Environ. Sci. Technol. 1991, 25, 1612- 1618
Role of Dissolved Oxygen on the Adsorptive Capacity of Activated Carbon for Synthetic and Natural Organic Matter Radlsav D. Vldlc and Makram T. Suidan"
Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 4522 1-0071 Parameters affecting the adsorptive capacity of granular activated carbon (GAC) for o-cresol, phenol, o-chlorophenol, 3-ethylphenol, trichloroethylene, and natural organic matter are investigated in this study. Experimental results prove that the presence of molecular oxygen significantly increases the adsorptive capacity of GAC for five of these six compounds. The oxic GAC adsorptive capacity for o-cresol, for example, can be up to 2.6-fold the capacity that is attainable under anoxic conditions. Experimental data also prove that there was no biological degradation of these compounds in the presence of oxygen; consequently, biological activity was not responsible for the increased adsorbate removal from the liquid phase. Presumably, the increase in the adsorptive capacity under oxic conditions is due to some polymerization of adsorbate on the surface of the carbon. Naturally occurring organic matter was also adsorbed to a greater extent when molecular oxygen was present in the test environment. However, the adsorptive capacity of GAC for aliphatic organic compounds, such as trichloroethylene, is not significantly influenced by the presence of molecular oxygen. ~
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Introduction Adsorption on activated carbon has been demonstrated in numerous studies to represent one of the better available technologies for the removal of synthetic organic chemicals (SOCs) from water. It has also been acknowledged as a very efficient unit process for removing refractory organic compounds that persist in the environment or resist removal by conventional treatment. New applications of activated carbon for water purification usually involve extensive pilot-scale studies, although several single and multicomponent adsorption models have been developed for predicting the performance of granular activated carbon (GAC) adsorbers (1-3). Both experimental and modeling procedures, however, require knowledge of the capacity of activated carbon to adsorb the organic substances of interest. This capacity, given in terms of the mass of compound adsorbed per mass of activated carbon as a function of the equilibrium concentration of adsorbate in the liquid phase, is expressed as an adsorption isotherm. Adsorption isotherm data are crucial for the design of GAC adsorbers, the scaling-up of experimental data, or simply for research purposes. The literature contains many contradictory findings regarding parameters pertinent to the adsorptive capacity of GAC. Weber and Morris ( 4 ) reported that the adsorptive capacity of GAC for 3-dodecylbenzenesulfonate was influenced by particle size. However, Martin and Al-Bahrani (51, Peel and Benedek (6),and Randtke and Snoeyink (7) found that particle size affects the capacity only if true equilibrium is not reached. Crittenden and Weber ( I ) and van Vliet et al. (8)found that the adsorptive capacity of GAC for phenol and p-bromophenol increased when the initial concentrations of these compounds were decreased. On the other hand, Peel and Benedek (6), Yonge et al. (9), and Zogorski et al. (10) experienced no such effects for the adsorption of phenol and o-cresol. Isotherm data are traditionally obtained by the, socalled, "bottle-point'' technique. Although this is a very 1612 Environ. Sci. Technol., Vol. 25, No. 9, 1991
simple laboratory test, there is yet no unified procedure for conducting the experiment, which may explain some of the diversity in published data. Almost every researcher employs different experimental procedures with respect to carbon preparation, carbon particle size, fraction of the bottles filled with sorbate solution, buffer application, and equilibration time (1-12). Moreover, many researchers have reported disagreement between the adsorptive capacities obtained from the bottle-point tests and those obtained from column experiments (I, 8, 9, 13, 14). There has been no systematic study of the effects of oxygen on the adsorption process. Prober et al. (15) showed that dissolved oxygen adsorbs on GAC in the range of 10-40 mg/g. This adsorption of oxygen increases the concentrations of acidic oxide groups on the carbon surface, resulting in an increased base sorption capacity, particularly in the pH ranges of the carboxylic acid group. Coughlin and Ezra (16) and Magne and Walker (17) also demonstrated changes in the GAC adsorptive capacity for nitrobenzene and phenol as a result of chemisorption of oxygen. The same phenomenon of a decrease in adsorptive capacity of activated carbon as a result of an increase in acidic surface oxides was demonstrated for many other compounds (benzene, nitrobenzene, p-hydroxybenzaldehyde, sodium benzenesulfonate,phenol, p-nitrophenol, chloroform,nitromethane, methyl ethyl ketone, n-butanol, l,li-dioxane, dextrose, oxalic acid, and succinic acid) (18). This paper presents data that provide further insight into the single-solute adsorption process with particular attention given to the influence of molecular oxygen, which may be entrapped inside the carbon pores, sealed in the headspace of isotherm bottles, and dissolved in adsorbate solutions. It will be shown in this study that the presence of this molecular oxygen causes a tremendous increase in the removal of several organic compounds by GAC adsorption. This phenomenon is in no way associated with the adsorption behavior resulting from an increase in acidic surface oxides since the sources of oxygen are totally different and the effects of the two types of oxygen are completely opposite. Discoveries made in this study offer a new explanation for previously observed discrepancies. Experimental Methods The adsorbent used in this study was Filtrasorb 400 GAC (Calgon Carbon Co., Pittsburgh, PA) supplied in 12 X 40 U S . mesh size. Fractions used for the experiments were 12 X 14,16 X 20, and 20 X 30, with geometric mean particle diameters of 0.154, 0.100, and 0.071 cm, respectively. Carbon was thoroughly washed with deionized water, dried at 110 "C for 2 days, and stored in a desiccator until use. All the experiments were performed using one batch of activated carbon. Reagent grade phenol, o-cresol, o-chlorophenol (OCP), 3-ethylphenol, and trichloroethylene (TCE) (Aldrich Chemical Co., Inc.) were used as adsorbates in this study. Milli-Q water (deionized tap water filtered through Millipore purification system, Millipore Corp., Bedford, MA) containing 0.01 M phosphate buffer and adjusted to a pH of 7.0 was used for preparation of the sorbate solutions. The adsorptive capacity of GAC for natural organic matter
0013-936X/91/0925-1612$02.50/0
0 1991 American Chemical Society
(NOM) was evaluated by using prefitered water from Lake Decatur (Illinois) as the source of NOM. This water, which was filtered through 0.45-pm membrane filter (Micron Separations Inc., Westboro, MA), had a dissolved organic carbon (DOC) level of 7.6 mg/L. The bottle-point technique was used for all adsorption isotherm tests. However, two different procedures, denoted henceforth as "oxic" and "anoxic", were employed in conducting these tests. The oxic procedure is very similar to the traditionally used techniques and involves placing accurately weighed portions of GAC (fO.l mg) into a series of 160-mL bottles. After the addition of 100 mL of sorbate solution, bottles were sealed with rubber stoppers and aluminum caps. This procedure is denoted as oxic since it allows for three different sources of molecular oxygen in the bottles, namely, air in the headspace of isotherm bottles, air associated with the carbon particles, and dissolved oxygen (DO) present in the solution water. It will be proven that the amount of oxygen present in an isotherm bottle has a significant influence on the adsorptive capacity of GAC. Better control of the mass of oxygen present during the isotherm experiment was achieved by completely filling the isotherm bottles with an adsorbate solution having a known DO content. The anoxic procedure, on the other hand, requires the absence of molecular oxygen from isotherm bottles. This was achieved by displacing the air associated with carbon particles with nitrogen gas, stripping the oxygen from the water with nitrogen prior to the addition of sorbate, and purging the headspace in the bottle (if any was used) with nitrogen gas prior to sealing. To ensure maximum displacement of the oxygen entrapped in carbon pores, GAC was purged with nitrogen gas twice each day for a period of 3 days prior to the addition of adsorbate solution. It is possible that not all of the oxygen adsorbed on the surface of GAC particles during preparation will be removed just by purging with nitrogen gas. However, it was not the intention of this study to evaluate the effects of irreversibly adsorbed or chemisorbed oxygen, but rather the effect of molecular oxygen present in air associated with carbon particles or isotherm bottle headspace and oxygen dissolved in the sorbate solution. Furthermore, a series of tests were conducted without the carbon being purged prior to the start of the anoxic isotherm tests. These tests were designed to provide a base line for the initial state of the carbon. Adsorbate sollutions used in the adsorption isotherm tests with NOM and TCE were prepared by somewhat different experimental procedures. Lake Decatur water used in oxic isotherm experiments with NOM was purged with oxygen gas for 4 h. The dissolved oxygen level in that water reached 35 mg/L. On the other hand, the water used in anoxic isotherm experiments was first oxygenated for 1 h and then purged with nitrogen gas for an additional 3 h to remove previously introduced oxygen. For the adsorption isotherm tests with TCE, a stock solution having a high concentration of TCE was first prepared in oxygen-free Milli-Q water. Dilution water used for the oxic adsorption isotherm test contained 35 mg/L dissolved oxygen while dilution water used for the anoxic isotherm tests was free of oxygen. Isotherm bottles were completely fiiled with sorbate solution, thus eliminating the possibility of TCE volatilization into the bottle headspace. Isotherm bottles were placed in a rotary shaker and allowed to equilibrate for at least 2 weeks. Included in each set of isotherm bottles were two control bottles, which contained sorbate solution but no GAC. The control bottles were used to check for sorbate volatilization and/or
adsorption onto the walls of the container during the equilibration period. Experiments conducted at 35 "C were carried out in a constant-temperature room while the remaining tests were performed at room temperature. Room temperature varied in a very narrow range of 21 f 1 "C. Concentration measurements for the phenolic compounds were performed on a Perkin-Elmer Lambda 3B UV/vis spectrophotometer using l-cm quartz cell. For more accurate measurements at low sorbate concentrations, a 5-cm quartz cell was employed. Wavelengths used were as follows: 268 nm for phenol, 269 nm for o-cresol, 270 nm for 3-ethylphenol,and 272.5 nm for OCP. Periodic photometric scans in the range of 230-300 nm were also performed. These scans were always identical with scans obtained for standard solutions prepared for the calibration of the instrument. TCE concentrations were measured according to EPA method 502.2, modified as follows: (1) A 30-m-long DB624 mega-bore capillary column was used with a carrier flow of 8 mL/min. The oven temperature was programmed from 35 to 90 "C at 5 "C/min with a 5-min hold at 35 "C and then from 90 to 150 "C at 10 "C/min. (2) A Tenax trap was used with a He purge flow of 40 mL/min and a desorption time of 8 min. (3) An electron capture detector was used with P-5 make-up gas at a flow of 40 mL/min. NOM concentrations were determined with an Envirotech Dohrmann, Model DC-80, DOC analyzer. It is important to note that the liquid samples from all isotherm tests were first filtered through 0.45-pm Nylon filters (Micron Separation Inc.) to eliminate the interference of carbon fines. Preliminary tests showed that none of the compounds used in this study adsorb on the filters. DOC and inorganic carbon analyses were performed on liquid samples randomly selected from isotherm bottles that were filled to the top with sorbate solution. DOC concentrations were compared with theoretical values that were calculated from concentration measurements of phenols obtained by UV spectroscopy. Inorganic carbon analyses were performed at the beginning and end of each isotherm run to check for possible biological activity in the isotherm bottles. GAC samples used in the o-cresol isotherm tests were extracted in a Soxhlet extraction apparatus. GAC samples were first extracted for 1day with methanol and then with methylene chloride for an additional 3 days. Extracts were analyzed with a Hewlett-Packard gas chromatograph-mass spectrometer (GC-MS Model 5985A), using the electron-impact positive-ion mode. The gas chromatograph was equipped with a DB-1 30-m fused-silica capillary column. The oven temperature was programmed from 40 to 280 "C at 10 "C/min with a 5-min hold at 40 "C.
Results and Discussion Adsorption of Phenolics. An extensive study of the effect of oxygen on the adsorptive capacity of GAC was performed using o-cresol (OC) as an adsorbate. o-Cresol adsorption isotherm tests were performed at 35 and 21 "C. Experimental conditions for isotherm tests, conducted at 35 "C, are listed in Table I. Isotherm data obtained from five experimental runs conducted in the presence of molecular oxygen and eight experimental runs conducted in the absence of molecular oxygen at 35 "C are presented in Figure 1. Bottles used in all isotherm tests had a total volume of 160 mL. Only 100 mL of sorbate solution was added to each bottle in these experiments. Therefore, in the oxic procedure, a 60-mL headspace of air provided a total of approximately 18 mg of oxygen. Additional sources of Environ. Sci. Technol., Voi. 25, No. 9, 1991
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Table I. Experimental Conditions for o-Cresol Isotherm Tests at 35 OC isotherm run
initial concn," mg/L
OC1 OC2 OC3 OC4 OC5 OC6 OC7* Ocab
loo0 (2) 1000 (1) 200 (2) 1000 (2) 200 (2) 50 (2) 1000c 1000d
particle size (U.S. mesh) 12 X 16 X 12 X 16 X 16 X 16 X 16 X
16 X
14 20 14 20 20 20 20 20
headspace
isotherm type
air air air
oxic oxic oxic anoxic anoxic anoxic anoxic anoxic
N2 N2
N2 N2 N2
Table 11. Experimental Conditions for o-Cresol Isotherm Tests at 21 OC initial concn: mg/L 1000 (2) 150 100 (2) 1000 1000 (2) 200 150 50 (3)
isotherm run OC9 OClO OCll oc12 OC13 OC14 OC15 OC16
Values in parentheses are the number of runs. * GAC was not purged with nitrogen prior to the addition of adsorbate solution. 701 mg of GAC. 334 mg of GAC.
0
0x1'2
0
anox>c OC? OCB
. 10'
1
100
.
headspace pure O2 pure O2 pure O2 absent N2 N2 absent N2
isotherm type oxic oxic oxic anoxic anoxic anoxic anoxic anoxic
Values in parentheses are the number of runs. Table 111. Freundlich Isotherm Equation Parameters for o-Cresol isotherm type oxic anoxic oxic anoxic
t
particle (U.S. mesh) 16 X 20 16 X 20 16 X 20 16 X 20 16 X 20 16 X 20 16 X 20 16 X 20
K,
temp, "C 35 35 21 21
(mg/g) (L/mg)'/" 227.3 i 3.3 80.9 f 1.89 241.3 f 3.6 94.1 f 1.92
1/n
0.080 f 0.003 0.201 f 0.005 0.079 f 0.003 0.165 i 0.004
I
t
1 10'
io3
108
e , mg/L Figure 1. Adsorption isotherms for o-cresol at 35
oxygen in these experiments came from air associated with carbon particles and from DO in the solution water. The fact that the initial adsorbate concentration and the particle size had no detectable influence on either oxic or anoxic GAC adsorptive capacity, as is apparent from Figure 1, indicated that the period of 2 weeks was sufficient for establishing true equilibrium between adsorbed and liquid-phase concentrations of o-cresol (6, 7). To experimentally reinforce this hypothesis, several sets of isotherm bottles were allowed to equilibrate for 3 and 4 weeks, and no additional removal of o-cresol from the liquid phase was detected in these sets even after 4 weeks of equilibration. The adsorptive capacity of GAC obtained from the isotherm experiments in which GAC was not repeatedly purged with nitrogen gas prior to the addition of adsorbate solution (isotherm runs OC7 and OC8) was identical with the anoxic GAC adsorptive capacity. Therefore, purging the carbon with nitrogen gas did not cause any changes in the carbon surface or the adsorptive properties of F-400 GAC. Another set of o-cresol isotherm tests was conducted at 21 "C under the experimental conditions given in Table 11. Most of the isotherm bottles in the oxic adsorption isotherm experiments had a headspace of 60 mL filled with pure oxygen, which provided a minimum of 86 mg of oxygen. At the end of the isotherm experiments, DO level in the solution was above 16 mg/L in each bottle. Therefore, the presence of molecular oxygen was not in any way a limiting factor in these experiments. Some of the isotherm bottles used for the oxic experiments were filled completely with adsorbate solution that was saturated with oxygen by bubbling pure oxygen gas through the solution. These bottles were used to measure the changes in inor1614
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Environ. Sci. Technoi., Voi. 25, No. 9, 1991
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