|HAN, LANFANG - Beijing Normal University|
|SUN, KE - Beijing Normal University|
|SUN, HAORAN - Beijing Normal University|
|LIBRA, JUDY - Leibniz Institute|
|XING, BAOSHAN - University Of Massachusetts|
Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2017
Publication Date: 11/7/2017
Citation: Han, L., Ro, K.S., Sun, K., Sun, H., Libra, J.A., Xing, B. 2017. Oxidation resistance of biochars as a function of feedstock and pyrolysis condition. Science of the Total Environment. https://doi.org/10.1016/j.scitotenv.2017.11.014.
Interpretive Summary: Recently, scientists reported that biochar, a carbonaceous solid made from heating biomass without air, can be used to improve soil quality, remove pollutants from water and air, and increase soil carbon. However, when biochar is used to sequester carbon in soil, its stability is largely unknown. Researchers reported decadal to century time scales for the stability of biochar in soil. The stability of biochar depends on types of original biomass feedstock and the thermal processing conditions it has been gone through. In this study, we determined chemical stability of biochar made from different biomass and thermal processing conditions to infer the stability of biochar in soil. Pinewood, rice straw, swine solids, and chicken litter were used as feedstocks in making biochar. These carbonaceous biochars were subjected to chemical oxidation with hydrogen peroxide at various intensities and durations. Chemical structures of oxidized biochar were examined using various state-of-art analytical instruments. We found that the stability of biochar made from ash-poor feedstock (pinewood) was primarily affected by its aromaticity, i.e., fused carbon rings. The chemical stability of other biochars made from ash-rich feedstocks (rice straw, swine solids, and chicken litter) was determined by minerals, especially Silicon (Si). The findings of this study will be useful in designing biochar with desired stability.
Technical Abstract: This study investigated the chemical stability of a wide range of biochar samples (pyrochars from ash-poor (pine wood) and ash-rich feedstocks (rice straw, swine solids and poultry litter) at 250, 450 and 600 Celsius, and 250 °C hydrochars from swine solids and poultry litter) to short- and long-term oxidative degradation in acidic hydrogen peroxide. The results showed that organic carbon (OC) of the 250 Celsius pyrochars (P250) was mainly labile carbon (C). By contrast, hydrochars and 450 Celsius pyrochars (P450) from ash-rich feedstocks contained roughly 5.9-18.3% labile C, 43.2-56.5% semi-labile C, and 26.9-45.9% stable C. The OC loss trend of 600 Celsius pyrochars (P600) suggested that a fraction of aromatic C within P600 was easily oxidizable C. The amorphous C peak loss and the sharper graphitic C peak of P600 after being treated with high H2O2 concentration clearly suggested that the easily oxidizable aromatic C of P600 was amorphous, and the stable C of P600 mostly consisted of highly condensed aromatic C. Additionally, the OC loss of pyrochar from ash-poor feedstocks decreased with the increase of pyrolysis temperatures and aromaticity. However, this trend did not occur for biochar (both pyrochars and biochars) from ash-rich feedstocks, implying that the stability of biochars made from ash-rich feedstocks in soils may not only be influenced by aromaticity, but also by their mineral components. Mineral encapsulation was found to be regulated by pyrolysis conditions. Several characterization methods, including scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), X-ray photoelectron spectra (XPS), Raman and X-ray diffraction (XRD) showed that the surface of biochars from rice straw and poultry litter contained comparatively abundant silicon, and its speciation changed from amorphous to crystalline matter with increasing pyrolytic temperature. The amorphous silicon within hydrochar and P450 possibly interacted with OC, preventing OC from being oxidized, to some extent. Nevertheless, this type of chemical oxidation protection only occurred for hydrochar and P450, not for P250 and P600.