Environ. Sci. Technol. 1994, 28, 1929-1933
Cross-Correlation of Polarity Curves To Predict Partition Coefficients of Nonionic Organic Contaminants Baoshan Xlng, Wllliam B. McGiII,' and Marvin J. Dudas
Department of Soil Science, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 Accurate sorption coefficients are important for models to predict fate and movement of organic chemicals in soils or sediments. We report here on a new method for predicting partition coefficients (Kd)of nonionic chemicals onto soils and geological materials. It includes properties of sorbents and of sorbates, thereby yielding more accurate organic carbon-normalized partition coefficients (K,) than a single value derived from an octanol-water partition coefficient (Kc,,). A regression of log KO,on log KO, and polarity index (PI: [(O N)/Cl) was established using benzene, toluene, and o-xylene as sorbates and using model organic polymers as sorbents. KO,data for sorption of o-xylene onto soils and geological materials were used in arearranged form of the regression equation to determine effective polarity of the soils. Effective polarity was then used together with the respective KO, values to predict K, of benzene and toluene on each soil. This method provides a range of KO,values that are soil sensitive. The effective polarity provides a new parameter to describe soil organic sorbents and includes any influence that mineral components may exert on the sorptive properties of organic matter.
+
Introduction The organic phase of soils and sediments is the main sorbent of nonionic organic sorbates from aqueous solutions (1-3). Linear isotherms, low heat release, and the absence of competition among sorbates suggest that sorption by soils and sediments is through partitioning ( I ) . It is characterized by the linear relation ( I , 4 ) X/m
= KdCe
(1)
where x is the quantity of sorbate (pmol), m is the mass of sorbent (g), C, is the equilibrium concentration of the sorbate (pmol/mL), and, Kd is the partition coefficient (mL/g) or
where KO,is the carbon-normalized partition coefficient (mL/g) and foc is the mass fraction of organic carbon. Readily available properties of the sorbate have been used to predict KO,(1, 2, 5 , 6 ) using linear equations of the form: log KO,= a
+ b log KO,
(3)
log KO,= c
+ d log S,
(4)
where KO,is the octanol-water partition coefficient, S, is the solubility in water, and a-d are empirical constants. Values of KO,or S, for each sorbate may be measured or derived from quantitative structure-activity relations (e.g.,
* Corresponding author. Telephone:
(403) 492-5397; Fax: (403) 492-1767; E-mail address:
[email protected]. 0013-936X/94/0928-1Q29$04.50/0
0 1994 American Chemical Society
Table 1, Properties (%) of Model Organic Sorbents Used To Establish K,-Polarity Curves
biopolymers
C
H
N
0
ash
lignin 0 lignin A humicacidB humic acidD chitin cellulose
65.8 57.1 53.0 50.5 44.6 44.4
5.37 5.03 3.96 4.16 6.82 6.20
0.08 0.09 3.54 4.25 6.74 0.0
28.7 30.6 38.6 38.4 38.1 49.4
0.0 7.1 0.6 2.8
3.7 0.0
refs 7 and 8). Equations 3 and 4 imply uniformity of composition and sorptive characteristics of soils (sorbents) regardless of age, origin, and mode or extent of transformation. In contrast, properties of the sorbent, in addition to mass fraction of organic C, are reported to influence sorption (9-15). For example, the KO,of a-naphthol (14) and the affinity of pyrene for dissolved humic acid increased with increasing aromaticity (15). Consequently, the assumption of uniformity of sorbents in eqs 3 and 4 may lead to errors in predictions of sorption by soils and in retardation coefficients used in transport and fate models. Can sorption by soils be predicted more accurately? Rutherford et al. (11) using benzene and carbon tetrachloride and Xing et al. using a-naphthol (14)and benzene, toluene, and xylene (BTX) (16)reported that K, decreased with increasing mass ratio [(O N)/Cl of nonprotein organic sorbents. They proposed the term polarity index (PI)for this mass ratio, recognizingthat it ignores polarity that may arise from configuration and structure. Consequently, the partition coefficient of a sorbate in a soil might be determined from the relationship between K , and the polarity of the soil organic phase. To do so requires that the polarity of the participating organic sorbents in soils or geologic materials (effective polarity) be known or measurable. The polarity of extracted soil organic phase cannot be used because extraction is incomplete, and we expect the effective polarity of the soil organic phase is influenced by its association with soil minerals. Nor can effective polarity be calculated from total 0,N, and organic C contents, because soil minerals contain large quantities of 0 and in some cases N. The effective polarity of soils or geologic materials must therefore be measured on unaltered samples. We have reportedK,-polarity curves for the sorption of BTX by several biopolymers (16). We now extend the work to soils and geologic materials of unknown polarity and report on a method to determine the effective polarity of soils for use in predicting K,.
+
Materials and Methods Commercial biopolymers (lignins, chitin, cellulose) and humic acids extracted from soils (Table 1) were used to develop the K,-polarity relationships. Lignin organosolv, Environ. Sci. Technol., Voi. 28, No. 11, 1994
1929
Table 2. Properties of Soils and of Their Organic Phases
percent of C as C carboxyl+ pHin clay sand ash soil (%) aromatic aliphatic carbonyl CaClt (%) (%) (%) 1 54.3 2 32.2
56 20 28 33 ndb
29 70 58 50 nd
15
4.6 naa na 16 5.4 na na 41 3 7.38 14 4.9 25 9 na 4 4.53 6.6 22 17 16 na 5 2.55 nd 5.5 10 40 na na = not applicable. nd = not determined due to low signal/ noise ratio of CPMAS spectra. 10
*
Sorbent: Humic Acid 6 ;,f
120
1930 Environ. Scl. Technol., Vol. 28, No. 11, 1994
/
h
0
‘0 E a
loo-:
Em
/?
V/FI
15.42
? = 0.994
= 0,998/
9 = 0.997
C
6
K,
K, = 34.13 K, = 78.44
.0 .Id
.Id
lignin alkali, and cellulose were obtained from Aldrich Chemical Co., and chitin was from Sigma Chemical Co. Humic acids were extracted from A horizons of two Chernozemic soils following Schnitzer et al. (17). Humic acid B was from a Black Chernozem near Millet (53O06’ N, 113’28’ W), Alberta, Canada; humic acid D was from a Dark Brown Chernozem near Consort (52’01’N, 110O46’ W). Mass percent of C, H, N, and 0 for cellulose was calculated from the formula (CeH100&; for other materials, it was determined by an EA 1108 elemental analyzer (Carlo Erba Instruments). The percent ash was determined by mass loss after combustion at 750 “ C for 4 h. We determined effective polarity on samples of five soils or geological materials, hereafter called “soils” (Table 2). They were air-dried and ground to pass a 100-pm sieve before analyses and sorption experiments. The samples included one Cretaceous aged soft coal mixed with weathered shale (soil 1)and a sedge peat sample that was well humified but of recent (a few hundred years) origin (soil 2). Soils 1 and 2 were sampled near Edmonton, Alberta (53O33’N, 113’28’W). The three remaining soils were a White Clay soil formed on Quaternary aged sediments (soil 3; 45O33’ N, 131O57’ W) from Heilongjiang Province, the People’s Republic of China (18), a Black Chernozemic (soil 4; 53O06’ N, 113’28’ W), and a Brown Chernozemic(soil 5; 4g023’N, 111O15’W)formedonglacial deposits from Alberta, Canada (19). These five soils varied in age, diagenesis, kind, and amount of organic matter. The solid-state 13C NMR spectra were obtained in the Department of Chemistry, University of Alberta. The NMR spectrometer is a Brucker AM 300 instrument with cross-polarization (CP, contact time of 2 ms), magic-angle spinning (MAS, spinning rate of 4.5 kHz), and an H P WP 73A probe. Percentage of aromatic C (i.e., aromaticity) for the soil materials was calculated from the CPMAS I3C NMR spectra using the ratio of peak area of 106-165 ppm to the total area of 0-230 ppm. Similarly, percentages of aliphatic C (0-106 ppm), carboxyl C (165-190 ppm), and carbonyl C (190-230 ppm) were calculated from their respective ratios (17, 20). Benzene, toluene, and xylene isomers comprise 55,15, and IO%, respectively, of light oil and are possibly carcinogenic,teratogenic, and mutagenic (21). Accidental introductions of BTX into the environment contribute to concerns about their fate in soils, sediments, and groundwater. Therefore, benzene, toluene, and o-xylene were selected as sorbates in this study and were purchased from BDH Chemical Co. Sorption isotherms were obtained using batch equilibration. The background ionic strength was 0.03 M CaC12, with a 105M HgC12 solution to minimize biological activity. Equilibration pH was maintained near 6 for all sorbents
I
= 0.53
60-40-1
n 2 20-5
Benzene
Equilibrium Concentration of Sorbate (pmol/ml) Figure 1. Linear isotherms of BTX for humic acid 6 .
except for humic acid, which was maintained at a pH of 4 to avoid dissolution. The so1id:solutionratio varied from 1:25 to 1:200 to ensure 2 5 4 0 % uptake by each sorbent. Sorbents (in triplicate) were weighed into 120-mLWheaton serum bottles containing several glass beads (3 mm in diameter) to facilitate mixing between sorbent and solution. The bottles were filled to minimal headspace (115 mL) with sorbate solution, sealed with a crimped Teflonlined cap, and equilibrated for 48 h in a reciprocal shaker at 23 f 1 OC. We confirmed that the bottle and glass beads did not sorb the chemicals and that equilibrium was reached in 36 h except for lignin (organosolv), which required about 10 days (14). Consequently, a 12-day equilibration period was employed for lignin (organosolv). After equilibration, bottles were centrifuged at 4800g for 25 min, and a l-mL aliquot of supernatant solution was taken by syringe for gas chromatography (GC) analysis. We detected no biodegradation products of the sorbates in any of the chromatograms. The differences between initial and final concentrations of sorbates were attributed to sorption onto the sorbents. Sorbate analyses used an HP 5890 Series I1 GC equipped with a Tekmar purge-trap system, an HP-17 (cross-linked 50% phenylmethyl silicone) 10 m X 0.53 mm capillary column, and a flame ionization detector. Purge-trap conditions were set as follows: solution purge for 5 min, dry purge for 2 min, trap preheated to 175OC and desorbed at 180 “C for 2 min into the GC column, and trap baked at 225 “C for 4 min after desorption. The GC was interfaced with an H P 3365 Chem Station, and peak area ratios were converted to mole ratios for quantitative determination using internal standard calibration curves constructed from known mixtures of sorbates with toluene or o-xylene as internal standards. Results and Discussion
Sorption of BTX by all the biopolymers and humic acids followed linear isotherms; sorption by humic acid B is shown as an example in Figure 1. Linear isotherms suggest partitioning as a sorption mechanism ( I ) , and are consistent with the literature (1,22,23). For all three sorbates,
600
4
L
\
>
\
l o o --
E
Lignin (0)
a
BO-:
.0 -Id
5
\
Lignin (A)
60-r
3 0-
2
\
m
40-1
c . '
Q
=
Humic Acids 0
rn
Toluene Benzene
20-1 0
Chitin
--
Cellulose
Equilibrium concentration of o-Xylene (pmol/ml)
I I I I I I I . l l l l l . I I I I . I I I I I . ~ . ~ . . ~ . ~ ~ .
Flgure 4. Isotherms of &xylene for the five soils varying in properties of the organic phase. ~~
Table 3. KO,and Polarity Index for Five Soils and Estimated KO,Values Derived from Published K,-KOw Relations hips
sorbent n
.-3 -Id
120
A
w
I
= 0.322
K, = 27.5, ,f
a a,
&
m
K,=
.L
Q
3
9.66, f,=
0.0738
40 K,= 6.80,,f = 0.0453 U K.~ . = 27:1, f,, = 0.0255
I
C
1
0
.
j
~
'
"
1
"
'
~
2
'
"
'
~
3
'
'
'
*I " " [
4
66 42 55 69 51
soil 1 soil 2 soil 3 soil 4 soil 5
soil 3 soil 4 soil 5
E
measured K, (mL/g) benzene toluene o-xylene
5
153 85 131 150 107
equation log Komb= 0.52 log KO, + 0.64 log Komb= 0.904 log KO, - 0.779 log K, = 1.0 log KO, - 0.21 log K, = log KO, - 0.317 log KO, values used above
polarityindex4
409 209 228 285 225
0.51 0.63 0.61 0.57 0.62
estimated K , (mL/g) benzene toluene o-xylene ref 96 24 83 65 2.13
188 77 302 236 2.69
326 201 870 680 3.15
6 1 2 5 25
a Calculated from eq 6 using K, values for the sorption of o-xylene 1.72Kom. by each soil. K,
*
Equilibrium Concentration of Toluene (!.tmol/ml) Figure 3. Isotherms of toluene for the five soils varying in properties of the organic phase.
the Kd values decreased in order of lignin 0 > lignin A > humic acid B > humic acid D > chitin > cellulose (data not shown). However, the KO,values of BTX for these sorbents decreased with increasing polarity of sorbents (Figure 2), which agrees with the work by Rutherford et al. (11)and our work with phenol and a-naphthol (10,14). Decreasing K,,, with an increasing ratio of [ ( O + N)/Cl suggests that the KO,of soils might vary if they differ in age and diagenesis of the organic phase. Sorption isotherms of BTX for the five soils were linear, and Kd decreased as their foc decreased (Table 2; Figures 3 and 4). Normalization to organic C reduced variability between sorbents, but the KO,of BTX for the five soils still varied by factors of about 2 (Table 3). Rutherford et al. (11) analyzed KO,values for many diverse soils and reported KO,to vary by a factor of 3. Similarly, Grathwohl (12)reported that Ko,values for the sorption of chlorinated aliphatic hydrocarbons by organic matter varied by a factor of 20. Consequently, in addition to being a function of
the sorbate, KO,is also a function of the sorbent as seen here with biopolymers, humic acids, and soils. Several equations are available to estimate KO,from KO,. Depending on which of the several estimates of KO, (24, 25) are used, they yield one value for each sorbate regardless of sorbent, and it is instructive to ask how close their predictions are to the values reported here. For each sorbate, the estimates were both above and below our observations and varied among equations by a factor of 4 (Table 3). Given that KO,varies with [ ( O + N)/Cl and that published Ko,-Koc equations yield diverse results, we sought to calculate KO,by combining properties of the sorbent, such as KO,, with those of the sorbate, such as polarity index. Attempts to include properties of the soil organic phase in predictive equations have been made in the past. The oxygen content of organic materials was one of the variables in equations developed by Garbarini and Lion (26) and Grathwohl(12). We have chosen to work with whole soils however. We describe below how we obtained the effective polarity of soils and estimated KO,from a combination of PI of the sorbent and KO, of the sorbate. Envlron. Sci. Technol., Vol. 28, No. 11, 1994
1931
100
50
0
Benzene
Toluene
Flpui 5. Values of K, for toluene and benzene as observed or as predicted using eq 5. The values of PI that were used In eq yere V e d from eq 6 using K.. values for the sorptlon of *xylene by each soil (Table3). Panel A shows the correlation of observed K.. values with those
predicted by eq 5. The 1:i line is an eMCt correspondence of observed and predicted values. and 6 is the simple linear cwrelatlon coefficient between observed and Predicted values. Panel B shows observed values for benzene and toluene separately (obs K..) in comparison with those predicted by eq 5 (pred K..) and by two equations from the literature (refs 1 and 6).
The following equation was fitted to values for log K , and PI from Figure 2 and K , from Table 3: log K , = 1.83(&0.396)+ 0.625(i0.131) X log KO,- 2.34(f0.229)PI (5) (r2= 0.895; p
< 0.0001)
Equation 5 yields a value for K , provided PI and K , are known. Values for P I are not known for soils, so eq 5 was rearranged (eq 6) and used to calculate PI (Table 3) for each soil using the data from Table 3 for K , and KOwof o-xylene: P I = (1.83 + 0.625 log K , - log K,)/2.34
(6)
We used measured K , values of o-xylene in eq 6 to obtain the PI of each soil because o-xylene has the highest K,and is the most sensitive to changes in sorbent polarity among the three chemicals (Figure 2). We hypothesized that the K , for benzene and toluene oneachsoilshouldbepredictedfromtheeffectivepolarity of that soil using eq 5. Predicted values for K , using the proposed cross-correlation method were closer ta the observedvalues (r2= 0 . 8 4 ; ~< 0.000178) than are obtained withequations that includeonlyK, (Figure 5). Therefore, it appears that K , obtained using one sorbate when 1932 Enviran. Sd. Technol.. Vol. 28. NO. 11, 1994
coupled with K,-polarity curves (eq 6)provides a soil's effective polarity. This estimate of its effective polarity and published values of KO,can be used in eq 5 to predict the K , values for groups of related nonionic sorbates on it. Can effective polarity be obtained independently by using a soil parameter that contributes to or correlates with it? Our work with a-naphthol suggests that K , is related to polarity and to aromaticity (14). Both polarity and proportion of aromatic C varied among the soils used (Tables 2 and 3). They are strongly correlated, such that PI = 0.702(*0.0166) - 0.00353(40.00045)AR
(7)
(r2= 0.969; p < 0.015862) where AR = percent of C present as aromatic C as measured using CPMAS I3C NMR. The sample size is small, and more work is required to confirm this relationship, but it suggests that the aromaticity either influences sorption or is a surrogate for soil effective polarity. There are two limitations associated with the proposed cross-correlationmethod. First, a polarity curve is required for each organic chemical. Second, it is not appropriate for free flexible sorbents that may reorganize to change sorptive behavior during sorption experiments (16). We have presented a method and two equations for relating the K , of nonionic organic sorbates and the
polarity index of sorbents to Koc. Equations 5 and 6 should not be extrapolated beyond BTX without further testing. Future studies should add organic sorbents with lower polarity than lignins to the KO,-polarity curves to accommodate a wider range of soil and sediment samples. A wider range of soils and sorbents should also be used to test the cross-correlation method. Acknowledgments
The Institute for Chemical Science and Technology (ICST) and the Natural Sciences and Engineering Research Council of Canada (NSERC) provided financial support for this research. The Alberta Heritage Scholarship Fund provided a Queen Elizabeth I1 Doctoral Fellowship in Environmental Studies to B.X. We appreciate the interest, comments, and support provided by Dr. S. A. Abboud and Dr. L. J. Lawlor. Author Supplied Registry Numbers: Benzene, 7143-2; toluene, 108-88-3; o-xylene, 95-47-6; lignin (organosolv), 8068-03-9;lignin (alkali), 8068-05-1;cellulose, 900434-6; chitin, 1398-61-4. Literature Cited Chiou. C. T. In Reactions and Movement of Organic Chemicals in Soils; Sawhney, B. L., Brown, K.,’Eds: Soil Science Society of America: Madison, WI, 1989; Chapter i
1.
Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979,13, 241-248. Sun, S.; Boyd, S. A. J. Environ. Qual. 1991, 20, 557-571. Xing, B. Ph.D. Dissertation, Department of Soil Science, University of Alberta, Edmonton, 1994. Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Enuiron. Sci. Technol. 1980, 14, 1524-1528. Briggs, G. G. J. Agric. Food Chem. 1981, 29, 1050-1059. Hansch, C.; Leo, A. Substituent constants for correlation analysis in chemistry and biology;John Wiley& Sons: New York, 1979. Warne, M. St. J.; Connell, D. W.; Hawker, D. W.; Schuiirmann, G. Chemosphere 1990,21, 877-888. Xing, B.; McGill, W. B.; Dudas, M. J.; Maham, Y.; Hepler, L. G. Preprints; 205th ACS Conference, Division of
Environmental Chemistry; American Chemical Society: Washington, DC, 1993; pp 33, 57-60. (10) Xing, B.; McGill, W. B.; Dudas, M. J.; Maham, Y.; Hepler, L. G. Environ. Sci. Technol. 1994, 28, 466-473. (11) Rutherford, D. W.; Chiou, C. T.; Kile, D. E. Environ. Sci. Technol. 1992,26, 336-340. (12) Grathwohl, P. Environ. Sci. Technol. 1990,24,1687-1693. (13) Gerstl, Z.J. Contam. Hydrol. 1990, 6, 357-375. (14) Xing, B.; McGill, W. B.; Dudas, M. J. Chemosphere 1994, 28, 145-153. (15) Gauthier, T. D.; Seitz, W. R.; Grant, C. L. Environ. Sci. Technol. 1987,21,243-248. (16) Xing, B.; McGill, W. B.; Dudas, M. J. Can. J. Soil Sci., in press. (17) Schnitzer, M.; Kodama, H.; Ripmeester, J. A. Soil Sci. SOC. Am. J. 1991,55, 745-750. (18) Xing, B.; Dudas, M. J. Ceoderma 1992,54, 189-211. (19) Gajdostik, A. B. M.Sc. Thesis, Department of Soil Science, University of Alberta, Edmonton, 1992. (20) Wilson, M. A. In Soil analysis: Modern instrumental techniques, 2nd ed.; Smith, K. A., Ed.; Marcel Dekker Inc.: New York, 1991; Chapter 14. (21) Buikema, A. L.; Hendricks, A. C. Benzene, xylene and toluene in aquatic systems: a review;American Petroleum Institute: Washington, DC, 1980. (22) Weber, J. B.; Miller, C. T. In Reactions and Movement of Organic Chemicals in Soils; Sawhney, B. L., Brown, K., Eds.; Soil Science Society of America: Madison, WI, 1989; Chapter 12. (23) Brown, D. s.;Flagg, E. W. J. Environ. Qual. 1981,10,382386. (24) Handbook of RCRA ground-water monitoring constituents: Chemical bphysicalproperties;U.S. Environmental Protection Agency, NTIS: Springfield, Sep 1992; 40 CFR Part 264, Appendix IX. (25) Mackay, D.; Shiu, W. Y.; Ma, K. C. Illustrated handbook of physical-chemical properties and environmental fate for organic chemicals;Lewis Publishers: Boca Raton, 1992; Vol. I, p 141. (26) Garbarini, D. R.; Lion, L. W. Environ. Sci. Technol. 1986, 20,1263-1269. I .
Received for review February 8, 1994. Revised manuscript received June 24,1994. Accepted June 28, 1994.’ Abstract published in Advance ACS Abstracts,August 1,1994.
Environ. Sci. Technol., Vol. 28, No. 11, 1994 I833
