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Evaluation of the partitioning of hydrophobic pollutants between aquatic and solid phases in natural systems
the Science of the TotaiEmironment Yr--C-R~h m.~-mldIDlld.~~ ELSEVIER
The Scienceof the Total Environment 177 (1996) l-7
Evaluation of the partitioning of hydrophobic pollutants between aquatic and solid phases in natural systems Yuan-Hui Zhao” , Pei-Zhen Lang Department
of Enkorunental
Science, Northeast
Normal
Unirwsiy,
Changchun
City, People’s Republic
of China
130024
Received 28 February 1985;accepted 1 April 1995
Abstract Sorption partition coefficients are commonly used to quantify the distribution of organic pollutants between the aqueousand particulate phaseof natural aqueoussystems.Lotse et al. [6] found a significantincreasein partitioning as the solid concentration decreases.Pollutants with larger partition coefficients have a more pronouncedeffect on the observed partitioning with changes in solid concentration. In the present study, partition coefficients for sediment-watersystemswere determined using Second SonghuaRiver sediment.The effect of added DOC (using 12.5 ppm fulvic acid) and centrifuge speedon the partitioning of pollutants were studied. Dissolvedorganic carbon (DOC), nonsettlingmicro-particles(NSPs)and light absorptionwere alsoanalyzed. From the results,the solidseffect can be attributed to a transfer of solute-bindingmaterial from the solid phaseto the liquid phaseduring the course of the partitioning procedure. This material, whether DOC or NSPsin nature, was not removed from the liquid phase during the separation procedure and possessed a higher capacity for the solute than the water itself. It increasesthe amount of solute in the liquid phaseand makesthe observedpartition coefficients decrease.The true partition coefficients (K&) of 11 constituents have been calculated from the observed partition coefficients at different solid concentrations. The values of K& approach a constant and cannot be affected by the solid concentration. It supportsthe explanation describingthe effect of solids. Hydrophobic pollutants; Sediments;Partition coefficients; DissolvedOrganic Carbon(DOC); Nonsettling microparticles(NSPs) Keywords:
1. Introduction
Theoretical and experimental evidence relating to the sorption of hydrophobic compounds by
* Corresponding author.
sediments and suspended solids have been reviewed in previous publications [l-4]. Generally, it has been concluded that the extent of such reactions is proportional to the octanol-water partition coefficient (K,) of the solute and the organic carbon content of the adsorbent
OCM8-9697/96/$09.50 0 1996ElsevierScienceBV. All rightsreserved. SSDI
0048-9697(95)04853-S
2
Y.-H. Zhao,
P.-Z.
gression equation for the K,,,-K, logI&
= ZogK,, - 0.21
K, = q&c
Lang / The Science
relationship (1) (2)
where K, is the sorption partition coefficient and K, is the organic carbon normalized sorption partition coefficient. Recently, however, the concept of linear partitioning has come into question. Some scientists have found a significant increase in partitioning as the solid concentration decreases. Pollutants with larger partition coefficients have a more pronounced effect on the observed partitioning with changes in solid concentration. Studies on lindane sorption to lake sediments [61 suggested that partitioning of these compounds may vary as a function of solid concentration. However, Karickhoff et al. [S] did not find evidence that changes in solid concentration significantly affected pyrene and methoxychlor partitioning. A similar effect has been observed for the sorption of dieldrin to clay [7] and it was suggested that the solids effect possibly resulted from the loss of dieldrin onto the glass surface of the experimental vessel. However, a variety of studies have suggested that solid concentration significantly affected compound partitioning. Choi and Chen [S] reported that the partition coefficients were linearly related to organic carbon and clay content. Horzempa and DiToro [lo] found that solid concentration affected HCBP (2,4,5,2’,4’,5’-hexachlorobiphenyl) sorption by montmorillonite clay and Saginaw Bay sediment. They suggested that the observed variation in partitioning with solid concentration may result from interactions occurring between suspended sediment particles (for instance, electrostatic interactions). Interactions of this nature could result in a reduction in the number of solid phase binding sites accessible for sorption of a molecule such as HCBP. Linear increases in solid concentration may not necessarily result in analogous linear increases in surface area. O’Connor and Connolly [ll] have presented a functional correlation between the partition coefficient and solid concentration. Voice et al. [121 found that the solids effect appeared to result
of the
Total Enrironment
177 (1996)
I-7
from the presence of microparticles contributed by the solids, which were not removed from suspension in the separation procedure. To investigate this explanation, the liquid phase was analyzed after separation for total organic carbon (TOC), turbidity, W adsorption at 254 run, and fluorescence (excitation at 365 nm, emission at 415 mu). They found a significant trend of increased turbidity and TOC with increased solid concentration. Gschwend and Wu [13] reported similar results for the sorption of polychlorinated biphenyl isomers (PCB) to sediment and predicted the observed sorption partition coefficients (K,O) with theoretical equation. The purpose of this investigation was to study the behaviour of pollutants in the Second Songhua River; specifically towards obtaining a quantitative assessment of the effect of solid concentration on the partitioning between solid and liquid phases of hydrophobic compounds. Sediment from the river and 11 hydrophobic pollutants in this system were selected for the study. 2. Materials
and methods
Near-surface sediment (Wukeshu, Second Songhua River, September 1993) was collected, dried naturally, homogenised by hand and sieved to remove particles > 1 mm. The organic carbon content determined by the K&O, method is 0.85%. Stock solutions of hydrophobic pollutants were prepared in acetone. Sorption isotherms were obtained on various combinations of solids and solutes. Each isotherm was conducted according to the following procedure. A constant mass of sediment was weighed into each of five lo-ml centrifuge tubes fitted with ground-glass stoppers. Different volumes of stock solution were added to each tube using a microinjector. Distilled water was also added to each tube to ensure the same solution volume (10 ml>. Five tubes containing solute, but no sediment, were used as blanks. All sorption experiments were carried out in duplicate. The tubes were sealed and placed on a thermostatic shaker at 20 f 0.5”C. Sorption kinetic studies showed that the equilibrium was quickly reached between 4-6 h,
Y.-H. Zhao,
P.-Z. Lang/The
Science
so that a contact time of 6 h was arbitrarily selected to ensure attainment of sorption equilibrium. After 6 h of shaking, the tubes were removed and centrifuged at 4000 rev./min for 20 min. Nine millilitres of the supernatant were then carefully transferred to a separator funnel containing 1 g of NaCl and extracted three times with 1 ml of petroleum ether for 5 min. The extracts were analyzed by gas chromatography (Shimadzu GC-7AG) equipped with an electron capture detector. The concentration of solutes in the solid phase was computed by difference. Partition coefficients of added DOC (using 12.5 ppm fulvic acid (FA), from the Institute of Geography of Academia Sinica, Changchun, China) and different centrifuge speeds were determined to assess the impact of DOC and NSPs. DOC measurements were made in the supernatant by TOC-500 Total Organic Carbon Analyzer. The contents of NSPs, average diameters of NSPs and light adsorption in the supernatant were also analyzed with a CAPAParticle Analyzer. 3. Results and discussion It was observed that for each solid concentration, the isotherms were linear over the range of solution concentrations with near zero intercepts (Fig. 1). K,, values were obtained from the slopes of isotherms divided by the organic carbon content of the sediment. Fig. 2 shows the dependence of normalized partition coefficients on the solid concentration for five different solutes. In all cases, the partition coefficients increase significantly as the solid concentration in the system is decreased. Pollutants with larger partition coefficients have a more pronounced effect on the observed partitioning with changes in solid concentration. The effect of solid concentration is less pronounced for substances with low partitioning coefficients since values are essentially constant. Data are given in Table 1 to show the effect of added DOC (using 12.5 ppm FA) on the partitioning of hexachloroethane between solid and aqueous phases. The results indicate a significant reduction in hexachloroethane sorption as the DOC concentration is increased. Table 2 shows the
of the Total Environment
177 (1996)
0
PI
3
l-7
4,
am
l e
conoontration
aqueous
1.0
fug/l)
Fig. 1. Hexachloroethane isotherms for different solid concentrations. Solid concentration CC,): a, 25.0 (mg/ml); b, 50.0 (mg/ml); c, 75.0 (mg/ml).
results of the normalized partition coefficients for hexachloroethane determined at different rev./min. Clearly, the K,, values increase as the rev./min are increased. Also shown are the results of two experiments in which light absorption and average diameter of NSPs are decreased with increased rev./mm (Table 2). On the basis of these results, the observed change in partition behaviour with changes in solid concentration can be attributed to a transfer
‘=m
0
PI
solid
40
concentration
00
00
(mg/ml 1
Fig. 2. Normalized partition coefficients as a function of the solid concentration. 1, hexachlorobutadiene; 2, 1,2,3-trichlorobenzene; 3, hexachloroethane; 4, (~-666; 5, m-nitroanisole.
4 Table 1 Hexachloroethane
Y.-H. Zhao,
P.-Z. Lang / The Science
of the
Total Environment
normalized partition coefficient of added 12.5 ppm FA
Solid concentration (mg/ml)
Supematant
62.5 62.5
Distilled water 12.5 ppm FA solution
of solute-binding material from the solid phase to the liquid phase during the course of the partition experiment. It is suggested that this material, whether DOC or NSPs in nature, was not removed from the liquid phase during the separation procedure and possessed a higher capacity for the solute than the water itself. The contents DOC and NSPs in supernatant increase as the solid concentration is increased (Table 3). It increases the amount of solute in the liquid phase and makes the observed partition coefficient decrease. Further experiments were carried out to assess the role of DOC and NSPs in the partitioning process. The supematant was analyzed after separation for DOC concentration, light adsorption, contents and average diameter of NSPs. A trend of increased light absorption, increased contents of NSPs and DOCs with increased solid concentration can be observed (Table 3). In order to assess the effect of DOC and NSPs upon the sorption of solute, excess hexachlorobutadiene was added separately to three flasks containing distilled water and 12.5 ppm FA solution and the supematant centrifuged at 4000 rev./mm after 6 h of equilibration time. The results given in Table 4 show that the solubility of 12.5 ppm FA solution and the supematant are greater than distilled water. The data suggest that DOC and NSPs
Table 2 Hexachloroethane Solid concentration
2360 1380
possess a higher capacity for solute than for water. Based upon the above results, we assume that the system consists of water phase, solid phase, DOC, NSPs and sorption is a partition process [9,131 for non-ionic organic compounds. We can obtain Eqs. 3 and 4 (3) (4)
where Ki is the observed sorption partition coefficient, Ki is the true sorption partition coefficient, V is the volume of water, P is the mass of compound adsorbed to settleable particles (solid phase), D is the mass of compound dissolved, N is the mass of compound adsorbed to NSPs, DOC is the mass of compound adsorbed to DOC, and M is the mass of settleable particles. Combining Eqs. 3 and 4, we obtain Eq. 5 (5)
Karickhoff and others [5,8] found that pollutants adsorbed more strongly on finer particle sires than on large particle sizes. Similar results
normalized partition coefficients, light adsorption and average diameter at different rev./min Rev./min
kc
Light adsorption
Average diameter
(pm)
(mg/ml) 62.5 62.5 62.5
I77 (I 996) I - 7
3000 4000 5ooo
1980 2360 2820
0.10 0.06 0.04
0.17 0.15 0.12
Y.-H. Zhao,
P.-Z.
Lang/The
Science
of the
Total Enuironment
177 (1996)
5
l-7
Table 3 Some properties in supematant centrifuged at 4000 rev./min
Content of NSPs ( pg/ml) DOC concentration (ppm) Light absorption
Solid concentration (mg/ml) 12.5 37.5
62.5
87.5
56 1.52 0.01
381 3.34 0.05
483 3.59 0.07
224 2.62 0.02
9, we obtain Eq. 10
have been observed in investigations on the sorption of organophosphorus and carbamate insecticides to soils [14-161. They suggested that NSPs and DOC should be treated with enhanced K,s. If we use Ki-, to express the partition coefficients of compounds between NSPs and water, K’ p-D to express the partition coefficients of compounds between DOC and water and assume that K’ P-N and Ki-,, are larger than Ki (sediment/water partition coefficient), that is Ki = fo,K:, s foe-NK;c-N = A,fo, Kh, K; = f,Ki, I focvDK&, = Azfoc K& (Al 2 1 and A, 2 1 are constants), we can obtain Eqs. 6 and
KG=
=A,focK:,
K;-D =fo,.DK:c-,
=AzfocK:c
N/MN = - D/V DOC/M, = D/V
(6) (7)
From Eqs. 6 and 7, we obtain Eqs. 8 and 9
DOC=A2fo,K$MD
K,“, = (8)
N=nlfo,K:c;M,v =A,K$M,
K& 1 +K:,C,(A,B,f,,,
(111
+A,B,)
(9)
solubility in different conditions
-
Distilled water FA solution Supematant
(10)
Eq. 11 explains the experimental phenomena concerning the effect of solid concentration on sorption partition coefficients which we have observed. Pollutants with larger partition coefficients have a more pronounced effect on the observed partitioning with changes in solid con-
where M, is the mass of NSPs, M,, is the mass of DOC, and foe-n = 100%. From Eqs. 5, 7 and Table 4 Hexachlorobutadiene
+A$@/V
Voice et al. [12,13] found that the amount of materials (DOC and NSPs) contributed to the liquid phase were most likely proportional to the solid phase present. Our experimental results show also that the weight of NSPs CM,) and weight of DOC CM,,) are proportional to the weight of sediment (M = C,V>. That is MN = B&V and M, = B,C,V (B, and B,, the weight of DOC and NSPs/per weight of sediment, are constants, related to properties of sediment. For the Songhua River sediment, B, = 5.77 x 10m3, B, = 2.77 x 10e5). Introducing into Eq. 10, we obtain Eq. 11
7
K;zv =foc-,vK&-,v
Klic 1 +K;(A,f,,M,
DOC (wd
Content of NSPs ( pg/ml)
Solubility @Pm)
0.0 12.5 3.3
0.0 0.0 387.0
1.8 2.1 2.4
6
Y.-H. Zhuo,
Table 5 Eleven organic
compounds
Solid concentration
K&
P.-Z. Lung/The
values
(mg/ml)
K;
1,2,4,5-Tetrachlorobenzene Hexachlorobutadiene Hexachloroethane 1,2,3-Trichlorobenzene 6666 P-666 y-666 a-666 2,6-Dinitrotoluene m-Nitroanisole p-Nitrotoluene
calculated
Science
from
x lo3 (ml/g)
values
Kk
sediment
X lo3 (ml/g)
25.0
50.0
75.0
6.25
12.5
25.0
50.0
75.0
11.63 12.04 6.84 8.16 7.29 4.40 3.71 3.64
9.50 9.64 4.90 6.71 5.33 4.13 3.04 2.74 0.74 0.73
5.75 7.45 2.87 4.22 4.17 2.87 2.39 2.06
4.61 5.73 1.82 3.25 2.95 2.01 1.88 1.53 0.56 0.59 0.53
3.24 5.02 1.30 2.39 2.34 1.59 1.37 1.03 0.55 0.56 0.51
15.20 13.80 11.10 10.60 9.03 5.24 4.39 4.73
15.40 13.80 11.10 10.50 7.24 4.35 4.08 4.73 0.80 0.77
10.80 10.90 8.20 7.80 7.54 4.94 3.98 4.30
18.10 11.80 10.30 10.70 10.36 4.86 5.07 6.78 0.71 0.74 0.65
15.20 13.80 11.10 10.60 9.03 5.24 4.39 4.73 0.80 0.77 0.68
0.69 0.61
+A,B,)
cc
= 1 - CK&C,
K0oc2 = 1 - CK&,,C,, calculated
River
1-7
12.5
c = A,B, f,, + A$,, is constant. Eq. 12 allows us to predict the true partition coefficient (K&j, if the values of K,“, f,,, A and B are given. But the determination of KheN and K& is difficult. In another words, we cannot obtain A r and A, values accurately. If Eq. 12 is an ‘ideal’ equation, the values of K& should be a constant calculated at different solid concentration and caMot be affected by the solid concentration. From Eq. 12, we obtain Eqs. 13 and 14
Table 6 HCBP KL
177 (1996)
6.25
(12)
K”OCl 1 - CK&,C,,
Total Entirontnent
Eqs. 12 and 14 on Songhua
centration and the solid concentration does not affect the partition coefficient when f,,,KA, C, (A, B, f,, + AZ&) < 1. Modifying Eq. 11, we obtain Eq. 12 KAc = 1 - K,O,C,(A,B,f,,
of the
from
(13)
0.78 0.68
K” -KgOc, ’ = K,“,,K:2,(C,, - Cs2>
(14)
Using Eqs. 12 and 14 and the observed partition coefficients at different solid concentration, we can calculate the true partition coefficients. Table 5 shows the values of K& calculated for 11 different compounds. Also, listed in Table 6 are K& values for HCBP calculated for hvo different sediments using experimental data obtained elsewhere [lo]. The data in Tables 5 and 6 indicate that the Kk values are approximately a constant across the range of solid concentrations (0.05-75 mg/ml) used in these investigations. This further supports the hypothesis that observed changes in partition behaviour due to solid concentration may be attributed to the transfer of solute-binding material from solid phase to the liquid phase during the course of the partitioning experiment. 4. Conclusion
Our results support the view of Voice and others that a solids effect can be attributed to
Eqs. 12 and 14
Solid concentration (mg/ml)
Montmorillonite clay KC
Kk
0.05 0.20 1.00
10 600 6690 2900
12300 10300 12300
Solid concentration (mg/ml) 0.055 0.220 1.100
Saginaw sediment
Bay
%C
CC
17100 12300 9900
17800 15800 17800
Y.-H. Zhao,
P.-Z. Lang/TheScienceofthe
dissolved organic carbon DOC and nonsettling microparticles NSPs which transfers from solid phase to liquid phase during the course of the partitioning experiment. The derived formula was used to calculate the true partition coefficient (K:,) values based upon our experiments and those of Gschwend and Wu [13]. The K& values were calculated at different solid concentrations for 11 organic compounds. The results indicate that KL values are approximately constant across the range of solid concentrations. Our results explain the experimental phenomena that pollutants with larger partition coefficients have a more pronounced effect and pollutants with lower partition coefficients have a less pronounced effect on the observed partitioning with changes in solid concentration. References Ul T.C. Voice and W.J. Weber, Jr., Sorption of hyDl
[31
[41
151
drophobic compounds by sediments, soils and suspended solids - I. Water Res., 17 (19831 143331441. J.W. Weber, Jr., T.C. Voice, M. Pirbazari, GE. Hunt and D.M. Vlanoff, Sorption of hydrophobic compounds by sediments, soils and suspended solids - II. Water Res., 17 (1983) 1443-1452. C.H. Giles and D. Smith, A general treatment and classification of the solute adsorption isotherm Part I. Theoretical. J. Colloid Interface Sci., 47 (1974) 755-765. C.H. Giles, A.P. D’Silva and L.A. Easton, A general treatment and classification of the solute adsorption isotherm Part II. Experimental interpretation, J. Colloid Interface Sci., 47 (1974) 766-778. S.W. Karickhoff, D.B. Brown and T.A. Scott, Sorption of
TotalEnbonment177(1996)1-7
I
hydrophobic pollutants on natural sediment. Water Res., 13 (1979) 241-248. [‘51 E.G. Lotse, D.A. Gratz, G. Chesters, G.B. Lee and L.W. Newland, Lindane adsorption by lake sediment. Environ. Sci. Technol., 2 (1968) 353-357. [71 B.T. Bowman and W.W. Sans, Partitioning behaviour of insecticides in soil-water system: I. Adsorbent concentration effects. J. Environ. Qual., 14 (1985) 265-269. [81 W.W. Choi and K.Y. Chen, Associations of chlorinated hydrocarbons with fine particles and humic substances in nearshore surficial sediments. Environ. Sci. Technol., 10 (1976) 782-786. [91 C.T. Chiou, P.E. Porter and D.W. Schmedding, Partition equilibria of nonionic organic compounds between soil organic matter and water. Environ. Sci. Technol., 17 (1983) 227-231. DO1 L.M. Horzempa and D.M. DiToro, PCB partitioning in sediment-water systems: the effect of sediment concentration. J. Environ. Qual., 12 (1983) 373-380. ill1 D.J. O’Connor and J.P. Connolly, The effect of concentration of adsorbing solids on the partition coefficient. Water Res., 14 (1980) 1517-1523. D21 T.C. Voice, C.P. Rice and W.J. Weber, Effect of solids concentration on the sorption partitioning of hydrophobic pollutant in aquatic systems. Environ. Sci. Technol., 17 (1983) 513-518. [131 P.M. Gschwend and S.C. Wu, On the constancy of sediment-water partition coefficients of hydrophobic organic pollutants. Environ. Sci. Technol., 19 (1985) 90-96. 1141 A. Felsot and P.A. Dahm, Sorption of organophosphoNS and carbamate insecticides by soil. J. Agric. Food Chem., 27 (1979) 557-563. [151 A. Khan, J.J. Hassett and W.L. Banwart, Sorption of acetophenone by sediments and soils. Soil Sci., 128 (1979) 297-302. [I61 D.L. Zierath, J.J. Hassett and W.L. Banwart, Sorption of benzindine by sediments and soils. Soil Sci., 129 (1980) 217-281.
The Scienceof the Total Environment 177 (1996) l-7
Evaluation of the partitioning of hydrophobic pollutants between aquatic and solid phases in natural systems Yuan-Hui Zhao” , Pei-Zhen Lang Department
of Enkorunental
Science, Northeast
Normal
Unirwsiy,
Changchun
City, People’s Republic
of China
130024
Received 28 February 1985;accepted 1 April 1995
Abstract Sorption partition coefficients are commonly used to quantify the distribution of organic pollutants between the aqueousand particulate phaseof natural aqueoussystems.Lotse et al. [6] found a significantincreasein partitioning as the solid concentration decreases.Pollutants with larger partition coefficients have a more pronouncedeffect on the observed partitioning with changes in solid concentration. In the present study, partition coefficients for sediment-watersystemswere determined using Second SonghuaRiver sediment.The effect of added DOC (using 12.5 ppm fulvic acid) and centrifuge speedon the partitioning of pollutants were studied. Dissolvedorganic carbon (DOC), nonsettlingmicro-particles(NSPs)and light absorptionwere alsoanalyzed. From the results,the solidseffect can be attributed to a transfer of solute-bindingmaterial from the solid phaseto the liquid phaseduring the course of the partitioning procedure. This material, whether DOC or NSPsin nature, was not removed from the liquid phase during the separation procedure and possessed a higher capacity for the solute than the water itself. It increasesthe amount of solute in the liquid phaseand makesthe observedpartition coefficients decrease.The true partition coefficients (K&) of 11 constituents have been calculated from the observed partition coefficients at different solid concentrations. The values of K& approach a constant and cannot be affected by the solid concentration. It supportsthe explanation describingthe effect of solids. Hydrophobic pollutants; Sediments;Partition coefficients; DissolvedOrganic Carbon(DOC); Nonsettling microparticles(NSPs) Keywords:
1. Introduction
Theoretical and experimental evidence relating to the sorption of hydrophobic compounds by
* Corresponding author.
sediments and suspended solids have been reviewed in previous publications [l-4]. Generally, it has been concluded that the extent of such reactions is proportional to the octanol-water partition coefficient (K,) of the solute and the organic carbon content of the adsorbent
OCM8-9697/96/$09.50 0 1996ElsevierScienceBV. All rightsreserved. SSDI
0048-9697(95)04853-S
2
Y.-H. Zhao,
P.-Z.
gression equation for the K,,,-K, logI&
= ZogK,, - 0.21
K, = q&c
Lang / The Science
relationship (1) (2)
where K, is the sorption partition coefficient and K, is the organic carbon normalized sorption partition coefficient. Recently, however, the concept of linear partitioning has come into question. Some scientists have found a significant increase in partitioning as the solid concentration decreases. Pollutants with larger partition coefficients have a more pronounced effect on the observed partitioning with changes in solid concentration. Studies on lindane sorption to lake sediments [61 suggested that partitioning of these compounds may vary as a function of solid concentration. However, Karickhoff et al. [S] did not find evidence that changes in solid concentration significantly affected pyrene and methoxychlor partitioning. A similar effect has been observed for the sorption of dieldrin to clay [7] and it was suggested that the solids effect possibly resulted from the loss of dieldrin onto the glass surface of the experimental vessel. However, a variety of studies have suggested that solid concentration significantly affected compound partitioning. Choi and Chen [S] reported that the partition coefficients were linearly related to organic carbon and clay content. Horzempa and DiToro [lo] found that solid concentration affected HCBP (2,4,5,2’,4’,5’-hexachlorobiphenyl) sorption by montmorillonite clay and Saginaw Bay sediment. They suggested that the observed variation in partitioning with solid concentration may result from interactions occurring between suspended sediment particles (for instance, electrostatic interactions). Interactions of this nature could result in a reduction in the number of solid phase binding sites accessible for sorption of a molecule such as HCBP. Linear increases in solid concentration may not necessarily result in analogous linear increases in surface area. O’Connor and Connolly [ll] have presented a functional correlation between the partition coefficient and solid concentration. Voice et al. [121 found that the solids effect appeared to result
of the
Total Enrironment
177 (1996)
I-7
from the presence of microparticles contributed by the solids, which were not removed from suspension in the separation procedure. To investigate this explanation, the liquid phase was analyzed after separation for total organic carbon (TOC), turbidity, W adsorption at 254 run, and fluorescence (excitation at 365 nm, emission at 415 mu). They found a significant trend of increased turbidity and TOC with increased solid concentration. Gschwend and Wu [13] reported similar results for the sorption of polychlorinated biphenyl isomers (PCB) to sediment and predicted the observed sorption partition coefficients (K,O) with theoretical equation. The purpose of this investigation was to study the behaviour of pollutants in the Second Songhua River; specifically towards obtaining a quantitative assessment of the effect of solid concentration on the partitioning between solid and liquid phases of hydrophobic compounds. Sediment from the river and 11 hydrophobic pollutants in this system were selected for the study. 2. Materials
and methods
Near-surface sediment (Wukeshu, Second Songhua River, September 1993) was collected, dried naturally, homogenised by hand and sieved to remove particles > 1 mm. The organic carbon content determined by the K&O, method is 0.85%. Stock solutions of hydrophobic pollutants were prepared in acetone. Sorption isotherms were obtained on various combinations of solids and solutes. Each isotherm was conducted according to the following procedure. A constant mass of sediment was weighed into each of five lo-ml centrifuge tubes fitted with ground-glass stoppers. Different volumes of stock solution were added to each tube using a microinjector. Distilled water was also added to each tube to ensure the same solution volume (10 ml>. Five tubes containing solute, but no sediment, were used as blanks. All sorption experiments were carried out in duplicate. The tubes were sealed and placed on a thermostatic shaker at 20 f 0.5”C. Sorption kinetic studies showed that the equilibrium was quickly reached between 4-6 h,
Y.-H. Zhao,
P.-Z. Lang/The
Science
so that a contact time of 6 h was arbitrarily selected to ensure attainment of sorption equilibrium. After 6 h of shaking, the tubes were removed and centrifuged at 4000 rev./min for 20 min. Nine millilitres of the supernatant were then carefully transferred to a separator funnel containing 1 g of NaCl and extracted three times with 1 ml of petroleum ether for 5 min. The extracts were analyzed by gas chromatography (Shimadzu GC-7AG) equipped with an electron capture detector. The concentration of solutes in the solid phase was computed by difference. Partition coefficients of added DOC (using 12.5 ppm fulvic acid (FA), from the Institute of Geography of Academia Sinica, Changchun, China) and different centrifuge speeds were determined to assess the impact of DOC and NSPs. DOC measurements were made in the supernatant by TOC-500 Total Organic Carbon Analyzer. The contents of NSPs, average diameters of NSPs and light adsorption in the supernatant were also analyzed with a CAPAParticle Analyzer. 3. Results and discussion It was observed that for each solid concentration, the isotherms were linear over the range of solution concentrations with near zero intercepts (Fig. 1). K,, values were obtained from the slopes of isotherms divided by the organic carbon content of the sediment. Fig. 2 shows the dependence of normalized partition coefficients on the solid concentration for five different solutes. In all cases, the partition coefficients increase significantly as the solid concentration in the system is decreased. Pollutants with larger partition coefficients have a more pronounced effect on the observed partitioning with changes in solid concentration. The effect of solid concentration is less pronounced for substances with low partitioning coefficients since values are essentially constant. Data are given in Table 1 to show the effect of added DOC (using 12.5 ppm FA) on the partitioning of hexachloroethane between solid and aqueous phases. The results indicate a significant reduction in hexachloroethane sorption as the DOC concentration is increased. Table 2 shows the
of the Total Environment
177 (1996)
0
PI
3
l-7
4,
am
l e
conoontration
aqueous
1.0
fug/l)
Fig. 1. Hexachloroethane isotherms for different solid concentrations. Solid concentration CC,): a, 25.0 (mg/ml); b, 50.0 (mg/ml); c, 75.0 (mg/ml).
results of the normalized partition coefficients for hexachloroethane determined at different rev./min. Clearly, the K,, values increase as the rev./min are increased. Also shown are the results of two experiments in which light absorption and average diameter of NSPs are decreased with increased rev./mm (Table 2). On the basis of these results, the observed change in partition behaviour with changes in solid concentration can be attributed to a transfer
‘=m
0
PI
solid
40
concentration
00
00
(mg/ml 1
Fig. 2. Normalized partition coefficients as a function of the solid concentration. 1, hexachlorobutadiene; 2, 1,2,3-trichlorobenzene; 3, hexachloroethane; 4, (~-666; 5, m-nitroanisole.
4 Table 1 Hexachloroethane
Y.-H. Zhao,
P.-Z. Lang / The Science
of the
Total Environment
normalized partition coefficient of added 12.5 ppm FA
Solid concentration (mg/ml)
Supematant
62.5 62.5
Distilled water 12.5 ppm FA solution
of solute-binding material from the solid phase to the liquid phase during the course of the partition experiment. It is suggested that this material, whether DOC or NSPs in nature, was not removed from the liquid phase during the separation procedure and possessed a higher capacity for the solute than the water itself. The contents DOC and NSPs in supernatant increase as the solid concentration is increased (Table 3). It increases the amount of solute in the liquid phase and makes the observed partition coefficient decrease. Further experiments were carried out to assess the role of DOC and NSPs in the partitioning process. The supematant was analyzed after separation for DOC concentration, light adsorption, contents and average diameter of NSPs. A trend of increased light absorption, increased contents of NSPs and DOCs with increased solid concentration can be observed (Table 3). In order to assess the effect of DOC and NSPs upon the sorption of solute, excess hexachlorobutadiene was added separately to three flasks containing distilled water and 12.5 ppm FA solution and the supematant centrifuged at 4000 rev./mm after 6 h of equilibration time. The results given in Table 4 show that the solubility of 12.5 ppm FA solution and the supematant are greater than distilled water. The data suggest that DOC and NSPs
Table 2 Hexachloroethane Solid concentration
2360 1380
possess a higher capacity for solute than for water. Based upon the above results, we assume that the system consists of water phase, solid phase, DOC, NSPs and sorption is a partition process [9,131 for non-ionic organic compounds. We can obtain Eqs. 3 and 4 (3) (4)
where Ki is the observed sorption partition coefficient, Ki is the true sorption partition coefficient, V is the volume of water, P is the mass of compound adsorbed to settleable particles (solid phase), D is the mass of compound dissolved, N is the mass of compound adsorbed to NSPs, DOC is the mass of compound adsorbed to DOC, and M is the mass of settleable particles. Combining Eqs. 3 and 4, we obtain Eq. 5 (5)
Karickhoff and others [5,8] found that pollutants adsorbed more strongly on finer particle sires than on large particle sizes. Similar results
normalized partition coefficients, light adsorption and average diameter at different rev./min Rev./min
kc
Light adsorption
Average diameter
(pm)
(mg/ml) 62.5 62.5 62.5
I77 (I 996) I - 7
3000 4000 5ooo
1980 2360 2820
0.10 0.06 0.04
0.17 0.15 0.12
Y.-H. Zhao,
P.-Z.
Lang/The
Science
of the
Total Enuironment
177 (1996)
5
l-7
Table 3 Some properties in supematant centrifuged at 4000 rev./min
Content of NSPs ( pg/ml) DOC concentration (ppm) Light absorption
Solid concentration (mg/ml) 12.5 37.5
62.5
87.5
56 1.52 0.01
381 3.34 0.05
483 3.59 0.07
224 2.62 0.02
9, we obtain Eq. 10
have been observed in investigations on the sorption of organophosphorus and carbamate insecticides to soils [14-161. They suggested that NSPs and DOC should be treated with enhanced K,s. If we use Ki-, to express the partition coefficients of compounds between NSPs and water, K’ p-D to express the partition coefficients of compounds between DOC and water and assume that K’ P-N and Ki-,, are larger than Ki (sediment/water partition coefficient), that is Ki = fo,K:, s foe-NK;c-N = A,fo, Kh, K; = f,Ki, I focvDK&, = Azfoc K& (Al 2 1 and A, 2 1 are constants), we can obtain Eqs. 6 and
KG=
=A,focK:,
K;-D =fo,.DK:c-,
=AzfocK:c
N/MN = - D/V DOC/M, = D/V
(6) (7)
From Eqs. 6 and 7, we obtain Eqs. 8 and 9
DOC=A2fo,K$MD
K,“, = (8)
N=nlfo,K:c;M,v =A,K$M,
K& 1 +K:,C,(A,B,f,,,
(111
+A,B,)
(9)
solubility in different conditions
-
Distilled water FA solution Supematant
(10)
Eq. 11 explains the experimental phenomena concerning the effect of solid concentration on sorption partition coefficients which we have observed. Pollutants with larger partition coefficients have a more pronounced effect on the observed partitioning with changes in solid con-
where M, is the mass of NSPs, M,, is the mass of DOC, and foe-n = 100%. From Eqs. 5, 7 and Table 4 Hexachlorobutadiene
+A$@/V
Voice et al. [12,13] found that the amount of materials (DOC and NSPs) contributed to the liquid phase were most likely proportional to the solid phase present. Our experimental results show also that the weight of NSPs CM,) and weight of DOC CM,,) are proportional to the weight of sediment (M = C,V>. That is MN = B&V and M, = B,C,V (B, and B,, the weight of DOC and NSPs/per weight of sediment, are constants, related to properties of sediment. For the Songhua River sediment, B, = 5.77 x 10m3, B, = 2.77 x 10e5). Introducing into Eq. 10, we obtain Eq. 11
7
K;zv =foc-,vK&-,v
Klic 1 +K;(A,f,,M,
DOC (wd
Content of NSPs ( pg/ml)
Solubility @Pm)
0.0 12.5 3.3
0.0 0.0 387.0
1.8 2.1 2.4
6
Y.-H. Zhuo,
Table 5 Eleven organic
compounds
Solid concentration
K&
P.-Z. Lung/The
values
(mg/ml)
K;
1,2,4,5-Tetrachlorobenzene Hexachlorobutadiene Hexachloroethane 1,2,3-Trichlorobenzene 6666 P-666 y-666 a-666 2,6-Dinitrotoluene m-Nitroanisole p-Nitrotoluene
calculated
Science
from
x lo3 (ml/g)
values
Kk
sediment
X lo3 (ml/g)
25.0
50.0
75.0
6.25
12.5
25.0
50.0
75.0
11.63 12.04 6.84 8.16 7.29 4.40 3.71 3.64
9.50 9.64 4.90 6.71 5.33 4.13 3.04 2.74 0.74 0.73
5.75 7.45 2.87 4.22 4.17 2.87 2.39 2.06
4.61 5.73 1.82 3.25 2.95 2.01 1.88 1.53 0.56 0.59 0.53
3.24 5.02 1.30 2.39 2.34 1.59 1.37 1.03 0.55 0.56 0.51
15.20 13.80 11.10 10.60 9.03 5.24 4.39 4.73
15.40 13.80 11.10 10.50 7.24 4.35 4.08 4.73 0.80 0.77
10.80 10.90 8.20 7.80 7.54 4.94 3.98 4.30
18.10 11.80 10.30 10.70 10.36 4.86 5.07 6.78 0.71 0.74 0.65
15.20 13.80 11.10 10.60 9.03 5.24 4.39 4.73 0.80 0.77 0.68
0.69 0.61
+A,B,)
cc
= 1 - CK&C,
K0oc2 = 1 - CK&,,C,, calculated
River
1-7
12.5
c = A,B, f,, + A$,, is constant. Eq. 12 allows us to predict the true partition coefficient (K&j, if the values of K,“, f,,, A and B are given. But the determination of KheN and K& is difficult. In another words, we cannot obtain A r and A, values accurately. If Eq. 12 is an ‘ideal’ equation, the values of K& should be a constant calculated at different solid concentration and caMot be affected by the solid concentration. From Eq. 12, we obtain Eqs. 13 and 14
Table 6 HCBP KL
177 (1996)
6.25
(12)
K”OCl 1 - CK&,C,,
Total Entirontnent
Eqs. 12 and 14 on Songhua
centration and the solid concentration does not affect the partition coefficient when f,,,KA, C, (A, B, f,, + AZ&) < 1. Modifying Eq. 11, we obtain Eq. 12 KAc = 1 - K,O,C,(A,B,f,,
of the
from
(13)
0.78 0.68
K” -KgOc, ’ = K,“,,K:2,(C,, - Cs2>
(14)
Using Eqs. 12 and 14 and the observed partition coefficients at different solid concentration, we can calculate the true partition coefficients. Table 5 shows the values of K& calculated for 11 different compounds. Also, listed in Table 6 are K& values for HCBP calculated for hvo different sediments using experimental data obtained elsewhere [lo]. The data in Tables 5 and 6 indicate that the Kk values are approximately a constant across the range of solid concentrations (0.05-75 mg/ml) used in these investigations. This further supports the hypothesis that observed changes in partition behaviour due to solid concentration may be attributed to the transfer of solute-binding material from solid phase to the liquid phase during the course of the partitioning experiment. 4. Conclusion
Our results support the view of Voice and others that a solids effect can be attributed to
Eqs. 12 and 14
Solid concentration (mg/ml)
Montmorillonite clay KC
Kk
0.05 0.20 1.00
10 600 6690 2900
12300 10300 12300
Solid concentration (mg/ml) 0.055 0.220 1.100
Saginaw sediment
Bay
%C
CC
17100 12300 9900
17800 15800 17800
Y.-H. Zhao,
P.-Z. Lang/TheScienceofthe
dissolved organic carbon DOC and nonsettling microparticles NSPs which transfers from solid phase to liquid phase during the course of the partitioning experiment. The derived formula was used to calculate the true partition coefficient (K:,) values based upon our experiments and those of Gschwend and Wu [13]. The K& values were calculated at different solid concentrations for 11 organic compounds. The results indicate that KL values are approximately constant across the range of solid concentrations. Our results explain the experimental phenomena that pollutants with larger partition coefficients have a more pronounced effect and pollutants with lower partition coefficients have a less pronounced effect on the observed partitioning with changes in solid concentration. References Ul T.C. Voice and W.J. Weber, Jr., Sorption of hyDl
[31
[41
151
drophobic compounds by sediments, soils and suspended solids - I. Water Res., 17 (19831 143331441. J.W. Weber, Jr., T.C. Voice, M. Pirbazari, GE. Hunt and D.M. Vlanoff, Sorption of hydrophobic compounds by sediments, soils and suspended solids - II. Water Res., 17 (1983) 1443-1452. C.H. Giles and D. Smith, A general treatment and classification of the solute adsorption isotherm Part I. Theoretical. J. Colloid Interface Sci., 47 (1974) 755-765. C.H. Giles, A.P. D’Silva and L.A. Easton, A general treatment and classification of the solute adsorption isotherm Part II. Experimental interpretation, J. Colloid Interface Sci., 47 (1974) 766-778. S.W. Karickhoff, D.B. Brown and T.A. Scott, Sorption of
TotalEnbonment177(1996)1-7
I
hydrophobic pollutants on natural sediment. Water Res., 13 (1979) 241-248. [‘51 E.G. Lotse, D.A. Gratz, G. Chesters, G.B. Lee and L.W. Newland, Lindane adsorption by lake sediment. Environ. Sci. Technol., 2 (1968) 353-357. [71 B.T. Bowman and W.W. Sans, Partitioning behaviour of insecticides in soil-water system: I. Adsorbent concentration effects. J. Environ. Qual., 14 (1985) 265-269. [81 W.W. Choi and K.Y. Chen, Associations of chlorinated hydrocarbons with fine particles and humic substances in nearshore surficial sediments. Environ. Sci. Technol., 10 (1976) 782-786. [91 C.T. Chiou, P.E. Porter and D.W. Schmedding, Partition equilibria of nonionic organic compounds between soil organic matter and water. Environ. Sci. Technol., 17 (1983) 227-231. DO1 L.M. Horzempa and D.M. DiToro, PCB partitioning in sediment-water systems: the effect of sediment concentration. J. Environ. Qual., 12 (1983) 373-380. ill1 D.J. O’Connor and J.P. Connolly, The effect of concentration of adsorbing solids on the partition coefficient. Water Res., 14 (1980) 1517-1523. D21 T.C. Voice, C.P. Rice and W.J. Weber, Effect of solids concentration on the sorption partitioning of hydrophobic pollutant in aquatic systems. Environ. Sci. Technol., 17 (1983) 513-518. [131 P.M. Gschwend and S.C. Wu, On the constancy of sediment-water partition coefficients of hydrophobic organic pollutants. Environ. Sci. Technol., 19 (1985) 90-96. 1141 A. Felsot and P.A. Dahm, Sorption of organophosphoNS and carbamate insecticides by soil. J. Agric. Food Chem., 27 (1979) 557-563. [151 A. Khan, J.J. Hassett and W.L. Banwart, Sorption of acetophenone by sediments and soils. Soil Sci., 128 (1979) 297-302. [I61 D.L. Zierath, J.J. Hassett and W.L. Banwart, Sorption of benzindine by sediments and soils. Soil Sci., 129 (1980) 217-281.