Iron oxyhydroxide catalyzes production of artificial humic substances from waste biomass.

Production of artificial humic substances (AHS) from waste biomass will contribute to environmental protection and agricultural productivity. However, there is still a lack of a faster, more efficient and eco-friendly way for sustainable production. In this study, we proposed a method to accelerate the production of AHS from cotton stalks by mild pyrolysis and HO oxidation in only 4 hours, and investigated the formation of AHS during biomass transformation. We found that the process increased the aromatic matrix and facilitated biomass transformation by enhancing the depolymerization of lignin into micromolecular phenolics (e.g., guaiacol, p-ethyl guaiacol, etc.). The optimum conditions of pyrolysis at 250 °C and oxidation with 6 mL HO (5 wt%) yielded up to 19.28 ± 1.30 wt% artificial humic acid (AHA) from cotton stalks. In addition, we used iron oxyhydroxide (FeOOH) to catalyze biomass transformation and investigated the effect of FeOOH on the composition and properties of AHS. 1.5 wt% FeOOH promoted the increased content of artificial fulvic acid (AFA) in AHS from 10.1% to 26.5%, eventually improving the activity of AHS. FeOOH raised the content of oxygen-containing groups, such as carboxylic acids and aldehyde, and significantly increased polysaccharide (10.94%-18.95%) and protein (1.95%-2.18%) derivatives. Polymerization of amino acid analogs and many small-molecule carbohydrates (e.g., furans, aldehydes, ketones, and their derivatives) promoted AFA formation. Finally, carbon flow analysis and maize incubation tests confirmed that AHS were expected to achieve carbon emission reductions and reduce environmental pollution from fertilizers. This study provides a sustainable strategy for the accelerated production of AHS, which has important application value for waste biomass resource utilization.