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| Titel |
Probing sorption selectivity of neutral organic compounds to Soil/Sediment Organic Matter through advanced Solid State NMR and nuclear paramagnetic relaxation probes |
| VerfasserIn |
Charisma Lattao, Yuan Li, Jingdong Mao, Mark Chappell, Lesley Miller, Albert Leo Dela Cruz, Joseph Pignatello |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046214
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| Zusammenfassung |
| One of the widely recognized functions of Soil/Sediment Organic Matter (SOM) is sorption
of organic pollutants. SOM-pollutant interaction influences contaminant bioavailability and
mobility, which in turn dictates fate and persistence of these contaminants in the environment.
Thus, a better understanding of the nature of SOM as well as pollutant retention mechanism
is essential in estimating or modeling soil and water systems contamination, pollutant risk
assessment and remediation designs. A molecular level understanding of SOM-pollutant
interaction is limited.
The main objective of this study is to probe at the molecular level, the question of whether
or not there are preferred sites of sorption of organic compounds to SOM. A novel approach
was employed, which involved the use of Solid State Nuclear Magnetic Resonance (NMR)
techniques and nuclear paramagnetic relaxation probes as sorbates. It takes advantage of the
spectral changes caused by the highly efficient relaxation afforded by these relaxation probes
near the site of sorption. As a result, line-broadening and reduction of spin-lattice relaxation
rate are expected.
Stable organic nitroxyl free radical relaxation probes of different polarities
namely, HTEMPO (1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine) and TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl) were used as models for organic compounds. A high
organic soil Pahokee Peat and a high organic soft coal Beulah Zap lignite were used as
sorbents. In addition, a polystyrene- polyvinylmethylether (PS/PVME) polymer blend was
synthesized and used as an example of an intimate blend of aromatic and aliphatic
microdomains. Sorption kinetics revealed that pseudo-equilibrium was reached within ~4
days for all sorbate-sorbent combination. Sorption isotherms were then constructed with an
equilibration time of at least 6 days, and with concentrations spanning at least two orders of
magnitude. The isotherms were corrected with respect to relaxation probe recovery in order
to account for the physisorbed molecules only. Freundlich sorption capacity (KF) and
nonlinearity values (N) for TEMPO were as follows: Pahokee Peat (KF=1.91, N=0.68);
Beulah Zap (KF=2.62, N=0.54); PS/PVME blend (KF=1.26, N=0.99). Sorption constants for
HTEMPO were as follows: Pahokee Peat (KF=1.20, N=0.75); Beulah Zap (KF=1.89,
N=0.68). After equilibration, samples were freeze dried and analyzed using 13C
Cross Polarization/Total Sideband Suppression (CP/TOSS) Solid State NMR and T2
filter. Peak suppression in terms of percent peak area or peak intensity reduction
for TEMPO and HTEMPO in Pahokee Peat and Beulah Zap reveals little to no
selectivity with the different SOM functional groups based on 13C chemical shift.
Electron paramagnetic resonance as well as partition coefficients of TEMPO and
HTEMPO between water and different organic compunds (i.e., n-octanol, anisole,
toluene, hexadecane, cellulose) was also determined in order to help explain this
phenomenon.
In conclusion, there is little to no selectivity of probes to SOM on the basis of
functional group chemistry. This contradicts previous findings on preferential sorption,
which were based on functional group correlations with sorption coefficients, i.e.,
macroscopic data. Furthermore, there seems to be no preference for sorption to
the small amount of black carbon present in these SOM samples represented by
aromatic region centered at 128 ppm. Thus, the implication of these results is that site
selectivity may be due to factors other than functional group composition. This work
demonstrates for the first time the use of molecular probes to study sorption specifity. |
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