Environ. Sci. Techno/. 1995, 29, 941-950
Introduction
Anthropogenichmic Matter F R A N K - D I E T E R KOPINKE,* J U R G E N PORSCHMANN, A N D ULRICH STOTTMEISTER Department of Remediation Research, Center for Environmental Research, Permoserstrasse 15, 0431 8 Leipzig, Germany
Anthropogenic and natural humic materials were investigated as sorbents for hydrophobic solutes in aqueous solution. The first was taken from a brown coal wastewater pond. The measured sorption coefficients KOCand KDOMare close to literature values for both sorbents. This holds for particulate (sediments) as well as for dissolved materials (humic and fulvic acids). A modified solubility parameter concept is proposed to better understand the KOC-KOW correlation. It makes it possible to estimate sorption coefficients for nonpolar solutes based on their KOW,if the d value of the sorbent is known. On the other hand, measured sorption coefficients permit the calculation of this value, which can be considered a feature of the humic fraction under study. The mean 6 value of humic organic matter estimated from sorption data in this paper is 12.5 f 0.5 (cal ~ m - ~ ) " * .
Coal conversion processes give rise to large effluent streams, which are highly contaminated with organic pollutants. The dischargeof the effluent streams into the environment results in the exposure of these organics to sediments,which in tum may result in subsequent sorption processes. The extent of sorption is either chemically or physically dictated, depending on the forces interacting between the sorbent (sediment) and sorbate phase (organic pollutant). In the case of largely nonpolar sorbates, the partitioning phenomena may play the major role (1-3). However, the theoretical description of sorption processes by means of parameters familiar to environmentalchemistsis not highly sophisticated. The object of this study includes brown coal wastewaters and sediments originating from a pond near Leipzig with an area of about 9 x lo4 m2,which is filled with nearly 2 x lo6 m3 of brown coal wastewater highly contaminated with organic compounds. The coal wastewater under study is dark brown due to the presence of humine-like organic matter (about 400 mg L-l), the latter being formed spontaneously as a result of oxidation processes of susceptible, anthropogenic compounds, mainly including monomeric mono- and diphenols. While the interaction of natural humic organic material with organic pollutants has frequently been a matter of concern (e.g., refs 1-30), less attention has been paid to anthropogenic humic material (31). Using these water and sediment matrices, we aimed our efforts at investigatingcorrelations between the sorption of organic pollutants on humic organic material originated from both coal wastewater and sediment, on the one hand, and structural features of these polymers on the other. The results will be published elsewhere (32). The subject of the present paper is a comparison between sorption properties of natural and anthropogenic humic materials. It includes a theoretical interpretation of experimental results using the concept of solubility parameters that has been familiar mainly to polymer chemists up to now.
Experimental Section Isolation of Humic and Fulvic Acids. The isolation procedure is described in ref 33. Humic and fulvic acids from bog water were kindly supplied by K. Wienhold (University of Leipzig). We used the following acids for investigations: humic and fulvic acids from surface water of a coal wastewater dump, humic and fulvic acids from bog water, and humic and fulvic acids from sediment of the coal wastewater dump. Determination of Organic Pollutants in Coal Wastewater. One liter of wastewater to which internal standards were added was shaken vigorously for 1 h with 50 mL of benzene p.a. The procedure was repeated twice. The combined extracts were dried with sodium sulfate, purified by means of silica gel solid-phase extraction (Baker), and then carefully evaporated to a volume of 1 mL. Determination of Organic Pollutants in Sediments. About a 1.5 g of air-dried sediment sample was inserted into a 2.5-mL SFE (supercritical fluid extraction) cartridge and extracted with supercritical COz at 35 MPa and 80 "C for 30 min. A polar modifier, methanol, was added in the
0013-936~95/0929-0941$09.00/0Q 1995 American Chemical Society
VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1941
range of 0.5% at the beginning of the extraction, rising to 4% after an extractiontime of 15min. Then the temperature
and the pressure were raised to 120 "C and 48 MPa, respectively. Recoveries of alkanes and polycyclic aromatic hydrocarbons (PAHs) turned out to be in the range of 90 f lo%, those of phenols about 85 It 10%. Determination of Sorption Behavior of Sediments. A total of 20-1000 mg of air-dried sediment was washed several times with water in order to remove the portion of soluble and nonsettling organic matter (14). Then the sample was added with distilled water up to 10 mL, the latter containing 1 mg of aniline-d5,phenol-&, o-cresol-d8, nitrobenzene-d5, 2-nitrophenol-d~, and 2,4-dimethylphenol-& each. Sodium azide (200mg L-I) was added to inhibit biodegradation processes (22). The application of isotopically labeled compounds as sorbates was necessary to exclude any interference with desorption of these compounds from the original coal sediment. The flasks were vigorously shaken at room temperature for 72 h in the dark. The mixture was allowed to stand for some hours until the sediment settling was complete. An aliquot of 5 mL of the clear water phase was transferred into a round-bottom flask with a small neck. The aliquot was vigorously shaken with 200 yL of benzene for 2 min, and the supernatant was immediately subjected to GUMS analysis. Quartz sand instead of sediment served as a blank. The procedure implies that the decrease in solute concentration between the blank experiment and the sediment experiments is due to sorption on sediments exclusively. Determination of Sorption by Means of 14C-Labeled Compounds. The sorption experiments with [l4C1benzene, -toluene, and -naphthalene (specificactivity about 100kBq mg-') were conducted as described above. The concentration of the labeled compound in the water phase was measured as activity of an aliquot (100 ,uL) by liquid scintillation counting. The small loss of activity in blank runs (1-3%) was mainly due to evaporation, sorption on glass walls, and sorption on quartz sand. Sorption Properties of Isolated Humic and Fulvic Acids. A mixture containing 25 phenols, PAHs, nitroaromatics, and aniline was added to 10 mL of an aqueous solution containing 5 mg of either a fulvic or a humic acid in a round-bottom flask. The concentration of individual pollutants ranged from 0.1 (each PAH) to 1.0 ppm (each phenol). The mixtures were stored in the dark for 72 h and then extractedwith 200,uLofbenzene byshakingfor 1min. The organic phase was directly subjected to GUMS analysis. Data acquisition was performed in the full-scan mode combined with the integration of selected ion signals in a defined retention time window. We estimated the sorption behavior of the acid under study by means of the reduction of the solute concentration in the benzene extract of the aqueous solution spiked with humic or fulvic acid in comparison with blank water. This procedure implies the preconditions that all the missing solute is sorbed on the dissolved organic matter (DOM) exclusively and that the presence of both fulvic and humic acids, which are surfactants, does not profoundly influence the extractibility of the pollutants. The applied analytical method is based on the assumption that a fast liquid-liquid extraction is able to remove the free solutes and possibly loosely bound sorbates from the aqueous phase without destroying the relatively stable DOM-solute complex (34,3 3 . This was verified by a subsequent fast extraction immediately following the first, which did not yield more solute in the 942 * ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995
organic phase. Apparentlythe reequilibrationof the DOMsolute system is sufficiently slow to justify our approach. Devices. Capillary gas chromatograph HP 5890A, GC/ MS coupling HP 5880Awith MSD HP 5790A, Tri-Carb 3255 liquid scintillation spectrometer (Packard), and SFE ISCO 2200 were used.
Background of Hydrophobic Sorption As known from the literature (see refs 1-3 and references cited therein), the sorption of hydrophobic compounds on soil or sediment organic matter (SOM) is considered to consist of both an adsorption and a partition term, the latter being described by the KOC value: &C
= ci in solid phase 100% ci in aqueous phase wt % organic carbon in solid phase
(1) On the premise that hydrophobic interaction forces dominate, this concept is valid and there is a correlation between the sorption coefficient normalized to organic carbon (OC) content and the octanol-water partition coefficient ( & , i = Ci in l-octanoilci in ~ 2 0 1 :
log K& = a log K&
+b
(2)
There is a lot of GC-KOWcorrelations in the literature, most of them as linear relations between the logarithms of the coefficients (2,4-6,8-10, 12,13, 15,20,24,36-38,51). The regression coefficients a and b are empirical in nature, without a defmite physical or chemical meaning. The KOCKoW concept is a useful tool for reflecting sorption phenomena, provided that nonpolar hydrophobic interactions rather than strong polar interactions prevail between solutes and SOM. However,there is some evidence in the literature that in some respects the KOCconcept fails to do the following: (a) explain the differences of organic carbonrelated PAH contents in the grain size fractions of a natural river sediment ( 19); (b) explain widely different KOCvalues of different grain size fractions from one or different soils (22);(c) explain KOCvalues of soil fractions separated by manual sorting (visual observation), which differ by more than 1 order of magnitude (29, 30); or (d) explain the experimental findings observed in ref 24: The obvious rule saying that the higher the oxygen content in the organic matter the lower the sorption of nonpolar organic compounds is not valid for some polymers rich in oxygen. To cite an example, cellulose and a typical humic acid (either of them having a similiar O/C molar ratio of 0.8 and a H/O ratio in the range of 2.0) differ in the sorption of trichloroethylene by a factor of almost 30. Weber et al. (29) observed a great heterogeneity on a particle scale of subsurface soils and sediments with respect to sorption capacity. They found shale fractions to be by far the most reactive ones for hydrophobic solutes with sorption capacities per unit weight more than 2 orders of magnitude greater than the bulk soils. This means the sorption capacity of a soil sample can be concentrated in averysmallmassfraction (0.1-1%),whichisnot composed of base-extractable materials. To sum up, no parameter just mentioned is capable of reflecting the distribution and extension of hydrophilic and hydrophobic moieties in the polymer backbone. Among other reasons, the present work has been stimulated by this very lack of a parameter characterizing the ability of
TABLE 1
S Values [(cal ~ m - ~ ) of ~ ’ Some ~ ] Solvents and Polymers at 25 “C (39-41) solvent s solvent s n-hexane n-dodecane cyclohexane benzonitrile n-butylacetate diethylketone toluene benzene styrene n-dodecanol naphthalene anthracene a
7.3 7.9 8.2 8.4 8.5 8.8 8.9 9.2 9.3 9.8 9.9 9.9
nitrobenzerle o-cresol aniline I-octanol 1-pentanol 1-butanol I-propanol ethanol meth ano I ammonia g Iyce rol water
10.0 10.2 10.3 10.3 10.9 11.4 11.9 12.7 14.5 16.3 16.5 23.4
polymer
sa
poly(tetrafluoroethy1ene) poly(ethylene) poly( butadiene) poly(styrene) poly(viny1acetate) ethyl cellulose lignin nylon 66 phenolic resin cellulose acetate poly(acrylonitri1e) cellulose
6.2 8.0 8.4 9.2 9.8 10.3 10.5 11.2 12.5 13.4 14.0 15.7
A literature search revealed that the d values of the polymers may scatter within a range of 3 ~ 1unit.
a definite SOM to bind hydrophobic compounds with which the applied chemist has familiarized himself. Freeman and Cheung (7) were the first to apply the solubility parameter 6 to organic carbon-controlledsorption to natural sediments. They proposed the following relation to describe the extraction of sorbates i with a solvent:
withA and B as sediment-specificparameters, L.,as the size of the solute i, vi as its molar volume, and 6 as the solubility parameters of the corresponding components. K , a general sorption coefficient, is considered in eq 3 to consist of kinetic (diffusion) and equilibrium inputs. Chiou et al. (12)were the first to apply the Flory-Huggins treatment to account for the solute activity in the amorphous polymeric humic phase. They used the correlation between the molar solubility S of a hydrophobic solute in water and the sorption equilibrium constant &M In GM = -In (Sv) - In e - (1
+ x)
It was found empirically that two substances are all the more compatible the closer their cohesive energy densities are. Table 1 presents a compilation of 6 values of some solvents and polymers from the literature. There is evidence from the data shown in Table 1 that 6 can be considered a polarity parameter in the first approximation because the cohesive energy of a material is strongly increased by polar interactions. Although the water solubility (or better incompatibility) of an organic solute appears to be the decisive factor which governs KOM,there is a significant contribution by the organic matter as well. Our interest is focused on the investigation of how different organic materials vary in different soils or sediments with respect to their sorption potentials. Startingfrom these considerations,we propose to describe the partition properties of a certain SOM in competition with that of 1-octanol in the following way:
(4)
is the density of the organic matter and x is the dimensionless Flory-Huggins interaction parameter. In this respect, the total amount of organic matter (OM) is a more appropriate figure of the sorbent as compared with the commonly used organic carbon content (KOM= KOCx weight fraction of OC in the OM). The enthalpiccomponent XH ofX =XH + xs can-depending on certain preconditionsbe approximated by Q
Using benzene and some nonprotic benzene derivatives including anisole,ethylbenzene, chlorobenzenes, and PCBs as solutes under study, Chiou et al. (12) estimated &OM =13.0 f0.5 (cal for a Woodburn silt loam soilwith an organic matter content of 1.9 wt %. Despite the fact that the value measured applies to water-saturated SOM, which is more polar than the dry polymer, this figure is an appropriatemeasure for SOM polarity in the region between phenolic resin and cellulose acetate (cf. Table 1). The Hildebrand or one-component solubility parameter 6 deserves some additional comment. Originally, it was defined in terms of the molar cohesive energy ( - E ) per unit volume (v) of the material under consideration (39-41):
A formal way of deriving eq 7 makes use of eq 8, derived by Chin and Weber (,?I), for the partitioning of a solute i between a polymer and water. yw,iis the activity coefficient of the solute in the water phase.
In K(polymer/water), = In (yWJ + In (VWlVJ - (1 + Xpolymer,i) - In @polymer (8) Considering the humic organic matter and 1-octanolas ‘polymer’ phases, subtracting both formulas from each other, and substituting x by eq 5, eq 7 is obtained. The advantage of eq 7 over eq 8 consists of the elimination of activity coefficients. We are aware of the fact that this approach can hardly be put on an exact basis (e.g., 6 refers to enthalpic effects only), but this cannot be expected of a usable one-parameter formula. If the partition properties of SOM are equivalent to 1-octanol,the term in parentheses disappears, which results in KOM= KOW~QOM. The density term in eq 7 is introduced to adapt the dimensions common for Ci in KOW(g mL-’1 and in &M (gg-l). The ratio &M/&w measured for any compound i can formally be attributed to a difference in the 6 values of SOM and 1-octanol. The VOL. 29, NO. 4, 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY rn 943
TABLE 2
Sorption of Phenols and PAHs by Corl Wttstewater Sedimeiit under Natural Confitions (52 wt YO OM Inclusive 23.5 wt % 06) or anic poautsnt
.hJ Klc
c in wa a, G i aadi sm log I(0c literature IO h w KoJ tppm) [ppmT determined (4, le) (A-46) KOW
phenol 164 methylphenols 79 CZ-phenols 32 naphthalene 0.21 fluorene 0.026 phenanthrene 0.023 pyrene 0.002
315 288 260 49 34 81 25
0.91 1.19 1.54 3.00 3.75 4.18 4.73
1.18 1.65 3.1 4.35 4.9
1.49 1.94 2.35 3.35 4.18 4.57 5.18
0.26 0.18 0.16 0.44 0.37 0.40 0.35
direction in which this difference moves is, however, ambiguous due to mathematical operations (the sign of the difference of the 6 values is lost because of the formation of the squares). It can be determined by means of solutes with significantly different 6 values. In the particular case of SOM, a 6 value lower than that of 1-octanol is, however, unlikely. Below, the usefulness of eq 7 suggested by us is demonstrated: As revealed by the literature (12,21,39-411, a lot of representative hydrophobic organic pollutants, among them PAHs and PCBs, possess 6 values and molar volumes in the following region:
6 = 10.0 f 0.3 (cal ~rn-~)’’*V = 150 f 50 cm3 mol-’ Taking the values BOM = 13 (cal cm -3)1/2 (estimated by Chiou et d. (12)),61.0ctanol= 10.3 ( C d cm -3)112, and @OM =1.2 g ~ m - it~ follows , according to eq 7 that
KoMIK&
0.135
For reasons of simplicity we picked up two linear correlations, those from Kdckhoff et al. (4)KOCIKOW= 0.63 and those from Means et al. (5,6) KoclKow = 0.48 out of the large number of correlations published. Under the assumption that the content of organic carbon in the SOM is about 50 wt % we obtain = 0.28
f 0.04
This is a reasonable agreement between predicted KOM based on a BoMvalue of some 13 (cal ~ m -112~ in ) eq 7 and the experimental values given in the special literature. Furthermore in this paper, we will verlfy the modified solubility parameter concept by means of some own sorption data.
Results and Discussion Sorptive Behavior of Sediments. Under the assumption there is an equilibrium state at the bottom of the coal wastewater pond, a calculationof partition coefficientsfrom the measured concentrations of solutes in the sediment and in the surrounding water phase can be performed straightfonvardly. The results are presented in Table 2. It follows that the apparent &C values correlate well with the hydrophobicity of the most abundant solutes, mainly phenols and PAHs. The estimated KOCvalues of PAHs are slightly lower than those from the literature. However, care has to be taken with respect to the high DOC content in the deep coal wastewater (690 mg L-l), which can bind a significant portion of hydrocarbons. 944 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4,1995
In order to reveal differences in the sorption properties of natural and anthropogenic sediments, we followed the conclusions of Cooper et al. (18,23),who found out from their sorption experiments that the less hydrophobic the solute is, the more its affinity to SOM becomes the driving force for solute sorption. In other words, to characterize sorption properties of SOMs, it is reasonable to choose less hydrophobic solutes as probes, such as simple aromatic hydrocarbons, because otherwise when very pronounced hydrophobic solutes are used, minor differences in sorption behavior of organic matrices can be flattened or obscured. Two typical isotherms for the sorption of naphthalene (which represents a medium hydrophobic compound) by sediments are shown in Figure 1. The dataobtainedwith 14C-labelednaphthalenefor river and coal sediments refer to different grain-size fractions. The adsorption step was followed by a desorption step implemented by diluting the equilibratedsystem with water. Both sorption modes-adsorption and desorption-produced the same value of the partition coefficient. This finding confirms the complete reversibility of the sorption process for nonfunctionalized aromatic hydrocarbons under the experimental conditions applied. The size of the sediment particles has no significant influence on the partition coefficient within the range studied VO.1 and 0.25-0.5 mm). The linearity of the isotherm up to a sorbate concentration of lo4 ppm clearly favors partitioning processes over surface sorption as the dominant mechanism. Otherwise, an isotherm of the Freundlich type would have to be expected (3). Table 3 and Figure 2 present log KOC versus log KOW for some benzene derivatives. The differences in KOCof the river and the coal sediment are small, despite their different origin and organic matter contents (the latter being 6.516.9 and 52 wt %, respectively). The relation between KOC and KOWis in good accordance with that described in the literature (KOCx O.5Gw (4-6)). This clearly confirms the dominance of hydrophobic interactions in the sorption process. The position of phenols in Figure 2, closely above that of nonfunctionalized benzenes, indicates some contribution of polar interactions to sorptive binding. Apparently, a nitro group causes a stronger binding than the hydroxyl group. Charge transfer interactions are likely to occur. The common presence of both a nitro group and a hydroxyl group in one molecule backbone leads to stronger interactions with the organic matter of the sediment. Generally, the most significant deviation between hydrophobicity of neutral molecules and their sorption is observed for aromatic amines, where sorption typically exceeds estimated hydrophobic bonding5-10-fold (2,111. The extra contribution of an aromatic NH2 group to log KOC on sediments was found at about 0.65 unit for anthracene and chrysene. Surprisingly,Means et al. (11)did not observe significant pH effects, which are expected of an ionic sorption mechanism. Aniline, the most hydrophilic solute among those investigated here, can interact as a weak base (pK, = 4.87 (42))with acidic centers of the sediment material, or according to Paris (431, it reacts with humic carbonyl groups to yield imines. Therefore, no correlation between &C and KOWcan be expected for aniline. Apparently, the coal sediment has more strongly acidic centers or reactive carbonyl groups than the river sediment has. This is the onlypoint that could be revealed as a significant difference
f
10000
-k
8000
a
0
9000
7000
0
6000 E U
%
5000
c_
$
-5
4000
r 0 . c 0
3000
\,
0
2000
1000 n
0
1
2
3
4
5
6
C(naphtha1ene in water, ppm)
FIGURE 1. Sorption isotherms of naphthalene on sediments. TABLE 3
SOU- Water Partition Coefficients for Sediment Organic Materials solute benzene toluene o-xylene naphthalene phenol o-cresol 2,4-dimethylphenol nitrobenzene o-nitrophenol aniline
log Koc log KOW river sediment 2.13 2.73 3.12 3.35 1.49 1.95 2.30 1.85 1.79 0.90
1.73 2.08 2.32 3.08 1.4 1.7 2.08 1.95 2.32 1.96
log Koc
coal sediment 1.70 2.03 2.40 3.28 1.6 1.75 2.02 2.0 2.38 3.4
in the sorption properties of both sediments as related to an organic carbon base. Sorption Behavior of Humic and Fulvic Acids. Unfortunately,the experimentalapproach applied to sorption studies with particulate OM cannot be applied to DOM. The reason is the limited solubility of fulvic and humic acids in neutral water. This renders it very difficult to measure the complexation of less hydrophobic solutes because there are no pronounced interactions at all. Moreover, high DOM concentrations can change the complexation behavior of DOM in a nonrepresentative manner, e.g., by micelle formation. Under the assumption that the concentration of DOM amounts to 500 mg L-l, as used in our methodological studies, a portion of 33% complexation is produced for a solute with log KDOM = 3.0. If the noncomplexed portion of solute is determined by the analytical method and the complexed portion is calculated from the balance, as it is common practice, lower complexation ratios can hardly be detected reliably due to subtraction. The direct determination of the complexed portion after the release from the complex is a good alternative. In the case of particulate OM, a low partition coefficient can be compensated for by a high loading of the
7
8
9
water phase with solid material. The high DOM concentration used here favours extensive interactions between dissolved humic molecules themselves (47). The resulting DOM micelles possess a hydrophobic interior which may play the role of a hydrohpobic partitioning phase. This may be the reason why a fast liquid-liquid extraction does not remove this sorbate fraction, whereas the surfacebonded sorbate is assumed to be at least partially extracted. Nevertheless, the DOM concentration in our sorption experiments is close to that in the original coal wastewater under study. In the present study, a mixture containing 25 components, among them alkylated, chlorinated, and nitrated phenols as well as PAHs, was used for complexation experiments with different humic and fulvic acids of both anthropogenic and natural orign. The solute components covered a range in log &W between 0.90 for aniline and 5.24 for pentachlorobenzene. For most of them, which possess a log KOWI3.0, no significant complexation could be observed. We chose two homologous series of sufficiently hydrophobic compounds to probe hydrophobic complexation affinity of DOMs. The first series involved four chlorinated phenols with 2-5 chlorine atoms (log&w = 3.06 ... 5-24], the second series involved five PAHs from naphthalene to fluoranthene (log&, = 3.35 ... 5.16). The correlation log KDoM versus log &w for both series of solutes and the DOMs from the coal wastewater dump is presented in Figures 3 and 4. For the purpose of comparison, the straight line which corresponds to the relation
cited by Hutchins et al. (15) and Enfield et al. (20) was inserted in Figure 3. Although the KDOM value increases with enhanced hydrophobicity of the solutes, there is no common correlation for chlorophenols and PAHs. Beside, the scattering in our complexation data (h0.2unit in log &OM) does not permit the calculation of reliable regression VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
945
..Q . river sediment. ammatica
3.5
nitrammatics
-0-
3
c a i Mdlmsnt, ammatlca
+-phenols
nnmaromatics
- K o =~ 0.6 Kow Ig Koc 2
1.5
./
I
13
1
23
2
IgKow
3
3.5
__C
FIGURE 2. Log KOCversus log K, lor sorption of some aromatics on different sediments 43
4,3
41
t
3,s
1
3.7
Ig K(D0M) 3,s 3,3
0
3.1
2,s PAHs + humic acid
Ig K(DOC)-0.709 Ig
2.7 2s
3
33
4
-
485
IgKow
5
53
FIGURE 3. Log Koom versus log KOWfor PAHs with fulvic and humic acids from coal wastewater (surface water).
coefficients for each correlation. It is, however, obvious from our data that the slopes of such regression lines are significantly below 1.0 (in the order of 0.6 for humic acids and 0.4 for fulvic acids). This means there is no Linear relation between Kom andhw. Inotherwords, anincrease in the hydrophobicity of the solute does not produce a proportional increase in the complexation by humic and Mvic acids. This tendency is more pronounced for the more polar fulvic acids than for the less polar humic acids. Intheliterature,itispossibletofindvaluesofthecoefficient a in eq 2 in the range of 0.5 (9) up to 1.0 (4-6). 946 m ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29.
NO. 4.1995
We decided to evaluate the hydrophobic complexation afiinity ofDOMs by usingthe log KoaMoffluoranthene and pentachlorophenol, the most hydrophobic solutes in our “homologous series”, because they give the most reliable sorption data. The data in Table 4 permit us to establish a rank of hydrophobicitiesof the DOMs studied hereJhe Mvic acids are generally less hydrophobic than the corresponding humic acids. That is what can be expected. The distance amounts to a factor of about 2-3 in absolute KDoMvalues. This is very close to literature results (16, 17,27,31). The
415
1
t Ig K(D0M)
+
2,s
!
/
3
PCPs + humlc acid
- -10 K(DOC)e0.70gIg K(OW)+0.748
-
I
I
1
395
4
4,5
IgKow
I
1
5
5,5
FIGURE 4. Log KOOMversus log KOWfor chlorophenols with fulvic and humic acids from coal wastewater (surface water). TABLE 4
Hydrophobic Complexation of Fluoranthene and Pentachlorophenol by Different Fulvic and Humic Acids
DOM bog water
fulvic acid humic acid coal sediment fulvic acid humic acid coal waste water fulvic acid (surface water) humic acid river sediment humic acid
log KDOM pentafluor- chloroSOOM anthene phenol [(cal CIII-~)-~~]
3.8 4.05 4.25 4.4 3.9 4.35 3.8
3.5 4.15 4.05 4.3 3.65 4.05 3.5
13.01 12.74 12.45 12.20 12.94 12.29 12.74
calculation of sorption coefficients by means of correlations between KDOCand KOWfrom the literature yields a good fit to the experimentally measured values for the humic acid from bog water. Recently, Gremm (31)determined partition coefficients of some PAHs on different kinds of DOM by means of fluorescence quenching. For pyrene (log KOW= 4.881, he obtained log KDOM in the range of 4.0-4.9. Maxim (25,26) gives log KDOM= 3.1 for the interaction between pyrene and DOM isolated from soil. Complexation was measured by means of fast solid-phase extraction. These two results point to the sustaininglarge range of scatteringin literature data on DOM sorption properties. This suggests that it would be appropriate to compare sorption data of such DOMs that are isolated and characterized by identical methods instead of cross comparisons between results obtained by different researcher groups. Regarding the different sources of DOM investigated in our study, the humic acids isolated from the coal wastewater and its sediment possess the most hydrophobic properties, but the difference to humic acid originating from bog water is less than a factor of 2 in terms of KDOMfor the probe
components pentachlorophenol and fluoranthene. Thus, taking together all the findings, including those obtained from our studies with sediments, we can conclude: The organic matter-both the particulate and the dissolved one-from the coal wastewater pond possess a hydrophobic sorption affinity,which is comparable to or slightly higher than that of natural OM. Three further findings obtained from sorption experiments with the 25-component mixture used should be briefly highlighted: (i) rn-Dinitrobenzene was found to be complexed by most of the DOMs (log KDOMRZ 3 ... 4) despite its low hydrophobicity (logKOW= 1.49). This is in agreement with the preferred sorption of nitrobenzene and nitrophenol on SOM (cf. Figure 2). (ii) Acenaphthylene is complexed to a significantly greater degree than its position among the other PAHs would warrant (loghw = 3.94 (48)).Based on acomparison with acenaphthene (log KOW= 3.92 (4811, the additional contribution of the double bond to an enhanced partition coefficient of acenaphthylene is about 0.3 unit in log KDOM. Apparently, the activated double bond in acenaphthylene plays a role in its interactions with DOM. (iii) Aniline is bound completelyby all of the investigated DOMs at a pH close to 7. Aplausible reason is salt formation with acidic groups,which are available abundantly in fulvic and humic acids. Interpretation of Sorption Data in View of Solubility Parameter Concept. Table 5 summarizes a set of relevant data on solute components. Their 6 values were taken from the literature or, if data were not available, estimated from the heat of vaporization and by means of incremental methods (39-41). Eight compounds (BTX,naphthalene, phenols) were used to calculate &OM of the two sediments. The meanvalue is 12.5 f.0.2 (cal~ m - ~ ) ’for / *both sediments under study. The range of error corresponds to one standard deviation of the single 6 values clustered around the mean value. VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1947
TABLE 5
Solubility Parameters 6 [(cal ~ m - ~ ) - ~of’ ~ ] Sediments According to Eq 7 from Sorption Experiments solute
6i
benzene toluene o-xylene naphthalene phenol o-cresol 2,4-dimethylphenol nitrobenzene o-nitrophenol aniline av (6 from 8)
9.2 8.9 8.8 9.9 11.1 10.2 9.9 10.0 12.6 10.3
a
[rnL
vi
89.4 106.8 121.2 111.5 87.7 104.7 117.3 102.2 106.0 91.5
!OM
river sediment 12.47 12.52 12.51 12.39 13.44 12.68 12.20 a 1 1.85 a
SOM
coal sediment 12.55 12.61 12.38 11.83 12.64 12.55 12.35 a 11.87 a
12.46 i 0.16 12.51 i 0.12
E q 7 yields a complex number for doM.
The binding behavior of aniline and nitrobenzene cannot sufficiently be described by the one-component solubility parameter. Most likely, the aniline is bound by specific interactions as discussed above. The application of eq 7 to nitrobenzene using the mean BSOM values produces an underestimation of the sorption coefficients by 0.3 and 0.5 unit in log KOCfor the river sediment and the wastewater sediment, respectively. The sorption isotherms of nitrobenzene, however, were found not quite linear but slightly curved (Freundlich type). This is characteristic of a surface sorption instead of a pure partitioning mechanism or it is due to an experimental artifact. This cannot be met by the 6 concept. The generalizable conclusion is the close similarity between both types of sediments with respect to hydrophobic interactions. The solubility parameter of the “polymer” SOM is close to 12.5 (cal ~ m - ~ ) instead l/* of 13 proposed earlier by Chiu et al. (12). Unfortunately, we could neither find nor estimate reliable 6 values of the polychlorinated phenols. Therefore, the discussion of solute binding by DOM is restricted to PAHs, namely, fluoranthene (log KOW= 5.16). Its 6 value was assumed to be close to those of other PAHs, Le., 9.8 f 0.1 (cal and its molar volume was about 176 cm3. The calculated 6 values of DOMs have been compiled in Table 4. They are in the range of 12.6& 0.4 (cal cm-3)112. This is very close to that of particulate SOMs. The 6 values of fulvic acids are about 0.4 unit higher than those of the corresponding humic acids, that means they have a slightly higher polarity, as expected from their greater water solubility. It should be noted that the 6 concept produces consistent results for very different hydrophobic solutes, such as benzene and fluoranthene. However, it fails to explain the observed binding of dinitrobenzene on DOMs. The calculation yields KDOM = 2Kowin contrast to the experimental results, which indicate KDOM > 1OKoW. Specificinteraction between nitro groups and SOM or DOM, e.g., formation of charge-transfer complexes, might be an explanation. Formula 7 may be rearranged so that the solubility parameter BOM can be calculated explicitly with known properties (V, KOW,KOM,6 ) of the solute i. Provided that specific interactions are absent, the value of BOM should not depend on the solute that is applied as a probe in sorption experiments. Figure 5 presents the result of this 948
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995
calculation for a commercial humic acid (data from Chin and Weber (21))and for the humic substances investigated in the present study. Amean 6 0value ~ of 12.5 f 0.5 (cal cm-3)1’2is obtained that is approximately constant through 4.5 orders of magnitude in KOWof solutes. Differences between particulate and dissolved organic matters are small. Rav-Acha and Rebhun (50)concluded from similar slopes in eq 2 for sorption of PAHs on solid soil humics and dissolved humic substances that the binding to both types of sorbents is controlled by similar thermodynamics. This conclusion is in line with the results of our treatment. They do not, however, confirm a 20 times higher sorption coefficient on DOM compared with SOM as claimed by Rav-Acha and Rebhun for fluoranthene (178versus 8 L g-l). Garbarini and Lion (49)determined sorption coefficients for toluene and trichloroethylene on several fractions of soil organic matter as well as on some model compounds. We used eq 7 to calculate solubility parameters of these fractions with the approximation KOMx 0 . 5 6 ~ The . results are presented in Table 6. Both solutes give very similar 6 values for the same sorbent. The scattering is the greatest for cellulose due to verylow sorption coefficients (e.g.,KOC= 0.05 for toluene). Nevertheless,the mean calculated value (15.6)is very close to the literature value of cellulose (15.7, cf. Table 1). In summary, the obtained values are reasonable. They reflect an increasing hydrophobicity of soil organic fractions in the order lipophilic fraction > humin > complete soil > humic acid > fulvic acid. The 6 values of complete soil, soil humic acid, and Aldrich humic acid are in the range 12.5f0.3 (cal ~ m - ~ ) ”which * , is identicalwith thosevalues for sediments and dissolved humic matters determined in this paper. The fulvic acids isolated from soil by Garbarini and Lion appear significantly more polar (6 = 13.8-14.6) than ours from an aquatic environment (6 = 12.5-13.0). Another example for verifylng our concept includes recent data of Weber et al. (291,who measured the sorption of three chlorinated hydrocarbons, tetrachloroethylene (log KOW = 2.88,6 = 9.31, 1,4-dichlorobenzene (3.38, 10.01, and 1,2,4-trichlorobenzene (3.98, 10.2) on six subsurface soils. We picked up the sorption coefficients for that soil having the most linear isotherms (AnnArbor Ih0.58 wt % OC; log KOC= 2.26, 2.90, and 3.55, respectively). Application of eq 7 yields &OM = 12.8 & 0.1 for all the three solutes. This is in good conformity with our concept. If the solubility parameter is taken as a measure of polarity of a compound, it is useful to compare the value for humic substances with those of well-defined polymers. Humic substances are placed in the rank between ethyl cellulose (6 = 10.3)and cellulose acetate (6 = 13.4)and are very close to phenolic resin (6 = 12.5). This is a plausible rank. Chin and Weber (21) proposed methyl salicylate (6 = 10.6)and polymaleic acid (6 = 13.6)as suitable surrogates for commercial humic acids and more polar natural fulvic acids, respectively. The estimated 6 value of polymaleic acid, however, seems too low to us. It was based on the 6 value of maleic anhydride, which is hardly a good representative of a polyacid. Irrespective of the surrogate structure, the proposed 6 value of fulvic acids is in a range close to experimental values (see above). The solubility parameter of lignin deduced from sorption coefficients of toluene and trichloroethylene (49) amounts to 12.15 in comparison to 10.5 f 1 from the literature (40).
0 13
* -IPolyethylenel
ACoal wastewater dump ORlver sediment
7
1
TABLE 6
Solubility Parameters of Soil Fractions and Some Model Compounds from Sorption Coefficients (49) [a as (cal sorbent/solute soil humin humic acid fulvic acid lipophilic fraction lignin zein Aldrich-HA tannic acid cellulose
toluene trichloroethylene ( V = 106.8 mL 6 = 8.9, ( V = 90.2 mL, 6 = 9.2, log Kow = 2.69) log I& = 2.35) 12.2 11.7 12.65 13.75 n.d. 12.2 12.35 12.6 13.6 16.2
12.2 11.85 12.8 14.6 9.2 12.1 12.15 12.65 13.7 15.0
+ Cnitrogen
log k& = 1.83
+ 0.625 log GW- 2.34PI (? = 0.895) (11)
In principle, this correlation could be applied to any SOM, but there are some difficulties in determining polarity indices of soils directly. It is obvious that the elemental composition is a rough measure of the polarity of a substance only. As an example, an oxygen atom in an ether bridge or in a hydroxyl group effects quite different contributions to the polarity of the compound. Therfore, the authors defined an ‘effective polarity’ for soils by rearranging eq 11
An extension of the one-component solubilityparameter to composed solubility parameters, as it was proposed by Chin and Weber (211,will surely improve the precision of predicted partition coefficients. Nevertheless,the concept will be unable to describe more than an unspecific, hydrophobic sorption. Therefore, we preferred the easily applicable one-component parameter, which can be determined more reliably than a multicomponent one. Very recently, Xing et al. proposed in this journal (51) a ‘cross correlation of polarity curves to predict partition coefficients of nonionic organic contaminants’, which is very close to the subject of the present paper. The authors defined a polarity index Coxycygen
measured partition coefficients KOCfor benzene, toluene, and o-xylene between water and some model polymers (lignins,humic acids, chitin, cellulose). The obtained KOC values were correlated with the corresponding KOWvalues of the sorbates and the PIS of the sorbents
PI,,, =
1.83
+ 0.625 log &, 2.34
- log 1;6,
(12)
The effective polarity of a soil obtained from at least one measured KOCcan be used now to calculate KOCvalues for any sorbate on the basis of its tabulated KOWvalues. The advantage of the effective polarity is that it includes a lot of soil properties beyond its elemental composition. It is, however, an completely empirical term. It correlates with the proportion of aromatic carbon in SOM
PI,,, = 0.702 - 0.00353
% Carornatic %
(? = 0.969)
Ctotal
(10)
(13)
for organic polymers, with ci being the weight portions of oxygen, nitrogen, and carbon in the polymer. Then, they
This correlation suggests that the aromaticity either influences sorption or is a surrogate for soil polarity. The importance of aromaticity in dissolved humic matter for
PI =
Ccarbon
VOL. 29, NO. 4, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1949
interaction with nonionic solutes has been suggested by Gremm (31). The approach of Xing et al. (51)is a one-parameter concept too, but it does not consider the sensitivity of different sorbates for sorbent’s polarity, as included in our concept by the term OM - ai)’. Every new group of sorbents needs a separate, empirical, Koc/&w/PI correlation. Nevertheless, the sorption coefficients ofXing et al. can be used for recalculating some solubility parameters. We obtained for the two humic acids 6 = 12.80 and 12.85 (cal ~ m - ~ )for ” ~the , two lignins 6 = 11.9 and 12.3, and for the investigated soils ~ S O Mbetween 12.2 and 12.65. The 6 values of all the sorbents are independent of the solutes within f O . l unit. The obtained 6 values are fully in line with our own data and those presented in Table 6. It is noteworthy that the mean 6 value of lignin (12.1) is identical with that in Table 6 (12.15).
Acknowledgments This work was supported financially by the Deutsche Forschungsgemeinschaft (Project KO 133412-1).
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Received for review June3,1994. Revised manuscript received December 1, 1994. Accepted December 13, 1994.@
ES940341A ~
~~~
@Abstractpublished in Advance ACS Abstracts, February 1, 1995.
