Solubilization of hemicellulose and lignin from wheat straw through microwave-assisted alkali treatment

The saccharide- and lignin-releasing potential of microwave-assisted alkali pretreatment on wheat straw was examined using a dedicated laboratory microwave reactor with two magnetrons á 500W and magnetic stirring devices. 2–5 wt% NaOH solutions at a high solid loading of 10 wt% biomass and temperature–time settings of 60–140 °C and 10–60 min were tested. NaOH concentrations of at least 4–5 wt% (around 0.4 g/g biomass) were required to dissolve high amounts of hemicellulose (xylan) in the supernatant. A maximum xylan recovery of 73% could be achieved. More diluted NaOH solutions of 2 wt% yielded only about 30% xylan and led to high losses of saccharides. The average microwave energy input was calculated for the suspensions containing 10% wheat straw dry mass and ranged between 120 and 735 J/g dry mass depending on the severity of the treatment. The solubilization of lignin, estimated as reduction of acid insoluble lignin in the residue, was found to be highly dependent on the microwave energy input, while the NaOH load exhibited only minor effects on lignin solubilization within the test range. More than 80% of the hemicellulose and 90% of the lignin could be removed from the solid wheat straw matter without excessive saccharide degradation or solubilizing high amounts of cellulose. This makes microwave-assisted alkali pretreatment an effective method for the pretreatment of wheat straw.

► High yields of xylan in supernatant at short residence times.
► Xylan degradation at low NaOH load (2 wt%) in solution higher than at high NaOH loads.
► High NaOH loads enable high xylan yields without excessive lignin solubilization.
► Lignin solubilization strictly influenced by energy input but hardly by NaOH load.