Oxidation-alkaline-enhanced abiotic humification valorizes lignin-rich biogas digestate into artificial humic acids

This study introduces a cost-effective, mild thermal abiotic humification method for producing highly humified artificial humic acids (AHAs) from lignin-rich biogas digestate. Derived from the co-anaerobic digestion of cattle manure-wheat straw, the digestate slurry undergoes an MnO2-KOH-urea-enhanced humification reaction. Key process variables, such as MnO2 dose (0–15 mg/L), KOH dose (0–1 mol/L), urea dose (0–1.2 mol/L), reaction time (30–150 min), and temperature (25–85 °C), were systematically explored to optimize AHA yield, carboxylic acid content, and lignin removal. Optimal conditions, employing 7.69 mg/L of MnO2, 0.57 mol/L of KOH, and 0.63 mol/L of urea at 85 °C for 106 min, resulted in an impressive AHA yield of 32.15%, featuring a significant carboxylic acid content of 3.325 mmol/g. Under these conditions, up to 65% of lignin was effectively removed, accompanied by the release of orthophosphate up to 258 mg/L. The produced AHAs exhibited reduced toxicity, as demonstrated by substantial reductions of 54.13%, 57.14%, and 42.50% in phenols, furfural, and hydroxymethylfurfural, respectively. Notably, the AHAs displayed favorable characteristics, including a lower molecular weight (760.49 g/mol), diminished aromaticity (66.25% reduction), higher humification degree (lower C/N ratio of 8.79), increased oxidation degree (higher O/C ratio of 0.6), and elevated spectral index of humification (higher E4/E6 of 4.58). Analytical techniques, such as FT-IR, XPS, and TGA-MS, revealed chemical resemblance and enhanced functionality of AHAs compared to natural counterparts. Differentiation between AHA, lignin, and humification residues was confirmed through SEM-EDX, ICP-OES, and organic/inorganic carbon analyses. The obtained AHAs exhibit promising characteristics suitable for diverse applications in sustainable agriculture and environmental management.