Organics in soda-anthraquinone black liquors from hot-water-extracted non-wood feedstocks

The chemical compositions of black liquors (BLs) obtained from the soda-anthraquinone (AQ) pulping of untreated and hot-water-extracted non-wood feedstocks (okra stalk, miscanthus stalk, and wheat straw) were determined. These under-utilized renewable and widely available feedstocks could provide a cheaper source for producing fiber and precursors for manufacturing green chemicals. These BLs were mainly characterized in terms of carbohydrate-derived volatile carboxylic acids (acetic and formic acids) and non-volatile carboxylic acids (hydroxy acids) as well as lignin. Additionally, in each case, the average molar mass and molar mass distribution of the dissolved lignin were measured. Results indicated typical effects of hot-water extraction; for example, in the case of hot-water-extracted feedstocks, compared to untreated feedstocks, more aliphatic carboxylic acids were formed, whereas in both cases the removal of lignin was similar. The maximum total amounts of aliphatic carboxylic acids were in the following order: okra/150 °C/P200 (39 g L−1) > miscanthus/150 °C/P200 (31 g L−1) > wheat/150 °C/P200 (15 g L−1). On the other hand, the total dissolved lignin was in the following order: wheat/150 °C/P200 (45 g L−1) > miscanthus/150 °C/P200 (35 g L−1) ≈ okra/140 °C/P50 (35 g L−1). The hot-water extraction also affected weight average molar mass (M¯w) values of the dissolved lignin in BLs. When the pre-treatment conditions became harsher, the M¯w values increased; the lowest value detected was 4160 g mol−1 for lignin in the okra BL from the untreated feedstock, and the highest value, 8730 g mol−1, was obtained for lignin in the wheat BL from pre-treated feedstock (150 °C/P200). All the data obtained are of importance when planning the overall strategy for recovering the non-wood-derived aliphatic carboxylic acids and lignin formed in this kind of integrated biorefinery approach.