ARS | Publication request: Qualitative analysis of volatile organic compounds on biochar

Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 30, 2011
Publication Date: October 20, 2011
Citation: Spokas, K.A., Novak, J.M., Stewart, C.E., Cantrell, K.B., Uchimiya, S.M., Dusaire, M.G., Ro, K.S. 2011. Qualitative analysis of volatile organic compounds on biochar. Chemosphere. 85(5):869-882.

Interpretive Summary: A potential abatement strategy to decrease atmospheric levels of carbon dioxide (CO2) is to sequester that CO2 into more stable forms. Biomass feed stocks are used to generate more stable carbon forms (e.g., biochar) that can be returned to soil. Biochar contain C forms capable of entering into a slower cycling pool, resulting in alterations in plant growth/yield and microbial soil processes. However, the mechanisms behind the “biochar effect” have not been fully explained. In the present work, we have determined that the sorbed volatile organic “fingerprints” produced from various plant and manure sources under different pyrolysis conditions, using gas chromatographic techniques. In general, we found that higher pyrolysis temperatures will lead to lower amounts of volatile organic components being sorbed to the biochar. We found that the production of volatile compounds is highly variable across different pyrolysis units, despite similarity in production temperature, feedstock and residency times. Understanding the chemistry of these sorbed volatiles could provide additional insight into how the soil and plant system will response after biochar additions. These results should be significant to farmers and policy makers and may assist scientists and engineers with developing improved biochars based on properties to minimize greenhouse gas implications while also improving soil carbon management.

Technical Abstract: Qualitative identification of sorbed volatile organic compounds (VOCs) on biochar was conducted by headspace thermal desorption coupled to capillary gas chromatographic-mass spectrometry. VOCs may have a mechanistic role influencing plant and microbial responses to biochar amendments, since VOCs can directly inhibit/stimulate microbial and plant processes. Over 70 biochars encompassing a variety of parent feedstocks and manufacturing processes were evaluated and were observed to possess diverse sorbed VOC composition. There were over 140 individual chemical compounds thermally desorbed from some biochars, with hydrothermal carbonization (HTC) and fast pyrolysis biochars typically possessing the greatest number of sorbed volatiles. In contrast, gasification, thermal or chemical processed biochars, soil kiln mound, and open pit biochars possessed low to non-detectable levels of VOCs. Slow pyrolysis biochars were highly variable in terms of their sorbed VOC content. There were no clear feedstock dependencies to the sorbed VOC composition, suggesting a stronger linkage with biochar production conditions coupled to post-production handling and processing. Lower pyrolytic temperatures (=350 oC) produced biochars with sorbed VOCs consisting of short carbon chain aldehydes, furans and ketones; elevated temperature biochars (>350 oC) typically were dominated by sorbed aromatic compounds and longer carbon chain hydrocarbons. The presene of oxygen during pyrolysis also reduced sorbed VOCs. These compositional results suggest that sorbed VOCs are highly variable and that their chemical dissimilarity could play a role in the wide variety of plant and soil microbial responses to biochar soil amendment noted in the literature. This variability in VOC composition may argue for VOC characterization before land application to predict possible agroecosystem effects.