Activated Carbon and Its Application
Activated Carbon and Its Application...
Activated Carbon and its Application for Monitoring o£ Volatile Organic Compounds : A Case Study Dr. S.P. Singh Scientist
Central Fuel Research Institute Nagpur Unit, Nagpur and
Ms. Sarika N. Deole National Environmental Engineering Research Institute, Nagpur
Introduction Due to rapid development and industrialization in many countries, the levels of industrial pollution to the atmosphere have been steadily rising. One of these problems is the emission of toxic gases, which can have catastrophic effects on the environment and the general populace. Examples of some of these sources of toxic gas pollutants are volatile organic compounds emission from crystallized organic chemicals, printed materials, paints and coatings and dry cleaning fluids. The availability of variety of Granular Activated Carbon (GAC) samples in several countries, its versatility as an adsorbent and the recent inroads into the regeneration of spent GAC samples for many cycles (Skinner et. al.) have further boosted its application in air pollution monitoring and control.
Preparation of activated carbon The principle of manufacturing carbonaceous adsorbents is the selective removal of some groups of compounds from the suitable carbon containing materials such as wood, peat, lignite (Allen et. al.), bituminous coal, coconut shell, walnut shell, coke fines etc (Merchant et. al.). These raw materials are first subjected to carbonisation, which is usually carried out in the absence of air at
19 6. A
600 - 900 °C. However, this is not a universal temperature range. After carbonisation the product is subjected to physical or chemical activation.
In physical activation the raw material is first carbonised at high temperature in a high temperature carbonisation unit and later the carbonised or coked material is subjected to the action of an oxidising agent such as steam or carbon dioxide or the mixture of two. At activation temperature between 8001000 °C, part of the carbon gets gasified according to the following reactions (Faust et. al.):
H 2 0 + Cx
> H + CO + C ( x -i)
C 0 2 + Cx
> 2 CO + C( X _i)
0 2 + Cx
> C 0 2 + C(x - 2 )
(at 800 to 900 °C) — (1) (at 800 to 900 °C) — (2) (at 800 to 900 °C) — (3)
The oxidation step selectively erodes the surface, increasing the surface area and porosity leaving the carbon surface full of surface groups having specific affinities and a highly porous carbon skeleton with numerous capillaries, which provide an immense internal surface area (Hashimoto et. al.).
In the chemical activation process, the source material is mixed with dehydrating or oxidising agents like zinc chloride, phosphoric acid, sulphuric acid or KOH (Naidu et. al., Toshiro Otowa et. al. and Lane Jorge) . The mixture is then subjected to activation at temperatures between 400 and 950 °C. In the final step, chemicals used for activation are reclaimed by washing the final product. Chemically activated carbons feature an open pore structure, which makes them particularly suitable for the adsorption of large molecules. Steam activated carbon typically exhibits a narrow pore structure and is preferably used for air purification and adsorbing low molecular substances. The manufacture of high quality activated carbon with defined and uniform characteristics requires a sound knowledge and years of experience.
19 6. A
There are many existing companies all over the world, which are producing a wide range of activated carbons for a variety of gas phase and liquid phase applications such as gas separation, gas purification, solvent recovery, colour and odour removal and purification. Some of the internationally known producers of PAC and GAC are Pittsburgh Activated Carbon, Union Carbide and Westvaco
(Jungeten et. al.), LURGI & Degussa (Germany) and Norit Activated Carbon (Neatherland). The raw material used by most of these producers is bituminous and anthracite coals. The general steps involved in the GAC or PAC production are:
Preparation of Activated carbon. In
Institute of Chemical Technology,
developed granular activated carbon from coconut shells. The product granules were irregular in shape with N 2 -BET surface area of 900 m2/g and particle density of 0.802 g/cm3. Central Fuel Research Institute, Dhanbad (Choudhury et. al.) too has produced granular and powdered activated carbons from various raw materials such as coal (Samla), South Arcot lignite (SAL), Kutch lignite, Rajpasdi lignite, pine wood, soft wood saw dust, coconut shells, walnut shells, groundnut shells at laboratory and pilot plant scale by both physical and chemical activation methods. Granular or palletised active carbons prepared from raw materials like
19 6. A
coconut shells, saw dust and rice husk were found to be suitable for gas phase adsorption whereas those prepared from South Arcot lignite , Kutch lignite , non caking
decolourising properties. These carbons had comparatively lower surface area than the commercially available carbons but their decolourising power was comparable. Naidu et.al.
at National Environmental Engineering Research
Institute, Nagpur, obtained activated carbon from coke fine having N 2 -BET surface area of 526 m 2 /g and bulk density of 0.660 g/cm . It showed a good decolourising power but required much higher contact time for maximum removal of organic carbon from wastewater. Srivastava and co-workers prepared cheap carbonaceous adsorbent material from fertiliser waste and the product was found to have a very good adsorption potential form substituted phenols.
Characterization of Activated carbon Molecular and crystalline structure of activated carbon An understanding of the molecular and crystalline structure of activated carbon is necessary to discuss the surface chemistry of this material. The only apparent difference between activated carbon and carbon black is the small surface area of the latter. The basic structural unit of activated carbon and carbon black is closely approximated by the structure of pure graphite (Walker et. al.). The graphite crystal structure is composed of layers of fused hexagons held approximately 3.35 A 0 apart. The carbon-carbon bond distance with each layer is 1.415 A 0 . Three out of four electrons form regular covalent bonds with adjacent atoms, and the fourth electron resonates between several valence bond structures. This gives each carbon-carbon bond a one third double bond character. The carbon layers are so arranged that one half of the carbon atoms in any plane lie above the centre of the hexagons in the layer immediately below it. X-ray diffraction data indicates that this structure predominates for graphite (Heckman et. al.). During the carbonisation, several aromatic nuclei having a
19 6. A
structure similar to graphite are formed. The X-ray spectrographs interpreted these structures as micro crystallites consisting of fused hexagonal rings of carbon atoms (Wolff W. F.). The diameter of the planes making up the micro crystallite is estimated to be 150 A0 and the distance between micro crystallites ranges from 20-50 °A (Garten et. al.).
Porous structure and surface area The structure of granular activated carbon is highly heterogeneous and porous. The pores are widely dispersed and are classified in three categories, micro, meso and macro pores. In micro pores the effective pore width is less than 2 nm, but the interaction potential is significantly higher than the wider pores owing to the proximity to the walls and the amount adsorbed at a given relative pressure is correspondingly enhanced. Micro pores are of greatest significance for adsorption due to their very large pore volume and about 97% contribution to the total GAC surface area (Yenkie et. al. and Smisek et. al.).
Chemical Structure The chemical composition of activated carbon also affects its adsorptive properties significantly. Activated carbon contains chemically bonded elements such as oxygen and hydrogen. The hydrogen not only forms part of the oxygen functional groups but also directly combines with carbon atoms. Hydrogen is more chemisorbed than oxygen and hence is very difficult to be removed from the surface of the carbon.
Surface Chemistry The
shows the presence of many functional groups on the surface of activated carbon. They are carboxyl groups, phenolic hydroxyl groups, quinone type carbonyl groups, normal lactones, fluorescien type lactones, cyclic peroxides,
19 6. A
carboxylic acid, anhydrides etc. Observations of (Mattson et. a!.) the internal reflectance spectroscopic examination of activated sugar carbons, suggested the presence of a pair of adjacent carboxylic acid groups.
But according to Gorten
and Weiss, no acidic group of such strength as a carboxylic group is present on the surface of activated carbon. They proposed the presence of lactones of the fluorescien type to explain the reaction of diazomethane to form hydrolyzable esters.
Preservation of Carbon The preservation of carbon after the activation process is another significant aspect. Activated carbon containing carboxyl type groups, kept in an oxygen atmosphere will bring about an ageing process. Atmospheric oxygen reacts with the carbon surface to form primarily carboxyl or lactone groups. Hence storage of activated
atmosphere results in the degradation of the adsorption capacity for a large number of organic compounds (Coughlin et. al.). In addition to the effect of storing carbon under normal atmospheric conditions, it must be recognised that carbon exposed to any oxidising solution generally results in the formation of oxygen complexes that thermally decompose as carbon dioxide. This means that activated carbon in contact with chlorine water will revert to a surface chemistry containing excessive carbonyl and lactone-type groups. As discussed above, this carbon would have a reduced capacity for adsorbing most organics found in water.
activated carbon Air sample at a flow rate of 200 ml/min is passed through a glass tube packed with activated charcoal. The organic vapours were absorbed onto the charcoal. The collected vapours are desorbed using carbon disulfide (CS2) and
19 6. A
SEM of F-300 Grade Activated Carbon
Space ,1 ccotstblv to both MHorb.nu ,mct Solvent Spjcv ,iei;«',s&i()/t' 10
solve/it ami smaller •idsoibntv miili'ciilos
9 Jf -j
Spaca ' accessible only lo the solvent
Concept of sorption in Idealised Activated Carbon Pore Structure
19 6. A
RSIC,NAGPUR SAMPLE CODE - F300 Thu Sep 26 16!l7:29 1996 Number of (ample scans: 32 Number of background scans: Resolution: 4.000 Sample gain: 1.0 Mirror velocity: 0.6329 Aperture: 69.08 Detector: DTGS KBr Beamsplitter! KBr Source: IR
88 87 4000
Wavenumbers (cm-1) FT1R Sncctrn of C.AC. Filtrasorb 300
iMJ 3 JUW
analysed with a gas chromatograph equipped with flame ionization detector for Benzene, Toluene and Xylene (m,p,o). Activated charcoal with particle size of 0.35 - 0.85 mm is used for packing the absorption tube. However, before packing the tubes, the charcoal is heated in an inert atmosphere e.g. high purity N2 at 600°C for 1 hr. To prevent recontamination of the charcoal, it is kept in a clean atmosphere during its cooling to room temperature, storage & loading into the tubes. Analytical grade reagents are used for the analysis. Standard solutions of VOCs of interest are required as reagents for calibration purposes. Carbon disulphide of chromatograph quality is used as desorption or elution solvent. It is free from the compounds that may co-elute with the substances of interest. Carbon disulphide is normally recommended for activated carbon.
The Organic Vapour Sampler The Organic Vapour Sampler model Envirotech APM 850 used for collecting the samples is described below. Sampling Assembly: Activated charcoal tube The charcoal tube is made of a glass with both ends flame sealed, 70 mm long with an outside diameter of 6 mm & an inside diameter of 4 mm containing two sections of 0.35 mm to 0.85 mm of activated charcoal.
section contains 100 mg of charcoal & the back up section 50-mg, specially designed for large loading capacity of Volatile Organic Compounds. Provided two sections separated by intent glass tape plugs. Glass beads are packed before front section of the tube for proper distribution of air, entering into the tube. This tube has one inlet and two outlets. While sampling inlet is attached to filter adopter and outlet is connected to suction pump through orifice controlled Teflon nozzle while other outlet connected to 'u' tube manometer through
provided nozzles for pressure measurement. All connections of the tube are done with the help of silicon tubes. Two sections provided in this tube can be separated out by removing springs for desorption/extraction of adsorbed organic compounds separately. Ground glass joints male and female type have been provided to facilitate assembling and dissembling of the tube.
Rubber caps need to be used for sealing all three opening of the tubes after sampling and regeneration.
The pressure drop across the tube shall not exceed 3 Kpa (25 mm Hg) at the flow rate of 200 ml/min recommended for sampling.
Extraction Assembly :The extraction assembly consists of the following parts:
Desorption Bulb : A round bottom glass flask of 25-ml capacity provided with ground glass connector suitable to fit into both sections of the charcoal tube.
Sample vial & funnel : While agitating charcoal with elutant in desorption bulb fine carbon particles are observed in liquid phase. These need to be removed by filtration. Funnel loaded with filter is attached too sample vial. A 25mm diameter Whatman glass fibre filter with funnel should be used for filteration of extracted elutant.
Syringes : Syringe of capacity 10pl & 50pl graduated in 0.1 pi
19 6. A
Sample Analysis In general a gas liquid chromatograph (GLC) fitted with a flame ionization, photoionization, mass spectrometric or other suitable detector; capable of detecting the organic compound in an injection of 0.5 ng of sample with a signal to noise ratio of at least 5 to 1 can be used. In this study, Perkin Elmer Autosystem XL gas chromatograph equipped with Flame Ionization Detector (FID) and Nitrogen as a carrier gas has been used for determination of Benzene, Toluene & Xylene. The hydrocarbons are determined on 3% Dexsil-300 column. The column and its operating conditions: Column
S.S. 1/8" O.D., 6" feet 3% Dexsil on 80/100 chromosorb W-AW
Nitrogen 15 ml/min
Isothermal Oven Temp.
Operating conditions on suitable column for temperature programming may vary from 60-120 °C at 4°C/min with a carrier gas flow 10 ml/min to 20 ml/min. Examples of suitable choices with different stationary phases are given in following Table:
19 6. A
Equivalence of Gas Chromatograph Stationary Phases Company
SGE Chrompack J &W Supelco Hewlwtt-Packard Restek Quadrex
BP-1 CP-Sil 5 CB DB-1 SPB-1 HP-1 Rtx-1 007-1
BP-10 BP-10 DB-1701 SPB-1701 HP-1701 Rtx-1701 007-1701
SP-2330 and SP-1000 columns can also be used for the determination of Volatile Organic Compounds. The suitability of the column shall be verified by testing with 2 or more columns of dissimilar packing to ensure the absence of interferences.
Sampling Connect mains cable in the socket provided on the rear side of the sampler. Remove rubber caps of Activated Charcoal tube and fit it in the "hydrocarbon nozzle" with the help of silicon tube. Silicon tube of nozzle marked as "pressure" is connected to 2 nd connection of the tube. Take carbon monoxide or other gases indicator tube and break glass seal from both end of the tube and fit it in the nozzle marked with carbon monoxide. Use metallic folding stand for holding the tubes straight. Take initial reading of the time totalizer. Now switch on the power supply and see that indicator ON/OFF switch glows. Switch 'ON' the equipment and check flow in both the tubes with the help of rotameter by connecting silicon tube of nozzle marked with the flow to the tip of connected hydrocarbon/ carbon monoxide tube. Record flow reading on the note sheet. Flow is maintained constant in the equipment with the help of needle type orifices. Generally flow rate 100 to 200 ml/min is maintained in both the tubes. Higher flows can also be maintained by choosing different size of needle orifices. After noting the flow, attach filter adopter loaded with filter on each tube to avoid entry of particulates in the tubes, which may increase pressure drop and reduce
19 6. A
the adsorption efficiency
beyond acceptable limits.
Sampling frequency may
vary from 3-8 hrs at traffic junctions/sampling site depending on vehicular density/peak hours. While operating if no flow is observed check needle fitted in the Teflon nozzle. It is expected that needle may be chocked, replace needle in such cases. After operation for desired period check the flow reading and switch off the suction pump and note the final time totalizer reading. Now take out the charcoal tube and seal all three openings with the help of supplied plugs after marking sampling date, time and location on sealed tube. Now this tube needs to be sent to lab for analysis. Ensure storing of tube at low temperature (sub ambient temperature) to avoid migration of sorbed compounds from one section to another. CO tube is removed and concentration of CO is noted by comparing the colour with provided chart.
Extraction Open the sealed exposed Activated charcoal tube and separate the sampling tube's back up section and front section of modular activated charcoal tube. Extract sampled organic compounds separately from front section & backup section. Desorption need to be carried out using desorption bulbs. Take 10 ml of AR Grade CS 2 or suitable elutant as prescribed in standard analysis method in desorption bulb. Remove the glass tape from sampling tube and attach with desorption bulb with the help of provided male-female glass connector. Extraction is carried out by ultrasonication or it can be carried by shaking. The desorption bulb containing 10 ml elutant CS 2 & exposed activated charcoal is sonicated for 15 min. Repeat the same procedure for the backup section using different desorption bulb. Filter extracted solvent if particles are visible after desorption use 25-mm diameter glass fibre filter & provided funnel and
Use only transparent elutant for injecting
19 6. A
Analysis The transparent elutant obtained after extraction was used for gas chromatograph analysis. A Gas Liquid Chromatograph equipped with Flame Ionization Detector (FID) and Nitrogen as a carrier gas was kept ready for analysis of VOCs. The optimized operating conditions were set up as
. 3% Dexsil packed column was
used for determination of Benzene, Toluene and Xylene (BTX). Total time required for complete separation of BTX was about 10 minutes. Retention time in minutes was measured from the start up of solvent peak. After every sample a solvent was injected to flush out the column. The choice of column will largely depend upon compounds and interfering compounds present.
1 pi of Mix standard (BTX = 100 ng/pl) was injected into the Gas Chromatograph. Replicates of fixed volume of Mix standard was introduced to get a chromatograph for evaluating precision.
Mix standard was diluted with carbon disulphide (CS2) in 1:1 proportion and injected under the same GC condition in triplicate to get reproducibility.
In a similar way sample and spiked sample was also injected to get a repeatable peak pattern.
Results and discussion: The concentrations of analyte are calculated by measuring peak areas of respective compounds. Identification of compounds in a sample are done from the retention time of the standard. Area of Peak is calculated by height times width at half height.
19 6. A
Area = h W
/2 x h x W (mm2)
Height of a peak = Width of peak at half height
In case of very narrow or sharp peaks only height was considered and taken as area of a peak. The concentration of the compounds in desorbed sample in (pg/m 3 ) is calculated by following formula :
Cone, in (ng/m3)
Cone, of = standard x (in ng)
Peak height of sample (mm) x T e a k height of standard (mm)
Volume of sample (pi) Volume of sample injected (pi)
1 x Volume of air sampled (m3)
The statistical evaluations of results are shown in Table 1. The differences observed in Average Mean, Standard Deviation and % Error in replicate samples show that these errors may be caused by evaporation losses of standard and sample, varying G.C. conditions and mannual error during injection. Further studies are going on at the time of preparing this writeup.
19 6. A
Table 1 GC response for standard solution (100 ng/pl) Compounds
Peak ht. in mm
(+/-) % Error
GC response for Standard + Solvent (1.1) Compounds
Peak ht. in mm
(+/-) % Error
(+/-) % Error
GC response for Sample (2|jl) Compounds
Peak ht. in mm
19 6. A
GC response for Standard + Sample (1.1)
Peak ht. in mm
(+/-) % Error
GC response for Standard + Sample (pg/m3)
Peak ht. in mm
(+/-) % Error
19 6. A
Precautions : 1) During the sampling if the pressure drop increases more than 25 mm Hg, check the activated charcoal tube packing, loosen the glass beads and glass tube plug. 2) The desorption solvent carbon disulphide CS 2 is toxic and highly flammable. Avoid any exposure by inhalation or skin contact. Use only in a well ventilated fume cupboard. A carbon dioxide fire extinguisher should be available at all the times. Dispose of small waste quantities of CS2 in accordance with local regulations & accepted lab particles. 3) After the extraction of the sample, the concentrated tubes (sample tubes) should be sealed properly with Aluminium foil to keep the sample volume constant. 4) Extraction and analysis of the sample should be done on the same day. 5) If the samples are not to be analysed within 8 hrs, store tem in a sealed metal or glass container placed either in dry ice or in a freezer maintained at -20°C, in order to minimize migration of analyte from one section of the tube to another.
19 6. A
References : Skinner, J.H. and Bassin, N.J.; The Environmental Protection Agency's Hazardous Waste Research and Development Program, J APCA, 38(4), 377387 (1988) Allen, S.J., Balasundaram, V., and Chowdhury, M.,
characterisation of specialty activated carbon from lignite", pp. 35-42, in "The fundamentals of adsorption - Proceedings of the Vth International Conference on Fundamentals of Adsorption, Editor: M. Douglas LeVan, Kluwer Academic Publishers (1996). Merchant, A.A., and Petrich, M.A., A I Ch E J., 39(8), 1370-1376 (1993) Faust, S.D. and Aly, O.M.; in "Adsorption processes for water treatment" pp. 168-170, Butterworth Publishers (1987). Hashimoto Kenji, Miura, Yoshikawa Fumiaki ,and Imai Ichiro; l&EC Process Design & Development ;18 , 72 (1979). Naidu P.S., Ramteke D.S., Rao, K.S.M. and Kumaran P, J.; Research and Industry , 35 ,.237-241 (1990) Toshiro Otowa, Yutaka, Nojima and M. Itoh in "Activation mechanism, surface properties and adsorption characterisitcs of KOH activated high surface area carbon" pp. 709-716, in "The fundamentals of adsorption - Proceedings of the Vth International Conference on Fundamentals of Adsorption, Editor : M. Douglas LeVan, Kluwer Academic Publishers (1996). Lane Jorge , Carbon, 30(4), 601-604(1992). Juntgen H., Klein Jurgen, Knoblauch K., Schroten Hans-Jurgen, and Schulze Joachim in Chemistry of Utilization, llnd Supplementary Volume, Chapter -30, Editor. Elliott Martin A.; 2144-2145 (1981).
19 6. A
19 6. A
Choudhury, S.B., Banerjee, D.K., Dutta A.C., Mazumdar S., Ray A.K. and Prasad M.; Fuel Science and Technology, 4, 129-133 (1990). Srivastava S.K. and
Tyagi, Renu ; Fresenius Environ. Bull., 3(1), 12-17
(1994) Walker, P.L., Jr., "Carbon An Old But New Material", Am. Scientist, 50, 250 (1962) Heckman, F.A.; Rubber Chem. Technol., 37,1245 (1964) Wolff, W.F.; J. Phys. Chem., 63, 653 (1959) Garten, V.A. and Weiss, D.E., Rev. Pure Appl. Chem., 7, p 69, (1957) Yenkie, M.K.N, and Natarajan, G.S.; Sep. Sci. & Technol. 28(5), 1177-1190 (1993) Smisek, M. and. Cerny, S. ; Active Carbon ( Elsevier Publishing Company, Amsterdam 1970) Mattson, J.S., Mark, H.B. Jr., Malbin, M.D., Weber, W.J. Jr. and Crittenden, J.C.,
J. Colloid Interface Sci., 31, 116 (1969)
Coughlin, R.W., Ezra, F.S. and Tan, R.N. ; J Colloid Interface Sci., 28, 386 (1968). Commuter exposures to VOCs under different driving conditions. Atmo. Env. Vol.33, Nov 1998, pg. No. (409-417) Organic emissions profile for a light duty diesel vehicle. Atmo. Env. Vol.33 (Nov 1998) No.5, pg. No. 797 Fast & quantitative measurement of benzene, toluene &
automotive exhaust during transient engine operation with & without catalytic exhaust gas treatment. Atmo. Env. Vol.33 No.2 (Nov 1998) pg. No.205
Pollutant formation & control Edited by Eran Sher. (Academic Press) Characterisation & control of odours & VOC in the process industries Edited by S. Vigneron, J.Hermia, J.Chaauki Historical Accuracy measurement of VOCs in ambient air using compendium method T014/15 : Reference samples, canister surrogates & Green house gases. By Michael G.Winslow, D.F. Roberts. From : Book "Measurement of Toxic & Related Air Pollutants", Sponsored by Air & Waste Management Association
19 6. A
ORGANIC VAPOUR SAMPLER Code
© © © © ©
© © ©
Handle On/off Switch Time Totalizer Flow Measurment Nozze! Outlet Nozzel for collecting sample Rotameter -
Detai Is Teflon Nozzel for HC Supply Tef Ion Nozzel for CO Supply Mercury Manometer Nozzel for pressure measuring Stand
OutleM, for suction to pump
Outlet-2,for pressure drop-to 'U' Manometer
Inlet for air attach filter adopter
FIG. 1: ACTIVATED CHARCOAL TUBE
Details Outlet-1, for sucktionto pump Outlet-2, for pressure drop-to 'U' Manomete Spring Glass Tape - Plug
Details Activated Charcoal Glass Beads Inlet for air attach filter adopter
ACTIVATED CHARCOAL TUBE
FIG. 2 :CHRQMATQGRAM OF VQCs VQCs :1. BENZENE 2. T O L U E N E 3. p - X Y L E N E A. tn-XYLENE 5. o - X Y L E N E
FlG,2(g):CHR0MAT0GRAM FOR MIX STANDARD FlG.2 (b): C HROMATOGR AM FOR MIX STANDARD-*- SOLVENT(1-1)
F1G.2: (C):CHR0MAT0GRAM FOR AIR SAMPLE FIG.2 ( d) '• C H ROM ATOGRA M FOR STANDARD* SAM PL E( 1:1) 19 6. A