Paper No. 8
Presentation Time: 8:00 AM-6:00 PM
CARBON ISOTOPIC FRACTIONATION AS A FUNCTION OF CHARRING TEMPERATURE DURING THE PRODUCTION OF BIOCHAR
WERSHAW, Robert L.1, RUTHERFORD, David W.
2, ROSTAD, Colleen E.
2 and SAAD, Nabil M.R.
3, (1)Water Resources Discipline, National Research Program, US Geological Survey, Denver Federal Center, PO Box 25046, Building 95, MS 408, Denver, CO 80225, (2)National Research Program-Central Branch, U.S. Geological Survey, Box 25046, Building 95, MS 408, Denver Federal Center, Denver, CO 80225, (3)Picarro, Inc, 480 Oakmead Parkway, Sunnyvale, CA 94085, rwershaw@usgs.gov
The properties of biochars are likely to differ substantially from those of activated carbons because typically they are prepared at much lower temperatures and are being made from a wider variety of feedstocks. We have undertaken a systematic study of the physical and chemical properties of biochars in relation to charring temperatures (250
o to 900
o Celsius) and report carbon isotopic fractionation here, as part of this on-going study. Specifically, we show that carbon isotopic fractionation data provides information that can elucidate the biochar formation reaction mechanisms. Biochars were produced by heating 20 to 40 g samples of pine wood or switch grass in covered porcelain crucibles for eight hours at controlled temperatures in a preheated muffle furnace under an ultra-high purity nitrogen atmosphere.
13C/
12C isotopic ratios of aliquots of the biochars were measured by combusting the samples and measuring the
13C/
12C isotopic ratios of the resulting CO
2 in a Wavelength-Scanning Cavity Ring Down (WS-CRD) spectrometer.
13C concentration decreases in both the switch grass and pine wood biochars as formation temperature increase from 250o to 600oC. The 13C concentration then increases as charring temperature increases from 700o to 900oC. These changes are most likely due to the kinetic isotope effect. From 250o to 600oC, carbon atoms in the original cellulose, hemicellulose, and lignin structural components react to form graphene structures. Under these conditions, the 13C atoms will react slightly more slowly that the 12C atoms and therefore, there will be a depletion of 13C in the resulting graphene structures. At the higher temperatures increases in 13C concentrations in the chars indicate that the reverse is taking place and that the graphene structures are being degraded to volatile compounds that are depleted in 13C. These results are consistent with earlier reported results on the formation of aromatic carbons during initial charring followed by loss of aromatic carbon with prolonged charring.
