects of various sub-stituent groups. For example, the pKa value (i.e., negative logarithm of acid dis-sociation constant) for phenol is 9.3, whereas for pentachlorophenol or PCPpKa=4.75. As a result, PCP, which is extensively used in the wood preservationindustry, exists as an anion in contaminated surface water or groundwater atneutral pH. Contrary to other non-ionized hydrophobic aromatic compounds,pentachlorophenate or PCP is therefore more mobile in the natural environment85and not amenable to efcient removal by conventional hydrophobic sorbents such asactivated carbon. Like PCP, many other industrially signicant aromatic com-pounds, namely, naphthalenesulfonates and quaternary ammonium compounds,tend to exist as ions in the aqueous phase and are commonly referred to as hydro-phobic ionizable organic compounds or HIOCs [1,2]. While the aromaticityimparts hydrophobic or nonpolar characteristics, the ionic charge of these com-pounds enhances hydrophilicity through iondipole interaction with water mole-cules. The solubilities of weak-acid type HIOC compounds, therefore, increasesignicantly at pH values greater than the pKa.The aromatic anions have hydrophobic characteristics as well as ionic charac-teristics due to their nonpolar moieties (NPMs). Understandably, the sorptionbehaviors of such aromatic anions will be greatly inuenced by both hydrophobicand ionic properties. Unlike nonionized hydrophobic aromatic compounds, thesorption of these aromatic anions is not a physical sorption process. Such processesare characterized by equivalent exchange of ionic species between the liquid phaseand ion-exchanger solid phase, but ion-exchange selectivity is often determined byconcurrent interactions other than electrostatic ones [3].This study investigates favorable sorption behaviors of several environmen-tally signicant aromatic anions, e.g., chlorophenates and benzene- and naphtha-lenesulfonates, onto polymeric anion exchangers. Such favorable sorption equilibriaare, however, distinctively unique because they are all endothermic processesand accompanied by highly positive entropy changes. The solvent dielectric con-stant, polarity, or moisture content of the ion-exchanger matrix and the nonpolarmoiety (NPM) of the aromatic anion are the three fundamental process variablesthat govern the overall sorption equilibrium.Figure 1 A schematic drawing illustrating an exothermic and enthalpy-driven sorptionprocess.86 Li and SenGuptaII. NATURE OF SOLUTESORBENT ANDSOLUTESOLVENT INTERACTIONSA polymeric anion exchanger with xed positive charges will sorb aromatic anionssuch as pentachlorophenate and naphthalenesulfonate. A typical anion-exchangereaction between pentachloronate (PCP) and chloride (Cl) can be presented asfollows:RCl PCP)* RPCPCl (1)where the overbar represents the exchanger phase and R is an anion exchangerwith xed positive charges. Chloride (Cl) and pentachlorophenate (PCP) areidentical electrostatically; they each have one negative charge. Strictly from an elec-trostatic or Coulombic interaction viewpoint, the sorption of PCP onto a poly-meric anion exchanger in the presence of competing chloride ion is unlikely tobe a selective process. Previous studies, however, have shown very favorable sorptionbehaviors of chlorinated phenols and aromatic anions onto polymeric exchangers inpreference to chloride and other inorganic anions [46]. High ion-exchange selec-tivities have also been reported for aliphatic anions with long alkyl chains [7,8].Such high sorption afnities have, in general, been attributed to hydrophobic inter-actions resulting from the NPM of the aromatic anions. From a phenomenologicalviewpoint, the NPMsolvent and NPMmatrix interactions are recognized as thetwo primary contributors to high sorption afnity of aromatic ions in ion-exchangeprocesses. The matrix represents the skeletal organic component in the polymericion exchanger other than the charged functional groups. Assuming insignicantchange in the hydration of chloride ion between the aqueous and ion-exchangerphases, the following ion-exchange half-reaction is the primary determinant ofthe overall equilibrium of the reaction in Eq. (1):R PCP(aq))*RPCPwater (2)Because PCP sorption is favorable, the overall free energy change for Eq. (2) isnegative. The free energy change at the standard state of choice (DG0) is given byDG0= DH0T DS0(3)Therefore, both enthalpic (DH0) and entropic (DS0) changes help decidethe overall selectivity of the ion-exchange process. Note that the denition of thestandard state in the ion-exchanger phase may alter the signicance of exchanger-phase activity coefcients but in no way alters the relative enthalpic and entropiccontributions to the overall equilibrium. To elucidate interactions associated withPCP sorption in Eq. (2), the sorption process can be broken down into twoEntropy-Driven SIE for HIOCs 87consecutive steps: (1) desolvation of PCP and (2) PCP sorption onto the anionexchanger.Interaction during desolvation of PCP. A nonpolar moiety (NPM) is notcapable of forming hydrogen bonds with polar water molecules. Thus,when an ion with NPM is introduced into water (a polar solvent), thewater molecules tend to turn away from the NPM and reorganize them-selves into clusters through hydrogen bonding. Consequently, there is anoverall entropy decrease in the system due to reduced degrees of freedomof these self-associated water molecules. The concept of clusterlike forma-tion of structured water molecules around a hydrophobic solute was rstdiscussed by Frank and Wen [9] and later elaborated by Nemethy andScheraga [10] and others [1113]. As PCP leaves the aqueous phaseduring the course of the ion-exchange process, an overall increase inentropy will therefore result. Also, the solvent phase needs to absorb heatto break the highly associated clusterlike structure of water molecules; i.e.,the process is endothermic.Interaction during PCP sorption onto the polymeric exchanger. Once a PCPmolecule enters the exchanger phase and binds to the xed positivecharge, its NPM tends to be in direct contact with the nonpolar matrixof the ion exchanger. This results in expulsion of polar water moleculesfrom the exchanger phase, which are present primarily because of theosmotic pressure difference between the exchanger phase and the solvent.Although thermal energy is required for such localized dehydrationwithin the exchanger, the resulting increase in overall entropy due tothe direct contact between these two nonpolar substances (matrix andNPM of PCP) makes such a binding energetically advantageous [14].Figure 2 illustrates a mechanistic interpretation of the foregoing two steps ofthe sorption process. Note that hydrophobic interactions energetically compriseboth NPMsolvent and NPMmatrix interactions. Although not explicit, the effectof solventmatrix interaction is also included in Fig. 2. The weaker the solventmatrix interaction, the smaller will be the energy required to expel the solventmolecules from the matrix and hence the more favorable will be the sorption processand vice versa. For negligible swelling and=or shrinking of the polymeric exchanger,the overall free energy change for an ion-exchange reaction involving a counter ionwith an NPM is thus contributed by electrostatic (el), NPMsolvent, and NPMmatrix interactions.DG0overall = DG0elDG0NPMsolventDG0NPMmatrix (4)When Eq. (4) is applied to homovalent PCPCl exchange in Eq. (1), the freeenergy changes due to electrostatic interaction cancel out, and we get:88 Li and SenGuptaFigure 2 A schematic illustrating NPMsolvent, NPMmatrix, and electrostatic inter-actions during sorption of the aromatic anion from the aqueous phase.Entropy-Driven SIE for HIOCs 89DG0overall = DG0NPMsolventDG0NPMmatrix= (DH0NPMsolventDH0NPMmatrix)(TDS0NPMsolventTDS0NPMmatrix)= DH0overallTDS0overall (5)Note that only overall enthalpic and entropic changes during the sorptionprocess can be determined experimentally. However, by changing the nonpolarmoiety of the solute, the dielectric constant of the solvent, and the polarity ofthe matrix, one can assess the relative contributions of NPMsolvent andNPMmatrix interactions to the overall free energy change. The overall freeenergy change is again related to the equilibrium constant K of the reaction inEq. (1) as follows:DG0overall = RT ln K (6)where R is the universal gas constant and T is the temperature in kelvins. Forhomovalent PCPCl exchange, the equilibrium constant K is given byKPCP=Cl =yPCPfPCPyClfClxClgClxPCPgPCP_ _ (7)where yi and xi represent equivalent fractions of counter ion i in the exchangerphase and in the aqueous phase, respectively, and fi and gi represent activity coef-cients in the corresponding two phases. For ions with identical charges, the activitycoefcients in dilute aqueous solutions tend to be equal, i.e., gPCP=gCl is unity[15]. The separation factor for PCPCl exchange can be determined experi-mentally at a particular resin loading and is given byaPCP=Cl =yPCPxClyClxPCP(8)The variation in exchanger phase loading for PCPCl exchange is, how-ever, contained between yPCP=0 and yPCP=1.0. For homovalent ion exchange,the equilibrium constant can then be approximated as the average separation factorvalue integrated over the entire exchanger phase composition, i.e.,ln KPCP=Cl =_yPCP=1yPCP=0 ln aPCP=Cl dyPCP_yPCP=1yPCP=0 dyPCP=_yPCP=1yPCP=0 ln aPCP=Cl dyPCP1=_ yPCP=1yPCP=0ln aPCP=Cl dyPCP (9)90 Li and SenGuptaThe overall free energy change for PCPCl exchange is nowDG0overall = RT ln K = RT_10ln aPCP=Cl dyPCP= RT_10lnyPCP(1 xPCP)(1 yPCP)xPCPdyPCP (10)The above integral can now be computed from the binary sorption isothermdata.If the equilibrium constant values are determined at different temperaturesaround 298 K where standard enthalpy change (DH0) may be assumed to be con-sidered constant, the vant Hoff equation givesd(log K)d(1=T) = DH02:3R (11)where T is the absolute temperature in kelvins. The standard enthalpy change canbe computed from the slope of the log K vs. 1=T plot. Similar approaches have beenused successfully to determine DH0values during sorption processes at ambienttemperature [7,16]. Enthalpic changes thus determined agreed well with the valuesobtained independently using microcalorimetric techniques [16]. The standardentropic contribution at 298 K [TDS0] can subsequently be determined fromthe relationshipT DS0= DH0DG0(12)High selectivity of counter ions with nonpolar moieties results from hydro-phobic interactions, which are again manifested in enthalpic and entropic changes.Altogether, there are three independent process variableshydrophobicity of thesolute, polarity of the ion-exchanger matrix, and the dielectric constant of thesolventinuencing the selectivity of a specic aromatic anion.III. EXPERIMENTAL SECTIONA. Aromatic AnionsTwo types of aromatic anions were investigated: chlorophenols and sulfonated aro-matic anions. The chlorophenols were pentachlorophenol, 2,4,6-trichlorophenol,and 2,6-dichlorophenol. The sulfonated aromatic anions were naphthalene-1-sulfonate, naphthalene-1,5-disulfonate, and benzenesulfonate. Tables 1 and 2include salient information about chlorophenols and sulfonated aromatic acids.Entropy-Driven SIE for HIOCs 91Because of the electron-withdrawing effect (or inductive effect) of Clsubstituents, the pKa values of the phenols decrease as more Cl substituents areintroduced into benzene rings. In Table 1, the values of the octanol=water partitioncoefcient (KO=W) increase with increasing number of Cl substituents for undisso-ciated acids. KO=W is a measure of hydrophobicity. Note that the hydrophobicity ofthe phenols is enhanced with an increase in substituent Cl atoms. Naphthalenesul-fonic and benzenesulfonic acids are strong acids, and their pKa values are very low.Whereas naphthalene-1-sulfonate and benzenesulfonate are monovalent anions,naphthalene-1,5-disulfonate is a divalent anion.B. Ion ExchangersTwo types of ion-exchange resins, namely, IRA-900 and IRA-958, were used. Thesalient properties of the ion exchangers are presented in Table 3. IRA-900 and IRA-958 are strong-base anion exchangers. Both of the anion exchangers have quaternaryammonium functional groups. The matrix of IRA-900 is polystyrene, whereas thematrix of IRA-958 is polyacrylic. The polystyrene matrix is more nonpolar andhydrophobic than the polyacrylic matrix.Table 1 Properties of ChlorophenolsChlorophenol Molecular formula Mol wt pKa log KO=WaPentachlorophenol 266.5 4.8 5.22,4,6-Trichlophenol 197.5 6.1 3.72,6-Dichlorophenol 163 6.9 2.6aUndissociated phenols.Source : Data from Ref. 17.92 Li and SenGuptaC. Column RunsThe xed-bed column runs were carried out using a glass column (11 mm diameterand 250 mm length), a constant-ow pump, and an ISCO fraction collector. Allcolumn runs were performed under essentially the same hydraulic conditions; thesupercial liquid velocity (SLV) and the empty-bed contact time (EBCT) wereidentical, 1.2 m=h and 1.8 min, respectively. The efuent samples were collectedcontinuously with the fraction collector, and the concentrations of the differentspecies in efuent samples were analyzed. After the xed-bed column runs, theion-exchanger materials were divided into three equal portions and used in regen-eration tests using different regenerant media. The regeneration tests were per-formed with the same setup as that used in the column runs.Table 2 Properties of Sulfonated Aromatic AcidsCompound Molecular formula Mol wt pKaNaphthalene-1-sulfonic acid208 0.57 aNaphthalene-15- disulfonic acid284 b;pKa0) and a positive enthalpy change (DH0> 0) may beviewed as signals of the collapse of more structured water clusters and the predomi-nance of hydrophobic interaction during the sorption of aromatic anions. The esti-mated thermodynamic parameters substantiate the predominance of hydrophobicinteraction in determining th