Environ. Sci. Techno/. 1995, 29, 2243-2251
Sorption of Organic Contaminants by Fly Ash in a Single Solute System KASHI BANERJEE,*,+ PAUL N. CHEREMISINOFF,* AND S U L I N G CHENG’ Chester Environmental, 600 Clubhouse Drive,
Moon Township, Pennsylvania 15108, and Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102
A treatment process is developed using fly ash as a sorbent material to isolatelimmobilize organic contaminants from aqueous solution. Batch as well as dynamic studies were conducted during the investigation. The results of this research demonstrate that fly ash has a significant capacity for adsorption of organic compounds from the aqueous solution. The carbon content of fly ash plays a significant role during the sorption process. A significant correlation was observed between the Freundlich sorption capacity parameter and various properties of the organic contaminant such as molar volume, parachor, octanolwater partition coefficient, and electronic polarizability. The sorption of organic compounds onto fly ash is believed to occur principally via the weak induction forces of London or dispersion forces, which are characteristic of the physical adsorption process.
Introduction Industrial processes produce a million tons of nonradioactive sludges and solid wastes everyyear. Generally, these wastes are disposed on the earth’s crust in landfills. In recent years, disposal of such wastes on land has increased, but under growing pressure, more stringent controls have been implemented. One of the goals of the Federal Hazardous Waste Management Program is to reduce dependence on land disposal. Power plants that burn fossil fuels generate large quantities of solid residues as byproduct materials. Reliance on coal as an energy source has led to significantbyproduct management problems related to the storage or disposal of fly ash generated as a result of combustion. Therefore, an inexpensive byproduct management technology is needed for the environmentally safe disposal or storage of this material until beneficial uses for this byproduct are developed. Fly ash has been used in a variety of different applications, including construction of roads, dams, and bridges ( I ) . It was successfully used to remove phenol from an +
Chester Environmental. New Jersey Institute of Technology.
0013-936X/95/0929-2243509,00/0
3 1995 American Chemical Society
industrial wastewater (2). Fly ash contains many trace elements that accelerateplant growth, and it has some value as a fertilizer when mixed with sewage sludge (3). The results of a study conducted at the University of Cincinnati indicated that fly ash is capable of removing refractory organics from wastewater (4). Nelson and Guarino (5) reported that fly ash can be used to remove appreciable quantities of BOD and COD. Fly ash has proven to be a useful agent in conditioning sludge prior to vacuum filtration (6). Liskowitz et al. (7, 8) reported that a combination of illite/fly ashlzeolite can efficiently remove heavy metals and fluoride ions from petroleum sludge leachate. Mott and Weber (9) indicated that fly ash has a significant capacity for sorption of low molecular weight organic compounds in aqueous solutions. They also found that adding fly ash into soil-bentonite cutoff barriers can significantlyretard contaminant migration in these barriers. This research affords the opportunity to study the adsorption process as a possible technique for waste immobilization using the byproduct of combustion. The objectives of this research were to identify fly ash as an alternative treatment source; to investigate the adsorption mechanism; and to measure, describe, and explain the equilibrium sorption characteristics of organic compounds using fly ash. The predominant forces behind the sorption mechanism were also investigated.
Experimental Section Materials. Sorbent. The sorbent used in this research was fly ash, which is a byproduct of electric power plants using coal as fuel in combustion. It is the fine particulate matter, a sandy material dark gray in color, that escapes from the chimney stack. The individual particle size of fly ash ranges from 0.5 to 100 pm (8). Physically, fly ash is characterized by its fineness, its large surface area (1-6 m2/g) (IO),and its wide particle size distribution. The major components of fly ash are alumina, silica, iron oxide, calcium oxide, and residual carbon. The exact chemical composition of fly ash is dictated by the characteristics of the coal and the combustion temperature. Seven different types of fly ashes were used in this research, including Militant, Conemough, Wellmore Cactus, Deep Hollow, Blender, Keystone, and Upshore. Fly ashes are identified by the name of the mine from which the coal is extracted. The characteristics and major chemical constituents of these fly ashes are summarized in Table 1. Sorbate. The following organic compounds from three different functional groups commonly found in industrial wastewater were selected as “target” compounds: (a) alcohols: methanol, ethanol, propanol, butanol, and hexanol; (b) aromatics: phenol, rn-cresol, nitrobenzene, ethylbenzene, and o-xylene; and (c)ketones: acetone, methyl ethyl ketone (MEK), cyclohexanone, and methyl isobutyl ketone (MIBK). All of these compounds were laboratory reagent-grade (99.99%pure) and were obtained from Fisher Scientific, Springfield, NJ. Phase I: Adsorption EquilibridhothermStudies. Fly ash obtained from thermal power plants contains dust and fine particulate matter, a major portion of which is watersoluble material. It was reported in our earlier work (11) that the adsorption capacity offly ash increases significantly
VOL. 29, NO. 9, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY a 2243
Characteristics and Chemical Composition of Fly ASP fly ashl coal mine localion
Militant/ Clearfield Countq, PA
2555
A1203
I%)
CaO (%)
s (%I
carbon 1%)
3.6 -7.0 37.80 36.60 2.95 14.60 1.52
Conemoughl Conemough. PA 2125 7.0-8.5 57.00 29.30 2.46 6.32 2.50
Wellmon Cactus/ Buchenan Countq. PA 2155 8.0-9.0 36.40 36.10 6.14 15.90 3.73
Deep Hollow/ Preston County. PA
Blender/ mixed fly ish
Keqstonel Keystone. PA
Upshoral Upshors Countq, W
2575
2189
2163
2700
3.8-7.0 29.60 55.20 2.65 9.31 1.17
7.0-7.5 38.30 29.90 7.57 15.90 2.14
6.5-7.5 38.70 36.40 2.61 10.70 1.71
* Liskowitz et SI. (121
if it is washed prior to use. Hence, all the fly ashes used in this research were washed with distilled water for a period of 24 h and dried overnight at 103 "C before use. The isotherm studies were performed using Militant fly ash as a sorbent material because the quantities of other fly ashes in stock at the time of the experiment were not adequate to conduct both batch and dynamic studies. The experiment was conducted using a tumbled-bottle batch reactor. For each sorption data point, three vials were used. Accurately weighed dry Militant fly ash was placed into two of the three glass vials equipped with a Nbber-lined septum plastic screw cap. A known volume of buffered deionized distilled water was added to M y wet the fly ash prior to the introduction of the aqueous solution containing the "target" compounds. Sodium azide was added to the distilled water to prevent biological activity. Thethreevialswerethenfilledwiththetestsolution (leaving no head space), sealed immediately, and allowed to equilibrateusing agyratorshaker equippedwith a constant temperature bath. The temperature of the bath was adjusted to 20 "C. The vials containing fly ash yielded a replicate, and the vial without fly ash was used as a control, which revealed the initial concentration of the Constituents in the aqueous solution. In our earlier work (111, we reported that most of the organic compounds were removed within the fust 2.5 h of adsorptionand that the removal rate graduallyapproached a plateau. Similar results were obtained by Mancy (10) during the adsorption of ABS by fly ash. Based on that information, it was decided that 72 h of sorbate-sorbent contact is a reasonable time for achieving equilibrium. After 72 h of agitation, the samples were allowed to settle overnight within the bath. A 10-pL sample of the clear supernate was withdrawn from each of the vials without opening the caps and was injected immediately into a Perkin-Elmer flameionization detector gas chromatograph (GC) for analysis. Gas Chromatograph Operating Condition for Solute Analysis. The flame ionization detector (FID)required the followingthree gas streams: (a)carrier gas, which is helium or nitrogen, at the rate of 1 mL/min; (h) hydrogen at the rate of 30 mL/min; and (c) air at the rate of 250-300 mL/ min. The column used in this research had the following specifications: GP 80/100 Carbopack, C 0.1% SP-1000,6 ft x 2 mm i.d. glass column. The column was operated under an isothermal condition by maintaining the temperature at 220 "C. Phase II: Adsorption DynamicslContlnuous Column Tests. A schematic of the continuous column test unit is 2244 m ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 9.1995
I
Flyash
2.5-4.5 47.90 42.30 1 .87 3.94 0.81
TABLE 2
Freundlich Sorption Capacity (k) and Intensity (lln) Parameters for Sorption of Organic Compounds by Militant Fly Ash solute
P
95% Clb
CVc for k
lln
95% CIb
cvc coeff of for l/n determination( R z ) significanced(%)
methanol ethanol propanol butanol hexanol
5.00 4.566-5.434 6.50 6.290-6.710 8.30 8.279-8.400 15.74 15.677- 15.813 20.00 19.795-20.230
Alcohols 8.20 0.69 3.04 0.69 1.96 0.70 0.65 0.71 8.3 0.71
0.669-0.715 0.665-0.720 0.674-0.726 0.699-0.720 0.664-0.756
2.90 3.00 3.50 2.00 6.08
0.98 0.98 0.98 0.94 0.98
phenol rn-cresol nitrobenzene ethylbenzene 0-xy Ie ne
9.52 21.00 22.00 30.20 31.11
9.490-9.540 19.879-22.120 21.371 -22.628 28.858-31.142 30.900-31.310
Aromatics 0.31 0.70 5.10 0.72 2.80 0.70 3.62 0.90 1.00 0.91
0.665-0.744 0.709-0.731 0.684-0.735 0.866-0.933 0.899-0.916
4.85 1.50 3.45 3.50 1.50
0.98 0.98 0.92 0.94 0.98
acetone methyl ethyl ketone (MEK) cyclohexanone methyl isobutyl ketone (MIBK)
11.60 13.16 19.00 20.42
10.225-12.974 12.817-13.504 18.457-19.542 20.279-20.562
Ketones 11.25 0.60 2.50 0.61 2.80 0.64 0.97 0.65
0.587-0.613 0.582-0.632 0.623-0.657 0.639-0.670
2.00 3.50 2.50 3.35
0.90 0.92 0.94 0.98
a Units of k correspond t o mg/L and p g / g for aqueous-phase and solid-phase concentrations, respectively. variation. Number of observations = 10.
void out of the column packing. A solution of sodium azide (200 mg/L) was passed through each of the columns to prevent biological activity. After a saturation period of at least 24 h, the columns were filled with a solution of organic compounds. An overhead tank was used as a wastewater feed tank, which helped to maintain a constant head condition within the columns. The influent solution was fed to the columns from the overhead tank through a valve manifold that distributed the solution to the different columns. The volume of the effluent solution passing through the column was continuously monitored. Samples were collected periodicallyuntil the effluent concentrations were the same as their respective influent concentrations. Influent and effluent samples were analyzed using the gas chromatograph. Phase 111: Effect of Temperature on Adsorption. The experiment was conducted on representative organic compounds from the three functional groups that were studied in the adsorption equilibria experiment at 20 "C. From each functional group, the two compounds that were amenable to adsorption by fly ash were selected: butanol and hexanol, o-xylene and ethylbenzene, and MEK and MIBK, representing the alcohols, aromatics, and ketones, respectively. Militant fly ash was used as the sorbent material during this phase of the experiment. The experimental procedure was exactly identical to that which was followed during the adsorption equilibria study at 20 "C, except that the bath temperature was adjusted to 50 "C.
Results and Discussion Phase I: Adsorption EquilibridIsotherm Studies. The adsorption characteristics of fly ash were analyzed and evaluated using the Freundlich isotherm equation. The equation for the isotherm is X I M = kC"" where X / M is the loading of impurity on a unit weight of sorbent material; Xis the weight of the substance adsorbed;
1
1 1 1 1
Cherernisinoff (231. Coefficient of
M is the weight of the sorbent used; C is the equilibrium concentration of solute remaining in the solution; k is the empirical constant, known as capacity factor; and l l n is the slope of the line on a log-log plot, known as intensity factor. The data were fit into the Freundlich adsorption isotherm equation. The experimental values were transformed by taking the logarithm of C and X / M , and the resulting values were fit by the least squares method. Calculated values for k and l / n are presented in Table 2, along with the statistical analysis of the data. The high coefficient of determination values (R2> 0.90) show that the Freundlich isotherm model fits the experimental data reasonably well. The relative amenability of the target compounds to adsorption by fly ash is illustrated as follows:
alcohol group hexanol > butanol > propanol > ethanol > methanol aromatic group o-xylene > ethylbenzene > nitrobenzene > rn-cresol > phenol ketone group MIBK > cyclohexanone > MEK > acetone The above results indicate that all the target compounds can be treated to some degree using fly ash as a sorbent material. One should be careful when using fly ash that generates an acidic leachate. Because of the acidic characteristics,the leachate may contain contaminants such as boron and trace metals, which could also contaminate the wastewater. However, the literature data (12, 13) indicate that the desorption of trace metals from fly ash surfaces in aqueous solution decreases significantly with the increasing pH. An attempt was made to establish a correlation between the adsorption capacity parameter (k) and various properties of solute, such as molar volume fv), parachor (PI, VOL. 29, NO. 9. 1995 / E N V I R O N M E N T A L SCIENCE &TECHNOLOGY
2245
TABLE 3
Physical Properties of Organic Compounds mol wta compounds
(M, g/moll
dipole momenta (p,D)
methanol ethanol propanol butanol hexanol phenol m-cresol nitrobenzene ethyl benzene o-xylene acetone MEK cyclohexanone MlBK
32.04 46.07 60.09 74.10 102.20 94.11 108.13 123.11 106.17 106.16 58.10 72.10 98.14 100.16
1.70 1.67 1.67 1.67 1.30 1.45 1.30 4.22 0.59 0.62 2.89 2.72 3.25 3.08
a
electronicC polarizability (a,10P4 cm3)
densitya (g/cm3) at 20 "C
refractive indexa at 20 "C
octanol-water partition coeffb (Kow)
41 .OO 64.34 89.98 110.30 157.60 139.46 163.43 163.80 178.10 175.11 80.52 103.62 132.30 150.00
0.7914 0.7893 0.7796 0.8098 0.8136 1.0722 1.0336 1.2037 0.8672 0.8968 0.7908 0.8054 0.9978 0.8010
1.3288 1.361 1 1.3850 1.3992 1.4178 1.5509 1.5398 1.5562 1.4959 1.5058 1.3588 1.3814 at 15 "C 1.4522 1.3960
0.22 0.48 2.18 7.60 107.24 28.86 91.20 70.85 660.00 589.50 0.58 3.22 6.45 29.20
Weast (24). Leo et al. (25).Electronic polarizability values calculated using the Lorentz-Lorentz equation.
octanol-water partition coefficient (KO,,,),dipole moment @I, and electronic polarizability (a).The physical properties of the compounds are summarized in Table 3 and explained below: Molar volume (V)is the volume of 1 mol of compound, which is calculated from molecular weight and density data as
order to test the possible mutual correlation between the adsorption capacity parameter and the solute's various properties, a linear regression of each variable with the capacity parameter ( k ) was employed using the following regression equation:
V = MIQ
where P is the solute's property in question, and A and B are the coefficients. The most important coefficient determined in the regressions is the linear correlation coefficient R, which is a measure of how well the two variables are linearly correlated. The coefficient of determination (R2)and the significance level of the data for the linear regression were also determined. The values of A and B are not furnished in this paper because these are not pertinent to this discussion. The results of these calculations indicated the characteristics described below for each group of the organic compounds. Alcohol Group. Hexanol was the most favorably adsorbed material among all the alcohols, followed by butanol, propanol, ethanol, and methanol, which was the least adsorbed compound. Highly water-soluble low molecular weight alcohols had relativelylow amenability to adsorption onto fly ash. However, as the molecular weight increased, a corresponding increase in adsorption amenability was noted. This is because the oxygen atom present in the hydroxyl group in alcohol forms a hydrogen bond with the water molecules. The influence of the hydroxyl group is more significant in alcohols of lower molecular weight because it constitutes alarger part of the molecule, whereas the effect becomes less significant as the molecular weight of the alcohol increases. It appears that the hydrogen bonding between the solute and the water molecules has a significant effect on the adsorption of alcohols by fly ash. The values of the adsorption capacity parameter ( k ) for each of the solutes shown in Table 2 were plotted against the respective octanol-water partition coefficient (KO,,,)in Figure 2. Note that as the octanol-water partition coefficient values of alcohols increased, a corresponding increase in adsorption capacity was achieved. Increased adsorption capacitywith increasing octanol-water partition
where Mis the molecular weight of the compound (glmol) and e is the density of the compound (g/cm3). Parachor (PI is a function of the compound's molecular structure and is defined as (14):
p=- MU"^ e1
-e,
where e1 is the density of liquid (glcm3),eUis the density of vapor (g/cm31,and u is the surface tension (dynlcm). Since el >> e", p = - Mull4
e, p =v p (11 The electronic polarizability (a)of an atom is a measure of how "loosely" the nucleus controls its electron distribution under the influence of an electric field. It is the proportionality constant in the following equation:
where /induced is the dipole induced by the applied electric field, E. Polarizability (a),expressed in cm3,is calculated using the Lorentz-Lorentz equation (15):
where A4 is the molecular weight of the compound (g), e is the density of the compound at 20 "C (g/cm3)),NA is Avogadro's number (6.022 x lo2" molecules/mol), and ND is the index of refraction for the compound at 20 "C. In 2246
1
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 9, 1995
k=AP+B
s
&
t
nL p!
B3
10-
0 c .Q
Legend 0 AlcaholS (R* I 0.94) Armalics (R* I0.80) KBtOneD (R* 0.85)
0
-
P
1
i
I
I
TABLE 4
Coefficient of Determination (I?*)for Regressions of Sorption Capacity Parameter (k) with Various Properties of Solutea solute's
molar vol
parachor
electronic polarizability
dipole moment
functional coeff of significance coeff of significance coeff of significance coeff of significance (%) determination ( R 2 ) (YO) determination ( R 2 ) (YO) determination ( R z ) (YO) group determination ( R 2 ) alcohols aromatics ketones
0.92 0.96 0.83
Ib Ib
5c
0.92 0.96 0.85
Ib 16
5c
0.92 0.98 0.96
16 16 IC
0.62 0.08 0.64
>lob
NAd
'10C
BThesedata were derived from the adsorption isotherm study using Militant fly ash. Number of observations = 5. Number of observations = 4. NA, no association between the variables.
coefficient reveals that fly ash favors adsorption of compounds with low water solubility. The high coefficient of determination values listed in Table 4 indicate that a compound's molar volume, parachor, and electronic polarizability establish a strong correlation (R2> 0.90) with the adsorption capacity parameter. The dipole moment did not establish a strong correlation with the adsorption capacity parameter. The correlation of the dipole moment with the capacity parameter (R2 = 0.62) indicates that the influence of dipole moment during the adsorption of alcohol compounds onto fly ash is less significant than the other properties of the compounds. Aromatic Group. Molar volume, parachor, and electronic polarizability of the aromatic hydrocarbon compounds showed a strong correlation (R2 > 0.90) with the adsorption capacity parameter. No significant correlation was observed between the dipole moment and the adsorption capacity parameter. The low polarity and the consequent insolubility of the aromatic compounds explain the adsorption amenability. Ketone Group. The ketones, like the alcohols, are highly polar compounds, and the polarity of the carbonyl group becomes less significant as the molecular weight increases. Again, the influence of the solute's electronic polarizability on the adsorption capacity is clear. This characteristic established a strong correlation (R2 > 0.90) with the adsorption capacity parameter. A corresponding increase in adsorption capacity was observed with increasing octanol-water partition coefficient values also. As in the other groups, the dipole moment, with respect to the other
physical characteristics of the solute, did not correlate well (R2 = 0.64) with the adsorption capacity parameter. In the experiments described above, the electronic polarizability of the each solute demonstrated a strong correlation with the adsorption capacity parameter (k) of fly ash, while the dipole moment did not correlate as well. Therefore,it appears that electronic polarizability has more influence on the adsorption of organic compounds onto fly ash than the dipole moment. These two properties (electronic polarizability and dipole moment) are of particular interest because they are the only properties of the solute that appear in the fundamental equations of intermolecular forces, as described in the section entitled Intermolecular Forces and Adsorption Mechanism. Phase II: Adsorption DynamicsIContinuous Column Tests. The purpose of performing column studies was to establish data on a dynamic system that would eventually determine whether the effectiveness of the adsorbents, as predicted from the isotherms, would be observed in the column application. A continuous column experiment was conducted using a dilute solution of methanol, ethanol, butanol, MEK, MIBK, o-xylene,ethylbenzene, and m-cresol. The influent concentration of each of the compounds was maintained at 30 mglL. Figure 3 indicates that, of all the alcohols, butanol took the most time to reach the breakthrough point, while methanol was the first to reach the breakthrough point. Ethylbenzene and o-xylene reached the breakthrough point almost at the same time. Of all the aromatic hydrocarbon compounds, a relatively faster breakthrough was observed in the case of rn-cresol (see VOL. 29, NO. 9,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
2247
Volume, L
1
2
3
4
5
6
FIGURE 5. Fixed-bed breakthrough profiles for MEK and MIBK by Militant fly ash.
Volume, L
FIGURE 3. Fixed-bed breakthrough profiles for alcohols by Militant fly ash.
TABLE 5
Adsorption Capacity of Militant fly Ash on Selected Organic Compounds during Batch and Dynamic Study adsorption capacity (pg/g) compound
adsorption equilibria study'
methanol ethanol butanol m-cresol ethyl benzene 0-xy Ien e methyl ethyl ketone (MEK) methyl isobutyl ketone (MIBK)
52 68 175 243 645 682 105 1 86
dynamic studyb
69 100 200
310
aoo
883 125 220
Adsorption capacity from the equilibria study has been calculated using an equilibrium concentration of 30 mg/L and the Freundlich isotherm parameters (kand l/n)oftherespectivecompounds. Influent concentration of each of the compounds was maintained at 30 mg/L. Adsorption capacityfrom the dynamic study has been calculated from the breakthrough curve area under a complete exhaustion condition.
4
8
12
16
20
24
Volume, L
FIGURE 4. Fixed-bed breakthrough profiles for aromatic hydrocarbon compounds by Militant fly ash.
Figure 4). A relatively faster breakthrough was observed during the adsorption of MEK by fly ash (see Figure 5). The adsorption amenability of the compounds can be summarized as follows: alcohol group butanol
>
ethanol
>
methanol
aromatic group o-xylene > ethylbenzene
>
m-cresol
ketone group MIBK > MEK As indicated in the Experimental Section, a control column
was used to determine the other losses from the system. 2248
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 9.1995
The data show that the other losses are insignificant. The adsorption capacities of fly ash were computed for each of the compounds based on the breakthrough curves. The maximum amount of solute adsorbed was calculated from the breakthrough curve area defined by the column exhaustion condition. The adsorption capacity of the fly ash, in terms of micrograms of solute adsorbed per gram of sorbent, was obtained in the dynamic study by dividing the total weight of solute adsorbed by the total weight of fly ash used. The calculated data are shown in Table 5. For comparison purposes, the capacities determined from the adsorption equilibria experiment at an equilibrium concentration of 30 mg/L (byusing the respective values of the Freundich isotherm parameters) are also included in the table. The adsorption capacities of fly ash determined from the continuous column experiment were higher than those obtained from the batch equilibria study. The apparent capacitydifference noted between the batch equilibria study and the column study may have been caused by a somewhat less than complete equilibrium condition in the batch study.
TABLE 6
Adsorption Capacitya of Seven Different Types of Fly Ash on Butanol, Methyl Isobutyl Ketone (MIBK), and o=Xylene in a Dynamic Systed fly ash
Militant
a d s o r p t i o n capacity (pglg)
200
adsorption c a p a c i t y ( p g l g )
220
adsorption capacity (pglg)
883
Deep Hollow
Blender
Keystone
Upshore
140
262
218
87
Organic Compound: Methyl Isobutyl Ketone (MIBK) 150 546 370
300
240
105
1070
943
562
Conemough
Wellmore Cactus
Organic Compound: Butanol 496 340
Organic Compound: &Xylene 1107 1346
600
a Adsorption capacity of each variety of fly ash was determined from the breakthrough curve area under a complete exhaustion condition using an influent concentration of 30 mg/L. The above results were used to establish a correlation between the fly ash adsorption capacity and its major chemical constituents.
TABLE 7
Coefficient of Determination (R2)for Regression of Adsorption Capacity of Fly Ash versus Fly Ash Compositiona adsorption capacity residual carbon significance compound butanol
methyl isobutyl ketone o-xylene
R2
(%)b
A1203 R2
CaO RZ
0.99 0.99
1
0.28
1
0.30
0.33 0.27 0.008 0.35 0.28 0.01
0.91
1
0.52
0.42 0.37 0.01
RZ
RZ
The above results are based on the single-point sorption relationships developed from the dynamic sorption experiments. The significance levelsfor otherconstituentsof fly ash with the sorption capacity are not included because no significant association was found between the variables. Number of observations = 7. a
Because fly ash is a heterogeneous material, one of the primary objectives of this research was to explore the influence of the chemical constituents of fly ash on its adsorptive property. An attempt was made to establish a relationship between the adsorption capacity of fly ash and its major chemical constituents of aluminum oxide, calcium oxide, silica, sulfur, and carbon. Butanol, MIBK, and o-xylene were adsorbed separately in a continuous system by the followingfly ashes: Militant, Conemough, Wellmore Cactus, Deep Hollow, Blender, Keystone, and Upshore. The adsorption capacities summarized in Table 6 revealed that Wellmore Cactus fly ash was the most efficient
sorbent material of all the fly ashes used in this research. This was followed by Conemough, Blender, Keystone, Militant, Deep Hollow, and the least efficient sorbent material, Upshore fly ash. A regression analysis was performed to establish a correlation between the adsorption capacity of the fly ash and its major chemical constituents. The results summarized in Table 7 demonstrate a strong correlation (R2 > 0.90) between the adsorption capacity and the carbon content of the fly ash for all three of the compounds tested. Except for carbon, the other major chemical constituents of fly ash did not correlate well with the adsorption capacity. These results show that the carbon content of fly ash plays a significant role during the adsorption process. The higher the carbon content, the better the removal will be. An identical trend was observed by Mott and Weber (9) and Mancy (10). Phase III: Effect of Temperature on Adsorption. One of the vital objectives of this research was to determine the predominant mechanisms and forces involved in the adsorption process. In addition to establishing the correlations based on the experiments discussed above, the thermodynamic characteristics of the adsorption process were investigated. This phase of the study explored the effect of temperature on the adsorption process. Adsorption isotherm studies were conducted at 20 and 50 "C using the following compounds: hexanol, butanol, o-xylene, ethylbenzene, MIBK, and MEK. The Freundlich isotherm parameters are presented in Table 8, along with a statistical analysis of the data. The thermodynamic properties of adsorption predict the slight decrease observed
TABLE 8
Effect of Temperature on Sorption Capacity (k) and In.,nsity (lln) Parametersa temp = 20 "C
temp = 50 "C
AH hexanol butanol o-xylene ethylbenzene m e t h y l ethyl ketone (MEK) m e t h y l isobutyl ketone (MIBK)
20.00 19.795-20.230 8.3 0.710 0.664-0.756 6.08 17.25 16.945-17.614 1.69 0.730 15.74 15.667-15.813 0.65 0.710 0.699-0.720 0.65 14.25 13.495-15.000 4.61 0.720 31.11 30.900-31.310 1.00 0.910 0.899-0.916 1.50 26.50 24.400-28.600 6.88 0.915 30.20 28.858-31.142 3.62 0.900 0.866-0.933 3.50 25.80 24.343-27.257 4.91 0.907 13.16 12.817-13.504 2.50 0.610 0.582-0.632 2.50 11.75 11.229-12.271 3.85 0.625
0.718-0.741 1.34 0.713-0.726 0.78 0.904-0.926 1.05 0.859-0.954 4.54 0.609-0.640 2.14
-0.92 -0.62 -1.00 -0.98 -0.72
20.42 20.279-20.562 0.97 0.650 0.639-0.670 3.35 18.20 17.192-19.208 4.82 0.670 0.625-0.715 5.87
-0.70
a Militant fly ash was used as the sorbent material during this phase of the study. Units of kcorrespond t o mg/L and pglg for aqueous-phase and solid-phase concentrations, respectively. Cheremisinoff (23).Coefficient of variation. e AHvalues were computed based on two temperatures (20and 50 "0.
VOL. 29, NO. 9,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY rn 2249
in the adsorption capacityparameters (k)as the temperature increases. This trend indicates that the adsorption of organic compounds from an aqueous solution onto fly ash is an exothermic process. The differential heat of adsorption (AH) was determined using different values of k, the Freundlich isotherm capacity parameter, with their corresponding temperatures in the Van? Hoff-Arrhenius equation, as follows:
where AH is the differential heat of adsorption, TI and TZ are the temperature in degrees Kelvin at conditions 1 and 2, kl and k2 are the adsorption capacity factor at conditions 1 and 2, and R is the universal gas constant, 1.98 cal/mol. The differential heat of adsorption (AH) values are summarized in Table 8. The negative value of AH obtained in each case confirms that the adsorption of organic compounds onto fly ash is an exothermic process. The values, varying between 0.60 and 1.00 kcal/mol, are within the expectedrangefor physical adsorption (16-18). Thevalues of differential heat of adsorption computed here have no statistical significance because onlytwo temperatures were investigated.
Intermolecular Forces and Msorptien Mechanism The relatively low values for the heat of adsorption determined in this research are characteristic of the weaker physical adsorption processes, and therefore chemisorption of these compounds onto fly ash can be ruled out. The forces involved in physical adsorption include Van der Waals-London (dispersion) forces, predominantly polarizability and dipole interactions. The Van der Waals-London forces are comprised of three distinct interactions: (a) induced dipole-induced dipole interaction, (b) dipole-dipole interaction, and (c) dipole-induced dipole interaction. The first interaction (induced dipole-induced dipole) is known as the London or “dispersion”force. The London interaction is present between all atoms and molecules in close proximity. The forces originate from the oscillating motion of electrons in their orbitals around the atoms/ molecules, which results in an instantaneous dipole. This instantaneous dipole of one atom/molecule will induce a synchronous dipole in a nearby atomlmolecule, and an attractive energy will result. This energy E(r) between two atoms is calculated by the following equation (191:
E(r) = -
3
~
2
2r6[1/hv,+ l/hv,l
(4)
wherep~andpz are the dipole moment of molecules 1 and 2, respectively; EO is the permittivity of vacuum: E is the permittivity of medium; KB is the Boltzmann constant; T is the temperature; and r is the distance between the two molecules. The dipole-induced dipole interaction results when a molecule with a permanent dipole moment is in the vicinity of another molecule, which may itself be polar or nonpolar. The average interaction energy is (20)
(The symbols are as previously defined.) As discussed in Results and Discussion, the adsorption capacity factor (k)correlated very well with the solute’s polarizability (a),but correlated poorly with the dipole moment (4values. Since a andp are the only two sorbate properties that enter into the fundamental equations of intermolecular forces, it is apparent that the induced dipole-induced dipole or London (dispersion) interaction and the dipole-induced dipole interaction predominate over the dipole-dipole interaction. The above equations show that in order for the dipole-dipole interaction forces to predominate the adsorption system, a strong correlation of adsorption capacity with dipole moment is required; however, the results of this research revealed that the dipole moment always demonstrated poor correlation with the adsorption capacity (as compared to the solutes’ polarizability). Therefore, it is highly likely that the influence of dipole-dipole intermolecular forces is relatively insignificant during the adsorption of organic compounds onto fly ash as compared to induction forces. The adsorption of organic compounds onto fly ash is believed to occur principally via the weak induction (dispersion) forces of London and/or dipole-induced dipole interaction. However, since the London (dispersion) force is typicallymuch stronger than dipole-induced dipole interaction (211,it can be surmised that the dispersion force is primarily involved in the adsorption process.
Acknowledgments We particularly acknowledge Dr. John W. Liskowitz, Dr. Mung Sheih, and Dr. Pai Yuan Horng of the Department of Civil and Environmental Engineering at the New Jersey Institute of Technology and Walter Zabban and Carla Robinson of Chester Environmental for their input to this work. This work was supported by the Hazardous Substance Management Research Center, Newark, NJ, and was performed at the Water Pollution Control Research Laboratory at the New Jersey Institute of Technology, Newark, NJ.
where a is the polarizability of atoms 1 and 2, respectively; r is the distance between the two atoms; V I and Y Z are the oscillating frequency of the electron-nucleus system for atoms 1 and 2, respectively: and h is the Plank constant. The dipole-dipole interaction results when two polar molecules approach each other. The average interaction energy between two molecules is calculated by the following equation (20): (5)
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Received for review October 31, 1994. Revised manuscript received May 12, 1995. Accepted June I , 1995.@
ES940678C @Abstractpublished in Advance ACS Abstracts, July 15, 1995
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