Chapter 16
Landfill Leachate Effects on Transport of Organic Substances in Aquifer Materials
Downloaded by PENNSYLVANIA STATE UNIV on September 19, 2012 | http://pubs.acs.org Publication Date: May 13, 1992 | doi: 10.1021/bk-1992-0491.ch016
F. M . Pfeffer and C. G. Enfield Robert S. Kerr Environmental Research Laboratory, U.S. Environmental Protection Agency, Ada, OK 74820
The effect of dissolved organic carbon (DOC) in landfill leachate on the transport of a hydrophobic organic compound through saturated aquifer material was investigated. Leachate DOC was found to be complex; attempts to characterize the organic matrix were not successful. Two hydrophobic compounds of environmental significance were evaluated for 1) their partitioning to DOC in leachate, and 2) the effects of leachate on their partitioning to aquifer material. Results showed that the leachate reacts with aquifer material to increase partitioning to the stationary phase and that the DOC in the landfill leachate enhances mobility of the hydrophobic compounds. The conclusion was that at present, we do not know how to predict the impact of a given leachate on a given aquifer material without experimental measurements.
The transport of organic chemicals through saturated aquifer material is governed by many physical, chemical, and biological processes. The degree of organic chemical sorption, and therefore the retardation relative to the movement of water, is a function of solute hydrophobicity and the amount of sorbent hydrophobic phase. Properties that reflect chemical hydrophobicity, such as aqueous solubility (S), octanol: water partition coefficient ( K ) , and reverse-phase chromatographic retention time, have been successfully used to predict the magnitude of sorption ( 1 -6). In addition, the importance of soil organic carbon in hydrophobic sorption has been repeatedly demonstrated (7,8). Enfield et al. (9) questioned the assumption that chemicals in solution act independently of each other and demonstrated that dissolved organic carbon (DOC) in the fluid phase could increase the mobility of hydrophobic compounds, while Bouchard et al. (10) and Lee et al. (11 ) questioned the assumption of a stationary and unchanging hydrophobic phase and demonstrated that cationic surfactants could significantly increase the partitioning of neutral organics in an aquifer material. q w
This chapter not subject to U.S. copyright Published 1992 American Chemical Society
In Transport and Remediation of Subsurface Contaminants; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by PENNSYLVANIA STATE UNIV on September 19, 2012 | http://pubs.acs.org Publication Date: May 13, 1992 | doi: 10.1021/bk-1992-0491.ch016
16. PFEFFER & ENFIELD
Transport of Organic Substances
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Landfill leachate is a complicated mixture of organic chemicals which has been extremely difficult to characterize (12-14). We are proposing three conceptual mechanisms for the DOC in landfill leachate to affect the transport of hydrophobic compounds. First, the DOC in the leachate may increase the solubility of hydrophobic chemicals in the mobile phase where the DOC might behave like a cosolvent or may act like a surfactant partitioning hydrophobic compounds into a mobile organic phase. Second, the leachate may extract a portion of the soil's organic carbon, reducing the partitioning compared to that observed without leachate. Third, if polar or ionized organic material is in the leachate, the partition coefficient to the stationary (soil) phase may increase by replacement of inorganic ions with polar or ionizable organic compounds. To evaluate the significance of these potential mechanisms, three independent types of analyses were performed. First, a vapor phase transfer study was performed demonstrating the ability of the leachate to concentrate hydrophobic chemicals suggesting an increased potential for contaminant mobility. Second, batch sorption isotherms were measured. The soil in the batch studies was pretreated with either CaC^ or leachate. This was for the purpose of determining if there was an interaction between chemicals in the leachate and the soil. Finally, in anticipation of future studies, an attempt was made to produce a chemical fingerprint of the organic matrix in the leachate. This was to determine if changes in the matrix were occurring during the study. M
M
Experimental Section Materials and Instrumentation. The hydrophobic compounds studied were hexachlorobenzene (HCB) and benzo(a)pyrene (BaP). For HCB and BaP, the aqueous solubilities (S) are 6.0 χ 10" mgA and 3.8 χ 10" mgA, respectively, and the octanolrwater partition coefficients ( K ) are 2.60 χ 10 and 1.15 χ 10 , respectively (15). Because of the low solubilities of these study compounds it was necessary for purposes of quantitation in partitioning experiments to use C-labeled compounds obtained from Sigma. The activities of the compounds were: HCB=10.76mCi/mmol; BaP=5.0mCi/ mmol. Each C-labeled compound was initially dissolved in a very small amount of hexane and then diluted with methanol to make a spiking solution for the aqueous experiments. Three leachates were used in the study: one from Denmark collected in 1987 and two from Oklahoma. The Oklahoma samples were collected in 1985 and 1987 in Norman, Oklahoma from a well located in a closed municipal landfill. The Denmark leachate was preserved with mercuric chloride (0.2 g HgClJl) before shipment A l l leachates were stored at 4°C in glass containers with minimum neadspace. Leachate total organic carbon (TOC), reported as mgC/1, was measured with a Dohrman DC-80 analyzer. The leachate TOC values were: Denmark (355); Norman 1985 (200); Norman 1987 (142). The aquifer material was collected at a depth of 5 metersfroma location near Lula, Oklahoma. The material was air dried and sieved to < 2mm. The TOC of this material was 0.36 ± 0.08 g/kg (12 replicates) as measured with a Leco WR-12 analyzer. HCB and BaP were quantitated with a Beckman LS7800 liquid scintillation counter using 1 ml aqueous sample and 6 ml Beckman HP cocktail. Extractable organics were identified by GC/MS using a Finnigan 4600 system. Purgeable organics were identified by GC/MS using a Tekmar purge and trap system and a Finnigan 4500 system. PyrolysisGC/MS was performed using a CDS Pyroprobe 120 and a Finnigan Ion Trap Detector 700. Infrared spectra were obtained with a Bio-Rad FTS-45 using a diffuse reflectance 3
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6
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14
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In Transport and Remediation of Subsurface Contaminants; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
196
TRANSPORT AND REMEDIATION OF SUBSURFACE CONTAMINANTS
(DRIFT) accessory. A metals scan was performed using a Thermo Jarrell-Ash 975ICP spectrometer. Procedures. Experiments were conducted to measure the equilibrium partition coefficient Κ , defined as C / C , where C is the concentration of the study compound on DOC in the leachate and C is the corresponding aqueous concentration. A vapor phase transfer apparatus shown in Figure 1 was used, consisting of boiling flasks connected by an adapter. Leachate (50 ml) was placed in one flask and deionized water (50 ml) was placed in the other flask. Each flask was spiked at
