SEM study of phenolphthalein adsorption on granula

1093-0191/04/$-see front matter ᮊ2002Elsevier Science Ltd.All rights reserved.PII:S1093-0191Ž02.00126-0

SEM study of phenolphthalein adsorption on granular activated

carbon

Berrin Tansel*,Pradeep Nagarajan

Civil and Environmental Engineering Department,Florida International University,Miami,FL 33199,USA

Accepted 20October 2002

Abstract

Activated carbons are characterized by their high adsorption capacities.Although the highly active surface properties of the activated carbon are often attributed to the chemical functional groups,surface morphology plays a significant role in determining the surface availability.Depending on the preparation methods used,activated carbon contains micropores,mesopores,and macropores.The pore size defines the types of molecules that could be adsorbed on the carbon surface.The objectives of this study were to evaluate the effect of surface morphology on the adsorption of the contaminants on the carbon surface.The surface characteristics of the clean and saturated granular activated carbon particles were analyzed by scanning electron microscope (SEM ).The SEM images were digitized and analyzed by Image-Pro Plus software.Examination of the SEM micrographs of the clean activated carbon particles showed two types of surfaces:smooth surfaces with long linear ridges and rough surfaces with oval ridges and micropores.Based on the analysis of the images taken by SEM before and after the adsorption process,two distinct types of adsorption patterns were observed.On the smooth surfaces,the adsorption was observed to be in the form of a thin uniform layer.On the rough surfaces,linear accumulation patterns following the shapes of the oval patterns were observed.The linear accumulation on the rough surface showed a more distinct density difference,indicative of a thicker layer than those observed on the smooth surfaces.ᮊ2002Elsevier Science Ltd.All rights reserved.

Keywords:Activated carbon;Electron microscope;Image processing;Carbon surface;Microporous carbons

1.Introduction

Activated carbon is one of the most widely used materials to remove organic contaminants from water or air due to its high adsorption capacity.Industrial significance of the activated carbon include both gas phase adsorption (i.e.air purification,solvent recovery systems,and respirators )and liquid phase adsorption (i.e.drinking water treatment,groundwater treatment,and soft drink industry )applications (Neely and Isacoff,1982).Activated carbons are the carbonization products from wood,fruit shells,brown coals,lignites,and highly

*Corresponding author.Tel.:q 1-305-348-2928;fax:q 1-305-348-2808.

E-mail address:tansel@eng.fiu.edu (B.Tansel ).volatile bituminous coals (Rodriguez-Reinoso and Moli-na-Sabio,1998).The carbonization process involves heating the source material up to 6008C,usually in the absence of air,to drive out the volatile substances and leaving a charred residue.In order to increase the surface area,the carbon is further activated by steam,air,or carbon dioxide at elevated temperatures.The activation process often includes the use of metal salts such as calcium chloride,magnesium chloride,and zinc chlo-ride.The carbon then acquires an extensive internal surface area with high porosity.

The effectiveness of the activated carbon to remove specific contaminants depends on the presence of the functional groups on the carbon surface,which strongly influences the adsorption.In addition,it is possible that the defects due to missing units in the carbon-hexagon

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Fig.1.SEM micrographs of the smooth and rough areas of the surface from a granular activated carbon at 500=magnification:(a )smooth surface,and (b )rough surface.

networks are actually responsible for adsorption of certain contaminants (Cheremisinoff and Ellerbusch,1980).For example,a hole defect may retain metals or small molecules that may not otherwise be captured by the functional groups.The dislocation of groups with large number of carbons such as C60,C70and higher fullerenes creates significant numbers of gaps within the carbon matrix.Although the activated carbons have high porosities and large internal surface area,the characteristics of the exterior surface play a significant role in defining the effectiveness of the activated carbon particles in removing specific contaminants.The reac-tive groups on the exterior surface of the carbon particles are easily accessible.Also,the molecular size of the contaminants,contact time for adsorption as well as the tortuosity characteristics of the pores may limit the accessibility of the interior surfaces.

Pore size distribution has been used to describe the internal structures and adsorption capacities of activated carbons (Davies et al.,1999).The highly active surface properties of the activated carbon are attributed to the chemical functional groups and the internal surface areas,which typically range from 500to 3000m y g.2Depending on the preparation methods used,the pore sizes of the activated carbon could be categorized as follows (Bansal et al.,1988):●Micropores (width s -2nm )●Mesopores (width s 2–50nm )●Macropores (width s )50nm )

The difference between meso and macropores is based on the width of the openings,which represents the distance between the walls of a slit shaped pore and the radius of a cylindrical pore.

Image analysis techniques have been used to charac-terize the pore structures of activated carbons using transmission electron microscopy (Oshida et al.,1995;Endo et al.,1995);and scanning tunneling microscopy (Economy et al.,1995).The recent developments in image analysis techniques could provide more in-depth analysis and interpretation of the microscopic images of the carbon surfaces before and after the adsorption process.Objectives of this study were to characterize the surface structure of granular activated carbon by analysis of scanning electron microscope (SEM )images using image analysis techniques;and to identify the areas with high adsorption characteristics on the carbon surfaces.

2.Materials and methods

Short-term activated carbon adsorption experiments were conducted using PICA P104type granular activat-ed carbon that is provided by Pica USA,Inc.,Columbus,OH.The PICA P104is characterized by the manufac-turer as macro-microporous regenerable granular acti-vated carbon,designed to provide good adsorption of both small and large molecules.Phenolphthalein (C H COOC (C H OH ))with a molecular weight of 2318.33was used as the contaminant.The experiments

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Fig. 2.SEM micrograph of the rough surface at 5000=magnification.

Fig.3.Linear accumulation pattern of the contaminants on the edges of the rough surface.

Table 2

Adsorption on the external surface of activated carbon Surface characteristic Smooth surface (%)Rough surface (%)Edge area 4796Flat area

53

4

Table 1

Clean carbon external surface characteristics Surface characteristic Smooth surface

Rough surface

Edge area 10.7%

24.2%

Flat area

.3%(few macro pores )

75.8%(with macro pores )

were conducted by mixing 34ml of water with 0.17ml of phenolphthalein to make a solution of 0.005%.To this solution,1.05g of granular activated carbon was added and mixed for a period of 10min.After the mixing,the carbon granules were separated from the solution by filtration and dried for 5h at 1068C.After cooling,the carbon samples were visually examined by SEM.

Surface images of the granular activated carbon were captured before and after adsorption of phenolphthalein by SEM.The SEM used was Model JSM-35CF by JOEL,Tokyo,Japan.The characteristics of the SEM images were analyzed using Image-pro Plus (IPP )soft-ware by Media Cybernetics,Silver Spring,MD (Image-Pro Plus,1997).Enhancement techniques ranging from simple operations such as brightness and contrast adjust-ment to complex spatial and morphological filtering operations to improve and refine visual information were applied as necessary.The software was used for image enhancement using color or contrast filters including fast Fourier transforms;trace and count objects manually and automatically;measure object attributes such as area,perimeter,diameter,length,as

well as characteristics of the defined view area.Statis-tical analysis information of each image was automati-cally exported to spreadsheets.The change in pore characteristics of the activated carbon was analyzed in relation to the percent of adsorption capacity used during the development of adsorption isotherms.3.Results and discussion

Examination of the SEM micrographs of the clean activated carbon particles showed two types of surfaces:●smooth areas with long ridges

●rough areas with micropores and edges

Fig.1shows SEM micrographs of the smooth and rough areas of the surface from a granular activated carbon at 500=magnification.Smooth areas were characterized by a roughness structure resembling a series of parallel lines as shown in Fig.1a.A few macropores were clearly identifiable.The rough surface micrographs showed a distinct roughness with oval patterns as shown in Fig.1b.Within each oval section,

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Fig.4.Identification of areas with specific characteristics using image analysis.

presence of the macropores was clearly noticeable.Fig. 2shows the close up view of the macropores on the rough surface at5000=magnification.

The pore size and pore density of the smooth and rough surface as well as accumulation of contaminants on the carbon surface were analyzed by analysis of the SEM images using IPP software.Table1presents the percentage distribution of edge and flat areas on smooth and rough surfaces of the clean carbon particles.

After the carbon particles were allowed to adsorb the phenolic organic compound,the SEM images were taken and analyzed.Based on the analysis of the images taken by SEM before and after the adsorption process, two distinct types of adsorption patterns were observed. On the smooth surfaces,the adsorption was observed to be in the form of a thin uniform layer.On the rough surfaces,linear accumulation patterns on the exterior edges of the surfaces following the shapes of the oval patterns were observed.The linear accumulation on the rough surface showed a more distinct density difference, indicative of a thicker layer than those observed on the smooth surfaces.Fig.3presents the linear accumulation pattern of the contaminants on the edges of the rough surface.Fig.4presents the processed images for analysis of the visual characteristics of the carbon surface.Areas covered by the contaminants can be outlined or distin-guished in a different color,which could be used for area measurements or counting.Table2shows the percentage area of adsorption on the external surfaces of activated carbon.On the rough surface,the adsorption took place mainly on the edges.On the smooth surface, there was no significant difference in adsorption between edge and flat areas.

4.Conclusions

The image analysis is an effective tool to characterize the surface and adsorption properties of activated car-bon.Analysis of the images taken by SEM showed two different surface structures on the clean carbon particles:●smooth areas with long ridges

●rough areas with macropores and edges

The analysis of the images taken by SEM showed that macropores were not accessible by the large phe-nolic organic used in the study.Adsorption was more efficient on the edges of the carbon surface.The rough surfaces had a higher adsorption potential for the phe-nolic compound used.As a result,the rough surfaces, which showed a higher percentage of edge area,also had a higher adsorption potential.A significant fraction of the contaminant accumulation was observed on the rough surfaces as a linear pattern on the edges.The smooth surface showed a thin layer of adsorption.The linear accumulation on the rough surfaces was also significantly denser than the surface accumulation observed on the smooth surfaces.Analysis of the SEM images using image processing tools provides insight for understanding the changes in surface characteristics of the granular carbon before and after adsorption process.415 B.Tansel,P.Nagarajan/Advances in Environmental Research8(2004)411–415

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