Molecular drivers governing the bioavailability of fermentation liquid carbon source: deciphering thermodynamics and molecular profiles of dissolved organic matter under pH regulation and thermal pretreatment.

Abstract

Anaerobic fermentation liquid from organic solid wastes is a promising low-cost carbon source, yet conventional indicators (e.g., VFAs) fail to accurately reflect its biological availability. This study elucidated the molecular drivers governing the thermodynamics and molecular profiles of dissolved organic matter (DOM) by coupling 500 Da membrane fractionation with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Analyses were performed across variable pH regulation (uncontrolled vs. pH 10) for different feedstocks (carbohydrate-rich vs. protein-rich). Alkaline fermentation of carbohydrate-rich feedstock promotes the release of highly oxidized small molecules, which significantly enhances VFAs production. For protein feedstock, the 500 Da membrane effectively rejected highly negative nominal oxidation state of carbon (NOSC) macromolecular skeletons, while enriching active high-NOSC molecules, driving the BOD5/COD ratio from 0.35 (SCOD1000) to 0.69 (SCOD500). It is worth noting that thermal pretreatment feedstock yield distinct thermodynamic pathways: carbohydrate-rich feedstock underwent caramelization induced dehydration condensation, forming aggregates with elevated modified aromaticity index (AImod) and the double bond equivalent (DBE), whereas protein-rich feedstock suffered intense reductive aliphatic polymerization via deamination, leading to a surge in molecular weight (MW) and a sharp reduction in NOSC to -1.05. Furthermore, a multidimensional cross-validation framework integrating molecular drivers factors (NOSC, AImod, DBE, MW), excitation-emission matrix fluorescence spectroscopy, and BOD5/COD ratio was proposed to fractionate the DOM into bioavailable and recalcitrant molecular pools. This work provides a novel thermodynamic and molecular-fingerprinting foundation for the targeted and efficient reuse of fermentation liquid carbon source.

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