Effects of surface properties of activated carbons on adsorption behavior of selected aromatics

Effects of surface properties of activated carbons on adsorption behavior of selected aromatics

Carbon Vol. 35, No. 9, pp. 1375-1385, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/97 $17.00 + 0...

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Carbon Vol. 35, No. 9, pp. 1375-1385, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/97 $17.00 + 0.00

Pergamon PII: SOOOS-6223(97)00091-2

EFFECTS OF SURFACE PROPERTIES OF ACTIVATED CARBONS ON ADSORPTION BEHAVIOR OF SELECTED AROMATICS Department

of Chemical

C.-C. LENG and N. G. PINTO* Engineering, 697 Rhodes Hall, University OH 45221-0171, U.S.A.

(Received 30 June 1996; accepted in revisedform

of Cincinnati,

Cincinnati,

31 Murch 1997)

Abstract-Studies were undertaken to determine the role of surface oxygen complexes and metals on activated carbon on adsorption of selected aromatics. Three kinds of activated carbons (F400, MP and Darco G60) were used in the study. The F400 carbon was treated by oxygenation, deoxygenation and HCl-washing processes. Batch adsorption tests were used to evaluate the effects of surface oxygen complexes and metals on adsorption of phenol under oxic and anoxic conditions. The isotherm results showed that the removal of hydrophilic structures (carboxylic acid groups) of activated carbon increased physisorption and surface polymerization of phenol. It was also found that the metal (Fe) itself can not catalyze the surface polymerization of phenol on the carbon surface at room temperature. The findings of this study suggest that the life of a regenerated carbon adsorption bed could be extended by removing the metal content in activated carbon, and mildly oxidizing the surface. 0 1997 Elsevier Science Ltd Key Words-A.

Activatea

carbon,

B. oxidation,

C. adsorption,

1. INTRODUCTION

(DO) in the adsor-

The presence

of dissolved

oxygen

bate

can

the adsorption

solution

increase

capacity

of

This enhancement in adsorption capacity has been attributed to the surface polymerization of phenol on activated carbon. Grant and King [ 71 have shown that phenolic compounds can undergo oxidative coupling reactions on the surface of activated carbon, and these reactions produce polymeric compounds. The role of activated carbon in oxidative coupling of phenolic compounds is not well understood, despite a number of studies focused on this subject. The mechanism of surface polymerization of phenol on activated carbon has been postulated by Nakhla et al. [2]. Phenolate radicals are formed either by the loss of one electron from phenolate anions or one proton and one electron from phenol. The free radicals then produce stable polymer products by coupling reactions. Magne and Walker [8] have shown that chemisorption of phenol takes place on activated carbon regardless of purity, and is inhibited by the presence of oxygen surface complexes. Based on this they concluded that carbon sites are responsible for phenol chemisorption. Grant and King [7] have shown that the presence of Mn causes an increase in irreversible adsorption, but they have noted that ash is not necessary for irreversible adsorption to occur. In view of the present uncertainty on the roles of surface oxygen groups and metal impurities in the

phenol

on activated

carbon

[l-6].

*Corresponding author. Fax: + 1 513 556 3473; e-mail: [email protected] 1375

D. phase equilibria.

mechanism of oxidative coupling of phenol, a study has been undertaken to determine the dependence of the polymerization behavior of phenol on various surface properties of carbon. Moreover, F400 carbons were used as test adsorbents to investigate the validity of physisorption mechanisms proposed in a previous study [9]. Thus, the primary objective of this study was to establish the effects of metal impurity and surface oxygen groups on both physisorption and chemisorption of phenol on activated carbon.

2. EXPERIMENTAL

2.1 Preparation adsorben ts

PROCEDURES

and characterization

of

Three kinds of carbon, Kureha MP spherical activated carbon (SAC), a gift from the Kureha Chemical Company (New York, U.S.A.), F400 GAC supplied by the Calgon Carbon Corporation (Pittsburgh, U.S.A.), and Darco G-60 powder activated carbon (PAC) supplied by the Sigma Chemical Company (Saint Louis, U.S.A.) were used for all the experiments. Pretreatment of the carbon involved boiling in deionized water for 1 hour, followed by drying at 110°C for 24 hours. To study the influence of surface oxygen complexes on surface polymerization, the F400 carbon was oxygenated with air in a tubular furnace at 350°C. A 3 g sample of F400 carbon was placed in a quartz container, and the container was placed in the furnace. A constant flow of air was maintained through the tube. After 60 minutes at 35O”C, the sample was cooled to room temperature in air. The oxygenated F400 carbon will be referred to as F-Air.

C.-C LENG and N. G. PINTO

1376

Deoxygenation of the surface was achieved through a similar process, but in a stream of nitrogen at 850°C. After 60 minutes at this temperature, the sample was cooled to room temperature in a nitrogen atmosphere and stored at room temperature under nitrogen. This carbon will be called F-N,. Acid-washed carbons (F-HCI) were obtained by taking a 3 g sample of the carbon and placing it in 50 ml of concentrated hydrochloric acid for 12 hours. This procedure is known to remove metallic compounds from the surface [lo]. The acid-washed carbon was then thoroughly boiled in deionized water for 1 hour, and dried at 11O’C for 24 hours. The impregnation of iron into MP-DI carbon consisted of three steps. The first step was impregnation of the MP-DI carbon with ferric nitrate [ Fe(N0&.9H,O] in a 4:l mixture of benzene to ethanol. The iron concentration was adjusted so as to be 5% wt of carbon. A 5 g sample of carbon was put into 250 ml of solution, and shaken while nitrogen bubbled through the mixture at a flow rate of 500 cmm3 min- ‘. After 6 hours, the solution evaporated to dryness. The second step was drying the impregnated samples under vacuum at 113°C for 24 hours. The third step was a reduction of the sample in a flow of hydrogen ( 100 ml min-‘) at 500°C for 5 hours. This carbon is called MP-Fe. 2.2 FT-IR experiments Transmission FTIR spectra of F-N,, F-Air and F-D1 (unmodified carbon), were obtained on a BIORAD FTS-40 spectrophotometer. About 0.5% of carbon was used in each 0.18 g KBr pellet. The pellet was analyzed immediately after it was made.

2.3 Ash content measurements The ash content of the carbons was determined by burning approximately 5 g of carbon in a preweighed ceramic boat placed in a furnace at 850°C for 24 hours. The amount of ash observed in each case is reported in Table 1.

2.4 Determination

of surface areas

Surface area measurements for the variously treated carbons were made on a Micromeritics Gemini 2360 (Norcross, U.S.A.) BET apparatus. N, (77 K) was used as the adsorbate. The measured surface areas are reported in Table 1.

2.5 Proton induced X-ray emission (PIXE) The constituents of MP-DI and MP-Fe were measured using an Ion X Tandetron Model 41lOA PIXE. These experiments were performed by the Element Analysis Corporation (Lexington, Kentucky). 2.6 Adsorption isotherm measurements The equilibrium adsorption isotherms for phenol, o-cresol and benzoic acid from an aqueous solution on activated carbon were determined at 23 k 1°C using a bottle-point method. Different amounts of carbon were weighed and added into 15 bottles, and 100 ml of organic solution (250 mg L-‘) was added to each of the bottles (150ml). All solutions were prepared with deionized water, and were buffered with 0.05M Na2HP04/H3P04 at pH 7.0. The solution in the anoxic measurements was purged with nitrogen until oxygen was completely removed from the bottles, and quickly covered with parafilm and caps. Oxygen was purged into solution for oxic measurements, until dissolved oxygen saturated the solution. Each set of bottles included two bottles containing blank solutions to check for sorbate volatilization and adsorption of sorbate on the walls. The bottles were covered with parafilm and caps, and placed on a shaker for a period of 14 days. Subsequently, the concentration of organic solute in the liquid was determined using a Shimadzu UV 1604 UV-visible spectrophotometer. The UV wavelengths used for phenol, o-cresol and benzoic acid were 270, 270 and 250 nm, respectively.

2.7 Regeneration Methanol, 1% NaOH-methanol and formic acid have been reported to be effective regenerants for physisorbed phenol in a previous study [ 111, and were selected to regenerate carbons loaded with phenol under oxic and anoxic conditions. The regeneration experiments were carried out in three steps: ( 1) adsorption of phenol on F-D1 carbon under oxic and anoxic conditions; (2) regeneration with one of the selected regenerants; and (3) adsorption of phenol under anoxic conditions. The regeneration efficiency (RE) was calculated as a percentage by comparing the amount of phenol adsorbed in the first step to the amount of phenol adsorbed in the third step. The adsorption of the organic compounds from

Table 1, BET surface area and ash content of treated carbons Name F-D1 F-HCl F-N2 F-Air MP-DI Darco G-60

Treatments

BET surface area (rn’g-‘)

Ash (Wt%)

DI water washing HCl washing N,. 85O’C. 60 minutes Air, 35O’C, 30 minutes DI water washing HCI washing

940* 10 953+ IO 814k 10 965 k 10 1031*10 560+ 10

5.63 5.00 5.61 5.51 0 1.60

Effects of surface properties of activated carbons an aqueous solutions on F-D1 was conducted at 23 + 1°C using the procedure described earlier (adsorption isotherm measurements). Approximately 0.3 g of carbon were weighed and added into bottles, and 40 ml of phenol solution, either oxic or anoxic, was added to each of the bottles (50 ml). Initial concentrations of phenol were 350 and 300 mg L-r for the oxic and anoxic solutions, respectively. The bottle were placed on a shaker for a period of 7 days. The solution was then decanted, and the loaded carbons were transferred to a new bottle. The carbon was washed with about 200 ml deionized water prior to the addition of 40 ml of the regenerant of interest. The regenerant solution was mixed with the carbon in a shaker for 24 hours, after which it was decanted. Following the regeneration step, the carbons were transferred to a new bottle, and the adsorption step repeated. Anoxic solutions (300 mg L - ‘) were added into each bottle. The bottles were placed on a shaker for a period of 7 days. After 7 days, the liquid concentration was measured using the procedure described earlier (adsorption isotherm measurements).

1377

T 4000

I

/

3500

3000

1

2500 Wavenumber

2000

/

,

I

1500

1000

500

(em-l)

Fig. 1. FTIR spectra of F400 carbons.

3.2 EfSect of surface oxygen and metals on physisorption and chemisorption 2.8 GC-MS experiments Activated carbon samples were extracted in a Soxhlet extraction apparatus following the procedure of Vidic and Suidan [4]. The samples were made using the procedure described earlier (adsorption isotherm measurements). Once phenol was adsorbed on carbons (F-DI, MP-DI and MP-Fe), the carbons were first extracted with methanol for 1 day and then with methylene chloride for 3 days. The extract was then analyzed using a Hewlett-Packard 5970 mass spectrophotometer equipped with a Hewlett-Packard 589OA gas chromatograph. The column was a 30 m x0.32 mm ID fused capillary column (DBl) with a 0.25 pm film thickness.

3. RESULTS AND DISCUSSION

3.1 Surface oxygen characterization carbons

of the

Shown in Fig. 1 are the FTIR spectra of samples F-DI, F-N, and F-Air. Comparing the spectra of F-Air with F-DI, it is seen that adsorption bands at 1717 cm-’ and about 3400cm-’ increase with surface oxidation. The bands at 1717cm-’ and about 3400 cm-’ can be assigned to carboxylic acid groups [ 121. In contrast, the deoxidization treatment of the F400 carbon at 850°C results in a significant decrease in the surface oxygen complexes. Therefore, it can be concluded that the treatments conducted in this work give three distinctly different carbon surfaces with respect to their oxygen content. Furthermore, it appears that the change in surface oxygen content is primarily through the relative quantity of surface carboxylic groups.

Shown in Fig. 2 are the measured anoxic and oxic isotherms of phenol on F-DI, F-N,, F-Air and F-HCl carbons. The best-fit Freundlich isotherms for each case are also shown on these figures. Table 2 lists the corresponding best-fit Freundlich coefficients (q,=KCtI”). The 95% confidence intervals for the coefficients, and the coefficients of correlation are also shown in this table. From the R2 values it can be concluded that the Freundlich isotherm equation fits the experimental data very well. As can be seen from Fig. 2, under both anoxic and oxic conditions a distinct relationship is observed between the amount adsorbed and the surface oxygen content, As the surface oxygen increases, the amount adsorbed decreases. It should be noted that this trend is observed despite the N,-surface area increasing with increasing surface oxygen content (Table 1). Under anoxic conditions, it has been reported [5] that surface polymerization of phenol is absent or minor. Thus, it is appropriate to use these data to examine the general applicability of a model presented earlier for the physisorption of phenol on a different type of activated carbon (Kureha MP carbon) 191. In this model it was proposed that basal plane carbons are primarily responsible for phenol adsorption. However, surface oxygen groups were found to play an important role. In particular, it was found that an increased amount of carboxylic groups on the surface leads to increased water cluster formation [ 131. Also, this causes increased removal of x electrons from the basal planes [ 14,151, which results in weaker dispersion interactions with phenol. The combined influence of these two effects was proposed to underlie the reduction in the adsorption of phenol with increasing carboxylic oxygen. The trend

C.-C LENG

1378

and N. G. PINTO

0

10

Liquid Concentration

q

(mg/L)

A V

-

F-D1 F-HCI F-N2 F-Air Best Fits

10

Liquid Concentration

(mgiL)

Fig. 2. Phenol adsorption isotherms on various carbons observed with the anoxic data in Fig. 2 in combination with the FTIR spectra in Fig. 1 suggest that physisorption of phenol on F400 is consistent with this explanation. Fig. 2 also shows adsorption data for F-HCl. On comparison with F-D1 data it can be seen that acid treatment increases the adsorption capacity, though the N, surface area remains essentially the same. This observation can be explained on the basis of

the metal content of the two carbons. It has been suggested that metals on the surface act as sites for water adsorption [lo]. Acid washing reduces the ash content of F400 (Table l), and the correspondingly lower metal content will result in decreased water adsorption, and, hence, increased phenol capacity. As has been stated earlier, oxygen in solution has been observed to result in the formation of higher molecular weight products during the adsorption of

Table 2. Freundlich constants for phenol isotherms Carbon

Dissolved oxygen

F-D1 F-HCI F-N, F-Air

Anoxic Anoxic Anoxic Anoxic Oxic Oxic Oxic Oxic

F-D1 F-HCl F-N2 F-Air

K(mgg-‘) (Lmg-‘)“” (95% confidence interval) 49.3 56.2 55.6 35.0 75.2 75.5 96.0 54.0

(46.3-52.50 (49.0-64.4) (49.6-62.2) (30.1&40.7) (67.6-83.5) (69.8-81.6) (86.7-106.2) (56.7-63.9)

l/n (95% confidence

0.270 0.244 0.266 0.311 0.206 0.216 0.198 0.225

interval)

(0.253-0.288) (0.205-0.284) (0.234-0.298) (0.267-0.356) (0.177TO.237) (0.192ZO.226) (0.168-0.228) (0.183-0.266)

RZ

0.994 0.969 0.982 0.982 0.979 0.988 0.967 0.950

Etfects of surface properties

of activated

carbons

1379

0.5

c

g 0.4 E .s Z E 5 0.3 $ t rZ .g 0.2 t .I! f E i 0.1

0.4)

-o-f~-&-

F-D1 F-HCI F-N2

+-

F-Air

-0

20

40

60

80

100

120

Liquid Concentation (mg/L) Fig. 3. Effect of surface

properties

on the polymerization

factor

of phenol

Darco G-60 (Anoxic) Darco G-60 (Oric) MP-DI (Anoxic) MP-DI (Oh) Best Fits

100 Liquid Concentration (mg/L) Fig. 4. Phenol

adsorption

isotherms

on MP-DI

the polymerization

on carbon [l-6]. However, the role of surface groups in this polymerization is unclear. A comparison of the anoxic and oxic data in Fig. 2 gives valuable insight on the influence of surface oxygen and metals on the surface polymerization of phenol. A convenient way of analyzing the data is to define

phenol

Table 3. Metal contents Fe

C Carbon MP-DI MP-Fe

Al

Si

G-60 carbons.

factor (PF): PF=

(qOx/qAn)

-

(1)

1

where qox is the capacity under oxic conditions, and qAn is the capacity under anoxic conditions at the same liquid concentration. As can be seen from the of MP-DI S

and MP-Fe carbons K

(%) 99.9 96.6

and Darco

Ca

cu

Cr

Mn

Ni

2 16

5

0.8 _

3 _

(PPm) 0.008 1 3.3

70 66

376 136

77 75

_ 10

30 38

1380

C.-C LENG and

90 80

N. G.

PINTO

l-t 0

Attori:

0

Oxic Bset Fits

~

1

30 60

70

80

90

Liquid Concentration

100

(mg/L)

Fig. 5. Phenol adsorption isotherms on MP-Fe carbon at 23°C.

0 Olk

.I10

~0000~~ mm0000 lo*oooo 6OOW~H 6momomJOrnwl 7ootlmoll l00ll00ll ,D ll

12

II

26

24

Ilmo(mk.)

Fig. 6. CC-MS chromatograms

for oxic and anoxic phenol adsorption on F-D1 carbon

definition, the larger the absolute value of PF the larger is the polymerization. Fig. 3 shows polymerization factors calculated for F-N,, F-Air, F-D1 and F-HCl from the Freundlich

isotherm data in Table 2; these fit coefficients were used, in place of raw data, because they characterize the experimental information very well and provide the advantage of clarity. Three major trends are

Effects of surface properties

4 Olk*ll6

of activated

carbons

1381

II

1;

17

;

70

71

1(1**.(min.)

Fig. 7. GC-MS chromatograms

for oxic and anoxic phenol adsorption on MP-DI carbon.

l.oam*oa q.oom*OI

-

Fig. 8. GC-MS chromatograms

for oxic phenol adsorption on MP-Fe carbon.

revealed: 1) the PF decreases with an increase in liquid concentration; 2) the PF decreases with increase in oxidation of the carbon surface; and 3) the PF decreases with removal of metal(s). The first trend suggests that at higher concentrations the phenol is predominantly in monomeric form. A similar result has been reported by Nakhla et al. [2 J. The second trend suggests that the oxidative coupling reactions are inhibited by the presence of

surface oxygen groups. As stated earlier, in our previous study [9], it was proposed that the sites for physisorption of phenol occurred mostly in basal planes of activated carbon. Once phenol is adsorbed on basal planes, it appears that its polymerization is catalyzed by some specific, but unknown, sites, and polymeric products are formed by oxidative coupling. Therefore, one possibility is that these specific sites for oxidative coupling are somehow blocked by an

C.-C LENCand N. G.

1382

PINTO

2 ‘88 80 g 70 0 60 f $ 50 bl 7 40 E E

30

0

MP-DI (Amoric)

Cl

MP-DI (Oxic)

A

MP-Fe (Amoxie)

V

MF-Fe (Oric)

-

Bert Fits

5 20

I

, 10

100 Liquid Concentration

(mg/L)

Fig. 9. Phenol adsorption isotherms on MP-DI and MP-Fe carbons at 50°C.

0

0 -

20

30

40

so

Liquid Concentration

60

70

80

Anoric Oxic Best Fits

90 100

(mg/g)

Fig. 10. Phenol adsorption isotherms on F-N, (4 h, 1OOOT) carbon increase in surface oxygen complexes resulting from the oxidation process. An examination of PF curves in Fig. 3 for F-D1 and F-HCl along with the ash content data in Table 1 indicates that surface polymerization of phenol decreases with the removal of metallic compounds. Corapcioglu and Huang [16] have determined the composition of the ash in F400 carbon in Mg ( 1.1%) Al (lO.l%), Si(l6.0%), Ca (3.3%) and Fe (8.9%). Clearly, metals could play an important role in the surface polymerization. To test this hypothesis, isotherms were measured for MP-DI and Darco G-60 PAC. As can be seen in Fig. 4, the two isotherms of phenol (oxic and anoxic) on MP are identical, and isotherms on Darco G-60 show higher retention

capacity under oxic conditions. Since MP carbon is almost free of metals (Table 1) and the composition of the ash in Darco is Mg (1.5%) Al (1.9%), Si (28.5%), Ca (2.3%) and Fe (5.6%) [16], the data in Fig. 4 support the hypothesis that metals play an important role in surface polymerization of phenol. The role of metals was further investigated by impregnating MP-DI carbon with Fe and measuring the oxic and anoxic isotherms on this carbon. Table 3 shows the metal contents of MP-DI and MP-Fe by the PIXE analysis. Although MP-DI carbon contains some metals, these amounts are too small to be detected by ash measurements. It is clear that the amount of iron increases from 0.0081 to 3.3% by the impregnation process.

Effects of surface properties

of activated

carbons

1383

Phenol (Oxic) Benzoic Acid (Oric) O-Cresol (Oxic) Phenol (Anoric) Benzoic Acid (Anric) O-Cresol (Anric) Best Fits

Liquid Concentration (mg/L) Fig. 11. Effect of adsorbates

Shown in Fig. 5 are the phenol adsorption isotherms on MP-Fe under oxic and anoxic conditions. As can be seen in the figure, the two isotherms are identical. The results suggest that impregnated Fe in MP carbon can not catalyze the surface polymerization of phenol at 23°C. However, results of the GC-MS analyses conducted on the phenol extracts from the F-DI, MP-DI and MP-Fe, which are shown in Figs. 6-8, respectively, clearly show that in the case of the impregnated carbon (Fig. 8), more higher molecular weight products are formed. The peaks around 17 minute retention time in Figs. 6 and 7 were identified as 4-phenoxyphenol by MS software at 98% confidence. Since the matching qualities for other peaks in these GC-MS chromatograms are very poor, these could not be identified. It should be noted that Nakhla et al. [2] have also shown the presence of high molecular weight products in an analysis of the extract from F400; these have been identified as 2-dihydroxyl- 1, 1-biphenyl and 4-phenoxyphenol. These GC-MS results, which appear contradictory to the adsorption isotherm data (Fig. 5) can be explained on the basis of the high extraction temperature, which could facilitate polymerization. To test this hypothesis, phenol adsorption isotherms on MP-DI and MP-Fe at 50°C were obtained (Fig. 9). Figure 9 suggests that surface polymerization does occur on the surface of MP-DI at this higher temperature. An endothermic polymerization reaction or high activation energy can be used to rationalize the observed effects of temperature; it should be noted that this result is consistent with the observation of Grant and King [7] and Nakhla et al. [2]. This finding supports the hypothesis that some polymeric products found in the extract of MP-DI originated from the higher temperature used during Soxhlet extraction experiments.

on surface

polymerization.

The data in Fig. 9 also show that the enhancement of adsorption capacity due to polymerization is more pronounced for MP-Fe than MP-DI. This is consistent with the GC-MS chromatograms, which show much more significant quantities of higher molecular weight products in the case of MP-Fe (Fig. 8) than for MP-DI (Fig. 7). The formation of polymeric products with MP-Fe may originate from the transformation of phase structure of Fe or the increase of Fe in MP carbon. All of these results are consistent with the results of Grant and King [ 71. They have shown that carbon treated with aqueous KMnO, solutions can increase irreversible adsorption, and have concluded that ash is not necessary for irreversible adsorption to occur. The above experimental evidence suggests that the metal (Fe) itself can not catalyze the surface polymerization of phenol on the carbon surface at room temperature. Therefore, the catalytic properties may originate from other functional groups on activated carbon. It has been known for some time that activated carbon has a basic character. The possible structures of basic oxides are the chromene and pyrone groups [17]. However, the exact structures of the oxides responsible for basic character have not been elucidated. To test the impact of basic oxides on the surface polymerization, MP-DI carbon was outgassed at 1000°C for 4 hours in a nitrogen atmosphere. It is believed that all the acidic surface complexes are removed with this treatment [ 181, while some basic surface oxides are retained [ 191. This carbon will be referred to as F-N, (4 h). Shown in Fig. 10 are the phenol adsorption isotherms on F-N, (4 h). Clearly, surface polymerization still occurs on the carbon which is free of acidic surface complexes. This leads to the conclusion that acidic

1384

C.-CLENG andN.G.

70? e 2

O

0

PINTO

Methanol 0

65 60-

0 0

55 50 -

cl

0

45 -

cl

40 55

I 65

60

I 70

0

I 80

75

I 85

I 90

0 / 95

100

Amount Adsorbed (mpig)

. _. -__

00

Methanol-NaOH

0

0

0

0

65

70

75

80

85

90

95

Anoric Oric

100

Amount Adsorbed (mg/g) 75

0

I

0

0 0

0 Formic

55

60

65

/ 70

I 75

80

85

Acid

1 90

I 95

100

Amount Adsorbed (mg/g)

Fig. 12. Effects of regenerants

surface oxygen groups are not necessary for surface polymerization. Moreover, these groups may actually suppress polymerization (Fig. 3). This is consistent with the recent results reported by Vidic [20], who postulated that the catalytic properties of activated carbon may originate from undetected basic oxides. Garten and Weiss [ 211 have suggested a mechanism that is consistent with the observed surface polymerization of phenol on carbon surfaces. They proposed that acid is adsorbed partly chemically and partly physically. The chemical portion of adsorption is considered to be due to a primary reaction between carbon and oxygen resulting in the adsorption of acid and the liberation of hydrogen peroxide. Thus, for example, if chromene groups are present on the surface, it is possible that phenol reacts with hydrogen peroxide released to form phenoxy radicals that subsequently initiate the surface polymerization. Regarding the role of iron in oxidative reactions of some dibasic organic acids on carbon, it is known

and DO on regeneration

efficiency

that the carbon itself can function as a catalyst for the oxidation of organic acids, and that iron functions as a promoter. Ironcarbon centers possess an activity fifty times that of the ordinary carbon surface, whereas the ironcarbon-nitrogen sites are eight hundred times more active. The experimental results of this study along with findings in the literature suggest that basic oxides are the key to surface polymerization, and metal(s) promote the polymerization, although metal(s) alone cannot catalyze the reaction. 3.3 Effect of substituent polymerization

groups on

Shown in Fig. 11 are adsorption isotherms of phenol, o-cresol and benzoic acid on F-D1 carbon. The magnitude of the difference between the oxic and anoxic isotherms of each of these adsorbates indicates that the degree of surface polymerization is: o-cresol ( 1.040) > phenol ( 1.089) > benzoic acid

Effects of surface properties of activated carbons (NA); the values in the parenthesis are the critical oxidation potentials (COP) [22,23]. Grant and King [7] have argued that a correlation between COP and degree of irreversible adsorption is indicative of oxidative polymerization. The absence of polymerization in the benzoic acid case can be explained on the basis of results obtained by Lim et al. [ 241. They have shown that an activation energy of 12.5 kcal mol-’ is required for oxidative coupling of phenol in solution in the presence of the strong catalyst cuprous chloride. It is to be expected that with the weaker carbon catalyst an even higher energy of activation is involved for phenol oxidation. It is therefore speculated that carbon is not a strong enough catalyst to catalyze the polymerization of benzoic acid on the surface.

3.4 Effect of polymerization

on regeneration

Surface polymerization of adsorbates makes regeneration more difficult and reduces the life of activated carbon. Nakhla et al. [2] have shown that phenol extraction efficiency offered by solvents for the anoxic condition was about SO%, but it was only 25% under oxic conditions. Similar experiments conducted by Vidic and Suidan [4] observed between 85-95% and 12-35% regeneration efficiencies for the anoxic and oxic experiments, respectively, depending on the sorption capacity [4]. Significant differences in the regeneration efficiency were also observed in this study between oxic and anoxic conditions. Shown in Fig. 12 are regeneration efficiencies of three regenerants. Clearly, the regeneration efficiency of all the regenerants is higher under anoxic conditions. Formic acid was found to be the strongest regenerant for both oxic and anoxic conditions. This regenerant also shows very little dependency on the amount of phenol adsorbed on the carbon. The average regeneration efficiencies under oxic conditions are 55, 46 and 44% for formic acid, methanol and 1% NaOH-methanol, respectively. 4. CONCLUSIONS The adsorption of phenol on activated carbon has been found to be a combination of physisorption and surface polymerization. Physisorption appears to be dominated by interactions with the basal planes of carbon. However, surface oxygen content also influences behavior. Increased concentrations of surface carboxylic groups are found to decrease physisorption capacity, and this is suspected to occur because of increased water adsorption and weaker dispersion interactions with basal plane carbons. Metal impurities on the surface were found to play an important role in both physisorption and surface

1385

polymerization of phenol. In the former case, higher concentrations of metal are found to decrease adsorption capacity. In the latter case, metals are suspected to act as promoters, but are not essential for polymerization. As with previous studies, it was observed that oxygen in solution was necessary for surface polymerization to occur. Additionally, however, it was found that the concentration of acidic oxygen groups on the surface strongly influences the polymerization, with increased concentrations suppressing this reaction. Preliminary results indicate that basic oxides catalyze the polymerization reaction.

REFERENCES 1. Nakhla, G., Abu-zaid, N., Farooq, S. and Ala’ama, S., Environ. Technol., 1992, 13, 181. 2. Nakhla, G., Abu-zaid, N. and Farooq, S., War. Environ. Res., 1994, 66, 842. 3. Vidic, R. D., Suidan, M. T., Traegner, U. K. and Nakhla. G. F.. War. Res.. 1990. 24. 1187. 4. Vidic, R. D. and Suidan, M. T.; Environ. Sci. Technol., 1991,25, 1612. 5. Vidic, R. D., Suidan, M. T. and Brenner, R. C., Environ. Sci. Technol.,

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