Indi an Journal "Of Chemical Technology Vol. 9, July 2002, pp. 290-296

Articles

Influence of carbon-oxygen surface groups on the adsorption of phenol by activated carbons

R C Bansal*', Diksha Aggarwal, Meenakshi Goyal & B C Kaistha

Department of Chemical Engineering and Technology, Punjab University, Chandigarh 160016, India

Received 18 Jalluary 2001; revised received 2 May 2002; accepted 9 May 2002

Adsorption isotherms of phenol from its aqueous solutions on four samples of activated carbons having different surface areas and associated with varying amounts of carbon-oxygen surface groups have been determined in the concentration range 20-1000 mgIL. The adsorption of phenol does not depend upon surface area alone but is also influenced by the presence of carbon-oxygen surface groups. The amount of these surface groups was enhanced by oxidation with nitric acid, ammonium persulphate and hydrogen peroxide and was decreased by degassing the activated carbons at gradually increasing temperatures of 400, 650° and 950°C. The oxidation of the carbons decreases the adsorption of phenol, the extent of decrease depending upon the nature of the oxidative treatment. The adsorption increases on degassing of the carbon samples, the increase depending upon the nature of the carbon­oxygen surface groups being eliminated at that temperature on degassing. The results indicate that while the presence of acidic carbon-oxygen surface groups which are evolved as COl suppresses the adsorption of phenol, the presence of non-acidic surface groups which are evolved as CO tends to enhance the adsorption of phenol.

Phenols and their derivatives are invariably present in the effluents from industries engaged in the manufacture of a variety of chemicals such as plastics, dyes and in plants used for thermal processing of coal. Many of these phenols are carcinogenic even when present in low concentrations. The presence of phenols in water also produces foul smelling chlorophenols during chlorination treatment of water for domestic supply. Thus, investigations relating to the removal of phenols from water have engaged the attention of a large number of investigators. Activated carbons, because of their large surface area and a high degree of surface reactivity, are potential adsorbents for the removal of organics in general and have been found to have a high efficiency for the removal of phenols from waste waters.

laroniec and co-workers, ,2, Enrique et at. 3, Worch

and Zakke4 and Magne and WalkerS studied the adsorption of several phenols from aqueous solutions and found that the adsorption was partly physical and pal11y chemical in character. Aytekin6

, Chaplin7 and Kiselev and Krasilinkov8 observed that the adsorption isotherms of phenol from aqueous solution were step wise suggesting the possibility of rearrangement of

' For Correspondence: (E-mail : ballsalpu@yailoo.com; Fax: 91 ( 172) 779 173; Present address: Department of Chemistry & Biochemi stry, H.P. Agriculture University, Palampur, H.P. )

phenol molecules in the adsorbed phase and their interaction with active sites on the carbon surface. Morris and Weber9

, however, observed that the adsorption isotherms of phenols on active carbons show two plateaus, even before the completion of the monolayer. The upper plateau was observed at three times the concentration for the lower plateau.

Singer and Yen lO, while studying the adsorption of

phenol and alkyl substituted phenols on carbons, observed that the alkyl substituted phenols were more strongly adsorbed than phenol itself and that the adsorption increased with the length of the chain as well as when the number of substituents on the phenol molecule was increased. This was attributed to the fact that the substitution of the alkyl group on the phenol molecule decreased its solubility by rendering it less polar". These workers also found that the position of the alkyl group did not effect the extent of adsorption.

Abuzaid and Nakhla'2 and Vidic et al. ' 3 studied the adsorption of phenol on activated carbons from aqueous solutions and observed that the presence of molecular oxygen in the test environment causes up to a three-fold increase in the adsorptive capacity of the carbon. This has been attributed to the oxygen induced polymerization reactions on the surface of the carbon. luang et al.'4 studied liquid phase adsorption of eight phenolic compounds on a PAN based activated carbon fibre in the concentration range 40-500 mgIL and

Bansal el al.: Innuence of barbon-oxygen surface groups on adsorption of phenol by activated carbons Articles

observed that chlorinated phenols showed better adsorption than methyl substituted phenols.

Moreno-Castilla et at. 15 studied the adsorption of several phenols from aqueous solutions on activated carbons prepared from original and demineralised bituminous coal and found that the adsorption capacity depended upon the surface area and the porosity of the carbon, the solubility of the organic compound and hydrophobicity of the substituent. The adsorption was attributed to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol.

The adsorption of phenol on activated carbon has also been found to be influenced by the presence of carbon-oxygen surface structures on carbons. Radeke et at. 16 and Hanmin and Yigun l 7 observed that the adsorption of phenol increased on degassing of the carbon which reduced the surface acidity without any appreciable change in the physical properties of the carbon. The phenol uptake by porous carbons decreased sharply on surface oxidation and it increased when the chemisorbed oxygen was removed on heat treatment in nitrogen I8

, 19. The phenol adsorption also increased considerably on activation of the carbon with carbon dioxide or with water vapours. Graham20 and Coughlin et at. 21 studied the adsorption at lower concentrations of phenol and reported negative influence of surface oxide groups on the adsorption of phenol. However, Clauss et at. 22 while working at relatively higher concentrations did not observe any such effect. The negative effect of the surface oxides was attributed to the depletion of 1t electron bond of graphite like layers as a result of lowering of Van der Walls forces of attraction.

Above perusal of the literature shows that although the presence of oxygen on the carbon surface has been found to influence the adsorption of phenol, it has not been elucidated as to what type of carbon-oxygen surface groups are involved in the adsorption of phenol by activated carbons. The present work was, therefore, undertaken.

In the present studies the adsorption of phenol from aqueous solution on activated carbons having different surface areas and associated with varying amounts of different types of carbon-oxygen surface groups have been used. The amounts of these surface groups have been enhanced by oxidation with mtnc ac id, ammonium persulphate and hydrogen peroxide. The activated carbons have also been degassed at 400°, 650° and 950°C to eli minate different amounts of these different types of surface groups.

Experimental Procedure

Materwls Two samples of granulated activated carbons

GAC-E and GAC-S and two samples of activated carbon fibers ACF-307 and ACF-31O also referred to as fibrous activated carbons, have been used in these investigations. The granulated activated carbons are peat based materials activated with steam while the fibrous activated carbons are pitch based material s.

The chemicals used were of chemically pure grade and distilled water was used as a solvent.

Oxidation with nitric acid 5.0 g of the activated carbon sample was heated

with 150 mL of pure nitric acid in borosil beaker of 250 mL capacity in a water-bath maintained at about SO°e. When all but about 10 mL of the acid had evaporated, the contents were cooled, diluted with water and transferred over a filter paper. The carbon sample was washed exhaustively with hot distilled water until the filtrate was free of nitrate ions. This oxidation and washing resulted in the loss of about 10% carbon. A small amount of the oxidised carbon also passed through the filter paper. The washed carbon sample was dried first in air and then in an electric oven at 120°C and stored in stoppered glass bottles flushed with nitrogen.

Oxidation with ammonium persulphate 5.0 g of each activated carbon sample was mixed

with 500 mL of saturated solution of ammonium persulphate in stoppered reagent bottles. The contents were kept for 24 h, shaken occasionally and then filtered and washed with distilled water until free of sulphate ions. The sample was dried in an oven at 120°C and stored in stoppered bottles flushed with nitrogen gas.

Oxidation with hydrogen peroxide 5.0 g of each carbon sample was mixed with 500

mL of 3 N hydrogen peroxide solution and the suspension was shaken for 24 h in a mechanical shaker. The contents were filtered, dried in an electric oven at 120°C and stored in stoppered bottles flushed with nitrogen gas.

Degassing of the carbons About 5 g of each of the activated carbon sample

was spread in a thin layer about 5 in. long in a tube furnace. It was kept in position by means of porous copper gauge plugs. The tube furnace was connected to a Hyvac Cenco vacuum pump capable of giving a

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3 . vacu um to the order of 3xlO- mmlHg The temperature of the furnace was raised to the required level slowly. The gases began to evolve soon after. The temperature was a llowed to rise gradually and before it was raised by another 50°C, complete e limination of the gases, at the preceding temperature was ensured. After degassing at the required temperature, the sample was allowed to cool to room temperature in vac uum to avoid reformation of the carbon-oxygen surface groups and was then transferred to stoppered bottles flushed with nitrogen . These sampl es are referred to as "degassed samples" in the text.

Estimation of carbon-oxygen surface groups The amounts of surface oxygen groups were

determined by evacuating each carbon sample at gradually increasing temperatures up to 950°C and analysing the gases evolved. The details of the procedure have already been published24

,25 . 1 g of each carbon sample was taken in a platinum boat (4"x 1l2") which was covered with a platinum cap with small holes to let the evolved gases to escape. The boat was then placed in a quartz tube which was fUl1her placed in a controlled temperature tube furnace . One end of the quartz tube was closed with a ground glass stopper and the other end was connected to a Cenco vacuum pump through a series of weighed J..l-tubes containing anhydrous calcium chloride to adsorb water vapours and a series of Erlenmeyer fl asks containing a known volume of standard barium

0> -0>

0---0 ACF-307 • • ACF-310

250 ~ GAC-S x----x GAC- E

E 200

'0 QJ

.D L-

a Ul '0 <!

C :::J a E <!

x

200 400 600 BOO

Concentration (mg / U 1000

Fig. I - Adsorption isotherms of phenol on different as-received activated carbons.

292

Indian J. Chem. Techno!. , Jul y 200

hydroxide solution to absorb CO2. The evolved CO was collected over water in a measuring cylinder and the quantity measured using Orsat Lunge gas analysi s apparatus3 1

.32

.

Adsorption of phenol 0.2 g of each carbon sample, oven dried at 120°C

for 3 h was placed in contact with 20 mL solution of phenol of a given concentration in a borosil glass tube. The contents were placed in a thermostat at 25°C and shaken occasionally. After 24 h the concentration of phenol was determined spectrophotometrically at a wavelength of 270 nm. A blank wa run for each concentration.

Results and Discussion Adsorption isotherms of phenol from its aqueous

solutions on the four as-received samples of activated carbons in the concentration range 20-1000 mg/L are shown in Fig. 1. The adsorption isotherms are type I of the BET classification showing an increase in adsorption with increasing concentration of the adsorbate and tending to level off at higher concentrations. The linear Langmuir plots are presented in Fig. 2 and the Langmuir constants are recorded in Table 1.

All the carbons adsorb appreciable amounts of phenol although the amount adsorbed is different in

0----<l ACF-307 ____ ACF-310

6·0 0---0 GAC-S ~ ~ GAC-E ....... 01

-g 5·0 .D L o Ul '0

o 4.0 1: ::J

E ~ 3·0 x o E "- x .2 2·0 +­o L ...­C QJ V C o

U

100 200 300 400

Concentration (mg / L )

500

Fig. 2 - Langmuir adsorption isotherms of phenol on di fferent as­received activated carbons.

Bansal et at.: Influence of barbon-oxygen surface groups on adsorption of phenol by acti vated carbons Articles

different carbons. Activated carbon fibers ACF-307 and ACF-31O adsorb considerably larger amounts of phenol compared to granulated activated carbons GAC-S and GAC-E. The maximum amount adsorbed at the highest concentration in the case of fibrous activated carbons is between 20-22% while in the case of granulated carbons it varies between 9-12% . This difference in the uptake of phenol cannot be explained on the basis of surface area alone as ACF-307 which has a smaller surface area (910 m2/g) than GAC-E (1190 m2/g) adsorbs larger amounts of phenol. As these carbons have been prepared using different raw materials and using different preparation procedures, these carbons are expected to have different chemical structures of their surface. This indicates that, as shown by several earlier workersI8.19.23, the surface chemical structure of these carbons also influences the adsorption of phenol.

The chemical structure of a carbon surface is due to the presence of associated oxygen which is present in the form of two types of carbon-oxygen surface groups: one which are evolved as CO2 on degassing in the temperature range 350-750°C and the second type24

.25 which are evolved as CO in the temperature

range 500-900°C. The amount of these two types of surface groups was determined by degassing the carbons at 950°C raising the temperature gradually and measuring the amounts of CO2 and CO evolved using standard evacuation procedures24-27. The results of such evacuation studies are presented in Table 2.

It is seen that while the granulated activated carbons are associated with larger amounts of oxygen groups evolved as CO2, the fibrous activated carbons contain larger amounts of oxygen groups evolved as CO. The former groups have been postulated as carboxylic or

lactone groups28-30 and render the carbon surface hydrophillic in character. Thus, these surface groups will tend to enhance the preference of the carbon surface for adsorption of water than that of phenol from aqueous solutions I8,19.31.32. The carbon-oxygen surface groups which are evolved as CO on degassing have been postulated as quinones which are non-acidic in nature and tend to make the carbon surface hydrophobic23.29.33.

In order to examine the influence of these two types of surface groups on the adsorption of phenol more clearly, ACF-307 and ACF-31O were oxidised with nitric acid, ammonium persulphate and hydrogen peroxide solutions. These oxidative treatments are known to enhance the amounts of these carbon­oxygen surface groups28.29.31.34. The adsorption isotherms of phenol on these oxidised samples are presented in Figs 3 & 4. The adsorption isotherms on the as-received carbon samples are also reproduced in these figures for the sake of easy comparison. It is interesting to note that the adsorption of phenol decreases on each oxidative treatment. Furthermore, the decrease in adsorption is much larger in case of the samples oxidised with nitric acid and much smaller when the oxidation is carried out with hydrogen peroxide. In case of the treatment with nitric acid, the maximum uptake of phenol at the highest concentration (1000 mglL), decreases from 20-22% to as low as 10% on both the samples, while the decrease is from 20-22% to 15-17% in case of the two samples oxidised with hydrogen peroxide. In other words the decrease in adsorption of phenol in case of the different oxidative treatments is in the order.

Nitric acid> ammonium persulphate > hydrogen peroxide

Table I - Langmuir adsorption isotherm constants of phenol on four different as-received activated carbons

Sample Xm (mglg) K(Ug)

ACF-307 239.2 0.12

ACF-310 263.1 0.17

GAC-S 142.8 0. 10

GAC-E 10.44 0.09

Table 2 - Surface areas and amounts of oxygen evolved on degassing different as-received activated carbons at 950°C

Sample Surface area (m2/g) Oxygen evolved (glIOOg) as

CO2 CO H2O Total

ACF-307 910 1.00 5.30 1.30 7.60

ACF-310 1184 1.90 4.20 1.40 7.50

GAC-S 1256 2. 10 1.05 1.24 4.39

GAC-E 1190 2. 13 1.66 1.33 5.12

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The amount of the two types of surface groups present on the oxidised samples, as determined by degassi ng at 950°C, are given in Table 3. It is interesting to note that, out of the oxidised samples, the carbon samples oxidised with nitric acid have the largest amount of acidic surface groups while the samples oxidised with hydrogen peroxide have the smallest amount of acidic surface groups. Furthermore, it is also interesting that the amounts of non-acidic surface groups (evolved as CO) in all these oxidised ACF-307 and oxidised ACF-31O samples are of about the same order of magnitude.

It is apparent from these results that the adsorption of phenol from aqueous solutions is suppressed by the presence of acidic surface oxygen groups. As mentioned earlier this decrease in adsorption can be

250

en -;; 200 E -0 Q.I

.D 150 L-a til -0

<l: 100

C ::J a E <l:

0---0 As-received • • Oxidised with HN03

&-----8 Oxidised with {NH4)25208 8------<:J Oxidised with H2 0 2

•

200 400 600 800

Concentration (mg/L) 1000

Fig. 3 - Adsorption isotherms of phenol on ACF-307 before and after oxidation.

Indi an 1. Chern. Techno!. , July 200

attributed to the fact that these surface groups render the carbon surface hydrophillic in character thereby enhancing the preference of the surface for water.

The amount of these two types of surface groups on the surface of ACF-307 and ACF-31O was decreased by degassing these carbons at 400°, 650° and 950°C. The adsorption isotherms of phenol on the degassed samples are shown in Fig. 5 for ACF-307 and in Fig. 6 for ACF-31O samples. It is seen that the adsorption of phenol increases on degassing at all temperatures and in both the carbons. However, the increase in adsorption is maximum in case of the samples degassed at 650°C. Table 4 gives the amounts of the two types of carbon-oxygen surface groups in case of the degassed samples of ACF-307 and ACF-310. It is evident that the 400°-degassed samples have

0----0 As-received a-o oxidised with H202

250 -- Oxidised with HN03 t::..----6 Oxidised with ( NH412 52°8

m -;;; 200 E

" Q)

.D 150 L-o til -0 o +' 100 c ::J o E <l: 50

o __ -o-<!ro

200 400 600 8to--l~6-:-:00:---' Concentration (mgfL.)

Fig. 4 - Adsorption isotherms of phenol on ACF-31 0 before and after oxidation.

Table 3 - Amount of oxygen evolved on degassing different oxidised samples at 950°C

294

Sample

As-received Oxidised with -HN03

(NH4hS20 g

H20 2

As-received Oxidi sed with­HN03

(NH4) 2S20 g

H20 2

CO2

1.00

12.90 5.40 2.55

1.90

11.96 4.70 2.1

Ox ygen evol ved (gil OOg) as

CO H2O Total

ACF-307

5 .30 1.30 7.60

7.47 2.40 22.77 7.51 4 .91 17.82 7.42 2. 10 12.07

ACF-3 10

4.20 1.40 7.50

7.20 2.20 21.36 6.30 2.10 13.1 0 6.20 2. 10 10.40

Bansal el at.: lnnuence of barbon-oxygen surface groups on adsorption of phenol by activated carbons Articles

lost only a small portion (about 18-20%) of the acidic surface groups and retained most of their non-acidic surface groups. The 650°-degassed samples, on the other hand, have lost most (- 80%) of the acidic surface groups. The 950°-degassed samples are almost free of all the oxygen surface groups.

Thus, a small increase in phenol adsorption in case of the 400°-degassed samples can be attributed to a small decrease in the amount of ac idic surface groups. In case of the 650°-degassed sample (Figs 5 & 6), the adsorption of phenol is the largest because most of the acidic surface groups have been removed and the only dominating surface groups are the non-acidic surface groups. The above results clearly show that it is the oxygen groups evolved as CO2 (acidic surface groups)

250

-;;200 -en E

~ 150 .D L..

0 U1 "0 « 100

C :J 0 E «

~ ACF-307 ®-----® DegaSSed at 400

0

~ Degassed at 6500

+-+ Degassed at 9500

Concentration (mg / L)

1000

Fig. 5 - Adsorption isotherms of phenol on ACF-307 before and after degassing.

which tend to suppress the adsorption of phenol. When these surface groups are largely removed and when non-acidic surface groups (evolved as CO on degassing) become the domjnating surface oxygen groups, as in the case of the 650°-degassed samples, the adsorption of phenol is the largest. Further degassing at 950°C results in a decrease in the adsorption of phenol because this treatment removes almost completely both the acidic and non-acidic surface groups.

The adsorption of phenol by carbons from aqueous solutions has been studied by a number of workers but the results have been interpreted differently. Mattson et al. 35 attribute adsorption of phenol on the carbon

300

~ 250 en '-en E "0 200 CII .D 1-0

~ 150 Cl +' c ::J

E 100 «

~ As-received ___ oegassed at 4000

'i}---'W Degassed at 650: ~ Degassed at 950

200 400 600 800

Concentration (mg/U 1000

Fig. 6 - Adsorption isotherms of phenol on ACF-3\ 0 before and after degassi ng.

Table 4-Amount of oxygen evolved on evacuating different degassed samples at 950°C

Sample

As-recei ved 400°-degassed 650°-degassed 950°-degassed

As-received 400°-degassed 6500 -degassed 950°-degassed

CO2

1.00 0.82 0.2 1 Tr

1.90 1.47 0.32 Tr

Oxygen evolved (g1100g) as

CO H2O Total

ACF-307

5.30 1.30 7.60 5.14 2. 11 8.07 4.23 Tr 4.44 0.07 Tr 0.07

ACF-310

4.20 1.40 7.50 4 .03 1.92 7.42 3.17 Tr 3.49 0.03 Tr 0.03

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Article

surface to charge transfer interactions between the adsorbate and the surface carbonyl groups or between the adsorbate and the fused ring system of the basal planes of the carbon . Carbonyl oxygen groups on the carbon surface act as electron donors and the aromatic ring of the solute is the acceptor. Graham2o

, Coughlin et al. 21 and Puri et al. 23 are of the view that the presence of carbon-oxygen surface groups also in fI uences the adsorption of phenol through the formation of a bond with the non-acidic oxygen surface groups. The results presented in these in vestiga tions show that the oxidation of carbon with nitric acid which preferably creates carboxylic or lactonic acidic groups suppresses the adsorption of phenol. However, when these surface groups are removed by degassing, the adsorption of phenol tends to increase. The adsorption of phenol is maximum when the carbon surface is almost completely free of the ac idic groups and when carbonyl groups are the dominating surface groups. These electrophilic quinone groups present on the carbon surface are e lectron donors and may be involved in a strong bonding with the 1t electrons of the benzene ring. Thus, it appears that the phenol is adsorbed by the quinone groups by interaction between 1t electrons of the benzene ring and the partial positive charge on the carbonyl carbon atom in agreement.

These studies clearly show that activated carbons which are devoid of any acidic surface groups and which are associated with quinonic surface groups are the best adsorbents for the removal of phenol from water.

Acknowledgement The authors are thankful to the Ashland Petroleum

Company, Kentucky, USA for the gift of activated carbon fibers and to Norit N.V. Netherlands for the supply of granulated activated carbons, to CSIR for fi nancial grant and to AICTE for the grant of Emeritus Fellowship to R.C. Bansal.

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