Sorption of Polycyclic Aromatic Compounds to Humic Acid As Studied

Environ. Sci. Technol. 1997, 31, 1102-1108

Sorption of Polycyclic Aromatic Compounds to Humic Acid As Studied by High-Performance Liquid Chromatography T O R B E N N I E L S E N , * ,† K A T R I N S I I G U R , †,‡ CHRISTIAN HELWEG,† OLE JØRGENSEN,† POUL ERIK HANSEN,§ AND UUVE KIRSO‡ Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark, Institute of Chemical Physics and Biophysics, Akadeemia tee 23, EE-0026, Tallinn, Estonia, and Department of Life Sciences, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark

Aldrich humic acid was chemically immobilized to the silanol surface of a column material to be used for highperformance liquid chromatography (HPLC). The retention factors to the humic acid column material of 45 polycyclic aromatics compounds (PAC) were determined by HPLC. The PAC include PAH, N-, S-, O-PAC and substituted PAC (9substituted anthracenes, bromopyrenes, and quinoline derivatives). The sorption coefficient of quinoline to humic acid was directly determined at different pH. The good correlation achieved between the HPLC retention factors and literature Koc values, including the presented one of quinoline, was applied to determine Koc of 39 other PAC. The determined Koc values were parametrized with regard to size, ring heteroatoms, and steric and substituent effects and were compared with literature values of water solubility and recently determined octanol-water partition coefficients. It is shown that the sorption of PAC to humic acid is not only affected by hydrophobic interactions but also by hydrogen and especially ionic bonds. The investigation shows that the application of humic acid stationary HPLC phases is a valuable supplement to other techniques for determination of Koc.

Introduction The contamination of soil with creosote and coal tar is a widespread problem all over the industrialized world. The best known group of PAC is polycyclic aromatic hydrocarbons (PAH). However, the N-, O-, and S-PAC appear to be present in amounts of 1-10% of those of the analog PAH (1, 2). The N-PAC are much more water soluble than the hydrophobic PAH (3); therefore, their environmental impact may be more important than the PAH (4, 5). In addition to the use of tar and creosote (6, 7), other sources for pollution of the environment with PAC are combustion processes (8-10). The major concern in relation to pollution with PAC is that many are considered to be carcinogenic (11). Recently, acridine, a N-PAC, has been shown to be a strong phytotoxin toward land plants (5), and some photooxidation products of PAH have been shown to be toxic toward water plants (12). * Author to whom correspondence should be addressed. Telephone: +45-4677-4216; fax: +45-4237-0403; e-mail address: [email protected]. † Risø National Laboratory. ‡ Institute of Chemical Physics and Biophysics. § Roskilde University.

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 4, 1997

Microbiological oxidation may transform unsubstituted PAC to their hydroxy derivatives (13), but also O-methyl and N-methyl derivatives have been observed (14). Recently, quinoline N-oxide has been shown to be a microbiological oxidation product of quinoline (15). Photolytic reactions appear to be a possible source for chlorinated PAH in the marine environment and on the soil surface (16). Combustion processes and photochemical reactions in the atmosphere and the aquatic environment may produce a number of different PAC derivatives, e.g., nitro (9, 17, 18), cyano (19), chloro, bromo (20), methoxy (21) and some oxygenated derivatives, phenols, quinones, ketones, aldehydes, and carboxylic acid derivatives (9, 17, 22). The transport, fate, and bioavailability of PAC in the aquatic and terrestrial environment is very dependent on the partitioning of the PAC between water, dissolved organic matter, and soil or sediment organic matter (23). Several studies have dealt with the sorption of the hydrophobic PAH (24-34). Considering the huge number of polluting PAC having great variation in their physical-chemical properties as well as considering the variation in the properties of humic substances being present in the environment (26, 35), there is a great need of data to evaluate the sorption of PAC to organic matter. This paper describes the sorption of 45 PAC to chemically immobilized humic acid bonded to a highperformance liquid chromatography (HPLC) column. The retention factor of the PAC is compared with known Koc values. Most of the variation in the determined sorption coefficients can be parametrized by means of size, ring heteroatoms, and steric and substituent effects. The limitations for applying octanol-water partition coefficients, Kow (36), and water solubility (Sw) to estimate Koc are discussed.

Materials and Methods Preparation of Chemically Bonded Humic Acid Silica Gel for HPLC. The silica gel with chemically bonded humic acid was prepared in a number of stages using the method of Szabo´ and Bulman (37). Briefly, a suspension of silica gel (10 g) [Nucleosil-Si-50-10 (Macherey-Nagel)], toluene (50 mL), and 3-aminopropyl triethoxysilane (2.5 mL) (Lancaster, 97%) was refluxed by stirring for 4 h under argon. The crude aminopropyl silica gel was removed by filtration through a sintered glass filter (G4). The product was washed successively with 2 × 50 mL of each of the following solvents: toluene, methanol, water, and methanol and dried at 50 °C. The resulting suspension of the aminopropyl silica gel (10.47 g) and a 5% water solution of glutardialdehyde (230 mL) (Merck zur Synthesis) was stirred at room temperature under argon in a 500 mL-round-bottomed flask. The reaction mixture was filtered through a sintered glass filter (G4) and washed with water (350 mL) several times. The activated silica gel was dried at 50 °C overnight, and the yield was 11.98 g. Subsequently, humic acid (1.0 g) (Aldrich) was dissolved in water (100 mL), the activated silica gel was added, and the reaction suspension was stirred at room temperature under argon for 7.5 h. The product was removed by filtration through a sintered glass filter (G4) and washed with 0.5 M phosphate buffer (pH 7.5) (10 × 25 mL) and water (4 × 50 mL). The product was pressed as dry as possible and suspended in a 0.1 M water solution of 2-aminoethanol (Merck, p.a.) buffered with phosphoric acid to pH 7.5 (total volume 100 mL). The suspension was stirred at room temperature under argon for 3.5 h and filtered through a sintered glass filter (G4). The filtrate was washed with water (5 × 50 mL) and dried at 50 °C overnight yielding 11.7 g. The HPLC humic acid column was packed in methanol at 350 bar.

S0013-936X(96)00620-7 CCC: $14.00

 1997 American Chemical Society

TABLE 1. Measured Capacity Retention Factors, k′, to Humic Acid HPLC Column, Estimated Log Koc Values Directly Measured Log Koc, Octanol-Water Partition Coefficients, Log Kow, Difference Log Koc - Log Kow, and πAr for Aromatic Substituents compound N-PAC quinoline isoquinoline 4-azaflourene acridine 5,6-benzoquinoline 7,8-benzoquinoline phenantridine 1,2-benzacridine 10-azabenz[a]pyrene 1,2,3,4-dibenzacridine 1,2,5,6-dibenzacridine 1,2,7,8-dibenzacridine 3,4,5,6-dibenzacridine carbazole PAH naphthalene flourene anthracene phenanthrene benz[a]anthracene benzo[a]pyrene 1,2,3,4-dibenzanthracene 1,2,5,6-dibenzanthracene 1,2,7,8-dibenzanthracene O,S-PAC dibenzofuran dibenzothiophene X-N-PAC 2-hydroxyquinoline quinoline N-oxide N-methyl quinolinium iodide X-PAH 9-acetylanthracene 9-anthracene carboxamide 9-anthracene carboxylic acid methyl ester 9-bromoanthracene 9-chloroanthracene 9-cyanoanthracene 9-formylanthracene 9-methoxyanthracene 9-methylanthracene 9-nitroanthracene anthraquinone 1-bromopyrene 2-bromopyrene 4-bromopyrene 1,3-dibromopyrene 1,6-dibromopyrene 1,8-dibromopyrene a Log K ) 1.5 × log k′ + 4.16. oc refs 39 and 40. πAr(Ar-H) ) 0.

b

k′

log Koca

0.141 0.194 0.443 0.781 0.868 0.950 0.856 3.665 13.79 21.96 18.35 13.78 25.24 2.455

2.89 3.09 3.63 4.00 4.07 4.13 4.06 5.00 5.86 6.17 6.05 5.86 6.26 4.74

0.527 2.231 2.122 2.139 9.487 25.76 39.25 33.40 41.66

3.74 4.68 4.65 4.65 5.62 6.27 6.54 6.44 6.58

0.986 1.930

4.15 4.59

0.208 0.104 8.73

2.7c 2.0c 3.1c

1.085 0.899 1.605 4.584 3.404 2.307 0.824 1.528 2.952 2.250 1.273 10.24 10.06 10.95 23.13 17.94 22.59

4.21 4.09 4.47 5.15 4.95 4.70 4.03 4.43 4.86 4.69 4.32 5.67 5.66 5.71 6.20 6.04 6.18

log Kowb

log Koc - log Kow or πAr

ref

3.05

2.07 2.08 2.96 3.27 3.40 3.60 3.44 4.48 5.53 5.66 5.73 5.63 6.45 3.51

0.82 1.01 0.67 0.73 0.67 0.53 0.62 0.52 0.33 0.51 0.32 0.23 -0.19 1.23

this work

3.32

3.40 4.32 4.48 4.46 5.54 6.02 6.54 6.40 6.54

0.34 0.36 0.17 0.19 0.08 0.25 0.00 0.04 0.04

4.12 4.49

0.03 0.10

log Koc (lit.)

4.87 ( 0.26 4.81 ( 0.16 5.48 6.30

24 24, 25, 33 25, 33, 38 24 24, 27

πArd -0.55 -1.49 -0.01 0.86 0.71 -0.57 -0.65 -0.02 0.56 -0.28

From ref 36. c From log Koc - log Koc(naphthalene(Naph)) + 1.5 × (log k′(X)95%water - log k′(Naph)95%water).

Test Compounds. The test compounds (Table 1) were dissolved in methanol (Lichrosolv 99.8% or Lab Scan HPLC) with a typical concentration of 0.04 g/L. 1-Bromo-, 1,6dibromo-, and 1,8-dibromopyrene were prepared according to Vollman et al. (41). 1,3-Dibromopyrene was prepared from methyl 1,3-dibromopyrene-2-carboxylate (42). The latter was achieved from the 2-ester by bromination in acetic acid, which worked better than bromination in CCl4 as suggested by Flammang (42). The methyl 1,3-dibromopyrene-2-carboxylate was converted into the acid by alkaline hydrolysis in methanol for 15 d. The sodium salt was filtered off and made into the acid by treatment with concentrated hydrochloric acid. Subsequently 230 g of the acid was decarboxylated by heating 1.5 h with 100 mg of copper powder in 10 mL of refluxing quinoline. The 1,3-dibromopyrene was extracted with benzene, and the quinoline was removed by washing with dilute hydrochloric acid. The raw yield of 1,3-dibromopyrene was 120 mg. The melting point was 210 °C after

d

From

recrystallization from benzene. For IR data, see ref 43. All other compounds were obtained from commercial sources. Indirect Koc Determinations by HPLC. The HPLC system used was a low-pressure Shimadzu LC-10HPLC system with photodiodearray (PDA) detector, thermostated (30 °C) column oven, and autoinjector. The dimensions of the column were 12.5 cm × 4.6 mm (i.d.). The eluent consisted of 65% methanol and 35% of phosphate buffer (0.01 M, pH 7.0) flowing at 1 mL/min. The water was laboratory grade ion-exchanged water extra-purified on a Millipore-Q water purification system. Appropriate mixtures of the compounds were chromatographed, and the retention time (vr) of each compound was recorded. All measurements were repeated at least two times. The dead volume (v0) of the system, used for calculating the capacity coefficient, k′ ) (vr - v0)/v0, was determined by chromatographing water five times. The reproducibility of the retention time of water was 1.3%, and that of the times for the 45 PAC was 0.9 ( 0.6%.

VOL. 31, NO. 4, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Results and Discussion

FIGURE 1. Example of determination of sorption coefficient, Koc, of quinoline to humic acid by the dialysis tubing method. pH was 11.7. Koc is equal to the slope of the line (1050 ( 22 L/kg, r 2 ) 0.992; SE ) 0.01). TOC was the content of organic carbon in the humic acid solutions. C0 was the concentration of free dissolved quinoline in the blank experiments, and Cfree was that in presence of humic acid.

FIGURE 2. Influence of pH on the sorption coefficient, Koc, of quinoline to humic acid.

Direct Koc Determination of Quinoline. The quinoline (Merck zur Synthesis) was distilled twice before use. The humic acid (HA) (Aldrich) was dissolved in water, centrifuged at 10000g for 30min and then filtered through 0.3 µm glassfiber filter to remove particulate material. The dialysis tubing (Spectra Por 6; molecular weight cutoff of 1000) (24) was washed in laboratory grade ionized water to remove the sodium azide preservative. The equilibrium dialysis experiments to measure the binding of quinoline to HA were performed by placing 5 mL of a HA solution having varying HA concentrations (Figure 1) in dialysis tubing and clamping the ends. The dialysis bag was then placed in a 100-mL glass bottle containing a buffered (0.01 M phosphate) solution of quinoline (8.1 µM). The bottle was shaken in the dark for 24 h at 25 °C. Control experiments demonstrated that this was sufficient to achieve a steady-state. In each series, two blank experiments without HA were performed in order to correct for adsorption of quinoline to the surfaces. The quinoline concentration in the outer solution was determined by HPLC. A Waters Ultrahydrogel 250 size exclusion column was used with a mixture of 10% methanol and 90% 0.01 M phosphoric acid (pH 2.5) as eluent. The Koc value was derived from the relationship between the ratio of the concentration of quinoline in the blank experiment (C0) to that in the sorption experiments (Cfree) and the HA total organic carbon concentration (TOC): C0/Cfree ) Koc × [TOC] + 1 (Figure 1) (33), at six different pHs in the range 2-12 (Figure 2).

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Direct Determination of Koc of a Polar PAC, Quinoline. The sorption of quinoline to humic acid was determined in order to obtain a reference Koc value for a polar PAC. A number of investigations have determined the sorption coefficient between the Aldrich humic acid and the more hydrophobic PAH (Table 1). Quinoline is a base with pKa ) 4.92 (44). Therefore, the pH dependency of Koc was also investigated. Previously, it has been found, that the sorption to organic matter of protonated N-PAC is much less than that of the neutral base form (45). For example, the sorption of the neutral quinoline to sludge from a wastewater treatment plant was found to be 20 times higher than that of the quinolinium ion (45). The Koc of quinoline to humic acid was determined to be 1130 ( 80 L/kg at pH 7.0. In neutral and basic solutions (Figure 2), Koc was almost constant, while the variation of Koc in acidic solutions was more complex with a maximum at pH 4.8 or at pH ∼ pKQH+, and even at pH 2.5 at which almost all the quinoline is at the protonated acid form, Koc was about two times higher than in neutral or basic solutions. Two factors may contribute to the variation of Koc: (1) Ion and/or hydrogen bonds are formed between the protonated and positively charged nitrogen atom in quinoline and negatively charged carboxylate groups in humic acid (46). (2) The humic acid changes from an extended chain to a coil structure giving better a possibility for formation of micelle structures when pH decreases (33). Concerning the former possibility, aromatic carbon makes up about 25% and carboxylic carbon (mainly COOH) makes up almost 10% of the carbon in the Aldrich humic acid (35). The pKa values of the carboxylic acid groups are expected to cover the range 2.5-5.5 (47, 48). Thus, Koc increased as pH decreased from 7 to 5 when an increasing part of quinoline would be protonated. As pH decreased from 5 to 2.5, an increasing part of the carboxylate groups in the humic acid would be neutralized causing a decrease of Koc. The better binding at pH 2.5 as compared to neutral or basic conditions suggests that either the amount of charged carboxylate groups at pH 2.5 is high enough to bind the quinolinium ion or the humic acid coil structure is much more efficient in adsorbing quinoline than the long-chain structure, no matter whether quinoline is protonated or not. Kumke et al. (33) observed that the sorption of pyrene to Aldrich humic acid increased by a factor of 3, going from pH 11.5 to pH 2.0. The Koc of pyrene appeared to be linearly dependent of pH. Schlautman and Morgan (49) have observed a similar influence of pH on the sorption of PAH to other humic and fulvic acid materials. For phenanthrene, the influence of pH on Koc is very small (33). Kumke et al. (33) suggested that the pH effect depends on the water solubility of the PAH and that the pH effect is diminished with increasing solubility. Consistent with this, Traina et al. (50) did not observe any pH effect on the sorption of naphthalene to an extract of muck soil organic matter. If solubility is the determining factor for pH effects on Koc, the chain/coil structure of the HA will not contribute to the variation of Koc of quinoline considering that the water solubility of quinoline is almost 7000 times higher than that of phenanthrene (3). Furthermore, the interiors of the humic acid micelles at low pH are expected to be hydrophobic (51). Therefore, it appears that ionic binding is the determining factor for the variation of Koc of quinoline at pH below 7. Comparing our findings with previous ones (45), it appears that the “quality” of the carbon is a factor affecting the sorption behavior of the neutral N-PAC compounds and their protonated cationic form in addition to the amount of organic carbon (52). Humic and fulvic acids may show great variations in their composition and properties depending not only on their origin and history (53) but also on the applied isolation procedure. The Aldrich humic acid was preferred in this investigation, as it has been applied in other investigations to determine

the sorption of PAH (Table 1) and as it is available in bulk amounts. Gauthier et al. (26) and McCarthy et al. (27) have compared the sorption of pyrene and benzo[a]pyrene to Aldrich humic acid with the sorption to soil humic acids, fulvic acids, and water-dissolved organic matter. Applying the results from our investigation one should consider that Koc may vary by a factor of 15 depending on the properties of the humic and fulvic acids (26, 37). Based on the values of Gauthier et al. (26) and McCarthy et al. (27), it appears that Koc decreases in the range soil humic acid > Aldrich humic acid > soil fulvic acid > water-dissolved organic matter. Correlation between Sorption Coefficients and HPLC Capacity Coefficients. The HPLC capacity coefficient, k′ ) (vr - v0)/v0, of a compound is a relative measure for the sorption coefficient, K ) Cs/Cw, of the compound. (vr - v0)/ vr is a measure for the proportion of the compound being adsorbed to the surface of the column material, and v0/vr is a measure for that part being dissolved in the eluent. Thus, the correlation between experimentally determined sorption coefficients, Koc, to Aldrich humic acid for five PAH and the more polar quinoline and their capacity coefficients on the humic acid HPLC column

log Koc ) (1.50 ( 0.15) × log k′ + (4.16 ( 0.12) r 2 ) 0.980, SE ) 0.27 (1) indicates that the method can be applied to estimate log Koc for other PAC compounds by means of their capacity coefficients (Table 1), as the polar quinoline fitted well into the correlation with the more hydrophobic PAH. The fact that the Koc values were determined in different laboratories using different batches and applying different techniques also supports the general validity of the correlation. The capacity coefficients used in determining Koc (see above and Table 1) are determined using 35% water (buffer). However, the capacity coefficients of naphthalene (Naph), quinoline, isoquinoline, 2-hydroxyquinoline, and quinoline N-oxide increased with increasing water content. Thus k′Naph increased from 0.527 (35% water) to 37.2 (95% water), and k′2-hydroxyquinoline increased from 0.208 to 7.36, whereas k′(N-methylquinolinium iodide) was constant (mean ) 7.79 ( 0.38) in the range of 35-85% water and increased to 14.1 at 95% water. The capacity coefficient ratios of quinoline to Naph and isoquinoline to Naph did not show any significant variation with the water content, as the mean log (k′quinoline/k′Naph) ) -0.52 ( 0.03 and the mean log (k′isoquinoline/k′Naph) ) -0.47 ( 0.02 (Figure 3). In contrast to this, the capacity coefficient ratios of 2-hydroxyquinoline to Naph, quinoline N-oxide to Naph, and especially N-methylquinolinium iodide to Naph decreased with increasing water content (Figure 3). Thus, the correlation of eq 1 cannot be applied to very polar organic compounds, such as N-methylquinolinium iodide, and will also give too high Koc values, if it is applied to 2-hydroxyquinoline and quinoline N-oxide. Therefore, the Koc values of these three compounds were estimated by means of the capacity coefficients at 95% water (see Table 1) assuming

log Koc(X) - log Koc(Naph) ) 1.50 × (log k′(X)95%water - log k′(Naph)95%water) (2) The validity of eq 2 is supported by it giving the same Koc value for quinoline and isoquinoline as eq 1. Applying eq 2 instead of eq 1 log Koc of 2-hydroxyquinoline is estimated to be 2.7 instead of 3.1 and quinoline N-oxide is estimated to be 2.0 instead of 2.7. The corrected values are those given in Table 1. Table 1 also contains k′ and Koc values for a number of substituted anthracenes. Equation 1 was applied as none of these can be considered to be very polar, e.g., the influence of the substituent was lower than the combined effect of the hydroxy group and the nitrogen atom, i.e., (k′x)/k′anthracene > k′2-hydroxyquinoline/k′Naph.

FIGURE 3. Influence of the eluent water content on the capacity coefficient, k′, of bicyclic aromatics relative to that of naphthalene. Identity of the compounds: Naph, naphthalene; Qu, quinoline; isoQ, isoquinoline; 2-HQ, 2-hydroxyquinoline; NMQ+, N-methylquinolinium iodide; QNO, quinoline N-oxide. Influence of Size, Composition of Rings, and Steric Effects. Log Koc increased with the size of the molecule both for PAH and the basic N-PAC. For PAH very little variation in the sorption coefficient for compounds having the same molecular weight was found. Thus the log Koc for the three dibenzanthracenes are within the range 6.44-6.58. In contrast, log Koc of the four dibenzacridines (Table 1) varied by a greater amount. Correspondingly, Koc for 7,8-benzoquinoline was larger than those for acridine, phenanthridine, and 5,6-benzoquinoline. The reason for this appeared to be that the nitrogen atom can be shielded by the benzene rings. In 3,4,5,6-dibenzacridine the two rings in the 3,4- and 5,6position are pointing in the same direction as the nitrogen atom and thereby shielding it. This appears to decrease the ability of the nitrogen atom to form hydrogen bonds. Therefore, 3,4,5,6-dibenzacridine is expected to be more hydrophobic. In 1,2,7,8-dibenzacridine the two rings point in the opposite direction of the nitrogen atom and do not have any shielding effect on the nitrogen atom. 1,2,3,4- and 1,2,5,6-dibenzacridine are in-between these two as both have one ring shielding for the nitrogen atom. The shielding rings should also imply that the proton affinity of the nitrogen atom decreases, and the basicity of 1,2,5,6-dibenzacridine (pKa ) 3.6) (45) and 7,8-benzoquinoline (pKa ) 4.21) (54) is lower than those of pyridine, quinoline, isoquinoline, acridine, 5,6-benzoquinoline, phenanthridine, and 1,2-benzacridine (pKa ) 4.61-5.68) (44, 45, 54-57). It is possible to parametrize the effects of the size, rings nitrogen atom, and shielding rings (Figure 4). The former one is illustrated in that for pyrene the predicted log Koc value is 5.14, which is in good agreement with the experimental ones of 5.02 by Gauthier et al. (27) and 5.28 by Kumke et al. (33). The relationship predicts that log Koc increases by 0.97 with an addition of an extra ring (C4H2) to a PAH or basic N-PAC or decreases by 0.67 with replacing C-H in a PAH with N. Influence of Ring Heteroatoms. The results in Table 1 for the unsubstituted tricyclic aromatics show that log Koc decreases in the following range: carbazole > fluorene ∼ anthracene ) phenanthrene > dibenzothiophene > dibenzofuran > 7,8-benzoquinoline > 5,6-benzoquinoline ∼ phenanthridine > acridine > 4-azafluorene. The position of carbazole in this range suggests that hydrogen bonds between the NH group at the adsorbed compound and oxygen atoms in the humic acid increase the sorption significantly. However, the ranking of Koc appears to be affected by the humic


9

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FIGURE 4. Illustration of the relationship between log Koc and size, composition, and steric effects for basic N-PAC and PAH: predicted log Koc ) (0.01945 ( 0.00060) × MW - (0.670 ( 0.068) × NNA + (0.134 ( 0.069) × NSR + (1.214 ( 0.140)

r 2 ) 0.988, SE ) 0.14

which is the best fit between the log Koc from Table 1 and the molecular weight (MW), the number of ring nitrogen atoms (NNA), and the number of rings shielding the nitrogen atom (NSR).

FIGURE 5. Comparison of the relation between log Koc and water solubility (Sw (ppm)) for unsubstituted basic N-PAC and PAH: (1) N-PAC: predicted log Koc ) -(0.37 ( 0.03) × log Sw + (4.66 ( 0.09)

r 2 ) 0.958, SE ) 0.22

(2) PAH: acid composition. Thus, Koc to a peat humic acid having a high content of aromatic carbon decreased in the following range: anthracene > dibenzothiophene > carbazole (58). Effect of PAH Substituents. The sorption of anthracenes substituted in the 9-position are correlated with the partitioning substituent constant, πAr (39, 40):

log Koc ) (0.442 ( 0.092)πAr + (4.624 ( 0.062) r 2 ) 0.723, SE ) 0.20 (3) The πAr substituent constants are derived from partition coefficients for substituted benzenes for the octanol-water system [πAr = log Kow(PhX) - log Kow(benzene)] (39, 40). The lipophilic substituents (Br, Cl and CH3) increase the sorption, and most of the polar substituents [CHO, C(O)NH2, C(O)CH3, OCH3, and C(O)OCH3] decrease the sorption. The substituent effect on the sorption to humic acid was less than half (44 ( 9%) of that on the partitioning between octanol and water, provided it is justified to neglect differences in the aromatic system in the two sets of experiments. The substituent effect appears to be independent of the aromatic system, as the effect of the bromo-substituent in 9-bromoanthracene and 1-, 2-, and 4-bromopyrene was the same (0.53 ( 0.02). The substituent effect also appears to be additive, as the increase of log Koc was very close to a 2 times higher value (1.00 ( 0.07) for the three dibromo derivatives (1,3-, 1,6-, and 1,8-dibromopyrene) than the increase for 9-bromoanthracene and 1-, 2-, and 4-bromopyrene. However, as log Koc for 1,6-dibromopyrene (6.04) is less than those of the two other dibromopyrenes (6.20 and 6.18), other effects, e.g., steric effects, may also affect the sorption. The additive effect of the substituents were also confirmed by a positive correlation (r ) 0.95; p < 0.05) between the Koc values for the 9-substituted anthracenes (Table 1) and literature Koc values for substituted benzenes (59). Substituent Effects on Quinolines. The sorption coefficients of quinoline and the three quinoline derivatives only showed minor variations (Table 1). Koc for N-methylquinolinium iodide was of the same magnitude as that of the neutral quinoline. This result confirms that the sorption of the quinolinium ion is not minor than that of the neutral quinoline (see Figure 2). The lack of variation in the sorption coef-

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predicted log Koc ) -(0.57 ( 0.11) × log Sw + (5.19 ( 0.22)

r 2 ) 0.903, SE ) 0.44

(3) N-PAC + PAH: predicted log Koc ) -(0.46 ( 0.05) × log Sw + (4.97 ( 0.13)

r 2 ) 0.879, SE ) 0.46

which are the best fits between log Koc from Table 1 and the water solubility (3, 60, 61). ficients, especially of N-methylquinolinium iodide, quinoline, and 2-hydroxyquinoline, indicate that the sorption is dominated by the hydrogen bonds and especially ionic bonds. Quinoline N-oxide does not have the same possibilities for making ion bonds as N-methylquinolinium iodide, as the nitrogen cation is protected by the negatively charged oxygen. This is probably the reason that Koc of this compound is much lower than the other quinolines. Relation between Sorption and Water Solubility. Previously, it has been shown that the sorption of unsubstituted PAC is strongly related to their water solubility, provided that size is the main factor controlling both the sorption and the solubility (52). The results of this investigation do not contradict this observation. Most of the variation of log Koc for unsubstituted PAH and basic N-PAC can be explained by means of the variations in their water solubility (Figure 5). The effect on the relation of ring nitrogen atoms was only small. A slight but significant improvement (t-test, p < 0.05) in the predictions of log Koc was achieved if the data for N-PAC and PAH were treated separately:

N-PAC and PAH separate: mean|predicted log Koc - log Koc (Table 1)| ) 0.19 ( 0.10 N-PAC and PAH together: mean|predicted log Koc - log Koc (Table 1)| ) 0.31 ( 0.17 The sorption of carbazole is also higher than predicted from the relation between log Koc and water solubility comparing

progress in order to compare log Koc and log Kow also for substituted PAC.

Acknowledgments Funding from the Center for Ecotoxicological Research under the Danish Environmental Research Program and the Danish Science Research Council are gratefully appreciated. Dr. Christian Grøn, Risø National Laboratory, is thanked for the TOC analysis of the humic acid solution applied for the direct determinations of Koc of quinoline.

Literature Cited

FIGURE 6. Difference between the log Koc values predicted by means of the water solubility [Sw (ppm)] and the log Koc in Table 1 of unsubstituted tricyclic aromatics: predicted log Koc ) - (0.26 ( 0.07 × log Sw) + (4.56 ( 0.07)

r 2 ) 0.723, SE ) 0.18

which is the best fit between the log Koc from Table 1 and the water solubility (3, 61). Identity of the compounds: An, anthracene; Phen, phenanthrene; Fl, fluorene; Acr, acridine; BQ, 5,6-benzquinoline; Carb, carbazole; DbF, dibenzofuran; DbT, dibenzothiophene. the sorption of tricyclic aromatics (Figure 6). Thus, hydrogen bonds between N-H in carbazole and oxygen atoms in humic acid increase the sorption. Nevertheless, the water solubility is not a poor parameter to characterize the variation of the sorption of compounds being similar in size as SE for the correlation in the caption of Figure 6 was not higher than 0.18. Relation between Sorption and Octanol-Water Partitioning. A number of investigations have compared Koc and Kow (62, 63). The difference log Koc - log Kow decreases in the range: neutral N-PAC > basic N-PAC > PAH ∼ S-PAC ∼ O-PAC (Table 1). Our recent determinations of octanolwater partition coefficients of unsubstituted PAC show that the Kow values for N-PAC are affected not only by the hydrophobic interactions between the PAC and the alkyl group in octanol but also by hydrogen bonds between the nitrogen atom and the hydroxyl group in octanol (36). The top ranking of the neutral N-PAC, carbazole, indicated that the hydrogen bonds between N-H in carbazole and oxygen atoms in the humic acid were stronger than those between N-H and the hydroxyl group in octanol. Between log Koc and log Kow for the other unsubstituted PAC the following correlation was achieved:

predicted log Koc ) (0.889 ( 0.031) × log Kow + (0.276 ( 0.086) × NNA - (0.129 ( 0.084) × NSR + (0.716 ( 0.166)

r 2 ) 0.982, SE ) 0.17 (4)

where NNA refers to the number of ring nitrogen atoms and NSR is the number of rings shielding the nitrogen atom. Dibenzofuran and dibenzothiophene, and probably also other O- and S-PAC, behave like the analog PAH. This is consistent with the fact that O- and S-PAC are present in the PAH fraction when environmental PAC samples are fractionated for chemical analysis (64). The replacement of C-H with a nitrogen atom increased the log Koc - log Kow difference by 0.45 ( 0.07, showing that the hydrogen bonds between the PAC nitrogen and phenol groups in humic acid were stronger than those between the nitrogen atom and the OH group in octanol. The shielding effect on the availability of the nitrogen lone pair in, for example, 3,4,5,6-dibenzacridine was less important in the humic acid sorption process than in the octanol-water partitioning process (36). Work is in

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Received for review July 16, 1996. Revised manuscript received November 13, 1996. Accepted November 18, 1996.X ES960620T X

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

Sorption of Polycyclic Aromatic Compounds to Humic Acid As Studied

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