Grantee Research Project Results
Final Report: Analysis of Halogenated Organic Particle-Scale Desorption via Column Studies and 13C Solid State NMR Spectroscopy
EPA Grant Number: R822626Title: Analysis of Halogenated Organic Particle-Scale Desorption via Column Studies and 13C Solid State NMR Spectroscopy
Investigators: Reinhard, Martin
Institution: Stanford University
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 1995 through August 1, 1998 (Extended to August 31, 2000)
Project Amount: $177,916
RFA: Exploratory Research - Chemistry and Physics of Water (1995) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Safer Chemicals
Objective:
The overall objectives of this project were to: (1) identify and quantify the factors that govern the slow desorption of halogenated organic compounds (HOCs), such as trichloroethylene (TCE) and perchloroethylene (PCE), from natural soils, sediments, and microporous model solids; and (2) develop and evaluate models that describe the process. The basic hypothesis tested was that slow contaminant desorption is due to the sorption and sterically hindered diffusion in micropores. Specific objectives included: (1) development and evaluation of thermodynamic and kinetic sorption models based on single compound isotherm data; and (2) evaluation of the applicability of the ideal adsorbed solution theory (IAST) to the binary desorption isotherms obtained with TCE and perchloroethylene (PCE).The experimental approach involved packing columns with moist soils, sediments, and a microporous silica gel, and equilibrating the sorbents with TCE vapor for weeks to months. The sorbents used were Norwood soil, a Livermore sand fraction, a Livermore clay and silt fraction, Santa Clara sediment, and a microporous silica gel, all at 100 percent relative humidity. Isotherms were developed in the desorption-mode by measuring the TCE vapor phase concentration following purging TCE from the column pore space and re-equilibration. Desorption rate data were developed isothermally at 15o, 30o, and 60o C and after a temperature-step. The latter methodology was referred to as temperature-stepped desorption (TSD). In TSD, following equilibration at 30o C and 17 hours of slow desorption, columns were heated instantaneously to 60o C. Solid-state 13C NMR allowed the selective identification of "bulk" phase TCE in silica gel but was not sufficiently sensitive to detect sorbed TCE. The results obtained in these studies are useful for assessing the transport of chlorinated solvents in the subsurface environment. Assessing ecotoxicological risks, and the bioavailability of sorbed contaminants. The data and concepts developed during this project will assist decision-makers to gauge the viability of enhanced thermal recovery methods with respect to slow desorption.
Summary/Accomplishments (Outputs/Outcomes):
First, the TCE isotherms were developed at 15, 30, and 60o C. It was observed that the isotherms consisted of two different regimes: one regime in which the data could be described with a Freundlich isotherm, and a second non-Freundlich regime. From these isotherms, isosteric heats of adsorption (Qst(q)) were calculated and examined with respect to adsorption on water wet mineral surfaces, sorption in amorphous organic matter (AOM), and adsorption in hydrophobic micropores. For the silica gel, values of Qst(q) (9.5-45 kJ/mol) in both Freundlich and non-Freundlich regions were consistent with adsorption in hydrophobic micropores. For the natural solids, values of Qst(q) in the Freundlich regions were less than or equal to zero and are consistent with sorption on water wet mineral surfaces and in AOM. In the non-Freundlich regions, diverging different temperature isotherms with decreasing q and a Qst(q) value of 34 kJ/mol for the clay & silt fraction suggest adsorption is occurring in hydrophobic micropores.Desorption data indicated that the TCE desorbs from the columns in two steps, a fast-eluting fraction and a slowly-eluting fraction. Modeling of the fast eluting TCE fraction shows that fast desorption was controlled by diffusion through water-filled mesopores. Rates predicted from diffusion and surface-barrier models were compared to slow isothermal and TSD rates. Diffusion model fits are superior to surface-barrier model fits in all cases. Slow diffusion coefficients and a high activation energy calculated from silica gel data (~34 kJ/mol) indicate that slow desorption is controlled by activated diffusion in micropores. Initial amounts of slow desorbing TCE did not affect these rates and were found to obey Polanyi's equation. The mass adsorbed in non-Freundlich isotherm regions, where micropores are hypothesized to control adsorption, was significantly less than the mass adsorbed at the onset of slow desorption, suggesting these pores are undulating in nature. TSD column results were consistent with a mechanism where slow diffusion rates are controlled by sorptive forces at hydrophobic micropore constrictions.
A model (the Distributed Dual Diffusion Model) that was developed under separate funding (R-824768) was experimentally verified as part of this study. The model attributes non-equilibrium sorption of moderately hydrophobic, volatile organic compounds to diffusion. The model differs from those of previous researchers in that it considers: (1) two distinct intra-granular rate-limiting diffusion processes occurring in series and at widely different time scales; and (2) the slow diffusion rates to be gamma distributed. The disparity of time scales of the two processes was used to approximate a boundary condition for the distributed diffusion equation, allowing it to be solved analytically. The slower diffusion process is attributed to activated diffusion through micropores.
TCE elution profiles for purged and unswept columns were presented and simulated with the Distributed Dual Diffusion Model (DDDM). Elution profiles were measured at 15, 22, 30, and 60oC for the different solids, all at 100% relative humidity. Advection and dispersion control TCE transport though the vapor phase in purged columns. Diffusion controls the TCE transport in unswept columns. For both swept and unswept columns, a fast and a slow desorbing fraction of the TCE were observed. The DDDM effectively simulated both of these fractions. For the fast fraction, the DDDM predicted desorption with no fitting parameters. For the slow fraction, the DDDM was not predictive but it simulated desorption using either a single (for silica gel) or gamma distribution (for soils and sediments) of micropore diffusion constant(s) and a micropore capacity factor. Micropore capacity factors obtained by fitting the DDDM were used to predict the onset of slow desorption in unswept columns of the same solid.
To investigate counter-diffusion in microporous sorbents, the rate of exchange between deuterated trichloroethylene (DTCE) in fast desorbing sites and non-deuterated TCE (HTCE) in slow desorbing sites was measured. Exchange rates were measured for a silica gel, Santa Clara sediment, and a Livermore clay/silt fraction, all at 100 percent relative humidity and 30 ?C. Initially; solids were packed into stainless steel columns and incubated with HTCE for 1?3 weeks. After incubation, HTCE was replaced with DTCE in fast desorbing sites. Next, columns were capped (i.e., sealed), and DTCE was allowed to exchange with HTCE in slow desorbing sites for 1, 3, or 30 days. Elution profiles were then measured to determine the extent of exchange that occurred while the columns were capped. Results from experiments conducted with different exchange times supported the hypothesis that slow sorption kinetics is controlled by diffusion in micropores. For the silica gel and the Santa Clara sediment, HTCE was incompletely exchanged with DTCE after 30 days (a time period that was sufficient for apparent equilibrium of a single sorbate). This indicated that the counter-diffusion rate of DTCE into HTCE-filled micropores was less than the diffusion rate of HTCE into micropores not filled with TCE.
The competitive desorption of trichloroethylene (TCE) and tetrachloroethylene (PCE) was studied using a silica gel, Norwood soil, and Santa Clara aquifer material, all at 100 percent relative humidity. Results indicated that the IAST was able to describe desorption isotherms for the silica gel. For the natural solids, IAST was not able to describe desorption isotherms for the full concentration range examined. Failure of the IAST was greatest for the most heterogeneous sorbent, even when considering multiple sorption domains. In addition, IAST predictions worsened as non-linear uptake mechanisms began to dominate. Several explanations for the failure of the IAST are possible, including the possibility that complex interactions between the sorbing solutes and the sorbent may exist, causing deviations from ideal sorption behavior.
Conclusions:
This research has evaluated different mechanisms of sorption and quantified the rate with which natural solids (soils, aquifer material) release HOCs based on macroscopic data. TCE was used as the model compound. Isotherms and kinetic desorption data was found to be consistent with HOC sorption in micropores. The frequently found slow desorption of HOCs was attributed to sorption in hydrophobic micropores. Isotherms measured over a very broad concentration range may not fit the conventionally used Freundlich model and may require using the General Adsorption Isotherm (GAI) with greater than one local isotherm. Thermodynamic data are consistent with sorption in amorphous organic matter or at mineral surfaces and in hydrophobic micropores. Modeling desorption data suggests that desorption is more likely controlled by sterically hindered diffusion rather than by a one-step surface-barrier process. Hindered diffusion was also shown to control the exchange between fast and slow sites. Sorption of multiple sorbates on natural soils and sediments is complex and does not follow the IAST, an issue that requires further research. A kinetic desorption model was evaluated (DDDM) that considers diffusion to occur via slow and fast diffusion in series. Fast desorption could be described based on a priori measurable parameters, whereas the parameters to predict slow desorption are not experimentally accessible. The significance of these findings merits further investigation, especially with respect to the problem of setting of environmentally acceptable clean up goals for soils and sediments.Journal Articles on this Report : 7 Displayed | Download in RIS Format
Other project views: | All 11 publications | 7 publications in selected types | All 7 journal articles |
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Type | Citation | ||
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Cunningham JA, Werth CJ, Reinhard M, Roberts PV. Effects of grain-scale mass transfer on the transport of volatile organics through sediments. 1. Model development. Water Resources Research 1997;33(12):2713-2726. |
R822626 (1999) R822626 (Final) |
not available |
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Luthy RG, Aiken GR, Brusseau ML, Cunningham SD, Gschwend PM, Pignatello JJ, Reinhard M, Traina S, Weber WJ, Westall JC. Sequestration of hydrophobic organic contaminants by geosorbents. Environmental Science & Technology 1997;31(12):3341-3347. |
R822626 (Final) |
not available |
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Schaefer CE, Schuth C, Werth CR, Reinhard M. Binary desorption isotherms of TCE and PCE from silica gel and natural solids. Environmental Science & Technology 2000;34(20):4341-4347. |
R822626 (1999) R822626 (Final) |
not available |
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Werth CJ, Cunningham JA, Roberts PV, Reinhard M. Effects of grain-scale mass transfer on the transport of volatile organics through sediments 2: Column results. Water Resources Research 1997;33(12):2727-2740. |
R822626 (1999) R822626 (Final) |
not available |
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Werth CJ, Reinhard M. Effects of temperature on trichloroethylene desorption from silica gel and natural sediments. 1. Isotherms. Environmental Science & Technology 1997;31(3):689-696. |
R822626 (1999) R822626 (Final) |
not available |
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Werth CJ, Reinhard M. Effects of temperature on trichloroethylene desorption from silica gel and natural sediments. 2. Kinetics. Environmental Science & Technology 1997;31(3):697-703. |
R822626 (1999) R822626 (Final) |
not available |
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Werth CJ, Reinhard M. Counter diffusion of isotopically labeled trichloroethylene in silica gel and geosorbent micropores: Column results. Environmental Science & Technology 1999;33(5):730-736. |
R822626 (1999) R822626 (Final) |
not available |
Supplemental Keywords:
competitive sorption, micropores, steric hindrance, diffusion, groundwater, soil, sediments, environmental chemistry, VOC, TCE, sorption, competitive desorption, modeling, PCE, remediation., Scientific Discipline, Toxics, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Contaminated Sediments, Environmental Chemistry, Physics, HAPS, Chemistry, Fate & Transport, Engineering, Chemistry, & Physics, fate and transport, mass spectrometry, zeolites, contaminated sediment, spectroscopic studies, VOCs, particle scale desorption, Trichloroethylene, chemical composition, chemical detection techniques, mass transfer, spectroscopy, chemical kinetics, column studiesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.