Thermal degradation features of soil humic acid sub-fractions in pyrolytic treatment and their relation to molecular signatures

Fei Guo; Shuai Qin; Lei Xu; Yingchen Bai; Baoshan Xing

doi:10.1016/j.scitotenv.2020.142318

Thermal degradation features of soil humic acid sub-fractions in pyrolytic treatment and their relation to molecular signatures

Sci Total Environ. 2020 Dec 20:749:142318. doi: 10.1016/j.scitotenv.2020.142318. Epub 2020 Sep 9.

Authors

Fei Guo¹, Shuai Qin², Lei Xu², Yingchen Bai³, Baoshan Xing⁴

Affiliations

¹ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China; Guangdong Provincial Key Laboratory of Environmental Pollution and Health, School of Environment, Jinan University, Guangzhou 510632, China.
² State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China.
³ State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China. Electronic address: baiyc@craes.org.cn.
⁴ Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA.

PMID: 33370911
DOI: 10.1016/j.scitotenv.2020.142318

Abstract

Pyrolysis is a promising treatment for soil remediation for rapidity and fertility preservation. But it is difficult to establish the relationship between pyrolysis behaviors and soil organic matter (SOM) structures, for SOM is a mixture of heterogeneous compounds. HA sub-fractions from the same soil source may provide a series of promising objects to understand SOM at molecular level and the resulting patterns in SOM pyrolysis. We first propose a novel insight into pyrolysis mechanism response to molecular signatures using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with thermogravimetric analysis (TGA) to study six humic acid (HA) sub-fractions extracted from a forest soil. The findings indicate that decomposition of soil HA occurs systematically due to molecular signatures. The decomposition can be categorized as carboxyl controlled (below 280 °C), lipid-dominated (280-450 °C) and condensed aromatics-dominated processes (450-700 °C). Predominant reaction mechanism of all HA sub-fractions was random nucleation (α > 0.25). Lipid in HA tend to initiate multiple nuclei in thermal degradation, while condensed aromatics tend to initiate and grow centering single random point in higher conversion rate (α > 0.75). Bridging the molecular signature and thermogravimetry reveals that the pyrolysis stage below 350 °C should be divided into two distinct processes related to the carboxylic group and lipid compounds, although this stage has conventionally been considered as a single process. The N element of HA was mostly preserved in the condensed aromatics which was mainly pyrolyzed above 450 °C, suggesting that pyrolysis below 450 °C is a preferable remediation treatment considering nitrogen fertility preservation. The observed molecular-level pyrolysis patterns can be applied as a targeted remediation procedure for contaminated soils and can improve the understanding of SOM thermal behaviors at the molecular level.

Keywords: FT-ICR MS; Humic acid sub-fractions; Oil spill; Pyrolytic remediation; Soil; Thermal feature.