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Binding of methylated naphthalenes to concentrated aqueous humic acid
Advances in Environmental Research 6 Ž2002. 495᎐504
Binding of methylated naphthalenes to concentrated aqueous humic acid 夽 Dale R. Van StempvoortU , Suzanne Lesage National Water Research Institute, P.O. Box 5050, Burlington, ON, Canada L7R 4A6
Abstract In laboratory batches of 1 grl Aldrich humic acid ŽHA., apparent binding coefficients Ž K OC,app . for 1-methylnaphthalene Ž1-MN. and 1,3-dimethylnaphthalene Ž1,3-DMN. were determined by apparent solubility Žcomparison of solubility in water and HA solution.. For the HA batches with 1-MN, two trends were observed: delayed sorption Žup to 12 days., and an increase in the apparent solubility of 1-MN as the amount of this hydrocarbon as a non-aqueous liquid was increased. These two trends suggest that formation of a colloidal phase may have interfered with our apparent solubility tests. Alternatively, the non-aqueous 1-MN may have caused a slow change in the conformation andror aggregation of the humic acid and its binding properties. Other batch tests, using solid phase microextraction ŽSPME., did not have a non-aqueous hydrocarbon phase. Compared with the apparent solubility results, the SPME tests indicated similar but lower K OC,app values for binding of 1-MN and 1,3-DMN to Aldrich HA at 1 grl HA. Also, K OC,app values for 2-MN, 1,7-DMN and 2,3,5-trimethylnaphthalene in 1 grl HA were obtained by SPME. Other SPME tests indicated that the K OC,app values for 1-MN and 1,3-DMN in 3 grl Aldrich HA were lower, 35᎐50% of those in 1 grl HA. These significant differences are probably related to changes in the conformation andror aggregation of the HA. With an increase in HA from 3 and 10 grl, no significant change in K OC,app was observed for 1,3-DMN and 2,3,5-TMN, but the K OC,app value for 1-MN was slightly larger at 10 grl. These data do not support an earlier report that HA micelles form above 7.4 grl Aldrich HA. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Humic; Methylated naphthalenes; Binding; Solubility; Solid-phase microextraction
1. Introduction Humic substances are generally the dominant organic compounds in natural waters. The chemical binding of other organic compounds to aqueous humic substances is a key environmental process. By this
夽
NWRI Contribution No. 99-236. Corresponding author. Tel.: q1-905-319-6917; fax: q1905-336-6430. E-mail address: [email protected] ŽD.R. Van Stempvoort.. U
mechanism, natural humic substances play an important role in controlling the mobility ŽMcCarthy and Zachara, 1989; Dunnivant et al., 1992. and bioavailability ŽFriedig et al., 1998; Ma et al., 1999. of hydrophobic contaminants in the environment. Consequently, some studies have examined the potential use of concentrated commercial humic acids ŽHAs. as flushing agents to remove hydrophobic organic contaminants from soils or aquifers ŽAbdul et al., 1990; Xu et al., 1994; Lesage et al., 1995; Rebhun et al., 1996; Kurz et al., 1998; Johnson and John, 1999; Boving and Brusseau, 2000; Van Stempvoort et al., 2001.. The concentrations of humic substances in natural
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 0 7 6 - 4
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
496
waters typically range between 0.5 and 100 mgrl as organic C ŽFrimmel, 1998.. Accordingly, previous laboratory studies have usually looked at binding of other organic chemicals to HA over a similar range of HA concentrations Ž- 100 mgrl as C.. These studies have usually reported K OC values, which are binding coefficients, assuming equilibrium and linear partitioning: K OC s cOCrc w
Ž l r kg.
Ž1.
where cOC is the bound chemical per mass of organic C in the humic substance Žmgrkg.; c w is the dissolved chemical concentration Žmgrl.. Alternatively, the apparent binding coefficient, K OC,app can be calculated in the same way: K OC,app s cOCrc w
Ž l r kg.
Ž2.
This parameter reflects the distribution after a given time of contact and at the concentration of study. Thus, it does not necessarily reflect either a linear sorption isotherm or equilibrium Žcf. K d,app of Cornelissen et al., 1998.. Recently, Burkhard Ž2000. reviewed information on the relationship between published K OC values of non-ionic organic chemicals and their hydrophobicity, as measured by the 1-octanol-water partition coefficient Ž K OW .. Burkhard combined K OC values reported in 73 references for various non-ionic organic chemicals, and developed equations that predict K OC Žhis K DOC . as a function of K OW . In the case of Aldrich humic acid, which is most relevant to this study, the equation given by Burkhard is Žleaving out the "standard deviations.: log Ž K OC . s 0.85= log Ž K OW . q 0.27
Ž3.
The above equation may be useful as a general guide for the binding behavior of non-ionic organic compounds, particularly those that are members of the classes that are most strongly represented in Burkhard’s database Že.g. PCBs, PAHs.. However, recent studies indicate that, in the future, a more accurate modeling of interactions of non-ionic organic chemicals with humic acids must take into account the different types of non-covalent bonds, which vary with the classes of compounds that are considered ŽGoss and Schwarzenbach, 2001; Nanny and Maza, 2001.. Other studies have shown that K OC is not closely dependent on K OW for ionizable organic compounds Že.g. Holten Lutzhøft et ¨ al., 2000.. Little information is available on how K OC values vary as aqueous humic concentrations are increased above 100 mgrl Žas C., levels which might be suitable for remediation applications Ži.e. flushing of soils,
aquifers.. As noted, the vast majority of previous binding tests that have been reported in the literature were conducted at lower humic concentrations, representative of natural conditions. In the experiments reported here, the aqueous concentrations of Aldrich HA were varied between 1 and 10 grl Ž301᎐3145 mgrl as C.. Only a few previous publications have provided data for binding of contaminants to HA at similar elevated concentrations, relevant to remediation applications. Guetzloff and Rice Ž1994, 1996. examined the binding of DDT and pyrene in batch solutions containing up to 10 grl Aldrich HA. Kurz et al. Ž1998. looked at binding of trichloroethylene ŽTCE., tetrachloroethylene ŽPCE., 4-bromobiphenyl ether and carbon tetrachloride in batch solutions containing up to 75 grl HA extracted from leonardite Žlow grade coal. or peat. Johnson and John Ž1999. examined binding of PCE in batches that contained up to 12 grl Aldrich HA. Others have used column studies to examine the ability of aqueous Aldrich HA to facilitate the solubilization andror mobilization of contaminants in soil or aquifer materials. Ding and Wu Ž1997. tested mobilization of aldrin and DDT in soil, using 950 mgrl Aldrich HA Žas C.. Johnson and John Ž1999. studied solubilization of residual liquid phase PCE in sand with up to 12 grl Aldrich HA as the flushing solution. Boving and Brusseau Ž2000. investigated solubilization of residual trichloroethene ŽTCE. in sand, using flushing solutions containing 5% by weight Aldrich HA. Lesage and coworkers ŽLesage et al., 1995; Van Stempvoort et al., 2001. examined the solubilization of residual diesel in a pilot scale model sand aquifer, using 1 grl Aldrich HA as the flushing solution. Collectively, the above laboratory studies demonstrate that it may be feasible to use concentrated commercial HAs to solubilize andror mobilize organic contaminants in subsurface environments. As in Eq. Ž1., the binding of hydrophobic organic compounds by aqueous humic substances is often modeled as partitioning Ži.e. solid-phase dissolution., following a linear isotherm ŽChiou et al., 1986.. Changes in the extent of binding with pH and other chemical parameters have been documented Že.g. Carter and Suffet, 1982; Schlautman and Morgan, 1993.. Non-linear sorption of hydrophobic compounds to humic substances is often modeled using the Freundlich equation Že.g. Xing and Pignatello, 1997; Chiou et al., 2000.. The binding may be competitive ŽXing and Pignatello, 1997; Chiou et al., 2000.. Various methods have been used to quantify the binding of hydrophobic compounds by aqueous humic substances and the advantages and disadvantages of the various techniques have been described ŽLandrum et al., 1984; Laor and Rebhun, 1997; Poerschmann et al., 1997; Ramos et al., 1998.. For binding of PAHs, previous investigators have employed the fluorescence
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
quenching technique ŽLaor and Rebhun, 1997., but it is doubtful that this technique could be used at the elevated HA concentrations Ž1᎐10 grl. in this study. Further, the fluorescence quenching technique ‘is hindered by experimental difficulties’ ŽPoerschmann et al., 1997. and it may overestimate the binding coefficient ŽLaor and Rebhun, 1997.. Another approach to measuring the extent of binding is to compare the aqueous solubility of a hydrophobic compound, S w , and its ‘apparent solubility’, Ž SU w , in the presence of aqueous humic substances Chiou et al., 1986.. In this paper this approach will be referred to as the apparent solubility method. The apparent binding coefficient Ž K OC,app , lrkg. can be calculated, based on Eqs. Ž2., Ž4. and Ž5.: cOC s Ž S Uw y S w . r Ž c HA = fC .
Ž4.
c w s Sw
Ž5.
where c w , the dissolved hydrophobic compound Žmgrl., is assumed to be at saturation; c HA is the HA concentration Žgrl. and fC is the weight fraction of carbon in Aldrich HA. The solid-phase microextraction ŽSPME. technique ŽPoerschmann et al., 1997; Ramos et al., 1998; Pawliszyn, 2000. can be used to measure the free dissolved hydrophobic compound in water, c w , in the presence of other phases, such as aqueous humic substances. In this technique, a coated, fused-silica fiber Žsolid phase. is inserted in each test solution, then withdrawn, and the quantity of contaminant that has sorbed to the fiber is then desorbed and measured by GC or another technique, and calibrated to stock solutions. The total aqueous concentration of the same compound in the test solution, c t , can also be measured Žby HPLC or other technique ., and used to infer cOC : ct s c w q c b
Ž6.
cOC s c br Ž c HA = fC .
Ž7.
where c b is the bulk aqueous concentration of bound hydrophobic compound Žmgrl.. The c w and cOC values can then be substituted into Eq. Ž2. to calculate K OC,app . The use of the SPME technique appears to offer several key advantages. Unlike apparent solubility, it does not require the use of an immiscible liquid mixture, where tiny suspended droplets may be inadvertently measured as dissolved phase. The SPME tests can be conducted quickly, without requiring equilibrium in the concentrations of hydrophobic compound between the fiber and the solution. Some studies sug-
497
gest that the SPME measurements are free of interference that might be caused by binding of HA to the SPME fibers ŽRamos et al., 1998.. However, other researchers have concluded that high concentrations of aqueous HA may have an adverse, fouling effect on the SPME fibers ŽPoerschmann et al., 1999; Pawliszyn, 2000., In this study, the apparent solubility method and SPME were used to measure the binding of methylated naphthalenes by Aldrich HA. Methylated naphthalenes are some of the most abundant water-soluble PAH components of diesel ŽThomas and Delfino, 1991., and can occur at aqueous concentrations of several hundred grl in the presence of this fuel. Hence, methylated naphthalenes pose a major portion of risk associated with groundwater contamination by diesel. To our knowledge, this study provides the first published coefficients for binding of methylated naphthalenes to aqueous humic acid. Previous investigators have focused on higher molecular weight PAHs such as phenanthrene, pyrene, fluoranthene and anthracene.
2. Experimental procedures All experiments were conducted at room temperature Ž23 " 2⬚C. using analytical grade reagents. The apparent solubility tests were used to investigate binding of two pure phase isomers, 1-MN Žmethylnaphthalene . and 1,3-DMN Ždimethylnaphthalene .. SPME was used with GCrMS to examine the binding of the same isomers and three others, 2-MN, 1-7 DMN and trimethylnaphthalene Ž2,3,5-TMN..
2.1. Preparation of humic acid solutions Nominal Aldrich HA ‘solutions’ Ž1, 3 and 10 grl. were prepared by adding humic acid Žsodium salt, tech., product no. H16752, Lot No. 16206AN, Aldrich Chemicals, Milwaukee. to Milli-Q water. The 1 grl HA solution is the flushing solution that was tested at the pilot-scale by Lesage and coworkers ŽLesage et al., 1995; Van Stempvoort et al., 2001.. Aldrich HA was selected to be a well-documented, representative commercial humic product that could easily be prepared in aqueous form, in large volume and high concentration, for field-scale remediation applications. Based on information provided by Aldrich Chemicals, this product is derived from lignite obtained by open pit mining in Germany ŽOberhessen, Kassel., and contains approximately 9.4% Fe oxides, 3% S, 2% Na, 0.5% Ca, 0.4% Al, 0.05% Mg and 0.04% K. We measured the ash component of this product to be 64.6 Ž s 0.68.% by weight. The HA solutions were stirred for ; 12 h. Minor
498
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
particulate HA settled over the next 2᎐4 days and the supernatants of each were transferred by a peristaltic pump and stored for use in batch tests, as described below. Based on filtration and centrifugation tests, the HA solutions contained both dissolved and colloidal HA, and perhaps some remaining suspended, particulate HA. Some of the particulate matter that settled out might have been mineral impurities. A sample of each batch of aqueous HA was analyzed for total organic carbon by a Shimadzu TOC-5050 analyzer. These analyses indicated that the nominal aqueous HA concentrations of these batches corresponded to the following organic C concentrations: 1 grl HA, 301 mg Crl; 3 grl, 967 mg Crl; 10 grl, 3145 mg Crl. These values were used for subsequent K OC,app determinations. Given the fact that the batch solutions were mixtures of Aldrich HA and Milli-Q water, their physical chemistry Žionic strength, pH. was dependent on the chemistry and buffering capacity of the HA. The pH measurements indicated a range of 7.3᎐8.4.
2.2. Solubility of methylated naphthalenes in water and Aldrich HA: apparent solubility results Milli-Q water and the Aldrich HA solutions were sparged with Ar Ž; 1rmin per 100 ml. to remove O 2 , and placed in an anaerobic chamber Ž5% CO 2 , 10% H 2 and 85% N2 .. The O 2 was removed to minimize subsequent biodegradation and photooxidation of PAHs. Then, for each test, 100 ml of either HA and Milli-Q was transferred to a glass serum bottle, and pure phase Žliquid. 1-MN or 1,3-DMN ŽSigma-Aldrich Canada Ltd., Mississauga, ON. was added in excess of the anticipated saturation. The mixtures were sealed with Teflon-lined septa, placed on an orbital shaker for 16᎐24 h, returned to the anaerobic chamber, then sampled by glass syringe Žafter 1, 2, 5 and 12 days. and passed through columns of glass beads to trap and remove residual pure phase Žliquid. methylated naphthalenes. These columns had been prepared by packing glass beads Ž0.60᎐0.85-mm diam.. between silanized glass wool plugs in a disposable Pasteur pipette. During preliminary testing, the first six successive passes of the same sample showed gradual increases in concentrations, apparently due to sorption of some of the dissolved methylated naphthalenes to the packed glass bead column. Thus, the first six passes were discarded. Subsequent samples were diluted 50% in methanol ŽHPLC grade, Ar-sparged. and analyzed by HPLC, using a Waters 䊛 system Ž600E multisolvent delivery system, 700 WISP autosampler, and 470 Scanning Fluorescence Detector.. The chromatographic column selected for these analyses was a RP-8 Spheri-10 Brownlee 䊛 cartridge ŽPerkin-Elmer Corp., Norwalk, CT.. The
eluent was a mixture of methanol and Milli-Q water Ž65r35% by vol... Analyses of standards with increasing HA concentrations showed that there was no interference with the HPLC analyses of total methylated naphthalenes.
2.3. SPME-GCr MS tests of the binding of methynaphthalenes by HA The SPME method for the selective analysis of dissolved phase organic compounds in the presence of other phases, such as humic compounds, is well established ŽPawliszyn, 2000.. SPME fiber assemblies Žwith film of 100 m polydimethylsiloxane. and a manual sampling holder were purchased from SUPELCO Chromatography Products ŽOakville, ON.. The fibers were cut to reduce the length from 1 to 0.2 cm. In the anaerobic chamber, methylated naphthalenes were added from a stock Ž50 mgrl each of 1-MN, 1,3-DMN and 2,3,5-TMN or 50 mgrl each of 2-MN and 1,7-TMN, in methanol. to 100 ml of either Milli-Q water ŽpH adjusted with NaOH to 8, Ar-purged. or Aldrich HA solution Žnominal 3 or 10 grl, Ar-purged. in 120-ml glass serum bottles, to prepare final concentrations of between 200 and 7500 grl of each MN. These bottles were sealed and placed on an orbital shaker for 3 h Ž1 grl HA. to 15 h Ž3᎐10 grl HA.. The SPME-GCrMS analyses of methylated naphthalenes in Milli-Q were used as standards for calibration of the HA batch results. As a check for negligible depletion of the aqueous phase methylated naphthalenes by SPME, 1-l volumes of MN in hexane were injected manually into the GC, and the detector response per mass of MN determined. The fractions of each aqueous MN that had sorbed to the fiber per SPME-GCrMS test were on the order of 1᎐2% Ž1-MN, 1,3-DMN, 2,3,5-TMN.. For a check on the total methylated naphthalenes in the Aldrich HA batches, samples were diluted in methanol Ž1:5, H 2 O:MeOH. prior to HPLC analyses. This resulted in quantitative Ž) 96% for the majority of analyses . recovery of total methylated naphthalenes. Preliminary tests indicated that the free dissolved methylated naphthalene concentrations declined noticeably over the first 6᎐10 h in the presence of humic acid, then appeared to reach relatively stable values. After shaking, portions Ž20 ml. of these solutions were transferred to glass vials Ž25 ml. in the anaerobic chamber and capped by septa for storage prior to analysis. Then, for SPME analysis, each 20 ml batch was placed on a magnetic stirrer and mixed at ; 500 rev.rmin Žmaximum speed without a vortex forming on surface of batch solution.. The SPME fiber assembly was inserted through the septum and the fiber was exposed to the stirred solution for 10 min. After exposure was completed, within 1 min the fiber
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
was manually inserted into the port of a HewlettPackard GCrMSD ŽModel 5890A GC, 5970 MSD. and the accumulated methylated naphthalenes were desorbed at 250⬚C. The GC column was a DB-1 ŽJ & W Scientific Inc., Folsom, CA: 30 m long, 0.32 mm OD, 0.25 m film thickness ., and the carrier gas was helium. The GC oven was held at 50⬚C for 5 min, then increased at a rate of 20⬚Crmin to 250⬚C.
3. Results and discussion 3.1. Apparent solubility of methylated naphthalenes in the presence of Aldrich HA For 1-MN, the aqueous solubility, S w , was found to be 30.0" 1.7 mgrl Ž n s 40.. This result compares well with published values compiled by Mackay et al. Ž1992., which range from 26 to 32 mgrl. For 1,3-DMN, the S w was found to be 8.2" 0.4 mgrl Ž n s 24., similar to values compiled by Mackay et al. Ž1992., the majority of which range from 7.8 to 8.0 mgrl. The apparent solubilities, SU w , of 1-MN and 1,3-DMN in 301 mgrl HA Žas TOC. solutions were measured by HPLC. In the first apparent solubility batch test, a large amount of pure phase 1-MN was added Ž2 mg 1-MN per ml Milli-Q water. yielding very high SU w results Ž119᎐196 mgrl., with values increasing over 5 days ŽTable 1.. In the second apparent solubility test, a much smaller amount of 1-MN was used Ž0.4 mgrml.. After 1 day the SU w was 56.5 mgrl and it increased further over a period of 5᎐12 days ŽTable 1.. The SU w of 1,3-DMN in 1 grl HA as measured by HPLC was 50.4 mgrl after 1 day, and then decreased to 45.1 mgrl after 2 days ŽTable 1..
499
Table 1 provides the K OC,app values that were calculated for 1-MN and 1,3-DMN using Eqs. Ž3. ᎐ Ž5.. Table 1 also shows the octanol᎐water partial coefficients Ž K OW ., and K OC values obtained using Eq. Ž3., which relates K OC values to K OW . As explained in Section 1, Eq. Ž3. is based on published data for experiments with various non-ionic organic compounds at lower concentrations of Aldrich HA. Excepting our first test, where a large amount of 1-MN was added Ž2 mgrml., there was relatively close agreement between the values of K OC,app based on our tests and the values of K OC based on Eq. Ž3.. This comparison suggests that the binding of the methylated naphthalenes to Aldrich HA follows the general pattern reported by Burkhard Ž2000., and that the binding is not strongly affected by the concentration of HA. The K OC,app values shown in Table 1 for 1-MN and 1,3-DMN are approximately an order of magnitude smaller than binding coefficients that have been inferred for more strongly hydrophobic PAHs, such as fluoranthene wlogŽ K OW . s 5.22, K OC s 0.93᎐1.15 = 10 5 x and pyrene wlogŽ K OW . s 5.18, K OC s 1.05᎐1.19= 10 5 x, at lower concentrations of Aldrich HA, typically F 100 mgrl ŽLaor and Rebhun, 1997.. However, the K OC,app determined for 1,3-DMN in this study Ž1.50 to 1.71= 10 4 lrkg. is similar to previous K OC values for binding of similarly hydrophobic phenanthrene ŽlogŽ K OW . s 4.57, K OC s 1.47 to 1.66= 10 4 . and anthracene ŽlogŽ K OW . s 4.54, K OC s 2.71 = 10 4 . to Aldrich HA, as determined by complexation-flocculation ŽLaor and Rebhun, 1997.. The apparent solubility tests with 1-MN indicated that a large proportion of the apparent sorption was a slow-phase or ‘delayed’ component. Xu et al. Ž1994.
Table 1 Apparent solubilities Ž SUw . and apparent binding coefficients Ž K OC,app as lrkg organic C in HA. for methylated naphthalenes in 1 grl HA Isomer starting ratio: methylated naphthalene to water
1-MN 2 mgrml
0.4 mgrml
1,3-DMN 0.1 mgrml a
MacKay et al., 1992.
Aging of mixture Ždays.
SU w Žmgrl. "
KOC,app Žlrkg. " by apparent solubility
Ž n.
1 2 5 1 2 5 12
119.2" 5.7 127.3" 22.0 196.2" 16.3 56.5" 1.9 53.6" 2.2 63.1" 3.3 87.5" 3.8
9.87 Ž"0.66. = 103 1.08 Ž"0.25. = 104 1.84 Ž"0.18. = 104 2.92 Ž"0.28. = 103 2.61 Ž"0.30. = 103 3.66 Ž"0.41. = 103 6.37 Ž"0.46. = 103
Ž36. Ž36. Ž24. Ž36. Ž36. Ž36. Ž18.
1 2
50.4" 1.6 45.1" 1.4
1.71 Ž"0.06. = 104 1.50 Ž" 0.06. = 104
Ž36. Ž36.
logŽ KOW . a
KOC predicted by Eq. Ž3.
3.87
Žbased on KOW . 3.63= 103
4.42
1.06= 104
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
500
also used the apparent solubility technique and observed a similar delayed component for the binding of phenanthrene to Aldrich HA over a 36-day period. One possible explanation of this trend is that the delayed sorption is related to the slow rate of diffusion within the humic acid molecules andror to activation energies of sorption and desorption ŽPignatello and Xing, 1996.. However, the inferred delayed sorption in the apparent solubility tests occurs at a much slower rate Ždays to completion. than the delayed sorption in the SPME-GC tests Žhours, see Section 2.3.. This comparison suggests that the excess non-aqueous liquid 1-MN in the apparent solubility tests played a crucial role in causing what appears to be delayed Ži.e. slow. sorption. It has been suggested that the bulk of the apparent slow sorption Žup to 12 days or longer. in the apparent solubility tests may be an artifact, due to breakthrough of non-aqueous 1-MN through the glass bead columns. Perhaps this non-aqueous 1-MN was a colloidal phase ŽPage et al., 2000.. The breakthrough of such a nonaqueousrcolloidal phase might also explain our observation that the apparent binding coefficient for 1-MN increased as a function of the amount of non-aqueous phase 1-MN that we added ŽTable 1.. There is a weakness in this explanation: as indicated above, the tests without humic acid yielded solubility values very similar to those by other methods employed in previous studies, suggesting that a non-aqueousrcolloidal phase
did not interfere with these tests. However, perhaps the development of a colloidal phase was only important in the presence of the humic acid Že.g. a mixed, hydrocarbon᎐humic colloidal phase.. Alternative explanations of our observations cannot be excluded at this time. If the slow phase sorption of 1-MN in our apparent solubility tests was not an artifact, it might have been related to ongoing changes in aggregation or association of the HA molecules, influenced by the presence of non-aqueous 1-MN. The non-aqueous phase 1-MN may have interacted with the aqueous Aldrich HA, changing the conformation andror aggregation properties of the latter, which in turn changed the extent of binding of dissolved phase 1-MN by the HA. Perhaps, similar to the reported effects by other added organic phases ŽConte and Piccolo, 1999., the presence of 1-MN may cause a reduction in HA aggregation, which may lead to enhanced binding.
3.2. Binding of methylated naphthalenes by Aldrich HA-SPME results Table 2 provides K OC,app values for 1-MN, 2-MN, 1,3-DMN, 1,7-DMN and 2,3,5-TMN, based on SPMEGCrMS analyses. Compared with apparent solubility results, SPME-GCrMS analyses yielded lower but similar K OC,app values for 1-MN and 1,3-DMN in 1 grl Aldrich HA ŽTables 1 and 2.. We emphasize that this is
Table 2 Apparent binding coefficients in Aldrich HA based on SPME-GCrMS Ž K OC ,app as lrkg organic C in HA. Test
Aldrich HA conc. Žgrl.
Equilibration period Ždays.
Final dissolved methylated naphthalene conc. Žgrl.
KOC ,app Žlrkg. "
Ž n.
1-MN
1 3 3 10 10
1 1 3 1 3
309᎐1079 1269᎐1468 1325᎐1567 1794᎐1804 1507᎐1881
2.16 Ž"0.45. = 103 8.44 Ž"1.41. = 102 7.36 Ž"1.59. = 102 1.16 Ž"0.02. = 103 1.26 Ž"0.19. = 103
Ž8. Ž3. Ž3. Ž3. Ž3.
2-MN
1
1
664᎐977
1.69 Ž"0.27. = 103
Ž8.
1,3-DMN
1 3 3 10 10
1 1 3 1 3
359᎐704 737᎐860 761᎐981 899᎐965 799᎐964
5.04 Ž"0.69. = 103 2.15 Ž"0.28. = 103 1.87 Ž"0.42. = 103 2.59 Ž"0.11. = 103 2.75 Ž"0.31. = 103
Ž8. Ž3. Ž3. Ž3. Ž3.
1,7-DMN
1
1
460᎐993
3.54 Ž"0.41. = 103
Ž8.
1 3 3 10 10
1 1 3 1 3
233᎐439 468᎐539 474᎐652 573᎐622 481᎐566
9.81 Ž"1.00. = 103 3.96 Ž"0.45. = 103 3.42 Ž"0.86. = 103 4.27 Ž"0.19. = 103 4.83 Ž"0.49. = 103
Ž8. Ž3. Ž3. Ž3. Ž3.
2,3,5-TMN
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
not a direct comparison between SPME-GCrMS and the apparent solubility method, because the aqueous concentrations of methylated naphthalenes were different for these two types of tests, and there was no excess non-aqueous liquid phase of methylated naphthalenes in the SPME batches. These differences were intentional: the apparent solubility method requires the addition of an excess pure phase, with its inherent problems as noted above, whereas this is avoided when using the SPME technique. The K OC,app values determined for 1-MN, 1,3-DMN and 2,3,5-TMN in 3 grl Aldrich HA were 35᎐50% of the K OC,app values measured in 1 grl HA with the same SPME method ŽTable 2.. Student’s t-tests indicated that the differences were significant at the 95% CI. This result is in agreement with published results ŽCarter and Suffet, 1982; Landrum et al., 1984; Li et al., 1997., that have indicated that the binding of hydrophobic compounds by HA decreases as the HA concentration increases. Previous observations of such a relationship were conducted at much lower HA concentrations, typically - 50 mgrl. Landrum et al. Ž1984. suggested that this K OC,app vs. HA concentration trend might be related to the formation of larger aggregates or associations of humic acid at higher HA concentrations. Similarly, Li et al. Ž1997. attributed this trend to increased aggregation at higher HA concentrations, leading to fewer available exposed binding sites. In contrast to the above trend for the range 1᎐3 grl HA, the values of K OC,app for 1,3-DMN and 2,3,5-TMN at 10 grl HA were very similar to those at 3 grl ŽTable 2.. Student’s t-tests indicated that there were no significant differences in the K OC,app values for these compounds in 3 or 10 grl HA, at the 95% CI. Similarly, the value of K OC,app for 1-MN was only slightly higher in 10 grl than in 3 grl. These results suggest that the concentration effect on binding is important for the range 1᎐3 grl Žand perhaps below 1 grl., but becomes negligible at higher levels of Aldrich HA Ž3᎐10 grl.. Confirmation and explanation of this pattern requires further investigation. In particular, the concerns that some earlier researchers have raised about fouling of the SPME fibers at high aqueous HA concentrations should be examined ŽPoerschmann et al., 1999; Pawliszyn, 2000.. Perhaps a membrane-protected SPME procedure could be applied ŽPawliszyn, 2000.. Guetzloff and Rice Ž1994. reported that above 7.4 grl, aqueous Aldrich HA formed micelles, which greatly enhanced its ability to dissolve DDT. In contrast, Guetzloff and Rice Ž1996. and Johnson and John Ž1999. found no evidence for enhanced solubilization of pyrene and PCE, respectively, by micellar Aldrich HA at concentrations ) 7.4 grl. The SPME data obtained by Kurz et al. Ž1998. for binding of TCE and other compounds to leonardite and peat humic acids indicated
501
relatively constant binding coefficients over a large range in humic concentrations Žup to several 10s of grl., or a decrease in the strength of binding at the highest humic concentrations. Kurz et al. suggested that the latter trend might be an artifact. With the exception of Guetzloff and Rice Ž1994., the other above cited studies are consistent with our results in that they do not support the inferred critical micellar concentration Ž7.4 grl. for Aldrich HA, or other humic acids tested ŽKurz et al., 1998.. As noted, we found that the coefficients Žapparent. for binding of methylated naphthalenes to Aldrich HA are nearly identical for 3 and 10 grl HA. As indicated in Section 3.1, the apparent solubility tests suggested that a secondary, slow sorption phase required several days or more for equilibration between HA-bound and free aqueous methylated naphthalenes. In contrast, the SPME-GCrMS results suggest a much shorter period for equilibration. In SPME-GCrMS tests, the coefficients for binding of methylated naphthalenes to Aldrich HA in 3 and 10 grl HA were virtually identical at 1 and 3 days. In preliminary SPME tests, the free dissolved concentrations of methylated naphthalenes declined noticeably over the first 6 to 10 h. Using the complexation-flocculation technique, Laor and Rebhun Ž1997. reported similar trends for time-dependent binding of pyrene and phenanthrene to Aldrich HA Žfiltered 0.45 m. and inferred approximate equilibrium after 20 h. Some previous studies have indicated very rapid sorption kinetics, reaching apparent equilibrium in several minutes, for the binding of hydrophobic compounds to natural humic substances ŽSchlautman and Morgan, 1993; Poerschmann et al., 1997., and for binding of benzow axpyrene to aqueous Aldrich HA Žfiltered 0.3 m. ŽMcCarthy and Jimenez, 1985.. In this study, the standard deviations for the K OC,app values determined by SPME-GCrMS were somewhat higher than those determined for apparent solubility. We suggest that the larger analytical errors associated with the SPME-GCrMS technique were due to the following factors: Ži. the injections for this method were performed manually; Žii. the drift for the SPMEGCrMS analyses was greater, possibly related to fouling of the SPME fibers.
3.3. Comparison to pilot scale experimental data The different K OC,app values that were obtained by the apparent solubility and SPME-GCrMS techniques in this study reflect the differences in the experimental conditions. Combined, these two types of tests may provide a useful range of conditions that are representative of contaminated soils and aquifers that are being flushed with concentrated commercial humic acids. Van Stempvoort et al. Ž2001. have reported a pilot
502
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
scale in-situ flushing experiment, in which 1 grl Aldrich HA was used as a flushing solution to remove hydrocarbons from an artificial, non-aqueous liquid diesel source. Based on monitoring data, Van Stempvoort et al. estimated K OC,app Žlrkg. values for the binding of diesel-derived methylated naphthalenes to the aqueous Aldrich HA as follows: 2.74 Žqry0.85. = 10 3 for methylnaphalenes ŽMNs s 1-MN and 2-MN., 7.85 Žqry2.56. = 10 3 for dimethylnaphthalenes ŽDMNs, all isomers combined., and 21.6 Žqry6.0. = 10 3 for trimethylnaphthalenes ŽTMNs, all isomers combined.. In the following text, these are referred to as ‘diesel’ values. Some of the K OC,app values we obtained in bench scale batch tests, as reported in Tables 1 and 2, are close to the diesel values. The apparent solubility tests with 1-MN that had minimal excess non-aqueous phase, and were 1 to 2 days in duration, yielded K OC,app values that were nearly identical to the diesel value for MNs ŽFig. 1.. However, the K OC,app values obtained by this batch method for 1,3-DMN were considerably higher than the diesel values ŽFig. 1.. In part, these differences may reflect differences in contact times. In the diesel flushing experiment, the mean contact time between the flushing solution and the diesel source was
approximately 10 h ŽVan Stempvoort et al., 2001., whereas in the apparent solubility batch tests, the contact time was 1᎐12 days. The K OC,app values obtained by SPME-GCrMS ŽTable 2. using pure phase 1-MN and 1,3-DMN in 1 grl Aldrich HA ŽTable 2. are similar, though lower, than the values for MNs and DMNs from diesel fuel ŽFig. 1.. The lower K OC,app values for 2-MN and 1,7DMN ŽTable 2. are less similar to the corresponding diesel values. The much lower K OC,app value for 2,3,5TMN than the diesel value for TMNs ŽFig. 1. suggests that 2,3,5-TMN is not representative of the mixture of TMN isomers in the diesel studied by Van Stempvoort et al. Ž2001.. It is useful to note that the laboratory batch tests and analyses reported here required few resources compared with the detailed pilot scale study reported by Van Stempvoort et al. Ž2001..
4. Conclusions 1. The coefficients Ž K OC,app , lrkg. for binding of methylated naphthalenes to concentrated Aldrich
Fig. 1. Values of K OC ,app for binding of methylated naphthalenes in 1 grl Aldrich HA, as measured by apparent solubility ŽTable 1, minimal excess non-aqueous phase. and SPME-GCrMS ŽTable 2., and ‘diesel values’, based on a pilot scale diesel flushing experiment as reported elsewhere ŽVan Stempvoort et al., 2001..
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
2.
3.
4.
5.
humic acid ŽHA. tend to be higher with the apparent solubility method compared to solid-phase microextraction ŽSPME.. This reflects differences in the conditions of the two tests: the apparent solubility tests have only one methylated naphthalene per batch, a residual non-aqueous phase, and a higher dissolved concentration Žsaturated in water. of each methylated naphthalene. Potential artifacts for both methods, as discussed in this paper, should be investigated further. An increase in inferred binding over a 12 day period was observed in the apparent solubility tests with 1-MN. In contrast, in the absence of residual non-aqueous phase, the SPME-GCrMS tests showed negligible delayed binding of methylated naphthalenes between 1 and 3 days. Based on the SPME-GCrMS analyses, the K OC,app values for binding of methylated naphthalenes to Aldrich HA in 3 grL HA were 35᎐50% of their values in 1 grl HA. In contrast, the K OC,app values were very similar at HA concentrations of 3 and 10 grl. These observations indicate that within the 1 to 10 grl HA range tested, there was no significant formation of HA micelles, which would have resulted in a dramatic increase in the solubilization of methylated naphthalenes. Although our data also indicate some differences in binding strength at 1 and 3 grl HA, the K OC,app values we obtained for methylated naphthalenes in concentrated Aldrich HA Ž1᎐10 grl. are similar Žwithin an order in magnitude. to predicted K OC values as a function of K OW . The latter are based on experiments with various non-ionic chemicals at low levels of Aldrich HA ŽEq. Ž3. of this paper, from Burkhard, 2000.. This result suggests that the binding of methylated naphthalenes to concentrated Aldrich HA ŽG 1 grl. follows the general pattern for binding of non-ionic chemicals to relatively dilute Aldrich HA Ž- 0.1 grl.. The batch and analysis techniques reported in this paper provide a relatively inexpensive and rapid way to quantify the binding of organic contaminants to concentrated humic acid solutions. These data can then be used in numerical simulations for the application of concentrated commercial humic acids as flushing agents in subsurface remediation.
Acknowledgements Laboratory assistance was provided by Susan Brown, Dan Banks and Tomas Chihula. Funding was provided by Environment Canada, the Program of Energy Research and Development ŽPERD. and the Centre for
503
Research in Earth & Space Technology ŽCRESTech.. This paper was improved by the comments of three anonymous reviewers. References Abdul, A.S., Gibson, T.L., Rai, D.N., 1990. Use of humic acid solution to remove organic contaminants from hydrogeologic systems. Environ. Sci. Technol. 24, 328᎐333. Boving, T.B., Brusseau, M.L., 2000. Solubilization and removal of residual trichloroethene from porous media: comparison of several solubilization agents. J. Contam. Hydrol. 42, 51᎐67. Burkhard, L.P., 2000. Estimating dissolved organic carbon partition coefficients for nonionic organic chemicals. Environ. Sci. Technol. 34, 4663᎐4668. Carter, C.W., Suffet, I.H., 1982. Binding of DDT to dissolved humic materials. Environ. Sci. Technol. 16, 735᎐740. Chiou, C.T., Malcolm, R.L., Brinton, T.I., Kile, D.E., 1986. Water solubility enhancement of some organic pollutants and pesticides by dissolved humic and fulvic acids. Environ. Sci. Technol. 20, 502᎐508. Chiou, C.T., Kile, D.E., Rutherford, D.W., Sheng, G., Boyd, S.A., 2000. Sorption of selected organic compounds from water to a peat soil and its humic-acid and humin fractions: potential sources of the sorption nonlinearity. Environ. Sci. Technol. 34, 1254᎐1258. Conte, P., Piccolo, A., 1999. Conformational arrangement of dissolved humic substances. Influence of solution composition on association of humic molecules. Environ. Sci. Technol. 33, 1682᎐1690. Cornelissen, G., van Noort, P.C.M., Govers, H.A.J., 1998. Mechanism of slow desorption of organic compounds from sediments: a study using model sorbents. Environ. Sci. Technol. 32, 3124᎐3131. Ding, J.-Y., Wu, S.-C., 1997. Transport of organochlorine pesticides in soil columns enhanced by dissolved organic carbon. Water Sci. Technol. 35, 139᎐145. Dunnivant, F.M., Jardine, P.M., Taylor, D.L., McCarthy, J.F., 1992. Cotransport of cadmium and hexachlorobiphenyl by dissolved organic carbon through columns containing aquifer material. Environ. Sci. Technol. 26, 360᎐368. Friedig, A.P., Garicano, E.A., Busser, F.J.M., Hermens, J.L.M., 1998. Estimating impact of humic acid on bioavailability and bioaccumulation of hydrophobic chemicals in guppies using kinetic solid-phase extraction. Environ. Toxicol. Chem. 17, 1998᎐2004. Frimmel, F.H., 1998. Characterization of natural organic matter as major constituents in aquatic systems. J. Contam. Hydrol. 35, 201᎐216. Goss, K.-U., Schwarzenbach, R.P., 2001. Linear free energy relationships used to evaluate equilibrium partioning of organic compounds. Environ. Sci. Technol. 35, 1᎐9. Guetzloff, T.F., Rice, J.A., 1994. Does humic acid form a micelle? Sci. Total Environ. 152, 31᎐35. Guetzloff, T.F., Rice, J.A., 1996. Micellar nature of humic colloids. In: Gaffney, J.S., Marley, N.A., Clark, S.B. ŽEds.., Humic and Fulvic Acids, Isolation, Structure and Environmental Role. ACS Symposium Series, 651. American Chemical Society, Washington, DC, pp. 18᎐25.
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Holten Lutzhøft, H.-C., Vaes, W.H.J., Freidig, A.P., Halling¨ Sørensen, B., Hermens, J.L.M., 2000. Influence of pH and other modifying factors on the distribution behavior of 4-quinolones to solid phases and humic acids studied by ‘negligible-depletion’ SPME-HPLC. Environ. Sci. Technol. 34, 4989᎐4994. Johnson, W.P., John, W.W., 1999. PCE solubilization by commercial humic acid. J. Contamin. Hydrol. 35, 343᎐362. Kurz, M.D., Olson, E.S., Gallagher, J.R., 1998. Task 1.16 Enhanced Mobility of Dense Nonaqueous-phase Liquids ŽDNAPLs. Using Dissolved Humic Acids. Final Topical Report Prepared for US Dept. Energy by the Energy & Environmental Research Center, University of North Dakota. Publication 噛 98-EERC-10-03, 13 pp. Landrum, P.F., Nihart, S.R., Eadie, B.J., Gardner, W.S., 1984. Reverse-phase separation method for determining pollutant binding to Aldrich humic acid and dissolved organic carbon of natural waters. Environ. Sci. Technol. 18, 187᎐192. Laor, Y., Rebhun, M., 1997. Complexation-flocculation: a new method to determine binding coefficients of organic contaminants to dissolved humic substances. Environ. Sci. Technol. 31, 3558᎐3564. Lesage, S., Xu, H., Novakowski, K.S., Brown, S., Durham, L., 1995. Use of humic acids to enhance the removal of aromatic hydrocarbons from contaminated aquifers, part II: pilot scale. Fifth Annual Symposium on Groundwater and Soil Remediation, Toronto, Ontario, October, 2᎐6 Proceedings on CDrROM, 10 pp. Li, A.Z., Marx, K.A., Walker, J., Kaplan, D.L., 1997. Trinitrotoluene and metabolites binding to humic acid. Environ. Sci. Technol. 31, 584᎐589. Ma, H., Kim, S.D., Cha, D.K., Allen, H.E., 1999. Effects of kinetics of complexation by humic acid on toxicity of copper to Ceriodaphnia dubia. Environ. Toxicol. Chem. 18, 828᎐837. Mackay, D., Shiu, W.Y., Ma, K.C., 1992. Illustrated Handbook of Physical-Chemical Properties and Environmental Rate for Organic Chemicals, Vol. II. Lewis Publishers, USA, Chelsea, MI. McCarthy, J.F., Jimenez, B.D., 1985. Interactions between polycyclic aromatic hydrocarbons and dissolved humic material: binding and dissociation. Environ. Sci. Technol. 19, 1072᎐1076. McCarthy, J.F., Zachara, J.M., 1989. Subsurface transport of contaminants. Environ. Sci. Technol. 23, 496᎐502. Nanny, M.A., Maza, J.P., 2001. Noncovalent interactions between monoaromatic compounds and dissolved humic acids: a deuterium NMR T1 relaxation study. Environ. Sci. Technol. 35, 379᎐384. Page, C.A., Bonner, J.S., Sumner, P.L., Autenrieth, R.L., 2000. Solubility of petroleum hydrocarbons in oilrwater systems. Marine Chem. 70, 79᎐87. Pignatello, J.J., Xing, B., 1996. Mechanism of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 30, 1᎐11. Pawliszyn, J., 2000. Theory of solid-phase microextraction. J. Chromatogr. Sci. 38, 270᎐278.
Poerschmann, J., Zhang, Z., Kopinke, F.D., Pawliszyn, J., 1997. Solid phase microextraction for determining the distribution of chemicals in aqueous matrices. Anal. Chem. 69, 597᎐600. Poerschmann, J., Kopinke, F.-D., Plugge, J., Georgi, A., 1999. Interaction of organic chemicals ŽPAH, PCB, triazines, nitroaromatics and organotin compounds. with dissolved humic organic matter. In: Ghabbour, E.A., Davies, G. ŽEds... Understanding Humic Substances, Advanced Methods, Properties and Applications. Roy. Soc. Chemistry, Cambridge, UK, pp. 223᎐240. Ramos, E.U., Meijer, S.N., Vaes, W.H.J., Verhaar, H.J.M., Hermens, J.L.M., 1998. Using solid-phase microextraction to determine partition coefficients to humic acids and bioavailable concentrations of hydrophobic chemicals. Environ. Sci. Technol. 32, 3430᎐3435. Rebhun, M., de Smedt, F., Rwetabula, J., 1996. Dissolved humic substances for remediation of sites contaminated by organic pollutants. Binding-desorption model predictions. Water Res. 30, 2027᎐2038. Schlautman, M.A., Morgan, J.J., 1993. Effects of aqueous chemistry on the binding of polycyclic aromatic hydrocarbons by dissolved humic materials. Environ. Sci. Technol. 27, 961᎐969. Thomas, D.H., Delfino, J.J., 1991. A gas chromatographicrchemical indicator approach to assessing groundwater contamination by petroleum products. Ground Water Monitor. Rev. Fall, 90᎐100. Van Stempvoort, D.R., Lesage, S., Novakowski, E.K., Millar, K. and Brown, S. 2001. Humic acid-enhanced remediation of an emplaced diesel source in groundwater: 1. Laboratory-based pilot scale test, J. Contam. Hydrol. Žaccepted.. Xing, B., Pignatello, J.J., 1997. Dual-mode sorption of lowpolarity compounds in glassy polyŽvinyl chloride. and soil organic matter. Environ. Sci. Technol. 31, 792᎐799. Xu, H., Lesage, S., Durham, L., 1994. The use of humic acids to enhance removal of aromatic hydrocarbons from contaminated aquifers. Proceedings, 4th Annual Symposium on Groundwater & Soil Remediation, Calgary, AB, Canada, September 21᎐23, 1994, 635᎐645. Dale Van Stemp¨ oort, Ph.D., is a Research Scientist with Environment Canada. Dale’s research focuses on groundwater chemistry and contaminant remediation. Previous positions were at the Saskatchewan Research Council, Saskatoon, and as a private sector consultant. He received his Ph.D. in Earth Sciences at the University of Waterloo in 1989. Suzanne Lesage, Ph.D., is chief of the Aquatic Ecosystem Remediation Project at the National Water Research Institute, Environment Canada, Burlington, Ontario, Canada. She is also an adjunct professor, Department of Earth Sciences, University of Waterloo. She has a B.Sc. in Biochemistry ŽUniversity of Ottawa. and a Ph.D. in Chemistry ŽMcGill University..
Binding of methylated naphthalenes to concentrated aqueous humic acid 夽 Dale R. Van StempvoortU , Suzanne Lesage National Water Research Institute, P.O. Box 5050, Burlington, ON, Canada L7R 4A6
Abstract In laboratory batches of 1 grl Aldrich humic acid ŽHA., apparent binding coefficients Ž K OC,app . for 1-methylnaphthalene Ž1-MN. and 1,3-dimethylnaphthalene Ž1,3-DMN. were determined by apparent solubility Žcomparison of solubility in water and HA solution.. For the HA batches with 1-MN, two trends were observed: delayed sorption Žup to 12 days., and an increase in the apparent solubility of 1-MN as the amount of this hydrocarbon as a non-aqueous liquid was increased. These two trends suggest that formation of a colloidal phase may have interfered with our apparent solubility tests. Alternatively, the non-aqueous 1-MN may have caused a slow change in the conformation andror aggregation of the humic acid and its binding properties. Other batch tests, using solid phase microextraction ŽSPME., did not have a non-aqueous hydrocarbon phase. Compared with the apparent solubility results, the SPME tests indicated similar but lower K OC,app values for binding of 1-MN and 1,3-DMN to Aldrich HA at 1 grl HA. Also, K OC,app values for 2-MN, 1,7-DMN and 2,3,5-trimethylnaphthalene in 1 grl HA were obtained by SPME. Other SPME tests indicated that the K OC,app values for 1-MN and 1,3-DMN in 3 grl Aldrich HA were lower, 35᎐50% of those in 1 grl HA. These significant differences are probably related to changes in the conformation andror aggregation of the HA. With an increase in HA from 3 and 10 grl, no significant change in K OC,app was observed for 1,3-DMN and 2,3,5-TMN, but the K OC,app value for 1-MN was slightly larger at 10 grl. These data do not support an earlier report that HA micelles form above 7.4 grl Aldrich HA. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Humic; Methylated naphthalenes; Binding; Solubility; Solid-phase microextraction
1. Introduction Humic substances are generally the dominant organic compounds in natural waters. The chemical binding of other organic compounds to aqueous humic substances is a key environmental process. By this
夽
NWRI Contribution No. 99-236. Corresponding author. Tel.: q1-905-319-6917; fax: q1905-336-6430. E-mail address: [email protected] ŽD.R. Van Stempvoort.. U
mechanism, natural humic substances play an important role in controlling the mobility ŽMcCarthy and Zachara, 1989; Dunnivant et al., 1992. and bioavailability ŽFriedig et al., 1998; Ma et al., 1999. of hydrophobic contaminants in the environment. Consequently, some studies have examined the potential use of concentrated commercial humic acids ŽHAs. as flushing agents to remove hydrophobic organic contaminants from soils or aquifers ŽAbdul et al., 1990; Xu et al., 1994; Lesage et al., 1995; Rebhun et al., 1996; Kurz et al., 1998; Johnson and John, 1999; Boving and Brusseau, 2000; Van Stempvoort et al., 2001.. The concentrations of humic substances in natural
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 0 7 6 - 4
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
496
waters typically range between 0.5 and 100 mgrl as organic C ŽFrimmel, 1998.. Accordingly, previous laboratory studies have usually looked at binding of other organic chemicals to HA over a similar range of HA concentrations Ž- 100 mgrl as C.. These studies have usually reported K OC values, which are binding coefficients, assuming equilibrium and linear partitioning: K OC s cOCrc w
Ž l r kg.
Ž1.
where cOC is the bound chemical per mass of organic C in the humic substance Žmgrkg.; c w is the dissolved chemical concentration Žmgrl.. Alternatively, the apparent binding coefficient, K OC,app can be calculated in the same way: K OC,app s cOCrc w
Ž l r kg.
Ž2.
This parameter reflects the distribution after a given time of contact and at the concentration of study. Thus, it does not necessarily reflect either a linear sorption isotherm or equilibrium Žcf. K d,app of Cornelissen et al., 1998.. Recently, Burkhard Ž2000. reviewed information on the relationship between published K OC values of non-ionic organic chemicals and their hydrophobicity, as measured by the 1-octanol-water partition coefficient Ž K OW .. Burkhard combined K OC values reported in 73 references for various non-ionic organic chemicals, and developed equations that predict K OC Žhis K DOC . as a function of K OW . In the case of Aldrich humic acid, which is most relevant to this study, the equation given by Burkhard is Žleaving out the "standard deviations.: log Ž K OC . s 0.85= log Ž K OW . q 0.27
Ž3.
The above equation may be useful as a general guide for the binding behavior of non-ionic organic compounds, particularly those that are members of the classes that are most strongly represented in Burkhard’s database Že.g. PCBs, PAHs.. However, recent studies indicate that, in the future, a more accurate modeling of interactions of non-ionic organic chemicals with humic acids must take into account the different types of non-covalent bonds, which vary with the classes of compounds that are considered ŽGoss and Schwarzenbach, 2001; Nanny and Maza, 2001.. Other studies have shown that K OC is not closely dependent on K OW for ionizable organic compounds Že.g. Holten Lutzhøft et ¨ al., 2000.. Little information is available on how K OC values vary as aqueous humic concentrations are increased above 100 mgrl Žas C., levels which might be suitable for remediation applications Ži.e. flushing of soils,
aquifers.. As noted, the vast majority of previous binding tests that have been reported in the literature were conducted at lower humic concentrations, representative of natural conditions. In the experiments reported here, the aqueous concentrations of Aldrich HA were varied between 1 and 10 grl Ž301᎐3145 mgrl as C.. Only a few previous publications have provided data for binding of contaminants to HA at similar elevated concentrations, relevant to remediation applications. Guetzloff and Rice Ž1994, 1996. examined the binding of DDT and pyrene in batch solutions containing up to 10 grl Aldrich HA. Kurz et al. Ž1998. looked at binding of trichloroethylene ŽTCE., tetrachloroethylene ŽPCE., 4-bromobiphenyl ether and carbon tetrachloride in batch solutions containing up to 75 grl HA extracted from leonardite Žlow grade coal. or peat. Johnson and John Ž1999. examined binding of PCE in batches that contained up to 12 grl Aldrich HA. Others have used column studies to examine the ability of aqueous Aldrich HA to facilitate the solubilization andror mobilization of contaminants in soil or aquifer materials. Ding and Wu Ž1997. tested mobilization of aldrin and DDT in soil, using 950 mgrl Aldrich HA Žas C.. Johnson and John Ž1999. studied solubilization of residual liquid phase PCE in sand with up to 12 grl Aldrich HA as the flushing solution. Boving and Brusseau Ž2000. investigated solubilization of residual trichloroethene ŽTCE. in sand, using flushing solutions containing 5% by weight Aldrich HA. Lesage and coworkers ŽLesage et al., 1995; Van Stempvoort et al., 2001. examined the solubilization of residual diesel in a pilot scale model sand aquifer, using 1 grl Aldrich HA as the flushing solution. Collectively, the above laboratory studies demonstrate that it may be feasible to use concentrated commercial HAs to solubilize andror mobilize organic contaminants in subsurface environments. As in Eq. Ž1., the binding of hydrophobic organic compounds by aqueous humic substances is often modeled as partitioning Ži.e. solid-phase dissolution., following a linear isotherm ŽChiou et al., 1986.. Changes in the extent of binding with pH and other chemical parameters have been documented Že.g. Carter and Suffet, 1982; Schlautman and Morgan, 1993.. Non-linear sorption of hydrophobic compounds to humic substances is often modeled using the Freundlich equation Že.g. Xing and Pignatello, 1997; Chiou et al., 2000.. The binding may be competitive ŽXing and Pignatello, 1997; Chiou et al., 2000.. Various methods have been used to quantify the binding of hydrophobic compounds by aqueous humic substances and the advantages and disadvantages of the various techniques have been described ŽLandrum et al., 1984; Laor and Rebhun, 1997; Poerschmann et al., 1997; Ramos et al., 1998.. For binding of PAHs, previous investigators have employed the fluorescence
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
quenching technique ŽLaor and Rebhun, 1997., but it is doubtful that this technique could be used at the elevated HA concentrations Ž1᎐10 grl. in this study. Further, the fluorescence quenching technique ‘is hindered by experimental difficulties’ ŽPoerschmann et al., 1997. and it may overestimate the binding coefficient ŽLaor and Rebhun, 1997.. Another approach to measuring the extent of binding is to compare the aqueous solubility of a hydrophobic compound, S w , and its ‘apparent solubility’, Ž SU w , in the presence of aqueous humic substances Chiou et al., 1986.. In this paper this approach will be referred to as the apparent solubility method. The apparent binding coefficient Ž K OC,app , lrkg. can be calculated, based on Eqs. Ž2., Ž4. and Ž5.: cOC s Ž S Uw y S w . r Ž c HA = fC .
Ž4.
c w s Sw
Ž5.
where c w , the dissolved hydrophobic compound Žmgrl., is assumed to be at saturation; c HA is the HA concentration Žgrl. and fC is the weight fraction of carbon in Aldrich HA. The solid-phase microextraction ŽSPME. technique ŽPoerschmann et al., 1997; Ramos et al., 1998; Pawliszyn, 2000. can be used to measure the free dissolved hydrophobic compound in water, c w , in the presence of other phases, such as aqueous humic substances. In this technique, a coated, fused-silica fiber Žsolid phase. is inserted in each test solution, then withdrawn, and the quantity of contaminant that has sorbed to the fiber is then desorbed and measured by GC or another technique, and calibrated to stock solutions. The total aqueous concentration of the same compound in the test solution, c t , can also be measured Žby HPLC or other technique ., and used to infer cOC : ct s c w q c b
Ž6.
cOC s c br Ž c HA = fC .
Ž7.
where c b is the bulk aqueous concentration of bound hydrophobic compound Žmgrl.. The c w and cOC values can then be substituted into Eq. Ž2. to calculate K OC,app . The use of the SPME technique appears to offer several key advantages. Unlike apparent solubility, it does not require the use of an immiscible liquid mixture, where tiny suspended droplets may be inadvertently measured as dissolved phase. The SPME tests can be conducted quickly, without requiring equilibrium in the concentrations of hydrophobic compound between the fiber and the solution. Some studies sug-
497
gest that the SPME measurements are free of interference that might be caused by binding of HA to the SPME fibers ŽRamos et al., 1998.. However, other researchers have concluded that high concentrations of aqueous HA may have an adverse, fouling effect on the SPME fibers ŽPoerschmann et al., 1999; Pawliszyn, 2000., In this study, the apparent solubility method and SPME were used to measure the binding of methylated naphthalenes by Aldrich HA. Methylated naphthalenes are some of the most abundant water-soluble PAH components of diesel ŽThomas and Delfino, 1991., and can occur at aqueous concentrations of several hundred grl in the presence of this fuel. Hence, methylated naphthalenes pose a major portion of risk associated with groundwater contamination by diesel. To our knowledge, this study provides the first published coefficients for binding of methylated naphthalenes to aqueous humic acid. Previous investigators have focused on higher molecular weight PAHs such as phenanthrene, pyrene, fluoranthene and anthracene.
2. Experimental procedures All experiments were conducted at room temperature Ž23 " 2⬚C. using analytical grade reagents. The apparent solubility tests were used to investigate binding of two pure phase isomers, 1-MN Žmethylnaphthalene . and 1,3-DMN Ždimethylnaphthalene .. SPME was used with GCrMS to examine the binding of the same isomers and three others, 2-MN, 1-7 DMN and trimethylnaphthalene Ž2,3,5-TMN..
2.1. Preparation of humic acid solutions Nominal Aldrich HA ‘solutions’ Ž1, 3 and 10 grl. were prepared by adding humic acid Žsodium salt, tech., product no. H16752, Lot No. 16206AN, Aldrich Chemicals, Milwaukee. to Milli-Q water. The 1 grl HA solution is the flushing solution that was tested at the pilot-scale by Lesage and coworkers ŽLesage et al., 1995; Van Stempvoort et al., 2001.. Aldrich HA was selected to be a well-documented, representative commercial humic product that could easily be prepared in aqueous form, in large volume and high concentration, for field-scale remediation applications. Based on information provided by Aldrich Chemicals, this product is derived from lignite obtained by open pit mining in Germany ŽOberhessen, Kassel., and contains approximately 9.4% Fe oxides, 3% S, 2% Na, 0.5% Ca, 0.4% Al, 0.05% Mg and 0.04% K. We measured the ash component of this product to be 64.6 Ž s 0.68.% by weight. The HA solutions were stirred for ; 12 h. Minor
498
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
particulate HA settled over the next 2᎐4 days and the supernatants of each were transferred by a peristaltic pump and stored for use in batch tests, as described below. Based on filtration and centrifugation tests, the HA solutions contained both dissolved and colloidal HA, and perhaps some remaining suspended, particulate HA. Some of the particulate matter that settled out might have been mineral impurities. A sample of each batch of aqueous HA was analyzed for total organic carbon by a Shimadzu TOC-5050 analyzer. These analyses indicated that the nominal aqueous HA concentrations of these batches corresponded to the following organic C concentrations: 1 grl HA, 301 mg Crl; 3 grl, 967 mg Crl; 10 grl, 3145 mg Crl. These values were used for subsequent K OC,app determinations. Given the fact that the batch solutions were mixtures of Aldrich HA and Milli-Q water, their physical chemistry Žionic strength, pH. was dependent on the chemistry and buffering capacity of the HA. The pH measurements indicated a range of 7.3᎐8.4.
2.2. Solubility of methylated naphthalenes in water and Aldrich HA: apparent solubility results Milli-Q water and the Aldrich HA solutions were sparged with Ar Ž; 1rmin per 100 ml. to remove O 2 , and placed in an anaerobic chamber Ž5% CO 2 , 10% H 2 and 85% N2 .. The O 2 was removed to minimize subsequent biodegradation and photooxidation of PAHs. Then, for each test, 100 ml of either HA and Milli-Q was transferred to a glass serum bottle, and pure phase Žliquid. 1-MN or 1,3-DMN ŽSigma-Aldrich Canada Ltd., Mississauga, ON. was added in excess of the anticipated saturation. The mixtures were sealed with Teflon-lined septa, placed on an orbital shaker for 16᎐24 h, returned to the anaerobic chamber, then sampled by glass syringe Žafter 1, 2, 5 and 12 days. and passed through columns of glass beads to trap and remove residual pure phase Žliquid. methylated naphthalenes. These columns had been prepared by packing glass beads Ž0.60᎐0.85-mm diam.. between silanized glass wool plugs in a disposable Pasteur pipette. During preliminary testing, the first six successive passes of the same sample showed gradual increases in concentrations, apparently due to sorption of some of the dissolved methylated naphthalenes to the packed glass bead column. Thus, the first six passes were discarded. Subsequent samples were diluted 50% in methanol ŽHPLC grade, Ar-sparged. and analyzed by HPLC, using a Waters 䊛 system Ž600E multisolvent delivery system, 700 WISP autosampler, and 470 Scanning Fluorescence Detector.. The chromatographic column selected for these analyses was a RP-8 Spheri-10 Brownlee 䊛 cartridge ŽPerkin-Elmer Corp., Norwalk, CT.. The
eluent was a mixture of methanol and Milli-Q water Ž65r35% by vol... Analyses of standards with increasing HA concentrations showed that there was no interference with the HPLC analyses of total methylated naphthalenes.
2.3. SPME-GCr MS tests of the binding of methynaphthalenes by HA The SPME method for the selective analysis of dissolved phase organic compounds in the presence of other phases, such as humic compounds, is well established ŽPawliszyn, 2000.. SPME fiber assemblies Žwith film of 100 m polydimethylsiloxane. and a manual sampling holder were purchased from SUPELCO Chromatography Products ŽOakville, ON.. The fibers were cut to reduce the length from 1 to 0.2 cm. In the anaerobic chamber, methylated naphthalenes were added from a stock Ž50 mgrl each of 1-MN, 1,3-DMN and 2,3,5-TMN or 50 mgrl each of 2-MN and 1,7-TMN, in methanol. to 100 ml of either Milli-Q water ŽpH adjusted with NaOH to 8, Ar-purged. or Aldrich HA solution Žnominal 3 or 10 grl, Ar-purged. in 120-ml glass serum bottles, to prepare final concentrations of between 200 and 7500 grl of each MN. These bottles were sealed and placed on an orbital shaker for 3 h Ž1 grl HA. to 15 h Ž3᎐10 grl HA.. The SPME-GCrMS analyses of methylated naphthalenes in Milli-Q were used as standards for calibration of the HA batch results. As a check for negligible depletion of the aqueous phase methylated naphthalenes by SPME, 1-l volumes of MN in hexane were injected manually into the GC, and the detector response per mass of MN determined. The fractions of each aqueous MN that had sorbed to the fiber per SPME-GCrMS test were on the order of 1᎐2% Ž1-MN, 1,3-DMN, 2,3,5-TMN.. For a check on the total methylated naphthalenes in the Aldrich HA batches, samples were diluted in methanol Ž1:5, H 2 O:MeOH. prior to HPLC analyses. This resulted in quantitative Ž) 96% for the majority of analyses . recovery of total methylated naphthalenes. Preliminary tests indicated that the free dissolved methylated naphthalene concentrations declined noticeably over the first 6᎐10 h in the presence of humic acid, then appeared to reach relatively stable values. After shaking, portions Ž20 ml. of these solutions were transferred to glass vials Ž25 ml. in the anaerobic chamber and capped by septa for storage prior to analysis. Then, for SPME analysis, each 20 ml batch was placed on a magnetic stirrer and mixed at ; 500 rev.rmin Žmaximum speed without a vortex forming on surface of batch solution.. The SPME fiber assembly was inserted through the septum and the fiber was exposed to the stirred solution for 10 min. After exposure was completed, within 1 min the fiber
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
was manually inserted into the port of a HewlettPackard GCrMSD ŽModel 5890A GC, 5970 MSD. and the accumulated methylated naphthalenes were desorbed at 250⬚C. The GC column was a DB-1 ŽJ & W Scientific Inc., Folsom, CA: 30 m long, 0.32 mm OD, 0.25 m film thickness ., and the carrier gas was helium. The GC oven was held at 50⬚C for 5 min, then increased at a rate of 20⬚Crmin to 250⬚C.
3. Results and discussion 3.1. Apparent solubility of methylated naphthalenes in the presence of Aldrich HA For 1-MN, the aqueous solubility, S w , was found to be 30.0" 1.7 mgrl Ž n s 40.. This result compares well with published values compiled by Mackay et al. Ž1992., which range from 26 to 32 mgrl. For 1,3-DMN, the S w was found to be 8.2" 0.4 mgrl Ž n s 24., similar to values compiled by Mackay et al. Ž1992., the majority of which range from 7.8 to 8.0 mgrl. The apparent solubilities, SU w , of 1-MN and 1,3-DMN in 301 mgrl HA Žas TOC. solutions were measured by HPLC. In the first apparent solubility batch test, a large amount of pure phase 1-MN was added Ž2 mg 1-MN per ml Milli-Q water. yielding very high SU w results Ž119᎐196 mgrl., with values increasing over 5 days ŽTable 1.. In the second apparent solubility test, a much smaller amount of 1-MN was used Ž0.4 mgrml.. After 1 day the SU w was 56.5 mgrl and it increased further over a period of 5᎐12 days ŽTable 1.. The SU w of 1,3-DMN in 1 grl HA as measured by HPLC was 50.4 mgrl after 1 day, and then decreased to 45.1 mgrl after 2 days ŽTable 1..
499
Table 1 provides the K OC,app values that were calculated for 1-MN and 1,3-DMN using Eqs. Ž3. ᎐ Ž5.. Table 1 also shows the octanol᎐water partial coefficients Ž K OW ., and K OC values obtained using Eq. Ž3., which relates K OC values to K OW . As explained in Section 1, Eq. Ž3. is based on published data for experiments with various non-ionic organic compounds at lower concentrations of Aldrich HA. Excepting our first test, where a large amount of 1-MN was added Ž2 mgrml., there was relatively close agreement between the values of K OC,app based on our tests and the values of K OC based on Eq. Ž3.. This comparison suggests that the binding of the methylated naphthalenes to Aldrich HA follows the general pattern reported by Burkhard Ž2000., and that the binding is not strongly affected by the concentration of HA. The K OC,app values shown in Table 1 for 1-MN and 1,3-DMN are approximately an order of magnitude smaller than binding coefficients that have been inferred for more strongly hydrophobic PAHs, such as fluoranthene wlogŽ K OW . s 5.22, K OC s 0.93᎐1.15 = 10 5 x and pyrene wlogŽ K OW . s 5.18, K OC s 1.05᎐1.19= 10 5 x, at lower concentrations of Aldrich HA, typically F 100 mgrl ŽLaor and Rebhun, 1997.. However, the K OC,app determined for 1,3-DMN in this study Ž1.50 to 1.71= 10 4 lrkg. is similar to previous K OC values for binding of similarly hydrophobic phenanthrene ŽlogŽ K OW . s 4.57, K OC s 1.47 to 1.66= 10 4 . and anthracene ŽlogŽ K OW . s 4.54, K OC s 2.71 = 10 4 . to Aldrich HA, as determined by complexation-flocculation ŽLaor and Rebhun, 1997.. The apparent solubility tests with 1-MN indicated that a large proportion of the apparent sorption was a slow-phase or ‘delayed’ component. Xu et al. Ž1994.
Table 1 Apparent solubilities Ž SUw . and apparent binding coefficients Ž K OC,app as lrkg organic C in HA. for methylated naphthalenes in 1 grl HA Isomer starting ratio: methylated naphthalene to water
1-MN 2 mgrml
0.4 mgrml
1,3-DMN 0.1 mgrml a
MacKay et al., 1992.
Aging of mixture Ždays.
SU w Žmgrl. "
KOC,app Žlrkg. " by apparent solubility
Ž n.
1 2 5 1 2 5 12
119.2" 5.7 127.3" 22.0 196.2" 16.3 56.5" 1.9 53.6" 2.2 63.1" 3.3 87.5" 3.8
9.87 Ž"0.66. = 103 1.08 Ž"0.25. = 104 1.84 Ž"0.18. = 104 2.92 Ž"0.28. = 103 2.61 Ž"0.30. = 103 3.66 Ž"0.41. = 103 6.37 Ž"0.46. = 103
Ž36. Ž36. Ž24. Ž36. Ž36. Ž36. Ž18.
1 2
50.4" 1.6 45.1" 1.4
1.71 Ž"0.06. = 104 1.50 Ž" 0.06. = 104
Ž36. Ž36.
logŽ KOW . a
KOC predicted by Eq. Ž3.
3.87
Žbased on KOW . 3.63= 103
4.42
1.06= 104
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
500
also used the apparent solubility technique and observed a similar delayed component for the binding of phenanthrene to Aldrich HA over a 36-day period. One possible explanation of this trend is that the delayed sorption is related to the slow rate of diffusion within the humic acid molecules andror to activation energies of sorption and desorption ŽPignatello and Xing, 1996.. However, the inferred delayed sorption in the apparent solubility tests occurs at a much slower rate Ždays to completion. than the delayed sorption in the SPME-GC tests Žhours, see Section 2.3.. This comparison suggests that the excess non-aqueous liquid 1-MN in the apparent solubility tests played a crucial role in causing what appears to be delayed Ži.e. slow. sorption. It has been suggested that the bulk of the apparent slow sorption Žup to 12 days or longer. in the apparent solubility tests may be an artifact, due to breakthrough of non-aqueous 1-MN through the glass bead columns. Perhaps this non-aqueous 1-MN was a colloidal phase ŽPage et al., 2000.. The breakthrough of such a nonaqueousrcolloidal phase might also explain our observation that the apparent binding coefficient for 1-MN increased as a function of the amount of non-aqueous phase 1-MN that we added ŽTable 1.. There is a weakness in this explanation: as indicated above, the tests without humic acid yielded solubility values very similar to those by other methods employed in previous studies, suggesting that a non-aqueousrcolloidal phase
did not interfere with these tests. However, perhaps the development of a colloidal phase was only important in the presence of the humic acid Že.g. a mixed, hydrocarbon᎐humic colloidal phase.. Alternative explanations of our observations cannot be excluded at this time. If the slow phase sorption of 1-MN in our apparent solubility tests was not an artifact, it might have been related to ongoing changes in aggregation or association of the HA molecules, influenced by the presence of non-aqueous 1-MN. The non-aqueous phase 1-MN may have interacted with the aqueous Aldrich HA, changing the conformation andror aggregation properties of the latter, which in turn changed the extent of binding of dissolved phase 1-MN by the HA. Perhaps, similar to the reported effects by other added organic phases ŽConte and Piccolo, 1999., the presence of 1-MN may cause a reduction in HA aggregation, which may lead to enhanced binding.
3.2. Binding of methylated naphthalenes by Aldrich HA-SPME results Table 2 provides K OC,app values for 1-MN, 2-MN, 1,3-DMN, 1,7-DMN and 2,3,5-TMN, based on SPMEGCrMS analyses. Compared with apparent solubility results, SPME-GCrMS analyses yielded lower but similar K OC,app values for 1-MN and 1,3-DMN in 1 grl Aldrich HA ŽTables 1 and 2.. We emphasize that this is
Table 2 Apparent binding coefficients in Aldrich HA based on SPME-GCrMS Ž K OC ,app as lrkg organic C in HA. Test
Aldrich HA conc. Žgrl.
Equilibration period Ždays.
Final dissolved methylated naphthalene conc. Žgrl.
KOC ,app Žlrkg. "
Ž n.
1-MN
1 3 3 10 10
1 1 3 1 3
309᎐1079 1269᎐1468 1325᎐1567 1794᎐1804 1507᎐1881
2.16 Ž"0.45. = 103 8.44 Ž"1.41. = 102 7.36 Ž"1.59. = 102 1.16 Ž"0.02. = 103 1.26 Ž"0.19. = 103
Ž8. Ž3. Ž3. Ž3. Ž3.
2-MN
1
1
664᎐977
1.69 Ž"0.27. = 103
Ž8.
1,3-DMN
1 3 3 10 10
1 1 3 1 3
359᎐704 737᎐860 761᎐981 899᎐965 799᎐964
5.04 Ž"0.69. = 103 2.15 Ž"0.28. = 103 1.87 Ž"0.42. = 103 2.59 Ž"0.11. = 103 2.75 Ž"0.31. = 103
Ž8. Ž3. Ž3. Ž3. Ž3.
1,7-DMN
1
1
460᎐993
3.54 Ž"0.41. = 103
Ž8.
1 3 3 10 10
1 1 3 1 3
233᎐439 468᎐539 474᎐652 573᎐622 481᎐566
9.81 Ž"1.00. = 103 3.96 Ž"0.45. = 103 3.42 Ž"0.86. = 103 4.27 Ž"0.19. = 103 4.83 Ž"0.49. = 103
Ž8. Ž3. Ž3. Ž3. Ž3.
2,3,5-TMN
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
not a direct comparison between SPME-GCrMS and the apparent solubility method, because the aqueous concentrations of methylated naphthalenes were different for these two types of tests, and there was no excess non-aqueous liquid phase of methylated naphthalenes in the SPME batches. These differences were intentional: the apparent solubility method requires the addition of an excess pure phase, with its inherent problems as noted above, whereas this is avoided when using the SPME technique. The K OC,app values determined for 1-MN, 1,3-DMN and 2,3,5-TMN in 3 grl Aldrich HA were 35᎐50% of the K OC,app values measured in 1 grl HA with the same SPME method ŽTable 2.. Student’s t-tests indicated that the differences were significant at the 95% CI. This result is in agreement with published results ŽCarter and Suffet, 1982; Landrum et al., 1984; Li et al., 1997., that have indicated that the binding of hydrophobic compounds by HA decreases as the HA concentration increases. Previous observations of such a relationship were conducted at much lower HA concentrations, typically - 50 mgrl. Landrum et al. Ž1984. suggested that this K OC,app vs. HA concentration trend might be related to the formation of larger aggregates or associations of humic acid at higher HA concentrations. Similarly, Li et al. Ž1997. attributed this trend to increased aggregation at higher HA concentrations, leading to fewer available exposed binding sites. In contrast to the above trend for the range 1᎐3 grl HA, the values of K OC,app for 1,3-DMN and 2,3,5-TMN at 10 grl HA were very similar to those at 3 grl ŽTable 2.. Student’s t-tests indicated that there were no significant differences in the K OC,app values for these compounds in 3 or 10 grl HA, at the 95% CI. Similarly, the value of K OC,app for 1-MN was only slightly higher in 10 grl than in 3 grl. These results suggest that the concentration effect on binding is important for the range 1᎐3 grl Žand perhaps below 1 grl., but becomes negligible at higher levels of Aldrich HA Ž3᎐10 grl.. Confirmation and explanation of this pattern requires further investigation. In particular, the concerns that some earlier researchers have raised about fouling of the SPME fibers at high aqueous HA concentrations should be examined ŽPoerschmann et al., 1999; Pawliszyn, 2000.. Perhaps a membrane-protected SPME procedure could be applied ŽPawliszyn, 2000.. Guetzloff and Rice Ž1994. reported that above 7.4 grl, aqueous Aldrich HA formed micelles, which greatly enhanced its ability to dissolve DDT. In contrast, Guetzloff and Rice Ž1996. and Johnson and John Ž1999. found no evidence for enhanced solubilization of pyrene and PCE, respectively, by micellar Aldrich HA at concentrations ) 7.4 grl. The SPME data obtained by Kurz et al. Ž1998. for binding of TCE and other compounds to leonardite and peat humic acids indicated
501
relatively constant binding coefficients over a large range in humic concentrations Žup to several 10s of grl., or a decrease in the strength of binding at the highest humic concentrations. Kurz et al. suggested that the latter trend might be an artifact. With the exception of Guetzloff and Rice Ž1994., the other above cited studies are consistent with our results in that they do not support the inferred critical micellar concentration Ž7.4 grl. for Aldrich HA, or other humic acids tested ŽKurz et al., 1998.. As noted, we found that the coefficients Žapparent. for binding of methylated naphthalenes to Aldrich HA are nearly identical for 3 and 10 grl HA. As indicated in Section 3.1, the apparent solubility tests suggested that a secondary, slow sorption phase required several days or more for equilibration between HA-bound and free aqueous methylated naphthalenes. In contrast, the SPME-GCrMS results suggest a much shorter period for equilibration. In SPME-GCrMS tests, the coefficients for binding of methylated naphthalenes to Aldrich HA in 3 and 10 grl HA were virtually identical at 1 and 3 days. In preliminary SPME tests, the free dissolved concentrations of methylated naphthalenes declined noticeably over the first 6 to 10 h. Using the complexation-flocculation technique, Laor and Rebhun Ž1997. reported similar trends for time-dependent binding of pyrene and phenanthrene to Aldrich HA Žfiltered 0.45 m. and inferred approximate equilibrium after 20 h. Some previous studies have indicated very rapid sorption kinetics, reaching apparent equilibrium in several minutes, for the binding of hydrophobic compounds to natural humic substances ŽSchlautman and Morgan, 1993; Poerschmann et al., 1997., and for binding of benzow axpyrene to aqueous Aldrich HA Žfiltered 0.3 m. ŽMcCarthy and Jimenez, 1985.. In this study, the standard deviations for the K OC,app values determined by SPME-GCrMS were somewhat higher than those determined for apparent solubility. We suggest that the larger analytical errors associated with the SPME-GCrMS technique were due to the following factors: Ži. the injections for this method were performed manually; Žii. the drift for the SPMEGCrMS analyses was greater, possibly related to fouling of the SPME fibers.
3.3. Comparison to pilot scale experimental data The different K OC,app values that were obtained by the apparent solubility and SPME-GCrMS techniques in this study reflect the differences in the experimental conditions. Combined, these two types of tests may provide a useful range of conditions that are representative of contaminated soils and aquifers that are being flushed with concentrated commercial humic acids. Van Stempvoort et al. Ž2001. have reported a pilot
502
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
scale in-situ flushing experiment, in which 1 grl Aldrich HA was used as a flushing solution to remove hydrocarbons from an artificial, non-aqueous liquid diesel source. Based on monitoring data, Van Stempvoort et al. estimated K OC,app Žlrkg. values for the binding of diesel-derived methylated naphthalenes to the aqueous Aldrich HA as follows: 2.74 Žqry0.85. = 10 3 for methylnaphalenes ŽMNs s 1-MN and 2-MN., 7.85 Žqry2.56. = 10 3 for dimethylnaphthalenes ŽDMNs, all isomers combined., and 21.6 Žqry6.0. = 10 3 for trimethylnaphthalenes ŽTMNs, all isomers combined.. In the following text, these are referred to as ‘diesel’ values. Some of the K OC,app values we obtained in bench scale batch tests, as reported in Tables 1 and 2, are close to the diesel values. The apparent solubility tests with 1-MN that had minimal excess non-aqueous phase, and were 1 to 2 days in duration, yielded K OC,app values that were nearly identical to the diesel value for MNs ŽFig. 1.. However, the K OC,app values obtained by this batch method for 1,3-DMN were considerably higher than the diesel values ŽFig. 1.. In part, these differences may reflect differences in contact times. In the diesel flushing experiment, the mean contact time between the flushing solution and the diesel source was
approximately 10 h ŽVan Stempvoort et al., 2001., whereas in the apparent solubility batch tests, the contact time was 1᎐12 days. The K OC,app values obtained by SPME-GCrMS ŽTable 2. using pure phase 1-MN and 1,3-DMN in 1 grl Aldrich HA ŽTable 2. are similar, though lower, than the values for MNs and DMNs from diesel fuel ŽFig. 1.. The lower K OC,app values for 2-MN and 1,7DMN ŽTable 2. are less similar to the corresponding diesel values. The much lower K OC,app value for 2,3,5TMN than the diesel value for TMNs ŽFig. 1. suggests that 2,3,5-TMN is not representative of the mixture of TMN isomers in the diesel studied by Van Stempvoort et al. Ž2001.. It is useful to note that the laboratory batch tests and analyses reported here required few resources compared with the detailed pilot scale study reported by Van Stempvoort et al. Ž2001..
4. Conclusions 1. The coefficients Ž K OC,app , lrkg. for binding of methylated naphthalenes to concentrated Aldrich
Fig. 1. Values of K OC ,app for binding of methylated naphthalenes in 1 grl Aldrich HA, as measured by apparent solubility ŽTable 1, minimal excess non-aqueous phase. and SPME-GCrMS ŽTable 2., and ‘diesel values’, based on a pilot scale diesel flushing experiment as reported elsewhere ŽVan Stempvoort et al., 2001..
D.R. Van Stemp¨ oort, S. Lesage r Ad¨ ances in En¨ ironmental Research 6 (2002) 495᎐504
2.
3.
4.
5.
humic acid ŽHA. tend to be higher with the apparent solubility method compared to solid-phase microextraction ŽSPME.. This reflects differences in the conditions of the two tests: the apparent solubility tests have only one methylated naphthalene per batch, a residual non-aqueous phase, and a higher dissolved concentration Žsaturated in water. of each methylated naphthalene. Potential artifacts for both methods, as discussed in this paper, should be investigated further. An increase in inferred binding over a 12 day period was observed in the apparent solubility tests with 1-MN. In contrast, in the absence of residual non-aqueous phase, the SPME-GCrMS tests showed negligible delayed binding of methylated naphthalenes between 1 and 3 days. Based on the SPME-GCrMS analyses, the K OC,app values for binding of methylated naphthalenes to Aldrich HA in 3 grL HA were 35᎐50% of their values in 1 grl HA. In contrast, the K OC,app values were very similar at HA concentrations of 3 and 10 grl. These observations indicate that within the 1 to 10 grl HA range tested, there was no significant formation of HA micelles, which would have resulted in a dramatic increase in the solubilization of methylated naphthalenes. Although our data also indicate some differences in binding strength at 1 and 3 grl HA, the K OC,app values we obtained for methylated naphthalenes in concentrated Aldrich HA Ž1᎐10 grl. are similar Žwithin an order in magnitude. to predicted K OC values as a function of K OW . The latter are based on experiments with various non-ionic chemicals at low levels of Aldrich HA ŽEq. Ž3. of this paper, from Burkhard, 2000.. This result suggests that the binding of methylated naphthalenes to concentrated Aldrich HA ŽG 1 grl. follows the general pattern for binding of non-ionic chemicals to relatively dilute Aldrich HA Ž- 0.1 grl.. The batch and analysis techniques reported in this paper provide a relatively inexpensive and rapid way to quantify the binding of organic contaminants to concentrated humic acid solutions. These data can then be used in numerical simulations for the application of concentrated commercial humic acids as flushing agents in subsurface remediation.
Acknowledgements Laboratory assistance was provided by Susan Brown, Dan Banks and Tomas Chihula. Funding was provided by Environment Canada, the Program of Energy Research and Development ŽPERD. and the Centre for
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Poerschmann, J., Zhang, Z., Kopinke, F.D., Pawliszyn, J., 1997. Solid phase microextraction for determining the distribution of chemicals in aqueous matrices. Anal. Chem. 69, 597᎐600. Poerschmann, J., Kopinke, F.-D., Plugge, J., Georgi, A., 1999. Interaction of organic chemicals ŽPAH, PCB, triazines, nitroaromatics and organotin compounds. with dissolved humic organic matter. In: Ghabbour, E.A., Davies, G. ŽEds... Understanding Humic Substances, Advanced Methods, Properties and Applications. Roy. Soc. Chemistry, Cambridge, UK, pp. 223᎐240. Ramos, E.U., Meijer, S.N., Vaes, W.H.J., Verhaar, H.J.M., Hermens, J.L.M., 1998. Using solid-phase microextraction to determine partition coefficients to humic acids and bioavailable concentrations of hydrophobic chemicals. Environ. Sci. Technol. 32, 3430᎐3435. Rebhun, M., de Smedt, F., Rwetabula, J., 1996. Dissolved humic substances for remediation of sites contaminated by organic pollutants. Binding-desorption model predictions. Water Res. 30, 2027᎐2038. Schlautman, M.A., Morgan, J.J., 1993. Effects of aqueous chemistry on the binding of polycyclic aromatic hydrocarbons by dissolved humic materials. Environ. Sci. Technol. 27, 961᎐969. Thomas, D.H., Delfino, J.J., 1991. A gas chromatographicrchemical indicator approach to assessing groundwater contamination by petroleum products. Ground Water Monitor. Rev. Fall, 90᎐100. Van Stempvoort, D.R., Lesage, S., Novakowski, E.K., Millar, K. and Brown, S. 2001. Humic acid-enhanced remediation of an emplaced diesel source in groundwater: 1. Laboratory-based pilot scale test, J. Contam. Hydrol. Žaccepted.. Xing, B., Pignatello, J.J., 1997. Dual-mode sorption of lowpolarity compounds in glassy polyŽvinyl chloride. and soil organic matter. Environ. Sci. Technol. 31, 792᎐799. Xu, H., Lesage, S., Durham, L., 1994. The use of humic acids to enhance removal of aromatic hydrocarbons from contaminated aquifers. Proceedings, 4th Annual Symposium on Groundwater & Soil Remediation, Calgary, AB, Canada, September 21᎐23, 1994, 635᎐645. Dale Van Stemp¨ oort, Ph.D., is a Research Scientist with Environment Canada. Dale’s research focuses on groundwater chemistry and contaminant remediation. Previous positions were at the Saskatchewan Research Council, Saskatoon, and as a private sector consultant. He received his Ph.D. in Earth Sciences at the University of Waterloo in 1989. Suzanne Lesage, Ph.D., is chief of the Aquatic Ecosystem Remediation Project at the National Water Research Institute, Environment Canada, Burlington, Ontario, Canada. She is also an adjunct professor, Department of Earth Sciences, University of Waterloo. She has a B.Sc. in Biochemistry ŽUniversity of Ottawa. and a Ph.D. in Chemistry ŽMcGill University..