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CONTENTS Preface Introduction Chapter 1 ACTIVATED CARBON AND ITS SURFACE STRUCTURE 1.1 Crystalline Structure of Activated Carbons 3 1.2 Porous Structure of Activated Carbons 5 1.3 Chemical Structure of the Carbon Surface 8 1.3.1 Carbon - Oxygen Surface Groups 10 1.3.2 Characterisation of Carbon - Oxygen Surface Groups 13 1.3.2.1 Thermal Desorption Studies 13 1.3.2.2 Neutralisation of Alkalies 16 1.3.2.3 Specific Chemical Reactions 19 1.3.2.4 Spectroscopic Methods 19 1.4 Influence of Carbon - Oxygen Surface Groups on Surface Characteristics of Carbons 28 1.4.1 Surface Acidity 28 1.4.2 Hydrophobicity 29 1.4.3 Adsorption of Polar Vapours 30 1.4.4 Adsorption of Benzene Vapours 32 1.4.5 Immersional Heats of Wetting 32 1.4.6 Adsorption from Solutions 34 1.4.7 Preferential Adsorption 35 1.4.8 Catalytic Reactions 36 1.4.9 Resistivity 36 1.5 Active Sites on Carbon Surfaces 37 1.6 Modification of Activated Carbon Surface 41 1.6.1 Modification of Activated Carbon Surface by Nitrogenation 42 1.6.2 Modification of Carbon Surface by Halogenation 43 1.6.3 Modification of Carbon Surface by Sulphurisation 44 1.6.4 Activated Carbon Modification by Impregnation 46 CHAPTER 2 ADSORPTION ENERGETICS, MODELS AND ISOTHERM EQUATIONS 2.1 Adsorption on a Solid Surface 1 2.2 Adsorption Equilibrium 3 2.3 Energetics of Adsorption 5 2.4 Adsorption Isotherm Equations 12 2.4.1 Langmuir Isotherm Equation 12 2.4.1.1 Langmuir Isotherm for Dissociative Adsorption 16 2.4.1.2 Langmuir Isotherm for Simultaneous Adsorption of Two Gases 17 2.4.1.3 Applicability of the Langmuir Isotherm 18 2.4.2 Brunauer, Emmett and Teller (BET) Isotherm Equation 19 2.4.2.1 Derivation of the BET Equation 20 2.4.2.2 Applicability of the BET Equation to Activated Carbon 26 2.4.2.3 Criticism of the BET Equation, Alternative Approach to 27 Linearlisation of the BET Equation 2.4.2.4 Classification of Adsorption Isotherms 31 2.4.3 Potential Theory of Adsorption 39 2.4.3.1 Dubinin Equation for Potential Theory 43 2.4.4 Freundlich Adsorption Isotherm 47 2.4.5 Temkin Adsorption Isotherm 49 2.4.6 Capillary Condensation Theory 51 2.4.6.1 Evidence in Support of Capillary Condensation Theory 53 2.4.7 Applicability of Langmuir, Freundlich or Temkin isotherms to Adsorption Data 2.4.8 Adsorption Hysteresis 2.4.9 Theory of Volume Filling of Micropore (TVFM) CHAPTER 3 ACTIVATED CARBON ADSORPTION FROM SOLUTIONS 3.1 Types of Adsorption from Solution Phase 2 3.2 Types of Adsorption Isotherms 3 3.2.1 Adsorption from Dilute Solutions 5 3.2.1.1 Potential Theory of Adsorption from Dilute Solutions 13 3.2.2 Adsorption from Solutions of Higher Concentrations 15 (Completely Miscible Solutions) 3.2.3 Derivation of Composite Isotherm 16 3.2.4 Classification of Composite Isotherms 19 3.3 Factors Influencing Adsorption from Binary Solutions 20 3.3.1 Adsorbate-Adsorbent Interaction 21 3.3.2 Departure from Usual Composite Isotherm Shapes 29 3.3.3 Porosity of the Adsorbent 30 3.3.4 Surface Heterogeniety 31 3.3.5 Steric Effects 31 3.3.6 Orientation of Adsorbed Molecules 32 3.4 Determination of Individual Adsorption Isotherms from 33 Composite Isotherms 3.5 Thickness of the Adsorbed Layer 36 3.6 Chemisorption from Binary Solutions 39 3.7 Traube's Rule 42 CHAPTER 4 CARBON MOLECULAR SIEVES 4.1 Preparation of Carbon Molecular Sieves 3 4.2 Characterisation of Carbon Molecular Sieves 13 4.2.1 Characterisation by Adsorption of Organic Vapours 14 4.2.2 Characterisation by Immersional Heats of Wetting 18 4.3 Adsorption by Carbon Molecular Sieve CHAPTER 5 ACTIVATED CARBON ADSORPTION APPLICATIONS 5.1 Liquid Phase Applications 2 5.1.1 Food Processing 2 5.1.2 Preparation of Alcoholic Beverages 3 5.1.3 Decolorisation of Oils and Fats 5 5.1.4 Activated Carbon Adsorption in Sugar Industry 5 5.1.4.1 Decolorisation with Powdered Activated Carbons 8 5.1.4.2 Decolorisation with Granulated Activated Carbons 9 5.1.5 Applications in Chemical and Pharmaceutical Industries 11 5.1.6 Activated Carbon for the Recovery of Gold 13 5.1.6.1 Mechanism of Gold Recovery 15 5.1.6.2 Desorption of Gold from Active Carbon Surface 22 5.1.7 Purification of Electrolytic Baths 24 5.1.8 Refining of Liquid Fuels 25 5.2 Gas Phase Applications 5.2.1 Recovery of Organic Solvents 26 5.2.2 Removal of Sulphur Containing Toxic Components from Exhaust Gases and Recovery of Sulphur 29 5.2.2.1 Removal of Sulphur Dioxide from Waste Gases 30 5.2.2.2 Removal of Hydrogen Sulphide and Carbon Disulphide 35 5.3 Activated Carbon Adsorption in Nuclear Technology 39 5.4 Activated Carbon Adsorption in Vacuum Technology 40 5.5 Medicinal Applications of Activated Carbon Adsorption 41 CHAPTER 6 ACTIVATED CARBON ADSORPTION AND ENVIRONMENT 1.Removal of Inorganics from Waste Water 6.1 Activated Carbon Adsorption of Inorganics from Aqueous Phase (General) 4 6.2 Activated Carbon Adsorption of Copper 11 6.2.1 Mechanism of Copper Adsorption 17 6.3 Activated Carbon Adsorption of Chromium 18 6.3.1 Mechanism of Adsorption of Cr (III) ions 26 6.4 Activated Carbon Adsorption of mercury 27 6.5 Adsorptive Removal of Cadmium from aqueous solutions 34 6.6 Activated Carbon Adsorption of Cobalt 39 6.7 Activated Carbon Adsorption of Nickel 43 6.8 Removal of Lead from Water 46 6.9 Adsorptive Removal of Zinc 49 6.10 Activated Carbon Adsorption of Arsenic 51 6.11 Adsorptive Separation of Cations in Trace Amounts from Aqueous Solutions 53 6.12 Mechanism of Metal Ion Adsorption by Activated carbons 56 CHAPTER 7 ACTIVATED CARBON ADSORPTION AND ENVIRONMENT 2. Adsorptive Removal of Organic Compounds 7.1 Activated Carbon Adsorption of Halogenated Organic Compounds 2 7.2 Activated Carbon Adsorption of Natural Organic Matter (NOM) 8 7.3 Activated Carbon Adsorption of Phenolic Compounds 12 7.4 Adsorption of Nitro and Amino Compounds 25 7.5 Adsorption of Pesticides 30 7.6 Adsorption of Dyes 34 7.7 Activated Carbon Adsorption of Drugs and Toxins 42 7.8 Adsorption of Miscellaneous Organic Compounds 44 7.9 Mechanism of Adsorption of Organics by Activated Carbons 49 CHAPTER 8 ACTIVATED CARBON ADSORPTION AND ENVIRONMENT 3. Removal of Hazardous Gases and Vapors 8.1 Removal of Volatile Organic Compounds (VOC) at Low Concentrations 1 8.2 Removal of Oxides of Nitrogen from Flue Gases 3 8.3 Removal of Sulphur Dioxide from Flue Gases 8 8.4 Evaporated Loss Control Devices 9 8.5 Protection of upper Respiratory Tract in Hazardous Environment 10 8.6 Activated Carbon Adsorption of Mercury vapors 17 8.7 Removal of Organic Sulphur Compounds 8.8 Adsorptive Removal of Miscellaneous Vapors and Gases 19 Author Index Subject Index Preface: Activated carbons are versatile adsorbents. Their adsorptive properties are due to their high surface area, a microporous structure and a high degree of surface reactivity. They are therefore, used to purify, decolorise, deodorize, dechlorinate, separate and concentrate in order to permit recovery and to filter, remove or modify the harmful constituents from gases and liquid solutions. Consequently activated carbon adsorption is of interest to many economic sectors and concern areas as diverse as food, pharmaceutical, chemical, petroleum, nuclear, automobile and vacuum industries as well as for the treatment of drinking water, industrial and urban waste water and industrial flue gases. Interest in activated carbon adsorption of gases and vapors received a big boost during and after the first World War while the pollution of the environment, which includes air and water, due to rapid industrialization and ever increasing use of the amount and the variety of chemicals in almost every facet of life has initiated increasing attention to the activated carbon adsorption from aqueous solutions. It was, therefore, thought worthwhile and opportune to prepare a text that describes the surface structure of activated carbons, the adsorption phenomenon and the activated carbon adsorption of organics and inorganics from gaseous and aqueous phase. A vast amount of research has been carried out in the area of activated carbon adsorption during the last four or five decades and the research publications are scattered in different journals published in different countries and in the proceedings and abstracts of the International Conferences and Symposia on the science and technology of activated carbon adsorbents. The book critically reviews the available literature and tries to offer suitable interpretations of the surface related interactions of the activated carbons. The book also contains consistent explanations for surface interactions applicable to the adsorption of a wide variety of adsorbates which could be strong or weak electrolytes. The book has been written with a view to equip the surface scientists (chemists, physicists, and technologists) with the surface processes, their energetics, and with the adsorption isotherm equations, their applicability to and deviations from the adsorption data both for gases as well as for solutions. To the carbon scientists and technologists the book should help understand the parameters and the mechanisms involved in the activated carbon adsorption of organic and inorganic compounds. The book thus combines in one volume the surface physical and chemical structure of activated carbons, the surface phenomenon at solid-gas and solid-liquid interfaces and the activated carbon adsorption of gaseous adsorbates and solutes from solutions. This unified approach will provide the reader access to the relevant literature and promote further research towards improving and developing newer activated carbon adsorbents and develop processes for the efficient removal of pollutants from drinking water and industrial effluents. The book can also serve as a text material for studies relating to adsorption and adsorption processes occurring on solid surfaces. The authors are grateful to Elsevier, Ann Arbor Science publishers, South African Institute of Mining and Metallurgy, Marcel Dekker, Multi-Science publishing Co, Society of Chemistry and Industry and various authors for permission to reproduce certain Figures and Tables. Professor Bansal also acknowledges the understanding, the cooperation, and the encouragement of his wife Rajesh. Dr. Meenakshi Goyal is grateful to her husband Er. Arvinder Goyal for his patience and her son Nikhil and daughter Mehak who continued to attain excellence in their schools. We also thank Mr. Tulsi Ram and Ms. Ruby Singh for typing the manuscript and preparing figures and tables. Roop Chand Bansal Meenakshi Goyal INTRODUCTION Activated Carbons: Activated carbon in its broadest sense includes a wide range of processed amorphous carbon-based materials. It is not truly an amorphous material but has a microcrystalline structure. Activated carbons have a highly developed porosity and an extended interparticulate surface area. Their preparation involves two main steps: the carbonization of the carbonaceous raw material at temperatures below 8000 C in an inert atmosphere and the activation of the carbonized product. Thus all carbonaceous materials can be converted into activated carbon, although the properties of the final product will be different, depending on the nature of the raw material used, the nature of the activating agent, and the conditions of the carbonization and activation processes. During the carbonization process most of the noncarbon elements such as oxygen, hydrogen and nitrogen are eliminated as volatile gaseous species by the pyrolytic decomposition of the starting material. The residual elementary carbon atoms group themselves into stacks of flat aromatic sheets crosslinked in a random manner. These aromatic sheets are irregularly arranged which leaves free interstices. These interstices give rise to pores which make activated carbons excellent adsorbents. During carbonization these pores are filled with the tarry matteer or the products of decomposition or atleast blocked partially by disorganized carbon. This pore structure in carbonized char is further developed and enhanced during the activation process which converts the carbonized raw material into a form that contains the greatest possible number of randomly distributed pores of various sizes and shapes giving rise to an extended and extremely high surface area of the product. The activation of the char is usually carried out in an atmosphere of air, CO2 or steam in the temperature range 800-9000 C. This results in the oxidation of some of the regions within the char in preference to others so that as combustion proceeds a preferential etching takes place. This results in the development of a large internal surface which in some cases may be as high as 2500 m2/g. Activated carbons have a microcrystalline structure. But this microcrystalline structure differs from that of graphite with respect to interlayer spacing which is 0.335 nm in case of graphite and ranges between 0.34-0.35 nm in activated carbons. The orientations of the stacks of aromatic sheets is also different being less ordered in activated carbons. ESR studies have shown that the aromatic sheets in activated carbons contain free radical structure or structure with unpaired electrons. These unpaired electrons are resonance stabilized which are trapped during the carbonisation process due to the breaking of bonds at the edges of the aromatic sheets, thus creating edge carbon atoms. These edge carbon atoms have unsaturated valencies and can therefore interact with heteroatoms such as oxygen, hydrogen, nitrogen and sulphur giving rise to different types of surface groups. The elemental composition of a typical activated carbon has been found to be 88% C, 0.5% H, 0.5% N, 1.0%S and 6-7%O, the balance representing the inorganic ash constituents. The oxygen content of an activated carbon can, however, vary depending on the type of the source raw material and the conditions of the activation process. The activated carbons in general, have a strongly developed internal surface and are usually characterized by a polydisperse porous atructure consisting of pores of different sizes and shapes. Several different methods used to determine the shapes of the pores have indicated ink bottle shape, regular slit shaped, V-shaped, capillaries open at both ends or with one end closed and many more. However, it has been difficult to obtain an accurate information on the actual shape of the pores. It is now well accepted that activated carbons contain pores from less than a nanometer to several thousand nanometers. The classification of pores suggested by Dubinin and accepted by International Union of Pure and Applied Chemistry (IUPAC) is based on their width which represents the distance between the walls of a slit shaped pore or the radius of a cylindrical pore. The pores in activated carbons are divided into three groups: the micropores with diameters less than 2 nm, mesopores with diameters between 2 and 50 nm, and macropores with diameters greater than 50 nm. The micropores constitute a large surface area (about 95% of the total surface area of the activated carbon) and micropore volume and therefore, determine to a considerable extent the adsorption capacity of a given activated carbon provided, however, that the molecular dimensions of the adsorbate are not too large to enter the micropores. The micropores are filled at low relative vapour pressure before the commencement of capillary condensation. The mesopores contribute to about 5% of the total surface area of the carbon and are filled at higher relative pressure with the occurrence of capillary condensation. Attempts, however, are now on to prepare mesoporous carbons. The macropores are not of considerable importance to the process of adsorption in activated carbons as their contribution to surface area does not exceed 0.5 m2/g. They act as conduits for the passage of adsorbate molecules into the micro and mesopores. As all the pores have walls, they will comprise two types of surfaces: the internal or the microporous surface and the external surface. The former represents the walls of the pores and has a high surface area which may be several thousands in many activated carbons while the latter constitutes the walls of the meso and macropores as well as the edges of the outerfacing aromatic sheets and is comparatively much smaller and may vary between 10-200 m2/g for many of the activated carbons. Besides the crystalline and porous structure activated carbon surface has a chemical structure. The adsorption capacity of an activated carbon is determined by their physical or porous structure but is strongly influenced by the chemical structure of the carbon surface. In graphites which have a highly ordered crystalline structure, the adsorption capacity is determined mainly by the dispersion component of the van der walls forces. But the random ordering of the aromatic sheets in activated carbons causes a variation in the arrangement of electron clouds in the carbon skeleton and results in the creation of unpaired electrons and incompletely saturated valencies which would undoubtedly influence the adsorption properties of activated carbons. Activated carbons are invariably associated with certain amounts of oxygen and hydrogen. In addition they may contain small amounts of nitrogen. X-ray diffraction studies have shown that these heteroatoms are bonded at the edges and corners of the aromatic sheets or to carbon atoms at defect positions giving rise to carbon-oxygen, carbon-hydrogen and carbon-nitrogen surface compounds. As these edges constitute the main adsorbing surface the presence of these surface compounds modifies the surface characteristics and surface properties of activated carbons. Carbon-oxygen surface groups are by far the most important surface group which influence the surface characteristics such as the wettability, the polarity, the acidity and the physico-chemical properties such as catalytic, electrical and chemical reactivity of these materials. In fact the combined oxygen has often been found to be the source of the property by which a carbon becomes useful and effective in certain respects. For example the presence of oxygen on the carbon surface has an important effect on the adsorption capacity of water and other polar gases and vapors, on their ageing during storage, on the adsorption of electrolytes; on the properties of carbon blacks as fillers in rubber and plastics; on the lubricating properties of graphite as well as on its properties as a moderator in nuclear reactors. In the case of carbon fibres these surface oxygen groups determine their adhesion to plastic matrices and consequently improve their composite properties. Although the identification and the estimation of the carbon-oxygen surface groups has been carried out using several physical, chemical and physio-chemical techniques which include their desorption, neutralization with alkalies, potential, thermometric and radiometric titrations and spectroscopic methods such as IR spectroscopy and x-ray photoelectron spectroscopy, the precise nature of the chemical groups is not entirely established. The estimations obtained by different workers using varied techniques differ considerably because the activated carbon surface is very complex and difficult to reproduce. The surface groups can not be treated as ordinary organic compounds because they interact differently in different environments. They behave as complex structures presenting numerous mesomeric forms depending upon their location on the same polyaromatic frame. The aromatic sheets constituting the activated carbon structure have limited dimensions and therefore have edges. In addition these sheets are associated with defects, dislocations and discontinuities. The carbon atoms at these places have unpaired electrons and residual valencies and are richer in potential energy. These carbon atoms are highly reactive and are called active sites or active centers and determine the surface reactivity, surface reactions and catalytic reactions of carbons. The impregnation of activated carbons with metals and their oxides, dispersed as fine particles makes them extremely good catalysts for certain industrial processes. The impregnation of metals also modifies the gasification characteristics and varies the porous structure of the final product. Several inorganic and organic reagents when present on the carbon surface also modify the surface behaviour and adsorption characteristics of activated carbons and make them useful for the removal of hazardous gases and vapours by chemisorption and catalytic decomposition. Adsorption: Adsorption arises as a result of the unsaturated and unbalanced molecular forces which are present on every solid surface. Thus when a solid surface is brought into contact with a liquid or a gas, there is an interaction between the fields of forces of the surface and that of the liquid or the gas. The solid surface tends to satisfy these residual forces by attracting and retaining on its surface the molecules, atoms or ions of the gas or the liquid. This results in a greater concentration of the gas or the liquid in the near vicinity of the solid surface than in the bulk gas or vapor phase irrespective of the nature of the gas or the vapor. The process by which this surface excess is caused is called adsorption. The adsorption involves two types of forces: physical forces which may be dipole moments, polarization forces, dispersive forces or short range repulsive interactions and chemical forces which are valency forces arising out of the redistribution of electrons between the solid surface and the adsorbed atoms. Depending upon the nature of the forces involved, the adsorption is of two types: physical adsorption and chemisorption. In the case of physical adsorption, the adsorbate is bound to the surface by relatively weak van der walls forces which are similar to the molecular forces of cohesion and are involved in the condensation of vapors into liquids. Chemisorption, on the other hand involves exchange or sharing of electrons between the adsorbate molecules and the surface of the adsorbent resulting in a chemical reaction. The bond formed between the adsorbate and the adsorbent is essentially a chemical bond and is thus much stronger than in the physisorption. Two types of adsorptions differ in several ways. The most important difference between the two kinds of adsorption is the magnitude of the enthalpy of adsorption. In physical adsorption, the enthalpy of adsorption is of the same order as the heat of liquefaction and does not usually exceed 10-20 KJ per mol whereas in chemisorption the enthalpy change is generally of the order of 40-400 KJ per mol. Physical adsorption is non specific and occurs between any adsorbate-adsorbent system while chemisorption is specific. Another important point of difference between physisorption and chemisorption is the thickness of the adsorbed phase. Whereas it is multimolecular in physisorption the thickness is unimolecular in chemisorption. The type of adsorption that takes place in a given adsorbate-adsorbent system depends on the nature of the adsorbate, the nature of the adsorbent, the reactivity of the surface, the surface area of the adsorbate, and the temperature and pressure of adsorption. When a solid surface is exposed to a gas, the molecules of the gas strike the surface of the solid when some of these striking molecules stick to the solid surface and become adsorbed while some others rebound back. Initially the rate of adsorption is large as the whole surface is bare but the rate of adsorption continues to decrease as more and more of the solid surface becomes covered by the adsorbate molecules. However, the rate of desorption which is the rate at which the adsorbed molecules rebound from the surface increases because desorption takes place from the covered surface. With the passage of time the rate of adsorption continues to decrease while the rate of desorption continues to increase until an equilibrium is reached where the rate of adsorption is equal to the rate of desorption. At this point the solid is in adsorption equilibrium with the gas. It is a dynamic equilibrium because the number of molecules sticking to the surface is equal to the number of molecules rebounding from the surface. As the amount adsorbed at the equilibrium for a given adsorbate-adsorbent system depends upon the pressure of the gas and the temperature of adsorption, the adsorption equilibrium can be represented as an adsorption isotherm at constant temperature, adsorption bar at constant pressure and adsorption isostere for a constant equilibrium adsorption. In actual practice the determination of adsorption at constant temperature is most convenient and therefore, the adsorption isotherm is the most extensively employed method for representing the equilibrium states of an adsorption system. The adsorption isotherm gives useful information regarding the adsorbate, the adsorbent and the adsorption process. It helps in the determination of the surface area of the adsorbent, the volume of the pores and their size distribution. It also provides important information regarding the magnitude of the enthalpy of adsorption and the relative adsorbility of a gas or a vapor or a given adsorbent with respect to chosen standards. The adsorption data can be represented by several isotherm equation the more important being the Langmuir, the Freundlich, the Brunauer-Emmett-Teller (BET) and Dubinin equations. The first two isotherm equations apply equally to physisorption as well as chemisorption. The BET and Dubinin equations are most important for the analysis of physical adsorption of gases and vapors on porous carbons. Langmuir isotherm equation is the first theoretically developed adsorption isotherm which was derived using thermodynamic and statistical approaches. The applicability of the equation to the experimental data was carried out by a large number of investigators but deviations were often noticed. According to this isotherm equation the plot of p/v against p should be linear from ? = 0 to ? = ? and it should give a reasonable value of Vm (the monolayer capacity) which should be temperature independent. However, few data conform to this criteria. Similarly several chemisorption results are known where the Langmuir equation is valid only with in a small restricted range. Thus although Langmuir isotherm equation is of limited significance for the interpretation of the adsorption data because of its idealized character, the equation remains of basic importance for expressing dynamic adsorption equilibrium. Furthermore, it has provided a good basis for the derivation of other more complex models. The assumptions that the adsorption sites on solid surfaces are energetically homogeneous and that there are no lateral interactions between the adsorbed molecules are the weak points of this model. Brunauer, Emmet and Teller derived the BET equation for multimolecular adsorption by a method which is the generalization of the Langmuir treatment of unimolecular adsorption. These workers proposed that the forces acting in multimolecular adsorption are the same as those acting in the condensation of vapors. Only the first layer of adsorbed molecules, which is in direct contact with the adsorbent surface is bound by adsorption forces originating from the interaction between the adsorbate and the adsorbent. Thus the molecules in the second and subsequent layers have the same properties as in the liquid or gaseous phase. The BET equation has played a significant role in studies of adsorption because it represents the shapes of the actual isotherms. It also gives reasonable values for the average enthalpy of adsorption in the first layer and satisfactory values for Vm the monolayer capacity of the adsorbate which can be used to calculate the specific area of the solid adsorbent. The BET equation is applicable within the relative pressure range of 0.05-0.35. The failure of the equation below and above this range of relative pressures has been attributed to the faulty and simplifying assumptions of the theory. The failure below a relative pressure of 0.05 is due to the heterogeneity of the adsorbent surface. Activated carbon and inorganic gel surfaces which are important adsorbents are generally energetically heterogenous i.e. the enthalpy of adsorption varies from one part of the surface to another. At higher relative pressures, the BET equation loses its validity because adsorption by capillary condensation along with physical adsorption also takes place. The assumption that the adsorbate has liquid-like properties after the first layer is difficult to reconcile because both porous and nonporous adsorbents exposed to a saturated vapor sometimes adsorb strictly a limited amount and not the infinitely large quantity as postulated by the BET model. Thus the limited validity of the BET equation is due to the shortcomings in the model itself rather than to our lack of knowledge of the various parameters such as the number of layers, the heat of adsorption or the evaporation constant etc. in the higher layers. The potential theory of adsorption and the Dubinin equation, which is based on it, have been developed primarily for microporous adsorbents for which they have proved to be better than all other theories. Dubinin and coworkers, while investigating the effect of surface structure of activated carbons on the adsorbability of different vapors and of different solutes from solutions on active carbons, observed that over a wide ranges of values of adsorption, the characteristic curves of different vapors on the same adsorbent were related to each other. In fact, it was observed that if the adsorption potential corresponding to a certain volume of adsorption space on the characteristic curve for one vapor was multiplied by a constant called the affinity coefficient, the adsorption potential corresponding to the same value of adsorption space on the characteristic curve of another vapor was obtained. Based on these observations the characteristic curves for microporous activated carbons were expressed analytically by a Gaussian distribution equation between the total limiting volume of the adsorption space and the adsorption potential. This further made it possible to obtain an equation of the adsorption isotherm and to calculate the appropriate micropore volume. The Dubinin equation is valid over the range of relative pressures from 1x10-5 to 0.2 or 0.4 which corresponds to about 85-95% filling of the micropores. At relative pressures below 10-5 extremely ultra fine micropores which are not accessible to larger molecules are filled. Thus the potential theory of adsorption together with the Dubinin equation represent the temperature dependence of adsorption and enable calculation of important thermodynamic functions such as the heat and entropy of adsorption. The Dubinin equation has been further modified by Kagner to yield a method for calculating the specific surface area from these isotherms. He confined his attention to monolayer region and assumed that adsorption at very low relative pressures results in the formation of a unimolecular layer on the walls of all the pores. The method thus yields monolayer capacity rather than the micropore volume. The method is applicable in the low pressure region of the isotherm (below relative pressure of 10-4). The surface areas calculated by Kaganer method for activated carbons were within few percent of those calculated from the BET equation. The Freundlich isotherm equation is a limiting form of the Langmuir isotherm and is applicable only in the middle ranges of vapor pressure. The equation is of greater significance for chemisorption although some physical adsorption data have also been found to fit this equation. Adsorption from solutions on activated carbons has wide applications in food, pharmaceutical and other process industries to remove unwanted components from the solution. However, a theoretical analysis of adsorption from solution and the derivation of a suitable adsorption equation have been comparatively difficult because both the components of a solution compete each other for the available surface. Furthermore, the thermal motion of the molecules in the liquid phase and their mutual interactions are much less well understood. It is, therefore, difficult to correctly assess the nature of the adsorbed phase whether unimolecular or multimolecular. The adsorption of a solute from a solution is usually determined by the porosity and the chemical nature of the adsorbent, the nature of the components of the solution, the concentration of the solution and its pH and the mutual solubility of the components in the solution. The adsorption of a nonpolar solute will be higher on a nonpolar adsorbent. But since there is competition between the solute and the solvent, the solvent should be polar in nature for the solute to be adsorbed preferentially. The other factor that also determines the adsorption from solutions is the steric arrangement or the chemical structure of the adsorbate molecule. As the activated carbons have a highly microporous structure, some of the pores may be inaccessible to larger molecules of the adsorbate. Thus the experimentally simple technique of adsorption from solution can be developed into a method to determine surface area, microporosity, oxygen content and the hydrophobicity of the carbon surface. The adsorption from solutions is also receiving further attention because of the growing importance of environmental control involving purification of waste water using activated carbons. Adsorption from solutions can be classified into adsorption of solutes which have a limited solubility (i.e. from dilute solutions) and adsorption of solutes which are completely miscible with the solvent in all proportions. In the former case, the adsorption of the solvent is of little consequence and is generally neglected while in the latter case, the adsorption of both the components of the solution plays its part and has to be considered. The adsorption in such a system is the resultant of the adsorption of both the components of the solution. The adsorption from such solutions is represented in the form of a composite isotherm which is a combination of the isotherms for the individual components. Activated Carbon Adsorption: Carbon surface has a unique character. It has a porous structure which determines its adsorption capacity, it has a chemical structure which influences its interaction with polar and nonpolar adsorbates, it has active sites in the form of edges, dislocations and discontinuities which determine its chemical reactions with other atoms. Thus the adsorption behaviour of an activated carbon can not be interpreted on the basis of surface area and pore size distribution alone. Activated carbons having equal surface area but prepared by different methods or given different activation treatments show markedly different adsorption properties. The determination of a correct model for adsorption on activated carbon adsorbents with complex chemical structure is therefore, a complicated problem. A proper model must take into consideration both the chemical and the porous structure of the carbon which includes the nature and the concentration of the surface chemical groups, the polarity of the surface, the surface area and the pore size distribution as well as the physical and chemical characteristics of the adsorbate such as its chemical structure, polarity and molecular dimensions. In the case of adsorption from solutions, the concentration of the solution and its pH are also important additional factors. Thus activated carbons are excellent and versatile adsorbents. Their important applications are the adsorptive removal of color, odor and taste and other undesirable organic and inorganic pollutants from drinking water, in the treatment of industrial waste water, air purification in inhabited spaces such as in restaurants, food processing and chemical industries, for the purification of many chemical, food and pharmaceutical products, in respirators for work under hostile environments and in a variety of gas phase applications. Their use in medicine and health applications to combat certain types of bacterial ailments and for the adsorptive removal of certain toxins and poisons and for the purifications of blood is being fast developed. Activated carbons can be used in various forms: the powdered form, the granulated form and now in the fibrous form. Powdered activated carbons (PAC) generally have a finer particle size of about 44 ?m which permits faster adsorption but they are difficult to handle when used in fixed adsorption beds. They also cause a high pressure drop in fixed beds which are difficult to regenerate. The granulated activated carbon (GAC) have granules of 0.6-4.0 mm size and are hard, abrasion resistant resistant and relatively dense to withstand operating conditions. Although more expensive than PAC, they cause low hydrodynamic resistance and can be conveniently regenerated. GAC can be formulated into a module which can be removed after saturation, regenerated by heat treatment in steam and used again. The fibrous activated carbon fibres (ACF) are expensive materials for waste water treatment but they have the advantage of capability to be molded easily into the shape of the adsorption system and produce low hydrodynamic resistant to flow. The most important application of activated carbon adsorption where large amounts of activated carbons are being consumed and where the consumption is ever increasing are the purification of air and water. There are two types of adsorption systems for the purification of air. One is the purification of air for immediate use in inhabited spaces where free and clean air is a requirement. The other system prevents air pollution of the atmosphere from industrial exhaust streams. The former operates at pollutant concentrations below 10 ppm, generally about 2-3 ppm. As the concentration of the pollutant is low, the adsorption filters can work for a long time and the spent carbon can be discarded since regeneration may be expensive. Air pollution control requires a different adsorption set up to deal with larger concentrations of the pollutants. The saturated carbon needs to be regenerated by steam, air or nontoxic gaseous treatments. These two applications require activated carbons with different porous structures. The carbons required for the purification of air in inhabited spaces should be highly microporous to affect greater adsorption at lower concentrations. In the case of activated carbons for air pollution control, the pores should have higher adsorption capacity in the concentration range 10-500 ppm. It is difficult to specify the pore diameters exactly but generally in the micro and meso range are preferred because they fill in this concentration range. The effluent gases from industry and processing units contain a large number of pollutants such as oxides of nitrogen and sulphur, H2S and vapors of CS2, styrene and several solvents such as ethanol, toluene etc. Many of these compounds can be economically recovered when present in large amounts. However, when present in low concentrations, these volatile organic compounds need to be removed from the flue gases before they are mixed with air. Activated carbon is one of the important adsorbent which is used for the recovery of useful compounds when economically viable and for adsorptive removal of the pollutant gases and vapors when present in small amounts. In addition many of these VOC are released from the exhaust of automobiles on the roads. In order to reduce this VOC release, catalyst converters are being used to convert VOC into CO2 and water vapors. The release of these VOCs can be further decreased by fitting the automobiles with activated carbon canisters. However, in addition to the porous structure of activated carbons, their surface chemistry is also of considerable interest. For personal protection when working under hostile environment, the activated carbons used in respirators are also different. When working in chemical industry the respirators can use ordinary activated carbons because the pollutants are generally of low toxicity. However, for protection against warfare gases such as chloropicrin, cynogen chloride, hydrocynic acid and nerve gases, special types of impregnated activated carbons are used in respirators and body garments. These activated carbons can protect by physical adsorption, chemisorption and catalytic decomposition of the hazardous gases. More than 800 specific organic and inorganic chemical compounds have been identified in drinking water. These compounds are derived from industrial and municipal discharge, urban and rural runoff, natural decomposition of vegetable and animal matter as well as from water and waste water chlorination practices. Liquid effluents from industry also discharge varying amounts of a variety of chemicals into surface and ground water. Many of these chemicals are carcinogenic and cause many other ailments of varying intensity and character. Several methods such as coagulation, oxidation, aeration, ion exchange and activated carbon adsorption have been used for the removal of these chemical compounds. Many studies including laboratory tests and field operations have indicated that the activated carbon adsorption is perhaps the best broad spectrum control technology available at the present moment. An activated carbon in contact with a salt solution is a two phase system consisting of a solid phase which is the activated carbon surface and a liquid phase which is the salt solution containing varying amounts of different ionic and molecular species and their complexes. The interface between the two phases acts as an electrical double layer and determines the adsorption processes. The adsorption capacity of an activated carbon for metal cations from the aqueous solutions generally depends on the physico-chemical characteristics of the carbon surface which include surface area, pore size distribution, electro-kinetic properties and the chemistry of the carbon surface as well as the nature of the metal ions in the solution. Activated carbons are invariably associated with acidic and basic carbon-oxygen surface groups. The acidic groups which have been postulated as carboxyls, lactones and phenols render the carbon surface polar and hydrophilic while the basic groups have been postulated as pyrones and chromenes structures. A perusal of the literature indicates that the more important parameters which influence and determine the adsorption of metal ions from aqueous solutions are the carbon-oxygen functional groups present on the carbon surface and the pH of the solution. These two parameters determine the nature and the concentration of the ionic and molecular species in the solution. Electrokinetic studies have shown that the nature and the concentration of the carbon surface charge can be modified by changing the pH of the carbon-solution system. The activated carbon surface has a positive charge below pHzpc (zero point charge) and a negative charge above ZPC upto a certain range of pH values. The origin of the positive charge on the activated carbon surface has been attributed to the presence of basic surface groups, the excessive protonation of the surface at low pH values and to graphene layers which act as Lewis bases resulting in the formation of acceptor-donor complexes important for the adsorption of many organic compounds from aqueous solutions. At higher pH values, the carbon surface has a negative charge due to the ionization of acidic carbon-oxygen surface groups. Thus the adsorption of metal ions mainly involves electrostatic attractive and repulsive interactions between metal ionic species in the solution and the negative sites on the carbon surface produced by the ionization of acidic groups. The dispersive interactions between the ionic species in the solution and the graphene layers and the surface area of the carbon surface play a smaller role in the adsorption of inorganics. In the adsorption of organics, however, the situation is quite different. The organic compounds present in water can be polar or nonpolar so that not only electrostatic interactions but dispersive interactions will also play an important role. In addition the hydrogen bonding is also an important consideration in the adsorption of certain polar organic molecules. The molecular dimensions of the organic molecules also have a wide variation. Thus the porous structure of the activated carbon which includes the existence of mesopores shall also have an important consideration for the adsorption of essentially nonpolar organic molecules, because a certain proportion of the microporosity may not be accessible to very large organic molecules. The book has been written in eight chapters which covers activated carbons, their surface structure, the adsorption on solid surfaces and the models of adsorption, adsorption from solution phase, the preparation, characterization of and adsorption by carbon molecular sieves, important applications of activated carbons with special emphasis on medicinal and health applications and the use of activated carbons in environmental clean up. The crystalline, the microporous and the chemical structure of the activated carbon surface has been dealt with in Chapter 1. The classification of pores and their contribution to surface area and adsorption capacity; the nature and characteristics of carbon-oxygen surface groups, the methods of their identification and estimation using physical, chemical and physico-chemical methods which include XPS and latest innovations in infra red spectroscopy have been discussed. The influence of these surface groups on the adsorption characteristics and adsorption properties has been delineated. The adsorption on a solid surface, the types of adsorption, the energetics of adsorption, the theories of adsorption and the adsorption isotherm equations such as the Langmuir equation, the BET equation, the Dubinin equation, Temkin equation and the Freundlich equation are the subject matter of Chapter 2. The validity of each adsorption isotherm equation to the adsorption data has been examined. The theory of capillary condensation and the adsorption-desorption hysteresis and the Dubinin theory of volume filling of micropores (TVFM) for microporous activated carbons also find place in this chapter. The adsorption from binary solutions on solid adsorbents in general and on activated carbons in particular is discussed in Chapter 3. The nature and the types of adsorption and adsorption isotherms from dilute solutions and from completely miscible binary solutions have been described. The composite isotherm equation has been derived. The shapes and the classification of composite isotherms and the influence of adsorbate-adsorbent interactions, the heterogeneity of the carbon surface, the size and orientation of the adsorbed molecules on the shapes have been examined. The thickness of the adsorbed layer and the determination of individual adsorption isotherms from a composite isotherm has also been described. The fourth chapter describes briefly the preparation of carbon molecular sieves by pore blocking of activated carbons, decomposition of H2S or CS2 and depositing sulphur, by decomposition of benzene or other hydrocarbons and deposition of carbon and by impregnation of PVC followed by its decomposition. The characterization of carbon molecular sieves by molecular probe methods using adsorption of inorganic gases and organic vapors varying in size and shape and by immersional heats of wetting in liquids of varying sizes has been discussed. The applications of CMS for the separation of different gaseous mixtures have also been examined. The last four chapters (Chapters 5 to 8) have been devoted to important applications of activated carbon adsorption. The most general liquid phase and gas phase applications of activated carbons with special reference to the nature of the carbon surface and the form of the activated carbon have been discussed in Chapter 5. Special emphasis has been directed towards the medicinal and health applications. Different types of carbons prepared from different source raw materials and using different activation treatments have been examined for the control of drug overdose, control of antibacterial activities against certain bacteria and to remove toxins and poisons from human body and for the purification of blood by hemoperfusion. The next two chapters are concerned with the adsorptive removal of inorganic (Chapter 6) and organic (Chapter 7) pollutants from drinking and waste waters. The various parameters which are involved in the removal of hazardous organics and inorganics have been reviewed and the mechanisms involved have been suggested. The subject matter of the eighth chapter is the adsorptive removal of hazardous gases and vapors from the industrial flue gases and the automobile exhaust. The use of activated carbon in respirators for work under hostile environments has also been discussed.
Library of Congress Subject Headings for this publication:
Carbon, Activated.
Carbon -- Asorption and adsorption.