Environ. Sci. Technol. 1987, 21, 562-568
Sorption of Water-Soluble Oligomers on Sediments R. Thomas Podoll,” Katherine C. Irwin, and Sheryl Brendlinger Physical Chemistry Department, SRI International, Menlo Park, California 94025
The adsorption and desorption of oligomers of nonionic poly(ethy1ene glycol) (PEG) and cationic poly(ethy1enimine) (PEI) were measured on several sediments. These oligomers (M, 53400) adsorb strongly, particularly at low aqueous concentration, even though they are water-soluble. The adsorption isotherms have a Langmuir shape. Sorption capacity correlates with the cation-exchange capacity for PEI and the sediment clay content for PEG but not with the sediment organic carbon content. Sorption increases for a given oligomer with increasing molecular weight. Lowering the solution pH increases PEI adsorption and decreases PEG adsorption. Adsorption/desorption reversibility is less than 50% a t high aqueous concentrations and diminishes to 10% or less at low aqueous concentrations. At the same molecular weights, PEI adsorbs more strongly and less reversibly on a given sediment than does PEG-presumably because of the strong Coulombic attraction between PEI and the negatively charged sediment surface. W
Introduction A substantial amount of research has shown that polymers with molecular weights greater than about 10000 adsorb very strongly from water on clay minerals (I,2) and other adsorbents (3,4). In general, the adsorption isotherm rises very steeply a t low concentrations to a limiting plateau; adsorption increases with an increase in molecular weight and is predominantly irreversible. These effects have been attributed to multisegment attachment of the polymer to the surface and the resulting statistical improbability of all segments disengaging from the surface a t the same time for desorption to occur (5). Little research has been reported on the sorption of low molecular weight polymers or oligomers because they are not commercially important. However, they are produced as byproducts in the synthesis of high molecular weight, water-soluble polymers whose industrial output is rapidly growing, and there are indications that these low molecular weight byproducts can be very toxic to aquatic life (6, 7). The work reported here was initiated to aid the Office of Toxic Substances of the US. Environmental Protection Agency in its assessment of the fate and effects of water-soluble oligomers. It also has importance in developing an understanding of the sorption of oligomers. These chemicals are larger than low molecular weight organics, where molecular size has only a minor effect on sorption, and smaller than polymers, where size and conformational effects have a major impact on sorption. To better understand the experimental work reported here, it is useful to review the sorption behavior of organic solutes with molecular weights both lower (10000) than those of these oligomers. Background The sorption of low molecular weight organics from water on sediments (and soils) depends largely on the aqueous solubility of the organic and on the organic carbon content of the sediment (8,9). In general, sorption of these solutes increases with a decrease in solubility and is directly proportional to the organic carbon content of the sediment. Adsorption isotherms for hydrophobic solutes are typically 562
Environ. Sci. Technol., Vol. 21, No. 6, 1987
linear, and the sorption partition coefficient Kp
=
cs/cw
(1)
where C, is the mass of chemical adsorbed per gram of sediment and Cw is the aqueous concentration of the chemical, is thus constant as a function of concentration. The sorption of polymers from solution is very different from the sorption of small molecules. The difference is due to the large number of configurations that a polymer can assume in solution and in the adsorbed state. The most important trends that have been observed for polymer sorption are as follows (5): (1)The amount adsorbed rises very sharply a t low concentrations and starts to level out at higher concentrations. (2) Adsorption increases with molecular weight. (3) For polydisperse polymers the transition between the low- and high-concentration regions of the adsorption isotherm becomes less sharp, the isotherm depends on the adsorbent mass/solution volume ratio, and adsorption equilibrium is reached more slowly. (4) It is difficult or impossible to desorb polymers by dilution. In addition, water-soluble polymers are polar, whereas hydrophobic solutes are nonpolar. Thus, electrostatic interactions of the sorbate with charged or polar surface sites should be important for the sorption of water-soluble polymers whereas nonspecific dispersion interactions are known to dominate the sorption of hydrophobic solutes on sediments. As mentioned above, the strong irreversible sorption of high molecular weight polymers is due in large part to multisegment adsorption. At low concentrations, many segments of an individual polymer molecule can attach to the surface, and once they are adsorbed, it is statistically improbable that all segments will detach simultaneously for the polymer to desorb. A t higher concentrations, adsorption sites are filled, and the amount adsorbed tends to plateau or increase slightly with multilayer adsorption. Figure 1 illustrates two effects of molecular weight on polymer adsorption. First, for a given total polymer mass and number of segments on the surface, it is more difficult to desorb larger molecules because more segments must detach simultaneously for the larger molecule to desorb. Thus at low solution concentrations the isotherm is steeper for higher molecular weight polymers. Moreover, as shown also in Figure 1,for a given fraction of surface area covered by adsorbed polymer segments, larger polymers will have a greater mass in loops and tails, and therefore, the amount adsorbed will be larger on a mass basis. Because high molecular weight polymers preferentially adsorb over smaller ones, the approach to adsorption equilibrium for polydisperse polymers can be slow. Adsorption of low molecular weight polymers occurs first, because these polymers diffuse faster to the surface. However, larger molecules replace smaller molecules from the surface, and this redistribution can be slow. Polydisperse polymer solution adsorption also tends to give a more rounded knee to the isotherm a t a given ratio of adsorbent mass to solution volume. As the sediment concentration is lowered, however, the isotherm becomes sharper, and more polymer is adsorbed on a mass-to-mass basis. This is because adsorption from a polydisperse
0013-936X/87/092 1-0562$01.50/0
0 1987 American Chemical Society
~~
~
Table I. Characteristics of Sedimentsn sediments
(a)
and therefore ' monolayer" capacity will be larger on mass basis
(b) Moreover. for a given total Polymer mais and number of segments on the surface it is more difficult to desorb larger molecules because of "octopus" effect
Effects of
on polymer sorption.
For a given surface area larger polymers will have greater mass in loops and tails
Figure 1.
molecular weight
mixture results in a lower molecular weight distribution in solution (IO). Because of these differences between hydrophobic solute and polymer adsorption from water, our research focused on investigating (1)the extent of adsorption of oligomers of molecular weights less than 3400 from water on characterized sediments; (2) the effect of oligomer molecular weight and charge on adsorption; (3) the effect of sediment characteristics such as cation-exchange capacity, particle size distribution, clay mineral content, pH, and organic carbon content on sorption; (4) the effect of solution pH on sorption; and (5) the extent of adsorption/desorption reversibility Poly(ethy1ene glycol) (PEG) and poly(ethy1enimine) (PEI) were chosen for study principally because they were commercially available at low molecular weights and low polydispersities. PEG is nonionic, and PEI is cationic; thus, we were able to investigate the effect of oligomer charge on sorption. Moreover, these oligomers have simple functionalities, which helps to simplify the inherent complexity of oligomer sorption on sediments.
.
Experimental Section Polymers. Samples of poly(ethy1ene glycol) [HO(CH2CH20),H,Polysciences] were obtained with molecular weights (M,) from 194 (n = 4) to 3400 (n 77). Samples with M , 194, 618, and 960 had polydispersities 11.1. Diethylene glycol and triethylene glycol were obtained from Aldrich Chemicals. Poly(ethy1enimine) samples with M , 600,1200, and 1800 (n = 14-42) were obtained from Polysciences. Commerically available PEI, (-CH2CH,NH-),, is a highly branched polymer and therefore has primary, secondary, and tertiary amine groups. Sediments. Characterized air-dried sediments (Table I) were obtained from the Environmental Research Laboratory, Athens, GA. The dry sediments were equilibrated with 0.01 M CaSO, for 5 days at 4 "C before they were used in sorption experiments. So that sediment properties could be correlated with sorption capacity, the sediments were chosen with a range of cation-exchange capacities from 13 to 33 mequiv/100 g, sediment pH from 4.3 to 8.2, organic carbon content from 0.5 to 2.3%, and particle size distribution. Adsorption Equilibrium Isotherms. Adsorption isotherms were determined by batch equilibration. The wet sediment was weighed into 25-mL glass centrifuge tubes with Teflon-lined screw caps. Stock solutions of the polymers were made up in 0.01 M CaS04. Sorbate solutions were prepared from these by dilution with 0.01 M CaSO,, and 25-mL aliquots of the sorbate solutions were pipetted into the centrifuge tubes containing the sediment. Sorbent blanks were prepared with each sediment and calcium sulfate solvent. The sorption tubes were rotated end-over-end for approximately 16 h in a thermostated box at 25 "C. Following equilibration the samples were cen-
-
EPA-5 EPA-6 EPA-12 EPA-14 EPA-18 2.0 34.4 63.6 37 13 14
34.6 25.8 39.5 7.0 2.8 30
2.33 13.5
0.48 18.9
0.66 15.4
7.55
4.30
7.79
sand, % silt, % clay, % kaolinite, % illite, % vermiculite/montmorillonite, %
33.6 35.4 31.0 3.5 1.8 26
0.2
0
31.2 68.6 3.2 4.6 61
64.6 35.4
organic carbon, % cation-exchange capacity (mequiv/100 g) pH (1:2)
2.28 19.0
0.72 33.0
7.20
8.23
a
Characterized sediments provided by Dr. Samuel Karickhoff of the
US.Environmental Protection Agency, Athens, GA.
trifuged for 1h at 10000 rpm and 25 "C. Aliquots of the supernatant were removed and analyzed to determine the final supernatant concentration. Differential refractometry was used to analyze the supernatant solutions of the ethylene glycol oligomers. Because this method is nonspecific, error is introduced if materials are displaced and desorbed from the sediment during solute adsorption. The uncertainty at lower aqueous concentrations is also magnified in batch sorption if the sediment blank is high. Poly(ethy1enimine) concentrations were determined either by differential refractometry or spectrophotometrically by formation of its copper chelate (11). A 1-mL aliquot of 0.01 M cupric acetate in 0.01 M HC1 was added to 5 mL of the polymer solution, and the absorbance was measured at 269 nm with a Beckman DU spectrophotometer. The calibration was linear over the range of 2-100 ppm PEI. Calcium sulfate did not interfere with the method. Desorption experiments were done by continuous-flow frontal analysis. The sediment column (4mm i.d., stainless steel, 50 mm long) was packed with approximately 0.4 g of sorbent between silanized glass wool and 10-pm stainless steel frits. The packed column was conditioned with 0.01 M CaS04 for 5 days to wet the sediment and remove leachates, which would have interfered with refractive index detection. The sorbate solution and/or the calcium sulfate desorption solvent was percolated through the column at a constant flow rate (0.5 mL/min) with two HPLC syringe pumps (ISCO) connected to the column by a four-way valve. The effluent flowed directly into the differential refractive index detector (IBM Model 9525). The reference cell of the detector contained 0.01 M CaS04. The raw data were transferred by an analog/digital interface to a personal computer. A computer program was written to integrate the areas above the breakthrough and elution curves and to calculate the amounts adsorbed and desorbed, respectively. Adsorption or desorption experiments were usually completed within 3 or 4 h. Sorption kinetics were apparently very fast, and the approach to sorption equilibrium was limited by the volumetric flow of solution or solvent through the sediment column. Several experiments were conducted where the flow was stopped, when apparent desorption equilibrium was reached, and then were restarted 16 h later. The incremental amount desorbed during the stop-flow period was always less than 10% of the total initially desorbed and less than 2% of the amount initially adsorbed. The flow method was used instead of the batch dilution method for two principal reasons. First, sample handling Environ. Sci. Technol., Vol. 21, No. 6, 1987
563
20
,
I
I
4.0
I
1
A
I
I
I
~~
PEG3400
A
PEG 3400
z
c
PEG 600
~
3.0
s
Diethylene Glycol
0
I
I
I
I
500
1000
1500
2000
2500
SOLUTION CONCENTRATION (ppml
Flgure 2. Molecular weight dependence of PEG'adsorptlonIsotherms on EPA-5 sediment.
is minimized with frontal analysis, particularly where the effluent from the column can be monitored continuously by an on-line detector such as a differential refractometer. Desorption is initiated simply by switching the flow stream to solvent. If the desorption experiment is completed to equilibrium (back to zero concentration of adsorbate in the effluent), the amount irreversibly sorbed can be directly calculated. Second, if desorption is measured in the plateau region of the adsorption isotherm for the polymer, sorption may appear to be irreversible, even when it is not, because the amount of sorbed per gram of sediment remains constant with batch dilution until the knee of the isotherm is reached. In flow desorption experiments the adsorbent is equilibrated with solvent, and this problem is eliminated.
Results and Discussion Poly(ethy1ene glycol) (PEG) Sorption. Adsorption isotherms for ethylene glycol oligomers were measured (using the batch method) as a function of oligomer molecular weight on several sediments. The sediments were chosen to examine the effects of variable sediment organic carbon content, particle size distribution, cation-exchange capacity, clay composition, and pH on adsorption. Desorption experiments were also conducted with continuous-flow frontal analysis. The molecular weight dependence of PEG adsorption on EPA-5 sediment is illustrated in Figure 2. Note that as the molecular weight of the oligomers increases the isotherm rises more steeply at low concentrations and reaches a higher quasi-plateau region of adsorption. Thus these isotherms, at least qualitatively, conform to the Langmuir model in which the slope of the isotherm at low concentrations is constant and proportional to the monolayer capacity of the adsorbent. The adsorption of diethylene glycol (DEG) on EPA-6 was surprisingly strong given the miscibility of this dimer in water and given its molecular weight of only 106. The plateau region of the isotherm was apparently reached at a lower concentration than we could detect ( a ,IC *).
should increase if b is constant. The solid curve shown in Figure 6, together with the desorption data for PEI, was calculated from eq 3 assuming that b = 5 mL/g. The experimental data points are fitted reasonably well by the curve, considering the uncertainty in the data points, particularly at high Kpvalues and low fractional desorption, and considering that the data were collected for two separate sediments. The data for PEG desorption are not as well fitted by eq 3, possibly because the data are more uncertain and there are fewer data points. Nevertheless, the fractional amount desorbed does decrease with increasing values of Kp in accordance with model predictions. Conclusions The major conclusions of this work are as follows. (1)Water-soluble oligomers are adsorbed very strongly on sediments despite their water solubility. Site-specific adsorption of oligomers is indicated by the Langmuir shape of the adsorption isotherms, the correlation of PEI sorption with cation-exchange capacity, and solution and sediment pH effects on PEG adsorption. PEI cations apparently bind to negatively charged sites, and PEG apparently forms hydrogen bonds with negatively charged sediment sites. This site-specific adsorption is in sharp constrast to the nonspecific adsorption of hydrophobic organic chemicals on sediments where the water solubility of the chemical is critical in determining the sorption. (2) Adsorption isotherms are nonlinear, except possibly in the very dilute concentration range, and PEI adsorption can be fitted well with the Langmuir adsorption model. The Langmuir model works better for PEI adsorption because sorption is site-specific on cation-exchange sites and the strength of adsorbate/sediment interactions is probably much greater than adsorbate/adsorbate interactions. The Langmuir model works less well for PEG adsorption because adsorbatelsediment interactions are weaker and nonspecific adsorbatelsediment and adsorbateladsorbate interactions are probably more important. (3) Sorption is only partly reversible, and the degree of reversible sorption is inversely related to the adsorption partition coefficient. Sorption irreversibility is highest in the low concentration range, where sorption is strongest and multisegment adsorption of an oligomer is most likely.
Sorption irreversibility decreases in the plateau region of the adsorption isotherm, where the fractional portion of an adsorbed molecule directly attached to the surface decreases. (4)Adsorption increases for a given oligomer with increasing molecular weight. The reasons for this effect are illustrated in Figure 1. The effect is greater for nonionic PEG, presumably because PEG adsorption has less specificity than PEI adsorption. Adsorption of PEG is probably enhanced by dispersive interactions of the methylene segments of PEG with each other, in the loops and tails of adsorbed molecules, and with the sediment surface. PEI adsorption occurs by cation exchange, and the specificity of this adsorption probably limits multisegment adsorption of a single PEI molecule. This, in addition to the repulsive interaction of positive charges in the loops and/or tails of adsorbed PEI oligomers, limits the molecular weight effect for PEL ( 5 ) Adsorption correlates with the cation-exchange capacity for polycationic PEI and with the clay content and composition for nonionic PEG. The adsorption of PEG or PEI does not correlate with organic carbon content. Whereas the correlation of PEI adsorption with the CEC of the sediment appears straightforward, the correlation of PEG adsorption with clay content is complicated by the variation of clay types and the sediment pH values of the sediments studied. The presence of expandable clay minerals such as vermiculite and smectite appears to enhance the adsorption of PEG. However, the sediments with a high expandable clay content also had higher pH values. At low pHs, metal oxides on the clay mineral surfaces become protonated and thus are less likely to form hydrogen bonds with the hydroxyls on PEG. (6) Polycationic PEI adsorbs more strongly and irreversibly than nonionic PEG at the same molecular weight. The stronger adsorption of PEI is likely to be caused by the much stronger Coulombic interaction of PEI cations with negatively charged sediment surface sites. Because this work was limited to two types of oligomers and five different sediments, it is likely that these conclusions will be refined after further study. It would be interesting to extend these sorption measurements to low enough solution concentrations to satisfactorily determine the limiting linear adsorption of these and other oligomers. The analytical sensitivity required for these measurements would necessitate the use of radiolabeled oligomers and thus would add to the cost of research-but this work is clearly a necessary step in developing a predictive model for oligomer sorption. Acknowledgments We acknowledge David G. Lynch of the U.S. Environmental Protection Agency, Office of Toxic Substances, for helpful discussions and Dr. Samuel Karickhoff, U S . Environmental Protection Agency, Environmental Research Laboratory; Athens, GA, for providing characterized sediment samples. Redstry No. PEG, 25322-68-3; PEI, 9002-98-6; HO(CH,),O(CHz)zOH, 111-46-6; HO(CH~)20(CH2)20(CH2)2OH,112-27-6.
Literature Cited (1) Burchill, S.; Hayes, M. H. B.; Greenland, D. J. In The Chemistry of Soil Processes; Greenland, D. f.;Hayes, M. H. B., Eds.; Wiley: New York, 1981; pp 355-378. (2) Dodson, P. J.; Somasundaran, P. J. Colloid Interface Sci. 1984, 97, 481-487. (3) Killmann, E.; Korn, M.; Bergmann, M. In Adsorption from Solution; Ottewill, R. H.; Rochester, C. H.; Smith, A. L., Eds.; Academic: New York, 1983; pp 259-272. Environ. Sci. Technol., Vol. 21, No. 6, 1987
567
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Hesselink, F. Th. In Adsorption from Solution at the SolidlLiquid Interface; Parfitt, G. D.; Rochester, C. H., Eds.; Academic: New York, 1983; pp 377-412. Fleer, G. J.; Lyklema, J. In Adsorption from Solution at the SolidlLiquid Interface; Parfitt, G. D.; Rochester, C. H., Eds.; Academic: New York, 1983; pp 153-220. Bresinger, K. E.; Lemke, A. E.; Smith, W. E.; Tyo, R. M. J. Water Pollut. Control Fed. 1976, 48, 183-187. Letter and data submitted to US. Environmental Protection Agency; Petrolite Corp., Tretolite Division; 1982, St. Louis, MO (Fed. Reg. 1982,47, 33924-33948, August 4).
(8) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979,13, 241-248. (9) Chiou, C. T.; Peters, L. J.; Freed, V. H. Science (Washington, D.C.) 1979, 206, 831-832. (10) Cohen Stuart, M. A.; Scheutjens,J. M. H. M.; Fleer, G. J. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 559-573. (11) Perrine, T. D.; Landis, W. R. J.Polym. Sci., Part A-1 1967,
5 , 1993-2003. Received for review March 7,1986. Accepted December 4,1986. This work was supported by the U S . Environmental Protection Agency, Contract 68-02-3968.
Allyl Chloride: The Mutagenic Activity of Its Photooxidation Products Paul B. Shepson," Tadeusz E. Kleindlenst, Chris M. Nero, and Dennis N. Hodges Northrop Services, 1nc.-Environmental
Sciences, Research Triangle Park, North Carolina 27709
Larry T. Cupitt Atmospheric Sciences Research Laboratory, US. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
Larry D. Claxton Health Effects Research Laboratory, US. Environmental Protection Agency, Research Triangle Park, North Carolina 277 11
Irradiations of C3H5Cl/N0,, C3H5CI/C2H6/N0,, and C2H6/N0, mixtures were conducted in a 22.7-m3Teflon smog chamber, operated in a static mode. The irradiated mixtures were tested for mutagenic activity by periodically exposing Salmonella t y p h i m u r i u m strain TAlOO to the smog chamber effluent during the irradiation. The allyl chloride photooxidation products' total mutagenic activity was found to be dramatically dependent on the presence of C1 atom reaction products. In the absence of C2H6, which is used as a CI atom scavenger, the observed mutagenic activity of the irradiated C3H5C1/N0, mixture at long extent of reaction was 13 revertants.plate-'.h-l.(ppb C3H6CIconsumed)-'. However, when sufficient C&&was present to remove all C1 atoms, the observed mutagenic activity for C3H&1 photooxidation products was 1.4 revertants.plate-l.h-'.(ppb C3H5Cl consumed)-'. Under conditions of excess CzHs, the mixture is approximately 30 times more mutagenic than that previously observed for the mutagenic activity of the photooxidation products of propylene, the nonchlorinated analogue of allyl chloride. The observed mutagenic activity in the presence of excess C2H6 is consistent with the total response caused by chloroacetaldehyde, a primary C3H5Cl photooxidation product I
Introduction
As part of a long-term research effort aimed a t identifying mechanisms for the production of hazardous species in the atmosphere through reactions of OH, 03,and NO3 with reactive pollutants, we have reported that simple atmospheric hydrocarbons can be converted into mutagenic products through atmospheric photochemistry (1-3). In these studies we demonstrated that through reaction with the hydroxyl radical and in the presence of oxides of nitrogen (NO,), nonmutagenic hydrocarbons, such as acetaldehyde, can be converted into mutagenic products ( 4 ) , such as peroxyacetyl nitrate (PAN), as determined with the Ames test. Although most of the reactive hydrocarbon concentration in urban atmospheres is represented by carbon- and 568
Environ. Sci. Technol., Vol. 21, No. 6, 1987
hydrogen-containing (only) organic compounds, there are
a variety of chlorinated solvents present as well that can play a role in urban photochemistry. Many of these chlorinated hydrocarbons (including allyl chloride) are considered hazardous air pollutants (HAPs) and are currently being considered for regulatory action (5) because of their potential human health effects. Although HAPs, by definition, are considered to have some potential human health effects, it is possible that many of the chlorinated HAPs can be converted to products that are more (or less) mutagenic than the reactant HAP. For some oxygenated species, the chlorine-substituted analogues are more mutagenic. For example, chloroethylene oxide (a possible oxidation product of vinyl chloride) is 10000-15 000 times more mutagenic than is ethylene oxide (6). As a first step in investigating the potential for production of mutagenic products as a result of the photooxidation of chlorinated hydrocarbons, we report the results of a series of exposures of the bacteria Salmonella typhimurium, strain T A W (without metabolic activation), to the photooxidation products of allyl chloride (3chloropropene). This particular HAP was chosen in large part due to the fact that it is the chlorinated analogue of propylene, a hydrocarbon that we previously studied in detail regarding its mutagenic photooxidation products (2). Our previous experience with propylene, along with the fact that the chemical mechanism and reaction kinetics for allyl chloride are now fairly well understood (7), should facilitate the identification of individual mutagenic products for this HAP. In this paper we report the results of four static-mode smog chamber irradiations of C3H5Cl/ NO, and C3H5C1/C2H6/N0, mixtures, in which S. t y p h imurium was periodically exposed to the chamber effluent during the reaction profiles. Since the photooxidation of allyl chloride proceeds through OH and C1 atom chain reactions (7), the experiments with ethane were conducted to enable measurement of the mutagenic activity of the allyl chloride photooxidation products in the presence and absence of C1 atom reaction products. The results of these experiments will be interpreted in terms of the dependence of the observed mutagenic activities on the reaction con-
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