Multiple insights into the molecular heterogeneity of artificial humic acids derived from alkali-assisted hydrothermal humification of real-world biowastes

Artificial humic acids (AHAs) exhibit superior performance in enhancing crop yields and promoting environmental remediation. However, research into the compositional and property differences of AHAs derived from diverse real-world biowastes remains inadequate, thereby hindering their controlled preparation and practical applications. To address this gap, this study utilized a multifaceted analytical approach, notably incorporating electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), to systematically elucidate the molecular-level heterogeneity of AHAs from a specific set of real-world biowastes and link this diversity back to the composition of their precursors. AHAs produced from diverse precursors via hydrothermal humification consistently exhibited low yields and shared similarities in their functional groups and thermal stability. In contrast, fluorescence spectroscopy analysis revealed distinct fluorescent components and intensities, such as fulvic acid-like substances. ESI-FT-ICR-MS analysis revealed up to 2000 molecular compositional differences among the different AHAs, which were primarily attributed to CHON compounds. Lignin/CRAM-like compounds constituted the major components and were the primary determinants of AHAs heterogeneity. Compared with other AHAs, those derived from invasive plants presented greater heterogeneity, characterized by a lower molecular weight, reduced aromaticity, and a greater abundance and diversity of heteroatoms. Notably, substantial AHAs yields and significant molecular diversity were achieved even from precursors with low lignin, cellulose, and hemicellulose contents. This study provides a molecular-scale perspective on the similarities and differences in AHAs, offering new insights into their controlled synthesis and understanding their environmental behavior.