Structure of biochars produced from hydrothermal carbonization and pyrolisis of organic waste and their potentiall use as N fertilizers

Poster presentado en el XVIII Meeting of the Spanish Society of Chromatography and Related Techniques (SECyTA 2018), held in Granada, October 2nd - 4th, 2018.

There is an urgent necessity of a sustainable management for the increasing quantity of organic waste such as sewage sludge (SS) or agricultural residues. Biochar (BC) is the carbonaceous solid residue produced through the thermochemical conversion (pyrolysis) of biomass under low O2 atmosphere [1]. BC can act as a soil conditioner by retaining nutrients and improving soil physical and biological properties. Furthermore, the use of BC for managing urban wastes and generating added value products is gradually being more recognized. Nevertheless, char properties are highly dependent upon feedstock and pyrolysis process. Thus, if the mechanisms involved in the transformation of biomass into BCs are known it would be possible to infer the process to functionalize the product according to the intended final use. This hypothesis has remarkable implications for the potential use of BCs as N-fertilizer and soil conditioner. De la Rosa and Knicker [2] demonstrated that a significant part of the organic nitrogen contained in pyrogenic organic matter (OM) from plant residues was bioavailable. A comparable behavior for chars produced from other N-rich sources like SS is expected. This study attempts i) understanding the reactions/mechanisms involved in the formation of BCs from organic waste, and ii) assessing the applicability of chars as N-rich soil ameliorants. Thus, biochar from SS and rice husks (RH) were produced by applying dry pyrolysis (PY-600 ºC; 20 min) and hydrothermal carbonization (HTC; 200 ºC, 30 min; p=10 psi). In addition, 13C and 15N-enriched SS was used to produce HTC and PY 13C and 15N labeled BCs (samples 13C-15N-SS-Py600 and 13C-15N-SS-HTC200). Total C and N content decreased during pyrolysis of SS, whereas maintained almost constant for RH. H/Cat ratios decreased (˂0.7) significantly for Py-chars pointing to a significant increase of the condensation degree. The isotopic signature of light elements (δ13C and δ15N) determined different turnovers and dynamics of biomass transformation during the pyrolysis process for HTC and PY processes. Analytical pyrolysis (Py-GC/MS) was able to discern OM transformations that occurred during the pyrolysis process (dry and hydrothermal) at a molecular level. The pyrogram obtained for SSHTC200 was similar than the original feedstock (SS). It was dominated by N-containing heterocycles, fatty acids and sterols. Dry PY at 600ºC removed the cholestanes but increased the abundance of polycyclic aromatic hydrocarbons. Finally, pyrolysis δ13C compound specific isotopic analysis (Py-CSIA) was used to monitor the dynamics of organic compounds during the pyrolysis process and to discern the origin of the newly formed structures.

[1] M. Paneque, JM. De la Rosa, et al. J. Anal. Appl. Pyrol. 128 (2017) 314-323. [2] JM. De la Rosa, H. Knicker. Soil Biol. Biochem. 43 (2011) 2368-2373.

MINECO and AEI/FEDER are thanked for funding the projects CGL2016-76498-R and CGL2015-64811-P

Peer Reviewed