Impact of Oxygen-Containing Surface Functional Groups on Activated

LOIS J. URANOWSKI. Department of Civil and Environmental Engineering,. 943 Benedum Hall, University of Pittsburgh,. Pittsburgh, Pennsylvania 15261-229...
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Environ. Sci. Technol. 1997, 31, 1872-1878

Impact of Oxygen-Containing Surface Functional Groups on Activated Carbon Adsorption of Phenols CHARLES H. TESSMER, RADISAV D. VIDIC,* AND LOIS J. URANOWSKI Department of Civil and Environmental Engineering, 943 Benedum Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15261-2294

The adsorptive capacity of activated carbons for phenolic compounds increases significantly in the presence of dissolved oxygen (oxic conditions) due to the oligomerization of these compounds through oxidative coupling reactions. With increased capacity comes an increase in the amount of irreversible adsorption, which is defined as adsorbate that cannot be recovered by solvent extraction. The objective of this study was to determine the impact of oxygencontaining functional groups on activated carbon surface on the irreversible adsorption of phenolic compounds in the presence of dissolved oxygen. The adsorptive capacities and surface functional group (SFG) content were evaluated for seven commercially available activated carbons and an activated carbon whose SFG content was modified by outgassing. This study demonstrated that the presence of acidic surface functional groups hinders the ability of activated carbon to adsorb phenolic compounds under oxic conditions by reducing its effectiveness in promoting adsorption via oxidative coupling reactions. The catalytic ability of activated carbon may be enhanced by eliminating the acidic functional groups and encouraging formation of basic groups by outgassing at 900 °C. Re-introduction of oxygen-containing acidic surface functional groups onto the surface of outgassed GAC negates any gains in catalytic ability produced by the outgassing procedure. Therefore, outgassing affects the adsorption of phenolic compounds only by changing the amount and composition of oxygen complexes. Outgassing at higher temperatures (e.g., 1200 °C) causes the elimination of oxygen complexes, resulting in a more basic carbon that does not contain oxygenated basic groups. Greater structural ordering and delocalized electrons on the carbon surface may increase the basicity of the carbon but do not enhance its ability to promote irreversible adsorption. The presence of oxygen-containing basic groups (e.g., chromene-type, pyrone-type) is likely a key factor in promoting irreversible adsorption.

Introduction A clear understanding of the reactions taking place on the surface of granular activated carbon (GAC) is crucial to the design of activated carbon adsorbers. These reactions may determine both the adsorptive capacity and regeneration efficiency of carbon beds and ultimately their economic viability. A thorough knowledge of carbon surface chemistry may also lead to the development of more specialized and * Corresponding author phone: 412-6124-1307; fax: 412-624-0135; e-mail: [email protected].

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effective sorbents. A study by Vidic et al. (1) found that the adsorptive capacity of GAC for phenolic compounds may increase up to 3-fold in the presence of molecular oxygen. They discovered that these compounds undergo oxidative coupling reactions in the presence of dissolved oxygen and that the oligomers formed by these reactions are very difficult to desorb from the carbon surface. Vidic et al. (1) also revealed that an appreciable quantity of oxygen is consumed during the adsorption of phenolics. Oxygen chemisorbs on the surface of activated carbon forming carbon-oxygen functional groups that may be acidic, neutral, or basic. The temperature at which the freshly activated carbon is exposed to oxygen generally determines the nature of these oxygen complexes (2, 3). The acidic groups include carboxyl, lactone, and hydroxyl functionalities. Carbon basic properties are believed to arise from two different types of actives sites. The existence of pyrone-type (4, 5) and chromene-type (6) structures was postulated to account for the basic nature of the carbon surface. Carbons free of oxygen may also have basic sites arising from delocalized electrons (7, 8). Basic group content is difficult to determine since acidic groups are more numerous and may diminish their reactivity (5). Functional groups can be desorbed from the carbon surface upon heating in the absence of oxygen. Carboxyl and lactol groups evolve as carbon dioxide at temperatures between 250 and 600 °C, while hydroxyl and carbonyl groups evolve as carbon monoxide at temperatures between 500 and 1000 °C (9). Oxygen-containing surface functional groups are known to affect the adsorptive properties of activated carbons. Magne et al. (10) found that acidic oxygen surface complexes decrease the chemisorption of phenols. Coughlin et al. (11) suggested that phenol and nitrobenzene adsorption was hampered by the hydration of these groups, resulting in water complexes blocking pore entrances and reducing the surface area available for adsorption. They also proposed that oxygen-containing functional groups reduced adsorptive capacity by localizing free electrons of the carbon basal planes. This theory was corroborated by the work of Mahajan et al. (12), who tested phenol adsorption on graphite and borondoped graphite. Other studies by these authors demonstrated that the complexes desorbed as CO do not affect adsorption of phenols, while the more acidic complexes desorbed as CO2 hindered phenolic adsorption. Furthermore, Mattson et al. (13, 14) found the adsorption of p-nitrophenol to be enhanced by the presence of carbonyl groups, which is believed to form electron donor-acceptor complexes with the aromatic ring of the nitrophenol. Surface functional groups may also affect the catalytic properties of GAC. Voudrias et al. (15) reported that the removal of functional groups prior to oxidation with aqueous free chlorine resulted in a carbon less effective in the dimerization of 2,4-dichlorophenol. Cookson et al. (16) attributed the catalytic ability of GAC to oxidize mercaptans to disulfide in the presence of dissolved oxygen to quinone groups on the carbon surface. Recent studies by Chandran et al. (17) show that basic type (H) carbons promote oxidative coupling, while acidic type (L) carbons do not. However, the relationship between oxygen-containing surface functional groups and the ability of activated carbon to promote oxidative coupling (irreversible adsorption) of phenols has not yet been established and was the main objective of this study. Detailed investigation of the impact of oxygen-containing functional groups was motivated by the findings of Uranowski et al. (18) that metals and metal oxides present on the carbon surface are not the key factor responsible for catalyzing oxidative coupling of phenols under oxic conditions.

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Experimental Methods Reagent grade 2-methylphenol and 2-chlorophenol (Aldrich Chemical Co. Inc., Milwaukee, WI) were used as model adsorbates in this study. The water used in all experiments had been purified using reverse osmosis (ROpure LP, Sybron/ Barnstead Inc., Boston, MA) and deionization (Nanopure II, Sybron Barnstead Inc., Boston, MA). The RO/DI water containing 0.01 M phosphate buffer adjusted to a pH 7.0 with 10 N NaOH was used for the preparation of the sorbate solutions. Adsorbate concentration measurements were performed on a Perkin Elmer Lambda 2B UV/VIS spectrophotometer (Bodenseewerk Perkin Elmer, West Germany) using a 1-cm quartz cell. The wavelength used for 2-methylphenol analyses was 270 nm, while a wavelength of 273.5 nm was used for 2-chlorophenol analyses. The extracts from the GAC used in the adsorption isotherm tests were analyzed using a Hewlett-Packard HP 5890 Series II GC (HewlettPackard Co., Palo Alto CA) equipped with an FID detector. Chromatographic separation was performed using an HP-1 30-m crosslinked methyl silicone gum capillary column (Hewlett-Packard Co., Palo Alto CA). The oven temperature was held constant at 110 °C. Surface areas were determined by nitrogen adsorption using an Orr surface-area pore-volume analyzer Model 2100 (Micrometrics Instrument Corporation, Atlanta, GA) and calculated using the BET isotherm method. X-ray diffraction analysis of activated carbons was performed using the powder method in an X’pert diffractometer (Philips, Almelo, Holland) using Cu KR radiation with a graphite curved monochromator. The tube voltage was 40 kV, and the tube current was 30 mA. Detector and beam slits of 1° and 0.3°, respectively, were used in scanning 2θ from 10° to 90° at a rate of 2.4°/min. Activated Carbons. Seven varieties of commercially available activated carbon were used as adsorbents in this study. Three carbons were wood-based and are designated as W1 (BD, Calgon Carbon Corporation, Pittsburgh, PA), W2 (BGP, Elf-Atochem, Philadelphia, PA), and W3 (WSIV, Calgon Carbon Corporation, Pittsburgh, PA). Carbon W3 is chemically activated using KOH and extruded into pellets while carbons W1 and W2 are steam activated. A coconut-based carbon was also studied and is designated as carbon C (PCB, Calgon Carbon Corporation, Pittsburgh, PA). The final three were bituminous coal-based carbons, referred to as B1 (F400, Calgon Carbon Corporation, Pittsburgh, PA), B2 (1240, ElfAtochem, Philadelphia, PA), and B2+. B2+ is a B2 carbon that was acid-washed by the manufacturer. The 16 × 20 U.S. mesh size particles were used for all experiments. Carbon was thoroughly washed with RO/DI water, dried at 105 °C overnight, and stored in a desiccator until use. GAC Outgassing. Four additional varieties of activated carbon adsorbents were created by outgassing virgin B2 carbon. The outgassing was performed under argon atmosphere using a Lindberg Hevi-Duty furnace (Lindberg, Watertown, WI) and a mullite tube. The carbon was heated at a rate of 200 °C/h to a maximum temperature of 500, 900, or 1200 °C. The carbon was exposed to the maximum temperature for 16 h and then allowed to cool at a rate of 200 °C/h. The carbon was used immediately after preparation to reduce oxygen chemisorption that normally occurs during storage. These outgassed carbons are designated as 500 OG, 900 OG, and 1200 OG. A sample of activated carbon outgassed at 900 °C (900 OG) was soaked for 2 weeks in RO/DI water that was exposed to a headspace of pure oxygen in order to enhance the oxygen-containing surface functional group content of the carbon. This carbon is designated as 900 OGS. Similarly, 1200 OGS designates an activated carbon sample that was first outgassed at 1200 °C and then soaked in oxygenated water for 2 weeks. Surface Functional Group Determination. The determination of acidic surface functional group content was performed using the methods proposed by Boehm (3, 19).

Solutions of NaHCO3, Na2CO3, NaOH (0.05 and 0.25 N), and H2SO4 were prepared using RO/DI water. Seventy milliliter of base solution was added to 2 g of carbon in a 75-mL glass bottle. The bottle was sealed with a Teflon-lined stopper and aluminum cap and allowed to equilibrate for 3 days in a rotary shaker. Each set of bottles included four blanks of base solution without carbon. At the end of the equilibration period, the carbon was separated from solution using a vacuum filter, and the filtrate was titrated using standardized sulfuric acid solution. The amount of base consumed by the functional groups on the carbon surface was calculated as the difference in the amount of acid required to titrate the blank to pH 4.5 and the amount of acid required to titrate the filtrate to the same end point. Each determination was performed in triplicate. Standardization of the acid solution was performed using sodium carbonate according to Method 2320 (20). The free carboxyl group concentration on the carbon surface was determined as the amount of 0.05 N sodium bicarbonate consumed by the carbon sample. Lactone content was calculated as the difference between the amounts of 0.05 N sodium carbonate and the 0.05 N sodium bicarbonate consumed by the carbon. Phenolic hydroxyl group content was determined as the difference between the amounts of 0.05 N sodium hydroxide and the 0.05 N sodium carbonate consumed by that carbon. Carbonyl group content was found by subtracting the amount of 0.05 N sodium hydroxide consumed by the carbon from its 0.25 N sodium hydroxide consumption. Basic group content was evaluated by allowing 2 g of GAC to equilibrate with 70 mL of 0.05 N HCl for 3 days in a 75-mL glass bottle sealed with a Teflon-lined stopper and aluminum cap. The carbon was separated from solution using vacuum filtration, and the filtrate was titrated to pH 11.5 using 0.05 N NaOH. The basic group content was calculated as the difference between the amounts of NaOH required to reach the end point for the HCl blank and for the filtrate. The NaOH solution was standardized with potassium hydrogen phthalate according to Method 2310 (20). All group content values are expressed as micro-equivalents per gram of activated carbon. The determination of iron content in the filtrates from the basic group content experiments was performed using a Perkin Elmer 1100B atomic adsorption spectrophotometer (Perkin Elmer, Norwalk, CT) equipped for flame analysis. All pH measurements were determined using a Fisher Scientific Accumet pH meter 25 and Accumet glass body combination electrode with an Ag/AgCl reference element (Fisher Scientific, Pittsburgh, PA). Acidity/Basicity Determinations. Activated carbons were classified as either acidic (L) or basic (H) by their ability to affect the pH of the RO/DI water. Two grams of activated carbon and 70 mL of RO/DI water were equilibrated for 3 days in a sealed 75-mL glass bottle in a rotary shaker. At the end of this period, the pH of the slurry was determined allowing 5 min for the pH probe to equilibrate. Carbons whose slurries had pH values greater than that of the RO/DI blank were classified as basic carbons, while those whose slurries had pH values lower than that of the RO/DI blank were classified as acidic. Oxygen Uptake Studies. The uptake of oxygen during adsorption was determined by an N-CON Comput-OX Model WB512 computerized respirometer (N-CON Systems Co. Inc., Larchmont, NY). Previously weighed carbon samples were purged with nitrogen twice daily for 3 days to eliminate any oxygen physically bound to the carbon surface. The adsorbate solutions were saturated with air and had a DO level of approximately 8.5 mg/L. The solutions were 350 mL in volume, and adsorption was allowed to proceed for 2 weeks before analysis. These experiments followed the procedure of Vidic and Suidan (1, 21) except as noted above.

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TABLE 1. Activated Carbon Adsorptive Capacity for 2-Methylphenol carbon

isotherm type

K (mg/g) (L/mg)1/n

1/n

adsorbate recovery (%)

irreversible adsorption (mg/g)

W1

oxic anoxic oxic anoxic oxic anoxic oxic anoxic oxic anoxic oxic anoxic oxic anoxic

104.9 ( 3.1 79.4 ( 3.6 130.0 ( 7.3 93.6 ( 2.7 288.4 ( 1.2 110.8 ( 2.8 215.6 ( 10.3 142.0 ( 13.3 288.4 ( 12.2 79.0 ( 3.0 251.7 ( 10.0 95.5 ( 2.6 251.6 ( 5.8 100.6 ( 3.5

0.073 ( 0.007 0.119 ( 0.011 0.077 ( 0.015 0.128 ( 0.007 0.054 ( 0.010 0.170 ( 0.006 0.090 ( 0.010 0.100 ( 0.022 0.054 ( 0.010 0.190 ( 0.008 0.073 ( 0.009 0.170 ( 0.007 0.072 ( 0.006 0.157 ( 0.008

26-43 77-92 31-43 80-90 10-28 88-94 21-35 84-100 06-26 84-93 13-27 85-93 15-27 80-90

86.7

W2 W3 C B1 B2 B2+

104.0 273.0 228.3 288.3 276.7 255.7

Adsorption and Extraction Studies. A detailed description of the experimental protocol used to evaluate adsorptive capacity (oxic and anoxic) and irreversible adsorption is provided elsewhere (1, 21, 22) and will not be repeated here. All isotherm tests were performed at 24 °C.

Results and Discussion The adsorptive capacities of seven virgin activated carbons for 2-methylphenol measured in the presence and absence of dissolved oxygen are presented in Table 1. The isotherm data are modeled using the Freundlich isotherm equation, qe ) KCe1/n (qe ) equilibrium surface concentration; Ce ) equilibrium liquid-phase concentration), and the Freundlich parameters K and 1/n are reported together with standard deviation for each parameter. A brief inspection of the data reveals the oxic K values to be higher than their anoxic counterparts. This demonstrates the ability of dissolved oxygen to enhance the adsorptive capacity of activated carbon for phenolic compounds (1, 21). The anoxic 1/n parameter is always greater than its oxic counterpart, revealing that the effect of dissolved oxygen diminishes as 2-methylphenol equilibrium concentration in the aqueous phase increases. At higher adsorbate concentrations, the interactions between the adsorbate molecules are believed to affect the adsorption process more than the adsorbent-adsorbate forces (11). The increase in activated carbon capacity occurs at the expense of adsorbate recovery, which is much lower for activated carbons loaded in the presence of dissolved oxygen than for the carbons loaded in its absence. Irreversible adsorption indicates the degree to which the GAC surface promotes oxidative coupling and is shown in the last column of Table 1. It is defined as the amount of adsorbate that cannot be recovered from activated carbon subjected to the methanol/methylene chloride Soxhlet extraction procedure (1). Figure 1 shows the irreversible adsorption of 2-methylphenol on the bituminous, wood, and coconut-based carbons over the range of equilibrium conditions employed in this study. Each data point represents the result of an extraction experiment performed on a carbon sample loaded with adsorbate during isotherm tests. After measuring the equilibrium concentration of 2-methylphenol in the aqueous phase, the carbon was separated from solution using vacuum filtration and subjected to the solvent extraction procedure. As shown in Figure 1, irreversible adsorption varies only slightly over the range of equilibrium conditions studied, and each activated carbon has a distinct ability to promote oxidative coupling. Irreversible adsorption corresponds well with the increase in capacity associated with the presence of molecular oxygen since the carbons that exhibited a large increase in capacity under oxic conditions also exhibited a high degree of irreversible adsorption.

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FIGURE 1. Irreversible adsorption of 2-methylphenol on bituminous, wood, and coconut-based activated carbons.

TABLE 2. Functional Group Content of Virgin Activated Carbons, (µequiv/g) total total RO/DI type type type type acidic basic final carbon a b c d carbon I II III IV groups groups pH type W1 W2 W3 C B1 B2 B2+

56 0 0 0 80 10 38

48 32 43 18 102 17 12

100 81 121 97 125 33 54

37 43 119 23 9 11 0

242 156 283 139 315 71 107

118 126 159 37 145 111 107

5.74 8.08 8.98 9.42 5.56 7.17 6.98

L H H H L N L

a Carboxyl-type structure. b Lactone-type structure. c Hydroxyl-type structure. d Carbonyl-type structure.

The results of the surface functional group (SFG) determinations and the ability of the activated carbons to affect the pH of RO/DI water are given in Table 2. The functional group content of each carbon was compared with its ability to promote irreversible adsorption. Such comparisons were made for individual SFG types and the sums of the more acidic groups (i.e. I and II; I, II, and III; etc.). Comparisons were also made for the basic surface functional group content and for the differences between the basic and acidic groups. Based on poor statistical correlations among the parameters investigated, it was concluded that there is no relationship between surface functional group content determined by the titration method and irreversible adsorption on virgin activated carbons. The ability of an activated carbon to affect the pH of RO/ DI water also did not correlate well with the ability of the carbon to promote irreversible adsorption. Both acidic and basic carbons had the ability to promote polymerization. However, among the wood-based carbons, irreversible ad-

TABLE 3. Effect of Outgassing on Activated Carbon Adsorption of 2-Chlorophenol carbon B2 500 OG 900 OG 1200 OG 900 OGS

isotherm type

K (mg/g) (L/mg)1/n

1/n

adsorbate recovery (%)

irreversible adsorption (mg/m2)

O2 consumption/irrev. 2-CP adsorption

oxic anoxic oxic anoxic oxic anoxic oxic anoxic oxic anoxic

246.0 ( 4.1 128.2 ( 6.5 270.4 ( 6.7 115.8( 5.7 297.1 ( 7.1 115.0 ( 4.7 111.2 ( 6.9 88.3 ( 6.6 240.8 ( 8.2 117.7 ( 4.2

0.095 ( 0.004 0.156 ( 0.012 0.084 ( 0.005 0.184 ( 0.014 0.079 ( 0.006 0.185 ( 0.009 0.117 ( 0.011 0.147 ( 0.015 0.077 ( 0.007 0.158 ( 0.008

6-38 82-100 20-43 80-92 13-31 94-100 54-60 82-99 29-40 87-99

0.284

0.306

0.345

0.345

0.424

0.372

0.199 0.325

TABLE 4. Functional Group Content of Outgassed Activated Carbons (µequiv/g)

carbon

type Ia

type IIb

type IIIc

type IVd

total acidic groups

B2 500 OG 900 OG 1200 OG 900 OGS 1200 OGS

10 0 0

17 0 0

33 0 0

11 41 0

71 41 0

23 0

48 46

34 59

29 145

134 250

total basic groups

corrected basic groups

RO/DI final pH

carbon type

BET area (m2/g)

GAC mass reduction (%)

111 148 85 110 246 173

122 159 188 213 392 197

7.2e 9.8e 11.0f 11.1g 6.2h 6.7i

L H H H N N

818 735 716 471

0.8 4.5 8.8

a Carboxyl-type structure. b Lactone-type structure. c Hydroxyl-type structure. pH ) 7.0. h Initial pH ) 6.2. i Initial pH ) 6.7.

sorption did increase with increasing basicity. Another important conclusion from the data given in Table 2 is that the ability of the GAC to influence pH does not reflect the functional group content. Further evaluations of additional activated carbons in a manner similar to that described above would be of little use since factors like pore sizes distribution, crystalline structure, and inorganic content could easily mask the effect of surface functional groups on the ability of activated carbon to promote irreversible adsorption. Additionally, the determination of basic group content cannot be accurately performed on virgin activated carbons (5). Therefore, carbon B2 was selected to be modified by outgassing under an argon atmosphere at varying temperatures to selectively remove certain functional groups. According to Puri (9), outgassing at 500 °C would cause the carboxyl and lactone groups to desorb as CO2, while outgassing at 900 °C would cause the hydroxyl and carbonyl groups to desorb as CO. Outgassing was also performed at 1200 °C to ensure complete removal of carbonyl groups, which are resistant to thermolysis and may form basic sites upon exposure to oxygen (8), and to fully evaluate changes in the activated carbon crystalline structure. In the adsorption experiments with outgassed carbons, 2-chlorophenol was used as a model adsorbate. The adsorptive capacities of carbon B2 and its outgassed varieties for 2-chlorophenol in the presence and absence of molecular oxygen are shown in Table 3 together with standard deviations for each of the Freundlich isotherm parameters. As with the adsorption of 2-methylphenol on virgin activated carbons, an increase in adsorptive capacity for 2-chlorophenol was observed in the presence of dissolved oxygen, and the effect is reduced with greater concentrations of adsorbate in the solution. The average irreversible adsorption associated with these activated carbons is also noted in Table 3. The amount of oxygen consumed during the adsorption of 2-chlorophenol onto the surface of the B2, 500 OG, and 900 OG carbons was determined using a computerized respirometer. By combining these results with the irreversible adsorption data provided by the isotherm studies, the molar ratio of consumed oxygen and irreversibly adsorbed 2-chlorophenol was determined for each carbon and is presented

d

Carbonyl-type structure. e Initial pH ) 7.6. f Initial pH ) 7.9. g Initial

in the last column in Table 3. As the outgassing temperature increased, the molar ratio of consumed oxygen to irreversibly adsorbed 2-chlorophenol also increased, despite the greater irreversible adsorption exhibited at higher outgassing temperature. This increase in the molar ratio is most likely due to additional oxygen consumption by activated carbon for the recreation of oxygen-containing surface functional groups. It is also important to note that outgassing at 1200 °C caused appreciable decrease in the anoxic adsorptive capacity of B2 activated carbon for 2-chlorophenol. This behavior is partially due to the reduction in surface area as measured by the BET method (Table 4) but is also due to substantial changes in surface functional group content as discussed later in the manuscript. The effect of outgassing on the surface functional group content is presented in Table 4. The virgin carbon (B2) has significant quantities of each type of surface functional group. Outgassing of activated carbon at 500 °C removed type I, II, and III functional groups, while the number of carbonyl-type groups on the carbon surface increased, which may be due to the removal of the hydroxyl and carboxyl groups since carbonyl groups in their close proximity might condense to form lactones or lactols (23). Outgassing at 900 °C removed all SFGs from the carbon surface that can be detected by the titration method. It must be noted that the acidic functional group determinations by the base neutralization method account for only about 50% of the total oxygen content of the carbon and that the remainder of the total oxygen content may be in the form of ether-type oxygen or less reactive carbonyl groups (24). Papirer et al. (25) have shown that carbonyl groups may continue to evolve from carbon blacks even at heat treatment temperatures of 950 °C. Therefore, the adsorptive behavior of the bituminous coal-based activated carbon (B2) was also evaluated after it had been outgassed at 1200 °C (1200 OG). The SFG content of this carbon was not evaluated using the base neutralization method and was assumed to be zero. The effect of outgassing on activated carbon surface area as determined by nitrogen adsorption and the BET isotherm method is shown in Table 4, along with the corresponding loss in carbon mass upon outgassing. As outgassing tem-

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FIGURE 2. Effect of outgassing on activated carbon crystalline structure. perature increases, the carbon surface area diminishes due to sintering of pore structure (widening of individual pores) most likely and is most evident for the 1200 OG carbon, which experiences a 42% loss in surface area but only an 8.8% loss in mass. X-ray diffraction measurements performed on powdered samples of carbons B2, 900 OG, and 1200 OG show that changes in the crystalline structure of activated carbon also occur during outgassing. X-ray diffraction scans shown in Figure 2 confirmed that outgassing at higher temperatures not only removes oxygen-containing surface functional groups but also induces a more ordered carbon structure, as evidenced by the growth of the XRD peak at 2θ ) 26°. This is consistent with the studies of Leon y Leon et al. (8), who found that reducing carbon oxygen content resulted in carbon with higher ring densities. The total acidic SFG content of the activated carbon that was first outgassed at 900 °C and then soaked in oxygensaturated water (900 OGS) was found to be almost 2-fold the total acidic SFG group content of its virgin predecessor (B2), while the 1200 OGS had almost four times the total acidic group content of the virgin (B2) carbon. This was expected since the outgassing procedure creates carbons with a greater reactivity toward aqueous oxygen (8, 24). The procedure of fixing SFGs on the surface of outgassed carbons using oxygenated water is preferable to the use of stronger oxidizing agents, such as nitric acid, since water does not create oxidation byproducts that must be washed from the carbon surface. The basic nature of the carbon surface also changed with outgassing and was affected by the outgassing temperature. Using the neutralization of HCl to track the formation of basic surface functional groups, Papirer et al. (25) found that increasing the heat treatment temperatures causes greater acid consumption by carbon blacks. The data presented in the “basic” column of Table 4 do not follow this trend due to the evolution of iron upon the outgassing of the activated carbon B2. The presence of iron was evidenced by the formation of a visible precipitate during titration of HCl filtrates that were in contact with the 900 OG and 1200 OG carbons and was verified by AA spectroscopy. The evolution of iron from outgassed carbons was also noted by Cookson et al. (26). The HCl filtrates that were in contact with the virgin and 500 OG carbons contained only 8.0 mg/L iron, while filtrates that were contacted with 900 OG, 1200 OG, 900 OGS, and 1200 OGS carbons contained 76.9, 76.4, 108.6, and 17.7 mg/L iron, respectively. The formation of Fe(OH)3 during the titration of the filtrates caused an increase in the amount of NaOH required to reach the titration end point. The NaOH consumption for Fe(OH)3 formation has been subtracted from the total consumption, and the corrected values appear under the heading “corrected basic groups” in Table 4. The ability

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of activated carbon to raise the pH of RO/DI water also increased with an increase in outgassing temperature. This trend is shown in Table 4 and is corroborated by the work of Puri (9) and Leon y Leon et al. (8). The increases in carbon alkalinity upon outgassing is due to several factors. Initial gains may be attributed to the removal of the acidic groups. The destruction of these groups lowers the acidity of the carbon surface and eliminates the neutralization of the basic groups caused by their presence (5). This mechanism is primarily responsible for the increased basicity of the 500 OG carbon. Higher outgassing temperatures favor the formation of additional basic sites. Pyronetype structures are generated by thermal decomposition of oxygenated acidic groups, formation of active sites capable of fixing oxygen in ether form, and rearrangement with existing carbonyl groups that resist pyrolysis (5). These pyrone-type oxides are formed when a clean carbon surface, obtained by outgassing at 900-1000 °C, is exposed to oxygen in the presence of water after cooling to room temperature (24), as in the case of the 900 OGS carbon, which has the highest basic group content. The 1200 OGS carbon has half the basic group content of 900 OGS and about five times the type IV (carbonyl) group content. Outgassing at 1200 °C causes the complete evolution of oxygen as CO, CO2, and H2O (9). Thus, the formation of pyrone groups on the surface of the 1200 OG carbon may be restricted by the loss of the ether oxygen, resulting in fewer pyrone-type groups and the surplus of carbonyl-type groups. The same reasoning also applies for the chromene-type functional groups. The higher degree of alkalinity associated with the 1200 OG carbon is due to the lack of oxygen-containing functional groups that localize the free electrons associated with the basal planes of the carbon surface. Removal of oxygen from the carbon surface will delocalize these electrons, allowing them to behave as Lewis base sites (8). Unfortunately, acid titration technique does not differentiate between oxygen-containing basic surface functional groups and delocalized electrons (Lewis basis); and the resulting increase in the amount of acid consumed by activated carbons outgassed at higher temperatures is denoted by a general term like “total/corrected basic groups” in Table 4. The effect of the changes in surface functional group content due to outgassing at different temperatures on adsorptive capacity and irreversible adsorption is shown in Figures 3 and 4. The capacities of the outgassed carbons are shown in terms of mass of adsorbate per unit area to compensate for the reduction in surface area that accompanies heat treatment although the true surface area that is accessible to adsorbate molecules might be quite different from the BET area. Figure 3 illustrates the effect of outgassing at 500 and 900 °C on carbon loading (straight lines represent oxic and anoxic Freundlich isotherms for virgin carbon), while Figure 4 provides data on the irreversible adsorption on the surface of these outgassed carbons. Outgassing at 500 °C eliminates the more acidic groups (type I, II, and III), allowing the basic groups to become more active. The 500 OG carbon has a greater oxic capacity for 2-chlorophenol and exhibits greater irreversible adsorption than the virgin GAC (Figure 3a). Raising the outgassing temperature to 900 °C creates optimum conditions for the re-formation of the chromeneor pyrone-type basic groups. Hence the 900 OG carbon has a higher basic group content, as shown by the acid neutralization and pH tests presented in Table 4. The increased basic group content results in the highest oxic capacity and the greatest irreversible adsorption for 900 OG carbon. Adsorption isotherm data for 900 OGS are shown in Figure 3b, together with the isotherm data for the 900 OG and Freundlich isotherms for virgin carbon. As is apparent from Figure 3b, allowing surface functional groups to reform on the GAC surface prior to adsorption experiments decreases the oxic capacity of the outgassed carbon for 2-chlorophenol.

FIGURE 3. Effect of outgassing on activated carbon adsorptive capacity for 2-chlorophenol.

Outgassing at 1200 °C removes all oxygen complexes from the carbon surface and produces the most basic B2 carbon variety. Leon y Leon et al. (8) demonstrated that increased graphitization, as shown in Figure 2, enhances carbon basicity primarily due to delocalization of electrons associated with carbon’s basal planes that can act as Lewis basis. Figures 3c and 4 also compare the adsorptive capacities and irreversible adsorption of the virgin, 900 OG, and 1200 OG carbons. The adsorptive capacity of the 1200 OG carbon exhibited under anoxic conditions is slightly higher than the capacity measured for the virgin carbon, which is probably due to the removal of impurities from the carbon surface. On the other hand, the oxic capacity (Figure 3c) and irreversible adsorption (Figure 4) for the 1200 OG are the lowest among all carbons investigated in this study. The reduced ability of 1200 OG carbon to promote irreversible adsorption may be due to the lack of oxygencontaining basic functional sites (e.g., chromene- or pyronetype). Anoxic adsorptive capacity of 1200 OG carbon was very similar to that exhibited by the virgin carbon in spite of the fact that the total basic group content, as determined by acid titration method, was almost twice that of the virgin carbon and that the ability to raise the pH of RO/DI water was markedly increased by outgassing (Table 4). This finding indicate that greater structural perfection and the increased basicity due to these structural changes [increased presence of delocalized electrons (8)] are not the key factors influencing the adsorptive properties of this bituminous coal-based activated carbon. Although oxygen is completely removed from the GAC surface by heat treatment at 1200 °C, the re-formation of these groups upon exposure to dissolved oxygen cannot be ruled out. However, the dissolved oxygen must compete with the adsorbate for the active sites involved in the formation of basic SFGs. The complete removal of oxygenated SFGs may slow their re-formation and allow the majority of adsorption to take place before the catalytic effects of the basic groups are realized. The slower formation of surface oxides with increasing heat treatment temperature has been noted by Donnet (2), while Boehm et al. (19) have shown that increased structural perfection produces conditions unfavorable to the formation of functional groups on the carbon surface. The small difference between the oxic and anoxic capacity of the 1200 OG carbon for 2-chlorophenol and the low irreversible adsorption illustrate the importance of oxygen-containing basic surface functional groups in promoting irreversible adsorption.

Literature Cited

FIGURE 4. Irreversible adsorption of 2-chlorophenol on virgin and outgassed activated carbons. The 900 OGS carbon appears to have similar oxic and anoxic adsorptive capacity as the virgin GAC. Similarly, this carbon exhibited lower irreversible adsorption than the 900 OG carbon as can be seen in Figure 4. These findings clearly indicate that adsorptive capacity and irreversible adsorption are influenced by the presence or absence of functional groups and not by other outgassing effects such as pore destruction, evolution of impurities, or graphitization.

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Received for review June 3, 1996. Revised manuscript received March 3, 1997. Accepted March 6, 1997.X ES960474R X

Abstract published in Advance ACS Abstracts, May 1, 1997.