Two-step pyrolysis enabled by bentonite catalysis for the co-production of bioactive humic acid and functional carbon-clay composites.

Converting biomass waste into artificial humic substances (AHS) is critical for sustainable waste management and for mitigating soil organic matter depletion. However, conventional artificial humification routes typically couple depolymerization, condensation and oxidation, which limits yield, structural control and by-product utilization. Here we develop an atmosphere-programmed two-step pyrolysis strategy, assisted by bentonite catalysis, that decouples biomass depolymerization under an inert atmosphere from subsequent oxidative reconstruction under short-duration oxygen-limited conditions. The combined process affords 48.46% AHS, including 37.50% artificial humic acid (AHA) and 10.96% artificial fulvic acid (AFA). Structural characterization reveals controlled condensation with pronounced enrichment of acidic functional groups. Total acidity increases from 2.64 to 4.16 meq g-1, accompanied by enhanced aromaticity. The resulting AHA exhibits markedly improved bioactivity, increasing wheat root dry weight by more than 170%. Meanwhile, the post-extraction residue is valorized as a carbon-clay composite. This composite alleviates drought stress in chrysanthemum by enhancing antioxidant enzyme activities and scavenging reactive oxygen species. Overall, this work establishes an integrated pathway for the co-producing functional AHA and value-added composite materials from lignocellulosic biomass, with potential benefits for soil health improvement and plant stress tolerance.