Environ. Sci. Technol. 1994, 28, 466-473
Sorption of Phenol by Selected Biopolymers: Isotherms, Energetics, and Polarity Baoshan Xlng, William B. McGIII,' and Marvin J. Dudas Department of Soil Science, University of Alberta, Edmonton, Alberta, Canada T6G 2E3
Yadollah Maham and Loren Hepler Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
The behavior of phenol in the terrestrial environment is strongly regulated by its reaction with soil components. We report here on the uptake of phenol by soil minerals (goethite, kaolinite, and montmorillonite) and by organics that may occur naturally in or be added to soil (two lignins, chitin, cellulose, collagen, and activated carbon). Our objectives were to determine the energetics and capacity for their uptake of phenol using batch equilibration, calorimetry, and CPMAS 13C NMR and to evaluate the relation of organic carbon referenced sorption coefficient (KO,) with the polarity of biopolymers. The biopolymers sorbed 2-45-fold more phenol than did the minerals. The Kd for phenol uptake by lignins with high aromaticity and low polarity were 4-6.5-fold higher than that for chitin and 13-22-fold higher than that for cellulose. Energy released during the formation of a putative donor-acceptor complex on activated carbon was about 30 kJ/mol. Energy released for sorption of phenol by biopolymers ranged from 5 to 20 kJ/mol. The KO, of phenol decreased with increasing polarity: (N O)/C of the biopolymers. We infer that the KO,of organic compounds may not be adequately predicted from their K , without considering the nature of organic matter.
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Introduction
Phenol and its derivatives are the basic structural unit for a wide variety of synthetic organics including many pesticides. Therefore, phenol has been introduced into the environment including soils, through the application of pesticides, accidental spills, or unintentional release associated with manufacturing processes and waste disposal. Phenol has been listed as a priority pollutant by the US.Environmental Protection Agency ( I ) . Attenuation of pollutants by soils, either through decomposition or retention by the solid phase prevents transport. Retention by the solid phase, however, is the key process in soils and sediments because it regulates both leaching and biodegradation. Several terms are used to describe categories of mechanisms for retention by the solid phase. In this paper, we use sorption as a generic term for uptake of a solute, for which we use the generic term sorbate, without reference to a specific mechanism. Similarly, we use sorbent as the generic term for solid materials onto which sorption occurs without connotation of uptake mechanism. In addition, we refer to adsorption as surface accumulation of a solute by physical or chemical bonds, in contrast to partition or partitioning which we use to denote uptake of a solute into a network or matrix of organic solids by forces common to solution such as van der Waals forces (2). ~
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* Corresponding author; Fax:
(403) 492-1767; e-mail: wmcgill@
vm.ucs.ua1berta.ca. 466
Environ. Sci. Technol., Voi. 28, No. 3, 1994
Adsorption of phenol onto montmorillonite from aqueous suspension (31, gaseous phase ( 4 ) ,or organic solvents (5) has been reported. From Zhang et ala's work (3), adsorption of phenol onto montmorillonite under aqueous conditions is expected to be low, with a Kd of about 1.0 mL/g. Adsorption of phenol by montmorillonite was increased, however, by treatment with hexadecyltrimethylammonium (HDTMA) (6). Similarly, no adsorption of naphthalene or anthracene by montmorillonite and kaolinite was observed in aqueous systems (7). The sorption coefficient (Kd) for phenol by soils has been reported to increase with increasing amount of organic matter (8,9). Consequently, soil organic matter may be the dominant sorbent for phenol in soils. On the basis of evidence such as linear isotherms, low heat released, and the absence of competition among organic chemicals (2), sorption of organic contaminants by soil organic matter has been reported as primarily through partitioning. Partitioning of organic chemicals is frequently described by the following equation (10-12): log KO,= a log KO,(or S ) + b where K , (mL/g) is the organic carbon referenced partition coefficient, KO, is the octanol-water distribution coefficient, and S (mg/L) is solubility in water, a and b are empirical constants. Although Ko,-Ko, or Ko,-S relationships account for the quantity of organic matter and the nature of sorbate well, they ignore polarity, structure, or molecular size of the organic sorbent. The KO, of benzene and carbon tetrachloride decreased with the ratio of polar groups to nonpolar groups of organic matter (131, and values of KO,are higher for shales than for surface soils (14,15),leading to the hypothesis that KO,is a joint function of the nature of both the sorbent and the sorbate. The use of activated carbon for the removal of phenol from aqueous systems has grown over the past decade. However, there is only limited information available on the energetics of sorption of organic chemicals by activated carbon. Accordingly, the objectives of this research are (1)to determine the contribution of selected minerals and biopolymers to the uptake of phenol; (2) to determine the relation of polarity of biopolymers to the sorption of phenol; and (3) to evaluate the mechanisms and energetics of the uptake of phenol by selected biopolymers and activated carbon. Materials and Methods
Lignin (organosolv, lot no. = 04525BY), lignin (alkali, lot no. = 04425BY), and cellulose (lot no. = 00120PY) were obtained from Aldrich Chemical Co., and chitin (practical grade, lot no. = 10H7250) and collagen (type 11, lot no. = lllH7180) were from Sigma Chemical Co. These 0013-936X/94/0928-0466$04.50/0
0 1994 American Chernlcal Society
Table 1. Surface Areas, % Elemental Contents, and Ash Contents of Biopolymers and Activated Carbon (Oven-Dry Basis) biopolymers
surface area (m2/g)
C
H
N
0
ash
lignin (0)" lignin (A)b chitin cellulose collagen carbon
1.8 5.2 5.1 2.4 NDC 985
65.8 57.1 44.6 44.4 56.0 91.3
5.37 5.03 6.82 6.20 8.48 0.42
0.08 0.09 6.74 0.0 12.5 0.54
28.7 30.6 38.1 49.4 23.0 4.24
0.0 7.1 3.7 0.0 0.0 3.2
Lignin (organosolv). b Lignin (alkali). Surface area is less than 0.5 m2/g.
materials represent common input sources for soil organic matter and serve as model compounds for the more complex organic matter fraction of soils and sediments. Powdered activated carbon (catalog no. E345-07) was obtained from J. T. Baker Co. Air-dried samples of the biopolymers and the carbon were used without further treatment to minimize any changes in their uptake properties. Montmorillonite (Cheto, SAz-l), kaolinite (Oneal Pit, Macon, GA), and goethite (Biwabik, MN) were received from the Source Clays Repository (University of Missouri, Columbia, MO). Ca-saturated minerals were prepared by the procedure of Xing et al. (16) prior to use in sorption studies. Phenol was 99+ % analytical grade from Aldrich Chemical Co.; [l4C]phenol was purchased from Sigma Chemical Co. Elemental composition (C, H, N, 0)of the carbon and the biopolymers other than cellulose was determined by an EA 1108-elemental analyzer (Carlo Erba Instruments). The composition of cellulose was taken from the formula (C,3HloO& The BET surface areas were measured as described by Pieters and Venero (17) using an Omnisorp 360 analyzer, which uses the continuous addition of nitrogen, rather than the usual dosing procedure, for obtaining nitrogen uptake. Biopolymers were heated a t 50 "C for 4 h, and activated carbon was heated a t 300 "C for 4 h before the actual measurement. The percent ash determinations were by mass difference upon combustion a t 750 "C for 4 h (18). Characteristics of the sorbents are shown in Table 1. Isotherms for both uptake and release were obtained using batch equilibration a t pH 7 (0.002 M phosphate buffer) and background ionic strength of 0.03 M CaC12. At pH 7, phenol mainly exists in the nonionizable form (99.9%). So1ution:solid ratio was 1O:l for minerals, 20:l for biopolymers, and 40:l for activated carbon. Labeled phenol solutions were prepared by adding [14C]phenol with a specific activity of 3.33 X loll Bq mol-l to the phenol solutions (0.1-0.7 MI in a sufficient amount to produce activity of 100 Bq mL-l(-6000 cpm mL-l). Sorbents (in triplicate) were weighed into glass centrifuge tubes, which were then filled to minimal headspace with the labeled solutions. The tubes were immediately sealed with screwcaps having Teflon liners and shaken horizontally in a LAB-line Orbit Environ-shaker a t 300 rpm for 30 h a t 25 f 1 "C. A sum of M HgCl2 was used to minimize biological activity during equilibration. After centrifuging at 2000 rpm for 30 min, a 1-mL aliquot of the supernatant solution was removed for phenol analyses by liquid scintillation counting. The difference between the initial and final concentrations of phenol was attributed to uptake onto the test sorbents. Preliminary tests showed that glass
centrifuge tubes did not adsorb phenol and that uptake results from centrifuging at 4500 rpm were not significantly different from those centrifuged at 2000 rpm. Single-step desorption was used for constructing desorption (release) isotherms. After 30-h uptake and 30-min centrifugation, 2-mL aliquots were removed from the 10-mL supernatant solution, after which time, 2 mL of 0.01 M CaC12 solution was added to the tubes. After the test sorbent and solution were shaken by hand, the tubes were put back into the Environ-shaker a t 300 rpm for 30 h. A 1-mL aliquot of supernatant solution was removed for the phenol analysis after centrifugation. The rate of uptake of phenol by lignin (organosolv) was measured with initial phenol concentrations of 0.1,0.3, or 0.5 M. The so1ution:solid ratio was kept at 20:l. After being mixed for the required time at 25 f 1"C, the phenol solution was separated from the lignin by filtration with Teflon filters (All Tech) rather than centrifugation, because centrifugation takes 30 min, which cannot be properly used in kinetic calculations, especially for 10min to 1-h reaction times. Phenol in the filtered solution was analyzed using liquid scintillation counting. We did not observe sorption of phenol on Teflon filters. Heat released from phenol uptake and from wetting of sorbents was measured using a C-80 heat flow calorimeter manufactured by SETARAM of Lyon, France. Two identical calorimetric cells (about 4 mL in volume) were used for the heat measurements. One cell is for the addition of a test sorbent into the phenol solution and the other is for the addition of a test sorbent into the background solution. The two cells were equilibrated in a thermostat a t 25.0 f 0.1 "C. After thermal equilibrium was obtained, the thermostat was rotated vertically, thereby introducing the test sorbent to the solutions. The difference in heat produced in the two cells was determined from the resulting thermogram and was attributed to the heat of reaction of phenol with the test sorbent. Additional sources of heat released in the sample cell were canceled by the same sources of heat produced in the reference cell. The details of the operating procedure, calibration, and designs of cells are given by Xu (19). After heat measurement, the solution was centrifuged and analyzed for phenol concentration. Molar heat (enthalpy change), thermodynamic equilibrium constant, free energy change (AGO), and entropy change (AS") were calculated by the method outlined by Xing et al. (16). X-ray diffraction was used to test if phenol molecules in vapor could enter the interlayer space of dry montmorillonite. Uptake mechanisms were inferred from Fourier transformed infrared spectra and values of thermodynamic parameters. UV/visible spectrophotometry was used to check if biodegradation had occurred after equilibration. Periodic scans of samples both before and after experiments revealed no change in spectrum, from which we inferred that there was no biodegradation, Percentage aromaticity of biopolymers was calculated from solid-state cross-polarization and magic-angle spinning (CPMAS) 13C NMR spectra (20).
Results Reaction of Phenol with Minerals. Sorption and desorption isotherms were indistinguishable straight lines for goethite (Figure 1). The amount of phenol adsorbed onto goethite was low and the sorption coefficient (Kd) Environ. Sci. Technol., Vol. 28, No. 3, 1994
467
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phenol uptake between 20 and 28 h. For a 0.3 or 0.5 M phenol solution, lignin (organosolv) quickly aggregated into black, dense flocs upon mixing. Curvilinear isotherms of uptake and release were also observed for both collagen (shown in Figure 4 as an example) and chitin. Release curve started to separate
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from uptake curve a t equilibrium concentrations of 0.33 M for collagen and 0.45 M for chitin. Again, flocs of both biopolymers formed a t higher phenol concentrations, but the flocs were not as dense as the floc of lignin (organosolv). Both sorption isotherms were described by the Freundlich equation:
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For comparison purpose, the Kd values were calculated from the linear partition of each uptake isotherm. For collagen, the Kd is 7.3 mL/g (0.042-0.33 M; r = 0.99), and for chitin, it is 1.7 mL/g (0.092-0.36 M; r = 0.99). In contrast to lignin (organosolv), collagen, and chitin, the complete isotherms for bothlignin (alkali) and cellulose were linear (Figure 5): for lignin (alkali):
for cellulose:
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x / m = 2.0 x 10-~(&1.0 x 5.9 X 10-4(f5.8 X 104)C,
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Figure 5. Isotherms of phenol uptake by lignin (alkali), cellulose, and the activated carbon. (a) Lignin (alkali) and cellulose; (b) activated carbon. '5
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There was no major difference between uptake and release isotherms for either polymers, nor were flocs formed in the presence of phenol. The Kd is 9.8 mL/g for lignin (alkali) and 0.59 mL/g for cellulose. There was no apparent difference between isotherms of phenol adsorption onto and desorption from activated carbon (Figure 5). Adsorption of phenol onto activated carbon fits the linear form of Langmuir model [ x l m = KbC,/(l+ KC,)] better than the Freundlich model. The fit by the linear BET model yielded an unrealistic negative monolayer coverage. The value of K (related to adsorption affinity) from the Langmuir model was 9.66 and b (maximum adsorption) was 4.38 mM/g. Relation of Phenol Uptake to Polarity and Heat of Wetting of Biopolymers. Organic carbon referenced partition coefficient (Koc)was calculated using partition coefficient (Kd)divided by the proportional organic carbon content. The degree of polarity of the biopolymers was
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taken to be directly proportional to the mass ratio of elements: (N + O)/C. The ratio of (N + O)/C ranged from 0.44 to 1.1for the five biopolymers. We hypothesized that uptake would be decreased with more polar sorbents. Consistent with our hypothesis, the KO,decreased from 20 mL/g to 1.3 mL/g with increasing [(N + O)/Cl (Figure 6). Aromaticity is 54% for lignin (organosolv) and 50% for lignin (alkali) (Figure 6). Other biopolymers have no aromatic constituents. Lignins had higher phenol uptake than chitin and cellulose with zero aromaticity. Heat of wetting is here defined as the energy released when a test sorbent is mixed with 0.01 M CaCl2 aqueous solution. Wetting was exothermic as indicated by the negative values for heat of wetting (Figure 7). K , increased as wetting became less exothermic (less negative) (Figure Environ. Scl. Technol., Vol. 28, No. 3, 1994
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Figure 9. Molar heat and standard entropy change for adsorption of phenol onto the activated carbon.
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7). Heat of wetting for collagen was not plotted because the measured heat was the sum of at least two processes: wetting and expansion. The measured combined heat was about -75 J/g for collagen. Energetics of Uptake of Phenol by Lignins and Activated Carbon. Uptake of phenol by lignins and activated carbon was exothermic (Figures 8 and 9). The energy released during uptake ranged from -12 to -5.7 kJ/mol for lignin (organosolv);from -22 to -5.2 kJ/mol for lignin (alkali); and from -28 to-17 kJ/mol for the activated carbon. Because of the small amount of phenol sorbed onto cellulose and chitin, the heat released from uptake was below the detection limit of the calorimeter. For uptake of phenol by collagen, the thermogram initially displayed an exothermic phase and later displayed an endothermic phase. We could not confirm the exact time when the endothermic process started, and likely the two processes overlapped for a period. Consequently, molar heat is not reported for the uptake of phenol by collagen. Using the method outlined by Biggar and Chueng (21) and Xing et al. (16), the thermodynamic equilibrium constant for the adsorption of phenol onto activated carbon was 146 and AGO was -12.3 kJ/mol. Using the value of AGO and molar heat, calculated entropy change (ASo) 470
Envlron. Sci. Technol., Vol. 28, No. 3, 1994
ranged from-56.2 to-16.5 J mol-l K-l (Figure9). Because the surface area of the biopolymers varied with their change in geometry during the experiments, AGO and ASo were not calculated for the biopolymers. FTIR Spectra. The five adjacent H out-of-plane bending bands of phenol (750 cm-1) were observed on the FTIR spectrum of lignin (organosolv) with phenol (Figure 10). Intensity of aromatic ring C=C (1600 cm-l), C-C (1500 cm-l), and C-0 (1220 cm-l) stretching bands was higher for the spectrum of lignin with phenol when compared to the spectrum of lignin alone. Although intensity increased, there was no shift of major bands when the two spectra of lignin were compared with or without phenol. Similar FTIR spectra were observed for lignin (alkali) with or without phenol. Due to the limited quantity of phenol sorbed by chitin or cellulose, there was no difference between the spectra of the test sorbents with or without phenol. Spectra of lignin (alkali), collagen, cellulose, and chitin are not presented here. The 1580-cm-l band was evident on the spectrum of activated carbon and shifted to a higher wavenumber (1620 cm-1) on the spectrum of the carbon with phenol (Figure 10). Also, the five adjacent H out-of-plane bending bands of phenol (about 750 cm-l) were observed on the spectrum. A broad band at the 1100 cm-l region showed up on both spectra.
Discussion Goethite had relatively low adsorption of phenol (slope of linear portion of isotherm = 0.3 mL/g). The similar adsorption and desorption isotherms are taken as evidence that adsorption of phenol onto goethite is a reversible process at the range of phenol concentration used in this study and that an inner-sphere coordination, as suggested for chlorophenols and iron oxides (221, may not form between phenol and the surface of goethite. Adsorption of phenol onto montmorillonite and kaolinite in aqueous phase may be prevented by strong polar interactions of water with mineral surfaces. Water is more polar than phenol, and thus water competes favorably for adsorption to mineral surfaces, especially in the presence of di- or trivalent exchangeable cations. In the study on the kinetics of phenol adsorption and desorption on an
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1200
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2000
1600
1200
800
400
WAVENUMBERS
Flgure 10. FTIR spectra of lignin (organosolv) and the carbon. (a) Lignin with or without phenol; (b) carbon with or without phenol.
organo clay, Zhang and Sparks (6) concluded that there was little, if any, adsorption of phenol on the montmorillonite without HDTMA treatment. After peroxide treatment, lake sediments were unable to sorb unsubstituted phenol (23). Zhang et al. (3) reported that phenol was adsorbed onto Ca-montmorillonite and that & was about 1.0 mL/g. They did not use CaClz as a background electrolyte in their study. Background ionic strength of 0.03 M in our study may reduce the number of sorption sites on montmorillonite through flocculation and the formation of microstructures involving edge-to-face interactions (card-house effect). We have previously shown that the amount of pentachlorophenol adsorbed onto Ca-montmorillonite varied with the concentration of Ca in solution (16). In the absence of water, however, phenol was adsorbed in the interlayer space of montmorillonite at 50 "C and was not removed by heating a t 105 "C for 24 h. A basal spacing increase of 0.5 nm after treatment of montmorillonite with phenol vapor suggests that phenol was laterally adsorbed in the interlayer space. Adsorption of
phenol vapor by minerals was also reported by Sawhney et al. (24). Isotherm shape and uptake capacity for an organic chemical depend on the properties of test sorbents. Lignin (organosolv), collagen, and chitin had similar types of isotherms for phenol: nonlinear and convex with respect to the abscissa as phenol concentration increases. One interpretation of convex isotherms is that phenol may be present as a discrete phase trapped within the biopolymer matrix at high phenol concentrations (3). The amount of such phenol increased as the concentration of phenol increased in the external solution. The net result would be an upward curvature to the isotherm. An alternative explanation is that uptake is by partition, and solubility parameters for the polymers are altered following dissolution of the sorbate into these polymers (25). The difference between uptake and release isotherms (Figure 4)were also associated with the formation of flocs in all experiments. Although we are confident that negligible polymer entered and remained in solution at the end of each equilibrium, we cannot say with absolute certainty Environ. Sci. Technol., Vol. 28, No. 3, 1994
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that none was in solution and, consequently, cannot distinguish among these two alternative hypotheses. Although isotherm shapes were similar for lignin (organosolv), collagen, and chitin, the amount of phenol uptake to these three sorbents was different. The Kd of collagen with a surface area of
