Porous adsorbent for chromium (vi) removal



Chromium (VI) compounds are widely used in industry such as electroplating, meal finishing, leather tanning, pigments, etc. In recent years ground water is the main source of domestic water supply. Chromate poisoning cause severe skin disorders allergic dermatitis, liver and kidney damage. Thus chromium causes great public concern. A wide range of separation process has been investigated for the removal of Chromium (VI) from water. Adsorption using activated carbons posed to be the efficient method for removal of Chromium ions from water.

To use carbon as an adsorbent requires activation or surface modification of carbon. Methods like thermal and chemical methods of activation are common, but some problems are associated with them. Electrochemical method as one of the method of surface modification of commercially available activated carbon is applied. The result is better adsorption of Chromium (VI) ions. The present thesis includes various adsorption techniques. A literature review of adsorption characteristics has been included. Our project work include fabrication of an experimental setup, surface modification (oxidation at anode) in 0.5 M KCl solution at various intensity of currents, and latter comparisons of BET surface area, porosity, FTIR analysis ( for identification of changes in bonds after electrochemical oxidation) and adsorption in spectrophotometer.

Surface area, pore volume and pore size all decrease with increase in current intensity. A significant loss in porosity and decrease in pore diameter were observed and is due to blockage of pores by formation of functional groups (carboxylic acidic groups, hydroxyl groups, lactonic grops, phenolic group) and aggregation of humic substances. As the intensity of oxidation is increased by increasing the intensity of source current the amount of adsorption also increases. Also it is observed that if the intensity of current is increased from 0.1 Amp to 2.1 Amp, the amount of adsorption increases. But as the current approaches to 2.1 Amp the adsorption amount doesnt change significantly. Sample oxidized at 2.1 ampere was analyzed in FTIR. In the FTIR spectra it is revealed that in the range of 3600-3200 cm-1 a dip in transmittance was observed. Suitable reasons were found out. This process of activation can be suitably applied for activation of carbon.



Pollution load of the environment is increasing day by day due to global rise in pollution and our quest to lead comfortable life resulting in explosive growth of industries, mining operation, and increase usage of natural resources. Chromium (a metal) compounds are widely used in industries such as electroplating, meal finishing, leather tanning, pigments, etc. Chromate poisoning causes severe skin disorders such as allergic dermatitis and liver and kidney damage.

Chromium salts are almost exclusively in the Chromium +3 oxidation state or the Chromium +6 oxidation state. In the environment Chromium +3 is typically not a problem; its relative toxicity is low. In contrast, Chromium +6 compounds are toxic chemicals and genotoxic carcinogens.

Currently USPEA has set a maximum contaminant level (MCL) for Chromium at 0.1 ppm in drinking water [1]. The increasing concern with Chromium pollution significantly motivates the investigation and development of new and improved materials to address the problems.

There are many types of process available for water filtration. A water filter removes impurities from water by means of a fine physical barrier, a chemical process or a biological process. Filters cleanse water to various extents for irrigation, drinking water, aquariums and swimming pools.

Filtrations include sieving, ion exchange, reverse osmosis and adsorption mechanism. For plant, water filters include screen filters, cloth filters and sand filters. For home filters include granular activated carbon fibers (GAC), micro porous ceramic filters and carbon black resin (CBR) and ultra filtration membrane. Revere osmosis is most common today (hyper filtration) but it has disadvantage of getting corroded by chlorine. It is not capable of removing hydrogen H2S and volatile organic compound (VOCs).Even though RO removes bacteria and viruses, it is not relied upon. Membranes are also costly.

Activated carbon is used in gas purification, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and filter masks, filters in compressed air and many other applications. One major industrial application involves use of activated carbon in the metal finishing. It is very widely employed for purification of electroplating solutions. It removes impurities and restores plating performance to the desired level.

Activation of carbon involves pyrolysis of the precursor in an inert environment followed by activation. Activation process in common includes physical and chemical method. The disadvantages of the chemical activation process are lower purity of activated carbon and environmental pollution from the steps used to wash the chemicals away. In physical activation, consumption of energy to maintain a high working temperature is required.

The advantage of using electrochemical method of oxidation is that it increases the metal binding capacity and tensile strength [2. The advantage of using electrochemical method of oxidation is that it increases the metal binding capacity and tensile strength [2].other advantages include: (i) one of the reagents is the electron, which can be easily supplied by a direct current (DC) source and do not need any transport or handling; (ii) they can be applied in situ; (iii)the treatment can be immediately interrupted and can be run at room temperature and atmospheric pressure; (iv) the reaction conditions can be very precisely reproduced; and(v) oxidation and reduction processes are more selective and easily controlled by means of the electrode potential[3].



2.1Activated carbon:

Activated carbon, also called activated charcoal or activated coal, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.

The word activated in the name is sometimes substituted by active. Active carbons are highly developed internal surface area and porosity, sometimes described as solid sponges. The large surface area results in a high capacity for adsorbing chemicals from gases and liquids. The most widely used commercial active carbons have a specific surface area of the order of 800-1500 m2/g, as determined typically by nitrogen gas adsorption. Sufficient activation for useful applications may come solely from the high surface area, though further chemical treatment often enhances the adsorbing properties of the material. Difference in pore size affects the adsorption capacity for molecules of different shapes and sizes, and thus is one of the criteria by which carbons are selected for a specific application. Porosity is classified by IUPAC into three different groups of pore sizes. They are:

  • Micropores: width less than 2 nm
  • Mesopores: width between 2 and 50 nm
  • Macropores: width greater than 50 nm

Micropores dont help much in adsorption process because of its small sizes to accommodate large molecules. So, to widen them different activation process are there which are as follows.

2.2 Production:

Activated carbon is produced from carbonaceous source materials like nutshells, peat, wood, lignite and coal. It can be produced by one of the following processes:

2.2.1 Physical activation: The precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:

  • Carbonization: Material with carbon content is pyrolyzed at temperature in the range 600-900 C, in absence of air (usually in inert atmosphere with gases like argon or nitrogen)
  • Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (carbon dioxide, oxygen, or steam) at temperatures above 250 C, usually in the temperature range of 600-1200 C.

2.2.2 Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Then, the raw material is carbonized at lower temperatures (450-900 C). It is believed that the carbonization / activation step proceeds simultaneously with the chemical activation. This technique can be problematic in some cases, because, for example, zinc trace residues may remain in the end product. However, chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.

2.2.3 Electrochemical activation: It includes the oxidation of carbon by passage of electricity. It is a process of electrolysis where cathode is copper plate and carbon fibers as anode (which is in contact with another copper plate) .On application of power anodic oxidation of carbon fibers takes place .it introduces the new functional groups, such as COOH, and OH, onto carbon surfaces to increase the adsorption capacity and rate of the adsorption in the liquid phase.

2.3 Classifications:

Activated carbons are complex products which are difficult to classify on the basis of their behavior, surface characteristics and preparation methods. Classification is made for general purpose based on their physical characteristics.

2.3.4 Powdered activated carbon (PAC)

Traditionally, active carbons are made in particular form as powders or fine granules less than 1.0mm in size with an average diameter between .15 and .25mm. PAC is made up of crushed or ground carbon particles. PAC is not commonly used in a dedicated vessel, owing to the high head loss. PAC is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters.

2.3.5 Granular activated carbon (GAC)

Granular activated carbon has a relatively larger particle size compared to powdered activated carbon. Diffusion of the adsorbate is thus an important factor. These carbons are therefore preferred for all adsorption of gases and vapors as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodorization and separation of components of flow system. GAC is designated by sizes such as 8x20, 20x40, or 8x30 for liquid phase applications and 4x6, 4x8 or 4x10 for vapor phase applications.

2.3.6 Extruded activated carbon (EAC)

Cylindrical shaped activated carbon with diameters from 0.8 to 45mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content.

2.3.7 Impregnated carbon

Porous carbons containing several types of inorganic impregnate such as iodine, silver, cation such as Al, Mn, Zn, Fe, Li, and Ca. Due to antimicrobial/antiseptic properties, silver loaded activated carbon is used as an adsorbent for purifications of domestic water. Impregnated carbons are also used for the adsorption of H2S and mercaptans.

2.3.8 Polymers coated carbon

This is a process by which a porous carbon can be coated with a biocompatible polymer to give a smooth and permeable coat without blocking the pores.

2.4 Characterization of activated carbon

2.4.1 Gas adsorption [4]

It is one of the many experimental methods (SAXS, SANS. Mercury porosimetry, SEM, STM thermoporometry, NMR methods and others) available for the surface and pore size distribution. The amount of adsorbate adsorbed on a solid surface depend upon absolute temperature T, the pressure P, and the potential E between the vapors (adsorbate), the surface (adsorbent). Therefore at some equilibrium pressure and temperature the weight W of gas adsorbed on a unit weight of adsorbent is given by: W=f(P,T,E)

A plot of W versus P, at constant T, is referred as the sorption isotherm of a particular gas-solid interface. Physical and chemical adsorption

Depending upon the strength of the interaction, all adsorption process can be divided into two categories: chemical (irreversible) and physical adsorption. Chemical adsorption is characterized mainly by large interaction potential, which leads to high heat of adsorption often approaching to chemical bonds. This fact, coupled with other spectroscopic, electron spin resonance and magnetic susceptibility measurements confirms that chemisorptions involves true chemical bonding of the gas or vapor with the interface It is also associated with an activation energy[4]. Molecules that are chemical adsorbed are more localized than physical adsorption. Physical adsorption (or reversible) makes it suitable for surface adsorption by the following way:

  1. Physical adsorption is accompanied by low heat of adsorption.
  2. In physical adsorption more than one layer of molecules can be adsorbed
  3. Equilibrium reached is fast, as activation energy is not required.
  4. Adsorbed molecules are restricted to sites.
  5. Pores can be filled completely. Physical adsorption force

Adsorption of gas leads to decrease in entropy. Condensed state is more ordered than gaseous state. To have spontaneity of adsorption process Gibbs free energy, ?G should be negative. Assuming entropy change is not much and enthalpy change, ?H always negative, makes ?G always negative [25].

Vander wall force of attraction is responsible for physical adsorption

  1. Dispersion forces are present regardless of the nature of other interactions and responsible for major part of the adsorptive-adsorbate potential
  2. Ion-dipole: present in an ionic solid and a polar adsorbate
  3. Ion-induced dipole: preset between an polar solid and polarizable molecule
  4. Dipole-dipole: a polar solid and polarizable molecule
  5. Quadruple interactions: symmetrical molecules, such as nitrogen and carbon dioxide have this kind of interactions Physical adsorption on macropores (assumed as a planar molecule)

The London-vander walls interaction energy Us(z) of a gas molecule with a planar surface is given by: Us(z)=C1z-9-C2z-3

Where C1 and C2 are constants and z is the distance between gas molecules from the surface. The first term express repulsive force while second term express fluid wall interactions. The attractive interaction energy at the minimum relative close distance is ten times greater than the thermal energy ( KbT) [4] Physical adsorption on mesopores

Adsorption behavior depends upon the fluid wall interaction as well as attractive interactions between fluid molecules. Physical adsorption on micropores

As the pores are very small the interactions is dominant by fluid-wall interactions

2.4.2 Isotherms

Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. The quantity adsorbed is nearly always normalized by the mass of the adsorbent to allow comparison of different materials [5] Freundlich Isotherms

The first mathematical fit to an isotherm was published by Freundlich and Kster (1894) and is a purely empirical formula for gaseous adsorbates, \frac{x}{m}=kP^{\frac{1}{n}}

where x is the quantity adsorbed, m is the mass of the adsorbent, P is the pressure of adsorbate and k and n are empirical constants for each adsorbent-adsorbate pair at a given temperature. The function has an asymptotic maximum as pressure increases without bound. As the temperature increases, the constants k and n change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface. Langmuir theory

The problem encountered in Freundlich isotherm is that the calculated surface area will be less than the actual surface area. It is because, molecules (adsorbate) adheres tightly to the surface without allowing rearrangement of themselves [4]. Langmuir adsorption isotherm was based on monolayer physical adsorption. It is based on four assumptions:

  1. The surface of the adsorbent is uniform, that is, all the adsorption sites are equivalent.
  2. Adsorbed molecules do not interact.
  3. All adsorption occurs through the same mechanism.
  4. At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.

According to kinetic theory number of molecules striking unit are of surface per second

If ?o is the fraction of the surface unoccupied then number of collisions with bare surface per unit area per second is dNDt=kp? where k=N2MRT

The no of molecules striking and adhering to each unit area of surface is Nads =kP?oA1

The rate at which adsorbed molecules leave Ndes=Nm?1v1e-E/RT

Where, Nm is the number of molecules in a completed monolayer of unit area. At equilibrium

Where, N is incomplete monolayer. W/Wm is the weight adsorbed relative to the weight adsorbed in a completed monolayer Brunauer, Emmett and Teller (BET) Theory

BET theory is a rule for the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of a material.

The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses:

(a) gas molecules physically adsorb on a solid in layers infinitely;

(b) there is no interaction between each adsorption layer; and

(c) the Langmuir theory can be applied to each layer. The resulting BET equation is expressed by (1):

P and P0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, v is the adsorbed gas quantity (in volume units), and vm is the monolayer adsorbed gas quantity. c is the BET constant, which is expressed by (2):

E1 is the heat of adsorption for the first layer, and EL is that for the second and higher layers and is equal to the heat of liquefaction.

Equation (1) is an adsorption isotherm and can be plotted as a straight line with 1 / v[(P0 / P) - 1] on the y-axis and f = P / P0 on the x-axis according to experimental results. This plot is called a BET plot. The linear relationship of this equation is maintained only in the range of 0.05 < P / P0 < 0.35. The value of the slope A and the y-intercept I of the line are used to calculate the monolayer adsorbed gas quantity vm and the BET constant c. The following equations can be used:

N: Avogadro's number,

s: adsorption cross section,

V: molar volume of adsorbent gas

a: molar weight of adsorbed species

2.4.3 Classification of adsorption isotherm

Based on extensive literature survey, performed by Brunauer, Demming, Demming and Teller, the IUPAC published a classification of six adsorption isotherm:

Type 1: the curve is concave to P/Po axis and the amount adsorbed reaches to a limiting value as P/Po?1. It indicates adsorption to a few molecular layers. Chemical adsorption often shows this kind of isotherm. If physical adsorption is taking place then adsorption on micropores layer takes place.

Type2: First unrestricted monolayer adsorption takes place and then multilayer adsorption takes place. After inflection point molecular force of attraction is more than adsorbate-adsorbent force of attraction.

Type 3: adsorbate-adsorbate force of attraction is predominant over adsorbate-adsorbent force of attraction. fig25.3.2.6

Type 4: this indicates mesopores structures.

The line PQ describes adsorption in the microporosity. The smaller the micropore size, the steeper is the line PQ. The line QR continues the adsorption process in smallest mesopores. RS indicates progressive feeling of the mesopores concluding the largest of the mesopores is at position S. On lowering the pressure of the nitrogen in the equipment , the equilibrium positions, i.e. desorption isotherm, do not follow the line SR but the line SUR to create hysteresis loop closing at point R. This indicates desorption line equilibria follows a curved meniscus of desorption. If the line RS is vertical then it indicates mesopores are of equal sizes. If the value of P/Po is 0.90 then it correspond to pore size of 10 nm. Similarly for P/Po= 0.95 then pore size is of 20 nm, and P/Po= .995 indicates pore sizes is of 200 nm.[21]

Type 5: It shows pore condensation and hysteresis. Initial part shows weak attractive interactions between the adsorbent and the adsorbate.

Type 6: It represent stepwise multilayer adsorption on a uniform, non-porous surface [30.1Hill T. L.(1955) J. phys. Chem., 59,1065]particularly by spherically symmetrical, non polar adsorptive. The sharpness of steps depends upon on the homogeneity of the adsorbent surface, the adsorptive and the temperature.[25]

2.5 Spectrophotometer

A spectrophotometer is an instrument designed to detect the amount of radiant light energy absorbed by molecules. Light is allowed to fall on it. Light that passes through the slit travels to the phototube, where it creates an electric current proportional to the number of photons striking the phototube. If a digital meter is attached to the phototube, the electric current output can be measured and recorded. The scale is usually calibrated in two ways: percent transmittance, which runs on a scale from 0 to 100; and absorbance, or optical density units, which runs from 0 to 2 [6]. The amount of light absorbed is proportional to the concentration of a compound Beer's Law explains the relationship between absorbance, at a given wavelength and concentration.

The relationship between absorbance and concentration is linear. As concentration increase the absorbance also increases. This relationship allows one to convert an absorbance value into a concentration.

2.6 Quantachrome BET surface area analyzer

The principle of the Quantasorb surface area analyzer is based on the B.E.T. theory which is a commonly used method for determining the specific surface area of solids by the physical adsorption of a gas on the surface of a solid. To determine the monolayer capacity and in return the surface area is given by BET theory. Usually a second layer may be forming before the monolayer is complete. Total monolayer capacity is determined from the isotherm equation irrespective of the influence of multilayer. Another model is the Langmuir. In each of these approaches the output will results in a series of calculations and the main output is surface area range, adsorption isotherm [7].

2.7 FTIR analyzer

Infrared (IR) spectroscopy is a chemical analytical technique, which measures the infrared intensity versus wavelength (wave number) of light. Based upon the wave number, infrared light can be categorized as far infrared (4 ~ 400cm-1), mid infrared (400 ~ 4,000cm-1) and near infrared (4,000 ~ 14,000cm-1).

It detects the vibration characteristics of chemical functional groups in a sample. When an infrared light interacts with the matter, chemical bonds will stretch, contract and bend. As a result, a chemical functional group tends to adsorb infrared radiation in a specific wave number range regardless of the structure of the rest of the molecule. The correlation of the band wave number position with the chemical structure is used to identify a functional group in a sample [8].

An interferometer utilizes a beam splitter to split the incoming infrared beam into two optical beams. One beam reflects off of a flat mirror which is fixed in place. Another beam reflects off of a flat mirror which travels a very short distance (typically a few millimeters) away from the beam splitter. The two beams reflect off of their respective mirrors and are recombined when they meet together at the beam splitter. The re-combined signal results from the interfering with each other. The resulting signal is called interferogram,. When the interferogram signal is transmitted through the sample surface, the specific frequencies of energy are adsorbed by the sample due to the excited vibration of function groups in molecules. The beam finally arrives at the detector and is measured by the detector. The computer can perform the Fourier transformation calculation and present an infrared spectrum, which plots transmittance versus wave number.

The final transmittance should be devoid of all instrumental and environmental contributions.

2.7.1 Spectra of transmittance with different wave numbers

The spectra of different functional groups are as follows: [9] Alkynes

  • CH stretch from 30002850 cm-1 Alkenes: Stretching vibrations of the C=CH bond are of higher frequency (higher wave number) than those of the CCH bond in alkanes.
  • C=C stretch from 1680-1640 cm-1
  • =CH stretch from 3100-3000 cm-1
  • =CH bend from 1000-650 cm-1


  • C=C stretch from 2260-2100 cm-1
  • C=CH: CH stretch from 3330-3270 cm-1
  • C=CH: CH bend from 700-610 cm-1

Alkyl Halides

  • CCl stretch 850-550 cm-1
  • CBr stretch 690-515 cm


Aromatic hydrocarbons show absorptions in the regions 1600-1585 cm-1 and 1500-1400 cm-1 due to carbon-carbon stretching vibrations in the aromatic ring. It distinguishes from non aromatics due to vibrations at:

  • 2000-1665 cm-1 (weak bands known as "overtones")
  • 900-675 cm-1 (out-of-plane or "oop" bands)


  • OH stretch, hydrogen bonded 3500-3200 cm-1
  • CO stretch 1260-1050 cm-1 (s)


C=O stretch:

  • aliphatic Ketones 1715 cm-1
  • a, -unsaturated Ketones 1685-1666 cm-


  • HC=O stretch 2830-2695 cm-1
  • C=O stretch:

o aliphatic aldehydes 1740-1720 cm-1

o alpha, beta-unsaturated aldehydes 1710-1685 cm-1

Carboxylic acids

Carboxylic acids show a strong, wide band for the OH stretch. Unlike the OH stretch band observed in alcohols, the carboxylic acid OH stretch appears as a very broad band in the region 3300-2500 cm-1, centered at about 3000 cm-1. This is in the same region as the CH stretching bands of both alkyl and aromatic groups. Thus a carboxylic acid shows a somewhat "messy" absorption pattern in the region 3300-2500 cm-1.

The carbonyl stretch C=O of a carboxylic acid appears from 1760-1690 cm-1. The exact position of this broad band depends on whether the carboxylic acid is saturated or unsaturated, dimerized, or has internal hydrogen bonding.

  • OH 3300-2500 cm-1
  • -COOH 3300-2500 cm-1.


  • C=O stretch
  • o aliphatic from 1750-1735 cm-1

    o a, -unsaturated from 1730-1715 cm-1

  • CO stretch from 1300-1000 cm-1


  • NH stretch 3400-3250 cm-1
  • o 1 amine: two bands from 3400-3300 and 3330-3250 cm-1

    o 2 amine: one band from 3350-3310 cm-1

    o 3 amine: no bands in this region

  • NH bend (primary amines only) from 1650-1580 cm-1
  • CN stretch (aromatic amines) from 1335-1250 cm-1
  • CN stretch (aliphatic amines) from 12501020 cm-1
  • NH wag (primary and secondary amines only) from 910-665 cm-1

2.8 Previous work

On application of positive potential on carbon, anodic oxidation of carbon fibers takes place. It introduces new functional groups, such as COOH, and OH, onto the carbon surfaces to increase the adsorption capacity and rate of the adsorption in the liquid phase. Generally it increases the adhesive property and strength.

Carbon fibers were subjected to electrochemical oxidation (25 oC, 0-450 mAmp, NaOH as electrolytic solution) and then dried (110 C for 6 hr). Nitrogen adsorption isotherms for BET surface area and pore volumes were measured at 77K [10]

Result shows specific surface area, average pore diameter, micropore volume does not change a much. Surface acidity of ACFs was determined by titration with hydrochloric acid. There was a significant increase in acidity. Result also shows that the amount of adsorption and the adsorption rate of Chromium from the aqueous solution increase with increase electrochemical oxidation of ACF.

GACs (170-210 M) were washed and dried (at 378 K) until no change in weight was noticed. It was then electrochemically oxidized at 3mAmp/m2 (range 0.01 to 3mAmpm-2) for 3 hrs in an electrolyte solution (0.5 M. Conc.) at 293-295 K. Surface area, acidic sites, batch and continuous sorption experiment and kinetics were analyzed[11].

Result shows BET surface area decreases, total oxygen containing functional groups increased by 3.36 times. This increased sorption capacity by 16.5 times. Cadmium uptake increased as PH increased (4-6).adsorption rate also increased4. Kinetic experiment shows that the adsorption rate for cadmium was rapid and 96% of fractional approach to equilibrium was attained in 12 minutes using both unoxidized electrochemical oxidized carbon.

The pitch based (bundle type) were washed with deionized water, then dried(24 hrs, 80 oC), and then subjected to electrochemical oxidation in the aqueous solution of 10 wt wt% H3PO4 and NH4OH, whereby negative ions were attracted to the surface (7 A , 10 min). It was then washed and dried (6 hr, 110 oC) [12]

Result shows total pore volume and micropore volume decreases. This is due to increase in oxygen containing functional groups and blockage of the pores. BET surface area decreased. Adsorption isotherm (% change in concentration vs. time) of Cr (VI), Cu (II), and Ni (II) from 0-180 minutes shows that the initial adsorption rate of Cr (VI) ion on the ACFs increases rapidly, especially due to molecular sieve structure of the ACFs12.Also the amount of Chromium (VI) adsorbed is comparatively much larger than the amount of Copper (II) and Nickel (II) adsorbed under similar conditions. This can be imagined due to high ionic radius of copper (II) 0.70 Ao and Nickel (II) 0.69 Ao compared to that of Chromium (VI) 0.52 Ao, induces a quick saturation of adsorption sites because of steric crowding.



This chapter describes materials used, fabrication and outlines the experimental procedure.

3.1 Chemicals used

Pure and analytical grade chemicals were used in all experiments. Potassium dichromate and 1,5-Diphenylcarbazide was procured from Merck, Chemical, India. Fresh stock solution for Cr (VI) solution was prepared following standard procedure. Others chemical used are sulphuric acid (reagent used during absorbance analyses in spectrophotometer), acetone (for washing) and potassium chloride (for preparation of electrolytic solution). Besides that distilled water is used for washing.

3.2 Sample used

The carbon that is taken for electrochemical oxidation for surface modification is commercially available chemically activated carbon under the brand name Kalpaka chemicals KALBON GAQ Water Treatment. This granular activated carbon is used for water treatment applications in fixed bed media filters. It is used for removal of taste, dour, chlorine, and dissolved organic contaminants from potable and process water. It is also used in DM plant filters, Tap water filters, Desalination plants and Aquariums [13].


Size 12x40 ASTM, oversize 0.6%, undersize 0.3%

Iodine No. 1050 mgram/gm

Hardness 98.60%

Moisture 2.677%

Ash 2.15%

3.3 Glassware and Apparatus used

All glass wares (Conical flasks, Measuring cylinders, Beakers, Petri plates and Test tubes etc.) used are of Borosil/Rankem. The instruments and apparatus used throughout the experiment are listed below:

3.4 Fabrication

To perform the electrochemical oxidation process of commercially activated carbon following materials are required for fabrication

  1. Perplex column ( I. D. 2.75)
  2. Plastic net
  3. Anode copper plate( 4x4x1.968)
  4. Plastic stands
  5. GI sheet
  6. Epoxy Adhesive (araldite)
  7. Liquid gasket sealant (Autobond)
  8. Transformer (A.C. to D.C.)
  9. D.C. Gear motor
  10. Cathode copper plate ( 2x12)

3.4.1 Fabrication of anode copper plate

To cut the anode side of copper plate which is 50 mm thick requires an electrically operated hex saw. The metal plate was fitted into bench vice. This work was carried out in the workshop department. With a glass marking pen the plate was marked so that it just fits into the perplex column. The plate was cut near to a circular shape. The instruments used area shown below.

To bring it into perfect circular shape the edges were polished in a grinder.

3.4.2 Fabrication of perplex column

From a large perplex column (1.5 x 2 .75 I.D.) two equal length (20 mm) of perplex column were cut manually in the fluid flow lab.

First a plastic net is wrapped around Perspex column. It is tightly tied on another side. This net is then glued with one side (edges) of the column with the help of epoxy adhesive. To another column liquid gasket sealant is applied on its edges. These were then dried for 2 days. Extra net was cut. Three legs were glued to the column at a uniform height. Finally an arrangement as shown in figure is got.

3.4.3 Fabrication of stand (support to the perplex columns)

Three equal size plastic stands (3mm x 20 mm x 55 mm) were cut from a large cuboidal plastic fiber in the fluid flow lab manually with a help of a hex saw.

3.4.4 Fabrication of clamps (to hold the columns)

To hold the perplex column tightly clamps were required. No clamps were available in the market as required. So from G.I. plates clamps were made manually. G.I. sheets were cut of dimensions 7mm x 60mm. These were later bend by an angle of 90o at both sides. The edges were also bent by 90o so that it doesnt slips out from the column. The screws were drilled at one end to have screws in them.

3.4.5 Fabrication of stirrer

The stirrer was powered by 12 V D.C. supply. Motor was held by a stand. It was dipped into the water bath (not in the electrolyte). This was used if sufficient rise in temperature was detected

3.5 Experimental setup

Setup consists of a plastic tub of 5000 ml capacity. Inside it, there is a cathode plate, stirrer, thermometer and an arrangement for oxidation of activated carbon. This arrangement consists of a three legged perplex column of 2 cm height (dia 2.75) with a net covering the whole area inside it. Above it is a filter paper that rests on the net. This holds the chemically activated carbon. To restrict the flow of electrolytic solution from sideway another perplex column (2 cm height x 2.75dia) with a gasket sealant is kept over it. To have a tight fitting between these two perplex column three clamps (made GI sheet) is used. This has screws to alter the degree of tightening. This allows solution to come to the activated carbon through filter paper. A circular copper plate of 5 mm thickness, just fitting into the perplex column act as anode. Thick copper plate is taken to restrict electrolytic solution to come to top of it and inhibit oxidation there.

The top anode is connected to positive potential of power supply. This can give current ranging from 0 Amp to 4.5 Amp. The cathode is connected to negative potential of power supply. In between these is an electronic multi meter. It measures the applied current. If high current is applied, temperature rise may be encountered. So an arrangement of stirrer is done. This is there to stir the water bath and maintain a uniform temperature throughout the oxidation process.

3.6 Preparation of adsorbent

In between the two columns a filter paper was kept. It is then held tight by the help of clamps. It has screws to tighten them. Now 10 grams of the sample (carbon) is taken and spread over the filter paper. Filter paper doesnt allow granular carbon to move into solutions. Electrolyte was then added so that it just immerses the sample. Above the sample the anode was placed. Below it cathode plate was placed.

3.6.1 Preparation of electrolytic solution

The solution is of potassium chloride. 0.5% M KCl solution is prepared. At a time 2 liters of solution were required. So in 2 liters of distilled water 74.56 grams of KCl salt was added.

3.6.2 Oxidation of activated carbon

Before start of oxidation the sample was allowed to wet with the solution for 20 minutes. Then oxidation of sample was done for 20 minutes. Current intensity was adjusted by the variac (of the power supply). After oxidation the sample was taken in a beaker and washed continuously in distilled water for 20 minutes.

3.6.3 Drying of oxidized (surface modified sample)

The sample was dried in oven at 85oC for 24 hours.

3.7 Method for surface area and porosity analyses

Sample of known weight (approx. 0.100 gm) is taken in the glass tube of degasser. Degassing is done (for 2 to 3 hours, depends upon the sample) at 250oC. It helps in removal of any adsorbed gas etc. from the sample. After that it is fitted into the quantachrome analyzer (dipped in liquid nitrogen) to analyze BET surface area and porosity.

3.8 Method of absorbance analyses in spectrophotometer

3.8.1 Preparation of potassium dichromate solution

28.23 grams of K2CR2O7 salt was taken in 1 liter of distilled water to prepare standard solution of 10 milligrams of chromium solution in 1 liter of water.

3.8.2 Preparation of standard indicator

The indicator for absorbance analyses is prepared by adding 250 milligrams of 1.5 diphenylcarbazide in 50 ml of acetone. It should be always freshly prepared. If Indicator is kept open in light, it undergoes photo reaction. So it should be always kept in a dark bottle.

3.8.3 Adsorbance of chromium

At first 2 mg/L, 4 mg/L, 6mg/L, 8mg/L and 10 mg/L was adsorbed on the carbon (1 gram) at a PH 5. 2 mg/L was made by adding 20 ml of 10 mg/L of Cr solution and 80 ml of distilled water (total solution is then 100 ml). The activated sample was proportionally taken as 100 milligram. Similarly 2 mg/L, 4 mg/L, 6mg/L, 8mg/L and 10 mg/L was made. They were then shacked and allowed to adsorb for 2 hours. Their adsorption on carbon was analyzed in spectrophotometer. Then all the 6 samples (0.1 Amp. 0.5 Amp, 0.9 Amp, 1.3 Amp, 1.7 Amp and 2.1 Amp) were analyzed in similar manner.

3.8.4 Method for spectrophotometeric analysis

1 ml of sample was taken in a cleaned testube. To it 9 ml of distilled water was added. So dilution factor is 10. To it 1 drop concentrated sulphuric acid and 1 ml of indicator was added. This was then well shacked. A purple color comes. In the spectrophotometer both the analyzer was kept initially with distilled water. In the sample side, sample was taken. Absorbance was then studied for each sample.

3.9 Method for FTIR analysis

The oxidized activated carbon sample (at 2.1 Amp) was analyzed in FTIR. This was done to know the changes in functional groups that took place during the course of oxidation.



4.1 Absorbance analyses

To know the optimum current that has to be supplied to oxidize the commercially available activated carbon, absorbance of adsorbed chromium solution was analyzed for five different concentration (2 mg/L, 4 mg/L, 6mg/L, 8mg/L and 10 mg/L). After 2 hours absorbance were analyzed in spectrophotometer. The table shows the absorbance of different samples at different current of oxidation. A standard calibration curve y=0.00058 x- 0.0091 is taken from R. Gottipati and S. Mishra (2010), where y is absorbance in spectrophotometer and x is concentration of chromium in solution.

A graph is plotted of this absorbance with X axis as concentration of the adsorbed sample of different initial concentration with Y axis showing the current.

From the graph, it is concluded that as the intensity of current is increased from 0.1 Amp to 2.1 Amp, the amount of adsorption increases. But as the current approaches to 2.1 Amp the adsorption amount doesnt change significantly. It is also supported from previous findings [14]. If it is considered that, 2.1 Amp as the final optimum current of oxidation then we can study adsorption characteristics and FTIR analysis of the sample oxidized at 2.1 Amp.

4.2 FTIR analysis

Infrared (IR) spectroscopy is a chemical analytical technique, which measures the infrared intensity versus wavelength (wave number) of light. It detects the vibration characteristics of chemical functional groups in a sample. The sample oxidized at 2.1 Amp for 20 minutes in 0.5 M KCl solution was sent to Metallurgy and Materials Engg. Department for analysis. A single FTIR report is of 60 pages and cannot be accommodated in this thesis. A short FTIR report containing 22 values is given below:

In the FTIR spectra it is revealed that in the range of 3600-3200 cm-1 a dip in transmittance was observed. It is because of stretching of O-H vibrations. Similarly a dip in transmittance was observed in 2500-2300 cm-1. It reveals of the presence of -C=C- groups. The adsorption bands in 1640-1500 cm-1 region suggests the overlapping of aromatic ring bands and double bands(C=C) vibrations with the bands of C=O moieties [10].

4.3 Surface area and porosity

BET surface area was analyzed in quantachrome autosorb-1. The table below shows the analysis of raw sample (unoxidized) and oxidized samples. P/Po is the ratio of pressure of N2 over carbon to that of atmospheric pressure. At first adsorption is done and then desorption is done.

This satisfies the decreases in specific surface area. As surface area decreases the pore volume must decrease. A significant loss in porosity were observed and attributed to blockage of pores through formation of functional groups (carboxylic acidic groups, hydroxyl groups, lactonic grops, phenolic group) and humic substances [16],[17].

4.4 Analysis of pore diameter with current

The presence of micropores is not easily determined. Their contribution to adsorption is less.Table 4.4: Current and pore size relationship

There is a large contribution of mesopores and macropores The pore diameter which is measured by analyzer is mainly due to contribution of mesopores and macropores. The pore diameter of samples before and after oxidation is given below.

Decrease in pore volume support decrease in pore size. Strict decrease in pore size may be attributed due to presence of some amount of humic substances in micro & mesopores [10]. Humic substances are generated during the course of experiment. These are some salts of metals like copper and potassium that are used during the course of experiment.



The electrochemical method of activation (surface modification) of commercially available activated carbon was studied.

  1. Fabrication of setup was done. Fabrication was done in a manner to have large surface area available for carbon to undergo oxidation.
  2. The parameters on which adsorption study was done include the affect of current on surface area, pore volume and pore size. Then optimum current for oxidation was found out. Sample oxidized at optimum current was analyzed in FTIR to know the reasons

  3. Surface area, pore volume and pore size all decrease with increase in current intensity. Decrease in surface area is resulted from pore blockage by formation of functional groups. The oxidation process also causes the destruction of pore resulting in final decrease in surface area.
  4. A significant loss in porosity were observed and is due to blockage of pores by formation of functional groups (carboxylic acidic groups, hydroxyl groups, lactonic grops, phenolic group) and aggregation of humic substances.
  5. Decrease in pore volume support decrease in pore size. Strict decrease in pore size may be attributed due to presence of some amount of humic substances in micro & mesopore[7]. Humic substances are generated during the course of experiment. These are some salts of copper and potassium that are generated during the course of experiment.
  6. As the intensity of oxidation is increased by increasing the intensity of source current the amount of adsorption also increases. This concludes that degree of formation of functional groups (that are responsible for adsorption) increases with increase in current.
  7. Also it is concluded that as the intensity of current is increased from 0.1 Amp to 2.1 Amp, the amount of adsorption increases. But as the current approaches to 2.1 Amp the adsorption amount doesnt change significantly. Therefore the optimum current for oxidation is 2.1 Ampere.
  8. Sample oxidized at 2.1 ampere was analyzed in FTIR. In the FTIR spectra it is revealed that in the range of 3600-3200 cm-1 a dip in transmittance was observed. It is because of stretching of O-H vibrations. Similarly a dip in transmittance was observed in 2500-2300 cm-1. It reveals of the presence of -C=C- groups. The adsorption bands in 1640-1500 cm-1 region suggests the overlapping of aromatic ring bands and double bands (C=C) vibrations with the bands of C=O moieties.
  9. In our project we could reduce the concentration of chromium from 10 mg/L to 0.6243 gm/L. Adsorption capability is much greater than the carbon activated bye other mode of activation. Conclusion is that it will be a better mode of activation of carbon.



[1] Zhongren Yue, Samantha E. Bender, Jinwen wang, James Economy, journal of hazardous material 166(2009)74-78.

[2] J. R. Rangel-Mendez, M. H. Tai, M. Streat, Ichem, Vol 78.

[3] R. Berenguera, J.P. Marco-Lozarb, C. Quijadac, D. Cazorla-Amorosb, E. Morallona, Carbon 47 (2009) 1018-1027.

[4] Characterization of porous solids and powders: surface area, pore size and density, by S. Lowell, Joan E. Sheilds, Martin A. Thomas and Matthias Thommes.

[5] www.wikipedia.org/wiki/Adsorption

[6] www.biology.lsu.edu/introbio/tutorial/Spec/spectrophotometry

[7] www.thefreelibrary.com



[10] Soo-Jin Park, Byung-Jae Park, Seung-Kon Ryu, Carbon 37(1999),1223.

[11] 4 J. R. Rangel-Mendez, M. H. Tai, M. Streat, Ichem, Vol 78.

[12] Soo-Jin Park*, Young-Mi Kim, Materials Science and Engineering A 391 (2005) 121.

[13] www.kalpakachemicals.com

[14] Z. R. Yue, W. Jiang, L. Wang, H. toghiani, S. D. Gardner, C. U. Pittman Jr.,Carbon 37(1999); 1607.

[15] Piotr A. Gauden, Artur P. Terzyk, Piotr Kowalczyk, Science, Volume 300, Issue 2, 15 August 2006, Pages 453-474.

[16] Soo-Jin Park*, Young-Mi Kim, Materials Science and Engineering A 391 (2005) 121.

[17] C.U. Pittman Jr.,W. Jianga, Z.R. Yue, S. Gardner, L. Wang, Carbon 37 (1999) ,1797.

Please be aware that the free essay that you were just reading was not written by us. This essay, and all of the others available to view on the website, were provided to us by students in exchange for services that we offer. This relationship helps our students to get an even better deal while also contributing to the biggest free essay resource in the UK!