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NMR Characterization of ¹³C-Benzene Sorbed to Natural and Prepared Charcoals

Ronald J. Smernik,*^† Rai S. Kookana,^‡ and Jan O. Skjemstad^‡

Soil and Land Systems, School of Earth and Environmental Sciences, The University of Adelaide, Waite Campus, Urrbrae, South Australia, 5064, Australia, and CSIRO, Land and Water, PMB 2, Glen Osmond, SA 5064 Australia

Environ. Sci. Technol., 2006, 40 (6), pp 1764–1769

DOI: 10.1021/es051895o

Publication Date (Web): February 11, 2006

Abstract

We investigated how the NMR properties of uniformly ¹³C-labeled benzene molecules are influenced by sorption to charcoals produced in the laboratory and collected from the field following wildfires. Uniformly ¹³C-labeled benzene was sorbed to two charcoals produced in the laboratory at 450 and 850 °C. The chemical shift of benzene sorbed to the higher-temperature charcoal was 5−6 ppm lower than that of benzene sorbed to the lower-temperature charcoal. This difference was attributed to stronger diamagnetic ring currents (which cause a shift to lower ppm values) in the more condensed or “graphitic” high-temperature charcoal. The chemical shift of benzene sorbed to two charcoals collected from the field following wildfires indicated a degree of charcoal graphitization intermediate between that of the two laboratory-prepared charcoals. Variable contact time and dipolar dephasing experiments showed that the molecular mobility of sorbed benzene molecules increased with increasing charcoal graphitization, and also increased with increasing benzene concentration. We propose that the chemical shift displacement of molecules sorbed to charcoal could be used to identify molecules sorbed to black carbon in heterogeneous matrixes such as soils and sediments, and to establish how condensed or “graphitic” the black carbon is.

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History

Published In Issue March 15, 2006
Received for review September 26, 2005
Revised manuscript received January 10, 2006
Accepted January 18, 2006

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Article

NMR Characterization of ¹³C-Benzene Sorbed to Natural and Prepared Charcoals