ENHANCING THE ECONOMIC LIABILITY OF BIOREFINERIES BY TRANSFORMING LIGNIN FROM LIGNOCELLULOSIC BIOMASSES INTO HUMIC-LIKE SUBSTANCES WITH PLANT PROMOTING BIOACTIVITY
In this study, lignin from three biomasses (miscanthus, giant reed and a pre-treated giant reed) has been isolated by two different methodologies and thoroughly characterized to produce humic-like substances, which were shown to positively affect plant growth and physiology. The techniques employed concerned the use of either sulfuric acid (SAL) or hydrogen peroxide at alkaline pH (Ox). Low yields were obtained by the Ox-method (less than 30% of Klason lignin), whereas the amount of materials coming from the SAL-extractive method was even greater than the Klason lignin, maybe due to condensation reaction between lignin and plant cell wall carbohydrates. An aqueous solubility test at pH 7 showed that a large depolymerization was obtained with the Ox-method, due to oxidation of isolated lignin material. This was also supported by elemental analyses, which showed that Ox samples had large oxygen content. Conversely, the lignin materials obtained with the SAL method were less than 6% water-soluble, thereby indicating lower depolymerization fragmentation. Thermal analyses and both infrared and nuclear magnetic resonance (NMR) spectroscopies confirmed the effectiveness of the two methods in separating cellulose from lignin. Size exclusion chromatography conducted on the Ox lignins from the three substrates, revealed nominal molecular sizes lower than 3000 Da for all substrates. Gas-chromatography Mass Spectrometry (GC-MS) analyses conducted on Ox-lignins extracted from raw miscanthus and giant reed biomasses, revealed a predominant abundance of guaiacyl-type molecule, though the small presence of sugars was also noted. Finally, different concentrations of aqueous solutions of lignins isolated by the Ox method from miscanthus and giant reed raw biomass were tested on the early stage of maize seedlings growth, in order to verify whether thse humic-like fragments displayed a plant biostimulation. Germination was not effected by any concentration of both lignin substrates, while root and shoot length were significantly enhanced at specific concentrations. These results are in accordance with some literature that indicated the bioativity of humic matter on plant growth parameters. These results suggest that water-soluble lignin fragments isolated from the lignocellulosic biomasses of this study exert hormone-like effects inasmuch as humic matter from soils of recycled biomasses. The employement of lignin residues from biorefineries in enhancing productivity of lignocellulosic biomasses in particular and agricultural crops in general appears to become an important contribution to the sustaiability of biorefineries
