Structural and Thermal Investigation of Three Agricultural Biomasses Following Mild-NaOH Pretreatment to Increase Anaerobic Biodegradability

In this work mild-alkaline semi-solid state pretreatments (NaOH: 2; 4; 6 %, at 25 °C, TS 10 %) were applied to Arundo donax, Sorghum bicolour L. and wheat straw (Triticum aestivum, L.) to test their efficiency on anaerobic biodegradability. The simple sugars (glucose, xylose, arabinose) from structural carbohydrate and the lignin content were analyzed in raw treated samples. Besides to this their structural variation were assessed by means of FT-IR and thermal analysis. The recovery of glucose (1–2 %), xylose (0.4–2.5 %), and arabinose (0.1–8.7 %) in the alkaline eluate showed the treatment produced moderate hydrolysis of the structural components. This was confirmed by the moderate variation of glucose, xylose, arabinose and lignin content determined in the solid residue. On the contrary the FT-IR analysis showed the solid fraction of samples had intense variations in the cellulose and hemicellulose structure (mainly saponification of ester bounds), also confirmed by thermal analysis. The maximum degradation rate (−dx/dtmax; s−1) of cellulose and hemicellulose increased in both Arundo and Sorghum, indicating a structural cleavage. Straw showed moderately increased resilience of hemicellulose by small temperature shifts. A general decrease of the apparent activation energy (E a ) was found in all samples as an indication of decreased structural order favourable to enzymatic hydrolysis and the additional anaerobic biodegradability, as proved by the anaerobic tests in which A. donax showed the highest increase of biodegradability at NaOH 6 % (+30 %) > wheat straw (+22 %) > Sorghum (11 %).