Hydrothermal humification of lignocellulosic components: Who is doing what?

The humification behavior of lignocellulosic components (i.e., cellulose, hemicellulose and lignin) under acid-alkali two-step hydrothermal treatment was investigated with particular interest of revealing the contribution of single components as well as their interaction to hydrothermal humic acid (HHA). Lignin with its rich aromatic structures, had the highest HHA contribution (51.8 ± 1.0 wt%), while cellulose and hemicellulose only contributed 8.8 ± 1.6 wt% and 20.1 ± 4.4 wt% HHA yields, respectively. The conversion pathway of these components toward humification indicated that the hydrochar (HC) rather than the hydrothermal solution (HS) had higher humification capability under alkaline hydrothermal conditions because the hydrochar-derived HHA yields of 40.8 ± 2.0 wt% (cellulose), 66.2 ± 6.4 wt% (hemicellulose) and 55.4 ± 6.1 wt% (lignin) were achieved. In addition, co-hydrothermal treatment of cellulose, hemicellulose and lignin under this process improved the HHA production ascribing to the increased HC yield (23.1–53.1 %). The chemical characteristics of HHA products from lignocellulosic components (cellulose, hemicellulose, lignin) and their combinations were similar to the acknowledged natural humic acids. The hydrothermal humification of biowaste mainly passes through a top-down pathway (i.e., HHA forms from the oxidation of large fragments of hydrochar) as well as two bottom-up pathways (i.e., the soluble chemicals conduct the polymerization reaction for HHA production and react with the fragments from HC to synthesize the HHA). This study filled the lack of knowledge in the field of biowaste humification.