A solid-state fluorine-NMR study on hexafluorobenzene sorbed by sediments, polymers, and active carbon

In sediments, distinct fractions of compound desorbing at different rates are encountered. These fractions desorb rapidly (desorption rate constants similar 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 different sorbate fractions in sediment has been investigated. Sediments with hexafluorobenzene (HFB) at either rapidly or (slowly + very slowly) desorbing sites were subjected to solid-state MAS F-19 NMR measurements. Active carbon, glassy polystyrene, and rubbery polyacetal with sorbed HFB were also tested, to establish possible similarities between sediments and model sorbents. 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 resonance 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 fraction 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, respectively), 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 the sediment samples, polystyrene and active carbon also show two populations of sorbed HFB, whereas we observe only one population in polyacetal. In the active carbon sample one F-19 chemical shift clearly deviates from the ones in sediment (-176.6 ppm), whereas the other active carbon signal is similar to sediment (-125.5 ppm). The chemical shift of HFB in polyacetal is -167.2 ppm. The chemical shifts of the two fluorine signals of HFB in polystyrene are comparable to the ones for sediment(-163.9 and -125.5 ppm, respectively), supporting the hypothesis that sediment organic matter is a nanoporous material similar to a glassy polymer.