Journal of Scientific & Industrial Research Vo1.58, November 1 999, pp 844-860

Adsorption Techniques - A Review

A K Bajpai' and M Rajpoot Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College,

Jabalpur 482 00 1 , India

Received : I I August 1 998; accepted : 1 0 August 1 999

The industrial significance of adsorption phenomenon and some i mportant aspects of the adsorption technqiue are discussed. A brief i ntroduction on some fundamentals of adsorption and a modern theory which is based on the concept of excess surface work is included. Some noble adsorbents, which are significant from industrial poitn of view, are reported. Some important spectroscopic techniques like IR, Raman, NMR, etc., along with other techniques are discussed' and some important results from spectroscopic investigation are also i ncluded. Also, the work done by the other workers in the past decade, specially in the adsorption of low molecular weight compounds onto metal oxide, activated carbon and some miscellaneous adsorbents are critically reviewed.

Introduction

Adsorption is surface phenomenon which may be de­fined in terms of an unit operation in the chemical engi­neering sense and that operation which deals primarily with the utilization of surface forces ; and concentration of materials on the surface of solid bodies referred to as adsorption. Alternatively, it may be defined as either. partitioning of chemical species between bulk phase and an interface, or accumulation of substances near the in­terface. An interface has the special properties of ad­sorption was first theoretically systematised by J Willard Gibbs (Figure 1 ).

Although adsorption may be classified in different manners such as localized, nonlocalized, negative-posi­tive, static-dynamic, etc. , the classification on the basis of the strength of the binding forces is more common, i .e. , physical and chemical adsorption. Physical adsorp­tion involves vander Waal 's forces, hydrogen bonding and ion-exchange processes while electrostatic, cova­lent and co-ordinate displacements are the main factors in chemical adsorption.

Modern Theory of Adsorption

The behaviour of an adsorbing substance onto sur­faces has been interpreted by thermodynamic, statisti­cal, mechanical, and more complicated mathematical treatments. More than forty theories have been de vel-

. Author for correspondence

GAS

(a)

(c)

- E

E

K K L

0 0

0 0

F

(b)

0 0 0 N

o n. 0 H

0 0 0 M

(d) Figure I - Diagrammatic representation of different types of Inter­

face : (a) Solid-gas interface at E, E; (b) Solid-liquid inter­face at F, F and liquid-vapour interface at G; (c) Solid-solid interface at K, K, etc. Lumps of solid L are embedded in solid J (e.g. graphite in cast iron); (d) Liquid-liquid inter­face at H, H Drops of liquid N in liquid M (e.g. oil in water)

oped so far to describe adsorption and reviewed earlierl . However, none of the theories could give complete de­scription of the adsorption behaviour. The very first re­quirement is that the proposed model should fit the ex­perimental isotherm.

.

A recent theory using the concept of excess surface work has been proposed by Adolphs and Setzer and it i s

�

t

)..

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 845

claimed that the theory can be used for adsorption from gas or liquid phases. Regarding the precision and range of validity, the proposed theory i s an improvement on other methods, specially the BET method. According to this theory the adsorption system where particles are adsorbed on a solid surface or on another phase should be considered as an open system in mechanical and thermoequi librium. It is worth mentioning here that physico-chemical equilibrium is not presumed. To such an open system extended Gibb's model proposed by Defay et at. 3 can be appl ied, which also includes multi­layered adsorption. Assuming the temperature (T) and chemical potential (/1) to be equal in the gas (or liquid) phase and the adsorbed phase of an adsorption system, we can write :

. . . ( i )

I t i s known that for a real gas, the chemical potential is defined as fugacity (j), i .e . ,

11 . = I1(T) + RT . inif/f) . . .. (2)

Here, the index s denotes the phase saturation . If pro­cess of adsorption is isothermal, then pressure may be used in place of fugacity (the simplification may create some problem for adsorption systems consisting of so­lution and ions) . If change in chemical potential during adsorption is expressed by the ratio of pressure of the gas (P) to saturation vapour pressure (P), then it fol­lows that :

L1/1 = RT . In(P/ PJ . . . (3)

A new adsorption parameter <I> (excess surface work) may be defined as :

. . . (4)

This function is homogeneous of first degree in the adsorbed amount (nad). Now, it is evident that in an ad­sorption process when the adsorbed amount i s positive the excess surface work wil l be negative. At saturation of the vapour pressure (Ps)' no further changes occur, also <I> becomes zero, both at zero coverage and at infi­nite coverage, when the change in the chemical poten­tial diminishes. It is evident that the plots drawn between the excess surface work (<1» and amount adsorbed should possess a minima. The theory was further tested by us­ing the data from experimental isotherms by different authors and it was observed that in majority of examples, a minimum was evident. To explain the origin of minima, it was supposed that with increasing amount of adsorbed molecules, there is an increase in the total interaction between adsorbate molecules themselves (vertical and lateral interactions). From this logic, it should be antici­pated that during adsorption process in the mean time, the optimal state of lowest energy will be attained. It has now been established by dielectric constant measure­ments and the latest computer simulation4 that after completion of first adsorbed layer, the adsorption be­gins to grow in the further layers. However the authors of the theory suggested that this type of adsorption mechanism cannot be general ized. With the help of pro­posed theory, it was shown possible to calculate the spe­cific surface area of an adsorbent related to its dry mass provided the monolayer capacity and the specific sur­face occupied by an adsorbed molecule are known. The monolayer capacity can be correctly evaluated from a slope and intercept of straight l ine, drawn in accordance with the proposed integral method.

Dr A K Bajpai is workillf? as all Assistant Professor ill Chelllistry Department, Government A utono­mOils Science College, Jabalpllr (MP) for the past 13 y. Dr Bajpai obtained his Masler's degree ill Physical Chemislly in 1979 and doctoral degree ill 1 984 from .Iabalpur University. Topic of his research being "Studies Oil Addi/ion Polymerization ". He has more thall fifty publications to his credit published in reputed journals. His currellt research inlerests include: Adsorption of low and high molecular //lass compounds and polymer sYlZlhesis. He is a member of Royal Society of Chem­istry ( UK). He is also Life Member of Indian Chemical Society, Indian Science Congress Association and Indian Association of Nuclear Chemisls alld A llied Sciellfisls.

Miss Manjulala Rajpoot obtained her Master 's d('gree ill Organic Chemislry in 1 992from .Iabalpur University. She has three year research experience and presenlly working as a Senior Research

Fellow (CSIR), perceiving doctoral studies IInder Ihe guidance of Dr A K Bajpai on AdsOlption of SulpllOnamides at the Solid-Liquid Interface ill Department of Chemistry, Government Autonomous Science College, Jaba/pU/: She has published Ihree research papers ill reputed journals.

846 J SCI IND RES VOL.58 NOVEMBER 1 999

0 1 2 3

"""' 4 � 5 '-' t9 6

1 8 9

0 1 4 e

b ) c ) d ) e ) f) �) h )

Figure 2 - Plot of the nonnalized excess suiface work (<<1» against the degree of coverage (e).The curves (b) and (c) represent two dimensional gas, (d), (e) and (f) physical adsorption, and (g) and (h) represent chemical adsorption

k2

kl k2 2 2

l-f 3 fJi \ A

( a ) (b) ( c )

Figure 3 - (a) Adsorption vessel used for studying the electrical conductivity of evaporated metal films : A = filament for the evapora­tion of a metal film; k ) and k2 = platinum contacts, (b) Adsorption vessel for studying the electrical conductivity of powders : A = powder adsorbent; k) and k2 = two gold electrodes, (c) Four-electrode system for measuring the electrical conductivity of semiconductors : I = electrodes supplying the electrical current; 2 = voltage-detecting electrodes; 3 = adsorbent semicon­ductors

Based on the above adsorption theory, Adolphs and Setzer made a c lassification of �dsorption isotherms which is categorized on the basis of the excess surface work. The above theory reveals that a plot between ex­.c ess surface work and the adsorbed amount gives a mini­mum in the monolayer capacity. Upon plotting surface coverage (8) and excess surface work for different types of adsorption systems, various adsorption isotherms are obtained, as shown in Figure 2. In the Figure 2, the curves (b) and (c) represent a two dimensional gas, (d), (e) and (f) physical adsorption, and (g) and (h) refers to chemi­cal adsorption. This classification refers to the strength

of the bonding energy. Thus, the new model allows an energetic classification of adsorption phenomena of sur­face film in the energetic range from weak physical ad­sorption to strong chemical adsorption . .

Experimental Techniques for Studying Adsorption The experimental techniques for studying adsorption

can be divided mainly into three classes : ( 1 ) Techniques based on the measurement of changes in the electrical, magnetic and work function properties of solid adsor­bent during adsorption, (2) Techniques based on the study of interaction of radiation, electrons and ions with the adsorbed layer, and (3) Spectroscopic methods.

. .

... . "

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 847

D-fype + 1 H "-tYpe - +

41

I �-. 8

7

( a )

4 Figure 4 - (a) The principle of Hall. effect measurements; H = direction of magnetic field intensity; A = adsorbent in the form of

a plate; I = direction of the electric current; U H = Hall voltage, (b) Adsorption vessel for measuring the Hall effect on thin layers; 1 and 2 = electric current inputs; R = resistor; 3 and 4 = electrodes for detecting the Hall voltage; 5 and 6 = shields limiting the size of the adsorbent layer; 7 = filaments for the film evaporation; 8 and 9 = poles of the magnet; 1 0 =

evaporated adsorbent layer

(1) Techniques Based on the Measurement of Elec­trical, Magnetic and Work Function Properties of Adsorbent

(a) By the Measurement of Electrical Conductivity of the

Adsorbent

For measuring the changes in electrical conductivity, caused due to adsorption of metals, polycrstalline mate­rials and semiconductors which are normally taken in the form of evaporated film, powder, and pressed tablet, respectively and special type of adsorption vessels are used (Figure 3).

The electrical conductivity of the metal is measured with the aid of d c wheatstone bridge, while in the case of semiconductor it is recommended that a four elec­trode system be used in order to avoid the influence of polarization in the region of the electrode-semiconduc­tor contact. Two electrodes are used to supply the cur­rent, while two other electrodes detect the voltage6•

Changes in the number of current carriers (electrons and holes) as well as variation in their mobility are re­flected as changes in the electrical conductivity. In real cases the theoretical interpretation of the results of con­ductivity measurement is difficult, since other factors may also be important, e.g., the tunnel effect between particles of the film or powder, variation in the specific resistance of the film and the influence of nonuniform coverage of the sample. As a consequence of these fac-

tors, calculation of the number of electrons participat­ing in the interaction of one adsorbent atom with one · adsorbate molecule, as used in the l iterature is not physi­call y justified.

(b) By the Measurement of Hall Effect

For measurement of electrical conductivity of semi­conductors, some authors used measurement of Hall volt­age.

When a plate-shaped semiconductor adsorbent with an electric current (1) passing through it, is placed in a magnetic field (B), an electric field (E) is induced in the direction perpendicular to both I and B and charge carri­ers (holes or electrons) will be deviated in the transverse direction. This phenomenon known as the Hall effect, is used to determine whether a semiconductor is n- or p­type and to find the current carrier concentration.

In a stationary state, a potential difference (between both side of plate-shaped adsorbent) can be measured in the direction perpendicular to the direction of electric current and is known as Hall voltage. From the polarity of this Hall voltage, the sign of the current carriers may be determined (Figure 4). The magnitude of Hall volt­age is proportional to the product of the intensities of the electric and magnetic fields and proportionality con­stant is called Hall constant, which is inversely propor-

848 J SCI IND RES VOL.58 NOVEMBER 1 999

tional to the concentration of current carriers : A posi­tive value of this constant corresponds to positive cur­rent carriers (holes) and a negative value corresponds to negative carriers (electrons). This relationship is useful in the adsorption study. However, in the case where both current carriers, electrons and holes at the same time contribute to the conductivity and for ferromagnetic and paramagnetic substances, the relation between the Hall constant and number of carriers becomes more complex.

(c) By the Measurement of Magnetic Properties

Determination of the degree of oxidation of transi­tion metal ions in ionic crystals or the number of un­paired electrons in transition metals gives useful infor­mation concerning the measurement of the magnetisation or of the magnetic susceptibility, most often used in ad­sorption studies, carried out in the form of evaporated film of adsorbent.

Following two methods have been used for this pur­pose : (i) The Gouy method, measuring the force attract­ing the sample into a homogeneous magnetic field and (ii) The Faraday method, measuring the force which at­tracts the sample into an inhomogeneous magnetic field. With both methods, forces are measured by means of torsion or spiral balance (Figure 5) , in which the magnetisation was determined by measuring the torsion forces which kept the sample (evaporated film) at an angle of 45" to the direction of the external magnetic field. With powdered sample, the results of magnetisation measurements are difficu't to interpret because of the problem of surface cleanliness?

(d) By the Measurement of Work Function

Thermionic work function (<1» is given by Eq.(5).

<1> = F + eX, x . . . (5)

where <1> being the work necessary to remove an elec­tron from the highest populated level in a metal to a point outside, X being the work to take the charge across the phase boundary into the interior, giving the surface po­tential jump (X) and Fx is the electrochemical potential, defined as the total work of bringing a species from vacuum into a phase. The work function across a phase boundary is strongly affected by the presence of adsorbed species . In general, emission from the regions with low work function can only be studied, since the work func­tion usually rises with adsorption. The thermionic work

N

Figure 5 - Vessel for studying the magnetic properties of thin metal­lic layers; A = substrate for the adsorbent; z = mirror; n = evaporation filament; N = winding spool (to move the carrier plate A in front of the filament n); I = iron core; M) and MI = magnets controlling the winding spool; m ) and m2 = magnets fixed to the ring K which rotate the torsion filament T; 11) and III magnet poles

function for metals may be measured quite accurately and an extensive l iterature exists on the subject. A metal spontaneously emits electrons than that of the positive ions of the metals. Alternatively, if a metal filament is negative charged, electrons will flow from it to the an­ode, the rate of emission is highly temperature depen­dence, and <1> may be calculated from this temperature dependence. Experimental determination of work func­tion in the case of polycrystalline material is compli­cated, while work function of individual macroscopic crystallographic planes may be determined with the aid of field emission and field ion microscope method.

Field Emission and Field Ion Microscope Method

The field emission microscope was invented by Muller and the technique and related developments have become a major contributions to the structural study of surfaces . Fowler and Nordheimx have proposed Eq.(6) for experi­mental measurement of work function which gives the effect of an applied field on the rate of electron emis­sion .

..

..

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 849

c

Figure 6 - Schematic diagram of one form of the field emission microscope; E = glass envelope; S = phosphorescent screen; M = metal backing; A = anode lead in; T = emitter tip; C = tip support structure; V = vacuum lead

i B � 3 / 2 -- = A exp - ---'--

V2 V . . . (6)

where i is the total emission current, which depends ex­ponentially on lIV and with a coefficient from which the work function can be evaluated, V is the applied volt­age and A and B are constants that depend on <I> and geometrical factors. Experimentally, it is possible to observe the electron emission from a particualr atom (i.e. the intensity of a particular spot on the fluorescent screen) as V is varied to obtain the work function for that atom. Also the change in work function, when the site is cov­ered by an adsorbent, can be determined.

The geometrical arrangement of field emission mi­croscope is diagrammatically represented in Figure 6. The only difference in field ion microscope is that the tip is cooled by immersion of the heavy supporting wire in l iquid hydrogen and a small pressure of gaseous he­lium is admitted to the tube. On applying high field strengths, hel ium atoms are ionized at the metal surface and the ions are accelerated to the screen while in field emission microscope, electrons are drawn from the sur­face of the tip and accerlated to the screen. Complete information about the adsorbed layer is always obtained by this method by studying the changes in the electron emission current from the solution. Since, however, nei­ther the total number of adsorbed particles on different crystallographic planes nor the emission current changes per adsorbed particles on various crystallographic planes

are known, this method gives only qualitative informa­tion . And the main disadvantage is the presence of a high electric field, which is able to influence the properties of the adsorbed layer.

(2) Techniques Based on the Study of Interaction of Radiation, Electrons and Ions with the Adsorbed Layer

When a solid surface is irradiated by photons, the ra-. diation is either reflected or adsorbed. Depending on the incident radiation, either the radiation itself will change in a characteristic manner, while the properties of the adsorbed layer remain practically unchanged, or with high energy radiation, surface atoms or molecules may be excited or electron emission takes place. Other pos­sible effect is the decomposition of the adsorbed mol­ecules . Ellipsometry is one of the classical optical meth­ods of studying the interaction of the radiation with the solid surface. While studying the elastic scattering of electrons on surface, either the low-energy electron dif­fraction or high-energy electron diffraction methods can be used. Scanning tunneling microscopy (STM) is widely used to examine surfaces and what is observed with STM is a current flow between the probe and the surface due to an overlap of the respective wave functions, i .e . , to electron tunneling. Photon correlation spectroscopy is quite successfully used to give the surface properties of film-covered surfaces and the information may be ob-

850 J SCI IND RES VOL.58 NOVEMBER 1 999

Figure 7 - Ellipsometer block diagram : 1 = l inear He-Ne laser; 2 = quarter wave plate; 3 = polarizer; 4 = quarter wave plate; 5 = Langmuir trough; 6 = vibration isolation table; 7 = pin hole; 8 = analyzer; 9 = light baffle; 1 0 = photodiode; 1 1 = pin hole; 1 2 = multi meter; 1 3 = power supply; 1 4 = lock in amplifier

tained from the autocorrelation function of the scattered light. Thi s technique is called laser l ight scattering.

Ellipsometry

Ellipsometry i s a w idely used technique for measur­ing the thickness of an adsorbed film and the amount of adsorbate i n that film9• 1O• A schematic diagram of an el lipsometer is shown in Figure 7. It is based on the fact that the state of polarization of l ight changes after re­flection from a surface . When elliptically polarized l ight is reflected from a bare surface, there is a change in both the ampl itude ratio and the phase difference of the com­ponents. This can be expressed mathematically as given in Eq.(7) :

tan 'I/J . .

A l p / A l n . . . (7)

where, A fA i s the amplitude ratio of the l ight polarized p n

parallel (p) and normal (n) to the plane of incidence for the reflected (r) and incident (i) beams. The phase difference (8 - 8 ) of the two components changes on

p n reflection, so that the film thickness is given by :

t = (8' - 8' ) - (8i - 8i ) . p n p n . .. (8)

Detailed description concerning the theory and the apparatus have been described by De Feitze et al J J .

Low-energy Electron Diffraction (LEED)

The study of elastically reflected low-energy elec­tron provides information about the structure of the solid

surface and the adsorbed layers. The results of measurement of low-energy electrons

may be judged directly from the diffraction pattern. How­ever the location of the surface atoms w ith regard to the underlying bulk lattice is obtained on the basis of the intensity measurementsJ2• 1 3 •

An electron been accelerated by a voltage U(V), when falling onto a periodic lattice (crystal), behaves l ike elec­tromagnetic radiation of the wavelength A (A") accord­ing to the reiationshipJ4 :

"A = .J150.4 I U . . . (9)

When the direction of a primary electron beam is per­pendicular to the plane of the surface lattice, reflected electrons interfere in directions determined by the fol­lowing relationshipJ4.

n "A = d S i n \f' . . . ( 1 0)

where, n is the integer (the order of diffraction), d i s the lattice constant of the surface lattice, and \f' is the angle between the incident and reflected beam.

Presently, four types of apparatus are used in LEEd studies. The description of the first type of apparatus and method is presented in Figure 8 (ref. I S) . The ex­ample of the construction of the second type of appara­tllS has been discussed by Macrae. This technique has advantages in the study of rapid processes, e .g. , chemi­sorption. The main advantage of third type of apparatus, compared with the preceding two types, i s the abi lity to observe zero-order reflection. The fourth type of LEED

..

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 851

1 " \ \ g�� 2 "

�r�/ ·· fTl r-;:+­" �L=t.-

/1 / .

o

Figure 8 - (a) Apparatus for LEED study : V = sample; E = electron gun; G I = a grid (at the same potential as the sample), which screens the sample from influence of the accelerating voltage; G2 = retarding grid; L = fluorescent screen; 0 = optically polished wall for observing the diffraction pattern on the screen; 1 and 2 = direction of the incident and reflected electron beams respectively, (b) Diffraction pattern on the screen

apparatus is of rotational axis of the cylinderl4• Stem et al. 16 have employed the LEED technique to

determine the molecular orientation and array structure of adsorbed pyridine on a Pt(l 1 1 ) electrode surface.

Scanning Tunneling Microscopy (STM)

Scanning tunneling microscopy is the most recent technique, invented by B inning and Rohrer in 1 982, which provides new ins ight into the behaviour of adsorbed mono layers on surfaces. Developed by Por­terl7, the technique was used to reveal , at Angstrom-scale resolution, the packing arrangement of low molecular weight compounds such as n-alkanethiolate monolay­ers spontaneously adsorbed on gold films. Such organic monolayers have been studied as model system to help elucidate structure reactivity relationships at metal-liq­uid interfaces. These studies may be helpful in provid­ing details about macroscopic structure, electronic prop­erties, surface-free energy and imperfections of the lay­ers. The technique has been shown to be a powerful tool for obtaining structural information for both organic and inorganic adsorbents .

An alternative method of STM is the Atomic Force Microscopy (AFM) which yields almost the same infor­mation and resolution, as does the STMI8. One of the limiatations of STM is that conducting or semiconduct­ing surface is needed for STM. The AFM, introduced by Binning et al. IS, resolved this problem elegantly. Like

STM, AFM is also applicable in studying the solid sur­face under various environment including liquid phases 19 .

Photon Correlation Spectroscopy

This technique also measures the thickness of the ad­sorbate layer and is potentially one of the most reliable and accurate methods. It relies on the principle that if monochromatic light is passed through a dispersion of colloidal particles the scattered light wil l have its fre­quency changed by the motion of the particles, i .e. , there will be Doppler frequency shift between the incident and scattered beams. The combined scattered and incident beams fluctuate in intensity and these fluctuations have an autocorrelation function [g'(KTc)] ' where K = [41t1 A).Sin(9/2)], is the scattering vector, A is the wavelength of the light in the dispersion medium, 9 is the scattering angle, and Teo is the correlation time delay.

For monodispersed non-interacting particles, having a diffusion coefficient D , "

. . . ( 1 1 )

For spherical particles, D is related to the hydrody­namic thickness r by the Stokes-Einstein equation [Eq.( 1 2)] ,

D = K Tj6 1t 11 r, '" ( 1 2)

where, T\ is the viscosity of the solution and K is the Boltzmann's constant. Thus, if D for a particle is deter-

852 J SCI IND RES VOL.58 NOVEMBER 1 999

mined before and after adsorption, the changes can be attributed to an increase, Ilr, i n the thermodynamic ra­dius caused by the adsorbed particle. Further theoretical developments have beeJ;l discussed by Langvin2(). It is important that the state of aggregation of the particles (if any, as in dyes) does not change fol lowing addition of the particle. Sometimes .added particles cause some flocculation, particularly at low coverage, so care must be taken w ith particle concentration as well as with or­der of addition. It i s advisable to use low angle light scattering to determine whether slight changes in the state of aggregation have occurred during the measurement process.

(3) SpectrtJscopic Methods Spectral methods are one of the most efficient types

of methods for directly studying the adsorbed layer and adsorption centres.

Infra-red (fR) Spectroscopy

. IR technique is used to determine the nature of the adsorption centres and the mode of their interaction with the adsorbate, changes caused in the adsorbate molecule by the field of the adsorbent, and the nature of new chemi­cal compounds and bonds which are formed upon ad­sorption2 1 •

The theory of IR spectroscopy shows that a molecule, as a whole, carried out so-called normal vibrations, in which the amplitude of motion differs for different at­oms, while all atoms v ibrate at the same frequency. When the amplitude of one of the vibrations which form to­gether one of the normal v ibrations, is considerably greater than that of the others, it becomes the charac­teristic vibration of the particular bond or groups of at­oms (�CH3' >CH2, > CO), etc. (ref.2 1 ) . Among the dif­ferent possible vibrations, only those which have reason to occur in the dipole moment of the molecule will ap­pear in an adsorption spectrum2 1 . The instrument of the types single or double beam and the measuring cell used for adsorption analysis are described elsewhere2 1 •

The interaction o f NH3

and methanol with several kinds of alkali zeolites was studied by means of IR spec­troscopy by Kogelbauer et a/Y. IR spectra were re­corded in situ with a Bruker IFS 88 FTIR spectrometer using transmission-absorption technique . The IR cell was equipped with a heatable sample holder and evacuated for 1 0-4 Pa. The spectra (with a resolution of 4 cm- I ) were base l ine corrected in the range from 3800 to 1 300 cm-I . .

The FTIR spectroscopic results of alkali zeolites equili­brated with ammonia and methanol suggested that after adsorption all alkali zeolites showed simi lar type of IR spectra for a given adsorbate. For instance, assignment of the IR bands upon adsorption of ammonia revealed that ammonia adsorbed in the same way on all alkali zeolites, i .e . , interacting via the nitrogen lone pair elec­trons with the sodium cation of the zeo li tes. The higher intensity bands were observed for some zeoli tes indi­cating the h igher concentration of sodium cation in the zeolite than that present in others. For all zeolites, three bands observed for N-H stretching-vibration and a band around 1 640 cm-I attributed to N-H deformation v ibra­tions. In the case of the adsorption of methanol , rather complicated spectra were obtained. The cations were considered the primary adsorption s ites interacting with the lone pair electrons of the methanol oxygen. The spec­tra also displayed large differences in the O-H v ibration stretch ing of adsorbed methanol onto various zeolites. Besides the significant difference of the 0-H vibrations, a shift of the asymmetric and symmetric C-H stretch­ing-vibrations were also observed in the spectra. The authors with the aid ofIR spectroscopy also investigated the co-adsorption of methanol and ammonia. The ob­served spectra indicated a change in the environment of adsorbed methanol upon co-adsorption due to the for­mation of additional hydrogen bonding of methanol with ammonIa.

In the study of coke formation and its effects on shape selective adsorptive and catalytic properties of ferrierite in isomerization of butene to isobutylene, Xu et at. 23 have used FTIR technique to observe whether the nature of coke is aliphatic or aromatic. This coke formation con­trolled the s ide reaction (which h indered the skeletal isomerization reaction) by deposition around the acid sites so that they do not have enough space for s ide reac­tion l ike formation of l ight alkanes and dimerization of butene.

The FTIR experiments were done on Mattson GAL­AXY II spectrometer at a resolution of 4 cml using self­supported wafers ( 1 0 mg). Pyridine chemisorption ex­periments :vere done on self-supported wafer in an in situ IR cell . The sample C-2, C- I I , C- 1 8 and C- l 1 H, the coked ferrierite/alumina materials which were collected in n-butene isomerization reaction after 2, 1 I and 1 8 h , respectively, and sample C- l l heated a t 8 7 3 K in he­l ium (C- I 1 H), was dehydrated at 773 K for 5h under a vacuum of ] 0-5 Torr, followed by adsorption of purified

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 853

pyridine vapour at room temperature for 1 5 min. Then the system was evacuated at 423 K overnight to remove chemisorbed pyridine, and an IR spectrum was then re­corded.

For al l the three coked samples, C-2, C- I I , and C- 1 8, respectively, the IR bands were observed at 1 622, 1 589, 1 520, 1 47 1 , 1 437, and 1 352 cm· l . It was also observed that the intensities of these bands decreased with an in­crease in time on stream. After the sample C- I I was heated at 873 in vacuum, properties of this coke got changed and this is evidenced by disappearance of all bands in the IR spectrum. Pyridine adsorbed on the fresh ferrierite/alumina catalyst showed bands at 1 637, 1 600, 1 545, 1 489, 1 456, 1 390, and 1 330 cm- I . Among these absorption bands, 1 456 and 1 545 cm-I were due to pyri­dine molecules adsorbed on Lewis and Bronsted acid sites, respectively. Characteristics of IR bands for coke samples, obtained during butene isomerization to isobu­tylene catalysed by ferrierite/alumina catalyst, revealed that bands at 1 622 cm-I were assigned to C=C stretch­ing vibrations in n-butene24 . The 1 589 cm- I band was . due to microcrystalline polycyclic aromatics25. Coke with properties of aromatics also captures an IR absorption band for aromatics (C=C stretching vibration) at 1 520 cm-I (ref.24) . In addition, CH3 and CH

2 groups attached

to aromatics were also observed as evidenced by two IR bands at 1 47 1 cm-I (CHo deformation) and 1 437 cm-I (CH assymetric deformation)25 . Thus, IR data for coke 3 samples suggest that such coke i s aromatic in nature .

In order to develop a better understanding of methyl tert-butyl ether (MTBE) synthesis on zeolites, the co­adsorption of methanol and isobutene on zeolite was investigated by Kogelbauer and Goodwin2'i using IR spectroscopy. All IR spectra were recorded ill situ on a Bruker IFS88 FTIR spectrometer using the transmission­absorption technique. Typical ly, 1 50 scans were co-added for one spectrum at a resolution of 4 cm- I , and al l spec­tra were base l ine corrected in the range between 1 200 and 3800 cm- I . The IR cell was equipped with a heatable sample holder enabl ing in situ temperature treatment of the catalyst (zeol ites) and was connected to a h igh vacuum system achieving pressure of 1 0-6 mbar. A de­tailed description of the IR system used is given in ref.26.

In the spectrum of zeolites, the authors observed bands at 3634 and 3550 cm-I which were characteristics for Si­OH-AI hydroxyl groups located i n the supercage and in the sodalite cages, respectively. The band at 3734 cm-I was attributed to SiOH groups terminating the zeolite

lattice. By observing, mainly the disappearance of one or more than one of these bands and appearance of fur­ther new bands after adsorption of methanol, isobutene and MTBE and after co-adsorption of methanol onto the zeolite pre-equilibrated with isobutene and vice-versa, they concluded that in itial adsorption of isobutene alone led to rapid oligomerization and subsequent co-adsorp­tion of methanol did not induce any change,S in the zeo­lite-adsorbate complex. When methanol adsorbed first and isobutene was subsequently co-adsorbed, MTBE was formed on the catalyst (zeolite) surface and following reaction routes for methanol and isobutene on zeolite were proposed (I and II) :

2 � 2 - IB � 2 - DIfi MeOH ) No re(/clioll

IB - --. Z - M TB E

. . . ( I )

. . . ( I I)

IR reflection-absorption spectroscopy (lRAS) is an­other surface characterization IR spectroscopic technique used by Goodman et al. 27 to study the co-adsorption of nitric oxide and carbon monoxide on pal ladium cata­lysts between 1 00-500 K useful in minimizing the air pol lution . The experiments were carried out in two sepa­rate ultrahigh vacuum (UHV) chambers equipped with a Mattson Cygnus 1 00 FTIR spectrometer for IRAS stud­ies . The detai led of the c hambers are descri bed eleswhere2x•

Raman Spectrosocpy Raman spectroscopy offers distinct advantages over

the more direct IR absorption measurements. Raman spectroscopy can be used to detect and analyze molecules with IR inactive spectra, such as homonuclear diatomic molecules and complicated molecules whose low sym­metry does not forbid both Raman and IR activity, cer­tain vibrational modes are inherently stronger in the Raman effect and weaker in, or apparently absent from the IR spectrum2�. Raman activity tends to be a function of the covalent character of bonds. Hence, a Raman spec­trum reveals information regarding the backbone struc­ture of the molecule, whereas the strong IR features are indicative of polar segments. Raman spectra can be used to study materials in aqueous solution, a medium that transmits IR is poorly, and therefore, sample prepara-

854 J SCI IND RES VOL.58 NOVEMBER 1 999

tion for Raman is generally considerably simpler than for the IR29.

When monochromatic light is scattered by molecules, a small fraction of the scattered light is observed to have a different frequency from that of the irradiating l ight; this is known as Raman effect and it arises when a beam of intense monochromatic light passes through a sample that contains molecules which can undergo a change in molecular polarizabi lity as they vibrate. I t is strictly a quantum effect.

Surface enhanced Raman spectroscopy (SERS) tech­n ique has sprung in anal�tical field by the moderniza­tion in Raman spectroscopy, is useful in studying the characteristics of interface and surface either in dry or solution phase. The experimental facts related to SERS are believed to be the result of several mechanisms. There are two main types of mechanism that contribute to the SERS effect : (a) An electromagnetic effect associated with large local fields caused by electromagnetic reso­nance occurring near metal surface structures and (b) A chemical effect involving scattering process associated with chemical interactions between the molecule and the metal surface29.

Bradley et al. 30 have shown that a molecule adsorbed at an interface has also s ignificant interaction with other molecules present near the surface and the nature of the interaction was investigated by the SERS technique. The authors took three different but related colloidal prepa­rations of si lver, such as. dilute organo sols, di lute hy­drosols, and h ighly concentrated suspensions where the particles nearly touch each other. These col loidal par­t icles are simi lar but they do differ in local environments that can be reflected in different Raman spectra. For preparation of sols, silver salts were reduced chemically in noft-aqueous ·phase. In the presence of a dense or­ganic phase, certain additives, e.g., anisic acid anion were added . Also, high concentrated suspensions were called as metal liquid l ike fi lms (MELLFs) due to their un­usual visual and macroscopic properties. These colloi­dal sols which have the same origin but different envi­ronments were investigated by Raman spectroscopy. The high-pressure cell of SERS technique used in this study are shown in Figure 9. The observed Raman bands in organo sol indicated that anisate an ions (AA) were adsorbed onto si lver colloid resulting in a large surface enhancement. Thus the anisic acid acted as a stabi l izing agent also for the three colloidal sols. In all the three colloidal sols, the band in the 1 600 cm·1 was assigned to

2 . 50\\ Figure 9 - Schematic diagram of the high pressure cel l ; I = cell

body; 2 = electron port plug; 3 = bottom plug; 4 = pressure inlet and thermocouple ports

the in-plane ring C-C stretches3 1 and this indicated that the colloidal sols were environmentally identical. Indeed, the aromatic ring was found to be located in the adsorbed layer so that it was far of from the solution site. It was also concluded that the immediate environment of the adsorbed layer on the solution side can be probed. More­over the aqueous surrounding in the hydrosols, leading to flocculation, had recognizable high-pressure Raman features and these were quite distinct from those of a similar colloid in an organic liquid. The Raman behaviour of the two systems was different than the h ighly com­pact MELLFs.

Nuclear Magnetic Resonance (NMR)

Nuclei with a nucelar spin (generally nuclei with an odd number of protons) behave in a magnetic field like a magnetic dipole. The behaviour of a system of such dipoles, i .e. , variation of longitudinal and transverse mag­netization (magnetic moment per unit volume of the sample), of the sample, are studied experimentally. When the sample is placed in a magnetic field of intensity H, all of the energy levels are split into two sub-levels, of separation !1E = yhH When suitable energy quanta are supplied to the system, transitions occur between the levels. These transitions are also made possible by re­laxation processes, which are characterized by the rate constants I IT, and J IT2, T, being the t ime constant for longitudinal relaxation (spin-lattice, i .e . , energy trans­ferred from the nucleus in an upper energy state- is given

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 855

to the lattice as extra translational and rotational energy) and T2 is the time constant for transverse relaxation (spin­spin, i .e. , a nucleus in the upper energy state can transfer its energy to a neighbouring nucleus by a mutual ex­change of spin). The relaxation times are highly sensi­

. tive indicators of the properties of the environment of the nuclei which interact with the radiation. These are obtained from the shape and height of adsorption peaksJ2..

The absorption band of the spectrum widens, owing to the influence of the mututal interactions of the di­poles, and therefore, it is always wider in the solid state than in the liquid state. Thus, when an adsorbate mol­ecules are adsorbed at sol id-liquid interface, the molecu­lar motion of the adsorbate molecules make their high resolution spectrum too broad, which provides impor­tant information concerning the character of the region around the nuclei, thus the characteristics of the inter­face.

A modified NMR technique, i .e . , Pulsed Field Gradi­ent (PFG) NMR method has proved to be a rather sensi­t ive tool for studying structure-related transport proper­ties of adsorbate-adsorbent systems. To study the diffu­sion of water in zeolite NaX under equilibrium and non­equilibrium conditions the pulsed field gradient NMR (PFG-NMR) spectroscopy was applied by Bourdin et al. . The PFG-NMR method has proved to be a sensitive tool for studying structure-related transport properties of adsorbate-adsorbent system. The authors applied this technique in order to estimate the range and trends of the water diffusivities in zeolite NaX using samples ob­tained from different batches and under different pre­treatment conditions. The water diffusivities in differ­ent samples were studied as a function of the sorbate concentration . It was found that the observed mean dif­fus ion paths were much smaller than the mean crystal­l i tes diameter so that the obtained diffusivities are rep­resented genuinally in crystalline diffusivities. Moreover the NMR signals clearly indicated only one mode of molecule transport in these experiments. It was con­cluded that for zeol ite from differnt batches as well as after different mode of sample preparation, the water diffusivities did not differ from each other by more than a factor of about two. The results were also found to be in agreement with those of Thermal Frequency Response (TFR) method, employed in the same study.

Electron Paramagnetic Resonance (EPR)

The application of EPR method" is similar to that of NMR method, with the difference that the behaviour of

a system of non-pair electrons is studied. S ince the mag­netic moment of electrons is 1 0-3 - 1 0-4 times greater than that of nuclei, an increased splitting of this order is observed between the energy levels. In this case, M =

yhH = gJiBH, g being the spectroscopic splitting factor and IlB the Bohr magneton. With EPR, too, the structure and shape of the absorption bands are analysed. In this case, the bands are broadened mainly by the short life­time of the excited state and the strong dipole-dipole interaction between two dipoles . This undesirable broad­ening of bands is suppressed by lowering the tempera­ture and diluting the sample with an inert material. The data thus obtained are frequently analysed by analogy with the spectra of known compounds, by stu�ying the nature of temperature variation of the band shape and intensity. The main part of the experimental equipment for EPR is a microwave bridge made from two crossed wave guides3s.

Several methods have also come into use which pro­vide more information about the structure of the solid surface and energy states of the adsorbed species. To mention a few, Auger Electron Spectroscopy (AES)36.J7, High-Resolution Electron Energy Loss Spectroscopy (HREELS)3X,39, Ultra violet photoelectron and X-ray Photoelectron Spectroscopy (UPS and XPS)40.4 1 , Tem­perature Programmed Desorption Technique and Ther­mal Desorption Spectroscopy (TPD and TDS)42A3 are" some of the more intricate techniques being currently employed to study the adsorption phenomenon.

Survey on Adsorption

Adsorption on Metal Oxide

Modification of surfaces may bring a change in ad­sorption capacity of adsorbents, as shown by B idzilya et al. 44 in the study of adsorption of biological ly active amines on silica, it was reported that the adsorption ca­pacity of si lica increases by surface modification using crown ether which s lows down the desorption from the modified surface.

The adsorption of divalent heavy metal ions on the iron (III) oxide surface and the modelling of the process was investigated by Tamura et al.4S • In the proposed model, the authors assumed that : (i) There is formation of surface complex due to ( 1 : I ) and ( l :2) cations ex­change reactions with protons of acid surface hydroxyl sites and (ii) With increasing surface coverage the Gibb's free energy also increases l inearly. The adsorption af-

856 J SCI IND RES VOL.58 NOVEMBER 1 999

finities of the metal ions were found to increase in the fol lowing order C02+ � Zn2+ < Cu2+ < Pb2+. The ex­perimental results were found to support the surface com­plex model which assumes that metal ion adsorption is due to the donation of electron pairs from the lattice oxide ions to the metal ions.

Several interesting results were obtained by Huang et al.46 while studying the adsorption behaviour of system containing a cationic tetradecyl trimethyl ammonium chloride (TTAC) and non-ionic pentadecylethoxylated nonyl phenol (NP- 1 5) surfactants and alumina as an ad­sorbent. It was found that the cationic TTAC adsorbed on the alumina surface due to electrostatic attraction, whereas non-ionic NP- 1 5 did not. However, the authors noticed that from the mixture of the two surfactants the TTAC forced NP- 1 5 to be adsorbed as a result of hydro­phobic interactions between the adsorbed surfactant chains. The adsorption behaviour was found dependent upon the ratio onhe two surfactants in the mixture as well as the order of their addition. One more significant observation was also noted that the adsorption charac­teristics of the two surfactants were influenced by the factors such as competition for common adsorption sites, bulk iness of the adsorbed surfactants and synergistic interactions between the cationic and anionic heads of the surfactants.

Kataoka et al.47 had used iron oxide particles as ad­sorbent for the removal of phosphorous from river wa­ter by magnetic separation, in which the suspended par­ticles flow out from the flow of the river.

The affinity of formic acid for adsorption and decom­position on the polar oxide surfaces (NiO, MgO) was evaluated by Xu and Goodman4H using Temperature­Programmed Desorption (TPD) spectroscopy and High­Resolution Electron Energy Loss (HREEL) spectroscopy. The authors found that the surface of NiO showed higher adsorbabi l ity towards formic acid than MgO. At the same time, the NiO surface showed h igher reactivity towards the dissociation of formic acid to form a formate spe­cies than the MgO. It was also noted that the stabi l i ties of the formic acid on these two surfaces were qui te dif­ferent, on the NiO surface dissociation and adsorption of formic acid was found even at 1 00 K while on the MgO only dehydration was observed . The authors also proposed the fol lowing unimolecular decompos ition pathway :

H(a) + HCOO (a) --7 HP(g) + CO(g).

At higher temperature (560 K), the formate species showed decomposition via two competing pathway to

form H2, H20, CO and CO2. In contrast to these results

only, dehydration was observed for the MgO surface. The different behaviours of the two surfaces were ex­plained on the basis of the activity of the transition metal to form Nill-H-(-I ) type hydrides.

Rudzinski and Charmas4'J have critically discussed the effect of surface heterogeneity of metal ox ide on the adsorption of simple ions at oxide electrolyte interfaces. According to them the adsorption of ions occurs via for­mation of complexes with surface oxygen atom of the oxides because of the smal l degree of ox ide sUlface or­ganization. Different surface oxygens may have differ­ent status with respect to electrostatic interactions and the strength of the chemical bonds formed between the surface oxygen and the adsorbed proton or metal ions. It was shown that the equ i librium of formation of surface complexes can be conveniently transformed to Langmuir l ike equation for multicomponent adsorption. The au­thors also discussed various theoretical models for ad­sorption of simple ions and bivalent cations on various heterogeneous oxide surfaces .

Sulphonamides are significant antibacterial chemicals and possess a prime position in pharmaceutical indus­try. From point of view their separation, either from s ingle or from mUlti-component systems the adsorption technique may be a versati le tool for chemists and bio­chemists. Looking at this applied aspect of principle of adsorption, a systematic study was performed by Bajpai et al. 50 not only to study the adsorption response of sulphonamides but also to explore if any correlation ex­ists between the structure and the adsorption of these compounds . The authors used su lphan i l amide(s) , s u l phapyr id ine (SP) , s u l ph ad i az ine ( S O ) , and sulphamethoxazole (SM) as adsorbate compounds and studied their adsorption onto alumina at pH 7.2. The find­i ngs revealed that under ident i cal condi t ions the sulphonamides obey the fol lowing order of increasing adsorption, S < SP < SO < SM.

The results were explained on the bas is of pK" value and per cent ionization of the sulphonamide compounds at respective pH. It was concluded that lower the pK" value of sulphonamide, the greater would be i ts ioniza­tion in the solution and consequently more wi l l be the adsorption. The respective adsorbabi l ities of compounds were interpretted in terms of the greater extent of bind­ing forces between the ionized or unionized form of the compounds and aluminols (AIOH) of the alumina sur­faces. The study also comprised assessing effects of vari-

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 857

ous factors such as pH, temperature, the presence of salts and organic solvents on the adsorption characteristic of the four compounds.

Adsorption on Activated Carbons

Le Cloire and Delangh" patented a new separation method, based on adsorption process, for removal of organic and inorganic compounds from gaseous or aque­ous effluents. The method comprises membrane filtra­tion step fol lowed by activated charcoal membrane ad­sorption .

The removal of methylene blue dye from its aqueous solution was studied by Khali l and Girgis52 employing phosphoric acid treated activated carbon derived from rice husk. The authors found that the methylene blue dye showed high affinity towards phosphoric acid treated activated carbon rather than the untreated one. This also resulted from experiments that preimpregnation of rice husk with 50 per cent phosphoric acid and further car­bonization at 400"C proved to be most effective in pro­ducing active carbon w ith good adsorption capacity.

Two activated carbons from different sources were oxidized with nitric acid and hydrogen peroxide and the effect of this process on their surface structure and sorp­tion of water and methanol were studied by Bandosz et aU3. The activated carbons so prepared were character­ized and various microstmctural properties such as the surface acidity, pKa distri�ution, surface density of acidic group, were determined using techniques l ike Boehm titration and potentiometric titration, and diffelence re­flectance IR Fourier Transform. It was revealed from stmctural and surface chemical results that carbons de­rived from various sources had wide variations in their oxidation susceptibil ities which ultimately regulate the adsorption capacity of the adsorbent.

The efficiency of activated carbon in treating cyanide containing wastewater and the mechanisms involved in the process were studied by Wei et aU4 in an attempt to ascertain whether or not the process involved chemisorp­tion or catalysis of copper compounds in cyanide oxida­tion. The authors tested the effect of metal ions, dissolved oxygen and the pH of the solution on the treatment ca­pacity of activated carbon. The kinetic experiments were also performed and parameters l ike heat of adsorption and the activation energy were also evaluated. The ad­sorption mechanism confirmed it as a chemisorption

resulting from the interaction between complex ions and the surface groups of activated carbon. It was also found that the copper compounds which are adsorbed on the carbon surface served as catalyst in oxidation of cya­nide by adsorbed oxygen.

A two step process for biodegradation and activated carbon adsorption was applied by Uhl et ai. " for the removal of natural organics (dissolved organic carbon -DOC) and easi ly biodegradable Assimi lable Organic Carbon (AOC) from the potable water. These two pro­cesses were performed in two chambers of the purifica­tion system, viz. bioreactor and stand-alone-adsorber. In bioreactor, AOC was removed by biodegradation pro­cess and then in the next chamber water purified by the adsorpt ion of DOC . The authors observed that in bioreactor, 50 per cent purification was completed which increased the service time of adsorber because of the elimination of adsorbate compounds thereby preventing pre-loading of the adsorber. The authors also observed that results of this purificat ion process were of special interest for the effective removal of pollutant pesticides, which are easily biodegradable compounds and con­tribute to bacterial re-growth problems in potable water.

Saleem et ai. 56 studied the effect of pH on the adsorp­tion of Ce3+, Sm'+, Eu'+, and Gd3+ ions on activated char­coal and discussed the mechanism of these ions in terms of hydrolysed species formed in aqueous solution at dif­ferent pH.

Realizing the industrial and environmental signifi­cance of separation of gas mixtures, a novel theoretical and experimental study was performed by Cracknell et al. 57 compri s ing the Grand Canonical Monte Carlo method (GCMC) to study the adsorption and selectivity of mixtures carbon dioxide with methane and n itrogen at high ( i .e. ambient) temperatures in model sl it pores with graphitic surfaces. The comparison between simu­lation for the GCMC model and experimental data for carbon dioxide, methane and n itrogen was done and found to be in excellent correlation . The authors also demonstrated mixture simulations for gases and found that carbon dioxide is selectively adsorbed from equimo­lar mixtures in preference to both methane and nitrogen in slit shaped model carbon pores at ambient tempera­tures. It was also found that in very small pores with a physical width below 0.7 nm, methane or nitrogen are virtual ly excluded, compared to CO2 , The results were consistent with those reported earlier by other work­ers.

858 J SCI IND RES VOL.58 NOVEMBER 1 999

Adsorption on Miscellaneous Adsorbent

Zeng and Song5H studied the adsorption capacity of cationic starch (CST), specially for anionic dyes. They found that anioJ)ic dyes chemical ly adsorbed by CST and when the dye concentration was < 1 5m moll1, the adsorbed amount was proportional to the equil ibrium concentration. They also observed that at ambient tem­perature and wide range of pH value (pH = 2-8), CST showed good adsorption capacity for various anionic dyes.

In the study of the drug adsorption capacity of cotton, Gauze, Haize Gauze, and cellu lose systemized fiber (CSF), Kajimato and StrumiaSY observed that except CSF others can be efficiently applied for external u se of liq­uid medicine in patient treatment. The authors also in­vestigated the reason for this efficiency and improved it by making variation in experimental conditions and evaluated by its zeta-potential . They found that CSFs have a negative zeta-potential, resulting in good adsorp­tion of cationic drugs and adding salts and lowering the pH causes a reduction in the negative zeta-potential and decreases the adsorption of cationic drugs.

Besides the use of chitosin film in various biomedi­cal applications, it has potential ly been employed as an efficient adsorbent in many studies. In a simi lar type of investigation by Kishimoto et al. W the adsorption equi­libria of glutamic acid on crossl inked chitosin fibers were studied and the adsorption was monitored at different experimental conditions. The crossl inked chitosin fibre (ChF-B) is known to have fixed ammonium group and the adsorption of glutamic acid occurs via acid-base neu­tral ization type of reaction taking place between the car­boxylic groups of neutral glutamic acid and fixed am­monium group on the fibre surfaces. The adsorption was found independent of the initial concentration of L­glutamic acid, but greatly dependent on the pH of the solution. Moreover, the adsorption was controlled by the Langmuir-Frendl ich type of equation. The experimen­tal results were also found to be consistent with the theo­retical predictions.

Sakurai et al. � I patented manufacturing and deodor­izing characteristics of chitosan and polyurethane-coated acrylic fibres. Deodorizing fibre was manufactured by dipping fibres in m ixture of heat-reactive water solu­tion, polyurethanes and aqueous acidic solution of chitosan (1), squeezing the fibre to pick up 1 - 1 0 per cent (1) and then allowing for heat-treatment. The authors

found that coated fibres effectively adsorb aldehydes, especial ly methanal (MeCHO) of tobacco smoke.

Only l imited number of publications have appeared in the l iterature where co-ordination compounds of tran­sition metal ions have been employed as adsorbents and that also for the adsorption purpose of biomonomers. Vi l adkar et al. °2, have carried out the adsorpt ion of biomolecules such as adenine, adenosine, 5 ' -AMP, 5 ' ­ADP and 5 '-ATP onto Ni(II) hexacyanoferrate surfaces and the binding of the adsorbate molecules to the adsor­bent surface was explored by spectroscopic methods. The · adsorption was found to be of Langmuir type and on the basis of the surface coverage at saturation the adsorbing efficiency of the five biomonomers was obtained in the fol lowing pattern : 5 ' -ATP > 5 ' -ADP > adenine > 5 ' ­AMP > adenosine.

The adsorption abi l ity of adsorbent i s l inked with its complex abil ity and with the l igating nature of adsor­bate molecules since the biomonomers employed in the study exhibit ambivalent properties . They provide many active sites for metal binding. The adsorption of the ac­tive biomonomers was discussed, emphasising the num­ber and type of active sites of the adsorbent molecules involved in the adsorption process. The active sites re­sponsible for adsorption were further confirmed by IR spectroscopic studies. The adsorption was found sensi­tive to the pH of the solution and the isotherms showed a maxima at the pK" value of specific biomonomers. The authors have also shown , in their earl ier work, the adsorbing capacity of the co-ordination complexes of other metal ions l ike Cu2+, Zn2+, and Co'-+-.

Among various solid wastes, generated as effluents in several industries, solid spent catalysts form a major fraction and cause environmental problems in industrial sites. Lai and Liu et al. �3 have presented a technique of using spent catalyst as an adsorbent for fluoride removal from water. The authors u sed a spent catalyst from a petrochemical industry contain ing vanadium, iron, n ickel, etc . , of an average particle size of 1 1 .3 mm and surface area l 30m2/g. The results of fluoride removal indicated a greater uptake of fluoride by the catalyst at lower pH « 4.8) and this was attributed mainly to the formation of a positively charged fIuoro-alumino com­plex AIFnP-n). The adsorption process was also found to be of first order. The fol lowing important conclusions were drawn by the authors :

( i ) The adsorption capacity of spent catalyst is com­parable to that of activated alumina and can potential ly

BAJPAI & RAJPOOT : ADSORPTION TECHNIQUES 859

be employed as a seconary adsorbent in removing fluo­ride from aqueous solution.

(ii) The adsorption density of fluoride onto spent cata­lyst decreases w ith increasing pH.

(i i i) The adsorption reaction rate decreases with in­creasing surface loading.

(iv) The adsorption reaction i s endothermic and the activation energy i s very low.

(v) It is proposed that a fluoro-alumina complex and free fluoride ions are involved in the adsorption reac­tion. Additionally, both s ilica and alumina fractions of spent catalyst contribute sites for the adsorption reac­tion to occur. Coulombic force is the major driving force.

In an attempt to improve the well known carbon-in­pulp (CIP) process of gold recovery, Belfer and B inman64 employed hol low sulphochlorinated polyethylene fibres as adsorbent in place of activated carbon as commonly used in CIP process. The fibres were aminated with four different amines and it was found that the fibres treated with diaminopropane showed greatest gold cyanide re­moval from its alkal ine solution (pH I I ) and it was at­tributed to the anion exchanging property of the adsor­bent fibre. The authors also reported that when the diaminopropane treated fibres were quaternized to en­hance their basicity, they exhibited a greater and much faster recovery of gold from its solution. The fol lowing mechanism of gold cyanide uptake was proposed by the authors :

r- S02NH(CH2)3NH2H.x + Na[Au(CN)2Jaq 1- S02NH(CH2)3N(CH3)3X

'- S02NH(CH2)3NH2Na[Au(CN)21

S02NH(CH2)3N(CH3)3Au(CN)2

where 1- represents polyethylene .

•

A potential ly significant appl ication of adsorption process has been in monitoring heavy metal ion pol lu­t ion of drinking water, employing various low-cost adsorbents. While carrying out an investigation of this type, Pal et al.65 have studied the use of fly ash in re­moving hexavalent chromium from synthetic water samples. It was found that the separation of chromium ion was much favourable in acid pH range because of an ion exchange mechanism. The oxides of metals present in the adsorbent form aqua complexes with water and develop a charged surface through amphoteric dissocia-

tion at varying pH, as shown below :

The separation process was found partly diffusion controlled and endothermic in nature. The authors also calculated various kinetic and thermodynamic param­eters such as diffusion constant, mass transfer coeffi­cient, enthalpy and energy of activation, entropy and jus­tified the results.

The structure and sorption properties of partially hydrophobized si l icates [dodecyl-ammonium vermicu­l i te (DAV) and dodecyl d iammonium vermicu l ite (DDAV)] have been investigated in aqueous solution of i-butanol by Regdon et al. 66 using X-ray diffraction data for calculating the interlayer composition and flow sorp­tion microcolorimetry to determine the enthalpy of dis­placement of water by i-butanol. Results revealed that alcohol is preferentially adsorbed on the surface and pro­cess of displacement of water from butanol i s endother­mic. Results also indicated that in aqueous butanol solu­tion, basal spacing of DAV is increased with i ncrease in the extent of butanol adsorption while basal spacing of DDAV is independent which suggested that in aqueous butanol solution , DDAV si l icate formed relatively rigid structure than DAV sil icates.

Acknowledgement

The authors are grateful to Dr D D Mishra, Ex­Head and Prof, Chemi s try Department, Rani Durgavati University, Jabalpur, for useful discus­sions. M Rajpoot is grateful to the Council of Sci­entific and Industrial Research for financial support.

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