G. Cornelissen et al., A solid-state fluorine-NMR study on hexafluorobenzene sorbed by sediments,polymers, and active carbon, ENV SCI TEC, 34(4), 2000, pp. 645-649
Citations number
28
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
In sediments, distinct fractions of compound desorbing at different rates a
re encountered. These fractions desorb rapidly (desorption rate constants s
imilar to 10(-1) h(-1)), slowly (similar to 10(-3) h(-1)), or very slowly (
10(-5)-10(-4) h(-1)). The source of dissimilarity between kinetically diffe
rent sorbate fractions in sediment has been investigated. Sediments with he
xafluorobenzene (HFB) at either rapidly or (slowly + very slowly) desorbing
sites were subjected to solid-state MAS F-19 NMR measurements. Active carb
on, glassy polystyrene, and rubbery polyacetal with sorbed HFB were also te
sted, to establish possible similarities between sediments and model sorben
ts. HFB in sediment at rapidly desorbing sites showed a resonance at -125.5
ppm relative to CFCl3, whereas HFB at slowly desorbing sites showed a reso
nance at -165.6 ppm. We did not observe an NMR signal for the HFB fraction
at very slowly desorbing sites, probably because the amounts of this fracti
on were too small. The observations indicate that the kinetically different
fractions are probably present in different (physicochemical) environments
in the sediment. The lines observed for both rapidly and slowly desorbing
HFB were relatively narrow (similar to 1.5 and similar to 3 ppm, respective
ly), indicating that the molecules in both fractions are sorbed in a fairly
homogeneous way. However, the exact explanation of the chemical shifts and
line widths in terms of sorption mechanisms remains unclear. Similar to th
e sediment samples, polystyrene and active carbon also show two populations
of sorbed HFB, whereas we observe only one population in polyacetal. In th
e active carbon sample one F-19 chemical shift clearly deviates from the on
es in sediment (-176.6 ppm), whereas the other active carbon signal is simi
lar to sediment (-125.5 ppm). The chemical shift of HFB in polyacetal is -1
67.2 ppm. The chemical shifts of the two fluorine signals of HFB in polysty
rene are comparable to the ones for sediment(-163.9 and -125.5 ppm, respect
ively), supporting the hypothesis that sediment organic matter is a nanopor
ous material similar to a glassy polymer.